Adenovirus E4orf4 protein induces PP2A-dependent growth arrest in Saccharomyces cerevisiae and interacts with the anaphase-promoting complex/cyclosome

The adenovirus E4orf4 protein induces G2/M arrest and cell death by blocking protein phosphatase 2A activity regulated by the B55 subunit

Human adenovirus E4orf4 protein is toxic in human tumor cells. Its interaction with the B alpha subunit of protein phosphatase 2A (PP2A) is critical for cell killing; however, the effect of E4orf4 binding is not known. B alpha is one of several mammalian B-type regulatory subunits that form PP2A holoenzymes with A and C subunits. Here we show that E4orf4 protein interacts uniquely with B55 family subunits and that cell killing increases with the level of E4orf4 expression. Evidence suggesting that B alpha-specific PP2A activity, measured in vitro against phosphoprotein substrates, is reduced by E4orf4 binding was obtained, and two potential B55-specific PP2A substrates, 4E-BP1 and p70(S6K), were seen to be hypophosphorylated in vivo following expression of E4orf4. Furthermore, treatment of cells with low levels of the phosphatase inhibitor okadaic acid or coexpression of the PP2A inhibitor I(1)(PP2A) enhanced E4orf4-induced cell killing and G(2)/M arrest significantly. These results suggested that E4orf4 toxicity results from the inhibition of B55-specific PP2A holoenzymes, an idea that was strengthened by an observed growth arrest resulting from treatment of H1299 cells with B alpha-specific RNA interference. We believe that E4orf4 induces growth arrest resulting in cell death by reducing the global level of B55-specific PP2A activity, thus preventing the dephosphorylation of B55-specific PP2A substrates, including those involved in cell cycle progression.

Viral manipulation of DNA repair and cell cycle checkpoints

Recognition and repair of DNA damage is critical for maintaining genomic integrity and suppressing tumorigenesis. In eukaryotic cells, the sensing and repair of DNA damage are coordinated with cell cycle progression and checkpoints, in order to prevent the propagation of damaged DNA. The carefully maintained cellular response to DNA damage is challenged by viruses, which produce a large amount of exogenous DNA during infection. Viruses also express proteins that perturb cellular DNA repair and cell cycle pathways, promoting tumorigenesis in their quest for cellular domination. This review presents an overview of strategies employed by viruses to manipulate DNA damage responses and cell cycle checkpoints as they commandeer the cell to maximize their own viral replication. Studies of viruses have identified key cellular regulators and revealed insights into molecular mechanisms governing DNA repair, cell cycle checkpoints, and transformation.

Stress-induced ceramide-activated protein phosphatase can compensate for loss of amphiphysin-like activity in Saccharomyces cerevisiae and functions to reinitiate endocytosis

Saccharomyces cerevisiae cells lacking the amphiphysin-like orthologs, Rvs161 or Rvs167, are unable to thrive under many stress conditions. Here we show cells lacking Rvs161 require Cdc55, the B subunit of the yeast ceramide-activated protein phosphatase, for viability under heat stress. By using specific rvs mutant alleles, we linked this lethal genetic interaction to loss of Rvs161 endocytic domain function. Recessive mutations in the sphingolipid pathway, such as deletion of the very long-chain fatty acid elongase, Sur4, suppress the osmotic growth defect of rvs161 cells. We demonstrate that Cdc55 is required for sur4-dependent suppressor activity and that protein phosphatase activation, through overexpression of CDC55 alone, can also remediate this defect. Loss of SUR4 in rvs161 cells reinitiates Ste3 a-factor receptor endocytosis and requires Cdc55 function to do so. Moreover, overexpression of CDC55 reinitiates Ste3 endocytic-dependent degradation and restores fluid phase endocytosis in rvs161 cells. In contrast, loss of SUR4 or CDC55 overexpression does not remediate the actin polarization defects of osmotic stressed rvs161 cells. Importantly, remediation of rvs161 defects by protein phosphatase activation requires the ceramide-activated protein phosphatase catalytic subunit, Sit4, and the protein phosphatase 2A catalytic subunits, Pph21/Pph22. Finally, genetic analyses reveal a synthetic lethal interaction between loss of CDC55 and gene deletions lethal with rvs161, all of which function in endocytosis.

Localization and importance of the adenovirus E4orf4 protein during lytic infection

The human adenovirus type 5 (Ad5) E4orf4 product has been studied extensively although in most cases as expressed from vectors in the absence of other viral products. Thus, relatively little is known about its role in the context of an adenovirus infection. Although considerable earlier work had indicated that the E4orf4 protein is not essential for replication, a recent study using dl359, an Ad5 mutant believed to produce a nonfunctional E4orf4 protein, suggested that E4orf4 is essential for virus growth in primary small-airway epithelial cells (C. O'Shea, et al., EMBO J. 24:1211-1221, 2005). Hence, to examine further the role of E4orf4 during virus infection, we generated for the first time a set of E4orf4 virus mutants in a common Ad5 genetic background. Such mutant viruses included those that express E4orf4 proteins containing various individual point mutations, those defective entirely in E4orf4 expression, and a mutant expressing wild-type E4orf4 fused to the green fluorescent protein. E4orf4 protein was found to localize primarily in nuclear structures shown to be viral replication centers, in nucleoli, and in perinuclear bodies. Importantly, E4orf4 was shown not to be essential for virus growth in either human tumor or primary cells, at least in tissue culture. Unlike E4orf4-null virus, mutant dl359 appeared to exhibit a gain-of-function phenotype that impairs virus growth. The dl359 E4orf4 protein, which contains a large in-frame internal deletion, clustered in aggregates enriched in Hsp70 and proteasome components. In addition, the late viral mRNAs produced by dl359 accumulated abnormally in a nuclear punctate pattern. Altogether, our results indicate that E4orf4 protein is not essential for virus growth in culture and that expression of the dl359 E4orf4 product interferes with viral replication, presumably through interactions with structures in the nucleus.

Adenovirus E4orf4 induces HPV-16 late L1 mRNA production

The adenovirus E4orf4 protein regulates the switch from early to late gene expression during the adenoviral replication cycle. Here we report that overexpression of adenovirus E4orf4 induces human papillomavirus type 16 (HPV-16) late gene expression from subgenomic expression plasmids. E4orf4 specifically overcomes the negative effects of two splicing silencers at the two late HPV-16 splice sites SD3632 and SA5639. This results in the production of HPV-16 spliced L1 mRNAs. We show that the interaction of E4orf4 with protein phosphatase 2A (PP2A) is necessary for induction of HPV-16 late gene expression. Also an E4orf4 mutant that fails to bind the cellular splicing factor ASF/SF2 fails to induce L1 mRNA production. Collectively, these results suggest that dephosphorylation of SR proteins by E4orf4 activates HPV-16 late gene expression. Indeed, a mutant ASF/SF2 protein in which the RS-domain had been deleted could itself induce HPV-16 late gene expression, whereas wild type ASF/SF2 could not.

Manipulation of the ubiquitin-proteasome pathway by small DNA tumor viruses

Viruses have evolved to use cellular pathways to their advantage, including the ubiquitin-proteasome pathway of protein degradation. In several cases, viruses produce proteins that highjack cellular E3 ligases to modify their substrate specificity in order to eliminate unwanted cellular proteins, in particular inhibitors of the cell cycle. They can also inhibit E3 ligase to prevent specific protein degradation or even use the system to control the level of expression of their own proteins. In this review we explore the specific ways that small DNA tumor viruses exploit the ubiquitin-proteasome pathway for their own benefit.

The adenovirus E4orf4 protein induces growth arrest and mitotic catastrophe in H1299 human lung carcinoma cells

The human adenovirus E4orf4 protein, when expressed alone, induces p53-independent death in a wide range of cancer cells. Earlier studies by our groups suggested that although in some cases cell death can be associated with some hallmarks of apoptosis, it is not always affected by caspase inhibitors. Thus it is unlikely that E4orf4-induced cell death occurs uniquely through apoptosis. In the present studies using H1299 human lung carcinoma cells as a model system we found that death is induced in the absence of activation of any of the caspases tested, accumulation of reactive oxygen species, or release of cytochrome c from mitochondria. E4orf4 caused a substantial change in cell morphology, including vigorous membrane blebbing, multiple nuclei in many cells and increased cell volume. Most of these characteristics are not typical of apoptosis, but they are of necrosis. FACS analysis and western blotting for cell cycle markers showed that E4orf4-expressing cells became arrested in G(2)/M and also accumulated high levels of cyclin E. The presence of significant numbers of tetraploid and polyploid cells and some cells with micronuclei suggested that E4orf4 appears to induce death in these cells through a process resulting from mitotic catastrophe.

Adenovirus E4orf4 protein downregulates MYC expression through interaction with the PP2A-B55 subunit

The adenovirus E4 open reading frame 4 (E4orf4) protein is a multifunctional viral regulator that is involved in the temporal regulation of viral gene expression by modulating cellular and viral genes at the transcription and translation levels and by controlling alternative splicing of adenoviral late mRNAs. When expressed individually, E4orf4 induces apoptosis in transformed cells. Using oligonucleotide microarray analysis, validated by quantitative real time PCR, we found that MYC (also known as c-Myc) is downregulated early after the induction of E4orf4 expression. As a result, Myc protein levels are reduced in E4orf4-expressing cells. MYC downregulation is observed both when E4orf4 is expressed individually and within the context of viral infection. E4orf4 reduces MYC transcription but does not affect transcriptional elongation or RNA stability. An interaction with the PP2A-B55 subunit is required for the downregulation of MYC by E4orf4. Since Myc overexpression was previously shown to inhibit adenovirus replication, the downregulation of Myc by E4orf4 would contribute to efficient virus infection.

Apoptin: therapeutic potential of an early sensor of carcinogenic transformation

The avian virus-derived protein apoptin induces p53-independent apoptosis in a tumor-specific way. Apoptin acts as a multimeric complex and forms superstructures upon binding to DNA. In tumor cells, apoptin is phosphorylated and mainly nuclear, whereas in normal cells it is unphosphorylated, cytoplasmic, and becomes readily neutralized. Interestingly, apoptin phosphorylation, nuclear translocation, and apoptosis can transiently be induced in normal cells by cotransfecting SV40 large T oncogene, indicating that apoptin recognizes early stages of oncogenic transformation. In cancer cells, apoptin appears to recognize survival signals, which it is able to redirect into cell death impulses. Apoptin targets include DEDAF, Nur77, Nmi, Hippi, and the potential drug target APC1. Apoptin-transgenic mice and animal tumor models have revealed apoptin as a safe and efficient antitumor agent, resulting in significant tumor regression. Future antitumor therapies could use apoptin either as a therapeutic bullet or as an early sensor of druggable tumor-specific processes.

Cdc55p-mediated E4orf4 growth inhibition in Saccharomyces cerevisiae is mediated only in part via the catalytic subunit of protein phosphatase 2A

The adenovirus early region 4 open reading frame 4 (E4orf4) protein specifically induces p53-independent cell death of transformed but not normal human cells, suggesting that elucidation of its mechanism may provide important new avenues for cancer therapy. Wild-type E4orf4 and mutants that retain cancer cell toxicity also induce growth inhibition in Saccharomyces cerevisiae, which provides a genetically tractable system for studying E4orf4 function. Interaction with the protein phosphatase 2A (PP2A) B regulatory subunit is required for E4orf4's effects, suggesting that E4orf4 may function by regulating B subunit-containing heterotrimeric PP2A holoenzymes (PP2A(BAC)), which consist of a B subunit complexed with the PP2A structural (A) and catalytic (C) subunits. However, it is not known whether E4orf4-induced growth inhibition requires interaction with the PP2A C subunit or whether E4orf4 might have PP2A B subunit-dependent effects that are independent of PP2A(BAC) holoenzyme formation. To test these possibilities in S. cerevisiae, we disrupted the stable formation of PP2A(BAC) heterotrimers and thus E4orf4/C subunit association by PP2A C subunit point mutations or by deletion of the gene for the PP2A methyltransferase, Ppm1p, and assayed for effects on E4orf4-induced growth inhibition. Our results support a model in which E4orf4 mediates growth inhibition and cell killing both through PP2A(BAC) heterotrimers and through a B regulatory subunit-dependent pathway(s) that is independent of stable complex formation with the PP2A C subunit. They also indicate that Ppm1p has a function other than regulating the assembly of PP2A heterotrimers and suggest that selective PP2A trimer inhibitors and PP6 inhibitors may be useful as adjuvant anticancer therapies.

Accumulation of substrates of the anaphase-promoting complex (APC) during human cytomegalovirus infection is associated with the phosphorylation of Cdh1 and the dissociation and relocalization of APC subunits

Cell cycle dysregulation upon human cytomegalovirus (HCMV) infection of human fibroblasts is associated with the inactivation of the anaphase-promoting complex (APC), a multisubunit E3 ubiquitin ligase, and accumulation of its substrates. Here, we have further elucidated the mechanism(s) by which HCMV-induced inactivation of the APC occurs. Our results show that Cdh1 accumulates in a phosphorylated form that may prevent its association with and activation of the APC. The accumulation of Cdh1, but not its phosphorylation, appears to be cyclin-dependent kinase dependent. The lack of an association of exogenously added Cdh1 with the APC from infected cells indicates that the core APC also may be impaired. This is further supported by an examination of the localization and composition of the APC. Coimmunoprecipitation studies show that both Cdh1 and the subunit APC1 become dissociated from the complex. In addition, immunofluorescence analysis demonstrates that as the infection progresses, several subunits redistribute to the cytoplasm, while APC1 remains nuclear. Dissociation of the core complex itself would account for not only the observed inactivity but also its inability to bind to Cdh1. Taken together, these results illustrate that HCMV has adopted multiple mechanisms to inactivate the APC, which underscores its importance for a productive infection.

Viral protein apoptin as a molecular tool and therapeutic bullet: implications for cancer control

The chicken anemia virus-derived protein apoptin induces apoptosis in human tumor cells via a p53-independent pathway, while leaving normal cells intact. Moreover, apoptin treatment in preclinical animal studies leads to reduced tumor growth or remission without a detectable effect on healthy tissues. Apoptin is activated by a still unknown tumor-specific kinase activity. The mode of action of apoptin is under intense investigation, as certain features make it a promising tool for discovering early events in tumorigenesis, identifying druggable targets for antitumor treatment and possibly serving as an antitumor therapy in itself.

G2/M cell cycle arrest in the life cycle of viruses

There is increasing evidence that viral infection, expression of viral protein or the presence of viral DNA causes the host cell cycle to arrest during G2/M. The mechanisms used by viruses to cause arrest vary widely; some involve the activation of the cellular pathways that induce arrest in response to DNA damage, while others use completely novel means. The analysis of virus-mediated arrest has not been proven easy, and in most cases the consequences of arrest for the virus life cycle are not well defined. However, a number of effects of arrest are being investigated and it will be interesting to see to what extent perturbation of the G2/M transition is involved in viral infections.

Nuttingins A-F and malonganenones D-H, tetraprenylated alkaloids from the Tanzanian gorgonian Euplexaura nuttingi

Six new tetraprenylated purine alkaloids, designated nuttingins A-F (1-6), were isolated together with three known malonganenones, A-C (12-14), and five new closely related malonganenones, D-H (7-11), from the gorgonian Euplexaura nuttingi collected in Pemba Island, Tanzania. The structures of the compounds were elucidated by interpretation of 1D and 2D NMR data including 15N chemical shifts obtained from 1H-15N HMBC spectra. Nuttingins A-E (1-5) and malonganenones D-G (7-10) displayed inhibitory activity against both K562 and UT7 tumor cell lines, compounds 3-5 being the most active ones, approximately 3-fold more potent than the others. Compounds 1-5 and 7-11 also induce apoptosis in transformed mammalian cells at a concentration of 1.25 microg/mL.

Mitotic Cdc6 stabilizes anaphase-promoting complex substrates by a partially Cdc28-independent mechanism, and this stabilization is suppressed by deletion of Cdc55

Ectopic expression of Cdc6p results in mitotic delay, and this has been attributed to Cdc6p-mediated inhibition of Cdc28 protein kinase and failure to activate the anaphase-promoting complex (APC). Here we show that endogenous Cdc6p delays a specific subset of mitotic events and that Cdc28 inhibition is not sufficient to account for it. The depletion of Cdc6p in G(2)/M cells reveals that Cdc6p is rate limiting for the degradation of the APC/Cdc20 substrates Pds1p and Clb2p. Conversely, the premature expression of Cdc6p delays the degradation of APC/Cdc20 substrates. Abolishing Cdc6p/Cdc28p interaction does not eliminate the Cdc6-dependent delay of these anaphase events. To identify additional Cdc6-mediated, APC-inhibitory mechanisms, we looked for mutants that reversed the mitotic delay. The deletion of SWE1, RAD24, MAD2, or BUB2 had no effect. However, disrupting CDC55, a PP2A regulatory subunit, suppressed the Cdc6p-dependent delay of Pds1 and Clb2 destruction. A specific role for CDC55 was supported by demonstrating that the lethality of Cdc6 ectopic expression in a cdc16-264 mutant is suppressed by the deletion of CDC55, that endogenous Cdc6p coimmunoprecipitates with the Cdc55 and Tpd3 subunits of PP2A, that Cdc6p/Cdc55p/Tpd3 interaction occurs only during mitosis, and that Cdc6 affects PP2A-Cdc55 activity during anaphase. This demonstrates that the levels and timing of accumulation of Cdc6p in mitosis are appropriate for mediating the modulation of APC/Cdc20.

Cytotoxicity of a recombinant fusion protein of adenovirus early region 4 open reading frame 4 (E4orf4) and human epidermal growth factor on p53-deficient tumor cells

Adenovirus early region 4 open reading frame 4 (E4orf4) protein is a novel cell death factor that selectively induces p53-independent apoptosis in cancer cells, but not in normal human cells. This study presents an approach for inhibiting p53-deficient tumor cell growth by using protein-based E4orf4 that had been genetically fused to epidermal growth factor (EGF) to ensure selective targeting of EGF receptor-overexpressing tumor cells. EGF-E4orf4 enables binding onto the cell surface and is then internalized into Saos-2 cells. The success of the process had been demonstrated by immunofluorescence assay and confocal laser microscopy. After prolonged exposure, E4orf4 remained mostly in the nuclei. EGF-E4orf4 treatment of Saos-2 cells showed dose-dependent cytotoxicity. Nearly 50% of the Saos-2 cells were killed at a concentration of 250 nmol/l. In contrast, EGF-E4orf4 showed no significant inhibitory effect iresn primary cells of human umbilical vein endothelial cells. To confirm the ability of EGF-E4orf4 to induce apoptosis, DNA fragmentation was detected using BrdUTP end-labeling. Flow cytometric analysis revealed a significant increase of apoptotic cells in Saos-2 cells treated with EGF-E4orf4, but not in the case of cells cultured in plain medium (t=0.028, P<0.05). In conclusion, these preliminary results indicate that EGF-E4orf4 could show promise as a new reagent that is more efficient and less toxic in anti-cancer therapy.

Apoptin nucleocytoplasmic shuttling is required for cell type-specific localization, apoptosis, and recruitment of the anaphase-promoting complex/cyclosome to PML bodies

The chicken anemia virus protein Apoptin selectively induces apoptosis in transformed cells while leaving normal cells intact. This selectivity is thought to be largely due to cell type-specific localization: Apoptin is cytoplasmic in primary cells and nuclear in transformed cells. The basis of Apoptin cell type-specific localization and activity remains to be determined. Here we show that Apoptin is a nucleocytoplasmic shuttling protein whose localization is mediated by an N-terminal nuclear export signal (NES) and a C-terminal nuclear localization signal (NLS). Both signals are required for cell type-specific localization, since Apoptin fragments containing either the NES or the NLS fail to differentially localize in transformed and primary cells. Significantly, cell type-specific localization can be conferred in trans by coexpression of the two separate fragments, which interact through an Apoptin multimerization domain. We have previously shown that Apoptin interacts with the APC1 subunit of the anaphase-promoting complex/cyclosome (APC/C), resulting in G(2)/M cell cycle arrest and apoptosis in transformed cells. We found that the nucleocytoplasmic shuttling activity is critical for efficient APC1 association and induction of apoptosis in transformed cells. Interestingly, both Apoptin multimerization and APC1 interaction are mediated by domains that overlap with the NES and NLS sequences, respectively. Apoptin expression in transformed cells induces the formation of PML nuclear bodies and recruits APC/C to these subnuclear structures. Our results reveal a mechanism for the selective killing of transformed cells by Apoptin.

The anaphase promoting complex/cyclosome: a machine designed to destroy

The anaphase promoting complex/cyclosome (APC/C) is a ubiquitin ligase that has essential functions in and outside the eukaryotic cell cycle. It is the most complex molecular machine that is known to catalyse ubiquitylation reactions, and it contains more than a dozen subunits that assemble into a large 1.5-MDa complex. Recent discoveries have revealed an unexpected multitude of mechanisms that control APC/C activity, and have provided a first insight into how this unusual ubiquitin ligase recognizes its substrates.

Adenovirus E4orf4 hijacks rho GTPase-dependent actin dynamics to kill cells: a role for endosome-associated actin assembly

The adenovirus early region 4 ORF4 protein (E4orf4) triggers a novel death program that bypasses classical apoptotic pathways in human cancer cells. Deregulation of the cell cytoskeleton is a hallmark of E4orf4 killing that relies on Src family kinases and E4orf4 phosphorylation. However, the cytoskeletal targets of E4orf4 and their role in the death process are unknown. Here, we show that E4orf4 translocates to cytoplasmic sites and triggers the assembly of a peculiar juxtanuclear actin-myosin network that drives polarized blebbing and nuclear shrinkage. We found that E4orf4 activates the myosin II motor and triggers de novo actin polymerization in the perinuclear region, promoting endosomes recruitment to the sites of actin assembly. E4orf4-induced actin dynamics requires interaction with Src family kinases and involves a spatial regulation of the Rho GTPases pathways Cdc42/N-Wasp, RhoA/Rho kinase, and Rac1, which make distinct contributions. Remarkably, activation of the Rho GTPases is required for induction of apoptotic-like cell death. Furthermore, inhibition of actin dynamics per se dramatically impairs E4orf4 killing. This work provides strong support for a causal role for endosome-associated actin dynamics in E4orf4 killing and in the regulation of cancer cell fate.

Viral infections and cell cycle G2/M regulation

Progression of cells from G2 phase of the cell cycle to mitosis is a tightly regulated cellular process that requires activation of the Cdc2 kinase, which determines onset of mitosis in all eukaryotic cells. In both human and fission yeast (Schizosaccharomyces pombe) cells, the activity of Cdc2 is regulated in part by the phosphorylation status of tyrosine 15 (Tyr15) on Cdc2, which is phosphorylated by Wee1 kinase during late G2 and is rapidly dephosphorylated by the Cdc25 tyrosine phosphatase to trigger entry into mitosis. These Cdc2 regulators are the downstream targets of two well-characterized G2/M checkpoint pathways which prevent cells from entering mitosis when cellular DNA is damaged or when DNA replication is inhibited. Increasing evidence suggests that Cdc2 is also commonly targeted by viral proteins, which modulate host cell cycle machinery to benefit viral survival or replication. In this review, we describe the effect of viral protein R (Vpr) encoded by human immunodeficiency virus type 1 (HIV-1) on cell cycle G2/M regulation. Based on our current knowledge about this viral effect, we hypothesize that Vpr induces cell cycle G2 arrest through a mechanism that is to some extent different from the classic G2/M checkpoints. One the unique features distinguishing Vpr-induced G2 arrest from the classic checkpoints is the role of phosphatase 2A (PP2A) in Vpr-induced G2 arrest. Interestingly, PP2A is targeted by a number of other viral proteins including SV40 small T antigen, polyomavirus T antigen, HTLV Tax and adenovirus E4orf4. Thus an in-depth understanding of the molecular mechanisms underlying Vpr-induced G2 arrest will provide additional insights into the basic biology of cell cycle G2/M regulation and into the biological significance of this effect during host-pathogen interactions.

YND1 interacts with CDC55 and is a novel mediator of E4orf4-induced toxicity

Adenovirus E4orf4 (early region 4 open reading frame 4) protein induces protein phosphatase 2A-dependent non-classical apoptosis in mammalian cells and irreversible growth arrest in Saccharomyces cerevisiae. Oncogenic transformation sensitizes cells to E4orf4-induced cell death. To uncover additional components of the E4orf4 network required for induction of its unique mode of apoptosis, we used yeast genetics to select gene deletions conferring resistance to E4orf4. Deletion of YND1, encoding a yeast Golgi apyrase, conferred partial resistance to E4orf4. However, Ynd1p apyrase activity was not required for E4orf4-induced toxicity. Ynd1p and Cdc55p, the yeast protein phosphatase 2A-B subunit, contributed additively to E4orf4-induced toxicity. Furthermore, concomitant overexpression of one and deletion of the other was detrimental to yeast growth, demonstrating a functional interaction between the two proteins. YND1 and CDC55 also interacted genetically with CDC20 and CDH1/HCT1, encoding activating subunits of the anaphase-promoting complex/cyclosome. In addition to their functional interaction, Ynd1p and Cdc55p interacted physically, and this interaction was disrupted by E4orf4, which remained associated with both proteins. The results suggested that Ynd1p and Cdc55p share a common downstream target whose balanced modulation by the two E4orf4 partners is crucial to viability. Disruption of this balance by E4orf4 may lead to cell death. NTPDase-4/Lalp70/UDPase, the closest mammalian homologue of Ynd1p, associated with E4orf4 in mammalian cells, suggesting that the results in yeast are relevant to the mammalian system.

Nuclear localization of the adenovirus E4orf4 protein is mediated through an arginine-rich motif and correlates with cell death

The adenovirus E4orf4 protein induces p53-independent death of human cancer cells by a mechanism requiring interactions with the Balpha subunit of protein phosphatase 2A. When expressed alone E4orf4 localizes predominantly in the nucleus, although significant levels are also present in the cytoplasm. While tyrosine phosphorylation of E4orf4 and recruitment of Src have been linked with E4orf4 cytoplasmic cell death functions, little is known about the functions of E4orf4 in the nucleus. In this study, we identified an arginine-rich motif (E4ARM; residues 66-75) that is necessary and sufficient for nuclear and nucleolar localization. This motif, which is highly homologous to the arginine-rich nuclear and nucleolar localization motif of some lentiviral proteins, was shown to target heterologous proteins to the nucleus and to nucleoli, functions found to be dependent on the overall charge of the motif rather than on specific residues. Furthermore, mutation of arginine residues to alanines but not to lysines in E4ARM was shown to block such targeting activity and, when introduced into full-length E4orf4, to decrease induction of cell death. Finally, coexpression of the ARM motifs of E4orf4, HIV-1 Tat or Rev along with full-length E4orf4 was seen to decrease E4orf4-dependent cell killing. Thus it appears that targeting of E4orf4 to the nucleus and cell nucleoli by E4ARM is an important component of E4orf4-induced cell death.

The Scaffolding A/Tpd3 Subunit and High Phosphatase Activity Are Dispensable for Cdc55 Function in the Saccharomyces cerevisiae Spindle Checkpoint and in Cytokinesis

Protein serine/threonine phosphatase 2A (PP2A) is a multifunctional enzyme whose trimeric form consists of a scaffolding A subunit, a catalytic C subunit, and one of several regulatory B subunits (B, B', and B''). The adenovirus E4orf4 protein associates with PP2A by directly binding the B or B' subunits. An interaction with an active PP2A containing the B subunit, or its homologue in yeast, Cdc55, is required for E4orf4-induced apoptosis in mammalian cells and for induction of growth arrest in Saccharomyces cerevisiae. In this work, Cdc55 was randomly mutagenized by low-fidelity PCR amplification, and Cdc55 mutants that lost the ability to transduce the E4orf4 toxic signal in yeast were selected. The mutations obtained by this protocol inhibited the association of Cdc55 with E4orf4, or with the PP2A-AC subunits, or both. Functional analysis revealed that a mutant that does not bind Tpd3, the yeast A subunit, as well as wild type Cdc55 in a tpd3Delta background, can form a heterodimer with the catalytic subunit. This association requires C subunit carboxyl methylation. The residual phosphatase activity associated with Cdc55 in the absence of Tpd3 is sufficient to maintain a partially active spindle checkpoint and to prevent cytokinesis defects.

The viral protein Apoptin associates with the anaphase-promoting complex to induce G2/M arrest and apoptosis in the absence of p53

The chicken anemia virus protein Apoptin induces apoptosis in the absence of p53 by a mechanism that remains to be elucidated. Here we show that in transformed cells, Apoptin is associated with APC1, a subunit of the anaphase-promoting complex/cyclosome (APC/C). We demonstrate that Apoptin expression, or depletion of APC1 by RNA interference, inhibits APC/C function in p53 null cells, resulting in G2/M arrest and apoptosis. Our results explain the ability of Apoptin to induce apoptosis in the absence of p53 and suggest that the APC/C is an attractive target for anticancer drug development.

Activation of Adenovirus Type 2 Early Region 4 ORF4 Cytoplasmic Death Function by Direct Binding to Src Kinase Domain

Adenovirus type 2 (Ad2) early region 4 ORF4 (E4orf4) triggers a major death pathway that requires its accumulation in cellular membranes and its tyrosine phosphorylation. This program is regulated by Src family kinases and triggers a potent ZVAD (benzyloxycarbonyl-VAD)- and Bcl2-resistant cell death response in human-transformed cells. How E4orf4 deregulates Src-dependent signaling is unknown. Here we provide strong evidence that a physical interaction requiring the kinase domain of Src and the arginine-rich motif of E4orf4 is involved. The Src binding domain of E4orf4 overlaps with, but is distinct from that of the Balpha subunit of protein phosphatase 2A (PP2A-Balpha) and some E4orf4 complexes contain both PP2A and Src. Functional assays using mutant E4orf4 revealed that deregulation of Src signaling, activation of the Jun kinase pathway, and cell blebbing were all critically dependent on Src binding. In contrast, PP2A-Balpha binding per se was not required to engage the Src-dependent death pathway but was more critical for triggering a distinct death activity. Both E4orf4 death activities were manifested within a given cell population, were typified by distinct morphological features, and contributed to overall cell killing, although to different extents in various cell types. We conclude that E4orf4 binding to the Src kinase domain leads to deregulation of Src signaling and plays a crucial role in induction of the cytoplasmic death pathway. Nonetheless, both Src and PP2A enzymes are critical targets of E4orf4 that likely cooperate to trigger E4orf4-induced tumor cell killing and whose relative contributions may vary in function of the cellular background.

The role of phosphatases in TOR signaling in yeast

The TOR pathway controls cellular functions necessary for cell growth and proliferation of yeast and larger eukaryotes. The search for members of the TOR signaling cascade in yeast led to the discovery of type 2A protein phosphatases (PP2A) as important players within the pathway. We describe the roles in yeast of PP2A and the closely related phosphatase, Sit4, and then focus on complexes formed between the catalytic subunit of these phosphatases and Tap42, a direct target of the Tor protein kinases in yeast. Recent results suggest that Tap42 mediates many of the Tor functions in yeast, especially those involved in transcriptional modulation. However, whether Tap42 executes its function by inhibiting phosphatase activity or by activating phosphatases is still uncertain. In addition, Tor affects some transcriptional and physiological processes through Tap42 independent pathways. Thus, Tor proteins use multiple mechanisms to regulate transcriptional and physiological processes in yeast.

Apoptosis induced by environmental stresses and amphotericin B in Candida albicans

New antifungal agents are urgently required to combat life-threatening infections caused by opportunistic fungal pathogens like Candida albicans. The manipulation of endogenous fungal programmed cell death responses could provide a basis for future therapies. Here we assess the physiology of death in C. albicans in response to environmental stresses (acetic acid and hydrogen peroxide) and an antifungal agent (amphotericin B). Exposure of C. albicans to 40-60 mM acetic acid, 5-10 mM hydrogen peroxide, or 4-8 microg.ml-1 amphotericin B produced cellular changes reminiscent of mammalian apoptosis. Nonviable cells that excluded propidium iodide displayed the apoptotic marker phosphatidylserine (as shown by annexin-V-FITC labeling), were terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling (TUNEL)-positive (indicating nuclease-mediated double-strand DNA breakage), and produced reactive oxygen species. Ultrastructural changes in apoptotic cells included chromatin condensation and margination, separation of the nuclear envelope, and nuclear fragmentation. C. albicans cells treated at higher doses of these compounds showed cellular changes characteristic of necrosis. Necrotic cells displayed reduced TUNEL staining, a lack of surface phosphatidylserine, limited reactive oxygen species production, and an inability to exclude propidium iodide. Necrotic cells lacked defined nuclei and showed extensive intracellular vacuolization. Apoptosis in C. albicans was associated with an accumulation of cells in the G2/M phase of the cell cycle, and under some apoptosis-inducing conditions, significant proportions of yeast cells switched to hyphal growth before dying. This is a demonstration of apoptosis in a medically important fungal pathogen.

Remodelling of the host cell RNA splicing machinery during an adenovirus infection

Adenovirus makes extensive use of RNA splicing to produce a complex set of spliced mRNAs during virus replication. All transcription units, except pIX and IVa2, encode multiple alternatively spliced mRNAs. The accumulation of viral mRNAs is subjected to a temporal regulation, a mechanism that ensures that proteins that are needed at certain stages of the viral life cycle are produced. The complex interaction between host cell RNA splicing factors and viral regulatory elements has been studied intensely during the last decade. Such studies have begun to produce a picture of how adenovirus remodels the host cell RNA splicing machinery to orchestrate the shift from the early to the late profile of viral mRNA accumulation. Recent progress has to a large extent focused on the mechanisms regulating E1A and L1 alternative splicing. Here we will review the current knowledge of cis-acting sequence element, trans-acting factors and mechanisms controlling E1A and L1 alternative splicing.

Distinct cell death pathways triggered by the adenovirus early region 4 ORF 4 protein

In transformed cells, induction of apoptosis by adenovirus type 2 (Ad2) early region 4 ORF 4 (E4orf4) correlates with accumulation of E4orf4 in the cell membrane-cytoskeleton fraction. However, E4orf4 is largely expressed in nuclear regions before the onset of apoptosis. To determine the relative contribution of nuclear E4orf4 versus membrane-associated E4orf4 to cell death signaling, we engineered green fluorescent fusion proteins to target E4orf4 to specific cell compartments. The targeting of Ad2 E4orf4 to cell membranes through a CAAX-box or a myristylation consensus signal sufficed to mimic the fast Src-dependent apoptotic program induced by wild-type E4orf4. In marked contrast, the nuclear targeting of E4orf4 abolished the early induction of extranuclear apoptosis. However, nuclear E4orf4 still induced a delayed cell death response independent of Src-like activity and of E4orf4 tyrosine phosphorylation. The zVAD.fmk-inhibitable caspases were dispensable for execution of both cell death programs. Nevertheless, both pathways led to caspase activation in some cell types through the mitochondrial pathway. Finally, our data support a critical role for calpains upstream in the death effector pathway triggered by the Src-mediated cytoplasmic death signal. We conclude that Ad2 E4orf4 induces two distinct cell death responses, whose relative contributions to cell killing may be determined by the genetic background.

Cytoplasmic death signal triggered by SRC-mediated phosphorylation of the adenovirus E4orf4 protein

In transformed cells, the adenovirus E4orf4 death factor works in part by inducing a Src-mediated cytoplasmic apoptotic signal leading to caspase-independent membrane blebbing and cell death. Here we show that Src-family kinases modulate E4orf4 phosphorylation on tyrosine residues. Mutation of tyrosines 26, 42, and 59 to phenylalanines inhibited Src-induced phosphorylation of E4orf4 in vivo and in vitro but had no effect on the molecular association of E4orf4 with Src. However, in contrast to wild-type E4orf4, the nonphosphorylatable E4orf4 mutant was unable to modulate Src-dependent phosphorylation and was deficient in recruiting a subset of tyrosine-phosphorylated proteins. Indeed, the Src substrates cortactin and p62dok were found to associate with wild-type E4orf4 but not with the nonphosphorylatable E4orf4. Importantly, the nonphosphorylatable mutant E4orf4 was preferentially distributed in the cell nucleus, was unable to induce membrane blebbing, and had a highly impaired killing activity. Conversely, an activated form of E4orf4 was obtained by mutation of tyrosine 42 to glutamic acid. This pseudophosphorylated mutant E4orf4 was enriched in the cytoplasm and plasma membrane, showed increased binding to phosphotyrosine-containing proteins, and induced a dramatic blebbing phenotype associated with increased cell death. Altogether, our findings strongly suggest that Src-mediated phosphorylation of adenovirus type 2 E4orf4 is critical to promoting its cytoplasmic and membrane localization and is required for the transduction of E4orf4-Src-dependent induction of membrane blebbing. We propose that E4orf4 acts in part by uncoupling Src-dependent signals to drive the formation of a signaling complex that triggers a cytoplasmic death signal.

Apoptosis in yeast: a new model for aging research

Apoptosis is a form of programmed cell death with a central role in development and homeostasis of metazoan organisms. Recent research indicates the presence of an apoptotic cell death program in unicellular eukaryotes. Yeast can be killed by expression of mammalian proapoptotic genes or in response to oxygen stress, which is an inducer of mammalian apoptosis. The dying yeast cells show morphological alterations typical for apoptosis. Yeast provides a simple model for cellular aging. The observation that old yeast cells produce oxygen radicals and die apoptotically may provide clues to a similar sequence of events in mammalian aging.

Molecular regulation and biological function of adenovirus early genes: the E4 ORFs

Over the past few years there have been a number of interesting advances in our understanding of the functions encoded by the adenovirus early transcription unit 4 (Ad E4). A large body of recent data demonstrates that E4 proteins encompass an unexpectedly diverse collection of functions required for efficient viral replication. E4 gene products operate through a complex network of protein interactions with key viral and cellular regulatory components involved in transcription, apoptosis, cell cycle control and DNA repair, as well as host cell factors that regulate cell signaling, posttranslational modifications and the integrity of nuclear multiprotein complexes known as nuclear bodies (NBs) or PML oncogenic domains (PODs). As understood at present, some of the lytic functions overlap with roles in oncogenic transformation of primary mammalian cells. These observations, together with findings that E4 proteins substantially affect cell toxicity and the immune response of the host have profound implications for the development of Ad vectors for gene therapy. In this article we will summarize recent findings regarding the diverse functions of E4 gene products in the context of earlier work. We will emphasize the interaction of E4 proteins with cellular and viral interaction partners, the role of these interactions for lytic virus growth and how these interactions may contribute to viral oncogenesis. Finally, we will discuss their role in Ad vector and adeno-associated virus infections.