Amy McGough,Ý Brian Pope,* Wah Chiu,Ý & Alan Weeds* ÝVerna & Marrs McLean
Department of Biochemistry, Baylor College of Medicine, Houston, Texas, USA; *MRC Laboratory for Molecular Biology, Cambridge, England.
Cofilin is a member of a family of single domain actin-binding proteins which bind to either monomeric or filamentous actin. Initially, cofilin was thought to be an inefficient actin-severing protein whose preference for either G- or F-actin was regulated by pH. More recent work has cast doubt on cofilin's ability to sever filaments actively; nonetheless, it is generally accepted that cofilin regulates actin filament length. One curious feature of this molecule is its ability to recognize either F- or G-actin in a pH-dependent manner. At low pH (6.6), cofilin cosediments with long actin filaments; whereas at higher pH (8.0), it drives filament depolymerization and remains associated with monomeric actin. Because cofilin consists of a single domain, the switch from G- to F-actin binding must involve subtle differences in the cofilin/actin interface at the two pH values. To address this issue, we are studying cofilin/actin interactions using electron cryomicroscopy and image reconstruction. Our work has revealed a totally unexpected (and unique) property of cofilin, namely, its ability to change the twist of the actin filament. Three-dimensional reconstructions of F-actin and cofilin:F-actin show that cofilin binds two longitudinally-associated monomers in the filament. Although this site is distinct from the phalloidin binding site on F-actin, cofilin is known to compete with phalloidin for actin binding. Thus, one ramification of the change in twist induced by cofilin is to alter the phalloidin binding site. This is the first demonstration of an actin-binding protein which competes for binding in this way. The implications of this structure on cofilin's function in the cell and the structure of the actin filament will be discussed.
This work was supported by a Grant-in-Aid from the American Heart Association (to A.M.), an NIH NCRR grant (to W.C.), & the W. M. Keck Center for Computational Biology (Houston, TX).