KINETIC-STUDIES ON THE EFFECT OF YEAST COFILIN ON YEAST ACTIN POLYMERIZATION

The effect of yeast cofilin on the kinetics of polymerization of yeast actin has been examined at 20 degrees C at both pH 8.0 and 6.6. In th e absence of cofilin, the kinetic data may be described by a simple nu cleation-elongation mechanism. Kinetic data in the presence of cofilin suggests a complex dependence the cofilin concentration. At low cofil in-to-actin ratios, cofilin increases the rate of polymerization in a way best fit by assuming filament fragmentation. The apparent fragment ation rate constants increase with increasing cofilin concentration le veling off above a cofilin-to-actin ratio of 1:8 and are independent o f pH. At higher cofilin-to-actin ratios, a nonpolymerizable cofilin-G- actin complex forms resulting in a decreased rate of polymerization. T he data from fluorescence photobleaching recovery experiments at low c ofilin-to-actin ratios are consistent with the presence of severed fil aments at both pH 8 and 6.6. However, at pH 8 and a cofilin-to-actin r atio of 1:16, about 40-50% of the total actin is present as G-actin af ter polymerization while at pH 6.6 little or no G-actin is present at the same cofilin-to-actin ratio. The results suggest some cooperativit y with respect to cofilin binding to filamentous actin which may be pH dependent.