Determination of the structure of two novel echistatin variants and comparison of the ability of echistatin variants to inhibit aggregation of platelets from different species

Two new variants of short disintegrins were purified from the venom of Echis carinatus leakeyi and named echistatin β and γ. These proteins were found to be about 85% similar in amino acid sequence to echistatin α which has been well studied. The disulphide pattern of echistatin γ appeared to be identical with that of echistatin α. They all contain the adhesive recognition sequence Arg-Gly-Asp (RGD) but inhibit the aggregation of platelets from human and other mammals with different potencies. Echistatin β and α are far more effective on platelets from humans and guinea pigs than those from rabbits and rats whereas echistatin γ is less discriminating of the platelets of the species tested. This species-dependent platelet sensitivity to echistatin β and γ could be attributed to the variations in residues 15, 21, 22 and 27, which are close to or within the RGD loop, rather than to the C-terminal variations after residue 46. Taking advantage of the presence of methionine residues flanking both sides of the ARGDDM motif in echistatin γ, we deleted this hexapeptide by CNBr cleavage to produce des-(23-28)-echistatin γ. The modified protein showed c.d. and fluorescent spectra grossly similar to the intact echistatin but its antiplatelet potency decreased more than 200-fold. We thus propose that a favourable conformation of the RGD region is responsible mainly for the high-affinity binding of echistatin to the platelet glycoprotein IIb-IIIa as shown previously for the binding of medium-size disintegrin.