Formation of surface tubules and fluorescent foci in Arabidopsis thaliana protoplasts expressing a fusion between the green fluorescent protein and the cauliflower mosaic virus movement protein

The 28 Ser Amino Acid of Cucumber Mosaic Virus Movement Protein Has a Role in Symptom Formation and Plasmodesmata Localization

Cucumber mosaic virus (CMV, Cucumovirus, Bromoviridae) is an economically significant virus infecting important horticultural and field crops. Current knowledge regarding the specific functions of its movement protein (MP) is still incomplete. In the present study, potential post-translational modification sites of its MP were assayed with mutant viruses: MP/S28A, MP/S28D, MP/S120A and MP/S120D. Ser28 was identified as an important factor in viral pathogenicity on Nicotiana tabacum cv. Xanthi, Cucumis sativus and Chenopodium murale. The subcellular localization of GFP-tagged movement proteins was determined with confocal laser-scanning microscopy. The wild type movement protein fused to green fluorescent protein (GFP) (MP-eGFP) greatly colocalized with callose at plasmodesmata, while MP/S28A-eGFP and MP/S28D-eGFP were detected as punctate spots along the cell membrane without callose colocalization. These results underline the importance of phosphorylatable amino acids in symptom formation and provide data regarding the essential factors for plasmodesmata localization of CMV MP.

Key checkpoints in the movement of plant viruses through the host

Plant viruses cannot exploit any of the membrane fusion-based routes of entry described for animal viruses. In addition, one of the distinctive structures of plant cells, the cell wall, acts as the first barrier against the invasion of pathogens. To overcome the rigidity of the cell wall, plant viruses normally take advantage of the way of life of different biological vectors. Alternatively, the physical damage caused by environmental stresses can facilitate virus entry. Once inside the cell and taking advantage of the characteristic symplastic continuity of plant cells, viruses need to remodel and/or modify the restricted pore size of the plasmodesmata (channels that connect plant cells). In a successful interaction for the virus, it can reach the vascular tissue to systematically invade the plant. The connections between the different cell types in this path are not designed to allow the passage of molecules with the complexity of viruses. During this process, viruses face different cell barriers that must be overcome to reach the distal parts of the plant. In this review, we highlight the current knowledge about how plant RNA viruses enter plant cells, move between them to reach vascular cells and overcome the different physical and cellular barriers that the phloem imposes. Finally, we update the current research on cellular organelles as key regulator checkpoints in the long-distance movement of plant viruses.

Citrus Psorosis Virus Movement Protein Contains an Aspartic Protease Required for Autocleavage and the Formation of Tubule-Like Structures at Plasmodesmata

Plant virus cell-to-cell movement is an essential step in viral infections. This process is facilitated by specific virus-encoded movement proteins (MPs), which manipulate the cell wall channels between neighboring cells known as plasmodesmata (PD). Citrus psorosis virus (CPsV) infection in sweet orange involves the formation of tubule-like structures within PD, suggesting that CPsV belongs to "tubule-forming" viruses that encode MPs able to assemble a hollow tubule extending between cells to allow virus movement. Consistent with this hypothesis, we show that the MP of CPsV (MPCPsV) indeed forms tubule-like structures at PD upon transient expression in Nicotiana benthamiana leaves. Tubule formation by MPCPsV depends on its cleavage capacity, mediated by a specific aspartic protease motif present in its primary sequence. A single amino acid mutation in this motif abolishes MPCPsV cleavage, alters the subcellular localization of the protein, and negatively affects its activity in facilitating virus movement. The amino-terminal 34-kDa cleavage product (34KCPsV), but not the 20-kDa fragment (20KCPsV), supports virus movement. Moreover, similar to tubule-forming MPs of other viruses, MPCPsV (and also the 34KCPsV cleavage product) can homooligomerize, interact with PD-located protein 1 (PDLP1), and assemble tubule-like structures at PD by a mechanism dependent on the secretory pathway. 20KCPsV retains the protease activity and is able to cleave a cleavage-deficient MPCPsV in trans Altogether, these results demonstrate that CPsV movement depends on the autolytic cleavage of MPCPsV by an aspartic protease activity, which removes the 20KCPsV protease and thereby releases the 34KCPsV protein for PDLP1-dependent tubule formation at PD.IMPORTANCE Infection by citrus psorosis virus (CPsV) involves a self-cleaving aspartic protease activity within the viral movement protein (MP), which results in the production of two peptides, termed 34KCPsV and 20KCPsV, that carry the MP and viral protease activities, respectively. The underlying protease motif within the MP is also found in the MPs of other members of the Aspiviridae family, suggesting that protease-mediated protein processing represents a conserved mechanism of protein expression in this virus family. The results also demonstrate that CPsV and potentially other ophioviruses move by a tubule-guided mechanism. Although several viruses from different genera were shown to use this mechanism for cell-to-cell movement, our results also demonstrate that this mechanism is controlled by posttranslational protein cleavage. Moreover, given that tubule formation and virus movement could be inhibited by a mutation in the protease motif, targeting the protease activity for inactivation could represent an important approach for ophiovirus control.

The roles of membranes and associated cytoskeleton in plant virus replication and cell-to-cell movement

The infection of plants by viruses depends on cellular mechanisms that support the replication of the viral genomes, and the cell-to-cell and systemic movement of the virus via plasmodesmata (PD) and the connected phloem. While the propagation of some viruses requires the conventional endoplasmic reticulum (ER)-Golgi pathway, others replicate and spread between cells in association with the ER and are independent of this pathway. Using selected viruses as examples, this review re-examines the involvement of membranes and the cytoskeleton during virus infection and proposes potential roles of class VIII myosins and membrane-tethering proteins in controlling viral functions at specific ER subdomains, such as cortical microtubule-associated ER sites, ER-plasma membrane contact sites, and PD.

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.

Dual targeting of a virus movement protein to ER and plasma membrane subdomains is essential for plasmodesmata localization

Plant virus movement proteins (MPs) localize to plasmodesmata (PD) to facilitate virus cell-to-cell movement. Numerous studies have suggested that MPs use a pathway either through the ER or through the plasma membrane (PM). Furthermore, recent studies reported that ER-PM contact sites and PM microdomains, which are subdomains found in the ER and PM, are involved in virus cell-to-cell movement. However, functional relationship of these subdomains in MP traffic to PD has not been described previously. We demonstrate here the intracellular trafficking of fig mosaic virus MP (MPFMV) using live cell imaging, focusing on its ER-directing signal peptide (SPFMV). Transiently expressed MPFMV was distributed predominantly in PD and patchy microdomains of the PM. Investigation of ER translocation efficiency revealed that SPFMV has quite low efficiency compared with SPs of well-characterized plant proteins, calreticulin and CLAVATA3. An MPFMV mutant lacking SPFMV localized exclusively to the PM microdomains, whereas SP chimeras, in which the SP of MPFMV was replaced by an SP of calreticulin or CLAVATA3, localized exclusively to the nodes of the ER, which was labeled with Arabidopsis synaptotagmin 1, a major component of ER-PM contact sites. From these results, we speculated that the low translocation efficiency of SPFMV contributes to the generation of ER-translocated and the microdomain-localized populations, both of which are necessary for PD localization. Consistent with this hypothesis, SP-deficient MPFMV became localized to PD when co-expressed with an SP chimera. Here we propose a new model for the intracellular trafficking of a viral MP. A substantial portion of MPFMV that fails to be translocated is transferred to the microdomains, whereas the remainder of MPFMV that is successfully translocated into the ER subsequently localizes to ER-PM contact sites and plays an important role in the entry of the microdomain-localized MPFMV into PD.

The conundrum of a unique protein encoded by citrus tristeza virus that is dispensable for infection of most hosts yet shows characteristics of a viral movement protein

Citrus tristeza virus (CTV), one of the most economically important viruses, produces a unique protein, p33, which is encoded only in the genomes of isolates of CTV. Recently, we demonstrated that membrane association of the p33 protein confers virus ability to extend its host range. In this work we show that p33 shares characteristics of viral movement proteins. Upon expression in a host cell, the protein localizes to plasmodesmata and displays the ability to form extracellular tubules. Furthermore, p33 appears to traffic via the cellular secretory pathway and the actin network to plasmodesmata locations and is likely being recycled through the endocytic pathway. Finally, our study reveals that p33 colocalizes with a putative movement protein of CTV, the p6 protein. These results suggest a potential role of p33 as a noncanonical viral movement protein, which mediates virus translocation in the specific hosts.

Plasmodesmata: channels for viruses on the move

The symplastic communication network established by plasmodesmata (PD) and connected phloem provides an essential pathway for spatiotemporal intercellular signaling in plant development but is also exploited by viruses for moving their genomes between cells in order to infect plants systemically. Virus movement depends on virus-encoded movement proteins (MPs) that target PD and therefore represent important keys to the cellular mechanisms underlying the intercellular trafficking of viruses and other macromolecules. Viruses and their MPs have evolved different mechanisms for intracellular transport and interaction with PD. Some viruses move from cell to cell by interacting with cellular mechanisms that control the size exclusion limit of PD whereas other viruses alter the PD architecture through assembly of specialized transport structures within the channel. Some viruses move between cells in the form of assembled virus particles whereas other viruses may interact with nucleic acid transport mechanisms to move their genomes in a non-encapsidated form. Moreover, whereas several viruses rely on the secretory pathway to target PD, other viruses interact with the cortical endoplasmic reticulum and associated cytoskeleton to spread infection. This chapter provides an introduction into viruses and their role in studying the diverse cellular mechanisms involved in intercellular PD-mediated macromolecular trafficking.

Cell-to-cell movement of viruses via plasmodesmata

Plant viruses utilize plasmodesmata (PD), unique membrane-lined cytoplasmic nanobridges in plants, to spread infection cell-to-cell and long-distance. Such invasion involves a range of regulatory mechanisms to target and modify PD. Exciting discoveries in this field suggest that these mechanisms are executed by the interaction between plant cellular components and viral movement proteins (MPs) or other virus-encoded factors. Striking working analogies exist among endogenous non-cell-autonomous proteins and viral MPs, in which not only do they all use PD to traffic, but also they exploit same regulatory components to exert their functions. Thus, this review discusses on the viral strategies to move via PD and the PD-regulatory mechanisms involved in viral pathogenesis.

The Arabidopsis synaptotagmin SYTA regulates the cell-to-cell movement of diverse plant viruses

Synaptotagmins are a large gene family in animals that have been extensively characterized due to their role as calcium sensors to regulate synaptic vesicle exocytosis and endocytosis in neurons, and dense core vesicle exocytosis for hormone secretion from neuroendocrine cells. Thought to be exclusive to animals, synaptotagmins have recently been characterized in Arabidopsis thaliana, in which they comprise a five gene family. Using infectivity and leaf-based functional assays, we have shown that Arabidopsis SYTA regulates endocytosis and marks an endosomal vesicle recycling pathway to regulate movement protein-mediated trafficking of the Begomovirus Cabbage leaf curl virus (CaLCuV) and the Tobamovirus Tobacco mosaic virus (TMV) through plasmodesmata (Lewis and Lazarowitz, 2010). To determine whether SYTA has a central role in regulating the cell-to-cell trafficking of a wider range of diverse plant viruses, we extended our studies here to examine the role of SYTA in the cell-to-cell movement of additional plant viruses that employ different modes of movement, namely the Potyvirus Turnip mosaic virus (TuMV), the Caulimovirus Cauliflower mosaic virus (CaMV) and the Tobamovirus Turnip vein clearing virus (TVCV), which in contrast to TMV does efficiently infect Arabidopsis. We found that both TuMV and TVCV systemic infection, and the cell-to-cell trafficking of the their movement proteins, were delayed in the Arabidopsis Col-0 syta-1 knockdown mutant. In contrast, CaMV systemic infection was not inhibited in syta-1. Our studies show that SYTA is a key regulator of plant virus intercellular movement, being necessary for the ability of diverse cell-to-cell movement proteins encoded by Begomoviruses (CaLCuV MP), Tobamoviruses (TVCV and TMV 30K protein) and Potyviruses (TuMV P3N-PIPO) to alter PD and thereby mediate virus cell-to-cell spread.

Interference of Brefeldin A in viral movement protein tubules assembly

Plant virus genomes cross the barrier of the host cell wall and move to neighboring cells either in the form of nucleoprotein complex or encapsidated into virions. Virus transport is facilitated by virus-encoded movement proteins (MP), which are different from one another in number, size, sequence, and in the strategy used to overcome the size exclusion limit of plasmodesmata (PD). (1) A group of them forms tubules inside the lumen of highly modified PDs upon removal of the desmotubule. To date the molecular mechanism(s) and the host factors involved in the assembly of MP tubules as well as the mechanistic aspects of virus particle transport throughout them remain substantially unknown. In a recent study, we showed that Cauliflower mosaic virus (CaMV) MP traffics in the endocytic pathway with the help of 3 tyrosine-sorting signals, which are not required to target MP to the plasma membrane but are essential for tubule formation. (2) This evidence unravels a previously unknown connection between the plant endosomal system and tubule-mediated virus movement that is here supported by demonstration of hindrance of tubule assembly upon Brefeldin A (BFA) treatment. We discuss the implications of our data on the mechanisms of viral transport through tubules and draw parallels with plant mechanisms of polarized growth.

Cell-cell nanotubes: Tunneling through several types of synapses

Nanotube can be generally seen as a nanoscale cylindrical structure. Membrane (or tunneling) nanotube (TNT) is a cytoplasmic tunnel between two cells. Such direct cell-cell channel is used for a physical transport of biochemical cargo, whereas nanotubular networks between cells may be a novel principle of communicative and integrative biology. Recently, TNTs and their networks were discovered in plant cells and then they were reported also in animal cells. Just the reverse, a notion of plant synapse has been also proposed only recently, long after the corresponding notion of neuronal synapse in animals. However, both TNTs and synapses seem to be closely related and evolutionary conserved structures through different types of cells. Accordingly, this mini-review aims to demonstrate that TNTs may represent one of the deep functional similarities between neuronal, immune, viral and plant synapses.

The ORF3-encoded proteins of vitiviruses GVA and GVB induce tubule-like and punctate structures during virus infection and localize to the plasmodesmata

The genomic RNA of vitiviruses contains 5 open reading frames (ORF). ORF3 encodes a protein to which the function of a movement protein (MP) was assigned, based on sequence homology with other viral proteins. The aim of the research described in this paper was to gain further insight in distribution profile of the ORF3 product encoded by the vitiviruses Grapevine virus A (GVA) and Grapevine virus B (GVB). Expression of the GVA MP-GFP fusion protein via the virus genome in Nicotiana benthamiana leaves resulted in the formation of irregular spots and fibrous network structures on the outermost periphery of epidermal cells. Expression of GVA MP-GFP and GVB MP-GFP was involved in the formation of the tubule-like and punctate structures on the periphery of N. benthamiana and Vitis vinifera protoplasts. Co-expression of the GVA MP-GFP and GVA MP-RFP in protoplasts resulted in co-localization of these proteins into the same punctate structures, indicating that the MP is not accumulated randomly onto the cell surface, but targeted to particular sites at the cell periphery, where punctate and tubule-like structures are likely formed. With the use of cytoskeleton and secretory pathway inhibitors, we showed that the cytoskeletal elements are not likely to be involved in targeting of the MP-GFP to the punctate cellular structures. In addition to MP, a functional coat protein was found to be essential for virus spread within inoculated leaves.

Role of Rice Stripe Virus NSvc4 in Cell-to-Cell Movement and Symptom Development in Nicotiana benthamiana

Our previous work has demonstrated that the NSvc4 protein of Rice stripe virus (RSV) functions as a cell-to-cell movement protein. However, the mechanisms whereby RSV traffics through plasmodesmata (PD) are unknown. Here we provide evidence that the NSvc4 moves on the actin filament and endoplasmic reticulum network, but not microtubules, to reach cell wall PD. Disruption of cytoskeleton using different inhibitors altered NSvc4 localization to PD, thus impeding RSV infection of Nicotiana benthamiana. Sequence analyses and deletion mutagenesis experiment revealed that the N-terminal 125 amino acids (AAs) of the NSvc4 determine PD targeting and that a transmembrane domain spanning AAs 106-125 is critical for PD localization. We also found that the NSvc4 protein can localize to chloroplasts in infected cells. Analyses using deletion mutants revealed that the N-terminal 73 AAs are essential for chloroplast localization. Furthermore, expression of NSvc4 from a Potato virus X (PVX) vector resulted in more severe disease symptoms than PVX alone in systemically infected N. benthamiana leaves. Expression of NSvc4 in Spodoptera frugiperda 9 cells did not elicit tubule formation, but instead resulted in punctate foci at the plasma membrane. These findings shed new light on our understanding of the movement mechanisms whereby RSV infects host plants.

Parallels and distinctions in the direct cell-to-cell spread of the plant and animal viruses

The paradigm that viruses can move directly, and in some cases covertly, between contacting target cells is now well established for several virus families. The underlying mechanisms of cell-to-cell spread, however, remain to be fully elucidated and may differ substantially depending on the viral exit/entry route and the cellular tropism. Here, two divergent cell-to-cell spread mechanisms are exemplified: firstly by human retroviruses, which rely upon transient adhesive structures that form between polarized immune cells termed virological synapses, and secondly by herpesviruses that depend predominantly on pre-existing stable cellular contacts, but may also form virological synapses. Plant viruses can also spread directly between contacting cells, but are obliged by the rigid host cell wall to move across pore structures termed plasmodesmata. This review will focus primarily on recent advances in our understanding of animal virus cell-to-cell spread using examples from these two virus families to highlight differences and similarities, and will conclude by comparing and contrasting the cell-to-cell spread of animal and plant viruses.

Intracellular transport of plant viruses: finding the door out of the cell

Plant viruses are a class of plant pathogens that specialize in movement from cell to cell. As part of their arsenal for infection of plants, every virus encodes a movement protein (MP), a protein dedicated to enlarging the pore size of plasmodesmata (PD) and actively transporting the viral nucleic acid into the adjacent cell. As our knowledge of intercellular transport has increased, it has become apparent that viruses must also use an active mechanism to target the virus from their site of replication within the cell to the PD. Just as viruses are too large to fit through an unmodified plasmodesma, they are also too large to be freely diffused through the cytoplasm of the cell. Evidence has accumulated now for the involvement of other categories of viral proteins in intracellular movement in addition to the MP, including viral proteins originally associated with replication or gene expression. In this review, we will discuss the strategies that viruses use for intracellular movement from the replication site to the PD, in particular focusing on the role of host membranes for intracellular transport and the coordinated interactions between virus proteins within cells that are necessary for successful virus spread.

Cellular pathways for viral transport through plasmodesmata

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

Intracellular transport of viruses and their components: utilizing the cytoskeleton and membrane highways

Plant viruses are obligate organisms that require host components for movement within and between cells. A mechanistic understanding of virus movement will allow the identification of new methods to control virus systemic spread and serve as a model system for understanding host macromolecule intra- and intercellular transport. Recent studies have moved beyond the identification of virus proteins involved in virus movement and their effect on plasmodesmal size exclusion limits to the analysis of their interactions with host components to allow movement within and between cells. It is clear that individual virus proteins and replication complexes associate with and, in some cases, traffic along the host cytoskeleton and membranes. Here, we review these recent findings, highlighting the diverse associations observed between these components and their trafficking capacity. Plant viruses operate individually, sometimes within virus species, to utilize unique interactions between their proteins or complexes and individual host cytoskeletal or membrane elements over time or space for their movement. However, there is not sufficient information for any plant virus to create a complete model of its intracellular movement; thus, more research is needed to achieve that goal.

A family of plasmodesmal proteins with receptor-like properties for plant viral movement proteins

Plasmodesmata (PD) are essential but poorly understood structures in plant cell walls that provide symplastic continuity and intercellular communication pathways between adjacent cells and thus play fundamental roles in development and pathogenesis. Viruses encode movement proteins (MPs) that modify these tightly regulated pores to facilitate their spread from cell to cell. The most striking of these modifications is observed for groups of viruses whose MPs form tubules that assemble in PDs and through which virions are transported to neighbouring cells. The nature of the molecular interactions between viral MPs and PD components and their role in viral movement has remained essentially unknown. Here, we show that the family of PD-located proteins (PDLPs) promotes the movement of viruses that use tubule-guided movement by interacting redundantly with tubule-forming MPs within PDs. Genetic disruption of this interaction leads to reduced tubule formation, delayed infection and attenuated symptoms. Our results implicate PDLPs as PD proteins with receptor-like properties involved the assembly of viral MPs into tubules to promote viral movement.

The VP37 protein of Broad bean wilt virus 2 induces tubule-like structures in both plant and insect cells

VP37 protein of Broad bean wilt virus 2 (BBWV-2) is a multifunctional protein that binds single-strand nucleic acids, interacts with viral coat protein (CP) and potentiates the virus cell-to-cell movement in its host plant. In this study, tubule-like structures filled with virus-like particles were observed by Electron Microscopy in plasmodesmata in walls of Chenopodium quinoa leaf cells infected with BBWV-2. Immunogold labeling using VP37 protein specific antibody demonstrates that the VP37 is a component of the tubular structures. When VP37 was fused with the green fluorescent protein (VP37-GFP) and expressed in BY-2 protoplasts or in insect Tn cells, green fluorescent tubules of various lengths were produced, protruding from the surface of the expressing cells. These findings suggest that the movement of BBWV-2 between cells is mediated by the tubular structures that contain the VP37 protein, and the VP37 protein itself is capable of inducing these tubule-like structures in cells. Our results also suggest that the plant and insect cell factors involved in the tubule formation have conserved features.

Cellular remodeling during plant virus infection

This review focuses on the extensive membrane and organelle rearrangements that have been observed in plant cells infected with RNA viruses. The modifications generally involve the formation of spherules, vesicles, and/or multivesicular bodies associated with various organelles such as the endoplasmic reticulum and peroxisomes. These virus-induced organelles house the viral RNA replication complex and are known as virus factories or viroplasms. Membrane and organelle alterations are attributed to the action of one or two viral proteins, which additionally act as a scaffold for the assembly of a large complex of proteins of both viral and host origin and viral RNA. Some virus factories have been shown to align with and traffic along microfilaments. In addition to viral RNA replication, the factories may be involved in other processes such as viral RNA translation and cell-to-cell virus transport. Confining the process of RNA replication to a specific location may also prevent the activation of certain host defense functions.

Lettuce infectious yellows virus-encoded P26 induces plasmalemma deposit cytopathology

Lettuce infectious yellows virus (LIYV) encodes a 26 kDa protein (P26) previously shown to associate with plasmalemma deposits (PLDs), unique LIYV-induced cytopathologies located at the plasmalemma over plasmodesmata pit fields in companion cells and phloem parenchyma. To further characterize the relationship of P26 and PLDs, we assessed localization and cytopathology induction of P26 expressed from either LIYV or a heterologous Tobacco mosaic virus (TMV) vector using green fluorescent protein (GFP) fusions, immunofluorescence microscopy, biochemical fractionation, and transmission electron microscopy (TEM). TEM analyses demonstrated that P26 not only associated with, but induced formation of PLDs in the absence of other LIYV proteins. Interestingly, PLDs induced by P26-expressing TMV were no longer confined to phloem cells. Putative P26 orthologs from two other members of the genus Crinivirus which do not induce conspicuous PLDs exhibited fractionation properties similar to LIYV P26 but were not associated with any PLD-like cytopathology.

Translational control of recombinant human acetylcholinesterase accumulation in plants

Background

Codon usage differences are known to regulate the levels of gene expression in a species-specific manner, with the primary factors often cited to be mRNA processing and accumulation. We have challenged this conclusion by expressing the human acetylcholinesterase coding sequence in transgenic plants in its native GC-rich sequence and compared to a matched sequence with (dicotyledonous) plant-optimized codon usage and a lower GC content.

Results

We demonstrate a 5 to 10 fold increase in accumulation levels of the "synaptic" splice variant of human acetylcholinesterase in Nicotiana benthamiana plants expressing the optimized gene as compared to the native human sequence. Both transient expression assays and stable transformants demonstrated conspicuously increased accumulation levels. Importantly, we find that the increase is not a result of increased levels of acetylcholinesterase mRNA, but rather its facilitated translation, possibly due to the reduced energy required to unfold the sequence-optimized mRNA.

Conclusion

Our findings demonstrate that codon usage differences may regulate gene expression at different levels and anticipate translational control of acetylcholinesterase gene expression in its native mammalian host as well.

The spread of Rice dwarf virus among cells of its insect vector exploits virus-induced tubular structures

Various cytopathological structures, known as inclusion bodies, are formed upon infection of cultured leafhopper cells by Rice dwarf virus, a member of the family Reoviridae. These structures include tubules of approximately 85 nm in diameter which are composed of the nonstructural viral protein Pns10 and contain viral particles. Such tubular structures were produced in heterologous non-host insect cells that expressed Pns10 of the virus. These tubules, when associated with actin-based filopodia, were able to protrude from the surface of cells and to penetrate neighboring cells. A binding assay in vitro revealed the specific binding of Pns10 to actin. Infection of clusters of cells was readily apparent 5 days after inoculation at a low multiplicity of infection with the virus, even in the presence of neutralizing antibodies. However, treatment of host cells with drugs that inhibited the elongation of actin filaments abolished the extension of Pns10 tubules from the surface of cells, with a significant simultaneous decrease in the extent of infection of neighboring cells. These results together revealed a previously undescribed aspect of the intercellular spread of Rice dwarf virus, wherein the virus exploits tubules composed of a nonstructural viral protein and actin-based filopodia to move into neighboring cells.

A coiled-coil interaction mediates cauliflower mosaic virus cell-to-cell movement

The function of the virion-associated protein (VAP) of cauliflower mosaic virus (CaMV) has long been only poorly understood. VAP is associated with the virion but is dispensable for virus morphogenesis and replication. It mediates virus transmission by aphids through simultaneous interaction with both the aphid transmission factor and the virion. However, although insect transmission is not fundamental to CaMV survival, VAP is indispensable for spreading the virus infection within the host plant. We used a GST pull-down technique to demonstrate that VAP interacts with the viral movement protein through coiled-coil domains and surface plasmon resonance to measure the interaction kinetics. We mapped the movement protein coiled-coil to the C terminus of the protein and proved that it self-assembles as a trimer. Immunogold labeling/electron microscopy revealed that the VAP and viral movement protein colocalize on CaMV particles within plasmodesmata. These results highlight the multifunctional potential of the VAP protein conferred by its efficient coiled-coil interaction system and show a plant virus possessing a surface-exposed protein (VAP) mediating viral entry into host cells.

Subcellular distribution of mutant movement proteins of Cucumber mosaic virus fused to green fluorescent proteins

The subcellular distribution of the movement proteins (MPs) of nine alanine-scanning mutants of Cucumber mosaic virus (CMV), fused to the green fluorescent protein (GFP) and expressed from CMV, was determined by confocal microscopy of infected epidermal cells of Nicotiana tabacum and Nicotiana benthamiana, as well as infected N. benthamiana protoplasts. Only those mutant MPs that were functional for movement in all host species tested localized to plasmodesmata of infected epidermal cells and to tubules extending from the surface of infected protoplasts, as for wild-type CMV 3a MP. Various mutant MPs that were either conditionally functional for movement or dysfunctional for movement did not localize to plasmodesmata and did not form tubules on the surface of infected protoplasts. Rather, they showed distribution to different extents throughout the infected cells, including the cytoplasm, nucleus or the plasma membrane. The CMV 3a MP also did not associate with microtubules.

Not just another hole in the wall: understanding intercellular protein trafficking

Development and differentiation of multicellular organisms requires cell-to-cell communication. In plants direct signaling and exchange of macromolecules between cells is possible through plasmodesmata. Recently direct exchange of membrane-bound vesicles and organelles has been demonstrated between animal cells through formation of cytoplasmic bridges (tunneling nanotubes) in vitro. Here we review recent developments in cell-to-cell trafficking of macromolecules in plants and animals.

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.

Conformational analysis of hepatitis B surface antigen fusions in an Agrobacterium-mediated transient expression system

Vaccine antigens have been successfully produced in transgenic plants for oral immunization. Recently, a fusion strategy has been adopted to produce multicomponent vaccines and to target antigens to mucosal sites for enhanced oral immunogenicity. However, antigen fusions may not be folded correctly due to steric hindrance and may thus lose their potency. Here, we describe an Agrobacterium-mediated transient assay that provides enough antigen-expressing material at 2 days post-transfection to evaluate antigen conformation. Using the hepatitis B surface antigen (HBsAg) as a model antigen and the green fluorescent protein (GFP) as a model fusion partner, we showed that transiently expressed HBsAg and an HBsAg fusion with GFP at the N-terminus (GFP:HBsAg), but not the HBsAg fusion with GFP at the C-terminus (HBsAg:GFP), formed the 'a' determinant and virus-like particles (VLPs), similar to yeast-derived vaccine HBsAg. Thus, it is feasible to modify the HBsAg with an N-terminal fusion of up to 239 amino acids without altering its major antigenic properties. Our results also demonstrate that the Agrobacterium-mediated transient expression system can be used to evaluate the conformation of plant-based vaccines or other pharmaceutical proteins in a high-throughput manner.

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.

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).

The cauliflower mosaic virus virion-associated protein is dispensable for viral replication in single cells

Cauliflower mosaic virus (CaMV) open reading frame III (ORF III) codes for a virion-associated protein (Vap), which is one of two viral proteins essential for aphid transmission. However, unlike the aphid transmission factor encoded by CaMV ORF II, Vap is also essential for systemic infection, suggesting that it is a multifunctional protein. To elucidate the additional function or functions of Vap, we tested the replication of noninfectious ORF III-defective mutants in transfected turnip protoplasts. PCR and Western blot analyses revealed that CaMV replication had occurred with an efficiency similar to that of wild-type virus and without leading to reversions. Electron microscopic examination revealed that an ORF III frameshift mutant formed normally structured virions. These results demonstrate that Vap is dispensable for replication in single cells and is not essential for virion morphogenesis. Analysis of inoculated turnip leaves showed that the ORF III frameshift mutant does not cause any detectable local infection. These results are strongly indicative of a role for Vap in virus movement.

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.

Signal transduction in maize and Arabidopsis mesophyll protoplasts

Plant protoplasts show physiological perceptions and responses to hormones, metabolites, environmental cues, and pathogen-derived elicitors, similar to cell-autonomous responses in intact tissues and plants. The development of defined protoplast transient expression systems for high-throughput screening and systematic characterization of gene functions has greatly contributed to elucidating plant signal transduction pathways, in combination with genetic, genomic, and transgenic approaches.

Effects of movement protein mutations on the formation of tubules in plant protoplasts expressing a fusion between the green fluorescent protein and Cauliflower mosaic virus movement protein

Fusions between the green fluorescent protein (GFP) and the Cauliflower mosaic virus (CaMV) movement protein (MP) induce the formation of fluorescent foci and surface tubules in Arabidopsis thaliana leaf mesophyll protoplasts. Tubules elongate coordinately and progressively in an assembly process approximately 6 to 12 h following transfection of protoplasts with GFP-MP constructs. Tubules are not formed in protoplasts transfected by GFP-MP(ER2A), a MP mutation that renders CaMV noninfectious. A small number of short tubules are formed on protoplasts transfected by GFP-MP(N6) and GFP-MP(N13), two second-site revertants of ER2A that partially restore infectivity. Protoplasts cotransfected with cyan fluorescent protein (CFP)-MP(WT) and GFP-MP(ER2A) form tubules containing both MP fusions, indicating that although the GFP-MP(ER2A) cannot induce tubule formation, GFP-MP(ER2A) can coassemble or colocalize with CFP-MP(WT) in tubules. Thus, CaMV MP-induced tubule formation in protoplasts correlates closely with the infectivity of mutation ER2A and its revertants, suggesting that tubule-forming capacity in plant protoplasts reflects a process required for virus infection or movement.

Cucumber mosaic virus-plant interactions: identification of 3a protein sequences affecting infectivity, cell-to-cell movement, and long-distance movement

Mutants of the Cucumber mosaic virus (CMV) movement protein (MP) were generated and analyzed for their effects on virus movement and pathogenicity in vivo. Similar to the wild-type MP, mutants M1, M2, and M3, promoted virus movement in eight plant species. Mutant M3 showed some differences in pathogenicity in one host species. Mutant M8 showed some host-specific alterations in movement in two hypersensitive hosts of CMV. Mutant M9 showed altered pathogenicity on three hosts and was temperature sensitive for long-distance movement, demonstrating that cell-to-cell and long-distance movement are distinct movement functions for CMV. Four mutants (M4, M5, M6, and M7) were debilitated from movement in all hosts tested. Mutants M4, M5, and M6 could be complemented in trans by the wild-type MP expressed transgenically, although not by each other or by mutant M9 (at the restrictive temperature). Mutant M7 showed an inability to be complemented in trans. From these mutants, different aspects of the CMV movement process could be defined and specific roles for particular sequence domains assigned. The broader implications of these functions are discussed.