Structural protein VP2 of African horse sickness virus is not essential for virus replication in vitro.

The Reoviridae family consists of non-enveloped multi-layered viruses with a double stranded RNA genome consisting of 9 to 12 genome segments. The Orbivirus genus of the Reoviridae family contains African horse sickness virus (AHSV), bluetongue virus and epizootic haemorrhagic disease virus causing notifiable diseases and are spread by biting Culicoides species. Here, we used reverse genetics for AHSV to study the role of outer capsid protein VP2 encoded by genome segment 2 (Seg-2). Expansion of a previously found deletion in Seg-2 indicates that structural protein VP2 of AHSV is not essential for virus replication in vitro In addition, in-frame replacement of RNA sequences in Seg-2 for that of green fluorescence protein (GFP) resulted in AHSV expressing GFP which further confirmed that VP2 is not essential for virus replication. In contrast to virus replication without VP2 expression in mammalian cells, virus replication in insect cells was strongly reduced and virus release from insect cells was completely abolished. Further, the other outer capsid protein VP5 was not co-purified with virions for virus mutants without VP2 expression. AHSV without VP5 expression however could not be recovered, indicating that outer capsid protein VP5 is essential for virus replication in vitro. Our results demonstrate for the first time that a structural viral protein is not essential for orbivirus replication in vitro, which opens new possibilities for research on other members of the Reoviridae family.
IMPORTANCE:
Members of the Reoviridae family cause major health problems worldwide ranging from lethal diarrhoea by rotavirus in humans to economic losses in livestock production by different orbiviruses. The orbivirus genus contains many virus species of which bluetongue virus, epizootic haemorrhagic disease virus, and African horse sickness virus (AHSV) are causing notifiable diseases according to the World Organisation of Animal Health. Recently, it has been shown that non-structural proteins NS3/NS3a and NS4 are not essential for virus replication in vitro, whereas it is generally assumed that structural proteins VP1-7 of these non-enveloped architectural complex virus particles are essential. Here we demonstrate for the first time that structural protein VP2 of AHSV is not essential for virus replication in vitro Our findings are very important for virologists working in the field of non-enveloped viruses, in particular reoviruses.