Epstein-Barr Virus mRNA Export Factor EB2 Is Essential for Intranuclear Capsid Assembly and Production of gp350

Abstract

Most human herpesviruses, including Epstein-Barr virus (EBV), express a protein which functions primarily as an mRNA export factor. Previously, we deleted the gene for the Epstein-Barr virus mRNA export factor EB2 from the EBV genome and then introduced the mutated genome into 293 cells. Using a transcomplementation assay in which ectopic expression of the transcription factor EB1/ZEBRA was sufficient to induce the EBV productive cycle, we showed that Ori-Lyt-dependent replication of the EBV DNA occurs in the absence of EB2, indicating that EB2 is not essential for the expression and export of early mRNAs. However, in the absence of EB2, no infectious viral particles are produced (H. Gruffat, J. Batisse, D. Pich, B. Neuhierl, E. Manet, W. Hammerschmidt, and A. Sergeant, J. Virol. 76:9635-9644, 2002). In this report, we now show that EB2 is essential for the nuclear export of most, but not all, late mRNAs produced from intronless genes that translate into proteins involved in intranuclear capsid assembly and maturation. As a consequence, we show that EB2 is essential for the proper assembly of intranuclear capsids. Interestingly, the late BLLF1 gene contains an intron, and both unspliced and spliced mRNAs must be exported to the cytoplasm to be translated into gp350 and gp220, respectively (M. Hummel, D. A. Thorley-Lawson, and E. Kieff, J. Virol. 49: [413][414][415][416][417] 1984). Our results also demonstrate that although BLLF1 spliced mRNAs are exported to the cytoplasm independently of EB2, EB2 is essential for the nuclear export of unspliced BLLF1 mRNA. In the same assay, herpes simplex virus 1 ICP27 completely inhibited the nuclear export of BLLF1 spliced mRNAs whereas unspliced BLLF1 mRNAs were exported, confirming that in a physiological assay, ICP27 inhibits splicing.

Spironolactone blocks Epstein–Barr virus production by inhibiting EBV SM protein function

“…6). However, SPR did strongly inhibit expression of VCA, which is highly dependent on SM for efficient expression (4,9). Although the percentage of BMRF1 (EA-D)-, ZTA-, and SM-expressing cells was unaffected or actually increased slightly with SPR treatment, the percentage of VCA-positive cells decreased by 73% (P < 0.0001; Fig.…”

“…We have comprehensively characterized the specificity of SM effects on the EBV transcriptome by comparing the transcriptome of SMKO EBV-infected cells with that of cells infected with wt EBV and SM-rescued SMKO EBV (9). The effect of SM is highly gene-dependent and primarily enhances expression of several essential late genes, including VCA and gp350 (4,5,9). It was therefore possible to conduct a similar global comparison between the effects of SPR on WT EBV-infected cells and the effect of genetically knocking out SM function.…”

“…SM acts posttranscriptionally to enhance accumulation of many lytic EBV transcripts. SM activity is highly gene-specific and preferentially enhances expression of several late lytic transcripts, including those encoding the major viral capsid antigen (VCA) and gp350, which is required for infection of B lymphocytes (3)(4)(5). SM and its homologs in other herpesviruses may also enhance transcription and translation of specific viral genes (6).…”

Human cytomegalovirus UL69 protein facilitates translation by associating with the mRNA cap-binding complex and excluding 4EBP1

“…The most intensively studied activity of pUL69 is its ability to facilitate the export of unspliced mRNA from the nucleus to cytoplasm (12), a function that it shares with its counterparts in other herpesviruses (13)(14)(15). pUL69 shuttles between nucleus and cytoplasm (16), binds RNA (17), and interacts with the DExD/Hbox RNA helicase UAP56 or the related URH49 protein (12), cellular proteins involved in RNA transport.…”

The Varicella-Zoster virus IE4 protein: A conserved member of the herpesviral mRNA export factors family and a potential alternative target in antiherpetic therapies

“…Furthermore, EB2 has been shown to be essential for the production of infectious viral particles [45] and for the nuclear export of most, but not all, late mRNAs produced from genes implicated in the capsid assembly and maturation [46].…”