α\u3csub\u3ev\u3c/sub\u3eβ\u3csub\u3e3\u3c/sub\u3e integrin crystal structures and their functional implications

Many questions about the significance of structural features of integrin αVβ3 with respect to its mechanism of activation remain. We have determined and re-refined crystal structures of the αVβ3 ectodomain linked to C-terminal coiled coils (αVβ3-AB) and four transmembrane (TM) residues in each subunit (αVβ3-1TM), respectively. The αV and β3 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 the occupancy of three metal-binding sites in the βI domain. Occupancy appears to be related to the pH of crystallization, rather than to the physiologic regulation of ligand binding at the central, metal ion-dependent adhesion site. No electron density was observed for TM residues and much of the αV linker. αVβ3-AB and αVβ 3-1TM demonstrate flexibility in the linker between their extracellular and TM domains, rather than the previously proposed rigid linkage. A previously postulated interface between the αV and β3 subunits at their knees was also not supported, because it lacks high-quality density, required rebuilding in αVβ 3-1TM, and differed markedly between αVβ 3-1TM and αVβ3-AB. Together with the variation in domain-domain orientation within their bent ectodomains between αVβ3-AB and αVβ 3-1TM, these findings are compatible with the requirement for 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. © 2012 American Chemical Society