A common structure for the potexviruses

Inorganic-organic Nanomaterials for Therapeutics and Molecular Imaging Applications

Background::

Surface modification of nanoparticles with targeting moieties can be achieved through bioconjugation chemistries to impart new Functionalities. Various polymeric nanoparticles have been used for the formulation of nanoparticles such as naturally-occurring protein cages, virus-like particles, polymeric saccharides, and liposomes. These polymers have been proven to be biocompatible, side effects free and degradable with no toxicity.

Objectives::

This paper reviews available literature on the nanoparticles pharmaceutical and medical applications. The review highlights and updates the customized solutions for selective drug delivery systems that allow high-affinity binding between nanoparticles and the target receptors.

Methods::

Bibliographic databases and web-search engines were used to retrieve studies that assessed the usability of nanoparticles in the pharmaceutical and medical fields. Data were extracted on each system in vivo and in vitro applications, its advantages and disadvantages, and its ability to be chemically and genetically modified to impart new functionalities. Finally, a comparison between naturally occurring and their synthetic counterparts was carried out.

Results::

The results showed that nanoparticles-based systems could have promising applications in diagnostics, cell labeling, contrast agents (Magnetic Resonance Imaging and Computed Tomography), antimicrobial agents, and as drug delivery systems. However, precautions should be taken to avoid or minimize toxic effect or incompatibility of nanoparticles-based systems with the biological systems in case of pharmaceutical or medical applications.

Conclusion::

This review presented a summary of recent developments in the field of pharmaceutical nanotechnology and highlighted the challenges and the merits that some of the nanoparticles- based systems both in vivo and in vitro systems.

Plant Virus Nanoparticles for Vaccine Applications

In the rapidly evolving field of nanotechnology, plant virus nanoparticles (pVNPs) are emerging as powerful tools in diverse applications ranging from biomedicine to materials science. The proteinaceous structure of plant viruses allows the capsid structure to be modified by genetic engineering and/or chemical conjugation with nanoscale precision. This means that pVNPs can be engineered to display peptides and proteins on their external surface, including immunodominant peptides derived from pathogens allowing pVNPs to be used for active immunization. In this context, pVNPs are safer than VNPs derived from mammalian viruses because there is no risk of infection or reversion to pathogenicity. Furthermore, pVNPs can be produced rapidly and inexpensively in natural host plants or heterologous production platforms. In this review, we discuss the use of pVNPs for the delivery of peptide antigens to the host immune in pre-clinical studies with the final aim of promoting systemic immunity against the corresponding pathogens. Furthermore, we described the versatility of plant viruses, with innate immunostimulatory properties, in providing a huge natural resource of carriers that can be used to develop the next generation of sustainable vaccines.

Plant virus-based materials for biomedical applications: Trends and prospects

Nanomaterials composed of plant viral components are finding their way into medical technology and health care, as they offer singular properties. Precisely shaped, tailored virus nanoparticles (VNPs) with multivalent protein surfaces are efficiently loaded with functional compounds such as contrast agents and drugs, and serve as carrier templates and targeting vehicles displaying e.g. peptides and synthetic molecules. Multiple modifications enable uses including vaccination, biosensing, tissue engineering, intravital delivery and theranostics. Novel concepts exploit self-organization capacities of viral building blocks into hierarchical 2D and 3D structures, and their conversion into biocompatible, biodegradable units. High yields of VNPs and proteins can be harvested from plants after a few days so that various products have reached or are close to commercialization. The article delineates potentials and limitations of biomedical plant VNP uses, integrating perspectives of chemistry, biomaterials sciences, molecular plant virology and process engineering.

Small, Smaller, Nano: New Applications for Potato Virus X in Nanotechnology

Nanotechnology is an expanding interdisciplinary field concerning the development and application of nanostructured materials derived from inorganic compounds or organic polymers and peptides. Among these latter materials, proteinaceous plant virus nanoparticles have emerged as a key platform for the introduction of tailored functionalities by genetic engineering and conjugation chemistry. Tobacco mosaic virus and Cowpea mosaic virus have already been developed for bioimaging, vaccination and electronics applications, but the flexible and filamentous Potato virus X (PVX) has received comparatively little attention. The filamentous structure of PVX particles allows them to carry large payloads, which are advantageous for applications such as biomedical imaging in which multi-functional scaffolds with a high aspect ratio are required. In this context, PVX achieves superior tumor homing and retention properties compared to spherical nanoparticles. Because PVX is a protein-based nanoparticle, its unique functional properties are combined with enhanced biocompatibility, making it much more suitable for biomedical applications than synthetic nanomaterials. Moreover, PVX nanoparticles have very low toxicity in vivo, and superior pharmacokinetic profiles. This review focuses on the production of PVX nanoparticles engineered using chemical and/or biological techniques, and describes current and future opportunities and challenges for the application of PVX nanoparticles in medicine, diagnostics, materials science, and biocatalysis.

Systemic Infection of Nicotiana benthamiana with Potato virus X Nanoparticles Presenting a Fluorescent iLOV Polypeptide Fused Directly to the Coat Protein

Plant virus-based nanoparticles can be produced in plants on a large scale and are easily modified to introduce new functions, making them suitable for applications such as vaccination and drug delivery, tissue engineering, and in vivo imaging. The latter is often achieved using green fluorescent protein and its derivatives, but the monovalent fluorescent protein iLOV is smaller and more robust. Here, we fused the iLOV polypeptide to the N-terminus of the Potato virus X (PVX) coat protein, directly or via the Foot-and-mouth disease virus 2A sequence, for expression in Nicotiana benthamiana. Direct fusion of the iLOV polypeptide did not prevent the assembly or systemic spread of the virus and we verified the presence of fusion proteins and iLOV hybrid virus particles in leaf extracts. Compared to wild-type PVX virions, the PVX particles displaying the iLOV peptide showed an atypical, intertwined morphology. Our results confirm that a direct fusion of the iLOV fluorescent protein to filamentous PVX nanoparticles offers a promising tool for imaging applications.

Bioinspired Silica Mineralization on Viral Templates

Plant virus capsids are attractive entities for nanotechnological applications because of their variation in shape and natural assembly ability. This chapter describes the production and modification of three differently shaped plant virus capsids for silica mineralization purposes. The chosen plant viruses exhibit either an icosahedral (cowpea mosaic virus, CPMV), or a flexuous rod-like structure (potato virus X, PVX), or a rigid rod-like shape (tobacco mosaic virus, TMV), and are well-known and frequently used plant viruses for biotechnological applications. We describe the production (including genetic or chemical modification) and purification of the plant viruses or of empty virus-like particles in the case of CPMV, as well as the characterization of these harvested templates. The mineralization procedures and differences in the protocols specific to the distinct viruses are described, and the analyses of the mineralization results are explained.

Structural Homology Between Nucleoproteins of ssRNA Viruses

Our understanding of the viral world changed just after the first structures of icosahedral viral particles were unveiled. The structural similarities between capsid proteins of distant viral groups were not anticipated, and the findings suggested the existence of common ancestors for viruses with different host range, genomic structure and multiplication strategies. This way, diverse viruses with icosahedral particles can now be grouped based on the structural homology between their capsid proteins. In the last years, the presence of conserved folds between viral proteins in non-icosahedral viruses has also emerged. Viral particles with radically different morphologies, ranging from naked and filamentous to enveloped and pleomorphic, have shown structural homology between the nucleoproteins that bind directly to their genomes. This chapter overviews recent findings regarding the similar structure found between nucleoproteins of eukaryotic ssRNA viruses. The structural homology includes the coat proteins from all known families of flexible filamentous plant viruses, a group with monopartite (+)ssRNA genomes. Their coat proteins share a core domain with nucleoproteins of previously unrelated families of enveloped viruses that have segmented (-)ssRNA genomes. This last group consists of mostly animals viruses, including influenza virus.

Engineering Potato Virus X Particles for a Covalent Protein Based Attachment of Enzymes

Plant virus nanoparticles are often used to display functional amino acids or small peptides, thus serving as building blocks in application areas as diverse as nanoelectronics, bioimaging, vaccination, drug delivery, and bone differentiation. This is most easily achieved by expressing coat protein fusions, but the assembly of the corresponding virus particles can be hampered by factors such as the fusion protein size, amino acid composition, and post-translational modifications. Size constraints can be overcome by using the Foot and mouth disease virus 2A sequence, but the compositional limitations cannot be avoided without the introduction of time-consuming chemical modifications. SpyTag/SpyCatcher technology is used in the present study to covalently attach the Trichoderma reesei endoglucanase Cel12A to Potato virus X (PVX) nanoparticles. The formation of PVX particles is confirmed by western blot, and the ability of the particles to display Cel12A is demonstrated by enzyme-linked immunosorbent assays and transmission electron microscopy. Enzymatic assays show optimal reaction conditions of 50 °C and pH 6.5, and an increased substrate conversion rate compared to free enzymes. It is concluded that PVX displaying the SpyTag can serve as new scaffold for protein display, most notably for proteins with post-translational modifications.

Potyvirus virion structure shows conserved protein fold and RNA binding site in ssRNA viruses

Potyviruses constitute the second largest genus of plant viruses and cause important economic losses in a large variety of crops; however, the atomic structure of their particles remains unknown. Infective potyvirus virions are long flexuous filaments where coat protein (CP) subunits assemble in helical mode bound to a monopartite positive-sense single-stranded RNA [(+)ssRNA] genome. We present the cryo-electron microscopy (cryoEM) structure of the potyvirus watermelon mosaic virus at a resolution of 4.0 Å. The atomic model shows a conserved fold for the CPs of flexible filamentous plant viruses, including a universally conserved RNA binding pocket, which is a potential target for antiviral compounds. This conserved fold of the CP is widely distributed in eukaryotic viruses and is also shared by nucleoproteins of enveloped viruses with segmented (-)ssRNA (negative-sense ssRNA) genomes, including influenza viruses.

Chemical addressability of potato virus X for its applications in bio/nanotechnology

Potato virus X (PVX), a type member of the plant virus potexvirus group, offers a unique nanotechnology platform based on its high aspect ratio and flexible filamentous shape. The PVX platform has already been engineered and studied for its uses in imaging, drug delivery, and immunotherapies. While genetic engineering procedures are well established for PVX, there is limited information about chemical conjugation strategies for functionalizing PVX, partly due to the lack of structural information of PVX at high resolution. To overcome these challenges, we built a structural model of the PVX particle based on the available structures from pepino mosaic virus (PepMV), a close cousin of PVX. Using the model and a series of chemical conjugation experiments, we identified and probed the addressability of cysteine side chains. Chemical reactivity of cysteines was confirmed using Michael-addition and thiol-selective probes, including fluorescent dyes and biotin tags. LC/MS/MS was used to map Cys 121 as having the highest selectivity for modification. Finally, building on the availability of two reactive groups, the newly identified Cys and previously established Lys side chains, we prepared multifunctional PVX nanoparticles by conjugating Gd-DOTA for magnetic resonance imaging (MRI) to lysines and fluorescent dyes for optical imaging to cysteines. The resulting functionalized nanofilament could have applications in dual-modal optical-MRI imaging applications. These results further extend the understanding of the chemical properties of PVX and enable development of novel multifunctional platforms in bio/nanotechnology.

The near-atomic cryoEM structure of a flexible filamentous plant virus shows homology of its coat protein with nucleoproteins of animal viruses

Flexible filamentous viruses include economically important plant pathogens. Their viral particles contain several hundred copies of a helically arrayed coat protein (CP) protecting a (+)ssRNA. We describe here a structure at 3.9 Å resolution, from electron cryomicroscopy, of Pepino mosaic virus (PepMV), a representative of the genus Potexvirus (family Alphaflexiviridae). Our results allow modeling of the CP and its interactions with viral RNA. The overall fold of PepMV CP resembles that of nucleoproteins (NPs) from the genus Phlebovirus (family Bunyaviridae), a group of enveloped (-)ssRNA viruses. The main difference between potexvirus CP and phlebovirus NP is in their C-terminal extensions, which appear to determine the characteristics of the distinct multimeric assemblies – a flexuous, helical rod or a loose ribonucleoprotein. The homology suggests gene transfer between eukaryotic (+) and (-)ssRNA viruses.

DOI:http://dx.doi.org/10.7554/eLife.11795.001

A group of “flexible filamentous” viruses can cause serious diseases in a wide variety of crops and other plants. Each virus particle contains a single molecule called ribonucleic acid (RNA), which is protected by hundreds of copies of a coat protein. The RNA and coat proteins are arranged in a helical fashion to make a flexible rod-shaped particle.

The flexibility of these viruses makes it difficult to carry out in-depth studies of their three-dimensional structures. As a result, we do not know how the RNA and coat proteins interact to form the structure of each virus particle. Agirrezabala et al. used a technique called cryo-electron microscopy (or cryoEM for short) to generate a highly detailed three-dimensional model of a flexible filamentous virus called Pepino Mosaic Virus.

Agirezabala et al.’s findings reveal how the virus particles assemble, and the interactions between the coat protein and the ssRNA. Unexpectedly, the structure of the coat protein from Pepino Mosiac Virus is very similar to the structure of “nucleoproteins” from a group of viruses called the Phleboviruses, which infect animals. This similarity is striking and suggests that the gene that encodes these proteins has been transferred between the two groups of viruses during evolution. A future challenge is to find out whether this similarity extends to other groups of viruses.

DOI:http://dx.doi.org/10.7554/eLife.11795.002

The molecular basis for flexibility in the flexible filamentous plant viruses

Flexible filamentous plant viruses cause more than half the viral crop damage in the world but are also potentially useful for biotechnology. Structural studies began more than 75 years ago but have failed, owing to the virion's extreme flexibility. We have used cryo-EM to generate an atomic model for bamboo mosaic virus, which reveals flexible N- and C-terminal extensions that allow deformation while still maintaining structural integrity.

The coat protein of Alternanthera mosaic virus is the elicitor of a temperature-sensitive systemic necrosis in Nicotiana benthamiana, and interacts with a host boron transporter protein

Different isolates of Alternanthera mosaic virus (AltMV; Potexvirus), including four infectious clones derived from AltMV-SP, induce distinct systemic symptoms in Nicotiana benthamiana. Virus accumulation was enhanced at 15 °C compared to 25 °C; severe clone AltMV 3-7 induced systemic necrosis (SN) and plant death at 15 °C. No interaction with potexvirus resistance gene Rx was detected, although SN was ablated by silencing of SGT1, as for other cases of potexvirus-induced necrosis. Substitution of AltMV 3-7 coat protein (CPSP) with that from AltMV-Po (CP(Po)) eliminated SN at 15 °C, and ameliorated symptoms in Alternanthera dentata and soybean. Substitution of only two residues from CP(Po) [either MN(13,14)ID or LA(76,77)IS] efficiently ablated SN in N. benthamiana. CPSP but not CP(Po) interacted with Arabidopsis boron transporter protein AtBOR1 by yeast two-hybrid assay; N. benthamiana homolog NbBOR1 interacted more strongly with CPSP than CP(Po) in bimolecular fluorescence complementation, and may affect recognition of CP as an elicitor of SN.

Viral and nonviral elements in potexvirus replication and movement and in antiviral responses

In Potato virus X, a member of the genus Potexvirus, special sequences and structures at the 5' and 3' ends of the nontranslated region function as cis-acting elements for viral replication. These elements greatly affect interactions between viral RNAs and those between viral RNAs and host factors. The potexvirus genome encodes five open-reading frames. Viral replicase, which is required for the synthesis of viral RNA, binds viral RNA elements and host factors to form a viral replication complex at the host cellular membrane. The coat protein (CP) and three viral movement proteins (TGB1, TGB2, and TGB3) have critical roles in mediating cell-to-cell viral movement through plasmodesmata by virion formation or by nonvirion ribonucleoprotein (RNP) complex formation with viral movement proteins (TGBs). The RNP complex, like TGB1-CP-viral RNA, is associated with viral replicase and used for immediate reinitiation of viral replication in newly invaded cells. Higher plants have defense mechanisms against potexviruses such as Rx-mediated resistance and RNA silencing. The CP acts as an avirulence effector for plant defense mechanisms, while TGB1 functions as a viral suppressor of RNA silencing, which is the mechanism of innate immune resistance. Here, we describe recent findings concerning the involvement of viral and host factors in potexvirus replication and in antiviral responses to potexvirus infection.

Barley stripe mosaic virus: structure and relationship to the tobamoviruses

Barley stripe mosaic virus (BSMV) is the type member of the genus Hordeivirus, rigid, rod-shaped viruses in the family Virgaviridae. We have used fiber diffraction and cryo-electron microscopy to determine the helical symmetry of BSMV to be 23.2 subunits per turn of the viral helix, and to obtain a low-resolution model of the virus by helical reconstruction methods. Features in the model support a structural relationship between the coat proteins of the hordeiviruses and the tobamoviruses.

Partially disordered structure in intravirus coat protein of potyvirus potato virus A

Potyviruses represent the most biologically successful group of plant viruses, but to our knowledge, this work is the first detailed study of physicochemical characteristics of potyvirus virions. We measured the UV absorption, far and near UV circular dichroism spectra, intrinsic fluorescence spectra, and differential scanning calorimetry (DSC) melting curves of intact particles of a potato virus A (PVA). PVA virions proved to have a peculiar combination of physicochemical properties. The intravirus coat protein (CP) subunits were shown to contain an unusually high fraction of disordered structures, whereas PVA virions had an almost normal thermal stability. Upon heating from 20 °C to 55 °C, the fraction of disordered structures in the intravirus CP further increased, while PVA virions remained intact at up to 55 °C, after which their disruption (and DSC melting) started. We suggest that the structure of PVA virions below 55 °C is stabilized by interactions between the remaining structured segments of intravirus CP. It is not improbable that the biological efficiency of PVA relies on the disordered structure of intravirus CP.