Functional analysis of the short isoform of orf virus protein OV20.0.

The OV20.0 protein of orf virus (ORFV) has two isoforms and contributes to virulence, but the roles of the two forms are not known. This study shows that the shorter isoform (sh20) arises due to use of a downstream initiation codon and is amino-terminally truncated. The sh20 form also differs in expression kinetics and cellular localization from full-length OV20.0. Similar to the full-length isoform, sh20 is able to bind dsRNA and PKR, inactivate PKR, and thus act as an antagonist of the interferon response in vitro. In vivo, however, wild-type OV20.0 could not be replaced with sh20 alone without a loss of virulence, suggesting that the functions of the isoforms are not simply redundant.