Genome amplification and long-distance movement functions associated with the central domain of tobacco etch potyvirus helper component-proteinase

Functional replacement of the tobacco rattle virus cysteine-rich protein by pathogenicity proteins from unrelated plant viruses

Mutation of the 16K gene encoded by RNA1 of Tobacco rattle virus (TRV) greatly reduced the levels of viral RNA that accumulated in both infected protoplasts and plants, showing that the 16K cysteine-rich protein (CRP) is required for efficient multiplication of TRV. Overexpression of the 16K protein, either from an additional copy of the gene carried on TRV RNA2 or from a PVX vector, led to an increase in the severity of disease symptoms, suggesting that the protein has a role in the pathogenicity of the virus. Mutation of the 16K gene could be overcome by expression from RNA2 of the Cucumber mosaic virus 2b gene, the Soil-borne wheat mosaic virus 19K gene, or the Barley stripe mosaic virus gammab gene, indicating that the proteins encoded by these diverse genes may have similar functions. One known function of the CMV 2b gene is as a suppressor of posttranscriptional gene silencing, suggesting that the TRV 16K protein may also possess this activity.

Interaction between potyvirus helper component-proteinase and capsid protein in infected plants

Monoclonal antibodies were raised against helper component-proteinase (HcPro) purified from plants infected with the potyvirus Lettuce mosaic virus (LMV). These antibodies were used in a two-site triple antibody sandwich ELISA assay together with polyclonal antibodies directed against purified virions. An interaction between HcPro and the viral coat protein (CP) was demonstrated in extracts of LMV-infected leaves, as well as for two other potyviruses, Plum pox virus and Potato virus Y. The CP-HcPro interaction was not abolished in LMV derivatives with an HcPro GFP N-terminal fusion, or with a deletion from the CP of the amino acids involved in aphid transmission. Electron microscopy indicated that HcPro probably does not interact with the CP in the form of assembled virions or virus-like particles. Together, these results suggest that the interaction detected between CP and HcPro might be involved in a process of the potyvirus cycle different from aphid transmission.

Host-specific involvement of the HC protein in the long-distance movement of potyviruses

Plum pox virus (PPV) is a member of the Potyvirus genus that, in nature, infects trees of the Prunus genus. Although PPV infects systemically several species of the Nicotiana genus, such as N. clevelandii and N. benthamiana, and replicates in the inoculated leaves of N. tabacum, it is unable to infect systemically the last host. The long-distance movement defect of PPV was corrected in transgenic tobacco plants expressing the 5"-terminal region of the genome of tobacco etch virus (TEV), a potyvirus that infects systemically tobacco. The fact that PPV was unable to move to upper noninoculated leaves in tobacco plants transformed with the same TEV transgene, but with a mutation in the HC protein (HC-Pro)-coding sequences, identifies the multifunctional HC-Pro as the complementing factor, and strongly suggests that a defect in an HC-Pro activity is responsible for the long-distance movement defect of PPV in tobacco. Whereas PPV HC-Pro strongly intensifies the symptoms caused by potato virus X (PVX) in the PPV systemic hosts N. clevelandii and N. benthamiana, it has no apparent effect on PVX pathogenicity in tobacco, supporting the hypothesis that long-distance movement and pathogenicity enhancement are related activities of the potyviral HC proteins. The movement defect of PPV in tobacco could also be complemented by cucumber mosaic virus in a mixed infection, demonstrating that at least some components of the long-distance machinery of the potyviruses are not strictly virus specific. A general conclusion of this work is that the HC-Pro might be a relevant factor for controlling the host range of the potyviruses.

Viral genome-linked protein (VPg) controls accumulation and phloem-loading of a potyvirus in inoculated potato leaves

The viral protein covalently linked to the 5' end of the plus-sense, single-stranded RNA genome of potyviruses (genus Potyvirus) can be an avirulence determinant in incompatible potyvirus-host combinations in which the resistance prevents systemic virus infection. The mechanism is not well known. This study shows that virus strain-specific resistance to systemic infection with Potato virus A (PVA) in Solanum commersonii is overcome by a single amino acid (aa) substitution, His118Tyr, in the viral genome-linked protein (VPg). Virus localization and other experiments revealed that Tyr118, controls phloem loading of PVA. The critical boundary may be constituted in phloem parenchyma, companion cells, or both. Tyr118 also controls the cellular level of virus accumulation in infected leaves, including phloem cells. Amino acid substitutions at three additional positions of the central part (aa 116) and C terminus (aa 185) of the VPg and of the N terminus of the 6K2 protein (aa 5) affect virus accumulation and rate of systemic infection but are not sufficient for phloem loading of PVA. These data, together with previous studies, indicate that the PVA VPg aa residues crucial for systemic infection are host specific. Also, our data and previous studies on other potyvirus-host species combinations indicate that the central part of the VPg is a domain with universal importance to virus-host interactions required for systemic invasion of plants with potyviruses.

Hypoviruses and chestnut blight: exploiting viruses to understand and modulate fungal pathogenesis

Fungal viruses are considered unconventional because they lack an extracellular route of infection and persistently infect their hosts, often in the absence of apparent symptoms. Because mycoviruses are limited to intracellular modes of transmission, they can be considered as intrinsic fungal genetic elements. Such long-term genetic interactions, even involving apparently asymptomatic mycoviruses, are likely to have an impact on fungal ecology and evolution. One of the clearest examples supporting this view is the phenomenon of hypovirulence (virulence attenuation) observed for strains of the chestnut blight fungus, Cryphonectria parasitica, harboring members of the virus family Hypoviridae. The goal of this chapter is to document recent advances in hypovirus molecular genetics and to provide examples of how that progress is leading to the identification of virus-encoded determinants responsible for altering fungal host phenotype, insights into essential and dispensable elements of hypovirus replication, revelations concerning the role of G-protein signaling in fungal pathogenesis, and new avenues for enhancing biological control potential.

Transgenic plants expressing HC-Pro show enhanced virus sensitivity while silencing of the transgene results in resistance

Nicotiana benthamiana plants were engineered to express sequences of the helper component-proteinase (HC-Pro) of Cowpea aphid-borne mosaic potyvirus (CABMV). The sensitivity of the transgenic plants to infection with parental and heterologous viruses was studied. The lines expressing HC-Pro showed enhanced symptoms after infection with the parental CABMV isolate and also after infection with a heterologous potyvirus, Potato virus Y (PVY) and a comovirus, Cowpea mosaic virus (CPMV). On the other hand, transgenic lines expressing nontranslatable HC-Pro or translatable HC-Pro with a deletion of the central domain showed wild type symptoms after infection with the parental CABMV isolate and heterologous viruses. These results showed that CABMV HC-Pro is a pathogenicity determinant that conditions enhanced sensitivity to virus infection in plants, and that the central domain of the protein is essential for this. The severe symptoms in CABMV-infected HC-Pro expressing lines were remarkably followed by brief recovery and subsequent re-establishment of infection, possibly indicating counteracting effects of HC-Pro expression and a host defense response. One of the HC-Pro expressing lines (h48) was found to contain low levels of transgenic HC-Pro RNA and to be resistant to CABMV and to recombinant CPMV expressing HC-Pro. This indicated that h48 was (partially) posttranscriptionally silenced for the HC-Pro transgene inspite of the established role of HC-Pro as a suppressor of posttranscriptional gene silencing. Line h48 was not resistant to PVY, but instead showed enhanced symptoms compared to nontransgenic plants. This may be due to relief of silencing of the HC-Pro transgene by HC-Pro expressed by PVY.

Effects of inactivation of the coat protein and movement genes of Tomato bushy stunt virus on early accumulation of genomic and subgenomic RNAs

Accumulation of RNA of Tomato bushy stunt virus (TBSV) was examined within the first few hours after infection of Nicotiana benthamiana protoplasts to determine the influence of the coat protein (CP), the movement-associated proteins P22 and P19 and RNA sequences at very early stages of replication. The results showed that P19 had no effect on early RNA replication, whereas the absence of CP and/or P22 expression delayed RNA accumulation only marginally. Removal of CP-coding sequences had no added negative effects, but when the deletion extended into the downstream p22 gene, it not only eliminated synthesis of subgenomic RNA2 but also delayed accumulation of genomic RNA by 10 h. At times beyond 20 h post-transfection, RNA accumulated to normal high levels for all mutants. This illustrates that TBSV RNA sequences that have negligible impact on overall RNA levels observed late in infection can actually have pronounced effects at very early stages.

Functional specialization and evolution of leader proteinases in the family Closteroviridae

Members of the Closteroviridae and Potyviridae families of the plant positive-strand RNA viruses encode one or two papain-like leader proteinases. In addition to a C-terminal proteolytic domain, each of these proteinases possesses a nonproteolytic N-terminal domain. We compared functions of the several leader proteinases using a gene swapping approach. The leader proteinase (L-Pro) of Beet yellows virus (BYV; a closterovirus) was replaced with L1 or L2 proteinases of Citrus tristeza virus (CTV; another closterovirus), P-Pro proteinase of Lettuce infectious yellows virus (LIYV; a crinivirus), and HC-Pro proteinase of Tobacco etch virus (a potyvirus). Each foreign proteinase efficiently processed the chimeric BYV polyprotein in vitro. However, only L1 and P-Pro, not L2 and HC-Pro, were able to rescue the amplification of the chimeric BYV variants. The combined expression of L1 and L2 resulted in an increased RNA accumulation compared to that of the parental BYV. Remarkably, this L1-L2 chimera exhibited reduced invasiveness and inability to move from cell to cell. Similar analyses of the BYV hybrids, in which only the papain-like domain of L-Pro was replaced with those derived from L1, L2, P-Pro, and HC-Pro, also revealed functional specialization of these domains. In subcellular-localization experiments, distinct patterns were observed for the leader proteinases of BYV, CTV, and LIYV. Taken together, these results demonstrated that, in addition to a common proteolytic activity, the leader proteinases of closteroviruses possess specialized functions in virus RNA amplification, virus invasion, and cell-to-cell movement. The phylogenetic analysis suggested that functionally distinct L1 and L2 of CTV originated by a gene duplication event.

Arabidopsis RTM1 and RTM2 genes function in phloem to restrict long-distance movement of tobacco etch virus

Restriction of long-distance movement of tobacco etch virus (TEV) in Arabidopsis ecotype Col-0 plants requires the function of at least three genes: RTM1 (restricted TEV movement 1), RTM2, and RTM3. The mechanism of TEV movement restriction remains poorly understood, although it does not involve a hypersensitive response or systemic acquired resistance. A functional characterization of RTM1 and RTM2 was done. The RTM1 protein was found to be soluble with the potential to form self-interacting complexes. The regulatory regions of both the RTM1 and RTM2 genes were analyzed using reporter constructs. The regulatory sequences from both genes directed expression of beta-glucuronidase exclusively in phloem-associated cells. Translational fusion proteins containing the green fluorescent protein and RTM1 or RTM2 localized to sieve elements when expressed from their native regulatory sequences. Thus, components of the RTM system may function within phloem, and sieve elements in particular, to restrict TEV long-distance movement.

Plants expressing tomato golden mosaic virus AL2 or beet curly top virus L2 transgenes show enhanced susceptibility to infection by DNA and RNA viruses

The AL2 gene of the geminivirus tomato golden mosaic virus (TGMV) encodes a transcriptional activator protein (TrAP) that is required for efficient expression of the viral coat protein (CP) and BR1 gene promoters. In contrast, L2, the positional homolog of AL2 in the related beet curly top virus (BCTV), is not required for CP expression, raising questions about the functional relationship between the AL2 and L2 gene products. In this study, transgenic Nicotiana benthamiana and N. tabacum var. Samsun plants expressing a truncated AL2 gene (AL2(1-100), lacking the activation domain) or full-length L2 were prepared. These transgenic plants showed a novel enhanced susceptibility (ES) phenotype following inoculation with TGMV, BCTV, or tobacco mosaic virus (TMV), an unrelated RNA virus. ES is characterized by a reduction in the mean latent period (from 1 to 9 days) and by a decrease in the inoculum concentration required to infect transgenic plants (ID50 reduced 6- to 60-fold). However, ES does not result in an enhancement of disease symptoms, and viral nucleic acids do not accumulate to substantially greater levels in infected transgenic plants. That both viral transgenes condition ES suggests that their products share the ability to suppress a host stress or defense response that acts against DNA and RNA viruses. The data further indicate that the transcriptional activation activity of AL2 protein is not required for suppression. The nature of the response targeted by the AL2 and L2 gene products is discussed.

Long-distance movement and replication maintenance functions correlate with silencing suppression activity of potyviral HC-Pro

The tobacco etch potyviral protein, HC-Pro, is a multifunctional proteinase required for long-distance movement in plants and maintenance of genome replication at the single-cell level. It also functions in a counterdefensive capacity as a suppressor of posttranscriptional gene silencing (PTGS). To determine whether the requirements for HC-Pro during long distance movement and replication maintenance are due to the silencing suppressor function of the protein, a series of HC-Pro alanine scanning and other site-directed mutants were analyzed. Using a transient silencing suppression assay in Agrobacterium-injected leaf tissue, several suppression-defective mutants were identified. Each of six HC-Pro mutations, which were shown previously to confer long-distance movement and replication maintenance defects, conferred PTGS suppression defects. Interestingly, the genes encoding these defective HC-Pro derivatives were themselves susceptible targets of PTGS, resulting in low levels of mRNA and protein accumulation. Mutations that inactivated the proteinase domain active site had no effect on PTGS suppression function. The results are consistent with the hypothesis that the role of HC-Pro in long-distance movement and genome replication depends on PTGS suppression function and that this function is independent of HC-Pro proteolytic activity.

Potyviral helper-component proteinase expressed in transgenic plants enhances titers of Potato leaf roll virus but does not alleviate its phloem limitation

Coinfection of Nicotiana benthamiana with Potato virus A (PVA, a potyvirus) and Potato leaf-roll virus (PLRV, a luteovirus) induces a synergistic interaction manifested by enhanced titers of PLRV. The helper component proteinase (HC-Pro) of potyviruses is involved in viral vascular movement and suppression of an antiviral defense mechanism in plants. Data of our study showed that accumulation of PLRV in transgenic N. benthamiana expressing the PVA HC-Pro was enhanced on average by 4.5-fold, as compared to a 6.0-fold enhancement in wild-type N. benthamiana plants doubly infected with PVA and PLRV. Enhancement of PLRV accumulation was directly proportional to the concentration of the HC-Pro in leaves. In the HC-Pro-transgenic plants and wild-type plants, PLRV was almost exclusively confined to the phloem, but the HC-Pro-transgenic plants had a fourfold greater number of PLRV-infected cells within the phloem tissues, as revealed by immunohistochemical staining. In the leaves doubly infected with PLRV and PVA, PLRV was found to exit the phloem in 25.0% of the veins, infecting all types of leaf cells, but, on average, PLRV accumulation was not enhanced more than by sixfold at the whole-leaf level. Therefore, potyviral/luteoviral synergism seems to be based on two mechanisms. One of them is mediated by the HC-Pro and increases luteovirus accumulation without allowing detectable egress from vascular tissue. The other mechanism probably depends on additional potyviral proteins and alleviates the normal phloem limitation of PLRV.

Towards a protein interaction map of potyviruses: protein interaction matrixes of two potyviruses based on the yeast two-hybrid system

A map for the interactions of the major proteins from Potato virus A (PVA) and Pea seed-borne mosaic virus (PSbMV) (members of the genus POTYVIRUS:, family POTYVIRIDAE:) was generated using the yeast two-hybrid system (YTHS). Interactions were readily detected with five PVA protein combinations (HC-HC, HC-CI, VPg-VPg, NIa-NIb and CP-CP) and weak but reproducible interactions were detected for seven additional combinations (P1-CI, P3-NIb, NIaPro-NIb, VPg-NIa, VPg-NIaPro, NIaPro-NIa and NIa-NIa). In PSbMV, readily detectable interactions were found in five protein combinations (HC-HC, VPg-VPg, VPg-NIa, NIa-NIa and NIa-NIb) and weaker but reproducible interactions were detected for three additional combinations (P3-NIa, NIa-NIaPro and CP-CP). The self-interactions of HC, VPg, NIa and CP and the interactions of VPg-NIa, NIa-NIaPro and NIa-NIb were, therefore, common for the two potyviruses. The multiple protein interactions revealed in this study shed light on the co-ordinated functions of potyviral proteins involved in virus movement and replication.

Pathogenicity determinants in the complex virus population of a Plum pox virus isolate

Several subisolates were separated from a single Plum pox virus (PPV) isolate, PPV-PS. In spite of an extremely high sequence conservation (more than 99.9% similarity), different subisolates differed largely in pathogenicity in herbaceous hosts and infectivity in woody plants. The severity of symptomatology did not seem to correlate with virus accumulation. Sequence analysis and site-directed mutagenesis demonstrated that single amino acid changes in the helper component (HC) protein caused a drastic effect on virus symptoms in herbaceous hosts and notably modified virus infectivity in peach seedlings. These results indicate that HC variation might play an important role in virulence evolution of natural plant virus infections. Moreover, the analysis of Potato virus X (PVX)-HC chimeras showed that the identified HC amino acid changes had parallel effects on the severity of symptoms caused by PPV and on HC-induced enhancement of PVX pathogenicity, indicating that HC functions in potyvirus symptomatology and in synergism with other viruses have overlapping determinants.

Protection of rabbits against rabbit hemorrhagic disease virus by immunization with the VP60 protein expressed in plants with a potyvirus-based vector

A new plum pox potyvirus (PPV)-based vector has been constructed for the expression of full-length individual foreign proteins. The foreign sequences are cloned between the NIb replicase and capsid protein (CP) cistrons. The heterologous protein is split from the rest of the potyviral polyprotein by cleavage at the site that originally separated the NIb and CP proteins and at an additional NIa protease recognition site engineered at its amino-terminal end. This vector (PPV-NK) has been used to clone different genes, engendering stable chimeras with practical applications. We have constructed a chimera expressing high levels of jellyfish green fluorescent protein, which can be very useful for the study of PPV molecular biology. The VP60 structural protein of rabbit hemorrhagic disease virus (RHDV) was also successfully expressed by making use of the PPV-NK vector. Inoculation of extracts from VP60-expressing plants induced a remarkable immune response against RHDV in rabbits, its natural host. Moreover, these animals were protected against a lethal challenge with RHDV.

Breeding virus resistant potatoes (Solanum tuberosum): a review of traditional and molecular approaches

Tetraploid cultivated potato (Solanum tuberosum) is the World's fourth most important crop and has been subjected to much breeding effort, including the incorporation of resistance to viruses. Several new approaches, ideas and technologies have emerged recently that could affect the future direction of virus resistance breeding. Thus, there are new opportunities to harness molecular techniques in the form of linked molecular markers to speed up and simplify selection of host resistance genes. The practical application of pathogen-derived transgenic resistance has arrived with the first release of GM potatoes engineered for virus resistance in the USA. Recently, a cloned host virus resistance gene from potato has been shown to be effective when inserted into a potato cultivar lacking the gene. These and other developments offer great opportunities for improving virus resistance, and it is timely to consider these advances and consider the future direction of resistance breeding in potato. We review the sources of available resistance, conventional breeding methods, marker-assisted selection, somaclonal variation, pathogen-derived and other transgenic resistance, and transformation with cloned host genes. The relative merits of the different methods are discussed, and the likely direction of future developments is considered.

Virus-encoded suppressor of posttranscriptional gene silencing targets a maintenance step in the silencing pathway

Certain plant viruses encode suppressors of posttranscriptional gene silencing (PTGS), an adaptive antiviral defense response that limits virus replication and spread. The tobacco etch potyvirus protein, helper component-proteinase (HC-Pro), suppresses PTGS of silenced transgenes. The effect of HC-Pro on different steps of the silencing pathway was analyzed by using both transient Agrobacterium tumefaciens-based delivery and transgenic systems. HC-Pro inactivated PTGS in plants containing a preexisting silenced beta-glucuronidase (GUS) transgene. PTGS in this system was associated with both small RNA molecules (21-26 nt) corresponding to the 3' proximal region of the transcribed GUS sequence and cytosine methylation of specific sites near the 3' end of the GUS transgene. Introduction of HC-Pro into these plants resulted in loss of PTGS, loss of small RNAs, and partial loss of methylation. These results suggest that HC-Pro targets a PTGS maintenance (as opposed to an initiation or signaling) component at a point that affects accumulation of small RNAs and methylation of genomic DNA.

Leader proteinase of the beet yellows closterovirus: mutation analysis of the function in genome amplification

The beet yellows closterovirus leader proteinase (L-Pro) possesses a C-terminal proteinase domain and a nonproteolytic N-terminal domain. It was found that although L-Pro is not essential for basal-level replication, deletion of its N-terminal domain resulted in a 1, 000-fold reduction in RNA accumulation. Mutagenic analysis of the N-terminal domain revealed its structural flexibility except for the 54-codon-long, 5'-terminal element in the corresponding open reading frame that is critical for efficient RNA amplification at both RNA and protein levels.

Native electrophoresis and Western blot analysis (NEWeB): a method for characterization of different forms of potyvirus particles and similar nucleoprotein complexes in extracts of infected plant tissues

A combination of native electrophoresis and immunodetection (Western blot) was used for the characterization of nucleoprotein particles of the potyvirus Plum pox virus (PPV). Virus particles were electrophoresed directly from plant extracts in agarose or mixed acrylamide-agarose gels under native conditions, blotted on nitrocellulose membranes, and characterized with the aid of a coat protein-specific antibody. Using this combined methodology, called NEWeB (native electrophoresis and Western blotting), we could show that a population of particles that differ in their electrophoretic mobility can be detected in extracts of Nicotiana benthamiana, that two different strains of PPV can be distinguished in double infections of the same plant and that virus particles from leaves contain detectable levels of helper component proteinase molecules. The potential of the NEWeB method for the study of structure and function of virus particles and similar nucleoprotein complexes in single and mixed infections is discussed.

A Point Mutation in the FRNK Motif of the Potyvirus Helper Component-Protease Gene Alters Symptom Expression in Cucurbits and Elicits Protection Against the Severe Homologous Virus

Sequence comparison had previously shown three amino acid changes in conserved motifs in the 455-amino acid sequence of the helper component-protease (HC-Pro) between a severe field strain of Zucchini yellow mosaic virus (ZYMV-NAT) and a mild field strain of ZYMV (ZYMV-WK). In this study, exchange of fragments and site-directed mutagenesis within the HC-Pro gene in an infectious clone of ZYMV enabled the effects of the mutations on symptom expression to be mapped. The substitution of Ile for Arg at position 180 in the conserved motif Phe-Arg-Asn-Lys (FRNK) of potyviruses was found to affect symptom expression. Infection of cucurbits with the engineered ZYMV (ZYMV-AG) that contained this mutation caused a dramatic symptom change from severe to mild in squash and to a symptom-free appearance in cucumber, melon, and watermelon. The Ile to Arg mutation was found to be stable, and no revertant virus was found after several passages through plants after long incubation periods. The AG strain was detected 4 days postinoculation and accumulated in cucurbits to a level and with kinetics similar to that of the wild-type ZYMV-AT strain. Cucurbit plants infected with the AG strain were protected against infection by the severe strain.

Synergistic interactions of a potyvirus and a phloem-limited crinivirus in sweet potato plants

When infecting alone, Sweet potato feathery mottle virus (SPFMV, genus Potyvirus) and Sweet potato chlorotic stunt virus (SPCSV, genus Crinivirus) cause no or only mild symptoms (slight stunting and purpling), respectively, in the sweet potato (Ipomoea batatas L. ). In the SPFMV-resistant cv. Tanzania, SPFMV is also present at extremely low titers, though plants are systemically infected. However, infection with both viruses results in the development of sweet potato virus disease (SPVD) characterized by severe symptoms in leaves and stunting of the plants. Data from this study showed that SPCSV remains confined to phloem and at a similar or slightly lower titer in the SPVD-affected plants, whereas the amounts of SPFMV RNA and CP antigen increase 600-fold. SPFMV was not confined to phloem, and the movement from the inoculated leaf to the upper leaves occurred at a similar rate, regardless of whether or not the plants were infected with SPCSV. Hence, resistance to SPFMV in cv. Tanzania was not based on restricted virus movement, neither did SPCSV significantly enhance the phloem loading or unloading of SPFMV. It is also noteworthy that SPVD is an unusual synergistic interaction in that the potyvirus component is not the cause of synergism but is the beneficiary. It is hypothesized that SPCSV is able to enhance the multiplication of SPFMV in tissues other than where it occurs itself, perhaps by interfering with systemic phloem-dependent signaling required in a resistance mechanism directed against SPFMV.

Deletion mapping of the potyviral helper component-proteinase reveals two regions involved in RNA binding

The Potyvirus helper component-proteinase (HC-Pro) binds nonspecifically to single-stranded nucleic acids with a preference for RNA. To delineate the regions of the protein responsible for RNA binding, deletions were introduced into the full-length Potato potyvirus Y HC-Pro gene carried by an Escherichia coli expression vector. The corresponding proteins were expressed as fusions with the maltose-binding protein, purified, and assayed for their RNA-binding capacity. The results obtained by UV cross-linking and Northwestern blot assays demonstrated that the N- and C-terminal regions of HC-Pro are dispensable for RNA binding. They also revealed the presence of two independent RNA-binding domains (designated A and B) located in the central part of HC-Pro. Domain B appears to contain a ribonucleoprotein (RNP) motif typical of a large family of RNA-binding proteins involved in several cellular processes. The possibility that domain B consists of an RNP domain is discussed and suggests that HC-Pro could constitute the first example of a plant viral protein belonging to the RNP-containing family of proteins.

Multiple interactions among proteins encoded by the mite-transmitted wheat streak mosaic tritimovirus

The genome organization of the mite-transmitted wheat streak mosaic virus (WSMV) appears to parallel that of members of the Potyviridae with monopartite genomes, but there are substantial amino acid dissimilarities with other potyviral polyproteins. To initiate studies on the functions of WSMV-encoded proteins, a protein interaction map was generated using a yeast two-hybrid system. Because the pathway of proteolytic maturation of the WSMV polyprotein has not been experimentally determined, random libraries of WSMV cDNA were made both in DNA-binding domain and activation domain plasmid vectors and introduced into yeast. Sequence analysis of multiple interacting pairs revealed that interactions largely occurred between domains within two groups of proteins. The first involved interactions among nuclear inclusion protein a, nuclear inclusion protein b, and coat protein (CP), and the second involved helper component-proteinase (HC-Pro) and cylindrical inclusion protein (CI). Further immunoblot and deletion mapping analyses of the interactions suggest that subdomains of CI, HC-Pro, and P1 interact with one another. The two-hybrid assay was then performed using full-length genes of CI, HC-Pro, P1, P3, and CP, but no heterologous interactions were detected. In vitro binding assay using glutathione-S-transferase fusion proteins and in vitro translation products, however, revealed mutual interactions among CI, HC-Pro, P1, and P3. The failure to detect interactions between full-length proteins by the two-hybrid assay might be due to adverse effects of expression of viral proteins in yeast cells. The capacity to participate in multiple homomeric and heteromeric molecular interactions is consistent with the pleiotropic nature of many potyviral gene mutants and suggests mechanisms for regulation of various viral processes via a network of viral protein complexes.

Cloning of the Arabidopsis RTM1 gene, which controls restriction of long-distance movement of tobacco etch virus

The locus RTM1 is necessary for restriction of long-distance movement of tobacco etch virus in Arabidopsis thaliana without causing a hypersensitive response or inducing systemic acquired resistance. The RTM1 gene was isolated by map-based cloning. The deduced gene product is similar to the alpha-chain of the Artocarpus integrifolia lectin, jacalin, and to several proteins that contain multiple repeats of a jacalin-like sequence. These proteins comprise a family with members containing modular organizations of one or more jacalin repeat units and are implicated in defense against viruses, fungi, and insects.

Suppression of gene silencing: a general strategy used by diverse DNA and RNA viruses of plants

In transgenic and nontransgenic plants, viruses are both initiators and targets of a defense mechanism that is similar to posttranscriptional gene silencing (PTGS). Recently, it was found that potyviruses and cucumoviruses encode pathogenicity determinants that suppress this defense mechanism. Here, we test diverse virus types for the ability to suppress PTGS. Nicotiana benthamiana exhibiting PTGS of a green fluorescent protein transgene were infected with a range of unrelated viruses and various potato virus X vectors producing viral pathogenicity factors. Upon infection, suppression of PTGS was assessed in planta through reactivation of green fluorescence and confirmed by molecular analysis. These experiments led to the identification of three suppressors of PTGS and showed that suppression of PTGS is widely used as a counter-defense strategy by DNA and RNA viruses. However, the spatial pattern and degree of suppression varied extensively between viruses. At one extreme, there are viruses that suppress in all tissues of all infected leaves, whereas others are able to suppress only in the veins of new emerging leaves. This variation existed even between closely related members of the potexvirus group. Collectively, these results suggest that virus-encoded suppressors of gene silencing have distinct modes of action, are targeted against distinct components of the host gene-silencing machinery, and that there is dynamic evolution of the host and viral components associated with the gene-silencing mechanism.

Resistances to turnip mosaic potyvirus in Arabidopsis thaliana

The responses of a collection of Arabidopsis thaliana ecotypes to mechanical inoculation with turnip mosaic potyvirus were assessed. The virus induced characteristic severe symptoms of infection in systemically infected plants. Resistance was found in four ecotypes: Bay-0, Di-0, Er-0, and Or-0. Enzyme-linked immunosorbent assay results of the resistant ecotypes suggested that ecotypes Di-0, Er-0, and Or-0 actually consist of mixed genotypes with resistances acting at different levels in the virus life cycle. Another form of resistance was found in ecotype Bay-0, for which several lines of evidence indicated an interference with viral cell-to-cell movement in the inoculated leaves.

Mapping of a hypovirus p29 protease symptom determinant domain with sequence similarity to potyvirus HC-Pro protease

Hypovirus infection of the chestnut blight fungus Cryphonectria parasitica results in a spectrum of phenotypic changes that can include alterations in colony morphology and significant reductions in pigmentation, asexual sporulation, and virulence (hypovirulence). Deletion of 88% [Phe(25) to Pro(243)] of the virus-encoded papain-like protease, p29, in the context of an infectious cDNA clone of the prototypic hypovirus CHV1-EP713 (recombinant virus Deltap29) partially relieved virus-mediated suppression of pigmentation and sporulation without altering the level of hypovirulence. We now report mapping of the p29 symptom determinant domain to a region extending from Phe(25) through Gln(73) by a gain-of-function analysis following progressive repair of the Deltap29 deletion mutant. This domain was previously shown to share sequence similarity [including conserved cysteine residues Cys(38), Cys(48), Cys(70), and Cys(72)] with the N-terminal portion of the potyvirus-encoded helper component-proteinase (HC-Pro), a multifunctional protein implicated in aphid-mediated transmission, genome amplification, polyprotein processing, long-distance movement, and suppression of posttranscriptional silencing. Substitution of a glycine residue for either Cys(38) or Cys(48) resulted in no qualitative or quantitative changes in virus-mediated symptoms. Unexpectedly, mutation of Cys(70) resulted in a very severe phenotype that included significantly reduced mycelial growth and profoundly altered colony morphology. In contrast, substitution for Cys(72) resulted in a less severe symptom phenotype approaching that observed for Deltap29. The finding that p29-mediated symptom expression is influenced by two cysteine residues that are conserved in the potyvirus-encoded HC-Pro raises the possibility that these related viral-papain-like proteases function in their respective fungal and plant hosts by impacting ancestrally related regulatory pathways.

Biochemical and genetic evidence for interactions between potato A potyvirus-encoded proteins P1 and P3 and proteins of the putative replication complex

Interactions of the first and third proteins (P1 and P3) of the potato A potyvirus (PVA) with the other six main proteins of PVA were studied using Escherichia coli-expressed recombinant proteins in two in vitro interaction assays and a genetic assay yeast two-hybrid system (YTHS). In overlay blotting and binding assays in liquid, P1 and P3 interacted with each other and with proteins of the putative replication complex of potyvirus: RNA-helicase (CI), viral protein genome-linked (VPg), NIa proteinase part (NIaPro), and RNA-dependent-RNA-polymerase (NIb). In addition, P1 self-interaction and interaction with helper-component proteinase (HC-Pro) also were detected. Neither P1 nor P3 interact with coat protein (CP) or with various control proteins. In the YTHS, P1 interacted only with CI and P3 with NIb. The different results obtained using the two test systems may reflect changes in interactions at different stages of potyvirus infection: in the virus genome replication and the virion accumulation stages when nonstructural proteins form inclusions. Our data are consistent with previous functional data, indicating that P1 and P3 proteins are involved in potyvirus genome amplification and provide the first direct evidence that these proteins interact with the proteins that have been shown to be part of the replication complex.

Potyvirus helper component-proteinase self-interaction in the yeast two-hybrid system and delineation of the interaction domain involved

Using the yeast two-hybrid system, a screen was performed for possible interactions between the proteins encoded by the 5' region of potyviral genomes [P1, helper component-proteinase (HC-Pro), and P3]. A positive self-interaction involving HC-Pro was detected with lettuce mosaic virus (LMV) and potato virus Y (PVY). The possibility of heterologous interaction between the HC-Pro of LMV and of PVY was also demonstrated. No interaction involving either the P1 or the P3 proteins was detected. A series of ordered deletions from either the N- or C-terminal end of the LMV HC-Pro was used to map the domain involved in interaction to the 72 N-terminal amino acids of the protein, a region known to be dispensable for virus viability but necessary for aphid transmission. A similar but less detailed analysis mapped the interacting domain to the N-terminal half of the PVY HC-Pro.

Infectious virus in transgenic plants inoculated with a nonviable, P1-proteinase defective mutant of a potyvirus

A mutant (P1-616) of the tobacco vein mottling potyvirus that contains a four-codon insertion in the P1 protein coding region of the viral RNA is unable to infect the normal host plant of the virus. Processing of the P1/HC-Pro cleavage site does not occur during in vitro translation of the mutant viral RNA. When plants transformed with the P1/HC-Pro/P3 coding region of tobacco vein mottling potyvirus RNA were inoculated with P1-616, some of them became infected, although there was a delay in the production of disease symptoms. Virus isolated from these plants was able to infect nontransgenic plants. Two variants of the recovered, infectious virus contained single-nucleotide alterations in the four-codon insertion in the P1-616 genome. In vitro translation of the variant genomic RNAs resulted in partial processing of the P1/HC-Pro cleavage site, although serological analysis of infected tissue showed complete processing in vivo. These results indicate that limited complementation of P1-616 occurs in the transgenic plants and that eventually there arises one or more variants of the mutant sequence that can effect P1/HC-Pro processing and therefore be replicated.

Selectable viruses and altered susceptibility mutants in Arabidopsis thaliana

The genetic basis for susceptibility or nonsusceptibility of plants to viruses is understood poorly. Two selectable tobacco etch virus (TEV) strains were developed for identification of Arabidopsis thaliana mutants with either gain-of-susceptibility or loss-of-susceptibility phenotypes. These strains conferred a conditional-survival phenotype to Arabidopsis based on systemic expression of herbicide resistance or proherbicide sensitivity genes, thereby facilitating mass selections and screens for Arabidopsis mutants that enhance or suppress TEV replication, cell-to-cell movement, or long-distance movement. A multicomponent mechanism that restricts systemic invasion of TEV was identified through isolation of gain-of-susceptibility mutants with alterations at two loci.

Proteinases Involved in Plant Virus Genome Expression

This chapter discusses the proteinases involved in plant virus genome expression. The chapter focuses on virus-encoded proteinases. It gives an overall view of the use of proteolytic processing by different plant virus groups for the expression of their genomes. It also discusses that the development of full-length cDNA clones from which infectious transcripts can be produced either in vitro or in vivo, has facilitated the functional analysis of the plant virus proteinases. In spite of the high specificity of the viral proteinases, cellular substrates for animal virus proteinases have been described in this chapter. The activity of the viral proteinases can interfere with important cellular processes to favor virus replication. The recent use of proteinase inhibitors in AIDS therapy has emphasized the convenience of virus-encoded proteinases as targets of antiviral action. A mutant protein able to inhibit the activity of the TEV proteinase by manipulation of the α₂-macroglobulin bait region was designed by Van Rompaey.

Affinity purification of HC-Pro of potyviruses with Ni2+-NTA resin

The HC-Pro of zucchini yellow mosiac virus (ZYMV) was found to bind to Ni2+-NTA resin with or without His-tagging. The binding stringency was similar to that observed in proteins with a zinc finger motif like the HC-Pro. Using this characteristic we developed an efficient and rapid method (2-3 h) for purification of the HC-Pro of several potyviruses. A dominant protein of about 150 kDa was extracted and identified as the HC-Pro of ZYMV by means of immunoblotting. About 150 microg of HC-Pro were partially purified from the soluble fraction of 1 g of leaves. High titers of HC-Pro protein were obtained from plants infected with four potyviruses [ZYMV, watermelon mosaic virus II (WMVII), papaya ringspot virus (PRSV) and turnip mosaic virus (TuMV)]. The HC-Pros of potato virus Y (PVY) and tobacco vein mottling virus (TVMV) did not bind to the Ni2+-NTA resin. The ZYMV-HC-Pro purified by the Ni2+-NTA resin could bind in vitro to ZYMV virions blotted onto a membrane. All the HC-Pros which had been successfully purified by the Ni2+-NTA resin were bound in vitro to membrane-blotted ZYMV coat protein. However, only the HC-Pros of ZYMV and WMVII were able to mediate aphid transmission of purified ZYMV virions. The purification procedure described herein is efficient and convenient, and enables HC-Pro for a number of potyviruses to be obtained in larger amounts and at higher purity than possible by means of most existing methods, based on ultracentrifugation.

A counterdefensive strategy of plant viruses: suppression of posttranscriptional gene silencing

Posttranscriptional gene silencing (PTGS) in plants inactivates some aberrant or highly expressed RNAs in a sequence-specific manner in the cytoplasm. A silencing mechanism similar to PTGS appears to function as an adaptive antiviral response. We demonstrate that the P1/HC-Pro polyprotein encoded by tobacco etch virus functions as a suppressor of PTGS. A locus comprised of a highly expressed beta-glucuronidase (GUS) transgene was shown to exhibit PTGS. Genetic crosses and segregation analyses revealed that a P1/ HC-Pro transgene suppressed PTGS of the GUS sequence. Nuclear transcription assays indicated that the silencing suppression activity of P1/HC-Pro was at the posttranscriptional level. These data reveal that plant viruses can condition enhanced susceptibility within a host through interdiction of a potent defense response.

A viral suppressor of gene silencing in plants

Gene silencing is an important but little understood regulatory mechanism in plants. Here we report that a viral sequence, initially identified as a mediator of synergistic viral disease, acts to suppress the establishment of both transgene-induced and virus-induced posttranscriptional gene silencing. The viral suppressor of silencing comprises the 5'-proximal region of the tobacco etch potyviral genomic RNA encoding P1, helper component-proteinase (HC-Pro) and a small part of P3, and is termed the P1/HC-Pro sequence. A reversal of silencing assay was used to assess the effect of the P1/HC-Pro sequence on transgenic tobacco plants (line T4) that are posttranscriptionally silenced for the uidA reporter gene. Silencing was lifted in offspring of T4 crosses with four independent transgenic lines expressing P1/HC-Pro, but not in offspring of control crosses. Viral vectors were used to assess the effect of P1/HC-Pro expression on virus-induced gene silencing (VIGS). The ability of a potato virus X vector expressing green fluorescent protein to induce silencing of a green fluorescent protein transgene was eliminated or greatly reduced when P1/HC-Pro was expressed from the same vector or from coinfecting potato virus X vectors. Expression of the HC-Pro coding sequence alone was sufficient to suppress virus-induced gene silencing, and the HC-Pro protein product was required for the suppression. This discovery points to the role of gene silencing as a natural antiviral defense system in plants and offers different approaches to elucidate the molecular basis of gene silencing.

Viral invasion and host defense: strategies and counter-strategies

The outcome of infection of plants by viruses is determined by the net effects of compatibility functions and defense responses. Recent advances reveal that viruses have the capacity to modulate host compatibility and defense functions by a variety of mechanisms.

Genes required for replication of the 15.5-kilobase RNA genome of a plant closterovirus

A full-length cDNA clone of beet yellows closterovirus (BYV) was engineered and used to map functions involved in the replication of the viral RNA genome and subgenomic RNA formation. Among 10 open reading frames (ORFs) present in BYV, ORFs 1a and 1b suffice for RNA replication and transcription. The proteins encoded in these ORFs harbor putative methyltransferase, RNA helicase, and RNA polymerase domains common to Sindbis virus-like viruses and a large interdomain region that is unique to closteroviruses. The papain-like leader proteinase (L-Pro) encoded in the 5'-proximal region of ORF 1a was found to have a dual function in genome amplification. First, the autocatalytic cleavage between L-Pro and the remainder of the ORF 1a product was essential for replication of RNA. Second, an additional L-Pro function that was separable from proteolytic activity was required for efficient RNA accumulation. The deletion of a large, approximately 5.6-kb, 3'-terminal region coding for a 6-kDa hydrophobic protein, an HSP70 homolog, a 64-kDa protein, minor and major capsid proteins, a 20-kDa protein, and a 21-kDa protein (p21) resulted in replication-competent RNA. However, examination of mutants with replacements of start codons in each of these seven 3'-terminal ORFs revealed that p21 functions as an enhancer of genome amplification. The intriguing analogies between the genome organization and replicational requirements of plant closteroviruses and animal coronavirus-like viruses are discussed.

Infectivity of turnip mosaic potyvirus cDNA clones and transcripts on the systemic host Arabidopsis thaliana and local lesion hosts

Full-length cDNA clones of turnip mosaic virus were assembled under the control of T7 or 35S promoter. The 35S or nopaline synthase terminator signals were introduced downstream the full length cDNA controlled by 35S promoter. Both the capped in vitro transcripts from T7 controlled template, and the cDNAs from 35S controlled plasmids were infectious on Arabidopis thaliana plants according to systemically induced symptoms and to ELISA assays. The cDNAs from 35S controlled plasmids induced local lesions in Chenopodium amaranticolor and Chenopodium quinoa plants. A spontaneous silent C/T transition, giving rise to an additional SpeI restriction site, not present in the original viral RNA template, was used as a marker of the origin of infection.

Recombination of engineered defective RNA species produces infective potyvirus in planta

Recombination occurred between viral genomes when squash plants were cobombarded with mixtures of engineered disabled constructs of a zucchini yellow mosaic potyvirus. Single and double recombinants were detected in the progeny. Genes involved in the recombination process and the mechanisms of recombination were studied in potyviruses for the first time.

Secondary structures in the capsid protein coding sequence and 3' nontranslated region involved in amplification of the tobacco etch virus genome

The 3'-terminal 350 nucleotides of the tobacco etch potyvirus (TEV) genome span the end of the capsid protein (CP)-coding sequence and the 3' nontranslated region (NTR). The CP-coding sequence within this region contains a 105-nucleotide cis-active element required for genome replication (S. Mahajan, V. V. Dolja, and J. C. Carrington, J. Virol. 70:4370-4379, 1996). To investigate the sequence and secondary structure requirements within the CP cis-active region and the 3' NTR, a systematic linker-scanning mutagenesis analysis was done. Forty-six mutations, each with two to six nucleotide substitutions, were introduced at consecutive hexanucleotide positions in the genome of a recombinant TEV strain expressing a reporter protein (beta-glucuronidase). Genome amplification activity of each mutant in the protoplast cell culture system was measured. Mutations that severely debilitated genome amplification were identified throughout the CP-coding cis-active sequence and at several distinct locations within the 3' NTR. However, based on a computer model of RNA folding, mutations that had the most severe effects mapped to regions that were predicted to form base-paired secondary structures. Linker-scanning mutations predicted to affect either strand of a base-paired structure within the CP-coding cis-active sequence, a base-paired structure between two segments of the CP-coding cis-active sequence and a contiguous 14-nucleotide segment of the 3' NTR, and a base-paired structure near the 3' terminus of the 3' NTR inactivated genome amplification. Compensatory mutations that restored base pair interactions in each of these regions restored amplification activity, although to differing levels depending on the structure restored. These data reveal that the 3' terminus of the TEV genome consists of a series of functionally discrete sequences and secondary structures and that the CP-coding sequence and 3' NTR are coadapted for genome amplification function through a requirement for base pair interactions.

Genetic evidence for an essential role for potyvirus CI protein in cell-to-cell movement

The potyvirus cylindrical inclusion (CI) protein, an RNA helicase required for genome replication, was analyzed genetically using alanine-scanning mutagenesis. Thirty-one mutations were introduced into the CI protein coding region of modified tobacco etch virus (TEV) genomes expressing either beta-glucuronidase or green fluorescent protein reporters. Twelve of the mutants were replication-defective in protoplast inoculation assays. Among the 19 replication-competent mutants, several possessed cell-to-cell or long-distance movement defects in tobacco plants. Two mutants, AS1 and AS8, were restricted to single cells in inoculated leaves despite genome amplification levels that were equivalent to that of parental virus. Other mutants, such as AS9 and AS14, were able to move cell to cell slowly but were debilitated in long-distance movement. These data provide genetic evidence for a direct role of CI protein in potyvirus intercellular movement, and for distinct roles of the CI protein in genome replication and movement. In combination with high-resolution ultrastructural analyzes and previous genetic data, these results support a model in which CI protein interacts directly with plasmodesmata and capsid protein-containing ribonucleoprotein complexes to facilitate potyvirus cell-to-cell movement.

Identification and characterization of a locus (RTM1) that restricts long-distance movement of tobacco etch virus in Arabidopsis thaliana

Screens of Arabidopsis thaliana for susceptibility to tobacco etch virus (TEV) revealed that each of 10 ecotypes were able to support genome replication and cell-to-cell movement in inoculated leaves. However, only four ecotypes, including C24 and La-er, supported complete infections in which TEV was able to replicate and move from cell to cell and long distances through the vasculature. The rates of cell-to-cell movement of a reporter-tagged TEV strain (TEV-GUS) in inoculated leaves of C24 and Columbia (Col-3) were similar, and infection foci continued to expand in both ecotypes through 10 days post-inoculation. No visible or microscopic hypersensitive or cell death responses were evident in inoculated leaves of Col-3 plants. Infection of neither C24 nor Col-3 plants with TEV-GUS resulted in induction of PR-1a gene expression, which is normally associated with active defence responses and systemic acquired resistance. The genetic basis for the restriction of long-distance movement of TEV-GUS in Columbia was investigated using C24 x Col-3 crosses and backcrosses and using La-er x Col-0 recombinant inbred lines. A dominant locus conditioning the restricted TEV infection phenotype was identified on chromosome 1 between markers ATEAT1 and NCC1 at approximately 14 cM in both genetic analyses. This locus was designated RTM1 (restricted TEV movement 1). It is proposed that RTM1 mediates a restriction of long-distance movement through a mechanism that differs substantially from those conditioned by the dominant resistance genes normally associated with gene-for-gene interactions.

A single conserved amino acid in the coat protein gene of pea seed-borne mosaic potyvirus modulates the ability of the virus to move systemically in Chenopodium quinoa

Two isolates of pea seed-borne mosaic potyvirus, DPD1 and NY, which both infect pea (Pisum sativum) systemically, differ in their ability to move long distance in Chenopodium quinoa. DPD1 spreads to uninoculated leaves, whereas NY is restricted to the inoculated leaves. The NY isolate was found to move from cell to cell infecting all parts of the inoculated leaves, including the petiole. The coat protein (CP) coding region was identified as the determinant of long-distance movement. Virus chimeras containing the CP coding sequence of NY were restricted to inoculated leaves, whereas chimeras containing the CP coding sequence of DPD1 infected C. quinoa systemically. Mutational analysis of the CP demonstrated that changing the serine at position 47 of the NY CP to proline was sufficient to permit systemic spread of the NY(S47P) mutant. The reverse mutant, DPD1(P47S), in which the proline at position 47 of the CP was changed to serine, was restricted to inoculated leaves. The movement characteristics and CP sequences of 10 additional PSbMV isolates were determined. All isolates caused systemic infection in pea. In C. quinoa 6 of the isolates that were restricted to inoculated leaves had a serine at position 47. Two isolates that infected C. quinoa systemically had a proline at position 47. Two isolates, S6 and NEP-1, infected C. quinoa systemically, but had a serine at position 47 of the CP. This shows that although a proline/serine difference at position 47 of the CP determined systemic spread of the isolates DPD1 and NY, this amino acid alone does not govern the spread of PSbMV in C. quinoa.

VPg of tobacco etch potyvirus is a host genotype-specific determinant for long-distance movement

The V20 cultivar of Nicotiana tabacum was shown previously to exhibit a strain-specific restriction of long-distance movement of tobacco etch potyvirus (TEV). In V20, both TEV-HAT and TEV-Oxnard strains are capable of genome amplification and cell-to-cell movement, but only TEV-Oxnard is capable of systemic infection by vasculature-dependent long-distance movement. To investigate the basis for host-specific movement of TEV, chimeric virus genomes were assembled from TEV-HAT and TEV-Oxnard. Viruses containing the TEV-Oxnard coding regions for HC-Pro and/or capsid protein (CP), two proteins that are known to be essential for TEV long-distance movement, failed to infect V20 systemically. In contrast, chimeric viruses encoding the TEV-Oxnard VPg domain of NIa were able to infect V20 systemically. The critical region controlling the infection phenotype in V20 was mapped to a 67-nucleotide segment containing 10-nucleotide differences, but only five amino acid differences, between TEV-HAT and TEV-Oxnard. In V20 coinfection experiments, a restricted strain had no effect on systemic infection by a long-distance movement-competent chimeric strain, suggesting that the restricted strain was not inducing a generalized systemic resistance response. These data suggest that the VPg domain, which is covalently attached to the 5' end of genomic RNA, interacts either directly or indirectly with host components to facilitate long-distance movement.

Capsid protein and helper component-proteinase function as potyvirus cell-to-cell movement proteins

The role of bean common mosaic necrosis potyvirus (BCMNV) and lettuce mosaic potyvirus (LMV) proteins was investigated in terms of their capacity to function as viral movement proteins (MPs). Using Escherichia coli-expressed proteins and microinjection techniques, direct evidence was obtained that both the potyviral capsid protein (CP) and helper component- proteinase (HC-Pro) function in this capacity, in that both proteins (a) trafficked from cell to cell, (b) induced an increase in plasmodesmal size exclusion limit, and (c) facilitated cell-to-cell movement of viral RNA. CP and HC-Pro mutants were also produced and used in microinjection experiments. Mutations in the core region of the CP either impaired (single and double amino acid substitution mutants) or abolished (triple amino acid substitution mutant) cell-to-cell movement, as did C-terminal deletion mutants in HC-Pro. The BCMNV P1, CI, NIa, and NIb proteins did not exhibit viral MP properties, but NIa and NIb proteins were found to accumulate within the nuclei of injected cells. These results further establish the multifunctional nature of the potyvirus CP and HC-Pro.

Suppression of potyvirus infection by coexpressed closterovirus protein

A tobacco etch virus (TEV)-based expression vector has been used for insertion of several ORFs derived from the unrelated beet yellows virus (BYV). Hybrid TEV variants expressing the BYV capsid protein, 20-kDa protein, or HSP70 homolog systemically infected Nicotiana tabacum and stably retained BYV sequences. In contrast, insertion of the ORF encoding BYV leader proteinase (L-Pro) resulted in severely impaired systemic transport and accumulation of recombinant TEV. Progeny of this virus underwent various deletions affecting the L-Pro sequence and mitigating the defects in virus spread. Model experiments involving several spontaneous and engineered mutants indicated that the central domain of BYV L-Pro was responsible for the defect in hybrid virus accumulation, whereas full-size L-Pro was required for maximal debilitation of systemic transport. Strikingly, BYV L-Pro expression did not debilitate systemic infection of hybrid TEV in Nicotiana benthamiana plants. No major defects in replication or encapsidation of recombinant RNA were revealed in N. tabacum protoplasts. These results indicated that BYV L-Pro specifically interfered with TEV systemic transport and accumulation in a host-dependent manner and suggested a potential utility of closterovirus L-Pro as an inhibitor of potyvirus infection. In addition, it was demonstrated that the 107-amino-acid-residues-long N-terminal part of the TEV helper component proteinase is not essential for systemic infection.

Mutations in the region encoding the central domain of helper component-proteinase (HC-Pro) eliminate potato virus X/potyviral synergism

Coinfection of tobacco plants with potato virus X (PVX) and any of several members of the potyvirus group causes a synergistic disease characterized by a dramatic increase in symptom severity correlated with a 3- to 10-fold increase in the accumulation of PVX in the first systemically infected leaves. We have recently shown that PVX/potyviral synergistic disease is mediated by expression of potyviral 5'-proximal sequences encoding P1, helper component-proteinase (HC-Pro), and a fraction of P3 (termed P1/HC-Pro sequence). Here we report the effect of mutations in this potyviral sequence on the induction of synergistic disease. Three transgenic tobacco lines expressing the tobacco etch potyvirus (TEV) P1/HC-Pro sequence with mutations within the P1 coding region were not impaired in their ability to mediate synergism when infected with PVX. In contrast, two of three transgenic lines with mutations in the HC-Pro coding region were unable to induce the synergistic increases in either symptom severity or PVX accumulation. Loss of synergistic function was associated with mutations within the region encoding the central domain of HC-Pro, while the ability to induce synergism was retained in a transgenic line expressing HC-Pro with an alteration in the amino-terminal "zinc-finger domain." In coinoculation experiments, a TEV mutant lacking the sequence encoding the zinc-linger domain of HC-Pro induced a typical synergistic response in interaction with PVX. The results indicate that the zinc-finger domain comprising the first 66 amino acid residues of HC-Pro is dispensable for induction of synergistic disease and transactivation of PVX multiplication, while regions within the central domain of HC-Pro are essential for both of these responses.