title: Three-dimensional structure of the Fab from a human IgM cold agglutinin.
abstract: Cold agglutinins (CAs) are IgM autoantibodies characterized by their ability to agglutinate in vitro RBC at low temperatures. These autoantibodies cause hemolytic anemia in patients with CA disease. Many diverse Ags are recognized by CAs, most frequently those belonging to the I/i system. These are oligosaccharides composed of repeated units of N:-acetyllactosamine, expressed on RBC. The three-dimensional structure of the Fab of KAU, a human monoclonal IgM CA with anti-I activity, was determined. The KAU combining site shows an extended cavity and a neighboring pocket. Residues from the hypervariable loops V(H)CDR3, V(L)CDR1, and V(L)CDR3 form the cavity, whereas the small pocket is defined essentially by residues from the hypervariable loops V(H)CDR1 and V(H)CDR2. This fact could explain the V(H)4-34 germline gene restriction among CA. The KAU combining site topography is consistent with one that binds a polysaccharide. The combining site overall dimensions are 15 A wide and 24 A long. Conservation of key binding site residues among anti-I/i CAs indicates that this is a common feature of this family of autoantibodies. We also describe the first high resolution structure of the human IgM C(H)1:C(L) domain. The structural analysis shows that the C(H)1-C(L) interface is mainly conserved during the isotype switch process from IgM to IgG1.
authors.affiliation: Cátedra de Inmunología, Instituto de Estudios de la Inmunidad Humoral (IDEHU), Facultad de Farmacia y Bioquímica UBA, Buenos Aires, Argentina.
authors.forename: A
authors.initials: A
authors.lastname: Cauerhff
authors.forename: B C
authors.initials: BC
authors.lastname: Braden
authors.forename: J G
authors.initials: JG
authors.lastname: Carvalho
authors.forename: R
authors.initials: R
authors.lastname: Aparicio
authors.forename: I
authors.initials: I
authors.lastname: Polikarpov
authors.forename: J
authors.initials: J
authors.lastname: Leoni
authors.forename: F A
authors.initials: FA
authors.lastname: Goldbaum
mesh_terms: D000373:Agglutinins / Q000737:chemistry*; D000744:Anemia, Hemolytic, Autoimmune / Q000276:immunology; D000818:Animals; D001323:Autoantibodies / Q000737:chemistry; D003080:Cold Temperature; D003198:Computer Simulation; D003450:Cryoglobulins; D003460:Crystallization; D006388:Hemagglutinins / Q000737:chemistry*; D006801:Humans; D007127:Immunoglobulin Constant Regions / Q000737:chemistry; D007140:Immunoglobulin Fab Fragments / Q000737:chemistry*; D007143:Immunoglobulin Heavy Chains / Q000737:chemistry; D007132:Immunoglobulin Isotypes / Q000737:chemistry; D007147:Immunoglobulin Light Chains / Q000737:chemistry; D007075:Immunoglobulin M / Q000737:chemistry*; D007135:Immunoglobulin Variable Region / Q000737:chemistry; D051379:Mice; D008958:Models, Molecular
chemical_list: D000373:Agglutinins; D001323:Autoantibodies; D003450:Cryoglobulins; D006388:Hemagglutinins; D007127:Immunoglobulin Constant Regions; D007140:Immunoglobulin Fab Fragments; D007143:Immunoglobulin Heavy Chains; D007132:Immunoglobulin Isotypes; D007147:Immunoglobulin Light Chains; D007075:Immunoglobulin M; D007135:Immunoglobulin Variable Region; C029443:cold agglutinins