LAUSR

epatitis B virus (HBV) infection leads to chronic Hhepatitis B (CHB) in approximately 250 million people worldwide, putting them at high risk for developing cirrhosis and hepatocellular carcinoma. HBV is a partially double-stranded DNA virus that belongs to the Hepadnaviridae family. After entry into host cells, the viral genome is transported into the nucleus and converted to a covalently closed circular DNA (cccDNA), which serves as the template for all HBV viral RNAs. Currently available HBV antiviral drugs inhibit the reverse transcription of HBV pregenomic RNA but fail to suppress the established cccDNA reservoir in infected hepatocytes, resulting in viral rebound after therapy. In addition, HBV surface antigen hepatitis B surface antigen expressed from the cccDNA maintains immune tolerance to prevent induction of antibodies to hepatitis B surface antigen, which are critical for a functional cure of CHB in patients. The HBV regulatory protein hepatitis B virus X protein (HBx) is critical for HBV gene expression from the episomal cccDNA template via its interaction with cellular proteins. The best-characterized HBx binding partner is the damage-specific DNA binding protein 1 (DDB1). This binding is essential for HBx-enhanced HBV replication. The DNA repair factor DDB1 functions as a linker protein for the assembly of a number of Cullin 4-ROC1/RING E3 ubiquitin ligase. DDB1 bridges CUL4 to individual DDB1binding WD40 proteins (or DDB Cullin-associated factors), which in turn recruit substrates to the CUL4–ROC1 catalytic core for subsequent ubiquitination and degradation. A structural study has shown that HBx contains an H-box that is shared by several DDB1-binding WD40 proteins and directly binds to DDB1, suggesting that HBx functions as a DDB Cullin-associated factor to retarget the DDB1–Cullin 4-ROC1/RING E3 ubiquitin ligase to a new host factor. Two recent reports have shown that the HBx-DDB1-CUL4-ROC1 E3 ligase complex binds and degrades the structural maintenance of chromosomes (SMC) complex proteins SMC5/6 to enhance HBV replication. It is clear that disruption of the HBx/DDB1 interaction provides a strategy to develop novel therapeutics that inhibit HBV cccDNA activity. However, multiple efforts have failed to identify viable small molecules that can efficiently disrupt the interaction in vivo. Sekiba et al developed an elegant high-throughput screening assay to efficiently identify inhibitors of the HBx/DDB1 interaction. A novel split luciferase assay based on HBx–DDB1 interaction was used for screening compounds. The investigators