Turnip yellow mosaic virus isolates with experimentally produced recombinant virion proteins

Tobacco Necrosis Virus-AC Single Coat Protein Amino Acid Substitutions Determine Host-Specific Systemic Infections of Nicotiana benthamiana and Soybean

Plant viruses often infect several distinct host species. Sometimes, viruses can systemically infect a specific host whereas, in other cases, only local infections occur in other species. How viral and host factors interact to determine systemic infections among different hosts is largely unknown, particularly for icosahedral positive-stranded RNA viruses. The Tobacco necrosis virus-A Chinese isolate belongs to the genus Alphanecrovirus in the family Tombusviridae. In this study, we investigated variations in systemic infections of tobacco necrosis virus-AC (TNV-AC) in Nicotiana benthamiana and Glycine max (soybean) by alanine-scanning mutagenesis of the viral coat protein (CP), which is essential for systemic movement of TNV-AC. We found that three amino acids, R169, K177, and Q233, are key residues that mediate varying degrees of systemic infections of N. benthamiana and soybean. Further analysis revealed that variations in systemic trafficking of TNV-AC CP mutants in N. benthamiana and soybean are associated with virion assembly and stability. The CP amino acids K177 and Q233 are highly conserved among all TNV-A isolates and are replaced by Q and K in the TNV-D isolates. We demonstrated that systemic infectivity of either TNV-AC K177A and Q233A or K177Q and Q233K mutants are correlated with the binding affinity of the mutated CPs to the host-specific Hsc70-2 protein. These results expand our understanding of host-dependent long-distance movement of icosahedral viruses in plants.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Assembly of tomato blistering mosaic virus-like particles using a baculovirus expression vector system

The expression of several structural proteins from a wide variety of viruses in heterologous cell culture systems results in the formation of virus-like particles (VLPs). These VLPs structurally resemble the wild-type virus particles and have been used to study viral assembly process and as antigens for diagnosis and/or vaccine development. Tomato blistering mosaic virus (ToBMV) is a tymovirus that has a 6.3-kb positive-sense ssRNA genome. We have employed the baculovirus expression vector system (BEVS) for the production of tymovirus-like particles (tVLPs) in insect cells. Two recombinant baculoviruses containing the ToBMV wild-type coat protein (CP) gene or a modified short amino-terminal deletion (Δ_2-24CP) variant were constructed and used to infect insect cells. Both recombinant viruses were able to express ToBMV CP and Δ_2-24CP from infected insect cells that self-assembled into tVLPs. Therefore, the N-terminal residues (2-24) of the native ToBMV CP were shown not to be essential for self-assembly of tVLPs. We also constructed a third recombinant baculovirus containing a small sequence coding for the major epitope of the chikungunya virus (CHIKV) envelope protein 2 (E2) replacing the native CP N-terminal 2-24 amino acids. This recombinant virus also produced tVLPs. In summary, ToBMV VLPs can be produced in a baculovirus/insect cell heterologous expression system, and the N-terminal residues 2-24 of the CP are not essential for this assembly, allowing its potential use as a protein carrier that facilitates antigen purification and might be used for diagnosis.

Modification of Turnip yellow mosaic virus coat protein and its effect on virion assembly

Turnip yellow mosaic virus (TYMV) is a positive strand RNA virus. We have modified TYMV coat protein (CP) by inserting a c-Myc epitope peptide at the N- or C-terminus of the CP, and have examined its effect on assembly. We introduced the recombinant CP constructs into Nicotiana benthamiana leaves by agroinfiltration. Examination of the leaf extracts by agarose gel electrophoresis and Western blot analysis showed that the CP modified at the N-terminus produced a band co-migrating with wild-type virions. With C-terminal modification, however, the detected bands moved faster than the wild-type virions. To further examine the effect, TYMV constructs producing the modified CPs were prepared. With N-terminal modification, viral RNAs were protected from RNase A. In contrast, the viral RNAs were not protected with C-terminal modification. Overall, the results suggest that virion assembly and RNA packaging occur properly when the N-terminus of CP is modified, but not when the C-terminus is modified.

Two histidines of the coat protein of turnip yellow mosaic virus at the capsid interior are crucial for viability

RNA-coat protein interactions in turnip yellow mosaic virus (TYMV) have been shown to involve low pK proton-donating groups. Two different types of interaction have been proposed. In the so-called type I interaction, protonated C-residues interact with acidic amino acids at low pH, thereby providing a rationale for the high C-content (38%) of the genomic RNA. The type II interaction involves charged histidines interacting with phosphates of the RNA backbone. Site-directed mutagenesis of the TYMV coat protein and subsequent in vivo analysis were performed to distinguish between these two types of RNA-protein interaction. The results reveal a prominent role for the histidines H68 and H180, since mutation to an alanine residue inhibits symptom development on secondary leaves, indicating that spreading of the virus in the plant is blocked. Viral RNA and coat protein synthesis are not altered, showing that these two histidines may play a role in the process of RNA encapsidation. Overexpression of the TYMV coat protein in Escherichia coli leads to the formation of bona fide capsids, showing that the two histidines are not critical in capsid assembly. Mutagenesis of the acidic amino acids D11, E135, and D143 to alanine apparently did not interfere with virus viability. The functional role of the histidines during the infection cycle is discussed in terms of the structure of the coat protein, both at the level of amino acid sequence conservation among the members of the Tymoviridae family and as the three-dimensional structure of the coat protein.

The 5'-proximal hairpin of turnip yellow mosaic virus RNA: its role in translation and encapsidation

The RNA genome of turnip yellow mosaic virus (TYMV) consists of more than 6,000 nucleotides. During a study of the roles of the two hairpins located in its 90-nucleotide 5' untranslated region, it was observed that stabilization of the 5'-proximal hairpin leads to a delay in the development of symptoms on plants. This delay in symptom development for both locally and systemically infected leaves was found to be dependent on a change in the free energy of the hairpin caused by introduced mutations. A protoplast transfection assay revealed that the accumulation of plus-strand full-length RNA and subgenomic RNA, as well as protein expression levels, was affected by hairpin stability. Stabilization of this hairpin inhibited translation. A model is proposed in which a destabilized 5'-proximal hairpin allows maximal translation of the viral proteins. It is suggested that this hairpin may exist in close proximity to the 5' cap as long as its stability is low enough to enable translation. However, at an acidic pH, the hairpin structure becomes more stable and is functionally transformed into the initiation signal for viral packaging. Slightly acidic conditions can be found in chloroplasts, where TYMV assembly is driven by a low pH generated by active photosynthesis.

Identification of a discrete intermediate in the assembly/disassembly of physalis mottle tymovirus through mutational analysis

Assembly intermediates of icosahedral viruses are usually transient and are difficult to identify. In the present investigation, site-specific and deletion mutants of the coat protein gene of physalis mottle tymovirus (PhMV) were used to delineate the role of specific amino acid residues in the assembly of the virus and to identify intermediates in this process. N-terminal 30, 34, 35 and 39 amino acid deletion and single C-terminal (N188) deletion mutant proteins of PhMV were expressed in Escherichia coli. Site-specific mutants H69A, C75A, W96A, D144N, D144N-T151A, K143E and N188A were also constructed and expressed. The mutant protein lacking 30 amino acid residues from the N terminus self-assembled to T=3 particles in vivo while deletions of 34, 35 and 39 amino acid residues resulted in the mutant proteins that were insoluble. Interestingly, the coat protein (pR PhCP) expressed using pRSET B vector with an additional 41 amino acid residues at the N terminus also assembled into T=3 particles that were more compact and had a smaller diameter. These results demonstrate that the amino-terminal segment is flexible and either the deletion or addition of amino acid residues at the N terminus does not affect T=3 capsid assembly. In contrast, the deletion of even a single residue from the C terminus (PhN188Delta1) resulted in capsids that were unstable. These capsids disassembled to a discrete intermediate with a sedimentation coefficent of 19.4 S. However, the replacement of C-terminal asparagine 188 by alanine led to the formation of stable capsids. The C75A and D144N mutant proteins also assembled into capsids that were as stable as the pR PhCP, suggesting that C75 and D144 are not crucial for the T=3 capsid assembly. pR PhW96A and pR PhD144N-T151A mutant proteins failed to form capsids and were present as heterogeneous aggregates. Interestingly, the pR PhK143E mutant protein behaved in a manner similar to the C-terminal deletion protein in forming unstable capsids. The intermediate with an s value of 19.4 S was the major assembly product of pR PhH69A mutant protein and could correspond to a 30mer. It is possible that the assembly or disassembly is arrested at a similar stage in pR PhN188Delta1, pR PhH69A and pR PhK143E mutant proteins.