Mucin is a type of protein produced by the cells of the epithelium, or the tissue that lines the cavities and structures of the body. Mucins are found in all types of animals and about 19 different genes have been found to code for mucins in humans. The main role of these types of proteins is to produce and secrete gels into the body of the organism.
Mucins are characterized by their high molecular weight and the fact that they are highly glycosylated proteins. Proteins that are glycosylated have strands of carbohydrates attached to them. There are many different molecules that undergo glycosylation, but there are a few key features of the process. First of all, there is always an enzyme involved or the reaction will not take place. In addition, the molecule that donates the carbohydrates is usually a type of nucleotide sugar, and the reaction is very site specific, or the carbohydrate chain can only bond to a particular place on the receiving molecule.
A mucin molecule has two distinct regions that make up its structure. A very large central region is composed of repeating sequences that can be anywhere from 10 to 80 amino acids long. In the case of mucin molecules, at least half of these amino acids are serine or threonine. It is in this area that the glycosylation takes place with hundreds of carbohydrate chains bonding to the amino acids.
At either end of the mucin, there is very little glycosylation, but the areas have a large number of cysteines. Cysteine is a type of amino acid that is integral to two separate mucins binding together. Mucins bind together at each end through the production of disulfide bridges between a cysteine located on each mucin. A disulfide bridge is a type of bond that forms when a pair of sulfur atoms binds together.
Mucins are a key constituent of most gel-like secretions within the body, such as saliva. The functions of the different mucin molecules include lubrication; a method of signaling between cells; and a form of chemical barrier, often used for protection. In some animals, they also take part in the formation of bones. Finally, mucins work with the immune system and bind to pathogens, or cells that cause disease.
A link has been found between the over-expression of some types of mucins and different cancers. In particular, the over-expression of MUC1 is linked to a number of cancers including colon cancer. Breast, ovarian, pancreatic and lung cancer have also been associated with the over expression of mucin molecules. Some lung diseases, such as asthma, bronchitis and cystic fibrosis, have also been found to be related to mucin over-expression.