Abstract
The tyrosine kinase p72 Syk plays a critical role in platelet signal transduction. It associates with the platelet receptor for the Fc domain of IgGs, FcγRII, following stimulation by FcγRII cross-linking. Here, we show that p72 Syk and FcγRII tyrosine phosphorylation and association occured following platelet stimulation by: (1) two monoclonal antibodies, which form a bridge between a target antigen and FcγRII, and (2) the G-protein-coupled receptor agonist thrombin. The kinetics of the p72 Syk/FcγRII association depended on the signalling pathway (i.e., the antigen targeted or the thrombin receptor). We established a direct relationship between the level of FcγRII phosphorylation and the detection of its association with p72 Syk. Inhibition of p72 Syk by piceatannol resulted in partial or total inhibition of FcγRII phosphorylation, after immunological activation or addition of thrombin, respectively, suggesting that p72 Syk participates in FcγRII phosphorylation. The results provide evidence that p72 Syk/FcγRII association is not restricted to immunological activation.
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