Cross-talk between phosphorylation and SUMOylation regulates transforming activities of an adenoviral oncoprotein

The human adenovirus E1B-55K oncoprotein coordinates cell transformation through regulation of DNA-bound host transcription factors

The multifunctional adenovirus E1B-55K oncoprotein can induce cell transformation in conjunction with adenovirus E1A gene products. Previous data from transient expression studies and in vitro experiments suggest that these growth-promoting activities correlate with E1B-55K-mediated transcriptional repression of p53-targeted genes. Here, we analyzed genome-wide occupancies and transcriptional consequences of species C5 and A12 E1B-55Ks in transformed mammalian cells by combinatory ChIP and RNA-seq analyses. E1B-55K-mediated repression correlates with tethering of the viral oncoprotein to p53-dependent promoters via DNA-bound p53. Moreover, we found that E1B-55K also interacts with and represses transcription of numerous p53-independent genes through interactions with transcription factors that play central roles in cancer and stress signaling. Our results demonstrate that E1B-55K oncoproteins function as promiscuous transcriptional repressors of both p53-dependent and -independent genes and further support the model that manipulation of cellular transcription is central to adenovirus-induced cell transformation and oncogenesis.

SUMO Modification of Hepatitis B Virus Core Mediates Nuclear Entry, Promyelocytic Leukemia Nuclear Body Association, and Efficient Formation of Covalently Closed Circular DNA

Persistence of hepatitis B virus (HBV) infection is due to a nuclear covalently closed circular DNA (cccDNA), generated from the virion-borne relaxed circular DNA (rcDNA) genome in a process likely involving numerous cell factors from the host DNA damage response (DDR). The HBV core protein mediates rcDNA transport to the nucleus and likely affects stability and transcriptional activity of cccDNA. Our study aimed at investigating the role of HBV core protein and its posttranslational modification (PTM) with SUMO (small ubiquitin-like modifiers) during the establishment of cccDNA. HBV core protein SUMO PTM was analyzed in His-SUMO-overexpressing cell lines. The impact of HBV core SUMOylation on association with cellular interaction partners and on the HBV life cycle was determined using SUMOylation-deficient mutants of the HBV core protein. Here, we show that the HBV core protein is posttranslationally modified by the addition of SUMO and that this modification impacts nuclear import of rcDNA. By using SUMOylation-deficient HBV core mutants, we show that SUMO modification is a prerequisite for the association with specific promyelocytic leukemia nuclear bodies (PML-NBs) and regulates the conversion of rcDNA to cccDNA. By in vitro SUMOylation of HBV core, we obtained evidence that SUMOylation triggers nucleocapsid disassembly, providing novel insights into the nuclear import process of rcDNA. HBV core protein SUMOylation and subsequent association with PML bodies in the nucleus constitute a key step in the conversion of HBV rcDNA to cccDNA and therefore a promising target for inhibiting formation of the HBV persistence reservoir. IMPORTANCE HBV cccDNA is formed from the incomplete rcDNA involving several host DDR proteins. The exact process and the site of cccDNA formation are poorly understood. Here, we show that HBV core protein SUMO modification is a novel PTM regulating the function of HBV core. A minor specific fraction of the HBV core protein resides with PML-NBs in the nuclear matrix. SUMO modification of HBV core protein mediates its recruitment to specific PML-NBs within the host cell. Within HBV nucleocapsids, SUMOylation of HBV core induces HBV capsid disassembly and is a prerequisite for nuclear entry of HBV core. SUMO HBV core protein association with PML-NBs is crucial for efficient conversion of rcDNA to cccDNA and for the establishment of the viral persistence reservoir. HBV core protein SUMO modification and the subsequent association with PML-NBs might constitute a potential novel target in the development of drugs targeting the cccDNA.

Protein–Protein Interactions Facilitate E4orf6-Dependent Regulation of E1B-55K SUMOylation in HAdV-C5 Infection

The human adenovirus type C5 (HAdV-C5) E1B-55K protein is a multifunctional regulator of HAdV-C5 replication, participating in many processes required for maximal virus production. Its multifunctional properties are primarily regulated by post-translational modifications (PTMs). The most influential E1B-55K PTMs are phosphorylation at highly conserved serine and threonine residues at the C-terminus, and SUMO conjugation to lysines 104 (K104) and 101 (K101) situated in the N-terminal region of the protein, which have been shown to regulate each other. Reversible SUMO conjugation provides a molecular switch that controls key functions of the viral protein, including intracellular trafficking and viral immune evasion. Interestingly, SUMOylation at SUMO conjugation site (SCS) K104 is negatively regulated by another multifunctional HAdV-C5 protein, E4orf6, which is known to form a complex with E1B-55K. To further evaluate the role of E4orf6 in the regulation of SUMO conjugation to E1B-55K, we analyzed different virus mutants expressing E1B-55K proteins with amino acid exchanges in both SCS (K101 and K104) in the presence or absence of E4orf6. We could exclude phosphorylation as factor for E4orf6-mediated reduction of E1B-55K SUMOylation. In fact, we demonstrate that a direct interaction between E1B-55K and E4orf6 is required to reduce E1B-55K SUMOylation. Additionally, we show that an E4orf6-mediated decrease of SUMO conjugation to K101 and K104 result in impaired co-localization of E1B-55K and SUMO in viral replication compartments. These findings indicate that E4orf6 inhibits E1B-55K SUMOylation, which could favor assembly of E4orf6-dependent E3 ubiquitin ligase complexes that are known to degrade a variety of host restriction factors by proteasomal degradation and, thereby, promote viral replication.

E1B-55K is a phosphorylation-dependent transcriptional and post-transcriptional regulator of viral gene expression in HAdV-C5 infection

The multifunctional adenoviral E1B-55K phosphoprotein is a major regulator of viral replication and plays key roles in virus-mediated cell transformation. While much is known about its function in oncogenic cell transformation, underlying features and exact mechanisms that implicate E1B-55K in regulation of viral gene expression are less well understood. Therefore, this work aimed at unravelling basic intranuclear principles of E1B-55K-regulated viral mRNA biogenesis using wild type HAdV-C5 E1B-55K, a virus mutant with abrogated E1B-55K expression and a mutant that expresses a phosphomimetic E1B-55K. By subnuclear fractionation, mRNA, DNA and protein analyses as well as luciferase reporter assays, we show that (i) E1B-55K promotes efficient release of viral late mRNAs from their site of synthesis in viral replication compartments (RCs) to the surrounding nucleoplasm, that (ii) E1B-55K modulates the rate of viral gene transcription and splicing in RCs, that (iii) E1B-55K participates in the temporal regulation of viral gene expression, that (iv) E1B-55K can enhance or repress the expression of viral early and late promoters and that (v) the phosphorylation of E1B-55K regulates the temporal effect of the protein on each of these activities. Together, these data demonstrate that E1B-55K is a phosphorylation-dependent transcriptional and post-transcriptional regulator of viral genes during HAdV-C5 infection. Importance Human adenoviruses are useful models to study basic aspects of gene expression and splicing. Moreover, they are one of the most commonly used viral vectors for clinical applications. However, key aspects of the activities of essential viral proteins that are commonly modified in adenoviral vectors have not been fully described. A prominent example is the multifunctional adenoviral oncoprotein E1B-55K that is known to promote efficient viral genome replication and expression while simultaneously repressing host gene expression and antiviral host responses. Our study combined different quantitative methods to study how E1B-55K promotes viral mRNA biogenesis. The data presented here propose a novel role for E1B-55K as a phosphorylation-dependent transcriptional and post-transcriptional regulator of viral genes.

Ixovex-1, a novel oncolytic E1B-mutated adenovirus

There is a great demand for improved oncolytic viruses that selectively replicate within cancer cells while sparing normal cells. Here, we describe a novel oncolytic adenovirus, Ixovex-1, that obtains a cancer-selective replication phenotype by modulating the level of expression of the different, alternatively spliced E1B mRNA isoforms. Ixovex-1 is a recombinant adenovirus that carries a single point mutation in the E1B-93R 3' splice acceptor site that results in overexpression of the E1B-156R splice isoform. In this paper, we studied the characteristics of this novel oncolytic adenovirus by validating its in vitro behaviour in a panel of normal cells and cancer cells. We additionally studied its anti-tumour efficacy in vivo. Ixovex-1 significantly inhibited tumour growth and prolonged survival of mice in an immune-deficient lung carcinoma tumour implantation model. In complementation experiments, overexpression of E1B-156R was shown to increase the oncolytic index of both Ad5wt and ONYX-015. In contrast to prior viruses of similar type, Ixovex-1 includes a functional E3B region for better in vivo efficacy. Throughout this study, the Ixovex-1 virus has been proven to be superior in competency compared to a virus with multiple deletions.

Conserved E1B-55K SUMOylation in different human adenovirus species is a potent regulator of intracellular localization

Over the past decades, studies on the biology of human adenoviruses (HAdVs) mainly focused on the HAdV prototype species C type 5 (HAdV-C5) and revealed fundamental molecular insights into mechanisms of viral replication and viral cell transformation. Recently, other HAdV species are gaining more and more attention in the field. Reports on large E1B proteins (E1B-55K) from different HAdV species showed that these multifactorial proteins possess strikingly different features along with highly conserved functions. In this work, we identified potential SUMO-conjugation motifs (SCMs) in E1B-55K proteins from HAdV species A to F. Mutational inactivation of these SCMs demonstrated that HAdV E1B-55K proteins are SUMOylated at a single lysine residue that is highly conserved among HAdV species B to E. Moreover, we provide evidence that E1B-55K SUMOylation is a potent regulator of intracellular localization and p53-mediated transcription in most HAdV species. We also identified a lysine residue at position 101 (K101), which is unique to HAdV-C5 E1B-55K and specifically regulates its SUMOylation and nucleo-cytoplasmic shuttling. Our findings reveal important new aspects on HAdV E1B-55K proteins and suggest that different E1B-55K species possess conserved SCMs while their SUMOylation has divergent cellular effects during infection. Importance E1B-55K is a multifunctional adenoviral protein and its functions are highly regulated by SUMOylation. Although functional consequences of SUMOylated HAdV-C5 E1B-55K are well studied, we lack information on the effects of SUMOylation on homologous E1B-55K proteins from other HAdV species. Here, we show that SUMOylation is a conserved post-translational modification in most of the E1B-55K proteins, similar to what we know about HAdV-C5 E1B-55K. Moreover, we identify subcellular localization and regulation of p53-dependent transcription as highly conserved SUMOylation-regulated E1B-55K functions. Thus, our results highlight how HAdV proteins might have evolved in different HAdV species with conserved domains involved in virus replication and differing alternative functions and interactions with the host cell machinery. Future research will link these differences and similarities to the diverse pathogenicity and organ tropism of the different HAdV species.

A Tale of Usurpation and Subversion: SUMO-Dependent Integrity of Promyelocytic Leukemia Nuclear Bodies at the Crossroad of Infection and Immunity

Promyelocytic leukemia nuclear bodies (PML NBs) are multi-protein assemblies representing distinct sub-nuclear structures. As phase-separated molecular condensates, PML NBs exhibit liquid droplet-like consistency. A key organizer of the assembly and dynamics of PML NBs is the ubiquitin-like SUMO modification system. SUMO is covalently attached to PML and other core components of PML NBs thereby exhibiting a glue-like function by providing multivalent interactions with proteins containing SUMO interacting motifs (SIMs). PML NBs serve as the catalytic center for nuclear SUMOylation and SUMO-SIM interactions are essential for protein assembly within these structures. Importantly, however, formation of SUMO chains on PML and other PML NB-associated proteins triggers ubiquitylation and proteasomal degradation which coincide with disruption of these nuclear condensates. To date, a plethora of nuclear activities such as transcriptional and post-transcriptional regulation of gene expression, apoptosis, senescence, cell cycle control, DNA damage response, and DNA replication have been associated with PML NBs. Not surprisingly, therefore, SUMO-dependent PML NB integrity has been implicated in regulating many physiological processes including tumor suppression, metabolism, drug-resistance, development, cellular stemness, and anti-pathogen immune response. The interplay between PML NBs and viral infection is multifaceted. As a part of the cellular antiviral defense strategy, PML NB components are crucial restriction factors for many viruses and a mutual positive correlation has been found to exist between PML NBs and the interferon response. Viruses, in turn, have developed counterstrategies for disarming PML NB associated immune defense measures. On the other end of the spectrum, certain viruses are known to usurp specific PML NB components for successful replication and disruption of these sub-nuclear foci has recently been linked to the stimulation rather than curtailment of antiviral gene repertoire. Importantly, the ability of invading virions to manipulate the host SUMO modification machinery is essential for this interplay between PML NB integrity and viruses. Moreover, compelling evidence is emerging in favor of bacterial pathogens to negotiate with the SUMO system thereby modulating PML NB-directed intrinsic and innate immunity. In the current context, we will present an updated account of the dynamic intricacies between cellular PML NBs as the nuclear SUMO modification hotspots and immune regulatory mechanisms in response to viral and bacterial pathogens.

Inhibition of SENP6 restrains cerebral ischemia-reperfusion injury by regulating Annexin-A1 nuclear translocation-associated neuronal apoptosis

Rationale: Annexin-A1 (ANXA1) has previously been proposed to play a crucial role in neuronal apoptosis during ischemic stroke injury. Our recent study demonstrated that ANXA1 was modified by SUMOylation, and that this modification was greatly weakened after cerebral ischemia, but its effect on neuronal death and the underlying mechanism have not been fully elucidated. Methods: Mice subjected to middle cerebral artery occlusion were established as the animal model and primary cultured neurons treated with oxygen-glucose deprivation and reperfusion was established as the cell model of ischemic stroke. The Ni²⁺-NTA agarose affinity pull-down assay was carried out to determine the SUMOylation level of ANXA1. Co-immunoprecipitation assays was utilized to explore the protein interaction. Immunoblot analysis, quantitative real-time PCR, Luciferase reporter assay were performed to identify the regulatory mechanism. LDH release and TUNEL staining was performed to investigate the neuronal cytotoxicity and apoptosis, respectively. Results: In this study, we identified the deSUMOylating enzyme sentrin/SUMO-specific protease 6 (SENP6) as a negative regulator of ANXA1 SUMOylation. Notably, we found that SENP6-mediated deSUMOylation of ANXA1 induced its nuclear translocation and triggered neuronal apoptosis during cerebral ischemic injury. A mechanistic study demonstrated that SENP6-mediated deSUMOylation of ANXA1 promoted TRPM7- and PKC-dependent phosphorylation of ANXA1. Furthermore, blocking the deSUMOylation of ANXA1 mediated by SENP6 inhibited the transcriptional activity of p53, decreased Bid expression, suppressed caspase-3 pathway activation and reduced the apoptosis of primary neurons subjected to oxygen-glucose deprivation and reperfusion. More importantly, SENP6 inhibition by overexpression of a SENP6 catalytic mutant in neurons resulted in significant improvement in neurological function in the mouse model of ischemic stroke. Conclusions: Taken together, the results of this study identified a previously unidentified function of SENP6 in neuronal apoptosis and strongly indicated that SENP6 inhibition may provide therapeutic benefits for cerebral ischemia.

Functions of nuclear receptors SUMOylation

The nuclear receptor superfamily is a family of ligand-activated transcription factors that play a key role in cell metabolism and human diseases. They can be modified after translation, such as acetylation, ubiquitination, phosphorylation and SUMOylation. Crosstalk between SUMO and ubiquitin, phosphorylation and acetylation regulates a variety of metabolic and physiological activities. Nuclear receptors play an important role in lipid metabolism, inflammation, bile acid homeostasis and autophagy. SUMOylation nuclear receptors can regulate their function and affect cell metabolism. It also provides a potential therapeutic target for atherosclerosis, tumor and other metabolic and inflammation-related diseases. This review focuses on the function of SUMOylation nuclear receptors.

Double-edged role of PML nuclear bodies during human adenovirus infection

PML nuclear bodies are matrix-bound nuclear structures with a variety of functions in human cells. These nuclear domains are interferon regulated and play an essential role during virus infections involving accumulation of SUMO-dependent host and viral factors. PML-NBs are targeted and subsequently manipulated by adenoviral regulatory proteins, illustrating their crucial role during productive infection and virus-mediated oncogenic transformation. PML-NBs have a longstanding antiviral reputation; however, the genomes of Human Adenoviruses and initial sites of viral transcription/replication are found juxtaposed to these domains, resulting in a double-edged capacity of these nuclear multiprotein/multifunctional complexes. This enigma provides evidence that Human Adenoviruses selectively counteract antiviral responses, and simultaneously benefit from or even depend on proviral PML-NB associated components by active recruitment to PML track-like structures, that are induced during infection. Thereby, a positive microenvironment for adenoviral transcription and replication is created at these nuclear subdomains. Based on the available data, this review aims to provide a detailed overview of the current knowledge of Human Adenovirus crosstalk with nuclear PML body compartments as sites of SUMOylation processes in the host cells, evaluating the currently known principles and molecular mechanisms.

Cell transformation by the adenovirus oncogenes E1 and E4

Extensive studies on viral-mediated oncogenic transformation by human adenoviruses have revealed much of our current understanding on the molecular mechanisms that are involved in the process. To date, these studies have shown that cell transformation is a multistep process regulated by the cooperation of several adenoviral gene products encoded in the early regions 1 (E1) and 4 (E4). Early region 1A immortalizes primary rodent cells, whereas co-expression of early region protein 1B induces full manifestation of the transformed phenotype. Beside E1 proteins, also some E4 proteins have partial transforming activities through regulating many cellular pathways. Here, we summarize recent data of how adenoviral oncoproteins may contribute to viral transformation and discuss the challenge of pinpointing the underlying mechanisms.

The biology of the adenovirus E1B 55K protein

The adenovirus E1B 55K (E1B) protein plays major roles in productive adenoviral infection and cellular transformation. Interest in E1B increased because of the potential of adenoviruses as therapeutic vectors, and the E1B gene is commonly deleted from adenovirus vectors for anticancer therapy. E1B activities are spatiotemporally regulated through SUMOylation and phosphorylation, and through interactions with multiple partners that occur presumably at different intracellular sites and times postinfection. E1B is implicated in the formation of viral replication compartments and regulates viral genome replication and transcription, transcriptional repression, degradation of cellular proteins, and several intranuclear steps of viral late mRNA biogenesis. Here, we review advances in our understanding of E1B during productive adenovirus replication and discuss fundamental aspects that remain unresolved.

The human adenovirus type 5 E1B 55kDa protein interacts with RNA promoting timely DNA replication and viral late mRNA metabolism

The E1B 55kDa produced by human adenovirus type 5 is a multifunctional protein that participates in the regulation of several steps during the viral replication cycle. Previous studies suggest this protein plays an important role in postranscriptional regulation of viral and cellular gene expression, as it is required for the selective accumulation of maximal levels of viral late mRNA in the cytoplasm of the infected cell; however the molecular mechanisms that are altered or regulated by this protein have not been elucidated. A ribonucleoprotein motif that could implicate the direct interaction of the protein with RNA was initially predicted and tested in vitro, but the interaction with RNA could not be detected in infected cells, suggesting the interaction may be weak or transient. Here it was determined that the E1B 55kDa interacts with RNA in the context of the viral infection in non-transformed human cells, and its contribution to the adenovirus replication cycle was evaluated. Using recombinant adenoviruses with amino acid substitutions or a deletion in the ribonucleoprotein motif the interaction of E1B 55kDa with RNA was found to correlate with timely and efficient viral DNA replication and viral late mRNA accumulation and splicing.

PTM-ssMP: A Web Server for Predicting Different Types of Post-translational Modification Sites Using Novel Site-specific Modification Profile

Protein post-translational modifications (PTMs) are chemical modifications of a protein after its translation. Owing to its play an important role in deep understanding of various biological processes and the development of effective drugs, PTM site prediction have become a hot topic in bioinformatics. Recently, many online tools are developed to prediction various types of PTM sites, most of which are based on local sequence and some biological information. However, few of existing tools consider the relations between different PTMs for their prediction task. Here, we develop a web server called PTM-ssMP to predict PTM site, which adopts site-specific modification profile (ssMP) to efficiently extract and encode the information of both proximal PTMs and local sequence simultaneously. In PTM-ssMP we provide efficient prediction of multiple types of PTM site including phosphorylation, lysine acetylation, ubiquitination, sumoylation, methylation, O-GalNAc, O-GlcNAc, sulfation and proteolytic cleavage. To assess the performance of PTM-ssMP, a large number of experimentally verified PTM sites are collected from several sources and used to train and test the prediction models. Our results suggest that ssMP consistently contributes to remarkable improvement of prediction performance. In addition, results of independent tests demonstrate that PTM-ssMP compares favorably with other existing tools for different PTM types. PTM-ssMP is implemented as an online web server with user-friendly interface, which is freely available at http://bioinformatics.ustc.edu.cn/PTM-ssMP/index/.

Interaction of porcine reproductive and respiratory syndrome virus proteins with SUMO-conjugating enzyme reveals the SUMOylation of nucleocapsid protein

SUMOylation is a reversible post-translational modification that regulates the function of target protein. In this study, we first predicted by software that the multiple proteins of porcine reproductive and respiratory syndrome virus (PRRSV) could be sumoylated. Next, we confirmed that Nsp1β, Nsp4, Nsp9, Nsp10 and nucleocapsid (N) protein of PRRSV could interact with the sole SUMO E2 conjugating enzyme Ubc9, and Ubc9 could be co-localized with Nsp1β, Nsp4, Nsp9 and Nsp10 in the cytoplasm, while with N protein in both the cytoplasm and nucleus. Finally, we demonstrated that N protein could be sumoylated by either SUMO1 or SUMO2/3. In addition, the overexpression of Ubc9 could inhibit viral genomic replication at early period of PRRSV infection and the knockdown of Ubc9 by siRNA could promote the virus replication. These findings reveal the SUMOylation property of PRRSV N protein and the involvement of Ubc9 in PRRSV replication through interaction with multiple proteins of PRRSV. To our knowledge, this is the first study indicating the interplay between SUMO modification system and PRRSV.

Viral Interplay with the Host Sumoylation System

Viruses have evolved elaborate means to regulate diverse cellular pathways in order to create a cellular environment that facilitates viral survival and reproduction. This includes enhancing viral macromolecular synthesis and assembly, as well as preventing antiviral responses, including intrinsic, innate, and adaptive immunity. There are numerous mechanisms by which viruses mediate their effects on the host cell, and this includes targeting various cellular post-translational modification systems, including sumoylation. The wide-ranging impact of sumoylation on cellular processes such as transcriptional regulation, apoptosis, stress response, and cell cycle control makes it an attractive target for viral dysregulation. To date, proteins from both RNA and DNA virus families have been shown to be modified by SUMO conjugation, and this modification appears critical for viral protein function. More interestingly, members of the several viral families have been shown to modulate sumoylation, including papillomaviruses, adenoviruses, herpesviruses, orthomyxoviruses, filoviruses, and picornaviruses. This chapter will focus on mechanisms by which sumoylation both impacts human viruses and is used by viruses to promote viral infection and disease.

The Epstein-Barr virus miR-BHRF1-1 targets RNF4 during productive infection to promote the accumulation of SUMO conjugates and the release of infectious virus

Post-translational modification by the Small Ubiquitin-like Modifier (SUMO) regulates a variety of cellular functions, and is hijacked by viruses to remodel the host cell during latent and productive infection. Here we have monitored the activity of the SUMO conjugation machinery in cells productively infected with Epstein-Barr virus (EBV). We found that SUMO2/3 conjugates accumulate during the late phase of the productive virus cycle, and identified several viral proteins as bone fide SUMOylation substrates. Analysis of the mechanism involved in the accumulation of SUMOylated proteins revealed upregulation of several components of the SUMO-conjugation machinery and post-transcriptional downregulation of the SUMO-targeted ubiquitin ligase RNF4. The latter effect was mediated by selective inhibition of RNF4 protein expression by the viral miR-BHRF1-1. Reconstitution of RNF4 in cells expressing an inducible miR-BHRF1-1 sponge or a miR-BHRF1-1 resistant RNF4 was associated with reduced levels of early and late viral proteins and impaired virus release. These findings illustrate a novel strategy for viral interference with the SUMO pathway, and identify the EBV miR-BHRF1-1 and the cellular RNF4 as regulators of the productive virus cycle.

We have investigated the activity of the SUMOylation machinery in cells infected with Epstein-Barr virus (EBV), a human herpesvirus that infects B-lymphocytes and is associated with malignancies. We found that activation of the productive virus cycle is accompanied by accumulation of SUMO conjugates, upregulation of components of the SUMO conjugation machinery, and downregulation of the SUMO-targeted ubiquitin ligase RNF4. The decrease of RNF4 is due to post-transcriptional downregulation by miR-BHRF1-1, a member of the BHRF1 microRNA cluster that is upregulated during productive infection. The effect of miR-BHRF1-1 was confirmed in luciferase reported assays, by mutation of the RNF4 3’UTR seed site, by transfection of a synthetic miR-BHRF1-1 mimic, by ectopic expression of miR-BHRF1-1 and by the reversal of RNF4 downregulation in cells expressing a miR-BHRF1-1 sponge. We also found that several early and late viral proteins are bona fide SUMOylation substrates. Reconstitution of RNF4 in productively infected cells was accompanied by proteasome-dependent degradation of the SUMOylated viral protein and by a significantly reduced virus yield. These findings illustrate a new strategy for viral interference with the SUMO pathway, an unexpected contribution of miR-BHRF1-1 to the productive cycle of EBV and a previously unrecognized role of the RNF4 ligase in the regulation of virus production.

Automatic detection and measurement of viral replication compartments by ellipse adjustment

Viruses employ a variety of strategies to hijack cellular activities through the orchestrated recruitment of macromolecules to specific virus-induced cellular micro-environments. Adenoviruses (Ad) and other DNA viruses induce extensive reorganization of the cell nucleus and formation of nuclear Replication Compartments (RCs), where the viral genome is replicated and expressed. In this work an automatic algorithm designed for detection and segmentation of RCs using ellipses is presented. Unlike algorithms available in the literature, this approach is deterministic, automatic, and can adjust multiple RCs using ellipses. The proposed algorithm is non iterative, computationally efficient and is invariant to affine transformations. The method was validated over both synthetic images and more than 400 real images of Ad-infected cells at various timepoints of the viral replication cycle obtaining relevant information about the biogenesis of adenoviral RCs. As proof of concept the algorithm was then used to quantitatively compare RCs in cells infected with the adenovirus wild type or an adenovirus mutant that is null for expression of a viral protein that is known to affect activities associated with RCs that result in deficient viral progeny production.

Adenovirus Early Proteins and Host Sumoylation

The human adenovirus genome is transported into the nucleus, where viral gene transcription, viral DNA replication, and virion assembly take place. Posttranslational modifications by small ubiquitin-like modifiers (SUMOs) are implicated in the regulation of diverse cellular processes, particularly nuclear events. It is not surprising, therefore, that adenovirus modulates and utilizes the host sumoylation system. Adenovirus early proteins play an important role in establishing optimal host environments for virus replication within infected cells by stimulating the cell cycle and counteracting host antiviral defenses. Here, we review findings on the mechanisms and functional consequences of the interplay between human adenovirus early proteins and the host sumoylation system.

Sp100A is a tumor suppressor that activates p53-dependent transcription and counteracts E1A/E1B-55K-mediated transformation

Human adenoviruses (HAdV) are used as a model system to investigate tumorigenic processes in mammalian cells where the viral oncoproteins E1A and E1B-55K are absolutely required for oncogenic transformation, because they simultaneously accelerate cell cycle progression and inhibit tumor suppressor proteins such as p53, although the underlying mechanism is still not understood in detail. In our present study, we provide evidence that E1B-55K binding to the PML-NB component Sp100A apparently has an essential role in regulating adenovirus-mediated transformation processes. Specifically, when this E1B-55K/Sp100A complex recruits p53, Sp100A-induced activation of p53 transcriptional activity is effectively abolished. Hence, Sp100A exhibits tumor-suppressive activity, not only by stabilizing p53 transactivation but also by depressing E1A/E1B-55K-mediated transformation. E1B-55K counteracts this suppressive activity, inducing Sp100A SUMOylation and sequestering the modified cellular factor into the insoluble matrix of the nucleus or into cytoplasmic inclusions. These observations provide novel insights into how E1B-55K modulates cellular determinants to maintain growth-promoting activity during oncogenic processes and lytic infection.

KAP1 Is a Host Restriction Factor That Promotes Human Adenovirus E1B-55K SUMO Modification

Once transported to the replication sites, human adenoviruses (HAdVs) need to ensure decondensation and transcriptional activation of their viral genomes to synthesize viral proteins and initiate steps to reprogram the host cell for viral replication. These early stages during adenoviral infection are poorly characterized but represent a decisive moment in the establishment of a productive infection. Here, we identify a novel host viral restriction factor, KAP1. This heterochromatin-associated transcription factor regulates the dynamic organization of the host chromatin structure via its ability to influence epigenetic marks and chromatin compaction. In response to DNA damage, KAP1 is phosphorylated and functionally inactive, resulting in chromatin relaxation. We discovered that KAP1 posttranslational modification is dramatically altered during HAdV infection to limit the antiviral capacity of this host restriction factor, which represents an essential step required for efficient viral replication. Conversely, we also observed during infection an HAdV-mediated decrease of KAP1 SUMO moieties, known to promote chromatin decondensation events. Based on our findings, we provide evidence that HAdV induces KAP1 deSUMOylation to minimize epigenetic gene silencing and to promote SUMO modification of E1B-55K by a so far unknown mechanism.

IMPORTANCE

Here we describe a novel cellular restriction factor for human adenovirus (HAdV) that sheds light on very early modulation processes in viral infection. We reported that chromatin formation and cellular SWI/SNF chromatin remodeling play key roles in HAdV transcriptional regulation. We observed that the cellular chromatin-associated factor and epigenetic reader SPOC1 represses HAdV infection and gene expression. Here, we illustrate the role of the SPOC1-interacting factor KAP1 during productive HAdV growth. KAP1 binds to the viral E1B-55K protein, promoting its SUMO modification, therefore illustrating a crucial step for efficient viral replication. Simultaneously, KAP1 posttranslational modification is dramatically altered during infection. We observed an HAdV-mediated decrease in KAP1 SUMOylation, known to promote chromatin decondensation events. These findings indicate that HAdV induces the loss of KAP1 SUMOylation to minimize epigenetic gene silencing and to promote the SUMO modification of E1B-55K by a so far unknown mechanism.

Adenovirus E4-ORF3 Targets PIAS3 and Together with E1B-55K Remodels SUMO Interactions in the Nucleus and at Virus Genome Replication Domains

Adenovirus E4-ORF3 and E1B-55K converge in subverting critical overlapping cellular pathways to facilitate virus replication. Here, we show that E1B-55K and E4-ORF3 induce sumoylation and the assembly of SUMO2/3 viral genome replication domains. Using a conjugation-deficient SUMO2 construct, we demonstrate that SUMO2/3 is recruited to E2A viral genome replication domains through noncovalent interactions. E1B-55K and E4-ORF3 have critical functions in inactivating MRN and ATM to facilitate viral genome replication. We show that ATM kinase inhibitors rescue ΔE1B-55K/ΔE4-ORF3 viral genome replication and that the assembly of E2A domains recruits SUMO2/3 independently of E1B-55K and E4-ORF3. However, the morphology and organization of SUMO2/3-associated E2A domains is strikingly different from that in wild-type Ad5-infected cells. These data reveal that E1B-55K and E4-ORF3 specify the nuclear compartmentalization and structure of SUMO2/3-associated E2A domains, which could have important functions in viral replication. We show that E4-ORF3 specifically targets and sequesters the cellular E3 SUMO ligase PIAS3 but not PIAS1, PIAS2, or PIAS4. The assembly of E4-ORF3 into a multivalent nuclear matrix is required to target PIAS3. In contrast to MRN, PIAS3 is targeted by E4-ORF3 proteins from disparate adenovirus subgroups. Our studies reveal that PIAS3 is a novel and evolutionarily conserved target of E4-ORF3 in human adenovirus infections. Furthermore, we reveal that viral proteins not only disrupt but also usurp SUMO2/3 to transform the nucleus and assemble novel genomic domains that could facilitate pathological viral replication.

IMPORTANCE

SUMO is a key posttranslational modification that modulates the function, localization, and assembly of protein complexes. In the ever-escalating host-pathogen arms race, viruses have evolved strategies to subvert sumoylation. Adenovirus is a small DNA tumor virus that is a global human pathogen and key biomedical agent in basic research and therapy. We show that adenovirus infection induces global changes in SUMO localization and conjugation. Using virus and SUMO mutants, we demonstrate that E1B-55K and E4-ORF3 disrupt and usurp SUMO2/3 interactions to transform the nucleus and assemble highly structured and compartmentalized viral genome domains. We reveal that the cellular E3 SUMO ligase PIAS3 is a novel and conserved target of E4-ORF3 proteins from disparate adenovirus subgroups. The induction of sumoylation and SUMO2/3 viral replication domains by early viral proteins could play an important role in determining the outcome of viral infection.

Interplay between the virus and the ubiquitin-proteasome system: molecular mechanism of viral pathogenesis

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Many viruses have evolved to utilize the host UPS for their own benefit.

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Viruses subvert the UPS to maintain optimal level/function of viral proteins.

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Viruses exploit the UPS to degrade host proteins which impede viral growth.

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The UPS serves as an important host anti-viral defense mechanism.

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The UPS is inhibited by some viruses to prevent viral clearance.

The ubiquitin–proteasome system (UPS) plays a central role in a wide range of fundamental cellular functions by ensuring protein quality control and through maintaining a critical level of important regulatory proteins. Viruses subvert or manipulate this cellular machinery to favor viral propagation and to evade host immune response. The UPS serves as a double-edged sword in viral pathogenesis: on the one hand, the UPS is utilized by many viruses to maintain proper function and level of viral proteins; while on the other hand, the UPS constitutes a host defense mechanism to eliminate viral components. To combat this host anti-viral machinery, viruses have evolved to employ the UPS to degrade or inactivate cellular proteins that limit viral growth. This review will highlight our current knowledge pertaining to the different roles for the UPS in viral pathogenesis.

Viral Mimicry to Usurp Ubiquitin and SUMO Host Pathways

Posttranslational modifications (PTMs) of proteins include enzymatic changes by covalent addition of cellular regulatory determinants such as ubiquitin (Ub) and small ubiquitin-like modifier (SUMO) moieties. These modifications are widely used by eukaryotic cells to control the functional repertoire of proteins. Over the last decade, it became apparent that the repertoire of ubiquitiylation and SUMOylation regulating various biological functions is not restricted to eukaryotic cells, but is also a feature of human virus families, used to extensively exploit complex host-cell networks and homeostasis. Intriguingly, besides binding to host SUMO/Ub control proteins and interfering with the respective enzymatic cascade, many viral proteins mimic key regulatory factors to usurp this host machinery and promote efficient viral outcomes. Advanced detection methods and functional studies of ubiquitiylation and SUMOylation during virus-host interplay have revealed that human viruses have evolved a large arsenal of strategies to exploit these specific PTM processes. In this review, we highlight the known viral analogs orchestrating ubiquitin and SUMO conjugation events to subvert and utilize basic enzymatic pathways.

Adenovirus replaces mitotic checkpoint controls

Infection with adenovirus triggers the cellular DNA damage response, elements of which include cell death and cell cycle arrest. Early adenoviral proteins, including the E1B-55K and E4orf3 proteins, inhibit signaling in response to DNA damage. A fraction of cells infected with an adenovirus mutant unable to express the E1B-55K and E4orf3 genes appeared to arrest in a mitotic-like state. Cells infected early in G1 of the cell cycle were predisposed to arrest in this state at late times of infection. This arrested state, which displays hallmarks of mitotic catastrophe, was prevented by expression of either the E1B-55K or the E4orf3 genes. However, E1B-55K mutant virus-infected cells became trapped in a mitotic-like state in the presence of the microtubule poison colcemid, suggesting that the two viral proteins restrict entry into mitosis or facilitate exit from mitosis in order to prevent infected cells from arresting in mitosis. The E1B-55K protein appeared to prevent inappropriate entry into mitosis through its interaction with the cellular tumor suppressor protein p53. The E4orf3 protein facilitated exit from mitosis by possibly mislocalizing and functionally inactivating cyclin B1. When expressed in noninfected cells, E4orf3 overcame the mitotic arrest caused by the degradation-resistant R42A cyclin B1 variant.

IMPORTANCE

Cells that are infected with adenovirus type 5 early in G1 of the cell cycle are predisposed to arrest in a mitotic-like state in a p53-dependent manner. The adenoviral E1B-55K protein prevents entry into mitosis. This newly described activity for the E1B-55K protein appears to depend on the interaction between the E1B-55K protein and the tumor suppressor p53. The adenoviral E4orf3 protein facilitates exit from mitosis, possibly by altering the intracellular distribution of cyclin B1. By preventing entry into mitosis and by promoting exit from mitosis, these adenoviral proteins act to prevent the infected cell from arresting in a mitotic-like state.

PML isoforms IV and V contribute to adenovirus-mediated oncogenic transformation by functionally inhibiting the tumor-suppressor p53

Although modulation of the cellular tumor-suppressor p53 is considered to have the major role in E1A/E1B-55K-mediated tumorigenesis, other promyelocytic leukemia nuclear body (PML-NB)/PML oncogenic domain (POD)-associated factors including SUMO, Mre11, Daxx, as well as the integrity of these nuclear bodies contribute to the transformation process. However, the biochemical consequences and oncogenic alterations of PML-associated E1B-55K by SUMO-dependent PML-IV and PML-V interaction have so far remained elusive. We performed mutational analysis to define a PML interaction motif within the E1B-55K polypeptide. Our results showed that E1B-55K/PML binding is not required for p53, Mre11 and Daxx interaction. We also observed that E1B-55K lacking subnuclear PML localization because of either PML-IV or PML-V-binding deficiency was no longer capable of mediating E1B-55K-dependent SUMOylation of p53, inhibition of p53-mediated transactivation or efficiently transforming primary rodent cells. These results together with the observation that E1B-55K-dependent SUMOylation of p53 is required for efficient cell transformation, provides evidence for the idea that the SUMO ligase activity of the E1B-55K viral oncoprotein is intimately linked to its growth-promoting oncogenic activities.

Proteomic analysis of ubiquitin-like posttranslational modifications induced by the adenovirus E4-ORF3 protein

Viruses interact with and regulate many host metabolic pathways in order to advance the viral life cycle and counteract intrinsic and extrinsic antiviral responses. The human adenovirus (Ad) early protein E4-ORF3 forms a unique scaffold throughout the nuclei of infected cells and inhibits multiple antiviral defenses, including a DNA damage response (DDR) and an interferon response. We previously reported that the Ad5 E4-ORF3 protein induces sumoylation of Mre11 and Nbs1, which are essential for the DDR, and their relocalization into E4-ORF3-induced nuclear inclusions is required for this modification to occur. In this study, we sought to analyze a global change in ubiquitin-like (Ubl) modifications, with particular focus on SUMO3, by the Ad5 E4-ORF3 protein and to identify new substrates with these modifications. By a comparative proteome-wide approach utilizing immunoprecipitation/mass spectrometry, we found that Ubl modifications of 166 statistically significant lysine sites in 51 proteins are affected by E4-ORF3, and the proteome of modifications spans a diverse range of cellular functions. Ubl modifications of 92% of these identified sites were increased by E4-ORF3. We further analyzed SUMO3 conjugation of several identified proteins. Our findings demonstrated a role for the Ad5 E4-ORF3 protein as a regulator of Ubl modifications and revealed new SUMO3 substrates induced by E4-ORF3.

IMPORTANCE

The adenovirus E4-ORF3 protein induces dynamic structural changes in the nuclei of infected cells and counteracts host antiviral responses. One of the mechanisms that accounts for this process is the relocalization and sequestration of cellular proteins into an E4-ORF3 nuclear scaffold, but little is known about how this small viral protein affects diverse cellular responses. In this study, we analyzed for the first time the global pattern of ubiquitin-like (Ubl) modifications, with particular focus on SUMO3, altered by E4-ORF3 expression. The results suggest a role for the Ad5 E4-ORF3 protein as a regulator of Ubl modifications and reveal new SUMO3 substrates targeted by E4-ORF3. Our findings propose Ubl modifications as a new mechanism by which E4-ORF3 may modulate cellular protein functions in addition to subnuclear relocalization.

Sp100 isoform-specific regulation of human adenovirus 5 gene expression

Promyelocytic leukemia nuclear bodies (PML-NBs) are nuclear structures that accumulate intrinsic host factors to restrict viral infections. To ensure viral replication, these must be limited by expression of viral early regulatory proteins that functionally inhibit PML-NB-associated antiviral effects. To benefit from the activating capabilities of Sp100A and simultaneously limit repression by Sp100B, -C, and -HMG, adenoviruses (Ads) employ several features to selectively and individually target these isoforms. Ads induce relocalization of Sp100B, -C, and -HMG from PML-NBs prior to association with viral replication centers. In contrast, Sp100A is kept at the PML tracks that surround the newly formed viral replication centers as designated sites of active transcription. We concluded that the host restriction factors Sp100B, -C, and -HMG are potentially inactivated by active displacement from these sites, whereas Sp100A is retained to amplify Ad gene expression. Ad-dependent loss of Sp100 SUMOylation is another crucial part of the virus repertoire to counteract intrinsic immunity by circumventing Sp100 association with HP1, therefore limiting chromatin condensation. We provide evidence that Ad selectively counteracts antiviral responses and, at the same time, benefits from PML-NB-associated components which support viral gene expression by actively recruiting them to PML track-like structures. Our findings provide insights into novel strategies for manipulating transcriptional regulation to either inactivate or amplify viral gene expression.

IMPORTANCE

We describe an adenoviral evasion strategy that involves isoform-specific and active manipulation of the PML-associated restriction factor Sp100. Recently, we reported that the adenoviral transactivator E1A targets PML-II to efficiently activate viral transcription. In contrast, the PML-associated proteins Daxx and ATRX are inhibited by early viral factors. We show that this concept is more intricate and significant than originally believed, since adenoviruses apparently take advantage of specific PML-NB-associated proteins and simultaneously inhibit antiviral measures to maintain the viral infectious program. Specifically, we observed Ad-induced relocalization of the Sp100 isoforms B, C, and HMG from PML-NBs juxtaposed with viral replication centers. In contrast, Sp100A is retained at Ad-induced PML tracks that surround the newly formed viral replication centers, acting as designated sites of active transcription. The host restriction factors Sp100B, -C, and -HMG are potentially inactivated by active displacement from these sites, whereas Sp100A is retained to amplify Ad gene expression.

The Mre11 Cellular Protein Is Modified by Conjugation of Both SUMO-1 and SUMO-2/3 during Adenovirus Infection

The adenovirus type 5 (Ad5) E1B 55 kDa and E4 Orf6 proteins assemble a Cullin 5-E3 ubiquitin (Ub) ligase that targets, among other cellular proteins, p53 and the Mre11-Rad50-Nbs1 (MRN) complex for degradation. The latter is also inhibited by the E4 Orf3 protein, which promotes the recruitment of Mre11 into specific nuclear sites to promote viral DNA replication. The activities associated with the E1B 55 kDa and E4 Orf6 viral proteins depend mostly on the assembly of this E3-Ub ligase. However, E1B 55 kDa can also function as an E3-SUMO ligase, suggesting not only that regulation of cellular proteins by these viral early proteins may depend on polyubiquitination and proteasomal degradation but also that SUMOylation of target proteins may play a key role in their activities. Since Mre11 is a target of both the E1B/E4 Orf6 complex and E4 Orf3, we decided to determine whether Mre11 displayed similar properties to those of other cellular targets, in Ad5-infected cells. We have found that during Ad5-infection, Mre11 is modified by SUMO-1 and SUMO-2/3 conjugation. Unexpectedly, SUMOylation of Mre11 is not exclusively dependent on E1B 55 kDa, E4 Orf6, or E4 Orf3, rather it seems to be influenced by a molecular interplay that involves each of these viral early proteins.

Viral manipulation of cellular protein conjugation pathways: The SUMO lesson

Small ubiquitin-like modifier (SUMO)ylation is a key post-translational modification mechanism that controls the function of a plethora of proteins and biological processes. Given its central regulatory role, it is not surprising that it is widely exploited by viruses. A number of viral proteins are known to modify and/or be modified by the SUMOylation system to exert their function, to create a cellular environment more favorable for virus survival and propagation, and to prevent host antiviral responses. Since the SUMO pathway is a multi-step cascade, viral proteins engage with it at many levels, to advance and favor each stage of a typical infection cycle: replication, viral assembly and immune evasion. Here we review the current knowledge on the interplay between the host SUMO system and viral lifecycle.

Interplay between viruses and host sumoylation pathways

Post-translational modification by members of the small ubiquitin-like modifier (SUMO) family of proteins is important for the regulation of many cellular proteins and pathways. As obligate parasites, viruses must engage with the host cell throughout their replication cycles, and it is therefore unsurprising that there are many examples of interplay between viral proteins and the host sumoylation system. This article reviews recent advances in this field, summarizing information on sumoylated viral proteins, the varied ways in which viruses engage with SUMO-related pathways, and the consequences of these interactions for viral replication and engagement with innate and intrinsic immunity.

A ubiquitin-specific protease possesses a decisive role for adenovirus replication and oncogene-mediated transformation

Adenoviral replication depends on viral as well as cellular proteins. However, little is known about cellular proteins promoting adenoviral replication. In our screens to identify such proteins, we discovered a cellular component of the ubiquitin proteasome pathway interacting with the central regulator of adenoviral replication. Our binding assays mapped a specific interaction between the N-terminal domains of both viral E1B-55K and USP7, a deubiquitinating enzyme. RNA interference-mediated downregulation of USP7 severely reduced E1B-55K protein levels, but more importantly negatively affected adenoviral replication. We also succeeded in resynthesizing an inhibitor of USP7, which like the knockdown background reduced adenoviral replication. Further assays revealed that not only adenoviral growth, but also adenoviral oncogene-driven cellular transformation relies on the functions of USP7. Our data provide insights into an intricate mechanistic pathway usurped by an adenovirus to promote its replication and oncogenic functions, and at the same time open up possibilities for new antiviral strategies.

Adenoviral infections can result in severe outcomes leading to mortality especially in children undergoing immunosuppressive therapies. Unfortunately, no specific anti-adenoviral treatments are available to treat disseminated adenoviral infections. We have set out to identify host factors promoting adenoviral growth and could identify the cellular protein Ubiquitin-specific protease 7 (USP7) being central to adenoviral infection. Here we show that USP7 interacts with the viral protein E1B-55K, a central regulator of adenoviral replication and adenoviral oncogene-mediated cellular transformation. We demonstrate that USP7 ensures stability and/or proper expression levels of adenoviral proteins at early and late time points of infection. Consistent with this, small-molecule inhibitors of USP7 showed efficient reduction of capsid protein levels and viral progeny numbers. Thus, USP7 inhibition might be a useful treatment option in the context of disseminated adenoviral infections. Moreover, we were also able to show that adenoviral oncogene-mediated cellular transformation can be hampered by USP7 disruption. In summary, this study shows that two different adenoviral disease mechanisms can be inhibited by targeting one host cellular factor.

Role of E1B55K in E4orf6/E1B55K E3 ligase complexes formed by different human adenovirus serotypes

The E4orf6 protein of serotypes representing all human adenovirus species forms Cullin-based E3 ubiquitin ligase complexes that facilitate virus infection by inducing degradation of cellular proteins that impede efficient viral replication. This complex also includes the viral E1B55K product believed to bind and introduce substrates for ubiquitination. Heterogeneity in the composition of these ligases exists, as some serotypes form Cul5-based complexes whereas others utilize Cul2. Significant variations in substrate specificities also exist among serotypes, as some degrade certain substrates very efficiently whereas others induce more modest or little degradation. As E1B55K is believed to function as the substrate acquisition component of the ligase, we undertook studies to compare the ability of representative E1B55K proteins to bind substrates with the efficacy of degradation by their respective E4orf6-based ligases. Interestingly, although efficient degradation in some cases corresponded to the ability of E1B55K to bind to or relocalize substrates, there were several examples of substrates that bound efficiently to E1B55K but were not degraded and others in which substrates were degraded even though binding to E1B55K was low or undetectable. These results suggest that transient interactions with E1B55K may be sufficient for efficient substrate degradation and that binding alone is not sufficient, implying that the orientation of the substrate in the ligase complex is probably crucial. Nevertheless, we found that the substrate specificity of certain E4orf6-based ligases could be altered through the formation of hybrid complexes containing E1B55K from another serotype, thus confirming identification of E1B55K as the substrate acquisition component of the complex.

Aggresome formation by the adenoviral protein E1B55K is not conserved among adenovirus species and is not required for efficient degradation of nuclear substrates

Much of the work on the basic molecular biology of human adenoviruses has been carried out on a very limited number of the more than 60 serotypes, primarily the highly related species C viruses adenovirus type 5 (Ad5) and Ad2 and, to some extent, Ad12 of species A. Until recently, it has been widely assumed that insights obtained with these model viruses were representative of all human adenoviruses. Recent studies on the E3 ubiquitin ligase formed by the viral E1B55K and E4orf6 proteins with a cellular Cullin-based complex indicated that although all species form such a functional complex, significant variations exist in terms of complex composition and the substrates that are degraded. In the present report we conducted a comprehensive analysis of the localization of E1B55K products from representatives of six of the seven adenovirus species in the presence and the absence of the corresponding E4orf6 protein. We found that although in some species E1B55K localized in aggresomes, such was not always the case, suggesting that these structures are not necessary for the efficient degradation of substrates. In addition, differences were evident in the localization of E1B55K, although all forms readily associated with PML. Finally, Ad5 E1B55K was seen to localize in close proximity to Rab11, a marker for the endosomal recycling compartment, and both focused at the microtubule organizing center. These findings suggest that E1B55K from some species may employ the transport system utilized by the membrane recycling pathway to assemble aggresomes and the possibility that this structure might then affect recycling of cell surface components.

The adenoviral oncogene E1A-13S interacts with a specific isoform of the tumor suppressor PML to enhance viral transcription

PML nuclear bodies (PML NBs), also called ND10, are matrix-bound nuclear structures that have been implicated in a variety of functions, including DNA repair, transcriptional regulation, protein degradation, and tumor suppression. These domains are also known for their potential to mediate an intracellular defense mechanism against many virus types. This is likely why they are targeted and subsequently manipulated by numerous viral proteins. Paradoxically, the genomes of various DNA viruses become associated with PML NBs, and initial sites of viral transcription/replication centers are often juxtaposed to these domains. The question is why viruses start their transcription and replication next to their supposed antagonists. Here, we report that PML NBs are targeted by the adenoviral (Ad) transactivator protein E1A-13S. Alternatively spliced E1A isoforms (E1A-12S and E1A-13S) are the first proteins expressed upon Ad infection. E1A-13S is essential for activating viral transcription in the early phase of infection. Coimmunoprecipitation assays showed that E1A-13S preferentially interacts with only one (PML-II) of at least six nuclear human PML isoforms. Deletion mapping located the interaction site within E1A conserved region 3 (CR3), which was previously described as the transcription factor binding region of E1A-13S. Indeed, cooperation with PML-II enhanced E1A-mediated transcriptional activation, while deleting the SUMO-interacting motif (SIM) of PML proved even more effective. Our results suggest that in contrast to PML NB-associated antiviral defense, PML-II may help transactivate viral gene expression and therefore play a novel role in activating Ad transcription during the early viral life cycle.