alpha(2)beta(1) integrin is not recognized by rhodocytin but is the specific, high affinity target of rhodocetin, an RGD-independent disintegrin and potent inhibitor of cell adhesion to collagen

We have recombinantly expressed a soluble form of human alpha (2)beta (1) i ntegrin that lacks the membrane-anchoring transmembrane domains as well as the cytoplasmic tails of both integrin subunits. This soluble alpha (2)beta (1) integrin binds to its collagen ligands the same way as the wildtype al pha (2)beta (1) integrin. Furthermore, like the wild-type form, it can be a ctivated by manganese ions and an integrin-activating antibody. However, it does not bind to rhodocytin, a postulated agonist of alpha (2)beta (1) int egrin from the snake venom of Calloselasma rhodostoma, which elicits platel et aggregation, Tacking advantage of the recombinantly expressed, soluble a lpha (2)beta (1) integrin, an inhibition assay was established in which sam ples can be tested for their capability to inhibit binding of soluble alpha (2)beta (1) integrin to immobilized collagen. Thus, by scrutinizing the C. rhodostoma snake venom in this protein-protein interaction assay, we found a component of the snake venom that inhibits the interaction of soluble al pha (2)beta (1) integrin to type I collagen efficiently. N-terminal sequenc es identified this inhibitor as rhodocetin, a recently published antagonist of collagen-induced platelet aggregation. We could demonstrate that its in hibitory effect bases on its strong and specific binding to alpha (2)beta ( 1) integrin, proving that rhodocetin is a disintegrin. Standing apart from the growing group of RGD-dependent snake venom disintegrins, rhodocetin int eracts with alpha (2)beta (1) integrin in an RGD-independent manner. Furthe rmore, its native conformation, which is stabilized by disulfide bridges, i s indispensibly required for its inhibitory activity. Rhodocetin does not c ontain any major collagenous structure despite its high affinity to alpha ( 2)beta (1) integrin, which binds to collagenous molecules much more avidly than to noncollagenous Ligands, such as laminin. Blocking alpha (2)beta (1) integrin as the major collagen receptor on platelets, rhodocetin is respon sible for hampering collagen-induced, alpha (2)beta (1) integrin-mediated p latelet activation, leading to hemorrhages and bleeding disorders of the sn akebite victim. Moreover, having a widespread tissue distribution, alpha (2 )beta (1) integrin also mediates cell adhesion, spreading, and migration. W e showed that rhodocetin is able to inhibit alpha (2)beta (1) integrin-medi ated adhesion of fibrosarcoma cells to type I collagen completely.