Trypsin cleavage of the baculovirus occlusion-derived virus attachment protein P74 is prerequisite in per os infection

AcMNPV P74 is cleaved at R325 and R334 by proteinases of both OB and BBMV to expose a potential fusion peptide for oral infection

Baculoviruses enter insect midgut epithelial cells via a set of occlusion-derived virion (ODV) envelope proteins called per os infectivity factors (PIFs). P74 of Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV), which was the first identified PIF, is cleaved by an endogenous proteinase embedded within the occlusion body during per os infection, but the target site(s) and function of the cleavage have not yet been ascertained. Here, based on bioinformatics analyses, we report that cleavage was predicted at an arginine and lysine-rich region in the middle of P74. A series of recombinant viruses with site-directed mutants in this region of P74 were generated. R325 or R334 was identified as primary cleavage site. In addition, we showed that P74 is also cleaved by brush border membrane vesicles (BBMV) of the host insect at R325 or R334, instead of R195, R196, and R199, as previously reported. Simultaneous mutations in R195, R196, and R199 lead to instability of P74 during ODV release. Bioassays showed that mutations at both R325 and R334 significantly affected oral infectivity. Taken together, our data show that both R325 and R334 of AcMNPV P74 are the primary cleavage site for both occlusion body endogenous proteinase and BBMV proteinase during ODV release and are critical for oral infection.

IMPORTANCE

Cleavage of viral envelope proteins is usually an important trigger for viral entry into host cells. Baculoviruses are insect-specific viruses that infect host insects via the oral route. P74, a per os infectivity factor of baculoviruses, is cleaved during viral entry. However, the function and precise cleavage sites of P74 remain unknown. In this study, we found that R325 or R334 between the N- and C-conserved domains of P74 was the primary cleavage site by proteinase either from the occlusion body or host midgut. The biological significance of cleavage seems to be the release of the potential fusion peptide at the N-terminus of the cleaved C-terminal P74. Our results shed light on the cleavage model of P74 and imply its role in membrane fusion in baculovirus per os infection.

The Membrane-Anchoring Region of the AcMNPV P74 Protein Is Expendable or Interchangeable with Homologs from Other Species

Baculoviruses are insect pathogens that are characterized by assembling the viral dsDNA into two different enveloped virions during an infective cycle: occluded virions (ODVs; immersed in a protein matrix known as occlusion body) and budded virions (BVs). ODVs are responsible for the primary infection in midgut cells of susceptible larvae thanks to the per os infectivity factor (PIF) complex, composed of at least nine essential viral proteins. Among them, P74 is a crucial factor whose activity has been identified as virus-specific. In this work, the p74 gene from AcMNPV was pseudogenized using CRISPR/Cas9 technology and then complemented with wild-type alleles from SeMNPV and HearSNPV species, as well as chimeras combining the P74 amino and carboxyl domains. The results on Spodoptera exigua and Rachiplusia nu larvae showed that an amino terminal sector of P74 (lacking two potential transmembrane regions but possessing a putative nuclear export signal) is sufficient to restore the virus infectivity whether alone or fused to the P74 transmembrane regions of the other evaluated viral species. These results provide novel information about the functional role of P74 and delimit the region on which mutagenesis could be applied to enhance viral activity and, thus, produce better biopesticides.

Cross-talking between baculoviruses and host insects towards a successful infection

Baculoviridae is a family of large DNA viruses that infect insects. They have been extensively used as safe and efficient biological agents for the control of insect pests. As a result of coevolution with their hosts, baculoviruses developed unique life cycles characterized by the production of two distinctive virion phenotypes, occlusion-derived virus and budded virus, which are responsible for mediating primary infection in insect midgut epithelia and spreading systemic infection within infected insects, respectively. In this article, advances associated with virus-host interactions during the baculovirus life cycle are reviewed. We mainly focus on how baculoviruses exploit versatile strategies to overcome diverse host barriers and establish successful infections. For example, in the midgut, baculoviruses encode enzymes to degrade peritrophic membranes and use a series of per os infectivity factors to initiate primary infection. A viral fibroblast growth factor is expressed to attract tracheoblasts that spread the virus for systemic infection. Baculoviruses use different strategies to suppress host defence systems, including apoptosis, melanization and RNA interference. Additionally, baculoviruses can manipulate host physiology and induce 'tree-top disease' for optimal virus replication and dispersal. These advances in our understanding of baculoviruses will greatly inform the development of more effective baculoviral pesticides. This article is part of the theme issue 'Biotic signalling sheds light on smart pest management'.

Expression Profile of Glossina pallidipes MicroRNAs During Symptomatic and Asymptomatic Infection With Glossina pallidipes Salivary Gland Hypertrophy Virus (Hytrosavirus)

The Glossina pallidipes salivary gland hypertrophy virus (GpSGHV) infects tsetse flies predominantly asymptomatically and occasionally symptomatically. Symptomatic infections are characterized by overt salivary gland hypertrophy (SGH) in mass reared tsetse flies, which causes reproductive dysfunctions and colony collapse, thus hindering tsetse control via sterile insect technique (SIT). Asymptomatic infections have no apparent cost to the fly's fitness. Here, small RNAs were sequenced and profiles in asymptomatically and symptomatically infected G. pallidipes flies determined. Thirty-eight host-encoded microRNAs (miRNAs) were present in both the asymptomatic and symptomatic fly profiles, while nine host miRNAs were expressed specifically in asymptomatic flies versus 10 in symptomatic flies. Of the shared 38 miRNAs, 15 were differentially expressed when comparing asymptomatic with symptomatic flies. The most up-regulated host miRNAs in symptomatic flies was predicted to target immune-related mRNAs of the host. Six GpSGHV-encoded miRNAs were identified, of which five of them were only in symptomatic flies. These virus-encoded miRNAs may not only target host immune genes but may also participate in viral immune evasion. This evidence of differential host miRNA profile in Glossina in symptomatic flies advances our understanding of the GpSGHV-Glossina interactions and provides potential new avenues, for instance by utilization of particular miRNA inhibitors or mimics to better manage GpSGHV infections in tsetse mass-rearing facilities, a prerequisite for successful SIT implementation.

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.

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.

Autographa californica Multiple Nucleopolyhedrovirus AC83 is a Per Os Infectivity Factor (PIF) Protein Required for Occlusion-Derived Virus (ODV) and Budded Virus Nucleocapsid Assembly as well as Assembly of the PIF Complex in ODV Envelopes

Baculovirus occlusion-derived virus (ODV) initiates infection of lepidopteran larval hosts by binding to the midgut epithelia, which is mediated by per os infectivity factors (PIFs). Autographa californica multiple nucleopolyhedrovirus (AcMNPV) encodes seven PIF proteins, of which PIF1 to PIF4 form a core complex in ODV envelopes to which PIF0 and PIF6 loosely associate. Deletion of any pif gene results in ODV being unable to bind or enter midgut cells. AC83 also associates with the PIF complex, and this study further analyzed its role in oral infectivity to determine if it is a PIF protein. It had been proposed that AC83 possesses a chitin binding domain that enables transit through the peritrophic matrix; however, no chitin binding activity has ever been demonstrated. AC83 has been reported to be found only in the ODV envelopes, but in contrast, the Orgyia pseudotsugata MNPV AC83 homolog is associated with both ODV nucleocapsids and envelopes. In addition, unlike known pif genes, deletion of ac83 eliminates nucleocapsid formation. We propose a new model for AC83 function and show AC83 is associated with both ODV nucleocapsids and envelopes. We also further define the domain required for nucleocapsid assembly. The cysteine-rich region of AC83 is also shown not to be a chitin binding domain but a zinc finger domain required for the recruitment or assembly of the PIF complex to ODV envelopes. As such, AC83 has all the properties of a PIF protein and should be considered PIF8. In addition, pif7 (ac110) is reported as the 38th baculovirus core gene.IMPORTANCE ODV is essential for the per os infectivity of the baculovirus AcMNPV. To initiate infection, ODV binds to microvilli of lepidopteran midgut cells, a process which requires a group of seven virion envelope proteins called PIFs. In this study, we reexamined the function of AC83, a protein that copurifies with the ODV PIFs, to determine its role in the oral infection process. A zinc finger domain was identified and a new model for AC83 function was proposed. In contrast to previous studies, AC83 was found to be physically located in both the envelope and nucleocapsid of ODV. By deletion analysis, the AC83 domain required for nucleocapsid assembly was more finely delineated. We show that AC83 is required for PIF complex formation and conclude that it is a true per os infectivity factor and should be called PIF8.

Reaching the melting point: Degradative enzymes and protease inhibitors involved in baculovirus infection and dissemination

Baculovirus infection of a host insect involves several steps, beginning with initiation of virus infection in the midgut, followed by dissemination of infection from the midgut to other tissues in the insect, and finally culminating in "melting" or liquefaction of the host, which allows for horizontal spread of infection to other insects. While all of the viral gene products are involved in ultimately reaching this dramatic infection endpoint, this review focuses on two particular types of baculovirus-encoded proteins: degradative enzymes and protease inhibitors. Neither of these types of proteins is commonly found in other virus families, but they both play important roles in baculovirus infection. The types of degradative enzymes and protease inhibitors encoded by baculoviruses are discussed, as are the roles of these proteins in the infection process.

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.

In vitro antiviral activity of an alkaline trypsin from the digestive juice of Bombyx mori larvae against nucleopolyhedrovirus

Alkaline trypsin protein of molecular mass 25,436 Da purified from the digestive juice of Bombyx mori larvae indicated strong antiviral activity against Bombyx mori nucleopolyhedrovirus (BmNPV) under in vitro conditions. Partial N-terminal amino acid sequence of the protein was determined and the cDNA was cloned based on the amino acid sequence. A homology search of the deduced amino acid sequence of the cDNA showed 55% identity with Helicoverpa armigera trypsin and the active site of this protein was completely conserved. Hence, the protein was designated B. mori trypsin (Bmtryp). The results suggest that Bmtryp, an insect digestive enzyme, can be a potential antiviral factor against BmNPV at the initial site of viral infection.

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.

Sequence comparison between three geographically distinct Spodoptera frugiperda multiple nucleopolyhedrovirus isolates: Detecting positively selected genes

The complete genomic sequence of a Nicaraguan plaque purified Spodoptera frugiperda nucleopolyhedrovirus (SfMNPV) genotype SfMNPV-B was determined and compared to previously sequenced isolates from United States (SfMNPV-3AP2) and Brazil (SfMNPV-19). The genome of SfMNPV-B (132,954bp) was 1623bp and 389bp larger than that of SfMNPV-3AP2 and SfMNPV-19, respectively. Genome size differences were mainly due to a deletion located in the SfMNPV-3AP2 egt region and small deletions and point mutations in SfMNPV-19. Nucleotide sequences were strongly conserved (99.35% identity) and a high degree of predicted amino acid sequence identity was observed. A total of 145 open reading frames (ORFs) were identified in SfMNPV-B, two of them (sf39a and sf110a) had not been previously identified in the SfMNPV-3AP2 and SfMNPV-19 genomes and one (sf57a) was absent in both these genomes. In addition, sf6 was not previously identified in the SfMNPV-19 genome. In contrast, SfMNPV-B and SfMNPV-19 both lacked sf129 that had been reported in SfMNPV-3AP2. In an effort to identify genes potentially involved in virulence or in determining population adaptations, selection pressure analysis was performed. Three ORFs were identified undergoing positive selection: sf49 (pif-3), sf57 (odv-e66b) and sf122 (unknown function). Strong selection for ODV envelope protein genes indicates that the initial infection process in the insect midgut is one critical point at which adaptation acts during the transmission of these viruses in geographically distant populations. The function of ORF sf122 is being examined.

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.

A soluble form of P74 can act as a per os infectivity factor to the Autographa californica multiple nucleopolyhedrovirus

The baculovirus occlusion-derived virion (ODV) is required to spread virus infection among insect hosts via the per os route. The Autographa californica multicapsid nucleopolyhedrovirus P74 protein is an ODV envelope protein that is essential for ODVs to be infectious. P74 is anchored in the ODV envelope by a C-terminal transmembrane anchor domain and is N-terminally exposed on the ODV surface. In the present study, a series of N-terminal and C-terminal truncation mutants of P74 were evaluated for their ability to rescue per os infectivity of the P74-null virus, AcLP4. It was discovered that a P74 truncation mutant lacking the C-terminal transmembrane anchor domain of P74 was able to rescue per os infection. This result shows that a soluble form of P74 retains per os infectivity factor function and suggests that P74 may be complexed with other proteins in the ODV envelope.

Changes in gene expression in the permissive larval host lightbrown apple moth (Epiphyas postvittana, Tortricidae) in response to EppoNPV (Baculoviridae) infection

Host cell and virus gene expression were measured five days after per os inoculation of 3rd instar lightbrown apple moth (LBAM) larvae with the Epiphyas postvittana nucleopolyhedrovirus (EppoNPV). Microarray analysis identified 84 insect genes that were up-regulated and 18 genes that were down-regulated in virus-infected larvae compared with uninfected larvae. From the 134 viral open reading frames represented on the microarray, 81 genes showed strong expression. Of the 38 functionally identifiable regulated insect genes, 23 coded for proteins that have roles in one of five processes; regulation of transcription and translation, induction of apoptosis, and maintenance of both juvenility and actin cytoskeletal integrity. Of the 34 functionally identifiable viral genes that were most strongly expressed, 12 had functions associated with these five processes, as did a further seven viral genes which were expressed at slightly lower levels. A survey of the LBAM-expressed sequence tag library identified further genes involved in these processes. In total, 135 insect genes and 38 viral genes were analysed by quantitative polymerase chain reaction. Twenty-one insect genes were strongly up-regulated and 31 genes strongly down-regulated. All 38 viral genes examined were highly expressed. These data suggest that induction of apoptosis and regulation of juvenility are the major 'battlegrounds' between virus and insect, with the majority of changes observed representing viral control of insect gene expression. Transcription and translational effects seem to be exerted largely through modulation of mRNA and protein degradation. Examples of attempts by the insect to repel the infection via changes in gene expression within these same processes were, however, also noted. The data also showed the extent to which viral transcription dominated in the infected insects at five days post inoculation.

Mixtures of complete and pif1- and pif2-deficient genotypes are required for increased potency of an insect nucleopolyhedrovirus

The insecticidal potency of a nucleopolyhedrovirus population (SfNIC) that infects Spodoptera frugiperda (Lepidoptera) is greater than the potency of any of the component genotypes alone. Occlusion bodies (OBs) produced in mixed infections comprising the complete genotype and a deletion genotype are as pathogenic as the natural population of genotypes from the field. To test whether this increased potency was due to the deletion or to some other characteristic of the deletion variant genome, we used the SfNIC-B genome to construct a recombinant virus (SfNIC-B Delta 16K) with the same 16.4-kb deletion as that observed in SfNIC-C and another recombinant (SfNIC-B Delta pifs) with a deletion encompassing two adjacent genes (pif1 and pif2) that are essential for transmission per os. Mixtures comprising SfNIC-B and SfNIC-B Delta 16K in OB ratios that varied between 10:90 and 90:10 were injected into insects, and the progeny OBs were fed to larvae in an insecticidal potency assay. A densitometric analysis of PCR products indicated that SfNIC-B was generally more abundant than expected in mixtures based on the proportions of OBs used to produce the inocula. Mixtures derived from OB ratios of 10, 25, or 50% of SfNIC-B Delta 16K and the corresponding SfNIC-B proportions showed a significant increase in potency compared to SfNIC-B alone. The results of potency assays with mixtures comprising various proportions of SfNIC-B plus SfNIC-B Delta pifs were almost identical to the results observed with SfNIC-B Delta 16K, indicating that deletion of the pif gene region was responsible for the increased potency observed in mixtures of SfNIC-B and each deletion recombinant virus. Subsequently, mixtures produced from OB ratios involving 10 or 90% of SfNIC-B Delta 16K with the corresponding proportions of SfNIC-B were subjected to four rounds of per os transmission in larvae. The composition of each experimental mixture rapidly converged to a common equilibrium with a genotypic composition of approximately 85% SfNIC-B plus approximately 15% SfNIC-B Delta 16K. Nearly identical results were observed in peroral-passage experiments involving mixtures of SfNIC-B plus SfNIC-B Delta pifs. We conclude that (i) the deletion of the pif1 and pif2 region is necessary and sufficient to explain the increased potency observed in mixtures of complete and deletion genotypes and (ii) viral populations with decreased ratios of pif1- and pif2-deficient genotypes in the virus population increase the potency of genotypic mixtures and are likely to positively influence the transmission of this pathogen.