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

Integrins contain two structurally homologous but distantly related domains: an I-like domain that is present in all beta-subunits and an I domain that is present in some alpha-subunits. Atomic resolution and mutagenesis studies of a I domains demonstrate a C-terminal, axial displacement of the alpha7-helix that allosterically regulates the shape and affinity of the ligand-binding site. Atomic resolution studies of beta I-like domains have thus far demonstrated no similar a7-helix displacement; however, other studies are consistent with the idea that alpha I and beta I-like domains undergo structurally analogous rearrangements. To test the hypothesis that C-terminal, axial displacement of the alpha7-helix, coupled with beta6-alpha7 loop reshaping, activates beta I-like domains, we have mimicked the effect of a7-helix displacement on the beta6-alpha7 loop by shortening the a7-helix by two independent, four-residue deletions of about one turn of a-helix. In the case of integrin alpha(L)beta(2), each mutant exhibits constitutively high affinity for the physiological ligand intercellular adhesion molecule 1 and full exposure of a beta I-like domain activation-dependent antibody epitope. In the case of analogous mutants in integrin alpha(4)beta(7), 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 alpha I domains share a common pathway of beta I-like domain activation, and they suggest that beta I-like and alpha I domain activation involves structurally analogous alpha7-helix axial displacements.