ABSTRACT
The structural basis of the interaction of integrin heterodimers with their physiological ligands is poorly understood. We have used solution x-ray scattering to visualize the head region of integrin alpha 5 beta 1 in an inactive (Ca2+-occupied) state, and in complex with a fragment of fibronectin containing the RGD and synergy recognition sequences. Shape reconstructions of the data have been interpreted in terms of appropriate molecular models. The scattering data suggest that the head region undergoes no gross conformational changes upon ligand binding but do lend support to a proposed outward movement of the hybrid domain in the beta subunit. Fibronectin is observed to bind across the top of the head region, which contains an alpha subunit beta-propeller and a beta subunit vWF type A domain. The model of the complex indicates that the synergy region binds on the side of the beta-propeller domain. In support of this suggestion, mutagenesis of a prominent loop region on the side of the propeller identifies two residues (Tyr208 and Ile210) involved in recognition of the synergy region. Our data provide the first view of a complex between an integrin and a macromolecular ligand in solution, at a nominal resolution of approximately 10 A.
Subject(s)
Integrin alpha5/chemistry , Integrin alpha5/genetics , Integrin beta1/chemistry , Integrin beta1/genetics , Binding Sites , Fibronectins/chemistry , Fibronectins/metabolism , Humans , In Vitro Techniques , Integrin alpha5/metabolism , Integrin beta1/metabolism , Kinetics , Ligands , Macromolecular Substances , Models, Molecular , Mutagenesis, Site-Directed , Oligopeptides/chemistry , Peptide Fragments/chemistry , Peptide Fragments/metabolism , Protein Structure, Tertiary , Recombinant Proteins/chemistry , Recombinant Proteins/genetics , Recombinant Proteins/metabolism , Scattering, Radiation , Solutions , X-RaysABSTRACT
Historical data, available in the literature, concerning the sites responsible for integrin-ligand binding and activation may now be transposed onto recently solved crystal structures of integrin dimers and functional domains. The data were obtained through studies of natural and engineered mutations within integrins, and analyses of the epitopes for function-altering monoclonal antibodies. In the past two years, this combining of structure and function has led to the development of novel hypotheses for the mechanisms of integrin regulation.
