The Autographa californica multicapsid nucleopolyhedrovirus GP64 protein: analysis of transmembrane domain length and sequence requirements

Structural transition of GP64 triggered by a pH-sensitive multi-histidine switch

The fusion of viruses with cellular membranes is a critical step in the life cycle of enveloped viruses. This process is facilitated by viral fusion proteins, many of which are conformationally pH-sensitive. The specifics of how changes in pH initiate this fusion have remained largely elusive. This study presents the cryo-electron microscopy (cryo-EM) structures of a prototype class III fusion protein, GP64, in its prefusion and early intermediate states, revealing the structural intermediates accompanying the membrane fusion process. The structures identify the involvement of a pH-sensitive switch, comprising H23, H245, and H304, in sensing the low pH that triggers the initial step of membrane fusion. The pH sensing role of this switch is corroborated by assays of cell-cell syncytium formation and dual dye-labeling. The findings demonstrate that coordination between multiple histidine residues acts as a pH sensor and activator. The involvement of a multi-histidine switch in viral fusion is applicable to fusogens of human-infecting thogotoviruses and other viruses, which could lead to strategies for developing anti-viral therapies and vaccines.

Recombinant AcMNPV-gp64-EGFP and synergist triphenyl phosphate, an effective combination against Spodoptera frugiperda

OBJECTIVES

AcMNPV is a kind of microbial insecticide that can significantly relieve the resistance of Spodoptera frugiperda to chemical pesticides. TPP is a widely used synergist, which can reduce the use of pesticides by inhibiting carboxylesterase. It is emergently needed to develop a biological control way of Spodoptera frugiperda.

RESULTS

GP64 mediates low-pH-triggered membrane fusion during entry by endocytosis and participates in AcMNPV particle budding. We explored the synergistic anti-insect activity of AcMNPV-gp64-EGFP and TPP. AcMNPV-gp64-EGFP could increase progeny virus proliferation and accelerate the transcription of 38k and vp39 genes. TPP could inhibit the carboxylesterase activity in the midgut of Spodoptera frugiperda larvae infected with AcMNPV-gp64-EGFP and enhance the virulence of AcMNPV-gp64-EGFP to Spodoptera frugiperda.

CONCLUSIONS

TPP targeted carboxylesterase inhibition so that AcMNPV-gp64-EGFP could escape the antiviral response in insect hosts. It provided a novel strategy for the prevention of Spodoptera frugiperda.

Identification of Cellular Genes Involved in Baculovirus GP64 Trafficking to the Plasma Membrane

The baculovirus envelope protein GP64 is an essential component of the budded virus and is necessary for efficient virion assembly. Little is known regarding intracellular trafficking of GP64 to the plasma membrane, where it is incorporated into budding virions during egress. To identify host proteins and potential cellular trafficking pathways that are involved in delivery of GP64 to the plasma membrane, we developed and characterized a stable Drosophila cell line that inducibly expresses the AcMNPV GP64 protein and used that cell line in combination with a targeted RNA interference (RNAi) screen of vesicular protein trafficking pathway genes. Of the 37 initial hits from the screen, we validated and examined six host genes that were important for trafficking of GP64 to the cell surface. Validated hits included Rab GTPases Rab1 and Rab4, Clathrin heavy chain, clathrin adaptor protein genes AP-1-2β and AP-2μ, and Snap29. Two gene knockdowns (Rab5 and Exo84) caused substantial increases (up to 2.5-fold) of GP64 on the plasma membrane. We found that a small amount of GP64 is released from cells in exosomes and that some portion of cell surface GP64 is endocytosed, suggesting that recycling helps to maintain GP64 at the cell surface. IMPORTANCE While much is known regarding trafficking of viral envelope proteins in mammalian cells, little is known about this process in insect cells. To begin to understand which factors and pathways are needed for trafficking of insect virus envelope proteins, we engineered a Drosophila melanogaster cell line and implemented an RNAi screen to identify cellular proteins that aid transport of the model baculovirus envelope protein (GP64) to the cell surface. For this we developed an experimental system that leverages the large array of tools available for Drosophila and performed a targeted RNAi screen to identify cellular proteins involved in GP64 trafficking to the cell surface. Since viral envelope proteins are often critical for production of infectious progeny virions, these studies lay the foundation for understanding how either pathogenic insect viruses (baculoviruses) or insect-vectored viruses (e.g., flaviviruses, alphaviruses) egress from cells in tissues such as the midgut to enable systemic virus infection.

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.

Characterization of two monoclonal antibodies, 38F10 and 44D11, against the major envelope fusion protein of Helicoverpa armigera nucleopolyhedrovirus

The envelope fusion protein F of baculoviruses is a class I viral fusion protein which play a significant role during virus entry into insect cells. F is initially synthesized as a precursor (F₀) and then cleaved into a disulfide-linked F₁ and F₂ subunits during the process of protein maturation and secretion. To facilitate further investigation into the structure and function of F protein during virus infection, monoclonal antibodies (mAbs) against the F₂ subunit of Helicoverpa armigera nucleopolyhedrovirus (HearNPV) (HaF) were generated. Two kinds of mAbs were obtained according to their different recognition epitopes: one kind of mAbs, as represented by 38F10, recognizes amino acid (aa) 85 to 123 of F₂ and the other kind, represented by 44D11, recognizes aa 148 to 173 of F₂. Western blot and immunofluorescence assay confirmed that both of the mAbs recognized the F protein expressed in HearNPV infected cells, however, only 44D11 could neutralize HearNPV infection. The results further showed that 44D11 may not interact with a receptor binding epitope, rather it was demonstrated to inhibit syncytium formation in cells expressing the HaF protein. The results imply that the monoclonal antibody 44D11 recognizes a region within HaF2 that may be involved in the F-mediated membrane fusion process.

Herpesvirus gB: A Finely Tuned Fusion Machine

Enveloped viruses employ a class of proteins known as fusogens to orchestrate the merger of their surrounding envelope and a target cell membrane. Most fusogens accomplish this task alone, by binding cellular receptors and subsequently catalyzing the membrane fusion process. Surprisingly, in herpesviruses, these functions are distributed among multiple proteins: the conserved fusogen gB, the conserved gH/gL heterodimer of poorly defined function, and various non-conserved receptor-binding proteins. We summarize what is currently known about gB from two closely related herpesviruses, HSV-1 and HSV-2, with emphasis on the structure of the largely uncharted membrane interacting regions of this fusogen. We propose that the unusual mechanism of herpesvirus fusion could be linked to the unique architecture of gB.

Autographa californica multiple nucleopolyhedrovirus GP64 protein: Analysis of domain I and V amino acid interactions and membrane fusion activity

The Autographa californica multiple nucleopolyhedrovirus GP64 is a class III viral fusion protein. Although the post-fusion structure of GP64 has been solved, its pre-fusion structure and the detailed mechanism of conformational change are unknown. In GP64, domain V is predicted to interact with two domain I segments that flank fusion loop 2. To evaluate the significance of the amino acids involved in these interactions, we examined 24 amino acid positions that represent interacting and conserved residues within domains I and V. In several cases, substitution of a single amino acid involved in a predicted interaction disrupted membrane fusion activity, but no single amino acid pair appears to be absolutely required. We identified 4 critical residues in domain V (G438, W439, T452, and T456) that are important for membrane fusion, and two residues (G438 and W439) that appear to be important for formation or stability of the pre-fusion conformation of GP64.

Differential susceptibilities to BmNPV infection of two cell lines derived from the same silkworm ovarian tissues

We previously established and characterized two insect cell lines (BmN-SWU1 and BmN-SWU2) from Bombyx mori ovaries. Here, we examined their differential susceptibilities to Bombyx mori nucleopolyhedrovirus (BmNPV) despite having originated from the same tissue source. BmN-SWU1 cells were susceptible and supported high titers of BmNPV replication, while BmN-SWU2 cells were resistant to BmNPV infection. Subcellular localization analysis demonstrated that very few BmNPV particles could be imported into BmN-SWU2 cells. However, initiation of BmNPV DNA replication but not amplification was detected in BmN-SWU2 cells after transfection with vA4^prm-VP39-EGFP bacmid DNA. BmNPV transcription assays showed that late and very late but not early viral genes apparently were blocked in BmNSWU2 cells by unknown mechanisms. Further syncytium formation assays demonstrated that the BmNPV envelope fusion protein GP64 could not mediate BmN-SWU2 host cell-cell membrane fusion. Taken together, these results indicate that these two cell lines represent optimal tools for investigating host-virus interactions and insect antiviral mechanisms.

Functional analysis of the Autographa californica multiple nucleopolyhedrovirus GP64 terminal fusion loops and interactions with membranes

The Autographa californica multiple nucleopolyhedrovirus (AcMNPV) glycoprotein GP64 is the major envelope protein of the budded virus (BV). GP64 is a class III fusion protein that mediates BV attachment to the cell surface and low-pH-triggered membrane fusion between the BV envelope and the endosome membrane during entry. Class III fusion proteins contain terminal looped structures that are believed to interact with membranes. To examine the functions of 3 loops found at the apex of the GP64 postfusion structure, we generated 2-alanine substitutions that scanned the two so-called fusion loops (loop 1 and loop 2) plus an adjacent loop structure (loop 3) that is closely attached to loop 2 and is also found at the apex of the GP64 postfusion structure. We identified essential residues from Y75 to T86 (loop 1) and N149 to H156 (loop 2) that are required for fusion activity, but no essential residues in loop 3. Further analysis revealed that critical fusion loop residues fall within two groups that are associated with either membrane merger (hemifusion) or fusion pore expansion. We next examined the interactions of soluble GP64 proteins and BV with membranes composed of various phospholipids. BV interacted directly with small unilamellar vesicles (SUVs) comprised of phospholipids phosphatidylcholine and phosphatidic acid (PC/PA) or phosphatidylcholine and phosphatidylserine (PC/PS) under neutral and acidic pH. We also examined the interactions of soluble GP64 constructs containing substitutions of the most hydrophobic residues within each of the two fusion loops. We found that a 2-residue substitution in either single loop (loop 1 [positions 81 and 82] or loop 2 [positions 153 and 154]) was not sufficient to substantially reduce the GP64-liposome interaction, but the same substitutions in both fusion loops severely reduced the GP64-liposome association at neutral pH. These results suggest that critical hydrophobic residues in both fusion loops may be involved in the interaction of GP64 with host cellular membranes and direct GP64-membrane interactions may represent a receptor-binding step prior to a low-pH-triggered conformational change.

Autographa californica multiple nucleopolyhedrovirus GP64 protein: roles of histidine residues in triggering membrane fusion and fusion pore expansion

The Autographa californica multiple nucleopolyhedrovirus (AcMNPV) GP64 protein mediates membrane fusion during entry. Fusion results from a low-pH-triggered conformational change in GP64 and subsequent interactions with the membrane bilayers. The low-pH sensor and trigger of the conformational change are not known, but histidine residues are implicated because the pK(a) of histidine is near the threshold for triggering fusion by GP64. We used alanine substitutions to examine the roles of all individual and selected clusters of GP64 histidine residues in triggering and mediating fusion by GP64. Three histidine residues (H152, H155, and H156), located in fusion loop 2, were identified as important for membrane fusion. These three histidine residues were important for efficient pore expansion but were not required for the pH-triggered conformational change. In contrast, a cluster of three histidine residues (H245, H304, and H430) located near the base of the central coiled coil was identified as a putative sensor for low pH. Three alanine substitutions in cluster H245/H304/H430 resulted in dramatically reduced membrane fusion and the apparent loss of the prefusion conformation at neutral pH. Thus, the H245/H304/H430 cluster of histidines may function or participate as a pH sensor by stabilizing the prefusion structure of GP64.

A recombinant vesicular stomatitis virus bearing a lethal mutation in the glycoprotein gene uncovers a second site suppressor that restores fusion

Vesicular stomatitis virus (VSV), a prototype of the Rhabdoviridae family, contains a single surface glycoprotein (G) that is responsible for attachment to cells and mediates membrane fusion. Working with the Indiana serotype of VSV, we employed a reverse genetic approach to produce fully authentic recombinant viral particles bearing lethal mutations in the G gene. By altering the hydrophobicity of the two fusion loops within G, we produced a panel of mutants, W72A, Y73A, Y116A, and A117F, that were nonfusogenic. Propagation of viruses bearing those lethal mutations in G completely depended on complementation by expression of the glycoprotein from the heterologous New Jersey serotype of VSV. The nonfusogenic G proteins oligomerize and are transported normally to the cell surface but fail to mediate acid pH-triggered membrane fusion. The nonfusogenic G proteins also interfered with the ability of wild-type G to mediate fusion, either by formation of mixed trimers or by inhibition of trimer function during fusion. Passage of one recombinant virus, A117F, identified a second site suppressor of the fusion block, E76K. When analyzed in the absence of the A117F substitution, E76K rendered G more sensitive to acid pH-triggered fusion, suggesting that this compensatory mutation is destabilizing. Our work provides a set of authentic recombinant VSV particles bearing lethal mutations in G, confirms that the hydrophobic fusion loops of VSV G protein are critical for membrane fusion, and underscores the importance of the sequence elements surrounding the hydrophobic tips of the fusion loops in driving fusion. This study has implications for understanding dominant targets for inhibition of G-mediated fusion. Moreover, the recombinant viral particles generated here will likely be useful in dissecting the mechanism of G-catalyzed fusion as well as study steps of viral assembly.

Two conserved residues are important for inducing highly ordered membrane domains by the transmembrane domain of influenza hemagglutinin

The interaction with lipids of a synthetic peptide corresponding to the transmembrane domain of influenza hemagglutinin was investigated by means of electron spin resonance. A detailed analysis of the electron spin resonance spectra from spin-labeled phospholipids revealed that the major effect of the peptide on the dynamic membrane structure is to induce highly ordered membrane domains that are associated with electrostatic interactions between the peptide and negatively charged lipids. Two highly conserved residues in the peptide were identified as being important for the membrane ordering effect. Aggregation of large unilamellar vesicles induced by the peptide was also found to be correlated with the membrane ordering effect of the peptide, indicating that an increase in membrane ordering, i.e., membrane dehydration, is important for vesicle aggregation. The possibility that hydrophobic interaction between the highly ordered membrane domains plays a role in vesicle aggregation and viral fusion is discussed.

Helix-destabilizing, beta-branched, and polar residues in the baboon reovirus p15 transmembrane domain influence the modularity of FAST proteins

The fusogenic reoviruses induce syncytium formation using the fusion-associated small transmembrane (FAST) proteins. A recent study indicated the p14 FAST protein transmembrane domain (TMD) can be functionally replaced by the TMDs of the other FAST proteins but not by heterologous TMDs, suggesting that the FAST protein TMDs are modular fusion units. We now show that the p15 FAST protein is also a modular fusogen, as indicated by the functional replacement of the p15 ectodomain with the corresponding domain from the p14 FAST protein. Paradoxically, the p15 TMD is not interchangeable with the TMDs of the other FAST proteins, implying that unique attributes of the p15 TMD are required when this fusion module is functioning in the context of the p15 ecto- and/or endodomain. A series of point substitutions, truncations, and reextensions were created in the p15 TMD to define features that are specific to the functioning of the p15 TMD. Removal of only one or two residues from the N terminus or four residues from the C terminus of the p15 TMD eliminated membrane fusion activity, and there was a direct correlation between the fusion-promoting function of the p15 TMD and the presence of N-terminal, hydrophobic β-branched residues. Substitution of the glycine residues and triserine motif present in the p15 TMD also impaired or eliminated the fusion-promoting activity of the p15 TMD. The ability of the p15 TMD to function in an ecto- and endodomain-specific context is therefore influenced by stringent sequence requirements that reflect the importance of TMD polar residues and helix-destabilizing residues.

A single amino acid substitution modulates low-pH-triggered membrane fusion of GP64 protein in Autographa californica and Bombyx mori nucleopolyhedroviruses

We have previously shown that budded viruses of Bombyx mori nucleopolyhedrovirus (BmNPV) enter the cell cytoplasm but do not migrate into the nuclei of non-permissive Sf9 cells that support a high titer of Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) multiplication. Here we show, using the syncytium formation assay, that low-pH-triggered membrane fusion of BmNPV GP64 protein (Bm-GP64) is significantly lower than that of AcMNPV GP64 protein (Ac-GP64). Mutational analyses of GP64 proteins revealed that a single amino acid substitution between Ac-GP64 H155 and Bm-GP64 Y153 can have significant positive or negative effects on membrane fusion activity. Studies using bacmid-based GP64 recombinant AcMNPV harboring point-mutated ac-gp64 and bm-gp64 genes showed that Ac-GP64 H155Y and Bm-GP64 Y153H substitutions decreased and increased, respectively, the multiplication and cell-to-cell spread of progeny viruses. These results indicate that Ac-GP64 H155 facilitates the low-pH-triggered membrane fusion reaction between virus envelopes and endosomal membranes.

Baculovirus GP64 disulfide bonds: the intermolecular disulfide bond of Autographa californica multicapsid nucleopolyhedrovirus GP64 is not essential for membrane fusion and virion budding

The GP64 envelope glycoprotein of the Autographa californica nucleopolyhedrovirus (AcMNPV) is a class III viral membrane fusion protein that is triggered by low pH during entry. Unlike most other viral fusion protein trimers, the monomers of GP64 are covalently linked to each other within the trimer by a single intermolecular disulfide bond (Cys24 Cys372). Single or paired alanine substitutions for Cys24 and Cys372 resulted in lower-efficiency transport of GP64 to the cell surface. Surprisingly, these mutated GP64s induced syncytium formation, and normalized fusion activities were approximately 30% of that from wild-type (WT) GP64. Heat treatment (37 degrees C) did not further reduce fusion activity of GP64 constructs with a disrupted intermolecular disulfide bond, suggesting that the GP64 trimers were relatively thermostable in the absence of the intermolecular disulfide bond. In addition, analysis of binding by a conformation-specific monoclonal antibody (MAb) suggested that the low-pH-induced refolding of those GP64 constructs was generally similar to that of WT GP64. In addition to its critical role in membrane fusion, GP64 is also necessary for efficient budding. When GP64 constructs containing a disrupted intermolecular disulfide bond (Cys24 Cys372) were displayed at the cell surface at levels comparable to those of WT GP64, virion budding efficiency ranged from approximately 39 to 88%, indicating that the intermolecular disulfide bond is not required for virion budding. However, GP64 proteins with a disrupted intermolecular disulfide could not rescue a GP64-null bacmid. We also examined the 6 conserved intramolecular disulfide bonds using single and paired alanine substitution mutations. None of the GP64 constructs with disrupted intramolecular disulfide bonds were capable of mediating pH-triggered membrane fusion, indicating that the intramolecular disulfide bonds are all necessary for membrane fusion. Thus, while the intramolecular disulfide bonds of GP64 appear to serve critical roles in membrane fusion, the unusual intermolecular disulfide bond was not critical for membrane fusion or virion budding yet appears to play an unknown role in viral infectivity.

Identification of protein-protein interactions of the occlusion-derived virus-associated proteins of Helicoverpa armigera nucleopolyhedrovirus

The purpose of this study was to identify protein-protein interactions among the components of the occlusion-derived virus (ODV) of Helicoverpa armigera nucleopolyhedrovirus (HearNPV), a group II alphabaculovirus in the family Baculoviridae. To achieve this, 39 selected genes of potential ODV structural proteins were cloned and expressed in the Gal4 yeast two-hybrid (Y2H) system. The direct-cross Y2H assays identified 22 interactions comprising 13 binary interactions [HA9-ODV-EC43, ODV-E56-38K, ODV-E56-PIF3, LEF3-helicase, LEF3-alkaline nuclease (AN), GP41-38K, GP41-HA90, 38K-PIF3, 38K-PIF2, VP80-HA100, ODV-E66-PIF3, ODV-E66-PIF2 and PIF3-PIF2] and nine self-associations (IE1, HA44, LEF3, HA66, GP41, CG30, 38K, PIF3 and P24). Five of these interactions - LEF3-helicase and LEF3-AN, and the self-associations of IE1, LEF3 and 38K - have been reported previously in Autographa californica multiple nucleopolyhedrovirus. As HA44 and HA100 were two newly identified ODV proteins of group II viruses, their interactions were further confirmed. The self-association of HA44 was verified with a His pull-down assay and the interaction of VP80-HA100 was confirmed by a co-immunoprecipitation assay. A summary of the protein-protein interactions of baculoviruses reported so far, comprising 68 interactions with 45 viral proteins and five host proteins, is presented, which will facilitate our understanding of the molecular mechanisms of baculovirus infection.

Autographa californica multiple nucleopolyhedrovirus core gene ac96 encodes a per Os infectivity factor (PIF-4)

Autographa californica multiple nucleopolyhedrovirus (AcMNPV) ac96 is a core gene, but its role in virus replication is still unknown. To determine its role in the baculovirus life cycle, we used the AcMNPV bacmid system to generate an ac96-null virus (vAc(96)(null)). Our analyses showed that the absence of ac96 does not affect budded virus (BV) production or viral DNA replication in infected Sf9 cells. Western blotting and confocal immunofluorescence analysis showed that AC96 is expressed in both the cytoplasm and the nucleus throughout infection. In addition, AC96 was detected in the envelope fractions of both BV and occlusion-derived virus. Injection of vAc(96)(null) BV into the hemocoel killed Trichoplusia ni larvae as efficiently as repaired and control viruses; however, vAc(96)(null) was unable to infect the midgut tissue of Trichoplusia ni larvae when inoculated per os. Therefore, the results of this study show that ac96 encodes a new per os infectivity factor (PIF-4).

The pre-transmembrane domain of the Autographa californica multicapsid nucleopolyhedrovirus GP64 protein is critical for membrane fusion and virus infectivity

The envelope glycoprotein, GP64, of the baculovirus Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) is a class III viral fusion protein that mediates pH-triggered membrane fusion during virus entry. Viral fusion glycoproteins from many viruses contain a short region in the ectodomain and near the transmembrane domain, referred to as the pre-transmembrane (PTM) domain. In some cases, the PTM domain is rich in aromatic amino acids and plays an important role in membrane fusion. Although the 23-amino-acid (aa) PTM domain of AcMNPV GP64 lacks aromatic amino acids, we asked whether this region might also play a significant role in membrane fusion. We generated alanine scanning and single and multiple amino acid substitutions in the GP64 PTM domain. We specifically focused on amino acid positions conserved between baculovirus GP64 and thogotovirus GP75 proteins, as well as hydrophobic and charged amino acids. For each PTM-modified construct, we examined trimerization, cell surface localization, and membrane fusion activity. Membrane merger and pore formation were also examined. We identified eight aa positions that are important for membrane fusion activity. Critical positions were not clustered in the linear sequence but were distributed throughout the PTM domain. While charged residues were not critical or essential, three hydrophobic amino acids (L465, L476, and L480) played an important role in membrane fusion activity and appear to be involved in formation of the fusion pore. We also asked whether selected GP64 constructs were capable of rescuing a gp64null AcMNPV virus. These studies suggested that several conserved residues (T463, G460, G462, and G474) were not required for membrane fusion but were important for budding and viral infectivity.