title: Stable coordination of the inhibitory Ca2+ ion at the metal ion-dependent adhesion site in integrin CD11b/CD18 by an antibody-derived ligand aspartate: implications for integrin regulation and structure-based drug design.
abstract: A central feature of integrin interaction with physiologic ligands is the monodentate binding of a ligand carboxylate to a Mg(2+) ion hexacoordinated at the metal ion-dependent adhesion site (MIDAS) in the integrin A domain. This interaction stabilizes the A domain in the high-affinity state, which is distinguished from the default low-affinity state by tertiary changes in the domain that culminate in cell adhesion. Small molecule ligand-mimetic integrin antagonists act as partial agonists, eliciting similar activating conformational changes in the A domain, which has contributed to paradoxical adhesion and increased patient mortality in large clinical trials. As with other ligand-mimetic integrin antagonists, the function-blocking mAb 107 binds MIDAS of integrin CD11b/CD18 A domain (CD11bA), but in contrast, it favors the inhibitory Ca(2+) ion over the Mg(2+) ion at MIDAS. We determined the crystal structures of the Fab fragment of mAb 107 complexed to the low- and high-affinity states of CD11bA. Favored binding of the Ca(2+) ion at MIDAS is caused by the unusual symmetric bidentate ligation of a Fab-derived ligand Asp to a heptacoordinated MIDAS Ca(2+) ion. Binding of the Fab fragment of mAb 107 to CD11bA did not trigger the activating tertiary changes in the domain or in the full-length integrin. These data show that the denticity of the ligand Asp/Glu can modify the divalent cation selectivity at MIDAS and hence integrin function. Stabilizing the Ca(2+) ion at MIDAS by bidentate ligation to a ligand Asp/Glu may provide one approach for designing pure integrin antagonists.
authors.affiliation: Structural Biology Program, Division of Nephrology, Massachusetts General Hospital, Harvard Medical School, Charlestown, MA 02129, USA.
authors.forename: Bhuvaneshwari
authors.initials: B
authors.lastname: Mahalingam
authors.forename: Kaouther
authors.initials: K
authors.lastname: Ajroud
authors.forename: José Luis
authors.initials: JL
authors.lastname: Alonso
authors.forename: Saurabh
authors.initials: S
authors.lastname: Anand
authors.forename: Brian D
authors.initials: BD
authors.lastname: Adair
authors.forename: Alberto L
authors.initials: AL
authors.lastname: Horenstein
authors.forename: Fabio
authors.initials: F
authors.lastname: Malavasi
authors.forename: Jian-Ping
authors.initials: JP
authors.lastname: Xiong
authors.forename: M Amin
authors.initials: MA
authors.lastname: Arnaout
mesh_terms: D000818:Animals; D019138:Antibodies, Blocking / Q000737:chemistry / Q000378:metabolism*; D001224:Aspartic Acid / Q000276:immunology* / Q000378:metabolism; D001666:Binding Sites, Antibody / Q000276:immunology; D039481:CD11b Antigen / Q000737:chemistry / Q000378:metabolism*; D018821:CD18 Antigens / Q000737:chemistry / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism*; D002413:Cations, Divalent / Q000378:metabolism; D002448:Cell Adhesion / Q000276:immunology; D018360:Crystallography, X-Ray; D004306:Dose-Response Relationship, Immunologic; D015195:Drug Design; D006801:Humans; D007140:Immunoglobulin Fab Fragments / Q000737:chemistry / Q000378:metabolism; D020014:K562 Cells; D008024:Ligands; D009504:Neutrophils / Q000276:immunology / Q000378:metabolism; D051381:Rats
chemical_list: D019138:Antibodies, Blocking; D039481:CD11b Antigen; D018821:CD18 Antigens; D002413:Cations, Divalent; D007140:Immunoglobulin Fab Fragments; D008024:Ligands; D001224:Aspartic Acid; D002118:Calcium
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