P6 protein of Cauliflower mosaic virus, a translation reinitiator, interacts with ribosomal protein L13 from Arabidopsis thaliana

The interaction between the lemon ribosomal protein ClRPS9-2 and citrus yellow vein clearing virus coat protein affects viral infection and gene silencing suppressor activity

Citrus yellow vein clearing virus (CYVCV) is an emerging virus that causes serious economic damage to the lemon industry worldwide. The coat protein (CP) of CYVCV is a strong RNA silencing suppressor and is associated with the severity of symptoms in citrus, yet the interaction between CP and host factors remains unknown. In this study, the 40S ribosomal subunit protein S9-2 (ClRPS9-2) was identified as a CP-binding partner using the yeast two-hybrid system from a lemon (cv. Eureka) cDNA library, and the interaction between CP and ClRPS9-2 was demonstrated by in vivo methods. The results suggest that the N-terminal 8-108 amino acid sequence of ClRPS9-2 is crucial for its interaction with CP and may be associated with the nuclear localization of ClRPS9-2. The accumulation and silencing suppressor activity of CP were reduced by transient expression of ClRPS9-2 in Nicotiana benthamiana. Reverse transcription-quantitative PCR analysis showed that the content of CYVCV in ClRPS9-2 transgenic Eureka lemon plants was approximately 50% of that in CYVCV-infected wild-type plants 1 month after inoculation, and mild yellowing and vein clearing symptoms were observed in the transgenic plants. These findings demonstrate that ClRPS9-2 plays a role in host defensive reactions, and the enhanced resistance of transgenic plants to CYVCV may be associated with the up-regulation of salicylic acid-related and R genes.

Cauliflower mosaic virus protein P6 is a multivalent node for RNA granule proteins and interferes with stress granule responses during plant infection

Biomolecular condensation is a multipurpose cellular process that viruses use ubiquitously during their multiplication. Cauliflower mosaic virus replication complexes are condensates that differ from those of most viruses, as they are nonmembranous assemblies that consist of RNA and protein, mainly the viral protein P6. Although these viral factories (VFs) were described half a century ago, with many observations that followed since, functional details of the condensation process and the properties and relevance of VFs have remained enigmatic. Here, we studied these issues in Arabidopsis thaliana and Nicotiana benthamiana. We observed a large dynamic mobility range of host proteins within VFs, while the viral matrix protein P6 is immobile, as it represents the central node of these condensates. We identified the stress granule (SG) nucleating factors G3BP7 and UBP1 family members as components of VFs. Similarly, as SG components localize to VFs during infection, ectopic P6 localizes to SGs and reduces their assembly after stress. Intriguingly, it appears that soluble rather than condensed P6 suppresses SG formation and mediates other essential P6 functions, suggesting that the increased condensation over the infection time-course may accompany a progressive shift in selected P6 functions. Together, this study highlights VFs as dynamic condensates and P6 as a complex modulator of SG responses.

Functional analogs of mammalian 4E-BPs reveal a role for TOR in global plant translation

Mammalian/mechanistic target of rapamycin (mTOR) regulates global protein synthesis through inactivation of eIF4E-binding proteins (m4E-BPs) in response to nutrient and energy availability. Until now, 4E-BPs have been considered as metazoan inventions, and how target of rapamycin (TOR) controls cap-dependent translation initiation in plants remains obscure. Here, we present short unstructured 4E-BP-like Arabidopsis proteins (4EBP1/4EBP2) that are non-homologous to m4E-BPs except for the eIF4E-binding motif and TOR phosphorylation sites. Unphosphorylated 4EBPs exhibit strong affinity toward eIF4Es and can inhibit formation of the cap-binding complex. Upon TOR activation, 4EBPs are phosphorylated, probably when bound directly to TOR, and likely relocated to ribosomes. 4EBPs can suppress a distinct set of mRNAs; 4EBP2 predominantly inhibits translation of core cell-cycle regulators CycB1;1 and CycD1;1, whereas 4EBP1 interferes with chlorophyll biosynthesis. Accordingly, 4EBP2 overexpression halts early seedling development, which is overcome by induction of Glc/Suc-TOR signaling. Thus, TOR regulates cap-dependent translation initiation by inactivating atypical 4EBPs in plants.

A Capsid Protein Fragment of a Fusagra-like Virus Found in Carica papaya Latex Interacts with the 50S Ribosomal Protein L17

Papaya sticky disease is caused by the association of a fusagra-like and an umbra-like virus, named papaya meleira virus (PMeV) and papaya meleira virus 2 (PMeV2), respectively. Both viral genomes are encapsidated in particles formed by the PMeV ORF1 product, which has the potential to encode a protein with 1563 amino acids (aa). However, the structural components of the viral capsid are unknown. To characterize the structural proteins of PMeV and PMeV2, virions were purified from Carica papaya latex. SDS-PAGE analysis of purified virus revealed two major proteins of ~40 kDa and ~55 kDa. Amino-terminal sequencing of the ~55 kDa protein and LC-MS/MS of purified virions indicated that this protein starts at aa 263 of the deduced ORF1 product as a result of either degradation or proteolytic processing. A yeast two-hybrid assay was used to identify Arabidopsis proteins interacting with two PMeV ORF1 product fragments (aa 321-670 and 961-1200). The 50S ribosomal protein L17 (AtRPL17) was identified as potentially associated with modulated translation-related proteins. In plant cells, AtRPL17 co-localized and interacted with the PMeV ORF1 fragments. These findings support the hypothesis that the interaction between PMeV/PMeV2 structural proteins and RPL17 is important for virus-host interactions.

Analysis of Interaction Network Between Host Protein and M Protein of Swine Acute Diarrhea Syndrome Coronavirus

Swine acute diarrhea syndrome coronavirus (SADS-CoV) is an enterovirus that can cause acute diarrhea and death in piglets and cause serious economic losses to the pig industry. SADS-CoV membrane (M) protein mainly plays a key role in biological processes, such as virus assembly, budding, and host innate immune regulation. Understanding the interaction between M protein and host proteins is very important to define the molecular mechanism of cells at the protein level and to understand specific cellular physiological pathways. In this study, 289 host proteins interacting with M protein were identified by glutathione-S-transferase (GST) pull-down combined with liquid chromatography-mass spectrometry (LC-MS/MS), and the protein-protein interaction (PPI) network was established by Gene Ontology (GO) terms and Kyoto Encyclopedia of Gene and Genomes (KEGG) pathways analysis. Results showed that SADS-CoV M protein was mainly associated with the host metabolism, signal transduction, and innate immunity. The Co-Immunoprecipitation (CO-IP) validation results of six randomly selected proteins, namely, Rab11b, voltage-dependent anion-selective channel 1 (VDAC1), Ribosomal Protein L18 (RPL18), RALY, Ras Homolog Family Member A (RHOA), and Annexin A2 (ANXA2), were consistent with LC-MS results. In addition, overexpression of RPL18 and PHOA significantly promoted SADS-CoV replication, while overexpression of RALY antagonized viral replication. This work will help to clarify the function of SADS-CoV M protein in the life cycle of SADS-CoV.

RPLP1, an NS4B-interacting protein, enhances production of CSFV through promoting translation of viral genome

Classical swine fever virus (CSFV), the etiological agent of classical swine fever (CSF), causes serious financial losses to the pig industry. Using yeast two-hybrid screening, we have previously identified ribosomal protein RPLP1 as a potential binding partner of CSFV NS4B. In this study, the interaction between host RPLP1 and CSFV NS4B was further characterized by co-immunoprecipitation (co-IP), glutathione S-transferase (GST) pulldown, and confocal microscopy. In addition, lentivirus-mediated shRNA knockdown of RPLP1 drastically attenuated CSFV growth, while stable overexpression of RPLP1 markedly enhanced CSFV production. Moreover, cellular RPLP1 expression was found to be significantly up-regulated along with CSFV infection. Dual-luciferase reporter assay showed that depletion of RPLP1 had no effects on the activity of CSFV internal ribosome entry site (IRES). In the first life cycle of CSFV, further studies revealed that RPLP1 depletion did not influence the intracellular viral RNA abundance but diminished the intracellular and extracellular progeny virus titers as well as the viral E2 protein expression, which indicates that RPLP1 is crucial for CSFV genome translation. In summary, this study demonstrated that RPLP1 interacts with CSFV NS4B and enhances virus production via promoting translation of viral genome.

Proteome Mapping of South African Cassava Mosaic Virus-Infected Susceptible and Tolerant Landraces of Cassava

The production of cassava is threatened by the geminivirus South African cassava mosaic virus (SACMV), which causes cassava mosaic disease. Cassava landrace TME3 shows tolerance to SACMV, while T200 is highly susceptible. This study aimed to identify the leaf proteome involved in anti-viral defence. Liquid chromatography mass spectrometry (LC-MS) identified 2682 (54 differentially expressed) and 2817 (206 differentially expressed) proteins in both landraces at systemic infection (32 days post infection) and symptom recovery (67 days post infection), respectively. Differences in the number of differentially expressed proteins (DEPs) between the two landraces were observed. Gene ontology analysis showed that defence-associated pathways such as the chloroplast, proteasome, and ribosome were overrepresented at 67 days post infection (dpi) in SACMV-tolerant TME3. At 67 dpi, a high percentage (56%) of over-expressed proteins were localized in the chloroplast in TME3 compared to T200 (31% under-expressed), proposing that chloroplast proteins play a role in tolerance in TME3. Ribosomal_L7Ae domain-containing protein (Manes.12G139100) was over-expressed uniquely in TME3 at 67 dpi and interacts with the ribosomal protein Sac52 (RPL10). RPL10 is a known key player in the NIK1-mediated effector triggered immunity (ETI) response to geminivirus infection, indicating a possible role for Sac52 in SACMV recovery in TME3. In conclusion, differential protein expression responses in TME3 and T200 may be key to unravel tolerance to CMD.

Phosphorylation of a reinitiation supporting protein, RISP, determines its function in translation reinitiation

Reinitiation supporting protein, RISP, interacts with 60S (60S ribosomal subunit) and eIF3 (eukaryotic initiation factor 3) in plants. TOR (target-of-rapamycin) mediates RISP phosphorylation at residue Ser267, favoring its binding to eL24 (60S ribosomal protein L24). In a viral context, RISP, when phosphorylated, binds the CaMV transactivator/ viroplasmin, TAV, to assist in an exceptional mechanism of reinitiation after long ORF translation. Moreover, we show here that RISP interacts with eIF2 via eIF2β and TOR downstream target 40S ribosomal protein eS6. A RISP phosphorylation knockout, RISP-S267A, binds preferentially eIF2β, and both form a ternary complex with eIF3a in vitro. Accordingly, transient overexpression in plant protoplasts of RISP-S267A, but not a RISP phosphorylation mimic, RISP-S267D, favors translation initiation. In contrast, RISP-S267D preferentially binds eS6, and, when bound to the C-terminus of eS6, can capture 60S in a highly specific manner in vitro, suggesting that it mediates 60S loading during reinitiation. Indeed, eS6-deficient plants are highly resistant to CaMV due to their reduced reinitiation capacity. Strikingly, an eS6 phosphomimic, when stably expressed in eS6-deficient plants, can fully restore the reinitiation deficiency of these plants in cellular and viral contexts. These results suggest that RISP function in translation (re)initiation is regulated by phosphorylation at Ser267.

Cucumber Ribosomal Protein CsRPS21 Interacts With P22 Protein of Cucurbit Chlorotic Yellows Virus

Cucurbit chlorotic yellows virus (CCYV) is a cucurbit-infecting crinivirus. RNA silencing can be initiated as a plant defense against viruses. Viruses encode various RNA silencing suppressors to counteract antiviral silencing. P22 protein encoded by RNA1 of CCYV is a silencing suppressor, but its mechanism of action remains unclear. In this study, the cucumber ribosomal-like protein CsRPS21 was found to interact with P22 protein in vitro and in vivo. A conserved CsRPS21 domain was indispensable for its nuclear localization and interaction with P22. Transient expression of CsRPS21 in Nicotiana benthamiana leaves interfered with P22 accumulation and inhibited P22 silencing suppressor activity. CsRPS21 expression in N. benthamiana protoplasts inhibited CCYV accumulation. Increasing numbers of ribosomal proteins are being found to be involved in viral infections of plants. We identified a P22-interacting ribosomal protein, CsRPS21, and uncovered its role in early viral replication and silencing suppressor activity. Our study increases knowledge of the function of ribosomal proteins during viral infection.

Cauliflower mosaic virus P6 inclusion body formation: A dynamic and intricate process

During an infection, Cauliflower mosaic virus (CaMV) forms inclusion bodies (IBs) mainly composed of viral protein P6, where viral activities occur. Because viral processes occur in IBs, understanding the mechanisms by which they are formed is crucial. FL-P6 expressed in N. benthamiana leaves formed IBs of a variety of shapes and sizes. Small IBs were dynamic, undergoing fusion/dissociation events. Co-expression of actin-binding polypeptides with FL-P6 altered IB size distribution and inhibited movement. This suggests that IB movement is required for fusion and growth. A P6 deletion mutant was discovered that formed a single large IB per cell, which suggests it exhibited altered fusion/dissociation dynamics. Myosin-inhibiting drugs did not affect small IB movement, while those inhibiting actin polymerization did. Large IBs colocalized with components of the aggresome pathway, while small ones generally did not. This suggests a possible involvement of the aggresome pathway in large IB formation.

Fatal attraction: The roles of ribosomal proteins in the viral life cycle

Upon viral infection of a host cell, each virus starts a program to generate many progeny viruses. Although viruses interact with the host cell in numerous ways, one critical step in the virus life cycle is the expression of viral proteins, which are synthesized by the host ribosomes in conjunction with host translation factors. Here we review different mechanisms viruses have evolved to effectively seize host cell ribosomes, the roles of specific ribosomal proteins and their posttranslational modifications on viral RNA translation, or the cellular response to infection. We further highlight ribosomal proteins with extra-ribosomal function during viral infection and put the knowledge of ribosomal proteins during viral infection into the larger context of ribosome-related diseases, known as ribosomopathies. This article is categorized under: Translation > Translation Mechanisms Translation > Translation Regulation.

A conserved motif in three viral movement proteins from different genera is required for host factor recruitment and cell-to-cell movement

Due to their minimal genomes, plant viruses are forced to hijack specific cellular pathways to ensure host colonization, a condition that most frequently involves physical interaction between viral and host proteins. Among putative viral interactors are the movement proteins, responsible for plasmodesma gating and genome binding during viral transport. Two of them, DGBp1 and DGBp2, are required for alpha-, beta- and gammacarmovirus cell-to-cell movement, but the number of DGBp-host interactors identified at present is limited. By using two different approaches, yeast two-hybrid and bimolecular fluorescence complementation assays, we found three Arabidopsis factors, eIF3g1, RPP3A and WRKY36, interacting with DGBp1s from each genus mentioned above. eIF3g1 and RPP3A are mainly involved in protein translation initiation and elongation phases, respectively, while WRKY36 belongs to WRKY transcription factor family, important regulators of many defence responses. These host proteins are not expected to be associated with viral movement, but knocking out WRKY36 or silencing either RPP3A or eIF3g1 negatively affected Arabidopsis infection by Turnip crinkle virus. A highly conserved FNF motif at DGBp1 C-terminus was required for protein-protein interaction and cell-to-cell movement, suggesting an important biological role.

Regulation of Ribosomal Proteins on Viral Infection

Ribosomal proteins (RPs), in conjunction with rRNA, are major components of ribosomes involved in the cellular process of protein biosynthesis, known as "translation". The viruses, as the small infectious pathogens with limited genomes, must recruit a variety of host factors to survive and propagate, including RPs. At present, more and more information is available on the functional relationship between RPs and virus infection. This review focuses on advancements in my own understanding of critical roles of RPs in the life cycle of viruses. Various RPs interact with viral mRNA and proteins to participate in viral protein biosynthesis and regulate the replication and infection of virus in host cells. Most interactions are essential for viral translation and replication, which promote viral infection and accumulation, whereas the minority represents the defense signaling of host cells by activating immune pathway against virus. RPs provide a new platform for antiviral therapy development, however, at present, antiviral therapeutics with RPs involving in virus infection as targets is limited, and exploring antiviral strategy based on RPs will be the guides for further study.

Ribosome Shunting, Polycistronic Translation, and Evasion of Antiviral Defenses in Plant Pararetroviruses and Beyond

Viruses have compact genomes and usually translate more than one protein from polycistronic RNAs using leaky scanning, frameshifting, stop codon suppression or reinitiation mechanisms. Viral (pre-)genomic RNAs often contain long 5′-leader sequences with short upstream open reading frames (uORFs) and secondary structure elements, which control both translation initiation and replication. In plants, viral RNA and DNA are targeted by RNA interference (RNAi) generating small RNAs that silence viral gene expression, while viral proteins are recognized by innate immunity and autophagy that restrict viral infection. In this review we focus on plant pararetroviruses of the family Caulimoviridae and describe the mechanisms of uORF- and secondary structure-driven ribosome shunting, leaky scanning and reinitiation after translation of short and long uORFs. We discuss conservation of these mechanisms in different genera of Caulimoviridae, including host genome-integrated endogenous viral elements, as well as in other viral families, and highlight a multipurpose use of the highly-structured leader sequence of plant pararetroviruses in regulation of translation, splicing, packaging, and reverse transcription of pregenomic RNA (pgRNA), and in evasion of RNAi. Furthermore, we illustrate how targeting of several host factors by a pararetroviral effector protein can lead to transactivation of viral polycistronic translation and concomitant suppression of antiviral defenses. Thus, activation of the plant protein kinase target of rapamycin (TOR) by the Cauliflower mosaic virus transactivator/viroplasmin (TAV) promotes reinitiation of translation after long ORFs on viral pgRNA and blocks antiviral autophagy and innate immunity responses, while interaction of TAV with the plant RNAi machinery interferes with antiviral silencing.

Formation of large viroplasms and virulence of Cauliflower mosaic virus in turnip plants depend on the N-terminal EKI sequence of viral protein TAV

Cauliflower mosaic virus (CaMV) TAV protein (TransActivator/Viroplasmin) plays a pivotal role during the infection cycle since it activates translation reinitiation of viral polycistronic RNAs and suppresses RNA silencing. It is also the major component of cytoplasmic electron-dense inclusion bodies (EDIBs) called viroplasms that are particularly evident in cells infected by the virulent CaMV Cabb B-JI isolate. These EDIBs are considered as virion factories, vehicles for CaMV intracellular movement and reservoirs for CaMV transmission by aphids. In this study, focused on different TAV mutants in vivo, we demonstrate that three physically separated domains collectively participate to the formation of large EDIBs: the N-terminal EKI motif, a sequence of the MAV domain involved in translation reinitiation and a C-terminal region encompassing the zinc finger. Surprisingly, EKI mutant TAVm3, corresponding to a substitution of the EKI motif at amino acids 11-13 by three alanines (AAA), which completely abolished the formation of large viroplasms, was not lethal for CaMV but highly reduced its virulence without affecting the rate of systemic infection. Expression of TAVm3 in a viral context led to formation of small irregularly shaped inclusion bodies, mild symptoms and low levels of viral DNA and particles accumulation, despite the production of significant amounts of mature capsid proteins. Unexpectedly, for CaMV-TAVm3 the formation of viral P2-containing electron-light inclusion body (ELIB), which is essential for CaMV aphid transmission, was also altered, thus suggesting an indirect role of the EKI tripeptide in CaMV plant-to-plant propagation. This important functional contribution of the EKI motif in CaMV biology can explain the strict conservation of this motif in the TAV sequences of all CaMV isolates.

Setting Up Shop: The Formation and Function of the Viral Factories of Cauliflower mosaic virus

Similar to cells, viruses often compartmentalize specific functions such as genome replication or particle assembly. Viral compartments may contain host organelle membranes or they may be mainly composed of viral proteins. These compartments are often termed: inclusion bodies (IBs), viroplasms or viral factories. The same virus may form more than one type of IB, each with different functions, as illustrated by the plant pararetrovirus, Cauliflower mosaic virus (CaMV). CaMV forms two distinct types of IBs in infected plant cells, those composed mainly of the viral proteins P2 (which are responsible for transmission of CaMV by insect vectors) and P6 (required for viral intra-and inter-cellular infection), respectively. P6 IBs are the major focus of this review. Much of our understanding of the formation and function of P6 IBs comes from the analyses of their major protein component, P6. Over time, the interactions and functions of P6 have been gradually elucidated. Coupled with new technologies, such as fluorescence microscopy with fluorophore-tagged viral proteins, these data complement earlier work and provide a clearer picture of P6 IB formation. As the activities and interactions of the viral proteins have gradually been determined, the functions of P6 IBs have become clearer. This review integrates the current state of knowledge on the formation and function of P6 IBs to produce a coherent model for the activities mediated by these sophisticated virus-manufacturing machines.

Differential Requirement of the Ribosomal Protein S6 and Ribosomal Protein S6 Kinase for Plant-Virus Accumulation and Interaction of S6 Kinase with Potyviral VPg

Ribosomal protein S6 (RPS6) is an indispensable plant protein regulated, in part, by ribosomal protein S6 kinase (S6K) which, in turn, is a key regulator of plant responses to stresses and developmental cues. Increased expression of RPS6 was detected in Nicotiana benthamiana during infection by diverse plant viruses. Silencing of the RPS6 and S6K genes in N. benthamiana affected accumulation of Cucumber mosaic virus, Turnip mosaic virus (TuMV), and Potato virus A (PVA) in contrast to Turnip crinkle virus and Tobacco mosaic virus. In addition, the viral genome-linked protein (VPg) of TuMV and PVA interacted with S6K in plant cells, as detected by bimolecular fluorescence complementation assay. The VPg-S6K interaction was detected in cytoplasm, nucleus, and nucleolus, whereas the green fluorescent protein-tagged S6K alone showed cytoplasmic localization only. These results demonstrate that the requirement for RPS6 and S6K differs for diverse plant viruses with different translation initiation strategies and suggest that potyviral VPg-S6K interaction may affect S6K functions in both the cytoplasm and the nucleus.

Plant Translation Factors and Virus Resistance

Plant viruses recruit cellular translation factors not only to translate their viral RNAs but also to regulate their replication and potentiate their local and systemic movement. Because of the virus dependence on cellular translation factors, it is perhaps not surprising that many natural plant recessive resistance genes have been mapped to mutations of translation initiation factors eIF4E and eIF4G or their isoforms, eIFiso4E and eIFiso4G. The partial functional redundancy of these isoforms allows specific mutation or knock-down of one isoform to provide virus resistance without hindering the general health of the plant. New possible targets for antiviral strategies have also been identified following the characterization of other plant translation factors (eIF4A-like helicases, eIF3, eEF1A and eEF1B) that specifically interact with viral RNAs and proteins and regulate various aspects of the infection cycle. Emerging evidence that translation repression operates as an alternative antiviral RNA silencing mechanism is also discussed. Understanding the mechanisms that control the development of natural viral resistance and the emergence of virulent isolates in response to these plant defense responses will provide the basis for the selection of new sources of resistance and for the intelligent design of engineered resistance that is broad-spectrum and durable.

Mutations within A 35 amino acid region of P6 influence self-association, inclusion body formation, and Caulimovirus infectivity

Cauliflower mosaic virus gene VI product (P6) is an essential protein that forms cytoplasmic, inclusion bodies (IBs). P6 contains four regions involved in self-association, termed D1-D4. D3 binds to D1, along with D4 and contains a spacer region (termed D3b) between two RNA-binding domains. Here we show D3b binds full-length P6 along with D1 and D4. Full-length P6s harboring single amino acid substitutions within D3b showed reduced binding to both D1 and D4. Full-length P6s containing D3b mutations and fused with green fluorescent protein formed inclusion-like bodies (IL-Bs) when expressed in Nicotiana benthamiana leaves. However, mutant P6s with reduced binding to D1 and D4, showed smaller IL-Bs, than wild type. Likewise, viruses containing these mutations showed a decrease in inoculated leaf viral DNA levels and reduced efficiency of systemic infection. These data suggest that mutations influencing P6 self-association alter IB formation and reduce virus infection.

Tobacco etch virus protein P1 traffics to the nucleolus and associates with the host 60S ribosomal subunits during infection

The genus Potyvirus comprises a large group of positive-strand RNA plant viruses whose genome encodes a large polyprotein processed by three viral proteinases. P1 protein, the most amino-terminal product of the polyprotein, is an accessory factor stimulating viral genome amplification whose role during infection is not well understood. We infected plants with Tobacco etch virus (TEV; genus Potyvirus) clones in which P1 was tagged with a fluorescent protein to track its expression and subcellular localization or with an affinity tag to identify host proteins involved in complexes in which P1 also takes part during infection. Our results showed that TEV P1 exclusively accumulates in infected cells at an early stage of infection and that the protein displays a dynamic subcellular localization, trafficking in and out of the nucleus and nucleolus during infection. Inside the nucleolus, P1 particularly targets the dense granular component. Consistently, we found functional nucleolar localization and nuclear export signals in TEV P1 sequence. Our results also indicated that TEV P1 physically interacts with the host 80S cytoplasmic ribosomes and specifically binds to the 60S ribosomal subunits during infection. In vitro translation assays of reporter proteins suggested that TEV P1 stimulates protein translation, particularly when driven from the TEV internal ribosome entry site. These in vitro assays also suggested that TEV helper-component proteinase (HC-Pro) inhibits protein translation. Based on these findings, we propose that TEV P1 stimulates translation of viral proteins in infected cells.

IMPORTANCE

In this work, we researched the role during infection of tobacco etch virus P1 protease. P1 is the most mysterious protein of potyviruses, a relevant group of RNA viruses infecting plants. Our experiments showed that the viral P1 protein exclusively accumulates in infected cells at an early stage of infection and moves in and out of the nucleus of infected cells, particularly targeting the nucleolus. Our experiments also showed that P1 protein binds host ribosomes during infection. Based on these findings and other in vitro experiments we propose that P1 protein stimulates translation of viral proteins during infection.

Identification of the domains of cauliflower mosaic virus protein P6 responsible for suppression of RNA silencing and salicylic acid signalling

Cauliflower mosaic virus (CaMV) encodes a 520 aa polypeptide, P6, which participates in several essential activities in the virus life cycle including suppressing RNA silencing and salicylic acid-responsive defence signalling. We infected Arabidopsis with CaMV mutants containing short in-frame deletions within the P6 ORF. A deletion in the distal end of domain D-I (the N-terminal 112 aa) of P6 did not affect virus replication but compromised symptom development and curtailed the ability to restore GFP fluorescence in a GFP-silenced transgenic Arabidopsis line. A deletion in the minimum transactivator domain was defective in virus replication but retained the capacity to suppress RNA silencing locally. Symptom expression in CaMV-infected plants is apparently linked to the ability to suppress RNA silencing. When transiently co-expressed with tomato bushy stunt virus P19, an elicitor of programmed cell death in Nicotiana tabacum, WT P6 suppressed the hypersensitive response, but three mutants, two with deletions within the distal end of domain D-I and one involving the N-terminal nuclear export signal (NES), were unable to do so. Deleting the N-terminal 20 aa also abolished the suppression of pathogen-associated molecular pattern-dependent PR1a expression following agroinfiltration. However, the two other deletions in domain D-I retained this activity, evidence that the mechanisms underlying these functions are not identical. The D-I domain of P6 when expressed alone failed to suppress either cell death or PR1a expression and is therefore necessary but not sufficient for all three defence suppression activities. Consequently, concerns about the biosafety of genetically modified crops carrying truncated ORFVI sequences appear unfounded.

Plant science and agricultural productivity: why are we hitting the yield ceiling?

Trends in conventional plant breeding and in biotechnology research are analyzed with a focus on production and productivity of individual organisms. Our growing understanding of the productive/adaptive potential of (crop) plants is a prerequisite to increasing this potential and also its expression under environmental constraints. This review concentrates on growth rate, ribosome activity, and photosynthetic rate to link these key cellular processes to plant productivity. Examples of how they may be integrated in heterosis, organ growth control, and responses to abiotic stresses are presented. The yield components in rice are presented as a model. The ultimate goal of research programs, that concentrate on yield and productivity and integrating the panoply of systems biology tools, is to achieve "low input, high output" agriculture, i.e. shifting from a conventional "productivist" agriculture to an efficient sustainable agriculture. This is of critical, strategic importance, because the extent to which we, both locally and globally, secure and manage the long-term productive potential of plant resources will determine the future of humanity.

A plethora of virulence strategies hidden behind nuclear targeting of microbial effectors

Plant immune responses depend on the ability to couple rapid recognition of the invading microbe to an efficient response. During evolution, plant pathogens have acquired the ability to deliver effector molecules inside host cells in order to manipulate cellular and molecular processes and establish pathogenicity. Following translocation into plant cells, microbial effectors may be addressed to different subcellular compartments. Intriguingly, a significant number of effector proteins from different pathogenic microorganisms, including viruses, oomycetes, fungi, nematodes, and bacteria, is targeted to the nucleus of host cells. In agreement with this observation, increasing evidence highlights the crucial role played by nuclear dynamics, and nucleocytoplasmic protein trafficking during a great variety of analyzed plant-pathogen interactions. Once in the nucleus, effector proteins are able to manipulate host transcription or directly subvert essential host components to promote virulence. Along these lines, it has been suggested that some effectors may affect histone packing and, thereby, chromatin configuration. In addition, microbial effectors may either directly activate transcription or target host transcription factors to alter their regular molecular functions. Alternatively, nuclear translocation of effectors may affect subcellular localization of their cognate resistance proteins in a process that is essential for resistance protein-mediated plant immunity. Here, we review recent progress in our field on the identification of microbial effectors that are targeted to the nucleus of host plant cells. In addition, we discuss different virulence strategies deployed by microbes, which have been uncovered through examination of the mechanisms that guide nuclear localization of effector proteins.

Five proteins of Laodelphax striatellus are potentially involved in the interactions between rice stripe virus and vector

Rice stripe virus (RSV) is the type member of the genus Tenuivirus, which relies on the small brown planthopper (Laodelphax striatellus Fallén) for its transmission in a persistent, circulative-propagative manner. To be transmitted, virus must cross the midgut and salivary glands epithelial barriers in a transcytosis mechanism where vector receptors interact with virions, and as propagative virus, RSV need utilize host components to complete viral propagation in vector cells. At present, these mechanisms remain unknown. In this paper, we screened L. striatellus proteins, separated by two-dimensional electrophoresis (2-DE), as potential RSV binding molecules using a virus overlay assay of protein blots. The results, five L. striatellus proteins that bound to purified RSV particles in vitro were resolved and identified using mass spectrometry. The virus-binding capacities of five proteins were further elucidated in yeast two-hybrid screen (YTHS) and virus-binding experiments of expressed proteins. Among five proteins, the receptor for activated protein kinase C (RACK) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH3) did not interact with RSV nucleocapsid protein (NCP) in YTHS and in far-Western blot, and three ribosomal proteins (RPL5, RPL7a and RPL8) had specific interactions with RSV. In dot immunobinding assay (DIBA), all five proteins were able to bind to RSV particles. The five proteins' potential contributions to the interactions between RSV and L. striatellus were discussed. We proposed that RACK and GAPDH3 might be involved in the epithelial transcytosis of virus particles, and three ribosomal proteins probably played potential crucial roles in the infection and propagation of RSV in vector cells.

Viral factor TAV recruits TOR/S6K1 signalling to activate reinitiation after long ORF translation

The protein kinase TOR (target-of-rapamycin) upregulates translation initiation in eukaryotes, but initiation restart after long ORF translation is restricted by largely unknown pathways. The plant viral reinitiation factor transactivator-viroplasmin (TAV) exceptionally promotes reinitiation through a mechanism involving retention on 80S and reuse of eIF3 and the host factor reinitiation-supporting protein (RISP) to regenerate reinitiation-competent ribosomal complexes. Here, we show that TAV function in reinitiation depends on physical association with TOR, with TAV-TOR binding being critical for both translation reinitiation and viral fitness. Consistently, TOR-deficient plants are resistant to viral infection. TAV triggers TOR hyperactivation and S6K1 phosphorylation in planta. When activated, TOR binds polyribosomes concomitantly with polysomal accumulation of eIF3 and RISP--a novel and specific target of TOR/S6K1--in a TAV-dependent manner, with RISP being phosphorylated. TAV mutants defective in TOR binding fail to recruit TOR, thereby abolishing RISP phosphorylation in polysomes and reinitiation. Thus, activation of reinitiation after long ORF translation is more complex than previously appreciated, with TOR/S6K1 upregulation being the key event in the formation of reinitiation-competent ribosomal complexes.

Involvement of the plant nucleolus in virus and viroid infections: parallels with animal pathosystems

The nucleolus is a dynamic subnuclear body with roles in ribosome subunit biogenesis, mediation of cell-stress responses, and regulation of cell growth. An increasing number of reports reveal that similar to the proteins of animal viruses, many plant virus proteins localize in the nucleolus to divert host nucleolar proteins from their natural functions in order to exert novel role(s) in the virus infection cycle. This chapter will highlight studies showing how plant viruses recruit nucleolar functions to facilitate virus translation and replication, virus movement and assembly of virus-specific ribonucleoprotein (RNP) particles, and to counteract plant host defense responses. Plant viruses also provide a valuable tool to gain new insights into novel nucleolar functions and processes. Investigating the interactions between plant viruses and the nucleolus will facilitate the design of novel strategies to control plant virus infections.

A new plant protein interacts with eIF3 and 60S to enhance virus-activated translation re-initiation

The plant viral re-initiation factor transactivator viroplasmin (TAV) activates translation of polycistronic mRNA by a re-initiation mechanism involving translation initiation factor 3 (eIF3) and the 60S ribosomal subunit (60S). QJ;Here, we report a new plant factor-re-initiation supporting protein (RISP)-that enhances TAV function in re-initiation. RISP interacts physically with TAV in vitro and in vivo. Mutants defective in interaction are less active, or inactive, in transactivation and viral amplification. RISP alone can serve as a scaffold protein, which is able to interact with eIF3 subunits a/c and 60S, apparently through the C-terminus of ribosomal protein L24. RISP pre-bound to eIF3 binds 40S, suggesting that RISP enters the translational machinery at the 43S formation step. RISP, TAV and 60S co-localize in epidermal cells of infected plants, and eIF3-TAV-RISP-L24 complex formation can be shown in vitro. These results suggest that RISP and TAV bridge interactions between eIF3-bound 40S and L24 of 60S after translation termination to ensure 60S recruitment during repetitive initiation events on polycistronic mRNA; RISP can thus be considered as a new component of the cell translation machinery.

Differential requirement of ribosomal protein S6 by plant RNA viruses with different translation initiation strategies

Potyvirus infection has been reported to cause an increase in the mRNA transcripts of many plant ribosomal proteins (r-proteins). In this study, increased expression of r-protein mRNA transcripts was determined to occur in Nicotiana benthamiana during infection by potyviruses as well as a tobamovirus demonstrating that this response is not unique to potyviruses. Five r-protein genes, RPS6, RPL19, RPL13, RPL7, and RPS2, were silenced in N. benthamiana to test their roles in viral infection. The accumulation of both Turnip mosaic virus (TuMV), a potyvirus, and Tobacco mosaic virus (TMV), a tobamovirus, was dependent on RPL19, RPL13, RPL7, and RPS2. However, TMV was able to accumulate in RPS6-silenced plants while accumulation of TuMV and Tomato bushy stunt virus (TBSV) was abolished. These results demonstrate that cap-independent TuMV and TBSV require RPS6 for their accumulation, whereas accumulation of TMV is independent of RPS6.

The cauliflower mosaic virus protein P6 forms motile inclusions that traffic along actin microfilaments and stabilize microtubules

The gene VI product (P6) of Cauliflower mosaic virus (CaMV) is a multifunctional protein known to be a major component of cytoplasmic inclusion bodies formed during CaMV infection. Although these inclusions are known to contain virions and are thought to be sites of translation from the CaMV 35S polycistronic RNA intermediate, the precise role of these bodies in the CaMV infection cycle remains unclear. Here, we examine the functionality and intracellular location of a fusion between P6 and GFP (P6-GFP). We initially show that the ability of P6-GFP to transactivate translation is comparable to unmodified P6. Consequently, our work has direct application for the large body of literature in which P6 has been expressed ectopically and its functions characterized. We subsequently found that P6-GFP forms highly motile cytoplasmic inclusion bodies and revealed through fluorescence colocalization studies that these P6-GFP bodies associate with the actin/endoplasmic reticulum network as well as microtubules. We demonstrate that while P6-GFP inclusions traffic along microfilaments, those associated with microtubules appear stationary. Additionally, inhibitor studies reveal that the intracellular movement of P6-GFP inclusions is sensitive to the actin inhibitor, latrunculin B, which also inhibits the formation of local lesions by CaMV in Nicotiana edwardsonii leaves. The motility of P6 along microfilaments represents an entirely new property for this protein, and these results imply a role for P6 in intracellular and cell-to-cell movement of CaMV.

Cauliflower mosaic virus protein P6 is a suppressor of RNA silencing

We infected a transgenic Arabidopsis line (GxA), containing an amplicon-silenced 35S : : GFP transgene, with cauliflower mosaic virus (CaMV), a plant pararetrovirus with a DNA genome. Systemically infected leaves showed strong GFP fluorescence and amplicon transcripts were detectable in Northern blots, indicating that silencing of GFP had been suppressed during CaMV-infection. Transgenic Arabidopsis lines expressing CaMV protein P6, the major genetic determinant of symptom severity, were crossed with GxA. Progeny showed strong GFP fluorescence throughout and amplicon transcripts were detectable in Northern blots, indicating that P6 was suppressing local and systemic silencing. However, levels of 21 nt siRNAs derived from the GFP transgene were not reduced. In CaMV-infected plants, the P6 transgene did not reduce levels of CaMV leader-derived 21 and 24 nt siRNAs relative to levels of CaMV 35S RNA. These results demonstrate that CaMV can efficiently suppress silencing of a GFP transgene, and that P6 acts as a silencing suppressor.

Human T-cell leukemia virus type I p30 nuclear/nucleolar retention is mediated through interactions with RNA and a constituent of the 60 S ribosomal subunit

Human T-cell leukemia virus type I is the etiological agent of adult T-cell leukemia/lymphoma, an aggressive and fatal lymphoproliferative malignancy. The virus has evolved strategies to escape immune clearance by remaining latent in most infected cells in vivo. We demonstrated previously that virally encoded p30 protein is a potent post-transcriptional inhibitor of virus replication (Nicot, C., Dundr, M., Johnson, J. M., Fullen, J. R., Alonzo, N., Fukumoto, R., Princler, G. L., Derse, D., Misteli, T., and Franchini, G. (2004) Nat. Med. 10, 197-201). p30 is unable to shuttle out of the nucleus in heterokaryon assays, suggesting the existence of specific retention signals. Because suppression of virus replication relies on nuclear retention of the tax/rex mRNA by p30, determining the retention features of p30 will offer hints to break latency in infected cells and insights into new therapeutic approaches. In this study, we used live cell imaging technologies to study the kinetics of p30 and to delineate its retention signals and their function in virus replication. Notably, this is the first study to identify p30 nucleolar retention domains. Using mutants of p30 that localized in different cellular compartments, we show that post-transcriptional control of virus replication by p30 occurs in the nucleoplasm. We further demonstrate that p30 nuclear/nucleolar retention is dependent upon de novo RNA transcripts and interactions with components of the ribosomal machinery.

Translation reinitiation and leaky scanning in plant viruses

While translation of mRNAs in eukaryotic cells in general follows strict rules, viruses infecting these cells break those rules in various ways. Viruses are under high selection pressure to compete with the host, to economize genome size, and to accommodate signals for replication, virus assembly, etc., on their RNAs as well as using them for translation. The cornucopia of extraordinary translation strategies, such as leaky scanning, internal initiation of translation, ribosome shunt, and virus-controlled reinitiation of translation, evolved by viruses continues to surprise and inform our understanding of general translation mechanisms. While internal initiation is treated in another section of this issue, we concentrate on leaky scanning, shunt and reinitiation, with emphasis on plant pararetroviruses.

The open reading frame VI product of Cauliflower mosaic virus is a nucleocytoplasmic protein: its N terminus mediates its nuclear export and formation of electron-dense viroplasms

The Cauliflower mosaic virus (CaMV) open reading frame VI product (P6) is essential for the viral infection cycle. It controls translation reinitiation of the viral polycistronic RNAs and forms cytoplasmic inclusion bodies (viroplasms) where virus replication and assembly occur. In this study, the mechanism involved in viroplasm formation was investigated by in vitro and in vivo experiments. Far protein gel blot assays using a collection of P6 deletion mutants demonstrated that the N-terminal alpha-helix of P6 mediates interaction between P6 molecules. Transient expression in tobacco (Nicotiana tabacum) BY-2 cells of full-length P6 and P6 mutants fused to enhanced green fluorescent protein revealed that viroplasms are formed at the periphery of the nucleus and that the N-terminal domain of P6 is an important determinant in this process. Finally, this study led to the unexpected finding that P6 is a nucleocytoplasmic shuttle protein and that its nuclear export is mediated by a Leu-rich sequence that is part of the alpha-helix domain implicated in viroplasm formation. The discovery that P6 can localize to the nucleus opens new prospects for understanding yet unknown roles of this viral protein in the course of the CaMV infection cycle.