Echicetin, a GPIb-binding snake C-type lectin from Echis carinatus, also contains a binding site for IgMκ responsible for platelet agglutination in plasma and inducing signal transduction

Echicetin, a heterodimeric snake C-type lectin from Echis carinatus, is known to bind specifically to platelet glycoprotein (GP)Ib. We now show that, in addition, it agglutinates platelets in plasma and induces platelet signal transduction. The agglutination is caused by binding to a specific protein in plasma. The protein was isolated from plasma and shown to cause platelet agglutination when added to washed platelets in the presence of echicetin. It was identified as immunoglogulin M kappa (IgM kappa) by peptide sequencing and dot blotting with specific heavy and light chain anti-immunoglobulin reagents. Platelet agglutination by clustering echicetin with IgM kappa induced P-selectin expression and activation of GPIIb/IIa as well as tyrosine phosphorylation of several signal transduction molecules, including p53/56(LYN), p64, p72(SYK), p70 to p90, and p120. However, neither ethylenediaminetetraacetic acid nor specific inhibition of GPIIb/IIa affected platelet agglutination or activation by echicetin. Platelet agglutination and induction of signal transduction could also be produced by cross-linking biotinylated echicetin with avidin. These data indicate that clustering of GPIb alone is sufficient to activate platelets. In vivo, echicetin probably activates platelets rather than inhibits platelet activation, as previously proposed, accounting for the observed induction of thrombocytopenia. (Blood, 2001;97:2333-2341) (C) 2001 by The American Society of Hematology.