The potyviral silencing suppressor HCPro recruits and employs host ARGONAUTE1 in pro-viral functions.

Author summary In the course of an infection viral proteins come regularly into contact with host factors. Knowledge of these virus-host interactions in plants is essential for understanding the progress of infections and for addressing challenges in the production of healthy plants, which is important for global food security. In this study we report that the potyviral Helper component proteinase (HCPro) recruits host ARGONAUTE1 protein and engages it in pro-viral functions. We found that the interaction of potato virus A HCPro with ARGONAUTE1 benefited the infection by promoting the stability and accumulation of virus particles. When HCPro was engineered to disrupt the interaction, we detected very few virus particles and observed that the mutated virus did not spread efficiently. Thus, we propose that the interaction between HCPro and ARGONAUTE1 has biological relevance as it is important for formation of stable particles and achieving optimal systemic infection. In this study, we demonstrate a novel pro-viral role for theNicotiana benthamianaARGONAUTE 1 (AGO1) in potyvirus infection. AGO1 strongly enhanced potato virus A (PVA) particle production and benefited the infection when supplied in excess. We subsequently identified the potyviral silencing suppressor, helper-component protease (HCPro), as the recruiter of host AGO1. After the identification of a conserved AGO1-binding GW/WG motif in potyviral HCPros, we used site-directed mutagenesis to introduce a tryptophan-to-alanine change into the HCPro (HCPro(AG)) of PVA (PVA(AG)) and turnip mosaic virus (TuMV(AG)). AGO1 co-localization and co-immunoprecipitation with PVA HCPro was significantly reduced by the mutation suggesting the interaction was compromised. Although the mutation did not interfere with HCPro's complementation or silencing suppression capacity, it nevertheless impaired virus particle accumulation and the systemic spread of both PVA and TuMV. Furthermore, we found that the HCPro-AGO1 interaction was important for AGO1's association with the PVA coat protein. The coat protein was also more stable in wild type PVA infection than in PVA(AG)infection. Based on these findings we suggest that potyviral HCPro recruits host AGO1 through its WG motif and engages AGO1 in the production of stable virus particles, which are required for an efficient systemic infection.