Studies on group A streptococcal M-proteins: purification of type 5 M-protein and comparison of its amino terminal sequence with two immunologically unrelated M-protein molecules.

M-protein was isolated from group A, type 5, streptococci by limited proteolysis with pepsin and purified by chromatography on DEAE-Sephadex followed by gel filtration. The protein thus purified (Pep M5) was homogeneous by SDS-polyacrylamide gradient gel electrophoresis (apparent m.w.: 19,000), retained the capacity to remove opsonic antibodies from type 5 antiserum, and was capable of eliciting opsonic antibodies in rabbits. Its amino acid composition was very similar to that reported for M-proteins from other streptococcal types. The sequence of the first 29 amino terminal residues of Pep M5 was determined and compared with the reported amino terminal sequences of two immunologically unrelated M-proteins, namely, Pep M6 and Pep M24. The results revealed that, although the amino terminal sequences fo these three proteins differed from each other, some amino acid residues appeared to be conserved, suggesting a certain degree of structural relatedness among these M molecules. The possibility that this feature forms the molecular basis for the common antiphagocytic behavior of immunologically unrelated M-proteins is discussed.