The large gel-forming mucins, which form the major macromolecular components
of mucous secretions, are members of the mucin “superfamily.” Nine mucin genes
(MUC1–MUC4, MUC5AC, MUC5B, and MUC6–MUC8) have been identified (for
reviews see refs. 1 and 2), with each gene showing expression in several tissues. Only
the MUC1, MUC2, MUC4, MUC5, and MUC7 mucins have been sequenced com￾pletely (3–11) although large stretches of MUC5AC (12–15) as well as the C-terminal
sequences of MUC3 (16) and MUC6 (17) are now known.
A characteristic feature of mucins is the presence of one or more domains rich in
serine and/or threonine residues that, owing to a high degree of oligosaccharide substi￾tution, are resistant to proteolysis. Mucins comprise cell-associated, usually mono￾meric species, as well as those that are secreted; the latter can be subdivided into large,
gel-forming glycoproteins and smaller, monomeric ones. The gel-forming mucins
(Mr = 10–30 million Dalton) are oligomers formed by subunits (monomers) joined via
disulfide bonds (for a review see ref. 18), and treatment with reducing agents will
release the subunits and cause unfolding of regions stabilized by intramolecular disul￾fide bonds. Thus, after reduction, we term the monomers reduced subunits. Reduced
subunits are more sensitive to protease digestion than the intact mucin molecules