Orf virus (ORFV) OV20.0L is an ortholog of vaccinia virus (VACV) gene E3L. The function of VACV E3 protein as a virulencefactor is well studied, but OV20.0 has received less attention. Here we show that like VACV E3L, OV20.0L encodes two proteins, afull-length protein and a shorter form (sh20). The shorter sh20 is an N-terminally truncated OV20.0 isoform generated when adownstream AUG codon is used for initiating translation. These isoforms differed in cellular localization, with full-length
OV20.0 and sh20 found throughout the cell and predominantly in the cytoplasm, respectively. onetheless, both OV20.0 isoformswere able to bind double-stranded RNA (dsRNA)-activated protein kinase (PKR) and dsRNA. Moreover, both isoformsstrongly inhibited PKR activation as shown by decreased phosphorylation of the translation initiation factor eIF2_ subunit andprotection of Sindbis virus infection against the activity of interferon (IFN). In spite of this apparent conservation of function invitro, a recombinant ORFV that was able to express only the sh20 isoform was attenuated in a mouse model.
