P72(SYK) IS ACTIVATED BY VANADATE PLUS (HO2)-O-2 IN PORCINE PLATELETSAND PHOSPHORYLATES GTPASE-ACTIVATING PROTEIN ON TYROSINE RESIDUE(S)

Protein-tyrosine kinase p72(syk) exists abundantly in various hematopo ietic cells and is activated by physiological or non-physiological age nts. In this study we used vanadate, which is well known as a protein- tyrosine phosphatase inhibitor, to investigate the activity of p72(syk ) and a downstream substrate for p72(syk). Treatment with vanadate plu s H2O2 caused tyrosine phosphorylation of multiple cellular proteins a nd platelet activation, i.e. aggregation and secretion. During aggrega tion induced by this stimulant, p72(syk) was activated and GTPase acti vating protein (GAP) was phosphorylated on tyrosine residue(s). The ac tivation of p72(syk) was time- and dose-dependent. Also, the time cour se of activation of p72(syk) preceded that of tyrosine phosphorylation of GAP, and GAP was actually phosphorylated on tyrosine residue(s) by p72(syk) in vitro. These results suggest that p72(syk) is activated b y treatment with vanadate plus H2O2, and that GAP is one of the possib le substrates for p72(syk) in porcine platelets.