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Immunochemical properties of streptococcal M protein purified by isoelectric focusing.

Electrofucusing of an alkaline extract of type 24 streptococcal M protein yielded an antigenic fraction that was type specific and apparently homogeneous. The haptenic nature of this fraction was suggested by its inability to precipitate type-specific antiserum or to induce opsonic antibodies in rabbits, despite its ability to strongly inhibit opsonization of homologous-type streptococci by M antibody. The fraction migrated as a single band upon electrophoresis in sodium dodecyl sulfate (SDS) acrylamide gel. The mobility of the protein band was consistent with a molecular weight of 36,500 daltons. In some experiments using larger quantities of protein, a second faint protein band with an average molecular weight of 70,000 was observed, suggesting the presence of dimers of the 36,500-dalton protein. Amino acid analysis showed the predominant amino acid to be glycine followed by aspartic acid and glutamic acid. Moreover, this M protein fraction was free of non-type-specific immunotoxic properties in guinea pigs and in man. Although apparently not immunogenic, this nontoxic fraction may provide a useful tool to study the relationship of the type-specific protective moiety to potentially harmful "impurities" in M protein vaccines.