Autographa californica multiple nucleopolyhedrovirus core gene ac92 (p33) is required for efficient budded virus production

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.

AC81 Is a Putative Disulfide Isomerase Involved in Baculoviral Disulfide Bond Formation

The correct formation of native disulfide bonds is critical for the proper structure and function of many proteins. Cellular disulfide bond formation pathways commonly consist of two parts: sulfhydryl oxidase-mediated oxidation and disulfide isomerase-mediated isomerization. Some large DNA viruses, such as baculoviruses, encode sulfhydryl oxidases, but viral disulfide isomerases have not yet been identified, although G4L in poxvirus has been suggested to serve such a function. Here, we report that the baculovirus core gene ac81 encodes a putative disulfide isomerase. ac81 is conserved in baculoviruses, nudiviruses, and hytrosaviruses. We found that AC81 homologs contain a typical thioredoxin fold conserved in disulfide isomerases. To determine the role of AC81, a series of Autographa californica nucleopolyhedrovirus (AcMNPV) bacmids containing ac81 knockout or point mutations was generated, and the results showed that AC81 is essential for budded virus production, multinucleocapsid occlusion-derived virus (ODV) formation, and ODV embedding in occlusion bodies. Nonreducing Western blot analysis indicated that disulfide bond formation in per os infectivity factor 5 (PIF5), a substrate of the baculoviral sulfhydryl oxidase P33, was abnormal when ac81 was knocked out or mutated. Pulldown assays showed that AC81 interacted with PIF5 and P33 in infected cells. In addition, two critical regions that harbor key amino acids for function were identified in AC81. Taken together, our results suggest that AC81 is a key component involved in the baculovirus disulfide bond formation pathway and likely functions as a disulfide isomerase. IMPORTANCE Many large DNA viruses, such as poxvirus, asfarvirus, and baculovirus, encode their own sulfhydryl oxidase to facilitate the disulfide bond formation of viral proteins. Here, we show that AC81 functions as a putative disulfide isomerase and is involved in multiple functions of the baculovirus life cycle. Interestingly, AC81 and P33 (sulfhydryl oxidase) are conserved in baculoviruses, nudiviruses, and hytrosaviruses, which are all insect-specific large DNA viruses replicating in the nucleus, suggesting that viral disulfide bond formation is an ancient mechanism shared by these viruses.

Per Os Infectivity Factor 5 Identified as a Substrate of P33 in the Baculoviral Disulfide Bond Formation Pathway

Disulfide bonds are critical for the structure and function of many proteins. Some large DNA viruses encode their own sulfhydryl oxidase for disulfide bond formation. Previous studies have demonstrated that the baculovirus-encoded sulfhydryl oxidase P33 is necessary for progeny virus production, and its enzymatic activity is important for morphogenesis and oral infectivity of baculoviruses. However, the downstream substrates of P33 in the putative redox pathway of baculoviruses are unknown. In this study, we showed that PIF5, one of the per os infectivity factors (PIFs), contained intramolecular disulfide bonds and that the disulfide bond formation was interrupted in the absence of P33. In vivo pulldown and colocalization analyses revealed that PIF5 and P33 interacted with each other during virus infection. Further, in vitro assays validated that the reduced PIF5 proteins could be oxidized by P33. To understand the contribution of disulfide bonds to the function of PIF5, several cysteine-to-serine mutants were constructed, which all interfered with the disulfide bond formation of PIF5 to different extents. All the mutants lost their oral infectivity but had no impact on infectious budding virus (BV) production or virus morphogenesis. Taken together, our results indicated PIF5 as the first identified substrate of P33. Further, the disulfide bonds in PIF5 play an essential role in its function in oral infection.IMPORTANCE Similar to some large DNA viruses that encode their own disulfide bond pathway, baculovirus encodes a viral sulfhydryl oxidase, P33. Enzyme activity of P33 is related to infectious BV production, occlusion-derived virus (ODV) envelopment, occlusion body morphogenesis, and oral infectivity, suggesting that P33 is involved in disulfide bond formation of multiple proteins. A complete disulfide bond formation pathway normally contains a sulfhydryl oxidase, a disulfide-donating enzyme, and one or more substrates. In baculovirus, apart from P33, other components of the putative pathway remain unknown. In this study, we identified PIF5 as the first substrate of P33, which is fundamental for revealing the complete disulfide bond formation pathway in baculovirus. PIF5 is essential for oral infection and is absent from the PIF complex. Our study demonstrated that native disulfide bonds in PIF5 are required for oral infection, which will help us to reveal its mode of action.

Autographa californica Multiple Nucleopolyhedrovirus ac75 Is Required for the Nuclear Egress of Nucleocapsids and Intranuclear Microvesicle Formation

Autographa californica multiple nucleopolyhedrovirus (AcMNPV) orf75 (ac75) is a highly conserved gene of unknown function. In this study, we constructed an ac75 knockout AcMNPV bacmid and investigated the role of ac75 in the baculovirus life cycle. The expression and distribution of the Ac75 protein were characterized, and its interaction with another viral protein was analyzed to further understand its function. Our data indicated that ac75 was required for the nuclear egress of nucleocapsids, intranuclear microvesicle formation, and subsequent budded virion (BV) formation, as well as occlusion-derived virion (ODV) envelopment and embedding of ODVs into polyhedra. Western blot analyses showed that two forms, of 18 and 15 kDa, of FLAG-tagged Ac75 protein were detected. Ac75 was associated with both nucleocapsid and envelope fractions of BVs but with only the nucleocapsid fraction of ODVs; the 18-kDa form was associated with only BVs, whereas the 15-kDa form was associated with both types of virion. Ac75 was localized predominantly in the intranuclear ring zone during infection and exhibited a nuclear rim distribution during the early phase of infection. A phase separation assay suggested that Ac75 was not an integral membrane protein. A coimmunoprecipitation assay revealed an interaction between Ac75 and the integral membrane protein Ac76, and bimolecular fluorescence complementation assays identified the sites of the interaction within the cytoplasm and at the nuclear membrane and ring zone in AcMNPV-infected cells. Our results have identified ac75 as a second gene that is required for both the nuclear egress of nucleocapsids and the formation of intranuclear microvesicles.IMPORTANCE During the baculovirus life cycle, the morphogenesis of both budded virions (BVs) and occlusion-derived virions (ODVs) is proposed to involve a budding process at the nuclear membrane, which occurs while nucleocapsids egress from the nucleus or when intranuclear microvesicles are produced. However, the exact mechanism of virion morphogenesis remains unknown. In this study, we identified ac75 as a second gene, in addition to ac93, that is essential for the nuclear egress of nucleocapsids, intranuclear microvesicle formation, and subsequent BV formation, as well as ODV envelopment and embedding of ODVs into polyhedra. Ac75 is not an integral membrane protein. However, it interacts with an integral membrane protein (Ac76) and is associated with the nuclear membrane. These data enhance our understanding of the commonalities between nuclear egress of nucleocapsids and intranuclear microvesicle formation and may help to reveal insights into the mechanism of baculovirus virion morphogenesis.

Autographa californica multiple nucleopolyhedrovirus ac75 is required for egress of nucleocapsids from the nucleus and formation of de novo intranuclear membrane microvesicles

In this study, Autographa californica multiple nucleopolyhedrovirus ac75 was functionally characterized. Ac75 has homologs in all sequenced genomes of alphabaculoviruses, betabaculoviruses, and gammabaculoviruses. It was determined to encode a protein that is associated with the nucleocapsid of budded virus and with both envelope and nucleocapsids of occlusion-derived virus. Sf9 cells transfected by an ac75-knockout bacmid resulted in the infection being restricted to single cells. No budded virus were detected although viral DNA replication and late gene expression were unaffected. Electron microscopy revealed that the virogenic stroma, nucleocapsids and occlusion bodies appeared normal in the cells transfected by an ac75-knockout bacmid. However, the nucleocapsids were unenveloped, the occlusion bodies did not contain any virions or nucleocapsids, and no nucleocapsids were found outside the nucleus or spanning the nuclear membrane. In addition, de novo intranuclear membrane microvesicles that are the precursor of occlusion-derived virus envelopes were absent in the nuclei of transfected cells. Confocal microscopy showed that AC75 protein appeared in the cytoplasm as early as 6 hours post infection. It localized to the ring zone at the periphery of the nucleus from 15 to 24 hours post infection and demonstrated light blocky cloud-like distribution in the center of the nucleus. AC75 was found to co-immunoprecipitate with BV and ODV associated envelope protein ODV-E25. The data from this study suggest that ac75 is essential for induction of the intranuclear membrane microvesicles, it appears to be required for the intranuclear envelopment of nucleocapsids, and is also essential for egress of nucleocapsids from the nuclei, in infected cells.

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.

Autographa californica multiple nucleopolyhedrovirus orf132 encodes a nucleocapsid-associated protein required for budded-virus and multiply enveloped occlusion-derived virus production

Autographa californica multiple nucleopolyhedrovirus orf132 (named ac132) has homologs in all genome-sequenced group I nucleopolyhedroviruses. Its role in the viral replication cycle is unknown. In this study, ac132 was shown to express a protein of around 28 kDa, which was determined to be associated with the nucleocapsids of both occlusion-derived virus and budded virus. Confocal microscopy showed that AC132 protein appeared in central region of the nucleus as early as 12 h postinfection with the virus. It formed a ring zone at the periphery of the nucleus by 24 h postinfection. To investigate its role in virus replication, ac132 was deleted from the viral genome by using a bacmid system. In the Sf9 cell culture transfected by the ac132 knockout bacmid, infection was restricted to single cells, and the titer of infectious budded virus was reduced to an undetectable level. However, viral DNA replication and the expression of late genes vp39 and odv-e25 and a reporter gene under the control of the very late gene p10 promoter were unaffected. Electron microscopy showed that nucleocapsids, virions, and occlusion bodies were synthesized in the cells transfected by an ac132 knockout bacmid, but the formation of the virogenic stroma and occlusion bodies was delayed, the numbers of enveloped nucleocapsids were reduced, and the occlusion bodies contained mainly singly enveloped nucleocapsids. AC132 was found to interact with envelope protein ODV-E18 and the viral DNA-binding protein P6.9. The data from this study suggest that ac132 possibly plays an important role in the assembly and envelopment of nucleocapsids.

IMPORTANCE

To our knowledge, this is the first report on a functional analysis of ac132. The data presented here demonstrate that ac132 is required for production of the budded virus and multiply enveloped occlusion-derived virus of Autographa californica multiple nucleopolyhedrovirus. This article reveals unique phenotypic changes induced by ac132 deletion on the virus and multiple new findings on ac132.

The Trichoplusia ni single nucleopolyhedrovirus tn79 gene encodes a functional sulfhydryl oxidase enzyme that is able to support the replication of Autographa californica multiple nucleopolyhedrovirus lacking the sulfhydryl oxidase ac92 gene

The Autographa californica multiple nucleopolyhedrovirus ac92 is a conserved baculovirus gene with homology to flavin adenine dinucleotide-linked sulfhydryl oxidases. Its product, Ac92, is a functional sulfhydryl oxidase. Deletion of ac92 results in almost negligible levels of budded virus (BV) production, defects in occlusion-derived virus (ODV) co-envelopment and their inefficient incorporation into occlusion bodies. To determine the role of sulfhydryl oxidation in the production of BV, envelopment of nucleocapsids, and nucleocapsid incorporation into occlusion bodies, the Trichoplusia ni single nucleopolyhedrovirus ortholog, tn79, was substituted for ac92. Tn79 was found to be an active sulfhydryl oxidase that substituted for Ac92, resulting in the production of infectious BV, albeit about 10-fold less than an ac92-containing virus. Tn79 rescued defects in ODV morphogenesis caused by a lack of ac92. Active Tn79 sulfhydryl oxidase activity is required for efficient BV production, ODV envelopment, and their subsequent incorporation into occlusion bodies in the absence of ac92.

The baculovirus sulfhydryl oxidase Ac92 (P33) interacts with the Spodoptera frugiperda P53 protein and oxidizes it in vitro

The Autographa californica M nucleopolyhedrovirus (AcMNPV) sulfhydryl oxidase Ac92 is essential for production of infectious virions. Ac92 also interacts with human p53 and enhances human p53-induced apoptosis in insect cells, but it is not known whether any relationship exists between Ac92 and native p53 homologs from insect hosts of AcMNPV. We found that Ac92 interacted with SfP53 from Spodoptera frugiperda in infected cells and oxidized SfP53 in vitro. However, Ac92 did not interact with or oxidize a mutant of SfP53 predicted to lack DNA binding. Silencing Sfp53 expression did not rescue the ability of an ac92-knockout virus to produce infectious virus. Similarly, ac92 expression did not affect SfP53-stimulated caspase activity or the localization of SfP53. Thus, although Ac92 binds to SfP53 during AcMNPV replication and oxidizes SfP53 in vitro, we could not detect any effects of this interaction on AcMNPV replication in cultured cells.

Exploring ORFan domains in giant viruses: structure of mimivirus sulfhydryl oxidase R596

The mimivirus genome contains many genes that lack homologs in the sequence database and are thus known as ORFans. In addition, mimivirus genes that encode proteins belonging to known fold families are in some cases fused to domain-sized segments that cannot be classified. One such ORFan region is present in the mimivirus enzyme R596, a member of the Erv family of sulfhydryl oxidases. We determined the structure of a variant of full-length R596 and observed that the carboxy-terminal region of R596 assumes a folded, compact domain, demonstrating that these ORFan segments can be stable structural units. Moreover, the R596 ORFan domain fold is novel, hinting at the potential wealth of protein structural innovation yet to be discovered in large double-stranded DNA viruses. In the context of the R596 dimer, the ORFan domain contributes to formation of a broad cleft enriched with exposed aromatic groups and basic side chains, which may function in binding target proteins or localization of the enzyme within the virus factory or virions. Finally, we find evidence for an intermolecular dithiol/disulfide relay within the mimivirus R596 dimer, the first such extended, intersubunit redox-active site identified in a viral sulfhydryl oxidase.

Bm65 is essential for the propagation of Bombyx mori nucleopolyhedrovirus

Orf65 (Bm65) of Bombyx mori nucleopolyhedrovirus (BmNPV) is a highly conserved gene that encodes an unknown 104-amino acid protein. In the present study, we have shown the role of Bm65 in the baculovirus life cycle. 5'-RACE analysis showed that the transcription start site of Bm65 was 14 nucleotides upstream of the start codon ATG. The transcription profile of Bm65 was detected from 6 to 72 h postinfection (p. i.) by RT-PCR. A Bm65-knockout bacmid was constructed by homologous recombination to characterize the role of Bm65 in viral life cycle. Fluorescence microscopy showed that Bm65-knockout virus was unable to generate infectious budded virus in BmN cells. Furthermore, quantitative real-time PCR analysis demonstrated that Bm65 deletion did not affect the viral DNA replication. To conclude, Bm65 is essential for the propagation of BmNPV, but is unnecessary for the replication of viral DNA.

Use of bacterial artificial chromosomes in baculovirus research and recombinant protein expression: current trends and future perspectives

The baculovirus expression system is one of the most successful and widely used eukaryotic protein expression methods. This short review will summarise the role of bacterial artificial chromosomes (BACS) as an enabling technology for the modification of the virus genome. For many years baculovirus genomes have been maintained in E. coli as bacterial artificial chromosomes, and foreign genes have been inserted using a transposition-based system. However, with recent advances in molecular biology techniques, particularly targeting reverse engineering of the baculovirus genome by recombineering, new frontiers in protein expression are being addressed. In particular, BACs have facilitated the propagation of disabled virus genomes that allow high throughput protein expression. Furthermore, improvement in the selection of recombinant viral genomes inserted into BACS has enabled the expression of multiprotein complexes by iterative recombineering of the baculovirus genome.

Phylogeny and evolution of Hytrosaviridae

The Hytrosaviridae comprises a family of dsDNA viruses with a circular genome of 120-190 kb p. They are exclusively associated with Diptera, such as the tsetse fly, the house fly and the Narcissus bulb fly. Hytrosaviruses cause a very unique pathology including hypertrophy of salivary glands as well as testicular and ovarian malformation. On the other hand these viruses share a significant number of gene homologues with other dsDNA viruses, esp. baculoviruses and nudiviruses. These gene homologues include twelve so-called baculovirus core genes involved in transcription, DNA replication and the infection process. Most strikingly, the Musca domestica salivary gland hypertrophy virus (MdSGHV) encodes a homologue of a polyhedrin/granulin gene of Alpha-, Beta-, Gammabaculoviruses. Hence, it is proposed that hytrosaviruses are phylogenetically related to baculoviruses but evolved in a very close association with their dipteran hosts.

Crystal structure of Bombyx mori nucleopolyhedrovirus ORF75 reveals a pseudo-dimer of thiol oxidase domains with a putative substrate-binding pocket

Bombyx mori nucleopolyhedrovirus (BmNPV) triggers the global shutdown of host silkworm gene expression and protein synthesis approximately 12-18 h post-infection. Genome sequence analysis suggests that BmNPV ORF75 could be a flavin adenine dinucleotide (FAD)-linked thiol oxidase essential for virion assembly and virus propagation. Here, we report the crystal structure of BmNPV ORF75 at 2.1 Å (0.21 nm). The structure of BmNPV ORF75 resembles that of the thiol oxidase domain of human quiescin thiol oxidase (QSOX), displaying a pseudo-dimer of canonical and non-canonical thiol oxidase domains. However, BmNPV ORF75 is further dimerized by its C-terminal canonical thiol oxidase domain. Within the unique quaternary structural arrangement, the FAD-binding pocket and the characteristic CXXC motif from each monomer is 35 Å (3.5 nm) away from that of its corresponding molecule, which suggests that BmNPV ORF75 might adopt a deviant mechanism from that of QSOX to catalyse disulfide bond formation. Our thiol oxidase activity assay on the point mutations of the conserved residues participating in FAD recognition reveals an aromatic cage next to the FAD isoalloxazine moiety for substrate binding. These data suggest that the thiol oxidase activity of BmNPV ORF75 could be critical to catalyse the formation of the disulfide bonds of certain BmNPV proteins essential for BmNPV virion assembly.

Baculovirus infection induces a DNA damage response that is required for efficient viral replication

Several mammalian viruses have been shown to induce a cellular DNA damage response during replication, and in some cases, this response is required for optimal virus replication. However, nothing is known about whether a DNA damage response is stimulated by DNA viruses in invertebrates. Cell cycle arrest and apoptosis are two of the downstream effects of the DNA damage response, and both are stimulated by baculovirus infection, suggesting a possible relationship between baculoviruses and the DNA damage response. In the study described in this report, we found that replication of the baculovirus Autographa californica M nucleopolyhedrovirus (AcMNPV) in the cell line Sf9, derived from the lepidopteran insect Spodoptera frugiperda, stimulated a DNA damage response, as indicated by an increased abundance of the S. frugiperda P53 protein (SfP53) and phosphorylation of the histone variant protein H2AX. Stimulation of the DNA damage response was dependent on viral DNA replication. Inhibition of the DNA damage response prevented both the increase in SfP53 accumulation and H2AX phosphorylation and also caused a 10- to 100-fold reduction in virus production, along with decreased viral DNA replication and late gene expression. However, silencing of Sfp53 expression by RNA interference did not significantly affect AcMNPV replication or induction of apoptosis by a mutant of AcMNPV lacking the antiapoptotic gene p35, indicating that these processes are not dependent on SfP53 in Sf9 cells.

Identification of Autographa californica nucleopolyhedrovirus ac93 as a core gene and its requirement for intranuclear microvesicle formation and nuclear egress of nucleocapsids

Autographa californica nucleopolyhedrovirus (AcMNPV) orf93 (ac93) is a highly conserved uncharacterized gene that is found in all of the sequenced baculovirus genomes except for Culex nigripalpus NPV. In this report, using bioinformatics analyses, ac93 and odv-e25 (ac94) were identified as baculovirus core genes and thus p33-ac93-odv-e25 represent a cluster of core genes. To investigate the role of ac93 in the baculovirus life cycle, an ac93 knockout AcMNPV bacmid was constructed via homologous recombination in Escherichia coli. Fluorescence and light microscopy showed that the AcMNPV ac93 knockout did not spread by infection, and titration assays confirmed a defect in budded virus (BV) production. However, deletion of ac93 did not affect viral DNA replication. Electron microscopy indicated that ac93 was required for the egress of nucleocapsids from the nucleus and the formation of intranuclear microvesicles, which are precursor structures of occlusion-derived virus (ODV) envelopes. Immunofluorescence analyses showed that Ac93 was concentrated toward the cytoplasmic membrane in the cytoplasm and in the nuclear ring zone in the nucleus. Western blot analyses showed that Ac93 was associated with both nucleocapsid and envelope fractions of BV, but only the nucleocapsid fraction of ODV. Our results suggest that ac93, although not previously recognized as a core gene, is one that plays an essential role in the formation of the ODV envelope and the egress of nucleocapsids from the nucleus.

Characterization of cDNAs encoding p53 of Bombyx mori and Spodoptera frugiperda

Complementary DNAs encoding homologs of the tumor suppressor gene, p53, were characterized from two lepidopteran insects, Bombyx mori (Bm) and Spodoptera frugiperda (Sf). They encoded predicted proteins of 368 (41.2 kDa) (Bm) and 374 (42.5 kDa) (Sf) amino acids. The sequences shared 44% amino acid and 60% nucleotide sequence identity with each other, but exhibited less than 20% amino acid and 46% nucleotide sequence identity to Drosophila melanogaster p53. Despite the sequence diversity, conserved amino acids involved in DNA and zinc binding were present in the lepidopteran sequences. Expression of Sfp53-induced apoptosis in S. frugiperda cells, and antiserum made against recombinant Sfp53 recognized a protein whose abundance increased after treatment with DNA damaging agents.

Structure of a baculovirus sulfhydryl oxidase, a highly divergent member of the erv flavoenzyme family

Genomes of nucleocytoplasmic large DNA viruses (NCLDVs) encode enzymes that catalyze the formation of disulfide bonds between cysteine amino acid residues in proteins, a function essential for the proper assembly and propagation of NCLDV virions. Recently, a catalyst of disulfide formation was identified in baculoviruses, a group of large double-stranded DNA viruses considered phylogenetically distinct from NCLDVs. The NCLDV and baculovirus disulfide catalysts are flavin adenine dinucleotide (FAD)-binding sulfhydryl oxidases related to the cellular Erv enzyme family, but the baculovirus enzyme, the product of the Ac92 gene in Autographa californica multiple nucleopolyhedrovirus (AcMNPV), is highly divergent at the amino acid sequence level. The crystal structure of the Ac92 protein presented here shows a configuration of the active-site cysteine residues and bound cofactor similar to that observed in other Erv sulfhydryl oxidases. However, Ac92 has a complex quaternary structural arrangement not previously seen in cellular or viral enzymes of this family. This novel assembly comprises a dimer of pseudodimers with a striking 40-degree kink in the interface helix between subunits. The diversification of the Erv sulfhydryl oxidase enzymes in large double-stranded DNA viruses exemplifies the extreme degree to which these viruses can push the boundaries of protein family folds.

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.