Identification of two novel membrane proteins from the Tiger frog virus (TFV)

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.

Aspects of Biological Replication and Evolution Independent of the Central Dogma: Insights from Protein-Free Vesicular Transformations and Protein-Mediated Membrane Remodeling

Biological membrane remodeling is central to living systems. In spite of serving as “containers” of whole-living systems and functioning as dynamic compartments within living systems, biological membranes still find a “blue collar” treatment compared to the “white collar” nucleic acids and proteins in biology. This may be attributable to the fact that scientific literature on biological membrane remodeling is only 50 years old compared to ~ 150 years of literature on proteins and a little less than 100 years on nucleic acids. However, recently, evidence for symbiotic origins of eukaryotic cells from data only on biological membranes was reported. This, coupled with appreciation of reproducible amphiphilic self-assemblies in aqueous environments (mimicking replication), has already initiated discussions on origins of life beyond nucleic acids and proteins. This work presents a comprehensive compilation and meta-analyses of data on self-assembly and vesicular transformations in biological membranes—starting from model membranes to establishment of Influenza Hemagglutinin-mediated membrane fusion as a prototypical remodeling system to a thorough comparison between enveloped mammalian viruses and cellular vesicles. We show that viral membrane fusion proteins, in addition to obeying “stoichiometry-driven protein folding”, have tighter compositional constraints on their amino acid occurrences than general-structured proteins, regardless of type/class. From the perspective of vesicular assemblies and biological membrane remodeling (with and without proteins) we find that cellular vesicles are quite different from viruses. Finally, we propose that in addition to pre-existing thermodynamic frameworks, kinetic considerations in de novo formation of metastable membrane structures with available “third-party” constituents (including proteins) were not only crucial for origins of life but also continue to offer morphological replication and/or functional mechanisms in modern life forms, independent of the central dogma.

Tiger frog virus ORF104R interacts with cellular VDAC2 to inhibit cell apoptosis

Ranaviruses belong to the family Iridoviridae, and have become a serious threat to both farmed and natural populations of fish and amphibians. Previous reports showed that ranaviruses could encode viral Bcl-2 family-like proteins (vBcl-2), which play a critical role in the regulation of cell apoptosis. However, the mechanism of ranaviruses vBcl-2 interactions with host protein in mediating apoptosis remains unknown. Tiger frog virus (TFV) belonging to the genus Ranavirus has been isolated from infected tadpoles of Rana tigrina rugulosa, and it causes a high mortality rate among tiger frog tadpoles cultured in southern China. This study elucidated the molecular mechanism underlying the interaction of TFV ORF104R with the VDAC2 protein to regulate cell apoptosis. TFV ORF104R is highly similar to other ranaviruses vBcl-2 and host Mcl-1 proteins, indicating that TFV ORF104R is a postulate vBcl-2 protein. Transcription and protein expression levels showed that TFV orf104r was a late viral gene. Western blot results suggested that TFV ORF104R was a viral structural protein. Subcellular localization analysis indicated that TFV ORF104R was predominantly colocalized with the mitochondria. Overexpressed TFV ORF104R could suppress the release of cytochrome C and the activities of caspase-9 and caspase-3. These results indicated that TFV ORF104R might play an important role in anti-apoptosis. Furthermore, the interaction between TFV ORF104R and VDAC2 was detected by co-immunoprecipitation in vitro. The above observations suggest that the molecular mechanism of TFV-regulated anti-apoptosis is through the interaction of TFV ORF104R with the VDAC2 protein. Our study provided a mechanistic basis for the ranaviruses vBcl-2-mediated inhibition of apoptosis and improved the understanding on how TFV subverts host defense mechanisms in vivo.

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.

Identification and differential expression analysis of MicroRNAs encoded by Tiger Frog Virus in cross-species infection in vitro

Background

Tiger frog virus (TFV), dsDNA virus of the genus Ranavirus and family Iridoviridae, causes a high mortality of tiger frog tadpoles cultured in Southern China. MicroRNAs (miRNAs) have been identified in many viruses especially DNA viruses such as Singapore Grouper Iridoviruses (SGIV). MicroRNAs play important roles in regulating gene expression for virus subsistence in host. Considering that TFV infects cells of different species under laboratory conditions, we aim to identify the specific and essential miRNAs expressed in ZF4 and HepG2 cells.

Methods

We identified and predicted novel viral miRNAs in TFV-infected ZF4 and HepG2 cells by deep sequencing and software prediction. Then, we verified and described the expression patterns of TFV-encoded miRNAs by using qRT-PCR and Northern blot.

Results

Deep sequencing predicted 24 novel TFV-encoded miRNAs, and qRT-PCR verified 19 and 23 miRNAs in TFV-infected ZF4 (Group Z) and HepG2 (Group H) cells, respectively. Northern blot was performed to validate eight and five TFV-encoded miRNAs in Groups H and Z, respectively. We compared the expression of TFV-encoded miRNAs from two groups and defined TFV-miR-11 as the essential viral miRNA and TFV-miR-13 and TFV-miR-14 as the specific miRNAs that contribute to HepG2 cell infection.

Conclusions

We identified novel viral miRNAs and compared their expression in two host cells. The results of this study provide novel insights into the role of viral miRNAs in cross-species infection in vitro.

Electronic supplementary material

The online version of this article (doi:10.1186/s12985-016-0530-6) contains supplementary material, which is available to authorized users.

Tiger frog virus ORF080L protein interacts with LITAF and impairs EGF-induced EGFR degradation

Tiger frog virus (TFV) belongs to the genus Ranavirus, family Iridoviridae, and causes severe mortality in commercial cultures in China. TFV ORF080L is a gene homolog of lipopolysaccharide-induced TNF-α factor (LITAF), which is a regulator in endosome-to-lysosome trafficking through its function in the endosomal sorting complex required for transport machinery. The characteristics and biological roles of TFV ORF080L were identified. TFV ORF080L was predicted to encode an 84-amino acid peptide (VP080L). It had high-sequence identity with mammalian LITAF, but lacked the N-terminus of LITAF, which contains two PPXY motifs. Transcription and protein level analyses showed that TFV ORF080L was a late viral gene. Localization in the virons also showed that TFV VP080L was a viral structural protein. Immunofluorescence staining showed that TFV ORF080L was predominantly colocalized with plasma membrane and partly distributed with the late endosome in infected HepG2 cells. SiRNA-mediated TFV ORF080L silencing decreased viral reproduction. Moreover, TFV ORF080L interacted with human/zebrafish LITAF and impaired EGF-induced EGFR degradation, thereby indicating that TFV ORF080L played a role in endosome-to-lysosome trafficking. These findings suggested that TFV ORF080L might negate the function of cellular LITAF to impair endosomal sorting and trafficking. Results provide a clue to the link between the dysregulated endosomal trafficking and iridovirus pathogenesis.

Caveolae Restrict Tiger Frog Virus Release in HepG2 cells and Caveolae-Associated Proteins Incorporated into Virus Particles

Caveolae are flask-shaped invaginations of the plasma membrane. Caveolae play important roles in the process of viruses entry into host cells, but the roles of caveolae at the late stage of virus infection were not completely understood. Tiger frog virus (TFV) has been isolated from the diseased tadpoles of the frog, Rana tigrina rugulosa, and causes high mortality of tiger frog tadpoles cultured in Southern China. In the present study, the roles of caveolae at the late stage of TFV infection were investigated. We showed that TFV virions were localized with the caveolae at the late stage of infection in HepG2 cells. Disruption of caveolae by methyl-β-cyclodextrin/nystatin or knockdown of caveolin-1 significantly increase the release of TFV. Moreover, the interaction between caveolin-1 and TFV major capsid protein was detected by co-immunoprecipitation. Those results suggested that caveolae restricted TFV release from the HepG2 cells. Caveolae-associated proteins (caveolin-1, caveolin-2, cavin-1, and cavin-2) were selectively incorporated into TFV virions. Different combinations of proteolytic and/or detergent treatments with virions showed that caveolae-associated proteins were located in viral capsid of TFV virons. Taken together, caveolae might be a restriction factor that affects virus release and caveolae-associated proteins were incorporated in TFV virions.

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.

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.

Entry of tiger frog virus (an Iridovirus) into HepG2 cells via a pH-dependent, atypical, caveola-mediated endocytosis pathway

Tiger frog virus (TFV), in the genus Ranavirus of the family Iridoviridae, causes high mortality of cultured tiger frog tadpoles in China. To explore the cellular entry mechanism of TFV, HepG2 cells were treated with drugs that inhibit the main endocytic pathways. We observed that TFV entry was inhibited by NH(4)Cl, chloroquine, and bafilomycin, which can all elevate the pH of acidic organelles. In contrast, TFV entry was not influenced by chlorpromazine or overexpression of a dominant-negative form of Esp15, which inhibit the assembly of clathrin-coated pits. These results suggested that TFV entry was not associated with clathrin-mediated endocytosis, but was related to the pH of acidic organelles. Subsequently, we found that endocytosis of TFV was dependent on membrane cholesterol and was inhibited by the caveolin-1 scaffolding domain peptide. Dynamin and actin were also required for TFV entry. In addition, TFV virions colocalized with the cholera toxin subunit B, indicating that TFV enters as caveola-internalized cargo into the Golgi complex. Taken together, our results demonstrated that TFV entry occurs by caveola-mediated endocytosis with a pH-dependent step. This atypical caveola-mediated endocytosis is different from the clathrin-mediated endocytosis of frog virus 3 (FV3) by BHK cells, which has been recognized as a model for iridoviruses. Thus, our work may help further the understanding of the initial steps of iridovirus infection in lower vertebrates.