Identification and characterization of a filament-associated protein encoded by Amsacta moorei entomopoxvirus

In cultured cells the baculovirus P10 protein forms two independent intracellular structures that play separate roles in occlusion body maturation and their release by nuclear disintegration

P10 is a small, abundant baculovirus protein that accumulates to high levels in the very late stages of the infection cycle. It is associated with a number of intracellular structures and implicated in diverse processes from occlusion body maturation to nuclear stability and lysis. However, studies have also shown that it is non-essential for virus replication, at least in cell culture. Here, we describe the use of serial block-face scanning electron microscopy to achieve high-resolution 3D characterisation of P10 structures within Trichoplusia ni TN-368 cells infected with Autographa californica multiple nucleopolyhedrovirus. This has enabled unparalleled visualisation of P10 and determined the independent formation of dynamic perinuclear and nuclear vermiform fibrous structures. Our 3D data confirm the sequence of ultrastructural changes that create a perinuclear cage from thin angular fibrils within the cytoplasm. Over the course of infection in cultured cells, the cage remodels to form a large polarised P10 mass and we suggest that these changes are critical for nuclear lysis to release occlusion bodies. In contrast, nuclear P10 forms a discrete vermiform structure that was observed in close spatial association with both electron dense spacers and occlusion bodies; supporting a previously suggested role for P10 and electron dense spacers in the maturation of occlusion bodies. We also demonstrate that P10 hyper-expression is critical for function. Decreasing levels of p10 expression, achieved by manipulation of promoter length, correlated with reduced P10 production, a lack of formation of P10 structures and a concomitant decrease in nuclear lysis.

High-resolution 3D electron microscopy has revealed the complexity of structures formed by P10, a small 10kDa protein that accumulates to very high levels in baculovirus-infected cells. We demonstrate the formation and presence of two distinct, possibly unique, P10 structures that account for the diverse roles associated with this small protein. In the cytoplasm, a peri-nuclear cage-like structure matured into a polarised mass of P10. Remodelling of the cage provides evidence for a mechanism to effect nuclear lysis and release of occlusion bodies to promote dispersal. Over a similar time period, an independent vermiform P10 structure forms and matures within the cell nucleus. It is widely known that in the absence of P10, occlusion bodies do not fully mature. Our data suggest a mechanism for occlusion body maturation with P10 facilitating the envelopment of occlusion bodies with electron dense spacers. The P10 structures formed require vast quantities of P10 protein providing a rationale for the hyper-expression of this hitherto obscure viral protein.

Transcriptional analysis of the putative glycosyltransferase gene (amv248) of the Amsacta moorei entomopoxvirus

Amsacta moorei entomopoxvirus (AMEV), the most studied member of the genus Betaentomopoxvirus, was initially isolated from Red Hairy caterpillar larvae, Amsacta moorei. According to genome sequence and previous studies it was shown that amv248 encodes a putative glycosyltransferase that is the only conserved attachment protein in betaentomopoxviruses. Transcriptional analysis of the amv248 gene by RT-PCR and qPCR showed that transcription starts at 6h post infection (hpi). Also, transcription was not affected by a DNA replication inhibitor but was severely curtailed by a protein synthesis inhibitor. These results indicate that amv248 belongs to the intermediate class of gene expression. 5' and 3' untranslated regions analysis revealed that transcription initiates at position -126 relative to the translational start site, and ends between 50 and 83 bases after the stop codon. To narrow down the size and location of the gene's promoter, the upstream region as well as several different sized deletions thereof were generated and cloned upstream of a luciferase reporter gene. The constructs were used to measure the Firefly and Renilla luciferase activities in dual assays. The results showed that luciferase activity decreased when bases -198 to -235 of amv248 upstream region were missing. Sequence analysis among the intermediate gene promoters of AMEV showed that TTTAT(T/A)TT(T/A)₂TTA is possibly a common motif, however, further investigations are needed to confirm this conclusion.

Genome-wide analysis of differential mRNA expression of Amsacta moorei entomopoxvirus, mediated by the gene encoding a viral protein kinase (AMV197)

Insect-born entomopoxviruses (Fam: Poxviridae) are potentially important bio-pesticide against insect pests and expression vectors as well as vectors for transient human gene therapies including recombinant viral vaccines. For these reasons, it is necessary to understand the regulatory genes functions to improve its biotechnological potential. Here, we focused on the characterization of serine/threonine (Ser/Thr; ORF AMV197) protein kinase gene from the Amsacta moorei entomopoxvirus (AMEV), the type species of the genus Betaentomopoxvirus. Transcription of the parental and an amv197-null recombinant AMEV was compared by whole-genome gene expression microarray analysis. Blast2GO analysis reflected a broad diversity of upregulated and downregulated genes. Results showed that expression levels of 102 genes (45%) out of 226 tested genes changed significantly in the recombinant AMEV infected cells. Of these transcripts, 72 (70.58%) were upregulated and 30 (29.41%) were downregulated throughout the infection period. Genes involved in DNA repair, replication and nucleotide metabolism, transcription and RNA modification, and protein modification were mostly upregulated at different times in cells infected with the recombinant virus. Furthermore, transcription of all studied cellular genes including metabolism of apoptosis (Nedd2-like caspase, hemolin and elongation factor-1 alpha (ef1a) gene) was downregulated in the absence of amv197. Quantitative real time reverse transcription-PCR confirmed viral transcriptional changes obtained by microarray. The results of this study indicated that the product of amv197 appears to affect the transcriptional regulation of most viral and many cellular genes. Further investigations are, however, needed to narrow down the role of AMV197 throughout the infection process.

The complete genome sequence of the Alphaentomopoxvirus Anomala cuprea entomopoxvirus, including its terminal hairpin loop sequences, suggests a potentially unique mode of apoptosis inhibition and mode of DNA replication

Complete genome sequence of Anomala cuprea entomopoxvirus, which belongs to the genus Alphaentomopoxvirus, including its terminal hairpin loop sequences, is reported. This is the first genome sequence of Alphaentomopoxvirus reported, and hairpin loops in entomopoxviruses have not previously been sequenced. The genome is 245,717 bp, which is smaller than had previously been estimated for Alphaentomopoxvirus. The inverted terminal repeats are quite long, and experimental results suggest that one genome molecule has one type of hairpin at one end and another type at the other end. The genome contains unexpected ORFs, e.g., that for the ubiquitin-conjugating enzyme E2 of eukaryotes. The BIR and RING domains found in a single ORF for an inhibitor of apoptosis in baculoviruses and entomopoxviruses occurred in two different, widely separated ORFs. Furthermore, an ORF in the genome contains a serpin domain that was previously found in vertebrate poxviruses for apoptosis inhibition but not in insect viruses.

Induction of apoptosis by the Amsacta moorei entomopoxvirus

CF-70-B2 cells derived from the spruce budworm (Choristoneura fumiferana) undergo apoptosis when infected with Amsacta moorei entomopoxvirus (AMEV), as characterized by membrane blebbing, formation of apoptotic bodies, TdT-mediated dUTP nick-end labelling (TUNEL) staining, condensed chromatin and induction of caspase-3/7 activity. The apoptotic response was reduced when cells were infected with UV-inactivated AMEV, but not when infected in the presence of the DNA synthesis inhibitor, cytosine β-d-arabinofuranoside. Hence, only pre-DNA replication events were involved in inducing the antiviral response in CF-70-B2 cells. The virus eventually overcame the host’s antiviral response and replicated to high progeny virus titres accompanied by high levels of caspase-3/7 activity. The CF-70-B2 cells were less productive of progeny virus in comparison to LD-652, a Lymantria dispar cell line routinely used for propagation of AMEV. At late stages of infection, LD-652 cells also showed characteristics of apoptosis such as oligosomal DNA fragmentation, TUNEL staining, condensed chromatin and increased caspase-3/7 activity. Induction of apoptosis in LD-652 cells was dependent on viral DNA replication and/or late gene expression. A significantly reduced rate of infection was observed in the presence of general caspase inhibitors Q-VD-OPH and Z-VAD-FMK, indicating caspases may be involved in productive virus infection.

The heptad repeats region is essential for AcMNPV P10 filament formation and not the proline-rich or the C-terminus basic regions

Baculovirus P10 protein is a small conserved protein and is expressed as bundles of filaments in the host cell during the late phase of virus infection. So far the published results on the domain responsible for filament structural formation have been contradictory. Electron microscopy revealed that the C-terminus basic region was involved in filament structural formation in the Autographa californica multiple nucleocapsid nucleopolyhedrovirus (AcMNPV) [van Oers, M.M., Flipsen, J.T., Reusken, C.B., Sliwinsky, E.L., Vlak, J.M., 1993. Functional domains of the p10 protein of Autographa californica nuclear polyhedorsis virus. J. Gen. Virol. 74, 563-574.]. While in the Helicoverpa armigera nucleopolyhedrovirus (HearNPV), the heptad repeats region but not the C-terminus domain was proven to be responsible for filament formation [Dong, C., Li, D., Long, G., Deng, F., Wang, H., Hu, Z., 2005. Identification of functional domains required for HearNPV P10 filament formation. Virology 338, 112-120.]. In this manuscript, fluorescence confocal microscopy was applied to study AcMNPV P10 filament formation. A set of plasmids containing different P10 structural domains fused with a fluorescent protein were constructed and transfected into Sf-9 cells. The data indicated that the heptad repeats region, but not the proline-rich region or the C-terminus basic region, is essential for AcMNPV P10 filament formation. Co-transfection of P10s tagged with different fluorescent revealed that P10s with defective heptad repeats region could not interact with intact heptad repeats region or even full-length P10s to form filament structure. Within the heptad repeats region, deletion of the three amino acids spacing of AcMNPV P10 appeared to have no significant impact on the formation of filament structures, but the content of the heptad repeats region appeared to play a role in the morphology of the filaments.

Identification of functional domains required for HearNPV P10 filament formation

Baculovirus encoded P10 form fibrillar structures in infected cells. We have tried to identify the functional domains for the P10 filament formation by green fluorescence protein (GFP) tag. The p10 gene of Helicoverpa armigera single nucleocapsid nucleopolyhedrovirus (HearNPV) was the subject of these studies. Different lengths of HearNPV p10 gene were constructed with GFP fused in frame to the C-terminus. The constructs were transfected into insect and mammalian cells and observed by confocal microscopy. The results indicated that the first N-terminal 66 amino acids, which form the complete coiled-coil domain, were necessary for the aggregation and formation of filament structures of HearNPV P10. The proline-rich region and the C-terminal positively charged amino acids were not necessary for the formation of fibrillar structure but had some impact on the shape of the fibrillar structures. No other baculoviral proteins were needed for the formation of P10 filament structures in transfected cells.

Prediction of functional class of novel viral proteins by a statistical learning method irrespective of sequence similarity

The function of a substantial percentage of the putative protein-coding open reading frames (ORFs) in viral genomes is unknown. As their sequence is not similar to that of proteins of known function, the function of these ORFs cannot be assigned on the basis of sequence similarity. Methods complement or in combination with sequence similarity-based approaches are being explored. The web-based software SVMProt (http://jing.cz3.nus.edu.sg/cgi-bin/svmprot.cgi) to some extent assigns protein functional family irrespective of sequence similarity and has been found to be useful for studying distantly related proteins [Cai, C.Z., Han, L.Y., Ji, Z.L., Chen, X., Chen, Y.Z., 2003. SVM-Prot: web-based support vector machine software for functional classification of a protein from its primary sequence. Nucleic Acids Res. 31(13): 3692–3697]. Here 25 novel viral proteins are selected to test the capability of SVMProt for functional family assignment of viral proteins whose function cannot be confidently predicted on by sequence similarity methods at present. These proteins are without a sequence homolog in the Swissprot database, with its precise function provided in the literature, and not included in the training sets of SVMProt. The predicted functional classes of 72% of these proteins match the literature-described function, which is compared to the overall accuracy of 87% for SVMProt functional class assignment of 34 582 proteins. This suggests that SVMProt to some extent is capable of functional class assignment irrespective of sequence similarity and it is potentially useful for facilitating functional study of novel viral proteins.

Formation of P10 tubular structures during AcMNPV infection depends on the integrity of host-cell microtubules

During infection of insect cells with Autographa californica nucleopolyhedrovirus (AcMNPV), the very late protein P10 forms large fibrillar structures in the cytoplasm and nuclei of infected cells. In this study we have used confocal microscopy in association with a novel P10 antiserum to localise and study P10 in virus-infected cells. P10 was shown to be a component of tubular-like structures that spiralled throughout the cytoplasm and nucleus of AcMNPV-infected cells. These structures were observed to colocalise partly with cortical microtubules. When microtubules were depolymerised with the drug nocodazole, P10 tubules failed to form and the protein appeared concentrated in cytoplasmic foci. For the first time, we provide direct evidence using both antibody pulldown and yeast two-hybrid experiments for the interaction of P10 with host-cell tubulin. It is suggested that this interaction may be a critical factor in AcMNPV-induced cell lysis.

Sequence analysis of the Plutella xylostella granulovirus genome

The Plutella xylostella granulovirus (PxGV) genome DNA was sequenced and the predicted open reading frames (ORFs) were compared to genes of the first-sequenced GV, Xestia c-nigrum GV (XcGV), and those from other baculoviruses and organisms. PxGV DNA has a size of 100,999 bp with a G + C content of 40.7%. The analysis predicted 120 ORFs with a size of 150 nucleotides or larger that showed minimal overlap. Blast searches followed by a comparison of ORF arrangement with those of completely sequenced baculovirus genomes showed the presence of 102 homologs to other genes in the database. Among them, 74 and 100 were homologous to genes of Autographa californica NPV (AcMNPV) and XcGV, respectively. A striking feature of the relationship between the genomes of PxGV and XcGV was the conservation of the order and orientation of homologous genes. Even though the XcGV genome is much larger than that of PxGV (178 vs 101 kb) and had many more predicted ORFs (181 vs 120) with an average amino acid sequence relatedness of 42%, the order and orientation of almost all homologous genes was conserved. The PxGV genome contained four homologous regions (hrs), each with 10 to 23 repeated sequences of 101 to 105 nucleotides containing a 15-bp imperfect palindrome in the center of the repeats.

The carboxy-terminal acidic domain of Rift Valley Fever virus NSs protein is essential for the formation of filamentous structures but not for the nuclear localization of the protein

The ambisense S segment of Rift Valley fever (RVF) virus (a phlebovirus in the Bunyaviridae family) codes for two proteins: the viral complementary-sense RNA for the N nucleoprotein and the genomic-sense RNA for the nonstructural protein NSs. Except for the fact that the NSs protein is phosphorylated and forms filamentous structures in the nuclei of infected cells (R. Swanepoel and N. K. Blackburn, J. Gen. Virol. 34:557-561, 1977), its role is poorly understood, especially since the replication cycle of all these viruses takes place in the cytoplasm. To investigate the mechanisms involved in filament formation, we expressed NSs in mammalian cells via a recombinant Semliki Forest virus and demonstrated that the protein alone was able to form structures similar to those observed in RVF virus-infected cells, indicating that the presence of other RVF virus proteins is not required for filament formation. The yeast two-hybrid system was used to show that the protein interacts with itself and to map the interacting domains. Various deletion and substitution mutants were constructed, and the mutant proteins were analyzed by immunoprecipitation, Western blotting and immunofluorescence. These experiments indicated that the 10 to 17 amino acids of the carboxy-terminal domain were involved in self-association of the protein and that deletion of this acidic carboxy-terminal domain prevents the protein from forming filaments but does not affect its nuclear localization. The role of two phosphorylation sites present in this domain was also investigated, but they were not found to have a major influence on the formation of the nuclear filament.

The genome of Melanoplus sanguinipes entomopoxvirus

The family Poxviridae contains two subfamilies: the Entomopoxvirinae (poxviruses of insects) and the Chordopoxvirinae (poxviruses of vertebrates). Here we present the first characterization of the genome of an entomopoxvirus (EPV) which infects the North American migratory grasshopper Melanoplus sanguinipes and other important orthopteran pests. The 236-kbp M. sanguinipes EPV (MsEPV) genome consists of a central coding region bounded by 7-kbp inverted terminal repeats and contains 267 open reading frames (ORFs), of which 107 exhibit similarity to previously described genes. The presence of genes not previously described in poxviruses, and in some cases in any other known virus, suggests significant viral adaptation to the arthropod host and the external environment. Genes predicting interactions with host cellular mechanisms include homologues of the inhibitor of apoptosis protein, stress response protein phosphatase 2C, extracellular matrixin metalloproteases, ubiquitin, calcium binding EF-hand protein, glycosyltransferase, and a triacylglyceride lipase. MsEPV genes with putative functions in prevention and repair of DNA damage include a complete base excision repair pathway (uracil DNA glycosylase, AP endonuclease, DNA polymerase beta, and an NAD+-dependent DNA ligase), a photoreactivation repair pathway (cyclobutane pyrimidine dimer photolyase), a LINE-type reverse transcriptase, and a mutT homologue. The presence of these specific repair pathways may represent viral adaptation for repair of environmentally induced DNA damage. The absence of previously described poxvirus enzymes involved in nucleotide metabolism and the presence of a novel thymidylate synthase homologue suggest that MsEPV is heavily reliant on host cell nucleotide pools and the de novo nucleotide biosynthesis pathway. MsEPV and lepidopteran genus B EPVs lack genome colinearity and exhibit a low level of amino acid identity among homologous genes (20 to 59%), perhaps reflecting a significant evolutionary distance between lepidopteran and orthopteran viruses. Divergence between MsEPV and the Chordopoxvirinae is indicated by the presence of only 49 identifiable chordopoxvirus homologues, low-level amino acid identity among these genes (20 to 48%), and the presence in MsEPV of 43 novel ORFs in five gene families. Genes common to both poxvirus subfamilies, which include those encoding enzymes involved in RNA transcription and modification, DNA replication, protein processing, virion assembly, and virion structural proteins, define the genetic core of the Poxviridae.

Insect virus proteins (FALPE and p10) self-associate to form filaments in infected cells

Entomopoxviruses and baculoviruses are pathogens of insects which replicate in the cytoplasm and nuclei of their host cells, respectively. During the late stages of infection, both groups of viruses produce occlusion bodies which serve to protect virions from the external environment. Immunofluorescence and electron microscopy studies have shown that large bundles of filaments are associated with these occlusion bodies. Entomopoxviruses produce cytoplasmic fibrils which appear to be composed of the filament-associated late protein of entomopoxviruses (FALPE). Baculoviruses, on the other hand, yield filaments in the nuclei and cytoplasm of the infected cell which are composed of a protein called p10. Despite significant differences in their sequences, FALPE and p10 have similar hydrophilicity profiles, and each has a proline-rich stretch of amino acids at its carboxyl terminus. Evidence that FALPE and p10 could produce filaments in the absence of other viral proteins is presented. When FALPE was expressed in insect cells from a recombinant baculovirus, filaments similar to those produced by the wild-type Amsacta moorei entomopoxvirus were observed. In addition, when expression plasmids containing FALPE or p10 genes were transfected into Vero monkey kidney cells, filament structures similar to those found in infected insect cells were produced. The manner in which FALPE and p10 subunits interact to form polymers was investigated through deletion and site-specific mutagenesis in conjunction with immunofluorescence microscopy, yeast two-hybrid protein interaction analysis, and chemical cross-linking of adjacent molecules. These studies indicated that the amino termini of FALPE and p10 were essential for subunit interaction. Although deletion of the carboxy termini did not affect this interaction, it did inhibit filament formation. In addition, modification of several potential sites for phosphorylation also abolished filament assembly. We concluded that although the sequences of FALPE and p10 were different, the structural and functional properties of the two polypeptides appeared to be similar.