The catalytic engine of RNAi is the RNA-induced silencing complex (RISC), wherein the endoribonuclease Argonaute and single-stranded siRNA direct target mRNA cleavage. Here we have reconstituted long dsRNA- and duplex siRNA-initiated RISC activities using recombinant Drosophila Dicer-2, R2D2 and Ago2 proteins. We employ this core reconstitution system to purify an RNAi regulator-component 3 promoter of RISC (C3PO), a complex of Translin and Trax. C3PO is a Mg2+-dependent endoribonuclease that promotes RISC activation by removing siRNA passenger strand cleavage products. These studies establish an in vitro RNAi reconstitution system and identify C3PO as a key activator of the core RNAi machinery.

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Background: MicroRNA biogenesis is a multistep process regulated by RNA-binding proteins. Results: Sjögren syndrome antigen B (SSB)/La binds stem-loop precursor microRNAs (pre-miRNA) and is required for microRNA expression. Conclusion: La/SSB promotes global microRNA expression by stabilizing pre-miRNAs from nuclease-mediated decay. Significance: This study reveals a novel concept of pre-miRNA holding complex that protects and escorts pre-miRNAs in microRNA biogenesis pathway.

SUMMARY The effector of RNA interference (RNAi) is the RNA-induced silencing complex (RISC). C3PO promotes the activation of RISC by degrading Argonaute2 (Ago2)-nicked passenger strand of duplex siRNA. Active RISC is a multiple-turnover enzyme that uses the guide strand of siRNA to direct Ago2-mediated sequence-specific cleavage of complementary mRNA. How this effector step of RNAi is regulated is currently unknown. Here, we used human Ago2 minimal RISC system to purify Sjögren’s syndrome antigen B (SSB)/autoantigen La as an activator of the RISC-mediated mRNA cleavage activity. Our reconstitution studies showed that La could promote multiple-turnover RISC catalysis by facilitating the release of cleaved mRNA from RISC. Moreover, we demonstrated that La was required for efficient RNAi, antiviral defense, and transposon silencing in vivo. Taken together, the findings of C3PO and La reveal a general concept that regulatory factors are required to remove Ago2-cleaved products to assemble or restore active RISC.