Nucleolar Targeting by Oropouche Virus Nonstructural Protein Mediates RNA Polymerase II Degradation and Interferon...

The Oropouche virus (OROV) is an emerging orthobunyavirus responsible for significant outbreaks in South America. Its nonstructural protein, NSs, serves as a principal virulence factor by modulating host antiviral responses.

We investigated whether OROV NSs suppresses type I interferon (IFN-I) signaling downstream of IFNAR activation and examined its effects on host transcription. The function of NSs was analyzed by assessing RNA polymerase II (RNAPII) levels, phosphorylation, transcriptional activity, nucleolar localization, the effects of proteasome inhibition, and the characterization of NoLS-like basic residue motifs.

OROV NSs was found to attenuate IFN-I signaling in human cells without affecting the abundance, phosphorylation, or nuclear localization of STAT1/2. NSs significantly reduced RNAPII protein levels and RNAPII-driven transcription, with this reduction being reversed by proteasome inhibition, indicating proteasome-dependent degradation. NSs partially localized to the nucleolus and disrupted the nucleolar protein fibrillarin. Mutations in two basic residue clusters abolished nucleolar disruption, restored RNAPII abundance, and rescued transcriptional activity.

OROV NSs suppresses host transcription through nucleolar targeting and RNAPII degradation, thereby impairing IFN-I signaling downstream of IFNAR. These findings provide mechanistic insights into OROV immune evasion and underscore NSs-mediated transcriptional repression as a critical determinant of viral pathogenesis.