Taking RISCs with Ago hookers
Highlights
► Proteins containing evolutionarily conserved GW-rich motifs are implicated in Argonaute action. ► These Ago-hook factors are involved in diverse RNAi pathways. ► The AGO-hook motifs interact with the 5′ binding pocket of the mid-PIWI domain of Argonautes. ► Some plant pathogens encode silencing suppresors that hijack Ago-hooks to escape the host RNA silencing machinery.
Introduction
RNA silencing is an essential component of gene expression, widespread among eukaryotes. This process relies on sequence specific regulation triggered by a double strand RNA (dsRNA) molecule, and is involved in many developmental and biological functions [1, 2, 3]. DsRNA cleavage by Dicer-like (DCL) ribonucleases leads to the production of 20–30 nt small RNA carrying the sequence information that guides RNA silencing effector complexes to specific targets on DNA (TGS, Transcriptional Gene Silencing) or RNA (PTGS, Post-Transcriptional Gene Silencing). These complexes, also known as RITS (RNA-Induced Transcriptional Silencing) or RISC (RNA-Induced Post-Transcriptional Silencing) ensure modifications at the chromatin (histone modifications, DNA methylation) or mRNA (miRNA mediated translational repression or mRNA degradation) level [4, 5]. Up to now, Argonaute (AGO) proteins are the only factors common to all related RISC or RITS complexes and are key effectors of RNA silencing [6]. Although siRNA biogenesis and loading onto AGOs have been extensively described, the mode of action and protein composition of RITS/RISC complexes are still poorly explored. A recent breakthrough was the observation that certain novel components of these complexes in eukaryotes contain a striking, unusual, repetitive and conserved peptide sequence that binds AGO proteins: the GW/WG motif, defining an ‘Ago hook’ [7•, 8••, 9••].
This review offers a wide overview and characterization of Ago hook motifs. We will also address the question of how this interaction module is coordinated and, finally, emphasize the biological relevance of the conservation of this motif in evolution.
Section snippets
Identification of a new interfering component of RNA silencing: the ‘Ago hookers’
Glycine-tryptophan repeats were first described in the human autoantigen GW182, a protein presumed at that time to play a role in gene regulation by targeting a specific set of transcripts to cytoplasmic bodies designated as GW bodies [10]. Members of the GW182 family, including homologs in C. elegans and Drosophila, have since been shown to be key components in the miRNA pathway, in association with AGO proteins [10, 11, 12, 13, 14•]. Subsequent studies of two unrelated factors involved in the
A systematic search for Ago hookers
The Arabidopsis genome encodes 10 AGO proteins, while the rice genome contains 19 genes. This emphasises the important roles of these proteins in plants and suggested that many plant GW-containing proteins probably remained to be identified. The lack of conservation of primary sequence and the variable lengths of the motifs make these particularly refractory to identification by classical approaches. Apart from the conserved GW/WG sequences, comparison of repeat regions of GW182 and NRPE1
The binary interaction module involves the MID-PIWI domains of AGO proteins
An important feature of the interaction between AGO proteins and the GW AGO-anchor motif is its universality, as demonstrated by interaction assays performed with phylogenetically distant proteins. In particular, tryptophan-dependent interaction was reported between the prokaryotic A. fulgidus AGO and the GW-rich region of human GW182 and between the human AGO1/2 proteins and the GW-rich domain of Arabidopsis NRPE1, respectively [8••, 9••]. Overall, these observations support the idea that the
Hooking Argonautes, an input channel for pathogens to modulate host RNAi pathways?
The first biological role assigned for RNAi is defense against natural invaders. Hosts and parasites being in constant evolution, this raises the puzzling question as to whether pathogens have acquired Ago hook motifs similar to endogenous proteins to counteract host RNA silencing effectors. Viruses have evolved to escape the host RNAi machinery, deploying proteins acting as Suppressors of Viral Suppressor of RNA silencing (VSR) [2]. These are often multifunctional proteins also affecting key
Conclusions and perspectives
Ago hook proteins are now considered fundamental partners in RNA silencing pathways. Conserved AGO binding GW motifs may interfere positively or negatively with the last effector step, and also represent a by-pass RNAi mechanism for some pathogens. However, an intriguing question to address in future is whether potential AGO-binding GW motifs are involved in bacterial and viral diseases in animals. Although known GW/WG factors spread to both arms of RNA silencing in non-plant organisms, only
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
Work in our lab is supported by the Agence Nationale pour la Recherche (ANR-08-BLAN-0206) and CNRS.
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2015, Current Opinion in Plant BiologyCitation Excerpt :While retention of an AGO-hook motif is likely driven by a requirement for AGO interactions, factors driving the rapid evolution of the CTD as a whole remain unclear. One intriguing hypothesis suggests it might be driven by host-pathogen interactions, as several viral suppressor proteins with AGO-hook motifs have been identified [36,56,57]. However, there is currently no direct evidence that this phenomenon extends to AGOs that influence the RdDM pathway.
The role of long non-coding RNA in transcriptional gene silencing
2012, Current Opinion in Plant BiologyCitation Excerpt :In addition to binding lncRNAs AGO4 also directly interacts with the C-terminal domain of the largest subunit of Pol V [39]. Similarly, SPT5L interacts not only with lncRNA but also with AGO4 [28,29,40] (Figure 1b). This indicates that binding of silencing factors to specific genomic loci is enhanced by multiple protein–RNA and protein–protein interactions.
NERD, a Plant-Specific GW Protein, Defines an Additional RNAi-Dependent Chromatin-Based Pathway in Arabidopsis
2012, Molecular CellCitation Excerpt :Recruitment of the RISC complex onto chromatin also involves specific tryptophan-glycine/glycine-tryptophan (WG/GW)-rich motifs, or “AGO-hook” motifs, that are found in several RdDM pathway components. These include NRPE1, the largest subunit of PolV, NRPB2, the second largest subunit of PolII, and SPT5-like, a PolV-associated protein (Azevedo et al., 2011). Despite the significant progress in our understanding of RdDM, it remains unclear whether other RNAi-dependent pathways are also targeting chromatin in plants.
The unique dual targeting of AGO1 by two types of PRMT enzymes promotes phasiRNA loading in Arabidopsis thaliana
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