Two families of crystal structures of hDicer PAZ cassette-siRNAs complexes
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Dicer-specific loop region forms a knob-like α-helical protrusion
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Identification of a phosphate-binding pocket in the platform domain of hDicer
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Models of cleavage-competent and product release/transfer conformations
Summary
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 3′-overhang-binding pocket within the PAZ domain (3′ pocket) and a phosphate-binding pocket within the platform domain (phosphate pocket). One family of complexes contains a knob-like α-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 5′-phosphate from the phosphate pocket while retaining the 3′ overhang in the 3′ pocket.