Ataxin-10 Interacts with O-Linked β-N-Acetylglucosamine Transferase in the Brain

Abstract

Modification by O-GlcNAc involves a growing number of eucaryotic nuclear and cytosolic proteins. Glycosylation of intracellular proteins is a dynamic process that in several cases competes with and acts as a reciprocal modification system to phosphorylation. O-Linked β-N-acetylglucosamine transferase (OGT) levels are highest in the brain, and neurodegenerative disorders such as Alzheimer disease have been shown to involve abnormally phosphorylated key proteins, probably as a result of hypoglycosylation. Here, we show that the neurodegenerative disease protein ataxin-10 (Atx-10) is associated with cytoplasmic OGT p110 in the brain. In PC12 cells and pancreas, this association is competed by the shorter OGT p78 splice form, which is down-regulated in brain. Overexpression of Atx-10 in PC12 cells resulted in the reconstitution of the Atx-10-OGT p110 complex and enhanced intracellular glycosylation activity. Moreover, in an in vitro enzyme assay using PC12 cell extracts, Atx-10 increased OGT activity 2-fold. These data indicate that Atx-10 might be essential for the maintenance of a critical intracellular glycosylation level and homeostasis in the brain. Modification by O-GlcNAc involves a growing number of eucaryotic nuclear and cytosolic proteins. Glycosylation of intracellular proteins is a dynamic process that in several cases competes with and acts as a reciprocal modification system to phosphorylation. O-Linked β-N-acetylglucosamine transferase (OGT) levels are highest in the brain, and neurodegenerative disorders such as Alzheimer disease have been shown to involve abnormally phosphorylated key proteins, probably as a result of hypoglycosylation. Here, we show that the neurodegenerative disease protein ataxin-10 (Atx-10) is associated with cytoplasmic OGT p110 in the brain. In PC12 cells and pancreas, this association is competed by the shorter OGT p78 splice form, which is down-regulated in brain. Overexpression of Atx-10 in PC12 cells resulted in the reconstitution of the Atx-10-OGT p110 complex and enhanced intracellular glycosylation activity. Moreover, in an in vitro enzyme assay using PC12 cell extracts, Atx-10 increased OGT activity 2-fold. These data indicate that Atx-10 might be essential for the maintenance of a critical intracellular glycosylation level and homeostasis in the brain. Since its discovery (1.Torres C.R. Hart G.W. J. Biol. Chem. 1984; 259: 3308-3317Abstract Full Text PDF PubMed Google Scholar), the modification of intracellular proteins by a single GlcNAc moiety has emerged as a major signaling event involving a growing number of proteins (2.Vosseller K. Wells L. Hart G.W. Biochimie (Paris). 2001; 83: 575-581Crossref PubMed Scopus (73) Google Scholar, 3.Zachara N.E. Cheung W.D. Hart G.W. Methods Mol. Biol. 2004; 284: 175-194PubMed Google Scholar). Changes in O-GlcNAc glycosylation levels were shown to have a regulatory effect on diverse cellular processes such as proteasome activity (4.Zhang F. Su K. Yang X. Bowe D.B. Paterson A.J. Kudlow J.E. Cell. 2003; 115: 715-725Abstract Full Text Full Text PDF PubMed Scopus (352) Google Scholar), transcription (5.Gao Y. Miyazaki J. Hart G.W. Arch. Biochem. Biophys. 2003; 415: 155-163Crossref PubMed Scopus (136) Google Scholar), and enzyme function (6.Parker G.J. Lund K.C. Taylor R.P. McClain D.A. J. Biol. Chem. 2003; 278: 10022-10027Abstract Full Text Full Text PDF PubMed Scopus (145) Google Scholar). The enzyme OGT 2The abbreviations used are: OGT, O-linked β-N-acetylglucosamine transferase; Atx-10, ataxin-10; TPR, tetratricopeptide repeats; GST, glutathione S-transferase; MALDI-TOF, matrix-assisted laser desorption/ionization-time of flight; NTA, nickel-nitrilotriacetic acid; CKII, casein kinase. is encoded by a single gene, which is highly conserved among metazoans (7.Lubas W.A. Frank D.W. Krause M. Hanover J.A. J. Biol. Chem. 1997; 272: 9316-9324Abstract Full Text Full Text PDF PubMed Scopus (421) Google Scholar). Several splice forms have been described that vary in the length of the protein N terminus and subcellular localization (8.Hanover J.A. Yu S. Lubas W.B. Shin S.H. Ragano-Caracciola M. Kochran J. Love D.C. Arch. Biochem. Biophys. 2003; 409: 287-297Crossref PubMed Scopus (181) Google Scholar). Full-length OGT is a 110-kDa polypeptide consisting of two domains. The N-terminal half contains multiple tetratricopeptide repeats (TPR) that adopt a bent superhelical fold related to the armadillo repeat motif (9.Jinek M. Rehwinkel J. Lazarus B.D. Izaurralde E. Hanover J.A. Conti E. Nat. Struct. Mol. Biol. 2004; 11: 1001-1007Crossref PubMed Scopus (228) Google Scholar). The C-terminal portion shows glycosyltransferase activity, whereas the TPR domain is responsible for substrate binding and OGT oligomerization (9.Jinek M. Rehwinkel J. Lazarus B.D. Izaurralde E. Hanover J.A. Conti E. Nat. Struct. Mol. Biol. 2004; 11: 1001-1007Crossref PubMed Scopus (228) Google Scholar, 10.Iyer S.P. Hart G.W. J. Biol. Chem. 2003; 278: 24608-24616Abstract Full Text Full Text PDF PubMed Scopus (148) Google Scholar, 11.Kreppel L.K. Hart G.W. J. Biol. Chem. 1999; 274: 32015-32022Abstract Full Text Full Text PDF PubMed Scopus (338) Google Scholar). Originally, the enzyme was isolated from rat liver cytosol as an apparent heterotrimer consisting of two p110 subunits and one p78 subunit (12.Haltiwanger R.S. Blomberg M.A. Hart G.W. J. Biol. Chem. 1992; 267: 9005-9013Abstract Full Text PDF PubMed Google Scholar). The p110 subunit, which exerts full catalytic activity by itself, is ubiquitously expressed, whereas the p78 splice variant that lacks most of the TPR repeats appears to be restricted to certain tissues such as liver, kidney, and muscle (13.Kreppel L.K. Blomberg M.A. Hart G.W. J. Biol. Chem. 1997; 272: 9308-9315Abstract Full Text Full Text PDF PubMed Scopus (612) Google Scholar). Dysregulated OGT activity leading to hyper- or hypoglycosylation of target proteins is believed to be involved in the pathogenesis of disorders such as type II diabetes (6.Parker G.J. Lund K.C. Taylor R.P. McClain D.A. J. Biol. Chem. 2003; 278: 10022-10027Abstract Full Text Full Text PDF PubMed Scopus (145) Google Scholar, 14.Vosseller K. Wells L. Lane M.D. Hart G.W. Proc. Natl. Acad. Sci. U. S. A. 2002; 99: 5313-5318Crossref PubMed Scopus (393) Google Scholar) and Alzheimer disease (15.Robertson L.A. Moya K.L. Breen K.C. J. Alzheimer's Dis. 2004; 6: 489-495Crossref PubMed Scopus (77) Google Scholar). Protein glycosylation with GlcNAc may directly compete for serine and threonine residues with phosphorylation, thus creating a sensitive balance between positive and negative regulatory signals (16.Iyer S.P. Hart G.W. Biochemistry. 2003; 42: 2493-2499Crossref PubMed Scopus (70) Google Scholar). Perturbations of this reciprocal relationship might lead to cell degeneration as in the case of hyperphosphorylated tau (17.O'Donnell N. Zachara N.E. Hart G.W. Marth J.D. Mol. Cell Biol. 2004; 24: 1680-1690Crossref PubMed Scopus (343) Google Scholar, 18.Liu F. Iqbal K. Grundke-Iqbal I. Hart G.W. Gong C.X. Proc. Natl. Acad. Sci. U. S. A. 2004; 101: 10804-10809Crossref PubMed Scopus (594) Google Scholar). It has become evident from several observations that O-GlcNAc glycosylation is exceptionally important in neuronal tissues (17.O'Donnell N. Zachara N.E. Hart G.W. Marth J.D. Mol. Cell Biol. 2004; 24: 1680-1690Crossref PubMed Scopus (343) Google Scholar, 19.Khidekel N. Ficarro S.B. Peters E.C. Hsieh-Wilson L.C. Proc. Natl. Acad. Sci. U. S. A. 2004; 101: 13132-13137Crossref PubMed Scopus (254) Google Scholar). OGT expression is significantly up-regulated in the brain with the most prominent activity being localized in the cerebellar cortex (20.Akimoto Y. Comer F.I. Cole R.N. Kudo A. Kawakami H. Hirano H. Hart G.W. Brain Res. 2003; 966: 194-205Crossref PubMed Scopus (71) Google Scholar). Interestingly, cerebellar neurons have been shown to react directly to changes of protein kinase activity with reciprocal changes in OGT activity (21.Griffith L.S. Schmitz B. Eur. J. Biochem. 1999; 262: 824-831Crossref PubMed Scopus (96) Google Scholar). Here, we demonstrate binding of OGT p110 to the neurode-generative disease protein Atx-10 in the brain. Although ubiquitously expressed, both proteins show highest expression levels in the central nervous system, indicating a crucial regulatory effect of Atx-10 on intracellular glycosylation in the brain. Isolation of OGT-interacting Proteins Using the Cytotrap Screen—Atx10 was isolated as an OGT-interacting protein using the CytoTrap™ system according to the manufacturer's protocols (Stratagene) (22.Aronheim A. Zandi E. Hennemann H. Elledge S.J. Karin M. Mol. Cell Biol. 1997; 17: 3094-3102Crossref PubMed Scopus (391) Google Scholar, 23.Andrali S.S. Marz P. Ozcan S. Biochem. Biophys. Res. Commun. 2005; 337: 149-153Crossref PubMed Scopus (17) Google Scholar). For this purpose, the mitochondrial version of human OGT (7.Lubas W.A. Frank D.W. Krause M. Hanover J.A. J. Biol. Chem. 1997; 272: 9316-9324Abstract Full Text Full Text PDF PubMed Scopus (421) Google Scholar) was subcloned into the pSos vector (Stratagene) in-frame with human Sos protein. The screen was carried out with a mouse insulinoma (MIN6) cDNA library in the pMyr vector (Stratagene) (23.Andrali S.S. Marz P. Ozcan S. Biochem. Biophys. Res. Commun. 2005; 337: 149-153Crossref PubMed Scopus (17) Google Scholar, 24.Miyazaki J. Araki K. Yamato E. Ikegami H. Asano T. Shibasaki Y. Oka Y. Yamamura K. Endocrinology. 1990; 127: 126-132Crossref PubMed Scopus (1060) Google Scholar). After screening of about 1 × 106 colonies, Atx10 was isolated as an OGT-interacting protein in three independent experiments. Cell Culture and Transfection—Rat pheochromocytoma PC12 cells were obtained from ATCC (ATCC number: CRL-1721) and maintained as monolayer cultures in Dulbecco's modified Eagle's medium at 5% CO2 in a humidified atmosphere at 37 °C. The medium was changed every 3 days, and cells were subcultured once a week. Transient transfection of PC12 cells with a pCDM8-Atx-10 expression plasmid was performed according to the manufacturer's instructions using Lipofectamine 2000 (Invitrogen). On day 1 after transfection, increasing concentrations of UDP-GlcNAc substrate were added to the cells for 2 h. Preparation and Gel Filtration Analysis of PC12 Cell and Rat Tissue Lysates—3 × 107 PC12 cells were resuspended in 1 ml of phosphate-buffered saline containing protease inhibitor mix (Roche Applied Science). Cells were lysed on ice by passing several times through a 0.6-gauge syringe needle. Cell debris was removed by centrifugation, and the cytosolic fraction was passed through a 0.45-μm filter before being submitted to gel filtration. Gel filtration was performed using a prepacked Superdex S75 column (Amersham Biosciences) with were and by rat from were in and cells were using a The cell was in 1 ml of and by at for were submitted to gel filtration the as used for the PC12 cell For cell were with Atx-10 or protein for 1 at before gel filtration. The apparent were using a of proteins (Amersham Biosciences) that were the column in an independent of Atx-10, a a C-terminal of the protein was used P. A. S. M. S. U. S. J. Biol. Chem. 2004; Full Text Full Text PDF PubMed Scopus Google Scholar). OGT proteins were using a a the C-terminal residues to enzyme splice Proteins modified by O-GlcNAc were using a to was used for the were at For of were at of was performed using the of rat Atx-10 was in cells as described P. A. S. M. S. U. S. J. Biol. Chem. 2004; Full Text Full Text PDF PubMed Scopus Google Scholar). of cells were the cells and passed through a syringe medium was using according to the manufacturer's instructions and protein were in using a Cells were and in medium at 37 and expression was at by for h. Cells were and resuspended in and lysed by several and protein was using according to the manufacturer's instructions (Amersham The was by using as and as and with an N-terminal in E. cells using a vector After containing protein were in and using The protein with a of was to the by the to The protein was in were were with and with in The were in were with a and in were and with in phosphate-buffered saline for binding was with for 2 and were with Atx-10 or or OGT for at and the complex were for was with and in were on in with and in For the was from the were with an with an system of PC12 cells and rat brain were as for the gel filtration experiments. After protein were for at with an of Atx-10 protein in a of Atx-10 was from the using The were three times with phosphate-buffered saline and resuspended in OGT protein was by using OGT Atx-10 was with a protein to Atx-10 was with laser was performed in the positive with and using a 2The abbreviations used are: OGT, O-linked β-N-acetylglucosamine transferase; Atx-10, ataxin-10; TPR, tetratricopeptide repeats; GST, glutathione S-transferase; MALDI-TOF, matrix-assisted laser desorption/ionization-time of flight; NTA, nickel-nitrilotriacetic acid; CKII, casein kinase. and protein were from in was used as a on the were once with of the was to the at and the was with a of for after of the 1 of was added to the and was to at The proteins 1 of or were the and an 1 of was added to the protein protein was and a of the was to the OGT OGT assay was performed as described by S. T. T. M.A. J. S. Biochem. 2003; PubMed Scopus Google Scholar). of an OGT fraction obtained by gel filtration of PC12 cell was with of casein kinase and the was with of in a of For the of Atx-10, concentrations of Atx-10 were added before the The were at and the was by was from by The were once with resuspended in and in a OGT p110 with Atx-10 in the Brain in PC12 the screen assay with the mitochondrial splice of human OGT as we have isolated the Atx-10, as an of OGT from a mouse insulinoma (MIN6) cDNA In rat brain Atx-10 with the OGT p110 subunit gel filtration with the in a of about OGT and Atx-10 signals in this indicating that Atx-10 binding to OGT p110 is in the brain. Atx-10 be by a protein in in cytosol of PC12 Atx-10 was associated with OGT and in a which to its in P. A. S. M. S. U. S. J. Biol. Chem. 2004; Full Text Full Text PDF PubMed Scopus Google Scholar). OGT p110 with the OGT p78 subunit, which was in brain cell and binding of Atx-10 to OGT p110 in PC12 of OGT p110 from cell using Atx-10 binding activity for brain for PC12 cell the association between the two OGT as with the OGT of Atx-10 and OGT in of Atx-10 and OGT in neuronal tissues was by using mouse brain described for mouse and human brain P. A. S. M. S. U. S. J. Biol. Chem. 2004; Full Text Full Text PDF PubMed Scopus Google Scholar), Atx-10 protein is in the cerebellar system, the cerebellar cortex and the cerebellar the and the OGT with that of Atx-10 in the in the and in the with a In the cerebellar levels of Atx-10 were the of cells and OGT protein in the of the In of the for OGT for Atx-10, whereas cells in the cell were positive for both in the cerebellar and and in cerebellar as for the and for Atx-10 or OGT in a intracellular was in that the cytosolic of Atx-10 was by a of of OGT and Atx-10 in Rat Atx-10, OGT and OGT p78 were in tissues as with brain, we for the protein levels in multiple rat OGT p110 and Atx-10 were up-regulated in the brain and expression levels of OGT p78 were restricted to liver, most to OGT p78 be in by indicating that its expression is down-regulated levels of Atx-10 protein are highest in brain and and the of the as described P. A. S. M. S. U. S. J. Biol. Chem. 2004; Full Text Full Text PDF PubMed Scopus Google Scholar). OGT splice forms p110 or p78 be in this probably to and localization to subcellular Gel filtration of cytosol a result as that of PC12 cell that the of OGT p78 OGT association Interestingly, in pancreas, the cytoplasmic of OGT p78 that of OGT OGT p78 and Atx-10 for OGT p110 OGT p78 directly competes with Atx-10 for OGT p110 we PC12 cell cytosol with an of Atx-10 protein to gel filtration. shown in Atx-10 protein at concentrations OGT association by OGT p78 and binding to OGT the cytosolic of OGT p78 that of Atx-10 in PC12 we that OGT p110 to OGT p78 with a significantly to (9.Jinek M. Rehwinkel J. Lazarus B.D. Izaurralde E. Hanover J.A. Conti E. Nat. Struct. Mol. Biol. 2004; 11: 1001-1007Crossref PubMed Scopus (228) Google Scholar) have that the TPR repeats and function as the of Atx-10 and the OGT TPR domain a we that Atx-10 might the with have the in E. to was to compete the Atx-10-OGT p110 of the two major one at an of and the at an of The of is that for the protein (12.Haltiwanger R.S. Blomberg M.A. Hart G.W. J. Biol. Chem. 1992; 267: 9005-9013Abstract Full Text PDF PubMed Google Scholar, P. A. S. M. S. U. S. J. Biol. Chem. 2004; Full Text Full Text PDF PubMed Scopus Google Scholar), which is of laser was the to was the by the laser the These observations that the protein forms in as by the data (9.Jinek M. Rehwinkel J. Lazarus B.D. Izaurralde E. Hanover J.A. Conti E. Nat. Struct. Mol. Biol. 2004; 11: 1001-1007Crossref PubMed Scopus (228) Google Scholar). of rat brain with an of protein to a of the Atx-10 and OGT p110 signals gel indicating that Atx-10 the domain of OGT p110 for binding The protein was in containing both OGT p110 and Atx-10, indicating that associated with both proteins. The performed with PC12 cell to the of a OGT in a with to a of the complex result that the compete with OGT p78 for binding and that with OGT p110 in to OGT probably by of OGT the OGT is competed by might be that the of Atx-10 and OGT p78 for OGT p110 binding is to The protein was in this the OGT and Atx-10 with being in the of the column of of Atx-10 on binding of OGT p78 or Atx-10 the activity of OGT the expression levels of OGT p110 and Atx-10 are up-regulated in the brain, we that the OGT complex to glycosyltransferase activity, whereas of this by OGT p78 might a of intracellular this we PC12 cells with a cytoplasmic expression vector for Atx-10 and intracellular glycosylation in the of UDP-GlcNAc Cells were in medium before the as we that concentrations expression of OGT Atx-10 to a of several O-linked proteins in cells with the activity at UDP-GlcNAc concentrations of about and activity at substrate levels proteins in the in to In a PC12 cells to the of UDP-GlcNAc with increased levels of cellular proteins, indicating that the glycosyltransferase activity was at cellular Atx-10 levels and the in the cell medium The of OGT activity by Atx-10 be using as target and as substrate S. T. T. M.A. J. S. Biochem. 2003; PubMed Scopus Google Scholar). a of OGT activity obtained from PC12 cell that after as is in the of 1 Atx-10 in its Interestingly, the of levels at that which a balance of and most to the activity of the Atx-10 of this we PC12 with of Atx-10 for as in The data a with a of about which a binding of Atx-10 to OGT OGT activity from brain which OGT show by Atx-10 and is with the of OGT activity in PC12 cells obtained of Atx-10 modification with O-GlcNAc is an and dynamic process involving a growing number of target proteins N. Ficarro S.B. Peters E.C. Hsieh-Wilson L.C. Proc. Natl. Acad. Sci. U. S. A. 2004; 101: 13132-13137Crossref PubMed Scopus (254) Google Scholar). and O-linked glycosylation in the and intracellular glycosylation is and may directly the of a protein by with for the serine and threonine N.E. Hart G.W. Biophys. 2004; PubMed Scopus Google Scholar). is by the activity of a single OGT, which shows a substrate (9.Jinek M. Rehwinkel J. Lazarus B.D. Izaurralde E. Hanover J.A. Conti E. Nat. Struct. Mol. Biol. 2004; 11: 1001-1007Crossref PubMed Scopus (228) Google Scholar) and thus and signals to diverse cellular OGT p110 has been shown to the transferase activity in the OGT p78 is the OGT variant in the cytoplasmic and is by being the OGT splice with two TPR repeats (8.Hanover J.A. Yu S. Lubas W.B. Shin S.H. Ragano-Caracciola M. Kochran J. Love D.C. Arch. Biochem. Biophys. 2003; 409: 287-297Crossref PubMed Scopus (181) Google Scholar). The with a of appears to be restricted to D.C. J. Shin S.H. Hanover J.A. Kochran J. J. Cell Sci. 2003; PubMed Scopus Google Scholar). gel filtration OGT was as a p110 or heterotrimer complex (12.Haltiwanger R.S. Blomberg M.A. Hart G.W. J. Biol. Chem. 1992; 267: 9005-9013Abstract Full Text PDF PubMed Google Scholar). (9.Jinek M. Rehwinkel J. Lazarus B.D. Izaurralde E. Hanover J.A. Conti E. Nat. Struct. Mol. Biol. 2004; 11: 1001-1007Crossref PubMed Scopus (228) Google Scholar) have the of a OGT TPR domain with a central TPR repeats and that the domain is to in In (9.Jinek M. Rehwinkel J. Lazarus B.D. Izaurralde E. Hanover J.A. Conti E. Nat. Struct. Mol. Biol. 2004; 11: 1001-1007Crossref PubMed Scopus (228) Google Scholar) a of the OGT substrate binding domain with armadillo repeat proteins such as In the we demonstrate a association of OGT p110 with the Atx-10, in brain The shows an in from type T. T. A. R.P. K. M. L. M. E. T. Nat. PubMed Scopus Google Scholar). we have Atx-10 as a armadillo repeat protein that is in neuronal as as brain P. A. S. M. S. U. S. J. Biol. Chem. 2004; Full Text Full Text PDF PubMed Scopus Google Scholar). of expression by resulted in increased of rat cerebellar indicating that Atx-10 is essential for cell in this P. A. S. M. S. U. S. J. Biol. Chem. 2004; Full Text Full Text PDF PubMed Scopus Google Scholar). OGT expression a being highest in the brain and in cells Y. Kawakami H. K. E. T. Hirano H. Sci. 2003; PubMed Scopus Google Scholar). data that Atx-10 and OGT in the and associated to a major the OGT was as with the Atx-10 which might be to OGT that are associated with Y. Kawakami H. K. E. T. Hirano H. Sci. 2003; PubMed Scopus Google Scholar) that expression levels of OGT to levels of proteins in the neuronal indicating that modification with GlcNAc is critical for cellular in this OGT p110 was the cytosolic splice of the enzyme we in the brain, whereas in tissues that are involved in and such as liver and pancreas, OGT p78 was (13.Kreppel L.K. Blomberg M.A. Hart G.W. J. Biol. Chem. 1997; 272: 9308-9315Abstract Full Text Full Text PDF PubMed Scopus (612) Google Scholar) have shown that OGT p78 is in to a in muscle gel filtration that the of OGT p78 in the cell cytosol to a of the OGT PC12 cells as a system for tissues with OGT p78 levels in which Atx-10 is in of OGT activity. is the of OGT p78 expression on the function of the OGT have been shown to in N of TPR repeats (8.Hanover J.A. Yu S. Lubas W.B. Shin S.H. Ragano-Caracciola M. Kochran J. Love D.C. Arch. Biochem. Biophys. 2003; 409: 287-297Crossref PubMed Scopus (181) Google Scholar). The TPR domain forms a substrate binding in the of the whereas the at the (9.Jinek M. Rehwinkel J. Lazarus B.D. Izaurralde E. Hanover J.A. Conti E. Nat. Struct. Mol. Biol. 2004; 11: 1001-1007Crossref PubMed Scopus (228) Google Scholar). Several L.K. Hart G.W. J. Biol. Chem. 1999; 274: 32015-32022Abstract Full Text Full Text PDF PubMed Scopus (338) Google Scholar, W.A. Hanover J.A. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar) have shown that the of a number of TPR repeats glycosylation that the domain of OGT being of for UDP-GlcNAc binding by the catalytic for substrate In a Lazarus B.D. Love D.C. Hanover J.A. PubMed Scopus Google Scholar) have shown that OGT p78 show glycosylation activity. that OGT p78 expression might to a negative regulatory probably in to intracellular levels in the is by the that neuronal which show the highest levels of OGT activity, OGT p78 whereas pancreas, a regulatory for the most OGT p78 expression data that reconstitution of the OGT complex in a cell type with OGT p78 expression the to to levels of On the Atx-10 a of OGT activity at Interestingly, and Hart L.K. Hart G.W. J. Biol. Chem. 1999; 274: 32015-32022Abstract Full Text Full Text PDF PubMed Scopus (338) Google Scholar) have that subunit the transferase activity of OGT that Atx-10 and the OGT p78 splice are for OGT p110 Atx-10 binding to OGT p110 appears to be in tissues OGT p78 is whereas OGT p78 binding to OGT p110 is in tissues in the of and a of OGT p110 and Atx-10 by of OGT p110 Atx-10 shows a motif as the OGT TPR may be that Atx-10 as an which is to and the substrate of the intracellular glycosylation from a an for the of cerebellar cells in of expression by significantly intracellular glycosylation levels in cerebellar neurons indicating that Atx-10 has cellular is by the that Atx-10 levels are down-regulated in tissues in which Atx-10 and OGT p110 that the of the OGT p78 expression is a key intracellular glycosylation activity. The for be to the for the OGT and to the OGT p78 expression is on the for in the and for critical of the