The replicator of the Epstein-Barr virus latent cycle origin of DNA replication, oriP, is composed of multiple functional elements.

Replication of the Epstein-Barr virus genome initiates at one of several sites in latently infected, dividing cells. One of these replication origins is close to the viral DNA maintenance element, and, together, this replication origin and the maintenance element are referred to as oriP. The replicator of oriP contains four binding sites for Epstein-Barr virus nuclear antigen 1 (EBNA-1), the sole viral protein required for the replication and maintenance of oriP plasmids. We showed previously that these EBNA-1 sites function in pairs and that mutational inactivation of one pair does not eliminate replicator function. In this study we characterized the contribution of each EBNA-1 site within the replicator and flanking sequences through the use of an internally controlled replication assay. We present evidence that shows that all four EBNA-1 sites are required for an oriP plasmid to be replicated in every cell cycle. Results from these experiments also show that the paired EBNA-1 binding sites are not functionally equivalent and that the low affinity of sites 2 and 3 compared to that of sites 1 and 4 is not essential for replicator function. Our results suggest that a host cell protein(s) binds sequences flanking the EBNA-1 sites and that interactions between EBNA-1 and this protein(s) are critical for replicator function. Finally, we present evidence that shows that the minimal replicator of oriP consists of EBNA-1 sites 3 and 4 and two copies of a 14-bp repeat that is present in inverse orientation flanking these EBNA-1 sites. EBNA-1 sites 1 and 2, together with an element(s) within nucleotides 9138 to 9516, are ancillary elements required for full replicator activity.

Human p32: a coactivator for Epstein-Barr virus nuclear antigen-1-mediated transcriptional activation and possible role in viral latent cycle DNA replication.

The Epstein-Barr virus (EBV) nuclear antigen-1 (EBNA-1) is required for the maintenance of the viral chromosome in latently infected, proliferating cells and plays a role in latent cycle DNA replication. EBNA-1 also functions as a positive and negative regulator of EBV gene expression. We have investigated the interaction of EBNA-1 with p32, a host mitochondrial protein that associates with EBNA-1 in EBV-positive Burkitt's lymphoma cells. Using a chromatin immunoprecipitation assay, we found that a fraction of p32 localizes to the viral latent cycle origin of DNA replication oriP in vivo. p32 binds EBNA-1 independently of other proteins or DNA. EBNA-1 variants lacking one of two p32 binding elements did not interact stably with p32 in cultured cells and were defective for both transcriptional activation of a reporter gene linked to oriP FR and replication and/or maintenance of a plasmid bearing oriP. These results support a role for p32 in transcriptional activation by EBNA-1 and suggest that p32 plays a role in EBV latent cycle DNA replication.

EBNA-1, the major nuclear antigen of Epstein-Barr virus, resembles 'RGG' RNA binding proteins.

Nuclear antigen 1 (EBNA-1) is one of the key functions of the oncogenic DNA virus, Epstein-Barr virus (EBV), and is the only viral protein consistently expressed in EBV-associated malignancies. EBNA-1 binds in a site-specific manner to the viral DNA and is essential for viral replication, as well as for maintaining the genome as an extrachromosomal episome within infected cells. EBNA-1 is not recognized by the cellular immune system. Here we demonstrate that, in addition to its known DNA binding properties, EBNA-1 can also act as a strong RNA binding protein, interacting with diverse substrates in vitro, including the EBV-encoded RNA polymerase III transcript EBER1 and the HIV-encoded transactivation response (TAR) element. We also show that EBNA-1 can bind exon sequences derived from its own RNA expressed from the Fp promoter, as found in Burkitt's lymphoma-related cells and in nasopharyngeal carcinomas. EBNA-1 has been identified as a component in an RNA complex; moreover, an anti-EBNA-1 antibody 1H4-1, that does not inhibit DNA binding, blocks binding to RNA. Arginine/glycine-containing (so-called 'RGG') motifs have been found in an increasing number of proteins that interact with RNA. The EBV antigen contains three potential 'RGG' motifs located around an internal glycine/alanine-rich repetitive sequence in the protein, and outside the region of EBNA-1 mapped previously as essential for viral DNA replication and other functionally defined properties. These motifs could be involved in the observed binding between EBNA-1 and RNA.(ABSTRACT TRUNCATED AT 250 WORDS)

Sequence requirements of the Epstein-Barr virus latent origin of DNA replication.

The Epstein-Barr virus (EBV) latent origin of DNA replication (oriP) is composed of two elements that contain binding sites for the sole viral gene product required for latent cycle replication, EBNA-1. One of these elements, region I, functions as an EBNA-1-dependent enhancer for RNA polymerase II-transcribed genes, may play a role in plasmid segregation, and is required for origin function in B cells latently infected with EBV. The second element, region II, contains or is very near the site of initiation of DNA replication. A genetic approach was taken to determine the contribution of the EBNA-1 binding sites in oriP to origin function. Although region I is required for the transient replication of plasmids bearing region II in EBV-infected B cells, a plasmid lacking region I but containing region II, was observed to replicate transiently in both D98/Raji and HeLa cells expressing EBNA-1. Thus, binding of EBNA-1 to region I is not absolutely required for the molecular events that lead to initiation of DNA replication at region II. Site-directed mutagenesis of the four EBNA-1-binding sites in region II, individually and in various combinations, demonstrated that only two EBNA-1-binding sites are required for region II function. The results obtained with these mutants, together with the analysis of the replicative ability of plasmids containing insertions between EBNA-1-binding sites, suggested that the spatial relationship of the two sites is critical. Mutants that contain only two EBNA-1-binding sites separated by 26 to 31 bp in region II were not maintained as plasmids over many cell generations and were greatly reduced in their ability to replicate transiently in D98/Raji cells. The EBNA-1-induced bending or untwisting of the DNA in EBNA-1-binding sites 1 and 4 in region II did not, however, demonstrate this spatial constraint. It may be concluded from these results that specific protein-protein interactions between EBNA-1 and/or between EBNA-1 and a cellular protein(s) are required for origin function.

Interaction of Epstein-Barr virus nuclear antigen 1 with the viral latent origin of replication.

The Epstein-Barr virus latent origin of replication (oriP) requires only one viral protein, the Epstein-Barr virus nuclear antigen 1 (EBNA-1), for activity. oriP consists of two spatially separated, essential sequence elements, regions I and II, both of which contain multiple EBNA-1-binding sites. Region II contains, or is close to, the site at which DNA synthesis initiates. The role of region I, a transcriptional enhancer in cells that express EBNA-1, in replication is not understood. To determine how the binding of EBNA-1 to sites in region II leads to the initiation of DNA synthesis and to investigate the role of region I, EBNA-1 has been overproduced in insect cells by using a baculovirus vector and purified to homogeneity, and the interaction of EBNA-1 with oriP has been examined. Footprinting experiments demonstrated that EBNA-1 binds to oriP in a sequence-specific manner and bends or untwists the DNA at two symmetry-related sites in region II. Distortion of region I by EBNA-1 was not detected, suggesting that differences in the spacing of binding sites in regions I and II and resulting protein-protein interactions underlie differences in their biological properties. KMnO4 footprinting experiments did not reveal significant single-stranded structures in region II, suggesting that cellular proteins may recognize the EBNA--region II complex and unwind the DNA duplex. Region I did not quantitatively or qualitatively alter the interaction of EBNA-1 with region II. The contribution of an A + T-rich sequence in region II to replication was investigated by a mutational analysis. The results indicated that the overall A + T-rich nature of this sequence is not essential for replication of oriP-bearing plasmids. Nuclease protection experiments performed with these mutagenized plasmids provided additional evidence for protein-protein interactions in region II.

Expression of Epstein-Barr virus nuclear antigen 2 in insect cells from a baculovirus vector.

The open reading frame encoding the Epstein-Barr virus nuclear antigen 2 (EBNA-2) has been expressed in a recombinant baculovirus vector. The resulting product migrates with the same apparent molecular weight as EBNA-2 from latently infected or converted B cell lines. Rabbit antisera derived from the innoculation of this material immunoprecipitated EBNA-2 from cell extracts of EBV-containing cells. The high level of protein expression obtained in insect cells stands in sharp contrast to that seen in a number of mammalian cell lines using a variety of promoters including the endogenous EBNA-2 promoter, the Moloney MuLV LTR, the murine immunoglobulin heavy chain promoter, the human cytomegalovirus immediate early promoter, and the adenovirus major late promoter.

Inhibition of specific binding of EBNA 1 to DNA by murine monoclonal and certain human polyclonal antibodies.

EBNA 1 was expressed as a nonfusion protein in Escherichia coli under control of the lac promoter. The major immunoreactive EBNA 1 proteins migrated as two doublets with molecular masses of about 39/41 and 49/51 kDa. Gel mobility shift experiments showed that these products exhibit the sequence-specific DNA binding on ori P previously characterized for a 28-kDa lambda N-fusion protein encompassing the carboxy third of the EBNA 1 protein. Three monoclonal antibodies previously found to react with EBNA 1 were shown to block binding of DNA by the EBNA 1 products expressed in bacteria. The same monoclonal antibodies also blocked specific DNA binding by EBNA 1 produced in Burkitt lymphoma cells infected by EB virus. Fab fragments of two monoclonal antibodies inhibited DNA binding by EBNA 1, indicating that the antibodies recognize an epitope of the protein that is involved in the recognition of DNA, or another domain crucial for DNA binding such as a dimerization site. Some but not all human antisera with antibody to EBNA 1 neutralized specific binding of EBNA 1 to DNA. These findings will help to map the residues of the EBNA 1 protein which are essential for specific binding of DNA.

Isolation of Chinese hamster ovary cell lines temperature conditional for the cell-surface expression of integral membrane glycoproteins.

A procedure is described to select mutants of Chinese hamster ovary cells that are conditionally defective for the cell-surface expression of integral membrane glycoproteins, including the hemagglutinin (HA) of influenza virus. Using a combination of cell sorting and biochemical screening, seven cell lines were obtained that express more cell-surface HA at 32 degrees C than at 39 degrees C. The production of infectious vesicular stomatitis virus, whose growth requires insertion of an integral membrane protein into the plasma membrane, was also temperature conditional in the majority of these mutant cell lines. Five of the lines synthesized apparently normally core-glycosylated HA at the elevated temperature but the protein was neither displayed on the cell surface nor accumulated intracellularly. In these cell lines, little or no terminally glycosylated HA molecules were observed after synthesis at 39 degrees C. By contrast, the core glycosylation of HA and several other integral membrane proteins was abnormal in the remaining two cell lines at both permissive and restrictive temperatures, due to a lesion in a cellular gene(s) that affects the formation of and/or the addition of mannose-rich oligosaccharide chains to newly synthesized polypeptides. Although HA was transported to the plasma membrane at both 32 and 39 degrees C, it did not accumulate on the cell surface at the higher temperature, apparently because of an increased rate of degradation.

Addition of truncated oligosaccharides to influenza virus hemagglutinin results in its temperature-conditional cell-surface expression.

In the preceding paper (Hearing, J., E. Hunter, L. Rodgers, M.-J. Gething, and J. Sambrook. 1989. J. Cell Biol. 108:339-353) we described the isolation and initial characterization of seven Chinese hamster ovary cell lines that are temperature conditional for the cell-surface expression of influenza virus hemagglutinin (HA) and other integral membrane glycoproteins. Two of these cell lines appeared to be defective for the synthesis and/or addition of mannose-rich oligosaccharide chains to nascent glycoproteins. In this paper we show that at both 32 and 39 degrees C in two mutant cell lines accumulate a truncated version, Man5GlcNAc2, of the normal lipid-linked precursor oligosaccharide, Glc3Man9GlcNAc2. This is possibly due to a defect in the synthesis of dolichol phosphate because in vitro assays indicate that the mutant cells are not deficient in mannosylphosphoryldolichol synthase at either temperature. A mixture of truncated and complete oligosaccharide chains was transferred to newly synthesized glycoproteins at both the permissive and restrictive temperatures. Both mutant cell lines exhibited altered sensitivity to cytotoxic plant lectins when grown at 32 degrees C, indicating that cellular glycoproteins bearing abnormal oligosaccharide chains were transported to the cell surface at the permissive temperature. Although glycosylation was defective at both 32 and 39 degrees C, the cell lines were temperature conditional for growth, suggesting that cellular glycoproteins were adversely affected by the glycosylation defect at the elevated temperature. The temperature-conditional expression of HA on the cell surface was shown to be due to impairment at 39 degrees C of the folding, trimerization, and stability of HA molecules containing truncated oligosaccharide chains.

The Epstein-Barr virus nuclear antigen (BamHI K antigen) is a single-stranded DNA binding phosphoprotein.

The Epstein-Barr virus BamHI K nuclear antigen was shown to be phosphorylated in latently infected and virus-producing B-cell lines by in vivo labeling of cell cultures with [32P]orthophosphate and immunoprecipitation with anti-BamHI K antigen monoclonal antibody. Phosphoamino acid analysis of this protein isolated from a latently infected cell line demonstrated that the modified amino acid is phosphoserine. The BamHI K nuclear antigen transiently expressed in NIH 3T3 cells is also phosphorylated, as well as three truncated and deleted forms of the protein. Interaction of the Epstein-Barr virus BamHI K nuclear antigen with denatured DNA was examined by chromatography of wild-type and mutant forms of this protein on single-stranded DNA cellulose columns. The wild-type protein bound to denatured DNA cellulose but not cellulose alone. The BamHI K antigen remained bound to single-stranded DNA in 300 mM NaCl and eluted from the DNA at higher NaCl concentration. Similar results were obtained with 32P-labeled protein and total antigen as assayed by radioimmunoelectrophoresis. A mutant protein that lacks the glycine and alanine repeated amino acid domain and surrounding amino acids of this EBV polypeptide retained the ability to bind to denatured DNA, although it eluted at slightly lower NaCl concentration. One mutant protein that lacks the carboxyl-terminal third of the protein failed to bind to single-stranded DNA.

Structure of the Epstein-Barr virus nuclear antigen as probed with monoclonal antibodies.

Five hybridoma cell lines secreting antibodies directed against an Epstein-Barr virus (EBV) nuclear antigen (BamHI K antigen) have been isolated. All five antibodies detect this antigen in a variety of EBV-positive lymphoblastoid cell lines. The reaction of these antibodies with several mutant forms of this protein has allowed the antibody binding site(s) to be predicted.

Identification of Epstein-Barr virus sequences that encode a nuclear antigen expressed in latently infected lymphocytes.

The Epstein-Barr virus (EBV) BamHI restriction endonuclease fragment K (B95-8 strain) was introduced into a polyoma virus expression vector and used to transfect murine NIH 3T3 cells. An EBV-associated nuclear antigen was detected in these cells in an indirect immunofluorescence test using anti-EBV nuclear antigen-positive human sera. These sera recognized a Mr 88,000 polypeptide in 3T3 cells transfected with the BamHI fragment K-containing polyoma virus plasmid by radioimmunoelectrophoresis. A Mr 88,000 polypeptide also was detected in a B-cell line latently infected with the B95-8 strain of EBV. Plasmids containing insertion and deletion mutations in BamHI fragment K directed the synthesis of truncated forms of the Mr 88,000 polypeptide in 3T3 cells. These data directly demonstrate that the polypeptide identified in EBV-infected lymphocyte lines by anti-EBV nuclear antigen-positive human sera is encoded by the viral genome.