The packaging of different cargo into enveloped viral nanoparticles

Self-assembly of dengue virus empty capsid-like particles in solution

Nucleocapsid (NC) assembly is an essential step of the virus replication cycle. It ensures genome protection and transmission among hosts. Flaviviruses are human viruses for which envelope structure is well known, whereas no information on NC organization is available. Here we designed a dengue virus capsid protein (DENVC) mutant in which a highly positive spot conferred by arginine 85 in α4-helix was replaced by a cysteine residue, simultaneously removing the positive charge and restricting the intermolecular motion through the formation of a disulfide cross-link. We showed that the mutant self-assembles into capsid-like particles (CLP) in solution without nucleic acids. Using biophysical techniques, we investigated capsid assembly thermodynamics, showing that an efficient assembly is related to an increased DENVC stability due to α4/α4' motion restriction. To our knowledge, this is the first time that flaviviruses' empty capsid assembly is obtained in solution, revealing the R85C mutant as a powerful tool to understand the NC assembly mechanism.

Engineering Metastability into a Virus-like Particle to Enable Triggered Dissociation

For a virus-like particle (VLP) to serve as a delivery platform, the VLP must be able to release its cargo in response to a trigger. Here, we use a chemical biology approach to destabilize a self-assembling capsid for a subsequent triggered disassembly. We redesigned the dimeric hepatitis B virus (HBV) capsid protein (Cp) with two differentially addressable cysteines, C150 for reversibly crosslinking the capsid and C124 to react with a destabilizing moiety. The resulting construct, Cp150-V124C, assembles into icosahedral, 120-dimer VLPs that spontaneously crosslink via the C-terminal C150, leaving C124 buried at a dimer-dimer interface. The VLP is driven into a metastable state when C124 is reacted with the bulky fluorophore, maleimidyl BoDIPY-FL. The resulting VLP is stable until exposed to modest, physiologically relevant concentrations of reducing agent. We observe dissociation with FRET relaxation of polarization, size exclusion chromatography, and resistive-pulse sensing. Dissociation is slow, minutes to hours, with a characteristic lag phase. Mathematical modeling based on the presence of a nucleation step predicts disassembly dynamics that are consistent with experimental observations. VLPs transfected into hepatoma cells show similar dissociation behavior. These results suggest a generalizable strategy for designing a VLP that can release its contents in an environmentally responsive reaction.

The life cycle of the alphaviruses: From an antiviral perspective

The alphaviruses are a widely distributed group of positive-sense, single stranded, RNA viruses. These viruses are largely arthropod-borne and can be found on all populated continents. These viruses cause significant human disease, and recently have begun to spread into new populations, such as the expansion of Chikungunya virus into southern Europe and the Caribbean, where it has established itself as endemic. The study of alphaviruses is an active and expanding field, due to their impacts on human health, their effects on agriculture, and the threat that some pose as potential agents of biological warfare and terrorism. In this systematic review we will summarize both historic knowledge in the field as well as recently published data that has potential to shift current theories in how alphaviruses are able to function. This review is comprehensive, covering all parts of the alphaviral life cycle as well as a brief overview of their pathology and the current state of research in regards to vaccines and therapeutics for alphaviral disease.

Polymorphic self-assembly of helical tubules is kinetically controlled

In contrast to most self-assembling synthetic materials, which undergo unbounded growth, many biological self-assembly processes are self-limited. That is, the assembled structures have one or more finite dimensions that are much larger than the size scale of the individual monomers. In many such cases, the finite dimension is selected by a preferred curvature of the monomers, which leads to self-closure of the assembly. In this article, we study an example class of self-closing assemblies: cylindrical tubules that assemble from triangular monomers. By combining kinetic Monte Carlo simulations, free energy calculations, and simple theoretical models, we show that a range of programmable size scales can be targeted by controlling the intricate balance between the preferred curvature of the monomers and their interaction strengths. However, their assembly is kinetically controlled-the tubule morphology is essentially fixed shortly after closure, resulting in a distribution of tubule widths that is significantly broader than the equilibrium distribution. We develop a simple kinetic model based on this observation and the underlying free-energy landscape of assembling tubules that quantitatively describes the distributions. Our results are consistent with recent experimental observations of tubule assembly from triangular DNA origami monomers. The modeling framework elucidates design principles for assembling self-limited structures from synthetic components, such as artificial microtubules that have a desired width and chirality.

Protein Cages: From Fundamentals to Advanced Applications

Proteins that self-assemble into polyhedral shell-like structures are useful molecular containers both in nature and in the laboratory. Here we review efforts to repurpose diverse protein cages, including viral capsids, ferritins, bacterial microcompartments, and designed capsules, as vaccines, drug delivery vehicles, targeted imaging agents, nanoreactors, templates for controlled materials synthesis, building blocks for higher-order architectures, and more. A deep understanding of the principles underlying the construction, function, and evolution of natural systems has been key to tailoring selective cargo encapsulation and interactions with both biological systems and synthetic materials through protein engineering and directed evolution. The ability to adapt and design increasingly sophisticated capsid structures and functions stands to benefit the fields of catalysis, materials science, and medicine.

Multiple capsid protein binding sites mediate selective packaging of the alphavirus genomic RNA

The alphavirus capsid protein (Cp) selectively packages genomic RNA (gRNA) into the viral nucleocapsid to produce infectious virus. Using photoactivatable ribonucleoside crosslinking and an innovative biotinylated Cp retrieval method, here we comprehensively define binding sites for Semliki Forest virus (SFV) Cp on the gRNA. While data in infected cells demonstrate Cp binding to the proposed genome packaging signal (PS), mutagenesis experiments show that PS is not required for production of infectious SFV or Chikungunya virus. Instead, we identify multiple Cp binding sites that are enriched on gRNA-specific regions and promote infectious SFV production and gRNA packaging. Comparisons of binding sites in cytoplasmic vs. viral nucleocapsids demonstrate that budding causes discrete changes in Cp-gRNA interactions. Notably, Cp’s top binding site is maintained throughout virus assembly, and specifically binds and assembles with Cp into core-like particles in vitro. Together our data suggest a model for selective alphavirus genome recognition and assembly.

Alphaviruses need to selectively package genomic viral RNA for transmission, but the packaging mechanism remains unclear. Here, Brown et al. combine PAR-CLIP with biotinylated capsid protein (Cp) retrieval and identify multiple Cp binding sites on genomic viral RNA that promote virion formation.

Plant viral nanoparticles for packaging and in vivo delivery of bioactive cargos

Nanoparticles have unique capabilities and considerable promise for many different biological uses. One capability is delivering bioactive cargos to specific cells, tissues, or organisms. Depending on the task, there are multiple variables to consider including nanoparticle selection, targeting strategies, and incorporating cargo so it can be delivered in a biologically active form. One nanoparticle option, genetically controlled plant viral nanoparticles (PVNPs), is highly uniform within a given virus but quite variable between viruses with a broad range of useful properties. PVNPs are flexible and versatile tools for incorporating and delivering a wide range of small or large molecule cargos. Furthermore, PVNPs can be modified to create nanostructures that can solve problems in medical, environmental, and basic research. This review discusses the currently available techniques for delivering bioactive cargos with PVNPs and potential cargos that can be delivered with these strategies. This article is categorized under: Implantable Materials and Surgical Technologies > Nanomaterials and Implants Biology-Inspired Nanomaterials > Protein and Virus-Based Structures.

Removing the Polyanionic Cargo Requirement for Assembly of Alphavirus Core-Like Particles to Make an Empty Alphavirus Core

The assembly of alphavirus nucleocapsid cores requires electrostatic interactions between the positively charged N-terminus of the capsid protein (CP) and the encapsidated polyanionic cargo. This system differs from many other viruses that can self-assemble particles in the absence of cargo, or form “empty” particles. We hypothesized that the introduction of a mutant, anionic CP could replace the need for charged cargo during assembly. In this work, we produced a CP mutant, Minus 38 (M38), where all N-terminal charged residues are negatively-charged. When wild-type (WT) and M38 CPs were mixed, they assembled into core-like particles (CLPs). These “empty” particles were of similar size and morphology to WT CLPs assembled with DNA cargo, but did not contain nucleic acid. When DNA cargo was added to the assembly mixture, the amount of M38 CP that was assembled into CLPs decreased, but was not fully excluded from the CLPs, suggesting that M38 competes with DNA to interact with WT CPs. The composition of CLPs can be tuned by altering the order of addition of M38 CP, WT CP, and DNA cargo. The ability to produce alphavirus CLPs that contain a range of amounts of encapsidated cargo, including none, introduces a new platform for packaging cargo for delivery or imaging purposes.

Berberine Chloride is an Alphavirus Inhibitor That Targets Nucleocapsid Assembly

Alphaviruses are enveloped positive-sense RNA viruses that can cause serious human illnesses such as polyarthritis and encephalitis. Despite their widespread distribution and medical importance, there are no licensed vaccines or antivirals to combat alphavirus infections. Berberine chloride (BBC) is a pan-alphavirus inhibitor that was previously identified in a replicon-based small-molecule screen. This work showed that BBC inhibits alphavirus replication but also suggested that BBC might have additional effects later in the viral life cycle. Here, we show that BBC has late effects that target the virus nucleocapsid (NC) core. Infected cells treated with BBC late in infection were unable to form stable cytoplasmic NCs or assembly intermediates, as assayed by gradient sedimentation. In vitro studies with recombinant capsid protein (Cp) and purified genomic RNA (gRNA) showed that BBC perturbs core-like particle formation and potentially traps the assembly process in intermediate states. Particles produced from BBC-treated cells were less infectious, despite efficient particle production and only minor decreases in genome packaging. In addition, BBC treatment of free virus particles strongly decreased alphavirus infectivity. In contrast, the infectivity of the negative-sense RNA virus vesicular stomatitis virus was resistant to BBC treatment of infected cells or free virus. Together, our data indicate that BBC alters alphavirus Cp-gRNA interactions and oligomerization and suggest that this may cause defects in NC assembly and in disassembly during subsequent virus entry. Thus, BBC may be considered a novel alphavirus NC assembly inhibitor.IMPORTANCE The alphavirus chikungunya virus (CHIKV) is an example of an emerging human pathogen with increased and rapid global spread. Although an acute CHIKV infection is rarely fatal, many patients suffer from debilitating chronic arthralgia for years. Antivirals against chikungunya and other alphaviruses have been identified in vitro, but to date none have been shown to be efficacious and have been licensed for human use. Here, we investigated a small molecule, berberine chloride (BBC), and showed that it inhibited infectious virus production by several alphaviruses including CHIKV. BBC acted on a late step in the alphavirus exit pathway, namely the formation of the nucleocapsid containing the infectious viral RNA. Better understanding of nucleocapsid formation and its inhibition by BBC will provide important information on the mechanisms of infectious alphavirus production and may enable their future targeting in antiviral strategies.

Packaging of Genomic RNA in Positive-Sense Single-Stranded RNA Viruses: A Complex Story

The packaging of genomic RNA in positive-sense single-stranded RNA viruses is a key part of the viral infectious cycle, yet this step is not fully understood. Unlike double-stranded DNA and RNA viruses, this process is coupled with nucleocapsid assembly. The specificity of RNA packaging depends on multiple factors: (i) one or more packaging signals, (ii) RNA replication, (iii) translation, (iv) viral factories, and (v) the physical properties of the RNA. The relative contribution of each of these factors to packaging specificity is different for every virus. In vitro and in vivo data show that there are different packaging mechanisms that control selective packaging of the genomic RNA during nucleocapsid assembly. The goals of this article are to explain some of the key experiments that support the contribution of these factors to packaging selectivity and to draw a general scenario that could help us move towards a better understanding of this step of the viral infectious cycle.

Role of metallic core for the stability of virus-like particles in strongly coupled electrostatics

Electrostatic interactions play important roles in the formation and stability of viruses and virus-like particles (VLPs) through processes that often involve added, or naturally occurring, multivalent ions. Here, we investigate the electrostatic or osmotic pressure acting on the proteinaceous shell of a generic model of VLPs, comprising a charged outer shell and a metallic nanoparticle core, coated by a charged layer and bathed in an aqueous electrolyte solution. Motivated by the recent studies accentuating the role of multivalent ions for the stability of VLPs, we focus on the effects of multivalent cations and anions in an otherwise monovalent ionic solution. We perform extensive Monte-Carlo simulations based on appropriate Coulombic interactions that consistently take into account the effects of salt screening, the dielectric polarization of the metallic core, and the strong-coupling electrostatics due to multivalent ions. We specifically study the intricate roles these factors play in the electrostatic stability of the model VLPs. It is shown that while the insertion of a metallic nanoparticle by itself can produce negative, inward-directed, pressure on the outer shell, addition of only a small amount of multivalent counterions can robustly engender negative pressures, enhancing the VLP stability across a wide range of values for the system parameters.

Why Enveloped Viruses Need Cores-The Contribution of a Nucleocapsid Core to Viral Budding

During the lifecycle of many enveloped viruses, a nucleocapsid core buds through the cell membrane to acquire an outer envelope of lipid membrane and viral glycoproteins. However, the presence of a nucleocapsid core is not required for assembly of infectious particles. To determine the role of the nucleocapsid core, we develop a coarse-grained computational model with which we investigate budding dynamics as a function of glycoprotein and nucleocapsid interactions, as well as budding in the absence of a nucleocapsid. We find that there is a transition between glycoprotein-directed budding and nucleocapsid-directed budding that occurs above a threshold strength of nucleocapsid interactions. The simulations predict that glycoprotein-directed budding leads to significantly increased size polydispersity and particle polymorphism. This polydispersity can be explained by a theoretical model accounting for the competition between bending energy of the membrane and the glycoprotein shell. The simulations also show that the geometry of a budding particle leads to a barrier to subunit diffusion, which can result in a stalled, partially budded state. We present a phase diagram for this and other morphologies of budded particles. Comparison of these structures against experiments could establish bounds on whether budding is directed by glycoprotein or nucleocapsid interactions. Although our model is motivated by alphaviruses, we discuss implications of our results for other enveloped viruses.

Peptide-directed encapsulation of inorganic nanoparticles into protein containers

Biomolecules can be combined with inorganic compounds to unite biological features with the chemical and physical properties of abiotic materials. In particular, protein containers, with their inherent ability to encapsulate cargo molecules, are perfect platforms for the generation of multifunctional assemblies. However, encapsulation of foreign cargo is immensely challenging due to the lack of specific interactions between cargo and container. Here, we demonstrate that the highly specific cargo-loading mechanism of the bacterial nanocompartment encapsulin can be employed for encapsulation of artificial cargo like inorganic nanoparticles. For this purpose, container-filling gold nanoparticles were decorated with a small number of encapsulin cargo-loading peptides. By lock-and-key interaction between the peptides and the peptide-binding pockets on the inner container surface, the nanoparticles are encapsulated into encapsulin with extremely high efficiency. Most notably, peptide binding is independent from external factors such as ionic strength. Cargo-loading peptides may serve as generally applicable tool for efficient and specific encapsulation of cargo molecules into a proteinaceous compartment.

Self-assembly of convex particles on spherocylindrical surfaces

The precise control of assembly and packing of proteins and colloids on curved surfaces has fundamental implications in nanotechnology. In this paper, we describe dynamical simulations of the self-assembly of conical subunits around a spherocylindrical template, and a continuum theory for the bending energy of a triangular lattice with spontaneous curvature on a surface with arbitrary curvature. We find that assembly depends sensitively on mismatches between subunit spontaneous curvature and the mean curvature of the template, as well as anisotropic curvature of the template (mismatch between the two principal curvatures). Our simulations predict assembly morphologies that closely resemble those observed in experiments in which virus capsid proteins self-assemble around metal nanorods. Below a threshold curvature mismatch, our simulations identify a regime of optimal assembly leading to complete, symmetrical particles. Outside of this regime we observe defective particles, whose morphologies depend on the degree of curvature mismatch. To learn how assembly is affected by the nonuniform curvature of a spherocylinder, we also study the simpler cases of assembly around spherical and cylindrical cores. Our results show that both the intrinsic (Gaussian) and extrinsic (mean) curvatures of a template play significant roles in guiding the assembly of anisotropic subunits, providing a rich design space for the formation of nanoscale materials.

Protein Complexes and Virus-Like Particle Technology

Virus-like particle (VLP) technologies are based on virus-inspired artificial structures and the intrinsic ability of viral proteins to self-assemble at controlled conditions. Therefore, the basic knowledge about the mechanisms of viral particle formation is highly important for designing of industrial applications. As an alternative to genetic and chemical processes, different physical methods are frequently used for VLP construction, including well characterized protein complexes for introduction of foreign molecules in VLP structures.This chapter shortly discusses the mechanisms how the viruses form their perfectly ordered structures as well as the principles and most interesting application examples, how to exploit the structural and assembly/disassembly properties of viral structures for creation of new nanomaterials.

Length of encapsidated cargo impacts stability and structure of in vitro assembled alphavirus core-like particles

In vitro assembly of alphavirus nucleocapsid cores, called core-like particles (CLPs), requires a polyanionic cargo. There are no sequence or structure requirements to encapsidate single-stranded nucleic acid cargo. In this work, we wanted to determine how the length of the cargo impacts the stability and structure of the assembled CLPs. We hypothesized that cargo neutralizes the basic region of the alphavirus capsid protein and if the cargo is long enough, it will also act to scaffold the CP monomers together. Experimentally we found that CLPs encapsidating short 27mer oligonucleotides were less stable than CLPs encapsidating 48mer or 90mer oligonucleotides under different chemical and thermal conditions. Furthermore, cryo-EM studies showed there were structural differences between CLPs assembled with 27mer and 48mer cargo. To mimic the role of the cargo in CLP assembly we made a mutant (4D) where we substituted a cluster of four Lys residues in the CP with four Asp residues. We found that these few amino acid substitutions were enough to initiate CLP assembly in the absence of cargo. The cargo-free 4D CLPs show higher resistance to ionic strength and increased temperature compared to wild-type cargo containing CLPs suggesting their CLP assembly mechanism might also be different.

Emerging nanotechnologies for cancer immunotherapy

Founded on the growing insight into the complex cancer-immune system interactions, adjuvant immunotherapies are rapidly emerging and being adapted for the treatment of various human malignancies. Immune checkpoint inhibitors, for example, have already shown clinical success. Nevertheless, many approaches are not optimized, require frequent administration, are associated with systemic toxicities and only show modest efficacy as monotherapies. Nanotechnology can potentially enhance the efficacy of such immunotherapies by improving the delivery, retention and release of immunostimulatory agents and biologicals in targeted cell populations and tissues. This review presents the current status and emerging trends in such nanotechnology-based cancer immunotherapies including the role of nanoparticles as carriers of immunomodulators, nanoparticles-based cancer vaccines, and depots for sustained immunostimulation. Also highlighted are key translational challenges and opportunities in this rapidly growing field.

Virus-Based Nanoparticles as Versatile Nanomachines

Nanoscale engineering is revolutionizing the way we prevent, detect, and treat diseases. Viruses have played a special role in these developments because they can function as prefabricated nanoscaffolds that have unique properties and are easily modified. The interiors of virus particles can encapsulate and protect sensitive compounds, while the exteriors can be altered to display large and small molecules in precisely defined arrays. These properties of viruses, along with their innate biocompatibility, have led to their development as actively targeted drug delivery systems that expand on and improve current pharmaceutical options. Viruses are naturally immunogenic, and antigens displayed on their surface have been used to create vaccines against pathogens and to break self-tolerance to initiate an immune response to dysfunctional proteins. Densely and specifically aligned imaging agents on viruses have allowed for high-resolution and noninvasive visualization tools to detect and treat diseases earlier than previously possible. These and future applications of viruses have created an exciting new field within the disciplines of both nanotechnology and medicine.

Self-Assembly of an Alphavirus Core-like Particle Is Distinguished by Strong Intersubunit Association Energy and Structural Defects

Weak association energy can lead to uniform nanostructures: defects can anneal due to subunit lability. What happens when strong association energy leads to particles where defects are trapped? Alphaviruses are enveloped viruses whose icosahedral nucleocapsid core can assemble independently. We used a simplest case system to study Ross River virus (RRV) core-like particle (CLP) self-assembly using purified capsid protein and a short DNA oligomer. We find that capsid protein binds the oligomer with high affinity to form an assembly competent unit (U). Subsequently, U assembles with concentration dependence into CLPs. We determined that U-U pairwise interactions are very strong (ca. -6 kcal/mol) compared to other virus assembly systems. Assembled RRV CLPs appeared morphologically uniform and cryo-EM image reconstruction with imposed icosahedral symmetry yielded a T = 4 structure. However, 2D class averages of the CLPs show that virtually every class had disordered regions. These results suggested that irregular cores may be present in RRV virions. To test this hypothesis, we determined 2D class averages of RRV virions using authentic virions or only the core from intact virions isolated by computational masking. Virion-based class averages were symmetrical, geometric, and corresponded well to projections of image reconstructions. In core-based class averages, cores and envelope proteins in many classes were disordered. These results suggest that partly disordered components are common even in ostensibly well-ordered viruses, a biological realization of a patchy particle. Biological advantages of partly disordered complexes may arise from their ease of dissociation and asymmetry.

Alphavirus vectors as tools in neuroscience and gene therapy

Alphavirus-based vectors have been engineered for in vitro and in vivo expression of heterelogous genes. The rapid and easy generation of replication-deficient recombinant particles and the broad range of host cell infection have made alphaviruses attractive vehicles for applications in neuroscience and gene therapy. Efficient delivery to primary neurons and hippocampal slices has allowed localization studies of gene expression and electrophysiological recordings of ion channels. Alphavirus vectors have also been applied for in vivo delivery to rodent brain. Due to the strong local transient expression provided by alphavirus vectors a number of immunization and gene therapy approaches have demonstrated both therapeutic and prophylactic efficacy in various animal models.

The Role of Packaging Sites in Efficient and Specific Virus Assembly

During the life cycle of many single-stranded RNA viruses, including many human pathogens, a protein shell called the capsid spontaneously assembles around the viral genome. Understanding the mechanisms by which capsid proteins selectively assemble around the viral RNA amidst diverse host RNAs is a key question in virology. In one proposed mechanism, short sequences (packaging sites) within the genomic RNA promote rapid and efficient assembly through specific interactions with the capsid proteins. In this work, we develop a coarse-grained particle-based computational model for capsid proteins and RNA that represents protein-RNA interactions arising both from nonspecific electrostatics and from specific packaging site interactions. Using Brownian dynamics simulations, we explore how the efficiency and specificity of assembly depend on solution conditions (which control protein-protein and nonspecific protein-RNA interactions) and the strength and number of packaging sites. We identify distinct regions in parameter space in which packaging sites lead to highly specific assembly via different mechanisms and others in which packaging sites lead to kinetic traps. We relate these computational predictions to in vitro assays for specificity in which cognate viral RNAs compete against non-cognate RNAs for assembly by capsid proteins.

Alphaviruses in gene therapy

Alphavirus vectors present an attractive approach for gene therapy applications due to the rapid and simple recombinant virus particle production and their broad range of mammalian host cell transduction. Mainly three types of alphavirus vectors, namely naked RNA, recombinant particles and DNA/RNA layered vectors, have been subjected to preclinical studies with the goal of achieving prophylactic or therapeutic efficacy, particularly in oncology. In this context, immunization with alphavirus vectors has provided protection against challenges with tumor cells. Moreover, alphavirus intratumoral and systemic delivery has demonstrated substantial tumor regression and significant prolonged survival rates in various animal tumor models. Recent discoveries of the strong association of RNA interference and disease have accelerated gene therapy based approaches, where alphavirus-based gene delivery can play an important role.

Mechanisms of virus assembly

Viruses are nanoscale entities containing a nucleic acid genome encased in a protein shell called a capsid and in some cases are surrounded by a lipid bilayer membrane. This review summarizes the physics that govern the processes by which capsids assemble within their host cells and in vitro. We describe the thermodynamics and kinetics for the assembly of protein subunits into icosahedral capsid shells and how these are modified in cases in which the capsid assembles around a nucleic acid or on a lipid bilayer. We present experimental and theoretical techniques used to characterize capsid assembly, and we highlight aspects of virus assembly that are likely to receive significant attention in the near future.

Simulations show that virus assembly and budding are facilitated by membrane microdomains

For many viruses, assembly and budding occur simultaneously during virion formation. Understanding the mechanisms underlying this process could promote biomedical efforts to block viral propagation and enable use of capsids in nanomaterials applications. To this end, we have performed molecular dynamics simulations on a coarse-grained model that describes virus assembly on a fluctuating lipid membrane. Our simulations show that the membrane can promote association of adsorbed subunits through dimensional reduction, but it also introduces thermodynamic and kinetic effects that can inhibit complete assembly. We find several mechanisms by which membrane microdomains, such as lipid rafts, reduce these effects, and thus, enhance assembly. We show how these predicted mechanisms can be experimentally tested. Furthermore, the simulations demonstrate that assembly and budding depend crucially on the system dynamics via multiple timescales related to membrane deformation, protein diffusion, association, and adsorption onto the membrane.

Mutations in the alpha-helical region of the amino terminus of the Maize rayado fino virus capsid protein and CP:RNA ratios affect virus-like particle encapsidation of RNAs

Viral-based nanoplatforms rely on balancing the delicate array of virus properties to optimally achieve encapsidation of foreign materials with various potential objectives. We investigated the use of Maize rayado fino virus (MRFV)-virus-like particles (VLPs) as a multifunctional nanoplatform and their potential application as protein cages. MRFV-VLPs are composed of two serologically related, carboxy co-terminal coat proteins (CP1 and CP2) which are capable of self-assembling in Nicotiana benthamiana plants into 30nm particles with T=3 symmetry. The N-terminus of CP1 was targeted for genetic modification to exploit the driving forces for VLP assembly, packaging and retention of RNA in vivo and in vitro. The N-terminus of MRFV-CP1 contains a peptide sequence of 37 amino acids which has been predicted to have an alpha-helical structure, is rich in hydrophobic amino acids, facilitates CP-RNA interactions, and is not required for self-assembly. Amino acid substitutions were introduced in the 37 amino acid N-terminus by site-directed mutagenesis and the mutant VLPs produced in plants by a Potato virus X (PVX)-based vector were tested for particle stability and RNA encapsidation. All mutant CPs resulted in production of VLPs which encapsidated non-viral RNAs, including PVX genomic and subgenomic (sg) RNAs, 18S rRNA and cellular and viral mRNAs. In addition, MRFV-VLPs encapsidated GFP mRNA when was expressed in plant cells from the pGD vector. These results suggest that RNA packaging in MRFV-VLPs is predominantly driven by electrostatic interactions between the N-terminal 37 amino acid extension of CP1 and RNA, and that the overall species concentration of RNA in the cellular pool may determine the abundance and species of the RNAs packaged into the VLPs. Furthermore, RNA encapsidation is not required for VLPs stability, VLPs formed from MRFV-CP1 were stable at temperatures up to 70°C, and can be disassembled into CP monomers, which can then reassemble in vitro into complete VLPs either in the absence or presence of RNAs.

Dawn of advanced molecular medicine: nanotechnological advancements in cancer imaging and therapy

Nanotechnology plays an increasingly important role not only in our everyday life (with all its benefits and dangers) but also in medicine. Nanoparticles are to date the most intriguing option to deliver high concentrations of agents specifically and directly to cancer cells; therefore, a wide variety of these nanomaterials has been developed and explored. These span the range from simple nanoagents to sophisticated smart devices for drug delivery or imaging. Nanomaterials usually provide a large surface area, allowing for decoration with a large amount of moieties on the surface for either additional functionalities or targeting. Besides using particles solely for imaging purposes, they can also carry as a payload a therapeutic agent. If both are combined within the same particle, a theranostic agent is created. The sophistication of highly developed nanotechnology targeting approaches provides a promising means for many clinical implementations and can provide improved applications for otherwise suboptimal formulations. In this review we will explore nanotechnology both for imaging and therapy to provide a general overview of the field and its impact on cancer imaging and therapy.