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Bilinexin,a snake C-type lectin from Agkistrodon bilineatus venom agglutinates platelets via GPIb and α_2β_1

Kenneth J.Clemetson

   A new snake protein,named bilinexin,has been purified from Agkistrodon bilineatusvenom by ion-exchange chromatography and gel filtration chromatography.Under non-reducingconditions it has a mass of 110 kDa protein on SDS-PAGE.On reduction,it can be separated into fivesubunits with masses in the range of 13-25 kDa.The N-terminal sequences of these subunits are verysimilar to those of convulxin or the alboaggregins,identifying bilinexin as a new member of the snakeC-type lectin family,unusual in having multiple subunits.Unlike collagen-induced aggregation ofplatelets,bilinexin agglutinates fixed platelets,washed platelets and platelet rich plasma(PRP)withoutobvious activation(shape change),confirmed by light microscope examination.Both inhibitory andbinding studies indicate that antibodies against α_2β_1 inhibit not only platelet agglutination induced bybilinexin,but also bilinexin binding to platelets.VM16d,a monoclonal anti-GPIbαantibody,completely inhibits platelet agglutination induced by bilinexin,and polyclonal antibody against GPIbαprevents its binding to platelets.However,Neither convulxin,polyclonal anti-GPVI antibody norGPIIb/IIIa inhibitors affect its binding to and agglutination of platelets.Bilinexin neither activatesGPIIb/IIIa integrin on platelets and nor induces tyrosine phosphorylation of platelet proteins,norincreases intracellular Ca~(2+)in platelets.Like alboaggregin B,bilinexin agglutinates platelets,whichmakes it a good tool to investigate the differences in mechanism between snake C-type lectins causingplatelet agglutination and those that induce full activation.……