Small RNAs such as small interfering RNAs (siRNAs) and microRNAs (miRNAs) silence the expression of their complementary target messenger RNAs1,2 via the formation of effector RNA-induced silencing complexes (RISCs), which contain Argonaute (Ago) family proteins at their core. Although loading of siRNA duplexes into Drosophila Ago2 requires the Dicer-2–R2D2 heterodimer3,4,5 and the Hsc70/Hsp90 (Hsp90 also known as Hsp83) chaperone machinery6,7,8, the details of RISC assembly remain unclear. Here we reconstitute RISC assembly using only Ago2, Dicer-2, R2D2, Hsc70, Hsp90, Hop, Droj2 (an Hsp40 homologue) and p23. By following the assembly of single RISC molecules, we find that, in the absence of the chaperone machinery, an siRNA bound to Dicer-2–R2D2 associates with Ago2 only transiently. The chaperone machinery extends the dwell time of the Dicer-2–R2D2–siRNA complex on Ago2, in a manner dependent on recognition of the 5′-phosphate on the siRNA guide strand. We propose that the chaperone machinery supports a productive state of Ago2, allowing it to load siRNA duplexes from Dicer-2–R2D2 and thereby assemble RISC.
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We thank Q. Liu for providing vectors for Dicer-2 and R2D2 expression, M. Horwich and P. D. Zamore for Ago2 cDNA, and H. Siomi and M. C. Siomi for anti-Ago2 antibody and ago2414 flies. We are also grateful to P. B. Kwak and A. Tsutsumi for assistance with plasmid constructions, S. Katsuma for support for Dicer-2–R2D2 preparation, and A. Yamashita for advice on protein purification. We thank H. Taguchi and T. Ueda for fruitful discussions and P. D. Zamore, H. Seitz and the members of the Tomari laboratory for critical comments on the manuscript. This work was supported in part by Grants-in-Aid for Scientific Research on Innovative Areas ('Functional machinery for non-coding RNAs' and 'Non-coding RNA neo-taxonomy'), a Grant-in-Aid for Young Scientists (A) (to H.T. and Y.T.), a Grant-in-Aid for Research Activity start-up (to S.I.), and Grants-in-Aid for challenging Exploratory Research (to H.M.S.) from The Ministry of Education, Culture, Sports, Science and Technology in Japan.
The authors declare no competing financial interests.
Extended data figures and tables
a, CBB staining of recombinant proteins used in this study. b, Small RNA pull-down assay using immunopurified wild type (WT), catalytic mutant (Cmut; D965A) and 5′ nucleotide-binding pocket mutant (Pmut; Y897E) of Ago2. Ago2 alone (Buffer) could not load siRNA, but supplementation of lysate from ago2414 mutant flies directed efficient formation of pre- and mature Ago2–RISC. The catalytic mutant Ago2 predominantly formed pre-Ago2–RISC, while the 5′ nucleotide-binding pocket mutant failed to load siRNA. c, Western blotting of Ago2 and tubulin in lysate from S2 cells, wild-type embryo and ago2414 mutant embryo. d, Anti-Flag western blotting of wild-type and mutant Ago2 proteins in lysate. All the constructs are expressed at similar levels. e, Hsp70 inhibitor (PES) and p23 inhibitor (Celastrol) block duplex loading into Ago2.
a, Quantification of Fig. 1a. b, Quantification of Fig. 1b. c, Time course of target cleavage in the reconstituted system. Figure 1c represents the data at 30 min. d, Activities of the reconstituted system and ago2414 mutant lysate. e, Cleavage assay in a multiple turnover condition by Ago2–RISCs assembled in the reconstituted system or ago2414 mutant embryo lysate (d). ago2414 lysate was diluted to normalize the amount of the assembled Ago2–RISC. f, Estimated concentrations of mature single-strand-containing Ago2–RISCs by the pull-down assay (d) and the pre-steady-state 'burst' kinetics of target cleavage (e). Owing to a technical limitation in the quantification of small amounts of radiolabelled RNAs, the concentrations of RISCs estimated by the burst kinetics were higher than those estimated by the pull-down assay. However, the ratios between them were essentially identical between RISCs assembled in the reconstituted system and ago2414 mutant lysate, suggesting that the reconstituted Ago2–RISC is as fully active as the one assembled in lysate. g, Dicer-2–R2D2 and the chaperone machinery are dispensable for loading of single-stranded RNAs whereas they are essential for duplex loading. h, Quantification of g. Data represent mean ± s.d. (n = 3).
a, Quantification of Fig. 1d. b, Experimental strategy for c. c, The amount of pre-loaded CXCR4 siRNA did not change during new loading of let-7 siRNA. d, Quantification of c. Red, reconstituted system; black, GST. Data represent mean ± s.d. (n = 3).
Extended Data Figure 4 The reconstituted system faithfully recapitulates siRNA loading asymmetry and ATP dependency.
a, Ago2–RISC was assembled with siRNAs with either the guide strand or the passenger strand radiolabelled. Left, asymmetry-flipped let-7 siRNA. In contrast to the normal let-7 siRNA (Fig. 1e), the let-7 sequence in this siRNA duplex serves as the passenger strand due to the introduction of a mismatch at the opposite end. Middle, highly asymmetric CXCR4 siRNA. Right, relatively symmetric luc siRNA, in which the two strands can serve as the guide strand with similar chances. b, Small RNA pull-down assay was performed in the reconstituted system with 0.5 mM ATP, ATP-γS, AMP-PNP or ADP. c, Quantification of b. d, Quantification of Fig. 1f.
a–c, Halo-TEV-Flag–Ago2 was tethered to the surface as illustrated. For snapshot observation (a), the reaction mixture was infused into the observation chamber and incubated for 1 h. After washing of the surface, the images of the wash-resistant spots were acquired for 1 s. For snapshot observation after protease treatment (b), the protease mixture was infused into the chamber, which had been imaged and incubated for 30 min. For continuous monitoring (c), images were acquired for 20 min without washing immediately after the reaction was started on the surface.
Extended Data Figure 6 Halo-tagged Ago proteins and fluorescently labelled siRNAs used in single-molecule analysis.
a, Visualization of Halo-TEV-Flag-tagged wild type, catalytic mutant (Cmut; D965A) and 5′ nucleotide-binding pocket mutant (Pmut; Y897E) of Ago2 by a Halo-TMR ligand. b, RNA pull-down assay with siRNAs with or without 3′ fluorescent modifications. Red, guide strand; black, passenger strand. The 5′-phosphate of the guide strand was radiolabelled as denoted by yellow circles. Fluorescent labelling did not compromise the efficiency of RISC assembly in vitro. c, Time course of the guide-alone (red) or double-stranded (yellow) spot formation for wild-type Ago2 with the reconstituted system. d, TEV-protease-mediated cleavage of Halo-TEV-Flag–Ago2. After S2 lysate containing Halo-TEV-Flag–Ago2 was incubated with proteases (HRV3C or TEV) or lysis buffer for 30 min, western blotting was performed using anti-Halo and anti-Flag antibodies. e, f, Representative single-molecule images (e) and quantification (f) after protease treatment for 30 min (Extended Data Fig. 5b). Wash-resistant spots observed in the WT, +D/R and +Cpn conditions (Fig. 2b, c) largely disappeared after TEV-protease-mediated cleavage but not after the control treatment of HRV3C protease. The colour scheme is the same as in Fig. 2b, c. g, h, Validation of pre- and mature RISC formation with inversion of the strand colours. Representative single-molecule images (g) and quantification (h) of wash-resistant spots after 60 min incubation with the reconstituted system together with an siRNA with a 3′ Alexa555 (green)-labelled guide strand and a 3′ Alexa647 (red)-labelled passenger strand. Data represent mean ± s.d. (WT in e, n = 4; others, n = 3).
a, The data in Fig. 3c for short binding were accumulated and fitted to the following equation derived from the one-step reaction model: C˙[1 − exp(−t/τ)], where C is the normalized parameter, τ is the time constant, and t is the reaction time. b, The data in Fig. 3e for the yellow-to-red transition in wild type were accumulated and fitted to the following equation derived from the two-step reaction model: [C0/(C1 − C2)]˙[C1˙exp(−C2˙t) − C2˙exp(−C1˙t)], where C0 is the normalized parameter, C1 and C2 are the inverse of the time constants τ1 and τ2, respectively, and t is the reaction time. c–e, The data in Fig. 3f–h were accumulated and fitted to the following equation: C˙exp(−t/τ), where C is the normalized parameter, τ is the time constant, and t is the reaction time.
Extended Data Figure 8 The 5′-phosphate of the guide strand is dispensable for binding to Dicer-2–R2D2 but is essential for duplex loading into Ago2.
a, Gel shift assay with increasing concentrations of recombinant Dicer-2–R2D2 and passenger-strand-radiolabelled siRNA duplex with the 5′-phosphate (left) or 5′-hydroxyl group (right) on the guide strand. The 5′-phosphate of the guide strand is dispensable for the interaction with Dicer-2–R2D2. Red, guide strand; black, passenger strand. b, c, Small RNA pull-down assay with siRNA duplexes containing 5′-phosphate (left) or 5′-hydroxyl group (right) on the guide strand with wild-type or the catalytic mutant Ago2. The 3′ end of the guide strand (b) or the 5′ end of the passenger strand (c) of the siRNA duplex was radiolabelled, as denoted by yellow circles. The 5′-phosphate on the guide strand is essential for duplex loading into Ago2; in the absence of the guide 5′-phosphate, the siRNA duplex is hardly loaded while maintaining the functional asymmetry. d, Determination of the time constants of short binding in the absence of 5′-phosphate recognition. The data in Fig. 4d were accumulated and fitted to the one-step reaction model as in Extended Data Fig. 7a.
In the absence of the chaperone machinery, an siRNA bound to Dicer-2–R2D2 binds Ago2 only transiently, dissociating rapidly. The Hsc70/Hsp90 chaperone machinery extends their dwell time, presumably by inducing a productive state of Ago2, in a manner dependent on the recognition of the 5′-phosphate on the guide strand by Ago2. Successful anchoring of the 5′-phosphate would trigger loading of the rest of the siRNA duplex from Dicer-2–R2D2 into Ago2, leading to passenger ejection and mature RISC formation.
a, Representative time courses of one-step (left) and two-step (right) photobleaching for the guide (red) or passenger (green) strand. Spots for wild-type Ago2 with the reconstituted system were analysed. Arrows show the timings of photobleaching. b, ∼95% of the fluorescent spots showed one-step photobleaching for each colour, indicating that each spot contained a single RNA molecule. c, Photobleaching dependence on power density. Left, Alexa647. Right, Alexa555. The photobleaching time constants of Alexa647 and 555 under continuous monitoring condition were estimated as ∼22,800 and ∼5,900 s, respectively. Data represent mean ± s.d. (n = 3). d, Representative observed spots in snapshot observation without deconvolution (raw data). The distribution of fluorescence intensity was fitted to Gaussian function (bottom). The diameter of the spots is comparable to the diameter of the Airy disk (533 nm), indicating that the spots are diffraction limited. Scale bar, 500 nm. e, The signal intensity distributions of observed spots and single-molecule benchmarks for Alexa647 (red, left) and Alexa555 (green, right) in snapshot observation. Observed, fluorescent spots observed in snapshot observation (Fig. 2c; WT, +D/R, +Cpn, yellow). Benchmark, the catalytic mutant Ago2 programmed with siRNA duplex, of which the guide and passenger strands were labelled with Alexa647 and 555, respectively (1×, middle) or with the same dye (2×, bottom). Images of single-molecule benchmarks were taken under the same acquisition condition of snapshot observation. Background was subtracted from the raw data. f, Representative observed spots in continuous monitoring without deconvolution (raw data). The distribution of fluorescence intensity was fitted to Gaussian function (bottom). The diameter of the spots is comparable to the diameter of the Airy disk (533 nm), indicating that the spots are diffraction limited. Scale bar, 500 nm. g, The signal intensity distributions of observed spots and single-molecule benchmarks for Alexa647 (red, left) and Alexa555 (green, right) in continuous monitoring. Observed, fluorescent spots observed in continuous monitoring (Fig. 3d; WT, +D/R, +Cpn, yellow). Benchmark, the catalytic mutant Ago2 programmed with siRNA duplex, of which the guide and passenger strands were labelled with Alexa647 and 555, respectively (1×, middle) or with the same dye (2×, bottom). Images of single-molecule benchmarks were taken under the same acquisition condition of continuous monitoring. Background was subtracted from the raw data.
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Iwasaki, S., Sasaki, H., Sakaguchi, Y. et al. Defining fundamental steps in the assembly of the Drosophila RNAi enzyme complex. Nature 521, 533–536 (2015). https://doi.org/10.1038/nature14254
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