Cofilin Induces A Local Change In The Twist Of Actin Filaments

Cofilin is a small protein that binds and severs actin filaments near the boundaries between cofilin-decorated and bare actin segments. Cofilin binds cooperatively to actin by inducing conformational changes that propagate along the helical lattice to neighboring bare actin segments, favoring cofilin binding. However, the extent to which these cooperative conformational changes propagate remains unclear and prior estimates for the propagation length vary widely; it has been suggested that anywhere from 2 to 24 actin subunits extending into the bare region could be affected. We address this unresolved question by imaging partially cofilin-decorated actin filaments using cryo-electron microscopy and determining the location of boundaries between decorated and undecorated regions at the subunit level using a novel particle subtraction and 3D classification strategy. We used characteristic changes in filament twist induced by cofilin as a marker for cooperative changes in actin, and tracked these changes with respect to the boundary. Our results indicate that cofilin induces a change in twist that propagates at most 2 subunits away from the boundary, irrespective of the polarity of the boundary (i.e. whether the bare side of the boundary extends toward the pointed or barbed end of the actin filament). These observations provide direct experimental support for nearest-neighbor models describing cofilin-binding cooperativity, and inform mechanistic models for cofilin-mediated actin severing.