Hepatitis B Virus HBx Protein Mediates the Degradation of Host Restriction Factors through the Cullin 4 DDB1 E3 Ubiquitin Ligase Complex.

The hepatitis B virus (HBV) regulatory HBx protein is required for infection, and its binding to cellular damaged DNA binding protein 1 (DDB1) is critical for this function. DDB1 is an adaptor protein for the cullin 4A Really Interesting New Gene (RING) E3 ubiquitin ligase (CRL4) complex and functions by binding cellular DDB1 cullin associated factor (DCAF) receptor proteins that recruit substrates for ubiquitination and degradation. We compared the proteins found in the CRL4 complex immunoprecipitated from uninfected versus HBV-infected hepatocytes from human liver chimeric mice for insight into mechanisms by which HBV and the cell interact within the CRL4 complex. Consistent with its role as a viral DCAF, HBx was found in the HBV CRL4 complexes. In tissue culture transfection experiments, we showed that HBx expression led to decreased levels of known restriction factor structural maintenance of chromosomes protein 6 (SMC6) and putative restriction factors stromal interaction molecule 1 (STIM1, zinc finger E-box binding homeobox 2 (ZEB2), and proteasome activator subunit 4 (PSME4). Moreover, silencing of these proteins led to increased HBV replication in the HepG2-sodium taurocholate cotransporting polypeptide (NTCP) infection model. We also identified cellular DCAF receptors in CRL4 complexes from humanized mice. Increasing amounts of HBx did not reveal competitive DCAF binding to cullin4 (CUL4)-DDB1 in plasmid-transfected cells. Our results suggest a model in which HBx benefits virus replication by directly or indirectly degrading multiple cellular restriction factors.

Hepatitis B virus HBx protein interactions with the ubiquitin proteasome system.

The hepatitis B virus (HBV) causes acute and chronic hepatitis, and the latter is a major risk factor for the development of hepatocellular carcinoma (HCC). HBV encodes a 17-kDa regulatory protein, HBx, which is required for virus replication. Although the precise contribution(s) of HBx to virus replication is unknown, many viruses target cellular pathways to create an environment favorable for virus replication. The ubiquitin proteasome system (UPS) is a major conserved cellular pathway that controls several critical processes in the cell by regulating the levels of proteins involved in cell cycle, DNA repair, innate immunity, and other processes. We summarize here the interactions of HBx with components of the UPS, including the CUL4 adaptor DDB1, the cullin regulatory complex CSN, and the 26S proteasome. Understanding how these protein interactions benefit virus replication remains a challenge due to limited models in which to study HBV replication. However, studies from other viral systems that similarly target the UPS provide insight into possible strategies used by HBV.