Mucins (/ˈmjuːsɪn/) are a family of high
molecular weight, heavily
glycosylated proteins (
glycoconjugates) produced by
epithelial tissues in most
animals.[1] 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.[1] They often take an inhibitory role.[1] Some mucins are associated with controlling
mineralization, including
nacre formation in
mollusks,[2]calcification in
echinoderms[3] and bone formation in vertebrates.[4] They bind to pathogens as part of the immune system. Overexpression of the mucin proteins, especially
MUC1, is associated with many types of cancer.[5][6]
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]
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.[10]
Protein structure
Mature mammalian mucins are composed of two distinct regions:[7]
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-linkedoligosaccharides.
N-linkedoligosaccharides are also found on mucins, but in less abundance than O-linked sugars.
Evolutionary classification
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
otogelin and
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.
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
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.
Secretion
Upon stimulation,
MARCKS (myristylated alanine-rich C kinase substrate) protein coordinates the secretion of mucin from mucin-filled
vesicles within the specialized epithelial cells.[21] 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.[22][23]
Clinical significance
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]
Use of skincare products containing snail secretions of mucin have resulted in pain, swelling, and oozing.[31][32]. Counterfeit versions of a Korean snail mucin product called COSRX have been selling online, putting users at risk.[33]
^Liberelle M, Jonckheere N, Melnyk P, Van Seuningen I, Lebègue N (May 2020). "EGF-Containing Membrane-Bound Mucins: A Hidden ErbB2 Targeting Pathway?". Journal of Medicinal Chemistry. 63 (10): 5074–5088.
doi:
10.1021/acs.jmedchem.9b02001.
PMID32027502.
S2CID211044898.
^Singh AP, Chauhan SC, Bafna S, Johansson SL, Smith LM, Moniaux N, et al. (March 2006). "Aberrant expression of transmembrane mucins, MUC1 and MUC4, in human prostate carcinomas". The Prostate. 66 (4): 421–429.
doi:
10.1002/pros.20372.
PMID16302265.
S2CID21904013.