Amino acid sequence and physicochemical similarities between streptococcal M protein and mammalian tropomyosin.

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Abstract
The amino-terminal sequences of 2 peptides of type 24 streptococcal M protein show similarities with that of rabbit skeletal muscle tropomyosin, having up to 40% identical residues and probabilities of occurring by chance as low as P < 10-5. A hexapeptide (Glu-Ala-Glu-Lys-Ala-Ala) that is found 5 times in the M24 protein is identical to a sequence in tropomyosin. Similarities are also seen in the amino acid compositions and physicochemical properties of the 2 proteins. The amino-terminal sequences of peptides from another bacterial surface protein, staphylococcal protein A, are highly correlated with segments of 2 other myofibrillar proteins, rabbit actin (P < 10-7) and rabbit myosin A1 light chain (P < 10-6). The data presented suggest that a close structural relationship exists between mammalian muscle proteins and the biologically active surface proteins of staphylococci and streptococci. The correlation between sequences in M protein and tropomyosin represents direct evidence of a structural similarity at a molecular level between a streptococcal protein and a mammalian muscle component and may prove relevant to the pathogenicity of the streptococcus.