Nucleolar targeting by Oropouche virus nonstructural protein mediates RNA polymerase II degradation and interferon antagonism.

Abstract

The Oropouche virus (OROV) is an emerging orthobunyavirus that has caused outbreaks in South and Central America. Its nonstructural protein (NSs) is a key virulence factor that suppresses host antiviral responses, particularly type I interferon (IFN-I) signaling. We examined whether OROV NSs can modulate IFN-I signaling downstream of IFN alpha receptor (IFNAR) activation and assessed its effect on host transcription. NSs function was evaluated by analyzing RNA polymerase II (RNAPII) abundance, phosphorylation, nucleolar localization, effects of proteasome inhibition, and characterization of basic residue motifs resembling nucleolar localization signals. OROV NSs dampens IFN-I signaling downstream of IFNAR activation and suppresses host transcription through nucleolar targeting and RNAPII degradation. NSs expression reduced total and phosphorylated RNAPII levels in human cells, leading to global transcriptional suppression, which was reversed by proteasome inhibition, indicating proteasome-mediated degradation. NSs was partially localized to the nucleolus, where it disrupted the distribution of the nucleolar protein fibrillarin. Mutations in its two basic residue motifs restored fibrillarin localization and alleviated transcriptional repression. OROV NSs suppresses host transcription via nucleolar targeting and RNAPII degradation and partially modulates IFN-I signaling downstream of IFNAR activation, representing an evolutionarily acquired immune evasion mechanism. These findings enhance our understanding of OROV pathogenesis and help to identify NSs as a potential antiviral target.