Isolation of an apoptosis suppressor gene of the Spodoptera littoralis nucleopolyhedrovirus

Genomic sequencing of Troides aeacus nucleopolyhedrovirus (TraeNPV) from golden birdwing larvae (Troides aeacus formosanus) to reveal defective Autographa californica NPV genomic features

Background

The golden birdwing butterfly (Troides aeacus formosanus) is a rarely observed species in Taiwan. Recently, a typical symptom of nuclear polyhedrosis was found in reared T. aeacus larvae. From the previous Kimura-2 parameter (K-2-P) analysis based on the nucleotide sequence of three genes in this isolate, polh, lef-8 and lef-9, the underlying virus did not belong to any known nucleopolyhedrovirus (NPV) species. Therefore, this NPV was provisionally named “TraeNPV”. To understand this NPV, the nucleotide sequence of the whole TraeNPV genome was determined using next-generation sequencing (NGS) technology.

Results

The genome of TraeNPV is 125,477 bp in length with 144 putative open reading frames (ORFs) and its GC content is 40.45%. A phylogenetic analysis based on the 37 baculoviral core genes suggested that TraeNPV is a Group I NPV that is closely related to Autographa californica nucleopolyhedrovirus (AcMNPV). A genome-wide analysis showed that TraeNPV has some different features in its genome compared with other NPVs. Two novel ORFs (Ta75 and Ta139), three truncated ORFs (pcna, he65 and bro) and one duplicated ORF (38.7 K) were found in the TraeNPV genome; moreover, there are fewer homologous regions (hrs) than there are in AcMNPV, which shares eight hrs within the TraeNPV genome. TraeNPV shares similar genomic features with AcMNPV, including the gene content, gene arrangement and gene/genome identity, but TraeNPV lacks 15 homologous ORFs from AcMNPV in its genome, such as ctx, host cell-specific factor 1 (hcf-1), PNK/PNL, vp15, and apsup, which are involved in the auxiliary functions of alphabaculoviruses.

Conclusions

Based on these data, TraeNPV would be clarified as a new NPV species with defective AcMNPV genomic features. The precise relationship between TraeNPV and other closely related NPV species were further investigated. This report could provide comprehensive information on TraeNPV for evolutionary insights into butterfly-infected NPV.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5713-2) contains supplementary material, which is available to authorized users.

The complete genome sequence of a second alphabaculovirus from the true armyworm, Mythimna unipuncta: implications for baculovirus phylogeny and host specificity

The Mythimna unipuncta nucleopolyhedrovirus isolate KY310 (MyunNPV-KY310) is an alphabaculovirus isolated from a true armyworm (Mythimna unipuncta) population in Kentucky, USA. Occlusion bodies of this virus were examined by electron microscopy and the genome sequence was determined by 454 pyrosequencing. MyunNPV-KY310 occlusion bodies consisted of irregular polyhedra measuring 0.8-1.8 µm in diameter and containing multiple virions, with one to six nucleocapsids per virion. The genome sequence was determined to be 156,647 bp with a nucleotide distribution of 43.9% G+C. 152 ORFs and six homologous repeat (hr) regions were annotated for the sequence, including the 38 core genes of family Baculoviridae and an additional group of 26 conserved alphabaculovirus genes. BLAST queries and phylogenetic inference confirmed that MyunNPV-KY310 is most closely related to the alphabaculovirus Leucania separata nucleopolyhedrovirus isolate AH1, which infects Mythimna separata. In contrast, MyunNPV-KY310 did not exhibit a close relationship with Mythimna unipuncta nucleopolyhedrovirus isolate #7, an alphabaculovirus from the same host species. MyunNPV-KY310 lacks the gp64 envelope glycoprotein, which is a characteristic of group II alphabaculoviruses. However, this virus and five other alphabaculoviruses lacking gp64 are placed outside the group I and group II clades in core gene phylogenies, further demonstrating that viruses of genus Alphabaculovirus do not occur in two monophyletic clades. Potential instances of MyunNPV-KY310 ORFs arising by horizontal transfer were detected. Although there are now genome sequences of four different baculoviruses from M. unipuncta, comparison of their genome sequences provides little insight into the genetic basis for their host specificity.

Characterization and functional assay of apsup (Lyxy105) from Lymantria xylina multiple nucleopolyhedrovirus (LyxyMNPV)

The baculoviral anti-apoptotic genes, p35 and iap (inhibitor of apoptosis), play important roles in the initiation of viral infection. Recently, a new anti-apoptotic gene (apoptosis suppressor, apsup) was identified in Lymantria dispar multiple nucleopolyhedrovirus (LdMNPV). An apsup homolog gene, Lyxy105 (ly-apsup), was also predicted in the Lymantria xylina multiple nucleopolyhedrovirus (LyxyMNPV) genome. In this study, we attempt to perform a gene expression analysis and a functional assay of ly-apsup to demonstrate its anti-apoptotic activity and identify the functional domain of this protein. The transcription of the ly-apsup gene region was detected from 12 h post-infection (hpi) and increased significantly after 24-72 hpi. Comparison of the putative amino acid sequences to those of 18 baculoviral homolog proteins showed high amino acid identity to the LdMNPV sequences. Moreover, five conserved protein domains (named as domains I-V) were found. Therefore, protein functional assays were conducted on full-length proteins and different truncation clones. The overexpression of each clone was confirmed by western blot analysis, and the data revealed that a cleavage of ~ 5 kDa at the N-terminal region of the full-length, domains I-IV (1-241) and I-III (1-178), proteins occurred. The results of the functional analysis showed that full-length Ly-apsup and Ly-apsup with domain I (1-70) could inhibit Drosophila-RPR protein (D-RPR)-induced and actinomycin D (ActD)-induced apoptoses. In addition, the domains I and I-II (1-126) regions showed higher anti-apoptotic activity than the other domains in both D-RPR-induced and ActD-induced cell apoptoses. In conclusion, domain I of Ly-apsup may play an important role in the anti-apoptotic activity of this protein; cleavage of the Ly-apsup N-terminus may lead to decreased anti-apoptotic activity.

The interaction between baculoviruses and their insect hosts

Baculoviruses are double-stranded circular DNA viruses that infect arthropods via the midgut. Because of their superiority as eukaryotic expression systems and their importance as biopesticides, extensive research on the functions of baculovirus genes as well as on the host response to baculovirus infection has been carried out, including transcriptomic and proteomic analyses of the midgut. The morphological and cellular changes caused by baculovirus infection are also important to better understand the infection pathway. Thanks to these previous studies, we now have a clearer picture of the mechanisms of action of the virus and of host immunity. In this paper, we systematically reviewed studies on the interaction between baculoviruses and their insect hosts. By better understanding these interactions, baculoviruses can be developed for use as more efficient biopesticides to improve agricultural development in the future.

The protective effect of a novel antioxidant gene from Mycobacterium avium against nitrosative and oxidative stress in E. coli

The production of reactive oxygen intermediates (ROI) and reactive nitrogen intermediates (RNI) is an important host defense mechanism in response to infection by Mycobacterium tuberculosis. A variety of genes have been implicated in resistance to ROI and RNI, including noxR1. However, studies in Mycobacterium avium, an important pathogen among nontuberculous mycobacteria, are limited. We aim to investigate the role of a novel gene cloned from M. avium with high similarity to noxR1, noA, in resistance against RNI and ROI in M. tuberculosis. After subcloning noA into vector for expression in E. coli, we performed survival rate analysis in the bacteria transformed with noA (pET-noA) and without noA (pET-his) after exposure to nitrosative stresses by S-nitrosoglutathione (GSNO) and sodium nitrite, and oxidative stresses by H₂O₂. Compared with pET-his, the survival rate of pET-noA was 1 log₁₀-fold higher after exposure to GSNO and sodium nitrite. We observed 1 log₁₀-fold, 2 log₁₀-fold and 3 log₁₀-fold higher survival rate in pET-noA than pET-his after exposure to H₂O₂ for 3, 6 and 9 h, respectively. With the combined treatment of H₂O₂ and GSNO, we found more than 2 log₁₀-fold increase in survival rate in pET-noA comparing with pET-his, suggesting a possible synergistic effect. In summary, noA gene cloned from M. avium has been shown to protect E. coli from both RNI and ROI.

Using the Baculovirus/Insect Cell System to Study Apoptosis

Apoptosis is a physiological program of cell suicide conserved in invertebrates and vertebrates. Apoptosis is crucial to the normal development of organisms and in tissue homeostasis by promoting elimination of unwanted cells, including damaged or virus-infected cells. Due to the importance of programmed cell death for the survival of the organism, a tight regulation is exerted at various activation levels of the cell-death machinery. The utilization of the baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) to identify genes that inhibit the apoptotic process will be described using a transfection-based approach, illustrated by identification of the p49 gene.

Generating a host range-expanded recombinant baculovirus

As baculoviruses usually have a narrow insecticidal spectrum, knowing the mechanisms by which they control the host-range is prerequisite for improvement of their applications as pesticides. In this study, from supernatant of culture cells transfected with DNAs of an Autographa californica multiple nucleopolyhedrovirus (AcMNPV) mutant lacking the antiapoptotic gene p35 (vAc(∆P35)) and a cosmid representing a fragment of Spodoptera exigua nucleopolyhedrovirus (SeMNPV), a viral strain was plaque-purified and named vAcRev. vAcRev had a broader host range than either vAc(∆P35) or SeMNPV parental virus, being able to infect not only the permissive hosts of its parental viruses but also a nonpermissive host (Spodoptera litura). Genome sequencing indicated that vAcRev comprises a mixture of two viruses with different circular dsDNA genomes. One virus contains a genome similar to vAc(∆P35), while in the other viral genome, a 24.4 kbp-fragment containing 10 essential genesis replaced with a 4 kbp-fragment containing three SeMNPV genes including a truncated Se-iap3 gene. RNA interference and ectopic expression assays found that Se-iap3 is responsible for the host range expansion of vAcRev, suggesting that Se-iap3 inhibits the progression of apoptosis initiated by viral infection and promotes viral propagation in hosts both permissive and non-permissive for AcMNPV and SeMNPV.

Reaching the melting point: Degradative enzymes and protease inhibitors involved in baculovirus infection and dissemination

Baculovirus infection of a host insect involves several steps, beginning with initiation of virus infection in the midgut, followed by dissemination of infection from the midgut to other tissues in the insect, and finally culminating in "melting" or liquefaction of the host, which allows for horizontal spread of infection to other insects. While all of the viral gene products are involved in ultimately reaching this dramatic infection endpoint, this review focuses on two particular types of baculovirus-encoded proteins: degradative enzymes and protease inhibitors. Neither of these types of proteins is commonly found in other virus families, but they both play important roles in baculovirus infection. The types of degradative enzymes and protease inhibitors encoded by baculoviruses are discussed, as are the roles of these proteins in the infection process.

Novel apoptosis suppressor Apsup from the baculovirus Lymantria dispar multiple nucleopolyhedrovirus precludes apoptosis by preventing proteolytic processing of initiator caspase Dronc

We previously identified a novel baculovirus-encoded apoptosis suppressor, Apsup, from the baculovirus Lymantria dispar multiple nucleopolyhedrovirus (LdMNPV). Apsup inhibits the apoptosis of L. dispar Ld652Y cells triggered by infection with p35-defective Autographa californica MNPV (vAcΔp35) and exposure to actinomycin D or UV light. Here, we examined the functional role of Apsup in apoptosis regulation in insect cells. Apsup prevented apoptosis and the proteolytic processing of L. dispar initiator caspase Dronc (Ld-Dronc) in Ld652Y cells triggered by overexpression of Ld-Dronc, LdMNPV inhibitor-of-apoptosis 3 (IAP3), or Hyphantria cunea MNPV IAP1. In vAcΔp35-infected apoptotic Ld652Y cells, Apsup restricted apoptosis induction and prevented processing of endogenous Ld-Dronc. Conversely, upon RNA interference (RNAi)-mediated silencing of apsup, LdMNPV-infected Ld652Y cells, which typically support high-titer virus replication, underwent apoptosis, accompanied by the processing of endogenous Ld-Dronc. Furthermore, endogenous Ld-Dronc coimmunoprecipitated with transiently expressed Apsup, indicating that Apsup physically interacts with Ld-Dronc. Apsup prevented the apoptosis of Sf9 cells triggered by vAcΔp35 infection but did not inhibit apoptosis or activation of caspase-3-like protease in vAcΔp35-infected Drosophila melanogaster S2 cells. Apsup also inhibited the proteolytic processing of L. dispar effector caspase Ld-caspase-1 in the transient expression assay but did not physically interact with Ld-caspase-1. These results demonstrate that Apsup inhibits apoptosis in Ld652Y cells by preventing the proteolytic processing of Ld-Dronc. Together with our previous findings showing that Apsup prevents the processing of both overexpressed Ld-Dronc and Bombyx mori Dronc, these results also demonstrate that Apsup functions as an effective apoptotic suppressor in various lepidopteran, but not dipteran, insect cells.

p13 from group II baculoviruses is a killing-associated gene

p13 gene was first described in Leucania separata multinuclear polyhedrosis virus (Ls-p13) several years ago, but the function of P13 protein has not been experimentally investigated to date. In this article, we indicated that the expression of p13 from Heliothis armigera single nucleocapsid nucleopolyhedrovirus (Ha-p13) was regulated by both early and late promoter. Luciferase assay demonstrated that the activity of Ha-p13 promoter with hr4 enhancer was more than 100 times in heterologous Sf9 cells than that in nature host Hz-AM1 cells. Both Ls-P13 and Ha-P13 are transmembrane proteins. Confocal microscopic analysis showed that both mainly located in the cytoplasm membrane at 48 h. Results of RNA interference indicated that Ha-p13 was a killing-associated gene for host insects H. armigera. The AcMNPV acquired the mentioned killing activity and markedly accelerate the killing rate when expressing Ls-p13. In conclusion, p13 is a killing associated gene in both homologous and heterologous nucleopolyhedrovirus. [BMB Reports 2012; 45(12): 730-735]

Determination and analysis of the genome sequence of Spodoptera littoralis multiple nucleopolyhedrovirus

The Spodoptera littoralis multiple nucleopolyhedrovirus (SpliMNPV), a pathogen of the Egyptian cotton leaf worm S. littoralis, was subjected to sequencing of its entire DNA genome and bioassay analysis comparing its virulence to that of other baculoviruses. The annotated SpliMNPV genome of 137,998 bp was found to harbor 132 open reading frames and 15 homologous repeat regions. Four unique genes not present in SpltMNPV were identified, as were 14 genes that were absent or translocated by comparison. Bioassay analysis of experimentally infected Spodoptera frugiperda revealed an extended killing time for SpliMNPV as compared to S. frugiperda MNPV (SfMNPV), but a level of mortality similar to that caused by infection with SfMNPV and superior to that of Autographa californica MNPV (AcMNPV). Although extensive similarity was observed between the genome structure and predicted translation products of SpliMNPV and Spodoptera litura MNPV (SpltMNPV), genetic distances between isolates of SpliMNPV and SpltMNPV suggest that they are in fact different species of genus Alphabaculovirus.

Toward system-level understanding of baculovirus-host cell interactions: from molecular fundamental studies to large-scale proteomics approaches

Baculoviruses are insect viruses extensively exploited as eukaryotic protein expression vectors. Molecular biology studies have provided exciting discoveries on virus-host interactions, but the application of omic high-throughput techniques on the baculovirus-insect cell system has been hampered by the lack of host genome sequencing. While a broader, systems-level analysis of biological responses to infection is urgently needed, recent advances on proteomic studies have yielded new insights on the impact of infection on the host cell. These works are reviewed and critically assessed in the light of current biological knowledge of the molecular biology of baculoviruses and insect cells.

Baculovirus Lymantria dispar multiple nucleopolyhedrovirus IAP2 and IAP3 do not suppress apoptosis, but trigger apoptosis of insect cells in a transient expression assay

Ld652Y cells derived from the gypsy moth, Lymantria dispar, are permissive for productive infection with L. dispar multiple nucleopolyhedrovirus (LdMNPV), but undergo apoptosis upon infection with various other NPVs, including those isolated from Bombyx mori, Hyphantria cunea, Spodoptera exigua, Orgyia pseudotsugata, and Spodoptera litura. In this study, we examined whether LdMNPV-encoded inhibitor of apoptosis 2 (Ld-IAP2) and 3 (Ld-IAP3) are involved in apoptosis suppression in LdMNPV-infected Ld652Y cells. We found that neither Ld-IAP2 nor Ld-IAP3 was able to suppress the apoptosis of Ld652Y cells induced by p35-defective Autographa californica MNPV (vAcΔp35). However, both Ld-IAP2 and Ld-IAP3 induced apoptosis in Ld652Y cells in a transient expression assay. The apoptosis induced by Ld-IAP3 was accompanied by the stimulation of caspase-3-like protease activity and cleavage of the B. mori homolog of the initiator caspase Dronc, and was precluded by the LdMNPV-encoded apoptosis suppressor protein Apsup and H. cunea MNPV IAP3. Inconsistent with the results obtained previously in SpIm, Ld652Y and High Five cells infected with NPVs from H. cunea, O. pseudotsugata, and A. californica, respectively, considerable stimulation of caspase-3-like protease activity was not observed in LdMNPV-infected Ld652Y cells, likely due to the strong apoptosis suppression activity of Apsup. These results, together with the previous finding that RNAi-mediated silencing of apsup induces apoptosis of LdMNPV-infected Ld652Y cells, indicate that Apsup, but not Ld-IAP2 or Ld-IAP3, is primarily responsible for the suppression of apoptosis in LdMNPV-infected Ld652Y cells. However, it remains inconclusive whether Ld-IAP2 and Ld-IAP3 function as pro-apoptotic proteins in LdMNPV-infected Ld652Y cells.

Caspase inhibitors of the P35 family are more active when purified from yeast than bacteria

Many insect viruses express caspase inhibitors of the P35 superfamily, which prevent defensive host apoptosis to enable viral propagation. The prototypical P35 family member, AcP35 from Autographa californica M nucleopolyhedrovirus, has been extensively studied. Bacterially purified AcP35 has been previously shown to inhibit caspases from insect, mammalian and nematode species. This inhibition occurs via a pseudosubstrate mechanism involving caspase-mediated cleavage of a "reactive site loop" within the P35 protein, which ultimately leaves cleaved P35 covalently bound to the caspase's active site. We observed that AcP35 purifed from Saccharomyces cerevisae inhibited caspase activity more efficiently than AcP35 purified from Escherichia coli. This differential potency was more dramatic for another P35 family member, MaviP35, which inhibited human caspase 3 almost 300-fold more potently when purified from yeast than bacteria. Biophysical assays revealed that MaviP35 proteins produced in bacteria and yeast had similar primary and secondary structures. However, bacterially produced MaviP35 possessed greater thermal stability and propensity to form higher order oligomers than its counterpart purified from yeast. Caspase 3 could process yeast-purified MaviP35, but failed to detectably cleave bacterially purified MaviP35. These data suggest that bacterially produced P35 proteins adopt subtly different conformations from their yeast-expressed counterparts, which hinder caspase access to the reactive site loop to reduce the potency of caspase inhibition, and promote aggregation. These data highlight the differential caspase inhibition by recombinant P35 proteins purified from different sources, and caution that analyses of bacterially produced P35 family members (and perhaps other types of proteins) may underestimate their activity.

Genomic sequence analysis of granulovirus isolated from the tobacco cutworm, Spodoptera litura

BACKGROUND

Spodoptera litura is a noctuid moth that is considered an agricultural pest. The larvae feed on a wide range of plants and have been recorded on plants from 40 plant families (mostly dicotyledons). It is a major pest of many crops. To better understand Spodoptera litura granulovirus (SpliGV), the nucleotide sequence of the SpliGV DNA genome was determined and analyzed.

METHODOLOGY/PRINCIPAL FINDINGS

The genome of the SpliGV was completely sequenced. The nucleotide sequence of the SpliGV genome was 124,121 bp long with 61.2% A+T content and contained 133 putative open reading frames (ORFs) of 150 or more nucleotides. The 133 putative ORFs covered 86.3% of the genome. Among these, 31 ORFs were conserved in most completely sequenced baculovirus genomes, 38 were granulovirus (GV)-specific, and 64 were present in some nucleopolyhedroviruses (NPVs) and/or GVs. We proved that 9 of the ORFs were SpliGV specific.

CONCLUSIONS/SIGNIFICANCE

The genome of SpliGV is 124,121 bp in size. One hundred thirty-three ORFs that putatively encode proteins of 50 or more amino acid residues with minimal overlap were determined. No chitinase or cathepsin genes, which are involved in the liquefaction of the infected host, were found in the SpliGV genome, explaining why SpliGV-infected insects do not degrade in a typical manner. The DNA photolyase gene was first found in the genus Granulovirus. When phylogenic relationships were analyzed, the SpliGV was most closely related to Trichoplusia ni granulovirus (TnGV) and Xestia c-nigrum granulovirus (XecnGV), which belong to the Type I-granuloviruses (Type I-GV).

Cloning and characterization of a dronc homologue in the silkworm, Bombyx mori

We cloned and characterized a novel Bombyx mori homologue (bm-dronc) of Drosophila melanogaster dronc (dm-dronc), which could encode a polypeptide of 438 amino acid residues. Bm-Dronc shares relatively low amino acid sequence identities of 25% and 26% with Dm-Dronc and Aedes aegypti Dronc (Aa-Dronc), respectively. Bm-Dronc has the sequence QACRG surrounding the catalytic site (C), which is consistent with the QAC(R/Q/G)(G/E) consensus sequence in most caspases but distinct from the sequences PFCRG and SICRG of Dm-Dronc and Aa-Dronc, respectively. Bm-Dronc possesses a long N-terminal prodomain containing a caspase recruitment domain (CARD), a p20 domain and a p10 domain, exhibiting cleavage activities on synthetic substrates Ac-VDVAD-AMC, Ac-IETD-AMC and Ac-LEHD-AMC, which are preferred by human initiator caspases-2, -8 and -9, respectively. Bm-Dronc transiently expressed in insect cells and Escherichia coli cells underwent spontaneous cleavage and caused apoptosis and stimulation of caspase-3-like protease activity in various lepidopteran cell lines, but not in the dipteran cell line D. melanogaster S2. The apoptosis and the stimulation of caspase-3-like protease activity induced by Bm-Dronc overexpression were abrogated upon transfection with either a double-stranded RNA against bm-dronc or a plasmid expressing functional anti-apoptotic protein Hycu-IAP3 encoded by the baculovirus Hyphantria cunea multiple nucleopolyhedrovirus (MNPV). Apoptosis induction in BM-N cells by infection with a p35-defective Autographa californica MNPV or exposure to actinomycin D and UV promoted the cleavage of Bm-Dronc. These results indicate that Bm-Dronc serves as the initiator caspase responsible for the induction of caspase-dependent apoptosis.

Baculovirus genes affecting host function

Baculoviruses are insect-specific viruses. These large DNA viruses encode many genes in addition to those required to replicate and build new virions. These auxiliary genes provide selective advantages to the virus for invading and infecting host insects. Eight of these genes, which help the virus overcome insect defenses against invasion, are discussed. These include genes whose products help the virus traverse physical or physiological barriers and those that overcome host immune defenses.

Viral subversion of apoptotic enzymes: escape from death row

To prolong cell viability and facilitate replication, viruses have evolved multiple mechanisms to inhibit the host apoptotic response. Cellular proteases such as caspases and serine proteases are instrumental in promoting apoptosis. Thus, these enzymes are logical targets for virus-mediated modulation to suppress cell death. Four major classes of viral inhibitors antagonize caspase function: serpins, p35 family members, inhibitor of apoptosis proteins, and viral FLICE-inhibitory proteins. Viruses also subvert activity of the serine proteases, granzyme B and HtrA2/Omi, to avoid cell death. The combined efforts of viruses to suppress apoptosis suggest that this response should be avoided at all costs. However, some viruses utilize caspases during replication to aid virus protein maturation, progeny release, or both. Hence, a multifaceted relationship exists between viruses and the apoptotic response they induce. Examination of these interactions contributes to our understanding of both virus pathogenesis and the regulation of apoptotic enzymes in normal cellular functions.

Genomic and host range studies of Maruca vitrata nucleopolyhedrovirus

The complete genome of the Maruca vitrata nucleopolyhedrovirus (MaviNPV) isolated from the legume pod borer, Maruca vitrata (Lepidoptera: Pyralidae), was sequenced. It was found to be 111 953 bp in length, with an overall 39 % G+C content, and contained 126 open reading frames (ORFs) encoding predicted proteins of over 50 aa. The gene content and gene order of MaviNPV have the highest similarity to those of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) and their shared homologous genes are 100 % collinear. In fact, MaviNPV seems to be a mini-AcMNPV that is native to Taiwan and possesses a smaller genome with fewer auxiliary genes than the AcMNPV type species. Except for one ORF (Mv74), all of the MaviNPV ORFs have homologues in the AcMNPV genome. MaviNPV is the first lepidopteran-specific baculovirus to lack homologues of vfgf and odv-e66. In addition, MaviNPV lacks the baculovirus repeat ORF (bro) gene that corresponds to AcMNPV ORF2. Five homologous regions (hrs) were located within the MaviNPV genome, and these contained a total of 44 imperfect palindromes. Phylogenetic analysis of the whole genome revealed that MaviNPV was separated from the common ancestor of AcMNPV and Bombyx mori nucleopolyhedrovirus before these two viral species diverged from each other. Moreover, replication of MaviNPV in several cell lines and an egfp-MaviNPV infection assay revealed that IPLB-LD-652Y cells are only partially permissive to MaviNPV, which supports our conclusion that MaviNPV is a distinct species of the group I lepidopteran NPVs.

Functional analysis of the putative antiapoptotic genes, p49 and iap4, of Spodoptera litura nucleopolyhedrovirus with RNAi

A homology search of a public database revealed that Spodoptera litura nucleopolyhedrovirus (SpltNPV) possesses two putative, antiapoptotic genes, p49 and inhibitor of apoptosis 4 (iap4), but their function has not been investigated in its native host cells. In the present study, we used RNA interference (RNAi) to silence the expression of Splt-iap4 and Splt-p49, independently or together, to determine their roles during the SpltNPV life cycle. RT-PCR analysis and Western blot analysis showed the target gene expression had been knocked out in the SpltNPV-infected SpLi-221 cells after treatment with Splt-p49 or Splt-iap4 double-stranded RNA (dsRNA), respectively, confirming that the two genes were effectively silenced. In SpltNPV-infected cells treated with Splt-p49 dsRNA, apoptosis was observed beginning at 14 h, and almost all cells had undergone apoptosis by 48 h. In contrast, budded virus production and polyhedra formation progressed normally in infected cells treated with Splt-iap4 dsRNA. Cell viability analysis showed that Splt-IAP4 had no synergistic effect on the inhibition of apoptosis of SpLi-221 cells induced by SpltNPV infection. Interestingly, after Splt-iap4 dsRNA treatment, cells did not congregate like those infected with SpltNPV in the early infection phase, implying an unknown role of baculovirus iap4. Our results determine that Splt-p49 is necessary to prevent apoptosis; however, Splt-iap4 has no antiapoptotic function during SpltNPV infection.

Evolution and function of the p35 family of apoptosis inhibitors

Apoptosis is a common cellular response to virus infection. However, many viruses have evolved strategies, such as the expression of anti-apoptotic proteins, to combat this response. One such family of anti-apoptotic viral proteins is the p35 family of caspase inhibitors, which are expressed by certain insect viruses. Expression of p35 prevents the host cell from undergoing apoptosis, thereby allowing for propagation of the virus. p35 family members are potent inhibitors of caspases. Members of the family fall into one of three groups that inhibit different classes of caspases. Since the discovery of the first p35 gene in 1991, the anti-apoptotic function of this protein family has been studied extensively. This unique type of protease inhibitor has proven to be extremely useful in the study of apoptosis in experimental settings ranging from nematodes to mammals.

Reactive-site cleavage residues confer target specificity to baculovirus P49, a dimeric member of the P35 family of caspase inhibitors

Baculovirus proteins P49 and P35 are potent suppressors of apoptosis in diverse organisms. Although related, P49 and P35 inhibit initiator and effector caspases, respectively, during infection of permissive insect cells. The molecular basis of this novel caspase specificity is unknown. To advance strategies for selective inhibition of the cell death caspases, we investigated biochemical differences between these baculovirus substrate inhibitors. We report here that P49 and P35 use similar mechanisms for stoichiometric inhibition that require caspase cleavage of their reactive site loops (RSL) and chemical contributions of a conserved N-terminal cysteine to stabilize the resulting inhibitory complex. Our data indicated that P49 functions as a homodimer that simultaneously binds two caspases. In contrast, P35 is a monomeric, monovalent inhibitor. P49 and P35 also differ in their RSL caspase recognition sequences. We tested the role of the P(4)-P(1) recognition motif for caspase specificity by monitoring virus-induced proteolytic processing of Sf-caspase-1, the principal effector caspase of the host insect Spodoptera frugiperda. When P49's TVTD recognition motif was replaced with P35's DQMD motif, P49 was impaired for inhibition of the initiator caspase that cleaves and activates pro-Sf-caspase-1 and instead formed a stable inhibitory complex with active Sf-caspase-1. In contrast, the effector caspase specificity of P35 was unaltered when P35's DQMD motif was replaced with TVTD. We concluded that the TVTD recognition motif is required but not sufficient for initiator caspase inhibition by P49. Our findings demonstrate a critical role for the P(4)-P(1) recognition site in caspase specificity by P49 and P35 and indicate that additional determinants are involved in target selection.

Genome sequence of Leucania seperata nucleopolyhedrovirus

The nucleotide sequence of the Leucania seperata (Ls) Nucleopolyhedrovirus (LsNPV) genome has been determined and analyzed. The circular dsDNA genome contains 168041 bp, making it the largest NPV sequenced to date. The genome has a G + C content of 48.6% and encodes 169 predicted open reading frames (ORFs), one unique repeat region, and eight homologous repeat regions that are divided into two groups. Of the genome, 82.8% encodes predicted ORFs including five dispersal ORFs that have a large overlaps (range in 149 approximately 390 bp) with their adjacent ORFs, respectively such as expression factor 10, 11, 5, 2 (lef-10, lef-11, lef-5, lef-2), and telokin-like protein-20 (tlp-20); 4.4% is in repeat regions; the remaining 12.8% of the genome comprises nonrepeat intergenic regions. LsNPV encodes homologues of 133 ORFs identified previously in other baculoviruses. Other than 10 'baculovirus repeat ORFs' (bro) and two 'inhibitor of apoptosis' (iap) genes, no duplicated ORFs were found. LsNPV lacks a homologue of the ubiquitin gene, which has been found in all fully sequenced baculoviruses. Iap3 and p49, two genes were proven to be inhibitors of apoptosis by experiment, and are found in the LsNPV genome. It is not found in other baculoviruses that two kinds of inhibitors of apoptosis present in a baculovirus genome.

Baculovirus caspase inhibitors P49 and P35 block virus-induced apoptosis downstream of effector caspase DrICE activation in Drosophila melanogaster cells

Baculoviruses induce widespread apoptosis in invertebrates. To better understand the pathways by which these DNA viruses trigger apoptosis, we have used a combination of RNA silencing and overexpression of viral and host apoptotic regulators to identify cell death components in the model system of Drosophila melanogaster. Here we report that the principal effector caspase DrICE is required for baculovirus-induced apoptosis of Drosophila DL-1 cells as demonstrated by RNA silencing. proDrICE was proteolytically cleaved and activated during infection. Activation was blocked by overexpression of the cellular inhibitor-of-apoptosis proteins DIAP1 and SfIAP but not by the baculovirus caspase inhibitor P49 or P35. Rather, the substrate inhibitors P49 and P35 prevented virus-induced apoptosis by arresting active DrICE through formation of stable inhibitory complexes. Consistent with a two-step activation mechanism, proDrICE was cleaved at the large/small subunit junction TETD(230)-G by a DIAP1-inhibitable, P49/P35-resistant protease and then at the prodomain junction DHTD(28)-A by a P49/P35-sensitive protease. Confirming that P49 targeted DrICE and not the initiator caspase DRONC, depletion of DrICE by RNA silencing suppressed virus-induced cleavage of P49. Collectively, our findings indicate that whereas DIAP1 functions upstream to block DrICE activation, P49 and P35 act downstream by inhibiting active DrICE. Given that P49 has the potential to inhibit both upstream initiator caspases and downstream effector caspases, we conclude that P49 is a broad-spectrum caspase inhibitor that likely provides a selective advantage to baculoviruses in different cellular backgrounds.

P13 of Leucania separata multiple nuclear polyhedrosis virus affected the polyhedra and budded virions yields of AcMNPV

p13 gene was first described by our laboratory in Leucania separata multiple nuclear polyhedrovirus (Ls-p13, ORF114) back to 1995. However, the functions of Ls-P13 and its reported homologues remained unknown. In order to probe the function of Ls-P13, recombinant Autographa californica nucleopolyhedroviruses (rAcMNPVs) were constructed to express Ls-P13 in the Sf9 cells at early, late or early/late phase. Observations of microscope showed that the expression of Ls-P13 could decrease the yield of AcMNPV polyhedra in Sf9 cells, and early expressed Ls-P13 had stronger inhibition efficiency than that of the late expressed. Results of flow cytometry also indicated that Ls-P13 decreased the yield of AcMNPV polyhedra while increased those of budded virions (BVs) in Sf9 cells, but the efficacy was lost when its leucine zipper-like domain was mutated. Ls-P13 is a transmembrane protein, which was early located in the nucleus and late mainly in the cytoplasm membrane at 48 h. When its transmembrane domains were deleted, Ls-P13 distribution was dramatically diverted from cytoplasm membrane to nucleus, its corresponding efficacy on polyhedra yield was further increased while that on BVs was slightly weakened. Bioassay results indicated that Ls-P13 accelerated the larvae-killing rate. The mechanism might be that Ls-P13 increased BV yield.

Using the baculovirus/insect cell system to study apoptosis

Apoptosis is a physiological program of cell suicide conserved in invertebrates and vertebrates. Apoptosis is crucial to the normal development of organisms and in tissue homeostasis, by promoting elimination of unwanted cells including damaged- or virus-infected cells. Because of the importance of programmed cell death for the survival of the organism a tight regulation is exerted at various activation levels of the cell-death machinery. The utilization of the baculovirus Autographa californica multiple nucleopolyhedrovirus to identify genes that inhibit the apoptotic process will be described using a transfection-based approach, illustrated by identification of the p49 gene.

Genome of the most widely used viral biopesticide: Anticarsia gemmatalis multiple nucleopolyhedrovirus

The genome of Anticarsia gemmatalis multiple nucleopolyhedrovirus isolate 2D (AgMNPV-2D), which is the most extensively used virus pesticide in the world, was completely sequenced and shown to have 132 239 bp (G+C content 44.5 mol%) and to be capable of encoding 152 non-overlapping open reading frames (ORFs). Three ORFs were unique to AgMNPV-2D, one of which (ag31) had similarity to eukaryotic poly(ADP-ribose) polymerases. The lack of chiA and v-cath may explain some of the success and growth of the AgMNPV biological control programme, as it may explain the high recovery of polyhedra sequestered inside dead larvae in the field, which are collected and used for further application as biological pesticides in soybean fields. The genome organization was similar to that of the Choristoneura fumiferana defective MNPV (CfDefNPV). Most of the variation between the two genomes took place near highly repetitive regions, which were also closely associated with bro-coding regions. The separation of the NPVs into groups I and II was supported by: (i) a phenogram of the complete genomes of 28 baculovirus and Heliothis zea virus 1, (ii) the most parsimonious reconstruction of gene content along the phenograms and (iii) comparisons of genomic features. Moreover, these data also reinforced the notion that group I of the NPVs can be split further into the AgMNPV lineage (AgMNPV, CfDefNPV, Epiphyas postvittana NPV, Orgyia pseudotsugata MNPV and C. fumiferana MNPV), sharing eight defining genes, and the Autographa californica MNPV (AcMNPV) lineage (AcMNPV, Rachiplusia ou NPV and Bombyx mori NPV), sharing nine defining genes.

Identification and functional characterization of AMVp33, a novel homolog of the baculovirus caspase inhibitor p35 found in Amsacta moorei entomopoxvirus

Members of the baculovirus p35 gene family encode proteins that specifically inhibit caspases, cysteine proteases that are involved in apoptosis. To date, p35 homologs have only been found in baculoviruses. We have identified AMVp33, a gene from Amsacta moorei entomopoxvirus with low but significant homology to baculovirus p35 genes. Expression of AMVp33 blocked apoptosis in several different insect and human cell lines. Purified recombinant P33 protein was an efficient inhibitor of insect and human effector caspases, but not initiator caspases. P33 was cleaved by effector caspases, and the resulting cleavage fragments stably associated with the caspases. Mutation of the predicted caspase cleavage site in P33 eliminated cleavage, caspase inhibition and anti-apoptotic function. Thus, AMVp33 encodes a caspase inhibitor similar to baculovirus P35 with a preference for effector caspases. This is the first report of a p35 homolog from any viral or cellular genome outside of the baculovirus family.

Caspase inhibitors: viral, cellular and chemical

Caspases, key mediators of apoptosis, are a structurally related family of cysteine proteases that cleave their substrates at aspartic acid residues either to cause cell death or to activate cytokines as part of an immune response. They can be controlled upstream by the regulation of signals that lead to zymogen activation, or downstream by inhibitors that prevent them from reaching their substrates. This review specifically looks at caspase inhibitors as distinct from caspase regulators: those produced by the cell itself; those whose genes are carried by viruses; and artificial caspase inhibitors used for research and potentially as therapeutics.

Sequence analysis of the Choristoneura occidentalis granulovirus genome

The genome of the Choristoneura occidentalis granulovirus (ChocGV) isolated from the western spruce budworm, Choristoneura occidentalis, was sequenced completely. It was 104,710 bp long, with a 67.3% A+T content and contained 116 potential open reading frames (ORFs) covering 88.4% of the genome. Of these, 29 ORFs were conserved in all fully sequenced baculovirus genomes, 30 were GV-specific, 53 were present in some nucleopolyhedroviruses (NPVs) and/or GVs, three were common to ChocGV and Choristoneura fumiferana GV (ChfuGV) and one was so far unique. To date, ChocGV is the only GV identified that contains a homologue of the apoptosis inhibitor protein P35/P49, present in some group I NPVs. It is also the first GV without a Xestia c-nigrum GV ORF 26 homologue. Five homologous regions (hrs)/repeat regions, lacking typical NPV hr palindromes were identified. ChocGV hrs were similar to each other but not to other GV hrs. A 1.8 kb repeat region with a high A+T content (81%) and multiple repeats of 21-210 bp was found between choc36 and 37. This area resembled the non-homologous region origin of DNA replication (non-hr ori) identified in Cryptophlebia leucotreta GV (CrleGV) and Cydia pomonella GV (CpGV). Based on the mean amino acid identities of homologous proteins, ChocGV was closest to fully sequenced genomes CpGV (52.3%) and CrleGV (52.1%). The closest amino acid identity was to individual ORFs from the partially sequenced ChfuGV genome (97.2% in 38 ORFs). Phylogenetic analysis placed ChocGV in a clade with CrleGV and CpGV.

Response of immunocompetent and immunosuppressed Spodoptera littoralis larvae to baculovirus infection

The Mediterranean lepidopteran pest Spodoptera littoralis is highly resistant to infection with the Autographa californica multiple nucleopolyhedrovirus (AcMNPV) via the oral route, but highly sensitive to infection with budded virus (BV) via the intrahaemocoelic route. To study the fate of AcMNPV infection in S. littoralis, vHSGFP, an AcMNPV recombinant that expresses the reporter green fluorescent protein gene under the control of the Drosophila heat-shock promoter, and high-resolution fluorescence microscopy were utilized. S. littoralis fourth-instar larvae infected orally with vHSGFP showed melanization and encapsulation of virus-infected tracheoblast cells serving the midgut columnar cells. At 72 h post-infection, the viral foci were removed during the moult clearing the infection. Thus, oral infection was restricted by immune responses to the midgut and midgut-associated tracheal cells. By contrast, injection of BV into the haemocoel resulted in successful infection of tracheoblasts, followed by spread of the virus through the tracheal epidermis to other tissues. However, in contrast to fully permissive infections where tracheoblasts and haemocytes are equally susceptible to infection, a severe limitation to vHSGFP infection of haemocytes was observed. To investigate the resistance of S. littoralis haemocytes to BV infection with AcMNPV, the larval immune system was suppressed with the Chelonus inanitus polydnavirus or a putatively immunosuppressive polydnavirus gene, P-vank-1. Both treatments increased the susceptibility of S. littoralis larvae to AcMNPV. It is concluded that the resistance of S. littoralis to AcMNPV infection involves both humoral and cellular immune responses that act at the gut and haemocyte levels. The results also support the hypothesis that tracheolar cells mediate establishment of systemic baculovirus infections in lepidopteran larvae. The finding that polydnaviruses and their encoded genes synergize baculovirus infection also provides an approach to dissecting the responses of the lepidopteran immune system to viruses by using specific polydnavirus immunosuppressive genes.

Activation pathways and signal-mediated upregulation of the insect Spodoptera frugiperda caspase-1

Sf-caspase-1 is the most studied effector caspase of Lepidoptera. Its activation is believed to follow a two-step mechanism: The first step requires cleavage by an initiator caspase at D195 (between the large and small subunits) releasing the C-terminal small subunit. This is blocked by the baculovirus caspase inhibitor P49. The second step removes the N-terminal prodomain by cleavage at D28 to generate the large subunit that is blocked by the baculovirus caspase inhibitor P35. In this study, we identified an alternative mechanism of Sf-caspase-1 activation. This additional two-step mechanism involves first cleavage of pro-Sf-caspase-1 at D28 to remove the N-terminal prodomain and subsequently cleavage at D195 to generate the large and small subunits. Both mechanisms are triggered by apoptotic stimuli following a distinct pattern. We also showed that expression of Sf-caspase-1 was upregulated upon reception of apoptotic stimuli. Different from all published data, this upregulation occurred as a post-transcriptional event. Moreover, we proved that the stronger the stimuli, the higher the upregulation. And we demonstrated that P49 and P35 inhibited the cleavage at D28 and D195 respectively, independently of wether the first cleavage was at D195 or at D28.

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.

In vivo apoptosis induction and reduction of infectivity by an Autographa californica multiple nucleopolyhedrovirus p35− recombinant in hemocytes from the velvet bean caterpillar Anticarsia gemmatalis (Hübner) (Lepidoptera: Noctuidae)

Baculoviruses have long been shown to regulate apoptosis in cultured insect cells. Recently, this phenomenon was also reported to occur in vivo, reinforcing the importance of apoptosis in insect immunity against viruses. The vP35del virus, an Autographa californica multiple nucleopolyhedrovirus (AcMNPV) recombinant, was previously shown to induce apoptosis in Anticarsia gemmatalis cultured cells. In order to verify the AcMNPV interaction with hemocytes, apoptosis induction in vivo and its effects upon infectivity, we studied the course of intrahemocoelic infection of recombinant viruses (vHSGFP and vHSGFP/P35del) in A. gemmatalis larvae. Insect development and mortality were monitored and infection progress was followed by light, fluorescence and electron microscopy. For all doses tested, vHSGFP/P35del caused lower mortality than vHSGFP. Mortality of 95% occurred with a dose of 4x10(6) PFUs of vHSGFP, which was reduced to 60% for vHSGFP/P35del. GFP expression was first observed at 3 h p.i. for the two viruses, increasing for vHSGFP (40% at 120 h p.i.) and decreasing for vHSGFP/P35del (0% at 120 h p.i.). The virus vHSGFP/P35del induced apoptosis in hemocytes, with some budded virus being produced, and fragmented cells were observed between 24 and 72 h p.i. The recombinant vHSGFP induced typical wild-type cytopathic effects, with low production of occluded viruses until 120 h p.i. Plasmatocytes and granular hemocytes type 1 were the hemocytes most susceptible to both viruses. For these experimental conditions, we concluded that A. gemmatalis is a semi-permissive host to AcMNPV; moreover, apoptosis reduces AcMNPV infectivity and the p35 gene is essential for blocking apoptosis in this system.

Identification of the Apoptosis Inhibitor Gene p49 of Spodoptera litura Multicapsid Nucleopolyhedrovirus

Baculoviruses possess two types of genes that suppressed apoptosis, p35 and inhibitor of apoptosis (iap). Computer-assisted analysis indicated that Spodoptera litura multicapsid nucleopolyhedrovirus (SpltMNPV) ORF55 (designated as the p49 gene) display 79 and 31% amino acid identity with Spodoptera littoralis (Spli)MNPV P49 and Autographa californica (Ac)MNPV P35, respectively, Splt MNPV putative P49 contains a peptide cleavage site TVTDG recognized by death caspases. In marker rescue assay, Splt-p49 was able to suppress apoptosis induced by infection of a mutant AcMNPV deficient in p35 and rescue the mutant virus replication from apoptosis in Sf-9 cells.

Functional analysis of the inhibitor of apoptosis (iap) gene carried by the entomopoxvirus of Amsacta moorei

The entomopoxvirus from Amsacta moorei (AmEPV) contains none of the commonly recognized vertebrate poxvirus apoptotic suppressor genes. However, AmEPV carries a single inhibitor of apoptosis (iap) gene (AMViap) not present in vertebrate poxviruses. The AMViap gene was active when coexpressed with the Drosophila proapoptotic gene hid in Ld652 cells and can rescue cells from apoptosis as shown by increased number of surviving cells and reduced levels of caspase-3-like activity. We also showed that expression of the AMViap gene rescued polyhedron production in Autographa californica M nucleopolyhedrovirus (AcMNPV)Deltap35-infected Sf9 cells during an otherwise abortive infection induced by apoptosis. Surprisingly, deletion of the AMViap gene from the AmEPV genome led to only a modest (10-fold) loss of virion production in infected Ld652 cells, indicating that the AMViap gene is nonessential for virus replication under these conditions. However, infection of Ld652 cells by AmEPV lacking a functional iap gene led to a more rapid induction of cytotoxicity and increased levels of caspase-3-like activity. Similar results were observed and were more pronounced in infected Sf9 and S2 cells. The purified AMVIAP protein also inhibits the enzymatic activities of human caspase-9 and caspase-3 in vitro. Our results indicate that while the AMViap gene was active in controlling apoptosis through the intrinsic pathway, the virus likely encodes additional proteins that also regulate apoptosis.

ORF390 of white spot syndrome virus genome is identified as a novel anti-apoptosis gene

Apoptosis serves as an important defense strategy employed by host cells against viral invasion. Many viruses contain the anti-apoptotic genes to block the defense-by-death response of host cells. In this study, we tried to identify the putative anti-apoptotic genes in white spot syndrome virus (WSSV) genome. We confirmed that actinomycin D could induce apoptosis of shrimp primary cells. However, the apoptosis triggered by actinomycin D was inhibited by WSSV infection. As mutants of Autographa californica nucleopolyhedrovirus (AcMNPV), AcMNPVDelta35k/pol+ lacks a functional P35 gene undergoing apoptosis and its infection could induce Sf9 cell apoptosis. To identify the putative apoptotic suppressor gene of WSSV, overlapping cosmid clones representing the entire WSSV genome were individually cotransfected along with genome DNA of AcMNPVDeltaP35k/pol+. Using this marker rescue assay, a WSSV DNA fragment that was able to rescue AcMNPVDeltaP35k/pol+ infection in Sf9 cells was isolated. By further sequence analysis and rescue assay, the ORF390 was identified as a novel anti-apoptotic gene. The ORF displays two putative caspase9 cleavage sites LLVETDGPS, VKLEHDGSK, and a caspase3 cleavage site EEDEVDGVP. The ORF was cloned into the pIE1 vector and then the recombinant vector was transfected into Sf9 cells. The Sf9 cells did not show obvious characteristics of apoptosis when infected with AcMNPVDeltaP35k/pol+. And the transient expression of ORF390 allowed AcMNPVDeltaP35k/pol+ replication in Sf9 cells and resulted in the formation of polyhedra successfully. The results indicate that function of ORF390 in WSSV is a kind of apoptotic suppressor like P35 in AcMNPV.

Sequence analysis of the genome of the Neodiprion sertifer nucleopolyhedrovirus

The genome of the Neodiprion sertifer nucleopolyhedrovirus (NeseNPV), which infects the European pine sawfly, N. sertifer (Hymenoptera: Diprionidae), was sequenced and analyzed. The genome was 86,462 bp in size. The C+G content of 34% was lower than that of the majority of baculoviruses. A total of 90 methionine-initiated open reading frames (ORFs) with more than 50 amino acids and minimal overlapping were found. From those, 43 ORFs were homologous to other baculovirus ORFs, and 29 of these were from the 30 conserved core genes among all baculoviruses. A NeseNPV homolog to the ld130 gene, which is present in all other baculovirus genomes sequenced to date, could not be identified. Six NeseNPV ORFs were similar to non-baculovirus-related genes, one of which was a trypsin-like gene. Only one iap gene, containing a single BIR motif and a RING finger, was found in NeseNPV. Two NeseNPV ORFs (nese18 and nese19) were duplicates transcribed in opposite orientations from each other. NeseNPV did not have an AcMNPV ORF 2 homolog characterized as the baculovirus repeat ORF (bro). Six homologous regions (hrs) were located within the NeseNPV genome, each containing small palindromes embedded within direct repeats. A phylogenetic analysis was done to root the tree based upon the sequences of DNA polymerase genes of NeseNPV, 23 other baculoviruses, and other phyla. Baculovirus phylogeny was then constructed with 29 conserved genes from 24 baculovirus genomes. Culex nigripalpus nucleopolyhedrovirus (CuniNPV) was the most distantly related baculovirus, branching to the hymenopteran NeseNPV and the lepidopteran nucleopolyhedroviruses and granuloviruses.

Functional study on baculovirus anti-apoptosis genes

Apoptosis of bollworm cell line Hz-AM1 can be delayed by transient expression of AcNPV (Autographa californica Nuclear Polihedrosis Virus) p35 gene. Acp35Z, a p35 inactivated AcNPV by inserting with LacZ gene, cannot replicate in Hz-AM1 cells. However, the replication can be rescued by co-transfection with a plasmid containing AcNPV p35 gene. It is also realized that the transient expression of AcNPV p35 gene in Hela cells can put off cell apoptosis which is induced by adding actinomycin-D. Through co-transfection and G418 screening, two anti-apoptosis cell lines named Sf9-35 and Vero-35 are established by integrating AcNPV p35 and Neo expression cassette into the cell chromosomes. The Sf9-35 enhances the yield of budded virus of AcNPV, while the Vero-35 increases the propagation of measles virus.

The role of baculovirus apoptotic suppressors in AcMNPV-mediated translation arrest in Ld652Y cells

Infecting the insect cell line IPLB-Ld652Y with the baculovirus Autographa californica multinucleocapsid nucleopolyhedrovirus (AcMNPV) results in global translation arrest, which correlates with the presence of the AcMNPV apoptotic suppressor, p35. In this study, we investigated the role of apoptotic suppression on AcMNPV-induced translation arrest. Infecting cells with AcMNPV bearing nonfunctional mutant p35 did not result in global translation arrest. In contrast, global translation arrest was observed in cells infected with AcMNPV in which p35 was replaced with Opiap, Cpiap, or p49, baculovirus apoptotic suppressors that block apoptosis by different mechanisms than p35. These results indicated that suppressing apoptosis triggered translation arrest in AcMNPV-infected Ld652Y cells. Experiments using the DNA synthesis inhibitor aphidicolin and temperature shift experiments, using the AcMNPV replication mutants ts8 and ts8deltap35, indicated that translation arrest initiated during the early phase of infection, but events during the late phase were required for global translation arrest. Peptide caspase inhibitors could not substitute for baculovirus apoptotic suppressors to induce translation arrest in Ld652Y cells infected with a p35-null virus. However, if the p35-null-AcMNPV also carried hrf-1, a novel baculovirus host range gene, progeny virus was produced and treatment with peptide caspase inhibitors enhanced translation of a late viral gene transcript. Together, these results indicate that translation arrest in AcMNPV-infected Ld652Y cells is due to the anti-apoptotic function of p35, but suggests that rather than simply preventing caspase activation, its activity enhances signaling to a separate translation arrest pathway, possibly by stimulating the late stages of the baculovirus infection cycle.

Identification and functional analysis of Hyphantria cunea nucleopolyhedrovirus iap genes

Hyphantria cunea nucleopolyhedrovirus (HycuNPV) infection protected SpIm cells from actinomycin D (ActD)-induced apoptosis as early as 4 h postinfection. Analysis by Southern hybridization revealed that the HycuNPV genome possessed three members of inhibitor of apoptosis genes (iaps) that were designated as hycu-iap1, hycu-iap2, and hycu-iap3 because of their amino acid sequence homology with iaps identified in other baculoviruses. Functional analysis of Hycu-IAPs by transient expression assay in Sf9 cells revealed that Hycu-IAP3 blocked apoptosis induced by actinomycin D and rescued replication of p35 deficient-mutant AcMNPV, while Hycu-IAP1 and Hycu-IAP2 did not show any anti-apoptotic functions. Knockdown of hycu-iap3 expression by RNAi during HycuNPV infection in SpIm cells induced apoptosis. These results indicate that Hycu-IAP3 is essential for blockage of apoptosis during HycuNPV infection of permissive SpIm cells.

Baculovirus P35 interacts with a subunit of human RNA polymerase II and can enhance promoter activity in human cells

The early protein P35 from the baculovirus Autographa californica nucleopolyhedrovirus is a direct inhibitor of caspases and can block apoptosis in a wide variety of systems. In addition, it has been linked to the regulation of viral gene expression, shut-down of protein synthesis in infected insect cells and malignant transformation of mouse fibroblasts. By yeast-two-hybrid screening we identified the RPB11a subunit of human RNA polymerase II as an interaction partner of P35. Specificity of the interaction was confirmed by affinity blotting. By immunocytology, P35 was in part found in the nucleus of transfected cells. Homology searches further revealed that P35 has structural similarity with RPB3, the subunit of RNA polymerase II that has been demonstrated to interact directly with RPB11a. When transfected into human colon carcinoma cells, P35 was able to enhance the activity of E-cadherin and beta-actin promoters by about a factor of two as measured by luciferase reporter assay. P35 and hRPB11a together enhanced the E-cadherin activity about three- to fourfold. These data suggest an additional role for P35 in the regulation of cellular transcription.

Insect defenses against virus infection: the role of apoptosis

Insects, with their lack of an adaptive immune response, provide a unique animal model to examine the effects of apoptosis on viral infection. Several members of the baculovirus family of insect viruses have been shown to induce apoptosis during infection of cultured insect cells, and depending on the virus-host combination this apoptotic response can severely limit viral replication. In response to this evolutionary pressure, all baculoviruses studied to date carry antiapoptotic genes, including members of the p35 and IAP (inhibitor of apoptosis) gene families. Recent work has characterized the apoptotic response during infection of the host insect, and the results directly demonstrate the power of apoptosis as an antiviral response.

Identification and classification of the Spodoptera littoralis nucleopolyhedrovirus inhibitor of apoptosis gene

Baculoviruses possess two types of genes that suppressed apoptosis, p35 and inhibitor of apoptosis (iap). In this study we report the isolation and identification of an inhibitor of apoptosis gene Sliap in the genome of the Spodoptera littoralis nucleopolyhedrovirus (SINPV). The Sliap sequence predicted a 15 kDa polypeptide with only one BIR domain and a RING finger, both motifs characteristic of the IAP family of proteins, and a third specific acidic-rich motif. These characteristics, shared with the Spodoptera littura NPV IAP2/3, Epiphyas postvittana NPV IAP4, Lymantria dispar NPV IAP and Orgyia pseudotsugata NPV IAP4 (Orf 107) allowed us to classify them in a new homology group (IAP-4). Sliap was able to delay, but not to suppress, apoptosis induced by replication of a recombinant AcMNPV deficient in p35. In SINPV infected-SF9 cells Sliap was expressed earlier than sl-p49 suggesting that its role at the initiation of infection was to delay the apoptotic response of the host.