title: The integrin alpha-subunit leg extends at a Ca2+-dependent epitope in the thigh/genu interface upon activation.
abstract: Two activation-dependent Abs to the integrin alphaL-subunit were used to study conformational rearrangement of alphaLbeta2 on the cell surface. Activation lowered the concentration of Ca2+ required for maximal expression of each epitope. Each Ab requires the Ca2+-binding loop in the integrin genu and nearby species-specific residues in the thigh domain. Key thigh residues are shielded from Ab in the bent integrin conformation by the alpha-subunit calf-1 domain and the nearby bent beta leg, suggesting that extension at the genu is required for epitope exposure. Activating stimuli and alpha/beta I-like small molecule antagonists demonstrate that exposure of epitopes in the integrin alpha- and beta-subunit legs is coordinate during integrin activation. A coordinating residue donated by the calf-1 domain is as important as Ca2+ for mAb binding. Together with inspection of the alphaV structure, this result suggests that the genu/calf-1 interface is maintained in integrin activation, and that extension occurs by a rearrangement at the thigh/genu interface.
authors.affiliation: CBR Institute for Biomedical Research and Departments of Pathology and Anesthesia, Harvard Medical School, 200 Longwood Avenue, Boston, MA 02115, USA.
authors.forename: Can
authors.initials: C
authors.lastname: Xie
authors.forename: Motomu
authors.initials: M
authors.lastname: Shimaoka
authors.forename: Tsan
authors.initials: T
authors.lastname: Xiao
authors.forename: Pascale
authors.initials: P
authors.lastname: Schwab
authors.forename: Lloyd B
authors.initials: LB
authors.lastname: Klickstein
authors.forename: Timothy A
authors.initials: TA
authors.lastname: Springer
mesh_terms: D000595:Amino Acid Sequence; D002118:Calcium / Q000502:physiology*; D002460:Cell Line; D018604:Epitope Mapping; D000939:Epitopes / Q000276:immunology / Q000502:physiology*; D005434:Flow Cytometry; D005455:Fluorescent Antibody Technique; D006801:Humans; D039001:Integrin alpha Chains / Q000737:chemistry / Q000502:physiology*; D008969:Molecular Sequence Data; D017386:Sequence Homology, Amino Acid
chemical_list: D000939:Epitopes; D039001:Integrin alpha Chains; D002118:Calcium