Molecular Origins of Cofilin-Linked Changes in Actin Filament Mechanics

Abstract

The actin regulatory protein cofilin plays a central role in actin assembly dynamics by severing filaments and increasing the concentration of ends from which subunits add and dissociate. Cofilin binding modifies the average structure and mechanical properties of actin filaments, thereby promoting fragmentation of partially decorated filaments at boundaries of bare and cofilin-decorated segments. Despite extensive evidence for cofilin-dependent changes in filament structure and mechanics, it is unclear how the two processes are linked at the molecular level. Here, we use molecular dynamics simulations and coarse-grained analyses to evaluate the molecular origins of the changes in filament compliance due to cofilin binding. Filament subunits with bound cofilin are less flat and maintain a significantly more open nucleotide cleft than bare filament subunits. Decorated filament segments are less twisted, thinner (considering only actin), and less connected than their bare counterparts, which lowers the filament bending persistence length and torsional stiffness. Using coarse-graining as an analysis method reveals that cofilin binding increases the average distance between the adjacent long-axis filament subunit, thereby weakening their interaction. In contrast, a fraction of lateral filament subunit contacts are closer and presumably stronger with cofilin binding. A cofilactin interface contact identified by cryo-electron microscopy is unstable during simulations carried out at 310K, suggesting that this particular interaction may be short-lived at ambient temperatures. These results reveal the molecular origins of cofilin-dependent changes in actin filament mechanics that may promote filament severing.

Site-specific cation release drives actin filament severing by vertebrate cofilin

“…These observations suggest that stiffness cation occupancy is coupled to the D-loop conformation and radial distribution of long-axis contacts between adjacent filament subunits ( Fig. 1 D, E, G, and H), consistent with a role for the D-loop conformation in controlling filament mechanical properties (10,(24)(25)(26).…”

“…Several observations argue against this interpretation. Salts and electrolytes weaken ionic bridges between charged protein residues (36, 37), which would compromise intersubunit contacts and lower the filament stiffness (10,38). In contrast, Mg 2+ enhances intersubunit contacts (Fig.…”

“…Filaments with bound cofilin have altered twist (8,9) and are more compliant in both bending and twisting than bare filaments (10)(11)(12)(13). It has been suggested that deformations in filament shape promote fragmentation at or near regions of topological and mechanical discontinuities, such as boundaries between bare and cofilin-decorated segments along partially decorated filaments (5,12,(14)(15)(16)(17)(18).…”

Single-molecule imaging and kinetic analysis of cooperative cofilin–actin filament interactions

“…One of the remarkable and unusual properties of actin filaments is that cofilin bindings alter the helical structure of the filaments (26-28); they are more compliant in bending (29)(30)(31) and twisting (12,32). Because our recent study suggested that cofilin prefers to bind actin filaments with less tension and that actin filaments with less tension showed larger torsional fluctuations (33), we expected that the on-rate of cofilin bindings would be larger at more fluctuating regions of the actin filament.…”

“…The analysis of torsional fluctuations in actin filaments (12,32,37) showed that the amplitude of spontaneous fluctuations would twist actin filaments to a degree comparable with the twist when filaments are decorated with cofilin (4). These results suggest that spontaneous structural fluctuations of actin filaments enable cofilin binding to the filaments.…”