P33 of Helicoverpa armigera single nucleocapsid nucleopolyhedrovirus is a functional homolog of AcP33

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.

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.

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.

Three Conserved Regions in Baculovirus Sulfhydryl Oxidase P33 Are Critical for Enzymatic Activity and Function

Baculoviruses encode a conserved sulfhydryl oxidase, P33, which is necessary for budded virus (BV) production and multinucleocapsid occlusion-derived virus (ODV) formation. Here, the structural and functional relationship of P33 was revealed by X-ray crystallography, site-directed mutagenesis, and functional analysis. Based on crystallographic characterization and structural analysis, a series of P33 mutants within three conserved regions, i.e., the active site, the dimer interface, and the R127-E183 salt bridge, were constructed. In vitro experiments showed that mutations within the active site and dimer interface severely impaired the sulfhydryl oxidase activity of P33, while the mutations in the salt bridge had a relatively minor influence. Recombinant viruses containing mutated P33 were constructed and assayed in vivo Except for the active-site mutant AXXA, all other mutants produced infectious BVs, although certain mutants had a decreased BV production. The active-site mutant H114A, the dimer interface mutant H227D, and the salt bridge mutant R127A-E183A were further analyzed by electron microscopy and bioassays. The occlusion bodies (OBs) of mutants H114A and R127A-E183A had a ragged surface and contained mostly ODVs with a single nucleocapsid. The OBs of all three mutants contained lower numbers of ODVs and had a significantly reduced oral infectivity in comparison to control virus. Crystallographic analyses further revealed that all three regions may coordinate with one another to achieve optimal function of P33. Taken together, our data revealed that all the three conserved regions are involved in P33 activity and are crucial for virus morphogenesis and peroral infectivity.IMPORTANCE Sulfhydryl oxidase catalyzes disulfide bond formation of substrate proteins. P33, a baculovirus-encoded sulfhydryl oxidase, is different from other cellular and viral sulfhydryl oxidases, bearing unique features in tertiary and quaternary structure organizations. In this study, we found that three conserved regions, i.e., the active site, dimer interface, and the R127-E183 salt bridge, play important roles in the enzymatic activity and function of P33. Previous observations showed that deletion of p33 results in a total loss of budded virus (BV) production and in morphological changes in occlusion-derived virus (ODV). Our study revealed that certain P33 mutants lead to occlusion bodies (OBs) with a ragged surface, decreased embedded ODVs, and reduced oral infectivity. Interestingly, some P33 mutants with impaired ODV/OB still retained BV productivity, indicating that the impacts on BV and on ODV/OB are two distinctly different functions of P33, which are likely to be performed via different substrate proteins.