RNA interference is a conserved pathway of sequence-specific gene silencing that depends on small guide RNAs and the action of proteins assembled in the RNA-induced silencing complex (RISC). Minimally, the action of RISC requires the endonucleolytic slicer activity of Argonaute2 (Ago2) directed to RNA targets whose sequences are complementary to RISC-incorporated small RNA. To identify RISC components in human cells, we developed an affinity-purification strategy to isolate siRNA-programmed RISC. Here we report the identification of RNA helicase A (RHA) as a human RISC-associated factor. We show that RHA interacts in human cells with siRNA, Ago2, TRBP, and Dicer and functions in the RNAi pathway. In RHA-depleted cells, RNAi was reduced as a consequence of decreased intracellular concentration of active RISC assembled with the guide-strand RNA and Ago2. Our results identify RHA as a RISC component and demonstrate that RHA functions in RISC as an siRNA-loading factor.
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