Time-resolved profiling of RNA binding proteins throughout the mRNA life cycle.

mRNAs continually change their protein partners throughout their lifetimes, yet our understanding of mRNA-protein complex (mRNP) remodeling is limited by a lack of temporal data. Here, we present time-resolved mRNA interactome data by performing pulse metabolic labeling with photoactivatable ribonucleoside in human cells, UVA crosslinking, poly(A)+ RNA isolation, and mass spectrometry. This longitudinal approach allowed the quantification of over 700 RNA binding proteins (RBPs) across ten time points. Overall, the sequential order of mRNA binding aligns well with known functions, subcellular locations, and molecular interactions. However, we also observed RBPs with unexpected dynamics: the transcription-export (TREX) complex recruited posttranscriptionally after nuclear export factor 1 (NXF1) binding, challenging the current view of transcription-coupled mRNA export, and stress granule proteins prevalent in aged mRNPs, indicating roles in late stages of the mRNA life cycle. To systematically identify mRBPs with unknown functions, we employed machine learning to compare mRNA binding dynamics with Gene Ontology (GO) annotations. Our data can be explored at chronology.rna.snu.ac.kr.

Gene Architecture and Sequence Composition Underpin Selective Dependency of Nuclear Export of Long RNAs on NXF1 and the TREX Complex.

The core components of the nuclear RNA export pathway are thought to be required for export of virtually all polyadenylated RNAs. Here, we depleted different proteins that act in nuclear export in human cells and quantified the transcriptome-wide consequences on RNA localization. Different genes exhibited substantially variable sensitivities, with depletion of NXF1 and TREX components causing some transcripts to become strongly retained in the nucleus while others were not affected. Specifically, NXF1 is preferentially required for export of single- or few-exon transcripts with long exons or high A/U content, whereas depletion of TREX complex components preferentially affects spliced and G/C-rich transcripts. Using massively parallel reporter assays, we identified short sequence elements that render transcripts dependent on NXF1 for their export and identified synergistic effects of splicing and NXF1. These results revise the current model of how nuclear export shapes the distribution of RNA within human cells.

The mRNA Export Receptor NXF1 Coordinates Transcriptional Dynamics, Alternative Polyadenylation, and mRNA Export.

Alternative polyadenylation (APA) produces mRNA isoforms with different 3' UTR lengths. Previous studies indicated that 3' end processing and mRNA export are intertwined in gene regulation. Here, we show that mRNA export factors generally facilitate usage of distal cleavage and polyadenylation sites (PASs), leading to long 3' UTR isoform expression. By focusing on the export receptor NXF1, which exhibits the most potent effect on APA in this study, we reveal several gene features that impact NXF1-dependent APA, including 3' UTR size, gene size, and AT content. Surprisingly, NXF1 downregulation results in RNA polymerase II (Pol II) accumulation at the 3' end of genes, correlating with its role in APA regulation. Moreover, NXF1 cooperates with CFI-68 to facilitate nuclear export of long 3' UTR isoform with UGUA motifs. Together, our work reveals important roles of NXF1 in coordinating transcriptional dynamics, 3' end processing, and nuclear export of long 3' UTR transcripts, implicating NXF1 as a nexus of gene regulation.

Inhibition of mRNA nuclear export promotes SARS-CoV-2 pathogenesis.

The nonstructural protein 1 (Nsp1) of SARS-CoV-2 (Severe Acute Respiratory Syndrome Coronavirus 2) is a virulence factor that targets multiple cellular pathways to inhibit host gene expression and antiviral response. However, the underlying mechanisms of the various Nsp1-mediated functions and their contributions to SARS-CoV-2 virulence remain unclear. Among the targets of Nsp1 is the mRNA (messenger ribonucleic acid) export receptor NXF1-NXT1, which mediates nuclear export of mRNAs from the nucleus to the cytoplasm. Based on Nsp1 crystal structure, we generated mutants on Nsp1 surfaces and identified an acidic N-terminal patch that is critical for interaction with NXF1-NXT1. Photoactivatable Nsp1 probe reveals the RNA Recognition Motif (RRM) domain of NXF1 as an Nsp1 N-terminal binding site. By mutating the Nsp1 N-terminal acidic patch, we identified a separation-of-function mutant of Nsp1 that retains its translation inhibitory function but substantially loses its interaction with NXF1 and reverts Nsp1-mediated mRNA export inhibition. We then generated a recombinant (r)SARS-CoV-2 mutant on the Nsp1 N-terminal acidic patch and found that this surface is key to promote NXF1 binding and inhibition of host mRNA nuclear export, viral replication, and pathogenicity in vivo. Thus, these findings provide a mechanistic understanding of Nsp1-mediated mRNA export inhibition and establish the importance of this pathway in the virulence of SARS-CoV-2.

SUN5, a testis-specific nuclear membrane protein, participates in recruitment and export of nuclear mRNA in spermatogenesis.

SUN5, a testis-specific gene, is associated with acephalic spermatozoa syndrome (ASS). Here, we demonstrate that Sun5 is involved in mRNA export. In Sun5-knockout mice ( Sun5 -/-), poly(A) + RNA accumulates in the nuclei of germ cells, leading to reduced sperm counts, decreased sperm motility and disrupted sperm head-to-tail junctions. Additionally, in the GC-2 germ cell line with RNA interference of Sun5, heterogeneous nuclear ribonucleoproteins (hnRNPs) and poly (A) + RNA (mainly mRNA) are retained in the nucleus. Further mechanistic studies reveal that Sun5 interacts with Nxf1 (nuclear RNA export factor 1) and nucleoporin 93 (Nup93). Interference with Nup93 inhibits mRNA export. Treatment with leptomycin B to block the CRM1 pathway indicates that Sun5 regulates mRNA export through an Nxf1-dependent pathway. In Sun5 -/- mice, the binding of Nxf1 and Nup93 decreases due to loss of Sun5 function, and the process of submitting Nxf1-binding mRNPs to Nup93 is inhibited, resulting in abnormal spermatogenesis. Together, these data may elucidate a novel pathway for mRNA export in male germ cells.

Coupling pre-mRNA splicing and 3' end formation to mRNA export: alternative ways to punch the nuclear export clock.

How does a mammalian cell determine when newly synthesized mRNAs are fully processed and appropriate for nuclear export? Müller-McNicoll and colleagues (pp. 553-566) expand on mechanisms known to be mediated by nuclear export factor 1 (NXF1) by describing SR proteins as NXF1 adaptors that flag alternatively spliced and polyadenylated mRNA isoforms as cargo ready for the cytoplasm.

SR proteins are NXF1 adaptors that link alternative RNA processing to mRNA export.

Nuclear export factor 1 (NXF1) exports mRNA to the cytoplasm after recruitment to mRNA by specific adaptor proteins. How and why cells use numerous different export adaptors is poorly understood. Here we critically evaluate members of the SR protein family (SRSF1-7) for their potential to act as NXF1 adaptors that couple pre-mRNA processing to mRNA export. Consistent with this proposal, >1000 endogenous mRNAs required individual SR proteins for nuclear export in vivo. To address the mechanism, transcriptome-wide RNA-binding profiles of NXF1 and SRSF1-7 were determined in parallel by individual-nucleotide-resolution UV cross-linking and immunoprecipitation (iCLIP). Quantitative comparisons of RNA-binding sites showed that NXF1 and SR proteins bind mRNA targets at adjacent sites, indicative of cobinding. SRSF3 emerged as the most potent NXF1 adaptor, conferring sequence specificity to RNA binding by NXF1 in last exons. Interestingly, SRSF3 and SRSF7 were shown to bind different sites in last exons and regulate 3' untranslated region length in an opposing manner. Both SRSF3 and SRSF7 promoted NXF1 recruitment to mRNA. Thus, SRSF3 and SRSF7 couple alternative splicing and polyadenylation to NXF1-mediated mRNA export, thereby controlling the cytoplasmic abundance of transcripts with alternative 3' ends.

Evidence for Viral mRNA Export from Ebola Virus Inclusion Bodies by the Nuclear RNA Export Factor NXF1.

Many negative-sense RNA viruses, including the highly pathogenic Ebola virus (EBOV), use cytoplasmic inclusion bodies (IBs) for viral RNA synthesis. However, it remains unclear how viral mRNAs are exported from these IBs for subsequent translation. We recently demonstrated that the nuclear RNA export factor 1 (NXF1) is involved in a late step in viral protein expression, i.e., downstream of viral mRNA transcription, and proposed it to be involved in this mRNA export process. We now provide further evidence for this function by showing that NXF1 is not required for translation of viral mRNAs, thus pinpointing its function to a step between mRNA transcription and translation. We further show that RNA binding of both NXF1 and EBOV NP is necessary for export of NXF1 from IBs, supporting a model in which NP hands viral mRNA over to NXF1 for export. Mapping of NP-NXF1 interactions allowed refinement of this model, revealing two separate interaction sites, one of them directly involving the RNA binding cleft of NP, even though these interactions are RNA-independent. Immunofluorescence analyses demonstrated that individual NXF1 domains are sufficient for its recruitment into IBs, and complementation assays helped to define NXF1 domains important for its function in the EBOV life cycle. Finally, we show that NXF1 is also required for protein expression of other viruses that replicate in cytoplasmic IBs, including Lloviu and Junín virus. These data suggest a role for NXF1 in viral mRNA export from IBs for various viruses, making it a potential target for broadly active antivirals. IMPORTANCE Filoviruses such as the Ebola virus (EBOV) cause severe hemorrhagic fevers with high case fatality rates and limited treatment options. The identification of virus-host cell interactions shared among several viruses would represent promising targets for the development of broadly active antivirals. In this study, we reveal the mechanistic details of how EBOV usurps the nuclear RNA export factor 1 (NXF1) to export viral mRNAs from viral inclusion bodies (IBs). We further show that NXF1 is not only required for the EBOV life cycle but also necessary for other viruses known to replicate in cytoplasmic IBs, including the filovirus Lloviu virus and the highly pathogenic arenavirus Junín virus. This suggests NXF1 as a promising target for the development of broadly active antivirals.

GC-content biases in protein-coding genes act as an "mRNA identity" feature for nuclear export.

It has long been observed that human protein-coding genes have a particular distribution of GC-content: the 5' end of these genes has high GC-content while the 3' end has low GC-content. In 2012, it was proposed that this pattern of GC-content could act as an mRNA identity feature that would lead to it being better recognized by the cellular machinery to promote its nuclear export. In contrast, junk RNA, which largely lacks this feature, would be retained in the nucleus and targeted for decay. Now two recent papers have provided evidence that GC-content does promote the nuclear export of many mRNAs in human cells.

Virus Infection and mRNA Nuclear Export.

Gene expression in eukaryotes begins with transcription in the nucleus, followed by the synthesis of messenger RNA (mRNA), which is then exported to the cytoplasm for its translation into proteins. Along with transcription and translation, mRNA export through the nuclear pore complex (NPC) is an essential regulatory step in eukaryotic gene expression. Multiple factors regulate mRNA export and hence gene expression. Interestingly, proteins from certain types of viruses interact with these factors in infected cells, and such an interaction interferes with the mRNA export of the host cell in favor of viral RNA export. Thus, these viruses hijack the host mRNA nuclear export mechanism, leading to a reduction in host gene expression and the downregulation of immune/antiviral responses. On the other hand, the viral mRNAs successfully evade the host surveillance system and are efficiently exported from the nucleus to the cytoplasm for translation, which enables the continuation of the virus life cycle. Here, we present this review to summarize the mechanisms by which viruses suppress host mRNA nuclear export during infection, as well as the key strategies that viruses use to facilitate their mRNA nuclear export. These studies have revealed new potential antivirals that may be used to inhibit viral mRNA transport and enhance host mRNA nuclear export, thereby promoting host gene expression and immune responses.

The Great Escape: mRNA Export through the Nuclear Pore Complex.

Nuclear export of messenger RNA (mRNA) through the nuclear pore complex (NPC) is an indispensable step to ensure protein translation in the cytoplasm of eukaryotic cells. mRNA is not translocated on its own, but it forms ribonuclear particles (mRNPs) in association with proteins that are crucial for its metabolism, some of which; like Mex67/MTR2-NXF1/NXT1; are key players for its translocation to the cytoplasm. In this review, I will summarize our current body of knowledge on the basic characteristics of mRNA export through the NPC. To be granted passage, the mRNP cargo needs to bind transport receptors, which facilitate the nuclear export. During NPC transport, mRNPs undergo compositional and conformational changes. The interactions between mRNP and the central channel of NPC are described; together with the multiple quality control steps that mRNPs undergo at the different rings of the NPC to ensure only proper export of mature transcripts to the cytoplasm. I conclude by mentioning new opportunities that arise from bottom up approaches for a mechanistic understanding of nuclear export.

Nuclear retention of mRNAs - quality control, gene regulation and human disease.

Nuclear retention of incompletely spliced or mature mRNAs emerges as a novel, previously underappreciated layer of gene regulation, which enables the cell to rapidly respond to stress, viral infection, differentiation cues or changing environmental conditions. Focusing on mammalian cells, we discuss recent insights into the mechanisms and functions of nuclear retention, describe retention-promoting features in protein-coding transcripts and propose mechanisms for their regulated release into the cytoplasm. Moreover, we discuss examples of how aberrant nuclear retention of mRNAs may lead to human diseases.

NXF1 and CRM1 nuclear export pathways orchestrate nuclear export, translation and packaging of murine leukaemia retrovirus unspliced RNA.

Cellular mRNAs are exported from the nucleus as fully spliced RNAs. Proofreading mechanisms eliminate unprocessed and irregular pre-mRNAs to control the quality of gene expression. Retroviruses need to export partially spliced and unspliced full-length RNAs to the cytoplasm where they serve as templates for protein synthesis and/or as encapsidated RNA in progeny viruses. Genetically complex retroviruses such as HIV-1 use Rev-equivalent proteins to export intron-retaining RNA from the nucleus using the cellular CRM1-driven nuclear export machinery. By contrast, genetically simpler retroviruses such as murine leukaemia virus (MLV) recruit the NXF1 RNA export machinery. In this study, we reveal for the first time that MLV hijacks both NXF1 and CRM1-dependent pathways to achieve optimal replication capacity. The CRM1-pathway marks the MLV full-length RNA (FL RNA) for packaging, while NXF1-driven nuclear export is coupled to translation. Thus, the cytoplasmic function of the viral RNA is determined early in the nucleus. Depending on the nature of ribonucleoprotein complex formed on FL RNA cargo in the nucleus, the FL RNA will be addressed to the translation machinery sites or to the virus-assembly sites at the plasma membrane.

Epstein-Barr Virus-Induced Nodules on Viral Replication Compartments Contain RNA Processing Proteins and a Viral Long Noncoding RNA.

Profound alterations in host cell nuclear architecture accompany the lytic phase of Epstein-Barr virus (EBV) infection. Viral replication compartments assemble, host chromatin marginalizes to the nuclear periphery, cytoplasmic poly(A)-binding protein translocates to the nucleus, and polyadenylated mRNAs are sequestered within the nucleus. Virus-induced changes to nuclear architecture that contribute to viral host shutoff (VHS) must accommodate selective processing and export of viral mRNAs. Here we describe additional previously unrecognized nuclear alterations during EBV lytic infection in which viral and cellular factors that function in pre-mRNA processing and mRNA export are redistributed. Early during lytic infection, before formation of viral replication compartments, two cellular pre-mRNA splicing factors, SC35 and SON, were dispersed from interchromatin granule clusters, and three mRNA export factors, Y14, ALY, and NXF1, were depleted from the nucleus. During late lytic infection, virus-induced nodular structures (VINORCs) formed at the periphery of viral replication compartments. VINORCs were composed of viral (BMLF1 and BGLF5) and cellular (SC35, SON, SRp20, and NXF1) proteins that mediate pre-mRNA processing and mRNA export. BHLF1 long noncoding RNA was invariably found in VINORCs. VINORCs did not contain other nodular nuclear cellular proteins (PML or coilin), nor did they contain viral proteins (BRLF1 or BMRF1) found exclusively within replication compartments. VINORCs are novel EBV-induced nuclear structures. We propose that EBV-induced dispersal and depletion of pre-mRNA processing and mRNA export factors during early lytic infection contribute to VHS; subsequent relocalization of these pre-mRNA processing and mRNA export proteins to VINORCs and viral replication compartments facilitates selective processing and export of viral mRNAs.IMPORTANCE In order to make protein, mRNA transcribed from DNA in the nucleus must enter the cytoplasm. Nuclear export of mRNA requires correct processing of mRNAs by enzymes that function in splicing and nuclear export. During the Epstein-Barr virus (EBV) lytic cycle, nuclear export of cellular mRNAs is blocked, yet export of viral mRNAs is facilitated. Here we report the dispersal and dramatic reorganization of cellular (SC35, SON, SRp20, Y14, ALY, and NXF1) and viral (BMLF1 and BGLF5) proteins that play key roles in pre-mRNA processing and export of mRNA. These virus-induced nuclear changes culminate in formation of VINORCs, novel nodular structures composed of viral and cellular RNA splicing and export factors. VINORCs localize to the periphery of viral replication compartments, where viral mRNAs reside. These EBV-induced changes in nuclear organization may contribute to blockade of nuclear export of host mRNA, while enabling selective processing and export of viral mRNA.

It's Not the Destination, It's the Journey: Heterogeneity in mRNA Export Mechanisms.

The process of creating a translation-competent mRNA is highly complex and involves numerous steps including transcription, splicing, addition of modifications, and, finally, export to the cytoplasm. Historically, much of the research on regulation of gene expression at the level of the mRNA has been focused on either the regulation of mRNA synthesis (transcription and splicing) or metabolism (translation and degradation). However, in recent years, the advent of new experimental techniques has revealed the export of mRNA to be a major node in the regulation of gene expression, and numerous large-scale and specific mRNA export pathways have been defined. In this chapter, we will begin by outlining the mechanism by which most mRNAs are homeostatically exported ("bulk mRNA export"), involving the recruitment of the NXF1/TAP export receptor by the Aly/REF and THOC5 components of the TREX complex. We will then examine various mechanisms by which this pathway may be controlled, modified, or bypassed in order to promote the export of subset(s) of cellular mRNAs, which include the use of metazoan-specific orthologs of bulk mRNA export factors, specific cis RNA motifs which recruit mRNA export machinery via specific trans-acting-binding factors, posttranscriptional mRNA modifications that act as "inducible" export cis elements, the use of the atypical mRNA export receptor, CRM1, and the manipulation or bypass of the nuclear pore itself. Finally, we will discuss major outstanding questions in the field of mRNA export heterogeneity and outline how cutting-edge experimental techniques are providing new insights into and tools for investigating the intriguing field of mRNA export heterogeneity.

Influenza A Virus NS1 Protein Promotes Efficient Nuclear Export of Unspliced Viral M1 mRNA.

Influenza A virus mRNAs are transcribed by the viral RNA-dependent RNA polymerase in the cell nucleus before being exported to the cytoplasm for translation. Segment 7 produces two major transcripts: an unspliced mRNA that encodes the M1 matrix protein and a spliced transcript that encodes the M2 ion channel. Export of both mRNAs is dependent on the cellular NXF1/TAP pathway, but it is unclear how they are recruited to the export machinery or how the intron-containing but unspliced M1 mRNA bypasses the normal quality-control checkpoints. Using fluorescent in situ hybridization to monitor segment 7 mRNA localization, we found that cytoplasmic accumulation of unspliced M1 mRNA was inefficient in the absence of NS1, both in the context of segment 7 RNPs reconstituted by plasmid transfection and in mutant virus-infected cells. This effect was independent of any major effect on steady-state levels of segment 7 mRNA or splicing but corresponded to a ∼5-fold reduction in the accumulation of M1. A similar defect in intronless hemagglutinin (HA) mRNA nuclear export was seen with an NS1 mutant virus. Efficient export of M1 mRNA required both an intact NS1 RNA-binding domain and effector domain. Furthermore, while wild-type NS1 interacted with cellular NXF1 and also increased the interaction of segment 7 mRNA with NXF1, mutant NS1 polypeptides unable to promote mRNA export did neither. Thus, we propose that NS1 facilitates late viral gene expression by acting as an adaptor between viral mRNAs and the cellular nuclear export machinery to promote their nuclear export.IMPORTANCE Influenza A virus is a major pathogen of a wide variety of mammalian and avian species that threatens public health and food security. A fuller understanding of the virus life cycle is important to aid control strategies. The virus has a small genome that encodes relatively few proteins that are often multifunctional. Here, we characterize a new function for the NS1 protein, showing that, as well as previously identified roles in antagonizing the innate immune defenses of the cell and directly upregulating translation of viral mRNAs, it also promotes the nuclear export of the viral late gene mRNAs by acting as an adaptor between the viral mRNAs and the cellular mRNA nuclear export machinery.

The role of TREX in gene expression and disease.

TRanscription and EXport (TREX) is a conserved multisubunit complex essential for embryogenesis, organogenesis and cellular differentiation throughout life. By linking transcription, mRNA processing and export together, it exerts a physiologically vital role in the gene expression pathway. In addition, this complex prevents DNA damage and regulates the cell cycle by ensuring optimal gene expression. As the extent of TREX activity in viral infections, amyotrophic lateral sclerosis and cancer emerges, the need for a greater understanding of TREX function becomes evident. A complete elucidation of the composition, function and interactions of the complex will provide the framework for understanding the molecular basis for a variety of diseases. This review details the known composition of TREX, how it is regulated and its cellular functions with an emphasis on mammalian systems.

RNA Nuclear Export: From Neurological Disorders to Cancer.

The presence of a nuclear envelope, also known as nuclear membrane, defines the structural framework of all eukaryotic cells by separating the nucleus, which contains the genetic material, from the cytoplasm where the synthesis of proteins takes place. Translation of proteins in Eukaryotes is thus dependent on the active transport of DNA-encoded RNA molecules through pores embedded within the nuclear membrane. Several mechanisms are involved in this process generally referred to as RNA nuclear export or nucleocytoplasmic transport of RNA. The regulated expression of genes requires the nuclear export of protein-coding messenger RNA molecules (mRNAs) as well as non-coding RNAs (ncRNAs) together with proteins and pre-assembled ribosomal subunits. The nuclear export of mRNAs is intrinsically linked to the co-transcriptional processing of nascent transcripts synthesized by the RNA polymerase II. This functional coupling is essential for the survival of cells allowing for timely nuclear export of fully processed transcripts, which could otherwise cause the translation of abnormal proteins such as the polymeric repeat proteins produced in some neurodegenerative diseases. Alterations of the mRNA nuclear export pathways can also lead to genome instability and to various forms of cancer. This chapter will describe the molecular mechanisms driving the nuclear export of RNAs with a particular emphasis on mRNAs. It will also review their known alterations in neurological disorders and cancer, and the recent opportunities they offer for the potential development of novel therapeutic strategies.

The human TREX-2 complex is stably associated with the nuclear pore basket.

In eukaryotes, mRNA export involves many evolutionarily conserved factors that carry the nascent transcript to the nuclear pore complex (NPC). The THO/TREX complex couples transcription to mRNA export and recruits the mRNA export receptor NXF1 for the transport of messenger ribonucleoprotein particles (mRNP) to the NPC. The transcription and export complex 2 (TREX-2) was suggested to interact with NXF1 and to shuttle between transcription sites and the NPC. Here, we characterize the dynamics of human TREX-2 and show that it stably associates with the NPC basket. Moreover, the association of TREX-2 with the NPC requires the basket nucleoporins NUP153 and TPR, but is independent of transcription. Differential profiles of mRNA nuclear accumulation reveal that TREX-2 functions similarly to basket nucleoporins, but differently from NXF1. Thus, our results show that TREX-2 is an NPC-associated complex in mammalian cells and suggest that it is involved in putative NPC basket-related functions.

Insights into the nuclear export of murine leukemia virus intron-containing RNA.

The retroviral genome consists of an intron-containing transcript that has essential cytoplasmic functions in the infected cell. This viral transcript can escape splicing, circumvent the nuclear checkpoint mechanisms and be transported to the cytoplasm by hijacking the host machinery. Once in the cytoplasm, viral unspliced RNA acts as mRNA to be translated and as genomic RNA to be packaged into nascent viruses. The murine leukemia virus (MLV) is among the first retroviruses discovered and is classified as simple Retroviridae due to its minimal encoding capacity. The oncogenic and transduction abilities of MLV are extensively studied, whereas surprisingly the crucial step of its nuclear export has remained unsolved until 2014. Recent work has revealed the recruitment by MLV of the cellular NXF1/Tap-dependent pathway for export. Unconventionally, MLV uses of Tap to export both spliced and unspliced viral RNAs. Unlike other retroviruses, MLV does not harbor a unique RNA signal for export. Indeed, multiple sequences throughout the MLV genome appear to promote export of the unspliced MLV RNA. We review here the current understanding of the export mechanism and highlight the determinants that influence MLV export. As the molecular mechanism of MLV export is elucidated, we will gain insight into the contribution of the export pathway to the cytoplasmic fate of the viral RNA.