Gene silencing through RNA interference (RNAi) is carried out by RISC, the RNA-induced silencing complex. RISC contains two signature components, small interfering RNAs (siRNAs) and Argonaute family proteins. Here, we show that the multiple Argonaute proteins present in mammals are both biologically and biochemically distinct, with a single mammalian family member, Argonaute2, being responsible for messenger RNA cleavage activity. This protein is essential for mouse development, and cells lacking Argonaute2 are unable to mount an experimental response to siRNAs. Mutations within a cryptic ribonuclease H domain within Argonaute2, as identified by comparison with the structure of an archeal Argonaute protein, inactivate RISC. Thus, our evidence supports a model in which Argonaute contributes "Slicer" activity to RISC, providing the catalytic engine for RNAi.
The control of translation and mRNA degradation is an important part of the regulation of gene expression. It is now clear that small RNA molecules are common and effective modulators of gene expression in many eukaryotic cells. These small RNAs that control gene expression can be either endogenous or exogenous micro RNAs (miRNAs) and short interfering RNAs (siRNAs) and can affect mRNA degradation and translation, as well as chromatin structure, thereby having impacts on transcription rates. In this review, we discuss possible mechanisms by which miRNAs control translation and mRNA degradation. An emerging theme is that miRNAs, and siRNAs to some extent, target mRNAs to the general eukaryotic machinery for mRNA degradation and translation control.
Small RNAs, including small interfering RNAs (siRNAs) and microRNAs (miRNAs) can silence target genes through several different effector mechanisms1. Whereas siRNA-directed mRNA cleavage is increasingly understood, the mechanisms by which miRNAs repress protein synthesis are obscure. Recent studies have revealed the existence of specific cytoplasmic foci, referred to herein as processing bodies (P-bodies), which contain untranslated mRNAs and can serve as sites of mRNA degradation2-7. Here we demonstrate that Argonaute proteins -the signature components of the RNA interference (RNAi) effector complex, RISC -localize to mammalian P-bodies. Moreover, reporter mRNAs that are targeted for translational repression by endogenous or exogenous miRNAs become concentrated in P-bodies in a miRNA-dependent manner. These results provide a link between miRNA function and mammalian P-bodies and suggest that translation repression by RISC delivers mRNAs to P-bodies, either as a cause or as a consequence of inhibiting protein synthesis.RNAi was initially characterized as a post-transcriptional gene silencing mechanism in which the experimental introduction of long double-stranded RNAs (dsRNAs) induces sequencespecific destruction of homologous mRNAs (reviewed in ref. 1). RNAi pathways are initiated when dsRNAs are processed by Dicer into siRNAs of 21-26 nucleotides. siRNAs are incorporated into the effector complex RISC. In RISC, the siRNA is bound by an Argonaute protein, which uses the sequence of the siRNA to select and cleave complementary substrates (reviewed in ref. 8).RISC can also silence gene expression by preventing protein synthesis. Genetic studies of Caenorhabditis elegans that are mutant for Dicer forged the initial link between a previously known class of small regulatory RNAs, the stRNAs, and the RNAi pathway9-13. Subsequent studies showed that stRNAs are archetypes of a large class of regulatory RNAs, known as miRNAs (reviewed in ref. 14). Although miRNA and siRNA pathways can be biochemically compartmentalized, both types of RNAs enter RISC, bind to Argonaute proteins and identify their silencing targets in conceptually similar ways. They differ, at least in animals, in that miRNAs most often pair imperfectly with their targets and are thus unable to direct Argonautemediated cleavage15. Instead, miRNAs repress protein synthesis in a cleavage-independent fashion8,14,15.3 Correspondence should be addressed to G.J.H. (e-mail: hannon@cshl.org) and R.P. (e-mail: rrparker@u.arizona.edu).Note added in proof: After this study had been accepted for publication, Sen and Blau30 reported similar observations of the localization of human Ago2 to cytoplasmic P-bodies.Note: Supplementary Information is available on the Nature Cell Biology website. The mechanism by which miRNAs repress translation of their target mRNAs is unknown. Conceivably, RISC could prevent protein synthesis from miRNA targets in one of several ways. RISC could affect translation, per se, by altering rates of initiation or elongation by the riboso...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.