Abstract
Severe fever with thrombocytopenia syndrome virus YG1 strain was isolated from the first clinical case in Japan. We found that only some of the cells underwent low-pH-dependent cell fusion, though all of the cells were confirmed to have been infected with viruses, suggesting that YG1 strain consists of heterogeneous mixtures of related viruses. Here, we established three subclones, E3, A4 and B7, from YG1 strain by the limiting dilution method with pH-dependent cell fusion activities. Subclone E3 showed weak fusion activity and cytopathic effect (CPE) in Vero E6 cells. It has an amino acid sequence identical to the published sequence registered as YG1 strain, and it is thought to comprise a population of YG1 strain. Subclone A4 showed strong fusion activity under an acidic condition. In contrast, subclone B7 showed strong fusion activity and CPE under neutral and acidic conditions. Two amino acid differences common to B7 and A4 were found in the envelope glycoproteins. In addition, an amino acid variant of RNA-dependent RNA polymerase was found only in B7. These subclones will be valuable tools to elucidate cell fusion mechanisms of SFTSV and relations between viral proteins and their functions.
