PROTEIN-TYROSINE PHOSPHORYLATION IN HUMAN PLATELETS INDUCED BY INTERACTION BETWEEN GLYCOPROTEIN IB AND VON-WILLEBRAND-FACTOR

The interaction between von Willebrand factor (VWF) and glycoprotein I b (GPIb) induced by ristocetin or botrocetin resulted in associated pl atelet aggregation, protein tyrosine phosphorylation (PTP) of a 64 kDa protein, as detected by a monoclonal antibody against phosphotyrosine (PY-20), and intracellular Ca2+ elevation that is largely dependent u pon Ca2+ influx in human platelets. It is of interest that 75-80, 97 a nd 125 kDa proteins which are strongly tyrosine-phosphorylated in plat elet activation induced by thrombin and other agonists were not detect ed. Neither VWF nor a coaggregating agent (ristocetin or botrocetin) a lone induced aggregation, [Ca2+](i) elevation or the 64 kDa PTP. NMC-4 , an antibody which inhibits both ristocetin- or botrocetin-induced vW F binding to GPIb, abolished the appearance of the 64 kDa PTP as well as other responses, suggesting that it is specifically induced by the GPIb-vWF interaction. Aspirin, or ONO-3708, a competitive inhibitor of thromboxane A(2), did not modify the 64 kDa PTP, while [Ca2+](i) elev ation was moderately suppressed. Depletion of extracellular Ca2+ or RG D peptides suppressed neither the 64 kDa PTP nor aggregation. H-7, a p rotein kinase C inhibitor, did not inhibit the 64 kDa PTP, while staur osporine, a potent protein kinase inhibitor, inhibited the 64 kDa PTP and Ca2+ influx, but not aggregation, in a dose-dependent manner. It i s suggested that the 64 kDa PTP is associated with platelet aggregatio n induced by the interaction between GPIb and vWF.