title: Insights into IgA-mediated immune responses from the crystal structures of human FcalphaRI and its complex with IgA1-Fc.
abstract: Immunoglobulin-alpha (IgA)-bound antigens induce immune effector responses by activating the IgA-specific receptor FcalphaRI (CD89) on immune cells. Here we present crystal structures of human FcalphaRI alone and in a complex with the Fc region of IgA1 (Fcalpha). FcalphaRI has two immunoglobulin-like domains that are oriented at approximately right angles to each other. Fcalpha resembles the Fcs of immunoglobulins IgG and IgE, but has differently located interchain disulphide bonds and external rather than interdomain N-linked carbohydrates. Unlike 1:1 FcgammaRIII:IgG and Fc epsilon RI:IgE complexes, two FcalphaRI molecules bind each Fcalpha dimer, one at each Calpha2-Calpha3 junction. The FcalphaRI-binding site on IgA1 overlaps the reported polymeric immunoglobulin receptor (pIgR)-binding site, which might explain why secretory IgA cannot initiate phagocytosis or bind to FcalphaRI-expressing cells in the absence of an integrin co-receptor.
authors.affiliation: Division of Biology 114-96, California Institute of Technology, Pasadena, California 91125, USA.
authors.forename: Andrew B
authors.initials: AB
authors.lastname: Herr
authors.forename: Edward R
authors.initials: ER
authors.lastname: Ballister
authors.forename: Pamela J
authors.initials: PJ
authors.lastname: Bjorkman
mesh_terms: D000595:Amino Acid Sequence; D015703:Antigens, CD / Q000737:chemistry* / Q000276:immunology*; D018360:Crystallography, X-Ray; D006801:Humans; D007070:Immunoglobulin A / Q000737:chemistry* / Q000276:immunology*; D007073:Immunoglobulin E / Q000737:chemistry; D007128:Immunoglobulin Fragments / Q000737:chemistry / Q000276:immunology; D007074:Immunoglobulin G / Q000737:chemistry; D008969:Molecular Sequence Data; D020836:Protein Structure, Quaternary; D011961:Receptors, Fc / Q000737:chemistry* / Q000276:immunology*; D015398:Signal Transduction
chemical_list: D015703:Antigens, CD; C036832:Fc(alpha) receptor; C026930:Fch fragments; D007070:Immunoglobulin A; D007128:Immunoglobulin Fragments; D007074:Immunoglobulin G; D011961:Receptors, Fc; D007073:Immunoglobulin E