mucin

Mucin refers to a family of high molecular weight glycoproteins that are the major components of mucus, a viscous secretion that coats and protects mucosal surfaces. Mucins are produced by epithelial tissues, notably goblet cells and mucous glands, and exist in two main forms: secreted mucins that become part of the mucus gel, and membrane-bound mucins attached to the surface of epithelial cells.

Secreted mucins can be further categorized as gel-forming and non-gel-forming. Gel-forming mucins, such as MUC2 in the intestine and MUC5AC and MUC5B in the airways and stomach, create viscoelastic mucus networks. Non-gel-forming secreted mucins, including MUC7 found in saliva, contribute to lubrication and protective functions but do not form gels. Membrane-bound mucins, including MUC1, MUC4, and MUC16, are expressed on the cell surface and participate in signaling, barrier formation, and cell–cell interactions.

Structurally, mucins have a protein backbone rich in serine, threonine, and proline, with extensive O-linked glycosylation on serine and threonine residues. This glycosylation accounts for much of their hydration, charge, and gel-forming properties. The mucin network can be cross-linked and hydrated, enabling a protective mucus layer that traps particles and bacteria while preserving the underlying epithelium.

Functions of mucins include lubrication of mucosal surfaces, protection against chemical and mechanical damage, and entrapment of pathogens and particulates. They also help shape the composition of the local microbiota and support tissue hydration and barrier integrity. Altered mucin expression or glycosylation is associated with various diseases and can have diagnostic or prognostic significance.