[Latency vs lytic cycle of Epstein-Barr virus: regulation of a viral gene expression is the clue].
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B-Lymphocytes↗
Biomedical subjects
H Gruffat
Publications and source records attributed to H Gruffat.
MEF2-mediated recruitment of class II HDAC at the EBV immediate early gene BZLF1 links latency and chromatin remodeling.
In B lymphocytes induced to proliferate in vitro by the Epstein-Barr virus (EBV), extra-chromosomal viral episomes packaged in chromatin persist in the nucleus, and there is no productive cycle. A switch from this latency to the productive cycle is observed after induced expression of the EBV BZLF1 gene product, the transcription factor EB1. We present evidence that, during latency, proteins of the myocyte enhancer binding factor 2 (MEF2) family are bound to the BZLF1 promoter and recruit class II histone deacetylases. Furthermore, we propose that latency is determined primarily by a specific and local recruitment of class II histone deacetylase (HDAC) by MEF2D to the BZLF1 gene promoter. The switch from latency to the productive cycle could be due in part to post-translational modification of MEF2 proteins and changes in the local acetylation state of the chromatin.
B-Lymphocytes↗
Ubinuclein, a novel nuclear protein interacting with cellular and viral transcription factors.
The major target tissues for Epstein-Barr virus (EBV) infection are B lymphocytes and epithelial cells of the oropharyngeal zone. The product of the EBV BZLF1 early gene, EB1, a member of the basic leucine-zipper family of transcription factors, interacts with both viral and cellular promoters and transcription factors, modulating the reactivation of latent EBV infection. Here, we characterize a novel cellular protein interacting with the basic domains of EB1 and c-Jun, and competing of their binding to the AP1 consensus site. The transcript is present in a wide variety of human adult, fetal, and tumor tissues, and the protein is detected in the nuclei throughout the human epidermis and as either grainy or punctuate nuclear staining in the cultured keratinocytes. The overexpression of tagged cDNA constructs in keratinocytes revealed that the NH(2) terminus is essential for the nuclear localization, while the central domain is responsible for the interaction with EB1 and for the phenotype of transfected keratinocytes similar to terminal differentiation. The gene was identified in tail-to-tail orientation with the periplakin gene (PPL) in human chromosome 16p13.3 and in a syntenic region in mouse chromosome 16. We designated this novel ubiquitously expressed nuclear protein as ubinuclein and the corresponding gene as UBN1.
Adult↗
Kaposi's sarcoma-associated herpesvirus and Kaposi's sarcoma.
Kaposi's sarcoma-associated herpesvirus (KSHV) is present in all epidemiologic forms of Kaposi's sarcoma (KS). The KSHV genome contains several open reading frames which are potentially implicated in the development of KS. Some are unique to KSHV; others are homologous to cellular genes. The putative role of these genes in the genesis of KS is discussed.
Cell Transformation, Viral↗
Characterization of the epstein-barr virus BRRF1 gene, located between early genes BZLF1 and BRLF1.
The switch from latency to a productive cycle in Epstein-Barr virus (EBV)-infected B cells proliferating in vitro is thought to be due to the transcriptional activation of two viral genes, BZLF1 and BRLF1, encoding two transcription factors called EB1 and R respectively. However, a third gene, BRRF1 is contained in the BZLF1/BRLF1 locus, overlapping with BRLF1 but in inverse orientation. We have characterized the 5' end of the BRRF1 mRNA and the promoter, PNa, at which BRRF1 pre-mRNA is initiated. We show that although a single BRRF1 mRNA species is induced by 12-O-tetradecanoylphorbol 13-acetate/sodium butyrate in several EBV-infected B cell lines, in Akata cells treated with anti-IgG two BRRF1 mRNAs can be detected. Transcription initiated at the BRRF1 promoter was activated by EB1 but not by R, and EB1-binding sites which contribute to the EB1-activated transcription have been mapped to between positions -469 and +1. A 34 kDa protein could be translated from the BRRF1 mRNA both in vitro and in vivo, and was found predominantly in the nucleus of HeLa cells transfected with a BRRF1 expression vector. Thus there are three promoters in the region of the EBV chromatin containing the BZLF1/BRLF1 genes, two of which, PZ and PNa, potentially share regulatory elements.
Animals↗
Kaposi's sarcoma-associated herpesvirus (human herpesvirus-8) encodes a homologue of the Epstein-Barr virus bZip protein EB1.
Analysis of the recently completed genomic sequence of Kaposi's sarcoma-associated herpesvirus (human herpesvirus-8) revealed that ORF 50 encodes a protein with homology to the Epstein-Barr virus (EBV) transcription factor R. In this report, we show that ORF K8, contiguous to ORF 50, is interrupted by two introns and that the spliced RNA is translated into a bZip protein that has homology to the EBV transcription factor EB1. The newly characterized K8 protein forms homodimers but does not heterodimerize with other members of the bZip protein family.
Amino Acid Sequence↗
A particular DNA structure is required for the function of a cis-acting component of the Epstein-Barr virus OriLyt origin of replication.
OriLyt, thecis-acting element of Epstein-Barr virus lytic origin of replication, consists of upstream and downstream components. The upstream component plays a dual role in transcription and replication. The downstream component contains a homopurine-homopyrimidine sequence which forms an H palindrome. We show that the downstream component can adopt a triple helix structure in vitro, that the 5' border of the homopyrimidine sequence is sensitive to P1 nuclease when carried by a supercoiled plasmid and that an oligonucleotide complementary to the homopyrimidine strand is taken up by a plasmid carrying the OriLyt H palindrome. We also show that all mutations which alter the H palindrome impair both oligonucleotide uptake and OriLyt-dependent replication. Interestingly, compensatory mutations which restore an H palindrome also restore oligonucleotide uptake by the mutated plasmids and their OriLyt-dependent replication. Thus, there is a strong correlation between the inability of the OriLyt H palindrome to form a non-B-DNA structure in vitro and impairment of OriLyt-dependent replication. This suggests that the presence of a non-B-DNA structure in the OriLyt downstream component is required for OriLyt-dependent replication.
Base Sequence↗
Repression by RAZ of Epstein-Barr virus bZIP transcription factor EB1 is dimerization independent.
The hallmark of Epstein-Barr virus (EBV) infection is the establishment of a viral genome transcription pattern called latency. The EBV BZLF1 gene product EB1 (also known as ZEBRA or Zta) is a transcription factor which is essential for the switch from latency to the lytic cycle. It has been proposed that latency is maintained (i) by the inhibition of EB1 translation via antisense hybridization of EBNA1 and EB1 hnRNAs, or (ii) by the inactivation of the EB1 activating function via the direct interaction of EB1 with RelA, the retinoic acid receptor and p53, or via the titration of EB1 in RAZ:EB1 inactive heterodimers that are unable to bind to DNA. RAZ, a fusion protein which contains the EB1 C-terminal dimerization and DNA-binding domains fused to the N-terminal 86 amino acids of the EBV BRLF1 gene product R, has been described as a trans-dominant negative regulator of EB1-activated transcription. We demonstrate here that although RAZ efficiently represses EB1-mediated transcriptional activation, the amount of RAZ protein expressed is incompatible with repression through the titration of EB1 in inactive EB1:RAZ heterodimers. Furthermore, we also demonstrate that RAZ efficiently represses transcription activated by an EB1 mutant carrying the GCN4 homodimerization domain (EB1 gcn4), despite the inability of RAZ and EB1 gcn4 to form stable heterodimers.
Animals↗
Cellular proteins bind to the downstream component of the lytic origin of DNA replication of Epstein-Barr virus.
The lytic origin of DNA replication of Epstein-Barr virus, oriLyt, is a complex eukaryotic origin which is activated during the lytic phase of the viral life cycle. It consists of at least two independent cis-acting components, one of which plays a dual role in transcription and DNA replication. The binding of the viral factor BZLF1, a member of the AP1 family of transcription factors, to this upstream component is crucial for oriLyt function (A. Schepers, D. Pich, and W. Hammerschmidt, EMBO J. 12:3921-3929, 1993). The second cis-acting element, the downstream component of oriLyt, is equally indispensable; however, its function is unknown. In this study, the downstream component was found to be the binding target of several cellular proteins. One could be identified as Sp1 or as a related protein which binds twice to the downstream component of oriLyt. Mutational analysis indicated that Sp1 alone is not directly involved in mediating DNA replication; however, other factors which share the same binding sequence or bind closely to one of the Sp1 binding sites are likely candidates to contribute to a replication protein complex at the downstream component of oriLyt. The sequence requirements for the downstream component are remarkably stringent, indicating that at least one of the putative factors is a sequence-specific DNA-binding protein which is required for the activation of oriLyt.
Base Sequence↗
Characterization of the DNA-binding site repertoire for the Epstein-Barr virus transcription factor R.
The Epstein-Barr virus gene BRLF1 encodes the transcription factor R, which is a sequence-specific DNA-binding protein important for the switch from latency to a productive cycle. We have defined a repertoire of specific R-binding sites using a GST-R fusion protein and a pool of 23 bp random DNA sequences. The R-bound sequences were selected by several rounds of Electrophoretic Mobility Shift Assay (EMSA) and amplification by PCR. Among the 45 sites selected, some positions in the sequences were highly conserved, i.e., 5'-GTGCC N7GTGGTG-3'. The guanine methylation assay revealed that R simultaneously contacts guanines in the two conserved cores, defining the consensus binding site 5'-GNCC N9 GGNG-3', and 30 sites among the 45 selected have this sequence. This last result also suggests that R binds two adjacent major grooves of the DNA. As shown by EMSA assay, R binds to all the sites tested with a comparable affinity, and they all mediate R-induced transcriptional activation in a transient expression assay.
Base Sequence↗
Mechanisms of collagen trimer formation. Construction and expression of a recombinant minigene in HeLa cells reveals a direct effect of prolyl hydroxylation on chain assembly of type XII collagen.
Collagen types IX, XII, and XIV are characterized by the presence of a highly conserved region comprising the most C-terminal triple helical domain (COL1, approximately 100 residues/chain) and 2 cysteines separated by 4 amino acid residues at the junction between this COL1 domain and the C-terminal non-triple helical domain (NC1). In order to better understand the functions of this conserved domain, we have constructed a recombinant minigene, comprising the sequence coding for an unrelated signal peptide and for the COL1 and NC1 domains of type XII collagen. This construct was placed under the control of the cytomegalovirus promoter and transfected into HeLa cells. The cells expressed the transfected minigene and the secreted chain, called alpha 1 (mini XII), could be detected by immunotransfer with an anti-peptide antibody recognizing an epitope found in the NC1 domain. Under conditions preventing the hydroxylation of prolyl residues (absence of ascorbate or presence of alpha alpha'-dipyridyl), interchain disulfide bridges did not form, while in the presence of ascorbate, disulfide-bonded (alpha 1 (mini XII))3 molecules were secreted. The collagenous nature and triple helical conformation of the trimeric molecule were ascertained by the differential resistances of the COL1 and NC1 domains to trypsin and collagenase digestions, respectively. Our data demonstrate that the NC1 and COL1 domains of type XII collagen contain the information necessary for trimer formation and that, contrary to the fibrillar collagen types, posttranslational modification of the triple helical domain is essential for assembly and disulfide bonding of the chains.
Animals↗
The acidic activation domain of the Epstein-Barr virus transcription factor R interacts in vitro with both TBP and TFIIB and is cell-specifically potentiated by a proline-rich region.
In cells latently infected with Epstein-Barr virus (EBV), the expression of two viral transactivators, EB1 and R, is responsible for the switch from latency to a productive cycle. R contains a DNA-binding/dimerization domain localized at the N-terminus. The domain required for transcriptional activation is localized at the C-terminus and contains two regions of very different amino acid composition. The first is very rich in prolines, whereas the second is rich in acidic residues and contains two potential alpha-helices. We investigated the activation potential of these subregions when linked to the heterologous Gal4 DNA-binding domain. We found that the acidic region--more precisely, the second putative alpha-helix--is an activating domain. In contrast, the proline-rich region is insufficient by itself for activation but collaborates with the acidic region in a cell-specific manner to make transactivation more efficient. We demonstrated that R interacts in vitro with the basal transcription factors TBP and TFIIB, and that the acidic domain of R mediates these interactions.
Base Sequence↗
Characterization of an R-binding site mediating the R-induced activation of the Epstein-Barr virus BMLF1 promoter.
In cells latently infected with Epstein-Barr virus, the switch from latency to productive infection is linked to the expression of two Epstein-Barr virus transcription factors called EB1 and R. R is an enhancer factor, and an R-responsive element (RRE) has been identified in the BMLF1 promoter. In this study, we have used bidirectional deletion mutagenesis to delineate the BMLF1 RRE (RRE-M) to a 44-bp sequence. We also show that R expressed from a recombinant vaccinia virus protects RRE-M against digestion by DNase I. Using mobility shift assays and dimethyl sulfate interferences, we have characterized the contact points between in vitro-translated R and the DNA. R binds in vitro to one site by simultaneously contacting two sequences within the site, which are separated by 8 bp: 5'-catGTCCCtctatcatGGCGCagac-3'. Site-directed mutagenesis of this sequence completely impaired the binding of R in vitro and rendered the BMLF1 promoter nonresponsive to R. The results suggest that the R-inducible BMLF1 enhancer is composed of a single R-binding site, called RRE-M.
Binding Sites↗
Domains of the Epstein-Barr virus (EBV) transcription factor R required for dimerization, DNA binding and activation.
In cells latently infected with EBV, the switch from latency to a productive infection is linked to the expression of two transcriptional activators, the upstream element factor EB1 and the enhancer factor R. R activates by interacting directly with specific DNA sequences called RREs (R Responsive Elements). Each binding site covers about 18 bp, where R simultaneously contacts two core sequences separated by 5 to 7 bp (1). Here we show that R binds in vitro as a homodimer to an RRE, and that stable homodimers can also form in solution in the absence of DNA. By functional analysis of deletion and insertion mutants of R, we have localized the DNA binding region within the 280 N-terminal amino acids and the dimerization region within the 232 N-terminal amino acids. As no obvious homologies were detected with other known DNA binding or dimerization motifs, R could contain novel protein structures mediating these functions. The transcriptional activation domain has been located in the C-terminal half of the protein. This domain contains two regions with structures already identified in other transcription factors: one region is rich in proline, the other rich in acidic residues.
Base Sequence↗
The enhancer factor R of Epstein-Barr virus (EBV) is a sequence-specific DNA binding protein.
In cells latently infected with EBV, the switch from latency to productive infection is linked to the expression of two EBV transcription factors called EB1 (or Z) and R. EB1 is an upstream element factor which has partial homology to the AP1/ATF family, whereas R is an enhancer factor. In the R-responsive enhancer of the replication origin only active during the EBV lytic cycle (ORIIyt), R-responsive elements are located in a region of about 70 bp (RRE-DR). Here we show that R, produced either by in vitro translation, or present in nuclear extracts from HeLa cells constitutively producing R, binds directly to and protects against DNAase I digestion, two regions in RRE-DR. Using mobility shift assay and DMS interference, we have characterized the contact-points between R and the DNA. Two binding sites, RRE-DR1 and RRE-DR2, were characterized and are contiguous in RRE-DR. R binds to these two sites probably by simultaneously contacting two sequences within the sites, which are separated by 7 bp in RRE-DR1, cctGTGCCttgtcccGTGGACaatgtccc, and by 6bp in RRE-DR2, caatGTCCCtccagcGTGGTGgctg. Direct interaction of R with its cognate sequences is conferred by its N-terminal 355 amino-acids. Directed mutagenesis in RRE-DR, of either R-binding site, impaired binding of R in vitro and, as assayed by transient expression in HeLa cells, impaired R-activation by a factor of two. This suggests that RRE-DR1 and RRE-DR2 do not respond cooperatively to R.
Base Sequence↗
The Epstein-Barr virus (EBV) ORI1yt enhancer is not B-cell specific and does not respond synergistically to the EBV transcription factors R and Z.
The Epstein-Barr virus DR promoter is located upstream of the PstI repeats, and in addition to the TATA box, it contains an upstream region (positions -69 to -220) responsive to EB1 (Z) (the BZLF1-encoded transcription factor) and an enhancer with two functionally distinct domains, A and B. Domain B has been described as a B-cell-specific EB1-responsive element (P. M. Lieberman, J. M. Hardwick, and S. D. Hayward, J. Virol. 63:3040-3050, 1989) activated synergistically by EB1 and R, an EBV early product encoded by the open reading frame BRLF1 (M. A. Cox, J. Leahy, and J. M. Hardwick, J. Virol. 64:313-321, 1990). We show here that domain B is an R-responsive element in HeLa cells and is therefore not an EB1-responsive B-cell-specific element. However, there is an EB1-binding site (ZRE-B) located within the R-responsive enhancer region. ZRE-B can be deleted without affecting the R-dependent enhancer activity. Moreover, there is no cooperation or synergy between R and EB1 when activating the B domain (ZRE-B plus the R-responsive element) positioned as an enhancer. ZRE-B is therefore not part of the R-inducible enhancer. We have tested several subregions of the DR enhancer B domain, either alone or in combination, for their capacity to transmit the R-activating signal to the rabbit beta-globin promoter. We found that the R-responsive element is composed of four protoenhancers that span the whole B domain. These protoenhancers alone are weakly or not responsive to R. One of the protoenhancers contains the overlapping palindromes 5'-TTGTCCcgtGGACAAaTGTCC-3'. However, one palindrome, either alone or duplicated, or the overlapping palindromes did not respond to R.
B-Lymphocytes↗
Epstein-Barr virus bicistronic mRNAs generated by facultative splicing code for two transcriptional trans-activators.
The Epstein-Barr virus (EBV) genome codes for several transcriptional trans-activators. One of them, the BZLF1 open reading frame (ORF)-encoded product EB1, is able to induce the productive cycle in infected B cells. From the cloning and characterization of full-length cDNAs, we found that EB1 could be made from three overlapping messenger RNAs expressed under the control of two different promoters that we call P1 and P2. The first mRNA, 1 kb long, is made from the P1 promoter and codes for EB1 alone. The two other mRNAs, respectively 3 and 4 kb long and made by facultative splicing, are bicistronic mRNAs. They code not only for the trans-activator EB1 but also for a second EBV transcriptional trans-activator R, encoded by the BRLF1 ORF. In effect, authentic EB1 and R proteins are expressed from the 3 and 4 kb long cDNAs as demonstrated by identification of the proteins with specific antisera. In addition, EB1 and R expressed from the 3 and 4 kb cDNAs activate transcription from their specific targets in the EBV early promoter DR.
Cell Line, Transformed↗
The Epstein-Barr virus (EBV) early protein EB2 is a posttranscriptional activator expressed under the control of EBV transcription factors EB1 and R.
From the cloning and characterization of cDNAs, we found that the Epstein-Barr virus (EBV) open reading frame (ORF) BMLF1-BSLF2 coding for the early protein EB2 is present in several mRNAs generated by alternative splicing and expressed in the leftward direction from two promoters PM and PM1. The PM promoter controls the expression of two abundant mRNA species of 1.9 and 2 kilobases (kb), whereas the PM1 promoter controls the expression of at least three mRNAs 3.6, 4.0, and 4.4 kb long. The PM promoter probably overlaps with the PS promoter which controls the transcription of a 3.6-kb mRNA expressed in the rightward direction and containing the ORF BSRF1. Although it increases the amount of chloramphenicol acetyltransferase enzyme expressed from the chimeric pMCAT gene, EB2 is not a promiscuous trans-activator of gene expression and does not positively regulate its own expression from promoter PM. The EB2 activation is not promoter dependent but could possibly act by stabilizing mRNAs and increasing their translation. The PM promoter is, however, activated by the two EBV transcription trans-acting factors, EB1 and R, encoded by the EBV ORFs BZLF1 and BRLF1, respectively. EB1 activates the PM promoter from a consensus AP-1 binding site, and R activates the PM promoter from an enhancer.
Base Sequence↗