Cell locomotion results from the coordination of motions generated by
different parts of a cell. These motions are complex and difficult to describe, but
we can pick out their major features using powerful fluorescent antibody-labeling
methods combined with fluorescence microscopy. To provide a background for more
detailed discussions of the mechanism of cell locomotion later, we first briefly
describe movement by two types of crawling animal cells. One feature that
characterizes all moving cells is polarity; that is, certain structures always form
at the front of the cell, whereas others are found at the rear.
In the micrograph shown at the opening to this chapter, a skin cell (keratinocyte)
displays motions typical of a fast-moving cell like an ameba. Initially, the
keratinocyte forms a large, broad membrane protrusion, the
lamellipodium, which moves forward from the cell. After
contacting the substratum and forming special attachment structures, called
focal adhesions, the lamellipodium quickly fills with cytosol
and the rear of the cell retracts forward toward the body of the cell.
Figure 18-1
.
Actin structures in a fibroblast
(a) Scanning electron micrograph of a cultured fibroblast. At the front
of the cell, filopodia, lamellipodia, and ruffles project from the cell
membrane. At the rear of the cell, the tail is firmly attached to the
surface. The arrow indicates the direction of movement. (b) Fluorescence
micrograph of a fan-shaped fibroblast, stained with rhodamine
phalloidin. Visible are numerous actin bundles in the lamellipodia and
stress fibers in the cell body. [Part (a) courtesy of J. Heath; part (b)
courtesy of B. Hollifield.]
Locomotion of a slow-moving cell like a
fibroblast involves several structures not
seen in a fast-moving keratinocyte. As a
fibroblast moves forward, it extends
slender “fingers” of
membrane, called
filopodia, as well as lamellipodia (). Where the cell
membrane contacts the
substratum, focal adhesions assemble on the
ventral surface of the cell. As the cell
continues to travel, its tail is pulled forward, often leaving behind small patches
of the cell still firmly attached to the substratum through the focal adhesions.
Some areas of the cell
membrane do not form stable adhesions at the leading edge of
the cell; these areas project upward as a thin veil, or
ruffle,
that moves as an undulating ridge back along the
dorsal cell surface toward the cell
body.
The machinery that powers cell migration is built from the
actin cytoskeleton, which
is larger than any
organelle. When a
fibroblast is observed by fluorescence
microscopy after the
actin filaments are stained with a fluorescent dye, radially
oriented
actin filament bundles can be seen at the leading edge, and axial bundles,
called
stress fibers, are visible underlying the cell body (). In addition, a network of
actin
filaments fills the rest of the cell, but the individual filaments of this network
are difficult to resolve in the light microscope.
Much of the discussion in this section focuses on the ability of huge actin filament
structures to control the shape of a cell. Because the actin cytoskeleton is so big,
it can easily change cell morphology just by assembling or disassembling itself. In
previous chapters, we have seen examples of large protein complexes in which the
number and positions of the subunits are fixed. For example, all ribosomes have the
same number of protein and RNA components, and their three-dimensional geometry is
invariant. However, the actin cytoskeleton is
different — the lengths of filaments vary greatly,
the filaments are cross-linked into imperfect bundles and networks, and the ratio of
cytoskeletal proteins is not rigidly maintained. This organizational flexibility of
the actin cytoskeleton permits a cell to assume many shapes and to vary them easily.
In moving cells, the cytoskeleton must assemble rapidly and does not always have the
chance to form well-organized, highly ordered structures. Keeping this in mind, we
examine actin as a model for understanding how polymeric proteins form the
structural framework of a cell and how the cell tailors this framework to carry out
various tasks involving motion of the entire cell or subcellular parts.
Eukaryotic Cells Contain Abundant Amounts of Highly Conserved Actin
Actin is the most abundant intracellular protein in a eukaryotic cell. In muscle
cells, for example, actin comprises 10 percent by weight of the total cell
protein; even in nonmuscle cells, actin makes up
1 – 5 percent of the cellular protein. A typical
cytosolic concentration of actin in nonmuscle cells is 0.5 mM; in special
structures like microvilli, however, the local actin concentration can be
tenfold higher. To appreciate how much actin cells contain, consider a typical
liver cell, which has 20,000 insulin receptor molecules but approximately half a
billion (0.5 × 109) actin molecules.
This predominance of actin compared with other cell proteins is a common feature
of all cytoskeletal proteins. Because they form structures that must cover large
spaces in a cell, these proteins are among the most abundant proteins in a
cell.
A moderate-sized protein consisting of approximately 375 residues, actin is
encoded by a large, highly conserved gene family. Some single-celled eukaryotes
like yeasts and amebas have a single actin gene, whereas many multicellular
organisms contain multiple actin genes. For instance, humans have six actin
genes, which encode isoforms of the protein, and some plants
have as many as 60. Sequencing of these actins has revealed that it is one of
the most conserved proteins in a cell, comparable with histones, the structural
proteins of chromatin (Chapter 9).
Actin residues from amebas and from animals are identical at 80 percent of the
positions. In vertebrates, the four α-actin isoforms present in
various muscle cells and the β- and γ-actin isoforms present
in nonmuscle cells differ at only four or five positions. Although these
differences among isoforms seem minor, the isoforms have different functions:
α-Actin is associated with contractile structures, and
β-actin is at the front of the cell where actin filaments
polymerize.
Recently, a family of actin-related proteins (Arps), exhibiting
50 percent homology with actin, has been identified in many eukaryotic
organisms. As noted later, one group of Arps (Arp2/3) stimulates actin assembly;
intriguingly, another group (Arp1) is associated with microtubules and a
microtubule motor protein. They are discussed in the next chapter.
ATP Holds Together the Two Lobes of the Actin Monomer
Actin exists as a globular monomer called G-actin and as a
filamentous polymer called F-actin, which is a linear chain of
G-actin subunits. (The microfilaments visualized in a cell by electron
microscopy are F-actin filaments plus any bound proteins.) Each actin molecule
contains a Mg2+ ion complexed with either ATP or ADP. Thus
there are four states of actin: ATP – G-actin,
ADP – G-actin,
ATP – F-actin, and
ADP – F-actin. Two of these forms,
ATP – G-actin and
ADP – F-actin, predominate in a cell. We discuss
later how the interconversion between the ATP and ADP forms of actin is
important in the assembly of the cytoskeleton.
Figure 18-2
.
Structures of monomeric G-actin and F-actin filament
(a) Model of a β-actin monomer from a nonmuscle cell shows
it to be a platelike molecule (measuring
5.5 × 5.5 × 3.5
nm) divided by a central cleft into two approximately equal-sized
lobes and four subdomains, numbered
I – IV. ATP (red) binds at the
bottom of the cleft and contacts both lobes (the yellow ball
represents Mg2+). The N- and C-termini lie in
subdomain I. (b) In the electron microscope, negatively stained
actin filaments appear as long, flexible, and twisted strands of
beaded subunits. Because of the twist, the filament appears
alternately thinner (7 nm diameter) and thicker (9 nm diameter)
(arrows). (c) In one model of the arrangement of subunits in an
actin filament, the subunits lie in a tight helix along the
filament, as indicated by the arrow. One repeating unit consists of
28 subunits (13 turns of the helix), covering a distance of 72 nm.
Only 14 subunits are shown in the figure. The ATP-binding cleft is
oriented in the same direction (top) in all actin
subunits in the filament. As discussed later, this end of a filament
is designated the (−) end; the opposite end is the
(+) end. [Part (a) adapted from C. E. Schutt et al., 1993,
Nature
365:810, courtesy of M. Rozycki; part (b) courtesy of
R. Craig; part (c) see M. F. Schmid et al., 1994, J. Cell
Biol. 124:341, courtesy of M. Schmid.]
Although G-
actin appears globular in the electron microscope, x-ray
crystallographic analysis reveals that it is separated into two lobes by a deep
cleft (). The lobes and the
cleft compose the
ATPase fold, the site where ATP and
Mg
2+ are bound. In
actin, the floor of the cleft acts as
a hinge that allows the lobes to flex relative to each other. When ATP or ADP is
bound to G-
actin, the
nucleotide affects the
conformation of the molecule. In
fact, without a bound
nucleotide, G-
actin denatures very quickly.
G-Actin Assembles into Long, Helical F-Actin Polymers
The addition of ions — Mg2+,
K+, or Na+— to
a solution of G-actin will induce the polymerization of G-actin
into F-actin filaments. The process is also reversible: F-actin depolymerizes
into G-actin when the ionic strength of the solution is lowered. The F-actin
filaments that form in vitro are indistinguishable from microfilaments isolated
from cells. This indicates that other factors such as accessory proteins are not
required for polymerization in vivo. The assembly of G-actin into F-actin is
accompanied by the hydrolysis of ATP to ADP and Pi; however, as we
discuss later, ATP hydrolysis affects the kinetics of polymerization but is not
necessary for polymerization to occur.
When negatively stained by uranyl acetate for electron microscopy, F-
actin
appears as twisted strings of beads whose diameter varies between 7 and 9 nm
(). From x-ray
diffraction studies of
actin filaments and the
actin monomer structure shown in
, scientists have
produced a model of an
actin filament in which the subunits are organized as a
tightly wound helix (). In
this arrangement, each subunit is surrounded by four other subunits, one above,
one below, and two to one side. Each subunit corresponds to a bead seen in
electron micrographs of
actin filaments.
The ability of G-actin to polymerize into F-actin and of F-actin to depolymerize
into G-actin is an important property of actin. In this chapter, we will see how
the reversible assembly of actin lies at the core of many cell movements.
F-Actin Has Structural and Functional Polarity
All subunits in a filament point toward the same filament end (i.e., they have
the same
polarity). Consequently, at one end of the filament, by convention
designated the (−) end, the ATP-binding cleft of an
actin subunit is
exposed to the surrounding solution; at the opposite end, the cleft contacts the
neighboring
actin subunit (see ).
Figure 18-3
.
Experimental demonstration of polarity of an actin filament by
binding of myosin S1 head domains
When bound to all the
actin subunits, S1 appears to spiral around the
filament. This coating of
myosin heads produces a series of
arrowhead-like decorations, most easily seen at the wide views of
the filament. The
polarity in decoration defines a pointed
(−) end and a barbed (+) end; the former
corresponds to the top of the model in . [Courtesy of R. Craig.]
Without the atomic
resolution afforded by
x-ray crystallography, the cleft in an
actin subunit, and therefore the
polarity of a filament, is not detectable.
However, the
polarity of
actin filaments can be demonstrated by electron
microscopy in so-called “decoration” experiments, which
exploit the ability of
myosin to bind specifically to
actin filaments. In this
type of experiment, an excess of
myosin S1, the globular head
domain of
myosin,
is mixed with
actin filaments and binding is permitted to occur.
Myosin attaches
to the sides of a filament with a slight tilt. When all subunits are bound by
myosin, the filament appears coated (“decorated”) with
arrowheads that all point toward one end of the filament (). Because
myosin binds to
actin filaments and
not to
microtubules or
intermediate filaments, arrowhead decoration is one
criterion by which
actin filaments are identified among the other cytoskeletal
fibers in electron micrographs of thin-sectioned cells.
The Actin Cytoskeleton Is Organized into Bundles and Networks of
Filaments
Figure 18-4
.
Micrograph revealing bundles and networks of actin filaments in
the cytosol of a spreading platelet treated with detergent to remove
the plasma membrane
Actin bundles project from the cell to form the spikelike filopodia.
In the lamellar region of the cell, the actin filaments form a
network that fills the cytosol. In contrast to the roughly parallel
alignment of bundled filaments, the filaments in networks lie at
various angles approaching 90°. [Courtesy of J.
Hartwig.]
On first looking at an electron micrograph or immunofluorescence micrograph of a
cell, one is struck by the dense, seemingly disorganized mat of filaments
present in the
cytosol ().
However, a keen eye will
start to pick out areas, generally where the
membrane
protrudes from the cell surface, in which the filaments are concentrated into
bundles. From these bundles the filaments continue into the cell interior, where
they fan out and become part of a network of filaments. These two structures,
bundles and
networks, are the most common
arrangements of
actin filaments in a cell.
Functionally, bundles and networks have identical roles in a cell: both provide a
framework that supports the plasma membrane and, therefore, determines a
cell’s shape. Structurally, bundles differ from networks mainly in the
organization of actin filaments. In bundles the actin filaments are closely
packed in parallel arrays, whereas in a network the actin filaments crisscross,
often at right angles, and are loosely packed. Cells contain two types of actin
networks. One type, associated with the plasma membrane, is planar or
two-dimensional, like a net or a web; the other type, present within the cell,
is three-dimensional, giving the cytosol gel-like properties.
Figure 18-5
.
Actin cross-linking proteins bridging pairs of actin
filaments
(a) When cross-linked by fascin, a relatively short protein, actin
filaments form a bundle. (b) Long cross-linking proteins such as
filamin are flexible and thus can cross-link pairs of filaments
lying at various angles.
In all bundles and networks, the filaments are held together by
actin
cross-linking proteins. To connect two filaments, a cross-linking
protein must have two
actin-binding sites, one site for each filament. The
length and flexibility of a cross-linking
protein critically determine whether
bundles or networks are formed. Short cross-linking
proteins hold
actin
filaments close together, forcing the filaments into the parallel alignment
characteristic of bundles (). In contrast, long, flexible cross-linking
proteins are able to
adapt to any arrangement of
actin filaments and tether orthogonally oriented
actin filaments in networks ().
Table 18-1
Actin Cross-Linking Proteins
Many
actin cross-linking
proteins belong to the calponin
homology –
domain (CH-
domain) superfamily (
Table 18-1). Each of these
proteins has
a pair of
actin-binding
domains, whose sequence is homologous to calponin, a
muscle
protein. The
actin-binding
domains are separated by repeats of helical
coiled-coil or β-sheet immunoglobulin
motifs. Among the CH-
domain
proteins, the shortest (fimbrin and α-actinin) are found in
actin
bundles within cell extensions, and the longest (filamin, spectrin, and
dystrophin) are found in
actin networks in the cortical region adjacent to the
plasma membrane. A smaller number of cross-linking
proteins are classified into
two other groups; these bind to different sites on
actin than the CH-
domain
proteins.
Cortical Actin Networks Are Connected to the Membrane
The distinctive shape of a cell is dependent not only on the organization of
actin filaments but also on proteins that connect the filaments to the membrane.
These proteins, called membrane-microfilament binding proteins,
act as spot welds that tack the membrane sheet to the underlying cytoskeleton
framework. When attached to a bundle of filaments, the membrane acquires a
fingerlike shape, as we discuss later. When attached to a planar network of
filaments, the membrane is held flat. We focus on these network connections in
this section. The simplest connections entail binding of integral membrane
proteins directly to actin filaments. More common are complex linkages that
connect actin filaments to integral membrane proteins through peripheral
membrane proteins.
The richest area of actin filaments in a cell lies in the
cortex, a narrow zone just beneath the plasma membrane. In
this region, most actin filaments are arranged into a network that excludes most
organelles from the cortical cytoplasm. Several ways to organize the cortical
actin cytoskeleton are observed in different cell types. Perhaps the simplest
cytoskeleton is the two-dimensional network of actin filaments adjacent to the
erythrocyte plasma membrane. In more complicated cortical cytoskeletons, such as
those in platelets, epithelial cells, and muscle, actin filaments are part of a
three-dimensional network that fills the cytosol and anchors the cell to the
substratum. We first describe the erythrocyte cytoskeleton and its linkage to
the membrane and then examine the more complex cytoskeletons in platelets and
muscle.
Erythrocyte Cytoskeleton
Figure 18-6
.
Electron micrograph of human erythrocyte cytoskeleton
The long “spokes” are composed mainly of
spectrin and can be seen to intersect at the
“hubs,” or junctional complexes. The darker
spots along the spokes are ankyrin molecules, which cross-link
spectrin to integral membrane proteins.
Figure 18-7
.
The organization of the major erythrocyte cytoskeletal
proteins and their interactions with integral membrane
proteins
(Inset) Hypothetical arrangement of the
components of a junctional complex and their interactions with
the ends of spectrin tetramers. [Adapted from S. E. Lux, 1979,
Nature
281:426; E. J. Luna and A. L. Hitt, 1992,
Science
258:955.]
A red blood cell must squeeze through narrow blood capillaries without
rupturing its
membrane. The strength and flexibility of the erythrocyte
plasma membrane depends on a dense cytoskeletal network that underlies the
entire
membrane and is attached to it at many points. The primary component
of the erythrocyte
cytoskeleton is
spectrin, a long fibrous
protein. Two dimeric subunits of spectrin, each composed of an α
and β
polypeptide chain, associate to form head-to-head tetramers,
which are 200 nm long. The entire
cytoskeleton is arranged in a
spoke-and-hub network ().
Each spectrin tetramer comprises a spoke, extending from and cross-linking a
pair of hubs, called
junctional complexes. As illustrated
in , each junctional
complex is composed of a short (14-subunit)
actin filament plus adducin,
tropomyosin, and tropomodulin. The last two
proteins strengthen the network
by preventing the
actin filament from depolymerizing. Because several
spectrin molecules can bind the same
actin filament, the erythrocyte
cytoskeletal network has a spoke-and-hub organization. This polygonal
arrangement acts as a lamination of the
membrane.
To ensure that the erythrocyte retains its characteristic shape, the
spectrin-
actin cytoskeleton is firmly attached to the overlying erythrocyte
membrane by two
peripheral membrane proteins, each of which binds to a
specific
integral membrane protein (see ).
Ankyrin connects the center of
spectrin to band 3
protein, the anion-transporter
protein in the
membrane.
Band 4.1 protein, a component of the junctional
complex, binds to the
integral membrane protein glycophorin. This dual
binding ensures that the
membrane is connected to both the spokes and the
hubs of the spectrin-
actin cytoskeleton.
Platelet Cytoskeleton
Figure 18-8
.
Changes in shape of platelets during blood clotting
Resting cells have a discoid shape (left). When
exposed to clotting agents, the cells settle and spread out on
the substratum (center). During clot
retraction, the cells assume a stellate shape and extend
numerous filopodia (right). The changes in
morphology result from complex rearrangements of the actin
cytoskeleton, which is cross-linked to the plasma membrane.
[Courtesy of J. White.]
Isoforms of spectrin and
actin have been found in various nonerythroid cells,
suggesting that these cell types have a cortical spectrin-
actin cytoskeleton
like that present in the erythrocyte. In nonerythroid cells, however, the
actin cytoskeleton is more complicated than in erythrocytes. An example of
this more complicated structure is seen in the platelet, a small,
nonnucleated cell that is important in blood clotting and wound repair. The
platelet
cytoskeleton must undergo complicated rearrangements that are
responsible for a repertoire of changes in cell shape during a blood
clotting reaction ().
These changes in platelet shape could not be generated by the simple
cytoskeleton seen in the erythrocyte; thus additional components are needed
in the platelet
cytoskeleton. The
cytoskeleton of an unactivated platelet
consists of a rim of
microtubules (the
marginal band), a
cortical
actin network, and a cytosolic
actin network. The cortical
actin
network in platelets consists of
actin filaments cross-linked into a
two-dimensional network by a nonerythroid
isoform of spectrin and linked
through ankyrin to an anion transporter, the Na/K
ATPase, in the
membrane.
This network thus is somewhat similar to the erythrocyte
cytoskeleton. A
critical difference between the erythrocyte and platelet
cytoskeletons is
the presence in the platelet of the second network of
actin filaments, which
are organized by
filamin cross-links into a
three-dimensional gel (see ). The gel fills the
cytosol of a platelet and is anchored
by filamin to the
glycoprotein 1b-IX complex (Gp1b-IX) in the platelet
membrane. Although both spectrin and filamin are CH-
domain actin
cross-linking
proteins, filamin also can anchor the
cytoskeleton to the
plasma membrane.
Figure 18-9
.
Cross-linkage of actin filament networks to the plasma
membrane in platelets, muscle cells, and epithelial
cells
(a) In platelets a three-dimensional network of actin filaments
is attached to the integral membrane glycoprotein complex
Gp1b-IX by filamin. Gp1b-IX also binds to proteins in a blood
clot outside the platelet. Platelets also possess a
two-dimensional cortical network of actin and spectrin similar
to that underlying the erythrocyte membrane. (b) In muscle cells
dystrophin attaches actin filaments to an integral membrane
glycoprotein complex. This complex binds to laminin and agrin in
the extracellular matrix (ECM). (c) In epithelial cells, the ERM
protein, ezrin, and EBP50 crosslink an actin filament to the
cystic fibrosis transmembrane conductance receptor. After
activation, ezrin unfolds and oligomerizes to form head-to-tail
dimers. The head domain binds EBP50, while the tail domain binds
actin.
In order to close a wound, a platelet must be able to transmit cytoskeletal
changes within the cell to the blood clot outside the cell. This is
effectively accomplished by linking the
cytoskeleton to the same
proteins in
the
membrane that also bind to the clot. For example, Gp1b-IX not only binds
filamin but also is the
membrane receptor for two blood-clotting
proteins
(). Contraction of
the cell tightens the clot and closes the wound. The direct connection of
the
cytoskeleton to the
extracellular matrix through shared
membrane
proteins also occurs in muscle and many other cells.
Membrane-Cytoskeleton Linkage in Muscle Cells
The critical role of membrane-cytoskeleton linkages in muscle contraction was
uncovered by studies on Duchenne muscular dystrophy (DMD), a fatal,
degenerative sex-linked genetic disease of muscle that affects about 1 of
every 3500 males born. Patients with this disease have a defect in the gene
that encodes dystrophin, a very large protein that
constitutes 5 percent of the membrane-associated cytoskeleton in muscle
cells.
Like filamin in platelets, dystrophin cross-links
actin filaments into a
supportive cortical network and attaches this network to a
glycoprotein
complex in the muscle cell
membrane (). This
membrane complex also binds to
proteins in the
extracellular matrix. Thus the internal
cytoskeleton is connected to the
external matrix via the
dystrophin –
membrane glycoprotein linkage.
In individuals with DMD, who lack functional dystrophin, the
membrane of a
muscle cell is not supported by a cortical
cytoskeleton and presumably is
easily damaged by the stress of repeated muscle contraction.
ERMs: Conformationally Regulated Membrane-Microfilament Proteins
In addition to the
membrane linkages by CH
domain proteins, several
proteins
in the apical
membrane are cross-linked to
microfilaments through the ERM
(Ezrin, Radixin, and Moesin) family of
proteins. Found in a variety of cell
types, ERMs bind
membrane proteins, including the cystic fibrosis
transmembrane conductance regulator (CFTR) and β2-adrenergic
receptor. This interaction with different
membrane proteins requires an
adapter
protein, EBP50, and a Band 4.1
homology domain located at the
N-terminus. The crosslink to the
cytoskeleton is completed through an
actin-binding site located in a C-terminal
domain ().
Recent evidence suggests that a membrane-microfilament linkage represents an
open conformation of the molecule that is regulated by several signaling
pathways possibly through both serine/threonine and tyrosine kinases. In the
inactive state, the membrane and microfilament binding sites are masked by
an intermolecular association of the N- and C-terminal domains. In this
closed conformation, ezrin is unable to bind the cytoskeleton, and most of
the inactive protein is found in the cytosol. However, the membrane-binding
function is correlated with phosphorylation of specific serine and threonine
residues in the C-terminal domain.
Actin Bundles Support Projecting Fingers of Membrane
The surfaces of cells in multicellular organisms are studded with numerous
membrane projections. Two types of fingerlike membrane projections, microvilli
and filopodia, are supported by an internal actin bundle to which the
phospholipid membrane is anchored. Because it is held together by protein
cross-links, the actin bundle is stiff and provides a rigid structure that
reinforces the fragile projecting membrane, enabling it to maintain its long,
slender shape.
Figure 18-10
.
Micrograph of intestinal cell showing microvilli
At the core of each 2-μm-long microvillus, a bundle of actin
filaments, cross-linked by fimbrin and villin, stabilizes the
fingerlike structure. The plasma membrane surrounding a microvillus
is attached to the sides of the bundle by evenly spaced
membrane-microfilament linkages consisting of myosin I. Each bundle
continues into the cell as a 0.5-μm-long rootlet. The
rootlets are cross-braced by connecting fibers composed of an
intestinal isoform of spectrin, and the bases of the rootlets form
attachment sites for keratin filaments. These numerous connections
anchor the rootlets in a meshwork of filaments and thereby support
the upright orientation of the microvilli. [Courtesy of N.
Hirokawa.]
Microvilli range in length from 0.5 to 10 μm and are found
where the cell
membrane faces the fluid environment. As discussed in
Chapter 15, a dense carpet of
microvilli, the
brush border, covers the surface of intestinal
epithelial cells, greatly increasing the surface area of these cells, whose
primary function is to transport nutrients. The cross-linking
proteins that hold
actin filaments in the core bundle differ in microvilli found on different cell
types. shows the structure
of intestinal microvilli and the various connections supporting them. The
membrane is attached to the core bundle by
membrane-microfilament
linkages of
myosin I.
Filopodia, which are much less common than microvilli, attach cells to a solid
surface. These projections of the
membrane typically are found at the edge of
moving or spreading cells (see ). Filopodia are transient structures, present only during the
time required to establish a stable contact with the underlying substratum.
ǀ
