YEAST ACTIN IS RELATIVELY WELL BEHAVED

Actin from yeast has been reported previously to have unusual polymerization properties. Here we report a simple sensitive spot assay for actin and use it to develop a high-yield procedure for the purification of actin from the yeast Saccharomyces cerevisiae. The polymerization properties of purified yeast actin are quantitatively similar to all other characterized actins. We have characterized this actin with respect to its ability to interact with yeast profilin and tropomyosin, the only yeast actin-binding proteins so far purified and characterized. Yeast profilin can sequester yeast actin monomers and thereby reduce the ability of yeast actin to polymerize, whereas it has little effect on the degree of polymerization of rabbit skeletal muscle actin. By contrast, there is no apparent difference between the binding of yeast or smooth muscle tropomyosin to yeast or rabbit skeletal muscle actin. The availability of purified yeast actin should facilitate a detailed examination of its interaction with recently discovered yeast actin-binding proteins. Greer and Schekman (1982) [Greer, C. & Scheckman, R. (1982), Mol. Cell Biol. 2, 1279-1286] reported that an intrinsic property of yeast actin is a Ca2+ dependent increase in critical concentration with the formation of 15-50-nm particles. Our purified actin does not have this property. By modifying the purification protocol, we can obtain a preparation having a Ca2+-dependent change in polymerization properties. The Ca2+-dependent effect results in a slower polymerization rate as well as the formation of shorter filaments. Since this effect could be mediated by a protein present at a very low stoichiometry to actin, and we do not see any contaminating peptides, we have not pursued this effect further. We suggest that the Ca2+-dependent properties of the Greer and Schekman preparation are most likely due to a minor contaminant.