RAPID PROTEIN-TYROSINE PHOSPHORYLATION IN THE CYTOSKELETON OF STIMULATED HUMAN PLATELETS

Upon activation platelets show elevated protein tyrosine phosphorylation, and translocation of the protein tyrosine kinase pp60(c-src) from the plasma membrane to the cytoskeleton occurs. We therefore investigated whether tyrosine phosphorylation also increases in the cytoskeletal compartment. Here we show that almost identical patterns of phosphotyrosine-containing proteins are detectable in the cytoskeleton after platelet stimulation with compounds that directly (phorbol 12-myristate, 13-acetate) or indirectly (thrombin, vasopressin, collagen, ADP) activate protein kinase C. The apparent molecular masses of the proteins phosphorylated at tyrosine residues are 145, 130, 100, 85, 80, 60, 56, 54 and 38 kDa. Elevation of cyclic AMP by prostaglandin El had no effect. Concentrations of thrombin as low as 0.01 units per mi are able to cause tyrosine phosphorylation of multiple proteins. The time course of protein tyrosine phosphorylation for thrombin- and vasopressin-stimulated platelets revealed a rapid increase in the cytoskeleton within 5 to 20 s following activation consistent with a role in early events of platelet function.