Repeating covalent structure of streptococcal M protein.

We have attempted to identify the covalent structure of the M protein molecule of group A streptococci that is responsible for inducing type-specific, protective immunity. M protein was extracted from type 24 streptococci, purified, and cleaved with cyanogen bromide. Seven cyanogen bromide peptides were purified and further characterized. Together, the peptides account for the entire amino acid content of the M protein molecule. Each of the purified peptides possessed the type-specific determinant that inhibits opsonic antibodies for group A streptococci. The primary structures of the amino-terminal regions of each of the purified peptides was studied by automated Edman degradation. The partial sequences of two of the peptides were found to be identical to each other and to that of the uncleaved M protein molecule through at least the first 27 residues. The amino-terminal sequences of the remaining five peptides were identical to each other through the twentieth residue but completely different from the amino-terminal region of the other two peptides. However, the type-specific immunoreactivity and the incomplete analysis of the primary structure of the seven peptides suggest that the antiphagocytic determinant resides in a repeating amino acid sequence in the M protein molecule.