Aggretin, a Heterodimeric C-type Lectin from Calloselasma rhodostoma (Malayan Pit Viper), Stimulates Platelets by Binding to α2β1 Integrin and Glycoprotein Ib, Activating Syk and Phospholipase Cγ2, but Does Not Involve the Glycoprotein VI/Fc Receptor γ Chain Collagen Receptor

Aggretin, a potent platelet activator, was isolated from Calloselasma rhodostoma venom, and 30-amino acid N-terminal sequences of both subunits were determined. Aggretin belongs to the heterodimeric snake C-type lectin family and is thought to activate platelets by binding to platelet glycoprotein α 2 β 1 . We now show that binding to glycoprotein (GP) Ib is also required. Aggretin-induced platelet activation was inhibited by a monoclonal antibody to GPIb as well as by antibodies to α 2 β 1 . Binding of both of these platelet receptors to aggretin was confirmed by affinity chromatography. No binding of other major platelet membrane glycoproteins, in particular GPVI, to aggretin was detected. Aggretin also activates platelets from Fc receptor γ chain (Fcγ)-deficient mice to a greater extent than those from normal control mice, showing that it does not use the GPVI/Fcγ pathway. Platelets from Fcγ-deficient mice expressed fibrinogen receptors normally in response to collagen, although they did not aggregate, indicating that these platelets may partly compensate via other receptors including α 2 β 1 or GPIb for the lack of the Fcγ pathway. Signaling by aggretin involves a dose-dependent lag phase followed by rapid tyrosine phosphorylation of a number of proteins. Among these are p72 SYK , p125 FAK , and PLCγ2, whereas, in comparison with collagen and convulxin, the Fcγ subunit neither is phosphorylated nor coprecipitates with p72 SYK . This supports an independent, GPIb- and integrin-based pathway for activation of p72 SYK not involving the Fcγ receptor.