A rapid procedure to generate stably transfected HEK293 suspension cells for recombinant protein manufacturing: Yield improvements, bioreactor production and downstream processing

The human cell line HEK293 is one of the preferred choices for manufacturing therapeutic proteins and viral vectors for human applications. Despite its increased use, it is still considered in disadvantage in production aspects compared to cell lines such as the CHO cell line. We provide here a simple workflow for the rapid generation of stably transfected HEK293 cells expressing an engineered variant of the SARS-CoV-2 Receptor Binding Domain (RBD) carrying a coupling domain for linkage to VLPs through a bacterial transpeptidase-sortase (SrtA). To generate stable suspension cells expressing the RBD-SrtA, a single two plasmids transfection was performed, with hygromycin selection. The suspension HEK293 were grown in adherent conditions, with 20% FBS supplementation. These transfection conditions increased cell survival, allowing the selection of stable cell pools, which was otherwise not possible with standard procedures in suspension. Six pools were isolated, expanded and successfully re-adapted to suspension with a gradual increase of serum-free media and agitation. The complete process lasted four weeks. Stable expression with viability over 98% was verified for over two months in culture, with cell passages every 4-5 days. With process intensification, RBD-SrtA yields reached 6.4 μg/mL and 13.4 μg/mL in fed-batch and perfusion-like cultures, respectively. RBD-SrtA was further produced in fed-batch stirred tank 1L-bioreactors, reaching 10-fold higher yields than perfusion flasks. The trimeric antigen displayed the conformational structure and functionality expected. This work provides a series of steps for stable cell pool development using suspension HEK293 cells aimed at the scalable production of recombinant proteins.

A nanoparticle-based COVID-19 vaccine candidate elicits broad neutralizing antibodies and protects against SARS-CoV-2 infection

A vaccine candidate to SARS-CoV-2 was constructed by coupling the viral receptor binding domain (RBD) to the surface of the papaya mosaic virus (PapMV) nanoparticle (nano) to generate the RBD-PapMV vaccine. Immunization of mice with the coupled RBD-PapMV vaccine enhanced the antibody titers and the T-cell mediated immune response directed to the RBD antigen as compared to immunization with the non-coupled vaccine formulation (RBD + PapMV nano). Anti-RBD antibodies, generated in vaccinated animals, neutralized SARS-CoV-2 infection in vitro against the ancestral, Delta and the Omicron variants. At last, immunization of mice susceptible to the infection by SARS-CoV-2 (K18-hACE2 transgenic mice) with the RBD-PapMV vaccine induced protection to the ancestral SARS-CoV-2 infectious challenge. The induction of the broad neutralization against SARS-CoV-2 variants induced by the RBD-PapMV vaccine demonstrate the potential of the PapMV vaccine platform in the development of efficient vaccines against viral respiratory infections.

Une pandémie qui se prolonge : santé des dirigeants de TPE/PME

Depuis mars 2020, la pandémie de COVID-19 sévit durablement. Notre pays a connu trois confinements et des mesures restrictives mouvantes qui ont impacté durablement la vie sociale et l’économie avec des conséquences sanitaires non négligeables. En septembre 2020, le service de santé au travail interentreprises (SSTI) de Moselle, AGESTRA, a interrogé les dirigeants de TPE/PME pour connaître les effets du premier confinement sur leur santé. Ils étaient 38 % au printemps 2020 à déclarer une santé perçue moins bonne qu’avant la crise.

La persistance de la crise a motivé une seconde démarche afin de connaître l’effet de la durée de cette crise sur la santé des dirigeants d’entreprise.

Pour ce faire un nouveau questionnaire, reprenant les mêmes thèmes que celui d’AGESTRA, a été proposé aux 20 SSTI de l’association Grand Est Santé Travail (GEST). Douze ont sollicité par mailing les dirigeants (fonction publique exclue) de TPE et de PME dont ils assurent le suivi pour y répondre. L’enquête s’est déroulée du 13 avril au 17 mai 2021 et 934 dirigeants ont répondu.

Pour 81 %, l’activité de leur entreprise a été arrêtée ou diminuée pendant le premier confinement et 49 % déclarent la même situation au troisième confinement. Le secteur du commerce a été le plus impacté par la durée de la crise.

Quatre-vingt-quinze pour cent des dirigeants ont dû mettre en place des modifications de fonctionnement lors du premier confinement. Cinquante-deux pour cent ont eu des difficultés à le faire. Ces difficultés sont plus souvent rapportées par les dirigeants de PME. Vingt-trois pour cent des dirigeants mentionnent une augmentation de l’absentéisme, 34 % une diminution de la motivation des équipes et 48 % des résultats diminués.

Concernant la santé perçue, 47 % des répondants la déclarent moins bonne en mai 2021 qu’avant la pandémie. La dégradation est plus importante chez les femmes, lorsque l’arrêt ou la diminution de l’activité perdure et lorsque la motivation de leurs équipes est diminuée.

Les résultats montrent également une forte dégradation des santés mentale et physique, de la qualité du sommeil et des habitudes de vie chez les entrepreneurs.

La consommation de psychotropes, hypnotiques et antalgiques quant à elle est passée de 6 % avant la pandémie à 13 % au printemps 2021.

Ce travail confirme que la durée de la crise sanitaire et la dégradation de la santé des dirigeants sont fortement liées. Ils sont exposés à plusieurs facteurs de risques de dégradation de leur santé : durée de crise, résultats économiques incertains et confiance réduite en l’avenir.

Cette crise met en évidence l’impact fort de l’environnement socioéconomique sur la santé des dirigeants et nécessite donc une réflexion profonde en matière de prévention pour les entrepreneurs qui est actuellement négligée.

MHC class I antigen cross-presentation mediated by PapMV nanoparticles in human antigen-presenting cells is dependent on autophagy

Nanoparticles made of the coat protein of papaya mosaic virus (PapMV) and a single-strand RNA were previously shown to be an efficient antigen presentation system for the trigger of cellular immunity. Engineering of PapMV nano with a cytotoxic T lymphocyte epitope was previously shown activating specific T lymphocytes through a proteasome-independent major histocompatibility complex class I (MHC-I) cross-presentation. In this study, we provide new insights into the mechanism of the MHC-I cross-presentation mediated by PapMV nanoparticles. We demonstrate that PapMV nanoparticles do not require the transporter associated with antigen presentation (TAP), but rather depend on lysosome acidification and cathepsin S protease activity for presentation of the T cell epitope. We have also linked the induction of autophagy with this vacuolar MHC-I cross-presentation process. Interestingly, autophagy is induced in antigen-presenting cells after PapMV nanoparticles exposure and inhibition of autophagy reduce MHC-I cross-presentation. This study demonstrates that autophagy is associated with TAP- and proteasome-independent MHC-I cross-presentation. A deeper understanding of the autophagy-dependent MHC-I cross-presentation will be useful in designing vaccination platforms that aim to trigger an efficient cytotoxic T lymphocyte response.

Rapid High-Yield Production of Functional SARS-CoV-2 Receptor Binding Domain by Viral and Non-Viral Transient Expression for Pre-Clinical Evaluation

Vaccine design strategies against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are focused on the Spike protein or its subunits as the main antigen target of neutralizing antibodies. In this work, we propose rapid production methods of an extended segment of the Spike Receptor Binding Domain (RBD) in HEK293SF cells cultured in suspension, in serum-free media, as a major component of a COVID-19 subunit vaccine under development. The expression of RBD, engineered with a sortase-recognition motif for protein-based carrier coupling, was achieved at high yields by plasmid transient transfection or human type-5-adenoviral infection of the cells, in a period of only two and three weeks, respectively. Both production methods were evaluated in 3L-controlled bioreactors with upstream and downstream bioprocess improvements, resulting in a product recovery with over 95% purity. Adenoviral infection led to over 100 µg/mL of RBD in culture supernatants, which was around 7-fold higher than levels obtained in transfected cultures. The monosaccharide and sialic acid content was similar in the RBD protein from the two production approaches. It also exhibited a proper conformational structure as recognized by monoclonal antibodies directed against key native Spike epitopes. Efficient direct binding to ACE2 was also demonstrated at similar levels in RBD obtained from both methods and from different production lots. Overall, we provide bioprocess-related data for the rapid, scalable manufacturing of low cost RBD based vaccines against SARS-CoV-2, with the added value of making a functional antigen available to support further research on uncovering mechanisms of virus binding and entry as well as screening for potential COVID-19 therapeutics.

Increased Immunogenicity of Full-Length Protein Antigens through Sortase-Mediated Coupling on the PapMV Vaccine Platform

Background: Flexuous rod-shape nanoparticles—made of the coat protein of papaya mosaic virus (PapMV)—provide a promising vaccine platform for the presentation of viral antigens to immune cells. The PapMV nanoparticles can be combined with viral antigens or covalently linked to them. The coupling to PapMV was shown to improve the immune response triggered against peptide antigens (<39 amino acids) but it remains to be tested if large proteins can be coupled to this platform and if the coupling will lead to an immune response improvement. Methods: Two full-length recombinant viral proteins, the influenza nucleoprotein (NP) and the simian immunodeficiency virus group-specific protein antigen (GAG) were coupled to PapMV nanoparticles using sortase A. Mice were immunized with the nanoparticles coupled to the antigens and the immune response directed to the antigens were analyzed by ELISA and ELISPOT. Results: We showed the feasibility of coupling two different full-length proteins (GAG and NP) to the nanoparticle. We also showed that the coupling to PapMV nanoparticles improved significantly the humoral and the cytotoxic T lymphocyte (CTL) immune response to the antigens. Conclusion: This proof of concept demonstrates the versatility and the efficacy of the PapMV vaccine platform in the design of vaccines against viral diseases.

Efficacy of a Virus-Like Nanoparticle As Treatment for a Chronic Viral Infection Is Hindered by IRAK1 Regulation and Antibody Interference

Although vaccination has been an effective way of preventing infections ever since the eighteenth century, the generation of therapeutic vaccines and immunotherapies is still a work in progress. A number of challenges impede the development of these therapeutic approaches such as safety issues related to the administration of whole pathogens whether attenuated or inactivated. One safe alternative to classical vaccination methods gaining recognition is the use of nanoparticles, whether synthetic or naturally derived. We have recently demonstrated that the papaya mosaic virus (PapMV)-like nanoparticle can be used as a prophylactic vaccine against various viral and bacterial infections through the induction of protective humoral and cellular immune responses. Moreover, PapMV is also very efficient when used as an immune adjuvant in an immunotherapeutic setting at slowing down the growth of aggressive mouse melanoma tumors in a type I interferon (IFN-I)-dependent manner. In the present study, we were interested in exploiting the capacity of PapMV of inducing robust IFN-I production as treatment for the chronic viral infection model lymphocytic choriomeningitis virus (LCMV) clone 13 (Cl13). Treatment of LCMV Cl13-infected mice with two systemic administrations of PapMV was ineffective, as shown by the lack of changes in viral titers and immune response to LCMV following treatment. Moreover, IFN-α production following PapMV administration was almost completely abolished in LCMV-infected mice. To better isolate the mechanisms at play, we determined the influence of a pretreatment with PapMV on secondary PapMV administration, therefore eliminating potential variables emanating from the infection. Pretreatment with PapMV led to the same outcome as an LCMV infection in that IFN-α production following secondary PapMV immunization was abrogated for up to 50 days while immune activation was also dramatically impaired. We showed that two distinct and overlapping mechanisms were responsible for this outcome. While short-term inhibition was partially the result of interleukin-1 receptor-associated kinase 1 degradation, a crucial component of the toll-like receptor 7 signaling pathway, long-term inhibition was mainly due to interference by PapMV-specific antibodies. Thus, we identified a possible pitfall in the use of virus-like particles for the systemic treatment of chronic viral infections and discuss mitigating alternatives to circumvent these potential problems.

Activation of innate immunity in primary human cells using a plant virus derived nanoparticle TLR7/8 agonist

Rod-shaped virus-like nanoparticles (VLNP) made of papaya mosaic virus (PapMV) coat proteins (CP) self-assembled around a single stranded RNA (ssRNA) were showed to be a TLR7 agonist. Their utilization as an immune modulator in cancer immunotherapy was shown to be promising. To establish a clinical relevance in human for PapMV VLNP, we showed that stimulation of human peripheral blood mononuclear cells (PBMC) with VLNP induces the secretion of interferon alpha (IFNα) and other pro-inflammatory cytokines and chemokines. Plasmacytoid dendritic cells (pDCs) were activated and secreted IFN-α upon VLNP exposure. Monocyte-derived dendritic cells upregulate maturation markers and produce IL-6 in response to PapMV VLNP stimulation, which suggests the activation of TLR8. Finally, when co-cultured with NK cells, PapMV induced pDCs promoted the NK cytolytic activity against cancer cells. These data obtained with primary human immune cells further strengthen the clinical relevance of PapMV VLNPs as a cancer immunotherapy agent.

A versatile papaya mosaic virus (PapMV) vaccine platform based on sortase-mediated antigen coupling

Background

Flexuous rod-shaped nanoparticles made of the coat protein (CP) of papaya mosaic virus (PapMV) have been shown to trigger innate immunity through engagement of toll-like receptor 7 (TLR7). PapMV nanoparticles can also serve as a vaccine platform as they can increase the immune response to fused peptide antigens. Although this approach shows great potential, fusion of antigens directly to the CP open reading frame (ORF) is challenging because the fused peptides can alter the structure of the CP and its capacity to self assemble into nanoparticles—a property essential for triggering an efficient immune response to the peptide. This represents a serious limitation to the utility of this approach as fusion of small peptides only is tolerated.

Results

We have developed a novel approach in which peptides are fused directly to pre-formed PapMV nanoparticles. This approach is based on the use of a bacterial transpeptidase (sortase A; SrtA) that can attach the peptide directly to the nanoparticle. An engineered PapMV CP harbouring the SrtA recognition motif allows efficient coupling. To refine our engineering, and to predict the efficacy of coupling with SrtA, we modeled the PapMV structure based on the known structure of PapMV CP and on recent reports revealing the structure of two closely related potexviruses: pepino mosaic virus (PepMV) and bamboo mosaic virus (BaMV). We show that SrtA can allow the attachment of long peptides [Influenza M2e peptide (26 amino acids) and the HIV-1 T20 peptide (39 amino acids)] to PapMV nanoparticles. Consistent with our PapMV structural model, we show that around 30% of PapMV CP subunits in each nanoparticle can be fused to the peptide antigen. As predicted, engineered nanoparticles were capable of inducing a strong antibody response to the fused antigen. Finally, in a challenge study with influenza virus, we show that mice vaccinated with PapMV-M2e are protected from infection.

Conclusions

This technology will allow the development of vaccines harbouring long peptides containing several B and/or T cell epitopes that can contribute to a broad and robust protection from infection. The design can be fast, versatile and can be adapted to the development of vaccines for many infectious diseases as well as cancer vaccines.

Electronic supplementary material

The online version of this article (doi:10.1186/s12951-017-0289-y) contains supplementary material, which is available to authorized users.

Complement Component 3 Regulates IFN-α Production by Plasmacytoid Dendritic Cells following TLR7 Activation by a Plant Virus-like Nanoparticle

The increasing use of plant viruses for the development of new vaccines and immunotherapy approaches poses questions regarding the mechanism by which the mammalian immune system recognizes these viruses. For example, although natural Abs (NA) and complement are key components of the innate immune system involved in the opsonization, phagocytosis, and destruction of microorganisms infecting mammals, their implication in plant virus recognition and immunogenicity is not well defined. In this study, we address the involvement of NA and the complement system in the activation of innate immunity through engagement of TLR7 with papaya mosaic virus (PapMV)-like nanoparticles. We demonstrate that NA, although binding to PapMV, are not involved in its recognition by the immune system. On the other hand, C3 strongly binds to PapMV nanoparticles and its depletion significantly reduces PapMV's interaction with immune cells. Unexpectedly, however, we observed increased immune cell activation following administration of PapMV to complement-depleted mice. TLR7 activation by PapMV in the absence of C3 induced higher IFN-α production, resulting in superior immune cell activation and increased immunotherapeutic properties. In conclusion, in this study we established the involvement of the complement system in the recognition and the phagocytosis of PapMV nanoparticles and identified an unsuspected role for C3 in regulating the production of IFN-α following TLR7 activation.

Influence of PapMV nanoparticles on the kinetics of the antibody response to flu vaccine

BACKGROUND

The addition of an adjuvant to a vaccine is a promising approach to increasing strength and immunogenicity towards antigens. Despite the fact that adjuvants have been used in vaccines for decades, their mechanisms of action and their influence on the kinetics of the immune response are still not very well understood. The use of papaya mosaic virus (PapMV) nanoparticles-a novel TLR7 agonist-was recently shown to improve and broaden the immune response directed to trivalent inactivated flu vaccine (TIV) in mice and ferrets.

RESULTS

We investigated the capacity of PapMV nanoparticles to increase the speed of the immune response toward TIV. PapMV nanoparticles induced a faster and stronger humoral response to TIV that was measured as early as 5 days post-immunization. The addition of PapMV nanoparticles was shown to speed up the differentiation of B-cells into early plasma cells, and increased the growth of germinal centers in a CD4+ dependent manner. TIV vaccination with PapMV nanoparticles as an adjuvant protected mice against a lethal infection as early as 10 days post-immunization.

CONCLUSION

In conclusion, PapMV nanoparticles are able to accelerate a broad humoral response to TIV. This property is of the utmost importance in the field of vaccination, especially in the case of pandemics, where populations need to be protected as soon as possible after vaccination.

Potentiating Cancer Immunotherapy Using Papaya Mosaic Virus-Derived Nanoparticles

The recent development of novel immunotherapies is revolutionizing cancer treatment. These include, for example, immune checkpoint blockade, immunomodulation, or therapeutic vaccination. Although effective on their own, combining multiple approaches will most likely be required in order to achieve the maximal therapeutic benefit. In this regard, the papaya mosaic virus nanoparticle (PapMV) has shown tremendous potential as (i) an immunostimulatory molecule, (ii) an adjuvant, and (iii) a vaccine platform through its intrinsic capacity to activate the innate immune response in an IFN-α-dependent manner. Here, we demonstrate that intratumor administration of PapMV significantly slows down melanoma progression and prolongs survival. This correlates with enhanced chemokine and pro-inflammatory-cytokine production in the tumor and increased immune-cell infiltration. Proportions of total and tumor-specific CD8(+) T cells dramatically increase following PapMV treatment whereas those of myeloid-derived suppressor cells (MDSC) concomitantly decrease. Moreover, systemic PapMV administration prevents metastatic tumor-implantation in the lungs. Importantly, PapMV also synergistically improves the therapeutic benefit of dendritic cell (DC)-based vaccination and PD-1 blockade by potentiating antitumor immune responses. This study illustrates the immunostimulatory potential of a plant virus-derived nanoparticle for cancer therapy either alone or in conjunction with other promising immunotherapies in clinical development.

PapMV nanoparticles improve mucosal immune responses to the trivalent inactivated flu vaccine

Background

Trivalent inactivated flu vaccines (TIV) are currently the best means to prevent influenza infections. However, the protection provided by TIV is partial (about 50%) and it is needed to improve the efficacy of protection. Since the respiratory tract is the main site of influenza replications, a vaccine that triggers mucosal immunity in this region can potentially improve protection against this disease. Recently, PapMV nanoparticles used as an adjuvant in a formulation with TIV administered by the subcutaneous route have shown improving the immune response directed to the TIV and protection against an influenza challenge.

Findings

In the present study, we showed that intranasal instillation with a formulation containing TIV and PapMV nanoparticles significantly increase the amount of IgG, IgG2a and IgA in lungs of vaccinated mice as compared to mice that received TIV only. Instillation with the adjuvanted formulation leads to a more robust protection against an influenza infection with a strain that is lethal to mice vaccinated with the TIV.

Conclusions

We demonstrate for the first time that PapMV nanoparticles are an effective and potent mucosal adjuvant for vaccination.

A novel M2e based flu vaccine formulation for dogs

Background

The USA 2004 influenza virus outbreak H3N8 in dogs heralded the emergence of a new disease in this species. A new inactivated H3N8 vaccine was developed to control the spread of the disease but, as in humans and swine, it is anticipated that the virus will mutate shift and drift in the dog population. Therefore, there is a need for a vaccine that can trigger a broad protection to prevent the spread of the virus and the emergence of new strains.

Methodology and Principal Findings

The universal M2e peptide is identical in almost all the H3N8 influenza strains sequenced to date and known to infect dogs. This epitope is therefore a good choice for development of a vaccine to provide broad protection. Malva mosaic virus (MaMV) nanoparticles were chosen as a vaccine platform to improve the stability of the M2e peptide and increase its immunogenicity in animals. The addition of an adjuvant (OmpC) purified from Salmonella typhi membrane in the vaccine formulation increased the immune response directed to the M2e peptide significantly and enlarged the protection to include the heterosubtypic strain of influenza in a mouse model. An optimal vaccine formulation was also shown to be immunogenic in dogs.

Conclusions and Significance

The MaMV vaccine platform triggered an improved immune response directed towards the universal M2e peptide. The adjuvant OmpC increased the immune response to the M2e peptide and protection to a heterosubtypic influenza strain that harbors a different M2e peptide in a mouse model. Antibodies generated by the vaccine formulation showed cross-reactivity with M2e peptides derived from influenza strains H9N2, H5N1 and H1N1. The vaccine formulation shows a potential for commercialization of a new M2e based vaccine in dogs.

Engineering of papaya mosaic virus (PapMV) nanoparticles with a CTL epitope derived from influenza NP

Background

The ever-present threat of infectious disease, e.g. influenza pandemics, and the increasing need for new and effective treatments in immunotherapy are the driving forces that motivate research into new and innovative vaccine platforms. Ideally, such platforms should trigger an efficient CTL response, be safe, and easy to manufacture. We recently developed a novel nanoparticle adjuvant comprised of papaya mosaic virus (PapMV) coat protein (CP) assembled around an RNA. The PapMV nanoparticle is an efficient vaccine platform in which the peptide antigen is fused to the C-terminus of the PapMV CP, leading to nanoparticles presenting surface-exposed epitope. The fusion stabilizes the epitope and improves its immunogenicity. We found recently that C-terminal fusions are not always efficient, depending on the nature of the peptide fused to the platform.

Results

We chose a CTL epitope derived from the nucleocapsid (NP) of influenza virus (NP_147-155) for this proof-of-concept demonstration. Recombinant nanoparticles harbouring a fusion at the N-terminus were more efficient in triggering a CTL response. Efficacy appeared to be linked to the stability of the nanoparticles at 37°C. We also showed that discs—smaller than nanoparticles—made of 20 subunits of PapMV CP are less efficient for induction of a CTL response in mice, revealing that assembly of the recombinant PapMV CP into nanoparticles is crucial to triggering an efficient CTL response.

Conclusion

The point of fusion on the PapMV vaccine platform is critical to triggering an efficient CTL response. Efficacy is linked to nanoparticle stability; nanoparticles must be stable at 37°C but remain susceptible to cellular proteases to ensure efficient processing of the CTL epitope by cells of the immune system. The results of this study improve our understanding of the PapMV vaccine platform, which will facilitate the design of efficient vaccines to various infectious threats.

Induction of innate immunity in lungs with virus-like nanoparticles leads to protection against influenza and Streptococcus pneumoniae challenge

Nanoparticles composed of the coat protein of a plant virus (papaya mosaic virus; PapMV) and a single-stranded RNA (ssRNA) trigger a strong innate immune stimulation in the lungs of the animals a few hours following instillation. A rapid recruitment of neutrophils, monocytes/macrophages and lymphocytes follows. This treatment was able to provide protection to an influenza challenge that lasts at least 5 days. Protection could be recalled for longer periods by repeating the instillations once per week for more than 10 weeks. The treatment also conferred protection to a lethal challenge with Streptococcus pneumoniae--the major cause of bacterial pneumonia. Finally, we also showed that the nanoparticles could be used to treat mice infected with influenza and significantly decrease morbidity. These data strengthen the potential for using PapMV nanoparticles as non-specific inducers of the innate immune response in lungs during viral pandemics or to combat bioterrorist attack.

FROM THE CLINICAL EDITOR

In this study, virus-like nanoparticles were utilized to induce innate immune responses in a mouse model. They were also demonstrated to provide enhanced immune responses during actual pneumonia and ongoing viral infection. Strategies like this may become very helpful in human applications, including bioterrorism countermeasures.

Crystal structure of the coat protein of the flexible filamentous papaya mosaic virus

Papaya mosaic virus (PapMV) is a filamentous plant virus that belongs to the Alphaflexiviridae family. Flexible filamentous viruses have defied more than two decades of effort in fiber diffraction, and no high-resolution structure is available for any member of the Alphaflexiviridae family. Here, we report our structural characterization of PapMV by X-ray crystallography and cryo-electron microscopy three-dimensional reconstruction. We found that PapMV is 135Å in diameter with a helical symmetry of ~10 subunits per turn. Crystal structure of the C-terminal truncated PapMV coat protein (CP) reveals a novel all-helix fold with seven α-helices. Thus, the PapMVCP structure is different from the four-helix-bundle fold of tobacco mosaic virus in which helix bundling dominates the subunit interface in tobacco mosaic virus and conveys rigidity to the rod virus. PapMV CP was crystallized as an asymmetrical dimer in which one protein lassoes the other by the N-terminal peptide. Mutation of residues critical to the inter-subunit lasso interaction abolishes CP polymerization. The crystal structure suggests that PapMV may polymerize via the consecutive N-terminal loop lassoing mechanism. The structure of PapMV will be useful for rational design and engineering of the PapMV nanoparticles into innovative vaccines.

Autophagy is implicated in MHC class I antigen cross-presentation mediated by potexvirus VLP (106.10)

The development of vaccination platforms capable of triggering an efficient cytotoxic-T-lymphocyte (CTL) response is a priority for eliciting a protective cellular immune response. The proposed technology is based on the use of a new adjuvant composed of a virus-like particle (VLP) with the chimeric coat protein of papaya mosaic virus (PapMV). This VLP can trigger a CTL response through MHC class I antigenic cross-presentation of inserted epitopes. We have wanted to establish the cellular pathway leading to cross-presentation by those VLP. First, we chemically inhibit different molecular path that can be important for cross-presentation. We observed that cathepsin S and autophagy are implicated in cross-presentation by PapMV VLP. Also, we localized the VLP by fluorescence microscopy in antigen presenting cells (APC). To confirm the role of autophagy, we observed its induction by LC3 western blot and microscopy. When the APC are incubated with VLP we observed an activation of autophagy by LC3 lipidation. Finally for more specific inhibition, we used shRNA against ATG5 and cathepsin S in APC. Both ATG5 and cathepsin S reduced the cross-presentation by VLP In conclusion, the cross-presentation mediated by PapMV VLP is dependant on autophagy and degradation by cathepsin S. A better understanding of this mechanism would lead to the development of an efficient vaccine platform against cancer and infectious diseases

Mapping the surface-exposed regions of papaya mosaic virus nanoparticles

In general, the structure of the papaya mosaic virus (PapMV) and other members of the potexviruses is poorly understood. Production of PapMV coat proteins in a bacterial expression system and their self-assembly in vitro into nanoparticles is a very useful tool to study the structure of this virus. Using recombinant PapMV nanoparticles that are similar in shape and appearance to the plant virus, we evaluated surface-exposed regions by two different methods, immunoblot assay and chemical modification with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide or diethyl-pyrocarbonate followed by mass spectrometry. Three regions were targeted by the two techniques. The N- and C-termini were shown to be surfaced exposed as expected. However, the region 125-136 was revealed for the first time as the major surface-exposed region of the nanoparticles. The presence of linear peptides at the surface was finally confirmed using antibodies directed to those peptides. It is likely that region 125-136 plays a key role in the lifecycle of PapMV and other members of the potexvirus group.

Interfering with hepatitis C virus assembly in vitro using affinity peptides directed towards core protein

Viral assembly is a crucial key step in the life cycle of every virus. In the case of Hepatitis C virus (HCV), the core protein is the only structural protein to interact directly with the viral genomic RNA. Purified recombinant core protein is able to self-assemble in vitro into nucleocapsid-like particles upon addition of a structured RNA, providing a robust assay with which to study HCV assembly. Inhibition of self-assembly of the C170 core protein (first 170 amino acids) was tested using short peptides derived from the HCV core, from HCV NS5A protein, and from diverse proteins (p21 and p73) known to interact with HCV core protein. Interestingly, peptides derived from the core were the best inhibitors. These peptides are derived from regions of the core predicted to be involved in the interaction between core subunits during viral assembly. We also demonstrated that a peptide derived from the C-terminal end of NS5A protein moderately inhibits the assembly process.

Engineering of papaya mosaic virus (PapMV) nanoparticles through fusion of the HA11 peptide to several putative surface-exposed sites

Papaya mosaic virus has been shown to be an efficient adjuvant and vaccine platform in the design and improvement of innovative flu vaccines. So far, all fusions based on the PapMV platform have been located at the C-terminus of the PapMV coat protein. Considering that some epitopes might interfere with the self-assembly of PapMV CP when fused at the C-terminus, we evaluated other possible sites of fusion using the influenza HA11 peptide antigen. Two out of the six new fusion sites tested led to the production of recombinant proteins capable of self assembly into PapMV nanoparticles; the two functional sites are located after amino acids 12 and 187. Immunoprecipitation of each of the successful fusions demonstrated that the HA11 epitope was located at the surface of the nanoparticles. The stability and immunogenicity of the PapMV-HA11 nanoparticles were evaluated, and we could show that there is a direct correlation between the stability of the nanoparticles at 37°C (mammalian body temperature) and the ability of the nanoparticles to trigger an efficient immune response directed towards the HA11 epitope. This strong correlation between nanoparticle stability and immunogenicity in animals suggests that the stability of any nanoparticle harbouring the fusion of a new peptide should be an important criterion in the design of a new vaccine.

Structure and dynamics changes induced by 2,2,2-trifluoro-ethanol (TFE) on the N-terminal half of hepatitis C virus core protein

The Core protein of hepatitis C virus is involved in several interactions other than the encapsidation of viral RNA. We recently proposed that this is related to the fact that the N-terminal half of this protein (C82) is an intrinsically unstructured protein (IUP) domain. IUP domains can adopt a secondary structure when they are interacting with another molecule, such as a nucleic acid or a protein. It is also possible to mimic these conditions by modifying the environment of the protein. We investigated the propensity of this protein to fold as a function of salt concentration, detergent, pH, and 2,2,2-trifluoro-ethanol (TFE); only the addition of TFE resulted in a structural change. The effect of TFE addition was studied by circular dichroism, structural, and dynamic data obtained by NMR. The data indicate that C82 can adopt an alpha-helical structure; this conformation is likely relevant to one of the functional roles of the HCV Core protein.

Two distinct chimeric potexviruses share antigenic cross-presentation properties of MHC class I epitopes

Chimeric VLPs made of papaya mosaic virus (PapMV) trigger a CTL response through antigenic presentation of epitopes on MHC class I. Here, a chimeric VLP composed of malva mosaic virus (MaMV) was shown to share similar properties. We demonstrated the capacity of both VLPs to enter human APCs. The chimeric constructions were cross-presented in CD40-activated B lymphocytes leading to in vitro expansion of antigen-specific T lymphocytes. We showed that high concentrations of chimeric MaMV induced cell death, suggesting that some modifications can trigger collateral effects in vitro. Results suggest that potexvirus VLPs are an attractive vaccine platform for inducing a CTL response.

Palmitoylation of hepatitis C virus core protein is important for virion production

Hepatitis C virus core protein is the viral nucleocapsid of hepatitis C virus. Interaction of core with cellular membranes like endoplasmic reticulum (ER) and lipid droplets (LD) appears to be involved in viral assembly. However, how these interactions with different cellular membranes are regulated is not well understood. In this study, we investigated how palmitoylation, a post-translational protein modification, can modulate the targeting of core to cellular membranes. We show that core is palmitoylated at cysteine 172, which is adjacent to the transmembrane domain at the C-terminal end of core. Site-specific mutagenesis of residue Cys(172) showed that palmitoylation is not involved in the maturation process carried out by the signal peptide peptidase or in the targeting of core to LD. However, palmitoylation was shown to be important for core association with smooth ER membranes and ER closely surrounding LDs. Finally, we demonstrate that mutation of residue Cys(172) in the J6/JFH1 virus genome clearly impairs virion production.

The F13 residue is critical for interaction among the coat protein subunits of papaya mosaic virus

Papaya mosaic virus (PapMV) coat protein (CP) in Escherichia coli was previously showed to self-assemble in nucleocapsid-like particles (NLPs) that were similar in shape and appearance to the native virus. We have also shown that a truncated CP missing the N-terminal 26 amino acids is monomeric and loses its ability to bind RNA. It is likely that the N-terminus of the CP is important for the interaction between the subunits in self-assembly into NLPs. In this work, through deletion and mutation analysis, we have shown that the deletion of 13 amino acids is sufficient to generate the monomeric form of the CP. Furthermore, we have shown that residue F13 is critical for self-assembly of the CP subunits into NLPs. The replacement of F13 with hydrophobic residues (L or Y) generated mutated forms of the CP that were able to self-assemble into NLPs. However, the replacement of F13 by A, G, R, E or S was detrimental to the self-assembly of the protein into NLPs. We concluded that a hydrophobic interaction at the N-terminus is important to ensure self-assembly of the protein into NLPs. We also discuss the importance of F13 for assembly of other members of the potexvirus family.

Translating innate response into long-lasting antibody response by the intrinsic antigen-adjuvant properties of papaya mosaic virus

Identifying the properties of a molecule involved in the efficient activation of the innate and adaptive immune responses that lead to long-lasting immunity is crucial for vaccine and adjuvant development. Here we show that the papaya mosaic virus (PapMV) is recognized by the immune system as a pathogen-associated molecular pattern (PAMP) and as an antigen in mice (Pamptigen). A single immunization of PapMV without added adjuvant efficiently induced both cellular and specific long-lasting antibody responses. PapMV also efficiently activated innate immune responses, as shown by the induction of lipid raft aggregation, secretion of pro-inflammatory cytokines, up-regulation of co-stimulatory molecules on dendritic cells and macrophages, and long-lasting adjuvant effects upon the specific antibody responses to model antigens. PapMV mixed with Salmonella enterica serovar Typhi (S. typhi) outer membrane protein C increased its protective capacity against challenge with S. typhi, revealing the intrinsic adjuvant properties of PapMV in the induction of immunity. Antigen-presenting cells loaded with PapMV efficiently induced antibody responses in vivo, which may link the innate and adaptive responses observed. PapMV recognition as a Pamptigen might be translated into long-lasting antibody responses and protection observed. These properties could be used in the development of new vaccine platforms.

Effect of cAMP-dependent protein kinase A (PKA) on HCV nucleocapsid assembly and degradation

The primary function of the hepatitis C virus (HCV) core protein is genome encapsidation. Core protein is also subject to post-translational modifications that can impact on the assembly process. In this report, we have studied the effect of cAMP-dependent protein kinase A (PKA) phosphorylation on its assembly and stability in a yeast Pichia pastoris expression system. We have recently shown that co-expression of the human signal peptide peptidase and core protein (amino acids 1-191) in yeast leads to the formation of nucleocapsid-like particles (NLPs) that are morphologically similar to the wild-type HCV capsid. In this system, we expressed mutants S53A and S116A and mutants S53D and S116D to abolish or mimic PKA phosphorylation, respectively. None of these mutations affected HCV assembly, but S116D led to the degradation of core protein. We also showed that nonenveloped NLPs were labelled in vitro by PKA, suggesting that the phosphorylation sites are available at the surface of the NLPs. The co-expression of human PKA with core and human signal peptide peptidase in yeast did not produce phosphorylated NLPs and led to a decreased accumulation of nonenveloped particles. Mutation S116A restored the core protein content. These results suggest that PKA phosphorylation can modulate HCV core levels in infected cells.

Complete nucleotide sequence of Nootka lupine vein-clearing virus

The complete genome sequence of Nootka lupine vein-clearing virus (NLVCV) was determined to be 4,172 nucleotides in length containing four open reading frames (ORFs) with a similar genetic organization of virus species in the genus Carmovirus, family Tombusviridae. The order and gene product size, starting from the 5'-proximal ORF consisted of: (1) polymerase/replicase gene, ORF1 (p27) and ORF1RT (readthrough) (p87), (2) movement proteins ORF2 (p7) and ORF3 (p9), and, (3) the 3'-proximal coat protein ORF4, (p37). The genomic 5'- and 3'-proximal termini contained a short (59 nt) and a relatively longer 405 nt untranslated region, respectively. The longer replicase gene product contained the GDD motif common to RNA-dependent RNA polymerases. Phylogenetically, NLVCV formed a subgroup with the following four carmoviruses when separately comparing the amino acids of the coat protein or replicase protein: Angelonia flower break virus (AnFBV), Carnation mottle virus (CarMV), Pelargonium flower break virus (PFBV), and Saguaro cactus virus (SgCV). Whole genome nucleotide analysis (percent identities) among the carmoviruses with NLVCV suggested a similar pattern. The species demarcation criteria in the genus Carmovirus for the amino acid sequence identity of the polymerase (<52%) and coat (<41%) protein genes restricted NLVCV as a distinct species, and instead, placed it as a tentative strain of CarMV, PFBV, or SgCV when both the polymerase and CP were used as the determining factors. In contrast, the species criteria that included different host ranges with no overlap and lack of serology relatedness between NLVCV and the carmoviruses, suggested that NLVCV was a distinct species. The relatively low cutoff percentages allowed for the polymerase and CP genes to dictate the inclusion/exclusion of a distinct carmovirus species should be reevaluated. Therefore, at this time we have concluded that NLVCV should be classified as a tentative new species in the genus Carmovirus, family Tombusviridae.

Phosphorylation of the termini of Cauliflower mosaic virus precapsid protein is important for productive infection

Cauliflower mosaic virus (CaMV) coat protein precursor (pre-CP) has 489 amino acids (p57) and is processed by the viral proteinase into three major forms: p44, p39, and p37. The N- and C-terminal extensions of pre-CP are released during maturation by the virus-encoded proteinase. We showed that these extensions are phosphorylated at several sites by host casein kinase II (CKII). We have identified the phosphorylated amino acids using an in vitro phosphorylation assay and tested the effect of mutation of these sites on viral infectivity. Mutation of serines S66, S68, and S72 to alanine in the N-terminal extension abolished phosphorylation of the protein in vitro. Also, mutation of all S and T residues in the C-terminus (450 to 489) made this region insensitive to CKII. Amino acid substitutions also were introduced into a full-length infectious clone of CaMV. Mutated forms of the virus with S66, S68, and S72 substituted with A or D showed a delay in symptom development and affected the infectivity of the virus. However, a mutant with an A substitution of all the S and T residues of the C-terminal extension of CP was not infectious. These results suggest that phosphorylation of the N- and C-termini of CaMV pre-CP plays an important role in the initiation of viral infection.

A method for in vitro assembly of hepatitis C virus core protein and for screening of inhibitors

The assembly of hepatitis C virus (HCV) is not well understood. We investigated HCV nucleocapsid assembly in vitro and the role of electrostatic/hydrophobic interactions in this process. We developed a simple and rapid in vitro assay in which the progress of assembly is monitored by measuring an increase in turbidity, thereby allowing the kinetics of assembly to be determined. Assembly is performed using a truncated HCV core (C1-82), containing the minimal assembly domain, purified from Escherichia coli. The increase in turbidity is linked to the formation of nucleocapsid-like particles (NLPs) in solution, and nucleic acids are essential to initiate nucleocapsid assembly under the experimental conditions used. The sensitivity of NLP formation to salt strongly suggests that electrostatic forces govern in vitro assembly. Mutational analysis of C1-82 demonstrated that it is the global positive charge of C1-82 rather than any specific basic residue that is important for the assembly process. Our in vitro assembly assay provides an easy and efficient means of screening for assembly inhibitors, and we have identified several inhibitory peptides that could represent a starting point for drug design.

Immunogenicity of papaya mosaic virus-like particles fused to a hepatitis C virus epitope: evidence for the critical function of multimerization

Plant-virus-based vaccines have emerged as a promising avenue in vaccine development. This report describes the engineering of an innovative vaccine platform using the papaya mosaic virus (PapMV) capsid protein (CP) as a carrier protein and a C-terminal fused hepatitis C virus (HCV) E2 epitope as the immunogenic target. Two antigen organizations of the PapMV-based vaccines were tested: a virus-like-particle (VLP; PapMVCP-E2) and a monomeric form (PapMVCP(27-215)-E2). While the two forms of the vaccine were both shown to be actively internalized in vitro in bone-marrow-derived antigen presenting cells (APCs), immunogenicity was demonstrated to be strongly dependent on antigen organization. Indeed, C3H/HeJ mice injected twice with the multimeric VLP vaccine showed a long-lasting humoral response (more than 120 days) against both the CP and the fused HCV E2 epitope. The antibody profile (production of IgG1, IgG2a, IgG2b, IgG3) suggests a Th1/Th2 response. Immunogenicity of the PapMV vaccine platform was not observed when the monomer PapMVCP-E2 was injected. These results demonstrate for the first time the potential of the PapMV vaccine platform and the critical function of multimerization in its immunogenicity.

Proteasome-independent major histocompatibility complex class I cross-presentation mediated by papaya mosaic virus-like particles leads to expansion of specific human T cells

The development of versatile vaccine platforms is a priority that is recognized by health authorities worldwide; such platforms should induce both arms of the immune system, the humoral and cytotoxic-T-lymphocyte responses. In this study, we have established that a vaccine platform based on the coat protein of papaya mosaic virus (PapMV CP), previously shown to induce a humoral response, can induce major histocompatibility complex (MHC) class I cross-presentation of HLA-A*0201 epitopes from gp100, a melanoma antigen, and from influenza virus M1 matrix protein. PapMV proteins were able to assemble into stable virus-like particles (VLPs) in a crystalline and repetitive structure. When we pulsed HLA-A*0201+ antigen-presenting cells (APCs) with the recombinant PapMV FLU or gp100, we noted that antigen-specific CD8+ T cells were highly reactive to these APCs, demonstrating that the epitope from the VLPs were processed and loaded on the MHC class I complex. APCs were preincubated with two different proteasome inhibitors, which did not affect the efficiency of peptide presentation on MHC class I. Classical presentation from an endogenous antigen was abolished in the same conditions. Clearly, antigen presentation mediated by the PapMV system was proteasome independent. Finally, PapMV-pulsed APCs had the capacity to expand highly avid antigen-specific T cells against the influenza virus M1 HLA-A*0201 epitope when cocultured with autologous peripheral blood mononuclear cells. This study demonstrates the potential of PapMV for MHC class I cross-presentation and for the expansion of human antigen-specific T cells. It makes VLPs from PapMV CP a very attractive platform to trigger cellular responses for vaccine development against chronic infectious diseases and cancers.

High avidity binding of engineered papaya mosaic virus virus-like particles to resting spores of Plasmodiophora brassicae

Papaya mosaic virus (PapMV) like particles (VLPs) were used as a platform for fusion of affinity peptides binding to resting spores of Plasmodiophora brassicae-a major pathogen of crucifers. Three peptides with specific affinity to the target were isolated and cloned at the C-terminus of the PapMV coat protein (CP), generating three different high avidity VLPs. The peptides were exposed at the surface of the VLPs and their avidity to resting spores of P. brassicae was measured by flow cytometry. NLP-A, with the peptide DPAPRPR, showed the highest avidity. The binding avidity of NLP-A to P. brassicae spores was comparable to that of a polyclonal antibody. NLP-A was also shown to be more specific than the antibody. Fusion of the affinity peptide to a monomeric form (mCP) of the CP [Lecours, K., Tremblay, M.-H., Laliberté Gagné, M.-E., Gagné, S.M., Leclerc, D., 2006. Purification and biochemical characterization of a monomeric form of papaya mosaic potexvirus coat protein. Protein Express. Purific. 47, 273-280] generated a fusion protein that was unable to assemble into VLPs, and mCP-A fusions failed to bind resting spores. The avidity of VLP-A was increased by adding a glycine spacer between the C-terminus of the PapMV CP and the peptide, and improved even further by using a duplicated A peptide in the fusion protein. The use of high avidity VLPs has advantages over polyclonal antibodies because of target specificity. VLPs offers the specificity of monoclonal antibodies but can be more easily generated using the powerful selection of phage display.

Purification and biochemical characterization of a monomeric form of papaya mosaic potexvirus coat protein

Papaya mosaic virus (PapMV) is a flexuous rod shape virus made of 1400 subunits that assemble around a plus sense genomic RNA. The structure determination of PapMV and of flexuous viruses in general is a major challenge for both NMR and X-ray crystallography. In this report, we present the characterization of a truncated version of the PapMV coat protein (CP) that is suitable for NMR study. The deletion of the N-terminal 26 amino acids of the PapMV CP (CP27-215) generates a monomer that can be expressed to high level and easily purified for production of an adequate NMR sample. The RNA gel shift assay showed that CP27-215 lost its ability to bind RNA in vitro, suggesting that the multimerization of the subunit is important for this function. The fusion of a 6x His tag at the C-terminus improved the solubility of the monomer and allowed its concentration to 0.2 mM. The CD spectra of the truncated and the wild-type proteins were similar, suggesting that both proteins are well ordered and have a similar secondary structure. CP27-215 was 15N labeled for NMR studies and a 2D 1H-15N-HSQC spectrum confirmed the presence of a well-ordered structure and the monomeric form of the protein. These results show that CP27-215 is amenable to a complete and exhaustive NMR study that should lead to the first three-dimensional structure determination of a flexuous rod shape virus.

Signal peptide peptidase promotes the formation of hepatitis C virus non-enveloped particles and is captured on the viral membrane during assembly

The maturation of the core protein (C) of Hepatitis C virus (HCV) is controlled by the signal peptidase (sp) and signal peptide peptidase (spp) of the host. To date, it remains unknown whether spp cleavage influences viral infectivity and/or the assembly process. Here, evidence is provided that cleavage by spp is not required for assembly of nucleocapsid-like particles (NLPs) in yeast (Pichia pastoris). The immature NLPs (not processed by spp) show a density of 1.11 g ml(-1) on sucrose gradients and a diameter of 50 nm. Co-expression of human spp (hspp) with C generates the 21 kDa mature form of the protein and promotes the accumulation of non-enveloped particles. The amount of non-enveloped particles accumulating in the cell was correlated directly with the expression level of hspp. Furthermore, immunocapture studies showed that hspp was embedded in the membranes of enveloped particles. These results suggest that maturation of the C protein can occur after formation of the enveloped particles and that the abundance of hspp influences the types of particle accumulating in the cells.

Effect of mutations K97A and E128A on RNA binding and self assembly of papaya mosaic potexvirus coat protein

Papaya mosaic potexvirus (PapMV) coat protein (CP) was expressed (CPdeltaN5) in Escherichia coli and showed to self assemble into nucleocapsid like particles (NLPs). Twenty per cent of the purified protein was found as NLPs of 50 nm in length and 80% was found as a multimer of 450 kDa (20 subunits) arranged in a disk. Two mutants in the RNA binding domain of the PapMV CP, K97A and E128A showed interesting properties. The proteins of both mutants could be easily purified and CD spectra of these proteins showed secondary and tertiary structures similar to the WT protein. The mutant K97A was unable to self assemble and bind RNA. On the contrary, the mutant E128A showed an improved affinity for RNA and self assembled more efficiently in NLPs. E128A NLPs were longer (150 nm) than the recombinant CPdeltaN5 and 100% percent of the protein was found as NLPs in bacteria. E128A NLPs were more resistant to digestion by trypsin than the CPdeltaN5 but were more sensitive to denaturation by heat. We discuss the possible role of K97 and E128 in the assembly of PapMV.

A new sensitive and quantitative HTLV-I-mediated cell fusion assay in T cells

Similar to several other viruses, human T cell leukemia virus type I (HTLV-I) induces the formation of multinucleated giant cells (also known as syncytium) when amplified in tissue culture. These syncytia result from the fusion of infected cells with uninfected cells. Due to the intrinsic difficulty of infecting cells with cell-free HTLV-I virions, syncytium formation has become an important tool in the study of HTLV-I infection and transmission. Since most HTLV-I-based cell fusion assays rely on the use of non-T cells, the aim of this study was to optimize a new HTLV-I-induced cell fusion assay in which HTLV-I-infected T cell lines are co-cultured with T cells that have been transfected with an HTLV-I long terminal repeat (LTR) luciferase reporter construct. We demonstrate that co-culture of various HTLV-I-infected T cells with different transfected T cell lines resulted in induction of luciferase activity. Cell-to-cell contact and expression of the viral gp46 envelope protein was crucial for this induction while other cell surface proteins (including HSC70) did not have a significant effect. This quantitative assay was shown to be very sensitive. In this assay, the cell fusion-mediated activation of NF-kappaB and the HTLV-I LTR occurred through previously described Tax-dependent signaling pathways. This assay also showed that cell fusion could activate Tax-inducible cellular promoters. These results thus demonstrate that this new quantitative HTLV-I-dependent cell fusion assay is versatile, highly sensitive, and can provide an important tool to investigate cellular promoter activation and intrinsic signaling cascades that modulate cellular gene expression.

A coiled-coil interaction mediates cauliflower mosaic virus cell-to-cell movement

The function of the virion-associated protein (VAP) of cauliflower mosaic virus (CaMV) has long been only poorly understood. VAP is associated with the virion but is dispensable for virus morphogenesis and replication. It mediates virus transmission by aphids through simultaneous interaction with both the aphid transmission factor and the virion. However, although insect transmission is not fundamental to CaMV survival, VAP is indispensable for spreading the virus infection within the host plant. We used a GST pull-down technique to demonstrate that VAP interacts with the viral movement protein through coiled-coil domains and surface plasmon resonance to measure the interaction kinetics. We mapped the movement protein coiled-coil to the C terminus of the protein and proved that it self-assembles as a trimer. Immunogold labeling/electron microscopy revealed that the VAP and viral movement protein colocalize on CaMV particles within plasmodesmata. These results highlight the multifunctional potential of the VAP protein conferred by its efficient coiled-coil interaction system and show a plant virus possessing a surface-exposed protein (VAP) mediating viral entry into host cells.

Localization of the N-terminal domain of cauliflower mosaic virus coat protein precursor

Cauliflower mosaic virus (CaMV) open reading frame (ORF) IV encodes a coat protein precursor (pre-CP) harboring an N-terminal extension that is cleaved off by the CaMV-encoded protease. In transfected cells, pre-CP is present in the cytoplasm, while the processed form (p44) of CP is targeted to the nucleus, suggesting that the N-terminal extension might be involved in keeping the pre-CP in the cytoplasm for viral assembly. This study reports for the first time the intracellular localization of the N-terminal extension during CaMV infection in Brassica rapa. Immunogold-labeling electron microscopy using polyclonal antibodies directed to the N-terminal extension of the pre-CP revealed that this region is closely associated with viral particles present in small aggregates, which we called small bodies, adjacent to the main inclusion bodies typical of CaMV infection. Based on these results, we propose a model for viral assembly of CaMV.