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

Developments in the classification and nomenclature of arthropod-infecting large DNA viruses that contain pif genes

Viruses of four families of arthropod-specific, large dsDNA viruses (the nuclear arthropod large DNA viruses, or NALDVs) possess homologs of genes encoding conserved components involved in the baculovirus primary infection mechanism. The presence of such homologs encoding per os infectivity factors (pif genes), along with their absence from other viruses and the occurrence of other shared characteristics, suggests a common origin for the viruses of these families. Therefore, the class Naldaviricetes was recently established, accommodating these four families. In addition, within this class, the ICTV approved the creation of the order Lefavirales for three of these families, whose members carry homologs of the baculovirus genes that code for components of the viral RNA polymerase, which is responsible for late gene expression. We further established a system for the binomial naming of all virus species in the order Lefavirales, in accordance with a decision by the ICTV in 2019 to move towards a standardized nomenclature for all virus species. The binomial species names for members of the order Lefavirales consist of the name of the genus to which the species belongs (e.g., Alphabaculovirus), followed by a single epithet that refers to the host species from which the virus was originally isolated. The common names of viruses and the abbreviations thereof will not change, as the format of virus names lies outside the remit of the ICTV.

Identification and characterization of BmNPV Bm5 protein required for the formation of nuclear vesicle structures

BmNPV infection induces nuclear vesicle-like structures and its Bm5 protein mediates the intranuclear lipid accumulation, which is thought to participate in the formation of nuclear vesicles. However, the relationship between viral-induced nuclear vesicles and Bm5 protein is still unclear. Here, our results indicated that BmNPV Bm5 protein participated in the baculovirus infection-induced nuclear vesicle-like structures' invagination thereby influencing the production of occlusion-derived virion (ODV) and occlusion body (OB). The process of nuclear vesicle-like structures' formation was dispensable for the transport or recruitment of ODV major envelope proteins, such as P74 and Bm14. Furthermore, baculovirus-induced nuclear F-actin might provide a direct mechanical force to mediate the scission of large vesicle-like structures from the nuclear membrane. Collectively, these findings illustrated a BmNPV Bm5 protein-induced nuclear membrane invagination pathway and revealed the function of nuclear vesicle-like structures in ODV production.

Functional Peroral Infectivity Complex of White Spot Syndrome Virus of Shrimp

White spot syndrome virus (WSSV) is a major cause of disease in shrimp cultures worldwide. The infection process of this large circular double-stranded DNA virus has been well studied, but its entry mechanism remains controversial. The major virion envelope protein VP28 has been implicated in oral and systemic viral infection in shrimp. However, genetic analysis of viral DNA has shown the presence of a few genes related to proteins of per os infectivity factor (PIF) complex in baculoviruses. This complex is essential for the entry of baculoviruses, large terrestrial circular DNA viruses, into the midgut epithelial cells of insect larvae. In this study, we aimed to determine whether a PIF complex exists in WSSV, the components of this complex, whether it functions as an oral infectivity complex in shrimp, and the biochemical properties that contribute to its function in a marine environment. The results revealed a WSSV PIF complex (~720 kDa) comprising at least eight proteins, four of which were not identified as PIF homologs: WSV134, VP124 (WSV216), WSSV021, and WSV136. WSV134 is suggested to be a PIF4 homolog due to predicted structural similarity and amino acid sequence identity. The WSSV PIF complex is resistant to alkali, proteolysis, and high salt, properties that are important for maintaining infectivity in aquatic environments. Oral infection can be neutralized by PIF-specific antibodies but not by VP28-specific antibodies. These results indicate that the WSSV PIF complex is critical for WSSV entry into shrimp; the complex's evolutionary significance is also discussed. IMPORTANCE White spot disease, caused by the white spot syndrome virus (WSSV), is a major scourge in cultured shrimp production facilities worldwide. This disease is only effectively controlled by sanitation. Intervention strategies are urgently needed but are limited by a lack of appropriate targets. Our identification of a per os infectivity factor (PIF) complex, which is pivotal for the entry of WSSV into shrimp, could provide new targets for antibody- or dsRNA-based intervention strategies. In addition, the presence of a PIF complex with at least eight components in WSSV, which is ancestrally related to the PIF complex of invertebrate baculoviruses, suggests that this complex is structurally and functionally conserved in disparate virus taxa.

Structural Characterization of Per Os Infectivity Factor 5 (PIF5) Reveals the Essential Role of Intramolecular Interactions in Baculoviral Oral Infectivity

Baculoviruses initiate oral infection in the highly alkaline midgut of insects via a group of envelope proteins called per os infectivity factors (PIFs). To date, no high-resolution structural information has been reported for any PIF. Here, we present the crystal structure of the PIF5 ectodomain (PIF5e) from Autographa californica multiple nucleopolyhedrovirus (AcMNPV) at a 2.2-Å resolution. It revealed an open cavity between the N-terminal E1 domain and the C-terminal E2 domain and a cysteine-rich region with three pairs of disulfide bonds in the E2 domain. Multiple conserved intramolecular interactions within PIF5 are essential for maintaining its tertiary structure. Two conserved arginines (Arg8 and Arg74) play critical roles in E1-E2 interactions, and mutagenesis analysis supported their crucial role in oral infection. Importantly, the reduction in the oral infectivity of the Arg8, Arg74, or cysteine mutant viruses was related to the proteolytic cleavage of PIF5 by the endogenous protease embedded in occlusion bodies during alkaline treatment. This suggested that the structural stability of PIF5 under physiological conditions in the insect midgut is critical for baculoviral oral infectivity. IMPORTANCEPer os infection mediated by PIFs is the highly complex mechanism by which baculoviruses initiate infection in insects. Previous studies revealed that multiple PIF proteins form a large PIF complex on the envelope of virions, while PIF5 functions independently of the PIF complex. Here, we report the crystal structure of AcMNPV PIF5e, which, to our knowledge, is the first atomic structure reported for a PIF protein. The structure revealed the precise locations of three previously proposed disulfide bonds and other conserved intramolecular interactions, which are important for the structural stability of PIF5 and are also essential for oral infectivity. These findings advance our understanding of the molecular mechanism of baculovirus oral infection under alkaline conditions.

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.

Systematic Analysis of 42 Autographa Californica Multiple Nucleopolyhedrovirus Genes Identifies An Additional Six Genes Involved in the Production of Infectious Budded Virus

Baculoviruses have been widely used as a vector for expressing foreign genes. Among numerous baculoviruses, Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is the most frequently used and it encodes 155 open reading frames (ORFs). Here, we systematically investigated the impact of 42 genes of AcMNPV on the production of infectious budded viruses (BVs) by constructing gene-knockout bacmids and subsequently conducting transfection and infection assays. The results showed that among the 39 functionally unverified genes and 3 recently reported genes, 36 are dispensable for infectious BV production, as the one-step growth curves of the gene-knockout viruses were not significantly different from those of the parental virus. Three genes (ac62, ac82 and ac106/107) are essential for infectious BV production, as deletions thereof resulted in complete loss of infectivity while the repaired viruses showed no significant difference in comparison to the parental virus. In addition, three genes (ac13, ac51 and ac120) are important but not essential for infectious BV production, as gene-knockout viruses produced significantly lower BV levels than that of the parental virus or repaired viruses. We then grouped the 155 AcMNPV genes into three categories (Dispensable, Essential, or Important for infectious BV production). Based on our results and previous publications, we constructed a schematic diagram of a potential mini-genome of AcMNPV, which contains only essential and important genes. The results shed light on our understanding of functional genomics of baculoviruses and provide fundamental information for future engineering of baculovirus expression system.

Differential insecticidal properties of Spodoptera frugiperda multiple nucleopolyhedrovirus isolates against corn-strain and rice-strain fall armyworm, and genomic analysis of three isolates

The fall armyworm, Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae) is a destructive crop pest native to North, Central, and South America that recently has spread to Africa and Asia. Isolates of Spodoptera frugiperda multiple nucleopolyhedrovirus (SfMNPV) have the potential to be developed as low-risk biopesticides for management of fall armyworm, and a commercially available formulation has been developed for control of fall armyworm in North and South America. In this study, the virulence (LC₅₀ and LT₅₀) of several SfMNPV isolates towards larvae of both corn-strain and rice-strain fall armyworm was assessed. Bioassays with corn-strain larvae revealed that the isolates could be organized into fast-killing (LT₅₀ < 56 h post-infection) and slow-killing (LT₅₀ > 68 h post-infection) groups. Rice-strain larvae exhibited narrower ranges of susceptibility to baculovirus infection and of survival times in bioassays with different isolates. Two SfMNPV isolates with rapid speeds of kill (SfMNPV-459 from Colombia and SfMNPV-1197 from Georgia, USA) along with an isolate that killed corn-strain at relatively low concentrations (SfMNPV-281 from Georgia) were selected for the complete determination of their genome sequences. The SfMNPV-1197 genome sequence shared high sequence identity with genomes of a Nicaraguan isolate, while SfMNPV-281 formed a separate clade with a USA and a Brazilian isolate in phylogenetic trees. The SfMNPV-459 sequence was more divergent with the lowest genome sequence identities in pairwise alignments with other sequenced SfMNPV genomes, and was not grouped reliably with either the 1197 clade or the 281 clade. SfMNPV-459 contained homologs of two ORFs that were unique to another Colombian isolate, but these isolates were not placed in the same clade in phylogenetic trees. This study identifies isolates with superior properties for control of fall armyworm and adds to our knowledge of the genetics of SfMNPV.

Critical Residues and Contacts within Domain IV of Autographa californica Multiple Nucleopolyhedrovirus GP64 Contribute to Its Refolding during Membrane Fusion

Autographa californica multiple nucleopolyhedrovirus (AcMNPV) GP64 is a class III viral fusion protein that mediates low-pH-triggered membrane fusion during virus entry. Although the structure of GP64 in a postfusion conformation has been solved, its prefusion structure and the mechanism of how the protein refolds to execute fusion are unknown. In its postfusion structure, GP64 is composed of five domains (domains I to V). Domain IV (amino acids [aa] 374 to 407) contains two loops (loop 1 and loop 2) that form a hydrophobic pocket at the membrane-distal end of the molecule. To determine the roles of domain IV, we used alanine-scanning mutagenesis to replace each of the individual residues and the contact-forming residues within domain IV and evaluate their contributions to GP64-mediated membrane fusion and virus infection. In many cases, replacement of a single amino acid had no significant impact on GP64. However, replacement of R392 or disruption of the N381-N385, N384-Y388, N385-W393, or K389-W393 contact resulted in poor cell surface expression and fusion loss of the modified GP64, whereas replacement of E390 or G391 or disruption of the N381-K389, N381-Q401, or N381-I403 contact reduced the cell surface expression level of the constructs and the ability of GP64 to mediate fusion pore expansion. In contrast, replacement of N407 or disruption of contact D404-S406 appeared to restrict fusion pore expansion without affecting expression. Combined with the finding that these constructs remain in the prefusion conformation or have a dramatically less efficient transition from the prefusion to the postfusion state under acidic conditions, we proposed that domain IV is necessary for refolding of GP64 during membrane fusion.IMPORTANCE Baculovirus GP64 is grouped with rhabdovirus G, herpesvirus gB, and thogotovirus glycoproteins as a class III viral fusion protein. In their postfusion structures, these proteins contain five domains (domains I to V). Distinct from domain IV of rhabdovirus G and herpesvirus gB proteins, which is composed of β-sheets, domain IV of GP64 is a loop region; the same domain in thogotovirus glycoproteins has not been solved. In addition, domain IV is proximal to domain I (fusion domain) in prefusion structures of vesicular stomatitis virus (VSV) G and human cytomegalovirus (HCMV) gB but resides at the domain I-distal end of the molecule in a postfusion conformation. In this study, we identified that highly conserved residues and contacts within domain IV of AcMNPV GP64 are necessary for low-pH-triggered conformational change and fusion pore expansion. Our results highlight the roles of domain IV of class III viral fusion proteins in refolding during membrane fusion.

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'.

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.

The cysteine-rich region of a baculovirus VP91 protein contributes to the morphogenesis of occlusion bodies

The baculovirus core gene vp91 has been reported to be essential for nucleocapsid assembly and oral infection. Here, we studied the function of vp91 by analyzing its homologue, ha76, in Helicoverpa armigera nucleopolyhedrovirus (HearNPV). HA76 was expressed at the late stage of HearNPV infection; deletion of ha76 showed that the gene is required for budded virus production. A series of recombinants with truncated ha76 was constructed and analyzed in vitro and in vivo. The results showed that the region encoding the C-terminus of HA76 was essential for nucleocapsid assembly, whereas the N-terminal cysteine-rich region was responsible for oral infection. Electron microscope analyses further showed that the cysteine-rich region contributed to morphogenesis of occlusion bodies (OBs), with amino acids 136-223 of HA76 being critical for this function. The results revealed a novel function of VP91 and suggested that the impact on OB morphogenesis is partially related to oral infectivity.

Common themes in three independently derived endogenous nudivirus elements in parasitoid wasps

Endogenous Viral Elements (EVEs) are remnants of viral genomes that are permanently integrated into the genome of another organism. Parasitoid wasps have independently acquired nudivirus-derived EVEs in three lineages. Each parasitoid produces virions or virus-like particles (VLPs) that are injected into hosts during parasitism to function in subversion of host defenses. Comparing the inventory of nudivirus-like genes in different lineages of parasitoids can provide insights into the importance of each encoded function in virus or VLP production and parasitism success. Comparisons revealed the following conserved features: first, retention of genes encoding a viral RNA polymerase and infectivity factors; second, loss of the ancestral DNA polymerase gene; and third, signatures of viral ancestry in patterns of gene retention.

Baculovirus Per Os Infectivity Factor Complex: Components and Assembly

Baculovirus entry into insect midgut cells is dependent on a multiprotein complex of per os infectivity factors (PIFs) on the envelopes of occlusion-derived virions (ODVs). The structure and assembly of the PIF complex are largely unknown. To reveal the complete members of the complex, a combination of blue native polyacrylamide gel electrophoresis, liquid chromatography-tandem mass spectrometry, and Western blotting was conducted on three different baculoviruses. The results showed that the PIF complex has a molecular mass of ∼500 kDa and consists of nine PIFs, including a newly discovered member (PIF9). To decipher the assembly process, each pif gene was knocked out from the Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) genome individually by use of synthetic baculovirus technology, and the impact on PIF complex formation was investigated. Deletion of pif8 resulted in the formation of an ∼400-kDa subcomplex. Deletion of pif0, -4, -6, -7, or -9 resulted in a subcomplex of ∼230 kDa, but deletion of pif1, -2, or -3 abolished formation of any complex. Taken together, our data identified a core complex of ∼230 kDa, consisting of PIF1, -2, and -3. This revised the previous knowledge that the core complex was about 170 kDa and contained PIF1 to -4. Analysis of the PIF complex in cellular fractions suggested that it is assembled in the cytoplasm before being transported to the nucleus and subsequently incorporated into the envelopes of ODVs. Only the full complex, not the subcomplex, is resistant to proteolytic attack, indicating the essentiality of correct complex assembly for oral infection.IMPORTANCE Entry of baculovirus into host insects is mediated by a per os infectivity factor (PIF) complex on the envelopes of occlusion-derived viruses (ODVs). Knowledge of the composition and structure of the PIF complex is fundamental to understanding its mode of action. By using multiple approaches, we determined the complete list of proteins (nine) in the PIF complex. In contrast to previous knowledge in the field, the core complex is revised to ∼230 kDa and consists of PIF1 to -3 but not PIF4. Interestingly, our results suggest that the PIF complex is formed in the cytoplasm prior to its transport to the nucleus and subsequent incorporation into ODVs. Only the full complex is resistant to proteolytic degradation in the insect midgut, implying the critical role of the entire complex. These findings provide the baseline for future studies on the ODV entry mechanism mediated by the multiprotein complex.

Crustacean Genome Exploration Reveals the Evolutionary Origin of White Spot Syndrome Virus

White spot syndrome virus (WSSV) is a crustacean-infecting, double-stranded DNA virus and is the most serious viral pathogen in the global shrimp industry. WSSV is the sole recognized member of the family Nimaviridae, and the lack of genomic data on other nimaviruses has obscured the evolutionary history of WSSV. Here, we investigated the evolutionary history of WSSV by characterizing WSSV relatives hidden in host genomic data. We surveyed 14 host crustacean genomes and identified five novel nimaviral genomes. Comparative genomic analysis of Nimaviridae identified 28 "core genes" that are ubiquitously conserved in Nimaviridae; unexpected conservation of 13 uncharacterized proteins highlighted yet-unknown essential functions underlying the nimavirus replication cycle. The ancestral Nimaviridae gene set contained five baculoviral per os infectivity factor homologs and a sulfhydryl oxidase homolog, suggesting a shared phylogenetic origin of Nimaviridae and insect-associated double-stranded DNA viruses. Moreover, we show that novel gene acquisition and subsequent amplification reinforced the unique accessory gene repertoire of WSSV. Expansion of unique envelope protein and nonstructural virulence-associated genes may have been the key genomic event that made WSSV such a deadly pathogen.IMPORTANCE WSSV is the deadliest viral pathogen threatening global shrimp aquaculture. The evolutionary history of WSSV has remained a mystery, because few WSSV relatives, or nimaviruses, had been reported. Our aim was to trace the history of WSSV using the genomes of novel nimaviruses hidden in host genome data. We demonstrate that WSSV emerged from a diverse family of crustacean-infecting large DNA viruses. By comparing the genomes of WSSV and its relatives, we show that WSSV possesses an expanded set of unique host-virus interaction-related genes. This extensive gene gain may have been the key genomic event that made WSSV such a deadly pathogen. Moreover, conservation of insect-infecting virus protein homologs suggests a common phylogenetic origin of crustacean-infecting Nimaviridae and other insect-infecting DNA viruses. Our work redefines the previously poorly characterized crustacean virus family and reveals the ancient genomic events that preordained the emergence of a devastating shrimp pathogen.

Global Analysis of Baculovirus Autographa californica Multiple Nucleopolyhedrovirus Gene Expression in the Midgut of the Lepidopteran Host Trichoplusia ni

The baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is a large double-stranded DNA (dsDNA) virus that encodes approximately 156 genes and is highly pathogenic to a variety of larval lepidopteran insects in nature. Oral infection of larval midgut cells is initiated by the occlusion-derived virus (ODV), while secondary infection of other tissues is mediated by the budded virus (BV). Global viral gene expression has been studied in detail in BV-infected cell cultures, but studies of ODV infection in the larval midgut are limited. In this study, we examined expression of the ∼156 AcMNPV genes in Trichoplusia ni midgut tissue using a transcriptomic approach. We analyzed expression profiles of viral genes in the midgut and compared them with profiles from a T. ni cell line (Tnms42). Several viral genes (p6.9, orf76, orf75, pp31, Ac-bro, odv-e25, and odv-ec27) had high expression levels in the midgut throughout the infection. Also, the expression of genes associated with occlusion bodies (polh and p10) appeared to be delayed in the midgut in comparison with the cell line. Comparisons of viral gene expression profiles revealed remarkable similarities between the midgut and cell line for most genes, although substantial differences were observed for some viral genes. These included genes associated with high level BV production (fp-25k), acceleration of systemic infection (v-fgf), and enhancement of viral movement (arif-1/orf20). These differential expression patterns appear to represent specific adaptations for virus infection and transmission through the polarized cells of the lepidopteran midgut.IMPORTANCE Baculoviruses such as AcMNPV are pathogens that are natural regulators of certain insect populations. Baculovirus infections are biphasic, with a primary phase initiated by oral infection of midgut epithelial cells by occlusion-derived virus (ODV) virions and a secondary phase in which other tissues are infected by budded-virus (BV) virions. While AcMNPV infections in cultured cells have been studied extensively, comparatively little is known regarding primary infection in the midgut. In these studies, we identified gene expression patterns associated with ODV-mediated infection of the midgut in Trichoplusia ni and compared those results with prior results from BV-infected cultured cells, which simulate secondary infection. These studies provide a detailed analysis of viral gene expression patterns in the midgut, which likely represent specific viral strategies to (i) overcome or avoid host defenses in the gut and (ii) rapidly move infection from the midgut, into the hemocoel to facilitate systemic infection.

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.

Baculovirus Entry and Egress from Insect Cells

Baculoviruses are large DNA viruses of insects that are highly pathogenic in many hosts. In the infection cycle, baculoviruses produce two types of virions. These virion phenotypes are physically and functionally distinct, and each serves a critical role in the biology of the virus. One phenotype, the occlusion-derived virus (ODV), is occluded within a crystallized protein that facilitates oral infection of the host. A large complex of at least nine ODV envelope proteins called per os infectivity factors are critically important for ODV infection of insect midgut epithelial cells. Viral egress from midgut cells is by budding to produce a second virus phenotype, the budded virus (BV). BV binds, enters, and replicates in most other tissues of the host insect. Cell recognition and entry by BV are mediated by a single major envelope glycoprotein: GP64 in some baculoviruses and F in others. Entry and egress by the two virion phenotypes occur by dramatically different mechanisms and reflect a life cycle in which ODV is specifically adapted for oral infection while BV mediates dissemination of the infection within the animal.

The group I alphabaculovirus-specific protein, AC5, is a novel component of the occlusion body but is not associated with ODVs or the PIF complex

Autographa californica nucleopolyhedrovirus (AcMNPV) orf5 (ac5) is a group I alphabaculovirus-specific gene of unknown function, although the protein (AC5) was previously reported to be associated with the per os infectivity factor (PIF) complex. The purpose of this study was to study the dynamics of AC5 during AcMNPV infection and to verify whether it is indeed a component of the PIF complex. Transcription and expression analyses suggested that ac5 is a late viral gene. An ac5-deleted recombinant AcMNPV was generated by homologous recombination. A one-step growth curve assay indicated that ac5 was not required for budded virus (BV) production in Sf9 cells. Scanning electron microscopy and transmission electron microscopy demonstrated that the deletion of ac5 did not affect occlusion body (OB) morphology, and nor did it affect the insertion of occlusion-derived virus (ODV) into OBs. Partially denaturing SDS-PAGE and a co-immunoprecipitation assay clearly showed that AC5 was not a component of the PIF complex, while the deletion of ac5 did not affect the formation and presence of the PIF complex. Further analyses showed, however, that AC5 was an OB-specific protein, but it was not detected as a component of BVs or ODVs. Bioassay experiments showed that the oral infectivity of ac5-deleted AcMNPV to third instar Spodoptera exigua larvae was not significantly different from that of the ac5-repaired virus. In conclusion, AC5 is an intrinsic protein of OBs, instead of being a component of the PIF complex, and is not essential for either BV or ODV infection. AC5 is awaiting the assignment of another hitherto unknown function.