Autographa Californica Multiple Nucleopolyhedrovirus P48 (Ac103) Is Required for the Efficient Formation of Virus-Induced Intranuclear Microvesicles

CRISPR/Cas9-mediated disruption of orf76 as an antiviral therapy against BmNPV in the transgenic silkworm

Viral diseases pose a significant threat to livestock husbandry and plant cultivation. CRISPR/Cas9-mediated targeted editing of viral genes offers a promising approach to antiviral therapy. The silkworm, Bombyx mori, is an economically important insect susceptible to infection by B. mori nucleopolyhedrovirus (BmNPV), and viral outbreaks cause severe economic losses to the sericulture industry. Here, we identified BmNPV orf76 as a viral late gene that is highly similar to Autographa californica multiple nucleopolyhedrovirus Ac93. The deletion of orf76 abolished BmNPV proliferation and hindered the production of infectious budded viruses. We generated a transgenic line, Cas9(+)/sgorf76(+), that did not affect the growth or development of the silkworm and demonstrated that the transgenic line Cas9(+)/sgorf76(+) efficiently cleaved orf76 at the sgorf76 site, resulting in large deletions at 120 h post-infection, with no observed off-target effects. Survival analyses revealed that the transgenic line Cas9(+)/sgorf76(+) exhibited significantly higher survival rates than the control lines Cas9(-)/sgorf76(-), regardless of the BmNPV inoculation dose. Additionally, the number of BmNPV DNA copies and the expression levels of viral genes were markedly inhibited in the transgenic line Cas9(+)/sgorf76(+) compared with the control line Cas9(-)/sgorf76(-). The results provide a promising target for Cas9-mediated antiviral therapy against BmNPV, and the findings provide new insights for baculovirus gene function studies and lepidopteran pest control.

Multifaceted interactions between host ESCRT-III and budded virus-related proteins involved in entry and egress of the baculovirus Autographa californica multiple nucleopolyhedrovirus

The endosomal sorting complex required for transport (ESCRT) is a conserved protein machine mediating membrane remodeling and scission. In the context of viral infection, different components of the ESCRT-III complex, which serve as the core machinery to catalyze membrane fission, are involved in diverse viruses' entry, replication, and/or budding. However, the interplay between ESCRT-III and viral factors in the virus life cycle, especially for that of large enveloped DNA viruses, is largely unknown. Recently, the ESCRT-III components Vps2B, Vps20, Vps24, Snf7, Vps46, and Vps60 were determined for entry and/or egress of the baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV). Here, we identified the final three ESCRT-III components Chm7, Ist1, and Vps2A of Spodoptera frugiperda. Overexpression of the dominant-negative forms of these proteins or RNAi downregulation of their transcripts significantly reduced infectious budded viruses (BVs) production of AcMNPV. Quantitative PCR together with confocal and transmission electron microscopy analysis revealed that these proteins were required for internalization and trafficking of BV during entry and egress of nucleocapsids. In infected Sf9 cells, nine ESCRT-III components were distributed on the nuclear envelope and plasma membrane, and except for Chm7, the other components were also localized to the intranuclear ring zone. Y2H and BiFC analysis revealed that 42 out of 64 BV-related proteins including 35 BV structural proteins and 7 non-BV structural proteins interacted with single or multiple ESCRT-III components. By further mapping the interactome of 64 BV-related proteins, we established the interaction networks of ESCRT-III and the viral protein complexes involved in BV entry and egress.IMPORTANCEFrom archaea to eukaryotes, the endosomal sorting complex required for transport (ESCRT)-III complex is hijacked by many enveloped and nonenveloped DNA or RNA viruses for efficient replication. However, the mechanism of ESCRT-III recruitment, especially for that of large enveloped DNA viruses, remains elusive. Recently, we found the ESCRT-III components Vps2B, Vps20, Vps24, Snf7, Vps46, and Vps60 are necessary for the entry and/or egress of budded viruses (BVs) of Autographa californica multiple nucleopolyhedrovirus. Here, we demonstrated that the other three ESCRT-III components Chm7, Ist1, and Vps2A play similar roles in BV infection. By determining the subcellular localization of ESCRT-III components in infected cells and mapping the interaction of nine ESCRT-III components and 64 BV-related proteins, we built the interaction networks of ESCRT-III and the viral protein complexes involved in BV entry and egress. These studies provide a fundamental basis for understanding the mechanism of the ESCRT-mediated membrane remodeling for replication of baculoviruses.

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.

The amino acids of Autographa californica multiple nucleopolyhedrovirus P48 critical for the association with Ac93 are important for the nuclear egress of nucleocapsids and efficient formation of intranuclear microvesicles

Autographa californica multiple nucleopolyhedrovirus (AcMNPV) undergoes a biphasic life cycle with the production of two physically and functionally distinct virions: budded virions (BVs) and occlusion-derived virions (ODVs). Nuclear egress of nucleocapsids and intranuclear microvesicle formation are critical for the morphogenesis of BVs and ODVs, respectively, but the mechanisms and details of these two processes remain unknown. Our previous studies have shown that AcMNPV p48 (ac103) gene is essential for the nuclear egress of nucleocapsids and efficient formation of intranuclear microvesicles, and protein P48 associates with Ac93, which is also involved in the above processes in virion morphogenesis. In this study, we present evidence that alanine substitution for residues N318, V319, C320, R321, and I323 of P48 disrupted the association with Ac93. Moreover, mutation of these residues blocked the nuclear egress of nucleocapsids and efficient formation of intranuclear microvesicles, and subsequent BV formation, as well as ODV envelopment and embedding of ODVs into polyhedra. These results suggested that the association between P48 and Ac93 may be important for both BV and ODV morphogenesis.

A Conserved Phenylalanine Residue of Autographa Californica Multiple Nucleopolyhedrovirus AC75 Protein Is Required for Occlusion Body Formation

Autographa californica multiple nucleopolyhedrovirus (AcMNPV) orf75 (ac75) is a highly conserved gene that is essential for AcMNPV propagation. However, the key domains or residues of the AC75 protein that play a role in viral propagation have not been identified. In this study, sequence alignment revealed that residues Phe-54 and Gln-81 of AC75 were highly conserved among alphabaculoviruses and betabaculoviurses. Thus, Phe-54 and Gln-81 AC75 mutation bacmids were constructed. We found that Gln-81 was not required for viral propagation, whereas mutating Phe-54 reduced budded virus production by 10-fold and impaired occlusion body formation when compared with that of the wild-type AcMNPV. Electron microscopy observations showed that the Phe-54 mutation affected polyhedrin assembly and also occlusion-derived virus embedding, whereas western blot analysis revealed that mutating Phe-54 reduced the amount of AC75 but did not affect the localization of AC75 in infected cells. A protein stability assay showed that the Phe-54 mutation affected AC75 stability. Taken together, Phe-54 was identified as an important residue of AC75, and ac75 is a pivotal gene in budding virus production and occlusion body formation.