Mechanism of action of cytochalasin B on actin

Substoichiometric cytochalasin B (CB) inhibits both the rate of actin polymerization and the interaction of actin filaments in solution. The polymerization rate is reduced by inhibition of actin monomer addition to the "barbed" end of the filaments where monomers normally add more rapidly. 2 microM CB reduces the polymerization rate by up to 90%, but has little effect on the rate of monomer addition at the slow ("pointed") end of the filaments and no effect on the rate of filament annealing. Under most ionic conditions tested, 2 microM CB reduces the steady state high shear viscosity by 10-20% and increases the steady state monomer concentration by a factor of 2.5 or less. In addition to the effects on the polymerization process, 2 microM CB strongly reduces the low shear viscosity of actin filaments alone and actin filaments cross-linked by a variety of macromolecules. This may be due to inhibition of actin filament-filament interactions which normally contribute to network formation. Since the inhibition of monomer addition and of actin filament network formation have approximately the same CB concentration dependence, a common CB binding site, probably the barbed end of the filament, may be responsible for both effects.     

Effect of the structure of the N terminus of tropomyosin on tropomodulin function

Tropomodulins (Tmod) bind to the N terminus of tropomyosin and cap the pointed end of actin filaments. Tropomyosin alone also inhibits the rate of actin depolymerization at the pointed end of filaments. Here we have defined 1) the structural requirements of the N terminus of tropomyosin important for regulating the pointed end alone and with erythrocyte Tmod (Tmod1), and 2) the Tmod1 subdomains required for binding to tropomyosin and for regulating the pointed end. Changes in pyrene-actin fluorescence during polymerization and depolymerization were measured with actin filaments blocked at the barbed end with gelsolin. Three tropomyosin isoforms differently influence pointed end dynamics. Recombinant TM5a, a short non-muscle alpha-tropomyosin, inhibited depolymerization. Recombinant (unacetylated) TM2 and N-acetylated striated muscle TM (stTM), long alpha-tropomyosin isoforms with the same N-terminal sequence, different from TM5a, also inhibited depolymerization but were less effective than TM5a. All blocked the pointed end with Tmod1 in the order of effectiveness TM5a >stTM >TM2, showing the importance of the N-terminal sequence and modification. Tmod1-(1-344), lacking the C-terminal 15 residues, did not nucleate polymerization but blocked the pointed end with all three tropomyosin isoforms as does a shorter fragment, Tmod1-(1-92), lacking the C-terminal "capping" domain though higher concentrations were required. An even shorter fragment, Tmod1-(1-48), bound tropomyosin but did not influence actin filament elongation. Tropomyosin-Tmod may function to locally regulate cytoskeletal dynamics in cells by stabilizing actin filaments.     

Conformational dynamics of capping protein and interaction partners: simulation studies

Capping protein (CP) is important for the regulation of actin polymerization. CP binds to the barbed end of the actin filament and prevents actin polymerization. This interaction is modulated through competitive binding by regulatory proteins such as myotrophin (V-1) and the capping protein interacting (CPI) motif from CARMIL. The binding site of myotrophin overlaps with the region of CP that binds to the barbed end of actin filament, whereas CPI binds at a distant site. The binding of CPI to the myotrophin-CP complex dissociates myotrophin from CP. Detailed multicopy molecular dynamics simulations suggest that the binding of CPI shifts the conformational equilibria of CP away from states that favor myotrophin binding. This shift is underpinned by allosteric effects where CPI inhibits CP through suppression of flexibility and disruption of concerted motions that appear to mediate myotrophin binding. Accompanying these effects are changes in electrostatic interactions, notably those involving residue K142β, which appears to play a critical role in regulating flexibility. In addition, accessibility of the site on CP for binding the key hydrophobic residue W8 of myotrophin is modulated by CPI. These results provide insights into the modulation of CP by CPI and myotrophin and indicate the mechanism by which CPI drives the dissociation of the myotrophin-CP complex.     

Does actin bind to the ends of thin filaments in skeletal muscle?

We examined whether or not purified actin binds to the ends of thin filaments in rabbit skeletal myofibrils. Phase-contrast, fluorescence, and electron microscopic observations revealed that actin does not bind to the ends of thin filaments of intact myofibrils. However, in I-Z-I brushes prepared by dissolving thick filaments at high ionic strength, marked binding of actin to the free ends, i.e., the pointed ends, of thin filaments was observed when actin was added at an early phase of polymerization. As the polymerization of actin proceeded, the binding efficiency decreased. The critical actin concentration for this binding was higher than that for polymerization in solution. The binding of G-actin was not observed at low ionic strength. On the basis of these results, we suggest that a particular structure suppressing the binding of actin is present at the free ends of thin filaments in intact myofibrils and that a part of the end structure population is eliminated or modified at high ionic strength so that further binding of actin becomes possible. The myofibril and I-Z-I brush appear to be useful systems for studies aimed at elucidating the organizational mechanisms of actin filaments in vivo.     

Brevin and vitamin D binding protein: comparison of the effects of two serum proteins on actin assembly and disassembly

Actin depolymerizing activity in serum can be attributed to the two proteins brevin and vitamin D binding protein (DBP). To investigate their mechanisms of action, we used a number of techniques, including procedures involving the fluorescent pyrene-labeled actin probe, to compare the interaction of the two proteins with G- and F-actin in vitro. With a fluorescence enhancement assay, we determined that brevin forms a 1:2 complex and DBP forms a 1:1 complex with pyrene-G-actin. We also found that both proteins reduce the viscosity of F-actin measured with high-shear and low-shear viscometers, with brevin effective at much lower concentrations than DBP. In polymerization experiments, brevin inhibits filament elongation at substoichiometric levels by inhibiting monomer addition at the barbed end but can also accelerate polymerization by nucleating assembly of filaments which grow from the pointed end. DBP does not nucleate filament assembly and inhibits filament elongation at either end only at near-stoichiometric levels. Brevin, but not DBP, accelerates disassembly of filaments diluted into a depolymerizing medium. This is consistent with the capability of brevin to sever preformed filaments associated with erythrocyte membranes and to increase the number of filament ends as estimated by a cytochalasin binding assay. In steady-state experiments involving the use of pyrene-actin, brevin produces only a small increase in the apparent monomer concentration when the critical concentrations at the two ends of the filaments are the same (i.e., in 0.1 M KCl). However, when the critical concentration at the pointed end is higher than that at the barbed end (i.e., in 2 mM MgCl2), low molar ratios of brevin sharply increase the monomer concentration to the critical concentration of the pointed end. This allows substoichiometric amounts of brevin to completely depolymerize filaments when the total actin concentration is at or below that of the pointed end. In contrast to brevin, DBP increases the amount of nonfilamentous actin in a stoichiometric and dose-dependent manner regardless of the nature of the salt in the medium. We conclude from this study that brevin is similar in its mechanism of action to other proteins known to bind to the barbed end of filaments and that DBP is related in its action to proteins that complex monomers and prevent them from participating in the polymerization process.     

Direct measurement of actin polymerization rate constants by electron microscopy of actin filaments nucleated by isolated microvillus cores

We used actin filament bundles isolated from intestinal brush-border microvilli to nucleate the polymerization of pure muscle actin monomers into filaments. Growth rates were determined by electron microscopy by measuring the change in the length of the filaments as a function of time. The linear dependence of the growth rates on the actin monomer concentration provided the rate constants for monomer association and dissociation at the two ends of the growing filament. The rapidly growing ("barbed") end has higher association and dissociation rate constants than the slowly growing ("pointed") end. The values of these rate constants differ in 20 mM KCl compared with 75 mM KCl, 5 mM MgSO4. 2 microM cytochalasin B blocks growth entirely at the barbed end, apparently by reducing both association and dissociation rate constants to near zero, but inhibits growth at the pointed end to only a small extent.     

Structural insights into the tropomodulin assembly at the pointed ends of actin filaments

Tropomodulins are a family of important regulators of actin dynamics at the pointed ends of actin filaments. Four isoforms of tropomodulin, Tmod1‐Tmod4, are expressed in vertebrates. Binding of tropomodulin to the pointed end is dependent on tropomyosin, an actin binding protein that itself is represented in mammals by up to 40 isoforms. The understanding of the regulatory role of the tropomodulin/tropomyosin molecular diversity has been limited due to the lack of a three‐dimensional structure of the tropomodulin/tropomyosin complex. In this study, we mapped tropomyosin residues interacting with two tropomyosin‐binding sites of tropomodulin and generated a three‐dimensional model of the tropomodulin/tropomyosin complex for each of these sites. The models were refined by molecular dynamics simulations and validated via building a self‐consistent three‐dimensional model of tropomodulin assembly at the pointed end. The model of the pointed‐end Tmod assembly offers new insights in how Tmod binding ensures tight control over the pointed end dynamics.     

ATP hydrolysis by the gelsolin-actin complex and at the pointed ends of gelsolin-capped filaments

To obtain kinetic information about the pointed ends of actin filaments, experiments were carried out in the presence of gelsolin which blocks all events at the kinetically dominant barbed ends. The 1:2 gelsolin-actin complex retains 1 mol/mol of actin-bound ATP, but it neither hydrolyzes the ATP nor exchanges it with ATP free in solution at a significant rate. On the other hand, the actin filaments with their barbed ends capped with gelsolin hydrolyze ATP relatively rapidly at steady state, apparently as a result of the continued interaction of ATP-G-actin with the pointed ends of the filaments. ATP hydrolysis during spontaneous polymerization of actin in the presence of relatively high concentrations of gelsolin lags behind filament elongation so that filaments consisting of as much as 50% ATP-actin subunits are transiently formed. Probably for this reason, during polymerization the actin monomer concentration transiently reaches a concentration lower than the final steady-state critical concentration of the pointed end. At steady state, however, there is no evidence for an ATP cap at the pointed ends of gelsolin-capped filaments, which differs from the barbed ends which do have an ATP cap in the absence of gelsolin. As there is no reason presently to think that gelsolin has any effect on events at the pointed ends of filaments, the properties of the pointed ends deduced from these experiments with gelsolin-capped filaments are presumably equally applicable to the pointed ends of filaments in which the barbed ends are free.     

A 45,000-mol-wt protein-actin complex from unfertilized sea urchin egg affects assembly properties of actin

A one-to-one complex of a 45,000-mol-wt protein and actin was purified from unfertilized eggs of the sea urchin, Hemicentrotus pulcherrimus, by means of DNase l-Sepharose affinity and gel filtration column chromatographies. Effects of the complex on the polymerization of actin were studied by viscometry, spectrophotometry, and electron microscopy. The results are summarized as follows: (a) The initial rate of actin polymerization is inhibited at a very low molar ratio of the complex to actin. (b) Acceleration of the initial rate of polymerization occurs at a relatively high, but still substoichiometric, molar ratio of the complex to actin. (c) Annealing of F-actin fragments is inhibited by the complex. (d) The complex prevents actin filaments from depolymerizing. (e) Growth of the actin filament is inhibited at the barbed end. In all cases except b, a molar ratio of less than 1:100 of the 45,000-mol-wt protein-actin complex to actin is sufficient to produce these significant effects. These results indicate that the 45,000-mol-wt protein-actin complex from the sea urchin egg regulates the assembly of actin by binding to the barbed end (preferred end or rapidly growing end) of the actin filament. The 45,000-mol-wt protein-actin complex can thus be categorized as a capping protein.     

Rate of treadmilling of actin filaments in vitro

Actin filaments capped at the barbed ends were formed by polymerizing monomeric actin onto a gelsolin-actin complex. The rate of depolymerization and polymerization of the pointed ends was determined by diluting gelsolin-capped actin filaments into various concentrations of monomeric actin. Under the conditions of the experiments (100 mM-KCl, 2 mM-MgCl2 at 37 degrees C) the rate constant of dissociation of subunits both from a shortening and a lengthening filament was found to be 0.21 s-1. As the rate of dissociation of subunits from the slow pointed end determines the rate of treadmilling, it is concluded that actin filaments treadmill with a rate of about 2 micron/h.     

Phosphorylation by actin kinase of the pointed end domain on the actin molecule.

Fragmin from plasmodium of Physarum polycephalum binds G-actin and severs F-actin in the presence of Ca2+ over 10(-6) M. The fragmin-actin complex consisting of fragmin and G-actin nucleates actin polymerization and caps the barbed (fast growing) end of F-actin, regardless of the concentrations of Ca2+, and the actin filaments are shortened. Actin kinase purified from plasmodium abolishes the nucleation and capping activities of the complex by phosphorylating actin of the fragmin-actin complex (Furuhashi, K., and Hatano, S. (1990) J. Cell. Biol. 111, 1081-1087). This inactivation of the complex leads to production of long actin filaments. We obtained evidence that Physarum actin is phosphorylated by actin kinase at Thr-201, and probably at Thr-202 and/or Thr-203, with 1 mol of phosphate distributed among them. This finding raises the possibility that the site of phosphorylation, Thr-201 to Thr-203, is positioned on the pointed (slow growing) end domain of the actin molecule, because growth of actin filaments from the fragmin-actin complex occurs only from the pointed end. These observations are consistent with a model of the three-dimensional structure of G-actin. Inactivation of the fragmen-actin complex may follow phosphorylation of the pointed end domain of actin.     

Control of actin filament length and turnover by actin depolymerizing factor (ADF/cofilin) in the presence of capping proteins and ARP2/3 complex.

The effect of Arabidopsis thaliana ADF1 and human ADF on the number of filaments in F-actin solutions has been examined using a seeded polymerization assay. ADF did not sever filaments in a catalytic fashion, but decreased the steady-state length distribution of actin filaments in correlation with its effect on actin dynamics. The increase in filament number was modest as compared with the large increase in filament turnover. ADF did not decrease the length of filaments shorter than 1 micrometer. ADF promoted the rapid turnover of gelsolin-capped filaments in a manner dependent on the number of pointed ends. To explain these results, we propose that, as a consequence of the cooperative binding of ADF to F-actin, two populations of energetically different filaments coexist in solution pending a flux of subunits from one to the other. The ADF-decorated filaments depolymerize rapidly from their pointed ends, while undecorated filaments polymerize. ADF also promotes rapid turnover of gelsolin-capped filaments in the presence of the pointed end capper Arp2/3 complex. It is shown that the Arp2/3 complex steadily generates new barbed ends in solutions of gelsolin-capped filaments, which represents an important aspect of its function in actin-based motility.     

Difference in polymerization and steady-state dynamics of free and gelsolin-capped filaments formed by alpha- and beta-isoactins

The polymerization of scallop β-like actin is significantly slower than that of skeletal muscle α-actin. To reveal which steps of polymerization contribute to this difference, we estimated the efficiency of nucleation of the two actins, the rates of filament elongation at spontaneous and gelsolin-nucleated polymerization and the turnover rates of the filament subunits at steady-state. Scallop actin nucleated nearly twice less efficient than rabbit actin. In actin filaments with free ends, when dynamics at the barbed ends overrides that at the pointed ends, the relative association rate constants of α- and β-actin were similar, whereas the relative dissociation rate constant of β-ATP-actin subunits was 2- to 3-fold higher than that of α-actin. The 2- to 3-fold faster polymerization of skeletal muscle versus scallop Ca-actin was preserved with gelsolin-capped actin filaments when only polymerization at the pointed end is possible. With gelsolin-induced polymerization, the rate constants of dissociation of ATP-actin subunits from the pointed ends were similar, while the association rate constant of β-actin to the pointed filament ends was twice lower than that of α-actin. This difference may be of physiological relevance for functional intracellular sorting of actin isoforms.     

EPLIN regulates actin dynamics by cross-linking and stabilizing filaments

Epithelial protein lost in neoplasm (EPLIN) is a cytoskeleton-associated protein encoded by a gene that is down-regulated in transformed cells. EPLIN increases the number and size of actin stress fibers and inhibits membrane ruffling induced by Rac. EPLIN has at least two actin binding sites. Purified recombinant EPLIN inhibits actin filament depolymerization and cross-links filaments in bundles. EPLIN does not affect the kinetics of spontaneous actin polymerization or elongation at the barbed end, but inhibits branching nucleation of actin filaments by Arp2/3 complex. Side binding activity may stabilize filaments and account for the inhibition of nucleation mediated by Arp2/3 complex. We propose that EPLIN promotes the formation of stable actin filament structures such as stress fibers at the expense of more dynamic actin filament structures such as membrane ruffles. Reduced expression of EPLIN may contribute to the motility of invasive tumor cells.     

The Effect of Tropomyosin Mutations on Actin-Tropomyosin Binding: In Search of Lost Time

The initial binding of tropomyosin onto actin filaments and then its polymerization into continuous cables on the filament surface must be precisely tuned to overall thin-filament structure, function, and performance. Low-affinity interaction of tropomyosin with actin has to be sufficiently strong to localize the tropomyosin on actin, yet not so tight that regulatory movement on filaments is curtailed. Likewise, head-to-tail association of tropomyosin molecules must be favorable enough to promote tropomyosin cable formation but not so tenacious that polymerization precedes filament binding. Arguably, little molecular detail on early tropomyosin binding steps has been revealed since Wegner’s seminal studies on filament assembly almost 40 years ago. Thus, interpretation of mutation-based actin-tropomyosin binding anomalies leading to cardiomyopathies cannot be described fully. In vitro, tropomyosin binding is masked by explosive tropomyosin polymerization once cable formation is initiated on actin filaments. In contrast, in silico analysis, characterizing molecular dynamics simulations of single wild-type and mutant tropomyosin molecules on F-actin, is not complicated by tropomyosin polymerization at all. In fact, molecular dynamics performed here demonstrates that a midpiece tropomyosin domain is essential for normal actin-tropomyosin interaction and that this interaction is strictly conserved in a number of tropomyosin mutant species. Elsewhere along these mutant molecules, twisting and bending corrupts the tropomyosin superhelices as they “lose their grip” on F-actin. We propose that residual interactions displayed by these mutant tropomyosin structures with actin mimic ones that occur in early stages of thin-filament generation, as if the mutants are recapitulating the assembly process but in reverse. We conclude therefore that an initial binding step in tropomyosin assembly onto actin involves interaction of the essential centrally located domain.     

Conformational changes in actin filaments induced by formin binding to the barbed end

Formins bind actin filaments and play an essential role in the regulation of the actin cytoskeleton. In this work we describe details of the formin-induced conformational changes in actin filaments by fluorescence-lifetime and anisotropy-decay experiments. The results show that the binding of the formin homology 2 domain of a mammalian formin (mouse mDia1) to actin filaments resulted in a less rigid protein structure in the microenvironment of the Cys374 of actin, weakening of the interactions between neighboring actin protomers, and greater overall flexibility of the actin filaments. The formin effect is smaller at greater ionic strength. The results show that formin binding to the barbed end of actin filaments is responsible for the increase of flexibility of actin filaments. One formin dimer can affect the dynamic properties of an entire filament. Analyses of the results obtained at various formin/actin concentration ratios indicate that at least 160 actin protomers are affected by the binding of a single formin dimer to the barbed end of a filament.     

Regulation of Actin by Ion-Linked Equilibria

Actin assembly, filament mechanical properties, and interactions with regulatory proteins depend on the types and concentrations of salts in solution. Salts modulate actin through both nonspecific electrostatic effects and specific binding to discrete sites. Multiple cation-binding site classes spanning a broad range of affinities (nanomolar to millimolar) have been identified on actin monomers and filaments. This review focuses on discrete, low-affinity cation-binding interactions that drive polymerization, regulate filament-bending mechanics, and modulate interactions with regulatory proteins. Cation binding may be perturbed by actin post-translational modifications and linked equilibria. Partial cation occupancy under physiological and commonly used in vitro solution conditions likely contribute to filament mechanical heterogeneity and structural polymorphism. Site-specific cation-binding residues are conserved in Arp2 and Arp3, and may play a role in Arp2/3 complex activation and actin-filament branching activity. Actin-salt interactions demonstrate the relevance of ion-linked equilibria in the operation and regulation of complex biological systems.     

ADP-ribosylated actin caps the barbed ends of actin filaments

The mode of action on actin polymerization of skeletal muscle actin ADP-ribosylated on arginine 177 by perfringens iota toxin was investigated. ADP-ribosylated actin decreased the rate of nucleated actin polymerization at substoichiometric ratios of ADP-ribosylated actin to monomeric actin. ADP-ribosylated actin did not tend to copolymerize with actin. Actin filaments were depolymerized by the addition of ADP-ribosylated actin. The maximal monomer concentration reached by addition of ADP-ribosylated actin was similar to the critical concentration of the pointed ends of actin filaments. ADP-ribosylated actin had no effect on the rate of polymerization of gelsolin-capped actin filaments which polymerize at the pointed ends. The results suggest that ADP-ribosylated actin acts as a capping protein which binds to the barbed ends of actin filaments to inhibit polymerization. Based on an analysis of the depolymerizing effect of ADP-ribosylated actin, the equilibrium constant for binding of ADP-ribosylated actin to the barbed ends of actin filaments was determined to be about 10(8) M-1. As actin is ADP-ribosylated by perfringens iota toxin and by botulinum C2 toxin, it appears that conversion of actin into a capping protein by ADP-ribosylation is a pathophysiological reaction catalyzed by bacterial toxins which ultimately leads to inhibition of actin assembly.     

Mechanism of action of Acanthamoeba profilin: demonstration of actin species specificity and regulation by micromolar concentrations of MgCl2

Acanthamoeba profilin strongly inhibits in a concentration-dependent fashion the rate and extent of Acanthamoeba actin polymerization in 50 mM KCl. The lag phase is prolonged indicating reduction in the rate of nucleus formation. The elongation rates at both the barbed and pointed ends of growing filaments are inhibited. At steady state, profilin increases the critical concentration for polymerization but has no effect on the reduced viscosity above the critical concentration. Addition of profilin to polymerized actin causes it to depolymerize until a new steady-state, dependent on profilin concentration, is achieved. These effects of profilin can be explained by the formation of a 1:1 complex with actin with a dissociation constant of 1 to 4 microM. MgCl2 strongly inhibits these effects of profilin, most likely by binding to the high-affinity divalent cation site on the actin. Acanthamoeba profilin has similar but weaker effects on muscle actin, requiring 5 to 10 times more profilin than with amoeba actin.