Hepatitis B Virus X Protein Promotes Degradation of SMC5/6 to Enhance HBV Replication

The hepatitis B virus (HBV) regulatory protein X (HBx) activates gene expression from the HBV covalently closed circular DNA (cccDNA) genome. Interaction of HBx with the DDB1-CUL4-ROC1 (CRL4) E3 ligase is critical for this function. Using substrate-trapping proteomics, we identified the structural maintenance of chromosomes (SMC) complex proteins SMC5 and SMC6 as CRL4HBx substrates. HBx expression and HBV infection degraded the SMC5/6 complex in human hepatocytes in vitro and in humanized mice in vivo. HBx targets SMC5/6 for ubiquitylation by the CRL4HBx E3 ligase and subsequent degradation by the proteasome. Using a minicircle HBV (mcHBV) reporter system with HBx-dependent activity, we demonstrate that SMC5/6 knockdown, or inhibition with a dominant-negative SMC6, enhance HBx null mcHBV-Gluc gene expression. Furthermore, SMC5/6 knockdown rescued HBx-deficient HBV replication in human hepatocytes. These results indicate that a primary function of HBx is to degrade SMC5/6, which restricts HBV replication by inhibiting HBV gene expression. [Display omitted] •HBx interacts with SMC5/6 complex proteins•HBx/HBV infection leads to SMC5/6 degradation in human hepatocytes•HBx targets SMC5/6 for ubiquitylation and degradation by the DDB1-CUL4-ROC1 E3 ligase•Knockdown of SMC5/6 rescues replication of HBx-deficient HBV virus Murphy et al. find that HBx-redirected CRL4 E3 ligase activity targets SMC5/6. SMC5/6 inhibits HBV gene expression, and SMC5/6 depletion rescues HBx-deficient HBV replication, indicating that SMC5/6 is counteracted by CRL4HBx-mediated SMC5/6 degradation.