Adenovirus E1B 55-kilodalton oncoprotein binds to Daxx and eliminates enhancement of p53-dependent transcription by Daxx

Viruses and Cajal Bodies: A Critical Cellular Target in Virus Infection?

Nuclear bodies (NBs) are dynamic structures present in eukaryotic cell nuclei. They are not bounded by membranes and are often considered biomolecular condensates, defined structurally and functionally by the localisation of core components. Nuclear architecture can be reorganised during normal cellular processes such as the cell cycle as well as in response to cellular stress. Many plant and animal viruses target their proteins to NBs, in some cases triggering their structural disruption and redistribution. Although not all such interactions have been well characterised, subversion of NBs and their functions may form a key part of the life cycle of eukaryotic viruses that require the nucleus for their replication. This review will focus on Cajal bodies (CBs) and the viruses that target them. Since CBs are dynamic structures, other NBs (principally nucleoli and promyelocytic leukaemia, PML and bodies), whose components interact with CBs, will also be considered. As well as providing important insights into key virus-host cell interactions, studies on Cajal and associated NBs may identify novel cellular targets for development of antiviral compounds.

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.

DAXX in cancer: phenomena, processes, mechanisms and regulation

DAXX displays complex biological functions. Remarkably, DAXX overexpression is a common feature in diverse cancers, which correlates with tumorigenesis, disease progression and treatment resistance. Structurally, DAXX is modular with an N-terminal helical bundle, a docking site for many DAXX interactors (e.g. p53 and ATRX). DAXX's central region folds with the H3.3/H4 dimer, providing a H3.3-specific chaperoning function. DAXX has two functionally critical SUMO-interacting motifs. These modules are connected by disordered regions. DAXX's structural features provide a framework for deciphering how DAXX mechanistically imparts its functions and how its activity is regulated. DAXX modulates transcription through binding to transcription factors, epigenetic modifiers, and chromatin remodelers. DAXX's localization in the PML nuclear bodies also plays roles in transcriptional regulation. DAXX-regulated genes are likely important effectors of its biological functions. Deposition of H3.3 and its interactions with epigenetic modifiers are likely key events for DAXX to regulate transcription, DNA repair, and viral infection. Interactions between DAXX and its partners directly impact apoptosis and cell signaling. DAXX's activity is regulated by posttranslational modifications and ubiquitin-dependent degradation. Notably, the tumor suppressor SPOP promotes DAXX degradation in phase-separated droplets. We summarize here our current understanding of DAXX's complex functions with a focus on how it promotes oncogenesis.

Degradation of DAXX by adenovirus type 12 E1B-55K circumvents chemoresistance of ovarian cancer to cisplatin

Adenovirus E1B 55-kilodalton (E1B-55K) mediated DAXX degradation represents a potential mechanism by which E1B-55K sensitizes cancer cells to chemotherapy. Here we report the effects of E1B-55K-mediated DAXX degradation in chemoresistant ovarian cancer cells on response to chemotherapy. Cells with E1B-55K expression were more sensitive to cisplatin than cells without E1B-55K expression. In vivo C13* xenograft studies showed that the combination of cisplatin and E1B-55K was markedly more effective to slow tumor growth and to confer prolonged survival of tumor-bearing mice than either cisplatin or E1B-55K alone. Our studies show that DAXX plays an important role in cisplatin resistance in ovarian cancer, and strategies that promote DAXX degradation such as E1B-55K expression in combination with cisplatin can overcome drug resistance and improve responses to standard chemotherapy. These results also indicate that E1B-55K might be a novel agent for enhancing treatment responses for cisplatin-resistant ovarian cancer.

DNA damage-induced regulatory interplay between DAXX, p53, ATM kinase and Wip1 phosphatase

Death domain-associated protein 6 (DAXX) is a histone chaperone, putative regulator of apoptosis and transcription, and candidate modulator of p53-mediated gene expression following DNA damage. DAXX becomes phosphorylated upon DNA damage, however regulation of this modification, and its relationship to p53 remain unclear. Here we show that in human cells exposed to ionizing radiation or genotoxic drugs etoposide and neocarzinostatin, DAXX became rapidly phosphorylated in an ATM kinase-dependent manner. Our deletion and site-directed mutagenesis experiments identified Serine 564 (S564) as the dominant ATM-targeted site of DAXX, and immunofluorescence experiments revealed localization of S564-phosphorylated DAXX to PML nuclear bodies. Furthermore, using a panel of human cell types, we identified the p53-regulated Wip1 protein phosphatase as a key negative regulator of DAXX phosphorylation at S564, both in vitro and in cells. Consistent with the emerging oncogenic role of Wip1, its DAXX-dephosphorylating impact was most apparent in cancer cell lines harboring gain-of-function mutant and/or overexpressed Wip1. Unexpectedly, while Wip1 depletion increased DAXX phosphorylation both before and after DNA damage and increased p53 stability and transcriptional activity, knock-down of DAXX impacted neither p53 stabilization nor p53-mediated expression of Gadd45a, Noxa, Mdm2, p21, Puma, Sesn2, Tigar or Wip1. Consistently, analyses of cells with genetic, TALEN-mediated DAXX deletion corroborated the notion that neither phosphorylated nor non-phosphorylated DAXX is required for p53-mediated gene expression upon DNA damage. Overall, we identify ATM kinase and Wip1 phosphatase as opposing regulators of DAXX-S564 phosphorylation, and propose that the role of DAXX phosphorylation and DAXX itself are independent of p53-mediated gene expression.

Adenovirus type 12 E1B 55-kilodalton oncoprotein promotes p53-mediated apoptotic response of ovarian cancer to cisplatin

The tumor suppressor p53-mediated apoptotic response plays an important role in cisplatin resistant in ovarian cancer. The adenovirus (Ad) type 12 E1B 55-kDa protein binds to p53 and inactivates its transcriptional transactivation function. In this study, we test the hypothesis that Ad12 E1B 55-kDa oncoprotein promotes p53-mediated apoptotic response of ovarian cancer to cisplatin. First, we observed the upregulation protein level of p53 target genes in cisplatin-resistant or cisplatin-sensitive ovarian cancer by Western blotting. Second, after transfection of Ad12 E1b 55-kDa expression plasmid, the expressions of p53 target genes in A2780 cells were further enhanced. Co-IP experiment demonstrated Ad12 E1b 55 kDa associated with p53. MTT assay confirmed that the cell proliferation was enhanced after transfection, as well as the enhanced cell inhibitory rate in the presence of cisplatin. Using flow cytometry, transfection of Ad12 E1B 55-kDa protein induced apoptosis and promoted S-phase transition in proliferation. Finally, results showed that all these changes promoted by Ad12 E1b 55 kDa were attenuated by the exposure of specific inhibitor of p53 signaling, pifithrin-α. Taken together, we concluded that Ad E1B 55-kDa oncoprotein promotes p53-mediated apoptotic response of ovarian cancer to cisplatin.

Distribution and location of Daxx in cervical epithelial cells with high risk human papillomavirus positive

Aims

To provide the basis for further exploring the effect and its mechanism of Death domain associated protein (Daxx) on the progress of cervical carcinoma induced by human papillomavirus (HPV), the distribution and location of Daxx in cervical carcinoma with high risk HPV(HR-HPV) positive was analyzed.

Methods

The samples of normal cervical epithelial cells, cervical intraepithelial neoplasia grade I (CINI), CINII CINIII and cervical cancers were collected. Immunohistochemistry assay was used to analyze the distributions and locations of Daxx in the cervical tissue. Indirect immunoinfluorescence test was utilized to observe the locations of Daxx in Caski cells with HPV16 positive.

Results

Under the light microscopy, the brown signals of Daxx distributed in the nuclei of normal cervical epithelial cells; Daxx mainly distributed in nuclear membrane and there were a small amount of Daxx in the nuclei in CINI. Daxx intensively distributed in the cytoplasm and cell membrane in CINII, CINIII and cervical cancer. Under fluorescent microscopy, the distribution and location of Daxx in Caski cells was similarly to that in cervical cells of CINII, CINIII and cervical cancer.

Conclusion

In the progress of the cervical cancer, Daxx gradually translocates from nucleus into nuclear membrane, cytoplasm and cell membrane. Daxx locates in the cytoplasm and cell membrane in CINII, CINIII and cervical cancer.

Virtual slides

The virtual slide(s) for this article can be found here:

http://www.diagnosticpathology.diagnomx.eu/vs/4671548951113870.

Virion factors that target Daxx to overcome intrinsic immunity

PML nuclear bodies and their associated functions are part of an intrinsic cellular mechanism aimed at maintaining transcriptional control over viral gene expression and preventing replication of invading viruses. To overcome these barriers, many viruses express early nonstructural, multifunctional proteins to support the viral replication cycle or modulate host immune responses. Virion proteins constituting the invading particle are traditionally investigated for their role in transport during entry or egress and in the assembly of new virions. The additional functions of virion proteins have largely been ignored, in contrast to those of their nonstructural counterparts. A number of recent reports suggest that several virion proteins may also play vital roles in gene activation processes, in particular by counteracting intrinsic immune mechanisms mediated by the PML nuclear body-associated cellular factors Daxx, ATRX, and Sp100. These virion proteins share several features with their more potent nonstructural counterparts, and they may serve to bridge the gap in the early phase of an infection until immediate early viral gene expression is established. In this review, we discuss how virion proteins are an integral part of gene regulation among several viral families and to what extent structural proteins of incoming virions may contribute to species barrier, latency, and oncogenesis.

The repression domain of the E1B 55-kilodalton protein participates in countering interferon-induced inhibition of adenovirus replication

To begin to investigate the mechanism by which the human adenovirus type 5 E1B 55-kDa protein protects against the antiviral effects of type 1 interferon (IFN) (J. S. Chahal, J. Qi, and S. J. Flint, PLoS Pathog. 8:e1002853, 2012 [doi:10.1371/journal.ppat.1002853]), we examined the effects of precise amino acid substitution in this protein on resistance of viral replication to the cytokine. Only substitution of residues 443 to 448 of E1B for alanine (E1B Sub19) specifically impaired production of progeny virus and resulted in a large defect in viral DNA synthesis in IFN-treated normal human fibroblasts. Untreated or IFN-treated cells infected by this mutant virus (AdEasyE1Sub19) contained much higher steady-state concentrations of IFN-inducible GBP1 and IFIT2 mRNAs than did wild-type-infected cells and of the corresponding newly transcribed pre-mRNAs, isolated exploiting 5'-ethynyluridine labeling and click chemistry. These results indicated that the mutations created by substitution of residues 443 to 448 for alanine (Sub19) impair repression of transcription of IFN-inducible genes, by the E1B, 55-kDa protein, consistent with their location in a segment required for repression of p53-dependent transcription. However, when synthesized alone, the E1B 55-kDa protein inhibited expression of the p53-regulated genes BAX and MDM2 but had no impact whatsoever on induction of IFIT2 and GBP1 expression by IFN. These observations correlate repression of transcription of IFN-inducible genes by the E1B 55-kDa protein with protection against inhibition of viral genome replication and indicate that the E1B 55-kDa protein is not sufficient to establish such transcriptional repression.

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.

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.

Adenovirus degradation of cellular proteins

Eukaryotic cells orchestrate constant synthesis and degradation of intracellular components, including soluble proteins and organelles. The two major intracellular degradation pathways are the ubiquitin/proteasome system and autophagy. Whereas ubiquitin/proteasome system is involved in rapid degradation of proteins, autophagy selectively removes protein aggregates and damaged organelles. Failure of these highly adjusted proteolytic systems to maintain basal turnover leads to altered cellular homeostasis. During evolution, certain viruses have developed mechanisms to exploit their functions to facilitate their own replication, prevent viral clearance and promote the outcome of infection. In this article, we summarize the current opinion on adenoviruses (Ad) and molecular host cell targets, extending on recent evidences for protein degradation pathways in infected cells. We describe recently identified connections between Ad-mediated proteolysis and viral replication with main emphasis on the function of certain Ad proteins.

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 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.

Inhibition of p53 by adenovirus type 12 E1B-55K deregulates cell cycle control and sensitizes tumor cells to genotoxic agents

Adenovirus E1B-55K represses p53-mediated transcription. However, the phenotypic consequence of p53 inhibition by E1B-55K for cell cycle regulation and drug sensitivity in tumor cells has not been examined. In HCT116 cells with constitutive E1B-55K expression, the activation of p53 target genes such as the p21, Mdm2, and Puma genes was attenuated, despite markedly elevated p53 protein levels. HCT116 cells with E1B-55K expression displayed a cell cycle profile similar to that of the isogenic HCT116p53(-/-) cells, including unhindered S-phase entry despite DNA damage. Surprisingly, E1B-55K-expressing cells were more sensitive to drug treatment than parental cells. Compared to HCT116 cells, HCT116p53(-/-) cells were more susceptible to both doxorubicin and etoposide, and E1B-55K expression had no effects on drug treatment. E1B-55K expression increased the rate of cell proliferation in HCT116 but not in HCT116p53(-/-) cells. Thus, deregulation of p53-mediated cell cycle control by E1B-55K probably underlies sensitization of HCT116 cells to anticancer drugs. Consistently, E1B-55K expression in A549, A172, and HepG2 cells, all containing wild-type (wt) p53, also enhanced etoposide-induced cytotoxicity, whereas in p53-null H1299 cells, E1B-55K had no effects. We generated several E1B-55K mutants with mutations at positions occupied by the conserved Phe/Trp/His residues. Most of these mutants showed no or reduced binding to p53, although some of them could still stabilize p53, suggesting that binding might not be essential for E1B-55K-induced p53 stabilization. Despite heightened p53 protein levels in cells expressing certain E1B-55K mutants, p53 activity was largely suppressed. Furthermore, most of these E1B-55K mutants could sensitize HCT116 cells to etoposide and doxorubicin. These results indicate that E1B-55K might have utility for enhancing chemotherapy.

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.

DAXX is a new AIRE-interacting protein

The AIRE protein plays a remarkable role as a regulator of central tolerance by controlling the promiscuous expression of tissue-specific antigens in thymic medullary epithelial cells. Defects in the AIRE gene cause the autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy, a rare disease frequent in Iranian Jews, Finns, and Sardinian population. To this day, the precise function of the AIRE protein in regulating transcription and its interacting proteins has yet to be entirely clarified. The knowledge of novel AIRE interactors and their precise role will improve our knowledge of its biological activity and address some of the foremost autoimmunity-related questions. In this study, we have used a yeast two-hybrid system to identify AIRE-interacting proteins. This approach led us to the discovery of a new AIRE-interacting protein called DAXX. The protein is known to be a multifunctional adaptor with functions both in apoptosis and in transcription regulation pathways. The interaction between AIRE and DAXX has been validated by in vivo coimmunoprecipitation analysis and colocalization study in mammalian cells. The interaction has been further confirmed by showing in transactivation assays that DAXX exerts a strong repressive role on the transcriptional activity of AIRE.

The adenoviral E1B 55-kilodalton protein controls expression of immune response genes but not p53-dependent transcription

The human adenovirus type 5 (Ad5) E1B 55-kDa protein modulates several cellular processes, including activation of the tumor suppressor p53. Binding of the E1B protein to the activation domain of p53 inhibits p53-dependent transcription. This activity has been correlated with the transforming activity of the E1B protein, but its contribution to viral replication is not well understood. To address this issue, we used microarray hybridization methods to examine cellular gene expression in normal human fibroblasts (HFFs) infected by Ad5, the E1B 55-kDa-protein-null mutant Hr6, or a mutant carrying substitutions that impair repression of p53-dependent transcription. Comparison of the changes in cellular gene expression observed in these and our previous experiments (D. L. Miller et al., Genome Biol. 8:R58, 2007) by significance analysis of microarrays indicated excellent reproducibility. Furthermore, we again observed that Ad5 infection led to efficient reversal of the p53-dependent transcriptional program. As this same response was also induced in cells infected by the two mutants, we conclude that the E1B 55-kDa protein is not necessary to block activation of p53 in Ad5-infected cells. However, groups of cellular genes that were altered in expression specifically in the absence of the E1B protein were identified by consensus k-means clustering of the hybridization data. Statistical analysis of the enrichment of genes associated with specific functions in these clusters established that the E1B 55-kDa protein is necessary for repression of genes encoding proteins that mediate antiviral and immune defenses.

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.

Daxx mediates SUMO-dependent transcriptional control and subnuclear compartmentalization

SUMO (small ubiquitin-related modifier) modification is emerging as an important post-translational control in transcription. In general, SUMO modification is associated with transcriptional repression. Although many SUMO-modified transcription factors and co-activators have been identified, little is known about the mechanism underlying SUMOylation-elicited transcriptional repression. Here, we summarize that SUMO modification of transcription factors such as androgen receptor, glucocorticoid receptor, Smad4 and CBP [CREB (cAMP-response-element-binding protein)-binding protein] co-activator results in the recruitment of a transcriptional co-repressor Daxx, thereby causing transcriptional repression. Such a SUMO-dependent recruitment of Daxx is mediated by the interaction between the SUMO moiety of SUMOylated factors and Daxx SUMO-interacting motif. Interestingly, the transrepression effect of Daxx on these SUMOylated transcription factors can be relieved by SUMOylated PML (promyelocytic leukaemia) via altering Daxx partition from the targeted gene promoter to PML nuclear bodies. Because Daxx SUMO-interacting motif is a common binding site for SUMOylated factors, a model of competition for Daxx recruitment between SUMOylated PML and SUMOylated transcription factors was proposed. Together, our findings strongly suggest that Daxx functions as a SUMO reader in the SUMO-dependent regulation of transcription and subnuclear compartmentalization.

Daxx contains two nuclear localization signals and interacts with importin alpha3

Daxx plays a major role in several important signaling pathways including transcription and cell death. It has been postulated that Daxx regulates both events from the nucleus; however, the mechanism by which Daxx is localized in the nucleus remains obscure. Here we show that nuclear localization of Daxx is controlled by two independent signals and importin 3. Domain analysis reveals that Daxx contains two separate nuclear localizing domains. Site-directed mutagenesis reveals that the basic aa sequence RLKRK at residues 227-231 (NLS1) is responsible for nuclear localization of N-terminal domain, while aa sequence KKSRKEKK at residues 630-637 (NLS2) is responsible for nuclear localization of the C-terminal domain. Mutations of a NLS consensus sequence RKKRR at residues 391-395 and several other basic aa clusters have no effect on Daxx nuclear localization. In full-length Daxx, NLS1 contributes partially to nuclear localization, while NLS2 plays a major role. Markedly, it is essential to disrupt both NLS1 and NLS2 in order to completely block nuclear localization of the full-length protein and to prevent its association with PML nuclear bodies. Furthermore, Daxx interacts selectively with importin alpha3 through its NLS1 and NLS2 sequences. Conversely, importin alpha3 utilizes two NLS-binding sites for Daxx interaction, suggesting that the importin/mediates nuclear import of Daxx. Finally, we show that nuclear localization of Daxx is essential for its transcriptional effects on GR and p53. Together, these data unveil a molecular mechanism that controls nuclear localization of Daxx and support a nuclear role of Daxx in transcriptional regulation.

Adenovirus type 5 early region 1B 55-kDa oncoprotein can promote cell transformation by a mechanism independent from blocking p53-activated transcription

Inhibition of p53-activated transcription is an integral part of the mechanism by which early region 1B 55K oncoprotein (E1B-55K) from adenovirus type 5 (Ad5) contributes to complete cell transformation in combination with Ad E1A. In addition, more recent data suggest that the mode of action of the Ad protein during transformation may involve additional functions and other protein interactions. In the present study, we performed a comprehensive mutational analysis to assign further transforming functions of Ad5 E1B-55K to distinct domains within the viral polypeptide. Results from these studies show that the functions required for transformation are encoded within several patches of the 55K primary sequence, including several clustered cysteine and histidine residues, some of which match the consensus for zinc fingers. In addition, two amino-acid substitutions (C454S/C456S) created a 55K mutant protein, which had substantially reduced transforming activity. Interestingly, the same mutations neither affected binding to p53 nor inhibition of p53-mediated transactivation. Therefore, an activity necessary for efficient transformation of primary rat cells can be separated from functions required for inhibition of p53-stimulated transcription. Our data indicate that this activity is linked to the ability of the Ad5 protein to bind to components of the Mre11/Rad50/NBS1 DNA double-strand break repair complex, and/or its ability to assemble multiprotein aggregates in the cytoplasm and nucleus of transformed rat cells. These results introduce a new function for Ad5 E1B-55K and suggest that the viral protein contributes to cell transformation through p53 transcription-dependent and -independent pathways.

Adenovirus type 5 exerts genome-wide control over cellular programs governing proliferation, quiescence, and survival

The effects of the adenovirus Ad5 on basic host cell programs, such as cell-cycle regulation, were studied in a microarray analysis of human fibroblasts. About 2,000 genes were up- or down-regulated after Ad5 infection and Ad5 infection was shown to induce reversal of the quiescence program and recapitulation of the core serum response.

Background

Human adenoviruses, such as serotype 5 (Ad5), encode several proteins that can perturb cellular mechanisms that regulate cell cycle progression and apoptosis, as well as those that mediate mRNA production and translation. However, a global view of the effects of Ad5 infection on such programs in normal human cells is not available, despite widespread efforts to develop adenoviruses for therapeutic applications.

Results

We used two-color hybridization and oligonucleotide microarrays to monitor changes in cellular RNA concentrations as a function of time after Ad5 infection of quiescent, normal human fibroblasts. We observed that the expression of some 2,000 genes, about 10% of those examined, increased or decreased by a factor of two or greater following Ad5 infection, but were not altered in mock-infected cells. Consensus k-means clustering established that the temporal patterns of these changes were unexpectedly complex. Gene Ontology terms associated with cell proliferation were significantly over-represented in several clusters. The results of comparative analyses demonstrate that Ad5 infection induces reversal of the quiescence program and recapitulation of the core serum response, and that only a small subset of the observed changes in cellular gene expression can be ascribed to well characterized functions of the viral E1A and E1B proteins.

Conclusion

These findings establish that the impact of adenovirus infection on host cell programs is far greater than appreciated hitherto. Furthermore, they provide a new framework for investigating the molecular functions of viral early proteins and information relevant to the design of conditionally replicating adenoviral vectors.

Regulation of c-met expression by transcription repressor Daxx

The protooncogene c-met encodes the tyrosine kinase receptor for the hepatocyte growth factor/scatter factor (HGF/SF). While overexpression of c-met is documented in many types of tumors, the mechanism of c-met regulation remains elusive. Here, we demonstrate Daxx as a repressor of c-met transcription. The expression of c-met is elevated in Daxx knockout mouse cells and is reversed by Daxx reconstitution. C-met promoter analysis of Daxx-/- cells reveled changes in chromatin acetylation, but not in DNA methylation. Daxx binds to the mouse c-met promoter and Daxx-binding region is sufficient for transcription repression, while HDAC2 is associated with c-met promoter mostly in Daxx+/+ cells, pointing to Daxx-dependent HDAC2 recruitment as a potential mechanism of c-met repression. HGF-induced cell mobility and invasion confirmed augmented activity of c-Met/HGF pathway in Daxx-/- cells. Finally, inverse correlation between Daxx and c-Met in cancer cell lines and in metastatic breast cancer specimens suggests potential function of Daxx as a c-met repressor during cancer progression.

Role for KAP1 serine 824 phosphorylation and sumoylation/desumoylation switch in regulating KAP1-mediated transcriptional repression

As a multifunctional protein, KRAB domain-associated protein 1 (KAP1) is reportedly subjected to multiple protein posttranslational modifications, including phosphorylation and sumoylation. However, gaps exist in our knowledge of how KAP1 phosphorylation cross-talks with KAP1 sumoylation and what the biological consequence is. Here, we show that doxorubicin (Dox) treatment induces KAP1 phosphorylation at Ser-824 via an ataxia telangiectasia mutated (ATM)-dependent manner, correlating with the transcriptional de-repression of p21WAF1/CIP1 and Gadd45alpha. A S824A substitution of KAP1, which ablates the ATM-induced phosphorylation, results in an increase of KAP1 sumoylation and repression of p21 transcription in Dox-treated cells. By contrast, a S824D mutation of KAP1, which mimics constitutive phosphorylation of KAP1, leads to a decrease of KAP1 sumoylation and stimulation of p21 transcription before the exposure of Dox. We further provide evidence that SENP1 deSUMOylase is involved in activating basal, but not Dox-induced, KAP1 Ser-824 phosphorylation, rendering a stimulation of p21 and Gadd45alpha transcription. Moreover, KAP1 and differential sumoylation of KAP1 were also demonstrated to fine-tune the transcription of three additional KAP1-targeted genes, including Bax, Puma, and Noxa. Taken together, our results suggest a novel role for ATM that selectively stimulates KAP1 Ser-824 phosphorylation to repress its sumoylation, leading to the de-repression of expression of a subset of genes involved in promoting cell cycle control and apoptosis in response to genotoxic stresses.

Daxx-like protein of Drosophila interacts with Dmp53 and affects longevity and Ark mRNA level

Daxx-like protein (DLP), the Drosophila homolog of Daxx, binds Drosophila melanogaster p53 (Dmp53) through its C-terminal region. We generated DLP mutants and found that although DLP expression is developmentally regulated, it is not essential for the execution of the developmental program. The effects DLP mutations show in the loss of heterozygosity assay and on phenotypes resulting from Dmp53 overexpression indicate a genetic interaction between DLP and Dmp53. In contrast to Dmp53 mutants, however, loss of DLP does not result in radiosensitivity indicating that it does not play an essential role in the activation of Dmp53-dependent response after ionizing radiation, and DLP is also not required for the irradiation-induced activation of reaper. In contrast, DLP is involved in the transcriptional regulation of Ark, because Ark mRNA level is decreased in DLP mutants and increased upon ectopic overexpression of DLP. Interestingly, DLP mutants have reduced longevity and reduced female fertility. Altogether, our data suggest complex functions for DLP, which include an anti-apoptotic effect exerted through repression of some Dmp53 functions, and activation of some proapoptotic genes.

Adenoviral E1B55K oncoprotein sequesters candidate leukemia suppressor sequence-specific single-stranded DNA-binding protein 2 into aggresomes

Sequence-specific single-stranded DNA-binding protein 2 (SSBP2) is a candidate tumor suppressor for human acute myelogenous leukemia (AML). Inducible expression of SSBP2 causes growth arrest and partial differentiation in AML cells. Here, we report that the adenoviral oncoprotein E1B55K directly binds to endogenous SSBP2 protein and sequesters it into juxtanuclear bodies in adenovirally transformed human embryonic kidney (HEK) 293 cells. Similarly, transient expression of E1B55K in IMR90 fibroblasts and HeLa cells result in the formation of juxtanuclear bodies containing SSBP2. When nuclear export of E1B55K is prevented, SSBP2 remains associated with E1B55K in nuclear foci. A requirement for intact microtubules to retain the integrity of the juxtanuclear bodies suggests them to be E1B55K containing aggresomes. The adenoviral E1B55K protein has been shown to localize to the Mre11 complex and p53 to aggresome structures; together with the viral E4orf6 protein, E1B55K recruits a cellular E3 ubiquitin ligase that induces degradation of Mre11 and p53. However, our present studies reveal that E1B55K does not degrade SSBP2. These data demonstrate that E1B55K targets the candidate leukemia suppressor SSBP2 and suggest that subverting its function may contribute to cell transformation by viral oncoproteins.

Human cytomegalovirus gene expression is silenced by Daxx-mediated intrinsic immune defense in model latent infections established in vitro

In addition to productive lytic infections, herpesviruses such as human cytomegalovirus (HCMV) establish a reservoir of latently infected cells that permit lifelong colonization of the host. When latency is established, the viral immediate-early (IE) genes that initiate the lytic replication cycle are not expressed. HCMV IE gene expression at the start of a lytic infection is facilitated by the viral pp71 protein, which is delivered to cells by infectious viral particles. pp71 neutralizes the Daxx-mediated cellular intrinsic immune defense that silences IE gene expression by generating a repressive chromatin structure on the viral major IE promoter (MIEP). In naturally latently infected cells and in cells latently infected in vitro, the MIEP also adopts a similar silenced chromatin structure. Here we analyze the role of Daxx in quiescent HCMV infections in vitro that mimic some, but not all, of the characteristics of natural latency. We show that in these "latent-like" infections, the Daxx-mediated defense that represses viral gene expression is not disabled because pp71 and Daxx localize to different cellular compartments. We demonstrate that Daxx is required to establish quiescent HCMV infections in vitro because in cells that would normally foster the establishment of these latent-like infections, the loss of Daxx causes the lytic replication cycle to be initiated. Importantly, the lytic cycle is inefficiently completed, which results in an abortive infection. Our work demonstrates that, in certain cell types, HCMV must silence its own gene expression to establish quiescence and prevent abortive infection and that the virus usurps a Daxx-mediated cellular intrinsic immune defense mechanism to do so. This identifies Daxx as one of the likely multiple viral and cellular determinants in the pathway of HCMV quiescence in vitro, and perhaps in natural latent infections as well.

Intranuclear targeting and nuclear export of the adenovirus E1B-55K protein are regulated by SUMO1 conjugation

We have investigated the requirements for CRM1-mediated nuclear export and SUMO1 conjugation of the adenovirus E1B-55K protein during productive infection. Our data show that CRM1 is the major export receptor for E1B-55K in infected cells. Functional inactivation of the E1B-55K CRM1-dependent nuclear export signal (NES) or leptomycin B treatment causes an almost complete redistribution of the viral protein from the cytoplasm to the nucleus and its accumulation at the periphery of the viral replication centers. Interestingly, however, this nuclear restriction imposed on the wild type and the NES mutant protein is fully compensated by concurrent inactivation of the adjacent SUMO1 conjugation site. Moreover, the same mutation fully reverses defects of the NES mutant in the nucleocytoplasmic transport of Mre11 and proteasomal degradation of p53. These results show that nuclear export of E1B-55K in infected cells occurs via CRM1-dependent and -independent pathways and suggest that SUMO1 conjugation and deconjugation provide a molecular switch that commits E1B-55K to a CRM1-independent export pathway.

Focal adhesion kinase and p53 signaling in cancer cells

The progression of human cancer is characterized by a process of tumor cell motility, invasion, and metastasis to distant sites, requiring the cancer cells to be able to survive the apoptotic pressures of anchorage-independent conditions. One of the critical tyrosine kinases linked to these processes of tumor invasion and survival is the focal adhesion kinase (FAK). FAK was first isolated from human tumors, and FAK mRNA was found to be upregulated in invasive and metastatic human breast and colon cancer samples. Recently, the FAK promoter was cloned, and it has been found to contain p53-binding sites. p53 inhibits FAK transcription, and recent data show direct binding of FAK and p53 proteins in vitro and in vivo. The structure of FAK and p53, proteins interacting with FAK, and the role of FAK in tumorigenesis and FAK-p53-related therapy are reviewed. This review focuses on FAK signal transduction pathways, particularly on FAK and p53 signaling, revealing a new paradigm in cell biology, linking signaling from the extracellular matrix to the nucleus.

Oncolytic vesicular stomatitis virus induces apoptosis via signaling through PKR, Fas, and Daxx

Matrix (M) protein mutants of vesicular stomatitis virus (VSV) are promising oncolytic agents for cancer therapy. Previous research has implicated Fas and PKR in apoptosis induced by other viruses. Here, we show that dominant-negative mutants of Fas and PKR inhibit M protein mutant virus-induced apoptosis. Most previous research has focused on the adapter protein FADD as a necessary transducer of Fas-mediated apoptosis. However, the expression of dominant-negative FADD had little effect on the induction of apoptosis by M protein mutant VSV. Instead, virus-induced apoptosis was inhibited by the expression of a dominant-negative mutant of the adapter protein Daxx. These data indicate that Daxx is more important than FADD for apoptosis induced by M protein mutant VSV. These results show that PKR- and Fas-mediated signaling play important roles in cell death during M protein mutant VSV infection and that Daxx has novel functions in the host response to virus infection by mediating virus-induced apoptosis.

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.

Human cytomegalovirus (HCMV) UL82 gene product (pp71) relieves hDaxx-mediated repression of HCMV replication

This study examines the role of the cellular protein hDaxx in controlling human cytomegalovirus (HCMV) immediate-early (IE) gene expression and viral replication. Using permissive cell lines that either overexpress hDaxx or are depleted of hDaxx expression by the use of short hairpin RNA, we demonstrate that hDaxx functions as a repressor of HCMV IE gene expression and replication. In addition, we demonstrate that the impaired growth phenotype associated with the UL82 (pp71) deletion mutant is abolished when hDaxx knockdown cells are infected, suggesting that pp71 functions to relieve hDaxx-mediated repression during HCMV infection.

The cellular protein daxx interacts with avian sarcoma virus integrase and viral DNA to repress viral transcription

The cellular protein Daxx was identified as an interactor with avian sarcoma virus (ASV) integrase (IN) in a yeast two-hybrid screen. After infection, Daxx-IN interactions were detected by coimmunoprecipitation. An association between Daxx and viral DNA, likely mediated by IN, was also detected by chromatin immunoprecipitation. Daxx was not required for early events in ASV replication, including integration, as Daxx-null cells were transduced as efficiently as Daxx-expressing cells. However, viral reporter gene expression from ASV-based vectors was substantially higher in the Daxx-null cells than in Daxx-complemented cells. Consistent with this observation, histone deacetylases (HDACs) were found to associate with viral DNA in Daxx-complemented cells but not in Daxx-null cells. Furthermore, Daxx protein was induced in an interferon-like manner upon ASV infection. We conclude that Daxx interacts with an IN-viral DNA complex early after infection and may mediate the repression of viral gene expression via the recruitment of HDACs. Our findings provide a novel example of cellular immunity against viral replication in which viral transcription is repressed via the recruitment of antiviral proteins to the viral DNA.

Blockage of CRM1-dependent nuclear export of the adenovirus type 5 early region 1B 55-kDa protein augments oncogenic transformation of primary rat cells

The 55-kDa gene product from subgroup C adenovirus type 5 (Ad5) early region 1 (E1B-55kDa) plays a central role in the oncogenic transformation of primary rodent cells primarily by inactivating transcriptional and presumably other functional properties of the tumor suppressor protein p53. We have previously shown that Ad5 E1B-55kDa possesses a leucine-rich nuclear export signal (NES), which confers rapid nucleocytoplasmic shuttling via the CRM1-dependent export pathway. In this study we report that an export-deficient mutant of the viral protein (E1B-NES) substantially enhances focus formation of primary baby rat kidney cells in combination with Ad E1A. Transformed rat cells stably expressing the E1B-NES protein exhibited increased tumorigenicity and accelerated tumor growth in nude mice compared to transformants containing the wild-type E1B product. This 'gain of function' correlated with enhanced inhibition of p53 transactivation in transient reporter assays and the accumulation of the mutant protein and p53 in several dot-like subnuclear aggregates. Interestingly, these structures also contained a large fraction of cellular promyelocytic leukemia protein (PML), a known regulator of p53. These data indicate that E1B-NES promotes oncogenic transformation by combinatorial mechanisms that involve modulation of p53 in the context of PML nuclear bodies. In sum, these results extend our previous observation that inhibition of PML activities by E1B-55kDa is required for efficient focus formation and provide further support for the view that blocking p53 transcriptional functions is the principal mechanism by which the Ad protein contributes to complete cell transformation in conjunction with Ad E1A.

Negative regulation of p53 functions by Daxx and the involvement of MDM2

In normal cells p53 activity is tightly controlled and MDM2 is a known negative regulator. Here we show that via its acidic domain, Daxx binds to the COOH-terminal domain of p53, whose positive charges are critical for this interaction, as Lys to Arg mutations preserved, but Lys to Ala or Ser to Glu mutations abolished Daxx-p53 interaction. These results thus implicate acetylation and phosphorylation of p53 in regulating its binding to Daxx. Interestingly, whereas Daxx did not bind to p53 in cells as assessed by immunoprecipitation, MDM2 expression restored p53-Daxx interaction, and this correlated with deacetylation of p53. In p53/MDM2-null mouse embryonic fibroblasts (DKO MEF), Daxx repressed p53 target promoters whose p53-binding elements were required for the repression. Coexpression of Daxx and MDM2 led to further repression. p53 expression in DKO MEF induced apoptosis and Daxx expression relieved this effect. Similarly, in HCT116 cells, Daxx conferred striking resistance to 5-fluorouracil-induced apoptosis. As p53 is required for 5-fluorouracil-induced cell death, our data show that Daxx can suppress cell death induced by p53 overexpression and p53-dependent stress response. Collectively, our data reveal Daxx as a novel negative regulator of p53. Importantly, posttranslational modifications of p53 inhibit Daxx-p53 interaction, thereby relieving negative regulation of p53 by Daxx.

Diverse roles for E4orf3 at late times of infection revealed in an E1B 55-kilodalton protein mutant background

Species C human adenovirus mutants that fail to express open reading frame 3 of early region 4 (E4orf3) are phenotypically indistinguishable from the wild-type virus when evaluated in cells cultured in vitro. However, E4orf3 gene function has been productively studied in the context of additional viral mutations. This study identifies diverse roles for the E4orf3 protein that are evident in the absence of early region 1B 55-kDa protein (E1B-55K) function. In an E1B-55K-deficient background, the E4orf3 protein promotes viral replication by increasing both the burst size and the probability that an infected cell will produce virus. Early viral gene expression is not impaired in E1B-55K/E4orf3 double mutant virus-infected cells. Cells infected with the double mutant virus accumulated concatemers of viral DNA. However, the E1B-55K/E4orf3 double mutant virus did not replicate any better in MO59J cells, in which viral DNA concatemers did not accumulate, than in MO59K cells, in which viral DNA concatemers were produced, suggesting that viral DNA concatenation is not the primary growth defect of the E1B-55K/E4orf3 double mutant virus. Accumulation of viral mRNA in the nucleus and cytoplasm of E1B-55K/E4orf3 double mutant virus-infected cells was severely reduced compared to that on wild-type virus-infected cells. Thus, in an E1B-55K mutant background, the E4orf3 protein promotes the accumulation of late viral RNA and enhances late gene expression. Finally, within the context of an E1B-55K mutant virus, the E4orf3 protein acts to suppress host cell translation and preserve the viability of cells at moderately late times of infection.