Streptococcal M protein extracted by nonionic detergent. III. Correlation between immunological cross-reactions and structural similarities with implications for antiphagocytosis.

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Abstract
Three immunologically cross-reactive and non-cross-reactive streptococcal M proteins were analyzed by a chromatographic tryptic peptide mapping system. Cross-reactions correlated with the extent of structural similarity among the M protein molecules analyzed. Free lysine was released by the action of trypsin from these 3 M proteins, suggesting a common lys-lys or arg-lys sequence. In addition, only 1 peptide was common within all 3 M types. This limited structural relatedness among the 3 M proteins examined indicates that sequence variation plays a major role in the immunological specificity of the M antigens. However, despite sequence variation, all M protein molecules had a common antiphagocytic activity. The fact that no common opsonic antibody was yet found, even against limited M types, argues against this biological activity being solely the result of a common sequence. The antiphagocytic effect of M protein may be due to a conformationally created environment on the surface of the molecule which was selected by immunological and biological pressure.

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