Résumé
Two isolated recombinant fragments from human integrin α5β1 encompassing the FG-GAP repeats III to VII of α5 and the insertion-type domain from β1, respectively, are structurally well defined in solution, based on CD evidence. Divalent cation binding induces a conformational adaptation that is achieved by Ca2+ or Mg2+ (or Mn2+) with α5 and only by Mg2+ (or Mn2+) with β1. Mn2+ bound to β1 is highly hydrated (∼3 water molecules), based on water NMR relaxation, in agreement with a metal ion-dependent adhesion site-type metal coordination. Each fragment saturated with Mg2+ (or Mn2+) binds a recombinant fibronectin ligand in an RGD-dependent manner. A conformational rearrangement is induced on the fibronectin ligand upon binding to the α5, but not to the β1 fragment, based on CD. Ligand binding results in metal ion displacement from β1. Both α5 and β1 fragments form a stable heterodimer (α5β1 mini-integrin) that retains ligand recognition to form a 1:1:1 ternary complex, in the presence of Mg2+, and induces a specific conformational adaptation of the fibronectin ligand. A two-site model for RGD binding to both α and β integrin components is inferred from our data using low molecular weight RGD mimetics.