Abstract
The nuclear export of cellular mRNAs is a complex process that requires the orchestrated participation of many proteins that are recruited during the early steps of mRNA synthesis and processing. This strategy allows the cell to guarantee the conformity of the messengers accessing the cytoplasm and the translation machinery. Most transcripts are exported by the exportin dimer Nuclear RNA export factor 1 (NXF1)–NTF2-related export protein 1 (NXT1) and the transcription–export complex 1 (TREX1). Some mRNAs that do not possess all the common messenger characteristics use either variants of the NXF1–NXT1 pathway or CRM1, a different exportin. Viruses whose mRNAs are synthesized in the nucleus (retroviruses, the vast majority of DNA viruses, and influenza viruses) exploit both these cellular export pathways. Viral mRNAs hijack the cellular export machinery via complex secondary structures recognized by cellular export factors and/or viral adapter proteins. This way, the viral transcripts succeed in escaping the host surveillance system and are efficiently exported for translation, allowing the infectious cycle to proceed. This review gives an overview of the cellular mRNA nuclear export mechanisms and presents detailed insights into the most important strategies that viruses use to export the different forms of their RNAs from the nucleus to the cytoplasm.
Highlights
The hallmark difference between eukaryotic and prokaryotic life is the presence of the nucleus.Limited by the double lipidic bilayer nuclear envelope, the nucleus physically separates the genetic material from the cytoplasm, decoupling the transcriptional and translational steps of gene expression.The nucleus remains connected to the cytoplasm through the nuclear pore complex (NPC), the gatekeeper allowing the nucleocytoplasmic transport of selected macromolecules [1,2,3].Viruses represent the most abundant and genetically diverse biological entities on Earth and infect virtually every organism
Using Mason–Pfizer Monkey virus (MPMV) constitutive transport elements (CTE) RNA as bait for protein capture on an affinity column, Grüter et al discovered a strongly interacting protein, which they identified as human Nuclear RNA export factor 1 (NXF1) [149], a homolog of the yeast messenger RNAs (mRNAs) export factor 67 (Mex67) protein shown previously to be involved in mRNA export [150]
The kinases responsible for this phosphorylation vary from one subfamily of Herpesviridae to another, as does the nature and position of the RNA binding domain. These small but significant differences are exemplified by the fact that neither HSV1 infected cell protein 27 (ICP27) nor the Human cytomegalovirus’s unique long 69 gene product (UL69) can entirely complement the mutant Epstein–Barr virus EB2 protein, which is essential for viral mRNA export and virus production [259]
Summary
The hallmark difference between eukaryotic and prokaryotic life is the presence of the nucleus. The most prominent NTRs belong to the karyopherin family (reviewed in [27]) Within this family importin-α and importin-β are responsible for the nuclear import of proteins (reviewed in [28]), while exportin 1, known as Chromosome region maintenance 1 (CRM1), allows the export of proteins and the export of ribosomal subunits and small RNAs [29]. Together, these karyopherins transport over 1000 different cargoes to and from the nucleus [30,31].
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