title: A protein sensor for siRNA asymmetry.
abstract: To act as guides in the RNA interference (RNAi) pathway, small interfering RNAs (siRNAs) must be unwound into their component strands, then assembled with proteins to form the RNA-induced silencing complex (RISC), which catalyzes target messenger RNA cleavage. Thermodynamic differences in the base-pairing stabilities of the 5' ends of the two approximately 21-nucleotide siRNA strands determine which siRNA strand is assembled into the RISC. We show that in Drosophila, the orientation of the Dicer-2/R2D2 protein heterodimer on the siRNA duplex determines which siRNA strand associates with the core RISC protein Argonaute 2. R2D2 binds the siRNA end with the greatest double-stranded character, thereby orienting the heterodimer on the siRNA duplex. Strong R2D2 binding requires a 5'-phosphate on the siRNA strand that is excluded from the RISC. Thus, R2D2 is both a protein sensor for siRNA thermodynamic asymmetry and a licensing factor for entry of authentic siRNAs into the RNAi pathway.
authors.affiliation: Department of Biochemistry and Molecular Pharmacology, University of Massachusetts Medical School, Worcester, MA 01605, USA.
authors.forename: Yukihide
authors.initials: Y
authors.lastname: Tomari
authors.forename: Christian
authors.initials: C
authors.lastname: Matranga
authors.forename: Benjamin
authors.initials: B
authors.lastname: Haley
authors.forename: Natalia
authors.initials: N
authors.lastname: Martinez
authors.forename: Phillip D
authors.initials: PD
authors.lastname: Zamore
mesh_terms: D000818:Animals; D060565:Argonaute Proteins; D019281:Dimerization; D004330:Drosophila / Q000196:embryology / Q000378:metabolism; D029721:Drosophila Proteins / Q000378:metabolism*; D008027:Light; D008156:Luciferases / Q000235:genetics; D009690:Nucleic Acid Conformation; D020365:RNA Helicases / Q000378:metabolism*; D034622:RNA Interference; D012330:RNA, Double-Stranded / Q000737:chemistry / Q000378:metabolism; D034741:RNA, Small Interfering / Q000737:chemistry* / Q000378:metabolism*; D016601:RNA-Binding Proteins / Q000378:metabolism*; D034743:RNA-Induced Silencing Complex / Q000378:metabolism*; D043244:Ribonuclease III; D013482:Superoxide Dismutase / Q000235:genetics; D013816:Thermodynamics; D014498:Uracil / Q000031:analogs & derivatives* / Q000032:analysis
chemical_list: C433807:AGO2 protein, Drosophila; D060565:Argonaute Proteins; D029721:Drosophila Proteins; C478068:R2D2 protein, Drosophila; D012330:RNA, Double-Stranded; D034741:RNA, Small Interfering; D016601:RNA-Binding Proteins; D034743:RNA-Induced Silencing Complex; D014498:Uracil; D008156:Luciferases; D013482:Superoxide Dismutase; C478069:DCR-2 protein, Drosophila; D043244:Ribonuclease III; D020365:RNA Helicases; C048747:5-iodouracil