Characterization of an envelope protein 118L in invertebrate iridescent virus 6 (IIV6)

Invertebrate iridescent virus 6 (IIV6) is a nucleocytoplasmic insect virus and a member of the family Iridoviridae. The IIV6 genome consists of 212,482 bp of linear dsDNA with 215 non-overlapping and putative protein-encoding ORFs. The IIV6 118L ORF is conserved in all sequenced members of the Iridoviridae and encodes a 515 amino acid protein with three predicted transmembrane domains and several N-glycosylation/N-myristoylation sites. In this study, we characterized the 118L ORF by both deleting it from the viral genome and silencing its expression with dsRNA in infected insect cells. The homologous recombination method was used to replace 118L ORF with the green fluorescent protein (gfp) gene. Virus mutants in which the 118L gene sequence had been replaced with gfp were identified by fluorescence microscopy but could not be propagated separately from the wild-type virus in insect cells. Unsuccessful attempts to isolate the mutant virus with the 118L gene deletion suggested that the protein is essential for virus replication. To support this result, we used dsRNA to target the 118L gene and showed that treatment resulted in a 99% reduction in virus titer. Subsequently, we demonstrated that 118L-specific antibodies produced against the 118L protein expressed in the baculovirus vector system were able to neutralize the virus infection. All these results indicate that 118L is a viral envelope protein that is required for the initiation of virus replication.

Fusing a TurboID tag with the Andrias davidianus ranavirus 2L reduced virus adsorption efficiency

Andrias davidianus ranavirus (ADRV) is a member of the genus ranavirus (family Iridoviridae). ADRV 2L is an envelope protein that could be essential in viral infection. In the present study, the function of ADRV 2L was investigated by fusion with the biotin ligase TurboID tag. A recombinant ADRV with a V5-TurboID tag fused in the N-terminal of 2L (ADRVT-2L) and a recombinant ADRV expressing V5-TurboID (ADRVT) were constructed, respectively. Infection of the recombinant viruses and wild-type ADRV (ADRVWT) in the Chinese giant salamander thymus cell line (GSTC) showed that ADRVT-2L had reduced cytopathic effect and lower virus titers than the other two viruses, indicating the fusion of a big tag affected ADRV infection. Analysis of the temporal expression profile showed that the expression of V5-TurboID-2L was delayed than wild-type 2L. However, electron microscopy found that the virion morphogenesis was not affected in ADRVT-2L-infected cells. Furthermore, the virus binding assay revealed that the adsorption efficiency of ADRVT-2L was considerably decreased compared to the other two viruses. Therefore, these data showed that linking the TurboID tag to ADRV 2L affected virus adsorption to the cell membrane, which suggested an important role of 2L in virus entry into cells.

Genome Characteristics of Two Ranavirus Isolates from Mandarin Fish and Largemouth Bass

Ranaviruses are promiscuous pathogens that threaten lower vertebrates globally. In the present study, two ranaviruses (SCRaV and MSRaV) were isolated from two fishes of the order Perciformes: mandarin fish (Siniperca chuatsi) and largemouth bass (Micropterus salmoides). The two ranaviruses both induced cytopathic effects in cultured cells from fish and amphibians and have the typical morphologic characteristics of ranaviruses. Complete genomes of the two ranaviruses were then sequenced and analyzed. Genomes of SCRaV and MSRaV have a length of 99, 405, and 99, 171 bp, respectively, and both contain 105 predicted open reading frames (ORFs). Eleven of the predicted proteins have differences between SCRaV and MSRaV, in which only one (79L) possessed a relatively large difference. A comparison of the sequenced six ranaviruses from the two fish species worldwide revealed that sequence identities of the six proteins (11R, 19R, 34L, 68L, 77L, and 103R) were related to the place where the virus was isolated. However, there were obvious differences in protein sequence identities between the two viruses and iridoviruses from other hosts, with more than half lower than 55%. Especially, 12 proteins of the two isolates had no homologs in viruses from other hosts. Phylogenetic analysis revealed that ranaviruses from the two fishes clustered in one clade. Further genome alignment showed five groups of genome arrangements of ranaviruses based on the locally collinear blocks, in which the ranaviruses, including SCRaV and MSRaV, constitute the fifth group. These results provide new information on the ranaviruses infecting fishes of Perciformes and also are useful for further research of functional genomics of the type of ranaviruses.

Identification and characterization of an envelope protein 168L in Cherax quadricarinatus iridovirus (CQIV)

Cherax quadricarinatus iridovirus (CQIV), a new member of family Iridoviridae, mainly infects the shrimps and crayfish with a high mortality rate. Previous gel-based LC-MS/MS study on CQIV has identified 30 structural proteins. In this study, one of the structural proteins, CQIV-168L, was selected for further analysis. RT-PCR and Western-blotting (WB) detection revealed that the transcript and the protein appeared late during infection of C. quadricarinatus cells and that the transcript was blocked by viral DNA replication inhibitor, indicating that CQIV-168L is a late expression gene. The specific antiserum against CQIV-168L was raised and immunofluorescence analysis showed that CQIV-168L was localized in the cytoplasm and associated with virus factories. Western-blotting (WB) assay suggested that CQIV-168L antiserum bound specifically to a 57-kDa protein in both the intact virions and the envelope fraction. As revealed by immunogold labeling, CQIV-168L was a component of the viral envelope. Findings in this work help to further understand the structure and entry mechanism of CQIV.

ADRV 12L: A Ranaviral Putative Rad2 Family Protein Involved in DNA Recombination and Repair

The Andrias davidianus ranavirus (ADRV) is a member of the family Iridoviridae and belongs to the nucleocytoplasmic large DNA viruses. Based on genomic analysis, an ADRV-encoding protein, ADRV 12L, and its homologs from other iridoviruses were predicted as Rad2 family proteins based on the conserved amino acids, domains, and secondary structures. Expression analysis showed that the transcription of ADRV 12L started at 4 h post infection, and its expression was not inhibited by a DNA-replication inhibitor. Meanwhile, immunofluorescence localization showed that ADRV 12L mainly localized in viral factories and colocalized with the viral nascent DNA, which hinted at a possible role in DNA replication. Furthermore, a mutant ADRV lacking 12L (ADRV-Δ12L) was constructed. In both luciferase assays based on homologous recombination (HR) and double-strand break repair (DSBR) that followed, ADRV-Δ12L induced less luciferase activity than the wild-type ADRV, indicating that HR and DSBR were impaired in ADRV-Δ12L infected cells. In addition, infection with ADRV-Δ12L resulted in smaller plaque sizes and lower viral titers than that with wild-type ADRV, indicating an important role for 12L in efficient virus infection. Therefore, the results suggest that Rad2 homologs encoded by iridovirus have important roles in HR- and DSBR-process of the viral DNA and, thus, affect virus replication and the production of progeny virions.

Structural and Functional Diversity among Five RING Finger Proteins from Carassius Auratus Herpesvirus (CaHV)

Carassius auratus herpesvirus (CaHV) has been identified as a high-virulence pathogenic virus that infects aquatic animals, but the key factor for virus–host interaction is still unclear. Five Really interesting new genes (RING) finger proteins (39L, 52L, 131R, 136L, and 143R) of CaHV were screened to determine structural diversity. RING finger proteins were also predicted in other known fish herpesviruses, with an arrangement and number similar to CaHV. We performed multifaceted analyses of the proteins, including protein sizes, skeleton structures, subcellular localizations, and ubiquitination activities, to determine their precise roles in virus–host interactions. The five proteins were overexpressed and detected different levels of ubiquitination activities, and 143R showed the highest activity. Then, the prokaryotic expressed and purified full-length proteins (131R and 136L), RING domain isolates (131R_12–43 and 136L_45–87), and RING domain-deleted mutants (131RΔ_12–43 and 136LΔ_45–87) were prepared to detect their activities through ubiquitination assays. The results indicate that both full-length proteins and their isolates have activities that catalyze ubiquitination, and the full-length proteins possess higher activity than the isolates, but RING domain-deleted mutants lose their activities. Furthermore, the activities of the five proteins were verified as E3 ubiquitin ligase activity, showing that the RING domains determine the ubiquitination activity. These proteins present different subcellular localization. RING domain-deleted mutants showed similar subcellular localization with their full-length proteins, and all the isolates diffused in whole cells. The current results indicate that the sequence outside the RING domain determines subcellular localization and the level of ubiquitination activity, suggesting that the RING finger proteins of fish herpesviruses might have diverse functions in virus–host interaction.

A transmembrane domain of Andrias davidianus ranavirus 13R is crucial for co-localization to endoplasmic reticulum and viromatrix

13R, a core gene of Andrias davidianus ranavirus (ADRV), encoded a protein containing a transmembrane domain (TMD) and a restriction endonuclease-like domain. However, the characterization and function of 13R and the protein it encodes remain unclear. In this study, Chinese giant salamander thymus cell (GSTC) was used to investigate the function of 13R. The results showed that the 13R transcripts were detected first at 8 h post-infection (hpi) by RT-PCR and the protein was detected first at 24 hpi by western blot, but the transcription was inhibited by cycloheximide and cytosine arabinofuranoside, indicating that 13R is a viral late gene. Subcellular localization showed that the 13R was co-localized with endoplasmic reticulum (ER) in the cytoplasm, while 13R deleting TMD (13RΔTM) was distributed in cytoplasm and nucleus. During ADRV infection, 13R was observed first in the cytoplasm and nucleus, and later aggregated into the viromatrix, whereas 13RΔTM remain dispersed in cytoplasm and nucleus. Western blot analysis suggested that 13R was a viral non-structural protein and its overexpression did not affect the viral titer in GSTC. All these indicated that the TMD of 13R is crucial for the co-localization into the ER and the viromatrix.

Ranaviruses Bind Cells from Different Species through Interaction with Heparan Sulfate

Ranavirus cross-species infections have been documented, but the viral proteins involved in the interaction with cell receptors have not yet been identified. Here, viral cell-binding proteins and their cognate cellular receptors were investigated using two ranaviruses, Andrias davidianus ranavirus (ADRV) and Rana grylio virus (RGV), and two different cell lines, Chinese giant salamander thymus cells (GSTC) and Epithelioma papulosum cyprinid (EPC) cells. The heparan sulfate (HS) analog heparin inhibited plaque formation of ADRV and RGV in the two cell lines by more than 80% at a concentration of 5 μg/mL. In addition, enzymatic removal of cell surface HS by heparinase I markedly reduced plaque formation by both viruses and competition with heparin reduced virus-cell binding. These results indicate that cell surface HS is involved in ADRV and RGV cell binding and infection. Furthermore, recombinant viral envelope proteins ADRV-58L and RGV-53R bound heparin-Sepharose beads implying the potential that cell surface HS is involved in the initial interaction between ranaviruses and susceptible host cells. To our knowledge, this is the first report identifying cell surface HS as ranavirus binding factor and furthers understanding of interactions between ranaviruses and host cells.

Ubiquitin-Proteasome System Is Required for Efficient Replication of Singapore Grouper Iridovirus

The ubiquitin-proteasome system (UPS) serves as the major intracellular pathway for protein degradation and plays crucial roles in several cellular processes. However, little is known about the potential actions of the UPS during fish virus infection. In this study, we elucidated the possible roles of UPS in the life cycle of Singapore grouper iridovirus (SGIV); a large DNA virus that usually causes serious systemic diseases with high mortality in groupers. Data from transcriptomic analysis of differentially expressed genes illustrated that expression of 65 genes within the UPS pathway, including ubiquitin encoding, ubiquitination, deubiquitination, and proteasome, were up- or down-regulated during SGIV infection. Using different proteasome inhibitors, inhibition of the proteasome decreased SGIV replication in vitro, accompanied by inhibition of virus assembly site formation, and viral gene transcription and protein transportation. Over-expression of ubiquitin partly rescued the inhibitory effect of ubiquitin inhibitor on SGIV replication, suggesting that UPS was required for fish iridovirus infection in vitro. Viral or host proteins regulated by proteasome inhibition during SGIV infection were investigated with two-dimensional gel electrophoresis and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Sixty-two differentially expressed proteins, including 15 viral and 47 host proteins, were identified after SGIV infection. The host proteins were involved in ubiquitin-mediated protein degradation, metabolism, cytoskeleton, macromolecular biosynthesis, and signal transduction. Among them, 11 proteins were negatively regulated upon MG132 treatment during SGIV infection. This is believed to be the first study to provide evidence that UPS was essential for fish virus infection and replication.

Rana grylio virus 43R encodes an envelope protein involved in virus entry

Rana grylio virus (RGV), a member of genus Ranavirus in the family Iridoviridae, is a viral pathogen infecting aquatic animal. RGV 43R has homologues only in Ranavirus and contains a transmembrane (TM) domain, but its role in RGV infection is unknown. In this study, 43R was determined to be associated with virion membrane. The transcripts encoding 43R and the protein itself appeared late in RGV-infected EPC cells and its expression was blocked by viral DNA replication inhibitor, indicating that 43R is a late expressed protein. Subcellular localization showed that 43R-EGFP fusion protein distributed in cytoplasm of EPC cells and that TM domain is essential for its distribution in cytoplasm. 43R-EGFP fusion protein colocalized with viral factories in RGV-infected cells. A recombinant RGV deleting 43R (Δ43R-RGV) was constructed by homologous recombination to investigate its role in virus infection. Compared with wild type RGV, the ability of Δ43R-RGV to induce the cytopathic effect and its virus titers were significantly reduced. Furthermore, it is revealed that 43R deletion significantly inhibited viral entry but did not influence viral DNA replication by measuring and comparing the DNA levels of RGV and Δ43R-RGV in the infected cells at the early stage of infection. RGV neutralization with anti-43R serum reduced the virus titer. Therefore, these data showed that RGV 43R is a late gene that encodes an envelope protein involved in RGV entry.

Divergent transcriptomic responses underlying the ranaviruses-amphibian interaction processes on interspecies infection of Chinese giant salamander

Background

Ranaviruses (family Iridoviridae, nucleocytoplasmic large DNA viruses) have been reported as promiscuous pathogens of cold-blooded vertebrates. Rana grylio virus (RGV, a ranavirus), from diseased frog Rana grylio with a genome of 105.79 kb and Andrias davidianus ranavirus (ADRV), from diseased Chinese giant salamander (CGS) with a genome of 106.73 kb, contains 99% homologous genes.

Results

To uncover the differences in virus replication and host responses under interspecies infection, we analyzed transcriptomes of CGS challenged with RGV and ADRV in different time points (1d, 7d) for the first time. A total of 128,533 unigenes were obtained from 820,858,128 clean reads. Transcriptome analysis revealed stronger gene expression of RGV than ADRV at 1 d post infection (dpi), which was supported by infection in vitro. RGV replicated faster and had higher titers than ADRV in cultured CGS cell line. RT-qPCR revealed the RGV genes including the immediate early gene (RGV-89R) had higher expression level than that of ADRV at 1 dpi. It further verified the acute infection of RGV in interspecies infection.

The number of differentially expressed genes and enriched pathways from RGV were lower than that from ADRV, which reflected the variant host responses at transcriptional level. No obvious changes of key components in pathway “Antigen processing and presentation” were detected for RGV at 1 dpi. Contrarily, ADRV infection down-regulated the expression levels of MHC I and CD8. The divergent host immune responses revealed the differences between interspecies and natural infection, which may resulted in different fates of the two viruses. Altogether, these results revealed the differences in transcriptome responses among ranavirus interspecies infection of amphibian and new insights in DNA virus-host interactions in interspecies infection.

Conclusion

The DNA virus (RGV) not only expressed self-genes and replicated quickly after entry into host under interspecies infection, but also avoided the over-activation of host responses. The strategy could gain time for the survival of interspecies pathogen, and may provide opportunity for its adaptive evolution and interspecies transmission.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4596-y) contains supplementary material, which is available to authorized users.

Characterization of the VP39 envelope protein from Singapore grouper iridovirus

Singapore grouper iridovirus (SGIV) is a major pathogen that causes heavy economic losses to the grouper aquaculture industry in China and Southeast Asian countries. In the present study, a viral envelope protein, VP39, encoded by SGIV ORF39L, was identified and characterized. SGIV ORF39L was found in all sequenced iridoviruses and is now considered to be a core gene of the family Iridoviridae. ORF39L was classified as a late gene during in vitro infection using reverse transcription–polymerase chain reaction, western blotting, and a drug inhibition analysis. An indirect immunofluorescence assay revealed that the VP39 protein was confined to the cytoplasm, especially at viral assembly sites. Western blot and matrix-assisted laser desorption/ionization-time of flight tandem mass spectrometry analyses suggested that VP39 is an envelope protein. Immunogold electron microscopy further confirmed that VP39 is a viral envelope protein. Furthermore, a mouse anti-VP39 polyclonal antibody exhibited SGIV-neutralizing activity in vitro, suggesting that VP39 is involved in SGIV infection. Taken together, the current data suggest that VP39 represents a conserved envelope protein of iridoviruses that contributes to viral infection.

Whole-Genome Analysis of a Novel Fish Reovirus (MsReV) Discloses Aquareovirus Genomic Structure Relationship with Host in Saline Environments

Aquareoviruses are serious pathogens of aquatic animals. Here, genome characterization and functional gene analysis of a novel aquareovirus, largemouth bass Micropterus salmoides reovirus (MsReV), was described. It comprises 11 dsRNA segments (S1–S11) covering 24,024 bp, and encodes 12 putative proteins including the inclusion forming-related protein NS87 and the fusion-associated small transmembrane (FAST) protein NS22. The function of NS22 was confirmed by expression in fish cells. Subsequently, MsReV was compared with two representative aquareoviruses, saltwater fish turbot Scophthalmus maximus reovirus (SMReV) and freshwater fish grass carp reovirus strain 109 (GCReV-109). MsReV NS87 and NS22 genes have the same structure and function with those of SMReV, whereas GCReV-109 is either missing the coiled-coil region in NS79 or the gene-encoding NS22. Significant similarities are also revealed among equivalent genome segments between MsReV and SMReV, but a difference is found between MsReV and GCReV-109. Furthermore, phylogenetic analysis showed that 13 aquareoviruses could be divided into freshwater and saline environments subgroups, and MsReV was closely related to SMReV in saline environments. Consequently, these viruses from hosts in saline environments have more genomic structural similarities than the viruses from hosts in freshwater. This is the first study of the relationships between aquareovirus genomic structure and their host environments.

Rana grylio virus (RGV) envelope protein 2L: subcellular localization and essential roles in virus infectivity revealed by conditional lethal mutant

Rana grylio virus (RGV) is a pathogenic iridovirus that has resulted in high mortality in cultured frog. Here, an envelope protein gene, 2L, was identified from RGV and its possible role in virus infection was investigated. Database searches found that RGV 2L had homologues in all sequenced iridoviruses and is a core gene of iridoviruses. Western blotting detection of purified RGV virions confirmed that 2L protein was associated with virion membrane. Fluorescence localization revealed that 2L protein co-localized with viral factories in RGV infected cells. In co-transfected cells, 2L protein co-localized with two other viral envelope proteins, 22R and 53R. However, 2L protein did not co-localize with the major capsid protein of RGV in co-transfected cells. Meanwhile, fluorescence observation showed that 2L protein co-localized with endoplasmic reticulum, but did not co-localize with mitochondria and Golgi apparatus. Moreover, a conditional lethal mutant virus containing the lac repressor/operator system was constructed to investigate the role of RGV 2L in virus infection. The ability to form plaques and the virus titres were strongly reduced when expression of 2L was repressed. Therefore, the current data showed that 2L protein is essential for virus infection. Our study is the first report, to our knowledge, of co-localization between envelope proteins in iridovirus and provides new insights into the understanding of envelope proteins in iridovirus.

Identification and characterization of Rab7 from orange-spotted grouper, Epinephelus coioides

Rab7 is a small GTPase that regulates vesicular traffic from early to late endosomal stages of the endocytic pathway. During the virus-host co-evolution, host Rab7 was also exploited by virus to complete their life cycle. To date, however, the roles of fish Rab7 in virus infection remained largely unknown. Here, we cloned and characterized a Rab7 gene from grouper, Epinephelus coioides (Ec-Rab7). The full-length Ec-Rab7 cDNA was composed of 1182 bp and encoded a polypeptide of 207 amino acids which shared 99% identity with that from Anoplopoma fimbria or Oreochromis niloticus. Ec-Rab7 contained five conserved domains of Rab GTPase family including GTP-binding or GTPase regions as well as an effector site. RT-PCR analysis revealed that Ec-Rab7 ubiquitously expressed in all detected tissues and its transcript in spleen was up-regulated after challenge with Singapore grouper iridovirus (SGIV). Subcellular localization analysis revealed that Ec-Rab7 was distributed in the cytoplasm as spots and mostly colocalized with lysosomes. Notably, the ectopic expressed Ec-Rab7 partly aggregated into the viral factories in cells infected by SGIV. Furthermore, overexpression of Ec-Rab7 accelerated the occurrence of cytopathic effect (CPE) induced by SGIV infection and promoted viral gene transcription. In addition, far western blotting assay revealed that Ec-Rab7 might interact with viral proteins, including SGIV VP69 and VP101. Taken together, our data suggested that Ec-Rab7 might be potentially involved in SGIV replication.

Characterization of an envelope gene VP19 from Singapore grouper iridovirus

Background

Viral envelope proteins are always proposed to exert important function during virus infection and replication. Vertebrate iridoviruses are enveloped large DNA virus, which can cause great economic losses in aquaculture and ecological destruction. Although numerous iridovirus envelope proteins have been identified using bioinformatics and proteomic methods, their roles in virus infection remained largely unknown.

Methods

Using SMART and TMHMM programs, we investigated the structural characteristics of Singapore grouper iridovirus (SGIV) VP19. A specific antibody against VP19 was generated and the expression profile of VP19 was clarified. The subcellular localization of VP19 in the absence or presence of other viral products was determined via transfection and immune fluorescence assay. In addition, Western blot assay and electron microscopy examination were performed to demonstrate whether SGIV VP19 was an envelope protein or a capsid protein.

Results

Here, SGIV VP19 was cloned and characterized. Among all sequenced iridoviruses, VP19 and its orthologues shared common features, including 19 invariant cysteines, a proline-rich motif and a predicted transmembrane domain. Subsequently, the protein synthesis of VP19 was only detected at the late stage of SGIV infection and inhibited obviously by treating with AraC, confirming that VP19 was a late expressed protein. Ectopic expression of EGFP-VP19 in vitro displayed a punctate pattern in the cytoplasm. In SGIV infected cells, the newly synthesized VP19 protein was initially localized in the cytoplasm in a punctate pattern, and then aggregated into the virus assembly site at the late stage of SGIV infection, suggesting that other viral protein products were essential for VP19’s function during SGIV infection. In addition, Western blot assay and electron microscopy observation revealed that SGIV VP19 was associated with viral envelope, which was different from major capsid protein (MCP).

Conclusion

Taken together, the current data suggested that VP19 represented a conserved envelope protein in iridovirus, and might contribute greatly to virus assembly during virus infection.

Nonstructural protein NS80 is crucial in recruiting viral components to form aquareoviral factories

Background

Replication and assembly of vertebrate reoviruses occur in specific intracellular compartments known as viral factories. Recently, NS88 and NS80, the nonstructural proteins from aquareoviruses, have been proposed to share common traits with µNS from orthoreoviruses, which are involved in the formation of viral factories.

Methodology/Principal Findings

In this study, the NS80 characteristics and its interactions with other viral components were investigated. We observed that the NS80 structure ensured its self-aggregation and selective recruitment of viral proteins to viral factories like structures (VFLS). The minimum amino acids (aa) of NS80 required for VFLS formation included 193 aa at the C-terminal. However, this truncated protein only contained one aa coil and located in the nucleus. Its N-terminal residual regions, aa 1–55 and aa 55–85, were required for recruiting viral nonstructural protein NS38 and structural protein VP3, respectively. A conserved N-terminal region of NS38, which was responsible for the interaction with NS80, was also identified. Moreover, the minimal region of C-terminal residues, aa 506–742 (Δ505), required for NS80 self-aggregation in the cytoplasm, and aa 550–742 (Δ549), which are sufficient for recruiting viral structure proteins VP1, VP2, and VP4 were also identified.

Conclusions/Significance

The present study shows detailed interactions between NS80 and NS38 or other viral proteins. Sequence and structure characteristics of NS80 ensures its self-aggregation to form VFLS (either in the cytoplasm or nucleus) and recruitment of viral structural or nonstructural proteins.

Rana grylio virus (RGV) 50L is associated with viral matrix and exhibited two distribution patterns

BACKGROUND

The complete genome of Rana grylio virus (RGV) was sequenced and analyzed recently, which revealed that RGV 50 L had homologues in many iridoviruses with different identities; however, the characteristics and functions of 50 L have not been studied yet.

METHODOLOGY/PRINCIPAL FINDINGS

We cloned and characterized RGV50L, and revealed 50 L functions in virus assembly and gene regulation. 50 L encoded a 499-amino acid structural protein of about 85 kDa in molecular weight and contained a nuclear localization signal (NLS) and a helix- extension-helix motif. Drug inhibition assay demonstrated that 50 L was an immediate-early (IE) gene. Immuno-fluorescence assay revealed that 50 L appeared early and persisted in RGV-infected cells following two distribution patterns. One pattern was that 50 L exhibited a cytoplasm-nucleus- viromatrix distribution pattern, and mutagenesis of the NLS motif revealed that localization of 50 L in the nucleus was NLS-dependent; the other was that 50 L co-localized with viral matrix which plays important roles in virus assembly and the life circle of viruses.

CONCLUSIONS/SIGNIFICANCE

RGV 50L is a novel iridovirus IE gene encoded structural protein which plays important roles in virus assembly.

Antigenic identification of virion structural proteins from infectious spleen and kidney necrosis virus

Infectious spleen and kidney necrosis virus (ISKNV), belonging to the genus Megalocytivirus in the family Iridoviridae, is one of the major agents causing mortality and economic losses to the freshwater fish culture industry in Asian countries. Currently, little information regarding the antigenic properties of Megalocytivirus (especially ISKNV) is available. Our previous study using four different workflows with systematic and comprehensive proteomic approaches led to the identification of 38 ISKNV virion-associated proteins (J. Virol. 2869-2877, 2011). Thus, in this report, the antigenicity of 31 structural proteins from ISKNV virion was investigated. A one-dimensional gel electrophoresis immunoblot profile coupled with MALDI-TOF-TOF MS/MS was applied to identify six immunogenic viral proteins, namely, ORFs major capsid protein (006L), 054L, 055L, 101L, 117L, and 125L. Then, the antigenicity of 31 structural proteins was characterized by Western blot by using pooled sera from mandarin fish that survived ISKNV infection. Of the 31 viral proteins, 22 were recognized by the fish ISKNV antiserum. Furthermore, this antiserum neutralizes MFF-1 cells ISKNV infection. To our knowledge, this study is the first report on the immunogenicity of viral proteins and characterization of the proteome of megalocytivirus infective agents. Our findings are expected to promote the development of effective vaccine candidates.

The molecular biology of frog virus 3 and other iridoviruses infecting cold-blooded vertebrates

Frog virus 3 (FV3) is the best characterized member of the family Iridoviridae. FV3 study has provided insights into the replication of other family members, and has served as a model of viral transcription, genome replication, and virus-mediated host-shutoff. Although the broad outlines of FV3 replication have been elucidated, the precise roles of most viral proteins remain unknown. Current studies using knock down (KD) mediated by antisense morpholino oligonucleotides (asMO) and small, interfering RNAs (siRNA), knock out (KO) following replacement of the targeted gene with a selectable marker by homologous recombination, ectopic viral gene expression, and recombinant viral proteins have enabled researchers to systematically ascertain replicative- and virulence-related gene functions. In addition, the application of molecular tools to ecological studies is providing novel ways for field biologists to identify potential pathogens, quantify infections, and trace the evolution of ecologically important viral species. In this review, we summarize current studies using not only FV3, but also other iridoviruses infecting ectotherms. As described below, general principles ascertained using FV3 served as a model for the family, and studies utilizing other ranaviruses and megalocytiviruses have confirmed and extended our understanding of iridovirus replication. Collectively, these and future efforts will elucidate molecular events in viral replication, intrinsic and extrinsic factors that contribute to disease outbreaks, and the role of the host immune system in protection from disease.

Rana grylio virus as a vector for foreign gene expression in fish cells

In the present study, Rana grylio virus (RGV, an iridovirus) thymidine kinase (TK) gene and viral envelope protein 53R gene were chosen as targets for foreign gene insertion. ΔTK-RGV and Δ53R-RGV, two recombinant RGV, expressing enhanced green fluorescence protein (EGFP) were constructed and analyzed in Epithelioma papulosum cyprinid (EPC) cells. The EGFP gene which fused to the virus major capsid protein (MCP) promoter p50 was inserted into TK and 53R gene loci of RGV, respectively. Cells infected with these two recombinant viruses not only displayed plaques, but also emitted strong green fluorescence under fluorescence microscope, providing a simple method for selection and purification of recombinant viruses. ΔTK-RGV was purified by seven successive rounds of plaque isolation and could be stably propagated in EPC cells. All of the plaques produced by the purified recombinant virus emitted green fluorescence. However, Δ53R-RGV was hard to be purified even through twenty rounds of plaque isolation. The purified recombinant virus ΔTK-RGV was verified by PCR analysis and Western blotting. These results showed EGFP was expressed in ΔTK-RGV infected cells. Furthermore, one-step growth curves and electron microscopy revealed that infection with recombinant ΔTK-RGV and wild-type RGV are similar. Therefore, RGV was demonstrated could be as a viral vector for foreign gene expression in fish cells.

Construction of green fluorescent protein-tagged recombinant iridovirus to assess viral replication

Green fluorescent protein-tagged recombinant virus has been successfully applied to observing the infective dynamics and evaluating viral replication. Here, we identified soft-shelled turtle iridovirus (STIV) ORF55 as an envelope protein (VP55), and developed a recombinant STIV expressing an enhanced green fluorescent protein (EGFP) fused to VP55 (EGFP-STIV). Recombinant EGFP-STIV shared similar single-step growth curves and ultrastructural morphology with wild type STIV (wt-STIV). The green fluorescence distribution during EGFP-STIV infection was consistent with the intracellular distribution of VP55 which was mostly co-localized with virus assembly sites. Furthermore, EGFP-STIV could be used to evaluate viral replication conveniently under drug treatment, and the result showed that STIV replication was significantly inhibited after the addition of antioxidant pyrrolidine dithiocarbamate (PDTC). Thus, the EGFP-tagged recombinant iridovirus will not only be useful for further investigations on the viral replicative dynamics, but also provide an alternative simple strategy to screen for antiviral substances.

Global landscape of structural proteins of infectious spleen and kidney necrosis virus

Infectious spleen and kidney necrosis virus (ISKNV), the type species of the genus Megalocytivirus in the family Iridoviridae, causes severe damage to mandarin fish cultures in China. Little is known about the proteins of ISKNV virions. In this study, a total of 38 ISKNV virion-associated proteins were identified by four different workflows with systematic and comprehensive proteomic approaches. Among the 38 identified proteins, 21 proteins were identified by the gel-based workflows (one-dimensional [1-D] and two-dimensional [2-D] gel electrophoresis). Fifteen proteins were identified by 1-D gel electrophoresis, and 16 proteins were identified by 2-D gel electrophoresis, with 10 proteins identified by both methods. Another 17 proteins were identified only by liquid chromatography (LC)-based workflows (LC-matrix-assisted laser desorption ionization [MALDI] and linear trap quadrupole [LTQ]-Orbitrap). Among these 17 LC-identified proteins, 5 proteins were identified uniquely by the LC-MALDI workflow, whereas another 6 proteins were identified only by the LTQ-Orbitrap workflow. These results underscore the importance of incorporation of multiple approaches in identification of viral proteins. Based on viral genomic sequence, genes encoding these 38 viral proteins were cloned and expressed in vitro. Antibodies were produced against these 38 proteins to confirm the ISKNV structural proteins by Western blotting. Of the newly identified proteins, ORF 056L and ORF 118L were identified and confirmed as two novel viral envelope proteins by Western blotting and immunoelectron microscopy (IEM). The ISKNV proteome reported here is currently the only characterized megalocytivirus proteome. The systematic and comprehensive identification of ISKNV structural proteins and their localizations in this study will facilitate future studies of the ISKNV assembly process and infection mechanism.

Genome of turbot rhabdovirus exhibits unusual non-coding regions and an additional ORF that could be expressed in fish cell

Genomic sequence of Scophthalmus maximus rhabdovirus (SMRV) isolated from diseased turbot has been characterized. The complete genome of SMRV comprises 11,492 nucleotides and encodes five typical rhabdovirus genes N, P, M, G and L. In addition, two open reading frames (ORF) are predicted overlapping with P gene, one upstream of P and smaller than P (temporarily called Ps), and another in P gene which may encodes a protein similar to the vesicular stomatitis virus C protein. The C ORF is contained within the P ORF. The five typical proteins share the highest sequence identities (48.9%) with the corresponding proteins of rhabdoviruses in genus Vesiculovirus. Phylogenetic analysis of partial L protein sequence indicates that SMRV is close to genus Vesiculovirus. The first 13 nucleotides at the ends of the SMRV genome are absolutely inverse complementarity. The gene junctions between the five genes show conserved polyadenylation signal (CATGA(7)) and intergenic dinucleotide (CT) followed by putative transcription initiation sequence A(A/G)(C/G)A(A/G/T), which are different from known rhabdoviruses. The entire Ps ORF was cloned and expressed, and used to generate polyclonal antibody in mice. One obvious band could be detected in SMRV-infected carp leucocyte cells (CLCs) by anti-Ps/C serum via Western blot, and the subcellular localization of Ps-GFP fusion protein exhibited cytoplasm distribution as multiple punctuate or doughnut shaped foci of uneven size.

Frog virus 3 ORF 53R, a putative myristoylated membrane protein, is essential for virus replication in vitro

Although previous work identified 12 complementation groups with possible roles in virus assembly, currently only one frog virus 3 protein, the major capsid protein (MCP), has been linked with virion formation. To identify other proteins required for assembly, we used an antisense morpholino oligonucleotide to target 53R, a putative myristoylated membrane protein, and showed that treatment resulted in marked reductions in 53R levels and a 60% drop in virus titers. Immunofluorescence assays confirmed knock down and showed that 53R was found primarily within viral assembly sites, whereas transmission electron microscopy detected fewer mature virions and, in some cells, dense granular bodies that may represent unencapsidated DNA-protein complexes. Treatment with a myristoylation inhibitor (2-hydroxymyristic acid) resulted in an 80% reduction in viral titers. Collectively, these data indicate that 53R is an essential viral protein that is required for replication in vitro and suggest it plays a critical role in virion formation.

Proteomic analysis of Chilo iridescent virus

In this first proteomic analysis of an invertebrate iridovirus, 46 viral proteins were detected in the virions of Chilo iridescent virus (CIV) based on the detection of 2 or more distinct peptides; an additional 8 proteins were found based on a single peptide. Thirty-six of the 54 identified proteins have homologs in another invertebrate and/or in one or more vertebrate iridoviruses. The genes for 5 of the identified proteins, 22L (putative helicase), 118L, 142R (putative RNaseIII), 274L (major capsid protein) and 295L, are shared by all iridoviruses for which the complete nucleotide sequence is known and may therefore be considered as iridovirus core genes. Three identified proteins have homologs only in ascoviruses. The remaining 15 identified proteins are so far unique to CIV. In addition to broadening our insight in the structure and assembly of CIV virions, this knowledge is pivotal to unravel the initial steps in the infection process.

The type III secretion system of Vibrio alginolyticus induces rapid apoptosis, cell rounding and osmotic lysis of fish cells

Vibrio alginolyticus is a Gram-negative bacterium and has been recognized as an opportunistic pathogen in humans as well as marine animals. However, the virulence mechanisms for this species of Vibrio have not been elucidated. This study characterized multiple mechanisms that induce cell death in fish cells upon infection with a V. alginolyticus strain, ZJO. The bacterium required its type III secretion system (T3SS) to cause rapid death of infected fish cells. Dying cells exhibited some features of apoptotic cells, such as membrane blebbing, nuclear condensation and DNA fragmentation. Further studies showed that caspase-3 was activated by the T3SS of the ZJO strain, confirming that infection with V. alginolyticus rapidly induces T3SS-dependent apoptosis in fish cells. Infection with the ZJO strain also led to membrane pore formation and release of cellular contents from infected fish cells, as evidenced by lactate dehydrogenase release and the uptake of a membrane-impermeable dye. Importantly, inhibition of apoptosis did not prevent ZJO-infected cells from releasing cellular contents and did not block cell rounding. Taken together, these data demonstrate that infection with V. alginolyticus may promote at least three different T3SS-dependent events, which lead to the death of fish cells. This study provides an important insight into the mechanism used by Vibrio species to cause host-cell death.

Viral envelope protein 53R gene highly specific silencing and iridovirus resistance in fish Cells by AmiRNA

Background

Envelope protein 53R was identified from frog Rana grylio virus (RGV), a member of the family Iridoviridae, and it plays an important role in the virus assembly. Although inhibition of iridovirus major capsid protein (MCP) by small hairpin RNAs (shRNAs) has been shown to cause resistance to viral infection in vitro, RNA interference (RNAi) to inhibit aquatic animal virus envelope protein gene product has not been reported.

Methodology

We devised artificial microRNAs (amiRNAs) that target a viral envelope protein gene RGV 53R. By incorporating sequences encoding amiRNAs specific to 53R of RGV into pre-miRNA155 (pSM155) vectors, which use the backbone of natural miR-155 sequence and could intracellularly express 53R-targeted pre-amiRNAs. The pre-amiRNAs could be processed by the RNase III-like enzyme Dicer into 21–25 nt amiRNAs (amiR-53Rs) in fish cell lines. The levels of 53R expression were analyzed through real-time PCR and RGV virions assembly were observed by electronic microscopy in fish cells transfected with or without amiR-53Rs at 72 h of RGV infection.

Conclusion/Significance

The results argue that viral envelope protein RGV 53R can be silenced and the virions assembly was deficient by amiR-53R-1, and further identified the first amiRNA of envelope protein gene from iridovirus that was able to cause resistance to virus infection in fish cells. The data demonstrate that the viral infection is efficiently suppressed (58%) by amiR-53R-1 targeting positon 36–57 of RGV 53R. Moreover, electron microscopic observations revealed virion assembly defect or reduced virions assembly capacity was closely correlated to expression of amiR-53R-1. Based on real time PCR of the Mx gene, we found no evidence of activation of IFN by amiR-53R-1.

Identification and characterization of a novel capsid protein encoded by Singapore grouper iridovirus ORF038L

Singapore grouper iridovirus (SGIV) is an important pathogen isolated from grouper, Epinephelus tauvina, and characterized as a novel ranavirus. To better understand the function of viral structural genes involved in SGIV infection and virus-host interactions, a candidate gene, VP38 (ORF038L), was investigated in this study. SGIV VP38 was found to encode a 170-aa peptide containing an RGD motif, and it showed significant identity only to members of the genus Iridovirus, family Iridoviridae, except megalocytivirus. The VP38 gene was identified by temporal expression pattern analysis and drug inhibition assay as a late (L) gene. Immunofluorescence localization revealed that P38 was distributed predominately in the cytoplasm and that association of VP38 with viral factories increased at the late stage of SGIV infection. Consistent results from immunoelectron microscopy (IEM) and western blot analysis revealed that SGIV VP38 is a viral capsid protein. Furthermore, antibodies specific for SGIV VP38 exhibited substantial SGIV-neutralizing activity in vitro, suggesting that VP38 might play an important role in SGIV infectivity.