Regulation of plasmodesmal transport by phosphorylation of tobacco mosaic virus cell-to-cell movement protein

Potato leafroll virus: a classic pathogen shows some new tricks

SUMMARY Taxonomy: PLRV is the type species of the genus Polerovirus, in the family Luteoviridae. Isolates are known from most continents, presumably all spread in potato material derived from the Andean region of South America. Physical properties: PLRV particles are isometric and c. 25 nm in diameter. They contain one major (c. 23 kDa) and one minor (c. 80 kDa) protein. The genome is a single 5.8 kb positive sense RNA that has neither a 5'-cap nor 3' poly(A) but carries a VPg.

HOST RANGE

PLRV has a limited host range; about 20 largely solanaceous species have been infected experimentally. PLRV is a common pathogen of potato, and closely related isolates are occasionally found in tomato, but no other crops are affected.

SYMPTOMS

Infection, especially from infected seed potato stocks, causes leafrolling and stunting, the extent depending on the potato cultivar. Biological properties: The biology of PLRV is that of a classic luteovirus. Its isometric particles are persistently transmitted by aphids in a non-propagative manner, it multiplies largely in phloem tissue and disease symptoms reflect this localization. A decade or so of molecular study has revealed the many features of PLRV that are characteristic of its family. Key attractions: In recent years some interesting features of PLRV have emerged that are the focus of further investigation. These are, its phloem confinement, its movement in infected plants, its ability to suppress gene silencing and new ideas about the structure of its particles. This review describes the background to PLRV and points towards these new developments.

Dysfunctionality of a tobacco mosaic virus movement protein mutant mimicking threonine 104 phosphorylation

Replication of tobacco mosaic virus (TMV) is connected with endoplasmic reticulum (ER)-associated membranes at early stages of infection. This study reports that TMV movement protein (MP)-specific protein kinases (PKs) associated with the ER of tobacco were capable of phosphorylating Thr(104) in TMV MP. The MP-specific PKs with apparent molecular masses of about 45-50 kDa and 38 kDa were revealed by gel PK assays. Two types of mutations were introduced in TMV MP gene of wild-type TMV U1 genome to substitute Thr(104) by neutral Ala or by negatively charged Asp. Mutation of Thr(104) to Ala did not affect the size of necrotic lesions induced by the mutant virus in Nicotiana tabacum Xanthi nc. plants. Conversely, mutation of Thr to Asp mimicking Thr(104) phosphorylation strongly inhibited cell-to-cell movement. The possible role of Thr(104) phosphorylation in TMV MP function is discussed.

The catalytic subunit of protein kinase CK2 phosphorylates in vitro the movement protein of Tomato mosaic virus

The movement protein (MP) of Tomato mosaic virus (ToMV) was reported previously by us to be phosphorylated in vitro by a cellular protein kinase(s) that exhibited several characteristics of casein kinase 2 (CK2). To characterize further this CK2-like cellular kinase, we have cloned cDNAs encoding the CK2 catalytic subunit from tobacco and compared the properties of the recombinant protein with those of the CK2-like cellular kinase. The recombinant CK2 catalytic subunit formed a complex with ToMV MP and phosphorylated it, similar to the CK2-like cellular kinase. Phosphoamino acid analyses of various mutant MPs altered near the C terminus revealed that the recombinant CK2 catalytic subunit phosphorylated serine-261, while the CK2-like cellular kinase phosphorylated both serine-261 and threonine-256. Both kinases were suggested to phosphorylate an additional serine residue(s) in regions other than the C-terminal peptide. The results are consistent with our previous prediction of involvement of CK2 in phosphorylation of ToMV MP.

Defective tobamovirus movement protein lacking wild-type phosphorylation sites can be complemented by substitutions found in revertants

We reported previously that the movement protein (MP) of tomato mosaic tobamovirus is phosphorylated, and we proposed that MP phosphorylation is important for viral pathogenesis. Experimental data indicated that phosphorylation enhances the stability of MP in vivo and enables the protein to assume the correct intracellular location to perform its function. A mutant virus designated 37A238A was constructed; this virus lacked two serine residues within the MP, which prevented its phosphorylation. In the present study, we inoculated plants with the 37A238A mutant, and as expected, it was unable to produce local lesions on the leaves. However, after an extended period, we found that lesions did occur, which were due to revertant viruses. Several revertants were isolated, and the genetic changes in their MPs were examined together with any changes in their in vivo characteristics. We found that reversion to virulence was associated first with increased MP stability in infected cells and second with a shift in MP intracellular localization over time. In one case, the revertant MP was not phosphorylated in vivo, but it was functional.

Localization of a potyvirus and the viral genome-linked protein in wild potato leaves at an early stage of systemic infection

The upper noninoculated 'sink' leaves of the wild potato species, Solanum commersonii, were studied for distribution of Potato virus A (PVA) at an early stage of systemic infection. Viral RNA was detected by in situ hybridization, and five viral proteins were localized using immunohistochemical staining in leaf sections. Initial systemic infection foci were found at the vicinity of major and minor veins. In these infection foci, the viral coat protein, cylindrical inclusion protein, and helper component-proteinase colocalized with viral RNA in parenchyma and mesophyll cells, but none of these were detected in companion cells (CC). In contrast, VPg, which is the N-proximal half of the NIa protein (separated from the C-terminal proteinase domain, NIapro, by an autocatalytic cleavage) and acts as a viral genome-linked protein, was detected in CC in the infection foci, but only at an early stage of virus unloading. Outside the infection foci, conspicuous signals for VPg were readily and exclusively detected in CC of many veins in all vein classes in the absence of signals for NIapro, other viral proteins, and viral RNA. Taken together, our data indicate that both major and minor veins may unload PVA in the sink leaves of potato. The data suggest that VPg is translocated from inoculated source leaves to the sink leaves, where it accumulates in CC at an early stage of systemic infection. These findings suggest that VPg may be a 'phloem protein' that specifically acts in CC in the sink leaves to facilitate virus unloading.

Detection of the potyviral genome-linked protein VPg in virions and its phosphorylation by host kinases

The multifunctional genome-linked protein (VPg) of Potato virus A (PVA; genus Potyvirus) was found to be phosphorylated as a part of the virus particle by a cellular kinase activity from tobacco. Immunoprecipitation, immunolabeling, and immunoelectron microscopy experiments showed that VPg is exposed at one end of the virion and it is accessible to protein-protein interactions. Substitution Ser185Leu at the C-proximal part of VPg reduces accumulation of PVA in inoculated leaves of the wild potato species Solanum commersonii and delays systemic infection, which is not observed in tobacco plants. Our data show that kinases of S. commersonii differentially recognize the VPg containing Ser or Leu at position 185, whereas both forms of VPg are similarly recognized by tobacco kinases. Taken together, our data imply that the virion-bound VPg may interact with host proteins and that phosphorylation of VPg may play a role in the VPg-mediated functions during the infection cycle of potyviruses.

The dialogue between viruses and hosts in compatible interactions

Understanding the biological principles behind virus-induced symptom expression in plants remains a longstanding challenge. By dissecting the compatible host-virus relationship temporally and genetically, we have begun to map out the relationships of its component parts. The picture that emerges is one in which host gene expression and physiology are under tight temporal control during infection.

A novel plant homeodomain protein interacts in a functionally relevant manner with a virus movement protein

Tomato bushy stunt virus and its cell-to-cell movement protein (MP; P22) provide valuable tools to study trafficking of macromolecules through plants. This study shows that wild-type P22 and selected movement-defective P22 amino acid substitution mutants were equivalent for biochemical features commonly associated with MPs (i.e. RNA binding, phosphorylation, and membrane partitioning). This generated the hypothesis that their movement defect was caused by improper interaction between the P22 mutants and one or more host factors. To test this, P22 was used as bait in a yeast (Saccharomyces cerevisiae) two-hybrid screen with a tobacco (Nicotiana tabacum) cDNA library, which identified a new plant homeodomain leucine-zipper protein that reproducibly interacted with P22 but not with various control proteins. These results were confirmed with an independent in vitro binding test. An mRNA for the host protein was detected in plants, and its accumulation was enhanced upon Tomato bushy stunt virus infection of two plant species. The significance of this interaction was further demonstrated by the failure of the homeodomain protein to interact efficiently with two of the well-defined movement-deficient P22 mutants in yeast and in vitro. This is the first report, to our knowledge, that a new plant homeodomain leucine-zipper protein interacts specifically and in a functionally relevant manner with a plant virus MP.

The systemic movement of a tobamovirus is inhibited by a cadmium-ion-induced glycine-rich protein

Systemic movement is central to plant viral infection. Exposure of tobacco plants to low levels of cadmium ions blocks the systemic spread of turnip vein-clearing tobamovirus (TVCV). We identified a tobacco glycine-rich protein, cdiGRP, specifically induced by low concentrations of cadmium and expressed in the cell walls of plant vascular tissues. Constitutive cdiGRP expression inhibited systemic transport of TVCV, whereas suppression of cdiGRP production allowed TVCV movement in the presence of cadmium. cdiGRP exerted its inhibitory effect on TVCV transport by enhancing callose deposits in the vasculature. So cdiGRP may function to control plant viral systemic movement.

Phosphorylation of the movement protein of cucumber mosaic virus in transgenic tobacco plants

The 3a protein of Cucumber mosaic virus is essential for the cell-to-cell movement of the viral RNA through plasmodesmata. We have introduced an epitope peptide before the stop codon of the 3a protein and cloned the tagged ORF into a binary vector for Agrobacterium-mediated transformation. The established transgenic tobacco lines produced the 3a protein, which was specifically detected with anti-3a and anti-epitope antisera. Metabolic labeling and subsequent immunoprecipitation revealed that [32P]-orthophosphate was incorporated into the 3a protein. The phosphoamino acid analysis indicated that the 3a protein contained phosphoserine but not phosphothreonine or phosphotyrosine. This is the first demonstration of the 3a protein phosphorylation in planta.

Functional analysis of a DNA-shuffled movement protein reveals that microtubules are dispensable for the cell-to-cell movement of tobacco mosaic virus

Microtubules interact strongly with the viral movement protein (MP) of Tobacco mosaic virus (TMV) and are thought to transport the viral genome between plant cells. We describe a functionally enhanced DNA-shuffled movement protein (MP(R3)) that remained bound to the vertices of the cortical endoplasmic reticulum, showing limited affinity for microtubules. A single amino acid change was shown to confer the MP(R3) phenotype. Disruption of the microtubule cytoskeleton in situ with pharmacological agents, or by silencing of the alpha-tubulin gene, had no significant effect on the spread of TMV vectors expressing wild-type MP (MP(WT)) and did not prevent the accumulation of MP(WT) in plasmodesmata. Thus, cell-to-cell trafficking of TMV can occur independently of microtubules. The MP(R3) phenotype was reproduced when infection sites expressing MP(WT) were treated with a specific proteasome inhibitor, indicating that the degradation of MP(R3) is impaired. We suggest that the improved viral transport functions of MP(R3) arise from evasion of a host degradation pathway.

Analysis of the complexity of protein kinases within the phloem sieve tube system. Characterization of Cucurbita maxima calmodulin-like domain protein kinase 1

In angiosperms, functional, mature sieve elements lack nuclei, vacuoles, ribosomes, and most of the endomembrane network. In this study, the complexity, number, and nature of protein kinases within the phloem sap of Cucurbita maxima were investigated to test the hypothesis that the enucleate sieve tube system utilizes a simplified signal transduction network. Supporting evidence was obtained in that only five putative protein kinases (three calcium-independent and two calcium-dependent protein kinases) were detected within the phloem sap extracted from stem tissues. Biochemical methods were used to purify one such calcium-dependent protein kinase. The gene for this C. maxima calmodulin-like domain protein kinase 1 (CmCPK1), was cloned using peptide microsequences. A combination of mass spectrometry, peptide fingerprinting, and amino-terminal sequencing established that, in the phloem sap, CmCPK1 exists as an amino-terminally cleaved protein. A second highly homologous isoform, CmCPK2, was identified, but although transcripts could be detected in the companion cells, peptide fingerprint analysis suggested that CmCPK2 does not enter the phloem sap. Potential substrates for CmCPK1, within the phloem sap, were also detected using an on-membrane phosphorylation assay. Entry of CmCPK1 into sieve elements via plasmodesmata and the potential roles played by these phloem protein kinases are discussed.

Phosphorylation of cucumber mosaic virus RNA polymerase 2a protein inhibits formation of replicase complex

The 2a (polymerase) protein of cucumber mosaic virus (CMV) was shown to be phosphorylated both in vivo and in vitro. In vitro assays using 2a protein mutants and tobacco protein kinases showed that the 2a protein has at least three phosphorylation sites, one of which is located within the N-terminal 126 amino acid region. This region is essential and sufficient for interaction with the CMV 1a protein. When phosphorylated in vitro, the 2a protein N-terminal region failed to interact with the 1a protein. Since the 1a-2a interaction is essential for the replication of CMV, this suggests that phosphorylation of the N-terminal region of the 2a protein negatively modulates the interaction in vivo, and may have a regulatory role acting directly in viral infection.

Towards genomic and proteomic studies of protein phosphorylation in plant-pathogen interactions

Phosphorylation is an effective method of post-translational protein modification but understanding its significance is hindered by its biological complexity. Many protein kinases and phosphatases have been identified that connect signal perception mechanisms to plant defence responses. Recent studies of mitogen-activated protein kinases, calcium-dependent protein kinases and other kinases and phosphatases have revealed some important mechanisms, but have also raised new questions. The regulation of any phosphorylation pathway is complex and dynamic. There are many protein kinases and phosphatases in the plant genome, which makes it hard to delineate the phosphorylation machinery fully. Genomics and proteomics have already identified new components and will continue to influence the study of phosphorylation profoundly in plant-pathogen interactions.

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.

Conundrum of the lack of defective RNAs (dRNAs) associated with tobamovirus Infections: dRNAs that can move are not replicated by the wild-type virus; dRNAs that are replicated by the wild-type virus do not move

Two classes of artificially constructed defective RNAs (dRNAs) of Tobacco mosaic virus (TMV) were examined in planta with helper viruses that expressed one (183 kDa) or both (126 and 183 kDa) of the replicase-associated proteins. The first class of artificially constructed dRNAs had the helicase and polymerase (POL) domains deleted; the second had an intact 126-kDa protein open reading frame (ORF). Despite extremely high levels of replication in protoplasts, the first class of dRNAs did not accumulate in plants. The dRNAs with an intact 126-kDa protein ORF were replicated at moderate levels in protoplasts and in planta when supported by a TMV mutant that expressed the 183-kDa protein but not the 126-kDa protein (183F). These dRNAs were not supported by helper viruses expressing both replicase-associated proteins. De novo dRNAs were generated in plants infected by 183F but not in plants infected with virus with the wild-type replicase. These novel dRNAs each contained a new stop codon near the location of the wild-type stop codon for the 126-kDa protein and had most of the POL domain deleted. The fact that only dRNAs that contained a complete 126-kDa protein ORF moved systemically suggests that expression of a functional 126-kDa protein or the presence of certain sequences and/or structures within this ORF is required for movement of dRNAs. At least two factors may contribute to the lack of naturally occurring dRNAs in association with wild-type TMV infections: an inability of TMV to support dRNAs that can move in plants and the inability of dRNAs that can be replicated by TMV to move in plants.

Phosphorylation down-regulates the RNA binding function of the coat protein of potato virus A

Plant viruses encode movement proteins (MPs) to facilitate transport of their genomes from infected into neighboring healthy cells through plasmodesmata. Growing evidence suggests that specific phosphorylation events can regulate MP functions. The coat protein (CP) of potato virus A (PVA; genus Potyvirus) is a multifunctional protein involved both in virion assembly and virus movement. Labeling of PVA-infected tobacco leaves with [(33)P]orthophosphate demonstrated that PVA CP is phosphorylated in vivo. Competition assays established that PVA CP and the well characterized 30-kDa MP of tobacco mosaic virus (genus Tobamovirus) are phosphorylated in vitro by the same Ser/Thr kinase activity from tobacco leaves. This activity exhibits a strong preference for Mn(2+) over Mg(2+), can be inhibited by micromolar concentrations of Zn(2+) and Cd(2+), and is not Ca(2+)-dependent. Tryptic phosphopeptide mapping revealed that PVA CP was phosphorylated by this protein kinase activity on multiple sites. In contrast, PVA CP was not phosphorylated when packaged into virions, suggesting that the phosphorylation sites are located within the RNA binding domain and not exposed on the surface of the virion. Furthermore, two independent experimental approaches demonstrated that the RNA binding function of PVA CP is strongly inhibited by phosphorylation. From these findings, we suggest that protein phosphorylation represents a possible mechanism regulating formation and/or stability of viral ribonucleoproteins in planta.

Cell-to-cell movement of potato virus X involves distinct functions of the coat protein

Complementation of movement-deficient potato virus X (PVX) coat protein (CP) mutants, namely PVX.CP-Xho lacking the 18 C-terminal amino acid residues and PVX.DeltaCP lacking the entire CP gene, was studied by transient co-expression with heterologous proteins. These data demonstrated that the potyvirus CPs and both the major and minor CPs of beet yellows closterovirus could complement cell-to-cell movement of PVX.CP-Xho but not PVX.DeltaCP. These data also indicated that the C-terminally truncated PVX CP lacked a movement function which could be provided in trans by the CPs of other filamentous viruses, whereas another movement determinant specified by some region outside the most C-terminal part of the PVX CP could not be complemented either by potyvirus or closterovirus CPs. Surprisingly, the CP of spherical cocksfoot mottle sobemovirus rescued all of the PVX CP movement functions, complementing the spread of PVX.CP-Xho and, to a lesser extent, PVX.DeltaCP. Both these mutants were also rescued by the tobacco mosaic virus (TMV) movement protein (MP). To shed light on the movement function of PVX CP, attempts were made to complement PVX.CP-Xho by a series of TMV MP mutants. An internal deletion abolished complementation, suggesting that the internal region of TMV MP, which includes a number of overlapping functional domains important for cell-to-cell transport, provides an activity complementing movement determinant(s) specified by the C-terminal region of PVX CP.

Viral transport and the cytoskeleton

In the past decade, studies into the way in which intracellular bacterial pathogens hijack and subvert their hosts have provided many important insights into regulation of the actin cytoskeleton and cell motility, in addition to increasing our understanding of the infection process. Viral pathogens, however, may ultimately unlock more cellular secrets as they are even more dependent on their hosts during their life cycle.

Phosphorylation of viral movement proteins--regulation of cell-to-cell trafficking

In plants, proteins and nucleoprotein complexes can traffic from cell to cell, via plasmodesmata. Studies based on viral movement proteins (MP) have revealed that such trafficking events are likely to be regulated at the level of protein phosphorylation. Plasmodesmal-associated protein kinases could play a central role in plant defense, in addition to regulating the translatability of endogenous MP-mRNA complexes that function at a supracellular level.