Disintegrins are potent inhibitors of both platelet aggregation and integrin-dependent cell adhesion. A new disintegrin, salmosin, isolated from the venom of the Korean snake Agkistrodon halys brevicaudus, has been characterized by mass spectrometry and NMR spectroscopy, and its in vitro biological activity has been assessed. The IC(50) value of the purified salmosin was determined to be 2.2 nM in an assay for the inhibition of glycoprotein IIb-IIIa/fibrinogen interaction. Salmosin also inhibited the bovine capillary endothelial cell proliferation induced by bFGF in a dose-dependent manner. The NMR solution structures were well converged with a root-mean-square deviation of 0.76 A for backbone atoms among the 20 lowest energy structures, except for the arginylglycylaspartic acid (RGD) loop. The structure revealed that the conserved RGD motif with an unusual finger shape is distal from the rigid core of the C-terminal domain. Furthermore, even though the RGD motif did not interact with the hydrophobic core of the protein, it was stabilized by a network of molecular contacts through a small antiparallel beta-sheet comprising residues of Ile46-Ala50 and Asp54-Tyr58. Last, the electrostatic charge distribution on the surface of salmosin differs dramatically from that of other disintegrin proteins in that there is a cluster of negatively charged residues in close proximity to the RGD loop.