Autographa californica multiple nucleopolyhedrovirus nucleocapsid protein BV/ODV-C42 mediates the nuclear entry of P78/83

The conserved cysteines at position 18, 36, and 49 of Autographa californica multiple nucleopolyhedrovirus VP39 are essential for virus replication

Autographa californica nucleopolyhedrovirus orf89 (vp39) encodes the major capsid protein VP39. Multiple alignments of protein sequences showed that VP39 has 8 conserved cysteine (Cys) residues. Cysteine residues play an important role in proper function of a protein. To determine the importance of these conserved cysteine residues for virus proliferation, a series of recombinant viruses harboring VP39-Cys mutants were constructed. Viral growth curves and transmission electron microscopy showed that mutation of Cys29, Cys132, Cys169, Cys229, or Cys232 of VP39 to alanine did not affect budded virion production; however, the mutation of Cys18, Cys36, or Cys49 to alanine resulted in interruption of capsid assembly. Co-immunoprecipitation assays showed that mutations of these 8 cysteines individually or simultaneously had no effect on self-association of VP39. Immunofluorescence analysis by confocal microscopy revealed that the subcellular localization of VP39 with mutations in Cys18, Cys36 or Cys49 was exclusively distributed in the cytoplasm of a cell regardless of virus infection or not, while the wild-type VP39 or the VP39 carrying mutations in Cys29, Cys132, Cys169, Cys229, or Cys232 was distributed throughout the cytoplasm and the nucleus. Our results demonstrated that Cys18, Cys36, and Cys49 are essential for the proper localization of VP39, which is a prerequisite for successful nucleocapsid assembly of the virus.

Mitigation of viruses of concern and bacteriophage surrogates via common unit processes for water reuse: A meta-analysis

Water reuse is a growing global reality. In regulating water reuse, viruses have come to the fore as key pathogens due to high shedding rates, low infectious doses, and resilience to traditional wastewater treatments. To demonstrate the high log reductions required by emerging water reuse regulations, cost and practicality necessitate surrogates for viruses for use as challenge organisms in unit process evaluation and monitoring. Bacteriophage surrogates that are mitigated to the same or lesser extent than viruses of concern are routinely used for individual unit process testing. However, the behavior of these surrogates over a multi-barrier treatment train typical of water reuse has not been well-established. Toward this aim, we performed a meta-analysis of log reductions of common bacteriophage surrogates for five treatment processes typical of water reuse treatment trains: advanced oxidation processes, chlorination, membrane filtration, ozonation, and ultraviolet (UV) disinfection. Robust linear regression was applied to identify a range of doses consistent with a given log reduction of bacteriophages and viruses of concern for each treatment process. The results were used to determine relative conservatism of surrogates. We found that no one bacteriophage was a representative or conservative surrogate for viruses of concern across all multi-barrier treatments (encompassing multiple mechanisms of virus mitigation). Rather, a suite of bacteriophage surrogates provides both a representative range of inactivation and information about the effectiveness of individual processes within a treatment train. Based on the abundance of available data and diversity of virus treatability using these five key water reuse treatment processes, bacteriophages MS2, phiX174, and Qbeta were recommended as a core suite of surrogates for virus challenge testing.

The Bmtret1 Gene Family and Its Potential Role in Response to BmNPV Stress in Bombyx mori

Trehalose is a non-reducing disaccharide and participates in physiological activities such as organ formation, energy metabolism, and stress resistance in insects. The Bmtret1 gene family is mainly involved in in the sugar metabolism of silkworm. In the present study, phylogenetic analysis divided 21 Bmtret1 orthologs into three clades. These genes are equally distributed on the nine chromosomes. The cis-elements in the promoter regions of Bmtret1s indicated the possible function of Bmtret1s in response to hormones and environmental stimulus. The qPCR analysis showed the significantly different expression levels of Bmtret1s in different tissues and organs, indicating possible functional divergence. In addition, most Bmtret1s showed disturbed expression levels in response to silkworm nuclear polyhedrosis virus (BmNPV) stresses. Our results provide a clue for further functional dissection of the Tret1s in Bombyx mori and implicate them as potential regulators of antiviral responses.

Architecture of the baculovirus nucleocapsid revealed by cryo-EM

Baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) has been widely used as a bioinsecticide and a protein expression vector. Despite their importance, very little is known about the structure of most baculovirus proteins. Here, we show a 3.2 Å resolution structure of helical cylindrical body of the AcMNPV nucleocapsid, composed of VP39, as well as 4.3 Å resolution structures of both the head and the base of the nucleocapsid composed of over 100 protein subunits. AcMNPV VP39 demonstrates some features of the HK97-like fold and utilizes disulfide-bonds and a set of interactions at its C-termini to mediate nucleocapsid assembly and stability. At both ends of the nucleocapsid, the VP39 cylinder is constricted by an outer shell ring composed of proteins AC104, AC142 and AC109. AC101(BV/ODV-C42) and AC144(ODV-EC27) form a C14 symmetric inner layer at both capsid head and base. In the base, these proteins interact with a 7-fold symmetric capsid plug, while a portal-like structure is seen in the central portion of head. Additionally, we propose an application of AlphaFold2 for model building in intermediate resolution density.

Enhanced virulence of genetically engineered Autographa californica nucleopolyhedrovirus owing to accelerated viral DNA replication aided by inserted ascovirus genes

Genetic engineering technology is an ideal method to improve insecticidal efficiency by combining the advantages of different pathogenic microorganisms. Thus, six ascovirus genes were introduced into the genomic DNA of Autographa californica nucleopolyhedrovirus (AcMNPV) to possibly transfer the intrinsically valuable insecticidal properties from ascovirus to baculovirus. The viral budded virus (BV) production and viral DNA replication ability of AcMNPV-111 and AcMNPV-165 were significantly stronger than that of AcMNPV-Egfp (used as the wild-type virus in this study), whereas AcMNPV-33 had reduced ones. AcMNPV-111 and AcMNPV-165 also exhibited excellent insecticidal efficiency in the in vivo bioassays: AcMNPV-111 showed a 24.1% decrease in the LT₅₀ value and AcMNPV-165 exhibited a 56.3% decrease in the LD₅₀ value compared with AcMNPV-Egfp against the 3rd instar of Spodoptera exigua larvae, respectively. Furthermore, the size of the occlusion bodies (OBs) of AcMNPV-33, AcMNPV-111, and AcMNPV-165 were significantly increased compared to that of AcMNPV-Egfp. AcMNPV-111 and AcMNPV-165 had stable virulence against the 2nd to 4th instars tested larvae and higher OB yield than AcMNPV-Egfp in the 3rd and 4th instar larvae. Correlation and regression analyses indicated that it is better to use 5 OBs/larva virus to infect the 2nd instar larvae to produce AcMNPV-111 and 50 OBs/larva virus to infect the 3rd instar larvae to produce AcMNPV-165. The results of this study obtained recombinant viruses with enhanced virulence and exhibited a diversity of ascovirus gene function based on the baculovirus platform, which provided a novel strategy for the improvement of baculovirus as a biological insecticide.

Potential Proteins Interactions with Bombyx mori Nucleopolyhedrovirus Revealed by Co-Immunoprecipitation

Virus–host interactions are critical for virus replication, virulence, and pathogenicity. The Bombyx mori nucleopolyhedrovirus (BmNPV) is a typical model baculovirus, representing one of the most common and harmful pathogens in sericulture. Herein, we used co-immunoprecipitation to identify candidate proteins with potential interactions with BmNPV. First, a recombinant BV virus particle rBmBV-egfp-p64-3×flag-gp64sp was constructed using a MultiBac baculovirus multigene expression system. Co-immunoprecipitation experiments were then performed with the recombinant BV virus infected with BmN cells and Dazao silkworms. LC-MS/MS analysis revealed a total of 845 and 1368 candidate proteins were obtained from BmN cells and silkworm samples, respectively. Bioinformatics analysis (Gene Ontology, KEGG Pathway) was conducted for selection of proteins with significant enrichment for further confirmation of the effects on BmNPV replication. Overall, the results showed that SEC61 and PIC promoted the replication of BmNPV, while FABP1 inhibited the replication of BmNPV. In summary, this study reveals the potential proteins involved in BmNPV invasion and proliferation in the host and provides a platform for identifying the potential receptor proteins of BmNPV.

Nuclear Cytoskeleton in Virus Infection

The nuclear lamina is the main component of the nuclear cytoskeleton that maintains the integrity of the nucleus. However, it represents a natural barrier for viruses replicating in the cell nucleus. The lamina blocks viruses from being trafficked to the nucleus for replication, but it also impedes the nuclear egress of the progeny of viral particles. Thus, viruses have evolved mechanisms to overcome this obstacle. Large viruses induce the assembly of multiprotein complexes that are anchored to the inner nuclear membrane. Important components of these complexes are the viral and cellular kinases phosphorylating the lamina and promoting its disaggregation, therefore allowing virus egress. Small viruses also use cellular kinases to induce lamina phosphorylation and the subsequent disruption in order to facilitate the import of viral particles during the early stages of infection or during their nuclear egress. Another component of the nuclear cytoskeleton, nuclear actin, is exploited by viruses for the intranuclear movement of their particles from the replication sites to the nuclear periphery. This study focuses on exploitation of the nuclear cytoskeleton by viruses, although this is just the beginning for many viruses, and promises to reveal the mechanisms and dynamic of physiological and pathological processes in the nucleus.

Comparative Transcriptome Analysis of Bombyx mori (Lepidoptera) Larval Hemolymph in Response to Autographa californica Nucleopolyhedrovirus in Differentially Resistant Strains

Bombyx mori nucleopolyhedrovirus (BmNPV) is a kind of pathogen that causes huge economic losses to silkworm production. Although Autographa californica nucleopolyhedrovirus (AcMNPV) and BmNPV are both baculoviruses, the host domains of these two viruses have almost no intersection in nature. Recently, it has been found that some silkworms could be infected by recombinant AcMNPV through a puncture, which provided valuable material for studying the infection mechanism of baculovirus to silkworm. In this study, comparative transcriptomics was used to analyse the hemolymph of two differentially resistant strains following AcMNPV inoculation. There were 678 DEGs in p50 and 515 DEGs in C108 following viral infection. Among them, the upregulation and downregulation of DEGs were similar in p50; however, the upregulated DEGs were nearly twice as numerous as the downregulated DEGs in C108. The DEGs in different resistant strains differed by GO enrichment. Based on KEGG enrichment, DEGs were mainly enriched in metabolic pathways in p50 and the apoptosis pathway in C108. Moreover, 13 genes involved in metabolic pathways and 11 genes involved in the apoptosis pathway were analysed. Among the DEGs involved in apoptosis, the function of BmTex261 in viral infection was analysed. The BmTex261 showed the highest expression in hemolymph and a significant response to viral infection in the hemolymph of C108, indicating that it is involved in anti-AcMNPV infection. This was further validated by the significantly decreased expression of viral gene lef3 after overexpression of BmTex261 in BmN cells. The results provide a theoretical reference for the molecular mechanism of resistance to BmNPV in silkworms.

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.

Systematic analysis of nuclear localization of Autographa californica multiple nucleopolyhedrovirus proteins

Baculoviruses are large DNA viruses that replicate within the nucleus of infected host cells. Therefore, many viral proteins must gain access to the nucleus for efficient viral genome replication, gene transcription and virion assembly. To date, the global protein localization pattern of baculoviral proteins is unknown. In this study, we systematically analysed the nuclear localization of 154 ORFs encoded by the prototypic baculovirus, Autographa californica multiple nucleopolyhedrovirus (AcMNPV), either during transient expression or with super-infection of the virus. By transient expression of vectors containing egfp-fused ORFs, we found that in the absence of virus infection, 25 viral proteins were localized in the nucleus. Most of these, which we called ‘auto-nuclear localization’ proteins, are related to virus replication, transcription or virion structure, and 20 of them contain predicted classical nuclear localization signal. Upon virus infection, 11 proteins, which originally localized in the cytoplasm or both cytoplasm and nucleus in the transfection assays, were completely translocated into the nucleus, suggesting that their nuclear import is facilitated by other viral or host proteins. Further co-transfection experiments identified that four of the 11 proteins, including P143, P33, AC73 and AC114, were imported into the nucleus with the assistance of the auto-nuclear localization proteins LEF-3 (for P143), TLP (for P33) and VP80 (for both AC73 and AC114). This study presents the first global nuclear localization profile of AcMNPV proteins and provides useful information for further elucidation of the mechanisms of baculovirus nuclear entry and gene functions.

Heat Shock Protein 75 (TRAP1) facilitate the proliferation of the Bombyx mori nucleopolyhedrovirus

The viruses utilize multiple cellular proteins to facilitate their proliferation. The Heat Shock Protein (HSP), the highly conserved protein in eukaryotes and prokaryotes, plays a critical role in facilitating viral proliferation. However, less is known about the role of the HSPs in the life cycles of the Baculoviruses. We constructed recombinant Bombyx mori nucleopolyhedrovirus and discovered the Heat Shock Protein 75 (TRAP1) in the B. mori ovary (BmN) cells by the co-immunoprecipitation experiment using the GP64 (glycoprotein 64) as the bait protein. Tissue expression profile analysis of B. mori indicated that the TRAP1 gene has higher expression levels in the ovary, midgut, and hemolymph. Down-regulation of TRAP1 via RNA interference (RNAi) and geldanamycin (GA, a TRAP1 inhibitor) treatment can reduce the expression level of the major capsid protein VP39 (viral protein 39) of BmNPV. In contrast, the up-regulation of TRAP1 via overexpression can increase the expression level of the VP39. These results indicated that the TRAP1 of B. mori could facilitate the proliferation of the BmNPV. This study provided new insights into the function of TRAP1, and the basic mechanisms of the baculoviruses life cycle for disease prevention.

The Motif of 76KRKCSK in Bm65 Is an Efficient Nuclear Localization Signal Involved in Production of Infectious Virions

orf65 (Bm65) of Bombyx mori nucleopolyhedrovirus (BmNPV) codes for a putative 104-amino-acid protein containing three cysteine residues with a putative molecular mass of 12.2 kDa. Previous studies have showed that Bm65 accumulates mainly in nucleus and involved in the repair of UV-damaged DNA. However, the mechanism of nuclear import of Bm65 remains unclear. In this study, a SDS-stable Bm65 tetramer was found in BmNPV-infected BmN cells, and alanine substitutions for the three cysteine residues did not affect the formation of Bm65 tetramer. Additionally, a basic amino acid cluster of the Bm65 protein was identified as an efficient nuclear localization signal (NLS). Firstly, transient expression of GFP-fused truncated Bm65 variants revealed that the ⁷⁶KRKCSK motif functions as the NLS. This was also confirmed by alanine substitution in the ⁷⁶KRKCSK motif, which caused attenuated nuclear localization of Bm65. Next, the ⁷⁶KRKCSK motif-mutated bacmid was generated and the ⁷⁶KRKCSK motif was also found to be important for nuclear localization of Bm65 in BmNPV-infected conditions. Lastly, analyses of flag-tagged Bm65 expressing bacmids revealed that the mutations in ⁷⁶KRKCSK motif did not affect the synthesis of Bm65 tetramer, but severely impaired production levels of infectious virions. In conclusion, Bm65 exists in mainly a tetrameric form in virus-infected cells, which may be involved with production levels of infectious virions.

Mechanisms Mediating Nuclear Trafficking Involved in Viral Propagation by DNA Viruses

Typical viral propagation involves sequential viral entry, uncoating, replication, gene transcription and protein synthesis, and virion assembly and release. Some viral proteins must be transported into host nucleus to facilitate viral propagation, which is essential for the production of mature virions. During the transport process, nuclear localization signals (NLSs) play an important role in guiding target proteins into nucleus through the nuclear pore. To date, some classical nuclear localization signals (cNLSs) and non-classical NLSs (ncNLSs) have been identified in a number of viral proteins. These proteins are involved in viral replication, expression regulation of viral genes and virion assembly. Moreover, other proteins are transported into nucleus with unknown mechanisms. This review highlights our current knowledge about the nuclear trafficking of cellular proteins associated with viral propagation.

Major capsid protein of Autographa californica multiple nucleopolyhedrovirus contributes to the promoter activity of the very late viral genes

The baculovirus expression vector system (BEVS) is one of the most powerful eukaryotic expression systems. Recombinant protein expression is usually controlled by promoters of the baculovirus very late genes (i.e., polyhedrin and p10); therefore, identifying novel regulatory factors for these promoters is key to increasing BEVS productivity. Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is the viral vector most frequently used in BEVS. VP39 is the major nucleocapsid protein of AcMNPV and plays a pivotal role in nucleocapsid assembly in the nucleus. In this study, we found that knocking out vp39 from the AcMNPV genome resulted in decreased protein abundance of polyhedrin and P10. Further assays revealed that the mRNA transcripts and the promoter activities of polyhedrin and p10 were decreased in the absence of vp39, suggesting that VP39 contributes to the activity of the very late viral gene promoters and may represent a means of optimizing the current BEVS.

Nucleocapsid Assembly of Baculoviruses

The baculovirus nucleocapsid is formed through a rod-like capsid encapsulating a genomic DNA molecule of 80~180 kbp. The viral capsid is a large oligomer composed of many copies of various protein subunits. The assembly of viral capsids is a complex oligomerization process. The timing of expression of nucleocapsid-related proteins, transport pathways, and their interactions can affect the assembly process of preformed capsids. In addition, the selection of viral DNA and the injection of the viral genome into empty capsids are the critical steps in nucleocapsid assembly. This paper reviews the replication and recombination of baculovirus DNA, expression and transport of capsid proteins, formation of preformed capsids, DNA encapsulation, and nucleocapsid formation. This review will provide a basis for further study of the nucleocapsid assembly mechanism of baculovirus.

AcMNPV-miR-3 is a miRNA encoded by Autographa californica nucleopolyhedrovirus and regulates the viral infection by targeting ac101

MicroRNAs (miRNAs), which are small noncoding RNAs found in plants, animals, and many viruses, regulate various biological processes. Our group has previously reported the first miRNA encoded by Autographa californica multiple Nucleopolyhedrovirus (AcMNPV), AcMNPV-miR-1, which regulates the expression of three viral genes. This study characterizes another miRNA encoded by AcMNPV, AcMNPV-miR-3. This miRNA is located on the opposite strand of the viral gene ac101 coding sequence in the AcMNPV genome, and it can be detected at 6 h post-infection and accumulated to a peak around 12 h post-infection in AcMNPV infected Sf9 cells. Five viral genes (ac101, ac23, ac25, ac86, and ac98) were verified to be regulated by AcMNPV-miR-3. Ac101 was markedly down-regulated by AcMNPV-miR-3 that may be via a siRNA-like cleavage mode. Administrating excessive AcMNPV-miR-3 resulted in decreased production of infectious budded virions (BV) and accelerated the formation of occlusion-derived virions (ODV). These results suggest that AcMNPV-miR-3 may play a regulatory role in BV and ODV production.

The deficiency in nuclear localization signal of Neodiprion lecontei nucleopolyhedrovirus DNA polymerase prevents rescue of viral DNA replication and virus production in dnapol-null Autographa californica multiple nucleopolyhedrovirus

DNA polymerase (DNApol) is highly conserved in baculovirus and is required for viral DNA replication. However, little is known about gammabaculovirus DNApol. Here DNApol of the gammabaculovirus Neodiprion lecontei nucleopolyhedrovirus (NeleNPV) was cloned into a dnapol-null alphabaculovirus AcMNPV bacmid, creating Bac-GFP-AcΔPol-NlPol. The resulting recombinant bacmid did not spread to neighboring cells, virus growth curve and real-time PCR revealed that NeleNPV dnapol substitution did not rescue AcMNPV DNA replication and virus production. Immunofluorescence microscopy revealed that NeleNPV DNApol was expressed but could not localize to the nucleus. Subsequently NeleNPV DNApol was fused to SpltNPV DNApol nuclear localization signal (NLS) and the fused DNApol could import into nucleus. The NLS-fusing NeleNPV DNApol was further transposed into the dnapol-null AcMNPV bacmid, creating Bac-GFP-AcΔPol-HA:NlPol^NLS. The recombinant virus could replicate and produce infectious virus in Sf9 cells, albeit at reduced levels compared to wild type AcMNPV. Taken together, our results suggested that the NLS deficiency of NeleNPV DNApol blocked viral DNA replication and production of infectious virus in dnapol-null AcMNPV bacmid.

Identification of a novel bipartite nuclear localization signal in the DNA polymerase of the betabaculovirus Pieris rapae granulovirus

DNA polymerase (DNApol) is highly conserved in all baculoviruses and plays an essential role in viral DNA replication. Previous results showed that the DNApol of the betabaculovirus Pieris rapae granulovirus (PiraGV) can localize in the nucleus. However, it is not clear how the DNApol is transported into the nucleus. Bioinformatic and GFP localization analysis showed that PiraGV DNApol contains a nuclear localization signal (NLS) at aa 4-25 (LFKRKLDEPPTDHTLVKAIKLS) of the N-terminus that does not match either the classical monopartite or the bipartite NLS consensus sequence. Multiple-point-substitution analysis confirmed that the NLS is required for transport of PiraGV DNApol into the nucleus. We also substituted the NLS of the PiraGV DNApol for that of the alphabaculovirus Spodoptera litura nuclear polyhedrosis virus (SpltNPV) DNApol. A viral growth curve and quantitative real-time PCR revealed that the substitution impaired viral DNA replication and resulted in a reduction in virus production. Together, our results show that PiraGV contains a novel NLS and that the NLS cannot efficiently replace that of SpltNPV DNApol for viral DNA synthesis and virus production.

Ac102 Participates in Nuclear Actin Polymerization by Modulating BV/ODV-C42 Ubiquitination during Autographa californica Multiple Nucleopolyhedrovirus Infection

As a virus-encoded actin nucleation promoting factor (NPF), P78/83 induces actin polymerization to assist in Autographa californica multiple nucleopolyhedrovirus (AcMNPV) propagation. According to our previous study, although P78/83 actively undergoes ubiquitin-independent proteasomal degradation, AcMNPV encodes budded virus/occlusion derived virus (BV/ODV)-C42 (C42), which allows P78/83 to function as a stable NPF by inhibiting its degradation during viral infection. However, whether there are other viral proteins involved in regulating P78/83-induced actin polymerization has yet to be determined. In this study, we found that Ac102, an essential viral gene product previously reported to play a key role in mediating the nuclear accumulation of actin during AcMNPV infection, is a novel regulator of P78/83-induced actin polymerization. By characterizing an ac102 knockout bacmid, we demonstrated that Ac102 participates in regulating nuclear actin polymerization as well as the morphogenesis and distribution of capsid structures in the nucleus. These regulatory effects are heavily dependent on an interaction between Ac102 and C42. Further investigation revealed that Ac102 binds to C42 to suppress K48-linked ubiquitination of C42, which decreases C42 proteasomal degradation and consequently allows P78/83 to function as a stable NPF to induce actin polymerization. Thus, Ac102 and C42 form a regulatory cascade to control viral NPF activity, representing a sophisticated mechanism for AcMNPV to orchestrate actin polymerization in both a ubiquitin-dependent and ubiquitin-independent manner.IMPORTANCE Actin is one of the most functionally important proteins in eukaryotic cells. Morphologically, actin can be found in two forms: a monomeric form called globular actin (G-actin) and a polymeric form called filamentous actin (F-actin). G-actin can polymerize to form F-actin, and nucleation promoting factor (NPF) is the initiator of this process. Many viral pathogens harness the host actin polymerization machinery to assist in virus propagation. Autographa californica multiple nucleopolyhedrovirus (AcMNPV) induces actin polymerization in host cells. P78/83, a viral NPF, is responsible for this process. Previously, we identified that BV/ODV-C42 (C42) binds to P78/83 and protects it from degradation. In this report, we determined that another viral protein, Ac102, is involved in modulating C42 ubiquitination and, consequently, ensures P78/83 activity as an NPF to initiate actin polymerization. This regulatory cascade represents a novel mechanism by which a virus can harness the cellular actin cytoskeleton to assist in viral propagation.

Baculovirus AC102 Is a Nucleocapsid Protein That Is Crucial for Nuclear Actin Polymerization and Nucleocapsid Morphogenesis

The baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV), the type species of alphabaculoviruses, is an enveloped DNA virus that infects lepidopteran insects and is commonly known as a vector for protein expression and cell transduction. AcMNPV belongs to a diverse group of viral and bacterial pathogens that target the host cell actin cytoskeleton during infection. AcMNPV is unusual, however, in that it absolutely requires actin translocation into the nucleus early in infection and actin polymerization within the nucleus late in infection coincident with viral replication. Of the six viral factors that are sufficient, when coexpressed, to induce the nuclear localization of actin, only AC102 is essential for viral replication and the nuclear accumulation of actin. We therefore sought to better understand the role of AC102 in actin mobilization in the nucleus early and late in infection. Although AC102 was proposed to function early in infection, we found that AC102 is predominantly expressed as a late protein. In addition, we observed that AC102 is required for F-actin assembly in the nucleus during late infection, as well as for proper formation of viral replication structures and nucleocapsid morphogenesis. Finally, we found that AC102 is a nucleocapsid protein and a newly recognized member of a complex consisting of the viral proteins EC27, C42, and the actin polymerization protein P78/83. Taken together, our findings suggest that AC102 is necessary for nucleocapsid morphogenesis and actin assembly during late infection through its role as a component of the P78/83-C42-EC27-AC102 protein complex.IMPORTANCE The baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is an important biotechnological tool for protein expression and cell transduction, and related nucleopolyhedroviruses are also used as environmentally benign insecticides. One impact of our work is to better understand the fundamental mechanisms through which AcMNPV exploits the cellular machinery of the host for replication, which may aid in the development of improved baculovirus-based research and industrial tools. Moreover, AcMNPV's ability to mobilize the host actin cytoskeleton within the cell's nucleus during infection makes it a powerful cell biological tool. It is becoming increasingly clear that actin plays important roles in the cell's nucleus, and yet the regulation and function of nuclear actin is poorly understood. Our work to better understand how AcMNPV relocalizes and polymerizes actin within the nucleus may reveal fundamental mechanisms that govern nuclear actin regulation and function, even in the absence of viral infection.

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

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

Autographa californica Nucleopolyhedrovirus AC141 (Exon0), a Potential E3 Ubiquitin Ligase, Interacts with Viral Ubiquitin and AC66 To Facilitate Nucleocapsid Egress

During the infection cycle of Autographa californica multiple nucleopolyhedrovirus (AcMNPV), two forms of virions are produced, budded virus (BV) and occlusion-derived virus (ODV). Nucleocapsids that form BV have to egress from the nucleus, whereas nucleocapsids that form ODV remain inside the nucleus. The molecular mechanism that determines whether nucleocapsids remain inside or egress from the nucleus is unknown. AC141 (a predicted E3 ubiquitin ligase) and viral ubiquitin (vUbi) have both been shown to be required for efficient BV production. In this study, it was hypothesized that vUbi interacts with AC141, and in addition, that this interaction was required for BV production. Deletion of both ac141 and vubi restricted viral infection to a single cell, and BV production was completely eliminated. AC141 was ubiquitinated by either vUbi or cellular Ubi, and this interaction was required for optimal BV production. Nucleocapsids in BV, but not ODV, were shown to be specifically ubiquitinated by vUbi, including a 100-kDa protein, as well as high-molecular-weight conjugates. The viral ubiquitinated 100-kDa BV-specific nucleocapsid protein was identified as AC66, which is known to be required for BV production and was shown by coimmunoprecipitation and mass spectrometry to interact with AC141. Confocal microscopy also showed that AC141, AC66, and vUbi interact at the nuclear periphery. These results suggest that ubiquitination of nucleocapsid proteins by vUbi functions as a signal to determine if a nucleocapsid will egress from the nucleus and form BV or remain in the nucleus to form ODV.IMPORTANCE Baculoviruses produce two types of virions called occlusion-derived virus (ODV) and budded virus (BV). ODVs are required for oral infection, whereas BV enables the systemic spread of virus to all host tissues, which is critical for killing insects. One of the important steps for BV production is the export of nucleocapsids out of the nucleus. This study investigated the molecular mechanisms that enable the selection of nucleocapsids for nuclear export instead of being retained within the nucleus, where they would become ODV. Our data show that ubiquitination, a universal cellular process, specifically tags nucleocapsids of BV, but not those found in ODV, using a virus-encoded ubiquitin (vUbi). Therefore, ubiquitination may be the molecular signal that determines if a nucleocapsid is destined to form a BV, thus ensuring lethal infection of the host.

The role of viral protein Ac34 in nuclear relocation of subunits of the actin-related protein 2/3 complex

The actin nucleator actin-related protein complex (Arp2/3) is composed of seven subunits: Arp2, Arp3, p40/ARPC1 (P40), p34/ARPC2 (P34), p21/ARPC3 (P21), p20/ARPC4 (P20), and p16/ARPC5 (P16). Arp2/3 plays crucial roles in a variety of cellular activities through regulation of actin polymerization. Autographa californica multiple nucleopolyhedrovirus (AcMNPV), one of the beststudied alphabaculoviruses, induces Arp2/3 nuclear relocation and mediates nuclear actin polymerization to assist in virus replication. We have demonstrated that Ac34, a viral late-gene product, induces translocation of the P40 subunit of Arp2/3 to the nucleus during AcMNPV infection. However, it remains unknown whether Ac34 could relocate other Arp2/3 subunits to the nucleus. In this study, the effects of the viral protein Ac34 on the distribution of these subunits were studied by an immunofluorescence assay. Arp2, P34, P21, and P20 cloned from Spodoptera frugiperda (Sf9) cells showed mainly cytoplasmic localization and were relocated to the nucleus in the presence of Ac34. In addition, Arp3 was localized in the cytoplasm in both the presence and absence of Ac34, and P16 showed whole-cell localization. In contrast to Sf9 cells, all subunits of mammalian Arp2/3 showed no nuclear relocation in the presence of Ac34. Co-immunoprecipitation analysis of the interaction between Ac34 and Arp2/3 subunits revealed that Ac34 bound to P40, P34, and P20 of Sf9 cells. However, none of the subunits of mammalian Arp2/3 interacted with Ac34, indicating that protein-protein interaction is essential for Ac34 to relocate Arp2/3 subunits to the nucleus.

The 91-205 amino acid region of AcMNPV ORF34 (Ac34), which comprises a potential C3H zinc finger, is required for its nuclear localization and optimal virus multiplication

During baculovirus infection, most viral proteins must be imported to the nucleus to support virus multiplication. Autographa californica multiple nucleopolyhedrovirus (AcMNPV) orf34 (ac34) is an alphabaculovirus unique gene that is required for optimal virus production. Ac34 distributes in both the cytoplasm and the nuclei of virus-infected Sf9 cells, but contains no conventional nuclear localization signal (NLS). In this study, we investigated the nuclear targeting domains in Ac34. Transient expression assays showed that Ac34 localized in both the cytoplasm and the nuclei of Sf9 cells, indicating that no viral protein is required for Ac34 nuclear localization. Subcellular localization analysis of Ac34 truncations and internal deletions fused with green fluorescent protein in plasmid-transfected Sf9 cells identified that the 91-205 amino acid (aa) region is required for Ac34 nuclear localization. Mutations in a potential C3H zinc finger (aa 116-131) in Ac34 resulted in exclusive cytoplasmic distribution of GFP:Ac34, suggesting that the zinc finger is required for Ac34 nuclear localization. To assess the functional importance of Ac34 in the nucleus during virus replication, recombinant AcMNPV bacmids containing a series of Ac34 truncations, internal deletions, or site mutations fused with HA tags were constructed. Subcellular localization analysis showed that Ac34 with internal deletions in aa 91-205 or site mutations in the potential zinc finger was predominantly distributed in the cytoplasm. Viral plaque assays and virus growth curves indicated that disruption of Ac34 nuclear localization significantly impaired virus replication. Taken together, our findings demonstrated that the nuclear localization of Ac34 requires the 91-205 aa region and its nuclear localization is essential for optimal virus replication.

Autographa californica Multiple Nucleopolyhedrovirus Ac34 Protein Retains Cellular Actin-Related Protein 2/3 Complex in the Nucleus by Subversion of CRM1-Dependent Nuclear Export

Actin, nucleation-promoting factors (NPFs), and the actin-related protein 2/3 complex (Arp2/3) are key elements of the cellular actin polymerization machinery. With nuclear actin polymerization implicated in ever-expanding biological processes and the discovery of the nuclear import mechanisms of actin and NPFs, determining Arp2/3 nucleo-cytoplasmic shuttling mechanism is important for understanding the function of nuclear actin. A unique feature of alphabaculovirus infection of insect cells is the robust nuclear accumulation of Arp2/3, which induces actin polymerization in the nucleus to assist in virus replication. We found that Ac34, a viral late gene product encoded by the alphabaculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV), is involved in Arp2/3 nuclear accumulation during virus infection. Further assays revealed that the subcellular distribution of Arp2/3 under steady-state conditions is controlled by chromosomal maintenance 1 (CRM1)-dependent nuclear export. Upon AcMNPV infection, Ac34 inhibits CRM1 pathway and leads to Arp2/3 retention in the nucleus.

SfP53 and filamentous actin (F-actin) are the targets of viral pesticide AcMNPV-BmK IT (P10/PH) in host Spodoptera frugiperda 9 cells

OBJECTIVES

To analyze the anti-insect mechanism of viral pesticide AcMNPV-BmK IT(P10/PH) in the host Spodoptera frugiperda 9 (Sf9) cells.

RESULTS

Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV)- mediated expression of BmK IT, regulated by P10 protein promoter (P10) and polyhedrosis promoter (PH), promoted the replication of progeny virus in host Sf9 cells. AcMNPV-BmK IT(P10) could accelerate the budding process (or speed) of budded virus (BV) in Sf9 cells. The impact of AcMNPV-BmK IT(P10) on the nuclear polymerization of filamentous actin (F-actin) participated in regulating the accelerated budding process. Unexpectedly, both AcMNPV-BmK IT(P10) and AcMNPV-BmK IT(PH) delayed the nuclear polymerization of F-actin and promoted the clearance of F-actin in the nucleus. SfP53, an important apoptosis factor, was involved in the regulation of AcMNPV-BmK IT(P10/PH) in Sf9 cells. AcMNPV-BmK IT(P10/PH) could also delay and promote the nuclear recruitment of SfP53 after 27 h post infection (h p.i.).

CONCLUSION

SfP53 and F-actin are the targets of viral pesticide AcMNPV-BmK IT (P10/PH) in host Sf9 cells, which provides the experimental basis for the development of recombinant baculovirus biopesticides.

Rescue of dnapol-null Autographa californica multiple nucleopolyhedrovirus with DNA polymerase (DNApol) of Spodoptera litura nucleopolyhedrovirus (SpltNPV) and identification of a nuclear localization signal in SpltNPV DNApol

DNA polymerase (DNApol) is highly conserved in all baculoviruses and plays an essential role in viral DNA replication. It determines the fidelity of baculovirus DNA replication by inserting the correct nucleotides into the primer terminus and proofreading any mispaired nucleotides. DNApols of groups I and II of the genus Alphabaculovirus in the family Baculoviridae share many common structural features. However, it is not clear whether a group I Autographa californica multiple nucleopolyhedrovirus (AcMNPV) DNApol can be substituted by a group II NPV DNApol. Here we report the successful generation of AcMNPV dnapol-null virus being rescued by a group II Spodoptera litura NPV (SpltNPV) dnapol (Bac-AcΔPol : Slpol). Viral growth curves and quantitative real-time PCR showed that the dnapol replacement reduced the level of viral production and DNA replication of Bac-AcΔPol : SlPol compared with WTrep, a native dnapol insertion in an AcMNPV dnapol-null virus. Light microscopy showed that production of occlusion bodies for Bac-AcΔPol : Slpol was reduced. We also identified a nuclear localization signal (NLS) for the SpltNPV DNApol C terminus at residues 827-838 by mutational analysis and confocal microscopy. Multiple point substitution of SpltNPV DNApol NLS abrogated virus production and viral DNA replication. Overall, these data suggested that the NLS plays an important role in SpltNPV DNApol nuclear localization and that SpltNPV DNApol cannot efficiently substitute the AcMNPV DNApol in AcMNPV.

The Autographa californica Multiple Nucleopolyhedrovirus ac54 Gene Is Crucial for Localization of the Major Capsid Protein VP39 at the Site of Nucleocapsid Assembly

Baculovirus DNAs are synthesized and inserted into preformed capsids to form nucleocapsids at a site in the infected cell nucleus, termed the virogenic stroma. Nucleocapsid assembly ofAutographa californicamultiple nucleopolyhedrovirus (AcMNPV) requires the major capsid protein VP39 and nine minor capsid proteins, including VP1054. However, how VP1054 participates in nucleocapsid assembly remains elusive. In this study, the VP1054-encoding gene (ac54) was deleted to generate theac54-knockout AcMNPV (vAc54KO). In vAc54KO-transfected cells, nucleocapsid assembly was disrupted, leading to the formation of abnormally elongated capsid structures. Interestingly, unlike cells transfected with AcMNPV mutants lacking other minor capsid proteins, in which capsid structures were distributed within the virogenic stroma,ac54ablation resulted in a distinctive location of capsid structures and VP39 at the periphery of the nucleus. The altered distribution pattern of capsid structures was also observed in cells transfected with AcMNPV lacking BV/ODV-C42 or in cytochalasind-treated AcMNPV-infected cells. BV/ODV-C42, along with PP78/83, has been shown to promote nuclear filamentous actin (F-actin) formation, which is another requisite for nucleocapsid assembly. Immunofluorescence using phalloidin indicated that the formation and distribution of nuclear F-actin were not affected byac54deletion. However, immunoelectron microscopy revealed that BV/ODV-C42, PP78/83, and 38K failed to integrate into capsid structures in the absence of VP1054, and immunoprecipitation further demonstrated that in transient expression assays, VP1054 interacted with BV/ODV-C42 and VP80 but not VP39. Our findings suggest that VP1054 plays an important role in the transport of capsid proteins to the nucleocapsid assembly site prior to the process of nucleocapsid assembly.

IMPORTANCE

Baculoviruses are large DNA viruses whose replication occurs within the host nucleus. The localization of capsids into the capsid assembly site requires virus-induced nuclear F-actin; the inhibition of nuclear F-actin formation results in the retention of capsid structures at the periphery of the nucleus. In this paper, we note that the minor capsid protein VP1054 is essential for the localization of capsid structures, the major capsid protein VP39, and the minor capsid protein 38K into the capsid assembly site. Moreover, VP1054 is crucial for correct targeting of the nuclear F-actin factors BV/ODV-C42 and PP78/83 for capsid maturation. However, the formation and distribution of nuclear F-actin are not affected by the lack of VP1054. We further reveal that VP1054 interacts with BV/ODV-C42 and a capsid transport-related protein, VP80. Taken together, our findings suggest that VP1054 plays a unique role in the pathway(s) for transport of capsid proteins.

Nuclear Translocation Sequence and Region in Autographa californica Multiple Nucleopolyhedrovirus ME53 That Are Important for Optimal Baculovirus Production

Autographa californicamultiple nucleopolyhedrovirus (AcMNPV) is in the familyBaculoviridae, genusAlphabaculovirus AcMNPVme53is a highly conserved immediate early gene in all lepidopteran baculoviruses that have been sequenced and is transcribed up to late times postinfection. Althoughme53is not essential for viral DNA synthesis, infectious budded virus (BV) production is greatly attenuated when it is deleted. ME53 associates with the nucleocapsid on both budded virus and occlusion-derived virus, but not with the virus envelope. ME53 colocalizes in plasma membrane foci with the envelope glycoprotein GP64 in a GP64-dependent manner. ME53 localizes in the cytoplasm early postinfection, and despite the lack of a reported nuclear localization signal (NLS), ME53 translocates to the nucleus at late times postinfection. To map determinants of ME53 that facilitate its nuclear translocation, recombinant AcMNPV bacmids containing a series of ME53 truncations, internal deletions, and peptides fused with hemagglutinin (HA) or green fluorescent protein (GFP) tags were constructed. Intracellular-localization studies identified residues within amino acids 109 to 137 at the N terminus of ME53 that acted as the nuclear translocation sequence (NTS), facilitating its nuclear transport at late times postinfection. The first 100 N-terminal amino acids and the last 50 C-terminal amino acids of ME53 are dispensable for high levels of budded virus production. The region within amino acids 101 to 398, which also contains the NTS, is critical for optimal levels of budded virus production.

IMPORTANCE

Baculovirusme53is a conserved immediate early gene found in all sequenced lepidopteran alpha- and betabaculoviruses. We first identified residues within amino acids 109 to 137 at the N terminus that act as the ME53 nuclear translocation sequence (NTS) to facilitate its nuclear translocation and defined an internal region within amino acids 101 to 398, which includes the NTS, as being necessary for optimal budded virus production. Altogether, these results indicate a previously unidentified nuclear role that ME53 plays in virus replication.

Posttranslational Modifications of Baculovirus Protamine-Like Protein P6.9 and the Significance of Its Hyperphosphorylation for Viral Very Late Gene Hyperexpression

Many viruses utilize viral or cellular chromatin machinery for efficient infection. Baculoviruses encode a conserved protamine-like protein, P6.9. This protein plays essential roles in various viral physiological processes during infection. However, the mechanism by which P6.9 regulates transcription remains unknown. In this study, 7 phosphorylated species of P6.9 were resolved in Sf9 cells infected with the baculovirus type species Autographa californica multiple nucleopolyhedrovirus (AcMNPV). Mass spectrometry identified 22 phosphorylation and 10 methylation sites but no acetylation sites in P6.9. Immunofluorescence demonstrated that the P6.9 and virus-encoded serine/threonine kinase PK1 exhibited similar distribution patterns in infected cells, and coimmunoprecipitation confirmed the interaction between them. Upon pk1 deletion, nucleocapsid assembly and polyhedron formation were interrupted and the transcription of viral very late genes was downregulated. Interestingly, we found that the 3 most phosphorylated P6.9 species vanished from Sf9 cells transfected with the pk1 deletion mutant, suggesting that PK1 is involved in the hyperphosphorylation of P6.9. Mass spectrometry suggested that the phosphorylation of the 7 Ser/Thr and 5 Arg residues in P6.9 was PK1 dependent. Replacement of the 7 Ser/Thr residues with Ala resulted in a P6.9 phosphorylation pattern similar to that of the pk1 deletion mutant. Importantly, the decreases in the transcription level of viral very late genes and viral infectivity were consistent. Our findings reveal that P6.9 hyperphosphorylation is a precondition for the maximal hyperexpression of baculovirus very late genes and provide the first experimental insights into the function of the baculovirus protamine-like protein and the related protein kinase in epigenetics.

IMPORTANCE

Diverse posttranslational modifications (PTMs) of histones constitute a code that creates binding platforms that recruit transcription factors to regulate gene expression. Many viruses also utilize host- or virus-induced chromatin machinery to promote efficient infections. Baculoviruses encode a protamine-like protein, P6.9, which is required for a variety of processes in the infection cycle. Currently, P6.9's PTM sites and its regulating factors remain unknown. Here, we found that P6.9 could be categorized as unphosphorylated, hypophosphorylated, and hyperphosphorylated species and that a virus-encoded serine/threonine kinase, PK1, was essential for P6.9 hyperphosphorylation. Abundant PTM sites on P6.9 were identified, among which 7 Ser/Thr phosphorylated sites were PK1 dependent. Mutation of these Ser/Thr sites reduced very late viral gene transcription and viral infectivity, indicating that the PK1-mediated P6.9 hyperphosphorylation contributes to viral proliferation. These data suggest that a code exists in the sophisticated PTM of viral protamine-like proteins and participates in viral gene transcription.

Effects of recombinant baculovirus AcMNPV-BmK IT on the formation of early cables and nuclear polymerization of actin in Sf9 cells

Autographa californica nuclearpoly hedrosis virus (AcMNPV) is one of the most important baculoviridae. However, the application of AcMNPV as a biocontrol agent has been limited. Previously, we engineered Buthus martensii Karsch insect toxin (BmK IT) gene into the genome of AcMNPV. The bioassay data indicated that the recombinant baculovirus AcMNPV-BmK IT significantly enhanced the anti-insect efficacy of the virus. The actin cytoskeleton is the major component beneath the surface of eukaryotic cells. In this report, the effects of AcMNPV-BmK IT on the formation of early cables of actin and nuclear filamentous-actin (F-actin) were studied. The results indicated that these baculovirus induced rearrangement of the actin cytoskeleton of host cells during infection and actin might participate in the transportation of baculovirus from cytoplasm to the nuclei. AcMNPV-BmK IT delayed the formation of early cables of actin and nuclear F-actin and accelerated the clearance of actin in the nuclei.

Identification of a novel regulatory sequence of actin nucleation promoting factor encoded by Autographa californica multiple nucleopolyhedrovirus

Actin polymerization induced by nucleation promoting factors (NPFs) is one of the most fundamental biological processes in eukaryotic cells. NPFs contain a conserved output domain (VCA domain) near the C terminus, which interacts with and activates the cellular actin-related protein 2/3 complex (Arp2/3) to induce actin polymerization and a diverse regulatory domain near the N terminus. Autographa californica multiple nucleopolyhedrovirus (AcMNPV) nucleocapsid protein P78/83 is a virus-encoded NPF that contains a C-terminal VCA domain and induces actin polymerization in virus-infected cells. However, there is no similarity between the N terminus of P78/83 and that of other identified NPFs, suggesting that P78/83 may possess a unique regulatory mechanism. In this study, we identified a multifunctional regulatory sequence (MRS) located near the N terminus of P78/83 and determined that one of its functions is to serve as a degron to mediate P78/83 degradation in a proteasome-dependent manner. In AcMNPV-infected cells, the MRS also binds to another nucleocapsid protein, BV/ODV-C42, which stabilizes P78/83 and modulates the P78/83-Arp2/3 interaction to orchestrate actin polymerization. In addition, the MRS is also essential for the incorporation of P78/83 into the nucleocapsid, ensuring virion mobility powered by P78/83-induced actin polymerization. The triple functions of the MRS enable P78/83 to serve as an essential viral protein in the AcMNPV replication cycle, and the possible roles of the MRS in orchestrating the virus-induced actin polymerization and viral genome decapsidation are discussed.

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.

Autographa californica multiple nucleopolyhedrovirus DNA polymerase C terminus is required for nuclear localization and viral DNA replication

The DNA polymerase (DNApol) of the baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is essential for viral DNA replication. The DNApol exonuclease and polymerase domains are highly conserved and are considered functional in DNA replication. However, the role of the DNApol C terminus has not yet been characterized. To identify whether only the exonuclease and polymerase domains are sufficient for viral DNA replication, several DNApol C-terminal truncations were cloned into a dnapol-null AcMNPV bacmid with a green fluorescent protein (GFP) reporter. Surprisingly, most of the truncation constructs, despite containing both exonuclease and polymerase domains, could not rescue viral DNA replication and viral production in bacmid-transfected Sf21 cells. Moreover, GFP fusions of these same truncations failed to localize to the nucleus. Truncation of the C-terminal amino acids 950 to 984 showed nuclear localization but allowed for only limited and delayed viral spread. The C terminus contains a typical bipartite nuclear localization signal (NLS) motif at residues 804 to 827 and a monopartite NLS motif at residues 939 to 948. Each NLS, as a GFP fusion peptide, localized to the nucleus, but both NLSs were required for nuclear localization of DNApol. Alanine substitutions in a highly conserved baculovirus DNApol sequence at AcMNPV DNApol amino acids 972 to 981 demonstrated its importance for virus production and DNA replication. Collectively, the data indicated that the C terminus of AcMNPV DNApol contains two NLSs and a conserved motif, all of which are required for nuclear localization of DNApol, viral DNA synthesis, and virus production.

IMPORTANCE

The baculovirus DNA polymerase (DNApol) is a highly specific polymerase that allows viral DNA synthesis and hence virus replication in infected insect cells. We demonstrated that the exonuclease and polymerase domains of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) alone are insufficient for viral DNA synthesis and virus replication. Rather, we identified three features, including two nuclear localization signals and a highly conserved 10-amino-acid sequence in the AcMNPV DNApol C terminus, all three of which are important for both nuclear localization of DNApol and for DNApol activity, as measured by viral DNA synthesis and virus replication.

Toward system-level understanding of baculovirus-host cell interactions: from molecular fundamental studies to large-scale proteomics approaches

Baculoviruses are insect viruses extensively exploited as eukaryotic protein expression vectors. Molecular biology studies have provided exciting discoveries on virus-host interactions, but the application of omic high-throughput techniques on the baculovirus-insect cell system has been hampered by the lack of host genome sequencing. While a broader, systems-level analysis of biological responses to infection is urgently needed, recent advances on proteomic studies have yielded new insights on the impact of infection on the host cell. These works are reviewed and critically assessed in the light of current biological knowledge of the molecular biology of baculoviruses and insect cells.

AcMNPV core gene ac109 is required for budded virion transport to the nucleus and for occlusion of viral progeny

The Autographa californica multiple nucleopolyhedrovirus (AcMNPV) ac109 core gene has been previously characterized as an essential late gene. Our results showed that budded virions could be detected in supernatants of infected Sf-9 cells, even when ac109 knockout viruses displayed a single-cell infection phenotype. Moreover, confocal microscopy analysis revealed that budded virions can enter the cytoplasm but are unable to enter the cell nucleus. This defect could be repaired by complementing ac109 in trans. In addition, polyhedra of normal size could be detected in Sf-9 nuclei infected with ac109 knockout viruses. However, electron microscopy demonstrated that these occlusion bodies were empty. Altogether, these results indicate that ac109 is required for infectivity of both phenotypes of virus.

The ac53, ac78, ac101, and ac103 genes are newly discovered core genes in the family Baculoviridae

The family Baculoviridae is a large group of insect viruses containing circular double-stranded DNA genomes of 80 to 180 kbp, which have broad biotechnological applications. A key feature to understand and manipulate them is the recognition of orthology. However, the differences in gene contents and evolutionary distances among the known members of this family make it difficult to assign sequence orthology. In this study, the genome sequences of 58 baculoviruses were analyzed, with the aim to detect previously undescribed core genes because of their remote homology. A routine based on Multi PSI-Blast/tBlastN and Multi HaMStR allowed us to detect 31 of 33 accepted core genes and 4 orthologous sequences in the Baculoviridae which were not described previously. Our results show that the ac53, ac78, ac101 (p40), and ac103 (p48) genes have orthologs in all genomes and should be considered core genes. Accordingly, there are 37 orthologous genes in the family Baculoviridae.

Nuclear localization of actin requires AC102 in Autographa californica multiple nucleopolyhedrovirus-infected cells

Autographa californica multiple nucleopolyhedrovirus requires nuclear actin for progeny virus production and thereby encodes viral products that ensure actin's translocation to and retention within the nucleus. Current evidence suggests that the ie0-ie1 gene complex along with five nuclear localization of actin (NLA) genes are sufficient for NLA in transient transfection experiments. Here we report that, during infection, only one of the five NLA genes, Ac102, was essential for NLA, and that AC102 had at least one other activity critical for budded virus (BV) production. Viral deletion mutants in the other four NLA genes were viable, with only two having replication phenotypes different from that of the wild type. Infection with AcΔpe38 revealed a delay in both BV production and NLA. Infection with AcΔ152 revealed a delay in BV production, but no corresponding delay in NLA. Infection with either AcΔpe38 or AcΔ152 resulted in slightly reduced BV titres. Deletion of Ac004 or he65 had no impact on actin translocation kinetics, timing of BV production or BV titres. These results implicate AC102 as a key player in baculovirus manipulation of actin.

Helicoverpa armaigera nucleopolyhedrovirus ORF50 is an early gene not essential for virus propagation in vitro and in vivo

Homologs of Helicoverpa armigera nucleopolyhedrovirus ORF50 (HA50) are found in most alphabaculoviruses, but their functions remain unknown. Here, we characterized whether Ha50 is indispensable for virus progration. Ha50 transcript was first detected at 3 h post-infection from HearNPV-infected HzAM1 cells. 3'RACE analysis showed that Ha50 transcript was polyadenlylated. 5'RACE analysis revealed two transcription initiation sites, one of which was mapped to the canonical baculovirus early transcription initiator motif CAGT. HA50 protein could be detected from infected cells harvested at 12 h post-infection. Transient expression assays showed that GFP-fused HA50 localized in the cytoplasm and nucleus of HzAM1 cells with or without superinfection. To further examine the role of Ha50 in the virus life cycle, a Ha50 knockout bacmid and a repair bacmid carrying Ha50 under the control of its native promoter elements were constructed using bacmid technology. One-step growth curve analysis showed that the kinetics of infectious budded virus production of Ha50 knockout virus was similar to that of the parental virus or the repair virus. Analysis of the expression of viral early protein IE-1, late protein VP39 and very late protein suggested that viral protein expression was not affected by Ha50 inactivation. Electron microscopy revealed that HaBacΔ50-PH-G occluded viruses (ODVs) and occlusion bodies were indistinguishable from those of the wild-type virus. Similarly, bioassays showed no significant difference in the LC(50) values between Ha50 deletion virus and wild-type virus. Our results together demonstrate that Ha50 is an early gene dispensable for virus propagation in vitro and in vivo.

Nuclear actin and lamins in viral infections

Lamins are the best characterized cytoskeletal components of the cell nucleus that help to maintain the nuclear shape and participate in diverse nuclear processes including replication or transcription. Nuclear actin is now widely accepted to be another cytoskeletal protein present in the nucleus that fulfills important functions in the gene expression. Some viruses replicating in the nucleus evolved the ability to interact with and probably utilize nuclear actin for their replication, e.g., for the assembly and transport of capsids or mRNA export. On the other hand, lamins play a role in the propagation of other viruses since nuclear lamina may represent a barrier for virions entering or escaping the nucleus. This review will summarize the current knowledge about the roles of nuclear actin and lamins in viral infections.

The protamine-like DNA-binding protein P6.9 epigenetically up-regulates Autographa californica multiple nucleopolyhedrovirus gene transcription in the late infection phase

Protamines are a group of highly basic proteins first discovered in spermatozoon that allow for denser packaging of DNA than histones and will result in down-regulation of gene transcription[1]. It is well recognized that the Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) encodes P6.9, a protamine-like protein that forms the viral subnucleosome through binding to the viral genome[29]. Previous research demonstrates that P6.9 is essential for viral nucleocapsid assembly, while it has no influence on viral genome replication[31]. In the present study, the role of P6.9 in viral gene transcription regulation is characterized. In contrast to protamines or other protamine-like proteins that usually down-regulate gene transcription, P6.9 appears to up-regulate viral gene transcription at 12-24 hours post infection (hpi), whereas it is non-essential for the basal level of viral gene transcription. Fluorescence microscopy reveals the P6.9's co-localization with DNA is temporally and spatially synchronized with P6.9's impact on viral gene transcription, indicating the P6.9-DNA association contributes to transcription regulation. Chromatin fractionation assay further reveals an unexpected co-existence of P6.9 and host RNA polymerase II in the same transcriptionally active chromatin fraction at 24 hpi, which may probably contribute to viral gene transcription up-regulation in the late infection phase.

Putative phosphorylation sites on WCA domain of HA2 is essential for Helicoverpa armigera single nucleopolyhedrovirus replication

Protein phosphorylation is one of the most common post-translational modification processes that play an essential role in regulating protein functionality. The Helicoverpa armigera single nucleopolyhedrovirus (HearNPV) orf2-encoded nucleocapsid protein HA2 participates in orchestration of virus-induced actin polymerization through its WCA domain, in which phosphorylation status are supposed to be critical in respect to actin polymerization. In the present study, two putative phosphorylation sites ((232)Thr and (250)Ser) and a highly conserved Serine ((245)Ser) on the WCA domain of HA2 were mutated, and their phenotypes were characterized by reintroducing the mutated HA2 into the HearNPV genome. Viral infectivity assays demonstrated that only the recombinant HearNPV bearing HA2 mutation at (245)Ser can produce infectious virions, both (232)Thr and (250)Ser mutations were lethal to the virus. However, actin polymerization assay demonstrated that all the three viruses bearing HA2 mutations were still capable of initiating actin polymerization in the host nucleus, which indicated the putative phosphorylation sites on HA2 may contribute to HearNPV replication through another unidentified pathway.

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.

The putative pocket protein binding site of Autographa californica nucleopolyhedrovirus BV/ODV-C42 is required for virus-induced nuclear actin polymerization

Nuclear filamentous actin (F-actin) is essential for nucleocapsid morphogenesis of lepidopteran nucleopolyhedroviruses. Previously, we had demonstrated that Autographa californica multiple nucleopolyhedrovirus (AcMNPV) BV/ODV-C42 (C42) is involved in nuclear actin polymerization by recruiting P78/83, an AcMNPV orf9-encoded N-WASP homology protein that is capable of activating an actin-related-protein 2/3 (Arp2/3) complex to initiate actin polymerization, to the nucleus. To further investigate the role of C42 in virus-induced actin polymerization, the recombinant bacmid vAc(p78/83nls-gfp), with a c42 knockout, p78/83 tagged with a nuclear localization signal coding sequence, and egfp as a reporter gene under the control of the Pp10 promoter, was constructed and transfected to Sf9 cells. In the nuclei of vAc(p78/83nls-gfp)-transfected cells, polymerized F-actin filaments were absent, whereas other actin polymerization elements (i.e., P78/83, G-actin, and Arp2/3 complex) were present. This in vivo evidence indicated that C42 actively participates in the nuclear actin polymerization process as a key element, besides its role in recruiting P78/83 to the nucleus. In order to collect in vitro evidence for the participation of C42 in actin polymerization, an anti-C42 antibody was used to neutralize the viral nucleocapsid, which is capable of initiating actin polymerization in vitro. Both the kinetics of pyrene-actin polymerization and F-actin-specific staining by phalloidin indicated that anti-C42 can significantly attenuate the efficiency of F-actin formation compared to that with control antibodies. Furthermore, we have identified the putative pocket protein binding sequence (PPBS) on C42 that is essential for C42 to exert its function in nuclear actin polymerization.

Proteomics of the Autographa californica nucleopolyhedrovirus budded virions

Baculoviruses produce two progeny phenotypes during their replication cycles. The occlusion-derived virus (ODV) is responsible for initiating primary infection in the larval midgut, and the budded virus (BV) phenotype is responsible for the secondary infection. The proteomics of several baculovirus ODVs have been revealed, but so far, no extensive analysis of BV-associated proteins has been conducted. In this study, the protein composition of the BV of Autographa californica nucleopolyhedrovirus (AcMNPV), the type species of baculoviruses, was analyzed by various mass spectrometry (MS) techniques, including liquid chromatography-triple quadrupole linear ion trap (LC-Qtrap), liquid chromatography-quadrupole time of flight (LC-Q-TOF), and matrix-assisted laser desorption ionization-time of flight (MALDI-TOF). SDS-PAGE and MALDI-TOF analyses showed that the three most abundant proteins of the AcMNPV BV were GP64, VP39, and P6.9. A total of 34 viral proteins associated with the AcMNPV BV were identified by the indicated methods. Thirteen of these proteins, PP31, AC58/59, AC66, IAP-2, AC73, AC74, AC114, AC124, chitinase, polyhedron envelope protein (PEP), AC132, ODV-E18, and ODV-E56, were identified for the first time to be BV-associated proteins. Western blot analyses showed that ODV-E18 and ODV-E25, which were previously thought to be ODV-specific proteins, were also present in the envelop fraction of BV. In addition, 11 cellular proteins were found to be associated with the AcMNPV BV by both LC-Qtrap and LC-Q-TOF analyses. Interestingly, seven of these proteins were also identified in other enveloped viruses, suggesting that many enveloped viruses may commonly utilize certain conserved cellular pathways.

Identification of protein-protein interactions of the occlusion-derived virus-associated proteins of Helicoverpa armigera nucleopolyhedrovirus

The purpose of this study was to identify protein-protein interactions among the components of the occlusion-derived virus (ODV) of Helicoverpa armigera nucleopolyhedrovirus (HearNPV), a group II alphabaculovirus in the family Baculoviridae. To achieve this, 39 selected genes of potential ODV structural proteins were cloned and expressed in the Gal4 yeast two-hybrid (Y2H) system. The direct-cross Y2H assays identified 22 interactions comprising 13 binary interactions [HA9-ODV-EC43, ODV-E56-38K, ODV-E56-PIF3, LEF3-helicase, LEF3-alkaline nuclease (AN), GP41-38K, GP41-HA90, 38K-PIF3, 38K-PIF2, VP80-HA100, ODV-E66-PIF3, ODV-E66-PIF2 and PIF3-PIF2] and nine self-associations (IE1, HA44, LEF3, HA66, GP41, CG30, 38K, PIF3 and P24). Five of these interactions - LEF3-helicase and LEF3-AN, and the self-associations of IE1, LEF3 and 38K - have been reported previously in Autographa californica multiple nucleopolyhedrovirus. As HA44 and HA100 were two newly identified ODV proteins of group II viruses, their interactions were further confirmed. The self-association of HA44 was verified with a His pull-down assay and the interaction of VP80-HA100 was confirmed by a co-immunoprecipitation assay. A summary of the protein-protein interactions of baculoviruses reported so far, comprising 68 interactions with 45 viral proteins and five host proteins, is presented, which will facilitate our understanding of the molecular mechanisms of baculovirus infection.