Redistribution of Activated pp60^c-src to Integrin-Dependent Cytoskeletal Complexes in Thrombin-Stimulated Platelets

Abstract

Thrombin stimulation of platelets induces a transient increase in the specific activity of pp60^c-src followed by a redistribution of pp60^c-src to the Triton X-100-insoluble, cytoskeleton-rich fraction. Concomitant with the observed increase in pp60^c-src activity was a rapid dephosphorylation of tyrosine 527 in 10 to 15% of pp60^c-src molecules. In addition, we found that pp60^c-src from the Triton-insoluble fraction was phosphorylated on tyrosine 416, the autophosphorylation site which is phosphorylated in activated oncogenic variants of pp60src. Furthermore, in platelets from patients with Glanzmann's thrombasthenia (which are deficient in the integrin receptor GPIIb-IIIa), pp60^c-src was not translocated to the Triton-insoluble fraction, and there was a sustained increase in pp60^c-src activity following thrombin treatment. These results suggest that pp60^c-src is rapidly activated in thrombin-stimulated platelets, potentially by a protein tyrosine phosphatase, before it translocates to a cytoskeletal fraction, where many of its potential substrates are found. The evidence that the cytoskeletal association of pp60^c-src is dependent upon engagement of the integrin receptor GPIIb-IIIa suggests that integrin-cytoskeletal complexes may serve to compartmentalize and anchor activated enzymes involved in signal transduction.