Relative accessibility of N-acetylglucosamine in trimers of the adenovirus types 2 and 5 fiber proteins

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.

O-GlcNAcylation modulates HBV replication through regulating cellular autophagy at multiple levels

O-GlcNAcylation is a form of posttranslational modification, and serves various functions, including modulation of location, stability, and activity for the modified proteins. O-linked-N-acetylglucosamine (O-GlcNAc) transferase (OGT) is an essential cellular enzyme that posttranslationally modifies the cellular proteins with O-GlcNAc moiety. Early studies reported that the decreased O-GlcNAcylation regulates cellular autophagy, a process relevant for hepatitis B virus replication (HBV) and assembly. Therefore, we addressed the question how O-GlcNAcylation regulates cellular autophagy and HBV replication. Inhibition of OGT activity with a small molecule inhibitor OSMI-1 or silencing OGT significantly enhanced HBV replication and HBsAg production in hepatoma cells and primary human hepatocytes (PHHs). Western blotting analysis showed that inhibition of O-GlcNAcylation-induced endoplasmic reticulum (ER) stress and cellular autophagy, two processes subsequently leading to enhanced HBV replication. Importantly, the numbers of autophagosomes and the levels of autophagic markers LC3-II and SQSTM1/p62 in hepatoma cells were elevated after inhibition of O-GlcNAcylation. Further analysis revealed that inhibition of O-GlcNAcylation blocked autophagosome-lysosome fusion and thereby prevented autophagic degradation of HBV virions and proteins. Moreover, OSMI-1 further promoted HBV replication by inducing autophagosome formation via inhibiting the O-GlcNAcylation of Akt and mTOR. In conclusion, decreased O-GlcNAcylation enhanced HBV replication through increasing autophagosome formation at multiple levels, including triggering ER-stress, Akt/mTOR inhibition, and blockade of autophagosome-lysosome fusion.

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.

O-GlcNAc modification of the coat protein of the potyvirus Plum pox virus enhances viral infection

O-GlcNAcylation is a dynamic protein modification which has been studied mainly in metazoans. We reported previously that an Arabidopsis thaliana O-GlcNAc transferase modifies at least two threonine residues of the Plum pox virus (PPV) capsid protein (CP). Now, six additional residues were shown to be involved in O-GlcNAc modification of PPV CP. CP O-GlcNAcylation was abolished in the PPV CP7-T/A mutant, in which seven threonines were mutated. PPV CP7-T/A infected Nicotiana clevelandii, Nicotiana benthamiana, and Prunus persica without noticeable defects. However, defects in infection of A. thaliana were readily apparent. In mixed infections of wild-type arabidopsis, the CP7-T/A mutant was outcompeted by wild-type virus. These results indicate that CP O-GlcNAcylation has a major role in the infection process. O-GlcNAc modification may have a role in virion assembly and/or stability as the CP of PPV CP7-T/A was more sensitive to protease digestion than that of the wild-type virus.

O-GlcNAc transferase inhibits KSHV propagation and modifies replication relevant viral proteins as detected by systematic O-GlcNAcylation analysis

O-GlcNAcylation is an inducible, highly dynamic and reversible post-translational modification, mediated by a unique enzyme named O-linked N-acetyl-d-glucosamine (O-GlcNAc) transferase (OGT). In response to nutrients, O-GlcNAc levels are differentially regulated on many cellular proteins involved in gene expression, translation, immune reactions, protein degradation, protein-protein interaction, apoptosis and signal transduction. In contrast to eukaryotic cells, little is known about the role of O-GlcNAcylation in the viral life cycle. Here, we show that the overexpression of the OGT reduces the replication efficiency of Kaposi's sarcoma-associated herpesvirus (KSHV) in a dose-dependent manner. In order to investigate the global impact of O-GlcNAcylation in the KSHV life cycle, we systematically analyzed the 85 annotated KSHV-encoded open reading frames for O-GlcNAc modification. For this purpose, an immunoprecipitation (IP) strategy with three different approaches was carried out and the O-GlcNAc signal of the identified proteins was properly controlled for specificity. Out of the 85 KSHV-encoded proteins, 18 proteins were found to be direct targets for O-GlcNAcylation. Selected proteins were further confirmed by mass spectrometry for O-GlcNAc modification. Correlation of the functional annotation and the O-GlcNAc status of KSHV proteins showed that the predominant targets were proteins involved in viral DNA synthesis and replication. These results indicate that O-GlcNAcylation plays a major role in the regulation of KSHV propagation.

The viral transactivator HBx protein exhibits a high potential for regulation via phosphorylation through an evolutionarily conserved mechanism

BACKGROUND

Hepatitis B virus (HBV) encodes an oncogenic factor, HBx, which is a multifunctional protein that can induce dysfunctional regulation of signaling pathways, transcription, and cell cycle progression, among other processes, through interactions with target host factors. The subcellular localization of HBx is both cytoplasmic and nuclear. This dynamic distribution of HBx could be essential to the multiple roles of the protein at different stages during HBV infection. Transactivational functions of HBx may be exerted both in the nucleus, via interaction with host DNA-binding proteins, and in the cytoplasm, via signaling pathways. Although there have been many studies describing different pathways altered by HBx, and its innumerable binding partners, the molecular mechanism that regulates its different roles has been difficult to elucidate.

METHODS

In the current study, we took a bioinformatics approach to investigate whether the viral protein HBx might be regulated via phosphorylation by an evolutionarily conserved mechanism.

RESULTS

We found that the phylogenetically conserved residues Ser25 and Ser41 (both within the negative regulatory domain), and Thr81 (in the transactivation domain) are predicted to be phosphorylated. By molecular 3D modeling of HBx, we further show these residues are all predicted to be exposed on the surface of the protein, making them easily accesible to these types of modifications. Furthermore, we have also identified Yin Yang sites that might have the potential to be phosphorylated and O-β-GlcNAc interplay at the same residues.

CONCLUSIONS

Thus, we propose that the different roles of HBx displayed in different subcellular locations might be regulated by an evolutionarily conserved mechanism of posttranslational modification, via phosphorylation.

Cross talk between O-GlcNAcylation and phosphorylation: roles in signaling, transcription, and chronic disease

O-GlcNAcylation is the addition of β-D-N-acetylglucosamine to serine or threonine residues of nuclear and cytoplasmic proteins. O-linked N-acetylglucosamine (O-GlcNAc) was not discovered until the early 1980s and still remains difficult to detect and quantify. Nonetheless, O-GlcNAc is highly abundant and cycles on proteins with a timescale similar to protein phosphorylation. O-GlcNAc occurs in organisms ranging from some bacteria to protozoans and metazoans, including plants and nematodes up the evolutionary tree to man. O-GlcNAcylation is mostly on nuclear proteins, but it occurs in all intracellular compartments, including mitochondria. Recent glycomic analyses have shown that O-GlcNAcylation has surprisingly extensive cross talk with phosphorylation, where it serves as a nutrient/stress sensor to modulate signaling, transcription, and cytoskeletal functions. Abnormal amounts of O-GlcNAcylation underlie the etiology of insulin resistance and glucose toxicity in diabetes, and this type of modification plays a direct role in neurodegenerative disease. Many oncogenic proteins and tumor suppressor proteins are also regulated by O-GlcNAcylation. Current data justify extensive efforts toward a better understanding of this invisible, yet abundant, modification. As tools for the study of O-GlcNAc become more facile and available, exponential growth in this area of research will eventually take place.

Chemoselective attachment of small molecule effector functionality to human adenoviruses facilitates gene delivery to cancer cells

We demonstrate here a novel two-step "click" labeling process in which adenoviral particles are first metabolically labeled during production with unnatural azido sugars. Subsequent chemoselective modification allows access to viruses decorated with a broad array of effector functionality. Adenoviruses modified with folate, a known cancer-targeting motif, demonstrated a marked increase in gene delivery to a murine cancer cell line.

O-glycosylation of protein subpopulations in alcohol-extracted rice proteins

Mucin-type O-glycosylation has been well characterized in mammalian systems but not in plants. In this study, the purified alcohol-soluble, non-reduced protein (prolamin) fraction from rice seed was investigated for the occurrence of O-linked oligosaccharides. As storage prolamins are unlikely to be O-glycosylated, any O-glycosylation found was likely to belong to co-extracted proteins, whether because of association with the protein body or solubility. SDS-PAGE and MS analyses revealed 14 and 16kDa protein families in fractions that bound to the lectins peanut agglutinin (PNA), Vicia villosa lectin (VVL) and Jacalin, indicative of the presence of O-linked saccharides. Enzymatic cleavage, fluorescent labeling and high-performance liquid chromatography (HPLC) analysis demonstrated a peak consistent with Gal-beta-(1-->3)-GalNAc, with similar MS/MS fragmentation. Additionally, upon chemical analysis, a GlcNAc-containing O-linked carbohydrate moiety was discovered. Protein blotting with anti-O-GlcNAc antibody (clone CTD110.6) was positive in a subpopulation of the 14kDa alcohol-soluble protein fraction, but a hot capping experiment was negative. Therefore, the GlcNAc residue in this case is unlikely to be terminal. Additionally, a positive reaction with CTD110.6mAb cannot be taken as absolute proof of O-GlcNAc modification and further confirmatory experiments should be employed. We hypothesize that O-glycosylation may contribute to protein functionality or regulation. Further investigation is required to identify the specific proteins with these modifications. This 'reverse' approach could lead to the identification of proteins involved in mRNA targeting, signaling, translation, anchoring or maintenance of translational quiescence and may be applied to germinating rice seed extracts for further elucidation of protein function and regulation.

Dynamic interplay between O-linked N-acetylglucosaminylation and glycogen synthase kinase-3-dependent phosphorylation

O-GlcNAcylation on serine and threonine side chains of nuclear and cytoplasmic proteins is dynamically regulated in response to various environmental and biological stimuli. O-GlcNAcylation is remarkably similar to O-phosphorylation and appears to have a dynamic interplay with O-phosphate in cellular regulation. A systematic glycoproteomics analysis of the affects of inhibiting specific kinases on O-GlcNAcylation should help reveal both the global and specific dynamic relationships between these two abundant post-translational modifications. Here we report the O-GlcNAc perturbations in response to inhibition of glycogen synthase kinase-3 (GSK-3), a pivotal kinase involved in many signaling pathways. By combining immunoaffinity chromatography and SILAC (stable isotope labeling with amino acids in cell culture)-based quantitative mass spectrometry, we identified 45 potentially O-GlcNAcylated proteins. Quantitative measurements indicated that at least 10 proteins had an apparent increase of O-GlcNAcylation upon GSK-3 inhibition by lithium, whereas surprisingly 19 other proteins showed decreases. O-GlcNAcylation changes on a subset of the proteins were confirmed by follow-up experiments. By combining a new O-GlcNAc peptide enrichment method and beta-elimination followed by Michael addition with DTT, we also mapped the O-GlcNAc site (Ser-55) of vimentin, which showed an apparent increase of O-GlcNAcylation upon GSK-3 inhibition. Based on the MS data, we further investigated potential roles of O-GlcNAc on host cell factor-1, a transcription co-activator, and showed that dynamic regulation of O-GlcNAcylation on host cell factor-1 influenced its subcellular distribution. Taken together, these data indicated the complex interplay between phosphorylation and O-GlcNAcylation that occurs within signaling networks.

Characterization of the Adenovirus Fiber Protein

Entry of the adenovirus (Ad) capsids during the early stages of infection is a multistep process that includes initial attachment of the virus capsid to the cell surface followed by internalization of the virus into early endosomes. The Ad fiber protein, a complex of three apparently identical subunits, mediates the initial attachment step. In this chapter, methods for the purification and characterization of the Ad fiber protein are presented. Chromatographic methods for the isolation of the protein from infected cells can yield substantial quantities of protein for biochemical analysis. Protocols for characterization of the protein by Western blot and by indirect immunofluorescence of infected cells are also presented. The specificity of different monoclonal and polyclonal antibodies that recognize Ad fiber is also discussed. Ad fiber from a number of serotypes also contains a posttranslational modification, O-linked N-acetyl-glucosamine; methods for detection and characterization of this modification are also provided. With these tools and protocols, one can address important questions about this protein, which helps direct the tissue tropism of Ad.

Adenovirus type 5 fiber knob domain has a critical role in fiber protein synthesis and encapsidation

Adenovirus serotype 5 (Ad5) vectors carrying knobless fibers designed to remove their natural tropism were found to have a lower fiber content than recombinant Ad5 with wild-type (WT) capsid, implying a role for the knob-coding sequence or/and the knob domain in fiber encapsidation. Experimental data using a variety of fiber gene constructs showed that the defect did not occur at the fiber mRNA level, but at the protein level. Knobless fiber proteins were found to be synthesized at a significant slower rate compared with knob-carrying fibers, and the trimerization process of knobless fibers paralleled their slow rate of synthesis. A recombinant Ad5 diploid for the fiber gene (referred to as Ad5/R7-ZZwt/E1 : WT-fiber) was constructed to analyse the possible rescue of the knobless low-fiber-content phenotype by co-expression of WT fiber. Ad5/R7-ZZwt/E1 : WT-fiber contained a knobless fiber gene in its natural location (L5) in the viral genome and an additional WT fiber gene in an ectopic position in E1. Knobless fiber was still synthesized at low levels compared with the co-expressed E1 : WT fiber and the recovery of the two fiber species in virus progeny reflected their respective amounts in the infected cells. Our results suggested that deletion of the fiber knob domain had a negative effect on the translation of the fiber mRNA and on the intracellular concentration of fiber protein. They also suggested that the knob control of fiber protein synthesis and encapsidation occurred as aciseffect, which was not modified by WT fiber protein providedin transby the same Ad5 genome.

Mapping of two O-GlcNAc modification sites in the capsid protein of the potyvirus Plum pox virus

A large number of O-linked N-acetylglucosamine (O-GlcNAc) residues have been mapped in vertebrate proteins, however targets of O-GlcNAcylation in plants still have not been characterized. We show here that O-GlcNAcylation of the N-terminal region of the capsid protein of Plum pox virus resembles that of animal proteins in introducing O-GlcNAc monomers. Thr-19 and Thr-24 were specifically O-GlcNAcylated. These residues are surrounded by amino acids typical of animal O-GlcNAc acceptor sites, suggesting that the specificity of O-GlcNAc transferases is conserved among plants and animals. In laboratory conditions, mutations preventing O-GlcNAcylation of Thr-19 and Thr-24 did not have noticeable effects on PPV competence to infect Prunus persicae or Nicotiana clevelandii. However, the fact that Thr-19 and Thr-24 are highly conserved among different PPV strains suggests that their O-GlcNAc modification could be relevant for efficient competitiveness in natural conditions.

Identification and characterization of a host reversibly glycosylated peptide that interacts with the Tomato leaf curl virus V1 protein

Monopartite geminiviruses of the genus Begomovirus have two virion-sense genes, V1 and V2. V2 encodes the viral coat protein, but the function of V1 is largely unknown, although some studies suggest that it may play a role in cell-to-cell movement. Yeast two-hybrid technology was used to identify possible host binding partners of V1 from Tomato leaf curl virus (TLCV) to better understand its function. A protein closely related to a family of plant reversibly glycosylated peptides, designated SlUPTG1, was found to interact with V1 in yeast and in vitro. SlUPTG1 may function endogenously in the synthesis of cell wall polysaccharides, since a bacterially expressed form of the protein acted as an autocatalytic glycosyltransferase in vitro, a SlUPTG1:GFP fusion protein localized to the cell wall, and expression of SlUPTG1 appeared to be highest in actively dividing tissues. However, expression of SlUPTG1 in a transient TLCV replication assay increased the accumulation of viral DNA, suggesting that this host factor also plays a role in viral infection. Together, these data provide new insight into the role of V1 in TLCV infection and reveal another host pathway which geminiviruses may manipulate to achieve an efficient infection.

Identification of secret agent as the O-GlcNAc transferase that participates in Plum pox virus infection

Serine and threonine of many nuclear and cytoplasmic proteins are posttranslationally modified with O-linked N-acetylglucosamine (O-GlcNAc). This modification is made by O-linked N-acetylglucosamine transferases (OGTs). Genetic and biochemical data have demonstrated the existence of two OGTs of Arabidopsis thaliana, SECRET AGENT (SEC) and SPINDLY (SPY), with at least partly overlapping functions, but there is little information on their target proteins. The N terminus of the capsid protein (CP) of Plum pox virus (PPV) isolated from Nicotiana clevelandii is O-GlcNAc modified. We show here that O-GlcNAc modification of PPV CP also takes place in other plant hosts, N. benthamiana and Arabidopsis. PPV was able to infect the Arabidopsis OGT mutants sec-1, sec-2, and spy-3, but at early times of the infection, both rate of virus spread and accumulation were reduced in sec-1 and sec-2 relative to spy-3 and wild-type plants. By matrix-assisted laser desorption ionization-time of flight mass spectrometry, we determined that a 39-residue tryptic peptide from the N terminus of CP of PPV purified from the spy-3 mutant, but not sec-1 or sec-2, was O-GlcNAc modified, suggesting that SEC but not SPY modifies the capsid. While our results indicate that O-GlcNAc modification of PPV CP by SEC is not essential for infection, they show that the modification has a role(s) in the process.

Identification of the Glycosylation Site of the Adenovirus Type 5 Fiber Protein

The fiber protein purified from the pool of nonincorporated viral protein after infection of cells with adenovirus type 5 exists as two forms separable by reverse-phase HPLC. As determined by mass spectrometry, this heterogeneity results from a difference in one O-linked N-acetylglucosamine (GlcNac). A western blot analysis using a monoclonal antibody directed against the GlcNac motif showed that only one of the two forms reacted with the antibody, suggesting that one form carries a single GlcNac and the other form has none. The ratio of glycosylated to nonglycosylated forms of fiber, which is about 1, is conserved in assembled viruses. After digestion of glycosylated fiber with endoproteinase GluC, isolation of the glycosylated peptide by reverse-phase HPLC, and chemical derivatization using dimethylamine, the site of glycosylation was located in the fiber shaft at serine 109 by mass spectrometry. Elimination of glycosylation by site-directed mutagenesis of fiber should help to understand the function of this postranslational modification.

Proteomic approaches to analyze the dynamic relationships between nucleocytoplasmic protein glycosylation and phosphorylation

O-linked beta-N-acetylglucosamine (O-GlcNAc) is both an abundant and dynamic posttranslational modification similar to phosphorylation that occurs on serine and threonine residues of cytosolic and nuclear proteins in all metazoans and cell types examined, including cardiovascular tissue. Since the discovery of O-GlcNAc more than 20 years ago, the elucidation of O-GlcNAc as a posttranslational modification has been slow, albeit similar to the rate of acceptance of phosphorylation, because of the lack of tools available for its study. Identifying O-GlcNAc posttranslational modifications on proteins is a major challenge to proteomics. The recent development of mild beta-elimination followed by Michael addition with dithiothreitol has significantly improved the site mapping of both O-GlcNAc and O-phosphate in functional proteomics. beta-Elimination followed by Michael addition with dithiothreitol facilitates the study of the labile O-GlcNAc modification in the etiology of disease states. We discuss how recent technological innovations will expand our present understanding of O-GlcNAc and what the implications are for diabetes and cardiovascular complications.

Genetic modification of adenovirus 5 tropism by a novel class of ligands based on a three-helix bundle scaffold derived from staphylococcal protein A

The use of adenovirus (Ad) as an efficient and versatile vector for in vivo tumor therapy requires the modulation of its cellular tropism. We previously developed a method to genetically alter the tropism of Ad5 fibers by replacing the fiber knob domain by an extrinsic trimerization motif and a new cellular ligand. However, fibers carrying complex ligands such as single-chain antibody fragments did not assemble into functional pentons in vitro in the presence of penton base, and failed to be rescued into infectious virions because of their inability to fold correctly within the cytoplasm of Ad-infected cells. Here we show that the coding sequence for a disulfide bond-independent three-helix bundle scaffold Z, derived from domain B of Staphylococcal protein A and capable of binding to the Fc portion of immunoglobulin (Ig) G1, could be incorporated into modified knobless Ad fiber gene constructs with seven shaft repeats. These fiber gene constructs could be rescued into viable virions that were demonstrated to enter 293 cells engineered for IgG Fc surface expression but not unmodified 293 cells, via a mechanism that could be specifically blocked with soluble Fc target protein. However, the tropism modified viruses showed a slightly impaired cellular entry and a lower infectivity than wildtype (WT) virus. In addition, we generated recombinant fibers containing an IgA binding Affibody ligand, derived from combinatorial specificity-engineering of the Z domain scaffold. Such fiber constructs also showed the expected target specific binding, indicating that the affibody protein class is ideally suited for genetic engineering of Ad tropism.

Genetic retargeting of adenovirus vectors: functionality of targeting ligands and their influence on virus viability

BACKGROUND

We studied the ability of adenovirus type 5 (Ad5) to encapsidate new cellular ligands carried by their fibers to yield functional retargeted vectors for gene therapy. Recombinant Ad5 fibers containing shaft repeats 1 to 7 and an extrinsic trimerization motif, and terminated by its native knob or amino acid motifs containing RGD, have been rescued into infectious virions.

METHODS

Polypeptide ligands of cell surface molecules, including single-chain antibodies or epidermal growth factor, were cloned into recombinant fibers. Phenotypic analysis of fiber constructs and rescuing into the Ad5 genome were performed. Recombinant viruses were characterized with reference to fiber content, growth rate and infectivity.

RESULTS

A major limiting factor for recovering viable recombinant Ad5 carrying fiber-fused polypeptide ligands was apparently the ability of the ligand to fold correctly within the cellular cytoplasm. This constraint has previously not been systematically evaluated in the literature. Phenotypic analysis of the fiber-ligand fusions showed that their degree of cytoplasmic solubility correlated with their ability to yield viable Ad5 vectors. Our results suggested that the fiber manipulations diminish virus growth rate, probably through different, opposing effects: (i) the reduced shaft length increases fiber solubility in the absence of the knob but (ii) diminishes virus entry, and (iii) the absence of the knob alters the overall protein composition of the virion and decreases its fiber copy number.

CONCLUSIONS

Based on our findings, cytoplasmic solubility and cytoplasmic ligand reactivity of fiber-ligand fusion proteins are the best prediction criterion for viability and recovery of genetically retargeted Ad vectors.

The capsid protein of a plant single-stranded RNA virus is modified by O-linked N-acetylglucosamine

Plum pox virus (PPV) is a member of the Potyvirus genus of plant viruses. Labeling with UDP-[3H]galactose and galactosyltransferase indicated that the capsid protein (CP) of PPV is a glycoprotein with N-acetylglucosamine terminal residues. Mass spectrometry analysis of different PPV isolates and mutants revealed O-linked N-acetylglucosamination, a modification barely studied in plant proteins, of serine and/or threonine residues near the amino end of PPV CP. CP of PPV virions is also modified by serine and threonine phosphorylation, as shown by Western blot analysis with anti-phosphoserine and anti-phosphothreonine antibodies. Thus, "yin-yang" glycosylation and phosphorylation may play an important role in the regulation of the different functions in which the potyviral CP is involved.

Adenovirus serotype 30 fiber does not mediate transduction via the coxsackie-adenovirus receptor

Prior work by members of our laboratory and others demonstrated that adenovirus serotype 30 (Ad30), a group D adenovirus, exhibited novel transduction characteristics compared to those of serotype 5 (Ad5, belonging to group C). While some serotype D adenoviruses bind to the coxsackie-adenovirus receptor (CAR), the ability of Ad30 fiber to bind CAR is unknown. We amplified and purified Ad30 and cloned the Ad30 fiber by overlap PCR. Alignment of Ad30 fiber with Ad3, Ad35, Ad5, Ad9, and Ad17 revealed that Ad30, like Ad9 and Ad17, has a shortened fiber sequence relative to that of Ad5. The knob region of fiber was 45% identical to that of the Ad5 knob regions. We made a chimeric recombinant virus (Ad5GFPf30) in which the Ad5 fiber (amino acids [aa]47 to 582) was replaced with Ad30 fiber sequences (aa 46 to 372), and CAR-mediated viral entry was determined on CAR-expressing Chinese hamster ovary (CHO) cells. While CAR expression significantly increased Ad5GFP-mediated transduction in CHO cells (from 1 to 36%), it did not enhance Ad5GFPf30 gene transfer. Binding of radiolabeled Ad5GFPf30 or Ad30 wild-type virus was also not improved by the expression of CAR. These results suggest that Ad30 fiber is distinct from Ad5, Ad9, and Ad17 fibers in its inability to direct transduction via CAR.

Genetic retargeting of adenovirus: novel strategy employing "deknobbing" of the fiber

For efficient and versatile use of adenovirus (Ad) as an in vivo gene therapy vector, modulation of the viral tropism is highly desirable. In this study, a novel method to genetically alter the Ad fiber tropism is described. The knob and the last 15 shaft repeats of the fiber gene were deleted and replaced with an external trimerization motif and a new cell-binding ligand, in this case the integrin-binding motif RGD. The corresponding recombinant fiber retained the basic biological functions of the natural fiber, i.e., trimerization, nuclear import, penton formation, and ligand binding. The recombinant fiber bound to integrins but failed to react with antiknob antibody. For virus production, the recombinant fiber gene was rescued into the Ad genome at the exact position of the wild-type (WT) fiber to make use of the native regulation of fiber expression. The recombinant virus Ad5/FibR7-RGD yielded plaques on 293 cells, but the spread through the monolayer was two to three times delayed compared to WT, and the ratio of infectious to physical particles was 20 times lower. Studies on virus tropism showed that Ad5/FibR7-RGD was able to infect cells which did not express the coxsackie-adenovirus receptor (CAR), but did express integrins. Ad5/FibR7-RGD virus infectivity was unchanged in the presence of antiknob antibody, which neutralized the WT virus. Ad5/FibR7-RGD virus showed an expanded tropism, which is useful when gene transfer to cells not expressing CAR is needed. The described method should also make possible the construction of Ad genetically retargeted via ligands other than RGD.

Molecular and genetic evidence for a virus-encoded glycosyltransferase involved in protein glycosylation

The major capsid protein, Vp54, of chlorella virus PBCV-1 is a glycoprotein that contains either one glycan of approximately 30 sugar residues or two similar glycans of approximately 15 residues. Previous analysis of PBCV-1 antigenic mutants that contained altered Vp54 glycans led to the conclusion that unlike other glycoprotein-containing viruses, most, if not all, of the enzymes involved in the synthesis of the Vp54 glycan are probably encoded by PBCV-1 (I.-N. Wang et al., 1993, Proc. Natl. Acad. Sci. USA 90, 3840-3844). In this report we used molecular and genetic approaches to begin to identify these virus genes. Comparing the deduced amino acid sequences of the putative 375 PBCV-1 protein-encoding genes to databases identified seven potential glycosyltransferases. One gene, designated a64r, encodes a 638-amino-acid protein that has four motifs conserved in "Fringe type" glycosyltransferases. Analysis of 13 PBCV-1 antigenic mutants revealed mutations in a64r that correlated with a specific antigenic variation. Dual-infection experiments with different antigenic mutants indicated that viruses that contained wild-type a64r could complement and recombine with viruses that contained mutant a64r to form wild-type virus. Therefore, we conclude that a64r encodes a glycosyltransferase involved in synthesizing the Vp54 glycan. This is the first report of a virus-encoded glycosyltransferase involved in protein glycosylation.

Polypeptide composition of an adenovirus type 5 used in cancer gene therapy

For cancer gene therapy, a recombinant adenovirus serotype 5 named RPR/INGN201 has been constructed by susbtitution of the E1 region with human tumor suppressor gene p53. The protein components of RPR/INGN201 virions were separated by reversed-phase HPLC and were individually identified by electrospray time-of-flight mass spectrometry and N-terminal sequencing, both on intact proteins and on their proteolytic fragments after trypsin digestion. Twenty-five peptide components of the proteome (including fiber) with greater than 0.25-0.5% contribution to the protein content of the virus were identified and characterized. Fiber was confirmed to be partially glycosylated (both the non-glycosylated and the monoglycosylated states were identified), and two proteins were isolated and identified as phosphorylation derivatives, namely protein V (non-phosphorylated and monophosphorylated) and protein IIIa (mono- and diphosphorylated). This new analytical tool proved to be very useful not only for refining our current knowledge of the polypeptide repertoire of purified infectious virions but also for monitoring and very rapidly identifying structural modifications resulting from changes in the manufacturing process. It was also used successfully for the characterization of various adenoviral constructs.

Reversed-phase high-performance liquid chromatographic assay for the adenovirus type 5 proteome

An RP-HPLC assay was developed for a recombinant adenovirus type 5. During chromatography, intact adenovirus dissociated into its structural components (DNA and proteins) and the viral proteome was separated yielding a characteristic fingerprint. The individual components were identified by matrix-assisted laser desorption ionization time-of-flight mass spectroscopy, N-terminal sequencing and amino acid composition. The assay was utilized to measure adenovirus particle concentration through quantification of structural proteins. Each structural protein provided independent measurement of virus concentration allowing verification of accuracy. The assay sensitivity is at or below 2 x 10(8) particles. Contrary to the benchmark spectrophotometric assay, the RP-HPLC assay was shown to be insensitive to contaminants common for partially purified adenovirus preparations.

Unfolding studies of human adenovirus type 2 fibre trimers. Evidence for a stable domain

Adenovirus fibres are trimeric proteins that protrude from the 12 fivefold vertices of the virion and are the cell attachment organelle of the virus. They consist of three segments: an N-terminal tail, which is noncovalently attached to the penton base, a thin shaft carrying 15 amino acid pseudo repeats, and a C-terminal globular head (or knob) which recognizes the primary cell receptor. Due to their exceptional stability, which allows easy distinction of native trimers from unfolded forms and folding intermediates, adenovirus fibres are a very good model system for studying folding in vivo and in vitro. To understand the folding and stability of the trimeric fibres, the unfolding pathway of adenovirus 2 fibres induced by SDS and temperature has been investigated. Unfolding starts from the N-terminus and a stable intermediate accumulates that has the C-terminal head and part of the shaft structure (shown by electron microscopy). The unfolded part can be digested away using limited proteolysis, and the precise digestion sites have been determined. The remaining structured fragment is recognized by monoclonal antibodies that are specific for the trimeric globular head and therefore retains a native trimeric structure. Taken together, our results indicate that adenovirus fibres carry a stable C-terminal domain, consisting of the knob with five shaft-repeats.

Infectious pancreatic necrosis virus: identification of a VP3-containing ribonucleoprotein core structure and evidence for O-linked glycosylation of the capsid protein VP2

Virions of infectious pancreatic necrosis virus (IPNV) were completely disintegrated upon dialysis against salt-free buffers. Direct visualization of such preparations by electron microscopy revealed 5.0- to 6.5-nm-thick entangled filaments. By using a specific colloidal gold immunolabeling technique, these structures were shown to contain the viral protein VP3. Isolation by sucrose gradient centrifugation of the filaments, followed by serological analysis, demonstrated that the entire VP3 content of the virion was recovered together with the radiolabeled genomic material forming the unique threadlike ribonucleoprotein complexes. In a sensitive blotting assay, the outer capsid component of IPNV, i.e., the major structural protein VP2, was shown to specifically bind lectins recognizing sugar moieties of N-acetylgalactosamine, mannose, and fucose. Three established metabolic inhibitors of N-linked glycosylation did not prevent addition of sugar residues to virions, and enzymatic deglycosylation of isolated virions using N-glycosidase failed to remove sugar residues of VP2 recognized by lectins. However, gentle alkaline beta elimination clearly reduced the ability of lectins to recognize VP2. These results suggest that the glycosylation of VP2 is of the O-linked type when IPNV is propagated in RTG-2 cells.

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.

A subpopulation of estrogen receptors are modified by O-linked N-acetylglucosamine

Estrogen receptors (ER) are ligand-inducible transcription factors regulated by Ser(Thr)-O-phosphorylation. Many transcription factors and eukaryotic RNA polymerase II itself are also dynamically modified by Ser(Thr)-O-linked N-acetylglucosamine moieties (O-GlcNAc). Here we report that subpopulations of murine, bovine, and human estrogen receptors are modified by O-GlcNAc. O-GlcNAc moieties were detected on insect cell-expressed, mouse ER (mER) by probing with bovine milk galactosyltransferase, followed by structural analysis. Wheat germ agglutinin-Sepharose affinity chromatography also readily detected terminal GlcNAc residues on subpopulations of ER purified from calf uterus, from human breast cancer cells (MCF-7), or from mER produced by in vitro translation. These data suggest that greater than 10% of these populations of estrogen receptors bear O-GlcNAc. Site mapping of insect cell expressed mER localized one major site of O-GlcNAc addition to Thr-575, within a PEST region of the carboxyl-terminal F domain. Based upon their relative resistance to both hexosaminidase and to in vitro galactosylation, O-GlcNAc moieties appear to be largely buried on native mER. This dynamic saccharide modification, like phosphorylation, may play a role in modulating the dimerization, stability, or transactivation functions of estrogen receptors.

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.

Domains required for assembly of adenovirus type 2 fiber trimers

Entry of human adenovirus into cells is a two-step process, mediated in the first step by a specific interaction between the trimeric fiber protein and a specific receptor on the surface of susceptible cells. Because of the interest in human adenovirus as a vector for gene therapy, we have mapped domains in the fiber protein that are important for proper assembly of this trimeric structure and for proper addition of O-linked N-acetylglucosamine (0-GlcNAc). Mutants of adenovirus type 2 fiber in this study were expressed in human cells by use of a recombinant vaccinia virus expression system that yielded protein indistinguishable from the fiber produced during adenovirus infection. The N-terminal half of the protein did not appear to influence fiber trimer formation, since deletions up to 260 amino acids (aa) from the N-terminal end as well as in-frame deletions within the shaft of the molecule still allowed trimerization; internal deletions in the shaft between aa 61 and 260 appeared to alter addition of 0-GlcNAc, as judged by loss of reactivity to a monoclonal antibody specific for this carbohydrate addition. Deletions from the C terminus of the molecule (as small as 2 aa) appeared to prevent trimer formation. Additions of amino acids to the C-terminal end of the fiber showed variable results: a 6-aa addition allowed trimer formation, while a 27-aa addition did not. These trimer-defective mutants were also relatively less stable, as judged kV pulse-chase experiments. Taken together, our results indicate that trimerization of the fiber requires at least two domains, the entire head (aa 400 to 582), and at least the C-terminal-most 15 aa of the shaft.

Intracellular posttranslational modifications of S1133 avian reovirus proteins

Avian reovirus S1133 specifies at least 10 primary translation products, eight of which are present in the viral particle and two of which are nonstructural proteins. In the work presented here, we studied the covalent modifications undergone by these translation products in the infected cell. The structural polypeptide mu2 was shown to be intracellularly modified by both myristoylation and proteolysis. The site-specific cleavage of mu2 yielded a large carboxy-terminal fragment and a myristoylated approximately 5,500-Mr peptide corresponding to the amino terminus. Both mu2 and its cleavage products were found to be structural components of the reovirion. Most avian reovirus proteins were found to be glycosylated and to have a blocking group at the amino terminus. In contrast to the mammalian reovirus system, none of the avian reovirus polypeptides was found to incorporate phosphorus during infection. Our results add to current understanding of the similarities and differences between avian and mammalian reoviruses.

Site-specific glycosylation of the human cytomegalovirus tegument basic phosphoprotein (UL32) at serine 921 and serine 952

The virion basic phosphoprotein (BPP), UL32, of the human cytomegalovirus (HCMV) is a 149-kDa tegument protein that represents about 15% of the virion protein mass and is modified by O-linked N-acetylglucosamine (O-GlcNAc). O-GlcNAc has been postulated to mediate subunit-subunit interaction in many different types of intracellular protein complexes, while BPP may play a role in viral assembly and/or envelopment. This report describes the identification of the major O-GlcNAc attachment sites on the HCMV (AD169) BPP. Because the amount of BPP isolated from infectious virions was insufficient to determine the site(s) of glycosylation, the full-length protein has been characterized following overexpression in recombinant baculovirus-infected insect cells. The recombinant protein (rBPP) was electrophoretically (by sodium dodecyl sulfate-polyacrylamide gel electrophoresis) and immunologically (by Western immunoassaying) indistinguishable from the BPP isolated from HCMV virions. In addition, the rBPP was modified by O-GlcNAc, and a comparison of the tryptic glycopeptides from the rBPP and native virion BPP indicated that their O-GlcNAc sites are the same. Furthermore, the major sites of O-GlcNAc attachment to the rBPP were mapped on high-performance liquid chromatography-purified glycopeptides by gas phase microsequencing, manual Edman degradation, and electrospray-mass spectrometry. The results demonstrate that the major sites of O-GlcNAc attachment to the BPP are Ser-921 and Ser-952. Identification of these sites will now enable mutagenesis studies to determine the influence of O-GlcNAc on the intracellular location, protein-protein interaction, and biological function of BPP. Finally, the fidelity of the addition of O-GlcNAc to rBPP in insect cells compared with native virion BPP is documented to demonstrate the possible general applicability of the baculovirus expression system to study O-GlcNAc on other low-abundance proteins.

Human adenovirus type 41 contains two fibers

DNA sequencing of the subgroup F human adenovirus serotype 41 (TAK, Ad41) fiber gene revealed the presence of two adjacent open reading frames encoding information for proteins with molecular weights of 60.6 kDa and 41.4 kDa (Pieniazek, et al; Nucleic Acids Res. 18: p. 1901, 1990). In this paper, various approaches were used to characterize the two proteins and determine whether both fibers were expressed in infected cells as well as on viral particles. We initially used a reverse transcriptase-polymerase chain reaction with primers for the short and long fiber genes to amplify mRNA from Ad41 infected HEp-2 cells at 48 h post-infection. Two distinct DNA bands; one slightly larger than 1.1 kbp and the other at about 1.7 kbp were identified. Second, we used polyclonal anti-Ad41 virion and monoclonal anti-Ad5 fiber antibodies to demonstrate that at both 24 and 36 h post-infection, Ad41 expressed two fiber proteins of the expected size. Specifically, by SDS-PAGE, one fiber (short) had a molecular weight of 40 kDa, while the other (long) had a molecular weight of 60 kDa. Third, by electron microscopy, two sizes of fibers were released from CsCl purified virions, both having a characteristic adenovirus morphology, with a knob at one end. The long fiber measured 315A in length and the short fiber was 250A long. These measurements are consistent with the two Ad41 fibers being encoded by the above open reading frames. We also performed a computer search to compare fiber sequences from other human adenovirus serotypes with that of the Ad41 short and long fiber proteins. The primary structure of both Ad41 fibers were found to be similar in that they contained tail, shaft and knob regions. Further, the tail region of both fibers (amino acids 1-42) showed a 74% overall homology to each other and contained the Ad conserved sequence NH2-F-N-P-V-Y-P-Y-COOH. An interesting difference, however, was observed in the shaft region where the long fiber (amino acids 43-389) had twenty-two 16-amino acid repeat motifs, while the short fiber (amino acids 43-233) had only twelve. Finally, we noted that the long fiber knob region was about 15% longer than that of the short fiber, and showed little overall homology. In conclusion, human adenovirus subgroup F (type 41) virions appear to differ from those of all other human adenoviruses (subgenera A-E) in that they contain two fiber genes and correspondingly, two different sized fibers.(ABSTRACT TRUNCATED AT 400 WORDS)

Cell-binding domain of adenovirus serotype 2 fiber

The adenovirus fiber appears as a long, thin projection terminated by a knob (head). The fiber consists of a trimeric protein whose head domain is thought to interact with cell receptors. The head part (amino acids 388 to 582) of adenovirus type 2 fiber was produced in a baculovirus expression system. The purified protein was shown to cross-link into trimers. It was very resistant to proteolytic attack and seemed to attain a high degree of compactness. The head domain efficiently inhibited attachment of adenovirus to receptors on the surface of HeLa cells, thereby confirming the hypothesis that the head domain interacts with viral receptors.

The E3-14.5K integral membrane protein of adenovirus that is required for down-regulation of the EGF receptor and for prevention of TNF cytolysis is O-glycosylated but not N-glycosylated

The adenovirus E3-14.5K protein is a cytoplasmic integral membrane protein that functions in concert with the E3-10.4K protein to down-regulate the epidermal growth factor receptor and to prevent tumor necrosis factor cytolysis in adenovirus-infected cells. The 14.5K protein migrates as multiple bands in SDS-PAGE, indicating that it undergoes post-translational modification. The 14.5K protein is known to be phosphorylated on serine. We show here that 14.5K can be metabolically labeled with [3H]glucosamine, that the label is labile to alkali, and that the SDS-PAGE band pattern is simplified in a cell line that is defective in O-glycosylation. Thus, 14.5K is O-glycosylated, probably at a single site in the NH2-terminal lumenal domain. The protein was not metabolically labeled with [3H]mannose, and its SDS-PAGE band pattern was not affected by tunicamycin treatment in vivo or endo F treatment in vitro; thus, 14.5K is not N-glycosylated. There was no evidence that the 10.4K protein is glycosylated, and the 10.4K protein was not required for glycosylation of 14.5K. Virtually all 14.5K molecules appear to contain the core disaccharide Gal beta 1-3GalNAc alpha 1-Ser/Thr which is commonly found on mucin-type O-glycoproteins, and neuraminidase digestion experiments indicated that this disaccharide contains terminal sialic acid.

Deletion analysis of functional domains in baculovirus-expressed adenovirus type 2 fiber

Various forms of Ad2 fiber were expressed in insect cells using recombinant baculoviruses and phenotypically characterized with respect to the following properties: trimerization, binding to penton base, nuclear targeting, and glycosylation. The morphology and dimensions of full-length fiber produced by invertebrate cells were indistinguishable from those observed in extracts from lytically infected mammalian cells. The domain required for trimer formation was mapped to the C-terminus, between amino acids 541 and 582. The N-terminal domain, between amino acids 1 and 16, negatively influenced the trimerization efficiency. Fiber gene products reduced to the shaft portion of the fiber capsomer formed significant amounts of stable dimers. Recognition with penton base only occurred with trimeric forms of fiber and was apparently not affected by deletion of the first 60 amino acids from the N-terminus. Fiber deleted of the Met1-Gly60 sequence was found to localize within the nucleus at levels similar to those of full-length fiber. All recombinant fibers, including tail-and-know-deleted forms, were found to be glycosylated using three separate assays, (i) in vivo labeling with [3H]glucosamine, (ii) binding to WGA, and (iii) reaction with monoclonal antibody RL2 directed against O-GlcNAc-containing glycopeptide. This implied that Ad2 fiber is a substrate for GlcNAc O-seryl transferase in insect cell cytoplasm and that at least one major glycosylation site is located in the shaft domain, between Met61 and Asn410.