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.

Functional Characterization of the Group I Alphabaculovirus Specific Gene ac73

Baculoviridae is a family of large DNA viruses that specifically infect insects. It contains four genera, Alpha-, Beta-, Gamma-, and Deltabaculovirus. Alphabaculovirus is further divided into Group I and II, and Group I appears to be emerged most recently among all baculoviruses. Interestingly, there are 12 Group I specific genes that are only found in this lineage. Studying these genes is helpful to understand how baculoviruses evolved. Here, we reported the functional analyzing results of ac73, a function unknown Group I specific gene of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) which is the type species of baculovirus. The AC73 protein encoded by ac73 was found to be expressed during the late stage of infection and incorporated into the nucleocapsids of budded virus (BV) and occlusion-derived virus (ODV). In infected cells, AC73 resided mainly in the ring zone region of the nucleus, and appeared to be assembled into occlusion bodies (OBs). The ac73 knockout and repaired viruses were constructed and studied by in vitro and in vivo infection. Although ac73 was not essential for BV and ODV or OB formation, the BV titer and viral infectivity in insect larvae of ac73 knockout AcMNPV decreased by about 5-8 and 3-4 fold compared to those of wild type virus, respectively, suggesting ac73 contributed to infectious BV production and viral infectivity in vivo. This research provides new insight into the function of this Group I specific gene.

The deletion of the AcMNPV ac124 gene resulted in a decrease in chitinase transcription

The Autographa californica multiple nucleopolyhedrovirus (AcMNPV) ac124 gene has been previously characterized as a viral pathogenicity factor. In this study, an ac124-knockout virus (vAc124KO) was generated to examine the role of the ac124 gene in the context of the AcMNPV genome during infection. Our results showed that the absence of ac124 does not affect the production of budded virus (BV) and occlusion bodies (OBs) in infected Sf9 cells. Western blotting analysis showed that the deletion of ac124 does not affect the temporal expression and the relative levels of GP64, VP39, P6.9, and polyhedrin. qRT-PCR analysis showed that the transcription level of chitinase but not the adjacent cathepsin in vAc124KO infected cells was significantly lower than that of the vAcWT infected cells from 24 to 96 h p.i. Luciferase assays showed that the overexpression of Ac124 could significantly improve the ability of chitinase promoter to initiate reporter genes. Based on the above data, we hypothesize that Ac124 binds to the promoter of chitinase to regulate the expression of chitinase gene.

Baculovirus Entry and Egress from Insect Cells

Baculoviruses are large DNA viruses of insects that are highly pathogenic in many hosts. In the infection cycle, baculoviruses produce two types of virions. These virion phenotypes are physically and functionally distinct, and each serves a critical role in the biology of the virus. One phenotype, the occlusion-derived virus (ODV), is occluded within a crystallized protein that facilitates oral infection of the host. A large complex of at least nine ODV envelope proteins called per os infectivity factors are critically important for ODV infection of insect midgut epithelial cells. Viral egress from midgut cells is by budding to produce a second virus phenotype, the budded virus (BV). BV binds, enters, and replicates in most other tissues of the host insect. Cell recognition and entry by BV are mediated by a single major envelope glycoprotein: GP64 in some baculoviruses and F in others. Entry and egress by the two virion phenotypes occur by dramatically different mechanisms and reflect a life cycle in which ODV is specifically adapted for oral infection while BV mediates dissemination of the infection within the animal.

Bombyx mori nucleopolyhedrovirus ORF40 is essential for budded virus production and occlusion-derived virus envelopment

ORF40 (bm40) of the Bombyx mori nucleopolyhedrovirus (BmNPV) encodes a homologue of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) AC51 and is a highly conserved gene in sequenced alphabaculoviruses. To investigate the role of bm40 in the baculovirus infection cycle, a bm40 knockout BmNPV bacmid was constructed via homologous recombination in Escherichia coli. Western blotting analysis revealed that bm40 is a late gene during virus infection. Compared with wild-type and repair viruses, the knockout virus exhibited a single-cell infection phenotype. Titration assays confirmed that no infectious budded viruses (BVs) were produced due to the bm40 deletion, while there was no effect on viral DNA replication. Electron microscopy revealed that Bm40 is required for nucleocapsid egress from the nucleus to the cytoplasm, nucleocapsid envelopment to form occlusion-derived viruses (ODVs) and subsequent embedding of ODVs into polyhedra. Confocal microscopy showed that Bm40 was predominantly localized in the nuclei from 48 h post-infection and subsequently condensed on the nuclear membrane and polyhedra at the late phase of infection. Taken together, these results demonstrate that Bm40 plays an essential role in BV production and ODV envelopment in the BmNPV infection cycle.

The group I alphabaculovirus-specific protein, AC5, is a novel component of the occlusion body but is not associated with ODVs or the PIF complex

Autographa californica nucleopolyhedrovirus (AcMNPV) orf5 (ac5) is a group I alphabaculovirus-specific gene of unknown function, although the protein (AC5) was previously reported to be associated with the per os infectivity factor (PIF) complex. The purpose of this study was to study the dynamics of AC5 during AcMNPV infection and to verify whether it is indeed a component of the PIF complex. Transcription and expression analyses suggested that ac5 is a late viral gene. An ac5-deleted recombinant AcMNPV was generated by homologous recombination. A one-step growth curve assay indicated that ac5 was not required for budded virus (BV) production in Sf9 cells. Scanning electron microscopy and transmission electron microscopy demonstrated that the deletion of ac5 did not affect occlusion body (OB) morphology, and nor did it affect the insertion of occlusion-derived virus (ODV) into OBs. Partially denaturing SDS-PAGE and a co-immunoprecipitation assay clearly showed that AC5 was not a component of the PIF complex, while the deletion of ac5 did not affect the formation and presence of the PIF complex. Further analyses showed, however, that AC5 was an OB-specific protein, but it was not detected as a component of BVs or ODVs. Bioassay experiments showed that the oral infectivity of ac5-deleted AcMNPV to third instar Spodoptera exigua larvae was not significantly different from that of the ac5-repaired virus. In conclusion, AC5 is an intrinsic protein of OBs, instead of being a component of the PIF complex, and is not essential for either BV or ODV infection. AC5 is awaiting the assignment of another hitherto unknown function.

Viral mechanisms for docking and delivering at nuclear pore complexes

Some viruses possess the remarkable ability to transport their genomes across nuclear pore complexes (NPCs) for replication inside the host cell's intact nuclear compartment. Viral mechanisms for crossing the restrictive NPC passageway are highly complex and astonishingly diverse, requiring in each case stepwise interaction between incoming virus particles and components of the nuclear transport machinery. Exactly how a large viral genome loaded with accessory proteins is able to pass through the relatively narrow central channel of the NPC without causing catastrophic structural damage is not yet fully understood. It appears likely, however, that the overall structure of the NPC changes in response to the cargo. Translocation may result in nucleic acids being misdelivered to the cytoplasm. Here we consider in detail the diverse strategies that viruses have evolved to target and subvert NPCs during infection. For decades, this process has both captivated and confounded researchers in the fields of virology, cell biology, and structural biology.