Isolation and properties of a novel IgG-binding protein from streptococci of serological group U

A nonimmune binding of immunoglobulin (Ig) G has been detected in streptococci of group U. The group U Fc-binding site differed from the five previously known types of staphylococcal and streptococcal Fc-binding sites by its strong affinity for murine IgG, with dissociation constants in nanomolar range for rat and mouse IgG, as well as for mouse IgG subclasses 1, 2a, 2b and 3. It also differed from other binding sites by the high sensitivity towards trypsin. The Fc-binding protein could be solubilized from the streptococci of group U with papain and purified by gel filtration on sephacryl S-200 and by subsequent affinity chromatography on human IgG-Sepharose. The purified binding protein was homogeneous on SDS-polyacrylamide gel electrophoresis and had a molecular weight of approximately 58,000 daltons. It retained its binding activities for murine IgG subclasses as revealed by western blotting. Coupled to CNBr-activated sepharose, the purified Fc-binding protein could be effectively used for the isolation of murine IgG subclasses by affinity chromatography.