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M protein (Streptococcus)

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Title: M protein (Streptococcus)  
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Subject: Group A streptococcal infection, Streptococcus pyogenes, Superantigen, Bacterial capsule, M protein, MSCRAMM, Autoimmune disease
Publisher: World Heritage Encyclopedia

M protein (Streptococcus)

Symbol Gram_pos_anchor
Pfam Pfam clan InterPro PROSITE PDOC00373

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.


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