Tomari Yukihide, Matranga Christian, Haley Benjamin, Martinez Natalia, Zamore Phillip D
Department of Biochemistry and Molecular Pharmacology, University of Massachusetts Medical School, Worcester, MA 01605, USA.
Science. 2004 Nov 19;306(5700):1377-80. doi: 10.1126/science.1102755.
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.
为了在RNA干扰（RNAi）途径中发挥引导作用，小干扰RNA（siRNA）必须解旋成其组成链，然后与蛋白质组装形成RNA诱导沉默复合体（RISC），该复合体催化靶信使RNA的切割。两条约21个核苷酸的siRNA链5'端碱基配对稳定性的热力学差异决定了哪条siRNA链组装到RISC中。我们表明，在果蝇中，Dicer-2/R2D2蛋白异二聚体在siRNA双链体上的方向决定了哪条siRNA链与核心RISC蛋白AGO2结合。R2D2结合具有最大双链特征的siRNA末端，从而在siRNA双链体上定向异二聚体。R2D2的强结合需要siRNA链上的5'-磷酸，而该磷酸被排除在RISC之外。因此，R2D2既是siRNA热力学不对称的蛋白质传感器，也是真正的siRNA进入RNAi途径的许可因子。
