Self-assembly of tobacco mosaic virus: the role of an intermediate aggregate in generating both specificity and speed

Aggregation of TMV CP plays a role in CP functions and in coat-protein-mediated resistance

Tobacco mosaic virus (TMV) coat protein (CP) in absence of RNA self-assembles into several different structures depending on pH and ionic strength. Transgenic plants that produce self-assembling CP are resistant to TMV infection, a phenomenon referred to as coat-protein-mediated resistance (CP-MR). The mutant CP Thr42Trp (CP(T42W)) produces enhanced CP-MR compared to wild-type CP. To establish the relationship between the formation of 20S CP aggregates and CP-MR, virus-like particles (VLPs) produced by TMV variants that yield high levels of CP-MR were characterized. We demonstrate that non-helical structures are found in VLPs formed in vivo by CP(T42W) but not by wild-type CP and suggest that the mutation shifts the intracellular equilibrium of aggregates from low to higher proportions of non-helical 20S aggregates. A similar shift in equilibrium of aggregates was observed with CP(D77R), another mutant that confers high level of CP-MR. The mutant CP(D50R) confers a level of CP-MR similar to wild-type CP and aggregates in a manner similar to wild-type CP. We conclude that increased CP-MR is correlated with a shift in intracellular equilibrium of CP aggregates, including aggregates that interfere with virus replication.

Assembly of trans-encapsidated recombinant viral vectors engineered from Tobacco mosaic virus and Semliki Forest virus and their evaluation as immunogens

RNA virus vectors are attractive vaccine delivery agents capable of directing high-level gene expression without integration into host cell DNA. However, delivery of non-encapsidated RNA viral vectors into animal cells is relatively inefficient. By introducing the tobacco mosaic virus (TMV) origin of assembly into the RNA genome of Semliki Forest virus (SFV), we generated an SFV expression vector that could be efficiently packaged (trans-encapsidated) in vitro by purified TMV coat protein (CP). Using cellular assays, pseudovirus disassembly, RNA replication and reporter gene expression were demonstrated. We also evaluated the immune response to trans-encapsidated recombinant SFV carrying a model antigen gene (beta-galactosidase) in C57/B6 mice. Relative to RNA alone, vector encapsidation significantly improved the humoral and cellular immune responses. Furthermore, reassembly with recombinant TMV CPs permitted the display of peptide epitopes on the capsid surface as either genetic fusions or through chemical conjugation, to complement the immunoreactivity of the encapsidated RNA genetic payload. The SFV vector/TMV CP system described provides an alternative nucleic acid delivery mechanism that is safe, easy to manufacture in vitro and that also facilitates the generation of unique nucleic acid/protein antigen compositions.

Study and characterization of tobacco mosaic virus head-to-tail assembly assisted by aniline polymerization

One-dimensional composite nanofibres with narrow dispersity, high aspect ratio and high processibility have been fabricated by head-to-tail self-assembly of rod-like tobacco mosaic virus assisted by aniline polymerization, which can promote many potential applications including electronics, optics, sensing and biomedical engineering.

Physical regulation of the self-assembly of tobacco mosaic virus coat protein

We present a statistical mechanical model based on the principle of mass action that explains the main features of the in vitro aggregation behavior of the coat protein of tobacco mosaic virus (TMV). By comparing our model to experimentally obtained stability diagrams, titration experiments, and calorimetric data, we pin down three competing factors that regulate the transitions between the different kinds of aggregated state of the coat protein. These are hydrophobic interactions, electrostatic interactions, and the formation of so-called "Caspar" carboxylate pairs. We suggest that these factors could be universal and relevant to a large class of virus coat proteins.

Virus-like particles: models for assembly studies and foreign epitope carriers

Virus‐like particles (VLPs), formed by the structural elements of viruses, have received considerable attention over the past two decades. The number of reports on newly obtained VLPs has grown proportionally with the systems developed for the expression of these particles. The chapter outlines the recent achievements in two important fields of research brought about by the availability of VLPs produced in a foreign host. These are: (1) The requirements for VLP assembly and (2) the use of VLPs as carriers for foreign epitopes. VLP technology is a rapidly advancing domain of molecular and structural biology. Extensive progress in VLP studies was achieved as the insect cell based protein production system was developed. This baculovirus expression system has many advantages for the synthesis of viral structural proteins resulting in the formation of VLPs. It allows production of large amounts of correctly folded proteins while also providing cell membranes that can serve as structural elements for enveloped viruses. These features give us the opportunity to gain insights into the interactions and requirements accompanying VLP formation that are similar to the assembly events occurring in mammalian cells. Other encouraging elements are the ability to easily scale up the system and the simplicity of purification of the assembled VLPs. The growing number of VLPs carrying foreign protein fragments on their surface and studies on the successful assembly of these chimeric molecules is a promising avenue towards the development of a new technology, in which the newly designed VLPs will be directed to particular mammalian cell types by exposing specific binding domains. The progress made in modeling the surface of VLPs makes them to date the best candidates for the design of delivery systems that can efficiently reach their targets.

Proton dependence of tobacco mosaic virus dissociation by pressure

Tobacco mosaic virus (TMV) is an intensely studied model of viruses. This paper reports an investigation into the dissociation of TMV by pH and pressure up to 220 MPa. The viral solution (0.25 mg/ml) incubated at 277 K showed a significant decrease in light scattering with increasing pH, suggesting dissociation. This observation was confirmed by HPLC gel filtration and electron microscopy. The calculated volume change of dissociation (DeltaV) decreased (absolute value) from -49.7 ml/mol of subunit at pH 3.8 to -21.7 ml/mol of subunit at pH 9.0. The decrease from pH 9.0 to 3.8 caused a stabilization of 14.1 kJ/mol of TMV subunit. The estimated proton release calculated from pressure-induced dissociation curves was 0.584 mol H(+)/mol of TMV subunit. These results suggest that the degree of virus inactivation by pressure and the immunogenicity of the inactivated structures can be optimized by modulating the surrounding pH.

Phylogenomics and bioinformatics of SARS-CoV

Tracing the history of molecular changes in coronaviruses using phylogenetic methods can provide powerful insights into the patterns of modification to sequences that underlie alteration to selective pressure and molecular function in the SARS-CoV (severe acute respiratory syndrome coronavirus) genome. The topology and branch lengths of the phylogenetic relationships among the family Coronaviridae, including SARS-CoV, have been estimated using the replicase polyprotein. The spike protein fragments S1 (involved in receptor-binding) and S2 (involved in membrane fusion) have been found to have different mutation rates. Fragment S1 can be further divided into two regions (S1A, which comprises approximately the first 400 nucleotides, and S1B, comprising the next 280) that also show different rates of mutation. The phylogeny presented on the basis of S1B shows that SARS-CoV is closely related to MHV (murine hepatitis virus), which is known to bind the murine receptor CEACAM1. The predicted structure, accessibility and mutation rate of the S1B region is also presented. Because anti-SARS drugs based on S2 heptads have short half-lives and are difficult to manufacture, our findings suggest that the S1B region might be of interest for anti-SARS drug discovery.

Surface-exposed amino- and carboxy-terminal residues are crucial for the initiation of assembly in Pepper vein banding virus: a flexuous rod-shaped virus

The mechanism of assembly of flexuous viruses, such as potyviruses, is poorly understood. Using a recombinant system, we provide evidence that disassembly and reassembly of Pepper vein banding virus (PVBV), a member of the genus potyvirus, proceeds via a ring-like intermediate, and show that electrostatic interactions may be pivotal in stabilizing the particles. Although the surface-exposed N- and C-terminal residues can be removed from the virus-like particles (VLPs) by limited trypsinization without affecting their stability, such truncated CP subunits are unable to form VLPs. To further evaluate importance of these residues, N- and C-terminal deletion mutants were generated and their assembly behavior was investigated. N-terminal 53 and C-terminal 23 amino acids were found to be crucial for the intersubunit interactions involved in the initiation of virus assembly. These segments are surface exposed in the ring-like intermediate and dispensable for further interactions that result in the formation of the VLPs.

Self-organization: making complex infectious viral particles from purified precursors

Viruses have served as excellent model systems in which to study biological self-organization. Purified virion structural constituents have been shown to self-assemble into particles that can initiate a productive infection in the host cell resulting in the release of progeny virions. Accumulating information on virus structures and assembly principles has revealed unexpected similarities between viruses that infect hosts as diverse as bacteria and humans, suggesting that these viruses had an early common ancestor. I will describe, in more detail, the assembly pathway of a complex double-stranded RNA bacterial virus. In this system, infectious viral particles are produced starting from purified protein and nucleic acid constituents through an elaborate self-assembly, RNA-packaging and synthesis pathway.

Tobacco mosaic virus assembly and disassembly: determinants in pathogenicity and resistance

The structural proteins of plant viruses have evolved to self-associate into complex macromolecules that are centrally involved in virus biology. In this review, the structural and biophysical properties of the Tobacco mosaic virus (TMV) coat protein (CP) are addressed in relation to its role in host resistance and disease development. TMV CP affects the display of several specific virus and host responses, including cross-protection, systemic virus movement, hypersensitive disease resistance, and symptom development. Studies indicate that the three-dimensional structure of CP is critical to the control of these responses, either directly through specific structural motifs or indirectly via alterations in CP assembly. Thus, both the structure and assembly of the TMV CP function as determinants in the induction of disease and resistance responses.