O-Linked GlcNAc transferase is a conserved nucleocytoplasmic protein containing tetratricopeptide repeats

Uridine diphosphate release mechanism in O-N-acetylglucosamine (O-GlcNAc) transferase catalysis

O-linked N-acetylglucosamine transferase (OGT) is an essential enzyme that catalyzes the covalent bonding of N-acetylglucosamine (GlcNAc) to the hydroxyl group of a serine or threonine in the target protein. It plays an important role in many important cellular physiological catalytic reactions. Here, we determine the binding mode and the binding free energy of the OGT product (uridine diphosphate, UDP) as well as the hydrogen-bond-dependent release mechanism using extensive molecular dynamic simulations. The Lys634, Asn838, Gln839, Lys842, His901, and Asp925 residues were identified to play a major role in the UDP stabilization in the active site of OGT, where hydrogen bonding and π-π interactions mainly occur. The calculations on the mutant forms support our results. Sixteen possible release channels were identified while the two most favorable channels were determined using random acceleration molecular dynamics (RAMD) simulations combined with the constant velocity pulling (PCV) method. The thermodynamic and dynamic properties as along with the corresponding mechanism were determined and discussed according to the umbrella sampling technique. For the most optimal channel, the main free energy barrier is 13 kcal/mol, which probably originates from the hydrogen bonds between UDP and the Ala896 and Asp925 residues. Moreover, the unstable hydrogen bonds and the rollback of the ligand likely cause the other two small obstacles. This work clarifies the ligand transport mechanism in the OGT enzymatic process and is a great resource for designing inhibitors based on UDP or UDP-GlcNAc.

Critical observations that shaped our understanding of the function(s) of intracellular glycosylation (O-GlcNAc)

Almost 100 years after the first descriptions of proteins conjugated to carbohydrates (mucins), several studies suggested that glycoproteins were not restricted to the serum, extracellular matrix, cell surface, or endomembrane system. In the 1980s, key data emerged demonstrating that intracellular proteins were modified by monosaccharides of O-linked β-N-acetylglucosamine (O-GlcNAc). Subsequently, this modification was identified on thousands of proteins that regulate cellular processes as diverse as protein aggregation, localization, post-translational modifications, activity, and interactions. In this Review, we will highlight critical discoveries that shaped our understanding of the molecular events underpinning the impact of O-GlcNAc on protein function, the role that O-GlcNAc plays in maintaining cellular homeostasis, and our understanding of the mechanisms that regulate O-GlcNAc-cycling.

O-GlcNAc Modification During Pregnancy: Focus on Placental Environment

Successful placentation is a key event for fetal development, which commences following embryo implantation into the uterine wall, eliciting decidualization, placentation, and remodeling of blood vessels to provide physiological exchange between embryo-fetus and mother. Several signaling pathways are recruited to modulate such important processes and specific proteins that regulate placental function are a target for the glycosylation with O-linked β-N-acetylglucosamine (O-GlcNAc), or O-GlcNAcylation. This is a reversible post-translational modification on nuclear and cytoplasmic proteins, mainly controlled by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). O-GlcNAcylation has been implicated as a modulator of proteins, both in physiological and pathological conditions and, more recently, O-GlcNAc has also been shown to be an important modulator in placental tissue. In this mini-review, the interplay between O-GlcNAcylation of proteins and placental function will be addressed, discussing the possible implications of this post-translational modification through placental development and pregnancy.

Quantitative Profiling of Protein O-GlcNAcylation Sites by an Isotope-Tagged Cleavable Linker

Large-scale quantification of protein O-linked β- N-acetylglucosamine (O-GlcNAc) modification in a site-specific manner remains a key challenge in studying O-GlcNAc biology. Herein, we developed an isotope-tagged cleavable linker (isoTCL) strategy, which enabled isotopic labeling of O-GlcNAc through bioorthogonal conjugation of affinity tags. We demonstrated the application of the isoTCL in mapping and quantification of O-GlcNAcylation sites in HeLa cells. Furthermore, we investigated the O-GlcNAcylation sensitivity to the sugar donor by quantifying the levels of modification under different concentrations of the O-GlcNAc labeling probe in a site-specific manner. In addition, we applied isoTCL to compare the O-GlcNAcylation stoichiometry levels of more than 100 modification sites between placenta samples from male and female mice and confirmed site-specifically that female placenta has a higher O-GlcNAcylation than its male counterpart. The isoTCL platform provides a powerful tool for quantitative profiling of O-GlcNAc modification.

Pharmacological Inhibition of O-GlcNAcase Enhances Autophagy in Brain through an mTOR-Independent Pathway

The glycosylation of nucleocytoplasmic proteins with O-linked N-acetylglucosamine residues (O-GlcNAc) is conserved among metazoans and is particularly abundant within brain. O-GlcNAc is involved in diverse cellular processes ranging from the regulation of gene expression to stress response. Moreover, O-GlcNAc is implicated in various diseases including cancers, diabetes, cardiac dysfunction, and neurodegenerative diseases. Pharmacological inhibition of O-GlcNAcase (OGA), the sole enzyme that removes O-GlcNAc, reproducibly slows neurodegeneration in various Alzheimer's disease (AD) mouse models manifesting either tau or amyloid pathology. These data have stimulated interest in the possibility of using OGA-selective inhibitors as pharmaceuticals to alter the progression of AD. The mechanisms mediating the neuroprotective effects of OGA inhibitors, however, remain poorly understood. Here we show, using a range of methods in neuroblastoma N2a cells, in primary rat neurons, and in mouse brain, that selective OGA inhibitors stimulate autophagy through an mTOR-independent pathway without obvious toxicity. Additionally, OGA inhibition significantly decreased the levels of toxic protein species associated with AD pathogenesis in the JNPL3 tauopathy mouse model as well as the 3×Tg-AD mouse model. These results strongly suggest that OGA inhibitors act within brain through a mechanism involving enhancement of autophagy, which aids the brain in combatting the accumulation of toxic protein species. Our study supports OGA inhibition being a feasible therapeutic strategy for hindering the progression of AD and other neurodegenerative diseases. Moreover, these data suggest more targeted strategies to stimulate autophagy in an mTOR-independent manner may be found within the O-GlcNAc pathway. These findings should aid the advancement of OGA inhibitors within the clinic.

O-GlcNAc cycling in the developing, adult and geriatric brain

Hundreds of proteins in the nervous system are modified by the monosaccharide O-GlcNAc. A single protein is often O-GlcNAcylated on several amino acids and the modification of a single site can play a crucial role for the function of the protein. Despite its complexity, only two enzymes add and remove O-GlcNAc from proteins, O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). Global and local regulation of these enzymes make it possible for O-GlcNAc to coordinate multiple cellular functions at the same time as regulating specific pathways independently from each other. If O-GlcNAcylation is disrupted, metabolic disorder or intellectual disability may ensue, depending on what neurons are affected. O-GlcNAc's promise as a clinical target for developing drugs against neurodegenerative diseases has been recognized for many years. Recent literature puts O-GlcNAc in the forefront among mechanisms that can help us better understand how neuronal circuits integrate diverse incoming stimuli such as fluctuations in nutrient supply, metabolic hormones, neuronal activity and cellular stress. Here the functions of O-GlcNAc in the nervous system are reviewed.

A Conserved Splicing Silencer Dynamically Regulates O-GlcNAc Transferase Intron Retention and O-GlcNAc Homeostasis

Modification of nucleocytoplasmic proteins with O-GlcNAc regulates a wide variety of cellular processes and has been linked to human diseases. The enzymes O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) add and remove O-GlcNAc, but the mechanisms regulating their expression remain unclear. Here, we demonstrate that retention of the fourth intron of OGT is regulated in response to O-GlcNAc levels. We further define a conserved intronic splicing silencer (ISS) that is necessary for OGT intron retention. Deletion of the ISS in colon cancer cells leads to increases in OGT, but O-GlcNAc homeostasis is maintained by concomitant increases in OGA protein. However, the ISS-deleted cells are hypersensitive to OGA inhibition in culture and in soft agar. Moreover, growth of xenograft tumors from ISS-deleted cells is compromised in mice treated with an OGA inhibitor. Thus, ISS-mediated regulation of OGT intron retention is a key component in OGT expression and maintaining O-GlcNAc homeostasis.

The O-β-linked N-acetylglucosaminylation of the Lamin B receptor and its impact on DNA binding and phosphorylation

Lamin B Receptor (LBR) is an integral protein of the interphase inner nuclear membrane that is implicated in chromatin anchorage to the nuclear envelope. Phosphorylation of a stretch of arginine-serine (RS) dipeptides in the amino-terminal nucleoplasmic domain of LBR regulates the interactions of the receptor with other nuclear proteins, DNA and RNA and thus modulates tethering of heterochromatin to the nuclear envelope. While phosphorylation has been extensively studied, very little is known about other post-translational modifications of the protein. There is only one report on the O-β-linked N-acetyl-glucosaminylation (O-GlcNAcylation) of a serine residue downstream of the RS domain of rat LBR. In the present study we identify additional O-GlcNAcylation sites by using as substrates of O-β-N-acetylglucosaminyltransferase (OGT) a set of peptides containing the entire LBR RS domain or parts of it as well as flanking sequences. The in vitro activity of OGT was assessed by tandem mass spectrometry and NMR spectroscopy. Furthermore, we provide evidence that O-GlcNAcylation hampers DNA binding while it marginally affects RS domain phosphorylation mediated by SRPK1, Akt2 and cdk1 kinases.

GENERAL SIGNIFICANCE

Our methodology providing a quantitative description of O-GlcNAc patterns based on a combination of mass spectrometry and high resolution NMR spectroscopy on short peptide substrates allows subsequent functional analyses. Hence, our approach is of general interest to a wide audience of biologists aiming at deciphering the functional role of O-GlcNAc glycosylation and its crosstalk with phosphorylation.

The O-GlcNAc Transferase Intellectual Disability Mutation L254F Distorts the TPR Helix

O-linked β-N-acetyl-_D-glucosamine (O-GlcNAc) transferase (OGT) regulates protein O-GlcNAcylation, an essential post-translational modification that is abundant in the brain. Recently, OGT mutations have been associated with intellectual disability, although it is not understood how they affect OGT structure and function. Using a multi-disciplinary approach we show that the L254F OGT mutation leads to conformational changes of the tetratricopeptide repeats and reduced activity, revealing the molecular mechanisms contributing to pathogenesis.

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The intellectual disability L254F mutation in OGT affects activity

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The L254F mutation leads to shifts up to 12 Å in the OGT structure

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Thermal denaturing studies reveal reduction in TPR stability caused by L254F

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Simulations suggest the presence of alternate TPR_L254F conformations

Histone methylation regulates Hif-1 signaling cascade in activation of hepatic stellate cells

Liver fibrosis is characterized by deposition of excessive extracellular matrix (ECM). The major source of ECM is activated hepatic stellate cells (HSCs). Previously, we reported that hypoxia-inducible factor-1 (Hif-1) regulates activation of HSCs through autophagy. In current work, human HSC cell line LX-2 was used as cell model. It was determined that trimethylation of H3 histone on lysine 4 (H3K4me3) occurred in the Hif-1 transcriptional complex. Inhibition of modifications of histone methylation suppressed Hif-1 nuclear transport, autophagosome formation, and activation of LX-2 cells. These data suggest that histone methylation modification plays an important role in the Hif-1 signaling cascade regulating HSC activation.

O-GlcNAc as an Integrator of Signaling Pathways

O-GlcNAcylation is an important posttranslational modification governed by a single pair of enzymes-O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). These two enzymes mediate the dynamic cycling of O-GlcNAcylation on a wide variety of cytosolic, nuclear and mitochondrial proteins in a nutrient- and stress-responsive fashion. While cellular functions of O-GlcNAcylation have been emerging, little is known regarding the precise mechanisms how the enzyme pair senses the environmental cues to elicit molecular and physiological changes. In this review, we discuss how the OGT/OGA pair acts as a metabolic sensor that integrates signaling pathways, given their capability of receiving signaling inputs from various partners, targeting multiple substrates with spatiotemporal specificity and translocating to different parts of the cell. We also discuss how the pair maintains homeostatic signaling within the cell and its physiological relevance. A better understanding of the mechanisms of OGT/OGA action would enable us to derive therapeutic benefits of resetting cellular O-GlcNAc levels within an optimal range.

Detection of O-Linked-N-Acetylglucosamine Modification and Its Associated Enzymes in Human Degenerated Intervertebral Discs

Study Design

Human herniated discs were obtained from discectomy specimens for the immunohistochemical detection of O-GlcNAc and O-GlcNAcase (OGA)/O-GlcNAc transferase (OGT).

Purpose

This study aimed to quantify the extent of O-GlcNAcylation and its associated enzymes (OGT/OGA) in human degenerated intervertebral discs.

Overview of Literature

The O-GlcNAcylation of nuclear, cytoplasmic, and mitochondrial proteins as well as the effects of such post-translational modifications are currently the focus of extensive research. O-GlcNAcylation is believed to contribute to the etiology of chronic illnesses by acting as a nutrient and stress sensor in the cellular environment. Mature intervertebral disc cells are chondrocyte-like cells, and O-GlcNAc has been shown to promote chondrocyte apoptosis in vitro. We believe that O-GlcNAcylation is a key regulator of disc degeneration.

Methods

Fifty-six specimens were fixed for 24 hours in a 10% solution of neutral-buffered formaldehyde, dehydrated, and embedded in paraffin. Tissue slices (4-µm-thick) were used for hematoxylin-eosin staining and immunohistochemistry.

Results

We found that O-GlcNAcylation of cytoplasmic proteins was less than that of nuclear proteins in both single cells and cell clusters. Cytoplasmic O-GlcNAcylation occurs subsequent to nuclear O-GlcNAcylation and is directly proportional to disc degeneration. OGT and O-GlcNAc expression levels were identical in all specimens examined.

Conclusions

O-GlcNAc and OGA/OGT expression is shown to correlate for the first time with intervertebral disc cell degeneration. Increasing disc degeneration is associated with increasing O-GlcNAcylation in both nuclear and cytoplasmic proteins in human disc cells.

Functional significance of O-GlcNAc modification in regulating neuronal properties

Post-translational modifications (PTMs) covalently modify proteins and diversify protein functions. Along with protein phosphorylation, another common PTM is the addition of O-linked β-N-acetylglucosamine (O-GlcNAc) to serine and/or threonine residues. O-GlcNAc modification is similar to phosphorylation in that it occurs to serine and threonine residues and cycles on and off with a similar time scale. However, a striking difference is that the addition and removal of the O-GlcNAc moiety on all substrates are mediated by the two enzymes regardless of proteins, O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), respectively. O-GlcNAcylation can interact or potentially compete with phosphorylation on serine and threonine residues, and thus serves as an important molecular mechanism to modulate protein functions and activation. However, it has been challenging to address the role of O-GlcNAc modification in regulating protein functions at the molecular level due to the lack of convenient tools to determine the sites and degrees of O-GlcNAcylation. Studies in this field have only begun to expand significantly thanks to the recent advances in detection and manipulation methods such as quantitative proteomics and highly selective small-molecule inhibitors for OGT and OGA. Interestingly, multiple brain regions, especially hippocampus, express high levels of both OGT and OGA, and a number of neuron-specific proteins have been reported to undergo O-GlcNAcylation. This review aims to discuss the recent updates concerning the impacts of O-GlcNAc modification on neuronal functions at multiple levels ranging from intrinsic neuronal properties to synaptic plasticity and animal behaviors.

Nutrient-driven O-GlcNAc in proteostasis and neurodegeneration

Proteostasis is essential in the mammalian brain where post-mitotic cells must function for decades to maintain synaptic contacts and memory. The brain is dependent on glucose and other metabolites for proper function and is spared from metabolic deficits even during starvation. In this review, we outline how the nutrient-sensitive nucleocytoplasmic post-translational modification O-linked N-acetylglucosamine (O-GlcNAc) regulates protein homeostasis. The O-GlcNAc modification is highly abundant in the mammalian brain and has been linked to proteopathies, including neurodegenerative diseases such as Alzheimer's, Parkinson's, and Huntington's. C. elegans, Drosophila, and mouse models harboring O-GlcNAc transferase- and O-GlcNAcase-knockout alleles have helped define the role O-GlcNAc plays in development as well as age-associated neurodegenerative disease. These enzymes add and remove the single monosaccharide from protein serine and threonine residues, respectively. Blocking O-GlcNAc cycling is detrimental to mammalian brain development and interferes with neurogenesis, neural migration, and proteostasis. Findings in C. elegans and Drosophila model systems indicate that the dynamic turnover of O-GlcNAc is critical for maintaining levels of key transcriptional regulators responsible for neurodevelopment cell fate decisions. In addition, pathways of autophagy and proteasomal degradation depend on a transcriptional network that is also reliant on O-GlcNAc cycling. Like the quality control system in the endoplasmic reticulum which uses a 'mannose timer' to monitor protein folding, we propose that cytoplasmic proteostasis relies on an 'O-GlcNAc timer' to help regulate the lifetime and fate of nuclear and cytoplasmic proteins. O-GlcNAc-dependent developmental alterations impact metabolism and growth of the developing mouse embryo and persist into adulthood. Brain-selective knockout mouse models will be an important tool for understanding the role of O-GlcNAc in the physiology of the brain and its susceptibility to neurodegenerative injury.

The emerging link between O-GlcNAcylation and neurological disorders

O-linked β-N-acetylglucosaminylation (O-GlcNAcylation) is involved in the regulation of many cellular cascades and neurological diseases such as Alzheimer's disease (AD), Parkinson's disease (PD), and stroke. In the brain, the expression of O-GlcNAcylation is notably heightened, as is that of O-linked N-acetylglucosaminyltransferase (OGT) and β-N-acetylglucosaminidase (OGA), the presence of which is prominent in many regions of neurological importance. Most importantly, O-GlcNAcylation is believed to contribute to the normal functioning of neurons; conversely, its dysregulation participates in the pathogenesis of neurological disorders. In neurodegenerative diseases, O-GlcNAcylation of the brain's key proteins, such as tau and amyloid-β, interacts with their phosphorylation, thereby triggering the formation of neurofibrillary tangles and amyloid plaques. An increase of O-GlcNAcylation by pharmacological intervention prevents neuronal loss. Additionally, O-GlcNAcylation is stress sensitive, and its elevation is cytoprotective. Increased O-GlcNAcylation ameliorated brain damage in victims of both trauma-hemorrhage and stroke. In this review, we summarize the current understanding of O-GlcNAcylation's physiological and pathological roles in the nervous system and provide a foundation for development of a therapeutic strategy for neurological disorders.

Structural insights into the substrate binding adaptability and specificity of human O-GlcNAcase

The O-linked β-N-acetyl glucosamine (O-GlcNAc) modification dynamically regulates the functions of numerous proteins. A single human enzyme O-linked β-N-acetyl glucosaminase (O-GlcNAcase or OGA) hydrolyzes this modification. To date, it remains largely unknown how OGA recognizes various substrates. Here we report the structures of OGA in complex with each of four distinct glycopeptide substrates that contain a single O-GlcNAc modification on a serine or threonine residue. Intriguingly, these glycopeptides bind in a bidirectional yet conserved conformation within the substrate-binding cleft of OGA. This study provides fundamental insights into a general principle that confers the substrate binding adaptability and specificity to OGA in O-GlcNAc regulation.

O-linked β-N-acetyl glucosamine (O-GlcNAc) is an important protein modification that is hydrolyzed by O-GlcNAcase (OGA). Here the authors give insights into OGA substrate recognition by presenting four human OGA structures complexed with glycopeptide substrates containing a single O-GlcNAc modification on either a serine or threonine.

Quantitative time-resolved chemoproteomics reveals that stable O-GlcNAc regulates box C/D snoRNP biogenesis

O-linked GlcNAcylation (O-GlcNAcylation), a ubiquitous posttranslational modification on intracellular proteins, is dynamically regulated in cells. To analyze the turnover dynamics of O-GlcNAcylated proteins, we developed a quantitative time-resolved O-linked GlcNAc proteomics (qTOP) strategy based on metabolic pulse-chase labeling with an O-GlcNAc chemical reporter and stable isotope labeling with amino acids in cell culture (SILAC). Applying qTOP, we quantified the turnover rates of 533 O-GlcNAcylated proteins in NIH 3T3 cells and discovered that about 14% exhibited minimal removal of O-GlcNAc or degradation of protein backbones. The stability of those hyperstable O-GlcNAcylated proteins was more sensitive to O-GlcNAcylation inhibition compared with the more dynamic populations. Among the hyperstable population were three core proteins of box C/D small nucleolar ribonucleoprotein complexes (snoRNPs): fibrillarin (FBL), nucleolar protein 5A (NOP56), and nucleolar protein 5 (NOP58). We showed that O-GlcNAcylation stabilized these proteins and was essential for snoRNP assembly. Blocking O-GlcNAcylation on FBL altered the 2'-O-methylation of rRNAs and impaired cancer cell proliferation and tumor formation in vivo.

Stress-induced O-GlcNAcylation: an adaptive process of injured cells

In the 30 years, since the discovery of nucleocytoplasmic glycosylation, O-GlcNAc has been implicated in regulating cellular processes as diverse as protein folding, localization, degradation, activity, post-translational modifications, and interactions. The cell co-ordinates these molecular events, on thousands of cellular proteins, in concert with environmental and physiological cues to fine-tune epigenetics, transcription, translation, signal transduction, cell cycle, and metabolism. The cellular stress response is no exception: diverse forms of injury result in dynamic changes to the O-GlcNAc subproteome that promote survival. In this review, we discuss the biosynthesis of O-GlcNAc, the mechanisms by which O-GlcNAc promotes cytoprotection, and the clinical significance of these data.

Protein O-GlcNAcylation: emerging mechanisms and functions

O-GlcNAcylation - the attachment of O-linked N-acetylglucosamine (O-GlcNAc) moieties to cytoplasmic, nuclear and mitochondrial proteins - is a post-translational modification that regulates fundamental cellular processes in metazoans. A single pair of enzymes - O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) - controls the dynamic cycling of this protein modification in a nutrient- and stress-responsive manner. Recent years have seen remarkable advances in our understanding of O-GlcNAcylation at levels that range from structural and molecular biology to cell signalling and gene regulation to physiology and disease. New mechanisms and functions of O-GlcNAcylation that are emerging from these recent developments enable us to begin constructing a unified conceptual framework through which the significance of this modification in cellular and organismal physiology can be understood.

In Vitro Biochemical Assays for O-GlcNAc-Processing Enzymes

O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) are the only enzymes that regulate the dynamics of protein O-GlcNAcylation. Protein O-GlcNAcylation is an important post-translational modification (PTM) of nuclear and cytoplasmic proteins with O-linked β-N-acetyl-glucosamine (O-GlcNAc). O-GlcNAc and its enzymes are involved in a wide variety of cellular processes and are linked to the pathological progression of chronic diseases. Considering their emerging biological significance, systematic and rapid methods to determine the activities of OGT and OGA have become essential, and several chemical/biochemical methods for measuring the activities of these enzymes have been developed. This minireview mainly focuses on the various biochemical assay methods developed to date, while also providing a description of the fundamental principles underlying the monitoring of O-GlcNAc enzyme activities.

Characterization of tools to detect and enrich human and mouse O-GlcNAcase

O-linked β-N-acetylglucosamine (O-GlcNAc) is an essential regulatory post-translational modification of thousands of nuclear, cytoplasmic, and mitochondrial proteins. O-GlcNAc is dynamically added and removed from proteins by the O-GlcNAc transferase and the O-GlcNAcase (OGA), respectively. Dysregulation of O-GlcNAc-cycling is implicated in the etiology of numerous diseases including tumorigenesis, metabolic dysfunction, and neurodegeneration. To facilitate studies focused on the role of O-GlcNAc and OGA in disease, we sought to identify commercially available antibodies that enable the enrichment of full-length OGA from lysates of mouse and human origin. Here, we report that antibodies from Abcam and Bethyl Laboratories can be used to immunoprecipitate OGA to near-saturation from human and mouse cell lysates. However, Western blotting analysis indicates that both antibodies, as well as three non-commercially available antibodies (345, 346, 352), detect full-length OGA and numerous cross-reacting proteins. These non-specific signals migrate similarly to full-length OGA and are detected robustly, suggesting that the use of appropriate controls is essential to avoid the misidentification of OGA.

Mutations in N-acetylglucosamine (O-GlcNAc) transferase in patients with X-linked intellectual disability

N-Acetylglucosamine (O-GlcNAc) transferase (OGT) regulates protein O-GlcNAcylation, an essential and dynamic post-translational modification. The O-GlcNAc modification is present on numerous nuclear and cytosolic proteins and has been implicated in essential cellular functions such as signaling and gene expression. Accordingly, altered levels of protein O-GlcNAcylation have been associated with developmental defects and neurodegeneration. However, mutations in the OGT gene have not yet been functionally confirmed in humans. Here, we report on two hemizygous mutations in OGT in individuals with X-linked intellectual disability (XLID) and dysmorphic features: one missense mutation (p.Arg284Pro) and one mutation leading to a splicing defect (c.463–6T>G). Both mutations reside in the tetratricopeptide repeats of OGT that are essential for substrate recognition. We observed slightly reduced levels of OGT protein and reduced levels of its opposing enzyme O-GlcNAcase in both patient-derived fibroblasts, but global O-GlcNAc levels appeared to be unaffected. Our data suggest that mutant cells attempt to maintain global O-GlcNAcylation by down-regulating O-GlcNAcase expression. We also found that the c.463–6T>G mutation leads to aberrant mRNA splicing, but no stable truncated protein was detected in the corresponding patient-derived fibroblasts. Recombinant OGT bearing the p.Arg284Pro mutation was prone to unfolding and exhibited reduced glycosylation activity against a complex array of glycosylation substrates and proteolytic processing of the transcription factor host cell factor 1, which is also encoded by an XLID-associated gene. We conclude that defects in O-GlcNAc homeostasis and host cell factor 1 proteolysis may play roles in mediation of XLID in individuals with OGT mutations.

The Biochemistry of O-GlcNAc Transferase: Which Functions Make It Essential in Mammalian Cells?

O-linked N-acetylglucosamine transferase (OGT) is found in all metazoans and plays an important role in development but at the single-cell level is only essential in dividing mammalian cells. Postmitotic mammalian cells and cells of invertebrates such as Caenorhabditis elegans and Drosophila can survive without copies of OGT. Why OGT is required in dividing mammalian cells but not in other cells remains unknown. OGT has multiple biochemical activities. Beyond its well-known role in adding β-O-GlcNAc to serine and threonine residues of nuclear and cytoplasmic proteins, OGT also acts as a protease in the maturation of the cell cycle regulator host cell factor 1 (HCF-1) and serves as an integral member of several protein complexes, many of them linked to gene expression. In this review, we summarize current understanding of the mechanisms underlying OGT's biochemical activities and address whether known functions of OGT could be related to its essential role in dividing mammalian cells.

Protein O-GlcNAcylation: emerging mechanisms and functions

O-GlcNAcylation - the attachment of O-linked N-acetylglucosamine (O-GlcNAc) moieties to cytoplasmic, nuclear and mitochondrial proteins - is a post-translational modification that regulates fundamental cellular processes in metazoans. A single pair of enzymes - O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) - controls the dynamic cycling of this protein modification in a nutrient- and stress-responsive manner. Recent years have seen remarkable advances in our understanding of O-GlcNAcylation at levels that range from structural and molecular biology to cell signalling and gene regulation to physiology and disease. New mechanisms and functions of O-GlcNAcylation that are emerging from these recent developments enable us to begin constructing a unified conceptual framework through which the significance of this modification in cellular and organismal physiology can be understood.

Protein O-GlcNAcylation: emerging mechanisms and functions

O-GlcNAcylation - the attachment of O-linked N-acetylglucosamine (O-GlcNAc) moieties to cytoplasmic, nuclear and mitochondrial proteins - is a post-translational modification that regulates fundamental cellular processes in metazoans. A single pair of enzymes - O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) - controls the dynamic cycling of this protein modification in a nutrient- and stress-responsive manner. Recent years have seen remarkable advances in our understanding of O-GlcNAcylation at levels that range from structural and molecular biology to cell signalling and gene regulation to physiology and disease. New mechanisms and functions of O-GlcNAcylation that are emerging from these recent developments enable us to begin constructing a unified conceptual framework through which the significance of this modification in cellular and organismal physiology can be understood.

Insights into activity and inhibition from the crystal structure of human O-GlcNAcase

O-GlcNAc hydrolase (OGA) catalyzes removal of βα-linked N-acetyl-D-glucosamine from serine and threonine residues. We report crystal structures of Homo sapiens OGA catalytic domain in apo and inhibited states, revealing a flexible dimer that displays three unique conformations and is characterized by subdomain α-helix swapping. These results identify new structural features of the substrate-binding groove adjacent to the catalytic site and open new opportunities for structural, mechanistic and drug discovery activities.

Identification and characterization of a missense mutation in the O-linked β-N-acetylglucosamine (O-GlcNAc) transferase gene that segregates with X-linked intellectual disability

O-GlcNAc is a regulatory post-translational modification of nucleocytoplasmic proteins that has been implicated in multiple biological processes, including transcription. In humans, single genes encode enzymes for its attachment (O-GlcNAc transferase (OGT)) and removal (O-GlcNAcase (OGA)). An X-chromosome exome screen identified a missense mutation, which encodes an amino acid in the tetratricopeptide repeat, in OGT (759G>T (p.L254F)) that segregates with X-linked intellectual disability (XLID) in an affected family. A decrease in steady-state OGT protein levels was observed in isolated lymphoblastoid cell lines from affected individuals, consistent with molecular modeling experiments. Recombinant expression of L254F-OGT demonstrated that the enzyme is active as both a glycosyltransferase and an HCF-1 protease. Despite the reduction in OGT levels seen in the L254F-OGT individual cells, we observed that steady-state global O-GlcNAc levels remained grossly unaltered. Surprisingly, lymphoblastoids from affected individuals displayed a marked decrease in steady-state OGA protein and mRNA levels. We observed an enrichment of the OGT-containing transcriptional repressor complex mSin3A-HDAC1 at the proximal promoter region of OGA and correspondingly decreased OGA promoter activity in affected cells. Global transcriptome analysis of L254F-OGT lymphoblastoids compared with controls revealed a small subset of genes that are differentially expressed. Thus, we have begun to unravel the molecular consequences of the 759G>T (p.L254F) mutation in OGT that uncovered a compensation mechanism, albeit imperfect, given the phenotype of affected individuals, to maintain steady-state O-GlcNAc levels. Thus, a single amino acid substitution in the regulatory domain (the tetratricopeptide repeat domain) of OGT, which catalyzes the O-GlcNAc post-translational modification of nuclear and cytosolic proteins, appears causal for XLID.

Fatty acid synthase inhibits the O-GlcNAcase during oxidative stress

The dynamic post-translational modification O-linked β-N-acetylglucosamine (O-GlcNAc) regulates thousands of nuclear, cytoplasmic, and mitochondrial proteins. Cellular stress, including oxidative stress, results in increased O-GlcNAcylation of numerous proteins, and this increase is thought to promote cell survival. The mechanisms by which the O-GlcNAc transferase (OGT) and the O-GlcNAcase (OGA), the enzymes that add and remove O-GlcNAc, respectively, are regulated during oxidative stress to alter O-GlcNAcylation are not fully characterized. Here, we demonstrate that oxidative stress leads to elevated O-GlcNAc levels in U2OS cells but has little impact on the activity of OGT. In contrast, the expression and activity of OGA are enhanced. We hypothesized that this seeming paradox could be explained by proteins that bind to and control the local activity or substrate targeting of OGA, thereby resulting in the observed stress-induced elevations of O-GlcNAc. To identify potential protein partners, we utilized BioID proximity biotinylation in combination with stable isotopic labeling of amino acids in cell culture (SILAC). This analysis revealed 90 OGA-interacting partners, many of which exhibited increased binding to OGA upon stress. The associations of OGA with fatty acid synthase (FAS), filamin-A, heat shock cognate 70-kDa protein, and OGT were confirmed by co-immunoprecipitation. The pool of OGA bound to FAS demonstrated a substantial (∼85%) reduction in specific activity, suggesting that FAS inhibits OGA. Consistent with this observation, FAS overexpression augmented stress-induced O-GlcNAcylation. Although the mechanism by which FAS sequesters OGA remains unknown, these data suggest that FAS fine-tunes the cell's response to stress and injury by remodeling cellular O-GlcNAcylation.

Therapeutic Targeting of Epithelial Plasticity Programs: Focus on the Epithelial-Mesenchymal Transition

Mounting data points to epithelial plasticity programs such as the epithelial-mesenchymal transition (EMT) as clinically relevant therapeutic targets for the treatment of malignant tumors. In addition to the widely realized role of EMT in increasing cancer cell invasiveness during cancer metastasis, the EMT has also been implicated in allowing cancer cells to avoid tumor suppressor pathways during early tumorigenesis. In addition, data linking EMT to innate and acquired treatment resistance further points towards the desire to develop pharmacological therapies to target epithelial plasticity in cancer. In this review we organized our discussion on pathways and agents that can be used to target the EMT in cancer into 3 groups: (1) extracellular inducers of EMT, (2) the transcription factors that orchestrate the EMT transcriptome, and (3) the downstream effectors of EMT. We highlight only briefly specific canonical pathways known to be involved in EMT, such as the signal transduction pathways TGFβ, EFGR, and Axl-Gas6. We emphasize in more detail pathways that we believe are emerging novel pathways and therapeutic targets such as epigenetic therapies, glycosylation pathways, and immunotherapy. The heterogeneity of tumors and the dynamic nature of epithelial plasticity in cancer cells make it likely that targeting only 1 EMT-related process will be unsuccessful or only transiently successful. We suggest that with greater understanding of epithelial plasticity regulation, such as with the EMT, a more systematic targeting of multiple EMT regulatory networks will be the best path forward to improve cancer outcomes.

Production of O-GlcNAc Modified Recombinant Tau in E. coli and Detection of Ser400 O-GlcNAc Tau In Vivo

Assembly of the microtubule-associated protein tau (tau) into paired helical filaments that ultimately give rise to neurofibrillary tangles (NFTs) makes up one half of the two hallmark pathologies of Alzheimer's disease (AD). Tau has been shown to be modified with O-linked N-acetylglucosamine residues (O-GlcNAc), which is the modification of serine and threonine residues of nucleocytoplasmic proteins with N-acetyl-D-glucosamine (GlcNAc) moieties. Increasing O-GlcNAc in mouse models of tauopathy has been shown to hinder the progression of symptoms in these mice and impair the aggregation of tau into NFTs. In order to study how O-GlcNAc on tau may contribute to the protective effects observed in tauopathy mouse models, it is beneficial to study O-GlcNAc modified tau in vitro. Here we describe a method for producing, purifying and enriching recombinant tau that is O-GlcNAc modified. These methods have enabled the identification of O-GlcNAc modification sites on tau including Ser400. We further describe the detection of Ser400 O-GlcNAc on tau from brain lysates.

Insights into the role of maladaptive hexosamine biosynthesis and O-GlcNAcylation in development of diabetic cardiac complications

Diabetes mellitus significantly increases the risk of heart failure, independent of coronary artery disease. The mechanisms implicated in the development of diabetic heart disease, commonly termed diabetic cardiomyopathy, are complex, but much of the impact of diabetes on the heart can be attributed to impaired glucose handling. It has been shown that the maladaptive nutrient-sensing hexosamine biosynthesis pathway (HBP) contributes to diabetic complications in many non-cardiac tissues. Glucose metabolism by the HBP leads to enzymatically-regulated, O-linked attachment of a sugar moiety molecule, β-N-acetylglucosamine (O-GlcNAc), to proteins, affecting their biological activity (similar to phosphorylation). In normal physiology, transient activation of HBP/O-GlcNAc mechanisms is an adaptive, protective means to enhance cell survival; interventions that acutely suppress this pathway decrease tolerance to stress. Conversely, chronic dysregulation of HBP/O-GlcNAc mechanisms has been shown to be detrimental in certain pathological settings, including diabetes and cancer. Most of our understanding of the impact of sustained maladaptive HBP and O-GlcNAc protein modifications has been derived from adipose tissue, skeletal muscle and other non-cardiac tissues, as a contributing mechanism to insulin resistance and progression of diabetic complications. However, the long-term consequences of persistent activation of cardiac HBP and O-GlcNAc are not well-understood; therefore, the goal of this timely review is to highlight current understanding of the role of the HBP pathway in development of diabetic cardiomyopathy.

O-GlcNAcylation of 8-Oxoguanine DNA Glycosylase (Ogg1) Impairs Oxidative Mitochondrial DNA Lesion Repair in Diabetic Hearts

mtDNA damage in cardiac myocytes resulting from increased oxidative stress is emerging as an important factor in the pathogenesis of diabetic cardiomyopathy. A prevalent lesion that occurs in mtDNA damage is the formation of 8-hydroxy-2'-deoxyguanosine (8-OHdG), which can cause mutations when not repaired properly by 8-oxoguanine DNA glycosylase (Ogg1). Although the mtDNA repair machinery has been described in cardiac myocytes, the regulation of this repair has been incompletely investigated. Here we report that the hearts of type 1 diabetic mice, despite having increased Ogg1 protein levels, had significantly lower Ogg1 activity than the hearts of control, non-type 1 diabetic mice. In diabetic hearts, we further observed increased levels of 8-OHdG and an increased amount of mtDNA damage. Interestingly, Ogg1 was found to be highly O-GlcNAcylated in diabetic mice compared with controls. In vitro experiments demonstrated that O-GlcNAcylation inhibits Ogg1 activity, which could explain the mtDNA lesion accumulation observed in vivo Reducing Ogg1 O-GlcNAcylation in vivo by introducing a dominant negative O-GlcNAc transferase mutant (F460A) restored Ogg1 enzymatic activity and, consequently, reduced 8-OHdG and mtDNA damage despite the adverse hyperglycemic milieu. Taken together, our results implicate hyperglycemia-induced O-GlcNAcylation of Ogg1 in increased mtDNA damage and, therefore, provide a new plausible biochemical mechanism for diabetic cardiomyopathy.

Combined Antibody/Lectin Enrichment Identifies Extensive Changes in the O-GlcNAc Sub-proteome upon Oxidative Stress

O-Linked N-acetyl-β-d-glucosamine (O-GlcNAc) is a dynamic post-translational modification that modifies and regulates over 3000 nuclear, cytoplasmic, and mitochondrial proteins. Upon exposure to stress and injury, cells and tissues increase the O-GlcNAc modification, or O-GlcNAcylation, of numerous proteins promoting the cellular stress response and thus survival. The aim of this study was to identify proteins that are differentially O-GlcNAcylated upon acute oxidative stress (H₂O₂) to provide insight into the mechanisms by which O-GlcNAc promotes survival. We achieved this goal by employing Stable Isotope Labeling of Amino Acids in Cell Culture (SILAC) and a novel "G5-lectibody" immunoprecipitation strategy that combines four O-GlcNAc-specific antibodies (CTD110.6, RL2, HGAC39, and HGAC85) and the lectin WGA. Using the G5-lectibody column in combination with basic reversed phase chromatography and C₁₈ RPLC-MS/MS, 990 proteins were identified and quantified. Hundreds of proteins that were identified demonstrated increased (>250) or decreased (>110) association with the G5-lectibody column upon oxidative stress, of which we validated the O-GlcNAcylation status of 24 proteins. Analysis of proteins with altered glycosylation suggests that stress-induced changes in O-GlcNAcylation cluster into pathways known to regulate the cell's response to injury and include protein folding, transcriptional regulation, epigenetics, and proteins involved in RNA biogenesis. Together, these data suggest that stress-induced O-GlcNAcylation regulates numerous and diverse cellular pathways to promote cell and tissue survival.

Identification of the nuclear localisation signal of O-GlcNAc transferase and its nuclear import regulation

Nucleocytoplasmic O-GlcNAc transferase (OGT) attaches a single GlcNAc to hydroxyl groups of serine and threonine residues. Although the cellular localisation of OGT is important to regulate a variety of cellular processes, the molecular mechanisms regulating the nuclear localisation of OGT is unclear. Here, we characterised three amino acids (DFP; residues 451-453) as the nuclear localisation signal of OGT and demonstrated that this motif mediated the nuclear import of non-diffusible β-galactosidase. OGT bound the importin α5 protein, and this association was abolished when the DFP motif of OGT was mutated or deleted. We also revealed that O-GlcNAcylation of Ser389, which resides in the tetratricopeptide repeats, plays an important role in the nuclear localisation of OGT. Our findings may explain how OGT, which possesses a NLS, exists in the nucleus and cytosol simultaneously.

Functional analysis of recombinant human and Yarrowia lipolytica O-GlcNAc transferases expressed in Saccharomyces cerevisiae

O-linked β-N-acetylglucosamine (O-GlcNAc) glycosylation is an important post-translational modification in many cellular processes. It is mediated by O-GlcNAc transferases (OGTs), which catalyze the addition of O-GlcNAc to serine or threonine residues of the target proteins. In this study, we expressed a putative Yarrowia lipolytica OGT (YlOGT), the only homolog identified in the subphylum Saccharomycotina through bioinformatics analysis, and the human OGT (hOGT) as recombinant proteins in Saccharomyces cerevisiae, and performed their functional characterization. Immunoblotting assays using antibody against O-GlcNAc revealed that recombinant hOGT (rhOGT), but not the recombinant YlOGT (rYlOGT), undergoes auto-O-GlcNAcylation in the heterologous host S. cerevisiae. Moreover, the rhOGT expressed in S. cerevisiae showed a catalytic activity during in vitro assays using casein kinase II substrates, whereas no such activity was obtained in rYlOGT. However, the chimeric human-Y. lipolytica OGT, carrying the human tetratricopeptide repeat (TPR) domain along with the Y. lipolytica catalytic domain (CTD), mediated the transfer of O-GlcNAc moiety during the in vitro assays. Although the overexpression of full-length OGTs inhibited the growth of S. cerevisiae, no such inhibition was obtained upon overexpression of only the CTD fragment, indicating the role of TPR domain in growth inhibition. This is the first report on the functional analysis of the fungal OGT, indicating that the Y. lipolytica OGT retains its catalytic activity, although the physiological role and substrates of YlOGT remain to be elucidated.

O-GlcNAcylation and neurodegeneration

O-GlcNAcylation is a dynamic form of protein glycosylation which involves the addition of β-d-N-acetylglucosamine (GlcNAc) via an O-linkage to serine or threonine residues of nuclear, cytoplasmic, mitochondrial and transmembrane proteins. The two enzymes responsible for O-GlcNAc cycling are O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA); their expression and activities in brain are age dependent. More than 1000 O-GlcNAc protein targets have been identified which play critical roles in many cellular processes. In mammalian brain, O-GlcNAc modification of Tau decreases its phosphorylation and toxicity, suggesting a neuroprotective role of pharmacological elevation of brain O-GlcNAc for Alzheimer's disease treatment. Other observations suggest that elevating O-GlcNAc levels may decrease protein clearance or induce apoptosis. This review highlights some of the key findings regarding O-GlcNAcylation in models of neurodegenerative diseases.

O-GlcNAcylation: A regulator of tau pathology and neurodegeneration

O-GlcNAcylation is the posttranslational modification of intracellular proteins by O-linked β-N-acetylglucosamine (O-GlcNAc). The discovery of O-GlcNAc modification of tau and its impact on tau phosphorylation has attracted recent research interest in O-GlcNAc studies in the Alzheimer's disease (AD) field. Modification of proteins by O-GlcNAc occurs extensively in the brain. The expressions and activities of the enzymes catalyzing O-GlcNAc cycling are several-fold higher in the brain than in the peripheral tissues. The O-GlcNAcylation levels of brain proteins including tau are decreased in AD brain, probably due to decreased brain glucose metabolism. The reduction of brain O-GlcNAcylation appears to mediate the molecular mechanism by which decreased brain glucose metabolism contributes to neurodegeneration. Studies on mouse models of tauopathies suggest a neuroprotective role of pharmacological elevation of brain O-GlcNAc, which could potentially be a promising approach for treating AD and other neurodegenerative diseases.

Mutagenesis and Functional Analysis of the Bacterial Arginine Glycosyltransferase Effector NleB1 from Enteropathogenic Escherichia coli

Enteropathogenic Escherichia coli (EPEC) interferes with host cell signaling by injecting virulence effector proteins into enterocytes via a type III secretion system (T3SS). NleB1 is a novel T3SS glycosyltransferase effector from EPEC that transfers a single N-acetylglucosamine (GlcNAc) moiety in an N-glycosidic linkage to Arg(117) of the Fas-associated death domain protein (FADD). GlcNAcylation of FADD prevents the assembly of the canonical death-inducing signaling complex and inhibits Fas ligand (FasL)-induced cell death. Apart from the DXD catalytic motif of NleB1, little is known about other functional sites in the enzyme. In the present study, members of a library of 22 random transposon-based, in-frame, linker insertion mutants of NleB1 were tested for their ability to block caspase-8 activation in response to FasL during EPEC infection. Immunoblot analysis of caspase-8 cleavage showed that 17 mutant derivatives of NleB1, including the catalytic DXD mutant, did not inhibit caspase-8 activation. Regions of interest around the insertion sites with multiple or single amino acid substitutions were examined further. Coimmunoprecipitation studies of 34 site-directed mutants showed that the NleB1 derivatives with the E253A, Y219A, and PILN(63-66)AAAA (in which the PILN motif from residues 63 to 66 was changed to AAAA) mutations bound to but did not GlcNAcylate FADD. A further mutant derivative, the PDG(236-238)AAA mutant, did not bind to or GlcNAcylate FADD. Infection of mice with the EPEC-like mouse pathogen Citrobacter rodentium expressing NleBE253A and NleBY219A showed that these strains were attenuated, indicating the importance of residues E253 and Y219 in NleB1 virulence in vivo In summary, we identified new amino acid residues critical for NleB1 activity and confirmed that these are required for the virulence function of NleB1.

Role of O-linked N-acetylglucosamine modification in diabetic nephropathy

Increased O-linked β-N-acetylglucosamine glycosylation (O-GlcNAcylation) is a known contributor to diabetes; however, its relevance in diabetic nephropathy (DN) is poorly elucidated. Here, we studied the process and enzymes of O-GlcNAcylation with a special emphasis on Akt-endothelial nitric oxide synthase (eNOS) and heat shock protein (HSP)72 signaling. Since tubular injury is the prominent site of DN, the effect of hyperglycemia was first measured in proximal tubular (HK2) cells cultured in high glucose. In vivo O-GlcNAcylation and protein levels of O-GlcNAc transferase (OGT), O-GlcNAcase (OGA), phosphorylated (p)Akt/Akt, peNOS/eNOS, and HSP72 were assessed in the kidney cortex of streptozotocin-induced diabetic rats. The effects of various renin-angiotensin-aldosterone system (RAAS) inhibitors were also evaluated. In proximal tubular cells, hyperglycemia-induced OGT expression led to increased O-GlcNAcylation, which was followed by a compensatory increase of OGA. In parallel, peNOS and pAkt levels decreased, whereas HSP72 increased. In diabetic rats, elevated O-GlcNAcylation was accompanied by decreased OGT and OGA. RAAS inhibitors ameliorated diabetes-induced kidney damage and prevented the elevation of O-GlcNAcylation and the decrement of pAkt, peNOS, and HSP72. In conclusion, hyperglycemia-induced elevation of O-GlcNAcylation contributes to the progression of DN via inhibition of Akt/eNOS phosphorylation and HSP72 induction. RAAS blockers successfully inhibit this process, suggesting a novel pathomechanism of their renoprotective action in the treatment of DN.

Distributive O-GlcNAcylation on the Highly Repetitive C-Terminal Domain of RNA Polymerase II

O-GlcNAcylation is a nutrient-responsive glycosylation that plays a pivotal role in transcriptional regulation. Human RNA polymerase II (Pol II) is extensively modified by O-linked N-acetylglucosamine (O-GlcNAc) on its unique C-terminal domain (CTD), which consists of 52 heptad repeats. One approach to understanding the function of glycosylated Pol II is to determine the mechanism of dynamic O-GlcNAcylation on the CTD. Here, we discovered that the Pol II CTD can be extensively O-GlcNAcylated in vitro and in cells. Efficient glycosylation requires a minimum of 20 heptad repeats of the CTD and more than half of the N-terminal domain of O-GlcNAc transferase (OGT). Under conditions of saturated sugar donor, we monitored the attachment of more than 20 residues of O-GlcNAc to the full-length CTD. Surprisingly, glycosylation on the periodic CTD follows a distributive mechanism, resulting in highly heterogeneous glycoforms. Our data suggest that initial O-GlcNAcylation can take place either on the proximal or on the distal region of the CTD, and subsequent glycosylation occurs similarly over the entire CTD with nonuniform distributions. Moreover, removal of O-GlcNAc from glycosylated CTD is also distributive and is independent of O-GlcNAcylation level. Our results suggest that O-GlcNAc cycling enzymes can employ a similar mechanism to react with other protein substrates on multiple sites. Distributive O-GlcNAcylation on Pol II provides another regulatory mechanism of transcription in response to fluctuating cellular conditions.

Quantitative phosphoproteomics reveals crosstalk between phosphorylation and O-GlcNAc in the DNA damage response pathway

The modification of intracellular proteins by monosaccharides of O-linked β-N-acetylglucosamine (O-GlcNAc) is an essential and dynamic PTM of metazoans. The addition and removal of O-GlcNAc is catalyzed by the O-GlcNAc transferase (OGT) and O-GlcNAcase, respectively. One mechanism by which O-GlcNAc is thought to mediate proteins is by regulating phosphorylation. To provide insight into the pathways regulated by O-GlcNAc, we have utilized SILAC-based quantitative proteomics to carry out comparisons of site-specific phosphorylation in OGT wild-type and Null cells. Quantitation of the phosphoproteome demonstrated that of 5529 phosphoserine, phosphothreonine, and phosphotyrosine sites, 232 phosphosites were upregulated and 133 downregulated in the absence of O-GlcNAc. Collectively, these data suggest that deletion of OGT has a profound effect on the phosphorylation of cell cycle and DNA damage response proteins. Key events were confirmed by biochemical analyses and demonstrate an increase in the activating autophosphorylation event on ATM (Ser1987) and on ATM's downstream targets p53, H2AX, and Chk2. Together, these data support widespread changes in the phosphoproteome upon removal of O-GlcNAc, suggesting that O-GlcNAc regulates processes such as the cell cycle, genomic stability, and lysosomal biogenesis. All MS data have been deposited in the ProteomeXchange with identifier PXD001153 (http://proteomecentral.proteomexchange.org/dataset/PXD001153).

The active site of O-GlcNAc transferase imposes constraints on substrate sequence

O-GlcNAc transferase (OGT) glycosylates a diverse range of intracellular proteins with O-linked N-acetylglucosamine (O-GlcNAc), an essential and dynamic post-translational modification in metazoans. Although this enzyme modifies hundreds of proteins with O-GlcNAc, it is not understood how OGT achieves substrate specificity. In this study, we describe the application of a high-throughput OGT assay to a library of peptides. We mapped sites of O-GlcNAc modification by electron transfer dissociation MS and found that they correlate with previously detected O-GlcNAc sites. Crystal structures of four acceptor peptides in complex with Homo sapiens OGT suggest that a combination of size and conformational restriction defines sequence specificity in the -3 to +2 subsites. This work reveals that although the N-terminal TPR repeats of OGT may have roles in substrate recognition, the sequence restriction imposed by the peptide-binding site makes a substantial contribution to O-GlcNAc site specificity.