Glycome Atlas
Protein
Also known as mucin glycoproteins, MUC proteins, mucus glycoproteins
Plain-language answer
Mucins are large proteins carrying so many attached sugar chains that they are mostly sugar by weight. They are the main building blocks of mucus, the slippery gel that coats the gut, airways, and other wet surfaces of the body. Some mucins are secreted to form the gel itself, while others stay anchored in the cell membrane as part of a cell's outer sugar coat.12
Mucins create a protective barrier that keeps trillions of gut bacteria and swallowed irritants at arm's length from the delicate cells lining the intestine. When mucin production or structure is disturbed, that barrier can thin or become leaky, a pattern seen in inflammatory bowel disease and other gut conditions.1
Technical detail
Mucins are a family of high-molecular-weight glycoproteins defined by tandem-repeat protein domains rich in serine and threonine that carry dense clusters of GalNAc-initiated O-glycans, and they are subdivided into secreted gel-forming mucins that polymerize into the mucus barrier and membrane-bound mucins that form part of the apical glycocalyx.12
Mucin apoproteins contain variable-number tandem-repeat regions dense in serine, threonine, and proline that serve as attachment sites for hundreds of O-linked glycans. Each chain begins with N-acetylgalactosamine linked to serine or threonine, extended into core structures and capped with galactose, N-acetylglucosamine, fucose, and sialic acid. The resulting bottlebrush architecture holds the polypeptide in an extended, hydrated, protease-resistant conformation.21
Gel-forming secreted mucins such as MUC2, MUC5AC, and MUC5B contain cysteine-rich domains that support disulfide-linked oligomerization into large polymeric networks, forming the viscoelastic mucus gel at epithelial surfaces. Membrane-tethered mucins such as MUC1 and MUC16 possess a transmembrane domain and cytoplasmic tail, contributing to the apical glycocalyx and to cell-surface signaling and protection.1
At mucosal surfaces mucins lubricate, hydrate, and physically separate the epithelium from luminal contents, trapping particles and microbes for clearance. Their terminal glycans also serve as adhesion sites and as a carbohydrate nutrient reservoir for commensal bacteria, making mucin glycosylation a key determinant of mucosal microbial ecology.1
Human relevance
Altered mucin expression and a thinner, more penetrable colonic mucus layer are associated with ulcerative colitis and other inflammatory bowel disease, consistent with a weakened barrier that allows closer bacterial contact with the epithelium.1
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References