Vipera lebetina venom contains two disintegrins inhibiting laminin-binding β1 integrins

To explain the myotoxic effects of snake venoms, we searched for inhibitors of alpha(7)beta(1) integrin, the major laminin-binding integrin in skeletal muscle. We discovered two inhibitors in the venom of Vipera lebetina. One of them, lebein-1 ( known as lebein), has already been proposed to be a disintegrin because of its RGD-containing primary sequence. The other, lebein-2, is a novel protein that also interacts firmly with alpha(3)beta(1), alpha(6)beta(1), and alpha(7)beta(1) integrins, but not with the collagen-binding alpha(1)beta(1) and alpha(2)beta(1) integrins. Ligand binding of laminin-recognizing beta(1) integrins was efficiently blocked by both lebein-1 and lebein-2. In cell attachment assays, lebein-1 and lebein-2 inhibited myoblast attachment not only to laminin, but also to fibronectin. However, neither lebein-1 nor lebein-2 interacted with alpha(7)beta(1) integrin in an RGD-dependent manner, similar to the interaction of the laminin with alpha(7)beta(1) integrin. Identical divalent cation dependence of integrin binding to laminin and to either of the two inhibitors and their mutually exclusive binding suggest that both lebein-1 and lebein-2 interact with the ligand-binding site of laminin-binding beta(1) integrins by mimicking the yet unknown integrin-binding structure of laminins. Like lebein-1, lebein-2 is a soluble heterodimeric disintegrin of low molecular mass. Together with membrane-bound ADAM-2 and ADAM-9, the two inhibitors seem to form a small group of disintegrins that can bind to laminin-binding beta(1) integrins. Because of their inhibitory capability both in vitro and in vivo, lebein-1 and lebein-2 may be valuable tools in influencing laminin-induced, integrin-mediated cell functions such as cell anchorage, migration, and mechanical force transduction on laminin-rich basement membranes.