The Bombyx mori nucleopolyhedrovirus (BmNPV) ODV-E56 envelope protein is also a per os infectivity factor

Label-free proteomics analysis on the envelope of budded viruses of Bombyx mori nucleopolyhedrovirus harboring differential localized GP64

Bombyx mori nucleopolyhedrovirus (BmNPV) GP64 is the key membrane fusion protein that mediates budded virus (BV) infection. We recently reported that BmNPV GP64's n-region of signal peptide (SP) blocked the SP-cleavage and mediated GP64 localization on the plasma membrane (PM); n-region (SP^∆nGP64) absence caused GP64 intracellular localization, however, SP^∆nGP64 was still incorporated into virion to generate BVs with lower infectivity. To better understand the biogenesis of the envelope of BmNPV BV, we conducted a label-free ESI mass spectrometry analysis of the envelope of purified BVs harboring PM localized GP64 or intracellular localized SP^∆nGP64. The results indicated that 31 viral proteins were identified on the envelope, among which 15 were reported in other viruses. The other 16 proteins were first reported in BmNPV BV, including the BmNPV-specific protein BRO-A and proteins associated with vesicle transportation. Six proteins with significant intensity differences were detected in virions with differential localized GP64, and five specific proteins were identified in virions with GP64. Meanwhile, we identified 81 host proteins on the envelope, and seven lipoproteins were first identified in baculovirus virion; other 74 proteins are involved in the cytoskeleton, DNA-binding, vesicle transport, etc. In the meantime, eight and five specific host proteins were, respectively, identified in GP64 and SP^∆nGP64's virions. The two virions shared 68 common host proteins, and 8 proteins were identified on their envelopes with a significant difference. This study provides new insight into the protein composition of BmNPV BV and a clue for further investigation of the budding mechanism of BmNPV.

A selection marker-free method for gene deletion and editing in baculovirus genomes

Here, we develop a simple, efficient, bacmid-based, selection marker-free method for gene deletion and editing in baculovirus genomes. Specifically, based on pFastbac1, a donor plasmid with long left and right homology arms but without a reporter was constructed for disrupting ie1, an essential baculovirus gene. Instead of ligating with a plasmid, the homology arms were introduced to the polyhedrin locus of BmNPV bacmid using the BmNPV bac-to-bac expression system. Two viruses generated from the modified bacmid and unmodified BmNPV bacmid were then used to co-infect BmN cells in order that recombination takes place at the ie1 locus between them. Finally, without multiple rounds of purification, total cellular DNA was isolated, transformed into Cacl₂-treated competent DH10B cells, and then blue colonies were selected for PCR screening. Remarkably, the proportion of blue colonies containing ie1-disrupted bacmid was found to be around 7 %. Moreover, using primers flanking the homology arms further confirmed that all these positive recombinants were double crossovers. These findings indicate that our method is also capable of gene modification if inverse PCR or seamless cloning is used to construct the donor plasmid and sequencing is employed to select positive colonies.

Advances in the Arms Race Between Silkworm and Baculovirus

Insects are the largest group of animals. Nearly all organisms, including insects, have viral pathogens. An important domesticated economic insect is the silkworm moth Bombyx mori. B. mori nucleopolyhedrovirus (BmNPV) is a typical baculovirus and a primary silkworm pathogen. It causes major economic losses in sericulture. Baculoviruses are used in biological pest control and as a bioreactor. Silkworm and baculovirus comprise a well-established model of insect–virus interactions. Several recent studies have focused on this model and provided novel insights into viral infections and host defense. Here, we focus on baculovirus invasion, silkworm immune response, baculovirus evasion of host immunity, and enhancement of antiviral efficacy. We also discuss major issues remaining and future directions of research on silkworm antiviral immunity. Elucidation of the interaction between silkworm and baculovirus furnishes a theoretical basis for targeted pest control, enhanced pathogen resistance in economically important insects, and bioreactor improvement.

Baculovirus ODV-E66 degrades larval peritrophic membrane to facilitate baculovirus oral infection

ODV-E66 is a major envelope proteins of baculovirus occlusion derived virus (ODV) with chondroitinase activity. Here, we studied the roles of ODV-E66 during Helicoverpa armigera nucleopolyhedrovirus (HearNPV) primary infection. ODV-E66 is a late viral protein dispensable for BV production and ODV morphogenesis. Deletion of odv-e66 had a profound effect on HearNPV oral infectivity in 4th instar larvae with a 50% lethal concentration (LC₅₀) value of 26 fold higher than that of the repaired virus, compared to in 3rd instar larvae. Calcofluor white, an agent which destroys the peritrophic membrane (PM), could rescue the oral infectivity of odv-e66 deleted HearNPV, implying the PM may be the target of ODV-E66. In vitro assays showed HearNPV ODV-E66 has chondroitinase activity. Electron microscopy demonstrated that odv-e66 deletion alleviated the damage to the PM caused by HearNPV infection. These data suggest an important role of ODV-E66 in the penetration of the PM during oral infection.

Disruption of Autographa Californica Multiple Nucleopolyhedrovirus ac111 Results in Reduced per os Infectivity in a Host-Dependent Manner

The Autographa californica multiple nucleopolyhedrovirus (AcMNPV) ac111 gene is highly conserved in lepidopteran-specific baculoviruses, and its function in the AcMNPV life cycle is still unknown. To investigate the function of ac111, an ac111-knockout AcMNPV (vAc111KO) was constructed through homologous recombination in Escherichia coli. Viral growth curve analysis and plaque assays showed that the deletion of ac111 had no effect on infectious budded virion production. Quantitative real-time polymerase chain reaction analysis confirmed that viral DNA replication was unaffected in the absence of ac111. Electron microscopy revealed that the ac111 deletion did not affect nucleocapsid assembly, occlusion-derived virion formation, or the embedding of occlusion-derived virions into the occlusion bodies. However, in vivo bioassays showed that although the deletion of ac111 did not affect the per os infectivity of AcMNPV in Spodoptera exigua larvae, it led to an approximately five-fold reduction in infectivity of AcMNPV in Trichoplusia ni larvae, and vAc111KO took approximately 21 h longer to kill Trichoplusia ni larvae than the wild-type viruses. Taken together, our results demonstrated that although ac111 is not essential for virus replication in vitro, it plays an important role in the per os infectivity of AcMNPV in a host-dependent manner.

Bombyx mori nucleopolyhedrovirus orf133 and orf134 are involved in the embedding of occlusion-derived viruses into polyhedra

Bombyx mori nucleopolyhedrovirus (BmNPV) orf133 (bm133) and orf134 (bm134), the orthologues of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) ac4 and ac5, are two adjacent genes with opposite transcriptional orientations and are highly conserved in all sequenced group I nucleopolyhedroviruses (NPVs). A double bm133-bm134 knockout bacmid was generated to enable the functional study of each gene independently or together. Compared with wild-type and double-repair viruses, deletion of both bm133 and bm134 did not affect budded virus (BV) production or viral DNA replication in transfected BmN cells. Electron microscopy revealed that the double knockout did not affect nucleocapsid assembly, virus-induced intranuclear microvesicle formation or occlusion-derived virus (ODV) production, but the number of virions embedded in the polyhedra decreased significantly. Further investigations showed that disruption of either gene was unable to recover the defect of ODV occlusion, suggesting that Bm133 and Bm134 are indispensable to the embedding of ODVs into polyhedra. Confocal microscopy analysis showed that Bm133 and Bm134 distributed throughout the whole cell during viral infection and Bm134 concentrated on the mature polyhedra in lysed cells. These results suggest that although Bm133 and Bm134 are not essential for BV or ODV development, they play vital roles in polyhedra morphogenesis.

Discovery of anti-viral molecules and their vital functions in Bombyx mori

The silkworm Bombyx mori (B. mori), a lepidopteran model organism, has become an important model for molecular biology researches with its genome completely sequenced. Silkworms confront different types of virus diseases, mainly including those caused by Bombyx mori nucleopolyhedrovirus (BmNPV), Bombyx mori densovirus type 1 (BmDNV-1), Bombyx mori bidesovirus (BmBDV) which was termed as Bombyx mori densovirus type 2 (BmDNV-2) or Bombyx mori parvo-like virus (BmPLV) before in sericulture. B. mori offers excellent models to study the molecular mechanisms of insect innate immune responses to viruses. A variety of molecules and pathways have been identified to be involved in the immune responses in the silkworm to viruses, such as the antimicrobial peptides, prophenoloxidase-activating system, apoptosis, ROS, small RNA and related molecules. Here in this review, we summarize the current research advances in molecules involved in silkworm anti-virus pathways. Moreover, taking BmNPV as an example, we proposed a schematic model of molecules and pathways involved in silkworm immune responses against virus infection. We hope this review can facilitate further study of antiviral mechanisms in silkworm, and provide a reference for virus diseases in other organisms.

Protein composition analysis of polyhedra matrix of Bombyx mori nucleopolyhedrovirus (BmNPV) showed powerful capacity of polyhedra to encapsulate foreign proteins

Polyhedra can encapsulate other proteins and have potential applications as protein stabilizers. The extremely stable polyhedra matrix may provide a platform for future engineered micro-crystal devices. However, the protein composition of the polyhedra matrix remains largely unknown. In this study, the occlusion-derived virus (ODV)-removed BmNPV polyhedra matrix fraction was subjected to SDS-PAGE and then an LC-ESI-MS/MS analysis using a Thermo Scientific Q Exactive mass spectrometer. In total, 28 host and 91 viral proteins were identified. The host components were grouped into one of six categories, i.e., chaperones, ubiquitin and related proteins, host helicases, cytoskeleton-related proteins, RNA-binding proteins and others, according to their predicted Pfam domain(s). Most viral proteins may not be essential for polyhedra assembly, as evidenced by studies in the literature showing that polyhedra formation occurs in the nucleus upon the disruption of individual genes. The structural role of these proteins in baculovirus replication will be of significant interest in future studies. The immobilization of enhanced green fluorescent protein (eGFP) into the polyhedra by fusing with the C-terminus of BM134 that is encoded by open reading frame (ORF) 134 suggested that the polyhedra had a powerful capacity to trap foreign proteins, and BM134 was a potential carrier for incorporating proteins of interest into the polyhedra.

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.

Introduction to Baculovirus Molecular Biology

The development of baculovirus expression vector systems has accompanied a rapid expansion of our knowledge about the genes, their function and regulation in insect cells. Classification of these viruses has also been refined as we learn more about differences in gene content between isolates, how this affects virus structure and their replication in insect larvae. Baculovirus gene expression occurs in an ordered cascade, regulated by early, late and very late gene promoters. There is now a detailed knowledge of these promoter elements and how they interact first with host cell-encoded RNA polymerases and later with virus-encoded enzymes. The composition of this virus RNA polymerase is known. The virus replication process culminates in the very high level expression of both polyhedrin and p10 gene products in the latter stages of infection. It has also been realized that the insect host cell has innate defenses against baculoviruses in the form of an apoptotic response to virus invasion. Baculoviruses counter this by encoding apoptotic-suppressors, which also appear to have a role in determining the host range of the virus. Also of importance to our understanding of baculovirus expression systems is how the virus can accumulate mutations within genes that affect recombinant protein yield in cell culture. The summary in this chapter is not exhaustive, but should provide a good preparation to those wishing to use this highly successful gene expression system.

Analysis of the genome of a Korean isolate of the Pieris rapae granulovirus enabled by its separation from total host genomic DNA by pulse-field electrophoresis

BACKGROUND

Most traditional genome sequencing projects involving viruses include the culture and purification of the virus particles. However, purification of virions may yield insufficient material for traditional sequencing. The electrophoretic method described here provides a strategy whereby the genomic DNA of the Korean isolate of Pieris rapae granulovirus (PiraGV-K) could be recovered in sufficient amounts for sequencing by purifying it directly from total host DNA by pulse-field gel electrophoresis (PFGE).

METHODOLOGY/PRINCIPAL FINDINGS

The total genomic DNA of infected P. rapae was embedded in agarose plugs, treated with restriction nuclease and methylase, and then PFGE was used to separate PiraGV-K DNA from the DNA of P. rapae, followed by mapping of fosmid clones of the purified viral DNA. The double-stranded circular genome of PiraGV-K was found to encode 120 open reading frames (ORFs), which covered 92% of the sequence. BLAST and ORF arrangement showed the presence of 78 homologs to other genes in the database. The mean overall amino acid identity of PiraGV-K ORFs was highest with the Chinese isolate of PiraGV (~99%), followed up with Choristoneura occidentalis ORFs at 58%. PiraGV-K ORFs were grouped, according to function, into 10 genes involved in transcription, 11 involved in replication, 25 structural protein genes, and 15 auxiliary genes. Genes for Chitinase (ORF 10) and cathepsin (ORF 11), involved in the liquefaction of the host, were found in the genome.

CONCLUSIONS/SIGNIFICANCE

The recovery of PiraGV-K DNA genome by pulse-field electrophoretic separation from host genomic DNA had several advantages, compared with its isolation from particles harvested as virions or inclusions from the P. rapae host. We have sequenced and analyzed the 108,658 bp PiraGV-K genome purified by the electrophoretic method. The method appears to be generally applicable to the analysis of genomes of large viruses.

Identification of the epitopes of monoclonal antibodies against P74 of Helicoverpa armigera nucleopolyhedrovirus

P74 is a per os infectivity factor of baculovirus. Here, we report the production of three monoclonal antibodies (mAbs), denoted as 20D9, 20F9 and 21E1, raised against P74 of Helicoverpa armigera nucleopolyhedrovirus (HearNPV), and the identification of their recognition epitopes. The full-length P74, without the transmembrane domains at the C-terminus, was first divided into three segments (N, M and C, respectively), based on the proposed cleavage model for the protein, which were then expressed individually. Western blot analyses revealed specific cross-reactions with the N fragment, for both 20D9 and 21E1. Extensive truncation, followed by prokaryotic expression, of the P74 N fragment was then performed in order to screen for linear epitopes of P74. The recognition regions of 20D9 and 21E1 were revealed to be localized at R144-T153 and T199-C219, respectively. In addition, immunofluorescence microscopy indicated that 20D9 and 20F9 could recognize native P74 in HearNPV-infected cells. These findings will facilitate further investigations of the proteolytic processing of HearNPV P74, and of its involvement in virus-host interactions.

Bombyx mori nucleopolyhedrovirus BmP95 plays an essential role in budded virus production and nucleocapsid assembly

Bombyx mori nucleopolyhedrovirus (BmNPV) BmP95 is a highly conserved gene that is found in all of the baculovirus genomes sequenced to date and is also found in nudiviruses. To investigate the role of BmP95 in virus infection in vitro, a BmP95 deletion virus (vBmP95-De) was generated by homologous recombination in Escherichia coli. Fluorescence and light microscopy and titration analysis indicated that the BmP95 deletion bacmid led to a defect in production of infectious budded virus (BV). However, deletion of BmP95 did not affect viral DNA replication. Electron microscopy showed that masses of aberrant tubular structures were present in cells transfected with the BmP95 deletion bacmid, indicating that deletion of BmP95 affected assembly of the nucleocapsid. This defect could be rescued by insertion of full-length BmP95 into the polyhedrin locus of the BmP95-knockout bacmid but not the N-terminal domain of BmP95. Together, these results showed that full-length BmP95 is essential for BV production and is required for nucleocapsid assembly.

Characterization of Bombyx mori nucleopolyhedrovirus with a knockout of Bm17

Open reading frame 17 (Bm17) gene of Bombyx mori nucleopolyhedrovirus is a highly conserved gene in lepidopteran nucleopolyhedroviruses, but its function remains unknown. In this report, transient-expression and superinfection assays indicated that BM17 localized in the nucleus and cytoplasm of infected BmN cells. To determine the role of Bm17 in baculovirus life cycle, we constructed a Bm17 knockout virus and characterized its properties in cells. Analysis of the production and infection of budded virions, the level of viral DNA replication revealed showed that there was no significant difference among the mutant, the control, and the Bm17 repaired virus strains. These results suggest that BM17 is not essential for virus replication in cultured cells.

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.

Analysis of the autographa californica multiple nucleopolyhedrovirus overlapping gene pair lef3 and ac68 reveals that AC68 is a per os infectivity factor and that LEF3 is critical, but not essential, for virus replication

Autographa californica multiple nucleopolyhedrovirus ac68 is a core gene that overlaps lef3 which encodes the single-stranded DNA binding protein. A knockout (KO) virus lacking both lef3 and ac68 was generated (lef3-ac68 2×KO) to enable the functional study of ac68. To produce an ac68KO virus that did not impact lef3 expression, the lef3-ac68 2×KO virus was repaired with a DNA fragment containing lef3 and ac68, in which ac68 contained point mutations so that only LEF3 was expressed. Repair of lef3-ac68 2×KO with just ac68 generated an lef3KO virus. Analysis of the ac68KO virus showed that viral DNA replication and budded virus (BV) levels were unaffected compared to levels in the double-repair or wild-type (WT) control virus. Bioassay analyses of Trichoplusia ni larvae injected with BV directly into the hemolymph, bypassing the gut, showed no difference in mortality rates between the ac68KO and the WT viruses. However, in oral bioassays the ac68KO occlusion bodies failed to kill larvae. These results show that the core gene ac68 encodes a per os infectivity factor (pif6). The lef3KO virus was also analyzed, and virus replication was drastically reduced compared to WT virus, but very low levels of lef3KO virus DNA replication and BV production could be detected. In addition, in transfected cells P143 was transported to the nucleus in the absence of LEF3. This study therefore shows for the first time that even though the loss of LEF3 severely impairs virus replication, it is not absolutely essential for P143 nuclear import or viral replication.

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.

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.