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

Structural analysis and insertion study reveal the ideal sites for surface displaying foreign peptides on a betanodavirus-like particle

Betanodavirus infection causes fatal disease of viral nervous necrosis in many cultured marine and freshwater fish worldwide and the virus-like particles (VLP) are effective vaccines against betanodavirus. But vaccine and viral vector designs of betanodavirus VLP based on their structures remain lacking. Here, the three-dimensional structure of orange-spotted grouper nervous necrosis virus (OGNNV) VLP (RBS) at 3.9 Å reveals the organization of capsid proteins (CP). Based on the structural results, seven putative important sites were selected to genetically insert a 6× histidine (His)-tag for VLP formation screen, resulting in four His-tagged VLP (HV) at positions N-terminus, Ala220, Pro292 and C-terminus. The His-tags of N-terminal HV (NHV) were concealed inside virions while those of 220HV and C-terminal HV (CHV) were displayed at the outer surface. NHV, 220HV and CHV maintained the same cell entry ability as RBS in the Asian sea bass (SB) cell line, indicating that their similar surface structures can be recognized by the cellular entry receptor(s). For application of vaccine design, chromatography-purified CHV could provoke NNV-specific antibody responses as strong as those of RBS in a sea bass immunization assay. Furthermore, in carrying capacity assays, N-terminus and Ala220 can only carry short peptides and C-terminus can even accommodate large protein such as GFP to generate fluorescent VLP (CGV). For application of a viral vector, CGV could be real-time visualized to enter SB cells in invasion study. All the results confirmed that the C-terminus of CP is a suitable site to accommodate foreign peptides for vaccine design and viral vector development.

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

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

Baculovirus-expressed virus-like particles of Pea enation mosaic virus vary in size and encapsidate baculovirus mRNAs

Pea enation mosaic virus (PEMV: family Luteoviridae) is transmitted in a persistent, circulative manner by aphids. We inserted cDNAs encoding the structural proteins of PEMV, the coat protein (CP) and coat protein-read through domain (CPRT) into the genome of the baculovirus Autographa californica multiple nucleopolyhedrovirus with and without a histidine tag or an upstream Kozak consensus sequence. The Sf21 cell line provided the highest level of CP expression of the cell lines tested and resulted in production of virus-like particles (VLPs). The CPRT was not detected in recombinant baculovirus-infected cells by Western blot. Addition of a Kozak sequence increased the yield of baculovirus produced VLPs. Baculovirus-expressed VLPs purified on a nickel NTA column were of variable size (13-30 nm) and contained CP mRNA. The purified VLPs were also shown by RT-PCR to contain 70% of 154 baculovirus mRNAs, indicative of non-specific RNA encapsidation in the absence of viral RNA replication. When fed to the pea aphid, Acyrthosiphon pisum (Harris), the VLPs entered the aphid hemocoel, demonstrating that CPRT is not required for uptake of PEMV from the aphid gut. Baculovirus expression of PEMV VLPs provides a useful tool for future analysis of RNA encapsidation requirements and molecular aphid-virus interactions.

RNA-binding properties of Dendrolimus punctatus tetravirus p17 protein

The RNA-binding properties of the p17 protein of Dendrolimus punctatus tetravirus were analysed. We have demonstrated that p17 protein, a nonstructural protein with a potentially important role in viral replication, is a RNA-binding protein, which has not been previously described for any member of the family Tetraviridae. P17 protein was expressed in Escherichia coli and was used in UV cross-linking analysis, using a digoxigenin-UTP-labeled RNA probe and chemical cross-linking analysis. The analyses demonstrated that p17 protein could bind to RNA. When analysed for capacity of p17 to form oligomers, the protein could form dimers and tetramers. Furthermore, the circular dichroism spectrums of viral RNA 3'-UTR proved that their secondary structures were consistent with yeast tRNA.