Dynamic polymorphism of actin as activation mechanism for cell motility

Actin filament dynamics are crucial in cell motility. Actin filaments, and their bundles, networks, and gels assemble and disassemble spontaneously according to thermodynamic rules. These dynamically changing structures of actin are harnessed for some of its functions in cells. The actin systems respond to external signals, forces, or environments by biasing the fluctuation of actin assembly structures. In this study, dynamic conformation of actin molecules was studied by monitoring conformational dynamics of actin molecules at the single molecule level in real time. Actin conformation spontaneously fluctuates between multiple conformational states. Regarding myosin motility, the dynamic equilibrium of actin conformation was interpreted as between states that activates and inhibits the motility. The binding of myosin to actin filaments activates myosin motility by shifting the conformational fluctuation of actin towards the state that activates the motility. Thus, the activation mechanism based on thermal fluctuation is suggested at molecular level as well as at cellular level.     

The Role of Structural Dynamics of Actin in Class-Specific Myosin Motility

The structural dynamics of actin, including the tilting motion between the small and large domains, are essential for proper interactions with actin-binding proteins. Gly146 is situated at the hinge between the two domains, and we previously showed that a G146V mutation leads to severe motility defects in skeletal myosin but has no effect on motility of myosin V. The present study tested the hypothesis that G146V mutation impaired rotation between the two domains, leading to such functional defects. First, our study showed that depolymerization of G146V filaments was slower than that of wild-type filaments. This result is consistent with the distinction of structural states of G146V filaments from those of the wild type, considering the recent report that stabilization of actin filaments involves rotation of the two domains. Next, we measured intramolecular FRET efficiencies between two fluorophores in the two domains with or without skeletal muscle heavy meromyosin or the heavy meromyosin equivalent of myosin V in the presence of ATP. Single-molecule FRET measurements showed that the conformations of actin subunits of control and G146V actin filaments were different in the presence of skeletal muscle heavy meromyosin. This altered conformation of G146V subunits may lead to motility defects in myosin II. In contrast, distributions of FRET efficiencies of control and G146V subunits were similar in the presence of myosin V, consistent with the lack of motility defects in G146V actin with myosin V. The distribution of FRET efficiencies in the presence of myosin V was different from that in the presence of skeletal muscle heavy meromyosin, implying that the roles of actin conformation in myosin motility depend on the type of myosin.     

Electron microscopic study of actin polymerization in airway smooth muscle

Actin polymerization as part of the normal smooth muscle response to various stimuli has been reported. The actin dynamics are believed to be necessary for cytoskeletal remodeling in smooth muscle in its adaptation to external stress and strain and for maintenance of optimal contractility. We have shown in our previous studies in airway smooth muscle that myosins polymerized in response to contractile activation as well as to adaptation at longer cell lengths. We postulated that the same response could be elicited from actins under the same conditions. In the present study, actin filament formation was quantified electron microscopically in cell cross sections. Nanometer resolution allowed us to examine regional distribution of filaments in a cell cross section. Airway smooth muscle bundles were fixed in relaxed and activated states at two lengths; muscle preparations were also fixed after a period of oscillatory strain, a condition known to cause depolymerization of myosin filaments. The results indicate that contractile activation and increased cell length nonsynergistically enhanced actin polymerization; the extent of actin polymerization was substantially less than that of myosin polymerization. Oscillatory strain increased thin filament formation. Although thin filament density was found higher in cytoplasmic areas near dense bodies, contractile activation did not preferentially enhance actin polymerization in these areas. It is concluded that actin thin filaments are dynamic structures whose length and number are regulated by the cell in response to changes in extracellular environment and that polymerization and depolymerization of thin filaments occur uniformly across the whole cell cross section.     

Computer-assisted tracking of actin filament motility.

In vitro motility assays, in which fluorescently labeled actin filaments are propelled by myosin molecules adhered to a glass coverslip, require that actin filament velocity be determined. We have developed a computer-assisted filament tracking system that reduced the analysis time, minimized investigator bias, and provided greater accuracy in locating actin filaments in video images. The tracking routine successfully tracked filaments under experimental conditions where filament density, size, and extent of photobleaching varied dramatically. Videotaped images of actin filament motility were digitized and processed to enhance filament image contrast relative to background. Once processed, filament images were cross correlated between frames and a filament path was determined. The changes in filament centroid or center position between video frames were then used to calculate filament velocity. The tracking routine performance was evaluated and the sources of noise that contributed to errors in velocity were identified and quantified. Errors originated in algorithms for filament centroid determination and in the choice of sampling interval between video frames. With knowledge of these error sources, the investigator can maximize the accuracy of the velocity calculation through access to user-definable computer program parameters.     

Bending flexibility of actin filaments during motor-induced sliding

Muscle contraction and other forms of cell motility occur as a result of cyclic interactions between myosin molecules and actin filaments. Force generation is generally attributed to ATP-driven structural changes in myosin, whereas a passive role is ascribed to actin. However, some results challenge this view, predicting structural changes in actin during motor activity, e.g., when the actin filaments slide on a myosin-coated surface in vitro. Here, we analyzed statistical properties of the sliding filament paths, allowing us to detect changes of this type. It is interesting to note that evidence for substantial structural changes that led to increased bending flexibility of the filaments was found in phalloidin-stabilized, but not in phalloidin-free, actin filaments. The results are in accordance with the idea that a high-flexibility structural state of actin is a prerequisite for force production, but not the idea that a low-to-high flexibility transition of the actin filament should be an important component of the force-generating step per se. Finally, our data challenge the general view that phalloidin-stabilized filaments behave as native actin filaments in their interaction with myosin. This has important implications, since phalloidin stabilization is a routine procedure in most studies of actomyosin function.     

Structural dynamics of actin during active interaction with myosin: different effects of weakly and strongly bound myosin heads

We have used optical spectroscopy (transient phosphorescence anisotropy, TPA, and fluorescence resonance energy transfer, FRET) to detect the effects of weakly bound myosin S1 on actin during the actomyosin ATPase cycle. The changes in actin were reported by (a) a phosphorescent probe (ErIA) attached to Cys 374 and (b) a FRET donor-acceptor pair, IAEDANS attached to Cys 374 and a nucleotide analogue (TNPADP) in the nucleotide-binding cleft. Strong interactions were detected in the absence of ATP, and weak interactions were detected in the presence of ATP or its slowly hydrolyzed analogue ATP-gamma-S, under conditions where a significant fraction of weakly bound acto-S1 complex was present and the rate of nucleotide hydrolysis was low enough to enable steady-state measurements. The results show that actin in the weakly bound complex with S1 assumes a new structural state in which (a) the actin filament has microsecond rotational dynamics intermediate between that of free actin and the strongly bound complex and (b) S1-induced changes are not propagated along the actin filament, in contrast to the highly cooperative changes due to the strongly bound complex. We propose that the transition on the acto-myosin interface from weak to strong binding is accompanied by transitions in the structural dynamics of actin parallel to transitions in the dynamics of interacting myosin heads.     

Proteolytic cleavage of actin within the DNase-I-binding loop changes the conformation of F-actin and its sensitivity to myosin binding

Effects of subtilisin cleavage of actin between residues 47 and 48 on the conformation of F-actin and on its changes occurring upon binding of myosin subfragment-1 (S1) were investigated by measuring polarized fluorescence from rhodamine-phalloidin- or 1, 5-IAEDANS-labeled actin filaments reconstructed from intact or subtilisin-cleaved actin in myosin-free muscle fibers (ghost fibers). In separate experiments, polarized fluorescence from 1, 5-IAEDANS-labeled S1 bound to non-labeled actin filaments in ghost fibers was measured. The measurements revealed differences between the filaments of cleaved and intact actin in the orientation of rhodamine probe on the rhodamine-phalloidin-labeled filaments, orientation and mobility of the C-terminus of actin, filament flexibility, and orientation and mobility of the myosin heads bound to F-actin. The changes in the filament flexibility and orientation of the actin-bound fluorophores produced by S1 binding to actin in the absence of ATP were substantially diminished by subtilisin cleavage of actin. The results suggest that loop 38-52 plays an important role, not only in maintaining the F-actin structure, but also in the conformational transitions in actin accompanying the strong binding of the myosin heads that may be essential for the generation of force and movement during actin-myosin interaction.     

The effects of smooth muscle caldesmon on actin filament motility.

The movement of reconstituted thin filaments over an immobilized surface of thiophosphorylated smooth muscle myosin was examined using an in vitro motility assay. Reconstituted thin filaments contained actin, tropomyosin, and either purified chicken gizzard caldesmon or the purified COOH-terminal actin-binding fragment of caldesmon. Control actin-tropomyosin filaments moved at a velocity of 2.3 +/- 0.5 microns/s. Neither intact caldesmon nor the COOH-terminal fragment, when maintained in the monomeric form by treatment with 10 mM dithiothreitol, had any effect on filament velocity; and yet both were potent inhibitors of actin-activated myosin ATPase activity, indicating that caldesmon primarily inhibits myosin binding as reported by Chalovich et al. (Chalovich, J. M., Hemric, M. E., and Velaz, L. (1990) Ann. N. Y. Acad. Sci. 599, 85-99). Inhibition of filament motion was, however, observed under conditions where cross-linking of caldesmon via disulfide bridges was present. To determine if monomeric caldesmon could "tether" actin filaments to the myosin surface by forming an actin-caldesmon-myosin complex as suggested by Chalovich et al., we looked for caldesmon-dependent filament binding and motility under conditions (80 mM KCl) where filament binding to myosin is weak and motility is not normally seen. At caldesmon concentrations > or = 0.26 microM, actin filament binding was increased and filament motion (2.6 +/- 0.6 microns/s) was observed. The enhanced motility seen with intact caldesmon was not observed with the addition of up to 26 microM COOH-terminal fragment. Moreover, a molar excess of the COOH-terminal fragment competitively reversed the enhanced binding seen with intact caldesmon. These results show that tethering of actin filaments to myosin by the formation of an actin-caldesmon-myosin complex enhanced productive acto-myosin interaction without placing a significant mechanical load on the moving filaments.     

Formation and destabilization of actin filaments with tetramethylrhodamine-modified actin

Actin labeling at Cys(374) with tethramethylrhodamine derivatives (TMR-actin) has been widely used for direct observation of the in vitro filaments growth, branching, and treadmilling, as well as for the in vivo visualization of actin cytoskeleton. The advantage of TMR-actin is that it does not lock actin in filaments (as rhodamine-phalloidin does), possibly allowing for its use in investigating the dynamic assembly behavior of actin polymers. Although it is established that TMR-actin alone is polymerization incompetent, the impact of its copolymerization with unlabeled actin on filament structure and dynamics has not been tested yet. In this study, we show that TMR-actin perturbs the filaments structure when copolymerized with unlabeled actin; the resulting filaments are more fragile and shorter than the control filaments. Due to the increased severing of copolymer filaments, TMR-actin accelerates the polymerization of unlabeled actin in solution also at mole ratios lower than those used in most fluorescence microscopy experiments. The destabilizing and severing effect of TMR-actin is countered by filament stabilizing factors, phalloidin, S1, and tropomyosin. These results point to an analogy between the effects of TMR-actin and severing proteins on F-actin, and imply that TMR-actin may be inappropriate for investigations of actin filaments dynamics.     

Tropomyosin and actin isoforms modulate the localization of tropomyosin strands on actin filaments

Tropomyosin is present in virtually all eucaryotic cells, where it functions to modulate actin-myosin interaction and to stabilize actin filament structure. In striated muscle, tropomyosin regulates contractility by sterically blocking myosin-binding sites on actin in the relaxed state. On activation, tropomyosin moves away from these sites in two steps, one induced by Ca(2+) binding to troponin and a second by the binding of myosin to actin. In smooth muscle and non-muscle cells, where troponin is absent, the precise role and structural dynamics of tropomyosin on actin are poorly understood. Here, the location of tropomyosin on F-actin filaments free of troponin and other actin-binding proteins was determined to better understand the structural basis of its functioning in muscle and non-muscle cells. Using electron microscopy and three-dimensional image reconstruction, the association of a diverse set of wild-type and mutant actin and tropomyosin isoforms, from both muscle and non-muscle sources, was investigated. Tropomyosin position on actin appeared to be defined by two sets of binding interactions and tropomyosin localized on either the inner or the outer domain of actin, depending on the specific actin or tropomyosin isoform examined. Since these equilibrium positions depended on minor amino acid sequence differences among isoforms, we conclude that the energy barrier between thin filament states is small. Our results imply that, in striated muscles, troponin and myosin serve to stabilize tropomyosin in inhibitory and activating states, respectively. In addition, they are consistent with tropomyosin-dependent cooperative switching on and off of actomyosin-based motility. Finally, the locations of tropomyosin that we have determined suggest the possibility of significant competition between tropomyosin and other cellular actin-binding proteins. Based on these results, we present a general framework for tropomyosin modulation of motility and cytoskeletal modelling.     

G146V mutation at the hinge region of actin reveals a myosin class-specific requirement of actin conformations for motility

The G146V mutation in actin is dominant lethal in yeast. G146V actin filaments bind cofilin only minimally, presumably because cofilin binding requires the large and small actin domains to twist with respect to one another around the hinge region containing Gly-146, and the mutation inhibits that twisting motion. A number of studies have suggested that force generation by myosin also requires actin filaments to undergo conformational changes. This prompted us to examine the effects of the G146V mutation on myosin motility. When compared with wild-type actin filaments, G146V filaments showed a 78% slower gliding velocity and a 70% smaller stall force on surfaces coated with skeletal heavy meromyosin. In contrast, the G146V mutation had no effect on either gliding velocity or stall force on myosin V surfaces. Kinetic analyses of actin-myosin binding and ATPase activity indicated that the weaker affinity of actin filaments for myosin heads carrying ADP, as well as reduced actin-activated ATPase activity, are the cause of the diminished motility seen with skeletal myosin. Interestingly, the G146V mutation disrupted cooperative binding of myosin II heads to actin filaments. These data suggest that myosin-induced conformational changes in the actin filaments, presumably around the hinge region, are involved in mediating the motility of skeletal myosin but not myosin V and that the specific structural requirements for the actin subunits, and thus the mechanism of motility, differ among myosin classes.     

A mechanochemical model of actin filaments

In eukaryotic cells, actin filaments are involved in important processes such as motility, division, cell shape regulation, contractility, and mechanosensation. Actin filaments are polymerized chains of monomers, which themselves undergo a range of chemical events such as ATP hydrolysis, polymerization, and depolymerization. When forces are applied to F-actin, in addition to filament mechanical deformations, the applied force must also influence chemical events in the filament. We develop an intermediate-scale model of actin filaments that combines actin chemistry with filament-level deformations. The model is able to compute mechanical responses of F-actin during bending and stretching. The model also describes the interplay between ATP hydrolysis and filament deformations, including possible force-induced chemical state changes of actin monomers in the filament. The model can also be used to model the action of several actin-associated proteins, and for large-scale simulation of F-actin networks. All together, our model shows that mechanics and chemistry must be considered together to understand cytoskeletal dynamics in living cells.     

PC phosphorylation increases the ability of AFAP-110 to cross-link actin filaments

The actin filament-associated protein and Src-binding partner, AFAP-110, is an adaptor protein that links signaling molecules to actin filaments. AFAP-110 binds actin filaments directly and multimerizes through a leucine zipper motif. Cellular signals downstream of Src(527F) can regulate multimerization. Here, we determined recombinant AFAP-110 (rAFAP-110)-bound actin filaments cooperatively, through a lateral association. We demonstrate rAFAP-110 has the capability to cross-link actin filaments, and this ability is dependent on the integrity of the carboxy terminal actin binding domain. Deletion of the leucine zipper motif or PKC phosphorylation affected AFAP-110's conformation, which correlated with changes in multimerization and increased the capability of rAFAP-110 to cross-link actin filaments. AFAP-110 is both a substrate and binding partner of PKC. On PKC activation, stress filament organization is lost, motility structures form, and AFAP-110 colocalizes strongly with motility structures. Expression of a deletion mutant of AFAP-110 that is unable to bind PKC blocked the effect of PMA on actin filaments. We hypothesize that upon PKC activation, AFAP-110 can be cooperatively recruited to newly forming actin filaments, like those that exist in cell motility structures, and that PKC phosphorylation effects a conformational change that may enable AFAP-110 to promote actin filament cross-linking at the cell membrane.     

Myosin II filament assemblies in the active lamella of fibroblasts: their morphogenesis and role in the formation of actin filament bundles

The morphogenesis of myosin II structures in active lamella undergoing net protrusion was analyzed by correlative fluorescence and electron microscopy. In rat embryo fibroblasts (REF 52) microinjected with tetramethylrhodamine-myosin II, nascent myosin spots formed close to the active edge during periods of retraction and then elongated into wavy ribbons of uniform width. The spots and ribbons initially behaved as distinct structural entities but subsequently aligned with each other in a sarcomeric-like pattern. Electron microscopy established that the spots and ribbons consisted of bipolar minifilaments associated with each other at their head-containing ends and arranged in a single row in an "open" zig-zag conformation or as a "closed" parallel stack. Ribbons also contacted each other in a nonsarcomeric, network-like arrangement as described previously (Verkhovsky and Borisy, 1993. J. Cell Biol. 123:637-652). Myosin ribbons were particularly pronounced in REF 52 cells, but small ribbons and networks were found also in a range of other mammalian cells. At the edge of the cell, individual spots and open ribbons were associated with relatively disordered actin filaments. Further from the edge, myosin filament alignment increased in parallel with the development of actin bundles. In actin bundles, the actin cross-linking protein, alpha-actinin, was excluded from sites of myosin localization but concentrated in paired sites flanking each myosin ribbon, suggesting that myosin filament association may initiate a pathway for the formation of actin filament bundles. We propose that zig-zag assemblies of myosin II filaments induce the formation of actin bundles by pulling on an actin filament network and that co-alignment of actin and myosin filaments proceeds via folding of myosin II filament assemblies in an accordion-like fashion.     

Endothelial actin cytoskeleton remodeling during mechanostimulation with fluid shear stress

Fluid shear stress stimulation induces endothelial cells to elongate and align in the direction of applied flow. Using the complementary techniques of photoactivation of fluorescence and fluorescence recovery after photobleaching, we have characterized endothelial actin cytoskeleton dynamics during the alignment process in response to steady laminar fluid flow and have correlated these results to motility. Alignment requires 24 h of exposure to fluid flow, but the cells respond within minutes to flow and diminish their movement by 50%. Although movement slows, the actin filament turnover rate increases threefold and the percentage of total actin in the polymerized state decreases by 34%, accelerating actin filament remodeling in individual cells within a confluent endothelial monolayer subjected to flow to levels used by dispersed nonconfluent cells under static conditions for rapid movement. Temporally, the rapid decrease in filamentous actin shortly after flow stimulation is preceded by an increase in actin filament turnover, revealing that the earliest phase of the actin cytoskeletal response to shear stress is net cytoskeletal depolymerization. However, unlike static cells, in which cell motility correlates positively with the rate of filament turnover and negatively with the amount polymerized actin, the decoupling of enhanced motility from enhanced actin dynamics after shear stress stimulation supports the notion that actin remodeling under these conditions favors cytoskeletal remodeling for shape change over locomotion. Hours later, motility returned to pre-shear stress levels but actin remodeling remained highly dynamic in many cells after alignment, suggesting continual cell shape optimization. We conclude that shear stress initiates a cytoplasmic actin-remodeling response that is used for endothelial cell shape change instead of bulk cell translocation. atherosclerosis; cytoskeletal dynamics; endothelial cells; mechanotransduction     

Actin filament uncapping localizes to ruffling lamellae and rocketing vesicles

Regulated actin filament assembly is critical for eukaryotic cell physiology. Actin filaments are polar structures, and those with free high affinity or barbed ends are crucial for actin dynamics and cell motility. Actin filament barbed-end-capping proteins inhibit filament elongation after binding, and their regulated disassociation is proposed to provide a source of free filament ends to drive processes dependent on actin polymerization. To examine whether dissociation of actin filament capping proteins occurs with the correct spatio-temporal constraints to contribute to regulated actin assembly in live cells, I measured the dissociation of an actin capping protein, gelsolin, from actin in cells using a variation of fluorescence resonance energy transfer (FRET). Uncapping was found to occur in cells at sites of active actin assembly, including protruding lamellae and rocketing vesicles, with the correct spatio-temporal properties to provide sites of actin filament polymerization during protrusion. These observations are consistent with models where uncapping of existing filaments provides sites of actin filament elongation.     

The role of actin turnover in retrograde actin network flow in neuronal growth cones

The balance of actin filament polymerization and depolymerization maintains a steady state network treadmill in neuronal growth cones essential for motility and guidance. Here we have investigated the connection between depolymerization and treadmilling dynamics. We show that polymerization-competent barbed ends are concentrated at the leading edge and depolymerization is distributed throughout the peripheral domain. We found a high-to-low G-actin gradient between peripheral and central domains. Inhibiting turnover with jasplakinolide collapsed this gradient and lowered leading edge barbed end density. Ultrastructural analysis showed dramatic reduction of leading edge actin filament density and filament accumulation in central regions. Live cell imaging revealed that the leading edge retracted even as retrograde actin flow rate decreased exponentially. Inhibition of myosin II activity before jasplakinolide treatment lowered baseline retrograde flow rates and prevented leading edge retraction. Myosin II activity preferentially affected filopodial bundle disassembly distinct from the global effects of jasplakinolide on network turnover. We propose that growth cone retraction following turnover inhibition resulted from the persistence of myosin II contractility even as leading edge assembly rates decreased. The buildup of actin filaments in central regions combined with monomer depletion and reduced polymerization from barbed ends suggests a mechanism for the observed exponential decay in actin retrograde flow. Our results show that growth cone motility is critically dependent on continuous disassembly of the peripheral actin network.     

The tail of myosin reduces actin filament velocity in the in vitro motility assay

It has been observed that heavy meromyosin (HMM) propels actin filaments to higher velocities than native myosin in the in vitro motility assay, yet the reason for this difference has remained unexplained. Since the major difference between these two proteins is the presence of the tail in native myosin, we tested the hypothesis that unknown interactions between actin and the tail (LMM) slow motility in native myosin. Chymotryptic HMM and LMM were mixed in a range of molar ratios (0-5 LMM/HMM) and compared to native rat skeletal myosin in the in vitro motility assay at 30 degrees C. Increasing proportions of LMM to HMM slowed actin filament velocities, becoming equivalent to native myosin at a ratio of 3 LMM/HMM. NH4+ -ATPase assays demonstrated that HMM concentrations on the surface were constant and independent of LMM concentration, arguing against a simple displacement mechanism. Relationships between velocity and the number of available heads suggested that the duty cycle of HMM was not altered by the presence of LMM. HMM prepared with a lower chymotrypsin concentration and with very short digestion times moved actin at the same high velocity. The difference between velocities of actin filament propelled by HMM and HMM/LMM decreased with increasing ionic strength, suggesting that ionic bonds between myosin tail and actin filaments may play a role in slowing filament velocity. These data suggest the high velocities of actin filaments over HMM result from the absence of drag generated by the myosin tail, and not from proteolytic nicking of the motor domain.     

Crowded surfaces change annealing dynamics of actin filaments

Changes in cell shape that occur in many cellular processes are thought to arise from polymerization of actin filaments near the cell membrane. End-to-end annealing of actin filaments is believed to play only a minor role in this process, as annealing in solution was shown to be a slow process, which is not typical for a bimolecular reaction, its rate constant decreasing over time, being inversely proportional to the filament length. Furthermore, in vitro studies on f-actin solutions were found to display an exponential steady-state length distribution. In the cell, many physiologically important parameters, such as mechanical strength or viscoelastic response are a direct function of the physical properties of the underlying actin cytoskeleton, such as actin filament length distribution and dynamics. How the underlying physical parameters of the actin cytoskeleton may be influenced by the cell surface or molecular crowding remains poorly understood. Using total internal reflection fluorescence (TIRF) microscopy we reinvestigated actin end-to-end annealing in vitro in a more realistic environment. We studied the process near a hydrophilic surface together with crowding agents, in order to mimic the physiological media near the cell membrane, which has substantial amounts of macromolecules present. We find that actin end-to-end annealing changes in three ways near a crowded hydrophilic surface as compared to solution. First the annealing rate becomes a factor of 20 faster than in solution. Second the rate of annealing becomes typical of a bimolecular reaction, shows no length dependence and is basically just a function of the square of the concentration of ends. Lastly the length distribution is Gaussian throughout the entire annealing process. This implicates that dynamic rearrangement of actin filaments by annealing near the leading edge of the cell, could change physical parameters like the mechanical response and contribute significantly to cell motility.     

Cofilin increases the torsional flexibility and dynamics of actin filaments

We have measured the effects of cofilin on the conformation and dynamics of actin filaments labeled at Cys374 with erythrosin-iodoacetemide (ErIA), using time-resolved phosphorescence anisotropy (TPA). Cofilin quenches the phosphorescence intensity of actin-bound ErIA, indicating that binding changes the local environment of the probe. The cofilin concentration-dependence of the phosphorescence intensity is sigmoidal, consistent with cooperative actin filament binding. Model-independent analysis of the anisotropies indicates that cofilin increases the rates of the microsecond rotational motions of actin. In contrast to the reduction in phosphorescence intensity, the changes in the rates of rotational motions display non-nearest-neighbor cooperative interactions and saturate at substoichiometric cofilin binding densities. Detailed analysis of the TPA decays indicates that cofilin decreases the torsional rigidity (C) of actin, increasing the thermally driven root-mean-square torsional angle between adjacent filament subunits from approximately 4 degrees (C = 2.30 x 10(-27) Nm2 radian(-1)) to approximately 17 degrees (C = 0.13 x 10(-27) Nm2 radian(-1)) at 25 degrees C. We favor a mechanism in which cofilin binding shifts the equilibrium between thermal ErIA-actin filament conformers, and facilitates two distinct structural changes in actin. One is local in nature, which affects the structure of actin's C terminus and is likely to mediate nearest-neighbor cooperative binding and filament severing. The second is a change in the internal dynamics of actin, which displays non-nearest-neighbor cooperativity and increases the torsional flexibility of filaments. The long-range effects of cofilin on the torsional dynamics of actin may accelerate P(i) release from filaments and modulate interactions with other regulatory actin filament binding proteins.