Quality control of mRNA represents an important regulatory mechanism for gene expression in eukaryotes. One component of this quality control is the nuclear retention and decay of misprocessed RNAs. Previously, we demonstrated that mature mRNAs containing a 5’ splice site (5’SS) motif, which is typically found in misprocessed RNAs such as intronic polyadenylated (IPA) transcripts, are nuclear retained and degraded. Here we demonstrate that these transcripts require the zinc finger protein ZFC3H1 for their decay and nuclear retention into nuclear speckles. Furthermore, we find that U1-70K, a component of the U1 snRNP spliceosomal complex, is also required for their nuclear retention and likely functions in the same pathway as ZFC3H1. Finally, we show that the disassembly of nuclear speckles impairs the nuclear retention of mRNAs with 5’SS motifs. Together, our results suggest a model where mRNAs with 5’SS motifs are recognized by U1 snRNP, which then acts with ZFC3H1 to both promote their decay and prevent nuclear export of these mRNAs by sequestering them in nuclear speckles. Our results highlight a splicing independent role of U1 snRNP and indicate that it works in conjunction with ZFC3H1 in preventing the nuclear export of misprocessed mRNAs.

The nuclear pore complex is the sole gateway connecting the nucleoplasm and cytoplasm. In humans, the nuclear pore complex is one of the largest multiprotein assemblies in the cell, with a molecular mass of ∼110 MDa and consisting of 8 to 64 copies of about 34 different nuclear pore proteins, termed nucleoporins, for a total of 1000 subunits per pore. Trafficking events across the nuclear pore are mediated by nuclear transport receptors and are highly regulated. The nuclear pore complex is also used by several RNA viruses and almost all DNA viruses to access the host cell nucleoplasm for replication. Viruses hijack the nuclear pore complex, and nuclear transport receptors, to access the nucleoplasm where they replicate. In addition, the nuclear pore complex is used by the cell innate immune system, a network of signal transduction pathways that coordinates the first response to foreign invaders, including viruses and other pathogens. Several branches of this response depend on dynamic signaling events that involve the nuclear translocation of downstream signal transducers. Mounting evidence has shown that these signaling cascades, especially those steps that involve nucleocytoplasmic trafficking events, are targeted by viruses so that they can evade the innate immune system. This review summarizes how nuclear pore proteins and nuclear transport receptors contribute to the innate immune response and highlights how viruses manipulate this cellular machinery to favor infection. A comprehensive understanding of nuclear pore proteins in antiviral innate immunity will likely contribute to the development of new antiviral therapeutic strategies.

Mutations in RanBP2 (also known as Nup358), one of the main components of the cytoplasmic filaments of the nuclear pore complex, contribute to the overproduction of acute necrotizing encephalopathy (ANE1)-associated cytokines. Here we report that RanBP2 represses the translation of the interleukin 6 (IL6) mRNA, which encodes a cytokine that is aberrantly up-regulated in ANE1. Our data indicates that soon after its production, the IL6 messenger ribonucleoprotein (mRNP) recruits Argonautes bound to let-7 microRNA. After this mRNP is exported to the cytosol, RanBP2 sumoylates mRNP-associated Argonautes, thereby stabilizing them and enforcing mRNA silencing. Collectively, these results support a model whereby RanBP2 promotes an mRNP remodelling event that is critical for the miRNA-mediated suppression of clinically relevant mRNAs, such as IL6.

Ran Binding Protein 2 (RanBP2 or Nucleoporin358) is one of the main components of the cytoplasmic filaments of the nuclear pore complex. Mutations in the RANBP2 gene are associated with acute necrotizing encephalopathy type 1 (ANE1), a rare condition where patients experience a sharp rise in cytokine production in response to viral infection and undergo hyperinflammation, seizures, coma, and a high rate of mortality. Despite this, it remains unclear howRanBP2 and its ANE1-associated mutations contribute to pathology. Mounting evidence has shown that RanBP2 interacts with distinct viruses to regulate viral infection. In addition, RanBP2 may regulate innate immune response pathways. This review summarizes recent advances in our understanding of how mutations in RANBP2 contribute to ANE1 and discusses how RanBP2 interacts with distinct viruses and affects viral infection. Recent findings indicate that RanBP2 might be an important therapeutic target, not only in the suppression of ANE1-driven cytokine storms, but also to combat hyperinflammation in response to viral infections.

1RanBP2/Nup358 is one of the main components of the cytoplasmic filaments of the nuclear pore 2 complex. Four separate missense mutations in RanBP2 cause Acute Necrotizing Encephalopathy 1 3 (ANE1), which manifests as a sharp rise in cytokine production after common viral infections 4 such as influenza and parainfluenza. Infection in these individuals often leads to seizures, coma 5 and a high rate of mortality. However, how RanBP2 and its ANE1-associated mutations affect 6 cytokine production is not well understood. Here we report that RanBP2 represses the translation 7 of the interleukin-6 (IL6) mRNA, which encodes a cytokine that is aberrantly up-regulated in 8 ANE1. In particular, the SUMO E3-ligase activity of RanBP2 and the Let7 miRNA binding site 9 within the IL6 3′ untranslated region (UTR) are required for this repression, suggesting that 10 sumoylation promotes efficient miRNA-based silencing. Furthermore, our data indicates that 11 RanBP2-dependent sumoylation of the argonaute protein AGO1 inhibits its ubiquitination and its 12 degradation, and that overexpression of AGO1 partially restores the repression of IL6 in cells that 13 are defective in RanBP2-dependent sumoylation. Collectively, these results support a model 14 whereby RanBP2 promotes the sumoylation of AGO1, which stabilizes it, and ultimately enhances 15 the miRNA-mediated suppression of mRNAs such as IL6. 16 17 3 RanBP2 is required for Let7 mediated gene silencing (Sahoo et al, 2017). These observations 1 suggest that RanBP2 might impact the translation of the IL6 mRNA by post-translational 2 regulation of argonaute proteins. 3Here we present evidence that RanBP2 promotes the Let7-mediated suppression of IL6 4 protein production by sumoylating AGO1, which antagonizes AGO1 ubiquitination and thus 5 promotes its stability and its ability to translationally silence the IL6 mRNA. 6 7 6 RESULTS 1