ArhGAP15 and actin polymerization are parallel routes for Rac turnoff downstream of opto-PI3K. (A) ArhGAP15 could act downstream of actin assembly (left) or in a parallel pathway (right) to inhibit Rac*. (B) WT (black curve) and ARHGAP15-null cells (dark blue dashed curve) were first treated with an actin polymerization inhibitor (10 µM latrunculin B [Lat-B]) and then were treated with red light for 10 min. Rac* was measured as in Fig. 4 C. Each point represents a mean of three independent experiments. (C) ARHGAP15-null cells in the absence (light blue dashed curve) or presence of 10 µM latrunculin B (dark blue dashed curve) were treated with red light for 10 min. Rac* was measured as in Fig. 4 C. Each point represents a mean of three independent experiments. Our observation that disruption of ArhGAP15 and actin polymerization yielded compounded defects when combined suggests that they act in parallel pathways (Fig. 8 A, right). Error bars indicate SEM. *, P < 0.05 by unpaired t test. Immunoblots depict one representative experiment. Molecular masses are given in kilodaltons.