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

Abstract

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 X protein recruits methyltransferases to affect cotranscriptional N6-methyladenosine modification of viral/host RNAs

“…How m 6 A methylation affects RT priming activity remains to be characterized. The role of HBx protein in affecting m 6 A methylation occurring by recruiting the METTL3/14 complex onto the HBV episomal minichromosome supports the view of its function in regulating the viral life cycle in addition to its transactivating function and those affecting Smc5/6 complex and HBx-DDB-mediated degradation activities (29,30). Of relevance in this context are previous major findings that HBx directly binds to transcription factors and was found associated with the viral cccDNA, respectively; these form the basis of the various ways HBx may achieve these transactivating functions of gene expression (4, 7, 9, 10).…”

Innate immunity in hepatitis B and D virus infection: consequences for viral persistence, inflammation, and T cell recognition

“…The smallest subgenomic RNA produces the regulatory X protein (HBx), which was reported to interfere with several cellular pathways and transcription factors and to be recruited onto the cccDNA [32,34]. Moreover, a major function of HBx that has emerged in the last years regards its ability to hinder the host's attempts to silence cccDNA transcription [35][36][37]. By binding to the damaged DNA binding protein 1 (DDB1) [38], HBx triggers the degradation of the structural maintenance of chromosomes 5/6 complex (SMC5/6), a multi-functional DNA-binding complex involved in chromosome dynamics and stability [39].…”

Downregulation of HBx Restrains Proliferation, Migration, and Invasion of HepG2 Cells

“…However, the binding of HBx with other proteins has been reported before. For instance, HBx could bind to cellular damaged DNA binding protein 1 which is an adaptor protein for the cullin 4A Really Interesting New Gene E3 ubiquitin ligase complex [21]. Moreover, Lee et al reported that suppression of SELENBP1 by HBx might serve as one of the causes in the development of hepatocellular carcinoma caused by HBV infection [22].…”