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

Integrins contain two structurally homologous but distantly related domains: an I-like domain that is present in all -subunits and an I domain that is present in some -subunits. Atomic resolution and mutagenesis studies of I domains demonstrate a C-terminal, axial displacement of the 7-helix that allosterically regulates the shape and affinity of the ligand-binding site. Atomic resolution studies of I-like domains have thus far demonstrated no similar 7-helix displacement; however, other studies are consistent with the idea that I and I-like domains undergo structurally analogous rearrangements. To test the hypothesis that C-terminal, axial displacement of the 7-helix, coupled with 6– 7 loop reshaping, activates I-like domains, we have mimicked the effect of 7-helix displacement on the 6– 7 loop by shortening the 7-helix by two independent, four-residue deletions of about one turn of -helix. In the case of integrin L 2, each mutant exhibits constitutively high affinity for the physiological ligand intercellular adhesion molecule 1 and full exposure of a I-like domain activationdependent antibody epitope. In the case of analogous mutants in integrin 4 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 I domains share a common pathway of I-like domain activation, and they suggest that I-like and I domain activation involves structurally analogous 7-helix axial displacements.