Structure and Function
Mucin 16 is a membrane associated mucin that possesses a single transmembrane domain. A unique property of MUC16 is its large size. MUC16 is more than twice as long as MUC1 and MUC4 and contains about 22,000 amino acids, making it the largest membrane associated mucin. MUC16 is made of three different domains; an N-terminal domain, a tandem repeat domain, and a C-terminal domain. The N-terminal domain and tandem repeat domain are both entirely extracellular and highly O-glycosylated. All mucins contain a tandem repeat domain that has repeating sequences high in serine, threonine and proline. The C-terminal domain contains multiple extracellular SEA (sea urchin sperm protein, enterokinase, and agrin) modules, a transmembrane domain, and a cytoplasmic tail. The extracellular region of MUC16 can be released from the cell surface by undergoing proteolytic cleavage. MUC16 is thought to be cleaved at a site in the SEA modules.
MUC16 is a component of the ocular surface (including the cornea and conjunctiva) and the respiratory tract and female reproductive tract epithelia. Since MUC16 is highly glycosylated it creates a hydrophilic environment that acts as a lubricating barrier against foreign particles and infectious agents on the apical membrane of epithelial cells. The cytoplasmic tail of MUC16 has been shown to interact with cytoskeleton by binding members of the ERM protein family. The expression of mucin 16 has been shown to be altered in dry eye, cystic fibrosis, and several types of cancers.
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