Browsing by Subject "MOLECULAR INSIGHTS"

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  • Swarnalok, De; Pollari, Maija; Varjosalo, Markku; Mäkinen, Kristiina (2020)
    In this study, we investigated the significance of a conserved five-amino acid motif 'AELPR' in the C-terminal region of helper component-proteinase (HCPro) for potato virus A (PVA; genusPotyvirus) infection. This motif is a putative interaction site for WD40 domain-containing proteins, including VARICOSE (VCS). We abolished the interaction site in HCPro by replacing glutamic acid (E) and arginine (R) with alanines (A) to generate HCPro(WD). These mutations partially eliminated HCPro-VCS co-localization in cells. We have earlier described potyvirus-induced RNA granules (PGs) in which HCPro and VCS co-localize and proposed that they have a role in RNA silencing suppression. We now demonstrate that the ability of HCPro(WD)to induce PGs, introduce VCS into PGs, and suppress RNA silencing was impaired. Accordingly, PVA carrying HCPro(WD)(PVA(WD)) infectedNicotiana benthamianaless efficiently than wild-type PVA (PVA(WT)) and HCPro(WD)complemented the lack of HCPro in PVA gene expression only partially. HCPro was purified from PVA-infected leaves as part of high molecular weight (HMW) ribonucleoprotein (RNP) complexes. These complexes were more stable when associated with wild-type HCPro than with HCPro(WD). Moreover, VCS and two viral components of the HMW-complexes, viral protein genome-linked and cylindrical inclusion protein were specifically decreased in HCPro(WD)-containing HMW-complexes. A boost in translation of replication-deficient PVA (PVA(Delta GDD)) was observed only if viral RNA expressed wild-type HCPro. The role of VCS-VPg-HCPro coordination in PVA translation was further supported by results from VCS silencing and overexpression experiments and by significantly elevated PVA-derivedRenillaluciferase vs PVA RNA ratio upon VPg-VCS co-expression. Finally, we found that PVA(WD)was unable to form virus particles or to spread systemically in the infected plant. We highlight the role of HCPro-VCS containing multi-protein assemblies associated with PVA RNA in protecting it from degradation, ensuring efficient translation, formation of stable virions and establishment of systemic infection. Author summary This study revealed that the potyviral helper component proteinase (HCPro) and the host protein VARICOSE (VCS) are linked in a manner that is important for suppression of RNA silencing, formation of potyvirus-induced RNA granules, translation of viral proteins, stability of virions, and development of systemic potato virus A (PVA) infection. The results suggest that HCPro and VCS belong to the core components of large RNP complexes regulating PVA infection. We suggest that these complexes protect viral RNAs in the cytoplasm after release from the replication complex and direct them to translation and intact to the viral particles.
  • Pollari, Maija; Swarnalok, De; Wang, Aiming; Mäkinen, Kristiina (2020)
    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.