COFILIN CHANGES THE TWIST OF F-ACTIN - IMPLICATIONS FOR ACTIN FILAMENT DYNAMICS AND CELLULAR FUNCTION

Cofilin is an actin depolymerizing protein found widely distributed in animals and plants. We have used electron cryomicroscopy and helical reconstruction to identify its binding site on actin filaments. Cofili n binds filamentous (F)-actin cooperatively by bridging two longitudin ally associated actin subunits. The binding site is centered axially a t subdomain 2 of the lower actin subunit and radially at the cleft bet ween subdomains 1 and 3 of the upper actin subunit. Our work has revea led a totally unexpected (and unique) property of cofilin, namely, its ability to change filament twist. As a consequence of this change in twist, filaments decorated with cofilin have much shorter 'actin cross overs' (similar to 75% of those normally observed in F-actin structure s). Although their binding sites are distinct, cofilin and phalloidin do not bind simultaneously to F-actin. This is the first demonstration of a protein that excludes another actin-binding molecule by changing filament twist, Alteration of F-actin structure by cofilin/ADF appear s to be a novel mechanism through which the actin cytoskeleton may be regulated or re-modeled.