PLATELET ACTIVATION AND SIGNAL-TRANSDUCTION BY CONVULXIN, A C-TYPE LECTIN FROM CROTALUS-DURISSUS-TERRIFICUS (TROPICAL RATTLESNAKE) VENOM VIA THE P62 GPVI COLLAGEN RECEPTOR/

Convulxin, a powerful platelet activator, was isolated from Crotalus d urissus terrificus venom, and 20 amino acid N-terminal sequences of bo th subunits were deter mined. These indicated that convulxin belongs t o the heterodimeric C-type lectin family. Neither antibodies against G PIb nor echicetin had any effect on convulxin-induced platelet aggrega tion showing that, in contrast to other venom C-type lectins acting on platelets, GPIb is not involved in convulxin-induced platelet activat ion. In addition, partially reduced/denatured convulxin only affects c ollagen induced platelet aggregation. The mechanism of convulxin-induc ed platelet activation was examined by platelet aggregation, detection of time-dependent tyrosine phosphorylation of platelet proteins, and binding studies with I-125-convulxin. Convulxin induces signal transdu ction in part like collagen, involving the time-dependent tyrosine pho sphorylation of Fc receptor gamma chain, phospholipase C gamma 2, p72( SYK), c-Cbl, and p36-38. However, unlike collagen, pp125(FAK) and some other bands are not tyrosine-phosphorylated. Convulxin binds to a gly cosylated 62-kDa membrane component in platelet lysate and 60 p62/GPVI immunoprecipitated by human anti-p62/GPVI antibodies. Convulxin subun its inhibit both aggregation and tyrosine phosphorylation in response to collagen. Piceatannol, a tyrosine kinase inhibitor with some specif icity for p72(SYK), showed differential effects on collagen and convul xin-stimulated signaling. These results suggest that convulxin uses th e p62/GPVI but not the alpha(2) beta(1) part of the collagen signaling pathways to activate platelets. Occupation and clustering of p62/GPVI may activate Src family kinases phosphorylating Fc receptor gamma cha in and, by a mechanism previously described in T- and B-cells, activat e p72(SYK) that is critical for downstream activation of platelets.