New Insights into the Nucleolar Localization of a Plant RNA Virus-Encoded Protein That Acts in Both RNA Packaging and RNA Silencing Suppression: Involvement of Importins Alpha and Relevance for Viral Infection

Abstract

Despite the fact that replication of plus-strand RNA viruses takes place in the cytoplasm of host cells, different proteins encoded by these infectious agents have been shown to localize in the nucleus, with high accumulation at the nucleolus. In most cases, the molecular determinants or biological significance of such subcellular localization remains elusive. Recently, we reported that protein p37 encoded by Pelargonium line pattern virus (family Tombusviridae) acts in both RNA packaging and RNA silencing suppression. Consistently with these functions, p37 was detected in the cytoplasm of plant cells, although it was also present in the nucleus and, particularly, in the nucleolus. Here, we searched for further insights into factors influencing p37 nucleolar localization and into its potential relevance for viral infection. Besides mapping the protein region containing the nucleolar localization signal, we have found that p37 interacts with distinct members of the importin alpha family-main cellular transporters for nucleo-cytoplasmic traffic of proteins-and that these interactions are crucial for nucleolar targeting of p37. Impairment of p37 nucleolar localization through downregulation of importin alpha expression resulted in a reduction of viral accumulation, suggesting that sorting of the protein to the major subnuclear compartment is advantageous for the infection process.

Identification of the Potential Virulence Factors and RNA Silencing Suppressors of Mulberry Mosaic Dwarf-Associated Geminivirus

“…To explore which region(s) within MMDaV V2 is crucial for suppressing RNA silencing, we analyzed the MMDaV V2 sequence, and found that the RRLLRLIRRFSRVKDR motif between amino acid 61 and 76 defined a bipartite basic nuclear localization signal (NLS). Previous studies have indicated that the NLS of several VSRs is essential for the RNA silencing suppressor activity [ 31 , 37 , 38 , 39 ], prompting us to determine whether this motif is crucial for V2 to suppress PTGS. At a first step, we determined the subcellular localization of V2 via transient expression of eGFP-fused constructs into N. benthamiana leaves.…”

“…Despite the fact that deletion of the putative NLS from MMDaV V2 did not prevent translocation of the protein into the plant nucleus, mutagenesis experiment demonstrated that the predicted basic motif of MMDaV V2 is indispensable not only for subnuclear foci localization but also for silencing suppression. In previous studies, nuclear import was found mandatory for several VSRs, such as βC1 of tomato yellow leaf curl China betasatellite and tomato leaf curl China betasatellite, P37 of Pelargonium line pattern virus, and P6 of cauliflower mosaic virus (CaMV) and strawberry vein banding virus, to suppress RNA silencing [ 31 , 37 , 38 , 39 , 54 ]. Nuclear import of CaMV P6 is conducted via two importin-α-dependent NLSs, and it is required for CaMV infection and suppression of the nuclear RNA silencing factor dsRNA-binding protein 4 (DRB4) [ 54 ].…”

Importin-αs are required for the nuclear localization and function of the Plasmopara viticola effector PvAVH53

“…On the other hand, importin-αs also benefit pathogens, such as Pelargonium line pattern virus and Xanthomonas oryzae pv. oryzicola 9,10,49 .…”

African Swine Fever Virus MGF360-12L Inhibits Type I Interferon Production by Blocking the Interaction of Importin α and NF-κB Signaling Pathway

“…Karyopherins including karyopherin-α (KPNA) and karyopherin-β (KPNB) is a nuclear transport receptor that is responsible for transporting its specific cargo proteins, which harbor nuclear localization signals (NLSs) through the nuclear pore complex (NPCs) (Beaudet et al 2020 ; Hazawa et al 2020 ). Some viruses, such as Japanese encephalitis virus (JEV) (Ye et al 2017 ), Ebola virus (EBOV) (Reid et al 2006 ), pelargonium line pattern virus (Perez-Canamas and Hernandez 2018 ), hantavirus (HTNV) (Taylor et al 2009 ), prevent nuclear import of cellular cargo molecules by binding importin family proteins or down-regulating importin alpha expression. Porcine reproductive and respiratory syndrome virus (PRRSV) induces KPNA1 degradation and inhibits ISGF3 Nucleation (Wang et al 2013 ).…”