Expression of proteins encoded by Epstein-Barr virus trans-activator genes depends on the differentiation of epithelial cells in oral hairy leukoplakia

Targeting Metabolic Vulnerabilities in Epstein-Barr Virus-Driven Proliferative Diseases

The metabolism of cancer cells and Epstein-Barr virus (EBV) infected cells have remarkable similarities. Cancer cells frequently reprogram metabolic pathways to augment their ability to support abnormal rates of proliferation and promote intra-organismal spread through metastatic invasion. On the other hand, EBV is also capable of manipulating host cell metabolism to enable sustained growth and division during latency as well as intra- and inter-individual transmission during lytic replication. It comes as no surprise that EBV, the first oncogenic virus to be described in humans, is a key driver for a significant fraction of human malignancies in the world (~1% of all cancers), both in terms of new diagnoses and attributable deaths each year. Understanding the contributions of metabolic pathways that underpin transformation and virus replication will be important for delineating new therapeutic targets and designing nutritional interventions to reduce disease burden. In this review, we summarise research hitherto conducted on the means and impact of various metabolic changes induced by EBV and discuss existing and potential treatment options targeting metabolic vulnerabilities in EBV-associated diseases.

BRLF1-dependent viral and cellular transcriptomes and transcriptional regulation during EBV primary infection in B lymphoma cells

The immediate-early protein BRLF1 plays important roles in lytic infection of Epstein-Barr virus (EBV), in which it activates lytic viral transcription and replication. However, knowledge of the influence of BRLF1 on cellular gene expression and transcriptional reprogramming during the early lytic cycle remains limited. In the present study, deep RNA-sequencing analysis identified all differentially expressed genes (DEGs) and alternative splicing in B lymphoma cells subjected to wild-type and BRLF1-deficient EBV primary infection. The BRLF1-dependent cellular DEGs were annotated, and major differentially enriched pathways were related to DNA replication and transcription, immune and inflammatory responses, cytokine-receptor interactions and chemokine signaling and metabolic processes. Furthermore, analysis of BRLF1-binding proteins by mass spectrometry shows that BRLF1 binds to and cooperates with several transcription factors and components of the spliceosome and then influences both RNA polymerase II-dependent transcription and pre-mRNA splicing. The RTA-binding RRE motifs or specific motifs of unique cooperative transcription factors in viral and cellular DEG promoter regions indicate that BRLF1 employs different strategies for regulating viral and cellular transcription. Thus, our study characterized BRLF1-dependent cellular and viral transcriptional profile during primary infection and then revealed the comprehensive virus-cell interaction and alterations of transcription during EBV primary infection and lytic replication.

Clinical Manifestations and Epigenetic Regulation of Oral Herpesvirus Infections

The oral cavity is often the first site where viruses interact with the human body. The oral epithelium is a major site of viral entry, replication and spread to other cell types, where chronic infection can be established. In addition, saliva has been shown as a primary route of person-to-person transmission for many viruses. From a clinical perspective, viral infection can lead to several oral manifestations, ranging from common intraoral lesions to tumors. Despite the clinical and biological relevance of initial oral infection, little is known about the mechanism of regulation of the viral life cycle in the oral cavity. Several viruses utilize host epigenetic machinery to promote their own life cycle. Importantly, viral hijacking of host chromatin-modifying enzymes can also lead to the dysregulation of host factors and in the case of oncogenic viruses may ultimately play a role in promoting tumorigenesis. Given the known roles of epigenetic regulation of viral infection, epigenetic-targeted antiviral therapy has been recently explored as a therapeutic option for chronic viral infection. In this review, we highlight three herpesviruses with known roles in oral infection, including herpes simplex virus type 1, Epstein–Barr virus and Kaposi’s sarcoma-associated herpesvirus. We focus on the respective oral clinical manifestations of these viruses and their epigenetic regulation, with a specific emphasis on the viral life cycle in the oral epithelium.

The Role of EBV-Induced Hypermethylation in Gastric Cancer Tumorigenesis

Epstein–Barr-virus-associated Gastric Cancer (EBVaGC) comprises approximately 10% of global gastric cancers and is known to be the most hypermethylated of all tumor types. EBV infection has been shown to directly induce the hypermethylation of both the host and viral genome following initial infection of gastric epithelial cells. Many studies have been completed in an attempt to identify genes that frequently become hypermethylated and therefore significant pathways that become silenced to promote tumorigenesis. It is clear that EBV-induced hypermethylation silences key tumor suppressor genes, cell cycle genes and cellular differentiation factors to promote a highly proliferative and poorly differentiated cell population. EBV infection has been shown to induce methylation in additional malignancies including Nasopharyngeal Carcinoma and Burkitt’s Lymphoma though not to the same level as in EBVaGC. Lastly, some genes silenced in EBVaGC are common to other heavily methylated tumors such as colorectal and breast tumors; however, some genes are unique to EBVaGC and can provide insights into the major pathways involved in tumorigenesis.

Immunodeficiencies that predispose to pathologies by human oncogenic γ-herpesviruses

Human γ-herpesviruses include the closely related tumor viruses Epstein Barr virus (EBV) and Kaposi sarcoma-associated herpesvirus (KSHV). EBV is the most growth-transforming pathogen known and is linked to at least seven human malignancies. KSHV is also associated with three human cancers. Most EBV- and KSHV-infected individuals fortunately remain disease-free despite persistent infection and this is likely due to the robustness of the immune control that they mount against these tumor viruses. However, upon immune suppression EBV- and KSHV-associated malignancies emerge at increased frequencies. Moreover, primary immunodeficiencies with individual mutations that predispose to EBV or KSHV disease allow us to gain insights into a catalog of molecules that are required for the immune control of these tumor viruses. Curiously, there is little overlap between the mutation targets that predispose individuals to EBV versus KSHV disease, even so both viruses can infect the same host cell, human B cells. These differences will be discussed in this review. A better understanding of the crucial components in the near-perfect life-long immune control of EBV and KSHV should allow us to target malignancies that are associated with these viruses, but also induce similar immune responses against other tumors.

Epstein-Barr virus in the pathogenesis of oral cancers

Epstein-Barr virus (EBV) is a ubiquitous gamma-herpesvirus that establishes a lifelong persistent infection in the oral cavity and is intermittently shed in the saliva. EBV exhibits a biphasic life cycle, supported by its dual tropism for B lymphocytes and epithelial cells, which allows the virus to be transmitted within oral lymphoid tissues. While infection is often benign, EBV is associated with a number of lymphomas and carcinomas that arise in the oral cavity and at other anatomical sites. Incomplete association of EBV in cancer has questioned if EBV is merely a passenger or a driver of the tumorigenic process. However, the ability of EBV to immortalize B cells and its prevalence in a subset of cancers has implicated EBV as a carcinogenic cofactor in cellular contexts where the viral life cycle is altered. In many cases, EBV likely acts as an agent of tumor progression rather than tumor initiation, conferring malignant phenotypes observed in EBV-positive cancers. Given that the oral cavity serves as the main site of EBV residence and transmission, here we review the prevalence of EBV in oral malignancies and the mechanisms by which EBV acts as an agent of tumor progression.

Epstein-Barr Virus Gene BARF1 Expression is Regulated by the Epithelial Differentiation Factor ΔNp63α in Undifferentiated Nasopharyngeal Carcinoma

Epstein-Barr Virus (EBV) BamHI-A rightward frame 1 (BARF1) protein is considered a viral oncogene in epithelial cells and has immune-modulating properties. During viral lytic replication BARF1 is expressed as an early gene, regulated by the immediate early EBV protein R. However, in viral latency BARF1 is exclusively expressed in epithelial tumors such as nasopharyngeal (NPC) and gastric carcinoma (GC) but not in lymphomas, indicating that activation of the BARF1 promoter is cell type specific. Undifferentiated NPC is characterized by high expression of ΔNp63 isoforms of the epithelial differentiation marker p63, a member of the p53 family of transcription factors. Transcription factor binding site analysis indicated potential p53 family binding sites within the BARF1 promoter region. This study investigated ability of various p53 family members to transactivate the BARF1 promoter. Using BARF1 promoter luciferase reporter constructs we demonstrate that only p63 isoform ΔNp63α is capable of transactivating the BARF1 promoter, but not the TAp63 isoforms, p53 or p73. Direct promoter binding of ΔNp63α was confirmed by Chromatin Immune Precipitation (ChIP) analysis. Deletion mutants of the BARF1 promoter revealed multiple ΔNp63 response elements to be responsible for BARF1 promoter transactivation. However, ΔNp63α alone was not sufficient to induce BARF1 in tumor cells harboring full EBV genomes, indicating that additional cofactors might be required for full BARF1 regulation. In conclusion, in EBV positive NPC and GC, BARF1 expression might be induced by the epithelial differentiation marker ΔNp63α, explaining BARF1 expression in the absence of lytic reactivation.

Insights into Alternanthera mosaic virus TGB3 Functions: Interactions with Nicotiana benthamiana PsbO Correlate with Chloroplast Vesiculation and Veinal Necrosis Caused by TGB3 Over-Expression

Alternanthera mosaic virus (AltMV) triple gene block 3 (TGB3) protein is involved in viral movement. AltMV TGB3 subcellular localization was previously shown to be distinct from that of Potato virus X (PVX) TGB3, and a chloroplast binding domain identified; veinal necrosis and chloroplast vesiculation were observed in Nicotiana benthamiana when AltMV TGB3 was over-expressed from PVX. Plants with over-expressed TGB3 showed more lethal damage under dark conditions than under light. Yeast-two-hybrid analysis and bimolecular fluorescence complementation (BiFC) reveal that Arabidopsis thaliana PsbO1 has strong interactions with TGB3; N. benthamiana PsbO (NbPsbO) also showed obvious interaction signals with TGB3 through BiFC. These results demonstrate an important role for TGB3 in virus cell-to-cell movement and virus-host plant interactions. The Photosystem II oxygen-evolving complex protein PsbO interaction with TGB3 is presumed to have a crucial role in symptom development and lethal damage under dark conditions. In order to further examine interactions between AtPsbO1, NbPsbO, and TGB3, and to identify the binding domain(s) in TGB3 protein, BiFC assays were performed between AtPsbO1 or NbPsbO and various mutants of TGB3. Interactions with C-terminally deleted TGB3 were significantly weaker than those with wild-type TGB3, and both N-terminally deleted TGB3 and a TGB3 mutant previously shown to lose chloroplast interactions failed to interact detectably with PsbO in BiFC. To gain additional information about TGB3 interactions in AltMV-susceptible plants, we cloned 12 natural AltMV TGB3 sequence variants into a PVX expression vector to examine differences in symptom development in N. benthamiana. Symptom differences were observed on PVX over-expression, with all AltMV TGB3 variants showing more severe symptoms than the WT PVX control, but without obvious correlation to sequence differences.

The nuclear and adherent junction complex component protein ubinuclein negatively regulates the productive cycle of Epstein-Barr virus in epithelial cells

The Epstein-Barr Virus (EBV) productive cycle is initiated by the expression of the viral trans-activator EB1 (also called Zebra, Zta, or BZLF1), which belongs to the basic leucine zipper transcription factor family. We have previously identified the cellular NACos (nuclear and adherent junction complex components) protein ubinuclein (Ubn-1) as a partner for EB1, but the function of this complex has never been studied. Here, we have evaluated the consequences of this interaction on the EBV productive cycle and find that Ubn-1 overexpression represses the EBV productive cycle whereas Ubn-1 downregulation by short hairpin RNA (shRNA) increases virus production. By a chromatin immunoprecipitation (ChIP) assay, we show that Ubn-1 blocks EB1-DNA interaction. We also show that in epithelial cells, relocalization and sequestration of Ubn-1 to the tight junctions of nondividing cells allow increased activation of the productive cycle. We propose a model in which Ubn-1 is a modulator of the EBV productive cycle: in proliferating epithelial cells, Ubn-1 is nuclear and inhibits activation of the productive cycle, whereas in differentiated cells, Ubn-1 is sequestrated to tight junctions, thereby allowing EB1 to fully function in the nucleus.

Features distinguishing Epstein-Barr virus infections of epithelial cells and B cells: viral genome expression, genome maintenance, and genome amplification

Epstein-Barr virus (EBV) is associated with malignant diseases of lymphoid and epithelial cell origin. The tropism of EBV is due to B-cell-restricted expression of CD21, the major receptor molecule for the virus. However, efficient infection of CD21- epithelial cells can be achieved via transfer from EBV-coated B cells. We compare and contrast here the early events following in vitro infection of primary B cells and epithelial cells. Using sensitive, quantitative reverse transcription-PCR assays for several latent and lytic transcripts and two-color immunofluorescence staining to analyze expression at the single cell level, we confirmed and extended previous reports indicating that the two cell types support different patterns of transcription. Furthermore, whereas infection of B cells with one or two copies of EBV resulted in rapid amplification of the viral genome to >20 copies per cell, such amplification was not normally observed after infection of primary epithelial cells or undifferentiated epithelial lines. In epithelial cells, EBNA1 expression was detected in only ca. 40% of EBER+ cells, and the EBV genome was subsequently lost during prolonged culture. One exception was that infection of AGS, a gastric carcinoma line, resulted in maintenance of EBNA1 expression and amplification of the EBV episome. In contrast to B cells, where amplification of the EBV episome occurred even with a replication-defective BZLF1-knockout virus, amplification in AGS cells was dependent upon early lytic cycle gene expression. These data highlight the influence of the host cell on the outcome of EBV infection with regard to genome expression, amplification, and maintenance.

Virus and cell RNAs expressed during Epstein-Barr virus replication

Changes in Epstein-Barr virus (EBV) and cell RNA levels were assayed following immunoglobulin G (IgG) cross-linking-induced replication in latency 1-infected Akata Burkitt B lymphoblasts. EBV replication as assayed by membrane gp350 expression was approximately 5% before IgG cross-linking and increased to more than 50% 48 h after induction. Seventy-two hours after IgG cross-linking, gp350-positive cells excluded propidium iodide as well as gp350-negative cells. EBV RNA levels changed temporally in parallel with previously defined sensitivity to inhibitors of protein or viral DNA synthesis. BZLF1 immediate-early RNA levels doubled by 2 h and reached a peak at 4 h, whereas BMLF1 doubled by 4 h with a peak at 8 h, and BRLF1 doubled by 8 h with peak at 12 h. Early RNAs peaked at 8 to 12 h, and late RNAs peaked at 24 h. Hybridization to intergenic sequences resulted in evidence for new EBV RNAs. Surprisingly, latency III (LTIII) RNAs for LMP1, LMP2, EBNALP, EBNA2, EBNA3A, EBNA3C, and BARTs were detected at 8 to 12 h and reached maxima at 24 to 48 h. EBNA2 and LMP1 were at full LTIII levels by 48 h and localized to gp350-positive cells. Thus, LTIII expression is a characteristic of late EBV replication in both B lymphoblasts and epithelial cells in immune-comprised people (J. Webster-Cyriaque, J. Middeldorp, and N. Raab-Traub, J. Virol. 74:7610-7618, 2000). EBV replication significantly altered levels of 401 Akata cell RNAs, of which 122 RNAs changed twofold or more relative to uninfected Akata cells. Mitogen-activated protein kinase levels were significantly affected. Late expression of LTIII was associated with induction of NF-kappaB responsive genes including IkappaBalpha and A20. The exclusion of propidium, expression of EBV LTIII RNAs and proteins, and up-regulation of specific cell RNAs are indicative of vital cell function late in EBV replication.

Inhibition of the Epstein–Barr virus lytic cycle by Zta-targeted RNA interference

Epstein–Barr virus (EBV) reactivation into the lytic cycle plays certain roles in the development of EBV-associated diseases, so an effective strategy to block the viral lytic cycle may be of value to reduce the disease risk or to improve the clinical outcome. This study examined whether the EBV lytic cycle could be inhibited using RNA interference (RNAi) directed against the essential viral gene Zta. In cases of EBV reactivation triggered by chemicals or by exogenous Rta, Zta-targeted RNAi prevented the induction of Zta and its downstream genes and further blocked the lytic replication of viral genomes. This antiviral effect of RNAi was not likely to be mediated by activation of the interferon pathway, as phosphorylation of STAT1 was not induced. In addition, novel EBV-infected epithelial cells showing constitutive activation of the lytic cycle were cloned; such established lytic infection was also suppressed by Zta-targeted RNAi. These results indicate that RNAi can be used to inhibit the EBV lytic cycle effectively in vitro and could also be of potential use to develop anti-EBV treatments.

The BZLF1 promoter of Epstein-Barr virus is controlled by E box-/HI-motif-binding factors during virus latency

The BZLF1 open reading frame of Epstein-Barr virus (EBV) encodes an important transactivator of replication. During latency, transcription of this gene is switched off. HI motifs have been shown to cause negative regulation of the promoter. Using yeast one-hybrid assays, we isolated the E box-binding protein, E2-2, interacting with these motifs. Electrophoretic mobility shift assays demonstrated that E2-2 binds to HI alpha, HI beta and HI gamma, which contain E box consensus binding sites. Deletion of the HI-associated E boxes and overexpression of E2-2 in transfection assays revealed that these elements act as repressors in lymphoid cells. In contrast, in epithelial cells they contribute to the increased responsiveness of the promoter to transactivation by the BZLF1 protein. The data presented are in accord with an alternative and exclusive binding of different cell type- and differentiation-specific factors, such as E2-2, to the HI-associated E boxes in lymphoid and epithelial cells. This implies a role in cell type-specific virus replication.

CCAAT/enhancer binding protein alpha interacts with ZTA and mediates ZTA-induced p21(CIP-1) accumulation and G(1) cell cycle arrest during the Epstein-Barr virus lytic cycle

Cellular CCAAT/enhancer binding protein alpha (C/EBPalpha) promotes cellular differentiation and has antimitotic activities involving cell cycle arrest at G(1)/S through stabilization of p21(CIP-1)/WAF1 and through transcriptional activation of the p21 promoter. The Epstein-Barr virus lytic-cycle transactivator protein ZTA is known to arrest the host cell cycle at G(1)/S via a p53-independent p21 pathway, but the detailed molecular mechanisms involved have not been defined. To further evaluate the role of ZTA in cell cycle arrest, we constructed a recombinant adenovirus vector expressing ZTA (Ad-ZTA), whose level of expression at a low multiplicity of infection in normal human diploid fibroblast (HF) cells was lower than or equal to the physiological level seen in Akata cells lytically induced by EBV (EBV-Akata cells). Fluorescence-activated cell sorting analysis of HF cells infected with Ad-ZTA confirmed that G(1)/S cell cycle arrest occurred in the majority of ZTA-positive cells, but not with an adenovirus vector expressing green fluorescent protein. Double-label immunofluorescence assays (IFA) performed with Ad-ZTA-infected HF cells revealed that only ZTA-positive cells induced the expression of both endogenous C/EBPalpha and p21 and blocked the progression into S phase, as detected by a lack of incorporation of bromodeoxyuridine. The stimulation of endogenous ZTA protein expression either through treatment with tetradecanoyl phorbol acetate in D98/HR1 cells or through B-cell receptor cross-linking with anti-immunoglobulin G antibody in EBV-Akata cells also coincided with the induction of both C/EBPalpha and p21 and their mRNAs, as assayed by Northern blot, Western blot, and IFA experiments. Mechanistically, the ZTA protein proved to directly interact with C/EBPalpha by coimmunoprecipitation in EBV-Akata cells and with DNA-bound C/EBPalpha in electrophoretic mobility shift assay experiments, and the in vitro interaction domain encompassed the basic leucine zipper domain of ZTA. ZTA also specifically protected C/EBPalpha from degradation in a protein stability assay with a non-EBV-induced Akata cell proteasome extract. Furthermore, both C/EBPalpha and ZTA were found to specifically associate with the C/EBPalpha promoter in chromatin immunoprecipitation assays, but the interaction with ZTA appeared to be mediated by C/EBPalpha because it was abolished by clearing with anti-C/EBPalpha antibody. ZTA did not bind to or activate the C/EBPalpha promoter directly but cooperatively enhanced the positive autoregulation of the C/EBPalpha promoter by cotransfected C/EBPalpha in transient luciferase reporter gene assays with Vero and HeLa cells as well as with DG75 B lymphocytes. Similarly, ZTA alone had little effect on the p21 promoter in transient reporter gene assays, but in the presence of cotransfected C/EBPalpha, ZTA enhanced the level of C/EBPalpha activation. This effect proved to require a previously unrecognized region in the proximal p21 promoter that contains three high-affinity C/EBPalpha binding sites. Finally, in C/EBPalpha-deficient mouse embryonic fibroblasts (MEF), Ad-ZTA was unable to induce either p21 or G(1) arrest, whereas it was able to induce both in wild-type MEF. Overall, we conclude that C/EBPalpha is essential for at least one pathway of ZTA-induced G(1) arrest during EBV lytic-cycle DNA replication and that this process involves a physical piggyback interaction between ZTA and C/EBPalpha leading to greatly enhanced C/EBPalpha and p21 levels through both transcriptional and posttranslational mechanisms.

ZEB negatively regulates the lytic-switch BZLF1 gene promoter of Epstein-Barr virus

Epstein-Barr virus (EBV) is a human herpesvirus capable of establishing a latent state in B lymphocytes. The product of the immediate-early BZLF1 gene, Zta, is a transcriptional transactivator essential for viral DNA amplification and virion production. Previously, we identified a negative cis-acting element within the BZLF1 promoter termed ZV. ZV contains the sequence 5'-CAGGTA-3' located at nucleotides -17 to -12 relative to the transcription initiation site. It sequence specifically binds a cellular factor, ZVR. Based on sequence binding specificity, we postulated that ZVR may be zinc finger E-box binding factor (ZEB) or a related zinc finger/homeodomain family member. We show here by immunoshift assays that ZVR and human ZEB specifically cross-react with an antibody to deltaEF1, the chicken homolog of ZEB. Competition electrophoretic mobility shift assays confirmed that ZEB binds to the ZV element with the same binding specificity as ZVR. Overexpression of ZEB in either B-lymphocytic DG75 cells or mammary epithelial MCF-7 cells repressed Zta-induced activation of the BZLF1 promoter four- to fivefold via the ZV site. Thus, we conclude that the previously identified cellular repressor ZVR is, in fact, ZEB. We also present evidence that other cellular factors likely affect the transcriptional activity of ZEB. Lastly, we identify a ZEB-binding site within the promoter of the lytic BRLF1 gene of EBV. We postulate that ZEB likely plays an important role in regulating the life cycle of EBV.

Epstein-Barr virus BHRF1 functions downstream of Bid cleavage and upstream of mitochondrial dysfunction to inhibit TRAIL-induced apoptosis in BJAB cells

We have previously reported that TNF-related apoptosis inducing ligand (TRAIL) causes cleavage of Bid via activation of caspase-8 and the loss of mitochondrial membrane potential (DeltaPsim), resulting in apoptosis. Experiments with BJAB clones expressing Epstein-Barr virus (EBV) anti-apoptotic protein BHRF1 showed that BHRF1 drastically inhibited TRAIL-mediated apoptosis. Although Western blot analysis demonstrated that TRAIL-induced Bid cleavage was not inhibited by BHRF1, the decrease in DeltaPsim caused by TRAIL was effectively blocked by BHRF1. These findings suggest that in BJAB cells, BHRF1 acts downstream of Bid cleavage and upstream of mitochondrial damage, resulting in inhibition of TRAIL-induced apoptosis.

Cutaneous manifestations of Epstein-Barr virus infection

The most common cutaneous manifestations of EBV include IM, OHL, and cutaneous lymphoproliferative disorders. Infectious mononucleosis is a self-limited manifestation of acute EBV infection. The transient rash that occurs quite commonly in patients with IM who have received antibiotic therapy is an erythematous, maculopapular eruption, usually located on the trunk and upper extremities. Oral hairy leukoplakia occurs in immunosuppressed HIV-positive and HIV-negative individuals. In HIV-positive individuals, it serves as an indicator of disease severity and rapid progression to AIDS. The presence of OHL in an individual should prompt the clinician to perform a through history-taking and investigation of immune status. Cutaneous lymphoproliferative disorders associated with EBV occur in individuals with congenital or acquired immunodeficiency syndromes.

Cell cycle analysis of Epstein-Barr virus-infected cells following treatment with lytic cycle-inducing agents

While Epstein-Barr virus (EBV) latency-associated gene expression is associated with cell cycle progression, the relationship between the EBV lytic program and the cell cycle is less clear. Using four different EBV lytic induction systems, we address the relationship between lytic cycle activation and the cell cycle. In three of these systems, G0 or G1 cell growth arrest signaling is observed prior to detection of the EBV immediate-early gene product Zta. In tetradecanoyl phorbol acetate-treated P3HR1 cultures and in 5-iodo-2'-deoxyuridine-treated NPC-KT cultures, cell cycle analysis of Zta-expressing cell populations showed a significant G1 bias during the early stages of lytic cycle progression. In contrast, treatment of the cell line Akata with anti-immunoglobulin (Ig) results in rapid induction of immediate-early gene expression, and accordingly, activation of the immediate-early gene product Zta precedes significant anti-Ig-induced cell cycle effects. Nevertheless, cell cycle analysis of the Zta-expressing population following anti-Ig treatment shows a bias for cells in G1, indicating that anti-Ig-mediated induction of Zta occurs more efficiently in cells traversing G1. Last, although 5-azacytidine treatment of Rael cells results in a G1 arrest in the total cell population which precedes the induction of Zta, cell cycle analysis of the Zta-expressing population shows a significant bias for cells with an apparent G2/M DNA content. This bias may result, in part, from activation of Zta expression following demethylation of the Zta promoter during S-phase. Together, these studies indicate that induction of Zta occurs through several distinct mechanisms, some of which may involve checkpoint signaling.

Epstein-barr virus nuclear antigen 2 retards cell growth, induces p21(WAF1) expression, and modulates p53 activity post-translationally

The Epstein-Barr virus (EBV) nuclear antigen 2 (EBNA2) has been shown to be required for promotion of cell-cycle progression in EBV-immortalized B-lymphocytes. However, other studies have indicated that EBNA2 alone, in the absence of other EBV genes, may retard cell growth. To resolve this discrepancy, we investigated the effect of EBNA2 on the growth of various cells, including EBV target nasopharyngeal carcinoma cells, NPC-TW01 and NPC-TW04. We found that EBNA2 could retard cell growth, in stable Vero, HEp-2, and U2OS cell clones expressing EBNA2, and in Vero, 293, NPC-TW01, and NPC-TW04 cells transiently transfected with EBNA2. While investigating the mechanism underlying the growth-retarding function of EBNA2, we found that EBNA2 induced p21(WAF1) expression in these cells. This induction of p21(WAF1) expression was mediated through p53. EBNA2 was found to stimulate p53 to bind to the p53-response element within the p21(WAF1) promoter, possibly by promoting p53 phosphorylation. This enhancement of p53 sequence-specific DNA-binding activity may be the mechanism through which EBNA2 activates the expression of p53-regulated genes, including p21(WAF1) and mdm-2. Together, these studies reveal a possible intrinsic function of EBNA2 in cell-growth regulation and elucidate a novel mechanism by which EBNA2 modulates transcription.

Latency of Epstein-Barr virus is stabilized by antisense-mediated control of the viral immediate-early gene BZLF-1

The ability of the Epstein-Barr virus (EBV) to avoid lytic replication and to establish a latent infection in B-lymphocytes is fundamental for its lifelong persistence and the pathogenesis of various EBV-associated diseases. The viral immediate-early gene BZLF-1 plays a key role for the induction of lytic replication and its activity is strictly regulated on different levels of gene expression. Recently, it was demonstrated that BZLF-1 is also controlled by a posttranscriptional mechanism. Transient synthesis of a mutated competitor RNA saturated this mechanism and caused both expression of the BZLF-1 protein and the induction of lytic viral replication. Using short overlapping fragments of the competitor, it is shown that this control acts on the unspliced primary transcript. RT-PCR demonstrated unspliced BZLF-1 RNA in latently infected B-lymphocytes in the absence of BZLF-1 protein. Due to the complementarity of the gene BZLF-1 and the latency-associated gene EBNA-1 on the opposite strand of the genome, we propose an antisense-mediated mechanism. RNase protection assays demonstrated transcripts in antisense orientation to the BZLF-1 transcript during latency, which comprise a comparable constellation to other herpesviruses. A combined RNAse protection/RT-PCR assay detected the double-stranded hybrid RNA, consisting of the unspliced BZLF-1 transcript and a noncoding intron of the EBNA-1 gene. Binding of BZLF-1 transcripts is suggested to be an important backup control mechanism in addition to transcriptional regulation, stabilizing latency and preventing inappropriate lytic viral replication in vivo.

Genetic dissection of cell growth arrest functions mediated by the Epstein-Barr virus lytic gene product, Zta

Expression of the Epstein-Barr virus (EBV) latency-associated genes activates cell cycle progression and drives immortalization of the infected cell. In contrast, progression of the EBV replication program occurs most efficiently in growth-arrested cells. Previous studies showed that the EBV-encoded immediate-early transcription factor, Zta, can induce expression of the cyclin-dependent kinase inhibitors, p21 and p27, the tumor suppressor, p53, and cell growth arrest. Moreover, Zta-mediated induction of growth arrest occurs independently of its transcriptional transactivation function. Here we show that substitution of Zta's basic DNA binding domain with the analogous region of the Zta homologue, c-Fos, abrogates Zta's ability to induce growth arrest and to induce p21, p27, or p53 expression, suggesting that protein-protein interactions between this region of Zta and key cell cycle control proteins are involved in signaling cell cycle arrest. We also show that despite the crucial role for Zta's basic domain in eliciting cell growth arrest, its amino terminus is required for efficient induction of p27 and it modulates the level of p53 induction. Last, we provide evidence that Zta-mediated inductions of p21, p27, and p53 occur, at least in part, through distinct pathways. Therefore, Zta interacts with multiple growth arrest pathways, a property which may have evolved partly as a means to ensure that lytic replication occurs in a growth-arrested setting in multiple different tissues in various states of differentiation.

Epstein-Barr virus DNase contains two nuclear localization signals, which are different in sensitivity to the hydrophobic regions

The DNase of Epstein-Barr virus (EBV) is a 470-amino-acid protein which possesses both endonuclease and exonuclease activities and accepts both double-stranded DNA and single-stranded DNA as substrates. It has been reported that this protein may be found in the nucleus and/or cytoplasm of infected cells. In this study, using cell fractionation and immunoblotting to determine the distribution of EBV DNase in Akata cells stimulated with anti-human immunoglobulin G antibody (anti-IgG), the DNase was found to be located predominantly in the nucleus. To map the signals in DNase which mediate its nuclear localization, we monitored the nuclear transport of fusion proteins consisting of various fragments of EBV DNase linked to a cytoplasmic protein, beta-galactosidase (beta-Gal). The results demonstrated that two regions of the DNase with nuclear localization signal (NLS) activity, designated NLS-A (amino acids 239-266) and NLS-B (amino acids 291-306), were able independently to localize the beta-Gal to the nuclei of HEp-2 and HeLa cells. Five basic residues (R or K) were found in each NLS and distributed differently in primary structure. The basic domains and flanking residues of NLS-A and NLS-B are 250YKRPCKRSFIRFI262 and 294LKDVRKRKLGPGH306, respectively. Further examination of these sequences revealed that NLS-A contains bulky aromatic amino acids (Y and F) which may diminish its capacity to act as a strong NLS and lacks the typical proline and glycine helix-breakers. However, NLS-B contains typical proline and glycine helix-breakers and the histidine residue at amino acid 306 is required for NLS activity. In addition, two hydrophobic regions within the DNase were found to inhibit the function of NLS-A but not NLS-B, suggesting that these two domains are different types of NLSs and differ in their sensitivity to hydrophobic regions in the context of protein structure.

Epstein-Barr viral gene expression in B-lymphocytes

The strategy of the Epstein-Barr virus to persist lifelong in the host depends on establishing a reservoir, which cannot be detected by the immune system but allows reactivation of the virus for shedding and transmission to a new host. Epithelial cells and B-cells play a major role in this viral strategy of EBV, since differentiating epithelial tissues were shown to be permissive for lytic replication in vivo, whereas the B-lymphocytes become predominantly latently infected. However, which cells are the reservoir and which the sites of lytic replication are not quite clear. With the technique of reverse transcription, PCR and immunohistochemistry, we demonstrated that the B-cells of the peripheral blood are a major site of virus production during the primary infection during infectious mononucleosis. These permissive B-cells were also detected after convalescence, however, the absence of any lytic transcripts suggested an efficient immunological control very early in the viral lytic cycle. Serological data on reactivation of EBV correlated with the detection of lytic cycle transcripts in the blood and thus demonstrated that the site of virus production during infectious mononucleosis must be different from that of the persistent state. In those cases, where the infection takes a chronic active course, control of lytic replication is insufficient, either on the level of immune surveillance or of viral gene regulation. We have demonstrated a virus strain with a lytic phenotype in an individual suffering chronic active infection. The impaired capability of this virus to immortalise B-cells correlated with an enhanced expression of the lytic switch gene BZLF-1 and down-regulation of latent regulatory genes in the early phase of infection.

Viruses and the cell cycle

Viruses depend on the host's machineries to replicate and express their genome. Actively replicating cells have large pools of deoxynucleotides and high levels of key enzyme activities that viruses exploit to their own needs. Some viruses have developed strategies for driving quiescent cells into the S phase of the cell cycle, e.g. adenovirus, others, such as parvovirus, wait until the host itself begins to replicate. Viruses may also force the host cell to stay in a favourable phase, e.g. Epstein-Barr virus, or, if necessary, they may inhibit apoptotic cell death, e.g. human cytomegalovirus. In this review, we focus on the different strategies that viruses use to create in infected cells an environment favourable to the accomplishment of the viral life cycle through acting on cell cycle regulators.

Oral lesions in organ transplant patients

Patients who have undergone organ transplantation can present with a variety of oral lesions that appear to be related either directly to their medication or arise as a consequence of drug-induced immunosuppression. Such lesions include hairy leukoplakia, an increased propensity to both fungal and viral infections and a high incidence of malignant change, especially lip cancer. Cyclosporin remains the immunosuppressant of choice in most transplant patients. Gingival overgrowth is the main unwanted oral effect associated with cyclosporin. Some 30% of dentate transplant experience this problem, which is further compounded by concomitant medication with a calcium channel blocker. This review appraises the various oral problems that can arise in this group of patients and emphasises the importance of regular oral screening and the establishment of links with the various transplant teams.

Lytic Replication of Epstein-Barr Virus in the Peripheral Blood: Analysis of Viral Gene Expression in B Lymphocytes During Infectious Mononucleosis and in the Normal Carrier State

Epstein-Barr virus (EBV) has been shown to establish latency in resting B lymphocytes of the peripheral blood. This creates a virus reservoir in contrast to lytic virus replication, which is thought to be restricted to differentiated epithelial cells in vivo. So far, the route of transmission between B cells and the production of progeny virus in the epithelial tissue has remained unclear. Reverse transcriptase–polymerase chain reaction (RT-PCR) and immunohistochemistry analysis of 16 patients with acute infectious mononucleosis (IM) and 25 healthy seropositive donors was performed to detect lytic replication gene products in B lymphocytes of the peripheral blood. Transcriptional activity was found in peripheral blood B lymphocytes (PBLs) for BZLF1 in 88%, BALF2 in 50%, and BcLF1 in 25% of the tested IM patients. All positive results were further confirmed in enriched B-cell populations by antigen determination using immunostaining with the APAAP technique. Furthermore, we detected transcripts for BZLF1 in 72% and for BALF2 in 16% of peripheral B lymphocytes of healthy seropositive donors. In contrast to patients with IM, no signals for BcLF1 were ever found in healthy seropositive donors. In these individuals, lytic replication of EBV is probably restricted by immunologic and gene regulatory mechanisms, whereas in the absence of immunologic control, reflected here by IM patients, the production of infectious virus becomes visible in PBLs.

Lytic Replication of Epstein-Barr Virus in the Peripheral Blood: Analysis of Viral Gene Expression in B Lymphocytes During Infectious Mononucleosis and in the Normal Carrier State

Epstein-Barr virus (EBV) has been shown to establish latency in resting B lymphocytes of the peripheral blood. This creates a virus reservoir in contrast to lytic virus replication, which is thought to be restricted to differentiated epithelial cells in vivo. So far, the route of transmission between B cells and the production of progeny virus in the epithelial tissue has remained unclear. Reverse transcriptase–polymerase chain reaction (RT-PCR) and immunohistochemistry analysis of 16 patients with acute infectious mononucleosis (IM) and 25 healthy seropositive donors was performed to detect lytic replication gene products in B lymphocytes of the peripheral blood. Transcriptional activity was found in peripheral blood B lymphocytes (PBLs) for BZLF1 in 88%, BALF2 in 50%, and BcLF1 in 25% of the tested IM patients. All positive results were further confirmed in enriched B-cell populations by antigen determination using immunostaining with the APAAP technique. Furthermore, we detected transcripts for BZLF1 in 72% and for BALF2 in 16% of peripheral B lymphocytes of healthy seropositive donors. In contrast to patients with IM, no signals for BcLF1 were ever found in healthy seropositive donors. In these individuals, lytic replication of EBV is probably restricted by immunologic and gene regulatory mechanisms, whereas in the absence of immunologic control, reflected here by IM patients, the production of infectious virus becomes visible in PBLs.

The Epstein-Barr virus bZIP transcription factor Zta causes G0/G1 cell cycle arrest through induction of cyclin-dependent kinase inhibitors

While oncoproteins encoded by small DNA tumor viruses and Epstein-Barr virus (EBV) latent antigens facilitate G1/S progression, the EBV lytic switch transactivator Zta was found to inhibit growth by causing cell cycle arrest in G0/G1 in several epithelial tumor cell lines. Expression of Zta results in induction of the tumor suppressor protein, p53, and the cyclin-dependent kinase inhibitors, p21 and p27, as well as accumulation of hypophosphorylated pRb. Up-regulation of p53 and p27 occurs by post-transcriptional mechanisms while expression of p21 is induced at the RNA level in a p53-dependent manner. Inactivation of pRb by transient overexpression of the human papillomavirus E7 oncoprotein indicates that pRb or pRb-related proteins are key mediators of the growth-inhibitory function of Zta. These findings suggest that EBV plays an active role in redirecting epithelial cell physiology to facilitate the viral replicative program through a Zta-mediated growth arrest function.

The ecology and pathology of Epstein-Barr virus

Epstein-Barr virus achieves its ubiquitous and uniform epidemiological distribution by a dual strategy of latency to guarantee lifelong persistence and intermittent replication to guarantee transmission. These two functions appear to dictate residence in different cell types: latency in B lymphocytes and replication in epithelial cells. Both of these cell compartments are potential sites for EBV-associated malignancies.

Specific cytotoxic T lymphocytes recognize the immediate-early transactivator Zta of Epstein-Barr virus

We identified the immediate-early transactivator Zta of Epstein-Barr virus as a target for specific cytotoxic T lymphocytes (CTL). Cells pulsed with overlapping synthetic peptides representing the entire amino acid sequence of Zta proved to be efficient for the in vitro stimulation of Zta-specific CTL in several donors. With peptide-pulsed target cells, we found that CTL from several donors recognize a peptide comprising 15 amino acids. The immune response against this peptide exerted by CTL lines from different donors was found to be restricted by two different molecules of the major histocompatibility complex: HLA-B8 and HLA-Cw6. The latter molecule could for the first time be identified as a restricting element for a CTL response. The epitope of the HLA-B8-restricted CTL could be mapped to an octameric sequence between amino acid positions 190 and 197 of the Zta protein, whereas the minimal epitope of HLA-Cw6-restricted CTL consists of 11 to 15 residues between positions 187 and 201. Thus, the HLA-B8 and HLA-Cw6 epitopes widely overlap but are not completely identical. In vitro stimulation of blood lymphocytes from a panel of HLA-B8-positive or HLA-Cw6-positive virus carriers, using autologous cells pulsed with the Zta peptides comprising the HLA-B8 or HLA-Cw6 epitope, respectively, revealed in both cases that most of these donors developed a Zta-specific cytotoxic activity. These data, as well as the high spread of the major histocompatibility complex molecules HLA-B8 and HLA-Cw6 in most populations, suggest that an efficient CTL response directed against gene products of the immediate-early group of the lytic cycle exists in vivo in a considerable portion of virus carriers. A CTL response against proteins expressed immediately after the switch into the lytic cycle could eliminate lytically activated cells at an early stage and would thus efficiently prevent the production and release of progeny virions.

Identification of cellular target genes of the Epstein-Barr virus transactivator Zta: activation of transforming growth factor beta igh3 (TGF-beta igh3) and TGF-beta 1

The lytic switch transactivator Zta initiates the ordered cascade of Epstein-Barr virus gene expression that culminates in virus production. Zta is a sequence-specific DNA-binding protein that transactivates early viral promotes via cis-acting sequences. Activation of some of these genes is mediated through binding to consensus AP-1 promoter elements. This observation suggests that Zta may also regulate the expression of cellular genes. While many targets of Zta have been identified in the Epstein-Barr virus genome, putative host cell targets remain largely unknown. To address this issue, a tetracycline-regulated Zta expression system was generated, and differential hybridization screening was used to isolate Zta-responsive cellular genes. The major target identified by this analysis is a gene encoding a fasciclin-like secreted factor, transforming growth factor beta igh3 (TGF-beta igh3), that was originally identified as a gene that is responsive to the potent immunosuppressor TGF-beta 1. Northern (RNA) blot analysis demonstrated that induction of Zta expression results in a 10-fold increase in TGF-beta igh3 mRNA levels. Zta was also found to increase TGF-beta 1 mRNA levels as well as the amount of active TGF-beta 1 secreted into the medium. Interestingly, alpha 1-collagen IV, which has been shown to potentiate the effects of TGF-beta 1, is also a cellular target of Zta. These results suggest that Zta could play a role in modulating the host cell environment through activating the expression of secreted factors.

Epstein-Barr virus lytic replication is controlled by posttranscriptional negative regulation of BZLF1

Regulation of the immediate-early gene BZLF1 is assumed to play a key role in triggering the lytic replication of Epstein-Barr virus (EBV). The expression of BZLF1 is regulated on multiple levels, including control of transcription by several positive and negative cis-acting elements as well as posttranslational modifications and protein-protein interactions. Localization of BZLF1 on one strand of the genome and the latent EBNA1 transcription unit on the complementary strand suggests a regulatory mechanism via hybridization of antisense RNA. With a plasmid encoding a defective BZLF1 RNA, which could not be translated, we were able to induce expression of endogenous BZLF1 gene product Zta and other proteins of the lytic cycle. Our data show for the first time that latent replication is stabilized by negative regulation of an immediate-early gene of the lytic cycle by a posttranscriptional mechanism. This might be a common theme of herpes simplex virus and EBV latency.

Detection of Epstein-Barr virus DNA in tongue tissues from AIDS autopsies without clinical evidence of oral hairy leukoplakia

Epstein-Barr virus (EBV) DNA was detected by in situ hybridization at 3 sites of 30 samples taken from clinically normal lateral border of tongue mucosa from 15 AIDS autopsies and in none of 20 samples from 10 controls. The first positive case showed a thin layer of parakeratosis correlated with positive signals for EBV in one area and an adjacent area without obvious parakeratosis was also positive for EBV. These findings were present on both sides of the tongue. The second case was unilaterally positive for EBV and parakeratosis was absent. The hybridization signals were localised to koilocyte-like cells in the stratum spinosum, as in oral hairy leukoplakia (OHL). These observations suggest that the in situ hybridization technique can detect very early or subclinical OHL, and supports the role of EBV in the pathogenesis of this lesion.

Hairy leucoplakia and HIV-2--a case report and review of the literature

Type 2 human immunodeficiency virus (HIV-2), originally confined to West Africa, has lately appeared with increasing frequency in Europe. Oral lesions affect a large proportion of patients with AIDS. Hairy leucoplakia (HL), a clinical expression of Epstein-Barr virus (EBV), is a lesion of the oral mucosa (usually the lateral margin of the tongue) that is observed in patients who are immunocompromised due to HIV or, more rarely, due to immunosuppressive medication or other causes. We review the definition, clinical signs, histopathology, diagnosis, prognosis and treatment of HL, and report the (to our knowledge) first detailed description of an HIV-2-seropositive patient with HL, which affected the lateral and dorsal tongue and was the first clinical sign of HIV infection (this patient was at that time only the second native Spaniard in whom unique HIV-2 infection had been detected). EBV detection and subtyping by in situ hybridization and polymerase chain reaction were performed in paraffin-embedded tissue from the HL lesion of the dorsal tongue; EBV-1 and EBV-2 were detected by the latter technique.

Different activation of Epstein-Barr virus immediate-early and early genes in Burkitt lymphoma cells and lymphoblastoid cell lines

Specific expression of the Epstein-Barr virus (EBV) immediate-early and early gene products Zta, Rta, I'ta, and MSta by a recombinant vaccinia virus system allowed us to analyze the first steps in the induction of the lytic cycle in EBV-infected Burkitt lymphoma (BL) cells and lymphoblastoid cell lines (LCLs). Significant differences in the induction of early genes were found between these cell types: whereas in BL cells the trans activator Zta was found to induce key steps of the early lytic cycle, only minor activities of Zta were noted in LCLs. Contrary to Zta, the trans activator Rta was found to be highly effective in LCLs. These observations suggest that Rta may play an important role in the activation of the early lytic cycle in LCLs, although it cannot be activated by Zta. The latter may be a reason for the lower tendency of LCLs to switch into the lytic cycle compared with BL cells or differentiated epithelial cells.

Epstein-Barr virus and carcinomas

The Epstein-Barr virus, a human B lymphotropic herpes virus, is strongly associated with undifferentiated nasopharyngeal carcinoma, which is endemic in Southern China and North Africa. More recently, an association of the virus with certain other epithelial malignancies has been described, some of which are more common in Western countries. Also, the observation that oral hairy leukoplakia, an acquired immunodeficiency syndrome-related lesion of the tongue, supports Epstein-Barr virus replication in the epithelial cell compartment has further strengthened the notion that Epstein-Barr virus infection of epithelial cells may be an important phenomenon in vivo. The purpose of this article is to review the association of Epstein-Barr virus with nasopharyngeal carcinomas, to examine the evidence suggesting an association of the virus with other epithelial neoplasias and to discuss Epstein-Barr virus infection of non-neoplastic epithelial cells.

Epstein-Barr virus in lingual epithelium of liver transplant patients

Thirty-three liver transplant patients and 20 controls were examined for the presence of EBV in epithelium of the lateral border of the tongue by negative staining electron microscopy. Five of the specimens (15%) showed particles typical of the herpes virus family, while all controls were negative for EBV. In 3 of the 5 patients moderate oral hairy leukoplakia was observed clinically. Our results indicate that EBV may be expressed at the lateral border of the tongue of liver transplant patients, who in some cases show oral hairy leukoplakia clinically.

Identification and molecular characterization of two diagnostically relevant marker proteins of the Epstein-Barr virus capsid antigen complex

The molecular specificity of the IgG response against Epstein-Barr virus (EBV) was studied in 345 randomly collected sera of normal healthy individuals. The sera were tested on immunoblots containing antigens of the cell line HH514.c16 (a superinducible derivate of P3HR1), noninduced or induced for the expression of early antigens (EA) or viral capsid antigens (VCA), and from the EBV-negative cell line Ramos-Nut. This study reveals a remarkable similar antigen recognition pattern of IgG class antibodies in sera of healthy EBV carriers. The protein bands recognized predominantly have molecular weights of 18 kD, 36/38 kD, 40 kD, 72 kD, and 160 kD. The 72 kD and 36/38 kD bands were identified as EBNA1 and "Zebra," respectively, using reading frame-specific antisera. The bands at 160 kD (major capsid protein), 40 kD, and 18 kD were identified as VCA-class proteins. Of all EBV-seropositive sera tested, 98% reacted with either p18 or p40 or both. The synthesis of the antigens p18 and p40 was inhibited by phosphonoacetic acid, indicating that these were true late proteins. The detection of p18 and p40 in purified virion and capsid preparations confirms that these proteins are structural components of viral capsid antigen complex.

The lytic transition of Epstein-Barr virus is imitated by recombinant B-cells

Lytic transition of Epstein-Barr virus (EBV) is initiated by distinct immediate early regulators of the viral cycle, in synchronization to temporary, permissive conditions during host cell differentiation. We developed eukaryotic vectors suitable to imitate the processes involved in lytic transition in cell culture systems. Two stable B cell lines were established: R59Z activator cells were used to induce lytic EBV expression in a constitutive manner by the production of the BZLF 1 trans-activator (Zta). R7-57 reporter cells, on the other hand, signaled induced activity of the lytic origin of EBV replication (ori Lyt). Different modes, like chemical induction, lytic superinfection with EBV and single gene trans-activation converted the recombinant ori Lyt element in R7-57 reporter cells. BZLF 1, transiently expressed in R7-57 reporter cells, was the only EBV trans-activator found, sufficient in inducing the viral lytic cycle. Basing on these experiments, trans-cellular activation of EBV was tested by cocultivation of BZLF 1-expressing R59Z activator cells with the R7-57 reporter line. No lytic effect on the reporter cells could be measured, neither by cocultivation of activator cells nor by coincubation of BZLF 1-containing cell lysates. Latency breaking activity, however, was transferred from activator to reporter cells when active, exogenous virus was added. The cell system described in these experiments provides a tool for the detection of EBV reactivation and demonstrates the potential of the lytic regulatory gene BZLF 1.

The DNA-binding domain of two bZIP transcription factors, the Epstein-Barr virus switch gene product EB1 and Jun, is a bipartite nuclear targeting sequence

The Epstein-Barr virus BZLF1 gene product EB1 (also called ZEBRA and Zta), is a transcription factor belonging to the bZIP (basic domain leucine zipper) family of nuclear proteins. Translocation to the nucleus of EB1 (J. Becker, U. Leser, M. Marschall, A. Langford, W. Jilg, H. Gelderblom, P. Reichart, and H. Wolf, Proc. Natl. Acad. Sci. USA 88:8332-8336, 1991) and of two other bZIP proteins, c-Jun and c-Fos (P. Roux, J.-M. Blanchard, A. Fernandez, N. Lamb, P. Jeanteur, and M. Piechaczyk, Cell 63:341-351, 1990), has been shown to be subject to regulation. We show here that for both EB1 and Jun the nuclear targeting signals (NTS) in the proteins' primary sequences are two clusters of positively charged amino acids. These clusters, called BRA and BRB, are necessary and sufficient to direct beta-galactosidase to the nuclear compartment and act as a bipartite NTS. They are conserved among all the bZIP proteins, and although they are not identical, they probably share the same function. Site-directed mutagenesis studies made on these basic clusters suggest that they also act as a bipartite NTS in the EB1 protein. Our results also demonstrate that in EB1 and Jun, these bipartite NTS are superimposed with bipartite DNA-binding domains, since BRA and BRB are required in vitro for direct and specific contact between these proteins and their DNA-binding sites.