IE1 and hr facilitate the localization of Bombyx mori nucleopolyhedrovirus ORF8 to specific nuclear sites

Inhibitory role of the Bm8 protein in the propagation of Bombyx mori nucleopolyhedrovirus

Lepidopteran nucleopolyhedroviruses have distinct viral tissue tropisms in host larvae. We previously identified the Bm8 gene of Bombyx mori nucleopolyhedrovirus (BmNPV), the product of which inhibits viral propagation in the middle silk gland (MSG). However, it is unknown whether this inhibitory function of the Bm8 protein is specific to MSGs. Here we generated a Bm8-disrupted recombinant BmNPV expressing green fluorescent protein (GFP) and examined viral propagation in B. mori cultured cells and larvae. We found that Bm8-disrupted BmNPV produced fewer budded viruses and more occlusion bodies (OBs) than the wild-type virus in both cultured cells and larvae. Microscopic observation of OB production and GFP expression revealed that Bm8 disruption accelerated the progression of viral infection in various larval tissues. Furthermore, quantitative reverse transcription-polymerase chain reaction experiments showed that the loss of Bm8 enhanced viral gene expression in BmNPV-infected larval tissues. These results indicate that the Bm8 protein suppresses viral propagation to varying degrees in each larval tissue, which may establish BmNPV tissue tropisms in B. mori larvae.

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.

Characterization of the open reading frame 7a from Bombyx mori nucleopolyhedrovirus

The open reading frame 7a of the Bombyx mori nucleopolyhedrovirus (BmNPV orf7a, Bm7a) encodes a predicted polypeptide consisting of 53 amino acid residues. While this polypeptide's homologues were found in certain baculoviruses, its function has not yet been studied. A protein structural assay revealed that the polypeptide expressed from this gene contained a predicted signal peptide at its N-terminus. To investigate the expression of Bm7a, 3'rapid amplification of cDNA ends was used to detect its transcript. A series of recombinant viruses were also constructed to analyze the cellular localization of the Bm7a products. The results indicated that the Bm7a gene is co-transcribed with bv/odv-e26 and orf9. The function of BM7a was studied by using fused enhanced green fluorescence protein (eGFP) as a visual marker to trace the localization of the protein during infection. Fluorescence was observed in the cellular membrane of the infected cells by confocal microscopy. Western blot analysis using an eGFP-antibody also showed that the BM7a-eGFP fusion protein was present in the budded virions (BVs). These data implied that BM7a is a component of the BV. In this work, we identified the function of the Bm7a gene and suggested that it may play a role in BV packaging.

Baculovirus-encoded protein BV/ODV-E26 determines tissue tropism and virulence in lepidopteran insects

Lepidopteran nucleopolyhedroviruses (NPVs) show distinct tissue tropism in host insect larvae. However, the molecular mechanism of this tropism is largely unknown. We quantitatively investigated NPV tissue tropism by measuring mRNA levels of viral genes in 16 tissues from Bombyx mori NPV (BmNPV)-infected B. mori larvae and found clear tissue tropism, i.e., BmNPV replicates poorly in the silk glands, midgut, and Malpighian tubule compared with other larval tissues. We next identified the viral genes determining tissue tropism in NPV infection by investigating the phenotypes of larvae infected with 44 BmNPV mutants in which one gene was functionally disrupted by a LacZ cassette insertion. We found that occlusion body (OB) production was markedly enhanced compared with that of the wild type in the middle silk glands (MSGs) of larvae infected with three mutants in which one of three tandemly arrayed genes (Bm7, Bm8, and Bm9) was disrupted. We generated additional mutants in which one or two genes of this gene cluster were partially deleted and showed that Bm8, also known as BV/ODV-E26, was solely required for the suppression of OB production in the MSGs of BmNPV-infected B. mori larvae. Western blotting showed that a LacZ cassette insertion in Bm7 or Bm9 resulted in aberrant expression of Bm8, presumably leading to abnormal OB production in the MSGs. Larval bioassays also revealed that disruption of Bm8 accelerated the death of B. mori larvae. These results suggest that the group I NPV-specific protein BV/ODV-E26 determines tissue tropism and virulence in host lepidopteran insects.

Deletion of AcMNPV AC16 and AC17 results in delayed viral gene expression in budded virus infected cells but not transfected cells

This study investigated the combined function of the Autographa californica multiple nucleopolyhedrovirus overlapping genes ac16 (BV/ODV-E26, DA26) and ac17. Ac17 is a late gene and the promoter is within the ac16 open reading frame. A double ac16-ac17 knockout virus was generated to assess the function of each gene independently or together. Loss of ac17 did not affect viral DNA synthesis but budded virus (BV) production was reduced. Deletion of both ac16-ac17 resulted in reduced viral DNA synthesis and a further reduction in BV production. In BV infected Sf9 cells, viral gene expression was delayed up to 12 h in the absence of both AC16 and AC17 but not if either gene was present. Cells infected by transfecting viral DNA, by-passing the BV particle, exhibited no delay in gene expression from the double knockout virus. AC16 and AC17 are therefore required for rapid viral gene expression in cells infected by BV.

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.

AcMNPV AC16 (DA26, BV/ODV-E26) regulates the levels of IE0 and IE1 and binds to both proteins via a domain located within the acidic transcriptional activation domain

IE0 and IE1 are the primary viral regulatory proteins of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) involved in the transactivation of early genes, stimulation of late gene expression, and viral DNA replication. The protein interactions required for IE0 or IE1 to achieve these varied roles are not well defined, so to identify proteins that interact with IE0 and IE1, tandem affinity purification (TAP) and LC-MS/MS was used. Analysis of purified proteins identified AC16 (DA26, BV/ODV-E26) from TAP tagged IE0 virus infected Sf9 cells. Co-immunoprecipitation confirmed that AC16 interacts with both IE0 and IE1 and yeast 2-hybrid analysis mapped the domain required for interaction with AC16. Mutation of the AC16 binding domain enhanced BV production by viruses expressing only IE0 but had no effect if only IE1 is expressed. An ac16 deletion virus was constructed and was shown not to affect the temporal expression of IE0 and IE1; however the relative level of IE0 to IE1 was significantly increased.

Bombyx mori nucleopolyhedrovirus SNF2 global transactivator homologue (Bm33) enhances viral pathogenicity in B. mori larvae

The SNF2 global transactivator gene homologue (Bm33) of Bombyx mori nucleopolyhedrovirus (BmNPV) is one of the genes exclusive to group I NPVs, but its function remains unknown. This study describes the characterization of Bm33. Transcriptional analysis suggested that Bm33 is an early gene, as its transcript was observed at 4 h post-infection in BmNPV-infected BmN cells. To examine the role of Bm33 during BmNPV infection, a Bm33 deletion mutant (BmORF33D) was constructed and its infectivity was characterized in BmN cells and B. mori larvae. BmORF33D did not have any obvious defects in the production of budded viruses (BVs) or occlusion bodies (OBs) in BmN cells compared with wild-type BmNPV. Larval bioassays revealed that deletion of Bm33 did not reduce virus infectivity. However, BmORF33D took approximately 10–15 h longer than wild-type BmNPV to kill B. mori larvae when tested by either BV injection or OB ingestion. These results suggest that Bm33 is not essential for virus growth in vitro or in vivo, but that it accelerates the time of death of B. mori larvae.

Characterization of a late gene, ORF67 from Bombyx mori nucleopolyhedrovirus

Open reading frame 67 of Bombyx mori nucleopolyhedrovirus (BmORF67) is a homologue of Autographa californica multiple NPV ORF81. The gene is conserved among all baculoviruses and is thus considered a baculovirus core gene. The transcript of BmORF67 was detected at 18-72 h post-infection (p.i.). Polyclonal antiserum raised to a His-BmORF67 fusion protein recognized BmORF67 in infected cell lysates from 24 to 72 h p.i., suggesting that BmORF67 is a late gene. BmORF67 was not detected either in budded viruses or occlusion-derived virus. Immunofluoresence analysis showed that the protein located in the cytoplasm and interacted with host protein actin A3. In conclusion, BmORF67 is a late protein localized in the cytoplasm of infected cells that interacts with host protein.

BV/ODV-E26: a palmitoylated, multifunctional structural protein of Autographa californica nucleopolyhedrovirus

Autographa californica nucleopolyhedrovirus Ac16 is 1 of 17 genes conserved within Type 1 nucleopolyhedroviruses. This report demonstrates that multiple isoforms of the protein encoded by Ac16, BV/ODV-E26 (E26), are present in the infected cell. One form of E26 associates with viral DNA or DNA-binding proteins, while a second form associates with intracellular membranes and this is likely due to palmitoylation. The different forms of E26 present unique epitopes that can be discriminated by antiserum produced to bacterially or virally produced antigen. A summation of the data now available on E26 suggests that it is a multifunctional protein and the functional states assume unique conformations that can be discriminated by differing antisera.