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 isola ted from the venom of Crotalus durissus terrificus, with human platele ts. Cvx is a potent platelet agonist that induces an increase in the i ntracellular Ca2+ concentration ([Ca2+](i)), granule exocytosis and ag gregation. I-125-Labeled Cvx binds specifically and rapidly to platele ts at binding sites of high and moderate affinity. Platelets adhere to immobilized Cvx in a time-dependent but cation-independent manner. Pl atelet exocytosis and aggregation induced by Cvx were inhibited by an anti-integrin alpha(2) beta(1) monoclonal antibody (6F1) and by the Fa b 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 protei n with an electrophoretic mobility identical to that of GPVI. In addit ion, we observed the following: (i) I-125-CvX binds to GPVI immunoprec ipitated by the anti-GPVI antibody from a platelet lysate, and (ii) Cv x 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 o f Cvx-induced platelet activation, i.e. exocytosis and aggregation.