RDE-4 preferentially binds long dsRNA and its dimerization is necessary for cleavage of dsRNA to siRNA

In organisms ranging from Arabidopsis to humans, Dicer requires dsRNA-binding proteins (dsRBPs) to carry out its roles in RNA interference (RNAi) and micro-RNA (miRNA) processing. In Caenorhabditis elegans, the dsRBP RDE-4 acts with Dicer during the initiation of RNAi, when long dsRNA is cleaved to small interfering RNAs (siRNAs). RDE-4 is not required in subsequent steps, and how RDE-4 distinguishes between long dsRNA and short siRNA is unclear. We report the first detailed analysis of RDE-4 binding, using purified recombinant RDE-4 and various truncated proteins. We find that, similar to other dsRBPs, RDE-4 is not sequence-specific. However, consistent with its in vivo roles, RDE-4 binds with higher affinity to long dsRNA. We also observe that RDE-4 is a homodimer in solution, and that the C-terminal domain of the protein is required for dimerization. Using extracts from wild-type and rde-4 mutant C. elegans, we show that the C-terminal dimerization domain is required for the production of siRNA. Our findings suggest a model for RDE-4 function during the initiation of RNAi.     

Differential roles of human Dicer-binding proteins TRBP and PACT in small RNA processing

During RNA interference and related gene regulatory pathways, the endonuclease Dicer cleaves precursor RNA molecules to produce microRNAs (miRNAs) and short interfering RNAs (siRNAs). Human cells encode a single Dicer enzyme that can associate with two different double-stranded RNA (dsRNA)-binding proteins, protein activator of PKR (PACT) and trans-activation response RNA-binding protein (TRBP). However, the functional redundancy or differentiation of PACT and TRBP in miRNA and siRNA biogenesis is not well understood. Using a reconstituted system, we show here that PACT and TRBP have distinct effects on Dicer-mediated dsRNA processing. In particular, we found that PACT in complex with Dicer inhibits the processing of pre-siRNA substrates when compared with Dicer and a Dicer–TRBP complex. In addition, PACT and TRBP show non-redundant effects on the production of different-sized miRNAs (isomiRs), which in turn alter target-binding specificities. Experiments using chimeric versions of PACT and TRBP suggest that the two N-terminal RNA-binding domains of each protein confer the observed differences in dsRNA substrate recognition and processing behavior of Dicer–dsRNA-binding protein complexes. These results support the conclusion that in humans, Dicer-associated dsRNA-binding proteins are important regulatory factors that contribute both substrate and cleavage specificity during miRNA and siRNA production.     

Specific requirement of DRB4, a dsRNA-binding protein, for the in vitro dsRNA-cleaving activity of Arabidopsis Dicer-like 4

Arabidopsis thaliana Dicer-like 4 (DCL4) produces 21-nt small interfering RNAs from both endogenous and exogenous double-stranded RNAs (dsRNAs), and it interacts with DRB4, a dsRNA-binding protein, in vivo and in vitro. However, the role of DRB4 in DCL4 activity remains unclear because the dsRNA-cleaving activity of DCL4 has not been characterized biochemically. In this study, we biochemically characterize DCL4's Dicer activity and establish that DRB4 is required for this activity in vitro. Crude extracts from Arabidopsis seedlings cleave long dsRNAs into 21-nt small RNAs in a DCL4/DRB4-dependent manner. Immunoaffinity-purified DCL4 complexes produce 21-nt small RNAs from long dsRNA, and these complexes have biochemical properties similar to those of known Dicer family proteins. The DCL4 complexes purified from drb4-1 do not cleave dsRNA, and the addition of recombinant DRB4 to drb4-1 complexes specifically recovers the 21-nt small RNA generation. These results reveal that DCL4 requires DRB4 to cleave long dsRNA into 21-nt small RNAs in vitro. Amino acid substitutions in conserved dsRNA-binding domains (dsRBDs) of DRB4 impair three activities: binding to dsRNA, interacting with DCL4, and facilitating DCL4 activity. These observations indicate that the dsRBDs are critical for DRB4 function. Our biochemical approach and observations clearly show that DRB4 is specifically required for DCL4 activity in vitro.     

Structures of the first and second double-stranded RNA-binding domains of human TAR RNA-binding protein

The TAR RNA-binding Protein (TRBP) is a double-stranded RNA (dsRNA)-binding protein, which binds to Dicer and is required for the RNA interference pathway. TRBP consists of three dsRNA-binding domains (dsRBDs). The first and second dsRBDs (dsRBD1 and dsRBD2, respectively) have affinities for dsRNA, whereas the third dsRBD (dsRBD3) binds to Dicer. In this study, we prepared the single domain fragments of human TRBP corresponding to dsRBD1 and dsRBD2 and solved the crystal structure of dsRBD1 and the solution structure of dsRBD2. The two structures contain an α-β-β-β-α fold, which is common to the dsRBDs. The overall structures of dsRBD1 and dsRBD2 are similar to each other, except for a slight shift of the first α helix. The residues involved in dsRNA binding are conserved. We examined the small interfering RNA (siRNA)-binding properties of these dsRBDs by isothermal titration colorimetry measurements. The dsRBD1 and dsRBD2 fragments both bound to siRNA, with dissociation constants of 220 and 113 nM, respectively. In contrast, the full-length TRBP and its fragment with dsRBD1 and dsRBD2 exhibited much smaller dissociation constants (0.24 and 0.25 nM, respectively), indicating that the tandem dsRBDs bind simultaneously to one siRNA molecule. On the other hand, the loop between the first α helix and the first β strand of dsRBD2, but not dsRBD1, has a Trp residue, which forms hydrophobic and cation-π interactions with the surrounding residues. A circular dichroism analysis revealed that the thermal stability of dsRBD2 is higher than that of dsRBD1 and depends on the Trp residue.     

Substrate-specific kinetics of Dicer-catalyzed RNA processing

The specialized ribonuclease Dicer plays a central role in eukaryotic gene expression by producing small regulatory RNAs—microRNAs (miRNAs) and short interfering RNAs (siRNAs)—from larger double-stranded RNA (dsRNA) substrates. Although Dicer will cleave both imperfectly base-paired hairpin structures (pre-miRNAs) and perfect duplexes (pre-siRNAs) in vitro, it has not been clear whether these are mechanistically equivalent substrates and how dsRNA binding proteins such as trans-activation response (TAR) RNA binding protein (TRBP) influence substrate selection and RNA processing efficiency. We show here that human Dicer is much faster at processing a pre-miRNA substrate compared to a pre-siRNA substrate under both single and multiple turnover conditions. Maximal cleavage rates (V_max) calculated by Michaelis-Menten analysis differed by more than 100-fold under multiple turnover conditions. TRBP was found to enhance dicing of both substrates to similar extents, and this stimulation required the two N-terminal dsRNA binding domains of TRBP. These results demonstrate that multiple factors influence dicing kinetics. While TRBP stimulates dicing by enhancing the stability of Dicer-substrate complexes, Dicer itself generates product RNAs at rates determined at least in part by the structural properties of the substrate.     

Size-independent and noncooperative recognition of dsRNA by the Rice stripe virus RNA silencing suppressor NS3

Plant and animal viruses employ diverse suppressor proteins to thwart the host antiviral reaction of RNA silencing. Many suppressors bind dsRNA with different size specificity. Here, we examine the dsRNA recognition mechanism of the Rice stripe virus NS3 suppressor using quantitative biochemical approaches, as well as mutagenesis and suppression activity analyses in plants. We show that dimeric NS3 is a size-independent, rather than small interfering RNA-specific, dsRNA-binding protein that recognizes a minimum of 9 bp and can bind to long dsRNA with two or more copies. Global analysis using a combinatorial approach reveals that NS3 dimer has an occluded site size of ∼ 13 bp on dsRNA, an intrinsic binding constant of 1 × 10⁸ M^− 1, and virtually no binding cooperativity. This lack of cooperativity suggests that NS3 is not geared to target long dsRNA. The larger site size of NS3, compared with its interacting size, indicates that the NS3 structure has a border region that has no direct contact with dsRNA but occludes a ∼ 4-bp region from binding. We also develop a method to correct the border effect of ligand by extending the lattice length. In addition, we find that NS3 recognizes the helical structure and 2′-hydroxyl group of dsRNA with moderate specificity. Analysis of dsRNA-binding mutants suggests that silencing of the suppression activity of NS3 is mechanistically related to its dsRNA binding ability.     

Specific interactions between Dicer-like proteins and HYL1/DRB- family dsRNA-binding proteins in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>

Proteins that specifically bind double-stranded RNA (dsRNA) are involved in the regulation of cellular signaling events and gene expression, and are characterized by a conserved dsRNA-binding motif (dsRBM). Here we report the biochemical properties of nine such gene products, each containing one or two dsRBMs: four ArabidopsisDicer-like proteins (DCL1-4), ArabidopsisHYL1 and four of its homologs (DRB2, DRB4, DRB5 and OsDRB1). DCL1, DCL3, HYL1 and the four HYL1 homologs exhibit significant dsRNA-binding activity, indicating that these proteins are involved in RNA metabolism. The dsRBMs from dsRBM-containing proteins (dsRBPs) also function as a protein–protein interaction domain and homo- and heterodimerization are essential for biological functioning of these proteins. We show that DRB4 interacts specifically with DCL4, and HYL1 most strongly interacts with DCL1. These results indicate that each HYL1/DRB family protein interacts with one specific partner among the four Dicer-like proteins. Localization studies using GFP fusion proteins demonstrate that DCL1, DCL4, HYL1 and DRB4 localize in the nucleus, while DRB2 is present in the cytoplasm. Subcellular localizations of HYL1, DRB4, DCL1 and DCL4 further strengthen the notion that HYL1 and DCL1, and DRB4 and DCL4, exist as complexes. The presented data suggest that each member of the HYL1/DRB protein family may individually modulate Dicer function through heterodimerization with a Dicer-like protein in vivo.     

In C. elegans,High Levels of dsRNA Allow RNAi in the Absence of RDE-4

C. elegans Dicer requires an accessory double-stranded RNA binding protein, RDE-4, to enact the first step of RNA interference, the cleavage of dsRNA to produce siRNA. While RDE-4 is typically essential for RNAi, we report that in the presence of high concentrations of trigger dsRNA, rde-4 deficient animals are capable of silencing a transgene. By multiple criteria the silencing occurs by the canonical RNAi pathway. For example, silencing is RDE-1 dependent and exhibits a decrease in the targeted mRNA in response to an increase in siRNA. We also find that high concentrations of dsRNA trigger lead to increased accumulation of primary siRNAs, consistent with the existence of a rate-limiting step during the conversion of primary to secondary siRNAs. Our studies also revealed that transgene silencing occurs at low levels in the soma, even in the presence of ADARs, and that at least some siRNAs accumulate in a temperature-dependent manner. We conclude that an RNAi response varies with different conditions, and this may allow an organism to tailor a response to specific environmental signals.     

Antiviral RNA Silencing Initiated in the Absence of RDE-4, a Double-Stranded RNA Binding Protein,in Caenorhabditis elegans

Small interfering RNAs (siRNAs) processed from double-stranded RNA (dsRNA) of virus origins mediate potent antiviral defense through a process referred to as RNA interference (RNAi) or RNA silencing in diverse organisms. In the simple invertebrate Caenorhabditis elegans, the RNAi process is initiated by a single Dicer, which partners with the dsRNA binding protein RDE-4 to process dsRNA into viral siRNAs (viRNAs). Notably, in C. elegans this RNA-directed viral immunity (RDVI) also requires a number of worm-specific genes for its full antiviral potential. One such gene is rsd-2 (RNAi spreading defective 2), which was implicated in RDVI in our previous studies. In the current study, we first established an antiviral role by showing that rsd-2 null mutants permitted higher levels of viral RNA accumulation, and that this enhanced viral susceptibility was reversed by ectopic expression of RSD-2. We then examined the relationship of rsd-2 with other known components of RNAi pathways and established that rsd-2 functions in a novel pathway that is independent of rde-4 but likely requires the RNA-dependent RNA polymerase RRF-1, suggesting a critical role for RSD-2 in secondary viRNA biogenesis, likely through coordinated action with RRF-1. Together, these results suggest that RDVI in the single-Dicer organism C. elegans depends on the collective actions of both RDE-4-dependent and RDE-4-independent mechanisms to produce RNAi-inducing viRNAs. Our study reveals, for the first time, a novel siRNA-producing mechanism in C. elegans that bypasses the need for a dsRNA-binding protein.     

Structural basis of siRNA recognition by TRBP double‐stranded RNA binding domains

The accurate cleavage of pre‐micro(mi)RNAs by Dicer and mi/siRNA guide strand selection are important steps in forming the RNA‐induced silencing complex (RISC). The role of Dicer binding partner TRBP in these processes remains poorly understood. Here, we solved the solution structure of the two N‐terminal dsRNA binding domains (dsRBDs) of TRBP in complex with a functionally asymmetric siRNA using NMR, EPR, and single‐molecule spectroscopy. We find that siRNA recognition by the dsRBDs is not sequence‐specific but rather depends on the RNA shape. The two dsRBDs can swap their binding sites, giving rise to two equally populated, pseudo‐symmetrical complexes, showing that TRBP is not a primary sensor of siRNA asymmetry. Using our structure to model a Dicer‐TRBP‐siRNA ternary complex, we show that TRBP's dsRBDs and Dicer's RNase III domains bind a canonical 19 base pair siRNA on opposite sides, supporting a mechanism whereby TRBP influences Dicer‐mediated cleavage accuracy by binding the dsRNA region of the pre‐miRNA during Dicer cleavage.     

Backbone and sidechain methyl Ile (δ1), Leu and Val chemical shift assignments of RDE-4 (1–243), an RNA interference initiation protein in C. elegans

The RNAi pathway of several organisms requires presence of double stranded RNA binding proteins for functioning of Dicer in gene regulation. In C. elegans, a double stranded RNA binding protein, RDE-4 (385 aa, 44 kDa) recognizes long exogenous dsRNA and initiates the RNAi pathway. We have achieved complete backbone and stereospecific methyl sidechain Ile (δ1), Leu and Val chemical shifts of first 243 amino acids of RDE-4, namely RDE-4ΔC.     

siRNA repositioning for guide strand selection by human Dicer complexes

The human ribonuclease Dicer and its double-stranded RNA (dsRNA)-binding protein (dsRBP) partners TRBP and PACT play important roles in the biogenesis of regulatory RNAs. Following dicing, one dsRNA product strand is preferentially assembled into an RNA-induced silencing complex (RISC). The mechanism of strand selection in humans and the possible role of Dicer in this process remain unclear. Here we demonstrate that dsRNAs undergo significant repositioning within Dicer complexes following dicing. This repositioning enables directional binding of RNA duplexes, thereby biasing their orientation for guide strand selection according to the thermodynamic properties of the helix. Our findings indicate that Dicer is itself capable of sensing siRNA thermodynamic asymmetry regardless of the dsRBP to which it is bound. These results support a model in which Dicer employs two distinct RNA-binding sites-one for dsRNA processing and the other for sensing of siRNA thermodynamic asymmetry-during RISC loading in humans.     

RDE-2 interacts with MUT-7 to mediate RNA interference in Caenorhabditis elegans

In Caenorhabditis elegans, the activity of transposable elements is repressed in the germline. One of the mechanisms involved in this repression is RNA interference (RNAi), a process in which dsRNA targets cleavage of mRNAs in a sequence-specific manner. The first gene found to be involved in RNAi and transposon silencing in C.elegans is mut-7, a gene encoding a putative exoribonuclease. Here, we show that the MUT-7 protein resides in complexes of ~250 kDa in the nucleus and in the cytosol. In addition, we find that upon triggering of RNAi the cytosolic MUT-7 complex increases in size. This increase is independent of the presence of target RNA, but does depend on the presence of RDE-1 and RDE-4, two proteins involved in small interfering RNA (siRNA) production. Finally, using a yeast two-hybrid screen, we identified RDE-2/MUT-8 as one of the other components of this complex. This protein is encoded by the rde-2/mut-8 locus, previously implicated in RNAi and transposon silencing. Using genetic complementation analysis, we show that the interaction between these two proteins is required for efficient RNAi in vivo. Together these data support a role for the MUT-7/RDE-2 complex downstream of siRNA formation, but upstream of siRNA mediated target RNA recognition, possibly indicating a role in the siRNA amplification step.     

The role of PACT in the RNA silencing pathway

Small RNA-mediated gene silencing (RNA silencing) has emerged as a major regulatory pathway in eukaryotes. Identification of the key factors involved in this pathway has been a subject of rigorous investigation in recent years. In humans, small RNAs are generated by Dicer and assembled into the effector complex known as RNA-induced silencing complex (RISC) by multiple factors including hAgo2, the mRNA-targeting endonuclease, and TRBP (HIV-1 TAR RNA-binding protein), a dsRNA-binding protein that interacts with both Dicer and hAgo2. Here we describe an additional dsRNA-binding protein known as PACT, which is significant in RNA silencing. PACT is associated with an approximately 500 kDa complex that contains Dicer, hAgo2, and TRBP. The interaction with Dicer involves the third dsRNA-binding domain (dsRBD) of PACT and the N-terminal region of Dicer containing the helicase motif. Like TRBP, PACT is not required for the pre-microRNA (miRNA) cleavage reaction step. However, the depletion of PACT strongly affects the accumulation of mature miRNA in vivo and moderately reduces the efficiency of small interfering RNA-induced RNA interference. Our study indicates that, unlike other RNase III type proteins, human Dicer may employ two different dsRBD-containing proteins that facilitate RISC assembly.     

Structural determinants of RNA recognition and cleavage by Dicer

A hallmark of RNA interference is the production of short double-stranded RNA (dsRNA) molecules 21-28 nucleotides in length by the specialized RNase III protein Dicer. Dicer enzymes uniquely generate RNA products of specific lengths by mechanisms that have not been fully elucidated. Here we show that the PAZ domain responsible for dsRNA end recognition confers this measuring ability through both its structural position and RNA-binding specificity. Point mutations define the dsRNA-binding surface and reveal a protein loop important for cleavage of substrates containing perfect or imperfect base pairing. On the basis of these results, we reengineered Dicer with a U1A RNA-binding domain in place of the PAZ domain to create an enzyme with altered end-recognition specificity and RNA product length. These results explain how Dicer functions as a molecular ruler and provide a structural basis for modifying its activity in cells.     

Characterization of the TRBP domain required for Dicer interaction and function in RNA interference

Background  

Dicer, Ago2 and TRBP are the minimum components of the human RNA-induced silencing complex (RISC). While Dicer and Ago2 are RNases, TRBP is the double-stranded RNA binding protein (dsRBP) that loads small interfering RNA into the RISC. TRBP binds directly to Dicer through its C-terminal domain.