A cysteine-rich plant protein potentiates Potyvirus movement through an interaction with the virus genome-linked protein VPg

Interaction of Sesbania mosaic virus movement protein with VPg and P10: implication to specificity of genome recognition

Sesbania mosaic virus (SeMV) is a single strand positive-sense RNA plant virus that belongs to the genus Sobemovirus. The mechanism of cell-to-cell movement in sobemoviruses has not been well studied. With a view to identify the viral encoded ancillary proteins of SeMV that may assist in cell-to-cell movement of the virus, all the proteins encoded by SeMV genome were cloned into yeast Matchmaker system 3 and interaction studies were performed. Two proteins namely, viral protein genome linked (VPg) and a 10-kDa protein (P10) c v gft encoded by OFR 2a, were identified as possible interacting partners in addition to the viral coat protein (CP). Further characterization of these interactions revealed that the movement protein (MP) recognizes cognate RNA through interaction with VPg, which is covalently linked to the 5' end of the RNA. Analysis of the deletion mutants delineated the domains of MP involved in the interaction with VPg and P10. This study implicates for the first time that VPg might play an important role in specific recognition of viral genome by MP in SeMV and shed light on the possible role of P10 in the viral movement.

Cellular pathways for viral transport through plasmodesmata

Plant viruses use plasmodesmata (PD) to spread infection between cells and systemically. Dependent on viral species, movement through PD can occur in virion or non-virion form, and requires different mechanisms for targeting and modification of the pore. These mechanisms are supported by viral movement proteins and by other virus-encoded factors that interact among themselves and with plant cellular components to facilitate virus movement in a coordinated and regulated fashion.

Gene expression profiling of phytoplasma-infected Madagascar periwinkle leaves using differential display

Phytoplasmas are small (0.2-0.8 μm), wall-less, pleiomorphic prokaryotes responsible of numerous economically important plant diseases. They are characterized by a very small genome and are obligate parasites of phloem tissues and some insects that act as vectors of infection. To investigate molecular mechanisms involved in pathogenesis, the differential display technique was here applied to identify plant genes whose transcription was significantly altered in leaves of Madagascar periwinkle (Catharanthus roseus (L.) G.Don) infected by 'Candidatus Phytoplasma pyri'. We detected, reamplified, cloned, and sequenced 16 putative differentially expressed cDNA fragments. Northern blot analysis revealed that seven of the 16 genes identified were up-regulated following phytoplasma infection, while three genes were down-regulated. The remaining six genes did not show significant changes in the level of expression. Identified genes are mainly involved in plant defence/stress responses, protein metabolism and transport, transcriptional regulation, vesicle trafficking, and carbohydrate metabolism. The possible role played by these genes in the phytoplasma infection is discussed.

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 host RNA helicase-like protein, AtRH8, interacts with the potyviral genome-linked protein, VPg, associates with the virus accumulation complex, and is essential for infection

The viral genome-linked protein, VPg, of potyviruses is a multifunctional protein involved in viral genome translation and replication. Previous studies have shown that both eukaryotic translation initiation factor 4E (eIF4E) and eIF4G or their respective isoforms from the eIF4F complex, which modulates the initiation of protein translation, selectively interact with VPg and are required for potyvirus infection. Here, we report the identification of two DEAD-box RNA helicase-like proteins, PpDDXL and AtRH8 from peach (Prunus persica) and Arabidopsis (Arabidopsis thaliana), respectively, both interacting with VPg. We show that AtRH8 is dispensable for plant growth and development but necessary for potyvirus infection. In potyvirus-infected Nicotiana benthamiana leaf tissues, AtRH8 colocalizes with the chloroplast-bound virus accumulation vesicles, suggesting a possible role of AtRH8 in viral genome translation and replication. Deletion analyses of AtRH8 have identified the VPg-binding region. Comparison of this region and the corresponding region of PpDDXL suggests that they are highly conserved and share the same secondary structure. Moreover, overexpression of the VPg-binding region from either AtRH8 or PpDDXL suppresses potyvirus accumulation in infected N. benthamiana leaf tissues. Taken together, these data demonstrate that AtRH8, interacting with VPg, is a host factor required for the potyvirus infection process and that both AtRH8 and PpDDXL may be manipulated for the development of genetic resistance against potyvirus infections.

Control of nuclear and nucleolar localization of nuclear inclusion protein a of picorna-like Potato virus A in Nicotiana species

The multifunctional nuclear inclusion protein a (NIa) of potyviruses (genus Potyvirus; Potyviridae) accumulates in the nucleus of virus-infected cells for unknown reasons. In this study, two regions in the viral genome-linked protein (VPg) domain of NIa in Potato virus A (PVA) were found to constitute nuclear and nucleolar localization signals (NLS) in plant cells (Nicotiana spp). Amino acid substitutions in both NLS I (residues 4 to 9) and NLS II (residues 41 to 50) prevented nuclear localization, whereas mutations in either single NLS did not. Mutations in either NLS, however, prevented nucleolar localization and prevented or diminished virus replication in protoplasts, accumulation in infected plant tissues, and/or systemic movement in plants. One NLS mutant was partially complemented by the wild-type VPg expressed in transgenic plants. Furthermore, NLS I controlled NIa accumulation in Cajal bodies. The VPg domain interacted with fibrillarin, a nucleolar protein, and depletion of fibrillarin reduced PVA accumulation. Overexpression of VPg in leaf tissues interfered with cosuppression of gene expression (i.e., RNA silencing), whereas NLS I and NLS II mutants, which exhibited reduced nuclear and nucleolar localization, showed no such activity. These results demonstrate that some of the most essential viral functions required for completion of the infection cycle are tightly linked to regulation of the NIa nuclear and nucleolar localization.

Arabidopsis plant homeodomain finger proteins operate downstream of auxin accumulation in specifying the vasculature and primary root meristem

In Arabidopsis thaliana, auxin is a key regulator of tissue patterning in the developing embryo. We have identified a group of proteins that act downstream of auxin accumulation in auxin-mediated root and vascular development in the embryo. Combined mutations in OBERON1 (OBE1) and OBERON2 (OBE2) give rise to obe1 obe2 double mutant seedlings that closely phenocopy the monopteros (mp) mutant phenotype, with an absence of roots and defective development of the vasculature. We show that, in contrast to the situation in mp mutants, obe1 obe2 double mutant embryos show auxin maxima at the root pole and in the provascular region, and that the SCF(TIR1) pathway, which translates auxin accumulation into transcriptional activation of auxin-responsive genes, remains intact. Although we focus on the impact of obe mutations on aspects of embryo development, the effect of such mutations on a broad range of auxin-related gene expression and the tissue expression patterns of OBE genes in seedlings suggest that OBE proteins have a wider role to play in growth and development. We suggest that OBE1 and OBE2 most likely control the transcription of genes required for auxin responses through the action of their PHD finger domains.

The cytosolic protein response as a subcomponent of the wider heat shock response in Arabidopsis

In common with a range of environmental and biological stresses, heat shock results in the accumulation of misfolded proteins and a collection of downstream consequences for cellular homeostasis and growth. Within this complex array of responses, the sensing of and responses to misfolded proteins in specific subcellular compartments involves specific chaperones, transcriptional regulators, and expression profiles. Using biological (ectopic protein expression and virus infection) and chemical triggers for misfolded protein accumulation, we have profiled the transcriptional features of the response to misfolded protein accumulation in the cytosol (i.e., the cytoplasmic protein response [CPR]) and identified the effects as a subcomponent of the wider effects induced by heat shock. The CPR in Arabidopsis thaliana is associated with the heat shock promoter element and the involvement of specific heat shock factors (HSFs), notably HSFA2, which appears to be regulated by alternative splicing and non-sense-mediated decay. Characterization of Arabidopsis HSFA2 knockout and overexpression lines showed that HSFA2 is one of the regulatory components of the CPR.

Complementary functions of two recessive R-genes determine resistance durability of tobacco 'Virgin A Mutant' (VAM) to Potato virus Y

Tobaccos VAM and NC745 carry the recessive va gene that confers resistance to PVY(NN). However, they exhibit different levels of resistance durability. Upon virus inoculation, only NC745 developed sporadic systemic symptoms caused by emerging resistance-breaking variants that easily infected both NC745 and VAM genotypes. To identify the differential host conditions associated with this phenomenon, cellular accumulation, cell-to-cell movement, vascular translocation, and foliar content of PVY(NN) were comparatively evaluated. Virus cell-to-cell movement was restricted and its transit through the vasculature boundaries was completely blocked in both tobacco varieties. However, an additional defense mechanism operating only in tobacco VAM drastically reduced the in situ cellular virus accumulation. Genetic analyses of hybrid plant progenies indicate that VAM-type resistance was conditioned by at least two recessive genes: va and a newly reported va2 locus. Moreover, segregant plant progenies that restricted virus movement but permitted normal virus accumulation were prone to develop resistance-breaking infections.

Potato virus A genome-linked protein VPg is an intrinsically disordered molten globule-like protein with a hydrophobic core

Genome-linked protein VPg of Potato virus A (PVA; genus Potyvirus) has essential functions in all critical steps of PVA infection, i.e. replication, movement, and virulence. Structural features of the recombinant PVA VPg were investigated with the aim to create an outline for structure-function relationships. Circular dichroism data of PVA VPg revealed a distinct near-UV spectrum indicating that the environment around its aromatic residues is structured but rather flexible, and a far-UV spectrum that was characterized by features typical for intrinsically disordered proteins. Temperature-induced denaturation followed a typical all-or-none transition whereas urea- and GdmHCl-induced denaturation proceeded via a route best described by a three-state-model. The conclusion drawn was that the overall structure of PVA VPg is significantly unstable even in the absence of denaturants. Acrylamide fluorescence quenching and 1-anilino-8-naphthalene sulfonate binding experiments together with 1D and 2D NMR data further verified that PVA VPg behaves as a partially folded species that contains a hydrophobic core domain. Regions predicted to be disordered in PVA VPg were the ones that were cut the fastest by trypsin whereas regions predicted to be structured and to contain the most conserved amino acids among potyvirus VPgs were trypsin-resistant. Amino acid composition analysis of potyvirus VPgs revealed a clear enrichment of disorder and depletion of structure-promoting residues. Taken together it seems that the native structure of PVA VPg, and probably that of potyviral VPg in general, resembles a partially disordered molten globule. Further experimentation is required to understand the functional regulation achieved via this property.

Purification of viral genome-linked protein VPg from potato virus A-infected plants reveals several post-translationally modified forms of the protein

In order to be able to analyse post-translational modifications and protein interactions of viral genome-linked protein VPg taking place during potato virus A (PVA) infection, an affinity tag-based purification system was developed by inserting a sequence encoding a six-histidine and haemagglutinin (HisHA) tag to the 3' end of the VPg coding sequence within the infectious cDNA clone of PVA. The engineered virus was fully functional and the HisHA tag-encoding sequence remained stable in the PVA genome throughout the infection process. Purification under denaturing conditions resulted in a protein sample that contained multiple VPg and NIa forms carrying post-translational modifications that altered their isoelectric points. Non-modified tagged VPg (pI 8) was a minor product in the protein sample derived from total leaf proteins, but when the replication-associated membranes were used as starting material, its relative amount increased. Further characterization demonstrated that some of the PVA VPg isoforms were modified by multiple phosphorylation events. Purity of the proteins derived from the native purifications with either of the tags was evaluated. A clearly purer VPg sample was obtained by performing tandem affinity purification utilizing both tags sequentially. NIb, CI and HC-Pro co-purified in an affinity-tagged VPg-dependent manner, indicating that the system was able to isolate protein complexes operating during PVA infection.

Yeast as a model host to explore plant virus-host interactions

The yeast Saccharomyces cerevisiae is invaluable for understanding fundamental cellular processes and disease states of relevance to higher eukaryotes. Plant viruses are intracellular parasites that take advantage of resources of the host cell, and a simple eukaryotic cell, such as yeast, can provide all or most of the functions for successful plant virus replication. Thus, yeast has been used as a model to unravel the interactions of plant viruses with their hosts. Indeed, genome-wide and proteomics studies using yeast as a model host with bromoviruses and tombusviruses have facilitated the identification of replication-associated factors that affect host-virus interactions, virus pathology, virus evolution, and host range. Many of the host genes that affect the replication of the two viruses, which belong to two dissimilar virus families, are distinct, suggesting that plant viruses have developed different ways to utilize the resources of host cells. In addition, a surprisingly large number of yeast genes have been shown to affect RNA-RNA recombination in tombusviruses; this opens an opportunity to study the role of the host in virus evolution. The knowledge gained about host-virus interactions likely will lead to the development of new antiviral methods and applications in biotechnology and nanotechnology, as well as new insights into cellular functions of individual genes and the basic biology of the host cell.

Role of plant virus movement proteins

Plant viruses spread from the initially infected cells to the rest of the plant in several distinct stages. First, the virus (in the form of virions or nucleic acid protein complexes) moves intracellularly from the sites of replication to plasmodesmata (PD, plant-specific intercellular membranous channels), the virus then transverses the PD to spread intercellularly (cell-to-cell movement). Long-distance movement of virus occurs through phloem sieve tubes. The processes of plant virus movement are controlled by specific viral movement proteins (MPs). No extensive sequence similarity has been found in MPs belonging to different plant virus taxonomic groups. Moreover, different MPs were shown to use different pathways and mechanisms for virus transport. Some viral transport systems require a single MP while others require additional virus-encoded proteins to transport viral genomes. In this review, we focus on the functions and properties of different classes of MPs encoded by RNA containing plant viruses.

A developmentally linked, dramatic, and transient loss of virus from roots of Arabidopsis thaliana plants infected by either of two RNA viruses

Possible effects of host developmental stage on the amount of virus present in systemically infected plant tissues hitherto have received little attention. In this study, the pattern of virus accumulation over the plant lifespan has been examined in systemically invaded tissues of Arabidopsis thaliana infected by either of two distinct (+)RNA viruses: Turnip mosaic virus, a member of Potyvirus, and Oilseed rape mosaic virus, a member of Tobamovirus. Quantitative analyses of virus coat protein and virus genomic RNA in roots versus aerial plant parts revealed generally sinusoidal temporal patterns of virus accumulation. In noninoculated leaves, a time period was found during which no virus accumulation was detected. This period was coincident with the approximately 7 days of inflorescence bud formation and differentiation. In roots, virion content reached high levels a few days after inoculation, dropping dramatically during the period of bud formation and quickly recovering after it. These results, together with electron microscopy observations, are consistent with loss of virions due to disassembly. Fluorescence observations of green fluorescent protein-tagged virus-infected root tissue also were consistent with a net loss of virus-specified proteins. Inoculations performed after the emergence of the inflorescence and on A. thaliana flowering-time mutants support the temporal link between observed changes in virus content and inflorescence bud formation. Different host-involving biochemical processes can be invoked to provide mechanistic clues, but no one of them alone seems sufficient to explain the complex patterns of tight temporal regulation of virus accumulation observed in these experiments.

Potyvirus genome-linked protein, VPg, directly affects wheat germ in vitro translation: interactions with translation initiation factors eIF4F and eIFiso4F

Potyvirus genome linked protein, VPg, interacts with translation initiation factors eIF4E and eIFiso4E, but its role in protein synthesis has not been elucidated. We show that addition of VPg to wheat germ extract leads to enhancement of uncapped viral mRNA translation and inhibition of capped viral mRNA translation. This provides a significant competitive advantage to the uncapped viral mRNA. To understand the molecular basis of these effects, we have characterized the interaction of VPg with eIF4F, eIFiso4F, and a structured RNA derived from tobacco etch virus (TEV RNA). When VPg formed a complex with eIF4F, the affinity for TEV RNA increased more than 4-fold compared with eIF4F alone (19.4 and 79.0 nm, respectively). The binding affinity of eIF4F to TEV RNA correlates with translation efficiency. VPg enhanced eIFiso4F binding to TEV RNA 1.6-fold (178 nm compared with 108 nm). Kinetic studies of eIF4F and eIFiso4F with VPg show approximately 2.6-fold faster association for eIFiso4F.VPg as compared with eIF4F.VPg. The dissociation rate was approximately 2.9-fold slower for eIFiso4F than eIF4F with VPg. These data demonstrate that eIFiso4F can kinetically compete with eIF4F for VPg binding. The quantitative data presented here suggest a model where eIF4F.VPg interaction enhances cap-independent translation by increasing the affinity of eIF4F for TEV RNA. This is the first evidence of direct participation of VPg in translation initiation.

Capsid protein-mediated recruitment of host DnaJ-like proteins is required for Potato virus Y infection in tobacco plants

The capsid protein (CP) of potyviruses is required for various steps during plant infection, such as virion assembly, cell-to-cell movement, and long-distance transport. This suggests a series of compatible interactions with putative host factors which, however, are largely unknown. By using the yeast two-hybrid system the CP from Potato virus Y (PVY) was found to interact with a novel subset of DnaJ-like proteins from tobacco, designated NtCPIPs. Mutational analysis identified the CP core region, previously shown to be essential for virion formation and plasmodesmal trafficking, as the interacting domain. The ability of NtCPIP1 and NtCPIP2a to associate with PVY CP could be confirmed in vitro and was additionally verified in planta by bimolecular fluorescence complementation. The biological significance of the interaction was assayed by PVY infection of agroinfiltrated leaves and transgenic tobacco plants that expressed either full-length or J-domain-deficient variants of NtCPIPs. Transient expression of truncated dominant-interfering NtCPIP2a but not of the functional protein resulted in strongly reduced accumulation of PVY in the inoculated leaf. Consistently, stable overexpression of J-domain-deficient variants of NtCPIP1 and NtCPIP2a dramatically increased the virus resistance of various transgenic lines, indicating a critical role of functional NtCPIPs during PVY infection. The negative effect of impaired NtCPIP function on viral pathogenicity seemed to be the consequence of delayed cell-to-cell movement, as visualized by microprojectile bombardment with green fluorescent protein-tagged PVY. Therefore, we propose that NtCPIPs act as important susceptibility factors during PVY infection, possibly by recruiting heat shock protein 70 chaperones for viral assembly and/or cellular spread.

Virulence factor of potato virus Y, genome-attached terminal protein VPg, is a highly disordered protein

Potato virus Y (PVY) is a common potyvirus of agricultural importance, belonging to the picornavirus superfamily of RNA plus-stranded viruses. A covalently linked virus-encoded protein VPg required for virus infectivity is situated at the 5' end of potyvirus RNA. VPg seems to be involved in multiple interactions, both with other viral products and host proteins. VPgs of potyviruses have no known homologs, and there is no atomic structure available. To understand the molecular basis of VPg multifunctionality, we have analyzed structural features of VPg from PVY using structure prediction programs, functional assays, and biochemical and biophysical analyses. Structure predictions suggest that VPg exists in a natively unfolded conformation. In contrast with ordered proteins, PVY VPg is not denatured by elevated temperatures, has sedimentation values incompatible with a compact globular form, and shows a CD spectrum of a highly disordered protein, and HET-HETSOFAST NMR analysis suggests the presence of large unstructured regions. Although VPg has a propensity to form dimers, no functional differences were seen between the monomer and dimer. These data strongly suggest that the VPg of PVY should be classified among intrinsically disordered proteins. Intrinsic disorder lies at the basis of VPg multifunctionality, which is necessary for virus survival in the host.

The poly(A) binding protein is internalized in virus-induced vesicles or redistributed to the nucleolus during turnip mosaic virus infection

Poly(A) binding protein 2 (PABP2) of Arabidopsis thaliana was previously shown to interact with VPg-Pro of turnip mosaic virus (TuMV) and may consequently play an important role during infection. Subcellular fractionation experiments revealed that PABP2 was predominantly a cytoplasmic soluble protein in healthy plants. However, in TuMV-infected plants, a subpopulation of PABP2 was membrane associated or was localized in the nucleus. Confocal microscopy experiments indicated that PABP2 was partially retargeted to the nucleolus in the presence of TuMV VPg-Pro. In addition, the membrane association of PABP2 during TuMV infection resulted from the internalization of the host protein in 6K-VPg-Pro-induced vesicles, as shown by a combination of confocal microscopy and sucrose gradient fractionation experiments. This redistribution of an important translation initiation factor to the nucleolus and to membrane structure likely underlies two important processes of the TuMV replication cycle.

VPg of Potato virus A alone does not suppress RNA silencing but affects virulence of a heterologous virus

The viral genome-linked protein (VPg) is a well-known virulence factor in potyviruses (genus Potyvirus), including Potato virus A (PVA). Its ability to suppress onset and signalling of transgene-mediated RNA silencing and accumulation of small interfering RNA (siRNA) was studied using cross-protection and Agrobacterium infiltration assays and green fluorescent protein (GFP) and PVA VPg protein-expressing transgenic Nicotiana benthamiana plants. N. benthamiana plants were also transformed with a transgene comprising the cylindrical inclusion protein (CI), nuclear inclusion protein a (NIa) and coat protein (CP) encoding regions of PVA. This transgene mRNA was expressed in the T1 progeny of the transgenic lines but all were susceptible to PVA. This result contrasted the plants transformed with the PVA P1, VPg (N-proximal part of NIa) or CP encoding regions that expressed various forms of resistance. There was little evidence for direct involvement of VPg in suppression of silencing, while other mechanisms by which VPg might interfere with transgenic resistance could not be excluded. Expression of the wild-type PVA VPg from the genome of Potato virus X (PVX, genus Potexvirus) increased symptom severity in N. benthamiana, whereas a single point mutation introduced to the VPg enhanced accumulation of the PVX chimera. These data demonstrated previously unknown virulence functions controlled by the VPg of a potyvirus.

Central domain of a potyvirus VPg is involved in the interaction with the host translation initiation factor eIF4E and the viral protein HcPro

Using recombinant proteins produced in bacteria or in infected plants, interactions between the VPg and HcPro of Lettuce mosaic potyvirus (LMV) and between LMV VPg and the lettuce translation initiation factor 4E, the cap-binding protein (eIF4E), were demonstrated in vitro. Interaction with eIF4E and HcPro both involved the same VPg central domain. The structure of this domain in the VPg context was predicted to include an amphiphilic alpha-helix, with the amino acids related to biological functions in various potyviruses exposed at the hydrophilic side.

Tobacco etch virus mRNA preferentially binds wheat germ eukaryotic initiation factor (eIF) 4G rather than eIFiso4G

The 5'-leader of tobacco etch virus (TEV) genomic RNA directs the efficient translation from the naturally uncapped viral RNA. The TEV 143-nt 5'-leader folds into a structure that contains two domains, each of which contains RNA pseudoknots. The 5'-proximal pseudoknot 1 (PK1) is necessary to promote cap-independent translation (Zeenko, V., and Gallie, D. R. (2005) J. Biol. Chem. 280, 26813-26824). During the translation initiation of cellular mRNAs, eIF4G functions as an adapter that recruits many of the factors involved in stimulating 40 S ribosomal subunit binding to an mRNA. Two related but highly distinct eIF4G proteins are expressed in plants, animals, and yeast. The two plant eIF4G isoforms, referred to as eIF4G and eIFiso4G, differ in size (165 and 86 kDa, respectively) and their functional differences are still unclear. Although eIF4G is required for the translation of TEV mRNA, it is not known if eIF4G binds directly to the TEV RNA itself or if other factors are required. To determine whether binding affinity and isoform preference correlates with translational efficiency, fluorescence spectroscopy was used to measure the binding of eIF4G, eIFiso4G, and their complexes (eIF4F and eIFiso4F, respectively) to the TEV 143-nt 5'-leader (TEV1-143) and a shorter RNA that contained PK1. A mutant (i.e. S1-3) in which the stem of PK1 was disrupted resulting in impaired cap-independent translation, was also tested. These studies demonstrate that eIF4G binds TEV1-143 and PK1 RNA with approximately 22-30-fold stronger affinity than eIFiso4G. eIF4G and eIF4F bind TEV1-143 with similar affinity, whereas eIFiso4F binds with approximately 6-fold higher affinity than eIFiso4G. The binding affinity of eIF4G, eIF4F, and eIFiso4G to S1-3 was reduced by 3-5-fold, consistent with the reduction in the ability of this mutant to promote cap-independent translation. Temperature-dependent binding studies revealed that binding of the TEV 5'-leader to these initiation factors has a large entropic contribution. Overall, these results demonstrate the first direct interaction of eIF4G with the TEV 5'-leader in the absence of other initiation factors. These data correlate well with the observed translational data and provide more detailed information on the translational strategy of potyviruses.

Identification of a plum pox virus CI-interacting protein from chloroplast that has a negative effect in virus infection

The cylindrical inclusion (CI) protein of potyviruses is involved in virus replication and cell-to-cell movement. These two processes should rely on multiple plant-virus interactions; however, little is known about the host factors that are involved in, or that may interfere with, CI functions. By using a yeast two-hybrid system, the CI protein from Plum pox virus (PPV) was found to interact with the photosystem I PSI-K protein, the product of the gene psaK, of Nicotiana benthamiana. Coexpression of PPV CI was shown to cause a decrease in the accumulation level of PSI-K transiently expressed in N. benthamiana leaves. To test the biological relevance of this interaction, we have analyzed the infection of PPV in N. benthamiana plants in which psaK gene expression has been silenced by RNA interference, as well as in Arabidopsis thaliana psaK knockout plants. Our results show that downregulation of the psaK gene leads to higher PPV accumulation, suggesting a role for the CI-PSI-K interaction in PPV infection.

An Unusual Structure at One End of Potato Potyvirus Particles

The particles of potato virus Y (PVY) and potato virus A (PVA) potyviruses are helically constructed filaments that are thought to contain a single type of coat protein subunit. Examination of negatively stained virions by electron microscopy reveals flexuous rod-shaped particles with no obvious terminal structures. It is known that some helically constructed rod-shaped virus particles incorporate additional minor protein components, which form stable complexes that mediate particle disassembly, movement or transmission by vectors. Some of this information has been obtained using imaging techniques such as atomic force microscopy. The particles of PVY and PVA were examined by atomic force microscopy and immunogold labelling electron microscopy. Our results show that some of the potyvirus particles contain a protruding tip at one end of the virus particles, which is presumably associated with the 5'-end of viral RNA. The tip contains the virus-encoded proteins genome-linked protein and helper-component proteinase. The composition and possible roles of the terminal tip structures in virus biology are discussed.

Plant virus transport: motions of functional equivalence

Plant virus cell-to-cell movement and subsequent systemic transport are governed by a series of mechanisms involving various virus and plant factors. Specialized virus encoded movement proteins (MPs) control the cell-to-cell transport of viral nucleoprotein complexes through plasmodesmata. MPs of different viruses have diverse properties and each interacts with specific host factors that also have a range of functions. Most viruses are then transported via the phloem as either nucleoprotein complexes or virions, with contributions from host and virus proteins. Some virus proteins contribute to the establishment and maintenance of systemic infection by inhibiting RNA silencing-mediated degradation of viral RNA. In spite of all the different movement strategies and the viral and host components, there are possible functional commonalities in virus-host interactions that govern viral spread through plants.

Virus induction of heat shock protein 70 reflects a general response to protein accumulation in the plant cytosol

Different cytoplasmically replicating RNA viruses were shown to induce a specific subset of heat-inducible heat shock protein 70 (HSP70) genes in Arabidopsis (Arabidopsis thaliana). To identify the inducing principle, a promoterreporter system was developed for the facile analysis of differentially responding Arabidopsis HSP70 genes, by infiltration into Nicotiana benthamiana leaves. Through transient expression of individual viral cistrons or through deletion analysis of a viral replicon, we were unable to identify a unique inducer of HSP70. However, there was a positive correlation between the translatability of the test construct and the differential induction of HSP70. Since these data implied a lack of specificity in the induction process, we also expressed a random series of cytosolically targeted Arabidopsis genes and showed that these also differentially induced HSP70. Through a comparison of different promoterreporter constructs and through measurements of the steady-state levels of the individual proteins, it appeared that the HSP70 response reflected the ability of the cytosol to sense individual properties of particular proteins when expressed at high levels. This phenomenon is reminiscent of the unfolded protein response observed when the induced accumulation of proteins in the endoplasmic reticulum also induces a specific suite of chaperones.

Comparisons of the genetic structure of populations of Turnip mosaic virus in West and East Eurasia

The genetic structure of populations of Turnip mosaic virus in Eurasia was assessed by making host range and gene sequence comparisons of 142 isolates. Most isolates collected in West Eurasia infected Brassica plants whereas those from East Eurasia infected both Brassica and Raphanus plants. Analyses of recombination sites (RSs) in five regions of the genome (one third of the full sequence) showed that the protein 1 (P1 gene) had recombined more frequently than the other gene regions in both subpopulations, but that the RSs were located in different parts of the genomes of the subpopulations. Estimates of nucleotide diversity showed that the West Eurasian subpopulation was more diverse than the East Eurasian subpopulation, but the Asian-BR group of the genes from the latter subpopulation had a greater nonsynonymous/synonymous substitution ratio, especially in the P1, viral genome-linked protein (VPg) and nuclear inclusion a proteinase (NIa-Pro) genes. These subpopulations seem to have evolved independently from the ancestral European population, and their genetic structure probably reflects founder effects.