Collagen-induced tyrosine phosphorylation of WASP is inhibited by cytochalasin D or wortmannin. Platelet suspensions were incubated in the presence of RGDS peptide (200 μmol/L) for 5 minutes. After treatment with DMSO (0.1%) for 30 minutes, cytochalasin D (10 μmol/L) for 10 minutes, or wortmannin (50 nmol/L) for 30 minutes, collagen (50 μg/mL) was added to the suspension for 5 minutes as indicated (lanes 2 through 4). Platelets were lysed by the addition of an equal amount of a buffer containing 2% Triton X-100 before (resting, lane 1) and after the addition of collagen (lanes 2 through 4). Tyrosine phosphorylation of WASP was detected as described in Fig1.