α V

Many questions remain about the significance of structural features of integrin α(V)β(3) for its mechanism of activation. We have determined and re-refined, respectively, crystal structures of the α(V)β(3) ectodomain linked to C-terminal coiled-coils (α(V)β(3)-AB) or four transmembrane (TM) residues in each subunit (α(V)β(3)-1TM). The α(V) and β(3) subunits with four and eight extracellular domains, respectively, are bent at knees between the integrin headpiece and lower legs, and the headpiece has the closed, low-affinity conformation. The structures differ in occupancy of three metal binding sites in the βI domain. Occupancy appears related to the pH of crystallization, rather than to physiologic regulation of ligand binding at the central, metal-ion dependent adhesion site (MIDAS). No electron density was observed for TM residues and much of the α(V) linker. α(V)β(3)-AB and α(V)β(3)-1TM demonstrate flexibility in the linker between their extracellular and TM domains, rather than previously proposed rigid linkage. A previously postulated interface between the α(V) and β(3) subunits at their knees was also not supported, because it lacks high quality density, required rebuilding in α(V)β(3)-1TM, and differed markedly between α(V)β(3)-1TM and α(V)β(3)-AB. Together with variation in domain-domain orientation within their bent ectodomains between α(V)β(3)-AB and α(V)β(3)-1TM, the structures are compatible with the requirement of large structural changes, such as extension at the knees and headpiece opening, in conveying activation signals between the extracellular ligand-binding site and the cytoplasm