Protoplasts transiently expressing the 200K coding sequence of cowpea mosaic virus B-RNA support replication of M-RNA

Development and application of a full-length infectious clone of potato virus Y isolate belonging to SYR-I strain

Potato virus Y (PVY) causes huge damage to potato and tobacco production worldwide. The complete genome sequence of GZ, a PVY isolate (strain SYR-I) from Guizhou province, China, was cloned into the binary vector pCambia0390. Three introns were individually inserted into the P3 and CI ORFs to produce plasmid pCamPVY-GZ. The plasmid could infect plants of Nicotiana benthamiana, N. tabacum via agroinfiltration and plants of pepper and potato by mechanical inoculation. The green fluorescence protein gene of Aequoria victoriae was cloned into the encoding regions between nuclear inclusion body 'b' and coat protein genes in pCamPVY-GZ to produce pCamPVY-GZ-GFP, which could infect plants of N. benthamiana, N. tabacum, potato and tomato, and produce green fluorescence in the systemic leaves of inoculated plants. Mutations were introduced to pCamPVY-GZ to make the lysine (K) 391 and glutamic acid (E)410 of helper component-proteinase to arginine (R) and asparagic acid (E), respectively. Unlike wild type PVY-GZ, the mutant PVY-K391R/E410D could not induce veinal necrosis in N. tabacum plants. With an interval of 14 days, mutant PVY-K391R/E410D could protect N. tabacum plants from the infection of severe PVY strain. The results presented here provide a promising alternate for the prevention of diseases caused by PVY.

Morphological Characterization of the Self-Assembly of Virus Movement Proteins into Nanotubes in the Absence of Virus Particles

One infection mechanism of plant viruses is the generation of nanotubes by viral movement proteins, allowing cell-to-cell virus particle transport. Previously, it was assumed that viral nanotubes extend directly from the host-cell plasma membrane. In virus-infected plants, these nanotubes reach an extraordinary diameter:length ratio (≈100 nm:µm or mm range). Here, viral nanotubes are produced in a transient protoplast system; the coding sequence for alfalfa mosaic virus movement protein is translationally fused to green fluorescent protein. The maximum extension of viral nanotubes into the culture medium is achieved 24-48 h posttransfection, with lengths in the micro- and millimeter ranges. Scanning electron microscopy and transmission electron microscopy show that strong inhomogeneous viral nanotubes are formed compared to particle-filled systems. The nanotubes have similar length, but fluctuating wall thickness and diameter and are susceptible to entanglement and recombination. Indirect methods demonstrate that movement proteins assemble independently at the top of the nanotube. These viral nanotubes grow distinctly from previously known natural particle-filled systems and are a unique biological tubular nanomaterial that has the potential for micro- or nanoapplications as a mechanically stable structural component.

The role of plasmodesma-located proteins in tubule-guided virus transport is limited to the plasmodesmata

Intercellular spread of plant viruses involves passage of the viral genome or virion through a plasmodesma (PD). Some viruses severely modify the PD structure, as they assemble a virion carrying tubule composed of the viral movement protein (MP) inside the PD channel. Successful modulation of the host plant to allow infection requires an intimate interaction between viral proteins and both structural and regulatory host proteins. To date, however, very few host proteins are known to promote virus spread. Plasmodesmata-located proteins (PDLPs) localised in the PD have been shown to contribute to tubule formation in cauliflower mosaic virus and grapevine fanleaf virus infections. In this study, we have investigated the role of PDLPs in intercellular transport of another tubule-forming virus, cowpea mosaic virus. The MP of this virus was found to interact with PDLPs in the PD, as was shown for other tubule-forming viruses. Expression of PDLPs and MPs in protoplasts in the absence of a PD revealed that these proteins do not co-localise at the site of tubule initiation. Furthermore, we show that tubule assembly in protoplasts does not require an interaction with PDLPs at the base of the tubule, as has been observed in planta. These results suggest that a physical interaction between MPs and PDLPs is not required for assembly of the movement tubule and that the beneficial role of PDLPs in virus movement is confined to the structural context of the PD.

Proteomic analysis of the plasma membrane-movement tubule complex of cowpea mosaic virus

Cowpea mosaic virus forms tubules constructed from the movement protein (MP) in plasmodesmata (PD) to achieve cell-to-cell movement of its virions. Similar tubules, delineated by the plasma membrane (PM), are formed protruding from the surface of infected protoplasts. These PM-tubule complexes were isolated from protoplasts by immunoprecipitation and analysed for their protein content by tandem mass spectrometry to identify host proteins with affinity for the movement tubule. Seven host proteins were abundantly present in the PM-tubule complex, including molecular chaperonins and an AAA protein. Members of both protein families have been implicated in establishment of systemic infection. The potential role of these proteins in tubule-guided cell-cell transport is discussed.

Functional analysis by protein biochemistry

To date a number of cereal genomes are fully sequenced and more are near completion. The information within these genomes will be of most use to scientists when every gene has been functionally characterized leading to the complete annotation of these genomes. This chapter describes how functional characterization of plant proteins can be achieved via in vitro or in vivo methods. The first section of this chapter describes the use of Escherichia coli as a host for expression of plant genes, followed by purification and in vitro characterization of the resultant enzyme. The second section of this chapter details the methods involved in transient gene expression in Zea mays leaf protoplasts for in vivo functional characterization of protein localization.

Synergistic interactions between carotene ring hydroxylases drive lutein formation in plant carotenoid biosynthesis

Plant carotenoids play essential roles in photosynthesis, photoprotection, and as precursors to apocarotenoids. The plastid-localized carotenoid biosynthetic pathway is mediated by well-defined nucleus-encoded enzymes. However, there is a major gap in understanding the nature of protein interactions and pathway complexes needed to mediate carotenogenesis. In this study, we focused on carotene ring hydroxylation, which is performed by two structurally distinct classes of enzymes, the P450 CYP97A and CYP97C hydroxylases and the nonheme diiron HYD enzymes. The CYP97A and HYD enzymes both function in the hydroxylation of β-rings in carotenes, but we show that they are not functionally interchangeable. The formation of lutein, which involves hydroxylation of both β- and ε-rings, was shown to require the coexpression of CYP97A and CYP97C enzymes. These enzymes were also demonstrated to interact in vivo and in vitro, as determined using bimolecular fluorescence complementation and a pull-down assay, respectively. We discuss the role of specific hydroxylase enzyme interactions in promoting pathway flux and preventing the formation of pathway dead ends. These findings will facilitate efforts to manipulate carotenoid content and composition for improving plant adaptation to climate change and/or for enhancing nutritionally important carotenoids in food crops.

Plastid localization of the key carotenoid enzyme phytoene synthase is altered by isozyme, allelic variation, and activity

Plant carotenoids have unique physiological roles related to specific plastid suborganellar locations. Carotenoid metabolic engineering could enhance plant adaptation to climate change and improve food security and nutritional value. However, lack of fundamental knowledge on carotenoid pathway localization limits targeted engineering. Phytoene synthase (PSY), a major rate-controlling carotenoid enzyme, is represented by multiple isozymes residing at unknown plastid sites. In maize (Zea mays), the three isozymes were transiently expressed and found either in plastoglobuli or in stroma and thylakoid membranes. PSY1, with one to two residue modifications of naturally occurring functional variants, exhibited altered localization, associated with distorted plastid shape and formation of a fibril phenotype. Mutating the active site of the enzyme reversed this phenotype. Discovery of differential PSY locations, linked with activity and isozyme type, advances the engineering potential for modifying carotenoid biosynthesis.

Lycopene cyclase paralog CruP protects against reactive oxygen species in oxygenic photosynthetic organisms

In photosynthetic organisms, carotenoids serve essential roles in photosynthesis and photoprotection. A previous report designated CruP as a secondary lycopene cyclase involved in carotenoid biosynthesis [Maresca J, et al. (2007) Proc Natl Acad Sci USA 104:11784-11789]. However, we found that cruP KO or cruP overexpression plants do not exhibit correspondingly reduced or increased production of cyclized carotenoids, which would be expected if CruP was a lycopene cyclase. Instead, we show that CruP aids in preventing accumulation of reactive oxygen species (ROS), thereby reducing accumulation of β-carotene-5,6-epoxide, a ROS-catalyzed autoxidation product, and inhibiting accumulation of anthocyanins, which are known chemical indicators of ROS. Plants with a nonfunctional cruP accumulate substantially higher levels of ROS and β-carotene-5,6-epoxide in green tissues. Plants overexpressing cruP show reduced levels of ROS, β-carotene-5,6-epoxide, and anthocyanins. The observed up-regulation of cruP transcripts under photoinhibitory and lipid peroxidation-inducing conditions, such as high light stress, cold stress, anoxia, and low levels of CO(2), fits with a role for CruP in mitigating the effects of ROS. Phylogenetic distribution of CruP in prokaryotes showed that the gene is only present in cyanobacteria that live in habitats characterized by large variation in temperature and inorganic carbon availability. Therefore, CruP represents a unique target for developing resilient plants and algae needed to supply food and biofuels in the face of global climate change.

Role of N-glycosylation sites and CXC motifs in trafficking of medicago truncatula Nod factor perception protein to plasma membrane

The lysin motif receptor-like kinase, NFP (Nod factor perception), is a key protein in the legume Medicago truncatula for the perception of lipochitooligosaccharidic Nod factors, which are secreted bacterial signals essential for establishing the nitrogen-fixing legume-rhizobia symbiosis. Predicted structural and genetic analyses strongly suggest that NFP is at least part of a Nod factor receptor, but few data are available about this protein. Characterization of a variant encoded by the mutant allele nfp-2 revealed the sensitivity of this protein to the endoplasmic reticulum quality control mechanisms, affecting its trafficking to the plasma membrane. Further analysis revealed that the extensive N-glycosylation of the protein is not essential for biological activity. In the NFP extracellular region, two CXC motifs and two other Cys residues were found to be involved in disulfide bridges, and these are necessary for correct folding and localization of the protein. Analysis of the intracellular region revealed its importance for biological activity but suggests that it does not rely on kinase activity. This work shows that NFP trafficking to the plasma membrane is highly sensitive to regulation in the endoplasmic reticulum and has identified structural features of the protein, particularly disulfide bridges involving CXC motifs in the extracellular region that are required for its biological function.

In vivo hexamerization and characterization of the Arabidopsis AAA ATPase CDC48A complex using forster resonance energy transfer-fluorescence lifetime imaging microscopy and fluorescence correlation spectroscopy

The Arabidopsis (Arabidopsis thaliana) AAA ATPase CDC48A was fused to cerulean fluorescent protein and yellow fluorescent protein. AAA ATPases like CDC48 are only active in hexameric form. Förster resonance energy transfer-based fluorescence lifetime imaging microscopy using CDC48A-cerulean fluorescent protein and CDC48A-yellow fluorescent protein showed interaction between two adjacent protomers, demonstrating homo-oligomerization occurs in living plant cells. Interaction between CDC48A and the SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE1 (SERK1) transmembrane receptor occurs in very restricted domains at the plasma membrane. In these domains the predominant form of the fluorescently tagged CDC48A protein is a hexamer, suggesting that SERK1 is associated with the active form of CDC48A in vivo. SERK1 trans-phosphorylates CDC48A on Ser-41. Förster resonance energy transfer-fluorescence lifetime imaging microscopy was used to show that in vivo the C-terminal domains of CDC48A stay in close proximity. Employing fluorescence correlation spectroscopy, it was shown that CDC48A hexamers are part of larger complexes.

Plant G protein heterotrimers require dual lipidation motifs of Gα and Gγ and do not dissociate upon activation

In plants one bona fide Gα subunit has been identified, as well as a single Gβ and two Gγ subunits. To study the roles of lipidation motifs in the regulation of subcellular location and heterotrimer formation in living plant cells, GFP-tagged versions of the Arabidopsis thaliana heterotrimeric G protein subunits were constructed. Mutational analysis showed that the Arabidopsis Gα subunit, GPα1, contains two lipidation motifs that were essential for plasma membrane localization. The Arabidopsis Gβ subunit, AGβ1, and the Gγ subunit, AGG1, were dependent upon each other for tethering to the plasma membrane. The second Gγ subunit, AGG2, did not require AGβ1 for localization to the plasma membrane. Like AGG1, AGG2 contains two putative lipidation motifs, both of which were necessary for membrane localization. Interaction between the subunits was studied using fluorescence resonance energy transfer (FRET) imaging by means of fluorescence lifetime imaging microscopy (FLIM). The results suggest that AGβ1 and AGG1 or AGβ1 and AGG2 can form heterodimers independent of lipidation. In addition, FLIM-FRET revealed the existence of GPα1-AGβ1-AGG1 heterotrimers at the plasma membrane. Importantly, rendering GPα1 constitutively active did not cause a FRET decrease in the heterotrimer, suggesting no dissociation upon GPα1 activation.

Visualization of PtdIns3P dynamics in living plant cells

To investigate PtdIns3P localization and function in plants, a fluorescent PtdIns3P-specific biosensor (YFP-2xFYVE) was created. On lipid dot blots it bound specifically and with high affinity to PtdIns3P. Transient expression in cowpea protoplasts labelled vacuolar membranes and highly motile structures undergoing fusion and fission. Stable expression in tobacco BY-2 cells labelled similar motile structures, but labelled vacuolar membranes hardly at all. YFP-2xFYVE fluorescence strongly co-localized with the pre-vacuolar marker AtRABF2b, partially co-localized with the endosomal tracer FM4-64, but showed no overlap with the Golgi marker STtmd-CFP. Treatment of cells with wortmannin, a PI3 kinase inhibitor, caused the YFP-2xFYVE fluorescence to redistribute into the cytosol and nucleus within 15 min. BY-2 cells expressing YFP-2xFYVE contained twice as much PtdIns3P as YFP-transformed cells, but this had no effect on cell-growth or stress-induced phospholipid signalling responses. Upon treatment with wortmannin, PtdIns3P levels were reduced by approximately 40% within 15 min in both cell lines. Stable expression of YFP-2xFYVE in Arabidopsis plants labelled different subcellular structures in root compared with shoot tissues. In addition labelling the motile structures common to all cells, YFP-2xFYVE strongly labelled the vacuolar membrane in leaf epidermal and guard cells, suggesting that cell differentiation alters the distribution of PtdIns3P. In dividing BY-2 cells, YFP-2xFYVE-labelled vesicles surrounded the newly formed cell plate, suggesting a role for PtdIns3P in cytokinesis. Together, these data show that YFP-2xFYVE may be used as a biosensor to specifically visualize PtdIns3P in living plant cells.

Studies on the origin and structure of tubules made by the movement protein of Cowpea mosaic virus

Cowpea mosaic virus (CPMV) moves from cell to cell by transporting virus particles via tubules formed through plasmodesmata by the movement protein (MP). On the surface of protoplasts, a fusion between the MP and the green fluorescent protein forms similar tubules and peripheral punctate spots. Here it was shown by time-lapse microscopy that tubules can grow out from a subset of these peripheral punctate spots, which are dynamic structures that seem anchored to the plasma membrane. Fluorescence resonance energy transfer experiments showed that MP subunits interacted within the tubule, where they were virtually immobile, confirming that tubules consist of a highly organized MP multimer. Fluorescence recovery after photobleaching experiments with protoplasts, transiently expressing fluorescent plasma membrane-associated proteins of different sizes, indicated that tubules made by CPMV MP do not interact directly with the surrounding plasma membrane. These experiments indicated an indirect interaction between the tubule and the surrounding plasma membrane, possibly via a host plasma membrane protein.

Use of the fluorescent timer DsRED-E5 as reporter to monitor dynamics of gene activity in plants

Fluorescent proteins, such as green fluorescent protein and red fluorescent protein (DsRED), have become frequently used reporters in plant biology. However, their potential to monitor dynamic gene regulation is limited by their high stability. The recently made DsRED-E5 variant overcame this problem. DsRED-E5 changes its emission spectrum over time from green to red in a concentration independent manner. Therefore, the green to red fluorescence ratio indicates the age of the protein and can be used as a fluorescent timer to monitor dynamics of gene expression. Here, we analyzed the potential of DsRED-E5 as reporter in plant cells. We showed that in cowpea (Vigna unguiculata) mesophyll protoplasts, DsRED-E5 changes its fluorescence in a way similar to animal cells. Moreover, the timing of this shift is suitable to study developmental processes in plants. To test whether DsRed-E5 can be used to monitor gene regulation in plant organs, we placed DsRED-E5 under the control of promoters that are either up- or down-regulated (MtACT4 and LeEXT1 promoters) or constitutively expressed (MtACT2 promoter) during root hair development in Medicago truncatula. Analysis of the fluorescence ratios clearly provided more accurate insight into the timing of promoter activity.

Cowpea mosaic virus RNA-1 acts as an amplicon whose effects can be counteracted by a RNA-2-encoded suppressor of silencing

Lines of Nicotiana benthamiana transgenic for full-length copies of both Cowpea mosaic virus (CPMV) genomic RNAs, either singly or together, have been produced. Plants transgenic for both RNAs developed symptoms characteristic of a CPMV infection. When plants transgenic for RNA-1 were agro-inoculated with RNA-2, no infection developed and the plants were also resistant to challenge with CPMV. By contrast, plants transgenic for RNA-2 became infected when agro-inoculated with RNA-1 and were fully susceptible to CPMV infection. The resistance of RNA-1 transgenic plants was shown to be related to the ability of RNA-1 to self-replicate and act as an amplicon. The ability of transgenically expressed RNA-2 to counteract the amplicon effect suggested that it encodes a suppressor of posttranscriptional gene silencing (PTGS). By examining the ability of portions of RNA-2 to reverse PTGS in N. benthamiana, we have identified the small (S) coat protein as the CPMV RNA-2-encoded suppressor of PTGS.

Probing plasma membrane microdomains in cowpea protoplasts using lipidated GFP-fusion proteins and multimode FRET microscopy

Summary Multimode fluorescence resonance energy transfer (FRET) microscopy was applied to study the plasma membrane organization using different lipidated green fluorescent protein (GFP)-fusion proteins co-expressed in cowpea protoplasts. Cyan fluorescent protein (CFP) was fused to the hyper variable region of a small maize GTPase (ROP7) and yellow fluorescent protein (YFP) was fused to the N-myristoylation motif of the calcium-dependent protein kinase 1 (LeCPK1) of tomato. Upon co-expressing in cowpea protoplasts a perfect co-localization at the plasma membrane of the constructs was observed. Acceptor-photobleaching FRET microscopy indicated a FRET efficiency of 58% in protoplasts co-expressing CFP-Zm7hvr and myrLeCPK1-YFP, whereas no FRET was apparent in protoplasts co-expressing CFP-Zm7hvr and YFP. Fluorescence spectral imaging microscopy (FSPIM) revealed, upon excitation at 435 nm, strong YFP emission in the fluorescence spectra of the protoplasts expressing CFP-Zm7hvr and myrLeCPK1-YFP. Also, fluorescence lifetime imaging microscopy (FLIM) analysis indicated FRET because the CFP fluorescence lifetime of CFP-Zm7hvr was reduced in the presence of myrLeCPK1-YFP. A FRET fluorescence recovery after photobleaching (FRAP) analysis on a partially acceptor-bleached protoplast co-expressing CFP-Zm7hvr and myrLeCPK1-YFP revealed slow requenching of the CFP fluorescence in the acceptor-bleached area upon diffusion of unbleached acceptors into this area. The slow exchange of myrLeCPK1-YFP in the complex with CFP-Zm7hvr reflects a relatively high stability of the complex. Together, the FRET data suggest the existence of plasma membrane lipid microdomains in cowpea protoplasts.

The movement protein of cowpea mosaic virus binds GTP and single-stranded nucleic acid in vitro

The movement protein (MP) of Cowpea mosaic virus forms tubules in plasmodesmata to enable the transport of mature virions. Here it is shown that the MP is capable of specifically binding riboguanosine triphosphate and that mutational analysis suggests that GTP binding plays a role in the targeted transport of the MP. Furthermore, the MP is capable of binding both single-stranded RNA and single-stranded DNA in a non-sequence-specific manner, and the GTP- and RNA-binding sites do not overlap.

Identification of distinct steps during tubule formation by the movement protein of Cowpea mosaic virus

The movement protein (MP) of Cowpea mosaic virus (CPMV) forms tubules through plasmodesmata in infected plants thus enabling virus particles to move from cell to cell. Localization studies of mutant MPs fused to GFP in protoplasts and plants identified several functional domains within the MP that are involved in distinct steps during tubule formation. Coinoculation experiments and the observation that one of the C-terminal deletion mutants accumulated uniformly in the plasma membrane suggest that dimeric or multimeric MP is first targeted to the plasma membrane. At the plasma membrane the MP quickly accumulates in peripheral punctuate spots, from which tubule formation is initiated. One of the mutant MPs formed tubules containing virus particles on protoplasts, but could not support cell-to-cell movement in plants. The observations that this mutant MP accumulated to a higher level in the cell than wt MP and did not accumulate in the cell wall opposite infected cells suggest that breakdown or disassembly of tubules in neighbouring, uninfected cells is required for cell-to-cell movement.

The subcellular localization of plant protein phosphatase 5 isoforms is determined by alternative splicing

Protein serine/threonine phosphatase 5 (PP5) plays an important role in signal transduction in animal cells, but in plants, knowledge about PP5 is scarce. Here, we describe the isolation of a full-length cDNA encoding tomato (Lycopersicon esculentum) PP5 (LePP5) and its expression in Escherichia coli. Biochemical characterization showed that recombinant LePP5 has a low intrinsic protein phosphatase activity. This activity was increased 6- to 10-fold by either removal of the N-terminal tetratricopeptide repeat domain or by addition of fatty acids, indicating that biochemical features specific for PP5 homologs from other species are conserved in tomato. The single-copy LePP5 gene was cloned and shown to encode two mRNA species that arise by alternative pre-mRNA splicing. Similarly, Arabidopsis was found to express two PP5 transcripts, suggesting that alternative splicing of PP5 pre-mRNA is not specific for tomato. Alternative splicing results in a larger transcript containing an additional exon encoding two putative transmembrane domains and, hence, in a larger PP5 isoform. Subcellular fractionation studies on tomato protein lysates indicated that the majority of the 55-kD LePP5 isoform is soluble, whereas the 62-kD isoform is an integral membrane protein. Production of yellow fluorescent protein-PP5 chimeras in plant cells indicated that the 55-kD isoform is localized in both the nucleus and the cytoplasm, whereas the 62-kD isoform is targeted to the endoplasmic reticulum, including the nuclear envelope. Our findings show that alternative splicing generates two LePP5 isoforms with a different subcellular localization.

Alteration of microtubule dynamic instability during preprophase band formation revealed by yellow fluorescent protein-CLIP170 microtubule plus-end labeling

At the onset of mitosis, plant cells form a microtubular preprophase band that defines the plane of cell division, but the mechanism of its formation remains a mystery. Here, we describe the use of mammalian yellow fluorescent protein-tagged CLIP170 to visualize the dynamic plus ends of plant microtubules in transfected cowpea protoplasts and in stably transformed and dividing tobacco Bright Yellow 2 cells. Using plus-end labeling, we observed dynamic instability in different microtubular conformations in live plant cells. The interphase plant microtubules grow at 5 micro m/min, shrink at 20 micro m/min, and display catastrophe and rescue frequencies of 0.02 and 0.08 events/s, respectively, exhibiting faster turnover than their mammalian counterparts. Strikingly, during preprophase band formation, the growth rate and catastrophe frequency of plant microtubules double, whereas the shrinkage rate and rescue frequency remain unchanged, making microtubules shorter and more dynamic. Using these novel insights and four-dimensional time-lapse imaging data, we propose a model that can explain the mechanism by which changes in microtubule dynamic instability drive the dramatic rearrangements of microtubules during preprophase band and spindle formation in plant cells.

Cowpea mosaic virus: effects on host cell processes

SUMMARY Taxonomy: Cowpea mosaic virus (CPMV) is the type member of the Comoviridae and bears a strong resemblance to animal picornaviruses, both in gene organization and in the amino acid sequence of replication proteins. Little systematic work has been done to compare isolates of the virus from different parts of the world. Physical properties: Purified preparations of virus contain three centrifugal components; empty protein shells without RNA (T) and two nucleoprotein components (M and B), containing 24% and 34% RNA, respectively. The icosahedral particles have with a diameter of 28 nm, consist of 60 copies of two coat proteins, and are heat stable. Hosts: CPMV causes one of the most commonly reported virus diseases of cowpea (Vigna unguiculata), in which it produces chlorotic spots with diffuse borders in inoculated primary leaves. Trifoliate leaves develop a bright yellow or light green mosaic of increasing severity in younger leaves. The host range is rather limited, and few hosts are known outside the Leguminosae. The virus is transmitted by various beetles with biting mouthparts. Reported in Africa, the Philippines and Iran. Is apparently absent from North and South America. Useful website: http://mmtsb.scripps.edu/viper/1cpmv.html (structure); http://image.fs.uidaho.edu/vide/descr254.htm (general information).

Subcellular location of the helper component-proteinase of Cowpea aphid-borne mosaic virus

The helper component-proteinase (HC-Pro) of Cowpea aphid-borne mosaic virus (CABMV) was expressed in Escherichia coli and used to obtain HC-Pro antiserum that was used as an analytical tool for HC-Pro studies. The antiserum was used in immunofluorescence assays to study the subcellular location of HC-Pro expressed with other viral proteins in cowpea protoplasts in a natural CABMV infection, or in protoplasts transfected with a transient expression construct expressing HC-Pro separately from other viral proteins under the control of the 35S promoter. In both cases the protein showed a diffuse cytoplasmic location. Similar localisation patterns were shown in live protoplasts when the transient expression system was used to express HC-Pro as a fusion with the green fluorescent protein as a reporter. In an alternative expression system, the HC-Pro coding region was subcloned in-frame between the movement protein and large coat protein genes of RNA2 of Cowpea mosaic virus (CPMV). Upon transfection of protoplasts with this construct, HC-Pro was expressed as part of the RNA2 encoded polyprotein from which it was fully processed. In this case, the protein localised in broad cytoplasmic patches reminiscent of the typical CPMV induced cytopathic structures in which CPMV replication occurs, suggesting an interaction of HC-Pro with CPMV proteins or host factors in these structures. Finally, recombinant CPMV expressing HC-Pro showed a strongly enhanced virulence on cowpea and Nicotiana benthamiana consistent with the role of HC-Pro as a pathogenicity determinant, a phenomenon now known to be linked to its role as a suppressor of host defense responses based on post-transcriptional gene silencing.

Cowpea mosaic virus 32- and 60-kilodalton replication proteins target and change the morphology of endoplasmic reticulum membranes

Cowpea mosaic virus (CPMV) replicates in close association with small membranous vesicles that are formed by rearrangements of intracellular membranes. To determine which of the viral proteins are responsible for the rearrangements of membranes and the attachment of the replication complex, we have expressed individual CPMV proteins encoded by RNA1 in cowpea protoplasts by transient expression and in Nicotiana benthamiana plants by using the tobacco rattle virus (TRV) expression vector. The 32-kDa protein (32K) and 60K, when expressed individually, accumulate in only low amounts but are found associated with membranes mainly derived from the endoplasmic reticulum (ER). 24K and 110K are freely soluble and accumulate to high levels. With the TRV vector, expression of 32K and 60K results in rearrangement of ER membranes. Besides, expression of 32K and 60K results in necrosis of the inoculated N. benthamiana leaves, suggesting that 32K and 60K are cytotoxic proteins. On the other hand, during CPMV infection 32K and 60K accumulate to high levels without causing necrosis.

Phloem loading and unloading of Cowpea mosaic virus in Vigna unguiculata

Within their host plants, viruses spread from the initially infected cell through plasmodesmata to neighbouring cells (cell-to-cell movement), until reaching the phloem for rapid invasion of the younger plant parts (long-distance or vascular movement). Cowpea mosaic virus (CPMV) moves from cell-to-cell as mature virions via tubules constructed of the viral movement protein (MP). The mechanism of vascular movement, however, is not well understood. The characteristics of vascular movement of CPMV in Vigna unguiculata (cowpea) were examined using GFP-expressing recombinant viruses. It was established that CPMV was loaded into both major and minor veins of the inoculated primary leaf, but was unloaded exclusively from major veins, preferably class III, in cowpea trifoliate leaves. Phloem loading and unloading of CPMV was scrutinized at the cellular level in sections of loading and unloading veins. At both loading and unloading sites it was shown that the virus established infection in all vascular cell types with the exception of companion cells (CC) and sieve elements (SE). Furthermore tubular structures, indicative of virion movement, were never found in plasmodesmata connecting phloem parenchyma cells and CC or CC and SE. In cowpea, SE are symplasmically connected only to the CC and these results therefore suggest that CPMV employs a mechanism for phloem loading and unloading that is different from the typical tubule-guided cell-to-cell movement in other cell types.

Coalescence of the sites of cowpea mosaic virus RNA replication into a cytopathic structure

Cowpea mosaic virus (CPMV) replication induces an extensive proliferation of endoplasmic reticulum (ER) membranes, leading to the formation of small membranous vesicles where viral RNA replication takes place. Using fluorescent in situ hybridization, we found that early in the infection of cowpea protoplasts, CPMV plus-strand RNA accumulates at numerous distinct subcellular sites distributed randomly throughout the cytoplasm which rapidly coalesce into a large body located in the center of the cell, often near the nucleus. The combined use of immunostaining and a green fluorescent protein ER marker revealed that during the course of an infection, CPMV RNA colocalizes with the 110-kDa viral polymerase and other replication proteins and is always found in close association with proliferated ER membranes, indicating that these sites correspond to the membranous site of viral replication. Experiments with the cytoskeleton inhibitors oryzalin and latrunculin B point to a role of actin and not tubulin in establishing the large central structure. The induction of ER membrane proliferations in CPMV-infected protoplasts did not coincide with increased levels of BiP mRNA, indicating that the unfolded-protein response is not involved in this process.

The cytoskeleton and the secretory pathway are not involved in targeting the cowpea mosaic virus movement protein to the cell periphery

The movement protein (MP) of cowpea mosaic virus (CPMV) forms tubules on infected protoplasts and through plasmodesmata in infected plants. In protoplasts the MP fused to GFP (MP-GFP) was shown to localize in peripheral punctate structures and in long tubular structures extending from the protoplast surface. Using cytoskeletal assembly inhibitors (latrunculin B and oryzalin) and an inhibitor of the secretory pathway (brefeldin A), targeting of the MP to the peripheral punctate structures was demonstrated not to be dependent on an intact cytoskeleton or functional secretion pathway. Furthermore it was shown that a disrupted cytoskeleton had no effect on tubule formation but that the addition of brefeldin A severely inhibited tubule formation. The results presented in this paper suggest a role for a plasma membrane host factor in tubule formation of plant viral MPs.

Characterization of plant proteins that interact with cowpea mosaic virus '60K' protein in the yeast two-hybrid system

Cowpea mosaic virus (CPMV) replication occurs in close association with small membranous vesicles in the host cell. The CPMV RNA1-encoded 60 kDa nucleotide-binding protein ('60K') plays a role in the formation of these vesicles. In this study, five cellular proteins were identified that interacted with different domains of 60K using a yeast two-hybrid search of an Arabidopsis cDNA library. Two of these host proteins (termed VAP27-1 and VAP27-2), with high homology to the VAP33 family of SNARE-like proteins from animals, interacted specifically with the C-terminal domain of 60K and upon transient expression colocalized with 60K in CPMV-infected cowpea protoplasts. eEF1-beta, picked up using the central domain of 60K, was also found to colocalize with 60K. The possible role of these host proteins in the viral replicative cycle is discussed.

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.

Mutational analysis of the genome-linked protein of cowpea mosaic virus

In this study we have performed a mutational analysis of the cowpea mosaic comovirus (CPMV) genome-linked protein VPg to discern the structural requirements necessary for proper functioning of VPg. Either changing the serine residue linking VPg to RNA at a tyrosine or a threonine or changing the position of the serine from the N-terminal end to position 2 or 3 abolished virus infectivity. Some of the mutations affected the cleavage between the VPg and the 58K ATP-binding protein in vitro, which might have contributed to the lethal phenotype. RNA replication of some of the mutants designed to replace VPg with the related cowpea severe mosaic comovirus was completely abolished, whereas replication of others was not affected or only mildly affected, showing that amino acids that are not conserved between the comoviruses can be critical for the function of VPg. The replicative proteins of one of the mutants failed to accumulate in typical cytopathic structures and this might reflect the involvement of VPg in protein-protein interactions with the other replicative proteins.

Alfalfa mosaic virus replicase proteins P1 and P2 interact and colocalize at the vacuolar membrane

Replication of Alfalfa mosaic virus (AMV) RNAs depends on the virus-encoded proteins P1 and P2. P1 contains methyltransferase- and helicase-like domains, and P2 contains a polymerase-like domain. Coimmunoprecipitation experiments revealed an interaction between in vitro translated-P1 and P2 and showed that these proteins are present together in fractions with RNA-dependent RNA polymerase activity. A deletion analysis in the yeast two-hybrid system showed that in P1 the C-terminal sequence of 509 amino acids with the helicase domain was necessary for the interaction. In P2, the sequence of the N-terminal 241 aa was required for the interaction. In infected protoplasts, P1 and P2 colocalized at a membrane structure that was identified as the tonoplast (i.e., the membrane that surrounds the vacuoles) by using a tonoplast intrinsic protein as a marker in immunofluorescence studies. While P1 was exclusively localized on the tonoplast, P2 was found both at the tonoplast and at other locations in the cell. As Brome mosaic virus replication complexes have been found to be associated with the endoplasmic reticulum (M. A. Restrepo-Hartwig and P. Ahlquist, J. Virol. 70:8908-8916, 1996), viruses in the family Bromoviridae apparently select different cellular membranes for the assembly of their replication complexes.

Cowpea mosaic virus infection induces a massive proliferation of endoplasmic reticulum but not Golgi membranes and is dependent on de novo membrane synthesis

Replication of cowpea mosaic virus (CPMV) is associated with small membranous vesicles that are induced upon infection. The effect of CPMV replication on the morphology and distribution of the endomembrane system in living plant cells was studied by expressing green fluorescent protein (GFP) targeted to the endoplasmic reticulum (ER) and the Golgi membranes. CPMV infection was found to induce an extensive proliferation of the ER, whereas the distribution and morphology of the Golgi stacks remained unaffected. Immunolocalization experiments using fluorescence confocal microscopy showed that the proliferated ER membranes were closely associated with the electron-dense structures that contain the replicative proteins encoded by RNA1. Replication of CPMV was strongly inhibited by cerulenin, an inhibitor of de novo lipid synthesis, at concentrations where the replication of the two unrelated viruses alfalfa mosaic virus and tobacco mosaic virus was largely unaffected. These results suggest that proliferating ER membranes produce the membranous vesicles formed during CPMV infection and that this process requires continuous lipid biosynthesis.

Engineering cowpea mosaic virus RNA-2 into a vector to express heterologous proteins in plants

A series of new cowpea mosaic virus (CPMV) RNA-2-based expression vectors were designed. The jellyfish green fluorescent protein (GFP) was introduced between the movement protein (MP) and the large (L) coat protein or downstream of the small (S) coat protein. Release of the GFP inserted between the MP and L proteins was achieved by creating artificial processing sites each side of the insert, either by duplicating the MP-L cleavage site or by introducing a sequence encoding the foot-and-mouth disease virus (FMDV) 2A catalytic peptide. Eight amino acids derived from the C-terminus of the MP and 14-19 amino acids from the N-terminus of the L coat protein were necessary for efficient processing of the artificial Gln/Met sites. Insertion of the FMDV 2A sequence at the C-terminus of the GFP resulted in a genetically stable construct, which produced particles containing about 10 GFP-2A-L fusion proteins. Immunocapture experiments indicated that some of the GFP is present on the virion surface. Direct fusion of GFP to the C-terminus of the S coat protein resulted in a virus which was barely viable. However, when the sequence of GFP was linked to the C-terminus by an active FMDV 2A sequence, a highly infectious construct was obtained.

Mutational analysis of the cowpea mosaic virus movement protein

Cowpea mosaic virus moves from cell-to-cell in a virion form through tubular structures that are assembled in modified plasmodesmata. Similar tubular structures are formed on the surface of protoplasts inoculated with cowpea mosaic virus. The RNA 2-encoded movement protein (MP) is responsible for the induction and formation of these structures. To define functional domains of the MP, an alanine-substitution mutagenesis was performed on eight positions in the MP, including two conserved sequence motifs, the LPL and D motifs. Results show that these two conserved motifs as well as the central region of the MP are essential for cell-to-cell movement. Several viruses carrying mutations in the N- or C-terminal parts of their MP retained infectivity on cowpea plants. Coexpression studies revealed that mutant MPs did not interfere with the activity of wild-type MP and could not mutually complement their defects.

Studies on hybrid comoviruses reveal the importance of three-dimensional structure for processing of the viral coat proteins and show that the specificity of cleavage is greater in trans than in cis

A series of cowpea mosaic virus (CPMV)-based hybrid comoviral RNA-2 molecules have been constructed. In these, the region encoding both the large (L) and small (S) viral coat proteins was replaced by the equivalent region from bean pod mottle virus (BPMV). The hybrid RNA-2 molecules were able to replicate in cowpea protoplasts in the presence of CPMV RNA-1. Though processing of the hybrid polyproteins by the CPMV-specific 24K proteinase at the site between the 58/48K and L proteins could readily be achieved, no processing at the site between the L and S coat proteins could be obtained even when the sequence of amino acids between the two coat proteins was made CPMV-like. As a result, none of the hybrids was able to form functional virus particles, and they could not infect cowpea plants. Comparison with the processing of the L-S site in cis in reticulocyte lysates demonstrated that the requirements for processing are more stringent in trans than in cis. The results suggest that the L-S cleavage site is defined by more than just a linear sequence of amino acids and probably involves interactions between the L-S loop and the beta barrels of the viral coat proteins.

Faba bean necrotic yellows virus (genus Nanovirus) requires a helper factor for its aphid transmission

Purified faba bean necrotic yellows virus (FBNYV; genus Nanovirus) alone is not transmissible by its aphid vector, Acyrthosiphon pisum, regardless of whether it is acquired from artificial diets or directly microinjected into the aphid's hemocoel. The purified virus contains all of the genetic information required for its infection cycle as it readily replicated in cowpea protoplasts and systemically infected Vicia faba seedlings that were biolistically inoculated using gold particles coated with intact virions or viral DNA. The bombarded plants not only developed the typical disease syndrome, thus indicating that FBNYV is the sole causal agent of the disease, but also served as a source from which the virus was readily acquired and transmitted by A. pisum. The defect of the purified virus in aphid transmissibility suggests that FBNYV requires a helper factor (HF) for its vector transmission that is either nonfunctional or absent in purified virus suspensions. The requirement for an HF was confirmed in complementation experiments using two distinct isolates of the virus. These experiments revealed that aphids transmitted the purified virus isolate from artificial diets only when they had fed previously on plants infected with the other FBNYV isolate. Also, microinjected FBNYV, which persisted to the same extent in A. pisum as naturally acquired virus, was transmissible when aphids had acquired the HF from infected plants. This suggests that one of the functions of the HF in the transmission process is to facilitate virus transport across the hemocoel-salivary gland interface.

Studies on the movement of cowpea mosaic virus using the jellyfish green fluorescent protein

The jellyfish green fluorescent protein (GFP) coding sequence was used to replace the coat protein (CP) genes in a full-length cDNA clone of CPMV RNA-2. Transcripts of this construct were replicated in the presence of RNA-1 in cowpea protoplasts, and GFP expression could be readily detected by fluorescent microscopy. It was not possible to infect cowpea plants with these transcripts, but combined with a mutant RNA-2, in which the 48-kDa movement protein (MP) gene has been deleted infection did occur. With this tripartite virus (CPMV-TRI) green fluorescent spots were visible under UV light on the inoculated leaf after 3 days and a few days later on the higher leaves. These results show that the polyproteins encoded by RNA-2 do not possess an essential function in the virus infection cycle and that there is, contrary to what we have found so far for the proteins encoded by RNA-1, no need for a tight regulation of the amounts of MP and CPs produced in a cell. Subsequently, the GFP gene was introduced between the MP and CP genes of RNA-2 utilizing artificial proteolytic processing sites for the viral proteinase. This CPMV-GFP was highly infectious on cowpea plants and the green fluorescent spots that developed on the inoculated leaves were larger and brighter than those produced by CPMV-TRI described above. When cowpea plants were inoculated with CPMV RNA-1 and RNA-2 mutants containing the GFP gene but lacking the CP or MP genes, only single fluorescent epidermal cells were detected between 2 and 6 days postinoculation. This experiment clearly shows that both the capsid proteins and the MP are absolutely required for cell-to-cell movement.

Distinct functional domains in the cowpea mosaic virus movement protein

Cell-to-cell movement of cowpea mosaic virus particles in plants takes place with the help of tubules that penetrate presumably modified plasmodesmata. These tubules, which are built up by the virus-encoded 48-kDa movement protein (MP), are also formed on single protoplast cells. To determine whether the MP contains different functional domains, the effect of mutations in its coding region was studied. Mutations between amino acids 1 and 313 led to complete abolishment of the tubule-forming capacity, while a deletion in the C-terminal region resulted in tubules that could not take up virus particles. From these observations, it is concluded that the MP contains at least two distinct domains, one that is involved in tubule formation and that spans amino acids 1 and 313 and a second that is probably involved in the incorporation of virus particles in the tubule and that is located in the C terminus between amino acids 314 and 331.

Modification of phytohormone response by a peptide encoded by ENOD40 of legumes and a nonlegume

The gene ENOD40 is expressed during early stages of legume nodule development. A homolog was isolated from tobacco, which, as does ENOD40 from legumes, encodes an oligopeptide of about 10 amino acids. In tobacco protoplasts, these peptides change the response to auxin at concentrations as low as 10(-12) to 10(-16)M. The peptides encoded by ENOD40 appear to act as plant growth regulators.

Replication and translation of cowpea mosaic virus RNAs are tightly linked

The genome of cowpea mosaic virus (CPMV) is divided among two positive strand RNA molecules. B-RNA is able to replicate independently from M-RNA in cowpea protoplasts. Replication of mutant B-transcripts could not be supported by co-inoculated wild-type B-RNA, indicating that B-RNA cannot be efficiently replicated in trans. Hence replication of a B-RNA molecule is tightly linked to its translation and/or at least one of the replicative proteins functions in cis only. Remarkably also for efficient replication of M-RNA one of its translation products was found to be required in cis. This 58K protein possibly helps in directing the B-RNA-encoded replication complex to the M-RNA. In order to identify the viral polymerase the CPMV B-RNA-specific proteins have been produced individually in cowpea protoplasts using CaMV 35S promoter based expression vectors. Only protoplasts transfected with a vector containing the 200K coding sequence were able to support replication of co-transfected M-RNA. Despite this, CPMV-specific RNA polymerase activity could not be detected in extracts of these protoplasts using a poly(A)/oligo(U) assay. These results indicate that, in contrast to the poliovirus polymerase, the CPMV polymerase is not able to accept oligo(U) as a primer and in addition support the concept that translation and replication are linked.