Characterization of a disassembly deficient mutant of cowpea chlorotic mottle virus

Coupling Peptide Antigens to Virus-Like Particles or to Protein Carriers Influences the Th1/Th2 Polarity of the Resulting Immune Response

We have conjugated the S9 peptide, a mimic of the group B streptococcal type III capsular polysaccharide, to different carriers in an effort to elicit an optimal immune response. As carriers, we utilized the soluble protein keyhole limpet hemocyanin and virus-like particles (VLPs) from two plant viruses, Cowpea Chlorotic Mottle Virus and Cowpea Mosaic Virus. We have found that coupling the peptide to the soluble protein elicits a Th2 immune response, as evidenced by the production of the peptide-specific IgG1 antibody and IL-4/IL-10 production in response to antigen stimulation, whereas the peptide conjugated to VLPs elicited a Th1 response (IgG2a, IFN-γ). Because the VLPs used as carriers package RNA during the assembly process, we hypothesize that this effect may result from the presence of nucleic acid in the immunogen, which affects the Th1/Th2 polarity of the response.

Virus engineering: functionalization and stabilization

Chemically and/or genetically engineered viruses, viral capsids and viral-like particles carry the promise of important and diverse applications in biomedicine, biotechnology and nanotechnology. Potential uses include new vaccines, vectors for gene therapy and targeted drug delivery, contrast agents for molecular imaging and building blocks for the construction of nanostructured materials and electronic nanodevices. For many of the contemplated applications, the improvement of the physical stability of viral particles may be critical to adequately meet the demanding physicochemical conditions they may encounter during production, storage and/or medical or industrial use. The first part of this review attempts to provide an updated general overview of the fast-moving, interdisciplinary virus engineering field; the second part focuses specifically on the modification of the physical stability of viral particles by protein engineering, an emerging subject that has not been reviewed before.

Phosphorylation of hepatitis B virus core C-terminally truncated protein (Cp149) by PKC increases capsid assembly and stability

The HBV (hepatitis B virus) core is a phosphoprotein whose assembly, replication, encapsidation and localization are regulated by phosphorylation. It is known that PKC (protein kinase C) regulates pgRNA (pregenomic RNA) encapsidation by phosphorylation of the C-terminus of core, which is a component packaged into capsid. Neither the N-terminal residue phosphorylated by PKC nor the role of the C-terminal phosphorylation have been cleary defined. In the present study we found that HBV Cp149 (core protein C-terminally truncated at amino acid 149) expressed in Escherichia coli was phosphorylated by PKC at Ser106. PKC-mediated phosphorylation increased core affinity, as well as assembly and capsid stability. In vitro phosphorylation with core mutants (S26A, T70A, S106A and T114A) revealed that the Ser106 mutation inhibited phosphorylation of core by PKC. CD analysis also revealed that PKC-mediated phosphorylation stabilized the secondary structure of capsid. When either pCMV/FLAG-Cp149[WT (wild-type)] or pCMV/FLAG-S106A Cp149 was transfected into Huh7 human hepatoma cells, mutant capsid level was decreased by 2.06-fold with the S106A mutant when compared with WT, although the same level of total protein was expressed in both cases. In addition, when pUC1.2x and pUC1.2x/S106A were transfected, mutant virus titre was decreased 2.31-fold compared with WT virus titre. In conclusion, PKC-mediated phosphorylation increased capsid assembly, stability and structural stability.

Electrostatic properties of cowpea chlorotic mottle virus and cucumber mosaic virus capsids

Electrostatic properties of cowpea chlorotic mottle virus (CCMV) and cucumber mosaic virus (CMV) were investigated using numerical solutions to the Poisson-Boltzmann equation. Experimentally, it has been shown that CCMV particles swell in the absence of divalent cations when the pH is raised from 5 to 7. CMV, although structurally homologous, does not undergo this transition. An analysis of the calculated electrostatic potential confirms that a strong electrostatic repulsion at the calcium-binding sites in the CCMV capsid is most likely the driving force for the capsid swelling process during the release of calcium. The binding interaction between the encapsulated genome material (RNA) inside of the capsid and the inner capsid shell is weakened during the swelling transition. This probably aids in the RNA release process, but it is unlikely that the RNA is released through capsid openings due to unfavorable electrostatic interaction between the RNA and capsid inner shell residues at these openings. Calculations of the calcium binding energies show that Ca(2+) can bind both to the native and swollen forms of the CCMV virion. Favorable binding to the swollen form suggests that Ca(2+) ions can induce the capsid contraction and stabilize the native form.

Nanoindentation studies of full and empty viral capsids and the effects of capsid protein mutations on elasticity and strength

The elastic properties of capsids of the cowpea chlorotic mottle virus have been examined at pH 4.8 by nanoindentation measurements with an atomic force microscope. Studies have been carried out on WT capsids, both empty and containing the RNA genome, and on full capsids of a salt-stable mutant and empty capsids of the subE mutant. Full capsids resisted indentation more than empty capsids, but all of the capsids were highly elastic. There was an initial reversible linear regime that persisted up to indentations varying between 20% and 30% of the diameter and applied forces of 0.6-1.0 nN; it was followed by a steep drop in force that is associated with irreversible deformation. A single point mutation in the capsid protein increased the capsid stiffness. The experiments are compared with calculations by finite element analysis of the deformation of a homogeneous elastic thick shell. These calculations capture the features of the reversible indentation region and allow Young's moduli and relative strengths to be estimated for the empty capsids.

Interaction with capsid protein alters RNA structure and the pathway for in vitro assembly of cowpea chlorotic mottle virus

Viruses use sophisticated mechanisms to allow the specific packaging of their genome over that of host nucleic acids. We examined the in vitro assembly of the Cowpea chlorotic mottle virus (CCMV) and observed that assembly with viral RNA follows two different mechanisms. Initially, CCMV capsid protein (CP) dimers bind RNA with low cooperativity and form virus-like particles of 90 CP dimers and one copy of RNA. Longer incubation reveals a different assembly path. At a stoichiometry of about ten CP dimers per RNA, the CP slowly folds the RNA into a compact structure that can be bound with high cooperativity by additional CP dimers. This folding process is exclusively a function of CP quaternary structure and is independent of RNA sequence. CP-induced folding is distinct from RNA folding that depends on base-pairing to stabilize tertiary structure. We hypothesize that specific encapsidation of viral RNA is a three-step process: specific binding by a few copies of CP, RNA folding, and then cooperative binding of CP to the "labeled" nucleoprotein complex. This mechanism, observed in a plant virus, may be applicable to other viruses that do not halt synthesis of host nucleic acid, including HIV.

Pseudo-atomic models of swollen CCMV from cryo-electron microscopy data

The capsid of cowpea chlorotic mottle virus (CCMV) can reversibly switch between two forms that are contingent on the charge of acidic residues that are clustered at the quasi-threefold axes of the T=3 icosahedral particle. The quaternary structure conformations are dependent on divalent metal ions and pH and were previously analyzed by crystallography in the native, compact form, and by cryo-electron microscopy in the compact and swollen forms (Speir et al., 1995). In this report we use the atomic models of the three structurally unique viral subunits determined by crystallography for a detailed interpretation of the 28-A-resolution electron density of the swollen form and the production of a pseudo-atomic model of this particle. The model of the quaternary structure conforms with high fidelity to conventional geometric constraints, quasi-equivalence, intersubunit association energies, and the electron density. It was derived by conserving the pentamers and hexamers of subunits whose associated electron densities are strikingly similar in the two forms of the particles. Treating these as rigid units in the modeling implies that the particle flexibility is accommodated primarily by changes in dimer interactions, an observation that is consistent with the flexible C-terminal polypeptide extensions that stabilize this contact in the crystal structure. Because the hexamers and pentamers were incrementally translated and rotated in a screw motion, with energy minimization at each of 28 steps, a path for the expansion is also implied.

Effects of the cowpea chlorotic mottle bromovirus beta-hexamer structure on virion assembly

The X-ray crystal structure of Cowpea chlorotic mottle bromovirus (CCMV) revealed a unique tubular structure formed by the interaction of the N-termini from six coat protein subunits at each three-fold axis of the assembled virion. This structure, termed the beta-hexamer, consists of six short beta-strands. The beta-hexamer was postulated to play a critical role in the assembly and stability of the virion by stabilizing hexameric capsomers. Mutational analyses of the beta-hexamer structure, utilizing both in vitro and in vivo assembly assays, demonstrate that this structure is not required for virion formation devoid of nucleic acids in vitro or for RNA-containing virions in vivo. However, the beta-hexamer structure does contribute to virion stability in vitro and modulates disease expression in vivo. These results support a model for CCMV assembly through pentamer intermediates.

The structure of tomato aspermy virus by X-ray crystallography

The three-dimensional structure of tomato aspermy virus (TAV) has been solved by X-ray crystallography and refined to an R factor of 0.218 for 3.4-40 A data (effective resolution of 4A). Molecular replacement, using cucumber mosaic virus (Smith et al., 2000), provided phases for the initial maps used for model building. The coat protein of the 280 A diameter virion has the canonical "Swiss roll" beta-barrel topology with a distinctive amino-terminal alpha-helix directed into the interior of the virus where it interacts with encapsidated RNA. The N-terminal helices are joined to the beta-barrels of protein subunits by extended polypeptides of six amino acids, which serve as flexible hinges allowing movement of the helices in response to local RNA distribution. Segments of three nucleotides of partially disordered RNA interact with the capsid, primarily through arginine residues, at interfaces between A and B subunits. Side chains of cys64 and cys106 form the first disulfide observed in a cucumovirus, including a unique cysteine, 106, in a region otherwise conserved. A positive ion, putatively modeled as a Mg(+)ion, lies on the quasi-threefold axis surrounded by three quasi-symmetric glutamate 175 side chains.

Quasi-equivalent viruses: a paradigm for protein assemblies

The structure and assembly of icosahedral virus capsids composed of one or more gene products and displaying quasi-equivalent subunit associations are discussed at three levels. The principles of quasi-equivalence and the related geodesic dome formation are first discussed conceptually and the geometric basis for their construction from two-dimensional assembly units is reviewed. The consequences for such an assembly when three-dimensional protein subunits are the associating components are then discussed with the coordinates of cowpea chlorotic mottle virus (CCMV) used to generate hypothetical structures in approximate agreement with the conceptual models presented in the first section. Biophysical, molecular genetic, and atomic structural data for CCMV are then reviewed, related to each other, and incorporated into an assembly model for CCMV that is discussed with respect to the modular, chemical nature of the viral subunit structure. The concepts of quasi-equivalence are then examined in some larger virus structures containing multiple subunit types and auxiliary proteins and the need for additional control points in their assembly are considered. The conclusion suggests that some viral assembly principles are limited paradigms for protein associations occurring in the broader range of cell biology including signal transduction, interaction of transcription factors and protein trafficking.

Virion swelling is not required for cotranslational disassembly of cowpea chlorotic mottle virus in vitro

The mechanism by which virions of cowpea chlorotic mottle virus (CCMV) disassemble and allow for translation of the virion RNA is not well understood. Previous models have suggested that virion swelling is required to expose the virion RNA for translation in a process referred to as cotranslational disassembly (M. Brisco, R. Hull, and T. M. A. Wilson, Virology 148:210-217, 1986; J. W. Roenhorst, J. W. M. van Lent, and B. J. M. Verduin, Virology 164:91-98, 1988; J. W. Roenhorst, J. M. Verduin, and R. W. Goldbach, Virology 168:138-146, 1989). Previous work in our laboratory has identified point mutations in the CCMV coat protein which result in virions with altered swelling characteristics (J. Fox, F. G. Albert, J. Speir, and M. J. Young, Virology 227:229-233, 1997; J. M. Fox, X. Zhao, J. A. Speir, and M. J. Young, Virology 222:115-122, 1996). The wild-type and mutant CCMV virions were used to correlate virion swelling with the ability of virion RNA to be translated in a cell-free wheat germ extract. Mutant virions unable to swell (cpK42R) are as infectious as wild-type virions in vivo, and the levels of translated encapsidated virion RNA are similar to those of wild-type virions in vitro. Mutant virions capable of swelling but not of disassembling in vitro (cpR26C) are noninfectious and have severely reduced levels of translation of the encapsidated virion RNA in vitro. These studies suggest that virion swelling is not required for the cotranslational disassembly of CCMV. Additionally, the results indicate that there is a pH-dependent structural transition in the virion, other than swelling, that results in the RNA's being exposed for translation in vitro. An alternative model suggesting that cotranslational disassembly of CCMV involves presentation of the virion RNA through the virion fivefold axis is proposed.