2 CONFORMATIONS OF THE INTEGRIN A-DOMAIN (I-DOMAIN) - A PATHWAY FOR ACTIVATION

Background: Integrins are plasma membrane proteins that mediate adhesi on to other cells and to components of the extracellular matrix. Most integrins are constitutively inactive in resting cells, but are rapidl y and reversibly activated in response to agonists, leading to highly regulated cell adhesion. This activation is associated with conformati onal changes in their extracellular portions, but the nature of the st ructural changes that lead to a change in adhesiveness is not understo od. The interactions of several integrins with their extracellular lig ands are mediated by an A-type domain (generally called the I-domain i n integrins). Binding of the I-domain to protein ligands is dependent on divalent cations. We have described previously the structure of the I-domain from complement receptor 3 with bound Mg2+, in which the glu tamate side chain from a second I-domain completes the octahedral coor dination sphere of the metal, acting as a ligand mimetic. Results: We now describe a new crystal form of the I-domain with bound Mn2+, in wh ich water completes the metal coordination sphere and there is no equi valent of the glutamate ligand. Comparison of the two crystal forms re veals a change in metal coordination which is linked to a large (10 An gstrom) shift of the C-terminal helix and the burial of two phenylalan ine residues into the hydrophobic core oi the Mn2+ form. These structu ral changes, analogous to those seen in the signal-transducing G-prote ins, alter the electrophilicity of the metal, reducing its ability to bind ligand-associated acidic residues, and dramatically alter the sur face of the protein implicated in binding ligand. Conclusions: Our obs ervations provide the first atomic resolution view of conformational c hanges in an integrin domain, and suggest how these changes are linked to a change in integrin adhesiveness. We propose that the Mg2+ form r epresents the conformation of the domain in the active state and the M n2+ form the conformation in the inactive state of the integrin.