Actin and Actin-Binding Proteins

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CHAPTER 33 Actin and Actin-Binding Proteins

Actin filaments form a cytoskeletal and motility system in all eukaryotes (Fig. 33-1). Cross-linked actin filaments resist deformation, transmit forces, and restrict diffusion of organelles. A network of cortical actin filaments excludes organelles (Fig. 33-2C), reinforces the plasma membrane, and restricts the lateral motion of some integral membrane proteins. The cortex varies in thickness from a monolayer of actin filaments in red blood cells (see Fig. 7-10) to more than 1μm in amoeboid cells (Fig. 33-2C). Like fingers in a glove, bundles of actin filaments support slender protrusions of plasma membrane called microvilli or filopodia (Fig. 33-2B). Microvilli expand the cell surface for transport of nutrients and participate in sensory processes, including hearing. The actin cytoskeleton complements and interacts physically with cytoskeletal structures composed of microtubules (see Chapter 34) and intermediate filaments (see Chapter 35).

Figure 33-1 fluorescence micrographs illustrating the distribution of actin filaments in cells. A, Intestinal epithelial cells stained red with rhodamine-labeled phalloidin, a cyclic peptide that binds tightly to actin filaments. Actin filaments are concentrated in microvilli bordering the intestinal lumen. Nuclei are stained blue with DAPI.B, Cultured vascular smooth muscle cells. Actin filaments, stained red with a fluorescent antibody, are concentrated in stress fibers and in the cortex around the edges of these cells.C, Maize epidermis stained with rhodamine-labeled phalloidin. Actin filaments are concentrated in the cortex and in cytoplasmic bundles in these plant cells.D, Fission yeast Schizosaccharomyces pombe, stained with rhodamine-labeled phalloidin. Actin filaments are found in patches at the tips of growing cells and in the cleavage furrow of dividing cells. Scale bars are 10μm.

(A, Courtesy of C. Rahner, Yale University, New Haven, Connecticut. B, Courtesy of I. Herman, Tufts Medical School, Boston, Massachusetts. C, Courtesy of M. Frank, University of California, San Diego. D, Courtesy of W.-L. Lee, Salk Institute, La Jolla, California.)

Figure 33-2 electron micrographs of actin filaments. A, Filaments of purified actin prepared by negative staining.B, A thin section of an intestinal epithelial cell illustrating finger-like microvilli with tightly packed bundles of actin filaments linked to the surrounding plasma membrane by myosin-I. The barbed ends of these filaments (see Fig. 33-9) are located at the tips of the microvilli.C, A thin section of Acanthamoeba showing the actin filament meshwork in the cortex beneath the plasma membrane.D–E, Cultured fish scale keratocytes, fixed while actively migrating toward thetop of the figure. D, Electron micrograph of a whole mount of a cell illustrating the meshwork of branched filaments near the leading edge and longer, unbranched filaments deeper in the cytoplasm. Most filaments are oriented with their barbed ends forward.E, Fluorescence micrograph with phalloidin staining actin filaments (blue) and antibodies staining myosin II (red).

(A, Courtesy of U. Aebi, University of Basel, Switzerland. B, Courtesy of L. Tilney, University of Pennsylvania, Philadelphia, and M. Mooseker, Yale University, New Haven, Connecticut. D–E, Courtesy of T. Svitkina and G. Borisy, University of Wisconsin, Madison.)

Actin contributes to cell movements in two ways. First, polymerization and depolymerization of the network of actin filaments just inside the plasma membrane contribute to the extension of pseudopods, cell locomotion (Fig. 33-2D-E), and phagocytosis (see Fig. 22-3). Second, actin filaments are tracks for movements of the myosin family of motor proteins (see Fig. 36-7). Actin filaments and myosin filaments form the highly ordered, stable contractile apparatus of muscles (Fig. 33-3B; also see Fig. 39-3), as well as the transient contractile ring that pinches the two daughter cells apart at the end of mitosis (Fig. 33-3A; also see Fig. 44-23). Myosins also power movements of membranes and other cargo along actin filaments, complementing organelle movements by other motors along microtubules (see Fig. 37-1). Actin, myosin, and accessory proteins form intracellular bundles called stress fibers (Fig. 33-1B) that apply tension between adhesive junctions on the plasma membrane (see Fig. 30-11), where cells attach to each other or to the extracellular matrix. Stress fibers are prominent in tissue culture cells grown on glass or plastic and in endothelial cells lining major arteries.

Figure 33-3 micrographs of contractile bundles of actin filaments. A, Fluorescence micrograph of a dividing normal rat kidney cell stained with fluorescein-phalloidin. Actin filaments are concentrated in the contractile ring in the constricting cleavage furrow. The drawing illustrates the filaments in the contractile ring.B, Fluorescence micrograph of a myofibril isolated from skeletal muscle and stained with fluorescein-phalloidin for actin filaments (green) and rhodamine-antibody to α-actinin for Z disks (yellow). C, Electron micrograph of a thin section of skeletal muscle.

(A, Micrograph courtesy of Y.-L. Wang, University of Massachusetts, Worcester. B, Courtesy of V. Fowler, Scripps Research Institute, La Jolla, California. C, Courtesy of H. E. Huxley, Brandeis University, Waltham, Massachusetts.)

Actin and myosin are thought to be among the five most abundant eukaryotic proteins on the earth. Actin is often the most abundant protein in a cell, composing up to 15% of total protein, and the many types of actin-binding proteins may account for another 10% of cellular protein. In muscle, actin and myosin constitute more than 60% of the total protein. Given this abundance, it is curious that actin was discovered in muscle only in the 1940s and in nonmuscle cells in the late 1960s. Since the 1970s, scientists have discovered new actin-binding proteins every year, but the inventory is probably still incomplete. Genetic defects in components of the actin cytoskeletal and motility system cause many human diseases, including muscular dystrophy (see Table 39-2), hereditary fragility of red blood cells (i.e., hemolytic anemias, see Fig. 7-10), and hereditary heart diseases called cardiomyopathies (see Table 39-4).

Actin Molecule

Actin is folded into two domains that are stabilized by an adenine nucleotide lying in between (Fig. 33-4). The polypeptide of 375 residues crosses twice between the two domains, with the N- and C-termini located near each other. The two domains are folded similarly, suggesting that the actin gene arose by duplication of an ancestral gene. Remarkably, the adenine nucleotide binding site, fold, and overall shape of actin closely resemble those of two other proteins with very different functions: the glycolytic enzyme hexokinase (see Fig. 3-12) and the heat shock protein Hsc70. All three proteins might have evolved originally in prokaryotes from the same primordial nucleotide-binding protein.

Figure 33-4 atomic structure of actin. A, Ribbon model showing the polypeptide fold and the location of Mg-ATP, shown as space-filling. Numbers indicate the four subdomains.B, Surface rendering. ATP is almost completely buried in the cleft between the two lobes of the protein, where it makes extensive contacts with the protein. The barbed end of the molecule (Fig. 33-9) is at the bottom in this orientation.

(PDB file: 1ATN. Reference: Kabsch W, Mannherz HG, Suck D, et al: Atomic structure of the actin-DNase I complex. Nature 347:37–44, 1990.)

Actin binds adenosine triphosphate (ATP) or adenosine diphosphate (ADP) and a divalent cation, Mg²⁺ in cells, with nanomolar affinity. The affinity of actin for ATP is higher than that for ADP, so given the higher concentration of ATP in cells, unpolymerized actin is saturated with ATP. The bound nucleotide exchanges relatively slowly with nucleotide in the medium (Fig. 33-11). Actin monomer–binding proteins can inhibit or accelerate nucleotide exchange. Bound nucleotide stabilizes the molecule but is not required for polymerization in vitro. ATP-actin and ADP-actin polymerize at different rates.

Figure 33-11 regulation of actin nucleotide exchange by actinbinding proteins. The rate-limiting step is dissociation of the bound nucleotide, which requires opening the nucleotide cleft. ADF/cofilins and β-thymosins inhibit dissociation of both ATP and ADP. Profilin competes with both inhibitors for binding actin monomers and increases the rates of both nucleotide dissociation and binding. The ability of profilin to promote nucleotide exchange, the higher affinity of actin for ATP than for ADP, and the higher concentration of ATP than ADP in cytoplasm drive the reactions to the right, accounting for why essentially all unpolymerized actin in cells has bound ATP.

Posttranslational modifications of actins include acetylation of the N-terminus and (in most cases) methylation of histidine-68. In some insect flight muscles, the small protein ubiquitin (see Fig. 23-7) is attached covalently to about one in six actin molecules, yielding a 55-kD polypeptide that is incorporated with unmodified actin into filaments. Some invertebrate actins are phosphorylated on tyrosine-211. The functional significance of these modifications is still being investigated.

Actin genes originated in prokaryotes, where they are required for rod-shaped bacteria to maintain their asymmetric shapes. Other bacterial actins help to segregate DNA plasmids to the two daughters during cell division. Eukaryotic actin genes are highly conserved, but through divergent evolution, they encode subtly different proteins, some with novel functions. Most organisms have multiple actin genes, and all known actin isoform diversity arises from multiple genes rather than from alternative splicing of mRNAs. Humans have six actin genes; Dictyostelium has more than ten; but budding yeast has only one. Muscle actin genes diverged from cytoplasmic actins in primitive chordates (see Fig. 2-9). To fulfill special developmental functions, plant actin genes diverged among themselves more than animal actin genes.

The biochemical similarities of actin isoforms are more impressive than their differences (Fig. 33-5). The sequences of pairs of actins are generally more than 90% identical, even between highly divergent species. Humans express β and γ isoforms in nonmuscle cells and four different α and β isoforms in various muscle cells. Many nonmuscle cells express both the β and γ isoforms, but red blood cells use only β-actin.

Figure 33-5 sorting of actin isoforms in cells. Fluorescence micrograph of cultured cells doubly stained with fluorescent antibodies specific for β-actin concentrated at the leading edge (orange) and g-actin concentrated in stress fibers (green). Nuclei are stained blue with DAPI.

(Courtesy of I. Herman, Tufts Medical School, Boston, Massachusetts.)

In every case that was examined, actin isoforms copolymerize in the test tube, so it is remarkable that cells can sort actin isoforms into different structures. For example, b-actin is concentrated near the plasma membrane of cultured cells, whereas g-actin is concentrated in stress fibers (Fig. 33-6). In muscle, α-actin forms the thin filaments of the contractile apparatus, whereas g-actin localizes around mitochondria. Even the full array of known actin-binding proteins cannot yet explain how cells prevent copolymerization of the isoforms or concentrate isoforms at different locations.

Figure 33-6 comparison of actin and actin-related proteins. Space-filling models are based on the atomic structure of actin and the sequences of the Arps.Yellow residues are identical to actin,green residues are conservative substitutions,blue residues are nonconservative substitutions, and red residues are insertions. All of these proteins have similar internal architectures, including identical contacts with ATP, but their surfaces differ considerably. The phylogenetic tree, based on sequence comparisons, shows that the genes for actins and for all of the Arps had a common ancestor.

(From the work of J. Kelleher, Johns Hopkins Medical School, Baltimore, Maryland; illustration redrawn from Mullins D, Kelleher J, Pollard TD: Actin’ like actin. Trends Cell Biol 6:208–212, 1996.)

Actin-Related Proteins

After believing for 20 years that actins are one of the most evolutionarily conserved protein families, scientists discovered several families of highly divergent actin-related proteins (Arps) in the 1990s (Fig. 33-6). Genes for Arps diverged from actin genes after the earliest branches in the eukaryotic tree and are now found in eukaryotic species ranging from amoebas to humans. Arps share with actin the fold of the polypeptide chain and residues forming the nucleotide-binding site, but fewer than 60% of the residues are identical to actin. Divergent surface residues allow Arps to participate in molecular interactions different from actin. Arp1 forms a short filament as part of the dynactin complex that promotes cargo movement by the microtubule motor dynein (see Fig. 37-2). Arp2 and Arp3 are two of seven subunits in a protein complex that nucleates branched actin filaments in the cell cortex (Fig. 33-13). Eight additional types of Arps are widespread in eukaryotes. Several participate in complexes that regulate chromatin structure.

Figure 33-13 Nucleation of branched actin filaments by Arp2/3 complex.A, Ribbon diagram of the crystal structure of Arp2/3 complex. The seven subunits are color coded and labeled. Numbers label the subdomains of Arp2 and Arp3.B, Model of branch based on a 3D reconstruction from electron micrographs.C, Steps in branch formation (also refer back to panel A): A WASp/Scar nucleation promoting factor binds an actin monomer (1). This binary complex binds Arp2/3 complex, bringing together the actin subunit with Arp2 and Arp3 (2). The ternary complex binds to the side of an actin filament, completing the activation process (3). A new daughter filament grows at its barbed end from the side of the older mother filament (4). B is the barbed end. P is the pointed end.

(A, PDB file: 1K8K. References: Robinson R, Turbedsky K, Kaiser DA, et al: Crystal structure of Arp2/3 complex. Science 294:1679–1684, 2001. B, Based on the work of I. Rouiller, N. Volkmann, and D. Hanein, Burnham Institute, La Jolla, California. C, Marchand J-B, Kaiser DA, Pollard TD, Higgs HN: Interaction of WASp/Scar proteins with actin and vertebrate Arp2/3 complex. Nat Cell Biol 3:76–82, 2001.)

Actin Polymerization

Actin filaments are polarized, owing to the uniform orientation of the asymmetrical subunits along the polymer (Fig. 33-7). One end is called the barbed end, the other is called the pointed end. This nomenclature arises from the asymmetrical arrowhead pattern created when myosin bind along the length of actin filaments (Fig. 33-8). The helical arrangement of subunits in actin filaments was originally revealed in the 1960s by electron microscopy and X-ray fiber diffraction of whole muscle and actin gels. These low-resolution data are used to orient the atomic structure of the actin monomer in current models.

Figure 33-7 structure of the actin filament. A, Electron micrograph of a negatively stained actin filament. B, Reconstruction of an actin filament by image processing of electron micrographs (blue) with ribbon models of the subunits(gold) along one strand of the double helix. One subunit is enlarged to the right. The pointed end of the subunits with the nucleotide cleft is at thetop, and the faster-growing barbed end is at thebottom. This orientation of the actin molecule in the filament uniquely accounts for the X-ray fiber diffraction pattern of oriented filaments and agrees with electron microscopy with probes on specific actin residues and with chemical cross-linking between residues of adjacent subunits. C, Surface rendering of the molecular model. Subunits in the two long-pitch helices are shown asyellow-orange andblue-green (see Fig. 5-5 for nomenclature). The short pitch helix, including every subunit, follows a yellow-green-orange-blue pattern.D, Scale drawing used throughout this text.

(A–B, Courtesy of U. Aebi, University of Basel, Switzerland. C, Courtesy of R. Milligan, Scripps Research Institute, La Jolla, California.)

Figure 33-8 actin filament elongation. A, Electron micrograph of growth from an actin filament seed decorated with myosin heads to reveal the polarity. Growth is faster at the barbed end than at the pointed end.B, Rate constants for association (units: μM⁻¹s⁻¹) and dissociation (units: s⁻¹) for Mg-ATP-actin (T) and Mg-ADP-actin (D) were determined by measuring the rate of elongation at the two ends as a function of monomer concentration. Ratios of the rate constants yield critical concentrations (K, units: mM) for the various reactions. The critical concentrations at the two ends are the same for ADP-actin but differ for ATP-actin.

(A, Courtesy of M. Runge, Johns Hopkins Medical School, Baltimore, Maryland. B, Reference: Pollard TD: Rate constants for the reactions of ATP- and ADP-actin with the ends of actin filaments. J Cell Biol 103:2747–2754, 1986.)

Actin self-assembles into filaments by means of a series of bimolecular reactions (Fig. 33-9; see also Fig. 5-6). Actin is isolated from cells as a monomer at low salt concentrations. Physiological concentrations of monovalent and divalent cations bind to low-affinity sites on actin and promote polymerization. In vitro, actin trimers appear to be the nucleus that initiates polymer growth in the sense that the reactions that are required to form trimers are very unfavorable in comparison with reactions for elongation of polymers larger than trimers. To initiate new filaments, cells use regulatory proteins to overcome these unfavorable nucleation reactions.

Figure 33-9 actin filament nucleation, growth, and nucleotide hydrolysis. A, Nucleation. Formation of dimers and trimers is very unfavorable, owing to rapid dissociation of subunits. Actin trimers are called nuclei because they initiate the highly favorable elongation reactions. This mechanism is based on kinetic modeling of experimental data. Estimated rate constants have units of μM⁻¹s⁻¹ for association reactions and s⁻¹ for dissociation reactions.B, ATP hydrolysis by a polymer of ATP-actin(yellow subunits) is random and irreversible at a rate of 0.3s⁻¹, yielding subunits with bound ADP and inorganic phosphate (orange). Phosphate dissociates slowly at a rate of 0.002s⁻¹, converting half of the newly polymerized subunits to ADP-actin (pink) in 6 minutes. ADP bound to polymerized subunits does not exchange with nucleotide in the medium. Phosphate binding is reversible, but the affinity is low, so most subunits bind only ADP.

(Reference: Pollard TD, Blanchoin L, Mullins RD: Biophysics of actin filament dynamics in nonmuscle cells. Annu Rev Biophys Biomol Struct 29:545–576, 2000.)

Actin filaments grow and shrink by the addition and loss of subunits at the two ends of the polymer. The reactions at the two ends have different rate constants (Fig. 33-8). Association of subunits is rapid at both ends. Subunit association is a diffusion-limited reaction (see Chapter 4) at the rapidly growing barbed end and somewhat slower at the other end. Subunit dissociation is relatively slow at both ends, between 0.3 and 8 subunits per second. The rates of these reactions depend on the nucleotide bound to the monomer associating with or dissociating from a filament.

In the presence of ATP, purified actin assembles almost completely, leaving as monomers the critical concentration of about 0.1μM ATP-actin. The critical concentration is the monomer concentration giving equal rates of association and dissociation, 1.4s⁻¹ at the barbed end (see Figure 5-6). The critical concentration for ADP-actin is about 20 times higher than for ATP-actin.

Hydrolysis of bound ATP and dissociation of the g-phosphate during assembly modifies the behavior of actin filaments, including their affinity for regulatory proteins. Following incorporation of an ATP-actin subunit into a filament, bound ATP is hydrolyzed irreversibly to ADP and phosphate with a half time of 2s (Fig. 33-9). These ADP-Pi subunits behave much like ATP subunits. Phosphate dissociates slowly and reversibly over several minutes. This yields filaments with a core of subunits with tightly bound ADP. At the millimolar concentrations of phosphate in cytoplasm, phosphate rebinds to some ADP-actin subunits.

The critical concentrations for ATP-actin differ at the two ends of the filament. This results from differences in the probability of nucleotide hydrolysis and phosphate release at the two ends, which is more likely to expose ADP-subunits at the pointed end. At steady state in the presence of ATP, the actin monomer concentration falls between the critical concentrations at the two ends. Though the polymer and monomer concentrations remain constant, net addition of subunits at the barbed end and net loss of subunits at the pointed end result in the slow migration of subunits through the polymer from the barbed end to the pointed end. This process is called treadmilling. Neither end exhibits rapid fluctuations in length like those of microtubules (see Fig. 34-7).

In many cell types, the barbed ends of the filaments are associated with the plasma membrane (Fig. 33-2). Actin filaments in muscle are also anchored at their barbed ends (Fig. 33-3C; also see Fig. 39-3). This makes mechanical sense, as actin filaments sustain tension better than compression and because (with one interesting exception) all known myosins pull filaments in a direction away from the barbed end. The plasma membrane is a prominent actin filament–anchoring site, allowing force that is produced in the cytoplasmic actin network to change the shape of the membrane and to be transmitted to the substrate on which the cell sits or to adjacent cells (see Fig. 38-5).

Actin-Binding Proteins

In contrast to the slow treadmilling of filaments of purified actin, actin filaments in live cells can polymerize and depolymerize rapidly under the control of more than 60 families of actin-binding proteins (Fig. 33-10 and Appendix 33-1). Broadly, these proteins fall into families that bind monomers, sever filaments, cap filament ends, nucleate filaments, cross-link filaments, stabilize filaments, or move along filaments. Like actin, actin-binding proteins are ancient. Many families arose in early eukaryotes and are found in protozoa, yeast, plants, and vertebrates.

Figure 33-10 families of actin-binding proteins. Monomer-binding proteins generally favor either ATP-actin (profilin and thymosin-b₄) or ADP-actin (ADF/cofilins). Capping proteins bind to either the barbed end (capping protein, gelsolin) or the pointed end (tropomodulin, Arp2/3 complex) of filaments. Some severing proteins also cap (gelsolin, fragmin), whereas others do not (ADF/cofilins). Cross-linking proteins can form networks or bundles.

No actin-binding protein functions in isolation. Typically, two or more proteins collaborate to control each aspect of actin dynamics. This section introduces examples of each class of actin-binding protein. Following sections explain how ensembles of these proteins work together to regulate actin filament dynamics in cells.

Actin Monomer–Binding Proteins

Proteins that bind actin monomers cooperate with capping proteins to maintain a pool of unpolymerized actin in cells and regulate the nucleotide bound to actin.

Profilins are abundant proteins, found in all branches of the eukaryotic tree. Cells require three different profilin activities for viability: binding actin monomers, catalyzing the exchange of nucleotides bound to actin (Fig. 33-11) and binding to polyproline sequences on other protein such as formins (Fig. 33-12). Profilins also bind acidic membrane lipids (polyphosphoinositides). The nucleotide bound to actin monomers determines the affinity for profilin: highest for nucleotide-free actin monomers, followed by ATP-actin and ADP-actin. Profilins bind to the barbed end of actin monomers, thereby sterically blocking nucleation and pointed end elongation but not association of the profilin-actin complex with the barbed end of filaments. Profilin dissociates rapidly after the profilin-actin complex binds to a barbed end.

Figure 33-12 nucleation of actin filaments by formins. A, Domain structure of a generic formin similar to mouse mDia1. Rho-family GTPases activate formins by disrupting intramolecular associations that autoinhibit the FH2 domains.B, Ribbon diagram of the structure of the homodimer of FH2 domains of budding yeast Bni1p. The linker segments must be extended for the dimer to fit around an actin filament.C, Proposed pathway of actin filament nucleation and elongation in association with formin FH1 and FH2 domains. Elongation is favored by concentrating and orienting profilin-actin near the barbed end by binding to multiple polyproline sequences in FH1.

(B, PDB file: 1UX5. C, Reference: Kovar D, Harris ES, Mahaffy R, et al: Control of the assembly of ATP- and ADP-actin by formins and profilin. Cell 724:423–435, 2006.)

Found only in vertebrates, β-thymosins are peptides of 43 residues that bind ATP-actin monomers with higher affinity than ADP-actin monomers. They inhibit both actin polymerization and nucleotide exchange. Thymosin-b4 is the most abundant actin-binding protein in some animal cells, where it sequesters most of the unpolymerized actin.

Members of the ADF/cofilin family are essential for the viability of many eukaryotes. They bind ADP-actin monomers with higher affinity than ATP-actin and inhibit nucleotide exchange but not polymerization. Their major roles are to bind and destabilize ADP-actin filaments (Figs. 33-15 and 33-19) and to nucleate filaments.

Figure 33-15 actin filament–severing mechanisms. A, Severing by gelsolin. Ribbon model of the atomic structure of gelsolin, showing the six homologous domains labeled 1 to 6. When activated by Ca²⁺, gelsolin domains 2 to 6 bind to the side of an actin filament, increasing the likelihood that domain 1 will insert between actin subunits and disrupt the filament. The products are two filaments: one with a new pointed end and another with a new barbed end tightly capped by gelsolin.B, Severing by ADF/cofilins. ADF/cofilins bind to sites on ADP-actin filaments with a rare but naturally occurring tighter helical twist. Binding destabilizes and severs the filament. The products are two uncapped ends that are available for subunit association and dissociation.

(A, PDB file: 1DON. Reference: Robinson R, Mejillano M, Le VP, et al: Domain movement in gelsolin: A calcium-activated switch. Science 286:1939–1942, 1999. B, References: McGough A, Pope B, Chin W, Weeds AG: Cofilin changes the twist of F-actin. J Cell Biol 138:771–781, 1997; Blanchoin L, Pollard TD: Mechanism of interaction ofAcanthamoeba actophorin (ADF/cofilin) with actin filaments. J Biol Chem 274:15538–15546, 1999.)

Figure 33-19 actin polymerization cycle showing reaction rates adjusted for physiological concentrations (arrows). Note the main pathway followed by an actin molecule as it recycles between the monomer pool and filaments (shading). A pool of ATP-actin bound to profilin elongates actin filaments with free barbed ends. Profilin rapidly dissociates, and filamentous actin hydrolyzes ATP and dissociates phosphate. ADF/cofilins promote phosphate dissociation and sever ADP-actin filaments. ADP subunits dissociate from filaments. Profilin competes with ADF/cofilin for actin monomers and promotes the exchange of ADP for ATP. Thymosin-b₄, when present, sequesters a pool of unpolymerizable ATP-actin. Profilin transfers actin from this thymosin-actin pool to actin filaments. A, actin monomer; C, ADF/cofilin; D, ADP bound to actin; F, filamentous actin; P, profilin; Pi, phosphate bound to actin; T, ATP bound to actin.

Actin Filament Nucleation Factors

Spontaneous nucleation of new filaments from actin monomers is intrinsically unfavorable and further inhibited by profilin, so cells use nucleation factors to specify when and where new filaments form. The best characterized are formins, Arp2/3 complex, and spire. Each has a different evolutionary origin, mechanism of action, and physiological function.

Formins initiate unbranched actin filaments that are incorporated into the contractile ring and bundles of actin filaments such as cables in yeast (see Fig. 37-11) and stress fibers in animal cells (Fig. 33-1B). Each of the three formins in fission yeast assembles actin filaments for a specific function: the cytokinetic contractile ring, interphase actin cables, or mating structures. Thus, the 15 different mammalian formins are also likely to have specific functions. These proteins have in common a formin homology-2 (FH2) domain (Fig. 33-12). Pairs of FH2 domains form a doughnut-shaped ring that fits around the barbed end of an actin filament. Interactions with two actin monomers nucleate a filament that grows by adding subunits to the barbed end. An FH2 domain can track faithfully on the growing end as it adds thousands of new subunits. When associated with a membrane in a cell or a microscope slide in an experiment, a formin molecule can extrude a growing actin filament for many minutes. Profilin enhances elongation by targeting profilin-actin complexes to multiple polyproline sequences in the FH1 domain adjacent to FH2. Some well-studied formins are autoinhibited by interactions between parts of the polypeptide flanking the FH1FH2 domains. Rho family GTPases regulate foramins by overcoming this autoinhibition.

Arp2/3 complex consists of two actin-related proteins (Arp2 and Arp3) tightly bound to five novel proteins (Fig. 33-13). Arp2/3 complex binds to the side of actin filaments and forms branches by nucleating filaments that grow at their free barbed ends. The complex caps the pointed end of the new filament and attaches it to the side of the older filament. Growth of the free barbed ends of these branches produces the force that pushes the plasma membrane forward at the leading edge of motile cells (Fig. 33-2D).

Spire was discovered in Drosophila as a gene required for the development of eggs and embryos and later found in other metazoans but not fungi or protozoa. Spire proteins have multiple domains (corresponding to the V domains in Fig. 33-17A) that bind actin monomers and stabilize oligomers on the pathway to nucleation of unbranched filaments. The biological functions of these proteins are being investigated.

Figure 33-17 adapter proteins. A, WASp (Wiskott-Aldrich syndrome protein). Domains called V (for verprolin homology), C (for connecting), and A (for acidic) activate Arp2/3 complex. VC binds an actin monomer, and CA binds Arp2/3 complex. C also binds intramolecularly to the GBD (GTPase-binding domain), autoinhibiting VCA. Membrane-bound Rho-family GTPases and polyphosphoinositides compete C from GBD, releasing VCA to interact with actin and Arp2/3 complex. SH3 domain proteins such as Nck also activate WASp by binding the proline-rich domain. The N-terminal EVH1 (Enα-VASP homology) domain binds WIP (WASp interacting protein), the mammalian homolog of yeast verprolin.B, VASP (vasodilator-stimulated phosphoprotein). The EVH1 domain (see Fig. 25-11B) binds polyproline ligands, including vinculin and zyxin in focal contacts; the proline-rich domain binds profilin, profilin-actin complexes, and proteins with certain SH3 domains; the EVH2 domain binds both actin monomers and filaments; and the C-terminal coiled-coil (CC) mediates the formation of VASP tetramers.

Actin Filament–Capping Proteins

Capping proteins bind to either the barbed or pointed end of actin filaments, where they block subunit addition and dissociation (Fig. 33-14). Some capping proteins also stimulate the formation of new filaments and/or sever actin filaments (Fig. 33-15).

Figure 33-14 actin filament–capping proteins. Interactions of capping proteins with the ends of actin filaments. Most of these proteins bind with high affinity to a filament end. Tropomodulin requires tropomyosin for high-affinity binding.

Gelsolins consist of six domains with similar folds but different sequences and functions. They bind tightly to the sides and barbed ends of actin filaments, blocking both the dissociation and association of actin subunits. Gelsolin also binds actin dimers, forming a nucleus that grows at the pointed end. Phosphatidyl 4,5-bisphosphate (PIP₂) competes with actin for binding gelsolin. Three-domain capping proteins, such as fragmin and severin, are similar in structure and function to the first three domains of gelsolin. Genes for these capping proteins, found widely across the phylogenetic tree, are likely to have duplicated during evolution to give rise to gelsolin genes.

Heterodimeric capping proteins consist of two subunits of approximately 30 kD. They cap barbed ends with high affinity, independent of Ca²⁺, and promote nucleation of new pointed ends by stabilizing small actin oligomers. As with gelsolins, PIP₂ inhibits capping by these proteins. Heterodimeric capping proteins are found in most eukaryotic cells. In striated muscle, they cap the barbed end of actin filaments in the Z disk (see Fig. 39-5).

Tropomodulin caps the pointed end of stable actin filaments in muscle, red blood cells, and other cells of higher organisms. High-affinity binding to pointed ends requires tropomyosin, an α-helical protein that binds along the length of actin filaments (see Fig. 39-4).

Actin Filament–Severing Proteins

Three classes of proteins just introduced—gelsolin, fragmin/severin, and ADF/cofilin—also sever actin filaments into short fragments (Fig. 33-15). Domain 2 of gelsolin binds to the side of an actin filament, positioning domain 1 to bind between subunits and disrupt the filament. This requires micromolar concentrations of Ca²⁺. One gelsolin isoform is found inside cells; another is secreted into blood plasma, where it may sever actin filaments released from damaged cells. Fragmin and severin from slime molds (but not their vertebrate homologs) have similar Ca²⁺-dependent severing activity. Independent of Ca²⁺, ADF/cofilins bind ADP-actin subunits in filaments and promote severing and depolymerization.

Proteins That Bind the Side of Actin Filaments

Tropomyosin, nebulin, and caldesmon are extended proteins that bind along the sides of actin filaments. Tropomyosin increases the tensile strength of actin filaments; in striated muscles, it is also an essential component of the Ca²⁺-sensitive regulatory machinery that controls the interaction of myosin and actin (see Fig. 39-4). Nebulin may determine the length of the actin filaments in skeletal muscle (see Chapter 39). Caldesmon, together with tropomyosin and Ca²⁺-calmodulin, regulates interaction of actin and myosin in smooth muscle (see Fig. 39-21) and nonmuscle cells. Phosphorylation of caldesmon by cell cycle kinases and may play a role in the reorganization of actin filaments during mitosis.

Actin Filament Cross-Linking Proteins

Possession of two actin-binding sites enables cross-linking proteins (Fig. 33-16) to bridge filaments and to stabilize higher-order assemblies of actin filaments. Some have a greater tendency to cross-link filaments in regular bundles, like those in microvilli (Fig. 33-2A), but depending on protein concentrations and filament lengths, most of these proteins can promote the formation of both random networks and regular bundles of filaments.

Figure 33-16 Actin filament cross-linking proteins sharing ho-mologous actin-binding domains (red). Cross-linking requires two actin-binding sites, which can be part of one polypeptide (fimbrin) or on different subunits of dimeric proteins (α-actinin, filamin). Dystrophin has a second actin-binding site in the middle of the tail.

(Re-drawn from Matsudaira P: Mod-ular organization of actin cross-linking proteins. Trends Biochem Sci 16:87–92, 1991.)

Many of these proteins share a homologous actin binding domain associated with other domains that form dimers. α-Actinin is found in the cortical actin network, at intervals along stress fibers, on the cytoplasmic side of cell adhesion plaques (see Fig. 30-11), and in the Z-disk of striated muscles (see Fig. 39-5). Fimbrin and villin (a relative of gelsolin with an extra actin-binding site) stabilize the regular actin filament bundles in microvilli. Filamin cross-links filaments in the cortex of many cells and also anchors these filaments to an integrin, a plasma membrane receptor for adhesive glycoproteins (see Fig. 30-11). Actin filament cross-linking proteins of the plasma membrane skeleton, such as spectrin (see Fig. 7-10) and dystrophin (see Fig. 39-9) are anchored to integral membrane proteins. Relatively little is known about how cells regulate cross-linking proteins, although Ca²⁺ inhibits binding of some α-actinins to actin.

Adapter Proteins

Eukaryotic cells use multidomain proteins as adapters between signaling pathways and actin assembly (Fig. 33-17). The multidomain protein WASp is defective in the inherited immunodeficiency and bleeding disorder called Wiskott-Aldrich syndrome. The C-terminal domains of WASp (and related proteins N-WASp and Scar/WAVE) activate the Arp2/3 complex to nucleate new actin filaments on the side of existing filaments (Fig. 33-13). Intramolecular interactions autoinhibit WASp. Rho-family GTPases (guanosine triphosphatase), membrane polyphosphoinositides, and polyproline-binding proteins with SH3 domains cooperate to overcome the autoinhibition by binding to parts of WASp involved with the intramolecular interactions. Rac activates Scar by interacting with a complex of regulatoryproteins. An unrelated protein called VASP (vasodilator-stimulated phosphoprotein) also has EVH1 and proline-rich domains as well as an actin-binding domain. These domains allow VASP and related proteins to bind proline-rich ligands in focal adhesions and on the surface of the bacterium Listeria, profilin-actin complexes, and actin filaments. These interactions stimulate actin polymerization in filopodia and “comet tails” assembled by Listeria (see Fig. 37-12).

Functional Redundancy of Actin-Binding Proteins

The diversity and the apparent redundancy of actin-binding proteins are striking. Why should most organisms retain genes for 60 different actin-binding proteins if they have such a limited repertoire of functions: monomer binding, nucleation, capping, severing, cross-linking, stabilizing, and motility? Null mutations show that some proteins are essential for normal physiology. These include myosin-II, Arp2/3 complex, profilin, and cofilin in yeast. On the other hand, organisms can survive genetic deletion of some actin-binding proteins, suggesting that parts of the system are redundant. For example, in a laboratory environment, Dictyostelium tolerates the loss of cross-linking proteins (α-actinin or ABP-120), severin, or one of two profilin genes with only minor defects in behavior and growth. Humans who lack dystrophin develop and grow normally for a few years but later succumb to muscle wasting (see Table 39-1). Mice without their single gelsolin gene reproduce normally, with only mild defects in platelets and other cells.

The data suggest that each actin-binding protein has a distinct function, conferring a small selective advantage. Multiple proteins sharing overlapping functions make the actin system relatively fail-safe so that it is difficult to detect the phenotypic consequences of the loss of particular proteins in mutant animals. Alternatively, these proteins may have other unknown functions distinct from actin binding.

Actin Dynamics in Live Cells

Cellular actin filaments vary widely in stability. In muscle, the ends of actin filaments exchange subunits over hours. Four proteins stabilize these filaments (see Fig. 39-6). Tropomyosin and nebulin run along the length of the filament, CapZ binds to the barbed end, and tropomodulin binds to the pointed end. These proteins inhibit breakage of the filaments and allow limited exchange of actin subunits at the barbed ends. On the other hand, amoebae and white blood cells protrude and remodel actin-rich pseudopods on a time scale of seconds. Several methods are available to document actin filament dynamics in cells.

Although light microscopes lack the resolution to observe individual actin filaments crowded together in cytoplasm, assemblies of actin filaments can be imaged directly in favorable cases by DIC or phase contrast microscopy. For example, the parallel arrays of actin filaments in muscle are observed to be very stable, whereas networks of actin filaments in nerve growth cones constantly assemble and move away from the leading edge (Fig. 33-18).

Figure 33-18 actin filament dynamics at the leading edge of a giant growth cone of a neuron isolated from the mollusk aplysia. A network of actin filaments forms continuously at the leading edge of the growth cone, moves inward by retrograde flow, and disassembles near the central organelle-rich zone.A–C, Effect of the drug cytochalasin D on the growth cone. A, DIC micrograph before application of the drug, showing the broad lamellum at the leading edge (left). DIC (middle) and fluorescence (right) micrographs of this growth cone 60s after the application of the drug, which disrupts the network of actin filaments at the leading edge. The arrows mark the zone cleared of actin filaments, stained with rhodamine phalloidin in the right panel. A narrow rim of filaments survives at the leading edge.B, Time series of differential interference contrast micrographs at 6-s intervals, showing that cytochalasin blocks the formation of new filaments at the leading edge, but retrograde flow of existing filaments (toward the cell body) continues, separating the network from the leading edge. Small beads on the surface move at the same rate.C, This growth cone fixed after 60s and stained with rhodamine-phalloidin. The fluorescence micrograph shows that few filaments survive at the leading edge. If cytochalasin is removed from a live cell, the actin filament network recovers, beginning near the leading edge. D, Fluorescence micrographs of a growth cone injected with rhodamine-phalloidin to mark actin filaments.Left, Bundles of actin filaments arranged radially in the lamellum. Right, A time series of one actin filament bundle showing the steady retrograde flow of irregularities in the actin filament network at a rate of about 3 to 6μm/min (equivalent to filaments growing by 19 to 38 subunits per second) from the leading edge (top) to the organelle-rich central zone (bottom).

(Courtesy of Paul Forscher, Yale University, New Haven, Connecticut; based on Forscher P, Smith SJ: Actions of cytochalasins in a neuronal growth cone. J Cell Biol 107:1505–1516, 1988, by copyright permission of The Rockefeller University Press.)

A second approach is to observe the effects of drugs that bind actin such as cytochalasin or latrunculin (Box 33-1). Neither disassembles actin filaments directly, but they both interfere with assembly from monomers, so their effects reveal if filaments are turning over naturally. Muscle actin filaments are relatively resistant to these drugs, but they disrupt the cortical actin filament network in other cells in seconds (Fig. 33-18). When the drug is removed, the cortical actin network reforms rapidly from the leading edge. This response indicates that many cellular filaments turn over rapidly.

BOX 33-1 Tools to Study Actin Filaments: Natural Products Can Stabilize or Destabilize Actin Filaments

Destabilizers Cytochalasins, complex organic compounds synthesized by fungi, inhibit actin assembly in two ways. High-affinity binding to the barbed end of actin filaments inhibits subunit association and dissociation. Low-affinity binding to actin monomers promotes their dimerization and the hydrolysis of ATP bound to one subunit. In this way, cytochalasins catalyze the conversion of ATP-actin to ADP-actin. Cytochalasin (meaning “cell relaxing”) is so named because it causes regression of the cleavage furrow during cytokinesis and disrupts many structures containing actin filaments in cells. Cytochalasins are used to test for the participation of actin filaments in cellular processes, but observations must be interpreted cautiously, given the complicated mechanism of action. Sponges synthesize toxins that destabilize actin filaments in cells by sequestering actin monomers (latrunculin A and B) or severing actin filaments (swinholide A).

Stabilizers Phallotoxins (such as phalloidin) are cyclic peptides that are synthesized by poisonous mushrooms. They bind and stabilize actin filaments by reducing the rate of subunit dissociation to near zero at both ends of the polymer. When introduced into cells by microinjection, phallotoxins inhibit processes that depend on actin filament turnover, including amoeboid movement. They are toxic to humans because they interfere with bile secretion. Fluorescent derivatives of phallotoxins are widely used to localize actin filaments in cells and tissues (Fig. 33-1), as well as to quantify polymerized actin in cells and cell extracts. A sponge toxin, jasplakinolide, has effects similar to phallotoxins.

C2 toxin produced by Clostridium botulinum is an enzyme that catalyzes the ADP-ribosylation of cytoplasmic actins on arginine-177. Clostridium perfringens iota toxin does the same to muscle actin. ADP-ribosylated actin polymerizes poorly and caps the barbed end of actin filaments. The ability of these protein toxins to penetrate live cells, cap actin filaments, and alter actin polymerization accounts for their disruption of the actin cytoskeleton in cells and may contribute to their toxicity.

Actin can also be observed in live cells if labeled with a fluorescent probe. Purified actin can be labeled with a fluorescent dye and microinjected into live cells, where it is incorporated into the actin-containing structures. Expression of actin tagged with green fluorescent protein is even more convenient for following actin in live cells, although caution is required because the bulky green fluorescent protein can interfere with interactions with formins and perhaps other molecules. In muscle, fluorescent actin slowly incorporates, through exchange, at the pointed ends of the filaments. In nonmuscle cells, fluorescent actin is quickly incorporated into all of the filaments. If low levels of fluorescent actin are used, random incorporation into filaments can result in fluorescent “speckles” that can be used to follow the movements and turnover of these subunits (Fig. 33-18D). Figure 38-9 illustrates how bleaching or activating fluorescent actin in a live cell can reveal where filaments assemble and disassemble at the leading edge of motile cells. The associated text explains the biochemical mechanism.

Biochemical and genetic experiments identified the same minimal set of proteins that are essential for maintaining the pool of unpolymerized actin, initiating and terminating new filaments, and controlling disassembly (Fig. 33-19). For example, biochemical reconstitution of actin filament assembly and recycling in the comet tail of the intracellular bacterium Listeria (see Fig. 37-12) requires (in addition to a regulatory protein on the surface of the bacterium) only actin, Arp2/3 complex, ADF/cofilin, profilin, and a capping protein.

The Pool of Unpolymerized Actin

Cells can respond rapidly to stimuli such as chemoattractants by assembling actin filaments where needed, because they have a large pool of unpolymerized actin to grow the new filaments. Roughly half of the actin in the cytoplasm of resting cells is unpolymerized, corresponding to 50 to 100μM monomers, 500 to 1000 times higher than the critical concentration.

Two complementary mechanisms maintain the pool of unpolymerized actin: monomer binding to profilin and capping of actin filament ends. In vertebrate cells another protein, thymosin-b4, augments the effects of profilin. Thymosin-b4 buffers the free monomer concentration by sequestering actin monomers. Rapid exchange allows monomers to move between profilin and thymosin-b4. The concentrations of profilin and thymosin-b4 exceed the concentration of unpolymerized actin, and these proteins bind tightly enough to reduce the free monomer concentration to the micromolar level. Actin monomers bound to profilin or thymosin-b4 do not nucleate new filaments. However, for profilin to maintain a monomer pool, the barbed ends of most filaments must be capped, since rapid addition of actin-profilin complexes to free barbed ends would quickly deplete the pool of unpolymerized actin. Cells contain enough heterodimeric capping protein, gelsolin, or both to cap the barbed ends of most filaments. Together, monomer binding and capping allow cells to maintain a large pool of actin subunits ready to elongate any barbed ends created by uncapping, severing, or nucleation.

Initiation and Termination of Actin Filaments

A variety of external agonists and internal signals stimulate the conversion of actin from the unpolymerized pool into actin filaments. Examples include the ability of chemoattractants to direct pseudopod formation in slime molds (see Fig. 38-12) and white blood cells (see Fig. 30-13), and the influence of the mitotic spindle on the assembly of the cytokinetic contractile ring (see Fig. 44-22). Polymerization depends on creation of barbed ends, which grow rapidly at rates estimated to be 50 to 500 subunits per second, depending on the concentration of actin-profilin.

Three mechanisms are thought to create free barbed ends: uncapping, severing, and de novo formation of new barbed ends. In many cases, new barbed ends appear to form de novo. At the leading edge of motile cells, small Rho-family GTPases associated with the plasma membrane and polyphosphoinositides activate WASp/Scar proteins, which stimulate Arp2/3 complex to nucleate branches with free barbed ends on the side of older filaments (Figs. 33-2D and 33-13). Formins nucleate filaments for the cleavage furrow and appear to either initiate or sustain the growth of actin filaments in filopodia (see Fig. 38-2A). When thrombin activates platelets (see Fig. 30-14), plasma membrane polyphosphoinositides uncap barbed ends by dissociating gelsolin. Transient increases in the cytoplasmic Ca²⁺ concentration may also induce gelsolin to sever actin filaments, creating capped barbed ends. Dephosphorylation activates ADF/cofilin proteins, which can sever and nucleate filaments, creating free barbed ends.

The duration of growth depends on the nucleation mechanism and the local environment. At the leading edge, new branches nucleated by Arp2/3 complex grow rapidly but transiently, as the concentration of free capping protein is high enough to terminate growth by capping barbed ends in a few seconds. On the other hand, barbed ends growing in association with a formin are protected from capping and grow persistently, as is observed at the tips of filopodia and the barbed ends of actin cables located in the buds of yeast cells.

Actin Filament Turnover and Subunit Recycling

Actin filaments are long lived if protected by tropomyosin and capping, as in muscle and stress fibers, but many actin filaments, such as those at the leading edge of motile cells, turn over quickly. A possible mechanism involves the hydrolysis of ATP and dissociation of the g-phosphate, reactions that provide a timer to mark older filaments for depolymerization by regulatory proteins (Fig. 33-19). After phosphate dissociates, ADF/cofilin proteins bind ADP-actin subunits in filaments and sever these older filaments. After ADP-actin dissociates from filaments (the details are not yet clear), profilin replaces ADF/cofilin and stimulates the exchange of ADP for ATP. This process recycles actin back to the pool of ATP-actin monomers interacting with profilin. In cells with a high concentration of thymosin-b4, much of the ATP-actin is stored bound to thymosin. Profilin shuttles ATP-actin from this thymosin-b4 buffer to growing filaments. Tropomyosin stabilizes a subset of old filaments by protecting them against ADF/cofilins.

How Do Cells Organize Actin Assemblies?

Cells organize actin filaments in a variety of structures, including cortical networks, microvilli or filopodia, and contractile bundles (Figs. 33-1 to 33-3). Although each cell in a population is unique, all cells of a particular type achieve a similar pattern of organization. How are these patterns specified? Although not yet understood in detail, the mechanisms appear to depend on expression of an appropriate mixture of actin-binding proteins, a prerequisite for self-assembly of particular structures. For example, actin forms bundles similar to microvilli and filopodia when polymerized in the presence of fimbrin and villin, the two major cross-linking proteins found in microvilli. Overexpression of villin in cells induces extension of existing filopodia and formation of new filopodia. Thus, the pool of villin and fimbrin and other components sets the number of microvilli.

The Rho-family GTPases Cdc42, Rac, and Rho regulate the assembly of many actin filament structures (Fig. 33-20). A complex of proteins including Cdc42 in the bud of yeast cells anchors formins, which mediate the continuous assembly of a cable of actin filaments. The formins anchor the growing barbed ends as the pointed ends extend into the mother cell. These cables are tracks for myosin-V to move cargo into the bud. In motile cells, signals downstream of chemotactic receptors activate Cdc42 and Rac (see Fig. 38-8), which activate WASp/Scar proteins. They in turn stimulate Arp2/3 complex to generate the branched filament network that pushes the membrane forward. Listeria use a surface protein, ActA, to activate Arp2/3 complex, which generates a “comet tail” of actin filaments to push the bacterium through the cytoplasm (see Fig. 37-12). Proteins that mediate endocytosis activate homologs of WASp and other proteins to assemble actin patches in budding yeast (see Fig. 37-11) and fission yeast (Fig. 33-1D).

Figure 33-20 Rho-family GTPases promote the assembly of actin-based structures. Fluorescence micrographs of Swiss 3T3 fibroblasts stained with rhodamine-phalloidin to reveal actin filaments. A, Resting cells.B, Cells microinjected with activated Cdc42 form many filopodia.C, Cells microinjected with activated Rac have a thick cortical network of actin filaments around the periphery.D, Stress fibers anchored at their ends by focal contacts are abundant in cells microinjected with an activated form of Rho.

(Courtesy of Alan Hall, University of London, England.)

Physical forces also help to organize actin filaments. Bundles of actin filaments in stress fibers (Fig. 33-1) and the contractile ring during cytokinesis (Fig. 33-3; also see Fig. 44-23) appear to be aligned, at least in part, by tension generated by myosin motors. Activated Rho is required for cytokinesis and also stimulates formation of stress fibers (Fig. 33-20) by activating myosin II. Rho stimulates two kinases that phosphorylate the regulatory light chain and inhibit a phosphatase that reverses the phosphorylation of the light chain (see Fig. 39-21). The success of forces in organizing actin filaments depends on anchoring of the filaments to the plasma membrane—focal contacts in the case of stress fibers (see Fig. 30-11) and the equatorial plasma membrane for the contractile ring (see Fig. 44-23). Cross-linking proteins, such as α-actinin, help to maintain the integrity of these bundles under mechanical stress.

Mechanical Properties of Cytoplasm

Actin filaments provide the molecular basis for many of the mechanical properties of cytoplasm, a complicated, viscoelastic material.Viscoelastic means that cytoplasm can both resist flow, like a viscous liquid (e.g., molasses), and store mechanical energy when stretched or compressed, like a spring.

The physical properties of actin filaments depend on their lengths and their interactions. At physiological concentrations, purified actin filaments are viscoelastic. At high concentrations, actin filaments also align spontaneously into large parallel arrays called liquid crystals. Cross-linking networks of actin filaments increases both their viscosity and stiffness. Severing actin filaments decreases their viscoelasticity. On the other hand, shorter filaments have an increased tendency to form bundles in the presence of cross-linking proteins, so severing can actually promote the formation of rigid actin filament bundles.

Many cross-linking proteins, including α-actinin, have a low affinity for actin filaments with a K_d in the micromolar range. At steady state in vitro, bonds between these cross-linking proteins and actin filaments break and reform on a second or subsecond time scale. Consequently, gels of actin filaments and α-actinin are much more rigid when deformed rapidly than slowly (Fig. 33-21), because cross-links resisting the displacement of the filaments can rearrange if given sufficient time. Dynamic cross-links between filaments allow actin networks to remodel passively as cells move. Cells also remodel the actin cytoskeleton actively by nucleating, severing or depolymerizing filaments.

Figure 33-21 dynamic cross-linking of actin filaments. Rapid binding and dissociation of cross-linking proteins allow networks of actin filaments to resist rapid deformations but to change shape passively when force is applied for a prolonged time.A, Cross-linked network in a static region.B, Cross-linking proteins resist deformation if force is applied rapidly.C, Cross-linking proteins provide little resistance to deformation if force is applied slowly, since the cross-links rearrange faster than the filaments are displaced.

(Redrawn from Pollard TD, Satterwhite L, Cisek L, et al: Actin and myosin biochemistry in relation to cytokinesis. Ann N Y Acad Sci 582:120–130, 1990.)