p72syk Is Activated by Vanadate plus H2O2 in Porcine Platelets and Phosphorylates GTPase Activating Protein on Tyrosine Residue(s)1

Protein-tyrosine kinase p72syk exists abundantly in various hematopoietic cells and is activated by physiological or non-physiological agents. In this study we used vanadate, which is well known as a protein-tyrosine phosphatase inhibitor, to investigate the activity of p72syk and a downstream substrate for p72syk. Treatment with vanadate plus H2O2 caused tyrosine phosphorylation of multiple cellular proteins and platelet activation, i.e. aggregation and secretion. During aggregation induced by this stimulant, p72syk was activated and GTPase activating protein (GAP) was phosphorylated on tyrosine residue(s). The activation of p72syk was time- and dose-dependent. Also, the time course of activation of p72syk preceded that of tyrosine phosphorylation of GAP, and GAP was actually phosphorylated on tyrosine residue(s) by p72syk in vitro. These results suggest that p72syk is activated by treatment with vanadate plus H2O2, and that GAP is one of the possible substrates for p72syk in porcine platelets.