A Phosphate-Binding Pocket within the Platform-PAZ-Connector Helix Cassette of Human Dicer

Abstract

We have solved two families of crystal structures of the human Dicer ‘‘platform-PAZ-connector helix’’cassette in complex with small interfering RNAs(siRNAs). The structures possess two adjacently positioned pockets: a 2 nt 30-overhang-binding pocket within the PAZ domain (30 pocket) and a phosphate-binding pocket within the platform domain(phosphate pocket). One family of complexes contains a knob-like a-helical protrusion, designated ‘‘hDicer-specific helix,’’ that separates the two pockets and orients the bound siRNA away from the surface of Dicer, which could be indicative of a product release/transfer state. In the second complex, the helical protrusion is melted/disordered and the bound siRNA is aligned toward the surface of Dicer, suggestive of a cleavage-competent state. These structures allow us to propose that the transition from the cleavage-competent to the postulated product release/transfer state may involve release of the 50-phosphate from the phosphate pocket while retaining the 30 overhang in the 30 pocket.