title: Tropomyosin-like seven residue periodicity in three immunologically distinct streptococal M proteins and its implications for the antiphagocytic property of the molecule.
abstract: Partial sequences of three 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 three molecules. In addition, a common feature of the sequenced regions of these M proteins was their high alpha-helical potential and the presence of a repeating seven residue periodicity that is characteristic of the double helical coiled-coil molecule, tropomyosin. The existence of a tropomyosin-like seven residue periodicity strongly suggests that regions of these three 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 antiphagocytic property of the immunologically diverse M molecules. In addition to similarities in the secondary structure of M proteins and tropomyosin, significant sequence homology has also been 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.
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000276:immunology*; D006860:Hydrogen Bonding; D010446:Peptide Fragments; D010587:Phagocytosis; D011487:Protein Conformation; D013297:Streptococcus pyogenes / Q000276:immunology*; D013329:Structure-Activity Relationship; D014335:Tropomyosin / Q000276:immunology*
chemical_list: D001426:Bacterial Proteins; D010446:Peptide Fragments; D014335:Tropomyosin