Purification and immunobiological properties of R antigen and its relation to M protein of type 3 group A Streptococcus.

R protein was extracted from type 3 group A streptococci with hot (95 degrees C) HCl and was purified by ammonium sulfate precipitation followed by molecular-sieve and ion exchange chromatography. Although the R3 antigen was present in a heterogeneous population of proteins ranging from 78,000 to 100,000 daltons in size, we were able to separate an R-rich fraction that contained minimal amounts of heterogeneous proteins as indicated by electrophoresis in sodium dodecyl sulfate-polyacrylamide gels. The final yield of the purified R protein was approximately 15 mug (dry weight) per g (wet weight) of washed and sedimented streptococci. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicated a molecular size of approximately 78,000 daltons. Amino acid analysis showed lysine, glutamic acid, alanine, and aspartic acid as the predominant amino acids. A detailed comparison of the purified R3 protein with type 3 M protein indicated a similarity in composition and order of frequency of amino acids. However, the R3 antigen was found to be distinctive from the M3 antigen in agar gel diffusion tests. In addition, R3 and M3 proteins behaved differently in opsonophagocytosis tests and opsonization inhibition tests. Thus, R3 and M3 proteins produced precipitin lines of nonidentity with an unabsorbed antiserum against whole type 3 streptococci: M3-specific antiserum, but not R3-specific antiserum, enhanced the phagocytosis of type 3 streptococci. Purified M3 but not R3 protein was capable of inhibiting the type-specific opsonization of type 3 streptococci. The physicochemical resemblance between M and R proteins in general suggests a common genetic origin. Perhaps R proteins are variant forms of M proteins from which the antiopsonic determinant has been deleted.