STRUCTURE, FUNCTION AND GENE-EXPRESSION OF EPITHELIAL MUCINS

In this review the main characteristics, i.e., structure, function and gene expression, of the different mucins are discussed. Mucin-type mo lecules consist of a core protein moiety (apomucin) where a number of carbohydrate chains are attached to serines and threonines by glycosid ic bonds. O-linked carbohydrates form up to 80% of the molecule and th e length of the glucidic side chains varies from one to more than 20 r esidues. At least eight mucin-like genes have been isolated so far, an d the main characteristic is the presence of a central domain composed of a variable number of ''tandem repeats''. The sequence homology of the central domain among the different members of the mucin-type famil y is limited, indicating that this internal domain is unique for each mucin. Thanks to the integrated results of genetic, immunological and biochemical studies, it is now possible to identify eight apomucin gen es, namely MUC1, MUC2, MUC3, MUC4, MUC5AC, MUC5B, MUC6 and MUC7. MUC1 is the best characterized mucin and it is expressed on the apical surf ace of most polarized epithelial cells. The MUC1 gene has been cloned and sequenced. The MUC2 gene encodes a typical secretory gel-forming m ucin which represents the predominant form in human intestinal and col on tissues. Another intestinal mucin is MUC3. The MUC4, MUC5AC and MUC 5B genes have been isolated from a bronchial tissue cDNA library. The MUC4 and MUC5AC genes are mainly expressed in the respiratory tract, i n gastric and reproductive mucosa, while MUC5B is highly detectable on ly In the bronchial glands. The MUC6 gene is expressed by gastric tiss ue and, recently, MUC7 has been cloned and sequenced using a salivary cDNA library.