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

Opportunities for Therapeutic Modulation of O-GlcNAc

O-Linked β-N-acetylglucosamine (O-GlcNAc) is an essential, dynamic monosaccharide post-translational modification (PTM) found on serine and threonine residues of thousands of nucleocytoplasmic proteins. The installation and removal of O-GlcNAc is controlled by a single pair of enzymes, O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), respectively. Since its discovery four decades ago, O-GlcNAc has been found on diverse classes of proteins, playing important functional roles in many cellular processes. Dysregulation of O-GlcNAc homeostasis has been implicated in the pathogenesis of disease, including neurodegeneration, X-linked intellectual disability (XLID), cancer, diabetes, and immunological disorders. These foundational studies of O-GlcNAc in disease biology have motivated efforts to target O-GlcNAc therapeutically, with multiple clinical candidates under evaluation. In this review, we describe the characterization and biochemistry of OGT and OGA, cellular O-GlcNAc regulation, development of OGT and OGA inhibitors, O-GlcNAc in pathophysiology, clinical progress of O-GlcNAc modulators, and emerging opportunities for targeting O-GlcNAc. This comprehensive resource should motivate further study into O-GlcNAc function and inspire strategies for therapeutic modulation of O-GlcNAc.

Biological Functions and Potential Therapeutic Significance of O-GlcNAcylation in Hepatic Cellular Stress and Liver Diseases

O-linked-β-D-N-acetylglucosamine (O-GlcNAc) glycosylation (O-GlcNAcylation), which is dynamically regulated by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), is a post-translational modification involved in multiple cellular processes. O-GlcNAcylation of proteins can regulate their biological functions via crosstalk with other post-translational modifications, such as phosphorylation, ubiquitination, acetylation, and methylation. Liver diseases are a major cause of death worldwide; yet, key pathological features of the disease, such as inflammation, fibrosis, steatosis, and tumorigenesis, are not fully understood. The dysregulation of O-GlcNAcylation has been shown to be involved in some severe hepatic cellular stress, viral hepatitis, liver fibrosis, nonalcoholic fatty acid liver disease (NAFLD), malignant progression, and drug resistance of hepatocellular carcinoma (HCC) through multiple molecular signaling pathways. Here, we summarize the emerging link between O-GlcNAcylation and hepatic pathological processes and provide information about the development of therapeutic strategies for liver diseases.

Elucidating the Implications of Norovirus N- and O-Glycosylation, O-GlcNAcylation, and Phosphorylation

Norovirus is the most common cause of foodborne gastroenteritis, affecting millions of people worldwide annually. Among the ten genotypes (GI-GX) of norovirus, only GI, GII, GIV, GVIII, and GIX infect humans. Some genotypes reportedly exhibit post-translational modifications (PTMs), including N- and O-glycosylation, O-GlcNAcylation, and phosphorylation, in their viral antigens. PTMs have been linked to increased viral genome replication, viral particle release, and virulence. Owing to breakthroughs in mass spectrometry (MS) technologies, more PTMs have been discovered in recent years and have contributed significantly to preventing and treating infectious diseases. However, the mechanisms by which PTMs act on noroviruses remain poorly understood. In this section, we outline the current knowledge of the three common types of PTM and investigate their impact on norovirus pathogenesis. Moreover, we summarize the strategies and techniques for the identification of PTMs.

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.

The Cytomegalovirus Tegument Protein UL35 Antagonizes Pattern Recognition Receptor-Mediated Type I IFN Transcription

The rapid activation of pattern recognition receptor (PRR)-mediated type I interferon (IFN) signaling is crucial for the host response to infection. In turn, human cytomegalovirus (HCMV) must evade this potent response to establish life-long infection. Here, we reveal that the HCMV tegument protein UL35 antagonizes the activation of type I IFN transcription downstream of the DNA and RNA sensors cGAS and RIG-I, respectively. We show that ectopic expression of UL35 diminishes the type I IFN response, while infection with a recombinant HCMV lacking UL35 induces an elevated type I IFN response compared to wildtype HCMV. With a series of luciferase reporter assays and the analysis of signaling kinetics upon HCMV infection, we observed that UL35 downmodulates PRR signaling at the level of the key signaling factor TANK-binding kinase 1 (TBK1). Finally, we demonstrate that UL35 and TBK1 co-immunoprecipitate when co-expressed in HEK293T cells. In addition, we show that a previously reported cellular binding partner of UL35, O-GlcNAc transferase (OGT), post-translationally GlcNAcylates UL35, but that this modification is not required for the antagonizing effect of UL35 on PRR signaling. In summary, we have identified UL35 as the first HCMV protein to antagonize the type I IFN response at the level of TBK1, thereby enriching our understanding of how this important herpesvirus escapes host immune responses.

Nutrient regulation of signaling and transcription

In the early 1980s, while using purified glycosyltransferases to probe glycan structures on surfaces of living cells in the murine immune system, we discovered a novel form of serine/threonine protein glycosylation (O-linked β-GlcNAc; O-GlcNAc) that occurs on thousands of proteins within the nucleus, cytoplasm, and mitochondria. Prior to this discovery, it was dogma that protein glycosylation was restricted to the luminal compartments of the secretory pathway and on extracellular domains of membrane and secretory proteins. Work in the last 3 decades from several laboratories has shown that O-GlcNAc cycling serves as a nutrient sensor to regulate signaling, transcription, mitochondrial activity, and cytoskeletal functions. O-GlcNAc also has extensive cross-talk with phosphorylation, not only at the same or proximal sites on polypeptides, but also by regulating each other's enzymes that catalyze cycling of the modifications. O-GlcNAc is generally not elongated or modified. It cycles on and off polypeptides in a time scale similar to phosphorylation, and both the enzyme that adds O-GlcNAc, the O-GlcNAc transferase (OGT), and the enzyme that removes O-GlcNAc, O-GlcNAcase (OGA), are highly conserved from C. elegans to humans. Both O-GlcNAc cycling enzymes are essential in mammals and plants. Due to O-GlcNAc's fundamental roles as a nutrient and stress sensor, it plays an important role in the etiologies of chronic diseases of aging, including diabetes, cancer, and neurodegenerative disease. This review will present an overview of our current understanding of O-GlcNAc's regulation, functions, and roles in chronic diseases of aging.

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

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

Human Cytomegalovirus Nuclear Capsids Associate with the Core Nuclear Egress Complex and the Viral Protein Kinase pUL97

The nuclear phase of herpesvirus replication is regulated through the formation of regulatory multi-component protein complexes. Viral genomic replication is followed by nuclear capsid assembly, DNA encapsidation and nuclear egress. The latter has been studied intensely pointing to the formation of a viral core nuclear egress complex (NEC) that recruits a multimeric assembly of viral and cellular factors for the reorganization of the nuclear envelope. To date, the mechanism of the association of human cytomegalovirus (HCMV) capsids with the NEC, which in turn initiates the specific steps of nuclear capsid budding, remains undefined. Here, we provide electron microscopy-based data demonstrating the association of both nuclear capsids and NEC proteins at nuclear lamina budding sites. Specifically, immunogold labelling of the core NEC constituent pUL53 and NEC-associated viral kinase pUL97 suggested an intranuclear NEC-capsid interaction. Staining patterns with phospho-specific lamin A/C antibodies are compatible with earlier postulates of targeted capsid egress at lamina-depleted areas. Important data were provided by co-immunoprecipitation and in vitro kinase analyses using lysates from HCMV-infected cells, nuclear fractions, or infectious virions. Data strongly suggest that nuclear capsids interact with pUL53 and pUL97. Combined, the findings support a refined concept of HCMV nuclear trafficking and NEC-capsid interaction.

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.

Quantitative temporal viromics: an approach to investigate host-pathogen interaction

A systematic quantitative analysis of temporal changes in host and viral proteins throughout the course of a productive infection could provide dynamic insights into virus-host interaction. We developed a proteomic technique called “quantitative temporal viromics” (QTV), which employs multiplexed tandem-mass-tag-based mass spectrometry. Human cytomegalovirus (HCMV) is not only an important pathogen but a paradigm of viral immune evasion. QTV detailed how HCMV orchestrates the expression of >8,000 cellular proteins, including 1,200 cell-surface proteins to manipulate signaling pathways and counterintrinsic, innate, and adaptive immune defenses. QTV predicted natural killer and T cell ligands, as well as 29 viral proteins present at the cell surface, potential therapeutic targets. Temporal profiles of >80% of HCMV canonical genes and 14 noncanonical HCMV open reading frames were defined. QTV is a powerful method that can yield important insights into viral infection and is applicable to any virus with a robust in vitro model.

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>8,000 proteins quantified over eight time points, including 1,200 cell-surface proteins

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Temporal profiles of 139/171 canonical HCMV proteins and 14 noncanonical HCMV ORFs

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Multiple families of cell-surface receptors selectively modulated by HCMV

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Multiple signaling pathways modulated during HCMV infection

Dynamic O-GlcNAcylation and its roles in the cellular stress response and homeostasis

O-linked N-acetyl-β-D-glucosamine (O-GlcNAc) is a ubiquitous and dynamic post-translational modification known to modify over 3,000 nuclear, cytoplasmic, and mitochondrial eukaryotic proteins. Addition of O-GlcNAc to proteins is catalyzed by the O-GlcNAc transferase and is removed by a neutral-N-acetyl-β-glucosaminidase (O-GlcNAcase). O-GlcNAc is thought to regulate proteins in a manner analogous to protein phosphorylation, and the cycling of this carbohydrate modification regulates many cellular functions such as the cellular stress response. Diverse forms of cellular stress and tissue injury result in enhanced O-GlcNAc modification, or O-GlcNAcylation, of numerous intracellular proteins. Stress-induced O-GlcNAcylation appears to promote cell/tissue survival by regulating a multitude of biological processes including: the phosphoinositide 3-kinase/Akt pathway, heat shock protein expression, calcium homeostasis, levels of reactive oxygen species, ER stress, protein stability, mitochondrial dynamics, and inflammation. Here, we will discuss the regulation of these processes by O-GlcNAc and the impact of such regulation on survival in models of ischemia reperfusion injury and trauma hemorrhage. We will also discuss the misregulation of O-GlcNAc in diseases commonly associated with the stress response, namely Alzheimer's and Parkinson's diseases. Finally, we will highlight recent advancements in the tools and technologies used to study the O-GlcNAc modification.

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.

Tandem lectin weak affinity chromatography for glycoprotein enrichment

In this chapter we describe the application of lectin weak affinity chromatography (LWAC) for the enrichment of peptides modified by O-linked β-N-acetylglucosamine (O-GlcNAc). O-GlcNAc is a single carbohydrate moiety post-translational modification of intracellular proteins. The stoichiometry of the modification is low and identification of the sites of O-GlcNAc attachment is challenging. To map O-GlcNAc sites we use the approach where a protein sample of interest is digested with trypsin and subjected to LWAC, which employs weak interaction between lectin wheat germ agglutinin and O-GlcNAc. Obtained sample is enriched with O-GlcNAc-modified peptides, which can be identified by means of mass spectrometry.

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.

Proteomic profiling of the human cytomegalovirus UL35 gene products reveals a role for UL35 in the DNA repair response

Human cytomegalovirus infections involve the extensive modification of host cell pathways, including cell cycle control, the regulation of the DNA damage response, and averting promyelocytic leukemia (PML)-mediated antiviral responses. The UL35 gene from human cytomegalovirus is important for viral gene expression and efficient replication and encodes two proteins, UL35 and UL35a, whose mechanism of action is not well understood. Here, affinity purification coupled with mass spectrometry was used to identify previously unknown human cellular targets of UL35 and UL35a. We demonstrate that both viral proteins interact with the ubiquitin-specific protease USP7, and that UL35 expression can alter USP7 subcellular localization. In addition, UL35 (but not UL35a) was found to associate with three components of the Cul4(DCAF1) E3 ubiquitin ligase complex (DCAF1, DDB1, and DDA1) previously shown to be targeted by the HIV-1 Vpr protein. The coimmunoprecipitation and immunofluorescence microscopy of DCAF1 mutants revealed that the C-terminal region of DCAF1 is required for association with UL35 and mediates the dramatic relocalization of DCAF1 to UL35 nuclear bodies, which also contain conjugated ubiquitin. As previously reported for the Vpr-DCAF1 interaction, UL35 (but not UL35a) expression resulted in the accumulation of cells in the G(2) phase of the cell cycle, which is typical of a DNA damage response, and activated the G(2) checkpoint in a DCAF1-dependent manner. In addition, UL35 (but not UL35a) induced γ-H2AX and 53BP1 foci, indicating the activation of DNA damage and repair responses. Therefore, the identified interactions suggest that UL35 can contribute to viral replication through the manipulation of host responses.

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.

Yeast two hybrid analyses reveal novel binary interactions between human cytomegalovirus-encoded virion proteins

Human cytomegalovirus (HCMV) is the largest human herpesvirus and its virion contains many viral encoded proteins found in the capsid, tegument, and envelope. In this study, we carried out a yeast two-hybrid (YTH) analysis to study potential binary interactions among 56 HCMV-encoded virion proteins. We have tested more than 3,500 pairwise combinations for binary interactions in the YTH analysis, and identified 79 potential interactions that involve 37 proteins. Forty five of the 79 interactions were also identified in human cells expressing the viral proteins by co-immunoprecipitation (co-IP) experiments. To our knowledge, 58 of the 79 interactions revealed by YTH analysis, including those 24 that were also identified in co-IP experiments, have not been reported before. Novel potential interactions were found between viral capsid proteins and tegument proteins, between tegument proteins, between tegument proteins and envelope proteins, and between envelope proteins. Furthermore, both the YTH and co-IP experiments have identified 9, 7, and 5 interactions that were involved with UL25, UL24, and UL89, respectively, suggesting that these “hub” proteins may function as the organizing centers for connecting multiple virion proteins in the mature virion and for recruiting other virion proteins during virion maturation and assembly. Our study provides a framework to study potential interactions between HCMV proteins and investigate the roles of protein-protein interactions in HCMV virion formation or maturation process.

Biochemical and structural characterization of the capsid-bound tegument proteins of human cytomegalovirus

Human cytomegalovirus (HCMV) is the most genetically and structurally complex human herpesvirus and is composed of an envelope, a tegument, and a dsDNA-containing capsid. HCMV tegument plays essential roles in HCMV infection and assembly. Using cryo electron tomography (cryoET), here we show that HCMV tegument compartment can be divided into two sub-compartments: an inner and an outer tegument. The inner tegument consists of densely-packed proteins surrounding the capsid. The outer tegument contains those components that are loosely packed in the space between the inner tegument and the pleomorphic glycoprotein-containing envelope. To systematically characterize the inner tegument proteins interacting with the capsid, we used chemical treatment to strip off the entire envelope and most tegument proteins to obtain a tegumented capsid with inner tegument proteins. SDS-polyacrylamide gel electrophoresis analyses show that only two tegument proteins, UL32-encoded pp150 and UL48-encoded high molecular weight protein (HMWP), remains unchanged in their abundance in the tegumented capsids as compared to their abundance in the intact particles. Three-dimensional reconstructions by single particle cryo electron microscopy (cryoEM) reveal that the net-like layer of icosahedrally-ordered tegument densities are also the same in the tegumented capsid and in the intact particles. CryoET reconstruction of the tegumented capsid labeled with an anti-pp150 antibody is consistent with the biochemical and cryoEM data in localizing pp150 within the ordered tegument. Taken together, these results suggest that pp150, a betaherpesvirus-specific tegument protein, is a constituent of the net-like layer of icosahedrally-ordered capsid-bound tegument densities, a structure lacking similarities in alpha and gammaherpesviruses.

A role for the small GTPase Rab6 in assembly of human cytomegalovirus

In human-cytomegalovirus (HCMV)-infected cells, the localization of the viral protein pp150 to the virus assembly compartment (AC) is dependent on its direct interaction with the cellular protein Bicaudal D1 through a dynein- and microtubule-dependent mechanism. We found that the small GTPase Rab6 also interacts indirectly with pp150 through its interaction with Bicaudal D1. Inhibition of Rab6 activity in HCMV-infected cells interrupted the intracellular trafficking of pp150, significantly reducing infectious virus production without affecting the formation of the AC, arguing for an important function for this cellular GTPase in the intracellular localization of pp150 during virus assembly.

Tegument proteins of human cytomegalovirus

SUMMARY

Human cytomegalovirus (HCMV) is a common, medically relevant human herpesvirus. The tegument layer of herpesvirus virions lies between the genome-containing capsids and the viral envelope. Proteins within the tegument layer of herpesviruses are released into the cell upon entry when the viral envelope fuses with the cell membrane. These proteins are fully formed and active and control viral entry, gene expression, and immune evasion. Most tegument proteins accumulate to high levels during later stages of infection, when they direct the assembly and egress of progeny virions. Thus, viral tegument proteins play critical roles at the very earliest and very last steps of the HCMV lytic replication cycle. This review summarizes HCMV tegument composition and structure as well as the known and speculated functions of viral tegument proteins. Important directions for future investigation and the challenges that lie ahead are identified and discussed.

Glycomic Approaches to Study GlcNAcylation: Protein Identification, Site-mapping, and Site-specific O-GlcNAc Quantitation

Background

O-Linked β-N-acetylglucosamine (O-GlcNAc) is an enzyme-catalyzed posttranslational modification of serine or threonine side chains of nuclear and cytoplasmic proteins. O-GlcNAc is present in all metazoans and in viruses that infect eukaryotic cells. GlcNAcylation is dynamic and has a high cycling rate on many proteins in response to cellular metabolism and various environmental stimuli. The rapid cycling of O-GlcNAc modulates many biological processes, including transcriptional regulation, stress responses, cell cycle regulation, and protein synthesis and turnover.

Rationale

Despite the importance of O-GlcNAc, progress during the past two decades in this field has been slow. One of the major obstacles is the lack of simple and sensitive tools for efficient O-GlcNAc detection and localization. Recently developed O-GlcNAc derivatization and enrichment approaches, together with new techniques in mass spectrometric instrumentation and methods, have provided breakthroughs in O-GlcNAc site localization and site-specific quantitation. In this review, we will discuss how the current techniques are expanding our knowledge about O-GlcNAc proteomics/glycomics and functions.

Nuclear localization sequences in cytomegalovirus capsid assembly proteins (UL80 proteins) are required for virus production: inactivating NLS1, NLS2, or both affects replication to strikingly different extents

Scaffolding proteins of spherical prokaryotic and eukaryotic viruses have critical roles in capsid assembly. The primary scaffolding components of cytomegalovirus, called the assembly protein precursor (pAP, pUL80.5) and the maturational protease precursor (pPR, pUL80a), contain two nuclear localization sequences (NLS1 and NLS2), at least one of which is required in coexpression experiments to translocate the major capsid protein (MCP, pUL85) into the nucleus. In the work reported here, we have mutated NLS1 and NLS2, individually or together, in human cytomegalovirus (HCMV, strain AD169) bacmid-derived viruses to test their effects on virus replication. Consistent with results from earlier transfection/coexpression experiments, both single-mutant bacmids gave rise to infectious virus but the double mutant did not. In comparisons with the wild-type virus, both mutants showed slower cell-to-cell spread; decreased yields of infectious virus (3-fold lower for NLS1(-) and 140-fold lower for NLS2(-)); reduced efficiency of pAP, pPR, and MCP nuclear translocation (sixfold lower for NLS1(-) and eightfold lower for NLS2(-)); increased amounts of a 120-kDa MCP fragment; and reduced numbers of intranuclear capsids. All effects were more severe for the NLS2(-) mutant than the NLS1(-) mutant, and a distinguishing feature of cells infected with the NLS2(-) mutant was the accumulation of large, UL80 protein-containing structures within the nucleus. We conclude that these NLS assist in the nuclear translocation of MCP during HCMV replication and that NLS2, which is unique to the betaherpesvirus UL80 homologs, may have additional involvements during replication.

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.

A novel high-capacity ion trap-quadrupole tandem mass spectrometer

We describe a prototype tandem mass spectrometer that is designed to increase the efficiency of linked-scan analyses by >100-fold over conventional linked-scan instruments. The key element of the mass spectrometer is a novel high ion capacity ion trap, combined in tandem configuration with a quadrupole collision cell and a quadrupole mass analyzer (i.e. a TrapqQ configuration). This ion trap can store >10(6) ions without significant degradation of its performance. The current mass resolution of the trap is 100-450 full width at half maximum for ions in the range 800-4000 m/z, yielding a 10-20 m/z selection window for ions ejected at any given time into the collision cell. The sensitivity of the mass spectrometer for detecting peptides is in the low femtomole range. We can envisage relatively straightforward modifications to the instrument that should improve both its resolution and sensitivity. We tested the tandem mass spectrometer for collecting precursor ion spectra of all the ions stored in the trap and demonstrated that we can selectively detect a phosphopeptide in a mixture of non-phosphorylated peptides. Based on this prototype instrument, we plan to construct a fully functional model of the mass spectrometer for detecting modification sites on proteins and profiling their abundances with high speed and sensitivity.

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.

Betaherpesvirus-conserved cytomegalovirus tegument protein ppUL32 (pp150) controls cytoplasmic events during virion maturation

The UL32 gene of human cytomegalovirus (CMV) encodes a prominent betaherpesvirus-conserved virion tegument protein, called pp150 (basic phosphoprotein/ppUL32), that accumulates within a cytoplasmic inclusion adjacent to the nucleus at late times during infection. Using a UL32 deletion mutant (DeltaUL32-BAC) (where BAC is bacterial artificial chromosome), we demonstrate that pp150 is critical for virion maturation in the cytoplasmic compartment. Cotransfection of a pp150 expression plasmid with DeltaUL32-BAC DNA led to complementation of the replication defect with focus formation due to secondary spread. Deletion of the amino terminus of pp150 or disruption of the betaherpesvirus conserved regions, CR1 and CR2, revealed these regions to be critical for replication. In contrast, deletion of the carboxyl terminus only partially compromised maturation while disruption of glycosylation sites had no effect. An African green monkey CMV UL32 homolog complemented DeltaUL32-BAC replication but murine CMV M32 failed to complement, consistent with evolutionary divergence of rodent and primate cytomegaloviruses. Infection with DeltaUL32-BAC showed normal expression of all kinetic classes of viral genes and replication of viral DNA, with accumulation of viral DNA-containing particles in the cytoplasm; however, mutant virus did not spread to adjacent cells. In contrast to this block in virion infectivity, cell-to-cell transfer of pp65-containing particles was observed, suggesting that release of dense bodies continued in the absence of pp150. These observations demonstrate that pp150 is critical for virion egress, possibly at the stage of final envelopment.

O-linked N-acetylglucosamine proteomics of postsynaptic density preparations using lectin weak affinity chromatography and mass spectrometry

O-GlcNAc is a widespread dynamic carbohydrate modification of cytosolic and nuclear proteins with features analogous to phosphorylation. O-GlcNAc acts critically in many cellular processes, including signal transduction, protein degradation, and regulation of gene expression. However, the study of its specific regulatory functions has been limited by difficulties in mapping sites of O-GlcNAc modification. We report methods for direct enrichment and identification of in vivo O-GlcNAc-modified peptides through lectin weak affinity chromatography (LWAC) and mass spectrometry. The effectiveness of this strategy on complex peptide mixtures was demonstrated through enrichment of 145 unique O-GlcNAc-modified peptides from a postsynaptic density preparation. 65 of these O-GlcNAc-modified peptides were sequenced and belonged to proteins with diverse functions in synaptic transmission. Beta-elimination/Michael addition, MS(3) on O-GlcNAc neutral loss ions, and electron capture dissociation were shown to facilitate analysis of O-GlcNAc-modified peptides/sites from lectin weak affinity chromatography enriched postsynaptic density samples. Bassoon and Piccolo, proteins critical to synapse assembly and vesicle docking, were extensively modified by O-GlcNAc. In some cases, O-GlcNAc was mapped to peptides previously identified as phosphorylated, indicating potential interplay between these modifications. Shared substrate amino acid context was apparent in subsets of O-GlcNAc-modified peptides, including "PVST" and a novel "TTA" motif (two hydroxyl-containing amino acids adjacent to an alanine). The results suggest specific roles for O-GlcNAc modification in synaptic transmission, establish a basis for site-specific regulatory studies, and provide methods that will facilitate O-GlcNAc proteome analysis across a wide variety of cells and tissues.

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.

Quantitative analysis of both protein expression and serine / threonine post-translational modifications through stable isotope labeling with dithiothreitol

While phosphorylation and O-GlcNAc (cytoplasmic and nuclear glycosylation) are linked to normal and pathological changes in cell states, these post-translational modifications have been difficult to analyze in proteomic studies. We describe advances in beta-elimination / Michael addition-based approaches which allow for mass spectrometry-based identification and comparative quantification of O-phosphate or O-GlcNAc-modified peptides, as well as cysteine-containing peptides for expression analysis. The method (BEMAD) involves differential isotopic labeling through Michael addition with normal dithiothreitol (DTT) (d0) or deuterated DTT (d6), and enrichment of these peptides by thiol chromatography. BEMAD was comparable to isotope-coded affinity tags (ICAT; a commercially available differential isotopic quantification technique) in protein expression analysis, but also provided the identity and relative amounts of both O-phosphorylation and O-GlcNAc modification sites. Specificity of O-phosphate vs. O-GlcNAc mapping is achieved through coupling enzymatic dephosphorylation or O-GlcNAc hydrolysis with differential isotopic labeling. Blocking of cysteine labeling by prior oxidation of a cytosolic lysate from mouse brain allowed specific targeting of serine / threonine post-translational modifications as demonstrated through identification of 21 phosphorylation sites (5 previously reported) in a single mass spectrometry analysis. These results demonstate BEMAD is suitable for large-scale quantitative analysis of both protein expression and serine / threonine post-translational modifications.

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.

The coactivator of transcription CREB-binding protein interacts preferentially with the glycosylated form of Stat5

The signal transducer and activator of transcription (Stat) gene family comprises seven members with similarities in their domain structure and a common mode of activation. Members of this gene family mediate interferon induction of gene transcription and the response to a large number of growth factors and hormones. Extracellular ligand binding to transmembrane receptors causes the intracellular activation of associated tyrosine kinases, phosphorylation of Stat molecules, dimerization, and translocation to the nucleus. Prolactin-induced phosphorylation of Stat5 is a key event in the development and differentiation of mammary epithelial cells. In addition to the crucial phosphorylation at tyrosine 694, we have identified an O-linked N-acetylglucosamine (O-GlcNAc) as another secondary modification essential for the transcriptional induction by Stat5. This modification was only found on nuclear Stat5 after cytokine activation. Similar observations were made with Stat1, Stat3, and Stat6. Glycosylation of Stat5, however, does not seem to be a prerequisite for nuclear translocation. Mass spectrometric analysis revealed a glycosylated peptide in the N-terminal region of Stat5. Replacement of threonine 92 by an alanine residue (Stat5a-T92A) strongly reduced the prolactin induction of Stat5a glycosylation and abolished transactivation of a target gene promoter. Only the glycosylated form of Stat5 was able to bind the coactivator of transcription CBP, an essential interaction for Stat5-mediated gene transcription.

Microtubule network facilitates nuclear targeting of human cytomegalovirus capsid

We assessed the requirement of the host cytoskeleton for the intracytosolic transport of incoming human cytomegalovirus (HCMV) capsids. Treatments with microtubule (MT)-depolymerizing drugs nocodazole and colchicine led to a drastic decrease in levels of IE1 antigen, whereas cytochalasin B had no effect on the level of IE1 as determined by Western blot analyses. Sequential treatment including nocodazole washout and removal of cell surface virion revealed that HCMV entry into the cells occurred normally in the absence of the MT network. This finding was also supported by data obtained by monitoring pUL83 signals with an immunofluorescent assay (IFA). Furthermore, we demonstrated a close association of incoming HCMV capsids with MTs by IFA and ultrastructural analyses. In the absence of the MT network, the capsids which had entered the cytoplasm did not move to close proximity of the nucleus. These data suggest that HCMV capsids associate with the MT network to facilitate their own movement to the nucleus before the onset of immediate-early (IE) gene expression and that this association is required to start efficient IE gene expression.

The transcription factor PDX-1 is post-translationally modified by O-linked N-acetylglucosamine and this modification is correlated with its DNA binding activity and insulin secretion in min6 beta-cells

The pancreatic/duodenal homeobox-1 protein (PDX-1, also called STF-1, IPF-1) is a transcription factor that plays an important role in pancreatic function and development. Here, we have overexpressed and purified PDX-1 from baculovirus/sf-9 cells, transiently transfected Cos-7 cells and native Min6 cells and demonstrated that the protein is posttranslationally modified by O-linked N-acetylglucosamine (O-GlcNAc). The approaches we used include binding of the protein to the lectin WGA, labeling with galactosyltransferase and UDP-[(3)H]gal and probing with the O-GlcNAc-specific antibody, RL-2. PNGase F treatment and structural analysis indicate that the carbohydrate is beta-linked O-GlcNAc. Mapping of [(3)H]gal-labeled tryptic peptides indicates that PDX-1 has two major sites for O-GlcNAcylation. In Min6 cells, elevated glucose concentration leads to an increase in protein O-GlcNAcylation and this hyperglycosylation correlates with an increase in DNA binding activity of PDX-1 and insulin secretion. On the other hand, the GFAT inhibitor azaserine reduces intracellular O-GlcNAc levels and profoundly attenuates glucose-stimulated insulin secretion. These data suggest that O-GlcNAcylation may be involved in the regulation of PDX-1 DNA binding activity and in glucose-stimulated insulin secretion in beta-cells.

Identification and cloning of a novel family of coiled-coil domain proteins that interact with O-GlcNAc transferase

The abundant and dynamic post-translational modification of nuclear and cytosolic proteins by beta-O-linked N-acetylglucosamine (O-GlcNAc) is catalyzed by O-GlcNAc transferase (OGT). Here we used the yeast two-hybrid approach to identify and isolate GABA(A) receptor-associated protein, GRIF-1 (Beck, M., Brickley, K., Wilkinson, H. L., Sharma, S., Smith, M., Chazot, P. L., Pollard, S., and Stephenson, F. A. (2002) J. Biol. Chem. 277, 30079-30090), and its novel homolog, OIP106 (KIAA1042), as novel OGT-interacting proteins. The proteins are highly similar to each other but are encoded by two separate genes. Both GRIF-1 and OIP106 contain coiled-coil domains and interact with the tetratricopeptide repeats of OGT. GRIF-1 and OIP106 are modified by O-GlcNAc and therefore are substrates for OGT. However, unlike another high affinity protein substrate, such as nucleoporin p62, OIP106 and GRIF-1 co-immunoprecipitate with OGT, exhibiting stable in vitro and in vivo associations. Whereas GRIF-1 has been reported to be expressed only in excitable tissue, OIP106 is expressed in all human cell lines that were examined. Confocal and electron microscopy show that OIP106 localizes to nuclear punctae in HeLa cells and co-localizes with RNA polymerase II. Co-immunoprecipitation experiments confirm the presence of an in vivo RNA polymerase II-OIP106-OGT complex, suggesting that OIP106 may target OGT to transcriptional complexes for glycosylation of transcriptional proteins, such as RNA polymerase II, and transcription factors. Similarly, GRIF-1 may serve to target OGT to GABA(A) receptor complexes for mediating GABA signaling cascades.

Mapping sites of O-GlcNAc modification using affinity tags for serine and threonine post-translational modifications

Identifying sites of post-translational modifications on proteins is a major challenge in proteomics. O-Linked beta-N-acetylglucosamine (O-GlcNAc) is a dynamic nucleocytoplasmic modification more analogous to phosphorylation than to classical complex O-glycosylation. We describe a mass spectrometry-based method for the identification of sites modified by O-GlcNAc that relies on mild beta-elimination followed by Michael addition with dithiothreitol (BEMAD). Using synthetic peptides, we also show that biotin pentylamine can replace dithiothreitol as the nucleophile. The modified peptides can be efficiently enriched by affinity chromatography, and the sites can be mapped using tandem mass spectrometry. This same methodology can be applied to mapping sites of serine and threonine phosphorylation, and we provide a strategy that uses modification-specific antibodies and enzymes to discriminate between the two post-translational modifications. The BEMAD methodology was validated by mapping three previously identified O-GlcNAc sites, as well as three novel sites, on Synapsin I purified from rat brain. BEMAD was then used on a purified nuclear pore complex preparation to map novel sites of O-GlcNAc modification on the Lamin B receptor and the nucleoporin Nup155. This method is amenable for performing quantitative mass spectrometry and can also be adapted to quantify cysteine residues. In addition, our studies emphasize the importance of distinguishing between O-phosphate versus O-GlcNAc when mapping sites of serine and threonine post-translational modification using beta-elimination/Michael addition methods.

Cytosolic O-glycosylation is abundant in nerve terminals

Phosphorylation plays a key role in regulating growth cone migration and protein trafficking in nerve terminals. Here we show that nerve terminal proteins contain another abundant post-translational modification: beta-N-acetylglucosamine linked to hydroxyls of serines or threonines (O-GlcNAc(1)). O-GlcNAc modifications are essential for embryogenesis and mounting evidence suggests that O-GlcNAc is a regulatory modification that affects many phosphorylated proteins. We show that the activity and expression of O-GlcNAc transferase (OGT) and N-acetyl-beta-D-glucosaminidase (O-GlcNAcase), the two enzymes regulating O-GlcNAc modifications, are present in nerve terminal structures (synaptosomes) and are particularily abundant in the cytosol of synaptosomes. Numerous synaptosome proteins are highly modified with O-GlcNAc. Although most of these proteins are present in low abundance, we identified by proteomic analysis three neuron-specific O-GlcNAc modified proteins: collapsin response mediator protein-2 (CRMP-2), ubiquitin carboxyl hydrolase-L1 (UCH-L1) and beta-synuclein. CRMP-2, which is involved in growth cone collapse, is a major O-GlcNAc modified protein in synaptosomes. All three proteins are implicated in regulatory cascades that mediate intracellular signaling or neurodegenerative diseases. We propose that O-GlcNAc modifications in the nerve terminal help regulate the functions of these and other synaptosome proteins, and that O-GlcNAc may play a role in neurodegenerative disease.

Cytomegalovirus basic phosphoprotein (pUL32) binds to capsids in vitro through its amino one-third

The cytomegalovirus (CMV) basic phosphoprotein (BPP) is a component of the tegument. It remains with the nucleocapsid fraction under conditions that remove most other tegument proteins from the virion, suggesting a direct and perhaps tight interaction with the capsid. As a step toward localizing this protein within the molecular structure of the virion and understanding its function during infection, we have investigated the BPP-capsid interaction. In this report we present evidence that the BPP interacts selectively, through its amino one-third, with CMV capsids. Radiolabeled simian CMV (SCMV) BPP, synthesized in vitro, bound to SCMV B-capsids, and C-capsids to a lesser extent, following incubation with either isolated capsids or lysates of infected cells. Human CMV (HCMV) BPP (pUL32) also bound to SCMV capsids, and SCMV BPP likewise bound to HCMV capsids, indicating that the sequence(s) involved is conserved between the two proteins. Analysis of SCMV BPP truncation mutants localized the capsid-binding region to the amino one-third of the molecule--the portion of BPP showing the greatest sequence conservation between the SCMV and HCMV homologs. This general approach may have utility in studying the interactions of other proteins with conformation-dependent binding sites.

Analysis and characterization of the complete genome of tupaia (tree shrew) herpesvirus

The tupaia herpesvirus (THV) was isolated from spontaneously degenerating tissue cultures of malignant lymphoma, lung, and spleen cell cultures of tree shrews (Tupaia spp.). The determination of the complete nucleotide sequence of the THV strain 2 genome resulted in a 195,857-bp-long, linear DNA molecule with a G+C content of 66.5%. The terminal regions of the THV genome and the loci of conserved viral genes were found to be G+C richer. Furthermore, no large repetitive DNA sequences could be identified. This is in agreement with the previous classification of THV as the prototype species of herpesvirus genome group F. The search for potential coding regions resulted in the identification of 158 open reading frames (ORFs) regularly distributed on both DNA strands. Seventy-six out of the 158 ORFs code for proteins that are significantly homologous to known herpesvirus proteins. The highest homologies found were to primate and rodent cytomegaloviruses. Biological properties, protein homologies, the arrangement of conserved viral genes, and phylogenetic analysis revealed that THV is a member of the subfamily Betaherpesvirinae. The evolutionary lineages of THV and the cytomegaloviruses seem to have branched off from a common ancestor. In addition, it was found that the arrangements of conserved genes of THV and murine cytomegalovirus strain Smith, both of which are not able to form genomic isomers, are colinear with two different human cytomegalovirus (HCMV) strain AD169 genomic isomers that differ from each other in the orientation of the long unique region. The biological properties and the high degree of relatedness of THV to the mammalian cytomegaloviruses allow the consideration of THV as a model system for investigation of HCMV pathogenicity.

Glycosylation of the murine estrogen receptor-alpha

O-linked N-acetylglucosamine (O-GlcNAc) is a highly dynamic and abundant modification found on nuclear and cytoplasmic proteins of nearly all eukaryotes. O-GlcNAc addition is required for life at the single cell level and is analogous to protein phosphorylation in most respects. In a previous study (M.S. Jiang, G.W. Hart, A subpopulation of estrogen receptors are modified by O-linked N-acetylglucosamine. J. Biol. Chem. 270 (1997) 2421-2428), we demonstrated that a subpopulation of the murine estrogen receptor-alpha (mER-alpha) is modified by O-GlcNAc at Thr(575). Here we mutated mER-alpha to convert Thr(575) and Ser(576) to Val and Ala, respectively. Surprisingly, this glycosylation-site mutant is still extensively modified by O-GlcNAc. Analyses of glycopeptides identified two additional sites of modification on mER-alpha, at Ser(10) and Thr(50) near the N-terminus. The major glycosylation sites are within or near PEST regions, suggesting that O-GlcNAc may regulate mER-alpha turnover.

Simultaneous detection and identification of O-GlcNAc-modified glycoproteins using liquid chromatography-tandem mass spectrometry

Glycoproteins carrying O-linked N-acetylglucosamine (O-GlcNAc) modifications have been isolated from a wide range of organisms ranging from trypanosomes to humans. Interest in this modification is increasing as evidence accumulates that it is an abundant and transient modification that is dynamic and responsive to cellular stimuli. Concurrent advances in biological mass spectrometry (MS) have facilitated high-sensitivity protein identification by tandem MS. In this study, we show that the lability of the O-GlcNAc moiety to low-energy collision in tandem MS offers a means of distinguishing such peptides from others that are not modified. The differential between the energy required to remove the O-GlcNAc group and the energy required to fragment the peptide chain allows the O-GlcNAc group to be detected and the peptide sequence, and therefore the protein, to be identified. This technique thus allows the simultaneous detection and identification of O-GlcNAc-modified peptides, even when present at low levels in complex mixtures. The method was initially developed and validated using a synthetic O-GlcNAc-modified peptide and then applied to the detection of an extremely low abundance O-GlcNAc-modified peptide from bovine alpha-crystallin. We believe that with further development this assay system may prove to be a useful tool for the direct investigation of intracellular O-GlcNAc levels, thus providing valuable insights into the physiological role of O-GlcNAc modified proteins.

Alternative O-glycosylation/O-phosphorylation of the murine estrogen receptor beta

Estrogen receptor beta, a homologue to estrogen receptor alpha, is a new member of the steroid hormone receptor family. Recently, we documented that estrogen receptor alpha, like other transcription factors, is modified by O-linked N-acetylglucosamine (O-GlcNAc), a ubiquitous transitory posttranslational modification on nuclear and cytoplasmic proteins. Here, we report that estrogen receptor beta is alternatively modified by either O-GlcNAc or O-phosphate. Lectin chromatography of in vitro translated protein first suggested that murine estrogen receptor beta (mER-beta) is O-GlcNAcylated. Structural characterization of the carbohydrate moieties on mER-beta, overexpressed in insect Sf9 cells, confirmed the presence of O-GlcNAc. mER-beta, overexpressed in mammalian cells, is also O-GlcNAcylated. The major site of O-GlcNAc on mER-beta from Sf9 cells is Ser(16) near the N-terminus. Concomitant analyses also documented the O-phosphorylation of mER-beta at Ser(16). MALDI-TOF mass spectrometry showed alternative occupancy of this locus by these two abundant and dynamic posttranslational modifications. The localization of a major O-GlcNAc/O-phosphate site in proximity of the transactivation domain and as part of a PEST region (target sequences for rapid protein degradation) on mER-beta suggests that these modifications may play a role in regulating estrogen receptor beta transactivation and turnover.

The O-GlcNAc transferase gene resides on the X chromosome and is essential for embryonic stem cell viability and mouse ontogeny

Nuclear and cytoplasmic protein glycosylation is a widespread and reversible posttranslational modification in eukaryotic cells. Intracellular glycosylation by the addition of N-acetylglucosamine (GlcNAc) to serine and threonine is catalyzed by the O-GlcNAc transferase (OGT). This "O-GlcNAcylation" of intracellular proteins can occur on phosphorylation sites, and has been implicated in controlling gene transcription, neurofilament assembly, and the emergence of diabetes and neurologic disease. To study OGT function in vivo, we have used gene-targeting approaches in male embryonic stem cells. We find that OGT mutagenesis requires a strategy that retains an intact OGT gene as accomplished by using Cre-loxP recombination, because a deletion in the OGT gene results in loss of embryonic stem cell viability. A single copy of the OGT gene is present in the male genome and resides on the X chromosome near the centromere in region D in the mouse spanning markers DxMit41 and DxMit95, and in humans at Xq13, a region associated with neurologic disease. OGT RNA expression in mice is comparably high among most cell types, with lower levels in the pancreas. Segregation of OGT alleles in the mouse germ line with ZP3-Cre recombination in oocytes reveals that intact OGT alleles are required for completion of embryogenesis. These studies illustrate the necessity of conditional gene-targeting approaches in the mutagenesis and study of essential sex-linked genes, and indicate that OGT participation in intracellular glycosylation is essential for embryonic stem cell viability and for mouse ontogeny.

Accumulation of virion tegument and envelope proteins in a stable cytoplasmic compartment during human cytomegalovirus replication: characterization of a potential site of virus assembly

The assembly of human cytomegalovirus (HCMV) is thought to be similar to that which has been proposed for alphaherpesviruses and involve envelopment of tegumented subviral particles at the nuclear membrane followed by export from the cell by a poorly defined pathway. However, several studies have shown that at least two tegument virion proteins remain in the cytoplasm during the HCMV replicative cycle, thereby suggesting that HCMV cannot acquire its final envelope at the nuclear envelope. We investigated the assembly of HCMV by determining the intracellular trafficking of the abundant tegument protein pp150 (UL32) in productively infected human fibroblasts. Our results indicated that pp150 remained within the cytoplasm throughout the replicative cycle of HCMV and accumulated in a stable, juxtanuclear structure late in infection. Image analysis using a variety of cell protein-specific antibodies indicated that the pp150-containing structure was not a component of the endoplasmic reticulum, (ER), ER-Golgi intermediate compartment, cis or medial Golgi, or lysosomes. Partial colocalization of the structure was noted with the trans-Golgi network, and it appeared to lie in close proximity to the microtubule organizing center. Two additional tegument proteins (pp28 and pp65) and three envelope glycoproteins (gB, gH, and gp65) localized in this same structure late infection. This compartment appeared to be relatively stable since pp150, pp65, and the processed form of gB could be coisolated following cell fractionation. Our findings indicated that pp150 was expressed exclusively within the cytoplasm throughout the infectious cycle of HCMV and that the accumulation of the pp150 in this cytoplasmic structure was accompanied by at least five other virion proteins. These results suggested the possibility that this virus-induced structure represented a cytoplasmic site of virus assembly.