Protein H--a Novel IgG Binding Bacterial Protein

Mol Immunol. 1990 Jun;27(6):523-31. doi: 10.1016/0161-5890(90)90071-7.


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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Sequence
  • Antigens, Surface / metabolism
  • Bacterial Proteins / genetics
  • Bacterial Proteins / metabolism*
  • Blotting, Western
  • Cloning, Molecular
  • Genes, Bacterial
  • Humans
  • Immunoglobulin G / metabolism*
  • In Vitro Techniques
  • Molecular Sequence Data
  • Protein Binding
  • Restriction Mapping
  • Streptococcus pyogenes / immunology*


  • Antigens, Surface
  • Bacterial Proteins
  • Immunoglobulin G