There is growing evidence that
N-linked glycans play pivotal roles in protein folding and intra- and/or inter-cellular trafficking of
N-glycosylated proteins. It has been shown that during the N-glycosylation of proteins, significant amounts of free N-glycans are generated in the lumen of the endoplasmic reticulum (ER) by a mechanism which remains to be clarified. Free
N-glycans are also formed in the cytosol by deglycosylation of misfolded glycoproteins by cytoplasmic peptide:
N-glycanase (PNGase)(1), which are subjected to degradation by a cellular process called “ER-associated degradation” (ERAD). While the precise function of free
N-glycans remains unknown, biochemical studies have revealed that a novel cellular process enables them to be catabolized in a specialized manner, that involves pumping free
N-glycans in the lumen of the ER into the cytosol, where further processing occurs by endo-beta-
N-acetylglucosaminidase (ENGase)(2) and cytosolic alpha-mannosidase (Man2C1)(3). Recent studies also suggested that the structures and amount of free
N-glycans in tomovarious cancer-derived cell lines were quite different (4), prompting us to postulate that it can potentially serve as a biomarker of certain type of cancers and other diseases. There is also an evidence that the structure of free
N-glycans of cultured cells changes drastically under different culture conditions, indicating that the they can be regulated in response to the environmental conditions. In this symposium I will summarize the current knowledge on the molecular mechanism of formation and degradation of free
N-glycan, and also discuss its potential biological importance (5, 6).
References: (1) Suzuki, T. (2000) J. Cell Biol. 149, 1039; (2) Suzuki, T., et al. (2002) Proc. Natl. Acad. Sci. USA 99, 9691; (3) Suzuki, T., et al. (2006) Biochem. J. 400, 31; (4) Ishizuka, A., et al. (2008) Biochem. J. 413, 227; (5) Suzuki, T. and Funakoshi, Y. (2006) Glycoconj. J. 291; (6) Suzuki, T. (2007) Sem. Cell Dev. Biol. 18, 762.
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