BMB Rep. 2015; 48(12): 643-644  
Single-molecule fluorescence measurements reveal the reaction mechanisms of the core-RISC, composed of human Argonaute 2 and a guide RNA
Myung Hyun Jo1,2,3, Ji-Joon Song5 & Sungchul Hohng1,2,3,4,*
1Department of Physics and Astronomy, 2Institute of Applied Physics, 3National Center of Creative Research Initiatives, and 4Department of Biophysics and Chemical Biology, Seoul National University, Seoul 08826, 5Department of Biological Sciences, KAIST, Daejeon 34141, Korea
Correspondence to: E-mail: shohng@snu.ac.kr
Received: November 16, 2015; Published online: December 31, 2015.
© Korean Society for Biochemistry and Molecular Biology. All rights reserved.

Abstract
In eukaryotes, small RNAs play important roles in both gene regulation and resistance to viral infection. Argonaute proteins have been identified as a key component of the effector complexes of various RNA-silencing pathways, but the mechanistic roles of Argonaute proteins in these pathways are not clearly understood. To address this question, we performed single- molecule fluorescence experiments using an RNA-induced silencing complex (core-RISC) composed of a small RNA and human Argonaute 2. We found that target binding of core- RISC starts at the seed region of the guide RNA. After target binding, four distinct reactions followed: target cleavage, transient binding, stable binding, and Argonaute unloading. Target cleavage required extensive sequence complementarity and accelerated core-RISC dissociation for recycling. In contrast, the stable binding of core-RISC to target RNAs required seed-match only, suggesting a potential explanation for the seed-match rule of microRNA (miRNA) target selection.
Keywords: Argonaute, miRNA, siRNA, Single-molecule FRET


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