Structure of integrin α5β1 in complex with fibronectin

The membrane-distal headpiece of integrins has evolved to specifically bind large extracellular protein ligands, but the molecular architecture of the resulting complexes has not been determined. We used molecular electron microscopy to determine the three-dimensional structure of the ligand-binding headpiece of integrin alpha(5)beta(1) complexed with fragments of its physiological ligand fibronectin. The density map for the unliganded alpha(5)beta(1) headpiece shows a 'closed' conformation similar to that seen in the alpha(V)beta(3) crystal structure. By contrast, binding to fibronectin induces an 'open' conformation with a dramatic, similar to80degrees change in the angle of the hybrid domain of the beta subunit relative to its I-like domain. The fibronectin fragment binds to the interface between the beta-propeller and I-like domains in the integrin headpiece through the RGD-containing module 10, but direct contact of the synergy-region-containing module 9 to integrin is not evident. This finding is corroborated by kinetic analysis of real-time binding data, which shows that the synergy site greatly enhances k(on) but has little effect on the stability or k(off) of the complex.