Bombyx mori nucleopolyhedrovirus F-like protein Bm14 affects the morphogenesis and production of occlusion bodies and the embedding of ODVs

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

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

Label-free proteomics analysis on the envelope of budded viruses of Bombyx mori nucleopolyhedrovirus harboring differential localized GP64

Bombyx mori nucleopolyhedrovirus (BmNPV) GP64 is the key membrane fusion protein that mediates budded virus (BV) infection. We recently reported that BmNPV GP64's n-region of signal peptide (SP) blocked the SP-cleavage and mediated GP64 localization on the plasma membrane (PM); n-region (SP^∆nGP64) absence caused GP64 intracellular localization, however, SP^∆nGP64 was still incorporated into virion to generate BVs with lower infectivity. To better understand the biogenesis of the envelope of BmNPV BV, we conducted a label-free ESI mass spectrometry analysis of the envelope of purified BVs harboring PM localized GP64 or intracellular localized SP^∆nGP64. The results indicated that 31 viral proteins were identified on the envelope, among which 15 were reported in other viruses. The other 16 proteins were first reported in BmNPV BV, including the BmNPV-specific protein BRO-A and proteins associated with vesicle transportation. Six proteins with significant intensity differences were detected in virions with differential localized GP64, and five specific proteins were identified in virions with GP64. Meanwhile, we identified 81 host proteins on the envelope, and seven lipoproteins were first identified in baculovirus virion; other 74 proteins are involved in the cytoskeleton, DNA-binding, vesicle transport, etc. In the meantime, eight and five specific host proteins were, respectively, identified in GP64 and SP^∆nGP64's virions. The two virions shared 68 common host proteins, and 8 proteins were identified on their envelopes with a significant difference. This study provides new insight into the protein composition of BmNPV BV and a clue for further investigation of the budding mechanism of BmNPV.

Improvement of protein production in baculovirus expression vector system by removing a total of 10 kb of nonessential fragments from Autographa californica multiple nucleopolyhedrovirus genome

Baculovirus expression vector system (BEVS) is a powerful and versatile platform for recombinant protein production in insect cells. As the most frequently used baculovirus, Autographa californica multiple nucleopolyhedrovirus (AcMNPV) encodes 155 open reading frames (ORFs), including a considerable number of non-essential genes for the virus replication in cell culture. Studies have shown that protein production in BEVS can be improved by removing some viral dispensable genes, and these AcMNPV vectors also offer the possibility of accommodating larger exogenous gene fragments. In this study, we, respectively, deleted 14 DNA fragments from AcMNPV genome, each of them containing at least two contiguous genes that were known nonessential for viral replication in cell culture or functionally unknown. The effects of these fragment-deletions on virus replication and exogenous protein production were examined. The results showed that 11 of the 14 fragments, containing 43 genes, were dispensable for the virus replication in cultured cells. By detecting the expression of intracellularly expressed and secreted reporter proteins, we demonstrated that nine of the fragment-deletions benefited protein production in Sf9 cells and/or in High Five cells. After combining the deletion of some dispensable fragments, we obtained two AcMNPV vectors shortened by more than 10 kb but displayed an improved capacity for recombinant protein production. The deletion strategies used in this study has the potential to further improve the BEVS.

Functional analyses of inhibitor of apoptosis protein 1 (IAP1) of Antheraea pernyi multinucleocapsid nucleopolyhedrovirus (AnpeNPV) in viral replication and occlusion body production

Inhibitor of apoptosis protein 1 (IAP1) of Antheraea pernyi multinucleocapsid nucleopolyhedrovirus (AnpeNPV) belongs to the baculovirus IAP1 type. The function of AnpeNPV-IAP1 in viral replication and occlusion body (OB) production remains unknown. In this study, we demonstrated that AnpeNPV-iap1 is a late gene. AnpeNPV-IAP1 mainly localizes to the nuclear ring zone and exhibits dynamic distribution in the cytoplasm and the virogenic stroma during AnpeNPV infection. AnpeNPV-IAP1 impacted the expression of a variety of viral genes at the very late phase of infection in Tn-Hi5 cells. The deletion of AnpeNPV-iap1 caused decreased expression levels of polyhedrin, morphological changes to OBs and reduced OB production in A. pernyi pupae, along with a lengthening of the lethal time of A. pernyi larvae. These results suggest that AnpeNPV-iap1 is involved in regulating viral gene expression, OB production and morphogenesis in A. pernyi.

BmNPV p35 Reduces the Accumulation of Virus-Derived siRNAs and Hinders the Function of siRNAs to Facilitate Viral Infection

Antiviral immunity involves various mechanisms and responses, including the RNA interference (RNAi) pathway. During long-term coevolution, viruses have gained the ability to evade this defense by encoding viral suppressors of RNAi (VSRs). It was reported that p35 of baculovirus can inhibit cellular small interference RNA (siRNA) pathway; however, the molecular mechanisms underlying p35 as a VSR remain largely unclear. Here, we showed that p35 of Bombyx mori nucleopolyhedrovirus (BmNPV) reduces the accumulation of virus-derived siRNAs (vsiRNAs) mapped to a particular region in the viral genome, leading to an increased expression of the essential genes in this region, and revealed that p35 disrupts the function of siRNAs by preventing them from loading into Argonaute-2 (Ago2). This repressive effect on the cellular siRNA pathway enhances the replication of BmNPV. Thus, our findings illustrate for the first time the inhibitory mechanism of a baculovirus VSR and how this effect influences viral infection.

Bombyx mori nucleopolyhedrovirus F-like protein Bm14 is a factor for viral-induced cytopathic effects via regulating oxidative phosphorylation and cellular ROS levels

Bombyx mori nucleopolyhedrovirus (BmNPV) is highly pathogenic to Bombyx mori, silkworm, which causes serious cytopathic effects (CPEs) during infection. However, the role of viral protein in the virus-induced CPEs remains unclear. Here, we discovered that BmNPV infection induced severe CPEs including titer-dependent cell floating and changes in cellular surface morphology. Further explorations revealed the involvement of F-like protein (Bm14), a viral envelope protein, in inducing cytotoxicity and detachment of adherent BmN cells, and its disruption significantly impaired the virus infection-mediated CPEs. Intriguingly, transcriptomic analysis identified the tight association of Bm14 deletion with the activation of cellular oxidative phosphorylation pathway, consistent with the elevated mitochondrial membrane potential (MMP) levels and ATP concentrations as well as reduced ROS levels. Collectively, our results characterized for the first time the novel role of Bm14 in accelerating viral-induced cytopathogenicity via suppressing the cellular oxidative phosphorylation levels and upregulating the ROS levels.

Proliferation of Bombyx mori nucleopolyhedrovirus strain H4 in BmN cells is enhanced by exchange of the F gene sequence with type strain T3

The Bombyx mori nucleopolyhedrovirus (BmNPV)-based baculoviral expression vector system is among the most efficient expression vector systems for eukaryotic proteins especially when used in combination with silkworms as a host. We newly isolated a novel BmNPV strain (BmNPV H4) in Hokkaido, Japan that outperforms the type strain T3 in terms of both proliferation and expression of polyhedrin protein in silkworm larvae; however, it proliferates poorly in the BmN cell line. We inferred the gene responsible for the differences in proliferation between viral strains by quantifying amino acid similarity distances in protein functional domains and identifying highly divergent alleles between the H4 and T3 strains. Among proteins that differ markedly in functional domain sequence between H4 and T3, we identified the F gene, which encodes the F protein, as a putative cause of proliferative differences between the two strains. Using recombinant viruses with the F protein-coding sequence exchanged between H4 and T3, we determined that the T3 F protein increases H4 proliferation in BmN while the H4 F protein does not improve T3 proliferation in silkworm larvae. Our results suggest that the BmNPV F protein can strongly affect viral proliferation in a genetic background-specific manner and may be an important target for manipulating the proliferation characteristics of BmNPV-based expression vectors.

Bombyx mori nucleopolyhedrovirus Bm46 is essential for efficient production of infectious BV and nucleocapsid morphogenesis

Bombyx mori nucleopolyhedrovirus (BmNPV) orf46 (Bm46), the orthologues of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) ac57, is a highly conserved gene in group Ⅰ and group Ⅱ nucleopolyhedroviruses (NPVs). However, its function in viral life cycle is unclear. Our results indicated that Bm46 transcript was detected from infected cells at 12 h post infection, while Bm46 protein was detectable from 24 to 72 h post infection. Upon the deletion of Bm46, fewer infectious BVs were produced by titer assays, but neither viral DNA synthesis nor occlusion bodies (OBs) production was affected. Electron microscopy revealed that Bm46 knockout interrupted nucleocapsid assembly and occlusion-derived virus (ODV) embedding, resulting in aberrant capsid-like tubular structures accumulated in the RZ (ring zone). Interestingly, this abnormally elongated capsid structures were consistent with the immunofluorescence microscopy results showing that VP39 assembled into long filaments and cables in the RZ. Moreover, DNA copies decreased by 30 % in occlusion bodies (OBs) produced by Bm46-knockout virus. qRT-PCR and Western blot analysis showed that the expression of VP39 was affected by Bm46 disruption. Taken together, our findings clearly pointed out that Bm46 played an important role in BV production and the proper formation of nucleocapsid morphogenesis.