Mucin

Mucins (

Human mucins include genes with the HUGO symbol MUC 1 through 22. Of these mucins, the following classes have been defined by localization:^{[7][8][9][10]}

• Secreted mucins in humans, with their chromosomal location, repeat size in amino acids (aa), whether they are gel-forming (Y) or not (N), and their tissue expression.^[11]

• Membrane-bound (
  MUC21 (formerly C6orf205), MUC22 (highly polymorphic^[13]
  )

Mature mammalian mucins are composed of two distinct regions:[7]

• The
  carboxy-terminal regions are very lightly glycosylated, but rich in cysteines. The cysteine residues participate in establishing disulfide linkages within and among mucin monomers
  .
• A large central region ("PTS domain") formed of multiple tandem repeats of 10 to 80 residue sequences in which up to half of the
  oligosaccharides
  are also found on mucins, but in less abundance than O-linked sugars.

The functional classification does not correspond to an exact evolutionary relationship, which is still incomplete and ongoing.[10] Known-related groups include:

• The gel-forming mucins (2, 5AC, 5B, 6, 19) are related both to each other and to
  von Willebrand Factor (PTHR11339).^[14] Four of these occur in a well-conserved gene cluster (at 11p.15.5 in humans).^[15]
• The EGF-like domain containing mucins. These include MUC3(A,B), MUC4, MUC12, MUC13, and MUC17.^[16]
• Some EGF-like mucins, plus MUC1 and MUC16, carry SEA domains, a vertebrate invention. It is unclear whether this points to a common origin among these transmembrane mucins.^[14]
• MUC21 and MUC22 are related to each other by sharing a C-terminal domain (PF14654). They also occur in a human gene cluster on 6p21.33.
• MUC7 is a recent invention in placental mammals. It started as a copy in the
  secretory calcium-binding phosphoprotein (SCPP) gene cluster and rapidly gained PTS repeats.^[17]

Mucins have been found to have important functions in defense against bacterial and fungal infections. MUC5B, the predominant mucin in the mouth and female genital tract, has been shown to significantly reduce attachment and biofilm formation of Streptococcus mutans, a bacterium with the potential to form cavities.^[18] Unusually, MUC5B does not kill the bacteria but rather maintains it in the planktonic (non-biofilm) phase, thus maintaining a diverse and healthy oral microbiome.^[18] Similar effects of MUC5B and other mucins have been demonstrated with other pathogens, such as Candida albicans, Helicobacter pylori, and even HIV.^[19][20] In the mouth, mucins can also recruit anti-microbial proteins such as statherins and histatine 1, which further reduces risk of infection.^[20]

Eleven mucins are expressed by the eye surface epithelia, goblet cells and associated glands, even though most of them are expressed at very low levels. They maintain wetness, lubricate the blink, stabilize the tear film, and create a physical barrier to the outside world.^[12]

Glycosylation and aggregation

Mucin genes encode mucin monomers that are synthesized as rod-shaped apomucin cores that are post-translationally modified by exceptionally abundant glycosylation.

The dense "sugar coating" of mucins gives them considerable

Increased mucin production occurs in many adenocarcinomas, including cancers of the pancreas, lung, breast, ovary, colon and other tissues. Mucins are also overexpressed in lung diseases such as asthma, bronchitis, chronic obstructive pulmonary disease (COPD) or cystic fibrosis.^[24] Two membrane mucins, MUC1 and MUC4 have been extensively studied in relation to their pathological implication in the disease process.^[25][26][27] Mucins are under investigation as possible diagnostic markers for malignancies and other disease processes in which they are most commonly over- or mis-expressed.

Abnormal deposits of mucin are responsible for the non-pitting facial edema seen in untreated hypothyroidism. This edema is seen in the pretibial area as well.^[28]

Non-vertebrate mucins

Beyond the better-studied vertebrate mucins, other animals also express (not necessarily related) proteins with similar properties. These include:

• Drosophila is known to express mucin proteins containing PTS-rich repeats.^[29]
• Trypanosoma cruzi express cell-surface mucins (PF01456).^[30]

See also

• Bovine submaxillary mucin coatings
• Verotoxin-producing Escherichia coli

References