ACTIN-FILAMENTS IN YEAST ARE UNSTABLE IN THE ABSENCE OF CAPPING PROTEIN OR FIMBRIN

Many actin-binding proteins affect filament assembly in vitro and loca lize with actin in vivo, but how their molecular actions contribute to filament assembly in vivo is not understood well. We report here that capping protein (CP) and fimbrin are both important for actin filamen t assembly in vivo in Saccharomyces cerevisiae, based on finding decre ased actin filament assembly in CP and fimbrin mutants. We have also i dentified mutations in actin that enhance the CP phenotype and find th at those mutants also have decreased actin filament assembly in vivo. In vitro, actin purified from some of these mutants is defective in po lymerization or binding fimbrin. These findings support the con elusio n that CP acts to stabilize actin filaments in vivo. This conclusion i s particularly remarkable because it is the opposite of the conclusion drawn from recent studies in Dictyostelium (Hug, C., P. Y. Jay, I. Re ddy, J. G. McNally, P. C. Bridgman, E. L. Elson, and J. A. Cooper. 199 5. Cell. 81:591-600). In addition, we find that the unpolymerized pool of actin in yeast is very small relative to that found in higher cell s, which suggests that actin filament assembly is less dynamic in yeas t than higher cells.