The Integrin α9β1 Binds to a Novel Recognition Sequence (SVVYGLR) in the Thrombin-cleaved Amino-terminal Fragment of Osteopontin

Abstract

The integrin alpha(9)beta(1) mediates cell adhesion to tenascin-C and VCAM-1 by binding to sequences distinct from the common integrin-recognition sequence, arginine-glycine-aspartic acid (RGD). A thrombin-cleaved NH(2)-terminal fragment of osteopontin containing the RGD sequence has recently been shown to also be a ligand for alpha(9)beta(1). In this report, we used site-directed mutagenesis and synthetic peptides to identify the alpha(9)beta(1) recognition sequence in osteopontin. alpha(9)-transfected SW480, Chinese hamster ovary, and L-cells adhered to a recombinant NH(2)-terminal osteopontin fragment in which the RGD site was mutated to RAA (nOPN-RAA). Adhesion was completely inhibited by anti-alpha(9) monoclonal antibody Y9A2, indicating the presence of a non-RGD alpha(9)beta(1) recognition sequence within this fragment. Alanine substitution mutagenesis of 13 additional conserved negatively charged amino acid residues in this fragment had no effect on alpha(9)beta(1)-mediated adhesion, but adhesion was dramatically inhibited by either alanine substitution or deletion of tyrosine 165. A synthetic peptide, SVVYGLR, corresponding to the sequence surrounding Tyr(165), blocked alpha(9)beta(1)-mediated adhesion to nOPN-RAA and exposed a ligand-binding-dependent epitope on the integrin beta(1) subunit on alpha(9)-transfected, but not on mock-transfected cells. These results demonstrate that the linear sequence SVVYGLR directly binds to alpha(9)beta(1) and is responsible for alpha(9)beta(1)-mediated cell adhesion to the NH(2)-terminal fragment of osteopontin.