Mucin
Template:Short description Template:Infobox protein family
Mucins (Template:IPAc-en) are a family of high molecular weight, heavily glycosylated proteins (glycoconjugates) produced by epithelial tissues in most animals.<ref name="Marin2008">Template:Cite book</ref> Mucins' key characteristic is their ability to form gels; therefore they are a key component in most gel-like secretions, serving functions from lubrication to cell signalling to forming chemical barriers.<ref name=Marin2008/> They often take an inhibitory role.<ref name=Marin2008/> Some mucins are associated with controlling mineralization, including nacre formation in mollusks,<ref name=Marin2000>Template:Cite journal</ref> calcification in echinoderms<ref>Template:Cite journal</ref> and bone formation in vertebrates.<ref>Template:Cite journal</ref> They bind to pathogens as part of the immune system. Overexpression of the mucin proteins, especially MUC1, is associated with many types of cancer.<ref name="pmid18407586">Template:Cite journal</ref><ref name="pmid33704667">Template:Cite journal</ref>
Although some mucins are membrane-bound due to the presence of a hydrophobic membrane-spanning domain that favors retention in the plasma membrane, most mucins are secreted as principal components of mucus by mucous membranes or are secreted to become a component of saliva.
Genes and proteinsEdit
Human mucins include genes with the HUGO symbol MUC 1 through 22. Of these mucins, the following classes have been defined by localization:<ref name=pmid11578969/><ref>Template:Cite journal</ref><ref name="pmid21385362">Template:Cite journal</ref><ref name="pmid30875782">Template:Cite journal</ref>
- 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.<ref>Template:Cite journal</ref>
| Mucin | gel | chromosome | repeat size (aa) | tissue expression |
|---|---|---|---|---|
| MUC2 | Y | 11p15.5 | 23 | Jejunum, ileum, colon, endometrium |
| MUC5A | Y | 11p15.5 | 8 | Respiratory tract, stomach, conjunctiva, endocervix, endometrium |
| MUC5B | Y | 11p15.5 | 29 | Respiratory tract, submandibular glands, endocervix |
| MUC6 | Y | 11p15.5 | 169 | Stomach, ileum, gall bladder, endocervix, endometrium |
| MUC19 | Y | 12q12 | 19 | corneal and conjunctival epithelia; lacrimal gland<ref name=":2">Template:Cite journal</ref> |
| MUC7 | N | 4q13–q21 | 23 | Sublingual and submandibular glands |
| MUC8 | N | 12q24.3 | 13/41 | Respiratory tract, uterus, endocervix, endometrium |
| MUC9 | N | 1p13 | 15 | Fallopian tubes |
| MUC20 | N | 3 | 19 | kidney (high), moderately in placenta, lung, prostate, liver, digestive system |
- Membrane-bound (transmembrane) mucins: MUC1, MUC3A, MUC3B, MUC4, MUC12, MUC13, MUC15, MUC16, MUC17, MUC21 (formerly C6orf205), MUC22 (highly polymorphic<ref>Template:Cite journal</ref>)
The major secreted airway mucins are MUC5AC and MUC5B, while MUC2 is secreted mostly in the intestine but also in the airway. MUC7 is the major salivary protein.<ref name="pmid30875782"/>
Protein structureEdit
Mature mammalian mucins are composed of two distinct regions:<ref name=pmid11578969>Template:Cite journal</ref>
- The amino- and 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 amino acids are serine or threonine. This area becomes saturated with hundreds of O-linked oligosaccharides. N-linked oligosaccharides are also found on mucins, but in less abundance than O-linked sugars.
Evolutionary classificationEdit
The functional classification does not correspond to an exact evolutionary relationship, which is still incomplete and ongoing.<ref name="pmid30875782"/> Known-related groups include:
- The gel-forming mucins (2, 5AC, 5B, 6, 19) are related both to each other and to otogelin and von Willebrand Factor (PTHR11339).<ref name=pmid17911254>Template:Cite journal</ref> Four of these occur in a well-conserved gene cluster (at 11p.15.5 in humans).<ref>Template:Cite journal</ref>
- The EGF-like domain containing mucins. These include MUC3(A,B), MUC4, MUC12, MUC13, and MUC17.<ref>Template:Cite journal</ref>
- 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.<ref name=pmid17911254/>
- 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.<ref>Template:Cite journal</ref>
Function in humansEdit
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.<ref name=":0">Template:Cite journal</ref> 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.<ref name=":0" /> Similar effects of MUC5B and other mucins have been demonstrated with other pathogens, such as Candida albicans, Helicobacter pylori, and even HIV.<ref>Template:Cite journal</ref><ref name=":1">Template:Cite journal</ref> In the mouth, mucins can also recruit anti-microbial proteins such as statherins and histatine 1, which further reduces risk of infection.<ref name=":1" />
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.<ref name=":2" />
Glycosylation and aggregationEdit
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 water-holding capacity and also makes them resistant to proteolysis, which may be important in maintaining mucosal barriers.
Mucins are secreted as massive aggregates of proteins with molecular masses of roughly 1 to 10 million Da. Within these aggregates, monomers are linked to one another mostly by non-covalent interactions, although intermolecular disulfide bonds may also play a role in this process.
SecretionEdit
Upon stimulation, MARCKS (myristylated alanine-rich C kinase substrate) protein coordinates the secretion of mucin from mucin-filled vesicles within the specialized epithelial cells.<ref>Template:Cite journal</ref> Fusion of the vesicles to the plasma membrane causes release of the mucin, which as it exchanges Ca2+ for Na+ expands up to 600 fold. The result is a viscoelastic product of interwoven molecules which, combined with other secretions (e.g., from the airway epithelium and the submucosal glands in the respiratory system), is called mucus.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
Clinical significanceEdit
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.<ref>Template:Cite journal</ref> Two membrane mucins, MUC1 and MUC4 have been extensively studied in relation to their pathological implication in the disease process.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> 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.<ref>Hanberg, Allen "Medical Surgical Nursing: clinical management for positive outcomes" Black and Hawk (Eds.). ElSevier 2009.</ref>
Non-vertebrate mucinsEdit
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.<ref>Template:Cite journal</ref>
- Trypanosoma cruzi express cell-surface mucins (Template:Pfam).<ref>Template:Cite journal</ref>
Some other organisms produce mucilage that does not have a protein component, only polysacchides.
Cosmetic useEdit
Misuse of skincare products containing snail secretions of mucin have resulted in pain, swelling, and oozing.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Counterfeit versions of a Korean snail mucin product called COSRX have been selling online, putting users at risk.<ref>Template:Cite journal</ref> It’s important to follow manufacturer guidelines on their use and buy skin products from authorized vendors.