Vitold E. Galkin, Albina Orlova, Natalya Lukoyanova, Willy Wriggers, and Edward H. Egelman. ADF Stabilizes an Existing State of F-Actin and Can Change the Tilt of F-Actin Subunits. Journal of Cell Biology, 2001, Vol. 153, pp. 75-86.
Proteins in the ADF/cofilin family are essential for rapid F-actin turnover and most depolymerize actin in a pH-dependent manner. Complexes of human and plant ADF with F-actin at different pH were examined using electron microscopy and a novel method of image analysis for helical filaments. While ADF changes the mean twist of actin, we show that it does this by stabilizing a preexisting F-actin angular conformation. In addition, ADF induces a large, ~12 degree, tilt of actin subunits at high pH, where filaments are readily disrupted. A second ADF molecule binds to a site on the opposite side of F-actin from that of the previously described ADF binding site, and this second site is only largely occupied at high pH. All of these states display a high degree of cooperativity that appears to be an integral part of F-actin.
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