(D) Effect of WDR63 on Arp2/3-mediated branched actin polymerization was analyzed by electron microscopy. 0.6 μM WDR63 was incubated with 2 μM actin, 10 nM Arp2/3 and 400 nM GST-VCA as indicated. Actin structures were then stabilized with addition of 2μM phalloidin, followed by electron microscopy analysis. The arrows indicate actin branch junctions. The percentage of branched actin filaments was expressed as the ratio of the number of actin filaments with branch junctions to the total numbers of actin filaments. For each condition, over 150 actin filaments were counted. Data shown are mean ± SD; n = 3 independent experiments. ***, P ˂ 0.001; two-tailed Student's t-test. Scale bar: 200 nm. 