Viral proteins associated with the Epstein-Barr virus

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1 Proc. Nati. Acad. Sci. USA Vol. 89, pp , January 1992 Medical Sciences Viral proteins associated with the Epstein-Barr virus transactivator, ZEBRA DAVID AARON KATZ*, RAYMOND P. BAUMANN*, REN SUN*, JOHN L. KOLMAN*, NAOMI TAYLOR*, AND GEORGE MILLER*tt Departments of *Molecular Biophysics and Biochemistry, tpediatrics, and tepidemiology and Public Health, Yale University School of Medicine, New Haven, CT Contributed by Dorothy M. Horstmann, October 7, 1991 (received for review August 8, 1991) ABSTRACT The BamHI Z Epstein-Barr replication activator (ZEBRA) mediates disruption of latency and induction of Epstein-Barr virus (EBV) early gene expression in latently infected lymphocytes. Polyclonal rabbit sera raised against ZEBRA were used to immunoprecipitate ZEBRA-associated proteins (ZAPs). ZAPs of 19, 21, 23, and 42 kda were coimmunoprecipitated with ZEBRA from extracts of EBVproducing lymphoid cell lines. ZAPs were not recognized directly by the rabbit sera, but they were antigenic for EBV+ human sera. Immunoprecipitation of ZAPs by ZEBRA-specific antisera required the presence of ZEBRA. ZAPs were not coprecipitated with ZEBRA from mouse cells expressing only ZEBRA, from Raji (a cell line in which EBV is unable to complete lytic replication), or from cells treated with inhibitors of viral DNA synthesis. Thus, ZAPs are late EBV-encoded proteins. ZEBRA and ZAPs colocalized to a salt-insoluble nuclear fraction, and both were found extracellularly in crude preparations of virions. ZAPs might function to affect the cellular localization of ZEBRA, to alter its capacity to transactivate, or to influence its target gene specificity. The hallmark of Epstein-Barr virus (EBV) infection of B lymphocytes is establishment of latency. During latency the viral genome is maintained as a plasmid, there is limited expression of a few viral genes, and neither replicative proteins nor virions are produced. The switch between latency and lytic viral replication, in which many viral genes are expressed, can be triggered in vitro by external stimuli such as sodium butyrate or phorbol esters (1, 2). Study of the P3HR-1 human lymphocyte line, in which EBV replicates spontaneously, led to the identification of an EBV gene, BZLFJ, which mediates the switch between latency and lytic EBV replication (3). In a defective virus carried by the P3HR-1 cell line the BZLFJ gene is under aberrant positive control as the result of viral genome rearrangements (4, 5). In the standard EBV genome this same gene is unexpressed, possibly due to negative regulation (6, 7). This complex regulation probably involves interplay of viral and unidentified cellular factors. The product of the BZLFI gene, ZEBRA (BamHI Z-fragment-encoded Epstein-Barr replication activator), is a 34- to 38-kDa DNA-binding protein, which is polymorphic among viral strains (8). ZEBRA stimulates transcription from promoters of several EBV early genes, including that of BZLFI itself (9-13). Disruption of EBV latency in the absence of defective virus is thought to occur as the result of ZEBRA expression leading to transactivation of EBV early genes. It is not known what proteins ZEBRA must contact directly to activate transcription of early genes. ZEBRA may also play a role in lytic viral DNA replication. The origin of replication contains binding sites for ZEBRA, but it is The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C solely to indicate this fact. unknown if ZEBRA has a direct role in initiation of DNA synthesis. Nothing is known about a possible role of ZEBRA in late events leading to production of virus. In other herpesviruses, early regulatory proteins have been shown to play a role in later stages of the productive cycle (14). The work reported herein begins to explore protein-protein interactions necessary for ZEBRA's functions. METHODS Cells. Lymphocyte lines were grown in RPMI-1640 medium supplemented with 8% fetal bovine serum. B95-8 cells are EBV-transformed marmoset B lymphocytes (15). HR-1 clone 5 and HR-1 clone 16 are single-cell clones of an EBV-positive human B-lymphocyte line derived from a Burkitt lymphoma (16, 17). Raji is an EBV-positive Burkitt lymphoma cell line unable to produce viral particles (18). Zdex23 is a mouse fibroblast line that carries a dexamethasone-inducible ZEBRA expression plasmid (R.P.B., unpublished results). This line was grown in Eagle's minimal essential medium supplemented with 5% fetal bovine serum. Metabolic Labeling and Extract Preparation. Lymphocyte cell lines were split 7 days prior to harvesting. Four days later, EBV replication was induced by addition of 3 mm sodium butyrate alone (HR-1 clone 16) or in combination with 40 nm phorbol 12-tetradecanoate 13-acetate (PTA) (B95-8, Raji). Cultures were radiolabeled in RPMI-1640 medium without glutamine or methionine supplemented with 8% dialyzed fetal bovine serum and the appropriate inducing chemicals for 4 hr with 500 ACi of lysate from bacteria grown in 35S-containing medium (Tran35S; ICN; 1 Ci = 37 GBq). Labeled cultures were washed with TBS (40 mm Tris'HCI, ph 7.5/150 mm NaCl) and lysed in LB (50 mm Tris HCI, ph 7.5/0.25 M NaCI/0.1% Triton X-100/5 mm EDTA/50 mm NaF/0.1 mm Na3VO4/0.1 mg of phenylmethanesulfonyl fluoride per ml). Zdex23 cells were allowed to grow to confluency. ZEBRA expression was induced by addition of 0.2 AM dexamethasone. Cultures were incubated with Tran35S between 5 and 7 hr after induction. Unlabeled cells were identically induced, washed with TBS, and lysed in LB. Viral DNA synthesis in HR-1 clone 16 cells was inhibited by addition of 0.1 mm acyclovir or phosphonoacetic acid at 0.2 mg/ml simultaneously with sodium butyrate. Rabbit Antisera. Sera were raised against TrpE fusion proteins by using methods described previously (7). The proteins used as antigens contained amino acids (entire protein), (all of exon 1), or (6 amino acids of exon 2 plus all of exon 3) of ZEBRA. Immunoprecipitation and Electrophoresis. Immunoprecipitation was carried out at 4 C. Extract from approximately 2 x 107 cells was incubated for 30 min with 20 ul of formalin- Abbreviations: ZEBRA, BamHI Z-fragment-encoded Epstein-Barr replication activator; EBV, Epstein-Barr virus; ZAP, ZEBRAassociated protein; PTA, phorbol 12-tetradecanoate 13-acetate. 378

2 Medical Sciences: Katz et al. fixed Staphylococcus aureus cells (Pansorbin; Calbiochem) equilibrated in TBS/10% bovine serum albumin. In some experiments, this preclearing step also included preimmune serum from the appropriate rabbit. Cleared supernatant was incubated with serum for 30 min and subsequently with an additional 20 ul of Pansorbin for 60 min. Precipitates were washed with LB/0.1% SDS, and bound proteins were separated by SDS/polyacrylamide gel electrophoresis. Immunoblotting and Autoradiography. Separated proteins were transferred to nitrocellulose membranes, then immunoblotted with alkaline phosphatase-conjugated goat secondary antibodies (Bio-Rad) and developed with nitro blue tetrazolium and 5-bromo-4-chloro-3-indolyl phosphate, all according to instructions from Bio-Rad. Nitrocellulose membranes were placed on Kodak XAR-5 film in the presence of an intensifying screen for autoradiography. Cellular Fractionation, Virion Preparation, and Solubilization. Cells were washed with TBS, then fractionated as described by Sculley et al. (19). Virions were isolated from culture supernatants of induced B95-8 cells by overnight precipitation in the presence of 10% (wt/vol) PEG-8000 and 2% (wt/vol) NaCl followed by centrifugation through a 20% sucrose solution onto a 50% sucrose cushion. Isolated virions were resuspended in TBS and solubilized in LB for immunoprecipitation experiments. RESULTS A 21-kDa EBV Protein Is Coimmunoprecipitated with ZEBRA. To identify ZEBRA-associated proteins (ZAPs), ZEBRA was immunoprecipitated from metabolically labeled extracts of two cell lines: HR-1 clone 16, an EBV-positive human B-lymphocyte line in which viral production is chemically inducible (16); and Zdex23, a mouse fibroblast line that carries a dexamethasone-inducible ZEBRA expression plasmid. A monospecific rabbit serum raised against a bacterial fusion protein product of the BZLFI open reading frame (anti-trpe/bzlf1) (7) and a potent human EBV+ antiserum (SC) were used as precipitants. ZEBRA was immunoprecipitated from extracts of either cell line induced for ZEBRA expression. Additionally, a prominent protein of 21 kda (ZAP21) coimmunoprecipitated with ZEBRA from HR-1 clone 16 extracts (Fig. 1). Neither protein was precipitated by preimmune serum from the same rabbit, by an EBV- human serum (LH), or from extracts of uninduced cells, which do not express ZEBRA. Zdex Zdex CL 16 Proc. Natl. Acad. Sci. USA 89 (1992) 379 The presence of ZAP21 only in immunoprecipitates from an EBV+ cell line led to speculation that it was encoded or induced by EBV. Immunoprecipitates formed with rabbit anti-zebra or preimmune serum were analyzed by immunoblotting, using both EBV+ and EBV- human sera. ZAP21 was detected in immunoprecipitates of HR-1 clone 16 extracts containing ZEBRA by immunoblotting with EBV+ sera but not with an EBV- serum (Fig. 2 Upper). ZAP21 was not recognized in immunoblots of Zdex23 cells, even though these immunoblots did reveal ZEBRA (Fig. 2 Lower). RM, a human EBV+ antiserum that has little or no reactivity to ZEBRA, recognized ZAP21 strongly. This result suggested that ZAP21 was not a degradation product of ZEBRA. ZAPs Are a Group of Late Viral Polypeptides. To learn at what stage of the viral cycle ZAP21 was synthesized, immunoprecipitations were carried out from three additional B lymphocyte lines: HR-1 clone 5. which spontaneously produces virions; B95-8, in which EBV production was chemically enhanced; and Raji, which synthesizes only early viral products after chemical induction. Immunoprecipitated proteins were detected by immunoblotting with EBV+ antiserum SC (Fig. 3A). ZAP21 was coimmunoprecipitated with ZEBRA from HR-1 clone 5 and B95-8 extracts but not from Raji extracts. Additional polypeptides of 19 and 42 kda were coimmunoprecipitated with ZEBRA from B95-8 extracts. In some experiments, a protein of 23 kda was reactive with serum SC in immunoprecipitates from B95-8 (Fig. 5). These polypeptides were reactive with six other human EBV+ antisera on immunoblots but not with six human EBV- antisera (data not shown), and they are therefore likely encoded or induced by EBV. Raji cells have a block in lytic EBV DNA replication. The absence of ZAPs in Raji cells suggested that their synthesis occurs late in the viral replicative cycle. To confirm this, EBV DNA synthesis in induced HR-1 clone 16 cells was inhibited by treatment with acyclovir or phosphonoacetic acid (Fig. 3B). Both agents blocked the ability of anti-zebra serum to coimmunoprecipitate ZAP21. The ability of this CL16 ZAP21 -+ EBV+(SC) EBV+(RM) EBV-(LH) / loop= ZRR -ZEBRA> 6 Zdex " -44 (--ZAP21 Inducer-- dce dex none dex dex none but but Precipitant Ct pre uz SC LH SC C0 pre 02 FIG. 1. Coimmunoprecipitation of ZAP21 with ZEBRA from extracts of an EBV producer cell line. Immunoprecipitations were performed on metabolically radiolabeled extracts from Zdex23 (Zdex) treated with dexamethasone (dex), HR-1 clone 16 (CL16) treated with sodium butyrate (but), or untreated cells (-, none). The precipitant was rabbit anti-trpe/bzlf1 (az), preimmune rabbit serum (pre), human EBV+ serum SC, or human EBV- serum LH. Numbers are molecular mass in kda. Induced Precipitant 0XZ pre 0xZ az pre 0z az pre az FIG. 2. EBV+ sera recognize ZAP21. Immunoprecipitations were performed on extracts from HR-1 clone 16 induced for EBV replication (+), from Zdex23 cells induced for ZEBRA synthesis (+), or from uninduced cells (-). The precipitant was either rabbit Precip- anti-trpe/bzlf1 (az) or preimmune rabbit serum (pre). itated proteins were analyzed by immunoblotting with human EBV+ sera SC and RM and human EBV- serum LH.

3 380 Medical Sciences: Katz et al. ZA 4 13 R! Z~~~~~ FI1R\ ;_ ZA P2 I mom -7AP21> SR 111I (Li 1r u7 pre (Li hut d.a -ptj ("CH]1n (L B5-5s) Ral FIG. 3. Evidence that ZAP21 is a late protein. Immunoprecipitations were performed with rabbit anti-trpe/bzlf1 serum. Precipitated proteins were analyzed by immunoblotting with human serum SC. (A) Coimmunoprecipitation of ZAPs from various EBVcontaining cell lines. HR-1 clone 5 (CL5) and butyrate/pta-induced B95-8 are virus producers. Butyrate/PTA-induced Raji expresses early viral antigens. The precipitant was either rabbit anti-trpe/ BZLF1 (az) or preimmune rabbit serum (pre). (B) Inhibitors of viral DNA replication block coimmunoprecipitation of ZAP21. HR-1 clone 16 cells were treated with sodium butyrate alone (but), with both sodium butyrate and acyclovir (acv), or with both sodium butyrate and phosphonoacetic acid (paa). serum to immunoprecipitate ZEBRA, an early protein whose synthesis was not inhibited by the drugs, was unaffected. ZEBRA and ZAP21 Are Found Together in the Nucleus and in Virions. In cell fractionation experiments, Sculley et al. (19) found that a 22-kDa late viral polypeptide was not extracted from isolated nuclei by 2 M NaCl; in the same nuclear compartment was a then-unidentified 36-kDa early viral polypeptide. The size, antigenicity, and kinetic class of these proteins were consistent with their being ZAP21 and ZEBRA. To confirm this hypothesis, B95-8 cells were fractionated according to methods used previously (19). Coomassie blue staining of these fractions showed actin in the cytoplasm and histones in the salt-soluble compartment (data not shown). Quantitation of protein demonstrated that the salt-insoluble fraction contained about 5% of total cell protein. Immunoblotting was performed on cell fractions with an EBV+ serum (Fig. 4A). Proteins of the size expected for ZEBRA and ZAP21 were both enriched in the saltinsoluble nuclear compartment. Both ZEBRA and ZAP21 were immunoprecipitated by anti-zebra sera from this subcellular fraction (data not shown). EA-D, another EBV antigen, segregated primarily to the salt-soluble nuclear fraction and was not coimmunoprecipitated with ZEBRA. ZAP21 reacted strongly with EBV+ sera in immunoblots of a crude EB virion preparation (Fig. 4A). An immunoblot with EBV' serum was used to analyze proteins immunoprecipitated from solubilized virion preparations (Fig. 4B). ZAP21 E-A.\ 1.)-- ZI -'I IRA -.-j /AP2 ->~ -I,vl.i5 ol5 il)m vir t ZA P' I firc (f/ I FIG. 4. ZEBRA and ZAP21 in subcellular fractions and a crude virion preparation from butyrate/pta-induced B95-8 cells. (A) Western blot developed with human serum SC. Fractions: cyto, cytoplasm; sol, salt-soluble nucleus; insol, salt-insoluble nucleus; vir, crude virions. (B) Immunoprecipitation of ZAP21 by anti- ZEBRA serum from crude virion preparation. Precipitating sera were preimmune rabbit serum (pre), rabbit anti-trpe/bzlf1 (az), or rabbit anti-trpe (Trp). Precipitated proteins were analyzed by immunoblotting with human serum SC. was readily immunoprecipitated from virions by an anti- ZEBRA serum, but not by preimmune serum from the same rabbit. Serum from a rabbit immunized with TrpE also failed to precipitate ZAP21. In these experiments ZEBRA itself was undetectable. However, ZEBRA was detected in virion preparations by immunoblotting with a rabbit anti-zebra serum, which is able to detect lower levels of ZEBRA than human polyclonal EBV+ sera (R.S. and D.A.K., unpublished results). EA-D was not identified in the same virion preparations. Immunoprecipitation of ZAP21 Depends on Specific Intraceilular Association with ZEBRA. There are two trivial explanations for the foregoing results. First, ZAPs might be nonspecifically included in ZEBRA-containing immune complexes. Second, the precipitating serum might directly recognize ZAPs. The capacity of a panel of seven different rabbit anti-trpe/zebra sera to coimmunoprecipitate ZAPs from extracts of induced B95-8 cells was studied (Fig. 5A and data not shown). Only one of five sera (C, K, P, and two others) raised against the protein product of the BZLF1 open reading frame, which contains exon 1 and part of an intron of the ZEBRA gene, could immunoprecipitate ZAPs. Antiserum raised against the entire ZEBRA cdna product (Z) similarly failed to precipitate ZAPs. The best precipitant of ZAPs was an antiserum raised against the protein product of ZEBRA exon 3 (D. All seven rabbit antisera could immunoprecipitate ZEBRA, although the anti-exon 3 product serum was least efficient in this regard. The two antisera that coimmunoprecipitate ZAPs recognized ZEBRA, but not ZAP21, on immunoblots (Fig. 5B). These data indicate that ZAPs are not merely trapped in ZEBRA-containing immune complexes and suggest that the rabbit antisera do not recognize ZAP21 directly. To exclude the possibility that ZAP21 was recognized in solution by anti-zebra sera, immunoprecipitations were performed on extracts first depleted of ZEBRA. Extracts of B95-8 cells were depleted by repeated immunoprecipitation with two antisera, one (P) that coimmunoprecipitates ZAPs and another (K) that does not. The extracts, depleted respectively of both ZEBRA and ZAP21 or only ZEBRA, were finally immunoprecipitated with a rabbit antiserum that coimmunoprecipitates ZAPs (P) or with the EBV+ human serum RM, which recognizes ZAP21 but not ZEBRA. Precipitated proteins were analyzed by immunoblotting with the EBV+ human serum SC (Fig. 6). The rabbit antiserum was unable to precipitate ZAP21 from ZEBRA-depleted extracts (Fig. 6, lanes 2 and 3). The human serum, which recognizes ZAPs directly, precipitated ZAP21 from the same ZEBRA-depleted A Proc. Natl. Acad. Sci. USA 89 (1992) Rl /1 BRA - K -,!\p)4no~~k j_ "irr. ; EB FIG. 5. (A) ZAPs are specifically coimmunoprecipitated by some, but not all, anti-zebra rabbit sera. Immunoprecipitations were performed from extracts of butyrate/pta-induced B95-8 cells. Precipitating sera were raised against products of ZEBRA exon 1 + intron (C, K, P), ZEBRA cdna (Z), or ZEBRA exon 3 (D. Preimmune serum from rabbit P was used as a control (prep). Precipitated proteins were analyzed by immunoblotting with human serum SC. (B) ZAP21 is not recognized by anti-zebra rabbit serum by immunoblotting. Immunoprecipitations were performed on extracts from B95-8 induced for EBV replication (+) or from uninduced cells (-). The precipitant was either serum from rabbit ; (az) or preimmune rabbit serum (pre). Precipitated proteins were analyzed by immunoblotting with serum from rabbit

4 Medical Sciences: Katz et al. ZEBRA-+ - '-. -~~S u mm goo ZAP21- Precipitant ZEBRA - ZAPs - pe a a dazrm RM _ FIG. 6. Immunoprecipitation from B95-8 cell extracts depleted of ZEBRA or both ZEBRA and ZAP21. Extracts were depleted of ZEBRA by repeated immunoprecipitation using rabbit serum K, or of both ZEBRA and ZAP21 by repeated immunoprecipitation using rabbit serum P (see Fig. 5). Undepleted (-) or depleted (+) extracts were then immunoprecipitated with preimmune rabbit serum P (pre), rabbit serum P (az), or human serum RM. Precipitated proteins were analyzed by immunoblotting with human serum SC. extract (lane 5). If the monospecific anti-zebra serum bound directly to ZAP21, it would have been immunoprecipitated by that serum. This experiment indicated that coimmunoprecipitation of ZAP21 requires the presence of ZEBRA. DISCUSSION ZAPs are proteins that are immunoprecipitated by certain monospecific anti-zebra sera. ZAP21, the most prominent ZAP, is a nuclear protein expressed late in the EBV replicative cycle. ZAP21 is associated with the ZEBRA transactivator in a salt-insoluble nuclear fraction and in crude virion preparations. ZAPs are absent from mouse cells that express ZEBRA, but no other EBV proteins, and from Raji cells, which do not proceed to late EBV replicative events. ZAPs are not detectable in EBV-containing lymphoid cells that are not induced for virus production. ZAP21 was not detected in induced cells in which lytic viral DNA replication had been chemically inhibited. Since ZAPs are recognized as antigens by sera from EBV-seropositive but not EBV-seronegative humans, they are likely to be encoded by the EBV genome. Confirmation of this hypothesis requires identification of the gene(s) encoding ZAPs. What Is the Nature of the Association Between ZAP21 and ZEBRA? That ZAP21 and ZEBRA are distinct proteins is indicated by several lines of evidence. Rabbit antisera raised against ZEBRA do not recognize ZAP21 on immunoblots. Conversely, several human sera (such as RM) that react strongly with ZAP21 react weakly or not at all with ZEBRA. ZAP21 can be immunoprecipitated by rabbit antisera to ZEBRA only if ZEBRA is present in the cell extract. However, serum RM can immunoprecipitate ZAP21 in the absence of ZEBRA. ZAP21 was coimmunoprecipitated by two rabbit antisera raised against different portions of the ZEBRA protein that share no homology with each other. Not all rabbit antisera to ZEBRA coimmunoprecipitate ZAP21, suggesting that ZAP21 is not simply an abundant protein which is trapped in immunoprecipitates containing ZEBRA. Furthermore, EA-D, a group of abundant EBV-encoded nuclear products, do not coimmunoprecipitate or cofractionate with ZEBRA. It is not yet clear whether the association between ZEBRA and ZAP21 is direct or indirect. For example, ZEBRA and ZAP21 could coimmunoprecipitate because they are both -17 Proc. Natl. Acad. Sci. USA 89 (1992) 381 present in a macromolecular complex that is retained under the conditions used to prepare cell or virion extracts. The method used to show colocalization of ZEBRA and ZAP21 in the nucleus is similar to the methods used for isolation of the "nuclear matrix," the protein scaffold on which many nuclear processes are carried out (20). This structure is resistant to detergent and high salt concentration. Similarly, ZEBRA and ZAP21 may coprecipitate by virtue of their presence in immature virion particles within the nucleus. ZAP21 and ZEBRA may interact directly through proteinprotein contacts rather than indirectly through their common location in some larger structure. In favor of a direct association is the finding that many antisera raised against ZEBRA exon I are unable to coprecipitate ZAPs. These antisera may compete for an epitope in ZEBRA exon I that is necessary for binding ZAPs. Proof of a direct ZEBRA-ZAP association requires reconstruction of the interaction using purified proteins. The "epitope interference" hypothesis may also account for variable apparent molar ratios of ZEBRA and ZAP21 when different sera are used as immunoprecipitants. If ZAPs interact directly with ZEBRA exon 1, then P antibodies might be expected to compete partially for binding to ZEBRA. ; antibodies, which recognize a completely different portion of ZEBRA, would not. Thus the stoichiometry of ZEBRA-ZAP interaction will be better determined by using the latter serum. What Are the Potential Biologic Functions of ZAPs? One clue to possible functions of ZAPs is the colocalization of ZAP21 with ZEBRA in virions and in nuclear fractions that may contain virion precursors. Several functions of ZAP21 may then be envisioned. It may play a role in packaging ZEBRA in virions. Perhaps ZAP21 plays some additional role in the intracellular traffic of ZEBRA, such as delivery of ZEBRA to the nucleus upon de novo infection. Once ZEBRA is delivered to the nucleus of certain types of cells (such as epithelial cells), it could function to activate expression of EBV early lytic genes. An attractive alternate hypothesis is that ZAP21, by interacting with a domain of ZEBRA necessary for transactivation, inhibits the capacity of ZEBRA to transactivate early genes. ZEBRA complexed with ZAP21 would be expected to bind to ZEBRA-responsive genes, but since the activation domain is covered, the complex would repress transcription. In this fashion, ZAP21 could play a key role in the establishment of latency upon initial infection of lymphocytes. In epithelial cells there may be a mechanism to relieve this inhibition by dissociation of ZAP21 from ZEBRA. Apart from the possible interactions of ZAPs and ZEBRA in initial events following infection, their association may be important during the transition from early to late events in lytic EBV replication. ZAPs are evidently not important for transactivation of early viral gene transcription. Neither we nor others detect ZAPs in induced Raji cells, in which EBV replication is blocked. Previous work by Marschall et al. (21) showed that no proteins except ZEBRA and a 30-kDa polypeptide, assumed to be a degradation product of ZEBRA, were immunoprecipitated from Raji by a rabbit antiserum to ZEBRA. Finally, a ZEBRA-ZAP complex may be crucial in late events. This complex might recognize a different set of DNA sequences than does ZEBRA by itself. Thus ZEBRA could play a role in regulation of late viral genes. Alternatively, by binding to ZEBRA, ZAPs may inhibit ZEBRA action on early genes at late times in the viral replicative cycle. Identification of the ZAP genes, further characterization of the ZAP proteins, and elucidation of the functional consequences of ZEBRA-ZAP interaction should increase understanding of uncharacterized events in the EBV lytic replicative cycle.

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