Structure of Dicer and Mechanistic Implications for RNAi

Dicer is a specialized ribonuclease that processes double-stranded RNA (dsRNA) into small RNA fragments about 25 nucleotides in length during the initiation phase of RNA interference (RNAi). We previously determined the crystal structure of a Dicer enzyme from the diplomonad Giardia intestinalis and proposed a structural model for dsRNA processing. Here, we provide evidence that Dicer is composed of three structurally rigid regions connected by flexible hinges and propose that conformational flexibility facilitates dsRNA binding and processing. We also examine the role of the accessory domains found in Dicers of higher eukaryotes but absent in Giardia Dicer. Finally, we combine the structure of Dicer with published biochemical data to propose a model for the architecture of the RNA-induced silencing complex (RISC)-loading complex.