A 20-amino-acid Cardiovirus protein exhibits cytokine-mimicry activity to regulate viral replication.

Viral protein genome-linked (VPg) is a small protein encoded by several non-enveloped positive-sense RNA viruses, including Cardiovirus. Although VPg is widely recognized as essential for viral RNA synthesis, its additional roles remain largely unexplored. Here, we report an alternative function of Cardiovirus VPg as an interferon-γ receptor agonist. During encephalomyocarditis virus (EMCV) infection, a member of the Cardiovirus genus, we uncovered that VPg from EMCV (VPgEMCV) is released extracellularly from host cells. VPgEMCV is then sensed by uninfected host cells, activating the interferon-γ signaling cascade and ultimately producing nitric oxide (NO) through inducible NO synthase induction. NO accumulation is crucial for triggering potent cell death in cooperation with tumor necrosis factor α (TNF-α) signaling, with TNF-α also being induced as an antiviral host response during EMCV infection. We demonstrate that VPgEMCV, comprising merely 20 amino acids, can exhibit cytokine-mimicry activity, with the synergistic interplay between VPgEMCV and TNF-α signaling regulating viral replication and disease pathogenesis.