Mucin proteins are highly glycosylated extracellular proteins with characteristic gel-forming properties. Mucins are the major constitute of mucus, including saliva, and some transmembrane members of the protein family have been found to participate in cell signaling and cancer development.
Mucin Protein Composition
Mucins are 50% or more glycan by mass, usually in the form of sulfated polysaccharide or sialic acid, the latter being known as sialomucins. The proteins also contain a mucin domain rich in the amino acids threonine, serine, and proline. Like other glycoproteins, mucins are processed by the Golgi apparatus, and they are presented or secreted on the apical surface of epithelial cells.
Abnormal mucin glycosylation has been found to make some of the mucins tumor-specific antigens, particularly in pancreatic, breast, and ovarian cancers. Aberrant expression, such as over-expression of transmembrane forms, has also been identified as a potential mechanism affecting certain disease states, particularly in the gastrointestinal tract.
Function and Expression of the Human Mucin Genes
More than 20 human mucin genes have been catalogued by the National Library of Medicine ( MeSH; accessed March 30, 2010), National Center of Bioinformatics (NCBI; accessed March 30, 2010), and the UniProt database (accessed March 30, 2010):
- MUC1 is expressed by glandular epithelium and the protein is increased in breast ducts during lactation. Over-expression of the MUC1 gene has been associated with colorectal cancer, and reduced expression has been associated with prostate cancer.
- MUC2 encodes a gel-forming protein and is expressed in the small intestine and other mucus membrane-containing organs.
- MUC3 encodes a protein secreted by the intestinal mucosa, which occurs as closely related A and B subtypes.
- MUC4 encodes a transmembrane heterodimer found on a broad range of epithelial tissues. MUC4 is potentially involved in cell signaling, and down-regulation of the gene has been associated with prostate cancer.
- MUC5B encodes a gel-forming protein expressed by the sublingual gland and submucosal tissues. MUC5B is a principle component of saliva.
- MUC5AC encodes to gastric mucin proteins, MUC5A and MUC5C, which are gel-forming and found on the surface of respiratory epithelia.
- MUC6 encodes a gel-forming gastric mucin protein associated with the gastric epithelium.
- MUC7 encodes a protein secreted in saliva.
- MUC8 is expressed on tracheobronchial epithelium, which is found in the throat and windpipe.
- MUC9 encodes a protein known as oviductin, estrogen-dependent oviduct protein, or oviductal glycoprotein 1.
- MUC10 encodes a protein secreted by the submandibular gland into the saliva.
- MUC11/MUC12 are synonymous and encode a cell surface-associated protein with unknown function. The MUC12 precursor has ubiquitous tissue distribution with higher expression in the colon.
- MUC13 encodes a cell surface-associated protein expressed by ductal and glandular epithelia and the intestines.
- MUC14 encodes a protein known as endomucin 2 or gastric cancer antigen GA34, which interferes with the assembly of focal adhesion complexes.
- MUC15 encodes a cell surface-associated protein that appears to be expressed mainly in the placenta.
- MUC16 encodes a cell surface-associated protein known as CA-125 ovarian cancer antigen, which is expressed in the female reproductive system, respiratory tract, and ocular surface. Aberrant expression of this gene has also been associated with chronic dry eye.
- MUC17 encodes a cell surface-associated protein that confers anti-adhesive properties to cancer cells that lose their polarization. Expression of the gene also offers cytoprotection and maintains luminal structure in the small intestine. A secreted form has also been observed in the respiratory tract.
- MUC18 encodes a protein known as melanoma cell adhesion molecule (MCAM) or CD146.
- MUC19 expression is restricted to the glands around the oral cavity and trachea, but the encoded protein is not known to be a component of saliva.
- MUC20 encodes multiple isoforms of a cell surface-associated protein that suppresses the downstream signaling of the met proto-oncogene.
- MUC21 encodes a novel transmembrane mucin protein known as epiglycanin.
Additional information:
Alberts et al. Molecular Biology of the Cell, 4th edition. 2002
Cotran, Kumar, Collins. Robbins Pathologic Basis of Disease, 6th edition. 1999
Alicia Mae Prater - Alicia received her doctorate in Experimental Pathology in 2007. She has been a freelance writer and scientific editor since 2008.
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