The G-patch protein NF-κB-repressing factor mediates the recruitment of the exonuclease XRN2 and activation of the RNA helicase DHX15 in human ribosome biogenesis

Cellular functions of eukaryotic RNA helicases and their links to human diseases

RNA helicases are highly conserved proteins that use nucleoside triphosphates to bind or remodel RNA, RNA-protein complexes or both. RNA helicases are classified into the DEAD-box, DEAH/RHA, Ski2-like, Upf1-like and RIG-I families, and are the largest class of enzymes active in eukaryotic RNA metabolism - virtually all aspects of gene expression and its regulation involve RNA helicases. Mutation and dysregulation of these enzymes have been linked to a multitude of diseases, including cancer and neurological disorders. In this Review, we discuss the regulation and functional mechanisms of RNA helicases and their roles in eukaryotic RNA metabolism, including in transcription regulation, pre-mRNA splicing, ribosome assembly, translation and RNA decay. We highlight intriguing models that link helicase structure, mechanisms of function (such as local strand unwinding, translocation, winching, RNA clamping and displacing RNA-binding proteins) and biological roles, including emerging connections between RNA helicases and cellular condensates formed through liquid-liquid phase separation. We also discuss associations of RNA helicases with human diseases and recent efforts towards the design of small-molecule inhibitors of these pivotal regulators of eukaryotic gene expression.

DHX15 is involved in SUGP1-mediated RNA missplicing by mutant SF3B1 in cancer

SF3B1 is the most frequently mutated spliceosomal gene in cancer. Several hotspot mutations are known to disrupt the interaction of SF3B1 with another splicing factor, SUGP1, resulting in the RNA missplicing that characterizes mutant SF3B1 cancers. Properties of SUGP1, especially the presence of a G-patch motif, a structure known to function by activating DEAH-box RNA helicases, suggest the requirement of such an enzyme in SUGP1 function in splicing. However, the identity of this putative helicase has remained an important unanswered question. Here, using a variety of protein-protein interaction assays, we identify DHX15 as the critical helicase. We further show that depletion of DHX15 or expression of any of several DHX15 mutants, including one implicated in acute myeloid leukemia, partially recapitulates the splicing defects of mutant SF3B1. Moreover, a DHX15-SUGP1 G-patch fusion protein is able to incorporate into the spliceosome to rescue the splicing defects of mutant SF3B1. We also present the crystal structure of the human DHX15-SUGP1 G-patch complex, which reveals the molecular basis of their direct interaction. Our data thus demonstrate that DHX15 is the RNA helicase that functions with SUGP1 and additionally provide important insight into how mutant SF3B1 disrupts splicing in cancer.

Regulation of the DEAH/RHA helicase Prp43 by the G-patch factor Pfa1

The DEAH/RHA helicase Prp43 remodels protein-RNA complexes during pre-messenger RNA (mRNA) splicing and ribosome biogenesis. The helicase activity and ATP turnover are intrinsically low and become activated by G-patch (gp) factors in the specific cellular context. The gp motif connects the helicase core to the flexible C-terminal domains, but it is unclear how this affects RecA domain movement during catalysis and the unwinding of RNA substrates. We developed single-molecule Förster Resonance Energy Transfer (smFRET) reporters to study RecA domain movements within Prp43 in real time. Without Pfa1(gp), the domains approach each other adopting predominantly a closed conformation. The addition of Pfa1(gp) induces an open state, which becomes even more prevalent during interaction with RNA. In the open state, Prp43 has reduced contacts with bound nucleotide and shows rapid adenosine diphosphate (ADP) release accelerating the transition from the weak (ADP) to the strong (apo) RNA binding state. Using smFRET labels on the RNA to probe substrate binding and unwinding, we demonstrate that Pfa1(gp) enables Prp43(ADP) to switch between RNA-bound and RNA-unbound states instead of dissociating from the RNA. ATP binding to the apo-enzyme induces the translocation along the RNA, generating the unwinding force required to melt proximal RNA structures. During ATP turnover, Pfa1(gp) stimulates alternating of the RecA domains between open and closed states. Consequently, the translocation becomes faster than dissociation from the substrate in the ADP state, allowing processive movement along the RNA. We provide a mechanistic model of DEAH/RHA helicase motility and reveal the principles of Prp43 regulation by G-patch proteins.

Defining the Functional Interactome of Spliceosome-Associated G-Patch Protein Gpl1 in the Fission Yeast Schizosaccharomyces pombe

Pre-mRNA splicing plays a fundamental role in securing protein diversity by generating multiple transcript isoforms from a single gene. Recently, it has been shown that specific G-patch domain-containing proteins are critical cofactors involved in the regulation of splicing processes. In this study, using the knock-out strategy, affinity purification and the yeast-two-hybrid assay, we demonstrated that the spliceosome-associated G-patch protein Gpl1 of the fission yeast S. pombe mediates interactions between putative RNA helicase Gih35 (SPAC20H4.09) and WD repeat protein Wdr83, and ensures their binding to the spliceosome. Furthermore, RT-qPCR analysis of the splicing efficiency of deletion mutants indicated that the absence of any of the components of the Gpl1-Gih35-Wdr83 complex leads to defective splicing of fet5 and pwi1, the reference genes whose unspliced isoforms harboring premature stop codons are targeted for degradation by the nonsense-mediated decay (NMD) pathway. Together, our results shed more light on the functional interactome of G-patch protein Gpl1 and revealed that the Gpl1-Gih35-Wdr83 complex plays an important role in the regulation of pre-mRNA splicing in S. pombe.

DExH/D-box helicases at the frontline of intrinsic and innate immunity against viral infections

DExH/D-box helicases are essential nucleic acid and ribonucleoprotein remodelers involved in all aspects of nucleic acid metabolism including replication, gene expression and post-transcriptional modifications. In parallel to their importance in basic cellular functions, DExH/D-box helicases play multiple roles in viral life cycles, with some of them highjacked by viruses or negatively regulating innate immune activation. However, other DExH/D-box helicases have recurrently been highlighted as direct antiviral effectors or as positive regulators of innate immune activation. Innate immunity relies on the ability of Pathogen Recognition Receptors to recognize viral signatures and trigger the production of interferons (IFNs) and pro-inflammatory cytokines. Secreted IFNs interact with their receptors to establish antiviral cellular reprogramming via expression regulation of the interferon-stimulated genes (ISGs). Several DExH/D-box helicases have been reported to act as viral sensors (DDX3, DDX41, DHX9, DDX1/DDX21/DHX36 complex), and others to play roles in innate immune activation (DDX60, DDX60L, DDX23). In contrast, the DDX39A, DDX46, DDX5 and DDX24 helicases act as negative regulators and impede IFN production upon viral infection. Beyond their role in viral sensing, the ISGs DDX60 and DDX60L act as viral inhibitors. Interestingly, the constitutively expressed DEAD-box helicases DDX56, DDX17, DDX42 intrinsically restrict viral replication. Hence, DExH/D-box helicases appear to form a multilayer network of primary and secondary factors involved in both intrinsic and innate antiviral immunity. In this review, we highlight recent findings on the extent of antiviral defences played by helicases and emphasize the need to better understand their immune functions as well as their complex interplay.

The RNA helicase DHX15 is a critical regulator of natural killer-cell homeostasis and functions

The RNA helicase DHX15 is widely expressed in immune cells and traditionally thought to be an RNA splicing factor or a viral RNA sensor. However, the role of DHX15 in NK-cell activities has not been studied thus far. Here, we generated Dhx15-floxed mice and found that conditional deletion of Dhx15 in NK cells (Ncr1^CreDhx15^fl/fl mice) resulted in a marked reduction in NK cells in the periphery and that the remaining Dhx15-deleted NK cells failed to acquire a mature phenotype. As a result, Dhx15-deleted NK cells exhibited profound defects in their cytolytic functions. We also found that deletion of Dhx15 in NK cells abrogated their responsiveness to IL-15, which was associated with inhibition of IL-2/IL-15Rβ (CD122) expression and IL-15R signaling. The defects in Dhx15-deleted NK cells were rescued by ectopic expression of a constitutively active form of STAT5. Mechanistically, DHX15 did not affect CD122 mRNA splicing and stability in NK cells but instead facilitated the surface expression of CD122, likely through interaction with its 3'UTR, which was dependent on the ATPase domain of DHX15 rather than its splicing domain. Collectively, our data identify a key role for DHX15 in regulating NK-cell activities and provide novel mechanistic insights into how DHX15 regulates the IL-15 signaling pathway in NK cells.

A model for DHX15 mediated disassembly of A-complex spliceosomes

A critical step of pre-mRNA splicing is the recruitment of U2 snRNP to the branch point sequence of an intron. U2 snRNP conformation changes extensively during branch helix formation, and several RNA-dependent ATPases are implicated in the process. However, the molecular mechanisms involved remain to be fully dissected. We took advantage of the differential nucleotide triphosphates requirements for DExD/H-box enzymes to probe their contributions to in vitro spliceosome assembly. Both ATP and GTP hydrolysis support the formation of A-complex, indicating the activity of a DEAH-enzyme because DEAD-enzymes are selective for ATP. We immunodepleted DHX15 to assess its involvement, and although splicing efficiency decreases with reduced DHX15, A-complex accumulation incongruently increases. DHX15 depletion also results in the persistence of the atypical ATP-independent interaction between U2 snRNP and a minimal substrate that is otherwise destabilized in the presence of either ATP or GTP. These results lead us to hypothesize that DHX15 plays a quality control function in U2 snRNP's engagement with an intron. In efforts to identify the RNA target of DHX15, we determined that an extended polypyrimidine tract is not necessary for disruption of the atypical interaction between U2 snRNP and the minimal substrate. We also examined U2 snRNA by RNase H digestion and identified nucleotides in the branch binding region that become accessible with both ATP and GTP hydrolysis, again implicating a DEAH-enzyme. Together, our results demonstrate that multiple ATP-dependent rearrangements are likely involved in U2 snRNP addition to the spliceosome and that DHX15 may have an expanded role in maintaining splicing fidelity.

The RNA methyltransferase METTL8 installs m3C32 in mitochondrial tRNAsThr/Ser(UCN) to optimise tRNA structure and mitochondrial translation

Modified nucleotides in tRNAs are important determinants of folding, structure and function. Here we identify METTL8 as a mitochondrial matrix protein and active RNA methyltransferase responsible for installing m3C32 in the human mitochondrial (mt-)tRNAThr and mt-tRNASer(UCN). METTL8 crosslinks to the anticodon stem loop (ASL) of many mt-tRNAs in cells, raising the question of how methylation target specificity is achieved. Dissection of mt-tRNA recognition elements revealed U34G35 and t6A37/(ms2)i6A37, present concomitantly only in the ASLs of the two substrate mt-tRNAs, as key determinants for METTL8-mediated methylation of C32. Several lines of evidence demonstrate the influence of U34, G35, and the m3C32 and t6A37/(ms2)i6A37 modifications in mt-tRNAThr/Ser(UCN) on the structure of these mt-tRNAs. Although mt-tRNAThr/Ser(UCN) lacking METTL8-mediated m3C32 are efficiently aminoacylated and associate with mitochondrial ribosomes, mitochondrial translation is mildly impaired by lack of METTL8. Together these results define the cellular targets of METTL8 and shed new light on the role of m3C32 within mt-tRNAs.

DEAD/H-box helicases:Anti-viral and pro-viral roles during infections

DEAD/H-box RNA helicases make the prominent family of helicases super family-2 which take part in almost all RNA-related processes, from initiation of transcription to RNA decay pathways. In addition to these RNA-related activities, in recent years a certain number of these helicases are reported to play important roles in anti-viral immunity through various ways. Along with RLHs, endosomal TLRs, and cytosolic DNA receptors, many RNA helicases including DDX3, DHX9, DDX6, DDX41, DHX33, DDX60, DHX36 and DDX1-DDX21-DHX36 complex act as viral nucleic acid sensors or co-sensors. These helicases mostly follow RLHs-MAVS and STING mediated signaling cascades to trigger induction of type-I interferons and pro-inflammatory cytokines. Many of them also function as downstream adaptor molecules (DDX3), segments of stress and processing bodies (DDX3 and DDX6) or negative regulators (DDX19, DDX24, DDX25, DDX39A and DDX46). On the contrary, many studies indicated that several DEAD/H-box helicases such as DDX1, DDX3, DDX6, DDX24, and DHX9 could be exploited by viruses to evade innate immune responses, suggesting that these helicases seem to have a dual function as anti-viral innate immune mediators and viral replication cofactors. In this review, we summarized the current knowledge on several representative DEAD/H-box helicases, with an emphasis on their functions in innate immunity responses, involved in their anti-viral and pro-viral roles.

DHX15-independent roles for TFIP11 in U6 snRNA modification, U4/U6.U5 tri-snRNP assembly and pre-mRNA splicing fidelity

The U6 snRNA, the core catalytic component of the spliceosome, is extensively modified post-transcriptionally, with 2'-O-methylation being most common. However, how U6 2'-O-methylation is regulated remains largely unknown. Here we report that TFIP11, the human homolog of the yeast spliceosome disassembly factor Ntr1, localizes to nucleoli and Cajal Bodies and is essential for the 2'-O-methylation of U6. Mechanistically, we demonstrate that TFIP11 knockdown reduces the association of U6 snRNA with fibrillarin and associated snoRNAs, therefore altering U6 2'-O-methylation. We show U6 snRNA hypomethylation is associated with changes in assembly of the U4/U6.U5 tri-snRNP leading to defects in spliceosome assembly and alterations in splicing fidelity. Strikingly, this function of TFIP11 is independent of the RNA helicase DHX15, its known partner in yeast. In sum, our study demonstrates an unrecognized function for TFIP11 in U6 snRNP modification and U4/U6.U5 tri-snRNP assembly, identifying TFIP11 as a critical spliceosome assembly regulator.

Nuclear RNA Regulation by XRN2 and XTBD Family Proteins

XRN2 is a 5'-to-3' exoribonuclease that is predominantly localized in the nucleus. By degrading or trimming various classes of RNA, XRN2 contributes to essential processes in gene expression such as transcription termination and ribosome biogenesis. Despite limited substrate specificity in vitro, XRN2 targets a specific subset of RNA by interacting with other proteins in cells. Here we review the functions of proteins that have an evolutionarily conserved XRN2-binding domain, XTBD. These proteins modulate activity of XRN2 by stabilizing it, controlling its subcellular localization or recruiting it to specific RNA targets, and thereby impact on various cellular processes.Key words: RNA regulation, XRN2, XTBD, ribosome biogenesis, subcellular localization.

The RNA helicase Dbp7 promotes domain V/VI compaction and stabilization of inter-domain interactions during early 60S assembly

Early pre-60S ribosomal particles are poorly characterized, highly dynamic complexes that undergo extensive rRNA folding and compaction concomitant with assembly of ribosomal proteins and exchange of assembly factors. Pre-60S particles contain numerous RNA helicases, which are likely regulators of accurate and efficient formation of appropriate rRNA structures. Here we reveal binding of the RNA helicase Dbp7 to domain V/VI of early pre-60S particles in yeast and show that in the absence of this protein, dissociation of the Npa1 scaffolding complex, release of the snR190 folding chaperone, recruitment of the A3 cluster factors and binding of the ribosomal protein uL3 are impaired. uL3 is critical for formation of the peptidyltransferase center (PTC) and is responsible for stabilizing interactions between the 5′ and 3′ ends of the 25S, an essential pre-requisite for subsequent pre-60S maturation events. Highlighting the importance of pre-ribosome remodeling by Dbp7, our data suggest that in the absence of Dbp7 or its catalytic activity, early pre-ribosomal particles are targeted for degradation.

Early steps of large 60S ribosomal subunit biogenesis are not well understood. Here, the authors combine biochemical experiments with protein-RNA crosslinking and mass spectrometry to show that the RNA helicase Dbp7 is key player during early 60S ribosomal assembly. Dbp7 regulates a series of events driving compaction of domain V/VI in early pre60S ribosomal particles.

Dhx15 regulates zebrafish definitive hematopoiesis through the unfolded protein response pathway

Gene alterations are recognized as important events in acute myeloid leukemia (AML) progression. Studies on hematopoiesis of altered genes contribute to a better understanding on their roles in AML progression. Our previous work reported a DEAH box helicase 15 (DHX15) R222G mutation in AML patients, and we showed DHX15 overexpression is associated with poor prognosis in AML patients. In this work, we further study the role of dhx15 in zebrafish developmental hematopoiesis by generating dhx15^−/− zebrafish using transcription activator‐like effector nuclease technology. Whole‐mount in situ hybridization (WISH) analysis showed hematopoietic stem/progenitor cells were dramatically perturbed when dhx15 was deleted. Immunofluorescence staining indicated inhibited hematopoietic stem/progenitor cell (HSPC) proliferation instead of accelerated apoptosis were detected in dhx15^−/− zebrafish. Furthermore, our data showed that HSPC defect is mediated through the unfolded protein response (UPR) pathway. DHX15 R222G mutation, a recurrent mutation identified in AML patients, displayed a compromised function in restoring HSPC failure in dhx15^−/−; Tg (hsp: DHX15 R222G) zebrafish. Collectively, this work revealed a vital role of dhx15 in the maintenance of definitive hematopoiesis in zebrafish through the unfolded protein respone pathway. The study of DHX15 and DHX15 R222G mutation could hold clinical significance for evaluating prognosis of AML patients with aberrant DHX15 expression.

DHX15 is required to control RNA virus-induced intestinal inflammation

RNA helicases play critical roles in various biological processes, including serving as viral RNA sensors in innate immunity. Here, we find that RNA helicase DEAH-box helicase 15 (DHX15) is essential for type I interferon (IFN-I, IFN-β), type III IFN (IFN-λ3), and inflammasome-derived cytokine IL-18 production by intestinal epithelial cells (IECs) in response to poly I:C and RNA viruses with preference of enteric RNA viruses, but not DNA virus. Importantly, we generate IEC-specific Dhx15-knockout mice and demonstrate that DHX15 is required for controlling intestinal inflammation induced by enteric RNA virus rotavirus in suckling mice and reovirus in adult mice in vivo, which owes to impaired IFN-β, IFN-λ3, and IL-18 production in IECs from Dhx15-deficient mice. Mechanistically, DHX15 interacts with NLRP6 to trigger NLRP6 inflammasome assembly and activation for inducing IL-18 secretion in IECs. Collectively, our report reveals critical roles for DHX15 in sensing enteric RNA viruses in IECs and controlling intestinal inflammation.

RNA helicase-mediated regulation of snoRNP dynamics on pre-ribosomes and rRNA 2'-O-methylation

RNA helicases play important roles in diverse aspects of RNA metabolism through their functions in remodelling ribonucleoprotein complexes (RNPs), such as pre-ribosomes. Here, we show that the DEAD box helicase Dbp3 is required for efficient processing of the U18 and U24 intron-encoded snoRNAs and 2′-O-methylation of various sites within the 25S ribosomal RNA (rRNA) sequence. Furthermore, numerous box C/D snoRNPs accumulate on pre-ribosomes in the absence of Dbp3. Many snoRNAs guiding Dbp3-dependent rRNA modifications have overlapping pre-rRNA basepairing sites and therefore form mutually exclusive interactions with pre-ribosomes. Analysis of the distribution of these snoRNAs between pre-ribosome-associated and ‘free’ pools demonstrated that many are almost exclusively associated with pre-ribosomal complexes. Our data suggest that retention of such snoRNPs on pre-ribosomes when Dbp3 is lacking may impede rRNA 2′-O-methylation by reducing the recycling efficiency of snoRNPs and by inhibiting snoRNP access to proximal target sites. The observation of substoichiometric rRNA modification at adjacent sites suggests that the snoRNPs guiding such modifications likely interact stochastically rather than hierarchically with their pre-rRNA target sites. Together, our data provide new insights into the dynamics of snoRNPs on pre-ribosomal complexes and the remodelling events occurring during the early stages of ribosome assembly.

Ribosome-bound Get4/5 facilitates the capture of tail-anchored proteins by Sgt2 in yeast

The guided entry of tail-anchored proteins (GET) pathway assists in the posttranslational delivery of tail-anchored proteins, containing a single C-terminal transmembrane domain, to the ER. Here we uncover how the yeast GET pathway component Get4/5 facilitates capture of tail-anchored proteins by Sgt2, which interacts with tail-anchors and hands them over to the targeting component Get3. Get4/5 binds directly and with high affinity to ribosomes, positions Sgt2 close to the ribosomal tunnel exit, and facilitates the capture of tail-anchored proteins by Sgt2. The contact sites of Get4/5 on the ribosome overlap with those of SRP, the factor mediating cotranslational ER-targeting. Exposure of internal transmembrane domains at the tunnel exit induces high-affinity ribosome binding of SRP, which in turn prevents ribosome binding of Get4/5. In this way, the position of a transmembrane domain within nascent ER-targeted proteins mediates partitioning into either the GET or SRP pathway directly at the ribosomal tunnel exit.

The guided entry of tail-anchored proteins (GET) pathway assists in the delivery of such proteins to the ER. Here, the authors reveal that the pathway components Get4/5 probe a region near the ribosomal exit tunnel. Upon emergence of a client protein, Get4/5 recruits Sgt2 and initiates the targeting phase of the pathway.

Regulation of DEAH-box RNA helicases by G-patch proteins

RNA helicases of the DEAH/RHA family form a large and conserved class of enzymes that remodel RNA protein complexes (RNPs) by translocating along the RNA. Driven by ATP hydrolysis, they exert force to dissociate hybridized RNAs, dislocate bound proteins or unwind secondary structure elements in RNAs. The sub-cellular localization of DEAH-helicases and their concomitant association with different pathways in RNA metabolism, such as pre-mRNA splicing or ribosome biogenesis, can be guided by cofactor proteins that specifically recruit and simultaneously activate them. Here we review the mode of action of a large class of DEAH-specific adaptor proteins of the G-patch family. Defined only by their eponymous short glycine-rich motif, which is sufficient for helicase binding and stimulation, this family encompasses an immensely varied array of domain compositions and is linked to an equally diverse set of functions. G-patch proteins are conserved throughout eukaryotes and are even encoded within retroviruses. They are involved in mRNA, rRNA and snoRNA maturation, telomere maintenance and the innate immune response. Only recently was the structural and mechanistic basis for their helicase enhancing activity determined. We summarize the molecular and functional details of G-patch-mediated helicase regulation in their associated pathways and their involvement in human diseases.

The interaction of DNA repair factors ASCC2 and ASCC3 is affected by somatic cancer mutations

The ASCC3 subunit of the activating signal co-integrator complex is a dual-cassette Ski2-like nucleic acid helicase that provides single-stranded DNA for alkylation damage repair by the α-ketoglutarate-dependent dioxygenase AlkBH3. Other ASCC components integrate ASCC3/AlkBH3 into a complex DNA repair pathway. We mapped and structurally analyzed interacting ASCC2 and ASCC3 regions. The ASCC3 fragment comprises a central helical domain and terminal, extended arms that clasp the compact ASCC2 unit. ASCC2–ASCC3 interfaces are evolutionarily highly conserved and comprise a large number of residues affected by somatic cancer mutations. We quantified contributions of protein regions to the ASCC2–ASCC3 interaction, observing that changes found in cancers lead to reduced ASCC2–ASCC3 affinity. Functional dissection of ASCC3 revealed similar organization and regulation as in the spliceosomal RNA helicase Brr2. Our results delineate functional regions in an important DNA repair complex and suggest possible molecular disease principles.

The DNA helicase ASCC3 is the largest subunit of the activating signal co-integrator complex (ASCC), and its DNA unwinding activity is required for the AlkBH3/ASCC-dependent DNA de-alkylation repair pathway. Here, the authors identify a minimal stable complex of the two ASCC subunits ASCC2 and ASCC3, determine the complex crystal structure and further show that cancer-related mutations at the interface between both proteins reduce ASCC2–ASCC3 affinity.

The G-patch activators Pfa1 and PINX1 exhibit different modes of interaction with the Prp43 RNA helicase

Prp43 is a DEAH-box RNA helicase involved in both splicing and ribosome biogenesis. Its activities are directly stimulated by several co-activators that share a G-patch domain. The substrates of Prp43, its mechanism of action and the modes of interaction with and activation by G-patch proteins have been only partially characterized. We investigated how Pfa1 and PINX1, two G-patch proteins involved in ribosome biogenesis, interact with Prp43. We demonstrate that a protruding loop connecting the β4 and β5 strands of Prp43 OB fold is crucial for the binding of the G-patch domain of Pfa1. However, neither this loop nor the entire OB fold of Prp43 is essential for PINX1 binding. We conclude that the binding modes of Pfa1 and PINX1 G-patches to Prp43 are different. Nevertheless, stimulation of the ATPase and helicase activities of Prp43 by both full-length Pfa1 and PINX1 requires the β4-β5 loop. Moreover, we show that disruption of this loop completely abrogates Prp43 activity during yeast ribosome biogenesis but does not prevent its integration within pre-ribosomal particles. We propose that the β4-β5 loop plays a crucial role in the transmission of conformational changes induced by binding of the G-patch to Prp43 active site and substrate RNA.

The DExD box ATPase DDX55 is recruited to domain IV of the 28S ribosomal RNA by its C-terminal region

RNA helicases contribute to diverse aspects of RNA metabolism through their functions in re-arranging RNA structures. Identification of the remodelling targets of RNA helicases is a critical step in elucidating their cellular functions. Here, we show that, in contrast to many other ribosome biogenesis factors, the DExD box ATPase DDX55 predominantly localizes to the nucleoplasm and we identify a nuclear localization signal within the C-terminal region of the protein. DDX55 associates with pre-ribosomal subunits in human cells and is required for maturation of large subunit pre-rRNAs. Interestingly, in vitro analyses show that DDX55 selectively associates with double-stranded RNA substrates, which also stimulate its ATPase activity, and our data suggest that the C-terminal region of DDX55 contributes to this substrate specificity. The C-terminal region of DDX55 is also necessary for recruitment of the helicase to pre-ribosomes and, using in vivo crosslinking, we reveal a binding site for DDX55 in helix H62 of the 28S ribosomal RNA. Taken together, these data highlight the importance of the C-terminal region of DDX55 in substrate specificity and recruitment, and identify domain IV as a potential remodelling target of DDX55 during LSU biogenesis.

XRN2 interactome reveals its synthetic lethal relationship with PARP1 inhibition

Persistent R-loops (RNA-DNA hybrids with a displaced single-stranded DNA) create DNA damage and lead to genomic instability. The 5'-3'-exoribonuclease 2 (XRN2) degrades RNA to resolve R-loops and promotes transcription termination. Previously, XRN2 was implicated in DNA double strand break (DSB) repair and in resolving replication stress. Here, using tandem affinity purification-mass spectrometry, bioinformatics, and biochemical approaches, we found that XRN2 associates with proteins involved in DNA repair/replication (Ku70-Ku80, DNA-PKcs, PARP1, MCM2-7, PCNA, RPA1) and RNA metabolism (RNA helicases, PRP19, p54(nrb), splicing factors). Novel major pathways linked to XRN2 include cell cycle control of chromosomal replication and DSB repair by non-homologous end joining. Investigating the biological implications of these interactions led us to discover that XRN2 depletion compromised cell survival after additional knockdown of specific DNA repair proteins, including PARP1. XRN2-deficient cells also showed enhanced PARP1 activity. Consistent with concurrent depletion of XRN2 and PARP1 promoting cell death, XRN2-deficient fibroblast and lung cancer cells also demonstrated sensitivity to PARP1 inhibition. XRN2 alterations (mutations, copy number/expression changes) are frequent in cancers. Thus, PARP1 inhibition could target cancers exhibiting XRN2 functional loss. Collectively, our data suggest XRN2's association with novel protein partners and unravel synthetic lethality between XRN2 depletion and PARP1 inhibition.

Structural basis for DEAH-helicase activation by G-patch proteins

RNA helicases exert mechanical force that changes RNA configurations in many essential cellular pathways, e.g., during mRNA maturation or assembly of ribosomes. DEAH helicases work by translocating along RNA and thereby unwind RNA duplexes or dissociate bound proteins. Because DEAH proteins are poor enzymes without intrinsic selectivity for target RNAs, they require adapter proteins that recruit them to functional sites and enhance their catalytic activity. One essential class of DEAH activators is formed by G-patch proteins, which bind helicases via their eponymous glycine-rich motif. We solved the structure of a G-patch bound to helicase DHX15. Our analysis suggests that G-patches tether mobile sections of DEAH helicases together and activate them by stabilizing a functional conformation with high RNA affinity.

RNA helicases of the DEAH/RHA family are involved in many essential cellular processes, such as splicing or ribosome biogenesis, where they remodel large RNA–protein complexes to facilitate transitions to the next intermediate. DEAH helicases couple adenosine triphosphate (ATP) hydrolysis to conformational changes of their catalytic core. This movement results in translocation along RNA, which is held in place by auxiliary C-terminal domains. The activity of DEAH proteins is strongly enhanced by the large and diverse class of G-patch activators. Despite their central roles in RNA metabolism, insight into the molecular basis of G-patch–mediated helicase activation is missing. Here, we have solved the structure of human helicase DHX15/Prp43, which has a dual role in splicing and ribosome assembly, in complex with the G-patch motif of the ribosome biogenesis factor NKRF. The G-patch motif binds in an extended conformation across the helicase surface. It tethers the catalytic core to the flexibly attached C-terminal domains, thereby fixing a conformation that is compatible with RNA binding. Structures in the presence or absence of adenosine diphosphate (ADP) suggest that motions of the catalytic core, which are required for ATP binding, are still permitted. Concomitantly, RNA affinity, helicase, and ATPase activity of DHX15 are increased when G-patch is bound. Mutations that detach one end of the tether but maintain overall binding severely impair this enhancement. Collectively, our data suggest that the G-patch motif acts like a flexible brace between dynamic portions of DHX15 that restricts excessive domain motions but maintains sufficient flexibility for catalysis.

Full-length NF-κB repressing factor contains an XRN2 binding domain

NF-κB repressing factor (NKRF) was recently identified as an RNA binding protein that together with its associated proteins, the 5′–3′ exonuclease XRN2 and the helicase DHX15, is required to process the precursor ribosomal RNA. XRN2 is a multi-functional ribonuclease that is also involved in processing mRNAs, tRNAs and lncRNAs. The activity and stability of XRN2 are controlled by its binding partners, PAXT-1, CDKN2AIP and CDKN2AIPNL. In each case, these proteins interact with XRN2 via an XRN2 binding domain (XTBD), the structure and mode of action of which is highly conserved. Rather surprisingly, although NKRF interacts directly with XRN2, it was not predicted to contain such a domain, and NKRF's interaction with XRN2 was therefore unexplained. We have identified an alternative upstream AUG start codon within the transcript that encodes NKRF and demonstrate that the full-length form of NKRF contains an XTBD that is conserved across species. Our data suggest that NKRF is tethered in the nucleolus by binding directly to rRNA and that the XTBD in the N-terminal extension of NKRF is essential for the retention of XRN2 in this sub-organelle. Thus, we propose NKRF regulates the early steps of pre-rRNA processing during ribosome biogenesis by controlling the spatial distribution of XRN2 and our data provide further support for the XTBD as an XRN2 interacting motif.

Sgd1 is an MIF4G domain-containing cofactor of the RNA helicase Fal1 and associates with the 5' domain of the 18S rRNA sequence

Assembly of eukaryotic ribosomal subunits is a complex and dynamic process involving the action of more than 200 trans-acting assembly factors. Although recent cryo-electron microscopy structures have provided information on architecture of several pre-ribosomal particles and the binding sites of many AFs, the RNA and protein interactions of many other AFs not captured in these snapshots still remain elusive. RNA helicases are key regulators of structural rearrangements within pre-ribosomal complexes and here we have analysed the eIF4A-like RNA helicase Fal1 and its putative cofactor Sgd1. Our data show that these proteins interact directly via the MIF4G domain of Sgd1 and that the MIF4G domain of Sgd1 stimulates the catalytic activity of Fal1 in vitro. The catalytic activity of Fal1, and the interaction between Fal1 and Sgd1, are required for efficient pre-rRNA processing at the A₀, A₁ and A₂ sites. Furthermore, Sgd1 co-purifies the early small subunit biogenesis factors Lcp5 and Rok1, suggesting that the Fal1-Sgd1 complex likely functions within the SSU processome. In vivo crosslinking data reveal that Sgd1 binds to helix H12 of the 18S rRNA sequence and we further demonstrate that this interaction is formed by the C-terminal region of the protein, which is essential for its function in ribosome biogenesis.

Long Non-coding RNA JHDM1D-AS1 Interacts with DHX15 Protein to Enhance Non-Small-Cell Lung Cancer Growth and Metastasis

JHDM1D antisense 1 (JHDM1D-AS1), a long non-coding RNA (lncRNA), has been shown to promote pancreatic cancer growth by inducing an angiogenic response. However, its biological and clinical significance in non-small-cell lung cancer (NSCLC) is still unclear. In this study, we examined the expression and prognostic significance of JHDM1D-AS1 in NSCLC. The effects of JHDM1D-AS1 knockdown or overexpression on NSCLC growth and metastasis were investigated. We show that JHDM1D-AS1 is upregulated in NSCLC relative to adjacent normal lung tissues. High JHDM1D-AS1 expression is significantly correlated with advanced tumor, node, and metastasis (TNM) stage and lymph node metastasis. JHDM1D-AS1 expression serves as an independent prognostic factor for overall survival of patients with NSCLC. Functionally, JHDM1D-AS1 knockdown inhibits NSCLC cell aggressiveness both in vitro and in vivo, which is rescued by ectopic expression of JHDM1D-AS1. JHDM1D-AS1 binding stabilizes DHX15 protein in NSCLC cells. DHX15 overexpression enhances NSCLC cell proliferation and invasion, whereas knockdown of DHX15 exerts opposite effects. JHDM1D-AS1-mediated aggressive phenotype is impaired when DHX15 is silenced. Ectopic expression of DHX15 restores the defects in proliferation and invasion of JHDM1D-AS1-depleted NSCLC cells. Collectively, the interaction between JHDM1D-AS1 and DHX15 accounts for NSCLC growth and metastasis. This work provides potential additional therapeutic targets for treatment of NSCLC.

IT'S 2 for the price of 1: Multifaceted ITS2 processing machines in RNA and DNA maintenance

Cells utilize sophisticated RNA processing machines to ensure the quality of RNA. Many RNA processing machines have been further implicated in regulating the DNA damage response signifying a strong link between RNA processing and genome maintenance. One of the most intricate and highly regulated RNA processing pathways is the processing of the precursor ribosomal RNA (pre-rRNA), which is paramount for the production of ribosomes. Removal of the Internal Transcribed Spacer 2 (ITS2), located between the 5.8S and 25S rRNA, is one of the most complex steps of ribosome assembly. Processing of the ITS2 is initiated by the newly discovered endoribonuclease Las1, which cleaves at the C2 site within the ITS2, generating products that are further processed by the polynucleotide kinase Grc3, the 5'→3' exonuclease Rat1, and the 3'→5' RNA exosome complex. In addition to their defined roles in ITS2 processing, these critical cellular machines participate in other stages of ribosome assembly, turnover of numerous cellular RNAs, and genome maintenance. Here we summarize recent work defining the molecular mechanisms of ITS2 processing by these essential RNA processing machines and highlight their emerging roles in transcription termination, heterochromatin function, telomere maintenance, and DNA repair.

Uncovering the assembly pathway of human ribosomes and its emerging links to disease

The essential cellular process of ribosome biogenesis is at the nexus of various signalling pathways that coordinate protein synthesis with cellular growth and proliferation. The fact that numerous diseases are caused by defects in ribosome assembly underscores the importance of obtaining a detailed understanding of this pathway. Studies in yeast have provided a wealth of information about the fundamental principles of ribosome assembly, and although many features are conserved throughout eukaryotes, the larger size of human (pre-)ribosomes, as well as the evolution of additional regulatory networks that can modulate ribosome assembly and function, have resulted in a more complex assembly pathway in humans. Notably, many ribosome biogenesis factors conserved from yeast appear to have subtly different or additional functions in humans. In addition, recent genome-wide, RNAi-based screens have identified a plethora of novel factors required for human ribosome biogenesis. In this review, we discuss key aspects of human ribosome production, highlighting differences to yeast, links to disease, as well as emerging concepts such as extra-ribosomal functions of ribosomal proteins and ribosome heterogeneity.

NOL12 Repression Induces Nucleolar Stress-Driven Cellular Senescence and Is Associated with Normative Aging

The nucleolus is a subnuclear compartment with key roles in rRNA synthesis and ribosome biogenesis, complex processes that require hundreds of proteins and factors. Alterations in nucleolar morphology and protein content have been linked to the control of cell proliferation and stress responses and, recently, further implicated in cell senescence and ageing. In this study, we report the functional role of NOL12 in the nucleolar homeostasis of human primary fibroblasts. NOL12 repression induces specific changes in nucleolar morphology, with increased nucleolar area but reduced nucleolar number, along with nucleolar accumulation and increased levels of fibrillarin and nucleolin. Moreover, NOL12 repression leads to stabilization and activation of p53 in an RPL11-dependent manner, which arrests cells at G₂ phase and ultimately leads to senescence. Importantly, we found NOL12 repression in association with nucleolar stress-like responses in human fibroblasts from elderly donors, disclosing it as a biomarker in human chronological aging.

Interactions and activities of factors involved in the late stages of human 18S rRNA maturation

Ribosome production is an essential cellular process involving a plethora of trans-acting factors, such as nucleases, methyltransferases, RNA helicases and kinases that catalyse key maturation steps. Precise temporal and spatial regulation of such enzymes is essential to ensure accurate and efficient subunit assembly. Here, we focus on the maturation of the 3ʹ end of the 18S rRNA in human cells. We reveal that human RIO2 is an active kinase that phosphorylates both itself and the rRNA methyltransferase DIM1 in vitro. In contrast to yeast, our data confirm that human DIM1 predominantly acts in the nucleus and we further demonstrate that the 21S pre-rRNA is the main target for DIM1-catalysed methylation. We show that the PIN domain of the endonuclease NOB1 is required for site 3 cleavage, while the zinc ribbon domain is essential for pre-40S recruitment. Furthermore, we also demonstrate that NOB1, PNO1 and DIM1 bind to a region of the pre-rRNA encompassing the 3ʹ end of 18S and the start of ITS1, in vitro. Interestingly, NOB1 is present in the cell at higher levels than other pre-40S factors. We provide evidence that NOB1 is multimeric within the cell and show that NOB1 multimerisation is lost when ribosome biogenesis is blocked. Taken together, our data indicate a dynamic interplay of key factors associated with the 3ʹ end of the 18S rRNA during human pre-40S biogenesis and highlight potential mechanisms by which this process can be regulated.

The human RNA helicase DHX37 is required for release of the U3 snoRNP from pre-ribosomal particles

Ribosome synthesis is an essential cellular process, and perturbation of human ribosome production is linked to cancer and genetic diseases termed ribosomopathies. During their assembly, pre-ribosomal particles undergo numerous structural rearrangements, which establish the architecture present in mature complexes and serve as key checkpoints, ensuring the fidelity of ribosome biogenesis. RNA helicases are essential mediators of such remodelling events and here, we demonstrate that the DEAH-box RNA helicase DHX37 is required for maturation of the small ribosomal subunit in human cells. Our data reveal that the presence of DHX37 in early pre-ribosomal particles is monitored by a quality control pathway and that failure to recruit DHX37 leads to pre-rRNA degradation. Using an in vivo crosslinking approach, we show that DHX37 binds directly to the U3 small nucleolar RNA (snoRNA) and demonstrate that the catalytic activity of the helicase is required for dissociation of the U3 snoRNA from pre-ribosomal complexes. This is an important event during ribosome assembly as it enables formation of the central pseudoknot structure of the small ribosomal subunit. We identify UTP14A as a direct interaction partner of DHX37 and our data suggest that UTP14A can act as a cofactor that stimulates the activity of the helicase in the context of U3 snoRNA release.

RNA helicases mediate structural transitions and compositional changes in pre-ribosomal complexes

Production of eukaryotic ribosomal subunits is a highly dynamic process; pre-ribosomes undergo numerous structural rearrangements that establish the architecture present in mature complexes and serve as key checkpoints, ensuring the fidelity of ribosome assembly. Using in vivo crosslinking, we here identify the pre-ribosomal binding sites of three RNA helicases. Our data support roles for Has1 in triggering release of the U14 snoRNP, a critical event during early 40S maturation, and in driving assembly of domain I of pre-60S complexes. Binding of Mak5 to domain II of pre-60S complexes promotes recruitment of the ribosomal protein Rpl10, which is necessary for subunit joining and ribosome function. Spb4 binds to a molecular hinge at the base of ES27 facilitating binding of the export factor Arx1, thereby promoting pre-60S export competence. Our data provide important insights into the driving forces behind key structural remodelling events during ribosomal subunit assembly.

Elimination of 01/A'-A0 pre-rRNA processing by-product in human cells involves cooperative action of two nuclear exosome-associated nucleases: RRP6 and DIS3

Pre-rRNA processing generates mature 18S, 5.8S, and 28S/25S rRNAs through multistage removal of surrounding 5'-ETS/3'-ETS and intervening ITS1/ITS2 segments. Endonucleolytic activities release by-products, which need to be eliminated. Here, we investigated the interplay of exosome-associated 3'-5' exonucleases DIS3 and RRP6 in rRNA processing and by-product elimination in human cells. In agreement with previous reports, we observed accumulation of 5.8S and 18S precursors upon dysfunction of these enzymes. However, none of these phenotypes was so pronounced as previously overlooked accumulation of short RNA species derived from 5'-ETS (01/A'-A0), in cells with nonfunctional DIS3. We demonstrate that removal of 01/A'-A0 is independent of the XRN2 5'-3' exonucleolytic activity. Instead, it proceeds rapidly after A0 cleavage and occurs exclusively in the 3'-5' direction in several phases-following initiation by an unknown nuclease, the decay is executed by RRP6 with some contribution of DIS3, whereas the ultimate phase involves predominantly DIS3. Our data shed new light onto the role of human exosome in 5'-ETS removal. Furthermore, although 01/A'-A0 degradation involves the action of two nucleases associated with the exosome ring, similarly to 5.8S 3'-end maturation, it is likely that contrary to the latter process, RRP6 acts prior to or redundantly with DIS3.

Human RNA cap1 methyltransferase CMTr1 cooperates with RNA helicase DHX15 to modify RNAs with highly structured 5' termini

The 5′-cap structure, characteristic for RNA polymerase II-transcribed RNAs, plays important roles in RNA metabolism. In humans, RNA cap formation includes post-transcriptional modification of the first transcribed nucleotide by RNA cap1 methyltransferase (CMTr1). Here, we report that CMTr1 activity is hindered towards RNA substrates with highly structured 5′ termini. We found that CMTr1 binds ATP-dependent RNA DHX15 helicase and that this interaction, mediated by the G-patch domain of CMTr1, has an advantageous effect on CMTr1 activity towards highly structured RNA substrates. The effect of DHX15 helicase activity is consistent with the strength of the secondary structure that has to be removed for CMTr1 to access the 5′-terminal residues in a single-stranded conformation. This is, to our knowledge, the first demonstration of the involvement of DHX15 in post-transcriptional RNA modification, and the first example of a molecular process in which DHX15 directly affects the activity of another enzyme. Our findings suggest a new mechanism underlying the regulatory role of DHX15 in the RNA capping process. RNAs with highly structured 5′ termini constitute a significant fraction of the human transcriptome. Hence, CMTr1–DHX15 cooperation is likely to be important for the metabolism of RNA polymerase II-transcribed RNAs.

This article is part of the theme issue ‘5′ and 3′ modifications controlling RNA degradation’.

The m6A reader protein YTHDC2 interacts with the small ribosomal subunit and the 5'-3' exoribonuclease XRN1

N⁶-methyladenosine (m⁶A) modifications in RNAs play important roles in regulating many different aspects of gene expression. While m⁶As can have direct effects on the structure, maturation, or translation of mRNAs, such modifications can also influence the fate of RNAs via proteins termed "readers" that specifically recognize and bind modified nucleotides. Several YTH domain-containing proteins have been identified as m⁶A readers that regulate the splicing, translation, or stability of specific mRNAs. In contrast to the other YTH domain-containing proteins, YTHDC2 has several defined domains and here, we have analyzed the contribution of these domains to the RNA and protein interactions of YTHDC2. The YTH domain of YTHDC2 preferentially binds m⁶A-containing RNAs via a conserved hydrophobic pocket, whereas the ankyrin repeats mediate an RNA-independent interaction with the 5'-3' exoribonuclease XRN1. We show that the YTH and R3H domains contribute to the binding of YTHDC2 to cellular RNAs, and using crosslinking and analysis of cDNA (CRAC), we reveal that YTHDC2 interacts with the small ribosomal subunit in close proximity to the mRNA entry/exit sites. YTHDC2 was recently found to promote a "fast-track" expression program for specific mRNAs, and our data suggest that YTHDC2 accomplishes this by recruitment of the RNA degradation machinery to regulate the stability of m⁶A-containing mRNAs and by utilizing its distinct RNA-binding domains to bridge interactions between m⁶A-containing mRNAs and the ribosomes to facilitate their efficient translation.

DHX15 regulates CMTR1-dependent gene expression and cell proliferation

DHX15 helicase regulates CMTR1-dependent first transcribed nucleotide ribose O-2 methylation.

CMTR1 contributes to mRNA cap formation by methylating the first transcribed nucleotide ribose at the O-2 position. mRNA cap O-2 methylation has roles in mRNA stabilisation and translation, and self-RNA tolerance in innate immunity. We report that CMTR1 is recruited to serine-5–phosphorylated RNA Pol II C-terminal domain, early in transcription. We isolated CMTR1 in a complex with DHX15, an RNA helicase functioning in splicing and ribosome biogenesis, and characterised it as a regulator of CMTR1. When DHX15 is bound, CMTR1 activity is repressed and the methyltransferase does not bind to RNA pol II. Conversely, CMTR1 activates DHX15 helicase activity, which is likely to impact several nuclear functions. In HCC1806 breast carcinoma cell line, the DHX15–CMTR1 interaction controls ribosome loading of a subset of mRNAs and regulates cell proliferation. The impact of the CMTR1–DHX15 interaction is complex and will depend on the relative expression of these enzymes and their interactors, and the cellular dependency on different RNA processing pathways.

Nol12 is a multifunctional RNA binding protein at the nexus of RNA and DNA metabolism

To counteract the breakdown of genome integrity, eukaryotic cells have developed a network of surveillance pathways to prevent and resolve DNA damage. Recent data has recognized the importance of RNA binding proteins (RBPs) in DNA damage repair (DDR) pathways. Here, we describe Nol12 as a multifunctional RBP with roles in RNA metabolism and genome maintenance. Nol12 is found in different subcellular compartments-nucleoli, where it associates with ribosomal RNA and is required for efficient separation of large and small subunit precursors at site 2; the nucleoplasm, where it co-localizes with the RNA/DNA helicase Dhx9 and paraspeckles; as well as GW/P-bodies in the cytoplasm. Loss of Nol12 results in the inability of cells to recover from DNA stress and a rapid p53-independent ATR-Chk1-mediated apoptotic response. Nol12 co-localizes with DNA repair proteins in vivo including Dhx9, as well as with TOPBP1 at sites of replication stalls, suggesting a role for Nol12 in the resolution of DNA stress and maintenance of genome integrity. Identification of a complex Nol12 interactome, which includes NONO, Dhx9, DNA-PK and Stau1, further supports the protein's diverse functions in RNA metabolism and DNA maintenance, establishing Nol12 as a multifunctional RBP essential for genome integrity.

Human METTL16 is a N6-methyladenosine (m6A) methyltransferase that targets pre-mRNAs and various non-coding RNAs

N⁶-methyladenosine (m⁶A) is a highly dynamic RNA modification that has recently emerged as a key regulator of gene expression. While many m⁶A modifications are installed by the METTL3-METTL14 complex, others appear to be introduced independently, implying that additional human m⁶A methyltransferases remain to be identified. Using crosslinking and analysis of cDNA (CRAC), we reveal that the putative human m⁶A "writer" protein METTL16 binds to the U6 snRNA and other ncRNAs as well as numerous lncRNAs and pre-mRNAs. We demonstrate that METTL16 is responsible for N⁶-methylation of A43 of the U6 snRNA and identify the early U6 biogenesis factors La, LARP7 and the methylphosphate capping enzyme MEPCE as METTL16 interaction partners. Interestingly, A43 lies within an essential ACAGAGA box of U6 that base pairs with 5' splice sites of pre-mRNAs during splicing, suggesting that METTL16-mediated modification of this site plays an important role in splicing regulation. The identification of METTL16 as an active m⁶A methyltransferase in human cells expands our understanding of the mechanisms by which the m⁶A landscape is installed on cellular RNAs.

Noncoding RNA Surveillance: The Ends Justify the Means

Numerous surveillance pathways sculpt eukaryotic transcriptomes by degrading unneeded, defective, and potentially harmful noncoding RNAs (ncRNAs). Because aberrant and excess ncRNAs are largely degraded by exoribonucleases, a key characteristic of these RNAs is an accessible, protein-free 5' or 3' end. Most exoribonucleases function with cofactors that recognize ncRNAs with accessible 5' or 3' ends and/or increase the availability of these ends. Noncoding RNA surveillance pathways were first described in budding yeast, and there are now high-resolution structures of many components of the yeast pathways and significant mechanistic understanding as to how they function. Studies in human cells are revealing the ways in which these pathways both resemble and differ from their yeast counterparts, and are also uncovering numerous pathways that lack equivalents in budding yeast. In this review, we describe both the well-studied pathways uncovered in yeast and the new concepts that are emerging from studies in mammalian cells. We also discuss the ways in which surveillance pathways compete with chaperone proteins that transiently protect nascent ncRNA ends from exoribonucleases, with partner proteins that sequester these ends within RNPs, and with end modification pathways that protect the ends of some ncRNAs from nucleases.

The crystal structure of human DEAH-box RNA helicase 15 reveals a domain organization of the mammalian DEAH/RHA family

DEAH-box RNA helicase 15 (DHX15) plays important roles in RNA metabolism, including in splicing and in ribosome biogenesis. In addition, mammalian DHX15 also mediates the innate immune sensing of viral RNA. However, structural information on this protein is not available, although the structure of the fungal orthologue of this protein, Prp43, has been elucidated. Here, the crystal structure of the ADP-bound form of human DHX15 is reported at a resolution of 2.0 Å. This is the first structure to be revealed of a member of the mammalian DEAH-box RNA helicase (DEAH/RHA) family in a nearly complete form, including the catalytic core consisting of the two N-terminal RecA domains and the C-terminal regulatory domains (CTD). The ADP-bound form of DHX15 displayed a compact structure, in which the RecA domains made extensive contacts with the CTD. Notably, a potential RNA-binding site was found on the surface of a RecA domain with positive electrostatic potential. Almost all structural features were conserved between the fungal Prp43 and the human DHX15, suggesting that they share a fundamentally common mechanism of action and providing a better understanding of the specific mammalian functions of DHX15.