MUCIN STRUCTURE AND FUNCTION - INSIGHTS FROM MOLECULAR-BIOLOGY

Mucins are highly O-glycosylated glycoproteins implicated in the protection of cells from extracellular agents. Two general classes of mucins have been described: secreted and membrane. Most, but not all, mucin structures contain a central tandem repeat region which is rich in serine and threonine and is highly variable in size between different mucins; many also contain a cysteine-rich domain. Repeat sequences are often conserved within a single molecule, such as human MUC1 protein, but are poorly conserved between species, e.g. mouse and human MUC1 protein repeats. Many mucins are polymorphic due to variable numbers of repeats, and mucin transcripts are often heterogeneous. Although mucin expression is relatively tissue specific, some mucins, such as the MUC1 and MUC2 proteins, are found in multiple tissues. Moreover, a single tissue may express more than one mucin. Limited studies suggest that regulation of mucin expression is complex. Membrane mucins have been implicated in development and tumor progression, possibly by modulating cell-cell interactions. Some mucin cysteine-rich domains may also play a role in regulating cell proliferation. Undoubtedly, future studies using recombinant DNA probes will greatly expand our understanding of these complex molecules.