Characterization of the restricted component of Epstein-Barr virus early antigens as a cytoplasmic filamentous protein

Direct Evidence of Abortive Lytic Infection-Mediated Establishment of Epstein-Barr Virus Latency During B-Cell Infection

Viral infection induces dynamic changes in transcriptional profiles. Virus-induced and antiviral responses are intertwined during the infection. Epstein-Barr virus (EBV) is a human gammaherpesvirus that provides a model of herpesvirus latency. To measure the transcriptome changes during the establishment of EBV latency, we infected EBV-negative Akata cells with EBV-EGFP and performed transcriptome sequencing (RNA-seq) at 0, 2, 4, 7, 10, and 14 days after infection. We found transient downregulation of mitotic division-related genes, reflecting reprogramming of cell growth by EBV, and a burst of viral lytic gene expression in the early phase of infection. Experimental and mathematical investigations demonstrate that infectious virions were not produced in the pre-latent phase, suggesting the presence of an abortive lytic infection. Fate mapping using recombinant EBV provided direct evidence that the abortive lytic infection in the pre-latent phase converges to latent infection during EBV infection of B-cells, shedding light on novel roles of viral lytic gene(s) in establishing latency. Furthermore, we find that the BZLF1 protein, which is a key regulator of reactivation, was dispensable for abortive lytic infection in the pre-latent phase, suggesting the divergent regulation of viral gene expressions from a productive lytic infection.

Molecular diversity of IgG responses to Epstein-Barr virus proteins in asymptomatic Epstein-Barr virus carriers

The Epstein-Barr virus (EBV) is a ubiquitous pathogen that infects over 90 % of adults. EBV is the primary etiological agent of infectious mononucleosis and is closely associated with nasopharyngeal carcinoma, gastric carcinoma, Hodgkin lymphoma and Burkitt lymphoma. Clinical serological assays for EBV diagnosis only survey a small portion of the viral proteome, which does not represent the total antigenic breadth presented to the immune system during viral infection. In this study, we have generated an expression library containing the majority of EBV ORFs, and have systematically evaluated IgG responses to those EBV proteins in sera from EBV carriers. In addition to confirming previously recognized dominant EBV antigens, this study has identified additional immunodominant antigens, and has revealed a more expansive antigenic profile of the humoral responses to EBV in asymptomatic carriers. This EBV expression library will be deposited in a public repository with the goal of disseminating this new research tool for the application of identifying potential new biomarkers for EBV-associated diseases.

Epstein-Barr Virus-Specific Humoral Immune Responses in Health and Disease

Epstein-Barr virus (EBV) is widely distributed in the world and associated with a still increasing number of acute, chronic, malignant and autoimmune disease syndromes. Humoral immune responses to EBV have been studied for diagnostic, pathogenic and protective (vaccine) purposes. These studies use a range of methodologies, from cell-based immunofluorescence testing to antibody-diversity analysis using immunoblot and epitope analysis using recombinant or synthetic peptide-scanning. First, the individual EBV antigen complexes (VCA , MA, EA(D), EA(R) and EBNA) are defined at cellular and molecular levels, providing a historic overview. The characteristic antibody responses to these complexes in health and disease are described, and differences are highlighted by clinical examples. Options for EBV vaccination are briefly addressed. For a selected number of immunodominant proteins, in particular EBNA1, the interaction with human antibodies is further detailed at the epitope level, revealing interesting insights for structure, function and immunological aspects, not considered previously. Humoral immune responses against EBV-encoded tumour antigens LMP1, LMP2 and BARF1 are addressed, which provide novel options for targeted immunotherapy. Finally, some considerations on EBV-linked autoimmune diseases are given, and mechanisms of antigen mimicry are briefly discussed. Further analysis of humoral immune responses against EBV in health and disease in carefully selected patient cohorts will open new options for understanding pathogenesis of individual EBV-linked diseases and developing targeted diagnostic and therapeutic approaches.

Epstein-Barr virus early antigen diffuse (EBV-EA/D)-directed immunoglobulin A antibodies in systemic lupus erythematosus patients

OBJECTIVE

We sought to determine whether the serological response towards lytic cycle antigens of Epstein-Barr virus (EBV) is altered in systemic lupus erythematosus (SLE) patients.

METHOD

We used enzyme-linked immunosorbent assay (ELISA) to investigate the prevalence of EBV early antigen diffuse (EBV-EA/D) antibodies in sera from 60 patients with SLE, 40 with scleroderma (SSc), 20 with primary Sjögren's syndrome (pSS), 20 with rheumatoid arthritis (RA), 20 healthy controls, and also subjects with various circulating autoantibodies. Samples from patients were obtained from clinics specialized within the diseases in Denmark and Sweden and samples from healthy controls were obtained from volunteers.

RESULTS

A significant elevated titre of immunoglobulin (Ig)A, IgG, and IgM EBV-EA/D antibodies was found in SLE patients compared to healthy controls, a finding not explained by immunosuppressive treatment or disease activity. The largest difference was observed for IgA EBV-EA/D antibodies (p = 0.0013) with a seropositive rate of 58% in SLE patients and 0% in healthy controls. RA and SSc patients and individuals seropositive for anti-Scl-70 were additionally found to have elevated titres of IgA EBV-EA/D antibodies (40%, p = 0.014; 60%, p = 0.015; and 38.5%, p = 0.045, respectively). However, the titres were generally lower than in SLE patients.

CONCLUSION

Our findings support an association between EBV and SLE. The elevated titre of EBV-EA/D-directed IgA antibodies found in SLE patients could suggest reactivation of EBV in epithelial cells or reinfection of epithelial cells after reactivation in B cells, indicating lack of control of the latent infection.

Lytic but not latent replication of epstein-barr virus is associated with PML and induces sequential release of nuclear domain 10 proteins

Nuclear domains called ND10 (nuclear domain 10) are discrete nuclear protein aggregations characterized by a set of interferon-upregulated proteins including Sp100 and PML, where papova-, adeno-, and herpesviruses begin their transcription and DNA replication. Both the alpha- and betaherpesvirus subfamilies disrupt ND10 upon infection by dispersing and/or destroying ND10-associated proteins. We studied the effect of the gammaherpesvirus Epstein-Barr virus (EBV) on ND10 and its spatial distribution in the nucleus of cells during latency and lytic reactivation. In latently infected Burkitt's lymphoma, lymphoblastoid, and D98/HR1 cells, ND10 were intact, as judged by immunofluorescence localization of PML, Sp100, NDP55, and Daxx. Fluorescent in situ hybridization revealed no association between viral episomes and ND10 during latency, implying that the maintenance replication of EBV, which depends on host cell proliferation, occurs independent of ND10. As in mitosis, the EBV genomes were attached to interphase chromosomes, suggesting that they are unable to move freely within the interchromosomal space and thus unable to associate with the interchromosomally located ND10 or other nuclear domains. Upon lytic activation, ND10 became dispersed in cells expressing lytic proteins. Redistribution of ND10 proteins occurred sequentially at different stages of the lytic cycle, with Sp100, Daxx, and NDP55 dispersed before and PML dispersed after the onset of lytic replication. ND10 remnants were retained until the early stages of lytic replication, and replicating EBV genomes were frequently found beside this nuclear domain; the number of replication domains was usually lower than the average latent virus frequency. Thus, latency does not require or induce interaction of EBV with ND10 for transcription and replication, whereas lytic replication triggers dispersion of ND10 proteins and occurs in close association with PML aggregates. The required movement of chromosome-attached latent EBV episomes to ND10 after reactivation from latency might include physical release of the chromosome-bound episomes. Only episomes contacting ND10 after such a release might be able to begin lytic replication.

Detection of antitumor promoting activity in Raji cells carrying Epstein-Barr virus genome by immunoblotting analysis

Extract of Raji cells treated with sodium n-butyrate (1 mM) and a tumor promoter, 12-O-tetradecanoylphorbol-13-acetate (TPA, 40 ng/ml), was analyzed by immunoblotting using ten human sera with different antibody titers against Epstein-Barr virus early antigens. Two human sera reacted with one induced polypeptide of 48 kDa and its induction was inhibited by curcumin (4 micrograms/ml), an antitumor promoter from turmeric. A mouse antiserum against P3HR-1 cells treated with TPA and sodium n-butyrate also detected the 48-kDa polypeptide in Raji cells treated with TPA at concentrations of 2.5 to 80 ng/ml. These results indicate that the immunoblotting analysis can be used in a confirmation test for detection of antitumor promoting activity.

Radioimmunoprecipitation in the diagnosis of chronic active Epstein-Barr virus infection

Chronic active Epstein-Barr virus (EBV) infection occurs sporadically in a small fraction of individuals infected with EBV. A clear definition of the disease and an unambiguous diagnostic test are still lacking. In an attempt to identify a serologic marker to facilitate the diagnosis, immunoblot and radioimmunoprecipitation assay (RIPA) were compared with standard immunofluorescence on 39 available sera. Results by RIPA revealed that antibodies to a 120 kDa viral protein correlated with the presence of chronic active EBV infection; these antibodies were not detected in sera from other EBV-seropositive individuals, with the exception of one of two patients with ataxia telangiectasia. Also, RIPA was the most sensitive technique for detecting EBV antibodies in sera weakly or doubtfully positive for antibody to EB viral capsid antigen by indirect immunofluorescence. All these sera had antibodies to the 150 kDa protein, also known as p160, the major viral capsid antigen.

Activation of the Epstein-Barr virus replicative cycle by human herpesvirus 6

One common attribute of herpesviruses is the ability to establish latent, life-long infections. The role of virus-virus interaction in viral reactivation between or among herpesviruses has not been studied. Preliminary experiments in our laboratory had indicated that infection of Epstein-Barr virus (EBV) genome-positive human lymphoid cell lines with human herpesvirus 6 (HHV-6) results in EBV reactivation in these cells. To further our knowledge of this complex phenomenon, we investigated the effect of HHV-6 infection on expression of the viral lytic cycle proteins of EBV. Our results indicate that HHV-6 upregulates, by up to 10-fold, expression of the immediate-early Zebra antigen and the diffuse and restricted (85 kDa) early antigens (EA-D and EA-R, respectively) in both EBV producer and nonproducer cell lines (i.e., P3HR1, Akata, and Raji). Maximal EA-D induction was observed at 72 h post-HHV-6 infection. Furthermore, expression of late EBV gene products, namely, the viral capsid antigen (125 kDa) and viral membrane glycoprotein gp350, was also increased in EBV producer cells (P3HR1 and Akata) following infection by HHV-6. By using dual-color membrane immunofluorescence, it was found that most of the cells expressing viral membrane glycoprotein gp350 were also positive for HHV-6 antigens, suggesting a direct effect of HHV-6 replication on induction of the EBV replicative cycle. No expression of late EBV antigens was observed in Raji cells following infection by HHV-6, implying a lack of functional complementation between the deleted form of EBV found in Raji cells and the superinfecting HHV-6. The susceptibility of the cell lines to infection by HHV-6 correlated with increased expression of various EBV proteins in that B95-8 cells, which are not susceptible to HHV-6 infection, did not show an increase in expression of EBV antigens following treatment with HHV-6. Moreover, UV light-irradiated or heat-inactivated HHV-6 had no upregulating effect on the Zebra antigen or EA-D in Raji cells, indicating that infectious virus is required for the observed effects of HHV-6 on these EBV products. These results show that HHV-6, another lymphotropic human herpesvirus, can activate EBV replication and may thus contribute to the pathogenesis of EBV-associated diseases.

Detection of two early gene products of Epstein-Barr virus by fluorescence flow cytometry

The intracellular expression of two early gene products of Epstein-Barr virus (EBV) was evaluated by fluorescence flow cytometry. Two Burkitt's lymphoma cell lines were superinfected by EBV from the P3HR-1 cell line. Twelve to 96 hr after superinfection the cells were fixed with paraformaldehyde and made permeable by saponin treatment. Monoclonal antibodies to the diffuse (D) and restricted (R) components of the early antigen (EA) complex were applied to the cells followed by fluorochrome-conjugated goat antibodies specific for the heavy chain isotypes of the monoclonal antibodies. Fluorescence flow cytometry revealed a clear separation in fluorescence intensity between cells containing EA-R or EA-D and negative cells. A major proportion of EA-positive cells displayed both antigens. In addition, a significant fraction expressed either EA-R or EA-D but not both. Expression of EA occurred more rapidly and peaked earlier in Daudi cells than in Raji cells. This apparent difference in EA expression between the two cell lines, however, was much less pronounced when one considered the proportion of cells expressing EA-R only, EA-D only, or both EA-R and EA-D, as a percentage of the total EA expression. Parallel fluorescence microscopy experiments revealed that the variation of the ratio of EA-R to EA-D expression with time correlated well between the two methods.

Identification of a novel EBV-induced membrane glycoprotein of 43 kDa with H667 MAb

Using purified B95-8 Epstein-Barr virus (EBV), a MAb designated H667 was produced. We demonstrated by indirect membrane immunofluorescence (IF) on six EBV producer cell lines and by immunoelectron microscopy that H667 reacted with a membrane antigen. H667 recognized a 43-kDa EBV protein (p43) as determined by immunoblotting using purified EBV from the six producer cell lines. Phosphonoacetic acid treatment of B95-8 cells was associated with the disappearance of p43, indicating that it was a late antigen. This antigen was shown to be a glycoprotein by incorporation of [14C]glucosamine and was shown to contain an N-asparagine-linked glycosyl group by its sensitivity to tunicamycin. It was named gp43. The H667 MAb inhibited B95-8 EBV cord blood lymphocyte transformation only when a low inoculum was used but failed to inhibit EA induction in Raji cells by P3HR1 EBV. Human sera reactivity against the gp43 antigen was studied. By the immunoblotting method, using H667 immunoaffinity chromatography-purified gp43, we showed that 70.9% of the human sera tested had antibodies directed against gp43. By IF blocking tests, we found that only 12.5% of the sera tested were reactive, indicating that the epitope corresponding to the H667 MAb was not the most immunogenic gp43 epitope.

Up regulation of the Epstein-Barr virus (EBV)-encoded membrane protein LMP in the Burkitt's lymphoma line Daudi after exposure to n-butyrate and after EBV superinfection

The Burkitt's lymphoma line Daudi carries a nontransforming Epstein-Barr virus (EBV) strain that has a deletion in the BamHI WYH region of the genome coding for the EBV nuclear antigen 2 (EBNA-2). Daudi cells fail to express the EBV-encoded latent membrane protein (LMP) (D. Ghosh and E. Kieff, J. Virol. 64:1855-1858, 1990). We show that LMP expression can be up regulated by exposure to n-butyrate and by superinfection with the B95-8 (B virus)- and P3HR1 (P virus)-derived EBV strains. Two LMP polypeptides of 60 and 48 kilodaltons (kDa) were detected in immunoblots of Daudi cells that had been exposed to 3 mM n-butyrate for 24 h. The intensity of the 48-kDa LMP increased during 72 h, in parallel with the appearance of early antigen-positive cells. The 60-kDa LMP was expressed at a low level and remained constant. Superinfection of Daudi cells with B and P virus induced the 60-kDa LMP within 3 h. In addition, P virus induced the 48-kDa LMP at a low level. The B virus-encoded EBNA-2 and EBNA-5 were detected 12 h after superinfection. The B virus-encoded 63-kDa LMP was coexpressed with the endogenous LMP after 48 h. Inactivation of the virus by UV illumination abolished the expression of the B virus-encoded antigens but did not affect the induction of the endogenous LMP. The B-cell activation marker CD23 was up regulated by B virus superinfection but not by n-butyrate exposure. CD23 was also expressed at a higher level in a stable B virus-converted subline, E95A-Daudi, that was EBNA-2 positive and coexpressed the Daudi virus- and B virus-encoded LMP. The results suggest that LMP expression is regulated by the interaction of cellular and viral factors. Binding of the virus to its membrane receptor might be involved in the triggering of cellular control mechanisms. Viral gene products are not directly involved in this function but may contribute to create a permissive cellular environment for LMP expression.

High frequency of Epstein-Barr virus genome detection in Hodgkin's disease of HIV-positive patients

Lymph nodes obtained from 7 HIV-positive and 20 HIV-negative patients with Hodgkin's disease were examined for the presence of Epstein-Barr virus antigens and genome. EBV antigens were observed in only 2 out of 20 HIV-negative patients, whereas lymph nodes of HIV-positive patients did not reveal evidence of EBV antigens. By in situ hybridization and Southern blot analysis, EBV genome was found in 5 out of 7 HIV-positive patients; the EBV genome was detected in the nucleus of Reed-Sternberg and Hodgkin's cells. EBV DNA was observed by in situ hybridization and Southern blot analysis in only 3 out of 20 HIV-negative patients with Hodgkin's disease. In both groups, Reed-Sternberg and Hodgkin's cells were negative for C3d EBV receptor. Our results show a statistically significant increased expression of EBV DNA in HIV-positive patients with Hodgkin's disease, as compared with HIV-negative patients with HD.

Differences in the extent of activation of Epstein-Barr virus replicative gene expression among four nonproducer cell lines stably transformed by oriP/BZLF1 plasmids

Lymphoid cell lines were established which stably carry the Epstein-Barr viral (EBV) BZLF1 gene on an extrachromosomal plasmid. These lines, which spontaneously synthesize the BZLF1 gene product, ZEBRA, were examined for expression of EBV genes which were activated by ZEBRA. Cell lines which acquired oriP plasmids without BZLF1 served as controls. The extent of activation differed among derivatives of four cell lines. X50-7 cells, which harbor a standard latent EBV, could be induced by ZEBRA to produce transforming virus; a cellular subclone of this line was induced to express EBV late antigens but did not release transforming virus. In two other cell lines, Raji and ER, ZEBRA activated only a group of early antigens. Using immunofluorescence and immunoblotting with monoclonal antibodies and Northern analysis five EBV early genes were shown to be induced in cells stably transformed by oriP/BZLF1 plasmids. ZEBRA itself was activated; thus BZLF1 is autostimulatory. Four other activated genes were components of the diffuse (EA-D) and restricted (EA-R) early antigens (BMRF1, BMLF1, BHRF1, and BORF2). Stable cell lines with extrachromosomal BZLF1 expression vectors will ultimately be useful in a variety of experiments designed to study regulation of this gene, to analyze the effects of mutations on ZEBRA protein function, and to define the full spectrum of viral and cellular genes which are activated by and interact with the ZEBRA protein.

Antibodies against the large subunit of the EBV-encoded ribonucleotide reductase in patients with nasopharyngeal carcinoma

The open reading frame corresponding to BORF2 and encoding the large subunit of the Epstein-Barr virus (EBV) ribonucleotide reductase has been inserted into the prokaryotic expression vector pUC19. A 90-kDa protein was produced when the intact plasmid was used as a template for in vitro DNA-directed protein synthesis. Using templates generated by restriction digests within the BORF2 open reading frame, in the same system, truncated polypeptides resulted confirming the identity of the 90-kDa protein. The protein was then produced in a heterologous expression system and used in protein immunoblotting to screen for antibodies in sera from nasopharyngeal carcinoma (NPC), Burkitt's lymphoma (BL) or control subjects. Twenty out of 33 NPC sera were positive for antibodies against the large subunit, 13 of these were positive for both IgG and IgA, whilst 7 were positive for IgG only. Out of 15 BL sera and 10 control sera, none were positive. These results are similar to those observed for other EBV-encoded enzymes, including the DNase which had been used as an early marker for the development of NPC. The results presented here indicate that antibodies against the large subunit of ribonucleotide reductase could serve as an additional marker for NPC.

Characterization of a 75-kDa Epstein-Barr virus capsid protein using a new monoclonal antibody H250

A monoclonal antibody (mAb) designated H250, directed against an Epstein-Barr virus (EBV) capsid antigen, was obtained following immunization of BALB/c mice with naked particles from the producer cell line B95.8. This antigen was present in the producer lines B95.8, P3HR1, M81, RI and CA, and absent from the non-producer lines BJAB, Raji and 1022. H250 did not inhibit the transformation of cord blood lymphocytes by the B95.8 virus, nor did it inhibit EA induction on Raji cells by the P3HR1 virus. In addition, H250 showed no fluorescence on living B95.8 cells. This indicates that H250 does not recognize a membrane antigen. By indirect immunofluorescence, no fluorescence was observed on induced Raji cells or on PAA-treated B95.8 cells. Thus, H250 recognized a late antigen of the EBV virus replication cycle. Agglutination of naked virus by H250 showed it was directed against a capsid antigen. Positive fluorescence was observed on cells treated with tunicamycin, indicating that H250 recognized a protein. The molecular weight of this protein was obtained by Western blot and was approximately 75 kDa. The blocking tests carried out with H250 seemed to indicate that this Ab appeared late in patient sera during primary infection.

Neutralization of Epstein-Barr virus-induced ribonucleotide reductase with antibody to the major restricted early antigen polypeptide

A previous study had established a relationship between the major polypeptide of the Epstein-Barr Virus (EBV)-induced restricted (R) component of the early antigen (EA) complex and the large subunit of ribonucleotide reductase. This association was confirmed in this study by the observation that a monoclonal antibody prepared against the 85-kDa R component reacted in immunoblotting with the protein product (molecular weight approximately 93 kDa) encoded by the Xbal fragment of the B-95-8 strain of EBV. This fragment encodes for both the small and large subunits of this virus-induced enzyme. This size of the reactive protein indicated that it was the large subunit. Direct evidence that the 85-kDa EA-R component was associated with the viral-induced ribonucleotide reductase was provided by enzyme inhibition studies. Two monoclonal antibodies prepared against the tertiary structure of the 85-kDa EA-R antigen significantly inhibited the in vitro activity of the viral-specified enzyme but had no effect on ribonucleotide reductase activity derived from two different cellular preparations. A monoclonal antibody prepared against the denatured form of this antigen was ineffective in neutralizing the virus-specific ribonucleotide reductase as were antibodies to different EBV polypeptides. These results therefore conclusively demonstrate that the 85-kDa EA-R polypeptide is associated with the large subunit of this viral-specified enzyme and further suggest that the active site of the enzyme is associated with the tertiary structure of the large subunit.

Identification of an Epstein-Barr virus early gene encoding a second component of the restricted early antigen complex

When the latent Epstein-Barr virus (EBV) genome in B95-8 cells is induced into a replicative phase, two abundant early RNAs are transcribed rightward from the EBV BamHI H DNA fragment into BamHI F. Analysis of cDNA clones prepared from the RNA of cells replicating EBV revealed that both RNAs contain the BHRF1 open reading frame. Part of BHRF1, cloned into a prokaryotic fusion protein expression vector, expressed a fusion protein in Escherichia coli and the purified fusion protein was used to generate a monoclonal antibody against BHRF1. This antibody was then employed to characterize the protein encoded by BHRF1 in cells replicating EBV. The monoclonal antibody reacted with a 17-kDa protein component of the restricted early antigen (EA) complex. The distribution of the protein in cells was similar to that noted when sera from patients with African Burkitt's lymphoma were used to stain these cells. The protein was synthesized before the major 47-56 kDa protein associated with the diffuse component of EA in superinfected Raji cells. All human sera containing antibodies to EA as determined by immunofluorescence (IF) reacted with the protein as did some sera determined to be anti-VCA positive and anti-EA negative by IF. The predicted amino acid sequence of the protein has characteristics which suggest that it is a membrane protein. It also has significant homology with both the anchor region of polyoma middle T antigen and with the predicted protein product of the bcl-2 mRNA activated by the 14/18 chromosome translocation characteristic of follicular lymphomas. This latter homology is extensive and colinear, suggesting common evolution and function. However, neither a mRNA which could efficiently translate the BHRF1 protein nor the BHRF1 protein could be detected in latently infected cells. Thus, the bcl-2 predicted protein is similar to an EBV protein synthesized in the early phase of virus infection.

Identification of an Epstein-Barr virus glycoprotein which is antigenically homologous to the varicella-zoster virus glycoprotein II and the herpes simplex virus glycoprotein B

The Epstein-Barr virus (EBV) antigenic homologue of the varicella-zoster virus glycoprotein II and the herpes simplex virus (HSV) glycoprotein B (gB) was identified through cross-reactivity with anti-glycoprotein II and anti-glycoprotein B peptide sera. The homologue is the previously characterized EBV glycoprotein, with an apparent molecular weight of 125,000 Da, which is synthesized late during productive EBV infection and appears to be encoded by the BamHI A EBV fragment. This glycoprotein, but not other EBV proteins, reacted with the antisera in immunoprecipitation experiments and by ELISA. In addition, absorption of the sera with the purified EBV 125-kDa glycoprotein removed the cross-reacting antibody. Whether the EBV gB homologue has the same biological functions associated with HSV gB has yet to be determined.

A restricted component of the Epstein-Barr virus early antigen complex is structurally related to ribonucleotide reductase

The 85-kDa polypeptide previously shown to be associated with the restricted (R) component of the EBV-induced early antigen (EA) complex was subjected to amino acid sequencing analysis. This was accomplished by cyanogen bromide cleavage and by separation of individual peptides by high-pressure liquid chromatography employing reversed-phase C18 column techniques. Two of the isolated peptides, F11 and F13, were subjected to amino acid sequencing and both were found to have significant homology to the postulated protein encoded by the BamHI O right reading frame 2 (BamHI ORF2) of the B95-8 strain Epstein-Barr virus. Computer analysis revealed significant homology between the amino acid sequence of this polypeptide and ribonucleotide reductase, an enzyme previously mapped to this genomic fragment. Amino acid composition analysis also revealed a similar association. These results indicate that the 85-kDa EA(R) polypeptide is associated with a component of the EBV-induced ribonucleotide reductase.