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

Regulation of Glycogen Synthase Kinase-3β by Phosphorylation and O-β-Linked N-Acetylglucosaminylation: Implications on Tau Protein Phosphorylation

Glycogen synthase kinase 3 (GSK3) plays a pivotal role in signaling pathways involved in insulin metabolism and the pathogenesis of neurodegenerative disorders. In particular, the GSK3β isoform is implicated in Alzheimer's disease (AD) as one of the key kinases involved in the hyperphosphorylation of tau protein, one of the neuropathological hallmarks of AD. As a constitutively active serine/threonine kinase, GSK3 is inactivated by Akt/PKB-mediated phosphorylation of Ser9 in the N-terminal disordered domain, and for most of its substrates, requires priming (prephosphorylation) by another kinase that targets the substrate to a phosphate-specific pocket near the active site. GSK3 has also been shown to be post-translationally modified by O-linked β-N-acetylglucosaminylation (O-GlcNAcylation), with still unknown functions. Here, we have found that binding of Akt inhibits GSK3β kinase activity on both primed and unprimed tau substrates. Akt-mediated Ser9 phosphorylation restores the GSK3β kinase activity only on primed tau, thereby selectively inactivating GSK3β toward unprimed tau protein. Additionally, we have shown that GSK3β is highly O-GlcNAcylated at multiple sites within the kinase domain and the disordered N- and C-terminal domains, including Ser9. In contrast to Akt-mediated regulation, neither the O-GlcNAc transferase nor O-GlcNAcylation significantly alters GSK3β kinase activity, but high O-GlcNAc levels reduce Ser9 phosphorylation by Akt. Reciprocally, Akt phosphorylation downregulates the overall O-GlcNAcylation of GSK3β, indicating a crosstalk between both post-translational modifications. Our results indicate that specific O-GlcNAc profiles may be involved in the phosphorylation-dependent Akt-mediated regulation of GSK3β kinase activity.

OGT and OGA: Sweet guardians of the genome

The past 4 decades have witnessed tremendous efforts in deciphering the role of O-GlcNAcylation in a plethora of biological processes. Chemists and biologists have joined hand in hand in the sweet adventure to unravel this unique and universal yet uncharted post-translational modification, and the recent advent of cutting-edge chemical biology and mass spectrometry tools has greatly facilitated the process. Compared with O-GlcNAc, DNA damage response (DDR) is a relatively intensively studied area that could be traced to before the elucidation of the structure of DNA. Unexpectedly, yet somewhat expectedly, O-GlcNAc has been found to regulate various DDR pathways: homologous recombination, nonhomologous end joining, base excision repair, and translesion DNA synthesis. In this review, we first cover the recent structural studies of the O-GlcNAc transferase and O-GlcNAcase, the elegant duo that "writes" and "erases" O-GlcNAc modification. Then we delineate the intricate roles of O-GlcNAc transferase and O-GlcNAcase in DDR. We envision that this is only the beginning of our full appreciation of how O-GlcNAc regulates the blueprint of life-DNA.

Exploiting O-GlcNAc transferase promiscuity to dissect site-specific O-GlcNAcylation

Protein O-GlcNAcylation is an evolutionary conserved post-translational modification catalysed by the nucleocytoplasmic O-GlcNAc transferase (OGT) and reversed by O-GlcNAcase (OGA). How site-specific O-GlcNAcylation modulates a diverse range of cellular processes is largely unknown. A limiting factor in studying this is the lack of accessible techniques capable of producing homogeneously O-GlcNAcylated proteins, in high yield, for in vitro studies. Here, we exploit the tolerance of OGT for cysteine instead of serine, combined with a co-expressed OGA to achieve site-specific, highly homogeneous mono-glycosylation. Applying this to DDX3X, TAB1, and CK2α, we demonstrate that near-homogeneous mono-S-GlcNAcylation of these proteins promotes DDX3X and CK2α solubility and enables production of mono-S-GlcNAcylated TAB1 crystals, albeit with limited diffraction. Taken together, this work provides a new approach for functional dissection of protein O-GlcNAcylation.

O-GlcNAcylation determines the translational regulation and phase separation of YTHDF proteins

N6-methyladenosine (m6A) is the most abundant internal mRNA nucleotide modification in mammals, regulating critical aspects of cell physiology and differentiation. The YTHDF proteins are the primary readers of m6A modifications and exert physiological functions of m6A in the cytosol. Elucidating the regulatory mechanisms of YTHDF proteins is critical to understanding m6A biology. Here we report a mechanism that protein post-translational modifications control the biological functions of the YTHDF proteins. We find that YTHDF1 and YTHDF3, but not YTHDF2, carry high levels of nutrient-sensing O-GlcNAc modifications. O-GlcNAcylation attenuates the translation-promoting function of YTHDF1 and YTHDF3 by blocking their interactions with proteins associated with mRNA translation. We further demonstrate that O-GlcNAc modifications on YTHDF1 and YTHDF3 regulate the assembly, stability and disassembly of stress granules to enable better recovery from stress. Therefore, our results discover an important regulatory pathway of YTHDF functions, adding an additional layer of complexity to the post-transcriptional regulation function of mRNA m6A.

Cryo-EM structure of human O-GlcNAcylation enzyme pair OGT-OGA complex

O-GlcNAcylation is a conserved post-translational modification that attaches N-acetyl glucosamine (GlcNAc) to myriad cellular proteins. In response to nutritional and hormonal signals, O-GlcNAcylation regulates diverse cellular processes by modulating the stability, structure, and function of target proteins. Dysregulation of O-GlcNAcylation has been implicated in the pathogenesis of cancer, diabetes, and neurodegeneration. A single pair of enzymes, the O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), catalyzes the addition and removal of O-GlcNAc on over 3,000 proteins in the human proteome. However, how OGT selects its native substrates and maintains the homeostatic control of O-GlcNAcylation of so many substrates against OGA is not fully understood. Here, we present the cryo-electron microscopy (cryo-EM) structures of human OGT and the OGT-OGA complex. Our studies reveal that OGT forms a functionally important scissor-shaped dimer. Within the OGT-OGA complex structure, a long flexible OGA segment occupies the extended substrate-binding groove of OGT and positions a serine for O-GlcNAcylation, thus preventing OGT from modifying other substrates. Conversely, OGT disrupts the functional dimerization of OGA and occludes its active site, resulting in the blocking of access by other substrates. This mutual inhibition between OGT and OGA may limit the futile O-GlcNAcylation cycles and help to maintain O-GlcNAc homeostasis.

Phage display uncovers a sequence motif that drives polypeptide binding to a conserved regulatory exosite of O-GlcNAc transferase

The modification of nucleocytoplasmic proteins by O-linked N-acetylglucosamine (O-GlcNAc) is an important regulator of cell physiology. O-GlcNAc is installed on over a thousand proteins by just one enzyme, O-GlcNAc transferase (OGT). How OGT is regulated is therefore a topic of interest. To gain insight into these questions, we used OGT to perform phage display selection from an unbiased library of ~109 peptides of 15 amino acids in length. Following rounds of selection and deep mutational panning, we identified a high-fidelity peptide consensus sequence, [Y/F]-x-P-x-Y-x-[I/M/F], that drives peptide binding to OGT. Peptides containing this sequence bind to OGT in the high nanomolar to low micromolar range and inhibit OGT in a noncompetitive manner with low micromolar potencies. X-ray structural analyses of OGT in complex with a peptide containing this motif surprisingly revealed binding to an exosite proximal to the active site of OGT. This structure defines the detailed molecular basis driving peptide binding and explains the need for specific residues within the sequence motif. Analysis of the human proteome revealed this motif within 52 nuclear and cytoplasmic proteins. Collectively, these data suggest a mode of regulation of OGT by which polypeptides can bind to this exosite to cause allosteric inhibition of OGT through steric occlusion of its active site. We expect that these insights will drive improved understanding of the regulation of OGT within cells and enable the development of new chemical tools to exert fine control over OGT activity.

Promiscuous Enzymes for Residue-Specific Peptide and Protein Late-Stage Functionalization

The late-stage functionalization of peptides and proteins holds significant promise for drug discovery and facilitates bioorthogonal chemistry. This selective functionalization leads to innovative advances in in vitro and in vivo biological research. However, it is a challenging endeavor to selectively target a certain amino acid or position in the presence of other residues containing reactive groups. Biocatalysis has emerged as a powerful tool for selective, efficient, and economical modifications of molecules. Enzymes that have the ability to modify multiple complex substrates or selectively install nonnative handles have wide applications. Herein, we highlight enzymes with broad substrate tolerance that have been demonstrated to modify a specific amino acid residue in simple or complex peptides and/or proteins at late-stage. The different substrates accepted by these enzymes are mentioned together with the reported downstream bioorthogonal reactions that have benefited from the enzymatic selective modifications.

Investigation of the Catalytic Mechanism of a Soluble N-glycosyltransferase Allows Synthesis of N-glycans at Noncanonical Sequons

The soluble N-glycosyltransferase from Actinobacillus pleuropneumoniae (ApNGT) can establish an N-glycosidic bond at the asparagine residue in the Asn-Xaa-Ser/Thr consensus sequon and is one of the most promising tools for N-glycoprotein production. Here, by integrating computational and experimental strategies, we revealed the molecular mechanism of the substrate recognition and following catalysis of ApNGT. These findings allowed us to pinpoint a key structural motif (215DVYM218) in ApNGT responsible for the peptide substrate recognition. Moreover, Y222 and H371 of ApNGT were found to participate in activating the acceptor Asn. The constructed models were supported by further crystallographic studies and the functional roles of the identified residues were validated by measuring the glycosylation activity of various mutants against a library of synthetic peptides. Intriguingly, with particular mutants, site-selective N-glycosylation of canonical or noncanonical sequons within natural polypeptides from the SARS-CoV-2 spike protein could be achieved, which were used to investigate the biological roles of the N-glycosylation in membrane fusion during virus entry. Our study thus provides in-depth molecular mechanisms underlying the substrate recognition and catalysis for ApNGT, leading to the synthesis of previously unknown chemically defined N-glycoproteins for exploring the biological importance of the N-glycosylation at a specific site.

O-GlcNAc Modification Alters the Chaperone Activity of HSP27 Charcot-Marie-Tooth Type 2 (CMT2) Variants in a Mutation-Selective Fashion

Increased O-GlcNAc is a common feature of cellular stress, and the upregulation of this dynamic modification is associated with improved survival under these conditions. Likewise, the heat shock proteins are also increased under stress and prevent protein misfolding and aggregation. We previously linked these two phenomena by demonstrating that O-GlcNAc directly increases the chaperone of certain small heat shock proteins, including HSP27. Here, we examine this linkage further by exploring the potential function of O-GlcNAc on mutants of HSP27 that cause a heritable neuropathy called Charcot-Marie-Tooth type 2 (CMT2) disease. Using synthetic protein chemistry, we prepared five of these mutants bearing an O-GlcNAc at the major site of modification. Upon subsequent biochemical analysis of these proteins, we found that O-GlcNAc has different effects, depending on the location of the individual mutants. We believe that this has important implications for O-GlcNAc and other PTMs in the context of polymorphisms or diseases with high levels of protein mutation.

An O-GlcNAc transferase pathogenic variant linked to intellectual disability affects pluripotent stem cell self-renewal

O-linked β-N-acetylglucosamine (O-GlcNAc) transferase (OGT) is an essential enzyme that modifies proteins with O-GlcNAc. Inborn OGT genetic variants were recently shown to mediate a novel type of congenital disorder of glycosylation (OGT-CDG), which is characterised by X-linked intellectual disability (XLID) and developmental delay. Here, we report an OGTC921Y variant that co-segregates with XLID and epileptic seizures, and results in loss of catalytic activity. Colonies formed by mouse embryonic stem cells carrying OGTC921Y showed decreased levels of protein O-GlcNAcylation accompanied by decreased levels of Oct4 (encoded by Pou5f1), Sox2 and extracellular alkaline phosphatase (ALP), implying reduced self-renewal capacity. These data establish a link between OGT-CDG and embryonic stem cell self-renewal, providing a foundation for examining the developmental aetiology of this syndrome.

Structure and dynamics of the Arabidopsis O-fucosyltransferase SPINDLY

SPINDLY (SPY) in Arabidopsis thaliana is a novel nucleocytoplasmic protein O-fucosyltransferase (POFUT), which regulates diverse developmental processes. Sequence analysis indicates that SPY is distinct from ER-localized POFUTs and contains N-terminal tetratricopeptide repeats (TPRs) and a C-terminal catalytic domain resembling the O-linked-N-acetylglucosamine (GlcNAc) transferases (OGTs). However, the structural feature that determines the distinct enzymatic selectivity of SPY remains unknown. Here we report the cryo-electron microscopy (cryo-EM) structure of SPY and its complex with GDP-fucose, revealing distinct active-site features enabling GDP-fucose instead of UDP-GlcNAc binding. SPY forms an antiparallel dimer instead of the X-shaped dimer in human OGT, and its catalytic domain interconverts among multiple conformations. Analysis of mass spectrometry, co-IP, fucosylation activity, and cryo-EM data further demonstrates that the N-terminal disordered peptide in SPY contains trans auto-fucosylation sites and inhibits the POFUT activity, whereas TPRs 1-5 dynamically regulate SPY activity by interfering with protein substrate binding.

OGT Binding Peptide-Tagged Strategy Increases Protein O-GlcNAcylation Level in E. coli

O-GlcNAcylation is a single glycosylation of GlcNAc mediated by OGT, which regulates the function of substrate proteins and is closely related to many diseases. However, a large number of O-GlcNAc-modified target proteins are costly, inefficient, and complicated to prepare. In this study, an OGT binding peptide (OBP)-tagged strategy for improving the proportion of O-GlcNAc modification was established successfully in E. coli. OBP (P1, P2, or P3) was fused with target protein Tau as tagged Tau. Tau or tagged Tau was co-constructed with OGT into a vector expressed in E. coli. Compared with Tau, the O-GlcNAc level of P1Tau and TauP1 increased 4~6-fold. Moreover, the P1Tau and TauP1 increased the O-GlcNAc-modified homogeneity. The high O-GlcNAcylation on P1Tau resulted in a significantly slower aggregation rate than Tau in vitro. This strategy was also used successfully to increase the O-GlcNAc level of c-Myc and H2B. These results indicated that the OBP-tagged strategy was a successful approach to improve the O-GlcNAcylation of a target protein for further functional research.

Protein O-GlcNAcylation in Metabolic Modulation of Skeletal Muscle: A Bright but Long Way to Go

O-GlcNAcylation is an atypical, dynamic and reversible O-glycosylation that is critical and abundant in metazoan. O-GlcNAcylation coordinates and receives various signaling inputs such as nutrients and stresses, thus spatiotemporally regulating the activity, stability, localization and interaction of target proteins to participate in cellular physiological functions. Our review discusses in depth the involvement of O-GlcNAcylation in the precise regulation of skeletal muscle metabolism, such as glucose homeostasis, insulin sensitivity, tricarboxylic acid cycle and mitochondrial biogenesis. The complex interaction and precise modulation of O-GlcNAcylation in these nutritional pathways of skeletal muscle also provide emerging mechanical information on how nutrients affect health, exercise and disease. Meanwhile, we explored the potential role of O-GlcNAcylation in skeletal muscle pathology and focused on its benefits in maintaining proteostasis under atrophy. In general, these understandings of O-GlcNAcylation are conducive to providing new insights into skeletal muscle (patho) physiology.

Bioinformatic prediction of putative conveyers of O-GlcNAc transferase intellectual disability

Protein O-GlcNAcylation is a dynamic posttranslational modification that is catalyzed by the enzyme O-GlcNAc transferase (OGT) and is essential for neurodevelopment and postnatal neuronal function. Missense mutations in OGT segregate with a novel X-linked intellectual disability syndrome, the OGT congenital disorder of glycosylation (OGT-CDG). One hypothesis for the etiology of OGT-CDG is that loss of OGT activity leads to hypo-O-GlcNAcylation of as yet unidentified, specific neuronal proteins, affecting essential embryonic, and postnatal neurodevelopmental processes; however, the identity of these O-GlcNAcylated proteins is not known. Here, we used bioinformatic techniques to integrate sequence conservation, structural data, clinical data, and the available literature to identify 22 candidate proteins that convey OGT-CDG. We found using gene ontology and PANTHER database data that these candidate proteins are involved in diverse processes including Ras/MAPK signaling, translational repression, cytoskeletal dynamics, and chromatin remodeling. We also identify pathogenic missense variants at O-GlcNAcylation sites that segregate with intellectual disability. This work establishes a preliminary platform for the mechanistic dissection of the links between protein O-GlcNAcylation and neurodevelopment in OGT-CDG.

O-GlcNAcylation: The Underestimated Emerging Regulators of Skeletal Muscle Physiology

O-GlcNAcylation is a highly dynamic, reversible and atypical glycosylation that regulates the activity, biological function, stability, sublocation and interaction of target proteins. O-GlcNAcylation receives and coordinates different signal inputs as an intracellular integrator similar to the nutrient sensor and stress receptor, which target multiple substrates with spatio-temporal analysis specifically to maintain cellular homeostasis and normal physiological functions. Our review gives a brief description of O-GlcNAcylation and its only two processing enzymes and HBP flux, which will help to better understand its physiological characteristics of sensing nutrition and environmental cues. This nutritional and stress-sensitive properties of O-GlcNAcylation allow it to participate in the precise regulation of skeletal muscle metabolism. This review discusses the mechanism of O-GlcNAcylation to alleviate metabolic disorders and the controversy about the insulin resistance of skeletal muscle. The level of global O-GlcNAcylation is precisely controlled and maintained in the “optimal zone”, and its abnormal changes is a potential factor in the pathogenesis of cancer, neurodegeneration, diabetes and diabetic complications. Although the essential role of O-GlcNAcylation in skeletal muscle physiology has been widely studied and recognized, it still is underestimated and overlooked. This review highlights the latest progress and potential mechanisms of O-GlcNAcylation in the regulation of skeletal muscle contraction and structural properties.

Demystifying the O-GlcNAc Code: A Systems View

Post-translational modification with O-linked β-N-acetylglucosamine (O-GlcNAc), a process referred to as O-GlcNAcylation, occurs on a vast variety of proteins. Mounting evidence in the past several decades has clearly demonstrated that O-GlcNAcylation is a unique and ubiquitous modification. Reminiscent of a code, protein O-GlcNAcylation functions as a crucial regulator of nearly all cellular processes studied. The primary aim of this review is to summarize the developments in our understanding of myriad protein substrates modified by O-GlcNAcylation from a systems perspective. Specifically, we provide a comprehensive survey of O-GlcNAcylation in multiple species studied, including eukaryotes (e.g., protists, fungi, plants, Caenorhabditis elegans, Drosophila melanogaster, murine, and human), prokaryotes, and some viruses. We evaluate features (e.g., structural properties and sequence motifs) of O-GlcNAc modification on proteins across species. Given that O-GlcNAcylation functions in a species-, tissue-/cell-, protein-, and site-specific manner, we discuss the functional roles of O-GlcNAcylation on human proteins. We focus particularly on several classes of relatively well-characterized human proteins (including transcription factors, protein kinases, protein phosphatases, and E3 ubiquitin-ligases), with representative O-GlcNAc site-specific functions presented. We hope the systems view of the great endeavor in the past 35 years will help demystify the O-GlcNAc code and lead to more fascinating studies in the years to come.

The Beginner’s Guide to O-GlcNAc: From Nutrient Sensitive Pathway Regulation to Its Impact on the Immune System

The addition of N-acetyl glucosamine (GlcNAc) on the hydroxy group of serine/threonine residues is known as O-GlcNAcylation (OGN). The dynamic cycling of this monosaccharide on and off substrates occurs via O-linked β-N-acetylglucosamine transferase (OGT) and O-linked β-N-acetylglucosaminase (OGA) respectively. These enzymes are found ubiquitously in eukaryotes and genetic knock outs of the ogt gene has been found to be lethal in embryonic mice. The substrate scope of these enzymes is vast, over 15,000 proteins across 43 species have been identified with O-GlcNAc. OGN has been known to play a key role in several cellular processes such as: transcription, translation, cell signaling, nutrient sensing, immune cell development and various steps of the cell cycle. However, its dysregulation is present in various diseases: cancer, neurodegenerative diseases, diabetes. O-GlcNAc is heavily involved in cross talk with other post-translational modifications (PTM), such as phosphorylation, acetylation, and ubiquitination, by regulating each other’s cycling enzymes or directly competing addition on the same substrate. This crosstalk between PTMs can affect gene expression, protein localization, and protein stability; therefore, regulating a multitude of cell signaling pathways. In this review the roles of OGN will be discussed. The effect O-GlcNAc exerts over protein-protein interactions, the various forms of crosstalk with other PTMs, and its role as a nutrient sensor will be highlighted. A summary of how these O-GlcNAc driven processes effect the immune system will also be included.

The HLA Ligandome Comprises a Limited Repertoire of O-GlcNAcylated Antigens Preferentially Associated With HLA-B*07:02

Mass-spectrometry based immunopeptidomics has provided unprecedented insights into antigen presentation, not only charting an enormous ligandome of self-antigens, but also cancer neoantigens and peptide antigens harbouring post-translational modifications. Here we concentrate on the latter, focusing on the small subset of HLA Class I peptides (less than 1%) that has been observed to be post-translationally modified (PTM) by a O-linked N-acetylglucosamine (GlcNAc). Just like neoantigens these modified antigens may have specific immunomodulatory functions. Here we compiled from literature, and a new dataset originating from the JY B cell lymphoblastoid cell line, a concise albeit comprehensive list of O-GlcNAcylated HLA class I peptides. This cumulative list of O-GlcNAcylated HLA peptides were derived from normal and cancerous origin, as well as tissue specimen. Remarkably, the overlap in detected O-GlcNAcylated HLA peptides as well as their source proteins is strikingly high. Most of the O-GlcNAcylated HLA peptides originate from nuclear proteins, notably transcription factors. From this list, we extract that O-GlcNAcylated HLA Class I peptides are preferentially presented by the HLA-B*07:02 allele. This allele loads peptides with a Proline residue anchor at position 2, and features a binding groove that can accommodate well the recently proposed consensus sequence for O-GlcNAcylation, P(V/A/T/S)g(S/T), essentially explaining why HLA-B*07:02 is a favoured binding allele. The observations drawn from the compiled list, may assist in the prediction of novel O-GlcNAcylated HLA antigens, which will be best presented by patients harbouring HLA-B*07:02 or related alleles that use Proline as anchoring residue.

Cryo-EM structure provides insights into the dimer arrangement of the O-linked β-N-acetylglucosamine transferase OGT

The O-linked β-N-acetylglucosamine modification is a core signalling mechanism, with erroneous patterns leading to cancer and neurodegeneration. Although thousands of proteins are subject to this modification, only a single essential glycosyltransferase catalyses its installation, the O-GlcNAc transferase, OGT. Previous studies have provided truncated structures of OGT through X-ray crystallography, but the full-length protein has never been observed. Here, we report a 5.3 Å cryo-EM model of OGT. We show OGT is a dimer, providing a structural basis for how some X-linked intellectual disability mutations at the interface may contribute to disease. We observe that the catalytic section of OGT abuts a 13.5 tetratricopeptide repeat unit region and find the relative positioning of these sections deviate from the previously proposed, X-ray crystallography-based model. We also note that OGT exhibits considerable heterogeneity in tetratricopeptide repeat units N-terminal to the dimer interface with repercussions for how OGT binds protein ligands and partners.

Truncation of the TPR domain of OGT alters substrate and glycosite selection

O-GlcNAc transferase (OGT) is an essential enzyme that installs O-linked N-acetylglucosamine (O-GlcNAc) to thousands of protein substrates. OGT and its isoforms select from these substrates through the tetratricopeptide repeat (TPR) domain, yet the impact of truncations to the TPR domain on substrate and glycosite selection is unresolved. Here, we report the effects of iterative truncations to the TPR domain of OGT on substrate and glycosite selection with the model protein GFP-JunB and the surrounding O-GlcNAc proteome in U2OS cells. Iterative truncation of the TPR domain of OGT maintains glycosyltransferase activity but alters subcellular localization of OGT in cells. The glycoproteome and glycosites modified by four OGT TPR isoforms were examined on the whole proteome and a single target protein, GFP-JunB. We found the greatest changes in O-GlcNAc on proteins associated with mRNA splicing processes and that the first four TPRs of the canonical nucleocytoplasmic OGT had the broadest substrate scope. Subsequent glycosite analysis revealed that alteration to the last four TPRs corresponded to the greatest shift in the resulting O-GlcNAc consensus sequence. This dataset provides a foundation to analyze how perturbations to the TPR domain and expression of OGT isoforms affect the glycosylation of substrates, which will be critical for future efforts in protein engineering of OGT, the biology of OGT isoforms, and diseases associated with the TPR domain of OGT.

NleB/SseK-catalyzed arginine-glycosylation and enteropathogen virulence are finely tuned by a single variable position contiguous to the catalytic machinery

NleB/SseK effectors are arginine-GlcNAc-transferases expressed by enteric bacterial pathogens that modify host cell proteins to disrupt signaling pathways. While the conserved Citrobacter rodentium NleB and E. coli NleB1 proteins display a broad selectivity towards host proteins, Salmonella enterica SseK1, SseK2, and SseK3 have a narrowed protein substrate selectivity. Here, by combining computational and biophysical experiments, we demonstrate that the broad protein substrate selectivity of NleB relies on Tyr284^NleB/NleB1, a second-shell residue contiguous to the catalytic machinery. Tyr284^NleB/NleB1 is important in coupling protein substrate binding to catalysis. This is exemplified by S286Y^SseK1 and N302Y^SseK2 mutants, which become active towards FADD and DR3 death domains, respectively, and whose kinetic properties match those of enterohemorrhagic E. coli NleB1. The integration of these mutants into S. enterica increases S. enterica survival in macrophages, suggesting that better enzymatic kinetic parameters lead to enhanced virulence. Our findings provide insights into how these enzymes finely tune arginine-glycosylation and, in turn, bacterial virulence. In addition, our data show how promiscuous glycosyltransferases preferentially glycosylate specific protein substrates.

The NleB and SseK glycosyltransferases glycosylate arginine residues of mammalian proteins with different substrate specificities. We uncover that these differences rely on a particular second-shell residue contiguous to the catalytic machinery.

Advances in chemical probing of protein O-GlcNAc glycosylation: structural role and molecular mechanisms

The addition of O-linked-β-D-N-acetylglucosamine (O-GlcNAc) onto serine and threonine residues of nuclear and cytoplasmic proteins is an abundant, unique post-translational modification governing important biological processes. O-GlcNAc dysregulation underlies several metabolic disorders leading to human diseases, including cancer, neurodegeneration and diabetes. This review provides an extensive summary of the recent progress in probing O-GlcNAcylation using mainly chemical methods, with a special focus on discussing mechanistic insights and the structural role of O-GlcNAc at the molecular level. We highlight key aspects of the O-GlcNAc enzymes, including development of OGT and OGA small-molecule inhibitors, and describe a variety of chemoenzymatic and chemical biology approaches for the study of O-GlcNAcylation. Special emphasis is placed on the power of chemistry in the form of synthetic glycopeptide and glycoprotein tools for investigating the site-specific functional consequences of the modification. Finally, we discuss in detail the conformational effects of O-GlcNAc glycosylation on protein structure and stability, relevant O-GlcNAc-mediated protein interactions and its molecular recognition features by biological receptors. Future research in this field will provide novel, more effective chemical strategies and probes for the molecular interrogation of O-GlcNAcylation, elucidating new mechanisms and functional roles of O-GlcNAc with potential therapeutic applications in human health.

O-GlcNAcylation Prediction: An Unattained Objective

BACKGROUND

O-GlcNAcylation is an essential post-translational modification (PTM) in mammalian cells. It consists in the addition of a N-acetylglucosamine (GlcNAc) residue onto serines or threonines by an O-GlcNAc transferase (OGT). Inhibition of OGT is lethal, and misregulation of this PTM can lead to diverse pathologies including diabetes, Alzheimer's disease and cancers. Knowing the location of O-GlcNAcylation sites and the ability to accurately predict them is therefore of prime importance to a better understanding of this process and its related pathologies.

PURPOSE

Here, we present an evaluation of the current predictors of O-GlcNAcylation sites based on a newly built dataset and an investigation to improve predictions.

METHODS

Several datasets of experimentally proven O-GlcNAcylated sites were combined, and the resulting meta-dataset was used to evaluate three prediction tools. We further defined a set of new features following the analysis of the primary to tertiary structures of experimentally proven O-GlcNAcylated sites in order to improve predictions by the use of different types of machine learning techniques.

RESULTS

Our results show the failure of currently available algorithms to predict O-GlcNAcylated sites with a precision exceeding 9%. Our efforts to improve the precision with new features using machine learning techniques do succeed for equal proportions of O-GlcNAcylated and non-O-GlcNAcylated sites but fail like the other tools for real-life proportions where ~1.4% of S/T are O-GlcNAcylated.

CONCLUSION

Present-day algorithms for O-GlcNAcylation prediction narrowly outperform random prediction. The inclusion of additional features, in combination with machine learning algorithms, does not enhance these predictions, emphasizing a pressing need for further development. We hypothesize that the improvement of prediction algorithms requires characterization of OGT's partners.

O-GlcNAcylated peptides and proteins for structural and functional studies

O-GlcNAcylation is an enzymatic post-translational modification occurring in hundreds of protein substrates. This modification occurs through the addition of the monosaccharide N-acetylglucosamine to serine and threonine residues on intracellular proteins in the cytosol, nucleus, and mitochondria. As a highly dynamic form of modification, changes in O-GlcNAc levels coincide with alterations in metabolic state, the presence of stressors, and cellular health. At the protein level, the consequences of the sugar modification can vary, thus necessitating biochemical investigations on protein-specific and site-specific effects. To this end, enzymatic and chemical methods to 'encode' the modification have been developed and the utilization of these synthetic glycopeptides and glycoproteins has since been instrumental in the discovery of the mechanisms by which O-GlcNAcylation can affect a diverse array of biological processes.

Protein Substrates Engage the Lumen of O-GlcNAc Transferase's Tetratricopeptide Repeat Domain in Different Ways

Glycosylation of nuclear and cytoplasmic proteins is an essential post-translational modification in mammals. O-GlcNAc transferase (OGT), the sole enzyme responsible for this modification, glycosylates more than 1000 unique nuclear and cytoplasmic substrates. How OGT selects its substrates is a fundamental question that must be answered to understand OGT's unusual biology. OGT contains a long tetratricopeptide repeat (TPR) domain that has been implicated in substrate selection, but there is almost no information about how changes to this domain affect glycosylation of individual substrates. By profiling O-GlcNAc in cell extracts and probing glycosylation of purified substrates, we show here that ladders of asparagines and aspartates that extend the full length of OGT's TPR lumen control substrate glycosylation. Different substrates are sensitive to changes in different regions of OGT's TPR lumen. We also found that substrates with glycosylation sites close to the C-terminus bypass lumenal binding. Our findings demonstrate that substrates can engage OGT in a variety of different ways for glycosylation.

O-GlcNAcylation: the "stress and nutrition receptor" in cell stress response

O-GlcNAcylation is an atypical, reversible, and dynamic glycosylation that plays a critical role in maintaining the normal physiological functions of cells by regulating various biological processes such as signal transduction, proteasome activity, apoptosis, autophagy, transcription, and translation. It can also respond to environmental changes and physiological signals to play the role of "stress receptor" and "nutrition sensor" in a variety of stress responses and biological processes. Even, a homeostatic disorder of O-GlcNAcylation may cause many diseases. Therefore, O-GlcNAcylation and its regulatory role in stress response are reviewed in this paper.

C-Terminal Tag Location Hampers in Vitro Profiling of OGT Peptide Substrates by mRNA Display

O-GlcNAc transferase (OGT) is the only enzyme that catalyzes the post-translational modification of proteins at Ser/Thr with a single β-N-acetylglucosamine (O-GlcNAcylation). Its activity has been associated with chronic diseases such as cancer, diabetes and neurodegenerative disease. Although numerous OGT substrates have been identified, its accepted substrate scope can still be refined. We report here an attempt to better define the peptide-recognition requirements of the OGT active site by using mRNA display, taking advantage of its extremely high throughput to assess the substrate potential of a library of all possible nonamer peptides. An antibody-based selection process is described here that is able to enrich an OGT substrate peptide from such a library, but with poor absolute recovery. Following four rounds of selection for O-GlcNAcylated peptides, sequencing revealed 14 peptides containing Ser/Thr, but these were shown by luminescence-coupled assays and peptide microarray not to be OGT substrates. By contrast, subsequent testing of an N-terminal tag approach showed exemplary recovery. Our approach demonstrates the power of genetically encoded libraries for selection of peptide substrates, even from a very low initial starting abundance and under suboptimal conditions, and emphasizes the need to consider the binding biases of antibodies and both C- and N-terminal tags in profiling peptide substrates by high-throughput display.

Novel bisubstrate uridine-peptide analogues bearing a pyrophosphate bioisostere as inhibitors of human O-GlcNAc transferase

Protein O-linked β-D-N-acetylglucosamine (O-GlcNAc) modification (O-GlcNAcylation), an essential post-translational as well as cotranslational modification, is the attachment of β-D-N-acetylglucosamine to serine and threonine residues of nucleocytoplasmic proteins. An aberrant O-GlcNAc profile on certain proteins has been implicated in metabolic diseases such as diabetes and cancer. Inhibitors of O-GlcNAc transferase (OGT) are valuable tools to study the cell biology of protein O-GlcNAc modification. In this study we report novel uridine-peptide conjugate molecules composed of an acceptor peptide covalently linked to a catalytically inactive donor substrate analogue that bears a pyrophosphate bioisostere and explore their inhibitory activities against OGT by a radioactive hOGT assay. Further, we investigate the structural basis of their activities via molecular modelling, explaining their lack of potency towards OGT inhibition.

Elucidating the protein substrate recognition of O-GlcNAc transferase (OGT) toward O-GlcNAcase (OGA) using a GlcNAc electrophilic probe

The essential human O-linked β-N-acetylglucosamine (O-GlcNAc) transferase (OGT) is the sole enzyme responsible for modifying thousands of intracellular proteins with the monosaccharide O-GlcNAc. This unique modification plays crucial roles in human health and disease, but the substrate recognition of OGT remains poorly understood. Intriguingly, the only human enzyme reported to remove this modification, O-GlcNAcase (OGA), is O-GlcNAc modified. Here, we exploited a GlcNAc electrophilic probe (GEP1A) to rapidly screen OGT mutants in a fluorescence assay that can discriminate between altered OGT-sugar and -protein substrate binding to help elucidate the binding mode of OGT toward OGA protein substrate. Since OGT tetratricopeptide repeat (TPR) domain plays a key role in OGT-OGA binding, we screened 30 OGT TPR mutants, which revealed 15 "ladder like" asparagine or aspartate residues spanning TPRs 3-7 and 10-13.5 that affect OGA O-GlcNAcylation. By applying a truncated OGA construct, we found that OGA's N-terminal region or pseudo histone acetyltransferase domain is not required for its O-GlcNAcylation, suggesting OGT functionally interacts with OGA through its catalytic and/or stalk domains. This work represents the first effort to systemically investigate each OGT TPR and our findings will facilitate the development of new strategies to investigate the role of substrate-specific O-GlcNAcylation.

Regulating the Regulators: Mechanisms of Substrate Selection of the O-GlcNAc Cycling Enzymes OGT and OGA

Thousands of nuclear and cytosolic proteins are modified with a single β-N-acetylglucosamine on serine and threonine residues in mammals, a modification termed O-GlcNAc. This modification is essential for normal development and plays important roles in virtually all intracellular processes. Additionally, O-GlcNAc is involved in many disease states, including cancer, diabetes, and X-linked intellectual disability. Given the myriad of functions of the O-GlcNAc modification, it is therefore somewhat surprising that O-GlcNAc cycling is mediated by only two enzymes: the O-GlcNAc transferase (OGT), which adds O-GlcNAc, and the O-GlcNAcase (OGA), which removes it. A significant outstanding question in the O-GlcNAc field is how do only two enzymes mediate such an abundant and dynamic modification. In this review, we explore the current understanding of mechanisms for substrate selection for the O-GlcNAc cycling enzymes. These mechanisms include direct substrate interaction with specific domains of OGT or OGA, selection of interactors via partner proteins, posttranslational modification of OGT or OGA, nutrient sensing, and localization alteration. Altogether, current research paints a picture of an exquisitely regulated and complex system by which OGT and OGA select substrates. We also make recommendations for future work, toward the goal of identifying interaction mechanisms for specific substrates that may be able to be exploited for various research and medical treatment goals.

The human O-GlcNAcome database and meta-analysis

Over the past 35 years, ~1700 articles have characterized protein O-GlcNAcylation. Found in almost all living organisms, this post-translational modification of serine and threonine residues is highly conserved and key to biological processes. With half of the primary research articles using human models, the O-GlcNAcome recently reached a milestone of 5000 human proteins identified. Herein, we provide an extensive inventory of human O-GlcNAcylated proteins, their O-GlcNAc sites, identification methods, and corresponding references ( www.oglcnac.mcw.edu ). In the absence of a comprehensive online resource for O-GlcNAcylated proteins, this list serves as the only database of O-GlcNAcylated proteins. Based on the thorough analysis of the amino acid sequence surrounding 7002 O-GlcNAc sites, we progress toward a more robust semi-consensus sequence for O-GlcNAcylation. Moreover, we offer a comprehensive meta-analysis of human O-GlcNAcylated proteins for protein domains, cellular and tissue distribution, and pathways in health and diseases, reinforcing that O-GlcNAcylation is a master regulator of cell signaling, equal to the widely studied phosphorylation.

Novel nucleocytoplasmic protein O-fucosylation by SPINDLY regulates diverse developmental processes in plants

In metazoans, protein O-fucosylation of Ser/Thr residues was only found in secreted or cell surface proteins, and this post-translational modification is catalyzed by ER-localized protein O-fucosyltransferases (POFUTs) in the GT65 family. Recently, a novel nucleocytoplasmic POFUT, SPINDLY (SPY), was identified in the reference plant Arabidopsis thaliana to modify nuclear transcription regulators DELLAs, revealing a new regulatory mechanism for gene expression. The paralog of AtSPY, SECRET AGENT (SEC), is an O-link-N-acetylglucosamine (GlcNAc) transferase (OGT), which O-GlcNAcylates Ser/Thr residues of target proteins. Both AtSPY and AtSEC are tetratricopeptide repeat-domain-containing glycosyltransferases in the GT41 family. The discovery that AtSPY is a POFUT clarified decades of miss-classification of AtSPY as an OGT. SPY and SEC play pleiotropic roles in plant development, and the interactions between SPY and SEC are complex. SPY-like genes are conserved in diverse organisms, except in fungi and metazoans, suggesting that O-fucosylation is a common mechanism in modulating intracellular protein functions.

Semiprocessive Hyperglycosylation of Adhesin by Bacterial Protein N-Glycosyltransferases

Processivity is an important feature of enzyme families such as DNA polymerases, polysaccharide synthases, and protein kinases, to ensure high fidelity in biopolymer synthesis and modification. Here, we reveal processive character in the family of cytoplasmic protein N-glycosyltransferases (NGTs). Through various activity assays, intact protein mass spectrometry, and proteomics analysis, we established that NGTs from nontypeable Haemophilus influenzae and Actinobacillus pleuropneumoniae modify an adhesin protein fragment in a semiprocessive manner. Molecular modeling studies suggest that the processivity arises from the shallow substrate binding groove in NGT, which promotes the sliding of the adhesin over the surface to allow further glycosylations without temporary dissociation. We hypothesize that the processive character of these bacterial protein glycosyltransferases is the mechanism to ensure multisite glycosylation of adhesins in vivo, thereby creating the densely glycosylated proteins necessary for bacterial self-aggregation and adherence to human cells, as a first step toward infection.

Bisubstrate Ether-Linked Uridine-Peptide Conjugates as O-GlcNAc Transferase Inhibitors

The O-linked β-N-acetylglucosamine (O-GlcNAc) transferase (OGT) is a master regulator of installing O-GlcNAc onto serine or threonine residues on a multitude of target proteins. Numerous nuclear and cytosolic proteins of varying functional classes, including translational factors, transcription factors, signaling proteins, and kinases are OGT substrates. Aberrant O-GlcNAcylation of proteins is implicated in signaling in metabolic diseases such as diabetes and cancer. Selective and potent OGT inhibitors are valuable tools to study the role of OGT in modulating a wide range of effects on cellular functions. We report linear bisubstrate ether-linked uridine-peptide conjugates as OGT inhibitors with micromolar affinity. In vitro evaluation of the compounds revealed the importance of donor substrate, linker and acceptor substrate in the rational design of bisubstrate analogue inhibitors. Molecular dynamics simulations shed light on the binding of this novel class of inhibitors and rationalized the effect of amino acid truncation of acceptor peptide on OGT inhibition.

Molecular Interrogation to Crack the Case of O-GlcNAc

The O-linked β-N-acetylglucosamine (O-GlcNAc) modification, termed O-GlcNAcylation, is an essential and dynamic post-translational modification in cells. O-GlcNAc transferase (OGT) installs this modification on serine and threonine residues, whereas O-GlcNAcase (OGA) hydrolyzes it. O-GlcNAc modifications are found on thousands of intracellular proteins involved in diverse biological processes. Dysregulation of O-GlcNAcylation and O-GlcNAc cycling enzymes has been detected in many diseases, including cancer, diabetes, cardiovascular and neurodegenerative diseases. Here, recent advances in the development of molecular tools to investigate OGT and OGA functions and substrate recognition are discussed. New chemical approaches to study O-GlcNAc dynamics and its potential roles in the immune system are also highlighted. It is hoped that this minireview will encourage more research in these areas to advance the understanding of O-GlcNAc in biology and diseases.

Tools for functional dissection of site-specific O-GlcNAcylation

Protein O-GlcNAcylation is an abundant post-translational modification of intracellular proteins with the monosaccharide N-acetylglucosamine covalently tethered to serines and threonines. Modification of proteins with O-GlcNAc is required for metazoan embryo development and maintains cellular homeostasis through effects on transcription, signalling and stress response. While disruption of O-GlcNAc homeostasis can have detrimental impact on cell physiology and cause various diseases, little is known about the functions of individual O-GlcNAc sites. Most of the sites are modified sub-stoichiometrically which is a major challenge to the dissection of O-GlcNAc function. Here, we discuss the application, advantages and limitations of the currently available tools and technologies utilised to dissect the function of O-GlcNAc on individual proteins and sites in vitro and in vivo. Additionally, we provide a perspective on future developments required to decipher the protein- and site-specific roles of this essential sugar modification.

Control of protein palmitoylation by regulating substrate recruitment to a zDHHC-protein acyltransferase

Although palmitoylation regulates numerous cellular processes, as yet efforts to manipulate this post-translational modification for therapeutic gain have proved unsuccessful. The Na-pump accessory sub-unit phospholemman (PLM) is palmitoylated by zDHHC5. Here, we show that PLM palmitoylation is facilitated by recruitment of the Na-pump α sub-unit to a specific site on zDHHC5 that contains a juxtamembrane amphipathic helix. Site-specific palmitoylation and GlcNAcylation of this helix increased binding between the Na-pump and zDHHC5, promoting PLM palmitoylation. In contrast, disruption of the zDHHC5-Na-pump interaction with a cell penetrating peptide reduced PLM palmitoylation. Our results suggest that by manipulating the recruitment of specific substrates to particular zDHHC-palmitoyl acyl transferases, the palmitoylation status of individual proteins can be selectively altered, thus opening the door to the development of molecular modulators of protein palmitoylation for the treatment of disease.

Role of O-Linked N-Acetylglucosamine Protein Modification in Cellular (Patho)Physiology

In the mid-1980s, the identification of serine and threonine residues on nuclear and cytoplasmic proteins modified by a N-acetylglucosamine moiety (O-GlcNAc) via an O-linkage overturned the widely held assumption that glycosylation only occurred in the endoplasmic reticulum, Golgi apparatus, and secretory pathways. In contrast to traditional glycosylation, the O-GlcNAc modification does not lead to complex, branched glycan structures and is rapidly cycled on and off proteins by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), respectively. Since its discovery, O-GlcNAcylation has been shown to contribute to numerous cellular functions, including signaling, protein localization and stability, transcription, chromatin remodeling, mitochondrial function, and cell survival. Dysregulation in O-GlcNAc cycling has been implicated in the progression of a wide range of diseases, such as diabetes, diabetic complications, cancer, cardiovascular, and neurodegenerative diseases. This review will outline our current understanding of the processes involved in regulating O-GlcNAc turnover, the role of O-GlcNAcylation in regulating cellular physiology, and how dysregulation in O-GlcNAc cycling contributes to pathophysiological processes.

Synthetic Glycobiology: Parts, Systems, and Applications

Protein glycosylation, the attachment of sugars to amino acid side chains, can endow proteins with a wide variety of properties of great interest to the engineering biology community. However, natural glycosylation systems are limited in the diversity of glycoproteins they can synthesize, the scale at which they can be harnessed for biotechnology, and the homogeneity of glycoprotein structures they can produce. Here we provide an overview of the emerging field of synthetic glycobiology, the application of synthetic biology tools and design principles to better understand and engineer glycosylation. Specifically, we focus on how the biosynthetic and analytical tools of synthetic biology have been used to redesign glycosylation systems to obtain defined glycosylation structures on proteins for diverse applications in medicine, materials, and diagnostics. We review the key biological parts available to synthetic biologists interested in engineering glycoproteins to solve compelling problems in glycoscience, describe recent efforts to construct synthetic glycoprotein synthesis systems, and outline exemplary applications as well as new opportunities in this emerging space.

O-GlcNAcylation and its role in the immune system

O-linked-N-acetylglucosaminylation (O-GlcNAcylation) is a type of glycosylation that occurs when a monosaccharide, O-GlcNAc, is added onto serine or threonine residues of nuclear or cytoplasmic proteins by O-GlcNAc transferase (OGT) and which can be reversibly removed by O-GlcNAcase (OGA). O-GlcNAcylation couples the processes of nutrient sensing, metabolism, signal transduction and transcription, and plays important roles in development, normal physiology and physiopathology. Cumulative studies have indicated that O-GlcNAcylation affects the functions of protein substrates in a number of ways, including protein cellular localization, protein stability and protein/protein interaction. Particularly, O-GlcNAcylation has been shown to have intricate crosstalk with phosphorylation as they both modify serine or threonine residues. Aberrant O-GlcNAcylation on various protein substrates has been implicated in many diseases, including neurodegenerative diseases, diabetes and cancers. However, the role of protein O-GlcNAcylation in immune cell lineages has been less explored. This review summarizes the current understanding of the fundamental biochemistry of O-GlcNAcylation, and discusses the molecular mechanisms by which O-GlcNAcylation regulates the development, maturation and functions of immune cells. In brief, O-GlcNAcylation promotes the development, proliferation, and activation of T and B cells. O-GlcNAcylation regulates inflammatory and antiviral responses of macrophages. O-GlcNAcylation promotes the function of activated neutrophils, but inhibits the activity of nature killer cells.

Engineering a Proximity-Directed O-GlcNAc Transferase for Selective Protein O-GlcNAcylation in Cells

O-Linked β-N-acetylglucosamine (O-GlcNAc) is a monosaccharide that plays an essential role in cellular signaling throughout the nucleocytoplasmic proteome of eukaryotic cells. Strategies for selectively increasing O-GlcNAc levels on a target protein in cells would accelerate studies of this essential modification. Here, we report a generalizable strategy for introducing O-GlcNAc into selected target proteins in cells using a nanobody as a proximity-directing agent fused to O-GlcNAc transferase (OGT). Fusion of a nanobody that recognizes GFP (nGFP) or a nanobody that recognizes the four-amino acid sequence EPEA (nEPEA) to OGT yielded nanobody-OGT constructs that selectively delivered O-GlcNAc to a series of tagged target proteins (e.g., JunB, cJun, and Nup62). Truncation of the tetratricopeptide repeat domain as in OGT(4) increased selectivity for the target protein through the nanobody by reducing global elevation of O-GlcNAc levels in the cell. Quantitative chemical proteomics confirmed the increase in O-GlcNAc to the target protein by nanobody-OGT(4). Glycoproteomics revealed that nanobody-OGT(4) or full-length OGT produced a similar glycosite profile on the target protein JunB and Nup62. Finally, we demonstrate the ability to selectively target endogenous α-synuclein for O-GlcNAcylation in HEK293T cells. These first proximity-directed OGT constructs provide a flexible strategy for targeting additional proteins and a template for further engineering of OGT and the O-GlcNAc proteome in the future. The use of a nanobody to redirect OGT substrate selection for glycosylation of desired proteins in cells may further constitute a generalizable strategy for controlling a broader array of post-translational modifications in cells.

O-GlcNAcylation as a Therapeutic Target for Alzheimer's Disease

Alzheimer's disease (AD) is the most common cause of dementia and the number of elderly patients suffering from AD has been steadily increasing. Despite worldwide efforts to cope with this disease, little progress has been achieved with regard to identification of effective therapeutics. Thus, active research focusing on identification of new therapeutic targets of AD is ongoing. Among the new targets, post-translational modifications which modify the properties of mature proteins have gained attention. O-GlcNAcylation, a type of PTM that attaches O-linked β-N-acetylglucosamine (O-GlcNAc) to a protein, is being sought as a new target to treat AD pathologies. O-GlcNAcylation has been known to modify the two important components of AD pathological hallmarks, amyloid precursor protein, and tau protein. In addition, elevating O-GlcNAcylation levels in AD animal models has been shown to be effective in alleviating AD-associated pathology. Although studies investigating the precise mechanism of reversal of AD pathologies by targeting O-GlcNAcylation are not yet complete, it is clearly important to examine O-GlcNAcylation regulation as a target of AD therapeutics. This review highlights the mechanisms of O-GlcNAcylation and its role as a potential therapeutic target under physiological and pathological AD conditions.

Loss of CRMP2 O-GlcNAcylation leads to reduced novel object recognition performance in mice

O-GlcNAcylation is an abundant post-translational modification in the nervous system, linked to both neurodevelopmental and neurodegenerative disease. However, the mechanistic links between these phenotypes and site-specific O-GlcNAcylation remain largely unexplored. Here, we show that Ser517 O-GlcNAcylation of the microtubule-binding protein Collapsin Response Mediator Protein-2 (CRMP2) increases with age. By generating and characterizing a Crmp2S517A knock-in mouse model, we demonstrate that loss of O-GlcNAcylation leads to a small decrease in body weight and mild memory impairment, suggesting that Ser517 O-GlcNAcylation has a small but detectable impact on mouse physiology and cognitive function.

A missense mutation in the catalytic domain of O-GlcNAc transferase links perturbations in protein O-GlcNAcylation to X-linked intellectual disability

X‐linked intellectual disabilities (XLID) are common developmental disorders. The enzyme O‐GlcNAc transferase encoded by OGT, a recently discovered XLID gene, attaches O‐GlcNAc to nuclear and cytoplasmic proteins. As few missense mutations have been described, it is unclear what the aetiology of the patient phenotypes is. Here, we report the discovery of a missense mutation in the catalytic domain of OGT in an XLID patient. X‐ray crystallography reveals that this variant leads to structural rearrangements in the catalytic domain. The mutation reduces in vitro OGT activity on substrate peptides/protein. Mouse embryonic stem cells carrying the mutation reveal reduced O‐GlcNAcase (OGA) and global O‐GlcNAc levels. These data suggest a direct link between changes in the O‐GlcNAcome and intellectual disability observed in patients carrying OGT mutations.

Molecular mechanisms regulating O-linked N-acetylglucosamine (O-GlcNAc)-processing enzymes

The post-translational modification of proteins by O-linked N-acetylglucosamine (O-GlcNAc) dynamically programmes cellular physiology to maintain homoeostasis and tailor biochemical pathways to meet context-dependent cellular needs. Despite diverse roles of played by O-GlcNAc, only two enzymes act antagonistically to govern its cycling; O-GlcNAc transferase installs the monosaccharide on target proteins, and O-GlcNAc hydrolase removes it. The recent literature has exposed a network of mechanisms regulating these two enzymes to choreograph global, and target-specific, O-GlcNAc cycling in response to cellular stress and nutrient availability. Herein, we amalgamate these emerging mechanisms from a structural and molecular perspective to explore how the cell exerts fine control to regulate O-GlcNAcylation of diverse proteins in a selective fashion.

Catalytic deficiency of O-GlcNAc transferase leads to X-linked intellectual disability

O-GlcNAc transferase (OGT) is an X-linked gene product that is essential for normal development of the vertebrate embryo. It catalyses the O-GlcNAc posttranslational modification of nucleocytoplasmic proteins and proteolytic maturation of the transcriptional coregulator Host cell factor 1 (HCF1). Recent studies have suggested that conservative missense mutations distal to the OGT catalytic domain lead to X-linked intellectual disability in boys, but it is not clear if this is through changes in the O-GlcNAc proteome, loss of protein-protein interactions, or misprocessing of HCF1. Here, we report an OGT catalytic domain missense mutation in monozygotic female twins (c. X:70779215 T > A, p. N567K) with intellectual disability that allows dissection of these effects. The patients show limited IQ with developmental delay and skewed X-inactivation. Molecular analyses revealed decreased OGT stability and disruption of the substrate binding site, resulting in loss of catalytic activity. Editing this mutation into the Drosophila genome results in global changes in the O-GlcNAc proteome, while in mouse embryonic stem cells it leads to loss of O-GlcNAcase and delayed differentiation down the neuronal lineage. These data imply that catalytic deficiency of OGT could contribute to X-linked intellectual disability.