MUCUS GLYCOPROTEINS OF THE NORMAL GASTROINTESTINAL-TRACT

Gastrointestinal mucus glycoproteins (mucin, about 10 million Da) are large molecular weight polymers comprised of subunits joined together (except for submaxillary mucins) by disulphide bridges. The protein core of each subunit consists of two types of region: (1) regions containing large numbers of repeat sequences very rich in glycosylated serine and threonine residues which are resistant to proteolysis and (2) regions which are sparsely glycosylated, relatively rich in cysteine and susceptible to proteolysis. Cloning of mucin genes has provided much information about the sequence of the protein cores. Mucus, in contract to all other gastrointestinal secretions, is secreted as a viscoelastic gel which forms a protective barrier adherent to the mucosal surfaces. Polymeric mucins form a mucus gel by non-covalent interactions between the highly expanded molecules in solution.