A viral suppressor of RNA silencing inhibits ARGONAUTE 1 function by precluding target RNA binding to pre-assembled RISC

Abstract

In most eukaryotes, RNA silencing is an adaptive immune system regulating key biological processes including antiviral defense. To evade this response, viruses of plants, worms and insects have evolved viral suppressors of RNA silencing proteins (VSRs). Various VSRs, such as P1 from Sweet potato mild mottle virus (SPMMV), inhibit the activity of RNA-induced silencing complexes (RISCs) including an ARGONAUTE (AGO) protein loaded with a small RNA. However, the specific mechanisms explaining this class of inhibition are unknown. Here, we show that SPMMV P1 interacts with AGO1 and AGO2 from Arabidopsis thaliana, but solely interferes with AGO1 function. Moreover, a mutational analysis of a newly identified zinc finger domain in P1 revealed that this domain could represent an effector domain as it is required for P1 suppressor activity but not for AGO1 binding. Finally, a comparative analysis of the target RNA binding capacity of AGO1 in the presence of wild-type or suppressor-defective P1 forms revealed that P1 blocks target RNA binding to AGO1. Our results describe the negative regulation of RISC, the small RNA containing molecular machine.

Antiviral ARGONAUTEs Against Turnip Crinkle Virus Revealed by Image-Based Trait Analysis

“…One such mechanism is interference of AGOs that mediate antiviral silencing. Accumulating evidence indicates that various VSRs use diverse modes of action on AGO proteins, such as promoting AGO degradation (Pazhouhandeh et al, 2006;Baumberger et al, 2007;Bortolamiol et al, 2007;Chiu et al, 2010), inhibiting the slicing activities of AGOs (Zhang et al, 2006), or interfering with factors upstream of AGO activity such as RNA-dependent RNA polymerasedependent silencing , obstructing siRNA-loaded RNA-induced silencing complex activity (Giner et al, 2010;Kenesi et al, 2017), or indirectly repressing AGO protein level (Várallyay et al, 2010). Because functional VSRs can mask host antiviral silencing effects, VSR-defective mutant viruses that can only successfully infect immunocompromised plants have been constructed to identify antiviral roles of key components in the silencing pathway during virus infection (Qu et al, 2008;Garcia-Ruiz et al, 2015).…”

An original structural fold underlies the multitask P1, a silencing suppressor encoded by the Rice yellow mottle virus

“…Other VSR proteins were previously shown to be organized in modules, including ZnF recognized domains pointing to their involvement in diverse viral functions (López et al, 2000;Dong et al, 2003;Van Wezel et al, 2003;Trinks et al, 2005;Yang et al, 2007;Yambao et al, 2008;Valli et al, 2008;Chiba et al, 2013;Lukhovitskaya et al, 2013;Kenesi et al, 2017;Fujita et al, 2018). To our knowledge, we here report the first 3D structure revealing ZnF features within a plant VSR protein, with very few similarities with other proteins in the databases.…”

Antiviral Roles of Abscisic Acid in Plants

“…For example, the movement protein “triple gene block protein 1” in several potexviruses has VSR function along with its role in virus movement ( Senshu et al, 2009 ; Lim et al, 2010 ; Brosseau and Moffett, 2015 ). Viruses often encode one VSR that can interfere the RNA silencing pathway at different steps such as binding dsRNA, preventing siRNA translocation or RISC assembly, or interacting with AGO proteins and impairing their silencing function ( Li and Ding, 2006 ; Jin and Zhu, 2010 ; Burgyan and Havelda, 2011 ; Kenesi et al, 2017 ).…”