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A 6K-deletion variant of salmonid alphavirus is non-viable but can be rescued through RNA recombination.

ABSTRACT: Pancreas disease (PD) of Atlantic salmon is an emerging disease caused by Salmonid alphavirus (SAV) which mainly affects salmonid aquaculture in Western Europe. Although genome structure of SAV has been characterized and each individual viral protein has been identified, the role of 6K protein in viral replication and infectivity remains undefined. The 6K protein of alphaviruses is a small and hydrophobic protein which is involved in membrane permeabilization, protein processing and virus budding. Because these common features are shared across many viral species, they have been named viroporins. In the present study, we applied reverse genetics to generate SAV3 6K-deleted (?6K) variant and investigate the role of 6K protein. Our findings show that the 6K-deletion variant of salmonid alphavirus is non-viable. Despite viral proteins of ?6K variant are detected in the cytoplasm by immunostaining, they are not found on the cell surface. Further, analysis of viral proteins produced in ?6K cDNA clone transfected cells using radioimmunoprecipitation (RIPA) and western blot showed a protein band of larger size than E2 of wild-type SAV3. When ?6K cDNA was co-transfected with SAV3 helper cDNA encoding the whole structural genes including 6K, the infectivity was rescued. The development of CPE after co-transfection and resolved genome sequence of rescued virus confirmed full-length viral genome being generated through RNA recombination. The discovery of the important role of the 6K protein in virus production provides a new possibility for the development of antiviral intervention which is highly needed to control SAV infection in salmonids.

SUBMITTER: Guo TC

PROVIDER: S-EPMC4091863 | biostudies-literature | 2014

REPOSITORIES: biostudies-literature

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A 6K-deletion variant of salmonid alphavirus is non-viable but can be rescued through RNA recombination.

Guo Tz-Chun TC Johansson Daniel X DX Haugland Øyvind Ø Liljeström Peter P Evensen Øystein Ø

PloS one 20140710 7

Pancreas disease (PD) of Atlantic salmon is an emerging disease caused by Salmonid alphavirus (SAV) which mainly affects salmonid aquaculture in Western Europe. Although genome structure of SAV has been characterized and each individual viral protein has been identified, the role of 6K protein in viral replication and infectivity remains undefined. The 6K protein of alphaviruses is a small and hydrophobic protein which is involved in membrane permeabilization, protein processing and virus buddin ...[more]

PMID: 25009976

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Project description:The genus Alphavirus includes several potentially lethal human viruses. Additionally, species such as Sindbis virus and Semliki Forest virus are important vectors for gene therapy, vaccination and cancer research, and important models for virion assembly and structural analyses. The genome encodes nine known proteins, including the small '6K' protein. 6K appears to be involved in envelope protein processing, membrane permeabilization, virion assembly and virus budding. In protein gels, 6K migrates as a doublet--a result that, to date, has been attributed to differing degrees of acylation. Nonetheless, despite many years of research, its role is still relatively poorly understood.We report that ribosomal -1 frameshifting, with an estimated efficiency of approximately 10-18%, occurs at a conserved UUUUUUA motif within the sequence encoding 6K, resulting in the synthesis of an additional protein, termed TF (TransFrame protein; approximately 8 kDa), in which the C-terminal amino acids are encoded by the -1 frame. The presence of TF in the Semliki Forest virion was confirmed by mass spectrometry. The expression patterns of TF and 6K were studied by pulse-chase labelling, immunoprecipitation and immunofluorescence, using both wild-type virus and a TF knockout mutant. We show that it is predominantly TF that is incorporated into the virion, not 6K as previously believed. Investigation of the 3' stimulatory signals responsible for efficient frameshifting at the UUUUUUA motif revealed a remarkable diversity of signals between different alphavirus species.Our results provide a surprising new explanation for the 6K doublet, demand a fundamental reinterpretation of existing data on the alphavirus 6K protein, and open the way for future progress in the further characterization of the 6K and TF proteins. The results have implications for alphavirus biology, virion structure, viroporins, ribosomal frameshifting, and bioinformatic identification of novel frameshift-expressed genes, both in viruses and in cellular organisms.

Project description:BackgroundPancreas disease (PD) is a viral fish disease which in recent years has significantly affected Norwegian salmonid aquaculture. In Norway, the aetiological agent salmonid alphavirus (SAV) has been found to be represented by the subtype 3 only. SAV subtype 3 has in previous analyses been found to show a lower genetic divergence than the subtypes found to cause PD in Ireland and Scotland. The aim of this study was to evaluate the nucleotide (nt) and amino acid divergence and the phylogenetic relationship of 33 recent SAV subtype 3 sequences. The samples from which the sequences were obtained originated from both PD endemic and non-endemic regions in an attempt to investigate agent origin/spread. Multiple samples throughout the seawater production phase from several salmonid populations were included to investigate genetic variation during an outbreak. The analyses were mainly based on partial sequences from the E2 gene. For some samples, additional partial 6 K and nsP3 gene sequences were available.ResultsThe nucleotide divergence for all gene fragments ranged from total identity (0.0% divergence) to 0.45% (1103 nt fragment of E2), 1.11% (451 nt fragment of E2), 0.94% (6 K) and 0.28% (nsP3). This low nucleotide divergence corresponded well to previous reports on SAV 3 sequences; however the observed divergence for the short E2 fragment was higher than that previously reported. When compared to SAVH20/03 (AY604235), amino acid substitutions were detected in all assessed gene fragments however the in vivo significance of these on for example disease outbreak mortality could not be concluded on. The phylogenetic tree based on the 451 nt E2 fragment showed that the sequences divided into two clusters with low genetic divergence, representing only a single SAV subtype.ConclusionsThe analysed sequences represented two clusters of a single SAV subtype; however some of the observed sequence divergence was higher than that previously reported by other researchers. Larger scale, full length sequence analyses should be instigated to allow further phylogenetic and molecular epidemiology investigations of SAV subtype 3.

Project description:MicroRNAs (miRNAs) control multiple biological processes including the innate immune responses by negative post-transcriptional regulation of gene expression. As there were no studies on the role(s) of miRNAs in viral diseases in Atlantic salmon, we aimed to identify miRNAs responding to salmonid alphavirus (SAV) infection. Their expression were studied at different time points post infection with SAV isolates associated with different mortalities. Furthermore, the genome sequences of the identified miRNAs were analysed to reveal putative cis-regulatory elements, and, finally, their putative target genes were predicted.Twenty differentially expressed miRNAs (DE miRNAs) were identified. The expression of the majority of these increased post infection with maximum levels reached after the viral load were stabilized or decreasing. On the other hand, some miRNAs (e.g. the miRNA-21 family) showed decreased expression at the early time points post infection. There were significant differences in the temporal expression of individual miRNA associated with different SAV isolates. Target gene prediction in SAV responsive immune network genes showed that seventeen of the DE miRNAs could target 24 genes (e.g. IRF3, IRF7). Applying the Atlantic salmon transcriptome as input 28 more immune network genes were revealed as putative targets (e.g. IRF5, IRF4). The majority of the predicted target genes promote inflammatory response. The upstream sequences of the miRNA genes revealed a high density of cis-regulatory sequences known as binding sites for immune network transcription factors (TFs). A high expression in the late phase could therefore be due to increased transcription promoted by immune response activated TFs. Based on the in silico target predictions, we discuss their putative roles as early promotors or late inhibitors of inflammation. We propose that the differences in expressions associated with different SAV isolates could contribute to their differences in mortality rates.This study represents the first steps in exploring miRNAs important in viral-host interaction in Atlantic salmon. We identified several miRNAs responding to SAV infection. Some likely to prohibit harmful inflammation while other may promote an early immune response. Their predicted functions need to be validated and further studied in functional assays to fully understand their roles in immune homeostasis.

Dataset Information

A 6K-deletion variant of salmonid alphavirus is non-viable but can be rescued through RNA recombination.

Publications

A 6K-deletion variant of salmonid alphavirus is non-viable but can be rescued through RNA recombination.

Similar Datasets

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