Chapter 4: Cell Membrane and Cell Surface
I. Cell Membrane
II. Cell Junctions
III. Cell Adhesion
IV. Extracellular Matrix
http://www.cbi.pku.edu.cn/chinese/documents/chenjg/
I. Biomembranes: Their Structure,
Chemistry and Functions
Learning objectives:
1. A brief history of studies on the structrure of the
plasma membrane
2. Model of membrane structure: an experimental
perspective
3. The chemical composition of membranes
4. Characteristics of biomembrane
5. An overview of the functions of biomembranes
1. 1. A brief history of studies
on the structrure of the
plasmic membrane
A. Conception:
Plasma membrane(cell membrane),
Intracellular membrane,
Biomembrane.
B. The history of study
Overton(1890s):
Lipid nature of PM;
 J.D.Robertson(1959):
The TEM showing:the trilaminar appearance of PM;
Unit membrane model;
 S.J.Singer and G.Nicolson(1972):
fluid-mosaic model;
 K.Simons et al(1997):
lipid rafts model;
Functional rafts in Cell
membranes.
Nature 387:569-572
2. Singer and Nicolson’s Model of membrane
structure: The fluid-mosaic model is the “central
dogma” of membrane biology.
A. The core lipid bilayer exists in a fluid state, capable
of dynamic movement.
B. Membrane proteins form a mosaic of particles
penetrating the lipid to varying degrees.
The Fluid Mosaic
Model, proposed in
1972 by Singer and
Nicolson, had two
key features, both
implied in its name.
3. The chemical composition of membranes
A. Membrane Lipids: The Fluid Part of the Model
Membrane lipids are amphipathic.
There are three major classes of lipids:
Phospholipids:
Phosphoglyceride and sphingolipids
Glycolipids
Sterols ( is only found in animals)
Figure 10-2. The parts of a phospholipid molecule.
Phosphatidylcholine, represented schematically (A), in formula (B), as a
space-filling model (C), and as a symbol (D). The kink due to the cisdouble bond is exaggerated in these drawings for emphasis.
Figure 10-3. A lipid micelle and a lipid bilayer seen in cross-section.
Lipid molecules form such structures spontaneously in water. The shape
of the lipid molecule determines which of these structures is formed.
Wedge-shaped lipid molecules (above) form micelles, whereas cylindershaped phospholipid molecules (below) form bilayers.
Figure 10-4. Liposomes. (A) An electron
micrograph of unfixed, unstained
phospholipid vesicles (liposomes) in water.
The bilayer structure of the vesicles is
readily apparent. (B) A drawing of a small
spherical liposome seen in cross-section.
Liposomes are commonly used as model
membranes in experimental studies. (A,
courtesy of Jean Lepault.)
Figure 10-5. A cross-sectional view of a synthetic lipid bilayer, called
a black membrane. This planar bilayer is formed across a small hole in
a partition separating two aqueous compartments. Black membranes are
used to measure the permeability properties of synthetic membranes.
Figure 10-6. Phospholipid mobility. The types of movement possible
for phospholipid molecules in a lipid bilayer.
Figure 10-7. Influence of cis-double bonds in hydrocarbon chains.
The double bonds make it more difficult to pack the chains together and
therefore make the lipid bilayer more difficult to freeze.
Figure 10-8. The structure of cholesterol. Cholesterol is represented by
a formula in (A), by a schematic drawing in (B), and as a space-filling
model in (C).
Figure 10-9. Cholesterol in a lipid bilayer. Schematic drawing of a
cholesterol molecule interacting with two phospholipid molecules in one
leaflet of a lipid bilayer.
Figure 10-10. Four major phospholipids in mammalian plasma
membranes. Note that different head groups are represented by different symbols in
this figure and the next. All of the lipid molecules shown are derived from glycerol
except for sphingomyelin, which is derived from serine.
Figure 10-11. The asymmetrical distribution of phospholipids and
glycolipids in the lipid bilayer of human red blood cells. The symbols
used for the phospholipids are those introduced in Figure 10-10. In
addition, glycolipids are drawn with hexagonal polar head groups (blue).
Cholesterol (not shown) is thought to be distributed about equally in both
monolayers.
Figure 10-12. Glycolipid molecules. Galactocerebroside (A) is called a neutral
glycolipid because the sugar that forms its head group is uncharged. A ganglioside (B) always
contains one or more negatively charged sialic acid residues (also called N-acetylneuraminic acid,
or NANA), whose structure is shown in (C). Whereas in bacteria and plants almost all glycolipids
are derived from glycerol, as are most phospholipids, in animal cells they are almost always
produced from sphingosine, an amino alcohol derived from serine, as is the case for the
phospholipid sphingomyelin. Gal = galactose; Glc = glucose, GalNAc = N-acetylgalactos-amine;
these three sugars are uncharged.
Membrane proteins
Figure 10-13. Six ways in which membrane proteins associate with
the lipid bilayer. Most trans-membrane proteins are thought to extend across the
bilayer as a single a helix (1) or as multiple a helices (2); some of these "single-pass"
and "multipass" proteins have a covalently attached fatty acid chain inserted in the
cytoplasmic monolayer (1). Other membrane proteins are attached to the bilayer solely
by a covalently attached lipid - either a fatty acid chain or prenyl group - in the
cytoplasmic monolayer (3) or, less often, via an oligosaccharide, to a minor
phospholipid, phosphatidylinositol, in the noncytoplasmic monolayer (4). Finally, many
proteins are attached to the membrane only by noncovalent interactions with other
membrane proteins (5) and (6). How the structure in (3) is formed is illustrated in
Figure10-14.
Figure 10-14. The covalent attachment of either of two types of lipid groups can help
localize a water-soluble protein to a membrane after its synthesis in the cytosol. (A) A fatty
acid chain (either myristic or palmitic acid) is attached via an amide linkage to an amino-terminal
glycine. (B) A prenyl group (either farnesyl or a longer geranylgeranyl group - both related to
cholesterol) is attached via a thioether linkage to a cysteine residue that is four residues from the
carboxyl terminus. Following this prenylation, the terminal three amino acids are cleaved off and
the new carboxyl terminus is methylated before insertion into the membrane. The structures of
two lipid anchors are shown underneath: (C) a myristyl anchor (a 14-carbon saturated fatty acid
chain), and (D) a farnesyl anchor (a 15-carbon unsaturated hydrocarbon chain).
Figure 10-15. A segment of a transmembrane polypeptide chain
crossing the lipid bilayer as an a helix. Only the a-carbon backbone of the
polypeptide chain is shown, with the hydrophobic amino acids in green and yellow. (J.
Deisenhofer et al., Nature 318:618-624 and H. Michel et al., EMBO J. 5:1149-1158)
Figure 10-17. A typical
single-pass transmembrane
protein. Note that the
polypeptide chain traverses the
lipid bilayer as a right-handed
a helix and that the
oligosaccharide chains and
disulfide bonds are all on the
noncytosolic surface of the
membrane. Disulfide bonds do
not form between the
sulfhydryl groups in the
cytoplasmic domain of the
protein because the reducing
environment in the cytosol
maintains these groups in their
reduced (-SH) form.
Figure 10-18. A detergent micelle in water, shown in cross-section.
Because they have both polar and nonpolar ends, detergent molecules are
amphipathic.
Figure 10-19. Solubilizing membrane proteins with a mild detergent.
The detergent disrupts the lipid bilayer and brings the proteins into solution as proteinlipid-detergent complexes. The phospholipids in the membrane are also solubilized by
the detergent.
Figure 10-20. The structures
of two commonly used
detergents. Sodium dodecyl
sulfate (SDS) is an anionic
detergent, and Triton X-100 is
a nonionic detergent. The
hydrophobic portion of each
detergent is shown in green,
and the hydrophilic portion is
shown in blue. Note that the
bracketed portion of Triton X100 is repeated about eight
times.
Figure 10-21. The use of
mild detergents for
solubilizing, purifying, and
reconstituting functional
membrane protein systems.
In this example functional
Na+-K+ ATPase molecules
are purified and incorporated
into phospholipid vesicles.
The Na+-K+ ATPase is an ion
pump that is present in the
plasma membrane of most
animal cells; it uses the
energy of ATP hydrolysis to
pump Na+ out of the cell and
K+ in, as discussed in
Chapter 11.
Figure 10-22. A scanning electron micrograph of human red blood
cells. The cells have a biconcave shape and lack nuclei. (Courtesy of
Bernadette Chailley.)
Figure 10-24. SDS polyacrylamidegel electrophoresis pattern of the
proteins in the human red blood
cell membrane. The gel in (A) is
stained with Coomassie blue. The
positions of some of the major
proteins in the gel are indicated in the
drawing in (B); glycophorin is shown
in red to distinguish it from band 3.
Other bands in the gel are omitted
from the drawing. The large amount
of carbohydrate in glycophorin
molecules slows their migration so
that they run almost as slowly as the
much larger band 3 molecules. (A,
courtesy of Ted Steck.)
Figure 10-25. Spectrin molecules from human red blood cells. The
protein is shown schematically in (A) and in electron micrographs in (B).
Each spectrin heterodimer consists of two antiparallel, loosely
intertwined, flexible polypeptide chains called a and b these are attached
noncovalently to each other at multiple points, including both ends. The
phosphorylated "head" end, where two dimers associate to form a
tetramer, is on the left. Both the a and b chains are composed largely of
repeating domains 106 amino acids long. In (B) the spectrin molecules
have been shadowed with platinum. (D.W. Speicher and V.T. Marchesi,
Nature 311:177-180; B, D.M. Shotton et al., J. Mol. Biol. 131:303-329)
Figure 10-26. The spectrin-based cytoskeleton on the cytoplasmic side of the
human red blood cell membrane. The structure is shown schematically in (A) and in an
electron micrograph in (B). The arrangement shown in (A) has been deduced mainly from studies
on the interactions of purified proteins in vitro. Spectrin dimers associate head-to-head to form
tetramers that are linked together into a netlike meshwork by junctional complexes composed of
short actin filaments (containing 13 actin monomers), tropomyosin, which probably determines
the length of the actin filaments, band 4.1, and adducin. The cytoskeleton is linked to the
membrane by the indirect binding of spectrin tetramers to some band 3 proteins via ankyrin
molecules, as well as by the binding of band 4.1 proteins to both band 3 and glycophorin (not
shown). The electron micrograph in (B) shows the cytoskeleton on the cytoplasmic side of a red
blood cell membrane after fixation and negative staining. (B, courtesy of T. Byers and D. Branton,
PNSA. 82:6153-6157)
Figure 10-31. The three-dimensional structure of a bacteriorhodopsin molecule.
The polypeptide chain crosses the lipid bilayer as seven a helices. The location of the
chromophore and the probable pathway taken by protons during the light-activated
pumping cycle are shown. When activated by a photon, the chromophore is thought to
pass an H+ to the side chain of aspartic acid 85. Subsequently, three other H+ transfers
are thought to complete the cyclefrom aspartic acid 85 to the extra-cellular space, from
aspartic acid 96 to the chromophore, and from the cytosol to aspartic acid 96. (R.
Henderson et al. J. Mol. Biol.213:899-929)
Figure 10-32. The three-dimensional structure of a porin trimer of Rhodobacter
capsulatus determined by x-ray crystallography. (A) Each monomer consists of a 16stranded antiparallel b barrel that forms a transmembrane water-filled channel. (B) The
monomers tightly associate to form trimers, which have three separate channels for the
diffusion of small solutes through the bacterial outer membrane. A long loop of
polypeptide chain (shown in red), which connects two b strands, protrudes into the
lumen of each channel, narrowing it to a cross-section of 0.6 x 1 nm. (Adapted from
M.S. Weiss et al., FEBS Lett.280: 379-382)
Figure 10-33. The three-dimensional
structure of the photosynthetic
reaction center of the bacterium
Rhodopseudomonas viridis. The
structure was determined by x-ray
diffraction analysis of crystals of this
transmembrane protein complex. The
complex consists of four subunits, L,
M, H, and a cytochrome. The L and M
subunits form the core of the reaction
center, and each contains five a helices
that span the lipid bilayer. The
locations of the various electron carrier
coenzymes are shown in black.
(Adapted from a drawing by J.
Richardson based on data from J.
Deisenhofer et al., Nature 318:618-624)
4. Characteristics of biomembrane
A. Dynamic nature of biomembrane
Fluidity of membrane lipid. It give membranes the
ability to fuse, form networks, and separate charge;
Motility of membrane protein.
The lateral diffusion of membrane lipids can demonstrated
experimentally by a technique called Fluorescence Recovery After
Photobleaching (FRAP).
Figure 10-34. Experiment
demonstrating the mixing of
plasma membrane proteins on
mouse-human hybrid cells. The
mouse and human proteins are initially
confined to their own halves of the
newly formed heterocaryon plasma
membrane, but they intermix with time.
The two antibodies used to visualize the
proteins can be distinguished in a
fluorescence microscope because
fluorescein is green whereas rhodamine
is red. (Based on observations of L.D.
Frye and M. Edidin, J. Cell Sci. 7:319335)
Figure 10-35. Antibodyinduced patching and capping
of a cell-surface protein on a
white blood cell. The bivalent
antibodies cross-link the protein
molecules to which they bind.
This causes them to cluster into
large patches, which are actively
swept to the tail end of the cell to
form a "cap." The centrosome,
which governs the head-tail
polarity of the cell, is shown in
orange.
Figure 10-37. Diagram of an epithelial cell showing how a plasma
membrane protein is restricted to a particular domain of the
membrane. Protein A (in the apical membrane) and protein B (in the basal and
lateral membranes) can diffuse laterally in their own domains but are prevented from
entering the other domain, at least partly by the specialized cell junction called a tight
junction. Lipid molecules in the outer (noncytoplasmic) monolayer of the plasma
membrane are likewise unable to diffuse between the two domains; lipids in the inner
(cytoplasmic) monolayer, however, are able to do so.
Figure 10-38. Three
domains in the plasma
membrane of guinea pig
sperm defined with
monoclonal antibodies. A
guinea pig sperm is shown
schematically in (A), while each
of the three pairs of micrographs
shown in (B), (C), and (D)
shows cell-surface
immunofluorescence staining
with a different monoclonal
antibody (on the right) next to a
phase-contrast micrograph (on
the left) of the same cell. The
antibody shown in (B) labels
only the anterior head, that in (C)
only the posterior head, whereas
that in (D) labels only the tail.
(Courtesy of Selena Carroll and
Diana Myles.)
Figure 10-39. Four ways in
which the lateral mobility
of specific plasma
membrane proteins can be
restricted. The proteins can
self-assemble into large aggregates
(such as bacteriorhodopsin in the
purple membrane of
Halobacterium) (A); they can be
tethered by interactions with
assemblies of macromolecules
outside (B) or inside (C) the cell;
or they can interact with proteins
on the surface of another cell (D).
cell coat
Figure 10-41. Simplified diagram of the cell coat (glycocalyx). The cell
coat is made up of the oligosaccharide side chains of glycolipids and integral membrane
glycoproteins and the polysaccharide chains on integral membrane proteoglycans. In
addition, adsorbed glycoproteins and adsorbed proteoglycans (not shown) contribute to
the glycocalyx in many cells. Note that all of the carbohydrate is on the noncytoplasmic
surface of the membrane.
Figure 10-42. The protein-carbohydrate interaction that initiates the transient
adhesion of neutrophils to endothelial cells at sites of inflammation. (A) The lectin
domain of P-selectin binds to the specific oligosaccharide shown in (B), which is
present on both cell-surface glycoprotein and glycolipid molecules. The lectin domain
of the selectins is homologous to lectin domains found on many other carbohydratebinding proteins in animals; because the binding to their specific sugar ligand requires
extracellular Ca2+, they are called C-type lectins. A three-dimensional structure of one
of these lectin domains, determined by x-ray crystallography, is shown in (C); its bound
sugar is colored blue. Gal = galactose; GlcNAc = N-acetylglucosamine; Fuc = fucose;
NANA = sialic acid.
5. An Overview of membrane functions
1. Define the
boundaries of the cell
and its organelles.
2. Serve as loci for
specific functions.
3. provide for and
regulate transport
processes.
4. contain the receptors
needed to detect
external signals.
5. provide mechanisms
for cell-to-cell contact,
communication and
adhesion
II. Cell junction, Cell adhension
Extracellular matrix
Learning Objectives:
1. Integrating Cells into Tissues
2. Cell junctons: Cell-cell adhension and communication;
3. Cell-Matrix adhension;
4. Extracellular matrix: Components and Functions;
5. Cell Walls
Table 19-1 A Functional Classification of Cell Junctions
1. Occluding junctions (tight junctions)
2. Anchoring junctions
a. actin filament attachment sites
i. cell-cell adherens junctions (e.g., adhesion belts)
ii. cell-matrix adherens junctions (e.g., focal contacts)
iii. septate junctions (invertebrates only)
b. intermediate filament attachment sites
i. cell-cell (desmosomes)
ii. cell-matrix (hemidesmosomes)
3. Communicating junctions
a. gap junctions
b. chemical synapses
c. plasmodesmata (plants only)
Figure 19-1 Simplified drawing of a cross-section through
part of the wall of the intestine. This long, tubelike organ is
constructed from epithelial tissues (red), connective tissues (green), and
muscle tissues (yellow). Each tissue is an organized assembly of cells held
together by cell-cell adhesions, extracellular matrix, or both.
Tight junctions
Figure 19-2 The role of tight
junctions in transcellular
transport. Transport proteins are
confined to different regions of the
plasma membrane in epithelial cells of
the small intestine. This segregation
permits a vectorial transfer of nutrients
across the epithelial sheet from the gut
lumen to the blood. In the example
shown, glucose is actively transported
into the cell by Na+-driven glucose
symports at the apical surface, and it
diffuses out of the cell by facilitated
diffusion mediated by glucose carriers
in the basolateral membrane. Tight
junctions are thought to confine the
transport proteins to their appropriate
membrane domains by acting as
diffusion barriers within the lipid bilayer
of the plasma membrane; these
junctions also block the backflow of
glucose from the basal side of the
epithelium into the gut lumen.
Figure 19-3 Tight junctions allow cell sheets to serve as barriers to solute
diffusion. (A) Schematic drawing showing how a small extracellular tracer
molecule added on one side of an epithelial cell sheet cannot traverse the
tight junctions that seal adjacent cells together. (B) Electron micrographs of
cells in an epithelium where a small, extracellular, electron-dense tracer
molecule has been added to either the apical side (on the left) or the
basolateral side (on the right); in both cases the tracer is stopped by the tight
junction. (B, courtesy of Daniel Friend.)
Figure 19-4 Structure of a tight junction between epithelial cells of the
small intestine. The junctions are shown schematically in (A) and in freeze-fracture
(B) and conventional (C) electron micrographs. Note that the cells are oriented with their
apical ends down. In (B) the plane of the micrograph is parallel to the plane of the
membrane, and the tight junction appears as a beltlike band of anastomosing sealing
strands that encircle each cell in the sheet. The sealing strands are seen as ridges of
intramembrane particles on the cytoplasmic fracture face of the membrane (the P face)
or as complementary grooves on the external face of the membrane (the E face) (see
Figure 19-5). In (C) the junction is seen as a series of focal connections between the
outer leaflets of the two interacting plasma membranes, each connection corresponding
to a sealing strand in cross-section. (B and C, from N.B. Gilula, in Cell Communication
[R.P. Cox, ed.], pp. 1-29)
Figure 19-5 A current model of a tight
junction. It is postulated that the
sealing strands that hold adjacent
plasma membranes together are formed
by continuous strands of transmembrane
junctional proteins, which make contact
across the intercellular space and create
a seal. In this schematic the cytoplasmic
half of one membrane has been peeled
back by the artist to expose the protein
strands. Two peripheral proteins
associated with the cytoplasmic side of
tight junctions have been characterized,
but the putative transmembrane protein
has not yet been identified. In freezefracture electron microscopy the tightjunction proteins would remain with the
cytoplasmic (P face) half of the lipid
bilayer to give the pattern of
intramembrane particles seen in Figure
19-4B, instead of staying in the other half
as shown here.
Anchoring junctions
Figure 19-6 Anchoring junctions in an epithelial tissue. Highly
schematized drawing of how such junctions join cytoskeletal
filaments from cell to cell and from cell to extracellular matrix.
Figure 19-7 Construction of an anchoring junction.
Highly
schematized drawing showing the two classes of proteins that constitute such a
junction: intracellular attachment proteins and transmembrane linker proteins.
Adhesion belts
Figure 19-8 Adhesion belts between epithelial cells in the small intestine.
This beltlike anchoring junction encircles each of the interacting cells. Its most
obvious feature is a contractile bundle of actin filaments running along the
cytoplasmic surface of the junctional plasma membrane. The actin filaments
are joined from cell to cell by transmembrane linker proteins (cadherins),
whose extracellular domain binds to the extracellular domain of an identical
cadherin molecule on the adjacent cell.
Figure 19-9 The folding of an epithelial sheet to form an epithelial tube. It
is thought that the oriented contraction of the bundle of actin filaments running
along adhesion belts causes the epithelial cells to narrow at their apex and that
this plays an important part in the rolling up of the epithelial sheet into a tube
(although cellular rearrangements are also thought to play an important part).
An example is the formation of the neural tube in early vertebrate development
Septate junction
Figure 19-11 A septate junction. Electron micrograph of a
septate junction between two epithelial cells of a mollusk. The
interacting plasma membranes, seen in cross-section, are
connected by parallel rows of junctional proteins. The rows,
which have a regular periodicity, are seen as dense bars or
septa. (From N.B. Gilula, in Cell Communication [R.P. Cox, ed.],
pp. 1-29)
Desmosomes
Figure 19-12 Desmosomes.
(A) An electron micrograph of three desmosomes between
two epithelial cells in the intestine of a rat. (B) An electron micrograph of a single desmosome
between two epidermal cells in a developing newt, showing clearly the attachment of intermediate
filaments. (C) A schematic drawing of a desmosome. On the cytoplasmic surface of each
interacting plasma membrane is a dense plaque composed of a mixture of intracellular attachment
proteins (including plakoglobin and desmoplakins). Each plaque is associated with a thick network
of keratin filaments, which are attached to the surface of the plaque. Transmembrane linker
proteins, which belong to the cadherin family of cell-cell adhesion molecules, bind to the plaques
and interact through their extracellular domains to hold the adjacent membranes together by a
Ca2+-dependent mechanism. (A, from N.B. Gilula, in Cell Communication, pp. 1-29; B, from D.E.
Kelly, JCB. 28:51-59)
Figure 19-13 The distribution of desmosomes and hemidesmosomes in
epithelial cells of the small intestine. The keratin filament networks of
adjacent cells are indirectly connected to one another through desmosomes
and to the basal lamina through hemidesmosomes.
Table 19-2 Anchoring Junctions
Junction
Transmembrane Linker
Protein
Extracellular Ligand
Intracellular
Cytoskeletal
Attachment
Some Intracellular
Attachment Proteins
Adherens
(cell-cell)
cadherin (E-cadherin)
cadherin in neighboring
cell
actin
filaments
catenins, vinculin, actinin, plakoglobin
Desmosome
cadherin (desmogleins
& desmocollins)
cadherin in neighboring
cell
intermediate
filaments
desmoplakins,
plakoglobin
Adherens
(cell-matrix)
integrin
extracellular matrix
proteins
actin
filaments
talin, vinculin,
Hemidesmos
ome
integrin
extracellular matrix (basal
lamina) proteins
intermediate
filaments
desmoplakinlike protein
-actinin
Communicating junctions
gap-junction channel
Figure 19-14 Determining the size of a gap-junction channel. When
fluorescent molecules of various sizes are injected into one of two cells coupled
by gap junctions, molecules smaller than about 1000 daltons can pass into the
other cell but larger molecules cannot.
Figure 19-15 A model of a gap junction. The drawing shows the interacting
plasma membranes of two adjacent cells. The apposed lipid bilayers (red) are
penetrated by protein assemblies called connexons (green), each of which is
thought to be formed by six identical protein subunits (called connexins). Two
connexons join across the intercellular gap to form a continuous aqueous
channel connecting the two cells.
Figure 19-16 Gap junctions as seen in the electron microscope. Thinsection (A) and freeze-fracture (B) electron micrographs of a large and a small
gap junction between fibroblasts in culture. In (B) each gap junction is seen as
a cluster of homogeneous intramembrane particles associated exclusively with
the cytoplasmic fracture face (P face) of the plasma membrane. (From N.B.
Gilula, in Cell Communication [R.P. Cox, ed.], pp. 1-29)
Figure 11-33 Three classes of channel proteins. The
postulated relationship between the number of protein subunits
and pore diameter. (Adapted from B. Hille, Ionic Channels of
Excitable Membranes, 2nd ed. Sunderland, MA: Sinauer, 1992.)
Figure 19-17 A proposed model for how gap-junction channels may close
in response to a rise in Ca2+ or a fall in pH in the cytosol. A small rotation
of each subunit closes the channel. The model is based on an image analysis
of electron micrographs of rapidly frozen tissue in which the structure of gap
junction channels in their presumed open state was compared with their
structure in a Ca2+-induced closed state. It is possible that a similar mechanism
operates in the opening and closing of the gated ion channels discussed in
Chapter 11. (After P.N.T. Unwin and P.D. Ennis, Nature 307:609-613)
Figure 19-18 Summary of the various cell junctions found in
animal cell epithelia. This drawing is based on epithelial cells
of the small intestine.
Figure 19-19 Plasmodesmata. (A) The cytoplasmic channels of
plasmodesmata pierce the plant cell wall and connect all cells in a plant
together. (B) Each plasmodesma is lined with plasma membrane common to
two connected cells. It usually also contains a fine tubular structure (20-40nm),
the desmotubule, derived from smooth endoplasmic reticulum.
Figure 19-20 Plasmodesmata as seen in the
electron microscope. (A) Longitudinal
section of a plasmodesma from a water fern.
The plasma membrane lines the pore and is
continuous from one cell to the next.
Endoplasmic reticulum and its association
with the central desmotubule can be seen. (B)
A similar plasmodesma in cross-section.
(Courtesy of R. Overall.)
III. Cell Adhension
There Are Two Basic Ways in
Which Animal Cells Assemble
into Tissues
Figure 19-21 The simplest
mechanism by which cells
assemble to form a
tissue. The progeny of the
founder cell are retained in
the epithelial sheet by the
basal lamina and by cell-cell
adhesion mechanisms,
including the formation of
intercellular junctions.
Figure 19-22 An example of a more
complex mechanism by which cells
assemble to form a tissue. Neural crest
cells escape from the epithelium forming the
upper surface of the neural tube and migrate
away to form a variety of cell types and
tissues throughout the embryo. Here they
are shown assembling and differentiating to
form two collections of nerve cells in the
peripheral nervous system. Such a collection
of nerve cells is called a ganglion. Other
neural crest cells differentiate in the ganglion
to become supporting (satellite) cells
surrounding the neurons. Although it is not
shown, the neural crest cells proliferate
rapidly as they migrate.
Figure 19-23 Organ-specific
adhesion of dissociated
vertebrate embryo cells
determined by a radioactive
cell-binding assay. The rate of
cell adhesion can be measured by
determining the number of
radioactively labeled cells bound
to the cell aggregates after various
periods of time. The rate of
adhesion is greater between cells
of the same kind. In a commonly
used modification of this assay,
cells labeled with a fluorescent or
radioactive marker are allowed to
bind to a monolayer of unlabeled
cells in culture.
Cadherin
Figure 19-24 Schematic drawing of a typical
cadherin molecule. The extracellular part of
the protein is folded into five similar domains,
three of which contain Ca2+-binding sites. The
extracellular domain farthest from the
membrane is thought to mediate cell-cell
adhesion; the sequence His-Ala-Val in this
domain seems to be involved, as peptides with
this sequence inhibit cadherin-mediated
adhesion. The cytoplasmic tail interacts with
the actin cytoskeleton via a number of
intracellular attachment proteins, including
three catenin proteins. a-catenin is structurally
related to vinculin. X represents
uncharacterized attachment proteins involved
in coupling cadherins to actin filaments.
Figure 19-25 Distribution of Eand N-cadherin in the developing
nervous system. Immunofluorescence micrographs of a
cross-section of a chick embryo
showing the developing neural tube
labeled with antibodies against Ecadherin (A) and N-cadherin (B).
Note that the overlying ectoderm
cells express only E-cadherin,
while the cells in the neural tube
have lost E-cadherin and have
acquired N-cadherin. (Courtesy of
Kohei Hatta and Masatoshi
Takeichi.)
Selectin
Figure 10-42 The protein-carbohydrate interaction that initiates the
transient adhesion of neutrophils to endothelial cells at sites of
inflammation. (A) The lectin domain of P-selectin binds to the specific
oligosaccharide shown in (B), which is present on both cell-surface glycoprotein and
glycolipid molecules. The lectin domain of the selectins is homologous to lectin domains
found on many other carbohydrate-binding proteins in animals; because the binding to
their specific sugar ligand requires extracellular Ca2+, they are called C-type lectins. A
three-dimensional structure of one of these lectin domains, determined by x-ray
crystallography, is shown in (C); its bound sugar is colored blue. Gal = galactose;
GlcNAc = N-acetylglucosamine; Fuc = fucose; NANA = sialic acid.
Figure 19-26 Three
mechanisms by which cellsurface molecules can
mediate cell-cell
adhesion. Although all of
these mechanisms can
operate in animals, the one
that depends on an
extracellular linker molecule
seems to be least common.
NCAM
Figure 19-27 Schematic
drawing of four forms of
NCAM. The extracellular part
of the polypeptide chain in each
case is folded into five
immunoglobulinlike domains
(and one or two other domains
called fibronectin type III
repeats for reasons that will
become clear later). Disulfide
bonds (shown in red) connect
the ends of each loop forming
each Ig-like domain.
Figure 19-28 A summary of the
junctional and nonjunctional
adhesive mechanisms used by
animal cells in binding to one
another and to the extracellular
matrix. The junctional mechanisms are shown
in epithelial cells, while the nonjunctional
mechanisms are shown in nonepithelial cells. A
junctional interaction is operationally defined as
one that can be seen as a specialized region of
contact by conventional and/or freeze-fracture
electron microscopy. Note that the integrins and
cadherins are involved in both nonjunctional and
junctional cell-cell (cadherins) and cell-matrix
(integrins) contacts. The cadherins generally
mediate homophilic interactions, whereas the
integrins mediate heterophilic interactions. Both
the cadherins and integrins act as
transmembrane linkers and depend on
extracellular divalent cations to function; for this
reason, most cell-cell and cell-matrix contacts are
divalent-cation-dependent. The selectins and
integrins can also act as heterophilic cell-cell
adhesion molecules: the selectins bind to
carbohydrate, while the cell-binding integrins bind
to members of the immunoglobulin superfamily.
The integrins and integral membrane
proteoglycans that mediate nonjunctional
adhesion to the extracellular matrix are discussed
later.
Figure 19-29 Importance of the cytoskeleton in cell adhesion. This drawing
illustrates why cell-adhesion molecules must be linked to the cytoskeleton in
order to mediate robust cell-cell or cell-matrix adhesion. In reality, many
adhesion proteins would probably be pulled from the cell with bits of attached
membrane, and the holes left in the membrane would immediately reseal.
IV. Cell Coat and The Extracellular Matrix
Figure 19-30 Cells surrounded by
spaces filled with extracellular
matrix. The particular cells shown
in this low-power electron micrograph
are those in an embryonic chick limb
bud. The cells have not yet acquired
their specialized characteristics.
(Courtesy of Cheryll Tickle.)
Figure 19-31 The connective tissue underlying an epithelial
cell sheet. It consists largely of extracellular matrix that is
secreted by the fibroblasts.
Figure 19-32 Scanning electron micrograph of fibroblasts in connective
tissue. The tissue is from the cornea of a rat. The extracellular matrix surrounding the fibroblasts is composed largely of collagen fibrils. The glycoproteins,
glycosaminoglycans, and proteoglycans, which normally form a hydrated gel
filling the interstices of the fibrous network, have been removed by enzyme and
acid treatment. (T. Nishida et al. Invest. Ophthalmol. Vis. Sci. 29:1887-1890)
Figure 19-57 The comparative shapes and sizes of some of
the major extracellular matrix macromolecules. Protein is
shown in green, glycosaminoglycan in red.
Figure 19-33 The repeating disaccharide sequence of a dermatan sulfate
glycosaminoglycan (GAG) chain. These chains are typically 70 to 200
sugar residues long. There is a high density of negative charges along the
chain resulting from the presence of both carboxyl and sulfate groups.
Figure 19-35 The repeating disaccharide sequence in
hyaluronan, a relatively simple GAG. It consists of a single
long chain of up to 25,000 sugar residues. Note the absence of
sulfate groups.
Figure 19-36 The linkage between a GAG chain and its core protein in a
proteoglycan molecule. A specific link tetrasaccharide is first assembled on
a serine residue. In most cases it is not clear how the serine residue is selected,
but it seems to be a specific local conformation of the polypeptide chain, rather
than a specific linear sequence of amino acids, that is recognized. The rest of
the GAG chain, consisting mainly of a repeating disaccharide unit, is then
synthesized, with one sugar residue being added at a time. In chondroitin
sulfate the disaccharide is composed of D-glucuronic acid and N-acetyl-Dgalactosamine; in heparan sulfate it is D-glucosamine (or ¬-iduronic acid) and
N-acetyl-D-glucosamine; in keratan sulfate it is D-galactose and N-acetyl-Dglucosamine.
Figure 19-37 Examples of a large ( aggrecan ) and a small (decorin)
proteoglycan found in the extracellular matrix. They are compared to a
typical secreted glycoprotein molecule (pancreatic ribonuclease B). All are
drawn to scale. The core proteins of both aggrecan and decorin contain
oligosaccharide chains as well as the GAG chains, but these are not shown.
Aggrecan typically consists of about 100 chondroitin sulfate chains and about
30 keratan sulfate chains linked to a serine-rich core protein of almost 3000
amino acids. Decorin "decorates" the surface of collagen fibrils, hence its name.
Figure 19-38 An aggrecan aggregate from fetal bovine cartilage. (A)
Electron micrograph of an aggrecan aggregate shadowed with platinum. Many free
aggrecan molecules are also seen. (B) Schematic drawing of the giant aggrecan
aggregate shown in (A). It consists of about 100 aggrecan monomers (each like the one
shown in Figure 19-37) noncovalently bound to a single hyaluronan chain through two
link proteins that bind to both the core protein of the proteoglycan and to the hyaluronan
chain, thereby stabilizing the aggregate; the link proteins are members of the
hyaladherin family of hyaluronan-binding proteins discussed previously. The molecular
weight of such a complex can be 108 or more, and it occupies a volume equivalent to
that of a bacterium, which is about 2 x 10-12 cm. (A, courtesy of Lawrence Rosenberg.)
Figure 19-39 Electron micrograph of proteoglycans in the extracellular
matrix of rat cartilage. The tissue was rapidly frozen at -196°C and fixed
and stained while still frozen (a process called freeze substitution) to prevent
the GAG chains from collapsing. The proteoglycan molecules are seen to form
a fine filamentous network in which a single striated collagen fibril is embedded.
The more darkly stained parts of the proteoglycan molecules are the core
proteins; the faintly stained threads are the GAG chains. (E.B. Hunziker et al., J.
Cell Biol. 98:277-282.)
Collagen
Figure 19-40 The structure of a
typical collagen molecule. (A) A
model of part of a single collagen a chain
in which each amino acid is represented by
a sphere. The chain contains about 1000
amino acid residues and is arranged as a
left-handed helix with three amino acid
residues per turn and with glycine as every
third residue. Therefore an a chain is
composed of a series of triplet Gly-X-Y
sequences in which X and Y can be any
amino acid (although X is commonly
proline and Y is commonly hydroxyproline).
(B) A model of a part of a collagen
molecule in which three alpha chains, each
shown in a different color, are wrapped
around one another to form a triplestranded helical rod. Glycine is the only
amino acid small enough to occupy the
crowded interior of the triple helix. Only a
short length of the molecule is shown; the
entire molecule is 300 nm long. (From
model by B.L. Trus.)
Figure 19-41 Electron micrograph of fibroblasts surrounded by collagen
fibrils in the connective tissue of embryonic chick skin. The fibrils, which
are organized into bundles that run approximately at right angles to one another,
are produced by the fibroblasts. These cells contain abundant endoplasmic
reticulum, where secreted proteins such as collagen are synthesized. (C. Ploetz
et al. J. Struct. Biol. 106:73-81)
FIBRIL-FORMING
(FIBRILLAR)
Molecular Formula
Polymerized
Form
I
[ 1(I)]2 2(I)
fibril
bone, skin, tendon, ligaments, cornea, internal organs
(accounts for 90% of body collagen)
II
[ 1(II)]3
fibril
cartilage, intervertebral disc, notochord, vitreous
humor of the eye
III
[ 1(III)]3
fibril
skin, blood vessels, internal organs
V
[ 1(V)]2 2(V)
fibril (with
type I)
as for type I
fibril (with
type II)
as for type II
X
I
FIBRIL-ASSOCIATED
NETWORK-FORMING
Table 19-4 Some Types of Collagen and Their Properties
T
y
p
e
1(XI) 2(XI) 3(XI)
Tissue Distribution
I
X
1(IX) 2(IX) 3(IX) with
type II fibrils
lateral
association
cartilage
X
II
[ 1(XII)]3 with some type I
fibrils
lateral
association
tendon, ligaments, some other tissues
I
V
[ 1(IV)2 2(IV)
sheetlike
network
basal laminae
V
II
[ 1(VII)]3
anchoring
fibrils
beneath stratified squamous epithelia
Note that types I, IV, V, and XI are each composed of 2 or 3 types of alpha chain, whereas types II, III, VII, and XII are composed of
only 1 type of a chain each. Only 9 types of collagen are shown, but about 15 types of collagen and about 25 types of alpha chain have
been defined so far.
Figure 19-42 Hydroxylysine and hydroxyproline residues. These modified
amino acids are common in collagen; they are formed by enzymes that act after
the lysine and proline are incorporated into procollagen molecules
Figure 19-43 The intracellular and extracellular events involved in the
formation of a collagen fibril. Note that collagen fibrils are shown assembling in
the extracellular space contained within a large infolding in the plasma membrane. As
one example of how the collagen fibrils can form ordered arrays in the extracellular
space, they are shown further assembling into large collagen fibers, which are visible in
the light microscope.
Figure 19-44 How the staggered arrangement of collagen molecules gives rise to
the striated appearance of a negatively stained fibril. (A) Since the negative stain fills
only the space between the molecules, the stain in the gaps between the individual molecules in
each row accounts for the dark staining bands. An electron micrograph of a portion of a negatively
stained fibril is shown below (B). The staggered arrangement of the collagen molecules maximizes
the tensile strength of the aggregate. (B, courtesy of Robert Horne.)
Figure 19-45 The covalent intramolecular and intermolecular cross-links
formed between modified lysine side chains within a collagen fibril. The
cross-links are formed in several steps. First, certain lysine and hydroxylysine
residues are deaminated by the extracellular enzyme lysyl oxidase to yield
highly reactive aldehyde groups. The aldehydes then react spontaneously to
form covalent bonds with each other or with other lysine or hydroxylysine
residues. Most of the cross-links form between the short nonhelical segments at
each end of the collagen molecules.
Figure 19-46 Electron micrograph of a cross-section of tadpole
skin. Note the plywoodlike arrangement of collagen fibrils, in which
successive layers of fibrils are laid down nearly at right angles to each other.
This arrangement is also found in mature bone and in the cornea. (Courtesy of
Jerome Gross.)
Figure 19-47 Type IX collagen. (A) Schematic drawing of type IX collagen
molecules binding in a periodic pattern to the surface of a type-II-collagencontaining fibril. (B) Electron micrograph of a rotary-shadowed type-II-collagencontaining fibril in cartilage sheathed in type IX collagen molecules; an
individual type IX collagen molecule is shown in (C). (B and C, from L. Vaughan
et al., J. Cell Biol. 106:991-997.)
Figure 19-48 The shaping of the extracellular matrix by cells. This
micrograph shows a region between two pieces of embryonic chick heart (rich
in fibroblasts as well as heart muscle cells) that has grown in culture on a
collagen gel for four days. A dense tract of aligned collagen fibers has formed
between the explants, presumably as a result of the fibroblasts in the explants
tugging on the collagen. (D. Stopak and A.K. Harris, Dev. Biol. 90:383-398)
Elastin
Figure 19-49 A network of elastic fibers. These scanning electron
micrographs show a low-power view of a segment of a dog's aorta (A) and a
high-power view of the dense network of longitudinally oriented elastic fibers in
the outer layer of the same blood vessel (B). All of the other components have
been digested away with enzymes and formic acid. (K.S. Haas et al., Anat. Rec.
230:86-96.)
Figure 19-50 Stretching a network of elastin molecules. The molecules are
joined together by covalent bonds (indicated in red) to generate a cross-linked
network. In the model shown each elastin molecule in the network can expand
and contract as a random coil, so that the entire assembly can stretch and
recoil like a rubber band.
Fibronectin
Figure 19-51 The structure of a fibronectin dimer.
As shown schematically in (A), the
two polypeptide chains are similar but generally not identical. They are joined by two disulfide
bonds near the carboxyl terminus. Each chain is almost 2500 amino acid residues long and is
folded into five or six rodlike domains connected by flexible polypeptide segments. Individual
domains are specialized for binding to a particular molecule or to a cell, as indicated for three of the
domains. For simplicity, not all of the known binding sites are shown. (B) Electron micrographs of
individual molecules shadowed with platinum; arrows mark the carboxyl termini. (C) The threedimensional structure of a type III fibronectin repeat, as determined by nuclear magnetic resonance
studies. It is the main type of repeating module in fibronectin and is also found in many other
proteins. The Arg-Gly-Asp (RGD) sequence shown is part of the major cell-binding site (shown in
blue in [A]) that we discuss in the text. (B,J. Engel et al., J. Mol. Biol. 150:97-120; C, A.L. Main et al.,
Cell 71:671-678)
Figure 19-52 How type IV collagen
molecules are thought to
assemble into a multilayered
network. The model is based on
electron micrographs of rotaryshadowed preparations of these
molecules assembling in vitro.
(Based on P.D. Yurchenco et al., J.
Histochem. Cytochem. 34:93-102)
Figure 19-53 Three ways in which basal laminae (yellow lines) are
organized. They surround certain cells (such as muscle cells), underlie
epithelial cell sheets, and are interposed between two cell sheets (as in the
kidney glomerulus). Note that in the kidney glomerulus both cell sheets have
gaps in them, so that the basal lamina serves as the permeability barrier
determining which molecules will pass into the urine from the blood.
Figure 19-54 Scanning electron micrograph of a basal lamina in the cornea
of a chick embryo. Some of the epithelial cells (E) have been removed to
expose the upper surface of the matlike basal lamina (BL). A network of
collagen fibrils (C) in the underlying connective tissue interacts with the lower
face of the lamina. (Courtesy of Robert Trelstad.)
Figure 19-55 The structure of laminin.
A schematic drawing of a laminin molecule is shown
in (A), and electron micrographs of laminin molecules shadowed with platinum are shown in (B).
This multidomain glycoprotein is composed of three polypeptides (A, B1, and B2) that are disulfide
bonded into an asymmetric crosslike structure. Each of the polypeptide chains is more than 1500
amino acid residues long. Three types of Alpha chains, three types of B1 chains, and two types of
B2 chains have been identified, which in principle can associate to form 18 different laminin
isoforms. Several such isoforms have been found, each with a characteristic tissue distribution.
There are also several isoforms of type IV collegen, each with a distinctive tissue distribution. Thus
basal laminae are chemically diverse, which is not surprising in view of their functional diversity. (J.
Engel et al., J. Mol. Biol. 150:97-120)
Figure 19-56 A current model of the molecular structure of a basal
lamina. The basal lamina (A) is formed by specific interactions between the
proteins type IV collagen, laminin, and entactin plus the proteoglycan
perlecan (B). Arrows in (B) connect molecules that can bind directly to each
other. (Based on P.D. Yurchenco and J.C. Schittny, FASEB J. 4:1577-1590.)
Figure 19-58 Regeneration experiments indicating the special character of the
junctional basal lamina at a neuromuscular junction. When the nerve, but not the
muscle, is allowed to regenerate after both the nerve and muscle have been damaged
(upper part), the junctional lamina directs the regenerating nerve to the original synaptic
site. When the muscle, but not the nerve, is allowed to regenerate (lower part), the
junctional lamina causes newly made acetylcholine receptors to accumulate at the
original synaptic site. These experiments show that the junctional basal lamina controls
the localization of other components of the synapseon both sides of the lamina.
Figure 19-59 Importance of cell-surface-receptor-bound protease.
In (A) human
prostate cancer cells make and secrete the serine protease UPA, which binds to cell-surface UPA receptor
proteins. In (B) the same cells have been transfected with DNA that encodes an excess of an inactive form of
UPA, which binds to the UPA receptors but has no protease activity; by occupying most of the UPA receptors,
the inactive UPA prevents the active protease from binding to the cell surface. Both types of cells secrete
active UPA, grow rapidly, and produce tumors when injected into experimental animals. But the cells in (A)
metastasize widely, whereas the cells in (B) do not. In order to metastasize, tumor cells have to crawl through
basal laminae and other extracellular matrices on the way into and out of the bloodstream. This experiment
therefore suggests that proteases must be cell-surface bound to mediate migration through the matrix.
Figure 19-60 The subunit structure
of an integrin cell-surface matrix
receptor. Electron micrographs of
isolated receptors suggest that the
molecule has approximately the
shape shown, with the globular head
projecting more than 20 nm from the
lipid bilayer. By binding to a matrix
protein outside the cell and to the
actin cytoskeleton inside the cell, the
protein serves as a transmembrane
linker. The alpha and beta chains are
both glycosylated and are held
together by noncovalent bonds.
Figure 19-29 Importance of the cytoskeleton in cell adhesion. This
drawing illustrates why cell-adhesion molecules must be linked to the
cytoskeleton in order to mediate robust cell-cell or cell-matrix adhesion. In
reality, many adhesion proteins would probably be pulled from the cell with bits
of attached membrane, and the holes left in the membrane would immediately
reseal.
Figure 19-61 Coalignment of extracellular fibronectin filaments and
intracellular actin filament bundles. The fibronectin is visualized in two
rat fibroblasts in culture by the binding of rhodamine-coupled anti-fibronectin
antibodies (A). The actin is visualized by the binding of fluorescein-coupled
anti-actin antibodies (B). (R.O. Hynes and A.T. Destree, Cell 15:875-886)
Figure 19-62 How the
extracellular matrix could
propagate order from cell to
cell within a tissue. For
simplicity, the figure represents
a hypothetical scheme in which
one cell influences the
orientation of its neighboring
cells. It is more likely, however,
that the cells would mutually
affect one another's orientation.
Figure 19-63 Cells can regulate the activity of their
integrins. In (A) cell activation leads to a change in the extracellular binding
site of the integrin so that it can now mediate cell adhesion. In (B) the tyrosine
phosphorylation of the cytoplasmic tail of the integrins impairs their ability to
bind to the actin cytoskeleton. As integrins must bind to the cytoskeleton to
mediate robust cell-matrix adhesion, the phosphorylation causes the integrins
to relax their grip on the extracellular matrix.
Cell walls
A. Plant cell walls provide protection against abrasion,
osmotic stress, and pathogens.
B. Microfibrils of cellulose form the fibrous component
of the cell wall.
C. The matrix of
cell wall contains
hemicellulose,
pectins, and
hydroxyprolinerich,proline-rich,
and glycine-rich
structural proteins.
Figure 19-64 Plant cell walls. (A) Electron micrograph of the root tip of a rush,
showing the organized pattern of cells that results from an ordered sequence of
cell divisions in cells with rigid cell walls. (B) Section of a typical cell wall
separating two adjacent plant cells. The two dark transverse bands correspond
to plasmodesmata that span the wall. (A, courtesy of Brian Gunning; B,
courtesy of Jeremy Burgess.)
Figure 19-65 Scale model of a portion of a primary cell wall showing the
two major polysaccharide networks. The orthogonally arranged layers of
cellulose microfibrils (green) are cross-linked into a network by H-bonded hemicellulose
(red). This network is coextensive with a network of pectin polysaccharides (blue). The
cellulose and hemicellulose network provides tensile strength, while the pectin network
resists compression. Cellulose, hemicellulose, and pectin are typically present in roughly
equal quantities in a primary cell wall. The middle lamella is pectin rich and cements
adjacent cells together.
Figure 19-66 The orientation of cellulose microfibrils in the primary cell
wall of an elongating carrot cell. This electron micrograph of a shadowed
replica from a rapidly frozen and deep-etched cell wall shows the largely
parallel arrange-ments of cellulose microfibrils, oriented perpendicular to the
axis of cell elongation. The microfibrils are cross-linked by, and interwoven with,
a complex web of matrix molecules. (Brian Wells and Keith Roberts.)
Figure 19-68 The cortical array of microtubules in a plant cell. (A) A
grazing section of a root-tip cell from Timothy grass, showing a cortical array of
microtubules lying just below the plasma membrane. These microtubules are
oriented perpendicular to the long axis of the cell. (B) An isolated onion root-tip
cell. (C) The same cell stained by immunofluorescence to show the transverse
cortical array of microtubules. (A, courtesy of Brian Gunning; B and C, courtesy
of Kim Goodbody.)
Figure 19-69 One model of how the orientation of newly deposited
cellulose microfibrils might be determined by the orientation of cortical
microtubules. The large cellulose synthase complexes are integral membrane
proteins that continuously synthesize cellulose microfibrils on the outer face of the
plasma membrane. The distal ends of the stiff microfibrils become integrated into the
texture of the wall, and their elongation at the proximal end pushes the synthase
complex along in the plane of the membrane. Because the cortical array of
microtubules is attached to the plasma membrane in a way that confines this complex
to defined membrane channels, the microtubule orientation determines the axis along
which the microfibrils are laid down.
作 业
• 简述细胞黏附分子、细胞外基质
与肿瘤细胞迁移的关系。