Primary structural similarities between types 5 and 24 M proteins of [formula omitted

Abstract

Type 5 M protein was extracted from whole S. pyogenes organisms by limited digestion with pepsin. The peptic extract was purified to physicochemical homogeneity by ammonium sulfate fractionation and ion exchange chromatography. The purified product, designated pep M5, migrated as a single band in sodium dodecyl sulfate (SDS)-gels at a position consistent with a molecular weight of ∼ 21,000. Subcutaneous injections of rabbits with a single 100 μg dose of pep M5 emulsified in complete Freund's adjuvant resulted in the production of peak levels of type specific precipitating and opsonic antibodies in each of the animals at 8 weeks after the immunizing dose. The amino acid composition and the amino acid sequence of pep M5 showed repeating sequences and striking similarities with the known amino acid sequences of peptide fragments of the heterologous serotype 24 pep M protein. The antisera raised in rabbits against pep M5 reacted only with the homologous pep M5 and not with the heterologous pep M24 in enzyme linked immunosorbent assays (ELISA). One serum from a rabbit immunized with pep M24, however, exhibited a high titer of antibody against the heterologous pep M5. These results indicate that pep M5 similar to pep M24 is composed of repeating covalent structures and suggest that these heterologous serotypes of M protein share certain antigenic determinants which may reside in the regions of the molecule showing structural homologies.