Mutagenic analysis of potato virus X movement protein (TGBp1) and the coat protein (CP): in vitro TGBp1-CP binding and viral RNA translation activation

Potato virus X: A global potato-infecting virus and type member of the Potexvirus genus

TAXONOMY

Potato virus X is the type-member of the plant-infecting Potexvirus genus in the family Alphaflexiviridae.

PHYSICAL PROPERTIES

Potato virus X (PVX) virions are flexuous filaments 460-480 nm in length. Virions are 13 nm in diameter and have a helical pitch of 3.4 nm. The genome is approximately 6.4 kb with a 5' cap and 3' poly(A) terminus. PVX contains five open reading frames, four of which are essential for cell-to-cell and systemic movement. One protein encodes the viral replicase. Cellular inclusions, known as X-bodies, occur near the nucleus of virus-infected cells.

HOSTS

The primary host is potato, but it infects a wide range of dicots. Diagnostic hosts include Datura stramonium and Nicotiana tabacum. PVX is transmitted in nature by mechanical contact. USEFUL WEBSITE: https://talk.ictvonline.org/ictv-reports/ictv_online_report/positive-sense-rna-viruses/w/alphaflexiviridae/1330/genus-potexvirus.

Translational Cross-Activation of the Encapsidated RNA of Potexviruses

We had shown the genomic RNA of potexviruses potato virus X and the alternanthera mosaic virus to be inaccessible in vitro to ribosomes while in intact virion form, but the RNAs can be translationally activated following the binding of movement protein 1 (MP1) to virus particles. Here, we present the results of the follow-up study targeting two more potexvirus species - the Narcissus mosaic virus and the Potato aucuba mosaic virus. We found encapsidated potexviral RNA to share common translational features in vitro and the MP1 to be potent over homological virions of its "own" species and over heterological virions of other species, as well exhibiting selective specificity. Reciprocal cross-activation is observed among viral species phylogenetically either close or distant. There is direct evidence that MP1 binding to the end of the virion is necessary, but not sufficient, for translational activation of encapsidated RNA.

Cell-to-cell movement of viruses via plasmodesmata

Plant viruses utilize plasmodesmata (PD), unique membrane-lined cytoplasmic nanobridges in plants, to spread infection cell-to-cell and long-distance. Such invasion involves a range of regulatory mechanisms to target and modify PD. Exciting discoveries in this field suggest that these mechanisms are executed by the interaction between plant cellular components and viral movement proteins (MPs) or other virus-encoded factors. Striking working analogies exist among endogenous non-cell-autonomous proteins and viral MPs, in which not only do they all use PD to traffic, but also they exploit same regulatory components to exert their functions. Thus, this review discusses on the viral strategies to move via PD and the PD-regulatory mechanisms involved in viral pathogenesis.

Host and viral RNA-binding proteins involved in membrane targeting, replication and intercellular movement of plant RNA virus genomes

Many plant viruses have positive-strand RNA [(+)RNA] as their genome. Therefore, it is not surprising that RNA-binding proteins (RBPs) play important roles during (+)RNA virus infection in host plants. Increasing evidence demonstrates that viral and host RBPs play critical roles in multiple steps of the viral life cycle, including translation and replication of viral genomic RNAs, and their intra- and intercellular movement. Although studies focusing on the RNA-binding activities of viral and host proteins, and their associations with membrane targeting, and intercellular movement of viral genomes have been limited to a few viruses, these studies have provided important insights into the molecular mechanisms underlying the replication and movement of viral genomic RNAs. In this review, we briefly overview the currently defined roles of viral and host RBPs whose RNA-binding activity have been confirmed experimentally in association with their membrane targeting, and intercellular movement of plant RNA virus genomes.

Understanding the intracellular trafficking and intercellular transport of potexviruses in their host plants

The movement of potexviruses through the cytoplasm to plasmodesmata (PD) and through PD to adjacent cells depends on the viral and host cellular proteins. Potexviruses encode three movement proteins [referred to as the triple gene block (TGB1-3)]. TGB1 protein moves cell-to-cell through PD and requires TGB2 and TGB3, which are endoplasmic reticulum (ER)-located proteins. TGB3 protein directs the movement of the ER-derived vesicles induced by TGB2 protein from the perinuclear ER to the cortical ER. TGB2 protein physically interacts with TGB3 protein in a membrane-associated form and also interacts with either coat protein (CP) or TGB1 protein at the ER network. Recent studies indicate that potexvirus movement involves the interaction between TGB proteins and CP with host proteins including membrane rafts. A group of host cellular membrane raft proteins, remorins, can serve as a counteracting membrane platform for viral ribonucleoprotein (RNP) docking and can thereby inhibit viral movement. The CP, which is a component of the RNP movement complex, is also critical for viral cell-to-cell movement through the PD. Interactions between TGB1 protein and/or the CP subunit with the 5'-terminus of genomic RNA [viral RNA (vRNA)] form RNP movement complexes and direct the movement of vRNAs through the PD. Recent studies show that tobacco proteins such as NbMPB2C or NbDnaJ-like proteins interact with the stem-loop 1 RNA located at the 5'-terminus of Potato virus X vRNA and regulate intracellular as well as intercellular movement. Although several host proteins that interact with vRNAs or viral proteins and that are crucial for vRNA transport have been screened and characterized, additional host proteins and details of viral movement remain to be characterized. In this review, we describe recent progress in understanding potexvirus movement within and between cells and how such movement is affected by interactions between vRNA/proteins and host proteins.

Replication and trafficking of a plant virus are coupled at the entrances of plasmodesmata

Plant viruses use movement proteins (MPs) to modify intercellular pores called plasmodesmata (PD) to cross the plant cell wall. Many viruses encode a conserved set of three MPs, known as the triple gene block (TGB), typified by Potato virus X (PVX). In this paper, using live-cell imaging of viral RNA (vRNA) and virus-encoded proteins, we show that the TGB proteins have distinct functions during movement. TGB2 and TGB3 established endoplasmic reticulum-derived membranous caps at PD orifices. These caps harbored the PVX replicase and nonencapsidated vRNA and represented PD-anchored viral replication sites. TGB1 mediated insertion of the viral coat protein into PD, probably by its interaction with the 5' end of nascent virions, and was recruited to PD by the TGB2/3 complex. We propose a new model of plant virus movement, which we term coreplicational insertion, in which MPs function to compartmentalize replication complexes at PD for localized RNA synthesis and directional trafficking of the virus between cells.

Ser/Thr kinase-like protein of Nicotiana benthamiana is involved in the cell-to-cell movement of Bamboo mosaic virus

To investigate the plant genes affected by Bamboo mosaic virus (BaMV) infection, we applied a cDNA-amplified fragment length polymorphism technique to screen genes with differential expression. A serine/threonine kinase-like (NbSTKL) gene of Nicotiana benthamiana is upregulated after BaMV infection. NbSTKL contains the homologous domain of Ser/Thr kinase. Knocking down the expression of NbSTKL by virus-induced gene silencing reduced the accumulation of BaMV in the inoculated leaves but not in the protoplasts. The spread of GFP-expressing BaMV in the inoculated leaves is also impeded by a reduced expression of NbSTKL. These data imply that NbSTKL facilitates the cell-to-cell movement of BaMV. The subcellular localization of NbSTKL is mainly on the cell membrane, which has been confirmed by mutagenesis and fractionation experiments. Combined with the results showing that active site mutation of NbSTKL does not change its subcellular localization but significantly affects BaMV accumulation, we conclude that NbSTKL may regulate BaMV movement on the cell membrane by its kinase-like activity. Moreover, the transient expression of NbSTKL does not significantly affect the accumulation of Cucumber mosaic virus (CMV) and Potato virus X (PVX); thus, NbSTKL might be a specific protein facilitating BaMV movement.

The TGB1 movement protein of Potato virus X reorganizes actin and endomembranes into the X-body, a viral replication factory

Potato virus X (PVX) requires three virally encoded proteins, the triple gene block (TGB), for movement between cells. TGB1 is a multifunctional protein that suppresses host gene silencing and moves from cell to cell through plasmodesmata, while TGB2 and TGB3 are membrane-spanning proteins associated with endoplasmic reticulum-derived granular vesicles. Here, we show that TGB1 organizes the PVX "X-body," a virally induced inclusion structure, by remodeling host actin and endomembranes (endoplasmic reticulum and Golgi). Within the X-body, TGB1 forms helically arranged aggregates surrounded by a reservoir of the recruited host endomembranes. The TGB2/3 proteins reside in granular vesicles within this reservoir, in the same region as nonencapsidated viral RNA, while encapsidated virions accumulate at the outer (cytoplasmic) face of the X-body, which comprises a highly organized virus "factory." TGB1 is both necessary and sufficient to remodel host actin and endomembranes and to recruit TGB2/3 to the X-body, thus emerging as the central orchestrator of the X-body. Our results indicate that the actin/endomembrane-reorganizing properties of TGB1 function to compartmentalize the viral gene products of PVX infection.

Potato virus X movement in Nicotiana benthamiana: new details revealed by chimeric coat protein variants

Potato virus X coat protein is necessary for both cell-to-cell and phloem transfer, but it has not been clarified definitively whether it is needed in both movement phases solely as a component of the assembled particles or also of differently structured ribonucleoprotein complexes. To clarify this issue, we studied the infection progression of a mutant carrying an N-terminal deletion of the coat protein, which was used to construct chimeric virus particles displaying peptides selectively affecting phloem transfer or cell-to-cell movement. Nicotiana benthamiana plants inoculated with expression vectors encoding the wild-type, mutant and chimeric viral genomes were examined by microscopy techniques. These experiments showed that coat protein-peptide fusions promoting cell-to-cell transfer only were not competent for virion assembly, whereas long-distance movement was possible only for coat proteins compatible with virus particle formation. Moreover, the ability of the assembled PVX to enter and persist into developing xylem elements was revealed here for the first time.

Analysis of the role of the coat protein N-terminal segment in Potato virus X virion stability and functional activity

Previously, we have reported that intact Potato virus X (PVX) virions cannot be translated in cell-free systems, but acquire this capacity by the binding of PVX-specific triple gene block protein 1 (TGBp1) or after phosphorylation of the exposed N-terminal segment of intravirus coat protein (CP) by protein kinases. With the help of in vitro mutagenesis, a nonphosphorylatable PVX mutant (denoted ST PVX) was prepared in which all 12 S and T residues in the 20-residue-long N-terminal CP segment were substituted by A or G. Contrary to expectations, ST PVX was infectious, produced normal progeny and was translated in vitro in the absence of any additional factors. We suggest that the N-terminal PVX CP segment somehow participates in virion assembly in vivo and that CP subunits in ST virions may differ in structure from those in the wild-type (UK3 strain). In the present work, to test this suggestion, we performed a comparative tritium planigraphy study of CP structure in UK3 and ST virions. It was found that the profile of tritium incorporation into ST mutant virions in some CP segments differed from that of normal UK3 virions and from UK3 complexed with the PVX movement protein TGBp1. It is proposed that amino acid substitutions in ST CP and the TGBp1-driven remodelling of UK3 virions induce structural alterations in intravirus CPs. These alterations affect the predicted RNA recognition motif of PVX CP, but in different ways: for ST PVX, labelling is increased in α-helices 6 and 7, whereas, in remodelled UK3, labelling is increased in the β-sheet strands β3, β4 and β5.

Phosphorylation of bamboo mosaic virus satellite RNA (satBaMV)-encoded protein P20 downregulates the formation of satBaMV-P20 ribonucleoprotein complex

Bamboo mosaic virus (BaMV) satellite RNA (satBaMV) depends on BaMV for its replication and encapsidation. SatBaMV-encoded P20 protein is an RNA-binding protein that facilitates satBaMV systemic movement in co-infected plants. Here, we examined phosphorylation of P20 and its regulatory functions. Recombinant P20 (rP20) was phosphorylated by host cellular kinase(s) in vitro, and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and mutational analyses revealed Ser-11 as the phosphorylation site. The phosphor-mimic rP20 protein interactions with satBaMV-translated mutant P20 were affected. In overlay assay, the Asp mutation at S11 (S11D) completely abolished the self-interaction of rP20 and significantly inhibited the interaction with both the WT and S11A rP20. In chemical cross-linking assays, S11D failed to oligomerize. Electrophoretic mobility shift assay and subsequent Hill transformation analysis revealed a low affinity of the phospho-mimicking rP20 for satBaMV RNA. Substantial modulation of satBaMV RNA conformation upon interaction with nonphospho-mimic rP20 in circular dichroism analysis indicated formation of stable satBaMV ribonucleoprotein complexes. The dissimilar satBaMV translation regulation of the nonphospho- and phospho-mimic rP20 suggests that phosphorylation of P20 in the ribonucleoprotein complex converts the translation-incompetent satBaMV RNA to messenger RNA. The phospho-deficient or phospho-mimicking P20 mutant of satBaMV delayed the systemic spread of satBaMV in co-infected Nicotiana benthamiana with BaMV. Thus, satBaMV likely regulates the formation of satBaMV RNP complex during co-infection in planta.

Characteristics of Artificial Virus-like Particles Assembled in vitro from Potato Virus X Coat Protein and Foreign Viral RNAs

Potato virus X (PVX) and some other potexviruses can be reconstitutedin vitrofrom viral coat protein (CP) and RNA. PVX CP is capable of forming viral ribonucleoprotein complexes (vRNP) not only with homologous, but also with foreign RNAs. This paper presents the structure and properties of vRNP assembledin vitroupon incubation of PVX CP and RNAs of various plant and animal viruses belonging to different taxonomic groups. We have shown that the morphology and translational properties of vRNPs containing foreign (heterologous) RNA are identical to those of homological vRNP (PVX RNA - PVX CP). Our data suggest that the assembly of the "mixed" vRNPin vitrocould be started at the 5'-proximal region of the RNA, producing a helical structure of vRNPs with foreign nucleic acids. The formation of heterologous vRNPin vitrowith PVX CP appears not to require a specific 5' end RNA nucleotide sequence, and the PVX CP seems to be able to pack foreign genetic material of various sizes and compositions into artificial virus-like particles.

Mixed infections of Pepino mosaic virus strains modulate the evolutionary dynamics of this emergent virus

Pepino mosaic virus (PepMV) is an emerging pathogen that causes severe economic losses in tomato crops (Solanum lycopersicum L.) in the Northern hemisphere, despite persistent attempts of control. In fact, it is considered one of the most significant viral diseases for tomato production worldwide, and it may constitute a good model for the analysis of virus emergence in crops. We have combined a population genetics approach with an analysis of in planta properties of virus strains to explain an observed epidemiological pattern. Hybridization analysis showed that PepMV populations are composed of isolates of two types (PepMV-CH2 and PepMV-EU) that cocirculate. The CH2 type isolates are predominant; however, EU isolates have not been displaced but persist mainly in mixed infections. Two molecularly cloned isolates belonging to each type have been used to examine the dynamics of in planta single infections and coinfection, revealing that the CH2 type has a higher fitness than the EU type. Coinfections expand the range of susceptible hosts, and coinfected plants remain symptomless several weeks after infection, so a potentially important problem for disease prevention and management. These results provide an explanation of the observed epidemiological pattern in terms of genetic and ecological interactions among the different viral strains. Thus, mixed infections appear to be contributing to shaping the genetic structure and dynamics of PepMV populations.