Structural insights into RNA Processing by the Human RISC-Loading Complex

Targeted gene silencing by RNA interference (RNAi) requires loading of a short guide RNA (siRNA or miRNA) into an Argonaute protein to form the functional center of an RNA-induced silencing complex (RISC). In humans, Argonaute2 (Ago2) assembles with the guide RNA-generating enzyme Dicer and the RNA-binding protein TRBP to form a RISC-loading complex (RLC) necessary for efficient transfer of nascent siRNAs and miRNAs from Dicer to Ago2. Here we show, using single-particle electron microscopy analysis, that human Dicer exhibits an L-shaped structure. Withn the RLC Dicer's N-terminal DExH/D domain, located at the short base branch, interacts with TRBP, while its C-terminal catalytic domains in the main body are proximal to Ago2. A model generated by docking the available atomic structures of Dicer and Argonaute homologs into the RLC reconstruction suggests a mechanism for siRNA transfer from Dicer to Ago2.