Adhesion and activation of human platelets induced by convulxin involve glycoprotein VI and integrin alpha(2)beta(1)

We analyzed the interaction of convulxin (Cvx), a 72-kDa protein isolated from the venom of Crotalus durissus terrificus, with human platelets. Cvx is a potent platelet agonist that induces an increase in the intracellular Ca2+ concentration ([Ca2+](i)), granule exocytosis and aggregation. I-125-Labeled Cvx binds specifically and rapidly to platelets at binding sites of high and moderate affinity. Platelets adhere to immobilized Cvx in a time-dependent but cation-independent manner. Platelet exocytosis and aggregation induced by Cvx were inhibited by an anti-integrin alpha(2) beta(1) monoclonal antibody (6F1) and by the Fab fragments of a polyclonal antiglycoprotein VI (GPVI) antibody. Both the adhesion of platelets to Cvx and the Cvx-induced increase in [Ca2+](i) were inhibited by anti-GPVI Fab fragments but not by 6F1. Ligand blotting assay showed that I-125-CvX binds to a 57-kDa platelet protein with an electrophoretic mobility identical to that of GPVI. In addition, we observed the following: (i) I-125-CvX binds to GPVI immunoprecipitated by the anti-GPVI antibody from a platelet lysate, and (ii) Cvx inhibits the binding of anti-GPVI IgG to GPVI. Taken together, these results demonstrate that GPVI behaves as a Cvx receptor and that the alpha(2) beta(1) integrin appears to be involved in the later stages of Cvx-induced platelet activation, i.e. exocytosis and aggregation.