Adenovirus IVa2 protein plays an important role in transcription from the major late promoter in vivo

The protein composition of human adenovirus replication compartments

Human adenoviruses are double-stranded DNA viruses that replicate in the cell nucleus and induce the formation of replication compartments (RCs) that are critical in viral replication and control of virus-host interactions. RCs are specialized virus-induced subnuclear microenvironments where not only viral genome replication and expression are orchestrated but also host proteins that restrict viral replication are co-opted and subverted. The protein composition of these RCs remains largely unexplored. In this study, we isolated adenovirus RC-enriched fractions from infected cells at different times post-infection and employed a tandem mass tag-based quantitative mass spectrometry approach to identify proteins associated with RCs (data available via ProteomeXchange identifier PXD051745). These findings reveal an elaborate network of host and viral proteins potentially relevant for RC formation and function. To validate the RC-protein components identified by mass spectrometry, we employed immunofluorescence and immunoblotting techniques. Proteins previously described to colocalize in RCs in infected cells were identified in the isolated subnuclear fractions. In addition, we validated newly identified proteins associated with RCs, including the high mobility group box 1 (HMGB1), the SET nuclear proto-oncogene, the structure-specific recognition protein 1 (SSRP1), the CCCTC-binding protein (CTCF), and sirtuin 6 (SIRT6). We identified HMGB1 as a protein that binds to the viral DNA binding protein (DBP). Using shRNA knockdowns and inhibitors, we demonstrated that HMGB1 acts as a proviral factor, promoting efficient viral DNA synthesis and progeny production. Our data further suggest potential candidate targets for therapeutic intervention and provide mechanistic insights into the molecular basis of virus-host interactions.IMPORTANCEHuman adenoviruses serve as models for studying respiratory viruses and have provided critical insights into viral genome replication and gene expression, as well as the control of virus-host interactions. These processes are coordinated within virus-induced subnuclear microenvironments known as RCs. We conducted quantitative proteome analyses of RC-enriched subnuclear fractions at different times post-infection with human adenovirus species C type 5, revealing a multifaceted network of proteins that participate in the regulation of gene expression, DNA damage response, RNA metabolism, innate immunity, and other cellular antiviral defense mechanisms. Furthermore, we validated the localization of several host proteins to viral RCs using immunofluorescence microscopy and immunoblotting and identified cellular HMGB1 as a proviral factor late during infection. These findings represent the first analysis of the proteomes of isolated RCs and not only enhance our understanding of nuclear organization during infection but also shed light on the complex interplay between viral and host factors within RCs.

Stable expression of shRNA for the control of recombinant adenovirus replication

Preventing the replication of adenovirus could have practical uses, such as controlling infection with wild-type virus or in applications involving recombinant vectors. Mainly transient methods have been used to inhibit adenovirus replication, including siRNA or drugs. Here, we tested whether stable expression of shRNA designed to target hexon, Iva2, or pol can inhibit the replication of a recombinant adenoviral vector, Ad-LacZ (serotype 5, E1/E3 deleted), in 293T cells. Significant knockdown correlating with reduced Ad-LacZ replication was achieved only when hexon was targeted. Cell sorting and isolation of cellular clones further accentuated knockdown of the hexon transcript, reduced protein levels by more than 90%, and diminished adenovirus production. As visualized by transmission electron microscopy, the cellular clone expressing the hexon-specific shRNA yielded 89.2% fewer particles compared to the parental 293T cells. Full scale production followed by purification revealed a 90.2% reduction in Ad-LacZ biological titer. These results support the notion that stable expression of shRNA can be used as a means to control adenovirus replication.

E1B-55K is a phosphorylation-dependent transcriptional and post-transcriptional regulator of viral gene expression in HAdV-C5 infection

The multifunctional adenoviral E1B-55K phosphoprotein is a major regulator of viral replication and plays key roles in virus-mediated cell transformation. While much is known about its function in oncogenic cell transformation, underlying features and exact mechanisms that implicate E1B-55K in regulation of viral gene expression are less well understood. Therefore, this work aimed at unravelling basic intranuclear principles of E1B-55K-regulated viral mRNA biogenesis using wild type HAdV-C5 E1B-55K, a virus mutant with abrogated E1B-55K expression and a mutant that expresses a phosphomimetic E1B-55K. By subnuclear fractionation, mRNA, DNA and protein analyses as well as luciferase reporter assays, we show that (i) E1B-55K promotes efficient release of viral late mRNAs from their site of synthesis in viral replication compartments (RCs) to the surrounding nucleoplasm, that (ii) E1B-55K modulates the rate of viral gene transcription and splicing in RCs, that (iii) E1B-55K participates in the temporal regulation of viral gene expression, that (iv) E1B-55K can enhance or repress the expression of viral early and late promoters and that (v) the phosphorylation of E1B-55K regulates the temporal effect of the protein on each of these activities. Together, these data demonstrate that E1B-55K is a phosphorylation-dependent transcriptional and post-transcriptional regulator of viral genes during HAdV-C5 infection. Importance Human adenoviruses are useful models to study basic aspects of gene expression and splicing. Moreover, they are one of the most commonly used viral vectors for clinical applications. However, key aspects of the activities of essential viral proteins that are commonly modified in adenoviral vectors have not been fully described. A prominent example is the multifunctional adenoviral oncoprotein E1B-55K that is known to promote efficient viral genome replication and expression while simultaneously repressing host gene expression and antiviral host responses. Our study combined different quantitative methods to study how E1B-55K promotes viral mRNA biogenesis. The data presented here propose a novel role for E1B-55K as a phosphorylation-dependent transcriptional and post-transcriptional regulator of viral genes.

Adenovirus Core Proteins: Structure and Function

Adenoviruses have served as a model for investigating viral-cell interactions and discovering different cellular processes, such as RNA splicing and DNA replication. In addition, the development and evaluation of adenoviruses as the viral vectors for vaccination and gene therapy has led to detailed investigations about adenovirus biology, including the structure and function of the adenovirus encoded proteins. While the determination of the structure and function of the viral capsid proteins in adenovirus biology has been the subject of numerous reports, the last few years have seen increased interest in elucidating the structure and function of the adenovirus core proteins. Here, we provide a review of research about the structure and function of the adenovirus core proteins in adenovirus biology.

Enhanced oncolytic adenoviral production by downregulation of death-domain associated protein and overexpression of precursor terminal protein

Adequate viral replication in tumor cells is the key to improving the anti-cancer effects of oncolytic adenovirus therapy. In this study, we introduced short hairpin RNAs against death-domain associated protein (Daxx), a repressor of adenoviral replication, and precursor terminal protein (pTP), an initiator of adenoviral genome replication, into adenoviral constructs to determine their contributions to viral replication. Both Daxx downregulation and pTP overexpression increased viral production in variety of human cancer cell lines, and the enhanced production of virus progeny resulted in more cell lysis in vitro, and tumor regression in vivo. We confirmed that increased virus production by Daxx silencing, or pTP overexpression, occurred using different mechanisms by analyzing levels of adenoviral protein expression and virus production. Specifically, Daxx downregulation promoted both virus replication and oncolysis in a consecutive manner by optimizing IVa2-based packaging efficiency, while pTP overexpression by increasing both infectious and total virus particles but their contribution to increased viral production may have been damaged to some extent by their another contribution to apoptosis and autophagy. Therefore, introducing both Daxx shRNA and pTP in virotherapy may be a suitable strategy to increase apoptotic tumor-cell death and to overcome poor viral replication, leading to meaningful reductions in tumor growth in vivo.

Regions of bovine adenovirus-3 IVa2 involved in nuclear/nucleolar localization and interaction with pV

Bovine adenovirus-3 (BAdV-3) is a non enveloped, icosahedral DNA virus containing a genome of 34446 bps. The intermediate region of BAdV-3 encodes pIX and IVa2 proteins. Here, we report the characterization of BAdV-3 IVa2. Anti-IVa2 serum detected a 50 kDa protein at 24-48 h post infection in BAdV-3 infected cells. The IVa2 localizes to nucleus and nucleolus of BAdV-3 infected cells. Analysis of mutant IVa2 demonstrated that amino acids 1-25 and 373-448 are required for nuclear and nucleolar localization of IVa2, respectively. The nuclear import of IVa2 utilize importin α -1 of importin nuclear import pathway. Although deletion/substitution of amino acids 4-18 is sufficient to abrogate the nuclear localization of IVa2, amino acids 1-25 are required for nuclear localization of a cytoplasmic protein. Furthermore, we demonstrate that amino acids 1-25 and 120-140 of IVa2 interact with importin α-1 and pV proteins, respectively in BAdV-3 infected cells.

Formation of adenovirus DNA replication compartments

Adenoviruses induce an extensive reorganization of the host cell nucleus during replication. Such a process results in the assembly of viral and cellular macromolecules into nuclear structures called adenovirus replication compartments (AdRCs), which function as platforms for viral DNA replication and gene expression. AdRCs co-opt host proteins and cellular pathways that restrict viral replication, suggesting that the mechanisms that control AdRC formation and function are essential for viral replication and lay at the basis of virus-host interactions. Here, we review the hallmarks of AdRCs and recent progress in our understanding of the formation, composition, and function of AdRCs. Furthermore, we discuss how AdRCs facilitate the interplay between viral and cellular machineries and hijack cellular functions to promote viral genome replication and expression.

Haemorrhagic enteritis of turkeys - current knowledge

Haemorrhagic enteritis virus (HEV), an adenovirus associated with acute haemorrhagic gastro-intestinal disease of 6-11-week old turkeys predominantly hampers both humoral and cellular immunity. Affected birds are more prone to secondary complications (e.g. colibacillosis and clostridiosis) and failure to mount an effective vaccine-induced immune response. HEV belongs to the new genus Siadenovirus. Feco-oral transmission is the main route of entry of the virus and it mainly colonizes bursa, intestine and spleen. Both naturally occurring virulent and avirulent strains of HEVs are serologically indistinguishable. Recent findings revealed that ORF1, E3 and fib genes are the key factors affecting virulence. The adoption of suitable diagnostic tools, proper vaccination and biosecurity measures have restrained the occurrence of disease epidemics. For diagnostic purposes, the best source of HEV is either intestinal contents or samples from spleen. For rapid detection highly sensitive and specific tests such as quantitative real-time PCR based on Taq man probe has been designed. Avirulent strains of HEV or MSDV can be effectively used as live vaccines. Novel vaccines include recombinant hexon protein-based subunit vaccines or recombinant virus-vectored vaccines using fowl poxvirus (FPV) expressing the native hexon of HEV. Notably, subunit vaccines and recombinant virus vectored vaccines altogether offer high protection against challenge or field viruses. Herein, we converse a comprehensive analysis of the HEV genetics, disease pathobiology, advancements in diagnosis and vaccination along with appropriate prevention and control strategies.

Adenovirus with DNA Packaging Gene Mutations Increased Virus Release

Adenoviruses (Ads) have been extensively manipulated for the development of cancer selective replication, leading to cancer cell death or oncolysis. Clinical studies using E1-modified oncolytic Ads have shown that this therapeutic platform was safe, but with limited efficacy, indicating the necessity of targeting other viral genes for manipulation. To improve the therapeutic efficacy of oncolytic Ads, we treated the entire Ad genome repeatedly with UV-light and have isolated AdUV which efficiently lyses cancer cells as reported previously (Wechman, S. L. et al. Development of an Oncolytic Adenovirus with Enhanced Spread Ability through Repeated UV Irradiation and Cancer Selection. Viruses2016, 8, 6). In this report, we show that no mutations were observed in the early genes (E1 or E4) of AdUV while several mutations were observed within the Ad late genes which have structural or viral DNA packaging functions. This study also reported the increased release of AdUV from cancer cells. In this study, we found that AdUV inhibits tumor growth following intratumoral injection. These results indicate the potentially significant role of the viral late genes, in particular the DNA packaging genes, to enhance Ad oncolysis.

The repression domain of the E1B 55-kilodalton protein participates in countering interferon-induced inhibition of adenovirus replication

To begin to investigate the mechanism by which the human adenovirus type 5 E1B 55-kDa protein protects against the antiviral effects of type 1 interferon (IFN) (J. S. Chahal, J. Qi, and S. J. Flint, PLoS Pathog. 8:e1002853, 2012 [doi:10.1371/journal.ppat.1002853]), we examined the effects of precise amino acid substitution in this protein on resistance of viral replication to the cytokine. Only substitution of residues 443 to 448 of E1B for alanine (E1B Sub19) specifically impaired production of progeny virus and resulted in a large defect in viral DNA synthesis in IFN-treated normal human fibroblasts. Untreated or IFN-treated cells infected by this mutant virus (AdEasyE1Sub19) contained much higher steady-state concentrations of IFN-inducible GBP1 and IFIT2 mRNAs than did wild-type-infected cells and of the corresponding newly transcribed pre-mRNAs, isolated exploiting 5'-ethynyluridine labeling and click chemistry. These results indicated that the mutations created by substitution of residues 443 to 448 for alanine (Sub19) impair repression of transcription of IFN-inducible genes, by the E1B, 55-kDa protein, consistent with their location in a segment required for repression of p53-dependent transcription. However, when synthesized alone, the E1B 55-kDa protein inhibited expression of the p53-regulated genes BAX and MDM2 but had no impact whatsoever on induction of IFIT2 and GBP1 expression by IFN. These observations correlate repression of transcription of IFN-inducible genes by the E1B 55-kDa protein with protection against inhibition of viral genome replication and indicate that the E1B 55-kDa protein is not sufficient to establish such transcriptional repression.

Identification and characterization of a DNA binding domain on the adenovirus IVa2 protein

The adenovirus IVa2 protein has been implicated as a transcriptional activator of the viral major late promoter (MLP) and a key component in the packaging of the viral genome. IVa2 functions in packaging through its ability to form a complex with the viral L1 52/55kDa protein, which is required for encapsidation. IVa2, alone and in conjunction with another viral protein, the L4 22K protein, binds to the packaging sequence on the viral genome and to specific elements in the promoter. To define the DNA binding domain on IVa2 and determine its contribution to the viral life cycle, we created a mutant protein that lacks a putative helix-turn-helix motif at the extreme C-terminus. Characterization of this mutant protein showed that while MLP activity is relatively unaffected, it is unable to bind to and package DNA.

The adenovirus L4-22K protein is multifunctional and is an integral component of crucial aspects of infection

A variety of cellular and viral processes are coordinately regulated during adenovirus (Ad) infection to achieve optimal virus production. The Ad late gene product L4-22K has been associated with disparate activities during infection, including the regulation of late gene expression, viral DNA packaging, and infectious virus production. We generated and characterized two L4-22K mutant viruses to further explore L4-22K functions during viral infection. Our results show that L4-22K is indeed important for temporal control of viral gene expression not only because it activates late gene expression but also because it suppresses early gene expression. We also show that the L4-22K protein binds to viral packaging sequences in vivo and is essential to recruit two other packaging proteins, IVa2 and L1-52/55K, to this region. The elimination of L4-22K gave rise to the production of only empty virus capsids and not mature virions, which confirms that the L4-22K protein is required for Ad genome packaging. Finally, L4-22K contributes to adenovirus-induced cell death by regulating the expression of the adenovirus death protein. Thus, the adenovirus L4-22K protein is multifunctional and an integral component of crucial aspects of infection.

Identification of a previously unrecognized promoter that drives expression of the UXP transcription unit in the human adenovirus type 5 genome

We previously identified an adenovirus (Ad) protein named U exon protein (UXP) encoded by a leftward-strand (l-strand) transcription unit. Here we identify and characterize the UXP promoter. Primer extension and RNase protection assays mapped the transcription initiation site at 32 nucleotides upstream of the UXP gene initiation codon. A series of viral mutants with mutations at two putative inverted CCAAT (I-CCAAT) boxes and two E2F sites were generated. With mutants lacking the proximal I-CCAAT box, the UXP mRNA level decreased significantly to 30% of the Ad type 5 (Ad5) mRNA level as measured by quantitative reverse transcription-PCR. Decreased UXP was also observed by immunoblotting and immunofluorescence. UXP mRNA and protein levels were similar to those of Ad5 for mutants lacking the distal I-CCAAT box or both putative E2F sites. Ad DNA levels were similar in mutant- and wild-type Ad5-infected cells during the late stage of infection, strongly suggesting that the decreased UXP mRNA and protein from mutants lacking the proximal I-CCAAT box was due to decreased promoter activity. Electrophoretic mobility shift assays (EMSA) indicated that a cellular factor binds specifically to the proximal I-CCAAT box of the UXP promoter. An in vitro luciferase reporter assay demonstrated that basal promoter activity lies between bp -158 and +30 of the transcription initiation site. No E1A-mediated promoter transactivation was observed in 293 cells compared with A549 cells. Thus, we propose that there is a previously unidentified Ad5 promoter that drives expression of the UXP transcription unit. This promoter is embedded within the gene for fiber, and it contains a proximal I-CCAAT box critical for UXP mRNA transcription.

Adenovirus serotype 5 L4-22K and L4-33K proteins have distinct functions in regulating late gene expression

Adenoviruses express up to 20 distinct mRNAs from five major late transcription unit (MLTU) regions, L1 to L5, by differential splicing and polyadenylation of the primary transcript. MLTU expression is regulated at transcriptional and posttranscriptional levels. The L4-33K protein acts as a splicing factor to upregulate several MLTU splice acceptor sites as the late phase progresses. The L4 region also expresses a 22K protein whose sequence is related to the sequence of L4-33K. L4-22K is shown here also to have an important role in regulating the pattern of MLTU gene expression. An adenovirus genome containing a stop codon in the L4-22K open reading frame expressed low levels of both structural and nonstructural late proteins compared to the wild-type (wt) adenovirus genome; a decrease in intermediate proteins, IVa2 and IX, was also observed. However, early protein synthesis and replication were unaffected by the absence of L4-22K. Intermediate and late protein expression was restored to wt levels by L4-22K expressed in trans but not by L4-33K. Increased MLTU promoter activity, resulting from stabilization of the transcriptional activator IVa2 by L4-22K, made a small contribution to this restoration of late gene expression. However, the principal effect of L4-22K was on the processing of MLTU RNA into specific cytoplasmic mRNA. L4-22K selectively increased expression of penton mRNA and protein, whereas splicing to create penton mRNA is known not to be increased by L4-33K. These results indicate that L4-22K plays a key role in the early-late switch in MLTU expression, additional to and distinct from the role of L4-33K.

Adenoviruses: update on structure and function

Adenoviruses have been studied intensively for over 50 years as models of virus-cell interactions and latterly as gene vectors. With the advent of more sophisticated structural analysis techniques the disposition of most of the 13 structural proteins have been defined to a reasonable level. This review seeks to describe the functional properties of these proteins and shows that they all have a part to play in deciding the outcome of an infection and act at every level of the virus's path through the host cell. They are primarily involved in the induction of the different arms of the immune system and a better understanding of their overall properties should lead to more effective ways of combating virus infections.

Adenovirus IVa2 protein binds ATP

IVa2 is an essential, multifunctional protein of adenovirus (Ad) supporting packaging of the viral genome into the capsid, assisting in assembly of the capsid, and activating Ad late transcription. A comparison of IVa2 protein sequences from different species of Adenoviridae shows conserved motifs associated with binding and hydrolysis of ATP (Walker A and B motifs). ATPases are essential proteins of bacteriophage packaging motors, and such activity may be required for Ad packaging. Results presented here show that the Ad2 IVa2 protein binds ATP in vitro and that sequences in the Walker A and B motifs are necessary for this activity.

Control of adenovirus packaging

The results of studies of Adenovirus have contributed to our basic understanding of the molecular biology of the cell. While a great body of knowledge has been developed concerning Ad gene expression, viral replication, and effects on the infected host, the molecular details of the assembly process of Adenovirus particles are largely unknown. In this article, we would like to propose a theoretical model for the packaging and assembly of Adenovirus and present an overview of the studies that have contributed to our present understanding. In particular, we will summarize the molecular details of the process for packaging of viral DNA into virus particles and highlight the events in packaging and assembly that require further study.

Adenovirus type 5 exerts genome-wide control over cellular programs governing proliferation, quiescence, and survival

The effects of the adenovirus Ad5 on basic host cell programs, such as cell-cycle regulation, were studied in a microarray analysis of human fibroblasts. About 2,000 genes were up- or down-regulated after Ad5 infection and Ad5 infection was shown to induce reversal of the quiescence program and recapitulation of the core serum response.

Background

Human adenoviruses, such as serotype 5 (Ad5), encode several proteins that can perturb cellular mechanisms that regulate cell cycle progression and apoptosis, as well as those that mediate mRNA production and translation. However, a global view of the effects of Ad5 infection on such programs in normal human cells is not available, despite widespread efforts to develop adenoviruses for therapeutic applications.

Results

We used two-color hybridization and oligonucleotide microarrays to monitor changes in cellular RNA concentrations as a function of time after Ad5 infection of quiescent, normal human fibroblasts. We observed that the expression of some 2,000 genes, about 10% of those examined, increased or decreased by a factor of two or greater following Ad5 infection, but were not altered in mock-infected cells. Consensus k-means clustering established that the temporal patterns of these changes were unexpectedly complex. Gene Ontology terms associated with cell proliferation were significantly over-represented in several clusters. The results of comparative analyses demonstrate that Ad5 infection induces reversal of the quiescence program and recapitulation of the core serum response, and that only a small subset of the observed changes in cellular gene expression can be ascribed to well characterized functions of the viral E1A and E1B proteins.

Conclusion

These findings establish that the impact of adenovirus infection on host cell programs is far greater than appreciated hitherto. Furthermore, they provide a new framework for investigating the molecular functions of viral early proteins and information relevant to the design of conditionally replicating adenoviral vectors.

The adenovirus L4 33-kilodalton protein binds to intragenic sequences of the major late promoter required for late phase-specific stimulation of transcription

The adenovirus late IVa2 protein is required for maximally efficient transcription from the viral major late (ML) promoter, and hence, the synthesis of the majority of viral late proteins. This protein is a sequence-specific DNA-binding protein that also promotes the assembly of progeny virus particles. Previous studies have established that a IVa2 protein dimer (DEF-B) binds specifically to an intragenic ML promoter sequence necessary for late phase-specific stimulation of ML transcription. However, activation of transcription from the ML promoter correlates with binding of at least one additional infected-cell-specific protein, termed DEF-A, to the promoter. Using an assay for the DNA-binding activity of DEF-A, we identified the unknown protein by using conventional purification methods, purification of FLAG-tagged IVa2-protein-containing complexes, and transient synthesis of viral late proteins. The results of these experiments established that the viral L4 33-kDa protein is the only component of DEF-A: the IVa2 and L4 33-kDa proteins are necessary and sufficient for formation of all previously described complexes in the intragenic control region of the ML promoter. Furthermore, the L4 33-kDa protein binds to the promoter with the specificity characteristic of DEF-A and stimulates transcription from the ML promoter in transient-expression assays.

The L4 22-kilodalton protein plays a role in packaging of the adenovirus genome

Packaging of the adenovirus (Ad) genome into a capsid is absolutely dependent upon the presence of a cis-acting region located at the left end of the genome referred to as the packaging domain. The functionally significant sequences within this domain consist of at least seven similar repeats, referred to as the A repeats, which have the consensus sequence 5' TTTG-N(8)-CG 3'. In vitro and in vivo binding studies have demonstrated that the adenovirus protein IVa2 binds to the CG motif of the packaging sequences. In conjunction with IVa2, another virus-specific protein binds to the TTTG motifs in vitro. The efficient formation of these protein-DNA complexes in vitro was precisely correlated with efficient packaging activity in vivo. We demonstrate that the binding activity to the TTTG packaging sequence motif is the product of the L4 22-kDa open reading frame. Previously, no function had been ascribed to this protein. Truncation of the L4 22-kDa protein in the context of the viral genome did not reduce viral gene expression or viral DNA replication but eliminated the production of infectious virus. We suggest that the L4 22-kDa protein, in conjunction with IVa2, plays a critical role in the recognition of the packaging domain of the Ad genome that leads to viral DNA encapsidation. The L4 22-kDa protein is also involved in recognition of transcription elements of the Ad major late promoter.

A peptide inhibitor of exportin1 blocks shuttling of the adenoviral E1B 55 kDa protein but not export of viral late mRNAs

The human subgroup C adenoviral E1B 55 kDa and E4 Orf6 proteins are required for efficient nuclear export of viral late mRNAs, but the cellular pathway that mediates such export has not been identified. As a first step to develop a general approach to address this issue, we have assessed the utility of cell-permeable peptide inhibitors of cellular export receptors. As both E1B and E4 proteins have been reported to contain a leucine-rich nuclear export signal (NES), we synthesized a cell-permeable peptide containing such an NES. This peptide induced substantial inhibition of export of the E1B protein, whereas a control, non-functional peptide did not. However, under the same conditions, the NES peptide had no effect on export of viral late mRNAs. These observations establish that viral late mRNAs are not exported by exportin1, as well as the value of peptide inhibitors in investigation of mRNA export regulation in adenovirus-infected cells.