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

Sites and roles of phosphorylation of the human cytomegalovirus DNA polymerase subunit UL44

The human cytomegalovirus DNA polymerase subunit UL44 is a phosphoprotein, but its sites and roles of phosphorylation have not been investigated. We compared sites of phosphorylation of UL44 in vitro by the viral protein kinase UL97 and cyclin-dependent kinase 1 with those in infected cells. Transient treatment of infected cells with a UL97 inhibitor greatly reduced labeling of two minor UL44 phosphopeptides. Viruses containing alanine substitutions of most UL44 residues that are phosphorylated in infected cells exhibited at most modest effects on viral DNA synthesis and yield. However, substitution of highly phosphorylated sites adjacent to the nuclear localization signal abolished viral replication. The results taken together are consistent with UL44 being phosphorylated directly by UL97 during infection, and a crucial role for phosphorylation-mediated nuclear localization of UL44 for viral replication, but lend little support to the widely held hypothesis that UL97-mediated phosphorylation of UL44 is crucial for viral DNA synthesis.

Accumulation of Epstein-Barr virus (EBV) BMRF1 protein EA-D during latent EBV activation of Burkitt's lymphoma cell line Raji

As a new model to elucidate molecular mechanisms in Epstein-Barr virus (EBV) activation, we tested the tetracycline-inducible (Tet-On)/BZLF1-oriP plasmid system in Raji cells. Cells transfected with this Tet-On plasmid did not activate EBV by doxycycline and surprisingly EBV latency was disrupted with large amounts of BMRF1 protein (EA-D) being accumulated in the cells. Brilliant EA-D fluorescence was markedly condensed in small sized cells, intra-cellular vesicles, and extra-cellular particles. Scanning electron microscopy demonstrated the extra-cellular particles to be covered with a membrane. EA-D molecules of 58, 50, 48, and 44kDa were expressed in the cells. The high (58 and 50kDa) and low (48 and 44kDa) EA-D molecules appeared in the early and late stages, respectively. Low EA-D molecules were detected mostly in EA-D positive cells separated into the heaviest density layer of a discontinuous Percoll gradient. Such molecules could be created from high EA-D molecules by protein phosphatase treatment. The EA-D molecules that appeared similar were detected in EBV-activated P3HR-1 and Akata cells. Several hypotheses concerning the accumulation of EA-D molecules of various polymorphic forms and their phosphorylation/dephosphorylation in this model system are presented, with possible biological and clinical relevance.

Phosphorylation of the Epstein-Barr virus (EBV) DNA polymerase processivity factor EA-D by the EBV-encoded protein kinase and effects of the L-riboside benzimidazole 1263W94

A member of the family of L-riboside benzimidazole compounds, 1263W94, was shown recently to inhibit replication of Epstein-Barr virus (EBV) (V. L. Zacny, E. Gershburg, M. G. Davis, K. K. Biron, and J. S. Pagano, J. Virol. 73:7271-7277, 1999). In the present report the effect of 1263W94 on the phosphorylation pattern of the EBV DNA polymerase processivity factor, EA-D, during viral reactivation in latently EBV-infected Akata cells is analyzed. This pattern specifically changes with progression of cytolytic infection. In the presence of 1263W94 the appearance of the hyperphosphorylated form of EA-D is mainly affected. Next, coexpression of the cloned EBV-encoded protein kinase (EBV PK), BGLF4, with EA-D demonstrated the ability of EBV PK to phosphorylate EA-D to its hyperphosphorylated form in transient assays. However, the phosphorylation of EA-D was not directly inhibited by 1263W94 in these coexpression assays. The results indicate that the EBV PK appears to be responsible for the hyperphosphorylation of EA-D, imply that the phosphorylation status of EA-D is important for viral replication, and suggest that 1263W94 acts at a level other than direct inhibition of EA-D phosphorylation by EBV PK.

Characterization of human herpesvirus 8 ORF59 protein (PF-8) and mapping of the processivity and viral DNA polymerase-interacting domains

Human herpesvirus 8 (HHV-8) or Kaposi's sarcoma-associated herpesvirus (KSHV) ORF59 protein (PF-8) is a processivity factor for HHV-8 DNA polymerase (Pol-8) and is homologous to processivity factors expressed by other herpesviruses, such as herpes simplex virus type 1 UL42 and Epstein-Barr virus BMRF1. The interaction of UL42 and BMRF1 with their corresponding DNA polymerases is essential for viral DNA replication and the subsequent production of infectious virus. Using HHV-8-specific monoclonal antibody 11D1, we have previously identified the cDNA encoding PF-8 and showed that it is an early-late gene product localized to HHV-8-infected cell nuclei (S. R. Chan, C. Bloomer, and B. Chandran, Virology 240:118-126, 1998). Here, we have further characterized PF-8. This viral protein was phosphorylated both in vitro and in vivo. PF-8 bound double-stranded DNA (dsDNA) and single-stranded DNA independent of DNA sequence; however, the affinity for dsDNA was approximately fivefold higher. In coimmunoprecipitation reactions, PF-8 also interacted with Pol-8. In in vitro processivity assays with excess poly(dA):oligo(dT) as a template, PF-8 stimulated the production of elongated DNA products by Pol-8 in a dose-dependent manner. Functional domains of PF-8 were determined using PF-8 truncation mutants. The carboxyl-terminal 95 amino acids (aa) of PF-8 were dispensable for all three functions of PF-8: enhancing processivity of Pol-8, binding dsDNA, and binding Pol-8. Residues 10 to 27 and 279 to 301 were identified as regions critical for the processivity function of PF-8. Interestingly, aa 10 to 27 were also essential for binding Pol-8, whereas aa 1 to 62 and aa 279 to 301 were involved in binding dsDNA, suggesting that the processivity function of PF-8 is correlated with both the Pol-8-binding and the dsDNA-binding activities of PF-8.

Epstein-Barr virus uses HLA class II as a cofactor for infection of B lymphocytes

Infection of B lymphocytes by Epstein-Barr virus (EBV) requires attachment of virus via binding of viral glycoprotein gp350 to CD21 on the cell surface. Penetration of the cell membrane additionally involves a complex of three glycoproteins, gH, gL, and gp42. Glycoprotein gp42 binds to HLA-DR. Interference with this interaction with a soluble form of gp42, with a monoclonal antibody (MAb) to gp42, or with a MAb to HLA-DR inhibited virus infection. It was not possible to superinfect cells that failed to express HLA-DR unless expression was restored by transfection or creation of hybrid cell lines with complementing deficiencies in expression of HLA class II. HLA class II molecules thus serve as cofactors for infection of human B cells.

Compact Epstein-Barr virus diagnosis based on a defined antigen mix and specific IgA

An ELISA was evaluated to measure EBV-specific IgM, IgG and IgA using a test plate coated with a controlled mix of EA, VCA and EBNA. Using the IgM version, the multi-reactive surface reduced false-negative findings in cases of infectious mononucleosis (sensitivity 98%), but was confronted with polyclonal IgM responses (specificity 85% to 95% depending on the population). For the IgG version, a national research standard was involved, and in combination with the IgA estimation, a new diagnostic strategy became possible for chronic EBV infections and reactivations. Patients screened by enhanced IgG (> 650 U/ml) were confirmed by an elevated IgA signal (> 0.6 A), with an efficacy of 95%. A raised virus-specific IgA response was not observed in cases of infectious mononucleosis, but was seen for healthy HIV-positive persons. Superior to the traditional antibody puzzle for EBV diagnosis, it will now become possible, using this compact diagnostic concept, to reduce laboratory expenses without loss of reliable results.

Bipartite DNA-binding region of the Epstein-Barr virus BMRF1 product essential for DNA polymerase accessory function

The Epstein-Barr virus (EBV) BMRF1 gene product is necessary for DNA polymerase catalytic subunit (BALF5) activity in 100 mM ammonium sulfate. To map regions of BMRF1 necessary for polymerase accessory function, linker insertion and deletion mutant BMRF1 polypeptides were expressed by in vitro transcription-translation and assayed for DNA polymerase elongation activity and binding to double-stranded DNA (dsDNA)-cellulose. Amino-terminal deletions up to residue 303 were defective for stimulation of elongation. Deletions between residues 44 and 194 and residues 238 and 303 abolished binding to dsDNA-cellulose. The region from residues 194 to 238, therefore, is necessary for stimulation of BALF5 elongation but dispensable for dsDNA-cellulose binding. Deletion analysis also localized reactive epitopes of two neutralizing monoclonal antibodies to BMRF1 to a carboxy-terminal region which is dispensable for activity. These data suggest that a bipartite DNA-binding region is an essential component of the DNA polymerase accessory function and that the two noncontiguous regions are separated by a region (residues 194 to 217) which is essential for stimulation; therefore, it may interact with the BALF5 catalytic subunit of EBV DNA polymerase. Both EBV BMRF1 and herpes simplex virus UL42 gene products are DNA polymerase accessory proteins which bind dsDNA and increase the processivity of their corresponding catalytic components. Outstanding similarities between their primary amino acid sequences are not evident. However, it appears that their structural organizations are similar.

Purification and characterization of the DNA-binding activity of the Epstein-Barr virus DNA polymerase accessory protein BMRF1 gene products, as expressed in insect cells by using the baculovirus system

A recombinant baculovirus containing the complete sequence for the Epstein-Barr virus (EBV) BMRF1 gene product, the EBV DNA polymerase accessory protein, under the control of the polyhedrin promoter was constructed. Insect cells infected with the recombinant virus produced two phosphoproteins of 52 and 50 kDa and one unphosphorylated protein of 48 kDa, recognized by anti-BMRF1 protein-specific monoclonal antibody. The major protein bands were 50 and 48 kDa. The expressed BMRF1 gene products were purified to near homogeneity from the nuclear extract of the recombinant baculovirus-infected insect cells by double-stranded DNA-cellulose column chromatography followed by heparin-agarose column chromatography. The purified BMRF1 gene products exhibited higher binding affinity for double-stranded DNA than for single-stranded DNA without ATP hydrolysis. The protein-DNA interaction did not necessarily require a primer terminus. The present system will open the way for the biochemical characterization of the EBV DNA polymerase accessory protein.

Expression of the Epstein-Barr virus latent membrane protein (LMP) in insect cells and detection of antibodies in human sera against this protein

Recombinant baculoviruses containing the complete LMP and truncated LMP genes were generated and high levels of the LMP proteins were expressed in Spadoptera Frugiperda insect cells. A specific rabbit antiserum directed against the N-terminal part of LMP was obtained by immunizing the rabbits with Escherichia coli-expressed trpE-N-terminal part of LMP fusion protein. A total of 127 human sera were studied for their immune response to the recombinant full-length LMP. In immunofluorescence analysis, all sera tested showed no detectable reaction with the recombinant full-length LMP. In immunoprecipitation-immunoblotting analysis, however, sera from patients with nasopharyngeal carcinoma (5/22), patients with Hodgkin's disease (16/27), patients with other diseases exhibiting high EA-IgG titers (3/52), and VCA-IgG-positive healthy individuals (2/26) were shown to contain antibodies against this recombinant LMP. The expressed LMP proteins provided a sufficient and economic source of the proteins for further serological and biological studies.

Immunological characterization of the Epstein-Barr virus phosphoprotein PP58 and deoxyribonuclease expressed in the baculovirus expression system

The open reading frames of the phosphoprotein pp58 (BMRFI) and the deoxyribonuclease (BGLF5) of the Epstein-Barr-virus (EBV) strain M-ABA were cloned in the baculovirus expression vectors pAc373 and pAc360 and expressed in the Spodoptera frugiperda (SF158) insect cells. The recombinant phosphoprotein pp58 expressed in SF158 cells was recognized by the anti-pp58 rabbit anti-sera which were generated by immunizing rabbits with a TrpE-BMRFI fusion protein expressed in E. coli. DNA-cellulose chromatography showed that the recombinant pp58 exhibited DNA-binding activities. Immunofluorescence, immunoblot and ELISA analysis indicated that sera from patients with nasopharyngeal carcinoma (NPC) contained antibodies against pp58. The recombinant EBV DNase expressed in SF158 cells was recognized by the anti-EBV DNase rabbit anti-sera which were generated by immunizing rabbits with a TrpE-C-terminal part of BGLF5 fusion protein expressed in E. coli. The anti-EBV DNase rabbit anti-sera recognized also a protein of about 52 kDa in the EBV-harboring human B-cell lines Raji, Jijoye, B95-8, M-ABA and BL74 induced by TPA and n-butyrate. The recombinant EBV DNase exhibited exonuclease and endonuclease activities, a requirement for magnesium, and a high pH optimum (8.0). Its enzyme activities could be inhibited by sera from NPC patients and anti-EBV DNase rabbit anti-sera. Comparable studies of Raji EBV-DNase and recombinant EBV-DNase implied that recombinant EBV-DNase could also be used in the enzyme activity assay for the detection of NPC. In contrast to the enzyme inhibition test, immunofluorescence and immunoblot analysis demonstrated that the recombinant EBV DNase exhibited only a weak immunological reaction with NPC sera.

Biochemical characterization of Epstein-Barr virus nuclear antigen 2A

The Epstein-Barr virus nuclear antigen 2A (EBNA-2A) was immunoprecipitated from latently Epstein-Barr virus-infected lymphocytes with a polyclonal serum raised against the EBNA-2A C terminus. The nucleus contained three subfractions of EBNA-2A which could be distinguished by their resistance to salt extraction: (i) a nucleoplasmatic fraction that was solubilized at 50 mM NaCl, (ii) a chromatin-associated fraction extractable at 1.5 M NaCl, and (iii) a nuclear matrix-associated fraction solubilized only by boiling with buffer containing 2% sodium dodecyl sulfate. The three subfractions were phosphorylated; it was demonstrated that the nucleoplasmatic and the chromatin-associated fractions were phosphorylated at serine and threonine residues. The half-life of the EBNA-2A protein was determined by cycloheximide treatment and by pulse-chase experiments and was found to be at least 24 h. The turnover of the phosphate residues bound to the two salt-soluble subfractions was determined to be approximately 6 to 9 h, suggesting a possible role of the phosphorylation in the regulation of the biological activity of EBNA-2A. Dephosphorylation of EBNA-2A resulted in an increased mobility of the protein during sodium dodecyl sulfate-polyacrylamide gel electrophoresis and indicated the presence of differentially phosphorylated subclasses of the protein. Analysis of EBNA-2A by sucrose gradient centrifugation revealed the existence of two subclasses of complexed molecules which exhibited sedimentation coefficients of approximately 13S and 34S.

Identification and characterization of an Epstein-Barr virus early antigen that is encoded by the NotI repeats

The Epstein-Barr virus (EBV) genome is characterized by two regions carrying partially homologous clusters of short tandem repeats (NotI and PstI repeats) flanked by 1,044 and 1,045 base pairs with almost perfect homology (DL and DR, left and right duplications, respectively). Both repetitive regions are transcribed into poly(A)+ mRNA after induction of the productive EBV cycle with the tumor promoter 12-O-tetradecanoylphorbol-13-acetate and contain open reading frames. To identify the potential protein encoded by the NotI repeat open reading frame (BHLF1), two repeat units of EBV strain M-ABA were expressed using the tryptophan-regulated Escherichia coli expression vector pATH11. Rabbit antisera generated against the resulting fusion protein reacted specifically with a protein varying in molecular size between 70,000 and 90,000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, found after 12-O-tetradecanoyl-phorbol-13-acetate or n-butyrate induction in various cell lines harboring EBV. In immunofluorescence tests with the BHLF1-specific antiserum, an immunofluorescence with EA-D specificity could be observed. In addition, the BHLF1 protein is exhibiting polyanion-binding activity with a maximum for single-stranded DNA. Furthermore, the fusion protein is recognized by a number of human EBV-positive sera.

Characterization of a latent protein encoded by the large internal repeats and the BamHI Y fragment of the Epstein-Barr virus (EBV) genome

Analysis of EBV nuclear antigen 1 (EBNA 1) encoding transcripts by cDNA characterization revealed a potentially polycistronic message generated by long-range splicing of several exons (Speck, S., and Strominger, J., Proc. Natl. Acad. Sci. USA 82, 8305-8309, 1985). Besides the open reading frame encoding EBNA 1, two other open reading frames are found in the EBNA 1-specific cDNA. The first reading frame consists of several exons from BamHI W and Y viral genome fragments (W1, W2, Y1, and Y2). In our experiments, the W1 exon was expressed in the tryptophan-regulated expression vector pATH11. Rabbit sera, raised against the bacterial fusion protein, recognized one or two proteins of molecular weights between 30,000 and 100,000 in several EBV genome harboring Burkitt lymphoma and EBV immortalized peripheral blood cell lines. Although, in a few cell lines from both groups no specific protein could be detected. Immunofluorescence analysis and characterization of subcellular distribution demonstrated that this W/Y fragment encoded latent protein is located, in part, in the cytoskeleton fraction, and in the chromatin. In addition, 2-D immunoblot analysis revealed post-translational modifications of this latent protein, probably due to phosphorylation. In DNA-binding studies on DNA cellulose columns, this W/Y encoded latent protein exhibited specific DNA binding activities.

Expression of an early Epstein-Barr virus antigen (EA-D) in E. coli. Brief report

The 1.34 kb BcII-BgIII-fragment of the BamHI-M region of Epstein-Barr virus genome, comprising the complete BMRF1 open reading frame, was cloned into the tryptophan regulated E. coli expression vector pATH1. The resulting fusion protein, having a molecular weight of 80 kd, is recognized not only by anti-early antigen (EA)-positive human sera but also by the monoclonal antibody R3 directed against the diffuse component of EA (EA-D). A possible use for this fusion protein as an indicator protein in diagnosis of IgA antibodies against EA-D is presented.

Characterization of a cDNA clone corresponding to a transcript from the Epstein-Barr virus BamHI M fragment: evidence for overlapping mRNAs

A 1.95-kilobase cDNA clone was isolated by screening a size-selected lambda gt10 cDNA library prepared from an Epstein-Barr virus-transformed B-cell line, IB4, with the Epstein-Barr virus BamHI M fragment. Sequence analysis revealed that this clone contains about 75% of the BMRF1 and the complete BMRF2 open reading frames. The transcript is not spliced, and the polyadenylation signal at base pair 2641 of the BamHI M fragment is used. Northern blots (RNA blots) indicate that this polyadenylation signal is used for three overlapping mRNAs. The sizes of these transcripts are 3.5, 2.6, and 1.5 kilobases.

Characterization of an Epstein-Barr virus nuclear antigen 2 variant (EBNA 2B) by specific sera

The Jijoye Epstein-Barr virus (EBV) strain is characterized by a substitution of 1.8 kb in the C-terminal part of EBNA 2 gene compared to B95-8 or M-ABA virus. Protein immunoblot analysis using human sera against EBNA 2 indicated that an immunological variant to the EBNA 2 of B95-8 (type A) is encoded by the Jijoye virus (type B). In order to generate a specific EBNA 2B antiserum the NaeI/NsiI DNA fragment of the Jijoye virus containing 237 bp of the C-terminus from the EBNA 2B gene was cloned in an E. coli expression vector (pME3). The resulting fusion protein contained 79 C-terminal amino acids of the viral protein and a 37,000 Da part of the bacterial anthranilate synthase. Rabbit antisera generated against this fusion protein reacted specifically with two proteins of 73,000 and 77,000 Da from Jijoye cells and three other cell lines carrying type B virus, while no proteins could be identified in the type B cell line BL 29. In addition, using these sera directed against the pME3 fusion protein, no reaction could be observed with the EBNA 2A protein from the B95-8 and several other cell lines containing type A virus.

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.