SUMOylation and ubiquitination reciprocally regulate SMCHD1 antiviral activity against herpes simplex virus 1

Host restriction factors serve as intrinsic barriers against viral infection, and are frequently counteracted by viral antagonists. Previous studies, including our own, have identified SMCHD1 as a restriction factor that suppresses the replication of multiple viruses. Here, we reveal that the antiviral activity of SMCHD1 is dynamically regulated by two different post-translational modifications. SUMOylation of SMCHD1 promotes its association with the viral genome and enhances its antiviral activity. In contrast, during herpes simplex virus 1 (HSV-1) infection, the viral E3 ligase ICP0 induces SMCHD1 ubiquitination and proteasomal degradation, thereby relieving viral restriction. Loss of ICP0 stabilizes SMCHD1 and leads to marked accumulation of SUMOylated SMCHD1, rendering ICP0-deficient HSV-1 more sensitive to SMCHD1-mediated inhibition. Together, our findings uncover a reciprocal SUMO-ubiquitin regulatory mechanism that governs SMCHD1 antiviral activity and highlight a refined virus-host arms race centered on biphasic modification of a single restriction factor.