Jararhagin is a high-molecular-mass (52 kDa) haemorrhagic metalloproteinase from Bothrops jararacavenom and a member of the metalloproteinase/disintegrin/cysteine-rich protein family. The disintegrin domain of jararhagin has been implicated in the inhibition of platelet responses to collagen by a mechanism that is not entirely known. The present investigation demonstrates that both active and 1,10-phenanthroline-inactivated jararhagin inhibit platelet aggregation by collagen with an IC50 of 40 and 140 nM respectively. The apparently higher inhibitory effect of the active enzyme clearly indicates that, in addition to the disintegrin region, the metalloproteinase domain of jararhagin also participates in this inhibition. As collagen interacts with platelets via α2β1-integrin, we investigated the effects of jararhagin on this integrin using selected function-blocking monoclonal antibodies against both of its subunits. Flow cytometry of platelets treated with native jararhagin and immunoprecipitation of platelet surface glycoproteins from lysates after jararhagin treatment showed an apparently selective reduction of α2β1-integrin immunoreactivity with both anti-α2 and anti-β1 monoclonal antibodies. The loss of immunoreactivity was not due to integrin internalization, since it also took place in cytochalasin D-treated platelets. Here we show that jararhagin cleaved isolated α2β1-integrin resulting in the generation of a 115 kDa β1 fragment. We therefore propose that the inhibition by jararhagin of platelet response to collagen is mediated through the binding of jararhagin to platelet α2-subunit via the disintegrin domain, followed by proteolysis of the β1-subunit with loss of the integrin structure (conformation) necessary for the binding of macromolecular ligands.
Present address: Department of Haematology, Manchester Royal Infirmary, University of Manchester, Manchester, U.K.