Transport Via Macropinocytic Vesicles is Crucial for Productive Infection with Bombyx Mori Nucleopolyhedrovirus

Unveiling non-classical glycosylation patterns in Bombyx mori nucleopolyhedrovirus GP64: Insights into viral entry and fusion

The glycoprotein GP64 of alphabaculovirus is crucial for viral entry and fusion. Here, we investigated the N-glycosylation patterns of Bombyx mori nucleopolyhedrovirus (BmNPV) GP64 and its signal peptide (SP) cleaved form, SPΔnGP64, along with their impacts on viral infectivity and fusogenicity. Through deglycosylation assays, we confirmed N-glycosylation of BmNPV GP64 on multiple sites. Mutational analysis targeting predicted N-glycosylation sites revealed diverse effects on viral infectivity and cell fusion. Particularly noteworthy were mutations at sites 175, which resulted in complete loss of infectivity and fusion capacity. Furthermore, LC-MS/MS analysis uncovered unexpected non-classical N-glycosylation sites, including N252, N302, N367, and N471, with only N302 and N471 identified in SPΔnGP64. Subsequent investigation highlighted the critical roles of these residues in BmNPV amplification and fusion, underscoring the essentiality of N367 glycosylation for GP64 fusogenicity. Our findings provide valuable insights into the non-classical glycosylation landscape of BmNPV GP64 and its functional significance in viral biology.

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.

The Bombyx mori Nucleopolyhedrovirus GP64 Retains the Transmembrane Helix of Signal Peptide to Contribute to Secretion across the Cytomembrane

Bombyx mori nucleopolyhedrovirus (BmNPV) is the primary pathogen of silkworms that causes severe economic losses in sericulture. GP64 is the key membrane fusion protein that mediates budded virus (BV) fusion with the host cell membrane. Previously, we found that the n-region of the GP64 signal peptide (SP) is required for protein secretion and viral pathogenicity; however, our understanding of BmNPV GP64 remains limited. Here, we first reported that BmNPV GP64 retained its SP in the mature protein and virion in only host cells but did not retain in nonhost cells. Uncleaved SP mediates protein targeting to the cytomembrane or secretion in Bombyx mori cells. The exitance of the n-region extended the transmembrane helix length, which resulted in the cleavage site to be located in the helix structure and thus blocked cleavage from signal peptidase (SPase). Without the n-region, the protein fails to be transported to the cytomembrane, but this failure can be rescued by the cleavage site mutation of SP. Helix-breaking mutations in SP abolished protein targeting to the cytomembrane and secretion. Our results revealed a previously unrecognized mechanism by which SP of membrane fusion not only determines protein localization but also determines viral pathogenicity, which highlights the escape mechanism of SP from the cleavage by SPase. IMPORTANCE BmNPV is the primary pathogen of silkworms, which causes severe economic losses in sericulture. BmNPV and Autographa californica multiple nucleopolyhedrovirus (AcMNPV) are closely related group I alphabaculoviruses, but they exhibit nonoverlapping host specificity. Recent studies suppose that GP64 is a determinant of host range, while knowledge remains limited. In this study, we revealed that BmNPV GP64 retained its SP in host cells but not in nonhost cells, and the SP retention is required for GP64 secretion across the cytomembrane. This is the first report that a type I membrane fusion protein retained its SP in mature proteins and virions. Our results unveil the mechanism by which SP GP64 escapes cleavage and the role of SP in protein targeting. This study will help elucidate an important mechanistic understanding of BmNPV infection and host range specificity.

Identification of endoplasmic-reticulum-associated proteins involved in Bombyx mori nucleopolyhedrovirus entry by RNA-seq analysis

Membrane fusion is a key step in enveloped virus infection, releasing the viral genome into the cytoplasm to initiate infection. Bombyx mori nucleopolyhedrovirus (BmNPV) is an enveloped DNA virus that mainly infects silkworms. Information about membrane fusion of BmNPV with host cells is still limited. In this study, BmN cells were pretreated with ??ammonium chloride??, and infection with BmNPV was allowed to occur naturally through endocytosis or artificially through low-pH-induced fusion with the plasma membrane, after which the cells were subjected to RNA-seq. The results indicated that a few endoplasmic reticulum-associated proteins (ERAPs) were among the common upregulated DEGs, including BiP, CRT, and HSP90, and this upregulation was confirmed by q-PCR. Knockdown of BiP, CRT, and HSP90 expression by siRNA resulted in significant inhibition of BmNPV infection. This study suggests that ERAPs may be involved in the BmNPV membrane fusion process during infection.

Two Cholesterol Recognition Amino Acid Consensus Motifs of GP64 with Uncleaved Signal Peptide Are Required for Bombyx mori Nucleopolyhedrovirus Infection

The signal peptide (SP) of integrated membrane proteins is removed cotranslationally or posttranslationally in the endoplasmic reticulum, while GP64, a membrane fusion protein of Bombyx mori nucleopolyhedrovirus (BmNPV), retains its SP in the mature protein and virion. In this study, we revealed that uncleaved SP is a key determinant with additional functions in infection. First, uncleaved SP endows BmNPV with strong virulence; second, SP retention-induced BmNPV infection depends on cholesterol recognition amino acid consensus domain 1 (CRAC1) and CRAC2. In contrast, the recombinant virus with SP-cleaved GP64 has reduced infectivity, and only CRAC2 is required for BmNPV infection. Furthermore, we showed that cholesterol in the plasma membrane is an important fusion receptor that interacts with CRAC2 of GP64. Our study suggested that BmNPV GP64 is a key cholesterol-binding protein and uncleaved SP determines GP64's unique dependence on the CRAC domains. IMPORTANCE BmNPV is a severe pathogen that mainly infects silkworms. GP64 is the key membrane fusion protein that mediates BmNPV infection, and some studies have indicated that cholesterol and lipids are involved in BmNPV infection. A remarkable difference from other membrane fusion proteins is that BmNPV GP64 retains its SP in the mature protein, but the cause is still unclear. In this study, we investigated the reason why BmNPV retains this SP, and its effects on protein targeting, virulence, and CRAC dependence were revealed by comparison of recombinant viruses harboring SP-cleaved or uncleaved GP64. Our study provides a basis for understanding the dependence of BmNPV infection on cholesterol/lipids and host specificity.

18 Additional Amino Acids of the Signal Peptide of the Bombyx mori Nucleopolyhedrovirus GP64 Activates Immunoglobulin Binding Protein (BiP) Expression by RNA-seq Analysis

GP64 is the key membrane fusion protein of Group I baculovirus, and while the Bombyx mori nucleopolyhedrovirus (BmNPV) GP64 contains a longer n-region (18 amino acid) of the signal peptide than does the Autographa californica multiple nucleopolyhedrovirus (AcMNPV), the function of the n-region has not been determined. In this study, we first showed that n-region is required for membrane protein localization in BmN cells, then the transcriptome sequencing was conducted on proteins guided by different signal peptide regions, and the results were analyzed and validated by quantitative PCR and luciferase assays. The results indicated that 1049 differentially expressed genes (DEGs) were identified among the different region of signal peptides and the control. With the n-region, the protein export pathway was upregulated significantly, the Wnt-1 signaling pathway was downregulated, and BiP was significantly activated by the GP64 full-length signal peptide. Furthermore, RNA interference on BiP efficiently increased luciferase secretion. These results indicate that the GP64 n-region plays a key role in protein expression and regulation.

Special Issue "Transmission Dynamics of Insect Viruses"

At the close of this Special Issue of Viruses on the Transmission Dynamics of Insect Viruses, we would like to thank all of the authors for their submissions and the great work expanding our knowledge of insect virus biology and transmission [...].

Efficient Expression and Processing of Ebola Virus Glycoprotein Induces Morphological Changes in BmN Cells but Cannot Rescue Deficiency of Bombyx Mori Nucleopolyhedrovirus GP64

Ebola virus (EBOV) disease outbreaks have resulted in many fatalities, yet no licensed vaccines are available to prevent infection. Recombinant glycoprotein (GP) production may contribute to finding a cure for Ebola virus disease, which is the key candidate protein for vaccine preparation. To explore GP_1,2 expression in BmN cells, EBOV-GP_1,2 with its native signal peptide or the GP64 signal peptide was cloned and transferred into a normal or gp64 null Bombyx mori nucleopolyhedrovirus (BmNPV) bacmid via transposition. The infectivity of the recombinant bacmids was investigated after transfection, expression and localization of EBOV-GP were investigated, and cell morphological changes were analyzed by TEM. The GP64 signal peptide, but not the GP_1,2 native signal peptide, caused GP_1,2 localization to the cell membrane, and the differentially localized GP_1,2 proteins were cleaved into GP₁ and GP₂ fragments in BmN cells. GP_1,2 expression resulted in dramatic morphological changes in BmN cells in the early stage of infection. However, GP_1,2 expression did not rescue GP64 deficiency in BmNPV infection. This study provides a better understanding of GP expression and processing in BmN cells, which may lay a foundation for EBOV-GP expression using the BmNPV baculovirus expression system.

Methyl-Beta-Cyclodextrin-Induced Macropinocytosis Results in Increased Infection of Sf21 Cells by Bombyx Mori Nucleopolyhedrovirus

Bombyx mori nucleopolyhedrovirus (BmNPV) is closely related to Autographa californica multiple nucleopolyhedrovirus (AcMNPV) with over ~93% amino acid sequence identity. However, their host ranges are essentially nonoverlapping. The mechanism of BmNPV entry into host cells is completely different from that of AcMNPV, and whether the entry mechanism difference relates to the host range remains unclear. BmNPV produces an abortive infection in nonhost cells due to virion nuclear transportation failure. Here, we performed a detailed study by increasing BmNPV infection in Sf21 cells with the aid of methyl-beta-cyclodextrin (MβCD). We found that low-concentration MβCD incubation efficiently activates membrane ruffling in Sf21 cells, which mediates the increase in BmNPV infection. Interestingly, MβCD incubation after virion internalization also increases the infection, which suggests that macropinocytosis is involved in BmNPV infection in Sf21 cells after virion internalization. Further study revealed that clathrin-mediated endocytosis (CME) is employed by BmNPV to facilitate entry into Sf21 cells, and chlorpromazine application abolishes BmNPV infection in cells incubated both with and without MβCD. Based on these studies, we show that BmNPV enters Sf21 cells via CME and that parallel induction of macropinocytosis facilitates BmNPV infection in Sf21 cells. This study reveals the mechanism of BmNPV entry into Sf21 cells and provides clues for improving BmNPV infections in nonpermissive cells.