The GW182 RNA-binding protein was initially shown to associate with a specific subset of mRNAs and to reside within discrete cytoplasmic foci named GW bodies (GWBs). GWBs are enriched in proteins that are involved in mRNA degradation. Recent reports have shown that exogenously introduced human Argonaute-2 (Ago2) is also enriched in GWBs, indicating that RNA interference function may be somehow linked to these structures. In this report, we demonstrate that endogenous Ago2 and transfected small interfering RNAs (siRNAs) are also present within these same cytoplasmic bodies and that the GW182 protein interacts with Ago2. Disruption of these cytoplasmic foci in HeLa cells interferes with the silencing capability of a siRNA that is specific to lamin-A/C. Our data support a model in which GW182 and/or the microenvironment of the cytoplasmic GWBs contribute to the RNA-induced silencing complex and to RNA silencing.

There was an error published in J. Cell Sci. 120, 1317-1323.We apologise for two errors that occurred in the online and pdf versions of this article. The printed version is correct.On p. 1317, in the Summary, the sentence 'Formation of GW bodies appears to depend on both specific protein factors and RNA, in particular, microRNA.' appeared twice. The correct version of the summary is shown below. Summary GW bodies, also known as mammalian P-bodies, are cytoplasmic foci involved in the post-transcriptional regulation of eukaryotic gene expression. Recently, GW bodies have been linked to RNA interference and demonstrated to be important for short-interfering-RNA-and microRNA-mediated mRNA decay and translational repression. Evidence indicates that both passenger and guide strands of short-interfering RNA duplexes can localize to GW bodies, thereby indicating that RNA-induced silencing complexes may be activated within these cytoplasmic centers. Work over the past few years has significantly increased our understanding of the biology of GW bodies, revealing that they are specialized cell components that spatially regulate mRNA turnover in various biological processes. Formation of GW bodies appears to depend on both specific protein factors and RNA, in particular, microRNA. Here, we propose a working model for GW body assembly in terms of its relationship to RNA interference. In this process, one or more heteromeric protein complexes accumulate in successive steps into larger ribonucleoprotein structures.On p. 1319, right column, first paragraph, the word order of the penultimate sentence was incorrect and should read:In particular, studies in Drosophila indicate that GW182 interacts with Ago1 and promotes miRNA-mediated degradation of a subset of mRNA targets (Behm-Ansmant et al., 2006). Commentary IntroductionThe control of mRNA stability plays key roles in both the posttranscriptional regulation of eukaryotic gene expression (Keene and Lager, 2005;Wilusz and Wilusz, 2004) and mRNA quality control (Fasken and Corbett, 2005). The latter involves the recognition and rapid degradation of aberrant mRNAs and takes place when translation termination occurs too early (nonsense-mediated decay) or fails to occur (non-stop decay) (Fasken and Corbett, 2005) or when translation elongation stalls (no-go decay) (Doma and Parker, 2006). In eukaryotes, mRNA turnover is regulated by two major mechanisms. One involves the multisubunit exosome, where transcripts are degraded by 3Ј-to-5Ј exonucleases (for a review, see van Hoof and Parker, 1999). The second mechanism involves cytoplasmic compartments termed GW bodies (GWBs), which spatially control mRNA turnover by the 5Ј-to-3Ј mRNA decay machinery. These discrete cytoplasmic foci, also called Dcpcontaining bodies or processing (P)-bodies, constitute sites of mRNA degradation, storage and translational repression (Brengues

Gene silencing using small interfering RNA (siRNA) is a valuable laboratory tool and a promising approach to therapeutics for a variety of human diseases. Recently, RNA interference (RNAi) has been linked to cytoplasmic GW bodies (GWB). However, the correlation between RNAi and the formation of GWB, also known as mammalian processing bodies, remains unclear. In this report, we show that transfection of functional siRNA induced larger and greater numbers of GWB. This siRNA-induced increase of GWB depended on the endogenous expression of the target mRNA. Knockdown of GW182 or Ago2 demonstrated that the siRNA-induced increase of GWB required these two proteins and correlated with RNAi. Furthermore, knockdown of rck/p54 or LSm1 did not prevent the reassembly of GWB that were induced by and correlated with siRNA-mediated RNA silencing. We propose that RNAi is a key regulatory mechanism for the assembly of GWB, and in some cases, GWB may serve as markers for RNAi in mammalian cells. INTRODUCTIONGW bodies (GWB), also known as mammalian processing bodies (P bodies), are cytoplasmic foci that contain multiple decay factors and that are involved in the 5Ј33Ј mRNA degradation pathway. GWB are named from the marker protein GW182, which contains multiple glycine (G) and tryptophan (W) repeats and a classic RNA binding domain at the carboxyl terminus (Eystathioy et al., 2002a). The mRNA decay factors/complexes found in GWB include the deadenylase Ccr4, the decapping complex Dcp1a/1b/Dcp2, the LSm1-7 complex, Ge-1 (also known as Hedls), rck/p54, and exonuclease Xrn1 (Bashkirov et al., 1997;van Dijk et al., 2002;Ingelfinger et al., 2002;Lykke-Andersen, 2002;Eystathioy et al., 2003;Cougot et al., 2004;Andrei et al., 2005;Yu et al., 2005;Fenger-Gron et al., 2005). GWB are physically juxtaposed to and transiently interact with stress granules (SG). SG process cytoplasmic aggregates of stalled translational preinitiation complexes that accumulate during stress responses and that share certain components with GWB (Kedersha et al., 2005).In addition to mRNA decay, a crucial role of GWB and their components in RNA interference (RNAi) was recently uncovered (Anderson and Kedersha, 2006;Eulalio et al., 2007a;Jakymiw et al., 2007). RNAi is a posttranscriptional gene silencing mechanism that uses specific doublestranded RNA to silence genes in a sequence-specific manner Mello and Conte, 2004). In brief, the double-stranded RNA is processed by Dicer into small interfering RNA (siRNA) or microRNA (miRNA). The 21-to 26-nucleotide siRNA and miRNA are then incorporated in the effector complex, RNA-induced silencing complex (RISC), which either cleaves or inhibits translation of the target mRNA. In 2005, two key components of RISC, Argonaute2 (Ago2) and siRNA/miRNA, were found to be enriched in GWB (Sen and Blau, 2005;Pillai et al., 2005;Jakymiw et al., 2005;Liu et al., 2005b;Pauley et al., 2006). miRNA-targeted mRNA also localizes to GWB in a miRNAdependent manner (Liu et al., 2005b). These observations provide the first evidence that RNAi is ...

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