A virion-associated protein kinase induces apoptosis

Identification of the potential matrix protein of invertebrate iridescent virus 6 (IIV6)

Invertebrate iridescent virus 6 (IIV6) is a nucleocytoplasmic virus with a ∼212 kb linear dsDNA genome that encodes 215 putative open reading frames (ORFs). Proteomic analysis has revealed that the IIV6 virion consists of 54 virally encoded proteins. Interactions among the structural proteins were investigated using the yeast two-hybrid system, revealing that the protein of 415R ORF interacts reciprocally with the potential envelope protein 118L and the major capsid protein 274L. This result suggests that 415R might be a matrix protein that plays a role as a bridge between the capsid and the envelope proteins. To elucidate the function of 415R protein, we determined the localization of 415R in IIV6 structure and analyzed the properties of 415R-silenced IIV6. Specific antibodies produced against 415R protein were used to determine the location of the 415R protein in the virion structure. Both western blot hybridization and immunogold electron microscopy analyses showed that the 415R protein was found in virions treated with Triton X-100, which degrades the viral envelope. The 415R gene was silenced by the RNA interference (RNAi) technique. We used gene-specific dsRNA's to target 415R and showed that this treatment resulted in a significant drop in virus titer. Silencing 415R with dsRNA also reduced the transcription levels of other viral genes. These results provide important data on the role and location of IIV6 415R protein in the virion structure. Additionally, these results may also shed light on the identification of the homologs of 415R among the vertebrate iridoviruses.

Unity and diversity among viral kinases

Viral kinases are known to undergo autophosphorylation and also phosphorylate viral and host substrates. Viral kinases have been implicated in various diseases and are also known to acquire host kinases for mimicking cellular functions and exhibit virulence. Although substantial analyses have been reported in the literature on diversity of viral kinases, there is a gap in the understanding of sequence and structural similarity among kinases from different classes of viruses. In this study, we performed a comprehensive analysis of protein kinases encoded in viral genomes. Homology search methods have been used to identify kinases from 104,282 viral genomic datasets. Serine/threonine and tyrosine kinases are identified only in 390 viral genomes. Out of seven viral classes that are based on nature of genetic material, only viruses having double-stranded DNA and single-stranded RNA retroviruses are found to encode kinases. The 716 identified protein kinases are classified into 63 subfamilies based on their sequence similarity within each cluster, and sequence signatures have been identified for each subfamily. 11 clusters are well represented with at least 10 members in each of these clusters. Kinases from dsDNA viruses, Phycodnaviridae which infect green algae and Herpesvirales that infect vertebrates including human, form a major group. From our analysis, it has been observed that the protein kinases in viruses belonging to same taxonomic lineages form discrete clusters and the kinases encoded in alphaherpesvirus form host-specific clusters. A comprehensive sequence and structure-based analysis enabled us to identify the conserved residues or motifs in kinase catalytic domain regions across all viral kinases. Conserved sequence regions that are specific to a particular viral kinase cluster and the kinases that show close similarity to eukaryotic kinases were identified by using sequence and three-dimensional structural regions of eukaryotic kinases as reference. The regions specific to each viral kinase cluster can be used as signatures in the future in classifying uncharacterized viral kinases. We note that kinases from giant viruses Marseilleviridae have close similarity to viral oncogenes in the functional regions and in putative substrate binding regions indicating their possible role in cancer.

IIV-6 Inhibits NF-κB Responses in Drosophila

The host immune response and virus-encoded immune evasion proteins pose constant, mutual selective pressure on each other. Virally encoded immune evasion proteins also indicate which host pathways must be inhibited to allow for viral replication. Here, we show that IIV-6 is capable of inhibiting the two Drosophila NF-κB signaling pathways, Imd and Toll. Antimicrobial peptide (AMP) gene induction downstream of either pathway is suppressed when cells infected with IIV-6 are also stimulated with Toll or Imd ligands. We find that cleavage of both Imd and Relish, as well as Relish nuclear translocation, three key points in Imd signal transduction, occur in IIV-6 infected cells, indicating that the mechanism of viral inhibition is farther downstream, at the level of Relish promoter binding or transcriptional activation. Additionally, flies co-infected with both IIV-6 and the Gram-negative bacterium, Erwinia carotovora carotovora, succumb to infection more rapidly than flies singly infected with either the virus or the bacterium. These findings demonstrate how pre-existing infections can have a dramatic and negative effect on secondary infections, and establish a Drosophila model to study confection susceptibility.

Combinatory effects of vaccinia virus VG9 and the STAT3 inhibitor Stattic on cancer therapy

The recombinant vaccinia virus VG9 and the STAT3 inhibitor Stattic were combined to kill cancer cells via both oncolytic activity and inhibition of STAT3 phosphorylation in cells. The combinatory anti-tumour activity of these compounds was superior to the activity of VG9 or Stattic alone in vivo. The inhibition of tumour growth occurred via increased apoptosis and autophagy pathways. Furthermore, the combinatory anti-tumour activity was more efficient than that of VG9 or Stattic alone on xenografts, especially in nude mice.

Chilo iridescent virus encodes two functional metalloproteases

The genome of Chilo iridescent virus (CIV) has two open reading frames (ORFs) with matrix metalloprotease (MMP) domains. The protein encoded by ORF 136R contains 178 amino acids with over 40% amino acid sequence identity to hypothetical metalloproteases of other viruses, and the protein 165R contains 264 amino acids with over 40% amino acid sequence identity to metalloproteases of a large group of organisms, primarily including a variety of Drosophila species. These proteins possess conserved zinc-binding motifs in their catalytic domains. In this study, we focused on the functional analysis of these ORFs. They were cloned into the Autographa californica multiple nucleopolyhedrovirus (AcMNPV) Bac-to-Bac baculovirus expression-vector system, expressed in insect Sf9 cells with an N-terminal His tag, and purified to homogeneity at 72 hours postinfection using Ni-NTA affinity chromatography. Western blot analyses of purified 136R and 165R proteins with histidine tags resulted in 24- and 34-kDa protein bands, respectively. Biochemical assays with the purified proteins, performed using azocoll and azocasein as substrates, showed that both proteins have protease activity. The enzymatic activities were inhibited by the metalloprotease inhibitor EDTA. Effects of these proteins were also investigated on Galleria mellonella larvae. Insecticidal activity was tested by injecting the larvae with the virus derived from the AcMNPV bacmid carrying 136R or 165R ORFs. The results showed that the baculoviruses harbouring the iridoviral metalloproteases caused early death of the larvae compared to control group. These data suggest that the CIV 136R and 165R ORFs encode functional metalloproteases. This study expands our knowledge about iridoviruses, describes the characterization of CIV matrix metalloproteinases, and might ultimately contribute to the use of this virus as a research tool.

Baculovirus PTP2 Functions as a Pro-Apoptotic Protein

The family Baculoviridae encompasses a large number of invertebrate viruses, mainly infecting caterpillars of the order Lepidoptera. The baculovirus Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) induces physiological and behavioral changes in its host Spodoptera exigua, as well as immunological responses, which may affect virus transmission. Here we show that the SeMNPV-encoded protein tyrosine phosphatase 2 (PTP2) induces mild apoptosis in Spodoptera frugiperda (Sf) 21 cells upon transient expression. Transient expression of a catalytic-site mutant of ptp2 did not lead to apoptosis, indicating that the phosphatase activity of PTP2 is needed to induce apoptosis. We also found that the caspase level (indicator of apoptosis) was higher in cells transfected with the ptp2 gene than in cells transfected with the catalytic mutant. Adding a caspase inhibitor reduced the level of ptp2-induced apoptosis. Moreover, deletion of the ptp2 gene from the viral genome prevented the induction of apoptosis in S. exigua hemocytes. The virus titer and virulence indices (the viral infectivity and the time to death) were not affected by deletion of the ptp2 gene. However, the viral occlusion body yield from S. exigua larvae infected with the mutant virus lacking the ptp2 gene was much lower than the yield from larvae infected with the wild-type (WT) virus. We hypothesize that the observed pro-apoptotic effects of PTP2 are the result of PTP2-mediated immune suppression in larvae, which consequently leads to higher viral occlusion body yields.

Invertebrate Iridoviruses: A Glance over the Last Decade

Members of the family Iridoviridae (iridovirids) are large dsDNA viruses that infect both invertebrate and vertebrate ectotherms and whose symptoms range in severity from minor reductions in host fitness to systemic disease and large-scale mortality. Several characteristics have been useful for classifying iridoviruses; however, novel strains are continuously being discovered and, in many cases, reliable classification has been challenging. Further impeding classification, invertebrate iridoviruses (IIVs) can occasionally infect vertebrates; thus, host range is often not a useful criterion for classification. In this review, we discuss the current classification of iridovirids, focusing on genomic and structural features that distinguish vertebrate and invertebrate iridovirids and viral factors linked to host interactions in IIV6 (Invertebrate iridescent virus 6). In addition, we show for the first time how complete genome sequences of viral isolates can be leveraged to improve classification of new iridovirid isolates and resolve ambiguous relations. Improved classification of the iridoviruses may facilitate the identification of genus-specific virulence factors linked with diverse host phenotypes and host interactions.

Delineating the roles of cellular and innate antiviral immune parameters mediating ranavirus susceptibility using rainbow trout cell lines

Frog virus 3 is the type species of the Ranavirus genus and the causative agent of massive mortalities of aquatic species worldwide. A critical step in limiting virus replication, particularly early in infection, is the innate immune response. Presently, little is known regarding what innate immune strategies limit FV3 at the cellular level. To this end, the present study uses two rainbow trout cell lines, RTG-2 and RTgutGC, which demonstrate susceptible and relatively resistant phenotypes to FV3 infection, to elucidate susceptibility factors to FV3. RTG-2 demonstrated a lower LD₅₀ and significantly higher virus transcript production compared to RTgutGC. The mode of cell death appeared to be apoptosis for both cell lines; however, RTG-2 did not demonstrate fragmented nuclei typical of apoptosis in cell culture. Next, the source of RTG-2's enhanced susceptibility was pursued, in hopes of highlighting unique features of this virus-host interaction that would predispose a cell to susceptibility. The type I interferon (IFN) response is the keystone mechanism used by the innate immune system to limit virus replication. FV3 induced very low to no levels of IFNs and interferon stimulated genes (ISGs) in either cell line, nor did inducing IFNs prior to infection inhibit virus-induced cell death. A dsRNA-induced antiviral state did reduce virus replication however. UV-inactivated FV3 was also able to kill RTG-2; thus, susceptibility to FV3-induced cell death observed in RTG-2 was independent of virus replication or the cell's ability, or lack thereof, to produce an IFN response. Importantly, RTG-2 showed greater viral entry compared to RTgutGC, suggesting non-innate immune factors, such as surface receptor expression or endocytic mechanism rates, may be key contributors to FV3 susceptibility. These findings contribute to our understanding of cell-level susceptibility to this environmentally important aquatic animal pathogen.

Chilo iridescent virus (CIV) ORF 012L encodes a protein with both exonuclease and endonuclease functions

Chilo iridescent virus (CIV) is the type member of the genus Iridovirus within the family Iridoviridae. The virions of CIV contain a single linear dsDNA molecule that is circularly permuted and terminally redundant. The genome of CIV contains an open reading frame (ORF 012L) encoding a protein homologous to exonuclease II of Schizosaccharomyces pombe. In this study, we focused on the characterization of CIV ORF 012L. The target ORF was cloned into the pET28a vector, expressed in E. coli strain BL21 (DE3) pLysS with an N-terminal His tag and purified to homogeneity by using Ni-NTA affinity chromatography. Biochemical characterization of the purified CIV 012L confirmed that this viral protein is a functional 5'-3' exonuclease that digests 3'-biotin-labelled oligonucleotides and linear double-stranded DNA (dsDNA) molecules from their 5' termini in a highly processive manner. CIV 012L also has a potent endonuclease activity on dsDNA in vitro. In addition, CIV 012L converted supercoiled plasmid DNA (replicative form I, RFI) into the open circular form (RFII) and then open circular form into linear form (RFIII). Endonuclease activity of CIV 012L was optimal in the presence of 10 mM Mg(2+) or 30 mM Mn(2+) ions and at 150 mM NaCl or KCl salt concentrations. The highest endonuclease activity was obtained at pH 8, and it reached a maximum at 55 °C. The CIV 012L protein showed deficiencies for both double- and single-stranded RNAs.

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.

Construction and characterization of a recombinant invertebrate iridovirus

Chilo iridescent virus (CIV), officially named Insect iridescent virus 6 (IIV6), is the type species of the genus Iridovirus (family Iridoviridae). In this paper we constructed a recombinant CIV, encoding the green fluorescent protein (GFP). This recombinant can be used to investigate viral replication dynamics. We showed that homologous recombination is a valid method to make CIV gene knockouts and to insert foreign genes. The CIV 157L gene, putatively encoding a non-functional inhibitor of apoptosis (IAP), was chosen as target for foreign gene insertion. The gfp open reading frame preceded by the viral mcp promoter was inserted into the 157L locus by homologous recombination in Anthonomus grandis BRL-AG-3A cells. Recombinant virus (rCIV-Δ157L-gfp) was purified by successive rounds of plaque purification. All plaques produced by the purified recombinant virus emitted green fluorescence due to the presence of GFP. One-step growth curves for recombinant and wild-type CIV were similar and the recombinant was fully infectious in vivo. Hence, CIV157L can be inactivated without altering the replication kinetics of the virus. Consequently, the CIV 157L locus can be used as a site for insertion of foreign DNA, e.g. to modify viral properties for insect biocontrol.

First complete and productive cell culture model for members of the genus Iridovirus

Chilo iridescent virus (CIV; the type strain of the genus Iridovirus) replicates productively in larvae of the boll weevil, Anthonomus grandis. This study focuses on characterizing productive infections of a boll weevil cell line, BRL-AG-3A (AG3A), starting with CIV reared in the waxworm, Galleria mellonella. We show that CIV can be continually and productively passaged to high titer in AG3A cells. The replication of larval-derived CIV in AG3A was analyzed by observing viral DNA replication and restriction endonuclease digestion profiles, morphogenesis, and infectivity using TCID(50) assays with AG3A as an indicator cell line. The data showed that virus passaged in the AG3A host is stable. AG3A cells are more efficient than previously utilized CF-124T cells from Choristoneura fumiferana. This system constitutes a superior model for cellular and molecular studies on CIV; it represents the first complete, productive cell culture model for the replication of CIV or any member of the genus Iridovirus.