Physiol Rev
83: 731– 801, 2003; 10.1152/physrev.00029.2002.
The Calpain System
DARREL E. GOLL, VALERY F. THOMPSON, HONGQI LI, WEI WEI, AND JINYANG CONG
Muscle Biology Group, University of Arizona, Tucson, Arizona
I. Introduction
II. Background Information: The Well-Characterized Players, ␮-Calpain, m-Calpain, and Calpastatin
A. Properties of ␮- and m-calpain
B. Properties of calpastatin
C. Purification of the calpains and calpastatin and assay of their activity
III. Other Members of the Calpain Family: Calpain-like Molecules
A. Calpain-like molecules in invertebrates
B. Calpain-like molecules in vertebrates
C. Summary
D. Nomenclature
IV. Biochemical Properties and Regulation of Calpain Activity
A. Expression of proteolytically active molecules
B. Tissue localization
C. Regulation of calpain activity
V. Physiological Functions of the Calpain System
A. What do the transgenic mice show?
B. In vitro substrates
C. In vivo substrates and some proposed functions
VI. Pathological Implications of the Calpain System
VII. Summary and Conclusions
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Goll, Darrel E., Valery F. Thompson, Hongqi Li, Wei Wei, and Jinyang Cong. The Calpain System. Physiol Rev
83: 731– 801, 2003; 10.1152/physrev.00029.2002.—The calpain system originally comprised three molecules: two
Ca2⫹-dependent proteases, ␮-calpain and m-calpain, and a third polypeptide, calpastatin, whose only known
function is to inhibit the two calpains. Both ␮- and m-calpain are heterodimers containing an identical 28-kDa
subunit and an 80-kDa subunit that shares 55– 65% sequence homology between the two proteases. The crystallographic structure of m-calpain reveals six “domains” in the 80-kDa subunit: 1) a 19-amino acid NH2-terminal
sequence; 2) and 3) two domains that constitute the active site, IIa and IIb; 4) domain III; 5) an 18-amino acid
extended sequence linking domain III to domain IV; and 6) domain IV, which resembles the penta EF-hand family
of polypeptides. The single calpastatin gene can produce eight or more calpastatin polypeptides ranging from 17 to
85 kDa by use of different promoters and alternative splicing events. The physiological significance of these different
calpastatins is unclear, although all bind to three different places on the calpain molecule; binding to at least two of
the sites is Ca2⫹ dependent. Since 1989, cDNA cloning has identified 12 additional mRNAs in mammals that encode
polypeptides homologous to domains IIa and IIb of the 80-kDa subunit of ␮- and m-calpain, and calpain-like mRNAs
have been identified in other organisms. The molecules encoded by these mRNAs have not been isolated, so little
is known about their properties. How calpain activity is regulated in cells is still unclear, but the calpains ostensibly
participate in a variety of cellular processes including remodeling of cytoskeletal/membrane attachments, different
signal transduction pathways, and apoptosis. Deregulated calpain activity following loss of Ca2⫹ homeostasis results
in tissue damage in response to events such as myocardial infarcts, stroke, and brain trauma.
I. INTRODUCTION
A great deal of information has been obtained on
properties of the calpain system since purification in 1976
of the protein that was later named m-calpain (83, 84).
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Books dedicated to Ca2⫹-dependent proteases and the
calpains were published in 1990 (284) and 1999 (473), and
a monograph on calpain methods and protocols appeared
in 2000 (104). Two FASEB Summer Research Conferences devoted to the calpain system have been held: the
0031-9333/03 $15.00 Copyright © 2003 the American Physiological Society
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GOLL, THOMPSON, LI, WEI, AND CONG
first in Copper Mountain, CO in 1999 and the second in
2001 at Whitefish, MT. A web site that offers opportunities
for exchange of information and discussion has been
established: http://ag.arizona.edu/calpains/. A number of
shorter review articles on different aspects of the calpain
system have also appeared during the last 10 years (139,
196, 377, 418 – 420, 437, 438, 472 as examples of some of
these reviews). The state of the calpain system up to 1991
was summarized in a comprehensive review on the calpains in this journal (76). A number of important advances have occurred since this 1991 review, including 1)
cloning and successful expression of a proteolytically
active m-calpain from E. coli (142) or baculovirus (268)
expression systems; 2) crystallographic structures, first of
domain VI (39, 247) and then of the entire m-calpain
molecule in the absence of Ca2⫹ (171, 430); 3) identification of calpain-like Ca2⫹-activated proteases in invertebrate tissues and of a number of mRNAs encoding putative calpains, some of them evidently tissue specific (416,
417, 420, 438); and 4) generation of two knock-out mice
demonstrating that the calpain system is essential for life
(10) but that deletion of only one of the two “ubiquitous”
calpains, ␮-calpain, does not result in embryonic death
(14).
Despite this wealth of new information, two of the
primary questions concerning the calpain system remain
unanswered: 1) How is calpain activity regulated in living
cells? 2) What is the physiological function (or stated
differently, what are the physiological substrates) of the
calpain system in normal cells? The structural information obtained during the last 10 years has altered views as
to how calpain activity may be regulated in living cells and
has made it possible to propose specific structural
changes that must occur to convert a catalytically inactive
calpain molecule into an active proteolytic enzyme. On
the other hand, although information is accumulating rapidly, the physiological function(s) of the calpain system
remain poorly defined. This review summarizes the recent
structural findings and some of the evolving concepts
concerning regulation of calpain activity in cells and then
describes some of the wide variety of physiological functions in which the calpain system has been implicated.
The abundance of information on the calpain system
makes it impossible to completely review all facets of this
rapidly evolving area, and the review focuses on summarizing the structural information that has been firmly established and on those areas where the available information supports concrete concepts concerning regulation or
function. Areas where controversy still exists, where results must be interpreted cautiously, and where new information is needed will be indicated.
The review begins with two sections describing properties of the well-characterized ␮-calpain, m-calpain, and
calpastatin molecules and the identification and some
properties of the newest members of the calpain family,
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and follows with two sections on regulation and physiological function of the calpain system. The review ends
with a brief overview of some of the potential pathological involvements of the calpain system. A chapter describing the events that led to purification of m-calpain in 1976
and summarizing some of the questions regarding the
calpain system at that time was published in 1990 (135),
and readers should consult this chapter for information
on history of the calpain system.
II. BACKGROUND INFORMATION: THE WELLCHARACTERIZED PLAYERS ␮-CALPAIN,
m-CALPAIN, AND CALPASTATIN
Nomenclature of the calpain system will be discussed in section III after description of the recently identified “novel” or “atypical” calpains. The terms ␮-calpain
and m-calpain were first used in 1989 (62) to refer to the
micromolar Ca2⫹-requiring (␮-calpain) and millimolar
Ca2⫹-requiring (m-calpain) proteases, respectively, and it
has been recommended that these names be used to
distinguish these two calpains (434); ␮- and m-calpain are
sometimes referred to as the “ubiquitous,” “conventional,”
or “typical” calpains.
A. Properties of ␮- and m-Calpain
The calpains that have been isolated in a protein form
are Ca2⫹-dependent, cysteine proteases (Table 1). Calpastatin, the third well-characterized member of the calpain system, is a protein that specifically inhibits the
proteolytic activity of ␮- and m-calpain but of no other
proteases tested thus far. Some general properties of the
three well-characterized members of the calpain system
are summarized in Table 1.
1. cDNA sequences of ␮- and m-calpain
cDNAs encoding ␮-calpain and m-calpain have been
cloned and sequenced for human (433), monkey, mouse,
rat (91), bovine, porcine (411), rabbit (109, 111; partial),
and chicken (324) calpains, and a great deal is known
about the properties of these two proteins (Fig. 1). The
large subunit has a molecular mass near 80 kDa, with the
␮-calpain 80-kDa subunit usually being slightly larger than
the m-calpain 80-kDa subunit (81,889 Da compared with
79,900 Da for the human calpains for example; Refs. 8,
177). Also, molecular masses for the small subunit from a
number of species have ranged from 28,100 (381) to
28,235 Da (109). The amino acid sequences of calpains
from vertebrate species are highly conserved with over
90% homology among the mammalian calpains sequenced
thus far (91, 411, 433), and there is no evidence of any
alternative splicing during expression of the calpain
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THE CALPAIN SYSTEM
TABLE 1. Some general properties of the three
well-characterized members of the Ca2⫹-dependent
proteinase (calpain) system, ␮-calpain, m-calpain,
and calpastatin
Protein Type
[Ca2⫹] for Half-maximal Activity
Polypeptides
Properties
Proteases
␮-Calpain
m-Calpain
Inhibitor
Calpastatin
80, 28 kDa
80, 28 kDa
3–50 ␮M
400–800 ␮M
Variable, 46 kDa
70–76 kDa
87 kDa
Requires Ca2⫹ to bind to the
calpains; [Ca2⫹] requirement
depends on the calpain
Both calpains are Ca2⫹-dependent cysteine proteases with a pH
optimum of 7.2–8.2; calpastatin specifically inhibits the calpains and
does not inhibit any other protease that has been tested. Twelve
additional mRNAs encoding polypeptides having sequence
homology to the calpains have been identified in humans, many in
specific tissues, but with very few exceptions these polypeptides
have not been isolated thus far.
Occurrence
Found in all vertebrate cells that have been examined; calpain-like
proteases have been isolated from Drosophila and Schistosoma
mansoni, a blood fluke; mRNAs encoding polypeptides with
sequence homology and domain structures similar to the calpains
have been identified in Drosophila, C. elegans, and a number of
other organisms and as a transmembrane protein in plants; has not
been identified in bacteria.
Cellular distribution
Calpastatin and ␮- and m-calpain are located exclusively
intracellularly and seem to be associated primarily but not entirely
with subcellular organelles such as myofibrils in skeletal muscle,
the cytoskeleton in nonmuscle cells, vesicles, and the plasma
membrane.
genes. Hence, the large and small subunits for ␮- and
m-calpain have masses near 80 and 28 kDa, respectively,
at least among the vertebrate species, and in their native
form, the ␮- and m-calpain molecules are heterodimers.
Possible causes (for example, autolysis during purification) for the differences in molecular weights or subunit
composition of ␮- and m-calpain reported in some of the
early studies have been discussed in the earlier review in
this journal (76).
The 28-kDa subunit is identical in the ␮- and mcalpain molecules and is encoded by a single gene on
chromosome 19 in humans (326). The NH2-terminal region of this subunit, domain V (Fig. 1), is Gly rich; 40 of
the first 64 residues of this domain are Gly, and there are
stretches of 11 and 20 Gly residues in this region (all
numbers in this section are for the human calpains unless
otherwise noted). The first Gly is at residue 10 in the small
subunit, and residues 10 –20 are Gly. The stretch of 20 Gly
residues begins at residue 37 and ends at residue 56.
Domain V is often referred to as an hydrophobic domain,
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and it has been suggested that this domain binds phospholipid (180). Because Gly is near the center of the
hydrophobicity scale (103), however, the amino acid composition of domain V is not characteristic of a highly
hydrophobic domain, and recent studies have indicated
that domain III of the 80-kDa subunit binds phospholipid
(462) and has a role in interacting with cell membranes
(127). Of the 101 residues in domain V, 40 are Gly, 5 are
Pro, 30 are hydrophobic, and 26 are polar or charged. In
addition to the long stretches of Gly residues, a region
from residues 78 – 83 contains all five Pro residues
(PEPPPP), and a region between residues 91–97 contains
four Glu residues (EANESEE). Hence, the amino acid
sequence of domain V suggests an unordered structure
that could serve as a tether to other molecules or structures.
The COOH-terminal part of the 28-kDa subunit (residues 101–268 in Fig. 1), domain VI, is frequently termed a
calmodulin-like domain, because initial analyses of the
amino acid sequence of this domain suggested the presence of four EF hand Ca2⫹-binding sequences at residues
152–163, 182–193, 217–228, and 247–258 (325). The amino
acid sequence of domain VI is only marginally homologous to that of calmodulin, however (23% identity and 30%
similarity for the human molecules). Moreover, the X-ray
crystallographic structures of this subunit (39, 247) have
revealed the presence of a fifth Ca2⫹-binding site at residues 108 –119, thereby identifying the calpains as members of the penta-EF-hand family of proteins (261, 484).
Members of the penta-EF-hand family of proteins form
dimers that involve the fifth EF-hand and associate with
membranes (484), two properties that are also characteristic of ␮- and m-calpain. Subsequently in this review, the
five EF-hand sequences in the 28-kDa subunit will be
designated EF-1⬘, EF-2⬘, EF-3⬘, EF-4⬘ and EF-5⬘, although
EF-5⬘ and probably EF-4⬘ do not bind Ca2⫹ in the calpain
molecule.
Recently, cloning and expression studies have identified an intronless gene that is present in both human and
mice and that encodes a 248-amino acid polypeptide having a predicted molecular mass of 27,659 Da and 63%
amino acid sequence identity with small, 28-kDa subunit
of the calpains (393). This 248-amino acid polypeptide
differs from the classical 28-kDa small subunit in that it
does not have the two stretches of 11 and 20 Gly residues
in domain V that the 28-kDa subunit has, and it seems to
bind only weakly to the large, 80-kDa subunit in in vitro
assays (393). Coexpression of the 248-amino acid
polypeptide with the 80-kDa subunit of m-calpain, however, produces a proteolytically active enzyme having
⬃70% of the activity of an expressed m-calpain. As will be
described subsequently in this review, disruption of the
gene encoding the 28-kDa small subunit is embryonically
lethal in mice, even though the 248-amino acid polypep-
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GOLL, THOMPSON, LI, WEI, AND CONG
FIG. 1. Schematic diagram showing
the domain structure of human ␮- and
m-calpain as predicted by their amino acid
sequence and the domain structure of human m-calpain as determined from its
crystallographic structure. The six EFhand sequences in the 80-kDa subunit and
the five EF-and sequences in the 28-kDa
subunit are shown as vertical bars. The
crystallographic structure of m-calpain
shows that the sixth EF-hand predicted
from the amino acid sequence at the domain II/III boundary does not have an EFhand structure in the calpain molecule,
and it is unlikely that it binds Ca2⫹, at
least in m-calpain. Crystallographic analysis also indicates that the EF-hand at the
COOH-terminal ends of domains IV and VI
of the 28- and the 80-kDa subunits, respectively, is involved in association of the two
subunits and not in Ca2⫹ binding.
tide is expressed in these animals. Hence, the 248-amino
acid polypetide cannot substitute for the 28-kDa subunit
in cells, and the function of this polypeptide is presently
unclear.
The 80-kDa subunits of ␮- and m-calpain are different
gene products (genes on chromosomes 11 and 1, respectively, in the human; Ref. 326), but share 55– 65% sequence
homology within a given species (433). The 80-kDa subunits of ␮- and m-calpain were originally divided into four
domains on the basis of their amino acid sequence (433;
Fig. 1), but the recent crystallographic structure of mcalpain (171, 430; discussed in sect. IIA3) has shown that
the 80 kDa of this calpain has six “domains” (Fig. 1). Two
of these domains contain only 18 (the NH2-terminal domain; Fig. 1) or 17 (the linker domain; Fig. 1) amino acids,
and hence are not typical of domains as the term is
normally used. Because the crystallographic structure of
␮-calpain is not yet known, it is still unclear whether
␮-calpain also contains six domains similar to those in the
crystallographic structure of the Ca2⫹-free m-calpain molecule. In view of these uncertainties, the NH2-terminal 18
amino acids will be referred to as domain I and the 17
amino acids in the linker will be referred to as the linker
domain with the realization that this nomenclature may
change as additional information becomes available. This
nomenclature will be limited to discussion of m-calpain
and will be clearly designated as the “crystallographic
domain” structure to distinguish it from the domains predicted from amino acid sequence. The domain structure
based on amino acid sequence will be used in the following because it is available for both ␮- and m-calpain.
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A) DOMAIN I. The NH2-terminal domain has no sequence homology with any polypeptide sequenced thus
far; sequence homology of domain I among different species [human, chicken, rat, porcine, rabbit (partial)] is
72– 86%.
B) DOMAIN II. A domain with a Cys residue at position
115 (␮-calpain) or 105 (m-calpain) that is the active site
Cys and with a His residue at position 272 (␮-calpain) or
262 (m-calpain) and an Asn residue at position 296 (␮calpain) or 286 (m-calpain); these residues form a catalytic triad characteristic of cysteine proteases such as
papain or cathepsins B, L, or S. Domain II, however,
shares little sequence homology with these other cysteine
proteases, and it is likely that it evolved from a different
ancestral gene. Note that the active site Cys is in domain
IIa, whereas the His and Asn that constitute the remainder
of the catalytic triad are in domain IIb in the crystallographic structure of m-calpain (Fig. 1). Recent X-ray crystallographic results show that domains IIa and IIb each
bind one atom of Ca2⫹ in a peptide loop consisting of 8
(domain IIa) or 9 (domain IIb) amino acids (298). Sequence homology of domain II among different species is
high, ranging from 85 to 93%.
C) DOMAIN III. This domain has no sequence homology
with any polypeptide sequenced thus far. In addition to
linking the Ca2⫹-binding domains of the calpain molecule
to the catalytic domain (domain II), domain III may be
involved in binding phospholipids (462) and in regulating
calpain activity by its participation in critical electrostatic
interactions (171, 430). Analysis of the amino acid sequences indicates that this domain also contains two
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THE CALPAIN SYSTEM
potential EF-hand Ca2⫹-binding sequences, one at the
domain II/III boundary (residues 329 –341, for ␮-calpain;
residues 318 –338 for m-calpain; in domain IIb of the
crystallographic structure of m-calpain), and one at the
domain III/IV boundary (␮-calpain residues 554 –565; mcalpain residues 541–552; in domain IV of the crystallographic structure of m-calpain; shaded areas in Fig. 1).
The sequence at the domain II/III boundary does not have
an EF-hand conformation in the crystallographic structure of rat or human m-calpain, and this region does not
seem to bind Ca2⫹ in m-calpain. The EF-hand sequence at
the domain II/III boundary in the calpain isolated from
Schistosoma mansoni, however, binds Ca2⫹ (7; see sect.
IIIA1). Because the crystallographic structures of ␮-calpain and the other calpains that have been identified in
the past 10 years (see sect. III) are not yet available, it is
still unclear whether this sequence binds Ca2⫹ in these
other calpains. The EF-hand sequence at the domain II/III
boundary will be referred to as EF-IIb in this review with
the understanding that it may not bind Ca2⫹ in some
calpains..
D) DOMAIN IV. Like the sequence of domain VI, the
sequence of this domain is marginally homologous to
calmodulin (24 – 44% identity and 51–54% similarity for ␮or m-calpain) and contains four sets of sequences that
predict EF-hand Ca2⫹-binding sites (Fig. 1). In contrast to
the domain assignment based on amino acid sequence,
the crystallographic structure of m-calpain indicates that
domain IV of human m-calpain (430) begins at amino acid
residue 530 and thus includes the EF-hand at residues
541–552. Hence, domain IV of the 80-kDa subunit also has
five EF-hand sequences, with the fifth, COOH-terminal
EF-hand involved in dimerization of the 28- and 80-kDa
subunits. Subsequently in this review, the six EF-hand
sequences on the 80-kDa subunit will be referred to as
EF-IIb, EF-1, EF-2, EF-3, EF-4, and EF-5 from the NH2terminal to the COOH-terminal end of the 80-kDa subunit.
Sequence homology among the species ranges from 65 to
93% for this domain.
2. Genomic sequences of ␮- and m-calpain
Annotation of the genomic sequences for the human,
mouse, and other species whose genome has been/is being sequenced will soon make this information available
for all of the calpain polypeptides, including the recently
identified calpain-like molecules (see sect. III) from a number of species. Genomic DNA sequences for the 80-kDa
subunit of mouse ␮-calpain (28 exons, 21 kb; Ref. 14), the
80-kDa subunit of rat m-calpain (21 exons, ⬎33 kb), human calpain 3a (24 exons, 40 kb; Ref. 366), human calpain
4 (11 exons, 11 kb; Ref. 296), mouse calpain 8 (23 exons,
50 kb; Ref. 157), human calpain 10 (15 exons, 31 kb; Ref.
170), and the mouse calpain 12 (21 exons, 13 kb; Ref. 88)
have been determined. Studies of the promoter region of
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calpain genes have been done for only the chicken and
human m-calpain 80-kDa subunits (112, 155, 156) and the
human 28-kDa subunit (296). The human 28-kDa subunit
gene is ⬃11 kb and contains 11 exons. Exon 1 is a noncoding sequence, and translation starts at the 16th base in
exon 2. Each of the EF-2⬘, EF-3⬘, EF-4⬘, and EF-5⬘ sequences are encoded by one exon, exons 7, 8, 9, and 10,
respectively, but the EF-1⬘ sequence is split between exons 4 and 5. The positions of the intron/exon junctions are
identical in domain IV of the chicken m-calpain gene and
domain VI of the human 28-kDa gene. The 5⬘-upstream
region of the human 28-kDa gene lacks a TATA or CAAT
box sequence and is GC rich, containing three G-C boxes
(GGGCGG). Such 5⬘-upstream sequences are characteristic of “housekeeping” genes.
The gene for the chicken 80-kDa m-calpain subunit is
⬃10 kb and contains 11 exons (112). The 5⬘-upstream
sequence of both the chicken and the human 80-kDa
m-calpain subunit (only the 5⬘-upstream region of the
human gene was sequenced, so the size and number of
introns for the human 80-kDa m-calpain subunit gene are
unknown) also lack a TATA or CAAT box and are GC rich,
indicating that these genes also probably belong to the
“housekeeping” family of genes. CAT expression analysis
identified four negative elements in the 2,500 nt upstream
from the transcription start site of the human 80-kDa
m-calpain gene. Removal of these elements results in a
13-fold increase in CAT expression. All four negative enhancer elements respond to the same or similar cellular
trans-acting factor(s). The regions from nt ⫺202 to ⫺160
and from nt ⫺130 to ⫺80 contain two promoter elements.
Both the human 80-kDa m-calpain gene and the human
28-kDa gene contain AP-1 and Sp1 binding sites upstream
of the translation initiation site.
Incubation of HeLa cells with the tumor-promoting
phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA),
results in upregulation of expression of the m-calpain
80-kDa gene but has little or no effect on expression of the
genes for the ␮-calpain 80-kDa, the 28-kDa subunit, or
calpastatin (156). Assay of human 80-kDa m-calpain-CAT
constructs identified a cis-acting element in the ⫺202/⫺80
upstream region of the m-calpain gene. This region of the
human 80-kDa m-calpain gene contains a TGAATCA sequence at ⫺132, which is closely related to the collagenase TPA-responsive element. It has been suggested that
upregulation of both protein kinase C (PKC) and m-calpain expression by TPA is related to a role of m-calpain in
regulation of PKC activity in cells.
Limited sequence data (410) have not identified obvious consensus promoter elements in the 5⬘-untranslated
(UTR) region of the human ␮-calpain gene and has shown
that the 5⬘-UTR region of the human ␮-calpain gene is
highly homologous with the 5⬘-UTR regions of the bovine
(77% identity) and porcine ␮-calpain genes.
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GOLL, THOMPSON, LI, WEI, AND CONG
3. Crystallographic and solution structures
Crystallographic structural information on the calpains has become available in the past 5 years. First, the
crystallographic structures of expressed domain VI
polypeptides were solved to 2.3 Å (39) and 1.9 Å resolution (247). This was followed shortly afterward by the
crystallographic structure of expressed rat (171) or human (430) m-calpain at 2.6 and 2.3 Å, respectively.
The expressed domain VI polypeptides contained
amino acids Met-87/Ser-270 from the rat 28-kDa subunit
(39) or amino acids His-84/Ser-266 from the porcine 28kDa subunit (247; both expressed in Escherichia coli).
The structures obtained in the two studies were essentially identical; the domain VI polypeptide crystallized as
a homodimer with each monomer having five EF-hands.
EF-1⬘ (residues 108 –119) and EF-2⬘ (residues 152–163)
structures were paired and were in an “open” conformation, whereas the EF-3⬘/EF-4⬘ (residues 182–193 and 217–
228, respectively) pair was in a “closed” conformation.
The fifth EF-hand, EF-5⬘, was involved in forming the
interface between the two monomers in the homodimer.
Earlier studies (181, 320) indicated that the COOH-terminal ends of domains IV and VI (Fig. 1) were involved in the
noncovalent association of the 28- and 80-kDa subunits of
the calpain molecule, and the crystallographic structure
of the m-calpain molecule (171, 430) has now shown that
the fifth EF-hands of domains IV and VI are responsible
for this association (at least for m-calpain). Removal of
either 22 or 25 residues from the expressed domain VI,
i.e., the fifth EF-hand loop and the eighth ␣-helix, prevents
heterodimer formation (106, 294). The interactions between the two domain VI monomers in the crystal structure are principally hydrophobic and involve Ile-254, Val256, Ile-258, and interaction of Trp-261, Leu-262, Leu-264,
Met-266, and Tyr-267 on one polypeptide with Phe-243,
Phe-240, Met-239, and Leu-236 on the second polypeptide.
Interestingly, the same residues with only a few conservative substitutions are also present in corresponding
positions of the COOH terminus of domain IV from the
80-kDa subunits of both ␮- and m-calpain (Table 2). That
these residues are highly conserved in the calpains that
have been sequenced thus far (Table 2) indicates their
importance in association of the 28- and 80-kDa subunits
in both ␮- and m-calpain.
The EF-1⬘, EF-2⬘, and EF-3⬘ sites all contained a Ca2⫹
atom when crystallization was done at “low” Ca2⫹ (1 mM;
Ref. 39), whereas the EF-4⬘ site contained a Ca2⫹ atom
only when crystallization was done at higher Ca2⫹ concentrations (20 mM; Ref. 247; or 200 mM; Ref. 39). Moreover, the Ca2⫹ atom in the EF-4⬘ site was not located in
the loop of the EF-hand but rather at the COOH-terminal
end of the loop near the NH2 terminus of the seventh
␣-helix in this domain. Soaking crystals in ytterbium resulted in replacement of the Ca2⫹ atom in the EF-4⬘ site,
Physiol Rev • VOL
TABLE 2. Comparison of those amino acid residues that
are involved in dimerization of domain VI of the
28-kDa subunit with corresponding residues on the
80-kDa subunits of ␮- and m-calpain
Amino Acids Identified
in Dimerization
of Domain VI Crystalsa
Putative Corresponding
Amino Acids on
␮-Calpain 80-kDa Subunit
Putative Corresponding
Amino Acids on
m-Calpain 80-kDa Subunit
Leu-236
Met-239
Phe-240
Phe-243
Trp-261
Leu-262
Leu-264
Met-266
Tyr-267
Leu-682
Met-685
Phe-686
Phe-689
Trp-707
Leu-708
Leu-710
Met-712
Phe-713
Leu-669
Leu-672
Phe-673b
Phe-676
Trp-694
Leu-695
Phe-697c
Val-699d
Leu-700e
Except where otherwise noted, the amino acid residues for ␮-calpain are conserved among eight species (human, monkey, rabbit, bovine, pig, mouse, rat, and chicken) and those for m-calpain are conserved among seven species (human, monkey, rabbit, mouse, rat,
a
chicken, and frog).
From the crystal structures of domain VI
b
dimers from rat (39) or porcine (247).
Conservative substitution of
c
Tyr for Phe in the chicken.
Conservative substitution of Ala for Phe
d
in the frog.
Conservative substitution of Leu for Val in the
e
frog.
Conservative substitution of Val for Leu in the frog.
supporting the suggestion that this site does not bind
Ca2⫹ with an affinity as high as the EF-1⬘, EF-2⬘, and EF-3⬘
sites do (39). Ca2⫹-binding properties of the complete
calpain molecules are discussed in section IVC2.
Comparison of structures obtained in the presence
(Ca2⫹ in EF-1⬘, EF-2⬘, and EF-3⬘) or in the absence of
Ca2⫹ indicated that the Ca2⫹-induced structural changes
in domain VI crystals are very small. The largest Ca2⫹induced structural changes occur in the EF-1⬘ region of
the molecule (residues 98 –115, Ref. 39). For reasons that
presently are unclear, stability of the heterodimeric mcalpain molecule is reduced when the 28-kDa subunit of
the expressed molecule has been truncated up to residue
115 (removing the EF-1⬘ hand); possibly, removal of this
region of the domain VI polypeptide alters folding of the
remainder of the polypeptide (106).
Consequently, the crystallographic structures of domain VI suggest that binding of Ca2⫹ to the EF-1⬘, EF-2⬘,
and EF-3⬘ sites in this domain results in only a small
structural change at the EF-1⬘ region of this domain. If
Ca2⫹ binding to the EF-1, EF-2, and EF-3 sites in domain
IV results in a similar pattern of conformational changes,
and if Ca2⫹ binding to domains IV and VI is the trigger for
Ca2⫹-induced proteolytic activity of the calpains, then the
small conformational changes in the NH2-terminal region
of domain IV would need to be transmitted through domain III to the catalytic domain II (Fig. 1). This use of a
“lever” to amplify a small conformational change is reminiscent of the myosin head where small structural
changes associated with binding of ATP are amplified
through the “stalk” of the myosin head to produce a
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THE CALPAIN SYSTEM
movement of ⬃10 nm relative to the actin filament (497).
It is still unknown whether such an amplification occurs
upon Ca2⫹ binding to the calpains. Ca2⫹ binding to domains IV and VI also weakens the affinity of these two
subunits for each other, causing dissociation of the 28and 80-kDa subunits (see sect. IVC6). An inhibitor of the
calpains that does not bind to the active site and that has
an inhibition constant (Ki) of 0.3 ␮M (an ␣-mercaptoacrylate derivative) was shown to bind to a hydrophobic
pocket between helices B and D (2nd and 4th helices) in
domain VI. Inhibitor binding caused minimal conformational changes in the crystal structure of this domain
(247). It is possible that binding of the inhibitor “locks”
domain VI (and domain IV) in a structure that prevents
the small conformational changes in the NH2-terminal
region of this domain and thereby prevents transmission
and amplification of this signal to the remainder of the
molecule. Additional studies showing whether Ca2⫹ in the
presence of this inhibitor can elicit the large conformational changes that have been observed in ␮- and mcalpain in solution (later in this section) would confirm or
refute this possibility.
The X-ray crystallographic structure of rat (171) or
human (430) m-calpain that had been expressed in an E.
coli (171) or a baculovirus (430) expression system has
been solved to 2.6 or 2.3 Å, respectively (Fig. 2). The two
structures, which are in a Ca2⫹-free state, are essentially
identical. The 28-kDa subunit of rat m-calpain was truncated at amino acid 85 to facilitate high levels of expression in E. coli, but the first 85 amino acids of the small
subunit were not represented by clear electron density in
the X-ray diffraction pattern of the human m-calpain anyway. Hence, neither of the structures contains domain V.
Attempts to obtain the crystallographic structure of ␮-calpain have not yet been successful, although the similarity
in amino acid sequence suggests that the two structures
will be similar, and the structure of ␮-calpain has been
modeled on the basis of these similarities (365). m-Calpain is an elongated molecule with dimensions of ⬃100 ⫻
60 ⫻ 50 Å. Hydrodynamic measurements had earlier indicated that the calpain molecules were ellipsoidal (101),
but the hydrodynamic estimates of 20 ⫻ 76 Å were
smaller than those obtained from the X-ray structure. The
crystal structure suggests that the 80 kDa of m-calpain has
six “domains” (or, as discussed previously, four domains,
a NH2-terminal sequence and a linker) rather than the four
domains predicted from amino acid sequence (Fig. 1). The
crystallographic domain I (NH2-terminal sequence) is
short, 19 amino acids, and makes contact with domain VI
but not with any other parts of the calpain molecule.
Domain II, the catalytic domain, is divided in the crystallographic structure into two domains, domain IIa and
domain IIb, that contain the Cys and the His/Asn residues,
respectively, that constitute the catalytic triad of the calpains. The crystallographic domain III is shorter than the
Physiol Rev • VOL
737
FIG. 2. Crystallographic structure of human m-calpain. The domains, in different colors, are labeled dI, dIIa, dIIb, dIII, dIV, dV, and dVI.
I-II is the ␣-helix linking domains I and IIa. The linker “domain” is a red
line running from the gap between dIII and dIV to the bottom right of the
diagram and is labeled III-IV. Locations of EF-1, EF-2, and EF-3 hands in
domains IV and VI are indicated. The active site Cys, C105, is in gray as
are His-262, Asn-286, and Trp-288 at the top of domain IIb. [Adapted from
Reverter et al. (364).]
domain III predicted from the amino acid sequence and
ends with a stretch of 18 amino acids that form a “linker”
with domain IV. The structure of domain IV is very similar
to the structures of domain VI published previously (39,
247); indeed, the structure of domain IV in the human
m-calpain resembles the Ca2⫹-bound structure of domain
VI more closely than the Ca2⫹-free domain VI (430).
The crystallographic structure of m-calpain leads to
several important conclusions (these conclusions strictly
pertain only to the Ca2⫹-free form of m-calpain).
1) The active site, which is at the top of the structure
in Figure 2, is not sterically blocked by domain I (the
NH2-terminal, 18-amino acid domain) as has been widely
believed. Hence, removal of domain I by autolysis (see
sect. IIA4) does not activate a proenzyme by removal of a
peptide that blocks access to the active site as autolytic
activation of other proenzymes does.
2) The structure of the Ca2⫹-free enzyme shows that
the enzyme is catalytically inactive because the active site
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GOLL, THOMPSON, LI, WEI, AND CONG
Cys is 10.5 Å away from the His and Asn residues, a
distance too great to allow formation of a catalytically
functional complex. Hence, Ca2⫹ must induce a conformational change in m-calpain that reduces this distance
from 10.5 to ⬃3.7 Å, so that Cys-105 can interact effectively with His-262 and Asn-286 to form a catalytic triad.
This conformational change likely involves a rotation of
Trp-288 in domain IIb (Fig. 2). Earlier studies indicated
that mutation of Trp-288 to a Tyr residue reduced proteolytic activity of the mutated calpain to 5.5% of its activity
before mutation (11).
3) The autolytic cleavage of domain I at amino acids
19 –20 occurs at a point that is 40 Å away from the
catalytic site in the Ca2⫹-free enzyme, so this autolysis
likely is not an intramolecular event.
4) The crystallographic structure of the m-calpain
molecule could not be done in the presence of Ca2⫹, but
the very small conformational change resulting from binding of Ca2⫹ to domain VI suggests that the “Ca2⫹ switch”
for initiation of proteolytic activity may involve Ca2⫹
binding at some site in addition to or other than domains
IV and VI, the calmodulin-like domains. A group of acidic
residues, Glu-392, Glu-393, Glu-394, Asp-395, Glu-396,
Asp-397, Glu-398, Glu-399, and Glu-401, are highly conserved in the calpains, and some of these acidic residues
form salt bridges with Lys-226, Lys-230, Lys-234 and Lys354, Lys-355, Lys-357 in the crystallographic structure of
m-calpain (Fig. 2). Ca2⫹ would disrupt these salt bridges,
and this disruption could permit domain IIb to move
toward domain IIa, leading to formation of the catalytic
triad (430). Although this electrostatic mechanism
“switch” may have an important role in the Ca2⫹-induced
catalytic activity of the calpains (171), recent studies
(298) have shown that expressed domain IIa/IIb from
␮-calpain can each bind one atom of Ca2⫹ in a peptide
loop and that this binding induces a conformational
change that brings the catalytic residues in the expressed
domain IIa/IIb within 3.7 Å of each other. Hence, at least
part of the Ca2⫹ switch for activating the calpains may
reside in the catalytic domain itself (see sect. IVC6).
In the absence of structural information on the ␮- and
m-calpain molecules in the presence of Ca2⫹, limited
proteolytic hydrolysis with trypsin or chymotrypsin has
been used to compare the structures of the complete
heterodimeric calpain molecules in the presence or the
absence of Ca2⫹ in solution (297, 459). Moldoveanu et al.
(297) used an expressed m-calpain that had the active site
Cys mutated to Ser to prevent autolysis in the presence of
Ca2⫹, whereas Thompson et al. (459) used purified preparations of both ␮- and m-calpain and sodium tetrathionate oxidation (199) to reversibly inactivate calpains and
prevent their autolysis in the presence of Ca2⫹. The two
studies produced similar results.
1) Either trypsin or chymotrypsin digestion in the
absence of Ca2⫹ for periods as long as 120 min produces
Physiol Rev • VOL
a limited number of polypeptide fragments, suggesting
that both the ␮-calpain and m-calpain molecules have a
compact structure that limits the number of peptide
bonds accessible to proteolytic enzymes in the absence of
Ca2⫹. Moreover, similar proteolytic fragments were produced by these two proteases, which differ in their subsite specificity. Both trypsin and chymotrypsin rapidly
cleave either ␮- or m-calpain at a region near the COOHterminal end of domain II or IIb in the crystallographic
structure (residues 245 or 266 in ␮-calpain; residue 265 in
m-calpain) and at several sites in domain III (residues 363
or 472 in ␮-calpain; residues 383, 400, and 503 in mcalpain). Domain I is resistant to trypsin digestion for up
to 120 min in the absence of Ca2⫹, although chymotrypsin
removes small segments (6 amino acids from ␮-calpain; 9
amino acids from m-calpain) from the NH2 terminus of
domain I; the remainder of domain I and domain II is
resistant to tryptic or chymotryptic cleavage for 120 min,
suggesting that this part of the molecule (domain IIa in
the crystallographic structure) is in a compact conformation in the absence of Ca2⫹.
2) The COOH-terminal regions of domain III and all
of domain IV in either ␮- or m-calpain also are resistant to
proteolytic degradation, suggesting that these regions of
the calpain molecule are also in a compact conformation.
3) Both tryptic or chymotryptic treatment cleaved
the 28-kDa subunit of either ␮- or m-calpain at residues
59 – 61 and 85– 88 within 10 –20 min, leaving a 24-kDa or a
20-kDa fragment that was not degraded for 120 min.
4) The pattern of proteolytic digestion of ␮- and
m-calpain by trypsin or chymotrypsin changes significantly in the presence of 1 mM Ca2⫹, a cation that does
not directly affect trypsin or chymotrypsin activity. Both
␮- and m-calpain that had been proteolytically inactivated
were rapidly degraded within 5 min of incubation by
either trypsin or chymotrypsin to small polypeptides.
Only domains IV and VI remained after 30 min of digestion
in the presence of Ca2⫹. Indeed, the proteolytic fragments
of the 28-kDa subunit are identical whether proteolytic
digestion is done in the presence or the absence of Ca2⫹.
Chymotryptic digestion of either ␮- or m-calpain in the
presence of Ca2⫹ results in only one major degradation
product from the 80-kDa subunit; the NH2 terminus of this
fragment begins at residue 515 (␮-calpain) or 503 (mcalpain). Therefore, Ca2⫹ at the concentration of 1 mM
causes a substantial change in the conformation of the
calpain molecules and “opens” the molecule to make it
more susceptible to proteolytic degradation. Only the
conformations of domains IV and VI do not seem to be
significantly altered by Ca2⫹, a conclusion that was also
reached in the crystallographic studies of domain VI (39,
247).
In sum, the currently available evidence indicates
that Ca2⫹ binds to multiple sites on both the ␮- and
m-calpain molecules and that not all of these sites are in
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THE CALPAIN SYSTEM
domains IV and VI, the penta-EF hand (“calmodulin-like”)
domains. Because Ca2⫹ binding to domain VI (crystallographic evidence) and to domain IV (evidence from limited proteolysis) causes only very small conformational
changes in these domains, it seems likely that Ca2⫹ binding to regions other than or in addition to these two
domains is involved in the “switch” that initiates proteolytic activity in the calpains. It is clear that Ca2⫹ binding
to either ␮- or m-calpain causes significant conformational changes in the calpain molecules and that these
changes involve a “loosening” or partial unfolding of domain II (IIa, IIb) and the NH2-terminal part of domain III,
making these domains that were previously resistant to
proteolytic cleavage now highly susceptible. The exact
nature of these changes is unknown, but they must include a moving together of critical residues in domains IIa
and IIb to form a functional catalytic triad. Although the
structural information that has become available during
the last four to five years has changed the prevailing views
as to how Ca2⫹ initiates proteolytic activity of the calpains, the regions in addition to those in domains IV and
VI in the calpain molecule that bind Ca2⫹ and how this
binding affects conformation of the calpains in a way that
results in proteolytic activity remain unclear. Finally, it
should be remembered that these Ca2⫹ binding events
must occur at physiological Ca2⫹ concentrations, which
are in the range of 50 –300 nM (151, 192, 266). The Ca2⫹
binding properties of the calpains are discussed in section
IVC2.
4. Autolysis and the proenzyme question
It was discovered in 1981 that m-calpain autolyzes
rapidly in the presence of Ca2⫹ (439, 440), and it is now
known that both ␮- and m-calpain autolyze when incubated with Ca2⫹ (62, 81, 101, 373, 376). Although it is not
uncommon for proteolytic enzymes to autolyze, autolysis
of the calpains has several unique features (autoproteolysis may be grammatically more accurate than autolysis,
but the briefer autolysis will be used here with the understanding that the autolytic events are proteolytic). Brief
autolysis, up to 2–3 min at 25°C and in the presence of 1
mM or greater Ca2⫹, reduces the Ca2⫹ concentration
required for half-maximal proteolytic activity of ␮-calpain
from 3–50 to 0.5–2.0 ␮M and that of m-calpain from
400 – 800 to 50 –150 ␮M (137; Table 3) without affecting
the specific activity of either enzyme (101). This same
autolysis reduces mass of the 80-kDa subunit of ␮-calpain
to 76 kDa, mass of the 80-kDa subunit of m-calpain to 78
kDa, and mass of the 28-kDa subunit common to ␮- and
m-calpain to 18 kDa (SDS-PAGE; actual mass of this
fragment is 20.5 kDa; Ref. 142). Amino acid sequencing
studies show that the NH2-terminal 27 or 18 (␮- or mcalpain) amino acids are removed from the 80-kDa subunit and that the NH2-terminal 91 amino acids (hence, the
Physiol Rev • VOL
3. Estimates of Ca2⫹ concentrations required for
different properties of the calpains
TABLE
Calpain Property
Proteolytic activity
Binding to calpastatin
Autolysis without PL*
Autolysis with PL*
Autolyzed
␮-Calpain
0.5–2
0.042
␮-Calpain
3–50
40
50–150
0.8–50
Autolyzed
m-Calpain
50–150
25
m-Calpain
400–800
250–500
550–800
90–400
Numbers are Ca2⫹ concentrations (in ␮M) required for half-maximal activity, binding, or rate of autolysis and are based largely on
experiments with bovine skeletal calpains (62, 137, 199). The Ca2⫹
requirement of calpains from different species may differ slightly from
these for bovine skeletal muscle, but because these results are from the
same species and tissue, the relative Ca2⫹ requirements for different
properties of the calpains can be compared directly.
* PL is phospholipid; numbers are based largely on experiments using phosphatidylinositol or phosphatidylinositol 4,5-bisphosphate.
entire Gly-rich domain) are removed from the 28-kDa
subunit during this autolysis (433; residue numbers are
for the human calpains; the autolysis sites are similar but
not identical in calpains from other species).
Although autolysis occurs rapidly, it involves several
sequential steps: 1) for the 80-kDa subunit of ␮-calpain,
the NH2-terminal 14 amino acids are removed first to
produce a 78-kDa intermediate product followed by removal of an additional 12 amino acids to produce the
76-kDa autolytic fragment (504; molecular weights as estimated with SDS-PAGE); 2) for the 80-kDa subunit of
m-calpain, the NH2-terminal 9 amino acids are removed
first followed by removal of an additional 10 amino acids
to produce the 78-kDa autolytic fragment (45); and 3) for
the 28-kDa subunit, the NH2-terminal 26 amino acids are
removed to produce a 26- to 27-kDa fragment, an additional 37 amino acids (to Gly-64) are then removed to
produce a 22- to 23-kDa fragment, and finally 28 more
amino acids (to Ala-92) are removed to produce the 18kDa (20.5 kDa) autolytic fragment (272). The 28-kDa subunit of m-calpain is autolyzed more rapidly than the 80kDa subunit (45, 73), whereas autolysis of the 80-kDa
subunit in the ␮-calpain molecule seems to proceed as
rapidly as or even more rapidly than autolysis of the
28-kDa subunit in this molecule (45, 68, 504).
The small change in mass during autolysis of the
80-kDa subunit of m-calpain is difficult to detect in normal
SDS-PAGE analysis of the calpains, and claims that this
subunit has or has not undergone autolysis need to be
substantiated either by direct sequencing or by use of an
antibody that specifically recognizes the NH2 terminus of
the autolyzed molecule (78, 375). It seems likely that
claims that the 80-kDa subunit of m-calpain had not undergone autolysis in some early publications were incorrect because of the difficulty in resolving the 78-kDa
autolytic fragment from the 80-kDa native subunit in SDSPAGE.
Although the biochemical changes that accompany
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GOLL, THOMPSON, LI, WEI, AND CONG
autolysis have been well characterized, the physiological
significance of autolysis remains controversial. The Ca2⫹
concentrations required for autolysis are similar to or just
slightly greater than those required for proteolytic activity
(62, 218, 504; Table 3) and therefore are higher than would
be encountered in living cells. Because these two Ca2⫹
requirements are so similar, in vitro assays of proteolytic
activity are accompanied by autolysis unless the Ca2⫹
concentrations are chosen carefully to be just below
those required to initiate autolysis. Proteolytic activity is
less than half-maximal (usually much less) at these concentrations, and they are rarely used in in vitro assays.
Many members of the cysteine protease family are proenzymes that are activated by autolytic removal of a proenzyme polypeptide that blocks access to the active site of
the enzyme. That assays of proteolytic activity were always accompanied by autolysis raised the possibility that
the unautolyzed calpains also were proenzymes that required autolysis for activation (274, 435).
The concept that the calpains were proenzymes was
widely accepted in the 1990s, and papers have described
“activation” of the calpains as being synonymous with
autolysis. Although a number of studies reported that
both unautolyzed ␮-calpain (62, 139, 299) and unautolyzed m-calpain (63, 73, 139) were active proteases, other
kinetic studies (18, 68, 161, 218) found that autolysis
preceded proteolytic activity of the calpains. Mutation of
the residue that is cleaved during autolysis of m-calpain
prevented autolysis, but the resulting unautolyzed m-calpain was an active protease (107). Furthermore, SDSPAGE has been used to show that a casein substrate is
cleaved to small fragments by either unautolyzed ␮-calpain or unautolyzed m-calpain (69). Oxidation of ␮-calpain with 100 ␮M hydrogen peroxide or sodium hypochlorite does not prevent autolysis but completely inhibits
proteolytic activity, suggesting that the two are separate
events (147). Finally, the crystallographic structure of
m-calpain (171, 430) shows that autolysis of m-calpain
removes an ␣-helical NH2 terminus of the 80-kDa subunit
and that this NH2 terminus does not block the active site
in the inactive enzyme. The NH2-terminal part of the
28-kDa subunit is not visible in the X-ray pattern, but it
seems likely that if it were fixed in a position blocking the
active site of the enzyme, it would have diffracted sufficiently to have been detected. Consequently, autolysis of
m-calpain and, to the extent that structures of the two
molecules are similar, of ␮-calpain, does not unblock an
active site as autolysis of other cysteine proteases does.
Therefore, it presently seems likely that the unautolyzed calpains are capable of proteolytic activity and are
not proenzymes that require autolysis for their activation.
Whether the unautolyzed calpains are enzymatically active or not may be a moot question, however, because
autolysis seems to occur consistently under conditions
where the calpains are proteolytically active (139). ConPhysiol Rev • VOL
comitant autolysis and proteolytic activity are especially
well documented in activation of platelets (120, 217, 374,
378), where platelet activation is accompanied by cleavage of spectrin (118), talin, and filamin (117) in a calpainspecific manner and by autolysis of the ␮-calpain in platelets. Hence, autolysis can be used as an indicator that the
calpains have been proteolytically active in a cell, but the
absence of autolysis does not guarantee that the calpains
have not been active.
It seems, therefore, that autolysis has some important role in calpain function. The nature of this role,
however, remains a mystery. It has been reported that the
peptides released during autolysis of the large subunit of
␮-calpain (225) or autolysis of the 28-kDa small subunit
(226, 227) have chemotactic activities on neutrophils
(225, 226) or immunocytes (227) and that peptides released by autolysis of the small subunit have spasmogenic
activity on sections of guinea pig ileum (257). There are
numerous instances in biology where peptides released
during limited proteolysis have important physiological
functions related to the initial proteolysis, such as the
vasoconstrictive effects of the peptides released during
cleavage of fibrinogen to produce fibrin during blood
clotting. It is possible that the peptides released during
autolysis of the calpains may also have important as yet
undiscovered properties. Recent studies (discussed in
sect. IVC6) indicate that autolysis affects stability of the
calpains and may initiate dissociation of the two subunits
and inactivation. The possible role(s) of autolysis in the
physiological function of the calpains will remain an active area of research in the coming years.
B. Properties of Calpastatin
It was discovered during the initial studies on purification of m-calpain (83) that muscle extracts having calpain activity also contained an inhibitor of this activity
(see Ref. 135). Initial studies established that this inhibitor
was a heat-stable protein (to 100°C; Ref. 332), and it has
subsequently been shown that it is resistant to a wide
variety of denaturing agents such as urea, SDS, or trichloroacetic acid (126, 337). The name, calpastatin, was proposed for this inhibitor in 1979 by Takashi Murachi (309).
The early attempts to purify this inhibitor produced inconsistent and variable results, and the inhibitor was
described as a protein having molecular masses varying
from 34 to 280 –300 kDa (see Refs. 135, 259 for summaries). In retrospect, a number of factors likely contributed
to these inconsistencies.
First, calpastatin is labile to proteolytic degradation
(277, 283, 337), and it is likely that the harsh conditions
used in some of the early attempts to purify calpastatin,
many of which involved heating crude extracts that likely
contained a number of endogenous proteolytic enzymes,
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THE CALPAIN SYSTEM
resulted in varying degrees of proteolytic degradation
even though protease inhibitors were used.
Second, it is now known that the calpastatin polypeptide is nearly completely in a random coil conformation as
determined by circular dichroism (212, 403) or nuclear
magnetic resonance (212, 466). Because estimates of molecular weight using size exclusion chromatography are
invariably based on spherical molecules as standards, size
exclusion chromatography will greatly overestimate the
molecular weight of a molecule having a coil conformation. The studies that reported very large molecular
weights of over 200 kDa for calpastatin all used size
exclusion chromatography, and it seems likely that they
substantially overestimated the molecular weight of calpastatin because of this. Some of these studies also indicated that calpastatin was a dimer or tetramer in nondenaturing solutions because SDS-PAGE of their calpastatin
preparations contained polypeptides that were much
smaller than those estimated with size-exclusion chromatography. The available evidence indicates that calpastatin is a monomer in nondenaturing solvents (337) and
functions as a monomer in cells, although definitive sedimentation equilibrium studies have not yet been done.
Third, although only a single calpastatin gene exists
in humans (chromosome 5; Ref. 183), pigs (chromosome
2; Ref. 114), and bovine (chromosome 7; Ref. 38), studies
during the past 8 –9 years have shown that, because of the
use of different promoters (65, 178, 448, 471) or alternative splicing mechanisms (238, 239, 344, 443, 448), a large
number of different calpastatin isoforms ranging in molecular mass from 17.5 kDa (471) to 46.35 kDa (178) to 84
kDa (65) are produced from this single gene. At least eight
different calpastatin isoforms (Table 4) have been identified in tissues (126, 138, 344, 448). Frequently, more than
one isoform exists in a single tissue (288), and different
isoforms exist in erythrocyes from different species (185).
Fourth, calpastatin migrates anomalously in SDS-
TABLE
PAGE (259, 445), and it is difficult to relate a band migrating at a particular molecular weight in SDS-PAGE to a
known calpastatin isoform. For example, the 46.35-kDa
calpastatin in human erythrocytes, which was the first
calpastatin to be purified (446), migrates at 70 kDa in
SDS-PAGE. The expressed fragments of the calpastatin
molecule each migrate more slowly in SDS-PAGE than
would be predicted from their respective sizes (110, 445).
Hence, the anomalously slow migration of calpastatin in
SDS-PAGE is a property of the calpastatin polypeptide
itself and is not due to posttranslational modifications.
Consequently, although the multiple bands frequently observed in SDS-PAGE of calpastatin polypeptides are often
ascribed to proteolytic degradation, it is not always clear
without actually sequencing them whether the bands are
proteolytic products, whether they are different calpastatin isoforms resulting from some combination of alternative splicing or promoter usage, or whether they are produced by some combination of proteolysis and different
isoforms.
Calpastatin is the only known protein inhibitor specific for the calpains, although both L- and H-kininogen
(only the central cystatin domain of the kininogen heavy
chain) and ␣2-macroglobulin inhibit the calpains in addition to the other proteases they inhibit (72). The central
domain of kininogen has a Ki of 1.0 nM for m-calpain, and
the rate constant for inhibition of the calpains by ␣2macroglobin is 3 ⫻ 10⫺4 M⫺1䡠s⫺1, which is intermediate in
the range of values that have been reported for the inhibition of different proteases by ␣2-macroglobulin (72).
Calpastatin does not inhibit any other protease with
which it has been tested including the cysteine proteases,
papain, cathepsin B, bromelin, or ficin in addition to
proteases from other classes such as trypsin, chymotrypsin, plasmin, thrombin, pepsin, cathepsin D, or thermolysin (72). Neither calpastatin nor calpastatin-like activities
have been reported in invertebrate tissues, and genes
4. Summary of calpastatin isoforms
Nature of Transcript
Properties of Protein
Type I; contains 1xa, 1y, 1z, exons for domain L and all four repeats;
85-kDa polypeptide
Type II; contains 1xb, 1y, 1z, exons for domain L and all four repeats;
84-kDa polypeptide
Rate of migration in SDS-PAGE and sites of tissue expression are
unknown; 172 kDa in porcine skeletal muscle??
Migrates as 145 kDa in SDS-PAGE; XL domain contains three protein
kinase A phosphorylation sites in some species; present in cardiac
muscle; other tissues?
Migrates as 135 kDa in SDS-PAGE; mammalian skeletal muscle;
other distribution not characterized
Migrates as 125 kDa in SDS-PAGE; mammalian skeletal muscle;
other distribution not characterized
Rate of migration in SDS-PAGE unknown (110–115 kDa?); tissue
distribution has not been characterized
Migrates as 70 kDa in SDS-PAGE; human, pig, rabbit erythrocytes;
possibly also in cells
Rate of migration in SDS-PAGE is unknown; transcript found only in
testis
Rate of migration in SDS-PAGE unknown; found in human testis
Type III; prototypical; contains exons for domain L and all four
repeats; 77–78 kDa
Deletion of exon 3 in domain L; contains all four repeats; 74–75 kDa
Deletion of exons 3 and 6 in domain L; 72–73 kDa
Lacks domains L and I; 46-kDa polypeptide
Type IV; lacks domains L and I and part of domain II; ?? kDa
Lacks domain L and domains I, II, and III; 18.7 kDa
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GOLL, THOMPSON, LI, WEI, AND CONG
having sequence homologies to calpastatin have not been
detected in Drosophila, Caenorhabditis elegans, or Saccharomyces cerevisiae. Hence, calpastatin may be restricted to vertebrates.
1. DNA sequencing
cDNAs encoding the calpastatins for human (12),
monkey, mouse (443), rat (188), pig (445), bovine (65,
206), and rabbit (108) have been cloned and sequenced
(259). Although sequence homology among the calpastatins is not as high as it is among the calpains (188),
there is ⬎65% amino acid sequence identity among the
calpastatins sequenced thus far. The calpastatin amino
acid sequence is unique and is not homologous to any
polypeptide that has been sequenced thus far, including
the cystatins which inhibit many of the cysteine proteases
but not the calpains (72). The initial studies on calpastatin
cDNAs from rabbit lung or liver (108) or pig heart (445)
indicated that the calpastatin polypeptide consisted of
four repeating, marginally homologous (23 to 36%) domains of ⬃140 amino acids each, plus a NH2-terminal
domain, called domain L (Fig. 3), that varies in size due to
alternative splicing (238, 239). The predicted mass for
these calpastatins ranged from 68 to 78 kDa, depending
on the size of domain L. As indicated earlier, however, the
calpastatins migrate anomalously in SDS-PAGE, and relative molecular masses ranging from 107 kDa (179), to
125 kDa (93, 403), to 145 kDa (277), and to 172 kDa (241)
were reported for these different calpastatins. Protein
sequencing showed that the NH2 terminus of the 46.35kDa human erythrocyte calpastatin was Ser-Asp-Gln-AlaLeu-Glu-Ala-Ser-Ala-Ser-Leu (178), which corresponds to
the sequence of the human hepatic calpastatin beginning
at Ser-287 (Fig. 3). This Ser is the first residue of domain
II in human hepatic calpastatin and is located immediately
after a Met residue that is encoded in a acaATGa sequence, a good Kozak consensus sequence for translation
initiation. Hence, human erythrocyte calpastatin is an
example of a calpastatin isoform produced from an alternative transcription/translation start site.
Sequencing the mouse calpastatin gene has shown
that this gene contains 34 exons in ⬃60 kb of sequence
(448), including 5 exons upstream from exon 2, the exon
encoding the NH2 terminus of the “prototypical” calpastatin (human hepatic calpastatin in Fig. 3). The 5 upstream
exons were named 1xa, 1xb, 1y, 1z, and 1u (Fig. 4). Earlier
studies (65) had identified a 5⬘-region of the bovine calpastatin gene containing exons 1xb, 1y, and 1z. These
three exons encoded a 68-amino acid domain termed
domain XL (Figs. 3 and 4) that contained three protein
kinase A (PKA) phosphorylation sites. Incubation of an
expressed calpastatin containing domain XL with purified
PKA showed that these sites were phosphorylated in
vitro. Expressed calpastatin containing the XL domain
migrated as a polypeptide of ⬃145 kDa (molecular mass
of the predicted amino acid sequence is 84 kDa), and
antibodies specific for the XL domain showed that a 145kDa calpastatin containing the XL domain is expressed in
bovine liver and cardiac muscle extracts (65). These extracts also contained polypeptides of ⬃120 and 110 kDa
that were labeled by a monoclonal antibody specific for
domain IV of calpastatin (Fig. 3; Ref. 65). Hence, some
bovine tissues contain at least three different calpastatins.
The upstream AUG initiation site for the XL domain is in
a better Kozak consensus context than the second, downstream AUG (Met-1 in the human hepatic calpastatin in
Fig. 3) and was the preferred initiation site in transcription/translation assays (65).
FIG. 3. Schematic diagram showing
the domain structure of three different
calpastatin isoforms as determined from
their deduced amino acid sequences. The
species and tissues named under each calpastatin is the species and tissue from
which the calpastatin was cloned and sequenced. The calpastatin isoforms containing a L and/or a XL domain are found
in a number of tissues from a number of
different species and are not unique to the
species or tissue named. A, B, and C are
three subdomains within each domain:
the B subdomain from a number of species contains the conserved sequence indicated. *** Three sites on domain XL that
are phosphorylated by protein kinase A.
See text for additional details.
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743
FIG. 4. Genomic structure of the mouse
calpastatin gene and the relation of the 34
exons (29 numbered plus 1xa, 1xb, 1y, 1z, and
1u) in this gene to the predicted domains in
the mouse calpastatin protein. The mouse calpastatin gene is ⬃60 kb, and only the exons
are shown in this diagram. The 1u exon is
expressed only in the type III transcript, but
translation of this transcript begins in exon 2.
cDNAs encoding four different NH2-terminal sequences have been identified in the mouse (448); these
four calpastatin cDNAs were named type I, type II, type
III, and type IV (Fig. 4).
A) TYPE I. Type I calpastatin started at exon 1xa; type
I transcripts skip exon 1xb, continue with exons 1y and
1z, and skip exon 1u. Type I transcripts were identified in
mouse brain, liver, and testis.
B) TYPE II. Type II calpastatin started at exon 1xb and
continued with exons 1y and 1z before skipping exon 1u
and proceeding to exon 2. Type II transcripts therefore
encoded calpastatin containing the XL domain (Fig. 3).
Type II calpastatin was detected in skeletal and cardiac
muscle and in much lower levels in other mouse tissues.
C) TYPE III. Type III calpastatin mRNA started at exon
1u and encoded the prototypical calpastatin. Translation
of type III transcripts starts in exon 2, however. Type III
calpastatin was expressed ubiquitously in the mouse tissues tested.
D) TYPE IV. Type IV calpastatin expression was limited
to testis. Initiation of transcription of type IV calpastatin
mRNA began at a unique exon between exons 14 and 15
that was named exon 14t, and that was not expressed in
type I, II, or III calpastatin mRNAs. Expression of type IV
calpastatin therefore was initiated in domain II just after
subdomain IIA and produced a polypeptide that migrated
at ⬃68 kDa in SDS-PAGE.
The four mouse calpastatins are probably not unique
to the mouse; the human EST database includes sequences for partial cDNAs representing types I, II, III, and
IV, and calpastatin mRNA transcripts corresponding to
types I, II, and III calpastatins have been identified in
porcine cardiac tissue (344). Moreover, further examination of the bovine calpastatin gene (65) shows that this
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gene also includes a region corresponding to exon 1xa.
Although type II calpastatin from the mouse and human
contain the PKA phosphorylation sites described by Cong
et al. (65) in the XL domain, the porcine type II calpastatin
lacks these sites (344). Only type III calpastatin was detected in skeletal muscle from the pig (344).
It is unclear how many different start sites of transcription/translation may be used in expression of the
calpastatin gene in different tissues or under different
physiological states. In addition to the four ATGs at the
5⬘-ends of types I, II, III, and IV calpastatin, there is an
ATG that encodes Met-189 (numbering based on Met-1 at
the NH2 terminus of type I calpastatin), which is located
at the L domain/domain I junction (Figs. 3 and 4), and
another that encodes Met-322, which is at the NH2 terminus of the 46.35-kDa erythrocyte calpastatin that begins at
the domain II/III boundary of calpastatin (Fig. 3). A 17.5kDa glycoprotein with a predicted amino acid sequence
identical to the COOH-terminal 186 amino acids (beginning at Gln-559 sequence number based on type I calpastatin; domain III/IV in Fig. 3) of human hepatic calpastatin has been identified in human sperm (471). This
glycoprotein was not isolated, so its NH2-terminal amino
acid is unknown. Human hepatic calpastatin, however,
contains a Met residue at position 542, which could be the
NH2 terminus for this “sperm calpastatin.”
The 5⬘-promoter region of the calpastatin gene in a
number of species is GC rich and lacks a TATA box (65,
344), properties that were also observed for the 5⬘-promoter region of the m-calpain gene and that are characteristic of “housekeeping” genes. Both the bovine (65) and
the porcine (344) calpastatin promoter contain multiple
Sp-1 binding sites and putative CRE motifs. Earlier partial
sequencing of the human calpastatin gene (239, 260) and
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GOLL, THOMPSON, LI, WEI, AND CONG
the recent, more extensive sequencing of the mouse calpastatin gene (448) now makes it possible to assign exons
2– 8 to domain L and exons 9 –13 to domain I (Fig. 4). The
exon/intron boundaries of all four of the calpastatin inhibitory domains are highly conserved (448). With the
exception of domain IV, that has an additional sixth exon
at its COOH-terminal end, all inhibitory domains contain
five exons. Because each domain is capable of inhibiting
one calpain molecule (see sect. IIB2) and because removal
of either subdomain A or C within a domain does not
completely ablate the inhibitory ability of that domain
(see sect. IIB2), the use of multiple domains and multiple
exons within each domain may be a protective mechanism used to ensure that mutation within one or more
exons does not completely destroy the ability of calpastatin to inhibit the calpains (448).
Sequencing of calpastatin transcripts has shown that
exon 3, and less frequently, exons 4, 5, and 6, are often
deleted from calpastatin mRNAs by alternative splicing.
Sequences of rat calpastatin obtained in early studies of
this calpastatin all lacked exon 3 (94, 238), and it was
assumed initially that rodent calpastatins were unique in
that they lacked this exon. Several forms of mouse type I,
II, and III calpastatins, some of them containing exon 3
and some of them having exon 3 deleted (443) have been
identified, however; hence, some rodent calpastatins also
contain exon 3.
The use of at least four alternative promoters and the
ability to alter domain L by alternative splicing mechanisms make it possible to produce a number of different
calpastatin polypeptides from a single calpastatin gene.
The physiological significance of these different calpastatins remains a mystery. Deletion of exon 3 would remove
a stretch of basic amino acids, Lys-Lys-Arg-His-Lys-Lys
(residues 106 –111; residue numbers based on Met from
exon 1a as number 1), from the L domain of calpastatin.
This region of calpastatin has been reported to be involved in binding calpastatin to biological membranes
(275, 279, 281). Less than 2% of total cellular calpastatin,
however, is estimated to be bound to the sarcolemma in
cardiac muscle (279). Loss of the XL domain would remove several potential PKA sites (at least in the human,
mouse, and bovine calpastatins) and could therefore alter
regulation of calpastatin by this kinase. Although the
three PKA sites in the XL domain are phosphorylated in in
vitro assays (65), the effects of this phosphorylation on
calpastatin activity have not been studied. Calpastatin is
phosphorylated both in vitro by PKA (277, 384) and in
vivo (1, 384) by either PKA or PKC, but this phosphorylation had only small effects on the calpastatin inhibitory
properties that were measured. In vitro phosphorylation
with PKA had no effect on the measured properties of
calpastatin (277), whereas in vivo phosphorylation with
PKC or PKA resulted in an increase in the amount of
membrane-associated calpastatin from 6 to 30% (1). It is
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not known whether the 2% of membrane-bound calpastatin in bovine cardiac muscle (279) was phosphorylated,
but membrane-bound calpastatin can inhibit the calpains
as effectively as cytosolic calpastatin (280); hence, the
physiological significance of membrane-associated calpastatin is unclear. In vitro phosphorylation of rat brain
calpastatin by either PKA or PKC (382) or by PKC (13) has
been reported to decrease its efficiency (increase the
concentration required for half-maximal inhibition) in inhibiting either ␮- or m-calpain. The PKA phosphorylation
occurred in the NH2-terminal part of calpastatin, and the
PKC phosphorylation occurred on Ser-13 of rat calpastatin (rat calpastatin lacks the XL domain, so numbering
starts with Met-1 of the L-domain; Ref. 13). The S13S14
sequence for PKC is not conserved among all calpastatins,
however (e.g., the mouse, Refs. 238, 443; or the human or
monkey, Ref. 239; do not have this sequence), so it is
unclear whether phosphorylation at this site could serve
as a general mechanism of regulating calpastatin activity.
Thus, although calpastatin can be phosphorylated both in
vivo and in vitro by PKA or PKC, the effects of this
phosphorylation on calpastatin function are still unclear.
Calpastatin levels in skeletal muscle increase 52–
100% in response to ␤-adrenergic agonist administration
(23, 215, 342, 402), and the isoform that is affected primarily is the 135-kDa polypeptide (the prototypical calpastatin without the XL domain; Fig. 3), which is also the
predominant calpastatin in skeletal muscle (343, 344).
␤-Agonists bind to the ␤2-adrenergic receptor and activate
signaling cascades that involve the PKA pathway. Although Cong et al. (64) identified a dibutyryl-cAMP responsive element between nt ⫺76 to ⫺73 upstream of the
initiation of transcription of the DNA encoding the XL
domain, it is unclear if this element is involved in upregulation of the 135-kDa calpastatin observed by Parr and
co-workers (343, 344). Northern analysis of skeletal and
cardiac muscle tissue from a number of species has typically detected two to four calpastatin transcripts in these
tissues. These transcripts, however, cannot be directly
related to the type I, type II, or type III calpastatins
because the differences in their size evidently are due to
large differences in the lengths of their 3⬘-untranslated
regions. Again, the significance of this variability in size of
the 3⬘-UTRs of calpastatin transcripts is unknown.
A systematic study of six different calpastatin isoforms obtained from an E. coli expression system revealed up to an eightfold difference in the ability (as
measured by IC50 values) to inhibit the calpains (244).
Both ␮- and m-calpain and the autolyzed forms of these
two enzymes were included in the study. The six expressed calpastatin isoforms were as follows: domains
XL-IV, domains L-IV, domains I-IV, domains II-IV, domains
III-IV, and domain IV. Domain III-IV (IC50 values of 10 –34
nM) was the least effective isoform, and domain IV (IC50
values of 1.25–7.1 nM) was the most effective. Proteolytic
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THE CALPAIN SYSTEM
activity of autolyzed m-calpain was inhibited more effectively by all six calpastatin isoforms than activity of any of
the other calpains (unautolyzed m-calpain, autolyzed or
unautolyzed ␮-calpain) was. The results thus far indicate
that the different calpastatin isoforms do not differ greatly
(i.e., by several orders of magnitude or more) in their
ability to inhibit the calpains and do not suggest any
functional reason for the different calpastatin isoforms.
2. Domain structure of calpastatin
Expression of truncated cDNAs encoding portions of
the calpastatin polypeptide and assay of the expressed
polypeptides has indicated that each of the domains, I, II,
III, and IV (Fig. 3), can inhibit the proteolytic activity of
either ␮- or m-calpain (hence, theoretically, 4 calpains
inhibited by 1 “large” calpastatin; Refs. 110, 264; 3 calpains
inhibited by one “small” erythrocyte calpastatin; Ref. 178).
The L domain alone has no inhibitory activity (445). Ability of the individual domains to inhibit either ␮- or mcalpain has been reported to differ in the order, domain
I ⬎ domain IV ⬎ domain III ⬎ domain II, from most to
least effective (201). Although several studies using unpurified E. coli expression extracts found that the individually expressed domains inhibited m-calpain more effectively than they did ␮-calpain (262, 263), the relative ability to inhibit ␮- or m-calpain activity is not consistent
among the different calpastatin domains (201, 244, 264),
and the reported differences are less than an order of
magnitude. Other studies have reported that rat calpastatin (110-kDa so evidently lacking the XL domain and
perhaps exon 3) inhibited ␮-calpain more effectively than
m-calpain, but that this difference was eliminated by dephosphorylation (384). Hence, it is not clear that the
different calpastatin domains differ significantly in their
ability to inhibit ␮- or m-calpain; additional study will be
needed to clarify this question.
A conserved 12-amino acid sequence near the center
of each domain (B in Fig. 3) is essential for inhibitory
activity. Any combination of synthetic peptides or expressed polypeptides that lacked subdomain B had no
detectable inhibitory activity (201, 258, 264). Mutation of
two amino acids in this region almost completely abolishes inhibitory activity (264). Paradoxically, this 12amino acid residue alone has no inhibitory activity, although a 27-amino acid peptide containing this domain
and 11 amino acids NH2-terminal to the Gly residue at the
NH2 terminus of subdomain B (legend to Fig. 3) and four
amino acids added to the COOH terminus of subdomain B
(i.e., lacking both subdomain A and C) inhibited both ␮and m-calpain with an IC50 of ⬃25 nM (m-calpain) to 800
nM (␮-calpain) (258). Inhibition by this peptide was less
effective than inhibition with a complete domain. The
complete calpastatin molecule with four domains (Fig. 3)
is a very tightly binding inhibitor with a dissociation conPhysiol Rev • VOL
745
stant (Kd) ⬍3 nM (72). Because traditional kinetic analysis of inhibitor mechanisms is inappropriate for very
tightly binding inhibitors (233, 303), the early reports that
calpastatin was a noncompetitive inhibitor of the calpains
are inaccurate. The expressed domain III (residues 426 –
555), which completely inhibited calpain activity, bound
less tightly to the calpains than the complete calpastatin
molecule did (264), thereby permitting kinetic analysis of
calpastatin inhibition. Such analysis indicated that domain III is a competitive inhibitor of ␮-calpain with a Ki
value of 3 nM. The unmutated form of domain III bound
too tightly to m-calpain to permit accurate kinetic analysis, but mutated forms of domain III also inhibited mcalpain competitively with Ki values 8- to 10-fold less than
those for ␮-calpain (264), reflecting the greater sensitivity
of m-calpain to inhibition by this domain of calpastatin.
3. Binding of calpastatin to the calpains
It was recognized early that Ca2⫹ is required for
calpastatin to bind to and inhibit the calpains (70, 182,
337). Use of calpastatin-affinity columns (calpastatin coupled to agarose) indicated that the Ca2⫹ concentration
required for the calpains to bind to calpastatin depended
on the calpain molecule, that calpastatin inhibition was
reversible, and that bound calpain could be released in an
undegraded form by chelating Ca2⫹ with EDTA (199, 337).
In all instances except for unautolyzed ␮-calpain, the
Ca2⫹ concentration required for the calpains to bind to
calpastatin is significantly lower than that required to
initiate their proteolytic activity (Table 3; Ref. 199). There
is no evidence that calpastatin binds Ca2⫹, so the Ca2⫹
requirement for the calpastatin/calpain interaction must
originate from the calpain molecule.
Binding of autolytic fragments of the calpains to
calpastatin first indicated that calpastatin bound to both
domains IV and VI of the calpain molecule (320). Subsequent studies using expressed segments (subdomains) of
the calpastatin molecule showed that a 14-amino acid
subdomain that was conserved among the four domains
of the calpastatin molecule (subdomain A in Fig. 3) bound
specifically to domain IV of calpain (444, 489) in a Ca2⫹dependent manner with a Kd value of 3.1 nM. Further
studies using expressed segments (256) or synthetic peptides (444) representing subdomain C of calpastatin (Fig.
3) showed that this region of the calpastatin binds specifically to domain VI of calpain with a Kd of 31 nM. The 14
amino acids in subdomain C also are conserved among
the four domains in the calpastatin molecule. Neither
subdomain A nor subdomain C alone has any inhibitory
activity when incubated with the calpains.
Calpastatin peptides containing only subdomain B do
not block binding of the calpains to cell membranes,
although they reduce the rate of autolysis of the calpains
(202). Expressed calpastatin fragments containing only
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GOLL, THOMPSON, LI, WEI, AND CONG
subdomains A and C and not subdomain B, however,
block binding of the calpains to cell membranes, but do
not affect the rate of autolysis. Like the Ca2⫹ concentrations required for proteolytic activity of the calpains in in
vitro assays, the Ca2⫹ concentrations required in vitro for
binding of calpastatin to the calpains (Table 3) are also
much higher than 50 – 400 nM free Ca2⫹ concentrations
that normally exist in living cells (30, 32, 151, 228). Hence,
it is unclear whether calpastatin is bound to domains IV
and VI on calpain molecules in living cells, and thus is
“poised” to prevent any inappropriate or inadvertent calpain activity. Because the Ca2⫹ concentration required for
calpastatin to bind to the calpains is less than that required to initiate their proteolytic activity in vitro, and
because immunolocalization results suggest that the calpains and calpastatin are frequently colocalized in cells
(see sect. IVB), cells must possess some mechanism to
allow calpain activity in the presence of calpastatin. Otherwise, rising Ca2⫹ concentrations would cause calpastatin binding before the calpains could initiate proteolytic
activity. This mechanism may involve translocating the
calpains away from the calpastatin or reducing the Ca2⫹
concentration required for proteolytic activity of the calpains without affecting the Ca2⫹ concentration required
for calpastatin binding, or both these possibilities.
The available information leads to several conclusions about inhibition of the calpains by calpastatin and
also raises several questions concerning this inhibition.
1) Effective inhibition of the calpains by calpastatin
requires that calpastatin binds to the calpains at three
sites on the calpain molecule: Ca2⫹-dependent binding
involving subdomain A of calpastatin to domain IV in
calpain and subdomain C of calpastatin to domain VI in
calpain, as well as binding of subdomain B of calpastatin
to an area near the active site (domains IIa or IIb or both)
of calpain. Such binding of subdomain B would account
for the competitive inhibition observed in kinetic assays
of inhibition by individual calpastatin domains whose
binding to the calpains is sufficiently weak to allow kinetic analysis. Subdomain B binding was originally
thought to be Ca2⫹ independent, but there is no firm
evidence to support this assumption.
2) That binding of a calpastatin fragment containing
subdomains A and C to calpain prevents binding of calpain to membranes (202) suggests that binding of the
calpains to membranes involves domains IV and VI, the
“calmodulin-like” domains, in the calpain molecule, because calpastatin subdomains A and C bind to domains IV
and VI in the calpains, respectively. The presence of Ca2⫹
makes calmodulin a more hydrophobic molecule, and
Ca2⫹ may also increase the hydrophobicity of the “calmodulin-like” domains of the calpains. It is unclear
whether binding of the calpains to cell membranes is
related to binding of phospholipids to the calpains; different studies have found that phospholipids bind to a region
Physiol Rev • VOL
between amino acids 39 – 62 in the small subunit of the
calpains (9) or to domain III of the calpains (127, 462).
3) A 24-amino acid peptide synthesized to mimic the
calpastatin subdomain B sequence (contained three
amino acids added to the NH2 terminus of the 12-amino
acid subdomain B and 9 amino acids added to the COOH
terminus of this subdomain) inhibited both ␮- and mcalpain with IC50 values of 0.5–2 ␮M (77), somewhat
higher than the nanomolar IC50 values observed with the
complete domain III (264). Binding of this peptide, which
inhibited the calpains competitively, was greatly enhanced in the presence of Ca2⫹. The region of binding was
determined by covalently cross-linking the bound peptide
to a Cys residue in calpain, followed by tryptic digestion.
The cross-linking was not completely specific, and the
cross-linked Cys was determined by inference to be between residues 300 and 510 in bovine m-calpain (COOHterminal region of domain IIb and all of domain III) and
most likely to be Cys-497 in m-calpain. ␮-Calpain does not
have a Cys residue at this position, so no specific site of
cross-linking could be determined for ␮-calpain, although
the peptide also inhibited ␮-calpain activity. The residues
between 300 and 510 are some distance away from the
active site (Cys-105, His-262, and Asn-286) in the crystallographic structure of m-calpain, which was obtained in
the absence of Ca2⫹ (171, 430), and it is not known how
the peptide inhibited the calpains in a competitive manner. Binding of the peptide was nearly dependent on the
presence of Ca2⫹ (500 ␮M Ca2⫹); it is unclear whether
Ca2⫹ binding to domains IV and VI in the calpain molecule
caused a conformational change that permitted binding of
the inhibitory peptide, which then prevented subsequent
assembly of the catalytically active conformation, or
whether Ca2⫹ binding at some site other than domains IV
or VI was directly involved in binding of the inhibitory
peptide. Recent studies have identified two Ca2⫹-binding
sites in an expressed calpain domain II (298; see sect.
2⫹
IVC6); it is unclear whether these two Ca
binding sites
2⫹
could be involved in the Ca -dependent binding of the
24-amino acid inhibitory peptide to the calpains.
4) Because the most effective inhibition of the calpains by calpastatin requires all calpastatin subdomains,
A, B, and C, it seems likely that all three subdomains must
bind to calpain simultaneously for maximum inhibitory
efficiency. A number of calpain-like molecules have been
identified in the past 12 years (see sect. III). Several of
these recently identified calpains have been isolated and
have been shown to have no small, 28-kDa subunit (for
example, a Drosophila calpain and Lp82; see sects. IIIA2
and IIIB, respectively). Lp82 is inhibited very poorly by
calpastatin, possibly because of the lack of a 28-kDa small
subunit binding site. Also, the very weak proteolytic activity of the expressed domain II (298) is not inhibited by
calpastatin, suggesting that calpastatin cannot bind effectively to a calpain that lacks both domain IV of the 80-kDa
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THE CALPAIN SYSTEM
subunit and domain VI of the 28-kDa subunit. To permit
the simultaneous binding of all three calpastatin subdomains to the calpains and accepting the evidence that
subdomains A and C bind to domains IV and VI in the
calpains and that subdomain B binds at or near the active
site of the calpains, the distances between the sites of
calpastatin binding of domains IV and VI and the catalytic
site in domains IIa/IIb after the Ca2⫹-induced activation
must be the same as the distances between subdomains A
and B and subdomains C and B . Interestingly, the number
of amino acid residues between subdomains A and B and
B and C is remarkably conserved among the four domains
of all the calpastatins sequenced thus far (Table 5). The
length of a fully extended polypeptide chain is 7.23 Å/two
residues (102), so the maximum distance between the
calpastatin binding sites in domain IV or domain VI and
the catalytic site in domain IIa/IIb of the catalytically
active calpain is 105 and 83 Å, respectively (this assumes
that subdomain B binds at or near the active site). These
distances are nearly as large as the dimensions of the
calpain molecule in the absence of Ca2⫹ (171, 430), so it
should be easily possible for all three subdomains to bind
simultaneously to the calpain molecule in the presence of
Ca2⫹.
C. Purification of the Calpains and Calpastatin and
Assay of Their Activity
Although expression of catalytically active proteases
(142, 268) may eventually reduce the need for conventional purification of the calpains, it has not been possible
to obtain appreciable quantities of catalytically active
␮-calpain from expression systems, and catalytically active m-calpain can be obtained in significant quantities
from E. coli expression systems only if the 28-kDa subunit
is truncated at amino acid 85 (142). Hence, it likely will
TABLE 5. Number of amino acid residues between
subdomains A and B and between subdomains
B and C in the four domains of calpastatins
from different species
Domains
I. A
B
II. A
B
III. A
B
IV. A
B
3
3
3
3
3
3
3
3
B
C
B
C
B
C
B
C
Human, Monkey,
Porcine, and Rat
Calpastatins*
Bovine Cardiac
Calpastatin†
30
23
30
26
29
27
29
29
30
23
30
26
29
27
29
30
* From data in Maki et al. (259) and Lee et al. (239).
Cong et al. (65).
† From
Physiol Rev • VOL
747
remain necessary to purify the calpains from tissues or
cells for a number of years.
1. Purification of the calpains
Even though purification of m-calpain was first described over 25 years ago (83), it remains difficult to
purify the calpains from tissue sources. The initial purification of m-calpain required isoelectric precipitation at
pH 4.9 followed by five consecutive column chromatographic steps; 5 mg of purified enzyme was obtained from
12,000 g of porcine skeletal muscle. Purification of ␮-calpain was and still is more difficult than purification of
m-calpain. Some tissues or cells, such as human erythrocytes or human platelets, contain predominantly (probably entirely) ␮-calpain, whereas other tissues such as
vascular or gizzard smooth muscle contain predominantly
m-calpain (136, 452). Skeletal muscle and kidney of most
mammalian species contain approximately equal amounts
of ␮- and m-calpain. The reason for these differences in
␮-calpain/m-calpain ratios among different tissues/cells is
unknown, but it is much easier, for example, to purify
significant quantities of ␮-calpain from erythrocytes or
human platelets than it is from liver or smooth muscle. An
outline of different procedures that can be used to purify
the calpains and calpastatin, either individually or simultaneously from the same tissue source if necessary, has
been described (457), and this source should be consulted
for details concerning possible difficulties or precautions.
Only a general overview of the purification process will be
described here. The presence of Ca2⫹ results in rapid
autolysis of the calpains during purification, and it is
essential that the solution used to lyse/homogenize the
cells/tissue and the buffers used during purification contain EDTA or some Ca2⫹ chelator at a reasonably high
concentration (5–15 mM) to ensure that the free Ca2⫹
concentration in the homogenate is nearly zero. Some of
the initial purification protocols did not include Ca2⫹
chelators, and the 28-kDa subunit was autolyzed to the
18-kDa fragment and was not detected in SDS-PAGE of
the purified enzyme. The homogenizing/lysing solution
should also contain a cocktail of protease inhibitors; this
is especially important if calpastatin is going to be purified. An effective cocktail that uses less costly reagents
(desirable for large volumes) uses EDTA, which inhibits
metalloproteinases in addition to preventing autolysis;
phenylmethylsulfonyl fluoride (PMSF; a slow-acting but
irreversible inhibitor of serine proteases); ovomucoid,
partly purified containing ovoinhibitor (a fast acting inhibitor of trypsin-like serine proteases; the ovoinhibitor
also inhibits chymotrypsin-like serine proteases); and
E-64 (an irreversible inhibitor of cysteine proteases). This
cocktail contains inhibitors for each of three of the four
major classes of proteases, and homogenization at pH 8.0
inhibits aspartic proteases, the fourth class (pepstatin can
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GOLL, THOMPSON, LI, WEI, AND CONG
be added if the homogenization has to be done at a lower
pH); hence, the cocktail is more efficient than including
three or four serine protease inhibitors but omitting a
cysteine protease inhibitor, for example. More expensive
serine protease inhibitors such as pefabloc can be used
for smaller samples/homogenates. It is interesting that
large quantities of E-64, an irreversible inhibitor of cysteine proteases, can be included in the homogenization
buffer without adverse effects on activity of the calpains,
which are cysteine proteases. Purified calpains are not
inhibited by molar excess quantities of E-64 in the presence of EDTA, but are rapidly inhibited when Ca2⫹ is
added to the solution (458). Evidently, conformation of
the active site of the calpains in the absence of Ca2⫹
prevents or at least retards reaction of E-64 with the
active site Cys. Because all other cysteine proteases except the calpains are irreversibly inhibited when E-64 is
included in the homogenizing or lysing buffer, inclusion of
E-64 is an effective approach to inhibiting cysteine proteases when purifying the calpains. Curiously, iodoacetate (186 Da; E-64 is 367 Da) will slowly inhibit the
calpains in the absence of Ca2⫹ (481); it is not clear why
these two covalent inhibitors of cysteine proteases behave differently.
The tissue homogenate can be salted out between 0
and 48 –50% ammonium sulfate saturation (the calpains
salt out between 30 – 45% ammonium sulfate; the “larger”
calpastatins at slightly higher ammonium sulfate concentration 40 –50% saturation), or the homogenate can be
loaded directly onto a column if the volume of the extract
is not too large. The calpains can also be precipitated
from extracts between pH 6.2 and pH 4.9; this procedure
precipitates the calpains but leaves calpastatin in the
extract. The low pH seems to result in degradation of
calpastatin, so isoelectric precipitation is not an effective
way to purify calpastatin.
The first two column chromatographic steps in purification of the calpains are anion-exchange and hydrophobic interaction chromatography using phenyl Sepharose.
Either column can be used first; anion-exchange media
[either DEAE or a quaternary ion-exchange (QAE) are
effective] is generally less expensive than phenyl Sepharose, and recoveries of the calpains are greater off ionexchange than off phenyl Sepharose media (213). The
existence of different calpastatin isoforms has already
been described, and elution of calpastatin from anionexchange columns is variable; in some tissues, calpastatin
elution occurs just before or partly overlaps with ␮-calpain elution, and in other tissues, it almost completely
overlaps ␮-calpain elution with the result that neither
calpastatin nor ␮-calpain activity can be detected in the
column eluant. Some of the early reports indicating that
certain tissues did not contain ␮-calpain may have simply
failed to detect ␮-calpain activity because it coeluted with
calpastatin off the anion-exchange column used. To dePhysiol Rev • VOL
termine whether ␮-calpain is present, it is necessary to
collect the fractions eluting in this area of the gradient off
anion-exchange columns and apply them to a second
column that will separate calpastatin and ␮-calpain. PKC
coelutes with ␮-calpain off anion-exchange columns, and
this has led to some claims that ␮-calpain is associated
with PKC; the two activities can be separated, however.
The hydrophobic interaction media, phenyl Sepharose, was first used by Szpacenko et al. (441) in purification of ␮-calpain and is very effective in purification of the
calpains. Both ␮- and m-calpain bind to phenyl Sepharose
tightly, and ␮- and m-calpain containing fractions eluting
from an anion-exchange column (120 –180 and 180 –300
mM salt, respectively) or in a tissue homogenate containing 100 –150 mM salt can be applied directly to a phenyl
Sepharose column without dialysis or salting out. The
calpains are eluted with 1 mM EDTA, 2-mercaptoethanol
(but see Ref. 481). Calpastatin does not bind to phenyl
Sepharose under these conditions, so it is separated from
any ␮-calpain that coeluted with it from the anion-exchange column. A number of additional steps (described
in Ref. 457) are required to purify the calpains after anionexchange and phenyl Sepharose chromatography.
Several studies have used Ca2⫹-dependent affinity
chromatography, either with a calpastatin peptide that
contains the subdomains needed to bind calpains (5), or a
calmodulin-binding motif (301), or with a substrate such
as casein (74) to purify the calpains. These procedures all
involve binding of the calpains to the peptide/motif/substrate in the presence of Ca2⫹ and then elution with a
Ca2⫹ chelator such as EDTA. These Ca2⫹-dependent affinity procedures are effective in purifying the calpains,
but because the calpains autolyze rapidly in the presence
of Ca2⫹, calpains purified by using Ca2⫹-dependent affinity chromatography are, as pointed out by Croall (74),
almost invariably autolyzed to varying degrees. The small
subunit is especially sensitive to such autolysis, and the
28-kDa small subunit in calpains that have been purified
by using Ca2⫹-dependent affinity columns often has been
degraded.
Reactive Red dye affinity columns have been used to
purify the calpains (57, 74). Reactive Red binds the calpains in high (0.5 M) salt concentrations, and the calpains
are eluted at ionic strengths below 20 mM, so the binding
evidently involves hydrophobic interactions. Binding of
the calpains to Reactive Red also involves something in
addition to hydrophobic interactions because Reactive
Red chromatography provides purification that cannot be
obtained with phenyl Sepharose. Reactive Red purification is most effective if the calpains have been partly
purified by several chromatographic steps before loading
them onto a Reactive Red column. Elution of ␮-calpain off
Reactive Red columns is not always reproducible, and
Reactive Red is much more efficient in purifying m-calpain than it is in purifying ␮-calpain.
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THE CALPAIN SYSTEM
Recently, it was discovered that a monoclonal antibody specific for the 28-kDa subunit common to both ␮and m-calpain (epitope is between amino acids 92–104,
the NH2 terminus of domain VI; Fig. 1) can be used in an
immunoaffinity column to purify either ␮- or m-calpain
(61). The antibody labels the 28-kDa subunit of both ␮and m-calpain effectively in Western blots, but the antibody’s binding affinity for the calpains is very low when
linked covalently to an agarose matrix. An immunoaffinity
column containing the 5B9 monoclonal antibody will bind
either ␮- or m-calpain from extracts if the extracts are
loaded onto the column at neutral pH and very low ionic
strength. The bound calpain can be eluted either by flushing the column with high salt or at pH 9.0 –9.5, conditions
that do not affect activity of the calpains. Because the
binding is so weak, the column is much more effective if
the extracts have been partly purified by passing them
through successive phenyl Sepharose and anion-exchange columns before loading them onto the immunoaffinity column. The immunoaffinity column is more effective for purifying m-calpain than for purifying ␮-calpain; a
single pass of an extract that has been partly purified with
phenyl Sepharose and anion-exchange chromatography
through the immunoaffinity column results in a m-calpain
that is ⬎90% pure. A single pass through the column of a
partly purified ␮-calpain extract results in a ␮-calpain that
is ⬎80% pure, and two passes are required to achieve
⬎90% purity of ␮-calpain. Even so, the immunoaffinity
column is a much more efficient way to purify the calpains than the more traditional methods. The immunoaffinity column can be used to obtain partly purified calpain
from small samples of tissue by loading tissue extracts
directly onto the column. It is not clear why the 5B9
antibody binds the calpains so weakly. Evidently, the
epitope for the antibody is shielded in the native molecules.
749
pastatin isoforms that may be present in a given tissue. A
“purified” calpastatin may contain three to five or more
different polypeptides, and it is not always clear whether
these different polypeptides are due to proteolytic degradation of a native calpastatin polypeptide or whether they
are different calpastatin isoforms. The “large” form of
calpastatin (⬎100 kDa) but not the small erythrocyte
calpastatin will bind to phenyl Sepharose in the presence
of 1 M ammonium sulfate and is eluted between 1.0 and
0.5 M ammonium sulfate. The calpastatin can then be
purified to homogeneity by using size-exclusion chromatography (e.g., Sephacryl S-300) and anion-exchange chromatography with a shallow gradient (403). This latter
procedure avoids use of heating steps or denaturing solvents (457).
Calpastatin can also be purified by using an immunoaffinity column containing a monoclonal antibody specific
for domain IV of calpastatin (epitope between amino
acids 707–786 of bovine cardiac calpastatin; the area of
this epitope contains subdomain C of domain IV but not
subdomains A or B; Ref. 476). Because calpastatin is
resistant to many denaturing conditions including the pH
2.5 frequently used to elute molecules from immunoaffinity columns, it would seem straightforward to use immunoaffinity columns to purify calpastatin. For reasons that
are still not clear, however, calpastatin in solution does
not bind strongly to immunoaffinity columns containing
specific anti-calpastatin antibodies. Even with an antibody that specifically recognizes the COOH-terminal end
of calpastatin, binding is weak, and the calpastatin-containing extracts must be loaded onto the anti-calpastatin
immunoaffinity column in a low-ionic-strength solution
for calpastatin to bind. The calpastatin can be eluted from
this column at high ionic strength or at pH 2.5, because it
is stable at this pH.
3. Assays of calpain activity
2. Purification of calpastatin
Calpastatin is also difficult to purify, and a number of
different procedures for purifying calpastatin have been
published (see Refs. 126, 285). Many of these procedures
use heating to 90 –100°C after one or two initial chromatographic steps (e.g., after phenyl Sepharose chromatography) to denature and precipitate most of the proteins in
the fraction leaving the heat-stable calpastatin intact, or
precipitation with 15% trichloroacetic acid to denature
noncalpastatin polypeptides (126). Calpastatin can then
be purified with anion-exchange chromatography. Heating crude extracts, however, can lead to degradation of
the protease-sensitive calpastatin, whereas trichloroacetic acid precipitation decreases calpastatin activity, suggesting that 15% trichloroacetic acid may at least partly
denature calpastatin. Many of the difficulties in purifying
calpastatin are caused by the number of different calPhysiol Rev • VOL
Assays of proteolytic activity of the calpains usually
use a protein substrate; casein and myofibrils were used
in the initial purification of m-calpain (83), and casein is
still widely used. The cytoskeletal proteins, MAP-2, tau,
and spectrin, are also excellent calpain substrates, with
MAP-2 being degraded at a rate approximately twice that
of tau, and more than four times that of spectrin (148);
these proteins, however, are not as readily available and
are more expensive than casein. Because the calpains
evidently use a large area of the substrate for binding and
for subsite specificity (see sect. IVC3), synthetic peptides
are “poor substrates” in the sense that they have high
Michaelis constant (Km) values, on the order of 0.2–5 mM
(390), compared with Km values in the low micromolar
range for protein substrates (27). These peptides are also
poorly soluble in aqueous solvents, so assays using synthetic peptide substrates are usually done at substrate
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GOLL, THOMPSON, LI, WEI, AND CONG
concentrations of ⬃1–2 mM or even less; the calpains are
approximately half-maximally active under these conditions. Casein contains 10 –15 mol Ca2⫹/casein molecule
(equivalent to 1–1.5 mM in most calpain assay conditions), but this Ca2⫹ evidently is not available to the
calpains because its removal has no effect on the Ca2⫹
concentration required for half-maximal activity of the
calpains (27). It has been suggested occasionally that
substrate has a significant effect on the Ca2⫹ concentration required for half-maximal activity of the calpains and
that use of “physiological substrate(s)” of the calpains
might result in calpain activity at physiological Ca2⫹ concentrations (see sect. IVC1 for discussion of “the Ca2⫹
requirement problem”). A systematic comparison of four
protein substrates and three synthetic peptide substrates,
however, found no significant difference in Ca2⫹ concentration required for half-maximal hydrolysis for any of the
seven substrates (27).
Assays of calpastatin activity require calpain because
calpastatin specifically inhibits calpain proteoytic activity
but not the proteolytic activity of any other protease.
Either pure or impure calpain can be used, but if calpastatin activity is to be measured quantitatively, the degree of
inhibition must be “titered” to ensure that it does not
reach 100% inhibition.
Assay temperature has unusual effects on measurement of calpain activity. Both ␮- and m-calpain rapidly
lose proteolytic activity when assayed at temperatures
above 25°C (84; Fig. 4). If m-calpain activity is assayed for
50 min with casein as a substrate, the activity at 0°C is as
great as it is at 37°C because there is no proteolytic
activity after 10 min at 37°C (84). The rapid loss in calpain
activity also occurs at 30°C (Fig. 5), a temperature widely
used to assay calpain activity. Loss of calpain activity
occurs more rapidly at Ca2⫹ concentrations above those
required for half-maximal activity and is probably associated with autolysis of the calpains. Because most assays
of calpain activity use Ca2⫹ concentrations of 1–5 mM,
these assays should be done at temperatures below 30°C
and for periods of 20 –30 min or less. Assays at temperatures of 30°C or higher and at Ca2⫹ concentrations of 1–5
mM should not be run for longer than 10 –15 min or
accuracy is likely to be compromised.
III. OTHER MEMBERS OF THE CALPAIN
FAMILY: CALPAIN-LIKE MOLECULES
During the past 13 years, cloning and sequencing
DNA has led to identification of a number of calpain-like
genes in different organisms or in specific tissues. With
only two exceptions, a calpain from Drosophila (351, 352)
and a calpain in Schistosoma mansoni, a flatworm parasite that inhabits the portomesentric circulation of humans (409), the proteins encoded by these DNA sePhysiol Rev • VOL
quences have not been isolated from tissues, so very little
is known about the catalytic or other properties of the
proteins encoded by these genes. Complete genomic DNA
sequences have now been determined for a number of
organisms including E. coli (40), S. cerevisiae (58, 132),
Drosophila (3), and the human (187). Until annotation of
genomic DNA sequences of the higher organisms has
been completed, however, it is difficult to determine exactly how many sequences each organism has that encode
molecules having significant homology to the calpains (a
search of the published human genome sequence suggests that it contains 14 calpain large subunit genes;
Ref. 86).
Implied in the search for calpain-like DNA sequences
is that some definition exists of what a calpain-like molecule is. This is especially important for those molecules
that have not been isolated in protein form and that may
have only marginal amino acid sequence similarity/identity with the “ubiquitous” ␮- and m-calpains. Domain II of
␮- or m-calpain is the catalytic domain, but although it
contains the Cys, His, and Asn residues characteristic of a
cysteine protease, it has only marginal sequence homology to papain or other families of cysteine proteases.
Consequently, the calpains have been grouped in a class
of cysteine peptidases, CLAN CA, family C2, separate
from the other cysteine proteases (26). Even if sequence
homology with domain II of the ubiquitous calpains is
used as the criteria for being a member of the calpain
family of molecules, some choice must be made as to
what level of sequence identity/homology with domain II
of the ubiquitous calpains is needed to identify a molecule
as a member of the calpain family. Recent analysis (479)
has suggested that the percentage of proteins with the
same class of function decreases precipitously when sequence identity falls below 35%. Using homology with
domain II and ⬎ 20 –25% sequence identity in this domain
as criteria (an inclusive approach), 14 calpain-like genes
can be identified in mammals, 4 in Drosophila, 12 in C.
elegans, 2 in two species of fungi/yeast, 5 in the unicellular organism Trypanosoma brucei, and a transmembrane
calpain in a variety of plants (Tables 6 and 7, Ref. 420).
These 38 calpain-like genes have widely divergent properties outside the domain II homology and can be grouped
into two general categories of 11 “typical” calpains, defined as those calpains having a domain structure similar
to the 80-kDa subunit of ␮- or m-calpain including EFhand sequences in domain IV (8 identified in vertebrates,
3 in Drosophila), and 27 “atypical” calpains, which can be
further subdivided into six groups (Table 6). The “atypical” calpains do not have calmodulin-like EF-hand sequences in their domain IV (some are lacking a domain
IV), so it is unclear whether they are Ca2⫹-dependent
(although structures in addition to the EF-hand structures
in ␮- and m-calpain may be involved in the Ca2⫹-depen-
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751
THE CALPAIN SYSTEM
FIG. 5. Time course of hydrolysis of a casein
substrate by ␮-calpain at 50 ␮M Ca2⫹ (top panel) or
by m-calpain at 500 ␮M Ca2⫹ (bottom panel) at three
different temperatures of 25, 30, or 37°C as indicated.
Proteolytic activity was monitored continuously by
using the BODIPY-FL microplate assay (460). Note
that after 30 min, activity measured at 25°C is ⬃1.5–
1.7 times greater than activity measured at 30°C and
⬃2–3 times greater than activity measured at 37°C.
dent proteolytic activity of these enzymes; see sect. IVC6).
Six of the atypical calpains (5 genes in T. brucei and
calpain 6) and one typical calpain in Drosophila are lacking one or more of the Cys, His, or Asn residues at their
catalytic site (Table 6), so they probably are not proteolytic enzymes.
It is still uncertain how many “calpain-like” DNA
sequences will be detected in the genomes of different
organisms and what the properties of the molecules will
be. The existence of polypeptides possessing domains
that have 33–37% sequence identity to domain II of ␮-calpain (Tables 6 and 7) but lacking one or more of the
residues that constitute the catalytic triad of the calpains
implies that these calpain-like polypeptides are proteolytically inactive and that the calpains have a function other
than/in addition to their proteolytic activity. The nature of
Physiol Rev • VOL
this function is presently a complete mystery. Finally, it
should be indicated explicitly that all the sequence homologies are based on homology with the 80-kDa subunit
(more specifically, domain II/IIaIIb of the 80-kDa subunit),
and that with the exception of the Schistosoma calpain, it
is often assumed that the newly identified calpains function as a single polypeptide rather than as a heterodimer
as the ␮- and m-calpains do. This assumption has not been
proven, however, and it is possible that some “cofactor”
analogous to the 28-kDa subunit of the ␮- and m-calpains
(for example, other members of the penta-EF-hand family
of proteins such as ALG-2, peflin, sorcin, and grancalcin;
calmodulin plays this role for many proteins) is required
for activity of the newly identified calpains. Until the
properties of at least some of the proteins encoded by
these “calpain-like” DNA sequences have been deter-
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TABLE
GOLL, THOMPSON, LI, WEI, AND CONG
6. Subgroups within the calpain family of polypeptides
Calpain Subgroup
Catalytic Triad
Calmodulinlike Domain
Comments
Typical calpains
Mammalian calpains (8)
␮-Calpain, m-calpain
Calpains 3, 8, 9, 11, 12, 14
Drosophila calpains (3)
CALPA, CALPB
Drosophila CG 3692
C, H, N
Yes
Lp82 is alternative splicing product of calpain 3
C, H, N
R, I, D
Yes
CALPA’ is an alternative splicing product of CALPA with the
calmodulin domain deleted
Lacks catalytic triad
Atypical calpains
Group I (6)
Calpain 6
Trypanosoma brucei (five similar
genes)
Group II (7)
Calpains 10, 13
C. elegans (five similar genes)
Group III (2)
Calpain 5
C. elegans TRA-3
Group IV (7)
Calpain 15 (SOLH)
Drosophila SOL
C. elegans (5 similar genes)
Group V (4)
Calpain 7
Aspergillus nidulans (fungus)
C. elegans CE 01070
Saccharomyces cerevisiae
Group VI (1 family)
Arabidopsis thaliana (plant)
Maize, other plants
K, H, N (h calpain 6)
K, Y, N (m calpain 6)
S, Y, N (T. brucei)
No
Lack catalytic triad; proteolytically inactive?
C, H, N
No
C, H, N
No
Calcium dependent?
Calpain 10 has two domain III-like domains in tandum at COOH
terminus
COOH-terminal “T” domain in place of calmodulin-like domain
C, H, N
No
COOH-terminal “SOL” domain in place of calmodulin-like
domain; Zn-finger domains in NH2 terminus
C, H, N
No
PalB-like domain followed by a domain III-like domain in COOH
terminus
C, H, N
No
Transmembrane NH2-terminal domain
mined, it will be difficult to develop a system for classifying the different calpains.
Proteins having properties homologous to the calpains have not yet been unequivocally detected in prokaryotes. The protease from Porphyromonas gingivalis
(Table 7) illustrates some of the difficulties encountered
when assigning molecules to the calpain family on the
basis of cDNA-derived amino acid sequence alone. Although the predicted amino acid sequence of the P. gingivalis enzyme has 53.1% similarity (23.7% identity) to
domain IIa/IIb of human ␮-calpain (Table 7), the P. gingivalis enzyme has nearly the same homology (22.5%
identity) with the amino acid sequence of papain; the
expressed protease is not inhibited by leupeptin, an inhibitor of the calpains; is maximally active at 45°C, a
temperature that results in rapid autolysis of ␮- and mcalpain; degrades azocoll, a collagen substrate that is not
degraded by either ␮- or m-calpain; and the expressed
protease is inhibited, not activated, by 15 mM Ca2⫹ (44).
It seems unlikely that this enzyme is a member of the
calpain family.
Recent genomic sequencing efforts have identified a
family of transmembrane genes in Arabidopsis thaliana
Physiol Rev • VOL
(455), Zea mays (246), and other plants. The polypeptides
encoded by these genes all have a large transmembrane
domain and an intracellular domain with significant sequence homology to domain IIa/IIb of ␮- or m-calpain
(Table 7). Interestingly, the plant calpains also all have a
domain III whose sequence is homologous to domain III
of ␮- and m-calpain (homologies of domain III are approximately the same as the homologies of domains IIa/IIb). A
search of the available data bases indicates that all plants
that have significant portions of their genomes sequenced
have this transmembrane “calpain” gene and have only a
single copy of it (246). Because the protein form of this
gene has not been isolated, the function and properties of
this “plant calpain” remain unknown. A Ca2⫹-dependent
protease isolated from Arabidopsis thaliana was not labeled in Western blots by any of five different anti-calpain
antibodies (371) and was inhibited more strongly by pepstatin than by E-64 (361), whereas pepstatin has no effect
on the calpains (481). Because there is no sequence information available on the polypeptides studied by Safadi
et al. (371) and Reddy et al. (361), it is unclear whether
they are the same as or related to the polypeptide encoded by the At1g55350 gene. There have been several
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THE CALPAIN SYSTEM
TABLE 7. Amino acid sequence similarities and identities of some purported members of the calpain family
with domain II of the 80-kDa subunit of human ␮-calpain
Polypeptide
Species
Predicted Number of
Amino Acids/Massa
Calpain 2 (mCL)
Calpain 3 (p94)
Calpain 5 (htra-3)
Calpain 6 (calpamodulin)
Calpain 7 (PalBH)
Calpain 8 (nCL2)
Calpain 9 (GC36)c
Calpain 9 (nCL4)d
Calpain 10 (Type A)
Calpain 11
Calpain 12
Calpain 13
Calpain 14
Calpain 15 (SolH)
CALP A
CALP B
CG3692
SOL
(S. mansoni calpain)
(CAP5.5)e
(PalB)
(At1g55350)
Dek1
Phytocalpain
(Cp11p/YMR154C)
(Tpr)
Human
Human
Human
Human
Human
Mouse
Human
Human
Human
Human
Mouse
Human
Human
Human
Drosophila
Drosophila
Drosophila
Drosophila
Schistosoma mansoni
Trypansoma brucei
Aspergillus nidulans
Arabidopsis thaliana
Maize
Sugarcane
Saccharomyces cerevisiae
Porphyromonas gingivalis
700/80,006
821/94,084
639/72,950
641/74,576
813/92,652
703/79,335
664/76,168
690/79,096
672/74,938
702/80,582
720/80,588
557/63,600
663/76,700
1,086/117,313
828/93,962
791/90,132
1,043/118,957
1,597/174,695
758/86,863
853/94,772
847/94,129
2,143/236,779
2,159/238,999
1,160 (partial)f
727/83,123
482/55,104
Sequence Similarity
With ␮-Calpain
Domain II,b %
Sequence Identity
With ␮-Calpain
Domain II,b %
87.7
76.6
69.3
68.7
59.7
86.2
79.4
79.2
63.4
84.0
75.5
68.6
73.1
60.0
76.6
78.6
64.1
62.4
70.2
56.0
55.6
61.2
66.2
65.9
43.8
53.1
69.3
54.7
41.7
37.2
26.7
68.8
60.8
59.9
36.5
60.1
53.4
43.7
46.9
33.3
52.1
49.7
33.4
36.3
45.2
23.2
26.9
34.5
36.0
35.7
18.0
23.7
Similarities and identities were determined by using the Genestream Align program (346). Whenever possible for the vertebrate calpains,
a
b
comparisons were made between the human sequences. Exceptions are noted.
Mass is in Daltons.
Domain II in ␮-calpain was chosen to
contain amino acids 21–352. This is consistent with the crystallographic structure of m-calpain. The protease domains of the other molecules
(including calpain 2 which is included for comparison) were predicted by the Simple Modular Architecture Research Tool (SMART) (399, 400).
c
d
e
f
Yoshikawa et al. (493).
Lee et al. (235).
Hertz-Fowler et al. (165).
Full sequence not yet published.
other reports of Ca2⫹-dependent, cysteine proteolytic activity in plants such as Allomyces arbuscula (174, 327,
328, 329, 330, 331), or the algae Chara australis (302).
Thus far, none of these Ca2⫹-dependent plant cysteine
proteases has been cloned and sequenced, and their biochemical properties differ substantially from those of the
calpains. The algae protease was assayed only in cell
extracts, and its nature is unknown. Two proteases have
been purified from Allomyces arbuscula (328), one eluting at 0.07 mM (CDP I) and the other at 0.2 mM NaCl (CDP
II), similar to the elution pattern of ␮- and m-calpain off
anion-exchange columns. CDP I, however, has a molecular mass of 39 kDa, whereas CDP II is a dimer of 40- and
43-kDa subunits that degrades collagen rapidly and casein
slowly (331), just the opposite of the effects of ␮- and
m-calpain on these two protein substrates. Also, phosphatidylinositol inhibited activity of the Allomyces protease
(327), whereas it decreases the Ca2⫹ concentration required for activity of the calpains (373, 376). Consequently, it presently is uncertain whether any of these
Ca2⫹-dependent plant cysteine proteolytic activities are
members of the calpain family; a final conclusion will
need to wait until sequence information is available for
the plant proteases that have been isolated. An attempt to
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isolate calpain activity from a plant species rich in actin,
Elodea densa, failed to find any Ca2⫹-dependent proteolytic activity similar to the calpains (482).
A. Calpain-Like Molecules in Invertebrates
This section focuses on those molecules that have
been isolated in protein form, so there is some information on their catalytic properties. There are three classes
of such molecules: 1) the Schistosoma mansoni proteases, 2) the Drosophila proteases, and 3) the lobster
proteases. One calpain-like molecule from Caenorhabditis elegans has been expressed and its properties studied;
because this C. elegans calpain has unusual properties,
and because C. elegans is a genetically tractable organism, properties of the C. elegans calpains will also be
discussed. Interestingly, neither calpastatin activity nor a
calpastatin-like DNA sequence has yet been detected in
invertebrates; a search of the Drosophila genome failed to
detect a gene homologous to calpastatin (234).
1. Schistosoma mansoni calpains
Screening a cDNA library from adult Schistosoma
mansoni first indicated that these parasitic organisms
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GOLL, THOMPSON, LI, WEI, AND CONG
contained a 768-amino acid, 86.9-kDa molecule having the
four domains and EF-hand structures characteristic of ␮or m-calpain (7). The S. mansoni calpain had 62% amino
acid sequence similarity and 42% identity with human
␮-calpain (Table 7), clearly identifying it as a member of
the calpain family, although domain IV had only three
EF-hand sequences. The existence of an EF-hand sequence at the domain II/III boundary was first noticed in
the S. mansoni calpain, and assays indicated that this
EF-hand and all three EF-hands in domain IV of the S.
mansoni calpain bound Ca2⫹. A Ca2⫹-dependent protease
activity was subsequently purified from S. mansoni (409);
the purified S. mansoni calpain had two subunits of 78
and 28 kDa, although the 28-kDa polypeptide seemed to
be in excess of the 78-kDa polypeptide in a purified preparation (409). Two-dimensional electrophoresis indicated
that there were two 78-kDa polypeptides but only one
28-kDa polypeptide, suggesting that there may be two
Schistosoma calpains similar to the vertebrate ␮- and
m-calpain. The purified Schistosoma calpain was labeled
by antibodies elicited against either the polypeptide obtained by expression of the cDNA characterized earlier (7,
200) or the 78-kDa S. mansoni calpain (409). Antibodies
elicited against the expressed polypeptide labeled only
one of the two 78-kDa polypeptides separated by twodimensional electrophoresis (the one with the higher isoelectric point, possibly the “␮-calpain” equivalent). Neither of the S. mansoni antibodies labeled purified rabbit
calpain, indicating that the Schistosoma calpain was immunologically distinct from mammalian calpains. The
NH2-terminal sequence in domain I of the S. mansoni
calpain had a 28-amino acid extension that is not present
in the mammalian calpains; the NH2-terminal domain is
very heterogeneous among the different members of the
calpain family.
2. Drosophila calpains
A calpain-like activity was first identified in Drosophila in 1988 before any cDNA encoding a calpain-like molecule was identified (351). The following year, Müller and
Spatz (306) also identified a Ca2⫹-dependent proteolytic
activity in Drosophila that they partly purified, that required 500 ␮M Ca2⫹ for half-maximal activity, and that
degraded the regulatory subunit cAMP-dependent kinase.
Because it was inhibited by p-chloromercuribenzoate and
iodoacetate but not by various trypsin inhibitors, the proteolytic activity seemed to be due to a cysteine protease.
The activity eluted off a size-exclusion column at a position corresponding to an 83-kDa molecule (306). The
Drosophila Ca2⫹-dependent proteolytic activity was subsequently purified and was shown to be a single polypeptide chain of 94-kDa that was inhibited by calpastatin and
that required 600 ␮M Ca2⫹ for half-maximal proteolytic
activity (352), a Ca2⫹ requirement similar to mammalian
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m-calpain (cf. Table 3). The Drosophila calpain was the
first calpain and remains the only calpain that has been
demonstrated to be proteolytically active without a 28kDa small subunit. Yeast two-hybrid studies have suggested that calpain 3 (Table 7) does not bind a small
subunit (see sect. IIIB1), and it is assumed that many of
the calpains that have been identified as mRNAs also
function as a single polypeptide. In the absence of purified
enzymes, however, these assumptions remain unproven.
Assays of crude extracts of Drosophila heads (351)
indicated that these extracts contained a second Ca2⫹dependent proteolytic activity (␮-calpain?) and an inhibitor of the Ca2⫹-dependent proteolytic activity (probably
not analogous to calpastatin), but these activities have not
been purified. Antibodies raised against chicken m-calpain did not label the Drosophila calpain, again indicating
that these evolutionarily distant proteins are not immunologically similar.
Screening Drosophila cDNA libraries has identified
three cDNAs having homology to the calpain family (90,
113, 193, 456). The cDNAs encode polypeptides of 821
amino acids and a predicted molecular mass of 94 kDa
(CALPA; Ref. 456); of 791 amino acids and a predicted
molecular mass of 90,134 Da (CALPB; Ref. 193); and of
1,597 amino acids and a predicted molecular mass of 175
kDa (SOL protein for small optic lobes; Ref. 90). It is
unclear whether either CALPA or CALPB correspond to
the 94-kDa calpain that was purified in 1992 (352). The
encoded polypeptides for CALPA and CALPB have the
four domains characteristic of the 80-kDa subunits of ␮and m-calpain (Fig. 2), including the four EF-hand sequences in domain IV and at the domain II/III boundary,
and are therefore grouped with the “typical” calpains
(Table 6). The predicted amino acid sequences of CALPA
and CALPB are significantly homologous to those of the
vertebrate calpains: domain II was the most highly conserved, having 50 –52% sequence identity to domain II of
vertebrate calpains (Table 7); domains III and IV from
CALPA have 44 – 49 and 35– 42% sequence identity, respectively, with their counterparts from vertebrate calpains, whereas domain I is poorly conserved. Immunological and in situ hybridization studies showed that expression of Drosophila calpain is especially high in the central
nervous system (456). A CALPA transcript that lacked the
calmodulin-like EF-hand domain, CALPA⬘, is produced
from the CALPA gene by alternative splicing.
The third “calpain cDNA” in Drosophila encodes a
polypeptide differing in several ways from the other calpains (90). This DNA is cytogenetically located at the sol
(small optic lobe) locus of the Drosophila gene. Part of
this large 1,597-amino acid (175 kDa) polypeptide between amino acids 1017 and 1320 in the COOH-terminal
half of the polypeptide has 36.3% identity to domain II of
human ␮-calpain (Table 7). The sequence of the NH2terminal half of this molecule, however, has no homology
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with the calpains but instead predicts six motifs having
some similarity to zinc fingers found in steroid hormone
receptors and DNA-binding proteins. The SOL protein
does not have a domain IV containing EF-hand sequences
nor an EF-hand sequence at the domain II/III boundary.
Moreover, the sequence homology of SOL “domain II”
with domain II of ␮-calpain is only slightly better than
with the cathepsins (21% identity with mouse cathepsin
B). Hence, the Drosophila SOL calpain is an example of a
calpain homolog whose predicted amino acid sequence
differs considerably from the sequences of the classical ␮and m-calpains, and it will be interesting to learn about
the catalytic and other properties of the SOL protein.
3. Crustacean Ca2⫹-dependent proteases
Ca2⫹-dependent protease activity that was inhibited
by two inhibitors of cysteine proteases, leupeptin and
antipain, but not by an inhibitor of aspartic proteases,
pepstatin, was described in crustacean muscle (Bermuda
land crab) in 1982 (311). This Ca2⫹-dependent activity
was increased more than twofold during the muscle atrophy that occurs during molting and degraded the myofibrillar proteins in crab claw muscle (311).
Four different Ca2⫹-dependent proteolytic activities
have been isolated from the claw muscle of another crustacean, the lobster (312). These four proteases have molecular masses of 310 kDa (I), 125 kDa (IIa), 195 kDa (IIb),
and 59 kDa (III) as estimated by using size exclusion
chromatography and require Ca2⫹ concentrations of 1
mM (I), 1.5 mM (IIa), 2 mM (IIb), and 0.6 mM (III) for
half-maximal activity. Hence, the Ca2⫹ requirements for
these proteases resemble the Ca2⫹ requirement of mcalpain more closely than they resemble that of ␮-calpain.
Two of these proteases, IIa and IIb (34), have been partly
purified and have molecular masses of 60 and 90 kDa,
respectively, in SDS-PAGE. Because these SDS-PAGE molecular masses are approximately one-half of the molecular masses obtained with size exclusion chromatography, it was assumed that the native proteases are dimers
(33), although they could also be monomeric polypeptides
having an asymmetric shape. All four proteases degraded
the myofibrillar proteins in crustacean muscle including
actin and myosin to small trichloroacetic acid-soluble
peptides (270). Protease IIa resembled the vertebrate calpains in that it is not inhibited by PMSF or pepstatin,
whereas protease I is inhibited by PMSF (26% inhibition)
and proteases IIb and III are inhibited by pepstatin (26
and 90% inhibition, respectively). Hence, the lobster proteases differ from ␮- or m-calpain in several ways: 1) they
degrade the myofibrillar proteins, including myosin and
actin, to small peptides, whereas ␮- and m-calpain make
relatively few cleavages in myofibrillar proteins leaving
large fragments, do not degrade actin, and degrade myosin very slowly; 2) the lobster proteases do not possess a
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755
small subunit similar to the 28-kDa subunit and may be
homodimers instead of heterodimers; 3) autolysis of the
lobster proteases does not affect the Ca2⫹ concentration
required for their proteolytic activity (33); and 4) the
lobster proteases are partly inhibited by PMSF or pepstatin, which have no effect on the calpains. Despite these
differences, immunological evidence suggests that the
lobster proteases, at least proteases IIa and IIb, may be
members of the calpain family. An antibody raised against
a conserved sequence in domain II of the calpains (residues 101–121 of human ␮-calpain) labeled both ␮- and
m-calpain and the lobster IIa protein in a Western analysis, but did not label lobster IIb (33). Antibodies raised
against an expressed polypeptide representing the COOHterminal 70-kDa of Drosophila CALPA , however, labeled
lobster IIb in addition to the Drosophila CALPA, but did
not label the lobster IIa protease (33). Conversely, an
anti-lobster IIb antibody labeled the expressed Drosophila calpain but did not label either ␮- or m-calpain nor the
lobster IIa protease. Hence, the lobster IIa and IIb proteases may be members of the calpain family that are
evolutionarily so distant that they differ in several properties from the prototypical ␮- and m-calpains. Sequencing the cDNAs for the lobster IIa and IIb proteases should
eventually settle this issue.
4. Calpain-like molecules in C. elegans
Sequencing the C. elegans genome has led to identification of 12 genes, grouped in 4 classes (420), that
encode calpain-like polypeptides (48). None of the putative C. elegans calpains has been isolated in protein form,
so very little is known about the catalytic or other properties of these proteins.
One of the C. elegans calpain-like genes, tra-3, encodes a protein of 648 amino acids (73.6 kDa) having
significant sequence homology to domains II and III of ␮and m-calpain (Table 6; Ref. 24). This homology diverges
after domain III, and the COOH-terminal 142 amino acids
of tra-3 constitute a domain, called domain T, that has no
homology with domain IV of ␮- and m-calpain. The NH2terminal 41 amino acids of domain T, however, have 49%
similarity with the COOH-terminal 42 amino acids of mcalpain, which are those amino acids involved in the 28/80
subunit interaction. Genetic analysis indicates that the
tra-3 gene is involved in sex determination in C. elegans.
Transgenic nematodes overexpressing TRA-3 were used
to show that TRA-3 functions by selectively cleaving in a
Ca2⫹-dependent manner the membrane protein TRA-2A,
which binds FEM-3 and inhibits its ability to promote
male development in C. elegans (412). The T-domain of
TRA-3 does not possess the EF-hand sequences that exist
in domain IV, but the TRA-3 sequence predicts an EFhand sequence at the domain II/III boundary. Hence, the
results observed in vivo with overexpressed TRA-3 sug-
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GOLL, THOMPSON, LI, WEI, AND CONG
gest that the Ca2⫹-dependent proteolytic activity of this
molecule may be due to an EF-hand sequence at the
domain II/III boundary or to Ca2⫹ binding to domain II as
described by Moldoveanu et al. (298; see sect. IVC6). A
homolog of TRA-3 containing the T-domain has been detected in both the mouse (87) and the human (87, 304) and
has been given the name calpain 5 (Table 7). The role of
calpain 5 in mammals is a mystery; is it also involved in
sex determination in mammals? The highest levels of
calpain 5 mRNA in human tissues were in the testis, liver,
trachea, colon, and kidney (87). The ability to use a genetic approach with Drosophila and C. elegans provides a
powerful approach to studying the function of the calpains.
B. Calpain-Like Molecules in Vertebrates
The first report that the calpain system contained
molecules in addition to the three, by then well-characterized, proteins, ␮-calpain, m-calpain, and calpastatin,
appeared in 1989 when Sorimachi et al. (414) described
the presence in muscle of a mRNA that encoded a molecule having 51–54% sequence homology to the 80-kDa
subunits of the calpains (414). In addition to those molecules described in the preceding section and the 80-kDa
subunits of ␮- and m-calpain, 12 different mRNAs or
genes encoding polypeptides with sequence homology to
the calpains have been identified in vertebrates (Tables 6
and 7; Ref. 420). If the human genome contains only 14
genes that can be characterized as encoding molecules
belonging to the calpain family (86), the discovery of new
calpains, at least in mammals, may be nearing an end (as
indicated previously, identification of new calpains will
depend on what criteria are used to specify a molecule as
belonging to the calpain family). It will be necessary to
isolate the protein forms of these “new” calpains, most of
which have been identified only as DNA sequences, and to
characterize their catalytic and other properties before
the nature of the calpain family is fully understood.
Five of the putative calpains that have been identified
since 1989 seem to be tissue specific because their
mRNAs are expressed principally in skeletal (calpain 3a,
p94; Ref. 414) or smooth (calpain 8a, calpain 8b; Ref. 415)
muscle cells, in placenta (calpain 6; Ref. 87), in the testis
(calpain 11; Refs. 85, 89), in the skin during the first 16
days after birth (Capn12; Ref. 88), or in the testis and lung
(calpain 13; Ref. 86). Tissue expression is more wide-
spread for calpain 5 (269), calpain 7 (123), calpain 9 (236,
249), and calpain 10 (170). Calpain 14 mRNA could not be
detected in any of 76 tissues examined (86), and the
significance of the calpain 14 gene (Capn14) is unclear.
Some properties of the calpains that have been identified
since 1989 are summarized in Table 6. Calpain 6 is unique
in that its predicted amino acid sequence has a Lys residue in place of Cys at the active site, so this calpain likely
is not a proteolytic enzyme. Calpain 5 is the vertebrate
analog of the TRA3 calpain in C. elegans, calpain 7 (122)
is the vertebrate analog of the PalB calpain from the
fungus Aspergillus nidulans, and calpain 15 is the vertebrate analog of the SOL calpain from Drosophila. Other
than sequence homologies, very little is known about the
properties of these putative calpains, and the remainder
of this section will focus on calpain 3a (p94) and its splice
variant, Lp82 (calpain 3b); on calpain 9; on calpain 10; and
on an unusual ␮/m-calpain that has been identified only in
the chicken.
Of the 14 calpain-like DNA sequences that have been
identified in mammals, only three, ␮-calpain, m-calpain,
and Lp82 (calpain 3b), have been isolated in protein form.
Calpain 9 has been expressed in a proteolytically active
form in a baculovirus system (236). It was necessary to
coexpress the 80-kDa calpain 9 subunit and the 28-kDa
small subunit of ␮- or m-calpain to obtain a proteolytically
active form of calpain 9. The heterodimeric calpain 9
required 125 ␮M Ca2⫹ for half-maximal proteolytic activity; its proteolytic activity was inhibited by calpastatin
and E-64, but its specific activity was ⬍5% the specific
activity of ␮- or m-calpain (236). Expression of the Capn9
gene in the stomach was suppressed in gastric cancer
cells (493), and antisense-induced deficiency of calpain 9
resulted in cellular transformation and tumorigenesis of
mouse NIH3T3 fibroblasts (249). Hence, calpain 9 may
have antitumorigenic properties, but the inability to isolate a catalytically active calpain 9 protein from tissue
makes it difficult to determine how it might function in
this role. None of the other vertebrate “calpains” listed in
Tables 6 and 7 has been expressed in a proteolytically
active form, so little is known about their catalytic properties, although calpain 3a and to a lesser extent calpain
10 have been studied extensively.
Lp82 (calpain 3b) is a splice variant of the Capn3
gene that is produced by deletion of exons 6, 15, and 16,
which encode the two unique insertion sequences in calpain 3a (Fig. 6), and by use of a different exon 1 (253),
FIG. 6. Schematic diagram showing
the domain structure of calpain 3a as predicted from its amino acid sequence. The
six EF-hand sequences are shown as vertical bars, and the IS1 and IS2 domains
unique to calpain 3a are indicated.
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which results in a NH2 terminus different from calpain 3a.
Lp82 (calpain 3b) is found exclusively in the eye lenses of
a number of animal species but not the human (253, 316).
Lp82 was partly purified by anion-exchange chromatography and shown to have Ca2⫹-dependent proteolytic activity in casein zymograms (255). Lp82 seems to be proteolytically active as a single polypeptide chain, although this
has not been established conclusively because of the
inability to completely purify the enzyme. Proteolytic activity of Lp82 is poorly inhibited by calpastatin (315) but
is completely inhibited by cysteine protease inhibitors
such as E-64, suggesting that not all of the recently identified calpains may be subject to calpastatin inhibition as
␮- and m-calpain are. As indicated earlier in this review,
the weak inhibition of Lp82 activity by calpastatin may be
related to weak binding of calpastatin to a polypeptide
that lacks the small subunit and that therefore does not
have the third binding site of calpastatin (see sect. IIB3).
1. Calpain 3a (p94) and limb girdle muscular
dystrophy type 2A
Interest in calpain 3a (p94), the “skeletal musclespecific” calpain, increased dramatically when it was
demonstrated that disruption of the calpain 3a gene was
the cause of limb girdle muscular dystrophy type 2A
(LGMD2A; Ref. 366). Calpain 3a is by far the most extensively studied of the new calpains. Although Northern blot
analysis initially suggested that this calpain was expressed only in skeletal muscle (414), more sensitive
RT-PCR analyses have shown that cardiac muscle contains 46% and liver ⬃10% as much calpain 3a mRNA as
skeletal muscle (360). The Capn3 gene consists of 15
exons spanning ⬃45 kb; the calpain 3a cDNA predicts a
polypeptide of 821 amino acids having 54 and 51% sequence homology to the 80-kDa subunit of ␮- and mcalpain, respectively (Table 7, Fig. 6; Ref. 414). Mass of
the calpain 3a polypeptide is larger than mass of the
80-kDa large subunit of ␮- or m-calpain because of an
unique NH2-terminal domain I that contains 20 –30 additional amino acids not present in domain I of ␮- or mcalpain and the presence of two unique “insertion sequences” of 62 and 77 amino acids at the COOH-terminal
regions of domain II (IS1) and domain III (IS2), respectively (Fig. 6; Ref. 414). The function of these sequences is
unknown, although IS2 contains a basic PVKKKKNKP
sequence near its NH2-terminal end that has been suggested to enable occasional nuclear localization of calpain 3a.
Properties of the calpain 3a molecule remain an
enigma. First, it is puzzling that disruption of a gene
encoding a putative proteolytic enzyme should result in a
disease associated with loss of muscle mass. Second,
although the mRNA for calpain 3a is expressed at levels
⬃10 times higher than the levels of the mRNAs for the
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80-kDa subunits of ␮- or m-calpain (414), it has been
impossible to isolate the calpain 3a protein from muscle.
No proteolytic activity that might originate from calpain
3a has ever been detected in muscle homogenates, although countless assays of such homogenates have been
done during purifications of ␮- and m-calpain from muscle. An attempt to isolate calpain 3a from rabbit skeletal
muscle produced fragments of 60, 58, 55, and 31 kDa but
not a 94-kDa polypeptide (207). The evidence suggested
that the 60-, 58-, 55-, and 31-kDa polypeptides were produced by very rapid autolysis of the native 94-kDa
polypeptide, in spite of inclusion of EDTA/EGTA, leupeptin, and E-64 in the homogenizing buffer. Earlier studies
involving transfection of COS cells with a full-length
cDNA encoding calpain 3a had led to the conclusion that
calpain 3a could not be detected after transfection because it turned over very rapidly with a half-life of ⬃30
min (421). Deletion of the IS2 sequence resulted in a
stable polypeptide in the transfected cells (421). The purification studies, however, showed that the degradation
occurred in the IS1 sequence, with the 60-, 58- and finally
the 55-kDa polypeptide fragments being produced sequentially (207). Mutating Cys-129 (the active site Cys) to
Ser resulted in a stable polypeptide that could be purified
from COS cells and that eluted from a size-exclusion
column at a position corresponding to 180 kDa, suggesting that the mutated calpain 3a was a homodimer and that
the rapid autolysis was due to a cysteine protease that
should have been inhibited by E-64. It was inferred that
calpain 3a has proteolytic activity because partly purified
calpain 3a degraded mutated C129S calpain 3a to produce
a 55-kDa fragment similar to the 55-kDa autolytic fragment (207). The partly purified calpain 3a, however, was
proteolytically inactive against an exogenous substrate
(207). It seems that the two insertion sequences, IS1 and
IS2, are involved in the instability of the calpain 3a molecule; removal of these sequences results in a stable
protein (163), and Lp82 (calpain 3b), which lacks these
sequences, is stable.
A full-length calpain 3a has been stably overexpressed in transgenic mice at mRNA levels 25– 60 times
greater than the levels of endogenous calpain 3a mRNA;
proteolytic activity that was ascribed to calpain 3a was
detected by casein zymography in muscles from the transgenic mice but not in muscles from nontransgenic mice
(424). Western analysis detected polypeptides of 94, 60,
58, and 55 kDa in muscles from the transgenic mice, but
only a polypeptide of 94 kDa at a much reduced level in
muscles from the nontransgenic mice. Overexpression of
the full-length calpain 3a had little or no effect on skeletal
muscle morphology, and the mice had normal muscle
strength as measured in a grip test (424). Overexpression
of a calpain 3a isoform lacking the IS1 domain resulted in
a muscle morphology characteristic of developmental immaturity, and overexpression of a calpain 3a lacking the
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GOLL, THOMPSON, LI, WEI, AND CONG
IS2 domain affected morphology of the slow soleus muscle but not the other muscles in the transgenic mice. None
of the transgenic calpain 3 mice showed any signs of
elevated rates of skeletal muscle wasting. No attempt was
made to determine stability of the calpain 3a polypeptide
in muscles of the transgenic mice.
Yeast two-hybrid analysis (208, 417) indicated that
calpain 3a binds to two different sites along the large titin
polypeptide: 1) at intervening sequences called M-is7 at
the COOH-terminal end of the titin molecule (M-line region); this binding requires the full-length calpain 3a molecule; and 2) at the N2A region of the N2 line in skeletal
muscle myofibrils; this binding involves the IS2 sequence.
Binding to titin, which is part of the skeletal muscle
myofibril, indicates that not all the calpain 3a in muscle
cells is located in the nucleus, as was observed when
calpain 3a was overexpressed in COS cells (421). The
yeast two-hybrid studies did not detect any interaction of
calpain 3a with calpastatin or the 28-kDa small subunit.
The experimental results leave a number of questions
about the nature of the calpain 3a molecule. How can a
cysteine, putatively Ca2⫹-dependent protease, degrade itself uncontrollably in the presence of cysteine protease
inhibitors and Ca2⫹ chelators? If calpain 3a is degraded so
rapidly that it cannot be purified even with an immunoaffinity column that requires only minutes, why can it be
detected in isolated myofibrils (several hours to prepare;
Ref. 417) or in biopsies of human skeletal muscle 8 h after
their removal (6)? And finally, why would a cell contain
such large amounts of calpain 3a mRNA if the protein is
destroyed within minutes after its synthesis? If calpain 3a
is stabilized by its association with titin/myofibrils in muscle; it should be possible to isolate myofibrils and show
that calpain 3a is associated with them and that it has
proteolytic activity. This has not yet been done. Does the
function of calpain 3a require that it appear in large
quantities and then be immediately destroyed? Does the
lack of proteolytic activity of partly purified calpain indicate that it requires a cofactor for proteolytic activity that
is present in vivo but that is removed during anion-exchange chromatography?
Because calpain 3a is degraded so rapidly and/or is
proteolytically inactive after isolation, studies on calpain
3a have used antibodies elicited against synthetic peptides representing the IS1, IS2, or unique NH2-terminal
sequence of calpain 3a (Fig. 6). Although these peptide
antibodies recognize the expressed calpain 3a in transfected cells or in muscles from transgenic animals that are
overexpressing calpain 3a, the only way at present to
confirm that they label calpain 3a specifically in wild-type
skeletal muscle tissue is to isolate and sequence the
polypeptide(s) that are labeled in skeletal muscle extracts
by these antibodies to ensure that the sequence of these
polypeptides matches the sequence predicted for calpain
3a. It will be important to do this confirmation.
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Over 100 pathogenic mutations have been reported
for the calpain 3a gene. Analysis of nine of the point
mutations that have been identified in LGMD2A patients
indicated that all nine had lost the ability to degrade
fodrin in transfected COS7 cells, but that only seven had
lost their autolytic ability, three had lost the ability to bind
to the N2A region of titin, and six had lost the ability to
bind to the M-is7 region of titin (333). The results suggest
that LGMD2A is associated with loss of calpain 3a’s ability
to degrade a protein substrate. The crystallographic structure of calpain 3a has been modeled based on the similarity of its amino acid sequence to the amino acid sequence of m-calpain (195). These modeling studies suggest that many of the point mutations in calpain 3a that
are associated with LGMD2A may affect domain/domain
interactions in the calpain 3a structure and would be
expected to reduce the proteolytic activity of calpain 3a.
Based on in vivo studies, Baghdiguian et al. (16) suggested
that calpain 3a degrades I␬B␣, which binds to NF-␬B in
cells and prevents its translocation to the nucleus, where
it acts to induce expression of genes involved in the
inflammatory response and cell survival. According to this
hypothesis, the absence of proteolytically active calpain
3a prevents nuclear location of NF-␬B and results in
initiation of apoptosis in the affected nuclei. This mechanism would explain how loss of a putative proteolytic
enzyme could result in loss of muscle mass, but it does
not explain why skeletal muscle cells contain such large
amounts of calpain3a mRNA.
Suppressing calpain 3a expression in primary rat
myoblast cultures by antisense oligonucleotides does not
block fusion but interferes with organization of the myofibrillar lattice in developing muscle. Z-disks are primarily
affected and are wavy and disorganized (360). Calpain 3a
is expressed relatively late during muscle cell development after fusion of myoblasts, and calpain 3a antisense
oligonucleotides have little effect if administered before
myoblast fusion (360). Until the protein form of calpain 3a
has been isolated and its properties characterized, it will
be difficult to interpret these observations. Inhibiting proteolytic activity would be expected to result in enhanced
myofibrillar assembly, especially in the Z-disk area, which
is a target of ␮- and m-calpain degradation (and presumably, also any proteolytic activity of the homologous calpain 3a).
2. Other vertebrate calpain homologs
The human Capn10 gene maps to chromosome 2q in
humans and contains 15 exons spanning 31 kb; it is expressed as eight different isoforms that are named calpain
10a, calpain 10b, and so on to calpain 10h (170). The
calpain 10a isoform is the most abundant and is expressed
in heart (greatest levels), brain, placenta, lung, liver, skeletal muscle, kidney, pancreas, lens, and retina (170, 254).
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Calpain 10c and calpain 10g transcripts are detected in
most tissues, but calpain 10b, 10d, 10e, and 10f are less
abundant. The mechanisms regulating expression of the
different transcripts and the properties of the different
calpain 10 isoforms are unknown. An antibody elicited
against a 20-amino acid peptide from domain II of rat
calpain 10 was used to show that the calpain 10 protein
was expressed as a doublet of ⬃74.5 kDa in some tissues
that contained the calpain 10 transcripts; the calpain 10
protein was located in a water-insoluble fraction of the rat
tissues (254). Calpain 10 is an atypical calpain that has a
T domain at its COOH-terminal end, similar to calpain 5
and 6 (Table 6).
Interest in calpain 10 increased when it was learned
that a G/A polymorphism in intron 3 of the calpain 10 gene
is associated with insulin resistance and type 2 diabetes
(170, 490). The at-risk genotype was homozygous for the
UCSNP-43 G allele within the calpain 10 gene in MexicanAmerican populations but not in populations of Pima
Indians (17). Type 2 diabetes is a multifactorial disease,
and it is unclear how polymorphisms at the UCSNP-43
locus in intron 3 are related to type 2 diabetes. Intron 3
seems to have promoter activity, with the G allele having
greater promoter activity than the A allele (17). Calpain 10
may affect insulin secretion, insulin action, and hepatic
glucose production, each of which is altered in patients
with type 2 diabetes (170). It seems likely that the calpain
10 gene functions in some as yet unknown combination
with other genes in type 2 diabetes. The positional cloning
that led to identification of the association between the
calpain 10 gene and type 2 diabetes is a seminal example
of efforts to identify genes involved in multifactorial diseases such as diabetes, and illustrates the difficulties that
will be encountered in such studies.
The first calpain that was purified from chicken skeletal muscle (189) was originally believed to be a m-calpain, but recent cloning and sequencing analysis have
suggested that it is a unique calpain whose 80-kDa subunit
has a sequence intermediate (in terms of homology) between the 80-kDa subunits of ␮- and m-calpain in mammalian species (422). A chicken ␮-calpain cDNA has been
cloned and sequenced (194), and a ␮-calpain protein has
been purified from chicken skeletal muscle (481). It is
unclear, however, whether the “high m-calpain” that was
partly purified from this tissue (481) corresponds to the
m-calpain cDNA sequence obtained for the chicken (422).
The “intermediate” calpain, called ␮/m-calpain, is expressed in a number of different chicken tissues (422) and
is the predominant calpain in chicken skeletal muscle
(481). ␮/m-Calpain has not yet been identified at either the
DNA or protein level in mammalian tissues, and it presently is unclear whether it exists only in chickens or is
present in other species as well. Unlike many of the “other
calpains” or calpain-like polypeptides that have been discussed in this section, the ␮/m-calpain is well characterPhysiol Rev • VOL
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ized, is a heterodimer with a 28-kDa small subunit similar
to ␮- and m-calpain, and has a Ca2⫹ requirement of 420
␮M for half-maximal proteolytic activity (481) on the
lower edge of the Ca2⫹ requirements of the m-calpains
that have been studied thus far (Table 3).
C. Summary
Most of the calpains discussed in this section have
been identified only as genes or mRNAs having sequence
homology to the prototypal ␮- and m-calpain, or more
specifically to the protease domain of ␮- and m-calpain.
The crustacean proteases and the purified Drosophila
calpain do not have a small subunit, and many of the other
calpains either lack a domain IV or have an altered domain IV (e.g., domain T, PalB-like domain). Because domain IV contains the site(s) responsible for association of
the 80- and 28-kDa subunits in ␮- and m-calpain, it is
possible, perhaps even likely, that these other, atypical
calpains also will not have a small subunit when they
finally are isolated in protein form. It is still unclear
whether these atypical members of the calpain family
associate with a cofactor different from the 28-kDa
polypeptide that is essential for their proteolytic activity
or whether the large subunit alone is sufficient for proteolytic activity of these calpains.
The discovery of different calpain isoforms in addition to ␮- and m-calpain has clearly opened the possibility
that the calpains may participate in a wide range of hitherto unsuspected cellular functions. Does a calpain isoform that is involved in sex determination in C. elegans
and whose mammalian counterpart (calpain 5) is prevalent in the testis suggest some role in sexual maturation in
the human (mammals) for this calpain? The existence of
calpains that lack one or more of the residues involved in
the catalytic triad indicates that these calpains have some
function that does not involve proteolytic activity. Is this
same nonproteolytic function preserved in those calpains
that have proteolytic activity? Until the identification in
Arabidopsis thaliana and subsequently in Zea mays of a
gene having significant sequence homology to the calpains, it had been believed that the plant kingdom did not
have calpain-like molecules. Genomic sequencing now
indicates that all plants may have a single “calpain” gene
and that this gene encodes a polypeptide having a transmembrane domain in addition to the protease domain II
and a domain III that is homologous to domain III of ␮and m-calpain. A number of studies have found that the
animal calpains are translocated to the plasma membrane
where they degrade membrane-associated substrates.
Does the transmembrane domain in the plant calpains
suggest that the plant kingdom simply tethers their calpain to the membrane rather than translocating it there? It
may be anticipated that isolation and/or expression of the
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GOLL, THOMPSON, LI, WEI, AND CONG
protein forms of these new calpains and characterization
of their properties will lead to a more completely defined
role of the calpain system in general and to the physiological functions it has in cells. It is ironic in many ways
that the calpains currently are better known for their roles
in a variety of tissue pathologies than for what functions
they play in normal cells.
The procedures described in section IIC that have
been used to purify ␮- and m-calpain have also been used
in attempts to purify some of the recently identified calpains (calpain 3a is an example). No proteolytic activities
that might be ascribed to the molecules encoded by the
mRNAs for these recently discovered calpains have been
identified in tissue extracts, suggesting that these calpains
either have very low proteolytic activity (possibly because they are present in very small quantities) or that
they require for their proteolytic activity a cofactor that is
lost during tissue homogenation and fractionation. It may
be more productive when attempting to obtain the protein
forms of these new calpains to use expression systems
and/or to use antibodies to immunoprecipitate the proteins from tissue extracts.
Information on new members of the calpain family
has accumulated rapidly during the past 10 years, and it is
impossible to review this information in depth here. Readers are referred to several excellent recent reviews on the
“new” calpains (334, 416, 420, 438) for more detailed
information.
D. Nomenclature
As indicated earlier, it has been recommended that
the terms ␮-calpain and m-calpain be used to refer to the
well-characterized micromolar and millimolar Ca2⫹-requiring proteases, respectively (434). This recommendation has been widely adopted. The discoveries during the
past 13 years of putative calpain-like polypeptides in a
number of invertebrate species and of calpain-like genes,
sometimes expressed specifically in certain tissues in vertebrates, however, has complicated nomenclature of the
calpains. The situation is similar to myosin where 24
genes have been identified that encode molecules having
sequences with homology to the catalytic domain of myosin but that have very little similarity to the remainder of
the myosin molecule. It has been suggested (418, 438) that
the calpains can be divided into two general classes: 1)
“typical” calpains, those possessing a calmodulin-like domain at their COOH terminus, and 2) “atypical” calpains,
those lacking a calmodulin-like domain IV at their COOH
terminus, similar to the nomenclature that has been used
for the PKC family (205). Because the Ca2⫹ “switch” for
proteolytic activity of ␮- or m-calpain may reside in domain II or III of these enzymes (see sect. IIA3), it is
premature to assume that atypical calpains lacking domain IV are Ca2⫹ independent.
Physiol Rev • VOL
The recent increase in the number of calpain-like
molecules has led to a corresponding increase in the
number of names for the calpains. Unlike the myosin field
where many of the molecules have been expressed in a
catalytically active form, the lack of information on the
catalytic properties of many (most) of the calpain-like
molecules makes it difficult to develop a rational system
of nomenclature that reflects the properties of the molecules themselves. Nevertheless, it is probably useful to
discuss a systematic approach to calpain nomenclature in
the hope that this discussion will provide a framework for
a system that will eventually evolve. Table 8 lists most
(but not all) of the molecules that have been identified as
calpains or calpain-like molecules and proposes a name
for the polypeptides listed. The calpain genes have been
named numerically, Capn1 through Capn14, and it is
proposed that the polypeptide encoded by these genes
also be named numerically, calpain 1 through calpain 14
(Table 8). The polypeptides, calpain 1 and calpain 2, and
probably calpain 9, do not function as monomeric
polypeptides, so the names ␮-calpain and m-calpain are
used for molecules containing calpain 1 plus the 28-kDa
subunit and calpain 2 plus the 28-kDa subunit, respectively. To prevent confusion, the names calpain I and
calpain II that have sometimes been used to refer to
␮-calpain and m-calpain, respectively, should be avoided.
It is still unclear whether the other calpain polypeptides
function as monomers or whether they also function in
combination with some other accessory molecule. A number of small penta-EF-hand polypeptides such as ALG-2,
peflin, sorcin, and grancalin have been identified during
the last several years (210). These penta-EF-hand
polypeptides dimerize with their EF-5 hands involved in
the dimerization, similar to dimerization of the expressed
domain VI (39, 247). It has not yet been determined
whether these penta-EF-hand polypeptides can form
dimers with the large subunit of the typical calpains.
Would such dimers stabilize calpain 3? If the calpain
polypeptides function as monomers, the protein would
retain the name, calpain 3, calpain 4, etc. If they function
in combination with some other molecule, they can be
named once this identification has been made. The human
SOL calpain has been given the name calpain 15, and the
Drosophila calpains have been designated CALPA,
CALPB, CALPC, and dcalpain 15. It is proposed that calpains produced by alternative splicing be named calpain
10a, calpain 10b, and so on using letters to distinguish
newly discovered isoforms. The term CALPA⬘ has been
used to distinguish the alternatively spiced form of
CALPA, but if a number of isoforms are identified for the
invertebrate calpains, it may be more convenient to also
use letters to distinguish alternatively spliced forms of
invertebrate calpains: CALPAa, CALPAb, and so on.
The system in Table 8 is intended as a proposal for
discussion, and no attempts are made to name all the C.
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THE CALPAIN SYSTEM
TABLE
8. Some members of the calpain family, their gene names, and possible names for the polypeptides
Species
Name of Gene
Mammalian
Mammalian
Mammalian
Capn1
Capn2
Capn3
Mammalian
Mammalian
Mammalian
Mammalian
Mammalian
Mammalian
Mammalian
Capn4
Capn5
Capn6
Capn7
Capn8
Capn9
Capn10
Mammalian
Mammalian
Mammalian
Mammalian
Mammalian
Drosophila
Drosophila
Drosophila
Drosophila
C. elegans
C. elegans
C. elegans
Trypansoma brucei
Arabidopsis thaliana
Aspergillius nidulans
Capn11
Capn12
Capn13
Capn14
Capn15
CALPA
CALPB
CALPC
SOL
TRA3
CED6412 CET
TO4A8.16
F7A10.23
palB
Name of
Polypeptide
Calpain 1
Calpain 2
Calpain 3a
Calpain 3b
28-kDa subunit
Calpain 5
Calpain 6
Calpain 7
Calpain 8
Calpain 9
Calpain 10, 10a,
10b–10g
Calpain 11
Calpain 12
Calpain 13
Calpain 14
Calpain 15
CALPA
CALPB
CALPC
Dcalpain 15
CeCalpain 5
Phytocalpain
Acalpain 7
Previously
Used Name
Typical/
Atypical
␮CL
mCL
nCL-1, p94
Lp82
Typical
Typical
Typical
nCL-3, htra3
Calpamodulin
palBH
nCL-2
nCL-4
Atypical
Atypical
Atypical
Typical
Typical
Atypical
Typical
Typical
Atypical
Typical
Atypical
Typical
Typical
Typical
Atypical
Atypical
Atypical
Atypical
Atypical
Atypical
Atypical
SOLH
Calpain A
Calpain B
CG3692
SOL
TRA-3
Ce-p70
Ce-p92
Cap 5.5
DEK 1
PalB
elegans calpains or the calpains from the lower organisms, except to suggest that those calpains having homology to a mammalian calpain be named accordingly; for
example, acalpain 7 for the Aspergillus nidulans calpain
with a palB domain similar to calpain 7, dcalpain 15 for
CALPD. Hopefully, Table 8 can serve as a framework for
discussion of calpain nomenclature.
IV. BIOCHEMICAL PROPERTIES AND
REGULATION OF CALPAIN ACTIVITY
The preceding two sections have summarized the
structural aspects of the calpain system, and the last three
sections will discuss the enzymatic properties and physiological significance of the calpain system. The discussion
in the latter three sections will be devoted almost entirely
to ␮- and m-calpain and calpastatin because these are the
only members of the calpain system whose catalytic (and
other properties) have been studied extensively thus far.
The recent advances in calpain structure, the availability
of amino acid sequences for the calpains and calpastatin,
and the increasing information on properties of the calpains and calpastatin, all reviewed in section II, now make
it possible to propose clearly defined mechanisms for
regulation of calpain activity in cells, so regulation of
calpain activity will be reviewed in detail. Discussion of
the physiological function(s) of the calpains will focus on
those areas where there is definitive evidence implicating
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Protein/Comments
␮-Calpain (calpain 1 ⫹ 28 kDa)
m-Calpain (calpain 2 ⫹ 28 kDa)
Limb girdle muscular dystrophy type 2A
Not a member of the 80-kDa calpain family
Human homolog of C. elegans TRA3
Cys/Lys, His/Tyr, substitutions in the catalytic triad
Human homolog of Aspergillus Palb
Associated with gastric cancer
Associated with type 2 diabetes mellitus
Human homolog of Drosophila SOL
SOL domain (small optic lobe) in COOH terminus
Sex determination in C. elegans
SOL domain in COOH terminus
PBH domain in COOH terminus
Calpastatin-like domain in NH2 terminus
Transmembrane domain in NH2 terminus
PBH domain in COOH terminus
the calpains. There is an extensive literature suggesting
that calpains are involved in a variety of physiological
functions. In many instances, however, the studies have
used inhibitors having unproven or unsuspected effects,
and it is unclear whether the findings are the result of
calpain inhibition or inhibition of some other proteolytic
enzyme. Hence, review of the physiological functions of
the calpains will be relatively brief, considering the extensive literature in the area, and will emphasize those
areas where unambiguous evidence supports a clearly
defined role for the calpains. Similarly, there have been a
number of reviews on the pathological consequences of
unregulated calpain activity, and space restrictions (and
limits of readers’ patience) will limit the discussion in this
section to a listing of diseases/conditions that have been
ascribed to inappropriate calpain activity.
A. Expression of Proteolytically Active Molecules
The calpains have been difficult to study, partly because it has been difficult to purify quantities of them in
an undegraded form and partly because addition of Ca2⫹
to study their catalytic properties or their structural response to Ca2⫹ results almost immediately in self-destruction through autolysis. Therefore, the ability to express
catalytically active calpains would significantly enhance
study of the calpains by making quantities of these proteins available in purified form and making it possible to
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GOLL, THOMPSON, LI, WEI, AND CONG
use mutagenesis to determine the effects of modifying
specific residues/domains on the catalytic properties of
the expressed molecules. Catalytically active rat m-calpain was expressed in 1994 in an E. coli expression
system with yields of 0.6 mg/l (142); much higher yields
are now obtained from this expression system (104). Although a m-calpain containing the full-length 28-kDa subunit could be expressed in a catalytically active form, the
28-kDa subunit was truncated at amino acid 85 to obtain
reasonably high yields of expressed m-calpain. Catalytically active human m-calpain was subsequently expressed
in a soluble form in a baculovirus expression system with
yields of 20 mg/l Sf9 cell culture (268). The Ca2⫹ concentrations required for a half-maximal rate of proteolytic
activity for the two expressed calpains were the same as
those required by the calpains that had been purified from
tissues (400 ␮M for the baculovirus calpain; 350 ␮M for
the E. coli calpain; cf. Table 3).
Attempts to express catalytically active ␮-calpain
have not been as successful as expression of m-calpain. It
has been difficult to repeat the early studies reporting that
catalytically active ␮-calpain could be obtained from either E. coli (469) or baculovirus (292, 469) expression
systems. Both reports indicated that the 80-kDa subunit of
␮-calpain was proteolytically active when expressed
alone, and Vilei et al. (469) were able to express proteolytically active ␮-calpain mutants lacking domains III or
IV and a mutant ␮-calpain containing domains I, II, and III
from ␮-calpain and domain IV from m-calpain. When
␮-calpain is expressed in E. coli, the ␮-calpain is deposited in inclusion bodies and only very low yields of catalytically active ␮-calpain are obtained (104). Similar low
yields of proteolytically active ␮-calpain have been obtained when the baculovirus system is used (104). Expression of a catalytically inactive form of ␮-calpain with the
active site Cys-115 mutated to Ser has been reported using
a baculovirus system (167), but the yields of this expression were not specified. Wild-type and mutant expressed
m-calpain have already been used to study the effects of
site-directed mutagenesis and amino acid deletions on
properties of m-calpain (discussed in sect. IV, C2 and C6),
and the ability to express catalytically active ␮-calpain
would make such studies possible also with ␮-calpain.
B. Tissue Localization
1. Presence in different tissues
One or more of the three well-characterized members
of the calpain system, ␮-calpain, m-calpain, and calpastatin, has been detected in every vertebrate cell that has
been carefully examined for their presence (139, 196,
308). Different tissues/cells, however, differ widely in
their ratios of the three proteins. For example, human
platelets and erythrocytes have no detectable m-calpain
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(308, 452), whereas bovine platelets (452), turkey gizzard
smooth muscle (136), and vascular smooth muscle (271)
have no detectable ␮-calpain. In general, skeletal muscle
from domestic animals contains approximately equal
amounts of ␮-calpain and m-calpain and contains calpastatin activity in excess of the activity of the two calpains together. Calpastatin activity exceeds activity of the
two calpains together in most but not all cells/tissues. The
approximate relative proportions of ␮-calpain, m-calpain,
and calpastatin in 20 different mammalian tissues are
summarized in Thompson and Goll (457). As indicated
earlier in this review, calpastatin has not been detected in
invertebrate tissues, although Drosophila, Schistosoma
mansoni, and C. elegans contain calpain.
2. Immunolocalization
Immunolocalization studies have shown that both ␮and m-calpain and calpastatin are located exclusively intracellularly; the few reports of extracellular localization
have been attributed to poor fixation procedures or to
diseased tissues where the calpains/calpastatin had
“leaked” from cells (222, 436). Many of the immunolocalization studies have been done on skeletal or cardiac
muscle. In normal skeletal muscle, most of the calpain
and calpastatin is located on or next to the Z-disk with
smaller amounts in the I-band and very little in the A-band
area (134, 139, 222, 425). Immunogold electron microscope studies using an antibody specific to ␮-calpain also
found that ␮-calpain was localized primarily in the Z-disk/
I-band region of sections of rat skeletal muscle (494).
Most immunolocalization studies have found that the
calpains and calpastatin are colocalized in cells (134, 139,
222), and Barnoy et al. (25) found that they coprecipitate
from extracts of rat L8 myoblasts. Immunofluorescence
studies indicated that calpastatin was localized at the
Z-disk in sections of cardiac muscle (232), the same location as the calpains (222). Several studies reported that
m-calpain is localized at the plasma membrane in dividing
(397) or fusing (396) L6 or L8 myoblasts and during metaphase or interphase in cultured PtK1 cells (398). These
and similar results in other tissues (378) have led to the
suggestion that the calpains relocate from a widely dispersed distribution to a preferential location near the cell
periphery in response to some cellular signal, possibly a
Ca2⫹ flux. A recent study using COS 7 and LCLC 103H
cells supports this suggestion. Both the 28- and 80-kDa
calpain subunits and calpastatin were widely distributed
in the cytoplasm of unstimultated cells with a slight predominance around the nucleus (127). After stimulation
with a Ca2⫹ ionophore, ionomycin, the calpain subunits
redistributed to a predominately plasma membrane location, but the calpastatin localization did not change. No
such redistribution of the calpains has ever been observed, however, in sections of postnatal rat soleus mus-
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cle (134) or in neuronal tissue (20), although calpastatin
was localized at or near the sarcolemma in cultured cardiocytes (232). It is unclear whether the calpains and
calpastatin can be translocated to the plasma membrane
in cultured, mitotically active cells, but remain stationary
in mitotically inactive tissues such as striated muscle
fibers or neurons. Immunoelectron localization found that
93% of the calpain in human erythrocytes was cytosolic
and the remaining 7% was associated with the plasma
membrane (386). If the calpains are activated by association with the plasma membrane, as has been suggested
(184, 217, 218; see sect. IVC1), these results would suggest
that only 7% of the calpain in human erythrocytes was
enzymatically active. In dystrophic skeletal muscle, the
calpains seem to be distributed more uniformly throughout the muscle fiber with less concentration at the Z-disk
than in normal muscle (223, 425).
Several studies have suggested that a substantial proportion of the calpains are not free in the cell cytoplasm,
as has been widely assumed, but that they are associated
with subcellular structures (222). In skeletal muscle,
these structures are myofibrils (222), whereas in other
cells they may be cytoskeletal (actin) filaments (231). The
immunolocalization studies thus far have not provided
much insight into function of the calpain system because
most studies find a widespread cytosolic distribution,
rather than an association with a particular organelle.
C. Regulation of Calpain Activity
It can be estimated that all Z-disks in a skeletal
muscle cell/fiber would be destroyed in ⬍5 min if all the
calpain, which as indicated in the preceding section is
largely located directly on the Z-disk, in that fiber were
proteolytically active. Hence, most of the calpain in a cell
must be proteolytically inactive most of the time. Based
on the current knowledge of the catalytic properties of
the calpains, it seems that calpain activity in cells is
regulated by altering the Ca2⫹ concentration required for
its proteolytic activity, by calpastatin, and probably also
by the intracellular location (access to substrate) and the
seemingly fastidious subsite specificity of the calpains.
1. The Ca2⫹ requirement problem
Ever since the first partial characterization of mcalpain (84), it has been clear that the Ca2⫹ concentrations required for proteolytic and other activities of the
calpains were much higher than the 50 –300 nM Ca2⫹
concentrations that exist in living cells (192, 266; cf. Table
3). Only the Ca2⫹ concentrations required by autolyzed
␮-calpain are in the physiological range, but Ca2⫹ concentrations in the 800 –15,000 nM range are required to initiate autolysis of ␮-calpain (Table 3). Consequently, there
have been numerous attempts to identify a mechanism for
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reducing the Ca2⫹ requirements of the two ubiquitous
calpains. So far, these efforts have not produced any
conclusive answers.
Efforts at identifying a mechanism for reducing the
Ca2⫹ requirements of ␮- and m-calpain (an “activation”
mechanism) have focused on two areas: 1) finding a
mechanism(s) for reducing the Ca2⫹ concentration
needed to induce autolysis, with the assumption that the
autolyzed enzymes could be active at physiological Ca2⫹
concentrations, and 2) attempts to find molecules in cells
that would interact with/alter the calpains and reduce
their Ca2⫹ requirements.
It was discovered in 1984 that certain phospholipids,
with phosphatidylinositol (PI) being the most effective,
would lower the Ca2⫹ concentration required for autolysis of m-calpain by nearly eightfold (66). Subsequent studies have shown that several phospholipids, with phosphatidylinositol 4,5-bisphosphate (PIP2) being the most effective (373, 376), lower the Ca2⫹ concentration required for
autolysis of either ␮-calpain (62, 373, 376) or m-calpain
(62) by three- to sixfold (Table 3). Very high molar ratios
of 375– 4,100 phospholipid (PI or PIP2) to one calpain,
however, were necessary to reduce the Ca2⫹ concentration required for initiation of autolysis in in vitro assays,
and neither PIP2 nor PI had any effect below these concentrations. The PIP2 and PI were micellar at these concentrations, and it is uncertain whether PIP2 has to be in
a micellar form to induce a reduction in Ca2⫹ requirement. It seems unlikely that the calpains would ever
encounter PIP2 molar ratios of 375– 4,100 to one calpain
or PIP2 micelles in vivo. Finally, even in the presence of
phospholipid, the Ca2⫹ concentrations required for autolysis of m-calpain are far above those that exist in living
cells, and the Ca2⫹ concentrations required for autolysis
of ␮-calpain are at the upper limits of in vivo Ca2⫹ concentrations (Table 3). Hence, ␮-calpain would autolyze
only in the presence of Ca2⫹ “spikes” of 1–10 ␮M that
might occur in localized areas. It has been suggested that
the calpains interact with a phospholipid at the plasma
membrane (or other cellular membranes), perhaps in the
area of a Ca2⫹ channel, and that this interaction is of a
nature that would reduce the Ca2⫹ requirement for autolysis. Several studies, however, have reported that the
calpains bind to proteins and not phospholipids in the
plasma membrane (184, 186, 219), and other studies have
shown that when the calpains are incubated with insideout erythrocyte membranes, the Ca2⫹ concentration required for their autolysis is not affected (T. Zalewska, V.
Thompson, and D. Goll, unpublished data). If these latter
results are indeed accurate, it is unclear whether interaction of the calpains with the plasma membrane would
have any effect on autolysis. Studies using two irreversible inhibitors to label ␮-calpain in human platelets indicated that the calpain in these platelets was autolyzed in
the cytosol before being translocated to the membrane
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GOLL, THOMPSON, LI, WEI, AND CONG
(4). In sum, there are a number of uncertainties as to
whether phospholipids/autolysis could be the “activation”
mechanism sought for the calpains, although as indicated
earlier, autolysis and possibly also interaction with phospholipids likely has some as yet unknown role in calpain
function.
A number of studies have described molecules that
seemed to reduce the Ca2⫹ requirements of the calpains
in in vitro assays. It was reported that isovalerylcarnitine
reduced the Ca2⫹ concentration required for maximal
proteolytic activity of m-calpain to ⬍10 ␮M and increased
its specific activity by 1.5-fold, but did not affect either the
Ca2⫹ requirement or the specific activity of ␮-calpain
(354, 356). Other studies described a heat-stable polypeptide of ⬃40 – 45 kDa isolated from human neutrophils
(353) or rat skeletal muscle (357) that reduced the Ca2⫹
concentration required for maximal activity of m-calpain
to 20 ␮M or less without affecting its specific activity. The
40- to 45-kDa “activator” also had no effect on the catalytic properties of ␮-calpain. A similar 40-kDa “activator”
protein that was purified from erythrocytes (383) reduced
the Ca2⫹ concentration required for half-maximal activity
of erythrocyte calpain, which is a ␮-calpain, from 50 to
⬃0.2 ␮M; this activator also reduced the Ca2⫹ requirements of m-calpain similar to the previously isolated ones.
More recently, a protein having a monomeric mass of 15
kDa but functioning as a dimer of 30 kDa reduced the
Ca2⫹ requirement of ␮-calpain from a variety of sources
to 0.4 – 0.5 ␮M (289), but had no effect on m-calpain.
Sequence analysis indicated that this protein was very
similar to a goat liver protein, UK114. The UK114-like
protein was active at a 1:1 molar ratio of UK114:calpain. A
subsequent study identified a 10-kDa polypeptide that
also functioned as a dimer of 20 kDa and that reduced the
Ca2⫹ requirement of m-calpain from several sources to
6 –7 ␮M (290). Sequencing followed by cloning and expression identified this protein as acyl-CoA binding protein. The acyl-CoA binding protein was maximally active
at a 1:1 molar ratio, acyl-CoA binding protein:m-calpain,
and had only a minimal effect on the Ca2⫹ requirement of
␮-calpain.
The significance of these activator proteins is still
unclear. The isovalerylcarnitine studies required molar
ratios of nearly 20,000 isovalerylcarnitine:1 calpain for a
reduction in Ca2⫹ requirement, raising the possibility that
some contaminant was the active agent or that nonspecific effects were involved. It is difficult to determine the
molar ratios used in the studies describing the 40- to
45-kDa activators, but if 4 ␮g activator was required to
“activate” 3 ␮g m-calpain (353), a molar ratio of ⬃4 activator:1 m-calpain was required for activity of this activator. The 40- to 45-kDa activator bound to m-calpain in a
1:1 molar ratio (293), and it is unclear whether the 40- to
45-kDa activator reduced the Ca2⫹ requirement of mcalpain at this ratio. One to one molar ratios of either the
Physiol Rev • VOL
UK114-like or the acyl-CoA binding protein were required
to reduce the Ca2⫹ requirement of the calpains. Hence,
these two activators did not act catalytically (for example,
via phosphorylation or dephosphorylation) to change the
Ca2⫹ requirement of the calpains, but would need to bind
to the calpains in a manner similar to the calmodulin
binding that activates a number of different enzymes. The
UK114-like activator was reported to enhance dissociation of the two subunits of ␮-calpain (289), but recent
results have shown that dissociation of the two subunits
results in aggregation of the 80-kDa subunit and loss of
proteolytic activity, not a reduction in Ca2⫹ requirement
(245).
Several other calpain activator proteins have been
described. These proteins increased the catalytic activity
(Vm) of the calpains by 25- (92), 10- (449), or 2-fold (406)
without affecting the Ca2⫹ concentration required for
proteolytic activity. There have been no subsequent studies describing these activators, and their nature is unknown.
2. Ca2⫹-binding properties of ␮- and m-calpain
Attempts to determine the number and binding affinities of the Ca2⫹ bound to the calpains have encountered
several technical difficulties. First, as described in section
2⫹
IIA4, the calpains autolyze rapidly in the presence of Ca ,
so ways must be found to prevent autolysis. Second, the
calpains aggregate/precipitate at Ca2⫹ concentrations
above 1 mM (11, 98), and this precipitation is likely to
impact Ca2⫹ binding measurements.
To prevent difficulties due to autolysis, domain IV
(Fig. 1) from chicken ␮/m-calpain, rabbit ␮-calpain, or
rabbit m-calpain and domain VI of rabbit calpain were
expressed and were assayed for their Ca2⫹-binding properties (295). Chicken ␮/m-domain IV bound 2.1 Ca2⫹ with
a Kd of 61 ␮M; rabbit ␮-calpain domain IV bound 1.6 Ca2⫹
with a Kd of 35 ␮M; rabbit m-calpain domain IV bound 4.0
Ca2⫹ with a Kd of 130 ␮M; and the rabbit small subunit
bound 2.1 Ca2⫹ with a Kd of 150 ␮M. Site-directed mutagenesis studies of domain VI suggested that the EF-2⬘
and EF-5⬘ sites bound the two Ca2⫹ that were bound by
this domain and that the EF-4⬘ site did not bind Ca2⫹
(294). Binding to proteins enhances the fluorescence of
terbium, and this property was used to show that bovine
erythrocyte ␮-calpain bound 4 mol Tb3⫹/mol with Kd
values of 10⫺5 to 10⫺7 M and that bovine brain m-calpain
bound 6 mol Tb3⫹/mol with Kd values of 10⫺6 to 10⫺4 M
(503).
The studies using expressed domains IV and VI were
done before identification of the two Ca2⫹-binding sites in
domains IIa and IIb in the 80-kDa subunit (see sects. IIA1
and IVC6), and because the isolated domains IV and VI
were used, the EF-5 hands, which now are known to be
involved in association of the two subunits and do not
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THE CALPAIN SYSTEM
bind Ca2⫹ in the intact calpain molecule, bound Ca2⫹.
Calpain precipitates at 250 ␮M Tb3⫹, so accuracy of the
Tb3⫹-binding studies is uncertain.
Critical, bidentate Ca2⫹-ligating residues in individual
EF-hands were mutated in expressed rat m-calpain to
reduce or eliminate Ca2⫹ binding to the mutated EF-hand
(98). The mutations were done either in single EF-hands
or in various combinations of EF-hands. X-ray crystallography was used to confirm that the mutations abolished
Ca2⫹ binding at the mutated site, and proteolytic activity
was used to infer Ca2⫹ binding; direct measurements of
Ca2⫹ binding were not attempted. The results showed
that 1) mutations in EF-1 in either subunit had only small
effects on the Ca2⫹ concentration required for half-maximal activity (K0.5); changes in K0.5 ranged from ⫺115 to
⫹66 ␮M from the 367 ␮M Ca2⫹ K0.5 for the wild-type
calpain; 2) mutations in EF-2 in either subunit increased
the K0.5 slightly, from 14 to 232 ␮M above the 367 ␮M; 3)
mutations in EF-3 had the largest effects on K0.5, with
mutations in EF-3⬘ of the small subunit increasing K0.5 by
⬃330 ␮M and mutations in EF-3 of the large subunit
increasing K0.5 by 466 –590 ␮M; 4) mutations in EF-4 of
either subunit had little effect on the K0.5 of the enzyme,
in agreement with the crystallographic results showing
very weak and abnormal Ca2⫹-binding to this site (39,
247); 5) when three EF-hands (omitting EF-4) in either
subunit (but not both simultaneously) were mutated, the
K0.5 rose to 940 ␮M, indicating that Ca2⫹ binding to either
subunit was sufficient for proteolytic activity; and 6) mutation of EF-3 in both the large and small subunits increased the K0.5 from 367 to 1,699 ␮M, and mutation of
EF-1, EF-2, and EF-3 in both subunits simultaneously
increased the K0.5 from 367 to 7,410 ␮M, illustrating the
importance of EF-3 in either subunit (98). The results of
this study indicate that Ca2⫹ binding to EF-hands in the
calpain molecule is highly cooperative, with Ca2⫹ binding
at any one site (possibly excluding EF-4) being sufficient
to initiate proteolytic activity of the enzyme (98). The fifth
EF-hand in each domain (EF-5 and EF-5⬘) was not studied
because it is involved in subunit association and does not
bind Ca2⫹. That mutation of EF-4 in either the 28- or the
80-kDa subunit has almost no effect on the Ca2⫹ requirement of expressed m-calpain makes it unlikely that the
two EF-4 sites in the calpain molecule are involved in
functional Ca2⫹ binding. Similarly, the EF-hand-like sequence identified at the domain II/III boundary does not
have a EF-hand-like structure in the crystallographic
structures of m-calpain (171, 430), and it is uncertain
whether this site binds Ca2⫹, at least in the m-calpain
molecule (it does bind Ca2⫹ however, in the calpain isolated from Schistosoma mansoni). Recent studies have
identified two additional Ca2⫹-binding sites in domains IIa
and IIb; these Ca2⫹-binding sites are not EF-hand sequences but are in peptide loops (see sect. IVC6; Ref. 298).
Equilibrium dialysis and a membrane filtration assay
Physiol Rev • VOL
765
were used in an attempt to measure the Ca2⫹-binding
properties of intact, unmutated, unautolyzed calpains
(M. Matsuishi, V. Thompson, and D. Goll, unpublished
data). The membrane filtration assay was rapid, so autolysis was minimized in these experiments, and all studies
were done at 4°C, also to minimize autolysis. In addition,
a complete set of experiments was done using calpains
that had been reversibly inactivated by using tetrathionate oxidation (199) to prevent autolysis altogether. Oxidative inactivation had little effect on the Ca2⫹-binding
properties of the calpains. Scatchard analysis was used to
determine Kd values and the number of Ca2⫹ bound per
molecule. For ␮-calpain: membrane filter assay-YM-10
membranes, 7.3 Ca2⫹/molecule and Kd ⫽ 61.1 ␮M; membrane filter assay-PVDF membranes, 8.3 Ca2⫹/molecule
and Kd ⫽ 20.6 ␮M; equilibrium dialysis, 4.6 Ca2⫹/molecule
and Kd ⫽ 32 ␮M. For m-calpain: membrane filter assayYM-10 membranes, 15.9 Ca2⫹/molecule and Kd ⫽ 502 ␮M;
membrane assay-PVDF membranes, 20.3 Ca2⫹/molecule
and Kd ⫽ 1,300 ␮M; equilibrium dialysis, 11.0 Ca2⫹/molecule and Kd ⫽ 322 ␮M. Hence, based on this study,
␮-calpain binds ⬃5– 8 Ca2⫹/molecule with a Kd of ⬃
21– 61 ␮M, whereas m-calpain binds many more Ca2⫹,
⬃11–20/molecule with a much higher Kd, 322–1,300 ␮M.
The variability in the results attests to the inherent difficulty in measuring the Ca2⫹-binding properties of the
calpains. The data, however, indicate that m-calpain has
more Ca2⫹-binding sites than would be accounted for by
the six EF-hand structures in the molecule, although
these sites have low affinity for Ca2⫹, whereas the Ca2⫹
bound by ␮-calpain can be accounted for by the six
EF-hand Ca2⫹-binding sites and the two Ca2⫹-binding
sites in domains IIa and IIb. Also, the Kd values for the
two calpains reflect their respective Ca2⫹ requirements
for proteolytic activity. Hopefully, it will be possible to
find new approaches to accurately measure the Ca2⫹binding properties of the calpains with precision; such
measurements are needed to show whether Ca2⫹ in addition to those binding to the EF-hand structures in domains IV and VI are involved in the catalytic activity of the
calpains.
3. Subsite specificity
The initial studies showing that m-calpain degraded
tropomyosin, troponins T and I, and C-protein but did not
degrade myosin and actin, the two major proteins in
skeletal muscle myofibrils, implied that the calpains had a
limited and very specific subsite specificity (82). In general, the calpains cleave proteins at a limited number of
sites and produce large polypeptide fragments rather than
small peptides or amino acids (138, 139, 310). Early studies suggested that the calpains preferentially cleaved peptide bonds having a Leu or a Val residue in the P2 position
(reviewed in Ref. 310), although this was not an absolute
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766
GOLL, THOMPSON, LI, WEI, AND CONG
requirement. More detailed analysis, however, has indicated that subsite specificity of the calpains is not governed by amino acid sequence, but rather by conformation of the polypeptide chain (75, 116, 152, 428). A summary of 23 sites cleaved by m-calpain in 5 different
proteins indicates that only 3 of them have a Leu or Val in
the P2 position (Table 9; the cleavage sites listed are
intended only as an example; see Ref. 76 for a more
complete list). Indeed, the variety of amino acids in the P2
position ranging from negatively charged to positively
charged to polar suggests that the P2 amino acid has only
a marginal effect on the specificity of the calpains.
Brain ␣-spectrin (␣-fodrin) is cleaved at a site in
repeat 11 by ␮-calpain: Q-Q-E-V-Y(1176)—-G(1177)-M-MP-R, to produce fragments of ⬃150 and 145 kDa in SDSPAGE. This same site evidently is also cleaved by mcalpain (52). Site-directed mutagenesis was used to substitute 20 different amino acids into the P2 position (428).
The substitutions significantly affected the rate of digestion of the expressed polypeptide by ␮-calpain, with Arg,
Val, and Ala favoring cleavage and Gly, Asp, Pro, and His
protecting against cleavage (428). Those amino acid substitutions that increased rate of calpain cleavage also
disrupted the structure of the expressed polypeptide, as
indicated by molecular modeling studies and additional
proteolytic degradation of the expressed polypeptide,
whereas those amino substitutions that decreased rate of
calpain cleavage stabilized the expressed polypeptide; the
results suggested that rate of calpain cleavage is directly
linked to polypeptide conformation. Nine of the 11 cal-
TABLE
pain cleavages in the ␣-tropomyosin polypeptide are in
the COOH-terminal half of the molecule (Table 9). Although the tropomyosin polypeptide is 100% ␣-helical, the
COOH-terminal half of the helix is significantly less stable
than the NH2-terminal half, again suggesting that subsite
specificity of the calpains depends on the conformation of
the polypeptide, with a more open structure favoring
cleavage. An early study on the subsite specificity of
m-calpain found that m-calpain cleaved vimentin at 10
sites and that all 10 were in the nonhelical, unordered part
of the molecule (116). The very limited degradation of
most polypeptides by the calpains, however, indicates
that specificity is regulated by more than just an “open”
structure, but the nature of these additional regulatory
interactions is unknown. Small peptides (3–5 amino acids) are very poor substrates for the calpains with Km
values in the millimolar range (390), again indicating that
the subsite recognition by the calpains uses a fairly large
area of the polypeptide substrate.
The specificities of ␮- and m-calpain are very similar
if not identical (75; Thompson and Goll, unpublished
data). Both ␮- and m-calpain seem to cleave the same
peptide bonds in ␣-tropomyosin, although they may
cleave them at slightly different rates (Thompson and
Goll, unpublished data).
Rates of calpain digestion of some proteins are affected significantly by phosphorylation of the protein substrate, and in some instances, the sites that are cleaved
seem to be affected. PKA phosphorylation of troponin I
(TN I) reduced its susceptibility to degradation by ␮-cal-
9. Amino acid residues on either side of the peptide bonds cleaved by m-calpain
Protein
Rabbit skeletal ␣-tropomyosin*
EGF receptor kinase†
Clupenine Y11‡
Clupenine 2‡
p35§
Cleavage
Cleavage
Cleavage
Cleavage
Cleavage
Cleavage
Cleavage
Cleavage
Cleavage
Cleavage
Cleavage
Cleavage
Cleavage
Cleavage
Cleavage
Cleavage
Cleavage
Cleavage
Cleavage
Cleavage
Cleavage
Cleavage
Cleavage
1
2
3
4
5
6
7
8
9
10
11
1
2
3
4
5
6
7
1
2
1
2
1
P4 P3 P2 P1
P⬘1 P⬘2 P⬘3 P⬘4
Gln-Met-Leu-Lys (12)
Gln-Ala-Glu-Ala (27)
Glu-Glu-Arg-Ala (183)
Thr-Asn-Asn-Leu (204)
Asn-Asn-Leu-Lys (205)
Lys-Ser-Glu-Glu (208)
Ala-Gln-Ala-Glu (212)
Gln-Lys-Glu-Asp (219)
Lys-Tyr-Glu-Glu (223)
Arg-Ala-Glu-Phe (241)
Ile-Asp-Asp-Leu (256)
Arg-Leu-Leu (659)
Trp-Ile-Pro (709)
Ser-Thr-Ser (1006)
Ser-Cys-Pro (1035)
Asp-Thr-Phe (1062)
Ser-Thr-Phe (1127)
Pro-Asn-Gly (1161)
Pro-Val-Arg-Arg (14)
Arg-Val-Ser-Arg (22)
Pro-Val-Arg-Arg (15)
Arg-Val-Ser-Arg (23)
Leu-Ser-Thr-Phe (98)
Leu (13)-Glu-Lys-Glu
Asp (28)-Lys-Lys-Arg
Glu (184)-Leu-Ser-Glu
Lys (205)-Ser-Leu-Glu
Ser (206)-Leu-Glu-Ala
Ala (209) Gln-Ala-Glu
Lys (213) Tyr-Ser-Glu
Lys (220)-Tyr-Glu-Glu
Glu (224)-Glu-Ile-Val
Ala (242)-Glu-Arg-Ser
Glu (257)-Asp-Asp-Glu
Gln (660)-Glu-Arg
Glu (710)-Gly-Glu
Arg (1007)-Thr-Pro
Ile (1036)-Lys-Glu
Leu (1063)-Pro-Val
Asp (1128)-Ser-Pro
Ile (1162)-Phe-Lys
Arg (15)-Arg-Pro-Arg
Arg (23)-Arg-Arg-Ala
Arg (16)-Arg-Pro-Arg
Arg (24)-Arg-Arg-Ala
Ala (99)-Gln-Pro-Pro
* V. F. Thompson and D. E. Goll, unpublished results. Digestion at 25°C was allowed to go to completion, and the fragments were separated
and analyzed.
† From Gregoriou et al. (144).
‡ From Hayashi et al. (160).
§ From Lee et al. (237).
Physiol Rev • VOL
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THE CALPAIN SYSTEM
pain by approximately fivefold, whereas PKC phosphorylation increased susceptibility of TN I to ␮-calpain degradation by approximately twofold (95). In situ phosphorylation of platelet filamin (actin-binding protein) by PKA
decreased its susceptibility to degradation by ␮-calpain
by nearly twofold (53). The effect was specific for ␮-calpain because phosphorylation in vitro by PKA did not
affect filamin degradation by trypsin, papain, or thermolysin (500). Cleavage of phosphorylated and dephosphorylated filamin occurred at similar sites (500). PKA
phosphorylation of the microtubule-associated protein
tau decreased its susceptibility to m-calpain and also
altered the pattern of peptide fragments produced during
digestion (248). Binding of calmodulin may also affect
rate of cleavage by the calpains (191, 450).
That binding of calmodulin or phosphorylation can
alter the rate and in some instances the sites that calpains
cleave in proteins substantiates the conclusion that subsite specificity of the calpains is regulated by polypeptide
conformation and raises the possibility that degradation
of physiological substrates by the calpains may be regulated not only by modifying the calpains but also by
altering the substrates. It has been suggested that PEST
sequences favor calpain cleavage, but subsequent studies
have indicated that PEST sequences do not influence
susceptibility to the calpains (300).
4. Phosphorylation of the calpains
Although an early study found that the calpains were
not phosphorylated in vivo (2), reexamination of calpain
767
phosphorylation by using antibodies specific for phosphoSer, phospho-Thr, and phospho-Tyr and MALDI-TOF
mass spectrometry found that both ␮- and m-calpain are
phosphorylated at multiple sites in situ (60). Calpains
prepared from a variety of tissues including bovine skeletal muscle, bovine cardiac muscle, human placenta, and
human platelets were used. The calpains were isolated
from these tissues with no attempt to inhibit intracellular
phosphatases, and the results, therefore, are representative of the calpains that have been used in in vitro studies
during the past 35 years. Both types of assay indicated
that ␮-calpain is phosphorylated at nine sites and that
m-calpain is phosphorylated at eight sites (Fig. 7; Ref. 60).
All sites that have been identified by MALDI are on the
80-kDa subunit; in several instances (4 of 19), the antiphospho-antibodies have detected a phospho-Ser in domain V of the 28-kDa subunit. This phospho-Ser would be
either S65 or S68. Phosphorylated residues have never
been detected in domains IV or VI. There are two phospho-Tyr, three phospho-Thr, and four phospho-Ser in
␮-calpain and one less phospho-Ser in m-calpain (Fig. 7).
The phosphorylated residues are clustered in two areas of
the calpain molecule: 1) at the domain I/II junction (IIa)
where there are four phosphorylated residues in residues
77– 81 (␮-calpain) or residues 66 –70 (m-calpain) and 2) at
the NH2 terminus of domain II (IIb) where there are four
(␮-calpain) or three (m-calpain) phosphorylated residues
(Fig. 7). The phosphorylated residues are in equivalent
positions in the ␮- and m-calpain polypeptides, except for
S360 and T362 on ␮-calpain and T316 on m-calpain (Fig. 7).
FIG. 7. Schematic diagram showing
the locations of sites that are phosphorylated in the domains predicted from the
amino acid sequences of ␮- and m-calpain
and in the domains that are predicted from
the crystallographic structure of m-calpain. Yellow circles, phosphotyrosine resides; blue circles, phosphoserine residues; green circles, phosphothreonine residues. Because the crystallographic
structure of ␮-calpain is not yet available,
the domain structure as predicted from the
amino acid sequences is used to permit
comparison of the phosphorylated sites on
the two calpains.
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GOLL, THOMPSON, LI, WEI, AND CONG
The sequences surrounding the phosphorylated residues
suggest that there are two PKA sites, two (␮-calpain) or
one (m-calpain) PKC sites, two calmodulin kinase II sites,
a casein kinase I site, and a protein kinase G site.
Not all sites are phosphorylated in all molecules; the
sites at residues 379/380 (␮-calpain) or 369/370 (m-calpain) are phosphorylated in most calpains, but the other
sites are phosphorylated less frequently. Direct assays
indicated that, on average, the calpains contained 2– 4
phosphates/molecule. The phosphates cannot be removed
by various phosphatases unless the calpain is in a solution
containing 500 ␮M Ca2⫹ or more. Removal of all the
phospho-Ser and phospho-Thr residues with calcineurin
results in a calpain that is nearly inactive proteolytically
(⬍15% of its original activity).
At this point it is difficult to determine what the
physiological significance of calpain phosphorylation
might be. It evidently has some effect on catalysis, and
based on the demonstrated importance of phosphorylation in regulating activity of countless other enzymes,
some combination of phosphorylation might be the longsought activator. On the other hand, phosphorylation may
have a primarily structural role, and the phospho-Tyr
residues may target the calpains to some signal transduction function.
5. Calpastatin
The calpain/calpastatin interaction has been wellcharacterized (see sect. IIB3), and phosphorylation of calpastatin has been discussed in section IIB1. It is clear that
calpastatin has an important role in regulating activity of
␮- and m-calpain, but how calpastatin regulates calpain
activity in living cells is not well understood. Also, the role
of calpastatin in regulation of the “other calpains” that
have been identified as DNA sequences is unknown. Calpastatin does not inhibit the weak proteolytic activity of
expressed domain IIa/IIb and only weakly inhibits the
activity of LP82. Both the expressed domain IIa/IIb and
LP82 do not have a small subunit, and hence, one of the
three sites that calpastatin uses to bind to the ␮- or
m-calpain molecule is absent. These results suggest that
calpastatin requires all three binding sites to be an effective inhibitor and that calpastatin may not inhibit singlechain calpains. Any regulation of calpain activity by calpastatin is likely limited to the vertebrate calpains because calpastatin has not been identified in invertebrate
cells (the invertebrate calpains may be all single-chain
calpains anyway). There have been reports that the calpains can autolyze in the presence of levels of calpastatin
that completely inhibit the proteolytic activity of the calpains. It is possible to envision combinations of calpastatin phosphorylation/calpain phosphorylation that completely ablate the ability of calpastatin to bind to and
inhibit the calpains at Ca2⫹ concentrations sufficient for
Physiol Rev • VOL
maximal proteolytic activity of the calpains. Because
there is so little information on regulation of the calpain/
calpastatin interaction, a number of different possible
mechanisms can be proposed without knowing if any of
them is close to the truth. Regulation of the calpain/
calpastatin interaction will likely be an area that will
receive considerable attention in the coming years.
6. Mechanisms involved in catalysis
The 28-kDa small subunit of the ␮- and m-calpain
molecules has been referred to as the “regulatory subunit,” as having chaperone activity needed for proper
folding of the 80-kDa subunit (495), or as being essential
for proteolytic activity of ␮- or m-calpain, without any
clear indication of what its function(s) actually is. It was
first proposed in 1984 that activation of rat liver ␮- or
m-calpain occurred in the presence of substrate and Ca2⫹
and that it correlated with dissociation of the two calpain
subunits (291, 355). Eleven years later, Yoshizawa et al.
(496) reported that the subunits of m-calpain dissociated
reversibly in the presence of 1 mM Ca2⫹ and that the
80-kDa monomer had full catalytic activity and a reduced
Ca2⫹ requirement (495, 496). Other laboratories, however,
found that the two subunits of calpain coimmunoprecipitated under conditions where they were proteolytically
active (499); that an inactive mutant m-calpain,
C-105 3 S-105 did not dissociate during anion-exchange
chromatography in the presence of 3 mM Ca2⫹ (106) nor
during affinity chromatography in the presence of 5 mM
Ca2⫹ (97); and that expressed m-calpain was catalytically
inactive when the COOH-terminal 25 amino acids were
removed from the small subunit, thereby preventing association of the two subunits, whereas a fully active enzyme was obtained when the subunits were expressed in
a manner that allowed their association (106). Furthermore, mutation of the EF-1⬘, EF-2⬘, and EF-3⬘ sites in a
domain VI increased the Ca2⫹ concentration required for
proteolytic activity of the expressed m-calpain in the presence of Ca2⫹, which would be impossible if the small
subunit did not remain associated with the large subunit
during catalysis (98). Finally, fluoresecence resonance
energy transfer (FRET) fluorescence is not diminished
during hydrolysis in vivo of a synthetic calpain substrate,
Suc-Leu-Leu-Val-Tyr-AMC, in either COS 7 or LCLC 103H
cells (127). Moreover, immunofluorescence and confocal
microscopy showed that the two subunits of ␮-calpain
that had been transfected into the cells remained colocalized following activation of the calpain by a Ca2⫹ ionophore (127). Hence, it seems very likely that the subunits
of the calpains remain associated during proteolysis in
cells.
Although the two subunits of the calpains evidently
remain associated during proteolytic activity in cells, a
number of studies have indicated that the two calpain
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THE CALPAIN SYSTEM
subunits may indeed dissociate under relatively mild conditions in vitro (243, 245, 313, 338). The physiological
significance of this dissociation remains unclear. Sizeexclusion chromatography showed that the expressed
domains IV from either ␮- or m-calpain and domain VI
associated as homodimers in 20 mM Tris 䡠 HCl, pH 7.5,
and 1 mM dithiothreitol (431). The ␮-domain IV homodimers dissociated to monomers in the presence of 500
␮M Ca2⫹, whereas 5 mM Ca2⫹ was required to even partly
dissociate the m-domain IV and domain VI homodimers
(431). Because these studies involved homodimers and
not the domain IV/domain VI association that exists in ␮and m-calpain, it is unclear whether they relate to dissociation of the calpain molecules. Also, it should be noted
that all Ca2⫹ concentrations used in these studies were far
higher than the 50 –300 nM that exists in vivo.
Other studies have used complete 80/28-kDa or 80/
21-kDa m-calpain molecules and catalytically inactive mutants (C-105 3 S-105) of these molecules expressed in E.
coli (338) or in a Sf9 baculovirus system (313) to study
dissociation of the two calpain subunits. Subunit dissociation was determined by measuring exchange of a truncated 28-kDa subunit (beginning at residue 86) in 5 mM
Ca2⫹ (338) or by separation of the small and large subunits during chromatography of a His-tagged 28-kDa subunit/large subunit heterodimer in 1 mM Ca2⫹ on a Ni2⫹immobilized ion affinity column (313). Use of the catalytically inactive mutants prevented autolysis in the
presence of Ca2⫹. Dissociation of the 80- and 21-kDa
subunits occurs in the presence of 5 mM Ca2⫹, but the
extent of this dissociation, as measured by exchange of
the truncated 21-kDa subunits, is incomplete, with only
⬃30% of the subunits exchanging (338). Sedimentation
equilibrium and Rayleigh light scattering showed that the
expressed m-calpain aggregated in the presence of 200 –
2,000 ␮M Ca2⫹, which significantly complicates interpretation of studies using 1,000 –5,000 ␮M Ca2⫹ to dissociate
the calpain subunits (338). Pal et al. (338) suggested that
interaction of the NH2-terminal 19 amino acids on the
large subunit with domain VI in the small calpain subunit
stabilized the calpain molecule and limited the extent of
subunit dissociation. This suggestion was supported by
the results of Nakagawa et al. (313) who found that the
subunits of m-calpain dissociated in the presence of Ca2⫹
only if the interaction of the first 19 amino acids in domain
I with domain VI was disrupted. The crystallographic
structure of m-calpain in the absence of Ca2⫹ shows that
K7 of the 80-kDa subunit forms an electrostatic interaction with D154 in the EF-2⬘ sequence of the 28-kDa subunit
and that R12 of the large subunit forms an electrostatic
interaction with E260 in the EF-5⬘ sequence of the 28-kDa
subunit. Disruption of either one of these interactions
alone does not result in dissociation in the presence of 1
mM Ca2⫹, but disruption of both results in ⬃50% dissociation of the two subunits in 1 mM Ca2⫹. Autolysis rePhysiol Rev • VOL
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moves the NH2-terminal 19 amino acids of the 80-kDa
subunit of m-calpain, and Nakagawa et al. (313) found
that ⬃50% of the autolyzed form of m-calpain also dissociated in the presence of 1 mM Ca2⫹. It was proposed that
Ca2⫹ binding to EF-2⬘ disrupted the salt bridge between
K7 and D154 in the EF-2⬘ sequence, that this disruption
allowed domains IIa and IIb to move together to form a
catalytically active molecule, and therefore that Ca2⫹
binding to D154 in EF-2⬘ was a “calpain activation” step
(313). It is not clear, however, how such an activation
process can be reconciled with the Ca2⫹ binding studies
showing that mutation of EF-1⬘, EF-2⬘, and EF-4⬘ in the
small subunit to prevent Ca2⫹ binding to these sites produced a mutant m-calpain that was proteolytically active
and that required 542 ␮M Ca2⫹ for half-maximal proteolytic activity, only 1.5-fold more than the wild-type mcalpain (98).
Both Pal et al. (338) and Nakagawa et al. (313) reported that the dissociated calpains were “unstable” and
lost proteolytic activity rapidly. Several earlier reports
(124, 125) had indicated that autolyzed ␮-calpain lost its
proteolytic activity rapidly at ionic strengths above 100
mM but in the absence of the high Ca2⫹ concentrations
used in the subsequent studies (313, 338). A more detailed
examination has shown that the subunits of both autolyzed ␮- and autolyzed m-calpain but not the unautolyzed
forms of these molecules dissociate rapidly at ionic
strengths above 100 mM in the absence of Ca2⫹ and that
the dissociated 80-kDa subunits aggregate to form a larger
species, possibly a mixture of trimers and tetramers (243,
245). The aggregated forms of these 80-kDa subunits have
no proteolytic activity (243, 245). The salt-induced aggregation is not reversible by dialysis, occurs within 2–5 min
after exposure of the autolyzed calpain to salt, and the
aggregates seem stable after formation (i.e., larger aggregates do not form with increasing time). The extent and
rate of dissociation/subsequent aggregation as estimated
by loss of proteolytic activity depends on ionic strength;
after 45 min in 100 mM KCl, ⬃40 –50% of the proteolytic
activity of either autolyzed ␮- or autolyzed m-calpain is
lost; after 45 min in 300 mM KCl, ⬃55– 65% of the activity
is lost, and after 45 min in 500 mM KCl, ⬃75– 80% of the
proteolytic activity is lost (243, 245). It seems likely that
the loss of proteolytic activity observed by Pal et al. (338)
and Nakagawa et al. (313) also resulted from aggregation
of the dissociated 80-kDa subunit that was induced by
high concentrations of Ca2⫹ in the latter studies (313,
338). Evidently, removal of the NH2-terminal 27 (␮-calpain) or 18 (m-calpain) amino acids from the 80-kDa
subunit and possibly also the NH2-terminal 91 amino acids
from the 28-kDa subunit has a substantial effect on stability of the calpain molecules, such that salt-induced
disruption of ionic bonds present after autolytic removal
of the NH2-terminal amino acids from domains I and V
results in subunit dissociation and loss of proteolytic
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GOLL, THOMPSON, LI, WEI, AND CONG
activity. Thus the K7 (80 subunit)-D154 (28 subunit) and
R12 (80 subunit)-E260 (28 subunit) interactions are not
involved in this dissociation. Because interaction of domains IV and VI in the 80- and 28-kDa subunits, respectively, involves hydrophobic forces (see sect. IIA3 and
Table 2), it seems likely that dissociation of the two
subunits exposes a hydrophobic surface in domains IV
and VI and that the dissociated subunits then self-aggregate. Ionic strengths of 100 mM are similar to (slightly less
than) those in living cells, but the 1–5 mM Ca2⫹ concentrations (313, 338) are several orders of magnitude higher
than those that exist in living cells. Hence, the effect/role
of autolysis in functioning of the calpains continues to
mystify (if autolysis results in dissociation, subunit aggregation, and loss of proteolytic activity, what do assays of
autolyzed calpains measure?), although it is becoming
clear that autolysis results in a considerably less stable
molecule.
Several recent studies have shown that a polypeptide
corresponding to domain IIa/IIb has very weak, Ca2⫹dependent proteolytic activity (158, 298). Degradation of
casein by an expressed human m-calpain domain IIa/IIb
(amino acids 19 –342; see Fig. 1) was Ca2⫹ dependent but
was incomplete even after 14 h at 30°C in 5 mM Ca2⫹
(158). Activity of the expressed domain IIa/IIb was estimated to be less than 1/100 that of the complete m-calpain
TABLE
molecule, and the Ca2⫹ concentration required for halfmaximal activity was estimated to be 900 ␮M. The activity
was inhibited by cysteine protease inhibitors such as
E-64c but only weakly by calpastatin. Nishimura and Goll
(320) reported that the 34-/35-kDa (␮-/m-calpain) polypeptides obtained by extensive autolysis of ␮- or m-calpain
and that contained amino acids 28 –330 (␮-calpain) or
amino acids 20 –330 (m-calpain) had no proteolytic activity in either the absence or the presence of Ca2⫹, although
they did not run the assays for sufficiently long periods of
time to detect the small activity recorded by Hata et al.
(158). An expressed rat ␮-calpain domain IIa/IIb (amino
acids 29 –356) degraded a catalytically inactive mutant
m-calpain and a synthetic substrate in a Ca2⫹-dependent
manner with a Ca2⫹ concentration of 42 ␮M required for
half-maximal activity (298). Activity of the ␮-calpain domain IIa/IIb also was low: ⬃1/40 that of intact m-calpain.
The expressed domain IIa/IIb was not inhibited by calpastatin domain I but was inhibited by leupeptin, E-64,
and calpain inhibitor I (Table 10). Crystallographic analysis showed that the expressed ␮-domain IIa/IIb bound
two Ca2⫹, one in domain IIa and the other in domain IIb
(298). The two Ca2⫹ were not bound in EF-hand structures, but in two peptide loops, one providing eight coordinations to the bound Ca2⫹ (domain IIa) and the other
providing seven coordinations (domain IIb). Ca2⫹ binding
10. Some inhibitors of the proteolytic activity of the calpains
Type and Name
Comments
Protein inhibitors
Calpastatin
Kininogen
␣-Macroglobulin
Specific for the calpains; reversible; tight binding with IC50 in pM range, requires Ca2⫹ to bind to the calpains
Calpains are inhibited by both L- and H-kininogen (but only the central domain of H-kininogen); kininogens are
extracellular, but calpains are intracellular; Ki ⫽ 1.0 nM
Partially inhibits the calpains; rate constant of 30,000 M⫺1 s⫺1
Reversible inhibitors
Leupeptin
Chymostatin
Antipain
Calpain inhibitor I
Calpain inhibitor II
Calpeptin
Ac-Leu-Leu-arginal; Ki ⫽ 3.2 (␮-calpain) to 4.3 (m-calpain) ⫻ 10⫺7 M; inhibits a number of cysteine and some
serine proteases
[(S)-1-carboxy-2-phenylethyl]-carbamoyl-␣-[2-iminohexahydro-4(S)-pyrimidyl]-(S)-glycyl-X-phenylalaninal with X as
L-Leu, L-Val, or L-Ile; inhibits the calpains weakly; chymotrypsin, cathepsins A and B more strongly
[(S)-1-carboxy-2-phenylethyl]-carbamoyl-L-Arg-L-Val-argininal; for calpains, Ki ⫽ 1.4 ⫻ 10⫺6 M; inhibits cathepsins
A, B, papain, and trypsin
N-Ac-Leu-Leu-norleucinal; Ki ⫽ 0.12–0.23 ⫻ 10⫺6 M for ␮-calpain; Ki ⬃ 2-fold higher for m-calpain; inhibits
cathepsins B (Ki ⫽ 0.5 nM) and L, papain and the proteosome; membrane permeable
N-Ac-Leu-Leu-methioninal; Ki ⫽ 0.12–0.23 ⫻ 10⫺6 M for m-calpain; Ki ⬃ 2-fold higher for ␮-calpain; inhibits
cathepsins B and L, weak inhibition of the proteosome; membrane permeable
Carbobenzyloxy-Leu-normleucinal; IC50 ⫽ 0.0029 ␮M for ␮-calpain; 0.0028 ␮M for m-calpain; membrane permeable
Irreversible inhibitors
Iodoacetate
Iodoacetamide
N-ethylmaleimide
E-64
Diazomethylketones
General inhibitor of all cysteine proteases
General inhibitor of all cysteine proteases
General inhibitor of all cysteine proteases
L-3-Carboxy-trans-2,3-epoxypropionyl-L-leucylamido-(4-guanidino)butane; one of a class of epoxy inhibitors of
cysteine proteases; poor membrane permeability; second-order rate constant for inhibition 7,500 M⫺1s⫺1, E-64d
a derivative of E-64c (with 3-methylbutane in place of the 4-guanidino group) is a membrane permeable but less
effective calpain inhibitor
Carbobenzyloxy-Leu-Leu-Tyr diazomethyl ketone, second-order rate constant of inhibition 230,000 M⫺1s⫺1; poor
inhibitory of the proteosome; potent inhibition of cathepsin L as are all compounds of this class
There are over 50 reported inhibitors of the calpains, and this list includes only those most widely used. See Wang and Yuen (472) and Wells
and Bihovsky (477) for more complete lists.
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induced conformational changes that brought the catalytic residues to within 3.7 Å from each other compared
with the 10.5 Å separating them in the Ca2⫹-free structures of the intact m-calpain molecule (171, 430).
The ability to express a catalytically active calpain
was used in an attempt to learn what parts of the calpain
molecule were responsible for differences in the Ca2⫹
requirements of ␮- and m-calpain (99). Mutating the residues in each EF-hand individually to exchange residues
present in ␮-calpain for the corresponding ones in mcalpain had little effect on the Ca2⫹ concentration required for half-maximal proteolytic activity of the expressed m-calpain. Substituting residues 2–58 from the
␮-calpain 80-kDa subunit for residues 2– 49 from m-calpain but leaving the remainder of the 80-kDa subunit
unchanged in a recombinant m-calpain reduced the Ca2⫹
concentration required for half-maximal activity for the
expressed calpain by 23%, from 325 to 250 ␮M even
though this part of the calpain molecule ostensibly does
not bind Ca2⫹. Substituting residues 2– 498 from ␮-calpain
for residues 2– 487 from m-calpain (residue 487 is almost
to the COOH-terminal end of domain III of m-calpain; Fig.
1) had little additional effect on the Ca2⫹ concentration
required for half-maximal activity, reducing it from 250 to
230 ␮M. This substitution includes the structure in domain III of the calpains that is marginally homologous to
a C2 domain and that has been suggested to be involved
in binding phospholipid and reduction of the Ca2⫹ concentration required for autolysis. Substituting residues
2–581 from ␮-calpain for residues 2–569 from m-calpain
(this extends the substitution past the linker and EF-1
hand parts in the crystallographic structure of m-calpain;
Fig. 1) had a marked effect on the Ca2⫹ concentration
required for half-maximal activity, reducing it from 230 to
110 ␮M even though mutating the EF-1 hand alone actually increased the Ca2⫹ concentration required for halfmaximal activity from 325 to 398 ␮M. Additional substitutions involving residues 2– 649 from ␮-calpain for residues 2– 637 from m-calpain (this substitution includes
EF-2 and EF-3 but not EF-4 and EF-5 of the 80-kDa
subunit of m-calpain) reduced the Ca2⫹ concentration
required for half-maximal activity from 110 to 80 ␮M
(wild-type ␮-calpain required 25 ␮M Ca2⫹ for half-maximal activity). The results of this study indicate that the
Ca2⫹ concentration required for proteolytic activity of the
calpains is regulated by the entire calpain molecule and
not by a specific set of Ca2⫹-binding sites.
Consequently, the mechanisms regulating calpain activity in cells remain unknown despite the rapid accumulation of information on properties of the calpains and the
recent demonstration of weak Ca2⫹-dependent proteolytic activity of expressed domains IIa/IIb. The Ca2⫹ concentrations required by the expressed domains IIa/IIb are
similar to those required by the intact molecules and are
therefore much higher than those that exist in cells. It
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seems clear that reducing the Ca2⫹ concentration required for calpain activity and perhaps also modifying the
Ca2⫹ concentration needed for its inhibition by calpastatin are crucial regulators of calpain activity. It is unclear
how many Ca2⫹ atoms are bound by the calpains, and
until some way is found to circumvent the autolysis and
aggregation/precipitation that occurs in the presence of
Ca2⫹, it will be difficult to measure Ca2⫹ binding carefully.
Two recent studies have advanced the understanding of
calpain activity. The FRET studies indicate that ␮- and
m-calpains function as heterodimers during proteolytic
activity in cells, and the substitutions of ␮-calpain domains for m-calpain domains suggest that regulation and
Ca2⫹ activation are likely to involve interactions that
include the entire calpain molecule rather than only one
or two domains (365). If so, this adds to the complexity of
the regulation and the difficulty in eventually understanding it.
V. PHYSIOLOGICAL FUNCTIONS OF THE
CALPAIN SYSTEM
The physiological functions of the calpain system
remain unclear despite the efforts of many laboratories.
The calpain system almost certainly has a number of
different roles in cells, including but not limited to “remodeling” of cytoskeletal attachments to the plasma
membrane during cell fusion and cell motility, proteolytic
modification of molecules in signal transduction pathways, degradation of enzymes controlling progression
through the cell cycle, regulation of gene expression,
substrate degradation in some apoptotic pathways, and
an involvement in long-term potentiation. The discovery
of members of the calpain family that lack some residues
necessary for catalysis suggests that the calpains may in
some instances have functions that do not require proteolytic activity; the nature of these functions is completely
unknown. That the calpains cleave polypeptides at a limited number of sites leaving large, often catalytically active fragments indicates that the calpains have a regulatory or signaling function in cells rather than a digestive
function such as the lysosomal proteases or the proteasome do. It is unclear why different cells have such widely
differing ratios of ␮-calpain to m-calpain. It is reasonable
to suggest that the two calpains can perform identical
physiological functions but respond to different cellular
signals: 1) they have very similar if not identical subsite
specificities, and 2) human platelets contain only ␮-calpain (at least within the ability to detect any m-calpain)
but bovine platelets, on the other hand, contain only
m-calpain (452); it seems likely that the calpains perform
identical functions in the platelets from these two species.
As indicated in the introduction to section IV, many of
the studies attempting to determine the physiological
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GOLL, THOMPSON, LI, WEI, AND CONG
functions of the calpain system have used protease inhibitors with the aim of identifying which properties of cells
are affected and then using this information to infer how
the calpains are involved. There is, however, only one
specific inhibitor of the calpains, calpastatin, and the
other inhibitors that have been used have effects in addition to inhibition of the calpains. In some instances, these
other effects are known (for example, calpain inhibitor I
is also a good inhibitor of cathepsins B and L and the
proteasome), but in other instances, they are not. Hence,
the results of experiments that rely totally on the effects
of a “calpain inhibitor” must be interpreted cautiously.
Table 10 lists some of the compounds that inhibit proteolytic activity of the calpains. Many of the reversible inhibitors of the calpains are peptidyl aldehydes. Two recent
reviews summarize the properties and structures of 59
calpain inhibitors (477) and the properties and therapeutic values of some of these inhibitors (472); these reviews
should be consulted for a more complete list and information on the properties of these inhibitors.
The two best approaches currently available for identifying the actions of the calpains in cellular functions are
introduction of calpastatin or a fragment of calpastatin
into cells by transfection or some other procedure, or the
use of selective antibodies such as the one that specifically labels the calpain-cleaved fragment of spectrin
(379). Hopefully, additional antibodies that specifically
label the calpain-cleaved fragments from other putative
calpain substrates, such as talin, PKC, ppFAK125, and
some of the transcription factors that are cleaved by the
calpains in vitro, will become available in the future. The
availability of such antibodies will significantly enhance
the ability to define the roles of the calpains in cells. Until
then, studies will need to rely on a combination of approaches involving different inhibitors having different
known selectivities, sensitivity to changes in Ca2⫹ concentration, cleavage of polypeptides that are known calpain substrates in vitro, and autolysis of the calpains as an
indication that the calpains have been active. As will be
discussed in the following sections, it seems that not all
calpain-sensitive substrates are cleaved in a given cellular
event; in some instances, spectrin is cleaved and talin is
spared, and in other instances, talin is cleaved and another calpain-sensitive polypeptide is spared. Degradation
of talin and filamin in platelets from Capn⫺/⫺ mice was
normal, although these platelets lacked ␮-calpain. Consequently, clearly defining the role of the calpains in cellular
functions will likely continue to be difficult without the
availability of more specific assays.
A. What Do The Transgenic Mice Show?
The short answer to the question posed is: 1) knocking out both ␮- and m-calpain by interfering with expres-
Physiol Rev • VOL
sion of the small subunit is embryonically lethal (10, 502),
2) knocking out only ␮-calpain results in platelet dysfunction but otherwise the mice survive (14), and 3) disrupting
the gene for calpain 3a has minimal effect on the mice
(367, 442). The two studies involving disruption of expression of the small subunit had identical results: 1)
Capn4⫺/⫺ embryos died by 11.5 days of gestation and had
no detectable calpain activity (10), but Capn4⫹/⫺ mice
were viable, fertile, had normal calpain activity and were
phenotypically normal (10, 502); 2) the Capn4⫺/⫺ embryos appeared normal up to day 10.5 but began to show
cardiovascular defects at that stage (10); 3) RNA levels
for the 80-kDa subunits of ␮- and m-calpain were normal
in Capn4⫺/⫺ embryonic stem (ES) cells, but the 80-kDa ␮and m-calpain polypeptides are barely detectable in these
cells; and 4) growth rates of Capn4⫺/⫺ ES cells were
normal and fibroblasts from the Capn4⫺/⫺ embryos had
no calpain activity but grew and proliferated normally
(10). The results raise a number of questions. That lack of
a 28-kDa small subunit is embryonically lethal implies that
␮- and m-calpain function as heterodimers in cells, at
least to a considerable extent (see sect. IVC6), or that the
28-kDa subunit has some presently unknown function
that is vital to cell survival. That the embryos survived to
day 11.5 and that the Capn4⫺/⫺ fibroblasts divide mitotically but have no detectable calpain activity is counter to
a number of studies indicating that ␮- and/or m-calpain
have an essential role in the mitotic cycle and cell growth.
Can one or more of the “other calpains” described in
section IIIB compensate for the absence of ␮- and mcalpain activity, at least for the first 10 days of development and in fibroblast function? If so, this implies that
these calpains can function without the 28-kDa subunit, a
conclusion consistent with the predicted amino acid sequences of the atypical calpains. Or, can the results be
explained by the presence of small residual amounts of ␮and/or m-calpain activity or to the 80-kDa subunits having
a small amount of residual activity acting alone? The
Zimmerman et al. (502) knock-out experiment eliminated
exons 4 – 8 (EF-1⬘, EF-2⬘, EF -3⬘; amino acids 82–202) of
the 28-kDa subunit and resulted in lethality before day 9,
whereas the Arthur et al. (10) knock-out experiment disrupted the COOH-terminal 25 amino acids of the small
subunit and was lethal at day 10 –11. It is unclear whether
the differences in survival are due to a “more complete”
disruption of the Capn4 transcript in the Zimmerman
study (502).
The Capn1⫺/⫺ mice are viable and fertile, but they
have a significant reduction in platelet aggregation and
clot retraction; surprisingly, bleeding times were normal
(14). Although it has been widely accepted that one of the
physiological roles of the calpains is degradation of filamin, talin, and spectrin during platelet activation, the
cleavage patterns and kinetics of degradation of both talin
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THE CALPAIN SYSTEM
and filamin were normal in platelets from the Capn1⫺/⫺
mice. Levels of tyrosine phosphorylation were decreased
in Capn1⫺/⫺ platelets. Western and Northern analysis
confirmed that ␮-calpain expression was ablated in
Capn1⫺/⫺ mice. Again, the results raise several questions.
Evidently, the presence of m-calpain, which was normal
in the Capn1⫺/⫺ mice, compensates for the absence of
␮-calpain in these animals. This finding is consistent with
but does not prove the hypothesis that ␮- and m-calpain
can cleave the same substrates in cells and are capable of
performing the same physiological functions. Because
platelets contain only (or in mice, largely) ␮-calpain, is the
m-calpain present in these platelets sufficient to produce
normal bleeding times and cleavage of talin and filamin in
the absence of ␮-calpain? Or, does the seemingly normal
pattern of filamin and talin cleavage in the Capn1⫺/⫺
platelets suggest that one of the “accepted” roles of the
calpains requires reexamination?
Recent studies have examined the effects of disruption of the 80-kDa subunit of m-calpain on mouse physiology. Although still preliminary, there is strong evidence
that disruption of the Capn2 gene in mice is embryonically lethal at a very early stage. If this is correct, it would
suggest, when considering the lethality of the Capn4⫺/⫺
mice (10, 502) and the very mild effects in the Capn1⫺/⫺
mice, that m-calpain is absolutely essential for embryonic
development (P. Dutt, J. S. Elce, and P. A. Greer, personal
communication) and that m-calpain performs some function during development that ␮-calpain does not or cannot. The lack of effect of Capn1⫺/⫺ on embryonic development may be due to late expression of ␮-calpain during
normal development so that it is not involved in calpain
cleavages required for development or to the ability of
m-calpain to perform certain cleavages that ␮-calpain
cannot.
Two types of calpain 3a transgenic mice have been
studied: one in which expression of the Capn3 gene has
been disrupted (367) and one expressing an inactive mutant of calpain 3a (the active site Cys mutated to Ser)
(442). The phenotypes of both mice were nearly normal;
the Capn3⫺/⫺ mice had a postnatal appearance similar to
their wild-type littermates, exhibited normal behavior,
were fertile, and were kept alive for 1.5 years with no
increase in death rate (367). The Capn3⫺/⫺ mice had
some loss of sarcolemmal integrity as indicated by increased membrane permeability. Expression of the inactive/mutant calpain 3a was low in the transgenic Cys-129/
Ser-129 mice, but examination of old (2 years) mice indicated that they had slightly lower “grip strengths”and
greater levels of centrally placed nuclei, more tubular
aggregate-like structures, and more lobulated fibers than
their wild-type littermates (442); these differences were
evident only at advanced ages. As in the Capn3⫺/⫺ studies, the Cys-129/Ser-129 mice appeared normal and were
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773
actually heavier than the wild-type littermates at 20 and 32
wk of age.
In sum, the transgenic mice studies have raised a
number of questions concerning the physiological function(s) of the calpains; roles that various studies had
suggested for the calpains such as mitosis and cleavage of
some cytoskeletal proteins during platelet activation
seem to proceed normally in the absence of ␮- and mcalpain. Although it has been demonstrated that disruption of the Capn3 gene is responsible for LGMD2A, disrupting expression of this gene in mice has minimal consequences. Clearly, a great deal remains to be learned
about how the calpains function in different cells and the
signals that trigger calpain activity.
B. In Vitro Substrates
A large number of proteins (over 100 have been
reported) are cleaved by the calpains in in vitro assays
(see Refs. 76, 138 for proteins in muscle/cytoskeleton; see
Ref. 474 for others). Many but not all of these in vitro
substrates can be placed in one of four general categories:
1) cytoskeletal proteins, especially those involved in cytoskeletal/plasma membrane interactions; 2) kinases and
phosphatases; 3) membrane-associated proteins, including some receptors and ion-channel proteins; and 4) some
transcription factors (Table 11 lists some of the substrates
in the first two of these categories). It is important when
evaluating reports of proteins cleaved by the calpains to
ensure that the proteins were not denatured and to consider the rates of cleavage. For example, undenatured
sarcomeric myosin heavy chain is degraded very slowly
and undenatured sarcomeric actin is not degraded at all,
although denatured actin is degraded rapidly. Similarly,
undenatured ␣-actinin is degraded very slowly and in a
limited manner (133). In vitro degradation of many kinases or phosphatases by the calpains leaves large catalytically active fragments that lack the physiological controls of the undegraded molecule (Table 11). In addition
to the proteins listed in Table 11, m-calpain cleavage of
brain calmodulin-dependent, cyclic nucleotide phosphodiesterase leaves an enzyme that is active in the absence
of calmodulin (197), and calpain cleavage of p53 ablates
its ability to regulate gene expression (141, 216, 340).
Many of the cytoskeletal proteins involved in linking the
cytoskeleton to the plasma membrane are cleaved rapidly
by the calpains; among these proteins are talin, vinculin,
spectrin, filamin, band 4.1a, band 4.1b, ezrin, but not
␣-actinin, which is cleaved slowly, or the ERM proteins,
moesin and radixin, which are not cleaved (404). Similarly, most of the intermediate filament proteins such as
desmin and vimentin are cleaved rapidly by the calpains.
In most instances, calpain cleavage of the cytoskeletal
proteins severs their cross-linking ability.
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11. Some known substrates of the calpains
Polypeptide Class and
Name
Effects of Calpain Cleavage
Cytoskeletal proteins
Adducin
Ankyrin
Caldesmon
Cadherin
Calponin
Catenin
C-protein
Desmin
Dystrophin
Gelsolin
Filamin/actin-binding
protein
MAP1, MAP2
Myosin
Nebulin
Neurofilament proteins,
NFH, NFM, NFL
Protein 4.1a, 4.1b
␣II-Spectrin
␤-Spectrin
Synemin
Talin
Tau
Titin
Tropomyosin
Troponin I
Troponin T
Tubulin
Both the 103-kDa ␣-subunit and the 97-kDa ␤-subunit are degraded within 60 min to produce stable 57- and
49-kDa fragments, respectively; the 49-kDa ␤-subunit fragment does not bind calmodulin as its parent 97-kDa
does (392).
Both the brain (212 kDa) and erythrocyte (239 kDa) isoforms are rapidly degraded to a 160-kDa polypeptide
(150).
Very rapidly degraded to polypeptide fragments of 125, 115, 100, 105, and 88–90 kDa; the latter fragment is
stable (75).
E-cadherin is not degraded by calpain (47), whereas the intracellular domain of N-cadherin is degraded by ␮(71) or m-calpain (391).
Very rapidly degraded at approximately the same rate as caldesmon to 30-, 27-, and 19.5-kDa fragments; the
latter fragment is stable; rate of degradation is faster in the presence of calmodulin and slower when
associated with thin filaments (75, 465).
␣-Catenin is not degraded by calpain but calpain removes the NH2 terminus of ␤-catenin to produce 90- and
75-kDa fragments from the 97-kDa ␤-catenin.
Small fragment removed from the native 140-kDa polypeptide leaving a ⬃120-kDa fragment (SDS-PAGE); this
degradation can occur while the C-protein is bound to the myofibril (82).
Rapidly degraded to a 32- to 37-kDa stable fragment; an 18-kDa fragment appears after longer digestion (336);
a 9-kDa fragment is removed from the NH2 terminus leaving the central rod domain; calpain degradation
destroys the ability of desmin to self-assemble and to bind nucleic acids (318).
Degraded to a 30-kDa NH2-terminal fragment and several 50- to 140-kDa fragments (492).
Rapidly degraded to a 40-kDa fragment that contains the actin-binding segment of gelsolin and a 45-kDa
fragment that contains the Ca2⫹-binding properties of gelsolin.
Rapidly degraded to 240- and 10-kDa fragments that no longer have the cross-linking abilities of the undegraded
filamin (80); rate of calpain degradation is decreased by phosphorylation of the filamin.
Both are degraded to smaller fragments; MAP2 is more susceptible than MAP1 and is one of most calpainsensitive proteins known; rate of MAP2 degradation is decreased by phosphorylation of MAP2.
Very slow degradation of undenatured myosin; the 210-kDa large subunit is degraded to fragments of 150, 165,
and 180 kDa; slow degradation of LC2 light chain (347).
Very rapidly degraded to a series of smaller polypeptides ranging from 30 to several hundred kDa; fragments
produced by calpain may remain bound to actin (453); calpain degradation severs nebulin connection to
Z-disk.
The three neurofilament proteins, NFH (⬃200 kDa), NFM (⬃150 kDa), and NFL (⬃68 kDa), are all cleaved by
the calpains, in the order of NFM, NFL, and NFH (198). Phosphorylation increases rate and extent of NFH
degradion, has little effect on rate of NFM degradation, and does not affect NFL degradation (143). NFH is
degraded to ⬃190 kDa, NFM to ⬃68 kDa, 55 kDa, and 30 kDa, and NFL to ⬃57 kDa.
Both isoforms are degraded very rapidly to 43-, 40-, 38-, and 31-kDa fragments; the 31-kDa fragment is stable
and accumulates (75).
Rapidly degraded to 145- and 150-kDa fragments; the 150-kDa fragment may be detected by a specific antibody;
the 145- and 150-kDa fragments are then more slowly degraded to smaller fragments (491); the initial calpainsensitive site is at Tyr1176-Gly1177 (428).
All three isoforms are cleaved at a single site in helix B of repeat 17 at Trp-Ala (␤IE1), Thr-Ala (␤IE2), or AlaAla (␤IIE1) to produce fragments of 125- and 165-kDa; calpain cleavage seems to be modulated by binding of
calmodulin (250).
Very rapidly degraded to a large number of polypeptides ranging from 40- to 220-kDa with major fragments of
40 (stable), 45, 50, 66, and 200 kDa (36).
Very rapidly cleaved between Q433 and Q434 (chicken talin) to produce a 190-kDa COOH-terminal actin binding
fragment and a 47-kDa NH2-terminal fragment; after cleavage, can no longer cross-link integrin to cytoskeletal
elements (162, 305).
Very rapidly cleaved to major, stable fragments of 35 and 45 kDa and a minor fragment of 25 kDa; rate of
digestion is decreased by phosphorylation of tau (248).
The ⬃3000-kDa titin polypeptide is very rapidly cleaved to a large ⬃2,000-kDa fragment by removal of a 1,200kDa NH2-terminal fragment; this cleavage severs the connection of titin to the Z-disk; the 1,200-kDa fragment
is degraded to smaller fragments of 100–500 kDa, with the 500-kDa fragment being stable (453); chicken titin
is quickly degraded to a T2 (2,000 kDa) fragment which is then degraded to a 1,700- and a 400-kDa fragment;
the 1,700-kDa fragment is further degraded to a 1,400-kDa fragment (432).
Rapidly cleaved to a 14-kDa fragment and several smaller polypeptides (see Table 9); rate of degradation is
decreased greatly if tropomyosin is in the groove of the actin helix in myofibrils (82).
Degraded to smaller polypeptides (the 32-kDa cardiac troponin I is degraded to a 26-kDa fragment as measured
by SDS-PAGE) at a moderate rate; not protected while in the myofibrillar structure (82).
Very rapidly degraded to smaller fragments of ⬃35, 30, and 28 kDa as measured by SDS-PAGE; the 30-kDa
fragment is stable; a 15-kDa fragment is produced by longer digestion (168); not protected when in the
myofibrillar structure (82).
Very slowly degraded from a 55- to a 50- to 52-kDa fragment (37); or not degraded at all (388).
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TABLE
775
11—Continued
Polypeptide Class and
Name
Vimentin
Vinculin
Effects of Calpain Cleavage
Rapidly degraded to two 40- and 42-kDa fragments with stable 27-, 22-, 20-, 16-, and 15-kDa fragments being
produced subsequently; pattern and effects of calpain digestion are similar to those observed for calpain
degradation of desmin (318). Murine vimentin is cleaved at 10 sites that are all in the nonhelical region of the
molecule; 9 of these sites are in the NH2-terminal part of the molecule to produce the stable 42-kDa fragment
(116).
Very rapidly cleaved to a ⬃90-kDa fragment (453) that can no longer cross-link talin, paxillin, and ␣-actinin
filaments.
Kinases and phosphatases
Calcium/calmodulindependent protein
kinase
Doublecortin-like kinase
EGF receptor kinase
Myosin light-chain
kinase (MLCK)
ppFAK125
pp60src
Protein kinase C (PKC)
Calcineurin
Inositol polyphosphate
4-phosphatase
Protein tyrosine
phosphatase 1B
(PTP-1B)
The 52- to 54-kDa CaM kinase II is degraded to a major 35- to 40-kDa fragment which contains the entire
catalytic core and is active in the absence of calmodulin; degradation of the kinase occurs in situ in cultured
neurons and in vitro (149).
Calpain cleaves the 85-kDa kinase at two sites to produce a 35-kDa NH2-terminal fragment that retains the
microtubule-binding properties of the parent kinase and a 50-kDa COOH-terminal fragment that retains the
kinase activity. A similar cleavage occurs in vitro in embryonic mouse brain (46).
Degraded at seven sites, three of them in a “calpain-sensitive” hinge region between residues 996 and 1059 to
produce a 150-kDa fragment from the 170-kDa EGF receptor kinase (144). The 150-kDa fragment retains EGF
binding and EGF-stimulated kinase activity, but has reduced autophosphorylation activity.
In the presence of calmodulin, the 130-kDa MLCK is cleaved to a 62-kDa fragment that retains complete kinase
activity and that binds calmodulin; in the absence of calmodulin, MLCK is cleaved to a 60-kDa fragment that
has no kinase activity and that does not bind calmodulin (191).
Rapidly degraded to 90 kDa and the 90 kDa then to 45- and 40-kDa fragments with a concomitant loss of
autokinase activity; similar degradation occurs in platelets (67). Calpain degradation of ppFAK125 is associated
with its loss from the platelet cytoskeleton.
Cleaved to 52- and 47-Da fragments by removal of the NH2-terminal region of the kinase, which is the region
that binds the kinase to the membrane; pp60src calpain fragments are exclusively in the platelet cytoplasm;
calpain cleavage occurs in situ in platelets and results in decreased tyrosine kinase activity (323); cleaved
⬃50% as rapidly as ppFAK125 and protein tyrosine phosphatase 1B and less rapidly than talin or filamin in
platelets.
The PKC-␣, PKC-␤I, PKC-␤II, and PKC-␥ isoforms are all rapidly cleaved from a 82-kDa polypeptide to a 45- to
49-kDa fragment that is fully active as a kinase but that does not require Ca2⫹ or phospholipids for activity
and to a 36-kDa regulatory fragment (209); rate of cleavage is increased in the presence of phospholipid and
diacylglycerol, suggesting that the active form of the kinase is preferentially cleaved; cleavage occurs at one
or two sites in the third variable region of the PKC polypeptide. At least some of the novel PKC isoforms
(PKC-␦; Ref. 486; PKC-␧; Ref. 372) are also cleaved to a constitutively active form by the calpains.
The 60-kDa subunit is cleaved sequentially in the presence of calmodulin to 58-, 55-, and 48-kDa fragments; in
the absence of calmodulin, the 60-kDa subunit is cleaved sequentially to a 58- and then to a 45-kDa fragment;
the calpain-degraded phosphatase is 1.4–1.6 times more active than the native enzyme and does not require
calmodulin or Ca2⫹ for activity; cleavage is more rapid in the presence of calmodulin than in its absence; the
19-kDa Ca2⫹-binding subunit is not cleaved by the calpains (450).
The NH2 terminus of the 105-kDa phosphatase is clipped to produce a 104-kDa fragment that is then degraded
to a 39-kDa fragment; degradation occurs in vivo in platelets, and appearance of the 39-kDa fragment is
accompanied by loss of phosphatase activity (322).
The 50-kDa phosphatase is cleaved to a stable 42-kDa fragment, removing the COOH-terminal 60–70 amino acids
which are involved in binding PTP-1B to the plasma membrane; the 42-kDa fragment is located entirely in the
cytoplasm of platelets and has approximately twofold higher phosphatase activity than the native 50-kDa
polypeptide (368); PTP-1B cleavage occurs in situ in platelets (121); integrin signaling is necessary and
sufficient to induce calpain cleavage of PTP-1B in platelets; PTP-1B is cleaved at approximately the same rate
as ppFAK125.
Most of the masses listed are from SDS-PAGE and are known in some instances (for example, troponin T, troponin I, and synemin) to be
inaccurate. This table is a sampling of the more than 100 reported substrates for the calpains and is intended to provide examples of the limited
degradation resulting from calpain proteolysis; only two (cytoskeletal proteins; kinases and phosphatases) of the several classes of calpain
substrates are included.
C. In Vivo Substrates and Some Proposed
Functions
Calpain cleavage of a protein in vitro does not indicate that it is a substrate for the calpains in vivo. One
obvious example is the reports indicating that the calpains cleave certain extracellular proteins such as fi-
Physiol Rev • VOL
bronectin or factor V. The immunolocalization studies all
show that the calpains are located exclusively intracellularly except in diseased or injured tissues. Indeed, it
seems possible that one of the physiological reasons that
kininogen can inhibit the calpains (Table 10 and sect. IIB)
is to ensure that proteolytically active calpain is not
present in the circulatory system. On the other hand,
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GOLL, THOMPSON, LI, WEI, AND CONG
there is no conclusive evidence that the polypeptides
cleaved by the calpains in in vitro assays cannot also be
cleaved in vivo, although it is clear that not all potential
calpain substrates in a cell will be cleaved during a single
cellular event.
An antibody raised against a pentapeptide mimicking the NH2 terminus produced by calpain cleavage of
spectrin (G-M-M-P-R; Ref. 379) has been used to demonstrate that the calpains cleave spectrin in postischemic gerbil hippocampus (379) and during apoptosis in
a murine T-cell lymphoma (267). A number of studies
have used synthetic inhibitors of the calpains to conclude that the calpains are responsible for degradation
of various cytoskeletal proteins, p53, and different protein kinases. Although several of these inhibitors have
some selectivity for the calpains, there presently are no
synthetic inhibitors that are absolutely specific for the
calpains (see Table 10), and results of these studies
should be evaluated cautiously. If polypeptides that are
known to be highly susceptible to calpain cleavage are
degraded in response to a Ca2⫹ flux and this degradation is inhibited by synthetic calpain inhibitors, it is
reasonable to suggest that the cleavage is calpain mediated, but it does not prove that the calpains are
involved. In a novel attempt to determine calpain activity in living cells, a fusion protein containing enhanced yellow and enhanced cyan fluorescent protein
was linked by a peptide containing the ␣-spectrin cleavage sequence specific for calpain (468). The fusion
protein exhibited FRET when expressed in cells. FRET
was lost when the peptide linking the two fluorescent
proteins was cleaved, so calpain activity could be measured in vivo by loss of FRET. FRET decreased in
dendritic spines of cultured neurons when the neurons
were treated with glutamatergic agonists to increase
intracellular Ca2⫹ concentrations (468). The decrease
in FRET in the dendritic spines was paralleled by an
increase in the 150-kDa calpain fragment of ␣-spectrin,
implying a relationship between the 150-kDa calpain
fragment of ␣-spectrin and calpain activity. Thus far,
calpastatin is the only inhibitor known to be specific for
the calpains. Several studies have used expressed or
microinjected single domains (e.g., domain I) of calpastatin in cells to learn whether the intracellular proteolytic degradation observed was due to the calpains
(see Refs. 51, 345 as examples). Production of antibodies specific for other calpain-cleaved substrates, development of the long-sought synthetic inhibitor specific
for the calpains, and overexpression of calpastatin will
be needed, either singly or in combination, in future
studies to identify calpain cleavages in vivo with certainty.
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1. Cytoskeletal/membrane attachments: cell motility
Cleavage of cytoskeletal/membrane attachments is
likely the most thoroughly documented function of the
calpains. Some of the strongest evidence for calpain involvement in cleavage of cytoskeletal/membrane attachments originates from three systems: 1) fusion of skeletal
muscle myoblasts during muscle development, 2) fibroblasts from calpain knock-out mice (10), and 3) cell
spreading/motility and platelet activation. During skeletal
muscle development, embryonic myoblasts withdraw
from the mitotic cycle, align, and then fuse to form
multinucleated myotubes that accumulate skeletal muscle proteins such as actin, myosin, and titin to form
skeletal muscle fibers. Fusion of myoblasts requires extensive remodeling of the cytoskeletal/plasma membrane
attachments at the point of fusion. Microinjection of calpastatin into myoblasts just before fusion stops fusion
completely but does not kill the myoblasts, which can go
on to fuse after turning over the excess calpastatin (454).
An antisense oligodeoxyribonucleotide to m-calpain decreased the number of myoblasts that fused by ⬃50% (19),
and the membrane-permeable inhibitors calpeptin, Z-LeuMet-H (230), and E-64d (224) also nearly completely inhibited fusion, depending on the dosage applied. Although
the latter two inhibitors affect enzymes in addition to the
calpains, calpastatin and antisense oligos to m-calpain are
specific for the calpains. Hence, it seems likely that calpain activity is necessary to cut the cytoskeletal membrane attachments that must be severed to allow myoblast fusion.
Fibroblasts from Capn4⫺/⫺ embryos have no calpain
activity, are viable, and proliferate (10). These fibroblasts,
however, do not generate the calpain-specific degradation
products of spectrin that fibroblasts from wild-type animals do; do not degrade talin, a protein that is involved in
cytoskeletal/membrane attachments and that is degraded
rapidly in in vitro systems by the calpains; have decreased
migration rates and an abnormal cytoskeleton with loss of
central stress fibers; display delayed retraction of membrane protrusions and filopodia; and have a decreased
number of focal adhesions (96). These properties are
characteristic of a loss of ability of the cells to undergo
normal remodeling of cytoskeletal/membrane attachments. Expression of the rat small calpain subunit partially rescues the aberrant phenotype of the Capn4⫺/⫺
fibroblasts, indicating again that the 28-kDa subunit is
required for calpain activity in vivo (96).
Spreading and cell motility require calpain degradation of focal adhesions at attachment sites at both the
leading and the rear edges of cells (129, 348). Overexpression of calpastatin in NIH-3T3-derived clonal cells impairs
the ability of these cells to extend lamellipodia, reduces
by 90% the ability of the cells to spread, and results in an
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THE CALPAIN SYSTEM
increase in ezin content (suggesting that calpastatin overexpression prevents normal degradation of this calpainsensitive substrate; Ref. 359). Interestingly, levels of PKC␣/␤, ppFAK125, and talin, all calpain substrates in in vitro
assays and also in other in vivo situations, are not affected
in the calpastatin overexpressing cells, suggesting that
degradation of calpain-sensitive proteins in vivo may depend in part on the physiological signals that the cell has
received and possibly also the subcellular location of
calpain relative to the substrate proteins. Introduction of
a 24-amino acid sequence of calpastatin into platelets
inhibited thrombin-induced ␣-granule secretion, platelet
aggregation, and spreading of platelets on glass (79). The
24-amino acid calpastatin also inhibited Ca2⫹ ionophore
(A23187)-induced degradation of talin and filamin, which
have been shown to be degraded during platelet aggregation (117, 120). The results indicate that calpain activity is
involved in early events (the first 30 s) after platelet
activation, such as ␣-granule release, and also in aggregation. Earlier studies indicating that calpain activity was
not involved in platelet aggregation may have been the
result of poor membrane permeability of the ZLLY-CHN2
(benzyloxycarbonyl-Leu-Leu-Try-diazomethyl ketone) inhibitor used.
In sum, the calpains have a role in severing cytoskeletal/membrane attachments, although the exact nature of
this role remains unclear. Some functions that clearly
require extensive cytoskeletal/membrane remodeling,
such as cytokinesis for example, proceed in the Capn4⫺/⫺
fibroblasts that have no or very little calpain activity, and
some calpain-sensitive cytoskeletal proteins are degraded
in the presence of levels of calpastatin that are sufficient
to prevent cell spreading.
2. Signal transduction pathways
Calpain cleavage of PKC to produce a constitutively
active enzyme (209) led to suggestions that the calpains
were involved in signal transduction. In vitro studies
have shown that many of the kinases (e.g., ppFAK125,
pp60c-src), phosphatases (e.g., tyrosine phosphatase
PTP-1B), and cytoskeletal proteins (e.g., talin, filamin,
paxillin, vinculin) involved in signal transduction pathways are rapidly cleaved by the calpains, but it remains
difficult to identify specific calpain cleavages in vivo that
are related to specific signal transduction events. Recent
studies indicate that the calpains are involved in integrinmediated signal transduction pathways (119) and that
only the ␤-integrin family of integrins is cleaved by the
calpains (349). Calpain cleavage occurs at several sites in
the COOH-terminal half of the cytoplasmic domains of the
␤-integrins, specifically in regions flanking the conserved
NPXY/NXXY motifs; these motifs are involved in binding
of integrins to the cytoplasmic proteins, talin and filamin.
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777
The cleavage sites are not influenced by the linear sequence of amino acids on either side of the scissile bonds,
but rather by the structural framework provided by the
NXXY motifs.
Part of the role of the calpains in signal transduction
is related to their role in cytoskeletal/membrane interactions. Integrin clustering or binding of ligands to certain
receptors leads via some unknown mechanism to calpain
activation. Calpain cleavage of integrin/cytoskeletal proteins in focal adhesions severs the interactions among
these molecules, resulting in disassembly of the focal
adhesions, cell rounding, loss of submembraneous actin
filament networks, increased rates of cell spreading and
cell motility (129, 348), and, in platelets, to retraction of
fibrin clots (395). Calpain cleavage of proteins in focal
adhesions may be necessary for detachment at the trailing
edges of motile cells, and the breakdown and formation of
new focal contacts in extending lamellipodia allows cells
to spread (129, 175). A study using cultured smooth muscle cells showed that aggregation of integrin receptors
induced by collagen fragments that bind to the ␣2␤1integrin in these cells leads to a rapid cleavage of
pp125FAK, talin, and paxillin; this cleavage blocks downstream signal transduction signals and results in focal
adhesion disassembly, cell rounding, and increased motility (50). Cleavage was prevented by a synthetic calpain
inhibitor; neither ␣-actinin nor actin, which are poor calpain substrates, was cleaved.
The calpains also seem to have a role in signal transduction pathways in addition to cleaving integrin/cytoskeletal protein interactions. Synthetic inhibitors of the
calpains abrogate ability of bovine aortic endothelial cells
(BAEC) to spread and to form focal adhesions, actin
filament networks, and stress fibers. Cells expressing a
constitutively active form of RhoA, however, can still
spread and form focal adhesions when calpain inhibitors
are present; cells expressing active Rac1 can extend lamellipodia, form focal complexes and networks of subcutaneous actin fibers, but cannot form focal adhesions or
stress fibers (35, 221). The use of different inhibitors and
overexpression of a catalytically inactive form of ␮-calpain indicated that only ␮-calpain and not m-calpain was
involved in these changes in BAEC. The results suggest
that the calpains (␮-calpain in this instance) act in signal
transduction at a site upstream of both Rac1 and RhoA.
Western analysis showed that both talin and PKC were
cleaved in adherent (plated on fibronectin; ␣5␤1-integrin)
BAEC (221). Immunofluorescence studies using BAEC
have shown that integrin, calpain-induced clusters, assemble before formation of Rac1-containing focal complexes and the larger, RhoA-containing focal adhesions
(35). These calpain-induced clusters contain calpain, calpain-cleaved ␤-integrin subunit, and Rac1; require catalytically active calpain for their formation; and seem to be
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GOLL, THOMPSON, LI, WEI, AND CONG
involved in activation of Rac1. In contrast to the earlier
studies that found m-calpain in adhesion plaques in several cell types (BS-C-1; EBTr; MDBK; Ref. 31), calpain was
not detected immunologically in either focal complexes
or focal adhesions in BAEC. Do different focal adhesions/
focal complexes in different cells contain either ␮- or
m-calpain but not both? Transfection with constitutively
active Rac1 or RhoA showed that focal complexes and
focal adhesions can form in these cells via a mechanism
that does not require calpain. The results indicate that
calpain is involved, by some as yet unknown means, in
pathways that lead to activation of Rac1 and RhoA but
that focal complexes and focal adhesions can form independent of calpain if active Rac1 or RhoA is present.
RhoA activity is dynamic during transmembrane signaling. During the early stages of integrin-mediated cell
spreading, activity of RhoA is downregulated, but at a
later stage, RhoA activity is increased and focal adhesions
and stress fibers are formed. Downregulation of RhoA is
caused by specific cleavage of RhoA between Q180 and
A181 to produce a 20-kDa fragment that inhibits integrininduced stress fiber formation and focal adhesions (220).
The RhoA cleavage is inhibited by the synthetic calpain
inhibitors calpeptin and calpain inhibitor I, but not by
proteasome or caspase inhibitors (220). Neither Rac1 nor
cdc42 was cleaved when incubated with purified ␮-calpain, although RhoA was cleaved to a 20-kDa fragment.
Hence, the calpains (␮-calpain) can modulate RhoA activity directly.
The studies with BAEC suggest that several types of
integrin complexes exist: 1) calpain-induced clusters that
contain active calpain, calpain-cleaved ␤-integrin fragment, vinculin, phosphotyrosine, Rac1-binding protein,
␣-actinin, and skelemin but not talin; 2) Rac-induced focal
complexes that contain intact integrin, vinculin, and phosphotyrosine but not calpain and probably not skelemin;
and 3) RhoA-induced focal adhesions that contain intact
integrin, vinculin, phosphotyrosine, and talin, but not
␣-actinin, skelemin, or calpain (35, 362). These different
integrin complexes are formed by activation of different
signaling molecules and at different stages of spreading
and involve different integrin/cytoskeletal interactions.
Only ␣-actinin and skelemin can bind to the calpaincleaved, membrane-associated part of ␤3-integrin. A skelemin-based peptide that blocks the interaction of skelemin with ␤3-integrin also completely blocks formation of
focal complexes and focal adhesions, which form downstream from the calpain-induced clusters (362).
In sum, the studies thus far suggest a complex and
dynamic pattern of integrin signaling that may involve the
calpains in several ways ranging from cleavage of integrin/cytoskeletal proteins that ablates the interaction between them to activation of Rac1 and to direct inactiva-
Physiol Rev • VOL
tion RhoA. Other studies have indicated that epidermal
growth factor (EGF) receptor-mediated fibroblast motility requires activation of m-calpain downstream of ERK/
mitogen-activated protein kinase signaling (128, 131). Calpain activation in this system is triggered only by membrane-restricted EGF receptor, and not by the internalized
form of the receptor (131). EGF-induced activation in
fibroblasts (␣IIb␤3- and ␣5␤1-integrins) was specific for
m-calpain and did not involve ␮-calpain (128). Immunological studies have shown that m-calpain and not ␮-calpain is present in focal adhesions of several cell types (31)
and that m-calpain but not ␮-calpain is involved in adhesion and spreading of T-cell lymophocytes (368).
Transformation of cells by v-src results in deregulated cell growth, cytoskeletal disassembly, and loss of
integrin-linked focal adhesions, contributing to a highly
mitogenic and motile phenotype. Synthesis of m-calpain
was increased in chick embryo fibroblasts transformed by
v-src, and this increase was accompanied by degradation
of ppFAK125 and calpastatin, which is a calpain substrate
when calpains are in excess. Overexpressing calpastatin
in the v-src cells suppressed ppFAK125 degradation, morphological transformation, anchorage-independent growth, and
rate of progression through the G1 stage of the mitotic
cycle (51). The decreased rate of progression through the
cell cycle was accompanied by a decrease in hyperphosphorylation of the tumor suppressor protein pRb and
attenuation of the increases in cyclin A, cyclin D, and
cdk2 that normally accompany v-src transformation. The
v-src transformation, however, had little effect on the
phenotype of fibroblasts from Capn4⫺/⫺ mice, suggesting
that calpain activity is needed for v-src transformation of
cells (51).
Although it is becoming clear that the calpains are
activated in a number of signal transduction processes, it
will likely require a number of years before the precise
role of the calpains in signal transduction is clarified. It
presently seems that different cells and different signaling
pathways involve either ␮- or m-calpain but not both and
may involve different roles for the calpains in the same
cell depending on the signal the cell receives.
An unanswered question in the role of the calpains in
signal transduction is, how are the calpains activated by a
clustering of integrins or by binding of EGF to the EGF
receptor? The Ca2⫹ influx in response to ligand binding is
insufficient to activate ␮-calpain let alone m-calpain.
Glading et al. (130) reported that ERK kinase phosphorylated m-calpain on a Ser residue, probably Ser-50. The
phosphorylated m-calpain was fully active catalytically at
Ca2⫹ concentrations below 1 ␮M. It is tempting to speculate that the results suggest that some combination of
phosphorylation is the calpain “activator” that enables
calpain activity at physiological Ca2⫹ concentrations.
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THE CALPAIN SYSTEM
3. The cell cycle
A number of studies using synthetic inhibitors have
suggested that calpain activity is required for progression
through the cell cycle, specifically through the G1 to S
transition. As indicated previously, however, synthetic
inhibitors may have unexpected effects, such as inhibition
of protein tyrosine phosphatase (PTP) 1-B by calpeptin
(394), a calpain inhibitor that has been used widely. Also,
that fibroblasts from the Capn4⫺/⫺ mice proliferate normally but have no detectable calpain activity raises uncertainties as to the role of the calpains in the mitotic
cycle.
Nevertheless, several studies using combinations of
different inhibitors to minimize contributions of the proteasome or using the specific calpain inhibitor, calpastatin, have indicated that the calpains have a role in the cell
cycle. Cyclin D1 is rate limiting and is involved in progression through G1. Serum starvation of NIH 3T3 cells
results in a rapid loss of cyclin D1 that is prevented by
synthetic calpain inhibitors or by overexpressing calpastatin (55). Hyperphosphorylation of the retinoblastoma gene product pRb is associated with increased rate
of progression of cells through the G1 stage of mitosis;
overexpression of calpastatin represses progression of
v-src transformed chick embryo fibroblast through G1;
decreases the levels of cyclin A, cyclin D, and cdk2; and is
correlated with decreased levels of pRb phosphorylation
(51). Although cyclin D1 is also degraded by the proteasome, synthetic proteasome inhibitors did not prevent the
rapid loss of cyclin D1 following serum starvation. Progression of serum-starved WI-38 fibroblasts through G1
was partially inhibited by E-64d, a cell-permeant inhibitor
of calpain, whereas the proteasome inhibitor lactacystin
caused only modest inhibition of this degradation (276).
Addition of 20 ␮M benzyloxycarbonyl-Leu-Leu-Tyr-diazomethyl ketone (ZLLY-CHN2), a synthetic calpain inhibitor,
to cultures of WI-38 fibroblasts completely blocked their
proliferation at the late G1 stage and resulted in a twofold
increase in p53 levels (498). Immunofluorescence studies
showed that ␮-calpain appeared transiently in the nucleus
at late G1. A proteasome inhibitor also resulted in an
increase in p53 levels, so it seems that inhibition of either
protease alone can retard p53 degradation. In contrast to
the results of Choi et al. (55), ZLLY-CHN2 did not affect
the levels of cyclin D in WI-38 fibroblasts and also did not
affect the levels of c-Jun or c-Fos (498). Selection of a line
of Chinese hamster ovary (CHO) cells that were resistant
to ZLLY-CHN2 resulted in cells that had decreased levels
of ␮-calpain, unaltered levels of m-calpain and calpastatin, and a 50% increase in doubling time compared with
CHO cells before selection (282). The increased doubling
time was due to a prolonged G1 phase. Addition of ZLLYCHN2 to cultures of TE2 cells (simian virus 40-trans-
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formed human esophageal epithelial cells) or C-33A cells
(derived from human cervical carcinoma) inhibited
growth of both cell lines and lowered the Ca2⫹-dependent
proteolytic activity in both cell lines to 20% of the original
activity (286). Addition of ZLVG-CHN2, a synthetic inhibitor that has little effect on activity of the calpains, but
inhibits cathepsin L, however, had no effect on either the
growth or the calpain activity of these two cell lines,
suggesting that the effects of ZLLY-CHN2 were due to the
calpains.
A recent study found that both EcR-CHO cells overexpressing calpastatin and fibroblasts from Capn⫺/⫺ mice
proliferated at the same rate as mock-infected cells or
fibroblasts from Capn⫹/⫹ mice if the cells were plated at
densities of 1,500 cells/cm⫺2 (485). If the EcR-CHO cells
overexpressing calpastatin or the Capn⫺/⫺ fibroblasts
were plated at clonal densities (2– 4 cells/cm⫺2), however,
their rates of proliferation were only 20 –50% as great as
the rates of mock-infected cells or fibroblasts from
Capn⫹/⫹ mice (485). That both the calpastatin-overexpressing cells and the Capn⫺/⫺ fibroblasts proliferated,
albeit at a reduced rate, even at clonal densities indicates
that cells can proliferate in the absence of calpain activity.
Interestingly, the levels of ezrin, talin, and spectrin, all
purported in vivo substrates of the calpains and presumably proteins that would be cleaved as cytoskeletal/membrane attachments are broken during cytokinesis, were
the same in the calpastatin-overexpressing and the mockinfected cells (485). As noted previously, it seems that, for
reasons that are not yet clear, some calpain-susceptible
polypeptides in cells are spared from degradation in vivo
in circumstances where they would be expected to be
cleaved.
Several studies have implicated the calpains in meiosis. Calpain is present and undergoes autolysis and relocalization in concert with an increase in intracellular
Ca2⫹ concentration in rat eggs (265). Reinitiation of meiosis in starfish oocytes is accompanied by a Ca2⫹ transient, disassembly of the nuclear envelope, and degradation of the lamins, which are calpain substrates in vitro
(389). Microinjection of calpain into the nuclei of
prophase-arrested oocytes induced meiosis. Finally, Watanabe et al. (475) have shown that m-calpain degrades
the protein kinase pp39mos in Xenopus eggs, thereby allowing progression through metaphase.
In sum, the role of the calpains in mitosis/meiosis is
still poorly understood. The results thus far suggest that
calpain activity may not be essential for mitosis to occur.
The calpains may serve as an alternative to or have a
function supplemental to the proteasome in the proteolytic pathways that regulate cell proliferation. Although it
will likely be difficult to sort out what role the calpains
have in the mitotic cycle, recent advances in understanding the cell cycle and its regulation indicate that interest-
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ing new information of the role of the calpains in the cell
cycle may be expected in the future.
4. Regulation of gene expression
The exact nature of the role that the calpains may
have in regulation of gene expression also is presently
unclear. The concept that the calpains are involved in
regulation of gene expression seems to be based to a
considerable extent on ability of the calpains to cleave
several transcription factors such as c-Jun, c-Fos (166,
341), and p53 (141, 216, 340) in in vitro assays. These same
transcription factors are also degraded by the proteasome, however, and it seems likely that the proteasome is
the principal route of degradation of these transcription
factors in cells. Gonen et al. (141), for example, indicate
that, although they cannot absolutely rule out a role for
the calpains in degradation of p53, a rigorous proof for
their involvement is still missing. If the calpains have a
role in degradation of transcription factors, it is likely to
occur in the cell cytoplasm because immunolocalization
studies indicate that calpains are not generally located in
the nucleus.
On the other hand, overexpression of calpastatin in
NIH3T3 cells decreases and introduction of an anti-sense
calpastatin increases the rate of degradation of c-Jun in
these cells (161). Hence, the evidence suggests that c-Fos,
c-Jun, and p53 can be degraded via several different proteolytic pathways in vivo and that one of these pathways
may involve the calpains (385), although the proteasome
pathway is likely the predominant pathway in most instances. The factors regulating participation of the calpains in degradation of transcription factors are unknown. For example, addition of a calpain inhibitor to
cells expressing wild-type p53 resulted in stabilization of
the p53, whereas neither in vivo nor in vitro degradation
of p53 mediated by human papillomavirus E6 protein was
affected by the presence of the same calpain inhibitor
(216). The mutated transduced v-FosFBR but not the mutated transduced v-FosFBJ (transduced by Finkel-BiskisReilly murine sarcoma virus-FBR-MSV or by FinkelBiskis-Jenkins murine sarcoma virus-FBJ-MSV) was resistant to calpain cleavage, raising the possibility that
decreased sensitivity to the calpains might contribute to
the tumorigenic potential of FBR-MSV (429). Additional
work using specific calpain inhibitors such as calpastatin
will be needed to clarify the role of the calpains in gene
regulation.
5. Apoptosis
The term apoptosis was first used in 1972 (203) to
describe the process of programmed cell death. Reports
in 1993 first suggested that the calpains were involved in
apoptosis, but the exact role of the calpains in this process has been controversial. It presently seems that inPhysiol Rev • VOL
volvement of the calpains in apoptosis is limited to certain
cell types and to specific stimuli (204). There are several
difficulties in clearly defining the role of the calpains in
apoptosis. A number of proteolytic enzymes are involved
in apoptosis. It is clear that cysteine (but not Ca2⫹-dependent) proteases, the caspases, have a major role in apoptosis in most cell types, but lysosomal cathepsins and
the proteasome also seem to be involved in some apoptotic pathways. Some of the currently available “calpain
inhibitors” inhibit cathepsins B and L in addition to inhibiting the calpains, and calpain inhibitor I inhibits the
proteasome (Table 10). Hence, use of synthetic inhibitors
does not distinguish clearly among calpain, cathepsin,
and proteasome activity in apoptosis, and results of studies using calpain inhibitors should be interpreted cautiously. Some studies have used combinations of proteasome inhibitors, cathepsin inhibitors, and caspase inhibitors and differences in the abilities of the different
inhibitors to affect apoptosis in attempts to distinguish
the roles of the different proteolytic pathways in apoptosis. Although these studies provide suggestive evidence
for the role of the calpains in apoptosis, it remains difficult to be certain that the inhibitors used completely
inhibited their putative targets and that they did not have
unexpected effects.
Recent studies using injection or overexpression of
the calpain-specific inhibitor calpastatin or domains of
calpastatin have now shown that the calpains are clearly
involved in some types of apoptosis in specific cell types
and in response to certain apoptotic signals (54, 251, 369,
427). Other evidence supporting a role of the calpains in
apoptosis has relied on the ability to detect the calpainspecific 150/145-kDa degradation products of ␣-spectrin
(see sect. IVC3) in apoptotic cells by using the antibody
specific for the NH2 terminus of the 150-kDa calpain
fragment (379). Caspase degradation of ␣-spectrin produces 120- and 150-kDa fragments (267), so spectrin degradation by the calpains can be clearly distinguished from
spectrin degradation by the caspases.
Understanding the role of the calpains in apoptosis is
complicated by the ostensible ability of the calpains to
cleave the caspases themselves and to cleave several
proteins that regulate progression of apoptosis. Several
studies have found that the calpains cleave caspase-7, -8,
and -9 and that calpain cleavage of caspase-7 and -9
inactivates them (56). On the other hand, m-calpain has
been reported to cleave procaspase-12 to generate an
active caspase and to cleave the loop region of Bcl-xl to
change an antiapoptotic molecule into a proapoptotic
molecule (314). Therefore, it seems that in some instances, the calpains may act as negative regulators of
apoptosis by inactivating “upstream” caspases, whereas
in other instances, they may act as positive regulators of
apoptosis; these activities would be in addition to the
presumed direct role of the calpains in degradation of
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cytoskeletal proteins in some types of apoptosis. Studies
involving Chinese hamster cells overexpressing ␮-calpain
or calpastatin found that calpain actually protected
against tumor necrosis factor-␣-induced apoptosis,
whereas calpastatin protected against apoptosis induced
by thapsigargin, A23187, or serum deprivation in these
cells (251).
There are several other examples of “indirect” effects
of the calpains on apoptosis. The chaperone glycoprotein
GRP94 possesses antiapoptotic properties. During apoptosis induced in three different cell types by etoposide, a
topoisomerase II inhibitor, a fraction of total cellular
GRP94 that is associated with the endoplasmic reticulum
(⬃40% of total GRP94) is degraded by a protease that is
completely inhibited by a synthetic calpain inhibitor but
that is only partly inhibited by synthetic caspase inhibitors and is not affected by a proteasome inhibitor (363).
Immunolocalization studies using confocal microscopy
showed that calpain and GRP94 were colocalized in the
perinuclear region of Chinese hamster fibroblast K12 cells
after treatment with etoposide. The use of antisense
GRP94 to reduce cellular levels of GRP94 resulted in
initiation of apoptosis in Jurkat cells (363). Hence, calpain
cleavage of GRP94 may assist in progression of the apoptotic pathway independently of any direct effect of the
calpains on cleavage of cytoskeletal proteins. The synthetic calpain inhibitor calpeptin prevents cleavage of the proapoptotic protein Bax in a human hematopoietic cell line,
HL-60 (483). Both the caspases and the calpains can cleave
Bax, and cleavage enhances the proapoptotic properties
of Bax. In HL-60 cells induced to undergo apoptosis by the
topoisomerase I inhibitor 9-amino-20(s)-camptothecin (9AC), cleavage of ␣-spectrin to a 150-kDa polypeptide
(caspase fragment) and activation of caspases occurred
early in apoptosis, whereas activation of the calpains
seemed to be related to caspase activity and occurred
later, simultaneously with cleavage of Bax. The authors
speculate that calpain activation, as estimated by the
autolytic cleavage of its 28-kDa subunit, occurred downstream of caspase activity and only after the caspases had
degraded endogenous calpastatin. Pretreatment with calpeptin, which would inhibit the calpains, did not prevent
apoptosis in the HL-60 cells in this study, indicating that
apoptosis in HL-60 cells can proceed in the absence of
calpain activity.
Examination of apoptosis of senescent neutrophils
indicated that the caspases, calpains, and proteasome
function synergistically to complete apoptosis in these
cells and that both calpains and the proteasome act downstream from the caspases (211). On the other hand, apoptosis in human platelets, which contain caspase-9 and
caspase-3 and the caspase activators APAF-1 and cytochrome c, seems to be initiated by calpain (␮-calpain in
human platelets), and not by the caspases (480). Anti-IgMinduced apoptosis in an immature B-cell line was blocked
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781
completely by overexpression of calpastatin, but apoptosis induced by actinomycin D in these same cells was not
impeded by calpastatin overexpression (369), indicating
that the role of the calpains in apoptosis is cell and
apoptotic-signal specific.
Finally, studies on primary cultures of v-src-transformed and nontransformed chick embryo fibroblasts indicate that the calpains may regulate focal adhesion turnover but may not be involved in apoptosis in these cells.
Levels of m-calpain increased substantially during the
first 18 h after temperature-sensitive induction of v-src
expression, whereas the levels of ␮-calpain and calpastatin were unaffected (49). Induction of v-src resulted in
disassembly of focal adhesions, cell rounding, disorganization of the actin cytoskeleton, and increased cell motility. These changes were prevented by synthetic calpain
inhibitors and a calpastatin peptide, but not by a lysosomal inhibitor or by synthetic caspase or proteasome
inhibitors. ppFAK125, the focal adhesion kinase, was degraded during v-src transformation, but other calpainsensitive cytoskeletal proteins such as talin, paxillin, and
pp60c-src were not degraded. Rat-1 fibroblasts expressing
v-src undergo apoptosis when cultured in low serum; this
apoptosis is also accompanied by ppFAK125 degradation,
but in this instance, ppFAK125 degradation is prevented by
synthetic caspase inhibitors and is not affected by synthetic calpain inhibitors (49). Hence, the calpains seem to
be involved in disassembly of focal adhesions and loss of
substrate anchorage in v-Src-transformed fibroblasts, but
not in apoptosis of these cells.
The latter studies all used synthetic inhibitors of the
calpains and the proteasome and therefore are limited by
the uncertain specificity of these inhibitors. Nevertheless,
the results indicate that the role of the calpains in apoptosis is highly cell and apoptotic signal dependent. It is
unclear how much calpain degradation of the caspases
themselves and of some of the regulatory proteins of
apoptosis contribute to the progression of apoptosis and
whether this degradation is also cell and apoptotic signal
dependent. Apoptosis is a rapidly evolving area, and it can
be anticipated that a great deal of additional information
on the role of the calpains in apoptosis will become
available in the coming years.
6. Long-term potentiation
It was reported in 1981 that micromolar Ca2⫹ concentrations markedly enhanced glutamate binding to receptors in hippocampal synaptic membranes and that this
effect was partly irreversible (29). The increased binding
was associated with selective degradation of a 180-kDa
doublet by a Ca2⫹-dependent protease that was inhibited
by leupeptin and that had many of the properties of
m-calpain, whose purification had just been described
(29). It was proposed that Ca2⫹-dependent cleavage may
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GOLL, THOMPSON, LI, WEI, AND CONG
be a mechanism for the long-term potentiation of synaptic
transmission observed in the rat hippocampus. Since this
early report, evidence has continued to accumulate suggesting that calpains cleave cytoskeletal and other proteins that anchor the N-methyl-D-aspartate (NMDA) receptors in structures named postsynaptic densities (PSDs)
that underlie the postsynaptic membranes. In vitro studies have shown that ␮-calpain cleaves the NR2 subunits of
the NMDA receptor in their COOH-terminal regions (146)
and that m-calpain cleaves a protein, PSD-95, in the PSD
that binds to the NR2 subunits to anchor the receptor
(470). The current hypothesis suggests that proteolytically induced changes in the composition and the anatomy of NMDA and ␣-amino-3-hydroxy-5-methylisoxazole4-propionate (AMPA) receptors lead to long-term potentiation (252). These changes may involve changes in the
membrane environment of the receptors, a greater access
to modulatory proteins, a clustering of the receptors, a
modification of the space available for receptor insertion,
or any combination of these possibilities. The interest in
long-term potentiation is its purported role as a molecular
mechanism that underlies learning and memory formation. Theta pattern stimulation results in an influx of Ca2⫹
and activates calpain in the PSDs as determined by the
appearance of the calpain-specific 150-kDa fragment of
␣-spectrin (252). Many of the studies implicating the calpain system in long-term potentiation have used Ca2⫹dependent proteolysis in crude preparations and synthetic inhibitors of the calpains to infer a role for calpain
in glutamate receptor function, and the conclusions therefore should be interpreted cautiously. Nevertheless, it
seems likely that the calpains are responsible for degradation of key polypeptides in NMDA-induced potentiation, although the complexity of the system makes it
difficult to determine the exact nature of the role that the
calpains have.
VI. PATHOLOGICAL IMPLICATIONS OF THE
CALPAIN SYSTEM
The calpains have been implicated in a wide range of
pathological states in cells or tissues. Some but not all of
these “calpain-associated” pathologies are listed in Table
12. The putative role of the calpains in various pathologies
has been the subject of several recent reviews (173, 461,
472, 473), and the evidence linking the calpains to these
pathologies will not be reviewed in depth here. With three
exceptions, the role of calpain 3 in limb girdle muscular
dystrophy, the possible role of calpain 9 in tumorigenesis,
and the role of calpain 10 in type II diabetes (Table 12)
that have all been discussed earlier in this review, the
calpains are not the genetic cause of these diseases;
rather, conditions leading to loss of Ca2⫹ homeostasis in
cells are in many instances accompanied by degradation
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of proteins/polypeptides that are known substrates of the
calpains in in vitro assays. When synthetic inhibitors of
the calpains prevent or reduce this proteolytic degradation, it is frequently assumed that the increased intracellular Ca2⫹ concentrations have “activated” the calpains
and that the proteolytic degradation in these situations is
the result of increased calpain activity. Hence, the calpains become involved in a “guilt by association” process.
The vexing Ca2⫹ problem discussed earlier, however, appears again. The increases in intracellular Ca2⫹ concentration that occur in ischemic or other areas where Ca2⫹
homeostasis is impaired are not sufficiently great to activate the calpains directly (this is certainly true for mcalpain; in some instances, Ca2⫹ concentrations may increase to levels that could elicit minimal ␮-calpain activity, and it can be argued that such minimal and
unregulated ␮-calpain activity could over time result in
substantial tissue damage). A great deal of circumstantial
evidence implicating the calpains in the pathologies listed
in Table 12 has accumulated during the past 10 –15 years,
however, and it currently seems likely that the calpains
have some role in many of the conditions listed in Table
12. Rather than activating the calpains directly, however,
it seems probable that the increased Ca2⫹ concentrations
that occur during disruption of Ca2⫹ homeostasis in cells
alter regulation of calpain activity. For example, the increased Ca2⫹ concentration may affect activity of the
“activator” discussed in section IVC1, or it may alter regulation of calpain activity by calpastatin, phosphorylation,
or other mechanisms that are presently unknown.
The muscular dystrophies were the first disease to be
associated with inappropriate calpain activity (190).
Nearly all of the more than 20 different kinds of muscular
dystrophies involve some loss of Ca2⫹ homeostasis. The
Duchenne and Beckers muscular dystrophies are among
the most thoroughly studied of the muscular dystrophies;
both are caused by disruption of the gene encoding the
protein dystrophin. This protein is associated with the
skeletal muscle cell membrane and links the subsarcolemmal actin cytoskeleton to the extracellular matrix.
Loss of dystrophin leads to weakening of the plasma
membrane of skeletal muscle cells and “leakage” of extracellular Ca2⫹ into the muscle cell. Intracellular free
Ca2⫹ concentrations rise nearly two- to threefold up to
200 –500 nM in skeletal muscles of the mdx mouse (169),
an animal model of Duchenne muscular dystrophy that
also lacks dystrophin. There are several features of the
putative role of the calpains in muscle wasting that apply
also to many of the other “calpain pathologies.” These
features are summarized in the following model for turnover of myofibrillar proteins that was proposed a number
of years ago (82).
First, the specific subsite specificity of the calpains
indicates that inappropriate calpain activity is restricted
to a limited number of cleavages in a few proteins. Muscle
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TABLE
783
12. Some pathological conditions that have been linked to the calpains or to inappropriate calpain activity
Disease/Condition
Evidence/Comments
Genetic diseases
Limb girdle muscular
dystrophy type 2A
Gastric cancer
Type 2 diabetes mellitus
Caused by disruptions in the gene for calpain 3a (366); disease-related disruptions are linked to loss of calpain
3a proteolytic activity (333); see section IIIB1.
Downregulation of Capn9 gene is associated with human gastric cancer (493); antisense induced deficiency of
calpain 9 is associated with tumorigenesis in NIH3T3 fibroblasts (249).
Mutations in intron 3 of Capn10 gene associated with increased incidence of type 2 diabetes in some
populations; see section IIIB2.
Ca2⫹ homeostasis-linked pathologies
Alzheimer’s disease (AD)
Cataract formation
Muscular dystrophies
Myocardial infarcts
Multiple sclerosis (MS)
(demyelination)
Neuronal ischemia
(stroke)
Obsessive-compulsive
disorders
Traumatic spinal cord
(brain) injury
Amount of m-calpain in the cytosolic but not the membranous fractions (464) and in the neurofibrillary tangles
(145) of brain from Alzheimer’s patients is increased. Ratio of autolyzed to unautolyzed ␮-calpain is threefold
higher and amount of calpastatin is lower in AD prefrontal cortex than in normal brain (321). Calpain
evidently is not involved in cleavage of APP to produce amyloid ␤-peptides; see section VI.
Ca2⫹ influx of 1,000 ␮M or higher activates m-calpain, the predominant calpain in lens, and ␣- and ␤- but not
␥-crystallins are cleaved. The crystallin fragments aggregate to form cataracts. No Lp82 in human lens; see
section VI.
Many muscular dystrophies are accompanied by loss of Ca2⫹ homeostasis, either locally or involving entire
fibers; Duchenne and Beckers muscular dystrophy are caused by loss of a membrane-associated protein,
dystrophin, which results in increased Ca2⫹ concentrations in muscle, loss of Ca2⫹ homeostasis, and
inappropriate calpain activity (461); see section VI.
Ca2⫹ homeostasis is lost in ischemic areas, triggering inappropriate calpain activity. Desmin and ␣-spectrin are
degraded in ischemic hearts (339, 463, 491); this degradation occurs after an increase in [Ca2⫹] (463) and is
inhibited by synthetic calpain inhibitors. Protein and mRNA levels of first m-calpain and then ␮-calpain
increase after a myocardial infarction (387).
All major myelin proteins, the 68- and 200-kDa neurofilament proteins, myelin basic protein, and myelinassociated glycoprotein, are calpain substrates (407); m-calpain is the major calpain in myelin sheaths. Levels
of both calpain protein and calpain activity are increased in animal models of MS (408); in the demyelinating
disease, experimental allergic encephalomyelitis; and as measured immunologically, in astrocytes, activated
microglia, and activated T cells and macrophages that infiltrate sites of inflammation. The 150-kDa calpainspecific degradation product of ␣-spectrin increases 50% in human MS plaques (408). Degradation of the 68kDa neurofilament protein is inhibited by a synthetic calpain inhibitor (22).
Intracellular [Ca2⫹] increases in ischemic areas (487), partly due to overactivation of NMDA receptors; MAP2
and ␣-spectrin (fodrin) are degraded in animal models of cerebral ischemia with the calpain-specific 150-kDa
␣-spectrin fragment appearing 5 (28) or 60 min (319) after ischemia in rat brains; rate of expansion of the
area of spectrin degradation is reduced by synthetic calpain inhibitors, A275 and A295 (28). Calpastatin is
degraded by calpain to a membrane-bound 50-kDa polypeptide in ischemic brain (41). Tissue damage in
ischemic areas involves both apoptosis and necrosis and the calpains participate in both processes (42).
Erythrocytes from patients with obsessive-compulsive disorder have significantly higher calpain activities than
erythrocytes from normal patients; this difference could not be attributed to differences in memory function
(307). Calpastatin activities do not differ between obsessive-compulsive and normal patients.
Most studies done in the rat; injury results in increased intracellular [Ca2⫹] (370). Levels of calpain-specific 145kDa ␣-spectrin fragment increased up to 665% within 72 h after traumatic injury to rat brain (350). Loss of
neurofilament protein 68 (35% decrease), and NF200 (65% decrease) in ipsilateral rat cortex is prevented by
calpain inhibitor 2 (358); treatment with synthetic calpain inhibitor, AK295, improves motor scores and
performance on cognitive tests for rats after fluid percussion injury (370). Both ␮- and m-calpain activities
increase globally following brain injury (501). Spinal cord injury results in increased amounts of m-calpain
and degradation of NF68 and NF200; 20% loss in the first 30 min (21).
tissue contains three classes of proteins: 1) the sarcoplasmic or cytoplasmic proteins that constitute ⬃30 –35% of
total muscle protein; 2) the myofibrillar or contractile
proteins that constitute ⬃55– 60% of total muscle proteins; and 3) the stroma proteins that constitute 10 –15%
of total muscle protein and that include the plasma membranes and membrane receptors but that otherwise are
largely extracellular and are insoluble in most aqueous
solvents. Although the calpains can cleave many of the
kinases and phosphatases in the sarcoplasmic fraction
and some of the signal transduction molecules in the
stroma protein fraction, they do not catalyze bulk degradation of either the sarcoplasmic proteins (451) or the
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stroma proteins, many of which would be inaccessible to
the intracellular calpains. Hence, any role of the calpains
in muscle protein degradation is likely to involve principally the myofibrillar or the cytoskeletal proteins. Of the
known myofibrillar proteins, the calpains rapidly cleave
titin and nebulin at sites near the Z-disk in striated muscle
(Table 11), thereby severing their attachment to the proteins in the Z-disk. In addition, the calpains cleave the
intermediate filament protein desmin that attaches the
Z-disk to the sarcolemma; hence, the proteins constituting
the Z-disk, including ␣-actinin, are released, and the Zdisk disappears leaving a space in the myofibril (82, 84).
The released ␣-actinin can rebind to actin, indicating that
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GOLL, THOMPSON, LI, WEI, AND CONG
FIG. 8. Schematic diagram of a model for turnover/degradation of proteins assembled in myofibrillar structures in
striated muscle. The angled lines on the thick filaments represent bands of C-protein that are spaced at regular intervals
along the length of the thick filament. The myosin cross bridges and C-protein bands are not drawn to scale and represent
only a few of the cross bridges and C-protein bands in the thick filament. The lines distributed longitudinally on the
surface of the thin filaments represent troponin molecules, again not drawn to scale. Nebulin and titin filaments are not
shown to prevent the diagram from becoming overly complex. In this model, either ␮- or m-calpain degrade nebulin and
titin filaments on the surface of the myofibril at the point where they enter the Z-disk. This degradation severs the
attachments of Z-disk proteins to the remainder of the myofibril, releasing Z-disk proteins such as ␣-actinin and also
releasing thick and thin filaments from the surface of the myofibril. After the release of one “layer” of filaments by the
calpains, the remaining myofibril is narrower by two thick filaments and two thin filaments but is otherwise unchanged
and remains functionally capable of contraction. The released filaments can reassemble back onto the surface of the
myofibril, or additional degradation of troponin and tropomyosin on the thin filament and of C-protein on the thick
filament (these proteins are all rapidly cleaved by the calpains) will result in dissociation of the thick and thin filaments
to myosin and actin molecules, respectively. The actin and myosin molecules and the polypeptide fragments produced
by calpain degradation of titin, nebulin, desmin, troponin, tropomyosin, and C-protein can be ubiquitinated and degraded
to amino acids by the proteosome and cellular peptidases or perhaps also directly degraded by lysosomal cathepsins.
it has been released from the Z-disk in a substantially
undegraded form (133). Light and electron microscope
examination of muscle tissue in many conditions of muscle wasting, including the muscular dystrophies, show
that such muscle often has abnormal Z-disks, and in some
instances of severe muscle wasting, the Z-disks are missing altogether. The calpains also rapidly cleave troponins
T and I and tropomosin, and C-protein, which contribute
to stability of the thin and thick filaments, respectively,
but they cleave myosin and actin, the two major proteins
in striated muscle, very slowly if at all. Consequently, the
net effect of the calpains on striated muscle myofibrils is
progressive disruption of the Z-disk leading eventually to
its complete loss; the subsequent release of myosin thick
filaments and actin thin filaments from the surface of the
myofibril; and production of fragments of titin (some as
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large as 500 kDa), nebulin, desmin, and other calpainsusceptible proteins (Fig. 8). Because of the limited specificity of the calpains, further degradation of the titin,
nebulin, etc. fragments and of actin and myosin to amino
acids requires participation of other proteases. It seems
likely that the proteasome has a major role in degradation
of the released actin and myosin molecules and the other
myofibrillar protein fragments (100, 413, 478). The proteasome, on the other hand, cannot degrade intact myofibrils
(413) or cytoskeletal complexes, likely because the entrance to the central cavity of the proteasome containing
the active sites is only 10 –13 Å in diameter and is much
too narrow to allow entry of myofibrils that range from 10
to 100 ␮m in diameter. Hence, loss of myofibrillar proteins
in muscle wasting requires the concerted action of at least
two proteolytic systems: 1) the release of filaments from
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the myofibril, a process likely mediated by the calpains
(82, 138, 153, 478), and 2) degradation of the released
molecules and polypeptide fragements to amino acids/
small peptides, a process likely mediated by the proteasome (Fig. 8; Refs. 82, 100, 138). Rates of muscle wasting
are often estimated by measuring release of free amino
acids such as tyrosine or 3-methylhistidine; the latter
amino acid originates entirely from actin and myosin in
muscle (there are other sources of 3-methylhistidine in
animals, but actin and myosin are the only sources in
muscle). Such measurements estimate the rate of degradation of cytoskeletal (and other) polypeptides to amino
acids (i.e., the proteasome contribution) and do not include the effects of the calpains, because the calpains do
not degrade proteins to amino acids. Moreover, free
amino acids are also produced by degradation of the
sarcoplasmic proteins, which is probably due largely to
the proteasome or lysosomal cathepsins. It seems likely
that concerted action of the calpains and the proteasome
is also involved in other calpain pathologies such as neuronal ischemia and traumatic injury (Table 12) where
proteins that are known calpain substrates in vitro are
degraded first to large fragments and then to amino acids.
Several studies have used the antibody (379) that specifically labels the 150-kDa calpain degradation product of
␣-spectrin (fodrin) to detect calpain cleavage of spectrin
in rat heart that has been reperfused after ischemia (491)
or in postischemic gerbil hippocampus (379).
Second, although disruption of Ca2⫹ homeostasis in
dystrophic muscle, ischemic areas, or in other calpainrelated pathologies results in an increase in intracellular
Ca2⫹ concentration, this increase is not sufficient to support calpain activity directly; intracellular free Ca2⫹ concentrations of 10 –50 ␮M (␮-calpain; Table 3) or 400 – 800
␮M (m-calpain; Table 3) would lead to immediate cell
death regardless of any effect of the calpains. Hence, it
seems likely that the increase in free Ca2⫹ concentration
results in deregulation of one or more of the control
mechanisms that normally regulate calpain activity in
cells. Because the mechanisms that regulate calpain activity are still unknown, it is also unclear what the target(s) of this increase in free Ca2⫹ concentration are.
Third, measurements of dystrophic muscle, either
from the mdx mouse (59, 423, 425) or human dystrophic
muscle (223, 461), often indicate that calpain activity has
increased (423, 425, 467), that calpastatin has been degraded (317), that expression of mRNAs for the calpains
has increased (59, 467), or that the amount of calpain
protein has increased (223, 423, 425) in this muscle. These
measurements suggest that net calpain activity is increased (loss of calpastatin has the potential to effectively
increase net calpain activity) in dystrophic muscle and in
other “calpain pathologies.” Because the mechanisms
used to regulate calpain activity in situ are still unknown,
however, it is possible that calpain activity in diseased
Physiol Rev • VOL
785
cells could increase substantially with no increase in
amount of calpain protein (or decrease in amount of
calpastatin protein). Indeed, based on estimates that
⬍10% of the calpain in a muscle cell is active at any one
time, it is unclear why additional calpain would have any
effect on rate of substrate degradation unless the “extra
calpain” disrupts the normal regulation of calpain activity.
Fourth, intramuscular injection of the synthetic calpain inhibitor leupeptin to mdx mice prevents the decrease in muscle fiber diameter characteristic in such
mice; fiber diameters in the soleus and anterior tibialis
muscles from the injected mice were even larger than
those from the same muscles in a control line of mice.
Hence, a protease inhibitor that inhibits the calpains can
prevent loss of muscle mass in a dystrophic mouse (15).
Calpain activities were 44 – 84% lower in the injected muscles than in untreated muscles from mdx mice. Because
leupeptin does not inhibit proteasome activity, these results illustrate the importance of the calpain system as the
initiating step in myofibrillar protein turnover. Inhibiting
the first step in release of myofilaments would prevent
loss of myofibrils but would not necessarily affect the
release of amino acids from the sarcoplasmic proteins.
Although a large number of studies have shown that
calpain inhibitors prevent or retard proteolytic degradation in the pathologies listed in Table 12, these studies
have usually used synthetic inhibitors that are not completely specific for the calpains. Calpeptin, a synthetic
calpain inhibitor, prevents degradation of troponin I in
hypoxic neonatal cardiomyocytes (214). Intracellular protease activity increased by 80% after hypoxia in cultured
rat myocytes, and this protease activity and hypoxic cell
death were inhibited by E-64 and calpain inhibitor I (173).
The relative roles of the calpains and the proteasome
in muscle protein turnover have been debated for a number of years. A recent study using microarray assays
found that expression of two genes, MuRF1 (muscle ring
finger 1) and MAFbx (muscle atrophy F-box), was upregulated in three models of muscle wasting: denervation,
immobilization, and unweighting (43). A similar microarray assay independently found that expression of MuRF1
was upregulated in fasting, ureminc, and diabetic mice
(140). The proteins encoded by MuRF1 and MAFbx are
both skeletal muscle-specific ubiquitin ligases, implicating
the proteasome in these models of muscle wasting. Expression of neither of the calpains nor of any of the
subunits of the proteasome itself was notably upregulated
as measured in these microarray assays. As indicated
previously, however, the calpains are present in large
amounts in skeletal muscle, as is the proteasome (335).
Hence, the amounts of the calpains and the proteasome
do not need to change to increase the rate of muscle
protein degradation; regulation of the activity of these
two proteolytic systems needs to change. The rate at
which polypeptides are ubiquitinated has been consid-
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GOLL, THOMPSON, LI, WEI, AND CONG
ered to be the rate-limiting step for proteasome degradation (164, 335). Hence, it would be expected that components of the ubiquitinating system would be upregulated
in atrophying muscle with little or no change in expression of the proteasome or calpain subunits. It seems
unlikely that the myofibrillar proteins are ubiquitinated
while still assembled in the myofibrillar structure, and the
proteasome clearly cannot degrade myofibrillar proteins
until they have been disassembed from the myofibril (100,
413); hence, it is necessary that something dissociate
these proteins from the myofibril before they can be
degraded by either the proteasome or lysosomal proteases. The available evidence indicates that this “something” is the calpains (153, 154, 478). Because the calpains
do not degrade proteins to amino acids, estimating rate of
muscle protein turnover by measuring release of amino
acids such as tyrosine or 3-methylhistidine does not relate
directly to calpain activity.
The purported role of the calpains in Alzheimer’s
disease (AD) is more complex than simple unregulated/
excessive calpain activity in response to loss of Ca2⫹
homeostasis. AD is diagnosed by the presence of two
brain lesions: 1) the extracellular amyloid plaques that
contain deposits of the amyloid ␤-peptides, A␤40 and
A␤42; and 2) intracellular neurofibrillar tangles that are
composed principally of hyperphosphorylated tau, a microtubule-associated protein. The A␤40 and A␤42 peptides are produced by proteolytic cleavage of an amyloid
precursor protein, ␤-APP. These proteolytic cleavages are
mediated by a class of proteases called secretases: 1)
␣-secretase, which removes the large extracellular NH2
terminus of APP; 2) ␤-secretase, which also removes the
extracellular domain of APP; and 3) ␥-secretase, which
removes the COOH-terminal 60 or 58 amino acids of APP
and together with the ␤-secretase cleavage, produces the
A␤40 or A␤42 amyloid peptides. The ␣-, ␤-, and ␥-secretases are not related to the calpains, so the calpains are
not involved in A␤ peptide production (115). On the other
hand, Ca2⫹ concentrations are elevated in AD brains
(240), immunohistochemical studies indicate that m-calpain levels are increased in brains from AD patients (145,
464), and autolysis of ␮-calpain to its 76- and 78-kDa
forms is enhanced in brains from AD patients (380). It has
been suggested that abnormalities in calpain activity may
affect secretion of the ␤-amyloid peptides, possibly
through the conversion of PKC to a constitutively active
form by the calpains (321). The nature of such possible
indirect effects of the calpains on ␤-amyloid secretion,
however, is very poorly defined at present. The clearest
involvement of the calpains in AD is calpain cleavage of
p35, which activates the cyclin-dependent kinase 5
(CDK5), to a 25-kDa form (p25); the activated 25-kDa
form of the kinase then causes prolonged activation of
CDK5 (229). Tau is a substrate of CDK5, and p25 accumulates in brains of Alzheimer’s patients (237), suggesting
Physiol Rev • VOL
that calpain-cleaved p25 is responsible for the hyperphosphorylation of tau in the intracellular neurofibrillary tangles observed in AD brains (401). Hyperphosphorylation
makes tau, which normally is rapidly cleaved by the calpains (148), highly resistant to calpain degradation (248),
so the neurofibrillary tangles in AD are resistant to calpain
degradation. Finally, because the neurofilament proteins
are excellent substrates of the calpains (Table 11), the
calpains may have an important role in the necrotic neuronal death that accompanies AD. Diagnosis of AD involves structural analysis of brain tissue, and it is possible
(even likely) that not all neurofibrillary tangles or ␤-amyloid deposits originate from the same pathway, adding an
extra complicating factor in determining the role of the
calpains in AD.
More than 75% of human cataract cases involve elevated Ca2⫹ concentrations, and cataract formation in
young rat lens is probably one of the most compelling
instances of a relationship between inappropriate calpain
activity and tissue pathology (405). A wide variety of
insults can cause a large influx of Ca2⫹ into the lens, with
Ca2⫹ concentrations sometimes reaching 1,000 ␮M or
more (316), a concentration sufficient to directly initiate
proteolytic activity of m-calpain, which is the principal
ubiquitous calpain in lens. Hence, in cataract formation,
the increase in intracellular Ca2⫹ concentration is sufficient for direct activation of calpain activity, and it is not
necessary to propose some deregulation of a regulatory
mechanism. The m-calpain in lens cleaves the NH2-terminal region of ␣-crystallin to remove 4 – 49 amino acids and
cleaves 9 sites in the NH2 terminus of ␤-crystallin, but
does not degrade ␥-crystallin. The truncated crystallins
aggregate, are resistant to additional proteolysis, and
form cataracts that scatter light. Crystallin fragments produced in vivo during cataract formation in young rats are
identical in two-dimensional electrophoresis to the crystallin fragments produced by m-calpain in vitro. Lp82, a
splice variant of calpain 3 described in section IIIB, is
present in young rat lens at approximately the same levels
as m-calpain and degrades both ␣- and ␤-crystallins (316).
Expression of Lp82 decreases with advancing age, however, so its role in cataract formation in older animals is
uncertain. Although human lens also contain m-calpain,
they do not contain Lp82 because of a stop codon inserted
in exon 1 (316). The role of the calpains in human cataracts is not as well documented as it is in rat lens, although human lenses contain m-calpain and human crystallins are substrates for m-calpain in vitro.
The other “calpain pathologies” listed in Table 12 are
more directly linked to alterations in Ca2⫹ homeostasis
than AD or cataract formation, and the points made previously in the discussion of the muscular dystrophies
apply more directly to these pathologies.
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THE CALPAIN SYSTEM
VII. SUMMARY AND CONCLUSIONS
A great deal of information has been obtained on the
calpain system since the last comprehensive review in
1991 (76). The crystallographic structure of m-calpain
shows that m-calpain at least, and probably also ␮-calpain, is catalytically inactive in the absence of Ca2⫹ because the three residues constituting the calpain catalytic
triad, Cys, His, and Asn, are not sufficiently close in the
Ca2⫹-free state to assemble a functional active site.
Hence, the calpains are not proenzymes in the sense that
their active site is sterically blocked by a propeptide.
Generation of several “knock-out mouse” models has
shown that disrupting expression of the small subunit
common to both ␮- and m-calpain is embryonically lethal,
but that disrupting expression of ␮-calpain alone has
minimal consequences on survival. Evidently, m-calpain
can compensate for the absence of ␮-calpain, suggesting
that ␮- and m-calpain can cleave the same polypeptide
substrates, a conclusion consistent with the very similar if
not identical subsite specificities of the two calpains.
Beginning in 1989, a number of mRNAs encoding polypeptides having significant sequence homology to the catalytic domain of ␮- and m-calpain (domain II or IIa/IIb)
have been identified in mammals and in various other
species such as C. elegans, Drosophila, Aspergillus nidulans (a fungus), Arabidopsis thaliana (a plant), and
maize. In some instances, these calpain homologs are
expressed in specific tissues, and in several instances, the
calpain homolog does not have all three residues necessary for a catalytic triad. It is still unclear whether existence of calpains that lack one or more of the catalytic
residues indicates that the calpains have a function in
addition to their proteolytic activity. In almost all instances, however, these calpain homologs have been
identified only as mRNAs, and the proteins encoded by
these mRNAs have not been isolated. Hence, almost nothing is known about the catalytic properties of these calpain homologs, whether they function as monomers or
require a cofactor for proteolytic or other activity, or
whether they are inhibited by calpastatin, the specific
inhibitor of ␮- and m-calpain. Disruption of the gene for
one of these calpain homologs, calpain 3a, is the cause of
LGMD2A, and a mutation in the gene of another calpain
homolog, calpain 10, is associated with type 2 diabetes
mellitus. Suppression of expression of another calpain
homolog, calpain 9, has been associated with gastric cancer. The absence of information on properties of the
proteins encoded by these mRNAs makes it difficult to
infer how these calpain homologs are involved in these
diseases.
Although the past 11–12 years have produced a great
deal of new information on how activity of the calpains is
regulated in cells, many questions still remain. The high
Ca2⫹ concentrations required for in vitro proteolytic acPhysiol Rev • VOL
787
tivity of the calpains have been a central problem in
determining how calpain activity is regulated in cells, and
it remains a problem today, 26 years after the first characterization of purified m-calpain. Technical difficulties
have plagued attempts to determine how many Ca2⫹ atoms are bound by the calpains, and which Ca2⫹ atoms are
involved in inducing catalytic activity. The “EF-hand” predicted from the amino acid sequence at the boundary of
domains II/III in the 80-kDa calpain subunit does not have
an EF-hand structure in the crystallographic structure of
m-calpain, and it seems unlikely that this sequence binds
Ca2⫹ in m-calpain, although it does in a calpain that has
been isolated from a blood fluke, Schistosoma mansoni.
The fifth EF-hand in domains IV and VI is involved in
association of the 28- and 80-kDa subunits and does not
bind Ca2⫹ in the ␮- and m-calpain molecules. Ca2⫹-binding and crystallographic studies have shown that the
fourth EF-hand in domains IV and VI also binds Ca2⫹ very
weakly and has little influence on the Ca2⫹-dependent
proteolytic activity of the calpains, leaving 6 (or 7) of the
11 predicted (from amino acid sequence) EF-hands as
Ca2⫹-binding sites. Crystallographic and limited proteolytic digestion studies, however, indicate that the conformation of domains IV and VI changes very little when
Ca2⫹ is added, raising questions as to whether Ca2⫹ binding at some site in addition to or other than the EF-hands
is involved in inducing proteolytic activity of the calpains.
Recent studies have indicated that domain IIa/IIb (in the
crystallographic structure) of the 80-kDa subunit has very
weak, Ca2⫹-dependent proteolytic activity by itself and
that this domain binds one Ca2⫹ atom each in two peptide
loops that provide seven or eight coordinations to the
Ca2⫹ atom. The Ca2⫹ concentrations required for the
weak activity of domain IIa/IIb, like the Ca2⫹ concentrations required for the proteolytic activity of the entire
calpain molecules, are much higher than the Ca2⫹ concentrations that exist in living cells. The recent finding
that ␮- and m-calpain are phosphorylated at multiple sites
on the calpain molecule may provide a new approach to
studying the Ca2⫹-requirement problem. It is now clear
that calpastatin binds to the ␮- and m-calpain molecules
at three sites and that binding to at least two of these sites
is Ca2⫹ dependent. Two single-chain calpains that have
been recently isolated are not inhibited by calpastatin,
suggesting that removal of one of the three calpastatinbinding sites may weaken calpastatin binding to a point
where it can no longer inhibit calpain activity effectively.
The physiological significance of the eight or more calpastatin isoforms that are produced from the single calpastatin gene is still unclear. That knocking out expression of the 28-kDa subunit is embryonically lethal in mice
implies that catalytic activity of ␮- and m-calpain requires
the 28-kDa subunit, although recent studies have indicated that the 28-kDa subunit can be dissociated from the
80-kDa subunit and that this dissociation occurs much
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GOLL, THOMPSON, LI, WEI, AND CONG
more readily after the calpains have undergone autolysis.
Additional studies are needed to understand the physiological significance of autolysis and the role of the 28-kDa
subunit in calpain function.
The physiological functions of the calpain system
remain poorly defined, despite a great deal of work showing that the calpains are involved in functions as diverse
as cytoskeletal/plasma membrane attachments, cell motility, signal transduction pathways including activation of
some signaling molecules and assembly of focal adhesions, some aspects of the cell cycle and regulation of
gene expression, and some but not all apoptotic pathways, in addition to roles in other processes such as
long-term potentiation. Some additional reported functions of the calpain were not discussed in this review
simply because of space and time limitations, and because the evidence supporting these functions is not yet
conclusive. It seems likely that the calpains are capable of
cleaving a large number of different “physiological substrates” but that not all calpain-susceptible molecules may
be cleaved in a given cellular event; the molecules that are
degraded may depend on the signals that the cell has
received, the location of the calpains (and possibly calpastatin), and possibly other factors. This conclusion is
supported by the finding that overexpressing calpastatin
can prevent extension of lamellipodia and degradation of
ezrin, but has no effect on degradation of talin, PKC,
ppFAK125, and other calpain-sensitive polypeptides in
NIH3T3 cells. Additional information on the properties of
the recently identified calpain homologs will be needed
before the physiological functions of the calpain system
are fully understood. Finally, it is not clear why cells
would use proteolytic cleavage to regulate cytoskeletal/
plasma membrane interactions or signal transduction
pathways. It would seem much more efficient for cells to
use some easily reversible procedure such as phosphorylation rather than destroying an entire polypeptide and
then having to resynthesize it. Perhaps the polypeptide
fragments produced by limited calpain cleavage have
some physiological function essential to the cytoskeletal/
plasma membrane remodeling or to the particular signal
transduction event that cannot be performed by simple
phosphorylation/dephosphorylation.
The calpain system has been implicated in a number
of diseases or pathological conditions. In three instances,
abnormalities in expression of the genes for the newly
identified calpain homologs, calpain 3a, calpain 9, and
calpain 10, are related to LGMD2A, gastric cancer, and
type 2 diabetes, respectively. The other “calpain pathologies” involve loss of Ca2⫹ homeostasis by cells/tissues and
inappropriate/excessive degradation of proteins that are
known calpain substrates. It is widely assumed that loss
of Ca2⫹ homeostasis leads to increases in intracellular
Ca2⫹ concentrations that “activate” the calpains. Based
on the Ca2⫹ requirements of the calpains in in vitro asPhysiol Rev • VOL
says, however, the increases in intracellular Ca2⫹ concentrations are not sufficient to initiate proteolytic activity of
the calpains directly. It seems more likely that loss of
Ca2⫹ homeostasis results in disruption of some physiological process that regulates calpain activity in cells and
that disruption of this process results in unregulated calpain activity. Most of the studies implicating the calpains
in situations involving loss of Ca2⫹ homeostasis and degradation of calpain-susceptible molecules have used synthetic inhibitors of the calpains. Because none of the
presently available synthetic calpain inhibitors is completely specific for the calpains, the ability of these inhibitors to prevent inappropriate protein degradation does
not provide conclusive proof that the calpains are the
cause of the degradation. The use of calpastatin or calpastatin peptides, which are specific for inhibiting the
calpains, will provide more definitive evidence on the role
of the calpain system in these pathologies related to loss
of Ca2⫹ homeostasis.
We are very grateful to a number of individuals who read
sections of the draft of this review: to Dr. Zongchao Jia, Queens
University, Kingston, Canada, for his comments on the part on
structure of the calpains; to Dr. Masatoshi Maki, Nagoya University, Japan, for his comments on the section on calpastatin; to
Dr. Hiroyuki Sorimachi, University of Tokyo, Japan, for reviewing and sharing some of his preliminary results on the other
members of the calpain family; to Drs. Joan Fox and Sucheta
Kulkani from the Cleveland Clinic Foundation, Cleveland, OH,
for reviewing the section of calpains in signal transduction
pathways; to Dr. Ronald L. Mellgren, Medical College of Ohio,
Toledo, OH, for reading the entire section on physiological
functions of the calpains; and to Dr. John S. Elce, Queens
University, who read nearly the entire draft of the manuscript
and began his comments with “phew!,” our first indication that
someone might actually be able to read the entire thing. Any
omissions or errors are, of course, the responsibility of the
authors. We also thank Janet Christner for assistance in assembling the manuscript into a readable form. We apologize to those
individuals whose work was not cited; we have emphasized
references that have appeared since 1990 when the last major
review of the calpains was published. Medline lists 2,534 references for the calpains since 1990, and it was necessary to
restrict the number of citations severely.
The work described in this review that originated from the
authors’ laboratory was supported by grants from the National
Institutes of Health; the Muscular Dystrophy Association; the
American Heart Association, Arizona Affiliate; and National Research Initiative Competitive Grants 2001–35503–10776 and
2002–35206 –11630.
Address for reprint requests and other correspondence:
D. E. Goll, Muscle Biology Group, Univ. of Arizona, Tucson, AZ
85721 (E-mail: darrel.goll@arizona.edu).
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