Dicer's helicase domain discriminates dsRNA termini to promote an altered reaction mode

Kinetic Analysis of Small Silencing RNA Production by Human and Drosophila Dicer Enzymes In Vitro

Dicer enzymes produce small silencing RNAs such as microRNAs (miRNAs) and small interfering RNAs (siRNAs), which then are loaded into Argonaute proteins and act as sequence-specific guides. A powerful tool to understand the molecular mechanism of small silencing RNA production by Dicers is an in vitro RNA processing assay using recombinant Dicer proteins. Such biochemical analyses have elucidated the substrate specificities and kinetics of Dicers, the mechanism by which the length of small RNAs produced by Dicers is determined, and the effects of Dicer-partner proteins and endogenous small molecules such as ATP and inorganic phosphate on small RNA production by Dicers, among others. Here, we describe methods for in vitro small RNA production assay using recombinant human and Drosophila Dicer proteins.

DIP1 modulates stem cell homeostasis in Drosophila through regulation of sisR-1

Stable intronic sequence RNAs (sisRNAs) are by-products of splicing and regulate gene expression. How sisRNAs are regulated is unclear. Here we report that a double-stranded RNA binding protein, Disco-interacting protein 1 (DIP1) regulates sisRNAs in Drosophila. DIP1 negatively regulates the abundance of sisR-1 and INE-1 sisRNAs. Fine-tuning of sisR-1 by DIP1 is important to maintain female germline stem cell homeostasis by modulating germline stem cell differentiation and niche adhesion. Drosophila DIP1 localizes to a nuclear body (satellite body) and associates with the fourth chromosome, which contains a very high density of INE-1 transposable element sequences that are processed into sisRNAs. DIP1 presumably acts outside the satellite bodies to regulate sisR-1, which is not on the fourth chromosome. Thus, our study identifies DIP1 as a sisRNA regulatory protein that controls germline stem cell self-renewal in Drosophila.

Stable intronic sequence RNAs (sisRNAs) are by-products of splicing from introns with roles in embryonic development in Drosophila. Here, the authors show that the RNA binding protein DIP1 regulates sisRNAs in Drosophila, which is necessary for germline stem cell homeostasis.

Loquacious-PD facilitates Drosophila Dicer-2 cleavage through interactions with the helicase domain and dsRNA

Loquacious-PD (Loqs-PD) is required for biogenesis of many endogenous siRNAs in Drosophila In vitro, Loqs-PD enhances the rate of dsRNA cleavage by Dicer-2 and also enables processing of substrates normally refractory to cleavage. Using purified components, and Loqs-PD truncations, we provide a mechanistic basis for Loqs-PD functions. Our studies indicate that the 22 amino acids at the C terminus of Loqs-PD, including an FDF-like motif, directly interact with the Hel2 subdomain of Dicer-2's helicase domain. This interaction is RNA-independent, but we find that modulation of Dicer-2 cleavage also requires dsRNA binding by Loqs-PD. Furthermore, while the first dsRNA-binding motif of Loqs-PD is dispensable for enhancing cleavage of optimal substrates, it is essential for enhancing cleavage of suboptimal substrates. Finally, our studies define a previously unrecognized Dicer interaction interface and suggest that Loqs-PD is well positioned to recruit substrates into the helicase domain of Dicer-2.

New DRB complexes for new DRB functions in plants

Double-stranded RNA binding (DRB) proteins are generally considered as promoting cofactors of Dicer or Dicer-like (DCL) proteins that ensure efficient and precise production of small RNAs, the sequence-specificity guide of RNA silencing processes in both plants and animals. However, the characterization of a new clade of DRB proteins in Arabidopsis has recently challenged this view by showing that DRBs can also act as potent inhibitors of DCL processing. This is achieved through sequestration of a specific class of small RNA precursors, the endogenous inverted-repeat (endoIR) dsRNAs, thereby selectively preventing production of their associated small RNAs, the endoIR-siRNAs. Here, we concisely summarize the main findings obtained from the characterization of these new DRB proteins and discuss how the existence of such complexes can support a potential, yet still elusive, biological function of plant endoIR-siRNAs.

Overexpression and purification of Dicer and accessory proteins for biochemical and structural studies

The Dicer family of ribonucleases plays a key role in small RNA-based regulatory pathways by generating short dsRNA fragments that modulate expression of endogenous genes, or protect the host from invasive nucleic acids. Beginning with its initial discovery, biochemical characterization of Dicer has provided insight about its catalytic properties. However, a comprehensive understanding of how Dicer's domains contribute to substrate-specific recognition and catalysis is lacking. One reason for this void is the lack of high-resolution structural information for a metazoan Dicer in the apo- or substrate-bound state. Both biochemical and structural studies are facilitated by large amounts of highly purified, active protein, and Dicer enzymes have historically been recalcitrant to overexpression and purification. Here we describe optimized procedures for the large-scale expression of Dicer in baculovirus-infected insect cells. We then outline a three-step protocol for the purification of large amounts (3-4mg of Dicer per liter of insect cell culture) of highly purified and active Dicer protein, suitable for biochemical and structural studies. Our methods are general and are extended to enable overexpression, purification and biochemical characterization of accessory dsRNA binding proteins that interact with Dicer and modulate its catalytic activity.

The C-terminal dsRNA-binding domain of Drosophila Dicer-2 is crucial for efficient and high-fidelity production of siRNA and loading of siRNA to Argonaute2

Drosophila Dicer-2 efficiently and precisely produces 21-nucleotide (nt) siRNAs from long double-stranded RNA (dsRNA) substrates and loads these siRNAs onto the effector protein Argonaute2 for RNA silencing. The functional roles of each domain of the multidomain Dicer-2 enzyme in the production and loading of siRNAs are not fully understood. Here we characterized Dicer-2 mutants lacking either the N-terminal helicase domain or the C-terminal dsRNA-binding domain (CdsRBD) (ΔHelicase and ΔCdsRBD, respectively) in vivo and in vitro. We found that ΔCdsRBD Dicer-2 produces siRNAs with lowered efficiency and length fidelity, producing a smaller ratio of 21-nt siRNAs and higher ratios of 20- and 22-nt siRNAs in vivo and in vitro. We also found that ΔCdsRBD Dicer-2 cannot load siRNA duplexes to Argonaute2 in vitro. Consistent with these findings, we found that ΔCdsRBD Dicer-2 causes partial loss of RNA silencing activity in vivo. Thus, Dicer-2 CdsRBD is crucial for the efficiency and length fidelity in siRNA production and for siRNA loading. Together with our previously published findings, we propose that CdsRBD binds the proximal body region of a long dsRNA substrate whose 5'-monophosphate end is anchored by the phosphate-binding pocket in the PAZ domain. CdsRBD aligns the RNA to the RNA cleavage active site in the RNase III domain for efficient and high-fidelity siRNA production. This study reveals multifunctions of Dicer-2 CdsRBD and sheds light on the molecular mechanism by which Dicer-2 produces 21-nt siRNAs with a high efficiency and fidelity for efficient RNA silencing.

Nuclear phosphorylated Dicer processes double-stranded RNA in response to DNA damage

The endoribonuclease Dicer is a key component of the human RNA interference pathway and is known for its role in cytoplasmic microRNA production. Recent findings suggest that noncanonical Dicer generates small noncoding RNA to mediate the DNA damage response (DDR). Here, we show that human Dicer is phosphorylated in the platform-Piwi/Argonaute/Zwille-connector helix cassette (S1016) upon induction of DNA damage. Phosphorylated Dicer (p-Dicer) accumulates in the nucleus and is recruited to DNA double-strand breaks. We further demonstrate that turnover of damage-induced nuclear, double-stranded (ds) RNA requires additional phosphorylation of carboxy-terminal Dicer residues (S1728 and S1852). DNA damage-induced nuclear Dicer accumulation is conserved in mammals. Dicer depletion causes endogenous DNA damage and delays the DDR by impaired recruitment of repair factors MDC1 and 53BP1. Collectively, we place Dicer within the context of the DDR by demonstrating a DNA damage-inducible phosphoswitch that causes localized processing of nuclear dsRNA by p-Dicer to promote DNA repair.

Molecular mechanisms of Dicer: endonuclease and enzymatic activity

The enzyme Dicer is best known for its role as a riboendonuclease in the small RNA pathway. In this canonical role, Dicer is a critical regulator of the biogenesis of microRNA and small interfering RNA, as well as a growing number of additional small RNAs derived from various sources. Emerging evidence demonstrates that Dicer's endonuclease role extends beyond the generation of small RNAs; it is also involved in processing additional endogenous and exogenous substrates, and is becoming increasingly implicated in regulating a variety of other cellular processes, outside of its endonuclease function. This review will describe the canonical and newly identified functions of Dicer.

Switches in Dicer Activity During Oogenesis and Early Development

Dicer is a versatile protein regulating diverse biological processes via the production of multiple classes of small regulatory RNAs, including microRNAs (miRNAs) and small interfering RNAs (siRNAs). In this chapter, we will discuss roles for Dicer in driving temporal changes in activity of individual small RNA classes to support oogenesis and early embryogenesis. Genetic strategies that perturb particular functions of Dicer family proteins, such as ablation of individual Dicer paralogs or their binding partners as well as introduction of point mutations to individual domains, allowed the dissection of Dicer functions in diverse small RNA pathways. Evolutionary conservation and divergence of the mechanisms highlight the importance of Dicer versatility in supporting rapid changes in gene expression during oogenesis and early development. Furthermore, we will discuss potential roles of Dicer in transgenerational inheritance of small RNA-mediated gene regulation.

Phosphate-binding pocket in Dicer-2 PAZ domain for high-fidelity siRNA production

The enzyme Dicer produces small silencing RNAs such as micro-RNAs (miRNAs) and small interfering RNAs (siRNAs). In Drosophila, Dicer-1 produces ∼22-24-nt miRNAs from pre-miRNAs, whereas Dicer-2 makes 21-nt siRNAs from long double-stranded RNAs (dsRNAs). How Dicer-2 precisely makes 21-nt siRNAs with a remarkably high fidelity is unknown. Here we report that recognition of the 5'-monophosphate of a long dsRNA substrate by a phosphate-binding pocket in the Dicer-2 PAZ (Piwi, Argonaute, and Zwille/Pinhead) domain is crucial for the length fidelity, but not the efficiency, in 21-nt siRNA production. Loss of the length fidelity, meaning increased length heterogeneity of siRNAs, caused by point mutations in the phosphate-binding pocket of the Dicer-2 PAZ domain decreased RNA silencing activity in vivo, showing the importance of the high fidelity to make 21-nt siRNAs. We propose that the 5'-monophosphate of a long dsRNA substrate is anchored by the phosphate-binding pocket in the Dicer-2 PAZ domain and the distance between the pocket and the RNA cleavage active site in the RNaseIII domain corresponds to the 21-nt pitch in the A-form duplex of a long dsRNA substrate, resulting in high-fidelity 21-nt siRNA production. This study sheds light on the molecular mechanism by which Dicer-2 produces 21-nt siRNAs with a remarkably high fidelity for efficient RNA silencing.

Six Medicago truncatula Dicer-like protein genes are expressed in plant cells and upregulated in nodules

Here we report the existence of six putative Dicer-like genes in the Medicago truncatula genome. They are ubiquitously expressed throughout the plant and significantly induced in root nodules. Over the past decade, small noncoding RNAs (sncRNA) have emerged as widespread and important regulatory molecules influencing both the structure and expression of plant genomes. One of the key factors involved in sncRNA biogenesis in plants is a group of RNase III-type nucleases known as Dicer-like (DCL) proteins. Based on functional analysis of DCL proteins identified in Arabidopsis thaliana, four types of DCLs were distinguished (DCL1-4). DCL1 mainly produces 21 nt miRNAs. The products generated by DCL2, DCL3, and DCL4 belong to various classes of siRNAs that are 22, 24 and 21 nt in length, respectively. M. truncatula is a model legume plant closely related to many economically important cultivable species. By screening the recent M. truncatula genome assembly, we were able to identify three new DCL genes in addition to the MtDCL1-3 genes that had been earlier characterized. The newly found genes include MtDCL4 and two MtDCL2 homologs. We showed that all six M. truncatula DCL genes are expressed in plant cells. The first of the identified MtDCL2 paralogs encodes a truncated version of the DCL2 protein, while the second undergoes substantial and specific upregulation in the root nodules. Additionally, we identified an alternative splicing variant of MtDCL1 mRNA, similar to the one found in Arabidopsis. Our results indicate that DCL genes are differently activated during Medicago symbiosis with nitrogen fixing bacteria and upon pathogen infection. In addition, we hypothesize that the alternative splicing variant of MtDCL1 mRNA may be involved in tissue-specific regulation of the DCL1 level.

Helicases in Antiviral Immunity: Dual Properties as Sensors and Effectors

Many helicases have a unique ability to couple cognate RNA binding to ATP hydrolysis, which can induce a large conformational change that affects its interaction with RNA, position along RNA, or oligomeric state. A growing number of these helicases contribute to the innate immune system, either as sensors that detect foreign nucleic acids and/or as effectors that directly participate in the clearance of such foreign species. In this review, we discuss a few examples, including retinoic acid-inducible gene-I (RIG-I), melanoma differentiation-associated gene 5 (MDA5), and Dicer, focusing on their dual functions as both sensors and effectors. We will also discuss the closely related, but less understood, helicases, laboratory of genetics and physiology 2 (LGP2) and Dicer-related helicase-1 and -3 (DRH-1 and -3).

Reproducible features of small RNAs in C. elegans reveal NU RNAs and provide insights into 22G RNAs and 26G RNAs

Small RNAs regulate gene expression and most genes in the worm Caenorhabditis elegans are subject to their regulation. Here, we analyze small RNA data sets and use reproducible features of RNAs present in multiple data sets to discover a new class of small RNAs and to reveal insights into two known classes of small RNAs--22G RNAs and 26G RNAs. We found that reproducibly detected 22-nt RNAs, although are predominantly RNAs with a G at the 5' end, also include RNAs with A, C, or U at the 5' end. These RNAs are synthesized downstream from characteristic sequence motifs on mRNA and have U-tailed derivatives. Analysis of 26G RNAs revealed that they are processed from a blunt end of double-stranded RNAs and that production of one 26G RNA generates a hotspot immediately downstream for production of another. To our surprise, analysis of RNAs shorter than 18 nt revealed a new class of RNAs, which we call NU RNAs (pronounced "new RNAs") because they have a NU bias at the 5' end, where N is any nucleotide. NU RNAs are antisense to genes and originate downstream from U bases on mRNA. Although many genes have complementary NU RNAs, their genome-wide distribution is distinct from that of previously known classes of small RNAs. Our results suggest that current approaches underestimate reproducibly detected RNAs that are shorter than 18 nt, and theoretical considerations suggest that such shorter RNAs could be used for sequence-specific gene regulation in organisms like C. elegans that have small genomes.

A key gene of the small RNA pathway in the flounder, Paralichthys olivaceus: identification and functional characterization of dicer

Dicer is critical for producing mature microRNAs (miRNAs) from precursor molecules and small interfering RNAs and plays an important role in controlling development and metabolism. In the present study, we cloned the flounder dicer gene, which is 6585 nucleotides (nt), including a 5'-untranslated region (UTR) of 231 nt, a 3'-UTR of 663 nt and an open reading frame of 5691 nt encoding a polypeptide of 1897 amino acids, and analyzed the conservation and expression pattern of dicer. The tissue distribution analysis indicated that dicer is abundantly expressed in the brain, heart, liver, spleen, stomach, kidney, gill, muscle, intestine and gonad of adult fish. Temporal expression analysis indicated that dicer mRNA is highly expressed during the embryonic and early larval stages, and exhibits low expression during the metamorphic stages. Treatment with thyroid hormone (TH) or thiourea indirectly or directly up-regulated dicer mRNA levels at 17 and 23 dph, whereas treatment with TH down-regulated dicer mRNA levels at 36 dph. The dicer-specific siRNA significantly down-regulated dicer mRNA and pol-let-7d levels, while pol-let-7d precursor levels were not differentially changed compared with the control (NC). These results demonstrated that dicer plays a key role in development and metabolism through the production of mature miRNAs, providing basic information for further studies concerning the role of dicer in Paralichthys olivaceus development.

Drosophila dicer-2 cleavage is mediated by helicase- and dsRNA termini-dependent states that are modulated by Loquacious-PD

In previous studies we observed that the helicase domain of Drosophila Dicer-2 (dmDcr-2) governs substrate recognition and cleavage efficiency, and that dsRNA termini are key to this discrimination. We now provide a mechanistic basis for these observations. We show that discrimination of termini occurs during initial binding. Without ATP, dmDcr-2 binds 3' overhanging, but not blunt, termini. By contrast, with ATP, dmDcr-2 binds both types of termini, with highest-affinity binding observed with blunt dsRNA. In the presence of ATP, binding, cleavage, and ATP hydrolysis are optimal with BLT termini compared to 3'ovr termini. Limited proteolysis experiments suggest the optimal reactivity of BLT dsRNA is mediated by a conformational change that is dependent on ATP and the helicase domain. We find that dmDcr-2's partner protein, Loquacious-PD, alters termini dependence, enabling dmDcr-2 to cleave substrates normally refractory to cleavage, such as dsRNA with blocked, structured, or frayed ends.

Sensing viral RNAs by Dicer/RIG-I like ATPases across species

Induction of antiviral immunity in vertebrates and invertebrates relies on members of the RIG-I-like receptor and Dicer families, respectively. Although these proteins have different size and domain composition, members of both families share a conserved DECH-box helicase domain. This helicase, also known as a duplex RNA activated ATPase, or DRA domain, plays an important role in viral RNA sensing. Crystallographic and electron microscopy studies of the RIG-I and Dicer DRA domains indicate a common structure and that similar conformational changes are induced by dsRNA binding. Genetic and biochemical studies on the function and regulation of DRAs reveal similarities, but also some differences, between viral RNA sensing mechanisms in nematodes, flies and mammals.

A requirement for ERK-dependent Dicer phosphorylation in coordinating oocyte-to-embryo transition in C. elegans

Signaling pathways and small RNAs direct diverse cellular events, but few examples are known of defined signaling pathways directly regulating small RNA biogenesis. We show that ERK phosphorylates Dicer on two conserved residues in its RNase IIIb and double-stranded RNA (dsRNA)-binding domains and that phosphorylation of these residues is necessary and sufficient to trigger Dicer's nuclear translocation in worms, mice, and human cells. Phosphorylation of Dicer on either site inhibits Dicer function in the female germline and dampens small RNA repertoire. Our data demonstrate that ERK phosphorylates and inhibits Dicer during meiosis I for oogenesis to proceed normally in Caenorhabditis elegans and that this inhibition is released before fertilization for embryogenesis to proceed normally. The conserved Dicer residues, their phosphorylation by ERK, and the consequences of the resulting modifications implicate an ERK-Dicer nexus as a fundamental component of the oocyte-to-embryo transition and an underlying mechanism coupling extracellular cues to small RNA production.

DICER1: mutations, microRNAs and mechanisms

Dicer is central to microRNA-mediated silencing and several other RNA interference phenomena that are profoundly embedded in cancer gene networks. Most recently, both germline and somatic mutations in DICER1 have been identified in diverse types of cancer. Although some of the mutations clearly reduce the dosage of this key enzyme, others dictate surprisingly specific changes in select classes of small RNAs. This Review reflects on the molecular properties of the Dicer enzymes in small RNA silencing pathways, and rationalizes the newly discovered mutations on the basis of the activities and functions of its determinants.

The RNA-binding protein DDX1 promotes primary microRNA maturation and inhibits ovarian tumor progression

Posttranscriptional maturation is a critical step in microRNA (miRNA) biogenesis that determines mature miRNA levels. In addition to core components (Drosha and DGCR8 [DiGeorge syndrome critical region gene 8]) in the microprocessor, regulatory RNA-binding proteins may confer the specificity for recruiting and processing of individual primary miRNAs (pri-miRNAs). Here, we identify DDX1 as a regulatory protein that promotes the expression of a subset of miRNAs, including five members in the microRNA-200 (miR-200) family and four miRNAs in an eight-miRNA signature of a mesenchymal ovarian cancer subtype. A majority of DDX1-dependent miRNAs are induced after DNA damage. This induction is facilitated by the ataxia telangiectasia mutated (ATM)-mediated phosphorylation of DDX1. Inhibiting DDX1 promotes ovarian tumor growth and metastasis in a syngeneic mouse model. Analysis of The Cancer Genome Atlas (TCGA) reveals that low DDX1 levels are associated with poor clinical outcome in patients with serous ovarian cancer. These findings suggest that DDX1 is a key modulator in miRNA maturation and ovarian tumor suppression.

Identification and characterization of Dicer1e, a Dicer1 protein variant, in oral cancer cells

Background

The human dicer1 gene has been predicted to produce several mRNA variants that encode truncated Dicer1 proteins of varying lengths. One of these Dicer1 variants, Dicer1e, was recently found to be differentially expressed in breast cancer cells. Because the expression and function of the Dicer1e protein variant has not been well characterized and the underlying molecular mechanisms for the development of oral squamous cell carcinomas (OSCCs) are poorly understood, the present study sought to characterize the biological role of Dicer1e and determine its relationship, if any, to OSCC pathogenesis.

Methods

Western blot analyses were used to examine Dicer1e expression levels in a panel of oral cancer cells/tissues and during epithelial-mesenchymal transition (EMT), followed by 5′/3′-RACE analyses to obtain the full-length Dicer1e transcript. Biochemical fractionation and indirect immunofluorescent studies were performed to determine the cellular localization of Dicer1e and the effects of Dicer1e silencing on cancer cell proliferation, clonogenicity, and drug sensitivity were also assessed.

Results

Dicer1e protein levels were found to be overexpressed in OSCC cell lines of epithelial phenotype and in OSCC tissues with its levels downregulated during EMT. Moreover, the Dicer1e protein was observed to predominantly localize in the nucleus. 5′/3′-RACE analyses confirmed the presence of the Dicer1e transcript and silencing of Dicer1e impaired both cancer cell proliferation and clonogenicity by inducing either apoptosis and/or G2/M cell cycle arrest. Lastly, Dicer1e knockdown enhanced the chemosensitivity of oral cancer cells to cisplatin.

Conclusion

The expression levels of Dicer1e influence the pathogenesis of oral cancer cells and alter their response to chemosensitivity, thus supporting the importance of Dicer1e as a therapeutic target for OSCCs.

Electronic supplementary material

The online version of this article (doi:10.1186/1476-4598-13-190) contains supplementary material, which is available to authorized users.

A universal small molecule, inorganic phosphate, restricts the substrate specificity of Dicer-2 in small RNA biogenesis

The enzyme Dicer is central to the production of small silencing RNAs such as microRNAs (miRNAs) and small interfering RNAs (siRNAs). Like other insects, Drosophila melanogaster uses different Dicers to make siRNAs and miRNAs: Dicer-1 produces miRNAs from pre-miRNAs, whereas Dicer-2 generates siRNAs from long double-stranded RNA (dsRNA). How do the 2 Dicers achieve their substrate specificity? Here, we review recent findings that inorganic phosphate restricts the substrate specificity of Dicer-2 to long dsRNA. Inorganic phosphate inhibits Dicer-2 from binding and cleaving pre-miRNAs, without affecting the processing of long dsRNA. Crystal structures of a fragment of human Dicer in complex with an RNA duplex identify a phosphate-binding pocket that recognizes both the 5'-monophosphate of a substrate RNA and inorganic phosphate. We propose that inorganic phosphate occupies the phosphate-binding pocket in the fly Dicer-2, blocking binding of pre-miRNA and restricting pre-miRNA processing to Dicer-1. Thus, a small molecule can alter the substrate specificity of a nucleic acid-processing enzyme.

Inorganic phosphate blocks binding of pre-miRNA to Dicer-2 via its PAZ domain

In Drosophila, Dicer-1 produces microRNAs (miRNAs) from pre-miRNAs, whereas Dicer-2 generates small interfering RNAs from long double-stranded RNA (dsRNA), a process that requires ATP hydrolysis. We previously showed that inorganic phosphate inhibits Dicer-2 cleavage of pre-miRNAs, but not long dsRNAs. Here, we report that phosphate-dependent substrate discrimination by Dicer-2 reflects dsRNA substrate length. Efficient processing by Dicer-2 of short dsRNA requires a 5' terminal phosphate and a two-nucleotide, 3' overhang, but does not require ATP. Phosphate inhibits cleavage of such short substrates. In contrast, cleavage of longer dsRNA requires ATP but no specific end structure: phosphate does not inhibit cleavage of these substrates. Mutation of a pair of conserved arginine residues in the Dicer-2 PAZ domain blocked cleavage of short, but not long, dsRNA. We propose that inorganic phosphate occupies a PAZ domain pocket required to bind the 5' terminal phosphate of short substrates, blocking their use and restricting pre-miRNA processing in flies to Dicer-1. Our study helps explain how a small molecule can alter the substrate specificity of a nucleic acid processing enzyme.

Dicer-related helicase 3 forms an obligate dimer for recognizing 22G-RNA

Dicer is a specialized nuclease that produces RNA molecules of specific lengths for use in gene silencing pathways. Dicer relies on the correct measurement of RNA target duplexes to generate products of specific lengths. It is thought that Dicer uses its multidomain architecture to calibrate RNA product length. However, this measurement model is derived from structural information from a protozoan Dicer, and does not account for the helicase domain present in higher organisms. The Caenorhabditis elegans Dicer-related helicase 3 (DRH-3) is an ortholog of the Dicer and RIG-I family of double-strand RNA activated ATPases essential for secondary siRNA production. We find that DRH-3 specifies 22 bp RNAs by dimerization of the helicase domain, a process mediated by ATPase activity and the N-terminal domain. This mechanism for RNA length discrimination by a Dicer family protein suggests an alternative model for RNA length measurement by Dicer, with implications for recognition of siRNA and miRNA targets.

Biological mechanisms determining the success of RNA interference in insects

Insects constitute the largest group of animals on this planet, having a huge impact on our environment, as well as on our quality of life. RNA interference (RNAi) is a posttranscriptional gene silencing mechanism triggered by double-stranded (ds)RNA fragments. This process not only forms the basis of a widely used reverse genetics research method in many different eukaryotes but also holds great promise to contribute to the species-specific control of agricultural pests and to combat viral infections in beneficial and disease vectoring insects. However, in many economically important insect species, such as flies, mosquitoes, and caterpillars, systemic delivery of naked dsRNA does not trigger effective gene silencing. Although many components of the RNAi pathway have initially been deciphered in the fruit fly, Drosophila melanogaster, it will be of major importance to investigate this process in a wider variety of species, including dsRNA-sensitive insects such as locusts and beetles, to elucidate the factors responsible for the remarkable variability in RNAi efficiency, as observed in different insects. In this chapter, we review the current knowledge on the RNAi pathway, as well as the most recent insights into the mechanisms that might determine successful RNAi in insects.

The diversity, biogenesis, and activities of endogenous silencing small RNAs in Arabidopsis

In eukaryotic RNA silencing, RNase-III classes of enzymes in the Dicer family process double-stranded RNA of cellular or exogenous origin into small-RNA (sRNA) molecules. sRNAs are then loaded into effector proteins known as ARGONAUTEs (AGOs), which, as part of RNA-induced silencing complexes, target complementary RNA or DNA for silencing. Plants have evolved a large variety of pathways over the Dicer-AGO consortium, which most likely underpins part of their phenotypic plasticity. Dicer-like proteins produce all known classes of plant silencing sRNAs, which are invariably stabilized via 2'-O-methylation mediated by HUA ENHANCER 1 (HEN1), potentially amplified by the action of several RNA-dependent RNA polymerases, and function through a variety of AGO proteins. Here, we review the known characteristics and biochemical properties of the core silencing factors found in the model plant Arabidopsis thaliana. We also describe how interactions between these core factors and more specialized proteins allow the production of a plethora of silencing sRNAs involved in a large array of biological functions. We emphasize in particular the biogenesis and activities of silencing sRNAs of endogenous origin.

Parts, assembly and operation of the RIG-I family of motors

Host cell invasion is monitored by a series of pattern recognition receptors (PRRs) that activate the innate immune machinery upon detection of a cognate pathogen associated molecular pattern (PAMP). The RIG-I like receptor (RLR) family of PRRs includes three proteins--RIG-I, MDA5, and LGP2--responsible for the detection of intracellular pathogenic RNA. All RLR proteins are built around an ATPase core homologous to those found in canonical Superfamily 2 (SF2) RNA helicases, which has been modified through the addition of novel accessory domains to recognize duplex RNA. This review focuses on the structural bases for pathogen-specific dsRNA binding and ATPase activation in RLRs, differential RNA recognition by RLR family members, and implications for other duplex RNA activated ATPases, such as Dicer.

dsRNA sensing during viral infection: lessons from plants, worms, insects, and mammals

Host defense systems often rely on direct and indirect pattern recognition to sense the presence of invading pathogens. Patterns can be molecules directly produced by the pathogen or indirectly generated by changes in host parameters as a consequence of infection. Viruses are intracellular pathogens that hijack the cellular machinery to synthesize their own molecules making direct recognition of viral molecules a great challenge. Antiviral systems in prokaryotes and eukaryotes commonly exploit aberrant nucleic acid sensing to recognize virus infection as host and viral nucleic acid metabolism can greatly differ. Indeed, the generation of dsRNA is often associated with viral infection. In this review, we discuss current knowledge on the mechanisms of viral dsRNA sensing utilized by 2 important antiviral defense systems, RNA interference (RNAi) and the vertebrate immune system. The major viral sensors of the vertebrate immune systems are RIG-like receptors, while RNAi pathways depend on Dicer proteins. These 2 families of sensors share a similar helicase domain with high specificity for dsRNA, which is necessary, but not sufficient for efficient recognition by these receptors. Additional intrinsic features to the dsRNA molecule are also necessary for activation of antiviral systems. Studies utilizing synthetic ligands, in vitro biochemistry and reporter systems have greatly helped increase our knowledge on intrinsic features of dsRNA recognition. However, characteristics such as subcellular localization are extrinsic to the dsRNA itself, but certainly influence the recognition in vivo. Thus, mechanisms of viral dsRNA recognition must address how cellular sensors are recruited to nucleic acids or vice versa. Accessory proteins are likely important for in vivo recognition of extrinsic features of viral RNA, but have mostly remained undiscovered due to the limitations of previous strategies. Hence, the identification of novel components of antiviral systems must take into account the complexities involved in viral recognition in vivo.

Dicer's role as an antiviral: still an enigma

Dicer is a multifunctional protein that is essential across species for the generation of microRNAs, a function that is highly conserved across the plant and animal kingdoms. Intriguingly, Dicer exhibits antiviral functions in lower organisms including Drosophila melanogaster and Caenorhabditis elegans. Antiviral activity occurs via small interfering RNA production following cytoplasmic sensing of viral dsRNA. Notably, such antiviral activity has not yet been clearly demonstrated in higher organisms such as mammals. Here, we review the evidence for Dicer as an innate antiviral across species.

PHDcleav: a SVM based method for predicting human Dicer cleavage sites using sequence and secondary structure of miRNA precursors

BACKGROUND

Dicer, an RNase III enzyme, plays a vital role in the processing of pre-miRNAs for generating the miRNAs. The structural and sequence features on pre-miRNA which can facilitate position and efficiency of cleavage are not well known. A precise cleavage by Dicer is crucial because an inaccurate processing can produce miRNA with different seed regions which can alter the repertoire of target genes.

RESULTS

In this study, a novel method has been developed to predict Dicer cleavage sites on pre-miRNAs using Support Vector Machine. We used the dataset of experimentally validated human miRNA hairpins from miRBase, and extracted fourteen nucleotides around Dicer cleavage sites. We developed number of models using various types of features and achieved maximum accuracy of 66% using binary profile of nucleotide sequence taken from 5p arm of hairpin. The prediction performance of Dicer cleavage site improved significantly from 66% to 86% when we integrated secondary structure information. This indicates that secondary structure plays an important role in the selection of cleavage site. All models were trained and tested on 555 experimentally validated cleavage sites and evaluated using 5-fold cross validation technique. In addition, the performance was also evaluated on an independent testing dataset that achieved an accuracy of ~82%.

CONCLUSION

Based on this study, we developed a webserver PHDcleav (http://www.imtech.res.in/raghava/phdcleav/) to predict Dicer cleavage sites in pre-miRNA. This tool can be used to investigate functional consequences of genetic variations/SNPs in miRNA on Dicer cleavage site, and gene silencing. Moreover, it would also be useful in the discovery of miRNAs in human genome and design of Dicer specific pre-miRNAs for potent gene silencing.

Functional specialization of the small interfering RNA pathway in response to virus infection

In Drosophila, post-transcriptional gene silencing occurs when exogenous or endogenous double stranded RNA (dsRNA) is processed into small interfering RNAs (siRNAs) by Dicer-2 (Dcr-2) in association with a dsRNA-binding protein (dsRBP) cofactor called Loquacious (Loqs-PD). siRNAs are then loaded onto Argonaute-2 (Ago2) by the action of Dcr-2 with another dsRBP cofactor called R2D2. Loaded Ago2 executes the destruction of target RNAs that have sequence complementarity to siRNAs. Although Dcr-2, R2D2, and Ago2 are essential for innate antiviral defense, the mechanism of virus-derived siRNA (vsiRNA) biogenesis and viral target inhibition remains unclear. Here, we characterize the response mechanism mediated by siRNAs against two different RNA viruses that infect Drosophila. In both cases, we show that vsiRNAs are generated by Dcr-2 processing of dsRNA formed during viral genome replication and, to a lesser extent, viral transcription. These vsiRNAs seem to preferentially target viral polyadenylated RNA to inhibit viral replication. Loqs-PD is completely dispensable for silencing of the viruses, in contrast to its role in silencing endogenous targets. Biogenesis of vsiRNAs is independent of both Loqs-PD and R2D2. R2D2, however, is required for sorting and loading of vsiRNAs onto Ago2 and inhibition of viral RNA expression. Direct injection of viral RNA into Drosophila results in replication that is also independent of Loqs-PD. This suggests that triggering of the antiviral pathway is not related to viral mode of entry but recognition of intrinsic features of virus RNA. Our results indicate the existence of a vsiRNA pathway that is separate from the endogenous siRNA pathway and is specifically triggered by virus RNA. We speculate that this unique framework might be necessary for a prompt and efficient antiviral response.

The RNA interference (RNAi) pathway utilizes small non-coding RNAs to silence gene expression. In insects, RNAi regulates endogenous genes and functions as an RNA-based immune system against viral infection. Here we have uncovered details of how RNAi is triggered by RNA viruses. Double-stranded RNA (dsRNA) generated as a replication intermediate or from transcription of the RNA virus can be used as substrate for the biogenesis of virus-derived small interfering RNAs (vsiRNAs). Unlike other dsRNAs, virus RNA processing involves Dicer but not its canonical partner protein Loqs-PD. Thus, vsiRNA biogenesis is mechanistically different from biogenesis of endogenous siRNAs or siRNAs derived from other exogenous RNA sources. Our results suggest a specialization of the pathway dedicated to silencing of RNA viruses versus other types of RNAi silencing. The understanding of RNAi mechanisms during viral infection could have implications for the control of insect-borne viruses and the use of siRNAs to treat viral infections in humans.

A truncated form of dicer tilts the balance of RNA interference pathways

The RNase III enzyme Dicer is responsible for key steps in the biogenesis of small RNA species in multiple RNA interference pathways. Here, we show that, in the adult C. elegans soma, half of the total DCR-1 protein is expressed as a truncated, stable C-terminal fragment named small DCR-1 (sDCR-1). sDCR-1 operates independently of full-length DCR-1 in two distinct RNAi pathways; it enhances exogenous RNAi (exoRNAi) and concurrently acts as a negative regulator of microRNA (miRNA) biogenesis. Enhancement of exoRNAi relies on sDCR-1 catalytic activity, whereas impinging on miRNA processing does not. Instead, sDCR-1 competes with pre-miRNA processing by interacting with the miRNA-dedicated Argonautes ALG-1 and ALG-2. Finally, triggering a strong exoRNAi response in the presence of elevated levels of sDCR-1 exacerbates the miRNA processing defect. Our results unveil a surprising role for a truncated form of DCR-1 in the modulation of multiple RNAi activities and in the regulation of mechanistic boundaries between pathways.

Dissecting the interactions of SERRATE with RNA and DICER-LIKE 1 in Arabidopsis microRNA precursor processing

Efficient and precise microRNA (miRNA) biogenesis in Arabidopsis is mediated by the RNaseIII-family enzyme DICER-LIKE 1 (DCL1), double-stranded RNA-binding protein HYPONASTIC LEAVES 1 and the zinc-finger (ZnF) domain-containing protein SERRATE (SE). In the present study, we examined primary miRNA precursor (pri-miRNA) processing by highly purified recombinant DCL1 and SE proteins and found that SE is integral to pri-miRNA processing by DCL1. SE stimulates DCL1 cleavage of the pri-miRNA in an ionic strength-dependent manner. SE uses its N-terminal domain to bind to RNA and requires both N-terminal and ZnF domains to bind to DCL1. However, when DCL1 is bound to RNA, the interaction with the ZnF domain of SE becomes indispensible and stimulates the activity of DCL1 without requiring SE binding to RNA. Our results suggest that the interactions among SE, DCL1 and RNA are a potential point for regulating pri-miRNA processing.

Antagonistic and stimulative roles of ADAR1 in RNA silencing

Adenosine deaminases acting on RNA (ADARs) are involved in RNA editing that converts adenosine residues to inosine specifically in double-stranded RNAs (dsRNA). This A-to-I RNA editing pathway and the RNA interference (RNAi) pathway seem to interact antagonistically by competing for their common dsRNA substrates. For instance, A-to-I editing of certain microRNA (miRNA) precursors by ADAR1 and ADAR2 inhibits their processing to mature miRNAs. Recent studies unexpectedly revealed the presence of a completely different type of interaction between the RNA editing mechanism and the RNAi machinery. ADAR1 forms a complex via direct protein-protein interaction with Dicer, an RNase III gene family member involved in the RNAi mechanism. ADAR1 in the Dicer complex promotes pre-miRNA cleavage by Dicer and facilitates loading of miRNA onto RNA-induced silencing complexes, giving rise to an unsuspected stimulative function of ADAR1 on miRNA processing and RNAi mechanisms. ADAR1 differentiates its functions in RNA editing and RNAi by formation of either ADAR1-ADAR1 homodimer or Dicer-ADAR1 heterodimer complexes. Expression of miRNAs is globally inhibited in ADAR1-null mouse embryos, which, in turn, alters expression of their target genes and may contribute to their embryonic lethal phenotype.

Substrate-specific structural rearrangements of human Dicer

Dicer plays a central role in RNA interference pathways by cleaving double-stranded RNAs (dsRNAs) to produce small regulatory RNAs. Human Dicer can process long double-stranded and hairpin precursor RNAs to yield short interfering RNAs (siRNAs) or microRNAs (miRNAs), respectively. Previous studies have shown that pre-miRNAs are cleaved more rapidly than pre-siRNAs in vitro and are the predominant natural Dicer substrates. We have used electron microscopy and single particle analysis of Dicer–RNA complexes to gain insight into the structural basis for human Dicer’s substrate preference. Our studies show that Dicer traps pre-siRNAs in a non-productive conformation, while interactions of Dicer with pre-miRNAs and dsRNA binding proteins induce structural changes in the enzyme that enable productive substrate recognition in the central catalytic channel. These findings implicate RNA structure and cofactors in determining substrate recognition and processing efficiency by human Dicer.

The loop position of shRNAs and pre-miRNAs is critical for the accuracy of dicer processing in vivo

Short hairpin RNA (shRNA)-induced RNAi is used for biological discovery and therapeutics. Dicer, whose normal role is to liberate endogenous miRNAs from their precursors, processes shRNAs into different biologically active siRNAs, affecting their efficacy and potential for off-targeting. We found that, in cells, Dicer induced imprecise cleavage events around the expected sites based on the previously described 5'/3' counting rules. These promiscuous noncanonical cleavages were abrogated when the cleavage site was positioned 2 nt from a bulge or loop. Interestingly, we observed that the ~1/3 of mammalian endogenous pre-miRNAs that contained such structures were more precisely processed by Dicer. Implementing a "loop-counting rule," we designed potent anti-HCV shRNAs with substantially reduced off-target effects. Our results suggest that Dicer recognizes the loop/bulge structure in addition to the ends of shRNAs/pre-miRNAs for accurate processing. This has important implications for both miRNA processing and future design of shRNAs for RNAi-based genetic screens and therapies.

Molecular mechanisms of RNA interference

Small RNA molecules regulate eukaryotic gene expression during development and in response to stresses including viral infection. Specialized ribonucleases and RNA-binding proteins govern the production and action of small regulatory RNAs. After initial processing in the nucleus by Drosha, precursor microRNAs (pre-miRNAs) are transported to the cytoplasm, where Dicer cleavage generates mature microRNAs (miRNAs) and short interfering RNAs (siRNAs). These double-stranded products assemble with Argonaute proteins such that one strand is preferentially selected and used to guide sequence-specific silencing of complementary target mRNAs by endonucleolytic cleavage or translational repression. Molecular structures of Dicer and Argonaute proteins, and of RNA-bound complexes, have offered exciting insights into the mechanisms operating at the heart of RNA-silencing pathways.

Biology and Mechanisms of Short RNAs in Caenorhabditis elegans

The significance of noncoding RNAs in animal biology is being increasingly recognized. The nematode Caenorhabditis elegans has an extensive system of short RNAs that includes microRNAs, piRNAs, and endogenous siRNAs, which regulate development, control life span, provide resistance to viruses and transposons, and monitor gene duplications. Progress in our understanding of short RNAs was stimulated by the discovery of RNA interference, a phenomenon of sequence-specific gene silencing induced by exogenous double-stranded RNA, at the turn of the twenty-first century. This chapter provides a broad overview of the exogenous and endogenous RNAi processes in C. elegans and describes recent advances in genetic, genomic, and molecular analyses of nematode's short RNAs and proteins involved in the RNAi-related pathways.

The role of human Dicer-dsRBD in processing small regulatory RNAs

One of the most exciting recent developments in RNA biology has been the discovery of small non-coding RNAs that affect gene expression through the RNA interference (RNAi) mechanism. Two major classes of RNAs involved in RNAi are small interfering RNA (siRNA) and microRNA (miRNA). Dicer, an RNase III enzyme, plays a central role in the RNAi pathway by cleaving precursors of both of these classes of RNAs to form mature siRNAs and miRNAs, which are then loaded into the RNA-induced silencing complex (RISC). miRNA and siRNA precursors are quite structurally distinct; miRNA precursors are short, imperfect hairpins while siRNA precursors are long, perfect duplexes. Nonetheless, Dicer is able to process both. Dicer, like the majority of RNase III enzymes, contains a dsRNA binding domain (dsRBD), but the data are sparse on the exact role this domain plays in the mechanism of Dicer binding and cleavage. To further explore the role of human Dicer-dsRBD in the RNAi pathway, we determined its binding affinity to various RNAs modeling both miRNA and siRNA precursors. Our study shows that Dicer-dsRBD is an avid binder of dsRNA, but its binding is only minimally influenced by a single-stranded – double-stranded junction caused by large terminal loops observed in miRNA precursors. Thus, the Dicer-dsRBD contributes directly to substrate binding but not to the mechanism of differentiating between pre-miRNA and pre-siRNA. In addition, NMR spin relaxation and MD simulations provide an overview of the role that dynamics contribute to the binding mechanism. We compare this current study with our previous studies of the dsRBDs from Drosha and DGCR8 to give a dynamic profile of dsRBDs in their apo-state and a mechanistic view of dsRNA binding by dsRBDs in general.

Duplex RNA activated ATPases (DRAs): platforms for RNA sensing, signaling and processing

Double-stranded RNAs are an important class of functional macromolecules in living systems. They are usually found as part of highly specialized intracellular machines that control diverse cellular events, ranging from virus replication, antiviral defense, RNA interference, to regulation of gene activities and genomic integrity. Within different intracellular machines, the RNA duplex is often found in association with specific RNA-dependent ATPases, including Dicer, RIG-I and DRH-3 proteins. These duplex RNA-activated ATPases represent an emerging group of motor proteins within the large and diverse super family 2 nucleic acid-dependent ATPases (which are historically defined as SF2 helicases). The duplex RNA-activated ATPases share characteristic molecular features for duplex RNA recognition, including motifs (e.g., motifs IIa and Vc) and an insertion domain (HEL2i), and they require double-strand RNA binding for their enzymatic activities. Proteins in this family undergo large conformational changes concomitant with RNA binding, ATP binding and ATP hydrolysis in order to achieve their functions, which include the release of signaling domains and the recruitment of partner proteins. The duplex RNA-activated ATPases represent a distinct and fascinating group of nanomechanical molecular motors that are essential for duplex RNA sensing and processing in diverse cellular pathways.

Putative SF2 helicases of the early-branching eukaryote Giardia lamblia are involved in antigenic variation and parasite differentiation into cysts

Background

Regulation of surface antigenic variation in Giardia lamblia is controlled post-transcriptionally by an RNA-interference (RNAi) pathway that includes a Dicer-like bidentate RNase III (gDicer). This enzyme, however, lacks the RNA helicase domain present in Dicer enzymes from higher eukaryotes. The participation of several RNA helicases in practically all organisms in which RNAi was studied suggests that RNA helicases are potentially involved in antigenic variation, as well as during Giardia differentiation into cysts.

Results

An extensive in silico analysis of the Giardia genome identified 32 putative Super Family 2 RNA helicases that contain almost all the conserved RNA helicase motifs. Phylogenetic studies and sequence analysis separated them into 22 DEAD-box, 6 DEAH-box and 4 Ski2p-box RNA helicases, some of which are homologs of well-characterized helicases from higher organisms. No Giardia putative helicase was found to have significant homology to the RNA helicase domain of Dicer enzymes. Additionally a series of up- and down-regulated putative RNA helicases were found during encystation and antigenic variation by qPCR experiments. Finally, we were able to recognize 14 additional putative helicases from three different families (RecQ family, Swi2/Snf2 and Rad3 family) that could be considered DNA helicases.

Conclusions

This is the first comprehensive analysis of the Super Family 2 helicases from the human intestinal parasite G. lamblia. The relative and variable expression of particular RNA helicases during both antigenic variation and encystation agrees with the proposed participation of these enzymes during both adaptive processes. The putatives RNA and DNA helicases identified in this early-branching eukaryote provide initial information regarding the biological role of these enzymes in cell adaptation and differentiation.

Evolution of animal and plant dicers: early parallel duplications and recurrent adaptation of antiviral RNA binding in plants

RNA interference (RNAi) is a eukaryotic molecular system that serves two primary functions: 1) gene regulation and 2) protection against selfish elements such as viruses and transposable DNA. Although the biochemistry of RNAi has been detailed in model organisms, very little is known about the broad-scale patterns and forces that have shaped RNAi evolution. Here, we provide a comprehensive evolutionary analysis of the Dicer protein family, which carries out the initial RNA recognition and processing steps in the RNAi pathway. We show that Dicer genes duplicated and diversified independently in early animal and plant evolution, coincident with the origins of multicellularity. We identify a strong signature of long-term protein-coding adaptation that has continually reshaped the RNA-binding pocket of the plant Dicer responsible for antiviral immunity, suggesting an evolutionary arms race with viral factors. We also identify key changes in Dicer domain architecture and sequence leading to specialization in either gene-regulatory or protective functions in animal and plant paralogs. As a whole, these results reveal a dynamic picture in which the evolution of Dicer function has driven elaboration of parallel RNAi functional pathways in animals and plants.

A comprehensive analysis of precursor microRNA cleavage by human Dicer

Dicer cleaves double-stranded RNAs (dsRNAs) or precursor microRNAs (pre-miRNAs) to yield ≈ 22-nt RNA duplexes. The pre-miRNA structure requirement for human Dicer activity is incompletely understood. By large-scale in vitro dicing assays and mutagenesis studies, we showed that human Dicer cleaves most, although not all, of the 161 tested human pre-miRNAs efficiently. The stable association of RNAs with Dicer, as examined by gel shift assays, appears important but is not sufficient for cleavage. Human Dicer tolerates remarkable structural variation in its pre-miRNA substrates, although the dsRNA feature in the stem region and the 2-nt 3'-overhang structure in a pre-miRNA contribute to its binding and cleavage by Dicer, and a large terminal loop further enhances pre-miRNA cleavage. Dicer binding protects the terminal loop from digestion by S1 nuclease, suggesting that Dicer interacts directly with the terminal loop region.

Rarely at rest: RNA helicases and their busy contributions to RNA degradation, regulation and quality control

RNA helicases are compact, machine-like proteins that can harness the energy of nucleoside triphosphate binding and hydrolysis to dynamically remodel RNA structures and protein-RNA complexes. Through such activities, helicases participate in virtually every process associated with the expression of genetic information. Often found as components of multi-enzyme assemblies, RNA helicases facilitate the processivity of RNA degradation, the remodeling of protein interactions during maturation of structured RNA precursors, and fidelity checks of RNA quality. In turn, the assemblies modulate and guide the activities of the helicases. We describe the roles of RNA helicases with a conserved "DExD/H box" sequence motif in representative examples of such machineries from bacteria, archaea and eukaryotes. The recurrent occurrence of such helicases in complex assemblies throughout the course of evolution suggests a common requirement for their activities to meet cellular demands for the dynamic control of RNA metabolism.

Biogenesis and mechanism of action of small non-coding RNAs: insights from the point of view of structural biology

Non-coding RNAs are dominant in the genomic output of the higher organisms being not simply occasional transcripts with idiosyncratic functions, but constituting an extensive regulatory network. Among all the species of non-coding RNAs, small non-coding RNAs (miRNAs, siRNAs and piRNAs) have been shown to be in the core of the regulatory machinery of all the genomic output in eukaryotic cells. Small non-coding RNAs are produced by several pathways containing specialized enzymes that process RNA transcripts. The mechanism of action of these molecules is also ensured by a group of effector proteins that are commonly engaged within high molecular weight protein-RNA complexes. In the last decade, the contribution of structural biology has been essential to the dissection of the molecular mechanisms involved in the biosynthesis and function of small non-coding RNAs.

Effects of ADARs on small RNA processing pathways in C. elegans

Adenosine deaminases that act on RNA (ADARs) are RNA editing enzymes that convert adenosine to inosine in double-stranded RNA (dsRNA). To evaluate effects of ADARs on small RNAs that derive from dsRNA precursors, we performed deep-sequencing, comparing small RNAs from wild-type and ADAR mutant Caenorhabditis elegans. While editing in small RNAs was rare, at least 40% of microRNAs had altered levels in at least one ADAR mutant strain, and miRNAs with significantly altered levels had mRNA targets with correspondingly affected levels. About 40% of siRNAs derived from endogenous genes (endo-siRNAs) also had altered levels in at least one mutant strain, including 63% of Dicer-dependent endo-siRNAs. The 26G class of endo-siRNAs was significantly affected by ADARs, and many altered 26G loci had intronic reads and histone modifications associated with transcriptional silencing. Our data indicate that ADARs, through both direct and indirect mechanisms, are important for maintaining wild-type levels of many small RNAs in C. elegans.

Coordinated activities of human dicer domains in regulatory RNA processing

The conserved ribonuclease Dicer generates microRNAs and short-interfering RNAs that guide gene silencing in eukaryotes. The specific contributions of human Dicer's structural domains to RNA product length and substrate preference are incompletely understood, due in part to the difficulties of Dicer purification. Here, we show that active forms of human Dicer can be assembled from recombinant polypeptides expressed in bacteria. Using this system, we find that three distinct modes of RNA recognition give rise to Dicer's fidelity and product length specificity. The first involves anchoring one end of a double-stranded RNA helix within the PAZ domain, which can assemble in trans with Dicer's catalytic domains to reconstitute an accurate but non-substrate-selective dicing activity. The second entails nonspecific RNA binding by the double-stranded RNA binding domain, an interaction that is essential for substrate recruitment in the absence of the PAZ domain. The third mode of recognition involves hairpin RNA loop recognition by the helicase domain, which ensures efficient processing of specific substrates. These results reveal distinct interactions of each Dicer domain with different RNA structural features and provide a facile system for investigating the molecular mechanisms of human microRNA biogenesis.

The helicase and RNaseIIIa domains of Arabidopsis Dicer-Like1 modulate catalytic parameters during microRNA biogenesis

Dicer-Like1 (DCL1), an RNaseIII endonuclease, and Hyponastic Leaves1 (HYL1), a double-stranded RNA-binding protein, are core components of the plant microRNA (miRNA) biogenesis machinery. hyl1 null mutants accumulate low levels of miRNAs and display pleiotropic developmental phenotypes. We report the identification of five new hyl1 suppressor mutants, all of which are alleles of DCL1. These new alleles affect either the helicase or the RNaseIIIa domains of DCL1, highlighting the critical functions of these domains. Biochemical analysis of the DCL1 suppressor variants reveals that they process the primary transcript (pri-miRNA) more efficiently than wild-type DCL1, with both higher K(cat) and lower K(m) values. The DCL1 variants largely rescue wild-type miRNA accumulation levels in vivo, but do not rescue the MIRNA processing precision defects of the hyl1 null mutant. In vitro, the helicase domain confers ATP dependence on DCL1-catalyzed MIRNA processing, attenuates DCL1 cleavage activity, and is required for precise MIRNA processing of some substrates.