A nuclear envelop-associated baculovirus protein promotes intranuclear lipid accumulation during infection

Transcriptomics and interactomics during the primary infection of an SfNPV baculovirus on Spodoptera frugiperda larvae

The fall armyworm (FAW), Spodoptera frugiperda, has been the most devastating pest of corn as well as of other crops in America, and more recently in Africa and Asia. The development of resistance to chemical insecticides led the search for environmentally friendly biological alternatives such as baculoviruses. This study focuses on the primary infection of the baculovirus SfNPV-Ar in the FAW's midgut epithelium, by analyzing the differential expression of transcripts in excised midguts at 6, 12, and 24 h post-infection (hpi), and predicted their interactions. Interaction of viral factors with the infected midgut tissue could alters various cellular processes, such as the apoptotic system due to the up-regulation observed of FABP at 6 hpi and of HSP90 at 24 hpi, along with the down-regulated PRX at 6 hpi and FABP transcripts between 12 and 24 hpi. Changes in transcript regulation could affect the cellular architecture of infected cells due to up-regulation of ARP 2/3 at 6 and 12 hpi, followed by down-regulation at 24 hpi. In relation to protein folding proteins, HSP90 was up-regulated at 24 hpi and PDI was down-regulated between 6 and 12 hpi. With respect to metabolism and cellular transport, AcilBP and ATPS0 were up regulated at 6 hpi and 12 hpi, respectively. In reference to transcription and translation up-regulation of RPL11 at 6 hpi and of FPN32 and RPL19 at 24 hpi was detected, as well as the down-regulation of RPL19 at 6 hpi, of PDI and RPL7 at 12 hpi, and of FABP at 24 hpi. In conclusion, gene regulation induced by viral infection could be related to the cytoskeleton and cellular metabolism as well as to oxidative stress, apoptosis, protein folding, translation, and ribosomal structure. The results presented in this work are an approach to understanding how the virus takes control of the general metabolism of the insect host during the primary infection period.

A SLiM-dependent conformational change in baculovirus IE1 controls its focus formation ability

The baculovirus IE1 gene encodes a multifunctional protein that is essential for both DNA replication and RNA transcription of the virus. Prior to viral DNA replication, IE1 promotes early gene transcription when localized in hr-dependent foci. During viral DNA replication, the IE1 foci expand and fuse to generate the virogenic stroma (VS) with IE1 found in the VS reticulum. To explore the IE1 structural features essential for this coordinated localization, we constructed various IE1 mutants based on three putative domains (N, I, and C). We determined that a BDI motif located in the intrinsic disorder region (IDR) between the N and I domains acts as a nuclear localization signal, whereas BDII and HLH in the C domain are required for VS localization in infected cells or for chromosomal association in uninfected mitotic cells. Deletion of the SLiM (short linear motif) located in the I domain restrains both nuclear- and VS localization. Intra-molecular fluorescence resonance energy transfer (FRET) probes of IE1 mutants revealed a conformational change of the I-C two-domain fragment during infection, which was inhibited by aphidicolin, suggesting that IE1 undergoes a stage-dependent conformational change. Further, homo-dimerization of the I domain and stage-dependent conformational changes require an intact SLiM. Mutational analysis of SLiM revealed that VS localization and chromosomal association were retained following S291A and S291E substitutions, but hr-dependent focus formation differed between the two mutations. These results suggest that coordinated IE1 localization is controlled by SLiM-dependent conformational changes that are potentially switched by the phosphorylation state of the SLiM.

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.

Cryo-EM reveals binding of linoleic acid to SARS-CoV-2 spike glycoprotein, suggesting an antiviral treatment strategy

The COVID-19 pandemic and concomitant lockdowns presented a global health challenge and triggered unprecedented research efforts to elucidate the molecular mechanisms and pathogenicity of SARS-CoV-2. The spike glycoprotein decorating the surface of SARS-CoV-2 virions is a prime target for vaccine development, antibody therapy and serology as it binds the host cell receptor and is central for viral cell entry. The electron cryo-microscopy structure of the spike protein revealed a hydrophobic pocket in the receptor-binding domain that is occupied by an essential fatty acid, linoleic acid (LA). The LA-bound spike protein adopts a non-infectious locked conformation which is more stable than the infectious form and shields important immunogenic epitopes. Here, the impact of LA binding on viral infectivity and replication, and the evolutionary conservation of the pocket in other highly pathogenic coronaviruses, including SARS-CoV-2 variants of concern (VOCs), are reviewed. The importance of LA metabolic products, the eicosanoids, in regulating the human immune response and inflammation is highlighted. Lipid and fatty-acid binding to a hydrophobic pocket in proteins on the virion surface appears to be a broader strategy employed by viruses, including picornaviruses and Zika virus. Ligand binding stabilizes their protein structure and assembly, and downregulates infectivity. In the case of rhinoviruses, this has been exploited to develop small-molecule antiviral drugs that bind to the hydrophobic pocket. The results suggest a COVID-19 antiviral treatment based on the LA-binding pocket.

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.

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.

Role of Protein Glycosylation in Host-Pathogen Interaction

Host-pathogen interactions are fundamental to our understanding of infectious diseases. Protein glycosylation is one kind of common post-translational modification, forming glycoproteins and modulating numerous important biological processes. It also occurs in host-pathogen interaction, affecting host resistance or pathogen virulence often because glycans regulate protein conformation, activity, and stability, etc. This review summarizes various roles of different glycoproteins during the interaction, which include: host glycoproteins prevent pathogens as barriers; pathogen glycoproteins promote pathogens to attack host proteins as weapons; pathogens glycosylate proteins of the host to enhance virulence; and hosts sense pathogen glycoproteins to induce resistance. In addition, this review also intends to summarize the roles of lectin (a class of protein entangled with glycoprotein) in host-pathogen interactions, including bacterial adhesins, viral lectins or host lectins. Although these studies show the importance of protein glycosylation in host-pathogen interaction, much remains to be discovered about the interaction mechanism.