bZIP proteins of human gammaherpesviruses

Alternatively initiated gene 50/RTA transcripts expressed during murine and human gammaherpesvirus reactivation from latency

In the process of characterizing the requirements for expression of the essential immediate-early transcriptional activator (RTA) encoded by gene 50 of murine gammaherpesvirus 68 (MHV68), a recombinant virus was generated in which the known gene 50 promoter was deleted (G50pKO). Surprisingly, the G50pKO mutant retained the ability to replicate in permissive murine fibroblasts, albeit with slower kinetics than wild-type MHV68. 5'-rapid amplification of cDNA ends analyses of RNA prepared from G50pKO-infected fibroblasts revealed a novel upstream transcription initiation site, which was also utilized during wild-type MHV68 infection of permissive cells. Furthermore, the region upstream of the distal gene 50/RTA transcription initiation site exhibited promoter activity in both permissive NIH 3T12 fibroblasts as well as in the murine macrophage cell line RAW 264.7. In addition, in RAW 264.7 cells the activity of the distal gene 50/RTA promoter was strongly upregulated (>20-fold) by treatment of the cells with lipopolysaccharide. Reverse transcriptase PCR analyses of RNA prepared from Kaposi's sarcoma-associated herpesvirus- and Epstein-Barr virus-infected B-cell lines, following induction of virus reactivation, also revealed the presence of gene 50/RTA transcripts initiating upstream of the known transcription initiation site. The latter argues that alternative initiation of gene 50/RTA transcription is a strategy conserved among murine and human gammaherpesviruses. Infection of mice with the MHV68 G50pKO demonstrated the ability of this mutant virus to establish latency in the spleen and peritoneal exudate cells (PECs). However, the G50pKO mutant was unable to reactivate from latently infected splenocytes and also exhibited a significant reactivation defect from latently infected PECs, arguing in favor of a model where the proximal gene 50/RTA promoter plays a critical role in virus reactivation from latency, particularly from B cells. Finally, analyses of viral genome methylation in the regions upstream of the proximal and distal gene 50/RTA transcription initiation sites revealed that the distal promoter is partially methylated in vivo and heavily methylated in MHV68 latently infected B-cell lines, suggesting that DNA methylation may serve to silence the activity of this promoter during virus latency.

The reversal of epigenetic silencing of the EBV genome is regulated by viral bZIP protein

EBV (Epstein-Barr virus) alternates between latency and lytic replication. During latency, the viral genome is largely silenced by host-driven methylation of CpG motifs and in the switch to the lytic cycle this epigenetic silencing is overturned. A key event is the activation of the viral protein Zta with three ZREs (Zta-response elements) from the BRLF1 promoter (referred to as Rp). Two of these ZREs contain CpG motifs and are methylated in the latent genome. Biochemical analyses and molecular modelling of Zta bound to methylated RpZRE3 indicate the precise contacts made between a serine and a cysteine residue of Zta with methyl cytosines. A single point mutant of Zta, C189S, is defective in binding to the methylated ZREs both in vitro and in vivo. This was used to probe the functional relevance of the interaction. ZtaC189S was not able to activate Rp in a B-cell line, demonstrating the relevance of the interaction with methylated ZREs. This demonstrates that Zta plays a role in overturning the epigenetic control of viral latency.

Effect of phosphorylation on the transactivation activity of Epstein-Barr virus BMRF1, a major target of the viral BGLF4 kinase

Modification of human herpesvirus DNA polymerase processivity factors (PFs) by phosphorylation occurs frequently during viral lytic replication. However, functional regulation of the herpesvirus PFs through phosphorylation is not well understood. In addition to processivity, the PF BMRF1 of Epstein-Barr virus can function as a transactivator to activate the BHLF1 promoter within the lytic origin of replication (oriLyt), which is assumed to facilitate DNA replication through remodelling viral chromatin structure. BMRF1 is known to be phosphorylated by the viral BGLF4 kinase, but its impact on BMRF1 function is unclear. Seven candidate BGLF4 target sites were predicted within a proline-rich region between the DNA-processivity and nuclear-localization domains of BMRF1. We show that four of these residues, Ser-337, Thr-344, Ser-349 and Thr-355, are responsible for the BGLF4-induced hyperphosphorylation of BMRF1. In functional analyses, a phosphorylation-mimicking mutant of BMRF1 shows similar nuclear localization, as well as DNA-binding ability, to the wild type; however, it displays stronger synergistic activation of the BHLF1 promoter with Zta. Notably, BGLF4 downregulates BMRF1 transactivation and enhances the transactivation activity of Zta and the synergistic activation of BMRF1 and Zta on the BHLF1 promoter. Our findings suggest that BGLF4 may modulate the activation of the oriLyt BHLF1 promoter coordinately through multiple mechanisms to facilitate optimal oriLyt-dependent viral DNA replication.

Mutual inhibition between Kaposi's sarcoma-associated herpesvirus and Epstein-Barr virus lytic replication initiators in dually-infected primary effusion lymphoma

Background

Both Kaposi's sarcoma-associated herpesvirus (KSHV) and Epstein-Barr virus (EBV) are members of the human gamma herpesvirus family: each is associated with various human cancers. The majority of AIDS-associated primary effusion lymphoma (PEL) are co-infected with both KSHV and EBV. Dually-infected PELs selectively switch from latency to lytic replication of either KSHV or EBV in response to chemical stimuli. KSHV replication and transcription activator (K-RTA) is necessary and sufficient for the switch from KSHV latency to lytic replication, while EBV BZLF1 gene product (EBV-Z) is a critical initiator for induction of EBV lytic replication.

Methodology/Principal Findings

We show K-RTA and EBV-Z are co-localized and physically interact with each other in dually-infected PELs. K-RTA inhibits the EBV lytic replication by nullifying EBV-Z-mediated EBV lytic gene activation. EBV-Z inhibits KSHV lytic gene expression by blocking K-RTA-mediated transactivations. The physical interaction between K-RTA and EBV-Z are required for the mutual inhibition of the two molecules. The leucine heptapeptide repeat (LR) region in K-RTA and leucine zipper region in EBV-Z are involved in the physical interactions of the two molecules. Finally, initiation of KSHV lytic gene expression is correlated with the reduction of EBV lytic gene expression in the same PEL cells.

Conclusions/Significance

In this report, how the two viruses interact with each other in dually infected PELs is addressed. Our data may provide a possible mechanism for maintaining viral latency and for selective lytic replication in dually infected PELs, i.e., through mutual inhibition of two critical lytic replication initiators. Our data about putative interactions between EBV and KSHV would be applicable to the majority of AIDS-associated PELs and may be relevant to the pathogenesis of PELs.

Atypical bZIP domain of viral transcription factor contributes to stability of dimer formation and transcriptional function

The Epstein-Barr virus transcription factor Zta (encoded by BZLF1) is a bZIP protein containing an alpha-helical coiled-coil homodimerization motif (zipper). The Zta zipper forms less-stable dimers than other bZIP proteins, and an adjacent region (CT) interacts with the zipper to form a novel structure that is proposed to strengthen the dimer. Here we question the role of the CT region for Zta function. Cross-linking experiments demonstrate that the entire CT region lies adjacent to the zipper. Detailed analyses of Zta truncation mutations identify an involvement of the proximal CT region (221 to 230) in dimer formation with a further contribution from the distal region (236 to 243). Biophysical analyses reveal that residues 221 to 230 enhance the stability of the coiled coil. The ability of the Zta truncation mutants to interact with three Zta-binding sites also requires the proximal CT region. Fine mapping of DNA-binding requirements highlighted the contribution of these amino acids for Zta function. Thus, the proximal part of the CT region is required to aid the dimerization of Zta and thereby its DNA-binding ability. In contrast, although the distal part of the CT region aids dimerization, it promotes only a modest increase in DNA binding. To probe this further, we defined the contribution from the CT region for Zta to transactivate a promoter embedded within the viral genome. From this we conclude that the proximal part of the CT region is absolutely required, whereas the distal part is dispensable.

The porcine lymphotropic herpesvirus 1 encodes functional regulators of gene expression

The porcine lymphotropic herpesviruses (PLHV) are discussed as possible risk factors in xenotransplantation because of the high prevalence of PLHV-1, PLHV-2 and PLHV-3 in pig populations world-wide and the fact that PLHV-1 has been found to be associated with porcine post-transplant lymphoproliferative disease. To provide structural and functional knowledge on the PLHV immediate-early (IE) transactivator genes, the central regions of the PLHV genomes were characterized by genome walking, sequence and splicing analysis. Three spliced genes were identified (ORF50, ORFA6/BZLF1(h), ORF57) encoding putative IE transactivators, homologous to (i) ORF50 and BRLF1/Rta, (ii) K8/K-bZIP and BZLF1/Zta and (iii) ORF57 and BMLF1 of HHV-8 and EBV, respectively. Expressed as myc-tag or HA-tag fusion proteins, they were located to the cellular nucleus. In reporter gene assays, several PLHV-promoters were mainly activated by PLHV-1 ORF50, to a lower level by PLHV-1 ORFA6/BZLF1(h) and not by PLHV-1 ORF57. However, the ORF57-encoded protein acted synergistically on ORF50-mediated activation.

Latency and reactivation of human cytomegalovirus

Human cytomegalovirus (HCMV) persists as a subclinical, lifelong infection in the normal human host, maintained at least in part by its carriage in the absence of detectable infectious virus – the hallmark of latent infection. Reactivation from latency in immunocompromised individuals, in contrast, often results in serious disease. Latency and reactivation are defining characteristics of the herpesviruses and key to understanding their biology. However, the precise cellular sites in which HCMV is carried and the mechanisms regulating its latency and reactivation during natural infection remain poorly understood. This review will detail our current knowledge of where HCMV is carried in healthy individuals, which viral genes are expressed upon carriage of the virus and what effect this has on cellular gene expression. It will also address the accumulating evidence suggesting that reactivation of HCMV from latency appears to be linked intrinsically to the differentiation status of the myeloid cell, and how the cellular mechanisms that normally control host gene expression play a critical role in the differential regulation of viral gene expression during latency and reactivation.

Lytic replication of Epstein–Barr virus

Epstein–Barr virus (EBV) is a gammaherpesvirus with a 172kb genome and many genes encoding enzymes for lytic viral DNA replication. Recent observations indicate that an S-phase-like environment and the activated DNA repair system are required for viral lytic DNA replication. The virally encoded DNA replication-associated enzymes are then expressed in two clusters, suggesting their participation at different stages of replication. Simultaneously, EBV-encoded regulatory proteins may modulate cell-cycle control to enhance virus replication efficiency. The interactions among proteins in the viral replication complex and cellular proteins may either generate structural specificities for replication proteins or stabilize the protein complexes. During infection, EBV has evolved several strategies to overcome the host defense mechanism, such as interfering with innate immunity and withdrawing into a latent state. This review discusses the latest progress in viral control of lytic replication and the interactions among viral lytic replication compartment and cellular machineries. The possible contribution of EBV lytic gene products to human malignancy is also discussed.

Quantitative profiling of housekeeping and Epstein-Barr virus gene transcription in Burkitt lymphoma cell lines using an oligonucleotide microarray

Background

The Epstein-Barr virus (EBV) is associated with lymphoid malignancies, including Burkitt's lymphoma (BL), and can transform human B cells in vitro. EBV-harboring cell lines are widely used to investigate lymphocyte transformation and oncogenesis. Qualitative EBV gene expression has been extensively described, but knowledge of quantitative transcription is lacking. We hypothesized that transcription levels of EBNA1, the gene essential for EBV persistence within an infected cell, are similar in BL cell lines.

Results

To compare quantitative gene transcription in the BL cell lines Namalwa, Raji, Akata, Jijoye, and P3HR1, we developed an oligonucleotide microarray chip, including 17 housekeeping genes, six latent EBV genes (EBNA1, EBNA2, EBNA3A, EBNA3C, LMP1, LMP2), and four lytic EBV genes (BZLF1, BXLF2, BKRF2, BZLF2), and used the cell line B95.8 as a reference for EBV gene transcription. Quantitative polymerase chain reaction assays were used to validate microarray results. We found that transcription levels of housekeeping genes differed considerably among BL cell lines. Using a selection of housekeeping genes with similar quantitative transcription in the tested cell lines to normalize EBV gene transcription data, we showed that transcription levels of EBNA1 were quite similar in very different BL cell lines, in contrast to transcription levels of other EBV genes. As demonstrated with Akata cells, the chip allowed us to accurately measure EBV gene transcription changes triggered by treatment interventions.

Conclusion

Our results suggest uniform EBNA1 transcription levels in BL and that microarray profiling can reveal novel insights on quantitative EBV gene transcription and its impact on lymphocyte biology.

Unexpected structure of Epstein-Barr virus lytic cycle activator Zta

The key viral gene responsible for initiating the replicative cycle of Epstein-Barr virus (EBV), termed BZLF1, encodes the multifunctional protein Zta (ZEBRA or Z). It interacts with DNA as both a transcription and a replication factor, modulates both intracellular signal transduction and the DNA-damage response and manipulates cell cycle progression. Muller and colleagues have resolved the structure of Zta bound to DNA, which confirms some structural predictions but reveals an unexpected twist and a complex dimerization interface. Because EBV is associated with human disease, Zta presents a prime target for drug design.

Epstein-Barr virus immunossuppression of innate immunity mediated by phagocytes

Epstein-Barr virus (EBV) is an oncogenic human herpesvirus that persistently infects approximately 90% of the world's population. Such a remarkably sustained of viral infectivity relies on EBV's ability to evade the host immune defenses. A crucial part of this anti-EBV response is mediated by cytotoxic CD8+ T lymphocytes, which maintain a life-long control over proliferating latently-infected B cells in order to prevent these from giving rise to lymphomatous diseases. On the other hand, little has been done to assess the role of phagocytes-mediated innate immunity in the pathogenesis of EBV infection. In the course of primary EBV infection, episodes of neutropenia and monocytopenia can be observed during the acute phase of infection. According to the role of those cells in the non specific and specific immunity, such a decrease in circulating phagocytes may then temporarily affect the immune defense and potentially influence the outcome of EBV infection. Recent studies have demonstrated that EBV infects both neutrophils and monocytes and modulates several of their biological functions. This review covers the current state of our knowledge relative to the role of neutrophils and monocytes in EBV pathogenesis and describes the nature of countermeasures deployed by EBV against these cells.

DNA-damage response pathways triggered by viral replication

Many viruses, with distinct replication strategies, activate DNA-damage response pathways, including the lentivirus human immunodeficiency virus (HIV) and the DNA viruses Epstein-Barr virus (EBV), herpes simplex virus 1, adenovirus and SV40. DNA-damage response pathways involving DNA-dependent protein kinase, ataxia-telengiectasia mutated (ATM) and 'ataxia-telengiectasia and Rad3-related' (ATR) have all been implicated. This review focuses on the effects of HIV and EBV replication on DNA repair pathways. It has been suggested that activation of cellular DNA repair and recombination enzymes is beneficial for viral replication, as illustrated by the ability of suppressors of the ATM and ATR family to inhibit HIV replication. However, activation of DNA-damage response pathways can also promote apoptosis. Viruses can tailor the cellular response by suppressing downstream signalling from DNA-damage sensors, as exemplified by EBV. New small-molecule inhibitors of the DNA-damage response pathways could therefore be of value to treat viral infections.

A redox-sensitive cysteine in Zta is required for Epstein-Barr virus lytic cycle DNA replication

Epstein-Barr virus (EBV) reactivation from latency is known to be sensitive to redox regulation. The immediate-early protein Zta is a member of the basic-leucine zipper (bZIP) family of DNA binding proteins that stimulates viral and cellular transcription and nucleates a replication complex at the viral lytic origin. Zta shares with several members of the bZIP family a conserved cysteine residue (C189) that confers redox regulation of DNA binding. In this work, we show that replacement of C189 with serine (C189S) eliminated lytic cycle DNA replication function of Zta. The mechanistic basis for this replication defect was investigated. We show that C189S was not significantly altered for DNA binding activity in vitro or in vivo. We also show that C189S was not defective for transcription activation of EBV early gene promoters. C189S was deficient for transcription activation of several viral late genes that depend on lytic replication and therefore was consistent with a primary defect of C189S in activating lytic replication. C189S was not defective in binding methylated DNA binding sites and was capable of activating Rta from endogenous latent viral genomes, in contrast to the previously characterized S186A mutation. C189S was slightly impaired for its ability to form a stable complex with Rta, although this did not prevent Rta recruitment to OriLyt. C189S did provide some resistance to oxidation and nitrosylation, which potently inhibit Zta DNA binding activity in vitro. Interestingly, this redox sensitivity was not strictly dependent on C189S but involved additional cysteine residues in Zta. These results provide evidence that the conserved cysteine in the bZIP domain of Zta plays a primary role in EBV lytic cycle DNA replication.

Mutation of a single amino acid residue in the basic region of the Epstein-Barr virus (EBV) lytic cycle switch protein Zta (BZLF1) prevents reactivation of EBV from latency

Zta, the product of the BZLF1 gene carried by Epstein-Barr virus (EBV), is crucial for reactivation of EBV from latency. Zta is a member of the bZIP family of transcription factors, and in common with many of these, Zta possesses a conserved cysteine residue in its basic region (C189) and a further cysteine residue in its ZIP region (C222). We demonstrate that C189 is required to reactivate EBV from latency but C222 is not and that this single amino acid affects two independent functions of Zta, (i) binding to a Zta-responsive site and (ii) manipulating the cell cycle.

Cellular uptake of the EBV transcription factor EB1/Zta

A number of viral proteins have the property to penetrate into the cells when present in the extra-cellular compartment. Here, we report that the Epstein-Barr virus (EBV) transcriptional activator EB1/Zta, which is responsible for the activation of the EBV lytic replication, binds to lymphoid cells surface, is efficiently translocated and accumulates in the nucleus. The internalization of EB1/Zta is energy-dependent and shares common features with endocytosis. As the EB1/Zta was not degraded in the cells and reached the nucleus, the potential effect of its internalisation on viral reactivation was assessed.

Interferon regulatory factor 7 is negatively regulated by the Epstein-Barr virus immediate-early gene, BZLF-1

Virus infection stimulates potent antiviral responses; specifically, Epstein-Barr virus (EBV) infection induces and activates interferon regulatory factor 7 (IRF-7), which is essential for production of alpha/beta interferons (IFN-alpha/beta) and upregulates expression of Tap-2. Here we present evidence that during cytolytic viral replication the immediate-early EBV protein BZLF-1 counteracts effects of IRF-7 that are central to host antiviral responses. We initiated these studies by examining IRF-7 protein expression in vivo in lesions of hairy leukoplakia (HLP) in which there is abundant EBV replication but the expected inflammatory infiltrate is absent. This absence might predict that factors involved in the antiviral response are absent or inactive. First, we detected significant levels of IRF-7 in the nucleus, as well as in the cytoplasm, of cells in HLP lesions. IRF-7 activity in cell lines during cytolytic viral replication was examined by assay of the IRF-7-responsive promoters, IFN-alpha4, IFN-beta, and Tap-2, as well as of an IFN-stimulated response element (ISRE)-containing reporter construct. These reporter constructs showed consistent reduction of activity during lytic replication. Both endogenous and transiently expressed IRF-7 and EBV BZLF-1 proteins physically associate in cell culture, although BZLF-1 had no effect on the nuclear localization of IRF-7. However, IRF-7-dependent activity of the IFN-alpha4, IFN-beta, and Tap-2 promoters, as well as an ISRE promoter construct, was inhibited by BZLF-1. This inhibition occurred in the absence of other EBV proteins and was independent of IFN signaling. Expression of BZLF-1 also inhibited activation of IRF-7 by double-stranded RNA, as well as the activity of a constitutively active mutant form of IRF-7. Negative regulation of IRF-7 by BZLF-1 required the activation domain but not the DNA-binding domain of BZLF-1. Thus, EBV may subvert cellular antiviral responses and immune detection by blocking the activation of IFN-alpha4, IFN-beta, and Tap-2 by IRF-7 through the medium of BZLF-1 as a negative regulator.

Modeling early Epstein-Barr virus infection in Drosophila melanogaster: the BZLF1 protein

Epstein-Barr virus (EBV) is the causative agent of infectious mononucleosis and is associated with several forms of cancer, including lymphomas and nasopharyngeal carcinoma. The EBV immediate-early protein BZLF1 functions as a transcriptional activator of EBV early gene expression and is essential for the viral transition between latent and lytic replication. In addition to its role in the EBV life cycle, BZLF1 (Z) also has profound effects upon the host cellular environment, including disruption of cell cycle regulation, signal transduction pathways, and transcription. In an effort to understand the nature of Z interactions with the host cellular environment, we have developed a Drosophila model of early EBV infection, where we have expressed Z in the Drosophila eye. Using this system, we have identified a highly conserved interaction between the Epstein-Barr virus Z protein and shaven, a Drosophila homolog of the human Pax2/5/8 family of genes. Pax5 is a well-characterized human gene involved with B-cell development. The B-cell-specific Pax5 also promotes the transcription of EBV latent genes from the EBV Wp promoter. Our work clearly demonstrates that the Drosophila system is an appropriate and powerful tool for identifying the underlying genetic networks involved in human infectious disease.

Investigation of the multimerization region of the Kaposi's sarcoma-associated herpesvirus (human herpesvirus 8) protein K-bZIP: the proposed leucine zipper region encodes a multimerization domain with an unusual structure

The K8 gene of Kaposi's sarcoma-associated herpesvirus (human herpesvirus 8) shares many functional similarities with the BZLF1 gene of Epstein-Barr virus. The protein products of K8 and BZLF1, K-bZIP (RAP, K8) and Zta (BZLF1, ZEBRA, Z) have both been proposed to be members of the bZIP family of transcription factors, forming multimers via a coiled-coil motif termed a leucine zipper. Substantial evidence supporting this model for Zta is published. Here, we demonstrate that the proposed leucine zipper region of K-bZIP (amino acids 182 to 218) is required for multimer formation but that it does not fold as a coiled coil.

T-cell transformation and oncogenesis by gamma2-herpesviruses

gamma2-Herpesviruses, also termed rhadinoviruses, have long been known as animal pathogens causing lymphoproliferative diseases such as malignant catarrhal fever in cattle or T-cell lymphoma in certain Neotropical primates. The rhadinovirus prototype is Herpesvirus saimiri (HVS), a T-lymphotropic agent of squirrel monkeys (Saimiri sciureus); Herpesvirus ateles (HVA) is closely related to HVS. The first human rhadinovirus, human herpesvirus type 8 (HHV-8), was discovered a decade ago in Kaposi's sarcoma (KS) biopsies. It was found to be strongly associated with all forms of KS, as well as with multicentric Castleman's disease and primary effusion lymphoma (PEL). Since DNA of this virus is regularly found in all KS forms, and specifically in the spindle cells of KS, it was also termed KS-associated herpesvirus (KSHV). Several simian rhadinoviruses related to KSHV have been discovered in various Old World primates, though they seem only loosely associated with pathogenicity or tumor induction. In contrast, HVS and HVA cause T-cell lymphoma in numerous non-natural primate hosts; HVS strains of the subgroup C are capable of transforming human and simian T-lymphocytes to continuous growth in cell culture and can provide useful tools for T-cell immunology or gene transfer. Here, we describe their natural history, genome structure, biology, and pathogenesis in T-cell transformation and oncogenesis.

Kaposi's sarcoma-associated herpesvirus reactivation is regulated by interaction of latency-associated nuclear antigen with recombination signal sequence-binding protein Jkappa, the major downstream effector of the Notch signaling pathway

Kaposi's sarcoma-associated herpesvirus (KSHV) is the major biological cofactor contributing to development of Kaposi's sarcoma. KSHV establishes a latent infection in human B cells expressing the latency-associated nuclear antigen (LANA), a critical factor in the regulation of viral latency. LANA controls KSHV latent infection through repression of RTA, an activator of many lytic promoters. RTA activates the expression of several lytic viral genes by interacting with recombination signal sequence-binding protein Jkappa (RBP-Jkappa), a transcriptional repressor and the target of the Notch signaling pathway. The recognition that a number of KSHV lytic gene promoters, including RTA, contain RBP-Jkappa binding sites raised the possibility that RBP-Jkappa-mediated repression may be central to the establishment of latency. Here, we tested this hypothesis by examining the regulation of RTA by LANA through binding to RBP-Jkappa. This study demonstrates that LANA physically associates with RBP-Jkappa in vitro and in KSHV-infected cells, with the complex formed capable of binding to RBP-Jkappa cognate sequences. RBP-Jkappa binding sites within the RTA promoter have been found to be critical for LANA-mediated repression. Our study describes a novel mechanism through which LANA maintains KSHV latency by targeting a major downstream effector of the Notch signaling pathway.

Nuclear localization of HTLV-I bZIP factor (HBZ) is mediated by three distinct motifs

The genome of the human T-cell leukemia virus type I (HTLV-I) codes for a basic leucine zipper protein, HBZ, capable of repressing JUN activity and viral transcription. Transient expression in mammalian cells showed that HBZ was targeted to the nucleus, where it accumulated in nuclear speckles. By using a complementary set of deletion mutants, we report here that the nuclear targeting of HBZ is mediated by three distinct nuclear localization signals and that at least two are necessary for the translocation of HBZ to the nucleus. Moreover, the resulting mutant proteins distribute throughout the nucleoplasm and/or into the nucleoli, whereas the wild-type HBZ exclusively accumulates in nuclear speckles, suggesting that the integrity of the protein is required for its speckle localization. We also demonstrate that the HBZ-containing speckles do not correspond to Cajal bodies, splicing factor compartments, or promyelocytic leukemia oncoprotein bodies. Unexpectedly, by using immunogold electron microscopy, we found HBZ localized to heterochromatin. Until now, such characteristics had never been described for a transcription factor and could explain the inhibitory activity of HBZ.

p16(INK4A)-independence of Epstein-Barr virus-induced cell proliferation and virus latency

Epstein-Barr virus (EBV) has the ability to promote cell cycle progression following the initial infection of primary resting B-lymphocytes and to cause cell cycle arrest at the onset of the viral replicative cycle. Various mechanisms have been proposed for the proliferative effects, including the up-regulation of cyclin D2 by the viral EBNA-2 and EBNA-LP proteins, direct binding of EBNA3C to the retinoblastoma protein (pRb), and down-regulation of the p16(INK4A) tumour suppressor by the viral LMP1 product. To try to gain insight into the relative importance of these mechanisms, the ability of EBV to immortalize lymphocytes from an individual who is genetically deficient for p16(INK4A) was examined. From detailed analyses of the resultant lymphoblastoid cell lines it is concluded that p16(INK4A) status has little bearing on EBV's ability to manipulate the cell cycle machinery and a model to accommodate the previously proposed routes taken by EBV to bypass the restriction point is presented.

Glucocorticoid response and promoter occupancy of the mouse LXRalpha gene

The liver X receptors alpha and beta (LXRalpha and LXRbeta) are members of the nuclear receptor superfamily of proteins which are highly expressed in metabolically active tissues. They regulate gene expression of critical genes involved in cholesterol catabolism and transport, lipid and triglyceride biosynthesis, and carbohydrate metabolism in response to distinct oxysterol intermediates in the cholesterol metabolic pathway. Several LXR target genes have been identified, but there is limited information on how expression of the LXRs themselves is controlled. In this study we have characterized the upstream flanking region of the mouse LXRalpha gene. Transient transfections show that the LXRalpha promoter is able to drive transcription of a luciferase reporter gene, however, the transcriptional potential of the promoter in the cell lines used was low. The -2143 to -1513 region of the promoter mediates repression of reporter gene activity in all cells analyzed and multiple DNA-protein interactions were detected in this region by DNase I footprinting. The Zta, Ets, and Hes1 transcription factors were all shown to mediate alterations in reporter gene activity driven by LXRalpha promoter deletion constructs. These factors have been linked to cell cycle and differentiation processes suggesting that expression of LXRalpha might be under control of signalling mechanisms regulating cell proliferation. Several putative binding sites of the glucocorticoid receptor (GR) were identified in the LXRalpha promoter and transient cotransfections of the GR and LXRalpha promoter deletion constructs induced reporter gene activity. Addition of dexamethasone, a GR agonist, abolished this effect suggesting cross talk between GR and LXR signalling.

The zipper region of Epstein-Barr virus bZIP transcription factor Zta is necessary but not sufficient to direct DNA binding

The viral bZIP transcription factor Zta (BZLF1, EB1, ZEBRA) mediates the switch between the latent and lytic cycles of Epstein-Barr virus (EBV). In part, its activity requires the formation of homodimers and interaction with specific DNA sequence elements (ZREs). Zta has an atypical zipper motif that has a lower stability than do typical bZIP proteins. Here we show that a synthetic peptide directed against the zipper can disrupt the DNA-binding function of Zta. This highlights the relevance of this region for the function of Zta and demonstrates that the zipper region is a potential target for therapeutic agents. We also unmask the relevance of a region adjacent to the zipper (CT region), which is required to direct the interaction of Zta with DNA and to transactivate ZRE-dependent promoters in vivo.