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We previously reported that group D streptococci exhibited immunoglobulin G (IgG)-binding activity and that a 52-kDa IgG-binding protein was present in all Streptococcus suis strains examined (B. Serhir, R. Higgins, B. Foiry, and M. Jacques, J. Gen. Microbiol. 139:2953-2958, 1993). The objective of the present study was to purify and characterize this protein. Pig IgG were immobilized through their Fab fragments to ECH-Sepharose 4B, and the protein was purified by affinity chromatography. Electron microscopy observations of the purified material showed filamentous structures with a diameter of approximately 4 nm; these structures were not observed when the material was treated with either urea or ethanolamine. Electrophoretic and Western immunoblot analyses showed that the 52-kDa protein constituted the bulk of the recovered material. This protein was stained with either Coomassie brilliant blue or silver nitrate; it reacted with a large variety of mammalian IgG, human IgG (Fc) fragments, human IgA, and other human plasma proteins. The 52-kDa protein exhibited lower IgG-binding affinities than protein A and protein G. However, it was able to compete with protein A and protein G for binding to human IgG. In addition, it bound chicken IgG with high affinity. This last property differentiated the 52-kDa protein of S. suis from the six IgG-binding proteins described to date. The 52-kDa protein displayed similar affinities for untreated and deglycosylated pig IgG. The N-terminal amino acid sequence (SIITDVYAXEVLDSXGNPTLEV) revealed no homology with any bacterial proteins in the Swiss-Prot database. Its isoelectric point of approximately 4.6 and its amino acid composition, rich in aspartic and glutamic acids, showed that it had some similarities with other IgG-binding proteins. In this report, we have purified and characterized a 52-kDa IgG-binding protein from S. suis capsular type 2. Although this protein shares some similarities with other IgG- and/or IgA-binding proteins, it is unique in reacting with chicken IgG.

Purification and characterization of a 52-kilodalton immunoglobulin G-binding protein from Streptococcus suis capsular type 2