Tropomyosin-like seven residue periodicity in three immunologically distinct streptococal M proteins and its implications for the antiphagocytic property of the molecule.

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Abstract
Partial sequences of 3 immunologically distinct group A streptococcal M proteins (M5, M6, and M24) revealed significant homology with each other, certain amino acid residues being conserved within the 3 molecules. A common feature of the sequenced regions of these M proteins was their high .alpha.-helical potential and the presence of a repeating 7 residue periodicity that is characteristic of the double helical coiled-coil molecule, tropomyosin. Existence of a tropomyosin-like 7 residue periodicity strongly suggests that regions of these 3 M proteins may participate in intra- and/or intermolecular coiled-coil interactions. Because of the constraints imposed by such a repeating periodicity, certain conserved residues within the M proteins would occupy spatially equivalent positions in the tertiary structure of these molecules. This common characteristic could play an important role in the common anti-phagocytic property of the immunologically diverse M molecules. In addition to similarities in the secondary structure of M proteins and tropomyosin, significant sequence homology was observed between certain regions of these molecules with up to 50% identical residues. As a result of the striking structural similarity with tropomyosin, M proteins may play a regulatory role in the contractile mechanisms involved in phagocytosis.