“The Way Dicer Dices”

“The Way Dicer Dices”

Pick Wei Lau – Department of Cell Biology, The Scripps Research Institute, La Jolla, CA

Dicer is a specialized ribonuclease that initiates RNA interference (RNAi) by cleaving double-stranded RNA (dsRNA) into small RNA fragments about 22 nucleotides long. Dicer products are loaded into Argonaute proteins, which use the small RNAs as guides for the silencing of cognate genes. Like most Dicer enzymes, human Dicer is a large (220 kDa) multi-domain protein. Until recently, the only structural information describing a Dicer protein was the crystal structure of an atypically small (85 kDa) homolog from Giardia intestinalis. The additional domains found in human Dicer participate in dsRNA processing, regulate dicing activity and serve as molecular scaffolds for consolidating protein factors involved in the initiation of RNAi. In order to gain insight into the structure and mechanism of the large Dicer proteins found in multi-cellular organisms we generated a 3D reconstruction of human Dicer bound to the protein TRBP using high throughput electron microscopy and single particle analysis. Our analysis reveals that human Dicer is an L-shaped molecule with a long edge of 150 Å and a 100 Å extension on one end. A surface trench runs the length of the long edge of the molecule, defining a putative dsRNA-binding site. Docking the crystal structure of Giardia Dicer, which represents the nuclease core of human Dicer, into the EM map suggested two possible overall molecular architectures for human Dicer. We therefore generated additional 3D reconstructions of truncated Dicer proteins, lacking discrete domains. The combined results revealed an unexpected 3D architecture for human Dicer and offer new insights into possible roles of additional modular domains within dicer towards the processing of small RNAs during the initiation of RNAi.

Pick Wei Lau – Department of Cell Biology, The Scripps Research Institute, La Jolla, CA