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The N domain of Argonaute drives duplex unwinding during RISC assembly

Abstract

Small RNAs, such as microRNAs and small interfering RNAs, act through Argonaute (Ago) proteins as a part of RNA-induced silencing complexes (RISCs). To make RISCs, Ago proteins bind and subsequently unwind small RNA duplexes, finally leaving one strand stably incorporated. Here we identified the N domain of human AGO2 as the initiator of duplex unwinding during RISC assembly. We discovered that a functional N domain is strictly required for small RNA duplex unwinding but not for precedent duplex loading or subsequent target cleavage. We postulate that RISC assembly is tripartite, comprising (i) RISC loading, whereby Ago undergoes conformational opening and loads a small RNA duplex, forming pre-RISC; (ii) wedging, whereby the end of the duplex is pried open through active wedging by the N domain, in preparation for unwinding; and (iii) unwinding, whereby the passenger strand is removed through slicer-dependent or slicer-independent unwinding, forming mature RISC.

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Figure 1: N mutants are defective for siRNA, slicer-dependent unwinding.
Figure 2: N mutants cannot efficiently cleave the passenger strand of siRNA duplexes during RISC assembly.
Figure 3: N mutants are defective for unwinding, irrespective of slicer-dependency.
Figure 4: N mutants are defective for miRNA, slicer-independent unwinding.
Figure 5: Destabilizing small RNA duplexes rescues unwinding-defective N mutant.
Figure 6: N mutants are capable of target cleavage.
Figure 7: N mutant AGO2 is deficient for duplex unwinding in cells.
Figure 8: Models for tripartite RISC assembly and active versus passive wedging.

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Acknowledgements

We acknowledge T. Tuschl (Rockefeller University) for the pIRESneo-Flag-hemagglutinin-AGO2 plasmid and S. Katsuma (The University of Tokyo) for technical assistance. We are grateful to H. Seitz, T. Kawamata and members of the Tomari laboratory for helpful discussions and suggestions and critical comments on the manuscript. This work was supported in part by a Grant-in-Aid for Scientific Research on Innovative Areas (“Functional machinery for non-coding RNAs”) to Y.T. P.B.K. is a Prins Bernhard Cultuurfonds Fellow, supported by the Banning-de Jong Fonds.

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P.B.K. conducted all experiments, Y.T. supervised the study, and P.B.K. and Y.T. wrote the manuscript, discussed the results and approved the manuscript.

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Correspondence to Yukihide Tomari.

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The authors declare no competing financial interests.

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Supplementary Figures 1–8 and Supplementary Tables 1 and 2 (PDF 3536 kb)

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Kwak, P., Tomari, Y. The N domain of Argonaute drives duplex unwinding during RISC assembly. Nat Struct Mol Biol 19, 145–151 (2012). https://doi.org/10.1038/nsmb.2232

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