Mutant forms of vWF induce differential association between Syk and tyrosine phosphorylated FcR γ-chain.Platelets were stimulated with mutant vWF or pvWF in the presence of ristocetin for 45 seconds, thrombin for 45 seconds, or collagen for 120 seconds. Syk was immunoprecipitated, run on SDS-PAGE, and Western blotted with 4G10 [A(i) and B(i)] or with anti-Syk [A(ii) and B(ii)]. [A(i)]: lane 1, resting platelets; lane 2, pvWF (10 μg/mL) and ristocetin (1 mg/mL); lane 3, ▵A1-vWF (3 μg/mL) and ristocetin (1 mg/mL). [B(i)] Phosphorylation of FcR γ-chain which has been precipitated in association with Syk from basal platelets (lane 1); platelets stimulated with pvWF (10 μg/mL, lane 2), ▵A1-vWF (3 μg/mL, lane 3), RGGS-vWF (6 μg/mL, lane 4) in the presence of ristocetin (1 mg/mL); thrombin (1 U/mL, lane 5) or collagen (100 μg/mL, lane 6). Equal amounts of Syk were present in each lane [B(ii)]. Immunoblots shown are representative of 4 separate experiments.