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

The adenoviral E4orf3/4 is a regulatory polypeptide with cell transforming properties in vitro

The human adenovirus species C type 5 (HAdV-C5) early region 4 (E4) encodes several distinct polypeptides, defined as E4orf1 to E4orf6/7 according to the order and arrangement of the corresponding open reading frames (ORFs). All E4 gene products operate through a complex network of interactions with key viral and cellular regulatory proteins involved in transcription, apoptosis, cell cycle control, and DNA repair. Here, we generated a set of virus mutants carrying point mutations in the individual E4 genes. The phenotypic characterizations of these mutants revealed that mutations of these ORFs had no or only moderate effects on virus replication. Even a triple mutant that fails to produce E4orf3, E4orf4, and the yet uncharacterized alternatively spliced E4orf3/4 fusion protein, was replicating to wild type levels. The E4orf3/4 protein consists of the N-terminal 33 amino acid residues from E4orf3 and the C-terminal 28 amino acid residues from E4orf4. Intriguingly, we found that, similar to E4orf3, E4orf3/4 possesses properties that support the E1A/E1B-induced transformation of primary rodent cells. These results identify and functionally characterize E4orf3/4 and conclude that E4orf3/4 is another E4 region protein that is dispensable for virus replication but promotes the E1A/E1B-induced transformation of primary rodent cells.

Disruption of NBS1/MRN Complex Formation by E4orf3 Supports NF-κB That Licenses E1B55K-Deleted Adenovirus-Infected Cells to Accumulate DNA>4n

Cells increase their DNA content greater than the G2/M (DNA > 4n) phases along the path to cancer. The signals that support this increase in DNA content remain poorly understood. Cells infected with adenovirus (Ad) similarly develop DNA > 4n and share a need to bypass the DNA damage response (DDR) signals that trigger cell cycle arrest, and/or cell death. Ads with deletion in early region 1B55K (ΔE1B Ad) are oncolytic agents that are currently being explored for use in vaccine delivery. Interestingly, they promote higher levels of DNA > 4n than Ads that contain E1B55K. Existing in these and almost all Ads that are being explored for clinical use, is early region 4 (E4). The Ad E4 open reading frame 3 (E4orf3) is a viral oncogene that interferes with the ability of cells to respond to DNA damage by disrupting MRN complex formation. Our study reveals that E4orf3 is required for the enhanced fraction of ΔE1B Ad-infected cells with DNA > 4n. For that reason, we explored signaling events mediated by E4orf3. We found that in ΔE1B Ad-infected cells, E4orf3, as reported by others, isolates NBS1 in nuclear dots and tracks. This allows for elevated levels of phosphorylated ATM that is linked to transcriptionally active NF-κB. Pharmacological inhibition of NF-κB reduced the fraction of ΔE1B Ad-infected cells with DNA > 4n while pharmacological inhibition of ATM reduced the levels of nuclear NF-κB and the fraction of ΔE1B Ad-infected cells with DNA > 4n and increased the fraction of dead or dying cells with fragmented DNA. This ability of E4orf3 to disrupt MRN complex formation that allows cells to bypass the cell cycle, evade death, and accumulate DNA > 4n, may be linked to its oncogenic potential. IMPORTANCE Genome instability, a hallmark of cancer, exists as part of a cycle that leads to DNA damage and DNA > 4n that further enhances genome instability. Ad E4orf3 is a viral oncogene. Here, we describe E4orf3 mediated signaling events that support DNA > 4n in ΔE1B Ad-infected cells. These signaling events may be linked to the oncogenic potential of E4orf3 and may provide a basis for how some cells survive with DNA > 4n.

E4orf1 Suppresses E1B-Deleted Adenovirus Vaccine-Induced Immune Responses

As demonstrated by the recent COVID pandemic, vaccines can reduce the burden arising from infectious agents. Adenoviruses (Ads) with deletion of the early region 1B55K (ΔE1B Ad) are currently being explored for use in vaccine delivery. ΔE1B Ads are different from Ads with deletions in early region 1 and early region 3 (ΔE1/E3) used in most Ad vaccine vectors in that they contain the Ad early region 1A (E1A), and therefore the ability to replicate. Common to almost all Ads that are being explored for clinical use is the Ad early region 4 (E4). Among the E4 genes is open reading frame 1 (E4orf1), which mediates signals through the PI3-kinase/Akt pathway that is known to modulate immune responses. This suggests that E4orf1 might also modulate immune responses, although it has remained unexplored in ΔE1B Ad. Here, we show that cells infected with an E1B55K and E4orf1-deleted (ΔE41) Ad exhibited reduced levels of phosphorylated Akt (Ser473 and Thr308)) and expressed different intrinsic innate immune cytokines from those induced in cells infected with an E4orf1-containing, ΔE1B parental Ad that exhibited elevated levels of phosphorylated Akt. Rhesus macaques immunized with a ΔE41 Ad that expressed rhFLSC (HIV-1BaL gp120 linked to rhesus CD4 D1 and D2), exhibited higher levels of rhFLSC-specific interferon γ-producing memory T-cells, higher titers of rhFLSC-specific IgG1 binding antibody in serum, and antibodies able to mediate antibody-dependent cellular cytotoxicity (ADCC) with greater killing capacity than the ΔE1B Ad. Therefore, E4orf1, perhaps by acting through the PI3-kinase/Akt pathway, limits intrinsic innate and system-wide adaptive immune responses that are important for improved ΔE1B Ad-based vaccines.

A porcine xenograft-derived bone scaffold is a biocompatible bone graft substitute: An assessment of cytocompatibility and the alpha-Gal epitope

BACKGROUND

Xenografts are an attractive alternative to traditional bone grafts because of the large supply from donors with predictable morphology and biology as well as minimal risk of human disease transmission. Clinical series involving xenograft bone transplantation, most commonly from bovine sources, have reported poor results with frequent graft rejection and failure to integrate with host tissue. Failures have been attributed to residual alpha-Gal epitope in the xenograft which humans produce natural antibody against. To the authors' knowledge, there is currently no xenograft-derived bone graft substitute that has been adopted by orthopedic surgeons for routine clinical use.

METHODS

In the current study, a bone scaffold intended to serve as a bone graft substitute was derived from porcine cancellous bone using a tissue decellularization and chemical oxidation protocol. In vitro cytocompatibility, pathogen clearance, and alpha-Gal quantification tests were used to assess the safety of the bone scaffold intended for human use.

RESULTS

In vitro studies showed the scaffold was free of processing chemicals and biocompatible with mouse and human cell lines. When bacterial and viral pathogens were purposefully added to porcine donor tissue, processing successfully removed these pathogens to comply with sterility assurance levels established by allograft tissue providers. Critically, 98.5% of the alpha-Gal epitope was removed from donor tissue after decellularization as shown by ELISA inhibition assay and immunohistochemical staining.

CONCLUSIONS

The current investigation supports the biologic safety of bone scaffolds derived from porcine donors using a decellularization protocol that meets current sterility assurance standards. The majority of the highly immunogenic xenograft carbohydrate was removed from donor tissue, and these findings support further in vivo investigation of xenograft-derived bone tissue for orthopedic clinical application.

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.

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

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

IMPORTANCE

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

Adenovirus replaces mitotic checkpoint controls

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

IMPORTANCE

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

Replication of type 5 adenovirus promotes middle ear infection by Streptococcus pneumoniae in the chinchilla model of otitis media

Adenoviral infection is a major risk factor for otitis media. We hypothesized that adenovirus promotes bacterial ascension into the middle ear through the disruption of normal function in the Eustachian tubes due to inflammation-induced changes. An intranasal infection model of the chinchilla was used to test the ability of type 5 adenovirus to promote middle ear infection by Streptococcus pneumoniae. The hyperinflammatory adenovirus mutant dl327 and the nonreplicating adenovirus mutant H5wt300ΔpTP were used to test the role of inflammation and viral replication, respectively, in promotion of pneumococcal middle ear infection. Precedent infection with adenovirus resulted in a significantly greater incidence of middle ear disease by S. pneumoniae as compared to nonadenovirus infected animals. Infection with the adenovirus mutant dl327 induced a comparable degree of bacterial ascension into the middle ear as did infection with the wild-type virus. By contrast, infection with the nonreplicating adenovirus mutant H5wt300ΔpTP resulted in less extensive middle ear infection compared to the wild-type adenovirus. We conclude that viral replication is necessary for adenoviral-induced pneumococcal middle ear disease.

E1B and E4 oncoproteins of adenovirus antagonize the effect of apoptosis inducing factor

Adenovirus inundates the productively infected cell with linear, double-stranded DNA and an abundance of single-stranded DNA. The cellular response to this stimulus is antagonized by the adenoviral E1B and E4 early genes. A mutant group C adenovirus that fails to express the E1B-55K and E4orf3 genes is unable to suppress the DNA-damage response. Cells infected with this double-mutant virus display significant morphological heterogeneity at late times of infection and frequently contain fragmented nuclei. Nuclear fragmentation was due to the translocation of apoptosis inducing factor (AIF) from the mitochondria into the nucleus. The release of AIF was dependent on active poly(ADP-ribose) polymerase-1 (PARP-1), which appeared to be activated by viral DNA replication. Nuclear fragmentation did not occur in AIF-deficient cells or in cells treated with a PARP-1 inhibitor. The E1B-55K or E4orf3 proteins independently prevented nuclear fragmentation subsequent to PARP-1 activation, possibly by altering the intracellular distribution of PAR-modified proteins.

Effects of the deletion of early region 4 (E4) open reading frame 1 (orf1), orf1-2, orf1-3 and orf1-4 on virus-host cell interaction, transgene expression, and immunogenicity of replicating adenovirus HIV vaccine vectors

The global health burden engendered by human immunodeficiency virus (HIV)-induced acquired immunodeficiency syndrome (AIDS) is a sobering reminder of the pressing need for a preventative vaccine. In non-human primate models replicating adenovirus (Ad)-HIV/SIV recombinant vaccine vectors have been shown to stimulate potent immune responses culminating in protection against challenge exposures. Nonetheless, an increase in the transgene carrying capacity of these Ad vectors, currently limited to approximately 3000 base pairs, would greatly enhance their utility. Using a replicating, E3-deleted Ad type 5 host range mutant (Ad5 hr) encoding full-length single-chain HIVBaLgp120 linked to the D1 and D2 domains of rhesus macaque CD4 (rhFLSC) we systematically deleted the genes encoding early region 4 open reading frame 1 (E4orf1) through E4orf4. All the Ad-rhFLSC vectors produced similar levels of viral progeny. Cell cycle analysis of infected human and monkey cells revealed no differences in virus-host interaction. The parental and E4-deleted viruses expressed comparable levels of the transgene with kinetics similar to Ad late proteins. Similar levels of cellular immune responses and transgene-specific antibodies were elicited in vaccinated mice. However, differences in recognition of Ad proteins and induced antibody subtypes were observed, suggesting that the E4 gene products might modulate antibody responses by as yet unknown mechanisms. In short, we have improved the transgene carrying capacity by one thousand base pairs while preserving the replicability, levels of transgene expression, and immunogenicity critical to these vaccine vectors. This additional space allows for flexibility in vaccine design that could not be obtained with the current vector and as such should facilitate the goal of improving vaccine efficacy. To the best of our knowledge, this is the first report describing the effects of these E4 deletions on transgene expression and immunogenicity in a replicating Ad vector.

Adenoviral vectors stimulate glucagon transcription in human mesenchymal stem cells expressing pancreatic transcription factors

Viral gene carriers are being widely used as gene transfer systems in (trans)differentiation and reprogramming strategies. Forced expression of key regulators of pancreatic differentiation in stem cells, liver cells, pancreatic duct cells, or cells from the exocrine pancreas, can lead to the initiation of endocrine pancreatic differentiation. While several viral vector systems have been employed in such studies, the results reported with adenovirus vectors have been the most promising in vitro and in vivo. In this study, we examined whether the viral vector system itself could impact the differentiation capacity of human bone-marrow derived mesenchymal stem cells (hMSCs) toward the endocrine lineage. Lentivirus-mediated expression of Pdx-1, Ngn-3, and Maf-A alone or in combination does not lead to robust expression of any of the endocrine hormones (i.e. insulin, glucagon and somatostatin) in hMSCs. Remarkably, subsequent transduction of these genetically modified cells with an irrelevant early region 1 (E1)-deleted adenoviral vector potentiates the differentiation stimulus and promotes glucagon gene expression in hMSCs by affecting the chromatin structure. This adenovirus stimulation was observed upon infection with an E1-deleted adenovirus vector, but not after exposure to helper-dependent adenovirus vectors, pointing at the involvement of genes retained in the E1-deleted adenovirus vector in this phenomenon. Lentivirus mediated expression of the adenovirus E4-ORF3 mimics the adenovirus effect. From these data we conclude that E1-deleted adenoviral vectors are not inert gene-transfer vectors and contribute to the modulation of the cellular differentiation pathways.

Adenovirus regulates sumoylation of Mre11-Rad50-Nbs1 components through a paralog-specific mechanism

The Mre11-Rad50-Nbs1 (MRN) complex plays a key role in the DNA damage response, presenting challenges for DNA viruses and retroviruses. To inactivate this complex, adenovirus (Ad) makes use of the E1B-55K and E4-open reading frame 6 (ORF6) proteins for ubiquitin (Ub)-mediated, proteasome-dependent degradation of MRN and the E4-ORF3 protein for relocalization and sequestration of MRN within infected-cell nuclei. Here, we report that Mre11 is modified by the Ub-related modifier SUMO-2 and Nbs1 is modified by both SUMO-1 and SUMO-2. We found that Mre11 and Nbs1 are sumoylated during Ad5 infection and that the E4-ORF3 protein is necessary and sufficient to induce SUMO conjugation. Relocalization of Mre11 and Nbs1 into E4-ORF3 nuclear tracks is required for this modification to occur. E4-ORF3-mediated SUMO-1 conjugation to Nbs1 and SUMO-2 conjugation to Mre11 and Nbs1 are transient during wild-type Ad type 5 (Ad5) infection. In contrast, SUMO-1 conjugation to Nbs1 is stable in cells infected with E1B-55K or E4-ORF6 mutant viruses, suggesting that Ad regulates paralog-specific desumoylation of Nbs1. Inhibition of viral DNA replication blocks deconjugation of SUMO-2 from Mre11 and Nbs1, indicating that a late-phase process is involved in Mre11 and Nbs1 desumoylation. Our results provide direct evidence of Mre11 and Nbs1 sumoylation induced by the Ad5 E4-ORF3 protein and an important example showing that modification of a single substrate by both SUMO-1 and SUMO-2 is regulated through distinct mechanisms. Our findings suggest how E4-ORF3-mediated relocalization of the MRN complex influences the cellular DNA damage response.

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.

Biophysical and functional analyses suggest that adenovirus E4-ORF3 protein requires higher-order multimerization to function against promyelocytic leukemia protein nuclear bodies

The early region 4 open reading frame 3 protein (E4-ORF3; UniProt ID P04489) is the most highly conserved of all adenovirus-encoded gene products at the amino acid level. A conserved attribute of the E4-ORF3 proteins of different human adenoviruses is the ability to disrupt PML nuclear bodies from their normally punctate appearance into heterogeneous filamentous structures. This E4-ORF3 activity correlates with the inhibition of PML-mediated antiviral activity. The mechanism of E4-ORF3-mediated reorganization of PML nuclear bodies is unknown. Biophysical analysis of the purified WT E4-ORF3 protein revealed an ordered secondary/tertiary structure and the ability to form heterogeneous higher-order multimers in solution. Importantly, a nonfunctional E4-ORF3 mutant protein, L103A, forms a stable dimer with WT secondary structure content. Because the L103A mutant is incapable of PML reorganization, this result suggests that higher-order multimerization of E4-ORF3 may be required for the activity of the protein. In support of this hypothesis, we demonstrate that the E4-ORF3 L103A mutant protein acts as a dominant-negative effector when coexpressed with the WT E4-ORF3 in mammalian cells. It prevents WT E4-ORF3-mediated PML track formation presumably by binding to the WT protein and inhibiting the formation of higher-order multimers. In vitro protein binding studies support this conclusion as demonstrated by copurification of coexpressed WT and L103A proteins in Escherichia coli and coimmunoprecipitation of WT·L103A E4-ORF3 complexes in mammalian cells. These results provide new insight into the properties of the Ad E4-ORF3 protein and suggest that higher-order protein multimerization is essential for E4-ORF3 activity.

Timely synthesis of the adenovirus type 5 E1B 55-kilodalton protein is required for efficient genome replication in normal human cells

Previous studies have indicated that the adenovirus type 5 E1B 55-kDa protein facilitates viral DNA synthesis in normal human foreskin fibroblasts (HFFs) but not in primary epithelial cells. To investigate this apparent difference further, viral DNA accumulation was examined in primary human fibroblasts and epithelial cells infected by the mutant AdEasyE1Δ2347, which carries the Hr6 frameshift mutation that prevents production of the E1B 55-kDa protein, in an E1-containing derivative of AdEasy. Impaired viral DNA synthesis was observed in normal HFFs but not in normal human bronchial epithelial cells infected by this mutant. However, acceleration of progression through the early phase, which is significantly slower in HFFs than in epithelial cells, eliminated the dependence of efficient viral DNA synthesis in HFFs on the E1B 55-kDa protein. These observations suggest that timely synthesis of the E1B 55-kDa protein protects normal cells against a host defense that inhibits adenoviral genome replication. One such defense is mediated by the Mre11-Rad50-Nbs1 complex. Nevertheless, examination of the localization of Mre11 and viral proteins by immunofluorescence suggested that this complex is inactivated similarly in AdEasyE1Δ2347 mutant-infected and AdEasyE1-infected HFFs.

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.

Adenovirus E1B 55-kilodalton protein is a p53-SUMO1 E3 ligase that represses p53 and stimulates its nuclear export through interactions with promyelocytic leukemia nuclear bodies

Oncogenic transformation by adenovirus E1A and E1B-55K requires E1B-55K inhibition of p53 activity to prevent E1A-induced apoptosis. During viral infection, E1B-55K and E4orf6 substitute for the substrate-binding subunits of the host cell cullin 5 class of ubiquitin ligases, resulting in p53 polyubiquitinylation and proteasomal degradation. Here we show that E1B-55K alone also functions as an E3 SUMO1-p53 ligase. Fluorescence microscopy studies showed that E1B-55K alone, in the absence of other viral proteins, causes p53 to colocalize with E1B-55K in promyelocytic leukemia (PML) nuclear bodies, nuclear domains with a high concentration of sumoylated proteins. Photobleaching experiments with live cells revealed that E1B-55K tethering of p53 in PML nuclear bodies decreases the in vivo nuclear mobility of p53 nearly 2 orders of magnitude. E1B-55K-induced p53 sumoylation contributes to maximal inhibition of p53 function since mutation of the major p53 sumoylation site decreases E1B-55K-induced p53 sumoylation, tethering in PML nuclear bodies, and E1B-55K inhibition of p53 activity. Mutation of the E1B-55K sumoylation site greatly inhibits E1B-55K association with PML nuclear bodies and the p53 nuclear export to cytoplasmic aggresomes observed in E1A-E1B-transformed cells. Purified E1B-55K and p53 form high-molecular-weight complexes potentially through the formation of a network of E1B-55K dimers bound to the N termini of p53 tetramers. In support of this model, a p53 mutation that prevents tetramer formation greatly reduces E1B-55K-induced tethering in PML nuclear bodies and p53 nuclear export. These data indicate that E1B-55K's association with PML nuclear bodies inactivates p53 by first sequestering it in PML nuclear bodies and then greatly facilitating its nuclear export.

The adenovirus E1B 55-kilodalton and E4 open reading frame 6 proteins limit phosphorylation of eIF2alpha during the late phase of infection

During a productive infection, species C adenovirus reprograms the host cell to promote viral translation at the expense of cellular translation. The E1B 55-kilodalton (E1B-55K) and E4 open reading frame 6 (E4orf6) proteins are important in this control of gene expression. As part of a ubiquitin-protein ligase, these viral proteins stimulate viral mRNA export, inhibit cellular mRNA export, promote viral gene expression, and direct the degradation of certain host proteins. We report here that the E1B-55K and E4orf6 proteins limited phosphorylation of eIF2alpha and the activation of the eIF2alpha kinase PKR. Phospho-eIF2alpha levels were observed to rise and fall at least twice during infection. The E1B-55K and E4orf6 proteins prevented a third increase at late times of infection. PKR appeared to phosphorylate eIF2alpha only in the absence of E1B-55K/E4orf6 function. PKR activation and eIF2alpha phosphorylation was unrelated to the cytoplasmic levels of the adenovirus inhibitor of PKR, VA-I RNA. Nonetheless, expression of a PKR inhibitor, the reovirus double-stranded RNA-binding protein sigma 3, prevented PKR activation and eIF2alpha phosphorylation. The sigma 3 protein largely corrected the defect in viral late protein synthesis associated with the E1B-55K and E4orf6 mutant viruses without affecting cytoplasmic levels of the late viral mRNA. The ubiquitin-protein ligase activity associated with the E1B-55K/E4orf6 complex was necessary to prevent activation of PKR and phosphorylation of eIF2alpha. These findings reveal a new contribution of the E1B-55K/E4orf6 complex to viral late protein synthesis and the existence of multiple layers of regulation imposed on eIF2alpha phosphorylation and PKR activation in adenovirus-infected cells.

E4orf1 limits the oncolytic potential of the E1B-55K deletion mutant adenovirus

Clinical trials have shown oncolytic adenoviruses to be tumor selective with minimal toxicity toward normal tissue. The virus ONYX-015, in which the gene encoding the early region 1B 55-kDa (E1B-55K) protein is deleted, has been most effective when used in combination with either chemotherapy or radiation therapy. Therefore, improving the oncolytic nature of tumor-selective adenoviruses remains an important objective for improving this form of cancer therapy. Cells infected during the G(1) phase of the cell cycle with the E1B-55K deletion mutant virus exhibit a reduced rate of viral late protein synthesis, produce fewer viral progeny, and are less efficiently killed than cells infected during the S phase. Here we demonstrate that the G(1) restriction imposed on the E1B-55K deletion mutant virus is due to the viral oncogene encoded by open reading frame 1 of early region 4 (E4orf1). E4orf1 has been reported to signal through the phosphatidylinositol 3'-kinase pathway leading to the activation of Akt, mTOR, and p70 S6K. Evidence presented here shows that E4orf1 may also induce the phosphorylation of Akt and p70 S6K in a manner that depends on Rac1 and its guanine nucleotide exchange factor Tiam1. Accordingly, agents that have been reported to disrupt the Tiam1-Rac1 interaction or to prevent phosphorylation of the ribosomal S6 kinase partially alleviated the E4orf1 restriction to late viral protein synthesis and enhanced tumor cell killing by the E1B-55K mutant virus. These results demonstrate that E4orf1 limits the oncolytic nature of a conditionally replicating adenovirus such as ONYX-015. The therapeutic value of similar oncolytic adenoviruses may be improved by abrogating E4orf1 function.

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.

Differential requirements of the C terminus of Nbs1 in suppressing adenovirus DNA replication and promoting concatemer formation

Adenoviruses (Ad) with the early region E4 deleted (E4-deleted virus) are defective for DNA replication and late protein synthesis. Infection with E4-deleted viruses results in activation of a DNA damage response, accumulation of cellular repair factors in foci at viral replication centers, and joining together of viral genomes into concatemers. The cellular DNA repair complex composed of Mre11, Rad50, and Nbs1 (MRN) is required for concatemer formation and full activation of damage signaling through the protein kinases Ataxia-telangiectasia mutated (ATM) and ATM-Rad3-related (ATR). The E4orf3 and E4orf6 proteins expressed from the E4 region of Ad type 5 (Ad5) inactivate the MRN complex by degradation and mislocalization, and prevent the DNA damage response. Here we investigated individual contributions of the MRN complex, concatemer formation, and damage signaling to viral DNA replication during infection with E4-deleted virus. Using virus mutants, short hairpin RNA knockdown and hypomorphic cell lines, we show that inactivation of MRN results in increased viral replication. We demonstrate that defective replication in the absence of E4 is not due to concatemer formation or DNA damage signaling. The C terminus of Nbs1 is required for the inhibition of Ad DNA replication and recruitment of MRN to viral replication centers. We identified regions of Nbs1 that are differentially required for concatemer formation and inhibition of Ad DNA replication. These results demonstrate that targeting of the MRN complex explains the redundant functions of E4orf3 and E4orf6 in promoting Ad DNA replication. Understanding how MRN impacts the adenoviral life cycle will provide insights into the functions of this DNA damage sensor.

Cellular proteins PML and Daxx mediate an innate antiviral defense antagonized by the adenovirus E4 ORF3 protein

The adenovirus (Ad) E4 ORF3 protein is both necessary and sufficient to reorganize a nuclear subdomain, the PML nuclear body (PML-NB), from punctate structures into elongated nuclear tracks. PML-NB disruption is recapitulated by a variety of DNA viruses that encode proteins responsible for compromising PML-NB integrity through different mechanisms. PML-NB disruption has been correlated with the antagonism of both innate and intrinsic immune responses. The E4 ORF3 protein is required for adenoviral DNA replication in the interferon (IFN)-induced antiviral state. This may reflect the fact that PML itself, in addition to several other PML-NB proteins, is encoded by an interferon-stimulated gene. Here, we demonstrate that reorganization of the PML-NB by E4 ORF3 antagonizes an innate antiviral response mediated by both PML and Daxx. Reduction of either of these proteins is sufficient to restore the replicative capacity of virus with the E4 ORF3 protein deleted in the IFN-induced antiviral state. Further, we provide evidence that both the HSV1 ICP0 and HCMV IE1 proteins, which disrupt PML-NBs by mechanistically distinct strategies, behave in a manner functionally analogous to E4 ORF3 with respect to antagonizing the IFN-induced antiviral state. In addition, we assert that this innate antiviral strategy mediated by PML and Daxx does not involve transcriptional repression. While early gene transcription is modestly diminished in the absence of E4 ORF3 protein expression, this reduction does not affect early protein function. We propose that, in addition to its ability to repress gene expression, the PML-NB participates in additional innate immune activities.

Adenovirus E4 ORF3 protein inhibits the interferon-mediated antiviral response

The PML oncogenic domain (POD/ND10/PML body) is a common target of DNA viruses, which replicate their genomes in proximity to this nuclear structure. The adenovirus early protein E4 ORF3 is both necessary and sufficient to rearrange PODs from punctate bodies into track-like structures. Although multiple hypotheses exist, the precise reason for this activity has not yet been elucidated. PML, the protein responsible for nucleating PODs, is an interferon (IFN)-stimulated gene, implicating the participation of this nuclear body in an innate antiviral response. Here, we demonstrate that E4 ORF3 is critical to the replicative success of adenovirus during the IFN-induced antiviral state. When cells are pretreated with either IFN-alpha or IFN-gamma, a mutant virus that does not express E4 ORF3 is severely compromised for replication. This result suggests the functional significance of ORF3 track formation is the inhibition of a POD-mediated, antiviral mechanism. Replication of the E4 ORF3 mutant virus can be rescued following the introduction of E4 ORF3 from evolutionarily divergent adenoviruses, suggesting a conserved function for E4 ORF3 inhibition of the IFN-induced antiviral state. Furthermore, E4 ORF3 inhibition of an IFN-induced response is unrelated to the inhibition of adenovirus replication by the Mre11-Rad50-Nbs1 DNA repair complex. We propose that the evolutionarily conserved function of the adenovirus E4 ORF3 protein is the inhibition of a host interferon response to viral infection via disruption of the PML oncogenic domain.

The adenovirus E4 ORF3 protein binds and reorganizes the TRIM family member transcriptional intermediary factor 1 alpha

One of the most interesting functions attributed to the adenovirus early region 4 open reading frame 3 (E4 ORF3) protein is its reorganization of promyelocytic leukemia (PML) protein nuclear bodies. These normally punctate structures are reorganized by E4 ORF3 into tracks that eventually surround viral replication centers. PML rearrangement is an evolutionarily conserved function of E4 ORF3, yet its cause and functional relevance remain mysteries. The E4 ORF3 protein coimmunoprecipitates with the PML protein, yet E4 ORF3 still forms tracks in cells that lack PML. The PML protein is a member of a larger protein family termed tripartite motif (TRIM) proteins. TRIM proteins contain a tripartite domain structure in proximity to their N termini that consists of a RING finger domain, followed by one or two B box domains and a C-terminal coiled-coil domain (collectively termed the RBCC domain). The order and spacing of these domains are evolutionarily conserved and thought to mediate protein-protein interactions and other functions. We implemented a proteomic approach to isolate cellular proteins that bind to E4 ORF3. We identified a novel interaction between E4 ORF3 and another TRIM family member, transcriptional intermediary factor 1 alpha (TIF1alpha). TIF1alpha functions by recruiting coactivators and/or corepressors to modulate transcription. The interaction between E4 ORF3 and TIF1alpha was validated by coimmunoprecipitation and binding of recombinant proteins. Indirect immunofluorescence assays demonstrated that TIF1alpha is reorganized into track structures that contain PML upon E4 ORF3 expression. The RBCC domain of TIF1alpha is sufficient for E4 ORF3-induced rearrangement, and TIF1alpha reorganization is conserved across adenovirus serotypes.

Adenovirus ubiquitin-protein ligase stimulates viral late mRNA nuclear export

Theadenovirus type 5 (Ad5) E1B-55K and E4orf6 proteins are required together to stimulate viral late nuclear mRNA export to the cytoplasm and to restrict host cell nuclear mRNA export during the late phase of infection. Previous studies have shown that these two viral proteins interact with the cellular proteins elongins B and C, cullin 5, RBX1, and additional cellular proteins to form an E3 ubiquitin-protein ligase that polyubiquitinates p53 and probably one or more subunits of the MRE11-RAD50-NBS1 (MRN) complex, directing their proteasomal degradation. The MRN complex is required for cellular DNA double-strand break repair and induction of the DNA damage response by adenovirus infection. To determine if the ability of E1B-55K and E4orf6 to stimulate viral late mRNA nuclear export requires the ubiquitin-protein ligase activity of this viral ubiquitin-protein ligase complex, we designed and expressed a dominant-negative mutant form of cullin 5 in HeLa cells before infection with wild-type Ad5 or the E1B-55K null mutant dl1520. The dominant-negative cullin 5 protein stabilized p53 and the MRN complex, indicating that it inhibited the viral ubiquitin-protein ligase but had no effect on viral early mRNA synthesis, early protein synthesis, or viral DNA replication. However, expression of the dominant-negative cullin 5 protein caused a decrease in viral late protein synthesis and viral nuclear mRNA export similar to the phenotype produced by mutations in E1B-55K. We conclude that the stimulation of adenovirus late mRNA nuclear export by E1B-55K and E4orf6 results from the ubiquitin-protein ligase activity of the adenovirus ubiquitin-protein ligase complex.

Effects of adeno-associated virus on adenovirus replication and gene expression during coinfection

Adeno-associated virus (AAV) is a nonpathogenic parvovirus that requires adenovirus (Ad) or another helper virus for a fully permissive infection. AAV-mediated inhibition of Ad is well documented, yet many details of this interaction remain unclear. In this study, we observed a maximum 50-fold decrease in infectious virus production and a 10- to 40-fold reduction in Ad DNA synthesis during coinfections with AAV. With the exception of the E3 gene, AAV decreased all steady-state Ad mRNA levels at 24 h postinfection (hpi) in a dose-dependent manner. However, not all transcription units were affected equally. E4 and late transcription were the most strongly inhibited, and E1A and E2A were the least affected. The temporal effects of AAV on Ad mRNA transcript levels also varied among the Ad genes. Ad protein expression paralleled mRNA levels at 24 hpi, suggesting that coinfecting AAV does not exert substantial effects on translation. In plasmid transfection assays, Rep78 protein most effectively limited Ad amplification, while Rep40 had no effect. Since E2a and E4 proteins are essential for efficient Ad DNA amplification, we examined the relationship between reduced E2A and E4 expression and decreased DNA amplification. Transfected Rep78 did not reduce E2A and E4 transcript levels prior to DNA replication. Also, AAV-induced inhibition of E2A and E4 mRNA production did not occur in the presence of hydroxyurea. It is therefore unlikely that decreased early gene expression is solely responsible for AAV's suppression of Ad DNA replication. Our results suggest that AAV amplification and/or Rep gene expression inhibits Ad DNA synthesis.

Recent lessons in gene expression, cell cycle control, and cell biology from adenovirus

Adenovirus continues to be an important model system for investigating basic aspects of cell biology. Interactions of several cellular proteins with E1A conserved regions (CR) 1 and 2, and inhibition of apoptosis by E1B proteins are required for oncogenic transformation. CR2 binds RB family members, de-repressing E2F transcription factors, thus activating genes required for cell cycling. E1B-19K is a BCL2 homolog that binds and inactivates proapoptotic BAK and BAX. E1B-55K binds p53, inhibiting its transcriptional activation function. In productively infected cells, E1B-55K and E4orf6 assemble a ubiquitin ligase with cellular proteins Elongins B and C, Cullin 5 and RBX1 that polyubiquitinates p53 and one or more subunits of the MRN complex involved in DNA double-strand break repair, directing them to proteosomal degradation. E1A CR3 activates viral transcription by interacting with the MED23 Mediator subunit, stimulating preinitiation complex assembly on early viral promoters and probably also the rate at which they initiate transcription. The viral E1B-55K/E4orf6 ubiquitin ligase is also required for efficient viral late protein synthesis in many cell types, but the mechanism is not understood. E1A CR1 binds several chromatin-modifying complexes, but how this contributes to stimulation of cellular DNA synthesis and transformation is not clear. E1A CR4 binds the CtBP corepressor, but the mechanism by which this modulates the frequency of transformation remains to be determined. Clearly, adenovirus has much left to teach us about fundamental cellular processes.

Adenovirus exploits the cellular aggresome response to accelerate inactivation of the MRN complex

Results reported here indicate that adenovirus 5 exploits the cellular aggresome response to accelerate inactivation of MRE11-RAD50-NBS1 (MRN) complexes that otherwise inhibit viral DNA replication and packaging. Aggresomes are cytoplasmic inclusion bodies, observed in many degenerative diseases, that are formed from aggregated proteins by dynein-dependent retrograde transport on microtubules to the microtubule organizing center. Viral E1B-55K protein forms aggresomes that sequester p53 and MRN in transformed cells and in cells transfected with an E1B-55K expression vector. During adenovirus infection, the viral protein E4orf3 associates with MRN in promyelocytic leukemia protein nuclear bodies before MRN is bound by E1B-55K. Either E4orf3 or E4orf6 is required in addition to E1B-55K for E1B-55K aggresome formation and MRE11 export to aggresomes in adenovirus-infected cells. Aggresome formation contributes to the protection of viral DNA from MRN activity by sequestering MRN in the cytoplasm and greatly accelerating its degradation by proteosomes following its ubiquitination by the E1B-55K/E4orf6/elongin BC/Cullin5/Rbx1 ubiquitin ligase. Our results show that aggresomes significantly accelerate protein degradation by the ubiquitin-proteosome system. The observation that a normal cellular protein is inactivated when sequestered into an aggresome through association with an aggresome-inducing protein has implications for the potential cytotoxicity of aggresome-like inclusion bodies in degenerative diseases.

Genome concatenation contributes to the late gene expression defect of an adenovirus E4 mutant

Adenovirus mutants that lack the entire E4 region are severely defective for late gene expression. E4 mutant genomes are also concatenated by host double strand break repair (DSBR) proteins. We find that E4 mutant late gene expression improves in MO59J cells that fail to form genome concatemers. DSBR kinase inhibitors interfere with genome concatenation and also stimulate late gene expression. Concatenation of E4 mutant genomes interferes with cytoplasmic accumulation of viral late messages and leads to reduced late protein levels and poor viral yields following high multiplicity infection. However, failure to concatenate viral genomes did not rescue either the DNA replication defect or virus yield following low multiplicity E4 mutant infection. Our results indicate that if the E4 mutant DNA replication defect is overcome by high multiplicity infection, concatenation of the replicated genomes by host DSBR interferes with viral late gene expression.

Adenovirus type 5 E4orf3 protein targets the Mre11 complex to cytoplasmic aggresomes

Virus infections have dramatic effects on structural and morphological characteristics of the host cell. The gene product of open reading frame 3 in the early region 4 (E4orf3) of adenovirus serotype 5 (Ad5) is involved in efficient replication and late protein synthesis. During infection with adenovirus mutants lacking the E4 region, the viral genomic DNA is joined into concatemers by cellular DNA repair factors, and this requires the Mre11/Rad50/Nbs1 complex. Concatemer formation can be prevented by the E4orf3 protein, which causes the cellular redistribution of the Mre11 complex. Here we show that E4orf3 colocalizes with components of the Mre11 complex in nuclear tracks and also in large cytoplasmic accumulations. Rearrangement of Mre11 and Rad50 by Ad5 E4orf3 is not dependent on interactions with Nbs1 or promyelocytic leukemia protein nuclear bodies. Late in infection the cytoplasmic inclusions appear as a distinct juxtanuclear accumulation at the centrosome and this requires an intact microtubule cytoskeleton. The large cytoplasmic accumulations meet the criteria defined for aggresomes, including gamma-tubulin colocalization and formation of a surrounding vimentin cage. E4orf3 also appears to alter the solubility of the cellular Mre11 complex. These data suggest that E4orf3 can target the Mre11 complex to an aggresome and may explain how the cellular repair complex is inactivated during adenovirus infection.

Serotype-specific reorganization of the Mre11 complex by adenoviral E4orf3 proteins

The early transcriptional region 4 (E4) of adenovirus type 5 (Ad5) encodes gene products that modulate splicing, apoptosis, transcription, DNA replication, and repair pathways. Viruses lacking both E4orf3 and E4orf6 have a severe replication defect, partially characterized by the formation of genome concatemers. Concatemer formation is dependent upon the cellular Mre11 complex and is prevented by both the E4orf3 and E4orf6 proteins. The Mre11/Rad50/Nbs1 proteins are targeted for proteasome-mediated degradation by the Ad5 viral E1b55K/E4orf6 complex. The expression of Ad5 E4orf3 causes a redistribution of Mre11 complex members and results in their exclusion from viral replication centers. For this study, we further analyzed the interactions of E4 proteins from different adenovirus serotypes with the Mre11 complex. Analyses of infections with serotypes Ad4 and Ad12 demonstrated that the degradation of Mre11/Rad50/Nbs1 proteins is a conserved feature of the E1b55K/E4orf6 complex. Surprisingly, Nbs1 and Rad50 were localized to the replication centers of both Ad4 and Ad12 viruses prior to Mre11 complex degradation. The transfection of expression vectors for the E4orf3 proteins of Ad4 and Ad12 did not alter the localization of Mre11 complex members. The E4orf3 proteins of Ad4 and Ad12 also failed to complement defects in both concatemer formation and late protein production of a virus with a deletion of E4. These results reveal surprising differences among the highly conserved E4orf3 proteins from different serotypes in the ability to disrupt the Mre11 complex.