Formation of Potato Virus A-Induced RNA Granules and Viral Translation Are Interrelated Processes Required for Optimal Virus Accumulation

RNA granules are cellular structures, which play an important role in mRNA translation, storage, and degradation. Animal (+)RNA viruses often co-opt RNA granule proteins for viral reproduction. However, the role of RNA granules in plant viral infections is poorly understood. Here we use Potato virus A (PVA) as a model potyvirus and demonstrate that the helper component-proteinase (HCpro), the potyviral suppressor of RNA silencing, induces the formation of RNA granules. We used confocal microscopy to demonstrate the presence of host RNA binding proteins including acidic ribosomal protein P0, argonaute 1 (AGO1), oligouridylate-binding protein 1 (UBP1), varicose (VCS) and eukaryotic initiation factor iso4E (eIF(iso)4E) in these potyvirus-induced RNA granules. We show that the number of potyviral RNA granules is down-regulated by the genome-linked viral protein (VPg). We demonstrated previously that VPg is a virus-specific translational regulator that co-operates with potyviral RNA granule components P0 and eIF(iso)4E in PVA translation. In this study we show that HCpro and varicose, components of potyviral RNA granules, stimulate VPg-promoted translation of the PVA, whereas UBP1 inhibits this process. Hence, we propose that PVA translation operates via a pathway that is interrelated with potyviral RNA granules in PVA infection. The importance of these granules is evident from the strong reduction in viral RNA and coat protein amounts that follows knock down of potyviral RNA granule components. HCpro suppresses antiviral RNA silencing during infection, and our results allow us to propose that this is also the functional context of the potyviral RNA granules we describe in this study.RNA颗粒（RNA granules）是一类细胞结构，在信使RNA（mRNA）的翻译、储存与降解过程中发挥关键作用。动物正链RNA病毒（Animal (+)RNA viruses）通常会招募宿主的RNA颗粒蛋白以完成自身病毒增殖。然而，目前学界对于RNA颗粒在植物病毒侵染过程中的功能尚缺乏深入认知。本研究以马铃薯A病毒（PVA）作为马铃薯Y病毒属（potyvirus）的模型病毒，证实作为RNA沉默（RNA silencing）抑制因子的马铃薯Y病毒属辅助成分-蛋白酶（HCpro）能够诱导RNA颗粒的形成。研究人员借助共聚焦显微镜（confocal microscopy），在这类由马铃薯Y病毒属诱导形成的RNA颗粒中，检测到包括酸性核糖体蛋白P0、Argonaute蛋白1（AGO1）、寡尿苷酸结合蛋白1（UBP1）、varicose（VCS）以及真核起始因子iso4E（eIF(iso)4E）在内的多种宿主RNA结合蛋白。研究结果显示，基因组连接型病毒蛋白（VPg）可下调这类马铃薯Y病毒属RNA颗粒的数量。本团队此前已证实，VPg作为一种病毒特异性翻译调控因子，可与马铃薯Y病毒属RNA颗粒组分P0及eIF(iso)4E协同参与PVA的翻译过程。本研究进一步发现，HCpro与VCS这两种马铃薯Y病毒属RNA颗粒的组分，能够促进VPg介导的PVA翻译，而UBP1则对该过程起到抑制作用。据此，研究团队提出，在PVA侵染过程中，病毒的翻译通路与马铃薯Y病毒属RNA颗粒存在相互关联的调控网络。通过敲低马铃薯Y病毒属RNA颗粒的组分，可显著降低病毒RNA与衣壳蛋白的积累量，这直接印证了这类RNA颗粒的重要功能。HCpro在侵染过程中能够抑制宿主的抗病毒RNA沉默，结合本研究结果，我们认为本研究中所描述的马铃薯Y病毒属RNA颗粒的功能同样隶属于该抗病毒RNA沉默的调控背景。