Actin polymerization is essential for Rac turnoff downstream of opto-PI3K. (A and B, left) Schematics for experiments described in right panels. Stimulation using chemoattractant and opto-PI3K is represented by red circles and red lightning bolts, respectively. (A and B, right) WT cells in the absence (black curves) or presence of an actin polymerization inhibitor (10 µM latrunculin B [Lat-B], blue dashed curves) were treated with red light (A) or 10 nM fMLP (B) for 10 min. Rac* was measured as in Fig. 4 C. Each point represents a mean of four independent experiments. A loss of actin polymer significantly inhibited Rac turnoff downstream of opto-PI3K but had no significant effect on Rac activity for chemoattractant-stimulated cells. Error bars indicate SEM. *, P < 0.05 by unpaired t test. Immunoblots depict one representative experiment. Molecular masses are given in kilodaltons.