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Galkin, V. E., Orlova, A., Lukoyanova, N., Wriggers, W. & Egelman, E. H. (2001) Actin depolymerization factor stabilizes an existing state of F-actin and can change the tilt of F-actin subunits. J.Cell Biol. 153, 75-86.


Proteins in the actin depolymerizing factor (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. Although 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 ( approximately 12 degrees ) 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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