An Assembly-Activating Site in the Hepatitis B Virus Capsid Protein Can Also Trigger Disassembly

The Hepatitis B Virus (HBV) core protein homodimers self-assemble to form an icosahedral capsid that packages the viral genome. Disassembly occurs in the nuclear basket to release the mature genome to the nucleus. Small molecules have been developed that bind to a pocket at the interdimer interface to accelerate assembly and strengthen interactions between subunits; these are under development as antiviral agents. Here, we explore the role of the dimer–dimer interface by mutating sites in the drug-binding pocket to cysteine and examining the effect of covalently linking small molecules to them. We find that ligands bound to the pocket may trigger capsid disassembly in a dose-dependent manner. This result indicates that, at least transiently, the pocket adopts a destabilizing conformation. We speculate that this pocket also plays a role in virus disassembly and genome release by binding ligands that are incompatible with virus stability, “unwanted guests.” Investigating protein–protein interactions, especially large protein polymers, offers new and unique challenges. By using an engineered addressable thiol, we provide a means to examine the effects of modifying an interface without requiring drug-like properties for the ligand.