The Polerovirus F Box Protein P0 Targets ARGONAUTE1 to Suppress RNA Silencing

Abstract

Plants employ post-transcriptional gene silencing (PTGS) as an antiviral defense response. In this mechanism, viral-derived small RNAs are incorporated into the RNA-induced silencing complex (RISC) to guide degradation of the corresponding viral RNAs. ARGONAUTE1 (AGO1) is a key component of RISC: it carries the RNA slicer activity. As a counter-defense, viruses have evolved various proteins that suppress PTGS. Recently, we showed that the Polerovirus P0 protein carries an F box motif required to form an SCF-like complex, which is also essential for P0's silencing suppressor function. Here, we investigate the molecular mechanism by which P0 impairs PTGS. First we show that P0's expression does not affect the biogenesis of primary siRNAs in an inverted repeat-PTGS assay, but it does affect their activity. Moreover, P0's expression in transformed Arabidopsis plants leads to various developmental abnormalities reminiscent of mutants affected in miRNA pathways, which is accompanied by enhanced levels of several miRNA-target transcripts, suggesting that P0 acts at the level of RISC. Interestingly, ectopic expression of P0 triggered AGO1 protein decay in planta. Finally, we provide evidence that P0 physically interacts with AGO1. Based on these results, we propose that P0 hijacks the host SCF machinery to modulate gene silencing by destabilizing AGO1.

The 21-Nucleotide, but Not 22-Nucleotide, Viral Secondary Small Interfering RNAs Direct Potent Antiviral Defense by Two Cooperative Argonautes inArabidopsis thaliana

“…Induction of AGO1 mRNA accumulation is a general response to viral infection (Zhang et al, 2006;Csorba et al, 2007;Havelda et al, 2008). However, this induction is generally not accompanied by an increase in AGO1 activity because most viruses encode VSR (viral suppressors of RNA silencing) proteins that counteract either small RNA production, small RNA stability, small RNA loading on AGO1, AGO1 mRNA translation, AGO1 protein stability, or AGO1 activity (Chapman et al, 2004;Lakatos et al, 2006;Zhang et al, 2006;Baumberger et al, 2007;Bortolamiol et al, 2007;Csorba et al, 2007; Azevedo et al, 2010; Chiu et al, 2010;Giner et al, 2010;Vá rallyay et al, 2010). Induction of AGO2 mRNA and AGO2 protein accumulation in response to viral infection has been reported (Lewsey et al, 2010;Endres et al, 2010;Harvey et al, 2011), likely because AGO1 association with miR403 or AGO1/miR403-mediated regulation of AGO2 mRNA is impaired by VSR proteins.…”

Geminiviruses Subvert Ubiquitination by Altering CSN-Mediated Derubylation of SCF E3 Ligase Complexes and Inhibit Jasmonate Signaling inArabidopsis thaliana

“…It would be interesting to look for F-box proteins upregulated during geminivirus infection or in heterologous expression of geminiviral proteins to localize possible targets of this theoretical mechanism. Some plant viruses have been shown to co-opt the SCF machinery by encoding their own viral F-box proteins, which are assembled into plant SCF complexes (Baumberger et al, 2007;Bortolamiol et al, 2007;Lageix et al, 2007), triggering the ubiquitination of plant proteins that interfere with viral infection. Thus, it would be a tempting hypothesis that, to maximize the effect of their encoded F-box proteins, promoting their efficient incorporation in the maximum possible number of SCF complexes, these plant viruses might have developed means to interfere with the assembly/disassembly cycle of SCF complexes, which is based in rubylation and derubylation.…”

“…In plants, the Polerovirus P0 protein carries an F-box domain that allows its incorporation into an SCF complex to mediate degradation of AGO1, modulating gene silencing (Baumberger et al, 2007;Bortolamiol et al, 2007). The nanovirus Clink protein is also an F-box protein and can bind to both SKP1 and the cell cycle protein pRBR, affecting cell cycle regulation (Aronson et al, 2000).…”

In Planta Recognition of a Double-Stranded RNA Synthesis Protein Complex by a Potexviral RNA Silencing Suppressor

“…Moreover, CMV 2b protein inhibits AGO1 activity through a physical interaction with its PAZ and PIWI domains (Zhang et al, 2006). Furthermore, beet western yellows virus P0 protein and cymbidium ringspot virus (CymRSV) p19 protein can decrease the accumulation of AGO1 protein (Baumberger et al, 2007;Bortolamiol et al, 2007;Várallyay et al, 2010). Thus, siRNA binding and AGO1 inactivation have been regarded as general mechanisms of RNA silencing suppression and studied in detail.…”