The refined crystal structure of cowpea mosaic virus at 2.8 A resolution

Crystal Structure and Proteomics Analysis of Empty Virus-like Particles of Cowpea Mosaic Virus

Empty virus-like particles (eVLPs) of Cowpea mosaic virus (CPMV) are currently being utilized as reagents in various biomedical and nanotechnology applications. Here, we report the crystal structure of CPMV eVLPs determined using X-ray crystallography at 2.3 Å resolution and compare it with previously reported cryo-electron microscopy (cryo-EM) of eVLPs and virion crystal structures. Although the X-ray and cryo-EM structures of eVLPs are mostly similar, there exist significant differences at the C terminus of the small (S) subunit. The intact C terminus of the S subunit plays a critical role in enabling the efficient assembly of CPMV virions and eVLPs, but undergoes proteolysis after particle formation. In addition, we report the results of mass spectrometry-based proteomics analysis of coat protein subunits from CPMV eVLPs and virions that identify the C termini of S subunits undergo proteolytic cleavages at multiple sites instead of a single cleavage site as previously observed.

Protein cages and synthetic polymers: a fruitful symbiosis for drug delivery applications, bionanotechnology and materials science

Protein cages have become essential tools in bionanotechnology due to their well-defined, monodisperse, capsule-like structure. Combining them with synthetic polymers greatly expands their application, giving rise to novel nanomaterials fore.g.drug-delivery, sensing, electronic devices and for uses as nanoreactors.

Mechanisms of assembly and genome packaging in an RNA virus revealed by high-resolution cryo-EM

Cowpea mosaic virus is a plant-infecting member of the Picornavirales and is of major interest in the development of biotechnology applications. Despite the availability of >100 crystal structures of Picornavirales capsids, relatively little is known about the mechanisms of capsid assembly and genome encapsidation. Here we have determined cryo-electron microscopy reconstructions for the wild-type virus and an empty virus-like particle, to 3.4 Å and 3.0 Å resolution, respectively, and built de novo atomic models of their capsids. These new structures reveal the C-terminal region of the small coat protein subunit, which is essential for virus assembly and which was missing from previously determined crystal structures, as well as residues that bind to the viral genome. These observations allow us to develop a new model for genome encapsidation and capsid assembly.

Little is known about how the plant-infecting cowpea mosaic virus (CPMV)—an invaluable tool in several biotechnology applications—packages its single-strand RNA genome into the capsid. Here the authors present two high-resolution cryo-EM structures of CPMV and a new model for RNA recognition and capsid assembly.

Tropism of CPMV to Professional Antigen Presenting Cells Enables a Platform to Eliminate Chronic Infections

Chronic viral infections (e.g., HIV, HBV, HCV) represent a significant source of morbidity and mortality with over 500 million people infected worldwide. Dendritic cells (DCs) and macrophages are key cell types for productive viral replication and persistent systemic infection. We demonstrate that the plant virus cowpea mosaic virus (CPMV) displays tropism for such antigen presenting cells in both mice and humans, thus making it an ideal candidate for targeted drug delivery toward viral infections. Furthermore, we show inhibition of a key host protein for viral infection, site-1 protease (S1P), using the small molecule PF-429242 in the model pathogen arenavirus lymphocytic choriomeningitis virus (LCMV) limits viral growth. By packaging PF-429242 in CPMV, we are able to control drug release and efficiently deliver the drug. This sets the groundwork for utilizing the natural tropism of CPMV for a therapeutic approach that specifically targets cell types most commonly subverted by chronic viruses.

Application of the phase extension method in virus crystallography

The procedure for phase extension (PX) involves gradually extending the initial phases from low resolution (e.g., ~8Å) to the high-resolution limit of a diffraction data set. Structural redundancy present in the viral capsids that display icosahedral symmetry results in a high degree of non-crystallographic symmetry (NCS), which in turn translates into higher phasing power and is critical for improving and extending phases to higher resolution. Greater completeness of the diffraction data and determination of a molecular replacement solution, which entails accurately identifying the virus particle orientation(s) and position(s), are important for the smooth progression of the PX procedure. In addition, proper definition of a molecular mask (envelope) around the NCS-asymmetric unit has been found to be important for the success of density modification procedures, such as density averaging and solvent flattening. Regardless of the degree of NCS, the PX method appears to work well in all space groups, provided an accurate molecular mask is used along with reasonable initial phases. However, in the cases with space group P1, in addition to requiring a molecular mask, starting the phase extension at a higher resolution (e.g., 6Å) overcame the previously reported problems due to Babinet phases and phase flipping errors.

Viral nanoparticles, noble metal decorated viruses and their nanoconjugates

Virus-based nanotechnology has generated interest in a number of applications due to the specificity of virus interaction with inorganic and organic nanoparticles. A well-defined structure of virus due to its multifunctional proteinaceous shell (capsid) surrounding genomic material is a promising approach to obtain nanostructured materials. Viruses hold great promise in assembling and interconnecting novel nanosized components, allowing to develop organized nanoparticle assemblies. Due to their size, monodispersity, and variety of chemical groups available for modification, they make a good scaffold for molecular assembly into nanoscale devices. Virus based nanocomposites are useful as an engineering material for the construction of smart nanoobjects because of their ability to associate into desired structures including a number of morphologies. Viruses exhibit the characteristics of an ideal template for the formation of nanoconjugates with noble metal nanoparticles. These bioinspired systems form monodispersed units that are highly amenable through genetic and chemical modifications. As nanoscale assemblies, viruses have sophisticated yet highly ordered structural features, which, in many cases, have been carefully characterized by modern structural biological methods. Plant viruses are increasingly being used for nanobiotechnology purposes because of their relative structural and chemical stability, ease of production, multifunctionality and lack of toxicity and pathogenicity in animals or humans. The multifunctional viruses interact with nanoparticles and other functional additives to the generation of bioconjugates with different properties – possible antiviral and antibacterial activities.

Boosting Humoral Immune Responses to Tumor-associated Carbohydrate Antigens with Virus-like Particles

The development of carbohydrate-based anticancer vaccines is an attractive approach towards the prevention and treatment of cancer. The weak immunogenicity of carbohydrate antigens and tolerance by the immune system are major obstacles towards the design of effective cancer vaccines. Recently, virus-like particles have been shown to be a promising platform to overcome the aforementioned difficulties. In this chapter, we provide an overview on the structural and immunological features of virus-like particles in eliciting anti-carbohydrate antibody responses. The immuno-potentiating activities of several virus-like particle systems are compared, and insights into critical factors of virus-like particles that help shape the anti-carbohydrate responses are discussed.

Silica biomineralization via the self-assembly of helical biomolecules

The biomimetic synthesis of relevant silica materials using biological macromolecules as templates via silica biomineralization processes attract rapidly rising attention toward natural and artificial materials. Biomimetic synthesis studies are useful for improving the understanding of the formation mechanism of the hierarchical structures found in living organisms (such as diatoms and sponges) and for promoting significant developments in the biotechnology, nanotechnology and materials chemistry fields. Chirality is a ubiquitous phenomenon in nature and is an inherent feature of biomolecular components in organisms. Helical biomolecules, one of the most important types of chiral macromolecules, can self-assemble into multiple liquid-crystal structures and be used as biotemplates for silica biomineralization, which renders them particularly useful for fabricating complex silica materials under ambient conditions. Over the past two decades, many new silica materials with hierarchical structures and complex morphologies have been created using helical biomolecules. In this review, the developments in this field are described and the recent progress in silica biomineralization templating using several classes of helical biomolecules, including DNA, polypeptides, cellulose and rod-like viruses is summarized. Particular focus is placed on the formation mechanism of biomolecule-silica materials (BSMs) with hierarchical structures. Finally, current research challenges and future developments are discussed in the conclusion.

Structural insights into magnetic clusters grown inside virus capsids

Magnetic nanoparticles have multiple applications in materials science. In particular, virus capsids have been suggested as promising templates for building up nanometric-sized magnetic clusters by taking advantage of their inner cavity as a nanoreactor. In this study we investigate the magnetization of individual cobalt-filled cowpea mosaic virus empty virus-like particles using atomic force microscopy. We also combine the analysis of the effects of dehydration on the structure of virus particles with a comparison of their magnetic signal to that provided by commercially available magnetic nanoparticles of similar size. These two approaches allow the evaluation of the structure of the metallic cluster grown inside the virus capsid. We conclude that, rather than forming solid clusters, cobalt inside viruses forms a discontinuous structure that does not completely fill the virus cavity and reaches about 10% of its volume.

Self-Assembly in Biosilicification and Biotemplated Silica Materials

During evolution, living organisms have learned to design biomolecules exhibiting self-assembly properties to build-up materials with complex organizations. This is particularly evidenced by the delicate siliceous structures of diatoms and sponges. These structures have been considered as inspiration sources for the preparation of nanoscale and nanostructured silica-based materials templated by the self-assembled natural or biomimetic molecules. These templates range from short peptides to large viruses, leading to biohybrid objects with a wide variety of dimensions, shapes and organization. A more recent strategy based on the integration of biological self-assembly as the driving force of silica nanoparticles organization offers new perspectives to elaborate highly-tunable, biofunctional nanocomposites.

Virus-templated plasmonic nanoclusters with icosahedral symmetry via directed self-assembly

The assembly of plasmonic nanoparticles with precise spatial and orientational order may lead to structures with new electromagnetic properties at optical frequencies. The directed self-assembly method presented controls the interparticle-spacing and symmetry of the resulting nanometer-sized elements in solution. The self-assembly of three-dimensional (3D), icosahedral plasmonic nanosclusters (NCs) with resonances at visible wavelengths is demonstrated experimentally. The ideal NCs consist of twelve gold (Au) nanospheres (NSs) attached to thiol groups at predefined locations on the surface of a genetically engineered cowpea mosaic virus with icosahedral symmetry. In situ dynamic light scattering (DLS) measurements confirm the NSs assembly on the virus. Transmission electron micrographs (TEM) demonstrate the ability of the self-assembly method to control the nanoscopic symmetry of the bound NSs, which reflects the icosahedral symmetry of the virus. Both, TEM and DLS show that the NCs comprise of a distribution of capsids mostly covered (i.e., 6-12 NS/capsid) with NSs. 3D finite-element simulations of aqueous suspensions of NCs reproduce the experimental bulk absorbance measurements and major features of the spectra. Simulations results show that the fully assembled NCs give rise to a 10-fold surface-averaged enhancement of the local electromagnetic field.

Functionalization of protein-based nanocages for drug delivery applications

Traditional drug delivery strategies involve drugs which are not targeted towards the desired tissue. This can lead to undesired side effects, as normal cells are affected by the drugs as well. Therefore, new systems are now being developed which combine targeting functionalities with encapsulation of drug cargo. Protein nanocages are highly promising drug delivery platforms due to their perfectly defined structures, biocompatibility, biodegradability and low toxicity. A variety of protein nanocages have been modified and functionalized for these types of applications. In this review, we aim to give an overview of different types of modifications of protein-based nanocontainers for drug delivery applications.

Structure and shape effects of molecular glue on supramolecular tubulin assemblies

The possibility to arrange biological molecules into ordered nanostructures is an important issue in nano- and biotechnology. Nature offers a wide range of molecular "bricks" (e.g., proteins, oligonucleotides, etc.) that spontaneously assemble into more complex hierarchical systems with unique functionalities. Such molecular building blocks can be also used for the construction of nanomaterials with peculiar properties (e.g., DNA origami). In some cases, molecular glues able to bind biomolecules and to induce their assembly can be used to control the final structure and properties in a convenient way. Here we provide molecular-level description of how molecular glues designed to stick to the surface of microtubules (MTs) can control and transform the α/β-tubulin assembly upon temperature decreasing. By means of all-atom molecular dynamics (MD) simulations, we compared the adhesion to the MT surface of three molecular glues bearing the same guanidinium ion surface adhesive groups, but having different architecture, i.e., linear or dendritic backbone. Our evidence demonstrates that the adhesive properties of the different molecular glues are dependent on the shape they assume in solution. In particular, adhesion data from our MD simulations explain how globular- or linear-like molecular glues respectively stabilize MTs or transform them into a well-defined array of α/β-tubulin rings at 15 °C, where MTs naturally depolymerize. The comprehension of the MT transformation mechanism provides a useful rationale for designing ad hoc molecular glues to obtain ordered protein nanostructures from different biological materials.

Development of cowpea mosaic virus-based vectors for the production of vaccines in plants

Plant viruses are emerging as an attractive alternative to stable genetic transformation for the expression of foreign proteins in plants. The main advantages of using this strategy are that viral genomes are small and easy to manipulate, infection of plants with modified viruses is simpler and quicker than the regeneration of stably transformed plants and the sequence inserted into a virus vector will be highly amplified. One use of these virus expression systems is for vaccine production. Among plant viruses, cowpea mosaic virus makes an ideal candidate for the production of such vaccines because it grows extremely well in host plants, is very stable, and the purification of virus particles, if required, is straightforward. In this article, the authors review the progress made in the development of cowpea mosaic virus-based vectors for vaccine production, making use of two main approaches: epitope presentation and polypeptide expression.

Templated mineralization by charge-modified cowpea mosaic virus

Templated mineralization of virus particles provides routes to narrowly dispersed nanoparticles that are not readily prepared by other means. The templated mineralization of metal or metal oxide on the external surface of wild-type cowpea mosaic virus (CPMV), a plant virus, is facilitated by increasing the external surface negative charge. This is achieved by the chemical modification of surface lysine groups by succinic anhydride. Hence, for example, treatment of charge-modified CPMV succinamate with a 1:2 mixture of iron(II) and iron(III) salts, followed by raising the pH to 10.2, led to the formation of narrowly dispersed, CPMV-templated, magnetite (Fe3O4) nanoparticles.

Genetic engineering and characterization of Cowpea mosaic virus empty virus-like particles

The development of methods for the production of empty Cowpea mosaic virus (CPMV) virus-like particles (VLPs) that are devoid of RNA, eVLPs, has renewed promise in CPMV capsid technologies. The recombinant nature of CPMV eVLP production means that the extent and variety of genetic modifications that may be incorporated into the particles is theoretically much greater than those that can be made to infectious CPMV virions due to restrictions on viral propagation of the latter. Free of the infectious agent, the genomic RNA, these particles are now finding potential uses in vaccine development, in vivo imaging, drug delivery, and other nanotechnology applications that make use of internal loading of the empty particles. Here we describe methods for the genetic modification and production of CPMV eVLPs and describe techniques useful for their characterization.

Advancements in the mechanistic understanding of the copper-catalyzed azide-alkyne cycloaddition

The copper-catalyzed azide-alkyne cycloaddition (CuAAC) is one of the most broadly applicable and easy-to-handle reactions in the arsenal of organic chemistry. However, the mechanistic understanding of this reaction has lagged behind the plethora of its applications for a long time. As reagent mixtures of copper salts and additives are commonly used in CuAAC reactions, the structure of the catalytically active species itself has remained subject to speculation, which can be attributed to the multifaceted aggregation chemistry of copper(I) alkyne and acetylide complexes. Following an introductory section on common catalyst systems in CuAAC reactions, this review will highlight experimental and computational studies from early proposals to very recent and more sophisticated investigations, which deliver more detailed insights into the CuAAC's catalytic cycle and the species involved. As diverging mechanistic views are presented in articles, books and online resources, we intend to present the research efforts in this field during the past decade and finally give an up-to-date picture of the currently accepted dinuclear mechanism of CuAAC. Additionally, we hope to inspire research efforts on the development of molecularly defined copper(I) catalysts with defined structural characteristics, whose main advantage in contrast to the regularly used precatalyst reagent mixtures is twofold: on the one hand, the characteristics of molecularly defined, well soluble catalysts can be tuned according to the particular requirements of the experiment; on the other hand, the understanding of the CuAAC reaction mechanism can be further advanced by kinetic studies and the isolation and characterization of key intermediates.

A programmable fluorescent viral nanoblock: sensing made easy in a single step

Viral nanoblock provides specific positioning of recognition moieties and dye molecules which can be used as a signal-generating element and enhance detection sensitivity. The methods described herein use a 30 nm viral nanoblock to couple a variety of proteins and peptides for the incorporation of recognition elements along with a large number of dye molecules (200). The bioconjugation techniques were adapted and optimized over the years to fabricate nanoparticles that exhibit high fluorescence output while maintaining the selectivity of the target receptors. These complexes can be used for the detection of pathogens and toxins in a single step since both receptor and reporter are in the same viral nanoblock. Its stability and nanometer size allows for its utilization in well-established sensing platforms like microarrays.

Building a viral capsid in the presence of genomic RNA

Virus capsid assembly has traditionally been considered as a process that can be described primarily via self-assembly of the capsid proteins, neglecting interactions with other viral or cellular components. Our recent work on several ssRNA viruses, a major class of viral pathogens containing important human, animal, and plant viruses, has shown that this protein-centric view is too simplistic. Capsid assembly for these viruses relies strongly on a number of cooperative roles played by the genomic RNA. This realization requires a new theoretical framework for the modeling and prediction of the assembly behavior of these viruses. In a seminal paper Zlotnick [J. Mol. Biol. 241, 59 (1994)] laid the foundations for the modeling of capsid assembly as a protein-only self-assembly process, illustrating his approach using the example of a dodecahedral study system. We describe here a generalized framework for modeling assembly that incorporates the regulatory functions provided by cognate protein-nucleic-acid interactions between capsid proteins and segments of the genomic RNA, called packaging signals, into the model. Using the same dodecahedron system we demonstrate, using a Gillespie-type algorithm to deal with the enhanced complexity of the problem instead of a master equation approach, that assembly kinetics and yield strongly depend on the distribution and nature of the packaging signals, highlighting the importance of the crucial roles of the RNA in this process.

A cucumber mosaic virus based expression system for the production of porcine circovirus specific vaccines

Potential porcine circovirus type 2 (PCV2) capsid protein epitopes, suitable for expression on the surface of cucumber mosaic virus (CMV) particles were determined by a thorough analysis of the predicted PCV capsid protein structure. The ab initio protein structure prediction was carried out with fold recognition and threading methods. The putative PCV epitopes were selected on the basis of PCV virion models and integrated into the plant virus coat protein, after amino acid position 131. The recombinants were tested for infectivity and stability on different Nicotiana species and stable recombinant virus particles were purified. The particles were tested for their ability to bind to PCV induced porcine antibodies and used for specific antibody induction in mice and pigs. The results showed that PCV epitopes expressed on the CMV surface were recognized by the porcine antibodies and they were also able to induce PCV specific antibody response. Challenge experiment with PCV2 carried out in immunized pigs showed partial protection against the infection. Based on these results it was concluded that specific antiviral vaccine production for the given pathogen was feasible, offering an inexpensive way for the mass production of such vaccines.

Interior engineering of a viral nanoparticle and its tumor homing properties

The development of multifunctional nanoparticles for medical applications is of growing technological interest. A single formulation containing imaging and/or drug moieties that is also capable of preferential uptake in specific cells would greatly enhance diagnostics and treatments. There is growing interest in plant-derived viral nanoparticles (VNPs) and establishing new platform technologies based on these nanoparticles inspired by nature. Cowpea mosaic virus (CPMV) serves as the standard model for VNPs. Although exterior surface modification is well-known and has been comprehensively studied, little is known of interior modification. Additional functionality conferred by the capability for interior engineering would be of great benefit toward the ultimate goal of targeted drug delivery. Here, we examined the capacity of empty CPMV (eCPMV) particles devoid of RNA to encapsulate a wide variety of molecules. We systematically investigated the conjugation of fluorophores, biotin affinity tags, large molecular weight polymers such as poly(ethylene glycol) (PEG), and various peptides through targeting reactive cysteines displayed selectively on the interior surface. Several methods are described that mutually confirm specific functionalization of the interior. Finally, CPMV and eCPMV were labeled with near-infrared fluorophores and studied side-by-side in vitro and in vivo. Passive tumor targeting via the enhanced permeability and retention effect and optical imaging were confirmed using a preclinical mouse model of colon cancer. The results of our studies lay the foundation for the development of the eCPMV platform in a range of biomedical applications.

Structure of Sputnik, a virophage, at 3.5-Å resolution

"Sputnik" is a dsDNA virus, referred to as a virophage, that is coassembled with Mimivirus in the host amoeba. We have used cryo-EM to produce an electron density map of the icosahedral Sputnik virus at 3.5-Å resolution, sufficient to verify the identity of most amino acids in the capsid proteins and to establish the identity of the pentameric protein forming the fivefold vertices. It was also shown that the virus lacks an internal membrane. The capsid is organized into a T = 27 lattice in which there are 260 trimeric capsomers and 12 pentameric capsomers. The trimeric capsomers consist of three double "jelly-roll" major capsid proteins creating pseudohexameric capsomer symmetry. The pentameric capsomers consist of five single jelly-roll proteins. The release of the genome by displacing one or more of the pentameric capsomers may be the result of a low-pH environment. These results suggest a mechanism of Sputnik DNA ejection that probably also occurs in other big icosahedral double jelly-roll viruses such as Adenovirus. In this study, the near-atomic resolution structure of a virus has been established where crystallization for X-ray crystallography was not feasible.

Protein nanotubes, channels and cages

Proteins are the work-horses of life and excute the essential processes involved in the growth and repair of cells. These roles include all aspects of cell signalling, metabolism and repair that allow living things to exist. They are not only chemical catalysts and machine components, they are also structural components of the cell or organism, capable of self-organisation into strong supramolecular cages, fibres and meshes. How proteins are encoded genetically and how they are sythesised in vivo is now well understood, and for an increasing number of proteins, the relationship between structure and function is known in exquisite detail. The next challenge in bionanoscience is to adapt useful protein systems to build new functional structures. Well-defined natural structures with potential useful shapes are a good starting point. With this in mind, in this chapter we discuss the properties of natural and artificial protein channels, nanotubes and cages with regard to recent progress and potential future applications. Chemistries for attaching together different proteins to form superstructures are considered as well as the difficulties associated with designing complex protein structures ab initio.

Liquid viruses by nanoscale engineering of capsid surfaces

Surface engineering of plant virus capsids via cationization (1) and stoichiometric coupling of a polymer surfactant coronal layer (2) produces a highly concentrated, solvent-free liquid virus at 28 °C. These ionic bionanoconstructs are viscoelastic, retain plant infectivity and can be dispersed in a range of organic solvents for aerosol delivery.

Structures of giant icosahedral eukaryotic dsDNA viruses

In the last twenty years, numerous giant, dsDNA, icosahedral viruses have been discovered and assigned to the nucleocytoplasmic large dsDNA virus (NCLDV) clade. The major capsid proteins of these viruses consist of two consecutive jelly-roll domains, assembled into trimers, with pseudo 6-fold symmetry. The capsomers are assembled into arrays that have either p6 (as in Paramecium bursaria Chlorella virus-1) or p3 symmetry (as in Mimivirus). Most of the NCLDV viruses have a membrane that separates the nucleocapsid from the external capsid.

Guiding plant virus particles to integrin-displaying cells

Viral nanoparticles (VNPs) are structurally regular, highly stable, tunable nanomaterials that can be conveniently produced in high yields. Unmodified VNPs from plants and bacteria generally do not show tissue specificity or high selectivity in binding to or entry into mammalian cells. They are, however, malleable by both genetic and chemical means, making them useful scaffolds for the display of large numbers of cell- and tissue-targeting ligands, imaging moieties, and/or therapeutic agents in a well-defined manner. Capitalizing on this attribute, we modified the genetic sequence of the Cowpea mosaic virus (CPMV) coat protein to display an RGD oligopeptide sequence derived from human adenovirus type 2 (HAdV-2). Concurrently, wild-type CPMV was modified via NHS acylation and Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) chemistry to attach an integrin-binding cyclic RGD peptide. Both types of particles showed strong and selective affinity for several different cancer cell lines that express RGD-binding integrin receptors.

Recent Development in Carbohydrate Based Anti-cancer Vaccines

The development of carbohydrate based anti-cancer vaccines is of high current interests. Herein, the latest development in this exciting field is reviewed. After a general introduction about tumor associated carbohydrate antigens and immune responses, the review is focused on the various strategies that have been developed to enhance the immunogenecity of these antigens. The results from animal studies and clinical trials are presented.

Simultaneous gene transduction and silencing using stimuli-responsive viral/nonviral chimeric nanoparticles

Despite viral vectors' predominant use in clinical trials, due to higher gene delivery efficiency than nonviral counterparts, intrinsic immunogenicity and limited tunability for multi-modal effects are major concerns for their usage in gene therapy. An adeno-associated viral (AAV) particle was shielded with acid-degradable, siRNA-encapsulating polyketal (PK) shell, resulting in core-shell viral/nonviral chimeric nanoparticles (ChNPs). The AAV core of a ChNP is protected from immune responses by the PK shell which also facilitates the intracellular trafficking of the AAV core and efficiently releases the encapsulated siRNA into the cytoplasm. ChNPs led to significantly enhanced gene transduction, compared to unmodified free AAVs, and simultaneous silencing of a target gene, while avoiding inactivation by recognition from the immune system. Furthermore, conjugation of sialic acid (SA) on the surface of ChNPs enabled receptor-mediated targeted gene delivery to CD22-expressing cells. The ChNPs developed in this study combine the advantages of both viral and nonviral vectors and are a promising platform for targeted co-delivery of DNA and siRNA in inducing synergistic therapeutic effects by simultaneous expression and silencing of multiple genes.

Principles of virus structural organization

Viruses, the molecular nanomachines infecting hosts ranging from prokaryotes to eukaryotes, come in different sizes, shapes, and symmetries. Questions such as what principles govern their structural organization, what factors guide their assembly, how these viruses integrate multifarious functions into one unique structure have enamored researchers for years. In the last five decades, following Caspar and Klug's elegant conceptualization of how viruses are constructed, high-resolution structural studies using X-ray crystallography and more recently cryo-EM techniques have provided a wealth of information on structures of a variety of viruses. These studies have significantly -furthered our understanding of the principles that underlie structural organization in viruses. Such an understanding has practical impact in providing a rational basis for the design and development of antiviral strategies. In this chapter, we review principles underlying capsid formation in a variety of viruses, emphasizing the recent developments along with some historical perspective.

Not as they seem

I recount the early influences that directed me toward a career in research and then describe some efforts investigating Cowpea mosaic virus and the satellite RNA of Tobacco ringspot virus. These descriptions have a common theme of surprise, how things often can be not as they are expected to be. Finally, I examine the widely held belief that a plant transgene derived from a distant taxonomic source presents a greater risk than a transgene derived from a closely related plant and contend that this also is a situation in which things may not be as they initially seem.

Viruses as nanomaterials for drug delivery

Virus delivery vectors are one among the many nanomaterials that are being developed as drug delivery materials. This chapter focuses on methods utilizing plant virus nanoparticles (PVNs) synthesized from the Red clover necrotic mosaic virus (RCNMV). A successful vector must be able to effectively carry and subsequently deliver a drug cargo to a specific target. In the case of the PVNs, we describe two types of ways cargo can be loaded within these structures: encapsidation and infusion. Several targeting approaches have been used for PVNs based on bioconjugate chemistry. Herein, examples of such approaches will be given that have been used for RCNMV as well as for other PVNs in the literature. Further, we describe characterization of PVNs, in vitro cell studies that can be used to test the efficacy of a targeting vector, and potential routes for animal administration.

Virus templated metallic nanoparticles

Plant viruses are considered as nanobuilding blocks that can be used as synthons or templates for novel materials. Cowpea mosaic virus (CPMV) particles have been shown to template the fabrication of metallic nanoparticles by an electroless deposition metallization process. Palladium ions were electrostatically bound to the virus capsid and, when reduced, acted as nucleation sites for the subsequent metal deposition from solution. The method, although simple, produced highly monodisperse metallic nanoparticles with a diameter of ca. ≤35 nm. CPMV-templated particles were prepared with cobalt, nickel, iron, platinum, cobalt-platinum and nickel-iron.

Virus hybrids as nanomaterials for biotechnology

The current review describes advances in the field of bionanotechnology in which viruses are used to fabricate nanomaterials. Viruses are introduced as protein cages, scaffolds, and templates for the production of biohybrid nanostructured materials where organic and inorganic molecules are incorporated in a precise and a controlled fashion. Genetic engineering enables the insertion or replacement of selected amino acids on virus capsids for uses from bioconjugation to crystal growth. The variety of nanomaterials generated in rod-like and spherical viruses is highlighted for tobacco mosaic virus (TMV), M13 bacteriophage, cowpea chlorotic mottle virus (CCMV), and cowpea mosaic virus (CPMV). Functional biohybrid nanomaterials find applications in biosensing, memory devices, nanocircuits, light-harvesting systems, and nanobatteries.

Defining criteria for oligomannose immunogens for HIV using icosahedral virus capsid scaffolds

The broadly neutralizing antibody 2G12 recognizes a conserved cluster of high-mannose glycans on the surface envelope spike of HIV, suggesting that the "glycan shield" defense of the virus can be breached and may, under the right circumstances, serve as a vaccine target. In an attempt to recreate features of the glycan shield semisynthetically, oligomannosides were coupled to surface lysines on the icosahedral capsids of bacteriophage Q beta and cowpea mosaic virus (CPMV). The Q beta glycoconjugates, but not CPMV, presented oligomannose clusters that bind the antibody 2G12 with high affinity. However, antibodies against these 2G12 epitopes were not detected in immunized rabbits. Rather, alternative oligomannose epitopes on the conjugates were immunodominant and elicited high titers of anti-mannose antibodies that do not crossreact with the HIV envelope. The results presented reveal important design considerations for a carbohydrate-based vaccine component for HIV.

Redox-active ferrocene-modified Cowpea mosaic virus nanoparticles

A naturally occurring nanoparticle, the plant virus Cowpea mosaic virus, can be decorated with ferrocene derivatives, of various linker lengths with amine and carboxylate groups, on the external surface using a range of conjugation strategies. The multiple, organometallic, redox-active ferrocene moieties on the outer surface of the virus are electrochemically independent with reduction potentials that span a potential window of 0.16 V that are dependent on the site of modification and the nature of the ferrocene derivative. The number of ferrocenes coupled to each virus ranges from about 100 to 240 depending upon the conjugation site and the linker length and these redox active units can provide multielectron reservoirs.

Intravital imaging of embryonic and tumor neovasculature using viral nanoparticles

Viral nanoparticles are a novel class of biomolecular agents that take advantage of the natural circulatory and targeting properties of viruses to allow the development of therapeutics, vaccines and imaging tools. We have developed a multivalent nanoparticle platform based on the cowpea mosaic virus (CPMV) that facilitates particle labeling at high density with fluorescent dyes and other functional groups. Compared with other technologies, CPMV-based viral nanoparticles are particularly suited for long-term intravital vascular imaging because of their biocompatibility and retention in the endothelium with minimal side effects. The stable, long-term labeling of the endothelium allows the identification of vasculature undergoing active remodeling in real time. In this study, we describe the synthesis, purification and fluorescent labeling of CPMV nanoparticles, along with their use for imaging of vascular structure and for intravital vascular mapping in developmental and tumor angiogenesis models. Dye-labeled viral nanoparticles can be synthesized and purified in a single day, and imaging studies can be conducted over hours, days or weeks, depending on the application.