Mucin secretion

The coat mucins are thought to be secreted from two major glands in the larval body the oesophageal and secretory glands. The latter, previously termed the excretory cell, is directly connected to the cuticle by a duct opening at the secretory pore (Nichols, 1956). It is not certain whether the panel of secreted mucins are all represented in the coat, or whether there are important differences between the two compartments. 

Loukas, A.C., Hintz, M., Linder, D., Mullin, N.P., Parkinson, J., Tetteh, K.K.A. and Maizels, R.M. (2000) A family of secreted mucins from the parasitic nematode Toxocara canis bear diverse mucin domains but share similar flanking six-cysteine (SXC) repeat motiis. Journal of Biobgical Chemistry (ia press). 

Perez-Vilar J. and Hill R.L. (1998a). The carboxyl-terminal 90 residues of porcine submaxillary mucin are sufficient for forming disulfide-bonded dimers. J Biol Chem 273, 6982-6988 Perez-Vilar J. and Hill R.L. (1998b). Identification of the half-cystine residues in porcine submaxillary mucin critical for multimerization through the D-domains. Roles of the CGLCG motif in the D1 and D3-domains. J Biol Chem 273, 34527-34534 Perez-Vilar J. and Hill R.L. (1999). The structure and assembly of secreted mucins. J Biol Chem 274, 31751-31754... 

Selected human transmembrane-bound (TM) mucins and secreted mucins are listed, including those that form a mucus gel (Gel). The highly O-glycosylated Ser/Thr/Pro-rich variable number of tandem repeats (VNTR) show a representative amino acid sequence in one of the repeats. The underlined sequence in MUCl is a peptide epitope recognized by anti-MUC1 antibody SM3. Some of the tissues expressing these mucins are listed but most mucins are widely expressed and found in many other tissues and cell types. 

Figure 1 Model of a gel-forming mucin in the intestinai tract. A iarge secreted mucin is depicted, such as MUC2 in the coion. The VNTR (variabie number of tandem repeat) region is rich in Ser, Thr, and Pro and is highiy O-giycosyiated therefore, the peptide assumes an extended bottie brush -iike conformation. The majority of coionic mucin 0-giycans appear to have extended core 3 structures but extended core 1 and other O-giycan structures are also found. Both ends of the molecule have Cys-rich regions and other domains that may be involved in the polymerization to form large molecules of several million daltons.

Three major cell models obtained from human colon cancers have been used HT-29, and T84, and Caco-2. HT-29 is a cell line capable of secreting mucin it has been used to study the effect of mucin on drug absorption. The T84 cell line does not express biochemical differentiation markers and is not particularly useful for studying drug transport. However, the T84 ceU line does express P-glycoprotein and can be used to study the role of this efflux pump. ... 

Gel-forming secreted mucins, encoded by genes located at chromosome llplS and include MUC-2, MUG-5AC, MUC-5B and MUC-6. 

H03S-6]-GlcpNAc) on GI343AM-I and CD34, mucin-like glycoproteins expressed on HEVs, and involved in L-selectin binding. This enzyme is possibly involved in the Gal-6-O-sulfation of secreted mucins. 

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