Crystal structure of α5β1 integrin ectodomain: Atomic details of the fibronectin receptor

Integrin alpha 5 beta 1 is a major cellular receptor for the extracellular matrix protein fibronectin and plays a fundamental role during mammalian development. A crystal structure of the alpha 5 beta 1 integrin headpiece fragment bound by an allosteric inhibitory antibody was determined at a 2.9-angstrom resolution both in the absence and presence of a ligand peptide containing the Arg-Gly-Asp (RGD) sequence. The antibody-bound beta 1 chain accommodated the RGD ligand with very limited structural changes, which may represent the initial step of cell adhesion mediated by nonactivated integrins. Furthermore, a molecular dynamics simulation pointed to an important role for Ca2+ in the conformational coupling between the ligand-binding site and the rest of the molecule. The RGD-binding pocket is situated at the center of a trenchlike exposed surface on the top face of alpha 5 beta 1 devoid of glycosylation sites. The structure also enabled the precise prediction of the acceptor residue for the auxiliary synergy site of fibronectin on the alpha 5 subunit, which was experimentally confirmed by mutagenesis and kinetic binding assays.