Dual function for a unique site within the β 2I domain of integrin α Mβ 2

Abstract

Integrin activation has been postulated to occur in part via conformational changes in the I domain of the β subunit (the βI domain), especially near the F-α 7 loop, in response to "inside-out" signaling. However, direct evidence for a role of the F-α 7 loop in ligand binding and activity modulation is still lacking. Here, we report our finding that the F-α 7 loop (residues 344-358) within the β 2I domain has dual functions in ligand binding by α Mβ 2. On the one hand, it supports intercellular adhesion molecule 1 (ICAM-1) binding to α Mβ 2 directly as part of a recognition interface formed by five noncontiguous segments (Pro 192-Glu 197, Asn 213-Glu 220, Leu 225-Leu 230, Ser 324-Thr 329, and Glu 344-Asp 348) on the apex of the β 2I domain. On the other hand, it controls the open and closed conformation of the α Mβ 2 receptor, thereby indirectly affecting α Mβ 2 binding to other ligands. Switching the five constituent sequences of the ICAM-1-binding site within the β 2I domain to their β 1 counterparts destroyed ICAM-1 binding but had no effect on the gross conformations of the receptor. Of the five ICAM-1 binding-defective mutants, four had normal or even stronger interaction with Fg and C3bi, as reported in our previous study. Synthetic peptides derived from the identified site inhibited α Mβ 2-ICAM-1 interaction and supported direct binding to ICAM-1. Most importantly, perturbation of the F-α 7 loop caused conformational changes within the β 2I domain, which was further propagated to other regions of α Mβ 2. Altogether, our data demonstrate that inside-out signaling could modulate ligand binding directly by changing the ligand-binding pocket per se and/or indirectly by inducing multiple conformational changes within the receptor. © 2005 by The American Society for Biochemistry and Molecular Biology, Inc.