Protein H--a novel IgG binding bacterial protein.
Forty-eight group A streptococcal strains of different M types were screened for binding of human radiolabeled IgG. Three of the strains bound more than 80% of the added radioactivity and one of them, an M protein type 1 strain designated AP1, was selected for further analysis. Attempts were made to solubilize the IgG binding bacterial molecule, and small amounts of an IgG binding protein with a mol. wt of 40 kDa could be solubilized with mutanolysin, a muramolytic agent. The gene encoding this streptococcal protein was cloned and expressed in E. coli, and the E. coli-produced protein was purified in a single step by affinity chromatography on IgG-Sepharose. When tested with IgGs from different species, the molecule was found to bind human IgG almost exclusively. The N-terminal amino acid sequence was determined and showed no homology with previously isolated Ig binding proteins, and the name protein H (as in human IgG) is suggested for this novel Ig binding bacterial protein. Protein H showed preferential affinity for heavy chains and Fc fragments of human IgG, and did not bind Ig light chains. The affinity constant, determined by Scatchard plots, between protein H and human polyclonal IgG was 1.6 x 10(9). No binding was observed between protein H and IgM, IgA, IgD, or IgE. Finally, when tested against several additional proteins and human plasma, protein H only showed weak binding to alpha 2-macroglobulin, a proteinase inhibitor.
Akesson
P
P
Department of Medical and Physiological Chemistry, University of Lund, Sweden.
Cooney
J
J
Kishimoto
F
F
Björck
L
L
D000595:Amino Acid Sequence; D000954:Antigens, Surface / Q000378:metabolism; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D015153:Blotting, Western; D003001:Cloning, Molecular; D005798:Genes, Bacterial; D006801:Humans; D007074:Immunoglobulin G / Q000378:metabolism*; D066298:In Vitro Techniques; D008969:Molecular Sequence Data; D011485:Protein Binding; D015183:Restriction Mapping; D013297:Streptococcus pyogenes / Q000276:immunology*
D000954:Antigens, Surface; D001426:Bacterial Proteins; D007074:Immunoglobulin G