Activation of integrin β-subunit I-like domains by one-turn C-terminal α-helix deletions

Abstract

Integrins contain two structurally homologous but distantly related domains: an I-like domain that is present in all β-subunits and an I domain that is present in some α-subunits. Atomic resolution and mutagenesis studies of α I domains demonstrate a C-terminal, axial displacement of the α7-helix that allosterically regulates the shape and affinity of the ligand-binding site. Atomic resolution studies of β I-like domains have thus far demonstrated no similar α7-helix displacement; however, other studies are consistent with the idea that α I and β I-like domains undergo structurally analogous rearrangements. To test the hypothesis that C-terminal, axial displacement of the α7-helix, coupled with β6–α7 loop reshaping, activates β I-like domains, we have mimicked the effect of α7-helix displacement on the β6–α7 loop by shortening the α7-helix by two independent, four-residue deletions of about one turn of α-helix. In the case of integrin α_Lβ₂, each mutant exhibits constitutively high affinity for the physiological ligand intercellular adhesion molecule 1 and full exposure of a β I-like domain activation-dependent antibody epitope. In the case of analogous mutants in integrin α₄β₇, each mutant shows the activated phenotype of firm adhesion, rather than rolling adhesion, in shear flow. The results show that integrins that contain or lack α I domains share a common pathway of β I-like domain activation, and they suggest that β I-like and α I domain activation involves structurally analogous α7-helix axial displacements.