Maturation of a tetravirus capsid alters the dynamic properties and creates a metastable complex

A catalytic dyad modulates conformational change in the CO2-fixing flavoenzyme 2-ketopropyl coenzyme M oxidoreductase/carboxylase

2-Ketopropyl-coenzyme M oxidoreductase/carboxylase (2-KPCC) is a member of the flavin and cysteine disulfide containing oxidoreductase family (DSOR) that catalyzes the unique reaction between atmospheric CO2 and a ketone/enolate nucleophile to generate acetoacetate. However, the mechanism of this reaction is not well understood. Here, we present evidence that 2-KPCC, in contrast to the well-characterized DSOR enzyme glutathione reductase, undergoes conformational changes during catalysis. Using a suite of biophysical techniques including limited proteolysis, differential scanning fluorimetry, and native mass spectrometry in the presence of substrates and inhibitors, we observed conformational differences between different ligand-bound 2-KPCC species within the catalytic cycle. Analysis of site-specific amino acid variants indicated that 2-KPCC-defining residues, Phe501-His506, within the active site are important for transducing these ligand induced conformational changes. We propose that these conformational changes promote substrate discrimination between H+ and CO2 to favor the metabolically preferred carboxylation product, acetoacetate.

The Application of Porphyrins and Their Analogues for Inactivation of Viruses

The problem of treating viral infections is extremely relevant due to both the emergence of new viral diseases and to the low effectiveness of existing approaches to the treatment of known viral infections. This review focuses on the application of porphyrin, chlorin, and phthalocyanine series for combating viral infections by chemical and photochemical inactivation methods. The purpose of this review paper is to summarize the main approaches developed to date in the chemical and photodynamic inactivation of human and animal viruses using porphyrins and their analogues and to analyze and discuss the information on viral targets and antiviral activity of porphyrins, chlorins, of their conjugates with organic/inorganic compounds obtained in the last 10–15 years in order to identify the most promising areas.

High stability of plant-expressed virus-like particles of an insect virus in artificial gastric and intestinal fluids

The harsh conditions of the gastro-intestinal (GI) milieu pose a major barrier to the oral delivery of protein nanocages. Here we studied the stability of Nudaurelia capensis omega virus (NωV) virus-like particles (VLPs) in simulated GI fluids. NωV VLPs capsids and procapsids were transiently expressed in plants, the VLPs were incubated in various simulated GI fluids and their stability was determined by gel electrophoresis, density gradient ultracentrifugation and transmission electron microscopy (TEM). The results showed that the capsids were highly resistant to simulated gastric fluids at pH ≥ 3. Even under the harshest conditions, which consisted of a pepsin solution at pH 1.2, NωV capsids remained assembled as VLPs, though some digestion of the coat protein occurred. Moreover, 80.8% (±10.2%) stability was measured for NωV capsids upon 4 h incubation in simulated intestinal fluids. The high resistance of this protein cage to digestion and denaturation can be attributed to its distinctively compact structure. The more porous form of the VLPs, the procapsid, was less stable under all conditions. Our results suggest that NωV VLPs capsids are likely to endure transit through the GI tract, designating them as promising candidate protein nanocages for oral drug delivery.

Structural and Proteomic Characterization of the Initiation of Giant Virus Infection

Since their discovery, giant viruses have expanded our understanding of the principles of virology. Due to their gargantuan size and complexity, little is known about the life cycles of these viruses. To answer outstanding questions regarding giant virus infection mechanisms, we set out to determine biomolecular conditions that promote giant virus genome release. We generated four infection intermediates in Samba virus (Mimivirus genus, lineage A) as visualized by cryoelectron microscopy (cryo-EM), cryoelectron tomography (cryo-ET), and scanning electron microscopy (SEM). Each of these four intermediates reflects similar morphology to a stage that occurs in vivo. We show that these genome release stages are conserved in other mimiviruses. Finally, we identified proteins that are released from Samba and newly discovered Tupanvirus through differential mass spectrometry. Our work revealed the molecular forces that trigger infection are conserved among disparate giant viruses. This study is also the first to identify specific proteins released during the initial stages of giant virus infection.

Changes in the stability and biomechanics of P22 bacteriophage capsid during maturation

The capsid of P22 bacteriophage undergoes a series of structural transitions during maturation that guide it from spherical to icosahedral morphology. The transitions include the release of scaffold proteins and capsid expansion. Although P22 maturation has been investigated for decades, a unified model that incorporates thermodynamic and biophysical analyses is not available. A general and specific model of icosahedral capsid maturation is of significant interest to theoreticians searching for fundamental principles as well as virologists and material scientists seeking to alter maturation to their advantage. To address this challenge, we have combined the results from orthogonal biophysical techniques including differential scanning fluorimetry, atomic force microscopy, circular dichroism, and hydrogen-deuterium exchange mass spectrometry. By integrating these results from single particle and population measurements, an energy landscape of P22 maturation from procapsid through expanded shell to wiffle ball emerged, highlighting the role of metastable structures and the thermodynamics guiding maturation. The propagation of weak quaternary interactions across symmetric elements of the capsid is a key component for stability in P22. A surprising finding is that the progression to wiffle ball, which lacks pentamers, shows that chemical and thermal stability can be uncoupled from mechanical rigidity, elegantly demonstrating the complexity inherent in capsid protein interactions and the emergent properties that can arise from icosahedral symmetry. On a broader scale, this work demonstrates the power of applying orthogonal biophysical techniques to elucidate assembly mechanisms for supramolecular complexes and provides a framework within which other viral systems can be compared.

Photodynamic inactivation of bacteriophage MS2: The A-protein is the target of virus inactivation

Singlet oxygen mediated oxidation has been shown to be responsible for photodynamic inactivation (PDI) of viruses in solution with photosensitisers such as 5, 10, 15, 20-tetrakis (1-methyl-4-pyridinio) porphyrin tetra p-toluenesulfonate (TMPyP). The capsids of non-enveloped viruses, such as bacteriophage MS2, are possible targets for viral inactivation by singlet oxygen oxidation. Within the capsid (predominantly composed of coat protein), the A-protein acts as the host recognition and attachment protein. The A-protein has two domains; an α-helix domain and a β-sheet domain. The α-helix domain is attached to the viral RNA genome inside the capsid while the β-sheet domain, which is on the surface of the capsid, is believed to be the site for attachment to the host bacteria pilus during infection. In this study, 4 sequence-specific antibodies were raised against 4 sites on the A-protein. Changes induced by the oxidation of singlet oxygen were compared to the rate of PDI of the virus. Using these antibodies, our results suggest that the rate of PDI is relative to loss of antigenicity of two sites on the A-protein. Our data further showed that PDI caused aggregation of MS2 particles and crosslinking of MS2 coat protein. However, these inter- and intra-capsid changes did not correlate to the rate of PDI we observed in MS2. Possible modes of action are discussed as a means to gaining insight to the targets and mechanisms of PDI of viruses.

Breach: Host Membrane Penetration and Entry by Nonenveloped Viruses

Disruption of host membranes by nonenveloped viruses, which allows the nucleocapsid or genome to enter the cytosol, is a mechanistically diverse process. Although the membrane-penetrating agents are usually small, hydrophobic or amphipathic peptides deployed from the capsid interior during entry, their manner of membrane interaction varies substantially. In this review, we discuss recent data about the molecular pathways for externalization of viral peptides amidst conformational alterations in the capsid, as well as mechanisms of membrane penetration, which is influenced by structural features of the peptides themselves as well as physicochemical properties of membranes, and other host factors. The membrane-penetrating components of nonenveloped viruses constitute an interesting class of cell-penetrating peptides, and may have potential therapeutic value for gene transfer.

Hydrophobic pocket targeting probes for enteroviruses

Visualization and tracking of viruses without compromising their functionality is crucial in order to understand virus targeting to cells and tissues, and to understand the subsequent subcellular steps leading to virus uncoating and replication. Enteroviruses are important human pathogens causing a vast number of acute infections, and are also suggested to contribute to the development of chronic diseases like type I diabetes. Here, we demonstrate a novel method to target site-specifically the hydrophobic pocket of enteroviruses. A probe, a derivative of Pleconaril, was developed and conjugated to various labels that enabled the visualization of enteroviruses under light and electron microscopes. The probe mildly stabilized the virus particle by increasing the melting temperature by 1-3 degrees, and caused a delay in the uncoating of the virus in the cellular endosomes, but could not however inhibit the receptor binding, cellular entry or infectivity of the virus. The hydrophobic pocket binding moiety of the probe was shown to bind to echovirus 1 particle by STD and tr-NOESY NMR methods. Furthermore, binding to echovirus 1 and Coxsackievirus A9, and to a lesser extent to Coxsackie virus B3 was verified by using a gold nanocluster labeled probe by TEM analysis. Molecular modelling suggested that the probe fits the hydrophobic pockets of EV1 and CVA9, but not of CVB3 as expected, correlating well with the variations in the infectivity and stability of the virus particles. EV1 conjugated to the fluorescent dye labeled probe was efficiently internalized into the cells. The virus-fluorescent probe conjugate accumulated in the cytoplasmic endosomes and caused infection starting from 6 hours onwards. Remarkably, before and during the time of replication, the fluorescent probe was seen to leak from the virus-positive endosomes and thus separate from the capsid proteins that were left in the endosomes. These results suggest that, like the physiological hydrophobic content, the probe may be released upon virus uncoating. Our results collectively thus show that the gold and fluorescently labeled probes may be used to track and visualize the studied enteroviruses during the early phases of infection opening new avenues to follow virus uncoating in cells.

An encapsidated viral protein and its role in RNA packaging by a non-enveloped animal RNA virus

Alphatetraviruses are small (+) ssRNA viruses with non-enveloped, icosahedral, T=4 particles that assemble from 240 copies of a single capsid protein precursor. This study is focused on the mechanisms underlying selection and packaging of genomic vRNAs by Helicoverpa armigera stunt virus. We demonstrate that the viral protein, p17, is packaged at low levels (between 4 and 8 copies per capsid) raising the possibility of icosahedral asymmetry in wild-type particles. p17 promotes packaging of vRNA2 by virus-like particles (VLPs) generated from plasmid-expressed vRNA2. The 5' and 3' UTRs of RNA2 are not required for encapsidation. VLPs produced by recombinant baculoviruses package vRNA2 at detectable levels even in the absence of p17 and apparently excluding baculoviral transcripts. This suggests a role for p17 in vRNA selectivity. This is one of few examples of the packaging of a minor non-structural protein by (+) ssRNA animal viruses.

Comparative analysis of adeno-associated virus capsid stability and dynamics

Icosahedral viral capsids are obligated to perform a thermodynamic balancing act. Capsids must be stable enough to protect the genome until a suitable host cell is encountered yet be poised to bind receptor, initiate cell entry, navigate the cellular milieu, and release their genome in the appropriate replication compartment. In this study, serotypes of adeno-associated virus (AAV), AAV1, AAV2, AAV5, and AAV8, were compared with respect to the physical properties of their capsids that influence thermodynamic stability. Thermal stability measurements using differential scanning fluorimetry, differential scanning calorimetry, and electron microscopy showed that capsid melting temperatures differed by more than 20°C between the least and most stable serotypes, AAV2 and AAV5, respectively. Limited proteolysis and peptide mass mapping of intact particles were used to investigate capsid protein dynamics. Active hot spots mapped to the region surrounding the 3-fold axis of symmetry for all serotypes. Cleavages also mapped to the unique region of VP1 which contains a phospholipase domain, indicating transient exposure on the surface of the capsid. Data on the biophysical properties of the different AAV serotypes are important for understanding cellular trafficking and is critical to their production, storage, and use for gene therapy. The distinct differences reported here provide direction for future studies on entry and vector production.

Newly discovered insect RNA viruses in China

Insects are a group of arthropods and the largest group of animals on Earth, with over one million species described to date. Like other life forms, insects suffer from viruses that cause disease and death. Viruses that are pathogenic to beneficial insects cause dramatic economic losses on agriculture. In contrast, viruses that are pathogenic to insect pests can be exploited as attractive biological control agents. All of these factors have led to an explosion in the amount of research into insect viruses in recent years, generating impressive quantities of information on the molecular and cellular biology of these viruses. Due to the wide variety of insect viruses, a better understanding of these viruses will expand our overall knowledge of their virology. Here, we review studies of several newly discovered RNA insect viruses in China.

Epitope-distal effects accompany the binding of two distinct antibodies to hepatitis B virus capsids

Infection of humans by hepatitis B virus (HBV) induces the copious production of antibodies directed against the capsid protein (Cp). A large variety of anticapsid antibodies have been identified that differ in their epitopes. These data, and the status of the capsid as a major clinical antigen, motivate studies to achieve a more detailed understanding of their interactions. In this study, we focused on the Fab fragments of two monoclonal antibodies, E1 and 3120. E1 has been shown to bind to the side of outward-protruding spikes whereas 3120 binds to the "floor" region of the capsid, between spikes. We used hydrogen-deuterium exchange coupled to mass spectrometry (HDX-MS) to investigate the effects on HBV capsids of binding these antibodies. Conventionally, capsids loaded with saturating amounts of Fabs would be too massive to be readily amenable to HDX-MS. However, by focusing on the Cp protein, we were able to acquire deuterium uptake profiles covering the entire 149-residue sequence and reveal, in localized detail, changes in H/D exchange rates accompanying antibody binding. We find increased protection of the known E1 and 3120 epitopes on the capsid upon binding and show that regions distant from the epitopes are also affected. In particular, the α2a helix (residues 24-34) and the mobile C-terminus (residues 141-149) become substantially less solvent-exposed. Our data indicate that even at substoichiometric antibody binding an overall increase in the rigidity of the capsid is elicited, as well as a general dampening of its breathing motions.

Dynamics in cryo EM reconstructions visualized with maximum-likelihood derived variance maps

CryoEM data capture the dynamic character associated with biological macromolecular assemblies by preserving the various conformations of the individual specimens at the moment of flash freezing. Regions of high variation in the data set are apparent in the image reconstruction due to the poor density that results from the lack of superposition of these regions. These observations are qualitative and, to date, only preliminary efforts have been made to quantitate the heterogeneity in the ensemble of particles that are individually imaged. We developed and tested a quantitative method for simultaneously computing a reconstruction of the particle and a map of the space-varying heterogeneity of the particle based on an entire data set. The method uses a maximum likelihood algorithm that explicitly takes into account the continuous variability from one instance to another instance of the particle. The result describes the heterogeneity of the particle as a variance to be plotted at every voxel of the reconstructed density. The test, employing time resolved data sets of virus maturation, not only recapitulated local variations obtained with difference map analysis, but revealed a remarkable time dependent reduction in the overall particle dynamics that was unobservable with classical methods of analysis.

Assembly, stability and dynamics of virus capsids

Most viruses use a hollow protein shell, the capsid, to enclose the viral genome. Virus capsids are large, symmetric oligomers made of many copies of one or a few types of protein subunits. Self-assembly of a viral capsid is a complex oligomerization process that proceeds along a pathway regulated by ordered interactions between the participating protein subunits, and that involves a series of (usually transient) assembly intermediates. Assembly of many virus capsids requires the assistance of scaffolding proteins or the viral nucleic acid, which interact with the capsid subunits to promote and direct the process. Once assembled, many capsids undergo a maturation reaction that involves covalent modification and/or conformational rearrangements, which may increase the stability of the particle. The final, mature capsid is a relatively robust protein complex able to protect the viral genome from physicochemical aggressions; however, it is also a metastable, dynamic structure poised to undergo controlled conformational transitions required to perform biologically critical functions during virus entry into cells, intracellular trafficking, and viral genome uncoating. This article provides an updated general overview on structural, biophysical and biochemical aspects of the assembly, stability and dynamics of virus capsids.

Identification and characterization of RNA duplex unwinding and ATPase activities of an alphatetravirus superfamily 1 helicase

Dendrolimus punctatus tetravirus (DpTV) belongs to the genus omegatetravirus of the Alphatetraviridae family. Sequence analysis predicts that DpTV replicase contains a putative helicase domain (Hel). However, the helicase activity in alphatetraviruses has never been formally determined. In this study, we determined that DpTV Hel is a functional RNA helicase belonging to superfamily-1 helicase with 5′–3′ dsRNA unwinding directionality. Further characterization determined the length requirement of the 5′ single-stranded tail on the RNA template and the optimal reaction conditions for the unwinding activity of DpTV Hel. Moreover, DpTV Hel also contains NTPase activity. The ATPase activity of DpTV Hel could be significantly stimulated by dsRNA, and dsRNA could partially rescue the ATPase activity abolishment caused by mutations. Our study is the first to identify an alphatetravirus RNA helicase and further characterize its dsRNA unwinding and NTPase activities in detail and should foster our understanding of DpTV and other alphatetraviruses.

Dissecting quasi-equivalence in nonenveloped viruses: membrane disruption is promoted by lytic peptides released from subunit pentamers, not hexamers

Nonenveloped viruses often invade membranes by exposing hydrophobic or amphipathic peptides generated by a proteolytic maturation step that leaves a lytic peptide noncovalently associated with the viral capsid. Since multiple copies of the same protein form many nonenveloped virus capsids, it is unclear if lytic peptides derived from subunits occupying different positions in a quasi-equivalent icosahedral capsid play different roles in host infection. We addressed this question with Nudaurelia capensis omega virus (NωV), an insect RNA virus with an icosahedral capsid formed by protein α, which undergoes autocleavage during maturation, producing the lytic γ peptide. NωV is a unique model because autocatalysis can be precisely initiated in vitro and is sufficiently slow to correlate lytic activity with γ peptide production. Using liposome-based assays, we observed that autocatalysis is essential for the potent membrane disruption caused by NωV. We observed that lytic activity is acquired rapidly during the maturation program, reaching 100% activity with less than 50% of the subunits cleaved. Previous time-resolved structural studies of partially mature NωV particles showed that, during this time frame, γ peptides derived from the pentamer subunits are produced and are organized in a vertical helical bundle that is projected toward the particle surface, while identical polypeptides in quasi-equivalent subunits are produced later or are in positions inappropriate for release. Our functional data provide experimental support for the hypothesis that pentamers containing a central helical bundle, observed in different nonenveloped virus families, are a specialized lytic motif.

Structure of the Fab-labeled "breathing" state of native poliovirus

At 37°C, the structure of poliovirus is dynamic, and internal polypeptides VP4 and N terminus of VP1 (residues 1 to 53) externalize reversibly. An Fab fragment of a monospecific antibody, which binds to residues 39 to 55 of VP1, was utilized to locate the N termini of VP1 in native (160S) particles in this "breathing" state. Fab and virus were mixed and imaged via cryogenic electron microscopy. The resulting reconstruction showed the capsid expands similarly to the irreversibly altered cell entry intermediate (135S) particle, but the N terminus of VP1 is located near the 2-fold axes, instead of the "propeller tip" as in 135S particles.

Engineered biological entities for drug delivery and gene therapy protein nanoparticles

The development of genetic engineering techniques has speeded up the growth of the biotechnological industry, resulting in a significant increase in the number of recombinant protein products on the market. The deep knowledge of protein function, structure, biological interactions, and the possibility to design new polypeptides with desired biological activities have been the main factors involved in the increase of intensive research and preclinical and clinical approaches. Consequently, new biological entities with added value for innovative medicines such as increased stability, improved targeting, and reduced toxicity, among others have been obtained. Proteins are complex nanoparticles with sizes ranging from a few nanometers to a few hundred nanometers when complex supramolecular interactions occur, as for example, in viral capsids. However, even though protein production is a delicate process that imposes the use of sophisticated analytical methods and negative secondary effects have been detected in some cases as immune and inflammatory reactions, the great potential of biodegradable and tunable protein nanoparticles indicates that protein-based biotechnological products are expected to increase in the years to come.

Autoproteolytic activity derived from the infectious bursal disease virus capsid protein

Viral capsids are envisioned as vehicles to deliver the viral genome to the host cell. They are nonetheless dynamic protective shells, as they participate in numerous processes of the virus cycle such as assembly, genome packaging, binding to receptors, and uncoating among others. In so doing, they undergo large scale conformational changes. Capsid proteins with essential enzymatic activities are being described more frequently. Here we show that the precursor (pVP2) of the capsid protein VP2 of the infectious bursal disease virus (IBDV), an avian double-stranded RNA virus, has autoproteolytic activity. The pVP2 C-terminal region is first processed by the viral protease VP4. VP2 Asp-431, lying in a flexible loop preceding the C-terminal most alpha-helix, is responsible for the endopeptidase activity that cleaves the Ala-441-Phe-442 bond to generate the mature VP2 polypeptide. The D431N substitution abrogates the endopeptidase activity without introducing a significant conformational change, as deduced from the three-dimensional structure of the mutant protein at 3.1 A resolution. Combinations of VP2 polypeptides containing mutations affecting either the cleavage or the catalytic site revealed that pVP2 proteolytic processing is the result of a monomolecular cis-cleavage reaction. The D431N mutation does not affect the assembly of the VP2 trimers that constitute the capsid building block. Although VP2 D431N trimers are capable of assembling both pentamers and hexamers, expression of a polyprotein gene harboring the D431N mutation does not result in the assembly of IBDV virus-like particles. Reverse genetics analyses demonstrate that pVP2 self-processing is essential for the assembly of an infectious IBDV progeny.

Chemical modification of viruses and virus-like particles

Protein capsids derived from viruses may be modified by methods, generated, isolated, and purified on large scales with relative ease. In recent years, methods for their chemical derivatization have been employed to broaden the properties and functions accessible to investigators desiring monodisperse, atomic-resolution structures on the nanometer scale. Here we review the reactions and methods used in these endeavors, including the modification of lysine, cysteine, and tyrosine side chains, as well as the installation of unnatural amino acids, with particular attention to the special challenges imposed by the polyvalency and size of virus-based scaffolds.

Multi-disciplinary studies of viruses: the role of structure in shaping the questions and answers

This contribution to the 50th anniversary issue of the Journal of Structural Biology traces a path in which the author evolved from seeing macromolecular structure as end in it self to a means of organizing and correlating data from many sources. The author looks at where we have been and where we are going in this enterprise and the role that structure plays in defining ever more ambitious biological questions and testing and refining models that incorporate data from many techniques. In this, essentially, personal account, the author reflects on 35 years of structural virology and the stages experienced; from "stand alone" crystallography of virus particles to the study of virus assembly and maturation in vitro and eventually into the entire virus infection process from particle cell entry to egress. In the process data from many sources were incorporated into reasonable and testable models based on structures ranging in resolution from near-atomic determined by crystallography, to nanometer, determined by electron cryo-microscopy and image reconstruction, to five nanometer tomographic studies in the cell. The technological development over this period, for structural studies at all resolutions and functional studies that were unimaginable three decades ago, has been astonishing. Here we look at an aspect of this development applied to virology.

Plant viruses as biotemplates for materials and their use in nanotechnology

In recent years, plant virus capsids, the protein shells that form the surface of a typical plant virus particle, have emerged as useful biotemplates for material synthesis. All virus capsids are assembled from virus-coded protein subunits. Many plant viruses assemble capsids with precise 3D structures providing nanoscale architectures that are highly homogeneous and can be produced in large quantities. Capsids are amenable to both genetic and chemical modifications allowing new functions to be incorporated into their structure by design. The three capsid surfaces, the interior surface, the exterior surface, or the interface between coat protein subunits, can be independently functionalized to produce multifunctional biotemplates. In this review, we examine the recent advances in using plant virus capsids as biotemplates for nanomaterials and their potential for applications in nanotechnology, especially medicine.

Conformational equilibria and rates of localized motion within hepatitis B virus capsids

Functional analysis of hepatitis B virus (HBV) core particles has associated a number of biological roles with the C terminus of the capsid protein. One set of functions require the C terminus to be on the exterior of the capsid, while others place this domain on the interior. According to the crystal structure of the capsid, this segment is strictly internal to the capsid shell and buried at a protein-protein interface. Using kinetic hydrolysis, a form of protease digestion assayed by SDS-PAGE and mass spectrometry, the structurally and biologically important C-terminal region of HBV capsid protein assembly domain (Cp149, residues 1-149) has been shown to be dynamic in both dimer and capsid forms. HBV is an enveloped virus with a T=4 icosahedral core that is composed of 120 copies of a homodimer capsid protein. Free dimer and assembled capsid forms of the protein are readily hydrolyzed by trypsin and thermolysin, around residues 127-128, indicating that this region is dynamic and exposed to the capsid surface. The measured conformational equilibria have an opposite temperature dependence between free dimer and assembled capsid. This work helps to explain the previously described allosteric regulation of assembly and functional properties of a buried domain. These observations make a critical connection between structure, dynamics, and function: made possible by the first quantitative measurements of conformational equilibria and rates of conversion between protein conformers for a megaDalton complex.

Delivery methods for peptide and protein toxins in insect control

Since the introduction of DDT in the 1940s, arthropod pest control has relied heavily upon chemical insecticides. However, the development of insect resistance, an increased awareness of the real and perceived environmental and health impacts of these chemicals, and the need for systems with a smaller environmental footprint has stimulated the search for new insecticidal compounds, novel molecular targets, and alternative control methods. In recent decades a variety of biocontrol methods employing peptidic or proteinaceous insect-specific toxins derived from microbes, plants and animals have been examined in the laboratory and field with varying results. Among the many interdependent factors involved with the production of a cost-effective pesticide--production expense, kill efficiency, environmental persistence, pest-specificity, pest resistance-development, public perception and ease of delivery--sprayable biopesticides have not yet found equal competitive footing with chemical counterparts. However, while protein/peptide-based biopesticides continue to have limitations, advances in the technology, particularly of genetically modified organisms as biopesticidal delivery systems, has continually progressed. This review highlights the varieties of delivery methods currently practiced, examining the strengths and weaknesses of each method.

Surface display of an internal His-tag on virus-like particles of Nudaurelia capensis ω virus (NωV) produced in a baculovirus expression system

Nudaurelia capensis omega virus (NomegaV) is a member of the Tetraviridae, a family of small, icosahedral, non-enveloped, (+) sense single-stranded RNA insect viruses with T = 4 symmetry. NomegaV virus-like particles (VLPs), which are morphologically indistinguishable from native virions and capable of packaging heterologous RNA, may be produced in the baculovirus expression system. As a first step towards manipulating the tropism of tetraviral nanoparticles (Capsivectors), a (His)6-tag was inserted into the GH loop (between Ala 378 and Gly 379) of the surface-exposed Ig-like domain of NomegaV capsid protein (p70). His-tagged p70 produced in a baculovirus expression system self-assembled into omegaHis VLPs that exhibited similar morphological and RNA encapsidation properties as wild-type NomegaV VLPs produced in the same system. Two assays using paramagnetic pre-charged nickel beads confirmed that multiple affinity tags were present on the surface of omegaHis VLPs and were capable of binding. These results indicate that the GH loop is a suitable site for the retargeting of NomegaV particles for potential biotechnological applications.

Viruses: making friends with old foes

The study of viruses has traditionally focused on their roles as infectious agents and as tools for understanding cell biology. Viruses are now finding a new expanded role as nanoplatforms with applications in materials science and medicine. Viruses form highly symmetrical monodisperse architectures and are ideal templates for engineering multifunctionality, including multivalent display of surface ligands and encapsulation of inorganic and organic materials. These developments assure that viruses will find applications as versatile nanoscale materials.

Small RNA Viruses of Insects: Expression in Plants and RNA Silencing

Interest in insect small RNA viruses (SRVs) has grown slowly but steadily. A number of new viruses have been analyzed at the sequence level, adding to our knowledge of their diversity at the level of both individual virus species and families. In particular, a number of possible new virus families have emerged. This research has largely been driven by interest in their potential for pest control, as well as in their importance as the causal agents of disease in beneficial arthropods. At the same time, research into known viruses has made valuable contributions to our understanding of an emerging new field of central importance to molecular biology-the existence of RNA-based gene silencing, developmental control, and adaptive immune systems in eukaryotes. Subject to RNA-based adaptive immune responses in their hosts, viruses have evolved a variety of genes encoding proteins capable of suppressing the immune response. Such genes were first identified in plant viruses, but the first examples known from animal viruses were identified in insect RNA viruses. This chapter will address the diversity of insect SRVs, and attempts to harness their simplicity in the engineering of transgenic plants expressing viruses for resistance to insect pests. We also describe RNA interference and antiviral pathways identified in plants and animals, how they have led viruses to evolve genes capable of suppressing such adaptive immunity, and the problems presented by these pathways for the strategy of expressing viruses in transgenic plants. Approaches for countering these problems are also discussed.