Immunochemical characterization of an IgG-binding protein of Streptococcus suis

Several bacterial species express surface proteins with affinity for the constant region (Fc) of immunoglobulin (Ig) of different animal species. Previous studies from our group have reported the presence of an IgG-binding protein in various serotypes of Streptococcus suis. This molecule was also shown to bind in a non-immune fashion chicken IgY and to our knowledge this characteristic is unique. In the present study, by dot-blotting, we showed that the native protein, obtained by affinity chromatography, reacted more strongly with IgG from various animal species than the denatured material. Using a competitive enzyme-linked immunosorbent assay the affinity of the native 60-kDa protein (previously identified as a 52-kDa protein) towards IgG of various animal species was compared to pig IgG. Bovine, goat and human IgG were able to compete effectively with pig IgG whereas chicken IgY constituted a poor competitor. Peptide mapping analysis using denatured protein indicated that pig and bovine IgG recognized the same proteolytic fragment whereas chicken IgY did not. The smallest proteolytic fragment that retained the binding activity towards the IgG of the different animal species tested had a molecular mass of approximately 40 kDa. Fragments with Mr < 40 kDa showed specific binding activities. That is, the smallest fragment binding pig and bovine IgG had a Mr of 30 kDa whereas for goat and human IgG a fragment of less than 16 kDa still showed binding activity. Finally, we observed that antisera raised against a heat-shock protein of Pseudomonas aeruginosa reacted with the 60-kDa S. suis protein indicating that the S. suis 60-kDa protein is a member of the 60-kDa hsp family that possesses the characteristic of binding in a non-immune way mammalian IgG and chicken IgY.