THROMBIN AND THROMBIN RECEPTOR AGONIST PEPTIDE INDUCE TYROSINE PHOSPHORYLATION AND TYROSINE KINASES IN THE PLATELET CYTOSKELETON - TRANSLOCATION OF PP60(C-SRC) AND INTEGRIN ALPHA(IIB)BETA(3) (GLYCOPROTEIN IIB IIIA) IS NOT REQUIRED FOR AGGREGATION, BUT IS DEPENDENT ON FORMATION OF LARGE AGGREGATE STRUCTURES/

The maximal aggregation of platelets induced by alpha-thrombin or by t he receptor agonist peptide thrombin-(42-47)-peptide (TRP42/47) rapidl y increased the pp60c-src associated with the cytoskeleton fraction. T here was good correlation between the tyrosine kinase activity and the mass of pp60c-src. Tyrosine kinase activity associated with the cytos keleton phosphorylated several endogenous cytoskeleton-associated prot eins, as revealed by immunoblotting with anti-phosphotyrosine antibody following incubation with ATP in vitro. However, with the exception o f pp60c-src, few phosphotyrosine-containing proteins were retained in the cytoskeleton in intact platelets when compared with total platelet lysates. Translocation of pp60c-src to the cytoskeleton induced by al pha-thrombin and TRP42/47 is dependent on glycoprotein IIb/IIIa (GPIIb /IIIa)-fibrinogen-mediated aggregation, but does not occur when ristoc etin/von Willebrand factor produces GPIb-mediated platelet aggregation . The translocation of GPIIb/IIIa and pp60c-src to the cytoskeleton is not necessary for aggregation, as it is not seen when clearly visible small to moderate-sized aggregates are initially formed after exposur e to thrombin. The linkage of these proteins to the cytoskeleton occur s only after later extensive formation of large aggregates. Translocat ion of GPIIa/IIIa to the cytoskeleton is not sufficient for the cytosk eletal association of pp60c-src, as the former occurs independently in platelets stimulated with concanavalin A in the absence of aggregatio n. Linkage of the integrin GPIIb/IIIa and pp60c-src to the internal cy toskeleton structure, and the corresponding tyrosine phosphorylation o f certain proteins upon formation of large aggregates, may be an examp le of mechanochemical transduction by integrin receptors and may repre sent a structure with the requisite tensile strength to stabilize larg e platelet aggregates against high shear stresses.