Structural analysis of the adenovirus type 5 E1B 55-kilodalton-E4orf6 protein complex

Development of a novel adenovirus serotype 35 vector vaccine possessing an RGD peptide in the fiber knob and the E4 orf 4, 6, and 6/7 regions of adenovirus serotype 5

Adenovirus (Ad) vectors based on human adenovirus serotype 5 (Ad5) have attracted significant attention as vaccine vectors for infectious diseases. However, the effectiveness of Ad5 vectors as vaccines is often inhibited by the anti-Ad5 neutralizing antibodies retained by many adults. To overcome this drawback, we focused on human adenovirus serotype 35 (Ad35) vectors with low seroprevalence in adults. Although Ad35 vectors can circumvent anti-Ad5 neutralizing antibodies, vector yields of Ad35 vectors are often inferior to those of Ad5 vectors. In this study, we developed novel Ad35 vectors containing the Ad5 E4 orf 4, 6, and 6/7 or the Ad5 E4 orf 6 and 6/7 for efficient vector production, and compared their properties. These E4-modified Ad35 vectors efficiently propagated to a similar extent at virus titers comparable to those of Ad5 vectors. An Ad35 vector containing the Ad5 E4 orf 4, 6, and 6/7 mediated more efficient transduction than that containing the Ad5 E4 orf 6 and 6/7 in human cultured cells. Furthermore, insertion of an arginine-glycine-aspartate (RGD) peptide in the fiber region of an Ad35 vector containing the Ad5 E4 orf 4, 6, and 6/7 significantly improved the transgene product-specific antibody production following intramuscular administration in mice. The Ad35 vector containing the RGD peptide mediated efficient vaccine effects even in the mice pre-immunized with an Ad5.

Identification of Adenovirus E1B-55K Interaction Partners through a Common Binding Motif

The adenovirus C5 E1B-55K protein is crucial for viral replication and is expressed early during infection. It can interact with E4orf6 to form a complex that functions as a ubiquitin E3 ligase. This complex targets specific cellular proteins and marks them for ubiquitination and, predominantly, subsequent proteasomal degradation. E1B-55K interacts with various proteins, with p53 being the most extensively studied, although identifying binding sites has been challenging. To explain the diverse range of proteins associated with E1B-55K, we hypothesized that other binding partners might recognize the simple p53 binding motif (xWxxxPx). In silico analyses showed that many known E1B-55K binding proteins possess this amino acid sequence; therefore, we investigated whether other xWxxxPx-containing proteins also bind to E1B-55K. Our findings revealed that many cellular proteins, including ATR, CHK1, USP9, and USP34, co-immunoprecipitate with E1B-55K. During adenovirus infection, several well-characterized E1B-55K binding proteins and newly identified interactors, including CSB, CHK1, and USP9, are degraded in a cullin-dependent manner. Notably, certain binding proteins, such as ATR and USP34, remain undegraded during infection. Structural predictions indicate no conservation of structure around the proposed binding motif, suggesting that the interaction relies on the correct arrangement of tryptophan and proline residues.

The Adenovirus Vector Platform: Novel Insights into Rational Vector Design and Lessons Learned from the COVID-19 Vaccine

The adenovirus vector platform remains one of the most efficient toolboxes for generation of transfer vehicles used in gene therapy and virotherapy to treat tumors, as well as vaccines to protect from infectious diseases. The adenovirus genome and capsids can be modified using highly efficient techniques, and vectors can be produced at high titers, which facilitates their rapid adaptation to current needs and disease applications. Over recent years, the adenovirus vector platform has been in the center of attention for vaccine development against the ongoing coronavirus SARS-CoV-2/COVID-19 pandemic. The worldwide deployment of these vaccines has greatly deepened the knowledge on virus-host interactions and highlighted the need to further improve the effectiveness and safety not only of adenovirus-based vaccines but also of gene therapy and oncolytic virotherapy vectors. Based on the current evidence, we discuss here how adenoviral vectors can be further improved by intelligent molecular design. This review covers the full spectrum of state-of-the-art strategies to avoid vector-induced side effects ranging from the vectorization of non-canonical adenovirus types to novel genome engineering techniques.

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.

Murine Cytomegalovirus M25 Proteins Sequester the Tumor Suppressor Protein p53 in Nuclear Accumulations

To ensure productive infection, herpesviruses utilize tegument proteins and nonstructural regulatory proteins to counteract cellular defense mechanisms and to reprogram cellular pathways. The M25 proteins of mouse cytomegalovirus (MCMV) belong to the betaherpesvirus UL25 gene family that encodes viral proteins implicated with regulatory functions. Through affinity purification and mass spectrometric analysis, we discovered the tumor suppressor protein p53 as a host factor interacting with the M25 proteins. M25-p53 interaction in infected and transfected cells was confirmed by coimmunoprecipitation. Moreover, the proteins colocalized in nuclear dot-like structures upon both infection and inducible expression of the two M25 isoforms. p53 accumulated in wild-type MCMV-infected cells, while this did not occur upon infection with a mutant lacking the M25 gene. Both M25 proteins were able to mediate the effect, identifying them as the first CMV proteins responsible for p53 accumulation during infection. Interaction with M25 proteins led to substantial prolongation of the half-life of p53. In contrast to the higher abundance of the p53 protein in wild-type MCMV-infected cells, the transcript levels of the prominent p53 target genes Cdkn1a and Mdm2 were diminished compared to cells infected with the ΔM25 mutant, and this was associated with reduced binding of p53 to responsive elements within the respective promoters. Notably, the productivity of the M25 deletion mutant was partially rescued on p53-negative fibroblasts. We propose that the MCMV M25 proteins sequester p53 molecules in the nucleus of infected cells, reducing their availability for activating a subset of p53-regulated genes, thereby dampening the antiviral role of p53.IMPORTANCE Host cells use a number of factors to defend against viral infection. Viruses are, however, in an arms race with their host cells to overcome these defense mechanisms. The tumor suppressor protein p53 is an important sensor of cell stress induced by oncogenic insults or viral infections, which upon activation induces various pathways to ensure the integrity of cells. Viruses have to counteract many functions of p53, but complex DNA viruses such as cytomegaloviruses may also utilize some p53 functions for their own benefit. In this study, we discovered that the M25 proteins of mouse cytomegalovirus interact with p53 and mediate its accumulation during infection. Interaction with the M25 proteins sequesters p53 molecules in nuclear dot-like structures, limiting their availability for activation of a subset of p53-regulated target genes. Understanding the interaction between viral proteins and p53 may allow to develop new therapeutic strategies against cytomegalovirus and other viruses.

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.

A simian-adenovirus-vectored rabies vaccine suitable for thermostabilisation and clinical development for low-cost single-dose pre-exposure prophylaxis

BACKGROUND

Estimates of current global rabies mortality range from 26,000 to 59,000 deaths per annum. Although pre-exposure prophylaxis using inactivated rabies virus vaccines (IRVs) is effective, it requires two to three doses and is regarded as being too expensive and impractical for inclusion in routine childhood immunization programmes.

METHODOLOGY/ PRINCIPAL FINDINGS

Here we report the development of a simian-adenovirus-vectored rabies vaccine intended to enable cost-effective population-wide pre-exposure prophylaxis against rabies. ChAdOx2 RabG uses the chimpanzee adenovirus serotype 68 (AdC68) backbone previously shown to achieve pre-exposure protection against rabies in non-human primates. ChAdOx2 differs from AdC68 in that it contains the human adenovirus serotype 5 (AdHu5) E4 orf6/7 region in place of the AdC68 equivalents, enhancing ease of manufacturing in cell lines which provide AdHu5 E1 proteins in trans. We show that immunogenicity of ChAdOx2 RabG in mice is comparable to that of AdC68 RabG and other adenovirus serotypes expressing rabies virus glycoprotein. High titers of rabies virus neutralizing antibody (VNA) are elicited after a single dose. The relationship between levels of VNA activity and rabies virus glycoprotein monomer-binding antibody differs after immunization with adenovirus-vectored vaccines and IRV vaccines, suggesting routes to further enhancement of the efficacy of the adenovirus-vectored candidates. We also demonstrate that ChAdOx2 RabG can be thermostabilised using a low-cost method suitable for clinical bio-manufacture and ambient-temperature distribution in tropical climates. Finally, we show that a dose-sparing effect can be achieved by formulating ChAdOx2 RabG with a simple chemical adjuvant. This approach could lower the cost of ChAdOx2 RabG and other adenovirus-vectored vaccines.

CONCLUSIONS/ SIGNIFICANCE

ChAdOx2 RabG may prove to be a useful tool to reduce the human rabies death toll. We have secured funding for Good Manufacturing Practice- compliant bio-manufacture and Phase I clinical trial of this candidate.

Degradation of a Novel DNA Damage Response Protein, Tankyrase 1 Binding Protein 1, following Adenovirus Infection

Infection by most DNA viruses activates a cellular DNA damage response (DDR), which may be to the detriment or advantage of the virus. In the case of adenoviruses, they neutralize antiviral effects of DDR activation by targeting a number of proteins for rapid proteasome-mediated degradation. We have now identified a novel DDR protein, tankyrase 1 binding protein 1 (TNKS1BP1) (also known as Tab182), which is degraded during infection by adenovirus serotype 5 and adenovirus serotype 12. In both cases, degradation requires the action of the early region 1B55K (E1B55K) and early region 4 open reading frame 6 (E4orf6) viral proteins and is mediated through the proteasome by the action of cullin-based cellular E3 ligases. The degradation of Tab182 appears to be serotype specific, as the protein remains relatively stable following infection with adenovirus serotypes 4, 7, 9, and 11. We have gone on to confirm that Tab182 is an integral component of the CNOT complex, which has transcriptional regulatory, deadenylation, and E3 ligase activities. The levels of at least 2 other members of the complex (CNOT3 and CNOT7) are also reduced during adenovirus infection, whereas the levels of CNOT4 and CNOT1 remain stable. The depletion of Tab182 with small interfering RNA (siRNA) enhances the expression of early region 1A proteins (E1As) to a limited extent during adenovirus infection, but the depletion of CNOT1 is particularly advantageous to the virus and results in a marked increase in the expression of adenovirus early proteins. In addition, the depletion of Tab182 and CNOT1 results in a limited increase in the viral DNA level during infection. We conclude that the cellular CNOT complex is a previously unidentified major target for adenoviruses during infection.

IMPORTANCE Adenoviruses target a number of cellular proteins involved in the DNA damage response for rapid degradation. We have now shown that Tab182, which we have confirmed to be an integral component of the mammalian CNOT complex, is degraded following infection by adenovirus serotypes 5 and 12. This requires the viral E1B55K and E4orf6 proteins and is mediated by cullin-based E3 ligases and the proteasome. In addition to Tab182, the levels of other CNOT proteins are also reduced during adenovirus infection. Thus, CNOT3 and CNOT7, for example, are degraded, whereas CNOT4 and CNOT1 are not. The siRNA-mediated depletion of components of the complex enhances the expression of adenovirus early proteins and increases the concentration of viral DNA produced during infection. This study highlights a novel protein complex, CNOT, which is targeted for adenovirus-mediated protein degradation. To our knowledge, this is the first time that the CNOT complex has been identified as an adenoviral target.

Normal human cell proteins that interact with the adenovirus type 5 E1B 55kDa protein

Several of the functions of the human adenovirus type 5 E1B 55kDa protein are fulfilled via the virus-specific E3 ubiquitin ligase it forms with the viral E4 Orf6 protein and several cellular proteins. Important substrates of this enzyme have not been identified, and other functions, including repression of transcription of interferon-sensitive genes, do not require the ligase. We therefore used immunoaffinity purification and liquid chromatography-mass spectrometry of lysates of normal human cells infected in parallel with HAdV-C5 and E1B 55kDa protein-null mutant viruses to identify specifically E1B 55kDa-associated proteins. The resulting set of >90 E1B-associated proteins contained the great majority identified previously, and was enriched for those associated with the ubiquitin-proteasome system, RNA metabolism and the cell cycle. We also report very severe inhibition of viral genome replication when cells were exposed to both specific or non-specific siRNAs and interferon prior to infection.

Prime, Shock, and Kill: Priming CD4 T Cells from HIV Patients with a BCL-2 Antagonist before HIV Reactivation Reduces HIV Reservoir Size

Understanding how some HIV-infected cells resist the cytotoxicity of HIV replication is crucial to enabling HIV cure efforts. HIV killing of CD4 T cells that replicate HIV can involve HIV protease-mediated cleavage of procaspase 8 to generate a fragment (Casp8p41) that directly binds and activates the mitochondrial proapoptotic protein BAK. Here, we demonstrate that Casp8p41 also binds with nanomolar affinity to the antiapoptotic protein Bcl-2, which sequesters Casp8p41 and prevents apoptosis. Further, we show that central memory CD4 T cells (TCM) from HIV-infected individuals have heightened expression of BCL-2 relative to procaspase 8, possibly explaining the persistence of HIV-infected TCMdespite generation of Casp8p41. Consistent with this hypothesis, the selective BCL-2 antagonist venetoclax induced minimal killing of uninfected CD4 T cells but markedly increased the death of CD4 T cells and diminished cell-associated HIV DNA when CD4 T cells from antiretroviral therapy (ART)-suppressed HIV patients were induced with αCD3/αCD28 to reactivate HIVex vivo Thus, priming CD4 T cells from ART suppressed HIV patients with a BCL-2 antagonist, followed by HIV reactivation, achieves reductions in cell-associated HIV DNA, whereas HIV reactivation alone does not.

IMPORTANCE

HIV infection is incurable due to a long-lived reservoir of HIV(+)memory CD4 T cells, and no clinically relevant interventions have been identified that reduce the number of these HIV DNA-containing cells. Since postintegration HIV replication can result in HIV protease generation of Casp8p41, which activates BAK, causing infected CD4 T cell death, we sought to determine whether this occurs in memory CD4 T cells. Here, we demonstrate that memory CD4 T cells can generate Casp8p41 and yet are intrinsically resistant to death induced by diverse stimuli, including Casp8p41. Furthermore, BCL-2 expression is relatively increased in these cells and directly binds and inhibits Casp8p41's proapoptotic effects. Antagonizing BCL-2 with venetoclax derepresses this antagonism, resulting in death, preferentially in HIV DNA containing cells, since only these cells generate Casp8p41. Thus, BCL-2 antagonism is a clinically relevant intervention with the potential to reduce HIV reservoir size in patients.

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.

Interregional Coevolution Analysis Revealing Functional and Structural Interrelatedness between Different Genomic Regions in Human Mastadenovirus D

Human mastadenovirus D (HAdV-D) is exceptionally rich in type among the seven human adenovirus species. This feature is attributed to frequent intertypic recombination events that have reshuffled orthologous genomic regions between different HAdV-D types. However, this trend appears to be paradoxical, as it has been demonstrated that the replacement of some of the interacting proteins for a specific function with other orthologues causes malfunction, indicating that intertypic recombination events may be deleterious. In order to understand why the paradoxical trend has been possible in HAdV-D evolution, we conducted an interregional coevolution analysis between different genomic regions of 45 different HAdV-D types and found that ca. 70% of the genome has coevolved, even though these are fragmented into several pieces via short intertypic recombination hot spot regions. Since it is statistically and biologically unlikely that all of the coevolving fragments have synchronously recombined between different genomes, it is probable that these regions have stayed in their original genomes during evolution as a platform for frequent intertypic recombination events in limited regions. It is also unlikely that the same genomic regions have remained almost untouched during frequent recombination events, independently, in all different types, by chance. In addition, the coevolving regions contain the coding regions of physically interacting proteins for important functions. Therefore, the coevolution of these regions should be attributed at least in part to natural selection due to common biological constraints operating on all types, including protein-protein interactions for essential functions. Our results predict additional unknown protein interactions.

IMPORTANCE

Human mastadenovirus D, an exceptionally type-rich human adenovirus species and causative agent of different diseases in a wide variety of tissues, including that of ocular region and digestive tract, as well as an opportunistic infection in immunocompromised patients, is known to have highly diverged through frequent intertypic recombination events; however, it has also been demonstrated that the replacement of a component protein of a multiprotein system with a homologous protein causes malfunction. The present study solved this apparent paradox by looking at which genomic parts have coevolved using a newly developed method. The results revealed that intertypic recombination events have occurred in limited genomic regions and been avoided in the genomic regions encoding proteins that physically interact for a given function. This approach detects purifying selection against recombination events causing the replacement of partial components of multiprotein systems and therefore predicts physical and functional interactions between different proteins and/or genomic elements.

Insight into the mechanism of inhibition of adeno-associated virus by the Mre11/Rad50/Nbs1 complex

Adeno-associated virus (AAV) is a dependent virus of the family Parvoviridae. The gene expression and replication of AAV and derived recombinant AAV (rAAV) vectors are severely limited (>10-fold) by the cellular DNA damage-sensing complex made up of Mre11, Rad50, and Nbs1 (MRN). The AAV genome does not encode the means to circumvent this block to productive infection but relies on coinfecting helper virus to do so. Using adenovirus helper proteins E1B55k and E4orf6, which enhance the transduction of AAV via degradation of MRN, we investigated the mechanism through which this DNA damage complex inhibits gene expression from rAAV. We tested the substrate specificity of inhibition and the contribution of different functions of the MRN complex. Our results demonstrate that both single- and double-stranded rAAV vectors are inhibited by MRN, which is in contrast to the predominant model that inhibition is the result of a block to second-strand synthesis. Exploring the contribution of known functions of MRN, we found that inhibition of rAAV does not require downstream DNA damage response factors, including signaling kinases ATM and ATR. The nuclease domain of Mre11 appears to play only a minor role in inhibition, while the DNA binding domain makes a greater contribution. Additionally, mutation of the inverted terminal repeat of the rAAV genome, which has been proposed to be the signal for interaction with MRN, is tolerated by the mechanism of inhibition. These results articulate a model of inhibition of gene expression in which physical interaction is more important than enzymatic activity and several key downstream damage repair factors are dispensable.

IMPORTANCE

Many viruses modulate the host DNA damage response (DDR) in order to create a cellular environment permissive for infection. The MRN complex is a primary sensor of damage in the cell but also responds to invading viral genomes, often posing a block to infection. AAV is greatly inhibited by MRN and dependent on coinfecting helper virus, such as adenovirus, to remove this factor. Currently, the mechanism through which MRN inhibits AAV and other viruses is poorly understood. Our results reform the predominant model that inhibition of rAAV by MRN is due to limiting second-strand DNA synthesis. Instead, a novel mechanism of inhibition of gene expression independent of a block in rAAV DNA synthesis or downstream damage factors is indicated. These findings have clear implications for understanding this restriction to transduction of AAV and rAAV vectors, which have high therapeutic relevance and likely translate to other viruses that must navigate the DDR.

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.

Effect of Hepatitis B Virus X Gene on the Expression Level of p53 Gene using Hep G2 Cell Line

BACKGROUND

The HBV-X (HBX) protein is believed to contribute to the development of HCC. However, the molecular mechanisms involved in HBX- mediated hepatocarcinogenesis remain obscure. In this study, the effect of hepatitis B virus X gene and its protein product HBxAg on expression of p53 gene in Hep G2 cell line was investigated.

METHODS

Viral DNA extracted from HBV-positive serum and HBX gene region was amplified using polymerase chain reaction (PCR). Then, PCR product was cloned into the pcDNA3 vector. After confirmation of cloning, the recombinant plasmid pcDNA3-X was transfected into HepG2 cell line using lipid-mediated DNA-transfection procedure. SDS-PAGE and western blotting methods were used to identify expression of HBX protein. Relative quantification was used to analyze the p53gene expression using the 2-(ΔΔ Ct) method.

RESULTS

Recombinant plasmid pcDNA3-HBX was confirmed by restriction endonucleases digestion and colony-PCR. The results of SDS-PAGE and western blot assays showed that HBX gene could be expressed in Hep G2 cell line. There was no significant difference between the expression levels of p53 compared with GAPDH gene as housekeeping gene (p < 0.05).

CONCLUSION

There was no significant difference in the protein levels between the transfected cells with X gene containing HBX130 and HBX131 double mu-tations and p53 gene. It is necessary to do more studies on Hepatitis B virus to understand the role of HBX on the development of liver cancer and its function on p53 tumor suppressor protein.

SPOC1-mediated antiviral host cell response is antagonized early in human adenovirus type 5 infection

Little is known about immediate phases after viral infection and how an incoming viral genome complex counteracts host cell defenses, before the start of viral gene expression. Adenovirus (Ad) serves as an ideal model, since entry and onset of gene expression are rapid and highly efficient, and mechanisms used 24–48 hours post infection to counteract host antiviral and DNA repair factors (e.g. p53, Mre11, Daxx) are well studied. Here, we identify an even earlier host cell target for Ad, the chromatin-associated factor and epigenetic reader, SPOC1, recently found recruited to double strand breaks, and playing a role in DNA damage response. SPOC1 co-localized with viral replication centers in the host cell nucleus, interacted with Ad DNA, and repressed viral gene expression at the transcriptional level. We discovered that this SPOC1-mediated restriction imposed upon Ad growth is relieved by its functional association with the Ad major core protein pVII that enters with the viral genome, followed by E1B-55K/E4orf6-dependent proteasomal degradation of SPOC1. Mimicking removal of SPOC1 in the cell, knock down of this cellular restriction factor using RNAi techniques resulted in significantly increased Ad replication, including enhanced viral gene expression. However, depletion of SPOC1 also reduced the efficiency of E1B-55K transcriptional repression of cellular promoters, with possible implications for viral transformation. Intriguingly, not exclusive to Ad infection, other human pathogenic viruses (HSV-1, HSV-2, HIV-1, and HCV) also depleted SPOC1 in infected cells. Our findings provide a general model for how pathogenic human viruses antagonize intrinsic SPOC1-mediated antiviral responses in their host cells. A better understanding of viral entry and early restrictive functions in host cells should provide new perspectives for developing antiviral agents and therapies. Conversely, for Ad vectors used in gene therapy, counteracting mechanisms eradicating incoming viral DNA would increase Ad vector efficacy and safety for the patient.

Viruses have acquired functions that target and modulate host cell signaling and diverse regulatory cascades, leading to efficient viral propagation. During the course of productive infection, Ad gene products manipulate destruction pathways to prevent viral clearance or cell death prior to viral genome amplification and release of progeny. Recently, we reported that chromatin formation and cellular SWI/SNF chromatin remodeling processes play a key role in Ad transcriptional regulation. Here, we observe for the first time that SPOC1, identified as a regulator of DNA damage response and chromatin structure, plays an essential role in restricting Ad gene expression and progeny production. This host cell antiviral mechanism is efficiently counteracted by tight association with the major core protein pVII bound to the incoming viral genome. Subsequently, SPOC1 undergoes proteasomal degradation via the Ad E1B-55K/E4orf6-dependent, Cullin-based E3 ubiquitin ligase complex. We also show that other viruses from RNA and DNA families also induce efficient degradation of SPOC1. These analyses of evasion strategies acquired by viruses and other human pathogens should provide important insights into factors manipulating the epigenetic environment to potentially inactivate, or amplify host cell immune responses, since detailed molecular mechanisms and the full repertoire of cellular targets still remain elusive.

Control of human adenovirus type 5 gene expression by cellular Daxx/ATRX chromatin-associated complexes

Death domain-associated protein (Daxx) cooperates with X-linked α-thalassaemia retardation syndrome protein (ATRX), a putative member of the sucrose non-fermentable 2 family of ATP-dependent chromatin-remodelling proteins, acting as the core ATPase subunit in this complex, whereas Daxx is the targeting factor, leading to histone deacetylase recruitment, H3.3 deposition and transcriptional repression of cellular promoters. Despite recent findings on the fundamental importance of chromatin modification in host-cell gene regulation, it remains unclear whether adenovirus type 5 (Ad5) transcription is regulated by cellular chromatin remodelling to allow efficient virus gene expression. Here, we focus on the repressive role of the Daxx/ATRX complex during Ad5 replication, which depends on intact protein-protein interaction, as negative regulation could be relieved with a Daxx mutant that is unable to interact with ATRX. To ensure efficient viral replication, Ad5 E1B-55K protein inhibits Daxx and targets ATRX for proteasomal degradation in cooperation with early region 4 open reading frame protein 6 and cellular components of a cullin-dependent E3-ubiquitin ligase. Our studies illustrate the importance and diversity of viral factors antagonizing Daxx/ATRX-mediated repression of viral gene expression and shed new light on the modulation of cellular chromatin remodelling factors by Ad5. We show for the first time that cellular Daxx/ATRX chromatin remodelling complexes play essential roles in Ad gene expression and illustrate the importance of early viral proteins to counteract cellular chromatin remodelling.

The human adenovirus type 5 E1B 55 kDa protein obstructs inhibition of viral replication by type I interferon in normal human cells

Vectors derived from human adenovirus type 5, which typically lack the E1A and E1B genes, induce robust innate immune responses that limit their therapeutic efficacy. We reported previously that the E1B 55 kDa protein inhibits expression of a set of cellular genes that is highly enriched for those associated with anti-viral defense and immune responses, and includes many interferon-sensitive genes. The sensitivity of replication of E1B 55 kDa null-mutants to exogenous interferon (IFN) was therefore examined in normal human fibroblasts and respiratory epithelial cells. Yields of the mutants were reduced at least 500-fold, compared to only 5-fold, for wild-type (WT) virus replication. To investigate the mechanistic basis of such inhibition, the accumulation of viral early proteins and genomes was compared by immunoblotting and qPCR, respectively, in WT- and mutant-infected cells in the absence or presence of exogenous IFN. Both the concentration of viral genomes detected during the late phase and the numbers of viral replication centers formed were strongly reduced in IFN-treated cells in the absence of the E1B protein, despite production of similar quantities of viral replication proteins. These defects could not be attributed to degradation of entering viral genomes, induction of apoptosis, or failure to reorganize components of PML nuclear bodies. Nor was assembly of the E1B- and E4 Orf6 protein- E3 ubiquitin ligase required to prevent inhibition of viral replication by IFN. However, by using RT-PCR, the E1B 55 kDa protein was demonstrated to be a potent repressor of expression of IFN-inducible genes in IFN-treated cells. We propose that a primary function of the previously described transcriptional repression activity of the E1B 55 kDa protein is to block expression of IFN- inducible genes, and hence to facilitate formation of viral replication centers and genome replication.

The most frequently used therapeutic vectors for gene transfer or cancer treatment are derived from human adenovirus type 5 (Ad5). We have observed previously that the E1B 55 kDa protein encoded by a gene routinely deleted from these vectors represses expression of numerous cellular genes regulated by interferon (IFN) α and β, which are important components of the innate immune response to viral infection. We therefore compared synthesis of pre-mRNA from IFN-inducible genes, viral yields and early reactions in the infectious cycle in normal human cells exposed to exogenous IFN and infected by wild-type or E1B 55 kDa null-mutant viruses. We report that the E1B 55 kDa protein is a potent repressor of expression of IFN-regulated genes, and protects viral replication against anti-viral actions of IFN by blocking inhibition of formation of viral replication centers and genome replication. These observations provide the first information about the function of the transcription repression activity of E1B during the infectious cycle. Importantly, they also suggest new design considerations for adenoviral vectors that can circumvent induction of innate immune responses, currently a major therapeutic limitation.

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

Since the discovery of post-translational modification (PTM) by the small ubiquitin-related modifiers (SUMOs), a multitude of proteins have been described to be reversibly modified, resulting in the alteration of several cellular pathways. Interestingly, various pathogens gain access to this modification system, although the molecular mechanisms and functional consequences are barely understood. We show here that the adenoviral oncoprotein E1B-55K is a substrate of the SUMO conjugation system, which is directly linked to its C-terminal phosphorylation. This regulative connection is indispensable for modulation of the tumor suppressor p53/chromatin-remodeling factor Daxx by E1B-55K and, consequently, its oncogenic potential in primary mammalian cells. In virus infection, E1B-55K PTMs are necessary for localization to viral transcription/replication sites. Furthermore, we identify the E2 enzyme Ubc9 as an interaction partner of E1B-55K, providing a possible molecular explanation for SUMO-dependent modulation of cellular target proteins. In conclusion, these results for the first time provide evidence how E1B-55K PTMs are regulated and subsequently facilitate exploitation of the host cell SUMOylation machinery.

Functional cooperation between human adenovirus type 5 early region 4, open reading frame 6 protein, and cellular homeobox protein HoxB7

Human adenovirus type 5 (HAdV5) E4orf6 (early region 4 open reading frame 6 protein) is a multifunctional early viral protein promoting efficient replication and progeny production. E4orf6 complexes with E1B-55K to assemble cellular proteins into a functional E3 ubiquitin ligase complex that not only mediates proteasomal degradation of host cell substrates but also facilitates export of viral late mRNA to promote efficient viral protein expression and host cell shutoff. Recent findings defined the role of E4orf6 in RNA splicing independent of E1B-55K binding. To reveal further functions of the early viral protein in infected cells, we used a yeast two-hybrid system and identified the homeobox transcription factor HoxB7 as a novel E4orf6-associated protein. Using a HoxB7 knockdown cell line, we observed a positive role of HoxB7 in adenoviral replication. Our experiments demonstrate that the absence of HoxB7 leads to inefficient viral progeny production, as HAdV5 gene expression is highly regulated by HoxB7-mediated activation of various adenoviral promoters. We have thus identified a novel role of E4orf6 in HAdV5 gene transcription via regulation of homeobox protein-dependent modulation of viral promoter activity.

Adenovirus E4orf3 targets transcriptional intermediary factor 1γ for proteasome-dependent degradation during infection

The ability of adenovirus early region proteins, E1B-55K and E4orf6, to usurp control of cellular ubiquitin ligases and target proteins for proteasome-dependent degradation during infection is well established. Here we show that the E4 gene product, E4orf3 can, independently of E1B-55K and E4orf6, target the transcriptional corepressor transcriptional intermediary factor 1γ (TIF1γ) for proteasome-mediated degradation during infection. Initial mass spectrometric studies identified TIF1 family members-TIF1α, TIF1β, and TIF1γ-as E1B-55K-binding proteins in both transformed and infected cells, but analyses revealed that, akin to TIF1α, TIF1γ is reorganized in an E4orf3-dependent manner to promyelocytic leukemia protein-containing nuclear tracks during infection. The use of a number of different adenovirus early region mutants identified the specific and sole requirement for E4orf3 in mediating TIF1γ degradation. Further analyses revealed that TIF1γ is targeted for degradation by a number of divergent human adenoviruses, suggesting that the ability of E4orf3 to regulate TIF1γ expression is evolutionarily conserved. We also determined that E4orf3 does not utilize the Cullin-based ubiquitin ligases, CRL2 and CRL5, or the TIF1α ubiquitin ligase in order to promote TIF1γ degradation. Further studies suggested that TIF1γ possesses antiviral activity and limits adenovirus early and late gene product expression during infection. Indeed, TIF1γ knockdown accelerates the adenovirus-mediated degradation of MRE11, while TIF1γ overexpression delays the adenovirus-mediated degradation of MRE11. Taken together, these studies have identified novel adenovirus targets and have established a new role for the E4orf3 protein during infection.

The E3 ubiquitin ligase activity associated with the adenoviral E1B-55K-E4orf6 complex does not require CRM1-dependent export

The adenovirus type 5 (Ad5) E1B-55K and E4orf6 (E1B-55K/E4orf6) proteins are multifunctional regulators of Ad5 replication, participating in many processes required for virus growth. A complex containing the two proteins mediates the degradation of cellular proteins through assembly of an E3 ubiquitin ligase and induces shutoff of host cell protein synthesis through selective nucleocytoplasmic viral late mRNA export. Both proteins shuttle between the nuclear and cytoplasmic compartments via leucine-rich nuclear export signals (NES). However, the role of their NES-dependent export in viral replication has not been established. It was initially shown that mutations in the E4orf6 NES negatively affect viral late gene expression in transfection/infection complementation assays, suggesting that E1B-55K/E4orf6-dependent viral late mRNA export involves a CRM1 export pathway. However, a different conclusion was drawn from similar studies showing that E1B-55K/E4orf6 promote late gene expression without active CRM1 or functional NES. To evaluate the role of the E1B-55K/E4orf6 NES in viral replication in the context of Ad-infected cells and in the presence of functional CRM1, we generated virus mutants carrying amino acid exchanges in the NES of either or both proteins. Phenotypic analyses revealed that mutations in the NES of E1B-55K and/or E4orf6 had no or only moderate effects on viral DNA replication, viral late protein synthesis, or viral late mRNA export. Significantly, such mutations also did not interfere with the degradation of cellular substrates, indicating that the NES of E1B-55K or E4orf6 is dispensable both for late gene expression and for the activity associated with the E3 ubiquitin ligase.

Adenovirus type 5 early region 1B 55K oncoprotein-dependent degradation of cellular factor Daxx is required for efficient transformation of primary rodent cells

Early region 1B 55K (E1B-55K) from adenovirus type 5 (Ad5) is a multifunctional regulator of lytic infection and contributes in vitro to complete cell transformation of primary rodent cells in combination with Ad5 E1A. Inhibition of p53 activated transcription plays a key role in processes by which E1B-55K executes its oncogenic potential. Nevertheless, additional functions of E1B-55K or further protein interactions with cellular factors of DNA repair, transcription, and apoptosis, including Mre11, PML, and Daxx, may also contribute to the transformation process. In line with previous results, we performed mutational analysis to define a Daxx interaction motif within the E1B-55K polypeptide. The results from these studies showed that E1B-55K/Daxx binding is not required for inhibition of p53-mediated transactivation or binding and degradation of cellular factors (p53/Mre11). Surprisingly, these mutants lost the ability to degrade Daxx and showed reduced transforming potential in primary rodent cells. In addition, we observed that E1B-55K lacking the SUMO-1 conjugation site (SCS/K104R) was sufficient for Daxx interaction but no longer capable of E1B-55K-dependent proteasomal degradation of the cellular factor Daxx. These results, together with the observation that E1B-55K SUMOylation is required for efficient transformation, provides evidence for the idea that SUMO-1-conjugated E1B-55K-mediated degradation of Daxx plays a key role in adenoviral oncogenic transformation. We assume that the viral protein contributes to cell transformation through the modulation of Daxx-dependent pathways. This further substantiates the assumption that further mechanisms for efficient transformation of primary cells can be separated from functions required for the inhibition of p53-stimulated transcription.

Export of adenoviral late mRNA from the nucleus requires the Nxf1/Tap export receptor

One important function of the human adenovirus E1B 55-kDa protein is induction of selective nuclear export of viral late mRNAs. This protein interacts with the viral E4 Orf6 and four cellular proteins to form an infected-cell-specific E3 ubiquitin ligase. The assembly of this enzyme is required for efficient viral late mRNA export, but neither the relevant substrates nor the cellular pathway that exports viral late mRNAs has been identified. We therefore examined the effects on viral late gene expression of inhibition of the synthesis or activity of the mRNA export receptor Nxf1, which was observed to colocalize with the E1B 55-kDa protein in infected cells. When production of Nxf1 was impaired by using RNA interference, the efficiency of viral late mRNA export was reduced to a corresponding degree. Furthermore, synthesis of a dominant-negative derivative of Nxf1 during the late phase of infection interfered with production of a late structural protein. These observations indicate that the Nxf1 pathway is responsible for export of viral late mRNAs. As the infected-cell-specific E3 ubiquitin ligase targets its known substrates for proteasomal degradation, we compared the concentrations of several components of this pathway (Nxf1, Thox1, and Thoc4) in infected cells that did or did not contain this enzyme. Although the concentration of a well-established substrate, Mre11, decreased significantly in cells infected by adenovirus type 5 (Ad5), but not in those infected by the E1B 55-kDa protein-null mutant Hr6, no E1B 55-kDa protein-dependent degradation of the Nxf1 pathway proteins was observed.

Adenoviral producer cells

Adenovirus (Ad) vectors, in particular those of the serotype 5, are highly attractive for a wide range of gene therapy, vaccine and virotherapy applications (as discussed in further detail in this issue). Wild type Ad5 virus can replicate in numerous tissue types but to use Ad vectors for therapeutic purposes the viral genome requires modification. In particular, if the viral genome is modified in such a way that the viral life cycle is interfered with, a specific producer cell line is required to provide trans-complementation to overcome the modification and allow viral production. This can occur in two ways; use of a producer cell line that contains specific adenoviral sequences incorporated into the cell genome to trans-complement, or use of a producer cell line that naturally complements for the modified Ad vector genome. This review concentrates on producer cell lines that complement non-replicating adenoviral vectors, starting with the historical HEK293 cell line developed in 1977 for first generation Ad vectors. In addition the problem of replication-competent adenovirus (RCA) contamination in viral preparations from HEK293 cells is addressed leading to the development of alternate cell lines. Furthermore novel cell lines for more complex Ad vectors and alternate serotype Ad vectors are discussed.

Proteasome-dependent degradation of Daxx by the viral E1B-55K protein in human adenovirus-infected cells

The death-associated protein Daxx found in PML (promyelocytic leukemia protein) nuclear bodies (PML-NBs) is involved in transcriptional regulation and cellular intrinsic antiviral resistence against incoming viruses. We found that knockdown of Daxx in a nontransformed human hepatocyte cell line using RNA interference (RNAi) techniques results in significantly increased adenoviral (Ad) replication, including enhanced viral mRNA synthesis and viral protein expression. This Daxx restriction imposed upon adenovirus growth is counteracted by early protein E1B-55K (early region 1B 55-kDa protein), a multifunctional regulator of cell-cycle-independent Ad5 replication. The viral protein binds to Daxx and induces its degradation through a proteasome-dependent pathway. We show that this process is independent of Ad E4orf6 (early region 4 open reading frame 6), known to promote the proteasomal degradation of cellular p53, Mre11, DNA ligase IV, and integrin alpha3 in combination with E1B-55K. These results illustrate the importance of the PML-NB-associated factor Daxx in virus growth restriction and suggest that E1B-55K antagonizes innate antiviral activities of Daxx and PML-NBs to stimulate viral replication at a posttranslational level.

Adenovirus 12 E4orf6 inhibits ATR activation by promoting TOPBP1 degradation

Activation of the cellular DNA damage response is detrimental to adenovirus (Ad) infection. Ad has therefore evolved a number of strategies to inhibit ATM- and ATR-dependent signaling pathways during infection. Recent work suggests that the Ad5 E4orf3 protein prevents ATR activation through its ability to mislocalize the MRN complex. Here we provide evidence to indicate that Ad12 has evolved a different strategy from Ad5 to inhibit ATR. We show that Ad12 utilizes a CUL2/RBX1/elongin C-containing ubiquitin ligase to promote the proteasomal degradation of the ATR activator protein topoisomerase-IIbeta-binding protein 1 (TOPBP1). Ad12 also uses this complex to degrade p53 during infection, in contrast to Ad5, which requires a CUL5-based ubiquitin ligase. Although Ad12-mediated degradation of p53 is dependent upon both E1B-55K and E4orf6, Ad12-mediated degradation of TOPBP1 is solely dependent on E4orf6. We propose that Ad12 E4orf6 has two principal activities: to recruit the CUL2-based ubiquitin ligase and to act as substrate receptor for TOPBP1. In support of the idea that Ad12 E4orf6 specifically prevents ATR activation during infection by targeting TOPBP1 for degradation, we demonstrate that Ad12 E4orf6 can inhibit the ATR-dependent phosphorylation of CHK1 in response to replication stress. Taken together, these data provide insights into how Ad modulates ATR signaling pathways during infection.

Adenovirus type 5 early encoded proteins of the E1 and E4 regions induce oncogenic transformation of primary rabbit cells

Analysis of the molecular mechanisms of viral-mediated oncogenesis has contributed enormously to the understanding of the basic principles of normal/malignant cell growth. Transformation by human adenoviruses is a multi-step process involving the modulation of numerous cellular pathways, leading to inhibition of apoptosis and growth arrest. However, the molecular mechanism of how the adenovirus oncogenes facilitate transformation of rodent cells, while concurrently failing to do so for human cells, remains elusive. In this report, we demonstrate for the first time that the transformation capabilities of adenovirus type 5 oncogenes are not restricted to rodent cells, but include cells of the related mammalian order Lagomorpha, inducing considerable morphological alterations, enhanced cell growth and tumour induction in vivo. Furthermore, the established cell lines may represent a suitable tool for further development to generate E4-mutated adenoviruses, which has so far been difficult as mutations within the E4 region often prove to be lethal without a helper-cell system.

A 49-kilodalton isoform of the adenovirus type 5 early region 1B 55-kilodalton protein is sufficient to support virus replication

The adenovirus type 5 (Ad5) early region 1B 55-kDa (E1B-55K) protein is a multifunctional regulator of cell-cycle-independent virus replication that participates in many processes required for maximal virus production. As part of a study of E1B-55K function, we generated the Ad5 mutant H5pm4133, carrying stop codons after the second and seventh codons of the E1B reading frame, thereby eliminating synthesis of the full-length 55K product and its smaller derivatives. Unexpectedly, phenotypic studies revealed that H5pm4133 fully exhibits the characteristics of wild-type (wt) Ad5 in all assays tested. Immunoblot analyses demonstrated that H5pm4133 and wt Ad5 produce very low levels of two distinct polypeptides in the 48- to 49-kDa range, which lack the amino-terminal region but contain segments from the central and carboxy-terminal part of the 55K protein. Genetic and biochemical studies with different Ad5 mutants show that at least one of these isoforms consists of two closely migrating polypeptides of 433 amino acid residues (433R) and 422R, which are produced by translation initiation at two downstream AUG codons of the 55K reading frame. Significantly, a virus mutant producing low levels of the 433R isoform alone replicated to levels comparable to those of wt Ad5, demonstrating that this polypeptide provides essentially all functions of E1B-55K required to promote maximal virus growth in human tumor cells. Altogether, these results extend previous findings that the wt Ad5 E1B region encodes a series of smaller isoforms of E1B-55K and demonstrate that very low levels of at least one of these novel proteins (E1B-433R) are sufficient for a productive infection.

Arginine methylation of human adenovirus type 5 L4 100-kilodalton protein is required for efficient virus production

The adenovirus type 5 (Ad5) late region 4 (L4) 100-kDa nonstructural protein (L4-100K) mediates inhibition of cellular protein synthesis and selective translation of tripartite leader (TL)-containing viral late mRNAs via ribosome shunting. In addition, L4-100K has been implicated in the trimerization and nuclear localization of hexon protein. We previously proved that L4-100K is a substrate of the protein arginine methylation machinery, an emergent posttranslational modification system involved in a growing list of cellular processes, including transcriptional regulation, cell signaling, RNA processing, and DNA repair. As understood at present, L4-100K arginine methylation involves protein arginine methyltransferase 1 (PRMT1), which asymmetrically dimethylates arginines embedded in arginine-glycine-glycine (RGG) or glycine-arginine-rich (GAR) domains. To identify the methylated arginine residues and assess the role of L4-100K arginine methylation, we generated amino acid substitution mutations in the RGG and GAR motifs to examine their effects in Ad-infected and plasmid-transfected cells. Arginine-to-glycine exchanges in the RGG boxes significantly diminished L4-100K methylation in the course of an infection and substantially reduced virus growth, demonstrating that L4-100K methylation in RGG motifs is an important host cell function required for efficient Ad replication. Our data further indicate that PRMT1-catalyzed arginine methylation in the RGG boxes regulates the binding of L4-100K to hexon and promotes the capsid assembly of the structural protein as well as modulating TL-mRNA interaction. Furthermore, substitutions in GAR, but not RGG, regions affected L4-100K nuclear import, implying that the nuclear localization signal of L4-100K is located within the GAR sequence.

Development and evaluation of a novel gene delivery vehicle composed of adenovirus serotype 35

The capacity of gene delivery vehicles is considered to be a critical factor determining the success of gene therapy. To date, various types of gene delivery vehicle have been developed. Among them, recombinant adeno-virus (Ad) vectors have potential that has favored their worldwide use in vitro and in vivo. Conventional Ad vectors are composed of subgroup C Ad serotype 5 (Ad5), although it has been clarified that the drawbacks of Ad5 vectors are a high seroprevalence of Ad5 in adults and low transduction efficiencies in cells lacking the primary receptor for Ad5, coxsackievirus and adenovirus receptor. To overcome these problems, we developed a novel Ad vector fully composed of Ad serotype 35 (Ad35). Ad35 vectors show a wide tropism for human cells because Ad35 binds to human CD46, which is ubiquitously expressed on almost all human cells, as a primary receptor. In addition, anti-Ad5 antibodies do not inhibit Ad35 vector-mediated transduction and the seroprevalence of Ad35 in adults is lower than that of Ad5. This paper reviews our studies on the development and evaluation of Ad35 vectors. Ad vectors derived from other Ad serotypes different from Ad5, including Ad35, are expected to be gene delivery vehicles alternative to conventional Ad5 vectors.

An early function of the adenoviral E1B 55 kDa protein is required for the nuclear relocalization of the cellular p53 protein in adenovirus-infected normal human cells

It is well established that the human subgroup C adenovirus type 5 (Ad5) E1B 55 kDa protein can regulate the activity and concentration of the cellular tumor suppressor, p53. However, the contribution(s) of these functions of the E1B protein to viral reproduction remains unclear. To investigate this issue, we examined properties of p53 in normal human cells infected by E1B mutant viruses that display defective entry into the late phase or viral late mRNA export. The steady-state concentrations of p53 were significantly higher in cells infected by the E1B 55 kDa null mutant Hr6 or three mutants carrying small insertions in the E1B 55 kDa protein coding sequence than in Ad5-infected cells. Nevertheless, none of the mutants induced apoptosis in infected cells. Rather, the localization of p53 to E1B containing nuclear sites observed during infection by Ad5 was prevented by mutations that impair interaction of the E1B protein with p53 and/or with the E4 Orf6 protein. These results indicate that the E1B protein fulfills an early function that correlates efficient entry into the late phase with the localization of E1B and p53 in the nucleus of Ad5-infected normal human cells.

Distinct requirements of adenovirus E1b55K protein for degradation of cellular substrates

The E1b55K and E4orf6 proteins of adenovirus type 5 (Ad5) assemble into a complex together with cellular proteins including cullin 5, elongins B and C, and Rbx1. This complex possesses E3 ubiquitin ligase activity and targets cellular proteins for proteasome-mediated degradation. The ligase activity has been suggested to be responsible for all functions of E1b55K/E4orf6, including promoting efficient viral DNA replication, preventing a cellular DNA damage response, and stimulating late viral mRNA nuclear export and late protein synthesis. The known cellular substrates for degradation by E1b55K/E4orf6 are the Mre11/Rad50/Nbs1 DNA repair complex, the tumor suppressor p53, and DNA ligase IV. Here we show that the degradation of individual targets can occur independently of other substrates. Furthermore, we identify separation-of-function mutant forms of E1b55K that can distinguish substrates for binding and degradation. Our results identify distinct regions of E1b55K that are involved in substrate recognition but also imply that there are additional requirements beyond protein association. These mutant proteins will facilitate the determination of the relevance of specific substrates to the functions of E1b55K in promoting infection and inactivating host defenses.

Adenovirus type 5 early region 1B 156R protein promotes cell transformation independently of repression of p53-stimulated transcription

Early region 1B (E1B) of adenovirus type 5 (Ad5) encodes at least five different polypeptides generated by alternative splicing of a common mRNA precursor. Two of these gene products, E1B-19K and E1B-55K, are individually capable of cooperating with the Ad5 E1A proteins to completely transform rodent cells in culture. Substantial evidence suggests that these two E1B proteins contribute to cell transformation by antagonizing growth arrest and apoptosis. Here, we performed genetic and biochemical analyses to assess the attributes of the remaining E1B proteins (E1B-156R, E1B-93R, and E1B-84R). Our results show that E1B-156R, which comprises the 79 amino-terminal and 77 carboxy-terminal amino acids of E1B-55K, also enhances focal transformation of primary rat cells in cooperation with E1A. Since E1B-156R seemed unable to relocalize p53 and inhibit its transactivating function, it must be assumed that it contributes to transformation independently of repression of p53-stimulated transcription. Furthermore, we discovered that E1B-156R contains a functional transcriptional repression domain and binds Ad5 E4orf6 and the cellular apoptosis regulator Daxx. While the ability to bind E4orf6 could indicate further biological functions of E1B-156R in viral infection, the interaction with Daxx might also be linked to its transforming potential. Taken together, these analyses introduce E1B-156R as a novel transformation-promoting E1B protein that acts without repressing p53 transactivation. Moreover, identification of the interaction partners E4orf6 and Daxx provides a first glance of E1B-156R's potential functions.

Novel replication-incompetent adenoviral B-group vectors: high vector stability and yield in PER.C6 cells

Adenoviral vectors based on adenovirus type 35 (rAd35) have the advantage of low natural vector immunity and induce strong, insert-specific T- and B-cell responses, making them prime-candidate vaccine carriers. However, severe vector-genome instability of E1-deleted rAd35 vectors was observed, hampering universal use. The instability of E1-deleted rAd35 vector proved to be caused by low pIX expression induced by removal of the pIX promoter, which was located in the E1B region of B-group viruses. Reinsertion of a minimal pIX promoter resulted in stable vectors able to harbour large DNA inserts (>5 kb). In addition, it is shown that replacement of the E4-Orf6 region of Ad35 by the E4-Orf6 region of Ad5 resulted in successful propagation of an E1-deleted rAd35 vector on existing E1-complementing cell lines, such as PER.C6 cells. The ability to produce these carriers on PER.C6 contributes significantly to the scale of manufacturing of rAd35-based vaccines. Next, a stable rAd35 vaccine was generated carrying Mycobacterium tuberculosis antigens Ag85A, Ag85B and TB10.4. The antigens were fused directly, resulting in expression of a single polyprotein. This vaccine induced dose-dependent CD4+ and CD8+ T-cell responses against multiple antigens in mice. It is concluded that the described improvements to the rAd35 vector contribute significantly to the further development of rAd35 carriers for mass-vaccination programmes for diseases such as tuberculosis, AIDS and malaria.

Adenovirus E1B 55-kilodalton protein is required for both regulation of mRNA export and efficient entry into the late phase of infection in normal human fibroblasts

The human adenovirus type 5 (Ad5) E1B 55-kDa protein is required for selective nuclear export of viral late mRNAs from the nucleus and concomitant inhibition of export of cellular mRNAs in HeLa cells and some other human cell lines, but its contributions(s) to replication in normal human cells is not well understood. We have therefore examined the phenotypes exhibited by viruses carrying mutations in the E1B 55-kDa protein coding sequence in normal human fibroblast (HFFs). Ad5 replicated significantly more slowly in HFFs than it does in tumor cells, a difference that is the result of delayed entry into the late phase of infection. The A143 mutation, which specifically impaired export of viral late mRNAs from the nucleus in infected HeLa cells (R. A. Gonzalez and S. J. Flint, J. Virol. 76:4507-4519, 2002), induced a more severe defect in viral mRNA export in HFFs. This observation indicates that the E1B 55-kDa protein regulates mRNA export during the late phase of infection of normal human cells. Other mutants exhibited phenotypes not observed in HeLa cells. In HFFs infected by the null mutant Hr6, synthesis of viral late mRNAs and proteins was severely impaired. Such defects in late gene expression were the result of inefficient progression into the late phase of infection, for viral DNA synthesis was 10-fold less efficient in Hr6-infected HFFs than in cells infected by Ad5. Similar, but less severe, defects in viral DNA synthesis were induced by the insertion mutation H224, which has been reported to inhibit binding of the E1B 55-kDa protein to p53 (C. C. Kao, P. R. Yew, and A. J. Berk, Virology 179:806-814, 1990).

Both BC-box motifs of adenovirus protein E4orf6 are required to efficiently assemble an E3 ligase complex that degrades p53

Small DNA tumor viruses typically encode proteins that either inactivate or degrade p53. Human adenoviruses encode products, including E4orf6 and E1B55K, that do both. Each independently binds to p53 and inhibits its ability to activate gene expression; however, in combination they induce p53 degradation by the ubiquitin pathway. We have shown previously that p53 degradation relies on interactions of E4orf6 with the cellular proteins Cul5, Rbx1, and elongins B and C to form an E3 ligase similar to the SCF and VBC complexes. Here we show that, like other elongin BC-interacting proteins, including elongin A, von Hippel-Lindau protein, and Muf1, the interaction of E4orf6 is mediated by the BC-box motif; however, E4orf6 uniquely utilizes two BC-box motifs for degradation of p53 and another target, Mre11. In addition, our data suggest that the interaction of E1B55K with E4orf6 depends on the ability of E4orf6 to form the E3 ligase complex and that such complex formation may be required for all E4orf6-E1B55K functions.

Analysis of early region 4 of porcine adenovirus type 3

The early region 4 (E4) of porcine adenovirus (PAdV)-3, located at the right-hand end of the genome is transcribed in a leftward direction and has the potential to encode seven (p1-p7) open reading frames (ORFs). To determine the role of each protein in viral replication, we constructed full-length PAdV-3 genomic clones containing deletions of individual E4 ORF or combined deletions of the neighboring ORFs. Transfection of swine testicular (ST) cells with individual E4 mutant plasmid DNAs generated PAdV-3 E4 mutant viruses except with plasmids containing a deletion of ORF p3, ORF p2+ p3 or ORF p3+ p4. Each of the mutants was further analyzed for growth kinetics, and early/late protein synthesis. Mutant viruses carrying deletions in ORF p1, ORF p2 or ORF p4 showed growth characteristics similar to that of wild-type PAdV-3. Early/late protein synthesis was also indistinguishable from that of wild-type PAdV-3. However, mutant viruses carrying deletions in ORF p5, ORF p6 or ORF p7 showed a modest effect in their ability to grow in porcine cells and express early proteins. These results suggest that the E4 ORF p3 (showing low homology with non-essential human adenovirus (HAdV)-9-E4 ORF1 encoded proteins) is essential for the replication of PAdV-3 in vitro. In contrast, the E4 ORF p7 (showing homology to essential HAdV-2 34 kDa protein) is not essential for replication of PAdV-3 in vitro. Moreover, successful deletion of 1.957 kb fragment in E4 region increased the available capacity of replication-competent PAdV-3 (E3 + E4 deleted) to approximately 4.3 kb and that of replication-defective PAdV-3 (E1 + E3 + E4 deleted) to approximately 7 kb. This is extremely useful for the construction of PAdV-3 vectors that express multiple genes and/or regulatory elements for gene therapy and vaccination.

Replicating Adenoviruses in Cancer Therapy

The potential use of adenoviruses in therapy against cancer has evoked a rapidly moving field of research. Unlike conventional gene therapy vectors, oncolytic adenoviruses retain the ability to replicate. However, replication is restricted as much as possible to tumor cells, with the aim of eliminating these cells through viral cytotoxicity. The two key issues are to improve the efficiency of virus replication and cell killing while ensuring the specificity of these activities for tumor cells. Wild-type adenoviruses as such may already be usable for cancer therapy. Strategies to further improve efficiency and specificity include the partial or complete removal of viral genes. The idea is that functions carried out by the corresponding gene products are not required for replication in tumor cells, but are needed in normal cells. Accordingly, the removal of genes encoding E1B-55 kDa or E1B-19 kDa, or the mutation of E1A may improve the selective killing of tumor cells. On the other hand, the overexpression of the adenovirus death protein (ADP) may enhance viral spread and oncolytic efficiency. Other strategies to improve the specific oncolytic activity of replicating adenoviruses have been pursued. For instance, some promoters are active specifically in tumor cells, and these promoters were introduced into the viral genome, to regulate essential viral genes. Moreover, replicating viruses were engineered to express toxic proteins or drug converters. A number of these viruses have been tested successfully using tumor xenografts in nude mice as a model system. An oncolytic adenovirus lacking the E1B-55 kDa gene product, termed dl1520 or ONYX015, was injected into squamous cell carcinomas of head and neck in phase II clinical trials, and the results were encouraging when chemotherapy was applied in parallel. In the future, further progress might be achieved on the level of virus constructs, but also by refining and adjusting simultaneous conventional therapies, and by standardizing the assessment of the clinical outcome. Recent progress has been made towards the use of replicating virus constructs in cancer therapy. The goal of these developments is to remove cancerous cells from patients with the help of viruses that selectively replicate in these cells. These viruses are generally termed oncolytic viruses. Some convenient properties of adenovirus make this virus particularly useful for this purpose. It infects a large number of human cell types, especially epithelial cells, which give rise to the vast majority of human malignancies. It can be grown easily and to high titers, and the creation of virus recombinants is well established. Finally, a large body of basic research has already been carried out on this virus, facilitating its manipulation. Various approaches to use adenovirus as a cancer drug have been reviewed (Alemany et al. 1999a, 2000; Curiel 2000; Galanis et al. 2001b; Gromeier 2001; Heise and Kirn 2000; Kirn 2000a; Kirn et al. 2001; Kirn and McCormick 1996; Smith and Chiocca 2000; Sunamura 2000; Wells 2000; Wodarz 2001). The aim of this chapter is to provide an integrated overview of these strategies.

The Mre11 complex is required for ATM activation and the G2/M checkpoint

The maintenance of genome integrity requires a rapid and specific response to many types of DNA damage. The conserved and related PI3-like protein kinases, ataxia-telangiectasia mutated (ATM) and ATM-Rad3-related (ATR), orchestrate signal transduction pathways in response to genomic insults, such as DNA double-strand breaks (DSBs). It is unclear which proteins recognize DSBs and activate these pathways, but the Mre11/Rad50/NBS1 complex has been suggested to act as a damage sensor. Here we show that infection with an adenovirus lacking the E4 region also induces a cellular DNA damage response, with activation of ATM and ATR. Wild-type virus blocks this signaling through degradation of the Mre11 complex by the viral E1b55K/E4orf6 proteins. Using these viral proteins, we show that the Mre11 complex is required for both ATM activation and the ATM-dependent G(2)/M checkpoint in response to DSBs. These results demonstrate that the Mre11 complex can function as a damage sensor upstream of ATM/ATR signaling in mammalian cells.

Adenovirus E4-34kDa requires active proteasomes to promote late gene expression

A complex of the Adenovirus (Ad) early region 1b 55-kDa protein (E1b-55kDa) and the early region 4 ORF6 34-kDa protein (E4-34kDa) promotes viral late RNA accumulation in the cytoplasm while inhibiting the transport of most newly synthesized cellular mRNA. The E4 ORF3 11-kDa protein (E4-11kDa) functionally compensates for at least some of the activities of this complex. We find that the same large central region of E4-34kDa that is required for proteasome-mediated degradation of p53 (J. Virol. 75, (2001) 699-709) is also required to promote viral late gene expression in a complementation assay. E4-34kDa does not promote late gene expression in complementation assays performed in the presence of proteasome inhibitors. A proteasome inhibitor also dramatically reduced late gene expression by a virus that lacks the E4-11kDa gene and therefore relies on E4-34kDa for late gene expression. Our results suggest that E4-34kDa activity in promoting late gene expression depends on the proteasome.

An activity associated with human chromosome 21 permits nuclear colocalization of the adenovirus E1B-55K and E4orf6 proteins and promotes viral late gene expression

The adenovirus E1B-55K and E4orf6 proteins cooperate during virus infection while performing several tasks that contribute to a productive infection, including the selective nucleocytoplasmic transport of late viral mRNA. Previous studies have shown that the E4orf6 protein retains the E1B-55K protein in the nucleus of human and monkey cells, but not in those of rodents, suggesting that primate-specific cellular factors contribute to the E4orf6-mediated retention of the E1B-55K protein in the nucleus. In an effort to identify these proposed primate-specific cellular factors, the interaction of the E1B-55K and E4orf6 proteins was studied in a panel of stable human-rodent monochromosomal somatic cell hybrids. Analysis of this panel of cell lines has demonstrated the existence of an activity associated with human chromosome 21 that permits the E1B-55K and E4orf6 proteins to colocalize in the nucleus of a rodent cell. Additional hybrid cells bearing portions of human chromosome 21 were used to map this activity to a 10-megabase-pair segment of the chromosome, extending from 21q22.12 to a region near the q terminus. Strikingly, this region also facilitates the expression of adenovirus late genes in a rodent cell background while having little impact on the expression of early viral genes.

Regulation of mRNA production by the adenoviral E1B 55-kDa and E4 Orf6 proteins

The E1B 55-kDa and E4 Orf6 proteins of human subgroup C adenoviruses both counter host cell defenses mediated by the cellular p53 protein and regulate viral late gene expression. A complex containing the two proteins has been implicated in induction of selective export of viral late mRNAs from the nucleus to the cytoplasm, with concomitant inhibition of export of the majority of newly synthesized cellular mRNAs. The molecular mechanisms by which these viral proteins subvert cellular pathways of nuclear export are not yet clear. Here, we review recent efforts to identify molecular and biochemical functions of the E1B 55-kDa and E4 Orf6 proteins required for regulation of mRNA export, the several difficulties and discrepancies that have been encountered in studies of these viral proteins, and evidence indicating that the reorganization of the infected cell nucleus and production of viral late mRNA at specific intra-nuclear sites are important determinants of selective mRNA export in infected cells. In our view, it is not yet possible to propose a coherent molecular model for regulation of mRNA export by the E1B 55-kDa and E4 Orf6 proteins. However, it should now be possible to address specific questions about the roles of potentially relevant properties of these viral proteins.

Nuclear matrix localization and SUMO-1 modification of adenovirus type 5 E1b 55K protein are controlled by E4 Orf6 protein

Human adenovirus serotype 5 encodes three proteins, E1b 55K, E4 Orf3 and E4 Orf6, which interact with each other and with components of the nucleus to regulate mRNA processing and export, viral DNA replication and p53-dependent apoptosis. Previous studies have shown that, during wild-type infection, 55K associates initially with structures termed ND10, which are sites of localization of the promyelocytic leukaemia protein, and then moves, dependent upon its interaction with Orf6, to the establishing virus replication centres. Absence of either Orf3 or Orf6 affects the localization of 55K and so may affect its function. In this study, the influence of Orf3 and Orf6 expression on the association of 55K with the insoluble matrix fraction of the nucleus and with ND10 particularly was examined. Overexpression of Orf6 was sufficient to block the association of 55K with this fraction, irrespective of the presence of Orf3. This effect depended upon the two proteins being able to interact. However, the association of 55K with ND10, which persists throughout infection in the absence of Orf6, required Orf3 to be present, thus distinguishing two subsets of matrix-associated 55K. A modified form of 55K, formation of which was blocked by mutating the known site of SUMO-1 attachment, was more abundant in the absence of Orf6 but unaffected by the absence of Orf3. Thus, this modification is favoured when 55K remains associated with the matrix but does not correlate with its stable association with ND10, many components of which are modified by SUMO-1.

Adenoviral inhibitors of apoptotic cell death

Adenoviruses (Ads) are endemic in the human population and the well-studied group C Ads typically cause an acute infection in the respiratory epithelium. A growing body of evidence suggests that these viruses also establish a persistent infection. The Ad genome encodes several proteins that counteract the host anti-viral mechanisms, which function to limit viral infections. This review describes the adenovirus immuno-regulatory proteins and how they function to block apoptosis of infected cells. In addition to facilitating the successful completion of the viral replication cycle and spread of progeny virus, these functions may help maintain the virus in a persistent state.

Analysis of the adenovirus E1B-55K-anchored proteome reveals its link to ubiquitination machinery

During the early phase of infection, the E1B-55K protein of adenovirus type 5 (Ad5) counters the E1A-induced stabilization of p53, whereas in the late phase, E1B-55K modulates the preferential nucleocytoplasmic transport and translation of the late viral mRNAs. The mechanism(s) by which E1B-55K performs these functions has not yet been clearly elucidated. In this study, we have taken a proteomics-based approach to identify and characterize novel E1B-55K-associated proteins. A multiprotein E1B-55K-containing complex was immunopurified from Ad5-infected HeLa cells and found to contain E4-orf6, as well as several cellular factors previously implicated in the ubiquitin-proteasome-mediated destruction of proteins, including Cullin-5, Rbx1/ROC1/Hrt1, and Elongins B and C. We further demonstrate that a complex containing these as well as other proteins is capable of directing the polyubiquitination of p53 in vitro. These ubiquitin ligase components were found in a high-molecular-mass complex of 800 to 900 kDa. We propose that these newly identified binding partners (Cullin-5, Elongins B and C, and Rbx1) complex with E1B-55K and E4-orf6 during Ad infection to form part of an E3 ubiquitin ligase that targets specific protein substrates for degradation. We further suggest that E1B-55K functions as the principal substrate recognition component of this SCF-type ubiquitin ligase, whereas E4-orf6 may serve to nucleate the assembly of the complex. Lastly, we describe the identification and characterization of two novel E1B-55K interacting factors, importin-alpha 1 and pp32, that may also participate in the functions previously ascribed to E1B-55K and E4-orf6.

Adenovirus E1-transformed cells grow despite the continuous presence of transcriptionally active p53

The E1 region of adenovirus (Ad) type 5 is capable of transforming cells. According to current concepts, the Ad E1B 55 kDa (E1B 55K) protein enables transformed cells to grow by constantly binding and inactivating the p53 tumour suppressor protein. To test this model, the transcriptional activity of p53 was determined in Ad E1-transformed cells. Surprisingly, it was found that a p53-responsive promoter is highly active in Ad E1-transformed cells and further activated only 3- to 4-fold (compared to 200-fold in p53(-/-) cells) by exogenously expressed p53 or p53mt24-28, a p53 mutant that is transcriptionally active but unable to bind the E1B 55K. On the other hand, the transient overexpression of E1B 55K led to a strong downregulation of a p53-responsive promoter relative to its baseline activity in Ad E1-transformed cells but not in p53(-/-) cells. COS-7 cells, transformed by simian virus 40 (SV40), also showed constitutive p53 activity, whereas HeLa cells, transformed with oncogenic human papillomavirus, did not. Upon stable transfection, Ad E1-transformed cells but not p53(-/-) cells gave rise to colonies that expressed exogenous p53 or p53mt24-28 but, nonetheless, grew at near-wild-type rates. It is proposed that E1B 55K or the SV40 tumour antigen are saturated by the p53 protein, which accumulates in virus-transformed cells, leaving a proportion of active p53 molecules. The transformation of cells by the Ad E1 genes confers permissiveness for active p53, conceivably by inactivating the relevant products of p53 target genes that would otherwise prevent cell growth. Thus, Ad-transformed cells contain and tolerate active p53.

Effects of mutations in the adenoviral E1B 55-kilodalton protein coding sequence on viral late mRNA metabolism

The human subgroup C adenoviral E1B 55-kDa protein cooperates with the viral E4 Orf6 protein to induce selective export of viral, late mRNAs from the nucleus to the cytoplasm. Previous studies have suggested that such preferential transport of viral mRNA and the concomitant inhibition of export of cellular mRNAs are the result of viral colonization of specialized microenvironments within the nucleus. However, neither the molecular basis of this phenomenon nor the mechanism by which the E1B 55-kDa protein acts has been elucidated. We therefore examined viral late mRNA metabolism in HeLa cells infected with a series of mutant viruses that carry insertions at various positions in the E1B protein coding sequence (P. R. Yew, C. C. Kao, and A. J. Berk, Virology 179:795-805, 1990). All the mutations examined impaired cytoplasmic accumulation of viral L2 mRNAs and reduced L2 mRNA export efficiency. However, in most cases these defects could be ascribed to reduced E1B 55-kDa protein concentration or the unexpected failure of the altered E1B proteins to enter the nucleus efficiently. The latter property, the pleiotropic defects associated with all the mutations that impaired nuclear entry of the E1B protein, and consideration of its primary sequence suggest that these insertions result in misfolding of the protein. Insertion of four amino acids at residue 143 also inhibited viral mRNA export but resulted in increased rather than decreased accumulation of the E1B 55-kDa protein in the nucleus. This mutation specifically impaired the previously described association of the E1B protein with intranuclear structures that correspond to sites of adenoviral DNA replication and transcription (D. Ornelles and T. Shenk, J. Virol. 65:424-439, 1991) and the colocalization of the E1B and E4 Orf6 proteins. As this insertion has been shown to inhibit the interaction of the E1B with the E4 Orf6 protein in infected cell extracts (S. Rubenwolf, H. Schütt, M. Nevels, H. Wolf, and T. Dobner, J. Virol. 71:1115-1123, 1997), these phenotypes provide direct support for the hypothesis that selective viral mRNA export is determined by the functional organization of the infected cell nucleus.