Identify which of the follwing is not a property of secreted mucins

Mucins are major glycoprotein components of the mucous that coats the surfaces of cells lining the respiratory, digestive, and urogenital tracts, and in some amphibia, the skin. They function to protect epithelial cells from infection, dehydration, and physical or chemical injury, as well as to aid the passage of materials through a tract. Individual organisms make several structurally different mucins, and a given mucin may be found in more than one organ (see Supplemental Material). Members of the mucin family can differ considerably in size. Some are small, containing a few hundred amino acid residues, whereas others contain several thousands of residues and are among the largest known proteins. Irrespective of size, all mucin polypeptide chains have domains rich in threonine and/or serine whose hydroxyl groups are in O-glycosidic linkage with oligosaccharides. Moreover, these domains are composed of tandemly repeated sequences that vary in number, length, and amino acid sequence from one mucin to another . The carbohydrate content of a mucin may account for up to 90% of its weight. There are two types of mucins, membrane-bound and secreted. Of the human mucins, two are membrane-bound (MUC1 and MUC4) and four are secreted (MUC2, MUC5AC, MUC5B, and MUC7) . The three other mucins (MUC3, MUC6, and MUC8) cannot be classified. Each human mucin has a counterpart in other animals. Thus, porcine submaxillary mucin (PSM)1, one of the most thoroughly characterized mucins, has a tissue distribution and structure similar to MUC5B. An increasing number of proteins that are not mucins also contain highly O-glycosylated domains called “mucin-like domains.”

The functions of mucins are dependent on their ability to form viscous solutions or gels. Although the highly glycosylated domains of mucins are devoid of secondary structures, they are long extended structures that are much less flexible than unglycosylated random coils. The oligosaccharides contribute to this stiffness in two ways, by limiting the rotation around peptide bonds and by charge repulsion among the neighboring, negatively charged oligosaccharide groups . Such long, extended molecules have a much greater solution volume than native or denatured proteins with little or no carbohydrate and endow aqueous mucin solutions with a high viscosity. Mucins protect against infection by microorganisms that bind cell surface carbohydrates, and mucin genes appear to be up-regulated by substances derived from bacteria,e.g. lipopolysaccharides.

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