Two activities of cofilin, severing and accelerating directional depolymerization of actin filaments, are affected differentially by mutations aroundthe actin-binding helix

The biochemical activities of cofilin are controversial. We demonstrated th at porcine cofilin severs actin filaments and accelerates monomer release a t the pointed ends. At pH 7.1, 0.8 mu M cofilin cut filaments (2.2 mu M act in) about every 290 subunits and increased the depolymerization rate 6.4-fo ld. A kink in the major alpha-helix of cofilin is thought to constitute a c ontact site for actin, Side chain hydroxyl groups of Ser119, Ser120 and Tyr 82 in cofilin form hydrogen bonds with main chain carbonyl moieties from th e helix, causing the kink. We eliminated side chain hydroxyls by Ser-->Ala and/or Tyr-->Phe mutagenesis, Severing and depolymerization-enhancing activ ities were reduced dramatically in an Ala120 mutant, whereas the latter was decreased in a Phe82 mutant with a relatively small effect on severing, su ggesting different structural bases for the two activities of cofilin, The Ala120-equivalent mutation in yeast cofilin affected cell growth, whereas t hat of the Phe82-equivalent had no effect in yeast. These results indicate the physiological significance of the severing activity of cofilin that is brought about by the kink in the helix.