M protein is a

virulence factor Virulence factors (preferably known as pathogenicity factors or effectors in plant science) are cellular structures, molecules and regulatory systems that enable microbial pathogens (bacteria, viruses, fungi, and protozoa) to achieve the following ...

that can be produced by certain species of ''

Streptococcus ''Streptococcus'' is a genus of gram-positive ' (plural ) or spherical bacteria that belongs to the family Streptococcaceae, within the order Lactobacillales (lactic acid bacteria), in the phylum Bacillota. Cell division in streptococci occurs ...

''. Viruses,

parasite Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has ...

s and

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...

are covered in

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...

and sugar

molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...

s that help them gain entry into a host by counteracting the host's defenses. One such molecule is the M protein produced by certain streptococcal

. At its C-terminus within the cell wall, M

s embody a motif that is now known to be shared by many

Gram-positive In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall. Gram-positive bact ...

l surface proteins. The motif includes a conserved pentapeptide LPXTG, which precedes a

hydrophobic In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, t ...

C-terminal

membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. ...

spanning domain, which itself precedes a cluster of basic residues at the C-terminus. M protein is strongly anti-phagocytic and is the major virulence factor for group A streptococci (''Streptococcus pyogenes''). It binds to serum

factor H Factor H is a member of the regulators of complement activation family and is a complement control protein. It is a large (155 kilodaltons), soluble glycoprotein that circulates in human plasma (at typical concentrations of 200–300 micrograms ...

, destroying

C3-convertase C3 convertase (''C4bC2b'', formerly ''C4b2a'') belongs to family of serine proteases and is necessary in innate immunity as a part of the complement system which eventuate in opsonisation of particles, release of inflammatory peptides, C5 conv ...

and preventing

opsonization Opsonins are extracellular proteins that, when bound to substances or cells, induce phagocytes to phagocytose the substances or cells with the opsonins bound. Thus, opsonins act as tags to label things in the body that should be phagocytosed (i.e. ...

C3b C3b is the larger of two elements formed by the cleavage of complement component 3, and is considered an important part of the innate immune system. C3b is potent in opsonization: tagging pathogens, immune complexes (antigen-antibody), and apopto ...

. 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 neutrophils.

Cross-reactivity Cross-reactivity, in a general sense, is the reactivity of an observed agent which initiates reactions outside the main reaction expected. This has implications for any kind of test or assay, including diagnostic tests in medicine, and can be a c ...

of anti-M protein antibodies with heart muscle has been suggested to be associated in some way with rheumatic fever. It was originally identified by

Rebecca Lancefield Rebecca Craighill Lancefield (January 5, 1895 – March 3, 1981). p.227 was a prominent American microbiologist. She joined the Rockefeller Institute for Medical Research (now Rockefeller University) in New York in 1918, and was associated with ...

, 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.

Literature

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References

{{reflist Virulence factors Streptococcal proteins