pFocal adhesion kinase (125 kDa form; pp125supFAK/sup) is a widely expressed non-receptor tyrosine kinase that is implicated in integrin-mediated signal transduction. We have identified a novel means of pp125supFAK/sup regulation in human platelets, in which this kinase undergoes sequential proteolytic modification from the native 125 kDa form to 90, 45 and 40 kDa fragments in thrombin-, collagen- and ionophore A23187-stimulated platelets. The proteolysis of pp125supFAK/sup was prevented by pretreating platelets with the calpain inhibitors calpeptin or calpain inhibitor-1, and was reproduced iin vitro/i by incubating immunoprecipitated pp125supFAK/sup with purified calpain. Proteolysis of pp125supFAK/sup resulted in a dramatic reduction in its autokinase activity and led to its dissociation from the cytoskeletal fraction of platelets. These studies define a novel signal-terminating role for calpain, wherein proteolytic modification of pp125supFAK/sup attenuates its autokinase activity and induces its subcellular relocation within the cell./p
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