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Blood, 15 April 2001, Vol. 97, No. 8, pp. 2333-2341
HEMOSTASIS, THROMBOSIS, AND VASCULAR BIOLOGY
Echicetin, a GPIb-binding snake C-type lectin from Echis
carinatus, also contains a binding site for IgM responsible for
platelet agglutination in plasma and inducing signal
transduction
Alexei Navdaev,
Dagmar Dörmann,
Jeannine M. Clemetson, and
Kenneth J. Clemetson
From the Theodor Kocher Institute, University of Berne,
Switzerland.
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 (IgM) by peptide sequencing and
dot blotting with specific heavy and light chain anti-immunoglobulin reagents. Platelet agglutination by clustering echicetin with IgM
induced P-selectin expression and activation of GPIIb/IIIa as well as
tyrosine phosphorylation of several signal transduction molecules,
including p53/56LYN, p64, p72SYK, p70 to p90,
and p120. However, neither ethylenediaminetetraacetic acid nor specific
inhibition of GPIIb/IIIa 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.
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