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

Emergence of two distinct spatial folds in a pair of plant virus proteins encoded by nested genes

Virus genomes may encode overlapping or nested open reading frames that increase their coding capacity. It is not known whether the constraints on spatial structures of the two encoded proteins limit the evolvability of nested genes. We examine the evolution of a pair of proteins, p22 and p19, encoded by nested genes in plant viruses from the genus Tombusvirus. The known structure of p19, a suppressor of RNA silencing, belongs to the RAGNYA fold from the alpha+beta class. The structure of p22, the cell-to-cell movement protein from the 30K family widespread in plant viruses, is predicted with the AlphaFold approach, suggesting a single jelly-roll fold core from the all-beta class, structurally similar to capsid proteins from plant and animal viruses. The nucleotide and codon preferences impose modest constraints on the types of secondary structures encoded in the alternative reading frames, nonetheless allowing for compact, well-ordered folds from different structural classes in two similarly-sized nested proteins. Tombusvirus p22 emerged through radiation of the widespread 30K family, which evolved by duplication of a virus capsid protein early in the evolution of plant viruses, whereas lineage-specific p19 may have emerged by a stepwise increase in the length of the overprinted gene and incremental acquisition of functionally active secondary structure elements by the protein product. This evolution of p19 toward the RAGNYA fold represents one of the first documented examples of protein structure convergence in naturally occurring proteins.

Variability, Functions and Interactions of Plant Virus Movement Proteins: What Do We Know So Far?

Of the various proteins encoded by plant viruses, one of the most interesting is the movement protein (MP). MPs are unique to plant viruses and show surprising structural and functional variability while maintaining their core function, which is to facilitate the intercellular transport of viruses or viral nucleoprotein complexes. MPs interact with components of the intercellular channels, the plasmodesmata (PD), modifying their size exclusion limits and thus allowing larger particles, including virions, to pass through. The interaction of MPs with the components of PD, the formation of transport complexes and the recruitment of host cellular components have all revealed different facets of their functions. Multitasking is an inherent property of most viral proteins, and MPs are no exception. Some MPs carry out multitasking, which includes gene silencing suppression, viral replication and modulation of host protein turnover machinery. This review brings together the current knowledge on MPs, focusing on their structural variability, various functions and interactions with host proteins.

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 Strange Lifestyle of Multipartite Viruses

Multipartite viruses have one of the most puzzling genetic organizations found in living organisms. These viruses have several genome segments, each containing only a part of the genetic information, and each individually encapsidated into a separate virus particle. While countless studies on molecular and cellular mechanisms of the infection cycle of multipartite viruses are available, just as for other virus types, very seldom is their lifestyle questioned at the viral system level. Moreover, the rare available “system” studies are purely theoretical, and their predictions on the putative benefit/cost balance of this peculiar genetic organization have not received experimental support. In light of ongoing progresses in general virology, we here challenge the current hypotheses explaining the evolutionary success of multipartite viruses and emphasize their shortcomings. We also discuss alternative ideas and research avenues to be explored in the future in order to solve the long-standing mystery of how viral systems composed of interdependent but physically separated information units can actually be functional.

A Renaissance in Nepovirus Research Provides New Insights Into Their Molecular Interface With Hosts and Vectors

Nepoviruses supplied seminal landmarks to the historical trail of plant virology. Among the first agriculturally relevant viruses recognized in the late 1920s and among the first plant viruses officially classified in the early 1970s, nepoviruses also comprise the first species for which a soil-borne ectoparasitic nematode vector was identified. Early research on nepoviruses shed light on the genome structure and expression, biological properties of the two genomic RNAs, and mode of transmission. In recent years, research on nepoviruses enjoyed an extraordinary renaissance. This resurgence provided new insights into the molecular interface between viruses and their plant hosts, and between viruses and dagger nematode vectors to advance our understanding of some of the major steps of the infectious cycle. Here we examine these recent findings, highlight ongoing work, and offer some perspectives for future research.

6K2-induced vesicles can move cell to cell during turnip mosaic virus infection

To successfully infect plants, viruses replicate in an initially infected cell and then move to neighboring cells through plasmodesmata (PDs). However, the nature of the viral entity that crosses over the cell barrier into non-infected ones is not clear. The membrane-associated 6K₂ protein of turnip mosaic virus (TuMV) induces the formation of vesicles involved in the replication and intracellular movement of viral RNA. This study shows that 6K₂-induced vesicles trafficked toward the plasma membrane and were associated with plasmodesmata (PD). We demonstrated also that 6K₂ moved cell-to-cell into adjoining cells when plants were infected with TuMV. 6K₂ was then fused to photo-activable GFP (6K₂:PAGFP) to visualize how 6K₂ moved intercellularly during TuMV infection. After activation, 6K₂:PAGFP-tagged vesicles moved to the cell periphery and across the cell wall into adjacent cells. These vesicles were shown to contain the viral RNA-dependent RNA polymerase and viral RNA. Symplasmic movement of TuMV may thus be achieved in the form of a membrane-associated viral RNA complex induced by 6K₂.

Contribution of host intracellular transport machineries to intercellular movement of turnip mosaic virus

The contribution of different host cell transport systems in the intercellular movement of turnip mosaic virus (TuMV) was investigated. To discriminate between primary infections and secondary infections associated with the virus intercellular movement, a gene cassette expressing GFP-HDEL was inserted adjacent to a TuMV infectious cassette expressing 6K₂:mCherry, both within the T-DNA borders of the binary vector pCambia. In this system, both gene cassettes were delivered to the same cell by a single binary vector and primary infection foci emitted green and red fluorescence while secondarily infected cells emitted only red fluorescence. Intercellular movement was measured at 72 hours post infiltration and was estimated to proceed at an average rate of one cell being infected every three hours over an observation period of 17 hours. To determine if the secretory pathway were important for TuMV intercellular movement, chemical and protein inhibitors that blocked both early and late secretory pathways were used. Treatment with Brefeldin A or Concanamycin A or expression of ARF1 or RAB-E1d dominant negative mutants, all of which inhibit pre- or post-Golgi transport, reduced intercellular movement by the virus. These treatments, however, did not inhibit virus replication in primary infected cells. Pharmacological interference assays using Tyrphostin A23 or Wortmannin showed that endocytosis was not important for TuMV intercellular movement. Lack of co-localization by endocytosed FM4-64 and Ara7 (AtRabF2b) with TuMV-induced 6K₂-tagged vesicles further supported this conclusion. Microfilament depolymerizing drugs and silencing expression of myosin XI-2 gene, but not myosin VIII genes, also inhibited TuMV intercellular movement. Expression of dominant negative myosin mutants confirmed the role played by myosin XI-2 as well as by myosin XI-K in TuMV intercellular movement. Using this dual gene cassette expression system and transport inhibitors, components of the secretory and actomyosin machinery were shown to be important for TuMV intercellular spread.

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.

Plasmodesmata: the battleground against intruders

Plasmodesmata are intercellular channels that establish a symplastic communication pathway between neighboring cells in plants. Owing to this role, opportunistic microbial pathogens have evolved to exploit plasmodesmata as gateways to spread infection from cell to cell within the plant. However, although these pathogens have acquired the capacity to breach the plasmodesmal trafficking pathway, plants are unlikely to relinquish control over a structure essential for their survival so easily. In this review, we examine evidence that suggests plasmodesmata play an active role in plant immunity against viral, fungal and bacterial pathogens. We discuss how these pathogens differ in their lifestyles and infection modes, and present the defense strategies that plants have adopted to prevent the intercellular spread of an infection.

Cellular factors in plant virus movement: at the leading edge of macromolecular trafficking in plants

To establish systemic infection, plant viruses must be localized to the correct subcellular sites to accomplish replication and then traffic from initially infected cells into neighboring cells and even distant organs. Viruses have evolved various strategies to interact with pre-existing cellular factors to achieve these functions. In this review we discuss plant virus intracellular, intercellular and long-distance movement, focusing on the host cellular factors involved. We emphasize that elucidating viral movement mechanisms will not only shed light on the molecular mechanisms of infection, but will also contribute valuable insights into the regulation of endogenous macromolecular trafficking.

Plasmodesmata: gateways to local and systemic virus infection

As channels that provide cell-to-cell connectivity, plasmodesmata are central to the local and systemic spread of viruses in plants. This review discusses the current state of knowledge of the structure and function of these channels and the ways in which viruses bring about functional changes that allow macromolecular trafficking to occur. Despite the passing of two decades since the first identification of a viral movement protein that mediates these changes, our understanding of the relevant molecular mechanisms remains in its infancy. However, viral movement proteins provide valuable tools for the modification of plasmodesmata and will continue to assist in the dissection of plasmodesmal properties in relation to their core roles in cell-to-cell communication.

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.

Cell-to-cell trafficking, subcellular distribution, and binding to coat protein of Broad bean wilt virus 2 VP37 protein

Broad bean wilt virus 2 (BBWV 2) is a member of the genus Fabavirus of the family Comoviridae. To date, a movement protein (MP) of BBWV 2 has not been described. Here we demonstrate that the green fluorescent protein (GFP)-VP37 fusion protein can move from initial bombarded cells to neighboring cells in Nicotiana benthamiana epidermal leaves. In addition, the GFP-VP37 fusion protein localizes as a halo around the nucleus and as punctate spots on the cell periphery in N. benthamiana epidermal leaf cells and BY-2 suspension cells. Fluorescence near the nucleus also was co-localized with the endoplasmic reticulum in BY-2 cells. Fibrous networks were found in GFP-VP37 agro-infiltrated N. benthamiana epidermal leaf cells. Deletion analyses indicated that the C-terminal region of the VP37 protein is essential for localization at the cell periphery. Using a blot overlay assay and bimolecular fluorescence complementation assay, the purified 6xHis-tagged VP37 protein was shown to bind specifically to the small coat protein of BBWV 2. The above results indicate that VP37 is a movement protein.

Endothelial targeting of cowpea mosaic virus (CPMV) via surface vimentin

Cowpea mosaic virus (CPMV) is a plant comovirus in the picornavirus superfamily, and is used for a wide variety of biomedical and material science applications. Although its replication is restricted to plants, CPMV binds to and enters mammalian cells, including endothelial cells and particularly tumor neovascular endothelium in vivo. This natural capacity has lead to the use of CPMV as a sensor for intravital imaging of vascular development. Binding of CPMV to endothelial cells occurs via interaction with a 54 kD cell-surface protein, but this protein has not previously been identified. Here we identify the CPMV binding protein as a cell-surface form of the intermediate filament vimentin. The CPMV-vimentin interaction was established using proteomic screens and confirmed by direct interaction of CPMV with purified vimentin, as well as inhibition in a vimentin-knockout cell line. Vimentin and CPMV were also co-localized in vascular endothelium of mouse and rat in vivo. Together these studies indicate that surface vimentin mediates binding and may lead to internalization of CPMV in vivo, establishing surface vimentin as an important vascular endothelial ligand for nanoparticle targeting to tumors. These results also establish vimentin as a ligand for picornaviruses in both the plant and animal kingdoms of life. Since bacterial pathogens and several other classes of viruses also bind to surface vimentin, these studies suggest a common role for surface vimentin in pathogen transmission.

Interaction between a 54-kilodalton mammalian cell surface protein and cowpea mosaic virus

Cowpea mosaic virus (CPMV), a plant virus that is a member of the picornavirus superfamily, is increasingly being used for nanotechnology applications, including material science, vascular imaging, vaccine development, and targeted drug delivery. For these applications, it is critical to understand the in vivo interactions of CPMV within the mammalian system. Although the bioavailability of CPMV in the mouse has been demonstrated, the specific interactions between CPMV and mammalian cells need to be characterized further. Here we demonstrate that although the host range for replication of CPMV is confined to plants, mammalian cells nevertheless bind and internalize CPMV in significant amounts. This binding is mediated by a conserved 54-kDa protein found on the plasma membranes of both human and murine cell lines. Studies using a deficient cell line, deglycosidases, and glycosylation inhibitors showed that the CPMV binding protein (CPMV-BP) is not glycosylated. A possible 47-kDa isoform of the CPMV-BP was also detected in the organelle and nuclear subcellular fraction prepared from murine fibroblasts. Further characterization of CPMV-BP is important to understand how CPMV is trafficked through the mammalian system and may shed light on how picornaviruses may have evolved between plant and animal hosts.

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.

Cell wall localization of Red clover necrotic mosaic virus movement protein is required for cell-to-cell movement

The Red clover necrotic mosaic virus movement protein (MP) is essential for cell-to-cell movement. Eight previously characterized alanine-scanning mutants of the MP were fused to the green fluorescent protein (GFP) and expressed from viral infectious transcripts. Inoculated plants were assayed for movement and intracellular accumulation of MP by confocal laser-scanning microscopy. A strict correlation was observed between the targeting to the cell wall (presumably the plasmodesmata) and cell-to-cell movement. Complementation of dysfunctional MP mutants with either wild-type MP or other null mutants in some cases rescued intracellular targeting and movement. The data suggest the presence of distinct domains in the MP for virus movement (near residues 27-31), complementarity (near residues 122 and 128), and intracellular localization (near residue 161). These data support a model of MP interacting cooperatively with itself to bind viral RNA, localize to and modify plasmodesmata and effect virus movement.

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.

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.