Actin depolymerizing factor stabilizes an existing state of F-actin and can change the tilt of F-actin subunits

Proteins in the actin depolymerizing factor (ADF)/cofilin family are essent ial for rapid F-actin turnover, and most depolymerize actin in a pH-depende nt manner. Complexes of human and plant ADF with F-actin at different pH we re examined using electron microscopy and a novel method of image analysis for helical filaments. Although ADF changes the mean twist of actin, we sho w that it does this by stabilizing a preexisting F-actin angular conformati on. In addition, ADF induces a large (similar to 12 degrees) tilt of actin subunits at high pH where filaments are readily disrupted. A second ADF mol ecule binds to a site on the opposite side of F-actin from that of the prev iously described ADF binding site, and this second site is only largely occ upied at high pH. All of these states display a high degree of cooperativit y that appears to be an integral part of F-actin.