Phosphorylation of Syk and FcRγ is diminished when platelet-platelet contacts are prevented. Platelets from Sema4D^(−/−) and matched Sema4D^(+/+) mice were incubated with Integrilin (10μM) plus aspirin (1mM) and apyrase (2 U/mL) for 30 minutes followed by collagen (2.5 μg/mL) for 2 minutes. (A) Immunoblot with anti-pSyk Y519/520 is shown. (B) FcRγ phosphorylation in Sema4D^(+/+) platelets was detected by immunoprecipitating with anti-FcRγ followed by immunoblotting with anti-phosphotyrosine antibody, 4G10P. In both panels A and B, a representative experiment is shown and the results of 3 experiments are summarized (mean ± SEM). N.S. = not significant.