Induction of particle polymorphism by cucumber necrosis virus coat protein mutants in vivo

D'Ann Rochon (1955-2022), a life of passion for plant virology

D'Ann Rochon passed away on November 29th 2022. She is remembered for her outstanding contributions to the field of plant virology, her strong commitment to high quality science and her dedication to the training and mentorship of the next generation of scientists. She was a research scientist for Agriculture and Agri-Food Canada and an Adjunct Professor for the University of British Columbia. Her research program provided new insights on the infection cycle of tombusviruses and related viruses, including ground-breaking research on the structure of virus particles, the mechanisms of virus transmission by fungal zoospores, and the complexity of plant-virus interactions. She also developed diagnostic antibodies for plum pox virus and little cherry virus 2 that have had a significant impact on the management of these viruses.

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.

Targeting of cucumber necrosis virus coat protein to the chloroplast stroma attenuates host defense response

Cucumber necrosis virus (CNV) is a (+)ssRNA virus that elicits spreading local and systemic necrosis in Nicotiana benthamiana. We previously showed that the CNV coat protein (CP) arm functions as a chloroplast transit peptide that targets a CP fragment containing the S and P domains to chloroplasts during infection. Here we show that several CP arm mutants that inefficiently target chloroplasts, along with a mutant that lacks the S and P domains, show an early onset of more localized necrosis along with protracted induction of pathogenesis related protein (PR1a). Agroinfiltrated CNV CP is shown to interfere with CNV p33 and Tomato bushy stunt virus p19 induced necrosis. Additionally, we provide evidence that a CP mutant that does not detectably enter the chloroplast stroma induces relatively higher levels of several plant defense-related genes compared to WT CNV. Together, our data suggest that targeting of CNV CP to the chloroplast stroma interferes with chloroplast-mediated plant defense.

Structural Perspectives of Beak and Feather Disease Virus and Porcine Circovirus Proteins

Circoviruses represent a rapidly expanding group of viruses that infect both vertebrate and invertebrate hosts. Members are responsible for diseases of veterinary and economic importance, including postweaning multisystemic wasting syndrome in pigs, and beak and feather disease (BFD) in birds. These viruses are associated with lymphoid depletion and immunosuppressive conditions in infected animals leading to systemic illness. Circoviruses are small nonenveloped DNA viruses containing a single-stranded circular genome, encoding two major proteins: the capsid-associated protein (Cap), comprising the entirety of the viral capsid, and the replication-associated protein (Rep). Cap is the only protein component of the virion and plays crucial roles throughout the virus replication cycle, including viral attachment, cell entry, genome uncoating, and packaging of newly formed viral particles. Rep mediates recognition of replication origin motifs in the viral genome sequence and is responsible for endonuclease activity enabling nicking of the circular DNA and initiation of rolling-circle replication (RCR). Porcine circovirus 2 (PCV2) was the first circovirus capsid structure to be solved at atomic resolution using X-ray crystallography. The structure revealed an assembly comprising 60 monomeric subunits to form virus-like particles. Each Cap monomer harbors a canonical viral jelly roll domain composed of two, four-stranded antiparallel β-sheets. Crystal structures of two distinct macromolecular assemblies from BFD virus Cap were also resolved at high resolution. In these structures, the exposure of the N-terminal arginine-rich motif, responsible for DNA binding and nuclear localization is reversed. Additional structural investigations have also elucidated a PCV2 type-specific neutralizing epitope, and interaction between the PCV2 capsid and polymers such as heparin. In this review, we provide a snapshot of the structural and functional aspects of circovirus proteins.

Near-Atomic-Resolution Cryo-Electron Microscopy Structures of Cucumber Leaf Spot Virus and Red Clover Necrotic Mosaic Virus: Evolutionary Divergence at the Icosahedral Three-Fold Axes

Members of the Tombusviridae family have highly similar structures, and yet there are important differences among them in host, transmission, and capsid stabilities. Viruses in the Tombusviridae family have single-stranded RNA (ssRNA) genomes with T=3 icosahedral protein shells with a maximum diameter of ∼340 Å. Each capsid protein is comprised of three domains: R (RNA binding), S (shell), and P (protruding). Between the R domain and S domain is the "arm" region that studies have shown to play a critical role in assembly. To better understand how the details of structural differences and similarities influence the Tombusviridae viral life cycles, the structures of cucumber leaf spot virus (CLSV; genus Aureusvirus) and red clover necrotic mosaic virus (RCNMV; genus Dianthovirus) were determined to resolutions of 3.2 Å and 2.9 Å, respectively, with cryo-electron microscopy and image reconstruction methods. While the shell domains had homologous structures, the stabilizing interactions at the icosahedral 3-fold axes and the R domains differed greatly. The heterogeneity in the R domains among the members of the Tombusviridae family is likely correlated with differences in the sizes and characteristics of the corresponding genomes. We propose that the changes in the R domain/RNA interactions evolved different arm domain interactions at the β-annuli. For example, RCNMV has the largest genome and it appears to have created the necessary space in the capsid by evolving the shortest R domain. The resulting loss in RNA/R domain interactions may have been compensated for by increased intersubunit β-strand interactions at the icosahedral 3-fold axes. Therefore, the R and arm domains may have coevolved to package different genomes within the conserved and rigid shell.IMPORTANCE Members of the Tombusviridae family have nearly identical shells, and yet they package genomes that range from 4.6 kb (monopartite) to 5.3 kb (bipartite) in size. To understand how this genome flexibility occurs within a rigidly conserved shell, we determined the high-resolution cryo-electron microscopy (cryo-EM) structures of cucumber leaf spot virus and red clover necrotic mosaic virus. In response to genomic size differences, it appears that the ssRNA binding (R) domain of the capsid diverged evolutionarily in order to recognize the different genomes. The next region, the "arm," seems to have also coevolved with the R domain to allow particle assembly via interactions at the icosahedral 3-fold axes. In addition, there are differences at the icosahedral 3-fold axes with regard to metal binding that are likely important for transmission and the viral life cycle.

The atomic structures of shrimp nodaviruses reveal new dimeric spike structures and particle polymorphism

Shrimp nodaviruses, including Penaeus vannamei (PvNV) and Macrobrachium rosenbergii nodaviruses (MrNV), cause white-tail disease in shrimps, with high mortality. The viral capsid structure determines viral assembly and host specificity during infections. Here, we show cryo-EM structures of T = 3 and T = 1 PvNV-like particles (PvNV-LPs), crystal structures of the protrusion-domains (P-domains) of PvNV and MrNV, and the crystal structure of the ∆N-ARM-PvNV shell-domain (S-domain) in T = 1 subviral particles. The capsid protein of PvNV reveals five domains: the P-domain with a new jelly-roll structure forming cuboid-like spikes; the jelly-roll S-domain with two calcium ions; the linker between the S- and P-domains exhibiting new cross and parallel conformations; the N-arm interacting with nucleotides organized along icosahedral two-fold axes; and a disordered region comprising the basic N-terminal arginine-rich motif (N-ARM) interacting with RNA. The N-ARM controls T = 3 and T = 1 assemblies. Increasing the N/C-termini flexibility leads to particle polymorphism. Linker flexibility may influence the dimeric-spike arrangement.

Nai-Chi Chen et al. solved the structures of two shrimp nodaviruses, focusing on the major domains to improve understanding of capsid organization. By combining cryo-EM and x-ray crystallography, the authors were able to observe the structures at a high resolution.

Evidence for the role of basic amino acids in the coat protein arm region of Cucumber necrosis virus in particle assembly and selective encapsidation of viral RNA

Cucumber necrosis virus (CNV) is a T = 3 icosahedral virus with a (+)ssRNA genome. The N-terminal CNV coat protein arm contains a conserved, highly basic sequence ("KGRKPR"), which we postulate is involved in RNA encapsidation during virion assembly. Seven mutants were constructed by altering the CNV "KGRKPR" sequence; the four basic residues were mutated to alanine individually, in pairs, or in total. Virion accumulation and vRNA encapsidation were significantly reduced in mutants containing two or four substitutions and virion morphology was also affected, where both T = 1 and intermediate-sized particles were produced. Mutants with two or four substitutions encapsidated significantly greater levels of truncated RNA than that of WT, suggesting that basic residues in the "KGRKPR" sequence are important for encapsidation of full-length CNV RNA. Interestingly, "KGRKPR" mutants also encapsidated relatively higher levels of host RNA, suggesting that the "KGRKPR" sequence also contributes to selective encapsidation of CNV RNA.

A simple method to estimate the isoelectric point of modified Tomato bushy stunt virus (TBSV) particles

We present a simple method to estimate the isoelectric point (pI) of Tomato Bushy Stunt particles. We demonstrate that the combination of agarose gels with different pH buffers can be used to electrophorese the virus particles and their migration patterns can be compared. This method allows us to estimate the pI of the virus particles (wild type, wt, and genetically modified particles) and to monitor the effect of the pI of modified peptide side chains of the viral capsid subunit on the pI of the whole virus particle.

Dissecting the multifunctional role of the N-terminal domain of the Melon necrotic spot virus coat protein in RNA packaging, viral movement and interference with antiviral plant defence

The coat protein (CP) of Melon necrotic spot virus (MNSV) is structurally composed of three major domains. The middle S-domain builds a robust protein shell around the viral genome, whereas the C-terminal protruding domain, or P-domain, is involved in the attachment of virions to the transmission vector. Here, we have shown that the N-terminal domain, or R-domain, and the arm region, which connects the R-domain and S-domain, are involved in different key steps of the viral cycle, such as cell-to-cell movement and the suppression of RNA silencing and pathogenesis through their RNA-binding capabilities. Deletion mutants revealed that the CP RNA-binding ability was abolished only after complete, but not partial, deletion of the R-domain and the arm region. However, a comparison of the apparent dissociation constants for the CP RNA-binding reaction of several partial deletion mutants showed that the arm region played a more relevant role than the R-domain in in vitro RNA binding. Similar results were obtained in in vivo assays, although, in this case, full-length CPs were required to encapsidate full-length genomes. We also found that the R-domain carboxyl portion and the arm region were essential for efficient cell-to-cell movement, for enhancement of Potato virus X pathogenicity, for suppression of systemic RNA silencing and for binding of small RNAs. Therefore, unlike other carmovirus CPs, the R-domain and the arm region of MNSV CP have acquired, in addition to other essential functions such as genome binding and encapsidation functions, the ability to suppress RNA silencing by preventing systemic small RNA transport.

Evidence that Hsc70 Is Associated with Cucumber Necrosis Virus Particles and Plays a Role in Particle Disassembly

Uncoating of a virus particle to expose its nucleic acid is a critical aspect of the viral multiplication cycle, as it is essential for the establishment of infection. In the present study, we investigated the role of plant HSP70 homologs in the uncoating process of Cucumber necrosis virus (CNV), a nonenveloped positive-sense single-stranded RNA [(+)ssRNA] virus having a T=3 icosahedral capsid. We have found through Western blot analysis and mass spectrometry that the HSP70 homolog Hsc70-2 copurifies with CNV particles. Virus overlay and immunogold labeling assays suggest that Hsc70-2 is physically bound to virions. Furthermore, trypsin digestion profiles suggest that the bound Hsc70-2 is partially protected by the virus, indicating an intimate association with particles. In investigating a possible role of Hsc70-2 in particle disassembly, we showed that particles incubated with Hsp70/Hsc70 antibody produce fewer local lesions than those incubated with prebleed control antibody on Chenopodium quinoa In conjunction, CNV virions purified using CsCl and having undetectable amounts of Hsc70-2 produce fewer local lesions. We also have found that plants with elevated levels of HSP70/Hsc70 produce higher numbers of local lesions following CNV inoculation. Finally, incubation of recombinant Nicotiana benthamiana Hsc70-2 with virus particles in vitro leads to conformational changes or partial disassembly of capsids as determined by transmission electron microscopy, and particles are more sensitive to chymotrypsin digestion. This is the first report suggesting that a cellular Hsc70 chaperone is involved in disassembly of a plant virus.

IMPORTANCE

Virus particles must disassemble and release their nucleic acid in order to establish infection in a cell. Despite the importance of disassembly in the ability of a virus to infect its host, little is known about this process, especially in the case of nonenveloped spherical RNA viruses. Previous work has shown that host HSP70 homologs play multiple roles in the CNV infection cycle. We therefore examined the potential role of these cellular components in the CNV disassembly process. We show that the HSP70 family member Hsc70-2 is physically associated with CNV virions and that HSP70 antibody reduces the ability of CNV to establish infection. Statistically significantly fewer lesions are produced when virions having undetectable HSc70-2 are used as an inoculum. Finally incubation of Hsc70-2 with CNV particles results in conformational changes in particles. Taken together, our data point to an important role of the host factor Hsc70-2 in CNV disassembly.

Cucumber Necrosis Virus Recruits Cellular Heat Shock Protein 70 Homologs at Several Stages of Infection

RNA viruses often depend on host factors for multiplication inside cells due to the constraints of their small genome size and limited coding capacity. One such factor that has been exploited by several plant and animal viruses is heat shock protein 70 (HSP70) family homologs which have been shown to play roles for different viruses in viral RNA replication, viral assembly, disassembly, and cell-to-cell movement. Using next generation sequence analysis, we reveal that several isoforms of Hsp70 and Hsc70 transcripts are induced to very high levels during cucumber necrosis virus (CNV) infection of Nicotiana benthamiana and that HSP70 proteins are also induced by at least 10-fold. We show that HSP70 family protein homologs are co-opted by CNV at several stages of infection. We have found that overexpression of Hsp70 or Hsc70 leads to enhanced CNV genomic RNA, coat protein (CP), and virion accumulation, whereas downregulation leads to a corresponding decrease. Hsc70-2 was found to increase solubility of CNV CP in vitro and to increase accumulation of CNV CP independently of viral RNA replication during coagroinfiltration in N. benthamiana. In addition, virus particle assembly into virus-like particles in CP agroinfiltrated plants was increased in the presence of Hsc70-2. HSP70 was found to increase the targeting of CNV CP to chloroplasts during infection, reinforcing the role of HSP70 in chloroplast targeting of host proteins. Hence, our findings have led to the discovery of a highly induced host factor that has been co-opted to play multiple roles during several stages of the CNV infection cycle.

IMPORTANCE

Because of the small size of its RNA genome, CNV is dependent on interaction with host cellular components to successfully complete its multiplication cycle. We have found that CNV induces HSP70 family homologs to a high level during infection, possibly as a result of the host response to the high levels of CNV proteins that accumulate during infection. Moreover, we have found that CNV co-opts HSP70 family homologs to facilitate several aspects of the infection process such as viral RNA, coat protein and virus accumulation. Chloroplast targeting of the CNV CP is also facilitated, which may aid in CNV suppression of host defense responses. Several viruses have been shown to induce HSP70 during infection and others to utilize HSP70 for specific aspects of infection such as replication, assembly, and disassembly. We speculate that HSP70 may play multiple roles in the infection processes of many viruses.

Encapsidation of Host RNAs by Cucumber Necrosis Virus Coat Protein during both Agroinfiltration and Infection

Next-generation sequence analysis of virus-like particles (VLPs) produced during agroinfiltration of cucumber necrosis virus (CNV) coat protein (CP) and of authentic CNV virions was conducted to assess if host RNAs can be encapsidated by CNV CP. VLPs containing host RNAs were found to be produced during agroinfiltration, accumulating to approximately 1/60 the level that CNV virions accumulated during infection. VLPs contained a variety of host RNA species, including the major rRNAs as well as cytoplasmic, chloroplast, and mitochondrial mRNAs. The most predominant host RNA species encapsidated in VLPs were chloroplast encoded, consistent with the efficient targeting of CNV CP to chloroplasts during agroinfiltration. Interestingly, droplet digital PCR analysis showed that the CNV CP mRNA expressed during agroinfiltration was the most efficiently encapsidated mRNA, suggesting that the CNV CP open reading frame may contain a high-affinity site or sites for CP binding and thus contribute to the specificity of CNV RNA encapsidation. Approximately 0.09% to 0.7% of the RNA derived from authentic CNV virions contained host RNA, with chloroplast RNA again being the most prominent species. This is consistent with our previous finding that a small proportion of CNV CP enters chloroplasts during the infection process and highlights the possibility that chloroplast targeting is a significant aspect of CNV infection. Remarkably, 6 to 8 of the top 10 most efficiently encapsidated nucleus-encoded RNAs in CNV virions correspond to retrotransposon or retrotransposon-like RNA sequences. Thus, CNV could potentially serve as a vehicle for horizontal transmission of retrotransposons to new hosts and thereby significantly influence genome evolution.

IMPORTANCE

Viruses predominantly encapsidate their own virus-related RNA species due to the possession of specific sequences and/or structures on viral RNA which serve as high-affinity binding sites for the coat protein. In this study, we show, using next-generation sequence analysis, that CNV also encapsidates host RNA species, which account for ∼0.1% of the RNA packaged in CNV particles. The encapsidated host RNAs predominantly include chloroplast RNAs, reinforcing previous observations that CNV CP enters chloroplasts during infection. Remarkably, the most abundantly encapsidated cytoplasmic mRNAs consisted of retrotransposon-like RNA sequences, similar to findings recently reported for flock house virus (A. Routh, T. Domitrovic, and J. E. Johnson, Proc Natl Acad Sci U S A 109:1907-1912, 2012). Encapsidation of retrotransposon sequences may contribute to their horizontal transmission should CNV virions carrying retrotransposons infect a new host. Such an event could lead to large-scale genomic changes in a naive plant host, thus facilitating host evolutionary novelty.

Hibiscus chlorotic ringspot virus coat protein is essential for cell-to-cell and long-distance movement but not for viral RNA replication

Hibiscus chlorotic ringspot virus (HCRSV) is a member of the genus Carmovirus in the family Tombusviridae. In order to study its coat protein (CP) functions on virus replication and movement in kenaf (Hibiscus cannabinus L.), two HCRSV mutants, designated as p2590 (A to G) in which the first start codon ATG was replaced with GTG and p2776 (C to G) in which proline 63 was replaced with alanine, were constructed. In vitro transcripts of p2590 (A to G) were able to replicate to a similar level as wild type without CP expression in kenaf protoplasts. However, its cell-to-cell movement was not detected in the inoculated kenaf cotyledons. Structurally the proline 63 in subunit C acts as a kink for β-annulus formation during virion assembly. Progeny of transcripts derived from p2776 (C to G) was able to move from cell-to-cell in inoculated cotyledons but its long-distance movement was not detected. Virions were not observed in partially purified mutant virus samples isolated from 2776 (C to G) inoculated cotyledons. Removal of the N-terminal 77 amino acids of HCRSV CP by trypsin digestion of purified wild type HCRSV virions resulted in only T = 1 empty virus-like particles. Taken together, HCRSV CP is dispensable for viral RNA replication but essential for cell-to-cell movement, and virion is required for the virus systemic movement. The proline 63 is crucial for HCRSV virion assembly in kenaf plants and the N-terminal 77 amino acids including the β-annulus domain is required in T = 3 assembly in vitro.

Atomic structure of Cucumber necrosis virus and the role of the capsid in vector transmission

Cucumber Necrosis Virus (CNV) is a member of the genus Tombusvirus and has a monopartite positive-sense RNA genome packaged in a T=3 icosahedral particle. CNV is transmitted in nature via zoospores of the fungus Olpidium bornovanus. CNV undergoes a conformational change upon binding to the zoospore that is required for transmission, and specific polysaccharides on the zoospore surface have been implicated in binding. To better understand this transmission process, we have determined the atomic structure of CNV. As expected, being a member of the Tombusvirus genus, the core structure of CNV is highly similar to that of Tomato bushy stunt virus (TBSV), with major differences lying on the exposed loops. Also, as was seen with TBSV, CNV appears to have a calcium binding site between the subunits around the quasi-3-fold axes. However, unlike TBSV, there appears to be a novel zinc binding site within the β annulus formed by the N termini of the three C subunits at the icosahedral 3-fold axes. Two of the mutations causing defective transmission map immediately around this zinc binding site. The other mutations causing defective transmission and particle formation are mapped onto the CNV structure, and it is likely that a number of the mutations affect zoospore transmission by affecting conformational transitions rather than directly affecting receptor binding.

Non-encapsidation activities of the capsid proteins of positive-strand RNA viruses

Viral capsid proteins (CPs) are characterized by their role in forming protective shells around viral genomes. However, CPs have additional and important roles in the virus infection cycles and in the cellular responses to infection. These activities involve CP binding to RNAs in both sequence-specific and nonspecific manners as well as association with other proteins. This review focuses on CPs of both plant and animal-infecting viruses with positive-strand RNA genomes. We summarize the structural features of CPs and describe their modulatory roles in viral translation, RNA-dependent RNA synthesis, and host defense responses.

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We review regulatory activities of the capsid proteins of (+)-strand RNA viruses.

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Activities of capsid proteins due to RNA binding and protein binding.

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Effects of capsid proteins on viral processes.

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Effects of capsid proteins on cellular processes.

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Regulatory activities of the capsid proteins are affected by capsid concentrations.

Structure of hepatitis E viral particle

Hepatitis E is acute hepatitis caused by infection of hepatitis E virus (HEV) via a fecal-to-oral or zoonotic route. HEV is a small, non-enveloped virus containing positive strand RNA as a genome. Recently, the three-dimensional structures of the HEV-like particles and spike domain protruded from the surface of the particle expressed by recombinant baculovirus or bacteria have been revealed. Based on these reports, the structural features of the HEV capsid subunit and viral particle are reviewed to give insights to the mechanisms underlying the particle assembly, antigenicity, host cell attachment and native virion packaging.

Distinct regions at the N-terminus of the Cucumber necrosis virus coat protein target chloroplasts and mitochondria

Cucumber necrosis virus (CNV) is a spherical virus consisting of 180 identical coat protein (CP) subunits. The N-terminus of the CP subunit contains a 58aa RNA binding (R) domain and a 34aa arm that connects the R domain to the shell. These regions are known to play critical roles in virus assembly and disassembly. It has recently been shown that a region encompassing the arm can function as a chloroplast transit peptide (TP) in infected plants and that targeting may represent a means for virus particle disassembly. In this study, we further delineate the TP region and show that a 22aa sequence at the N-terminus of the shell enhances chloroplast targeting. We also demonstrate that R domain specifically co-localizes with mitochondria in agroinfiltrated plants. Deletion analyses show that the N-terminal 39 amino acids of the R domain are sufficient for mitochondrial targeting and that this region contains features typical of mitochondrial presequences. The R/arm region is found to be dually targeted to mitochondria and chloroplasts suggesting that this region of the CP plays a critical role in determining the fate of CP during the infection process.

A highly basic KGKKGK sequence in the RNA-binding domain of the Cucumber necrosis virus coat protein is associated with encapsidation of full-length CNV RNA during infection

The Cucumber necrosis virus particle is a T=3 icosahedron consisting of 180 identical coat protein (CP) subunits. The N-terminal 58 aa residue segment of the CP R domain is believed to bind viral RNA within virions and during assembly. We report results of in vivo experiments that examine the role of the R domain in assembly. Deletion analyses identified 3 conserved 5-10 aa regions as playing critical roles. A highly basic KGKKGK sequence was found to be both necessary and sufficient for encapsidation of the full-length genome and polymorphic virions were produced in mutants lacking the KGKKGK sequence. The amount of full-length RNA present in virions was substantially reduced in R domain mutants where 2 of the 4 lysine residues were substituted with alanine, whereas substitution of 4 lysines by arginine had only a modest effect. The potential role of the R domain in formation of a scaffold for particle assembly is discussed.

Biological and immunological characteristics of hepatitis E virus-like particles based on the crystal structure

Hepatitis E virus (HEV) is a causative agent of acute hepatitis. The crystal structure of HEV-like particles (HEV-LP) consisting of capsid protein was determined at 3.5-A resolution. The capsid protein exhibited a quite different folding at the protruding and middle domains from the members of the families of Caliciviridae and Tombusviridae, while the shell domain shared the common folding. Tyr-288 at the 5-fold axis plays key roles in the assembly of HEV-LP, and aromatic amino acid residues are well conserved among the structurally related viruses. Mutational analyses indicated that the protruding domain is involved in the binding to the cells susceptive to HEV infection and has some neutralization epitopes. These structural and biological findings are important for understanding the molecular mechanisms of assembly and entry of HEV and also provide clues in the development of preventive and prophylactic measures for hepatitis E.

Fungal transmission of plant viruses

Fungal zoospores of Olpidium species transmit several viruses in the family Tombusviridae as well as in the Ophio- and Varicosavirus genera. This unit describes procedures for virus transmission by Olpidium sp. The method is useful for assessing fungal transmissibility of a given virus as well as for further studies on molecular and biological aspects of virus/vector interaction.