Mucin
| Identifiers | |
|---|---|
| Symbol | Mucin |
| Membranome | 111 |
Mucins (/ˈmjuːsɪn/) are a family of high molecular weight, heavily glycosylated proteins (glycoconjugates) produced by epithelial tissues in most animals. Mucins are distinguished between membrane-bound mucins, forming the glycocalyx, or secreted mucins, forming mucus-gels. These mucus gels cover the wet epithelium, serving functions from lubrication (as seen e.g. in the eyes), to physico-chemical barrier formation, protecting against foreign particles and pathogens as seen in e.g. the lungs or intestine. Next to their physical properties, gel-forming and transmembrane mucins alike are also involved in cell signalling and immune responses. They often take an inhibitory role. Some mucins are associated with controlling mineralization, including nacre formation in mollusks, calcification in echinoderms and bone formation in vertebrates.
Besides their physicochemical barrier properties, mucins have been reported to function as receptors for various pathogens, facilitating their invasion into host cells. Thus they display a dual role in host defence, which is currently under investigation.
Dysregulation of mucin expression and secretion is closely associated with various diseases. Overexpression of the mucin proteins, especially MUC1, is associated with many types of cancer. An impaired mucus barrier integrity, showing decreased mucus layer thickness and, or increased permeability are closely linked to e.g. Crohn´s disease and Ulcerative colitis.
Human mucins include genes with the HUGO symbol MUC 1 through 22. Of these mucins, the following classes have been defined by localization:
- 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.
| 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 |
| 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)
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.