Promiscuous Metal Site in Hepatitis B Virus X Protein Binds an Fe-S Cluster.

The Hepatitis B virus (HBV) regulatory protein HBx is essential for viral replication and pathogenesis, yet its cofactor specificity and ligand environment remain poorly defined. Although HBx binds either an Fe-S cluster or Zn, its intrinsic disorder and mutational tolerance have hindered its precise characterization. Here, we integrate chemoproteomics with HYSCORE spectroscopy to identify the metal-coordinating ligands in HBx. Histidine coordination is excluded, while C61, C69, C143, and C148 emerge as primary cysteine ligands for the Fe-S cluster, with C137 acting as a conditional ligand. These residues also bind Zn and are associated with HBx transactivation and clinically relevant variants. HBx engages the host cytosolic Fe-S machinery and displays sensitivity to Fe-S-targeting reagents, behavior consistent with Fe-S cluster acquisition and lability. Together, these findings suggest that HBx functionally behaves as an Fe-S cluster-associated protein, highlighting a potentially druggable vulnerability in HBV replication.