Comparison of the yeast and human nuclear exosome complexes

Role of senataxin in R-loop-mediated neurodegeneration

Senataxin is an RNA:DNA helicase that plays an important role in the resolution of RNA:DNA hybrids (R-loops) formed during transcription. R-loops are involved in the regulation of biological processes such as immunoglobulin class switching, gene expression and DNA repair. Excessive accumulation of R-loops results in DNA damage and loss of genomic integrity. Senataxin is critical for maintaining optimal levels of R-loops to prevent DNA damage and acts as a genome guardian. Within the nucleus, senataxin interacts with various RNA processing factors and DNA damage response and repair proteins. Senataxin interactors include survival motor neuron and zinc finger protein 1, with whom it co-localizes in sub-nuclear bodies. Despite its ubiquitous expression, mutations in senataxin specifically affect neurons and result in distinct neurodegenerative diseases such as amyotrophic lateral sclerosis type 4 and ataxia with oculomotor apraxia type 2, which are attributed to the gain-of-function and the loss-of-function mutations in senataxin, respectively. In addition, low levels of senataxin (loss-of-function) in spinal muscular atrophy result in the accumulation of R-loops causing DNA damage and motor neuron degeneration. Senataxin may play multiple functions in diverse cellular processes; however, its emerging role in R-loop resolution and maintenance of genomic integrity is gaining attention in the field of neurodegenerative diseases. In this review, we highlight the role of senataxin in R-loop resolution and its potential as a therapeutic target to treat neurodegenerative diseases.

Hypoxia-driven deSUMOylation of EXOSC10 promotes adaptive changes in the transcriptome profile

Reduced oxygen availability (hypoxia) triggers adaptive cellular responses via hypoxia-inducible factor (HIF)-dependent transcriptional activation. Adaptation to hypoxia also involves transcription-independent processes like post-translational modifications; however, these mechanisms are poorly characterized. Investigating the involvement of protein SUMOylation in response to hypoxia, we discovered that hypoxia strongly decreases the SUMOylation of Exosome subunit 10 (EXOSC10), the catalytic subunit of the RNA exosome, in an HIF-independent manner. EXOSC10 is a multifunctional exoribonuclease enriched in the nucleolus that mediates the processing and degradation of various RNA species. We demonstrate that the ubiquitin-specific protease 36 (USP36) SUMOylates EXOSC10 and we reveal SUMO1/sentrin-specific peptidase 3 (SENP3) as the enzyme-mediating deSUMOylation of EXOSC10. Under hypoxia, EXOSC10 dissociates from USP36 and translocates from the nucleolus to the nucleoplasm concomitant with its deSUMOylation. Loss of EXOSC10 SUMOylation does not detectably affect rRNA maturation but affects the mRNA transcriptome by modulating the expression levels of hypoxia-related genes. Our data suggest that dynamic modulation of EXOSC10 SUMOylation and localization under hypoxia regulates the RNA degradation machinery to facilitate cellular adaptation to low oxygen conditions.

Surveillance of 3' mRNA cleavage during transcription termination requires CF IB/Hrp1

CF IB/Hrp1 is part of the cleavage and polyadenylation factor (CPF) and cleavage factor (CF) complex (CPF-CF), which is responsible for 3' cleavage and maturation of pre-mRNAs. Although Hrp1 supports this process, its presence is not essential for the cleavage event. Here, we show that the main function of Hrp1 in the CPF-CF complex is the nuclear mRNA quality control of proper 3' cleavage. As such, Hrp1 acts as a nuclear mRNA retention factor that hinders transcripts from leaving the nucleus until processing is completed. Only after proper 3' cleavage, which is sensed through contacting Rna14, Hrp1 recruits the export receptor Mex67, allowing nuclear export. Consequently, its absence results in the leakage of elongated mRNAs into the cytoplasm. If cleavage is defective, the presence of Hrp1 on the mRNA retains these elongated transcripts until they are eliminated by the nuclear exosome. Together, we identify Hrp1 as the key quality control factor for 3' cleavage.

Caught in the act-Visualizing ribonucleases during eukaryotic ribosome assembly

Ribosomes are essential macromolecular machines responsible for translating the genetic information encoded in mRNAs into proteins. Ribosomes are composed of ribosomal RNAs and proteins (rRNAs and RPs) and the rRNAs fulfill both catalytic and architectural functions. Excision of the mature eukaryotic rRNAs from their precursor transcript is achieved through a complex series of endoribonucleolytic cleavages and exoribonucleolytic processing steps that are precisely coordinated with other aspects of ribosome assembly. Many ribonucleases involved in pre-rRNA processing have been identified and pre-rRNA processing pathways are relatively well defined. However, momentous advances in cryo-electron microscopy have recently enabled structural snapshots of various pre-ribosomal particles from budding yeast (Saccharomyces cerevisiae) and human cells to be captured and, excitingly, these structures not only allow pre-rRNAs to be observed before and after cleavage events, but also enable ribonucleases to be visualized on their target RNAs. These structural views of pre-rRNA processing in action allow a new layer of understanding of rRNA maturation and how it is coordinated with other aspects of ribosome assembly. They illuminate mechanisms of target recognition by the diverse ribonucleases involved and reveal how the cleavage/processing activities of these enzymes are regulated. In this review, we discuss the new insights into pre-rRNA processing gained by structural analyses and the growing understanding of the mechanisms of ribonuclease regulation. This article is categorized under: Translation > Ribosome Biogenesis RNA Processing > rRNA Processing.

Ribosome biogenesis factors-from names to functions

The assembly of ribosomal subunits is a highly orchestrated process that involves a huge cohort of accessory factors. Most eukaryotic ribosome biogenesis factors were first identified by genetic screens and proteomic approaches of pre-ribosomal particles in Saccharomyces cerevisiae. Later, research on human ribosome synthesis not only demonstrated that the requirement for many of these factors is conserved in evolution, but also revealed the involvement of additional players, reflecting a more complex assembly pathway in mammalian cells. Yet, it remained a challenge for the field to assign a function to many of the identified factors and to reveal their molecular mode of action. Over the past decade, structural, biochemical, and cellular studies have largely filled this gap in knowledge and led to a detailed understanding of the molecular role that many of the players have during the stepwise process of ribosome maturation. Such detailed knowledge of the function of ribosome biogenesis factors will be key to further understand and better treat diseases linked to disturbed ribosome assembly, including ribosomopathies, as well as different types of cancer.

A guard protein mediated quality control mechanism monitors 5'-capping of pre-mRNAs

Efficient gene expression requires properly matured mRNAs for functional transcript translation. Several factors including the guard proteins monitor maturation and act as nuclear retention factors for unprocessed pre-mRNAs. Here we show that the guard protein Npl3 monitors 5'-capping. In its absence, uncapped transcripts resist degradation, because the Rat1-Rai1 5'-end degradation factors are not efficiently recruited to these faulty transcripts. Importantly, in npl3Δ, these improperly capped transcripts escape this quality control checkpoint and leak into the cytoplasm. Our data suggest a model in which Npl3 associates with the Rai1 bound pre-mRNAs. In case the transcript was properly capped and is thus CBC (cap binding complex) bound, Rai1 dissociates from Npl3 allowing the export factor Mex67 to interact with this guard protein and support nuclear export. In case Npl3 does not detect proper capping through CBC attachment, Rai1 binding persists and Rat1 can join this 5'-complex to degrade the faulty transcript.

RNA marker modifications reveal the necessity for rigorous preparation protocols to avoid artifacts in epitranscriptomic analysis

The accurate definition of an epitranscriptome is endangered by artefacts resulting from RNA degradation after cell death, a ubiquitous yet little investigated process. By tracing RNA marker modifications through tissue preparation protocols, we identified a major blind spot from daily lab routine, that has massive impact on modification analysis in small RNAs. In particular, m6,6A and Am as co-varying rRNA marker modifications, appeared in small RNA fractions following rRNA degradation in vitro and in cellulo. Analysing mouse tissue at different time points post mortem, we tracked the progress of intracellular RNA degradation after cell death, and found it reflected in RNA modification patterns. Differences were dramatic between liver, where RNA degradation commenced immediately after death, and brain, yielding essentially undamaged RNA. RNA integrity correlated with low amounts of co-varying rRNA markers. Thus validated RNA preparations featured differentially modified tRNA populations whose information content allowed a distinction even among the related brain tissues cortex, cerebellum and hippocampus. Inversely, advanced cell death correlated with high rRNA marker content, and correspondingly little with the naïve state of living tissue. Therefore, unless RNA and tissue preparations are executed with utmost care, interpretation of modification patterns in tRNA and small RNA are prone to artefacts.

Tracing Eukaryotic Ribosome Biogenesis Factors Into the Archaeal Domain Sheds Light on the Evolution of Functional Complexity

Ribosome assembly is an essential and carefully choreographed cellular process. In eukaryotes, several 100 proteins, distributed across the nucleolus, nucleus, and cytoplasm, co-ordinate the step-wise assembly of four ribosomal RNAs (rRNAs) and approximately 80 ribosomal proteins (RPs) into the mature ribosomal subunits. Due to the inherent complexity of the assembly process, functional studies identifying ribosome biogenesis factors and, more importantly, their precise functions and interplay are confined to a few and very well-established model organisms. Although best characterized in yeast (Saccharomyces cerevisiae), emerging links to disease and the discovery of additional layers of regulation have recently encouraged deeper analysis of the pathway in human cells. In archaea, ribosome biogenesis is less well-understood. However, their simpler sub-cellular structure should allow a less elaborated assembly procedure, potentially providing insights into the functional essentials of ribosome biogenesis that evolved long before the diversification of archaea and eukaryotes. Here, we use a comprehensive phylogenetic profiling setup, integrating targeted ortholog searches with automated scoring of protein domain architecture similarities and an assessment of when search sensitivity becomes limiting, to trace 301 curated eukaryotic ribosome biogenesis factors across 982 taxa spanning the tree of life and including 727 archaea. We show that both factor loss and lineage-specific modifications of factor function modulate ribosome biogenesis, and we highlight that limited sensitivity of the ortholog search can confound evolutionary conclusions. Projecting into the archaeal domain, we find that only few factors are consistently present across the analyzed taxa, and lineage-specific loss is common. While members of the Asgard group are not special with respect to their inventory of ribosome biogenesis factors (RBFs), they unite the highest number of orthologs to eukaryotic RBFs in one taxon. Using large ribosomal subunit maturation as an example, we demonstrate that archaea pursue a simplified version of the corresponding steps in eukaryotes. Much of the complexity of this process evolved on the eukaryotic lineage by the duplication of ribosomal proteins and their subsequent functional diversification into ribosome biogenesis factors. This highlights that studying ribosome biogenesis in archaea provides fundamental information also for understanding the process in eukaryotes.

SUMOylated Senataxin functions in genome stability, RNA degradation, and stress granule disassembly, and is linked with inherited ataxia and motor neuron disease

BACKGROUND

Senataxin (SETX) is a DNA/RNA helicase critical for neuron survival. SETX mutations underlie two inherited neurodegenerative diseases: Ataxia with Oculomotor Apraxia type 2 (AOA2) and Amyotrophic Lateral Sclerosis type 4 (ALS4).

METHODS

This review examines SETX key cellular processes and we hypothesize that SETX requires SUMO posttranslational modification to function properly.

RESULTS

SETX is localized to distinct foci during S-phase of the cell cycle, and these foci represent sites of DNA polymerase/RNA polymerase II (RNAP) collision, as they co-localize with DNA damage markers 53BP1 and H2AX. At such sites, SETX directs incomplete RNA transcripts to the nuclear exosome for degradation via interaction with exosome component 9 (Exosc9), a key component of the nuclear exosome. These processes require SETX SUMOylation. SETX was also recently localized within stress granules (SGs), and found to regulate SG disassembly, a process that similarly requires SUMOylation.

CONCLUSION

SETX undergoes SUMO modification to function at S-phase foci in cycling cells to facilitate RNA degradation. SETX may regulate similar processes in non-dividing neurons at sites of RNAP II bidirectional self-collision. Finally, SUMOylation of SETX appears to be required for SG disassembly. This SETX function may be crucial for neuron survival, as altered SG dynamics are linked to ALS disease pathogenesis. In addition, AOA2 point mutations have been shown to block SETX SUMOylation. Such mutations induce an ataxia phenotype indistinguishable from those with SETX null mutation, underscoring the importance of this modification.

The leucine-NH4+ uptake regulator Any1 limits growth as part of a general amino acid control response to loss of La protein by fission yeast

The sla1⁺ gene of Schizosachharoymces pombe encodes La protein which promotes proper processing of precursor-tRNAs. Deletion of sla1 (sla1Δ) leads to disrupted tRNA processing and sensitivity to target of rapamycin (TOR) inhibition. Consistent with this, media containing NH4⁺ inhibits leucine uptake and growth of sla1Δ cells. Here, transcriptome analysis reveals that genes upregulated in sla1Δ cells exhibit highly significant overalp with general amino acid control (GAAC) genes in relevant transcriptomes from other studies. Growth in NH4⁺ media leads to additional induced genes that are part of a core environmental stress response (CESR). The sla1Δ GAAC response adds to evidence linking tRNA homeostasis and broad signaling in S. pombe. We provide evidence that deletion of the Rrp6 subunit of the nuclear exosome selectively dampens a subset of GAAC genes in sla1Δ cells suggesting that nuclear surveillance-mediated signaling occurs in S. pombe. To study the NH4⁺-effects, we isolated sla1Δ spontaneous revertants (SSR) of the slow growth phenotype and found that GAAC gene expression and rapamycin hypersensitivity were also reversed. Genome sequencing identified a F32V substitution in Any1, a known negative regulator of NH4⁺-sensitive leucine uptake linked to TOR. We show that ³H-leucine uptake by SSR-any1-F32V cells in NH4⁺-media is more robust than by sla1Δ cells. Moreover, F32V may alter any1⁺ function in sla1Δ vs. sla1⁺ cells in a distinctive way. Thus deletion of La, a tRNA processing factor leads to a GAAC response involving reprogramming of amino acid metabolism, and isolation of the any1-F32V rescuing mutant provides an additional specific link.

Rrp6 Moonlights in an RNA Exosome-Independent Manner to Promote Cell Survival and Gene Expression during Stress

The nuclear RNA exosome is essential for RNA processing and degradation. Here, we show that the exosome nuclear-specific subunit Rrp6p promotes cell survival during heat stress through the cell wall integrity (CWI) pathway, independently of its catalytic activity or association with the core exosome. Rrp6p exhibits negative genetic interactions with the Slt2/Mpk1p or Paf1p elongation factors required for expression of CWI genes during stress. Overexpression of Rrp6p or of its catalytically inactive or exosome-independent mutants can partially rescue the growth defect of the mpk1Δ mutant and stimulates expression of the Mpk1 p target gene FKS2. The rrp6Δ and mpk1Δ mutants show similarities in deficient expression of CWI genes during heat shock, and overexpression of the CWI gene HSP150 can rescue the stress-induced lethality of the mpk1Δrp6Δ mutant. These results demonstrate that Rrp6p moonlights independently from the exosome to ensure proper expression of CWI genes and to promote cell survival during stress.

Wang et al. show that Rrp6 functions with the Slt2/Mpk1 and Paf1 elongation factors for the proper expression of CWI genes during heat stress. The role of Rrp6p in promoting heat-stress-induced gene expression does not require Rrp6 catalytic activity or interaction with the nuclear RNA exosome.

Interactome analysis of the Tudor domain-containing protein SPF30 which associates with the MTR4-exosome RNA-decay machinery under the regulation of AAA-ATPase NVL2

The AAA-ATPase NVL2 associates with an RNA helicase MTR4 and the nuclear RNA exosome in the course of ribosome biogenesis. In our proteomic screen, we had identified a ribosome biogenesis factor WDR74 as a MTR4-interacting partner, whose dissociation is stimulated by the ATP hydrolysis of NVL2. In this study, we report the identification of splicing factor 30 (SPF30), another MTR4-interacting protein with a similar regulatory mechanism. SPF30 is a pre-mRNA splicing factor harboring a Tudor domain in its central region, which regulates various cellular events by binding to dimethylarginine-modified proteins. The interaction between SPF30 and the exosome core is mediated by MTR4 and RRP6, a catalytic component of the nuclear exosome. The N- and C-terminal regions, but not the Tudor domain, of SPF30 are involved in the association with MTR4 and the exosome. The knockdown of SPF30 caused subtle delay in the 12S pre-rRNA processing to mature 5.8S rRNA, even though no obvious effect was observed on the ribosome subunit profile in the cells. Shotgun proteomic analysis to search for SPF30-interacting proteins indicated its role in ribosome biogenesis, pre-mRNA splicing, and box C/D snoRNA biogenesis. These results suggest that SPF30 collaborates with the MTR4-exosome machinery to play a functional role in multiple RNA metabolic pathways, some of which may be regulated by the ATP hydrolysis of NVL2.

RNA exosome mutations in pontocerebellar hypoplasia alter ribosome biogenesis and p53 levels

The RNA exosome is a ubiquitously expressed complex of nine core proteins (EXOSC1-9) and associated nucleases responsible for RNA processing and degradation. Mutations in EXOSC3, EXOSC8, EXOSC9, and the exosome cofactor RBM7 cause pontocerebellar hypoplasia and motor neuronopathy. We investigated the consequences of exosome mutations on RNA metabolism and cellular survival in zebrafish and human cell models. We observed that levels of mRNAs encoding p53 and ribosome biogenesis factors are increased in zebrafish lines with homozygous mutations of exosc8 or exosc9, respectively. Consistent with higher p53 levels, mutant zebrafish have a reduced head size, smaller brain, and cerebellum caused by an increased number of apoptotic cells during development. Down-regulation of EXOSC8 and EXOSC9 in human cells leads to p53 protein stabilisation and G2/M cell cycle arrest. Increased p53 transcript levels were also observed in muscle samples from patients with EXOSC9 mutations. Our work provides explanation for the pathogenesis of exosome-related disorders and highlights the link between exosome function, ribosome biogenesis, and p53-dependent signalling. We suggest that exosome-related disorders could be classified as ribosomopathies.

Pol5 is required for recycling of small subunit biogenesis factors and for formation of the peptide exit tunnel of the large ribosomal subunit

More than 200 assembly factors (AFs) are required for the production of ribosomes in yeast. The stepwise association and dissociation of these AFs with the pre-ribosomal subunits occurs in a hierarchical manner to ensure correct maturation of the pre-rRNAs and assembly of the ribosomal proteins. Although decades of research have provided a wealth of insights into the functions of many AFs, others remain poorly characterized. Pol5 was initially classified with B-type DNA polymerases, however, several lines of evidence indicate the involvement of this protein in ribosome assembly. Here, we show that depletion of Pol5 affects the processing of pre-rRNAs destined for the both the large and small subunits. Furthermore, we identify binding sites for Pol5 in the 5' external transcribed spacer and within domain III of the 25S rRNA sequence. Consistent with this, we reveal that Pol5 is required for recruitment of ribosomal proteins that form the polypeptide exit tunnel in the LSU and that depletion of Pol5 impairs the release of 5' ETS fragments from early pre-40S particles. The dual functions of Pol5 in 60S assembly and recycling of pre-40S AFs suggest that this factor could contribute to ensuring the stoichiometric production of ribosomal subunits.

The splicing factor SRSF3 is functionally connected to the nuclear RNA exosome for intronless mRNA decay

The RNA exosome fulfills important functions in the processing and degradation of numerous RNAs species. However, the mechanisms of recruitment to its various nuclear substrates are poorly understood. Using Epstein-Barr virus mRNAs as a model, we have discovered a novel function for the splicing factor SRSF3 in the quality control of nuclear mRNAs. We have found that viral mRNAs generated from intronless genes are particularly unstable due to their degradation by the nuclear RNA exosome. This effect is counteracted by the viral RNA-binding protein EB2 which stabilizes these mRNAs in the nucleus and stimulates both their export to the cytoplasm and their translation. In the absence of EB2, SRSF3 participates in the destabilization of these viral RNAs by interacting with both the RNA exosome and its adaptor complex NEXT. Taken together, our results provide direct evidence for a connection between the splicing machinery and mRNA decay mediated by the RNA exosome. Our results suggest that SRSF3 aids the nuclear RNA exosome and the NEXT complex in the recognition and degradation of certain mRNAs.

Distinct and evolutionary conserved structural features of the human nuclear exosome complex

The nuclear RNA exosome complex mediates the processing of structured RNAs and the decay of aberrant non-coding RNAs, an important function particularly in human cells. Most mechanistic studies to date have focused on the yeast system. Here, we reconstituted and studied the properties of a recombinant 14-subunit human nuclear exosome complex. In biochemical assays, the human exosome embeds a longer RNA channel than its yeast counterpart. The 3.8 Å resolution cryo-EM structure of the core complex bound to a single-stranded RNA reveals that the RNA channel path is formed by two distinct features of the hDIS3 exoribonuclease: an open conformation and a domain organization more similar to bacterial RNase II than to yeast Rrp44. The cryo-EM structure of the holo-complex shows how obligate nuclear cofactors position the hMTR4 helicase at the entrance of the core complex, suggesting a striking structural conservation from lower to higher eukaryotes.

Structural insights into the interaction of the nuclear exosome helicase Mtr4 with the preribosomal protein Nop53

The nuclear exosome and the associated RNA helicase Mtr4 participate in the processing of several ribonucleoprotein particles (RNP), including the maturation of the large ribosomal subunit (60S). S. cerevisiae Mtr4 interacts directly with Nop53, a ribosomal biogenesis factor present in late pre-60S particles containing precursors of the 5.8S rRNA. The Mtr4-Nop53 interaction plays a pivotal role in the maturation of the 5.8S rRNA, providing a physical link between the nuclear exosome and the pre-60S RNP. An analogous interaction between Mtr4 and another ribosome biogenesis factor, Utp18, directs the exosome to an earlier preribosomal particle. Nop53 and Utp18 contain a similar Mtr4-binding motif known as the arch-interacting motif (AIM). Here, we report the 3.2 Å resolution crystal structure of S. cerevisiae Mtr4 bound to the interacting region of Nop53, revealing how the KOW domain of the helicase recognizes the AIM sequence of Nop53 with a network of hydrophobic and electrostatic interactions. The AIM-interacting residues are conserved in Mtr4 and are not present in the related cytoplasmic helicase Ski2, rationalizing the specificity and versatility of Mtr4 in the recognition of different AIM-containing proteins. Using nuclear magnetic resonance (NMR), we show that the KOW domain of Mtr4 can simultaneously bind an AIM-containing protein and a structured RNA at adjacent surfaces, suggesting how it can dock onto RNPs. The KOW domains of exosome-associated helicases thus appear to have evolved from the KOW domains of ribosomal proteins and to function as RNP-binding modules in the context of the nuclear exosome.

Endurance training lowers ribosome density despite increasing ribosome biogenesis markers in rodent skeletal muscle

Objective

The purpose of this study was to examine if: (a) high sugar/high fat Western diet (WD)-feeding affects skeletal muscle ribosome biogenesis markers in hyperphagic, diabetic-prone Otsuka Long-Evans Tokushima Fatty (OLETF) rats, and (b) 12 weeks of treadmill training rescued potential detriments that WD feeding exerted on these markers.

Methods

Eight week-old male OLETF rats were fed a low-fat control diet (O-CON, n = 10) or high/sucrose/cholesterol Western diet (WD). At weeks 20–32 of age, WD-fed rats were divided into WD sedentary (O-WD/SED, n = 16), or WD treadmill trained (5 days/week, 60 min/day) (O-WD/EX, n = 10) conditions.

Results

Interestingly, total RNA (i.e., ribosome density) was 2.3-fold greater in O-WD/SED versus O-WD/EX rats (p = 0.003) despite levels of upstream binding factor protein, RNA polymerase I protein and pre-45S rRNA being greater in O-WD/EX rats. Ribophagy (USP10 and G3BP1) and TRAMP-exosome rRNA degradation pathway (EXOSC10 and SKIV2L2) proteins were assayed to determine if these pathways were involved with lower ribosome density in O-WD/EX rats. While USP10 was higher in O-CON versus O-WD/SED and O-WD/EX rats (p < 0.001 and p < 0.001, respectively), G3BP1, EXOSC10 and SKIV2L2 did not differ between groups. Nop56 and Ncl mRNAs, ribosome assembly markers, were highest in O-WD/EX rats. However, Fbl mRNA and 28S rRNA, downstream ribosome processing markers, were lowest in O-WD/EX rats. Collectively these data suggest that, in WD-fed rats, endurance training increases select skeletal muscle ribosome biogenesis markers. However, endurance training may reduce muscle ribosome density by interfering with rRNA processing and/or export through mechanisms independent of ribophagy or rRNA degradation.

Comparison of preribosomal RNA processing pathways in yeast, plant and human cells - focus on coordinated action of endo- and exoribonucleases

Proper regulation of ribosome biosynthesis is mandatory for cellular adaptation, growth and proliferation. Ribosome biogenesis is the most energetically demanding cellular process, which requires tight control. Abnormalities in ribosome production have severe consequences, including developmental defects in plants and genetic diseases (ribosomopathies) in humans. One of the processes occurring during eukaryotic ribosome biogenesis is processing of the ribosomal RNA precursor molecule (pre-rRNA), synthesized by RNA polymerase I, into mature rRNAs. It must not only be accurate but must also be precisely coordinated with other phenomena leading to the synthesis of functional ribosomes: RNA modification, RNA folding, assembly with ribosomal proteins and nucleocytoplasmic RNP export. A multitude of ribosome biogenesis factors ensure that these events take place in a correct temporal order. Among them are endo- and exoribonucleases involved in pre-rRNA processing. Here, we thoroughly present a wide spectrum of ribonucleases participating in rRNA maturation, focusing on their biochemical properties, regulatory mechanisms and substrate specificity. We also discuss cooperation between various ribonucleolytic activities in particular stages of pre-rRNA processing, delineating major similarities and differences between three representative groups of eukaryotes: yeast, plants and humans.

Insight into the RNA Exosome Complex Through Modeling Pontocerebellar Hypoplasia Type 1b Disease Mutations in Yeast

Pontocerebellar hypoplasia type 1b (PCH1b) is an autosomal recessive disorder that causes cerebellar hypoplasia and spinal motor neuron degeneration, leading to mortality in early childhood. PCH1b is caused by mutations in the RNA exosome subunit gene, EXOSC3 The RNA exosome is an evolutionarily conserved complex, consisting of nine different core subunits, and one or two 3'-5' exoribonuclease subunits, that mediates several RNA degradation and processing steps. The goal of this study is to assess the functional consequences of the amino acid substitutions that have been identified in EXOSC3 in PCH1b patients. To analyze these EXOSC3 substitutions, we generated the corresponding amino acid substitutions in the Saccharomyces cerevisiae ortholog of EXOSC3, Rrp40 We find that the rrp40 variants corresponding to EXOSC3-G31A and -D132A do not affect yeast function when expressed as the sole copy of the essential Rrp40 protein. In contrast, the rrp40-W195R variant, corresponding to EXOSC3-W238R in PCH1b patients, impacts cell growth and RNA exosome function when expressed as the sole copy of Rrp40 The rrp40-W195R protein is unstable, and does not associate efficiently with the RNA exosome in cells that also express wild-type Rrp40 Consistent with these findings in yeast, the levels of mouse EXOSC3 variants are reduced compared to wild-type EXOSC3 in a neuronal cell line. These data suggest that cells possess a mechanism for optimal assembly of functional RNA exosome complex that can discriminate between wild-type and variant exosome subunits. Budding yeast can therefore serve as a useful tool to understand the molecular defects in the RNA exosome caused by PCH1b-associated amino acid substitutions in EXOSC3, and potentially extending to disease-associated substitutions in other exosome subunits.

Purification and analysis of endogenous human RNA exosome complexes

As a result of its importance in key RNA metabolic processes, the ribonucleolytic RNA exosome complex has been the focus of intense study for almost two decades. Research on exosome subunit assembly, cofactor and substrate interaction, enzymatic catalysis and structure have largely been conducted using complexes produced in the yeast Saccharomyces cerevisiae or in bacteria. Here, we examine different populations of endogenous exosomes from human embryonic kidney (HEK) 293 cells and test their enzymatic activity and structural integrity. We describe methods to prepare EXOSC10-containing, enzymatically active endogenous human exosomes at suitable yield and purity for in vitro biochemistry and negative stain transmission electron microscopy. This opens the door for assays designed to test the in vitro effects of putative cofactors on human exosome activity and will enable structural studies of preparations from endogenous sources.

The 3'-5' exoribonuclease Dis3 regulates the expression of specific microRNAs in Drosophila wing imaginal discs

Dis3 is a highly conserved exoribonuclease which degrades RNAs in the 3'-5' direction. Mutations in Dis3 are associated with a number of human cancers including multiple myeloma and acute myeloid leukemia. In this work, we have assessed the effect of a Dis3 knockdown on Drosophila imaginal disc development and on expression of mature microRNAs. We find that Dis3 knockdown severely disrupts the development of wing imaginal discs in that the flies have a “no wing” phenotype. Use of RNA-seq to quantify the effect of Dis3 knockdown on microRNA expression shows that Dis3 normally regulates a small subset of microRNAs, with only 11 (10.1%) increasing in level ≥2-fold and 6 (5.5%) decreasing in level ≥2-fold. Of these microRNAs, miR-252–5p is increased 2.1-fold in Dis3-depleted cells compared to controls while the level of the miR-252 precursor is unchanged, suggesting that Dis3 can act in the cytoplasm to specifically degrade this mature miRNA. Furthermore, our experiments suggest that Dis3 normally interacts with the exosomal subunit Rrp40 in the cytoplasm to target miR-252–5p for degradation during normal wing development. Another microRNA, miR-982–5p, is expressed at lower levels in Dis3 knockdown cells, while the miR-982 precursor remains unchanged, indicating that Dis3 is involved in its processing. Our study therefore reveals an unexpected specificity for this ribonuclease toward microRNA regulation, which is likely to be conserved in other eukaryotes and may be relevant to understanding its role in human disease.

The Zinc-Finger Protein SOP1 Is Required for a Subset of the Nuclear Exosome Functions in Arabidopsis

Correct gene expression requires tight RNA quality control both at transcriptional and post-transcriptional levels. Using a splicing-defective allele of PASTICCINO2 (PAS2), a gene essential for plant development, we isolated suppressor mutations modifying pas2-1 mRNA profiles and restoring wild-type growth. Three suppressor of pas2 (sop) mutations modified the degradation of mis-spliced pas2-1 mRNA species, allowing the synthesis of a functional protein. Cloning of the suppressor mutations identified the core subunit of the exosome SOP2/RRP4, the exosome nucleoplasmic cofactor SOP3/HEN2 and a novel zinc-finger protein SOP1 that colocalizes with HEN2 in nucleoplasmic foci. The three SOP proteins counteract post-transcriptional (trans)gene silencing (PTGS), which suggests that they all act in RNA quality control. In addition, sop1 mutants accumulate some, but not all of the misprocessed mRNAs and other types of RNAs that are observed in exosome mutants. Taken together, our data show that SOP1 is a new component of nuclear RNA surveillance that is required for the degradation of a specific subset of nuclear exosome targets.

DGCR8 Acts as an Adaptor for the Exosome Complex to Degrade Double-Stranded Structured RNAs

The Microprocessor complex (DGCR8/Drosha) is required for microRNA (miRNA) biogenesis but also binds and regulates the stability of several types of cellular RNAs. Of particular interest, DGCR8 controls the stability of mature small nucleolar RNA (snoRNA) transcripts independently of Drosha, suggesting the existence of alternative DGCR8 complex(es) with other nucleases to process a variety of cellular RNAs. Here, we found that DGCR8 copurifies with subunits of the nuclear exosome, preferentially associating with its hRRP6-containing nucleolar form. Importantly, we demonstrate that DGCR8 is essential for the recruitment of the exosome to snoRNAs and to human telomerase RNA. In addition, we show that the DGCR8/exosome complex controls the stability of the human telomerase RNA component (hTR/TERC). Altogether, these data suggest that DGCR8 acts as an adaptor to recruit the exosome complex to structured RNAs and induce their degradation.

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DGCR8 forms an alternative complex with the hRRP6-containing form of the exosome

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DGCR8 acts as an adaptor to recruit the exosome to target structured RNAs

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The DGCR8/hRRP6 complex also controls the stability of human telomerase RNA

Rrp6: Integrated roles in nuclear RNA metabolism and transcription termination

The yeast RNA exosome is a eukaryotic ribonuclease complex essential for RNA processing, surveillance, and turnover. It is comprised of a barrel-shaped core and cap as well as a 3'-5' ribonuclease known as Dis3 that contains both endo- and exonuclease domains. A second exonuclease, Rrp6, is added in the nucleus. Dis3 and Rrp6 have both shared and distinct roles in RNA metabolism, and this review will focus primarily on Rrp6 and the roles of the RNA exosome in the nucleus. The functions of the nuclear exosome are modulated by cofactors and interacting partners specific to each type of substrate. Generally, the cofactor TRAMP (Trf4/5-Air2/1-Mtr4 polyadenylation) complex helps unwind unstable RNAs, RNAs requiring processing such as rRNAs, tRNAs, or snRNAs or improperly processed RNAs and direct it toward the exosome. In yeast, Rrp6 interacts with Nrd1, the cap-binding complex, and RNA polymerase II to aid in nascent RNA processing, termination, and polyA tail length regulation. Recent studies have shown that proper termination and processing of short, noncoding RNAs by Rrp6 is particularly important for transcription regulation across the genome and has important implications for regulation of diverse processes at the cellular level. Loss of proper Rrp6 and exosome activity may contribute to various pathologies such as autoimmune disease, neurological disorders, and cancer. WIREs RNA 2016, 7:91-104. doi: 10.1002/wrna.1317 For further resources related to this article, please visit the WIREs website.

Quality control of spliced mRNAs requires the shuttling SR proteins Gbp2 and Hrb1

Eukaryotic cells have to prevent the export of unspliced pre-mRNAs until intron removal is completed to avoid the expression of aberrant and potentially harmful proteins. Only mature mRNAs associate with the export receptor Mex67/TAP and enter the cytoplasm. Here we show that the two shuttling serine/arginine (SR)-proteins Gbp2 and Hrb1 are key surveillance factors for the selective export of spliced mRNAs in yeast. Their absence leads to the significant leakage of unspliced pre-mRNAs into the cytoplasm. They bind to pre-mRNAs and the spliceosome during splicing, where they are necessary for the surveillance of splicing and the stable binding of the TRAMP complex to spliceosome-bound transcripts. Faulty transcripts are marked for their degradation at the nuclear exosome. On correct mRNAs the SR proteins recruit Mex67 upon completion of splicing to allow a quality controlled nuclear export. Altogether, these data identify a role for shuttling SR proteins in mRNA surveillance and nuclear mRNA quality control.

The exosome controls alternative splicing by mediating the gene expression and assembly of the spliceosome complex

The exosome is a complex with exoribonuclease activity that regulates RNA surveillance and turnover. The exosome also plays a role in regulating the degradation of precursor mRNAs to maintain the expression of splicing variants. In Neurospora, the silencing of rrp44, which encodes the catalytic subunit of the exosome, changed the expression of a set of spliceosomal snRNA, snRNP genes and SR protein related genes. The knockdown of rrp44 also affected the assembly of the spliceosome. RNA-seq analysis revealed a global change in bulk splicing events. Exosome-mediated splicing may regulate alternative splicing of NCU05290, NCU07421 and the circadian clock gene frequency (frq). The knockdown of rrp44 led to an increased ratio of splicing variants without intron 6 (I-6) and shorter protein isoform small FRQ (s-FRQ) as a consequence. These findings suggest that the exosome controls splicing events by regulating the degradation of precursor mRNAs and the gene expression, assembly and function of the spliceosome.

Rapid, optimized interactomic screening

We must reliably map the interactomes of cellular macromolecular complexes in order to fully explore and understand biological systems. However, there are no methods to accurately predict how to capture a given macromolecular complex with its physiological binding partners. Here, we present a screen that comprehensively explores the parameters affecting the stability of interactions in affinity-captured complexes, enabling the discovery of physiological binding partners and the elucidation of their functional interactions in unparalleled detail. We have implemented this screen on several macromolecular complexes from a variety of organisms, revealing novel profiles even for well-studied proteins. Our approach is robust, economical and automatable, providing an inroad to the rigorous, systematic dissection of cellular interactomes.

Perspective: The RNA exosome, cytokine gene regulation and links to autoimmunity

The RNA exosome is a highly conserved exoribonuclease complex that is involved in RNA processing, quality control and turnover regulation. The exosome plays pleiotropic functions by recruiting different cofactors that regulate its target specificity. Recently, the exosome has been implicated in the regulation of immune processes including cytokine production and negative regulation of innate sensing of nucleic acids. Careful regulation of such mechanisms is critical to avoid a breakdown of self-tolerance and the pathogenesis of autoimmune disorders. This perspective briefly introduces the exosome, its its normal function in RNA biology and summarizes regulatory roles of the RNA exosome in immunity. Finally we discuss how dysregulation of exosome function can lead to autoimmune disease.

Nab3 facilitates the function of the TRAMP complex in RNA processing via recruitment of Rrp6 independent of Nrd1

Non-coding RNAs (ncRNAs) play critical roles in gene regulation. In eukaryotic cells, ncRNAs are processed and/or degraded by the nuclear exosome, a ribonuclease complex containing catalytic subunits Dis3 and Rrp6. The TRAMP (Trf4/5-Air1/2-Mtr4 polyadenylation) complex is a critical exosome cofactor in budding yeast that stimulates the exosome to process/degrade ncRNAs and human TRAMP components have recently been identified. Importantly, mutations in exosome and exosome cofactor genes cause neurodegenerative disease. How the TRAMP complex interacts with other exosome cofactors to orchestrate regulation of the exosome is an open question. To identify novel interactions of the TRAMP exosome cofactor, we performed a high copy suppressor screen of a thermosensitive air1/2 TRAMP mutant. Here, we report that the Nab3 RNA-binding protein of the Nrd1-Nab3-Sen1 (NNS) complex is a potent suppressor of TRAMP mutants. Unlike Nab3, Nrd1 and Sen1 do not suppress TRAMP mutants and Nrd1 binding is not required for Nab3-mediated suppression of TRAMP suggesting an independent role for Nab3. Critically, Nab3 decreases ncRNA levels in TRAMP mutants, Nab3-mediated suppression of air1/2 cells requires the nuclear exosome component, Rrp6, and Nab3 directly binds Rrp6. We extend this analysis to identify a human RNA binding protein, RALY, which shares identity with Nab3 and can suppress TRAMP mutants. These results suggest that Nab3 facilitates TRAMP function by recruiting Rrp6 to ncRNAs for processing/degradation independent of Nrd1. The data raise the intriguing possibility that Nab3 and Nrd1 can function independently to recruit Rrp6 to ncRNA targets, providing combinatorial flexibility in RNA processing.

Eukaryotic genomes from yeast to man express numerous non-coding RNAs (ncRNAs) that regulate the expression of messenger RNAs (mRNAs) encoding the proteins vital for cell and body function. As faulty ncRNAs impair mRNA expression and contribute to cancers and neurodegenerative disease, it is imperative to understand how ncRNAs are processed and/or degraded. In budding yeast, a conserved RNA shredding machine known as the exosome nibbles at or destroys ncRNAs. The exosome is assisted by a conserved TRAMP exosome cofactor that recruits the exosome to ncRNAs for processing/ degradation. To better understand TRAMP function, we performed a genetic screen to identify genes that improve the growth of TRAMP mutant yeast cells that grow poorly at high temperature. We find that overexpression of the Nab3 RNA binding protein, which belongs to another exosome cofactor, the Nrd1-Nab3-Sen1 (NNS) complex, improves the growth of TRAMP mutant cells. Importantly, Nab3 requires the exosome to improve the growth and ncRNA processing of TRAMP mutant cells. We therefore suggest that Nab3 facilitates TRAMP function by recruiting the exosome to ncRNAs for processing/degradation. We also show that the human RNA binding protein, RALY, like Nab3, can improve the growth of TRAMP mutant cells.

The human nuclear exosome targeting complex is loaded onto newly synthesized RNA to direct early ribonucleolysis

The RNA exosome complex constitutes the major nuclear eukaryotic 3'-5' exonuclease. Outside of nucleoli, the human nucleoplasmic exosome is directed to some of its substrates by the nuclear exosome targeting (NEXT) complex. How NEXT targets RNA has remained elusive. Using an in vivo crosslinking approach, we report global RNA binding sites of RBM7, a key component of NEXT. RBM7 associates broadly with RNA polymerase II-derived RNA, including pre-mRNA and short-lived exosome substrates such as promoter upstream transcripts (PROMPTs), enhancer RNAs (eRNAs), and 3'-extended products from snRNA and replication-dependent histone genes. Within pre-mRNA, RBM7 accumulates at the 3' ends of introns, and pulse-labeling experiments demonstrate that RBM7/NEXT defines an early exosome-targeting pathway for 3'-extended snoRNAs derived from such introns. We propose that RBM7 is generally loaded onto newly synthesized RNA to accommodate exosome action in case of available unprotected RNA 3' ends.

An overview of pre-ribosomal RNA processing in eukaryotes

Ribosomal RNAs are the most abundant and universal noncoding RNAs in living organisms. In eukaryotes, three of the four ribosomal RNAs forming the 40S and 60S subunits are borne by a long polycistronic pre-ribosomal RNA. A complex sequence of processing steps is required to gradually release the mature RNAs from this precursor, concomitant with the assembly of the 79 ribosomal proteins. A large set of trans-acting factors chaperone this process, including small nucleolar ribonucleoparticles. While yeast has been the gold standard for studying the molecular basis of this process, recent technical advances have allowed to further define the mechanisms of ribosome biogenesis in animals and plants. This renewed interest for a long-lasting question has been fueled by the association of several genetic diseases with mutations in genes encoding both ribosomal proteins and ribosome biogenesis factors, and by the perspective of new anticancer treatments targeting the mechanisms of ribosome synthesis. A consensus scheme of pre-ribosomal RNA maturation is emerging from studies in various kinds of eukaryotic organisms. However, major differences between mammalian and yeast pre-ribosomal RNA processing have recently come to light. WIREs RNA 2015, 6:225–242. doi: 10.1002/wrna.1269

Cotranscriptional events in eukaryotic ribosome synthesis

Eukaryotic ribosomes are synthesized in a complex, multistep pathway. This begins with transcription of the rDNA genes by a specialized RNA polymerase, accompanied by the cotranscriptional binding of large numbers of ribosome synthesis factors, small nucleolar RNAs and ribosomal proteins. Cleavage of the nascent transcript releases the early pre-40S and pre-60S particles, which acquire export competence in the nucleoplasm prior to translocation through the nuclear pore complexes and final maturation to functional ribosomal subunits in the cytoplasm. This review will focus on the many and complex interactions occurring during pre-rRNA synthesis, particularly in budding yeast in which the pathway is best understood.

Certain adenylated non-coding RNAs, including 5' leader sequences of primary microRNA transcripts, accumulate in mouse cells following depletion of the RNA helicase MTR4

RNA surveillance plays an important role in posttranscriptional regulation. Seminal work in this field has largely focused on yeast as a model system, whereas exploration of RNA surveillance in mammals is only recently begun. The increased transcriptional complexity of mammalian systems provides a wider array of targets for RNA surveillance, and, while many questions remain unanswered, emerging data suggest the nuclear RNA surveillance machinery exhibits increased complexity as well. We have used a small interfering RNA in mouse N2A cells to target the homolog of a yeast protein that functions in RNA surveillance (Mtr4p). We used high-throughput sequencing of polyadenylated RNAs (PA-seq) to quantify the effects of the mMtr4 knockdown (KD) on RNA surveillance. We demonstrate that overall abundance of polyadenylated protein coding mRNAs is not affected, but several targets of RNA surveillance predicted from work in yeast accumulate as adenylated RNAs in the mMtr4KD. microRNAs are an added layer of transcriptional complexity not found in yeast. After Drosha cleavage separates the pre-miRNA from the microRNA's primary transcript, the byproducts of that transcript are generally thought to be degraded. We have identified the 5′ leading segments of pri-miRNAs as novel targets of mMtr4 dependent RNA surveillance.

The ticking tail: daily oscillations in mRNA poly(A) tail length drive circadian cycles in protein synthesis

In this issue of Genes & Development, Kojima and colleagues (pp. 2724-2736) examined the impact of mRNA poly(A) tail length on circadian gene expression. Their study demonstrates how dynamic changes in transcript poly(A) tail length can lead to rhythmic protein expression, irrespective of whether mRNA accumulation is circadian or constitutive.