Structurally distinct disintegrins contortrostatin and multisquamatin differentially regulate platelet tyrosine phosphorylation.

Tyrosine phosphorylation of multiple platelet proteins is regulated by the integrin alpha IIb beta 3. In order to further examine integrin-regulated tyrosine phosphorylation, we have used small Arg-Gly-Asp-containing snake venom proteins (termed disintegrins) that inhibit platelet aggregation to competitively block the agonist-induced binding of fibrinogen to alpha IIb beta 3. One structurally unique disintegrin, contortrostatin (which appears to be a disulfide-linked dimer of 13.5 kDa with two Arg-Gly-Asp sites), was found to trigger signaling events typically mediated by fibrinogen cross-linking of alpha IIb beta 3, as demonstrated by tyrosine phosphorylation of the tyrosine kinase pp72syk and a 140-kDa protein. Contortrostatin and another disintegrin, multisquamatin (a monomer of 5.7 kDa with a single Arg-Gly-Asp site), did not affect thrombin-induced platelet shape change, secretion, or integrin-independent tyrosine phosphorylation; however, they inhibited aggregation and aggregation-dependent tyrosine phosphorylation of numerous proteins, including the focal adhesion kinase pp125FAK. Our results suggest that structurally distinct disintegrins have varying effects on tyrosine phosphorylation; while monomeric multisquamatin and dimeric contortrostatin both inhibit aggregation-dependent tyrosine phosphorylation, contortrostatin also possesses a unique functional activity that allows it to activate an intracellular signaling pathway leading to tyrosine phosphorylation. This activity may be involved in the function of this snake venom protein on hemostasis.