Fig. 5.

The effect on the polymerization kinetics of adding TMR-labeled actin to unlabeled actin is similar to the effect of cofilin, as judged by a light-scattering assay of filament polymerization growth. A.U., arbitrary units. Unlabeled actin (5 μM) has the most gradual slope (black trace), resulting from the kinetics of limited nucleation and few filament ends. In the presence of 0.083 μM cofilin (blue trace), extensive fragmentation of filaments occurs, which leads to a much greater increase in the rate of polymerization. The incorporation of TMR-labeled actin into copolymers with unlabeled actin has an effect similar to that of cofilin, as seen (red trace) when 0.5 μM TMR-labeled actin is mixed with 4.5 μM unlabeled actin. The addition of 0.083 μM cofilin to this same 9:1 unlabeled actin/TMR-labeled actin mixture (green trace) leads to an even further enhancement in the overall rate of polymerization.