CRITICAL AMINO-ACID-RESIDUES FOR LIGAND-BINDING ARE CLUSTERED IN A PREDICTED BETA-TURN OF THE 3RD N-TERMINAL REPEAT IN THE INTEGRIN ALPHA-4AND ALPHA-5 SUBUNITS

Integrin alpha 4 beta 1 is a receptor for vascular cell adhesion molec ule (VCAM)-1 and fibronectin (CS-1). The alpha 4 beta 1-ligand interac tion is involved in the pathogenesis of diseases and is, therefore, a therapeutic target, Here, we identified critical residues of alpha 4 f or ligand binding using alanine-scanning mutagenesis of the previously localized putative ligand binding sites (residues 108-268), Among 43 mutations tested, mutations of Tyr187, Trp188 and Gly190 significantly inhibited cell adhesion to both VCAM-1 and CS-1. This inhibition was not due to any gross structural changes of alpha 4 beta 1. These criti cal residues are clustered in a predicted beta-turn structure (residue s 181-190) of the third N-terminal repeat in alpha 4. The repeat does not contain divalent cation binding motifs, Notably, the mutations wit hin the corresponding region of alpha 5 significantly reduced fibronec tin-alpha 5 beta 1 interaction, These findings suggest that the predic ted beta-turn structure could be ubiquitously involved in ligand bindi ng of non-I domain integrins.