RELOCATION OF SYK PROTEIN-TYROSINE KINASE TO THE ACTIN FILAMENT NETWORK AND SUBSEQUENT ASSOCIATION WITH FAK

Previous studies demonstrated that Syk protein-tyrosine kinase (Syk) i s activated by thrombin in platelets. To elucidate the function of Syk in platelets, we have biochemically examined the intracellular locati on of Syk and the molecules associated with Syk, following platelet ac tivation. In human platelets, thrombin induces the relocation of Syk t o the cytoskeletal fraction presumably via Syk tyrosine phosphorylatio n. Relocated Syk is associated with the actin filament network, and th e early phase (10-90 s) of this association can be partially inhibited by the pretreatment of platelets with cytochalasin D, an inhibitor of actin polymerization. Upon thrombin stimulation, Syk becomes associat ed with Fak as demonstrated by co-immunoprecipitation. The association of both kinases can be inhibited by pretreatment of platelets with cy tochalasin D. Interestingly, reconstitution experiments, using COS cel ls transfected with various porcine Syk mutants, revealed that the kin ase domain, but not the kinase activity, of Syk is required for the as sociation of Syk with the actin filament network. These findings sugge st that thrombin-induced association of Syk with Fak correlates with t he state of actin polymerization, and may play an important role in pl atelet activation.