The RNA helicase Mtr4p is a duplex-sensing translocase

High-resolution fleezers reveal duplex opening and stepwise assembly by an oligomer of the DEAD-box helicase Ded1p

DEAD-box RNA helicases are ubiquitous in all domains of life where they bind and remodel RNA and RNA-protein complexes. DEAD-box helicases unwind RNA duplexes by local opening of helical regions without directional movement through the duplexes and some of these enzymes, including Ded1p from Saccharomyces cerevisiae, oligomerize to effectively unwind RNA duplexes. Whether and how DEAD-box helicases coordinate oligomerization and unwinding is not known and it is unclear how many base pairs are actively opened. Using high-resolution optical tweezers and fluorescence, we reveal a highly dynamic and stochastic process of multiple Ded1p protomers assembling on and unwinding an RNA duplex. One Ded1p protomer binds to a duplex-adjacent ssRNA tail and promotes binding and subsequent unwinding of the duplex by additional Ded1p protomers in 4-6 bp steps. The data also reveal rapid duplex unwinding and rezipping linked with binding and dissociation of individual protomers and coordinated with the ATP hydrolysis cycle.

Evolutionary and functional insights into the Ski2-like helicase family in Archaea: a comparison of Thermococcales ASH-Ski2 and Hel308 activities

RNA helicases perform essential housekeeping and regulatory functions in all domains of life by binding and unwinding RNA molecules. The Ski2-like proteins are primordial helicases that play an active role in eukaryotic RNA homeostasis pathways, with multiple homologs having specialized functions. The significance of the expansion and diversity of Ski2-like proteins in Archaea, the third domain of life, has not yet been established. Here, by studying the phylogenetic diversity of Ski2-like helicases among archaeal genomes and the enzymatic activities of those in Thermococcales, we provide further evidence of the function of this protein family in archaeal metabolism of nucleic acids. We show that, in the course of evolution, ASH-Ski2 and Hel308-Ski2, the two main groups of Ski2-like proteins, have diverged in their biological functions. Whereas Hel308 has been shown to mainly act on DNA, we show that ASH-Ski2, previously described to be associated with the 5'-3' aRNase J exonuclease, acts on RNA by supporting an efficient annealing activity, but also an RNA unwinding with a 3'-5' polarity. To gain insights into the function of Ski2, we also analyse the transcriptome of Thermococcus barophilus ΔASH-Ski2 mutant strain and provide evidence of the importance of ASH-Ski2 in cellular metabolism pathways related to translation.

Conserved Residues at the Mtr4 C-Terminus Coordinate Helicase Activity and Exosome Interactions

Mtr4 is an essential RNA helicase involved in nuclear RNA processing and degradation and is a member of the Ski2-like helicase family. Ski2-like helicases share a common core architecture that includes two RecA-like domains, a winged helix, and a helical bundle (HB) domain. In Mtr4, a short C-terminal tail immediately follows the HB domain and is positioned at the interface of the RecA-like domains. The tail ends with a SLYΦ sequence motif that is highly conserved in a subset of Ski2-like helicases. Here, we show that this sequence is critical for Mtr4 function. Mutations in the C-terminus result in decreased RNA unwinding activity. Mtr4 is a key activator of the RNA exosome complex, and mutations in the SLYΦ motif produce a slow growth phenotype when combined with a partial exosome defect in S. cerevisiae, suggesting an important role of the C-terminus of Mtr4 and the RNA exosome. We further demonstrate that C-terminal mutations impair RNA degradation activity by the major RNA exosome nuclease Rrp44 in vitro. These data demonstrate a role for the Mtr4 C-terminus in regulating helicase activity and coordinating Mtr4-exosome interactions.

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.

Challenges in Inferring the Directionality of Active Molecular Processes from Single-Molecule Fluorescence Resonance Energy Transfer Trajectories

We discuss some of the practical challenges that one faces in using stochastic thermodynamics to infer directionality of molecular machines from experimental single-molecule trajectories. Because of the limited spatiotemporal resolution of single-molecule experiments and because both forward and backward transitions between the same pairs of states cannot always be detected, differentiating between the forward and backward directions of, e.g., an ATP-consuming molecular machine that operates periodically, turns out to be a nontrivial task. Using a simple extension of a Markov-state model that is commonly employed to analyze single-molecule transition-path measurements, we illustrate how irreversibility can be hidden from such measurements but in some cases can be uncovered when non-Markov effects in low-dimensional single-molecule trajectories are considered.

Measuring the impact of cofactors on RNA helicase activities

RNA helicases are the largest class of enzymes in eukaryotic RNA metabolism. In cells, protein cofactors regulate RNA helicase functions and impact biochemical helicase activities. Understanding how cofactors affect enzymatic activities of RNA helicases is thus critical for delineating physical roles and regulation of RNA helicases in cells. Here, we discuss approaches and conceptual considerations for the design of experiments to interrogate cofactor effects on RNA helicase activities in vitro. We outline the mechanistic frame for helicase reactions, discuss optimization of experimental setup and reaction parameters for measuring cofactor effects on RNA helicase activities, and provide basic guides to data analysis and interpretation. The described approaches are also instructive for determining the impact of small molecule inhibitors of RNA helicases.

High-Resolution Optical Tweezers Combined with Multicolor Single-Molecule Microscopy

We present an instrument that combines high-resolution optical tweezers and multicolor confocal fluorescence spectroscopy. Biological macromolecules exhibit complex conformation and stoichiometry changes in coordination with their motion and activity. To further our understanding of the complex machinery of life, we need methods that can simultaneously probe more than one degree of freedom of single molecules and complexes. Fluorescence optical tweezers, or "fleezers," combine the capabilities of optical tweezers and single-molecule fluorescence microscopy into a single instrument. Here we present the latest generation of a high-resolution fleezers instrument integrated with multicolor fluorescence spectroscopy. The tweezers portion of the instrument can manipulate biological macromolecules with pN scale forces while measuring subnanometer distances. Simultaneous with tweezers measurements, the multicolor fluorescence capability allows the direct observation of multiple molecules or multiple degrees of freedom which allows, for example, the observation of multiple proteins simultaneously within a complex. The instrument incorporates three fluorescence excitation lasers, all sourced from a single-mode optical fiber allowing a reliable alignment scheme, that allows, for example, three independent fluorescent probes or fluorescence resonance energy transfer (FRET) measurements and also increases flexibility in the choice of fluorescent probes. To avoid photobleaching and improve tweezers stability, the instrument implements a timesharing (using a single trap laser to produce a pair of traps via rapid switching between two locations) and interlacing (turning the trapping beam off when the fluorescence excitation beams are on and vice versa) scheme using acousto-optic modulators (AOM) to rapidly and precisely modulate lasers. Our latest "random phase" trap AOM control method obliterates previous residual trap positioning and bead position measurement errors. Here we present the general design principles and detailed construction and testing protocols for the instrument.

Shaanxi Provincial Diagnosis and Treatment Center of Kawasaki Disease

Kawasaki disease is an acute, self-limited vascuitis that mainly occurs in children under 5 years of age. Intravenous immunoglobulin (IVIG) has become an effective treatment regimen, which can effectively reduce the incidence of cardiovascular complications. However, there has been no consensus or clinical guidelines for the application of IVIG in children with Kawasaki disease till now. This consensus is developed based on the current research progress on the application of IVIG in children with Kawasaki disease in China and overseas, with reference to the diagnosis and treatment guidelines for Kawasaki disease in China and overseas, and the opinions of experts. This consensus provides recommendations on the clinical application strategy of IVIG in children with Kawasaki disease and the prevention and treatment of its adverse reactions. Citation.

Large-scale ratcheting in a bacterial DEAH/RHA-type RNA helicase that modulates antibiotics susceptibility

Many bacteria harbor RNA-dependent nucleoside-triphosphatases of the DEAH/RHA family, whose molecular mechanisms and cellular functions are poorly understood. Here, we show that the Escherichia coli DEAH/RHA protein, HrpA, is an ATP-dependent 3 to 5' RNA helicase and that the RNA helicase activity of HrpA influences bacterial survival under antibiotics treatment. Limited proteolysis, crystal structure analysis, and functional assays showed that HrpA contains an N-terminal DEAH/RHA helicase cassette preceded by a unique N-terminal domain and followed by a large C-terminal region that modulates the helicase activity. Structures of an expanded HrpA helicase cassette in the apo and RNA-bound states in combination with cross-linking/mass spectrometry revealed ratchet-like domain movements upon RNA engagement, much more pronounced than hitherto observed in related eukaryotic DEAH/RHA enzymes. Structure-based functional analyses delineated transient interdomain contact sites that support substrate loading and unwinding, suggesting that similar conformational changes support RNA translocation. Consistently, modeling studies showed that analogous dynamic intramolecular contacts are not possible in the related but helicase-inactive RNA-dependent nucleoside-triphosphatase, HrpB. Our results indicate that HrpA may be an interesting target to interfere with bacterial tolerance toward certain antibiotics and suggest possible interfering strategies.

Regulation of RNA helicase activity: principles and examples

RNA helicases are a ubiquitous class of enzymes involved in virtually all processes of RNA metabolism, from transcription, mRNA splicing and export, mRNA translation and RNA transport to RNA degradation. Although ATP-dependent unwinding of RNA duplexes is their hallmark reaction, not all helicases catalyze unwinding in vitro, and some in vivo functions do not depend on duplex unwinding. RNA helicases are divided into different families that share a common helicase core with a set of helicase signature motives. The core provides the active site for ATP hydrolysis, a binding site for non-sequence-specific interaction with RNA, and in many cases a basal unwinding activity. Its activity is often regulated by flanking domains, by interaction partners, or by self-association. In this review, we summarize the regulatory mechanisms that modulate the activities of the helicase core. Case studies on selected helicases with functions in translation, splicing, and RNA sensing illustrate the various modes and layers of regulation in time and space that harness the helicase core for a wide spectrum of cellular tasks.

Mtr4 RNA helicase structures and interactions

Mtr4 is a Ski2-like RNA helicase that plays a central role in RNA surveillance and degradation pathways as an activator of the RNA exosome. Multiple crystallographic and cryo-EM studies over the past 10 years have revealed important insight into the Mtr4 structure and interactions with protein and nucleic acid binding partners. These structures place Mtr4 at the center of a dynamic process that recruits RNA substrates and presents them to the exosome. In this review, we summarize the available Mtr4 structures and highlight gaps in our current understanding.

Force-dependent stimulation of RNA unwinding by SARS-CoV-2 nsp13 helicase

The superfamily 1 helicase nonstructural protein 13 (nsp13) is required for SARS-CoV-2 replication. The mechanism and regulation of nsp13 has not been explored at the single-molecule level. Specifically, force-dependent unwinding experiments have yet to be performed for any coronavirus helicase. Here, using optical tweezers, we find that nsp13 unwinding frequency, processivity, and velocity increase substantially when a destabilizing force is applied to the RNA substrate. These results, along with bulk assays, depict nsp13 as an intrinsically weak helicase that can be activated >50-fold by piconewton forces. Such force-dependent behavior contrasts the known behavior of other viral monomeric helicases, such as hepatitis C virus NS3, and instead draws stronger parallels to ring-shaped helicases. Our findings suggest that mechanoregulation, which may be provided by a directly bound RNA-dependent RNA polymerase, enables on-demand helicase activity on the relevant polynucleotide substrate during viral replication.

Observation of processive telomerase catalysis using high-resolution optical tweezers

Telomere maintenance by telomerase is essential for continuous proliferation of human cells and is vital for the survival of stem cells and 90% of cancer cells. To compensate for telomeric DNA lost during DNA replication, telomerase processively adds GGTTAG repeats to chromosome ends by copying the template region within its RNA subunit. Between repeat additions, the RNA template must be recycled. How telomerase remains associated with substrate DNA during this critical translocation step remains unknown. Using a newly developed single-molecule telomerase activity assay utilizing high-resolution optical tweezers, we demonstrate that stable substrate DNA binding at an anchor site within telomerase facilitates the processive synthesis of telomeric repeats. The product DNA synthesized by telomerase can be recaptured by the anchor site or fold into G-quadruplex structures. Our results provide detailed mechanistic insights into telomerase catalysis, a process of critical importance in aging and cancer.

Approaches for measuring the dynamics of RNA-protein interactions

RNA-protein interactions are pivotal for the regulation of gene expression from bacteria to human. RNA-protein interactions are dynamic; they change over biologically relevant timescales. Understanding the regulation of gene expression at the RNA level therefore requires knowledge of the dynamics of RNA-protein interactions. Here, we discuss the main experimental approaches to measure dynamic aspects of RNA-protein interactions. We cover techniques that assess dynamics of cellular RNA-protein interactions that accompany biological processes over timescales of hours or longer and techniques measuring the kinetic dynamics of RNA-protein interactions in vitro. This article is categorized under: RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications RNA Interactions with Proteins and Other Molecules > Protein-RNA Recognition RNA Interactions with Proteins and Other Molecules > RNA-Protein Complexes RNA Evolution and Genomics > Ribonomics.

Combined High-Resolution Optical Tweezers and Multicolor Single-Molecule Fluorescence with an Automated Single-Molecule Assembly Line

We present an instrument that combines high-resolution optical tweezers and multicolor confocal fluorescence spectroscopy along with automated single-molecule assembly. The multicolor allows the simultaneous observation of multiple molecules or multiple degrees of freedom, which allows, for example, the observation of multiple proteins simultaneously within a complex. The instrument incorporates three fluorescence excitation lasers, with a reliable alignment scheme, which will allow three independent fluorescent probe or FRET measurements and also increases flexibility in the choice of fluorescent molecules. We demonstrate the ability to simultaneously measure angstrom-scale changes in tether extension and fluorescence signals. Simultaneous tweezers and fluorescence measurement are particularly challenging because of fluorophore photobleaching, even more so if multiple fluorophores are to be measured. Therefore, (1) fluorescence excitation and detection is interlaced with time-shared dual optical traps. (2) We investigated the photostability of common fluorophores. The mean number of photons emitted before bleaching was unaffected by the trap laser and decreased only slightly with increasing excitation laser intensity. Surprisingly, we found that Cy5 outperforms other commonly used fluorophores by more than fivefold. (3) We devised computer-controlled automation, which conserves fluorophore lifetime by quickly detecting fluorophore-labeled molecule binding, turning off lasers, and moving to add the next fluorophore-labeled component. The single-molecule assembly line enables the precise assembly of multimolecule complexes while preserving fluorophores.

Structural basis for MTR4-ZCCHC8 interactions that stimulate the MTR4 helicase in the nuclear exosome-targeting complex

Aberrant or unwanted transcripts can be degraded by the RNA exosome with the help of the nuclear exosome-targeting (NEXT) complex. NEXT, composed of RNA-binding protein RBM7, scaffold ZCCHC8, and helicase MTR4, is implicated in stress response, neurodegeneration, and viral ribogenesis. Here, we characterize the activities of NEXT that support its role in exosome-mediated decay. NEXT catalyzes 3′→5′ helicase activity and disrupts RNA:RNA and DNA:RNA duplexes more efficiently than MTR4. Optimal activity is observed when substrates include a uridine-rich motif, for interactions with RBM7, and a 3′ poly(A) tail. The ZCCHC8 C-terminal domain binds the helicase core and can stimulate MTR4 helicase/ATPase activities. Our results highlight the interplay among NEXT subunits to ensure effective targeting of substrates.

The nuclear exosome-targeting (NEXT) complex functions as an RNA exosome cofactor and is involved in surveillance and turnover of aberrant transcripts and noncoding RNAs. NEXT is a ternary complex composed of the RNA-binding protein RBM7, the scaffold zinc-knuckle protein ZCCHC8, and the helicase MTR4. While RNA interactions with RBM7 are known, it remains unclear how NEXT subunits collaborate to recognize and prepare substrates for degradation. Here, we show that MTR4 helicase activity is enhanced when associated with RBM7 and ZCCHC8. While uridine-rich substrates interact with RBM7 and are preferred, optimal activity is observed when substrates include a polyadenylated 3′ end. We identify a bipartite interaction of ZCCHC8 with MTR4 and uncover a role for the conserved C-terminal domain of ZCCHC8 in stimulating MTR4 helicase and ATPase activities. A crystal structure reveals that the ZCCHC8 C-terminal domain binds the helicase core in a manner that is distinct from that observed for Saccharomyces cerevisiae exosome cofactors Trf4p and Air2p. Our results are consistent with a model whereby effective targeting of substrates by NEXT entails recognition of elements within the substrate and activation of MTR4 helicase activity.

RNA surveillance by the nuclear RNA exosome: mechanisms and significance

The nuclear RNA exosome is an essential and versatile machinery that regulates maturation and degradation of a huge plethora of RNA species. The past two decades have witnessed remarkable progress in understanding the whole picture of its RNA substrates and the structural basis of its functions. In addition to the exosome itself, recent studies focusing on associated co-factors have been elucidating how the exosome is directed towards specific substrates. Moreover, it has been gradually realized that loss-of-function of exosome subunits affect multiple biological processes such as the DNA damage response, R-loop resolution, maintenance of genome integrity, RNA export, translation and cell differentiation. In this review, we summarize the current knowledge of the mechanisms of nuclear exosome-mediated RNA metabolism and discuss their physiological significance.

RNA helicases in RNA decay

RNA molecules have the tendency to fold into complex structures or to associate with complementary RNAs that exoribonucleases have difficulties processing or degrading. Therefore, degradosomes in bacteria and organelles as well as exosomes in eukaryotes have teamed-up with RNA helicases. Whereas bacterial degradosomes are associated with RNA helicases from the DEAD-box family, the exosomes and mitochondrial degradosome use the help of Ski2-like and Suv3 RNA helicases.

The DEAD-box protein DDX43 (HAGE) is a dual RNA-DNA helicase and has a K-homology domain required for full nucleic acid unwinding activity

The K-homology (KH) domain is a nucleic acid-binding domain present in many proteins but has not been reported in helicases. DDX43, also known as HAGE (helicase antigen gene), is a member of the DEAD-box protein family. It contains a helicase core domain in its C terminus and a potential KH domain in its N terminus. DDX43 is highly expressed in many tumors and is, therefore, considered a potential target for immunotherapy. Despite its potential as a therapeutic target, little is known about its activities. Here, we purified recombinant DDX43 protein to near homogeneity and found that it exists as a monomer in solution. Biochemical assays demonstrated that it is an ATP-dependent RNA and DNA helicase. Although DDX43 was active on duplex RNA regardless of the orientation of the single-stranded RNA tail, it preferred a 5' to 3' polarity on RNA and a 3' to 5' direction on DNA. Truncation mutations and site-directed mutagenesis confirmed that the KH domain in DDX43 is responsible for nucleic acid binding. Compared with the activity of the full-length protein, the C-terminal helicase domain had no unwinding activity on RNA substrates and had significantly reduced unwinding activity on DNA. Moreover, the full-length DDX43 protein, with single amino acid change in the KH domain, had reduced unwinding and binding activates on RNA and DNA substrates. Our results demonstrate that DDX43 is a dual helicase and the KH domain is required for its full unwinding activity.