FIGURE 4.

Actin nucleation activities of wild type and arc40-139, arc40-141, and arc40-140 Arp2/3 complexes. The rates of assembly were determined from the slopes of the curves at their steepest points, which was between 25 and 50% assembly. A, C, and E, actin assembly reactions containing 3 μm monomeric rabbit muscle actin (5% pyrene-labeled) with variable concentrations of wild type (blue) or mutant (red) Arp2/3 complex. B, D, and F, actin assembly reactions containing 2 μm monomeric rabbit muscle actin (5% pyrene-labeled), Las17/WASp (5 nm for arc40-139, arc40-141 and 10 nm for arc40-140), and variable concentrations of wild type (blue) or mutant (red) Arp2/3 complex. A higher concentration of monomeric actin was used for the reactions in A, C, and E compared with B, D, and F to more readily detect base-line Arp2/3 complex nucleation activity in the absence of Las17/WASp.