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Journal of Virology, October 1999, p. 8867-8872, Vol. 73, No. 10
Department of Medicine, Brigham and Women's
Hospital, Harvard Medical School, Boston, Massachusetts 02115
Received 11 May 1999/Accepted 15 July 1999
Latent membrane protein 2B (LMP2B) is expressed during latent
Epstein-Barr virus (EBV) infection, but little is known about its role.
The goal of this study was to determine whether an LMP2B homologue is
conserved in the rhesus monkey lymphocryptovirus (LCV). Both rhesus LCV
LMP2A and LMP2B genes were cloned and sequenced. The rhesus LCV LMP2B
gene is positionally conserved, and the EBNA-2 responsiveness and the
bidirectional nature of the LMP1-LMP2B promoter have also been
functionally conserved. However, this region of the genome encoding the
LMP1, LMP1-LMP2B promoter, and LMP2B first exon demonstrates the most
dramatic nucleotide sequence divergence between human and nonhuman LCV
observed to date. Evolution of the rhesus LCV LMP2B promoter and
transcript despite the dynamic nature of this genomic region reflects
strong selective pressure for a yet-to-be-identified LMP2B function.
Latent membrane protein 2B (LMP2B)
is one of the three membrane proteins expressed during latent
Epstein-Barr virus (EBV) infection (for a review, see reference
17). LMP2B is also expressed in EBV-related
malignancies (for reviews, see references 2 and
29), but the role of LMP2B in EBV infection and
pathogenesis in vivo remains unknown. The LMP2B gene is closely related
to the LMP2A gene. They share eight of nine exons and are identical except for their unique first exons (20, 28). Whereas the first exon of LMP2A encodes a 119-amino-acid cytoplasmic domain, the
LMP2B first exon is noncoding. LMP2B translation initiates from an ATG
codon at the beginning of the second exon. Thus, LMP2B is essentially a
deletion mutant of LMP2A consisting of the last 379 amino acids encoded
in the common second through ninth exons. Since the LMP2B gene is
located immediately upstream of the LMP1 gene and is transcribed in the
opposite direction as LMP1, this region serves as a bidirectional
promoter for both LMP1 and LMP2B (19, 28).
To date, reports of significant functional activity for the LMP2
proteins have been limited to LMP2A and the presence of the unique
LMP2A first exon. Tyrosine residues in the LMP2A first exon are
important for interaction with and constitutive phosphorylation by syk
and lyn protein tyrosine kinases (PTKs) (8, 24). B cells
immortalized with LMP2A-deleted EBV are more sensitive to B-cell
receptor cross-linking and induction of lytic cycle infection (24,
25). Since B-cell receptor activation and src kinase activation
can induce EBV reactivation (4, 30), the interaction of
LMP2A with these PTKs is likely to be important for inhibiting lytic
cycle reactivation and maintaining latent infection in EBV-infected cells. Neither LMP2 gene is required for EBV-induced B-cell
immortalization in vitro, indicating that the LMP2A interaction with
src kinases is not required for growth transformation (22,
23). As LMP2B lacks the amino terminal cytoplasmic domain, LMP2B
is unlikely to interact with PTKs and to have similar effects on B-cell
receptor signal transduction.
The lack of obvious functional activity, the noncoding nature of the
first exon, and the unusual bidirectional nature of the LMP2B promoter
raise the possibility that the evolution of LMP2B might have been a
fortuitous event. Other herpesviruses that naturally infect Old World
nonhuman primates have evolved similarly and are classified in the same
lymphocryptovirus (LCV) subgroup as EBV (for reviews, see references
1 and 5). These simian LCVs have
similar B-cell immortalizing properties in vitro and similar
pathogenesis in vivo as EBV (10, 26). Studies from our
laboratory and others indicate that homologues for latent infection
nuclear proteins (EBNA-LP, -1, -2, -3A, -3B, and -3C) and membrane
proteins (LMP1 and LMP2A) have been conserved in baboon and rhesus LCVs
(6, 7, 15, 21, 32). Interestingly, there was no evidence for
an LMP2B transcript on Northern blots of RNA from baboon LCV-infected
cells probed with a baboon LCV LMP2A cDNA probe (6). To test
whether the LMP2B gene was an unusual evolutionary event restricted to
EBV, we examined whether the rhesus LCV encoded both LMP2A and LMP2B homologues.
LMP2A is positionally and structurally conserved in rhesus
LCV.
The putative LMP2A first exon was sequenced from the
previously published rhesus LCV DNA fragment, RE1 (7). The
sequence of a potential LMP2A ninth exon was derived from a 2.5-kb
BamHI DNA fragment (CD1PR1) which was subcloned from a
rhesus LCV cosmid clone (RcosCD1) by cross-hybridization with an EBV
LMP2A cDNA probe. Reverse transcription (RT)-PCR amplification with a
5' primer in the LMP2A first exon and a 3' primer in the putative LMP2A
ninth exon revealed a 1.9-kb product from rhesus LCV-infected B-cell
RNA. The nucleotide sequence of the 1.9-kb RT-PCR product from four
independent clones confirmed that the transcript was homologous to the
baboon LCV and EBV LMP2A transcripts (62 and 66% nucleotide homology,
respectively; Fig. 1A).
0022-538X/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Strong Selective Pressure for Evolution of an Epstein-Barr Virus
LMP2B Homologue in the Rhesus Lymphocryptovirus
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FIG. 1.
(A) Nucleotide and amino acid sequence of the rhesus LCV
LMP2A cDNA. RT-PCR was done with 5' primer E1A
(5'-GGAATCCACCTCCTTACG-3') and 3' primer E9
(5'-GTGCTAATTTCGTGAACCCC-3'). The sequence was derived from
both strands from four independent clones (GenBank accession no.
AF148640). (B) Comparison of rhesus LCV, EBV, and baboon LCV LMP2A
protein sequences. :, amino acid identity; ., amino acid similarity.
The hydrophobic transmembrane domains (underlined) were identified with
the SOSUI program (3a). Phosphotyrosine motifs already
reported to be involved in the function of EBV LMP2A (ITAM, PPPY motif,
YEE motif) are starred.
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LMP2B first exon is positionally conserved despite divergence
between rhesus LCV and EBV LMP1-LMP2B promoter regions.
To test
whether a rhesus LCV LMP2B first exon was positionally conserved, we
cloned and sequenced 563 bases of the LMP1 upstream region (Fig.
3). Surprisingly, the rhesus LCV LMP1
promoter is not well conserved with the EBV LMP1 promoter (27%
nucleotide homology). This is significantly different from other latent
infection promoters from rhesus and baboon LCVs (Table
1). Despite the poor sequence homology,
RBP-J
/CBF-1 (12, 13) and PU.1/Spi1 (16, 18)
binding sites important for LMP1 transcriptional regulation have been
conserved (Fig. 3). Interestingly, the rhesus LCV LMP1 promoter
contains only one RBP-J
/CBF-1 binding site, compared to two in EBV,
and both the RBP-J
/CBF-1 and PU.1/Spi1 binding sites are located on
the opposite strand as compared to the EBV LMP1 promoter. These
findings suggest that despite the high degree of genetic evolution in
this region, there is strong selective pressure for conserving
important transcriptional regulatory motifs.
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476 relative
to the LMP1 translational initiation site and the splice acceptor site
in the second exon was identical to LMP2A. Longer RT-PCR products (1.5 to 1.6 kb) were also obtained with primers 4, 5, and 6 with a 3' primer
in the LMP2A ninth exon, suggesting that the rhesus LCV LMP2B shares
exons two through nine of LMP2A (data not shown). Finally, the RT-PCR
results also suggest that the 5' end of the LMP2B transcript is between
primers 3 and 4, approximately
337 to
361 nucleotides upstream of
the LMP1 translational initiation site. In that case, the LMP2B first exon (110 bp) should be shorter than the LMP2A first exon (350 bp),
suggesting that the failure to detect an LMP2B transcript on Northern
blots is due to a much lower level of expression for rhesus LCV LMP2B
than for LMP2A.
Transcriptional regulation of the LMP1-LMP2B bidirectional promoter is conserved in rhesus LCV. Reporter constructs were made to test whether the bidirectional nature and EBNA-2 responsiveness of the LMP1 promoter were functionally conserved in rhesus LCV. EBV and rhesus LCV DNA fragments containing the LMP1 promoter were cloned in either direction relative to a luciferase reporter gene (Fig. 4, panel I). Both EBV and rhesus LCV LMP1 promoters (parts A and C, open boxes; Fig. 4, panel II) showed three- to sixfold activation of the reporter gene relative to the luciferase gene alone (pGL2; Fig. 4, panel II) when transfected into EBV-negative human B lymphoma cells. When the promoter elements were cloned in the opposite orientation (parts B and D, open boxes; Fig. 4, panel II), similar levels of activity were detected, indicating that the EBV and rhesus LCV LMP1 promoters are bidirectional. To test whether EBNA-2 responsiveness had been conserved, promoter constructs were cotransfected with an EBV EBNA-2 expression construct. EBNA-2 cotransfection increased luciferase activity of the rhesus LCV promoter five- to sixfold in either the LMP1 or LMP2B orientation (Fig. 4, panel II, filled boxes). The level of LMP1-LMP2B promoter activity induced by EBNA-2 was comparable among the EBV and rhesus LCV promoters.
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ACKNOWLEDGMENTS |
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This work was funded by grants from the Public Health Service (CA68051 and CA65319). P.R. was a fellow of the Association pour la Recherche sur le Cancer.
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FOOTNOTES |
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* Corresponding author. Mailing address: Channing Laboratories, 181 Longwood Ave., Boston, MA 02115. Phone: (617) 525-4258. Fax: (617) 525-4257. E-mail: fwang{at}rics.bwh.harvard.edu.
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