AFM study of potato virus X disassembly induced by movement protein

Recently we have reported that a selective binding of potato virus X (PVX)-coded movement protein (termed TGBp1 MP) to one end of a polar coat protein (CP) helix converted viral RNA into a translatable form and induced a linear destabilization of the whole helical particle. Here, the native PVX virions, RNase-treated (PVX(RNA-DEG)) helical particles lacking intact RNA and their complexes with TGBp1 (TGBp1-PVX and TGBp1-PVX(RNA-DEG)), were examined by atomic force microscopy (AFM). When complexes of the TGBp1 MP with PVX were examined by means of AFM in liquid, no structural reorganization of PVX particles was observed. By contrast, the products of TGBp1-dependent PVX degradation termed "beads-on-string" were formed under conditions of AFM in air. The AFM images of PVX(RNA-DEG) were indistinguishable from images of native PVX particles; however, the TGBp1-dependent disassembly of the CP-helix was triggered when the TGBp1-PVX(RNA-DEG) complexes were examined by AFM, regardless of the conditions used (in air or in liquid). Our data supported the idea that binding of TGBp1 to one end of the PVX CP-helix induced linear destabilization of the whole helical particle, which may lead to its disassembly under conditions of AFM.     

Modified model of the structure of the potato virus X coat protein

A modified model was proposed for the tertiary structure of the coat protein (CP) molecules in potato virus X (PVX) virions, similar to the original model of 2001 describing the structure of CP of potato virus A, a member of another group of filamentous viruses. According to the new model, CP comprises two main structural domains, namely, a bundle of α-helices, located near the long axis of the virion, and the socalled RNP fold (or abCd fold), located in the vicinity of its surface. The model made it possible to suggest a possible mechanism of the PVX virion structural rearrangement (remodeling) resulting from translational activation of virions by the TGB1 movement protein according to Atabekov and colleagues.     

Regulation of RNA translation in potato virus X RNA-coat protein complexes: The key role of the N-terminal segment of the protein

The efficiency of in vitro translation of the potato virus X (PVX) RNA was studied for viral ribonucleoprotein complexes (vRNP) assembled from the genomic RNA and the viral coat protein (CP). In vRNP particles the 5′-proximal RNA segments were encapsidated into the CP, which formed helical headlike structures differing in length. Translation of the PVX RNA was completely suppressed upon incubation with PVX CP and was activated within vRNPs assembled in vitro with two CP forms, differing in the modification of the N-terminal peptide containing the main phosphorylation site(s) for Thr/Ser protein kinases. It was shown that CP phosphorylation activates RNA translation within vRNPs and that the removal of the N-terminal peptide of CP suppresses activation, but CP still acts as a translational suppressor. This fact made it possible to suppose that the replacement of Ser/Thr by amino acid residues that are not subject to phosphorylation in the N-terminal peptide of CP of the mutant PVX (PVX-ST) completely inhibits RNA translation within vRNP. However, experiments disproved this assumption: PVX-ST RNA was efficiently translated within native virions, RNA of the wild-type (wt) PVX was efficiently translated in heterogeneous vRNP (wtRNA + PVX-ST CP), and the opposite result (repression of translation) was obtained for another heterogeneous vRNP (PVX-ST RNA + wtCP). Therefore, the N-terminal CP peptide located on the surface of the PVX virion or vRNP particles plays a key role in the activation of viral RNA translation.     

Linear remodeling of helical virus by movement protein binding

Previously we have shown that encapsidated potato virus X (PVX) RNA was nontranslatable in vitro, but could be converted into a translatable form by binding of the PVX-coded movement protein (termed TGBp1) to one end of a polar helical PVX virion. We reported that binding of TGBp1 to coat protein (CP) subunits located at one extremity of the helical particles induced a linear destabilization of the CP helix, which was transmitted along the whole particle. Two model structures were used: (i) native PVX and (ii) artificial polar helical PVX-like particles lacking intact RNA (PVX(RNA-DEG)). Binding of TGBp1 to the end of either of these particles led to their destabilization, but no disassembly of the CP helix occurred. Influence of additional factors was required to trigger rapid disassembly of TGBp1-PVX and TGBp1-PVX(RNA-DEG) complexes. Thus: (i) no disassembly was observed unless TGBp1-PVX complex was translated. A novel phenomenon of TGBp1-dependent, ribosome-triggered disassembly of PVX was described: initiation of translation and few translocation steps were needed to trigger rapid (and presumably cooperative) disassembly of TGBp1-PVX into protein subunits and RNA. Importantly, the whole of the RNA molecule (including its 3'-terminal region) was released. The TGBp1-induced linear destabilization of CP helix was reversible, suggesting that PVX in TGBp1-PVX complex was metastable; (ii) entire disassembly of the TGBp1-PVX(RNA-DEG) complex (but not of the TGBp1-free PVX(RNA-DEG) particles) into 2.8S subunits was triggered under influence of a centrifugal field. To our knowledge, transmission of the linear destabilization along the polar helical protein array induced by a foreign protein binding to the end of the helix represents a novel phenomenon. It is tempting to suggest that binding of TGBp1 to the end of the PVX CP helix induced conformational changes in terminal CP subunits that can be linearly transferred along the whole helical particle, i.e. that intersubunit conformational changes may be transferred along the CP helix.     

Comparative structural stability of subunits of the potato virus X coat protein in solution and in virus particles

Several optical methods and differential scanning calorimetry were used to study the structure and stability of free coat protein (CP) molecules and CP molecules in the virion of the potato virus X (PVX), a filamentous plant virus. All criteria suggest that PVX CP (hereinafter, CP) subunits in solution at room temperature display a certain preserved tertiary structure; however, this structure is very unstable and already denatures at 35°C. Very low concentrations of sodium dodecylsulfate or cetyltrimethylammonium bromide also disrupt the CP tertiary structure, three-five molecules of these detergents per one protein molecule being sufficient. However, the secondary structure of CP molecules does not change under the same conditions. Once included into the virion, CP subunits become considerably more stable towards increased temperature and detergents. This combination of a highly labile tertiary structure and a fairly stable secondary structure of free CP can be a structural basis for the recently discovered ability of PVX CP to assume two distinct functional states within the virion.     

NbALY916 is involved in potato virus X P25-triggered cell death in Nicotiana benthamiana

Systemic necrosis often occurs during viral infection of plants and is thought mainly to be the result of long-term stress induced by viral infection. Potato virus X (PVX) encodes the P25 pathogenicity factor that triggers a necrotic reaction during PVX-potato virus Ysynergistic coinfection. In this study, we discovered that NbALY916, a multifunctional nuclear protein, could interact with P25. When NbALY916 expression was reduced by tobacco rattle virus (TRV)-based virus-induced gene silencing, the accumulation of P25 was increased, which would be expected to cause more severe necrosis. However, silencing of NbALY916 reduced the extent of cell death caused by P25. Furthermore, we found that overexpression of NbALY916 increased the accumulation of H₂O₂ and triggered more extensive cell death when coexpressed with P25, even though accumulation of P25 was itself reduced by the increased expression of NbALY916. Furthermore, transient expression of P25 specifically induced the expression of NbALY916 mRNA, but not the mRNAs of three other ALYs in Nicotiana benthamiana. In addition, we showed that silencing of NbALY916 or transient overexpression of NbALY916 affected the infection of PVX in N. benthamiana. Our results reveal that NbALY916 has an antiviral role that, in the case of PVX, operates by inducing the accumulation of H₂O₂ and mediating the degradation of P25.     

Plasmodesmata transport of GFP alone or fused to potato virus X TGBp1 is diffusion driven

Summary. Plasmodesmata (Pd) provide a pathway for exchanging various macromolecules between neighboring plant cells. Researchers routinely characterize the mobility of the green-fluorescent protein (GFP) and GFP fusions through Pd by calculating the proportion of sites in bombarded leaves which show fluorescence in multiple cell clusters (% movement). Here, the Arrhenius equation was used to describe the temperature dependence of GFP and GFP-TGBp1 (potato virus X triple gene block protein1) movement, using % movement values, and to calculate the activation energy for protein transport. The resulting low activation energy indicates GFP and GFP-TGBp1 movement are diffusion driven. Furthermore, GFP movement is inversely proportional to the leaf surface area of expanding leaves. The increase in leaf area results mainly from cell expansion during the sink–source transition. The increasing cell size results in lower Pd density, which decreases the probability that a GFP attains an open Pd by diffusion. The decline in GFP movement as leaf area expands indicates that, in addition to GFP diffusion through Pd, attaining an open Pd by undirected diffusion might be limiting for Pd transport. In summary, this report provides a new quantitative method for studying Pd conductivity. Correspondence: Jeanmarie Verchot Lubicz, Department of Entomology and Plant Pathology, 127 Noble Research Center, Oklahoma State University, Stillwater, OK 74078, U.S.A.     

Identification of Arabidopsis proteins that interact with the cauliflower mosaic virus (CaMV) movement protein

Gene I of cauliflower mosaic virus (CaMV) encodes a protein that is required for virus movement. The CaMV movement protein (MP) was used in a yeast 2-hybrid system to screen an Arabidopsis cDNA library for cDNAs encoding MP-interacting (MPI) proteins. Three different clones were found encoding proteins (MPI1, -2 and -7) that interact with the N-terminal third of the CaMV MP. The interaction in the 2-hybrid system between MPI7 and CaMV MP mutants correlated with the infectivity of the mutants. A non-infectious MP mutant, ER2A, with two amino acid changes in the N-terminal third of the MP failed to interact with MPI7, while an infectious second-site mutant, that differed from ER2A by only a single amino acid change, interacted in the 2-hybrid system. MPI7 is encoded by a member of a large, but diverse gene family in Arabidopsis. MPI7 is related in sequence, size and hydropathy profile to mammalian proteins (such as rat PRA1) described as a rab acceptor. The gene encoding MPI7 is expressed widely is Arabidopsis plants, and in transgenic plants the MPI7:GFP fusion protein is localized in the cytoplasm, concentrated in punctate spots. In protoplasts transfected with CFP:MP and MPI7:YFP, CFP:MP colocalized to some of the sites where MPI7:YFP is expressed. At these sites, fluorescence resonance energy transfer (FRET) between fluorophores was observed indicating an interaction in planta between the CaMV MP and MPI7.     

Complete sequencing of potato virus X new strain genome and construction of viral vector for production of target proteins in plants

The complete nucleotide sequence of the genome of a new potato virus X (PVX) strain Tula isolated by us has been determined. Based on comparison of the PVX Tula nucleotide sequence with the sequences of 12 other PVX strains, this strain was assigned to the European cluster of PVX strains. Phylogenetic analysis revealed the same phylogeny for both full genome sequences and nucleotide sequences of polymerase and coat protein genes, suggesting that the PVX evolution did not involve recombination between different strains. The full-size cDNA copy of the PVX Tula genome was cloned and the accumulation of the viral coat protein in infected Nicotiana benthamiana was shown to be about twofold higher than for the PVX strain UK3. Based on the PVX Tula genome, a new vector which contained the target gene instead of the removed triple transport gene block and the coat protein gene has been constructed for expression of target proteins in plants. The productivity of the new vector was about 1.5-2-fold higher than the productivity of the vector of the same structure based on the standard PVX strain genome. The new viral vector can be used for superproduction of recombinant proteins in plants.     

Constitutive gain-of-function mutants in a nucleotide binding site-leucine rich repeat protein encoded at the Rx locus of potato

Rx in potato encodes a protein with a nucleotide binding site (NBS) and leucine-rich repeats (LRR) that confers resistance against Potato virus X. The NBS and LRR domains in Rx are present in many disease resistance proteins in plants and in regulators of apoptosis in animals. To investigate structure-function relationships of NBS-LRR proteins we exploited the potential of Rx to mediate a cell death response. With wild-type Rx cell death is elicited only in the presence of the viral coat protein. However, following random mutagenesis of Rx, we identified mutants in which cell death is activated in the absence of viral coat protein. Out of 2500 Rx clones tested there were seven constitutive gain-of-function mutants carrying eight independent mutations. The mutations encoded changes in the LRR or in conserved RNBS-D and MHD motifs of the NBS. Based on these findings we propose that there are inhibitory domains in the NBS and LRR. The constitutive gain-of-function phenotypes would be due to deletion or modification of these inhibitory domains. However activation of Rx is not simply release of negative regulation by the LRR and adjacent sequence because deleted forms of Rx that lack constitutive gain of function mutations are not active unless the protein is overexpressed.     

日本脑炎病毒E蛋白在酵母双杂交系统中的表达及其对报告基因激活作用的检测

用日本脑炎病毒(JEV)E蛋白基因片段构建酵母双杂交诱饵载体,并检测其表达产物对酵母细胞有无毒性作用及对报告基因有无激活作用。用RT—PCR从JEV感染的鼠脑中扩增出JEV E蛋白基因片段,克隆入pUCl9质粒,经测序正确后,再亚克隆入酵母双杂交诱饵载体pGBKT7中。将重组质粒导入酵母菌AHl09,检测其表达产物在酵母细胞中对报告基因有无激活作用。成功获得JEV E蛋白基因片段,表达的E蛋白对酵母菌AHl09无毒性,对报告基因亦无激活作用。为利用酵母双杂交GAL4系统3进行JEV细胞受体蛋白的研究奠定了基础。     

乙型肝炎病毒X蛋白对细胞凋亡的影响存在剂量依赖性

乙型肝炎病毒(HBV)X蛋白(HBx)与HBV相关肝细胞肝癌(HCC)的发生和发展密切相关.深入研究HBx在HCC形成中的作用将为探索HBV致癌机制提供重要依据.HBx是多功能蛋白,其对细胞凋亡的影响至今仍存在分歧.许多研究表明,HBx既有促进细胞凋亡又有抑制细胞凋亡的功能,但原因不清楚.本研究中,将表达HBx的质粒短...     

Developmental changes in plasmodesmata in transgenic tobacco expressing the movement protein of tobacco mosaic virus

Summary Cell-to-cell communication in plants occurs through plasmodesmata, cytoplasmic channels that traverse the cell wall between neighboring cells. Plasmodesmata are also exploited by many viruses as an avenue for spread of viral progeny. In the case of tobacco mosaic virus (TMV), a virally-encoded movement protein (MP) enables the virus to move through plasmodesmata during infection. We have used thin section electron microscopy and immunocytochemistry to examine the structure of plasmodesmata in transgenic tobacco plants expressing the TMV MP. We observed a change in structure of the plasmodesmata as the leaves age, both in control and MP expressing [MP(+)] plants. In addition, the plasmodesmata of older cells of MP(+) plants accumulate a fibrous material in the central cavity. The presence of the fibers is correlated with the ability to label plasmodesmata with anti-MP antibodies. The developmental stage of leaf tissue at which this material is observed is the stage at which an increase in the size exclusion limit of the plasmodesmata can be measured in MP(+) plants. Using cell fractionation and aqueous phase partitioning studies, we identified the plasma membrane and cell wall as the compartments with which the MP stably associates. The nature of the interaction between the MP and the plasma membrane was studied using sodium carbonate and Triton X-100 washes. The MP behaves as an integral membrane protein. Identifying the mechanism by which the MP associates with plasma membrane and plasmodesmata will lead to a better understanding of how the MP alters the function of the plasmodesmata.Abbreviations MP movement protein - TMV tobacco mosaic virus     

Molecular interactions between a plant virus movement protein and RNA: force spectroscopy investigation

RNA-protein interactions are fundamental for different aspects of molecular biology such as gene expression, assembly of biomolecular complexes or macromolecular transport. The 3a movement protein (MP) of a plant virus, Cucumber mosaic virus (CMV), forms ribonucleoprotein (RNP) complexes with viral RNA, capable of trafficking from cell-to-cell throughout the infected plant only in the presence of the CMV capsid protein (CP). However, deletion of the C-terminal 33 amino acid residues of the CMV MP (in the mutant designated 3aDeltaC33 MP) resulted in CP-independent cell-to-cell movement. The biological differences in the behaviour of CMV wild type (wt) 3a MP and 3aDeltaC33 MP could have been a consequence of differences in the RNA-binding properties of the two MPs detected previously using biochemical assays on ensembles of molecules. To investigate the physical mechanisms of MP-RNA interactions at a single molecule level, we applied atomic force microscopy to measure for the first time unbinding forces between these individual binding partners. Minimal unbinding forces determined for individual interaction of the CMV RNA molecule with the CMV wt or truncated MPs were estimated to be approximately 45 pN and approximately 90 pN, respectively, suggesting that the distinct differences in the strength of MP-RNA interactions for the wt MP and truncated MP are attributable to the molecular binding mechanism. We also demonstrated that molecules of both CMV 3a MP and 3aDeltaC33 MP were capable of self-interaction with minimal unbinding forces of approximately 50 pN and approximately 70 pN, respectively, providing a physical basis for the cooperative mechanism of the RNA binding. The significance of intermolecular force measurements for understanding the structural and functional aspects of viral RNP formation and trafficking is discussed.     

Cell-to-cell movement of the 25K protein of potato virus X is regulated by three other viral proteins

The 25K, 12K, and 8K proteins and coat protein (CP) of Potato virus X (PVX) are required for virus cell-to-cell movement. In this study, experiments were conducted to determine whether the PVX 25K protein moves cell to cell and to explore potential interactions between the PVX 25K, 12K, and 8K proteins and CP. The PVX 25K gene was fused to the green fluorescent protein (GFP) gene and inserted into plasmids adjacent to the cauliflower mosaic virus 35S promoter. These plasmids were introduced by biolistic bombardment to transgenic tobacco expressing the PVX 12K, 8K, and CP genes. The GFP:25K fused proteins moved cell to cell on nontransgenic tobacco and tobacco expressing either the 12K or 8K proteins. However, the GFP:25K proteins did not move on transgenic tobacco expressing the combined 12K/8K genes or the CP gene. Thus, movement of the PVX 25K protein through plasmodesmata may be regulated by interactions with other PVX proteins.