Glycoprotein
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O-GlcNAc and Its Function

 Many nuclear and cytoplasmic proteins are glycosylated on serine or threonine residue with a monosaccharide, -acetylglucosamine, in an O-glycosidic linkage which is termed O-GlcNAc (Fig. 1). O-GlcNAc is present in all eukaryotes. O-GlcNAc modification (O-GlcNAcylation) is one of the post-translational modifications and involved in signal transduction (1). O-GlcNAc and O-phosphate alternatively occupy the same or adjacent site. For example, the C-terminal domain of RNA polymerase II, Thr58 of c-myc, which is a "hot spot" in mutation for lymphomas, and Ser16 of estrogen receptor are modified not only by O-GlcNAc but also a phosphate group. The half life of O-GlcNAc is shorter than that of the polypeptides. O-GlcNAc modification is changed dynamically by certain stimuli such as TPA, okadaic acid and others. Therefore, it is suggested that one of the functions of O-GlcNAc is regulation of transient phosphorylation. The glycosylation sites with O-GlcNAc have no obvious consensus sequences. Unlike phosphorylation, O-GlcNAcylation has not been observed in tyrosine residues.
 
Fig. 1 Schematic diagram of O-GlcNAc modification for proteins. The protein shown possesses a typical O-GlcNAc attachment site. However, the consensus sequences to be modified is unknown.
 
 
  Immunohistochemical analysis of the localization of O-GlcNAc in rat pancreas revealed that O-GlcNAc is expressed on proteins in the nucleus and cytoplasm of endocrine cells in the islets of Langerhans (2). To date, a large number of cytoplasmic and nuclear proteins such as transcription factors, nuclear pore proteins, oncogene products, tumor suppressors and cytoskeletal proteins have been shown to be modified by O-GlcNAc. They have two common features: (1) they are also phosphorylated and (2) they form multimeric complexes with other proteins reversibly. Thus, the O-GlcNAcylation might be a regulatory modification analogous to phosphorylation, involved in signal transduction and protein multimerization.

Enzymes for addition and for removal of the O-GlcNAc residue have been purified and cloned (3, 4). UDP-GlcNAc: polypeptide O-acetylglucosaminyltransferase (O-GlcNAc transferase) adds acetylglucosamine to the hydroxyl group of serine or threonine residue(s) of proteins. The O-GlcNAc transferase is itself modified by O-GlcNAc and the phosphate group, and has 11 tetratricopeptide repeats (TPR) which are involved in protein-protein interaction. Recently, using a yeast two-hybrid system we identified GABAA receptor-associated protein (GRIF-1) and its novel homolog, OIP106 as O-GlcNAc transferase interacting proteins(5). O-GlcNAc transferase is abundant in brain and pancreas. Disruption of O-GlcNAc transferase activity leads to the death of mouse embryonic stem cells. This suggests that the O-GlcNAc modification is essential in cells. O-GlcNAc specific acetylglucosaminidase (O-GlcNAcase) removes O-GlcNAc residues from proteins. O-GlcNAcase is a cytosolic neutral -acetylglucosaminidase, unlike the acidic lysosomal hexosaminidases. The regulation of O-GlcNAcylation by these two enzymes is analogous to that of phosphorylation by kinases and phosphatases (Fig. 2).

There is a growing body of evidence that the aberrant O-GlcNAc modification is correlated with diabetes, tumorigenesis, Alzheimer's disease (1, 6-9). More detailed studies are necessary for determining the functions of O-GlcNAc in a variety of biological systems.
 
Fig. 2 Reciprocal relationship between phosphorylation and O-GlcNAc modifications of hydroxyl group of serine or threonine residue. O-GlcNAc: O-linked acetylglucosamine. O-GlcNAcase: O-GlcNAc specific acetylglucosaminidase.
 
 
Yoshihiro Akimoto
(Department of Anatomy, Kyorin University School of Medicine)
References (1) Wells L, Vosseller K, Hart GW (2001) Glycosylation of nucleocytoplasmic proteins: Signal transduction and O-GlcNAc. Science 291:2376-2378.
(2) Akimoto Y, Kreppel LK, Hirano H, Hart GW (1999) Localization of the O-GlcNAc transferase in rat pancreas. Diabetes 48: 2407-2413.
(3) Kreppel LK, Blomberg MA, Hart GW. (1997) Dynamic glycosylation of nuclear and cytosolic proteins. Cloning and characterization of a unique O-GlcNAc transferase with multiple tetratricopeptide repeats. J Biol Chem 272: 9308-9315.
(4) Gao Y, Wells L, Comer FI, Parker GJ, Hart GW (2001) Dynamic O-glycosylation of nuclear and cytosolic proteins: cloning and characterization of a neutral, cytosolic beta-N-acetylglucosaminidase from human brain. J Biol Chem 276: 9838-9845.
(5) Iyer SP, Akimoto Y, Hart GW (2003) Identification and cloning of a novel family of coiled-coil domainÅ proteins that interact with O-GlcNAc transferase. J Biol Chem 278: 5399-5409.
(6) Akimoto Y, Kreppel LK, Hirano H, Hart GW (2000) Increased O-GlcNAc transferase in pancreas of rats with streptozotocin-induced diabetes. Diabetologia 43:1239-1247.
(7) Akimoto Y, Kreppel LK, Hirano H, Hart GW (2001) Hyperglycemia and the O-GlcNAc transferase in rat aortic smooth muscle cells: elevated expression and altered patterns of O-GlcNAcylation. Arch Biochem Biophys 389:166-175.
(8) Brownlee M (2001) Biochemistry and molecular cell biology of diabetic complications. Nature 414: 813-820.
(9) Vosseller K, Wells L, Lane MD, Hart GW (2002) Elevated nucleocytoplasmic glycosylation by O-GlcNAc results in insulin resistance associated with defects in Akt activation in 3T3-L1 adipocytes. Proc Natl Acad Sci USA 99:5313-5318.
Jan. 22, 2003

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