Evidence for independent binding domains within a group A streptococcal type IIo IgG-binding protein

The gene for a type IIo IgG-binding protein has previously been cloned and sequenced. The approximately 60,000 M(r) recombinant gene product binds all four human IgG subclasses and fibrinogen. Treatment of this recombinant protein with CNBr results in generation of a series of fragments. One fragment, an approximately 32,000 M(r) polypeptide, binds IgG1, IgG2, and IgG4 but neither IgG3 nor fibrinogen. N-terminal amino sequencing of this fragment indicated that this was an internal fragment of the protein starting at amino acid 186 of the mature protein. These findings provide evidence for two distinct domains for binding IgG1, IgG2, and IgG4 and binding IgG3 within a single bacterial IgG-binding protein.