Rhodocytin induces platelet aggregation by interacting with glycoprotein Ia/IIa (GPIa/IIa, integrin alpha(2)beta(1)) - Involvement of GPIa/IIa-associated Src and protein tyrosine phosphorylation

Although glycoprotein Ia/IIa (GPIa/IIa, integrin alpha (2)beta (1)) has est ablished its role as a collagen receptor, it remains unclear whether GPIa/I Ia mediates activation signals. In this study, we show that rhodocytin, pur ified from the Calloselasma rhodostoma venom, induces platelet aggregation, which can be blocked by anti-GPIa monoclonal antibodies. Studies with rhod ocytin-coupled beads and liposomes loaded with recombinant GPIa/IIa demonst rated that rhodocytin directly binds to GPIa/IIa independently of divalent cations, In vitro kinase assays and Western blotting of GPIa immunoprecipit ates revealed that Src and Lyn constitutively associate with GPIa/IIa and t hat Src activity increases transiently after rhodocytin stimulation. Src sp ecifically associates with p130 Crk-associated substrate (Cas) in a manner dependent upon Cas phosphorylation, suggesting that Src is responsible for Cas tyrosine phosphorylation. While all these phenomena occur early after r hodocytin stimulation in a cAMP-resistant manner, tyrosine phosphorylation of Syk and phospholipase C gamma2, intracellular Ca2+ mobilization, and pla telet aggregation occur later in a cAMP-sensitive manner. Cytochalasin D, w hich interferes with actin polymerization and blocks receptor clustering, i nhibits all the rhodocytin-mediated signals we examined in this study. We s uggest that rhodocytin, by clustering GPIalIIa, activates GPIa/IIa-associat ed Src, which then mediates downstream activation signals.