The CUL4-DDB1 ubiquitin ligase machinery regulates diverse cellular functions and is frequently subverted by pathogenic viruses. Here we report the crystal structure of DDB1 in complex with a central fragment of hepatitis B virus X protein (HBx), whose DDB1-binding activity is essential for viral infection. The structure reveals that HBx binds DDB1 through an α-helical motif, which is also found in the unrelated paramyxovirus SV5-V protein despite their sequence divergence. Our structure-based functional analysis shows that, like SV5-V, HBx captures DDB1 to exploit the ubiquitin ligase activity of the CUL4-DDB1 E3. Based on the shared action mechanisms of the two viral proteins, we further identify the same α-helical motif in the substrate-recruiting subunits of the cellular E3 complex, DCAFs, which are functionally mimicked by the viral hijackers. Together, our studies reveal a common yet promiscuous structural element important for the assembly of viral and cellular substrate receptors into the core complex of the CUL4-DDB1 ubiquitin ligase.
Chronic hepatitis B virus (HBV) infection is a leading cause of hepatocellular carcinoma (HCC), but its role in the transformation process remains unclear. HBV encodes a small protein, known as HBx, which is required for infection and has been implicated in hepatocarcinogenesis. Here we show that HBx induces lagging chromosomes during mitosis, which in turn leads to formation of aberrant mitotic spindles and multinucleated cells. These effects require the binding of HBx to UV-damaged DNA binding protein 1 (DDB1), a protein involved in DNA repair and cell cycle regulation, and are unexpectedly attributable to HBx interfering with S-phase progression and not directly with mitotic events. HBx also affects S-phase and induces lagging chromosomes when expressed from its natural viral context and, consequently, exhibits deleterious activities in dividing, but not quiescent, hepatoma cells. Conclusion: In addition to its reported role in promoting HBV replication, the binding of HBx to DDB1 may induce genetic instability in regenerating hepatocytes and thereby contribute to HCC development, thus making this HBV-host protein interaction an attractive target for new therapeutic intervention. (HEPATOLOGY 2008;48:1467-1476.)H epatocellular carcinoma (HCC) is the fifth most frequent cancer in humans, accounting for nearly 1 million deaths annually, and is mainly the consequence of chronic hepatitis B virus (HBV) infection. 1 HBV encodes a small regulatory protein, termed HBx, which is essential for virus replication in vivo and is expressed during chronic infection. 2 HBx has also been implicated in hepatocarcinogenesis. HBx is conserved among all mammalian hepatitis viruses that cause liver cancer in their hosts, whereas no counterpart exists in the non-oncogenic avian hepatitis viruses. Evidence derived from tumor sample analysis, cell culture, and transgenic animal studies collectively supports a role for HBx in HCC development. 3,4 However, the mechanism by which HBx may contribute to hepatocyte transformation remains obscure.In cell culture, HBx exhibits many activities, including an ability to stimulate HBV replication and to interfere with cell cycle progression, and it is believed to do so through interaction with cellular proteins. 2 Among these is UV-damaged DNA binding protein 1 (DDB1), a highly conserved 127-kDa protein that is also targeted by other viral regulatory proteins (see Discussion) and that functions as a subunit of an E3 ubiquitin ligase complex, in which it serves an adapter function to select specific targets for ubiquitin-dependent proteolysis. 5 Previous work demonstrated that HBx stimulates HBV genome replication by binding to DDB1 in the nuclear compartment of cells. 6 A nuclear interaction with DDB1 is also required for HBx to interfere with cell viability. 7 A similar DDB1-binding dependent cytotoxic activity has been re-
Chronic hepatitis B virus (HBV) infection is a major risk factor for liver cancer development. HBV encodes the hepatitis B virus X (HBx) protein that promotes transcription of the viral episomal DNA genome by the host cell RNA polymerase II. Here we provide evidence that HBx accomplishes this task by a conserved and unusual mechanism. Thus, HBx strongly stimulates expression of transiently transfected reporter constructs, regardless of the enhancer and promoter sequences. This activity invariably requires HBx binding to the cellular UV‐damaged DDB1 E3 ubiquitin ligase, suggesting a common mechanism. Unexpectedly, none of the reporters tested is stimulated by HBx when integrated into the chromosome, despite remaining responsive to their cognate activators. Likewise, HBx promotes gene expression from the natural HBV episomal template but not from a chromosomally integrated HBV construct. The same was observed with the HBx protein of woodchuck HBV. HBx does not affect nuclear plasmid copy number and functions independently of CpG dinucleotide methylation. Conclusion: We propose that HBx supports HBV gene expression by a conserved mechanism that acts specifically on episomal DNA templates independently of the nature of the cis‐regulatory sequences. Because of its uncommon property and key role in viral transcription, HBx represents an attractive target for new antiviral therapies. (HEPATOLOGY 2012;56:2116–2124)
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