The Molecular Architecture of Human Dicer

Dicer is a multi-domain enzyme that generates small RNAs for gene silencing in eukaryotes. Current understanding of Dicer structure is restricted to simple forms of the enzyme, while that of the large and complex Dicer, widespread in eukarya, is unknown. Here, we describe a novel domain localization strategy developed to determine the structure of human Dicer by electron microscopy. A rearrangement of the nuclease core, compared to the archetypal Giardia Dicer, explains how metazoan Dicers generate 21–23 nucleotide products. The helicase domains form a clamp-like structure adjacent to the RNase III active site, facilitating recognition of pre-miRNA loops or translocation on long dsRNAs. Drosophila Dicer-2 displays similar features, revealing that the three-dimensional architecture is conserved. These results illuminate the structural basis for small RNA production in eukaryotes and provide a versatile new tool for determining structures of large molecular machines.