Quantitative regulation of nuclear pore complex proteins by O-GlcNAcylation

Nuclear pore dysfunction and disease: a complex opportunity

The separation of genetic material from bulk cytoplasm has enabled the evolution of increasingly complex organisms, allowing for the development of sophisticated forms of life. However, this complexity has created new categories of dysfunction, including those related to the movement of material between cellular compartments. In eukaryotic cells, nucleocytoplasmic trafficking is a fundamental biological process, and cumulative disruptions to nuclear integrity and nucleocytoplasmic transport are detrimental to cell survival. This is particularly true in post-mitotic neurons, where nuclear pore injury and errors to nucleocytoplasmic trafficking are strongly associated with neurodegenerative disease. In this review, we summarize the current understanding of nuclear pore biology in physiological and pathological contexts and discuss potential therapeutic approaches for addressing nuclear pore injury and dysfunctional nucleocytoplasmic transport.

Understanding and exploiting the roles of O-GlcNAc in neurodegenerative diseases

O-GlcNAc is a common modification found on nuclear and cytoplasmic proteins. Determining the catalytic mechanism of the enzyme O-GlcNAcase (OGA), which removes O-GlcNAc from proteins, enabled the creation of potent and selective inhibitors of this regulatory enzyme. Such inhibitors have served as important tools in helping to uncover the cellular and organismal physiological roles of this modification. In addition, OGA inhibitors have been important for defining the augmentation of O-GlcNAc as a promising disease-modifying approach to combat several neurodegenerative diseases including both Alzheimer's disease and Parkinson's disease. These studies have led to development and optimization of OGA inhibitors for clinical application. These compounds have been shown to be well tolerated in early clinical studies and are steadily advancing into the clinic. Despite these advances, the mechanisms by which O-GlcNAc protects against these various types of neurodegeneration are a topic of continuing interest since improved insight may enable the creation of more targeted strategies to modulate O-GlcNAc for therapeutic benefit. Relevant pathways on which O-GlcNAc has been found to exert beneficial effects include autophagy, necroptosis, and processing of the amyloid precursor protein. More recently, the development and application of chemical methods enabling the synthesis of homogenous proteins have clarified the biochemical effects of O-GlcNAc on protein aggregation and uncovered new roles for O-GlcNAc in heat shock response. Here, we discuss the features of O-GlcNAc in neurodegenerative diseases, the application of inhibitors to identify the roles of this modification, and the biochemical effects of O-GlcNAc on proteins and pathways associated with neurodegeneration.

Overexpressed Nup88 stabilized through interaction with Nup62 promotes NF-κB dependent pathways in cancer

Bidirectional nucleo-cytoplasmic transport, regulating several vital cellular processes, is mediated by the Nuclear Pore Complex (NPC) comprising the nucleoporin (Nup) proteins. Nup88, a constituent nucleoporin, is overexpressed in many cancers, and a positive correlation exists between progressive stages of cancer and Nup88 levels. While a significant link of Nup88 overexpression in head and neck cancer exists but mechanistic details of Nup88 roles in tumorigenesis are sparse. Here, we report that Nup88 and Nup62 levels are significantly elevated in head and neck cancer patient samples and cell lines. We demonstrate that the elevated levels of Nup88 or Nup62 impart proliferation and migration advantages to cells. Interestingly, Nup88-Nup62 engage in a strong interaction independent of Nup-glycosylation status and cell-cycle stages. We report that the interaction with Nup62 stabilizes Nup88 by inhibiting the proteasome-mediated degradation of overexpressed Nup88. Overexpressed Nup88 stabilized by interaction with Nup62 can interact with NF-κB (p65) and sequesters p65 partly into nucleus of unstimulated cells. NF-κB targets like Akt, c-myc, IL-6 and BIRC3 promoting proliferation and growth are induced under Nup88 overexpression conditions. In conclusion, our data indicates that simultaneous overexpression of Nup62 and Nup88 in head and neck cancer stabilizes Nup88. Stabilized Nup88 interacts and activates p65 pathway, which perhaps is the underlying mechanism in Nup88 overexpressing tumors.

Spatial and temporal proteomics reveals the distinct distributions and dynamics of O-GlcNAcylated proteins

Protein O-GlcNAcylation plays critical roles in many cellular events, and its dysregulation is related to multiple diseases. Integrating bioorthogonal chemistry and multiplexed proteomics, we systematically and site specifically study the distributions and dynamics of protein O-GlcNAcylation in the nucleus and the cytoplasm of human cells. The results demonstrate that O-GlcNAcylated proteins with different functions have distinct distribution patterns. The distributions vary site specifically, indicating that different glycoforms of the same protein may have different distributions. Moreover, we comprehensively analyze the dynamics of O-GlcNAcylated and non-modified proteins in these two compartments, respectively, and the half-lives of glycoproteins in different compartments are markedly different, with the median half-life in the cytoplasm being much longer. In addition, glycoproteins in the nucleus are more dramatically stabilized than those in the cytoplasm under the O-GlcNAcase inhibition. The comprehensive spatial and temporal analyses of protein O-GlcNAcylation provide valuable information and advance our understanding of this important modification.

Xu et al. systematically and site specifically study the distribution and dynamics of O-GlcNAcylated proteins in the nucleus and the cytoplasm. O-GlcNAcylated proteins with different functions have distinct distribution patterns. The half-lives of glycoproteins in the two cellular compartments are markedly different, with the much longer median half-life in the cytoplasm.

The Nuclear Pore Complex: Birth, Life, and Death of a Cellular Behemoth

Nuclear pore complexes (NPCs) are the only transport channels that cross the nuclear envelope. Constructed from ~500–1000 nucleoporin proteins each, they are among the largest macromolecular assemblies in eukaryotic cells. Thanks to advances in structural analysis approaches, the construction principles and architecture of the NPC have recently been revealed at submolecular resolution. Although the overall structure and inventory of nucleoporins are conserved, NPCs exhibit significant compositional and functional plasticity even within single cells and surprising variability in their assembly pathways. Once assembled, NPCs remain seemingly unexchangeable in post-mitotic cells. There are a number of as yet unresolved questions about how the versatility of NPC assembly and composition is established, how cells monitor the functional state of NPCs or how they could be renewed. Here, we review current progress in our understanding of the key aspects of NPC architecture and lifecycle.

Structure, Maintenance, and Regulation of Nuclear Pore Complexes: The Gatekeepers of the Eukaryotic Genome

In eukaryotic cells, the genetic material is segregated inside the nucleus. This compartmentalization of the genome requires a transport system that allows cells to move molecules across the nuclear envelope, the membrane-based barrier that surrounds the chromosomes. Nuclear pore complexes (NPCs) are the central component of the nuclear transport machinery. These large protein channels penetrate the nuclear envelope, creating a passage between the nucleus and the cytoplasm through which nucleocytoplasmic molecule exchange occurs. NPCs are one of the largest protein assemblies of eukaryotic cells and, in addition to their critical function in nuclear transport, these structures also play key roles in many cellular processes in a transport-independent manner. Here we will review the current knowledge of the NPC structure, the cellular mechanisms that regulate their formation and maintenance, and we will provide a brief description of a variety of processes that NPCs regulate.

Regulation of O-Linked N-Acetyl Glucosamine Transferase (OGT) through E6 Stimulation of the Ubiquitin Ligase Activity of E6AP

Glycosyltransferase OGT catalyzes the conjugation of O-linked β-D-N-acetylglucosamine (O-GlcNAc) to Ser and Thr residues of the cellular proteins and regulates many key processes in the cell. Here, we report the identification of OGT as a ubiquitination target of HECT-type E3 ubiquitin (UB) ligase E6AP, whose overexpression in HEK293 cells would induce the degradation of OGT. We also found that the expression of E6AP in HeLa cells with the endogenous expression of the E6 protein of the human papillomavirus (HPV) would accelerate OGT degradation by the proteasome and suppress O-GlcNAc modification of OGT substrates in the cell. Overall, our study establishes a new mechanism of OGT regulation by the ubiquitin–proteasome system (UPS) that mediates the crosstalk between protein ubiquitination and O-GlcNAcylation pathways underlying diverse cellular processes.

What sugar does to your pores

FG-repeat nucleoporins at the center of the nuclear pore complex (NPC) are highly modified with O-GlcNAc. In this issue, Yoo and Mitchison (2021. J. Cell Biol.https://doi.org/10.1083/jcb.202010141) use optogenetic probes to show that O-GlcNAc enhances permeability of the NPC, accelerating transport in both directions.

O-GlcNAc modification of nuclear pore complexes accelerates bidirectional transport

Macromolecular transport across the nuclear envelope depends on facilitated diffusion through nuclear pore complexes (NPCs). The interior of NPCs contains a permeability barrier made of phenylalanine-glycine (FG) repeat domains that selectively facilitates the permeation of cargoes bound to nuclear transport receptors (NTRs). FG-repeat domains in NPCs are a major site of O-linked N-acetylglucosamine (O-GlcNAc) modification, but the functional role of this modification in nucleocytoplasmic transport is unclear. We developed high-throughput assays based on optogenetic probes to quantify the kinetics of nuclear import and export in living human cells. We found that increasing O-GlcNAc modification of the NPC accelerated NTR-facilitated transport of proteins in both directions, and decreasing modification slowed transport. Superresolution imaging revealed strong enrichment of O-GlcNAc at the FG-repeat barrier. O-GlcNAc modification also accelerated passive permeation of a small, inert protein through NPCs. We conclude that O-GlcNAc modification accelerates nucleocytoplasmic transport by enhancing the nonspecific permeability of the FG-repeat barrier, perhaps by steric inhibition of interactions between FG repeats.

O-GlcNAcylation of Thr12/Ser56 in short-form O-GlcNAc transferase (sOGT) regulates its substrate selectivity

O-GlcNAcylation is a ubiquitous protein glycosylation playing different roles on variant proteins. O-GlcNAc transferase (OGT) is the unique enzyme responsible for the sugar addition to nucleocytoplasmic proteins. Recently, multiple O-GlcNAc sites have been observed on short-form OGT (sOGT) and nucleocytoplasmic OGT (ncOGT), both of which locate in the nucleus and cytoplasm in cell. Moreover, O-GlcNAcylation of Ser³⁸⁹ in ncOGT (1036 amino acids) affects its nuclear translocation in HeLa cells. To date, the major O-GlcNAcylation sites and their roles in sOGT remain unknown. Here, we performed LC-MS/MS and mutational analyses to seek the major O-GlcNAcylation site on sOGT. We identified six O-GlcNAc sites in the tetratricopeptide repeat domain in sOGT, with Thr¹² and Ser⁵⁶ being two "key" sites. Thr¹² is a dominant O-GlcNAcylation site, whereas the modification of Ser⁵⁶ plays a role in regulating sOGT O-GlcNAcylation, partly through Thr¹² In vitro activity and pulldown assays demonstrated that O-GlcNAcylation does not affect sOGT activity but does affect sOGT-interacting proteins. In HEK293T cells, S56A bound to and hence glycosylated more proteins in contrast to T12A and WT sOGT. By proteomic and bioinformatics analyses, we found that T12A and S56A differed in substrate proteins (e.g. HNRNPU and PDCD6IP), which eventually affected cell cycle progression and/or cell proliferation. These findings demonstrate that O-GlcNAcylation modulates sOGT substrate selectivity and affects its role in the cell. The data also highlight the regulatory role of O-GlcNAcylation at Thr¹² and Ser⁵⁶.

The RNA helicase UPF1 associates with mRNAs co-transcriptionally and is required for the release of mRNAs from gene loci

UPF1 is an RNA helicase that is required for nonsense-mediated mRNA decay (NMD) in eukaryotes, and the predominant view is that UPF1 mainly operates on the 3'UTRs of mRNAs that are directed for NMD in the cytoplasm. Here we offer evidence, obtained from Drosophila, that UPF1 constantly moves between the nucleus and cytoplasm by a mechanism that requires its RNA helicase activity. UPF1 is associated, genome-wide, with nascent RNAs at most of the active Pol II transcription sites and at some Pol III-transcribed genes, as demonstrated microscopically on the polytene chromosomes of salivary glands and by ChIP-seq analysis in S2 cells. Intron recognition seems to interfere with association and translocation of UPF1 on nascent pre-mRNAs, and cells depleted of UPF1 show defects in the release of mRNAs from transcription sites and their export from the nucleus.

Evolutionary divergence of the nuclear pore complex from fungi to metazoans

Nuclear pore complex (NPC) is the largest multimeric protein assembly of the eukaryotic cell, which mediates the nucleocytoplasmic transport. The constituent proteins of this assembly (nucleoporins) are present in varying copy numbers to give a size from ~ 60 MDa (yeast) to 112 MDa (human) and share common ancestry with other membrane‐associated complexes such as COPI/COPII and thus share the same structural folds. However, the nucleoporins across species exhibit very low percentage sequence similarity and this reflects in their distinct secondary structure and domain organization. We employed thorough sequence and phylogenetic analysis guided from structure‐based alignments of all the nucleoporins from fungi to metazoans to understand the evolution of NPC. Through evolutionary pressure analysis on various nucleoporins, we deduced that these proteins are under differential selection pressure and hence the homologous interacting partners do not complement each other in the in vitro pull‐down assay. The super tree analysis of all nucleoporins taken together illustrates divergent evolution of nucleoporins and notably, the degree of divergence is more apparent in higher order organisms as compared to lower species. Overall, our results support the hypothesis that the protein–protein interactions in such large multimeric assemblies are species specific in nature and hence their structure and function should also be studied in an organism‐specific manner.

Epigenetic Contribution to the Development and Progression of Vascular Diabetic Complications

SIGNIFICANCE

The number of people suffering from diabetes worldwide is steadily rising. Complications from diabetes, including cardiovascular and renal disease, contribute to the high morbidity and mortality associated with this disease. Recent Advances: Hyperglycemia promotes tissue damage through diverse mechanisms involving increased production of reactive oxygen species. Increased oxidative stress drives changes in chromatin structure that mediate gene expression changes leading to the upregulation of proinflammatory and profibrotic mediators. The epigenetic contribution to diabetes-induced changes in gene expression is increasingly recognized as a key factor in the development and progression of vascular diabetic complications.

CRITICAL ISSUES

The mechanisms through which stimuli from the diabetic milieu promote epigenetic changes remain poorly understood. In addition, glycemic control constitutes an important factor influencing epigenetic states in diabetes, and the phenomenon of hyperglycemic memory warrants further research.

FUTURE DIRECTIONS

Knowledge of the molecular mechanisms underlying epigenetic changes in diabetes may allow the design of novel therapeutic strategies to reduce the burden of diabetic complications. Furthermore, certain epigenetic markers are detected early during the onset of diabetes and its complications and may prove useful as biomarkers for disease risk prediction.

Inhibition of O-GlcNAc transferase activity reprograms prostate cancer cell metabolism

Metabolic networks are highly connected and complex, but a single enzyme, O-GlcNAc transferase (OGT) can sense the availability of metabolites and also modify target proteins. We show that inhibition of OGT activity inhibits the proliferation of prostate cancer cells, leads to sustained loss of c-MYC and suppresses the expression of CDK1, elevated expression of which predicts prostate cancer recurrence (p=0.00179). Metabolic profiling revealed decreased glucose consumption and lactate production after OGT inhibition. This decreased glycolytic activity specifically sensitized prostate cancer cells, but not cells representing normal prostate epithelium, to inhibitors of oxidative phosphorylation (rotenone and metformin). Intra-cellular alanine was depleted upon OGT inhibitor treatment. OGT inhibitor increased the expression and activity of alanine aminotransferase (GPT2), an enzyme that can be targeted with a clinically approved drug, cycloserine. Simultaneous inhibition of OGT and GPT2 inhibited cell viability and growth rate, and additionally activated a cell death response. These combinatorial effects were predominantly seen in prostate cancer cells, but not in a cell-line derived from normal prostate epithelium. Combinatorial treatments were confirmed with two inhibitors against both OGT and GPT2. Taken together, here we report the reprogramming of energy metabolism upon inhibition of OGT activity, and identify synergistically lethal combinations that are prostate cancer cell specific.

A Sweet Embrace: Control of Protein-Protein Interactions by O-Linked β-N-Acetylglucosamine

O-Linked β-N-acetylglucosamine (O-GlcNAc) is a critical post-translational modification (PTM) of thousands of intracellular proteins. Reversible O-GlcNAcylation governs many aspects of cell physiology and is dysregulated in numerous human diseases. Despite this broad pathophysiological significance, major aspects of O-GlcNAc signaling remain poorly understood, including the biochemical mechanisms through which O-GlcNAc transduces information. Recent work from many laboratories, including our own, has revealed that O-GlcNAc, like other intracellular PTMs, can control its substrates' functions by inhibiting or inducing protein-protein interactions. This dynamic regulation of multiprotein complexes exerts diverse downstream signaling effects in a range of processes, cell types, and organisms. Here, we review the literature about O-GlcNAc-regulated protein-protein interactions and suggest important questions for future studies in the field.

WGA-based lectin affinity gel electrophoresis: A novel method for the detection of O-GlcNAc-modified proteins

Post-translational modification with O-linked β-N-acetylglucosamine (O-GlcNAc) occurs selectively on serine and/or threonine residues of cytoplasmic and nuclear proteins, and dynamically regulates their molecular functions. Since conventional strategies to evaluate the O-GlcNAcylation level of a specific protein require time-consuming steps, the development of a rapid and easy method for the detection and quantification of an O-GlcNAcylated protein has been a challenging issue. Here, we describe a novel method in which O-GlcNAcylated and non-O-GlcNAcylated forms of proteins are separated by lectin affinity gel electrophoresis using wheat germ agglutinin (WGA), which primarily binds to N-acetylglucosamine residues. Electrophoresis of cell lysates through a gel containing copolymerized WGA selectively induced retardation of the mobility of O-GlcNAcylated proteins, thereby allowing the simultaneous visualization of both the O-GlcNAcylated and the unmodified forms of proteins. This method is therefore useful for the quantitative detection of O-GlcNAcylated proteins.

O-GlcNAc cycling and the regulation of nucleocytoplasmic dynamics

The dynamic carbohydrate post-translational modification (PTM) O-linked β-N-acetyl glucosamine (O-GlcNAc) is found on thousands of proteins throughout the nucleus and cytoplasm, and rivals phosphorylation in terms of the number of substrates and pathways influenced. O-GlcNAc is highly conserved and essential in most organisms, with disruption of O-GlcNAc cycling linked to diseases ranging from cancer to neurodegeneration. Nuclear pore proteins were the first identified O-GlcNAc-modified substrates, generating intense and ongoing interest in understanding the role of O-GlcNAc cycling in nuclear pore complex structure and function. Recent advances in detecting and altering O-GlcNAcylation levels have provided insights into many mechanisms by which O-GlcNAcylation influences the nucleocytoplasmic localization and stability of protein targets. The emerging view is that the multifunctional enzymes of O-GlcNAc cycling are critical nutrient-sensing components of a complex network of signaling cascades involving multiple PTMs. Furthermore, O-GlcNAc plays a role in maintaining the structural integrity of the nuclear pore and regulating its function as the gatekeeper of nucleocytoplasmic trafficking.

Mutant Huntingtin Disrupts the Nuclear Pore Complex

Huntington's disease (HD) is caused by an expanded CAG repeat in the Huntingtin (HTT) gene. The mechanism(s) by which mutant HTT (mHTT) causes disease is unclear. Nucleocytoplasmic transport, the trafficking of macromolecules between the nucleus and cytoplasm, is tightly regulated by nuclear pore complexes (NPCs) made up of nucleoporins (NUPs). Previous studies offered clues that mHTT may disrupt nucleocytoplasmic transport and a mutation of an NUP can cause HD-like pathology. Therefore, we evaluated the NPC and nucleocytoplasmic transport in multiple models of HD, including mouse and fly models, neurons transfected with mHTT, HD iPSC-derived neurons, and human HD brain regions. These studies revealed severe mislocalization and aggregation of NUPs and defective nucleocytoplasmic transport. HD repeat-associated non-ATG (RAN) translation proteins also disrupted nucleocytoplasmic transport. Additionally, overexpression of NUPs and treatment with drugs that prevent aberrant NUP biology also mitigated this transport defect and neurotoxicity, providing future novel therapy targets.

Enhanced transfer of a photocross-linking N-acetylglucosamine (GlcNAc) analog by an O-GlcNAc transferase mutant with converted substrate specificity

O-Linked β-N-acetylglucosamine (O-GlcNAc) is a post-translational modification of proteins in multicellular organisms. O-GlcNAc modification is catalyzed by the O-GlcNAc transferase (OGT), which transfers N-acetylglucosamine (GlcNAc) from the nucleotide sugar donor UDP-GlcNAc to serine or threonine residues of protein substrates. Recently, we reported a novel metabolic labeling method to introduce the diazirine photocross-linking functional group onto O-GlcNAc residues in mammalian cells. In this method, cells are engineered to produce diazirine-modified UDP-GlcNAc (UDP-GlcNDAz), and the diazirine-modified GlcNAc analog (GlcNDAz) is transferred to substrate proteins by endogenous OGT, producing O-GlcNDAz. O-GlcNDAz-modified proteins can be covalently cross-linked to their binding partners, providing information about O-GlcNAc-dependent interactions. The utility of the method was demonstrated by cross-linking highly O-GlcNAc-modified nucleoporins to proteins involved in nuclear transport. For practical application of this method to a broader range of O-GlcNAc-modified proteins, efficient O-GlcNDAz production is critical. Here we examined the ability of OGT to transfer GlcNDAz and found that the wild-type enzyme (wtOGT) prefers the natural substrate, UDP-GlcNAc, over the unnatural UDP-GlcNDAz. This competition limits O-GlcNDAz production in cells and the extent of O-GlcNDAz-dependent cross-linking. Here we identified an OGT mutant, OGT(C917A), that efficiently transfers GlcNDAz and, surprisingly, has altered substrate specificity, preferring to transfer GlcNDAz rather than GlcNAc to protein substrates. We confirmed the reversed substrate preference by determining the Michaelis-Menten parameters describing the activity of wtOGT and OGT(C917A) with both UDP-GlcNAc and UDP-GlcNDAz. Use of OGT(C917A) enhances O-GlcNDAz production, yielding improved cross-linking of O-GlcNDAz-modified molecules both in vitro and in cells.

Inhibition of O-Linked N-Acetylglucosamine Transferase Reduces Replication of Herpes Simplex Virus and Human Cytomegalovirus

O-linked N-acetylglucosamine (O-GlcNAc) transferase (OGT) is an essential cellular enzyme that posttranslationally modifies nuclear and cytoplasmic proteins via O-linked addition of a single N-acetylglucosamine (GlcNAc) moiety. Among the many targets of OGT is host cell factor 1 (HCF-1), a transcriptional regulator that is required for transactivation of the immediate-early genes of herpes simplex virus (HSV). HCF-1 is synthesized as a large precursor that is proteolytically cleaved by OGT, which may regulate its biological function. In this study, we tested whether inhibition of the enzymatic activity of OGT with a small molecule inhibitor, OSMI-1, affects initiation of HSV immediate-early gene expression and viral replication. We found that inhibiting OGT's enzymatic activity significantly decreased HSV replication. The major effect of the inhibitor occurred late in the viral replication cycle, when it reduced the levels of late proteins and inhibited capsid formation. However, depleting OGT levels with small interfering RNA (siRNA) reduced the expression of HSV immediate-early genes, in addition to reducing viral yields. In this study, we identified OGT as a novel cellular factor involved in HSV replication. Our results obtained using a small molecule inhibitor and siRNA depletion suggest that OGT's glycosylation and scaffolding functions play distinct roles in the replication cycle of HSV.

IMPORTANCE

Antiviral agents can target viral or host gene products essential for viral replication. O-GlcNAc transferase (OGT) is an important cellular enzyme that catalyzes the posttranslational addition of GlcNAc sugar residues to hundreds of nuclear and cytoplasmic proteins, and this modification regulates their activity and function. Some of the known OGT targets are cellular proteins that are critical for the expression of herpes simplex virus (HSV) genes, suggesting a role for OGT in the replication cycle of HSV. In this study, we found that OGT is required for efficient expression of viral genes and for assembly of new virions. Thus, we identify OGT as a novel host factor involved in the replication of HSV and a potential target for antiviral therapy.

Post-translational O-GlcNAcylation is essential for nuclear pore integrity and maintenance of the pore selectivity filter

O-glycosylation of the nuclear pore complex (NPC) by O-linked N-acetylglucosamine (O-GlcNAc) is conserved within metazoans. Many nucleoporins (Nups) comprising the NPC are constitutively O-GlcNAcylated, but the functional role of this modification remains enigmatic. We show that loss of O-GlcNAc, induced by either inhibition of O-GlcNAc transferase (OGT) or deletion of the gene encoding OGT, leads to decreased cellular levels of a number of natively O-GlcNAcylated Nups. Loss of O-GlcNAc enables increased ubiquitination of these Nups and their increased proteasomal degradation. The decreased half-life of these deglycosylated Nups manifests in their gradual loss from the NPC and a downstream malfunction of the nuclear pore selective permeability barrier in both dividing and post-mitotic cells. These findings define a critical role of O-GlcNAc modification of the NPC in maintaining its composition and the function of the selectivity filter. The results implicate NPC glycosylation as a regulator of NPC function and reveal the role of conserved glycosylation of the NPC among metazoans.

Glycosylation of the nuclear pore

The O-linked β-N-acetylglucosamine (O-GlcNAc) posttranslational modification was first discovered 30 years ago and is highly concentrated in the nuclear pore. In the years since the discovery of this single sugar modification, substantial progress has been made in understanding the biochemistry of O-GlcNAc and its regulation. Nonetheless, O-GlcNAc modification of proteins continues to be overlooked, due in large part to the lack of reliable methods available for its detection. Recently, a new crop of immunological and chemical detection reagents has changed the research landscape. Using these tools, approximately 1000 O-GlcNAc-modified proteins have been identified. While other forms of glycosylation are typically associated with extracellular proteins, O-GlcNAc is abundant on nuclear and cytoplasmic proteins. In particular, phenylalanine-glycine nucleoporins are heavily O-GlcNAc-modified. Recent experiments are beginning to provide insight into the functional implications of O-GlcNAc modification on certain proteins, but its role in the nuclear pore has remained enigmatic. However, tantalizing new results suggest that O-GlcNAc may play roles in regulating nucleocytoplasmic transport.