The integrin ␣91 is expressed on epithelial cells, smooth muscle cells, skeletal muscle, and neutrophils and recognizes at least three distinct ligands: vascular cell adhesion molecule 1 (VCAM-1), tenascin-C, and osteopontin. The ␣9 subunit is structurally similar to the integrin ␣4 subunit, and ␣91 and ␣41 both recognize VCAM-1 as a ligand. We therefore examined whether the disintegrin EC3, which we have recently shown specifically inhibits the binding of ␣4 integrins to ligands, would also be a functional inhibitor of ␣91. EC3 and a novel heterodimeric disintegrin that we identified, EC6, both were potent inhibitors of ␣91-mediated adhesion to VCAM-1 and of neutrophil migration across tumor necrosis factor-activated endothelial cells. A peptide containing a novel MLDG motif shared by both of these disintegrins also inhibited ␣91-and ␣41-mediated adhesion to VCAM-1. Surprisingly though, concentrations of EC3 that completely inhibited adhesion of ␣9-transfected cells to VCAM-1 had little or no effect on adhesion to either of the other ␣91 ligands, osteopontin and tenascin-C. Furthermore, peptides AEIDGIEL and SV-VYGLR, which we have previously shown inhibit binding of ␣91-expressing cells to tenascin-C and osteopontin, respectively, had no effect on adhesion to VCAM-1. These data suggest that there are structurally distinct requirements for interactions of the ␣91 integrin with VCAM-1 and the extracellular matrix ligands osteopontin and tenascin-C.The integrin ␣9 subunit forms a single known heterodimer, ␣91, that is widely expressed in epithelia and smooth and skeletal muscle and on neutrophils (1, 2). We and others have identified three distinct ligands for ␣91: the extracellular matrix proteins tenascin-C (3) and osteopontin (4 -6) and the inducible endothelial immunoglobulin family member, VCAM-1 1 (1). We have mapped the ligand-binding site in tenascin-C to an exposed peptide loop in the third fibronectin type III repeat containing the sequence AEIDGIEL (7). We have also mapped the ␣91 ligand-binding site in osteopontin (6). Although an initial report suggested that ␣91 might bind to an RGD-containing sequence in osteopontin (5), we were able to demonstrate by extensive mutagenesis that the binding site is within the linear peptide sequence SVVYGLR immediately adjacent to the RGD site (6).Structurally, the ␣9 subunit is closely related to the ␣4 subunit, and on the basis of sequence homology ␣4 and ␣9 appear to be the only known members of a subfamily of integrin ␣ subunits that lack both an insertional domain and an extracellular disulfide-linked cleavage site (2). Furthermore, both subunits are expressed on leukocytes and mediate leukocyte migration (1, 8). Finally, both integrins recognize VCAM-1 as a ligand (1, 9). We have shown that both ␣41 and ␣91 contribute to the chemotactic migration of human neutrophils across endothelial cell monolayers activated by tumor necrosis factor-␣ (TNF␣), an effect that is due at least in part to interaction of these integrins with VCAM-1 induced in resp...
