Mammalian reproductive tract mucins.

Mucin glycoproteins are major constituents of the glycocalyx that covers mucosal epithelium. Two broad classes of mucins exist: membrane-associated and secretory. Of the secreted mucins, those with cysteine-rich regions are thought to polymerize through disulphide bonds. Among these gel-forming mucins are MUC2, MUC5AC, MUC5B and possibly MUC6. MUC7 lacks cysteine-rich domains and is thought to be secreted as a soluble monomer. Incomplete sequence information prevents classification of other mucins. Tandem repeats of amino acids rich in serine, threonine and proline are a common element in mucin core proteins, giving rise to relatively rigid, linear molecules with great potential for glycosylation. Ten distinct mucin genes have been identified in humans so far. Patterns of expression vary greatly. While MUC9, or oviductin, appears to be restricted to oviduct, the transmembrane mucin MUC1 is widely expressed. Proven functions for the different mucins are largely unknown, although potential functions are addressed in this review. Genetic and protein sequence information and expression profiles are also summarized, followed by a description of mucin assembly. Special attention is given to mucin expression in male and female reproductive tracts.