title: Binding of native alpha 2-macroglobulin to human group G streptococci.
abstract: Binding of human alpha 2-macroglobulin (alpha 2M) to group G streptococci and to their immunoglobulin G (IgG)-binding proteins (protein G) was investigated. Native alpha 2M bound specifically to strain G-148 with an apparent dissociation constant of (2.2 +/- 1.5) x 10(-9) M. Proteinase-complexed alpha 2M did not compete for the binding sites, and 125I-labelled proteinase-complexed alpha 2M did not bind to the bacteria. Binding of native alpha 2M to the cells was not affected by IgG or protein G consisting of only IgG-binding domains. 125I-labelled recombinant protein G did not bind to native or proteinase-complexed alpha 2M. However, a lysate of G-148 cells inhibited binding of alpha 2M to the bacteria, and immobilized wild-type protein G bound alpha 2M directly from fresh human plasma. In 13 group G streptococcal isolates, IgG-binding proteins were immunologically identified as protein G. In 11 isolates, these molecules reacted also with alpha 2M and human serum albumin (HSA). Western blots (immunoblots) of two wild-type protein G variants revealed identical bands reactive with goat IgG, HSA, and native alpha 2M. Digestion of wild-type protein G with clostripain destroyed in both variants the binding sites for alpha 2M but not for albumin and IgG. N-terminal fragments of protein G (lacking the IgG-binding region) bound both alpha 2M and HSA, whereas a similar HSA-binding peptide lacking the first 80 amino acids did not react with alpha 2M. Our findings are consistent with a specific binding site for native alpha 2M in the N-terminal region of protein G and suggest that binding of alpha 2M via IgG-binding proteins may be a general feature of human group G streptococci.
authors.affiliation: Department of Microbiology, Swedish University of Agricultural Sciences, Uppsala.
authors.forename: H P
authors.initials: HP
authors.lastname: Müller
authors.forename: L K
authors.initials: LK
authors.lastname: Rantamäki
mesh_terms: D001426:Bacterial Proteins / Q000378:metabolism*; D006801:Humans; D066298:In Vitro Techniques; D010446:Peptide Fragments / Q000378:metabolism; D011485:Protein Binding; D011994:Recombinant Proteins / Q000378:metabolism; D013291:Streptococcus / Q000276:immunology / Q000378:metabolism*; D000511:alpha-Macroglobulins / Q000378:metabolism*
chemical_list: D001426:Bacterial Proteins; C034284:IgG Fc-binding protein, Streptococcus; D010446:Peptide Fragments; D011994:Recombinant Proteins; D000511:alpha-Macroglobulins