Intracellular expression of cellular eIF-5A mutants inhibits HIV-1 replication in human T cells: a feasibility study

eIF5A is activated by virus infection or dsRNA and facilitates virus replication through modulation of interferon production

Active hypusine-modified initiation elongation factor 5A is critical for cell proliferation and differentiation, embryonic development, and innate immune response of macrophages to bacterial infection. Here, we demonstrate that both virus infection and double-stranded RNA viral mimic stimulation induce the hypusination of eIF5A. Furthermore, we show that activation of eIF5A is essential for the replication of several RNA viruses including influenza A virus, vesicular stomatitis virus, chikungunya virus, mayaro virus, una virus, zika virus, and punta toro virus. Finally, our data reveal that inhibition of eIF5A hypusination using the spermidine analog GC7 or siRNA-mediated downmodulation of eIF5A1 induce upregulation of endoplasmic reticulum stress marker proteins and trigger the transcriptional induction of interferon and interferon-stimulated genes, mechanisms that may explain the broad-spectrum antiviral activity of eIF5A inhibition.

Inhibition of HIV-1 gene expression by Ciclopirox and Deferiprone, drugs that prevent hypusination of eukaryotic initiation factor 5A

BACKGROUND

Eukaryotic translation initiation factor eIF5A has been implicated in HIV-1 replication. This protein contains the apparently unique amino acid hypusine that is formed by the post-translational modification of a lysine residue catalyzed by deoxyhypusine synthase and deoxyhypusine hydroxylase (DOHH). DOHH activity is inhibited by two clinically used drugs, the topical fungicide ciclopirox and the systemic medicinal iron chelator deferiprone. Deferiprone has been reported to inhibit HIV-1 replication in tissue culture.

RESULTS

Ciclopirox and deferiprone blocked HIV-1 replication in PBMCs. To examine the underlying mechanisms, we investigated the action of the drugs on eIF5A modification and HIV-1 gene expression in model systems. At early times after drug exposure, both drugs inhibited substrate binding to DOHH and prevented the formation of mature eIF5A. Viral gene expression from HIV-1 molecular clones was suppressed at the RNA level independently of all viral genes. The inhibition was specific for the viral promoter and occurred at the level of HIV-1 transcription initiation. Partial knockdown of eIF5A-1 by siRNA led to inhibition of HIV-1 gene expression that was non-additive with drug action. These data support the importance of eIF5A and hypusine formation in HIV-1 gene expression.

CONCLUSION

At clinically relevant concentrations, two widely used drugs blocked HIV-1 replication ex vivo. They specifically inhibited expression from the HIV-1 promoter at the level of transcription initiation. Both drugs interfered with the hydroxylation step in the hypusine modification of eIF5A. These results have profound implications for the potential therapeutic use of these drugs as antiretrovirals and for the development of optimized analogs.

Molecular cloning and functional expression of bovine deoxyhypusine hydroxylase cDNA and homologs

Deoxyhypusine hydroxylase is the second of the two enzymes that catalyzes the maturation of eukaryotic initiation factor 5A (eIF5A). The mature eIF5A is the only known protein in eukaryotic cells that contains the unusual amino acid hypusine (N(epsilon)-(4-amino-2(R)-hydroxybutyl)lysine). Synthesis of hypusine is essential for the function of eIF5A in eukaryotic cell proliferation and survival. Here, we describe the cloning and characterization of bovine deoxyhypusine hydroxylase cDNA and its homologs. The deduced bovine deoxyhypusine hydroxylase protein is 87% identical to human enzyme and 45% identical to yeast enzyme. The overexpressed enzyme showed activity in catalyzing the hydroxylation of the deoxyhypusine residue in the eIF5A intermediate. An amino acid substitution from Glu 57 to Gly located at one of the four conserved His-Glu (HE) pairs, the potential metal coordination sites, resulted in severe reduction of deoxyhypusine hydroxylase activity. A deletion at the HEAT-repeats 1-3 resulted in complete losses of deoxyhypusine hydroxylase activity.

hnRNP E1 and E2 have distinct roles in modulating HIV-1 gene expression

Pre-mRNA processing, including 5' end capping, splicing, and 3' end cleavage/polyadenylation, are events coordinated by transcription that can influence the subsequent export and translation of mRNAs. Coordination of RNA processing is crucial in retroviruses such as HIV-1, where inefficient splicing and the export of intron-containing RNAs are required for expression of the full complement of viral proteins. RNA processing can be affected by both viral and cellular proteins, and in this study we demonstrate that a member of the hnRNP E family of proteins can modulate HIV-1 RNA metabolism and expression. We show that hnRNP E1/E2 are able to interact with the ESS3a element of the bipartite ESS in tat/rev exon 3 of HIV-1 and that modulation of hnRNP E1 expression alters HIV-1 structural protein synthesis. Overexpression of hnRNP E1 leads to a reduction in Rev, achieved in part through a decrease in rev mRNA levels. However, the reduction in Rev levels cannot fully account for the effect of hnRNP E1, suggesting that hmRNP E1 might also act to suppress viral RNA translation. Deletion mutagenesis determined that the C-terminal end of hnRNP E1 was required for the reduction in Rev expression and that replacing this portion of hnRNP E1 with that of hnRNP E2, despite the high degree of conservation, could not rescue the loss of function.

Human immunodeficiency virus type 2 Gag interacts specifically with PRP4, a serine-threonine kinase, and inhibits phosphorylation of splicing factor SF2

Using a yeast two-hybrid screen of a T-cell cDNA library to identify cellular proteins that bind to the human immunodeficiency virus type 2 (HIV-2) Gag polyprotein, we identified PRP4, a serine-threonine protein kinase. Specific interaction of PRP4 and HIV-2 Gag was confirmed in in vitro and in vivo assays. The interacting region of HIV-2 Gag is located in the conserved matrix and capsid domains, while both the RS (arginine-serine-rich) domain and the KS (kinase) domain of PRP4 are able to bind to HIV-2 Gag. PRP4 is not incorporated into virus particles. HIV-2 Gag is able to inhibit PRP4-mediated phosphorylation of the splicing factor SF2. This is also observed with Gag from simian immunodeficiency virus, a closely related virus, but not with Gag from human T-cell lymphotropic virus type 1. Our results provide evidence for a novel interaction between Gag and a cellular protein kinase involved in the control of constitutive splicing in two closely related retroviruses. We hypothesize that as Gag accumulates in the cell, down regulation of splicing occurs through reduced phosphorylation of SF2. At late stages of infection, this interaction may replace the function of the early viral regulatory protein Rev.

Screening assay for the identification of deoxyhypusine synthase inhibitors

The 1st step in the posttranslational hypusine [N(epsilon)-(4-amino-2-hydroxybutyl)lysine] modification of eukaryotic translation initiation factor 5A (eIF5A) is catalyzed by deoxyhypusine synthase (DHS). The eIF5A intermediate is subsequently hydroxylated by deoxyhypusine hydroxylase (DHH), thereby converting the eIF5A precursor into a biologically active protein. Depletion of eIF5A causes inhibition of cell growth, and the identification of eIF5A as a cofactor of the HIV Rev protein turns this host protein and therefore DHS into an interesting target for drugs against abnormal cell growth and/or HIV replication. The authors developed a 96-well format DHS assay applicable for the screening of DHS inhibitors. Using this assay, they demonstrate DHS inhibition by AXD455 (Semapimod, CNI-1493). This assay represents a powerful tool for the identification of new DHS inhibitors with potency against cancer and HIV.

Molecular cloning of bovine eIF5A and deoxyhypusine synthase cDNA

Deoxyhypusine synthase is the first of the two enzymes that catalyzes the maturation of eukaryotic initiation factor 5A (eIF5A). The mature eIF5A is the only known protein in eukaryotic cells that contains the unusual amino acid hypusine (N(epsilon)-(4-amino-2(R)-hydroxybutyl)-lysine). Synthesis of hypusine is essential for the function of eIF5A in eukaryotic cell proliferation and survival. Here we describe the cloning and characterization of bovine eIF5A and bovine deoxyhypusine synthase. The deduced bovine eIF5A protein is 100% identical to human eIF5A-1, and the deduced bovine deoxyhypusine synthase protein showed a 93% identity to the human protein.

Effects of 1,8-diaminooctane on the FIV Rev regulatory system

Proper function of the Rev regulatory system is essential for the replication of lentiviruses, including feline immunodeficiency virus (FIV) and human immunodeficiency virus type 1 (HIV-1). Specifically, Rev affects the overall stability of viral mRNAs that encode necessary structural and enzymatic proteins. In turn, the eukaryotic initiation factor (eIF-5A) is indispensable for Rev function and is the only known protein whose biologically active form requires the unique amino acid, hypusine. Because 1,8-diaminooctane blocks the formation of hypusine by disrupting the cellular enzyme, deoxyhypusine synthase, thereby preventing activation of eIF-5A, we investigated the effects of 1,8-diaminooctane on posttranscriptional regulation. These are the first results to demonstrate that diaminooctane significantly reduced viral replication in a dose-dependent manner, even under conditions of contact inhibition, diminishing the compound's effect on cell proliferation. Similarly, the addition of increased concentrations of diaminooctane caused a reduction in the expression of a Rev-dependent CAT system without affecting a Rev-independent CAT system. At the RNA level, exposure of chronically infected CrFK cells to increasing concentrations of diaminooctane substantially decreased the levels of unspliced and singly spliced viral mRNAs and increased the relative amounts of multiply spliced transcripts in the cytoplasm. The findings of this study are the first demonstration that FIV, similar to HIV-1, requires eIF-5A for efficient Rev function and that small molecule intervention can indirectly target this lentivirus regulatory system.

Nucleocytoplasmic RNA transport in retroviral replication

Retroviral replication is highly dependent on post-transcriptional regulation because a single primary transcript directs synthesis of many viral proteins. The identification and characterization of two post-transcriptional regulatory systems (Rev/RRE and CTE) revealed the efficient use of cellular transport pathways by retroviruses to achieve production of infectious progeny virus. The Rev/RRE system of HIV-1 consists of the viral Rev protein which binds to its target sequence on incompletely spliced RNAs and channels these into the CRM1-dependent export pathway, which is normally used for export of cellular proteins and RNAs (U snRNAs and 5 S rRNA). The CTE, on the other hand, directly recruits the cellular mRNA export receptor TAP to the viral RNA. Both systems have in common that they recruit a key player of a specific cellular export pathway and this recruitment appears to out-compete the respective cellular target molecules. The fact that CTE can functionally substitute for Rev/RRE, yielding a replication-competent virus, indicates that very short sequence elements are sufficient for post-transcriptional control. The presence of short dominant export signals could relieve the selective pressure on the remainder of the genome to maintain a sequence that is easily exported. The resultant increase in permitted sequence space may increase the potential for immune escape, thereby providing a selective advantage for the virus. Replication of the CTE-dependent HIV-1 variant is significantly impaired compared with the wild-type virus. Considering that post-transcriptional control in the case of HIV is also used to provide a temporal switch from the early phase of regulatory protein expression to the late phase of virion production, one may suggest that the CRM1 export pathway is advantageous for the rapid delivery of large amounts of cargo (i.e. HIV RNA). This would be in accordance with its normal function because CRM1 has been shown to direct the nuclear export of cellular regulatory proteins which must be accomplished rapidly as well. In summary, retroviruses have evolved fascinating ways to deal with their cellular environment and to make use of cellular transport pathways, allowing nuclear export of intron-containing RNAs which are normally restricted to the nucleus. Specific signals on the viral RNAs recruit key factors of cellular export, thus bypassing these restrictions and ensuring efficient viral replication.

Isolation and characterization of senescence-induced cDNAs encoding deoxyhypusine synthase and eucaryotic translation initiation factor 5A from tomato

Full-length cDNA clones encoding deoxyhypusine synthase (DHS) and eucaryotic initiation factor 5A (eIF-5A) have been isolated from a cDNA expression library prepared from tomato leaves (Lycopersicon esculentum, cv. Match) exposed to environmental stress. DHS mediates the first of two enzymatic reactions that activate eIF-5A by converting a conserved lysine to the unusual amino acid, deoxyhypusine. Recombinant protein obtained by expressing tomato DHS cDNA in Escherichia coli proved capable of carrying out the deoxyhypusine synthase reaction in vitro in the presence of eIF-5A. Of particular interest is the finding that DHS mRNA and eIF-5A mRNA show a parallel increase in abundance in senescing tomato flowers, senescing tomato fruit, and environmentally stressed tomato leaves exhibiting programmed cell death. Western blot analyses indicated that DHS protein also increases at the onset of senescence. It is apparent from previous studies with yeast and mammalian cells that hypusine-modified eIF-5A facilitates the translation of a subset of mRNAs mediating cell division. The present study provides evidence for senescence-induced DHS and eIF-5A in tomato tissues that may facilitate the translation of mRNA species required for programmed cell death.

Cofactor requirements for nuclear export of Rev response element (RRE)- and constitutive transport element (CTE)-containing retroviral RNAs. An unexpected role for actin

Nuclear export of proteins containing leucine-rich nuclear export signals (NESs) is mediated by the export receptor CRM1/exportin1. However, additional protein factors interacting with leucine-rich NESs have been described. Here, we investigate human immunodeficiency virus type 1 (HIV-1) Rev-mediated nuclear export and Mason-Pfizer monkey virus (MPMV) constitutive transport element (CTE)-mediated nuclear export in microinjected Xenopus laevis oocytes. We show that eukaryotic initiation factor 5A (eIF-5A) is essential for Rev and Rev-mediated viral RNA export, but not for nuclear export of CTE RNA. In vitro binding studies demonstrate that eIF-5A is required for efficient interaction of Rev-NES with CRM1/exportin1 and that eIF-5A interacts with the nucleoporins CAN/nup214, nup153, nup98, and nup62. Quite unexpectedly, nuclear actin was also identified as an eIF-5A binding protein. We show that actin is associated with the nucleoplasmic filaments of nuclear pore complexes and is critically involved in export processes. Finally, actin- and energy-dependent nuclear export of HIV-1 Rev is reconstituted by using a novel in vitro egg extract system. In summary, our data provide evidence that actin plays an important functional role in nuclear export not only of retroviral RNAs but also of host proteins such as protein kinase inhibitor (PKI).

Lentivirus replication and regulation

Lentiviruses are associated with chronic diseases of the hematological and neurological systems in animals and man. In particular, human immunodeficiency virus type 1 (HIV-1) is the etiological agent of the global AIDS epidemic. The genomes of lentiviruses are complex, encoding a number of regulatory and accessory proteins not found in other retroviruses. This complexity is reflected in their replication cycle, which reveals intricate regulatory pathways and unique mechanisms for viral persistence. In this review, we highlight some of these unique features for HIV-1, with particular focus on the transcriptional and posttranscriptional control of gene expression. Although our understanding of the biology of HIV-1 is far from complete, the knowledge gained thus far has already led to novel strategies for both virus intervention and exploiting the lentiviruses for therapeutic applications.

Nuclear pore localization and nucleocytoplasmic transport of eIF-5A: evidence for direct interaction with the export receptor CRM1

Eukaryotic initiation factor 5A (eIF-5A) is the only cellular protein known to contain the unusual amino acid hypusine. The exact in vivo function of eIF-5A, however, is to date unknown. The finding that eIF-5A is an essential cofactor of the human immunodeficiency virus type 1 (HIV-1) Rev RNA transport factor suggested that eIF-5A is part of a specific nuclear export pathway. In this study we used indirect immunofluorescence and immunogold electron microscopy to demonstrate that eIF-5A accumulates at nuclear pore-associated intranuclear filaments in mammalian cells and Xenopus oocytes. We are able to show that eIF-5A interacts with the general nuclear export receptor, CRM1. Furthermore, microinjection studies in somatic cells revealed that eIF-5A is transported from the nucleus to the cytoplasm, and that this nuclear export is blocked by leptomycin B. Our data demonstrate that eIF-5A is a nucleocytoplasmic shuttle protein.

Evidence for specific nucleocytoplasmic transport pathways used by leucine-rich nuclear export signals

Various proteins with different biological activities have been observed to be translocated from the nucleus to the cytoplasm in an energy- and signal-dependent manner in eukaryotic cells. This nuclear export is directed by nuclear export signals (NESs), typically characterized by hydrophobic, primarily leucine, amino acid residues. Moreover, it has been shown that CRM1/exportin 1 is an export receptor for leucine-rich NESs. However, additional NES-interacting proteins have been described. In particular, eukaryotic initiation factor 5A (eIF-5A) has been shown to be a critical cellular cofactor for the nuclear export of the HIV type 1 (HIV-1) Rev trans-activator protein. In this study we compared the nuclear export activity of NESs of different origin. Microinjection of export substrates into the nucleus of somatic cells in combination with specific inhibitors indicated that specific nuclear export pathways exist for different NES-containing proteins. In particular, inhibition of eIF-5A blocked the nuclear export of NESs derived from the HIV-1 Rev and human T cell leukemia virus type I Rex trans-activators, whereas nucleocytoplasmic translocation of the protein kinase inhibitor-NES was unaffected. In contrast, however, inhibition of CRM1/exportin 1 blocked the nuclear export of all NES-containing proteins investigated. Our data confirm that CRM1/exportin 1 is a general export receptor for leucine-rich NESs and suggest that eIF-5A acts either upstream of CRM1/exportin 1 or forms a complex with the NES and CRM1/exportin 1 in the nucleocytoplasmic translocation of the HIV-1 Rev and human T cell leukemia virus type I Rex RNA export factors.

Effect of polyamine analogues on hypusine content in JURKAT T-cells

The availability of synthetic hypusine and deoxyhypusine has made it possible to develop analytical methods which allow for the measurement of these compounds in various tissues. The methods involve dansylation of extracts from the pellet remaining after perchloric acid precipitation of cell or tissue homogenates, followed by high-performance liquid chromatography. To demonstrate the utility of this approach, the impact of four polyamine analogues, N1,N11-diethylnorspermine (DENSPM), N1,N14-diethylhomospermine (DEHSPM), 1,6,12-triazadodecane [(4,5) triamine], and 1,7, 13-triazatridecane [(5,5) triamine], on hypusine levels in a human T-cell line (JURKAT) is evaluated. All four analogues are active in controlling cell growth and compete well with spermidine for the polyamine transport apparatus. After 144 h of exposure to JURKAT cells, DENSPM reduces putrescine to below detectable limits and spermidine to 10% of the level in control cells. The other three analogues diminish both putrescine and spermidine to below detectable limits. The effectiveness with which the compounds lower spermine levels is DENSPM > DEHSPM > (4,5) triamine > (5,5) triamine. The analogues decrease the activities of ornithine decarboxylase and S-adenosylmethionine decarboxylase in a similar fashion. Of the four polyamines, DENSPM and DEHSPM are potent at lowering intracellular hypusine levels after 144 h: 59 +/- 9% and 73 +/- 12% of control levels, respectively. The other two analogues have marginal effects.

Synthesis of reagents for the construction of hypusine and deoxyhypusine peptides and their application as peptidic antigens

Two new synthetic methods which allow access to (2S)-deoxyhypusine, natural (2S,9R)-hypusine, (2S,9S)-hypusine, and deoxyhypusine- and hypusine-containing peptides are described. The methods involve both the construction of a deoxyhypusine reagent in which the alpha-nitrogen protecting group is orthogonal to the N-7 and N-12 protecting groups and an alternate synthesis of our previous hypusine reagent, a synthesis which provides for better stereochemical control at C-9. Synthetic hypusine and deoxyhypusine can be generated from these reagents. The hypusine-containing hexapeptide (Cys-Thr-Gly-Hpu-His-Gly) is conjugated to ovalbumin (OVA), keyhole limpet hemocyanin (KLH), and a bis-maleimide; KLH conjugates are also made with the deoxyhypusine- and lysine-containing hexapeptides. Monoclonal antibodies are generated to the hypusine-containing hexapeptide-OVA conjugate in mice. These are isolated and screened against the hypusine-containing hexapeptide-KLH and hypusine-containing hexapeptide-bis-maleimide conjugates, as well as against the deoxyhypusine-containing and lysine-containing hexapeptide-KLH conjugates. These antibodies may be useful in localizing intracellular hypusine-containing peptides as well as peptides containing hypusine analogues.

Antiretroviral effects of deoxyhypusyl hydroxylase inhibitors: a hypusine-dependent host cell mechanism for replication of human immunodeficiency virus type 1 (HIV-1)

The HIV-1 protein Rev, critical for translation of incompletely spliced retroviral mRNAs encoding capsid elements, requires a host cell protein termed "eukaryotic initiation factor 5A" (eIF-5A). This is the only protein containing hypusine, a lysine-derived hydroxylated residue that determines its proposed bioactivity, the translation of a subset of cellular mRNAs controlling G1-to-S transit of the cell cycle. We postulated that inhibiting the hypusine-forming deoxyhypusyl hydroxylase (DOHH) should, by depleting eukaryotic initiation factor 5A, compromise Rev function and thus reduce HIV-1 multiplication. We now report that the alpha-hydroxypyridones, specifically mimosine, a natural product, and deferiprone, an experimental drug, inhibited deoxyhypusyl hydroxylase in T-lymphocytic and promonocytic cell lines and, in a concentration-dependent manner, suppressed replication of HIV-1. However, the alpha-hydroxypyridones did not affect the formation of unspliced or multiply spliced HIV-1 transcripts. Rather, these agents caused Rev-dependent incompletely spliced HIV-1 mRNA such as gag, but not cellular "housekeeping" mRNAs, to disappear from polysomes. Consequently, alpha-hydroxypyridone-mediated depletion of eIF-5A decreased biosynthesis of structural HIV-1 protein encoded by gag, measured as p24, whereas the induced formation of cellular protein like tumor necrosis factor alpha remained unaffected. By interfering with the translation of incompletely spliced retroviral mRNAs, these compounds restrict HIV-1 to the early, nongenerative phase of its reproductive cycle. In the inducibly HIV-1 expressing T-cell line ACH-2, the deoxyhypusyl hydroxylase inhibitors triggered extensive apoptosis, particularly of cells that actively produce HIV-1. Selective suppression of retroviral protein biosynthesis and preferential apoptosis of retrovirally infected cells by alpha-hydroxypyridones point to a novel mode of antiretroviral action.

Functional analysis of the human immunodeficiency virus type 1 Rev protein oligomerization interface

The expression of human immunodeficiency virus type 1 (HIV-1) structural proteins requires the action of the viral trans-regulatory protein Rev. Rev is a nuclear shuttle protein that directly binds to its cis-acting Rev response element (RRE) RNA target sequence. Subsequent oligomerization of Rev monomers on the RRE and interaction of Rev with a cellular cofactor(s) result in the cytoplasmic accumulation of RRE-containing viral mRNAs. Moreover, Rev by itself is exported from the nucleus to the cytoplasm. Although it has been demonstrated that Rev multimerization is critically required for Rev activity and hence for HIV-1 replication, the number of Rev monomers required to form a trans-activation-competent complex on the RRE is unknown. Here we report a systematic analysis of the putative multimerization domains within the Rev trans-activator protein. We identify the amino acid residues which are part of the proposed single hydrophobic surface patch in the Rev amino terminus that mediates intermolecular interactions. Furthermore, we show that the expression of a multimerization-deficient Rev mutant blocks HIV-1 replication in a trans-dominant (dominant-negative) fashion.

Interaction of the HIV-1 rev cofactor eukaryotic initiation factor 5A with ribosomal protein L5

It has previously been shown that interaction of eukaryotic initiation factor 5A (eIF-5A) with the Rev trans-activator protein of HIV-1 mediates the transport of unspliced or incompletely spliced viral mRNAs across the nuclear envelope. Consequently, mutants of eIF-5A block Rev function and thereby replication of HIV-1 in trans, indicating that eIF-5A is a crucial protein that connects the viral Rev regulator with cellular RNA transport systems. Here we show that the ribosomal protein L5, which is the central protein component of the 5S rRNA export system, is a cellular interaction partner of eIF-5A. Functional studies demonstrate that overexpression of L5 protein significantly enhances Rev activity. Furthermore, Rev nuclear export activity is inhibited in human somatic cells by antibodies that recognize eIF-5A or L5. Our data suggest that the Rev export pathway shares components of a cellular transport system involved in the intracellular trafficking of polymerase III (5S rRNA) transcripts.

The HIV-1 Rev protein

The nuclear export of intron-containing HIV-1 RNA is critically dependent on the activity of Rev, a virally encoded sequence-specific RNA-binding protein. Rev shuttles between the nucleus and the cytoplasm and harbors both a nuclear localization signal and a nuclear export signal. These essential peptide motifs have now been shown to function by accessing cellular signal-mediated pathways for nuclear import and nuclear export. HIV-1 Rev therefore represents an excellent system with which to study aspects of transport across the nuclear envelope.

Viral RNA export

Viruses can express intron-containing and intronless mRNAs, which are exported by alternative pathways. The study of the nuclear export of these unconventional mRNAs can provide key insights into the normal process of nuclear export and the alternative pathways provide an attractive target for the development of specific antiviral therapies.