Deciphering condensin’s actions during chromosome segregation
Sara Cuylen1 and Christian H. Haering1
1
European Molecular Biology Laboratory (EMBL), Meyerhofstrasse 1, 69117 Heidelberg, Germany
Corresponding author: Haering C.H. (christian.haering@embl.de)
The correct segregation of eukaryotic genomes requires the resolution of sister DNA molecules and their movement
into opposite halves of the cell prior to cell division. The dynamic changes chromosomes need to undergo during
these events depend on the action of a multi-subunit SMC (Structural Maintenance of Chromosomes) protein
complex named condensin, yet its molecular function in chromosome segregation is still poorly understood. Recent
studies suggest that condensin has a role in the removal of sister chromatid cohesion, in sister chromatid
decatenation by topoisomerases, and in the structural reconfiguration of mitotic chromosomes. In this review, we
discuss possible mechanisms that could explain the variety of condensin actions during chromosome segregation.
This is the accepted version of the manuscript published in final form in Trends in Cell Biology Volume 21, Issue 9,
552-559, 15 July 2011
The condensin complex - a key player for chromosome
segregation
The sudden splitting of sister chromatids and their
movement to the cell poles is one of the most dramatic
events of the cell division cycle and has fascinated cell
biologists for decades. The successful execution of this
segregation process requires the structural re-organization
of a cell’s genetic material into defined mitotic chromosomes
during prophase, the bi-orientation of the kinetochores of all
sister chromatid pairs on the mitotic spindle during
metaphase, and eventually the synchronous dissolution of
the connections between sisters to trigger their separation at
anaphase onset.
Two related chromosomal protein complexes named
cohesin and condensin play key roles in these steps (see
Textbox 1). While different models for the mechanisms
behind cohesin’s function in holding sister chromatids
together have been tested and discussed extensively [1-4],
the molecular basis for condensin’s role in chromosome
segregation is less well defined. In this brief review, we
summarize the advances made in deciphering the actions of
eukaryotic condensin complexes by recent cell biological,
biochemical, and biophysical studies and try to also
integrate novel insights gained from the investigation of
prokaryotic SMC complexes (see Textbox 2). We will
discuss three different scenarios that may explain the
severe chromosome segregation failures in the absence of
condensin function and in the end attempt to synthesize a
consensus model for how condensin’s action may allow the
correct production of two daughter cells that inherit one and precisely one - copy of the genome during every cell
division.
Scenario 1: Condensin is required for the complete
resolution of sister chromatid cohesion
A possible reason for the large number of unresolved sister
chromatids observed during anaphase in cells depleted of
condensin may be an inability to completely remove sister
chromatid cohesion. In metazoans, cohesin is released from
chromosomes in two waves, first during prophase through a
pathway that is regulated by phosphorylation of cohesin’s
SA-1/2 subunits and a cohesin-associated protein named
Wapl [reviewed in 5], and second through cleavage of
cohesin’s α-kleisin subunit by separase at the transition from
metaphase to anaphase. While the bulk of cohesin still
dissociates from chromatin incubated in condensin-depleted
mitotic Xenopus egg extract [6], small amounts of cohesin
can yet be detected on the arms of chromosomes isolated
from nocodazole-arrested HeLa cells after depletion of
condensin I (but not condensin II) [7]. As a result, the
resolution of chromosome arms that is normally observed
under the conditions of the arrest is impaired. It therefore
seems that complete removal of cohesin from chromosome
arms requires condensin. Whether the inability to release
cohesin from chromosome arms in these cells is also the
1
2
Box 1. Cohesin and condensin
Cohesin and condensin are two of the three multi-subunit SMC protein
complexes found in all eukaryotes. Both are built upon specific pairs of
long coiled-coil subunits, which heterodimerize via the central halfdoughnut shaped ‘hinge’ domain situated at one end of the coil. The
ATPase ‘head‘ domains formed by the N and C termini at the other end
of the antiparallel coiled coil bind to different parts of a so-called kleisin
subunit, which recruits to the complex additional subunits that are
largely composed of HEAT repeat domains (see Figure I). Like all
ATPases of the ABC (ATP Binding Cassette) family, the SMC head
domains can dimerize by sandwiching a pair of ATP molecules
between them. Hydrolysis of the bound ATP molecules is thought to
drive the heads apart again. While unicellular organisms express only a
single isoform of cohesin and condensin during vegetative growth,
metazoans express different variants of the non-SMC subunits that
assemble in specific combinations with the SMC dimer [reviewed in
53].
showed that chromosomes assembled after knock-down of condensin
subunits are impaired in their structural integrity, even though such
chromosomes eventually seem to condense to almost normal degrees.
When cells attempt to undergo anaphase after condensin depletion,
chromosomes frequently fail to segregate, which is apparent by a
significant number of lagging chromosomes and the formation of
chromatin bridges [reviewed in 53 and 65]. In addition to its role during
cell division, condensin has been implicated in various interphase
processes that have been addressed in a recent review [66].
Cohesin is responsible for holding together sister chromatids as soon
as they are generated by the DNA replication machinery. A significant
body of evidence suggests that cohesin does so by entrapping the two
sister DNAs inside the tripartite ring structure formed by its Smc1/Smc3
and α-kleisin subunits [reviewed in 4]. Once all sister chromatid pairs
have been bi-oriented on the mitotic spindle, proteolytic opening of
cohesin rings by separase-mediated cleavage of the α-kleisin subunit
triggers the segregation of sister chromatids at anaphase onset.
Condensin is essential for the structural organization of mitotic and
meiotic chromosomes. Pioneering work by Hirano and colleagues
demonstrated that the transformation of sperm chromatin into compact
mitotic-like chromosomes in Xenopus egg extract is impaired after
condensin depletion [21, 64]. Studies in a number of cultured cell lines
cause for the later anaphase segregation defects is however
not clear, since separase is evidently capable of removing
excess arm cohesion when cohesin release during
prophase is blocked by either preventing cohesin
phosphorylation or by depleting Wapl [8, 9].
Box 2. Prokaryotic SMC complexes
Most prokaryotic genomes encode only a single SMC protein, which
forms a complex with two additional subunits that have no apparent
overall homology to their eukaryotic counterparts [reviewed in 67].
Studies of the prokaryotic SMC complexes from Bacillus subtilis or
Escherichia coli showed that prokaryotic SMCs homodimerize via their
‘hinge’ domains (see Figure II). Recent crystal structures of MukB,
MukE, and MukF subcomplexes demonstrated that two MukF
protamers, each bound by a dimer of MukE molecules, dimerize via
their N-terminal domains and bind to the MukB ATPase head domains
via their C-terminal winged helix domains (WHDs) [50]. Similarly, the
ScpA subunit of the B. subtilis complex binds to the SMC ATPase
head and recruits the ScpB subunit [68, 69]. The MukF and ScpA
‘kleisin’ subunits [70] therefore connect the head domains of V-shaped
SMC dimers in an overall arrangement similar to eukaryotic cohesin
and condensin complexes. The stoichiometry of the non-SMC
subunits in the complexes may vary, potentially as a consequence of
ATP-mediated SMC head dimerization (see Figure 3c) [50].
Although prokaryotic SMC complexes are - in contrast to their
eukaryotic equivalents - not essential for cell viability under conditions
of slow growth, null mutations in genes encoding any of their subunits
nevertheless cause severe chromosome segregation defects in fast
proliferating cells. Such defects are evident by the appearance of
anucleate cells, cells whose DNA mass has been split by the septum
(‘cut’ phenotype), and cells with a mispositioned, extended, or
irregularly shaped nucleoid mass [reviewed in 71]. MukBEF
Figure I. Architecture of cohesin and condensin complexes. Yeast condensin mutants also seem to be incapable of
efficiently removing all cohesin from chromosome arms
during mitotic and meiotic divisions [10, 11]. The fact that
separase overexpression reduces the meiotic telomere
segregation defects in a condensin mutant suggests that
overexpression on the other hand causes a two-fold over-compaction of
the E. coli nucleoid [72]. These findings are consistent with a role of
prokaryotic SMC complexes in the structural organization, compaction,
and/or disentanglement of the replicated bacterial chromosome during
its segregation.
Figure II. Architecture of prokaryotic SMC complexes properties of mitotic chromosomes [7]. A recent study of the
anaphase movement dynamics of different fluorescently
labeled yeast chromosome loci supports this notion [11].
When sister centromeres separate and move towards the
poles, regions along the chromosome arm split with delays
that increase towards the telomere. Additional cohesin
removal (by inducing α-kleisin degradation) upon anaphase
onset abolishes this delay, suggesting that cohesindependent linkages along chromosome arms that have
escaped cleavage by separase cause a sequential
stretching of chromosome arm regions, followed by
‘recoiling’ when these links are broken. Strikingly, recoiling
is impaired in condensin mutants. Mathematic modeling
suggests that the condensin-dependent recoiling activity is
an active process [11], and it is therefore conceivable that
condensin-mediated conformational changes along the
chromatin fiber generate the force to break or otherwise
remove leftover cohesin bridges (Figure 1).
Scenario 2: Condensin promotes the decatenation of
sister DNA molecules
Figure 1. Removal of “leftover” cohesin bridges by chromosome
recoiling. Condensin could break cohesin rings at chromosome arms
that had escaped removal by the prophase pathway or separase
cleavage by either actively promoting the contraction of chromosome
arms, or by stiffening the chromatid fiber and thereby allowing the
transmission of mitotic spindle pulling forces along the chromatid axis.
some unresolved arm cohesion may persist beyond
anaphase onset, which requires resolution by separase in a
condensin-dependent fashion. While this effect may not only
be due to the enhanced cohesin removal but also possibly
due to other consequences of separase overexpression (like
over-stimulation of the FEAR network), an obvious
explanation could be that condensin might increase the
susceptibility of leftover cohesin to separase cleavage. It
has been previously shown that phosphorylation of
cohesin’s α-kleisin subunit by PLK1 renders it a better
substrate for separase in yeast [12]. The finding that
chromosome localization of PLK1 and consequently
phosphorylation of cohesin’s α-kleisin subunit are reduced in
condensin mutants undergoing the first meiotic division are
consistent with this possibility [10]. It is, however, unlikely
that condensin recruits PLK1 directly to cohesin, since for
the most part the two SMC complexes localize to different
sites on yeast chromosome arms [13-16].
An alternative possibility is that condensin indirectly
destabilizes cohesin binding by altering the structural
A second reason for why sister chromatids lacking
condensin frequently fail to resolve may be the persistence
of DNA catenations beyond anaphase onset. Such DNA
intertwinings result from the collision of replication forks [17]
and are normally removed by type-II topoisomerases. One
function of condensin could hence be to drive the DNA
decatenation activity of topoisomerases during chromosome
segregation.
Is there any evidence for a direct interplay between
topoisomerases and an SMC protein complex? Two recent
studies report that the E. coli SMC protein MukB directly
binds to and stimulates the activity of the type-II
topoisomerase topo IV [18, 19]. Addition of excess MukB to
topo IV promotes the relaxation of DNA supercoiling and - to
a lesser extent - the disentangling of concatenated DNA
circles in vitro. Mutations in the hinge domain of MukB or in
the C-terminal domain of the topo IV subunit ParC that
prevent their association abolish this stimulatory effect. A
simple explanation for these observations may be that the
interaction with MukB renders topo IV more active.
Alternatively, MukB may recognize sites of DNA catenation
and help to recruit topoisomerase to them (Figure 2a). The
formation of chiral knots into circular plasmid DNA by topo II
in the presence of MukB in vitro (see Textbox 3) could
indeed be the result of a specific binding of MukB to DNA
crossovers.
Does a similar interplay exist between condensin and
topoisomerases in eukaryotic cells? While such a link was
first suggested by the finding that mutations in the fission
yeast genes encoding condensin’s Smc4 subunit and topo II
Box 3. Reconfiguration of DNA topology by condensin and
prokaryotic SMC complexes in vitro
Several biochemical activities have been found for condensin and
prokaryotic SMC complexes in vitro that are presumably important
for their in vivo functions. Condensin isolated from mitotic Xenopus
extracts has the ability to promote the formation of positive
supercoils in closed circular DNAs in the presence of type I
topoisomerase (topo I) [27]. Such supercoiling may be the
consequence of wrapping DNA around the condensin complex in
two gyres, as electron spectroscopic images of condensin bound to
small DNA circles would suggest [42]. Since this activity is both
ATP-dependent and stimulated by phosphorylation of condensin by
mitotic kinases such as cyclin-dependent kinase 1 (CDK1) and
polo-like kinase 1 (PLK1) [73, 74], it may be conceivable that
condensin-mediated reconfiguration of chromosome topology
during mitosis is essential to drive chromosome segregation.
Similarly, the MukB subunit of the E. coli SMC complex can support
the formation of supercoils in circular plasmids in the presence of
topo I. This reaction is ATP-independent and the supercoils
produced in this assay are of opposite sign to the ones produced by
eukaryotic condensin [75].
A different change in DNA topology can be observed when
condensin holocomplexes immunopurified from Xenopus egg
extracts or isolated yeast Smc2/Smc4 dimers are incubated with
nicked circular DNA in the presence of ATP and topoisomerase II
(topo II). In this case the DNA circles are converted into trefoil
knots, which are the result of a topo II-catalyzed DNA strand
passage [37, 76]. Surprisingly, a mutant Smc2/Smc4 dimer
defective in ATP hydrolysis shows a similar if not identical knotting
activity [38]. In analogy to its eukaryotic counterpart, the E. coli
MukB dimer promotes the formation of right-handed DNA knots in
the presence of topo II [75].
Yet another ATPase-independent activity is the promotion of singlestranded DNA annealing by the fission yeast Smc2/Smc4
heterodimer [28, 77]. It was recently speculated that this activity
may be required to remove ‘leftover’ products of interphase
processes, for example RNA-DNA hybrids, from mitotic
chromosomes to allow their correct segregation [78]. The B. subtilis
SMC dimer was shown to promote DNA re-annealing in a similar
yet ATP-stimulated manner [79].
are synthetic lethal [20], there is very little evidence for a
direct interaction between the two proteins [21, 22].
Furthermore, the evidence that condensin could stimulate
the activity of topo II is limited. While extracts prepared from
Drosophila cells depleted of Smc4 fail to decatenate DNA
circles [23], mitotic frog extracts depleted of condensin show
in contrast no reduction in decatenation activity [24]. The
absence of condensin has apparently also no strong effect
on topo II activity in vivo, since the enzyme still efficiently
cleaves the DXZ1 α-satellite array in cells lacking Smc2
[25], and the fraction of concatenated forms of a 14 kb
circular minichromosome is not increased in a yeast
condensin mutant [26].
Even though condensin doesn’t seem to directly promote
the enzymatic activity of topo II, it might still be important for
topoisomerase recruitment to catenated sister chromatids.
The findings that condensins or Smc2/Smc4 dimers
promote knotting of plasmids (see Textbox 3) and
preferentially bind to structured DNA substrates in vitro [27,
28] support the hypothesis that they might have an affinity
for DNA crossover sites. If this hypothesis were true, it could
explain the reduction in topo II staining on mitotic
chromosome spreads in a condensin yeast mutant [29] and
the massive enrichment of condensin at highly repetitive
chromosome regions such as the yeast ribosomal DNA
cluster [15, 16, 29, 30], which may be particularly difficult to
disentangle due to a large number of catenations that
remain at this region through metaphase [31]. Condensin’s
function might, however, not be limited to topo II recruitment
at these sites, since topo II function is no longer required
during anaphase for rDNA segregation under certain
conditions, while condensin function still is [32]. Another
argument against a role of condensin in topo II recruitment
are the facts that condensin and topo II do not appear to colocalize on mitotic chromosomes [33] and that chromosomal
topo II levels are not significantly affected by mutation or
depletion of different condensin subunits in metazoans [7,
22-24, 34]. The finding that topo II localization is no longer
restricted to a central chromosome axis after Smc2 or Smc4
depletion [7, 23, 34] might be the consequence of a loss in
overall chromosome organization in the absence of
condensin function rather than a defect in topoisomerase
recruitment.
Scenario 3: Condensin reconfigures the topology of
mitotic chromosomes
Even if condensin does not directly interact with or recruit
topoisomerase, it may still promote disentangling sister
chromatids by shifting the reaction equilibrium of topo II
towards DNA decatenation through an action that alters
properties of the sister chromatid fibers. Condensin may, for
example, contract the sister DNAs to pull them apart once
they had been decatenated by topo II, making a reversal of
the reaction improbable (Figure 2b). Since folding up the
chromatin fiber is entropically unfavorable, such an activity
would presumably be an energy-dependent process. The
presence of ATPase domains in condensin’s SMC subunits
suggests that they could in principle act as engines that
drive active chromatin contraction. While purified condensin
complexes only display weak ATPase activities (5-20
molecules ATP hydrolyzed per minute, per complex), the
ATP turnover rates are stimulated two to five-fold by the
addition of DNA [27, 35, 36]. This stimulation is probably
mediated by the non-SMC subunits of the complex, since
ATP hydrolysis of isolated Smc2/Smc4 dimers is not
influenced by the presence of DNA [35, 37]. One can hence
imagine that upon chromosome binding, condensin is
converted into a motor that actively reconfigures
chromosomes. As expected if such an activity were
essential for chromosome segregation, mutations that
prevent ATP binding or hydrolysis eliminate condensin
function [38, 39].
What could be the mechanistic basis for re-shaping of
chromosome fibers by condensins? The finding that
condensin complexes are able to affect the superhelicity of
DNA circles in vitro (see Textbox 3) suggests one
possibility. A recent report describes an increase in positive
supercoiling of circular yeast minichromosomes preceding
their segregation, which depends on both the presence of
mitotic spindle microtubules and condensin function [40].
Surprisingly, positively supercoiled minichromosome dimers
isolated from topo II-deficient cells arrested in mitosis are
more efficiently decatenated in vitro by recombinant topo II
enzyme than negatively supercoiled dimers. This leads to
the suggestion that a condensin-dependent change in DNA
topology imposes a geometry on (mini)chromosomes that
promotes the decatenation of inter-sister DNA crossovers
[40]. While alternative causes for the changes in DNA
superhelicity, which may for example result from
overstretching of small catenated DNA circles under the
tension of the mitotic spindle, still need to be ruled out, this
model is in line with the previous idea that changes in DNA
coiling by condensin may be translated into a global reorganization of the chromosome fiber [27, 41].
It has been suggested that DNA supercoiling could be the
result of wrapping DNA around the SMC head domains (and
presumably also the non-SMC subunits) in two positive
turns [42]. In budding and fission yeasts, where condensin
binding sites have been mapped genome-wide, individual
binding sites are spaced in average by 10 or 40 kb DNA,
respectively [15, 16]. If condensin binding to DNA were to
change the global superhelicity of the DNA fiber in a
chromatin context, it would first need to overcome the large
number of negative turns introduced by the binding of ~50 to
250 nucleosomes to DNA regions of these lengths [43, 44].
While future experiments need to test whether the density of
condensin binding may be higher on vertebrate
chromosomes, which could account for their stronger
compaction during mitosis, the results from the yeast
studies suggest that condensin would rather need to use a
catalytic mechanism (probably by directing topoisomerases;
see above) if its mechanism lay in altering of the overall
superhelicity of a chromosomal DNA helix.
Alternatively, condensin may alter the structural properties
of sister chromatids by acting as a molecular linker that
fastens together different regions of a chromatid. Evidence
that condensin can indeed connect different segments
within a single DNA strand comes from single molecule
experiments. In a magnetic tweezers setup, a linear DNA
fragment is stretched between a glass surface and a
paramagnetic bead. Addition of condensin I immunopurified
from frog egg extract and ATP induces a rapid movement of
the bead towards the glass surface [45], suggesting that
condensin can support the contraction of linear DNA by
bringing together two segments of the DNA and looping the
DNA in-between. Importantly, this reaction depends on ATP
and can only be measured when condensin is isolated from
mitotic extract and not when it is isolated from interphase
extract.
Figure 2. Two possibilities for how prokaryotic SMC complexes and
condensin may promote DNA decatenation by type II
topoisomerases. (a) Prokaryotic SMC complexes could recruit
topoisomerase (green) to catenated DNA molecules by directly
binding the enzyme and its DNA substrate. (b) Alternatively,
condensin may shift the equilibrium between sister chromatid
decatenation and catenation towards decatenation by modifying the
structural organization of the chromatid fibers.
How might condensin link two segments of a chromatin
fiber? This requires either the presence of (at least) two
chromosome binding sites in one condensin complex, or the
association of two (or more) condensin complexes that each
bind to a different chromosome segment. Atomic force
microscopy (AFM) of fission yeast condensin bound to linear
DNA fragments show individual rod-shaped structures that
appear to associate with the DNA via their SMC hinge
domains [36]. Further evidence for an interaction of the
hinge domains with DNA comes from the findings that
addition of DNA protects the Smc2 hinge domain from
proteolytic cleavage in vitro [46] and that isolated
Smc2/Smc4 hinge domains shift DNA during gel
electrophoresis [47]. It could therefore be possible that a
condensin complex binds to one chromatin segment via its
hinge domain while making direct contacts to a second
chromatin segment via another part of the complex (Figure
3a).
Is there any evidence for the existence of a DNA binding
site in SMC protein complexes besides in their hinge
domains? Studies of prokaryotic SMCs identified a positively
charged patch not only in their hinge domains [48] but also
in the structures of dimerized head domains [49, 50].
Mutations that reverse the charges in either domain reduce
the electrophoretic mobility shift of plasmid DNA. It is
therefore possible that prokaryotic SMC proteins link two
DNA segments by binding one via their head and the other
via their hinge domains. Finding out whether a DNA binding
site also exists in the associated head domains of
condensin’s Smc2/Smc4 heterodimer might presumably
need to await the solution their structure at atomic
resolution.
One caveat of the in vitro DNA binding experiments is that
detection of electrophoretic mobility shifts so far always
required an excess of protein over DNA. It may hence be
possible that the observed interactions reflect only transient
binding of SMC proteins to DNA (e.g. during loading onto
chromosomes), but may not be required for stably holding
together segments of a chromatin fiber. Is there another
possibility how condensin could act as a molecular linker?
Since condensin’s kleisin and Smc2/Smc4 subunits form a
triangular ring structure similar to cohesin rings [46], it is
conceivable that condensin rings could encircle two
chromosomal DNA segments in an analogous manner as
cohesin rings entrap two sister chromatids (Figure 3b) [41,
51]. However, the coiled-coil arms of eukaryotic condensin
complexes appear closely attached in most electron or
atomic force micrographs [36, 52], and proteolytic cleavage
of Smc2’s coiled coils does not release condensin from
isolated chromosomes in vitro [39]. Both findings argue
against the idea that DNA could pass through condensin
rings, yet the first may be a consequence of attaching
condensin complexes onto mica surfaces for EM or AFM
imaging and the second may be hampered by the possibility
that cleavage of the two strands of the Smc2 coiled coil in
offset positions may not break ring integrity. Entrapment
within rings would not rule out any additional direct contacts
between condensin subunits and the chromatin fiber
discussed above.
Irrespective of how condensin complexes contact DNAs,
they might not act in isolation. Multiple condensin
complexes, each holding on to a single chromosome site,
may interact as dimers or higher order assemblies and
thereby generate a network of chromosomal linkages [53].
Condensin’s localization
Figure
3.
Three
different
possibilities for how the chromatin
fiber may be organized through
linkages
by
SMC
protein
complexes. (a) Condensin may
bridge two chromosome segments
by binding one segment via its SMC
hinge and the other via its SMC
head domain or the non-SMC
subunits. (b) Alternatively, different
chromosome segments may be
linked by their entrapment within the
same condensin ring structure in an
analogous
manner
to
the
entrapment of sister chromatids
within
cohesin
rings.
(c)
Biochemical and structural data
suggests that one of the MukE2F
protamers dissociates from the
MukB head domain upon ATPdependent head dimerization. The
free protamer may bind to the head
domain of another MukB dimer,
thereby forming multimers of SMC
complexes that could arrange into
rosette or spiral structures. Such
networks might form a central
scaffold for loops of DNA.
along the inner axes of mitotic chromosomes is indeed
consistent with the formation of a condensin chromosome
‘scaffold’ [7, 33, 54]. Such condensin networks may not be
static structures but could at least in part be quite dynamic,
given the rapid turnover of condensin I on chromosomes
measured in FRAP experiments [55, 56]. How condensin
complexes might form such networks is not known. It may
be possible that condensin multimerization could follow a
similar principle as the formation of linear or rosette-like
aggregates observed for prokaryotic SMC complexes in
electron and atomic force micrographs [57, 58]. Two recent
crystal structures of MukBEF complexes suggest a
molecular mechanism for the formation of such multimers
[50]. In the first structure, two MukB head domains dimerize
by sandwiching a pair of ATPγS molecules between them,
and each head binds to the C-terminal winged helix domain
(WHD) of one MukF kleisin molecule. In the second
structure, only one MukF WHD is bound to the MukB
homodimer, while the second MukF has been displaced by
the central region of the bound MukF. The WHD of the
displaced MukF subunit would therefore be free to bind
another MukB dimer (Figure 3c).
A consensus model for condensin function?
Which of the three scenarios we discussed - complete
cohesin removal, resolution of sister catenations, and
reconfiguration of chromosome topology - represents
condensin’s major role during chromosome segregation? It
is obvious that all three scenarios are interconnected. A
condensin-driven reconfiguration of chromosome topology
may, for example, be required to expose cohesin binding
sites that had previously been inaccessible to the separase
protease [7]. Stiffening of the chromatid fiber resulting from
linkage of different chromatin segments by condensin could
allow the transmission of mechanical forces generated by
the pulling spindle at centromeres to chromosome arms and
thereby tear apart cohesin linkages that had not been
removed by separase cleavage (Figure 1) [11]. A rigid
chromatin structure generated through condensin crosslinks may in particular be important at the large centromere
regions found in most cells to allow the correct attachment
of kinetochores to spindle microtubules [23, 55, 59-61].
Another effect of rigging up the chromatin fiber through
condensin linkages would be that sister chromatid DNAs are
pulled away from each other, which may be a prerequisite
for the efficient decatenation of chromosome arms by
topoisomerase II (Figure 2b). Most of the anaphase
segregation defects observed in cells depleted for
condensin can hence be explained by a decrease in the
structural coherence of mitotic chromosome fibers.
Finding out how condensin can reinforce mitotic
chromosomes on a mechanistic level is a challenge for
future studies. While drawing parallels between the work on
prokaryotic SMCs and condensin may be speculative at this
stage, it is conceivable that some of the insights gained from
the in vitro studies of the latter may also help to understand
condensin’s action. A key step forward would be the
correlation of the various biochemical activities observed for
SMC complexes on naked DNA substrates in vitro (see
Textbox 3) with condensin’s binding to a chromatin
substrate in vivo. This may require first the isola
tion and characterization of condensin-bound chromatin
using a combination of molecular biology, biochemistry, and
biophysical techniques, and then the reconstitution of this
interaction in vitro with defined components. Central to
understanding condensin’s molecular machinery will be to
explain the dynamic changes the complex undergoes
through of cycles of ATP binding and hydrolysis by its SMC
head domains, and how these changes may be controlled
by post-translational modifications such as phosphorylation
[reviewed in 62]. Selective inhibition of condensin’s ATPase
activity in vivo may prove a powerful approach towards this
goal. Deciphering the interplay of condensin with other
components of mitotic chromosomes, like INCENP,
KIF4A/chromokinesin [34, 39], or yet unknown partners, is
another priority. Finally, high resolution optical imaging
technologies that allow the localization of individual
condensin molecules on mitotic chromosomes [63] will be
essential for understanding the formation and maintenance
of a structure that is without a doubt one of the cell’s most
fascinating molecules, the mitotic chromosome.
Acknowledgements
We are grateful to Jan Ellenberg, Marko Kaksonen, and
Ilaria Piazza for suggestions and comments on the
manuscript. Work in the Haering lab is supported by EMBL
and the German Research Foundation (DFG) Priority
Programme SPP1384.
References
1
Shintomi, K. and Hirano, T. (2007) How are cohesin rings opened
and closed? Trends Biochem Sci 32, 154-157
2
Onn, I., et al. (2008) Sister Chromatid Cohesion: A Simple Concept
with a Complex Reality. Annu Rev Cell Dev Biol 24, 105-129
3
Peters, J., et al. (2008) The cohesin complex and its roles in
chromosome biology. Genes Dev 22, 3089-3114
4
Nasmyth, K. and Haering, C. (2009) Cohesin: Its Roles and
Mechanisms. Annu Rev Genet 43, 525-558
5
Shintomi, K. and Hirano, T. (2010) Sister chromatid resolution: a
cohesin releasing network and beyond. Chromosoma 119, 459-467
6
Losada, A., et al. (1998) Identification of Xenopus SMC protein
complexes required for sister chromatid cohesion. Genes Dev 12,
1986-1997
7
Hirota, T., et al. (2004) Distinct functions of condensin I and II in
mitotic chromosome assembly. J Cell Sci 117, 6435-6445
8
Hauf, S., et al. (2005) Dissociation of cohesin from chromosome
arms and loss of arm cohesion during early mitosis depends on
phosphorylation of SA2. PLoS Biol 3, e69
9
Kueng, S., et al. (2006) Wapl controls the dynamic association of
cohesin with chromatin. Cell 127, 955-967
28 Sakai, A., et al. (2003) Condensin but not cohesin SMC heterodimer
induces DNA reannealing through protein-protein assembly. EMBO
J 22, 2764-2775
29 Bhalla, N., et al. (2002) Mutation of YCS4, a budding yeast
condensin subunit, affects mitotic and nonmitotic chromosome
behavior. Mol Biol Cell 13, 632-645
30
10 Yu, H.G. and Koshland, D. (2005) Chromosome morphogenesis:
condensin-dependent cohesin removal during meiosis. Cell 123,
397-407
Freeman, L., et al. (2000) The condensin complex governs
chromosome condensation and mitotic transmission of rDNA. J Cell
Biol 149, 811-824
31
11 Renshaw, M.J., et al. (2010) Condensins promote chromosome
recoiling during early anaphase to complete sister chromatid
separation. Dev Cell 19, 232-244
Sullivan, M., et al. (2004) Cdc14 phosphatase induces rDNA
condensation and resolves cohesin-independent cohesion during
budding yeast anaphase. Cell 117, 471-482
32
12 Alexandru, G., et al. (2001) Phosphorylation of the cohesin subunit
Scc1 by Polo/Cdc5 kinase regulates sister chromatid separation in
yeast. Cell 105, 459-472
D'Amours, D., et al. (2004) Cdc14 and condensin control the
dissolution of cohesin-independent chromosome linkages at
repeated DNA. Cell 117, 455-469
33
13 Glynn, E.F., et al. (2004) Genome-wide mapping of the cohesin
complex in the yeast Saccharomyces cerevisiae. PLoS Biol 2, E259
Maeshima, K. and Laemmli, U.K. (2003) A two-step scaffolding
model for mitotic chromosome assembly. Dev Cell 4, 467-480
34
Hudson, D.F., et al. (2003) Condensin is required for nonhistone
protein assembly and structural integrity of vertebrate mitotic
chromosomes. Dev Cell 5, 323-336
14 Lengronne, A., et al. (2004) Cohesin relocation from sites of
chromosomal loading to places of convergent transcription. Nature
430, 573-578
15 Wang, B.-D., et al. (2005) Condensin binding at distinct and specific
chromosomal sites in the Saccharomyces cerevisiae genome. Mol
Cell Biol 25, 7216-7225
16 D'Ambrosio, C., et al. (2008) Identification of cis-acting sites for
condensin loading onto budding yeast chromosomes. Genes Dev
22, 2215-2227
17 Sundin, O. and Varshavsky, A. (1981) Arrest of segregation leads to
accumulation of highly intertwined catenated dimers: dissection of
the final stages of SV40 DNA replication. Cell 25, 659-669
18 Li, Y., et al. (2010) Escherichia coli condensin MukB stimulates
topoisomerase IV activity by a direct physical interaction. Proc Natl
Acad Sci U S A 107, 18832-18837
35 Kimura, K. and Hirano, T. (2000) Dual roles of the 11S regulatory
subcomplex in condensin functions. Proc Natl Acad Sci USA 97,
11972-11977
36 Yoshimura, S.H., et al. (2002) Condensin architecture and
interaction with DNA: regulatory non-SMC subunits bind to the head
of SMC heterodimer. Curr Biol 12, 508-513
37
Stray, J.E. and Lindsley, J.E. (2003) Biochemical analysis of the
yeast condensin Smc2/4 complex: an ATPase that promotes
knotting of circular DNA. J Biol Chem 278, 26238-26248
38 Stray, J.E., et al. (2005) The Saccharomyces cerevisiae Smc2/4
condensin compacts DNA into (+) chiral structures without net
supercoiling. J Biol Chem 280, 34723-34734
39
Hudson, D., et al. (2008) Molecular and Genetic Analysis of
Condensin Function in Vertebrate Cells. Mol Biol Cell 19, 3070-3079
40
Baxter, J., et al. (2011) Positive supercoiling of mitotic DNA drives
decatenation by topoisomerase II in eukaryotes. Science 331, 13281332
20 Saka, Y., et al. (1994) Fission yeast cut3 and cut14, members of a
ubiquitous protein family, are required for chromosome
condensation and segregation in mitosis. EMBO J 13, 4938-4952
41
Swedlow, J.R. and Hirano, T. (2003) The making of the mitotic
chromosome: modern insights into classical questions. Mol Cell 11,
557-569
21 Hirano, T. and Mitchison, T.J. (1994) A heterodimeric coiled-coil
protein required for mitotic chromosome condensation in vitro. Cell
79, 449-458
42 Bazett-Jones, D.P., et al. (2002) Efficient supercoiling of DNA by a
single condensin complex as revealed by electron spectroscopic
imaging. Mol Cell 9, 1183-1190
22 Bhat, M.A., et al. (1996) Chromatid segregation at anaphase
requires the barren product, a novel chromosome-associated
protein that interacts with Topoisomerase II. Cell 87, 1103-1114
43 Lee, W., et al. (2007) A high-resolution atlas of nucleosome
occupancy in yeast. Nat Genet 39, 1235-1244
19 Hayama, R. and Marians, K.J. (2010) Physical and functional
interaction between the condensin MukB and the decatenase
topoisomerase IV in Escherichia coli. Proc Natl Acad Sci U S A 107,
18826-18831
23 Coelho, P.A., et al. (2003) Condensin-dependent localisation of
topoisomerase II to an axial chromosomal structure is required for
sister chromatid resolution during mitosis. J Cell Sci 116, 4763-4776
24 Cuvier, O. and Hirano, T. (2003) A role of topoisomerase II in linking
DNA replication to chromosome condensation. J Cell Biol 160, 645655
25 Vagnarelli, P., et al. (2006) Condensin and Repo-Man-PP1 cooperate in the regulation of chromosome architecture during mitosis.
Nat Cell Biol 8, 1133-1142
26 Lavoie, B.D., et al. (2000) Mitotic chromosome condensation
requires Brn1p, the yeast homologue of Barren. Mol Biol Cell 11,
1293-1304
27 Kimura, K. and Hirano, T. (1997) ATP-dependent positive
supercoiling of DNA by 13S condensin: a biochemical implication for
chromosome condensation. Cell 90, 625-634
44 Lantermann, A.B., et al. (2010) Schizosaccharomyces pombe
genome-wide nucleosome mapping reveals positioning mechanisms
distinct from those of Saccharomyces cerevisiae. Nat Struct Mol Biol
17, 251-257
45
Strick, T.R., et al. (2004) Real-time detection of single-molecule
DNA compaction by condensin I. Curr Biol 14, 874-880
46
Onn, I., et al. (2007) Reconstitution and subunit geometry of human
condensin complexes. EMBO J 26, 1024-1034
47 Griese, J.J., et al. (2010) Structure and DNA binding activity of the
mouse condensin hinge domain highlight common and diverse
features of SMC proteins. Nucleic Acids Res 38, 3454-3465
48 Hirano, M. and Hirano, T. (2006) Opening closed arms: longdistance activation of SMC ATPase by hinge-DNA interactions. Mol
Cell 21, 175-186
49
Lammens, A., et al. (2004) Structural biochemistry of ATP-driven
dimerization and DNA-stimulated activation of SMC ATPases. Curr
Biol 14, 1778-1782
64 Hirano, T., et al. (1997) Condensins, chromosome condensation
protein complexes containing XCAP-C, XCAP-E and a Xenopus
homolog of the Drosophila Barren protein. Cell 89, 511-521
50
Woo, J.-S., et al. (2009) Structural studies of a bacterial condensin
complex reveal ATP-dependent disruption of intersubunit
interactions. Cell 136, 85-96
65 Hudson, D.F., et al. (2009) Condensin: Architect of mitotic
chromosomes. Chromosome Res 17, 131-144
51 Haering, C.H. and Nasmyth, K. (2003) Building and breaking
bridges between sister chromatids. Bioessays 25, 1178-1191
52
Anderson, D.E., et al. (2002) Condensin and cohesin display
different arm conformations with characteristic hinge angles. J Cell
Biol 156, 419-424
53 Hirano, T. (2006) At the heart of the chromosome: SMC proteins in
action. Nat Rev Mol Cell Biol 7, 311-322
54 Cabello, O.A., et al. (2001) Cell cycle-dependent expression and
nucleolar localization of hCAP-H. Mol Biol Cell 12, 3527-3537
55 Gerlich, D., et al. (2006) Condensin I stabilizes chromosomes
mechanically through a dynamic interaction in live cells. Curr Biol
16, 333-344
66 Wood, A.J., et al. (2010) Condensin and cohesin complexity: the
expanding repertoire of functions. Nat Rev Genet 11, 391-404
67 Rybenkov, V.V. (2009) Towards the architecture
chromosomal architects. Nat Struct Mol Biol 16, 104-105
of
the
68 Volkov, A., et al. (2003) A prokaryotic condensin/cohesin-like
complex can actively compact chromosomes from a single position
on the nucleoid and binds to DNA as a ring-like structure. Mol Cell
Biol 23, 5638-5650
69 Hirano, M. and Hirano, T. (2004) Positive and negative regulation of
SMC-DNA interactions by ATP and accessory proteins. EMBO J 23,
2664-2673
70 Schleiffer, A., et al. (2003) Kleisins: a superfamily of bacterial and
eukaryotic SMC protein partners. Mol Cell 11, 571-575
56 Oliveira, R.A., et al. (2007) Condensin I binds chromatin early in
prophase and displays a highly dynamic association with Drosophila
mitotic chromosomes. Chromosoma 116, 259-274
71 Graumann, P.L. and Knust, T. (2009) Dynamics of the bacterial
SMC complex and SMC-like proteins involved in DNA repair.
Chromosome Res 17, 265-275
57 Mascarenhas, J., et al. (2005) Dynamic assembly, localization and
proteolysis of the Bacillus subtilis SMC complex. BMC Cell Biol 6,
28
72 Wang, Q., et al. (2006) Chromosome condensation in the absence
of the non-SMC subunits of MukBEF. J Bacteriol 188, 4431-4441
58 Matoba, K., et al. (2005) Comparison of MukB homodimer versus
MukBEF complex molecular architectures by electron microscopy
reveals a higher-order multimerization. Biochem Biophys Res
Commun 333, 694-702
59 Ribeiro, S., et al. (2009) Condensin Regulates the Stiffness of
Vertebrate Centromeres. Mol Biol Cell 20, 2371-2380
60 Oliveira, R.A., et al. (2005) The condensin I subunit Barren/CAP-H
is essential for the structural integrity of centromeric
heterochromatin during mitosis. Mol Cell Biol 25, 8971-8984
61 Ono, T., et al. (2004) Spatial and temporal regulation of Condensins
I and II in mitotic chromosome assembly in human cells. Mol Biol
Cell 15, 3296-3308
62
Bazile, F., et al. (2010) Three-step model for condensin activation
during mitotic chromosome condensation. Cell Cycle 9, 3243-3255
63
Shtengel, G., et al. (2009) Interferometric fluorescent superresolution microscopy resolves 3D cellular ultrastructure. Proc Natl
Acad Sci U S A 106, 3125-3130
73 Kimura, K., et al. (1998) Phosphorylation and activation of 13S
condensin by Cdc2 in vitro. Science 282, 487-490
74 St-Pierre, J., et al. (2009) Polo kinase regulates mitotic chromosome
condensation by hyperactivation of condensin DNA supercoiling
activity. Mol Cell 34, 416-426
75 Petrushenko, Z.M., et al. (2006) DNA reshaping by MukB. Righthanded knotting, left-handed supercoiling. J Biol Chem 281, 46064615
76 Kimura, K., et al. (1999) 13S condensin actively reconfigures DNA
by introducing global positive writhe: implications for chromosome
condensation. Cell 98, 239-248
77 Sutani, T. and Yanagida, M. (1997) DNA renaturation activity of the
SMC complex implicated in chromosome condensation. Nature 388,
798-801
78 Yanagida, M. (2009) Clearing the way for mitosis: is cohesin a
target? Nat Rev Mol Cell Biol 10, 489-496
79
Hirano, M. and Hirano, T. (1998) ATP-dependent aggregation of
single-stranded DNA by a bacterial SMC homodimer. EMBO J 17,
7139-7148