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Abstract
Small silencing RNAs--small interfering RNAs (siRNAs) or microRNAs (miRNAs)--direct
posttranscriptional gene silencing of their mRNA targets as guides for the RNA-induced
silencing complex (RISC). Both siRNAs and miRNAs are born double stranded. Surprisingly,
loading these small RNA duplexes into Argonaute proteins, the core components of RISC,
requires ATP, whereas separating the two small RNA strands within Argonaute does not.
Here we show that the Hsc70/Hsp90 chaperone machinery is required to load small RNA
duplexes into Argonaute proteins, but not for subsequent strand separation or target
cleavage. We envision that the chaperone machinery uses ATP and mediates a conformational
opening of Ago proteins so that they can receive bulky small RNA duplexes. Our data
suggest that the chaperone machinery may serve as the driving force for the RISC assembly
pathway.
Copyright 2010 Elsevier Inc. All rights reserved.