Collagen, convulxin, and thrombin stimulate aggregation-independent tyrosine phosphorylation of CD31 in platelets. Evidence for the involvement of Src family kinases

Platelet endothelial cell adhesion molecule-1 (CD31) is a 130-kDa glycoprotein receptor present on the surface of platelets, neutrophils, monocytes, certain T-lymphocytes, and vascular endothelial cells. CD31 is involved in adhesion and signal transduction and is implicated in the regulation of a number of cellular processes. These include transendothelial migration of leukocytes, integrin regulation, and T-cell function, although its function in platelets remains unclear. In this study, we demonstrate the ability of the platelet agonists collagen, convulxin, and thrombin to induce tyrosine phosphorylation of CD31. Furthermore, we show that this event is independent of platelet aggregation and secretion and is accompanied by an increase in surface expression of CD31. A kinase capable of phosphorylating CD31 was detected in CD31 immunoprecipitates, and its activity was increased following activation of platelets. CD31 tyrosine phosphorylation was reduced or abolished by the Src family kinase inhibitor PP2, suggesting a role for these enzymes. In accordance with this, each of the Src family members expressed in platelets, namely Fyn, Lyn, Src, Yes, and Hck, was shown to co-immunoprecipitate with CD31. The involvement of Src family kinases in this process was confirmed through the study of mouse platelets deficient in Fyn.