title: α(V)β(3) integrin crystal structures and their functional implications.
abstract: Many questions about the significance of structural features of integrin α(V)β(3) with respect to its mechanism of activation remain. We have determined and re-refined crystal structures of the α(V)β(3) ectodomain linked to C-terminal coiled coils (α(V)β(3)-AB) and four transmembrane (TM) residues in each subunit (α(V)β(3)-1TM), respectively. 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 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 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 the 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 the variation in domain-domain orientation within their bent ectodomains between α(V)β(3)-AB and α(V)β(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.
authors.affiliation: Immune Disease Institute, Children's Hospital Boston, and Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, 3 Blackfan Circle, Boston, Massachusetts 02115, United States.
authors.forename: Xianchi
authors.initials: X
authors.lastname: Dong
authors.forename: Li-Zhi
authors.initials: LZ
authors.lastname: Mi
authors.forename: Jianghai
authors.initials: J
authors.lastname: Zhu
authors.forename: Wei
authors.initials: W
authors.lastname: Wang
authors.forename: Ping
authors.initials: P
authors.lastname: Hu
authors.forename: Bing-Hao
authors.initials: BH
authors.lastname: Luo
authors.forename: Timothy A
authors.initials: TA
authors.lastname: Springer
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D016466:CHO Cells; D006224:Cricetinae; D003460:Crystallization; D018360:Crystallography, X-Ray; D057809:HEK293 Cells; D006801:Humans; D039302:Integrin alphaVbeta3 / Q000737:chemistry*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011487:Protein Conformation; D017434:Protein Structure, Tertiary
chemical_list: D039302:Integrin alphaVbeta3