The α2β1 integrin inhibitor rhodocetin binds to the A-domain of the integrin α2 subunit proximal to the collagen-binding site

Rhodocetin is a snake venom protein that binds to alpha2beta1 integrin, inhibiting its interaction with its endogenous ligand collagen. We have determined the mechanism by which rhodocetin inhibits the function of alpha2beta1. The interaction of alpha2beta1 with collagen and rhodocetin differed: Ca(2+) ions and slightly acidic pH values increased the binding of alpha2beta1 integrin to rhodocetin in contrast with their attenuating effect on collagen binding, suggesting that rhodocetin preferentially binds to a less active conformation of alpha2beta1 integrin. The alpha2A-domain [von Willebrand factor domain A homology domain (A-domain) of the integrin alpha2 subunit] is the major site for collagen binding to alpha2beta1. Recombinant alpha2A-domain bound rhodocetin, demonstrating that the A-domain is also the rhodocetin-binding domain. Although the interaction of alpha2beta1 with rhodocetin is affected by altering divalent cations, the interaction of the A-domain was divalent-cation-independent. The rhodocetin-binding site on the alpha2A-domain was mapped first by identifying an anti-alpha2 antibody that blocked rhodocetin binding and then mapping the epitope of the antibody using human-mouse alpha2A-domain chimaeras; and secondly, by binding studies with alpha2A-domain, which bear point mutations in the vicinity of the mapped epitope. In this way, the rhodocetin-binding site was identified as the alpha3-alpha4 loop plus adjacent alpha-helices. This region is known to form part of the collagen-binding site, thus attaining a mainly competitive mode of inhibition by rhodocetin.