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

Nup358 and Nup153 Facilitate nuclear import of BmNPV nucleocapsids in Bombyx mori cells

Nuclear pore complexes (NPCs) are crucial for the nuclear import of viral genomes, serving as gateways for many viruses to deliver their genetic material into host cell nuclei. However, the role of NPCs in the entry of Bombyx mori nucleopolyhedrovirus (BmNPV) remains poorly understood. This study investigates the involvement of specific nucleoporins in the nuclear import of BmNPV nucleocapsids, a crucial step for viral replication in host cells. Using a combination of Importazole, wheat germ agglutinin (WGA), and small interfering RNAs (siRNAs), we demonstrate that BmNPV entry into BmN cells is mediated through the NPC. Importazole and WGA treatment significantly inhibited viral infection, highlighting the essential role of nucleoporins in BmNPV entry. Additionally, siRNA-mediated knockdown of Nup358 and Nup153 resulted in a marked accumulation of nucleocapsids in the cytoplasm. Overexpression of the N-terminal fragment of Nup358 (1-1127) enhanced nucleocapsid entry efficiency, whereas deletion of the phenylalanine-glycine (FG) repeats in Nup153 (Nup153ΔFG) reduced viral entry. These results confirm the pivotal roles of Nup358 and Nup153 in the nuclear import of BmNPV nucleocapsids. Our findings provide new insights into the molecular interactions between BmNPV and host NPCs, offering potential targets for controlling BmNPV infections in sericulture.

Unraveling dual fusion mechanisms in BmNPV GP64: critical roles of CARC motifs and signal peptide retention

Viral membrane fusion is a critical process enabling viruses to invade host cells, driven by viral membrane fusion proteins (MFPs). Cholesterol plays a pivotal role in this process, which is essential for the infectivity of many enveloped viruses. The interaction between MFPs and cholesterol is often facilitated by specific amino acid motifs known as cholesterol recognition/interaction amino acid consensus (CRAC) motifs and reverse CARC motifs. In a previous study, we demonstrated that CRAC1 and CRAC2 in GP64 are required for Bombyx mori nucleopolyhedrovirus (BmNPV) infection. This study further investigates the role of CARC in the GP64 protein of BmNPV, revealing their complex interaction with cholesterol and the influence of signal peptide (SP) retention on viral infectivity. We identified six putative CARC motifs in GP64 and generated mutants to assess their function. Our findings show that CARC1, CARC2, CARC3, and CARC4 are indispensable for viral fusion and infection when the SP is retained, whereas only CARC2 and CARC3 remain essential after SP cleavage. In contrast, CARC1 and CARC4 are necessary for viral infection through a cholesterol-independent mechanism resulting from double mutations in the CRAC1 and CRAC2 motifs of GP64. These insights not only deepen our understanding of BmNPV GP64-mediated fusion but also highlight potential antiviral targets, underscoring the adaptability and resilience of viral fusion mechanisms.IMPORTANCEUnderstanding viral membrane fusion mechanisms is crucial for developing antiviral strategies. This study provides novel insights into the intricate roles of CARC and CRAC motifs in the GP64 protein of BmNPV, particularly their interaction with cholesterol and the influence of signal peptide retention. The discovery that certain CARC motifs are essential for cholesterol-dependent fusion, whereas others function in a cholesterol-independent context advances our understanding of viral fusion processes. These findings emphasize the potential of targeting CARC motifs for therapeutic interventions and underline the importance of cholesterol interactions in viral infections. This research not only deepens our understanding of BmNPV fusion mechanisms but also has broader implications for other enveloped viruses.

The uncleaved signal peptide of Bombyx mori nucleopolyhedrovirus GP64 preferentially activates BmSpz7 expression, contributing to protein secretion

Bombyx mori nucleopolyhedrovirus (BmNPV) significantly impacts silkworm sericulture, causing substantial economic losses. The GP64 protein, a primary envelope protein of BmNPV budded virus (BV), retains its signal peptide (SP) in the mature form, crucial for its translocation to the plasma membrane (PM) and viral infectivity. This study investigates the role of the uncleaved SP of GP64 in activating the expression of BmSpz7, a novel Spätzle family member identified through RNA-seq analysis. We cloned and characterized BmSpz7, demonstrating its upregulated expression in BmN cells and silkworm larvae infected with BmNPV containing GP64 with an uncleaved SP. Additionally, transient expression of GP64's SP significantly enhanced BmSpz7 expression and protein secretion. These findings suggest that the uncleaved SP of GP64 plays a pivotal role in activating BmSpz7, providing new insights into the molecular interactions between BmNPV and its host, and revealing potential targets for antiviral strategies in sericulture.

Role of ubiquitin-proteasome pathway in budded virus egress and GP64 surface distribution in Bombyx mori nucleopolyhedrovirus

The Bombyx mori nucleopolyhedrovirus (BmNPV) is a DNA virus that affects the silkworm, B. mori, causing substantial economic losses in sericulture. This study investigates the mechanisms underlying budded virus egress, focusing on the roles of the ubiquitin-proteasome pathway (UPP) machinery. BmNPV produces two virion types: budded virions (BVs) and occlusion-derived virions (ODVs), which differ in their envelope origins and functions. Recent findings suggest similarities in the budding pathways of BmNPV and Autographa californica multiple nucleopolyhedrovirus (AcMNPV), involving plasma membrane budding and multivesicular body (MVB) pathways. The study reveals that specific UPP-related proteins, including 26S proteasome non-ATPase regulatory subunit 14 (PSMD14), polyubiquitin, proteasome alpha subunit 6 (PSMA6) and proteasome zeta subunit (PSMZ), are involved in BV egress. Using recombinant viruses and UPP inhibitors, we demonstrate the necessity of these proteins for GP64 secretion and effective BV release. RNA interference and cell surface display of GP64 analyses further validate the critical role of UPP in BmNPV BV egress and protein secretion. This research enhances our understanding of the mechanisms behind BmNPV MVB budding and GP64 secretion while also identifying potential targets for controlling the virus in sericulture.

The signal peptide of BmNPV GP64 activates the ERAD pathway to regulate heterogeneous secretory protein expression

As a powerful eukaryotic expression vector, the baculovirus expression vector system (BEVS) is widely applied to the production of heterogeneous proteins for research and pharmaceutical purposes, while optimization of BEVS remains a work in progress for membrane or secreted protein expression. In this study, the impact of the signal peptide (SP) derived from Bombyx mori nucleopolyhedrovirus (BmNPV) GP64 protein on protein expression, secretion, and the endoplasmic reticulum-associated degradation (ERAD) pathway were investigated in BmN cells and BEVS. Transient expression studies in BmN cells revealed that SP alters the localization and expression levels of recombinant proteins, reducing intracellular accumulation while enhancing secretion efficiency. Quantitative analysis demonstrated that SP-mediated secretion was markedly higher compared to controls, albeit with lower total expression levels. Further exploration into SP-mediated ERAD pathway activation showed increased expression of BiP and other ERAD-associated genes (PDI, UFD1, S1P, and ASK1), correlating with higher SP-driven protein expression levels. RNA interference (RNAi) experiments elucidated that knockdown of ERAD-associated genes enhances both the secretion efficiency of SP-guided proteins and the infectivity of BmNPV. Particularly, interference with BiP demonstrated the most pronounced effect on protein secretion enhancement. Viral infection experiments further supported these findings, showing upregulated ERAD-associated genes during BmNPV infection, indicating their role in viral protein processing and infectivity. In conclusion, this study elucidates the complex interplay between SP-mediated protein secretion, ERAD pathway activation, and viral infectivity in BmNPV-infected cells. These insights suggest strategies for optimizing recombinant protein production and viral protein processing in baculovirus expression systems, with potential implications for biotechnological and biomedical applications. Further research could refine our understanding and manipulation of protein secretion pathways in insect cell-based expression systems.

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.

Intracellular localized heterogeneous protein franking by a transmembrane domain of GP64 is sufficient to be assembled on budded virions of Bombyx mori nucleopolyhedrovirus

Baculovirus has been widely used for foreign protein expression in biomedical studies, and budded virus (BV) surface display has developed into an important research tool for heterogenous membrane protein studies. The basic strategy of surface display is to construct a recombinant virus where the target gene is fused with a complete or partial gp64 gene. In this study, we further investigate and develop this BV surface displaying strategy. We constructed stable insect cell lines to express the target protein flanking with different regions of signal peptide (SP) and GP64 transmembrane domain (TMD). Subsequently, recombinant BmNPV was used to infect the cell, and the integration of heterogeneous protein into BV was detected. The results indicated that deletion of the n-region of SP (SPΔn) decreased the incorporation rate more than that of the full-length SP. However, the incorporation rate of the protein fused with h and c-region deletion of SP (SPΔh-c) was significantly enhanced by 35-40 times compare to full-length SP. Moreover, the foreign protein without SP and TMD failed to display on the BV, while the integration of foreign proteins with GP64 TMD fusion at the c-terminal was significantly enhanced by 12-26 times compared to the control. Thus, these new strategies developed the BV surface display system further.

Innexin hemichannel activation by Microplitis bicoloratus ecSOD monopolymer reduces ROS

The extracellular superoxide dismutases (ecSODs) secreted by Microplitis bicoloratus reduce the reactive oxygen species (ROS) stimulated by the Microplitis bicoloratus bracovirus. Here, we demonstrate that the bacterial transferase hexapeptide (hexapep) motif and bacterial-immunoglobulin-like (BIg-like) domain of ecSODs bind to the cell membrane and transiently open hemichannels, facilitating ROS reductions. RNAi-mediated ecSOD silencing in vivo elevated ROS in host hemocytes, impairing parasitoid larva development. In vitro, the ecSOD-monopolymer needed to be membrane bound to open hemichannels. Furthermore, the hexapep motif in the beta-sandwich of ecSOD49 and ecSOD58, and BIg-like domain in the signal peptides of ecSOD67 were required for cell membrane binding. Hexapep motif and BIg-like domain deletions induced ecSODs loss of adhesion and ROS reduction failure. The hexapep motif and BIg-like domain mediated ecSOD binding via upregulating innexins and stabilizing the opened hemichannels. Our findings reveal a mechanism through which ecSOD reduces ROS, which may aid in developing anti-redox therapy.

K64 acetylation of heat shock protein 90 suppresses nucleopolyhedrovirus replication in Bombyx mori

HSP90 is a highly conserved chaperone that facilitates the proliferation of many viruses, including silkworm (bombyx mori) nucleopolyhedrovirus (BmNPV), but the underlying regulatory mechanism was unclear. We found that suppression of HSP90 by 17-AAG, a HSP90-specific inhibitor, significantly reduced the expression of BmNPV capsid protein gp64 and viral genome replication, whereas overexpression of B. mori HSP90(BmHSP90) promoted BmNPV replication. Furthermore, in a recent study of the lysine acetylome of B. mori infected with BmNPV, we focused on the reduced viral proliferation due to changes of BmHSP90 lysine acetylation. Site-directed introduction of acetylated (K/Q) or deacetylated (K/R) mimic mutations into BmHSP90 revealed that lysine 64 (K64) acetylation activated the JAK/STAT pathway and reduced BmHSP90 ATPase activity, leading to diminished chaperone activity and ultimately inhibiting BmNPV proliferation. In this study, a single lysine 64 acetylation change of BmHSP90 was elucidated as a model of posttranslational modifications occurring in the wake of host-virus interactions, providing novel insights into potential antiviral strategies.

Data mining reveals tissue-specific expression and host lineage-associated forms of Apis mellifera filamentous virus

Background

Apis mellifera filamentous virus (AmFV) is a large double-stranded DNA virus of uncertain phylogenetic position that infects honey bees (Apis mellifera). Little is known about AmFV evolution or molecular aspects of infection. Accurate annotation of open-reading frames (ORFs) is challenged by weak homology to other known viruses. This study was undertaken to evaluate ORFs (including coding-frame conservation, codon bias, and purifying selection), quantify genetic variation within AmFV, identify host characteristics that covary with infection rate, and examine viral expression patterns in different tissues.

Methods

Short-read data were accessed from the Sequence Read Archive (SRA) of the National Center for Biotechnology Information (NCBI). Sequence reads were downloaded from accessions meeting search criteria and scanned for kmers representative of AmFV genomic sequence. Samples with kmer counts above specified thresholds were downloaded in full for mapping to reference sequences and de novo assembly.

Results

At least three distinct evolutionary lineages of AmFV exist. Clade 1 predominates in Europe but in the Americas and Africa it is replaced by the other clades as infection level increases in hosts. Only clade 3 was found at high relative abundance in hosts with African ancestry, whereas all clades achieved high relative abundance in bees of non-African ancestry. In Europe and Africa, clade 2 was generally detected only in low-level infections but was locally dominant in some North American samples. The geographic distribution of clade 3 was consistent with an introduction to the Americas with 'Africanized' honey bees in the 1950s. Localized genomic regions of very high nucleotide divergence in individual isolates suggest recombination with additional, as-yet unidentified AmFV lineages. A set of 155 high-confidence ORFs was annotated based on evolutionary conservation in six AmFV genome sequences representative of the three clades. Pairwise protein-level identity averaged 94.6% across ORFs (range 77.1-100%), which generally exhibited low evolutionary rates and moderate to strong codon bias. However, no robust example of positive diversifying selection on coding sequence was found in these alignments. Most of the genome was detected in RNA short-read alignments. Transcriptome assembly often yielded contigs in excess of 50 kb and containing ORFs in both orientations, and the termini of long transcripts were associated with tandem repeats. Lower levels of AmFV RNA were detected in brain tissue compared to abdominal tissue, and a distinct set of ORFs had minimal to no detectable expression in brain tissue. A scan of DNA accessions from the parasitic mite Varroa destructor was inconclusive with respect to replication in that species.

Discussion

Collectively, these results expand our understanding of this enigmatic virus, revealing transcriptional complexity and co-evolutionary associations with host lineage.

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.

Bombyx mori Nucleopolyhedrovirus Hijacks Multivesicular Body as an Alternative Envelopment Platform for Budded Virus Egress

Baculovirus budded virus (BV) acquires its envelope and viral membrane fusion proteins from the plasma membrane (PM) of the host cell during the budding process. However, this classical BV egress pathway has been questioned because an intracellularly localized membrane fusion protein, SPΔnGP64 (glycoprotein 64 [GP64] lacking the signal peptide [SP] n region), was assembled into the envelope to generate infective BVs in our recent studies. Here, we identify an additional pathway for Bombyx mori nucleopolyhedrovirus (BmNPV) BV assembly and release that differs, in part, from the currently accepted model for the egress pathway of baculovirus. Electron microscopy showed that during infection, BmNPV-infected cells contained many newly formed multivesicular body (MVB)-like compartments that included mature virions at 30 h postinfection (p.i.). Immunoelectron microscopy demonstrated that the MVBs contained CD63, an MVB endosome marker, and GP64, a BmNPV fusion glycoprotein. MVB fusion with the PM and the release of mature virions, together with naked nucleocapsids, were observed at the cell surface. Furthermore, MVB egress mediated the translocation of SPΔnGP64 to the PM, which induced cell-cell fusion until 36 h p.i. This BV egress pathway can be partially inhibited by U18666A incubation and RNA interference targeting MVB biogenesis genes. Our findings indicate that BmNPV BVs are enveloped and released through MVBs via the cellular exosomal pathway, which is a subordinate BV egress pathway that produces virions with relatively inferior infectivity. This scenario has significant implications for the elucidation of the BmNPV BV envelopment pathway. IMPORTANCE BmNPV is a severe pathogen that infects mainly Bombyx mori, a domesticated insect of economic importance, and accounts for approximately 15% of economic losses in sericulture. BV production plays a key role in systemic BmNPV infection of larvae. Despite the progress made in the functional gene studies of BmNPV, BmNPV BV egress is ill-understood. This study reports a previously unreported MVB envelopment pathway in BmNPV BV egress. To our knowledge, this is the first report of a baculovirus using dual BV egress pathways. This specific BV egress mechanism explains the cause of the non-PM-localized SPΔnGP64-rescued gp64-null bacmid infectivity, elucidating the reason underlying the retention of SP by BmNPV GP64. The data obtained elucidate an alternate molecular mechanism of baculovirus BV egress.