Functional nuclear pores reconstituted with beta 1-4 galactose-modified O-linked N-acetylglucosamine glycoproteins

Glycosyltransferases as versatile tools to study the biology of glycans

All cells are decorated with complex carbohydrate structures called glycans that serve as ligands for glycan-binding proteins (GBPs) to mediate a wide range of biological processes. Understanding the specific functions of glycans is key to advancing an understanding of human health and disease. However, the lack of convenient and accessible tools to study glycan-based interactions has been a defining challenge in glycobiology. Thus, the development of chemical and biochemical strategies to address these limitations has been a rapidly growing area of research. In this review, we describe the use of glycosyltransferases (GTs) as versatile tools to facilitate a greater understanding of the biological roles of glycans. We highlight key examples of how GTs have streamlined the preparation of well-defined complex glycan structures through chemoenzymatic synthesis, with an emphasis on synthetic strategies allowing for site- and branch-specific display of glyco-epitopes. We also describe how GTs have facilitated expansion of glyco-engineering strategies, on both glycoproteins and cell surfaces. Coupled with advancements in bioorthogonal chemistry, GTs have enabled selective glyco-epitope editing of glycoproteins and cells, selective glycan subclass labeling, and the introduction of novel biomolecule functionalities onto cells, including defined oligosaccharides, antibodies, and other proteins. Collectively, these approaches have contributed great insight into the fundamental biological roles of glycans and are enabling their application in drug development and cellular therapies, leaving the field poised for rapid expansion.

Wheat Germ Agglutinin—From Toxicity to Biomedical Applications

Wheat germ agglutinin is a hevein class N-Acetylglucosamine–binding protein with specific toxicity and biomedical potential. It is extractable from wheat germ—a low-value byproduct of the wheat industry—using well–established extraction methods based on salt precipitation and affinity chromatography. Due to its N-Acetylglucosamine affinity, wheat germ agglutinin exhibits antifungal properties as well as cytotoxic properties. Its anticancer properties have been demonstrated for various cancer cells, and toxicity mechanisms are well described. Wheat germ agglutinin has been demonstrated as a viable solution for various biomedical and therapeutic applications, such as chemotherapy, targeted drug delivery, antibiotic-resistant bacteria monitoring and elimination. This is performed mostly in conjunction with nanoparticles, liposomes, and other carrier mechanisms via surface functionalization. Combined with abundant wheat byproduct sources, wheat germ agglutinin has the potential to improve the biomedical field considerably.

Intracellular trafficking of begomoviruses in the midgut cells of their insect vector

Begomoviruses are exclusively transmitted by whiteflies in a persistent circulative manner and cause considerable economic losses to crop production worldwide. Previous studies have shown that begomoviruses accumulate in vesicle-like structures in whitefly midgut cells and that clathrin-mediated endocytosis is responsible for their internalization. However, the process by which begomoviruses are trafficked within whitefly midgut cells remains largely unknown. In this study, we investigated the roles of vesicle trafficking in the transport of Tomato yellow leaf curl virus (TYLCV), a begomovirus that has spread to over 50 countries and caused extensive damage to a range of important crops, within midgut cells of whitefly (Bemisia tabaci). By disrupting vesicle trafficking using RNA silencing and inhibitors, we demonstrated that the early steps of endosomal trafficking are important for the intracellular transport of TYLCV in the whitefly midgut. In addition, our data show that, unlike many animal viruses, TYCLV is trafficked within cells in a manner independent of recycling endosomes, late endosomes, lysosomes, the Golgi apparatus and the endoplasmic reticulum. Instead, our results suggest that TYLCV might be transported directly from early endosomes to the basal plasma membrane and released into the hemolymph. Silencing of the sorting nexin Snx12, which may be involved in membrane tubulation, resulted in fewer viral particles in hemolymph; this suggests that the tubular endosomal network may be involved in the transport of TYLCV. Our results also support a role for the endo-lysosomal system in viral degradation. We further showed that the functions of vector early endosomes and sorting nexin Snx12 are conserved in the transmission of several other begomoviruses. Overall, our data indicate the importance of early endosomes and the tubular endosomal network in begomovirus transmission.

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.

O-GlcNAc-ylation in the Nuclear Pore Complex

O-GlcNAc-ylation is the post-translational addition of an O-linked β-N-acetylglucosamine to the serine and threonine residues of thousands of proteins in eukaryotic cells. Specifically, half of the thirty different types of protein components in the nuclear pore complex (NPC) are modified by O-GlcNAc, of which the majority are intrinsically disordered nucleoporins (Nups) containing multiple phenylalanine-glycine (FG) repeats. Moreover, these FG-Nups form a strict selectivity barrier with a high density of O-GlcNAc in the NPC to mediate bidirectional trafficking between the cytoplasm and nucleus. However, the roles that O-GlcNAc plays in the structure and function of the NPC remain obscure. In this review paper, we will discuss the current knowledge of O-GlcNAc-ylated Nups, highlight some new techniques used to probe O-GlcNAc's roles in the nuclear pore, and finally propose a new model for the effect of O-GlcNAc on the NPC's permeability.

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.

The hexosamine signaling pathway: O-GlcNAc cycling in feast or famine

The enzymes of O-GlcNAc cycling couple the nutrient-dependent synthesis of UDP-GlcNAc to O-GlcNAc modification of Ser/Thr residues of key nuclear and cytoplasmic targets. This series of reactions culminating in O-GlcNAcylation of targets has been termed the hexosamine signaling pathway (HSP). The evolutionarily ancient enzymes of O-GlcNAc cycling have co-evolved with other signaling effecter molecules; they are recruited to their targets by many of the same mechanisms used to organize canonic kinase-dependent signaling pathways. This co-recruitment of the enzymes of O-GlcNAc cycling drives a binary switch impacting pathways of anabolism and growth (nutrient uptake) and catabolic pathways (nutrient sparing and salvage). The hexosamine signaling pathway (HSP) has thus emerged as a versatile cellular regulator modulating numerous cellular signaling cascades influencing growth, metabolism, cellular stress, circadian rhythm, and host-pathogen interactions. In mammals, the nutrient-sensing HSP has been harnessed to regulate such cell-specific functions as neutrophil migration, and activation of B-cells and T-cells. This review summarizes the diverse approaches being used to examine O-GlcNAc cycling. It will emphasize the impact O-GlcNAcylation has upon signaling pathways that may be become deregulated in diseases of the immune system, diabetes mellitus, cancer, cardiovascular disease, and neurodegenerative diseases.

ER membrane-bending proteins are necessary for de novo nuclear pore formation

Nucleocytoplasmic transport occurs exclusively through nuclear pore complexes (NPCs) embedded in pores formed by inner and outer nuclear membrane fusion. The mechanism for de novo pore and NPC biogenesis remains unclear. Reticulons (RTNs) and Yop1/DP1 are conserved membrane protein families required to form and maintain the tubular endoplasmic reticulum (ER) and the postmitotic nuclear envelope. In this study, we report that members of the RTN and Yop1/DP1 families are required for nuclear pore formation. Analysis of Saccharomyces cerevisiae prp20-G282S and nup133Δ NPC assembly mutants revealed perturbations in Rtn1–green fluorescent protein (GFP) and Yop1-GFP ER distribution and colocalization to NPC clusters. Combined deletion of RTN1 and YOP1 resulted in NPC clustering, nuclear import defects, and synthetic lethality with the additional absence of Pom34, Pom152, and Nup84 subcomplex members. We tested for a direct role in NPC biogenesis using Xenopus laevis in vitro assays and found that anti-Rtn4a antibodies specifically inhibited de novo nuclear pore formation. We hypothesize that these ER membrane–bending proteins mediate early NPC assembly steps.

O-GlcNAc cycling: implications for neurodegenerative disorders

The dynamic post-translational modification of proteins by O-linked N-acetylglucosamine (O-GlcNAc), termed O-GlcNAcylation, is an important mechanism for modulating cellular signaling pathways. O-GlcNAcylation impacts transcription, translation, organelle trafficking, proteasomal degradation and apoptosis. O-GlcNAcylation has been implicated in the etiology of several human diseases including type-2 diabetes and neurodegeneration. This review describes the pair of enzymes responsible for the cycling of this post-translational modification: O-GlcNAc transferase (OGT) and beta-N-acetylglucosaminidase (OGA), with a focus on the function of their structural domains. We will also highlight the important processes and substrates regulated by these enzymes, with an emphasis on the role of O-GlcNAc as a nutrient sensor impacting insulin signaling and the cellular stress response. Finally, we will focus attention on the many ways by which O-GlcNAc cycling may affect the cellular machinery in the neuroendocrine and central nervous systems.

Cell signaling, the essential role of O-GlcNAc!

An increasing body of evidence points to a central regulatory role for glucose in mediating cellular processes and expands the role of glucose well beyond its traditional role(s) in energy metabolism. Recently, it has been recognized that one downstream effector produced from glucose is UDP-GlcNAc. Levels of UDP-GlcNAc, and the subsequent addition of O-linked beta-N-acetylglucosamine (O-GlcNAc) to Ser/Thr residues, is involved in regulating nuclear and cytoplasmic proteins in a manner analogous to protein phosphorylation. O-GlcNAc protein modification is essential for life in mammalian cells, highlighting the importance of this simple post-translational modification in basic cellular regulation. Recent research has highlighted key roles for O-GlcNAc serving as a nutrient sensor in regulating insulin signaling, the cell cycle, and calcium handling, as well as the cellular stress response.

The hexosamine signaling pathway: deciphering the "O-GlcNAc code"

A dynamic cycle of addition and removal of O-linked N-acetylglucosamine (O-GlcNAc) at serine and threonine residues is emerging as a key regulator of nuclear and cytoplasmic protein activity. Like phosphorylation, protein O-GlcNAcylation dramatically alters the posttranslational fate and function of target proteins. Indeed, O-GlcNAcylation may compete with phosphorylation for certain Ser/Thr target sites. Like kinases and phosphatases, the enzymes of O-GlcNAc metabolism are highly compartmentalized and regulated. Yet, O-GlcNAc addition is subject to an additional and unique level of metabolic control. O-GlcNAc transfer is the terminal step in a "hexosamine signaling pathway" (HSP). In the HSP, levels of uridine 5'-diphosphate (UDP)-GlcNAc respond to nutrient excess to activate O-GlcNAcylation. Removal of O-GlcNAc may also be under similar metabolic regulation. Differentially targeted isoforms of the enzymes of O-GlcNAc metabolism allow the participation of O-GlcNAc in diverse intracellular functions. O-GlcNAc addition and removal are key to histone remodeling, transcription, proliferation, apoptosis, and proteasomal degradation. This nutrient-responsive signaling pathway also modulates important cellular pathways, including the insulin signaling cascade in animals and the gibberellin signaling pathway in plants. Alterations in O-GlcNAc metabolism are associated with various human diseases including diabetes mellitus and neurodegeneration. This review will focus on current approaches to deciphering the "O-GlcNAc code" in order to elucidate how O-GlcNAc participates in its diverse functions. This ongoing effort requires analysis of the enzymes of O-GlcNAc metabolism, their many targets, and how the O-GlcNAc modification may be regulated.

O-GlcNAc glycosylation: a signal for the nuclear transport of cytosolic proteins?

Year 2004 marks the 20th anniversary of the discovery of O-linked N-acetylglucosamine (O-GlcNAc) by Gerald W. Hart. Despite interest for O-GlcNAc, the functions played by this single monosaccharide remain poorly understood, though numerous roles have been suggested, among which is the involvement of O-GlcNAc in the nuclear transport of cytosolic proteins. This idea was first sustained by studies on bovine serum albumin that showed that the protein could be actively carried to the nucleus when it was modified with sugars. In this paper, we will review data on this puzzling problem. We will first describe the well-established nuclear localisation signal (NLS)-dependent nuclear transport by presenting the different factors involved, and then, we will examine where and how O-GlcNAc could be involved in nuclear transport. Whereas it has been suggested that O-GlcNAc could interfere at two levels in the nuclear transport both by modifying proteins to be translocated to the nucleus and by modifying the nucleoporins of the nuclear pore complex, according to us, this second idea seems unlikely. Part of this study will also be dedicated to a relatively new concept in the nuclear transport: the role of the 70-kDa heat shock proteins (HSP70). The action of the chaperone in nuclear translocation was put forward 10 years ago, but new findings suggest that this mechanism could be linked to O-GlcNAc glycosylation.

Pores for thought: nuclear pore complex proteins

Nuclear pore complexes (NPCs) are enormous macromolecular structures that mediate the active exchange of proteins and RNPs between the nucleus and cytoplasm. Recent work has resulted in a windfall of identified NPC polypeptides, many with unique sequences. Several of the proteins have been shown to be part of extended cytoplasmic and nucleoplasmic NPC filaments. Biochemical, structural and genetic studies on NPC proteins are just beginning to allow an understanding of how they associate into a functional organelle.

Characterization of the O-GlcNAc protein modification in Xenopus laevis oocyte during oogenesis and progesterone-stimulated maturation

Little information exists about single N-acetylglucosamine modifications on proteins in growth and developmental model systems. To explore these phenomena, Xenopus laevis oocytes from stages I-VI of oogenesis were isolated and proteins analyzed on SDS-PAGE. The proteins were probed with antibodies specific for O-GlcNAc. Levels of the O-GlcNAc protein modification were highest in stages I and II, while decreasing in stages III-VI. The reduction in amount of O-GlcNAc-modified proteins was correlated to increases in apparent O-GlcNAcase (streptozotocin-inhibitable neutral hexosaminidase), activity involved in removing protein monoglycosylations. The O-GlcNAc modification was also characterized during progesterone-stimulated oocyte maturation. Although O-GlcNAcase activity appeared relatively constant between quiescent and matured stage VI oocytes, a small decrease in the levels of both total and specific O-GlcNAc-modified proteins was observed. Investigating the function of O-GlcNAc during maturation, oocytes were incubated with compounds known to modulate the levels of the O-GlcNAc protein modification and then stimulated to mature. Oocytes treated with compounds known to increase O-glycosylation consistently matured slower than non-treated controls, while oocytes treated with compounds that decrease O-glycosylation matured slightly faster than controls. The O-GlcNAc modification may play important roles in both the developmental and cell division processes of X. laevis oocytes.

An N-terminal truncated form of Orp150 is a cytoplasmic ligand for the anti-proliferative mushroom Agaricus bisporus lectin and is required for nuclear localization sequence-dependent nuclear protein import

Nuclear localization sequence-dependent nuclear protein import is essential for maintaining cell function and can be selectively blocked in epithelial cells by mushroom (Agaricus bisporus) lectin. Here we report that a major intracellular ligand for this lectin is an N-terminally truncated form of oxygen-regulated protein 150 (Orp150), which lacks the endoplasmic reticulum translocation signal peptide of full-length Orp150. This cytoplasmic form of Orp150 expresses the lectin carbohydrate ligand (sialyl-2,3-galactosyl-beta1,3-N-acetylgalactosamine-alpha) and is shown to be essential for nuclear localization sequence-dependent nuclear protein import.

Glycan-dependent signaling: O-linked N-acetylglucosamine

The addition of O-linked N-acetylglucosamine (O-GlcNAc) to target proteins may serve as a signaling modification analogous to protein phosphorylation. Like phosphorylation, O-GlcNAc is a dynamic modification occurring in the nucleus and cytoplasm. Various analytical methods have been developed to detect O-GlcNAc and distinguish it from glycosylation in the endomembrane system. Many target molecules have been identified; these targets are typically components of supramolecular complexes such as transcription factors, nuclear pore proteins, or cytoskeletal components. The enzymes responsible for O-GlcNAc addition and removal are highly conserved molecules having molecular features consistent with a signaling role. The O-GlcNAc transferase and O-GlcNAcase are likely to act in consort with kinases and phosphatases generating various isoforms of physiological substrates. These isoforms may differ in such properties as protein-protein interactions, protein stability, and enzymatic activity. Since O-GlcNAc plays a critical role in the regulation of signaling pathways of higher plants, the glycan modification is likely to perform similar signaling functions in mammalian cells. Glucose and amino acid metabolism generates hexosamine precursors that may be key regulators of a nutrient sensing pathway involving O-GlcNAc signaling. Altered O-linked GlcNAc metabolism may also occur in human diseases including neurodegenerative disorders, diabetes mellitus and cancer.

Use of galactosyltransferase to assess the biological function of O-linked N-acetyl-d-glucosamine: a potential role for O-GlcNAc during cell division

Many cytosolic and nuclear proteins are modified by monomeric O-linked N-acetyl-d-glucosamine (O-GlcNAc). The biological functions of this form of glycosylation are unclear but evidence suggests that it heightens regulation of protein function. To assess the biological function of O-GlcNAc addition, we examined the biological effects of galactosyltransferase (GalT) microinjected into the cytoplasm of Xenopus ovarian oocytes. GalT, which catalyzes beta1-4-galactose addition to O-GlcNAc, should inhibit deglycosylation and lectin-like interactions requiring unmodified O-GlcNAc residues. Although GalT injection into diplotene-arrested oocytes has no detectable effects on cell viability, it is toxic to oocytes entering meiosis. Cell-cycle-specific toxicity is recapitulated in vitro as GalT inhibits formation of nuclei and microtubule asters from cell-free extracts of ovulated frog eggs. These observations suggest that regulation of O-GlcNAc is important for cell cycle progression and may be important in diseases in which O-GlcNAc metabolism is abnormal. The methods described here outline a viable experimental scheme for ascribing a biological function to this form of glycosylation.

Airway epithelial cell wound repair mediated by alpha-dystroglycan

Dystroglycans (DGs) bind laminin matrix proteins in skeletal and cardiac muscle and are expressed in other nonmuscle tissues. However, their expression in airway epithelial cells has not been demonstrated. We examined expression of DGs in the human airway epithelial cell line 1HAEo(-), and in human primary airway epithelial cells. Expression of the common gene for alpha- and beta-DG was demonstrated by reverse transcriptase/ polymerase chain reaction in 1HAEo(-) cells. Protein expression of beta-DG was demonstrated by both Western blot and flow cytometry in cultured cells. Localization of alpha-DG, using both a monoclonal antibody and the alpha-DG binding lectin wheat-germ agglutinin (WGA), was to the cell membrane and nucleus. We then examined the function of DGs in modulating wound repair over laminin matrix. Blocking alpha-DG binding to laminin in 1HAEo(-) monolayers using either glycosyaminoglycans or WGA attenuated cell migration and spreading after mechanical injury. alpha-DG was not expressed in epithelial cells at the wound edge immediately after wound creation, but localized to the cell membrane in these cells within 12 h of injury. These data demonstrate the presence of DGs in airway epithelium. alpha-DG is dynamically expressed and serves as a lectin to bind laminin during airway epithelial cell repair.

Wheatgerm agglutinin-mediated toxicity in pancreatic cancer cells

Lectin binding specificities for carbohydrate allow phenotypic and functional characterization of membrane-associated glycoproteins expressed on cancer cells. This analysis examined wheatgerm agglutinin binding to pancreatic cancer cells in vitro and the resulting toxicity. Membrane preparations of nine human pancreatic carcinoma cell lines were studied for lectin binding using wheatgerm agglutinin (WGA), concanavalin A (ConA) and phytohaemagglutinin-L (PHA-L) in a lectin blot analysis. Cell proliferation in vitro was measured by thymidine incorporation in the absence or presence of lectins at various concentrations. Sialic acid binding lectins or succinyl-WGA (succWGA) served as controls. WGA toxicity was tested after swainsonine or neuraminidase pretreatment. Binding and uptake of fluorescein-labelled lectins was studied under fluorescence microscopy. All pancreatic cell lines displayed high WGA membrane binding, primarily to sialic acid residues. Other lectins were bound with weak to moderate intensity only. Lectin toxicity corresponded to membrane binding intensity, and was profound in case of WGA (ID50 at 2.5-5 microg ml(-1)). WGA exposure induced chromatin condensation, nuclear fragmentation and DNA release consistent with apoptosis. Important steps for WGA toxicity included binding to sialic acid on swainsonine-sensitive carbohydrate and lectin internalization. There was rapid cellular uptake and subsequent nuclear relocalization of WGA. In contradistinction to the other lectins studied, WGA proved highly toxic to human pancreatic carcinoma cells in vitro. WGA binding to sialic acid residues of N-linked carbohydrate, cellular uptake and subsequent affinity to N-acetyl glucosamine appear to be necessary steps. Further analysis of this mechanism of profound toxicity may provide insight relevant to the treatment of pancreatic cancer.

Phosphorylation and glycosylation of nucleoporins

The nuclear pore complex mediates macromolecular transport between the nucleus and cytoplasm. Many nuclear pore components (nucleoporins) are modified by both phosphate and O-linked N-acetylglucosamine (O-GlcNAc). Among its many functions, protein phosphorylation plays essential roles in cell cycle progression. The role of O-GlcNAc addition is unknown. Here, levels of nucleoporin phosphorylation and glycosylation during cell cycle progression are examined. Whereas nuclear pore glycoproteins are phosphorylated in a cell-cycle-dependent manner, levels of O-GlcNAc remain constant. The major nucleoporin p62 can be phosphorylated in vitro by protein kinase A and glycogen synthase kinase (GSK)-3alpha but not by cyclin B/cdc2 or GSK-3beta. The consensus sites of these kinases resemble sites which can be glycosylated by O-GlcNAc transferase. These data are consistent with a model that O-GlcNAc limits nucleoporin hyperphosphorylation during M-phase and hastens the resumption of regulated nuclear transport at the completion of cell division.

An evaluation of sialation of the nucleoporin p62

Many nuclear and cytosolic proteins are modified by single residues of O-linked N-acetyl-D-glucosamine. These include many proteins found in nuclear pore complexes required for transport of macromolecules between the nucleus and the cytoplasm. The best characterized pore glycoprotein, p62, mediates its function as one component of a protein complex essential for nuclear transport. Although p62 sugar residues are not essential for nuclear transport, they appear to oppose protein phosphorylation occurring at sites predicted to destabilize protein-protein interactions of the p62 complex. Recently, a p62-like protein isolated from mouse neuroblastoma cells was reported to be modified by both GlcNAc and sialic acid. As there is little precedent for nucleoplasmic sialation, the finding that a characterized nuclear pore protein is sialated is significant because it may regulate pore function. To assess the biological importance of p62 sialation, GlcNAc and sialic acid-specific lectins were used to examine the state of p62 glycosylation in cells commonly used to study nuclear transport: frog eggs and normal rat kidney and HeLa fibroblasts. In addition, four mouse neuroblastoma cell lines derived from the same tumor were examined. The glycosylation of p62 in these cells appears to involve only single O-linked GlcNAc moieties; no significant sialation was detected.

Nuclear and cytoplasmic glycosylation

O-GlcNAcylation is a form of cytoplasmic and nuclear glycosylation that is found on many diverse proteins of the cell including RNA polymerase II and its associated transcription factors, cytoskeletal proteins, nucleoporins, viral proteins, heat shock proteins, tumor suppressors, and oncogenes. It involves the attachment of a single, unmodified N-acetylglucosaminyl residue O-glycosidically linked to the hydroxyl groups of serine and threonine moieties of proteins. It is a highly abundant and dynamic form of posttranslational modification that appears to modulate function in a manner similar to phosphorylation. All O-GlcNAc-containing proteins are phosphoproteins that are involved in the formation of multimeric complexes, suggesting that O-GlcNAc may play a role in mediating protein-protein interactions. O-GlcNAc sites resemble phosphorylation sites and in many cases the two modifications are mutually exclusive; therefore, O-GlcNAcylation may act as an antagonist of phosphorylation and help to mediate many essential functions of the cell.

Nuclear assembly

We review old and new insights into the structure of the nuclear envelope and the components responsible for its dynamic reassembly during mitosis. New information is coming to light about several of the proteins that mediate nuclear reassembly. These proteins include the lamins and their emerging relationship with proteins such as otefin and the MAN antigens: peripheral proteins that might participate in lamina structure. There are four identified proteins localized to the inner nuclear membrane: the lamina-associated proteins LAP1 and LAP2, emerin, and the lamin B receptor (LBR). LBR can interact independently with lamin B and a chromodomain protein, Hp1, and appears to be a central player in targeting nuclear membranes to chromatin. Intermediates in the assembly of nuclear pore complexes (NPCs) can now be studied biochemically and visualized by high resolution scanning electron microscopy. We discuss the possibility that the filament-forming proteins Tpr/p270, NuMA, and perhaps actin may have roles in nuclear assembly.

Yeast genetics to dissect the nuclear pore complex and nucleocytoplasmic trafficking

Eukaryotic cells evolved when their genetic information was packed into the cell nucleus. DNA replication and RNA biogenesis occur inside the nucleus while protein synthesis takes place in the cytoplasm. Bi-directional trafficking between these two compartments is mediated by a single supramolecular assembly, the nuclear pore complex. Nucleocytoplasmic transport is signal mediated, energy dependent, and requires, besides nuclear pore proteins (nucleoporins), a number of soluble transport factors. We review here our current knowledge on the role of nucleoporins, and on the mechanism of nucleocytoplasmic transport, with emphasis on the yeast Saccharomyces cerevisiae.

Architecture of the yeast cell wall. Beta(1-->6)-glucan interconnects mannoprotein, beta(1-->)3-glucan, and chitin

In a previous study (Kollár, R., Petráková, E., Ashwell, G., Robbins, P. W., and Cabib, E. (1995) J. Biol. Chem. 270, 1170-1178), the linkage region between chitin and beta(1-->3)-glucan was solubilized and isolated in the form of oligosaccharides, after digestion of yeast cell walls with beta(1-->3)-glucanase, reduction with borotritide, and subsequent incubation with chitinase. In addition to the oligosaccharides, the solubilized fraction contained tritium-labeled high molecular weight material. We have now investigated the nature of this material and found that it represents areas in which all four structural components of the cell wall, beta(1-->3)-glucan, beta(1-->6)-glucan, chitin, and mannoprotein are linked together. Mannoprotein, with a protein moiety about 100 kDa in apparent size, is attached to beta(1-->6)-glucan through a remnant of a glycosylphosphatidylinositol anchor containing five alpha-linked mannosyl residues. The beta(1-->6)-glucan has some beta(1-->3)-linked branches, and it is to these branches that the reducing terminus of chitin chains appears to be attached in a beta(1-->4) or beta(1-->2) linkage. Finally, the reducing end of beta(1-->6)-glucan is connected to the nonreducing terminal glucose of beta(1-->3)-glucan through a linkage that remains to be established. A fraction of the isolated material has three of the main components but lacks mannoprotein. From these results and previous findings on the linkage between mannoproteins and beta(1-->6)-glucan, it is concluded that the latter polysaccharide has a central role in the organization of the yeast cell wall. The possible mechanism of synthesis and physiological significance of the cross-links is discussed.

The vertebrate GLFG nucleoporin, Nup98, is an essential component of multiple RNA export pathways

The 97-kD O-linked glycoprotein, Nup98, is a component of the Xenopus laevis nuclear pore complex and the only vertebrate GLFG nucleoporin identified (Powers, M.A., C. Macauley, F. Masiarz, and D.J. Forbes. 1995. J. Cell Biol. 128:721-736). We have investigated possible roles of xNup98 in the nucleocytoplasmic transport of proteins and RNAs by analyzing the consequences of injecting monospecific polyclonal antibodies to xNup98 into X. laevis oocytes. We show here that nuclear injection of anti-xNup98 inhibited the export of multiple classes of RNAs, including snRNAs, 5S RNA, large ribosomal RNAs, and mRNA. In contrast, the export of tRNA was unaffected. Injection of anti-xNup98 into the oocyte cytoplasm had no effect on export of any of the RNAs. Significantly, nuclear injection of anti-xNup98 antibodies did not inhibit import of either karyophilic proteins or snRNPs. This latter result is in agreement with our previous finding that Nup98 is not an essential element of the protein import pathway. Thus, Nup98 plays a role specifically in RNA export from the nucleus, and it appears to be an essential component of multiple RNA export pathways.

Dynamic O-linked glycosylation of nuclear and cytoskeletal proteins

Modification of Ser and Thr residues by attachment of O-linked N-acetylglucos-amine [Ser(Thr)-O-GlcNAcylation] to eukaryotic nuclear and cytosolic proteins is as dynamic and possibly as abundant as Ser(Thr) phosphorylation. Known O-GlcNAcylated proteins include cytoskeletal proteins and their regulatory proteins; viral proteins; nuclear-pore, heat-shock, tumor-suppressor, and nuclearoncogene proteins; RNA polymerase II catalytic subunit; and a multitude of transcription factors. Although functionally diverse, all of these proteins are also phosphoproteins. Most O-GlcNAcylated proteins form highly regulated multimeric associations that are dependent upon their posttranslational modifications. Evidence is mounting that O-GlcNAcylation is an important regulatory modification that may have a reciprocal relationship with O-phosphorylation and may modulate many biological processes in eukaryotes.

Nuclear pore complex proteins

The nuclear envelope forms the boundary between the nucleus and the cytoplasm and as such regulates the exchange of macromolecules between the two compartments. The channels through the nuclear envelop that actually mediate this macromolecular traffic are the nuclear pore complexes. These are extremely elaborate structures which in vertebrate cells exhibit a mass of approximately 120 MDa. They are thought to be composed of as many as 100 distinct polypeptide subunits. A major challenge in the field of nucleocytoplasmic transport is to identify these subunits and to determine their functions and interactions in the context of the three-dimensional structure of the nuclear pore complex. It is the aim of this review to summarize what is currently known of the 20 or so nuclear pore complex proteins that have been described in either vertebrate or yeast cells.

The nuclear pore complex protein p62 is one of several sialic acid-containing proteins of the nuclear envelope

While investigating the glycosylation of nuclear envelope proteins of neuroblastoma cells, we found several proteins that bound the sialic acid-specific Sambucus nigra agglutinin. The strongest signals were obtained for proteins with apparent molecular masses of 66 and 180 kDa. The specificity of the lectin binding was checked by acylneuraminyl hydrolase treatment of nuclear envelope proteins, which prohibited S. nigra agglutinin binding. Digestion of nuclear envelope proteins with the N-glycosidase F revealed that sialic acid was N-glycosidically linked to the 180-kDa protein and very probably O-glycosidically linked to the 66-kDa protein. Upon extraction, the latter behaved like the nucleoporin p62 in that it was partly extracted by high ionic strength buffers, could not be solubilized by nonionic detergent, and was completely removed from the nuclear envelope with urea. Two-dimensional gel electrophoretic comparison showed that the S. nigra agglutinin-binding protein and p62 have an identical isoelectric point of about 5.0 and an identical apparent molecular mass of 66 kDa. This, together with the binding of the anti-nucleoporin antibody, demonstrated the identity of the 66-kDa sialoprotein and p62. S. nigra agglutinin inhibits nuclear protein transport in neuroblastoma cells, strongly suggesting a functional significance of sialylation of p62.

Mechanisms of nuclear protein import

The past two years have seen a significant increase in our understanding of nuclear protein import. Five cytosolic import factors have been identified, two of which have been shown to directly interact with components of the nuclear pore complex. These findings enable refinement of previous models for steps in the nuclear import pathway, and provide a framework for future research.

Differential mitotic phosphorylation of proteins of the nuclear pore complex

During each cell cycle, the nucleus of higher eukaryotes undergoes a dramatic assembly and disassembly. These events can be faithfully reproduced in vitro using cell-free extracts derived from Xenopus eggs. Such extracts contain three major N-acetylglucosaminylated proteins, p200, p97, and p60. All three become assembled into reconstituted nuclear pores. Here we show that p200, p97, and p60 exist in eggs in soluble high molecular mass complexes of 1000, 450, and 600 kDA, respectively. The bulk of p60 is stably associated with proteins of 58 and 54 kDa, while p200 is associated with a fraction of p60 in a separate complex lacking p58 and p54. Upon examining the behavior of these proteins in the cell cycle, we find that p200 and p97 are highly phosphorylated at mitosis, both in vivo and in vitro. Moreover, in extracts that cycle between interphase and mitosis, p200 and p97 are specifically phosphorylated at mitosis. Corresponding with their mitotic phosphorylation, both p200 and p97 are specific substrates for purified mitotic Cdc2 kinase, whereas nucleoporin p60 is not. Analysis indicates that the size of the complexes containing the pore N-acetylglucosamine glycoproteins does not change during mitosis, suggesting that such complexes represent stable multicomponent modules into which the nucleus disassembles at mitosis.