Mechanisms of infection with Epstein-Barr virus. III. The synthesis of proteins in superinfected Raji cells

Characterization of the Epstein-Barr virus-induced early polypeptide complex p50/58 EA-D using rabbit antisera, a monoclonal antibody, and human antibodies

A polypeptide complex (p52) belonging to the D-subspecificity of the EBV-induced early antigens (EA-D) was purified from chemically induced P3HR-1 cells. Rabbit antisera raised against the isolated polypeptides reacted with components of EA-D as could be shown by indirect immunofluorescence and immunoperoxidase staining of IdU-induced EA positive Raji cells, ELISA, and immunoblotting. In one-dimensional immunoblots the rabbit antisera detected a predominant polypeptide complex of 52 kDa. Two-dimensional immunoblots prepared with proteins from IdU-induced Raji cells showed that the rabbit sera detect three series of polypeptides of 52 kDa (pl 8.5-6.2), 55-58 kDa (pl 6.2-4.5), and 48-50 kDa (pl 6.0-4.5). These three groups of polypeptides could also be identified by 50 high titered anti-EA-D positive human sera and a specific monoclonal antibody (R3) as being the main components of EA-D in Raji and B95-8 cells. All polypeptides of the p50/58 complex showed DNA binding properties either by themselves or by an interaction with other proteins. When TPA or IdU-induced Raji cells were labeled with 2Pi, two phosphorylated polypeptides pp50 and pp58 could be immunoprecipitated with the rabbit sera and a high anti-EA titered human serum. The time course of the synthesis of polypeptides associated with the EA-D complex was studied by 2-D immunoblots: EA polypeptides of 52 kDa appeared as early as 6 hr after the addition of IdU to Raji cells in culture, polypeptides of 55-58 and 48-50 kDa after 18 and 25 hr, respectively. The coordinated appearance of these groups of polypeptides and their similar size and reactivity with human sera and rabbit antisera produced against the isolated p52 as well as with a monoclonal antibody (R3) suggested that most of these polypeptides are derived from post-translational modifications of one or a few initially synthesized polypeptides, possibly p52. Phosphorylation seems to be at least one possibility of post-translational modification.

The regulated expression of Epstein-Barr virus: evidence that the transition from primary to secondary protein synthesis in Raji cells superinfected with Epstein-Barr virus requires the synthesis of new RNA

In the presence of canavanine and the absence of arginine, superinfected Raji cells synthesize a limited spectrum of proteins (primary proteins). After the removal of canavanine at 8 h post infection, the cells proceed after a lag phase of 2-3 h to synthesize a second group of proteins. The appearance of these proteins can be prevented by addition of actinomycin-D to the chase media, suggesting that active primary proteins are required for the synthesis of new mRNA coding for the secondary proteins.

Detection by monoclonal antibodies of an early membrane protein induced by Epstein-Barr virus

Two monoclonal antibodies, E8B3 and E8D2, were raised against Epstein-Barr virus (EBV)-producing cells and were shown to immunoprecipitate a protein with an approximate molecular weight of 105,000 (p105). The protein was detectable only in EBV-containing cells which were supporting the virus lytic cycle, and its synthesis increased after cells were induced with phorbol esters. The molecule was radiolabeled and immunoprecipitated from virus-producing cells that had been extrinsically labeled with 125I, and the antibodies E8B3 and E8D2 reacted in immunofluorescence assays with infected cells; the molecule was also associated with virion particles. Synthesis of p105 was not blocked by phosphonoacetic acid and could be induced in Raji cells by superinfection with virus derived from P3HR1 cells. These data support the conclusion that p105 is an EBV-specific early membrane protein.

Purification of a polypeptide complex (p52) belonging to the D-subspecificites of Epstein-Barr virus-induced early antigens

A two-dimensional immunoblot analysis of chemically induced EBV DNA carrying Burkitt's lymphoma cell lines shows besides a large number of minor components at least two major groups of polypeptides: the most prominent group of polypeptides is observed in the range of 48 to 58 kDa (pI 4.5 to 8.5) and another group at 38/36 kDa (pI 4.4). A polypeptide complex (p52) belonging to the Epstein-Barr virus (EBV)-induced early antigen complex (EA) has been isolated from IdU-induced Raji and B95-8 cells as well as from n-butyrate-induced P3HR-1 cells. The p52 polypeptides have been purified by chromatography on Blue-, DEAE-, CM-, and Phenyl-Sepharose. The purification of these components of the EA complex was monitored by ELISA and by two-dimensional immunoblots using a well-characterized high anti-EBV positive human serum. The isolated polypeptides have an apparent mol wt of about 52,000 Da as determined under nondenaturing conditions by gel filtration chromatography on Sephacryl S-300. One- and two-dimensional immunoblots show a major group of polypeptides of 52 kDa (pI 8.5 to 5.5) with EA activity and some minor components with smaller size up to 40 kDa. The latter seem to be generated by limited proteolysis of p52 polypeptides. The EA activity of the isolated polypeptides could be confirmed by their reaction with IgG anti-EA positive as well as IgA anti-EA positive sera by ELISA. The purified polypeptide complex did not react with anti-EA-D negative, anti-EA-R positive sera obtained from patients with African Burkitt's lymphoma, suggesting that these polypeptides belong to the EA-D complex. The monoclonal antibody R3 reacted with the isolated 52 kDa components of EA suggesting a common epitope present on these polypeptides, the same result was obtained with three rabbit sera produced against the isolated polypeptide complex.

Biochemical characterization of two Epstein-Barr virus early antigen-associated phosphopolypeptides

Epstein-Barr virus (EBV) nonproducer cells NC37 induced to viral early antigen (EA) synthesis by the tumor promotor 12-O-tetradecanoylphorbol-13-acetate (TPA) were labeled at Day 4 after induction with 32P, and were analyzed by immunoprecipitation with human EA-positive sera. By employing this method the appearance of two phosphopolypeptides of 50 and 58K (pp50 and pp58) was well correlated with EA complex. Partial V8 protease digestion and two-dimensional peptide analysis revealed that the polypeptides pp50 and pp58 are related. The analysis of phosphoamino acids indicated that pp58 contains phosphoserine and phosphothreonine to the same percentage, whereas in pp50 only phosphoserine was found. The analysis of the subcellular distribution revealed that pp50 and pp58 are located in the chromatin. Both phosphopolypeptides exhibit DNA-binding activity, and are recognized by two monoclonal antibodies described recently (R3 and 1108-1).

Induction of a deoxyuridine triphosphate nucleotidohydrolase activity in Epstein-Barr virus-infected cells

Superinfection of Raji cells with Epstein-Barr virus (EBV) or chemical induction of HR-1 cells with 12-O-tetradecanoyl-phorbol-13-acetate (TPA) results in the induction of a deoxyuridine triphosphate nucleotidohydrolase (dUTPase) which is not observed in mock-treated cells or TPA-treated EBV genome-negative BJAB cells. The EBV-induced dUTPase could be distinguished from the host dUTPase based upon differences in their migration in polyacrylamide gels and sensitivity to the 5-mercurithioguanosine derivitive of dUTP. The expression of the EBV-specified dUTPase is prevented by phosphonoacetic acid indicating that its expression is dependent upon EBV-DNA replication.

Qualitative and quantitative analyses of Epstein-Barr virus early antigen diffuse component by western blotting enzyme-linked immunosorbent assay with a monoclonal antibody

We report the use of monoclonal antibody against the early antigen diffuse component (anti-EA-D) of Epstein-Barr virus (EBV) to analyze, both qualitatively and quantitatively, the expression of EA-D in various human lymphoblastoid cell lines activated by chemical inducers. The kinetics of synthesis of EA-D in P3HR-1, B95-8, and Ramos/AW cells were similar in that they all reached the peak of synthesis on day 5 after induction. Surprisingly, no expression of EA-D was found in induced BJAB/GC, an EBV-genome-containing cell line. EBV-negative cell lines, BJAB and Ramos, were negative for EA-D. Raji cells had no detectable EA-D but responded rapidly to induction, reaching a peak on day 3. Superinfection of Raji cells also resulted in marked induction of EA-D, which reached a plateau between 8 to 12 h postinfection. Western blotting coupled with the enzyme-linked immunosorbent assay was employed to identify polypeptides representing EA-D. A family of four polypeptides with molecular weights of 46,000 (46K protein), 49,000, 52,000, and 55,000 were identified to be reactive with monoclonal anti-EA-D antiserum. The pattern of EA-D polypeptides expressed in each cell line was different. Of particular interest was the expression of a large quantity of 46K protein both in induced Raji and P3HR-1 cells, but not in superinfected Raji cells. A 49K doublet was expressed in activated p3HR-1, B95-8, and Ramos/AW cells and in superinfected Raji cells. In addition, two distinct 52K and 55K polypeptides were expressed in induced Ramos/AW and superinfected Raji cells. However, none of these EA-D polypeptides was detectable in BJAB/GC, BJAB, Ramos, and mock-infected Raji cells. To approximate relative concentrations of EA-D in cell extracts, we employed the enzyme-linked immunosorbent assay and immunoblot dot methods by using one of the purified EA-D components to construct a standard curve. Depending upon the cell lines, it was estimated that ca. 1 to 3% (determined by the enzyme-linked immunosorbent assay) and 0.8 to 1.6% (determined by immunoblot dot) of total proteins from maximally induced cells were EA-D. These results suggest that differential expression of EA-D polypeptides could be of importance in the diagnosis of state of EBV infection.

Identification of Epstein-Barr virus genes expressed during the early phase of virus replication and during lymphocyte immortalization

Transcription of the Epstein-Barr virus (EBV) genome in Raji cells superinfected with P3HR-1 EBV in the presence of cycloheximide was compared to transcription in human lymphocytes infected with transforming EBV (B95-8). This was done to identify regions of the EBV genome which contain genes that may mediate initiation of virus replication. Hybridization of 32P-labeled cDNA to cloned fragments of EBV DNA (dot blot hybridization) was employed to identify transcriptionally active regions of the viral genome in these cells. DNA in the BamHI A, F, H, and M restriction fragments was found to encode poly(A) RNA during the early phase of EBV replication. In the absence of cycloheximide the earliest detectable transcripts were transcribed from the BamHI M region. The most transcriptionally active region of the EBV genome in lymphocytes following infection with EBV (B95-8) was the BamHI W-Y-H-F region and, to a lesser extent, the K region. Transcription of the BamHI M region was not detected in these cells. The data suggest that expression of a gene or genes located in the BamHI M region of the EBV genome is an important event in the initiation of EBV replication, whereas expression of the genes in the BamHI W-Y-H-F and K regions may be important in the establishment of latency and cellular immortalization.

Immunobiochemical characterization with monoclonal antibodies of Epstein-Barr virus-associated early antigens in chemically induced cells

Five monoclonal antibodies which are reactive to early antigens of Epstein-Barr virus have been produced by using somatic cell hybridization techniques. The specificity of the monoclonal antibodies to early antigens was demonstrated by indirect immunofluorescence, which showed that the antigens were localized to the nucleus of early antigen-induced Raji cells. Additional indirect immunofluorescence studies showed that like patient antisera to diffuse-staining early antigen, the monoclonal antibodies gave positive staining reactions after methanol fixation. One of the antibodies, 1150-4, was positive by the anti-complement immunofluorescence technique but differed with Epstein-Barr virus-associated nuclear antigen-positive patient sera in that it only stained induced cells. Different fixation methods were found to alter dramatically the appearance of the nuclear staining reactions produced by the monoclonal antibodies. Immunoprecipitation and immunoblot experiments revealed that monoclonal antibodies 1108-1 and 1129-1 recognized two polypeptides of 55,000 and 50,000 daltons (p55;50), 1173-6 and 1180-2 recognized just p50, and 1150-4 identified a 65,000-dalton nuclear protein. Immunobiochemical characterization of these viral antigens showed that p55 is a phosphoprotein, and p55;50 has strong DNA-binding activity preferentially to single-stranded DNA. Elucidation of the role of these nuclear proteins in Epstein-Barr virus infection and the events associated with Epstein-Barr virus-directed lymphocyte transformation may provide significant information on the pathogenicity of this important human virus.

An immunoprecipitation blocking assay for the analysis of EBV induced antigens

A technique for the analysis of EBV antigens in extracts of unlabelled EBV infected cells has been developed. Using this technique we have demonstrated that the EBV early antigen complex consists of several proteins and is not completely expressed in chemically induced Raji cells. Studies with a range of sera from patients with infectious mononucleosis and nasopharyngeal carcinoma have shown that despite similar titers by the indirect fluorescent antibody test different populations of proteins were precipitated by different sera.

Purification of a protein (60K/58K) associated with the Epstein-Barr virus-induced early antigen complex in Raji cells

A double antibody sandwich ELISA has been established for the detection and quantitation of EBV-associated early antigens (EA) in IUdR-induced Raji cells. The EA complex extracted from Raji cells could be separated by ion exchange chromatography and isoelectric focusing into several components. One EA-associated subspecificity has been purified by DEAE-, CM-, and Blue-Sepharose chromatography followed by isoelectric focusing. The isolated protein has an apparent molecular weight of 240,000 +/- 20,000 daltons under non-dissociating conditions on Sephacryl S-300, an isoelectric point of 4.5, and seems to be composed of two polypeptides of 60,000 and 58,000 daltons as shown by SDS-gel electrophoresis and two-dimensional gel electrophoresis. Preliminary data indicate that the 58,000 polypeptide is generated by limited proteolysis of the 60,000 polypeptide. The EA activity of the isolated protein has been confirmed by the double antibody sandwich ELISA and its reactivity with anti-EA-positive sera in an ELISA for the detection of anti-EA antibodies.

Identification of polypeptide components of the Epstein-Barr virus early antigen complex with monoclonal antibodies

Three monoclonal antibodies were produced against the Epstein-Barr virus-induced early antigen complex. These antibodies were shown to be specific for the early antigen complex by the fact that they only reacted with cells supporting a permissive or abortive Epstein-Barr virus infection and their synthesis was not affected by inhibitors of viral DNA synthesis. One monoclonal antibody, designated R3, was directed against a diffuse component of the early antigen complex since it reacted by immunofluorescence with cells fixed in acetone or methanol. The other two monoclonal antibodies, designated K8 and K9, reacted with a methanol-sensitive restricted component of this complex. The appearance of the R3 antigen in P3HR-1 superinfected Raji cells occurred approximately 4 h earlier than the antigen detected by K8. By both sodium dodecyl sulfate-polyacrylamide gel electrophoresis and radioimmunoelectrophoresis, it was determined that the R3 monoclonal antibody recognized two major polypeptides with molecular weights of approximately 50,000 to 52,000, whereas K8 and K9 precipitated a protein of approximately 85,000. The R3 monoclonal antibody also immunoprecipitated an in vitro primary translation product. It was, therefore, possible to map this product to the Epstein-Barr virus DNA BamH1 M fragment. These in vitro products were slightly smaller than the in vivo proteins, suggesting that these proteins probably undergo posttranslational modification during the virus replication cycle.

p105, an Epstein-Barr virus-induced, phosphonoacetic acid-insensitive glycoprotein target of the anti-Epstein-Barr virus immune response

To describe structures and biological functions of targets for antibody-mediated immune responses to Epstein-Barr virus (EBV)-infected lymphoid cells, we have characterized a membrane-associated protein of 105,000 daltons, p105, which was prominently recognized in immunoprecipitates with some EBV antigen-reactive patients' sera. A rabbit antiserum to immunopurified p105 was developed. [35S]methionine-labeled p105 was specific to EBV-superinfected Raji cells, and its synthesis was not blocked with phosphonoacetic acid, indicating that it is an "early" viral antigen. Phosphonoacetic acid treatment of EBV-superinfected Raji cells was associated with the accumulation, mainly in the cytosol, of an 88,000-dalton protein, p88, which was also recognized with anti-p105 serum, but was not detected in superinfected cells which had not been treated with phosphonoacetic acid. Although anti-p105 serum immunoprecipitated a membrane fraction protein, it did not neutralize P3HR-1 virus and was not considered to be an exposed virion component. We conclude that p105 is an early, EBV-induced, membrane fraction antigen to which EBV-infected patients generate a substantial antibody response.

Epstein-Barr virus RNA. VIII. Viral RNA in permissively infected B95-8 cells

More than 50 RNAs expressed by Epstein-Barr virus late in productive infection have been identified. B95-8-infected cells were induced to a relatively high level of permissive infection with the tumor promotor 12-O-tetradecanoylphorbol-13-acetate. Polyadenylated RNAs were extracted from the cell cytoplasm, separated by size on formaldehyde gels, transferred to nitrocellulose, and hybridized to labeled recombinant Epstein-Barr virus DNA fragments. Comparison of RNAs from induced cultures with RNAs from induced cultures also treated with phosphonoacetic acid to inhibit viral DNA synthesis identifies two RNA classes: a persistent early class of RNAs whose abundance is relatively resistant to viral DNA synthesis inhibition and a late class of RNAs whose abundance is relatively sensitive to viral DNA synthesis inhibition. The persistent early and late RNAs are not clustered but are intermixed and scattered through most of segments UL and US. The cytoplasmic polyadenylated RNAs expressed during latent infection were not detected in productively infected cells, indicating that different classes of viral RNA are associated with latent and productive infection. Non-polyadenylated small RNAs originally identified in cells latently infected with Epstein-Barr virus are expressed in greater abundance in productively infected cells and are part of the early RNA class.

Filamentous structures associated with Epstein-Barr virus-infected cells

After the onset of Epstein-Barr virus DNA and protein synthesis 10 h after superinfection of Raji cells (a cell line containing Epstein-Barr virus DNA but not producing virus), filamentous structures 25 nm in diameter and 0.2 to 1.4 micrometers in length could be detected in the cell cytoplasm by electron microscopy. These structures banded in metrizamide gradients with viral DNA and proteins, but at a density different from that of virions or nucleocapsids. These filaments, enriched in a 155,000-dalton protein similar in size to a major nucleocapsid protein of Epstein-Barr virus, may represent intermediates in viral nucleocapsid assembly.

Effect of the arginine analog canavanine on the synthesis of Epstein-Barr virus-induced proteins in superinfected Raji cells

The addition of canavanine to cultures of superinfected Raji cells in the absence of arginine prevented the appearance of early antigens as defined by immunofluorescent staining. Addition of various amounts of arginine permitted the identification of at least three groups of proteins, each responding differently to the various concentrations of arginine-canavanine.

An Epstein--Barr virus early protein induces cell fusion

Superinfection of Raji cells with Epstein--Barr virus (EBV) leads to syncytium formation. Studies using metabolic inhibitors and amino acid analogues suggest that the fusion-inducing factor belongs to the early group of virus-specified proteins. Induction of early EBV protein synthesis in Raji cells by using various chemicals also leads to syncytium formation, indicating that the fusion process is not caused by a virion membrane protein introduced into the cells upon infection. The relevance of these findings to the association of EBV with carcinoma of the nasopharynx is discussed.

Epstein-Barr virus polypeptides: identification of early proteins and their synthesis and glycosylation

We have identified at least six early polypeptides induced by Epstein-Barr virus in cells or under conditions which are nonpermissive for Epstein-Barr virus DNA replication ranging in molecular weight from 140,000 to 26,000.

Epstein-Barr virus RNA. VI. Viral RNA in restringently and abortively infected Raji cells

Nuclear and polyadenylated RNA fractions of Raji cells are encoded by larger fractions of Epstein-Barr virus DNA (35 and 18%, respectively) than encode polyribosomal RNA (10%). Polyribosomal RNA is encoded by DNA mapping at 0.05 X 10(8) to 0.29 X 10(8), 0.63 X 10(8) to 0.66 X 10(8), and 1.10 X 10(8) to 0.03 X 10(8) daltons. An abundant, small (160-base), non-polyadenylated RNA encoded by EcoRI fragment J (0.05 X 10(8) to 0.07 X 10(8) daltons) is also present in the cytoplasm of Raji cells. After induction of early antigen in Raji cells, there was a substantial increase in the complexity of viral polyadenylated and polyribosomal RNAs. Thus, nuclear RNA was encoded by 40% of Epstein-Barr virus DNA, and polyadenylated and polyribosomal RNAs were encoded by at least 30% of Epstein-Barr virus DNA. Polyribosomal RNA from induced Raji cells was encoded by Epstein-Barr virus DNAs mapping at 0.05 X 10(8) to 0.29 X 10(8), 0.63 X 10(8) to 0.66 X 10(8), and 1.10 X 10(8) to 0.03 X 10(8) daltons and also by DNAs mapping within the long unique regions of Epstein-Barr virus DNA at 0.39 X 10(8) to 0.49 X 10(8), 0.51 X 10(8) to 0.59 X 10(8), 0.66 X 10(8) to 0.77 X 10(8), and 1.02 X 10(8) to 1.05 X 10(8) daltons.

Epstein-Barr virus polypeptides: effect of inhibition of viral DNA replication on their synthesis

After Epstein-Barr virus superinfection of the human lymphoblastoid cell line Raji, a Burkitt lymphoma-derived line that contains Epstein-Barr virus genomes in an episomal form, at least 40 polypeptides could be resolved by polyacrylamide gel electrophoresis. Eleven of the 40 polypeptides were immunoprecipitable by early antigen+/viral capsid antigen+ antiserum. The polypeptides could be divided into six classes, immediate-early, early, intermediate, late, very late, and persistent, depending upon the time of synthesis. Ten of the 40 polypeptides appeared to preexist before superinfection and persisted despite general cessation of host protein synthesis; none of the persistent proteins was immunoprecipitated by the Epstein-Barr virus antibody-containing serum. When viral DNA replication was blocked by a variety of inhibitors of DNA synthesis, a number of different patterns of polypeptide synthesis could be detected. The synthesis of six polypeptides was blocked by the most virus-specific inhibitors, acyclovir and phosphonoacetic acid. Additionally, in the presence of 1-beta-D-arabinofuranosylcytosine, 1-beta-D-arabinofuranosyladenine, and methotrexate, seven polypeptides showed oversynthesis.

Epstein--Barr virus-induced cell fusion

Serological and molecular biological studies have shown an association between Epstein--Barr virus (EBV) and nasopharyngeal carcinoma. Although it has been shown that the epithelioid tumour cells carry EBV genomes, they are apparently devoid of receptors for EBV (H.W., unpublished observations). Other have suggested that fusion of EBV carrying cells with epithelial cells may be the mode of entry of the virus into cells unable to absorb the virus and that this may be mediated by one of the known syncytium-forming viruses which inhabit the respiratory tract (for example, members of the paramyxovirus group). de Thé and colleagues suggested that intercellular bridges could be seen in NPC tumour material. We have developed a technique which permits the preparation of stable monolayers of viable human lymphoblastoid cell lines. Using this technique we have now demonstrated that EBV can induce fusion between EBV-superinfected lymphoblastoid cells and cells devoid of EBV receptors.

Polypeptide synthesis and phosphorylation in Epstein-Barr virus-infected cells

Epstein-Barr virus superinfection of the human lymphoblastoid cell line Raji, a Burkitt lymphoma-derived line that contains Epstein-Barr virus genomes in an episomal form, results in the sequential synthesis of 29 detectable proteins, which range in molecular weight from approximately 155,000 to 21,000, and in the shutoff of the bulk of host protein synthesis within 6 to 9 h after infection. There are three classes of virus-induced proteins; these are an early class, consisting of eight proteins synthesized by 6 h postinfection, an intermediate class, containing two proteins synthesized 9 h postinfection, and a late class, consisting of five proteins synthesized 12 h postinfection. In addition, there is a fourth class of polypeptides, called persistent, that are found both before and after superinfection. The rates of synthesis of the proteins fall into three patterns; these are pattern A, in which the rate of synthesis decreases, pattern B, in which the rate of synthesis remains steady, and pattern C, in which the rate of synthesis increases after the initial appearance of the polypeptide. Both 9-(2-hydroxy-ethoxymethyl)guanine (acyclovir) and phosphonoacetic acid inhibit the appearance of one intermediate protein and at least three late proteins. Seven polypeptides are phosphorylated at different times after infection.