M protein (Streptococcus)

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Symbol Gram_pos_anchor
Pfam PF00746
Pfam clan CL0501
InterPro IPR019948

M protein is a virulence factor that can be produced by certain species of Streptococcus.[1]

Viruses, parasites and bacteria are covered in protein and sugar molecules that help them gain entry into a host by counteracting the host's defences. One such molecule is the M protein produced by certain streptococcal bacteria. M proteins embody a motif that is now known to be shared by many Gram-positive bacterial surface proteins. The motif includes a conserved pentapeptide LPXTG, which precedes a hydrophobic C-terminal membrane anchor, which itself precedes a cluster of basic residues.[2][3]

M protein is strongly anti-phagocytic and is a major virulence factor. It binds to serum factor H, destroying C3-convertase and preventing opsonization by C3b. However plasma B cells can generate antibodies against M protein which will help in opsonization and further the destruction of the microorganism by the macrophages and neutrophilis. Cross-reactivity of anti-M protein antibodies with heart muscle is the basis for rheumatic fever.

It was originally identified by Rebecca Lancefield,[4] who also formulated the Lancefield classification system for streptococcal bacteria. Bacteria like S. pyogenes, which possess M protein are classified in group A of the Lancefield system.


  1. ^ Chanter N, Talbot NC, Newton JR, Hewson D, Verheyen K (June 2000). "Streptococcus equi with truncated M-proteins isolated from outwardly healthy horses". Microbiology (Reading, England). 146. ( Pt 6): 1361–9. PMID 10846214. 
  2. ^ Schneewind O, Jones KF, Fischetti VA (June 1990). "Sequence and structural characteristics of the trypsin-resistant T6 surface protein of group A streptococci". J. Bacteriol. 172 (6): 3310–7. PMC 209141. PMID 2188957. 
  3. ^ Fischetti VA, Pancholi V, Schneewind O (September 1990). "Conservation of a pentapeptide sequence in the anchor region of surface proteins from gram-positive cocci". Mol. Microbiol. 4 (9): 1603–5. doi:10.1111/j.1365-2958.1990.tb02072.x. PMID 2287281. 
  4. ^ "Streptococcal M protein: molecular design and biological behavior.". Retrieved 2009-06-21.