Identification of protein-protein interactions of the occlusion-derived virus-associated proteins of Helicoverpa armigera nucleopolyhedrovirus

An Advanced View on Baculovirus per Os Infectivity Factors

Baculoviruses are arthropod-specific large DNA viruses that orally infect the larvae of lepidopteran, hymenopteran and dipteran insect species. These larvae become infected when they eat a food source that is contaminated with viral occlusion bodies (OBs). These OBs contain occlusion-derived viruses (ODVs), which are released upon ingestion of the OBs and infect the endothelial midgut cells. At least nine different ODV envelope proteins are essential for this oral infectivity and these are denoted per os infectivity factors (PIFs). Seven of these PIFs form a complex, consisting of PIF1, 2, 3 and 4 that form a stable core complex and PIF0 (P74), PIF6 and PIF8 (P95) that associate with this complex with lower affinity than the core components. The existence of a PIF complex and the fact that the pif genes are conserved in baculovirus genomes suggests that PIF-proteins cooperatively mediate oral infectivity rather than as individual functional entities. This review therefore discusses the knowledge obtained for individual PIFs in light of their relationship with other members of the PIF complex.

HearNPV Pseudotyped with PIF1, 2, and 3 from MabrNPV: Infectivity and Complex Stability

Effective oral infection is set off by interaction of a group of conserved per os infectivity factors (PIFs) with larval midgut columnar epithelial cells. We constructed pseudotyped viruses by substituting pif1, pif2 or pif3 genes of Helicoverpa armigera nucleopolyhedrovirus (HearNPV) with their homologs from Mamestra bracissae multiple nucleopolyhedrovirus and tested their infectivity to tissue culture cells and to larvae. Transfection and infection assays revealed that all recombinant viruses generated infectious budded virus in both cell culture and in larvae. Electron microscopy showed synthesized occlusion body and occlusion derived virus (ODV) were morphologically indistinguishable from those of the parental virus. By contrast, feeding assays revealed that pseudotyped viruses could not rescue oral infectivity except for pif3 pseudotyped virus that only partially rescued oral infectivity but at a mortality rate much lower than that of the parental HearNPV. Consistent with the bioassay result, PIF complex was detected in ODVs of pif3 pseudotyped virus only but not in pif1 or pif2 pseudotyped viruses. Our results suggest that PIF complex is essential for oral infectivity, and in the formation of the PIF complex, PIF1, 2 are virus-specific while PIF3 does not appear to be as specific and can function in heterologous environment, albeit to a much more limited extent.

The protein-protein interactions between Amsacta moorei entomopoxvirus (AMEV) protein kinases (PKs) and all viral proteins

Entomopoxviruses are an important group of viruses infecting only insects. They belong to Poxviridae which infect both invertebrates and vertebrates, including humans. Protein kinases are known to have roles at virus morphogenesis, host selectivity, the regulation of cell division and apoptosis in some vertebrate poxviruses. In this study, 2 protein kinases (PKs) (AMV153 and AMV197) of Amsacta moorei entomopoxvirus (AMEV) were investigated for the interactions among 230 viral proteins using yeast two-hybrid system (Y2H). For this purpose, two protein kinases and 230 viral genes were cloned into the bait and prey vectors, respectively. Bait vectors were introduced into Saccharomyces cerevisiae AH109. Expression of the bait genes were confirmed by western blot analysis. Both yeast strains of bait were transformed individually with each prey clone and grown on a selective medium (minimal synthetic defined) to determine the protein-protein interactions between bait and prey proteins. Transformations identified totally 16 interactions among AMEV protein kinases and all viral proteins of which 5 belong to AMV153 and 11 belong to AMV197. One of the five interactions detected for AMV153 protein kinase is self-association. Its other four interactions are with two virus entry complex proteins (AMV035 and AMV083), a membrane protein (AMV165) and a subunit of RNA polymerase (AMV230). The other protein kinase, AMV197, interacted with two virus entry complex proteins (AMV035 and AMV083) as AMV153, a caspase-2 enzyme (AMV063), a Holliday junction resolvase (AMV162), a membrane protein (AMV165), a subunit of RNA polymerase (AMV230) and five other hypothetical proteins (AMV026, AMV040, AMV062, AMV069, AMV120) encoded by AMEV genome. Glutathione S-transferase (GST) pull-down assay was used to confirm all interactions described by Y2H analysis. In addition, the theoretical structures of the two of 16 interactions were interpreted by docking analysis. Consistent with Y2H and pull down assays, docking analysis also showed the interactions of AMV063 with AMV153 and AMV197. Detected interactions of the AMEV viral proteins with viral protein kinases could lead to the understanding of the regulation of the viral activities of interacted viral proteins.

Networks of protein-protein interactions among structural proteins of budded virus of Bombyx mori nucleopolyhedrovirus

The structural proteins of baculovirus are well studied, but the interactions between them remain unclear. In order to reveal protein-protein interactions among viral structural proteins and their associated proteins of the budded virus of Bombyx mori nucleopolyhedrovirus (BmNPV), the yeast two hybrid (Y2H) system was used to evaluate the interactions of 27 viral genes products. Fifty-seven interactions were identified with 51 binary interactions and 6 self-associations. Among them, 10 interactions were further confirmed by co-immunoprecipitation assays. Five interaction networks were formed based on the direct-cross Y2H assays. VP39, 38 K, and FP were identified to interact with most of the viral proteins, and may form major structural elements of the viral architecture. In addition, each envelope protein was detected to interact with more than one capsid protein. These results suggest how viral structural and structural associated proteins may assemble to form a complete virus through interacting with each other.

Distinct Roles of Cellular ESCRT-I and ESCRT-III Proteins in Efficient Entry and Egress of Budded Virions of Autographa californica Multiple Nucleopolyhedrovirus

The endosomal sorting complex required for transport (ESCRT) machinery is necessary for budding of many enveloped viruses. Recently, it was demonstrated that Vps4, the key regulator for recycling of the ESCRT-III complex, is required for efficient infection by the baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV). However, ESCRT assembly, regulation, and function are complex, and little is known regarding the details of participation of specific ESCRT complexes in AcMNPV infection. In this study, the core components of ESCRT-I (Tsg101 and Vps28) and ESCRT-III (Vps2B, Vps20, Vps24, Snf7, Vps46, and Vps60) were cloned from Spodoptera frugiperda Using a viral complementation system and RNA interference (RNAi) assays, we found that ESCRT-I and ESCRT-III complexes are required for efficient entry of AcMNPV into insect cells. In cells knocking down or overexpressing dominant negative (DN) forms of the components of ESCRT-I and ESCRT-III complexes, entering virions were partially trapped within the cytosol. To examine only egress, cells were transfected with the double-stranded RNA (dsRNA) targeting an individual ESCRT-I or ESCRT-III gene and viral bacmid DNA or viral bacmid DNA that expressed DN forms of ESCRT-I and ESCRT-III components. We found that ESCRT-III components (but not ESCRT-I components) are required for efficient nuclear egress of progeny nucleocapsids. In addition, we found that several baculovirus core or conserved proteins (Ac11, Ac76, Ac78, GP41, Ac93, Ac103, Ac142, and Ac146) interact with Vps4 and components of ESCRT-III. We propose that these viral proteins may form an "egress complex" that is involved in recruiting ESCRT-III components to a virus egress domain on the nuclear membrane.IMPORTANCE The ESCRT system is hijacked by many enveloped viruses to mediate budding and release. Recently, it was found that Vps4, the key regulator of the cellular ESCRT machinery, is necessary for efficient entry and egress of Autographa californica multiple nucleopolyhedrovirus (AcMNPV). However, little is known about the roles of specific ESCRT complexes in AcMNPV infection. In this study, we demonstrated that ESCRT-I and ESCRT-III complexes are required for efficient entry of AcMNPV into insect cells. The components of ESCRT-III (but not ESCRT-I) are also necessary for efficient nuclear egress of progeny nucleocapsids. Several baculovirus core or conserved proteins were found to interact with Vps4 and components of ESCRT-III, and these interactions may suggest the formation of an "egress complex" involved in the nuclear release or transport of viral nucleocapsids.

Roles of Cellular NSF Protein in Entry and Nuclear Egress of Budded Virions of Autographa californica Multiple Nucleopolyhedrovirus

In eukaryotic cells, the soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein receptor (SNARE) proteins comprise the minimal machinery that triggers fusion of transport vesicles with their target membranes. Comparative studies revealed that genes encoding the components of the SNARE system are highly conserved in yeast, insect, and human genomes. Upon infection of insect cells by the virus Autographa californica multiple nucleopolyhedrovirus (AcMNPV), the transcript levels of most SNARE genes initially were upregulated. We found that overexpression of dominant-negative (DN) forms of NSF or knockdown of the expression of NSF, the key regulator of the SNARE system, significantly affected infectious AcMNPV production. In cells expressing DN NSF, entering virions were trapped in the cytoplasm or transported to the nucleus with low efficiency. The presence of DN NSF also moderately reduced trafficking of the viral envelope glycoprotein GP64 to the plasma membrane but dramatically inhibited production of infectious budded virions (BV). Transmission electron microscopy analysis of infections in cells expressing DN NSF revealed that progeny nucleocapsids were retained in a perinuclear space surrounded by inner and outer nuclear membranes. Several baculovirus conserved (core) proteins (Ac76, Ac78, GP41, Ac93, and Ac103) that are important for infectious budded virion production were found to associate with NSF, and NSF was detected within the assembled BV. Together, these data indicate that the cellular SNARE system is involved in AcMNPV infection and that NSF is required for efficient entry and nuclear egress of budded virions of AcMNPV.IMPORTANCE Little is known regarding the complex interplay between cellular factors and baculoviruses during viral entry and egress. Here, we examined the cellular SNARE system, which mediates the fusion of vesicles in healthy cells, and its relation to baculovirus infection. Using a DN approach and RNA interference knockdown, we demonstrated that a general disruption of the SNARE machinery significantly inhibited the production of infectious BV of AcMNPV. The presence of a DN NSF protein resulted in low-efficiency entry of BV and the retention of progeny nucleocapsids in the perinuclear space during egress. Combined with these effects, we also found that several conserved (core) baculovirus proteins closely associate with NSF, and these results suggest their involvement in the egress of BV. Our findings are the first to demonstrate that the SNARE system is required for efficient entry of BV and nuclear egress of progeny nucleocapsids of baculoviruses.

Per os infectivity factors: a complicated and evolutionarily conserved entry machinery of baculovirus

Baculoviruses are a family of arthropod-specific large DNA viruses that infect insect species belonging to the orders Lepidoptera, Hymenoptera and Diptera. In nature, occlusion-derived viruses (ODVs) initiate baculovirus primary infection in the midgut epithelium of insect hosts, and this process is largely dependent on a number of ODV envelope proteins designated as per os infectivity factors (PIFs). Interestingly, PIF homologs are also present in other invertebrate large DNA viruses, which is indicative that per os infection is an ancient and phylogenetically conserved entry mechanism shared by these viruses. Here, we review the advances in the knowledge of the functions of individual PIFs and recent discoveries about the PIF complex, and discuss the evolutionary implications of PIF homologs in invertebrate DNA viruses. Furthermore, future research highlights on the per os infection mechanism are also prospected.

Protein-protein interactions of the baculovirus per os infectivity factors (PIFs) in the PIF complex

After ingestion of occlusion bodies, the occlusion-derived viruses (ODVs) of the baculoviruses establish the first round of infection within the larval host midgut cells. Several ODV envelope proteins, called per os infectivity factors (PIFs), have been shown to be essential for oral infection. Eight PIFs have been identified to date, including P74, PIFs 1-6 and Ac110. At least six PIFs, P74, PIFs 1-4 and PIF6, together with three other ODV-specific proteins, Ac5, P95 (Ac83) and Ac108, have been reported to form a complex on the ODV surface. In this study, in order to understand the interactions of these PIFs, the direct protein-protein interactions of the nine components of the Autographa californica multiple nucleopolyhedrovirus PIF complex were investigated using yeast two-hybrid (Y2H) screening combined with bimolecular fluorescence complementation (BiFC) assay. Six direct interactions, comprising PIF1-PIF2, PIF1-PIF3, PIF1-PIF4, PIF1-P95, PIF2-PIF3 and PIF3-PIF4, were identified in the Y2H analysis, and these results were further verified by BiFC. For P74, PIF6, Ac5 and Ac108, no direct interaction was identified. P95 (Ac83) was identified to interact with PIF1, and further Y2H analysis of the truncation and deletion mutants showed that the predicted P95 chitin-binding domain and amino acids 100-200 of PIF1 were responsible for P95 interaction with PIF1. Furthermore, a summary of the protein-protein interactions of PIFs reported so far, comprising 10 reciprocal interactions and two self-interactions, is presented, which will facilitate our understanding of the characteristics of the PIF complex.

The Host Specificities of Baculovirus per os Infectivity Factors

Baculoviruses are insect-specific pathogens with a generally narrow host ranges. Successful primary infection is initiated by the proper interaction of at least 8 conserved per os infectivity factors (PIFs) with the host's midgut cells, a process that remains largely a mystery. In this study, we investigated the host specificities of the four core components of the PIF complex, P74, PIF1, PIF2 and PIF3 by using Helicoverpa armigera nucleopolyhedrovirus (HearNPV) backbone. The four pifs of HearNPV were replaced by their counterparts from a group I Autographa californica multiple nucleopolyhedrovirus (AcMNPV) or a group II Spodoptera litura nucleopolyhedrovirus (SpltNPV). Transfection and infection assays showed that all the recombinant viruses were able to produce infectious budded viruses (BVs) and were lethal to H. armigera larvae via intrahaemocoelic injection. However, feeding experiments using very high concentration of occlusion bodies demonstrated that all the recombinant viruses completely lost oral infectivity except SpltNPV pif3 substituted pif3-null HearNPV (vHaBacΔpif3-Sppif3-ph). Furthermore, bioassay result showed that the median lethal concentration (LC50) value of vHaBacΔpif3-Sppif3-ph was 23-fold higher than that of the control virus vHaBacΔpif3-Hapif3-ph, indicating that SpltNPV pif3 can only partially substitute the function of HearNPV pif3. These results suggested that most of PIFs tested have strict host specificities, which may account, at least in part, for the limited host ranges of baculoviruses.

Baculovirus nucleocapsid aggregation (MNPV vs SNPV): an evolutionary strategy, or a product of replication conditions?

Lepidopteran nucleopolyhedroviruses are members of the Baculoviridae and have been categorized as having two morphotypes of occluded virions: multiple nucleocapsids or single nucleocapsids within the virion envelope. Although it is a definitive characteristic of specific viruses, it appears to lack a defined genetic basis and is independent of virus phylogeny. This review summarizes the factors that appear to influence this trait and the role that it may play in virus biology.

Genome scale transcriptomics of baculovirus-insect interactions

Baculovirus-insect cell technologies are applied in the production of complex proteins, veterinary and human vaccines, gene delivery vectors' and biopesticides. Better understanding of how baculoviruses and insect cells interact would facilitate baculovirus-based production. While complete genomic sequences are available for over 58 baculovirus species, little insect genomic information is known. The release of the Bombyx mori and Plutella xylostella genomes, the accumulation of EST sequences for several Lepidopteran species, and especially the availability of two genome-scale analysis tools, namely oligonucleotide microarrays and next generation sequencing (NGS), have facilitated expression studies to generate a rich picture of insect gene responses to baculovirus infections. This review presents current knowledge on the interaction dynamics of the baculovirus-insect system' which is relatively well studied in relation to nucleocapsid transportation, apoptosis, and heat shock responses, but is still poorly understood regarding responses involved in pro-survival pathways, DNA damage pathways, protein degradation, translation, signaling pathways, RNAi pathways, and importantly metabolic pathways for energy, nucleotide and amino acid production. We discuss how the two genome-scale transcriptomic tools can be applied for studying such pathways and suggest that proteomics and metabolomics can produce complementary findings to transcriptomic studies.

The ha72 core gene of baculovirus is essential for budded virus production and occlusion-derived virus embedding, and amino acid K22 plays an important role in its function

ha72 of Helicoverpa armigera nucleopolyhedrovirus (a homologue of ac78) was identified as a conserved late baculovirus gene and characterized. HA72 localizes in the intranuclear ring zone. By generating mutants, we showed that HA72 is essential for budded virus (BD) production and occlusion-derived virus (ODV) embedding. HA72 also interacted with P33, a baculoviral sulfhydryl oxidase. A point mutation of amino acid 22 from lysine to glutamic acid curtailed BV production and precluded ODV occlusion as well as interaction with P33.

Functional characterization of Bombyx mori nucleopolyhedrovirus CG30 protein

The baculovirus cg30 gene is present in the genomes of most alphabaculoviruses. The gene product, CG30, contains two putative functional domains, a RING finger motif and a leucine zipper motif. A gene-knockout study in Autographa californica nucleopolyhedrovirus (AcMNPV) revealed that a cg30-disrupted virus did not show any striking differences compared with wild-type virus. To determine the roles of cg30 in another alphabaculovirus, we constructed two Bombyx mori NPV (BmNPV) mutants lacking a functional cg30 by lacZ cassette insertion and characterized its infectivity in BmN cells and B. mori larvae. The mutants produced fewer budded viruses (BVs) in BmN cells and B. mori larvae compared with wild-type BmNPV. We also observed a decrease in the release of occlusion bodies (OBs) in the hemolymph of the larvae infected with the cg30 mutants. To investigate the role of a RING finger domain of CG30 during virus growth, we further constructed two mutants; one expressed a mutant CG30 where a RING finger motif is disrupted by a single amino acid substitution, whereas the other possessed a CG30 derivative that completely lacked the RING finger domain. Both produced fewer OBs in the hemolymph of B. mori larvae, demonstrating that a RING finger domain of CG30 is involved in maximum OB production in the hemolymph of B. mori larvae. We also revealed that CG30 is expressed as a nuclear protein with a molecular mass of 30kDa in BmNPV-infected cells.

AcMNPV core gene ac109 is required for budded virion transport to the nucleus and for occlusion of viral progeny

The Autographa californica multiple nucleopolyhedrovirus (AcMNPV) ac109 core gene has been previously characterized as an essential late gene. Our results showed that budded virions could be detected in supernatants of infected Sf-9 cells, even when ac109 knockout viruses displayed a single-cell infection phenotype. Moreover, confocal microscopy analysis revealed that budded virions can enter the cytoplasm but are unable to enter the cell nucleus. This defect could be repaired by complementing ac109 in trans. In addition, polyhedra of normal size could be detected in Sf-9 nuclei infected with ac109 knockout viruses. However, electron microscopy demonstrated that these occlusion bodies were empty. Altogether, these results indicate that ac109 is required for infectivity of both phenotypes of virus.

Dynamic interactions between Bombyx mori nucleopolyhedrovirus and its host cells revealed by transcriptome analysis

Although microarray and expressed sequence tag (EST)-based approaches have been used to profile gene expression during baculovirus infection, the response of host genes to baculovirus infection and the interaction between baculovirus and its host remain largely unknown. To determine the host response to Bombyx mori nucleopolyhedrovirus infection and the dynamic interaction between the virus and its host, eight digital gene expression libraries were examined in a Bm5 cell line before infection and at 1.5, 3, 6, 12, 24, 48, and 96 h postinfection. Gene set enrichment analysis of differentially expressed genes at each time point following infection showed that gene sets including cytoskeleton, transcription, translation, energy metabolism, iron ion metabolism, and the ubiquitin-proteasome pathway were altered after viral infection. In addition, a time course depicting protein-protein interaction networks between the baculovirus and the host were constructed and revealed that viral proteins interact with a multitude of cellular machineries, such as the proteasome, cytoskeleton, and spliceosome. Several viral proteins, including IE2, CG30, PE38, and PK-1/2, were predicted to play key roles in mediating virus-host interactions. Based on these results, we tested the role of the ubiquitin-proteasome pathway and iron ion metabolism in the viral infection cycle. Treatment with a proteasome inhibitor and deferoxamine mesylate in vitro and in vivo confirmed that these pathways regulate viral infection. Taken together, these findings provide new insights into the interaction between the baculovirus and its host and identify molecular mechanisms that can be used to block viral infection and improve baculovirus expression systems.

ORF85 of HearNPV encodes the per os infectivity factor 4 (PIF4) and is essential for the formation of the PIF complex

ORF85 of Helicoverpa armigera nucleopolyhedrovirus (HearNPV) encodes a homologue of the per os infectivity factor 4 (PIF-4) of Autographa californica multiple nucleocapsid nucleopolyhedrovirus (AcMNPV). In this paper, the functions of HA85, particularly in relation to oral infection and interactions with other PIFs were investigated. An ha85-disrupted recombinant HearNPV was generated and resulted in a complete loss of oral infectivity. Western blotting and co-immunoprecipitation (Co-IP) analyses suggested PIF1, PIF2, and PIF3 assemble into a PIF complex in HearNPV ODV. Although Western blotting and Co-IP did not show that HA85 is associated with the PIF complex, further analysis revealed the inactivation of ha85 led to the disruption of PIF complex. Yeast two hybridization analyses revealed that HA85 interacts with P74, PIF1, PIF2 and PIF3. In conclusion, HA85 is identified as the PIF4 of HearNPV and is proposed to participate in the formation of HearNPV PIF complex via associations with other PIFs.

In situ cleavage of baculovirus occlusion-derived virus receptor binding protein P74 in the peroral infectivity complex

Proteolytic processing of viral membrane proteins is common among enveloped viruses and facilitates virus entry. The Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) occlusion-derived virus (ODV) protein P74 is part of a complex of essential peroral infectivity factors (PIFs). Here we report that P74 is efficiently cleaved into two fragments of about equal size by an occlusion body (OB) endogenous alkaline protease during ODV release when AcMNPV OBs are derived from larvae. The cleavage is specific for P74, since the other known peroral infectivity factors in the same complex (PIF1, PIF2, and PIF3) were not cleaved under the same conditions. P74 cleavage was not observed in OBs produced in three different insect cell lines, suggesting a larval host origin of the responsible protease. P74 in OBs produced in larvae of two different host species was cleaved into fragments with the same apparent molecular mass, indicating that the virus incorporates a similar alkaline protease from different hosts. Coimmunoprecipitation analysis revealed that the two P74 subunit fragments remain associated with the recently discovered PIF complex. We propose that under in vivo ODV infection conditions, P74 undergoes two sequential cleavage events, the first one being performed by an ODV-associated host alkaline protease and the second carried out by trypsin in the host midgut.

Helicoverpa armigera nucleopolyhedrovirus occlusion-derived virus-associated protein, HA100, affects oral infectivity in vivo but not virus replication in vitro

ORF100 (ha100) of Helicoverpa armigera nucleopolyhedrovirus (HearNPV) has been reported as one of the unique genes of group II alphabaculoviruses encoding a protein located in the occlusion-derived virus (ODV) envelope and nucleocapsid. The protein consists of 510 aa with a predicted mass of 58.1 kDa and is a homologue of poly(ADP–ribose) glycohydrolase in eukaryotes. Western blot analysis detected a 60 kDa band in HearNPV-infected HzAM1 cells starting at 18 h post-infection. Transient expression of GFP-fused HA100 in HzAM1 cells resulted in cytoplasmic localization of the protein, but after superinfection with HearNPV, GFP-fused HA100 was localized in the nucleus. To study the function of HA100 further, an ha100-null virus was constructed using bacmid technology. Viral one-step growth curve analyses showed that the ha100-null virus had similar budded virus production kinetics to that of the parental virus. Electron microscopy revealed that deletion of HA100 did not alter the morphology of ODVs or occlusion bodies (OBs). However, bioassays in larvae showed that the 50 % lethal concentration (LC50) value of HA100-null OBs was significantly higher than that of parental OBs; the median lethal time (LT50) of ha100-null OBs was about 24 h later than control virus. These results indicate that HA100 is not essential for virus replication in vitro. However, it significantly affects the oral infectivity of OBs in host insects, suggesting that the association HA100 with the ODV contributes to the infectivity of OBs in vivo.

ODV-associated proteins of the Pieris rapae granulovirus

Alphabaculovirus (lepidopteran-specific nucleopolyhedroviruses, NPV) and Betabaculovirus (granuloviruses, GV) are two main genera of the family Baculoviridae. The virion proteomes of Alphabaculovirus have been well studied; however, the Betabaculovirus virion compositions remain unclear. Pieris rapae granulovirus (PrGV) can kill larvae of P. rapae, a worldwide and important pest of mustard family crops. In this study, the occlusion-derived virus (ODV)-associated proteins of PrGV were identified using three mass spectrometry (MS) approaches. The MS analyses demonstrated that 47 proteins were present in PrGV-ODV. Of the 47 PrGV-ODV proteins, 33 have homologues identified previously in other baculovirus ODV/BVs, whereas 14 (P10, Pr21, Pr29, Pr35, Pr42, Pr54, P45/48, Pr83, Pr84, Pr89, Pr92, Pr111, Pr114 and FGF3) were newly identified ODV proteins. Seven of the 14 newly identified ODV proteins are specific to Betabaculovirus, including Pr35, Pr42, Pr54, Pr83, Pr84, Pr111 and Pr114. Furthermore, the data derived from these MS approaches were validated by immunoblotting analysis using antisera prepared from 11 randomly selected recombinant PrGV-ODV proteins (including 5 Betabaculovirus-unique proteins). Comparison analyses revealed the similar and different compositions between Betabaculovirus and Alphabaculovirus virions, which deepen our understanding of the baculovirus virion structure and provide helpful information on Betabaculovirus--host interaction studies.

Baculovirus: molecular insights on their diversity and conservation

The Baculoviridae is a large group of insect viruses containing circular double-stranded DNA genomes of 80 to 180 kbp. In this study, genome sequences from 57 baculoviruses were analyzed to reevaluate the number and identity of core genes and to understand the distribution of the remaining coding sequences. Thirty one core genes with orthologs in all genomes were identified along with other 895 genes differing in their degrees of representation among reported genomes. Many of these latter genes are common to well-defined lineages, whereas others are unique to one or a few of the viruses. Phylogenetic analyses based on core gene sequences and the gene composition of the genomes supported the current division of the Baculoviridae into 4 genera: Alphabaculovirus, Betabaculovirus, Gammabaculovirus, and Deltabaculovirus.

Autographa californica multiple nucleopolyhedrovirus odv-e66 is an essential gene required for oral infectivity

Autographa californica multiple nucleopolyhedrovirus (AcMNPV) odv-e66 is a core gene and encodes an occlusion-derived virus (ODV)-specific envelope protein, ODV-E66. The N-terminal 23 amino acid of the envelope protein ODV-E66 are sufficient to direct native and fusion proteins to induced membrane microvesicles and the viral envelope during infection with AcMNPV. In this study, an odv-e66-knockout bacmid can not express N-terminal hydrophobic domains was constructed via homologous recombination in Escherichia coli. The odv-e66 deletion had no effect on budded virus (BV) production and viral DNA replication in infected Sf9 cells. Larval bioassays demonstrated that injection of odv-e66 deletion BV into the hemocoel could kill P. xylostella larvae as efficiently as repaired and control viruses; however, odv-e66 deletion mutant resulted in a 50% lethal dose that was 10(3) higher than that of the repaired and control viruses when inoculated per os. These results indicated that ODV-E66 envelope protein most likely played an important role in the oral infectivity of AcMNPV, but is not essential for virus replication.

Baculovirus VP80 protein and the F-actin cytoskeleton interact and connect the viral replication factory with the nuclear periphery

Recently, we showed that the Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) VP80 protein is essential for the formation of both virion types, budded virus (BV) and occlusion-derived virus (ODV). Deletion of the vp80 gene did not affect assembly of nucleocapsids. However, these nucleocapsids were not able to migrate from the virogenic stroma to the nuclear periphery. In the current paper, we constructed a baculovirus recombinant with enhanced-green fluorescent protein (EGFP)-tagged VP80, allowing visualization of the VP80 distribution pattern during infection. In baculovirus-infected cells, the EGFP-VP80 protein is entirely localized in nuclei, adjacent to the virus-triggered F-actin scaffold that forms a highly organized three-dimensional network connecting the virogenic stroma physically with the nuclear envelope. Interaction between VP80 and host actin was confirmed by coimmunoprecipitation. We further showed that VP80 is associated with the nucleocapsid fraction of both BVs and ODVs, typically at one end of the nucleocapsids. In addition, the presence of sequence motifs with homology to invertebrate paramyosin proteins strongly supports a role for VP80 in the polar transport of nucleocapsids to the periphery of the nucleus on their way to the plasma membrane to form BVs and for assembly in the nuclear periphery to form ODVs for embedding in viral occlusion bodies.

Protein composition of the occlusion derived virus of Chrysodeixis chalcites nucleopolyhedrovirus

Chrysodeixis chalcites nucleopolyhedrovirus (ChchNPV) is a group II NPV and its genome has 151 predicted open reading frames. In this study, the protein composition of ChchNPV occlusion derived virus (ODV) was determined by LC-MS/MS. Fifty-three proteins were identified in ChchNPV ODV particles. One ODV-protein is encoded by a gene so far unique to ChchNPV (Chch105). The two DNA photolyases PHR1 and PHR2, which are characteristic for ChchNPV and thought to be involved in repairing UV damage in viral DNA, were not detected in the ODVs. Comparison of the ODV proteins identified in ChchNPV and in three other baculoviruses enabled the identification of ten conserved ODV proteins (ODV-E18, ODV-E56, ODV-EC27, ODV-EC43, P6.9, P33, P49, P74, GP41, and VP39). In addition, the baculovirus per os infectivity factors PIF1, PIF2 and PIF3 were all detected in ChchNPV and these should be considered as conserved ODV proteins as well as they are absolutely required for oral infection. With the LC-MS/MS method used 22 viral proteins were detected, which were not identified as ODV proteins in previous studies.

A peptide with similarity to baculovirus ODV-E66 binds the gut epithelium of Heliothis virescens and impedes infection with Autographa californica multiple nucleopolyhedrovirus

Baculoviruses infect their lepidopteran hosts via the midgut epithelium through binding of occlusion-derived virus (ODV) and fusion between the virus envelope and microvillar membranes. To identify genes and sequences that are involved in this process, a random phage display library was screened for peptides that bound to brush border membrane vesicles (BBMV) derived from the midgut epithelium of Heliothis virescens. Seventeen peptides that bound to BBMV were recovered. Two of these, HV1 and HV2, had sequence similarity to the ODV envelope protein ODV-E66 that is found in five species of alphabaculoviruses. Chemically synthesized versions of HV1 and HV2, and two peptides (AcE66A and AcE66B) derived from similar sequences of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) ODV-E66, bound to unfixed cryosections of whole midgut tissues. AcE66A, but not HV1, bound to H. virescens gut BBMV proteins on a far-Western blot. Competition assays with HV1 and purified AcMNPV ODV resulted in decreased mortality of H. virescens larvae at a dose of 1 LD(50), and a significant increase in survival time at higher virus concentrations. These results suggest a role for ODV-E66 in baculovirus infection of lepidopteran larval midgut epithelium.

Baculovirus per os infectivity factors form a complex on the surface of occlusion-derived virus

Five highly conserved per os infectivity factors, PIF1, PIF2, PIF3, PIF4, and P74, have been reported to be essential for oral infectivity of baculovirus occlusion-derived virus (ODV) in insect larvae. Three of these proteins, P74, PIF1, and PIF2, were thought to function in virus binding to insect midgut cells. In this paper evidence is provided that PIF1, PIF2, and PIF3 form a stable complex on the surface of ODV particles of the baculovirus Autographa californica multinucleocapsid nucleopolyhedrovirus (AcMNPV). The complex could withstand 2% SDS-5% beta-mercaptoethanol with heating at 50 degrees C for 5 min. The complex was not formed when any of the genes for PIF1, PIF2, or PIF3 was deleted, while reinsertion of these genes into AcMNPV restored the complex. Coimmunoprecipitation analysis independently confirmed the interactions of the three PIF proteins and revealed in addition that P74 is also associated with this complex. However, deletion of the p74 gene did not affect formation of the PIF1-PIF2-PIF3 complex. Electron microscopy analysis showed that PIF1 and PIF2 are localized on the surface of the ODV with a scattered distribution. This distribution did not change for PIF1 or PIF2 when the gene for PIF2 or PIF1 protein was deleted. We propose that PIF1, PIF2, PIF3, and P74 form an evolutionarily conserved complex on the ODV surface, which has an essential function in the initial stages of baculovirus oral infection.