Functional analysis of a conserved region of the baculovirus envelope fusion protein, LD130

Deltabaculoviruses encode a functional type I budded virus envelope fusion protein

Envelope fusion proteins (F proteins) are major constituents of budded viruses (BVs) of alpha- and betabaculoviruses (Baculoviridae) and are essential for the systemic infection of insect larvae and insect cell culture. An f homologue gene is absent in gammabaculoviruses. Here we characterized the putative F-homologue (Cuni-F), encoded by (ORF) 104 of Culex nigripalpus nucleopolyhedrovirus (CuniNPV), the only deltabaculovirus member. When expressed alone, this protein seems to locate on the cell surface and is able to induce cell-cell fusion. When expressed by an alphabaculovirus (Autographa california nucleopolyhedrovirus), it was found to be incorporated into BVs. Western blot analyses detected the uncleaved Cuni-F0 and the furin-cleaved F1 forms. Treatment of infected cells with tunicamycin showed that Cuni-F contains N-glycans. Mutagenesis analysis identified the canonical furin cleavage site 126RARR129 as being responsible for the cleavage of Cuni-F in insect cells. The collective evidence suggests that CuniNPV encodes a functional F protein.

Autographa californica multiple nucleopolyhedrovirus ORF 23 null mutant produces occlusion-derived virions with fewer nucleocapsids

Two envelope fusion protein gene homologues have been identified in the baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV). AcMNPV GP64 protein is fusogenic and essential for propagation and pathogenicity. The F homologue (Ac23) is not essential, is fusion-incompetent in standard assays, but contributes to faster host death. Here, we show that occlusion bodies (OBs) from Ac23null mutants and control viruses do not differ significantly in size and the number of occlusion-derived virions (ODVs) contained; however, Ac23null OBs had a much higher percentage of ODVs with a single nucleocapsid (44.6 %) than the near-isogenic control (11.3 %). Infection of Sf9 cells with Ac23–green fluorescent protein (gfp)-expressing recombinant viruses showed Ac23–gfp fluorescence overlapping perinuclear DAPI staining at later times, a pattern not observed with GP64. These results suggest that F proteins have evolved functions beyond envelope fusion and play a different role from that of GP64 in viruses that contain both proteins.

Functional studies of per os infectivity factors of Helicoverpa armigera single nucleocapsid nucleopolyhedrovirus

A combined functional investigation on the four per os infectivity factors (PIFs) of Helicoverpa armigera single nucleocapsid nucleopolyhedrovirus (HearNPV) was conducted in this study. HearNPV bacmids with deletions of p74 (Ha20), pif1 (Ha111), pif2 (Ha132) and pif3 (Ha98) were constructed individually by homologous recombination in Escherichia coli cells. Repaired bacmids with respective pifs were also constructed. Western blot analyses revealed that all four PIFs were structural components of the envelope of HearNPV occlusion-derived virus (ODV). Electron microscopy showed that deletion of the pifs did not have any obvious effects on the morphology of the occlusion bodies (OBs). Bioassay analyses indicated that deletion of any of the above pifs resulted in loss of oral infectivity of OBs. The mixtures of the four pif-deletion mutants also resulted in deficiency of oral infectivity, implying that the four PIFs must be structural components of the same ODV to accomplish their function. Repairing of the respective genes into the pif-deletion bacmids could rescue the oral infectivity of the pif-deletion viruses. Calcofluor, which can damage the peritrophic membrane (PM), could not rescue the defects of the oral infectivity of the pif-deletion viruses, indicating that the PM is not likely to be the functional target of the PIFs.

The F-like protein Ac23 enhances the infectivity of the budded virus of gp64-null Autographa californica multinucleocapsid nucleopolyhedrovirus pseudotyped with baculovirus envelope fusion protein F

The GP64 and F proteins were previously identified as the sole functional envelope fusion proteins in Baculoviridae. F-like proteins, present only in group I nucleopolyhedroviruses (NPVs), are remnant, nonfunctional F proteins. In this report, we describe the effect of the presence or absence of the F-like protein Ac23 in a gp64-null Autographa californica multinucleocapsid NPV pseudotyped with the F protein from Spodoptera exigua multicapsid NPV (SeF). We found that the presence of Ac23 elevates the infectivity of the pseudotyped virus. This is in contrast to the results of Lung et al. (J. Virol. 76:5729-5736, 2002), who found no such effect. The possible reasons for the differing results are discussed.

A functional F analogue of Autographa californica nucleopolyhedrovirus GP64 from the Agrotis segetum granulovirus

The envelope fusion protein F of Plutella xylostella granulovirus is a computational analogue of the GP64 envelope fusion protein of Autographa californica nucleopolyhedrovirus (AcMNPV). Granulovirus (GV) F proteins were thought to be unable to functionally replace GP64 in the AcMNPV pseudotyping system. In the present study the F protein of Agrotis segetum GV (AgseGV) was identified experimentally as the first functional GP64 analogue from GVs. AgseF can rescue virion propagation and infectivity of gp64-null AcMNPV. The AgseF-pseudotyped AcMNPV also induced syncytium formation as a consequence of low-pH-induced membrane fusion.

Mutagenesis and nuclear magnetic resonance analyses of the fusion peptide of Helicoverpa armigera single nucleocapsid nucleopolyhedrovirus F protein

The entry of enveloped viruses into cells is normally mediated by fusion between viral and cellular membranes, in which the fusion peptide plays a crucial role. The fusion peptides of group II nucleopolyhedrovirus (NPV) F proteins are quite conserved, with a hydrophobic region located at the N terminal of the F(1) fragment. For this report, we used mutagenesis and nuclear magnetic resonance (NMR) to study the structure and function of the fusion peptide of the Helicoverpa armigera single-nucleocapsid NPV (HearNPV) F protein (HaF). Five mutations in the fusion peptide of HaF, N(1)G, N(1)L, I(2)N, G(3)L, and D(11)L, were generated separately, and the mutated f genes were transformed into the f-null HearNPV bacmid. The mutations N(1)L, I(2)N, and D(11)L were found to completely abolish the ability of the recombinant bacmids to produce infectious budded virus, while the mutations N(1)G and G(3)L did not. The low-pH-induced envelope fusion assay demonstrated that the N(1)G substitution increased the fusogenicity of HaF, while the G(3)L substitution reduced its fusogenicity. NMR spectroscopy was used to determine the structure of a synthetic fusion peptide of HaF in the presence of sodium dodecyl sulfate micelles at pH 5.0. The fusion peptide appeared to be an amphiphilic structure composed of a flexible coil in the N terminus from N(1) to N(5), a 3(10)-helix from F(6) to G(8), a turn at S(9), and a regular alpha-helix from V(10) to D(19). The data provide the first NMR structure of a baculovirus fusion peptide and allow us to further understand the relationship of structure and function of the fusion peptide.

Functional analysis of the transmembrane (TM) domain of the Autographa californica multicapsid nucleopolyhedrovirus GP64 protein: substitution of heterologous TM domains

GP64, the major envelope glycoprotein of the Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) budded virion, is important for host cell receptor binding and mediates low-pH-triggered membrane fusion during entry by endocytosis. In the current study, we examined the functional role of the AcMNPV GP64 transmembrane (TM) domain by replacing the 23-amino-acid GP64 TM domain with corresponding TM domain sequences from a range of viral and cellular type I membrane proteins, including Orgyia pseudotsugata MNPV (OpMNPV) GP64 and F, thogotovirus GP75, Lymantria dispar MNPV (LdMNPV) F, human immunodeficiency virus type 1 (HIV-1) GP41, human CD4 and glycophorin A (GpA), and influenza virus hemagglutinin (HA), and with a glycosylphosphatidylinositol (GPI) anchor addition sequence. In transient expression experiments with Sf9 cells, chimeric GP64 proteins containing either a GPI anchor or TM domains from LdMNPV F or HIV-1 GP41 failed to localize to the cell surface and thus appear to be incompatible with either GP64 structure or cell transport. All of the mutant constructs detected at the cell surface mediated hemifusion (outer leaflet merger) upon low-pH treatment, but only those containing TM domains from CD4, GpA, OpMNPV GP64, and thogotovirus GP75 mediated pore formation and complete membrane fusion activity. This supports a model in which partial fusion (hemifusion) proceeds by a mechanism that is independent of the TM domain and the TM domain participates in the enlargement or expansion of fusion pores after hemifusion. GP64 proteins containing heterologous TM domains mediated virion budding with dramatically differing levels of efficiency. In addition, chimeric GP64 proteins containing TM domains from CD4, GpA, HA, and OpMNPV F were incorporated into budded virions but were unable to rescue the infectivity of a gp64 null virus, whereas those with TM domains from OpMNPV GP64 and thogotovirus GP75 rescued infectivity. These results show that in addition to its basic role in membrane anchoring, the GP64 TM domain is critically important for GP64 trafficking, membrane fusion, virion budding, and virus infectivity. These critical functions were replaced only by TM domains from related viral membrane proteins.

Conserved leucines in N-terminal heptad repeat HR1 of envelope fusion protein F of group II nucleopolyhedroviruses are important for correct processing and essential for fusogenicity

The heptad repeat (HR), a conserved structural motif of class I viral fusion proteins, is responsible for the formation of a six-helix bundle structure during the envelope fusion process. The insect baculovirus F protein is a newly found budded virus envelope fusion protein which possesses common features to class I fusion proteins, such as proteolytic cleavage and the presence of an N-terminal open fusion peptide and multiple HR domains on the transmembrane subunit F(1). Similar to many vertebrate viral fusion proteins, a conserved leucine zipper motif is predicted in this HR region proximal to the fusion peptide in baculovirus F proteins. To facilitate our understanding of the functional role of this leucine zipper-like HR1 domain in baculovirus F protein synthesis, processing, and viral infectivity, key leucine residues (Leu209, Leu216, and Leu223) were replaced by alanine (A) or arginine (R), respectively. By using Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) as a pseudotype expression system, we demonstrated that all mutant F proteins incorporated into budded virus, indicating that leucine substitutions did not affect intercellular trafficking of F. Furin-like protease cleavage was not affected by any of the leucine substitutions; however, the disulfide bridging and N-linked glycosylation patterns were partly altered. Single substitutions in HR1 showed that the three leucine residues were critical for F fusogenicity and the rescue of AcMNPV infectivity. Our results support the view that the leucine zipper-like HR1 domain is important to safeguard the proper folding, glycosylation, and fusogenicity of baculovirus F proteins.

Positive selection of Iris, a retroviral envelope-derived host gene in Drosophila melanogaster

Eukaryotic genomes can usurp enzymatic functions encoded by mobile elements for their own use. A particularly interesting kind of acquisition involves the domestication of retroviral envelope genes, which confer infectious membrane-fusion ability to retroviruses. So far, these examples have been limited to vertebrate genomes, including primates where the domesticated envelope is under purifying selection to assist placental function. Here, we show that in Drosophila genomes, a previously unannotated gene (CG4715, renamed Iris) was domesticated from a novel, active Kanga lineage of insect retroviruses at least 25 million years ago, and has since been maintained as a host gene that is expressed in all adult tissues. Iris and the envelope genes from Kanga retroviruses are homologous to those found in insect baculoviruses and gypsy and roo insect retroviruses. Two separate envelope domestications from the Kanga and roo retroviruses have taken place, in fruit fly and mosquito genomes, respectively. Whereas retroviral envelopes are proteolytically cleaved into the ligand-interaction and membrane-fusion domains, Iris appears to lack this cleavage site. In the takahashii/suzukii species groups of Drosophila, we find that Iris has tandemly duplicated to give rise to two genes (Iris-A and Iris-B). Iris-B has significantly diverged from the Iris-A lineage, primarily because of the “invention” of an intron de novo in what was previously exonic sequence. Unlike domesticated retroviral envelope genes in mammals, we find that Iris has been subject to strong positive selection between Drosophila species. The rapid, adaptive evolution of Iris is sufficient to unambiguously distinguish the phylogenies of three closely related sibling species of Drosophila (D. simulans, D. sechellia, and D. mauritiana), a discriminative power previously described only for a putative “speciation gene.” Iris represents the first instance of a retroviral envelope–derived host gene outside vertebrates. It is also the first example of a retroviral envelope gene that has been found to be subject to positive selection following its domestication. The unusual selective pressures acting on Iris suggest that it is an active participant in an ongoing genetic conflict. We propose a model in which Iris has “switched sides,” having been recruited by host genomes to combat baculoviruses and retroviruses, which employ homologous envelope genes to mediate infection.

Mobile genetic elements have made homes within eukaryotic (host) genomes for hundreds of millions of years. These include retroviruses that integrate into host genomes as an essential step in their life cycle. While most such integration events are likely to be either deleterious or of little consequence to the host, on rare occasions host genomes can preserve and exploit capabilities of mobile elements for their own function. Especially intriguing are instances where host genomes have chosen to retain the envelope genes of retroviruses; the same envelope genes are responsible for conferring infectious ability to retroviruses. Primates and rodent genomes each have domesticated retroviral envelope genes (called “syncytin” genes) for important roles in placental function.

Now, Harmit Malik and colleagues show that a similar, ancient domestication event has taken place within the fruit fly Drosophila melanogaster. They identify a gene, Iris, which was acquired from an envelope gene of insect retroviruses, and has been maintained as a host gene for more than 25 million years. Unexpectedly, the authors find that Iris continues to evolve rapidly whereas previous studies have shown that mammalian syncytin genes do not. They suggest a model in which the Iris gene has “switched sides,” from its original role in causing infections to its current role in preventing them.

Envelope gene capture and insect retrovirus evolution: The relationship between errantivirus and baculovirus envelope proteins

In this report the evolution of insect retroviruses (errantiviruses) is reviewed with particular emphasis on the relationship between their env protein and a baculovirus envelope fusion protein. In addition, selected features of the env protein from the errantivirus Dme17.6V are examined. These include characterization of the 21 amino acid predicted fusion peptide sequence that is highly homologous to a region of baculovirus envelope fusion proteins. We found that, although this sequence could not substitute for the homologous sequence in the baculovirus LD130 envelope fusion protein, by changing four amino acids, the hybrid construct became active for low-pH induced cell fusion. In addition, a Dme17.6V env-egfp construct was found to localize to cell membranes.

Function, oligomerization and N-linked glycosylation of the Helicoverpa armigera single nucleopolyhedrovirus envelope fusion protein

In the family Baculoviridae, two distinct envelope fusion proteins are identified in budded virions (BVs). GP64 is the major envelope fusion protein of group I nucleopolyhedrovirus (NPV) BVs. An unrelated type of envelope fusion protein, named F, is encoded by group II NPVs. The genome of Helicoverpa armigera (Hear) NPV, a group II NPV of the single nucleocapsid or S type, also encodes an F-like protein: open reading frame 133 (Ha133). It was demonstrated by N-terminal sequencing of the major 59 kDa protein present in HearNPV BV that this protein is one of the two F subunits: F1 (transmembrane subunit of 59 kDa) and F2 (surface subunit of 20 kDa), both the result of cleavage by a proprotein convertase and disulfide-linked. The HearNPV F protein proved to be a functional analogue of GP64, as the infectivity of an AcMNPV gp64-deletion mutant was rescued by the introduction of the HearNPV F gene. It was also demonstrated by chemical cross-linking that HearNPV F is present in BVs as an oligomer whereby, unlike GP64, disulfide bonds are not involved. Deglycosylation assays indicated that both F1 and F2 possess N-linked glycans. However, when F was made in Hz2E5 cells, these glycans did not have an alpha-1-3 core fucose modification that usually occurs in insect cells. As alpha-1-3 core fucose is a major inducer of an allergic response in humans, the present observation makes the HearNPV-Hz2E5 system an attractive alternative for the production of recombinant glycoproteins for therapeutic use in humans.

Conserved molecular systems of the Baculoviridae

Although the Baculoviridae are a large and diverse family of viruses, they are united by a number of shared features that form the basis for their unique life cycle. These include the mechanism of cell entry, genome replication and processing, and late and very late gene transcription. In this review, the molecular systems that are conserved within the Baculoviridae and that are responsible these processes are described.

Molecular identification and phylogenetic analysis of baculoviruses from Lepidoptera

PCR amplification of the highly conserved baculovirus genes late expression factor 8 (lef-8), late expression factor 9 (lef-9) and polyhedrin/granulin (polh/gran) combined with molecular phylogenetic analyses provide a powerful tool to identify lepidopteran-specific baculoviruses and to study their diversity. In the present investigation, we have improved the degenerate oligonucleotides and corroborated the approach that was recently described by Lange et al. (Lange, M., Wang, H., Zhihong, H., Jehle, J.A., 2004. Towards a molecular identification and classification system of lepidopteran-specific baculoviruses. Virology 325, 36-47.). Baculovirus DNA was isolated from 71 uncharacterized historic baculovirus samples, and partial gene sequences were amplified by using gene-specific degenerate PCR primers. The obtained PCR products were directly sequenced, and the deduced amino acid sequences were compiled and aligned with published sequences of these target genes. A phylogenetic tree of 117 baculoviruses was inferred using maximum parsimony and distance methods. Based on the comprehensive phylogenetic analysis of the partial lef-8, lef-9 and polh/gran genes, we propose a phylogenetic species criterion for lepidopteran-specific baculoviruses that uses the genetic distances of these genes for species demarcation.

A cellular Drosophila melanogaster protein with similarity to baculovirus F envelope fusion proteins

Baculovirus F (fusion) proteins are found in the envelopes of budded virions. Recently a Drosophila melanogaster gene (CG4715) that encodes a protein with sequence similarity to baculovirus F proteins was discovered. To examine similarities and differences with baculovirus F proteins, we cloned the D. melanogaster cellular F (Dm-cF) protein gene and analyzed Dm-cF expression and localization. The predicted Dm-cF protein sequence lacks a furin cleavage site, and transiently expressed Dm-cF showed no protein cleavage and no detectable membrane fusion activity. In cell localization studies, transiently expressed Dm-cF was localized to intracellular organelles in D. melanogaster S2 cells, unlike baculovirus F proteins, which localize to cellular plasma membranes. Using reverse transcriptase PCR and Western blot analysis to examine Dm-cF expression in animals, we detected Dm-cF expression in both larval and adult D. melanogaster cells. However, Dm-cF expression was detected only in third instar larvae and adults, suggesting that Dm-cF expression may be developmentally regulated. We also identified genes related to Dm-cF in the genomes of two other Drosophila species, Drosophila yakuba and Drosophila pseudoobscura, and the mosquito Anopheles gambiae. These observations suggest that f genes may be present in the genomes of many insects. Conservation within and between 22 baculovirus and 4 insect F proteins was examined in detail. These studies demonstrate that Dm-cF is expressed in D. melanogaster and suggest that if baculovirus f genes are derived from a host cellular f gene, the function appears to have changed substantially upon adaptation to the baculovirus infection cycle.

Analysis of the Drosophila gypsy endogenous retrovirus envelope glycoprotein

gypsy is the only endogenous retrovirus of Drosophila whose infectious properties have been reported. Previous studies have shown an unexpected relationship between the gene encoding the putative envelope glycoprotein (Env) of gypsy and genes encoding the fusion protein of several baculoviruses. The fact that fusion proteins mediate membrane fusion suggests that Env of insect retroviruses might also have fusogenic properties. The results reported here indicate that gypsy Env mediates cell-to-cell fusion. Cleavage of the Env precursor was also studied; it is shown that this polypeptide is cleaved at a furin-like cleavage site. This is the first report that the env-like gene of insect retroviruses encodes a fusion protein.

Functional analysis of the putative fusion domain of the baculovirus envelope fusion protein F

Group II nucleopolyhedroviruses (NPVs), e.g., Spodoptera exigua MNPV, lack a GP64-like protein that is present in group I NPVs but have an unrelated envelope fusion protein named F. In contrast to GP64, the F protein has to be activated by a posttranslational cleavage mechanism to become fusogenic. In several vertebrate viral fusion proteins, the cleavage activation generates a new N terminus which forms the so-called fusion peptide. This fusion peptide inserts in the cellular membrane, thereby facilitating apposition of the viral and cellular membrane upon sequential conformational changes of the fusion protein. A similar peptide has been identified in NPV F proteins at the N terminus of the large membrane-anchored subunit F(1). The role of individual amino acids in this putative fusion peptide on viral infectivity and propagation was studied by mutagenesis. Mutant F proteins with single amino acid changes as well as an F protein with a deleted putative fusion peptide were introduced in gp64-null Autographa californica MNPV budded viruses (BVs). None of the mutations analyzed had an major effect on the processing and incorporation of F proteins in the envelope of BVs. Only two mutants, one with a substitution for a hydrophobic residue (F152R) and one with a deleted putative fusion peptide, were completely unable to rescue the gp64-null mutant. Several nonconservative substitutions for other hydrophobic residues and the conserved lysine residue had only an effect on viral infectivity. In contrast to what was expected from vertebrate virus fusion peptides, alanine substitutions for glycines did not show any effect.

Towards a molecular identification and classification system of lepidopteran-specific baculoviruses

Virus genomics provides novel approaches for virus identification and classification. Based on the comparative analyses of sequenced lepidopteran-specific baculovirus genomes, degenerate oligonucleotides were developed that allow the specific amplification of several regions of the genome using polymerase chain reaction (PCR) followed by DNA sequencing. The DNA sequences within the coding regions of three highly conserved genes, namely polyhedrin/granulin (polh/gran), late expression factor 8 (lef-8), and late expression factor 9 (lef-9), were targeted for amplification. The oligonucleotides were tested on viral DNAs isolated from historical field samples, and amplification products were generated from 12 isolated nucleopolyhedrovirus (NPV) and 8 granulovirus (GV) DNAs. The PCR products were cloned or directly sequenced, and phylogenetic trees were inferred from individual and combined data sets of these three genes and compared to a phylogeny, which includes 22 baculoviruses using a combined data set of 30 core genes. This method allows a fast and reliable detection and identification of lepidopteran-specific NPVs and GVs. Furthermore, a strong correlation of the base composition of these three genome areas with that of the complete virus genome was observed and used to predict the base composition of uncharacterized baculovirus genomes. These analyses suggested that GVs have a significantly higher AT content than NPVs.

Conservation of a proteinase cleavage site between an insect retrovirus (gypsy) Env protein and a baculovirus envelope fusion protein

The predicted Env protein of insect retroviruses (errantiviruses) is related to the envelope fusion protein of a major division of the Baculoviridae. The highest degree of homology is found in a region that contains a furin cleavage site in the baculovirus proteins and an adjacent sequence that has the properties of a fusion peptide. In this investigation, the homologous region in the Env protein of the gypsy retrovirus of Drosophila melanogaster (DmegypV) was investigated. Alteration of the predicted DmegypV Env proteinase cleavage site from RIAR to AIAR significantly reduced cleavage of Env in both Spodoptera frugiperda (Sf-9) and D. melanogaster (S2) cell lines. When the predicted DmegypV Env cleavage site RIAR was substituted for the cleavage sequence RRKR in the Lymantria dispar nucleopolyhedrovirus fusion protein (LD130) sequence, cleavage of the hybrid LD130 molecules still occurred, although at a reduced level. The conserved 21-amino acid sequence just downstream of the cleavage site, which is thought to be the fusion peptide in LD130, was also characterized. When this sequence from DmegypV Env was substituted for the homologous sequence in LD130, cleavage still occurred, but no fusion was observed in either cell type. In addition, although a DmegypV-Env-green fluorescent protein construct localized to cell membranes, no cell fusion was observed.