Visualization of protein-nucleic acid interactions in a virus. Refined structure of intact tobacco mosaic virus at 2.9 A resolution by X-ray fiber diffraction

3D Puzzle at the Nanoscale-How do RNA Viruses Self-Assemble their Capsids into Perfectly Ordered Structures

The phenomenon of RNA virus self-organization, first observed in the mid-20th century in tobacco mosaic virus, is the subject of extensive research. Efforts to comprehend this process intensify due to its potential for producing vaccines or antiviral compounds as well as nanocarriers and nanotemplates. However, direct observation of the self-assembly is hindered by its prevalence within infected host cells. One of the approaches involves in vitro and in silico research using model viruses featuring a ssRNA(+) genome enclosed within a capsid made up of a single type protein. While various pathways are proposed based on these studies, their relevance in vivo remains uncertain. On the other hand, the development of advanced microscopic methods provide insights into the events within living cells, where following viral infection, specialized compartments form to facilitate the creation of nascent virions. Intriguingly, a growing body of evidence indicates that the primary function of packaging signals in viral RNA is to effectively initiate the virion self-assembly. This is in contrast to earlier opinions suggesting a role in marking RNA for encapsidation. Another noteworthy observation is that many viruses undergo self-assembly within membraneless liquid organelles, which are specifically induced by viral proteins.

A Combination of Flexible Modified Plant Virus Nanoparticles Enables Additive Effects Resulting in Improved Osteogenesis

Plant virus nanoparticles (VNPs) genetically engineered to present osteogenic cues provide a promising method for biofunctionalizing hydrogels in bone tissue engineering. Flexible Potato virus X (PVX) nanoparticles substantially enhance the attachment and differentiation of human mesenchymal stem cells (hMSCs) by presenting the RGD motif, hydroxyapatite-binding peptide (HABP), or consecutive polyglutamates (E8) in a concentration-dependent manner. Therefore, it is hypothesized that Tobacco mosaic virus nanoparticles, which present 1.6 times more functional peptides than PVX, will meliorate such an impact. This study hypothesizes that cultivating hMSCs on a surface coated with a combination of two VNPs presenting peptides for either cell attachment or mineralization can achieve additionally enhancing effects on osteogenesis. Calcium minerals deposited by differentiating hMSCs increases two to threefold for this combination, while the Alkaline Phosphatase activity of hMSCs grown on the PVX-RGD/PVX-HABP-coated surface significantly surpasses any other VNP combination. Superior additive effects are observed for the first time by employing a combination of VNPs with varying functionalities. It is found that the flexible VNP geometry plays a more critical role than the concentration of functional peptides. In conclusion, various peptide-presenting plant VNPs exhibit an additive enhancing effect offering significant potential for effectively functionalizing cell-containing hydrogels in bone tissue engineering.

Getting Hold of the Tobamovirus Particle-Why and How? Purification Routes over Time and a New Customizable Approach

This article develops a multi-perspective view on motivations and methods for tobamovirus purification through the ages and presents a novel, efficient, easy-to-use approach that can be well-adapted to different species of native and functionalized virions. We survey the various driving forces prompting researchers to enrich tobamoviruses, from the search for the causative agents of mosaic diseases in plants to their increasing recognition as versatile nanocarriers in biomedical and engineering applications. The best practices and rarely applied options for the serial processing steps required for successful isolation of tobamoviruses are then reviewed. Adaptations for distinct particle species, pitfalls, and 'forgotten' or underrepresented technologies are considered as well. The article is topped off with our own development of a method for virion preparation, rooted in historical protocols. It combines selective re-solubilization of polyethylene glycol (PEG) virion raw precipitates with density step gradient centrifugation in biocompatible iodixanol formulations, yielding ready-to-use particle suspensions. This newly established protocol and some considerations for perhaps worthwhile further developments could serve as putative stepping stones towards preparation procedures appropriate for routine practical uses of these multivalent soft-matter nanorods.

Structural Insights into Plant Viruses Revealed by Small-Angle X-ray Scattering and Atomic Force Microscopy

The structural study of plant viruses is of great importance to reduce the damage caused by these agricultural pathogens and to support their biotechnological applications. Nowadays, X-ray crystallography, NMR spectroscopy and cryo-electron microscopy are well accepted methods to obtain the 3D protein structure with the best resolution. However, for large and complex supramolecular structures such as plant viruses, especially flexible filamentous ones, there are a number of technical limitations to resolving their native structure in solution. In addition, they do not allow us to obtain structural information about dynamics and interactions with physiological partners. For these purposes, small-angle X-ray scattering (SAXS) and atomic force microscopy (AFM) are well established. In this review, we have outlined the main principles of these two methods and demonstrated their advantages for structural studies of plant viruses of different shapes with relatively high spatial resolution. In addition, we have demonstrated the ability of AFM to obtain information on the mechanical properties of the virus particles that are inaccessible to other experimental techniques. We believe that these under-appreciated approaches, especially when used in combination, are valuable tools for studying a wide variety of helical plant viruses, many of which cannot be resolved by classical structural methods.

Soft Sub-Structured Multi-Material Biosensor Hydrogels with Enzymes Retained by Plant Viral Scaffolds

An all-soft multi-material combination consisting of a hydrogel based on poly(ethylene glycol) (PEG) coated with spatially defined spots of gelatin methacryloyl (GM) containing selectively addressable viral nanorods is presented, and its basic application as a qualitative biosensor with reporter enzymes displayed on the tobacco mosaic virus (TMV) bioscaffolds within the GM is demonstrated. Biologically inert PEG supports are equipped with GM spots serving as biological matrix for enzymes clustered on TMV particles preventing diffusion out of the gel. For this multi-material combination, i) the PEG-based hydrogel surface is modified to achieve a clear boundary between coated and non-coated regions by introducing either isothiouronium or thiol groups. ii) Cross-linking of the GM spots is studied to achieve anchoring to the hydrogel surface. iii) The enzymes horseradish peroxidase or penicillinase (Pen) are conjugated to TMV and integrated into the GM matrix. In contrast to free enzymes, enzyme-decorated TMVs persist in GM spots and show sustained enzyme activity as evidenced by specific color reaction after 7 days of washing, and for Pen after 22 months after dry storage. Therefore, the integration of enzyme-coupled TMV into hydrogel matrices is a promising and versatile approach to obtaining reusable and analyte-specific sensor components.

Convenient site-selective protein coupling from bacterial raw lysates to coenzyme A-modified tobacco mosaic virus (TMV) by Bacillus subtilis Sfp phosphopantetheinyl transferase

A facile enzyme-mediated strategy enables site-specific covalent one-step coupling of genetically tagged luciferase molecules to coenzyme A-modified tobacco mosaic virus (TMV-CoA) both in solution and on solid supports. Bacillus subtilis surfactin phosphopantetheinyl transferase Sfp produced in E. coli mediated the conjugation of firefly luciferase N-terminally extended by eleven amino acids forming a 'ybbR tag' as Sfp-selective substrate, which even worked in bacterial raw lysates. The enzymes displayed on the protein coat of the TMV nanocarriers exhibited high activity. As TMV has proven a beneficial high surface-area adapter template stabilizing enzymes in different biosensing layouts in recent years, the use of TMV-CoA for fishing ybbR-tagged proteins from complex mixtures might become an advantageous concept for the versatile equipment of miniaturized devices with biologically active proteins. It comes along with new opportunities for immobilizing multiple functionalities on TMV adapter coatings, as desired, e.g., in handheld systems for point-of-care detection.

Ned Seeman and the prediction of amino acid-basepair motifs mediating protein-nucleic acid recognition

Fifty years ago, the first atomic-resolution structure of a nucleic acid double helix, the mini-duplex (ApU)2, revealed details of basepair geometry, stacking, sugar conformation, and backbone torsion angles, thereby superseding earlier models based on x-ray fiber diffraction, including the original DNA double helix proposed by Watson and Crick. Just 3 years later, in 1976, Ned Seeman, John Rosenberg, and Alex Rich leapt from their structures of mini-duplexes and H-bonding motifs between bases in small-molecule structures and transfer RNA to predicting how proteins could sequence specifically recognize double helix nucleic acids. They proposed interactions between amino acid side chains and nucleobases mediated by two hydrogen bonds in the major or minor grooves. One of these, the arginine-guanine pair, emerged as the most favored amino acid-base interaction in experimental structures of protein-nucleic acid complexes determined since 1986. In this brief review we revisit the pioneering work by Seeman et al. and discuss the importance of the arginine-guanine pairing motif.

Designer-length palladium nanowires can be templated by the central channel of tobacco mosaic virus nanorods

We have developed methods for the templated synthesis of palladium nanowires (Pd NWs) within the central channel of tobacco mosaic virus (TMV) nanorods of various lengths. We show that uniform 4 nm diameter Pd NWs can be produced by selective growth within these channels by including the capping reagent, poly(vinyl-pyrrolidone) (PVP30K) and reducing the metal precursor to metallic palladium with ascorbic acid. The length of the Pd NWs can be controlled either by varying the length of the nanorod templates and/or through alterations to the reaction conditions. We have also demonstrated bimetallic gold (Au)-palladium (Pd) in-situ metallization of TMV nanorods resulting in the production of Pd NWs 6 nm gold nanoparticles attached to their ends. The materials produced have many potential applications in the construction of nanoscale devices.

Tobacco Mosaic Virus and the History of Molecular Biology

The history of tobacco mosaic virus (TMV) includes many firsts in science, beginning with its being the first virus identified. This review offers an overview of a history of research on TMV, with an emphasis on its close connections to the emergence and development of molecular biology. Expected final online publication date for the Annual Review of Virology, Volume 9 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Isolation of Tobacco Mosaic Virus-Binding Peptides for Biotechnology Applications

Tobacco mosaic virus (TMV) was the first virus to be discovered and it is now widely used as a tool for biological research and biotechnology applications. TMV particles can be decorated with functional molecules by genetic engineering or bioconjugation. However, this can destabilize the nanoparticles, and/or multiple rounds of modification may be necessary, reducing product yields and preventing the display of certain cargo molecules. To overcome these challenges, we used phage display technology and biopanning to isolate a TMV-binding peptide (TBPT25 ) with strong binding properties (IC50 =0.73 μM, KD =0.16 μM), allowing the display of model cargos via a single mixing step. The TMV-binding peptide is specific for TMV but does not recognize free coat proteins and can therefore be used to decorate intact TMV or detect intact TMV particles in crude plant sap.

Structures of synthetic helical filaments and tubes based on peptide and peptido-mimetic polymers

Synthetic peptide and peptido-mimetic filaments and tubes represent a diverse class of nanomaterials with a broad range of potential applications, such as drug delivery, vaccine development, synthetic catalyst design, encapsulation, and energy transduction. The structures of these filaments comprise supramolecular polymers based on helical arrangements of subunits that can be derived from self-assembly of monomers based on diverse structural motifs. In recent years, structural analyses of these materials at near-atomic resolution (NAR) have yielded critical insights into the relationship between sequence, local conformation, and higher-order structure and morphology. This structural information offers the opportunity for development of new tools to facilitate the predictable and reproduciblede novodesign of synthetic helical filaments. However, these studies have also revealed several significant impediments to the latter process – most notably, the common occurrence of structural polymorphism due to the lability of helical symmetry in structural space. This article summarizes the current state of knowledge on the structures of designed peptide and peptido-mimetic filamentous assemblies, with a focus on structures that have been solved to NAR for which reliable atomic models are available.

Autoxidation Products of the Methanolic Extract of the Leaves of Combretum micranthum Exert Antiviral Activity against Tomato Brown Rugose Fruit Virus (ToBRFV)

Tomato brown rugose fruit virus (ToBRFV) is a new damaging plant virus of great interest from both an economical and research point of view. ToBRFV is transmitted by contact, remains infective for months, and to-date, no resistant cultivars have been developed. Due to the relevance of this virus, new effective, sustainable, and operator-safe antiviral agents are needed. Thus, 4-hydroxybenzoic acid was identified as the main product of the alkaline autoxidation at high temperature of the methanolic extract of the leaves of C. micranthum, known for antiviral activity. The autoxidized extract and 4-hydroxybenzoic acid were assayed in in vitro experiments, in combination with a mechanical inoculation test of tomato plants. Catechinic acid, a common product of rearrangement of catechins in hot alkaline solution, was also tested. Degradation of the viral particles, evidenced by the absence of detectable ToBRFV RNA and the loss of virus infectivity, as a possible consequence of disassembly of the virus coat protein (CP), were shown. Homology modeling was then applied to prepare the protein model of ToBRFV CP, and its structure was optimized. Molecular docking simulation showed the interactions of the two compounds, with the amino acid residues responsible for CP-CP interactions. Catechinic acid showed the best binding energy value in comparison with ribavirin, an anti-tobamovirus agent.

Advances and insights in the diagnosis of viral infections

Viral infections are the most common among diseases that globally require around 60 percent of medical care. However, in the heat of the pandemic, there was a lack of medical equipment and inpatient facilities to provide all patients with viral infections. The detection of viral infections is possible in three general ways such as (i) direct virus detection, which is performed immediately 1-3 days after the infection, (ii) determination of antibodies against some virus proteins mainly observed during/after virus incubation period, (iii) detection of virus-induced disease when specific tissue changes in the organism. This review surveys some global pandemics from 1889 to 2020, virus types, which induced these pandemics, and symptoms of some viral diseases. Non-analytical methods such as radiology and microscopy also are overviewed. This review overlooks molecular analysis methods such as nucleic acid amplification, antibody-antigen complex determination, CRISPR-Cas system-based viral genome determination methods. Methods widely used in the certificated diagnostic laboratory for SARS-CoV-2, Influenza A, B, C, HIV, and other viruses during a viral pandemic are outlined. A comprehensive overview of molecular analytical methods has shown that the assay's sensitivity, accuracy, and suitability for virus detection depends on the choice of the number of regions in the viral open reading frame (ORF) genome sequence and the validity of the selected analytical method.

Structure of filamentous viruses

Despite filamentous viruses represent an important portion of the universe of viruses, their 3D structures available are quite limited, particularly if compared to the large number of structures of icosahedral viruses present in the Protein Data Bank. As a matter of fact, flexible filamentous viruses cannot be grown as single crystals and past structural studies have mostly been limited to X-ray fiber diffraction or to the determination of the structure of isolated viral proteins. Only very recently, several structures of filamentous viruses have become available, owing to the recent development of cryo-electron microscopy. This technique has given a strong impulse to the field and has allowed the building of reliable molecular models of entire viruses, in some cases at a nearly atomic resolution level. In this paper we briefly describe the architecture of filamentous viruses that infect bacteria, archaea, plants and humans. It is easy to foresee that more new structures of filamentous viruses will become available soon and they will allow a better understanding of the rules underlying the structural organization of these organisms so relevant for the life on our planet.

Application of Plant Viruses in Biotechnology, Medicine, and Human Health

Plant-based nanotechnology programs using virus-like particles (VLPs) and virus nanoparticles (VNPs) are emerging platforms that are increasingly used for a variety of applications in biotechnology and medicine. Tobacco mosaic virus (TMV) and potato virus X (PVX), by virtue of having high aspect ratios, make ideal platforms for drug delivery. TMV and PVX both possess rod-shaped structures and single-stranded RNA genomes encapsidated by their respective capsid proteins and have shown great promise as drug delivery systems. Cowpea mosaic virus (CPMV) has an icosahedral structure, and thus brings unique benefits as a nanoparticle. The uses of these three plant viruses as either nanostructures or expression vectors for high value pharmaceutical proteins such as vaccines and antibodies are discussed extensively in the following review. In addition, the potential uses of geminiviruses in medical biotechnology are explored. The uses of these expression vectors in plant biotechnology applications are also discussed. Finally, in this review, we project future prospects for plant viruses in the fields of medicine, human health, prophylaxis, and therapy of human diseases.

Analysis of Engineered Tobacco Mosaic Virus and Potato Virus X Nanoparticles as Carriers for Biocatalysts

Plant virus nanoparticles are promising candidates for the development of novel materials, including nanocomposites and scaffolds/carriers for functional molecules such as enzymes. Their advantages for enzyme immobilization include a modular organization, a robust and programmable structure, and a simple, cost-effective production. However, the activity of many enzymes relies on posttranslational modification and most plant viruses replicate in the cytoplasm, so functional enzymes cannot be displayed on the virus surface by direct coat protein fusions. An alternative display system to present the Trichoderma reesei endoglucanase Cel12A on potato virus X (PVX) using SpyTag/SpyCatcher (ST/SC) technology was recently developed by the authors, which allows the carrier and enzyme to be produced separately before isopeptide conjugation. Although kinetic analysis clearly indicated efficient biocatalyst activity, the PVX carrier interfered with substrate binding. To overcome this, the suitability of tobacco mosaic virus (TMV) was tested, which can also accommodate a larger number of ST peptides. We produced TMV particles displaying ST as a new platform for the immobilization of enzymes such as Cel12A, and compared its performance to the established PVX-ST platform in terms of catalytic efficiency. Although more enzyme molecules were immobilized on the TMV-ST particles, we found that the rigid scaffold and helical spacing significantly affected enzyme activity.

A Replicating Viral Vector Greatly Enhances Accumulation of Helical Virus-Like Particles in Plants

The production of plant helical virus-like particles (VLPs) via plant-based expression has been problematic with previous studies suggesting that an RNA scaffold may be necessary for their efficient production. To examine this, we compared the accumulation of VLPs from two potexviruses, papaya mosaic virus and alternanthera mosaic virus (AltMV), when the coat proteins were expressed from a replicating potato virus X- based vector (pEff) and a non-replicating vector (pEAQ-HT). Significantly greater quantities of VLPs could be purified when pEff was used. The pEff system was also very efficient at producing VLPs of helical viruses from different virus families. Examination of the RNA content of AltMV and tobacco mosaic virus VLPs produced from pEff revealed the presence of vector-derived RNA sequences, suggesting that the replicating RNA acts as a scaffold for VLP assembly. Cryo-EM analysis of the AltMV VLPs showed they had a structure very similar to that of authentic potexvirus particles. Thus, we conclude that vectors generating replicating forms of RNA, such as pEff, are very efficient for producing helical VLPs.

Reprogramming Virus Coat Protein Carboxylate Interactions for the Patterned Assembly of Hierarchical Nanorods

The self-assembly system of the rod-shaped tobacco mosaic virus (TMV) has been studied extensively for nanoscale applications. TMV coat protein assembly is modulated by intersubunit carboxylate groups whose electrostatic repulsion limits the assembly of virus rods without incorporating genomic RNA. To engineer assembly control into this system, we reprogrammed intersubunit carboxylate interactions to produce self-assembling coat proteins in the absence of RNA and in response to unique pH and ionic environmental conditions. Specifically, engineering a charge attraction at the intersubunit E50-D77 carboxylate group through a D77K substitution stabilized the coat proteins assembly into virus-like rods. In contrast, the reciprocal E50K modification alone did not confer virus-like rod assembly. However, a combination of R46G/E50K/E97G substitutions enabled virus-like rod assembly. Interestingly, the D77K substitution displays a unique pH-dependent assembly-disassembly profile, while the R46G/E50K/E97G substitutions confer a novel salt concentration dependency for assembly control. In addition, these unique environmentally controlled coat proteins allow for the directed assembly and disassembly of chimeric virus-like rods both in solution and on substrate-attached seed rods. Combined, these findings provide a controllable means to assemble functionally discrete virus-like rods for use in nanotechnology applications.

The pharmacology of plant virus nanoparticles

The application of nanoparticles for medical purposes has made enormous strides in providing new solutions to health problems. The observation that plant virus-based nanoparticles (VNPs) can be repurposed and engineered as smart bio-vehicles for targeted drug delivery and imaging has launched extensive research for improving the therapeutic and diagnostic management of various diseases. There is evidence that VNPs are promising high value nanocarriers with potential for translational development. This is mainly due to their unique features, encompassing structural uniformity, ease of manufacture and functionalization by means of expression, chemical biology and self-assembly. While the development pipeline is moving rapidly, with many reports focusing on engineering and manufacturing aspects to tailor the properties and efficacy of VNPs, fewer studies have focused on gaining insights into the nanotoxicity of this novel platform nanotechnology. Herein, we discuss the pharmacology of VNPs as a function of formulation and route of administration. VNPs are reviewed in the context of their application as therapeutic adjuvants or nanocarrier excipients to initiate, enhance, attenuate or impede the formulation's toxicity. The summary of the data however also underlines the need for meticulous VNP structure-nanotoxicity studies to improve our understanding of their in vivo fates and pharmacological profiles to pave the way for translation of VNP-based formulations into the clinical setting.

An RNA-centric historical narrative around the Protein Data Bank

Some of the amazing contributions brought to the scientific community by the Protein Data Bank (PDB) are described. The focus is on nucleic acid structures with a bias toward RNA. The evolution and key roles in science of the PDB and other structural databases for nucleic acids illustrate how small initial ideas can become huge and indispensable resources with the unflinching willingness of scientists to cooperate globally. The progress in the understanding of the molecular interactions driving RNA architectures followed the rapid increase in RNA structures in the PDB. That increase was consecutive to improvements in chemical synthesis and purification of RNA molecules, as well as in biophysical methods for structure determination and computer technology. The RNA modeling efforts from the early beginnings are also described together with their links to the state of structural knowledge and technological development. Structures of RNA and of its assemblies are physical objects, which, together with genomic data, allow us to integrate present-day biological functions and the historical evolution in all living species on earth.

In vivo antiviral activity and disassembly mechanism of novel 1-phenyl-5-amine-4-pyrazole thioether derivatives against Tobacco mosaic virus

A series of novel 1-phenyl-5-amine-4-pyrazole thioether derivatives containing a 1,3,4-oxadiazole moiety was designed and synthesised. In vivo antiviral bioassay results showed that most of the target compounds exhibited excellent inactivation activity against Tobacco mosaic virus (TMV). The EC₅₀ values of the inactivation activities for T₂, T₇, T₉, T₂₄, T₂₅ and T₂₇ were 15.7, 15.7, 15.5, 11.9, 12.5 and 16.5 μg/mL, respectively, which were remarkably superior over that of the commercialised antiviral agent ningnanmycin (40.3 μg/mL). Morphological study using AFM and TEM of TMV treated with T₂₄ showed that T₂₄ could significantly shorten the polymerization length of TMV particles and formed a distinct break on the rod-shaped TMV. Investigations for virus infection efficiency on tobacco leaves demonstrated that infectivity of virion had been reduced obviously upon T₂₄ treatment. Subsequently, a strong interaction between T₂₄ and TMV-CP (K_d = 3.8 μM, score 6.11) was observed through MST experiments. Molecular docking study further revealed that target compounds interact with amino acid residue Glu50 in TMV CP, causing disassembly of virion, shorting the length of the virion and reducing the infectivity of virion, and resulting in high inactivating activity of target compounds. This study provides a new insight for discovery of antiviral compounds through a new action mechanism with a new binding site.

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.

Charge Calibration Standard for Atomic Force Microscope Tips in Liquids

An electric charge standard with nanoscale resolution is created using the known charge distribution of a single tobacco mosaic virus coat protein combined with the known packing of these proteins in the virus capsid. This advances the ability to measure charge on nanometric samples. Experimental atomic force microscope (AFM) force-distance curves are collected under aqueous conditions with controlled pH and ion concentration. A mathematical model that considers a polarizable dielectric tip immersed in an electrolyte is used to obtain charge density from the AFM measurements. Interactions between the tip and the sample are modeled using theory that includes monopolar electrostatic interactions, dipolar interactions, screening from both the dielectric nature of ambient water and solvated ions as described by the linear Poisson-Boltzmann equation, and hard-core repulsion. It is found that the tip charge density changes on a timescale of hours requiring recalibration of the tip for experiments lasting more than an hour. As an example of how a charge-calibrated tip may be used, the surface charge densities on 20 individual carboxylate-modified polystyrene (PS) beads are measured. The average of these AFM-measured bead charge densities is compared with the value obtained from conventional titration combined with electron microscopy. The two values are found to agree within 20%. While the comparison demonstrates similarity of the two charge measurements, hypotheses are put forward as to why the two techniques might be expected not to provide identical mean charge densities. The considerations used to build these hypotheses thus underscore the relevance of the method performed here if charge information is required on individual nanoparticles.

Conformational behavior of coat protein in plants and association with coat protein-mediated resistance against TMV

Tobacco mosaic virus (TMV) coat protein (CP) self assembles in viral RNA deprived transgenic plants to form aggregates based on the physical conditions of the environment. Transgenic plants in which these aggregates are developed show resistance toward infection by TMV referred to as CP-MR. This phenomenon has been extensively used to protect transgenic plants against viral diseases. The mutants T42W and E50Q CP confer enhanced CP-MR as compared to the WT CP. The aggregates, when examined, show the presence of helical discs in the case of WT CP; on the other hand, mutants show the presence of highly stable non-helical long rods. These aggregates interfere with the accumulation of MP as well as with the disassembly of TMV in plant cells. Here, we explored an atomic level insight to the process of CP-MR through MD simulations. The subunit-subunit interactions were assessed with the help of MM-PBSA calculations. Moreover, classification of secondary structure elements of the protein also provided unambiguous information about the conformational changes occurring in the two chains, which indicated toward increased flexibility of the mutant protein and seconded the other results of simulations. Our finding indicates the essential structural changes caused by the mutation in CP subunits, which are critically responsible for CP-MR and provides an in silico insight into the effects of these transitions over CP-MR. These results could further be utilized to design TMV-CP-based small peptides that would be able to provide appropriate protection against TMV infection.

Dynamic rotation of the protruding domain enhances the infectivity of norovirus

Norovirus is the major cause of epidemic nonbacterial gastroenteritis worldwide. Lack of structural information on infection and replication mechanisms hampers the development of effective vaccines and remedies. Here, using cryo-electron microscopy, we show that the capsid structure of murine noroviruses changes in response to aqueous conditions. By twisting the flexible hinge connecting two domains, the protruding (P) domain reversibly rises off the shell (S) domain in solutions of higher pH, but rests on the S domain in solutions of lower pH. Metal ions help to stabilize the resting conformation in this process. Furthermore, in the resting conformation, the cellular receptor CD300lf is readily accessible, and thus infection efficiency is significantly enhanced. Two similar P domain conformations were also found simultaneously in the human norovirus GII.3 capsid, although the mechanism of the conformational change is not yet clear. These results provide new insights into the mechanisms of non-enveloped norovirus transmission that invades host cells, replicates, and sometimes escapes the hosts immune system, through dramatic environmental changes in the gastrointestinal tract.

FiXR: a framework to reconstruct fiber cross-sections from X-ray fiber diffraction experiments

Ab initio reconstruction methods have revolutionized the capabilities of small-angle X-ray scattering (SAXS), allowing the data-driven discovery of previously unknown molecular conformations, exploiting optimization heuristics and assumptions behind the composition of globular molecules. While these methods have been successful for the analysis of small particles, their impact on fibrillar assemblies has been more limited. The micrometre-range size of these assemblies and the complex interaction of their periodicities in their scattering profiles indicate that the discovery of fibril structures from SAXS measurements requires novel approaches beyond extending existing tools for molecular discovery. In this work, it is proposed to use SAXS measurements, together with diffraction theory, to infer the electron distribution of the average cross-section of a fiber. This cross-section is modeled as a discrete electron density with continuous support, allowing representations beyond binary distributions. Additional constraints, such as non-negativity or smoothness/connectedness, can also be added to the framework. The proposed approach is tested using simulated SAXS data from amyloid β fibril models and using measured data of Tobacco mosaic virus from SAXS experiments, recovering the geometry and density of the cross-sections in all cases. The approach is further tested by analyzing SAXS data from different amyloid β fibril assemblies, with results that are in agreement with previously proposed models from cryo-EM measurements. The limitations of the proposed method, together with an analysis of the robustness of the method and the combination with different experimental sources, are also discussed.

Development of nanotubes coated with platinum nanodendrites using a virus as a template

We developed a facile method to fabricate platinum (Pt) porous nanotubes coated with interconnected Pt dendrites using the tobacco mosaic virus (TMV) as a template. The surface-exposed arginine residues of the TMV induced the selective deposition of Pt seeds on the TMV outside surface, and poly(sodium-p-styrenesulfonate) (PSS) was chosen to stabilize the dispersity of TMV coated with Pt seeds (TMV/SPt). The limited space between the Pt seeds and their uniform distribution on the TMV exterior confined the growth of Pt dendrites, resulting in continuous dendritic platinum nanotubes (TMV/DPtNT). The synergistic effects of porous dendrites and anisotropic structures of the TMV/DPtNTs provided an increase in the active sites, the enhancement of transport efficiency and long-distance electron transfer, which greatly improved the catalytic activity. We also demonstrated that such nanotubes could be used in the detection of H₂O₂ with good sensitivity.

Bayesian inference for three-dimensional helical reconstruction using a soft-body model

Estimation of the three-dimensional (3D) structure of a protein using cryo transmission electron microscopy (cryo-TEM) is an inverse problem, which aims to estimate the parameters of a specific physical process from observations. In general, we need to model the observation process to estimate a structure. However, the inconsistency between the model and a real observation process decreases the estimation accuracy. In cryo-TEM, the flexibility of a soft protein, including the bending of a helix, can lead to inconsistencies between the observations because of the assumption that there is a consistent 3D structure behind each observed image. In this paper, we propose a 3D reconstruction algorithm for helical structures using a parametric soft-body model that can represent continuous deformation. We performed an approximate Bayesian inference for unobservable (hidden) variables, such as the deformation parameters, projection angle, and two-dimensional origin offset (shift) of each protein in the 3D structure estimation problem. Our principled approach is not only beneficial to deal with the uncertainties in the estimation, but also beneficial to make the optimization algorithm convergent and efficient. Reconstructions with artificial molecules validated the advantage of the proposed method, particularly, when deformed helices were imaged under a low signal-to-noise ratio condition. Moreover, we confirmed that the proposed method successfully reconstructed a 3D structure from cryo-TEM images of the tobacco mosaic virus.

Functionalized Tobacco Mosaic Virus Coat Protein Monomers and Oligomers as Nanocarriers for Anti-Cancer Peptides

Components with self-assembly properties derived from plant viruses provide the opportunity to design biological nanoscaffolds for the ordered display of agents of diverse nature and with complementing functions. With the aim of designing a functionalized nanoscaffold to target cancer, the coat protein (CP) of Tobacco mosaic virus (TMV) was tested as nanocarrier for an insoluble, highly hydrophobic peptide that targets the transmembrane domain of the Neuropilin-1 (NRP1) receptor in cancer cells. The resulting construct CPL-K (CP-linker-“Kill”) binds to NRP1 in cancer cells and disrupts NRP1 complex formation with PlexA1 as well as downstream Akt survival signaling. The application of CPL-K also inhibits angiogenesis and cell migration. CP was also fused to a peptide that targets the extracellular domain of NRP1 and this fusion protein (CPL-F, CP-Linker-“Find”) is shown to bind to cultured cancer cells and to inhibit NRP1-dependent angiogenesis as well. CPL-K and CPL-F maintain their anti-angiogenic properties upon co-assembly to oligomers/nanoparticles together with CPL. The observations show that the CP of TMV can be employed to generate a functionalized nanoparticle with biological activity. Remarkably, fusion to CPL allowed us to solubilize the highly insoluble transmembrane NRP1 peptide and to retain its anti-angiogenic effect.

From stars to stripes: RNA-directed shaping of plant viral protein templates-structural synthetic virology for smart biohybrid nanostructures

The self-assembly of viral building blocks bears exciting prospects for fabricating new types of bionanoparticles with multivalent protein shells. These enable a spatially controlled immobilization of functionalities at highest surface densities-an increasing demand worldwide for applications from vaccination to tissue engineering, biocatalysis, and sensing. Certain plant viruses hold particular promise because they are sustainably available, biodegradable, nonpathogenic for mammals, and amenable to in vitro self-organization of virus-like particles. This offers great opportunities for their redesign into novel "green" carrier systems by spatial and structural synthetic biology approaches, as worked out here for the robust nanotubular tobacco mosaic virus (TMV) as prime example. Natural TMV of 300 x 18 nm is built from more than 2,100 identical coat proteins (CPs) helically arranged around a 6,395 nucleotides ssRNA. In vitro, TMV-like particles (TLPs) may self-assemble also from modified CPs and RNAs if the latter contain an Origin of Assembly structure, which initiates a bidirectional encapsidation. By way of tailored RNA, the process can be reprogrammed to yield uncommon shapes such as branched nanoobjects. The nonsymmetric mechanism also proceeds on 3'-terminally immobilized RNA and can integrate distinct CP types in blends or serially. Other emerging plant virus-deduced systems include the usually isometric cowpea chlorotic mottle virus (CCMV) with further strikingly altered structures up to "cherrybombs" with protruding nucleic acids. Cartoon strips and pictorial descriptions of major RNA-based strategies induct the reader into a rare field of nanoconstruction that can give rise to utile soft-matter architectures for complex tasks. This article is categorized under: Biology-Inspired Nanomaterials > Protein and Virus-Based Structures Nanotechnology Approaches to Biology > Nanoscale Systems in Biology Biology-Inspired Nanomaterials > Nucleic Acid-Based Structures.

Surface Charge Mapping on Virions and Virus-Like Particles of Helical Plant Viruses

Currently, the assembly of helical plant viruses is poorly understood. The viral assembly and infection may be affected by the charge distribution on the virion surface. However, only the total virion charge (isoelectric point) has been determined for most plant viruses. Here, we report on the first application of positively charged magnetic nanoparticles for mapping the surface charge distribution of helical plant viruses. The charge was demonstrated to be unevenly distributed on the surface of viruses belonging to different taxonomic groups, with the negative charge being predominantly located at one end of the virions. This charge distribution is mainly controlled by viral RNA.

A single-molecule atomic force microscopy study reveals the antiviral mechanism of tannin and its derivatives

Antiviral agents work by stopping or intervening the virus replication. Virus replication is a fast and multi-step process while effective antiviral intervention requires agents to interact with the protein coat, genetic RNA/DNA or both during virus replication. Thus, quantifying these interactions at the molecular level, although it is quite challenging, is very important for an understanding of the underlying molecular mechanism of antiviral intervention. Here, at the single molecule level, we employ single molecule force spectroscopy (SMFS) in combination with AFM imaging and choose tobacco mosaic virus (TMV)/tannin as a model system of tubular virus to directly study how the inhibitor influences the interactions of RNA and coat protein. We illustrated the antiviral mechanism of tannin during the three main stages of TMV infection, i.e., before the entry of cells, the disassembly of genetic RNA and reassembly of genetic RNA, respectively. Our SMFS results show that tannin and its derivatives can stabilize the TMV complex by enhancing the interactions between RNA and coat protein via weak interactions, such as hydrogen bonding and hydrophobic interactions. In addition, the stabilization effect showed molecular weight dependence, i.e., for higher molecular weight tannin the stabilization occurs after genetic RNA gets partially disassembled from the protein coat, while the lower molecular weight tannin hydrolyte starts experiencing the stabilization effect before the RNA disassembly. Furthermore, the cycling stretching-relaxation experiments in the presence/absence of tannin proved that tannin can prevent the assembling of RNA and coat protein. In addition, the AFM imaging results demonstrate that tannin can cause the aggregation of TMV particles in a concentration-dependent manner; a higher concentration of tannin will cause more severe aggregations. These results deepen our understanding of the antiviral mechanism of tannin and its derivatives, which facilitate the rational design of efficient agents for antiviral therapy.

Elucidation of the viral disassembly switch of tobacco mosaic virus

Stable capsid structures of viruses protect viral RNA while they also require controlled disassembly for releasing the viral genome in the host cell. A detailed understanding of viral disassembly processes and the involved structural switches is still lacking. This process has been extensively studied using tobacco mosaic virus (TMV), and carboxylate interactions are assumed to play a critical part in this process. Here, we present two cryo‐EM structures of the helical TMV assembly at 2.0 and 1.9 Å resolution in conditions of high Ca²⁺ concentration at low pH and in water. Based on our atomic models, we identify the conformational details of the disassembly switch mechanism: In high Ca²⁺/acidic pH environment, the virion is stabilized between neighboring subunits through carboxyl groups E95 and E97 in close proximity to a Ca²⁺ binding site that is shared between two subunits. Upon increase in pH and lower Ca²⁺ levels, mutual repulsion of the E95/E97 pair and Ca²⁺ removal destabilize the network of interactions between adjacent subunits at lower radius and release the switch for viral disassembly.

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.

Plant Viruses in Plant Molecular Pharming: Toward the Use of Enveloped Viruses

Plant molecular pharming has emerged as a reliable platform for recombinant protein expression providing a safe and low-cost alternative to bacterial and mammalian cells-based systems. Simultaneously, plant viruses have evolved from pathogens to molecular tools for recombinant protein expression, chimaeric viral vaccine production, and lately, as nanoagents for drug delivery. This review summarizes the genesis of viral vectors and agroinfection, the development of non-enveloped viruses for various biotechnological applications, and the on-going research on enveloped plant viruses.

Delivery of thrombolytic therapy using rod-shaped plant viral nanoparticles decreases the risk of hemorrhage

Cardiovascular thrombotic disease is an underlying cause of stroke, myocardial infarction and pulmonary embolism - some of the leading causes of death worldwide. Reperfusion therapy with anticoagulant, antiplatelet, and fibrinolytic agents has significantly reduced early mortality and morbidity from acute myocardial infarction and stroke. Nevertheless, bleeding side effects (e.g., intracranial hemorrhage) associated with the anti-thrombotic therapy can offset its benefits and limit its applicability to strictly defined short therapeutic windows. We have previously shown that elongated plant virus based nanoparticles can target cardiovascular thrombi and exhibited their utility for the delivery of streptokinase in an ex vivo model of thrombosis. Herein, we build upon our previous findings and demonstrate plant viral delivery of the current standard-of-care tissue plasminogen activator (tPA). Studies on a pre-clinical mouse model of arterial thrombosis indicate that while the therapeutic efficacy of free tPA and tPA-conjugated TMV are similar, the safety profile of the tPA-TMV formulation is improved, i.e. administration of the latter has less impact on hemostasis as demonstrated by decreased bleeding time. Thus, our data suggest that TMV-based delivery of thrombolytic therapies could be a promising and safer alternative to reperfusion therapy with the tPA.

Application of Plant Viruses as a Biotemplate for Nanomaterial Fabrication

Viruses are widely used to fabricate nanomaterials in the field of nanotechnology. Plant viruses are of great interest to the nanotechnology field because of their symmetry, polyvalency, homogeneous size distribution, and ability to self-assemble. This homogeneity can be used to obtain the high uniformity of the templated material and its related properties. In this paper, the variety of nanomaterials generated in rod-like and spherical plant viruses is highlighted for the cowpea chlorotic mottle virus (CCMV), cowpea mosaic virus (CPMV), brome mosaic virus (BMV), and tobacco mosaic virus (TMV). Their recent studies on developing nanomaterials in a wide range of applications from biomedicine and catalysts to biosensors are reviewed.

TMV Particles: The Journey From Fundamental Studies to Bionanotechnology Applications

Ever since its initial characterization in the 19th century, tobacco mosaic virus (TMV) has played a prominent role in the development of modern virology and molecular biology. In particular, research on the three-dimensional structure of the virus particles and the mechanism by which these assemble from their constituent protein and RNA components has made TMV a paradigm for our current view of the morphogenesis of self-assembling structures, including viral particles. More recently, this knowledge has been applied to the development of novel reagents and structures for applications in biomedicine and bionanotechnology. In this article, we review how fundamental science has led to TMV being at the vanguard of these new technologies.

Protein cage assembly across multiple length scales

Within the materials science community, proteins with cage-like architectures are being developed as versatile nanoscale platforms for use in protein nanotechnology. Much effort has been focused on the functionalization of protein cages with biological and non-biological moieties to bring about new properties of not only individual protein cages, but collective bulk-scale assemblies of protein cages. In this review, we report on the current understanding of protein cage assembly, both of the cages themselves from individual subunits, and the assembly of the individual protein cages into higher order structures. We start by discussing the key properties of natural protein cages (for example: size, shape and structure) followed by a review of some of the mechanisms of protein cage assembly and the factors that influence it. We then explore the current approaches for functionalizing protein cages, on the interior or exterior surfaces of the capsids. Lastly, we explore the emerging area of higher order assemblies created from individual protein cages and their potential for new and exciting collective properties.

Femtosecond X-ray coherent diffraction of aligned amyloid fibrils on low background graphene

Here we present a new approach to diffraction imaging of amyloid fibrils, combining a free-standing graphene support and single nanofocused X-ray pulses of femtosecond duration from an X-ray free-electron laser. Due to the very low background scattering from the graphene support and mutual alignment of filaments, diffraction from tobacco mosaic virus (TMV) filaments and amyloid protofibrils is obtained to 2.7 Å and 2.4 Å resolution in single diffraction patterns, respectively. Some TMV diffraction patterns exhibit asymmetry that indicates the presence of a limited number of axial rotations in the XFEL focus. Signal-to-noise levels from individual diffraction patterns are enhanced using computational alignment and merging, giving patterns that are superior to those obtainable from synchrotron radiation sources. We anticipate that our approach will be a starting point for further investigations into unsolved structures of filaments and other weakly scattering objects.

Building expanded structures from tetrahedral DNA branching elements, RNA and TMV protein

By combining both chemical and enzymatic ligation with procedures guiding the self-assembly of nanotubular tobacco mosaic virus (TMV)-like particles (TLPs), novel nucleoprotein structures based on DNA-terminated branching elements, RNA scaffolds and TMV coat protein (CP) are made accessible. Tetrahedral tetrakis(hydroxybiphenyl)adamantane cores with four 5'-phosphorylated dinucleotide arms were coupled to DNA linkers by chemical ligation. The resulting three-dimensional (3D) branching elements were enzymatically ligated to the 3' termini of RNA scaffolds either prior to or after the RNAs' incorporation into TLPs. Thus, architectures with interconnected nanotube domains in two different length classes were generated, each containing 70 CP subunits per 10 nm length. Short TMV origin-of-assembly-containing RNA scaffolds ligated to the DNA allowed the growth of protein-coated 34 nm tubes on the terminal RNA strands in situ. Alternatively, 290 nm pre-fabricated tubes with accessible RNA 3' termini, attained by DNA blocking elements hybridized to the RNAs, were ligated with the branched cores. Both approaches resulted in four-armed nanoobjects, demonstrating a so far unique combination of organic synthesis of branching elements, enzymatic modifications, nucleic acid-based scaffolding and RNA-guided and DNA-controlled assembly of tubular RNA-encapsidating protein domains, yielding a novel class of 3D nucleoprotein architectures with polyvalent protein elements. In the long term, the production route might give rise to supramolecular systems with complex functionalities, installed via the orthogonal coupling of effector molecules to TLP domains.

Microstructural metrology of tobacco mosaic virus nanorods during radial compression and heating

We determined stress-induced deformations and the thermal stability of nanorod-shaped tobacco mosaic virus (TMV) capsids in coffee-ring structures by X-ray nanodiffraction. The hexagonal capsids lattice transforms under compression in the outer boundary zone of the coffee-ring into a tetragonal lattice. The helical pitch of the nanorods increases by about 2.5% across the outer boundary zone while the lateral distance between nanorods decreases continuously across the whole coffee-ring structure by about 2% due to compressive forces. The diffraction patterns show a mixture of helical scattering and Bragg peaks attributed to a lattice of nanorods interlocked by their helical grooves. Thermo-nanodiffraction reveals water loss up to about 100 °C resulting in a reduction of the helical pitch by about 6% with respect to its maximum value and a reduction of the nanorods separation by about 0.5 nm. Up to about 200 °C the pitch is increasing again by about 2%. Secondary crystallization in the bulk reaches a maximum at 150-160 °C. At higher temperatures the crystallinity is continuously decreasing up to about 220 °C. Above about 200 °C and depending on the heating history, the nanorods start disintegrating into small, randomly oriented aggregates.

Bottom-Up Assembly of TMV-Based Nucleoprotein Architectures on Solid Supports

RNA-guided self-assembly of tobacco mosaic virus (TMV)-like nucleoprotein nanotubes is possible using 3'-terminally surface-linked scaffold RNAs containing the viral origin of assembly (OAS). In combination with TMV coat protein (CP) preparations, these scaffold RNAs can direct the growth of selectively addressable multivalent carrier particles directly at sites of interest on demand. Serving as adapter templates for the installation of functional molecules, they may promote an integration of active units into miniaturized technical devices, or enable their presentation on soft-matter nanotube systems at high surface densities advantageous for, for example, biodetection or purification applications. This chapter describes all procedures essential for the bottom-up fabrication of "nanostar" colloids with gold cores and multiple TMV-like arms, immobilized in a programmable manner by way of hybridization of the RNA scaffolds to oligodeoxynucleotides exposed on the gold beads.

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.

Dual Functionalization of Rod-Shaped Viruses on Single Coat Protein Subunits

Plant viruses are emerging as versatile tools for nanotechnology applications since it is possible to modify their multivalent protein surfaces and thereby introduce and display new functionalities. In this chapter, we describe a tobacco mosaic virus (TMV) variant that exposes two selectively addressable amino acid moieties on each of its 2130 coat protein (CP) subunits. A lysine as well as a cysteine introduced at accessible sites of every CP can be modified with amino- and/or thiol-reactive chemistry such as N-hydroxysuccinimide esters (NHS ester) and maleimide containing reagents alone or simultaneously. This enables the pairwise immobilization of distinct molecules in close vicinity to each other on the TMV surface by simple standard conjugation protocols. We describe the generation of the mutations, the virus propagation and isolation as well as the dual functionalization of the TMV variant with two fluorescent dyes. The labeling is evaluated by SDS-PAGE and spectrophotometry and the degree of labeling (DOL) calculated.

Fabrication of Tobacco Mosaic Virus-Like Nanorods for Peptide Display

Virus-like particles (VLPs) are genome-free protein shells assembled from virus coat proteins (CPs). The uniform and nanoscale structure of VLPs combined with their noninfectious nature have made them ideal candidates for the display of functional peptides. While the vast majority of VLPs are derived from spherical viruses, tobacco mosaic virus (TMV) produces a rod-shaped particle with a hollow central channel. However, under physiological conditions the TMV CP forms only disk-shaped macromolecules. Here, we describe the design, construction, purification, and processing of rod-shaped TMV-VLPs using a simple bacterial expression system. The robust nature of this system allows for the display of functional peptides and molecules on the outer surface of this novel VLP.