The identification of a second cofilin binding site on actin suggests a novel, intercalated arrangement of F-actin binding

The cofilins are members of a protein family that binds monomeric and filam entous actin, severs actin filaments, and increases monomer off-rate from t he pointed end. Here, we characterize the cofilin-actin interface. We confi rm earlier work suggesting the importance of the lower region of subdomain 1 encompassing the N and C termini (site 1) in cofilin binding. In addition , we report the discovery of a new cofilin binding site (site 2) from resid ues 112-125 that form a helix toward the upper, rear surface of subdomain 1 in the standard actin orientation (Kabsch, W., Mannherz, H. G., Suck, D., Pal, E. F., and Holmes, H. C. (1990) Nature 347, 37-44). We propose that co filin binds "behind" one monomer and "in front" of the other longitudinally associated monomer, accounting for the fact that cofilin alters the twist in the actin (McGough, A. Pope, B., Chin, W., and Weeds, A. (1997) J. Cell Biol. 138, 771-781). The characterization of the cofilin-actin interface wi ll facilitate an understanding of how cofilin severs and depolymerizes fila ments and may shed light on the mechanism of the gelsolin family because th ey share a similar fold with the cofilins (Hatanaka, H., Ogura, K., Moriyam a, K., Ichikawa, S., Yahara, I., and Inagiki, F. (1996) Cell 85, 1047-1055) .