O-Glycosylation of Nuclear and Cytosolic Proteins

serine/threonine-linked β-N-acetylglucosamine O-GlcNAc transferase β-N-acetylglucosaminidase eukaryotic initiation factor 2 C-terminal domain of RNA polymerase II glutamine:fructose-6-phosphate amidotransferase Despite the long held view that protein glycosylation occurs exclusively on extracellular or lumenal polypeptides (1Furukawa K. Kobata A. Curr. Opin. Biotechnol. 1992; 3: 554-559Crossref PubMed Scopus (34) Google Scholar), it is clear that many nuclear and cytoplasmic proteins are multiply O-glycosylated at specific serine or threonine hydroxyl groups by single β-N-acetylglucosamine moieties (O-GlcNAc)1(2Hart G.W. Haltiwanger R.S. Holt G.D. Kelly W.G. Annu. Rev. Biochem. 1989; 58: 841-874Crossref PubMed Scopus (322) Google Scholar, 3Hart G.W. Annu. Rev. Biochem. 1997; 66: 315-335Crossref PubMed Scopus (450) Google Scholar, 4Haltiwanger R.S. Busby S. Grove K. Li S. Mason D. Medina L. Moloney D. Philipsberg G. Scartozzi R. Biochem. Biophys. Res. Commun. 1997; 231: 237-242Crossref PubMed Scopus (95) Google Scholar). O-GlcNAc modification is common to nearly all eukaryotes, including filamentous fungi, plants, animals, and animal parasites, as well as viruses that infect eukaryotes. Mounting evidence suggests a direct role for O-GlcNAc in cellular regulation. For example, the α-toxin of the gangrene causing bacteriaClostridium novyi is an O-GlcNAc transferase that exerts its toxic effects by the addition of O-GlcNAc to proteins in the Rho subfamily (5Selzer J. Hofmann F. Rex G. Wilm M. Mann M. Just I. Aktories K. J. Biol. Chem. 1996; 271: 25173-25177Abstract Full Text Full Text PDF PubMed Scopus (123) Google Scholar). Thus, the disruption of normalO-GlcNAc-regulated pathways may be responsible for the pathology of some bacteria. Moreover, disruption of the gene forO-GlcNAc transferase demonstrates that O-GlcNAc modification is essential for life, even at the single cell level (6Shafi R. Iyer S.P. Ellies L.G. O'Donnell N. Marek K.W. Chui D. Hart G.W. Marth J.D. Proc. Natl. Acad. Sci. U. S. A. 2000; 97: 5735-5739Crossref PubMed Scopus (593) Google Scholar). O-GlcNAc appears to be both as abundant and as dynamic as protein phosphorylation. In several documented instances, phosphorylation and O-GlcNAc modification are reciprocal, occurring at the same or adjacent hydroxyl moieties (7Kelly W.G. Dahmus M.E. Hart G.W. J. Biol. Chem. 1993; 268: 10416-10424Abstract Full Text PDF PubMed Google Scholar, 8Chou T.-Y. Dang C.V. Hart G.W. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 4417-4421Crossref PubMed Scopus (183) Google Scholar). Furthermore, all O-GlcNAc-modified proteins identified to date also occur as phosphorylated proteins. Nevertheless, the interrelationship between Ser/Thr O-GlcNAc modification andO-phosphorylation appears to be complex. Although there are examples of mutually exclusive O-GlcNAc modification andO-phosphorylation, it is likely that all possible combinations are represented in the complex environment of a eukaryotic cell (Fig. 1). The specific addition and removal of these two differentially regulated post-translational modifications might allow for nearly infinite modulation of protein function. The immense task of coordinating cellular activities and responding to extracellular cues with both temporal and spatial accuracy is likely to require the concerted action of both of these regulatory modifications. Reports that alkali-induced β-elimination of adenovirus fiber proteins releases GlcNAcitol hinted at the existence ofO-linked GlcNAc (9Ishibashi M. Maizel Jr., J.V. Virology. 1974; 58: 345-361Crossref PubMed Scopus (58) Google Scholar). Subsequent analysis confirmed the presence of β-O-GlcNAc and suggested that the modification may be involved in adenovirus fiber assembly or stabilization (10Mullis K.G. Haltiwanger R.S. Hart G.W. Marchase R.B. Engler J.A. J. Virol. 1990; 64: 5317-5323Crossref PubMed Google Scholar). Although another study suggested the existence of O-glycosidically linked GlcNAc on extracellular proteins (11Lin T.S. Kolattukudy P.E. Eur. J. Biochem. 1980; 106: 341-351Crossref PubMed Scopus (53) Google Scholar), later structural analyses suggested that these workers were likely observing α-linked GlcNAc, a mucin-like modification common in primitive eukaryotes (12Jung E. Gooley A.A. Packer N.H. Karuso P. Williams K.L. Eur. J. Biochem. 1998; 253: 517-524Crossref PubMed Scopus (22) Google Scholar). Another early study showed that the GlcNAc-binding lectin wheat germ agglutinin blocked ATP-dependent RNA nuclear transport (13Baglia F. Maul G.G. Proc. Natl. Acad. Sci. U. S. A. 1983; 80: 2285-2289Crossref PubMed Scopus (46) Google Scholar). The characterization of β-O-GlcNAc in 1984 explained some of these preliminary observations and establishedO-GlcNAc as a major form of intracellular protein glycosylation (14Torres C.-R. Hart G.W. J. Biol. Chem. 1984; 259: 3308-3317Abstract Full Text PDF PubMed Google Scholar). The nuclear pore proteins were among the first structurally characterized O-GlcNAc proteins (15Holt G.D. Snow C.M. Senior A. Haltiwanger R.S. Gerace L. Hart G.W. J. Cell Biol. 1987; 104: 1157-1164Crossref PubMed Scopus (313) Google Scholar, 16Hanover J.A. Cohen C.K. Willingham M.C. Park M.K. J. Biol. Chem. 1987; 262: 9887-9894Abstract Full Text PDF PubMed Google Scholar). Since then, several laboratories have shown that hundreds, if not thousands, of proteins in the nucleus and cytoplasm are modified withO-GlcNAc (3Hart G.W. Annu. Rev. Biochem. 1997; 66: 315-335Crossref PubMed Scopus (450) Google Scholar, 4Haltiwanger R.S. Busby S. Grove K. Li S. Mason D. Medina L. Moloney D. Philipsberg G. Scartozzi R. Biochem. Biophys. Res. Commun. 1997; 231: 237-242Crossref PubMed Scopus (95) Google Scholar). Given the broad spectrum of proteins that contain this modification, there are likely to be many different functions for O-GlcNAc. Studies with the transcription factor Sp1 suggest that O-GlcNAc protects the protein from proteasome degradation (17Han I. Kudlow J.E. Mol. Cell. Biol. 1997; 17: 2550-2558Crossref PubMed Scopus (377) Google Scholar). Recent reports have shown that recognition of O-GlcNAc on peptides constitutes an important feature of major histocompatibility complex Class I antigen presentation (18Haurum J.S. Hoier I.B. Arsequell G. Neisig A. Valencia G. Zeuthen J. Neefjes J. Elliott T. J. Exp. Med. 1999; 190: 145-150Crossref PubMed Scopus (93) Google Scholar). Pulse-chase analyses have shown that the O-GlcNAc modification of some proteins is highly transient, with turnover rates similar to phosphorylation (19Chou C.-F. Smith A.J. Omary M.B. J. Biol. Chem. 1992; 267: 3901-3906Abstract Full Text PDF PubMed Google Scholar, 20Roquemore E.P. Chevrier M.R. Cotter R.J. Hart G.W. Biochemistry. 1996; 35: 3578-3586Crossref PubMed Scopus (151) Google Scholar). Another study found that dynamic changes of O-GlcNAc-modified proteins are associated with lymphocyte activation (21Kearse K.P. Hart G.W. Proc. Natl. Acad. Sci. U. S. A. 1991; 88: 1701-1705Crossref PubMed Scopus (191) Google Scholar). Several recent reports with phosphatase and kinase inhibitors have provided direct support for a relationship between O-phosphorylation and O-glycosylation of serine or threonine residues of some proteins (4Haltiwanger R.S. Busby S. Grove K. Li S. Mason D. Medina L. Moloney D. Philipsberg G. Scartozzi R. Biochem. Biophys. Res. Commun. 1997; 231: 237-242Crossref PubMed Scopus (95) Google Scholar, 22Hart G.W. Greis K.D. Dong L.Y.D. Blomberg M.A. Chou T.Y. Jiang M.S. Roquemore E.P. Snow D.M. Kreppel L.K. Cole R.N. Comer F.I. Arnold C.S. Hayes B.K. Adv. Exp. Med. Biol. 1995; 376: 115-123Crossref PubMed Scopus (121) Google Scholar, 23Griffith L.S. Schmitz B. Eur. J. Biochem. 1999; 262: 824-831Crossref PubMed Scopus (96) Google Scholar, 24Lefebvre T. Alonso C. Mahboub S. Dupire M.J. Zanetta J.P. Caillet-Boudin M.L. Michalski J.C. Biochim. Biophys. Acta. 1999; 1472: 71-81Crossref PubMed Scopus (60) Google Scholar). Consistent with the hypothesis that O-GlcNAc has a regulatory role, disruptions of the O-GlcNAc transferase homolog, SPY, inArabidopsis results in impaired gibberellin signal transduction (25Thornton T.M. Swain S.M. Olszewski N.E. Trends Plant Sci. 1999; 4: 424-428Abstract Full Text Full Text PDF PubMed Scopus (106) Google Scholar). It is clear that O-GlcNAc is involved in very diverse aspects of cellular physiology (Fig.2). The challenge for the coming years is to determine the precise contribution of O-GlcNAc in the regulation of these systems. A uridine diphospho-N-acetylglucosamine:polypeptide β-N-acetylglucosaminyltransferase (O-GlcNAc transferase (OGT); EC 2.4.1.94) was identified in extracts of rabbit reticulocyte membranes and rat liver using a synthetic peptide acceptor (26Haltiwanger R.S. Holt G.D. Hart G.W. J. Biol. Chem. 1990; 265: 1-6Abstract Full Text PDF PubMed Google Scholar). Latency studies confirmed that OGT is cytosolic and nuclear. OGT was subsequently purified to near homogeneity from rat liver (27Haltiwanger R.S. Blomberg M.A. Hart G.W. J. Biol. Chem. 1992; 267: 9005-9013Abstract Full Text PDF PubMed Google Scholar), and its gene was cloned from rat (28Kreppel L.K. Blomberg M.A. Hart G.W. J. Biol. Chem. 1997; 272: 9308-9315Abstract Full Text Full Text PDF PubMed Scopus (604) Google Scholar, 29Lubas W.A. Frank D.W. Krause M. Hanover J.A. J. Biol. Chem. 1997; 272: 9316-9324Abstract Full Text Full Text PDF PubMed Scopus (412) Google Scholar), Caenorhabditis elegans, and human (30Lubas W.A. Hanover J.A. J. Biol. Chem. 2000; 275: 10983-10988Abstract Full Text Full Text PDF PubMed Scopus (255) Google Scholar). Recent studies using the Cre-Lox system in the mouse have shown that a functional OGT is essential for viability of embryonic stem cells and for mouse ontogeny (6Shafi R. Iyer S.P. Ellies L.G. O'Donnell N. Marek K.W. Chui D. Hart G.W. Marth J.D. Proc. Natl. Acad. Sci. U. S. A. 2000; 97: 5735-5739Crossref PubMed Scopus (593) Google Scholar). Thus,O-GlcNAc modification appears to be essential for eukaryotic cellular physiology. Consistent with the essential role ofO-GlcNAc, there appear to be multiple mechanisms for the control of the O-GlcNAc transferase (Fig.3). OGT has 11 tetratricopeptide repeats at its N terminus, which mediate the trimerization of the catalytic subunit (31Kreppel L.K. Hart G.W. J. Biol. Chem. 1999; 274: 32015-32022Abstract Full Text Full Text PDF PubMed Scopus (335) Google Scholar) as well as the interactions of the enzyme with many other proteins. The tetratricopeptide repeat domain also appears to play a role in substrate selectivity. 2F. I. Comer and G. W. Hart, manuscript in preparation. OGT is a unique gene without other obvious family members but is very highly conserved from C. elegans to human. In addition to the cloned 110-kDa catalytic subunit of OGT, there is a highly related 78-kDa subunit in several tissues (28Kreppel L.K. Blomberg M.A. Hart G.W. J. Biol. Chem. 1997; 272: 9308-9315Abstract Full Text Full Text PDF PubMed Scopus (604) Google Scholar). This related isoform could be the product of proteolytic processing or alternative RNA transcript processing. TheO-GlcNAc transferase itself is modified withO-GlcNAc as well as tyrosine phosphate (28Kreppel L.K. Blomberg M.A. Hart G.W. J. Biol. Chem. 1997; 272: 9308-9315Abstract Full Text Full Text PDF PubMed Scopus (604) Google Scholar), suggesting possible regulation through post-translational modifications. The activity of OGT is potently inhibited by UDP, the by-product of the transfer reaction. The enzyme is sensitive to UDP-GlcNAc/UDP ratios over the entire physiologic range of sugar nucleotide concentration (micromolar to millimolar). In many cells the concentrations of UDP-GlcNAc approach that of ATP with as much as 2–5% of total glucose utilization channeled to making this sugar nucleotide (32Zhivkov V. Tosheva R. Zhivkova Y. Comp. Biochem. Physiol. 1975; 51B: 421-424Google Scholar). Thus, local or global perturbations in the UDP-GlcNAc or UDP levels can modulate the activity of OGT. These features of the O-GlcNAc transferase and its environment suggest that it may be subject to complex regulatory mechanisms. A cytosolic and nuclear β-N-acetylglucosaminidase (O-GlcNAcase) with a neutral pH optimum and selectivity toward O-linked GlcNAc has also been identified and purified (33Dong D.L.-Y. Hart G.W. J. Biol. Chem. 1994; 269: 19321-19330Abstract Full Text PDF PubMed Google Scholar). Several useful inhibitors, such as,O-(2-acetamido-2-deoxy-o-glucopyranosylidene)-amino-N-phenylcarbamate (PUGNAC) (34Horsch M. Hoesch L. Vasella A. Rast D.M. Eur. J. Biochem. 1991; 197: 815-818Crossref PubMed Scopus (134) Google Scholar), have been identified. Treatment of several different cell types with this inhibitor results in an overall increase in O-GlcNAc levels on numerous proteins (35Haltiwanger R.S. Grove K. Philipsberg G.A. J. Biol. Chem. 1998; 273: 3611-3617Abstract Full Text Full Text PDF PubMed Scopus (245) Google Scholar). Current data suggest that in many systems the regulation of O-GlcNAc cycling will have important consequences for the regulation of protein function. For example, a recent study showed that, when expressed in the cytoplasm and nucleus, a Golgi enzyme that caps O-GlcNAc and prevents its normal cycling is lethal to cells (36Snow D.M. Shaper J.H. Shaper N.L. Hart G.W. Mol. Biol. Cell. 1996; 6 (abstr.): 357Google Scholar). The cloning and expression of the O-GlcNAcase will help determine the interplay between the regulated addition and removal ofO-GlcNAc from proteins. Nucleoporins, which mediate the active transport of macromolecules into and out of the nucleus, are extensively modified withO-GlcNAc on their exposed surfaces. Several studies have shown that monoclonal antibodies or lectins that bindO-GlcNAc block nuclear transport of macromolecules at an energy-dependent step (37Finlay D.R. Newmeyer D.D. Price T.M. Forbes D.J. J. Cell Biol. 1987; 104: 189-200Crossref PubMed Scopus (376) Google Scholar). Some investigators have suggested that O-GlcNAc is an alternative nuclear transport signal on some proteins (38Hubert J. Sève A.P. Facy P. Monsigny M. Cell Differ. 1989; 27: 69-81Crossref Scopus (60) Google Scholar). It is also possible thatO-GlcNAc is directly involved in control of the translocation machinery. Nevertheless, the roles of O-GlcNAc in nuclear transport are presently unclear (39Miller M.W. Caracciolo M.R. Berlin W.K. Hanover J.A. Arch. Biochem. Biophys. 1999; 367: 51-60Crossref PubMed Scopus (71) Google Scholar). RNA polymerase II and many of its transcription factors are extensively modified with O-GlcNAc (40Jackson S.P. Tjian R. Cell. 1988; 55: 125-133Abstract Full Text PDF PubMed Scopus (649) Google Scholar, 41Comer F.I. Hart G.W. Biochim. Biophys. Acta. 1999; 1473: 161-171Crossref PubMed Scopus (149) Google Scholar). The O-GlcNAc modification and O-phosphorylation of the C-terminal repeat domain (CTD) appear to be reciprocal (7Kelly W.G. Dahmus M.E. Hart G.W. J. Biol. Chem. 1993; 268: 10416-10424Abstract Full Text PDF PubMed Google Scholar). The glycosylation of the CTD sequence induces a turnlike structure, which could dramatically alter the conformation of the domain (42Simanek E.E. Huang D.H. Pasternack L. Machajewski T.D. Seitz O. Millar D.S. Dyson H.J. Wong C.H. J. Am. Chem. Soc. 1998; 120: 11567-11575Crossref Scopus (70) Google Scholar). Such conformational changes in the CTD could affect the specificity of interactions with other components of the transcription machinery. Phosphorylation of the CTD is associated with transcript elongation and RNA processing (43Dahmus M.E. J. Biol. Chem. 1996; 271: 19009-19012Abstract Full Text Full Text PDF PubMed Scopus (462) Google Scholar, 44Proudfoot N. Trends Biochem. Sci. 2000; 25: 290-293Abstract Full Text Full Text PDF PubMed Scopus (140) Google Scholar). Therefore, it is plausible that O-GlcNAc on the CTD may be involved in events prior to the elongation phase of the transcription cycle. There is evidence that O-GlcNAc plays a role in controlling both the turnover and transactivation activities of key transcription factors, such as SP1 and estrogen receptors (41Comer F.I. Hart G.W. Biochim. Biophys. Acta. 1999; 1473: 161-171Crossref PubMed Scopus (149) Google Scholar). Some researchers have proposed that O-GlcNAc modulates or mediates the assembly of transcriptional complexes, leading to the activation of specific genes (7Kelly W.G. Dahmus M.E. Hart G.W. J. Biol. Chem. 1993; 268: 10416-10424Abstract Full Text PDF PubMed Google Scholar, 22Hart G.W. Greis K.D. Dong L.Y.D. Blomberg M.A. Chou T.Y. Jiang M.S. Roquemore E.P. Snow D.M. Kreppel L.K. Cole R.N. Comer F.I. Arnold C.S. Hayes B.K. Adv. Exp. Med. Biol. 1995; 376: 115-123Crossref PubMed Scopus (121) Google Scholar). Although it is clear thatO-GlcNAc is enriched on proteins involved in eukaryotic gene transcription, further work is necessary to elucidate its specific roles in this complex process. Gupta, Datta, and colleagues (45Datta B. Ray M.K. Chakrabarti D. Wylie D.E. Gupta N.K. J. Biol. Chem. 1989; 264: 20620-20624Abstract Full Text PDF PubMed Google Scholar) have shown that O-GlcNAc plays a direct role in regulating protein translation. The eukaryotic initiation factor 2 (eIF-2) is a translation initiation factor that is inactivated when phosphorylated by any of several eIF-2 kinases. TheO-GlcNAc-modified form of the eIF-2- associated protein, p67, binds to eIF-2 and prevents the action of the inhibitory eIF-2 kinases. Under conditions of serum starvation or heme depletion, a normally latent deglycosylase removes the O-GlcNAc from p67, which causes p67 to dissociate from eIF-2 and also accelerates the proteolytic degradation of p67. As a result of p67 dissociation, eIF-2 becomes subject to the action of eIF-2 kinases, hence shutting down protein translation. It will require the work of many laboratories to sort out the functions of O-GlcNAc in the regulation of protein synthesis, but it is clear that it plays a critical role in this process. Many actin and tubulin regulatory proteins as well as intermediate filament proteins are O-GlcNAc- modified (22Hart G.W. Greis K.D. Dong L.Y.D. Blomberg M.A. Chou T.Y. Jiang M.S. Roquemore E.P. Snow D.M. Kreppel L.K. Cole R.N. Comer F.I. Arnold C.S. Hayes B.K. Adv. Exp. Med. Biol. 1995; 376: 115-123Crossref PubMed Scopus (121) Google Scholar, 46Dong D.L.Y. Xu Z.S. Hart G.W. Cleveland D.W. J. Biol. Chem. 1996; 271: 20845-20852Abstract Full Text Full Text PDF PubMed Scopus (99) Google Scholar). Erythrocyte Band 4.1 associated with the plasma membrane appears to contain more O-GlcNAc than the total population. Many proteins that serve to bridge the cytoskeleton to cellular membranes are O-GlcNAc-modified, such as talin, vinculin, and the synapsins (3Hart G.W. Annu. Rev. Biochem. 1997; 66: 315-335Crossref PubMed Scopus (450) Google Scholar, 47Cole R.N. Hart G.W. J. Neurochem. 1999; 73: 418-428Crossref PubMed Scopus (95) Google Scholar). O-GlcNAc site-mapping studies on neurofilaments, together with limited mutagenesis studies, suggest that the saccharide might play a role in intermediate filament fibrillogenesis. Interestingly, O-phosphorylation andO-GlcNAc modification appear to occur on distinct subsets of cytokeratins (48Chou C.-F. Omary M.B. J. Biol. Chem. 1993; 268: 4465-4472Abstract Full Text PDF PubMed Google Scholar), with both changing during the cell cycle or after treatment with phosphatase inhibitors. Thus, on some proteins theO-GlcNAc and phosphate may not merely be reciprocal modifications but rather may represent functionally distinct isoforms of these proteins (Fig. 1). O-GlcNAc is abundant in the brain, particularly on cytoskeletal proteins such as neurofilaments, microtubule-associated proteins, clathrin assembly protein, and the β-amyloid precursor protein (46Dong D.L.Y. Xu Z.S. Hart G.W. Cleveland D.W. J. Biol. Chem. 1996; 271: 20845-20852Abstract Full Text Full Text PDF PubMed Scopus (99) Google Scholar, 49Arnold C.S. Johnson G.V.W. Cole R.N. Dong D.L.Y. Lee M. Hart G.W. J. Biol. Chem. 1996; 271: 28741-28744Abstract Full Text Full Text PDF PubMed Scopus (288) Google Scholar, 50Yao P.J. Coleman P.D. J. Neurosci. 1998; 18: 2399-2411Crossref PubMed Google Scholar, 51Griffith L.S. Mathes M. Schmitz B. J. Neurosci. Res. 1995; 41: 270-278Crossref PubMed Scopus (124) Google Scholar). Disruptions of O-GlcNAc modification of some of these proteins may contribute to certain neurodegenerative disorders. For example, the OGT gene itself maps on the X chromosome to the same locus as X-linked Parkinson dystonia (6Shafi R. Iyer S.P. Ellies L.G. O'Donnell N. Marek K.W. Chui D. Hart G.W. Marth J.D. Proc. Natl. Acad. Sci. U. S. A. 2000; 97: 5735-5739Crossref PubMed Scopus (593) Google Scholar). The microtubule-associated protein, Tau, which is a major component of neurofibrillary tangles in Alzheimer's diseased brains, is multiply O-GlcNAc-modified in normal brains (49Arnold C.S. Johnson G.V.W. Cole R.N. Dong D.L.Y. Lee M. Hart G.W. J. Biol. Chem. 1996; 271: 28741-28744Abstract Full Text Full Text PDF PubMed Scopus (288) Google Scholar). In contrast, Tau is in with Alzheimer's I. K. Med. 1996; PubMed Scopus Google Scholar). The β-amyloid precursor protein, which to the Alzheimer's β-amyloid peptide is L.S. Mathes M. Schmitz B. J. Neurosci. Res. 1995; 41: 270-278Crossref PubMed Scopus (124) Google Scholar). have that some neurodegenerative might result directly from glucose by which results in a modification of key cytosolic and nuclear proteins. This to O-GlcNAc could subsequently to the phosphorylation of many proteins, but the abundant cytoskeletal proteins the As O-GlcNAc is on many transcription regulatory proteins. of many of these proteins contribute to the Some of these may their effects in by disruption regulation of these proteins. For example, the is in its transactivation domain at which is also the found in a of from human Interestingly, this glycosylation is also an important phosphorylation that transcriptional activity T.-Y. Hart G.W. Dang C.V. J. Biol. Chem. 1995; Full Text Full Text PDF PubMed Scopus Google Scholar). the many mutagenesis studies that were to elucidate the roles of on this and other proteins, in between the and O-GlcNAc at these of reciprocal There also appears to be a reciprocal relationship between the O-phosphorylation andO-GlcNAc modification of the antigen L. Grove K. Haltiwanger R.S. 1998; PubMed Scopus Google Scholar). The which is the gene in a range of human is P. J. R. R. 1996; Google Scholar). There is preliminary evidence that the O-GlcNAc on its to Many essential of cellular are subject to complex pathways J. Sci. 1996; Scopus Google Scholar). O-GlcNAc modification another level of which could allow for control of cell regulatory mechanisms. Disruptions of of these post-translational modifications may with critical control leading to the Recent evidence to a between O-GlcNAc and Several studies have that the of glucose into which is by glutamine:fructose-6-phosphate amidotransferase EC is essential for the of 2 and J. M. in Scholar). The action of to the key precursor to the for O-GlcNAc cells be without a of of to and Jr., M.C. J.S. J. 1996; PubMed Scopus Google Scholar). of cells or to concentrations of results in the of Jr., J.C. D.E. J. Biol. Chem. 1993; 268: Full Text PDF PubMed Google Scholar, 1993; PubMed Scopus Google Scholar). It is not clear at this directly to O-GlcNAc or subsequently to but a recent study showed that and increase the of O-GlcNAc in proteins in A. M.C. D. W.K. 1998; Full Text PDF PubMed Scopus Google Scholar). a structural of the of the and induces in a single of is a inhibitor of O-GlcNAcase and O-GlcNAc levels in the L. and Hart, G. W. in Scholar, J.A. Lee G. W.A. S.M. Arch. Biochem. Biophys. 1999; PubMed Scopus Google Scholar, W. K. J.A. E. A.J. Kudlow J.E. Proc. Am. 1998; Google Scholar), suggesting that its of action may disruption Another recent study found that may by the transport of from the cytoskeleton to the plasma membrane J.S. J. L. J. 1995; PubMed Scopus Google Scholar). evidence for a role of O-GlcNAc in is Nevertheless, the key role of in the the of on O-GlcNAc and the presence of O-GlcNAc on many essential regulatory proteins, it is plausible that O-GlcNAc modification is directly involved in the of that the data that glucose or to cellular UDP-GlcNAc in modification of proteins. The of O-GlcNAc on certain proteins such as could the phosphorylation of to the from the their transport to the plasma The modification of key transcription factors or involved in the to could also contribute to Current work in several laboratories is these and other for the mechanisms of have been of the O-GlcNAc modification of nuclear and cytosolic proteins for over are to the functions of this This is of the in the modification using the of the and of theO-GlcNAc and the of laboratories that have their in this of The recent of monoclonal antibodies of the of O-GlcNAcase inhibitors, as well as in K.D. Hayes B.K. Comer F.I. M. S. Hart G.W. Biochem. 1996; PubMed Scopus Google Scholar) will analysis of O-GlcNAc on proteins. It is clear that O-GlcNAc modification plays a role in many of the cellular including transcription, nuclear and cytoskeletal in the control of O-GlcNAc modification in any of these systems is likely to contribute to any of a of As in the for protein phosphorylation 1993; PubMed Scopus Google Scholar, Biochem. Soc. 1993; PubMed Scopus Google Scholar), it will require the of many investigators in of these to elucidate the of O-GlcNAc in these critical