PROFILIN IS REQUIRED FOR THE NORMAL TIMING OF ACTIN POLYMERIZATION INRESPONSE TO THERMAL-STRESS

We have used a fluorometric assay to determine the relative amounts of polymerized actin (F-actin) in wild-type and profilin mutant yeast ce lls. Our results indicate that profilin plays a role in maintaining no rmal F-actin levels in response to shifts to high temperature. Cells l acking profilin display a greater drop in F-actin levels upon such tem perature shifts, and are slower to recover to initial F-actin levels t han are wild-type cells, Interestingly, shifts to cold temperatures re sult in rapid increases of F-actin levels in wild-type and profilin nu ll cells. We have further determined that shifting to high-osmolarity growth conditions causes a relatively slow decrease in F-actin levels in wild-type cells, and a small but rapid increase in the F-actin leve ls in profilin null cells. Profilin null cells contain normal concentr ations of F-actin while growing exponentially at room temperature, ind icating that profilin is not essential for maintaining F-actin concent rations during steady-state growth. Our data suggest that actin is inh erently unstable in vivo at high temperatures, and that profilin helps to maintain actin in its filamentous state at these temperatures, per haps by stimulating actin polymerization in a proper temporal and spat ial fashion.