title: Identification of non-immunoglobulin A-Fc-binding forms and low-molecular-weight secreted forms of the group B streptococcal beta antigen.
abstract: The beta antigen expressed on the surfaces of certain strains of group B streptococci has been reported to bind to the Fc region of human immunoglobulin A (IgA). In this study, we screened 100 isolates of group B streptococci for expression of both beta antigen and IgA-Fc-binding activity. We identified two isolates which expressed the beta antigen but could not bind human IgA Fc fragments and also observed variability in IgA-Fc-binding activity among other beta-antigen-expressing strains. Novel low-molecular-weight forms of beta antigen were secreted by four beta-antigen surface-negative isolates and included IgA-Fc-binding (Mrs, 55,000 and 53,000) and non-IgA-Fc-binding (Mr, 38,000) molecules. These results suggest that the IgA-Fc-binding site represents a unique domain of the beta antigen. The 55,000- and 53,000-Mr forms of secreted beta antigen were functionally and antigenically representative of the size-heterogeneous (Mr, up to 145,000) beta-antigen molecules expressed by surface-positive strains. The cell surface-localized IgA-Fc-binding molecules could bind only human serum IgA efficiently; however, once solubilized, these molecules could bind both human serum and secretory IgAs.
authors.affiliation: Department of Immunology and Medical Microbiology, University of Florida College of Medicine, Gainesville 32610.
authors.forename: L J
authors.initials: LJ
authors.lastname: Brady
authors.forename: M D
authors.initials: MD
authors.lastname: Boyle
mesh_terms: D000942:Antigens, Bacterial / Q000502:physiology*; D000954:Antigens, Surface / Q000502:physiology; D001426:Bacterial Proteins / Q000378:metabolism*; D007070:Immunoglobulin A / Q000378:metabolism*; D007071:Immunoglobulin A, Secretory / Q000378:metabolism; D007141:Immunoglobulin Fc Fragments / Q000378:metabolism*; D008970:Molecular Weight; D012995:Solubility; D013292:Streptococcus agalactiae / Q000276:immunology*
chemical_list: D000942:Antigens, Bacterial; D000954:Antigens, Surface; D001426:Bacterial Proteins; D007070:Immunoglobulin A; D007071:Immunoglobulin A, Secretory; D007141:Immunoglobulin Fc Fragments