The complete amino acid sequence of a biologically active 197-residue fragment of M protein isolated from type 5 group A streptococci

The complete amino acid sequence of a peptic fragment (Pep M5) of the group A streptococcal type 5 M protein, the antiphagocytic cell surface molecule of the bacteria, is described. This fragment, comprising nearly half of the native M molecule, is biologically active in that it has the ability to interact with opsonic antibodies as well as to evoke such an antibody response in rabbits. The sequence of Pep M5 was determined by automated Edman degradations of the uncleaved molecule and its enzymatically derived peptides. The primary peptides for Edman degradation were the arginine peptides obtained by tryptic digestion. The tryptic cleavage of Pep M5 was limited to the arginyl peptide bonds by derivatizing the ε-amino groups of lysine residues by reductive dihyroxypropylation. The overlapping peptides were generated by digestion of the unmodified Pep M5 with chymotrypsin, V8 protease, and subtilisin. The sequence thus established for the Pep M5 molecule consists of a total of 197 residues (M(r) = 22,705). The Pep M5 protein contains some identical, or nearly so, repeating sequences: four 7-residue segments and two 10-residue segments. However, extensive sequence repeats of the kind previously reported within the partial sequence of another M protein serotype, namely Pep M24, were absent. The Pep M5 sequence is distinct from, but exhibits some homology with, the partial sequences of two other M protein serotypes, namely, Pep M6 and Pep M24. Furthermore, the 7-residue periodicity of the nonpolar and charged residues, an α-helical coiled-coil structural characteristic that was previously observed within the partial sequences of M proteins, was found to extend over a significant part of the Pep M5 sequence. The implication of these results to the function and immunological diversity in M proteins is discussed.