FOCAL ADHESION KINASE (PP125(FAK)) CLEAVAGE AND REGULATION BY CALPAIN

Focal adhesion kinase (125 kDa form; pp125(FAK)) is a widely expressed non-receptor tyrosine kinase that is implicated in integrin-mediated signal transduction. We have identified a novel means of pp125(FAK) re gulation in human platelets, in which this kinase undergoes sequential proteolytic modification from the native 125 kDa form to 90, 45 and 4 0 kDa fragments in thrombin-, collagen- and ionophore A23187-stimulate d platelets. The proteolysis of pp125(FAK) was prevented by pretreatin g platelets with the calpain inhibitors calpeptin or calpain inhibitor -1, and was reproduced in vitro by incubating immunoprecipitated pp125 (FAK) with purified calpain. Proteolysis of pp125(FAK) resulted in a d ramatic reduction in its autokinase activity and led to its dissociati on from the cytoskeletal fraction of platelets. These studies define a novel signal-terminating role for calpain, wherein proteolytic modifi cation of pp125(FAK) attenuates its autokinase activity and induces it s subcellular relocation within the cell.