Mucins and secreted factors.

Mucin is a large glycoprotein (M up to 4-6.10(6)) with a high content of serine, threonine, and proline residues and numerous O-linked saccharides, often occurring in clusters on the polypeptide. Nine mucin genes exist in humans that encode an apomucin highly modified by O-glycans in forming epithelial mucins. These are variably expressed by epithelial cells of the gastrointestinal, tracheobronchial and reproductive tracts. They may be found either membrane-associated or secreted. As might be imagined from their ubiquitous and complex nature, the biological roles of glycans are quite varied. Protection against infection is an important biological role. Alterations in their carbohydrate moiety have been reported in airway and salivary mucins secreted by patients suffering of cystic fibrosis (CF). Moreover, changes in expression of glycans are also often reported in the setting of transformation and progression to malignancy. Due to the key roles played by glycans of glycoconjugates in both physiological and pathological events, glycobiology and carbohydrate chemistry have become of increasing importance in modern biotechnology.