Processing bodies (P-bodies) are cytoplasmic foci implicated in the regulation of mRNA translation, storage, and degradation. Key effectors of microRNA (miRNA)-mediated RNA interference (RNAi), such as Argonaute-2 (Ago2), miRNAs, and their cognate mRNAs, are localized to these structures; however, the precise role that P-bodies and their component proteins play in small interfering RNA (siRNA)-mediated RNAi remains unclear. Here, we investigate the relationship between siRNA-mediated RNAi, RNAi machinery proteins, and P-bodies. We show that upon transfection into cells, siRNAs rapidly localize to P-bodies in their native double-stranded conformation, as indicated by fluorescence resonance energy transfer imaging and that Ago2 is at least in part responsible for this siRNA localization pattern, indicating RISC involvement. Furthermore, siRNA transfection induces up-regulated expression of both GW182, a key P-body component, and Ago2, indicating that P-body localization and interaction with GW182 and Ago2 are important in siRNA-mediated RNAi. By virtue of being centers where these proteins and siRNAs aggregate, we propose that the P-body microenvironment, whether as microscopically visible foci or submicroscopic protein complexes, facilitates siRNA processing and siRNA-mediated silencing through the action of its component proteins.
INTRODUCTIONRNA interference (RNAi) is a powerful homology-based gene silencing mechanism directed by small RNAs, including small interfering RNAs (siRNAs) and microRNAs (miRNAs). RNAi is a posttranscriptional process, leading to either translational repression of the target mRNA, typical of miRNAmediated silencing, or degradation of the target via siRNAmediated silencing (Hammond, 2005). Recently, several components of the RNAi pathway, including Argonaute proteins (Sen and Blau, 2005), miRNAs, and their targets (Liu et al., 2005b;Pillai et al., 2005) have been localized to processing bodies (P-bodies). P-bodies are recognized as important cytoplasmic mRNA processing centers where nontranslating mRNA is sorted and either stored, repressed, or degraded (for review, see Eulalio et al., 2007a). Enzymes associated with mRNA degradation, such the CCR4-CAF-1-Not complex involved in deadenlyation (Cougot et al., 2004); DCP1 and -2 (van Dijk et al., 2002) involved in decapping; and XRN-1, a 5Ј-3Ј exonuclease (Ingelfinger et al., 2002), have all been localized to P-bodies. These foci also contain proteins involved in nonsense-mediated decay (Sheth and Parker, 2006) and AU-rich element-mediated mRNA decay pathways (Vasudevan and Steitz, 2007).Although it is clear that RNAi and P-bodies are associated, the precise role of P-bodies in RNAi remains unclear. There is evidence that P-body components are essential for miRNA-based gene silencing. Depletion of certain P-body components, including RCK/P54 (Chu and Rana, 2006) and GW182 in human (Liu et al., 2005a) and Drosophila (Rehwinkel et al., 2005) cells leads to a loss of silencing. Furthermore, the decay of miRNA targets seems to require the de...