Structural Basis for RNA Processing by Dicer

Abstract

Dicer enzymes, found in nearly all eukaryotic cell types, cleave double-stranded RNA substrates into the ~23 base-pair products that are the effectors of gene silencing by RNA interference (RNAi). We describe the 3 Å crystal structure of the intact Dicer from Giardia intestinalis, revealing the structural basis for RNA recognition, measuring and cleavage by this class of enzymes. The hatchet-shaped protein contains an RNA-binding PAZ domain at one end and two RNase III domains at the other, connected by an N-terminal domain and a long alpha-helix. The connector helix sets the 60 Å distance between the PAZ and RNase III domains that corresponds the length of the double-stranded RNA products of Dicer, implicating this structural element as the “ruler” that defines interfering RNA size. Other details of the structure and biochemical activity of Dicer, as well as implications for its catalytic mechanism and interactions with ligands, will be discussed.