Extraction and characterization of IgG Fc receptors from group C and group G streptococci

Immunoglobulin G (IgG) Fc receptors (FcRs) were extracted by proteolytic digestion of four strains each of group C and group G streptococci. The solubilized proteins were analyzed in Western blots and multiple IgG-binding bands were obtained. The banding patterns of some of the strains were very similar, but this property was independent of which streptococcal group the strains belonged to. Highly purified FcRs were prepared from one group C and one group G strain. The 13 N-terminal amino acids were determined, and found to be identical, whereas comparison with the sequence of staphylococcal protein A did not reveal any homology. The isolated streptococcal FcRs also appeared closely related antigenically and functionally. Thus, both molecules were capable of inhibiting each others binding to immobilized IgG, and the radiolabelled group G FcR was completely inhibited from binding to IgG by an antibody to the group C FcR. Finally, in a direct binding assay both proteins were capable of reacting to a similar degree with a wide variety of IgGs, thereby demonstrating the great potential of streptococcal FcRs as tools for binding and detection of IgG antibodies.