Mucins: structure, function, and role in pulmonary diseases.

Mucins, major components of the extracellular mucus blanket that protect and lubricate mammalian epithelia, are high-molecular-mass glycoconjugates (154 to > or = 7,000 kDa) with hundreds of oligosaccharide chains in O-glycosidic linkages to a protein backbone. The apparent expression of more than one type of oligosaccharide core structure in mucins isolated from pathological material may reflect either inherent limitations in analysis, disease-related alterations in parameters affecting glycosylation and post-translational modifications (e.g., nucleotide-sugar concentrations, expression of specific glycosyltransferases, rates of transport through the endoplasmic reticulum and Golgi) or the activation of mucin protein genes that are more highly expressed in disease states with different glycosylation patterns. Recent studies have revealed the existence of a family of at least four human mucin proteins; MUC1, MUC2, MUC3, MUC4, each of which contains a variable number of tandem repeats that differ in sequence and size. Full-length sequences of cDNA clones encoding human mucin proteins are currently available only for MUC1 which, in contrast to most airway and intestinal mucins, is membrane associated and not secreted. Current information on mucin oligosaccharides and proteins is reviewed herein. More detailed knowledge of the protein and oligosaccharide structures of mucins will be important in identifying specific role(s) in health and disease, i.e., in the physiological functions of mucus.