FIGURE 4.

Capping protein restores rapid Arp2/3 complex-mediated actin polymerization in the presence of Fim1. A, CP reduces Fim1-mediated bundling efficiency. Dependence of actin in the low speed sedimentation supernatant (non-cross-linked actin) on the concentration of Fim1 in the absence (●) or presence (□) of 100 nm CP is shown. Inset, plot with a magnified x axis. B, TIRF microscopy time-lapse micrographs of actin filament branching by 25 nm Arp2/3 complex and 100 nm Wsp1 in the presence of 100 nm CP (left), 250 nm Fim1 (center), or CP and Fim1 (right). Time in seconds is indicated at the upper right. Bar, 5 μm. Corresponding supplemental Movie S11 is available online. C, plot of branching rate (branches μm⁻¹ min⁻¹) in the presence of 25 nm Arp2/3 complex with 100 nm Wsp1 and the indicated combinations of 100 nm CP and 250 nm Fim1. Values are the mean ± S.D.