Linking gene regulation to mRNA production and export

eIF4E orchestrates mRNA processing, RNA export and translation to modify specific protein production

The eukaryotic translation initiation factor eIF4E acts as a multifunctional factor that simultaneously influences mRNA processing, export, and translation in many organisms. Its multifactorial effects are derived from its capacity to bind to the methyl-7-guanosine cap on the 5'end of mRNAs and thus can act as a cap chaperone for transcripts in the nucleus and cytoplasm. In this review, we describe the multifactorial roles of eIF4E in major mRNA-processing events including capping, splicing, cleavage and polyadenylation, nuclear export and translation. We discuss the evidence that eIF4E acts at two levels to generate widescale changes to processing, export and ultimately the protein produced. First, eIF4E alters the production of components of the mRNA processing machinery, supporting a widescale reprogramming of multiple mRNA processing events. In this way, eIF4E can modulate mRNA processing without physically interacting with target transcripts. Second, eIF4E also physically interacts with both capped mRNAs and components of the RNA processing or translation machineries. Further, specific mRNAs are sensitive to eIF4E only in particular mRNA processing events. This selectivity is governed by the presence of cis-acting elements within mRNAs known as USER codes that recruit relevant co-factors engaging the appropriate machinery. In all, we describe the molecular bases for eIF4E's multifactorial function and relevant regulatory pathways, discuss the basis for selectivity, present a compendium of ~80 eIF4E-interacting factors which play roles in these activities and provide an overview of the relevance of its functions to its oncogenic potential. Finally, we summarize early-stage clinical studies targeting eIF4E in cancer.

Nuclear mRNA export

In eukaryotic cells, gene expression begins with transcription in the nucleus, followed by the maturation of messenger RNAs (mRNAs). These mRNA molecules are then exported to the cytoplasm through the nuclear pore complex (NPC), a process that serves as a critical regulatory phase of gene expression. The export of mRNA is intricately linked to precursor mRNA (pre-mRNA) processing, ensuring that only properly processed mRNA reaches the cytoplasm. This coordination is essential, as recent studies have revealed that mRNA export factors not only assist in transport but also influence upstream processing steps, adding a layer of complexity to gene regulation. Furthermore, the export process competes with RNA processing and degradation pathways, maintaining a delicate balance vital for accurate gene expression. While these mechanisms are generally conserved across eukaryotes, significant differences exist between yeast and higher eukaryotic cells, particularly due to the more genome complexity of the latter. This review delves into the current research on mRNA export in higher eukaryotic cells, focusing on its role in the broader context of gene expression regulation and highlighting how it interacts with other gene expression processes to ensure precise and efficient gene functionality in complex organisms.

Inferring Stochastic Rates from Heterogeneous Snapshots of Particle Positions

Many imaging techniques for biological systems-like fixation of cells coupled with fluorescence microscopy-provide sharp spatial resolution in reporting locations of individuals at a single moment in time but also destroy the dynamics they intend to capture. These snapshot observations contain no information about individual trajectories, but still encode information about movement and demographic dynamics, especially when combined with a well-motivated biophysical model. The relationship between spatially evolving populations and single-moment representations of their collective locations is well-established with partial differential equations (PDEs) and their inverse problems. However, experimental data is commonly a set of locations whose number is insufficient to approximate a continuous-in-space PDE solution. Here, motivated by popular subcellular imaging data of gene expression, we embrace the stochastic nature of the data and investigate the mathematical foundations of parametrically inferring demographic rates from snapshots of particles undergoing birth, diffusion, and death in a nuclear or cellular domain. Toward inference, we rigorously derive a connection between individual particle paths and their presentation as a Poisson spatial process. Using this framework, we investigate the properties of the resulting inverse problem and study factors that affect quality of inference. One pervasive feature of this experimental regime is the presence of cell-to-cell heterogeneity. Rather than being a hindrance, we show that cell-to-cell geometric heterogeneity can increase the quality of inference on dynamics for certain parameter regimes. Altogether, the results serve as a basis for more detailed investigations of subcellular spatial patterns of RNA molecules and other stochastically evolving populations that can only be observed for single instants in their time evolution.

BRANEnet: embedding multilayer networks for omics data integration

Background

Gene expression is regulated at different molecular levels, including chromatin accessibility, transcription, RNA maturation, and transport. These regulatory mechanisms have strong connections with cellular metabolism. In order to study the cellular system and its functioning, omics data at each molecular level can be generated and efficiently integrated. Here, we propose BRANEnet, a novel multi-omics integration framework for multilayer heterogeneous networks. BRANEnet is an expressive, scalable, and versatile method to learn node embeddings, leveraging random walk information within a matrix factorization framework. Our goal is to efficiently integrate multi-omics data to study different regulatory aspects of multilayered processes that occur in organisms. We evaluate our framework using multi-omics data of Saccharomyces cerevisiae, a well-studied yeast model organism.

Results

We test BRANEnet on transcriptomics (RNA-seq) and targeted metabolomics (NMR) data for wild-type yeast strain during a heat-shock time course of 0, 20, and 120 min. Our framework learns features for differentially expressed bio-molecules showing heat stress response. We demonstrate the applicability of the learned features for targeted omics inference tasks: transcription factor (TF)-target prediction, integrated omics network (ION) inference, and module identification. The performance of BRANEnet is compared to existing network integration methods. Our model outperforms baseline methods by achieving high prediction scores for a variety of downstream tasks.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12859-022-04955-w.

Structural assembly of the nucleic-acid-binding Thp3-Csn12-Sem1 complex functioning in mRNA splicing

PCI domain proteins play important roles in post-transcriptional gene regulation. In the TREX-2 complex, PCI domain-containing Sac3 and Thp1 proteins and accessory Sem1 protein form a ternary complex required for mRNA nuclear export. In contrast, structurally related Thp3–Csn12–Sem1 complex mediates pre-mRNA splicing. In this study, we determined the structure of yeast Thp3^186–470–Csn12–Sem1 ternary complex at 2.9 Å resolution. Both Thp3 and Csn12 structures have a typical PCI structural fold, characterized by a stack of α-helices capped by a C-terminal winged-helix (WH) domain. The overall structure of Thp3^186–470–Csn12–Sem1 complex has an inverted V-shape with Thp3 and Csn12 forming the two sides. A fishhook-shaped Sem1 makes extensive contacts on Csn12 to stabilize its conformation. The overall structure of Thp3^186–470–Csn12–Sem1 complex resembles the previously reported Sac3–Thp1–Sem1 complex, but also has significant structural differences. The C-terminal WH domains of Thp3 and Csn12 form a continuous surface to bind different forms of nucleic acids with micromolar affinity. Mutation of the basic residues in the WH domains of Thp3 and Csn12 affects nucleic acid binding in vitro and mRNA splicing in vivo. The Thp3–Csn12–Sem1 structure provides a foundation for further exploring the structural elements required for its specific recruitment to spliceosome for pre-mRNA splicing.

Nup98-dependent transcriptional memory is established independently of transcription

Cellular ability to mount an enhanced transcriptional response upon repeated exposure to external cues is termed transcriptional memory, which can be maintained epigenetically through cell divisions and can depend on a nuclear pore component Nup98. The majority of mechanistic knowledge on transcriptional memory has been derived from bulk molecular assays. To gain additional perspective on the mechanism and contribution of Nup98 to memory, we used single-molecule RNA FISH (smFISH) to examine the dynamics of transcription in Drosophila cells upon repeated exposure to the steroid hormone ecdysone. We combined smFISH with mathematical modeling and found that upon hormone exposure, cells rapidly activate a low-level transcriptional response, but simultaneously begin a slow transition into a specialized memory state characterized by a high rate of expression. Strikingly, our modeling predicted that this transition between non-memory and memory states is independent of the transcription stemming from initial activation. We confirmed this prediction experimentally by showing that inhibiting transcription during initial ecdysone exposure did not interfere with memory establishment. Together, our findings reveal that Nup98's role in transcriptional memory is to stabilize the forward rate of conversion from low to high expressing state, and that induced genes engage in two separate behaviors - transcription itself and the establishment of epigenetically propagated transcriptional memory.

Targeting autophagy in neurodegenerative diseases: From molecular mechanisms to clinical therapeutics

Many neurodegenerative diseases are associated with pathological aggregation of proteins in neurons. Autophagy is a natural self-cannibalization process that can act as a powerful mechanism to remove aged and damaged organelles as well as protein aggregates. It has been shown that promoting autophagy can attenuate or delay neurodegeneration by removing protein aggregates. In this paper, we will review the role of autophagy in Alzheimer's disease (AD), Parkinson's Disease (PD), and Huntington's Disease (HD) and discuss opportunities and challenges of targeting autophagy as a potential therapeutic avenue for treatment of these common neurodegenerative diseases.

The Mammalian Ecdysoneless Protein Interacts with RNA Helicase DDX39A To Regulate Nuclear mRNA Export

The mammalian orthologue of ecdysoneless (ECD) protein is required for embryogenesis, cell cycle progression, and mitigation of endoplasmic reticulum stress. Here, we identified key components of the mRNA export complexes as binding partners of ECD and characterized the functional interaction of ECD with key mRNA export-related DEAD BOX protein helicase DDX39A. We find that ECD is involved in RNA export through its interaction with DDX39A. ECD knockdown (KD) blocks mRNA export from the nucleus to the cytoplasm, which is rescued by expression of full-length ECD but not an ECD mutant that is defective in interaction with DDX39A. We have previously shown that ECD protein is overexpressed in ErbB2⁺ breast cancers (BC). In this study, we extended the analyses to two publicly available BC mRNA The Cancer Genome Atlas (TCGA) and Molecular Taxonomy of Breast Cancer International Consortium (METABRIC) data sets. In both data sets, ECD mRNA overexpression correlated with short patient survival, specifically ErbB2⁺ BC. In the METABRIC data set, ECD overexpression also correlated with poor patient survival in triple-negative breast cancer (TNBC). Furthermore, ECD KD in ErbB2⁺ BC cells led to a decrease in ErbB2 mRNA level due to a block in its nuclear export and was associated with impairment of oncogenic traits. These findings provide novel mechanistic insight into the physiological and pathological functions of ECD.

Search for potential biomarkers for saxitoxin detection

The Neuro-2a cell assay has been a promising in vitro alternative for the detection of saxitoxin (STX)-like toxins. However, its application is problematic in samples with complex matrices containing different toxins, whose mechanisms of action could be antagonistic. In the search of alternative procedures that reduce or avoid this interference, we evaluated the transcriptional modulation produced by a 24-h exposure to STX in Neuro-2a cells under three conditions: exposure to STX (33 nM), a mussel meat matrix (12.5 mg meat/mL) and a fortified sample (STX-fortified matrix). Differential gene expression was evaluated by RNA-seq after Illumina high-throughput sequencing, and data were analyzed to identify genes differentially expressed regardless of the matrix. From the 9487 identified genes, 213 were differentially expressed of these, 10 genes were identified as candidate markers for STX detection due to their regulation by STX regardless of the matrix interference. Expression dynamics of 7 of these candidate genes (Fgf-1, Adgrb2, Tfpt, Zfr2, Nup 35, Fam195a, and Dusp7) was further evaluated by qRT-PCR analysis of cells exposed to different concentrations of STX for up to 24 h. A downregulation of some markers expression was observed, namely Nup35 (involved in nucleo-cytoplasmic transporter activity) and Zfr-2 (involved in nucleic acids binding), whereas Fgf-1 (apoptosis signaling) was significantly upregulated. Markers' expression was not influenced by the matrix, suggesting that gene expression variations are directly related to STX response. These results support the potential of these genes as biomarkers for the development of an alternative STX-like toxins screening method.

The Nuclear Pore Complex and mRNA Export in Cancer

Export of mRNAs from the nucleus to the cytoplasm is a key regulatory step in the expression of proteins. mRNAs are transported through the nuclear pore complex (NPC). Export of mRNAs responds to a variety of cellular stimuli and stresses. Revelations of the specific effects elicited by NPC components and associated co-factors provides a molecular basis for the export of selected RNAs, independent of bulk mRNA export. Aberrant RNA export has been observed in primary human cancer specimens. These cargo RNAs encode factors involved in nearly all facets of malignancy. Indeed, the NPC components involved in RNA export as well as the RNA export machinery can be found to be dysregulated, mutated, or impacted by chromosomal translocations in cancer. The basic mechanisms associated with RNA export with relation to export machinery and relevant NPC components are described. Therapeutic strategies targeting this machinery in clinical trials is also discussed. These findings firmly position RNA export as a targetable feature of cancer along with transcription and translation.

Multifaceted activities of the plant SAGA complex

From yeast to human, the Spt-Ada-GCN5-acetyltransferase (SAGA) gigantic complex modifies chromatin during RNA polymerase II initiation and elongation steps to facilitate transcription. Its enzymatic activity involves a histone acetyltransferase module (HATm) that acetylates multiple lysine residues on the N-terminal tails of histones H2B and H3 and a deubiquitination module (DUBm) that triggers co-transcriptional deubiquitination of histone H2B. With a few notable exceptions described in this review, most SAGA subunits identified in yeast and metazoa are present in plants. Studies from the last 20 years have unveiled that different SAGA subunits are involved in gene expression regulation during the plant life cycle and in response to various types of stress or environmental cues. Their functional analysis in the Arabidopsis thaliana model species is increasingly shedding light on their intrinsic properties and how they can themselves be regulated during plant adaptive responses. Recent biochemical studies have also uncovered multiple associations between plant SAGA and chromatin machineries linked to RNA Pol II transcription. Still, considerably less is known about the molecular links between SAGA or SAGA-like complexes and chromatin dynamics during transcription in Arabidopsis and other plant species. We summarize the emerging knowledge on plant SAGA complex composition and activity, with a particular focus on the best-characterized subunits from its HAT (such as GCN5) and DUB (such as UBP22) modules, and implication of these ensembles in plant development and adaptive responses.

A multi-omics dataset of heat-shock response in the yeast RNA binding protein Mip6

Gene expression is a biological process regulated at different molecular levels, including chromatin accessibility, transcription, and RNA maturation and transport. In addition, these regulatory mechanisms have strong links with cellular metabolism. Here we present a multi-omics dataset that captures different aspects of this multi-layered process in yeast. We obtained RNA-seq, metabolomics, and H4K12ac ChIP-seq data for wild-type and mip6Δ strains during a heat-shock time course. Mip6 is an RNA-binding protein that contributes to RNA export during environmental stress and is informative of the contribution of post-transcriptional regulation to control cellular adaptations to environmental changes. The experiment was performed in quadruplicate, and the different omics measurements were obtained from the same biological samples, which facilitates the integration and analysis of data using covariance-based methods. We validate our dataset by showing that ChIP-seq, RNA-seq and metabolomics signals recapitulate existing knowledge about the response of ribosomal genes and the contribution of trehalose metabolism to heat stress. Raw data, processed data and preprocessing scripts are made available.

Localization elements and zip codes in the intracellular transport and localization of messenger RNAs in Saccharomyces cerevisiae

Intracellular trafficking and localization of mRNAs provide a mechanism of regulation of expression of genes with excellent spatial control. mRNA localization followed by localized translation appears to be a mechanism of targeted protein sorting to a specific cell-compartment, which is linked to the establishment of cell polarity, cell asymmetry, embryonic axis determination, and neuronal plasticity in metazoans. However, the complexity of the mechanism and the components of mRNA localization in higher organisms prompted the use of the unicellular organism Saccharomyces cerevisiae as a simplified model organism to study this vital process. Current knowledge indicates that a variety of mRNAs are asymmetrically and selectively localized to the tip of the bud of the daughter cells, to the vicinity of endoplasmic reticulum, mitochondria, and nucleus in this organism, which are connected to diverse cellular processes. Interestingly, specific cis-acting RNA localization elements (LEs) or RNA zip codes play a crucial role in the localization and trafficking of these localized mRNAs by providing critical binding sites for the specific RNA-binding proteins (RBPs). In this review, we present a comprehensive account of mRNA localization in S. cerevisiae, various types of localization elements influencing the mRNA localization, and the RBPs, which bind to these LEs to implement a number of vital physiological processes. Finally, we emphasize the significance of this process by highlighting their connection to several neuropathological disorders and cancers. This article is categorized under: RNA Export and Localization > RNA Localization.

NUP37 silencing induces inhibition of cell proliferation, G1 phase cell cycle arrest and apoptosis in non-small cell lung cancer cells

NUP37 has been reported as a component of the nuclear pore complex, which may be involved in tumorigenesis. Previous reports have shown that NUP37 acts as an oncogene in the development of hepatocellular carcinoma. However, its role in lung cancer remains unknown. The present study demonstrated for the first time that NUP37 expression was overexpressed in non-small cell lung cancer (NSCLC) samples compared with the corresponding expression noted in normal tissues. The results were derived by analyzing public datasets. Moreover, it was shown that NUP37 was overexpressed in advanced stage NSCLC samples compared with the corresponding expression of this protein in early stage NSCLC samples. Higher expression levels of NUP37 correlated with lower overall survival (OS) in NSCLC samples. Bioinformatic analysis indicated that NUP37 was involved in regulating cell cycle progression in NSCLC. Furthermore, knockdown of NUP37 suppressed cell growth and proliferation in A549and H1299 cells as demonstrated with the Celigo Cell Counting method and the MTT assay. Flow cytometry analysis indicated that knockdown of NUP37 induced significant S phage cell arrest and apoptosis in A549 and H1299 cells. The results showed that knockdown of NUP37 remarkably induced the protein levels of cleaved PARP, P53 and BCL2 in A549 cells. Therefore, it was concluded that NUP37 serves a distinguished role in the growth of lung cancer cells and may be considered as a potential biomarker and therapeutic target for lung cancer.

Mip6 binds directly to the Mex67 UBA domain to maintain low levels of Msn2/4 stress-dependent mRNAs

RNA-binding proteins (RBPs) participate in all steps of gene expression, underscoring their potential as regulators of RNA homeostasis. We structurally and functionally characterize Mip6, a four-RNA recognition motif (RRM)-containing RBP, as a functional and physical interactor of the export factor Mex67. Mip6-RRM4 directly interacts with the ubiquitin-associated (UBA) domain of Mex67 through a loop containing tryptophan 442. Mip6 shuttles between the nucleus and the cytoplasm in a Mex67-dependent manner and concentrates in cytoplasmic foci under stress. Photoactivatable ribonucleoside-enhanced crosslinking and immunoprecipitation experiments show preferential binding of Mip6 to mRNAs regulated by the stress-response Msn2/4 transcription factors. Consistent with this binding, MIP6 deletion affects their export and expression levels. Additionally, Mip6 interacts physically and/or functionally with proteins with a role in mRNA metabolism and transcription such as Rrp6, Xrn1, Sgf73, and Rpb1. These results reveal a novel role for Mip6 in the homeostasis of Msn2/4-dependent transcripts through its direct interaction with the Mex67 UBA domain.

The nascent RNA binding complex SFiNX licenses piRNA-guided heterochromatin formation

The PIWI-interacting RNA (piRNA) pathway protects genome integrity in part through establishing repressive heterochromatin at transposon loci. Silencing requires piRNA-guided targeting of nuclear PIWI proteins to nascent transposon transcripts, yet the subsequent molecular events are not understood. Here, we identify SFiNX (silencing factor interacting nuclear export variant), an interdependent protein complex required for Piwi-mediated cotranscriptional silencing in Drosophila. SFiNX consists of Nxf2-Nxt1, a gonad-specific variant of the heterodimeric messenger RNA export receptor Nxf1-Nxt1 and the Piwi-associated protein Panoramix. SFiNX mutant flies are sterile and exhibit transposon derepression because piRNA-loaded Piwi is unable to establish heterochromatin. Within SFiNX, Panoramix recruits heterochromatin effectors, while the RNA binding protein Nxf2 licenses cotranscriptional silencing. Our data reveal how Nxf2 might have evolved from an RNA transport receptor into a cotranscriptional silencing factor. Thus, NXF variants, which are abundant in metazoans, can have diverse molecular functions and might have been coopted for host genome defense more broadly.

The Structure of the Nuclear Pore Complex (An Update)

The nuclear pore complex (NPC) serves as the sole bidirectional gateway of macromolecules in and out of the nucleus. Owing to its size and complexity (∼1,000 protein subunits, ∼110 MDa in humans), the NPC has remained one of the foremost challenges for structure determination. Structural studies have now provided atomic-resolution crystal structures of most nucleoporins. The acquisition of these structures, combined with biochemical reconstitution experiments, cross-linking mass spectrometry, and cryo-electron tomography, has facilitated the determination of the near-atomic overall architecture of the symmetric core of the human, fungal, and algal NPCs. Here, we discuss the insights gained from these new advances and outstanding issues regarding NPC structure and function. The powerful combination of bottom-up and top-down approaches toward determining the structure of the NPC offers a paradigm for uncovering the architectures of other complex biological machines to near-atomic resolution.

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.

A role for Mog1 in H2Bub1 and H3K4me3 regulation affecting RNAPII transcription and mRNA export

Monoubiquitination of histone H2B (to H2Bub1) is required for downstream events including histone H3 methylation, transcription, and mRNA export. The mechanisms and players regulating these events have not yet been completely delineated. Here, we show that the conserved Ran-binding protein Mog1 is required to sustain normal levels of H2Bub1 and H3K4me3 in Saccharomyces cerevisiae Mog1 is needed for gene body recruitment of Rad6, Bre1, and Rtf1 that are involved in H2B ubiquitination and genetically interacts with these factors. We provide evidence that the absence of MOG1 impacts on cellular processes such as transcription, DNA replication, and mRNA export, which are linked to H2Bub1. Importantly, the mRNA export defect in mog1Δ strains is exacerbated by the absence of factors that decrease H2Bub1 levels. Consistent with a role in sustaining H2Bub and H3K4me3 levels, Mog1 co-precipitates with components that participate in these modifications such as Bre1, Rtf1, and the COMPASS-associated factors Shg1 and Sdc1. These results reveal a novel role for Mog1 in H2B ubiquitination, transcription, and mRNA biogenesis.

Modulating the expression of Chtop, a versatile regulator of gene-specific transcription and mRNA export

Chtop binds competitively to the arginine methyltransferases PRMT1 and PRMT5, thereby promoting the asymmetric or symmetric methylation of arginine residues, respectively. In cooperation with PRMT1, Chtop activates transcription of certain gene groups, such as the estrogen-inducible genes in breast cancer cells, the 5-hydroxymethylcytosine-modified genes involved in glioblastomagenesis, or the Zbp-89-dependent genes in erythroleukemia cells. Chtop also represses expression of the fetal γ-globin gene. In addition, Chtop is a component of the TREX complex that links transcription elongation to mRNA export. The regulation of Chtop expression is, therefore, a key process during the expression of certain gene groups and pathogenesis of certain diseases. Our recent study revealed that cellular levels of Chtop are strictly autoregulated by a mechanism involving intron retention and nonsense-mediated mRNA decay. Here, we summarize roles of Chtop in gene-specific expression and highlight our recent findings concerning the autoregulation of Chtop.

Nuclear mRNA Surveillance Mechanisms: Function and Links to Human Disease

Production of export-competent mRNAs involves transcription and a series of dynamic processing and modification events of pre-messenger RNAs in the nucleus. Mutations in the genes encoding the transcription and mRNP processing machinery and the complexities involved in the biogenesis events lead to the formation of aberrant messages. These faulty transcripts are promptly eliminated by the nuclear RNA exosome and its cofactors to safeguard the cells and organisms from genetic catastrophe. Mutations in the components of the core nuclear exosome and its cofactors lead to the tissue-specific dysfunction of exosomal activities, which are linked to diverse human diseases and disorders. In this article, we examine the structure and function of both the yeast and human RNA exosome complex and its cofactors, discuss the nature of the various altered amino acid residues implicated in these diseases with the speculative mechanisms of the mutation-induced disorders and project the frontier and prospective avenues of the future research in this field.

The SAGA/TREX-2 subunit Sus1 binds widely to transcribed genes and affects mRNA turnover globally

BACKGROUND

Eukaryotic transcription is regulated through two complexes, the general transcription factor IID (TFIID) and the coactivator Spt-Ada-Gcn5 acetyltransferase (SAGA). Recent findings confirm that both TFIID and SAGA contribute to the synthesis of nearly all transcripts and are recruited genome-wide in yeast. However, how this broad recruitment confers selectivity under specific conditions remains an open question.

RESULTS

Here we find that the SAGA/TREX-2 subunit Sus1 associates with upstream regulatory regions of many yeast genes and that heat shock drastically changes Sus1 binding. While Sus1 binding to TFIID-dominated genes is not affected by temperature, its recruitment to SAGA-dominated genes and RP genes is significantly disturbed under heat shock, with Sus1 relocated to environmental stress-responsive genes in these conditions. Moreover, in contrast to recent results showing that SAGA deubiquitinating enzyme Ubp8 is dispensable for RNA synthesis, genomic run-on experiments demonstrate that Sus1 contributes to synthesis and stability of a wide range of transcripts.

CONCLUSIONS

Our study provides support for a model in which SAGA/TREX-2 factor Sus1 acts as a global transcriptional regulator in yeast but has differential activity at yeast genes as a function of their transcription rate or during stress conditions.

Messenger RNA Life-Cycle in Cancer Cells: Emerging Role of Conventional and Non-Conventional RNA-Binding Proteins?

Functional specialization of cells and tissues in metazoans require specific gene expression patterns. Biological processes, thus, need precise temporal and spatial coordination of gene activity. Regulation of the fate of messenger RNA plays a crucial role in this context. In the present review, the current knowledge related to the role of RNA-binding proteins in the whole mRNA life-cycle is summarized. This field opens up a new angle for understanding the importance of the post-transcriptional control of gene expression in cancer cells. The emerging role of non-classic RNA-binding proteins is highlighted. The goal of this review is to encourage readers to view, through the mRNA life-cycle, novel aspects of the molecular basis of cancer and the potential to develop RNA-based therapies.

The exon junction complex factor Y14 is dynamic in the nucleus of the beetle Tribolium castaneum during late oogenesis

Background

The oocyte chromosomes of the red flour beetle, Tribolium castaneum, are gathered into a knot, forming a karyosphere at the diplotene stage of meiotic prophase. Chromatin rearrangement, which is a characteristic feature of oocyte maturation, is well documented. The T. castaneum karyosphere is surrounded by a complex extrachromosomal structure termed the karyosphere capsule. The capsule contains the vast majority of oocyte RNA. We have previously shown using a BrUTP assay that oocyte chromosomes in T. castaneum maintain residual transcription up to the very end of oocyte maturation. Karyosphere transcription requires evidently not only transcription factors but also mRNA processing factors, including the components of the exon junction complex with its core component, the splicing factor Y14. We employed a gene engineering approach with injection of mRNA derived from the Myc-tagged Y14 plasmid-based construct in order to monitor the newly synthesized fusion protein in the oocyte nuclei.

Results

Our preliminary data have been presented as a brief correspondence elsewhere. Here, we provide a full-length article including immunoelectron-microscopy localization data on Y14–Myc distribution in the nucleus of previtellogenic and vitellogenic oocytes. The injections of the fusion protein Y14–Myc mRNA into the oocytes showed a dynamic pattern of the protein distribution. At the previtellogenic stage, there are two main locations for the protein: SC35 domains (the analogues of interchromatin granule clusters or nuclear speckles) and the karyosphere capsule. At the vitellogenic stage, SC35 domains were devoid of labels, and Y14–Myc was found in the perichromatin region of the karyosphere, presumably at the places of residual transcription. We show that karyosphere formation is accompanied by the movement of a nuclear protein while the residual transcription occurs during genome inactivation.

Conclusions

Our data indicate that the karyosphere capsule, being a destination site for a protein involved in mRNA splicing and export, is not only a specializes part of nuclear matrix separating the karyosphere from the products of chromosome activity, as believed previously, but represents a special nuclear compartment involved in the processes of gene expression in the case the karyosphere retains residual transcription activity.

Electronic supplementary material

The online version of this article (10.1186/s13039-017-0342-4) contains supplementary material, which is available to authorized users.

The interplay between transcription and mRNA degradation in Saccharomyces cerevisiae

The cellular transcriptome is shaped by both the rates of mRNA synthesis in the nucleus and mRNA degradation in the cytoplasm under a specified condition. The last decade witnessed an exciting development in the field of post-transcriptional regulation of gene expression which underscored a strong functional coupling between the transcription and mRNA degradation. The functional integration is principally mediated by a group of specialized promoters and transcription factors that govern the stability of their cognate transcripts by “marking” them with a specific factor termed “coordinator.” The “mark” carried by the message is later decoded in the cytoplasm which involves the stimulation of one or more mRNA-decay factors, either directly by the “coordinator” itself or in an indirect manner. Activation of the decay factor(s), in turn, leads to the alteration of the stability of the marked message in a selective fashion. Thus, the integration between mRNA synthesis and decay plays a potentially significant role to shape appropriate gene expression profiles during cell cycle progression, cell division, cellular differentiation and proliferation, stress, immune and inflammatory responses, and may enhance the rate of biological evolution.

5-methylcytosine promotes mRNA export - NSUN2 as the methyltransferase and ALYREF as an m5C reader

5-methylcytosine (m⁵C) is a post-transcriptional RNA modification identified in both stable and highly abundant tRNAs and rRNAs, and in mRNAs. However, its regulatory role in mRNA metabolism is still largely unknown. Here, we reveal that m⁵C modification is enriched in CG-rich regions and in regions immediately downstream of translation initiation sites and has conserved, tissue-specific and dynamic features across mammalian transcriptomes. Moreover, m⁵C formation in mRNAs is mainly catalyzed by the RNA methyltransferase NSUN2, and m⁵C is specifically recognized by the mRNA export adaptor ALYREF as shown by in vitro and in vivo studies. NSUN2 modulates ALYREF's nuclear-cytoplasmic shuttling, RNA-binding affinity and associated mRNA export. Dysregulation of ALYREF-mediated mRNA export upon NSUN2 depletion could be restored by reconstitution of wild-type but not methyltransferase-defective NSUN2. Our study provides comprehensive m⁵C profiles of mammalian transcriptomes and suggests an essential role for m⁵C modification in mRNA export and post-transcriptional regulation.

Deciphering the mRNP Code: RNA-Bound Determinants of Post-Transcriptional Gene Regulation

Eukaryotic cells determine the final protein output of their genetic program not only by controlling transcription but also by regulating the localization, translation and turnover rates of their mRNAs. Ultimately, the fate of any given mRNA is determined by the ensemble of all associated RNA-binding proteins (RBPs), non-coding RNAs and metabolites collectively known as the messenger ribonucleoprotein particle (mRNP). Although many mRNA-associated factors have been identified over the past years, little is known about the composition of individual mRNPs and the cooperation of their constituents. In this review we discuss recent progress that has been made on how this 'mRNP code' is established on individual transcripts and how it is interpreted during gene expression in eukaryotic cells.

Galectin-3 is a non-classic RNA binding protein that stabilizes the mucin MUC4 mRNA in the cytoplasm of cancer cells

Pancreatic cancer cells express high levels of MUC1, MUC4 and MUC16 mRNAs that encode membrane-bound mucins. These mRNAs share unusual features such as a long half-life. However, it remains unknown how mucin mRNA stability is regulated. Galectin-3 (Gal-3) is an endogenous lectin playing important biological functions in epithelial cells. Gal-3 is encoded by LGALS3 which is up-regulated in pancreatic cancer. Despite the absence of a RNA-recognition motif, Gal-3 interacts indirectly with pre-mRNAs in the nucleus and promotes constitutive splicing. However a broader role of Gal-3 in mRNA fate is unexplored. We report herein that Gal-3 increases MUC4 mRNA stability through an intermediate, hnRNP-L which binds to a conserved CA repeat element in the 3′UTR in a Gal-3 dependent manner and also controls Muc4 mRNA levels in epithelial tissues of Gal3^−/− mice. Gal-3 interacts with hnRNP-L in the cytoplasm, especially during cell mitosis, but only partly associates with protein markers of P-Bodies or Stress Granules. By RNA-IP plus RNA-seq analysis and imaging, we demonstrate that Gal-3 binds to mature spliced MUC4 mRNA in the perinuclear region, probably in hnRNP-L-containing RNA granules. Our findings highlight a new role for Gal-3 as a non-classic RNA-binding protein that regulates MUC4 mRNA post-transcriptionally.

RNP transport in cell biology: the long and winding road

Regulation of gene expression is key determinant to cell structure and function. RNA localization, where specific mRNAs are transported to subcellular regions and then translated, is highly conserved in eukaryotes ranging from yeast to extremely specialized and polarized cells such as neurons. Messenger RNA and associated proteins (mRNP) move from the site of transcription in the nucleus to their final destination in the cytoplasm both passively through diffusion and actively via directed transport. Dysfunction of RNA localization, transport and translation machinery can lead to pathology. Single-molecule live-cell imaging techniques have revealed unique features of this journey with unprecedented resolution. In this review, we highlight key recent findings that have been made using these approaches and possible implications for spatial control of gene function.

Structural and biochemical analyses of the DEAD-box ATPase Sub2 in association with THO or Yra1

mRNA is cotranscrptionally processed and packaged into messenger ribonucleoprotein particles (mRNPs) in the nucleus. Prior to export through the nuclear pore, mRNPs undergo several obligatory remodeling reactions. In yeast, one of these reactions involves loading of the mRNA-binding protein Yra1 by the DEAD-box ATPase Sub2 as assisted by the hetero-pentameric THO complex. To obtain molecular insights into reaction mechanisms, we determined crystal structures of two relevant complexes: a THO hetero-pentamer bound to Sub2 at 6.0 Å resolution; and Sub2 associated with an ATP analogue, RNA, and a C-terminal fragment of Yra1 (Yra1-C) at 2.6 Å resolution. We found that the 25 nm long THO clamps Sub2 in a half-open configuration; in contrast, when bound to the ATP analogue, RNA and Yra1-C, Sub2 assumes a closed conformation. Both THO and Yra1-C stimulated Sub2’s intrinsic ATPase activity. We propose that THO surveys common landmarks in each nuclear mRNP to localize Sub2 for targeted loading of Yra1.

DOI:http://dx.doi.org/10.7554/eLife.20070.001

Calpain-6 confers atherogenicity to macrophages by dysregulating pre-mRNA splicing

Macrophages contribute to the development of atherosclerosis through pinocytotic deposition of native LDL-derived cholesterol in macrophages in the vascular wall. Inhibiting macrophage-mediated lipid deposition may have protective effects in atheroprone vasculature, and identifying mechanisms that potentiate this process may inform potential therapeutic interventions for atherosclerosis. Here, we report that dysregulation of exon junction complex-driven (EJC-driven) mRNA splicing confers hyperpinocytosis to macrophages during atherogenesis. Mechanistically, we determined that inflammatory cytokines induce an unconventional nonproteolytic calpain, calpain-6 (CAPN6), which associates with the essential EJC-loading factor CWC22 in the cytoplasm. This association disturbs the nuclear localization of CWC22, thereby suppressing the splicing of target genes, including those related to Rac1 signaling. CAPN6 deficiency in LDL receptor-deficient mice restored CWC22/EJC/Rac1 signaling, reduced pinocytotic deposition of native LDL in macrophages, and attenuated macrophage recruitment into the lesions, generating an atheroprotective phenotype in the aorta. In macrophages, the induction of CAPN6 in the atheroma interior limited macrophage movements, resulting in a decline in cell clearance from the lesions. Consistent with this finding, we observed that myeloid CAPN6 contributed to atherogenesis in a murine model of bone marrow transplantation. Furthermore, macrophages from advanced human atheromas exhibited increased CAPN6 induction and impaired CWC22 nuclear localization. Together, these results indicate that CAPN6 promotes atherogenicity in inflamed macrophages by disturbing CWC22/EJC systems.

Deciphering the Structure and Function of Nuclear Pores Using Single-Molecule Fluorescence Approaches

Due to its central role in macromolecular trafficking and nucleocytoplasmic information transfer, the nuclear pore complex (NPC) has been studied in great detail using a wide spectrum of methods. Consequently, many aspects of its architecture, general function, and role in the life cycle of a cell are well understood. Over the last decade, fluorescence microscopy methods have enabled the real-time visualization of single molecules interacting with and transiting through the NPC, allowing novel questions to be examined with nanometer precision. While initial single-molecule studies focused primarily on import pathways using permeabilized cells, it has recently proven feasible to investigate the export of mRNAs in living cells. Single-molecule assays can address questions that are difficult or impossible to answer by other means, yet the complexity of nucleocytoplasmic transport requires that interpretation be based on a firm genetic, biochemical, and structural foundation. Moreover, conceptually simple single-molecule experiments remain technically challenging, particularly with regard to signal intensity, signal-to-noise ratio, and the analysis of noise, stochasticity, and precision. We discuss nuclear transport issues recently addressed by single-molecule microscopy, evaluate the limits of existing assays and data, and identify open questions for future studies. We expect that single-molecule fluorescence approaches will continue to be applied to outstanding nucleocytoplasmic transport questions, and that the approaches developed for NPC studies are extendable to additional complex systems and pathways within cells.

Bioavailable inhibitors of HIV-1 RNA biogenesis identified through a Rev-based screen

New antiretroviral agents with alternative mechanisms are needed to complement the combination therapies used to treat HIV-1 infections. Here we report the identification of bioavailable molecules that interfere with the gene expression processes of HIV-1. The compounds were detected by screening a small library of FDA-approved drugs with an assay based on measuring the displacement of Rev, and essential virus-encoded protein, from its high-affinity RNA binding site. The antiretroviral activity of two hits was based on interference with post-integration steps of the HIV-1 cycle. Both hits inhibited RRE-Rev complex formation in vitro, and blocked LTR-dependent gene expression and viral transcription in cellular assays. The best compound altered the splicing pattern of HIV-1 transcripts in a manner consistent with Rev inhibition. This mechanism of action is different from those used by current antiretroviral agents. The screening hits recognized the Rev binding site in the viral RNA, and the best compound did so with substantial selectivity, allowing the identification of a new RNA-binding scaffold. These results may be used for developing novel antiretroviral drugs.

Nucleoporin 35 regulates cardiomyocyte pH homeostasis by controlling Na+-H+ exchanger-1 expression

The mammalian nuclear pore complex is comprised of ∼ 30 different nucleoporins (Nups). It governs the nuclear import of gene expression modulators and the export of mRNAs. In cardiomyocytes, Na(+)-H(+) exchanger-1 (NHE1) is an integral membrane protein that exclusively regulates intracellular pH (pHi) by exchanging one intracellular H(+) for one extracellular Na(+). However, the role of Nups in cardiac NHE1 expression remains unknown. We herein report that Nup35 regulates cardiomyocyte NHE1 expression by controlling the nucleo-cytoplasmic trafficking of nhe1 mRNA. The N-terminal domain of Nup35 determines nhe1 mRNA nuclear export by targeting the 5'-UTR (-412 to -213 nt) of nhe1 mRNA. Nup35 ablation weakens the resistance of cardiomyocytes to an acid challenge by depressing NHE1 expression. Moreover, we identify that Nup35 and NHE1 are simultaneously downregulated in ischemic cardiomyocytes both in vivo and in vitro. Enforced expression of Nup35 effectively counteracts the anoxia-induced intracellular acidification. We conclude that Nup35 selectively regulates cardiomyocyte pHi homeostasis by posttranscriptionally controlling NHE1 expression. This finding reveals a novel regulatory mechanism of cardiomyocyte pHi, and may provide insight into the therapeutic strategy for ischemic cardiac diseases.

Control of mammalian gene expression by selective mRNA export

Nuclear export of mRNAs is a crucial step in the regulation of gene expression, linking transcription in the nucleus to translation in the cytoplasm. Although important components of the mRNA export machinery are well characterized, such as transcription-export complexes TREX and TREX-2, recent work has shown that, in some instances, mammalian mRNA export can be selective and can regulate crucial biological processes such as DNA repair, gene expression, maintenance of pluripotency, haematopoiesis, proliferation and cell survival. Such findings show that mRNA export is an unexpected, yet potentially important, mechanism for the control of gene expression and of the mammalian transcriptome.

Emerging properties of nuclear RNP biogenesis and export

RNA biology has recently seen an explosion of data due to advances in RNA sequencing, proteomic, and RNA imaging technologies. In this review, we highlight progress that has been made using these approaches in the area of nuclear RNP biogenesis and export. Excitingly, the ability to collect quantitative data at the 'omics' scale combined with measurements of transcription, decay, and transport kinetics is providing the information needed to address RNP biogenesis at a systems level. We believe this to be a necessary and critical next step that will lead to a better understanding of how RNP quality, diversity, and fate emerge from a defined set of nuclear RNP assembly and maturation steps.

Subcellular mRNA localisation at a glance

mRNA localisation coupled to translational regulation provides an important means of dictating when and where proteins function in a variety of model systems. This mechanism is particularly relevant in polarised or migrating cells. Although many of the models for how this is achieved were first proposed over 20 years ago, some of the molecular details are still poorly understood. Nevertheless, advanced imaging, biochemical and computational approaches have started to shed light on the cis-acting localisation signals and trans-acting factors that dictate the final destination of localised transcripts. In this Cell Science at a Glance article and accompanying poster, we provide an overview of mRNA localisation, from transcription to degradation, focusing on the microtubule-dependent active transport and anchoring mechanism, which we will use to explain the general paradigm. However, it is clear that there are diverse ways in which mRNAs become localised and target protein expression, and we highlight some of the similarities and differences between these mechanisms.

Nuclear export of messenger RNA

Transport of messenger RNA (mRNA) from the nucleus to the cytoplasm is an essential step of eukaryotic gene expression. In the cell nucleus, a precursor mRNA undergoes a series of processing steps, including capping at the 5' ends, splicing and cleavage/polyadenylation at the 3' ends. During this process, the mRNA associates with a wide variety of proteins, forming a messenger ribonucleoprotein (mRNP) particle. Association with factors involved in nuclear export also occurs during transcription and processing, and thus nuclear export is fully integrated into mRNA maturation. The coupling between mRNA maturation and nuclear export is an important mechanism for providing only fully functional and competent mRNA to the cytoplasmic translational machinery, thereby ensuring accuracy and swiftness of gene expression. This review describes the molecular mechanism of nuclear mRNA export mediated by the principal transport factors, including Tap-p15 and the TREX complex.

Invited review: decoding the pathophysiological mechanisms that underlie RNA dysregulation in neurodegenerative disorders: a review of the current state of the art

Altered RNA metabolism is a key pathophysiological component causing several neurodegenerative diseases. Genetic mutations causing neurodegeneration occur in coding and noncoding regions of seemingly unrelated genes whose products do not always contribute to the gene expression process. Several pathogenic mechanisms may coexist within a single neuronal cell, including RNA/protein toxic gain-of-function and/or protein loss-of-function. Genetic mutations that cause neurodegenerative disorders disrupt healthy gene expression at diverse levels, from chromatin remodelling, transcription, splicing, through to axonal transport and repeat-associated non-ATG (RAN) translation. We address neurodegeneration in repeat expansion disorders [Huntington's disease, spinocerebellar ataxias, C9ORF72-related amyotrophic lateral sclerosis (ALS)] and in diseases caused by deletions or point mutations (spinal muscular atrophy, most subtypes of familial ALS). Some neurodegenerative disorders exhibit broad dysregulation of gene expression with the synthesis of hundreds to thousands of abnormal messenger RNA (mRNA) molecules. However, the number and identity of aberrant mRNAs that are translated into proteins - and how these lead to neurodegeneration - remain unknown. The field of RNA biology research faces the challenge of identifying pathophysiological events of dysregulated gene expression. In conclusion, we discuss current research limitations and future directions to improve our characterization of pathological mechanisms that trigger disease onset and progression.

Role of omega-3 PUFA-derived mediators, the protectins, in influenza virus infection

Influenza A viruses are the causative agents of seasonal and pandemic infections. Influenza strains have recently emerged that show resistance to anti-viral drugs. Moreover, therapies in critically ill patients with severe influenza are limited, with the current anti-viral drugs showing disappointing results even in the absence of obvious viral resistance. Given the high mortality associated with avian H5N1 or H7N9 infections and the risk of pandemic potentials, effective drugs are needed for the treatment of severe influenza. A virus-host interaction is a multidimensional host response, in which not only genes and protein but also metabolites are up- or down-regulated, and cellular pathways and networks implicated in the viral pathogenesis are perturbed. Thus, it seems an attractive strategy to overcome influenza by targeting host metabolites and/or metabolic pathways involved in viral pathogenesis. Using lipidomics and lipid libraries screening, potectin D1 isomer (PDX) derived from the 15-lipoxygenase product 17S-H(p)DHA and/or 17HDHA precursor, has recently been identified, which suppresses influenza virus replication by inhibiting the nuclear export of viral mRNA rather than regulating resolution of inflammation. Contribution of the protectins to control influenza virus replication and their therapeutic potentials are reviewed here. This article is part of a Special Issue entitled "Oxygenated metabolism of PUFA: analysis and biological relevance".

How the nucleus copes with proteotoxic stress

The proper folding of proteins is continuously challenged by intrinsic and extrinsic stresses, and the accumulation of toxic misfolded proteins is associated with many human diseases. Eukaryotic cells have evolved a complex network of protein quality control pathways to protect the proteome, and these pathways are specialized for each subcellular compartment. While many details have been elucidated for how the cytosol and endoplasmic reticulum counteract proteotoxic stress, relatively little is known about the pathways protecting the nucleus from protein misfolding. Proper maintenance of nuclear proteostasis has important implications in preserving genomic integrity, as well as for aging and disease. Here, we offer a conceptual framework for how proteostasis is maintained in this organelle. We define the particular requirements that must be considered for the nucleus to manage proteotoxic stress, summarize the known and implicated pathways of nuclear protein quality control, and identify the unresolved questions in the field.

Labelling and imaging of single endogenous messenger RNA particles in vivo

RNA molecules carry out widely diverse functions in numerous different physiological processes in living cells. The RNA life cycle from transcription, through the processing of nascent RNA, to the regulatory function of non-coding RNA and cytoplasmic translation of messenger RNA has been studied extensively using biochemical and molecular biology techniques. In this Commentary, we highlight how single molecule imaging and particle tracking can yield further insight into the dynamics of RNA particles in living cells. In the past few years, a variety of bright and photo-stable labelling techniques have been developed to generate sufficient contrast for imaging of single endogenous RNAs in vivo. New imaging modalities allow determination of not only lateral but also axial positions with high precision within the cellular context, and across a wide range of specimen from yeast and bacteria to cultured cells, and even multicellular organisms or live animals. A whole range of methods to locate and track single particles, and to analyze trajectory data are available to yield detailed information about the kinetics of all parts of the RNA life cycle. Although the concepts presented are applicable to all types of RNA, we showcase here the wealth of information gained from in vivo imaging of single particles by discussing studies investigating dynamics of intranuclear trafficking, nuclear pore transport and cytoplasmic transport of endogenous messenger RNA.

Components and regulation of nuclear transport processes

The spatial separation of DNA replication and gene transcription in the nucleus and protein translation in the cytoplasm is a uniform principle of eukaryotic cells. This compartmentalization imposes a requirement for a transport network of macromolecules to shuttle these components in and out of the nucleus. This nucleo‐cytoplasmic transport of macromolecules is critical for both cell physiology and pathology. Consequently, investigating its regulation and disease‐associated alterations can reveal novel therapeutic approaches to fight human diseases, such as cancer or viral infection. The characterization of the nuclear pore complex, the identification of transport signals and transport receptors, as well as the characterization of the Ran system (providing the energy source for efficient cargo transport) has greatly facilitated our understanding of the components, mechanisms and regulation of the nucleo‐cytoplasmic transport of proteins in our cells. Here we review this knowledge with a specific emphasis on the selection of disease‐relevant molecular targets for potential therapeutic intervention.

Camello, a novel family of Histone Acetyltransferases that acetylate histone H4 and is essential for zebrafish development

In this study, we have investigated genome-wide occurrence of Histone Acetyltransferases (HATs) in genomes of Mus musculus and Danio rerio on the basis of presence of HAT domain. Our study identified a group of proteins that lacks characteristic features of known HAT families, relatively smaller in size and has no other associated domains. Most of the proteins in this unclassified group are Camello proteins, which are not yet known and classified as functional HATs. Our in vitro and in vivo analysis revealed that Camello family proteins are active HATs and exhibit specificity towards histone H4. Interestingly, Camello proteins are among the first identified HATs showing perinuclear localization. Moreover, Camello proteins are evolutionarily conserved in all chordates and are observed for the first time in cnidarians in phylogeny. Furthermore, knockdown of Camello protein (CMLO3) in zebrafish embryos exhibited defects in axis elongation and head formation. Thus, our study identified a novel family of active HATs that is specific for histone H4 acetylation, exhibits perinuclear localization and is essential for zebrafish development.

The contribution of co-transcriptional RNA:DNA hybrid structures to DNA damage and genome instability

Accurate DNA replication and DNA repair are crucial for the maintenance of genome stability, and it is generally accepted that failure of these processes is a major source of DNA damage in cells. Intriguingly, recent evidence suggests that DNA damage is more likely to occur at genomic loci with high transcriptional activity. Furthermore, loss of certain RNA processing factors in eukaryotic cells is associated with increased formation of co-transcriptional RNA:DNA hybrid structures known as R-loops, resulting in double-strand breaks (DSBs) and DNA damage. However, the molecular mechanisms by which R-loop structures ultimately lead to DNA breaks and genome instability is not well understood. In this review, we summarize the current knowledge about the formation, recognition and processing of RNA:DNA hybrids, and discuss possible mechanisms by which these structures contribute to DNA damage and genome instability in the cell.

Nucleoporin FG domains facilitate mRNP remodeling at the cytoplasmic face of the nuclear pore complex

Directional export of messenger RNA (mRNA) protein particles (mRNPs) through nuclear pore complexes (NPCs) requires multiple factors. In Saccharomyces cerevisiae, the NPC proteins Nup159 and Nup42 are asymmetrically localized to the cytoplasmic face and have distinct functional domains: a phenylalanine-glycine (FG) repeat domain that docks mRNP transport receptors and domains that bind the DEAD-box ATPase Dbp5 and its activating cofactor Gle1, respectively. We speculated that the Nup42 and Nup159 FG domains play a role in positioning mRNPs for the terminal mRNP-remodeling steps carried out by Dbp5. Here we find that deletion (Δ) of both the Nup42 and Nup159 FG domains results in a cold-sensitive poly(A)+ mRNA export defect. The nup42ΔFG nup159ΔFG mutant also has synthetic lethal genetic interactions with dbp5 and gle1 mutants. RNA cross-linking experiments further indicate that the nup42ΔFG nup159ΔFG mutant has a reduced capacity for mRNP remodeling during export. To further analyze the role of these FG domains, we replaced the Nup159 or Nup42 FG domains with FG domains from other Nups. These FG "swaps" demonstrate that only certain FG domains are functional at the NPC cytoplasmic face. Strikingly, fusing the Nup42 FG domain to the carboxy-terminus of Gle1 bypasses the need for the endogenous Nup42 FG domain, highlighting the importance of proximal positioning for these factors. We conclude that the Nup42 and Nup159 FG domains target the mRNP to Gle1 and Dbp5 for mRNP remodeling at the NPC. Moreover, these results provide key evidence that character and context play a direct role in FG domain function and mRNA export.