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Journal of Virology, July 2001, p. 6228-6234, Vol. 75, No. 13
Lineberger Comprehensive Cancer
Center1 and Department of Medicine and
Microbiology,2 University of North Carolina,
Chapel Hill, North Carolina 27599
Received 18 December 2000/Accepted 6 April 2001
The Epstein-Barr virus (EBV) immediate-early protein BRLF1 is a
transcriptional activator that mediates the switch from latent to lytic
viral replication. Many transcriptional activators function, in part,
due to an interaction with histone acetylases, such as CREB-binding
protein (CBP). Here we demonstrate that BRLF1 interacts with the amino
and carboxy termini of CBP and that multiple domains of the BRLF1
protein are necessary for this interaction. Furthermore, we show that
the interaction between BRLF1 and CBP is important for BRLF1-induced
activation of the early lytic EBV gene SM in Raji cells.
Epstein-Barr virus (EBV) is a human
herpesvirus that infects the majority of the world's population. EBV
is the causative agent of infectious mononucleosis, and it is
associated with a variety of disorders, including nasopharyngeal
carcinoma and Burkitt's lymphoma (31, 41). EBV infects
both epithelial cells and B lymphocytes. Infection of epithelial cells
results in lytic viral replication, with subsequent release of new
viral particles (41). Infection of B lymphocytes usually
results in viral latency, with only a small fraction of the viral genes
being expressed (41). Occasionally, latent EBV infection
can be reactivated to enter the viral lytic cycle.
Expression of either one of the EBV immediate-early (IE) proteins,
BRLF1 and BZLF1, is sufficient to induce the switch from latency to
lytic replication in the infected cell (9, 11, 39, 42, 47, 50,
51). BZLF1 and BRLF1 are both transcriptional activators
(12, 18, 21, 22-26, 30, 36, 38, 40, 46). Each IE protein
initially activates transcription of the other IE gene (3, 16,
34, 51), and both IE proteins together are required for
induction of the full complement of early viral genes necessary for
lytic viral replication (3, 16, 40).
Many transcriptional activators function through an interaction with
CREB-binding protein (CBP). CBP is a transcriptional activator,
regulating transcription via its histone acetylase activity
(6). Upon acetylation by CBP, interactions between histones and DNA weaken, thereby altering the conformation and stability of nucleosome core particles and enhancing transcription (32, 37, 49). CBP and its related family member p300 have been shown to interact with a variety of cellular and viral proteins and to enhance their transactivation functions. Cellular transcription factors with which CBP interacts include p53, NF- This laboratory (2) and others (53) have
shown that the EBV IE protein BZLF1 interacts with CBP and that this
interaction is important for efficient disruption of viral latency by
BZLF1. However, BRLF1 expression can also induce lytic EBV infection but must initially activate BZLF1 transcription to do so
(51). In addition, certain EBV lytic promoters have been
shown to be primarily BRLF1, rather than BZLF1, responsive (16,
40). Therefore, particularly in situations where BRLF1
expression precedes BZLF1 expression, BRLF1 association with CBP could
potentially enhance the ability of BRLF1 to induce viral reactivation
in the EBV-infected cell.
To determine if BRLF1 interacts with CBP, HeLa cells were mock infected
or infected with adenovirus vectors (constructed as previously
described [50]) expressing the BRLF1 (AdBRLF1) or beta-galactosidase (AdLacZ) genes at a multiplicity of infection of 50 and were harvested at 24 h postinfection. Cell extracts were
immunoprecipitated with an antibody directed against CBP (A-22; Santa
Cruz) or equal amounts of a control rabbit antibody (anti-ets 1-2, C-275; Santa Cruz). Complexes were separated on a polyacrylamide gel
and immunoblotted for the presence of BRLF1 (using a 1:100 dilution of
anti-BRLF1 antibody; Argene). As indicated in Fig.
1A, BRLF1 was coimmunoprecipitated with
CBP (lane 6) but not with the control antibody (lane 9). Similar
results were obtained using extracts from lytically infected,
EBV-positive Burkitt lymphoma cells (Akata) (Fig. 1B). The CBP antibody
did not immunoprecipitate a glutathione S-transferase
(GST)-BRLF1 fusion protein (Fig. 1C), confirming that it does not
cross-react with the BRLF1 protein. These results are consistent with a
direct interaction between BRLF1 and CBP in the host cell.
0022-538X/01/$04.00+0 DOI: 10.1128/JVI.75.13.6228-6234.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Epstein-Barr Virus Immediate-Early Protein BRLF1
Interacts with CBP, Promoting Enhanced BRLF1 Transactivation
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ABSTRACT
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TEXT
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B, p65, c-Fos, c-Jun, and c-Myb, among others (5, 7, 10, 13, 19, 33, 44,
45). Several key viral regulatory factors also interact with
CBP, including adenovirus E1A, simian virus 40 T antigen, human
immunodeficiency virus type 1 Tat, cytomegalovirus IE2-86, and herpes
simplex virus VP16 (4, 8, 14, 15, 27, 35, 43, 48).



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FIG. 1.
BRLF1 interacts with CBP in vivo. (A) HeLa cells were
mock infected or infected with adenovirus vectors expressing
beta-galactosidase (AdLacZ) or BRLF1 (AdBRLF1). Anti-CBP antibody (Ab)
or a control rabbit antibody were used to coimmunoprecipitate BRLF1
(300 µg; lanes 4 to 9). Immunocomplexes were electrophoresed on a
7.5% polyacrylamide gel, transferred to nitrocellulose, and
immunoblotted for BRLF1. Proteins were visualized by chemiluminescence
and autoradiography. Direct loads (30 µg; lanes 1 to 3) confirmed the
presence of BRLF1 in the HeLa cell extracts. (B) Coimmunoprecipitation
experiments (as described for panel A) were performed using extracts
from Akata cells with or without anti-immunoglobulin G (anti-IgG)
treatment (100 µg/ml; Sigma) for 4 h, which induces lytic EBV
infection. The direct load lane is lysate from Akata cells treated with
an anti-IgG antibody. (C) The same CBP and control rabbit antibodies
used for panels A and B as well as a BRLF1-reactive antibody were used
to immunoprecipitate GST or GST-BRLF1 fusion protein (GST-R), followed
by immunoblot analysis with a BRLF1 antibody.
To determine the domain(s) of CBP necessary for interaction with BRLF1,
mapping experiments were performed using a series of GST-CBP fusion
constructs (a gift from Michael Rosenfeld [29]) as shown
in the schematic diagram in Fig. 2A.
GST-CBP constructs, GST-BRLF1 (a gift from Alain Sergeant
[21]), and a GST control were incubated with in
vitro-translated 35S-labeled BRLF1, and glutathione bead
spin-down assays were subsequently performed as previously described
(2). As expected, in vitro-translated BRLF1 dimerized with
GST-BRLF1 but did not interact with control GST (Fig. 2B, lanes 2 and
3). BRLF1 associated with GST-CBP 1-721 and GST-CBP 1892-2441 (lanes 4 and 8) but did not interact with other GST-CBP constructs (lanes 5 to
7). This indicates that both the amino- and carboxy-terminal regions of
CBP are independently capable of interaction with BRLF1.
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To map the region(s) of BRLF1 necessary for interaction with CBP a
series of in vitro-translated BRLF1 mutants (a gift from Alain Sergeant
[36]) were examined for the ability to interact with the
GST constructs containing either the amino-terminal or carboxy-terminal
portions of CBP. A schematic diagram (Fig.
3A) shows the functional domains of
full-length BRLF1 protein and the BRLF1 mutants used in these studies.
The BRLF1 1-416 mutant is missing most of the transactivator domain
(located in the carboxy terminus of BRLF1), whereas both BRLF1
2-22
(amino acids 2 through 22 deleted) and BRLF1
81-184 (amino acids 81 through 184 deleted) contain deletions within the DNA binding and
dimerization domains (located in the amino terminus of BRLF1).
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In experiments performed using the amino-terminal CBP construct
(GST-CBP 1-721), only the full-length BRLF1 protein interacted efficiently with CBP (Fig. 3B, lanes 9 through 12). Thus, the DNA
binding, dimerization, and transactivator domains of BRLF1 are all
required for efficient interaction with the amino terminus of CBP. In
experiments performed with the carboxy-terminal CBP construct, the
BRLF1 transactivator domain as well as amino acids 2 through 22 were
shown to be necessary for efficient interaction (Fig. 3C, lanes 9 through 11). In contrast to the results seen with the amino terminus of
CBP, the BRLF1
81-184 mutant still interacted with GST-CBP 1982-2441 (Fig. 3C, lane 12), although somewhat less efficiently than the intact
BRLF1 protein. These results indicate that multiple domains of the
BRLF1 protein are necessary for efficient interaction with both the
amino and carboxy termini of GST-CBP.
To examine the functional significance of the interaction between BRLF1
and CBP, experiments were performed to determine whether CBP affects
BRLF1 transactivation in a transient-reporter-gene assay. HeLa cells
were transfected with a chloramphenicol acetyltransferase (CAT)
reporter plasmid (S-CAT) (30) driven by the EBV early SM
gene promoter in the presence or absence of cotransfected expression plasmids for CBP (a gift from Michael Rosenfeld [29])
alone, BRLF1 alone, or BRLF1 and CBP together. Cells were harvested at 2 days posttransfection and analyzed for CAT activity as previously described (20). CBP enhanced BRLF1-mediated activation of
the EBV early gene promoter, SM, in HeLa cells (Fig.
4A).
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To further confirm that an interaction between CBP and BRLF1 is
required for efficient BRLF1 transactivator function, we examined the
effect of the adenovirus protein E1A. It has previously been shown that
the amino-terminal portion of E1A interacts very strongly with CBP and
competitively inhibits the interaction of CBP with other transcription
factors (5, 8). As shown in Fig. 4B, wild-type E1A
completely inhibited BRLF1 transactivation of the SM reporter
construct, whereas an E1A mutant that is unable to bind CBP
(E1A
2-36) (8) had reduced inhibition. To ensure that the mutant E1A and BRLF1 proteins were expressed at an adequate level
in these experiments, immunoblot analysis was performed on the same
extracts used in the CAT assays in Fig. 4B. The mutant E1A protein was
expressed at a higher level than the wild-type E1A, and BRLF1
expression was not inhibited by wild-type E1A (Fig. 4C).
The major role of BRLF1 in the EBV life cycle is to mediate the switch between latent and lytic viral replication. However, BZLF1 and BRLF1 activate each other's promoters in most EBV-positive cell lines, and both the BZLF1 and BRLF1 transcriptional functions are required for activation of at least a portion of the viral early genes, such as BMRF1 (3, 40). Because the BZLF1 transactivator function has already been shown to require CBP (2, 53), it is difficult to examine the specific effect of CBP on BRLF1 (versus BZLF1) transactivator function in most EBV-positive cell lines. However, even though we and others have shown that BRLF1 cannot activate BZLF1 transcription in the Raji cell line (40, 51), the Miller group recently demonstrated that BRLF1 can activate the SM (but not BMRF1) early promoter in Raji cells (40).
We therefore examined the effect of CBP and E1A on the ability of BRLF1
to activate the early SM promoter from the endogenous EBV genome in
Raji cells. As shown in Fig. 5A, only
BZLF1 (but not BRLF1) induced expression of the early viral BMRF1
protein in Raji cells, whereas the BZLF1 and BRLF1 IE proteins induced equivalent expression of the early SM protein. Cotransfected CBP did
not significantly enhance the ability of BRLF1 to induce expression of
the SM early gene from the endogenous viral genome in Raji cells (Fig.
5B). Nevertheless, wild-type E1A, but not the mutant E1A (
2-36)
defective in CBP binding, significantly inhibited the ability of BRLF1
to activate SM expression (Fig. 5C), while not significantly affecting
the level of transfected BRLF1 protein (Fig. 5D). These results suggest
that while the level of constitutive CBP expression in Raji cells is
apparently already sufficient for maximal BRLF1 transactivator
function, the interaction between BRLF1 and CBP is nevertheless
essential.
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In this report we have demonstrated that BRLF1 directly interacts with the histone acetylase CBP and that this interaction is important for the ability of BRLF1 to activate transcription of at least one EBV early lytic gene, SM. BRLF1 induces lytic EBV infection in most (but not all) cell types (16, 39, 40, 50, 51) by transcriptionally activating lytic EBV genes through both direct binding and indirect mechanisms. The critical role of BRLF1 in lytic EBV infection was recently confirmed by the demonstration that an EBV mutant lacking BRLF1 is unable to undergo the lytic form of EBV infection (16).
The direct interaction between cellular and viral transcription factors
with histone acetylases such as CBP and p300 is an increasingly
well-recognized phenomenon. As shown in Fig. 2A, multiple different
domains of CBP are bound by various transcription factors. In the case
of BRLF1, we demonstrate in this report that BRLF1 can interact
independently with both the amino and carboxy termini of CBP. Several
other regulatory proteins, including CREB, NF-
B, c-Jun, and c-Myb,
have been found to interact with the amino-terminal region of CBP
(amino acids 1 to 721) (7, 10, 13, 19, 33). Viral proteins
adenovirus E1A, simian virus 40 T antigen, and human immunodeficiency
virus Tat, as well as cellular proteins Src-1, JunB, and Cdk, have
likewise been shown to bind to the carboxy terminus of CBP (amino acids
1892 to 2441) (4, 14, 15, 17, 27, 29, 33, 35). Interaction with both the amino- and carboxy-terminal regions of CBP is not unique
to BRLF1 in that the EBV BZLF1 protein and NF-
B (19, 53) can likewise bind to both the amino and carboxy termini of
CBP. The ability of BRLF1 to interact with both the amino and carboxy
termini of CBP may serve to strengthen the interaction between BRLF1
and CBP. In any event, the BRLF1-CBP interaction appears to be quite
strong in that this interaction was easily observed in
coimmunoprecipitation experiments in vivo, and BRLF1 was retained by
the amino- and carboxy-terminal GST-CBP fusion proteins as efficiently
as by the GST-BRLF1 fusion protein.
We demonstrate in this report that the carboxy terminus of BRLF1, which contains the transcriptional activation domain, is required for efficient interaction with both the amino and carboxy termini of CBP. However, as can be seen in Fig. 3, deletion of the BRLF1 transactivator domain, while completely abolishing interactions with the carboxy terminus of CBP, reduces but does not completely abolish interactions between BRLF1 and the amino terminus of CBP. The amino terminus of BRLF1, which contains the DNA binding and dimerization domains, is also required for efficient interaction with both the amino and carboxy termini of CBP. Nevertheless, there are also differences in the requirements for the BRLF1 amino-terminal domain in that deletion of BRLF1 amino acid residues 81 through 184 completely abolishes the interaction between BRLF1 and the amino terminus of CBP, while only reducing the efficiency of the interaction between BRLF1 and the carboxy terminus of CBP (Fig. 3).
The direct interaction between transcription factors and CBP is thought to enhance the function of these factors. In the case of BRLF1, it appears that the constitutive level of CBP in Raji cells is already sufficient for maximal BRLF1 transactivator function, since increasing the level of CBP did not augment BRLF1-dependent transactivation. Nevertheless, our finding that E1A inhibits BRLF1 transactivator function through a CBP binding-dependent mechanism suggests that BRLF1 does indeed require CBP to activate the SM promoter.
Histone acetylation of the BZLF1 IE EBV gene promoter was recently shown to be important during the reactivation of latent EBV (28). However, the mechanism by which BRLF1 enhances BZLF1 transcription remains somewhat obscure, since BRLF1 does not bind directly to either BZLF1 promoter (Zp and Rp) (21, 52) but instead induces BZLF1 transcription through an indirect mechanism involving activation of the stress mitogen-activated protein kinase pathways (1). The SM promoter is one of the few EBV promoters known to be directly bound by BRLF1 (22, 30, 38), and it thus remains possible that the ability of BRLF1 to activate a variety of other promoters through indirect mechanisms is not necessarily CBP dependent.
Interestingly, both EBV IE transactivators, BRLF1 and BZLF1, have now been shown to interact with CBP (2, 53; the present study). Similar to the effect of CBP on BRLF1, the interaction between BZLF1 and CBP likewise enhances BZLF1 transactivator activity and increases the ability of BZLF1 to induce the lytic form of EBV infection in latently infected cells (2, 53). CBP is thought to be limiting in cells, and we previously showed that BZLF1 is able to inhibit CREB transactivator function by interacting with CBP, thereby competing for limiting amounts of CBP (2). In the context of lytic EBV infection, BRLF1 (which appears to interact with CBP at least as strongly as BZLF1) and BZLF1 may thus regulate one another's function by competing for limiting quantities of CBP in the host cell.
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ACKNOWLEDGMENTS |
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This work was supported by Public Health Service grant RO1-CA58853 from the National Institutes of Health.
We thank the UNC Gene Therapy Center for construction of AdBRLF1 and AdLacZ, Amy Mauser for preparation of figures, Sankar Swaminathan for SM antibody, Alain Sergeant and Diane Hayward for BZLF1 and BRLF1 expression vectors, and Mary Paula Beckett for technical assistance.
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FOOTNOTES |
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* Corresponding author. Mailing address: Lineberger Comprehensive Cancer Center, University of North Carolina, Chapel Hill, NC 27599-7295. Phone: (919) 966-1248. Fax: (919) 966-8212. E-mail: shann{at}med.unc.edu.
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