A Betabaculovirus-Encoded gp64 Homolog Codes for a Functional Envelope Fusion Protein

Cross-talking between baculoviruses and host insects towards a successful infection

Baculoviridae is a family of large DNA viruses that infect insects. They have been extensively used as safe and efficient biological agents for the control of insect pests. As a result of coevolution with their hosts, baculoviruses developed unique life cycles characterized by the production of two distinctive virion phenotypes, occlusion-derived virus and budded virus, which are responsible for mediating primary infection in insect midgut epithelia and spreading systemic infection within infected insects, respectively. In this article, advances associated with virus-host interactions during the baculovirus life cycle are reviewed. We mainly focus on how baculoviruses exploit versatile strategies to overcome diverse host barriers and establish successful infections. For example, in the midgut, baculoviruses encode enzymes to degrade peritrophic membranes and use a series of per os infectivity factors to initiate primary infection. A viral fibroblast growth factor is expressed to attract tracheoblasts that spread the virus for systemic infection. Baculoviruses use different strategies to suppress host defence systems, including apoptosis, melanization and RNA interference. Additionally, baculoviruses can manipulate host physiology and induce 'tree-top disease' for optimal virus replication and dispersal. These advances in our understanding of baculoviruses will greatly inform the development of more effective baculoviral pesticides. This article is part of the theme issue 'Biotic signalling sheds light on smart pest management'.

Evaluation of the anti-apoptotic activity of bovine alphaherpesvirus type 5 US3 protein kinase in insect cells using a recombinant baculovirus

Bovine alphaherpesvirus type 5 (BoHV-5) is one of the main agents responsible for meningoencephalitis in cattle in Brazil, causing significant economic losses. It is known that other viruses of the Herpesviridae family such as Bovine alphaherpesvirus type 1, Swine alphaherpesvirus type 1, and the Human alphaherpesvirus types 1 and 2 encode genes homologous to BoHV-5, with recognized action in the control of apoptosis. The objective of this work was to express the BoHV-5 US3 gene in a baculovirus-based expression system for the production of the serine/threonine kinase protein and to evaluate its activity in the control of apoptosis in vitro. A recombinant baculovirus derived from the Autographa californica multiple nucleopolyhedrovirus (AcMNPV) containing the US3 gene and a deletion in the baculovirus anti-apoptotic p35 gene was constructed using the Bac-to-Bac™ system. This recombinant baculovirus was used to evaluate the anti-apoptotic activity of the recombinant US3 protein in insect cells comparing with two other AcMNPV recombinants, one containing a functional copy of the AcMNPV anti-apoptotic p35 gene and an AcMNPV p35 knockout virus with the anti-apoptotic iap-3 gene from Anticarsia gemmatalis multiple nucleopolyhedrovirus (AgMNPV). We found that the caspase level was higher in insect cells infected with the US3-contanining recombinant virus than in cells infected with the AcMNPV recombinants containing the p35 and iap-3 genes. These results indicate that the BoHV-5 US3 protein kinase gene is not able to block apoptosis in insect cells induced by the infection of a p35 knockout AcMNPV.

A Nymphalid-Infecting Group I Alphabaculovirus Isolated from the Major Passion Fruit Caterpillar Pest Dione juno juno (Lepidoptera: Nymphalidae)

Baculoviruses are capable of infecting a wide diversity of insect pests. In the 1990s, the Dione juno nucleopolyhedrovirus (DijuNPV) was isolated from larvae of the major passionfruit defoliator pest Dione juno juno (Nymphalidae) and described at ultrastructural and pathological levels. In this study, the complete genome sequence of DijuNPV was determined and analyzed. The circular genome presents 122,075 bp with a G + C content of 50.9%. DijuNPV is the first alphabaculovirus completely sequenced that was isolated from a nymphalid host and may represent a divergent species. It appeared closely related to Orgyia pseudotsugata multiple nucleopolyhedrovirus (OpMNPV) and other Choristoneura-isolated group I alphabaculoviruses. We annotated 153 open reading frames (ORFs), including a set of 38 core genes, 26 ORFs identified as present in lepidopteran baculoviruses, 17 ORFs unique in baculovirus, and several auxiliary genes (e.g., bro, cathepsin, chitinase, iap-1, iap-2, and thymidylate kinase). The thymidylate kinase (tmk) gene was present fused to a dUTPase (dut) gene in other baculovirus genomes. DijuNPV likely lost the dut portion together with the iap-3 homolog. Overall, the genome sequencing of novel alphabaculoviruses enables a wide understanding of baculovirus evolution.

Improving Baculovirus Transduction of Mammalian Cells by Incorporation of Thogotovirus Glycoproteins

Baculovirus can transduce a wide range of mammalian cells and is considered a promising gene therapy vector. However, the low transduction efficiency of baculovirus into many mammalian cells limits its practical application. Co-expressing heterologous viral glycoproteins (GPs), such as vesicular stomatitis virus G protein (VSV G), with baculovirus native envelope protein GP64 is one of the feasible strategies for improving virus transduction. Tick-borne thogotoviruses infect mammals and their GPs share sequence/structure homology and common evolutionary origins with baculovirus GP64. Herein, we tested whether thogotovirus GPs could facilitate the entry of the prototype baculovirus Autographa californica multiple multiple nucleopolyhedrovirus (AcMNPV) into mammalian cells. The gp genes of two thogotoviruses, Thogoto virus and Dhori virus, were inserted into the AcMNPV genome. Both GPs were properly expressed and incorporated into the envelope of the recombinant AcMNPVs. The transduction rates of recombinant AcMNPVs expressing the two thogotovirus GPs increased for approximately 4-12 fold compared to the wild type AcMNPV in six of the 12 tested mammalian cell lines. It seemed that thogotovirus GPs provide the recombinant AcMNPVs with different cell tropisms and showed better performance in several mammalian cells compared to VSV G incorporated AcMNPV. Further studies showed that the improved transduction was a result of augmented virus-endosome fusion and endosome escaping, rather than increased cell binding or internalization. We found the AcMNPV envelope protein GP64-mediated fusion was enhanced by the thogotovirus GPs at relatively higher pH conditions. Therefore, the thogotovirus GPs represent novel candidates to improve baculovirus-based gene delivery vectors.

Genome sequence analysis and organization of the Hyphantria cunea granulovirus (HycuGV-Hc1) from Turkey

The fall webworm (Hyphantria cunea) impacts a wide variety of crops and cultivated broadleaf plant species. The pest is native to North America, was introduced to Europe and has since spread further as far as central Asia. Despite several attempts to control its distribution, the pest continues to spread causing damage all over the world. A naturally occurring baculovirus, Hyphantria cunea granulovirus (HycuGV-Hc1), isolated from the larvae of H. cunea in Turkey appears to have a potential as microbial control agent against this pest. In this report we describe the complete genome sequence and organization of the granulovirus isolate (HycuGV-Hc1) that infects the larval stages and compare it to other baculovirus genomes. The HycuGV-Hc1 genome is a circular double-stranded DNA of 114,825 bp in size with a nucleotide distribution of 39.3% G + C. Bioinformatics analysis predicted 132 putative open reading frames of (ORFs) ≥ 150 nucleotides. There are 24 ORFs with unknown function. Seven homologous repeated regions (hrs) and two bro genes (bro-1 and bro-2) were identified in the genome. Comparison to other baculovirus genomes, HycuGV-Hc1 revealed some differences in gene content and organization. Gene parity plots and phylogenetics confirmed that HycuGV-Hc1 is a Betabaculovirus and is closely related to Plutella xylostella granulovirus. This study expands our knowledge on the genetic variation of HycuGV isolates and provides further novel knowledge on the nature of granuloviruses.

Genome analysis of a novel Group I alphabaculovirus obtained from Oxyplax ochracea

Oxyplax ochracea (Moore) is a pest that causes severe damage to a wide range of crops, forests and fruit trees. The complete genome sequence of Oxyplax ochracea nucleopolyhedrovirus (OxocNPV) was determined using a Roche 454 pyrosequencing system. OxocNPV has a double-stranded DNA (dsDNA) genome of 113,971 bp with a G+C content of 31.1%. One hundred and twenty-four putative open reading frames (ORFs) encoding proteins of >50 amino acids in length and with minimal overlapping were predicted, which covered 92% of the whole genome. Six baculoviral typical homologous regions (hrs) were identified. Phylogenetic analysis and gene parity plot analysis showed that OxocNPV belongs to clade “a” of Group I alphabaculoviruses, and it seems to be close to the most recent common ancestor of Group I alphabaculoviruses. Three unique ORFs (with no homologs in the National Center for Biotechnology Information database) were identified. Interestingly, OxocNPV lacks three auxiliary genes (lef7, ie-2 and pcna) related to viral DNA replication and RNA transcription. In addition, OxocNPV has significantly different sequences for several genes (including ie1 and odv-e66) in comparison with those of other baculoviruses. However, three dimensional structure prediction showed that OxocNPV ODV-E66 contain the conserved catalytic residues, implying that it might possess polysaccharide lyase activity as AcMNPV ODV-E66. All these unique features suggest that OxocNPV represents a novel species of the Group I alphabaculovirus lineage.

Autographa californica Nucleopolyhedrovirus AC141 (Exon0), a Potential E3 Ubiquitin Ligase, Interacts with Viral Ubiquitin and AC66 To Facilitate Nucleocapsid Egress

During the infection cycle of Autographa californica multiple nucleopolyhedrovirus (AcMNPV), two forms of virions are produced, budded virus (BV) and occlusion-derived virus (ODV). Nucleocapsids that form BV have to egress from the nucleus, whereas nucleocapsids that form ODV remain inside the nucleus. The molecular mechanism that determines whether nucleocapsids remain inside or egress from the nucleus is unknown. AC141 (a predicted E3 ubiquitin ligase) and viral ubiquitin (vUbi) have both been shown to be required for efficient BV production. In this study, it was hypothesized that vUbi interacts with AC141, and in addition, that this interaction was required for BV production. Deletion of both ac141 and vubi restricted viral infection to a single cell, and BV production was completely eliminated. AC141 was ubiquitinated by either vUbi or cellular Ubi, and this interaction was required for optimal BV production. Nucleocapsids in BV, but not ODV, were shown to be specifically ubiquitinated by vUbi, including a 100-kDa protein, as well as high-molecular-weight conjugates. The viral ubiquitinated 100-kDa BV-specific nucleocapsid protein was identified as AC66, which is known to be required for BV production and was shown by coimmunoprecipitation and mass spectrometry to interact with AC141. Confocal microscopy also showed that AC141, AC66, and vUbi interact at the nuclear periphery. These results suggest that ubiquitination of nucleocapsid proteins by vUbi functions as a signal to determine if a nucleocapsid will egress from the nucleus and form BV or remain in the nucleus to form ODV.IMPORTANCE Baculoviruses produce two types of virions called occlusion-derived virus (ODV) and budded virus (BV). ODVs are required for oral infection, whereas BV enables the systemic spread of virus to all host tissues, which is critical for killing insects. One of the important steps for BV production is the export of nucleocapsids out of the nucleus. This study investigated the molecular mechanisms that enable the selection of nucleocapsids for nuclear export instead of being retained within the nucleus, where they would become ODV. Our data show that ubiquitination, a universal cellular process, specifically tags nucleocapsids of BV, but not those found in ODV, using a virus-encoded ubiquitin (vUbi). Therefore, ubiquitination may be the molecular signal that determines if a nucleocapsid is destined to form a BV, thus ensuring lethal infection of the host.

Genomic analysis of a Trichoplusia ni Betabaculovirus (TnGV) with three different viral enhancing factors and two unique genes

The complete genome of a Trichoplusia ni granulovirus (TnGV) is described and analyzed. The genome contains 175,360 bp (KU752557), becoming the third largest genome within the genus Betabaculovirus, smaller only than the Xestia c-nigrum GV (XecnGV) (178,733 pb) and the Pseudaletia unipuncta GV (PsunGV) (176,677 pb) genomes. The TnGV genome has a 39.81% C+G content and a total of 180 ORFs were identified, 96 of them in the granulin gene direction and 84 in the opposite direction. A total of 94.38% of the ORFs showed high identity with those of ClanGV, HaGV, and SlGV. Eight homologous regions (hrs) were identified as well as one apoptosis inhibitor (IAP-3). Interestingly, three viral enhancing factors (VEFs) were located in TnGV genome: VEF-1 (orf153), VEF-3 (orf155), and VEF-4 (orf164), additional to another metalloprotease (orf37). Two ORFs were unique to TnGV (orf100 and orf101) and another one was shared by only TnGV and AgseGV (orf2). Eleven of the deduced proteins showed high identity with proteins from nucleopolyhedroviruses, three with proteins from ascoviruses, and one with an entomopoxvirus protein. The largest deduced protein contains 1,213 amino acids (orf43) and the smallest deduced protein contains only 50 amino acids (orf143). Sequence identity and phylogenetic analyses showed that the closest related genomes to TnGV are, to date, those of PsunGV and XecnGV. This genome analysis may contribute to functional research on TnGV, and may form the bases for the utilization of this betabaculovirus as a pest control agent.

A betabaculovirus DNA polymerase cannot substitute for the DNA polymerase of the alphabaculovirus Autographa californica nucleopolyhedrovirus

DNA polymerase (DNApol) is present in all baculoviruses and plays a crucial role in viral DNA replication. Previously we showed that the DNApol of the alphabaculovirus group II Spodoptera litura nucleopolyhedrovirus (SpltNPV) could partially substitute for the DNApol of a group I alphabaculovirus, Autographa californica multiple nucleopolyhedrovirus (AcMNPV). However, it is not known if a betabaculovirus DNApol could subsititute for the alphabaculovirus DNApol in AcMNPV. In this report, DNApol of the betabaculovirus Pieris rapae granulovirus (PiraGV) was inserted into a dnapol-null AcMNPV bacmid, creating Bac-AcΔpol:PrPol. The repair virus did not spread to neighboring cells; virus growth curve and real-time PCR revealed that the PiraGV dnapol substitution abrogated AcMNPV DNA replication and virus production. Immunofluorescence microscopy showed that PiraGV DNApol could be expressed and localized to the nucleus. Collectively, our results suggested that the alphabaculovirus AcMNPV DNApol could not be replaced by a DNApol from the betabaculovirus, PiraGV.

A betabaculovirus encoding a gp64 homolog

Background

A betabaculovirus (DisaGV) was isolated from Diatraea saccharalis (Lepidoptera: Crambidae), one of the most important insect pests of the sugarcane and other monocot cultures in Brazil.

Results

The complete genome sequence of DisaGV was determined using the 454-pyrosequencing method. The genome was 98,392 bp long, which makes it the smallest lepidopteran-infecting baculovirus sequenced to date. It had a G + C content of 29.7 % encoding 125 putative open reading frames (ORF). All the 37 baculovirus core genes and a set of 19 betabaculovirus-specific genes were found. A group of 13 putative genes was not found in any other baculovirus genome sequenced so far. A phylogenetic analysis indicated that DisaGV is a member of Betabaculovirus genus and that it is a sister group to a cluster formed by ChocGV, ErelGV, PiraGV isolates, ClanGV, CaLGV, CpGV, CrleGV, AdorGV, PhopGV and EpapGV. Surprisingly, we found in the DisaGV genome a G protein-coupled receptor related to lepidopteran and other insect virus genes and a gp64 homolog, which is likely a product of horizontal gene transfer from Group 1 alphabaculoviruses.

Conclusion

DisaGV represents a distinct lineage of the genus Betabaculovirus. It is closely related to the CpGV-related group and presents the smallest genome in size so far. Remarkably, we found a homolog of gp64, which was reported solely in group 1 alphabaculovirus genomes so far.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2408-9) contains supplementary material, which is available to authorized users.