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Role of Hepatitis B Virus Capsid Phosphorylation in Nucleocapsid Disassembly and Covalently Closed Circular DNA Formation- [url=]Jun Luo,[/url]
- [url=]Ji Xi,[/url]
- [url=]Lu Gao,[/url]
- [url=]Jianming Hu
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Role of Hepatitis B Virus Capsid Phosphorylation in Nucleocapsid Disassembly and Covalently Closed Circular DNA Formation
This is an uncorrected proof.
Abstract
Hepatitis B virus (HBV) delivers a partially double-stranded, relaxed circular (RC) DNA genome in complete virions to the host cell nucleus for conversion to the covalently closed circular (CCC) DNA, which establishes and sustains viral infection. An overlength pregenomic RNA (pgRNA) is then transcribed from CCC DNA and packaged into immature nucleocapsids (NCs) by the viral core (HBc) protein. pgRNA is reverse transcribed to produce RC DNA in mature NCs, which are then enveloped and secreted as complete virions, or delivered to the nucleus to replenish the nuclear CCC DNA pool. RC DNA, whether originating from extracellular virions or intracellular mature NCs, must be released upon NC disassembly (uncoating) for CCC DNA formation. HBc is known to undergo dynamic phosphorylation and dephosphorylation at its C-terminal domain (CTD) to facilitate pgRNA packaging and reverse transcription. Here, two putative phosphorylation sites in the HBc N-terminal domain (NTD), S44 and S49, were targeted for genetic and biochemical analysis to assess their potential roles in viral replication. The NTD mutant that mimics the non-phosphorylated state (N2A) was competent in all steps of viral replication tested from capsid assembly, pgRNA packaging, reverse transcription, to virion secretion, except for a decrease in CCC DNA formation. On the other hand, the phosphor-mimetic mutant N2E showed a defect in the early step of pgRNA packaging but enhanced the late step of mature NC uncoating and consequently, increased CCC DNA formation. N2E also enhanced phosphorylation in CTD and possibly elsewhere in HBc. Furthermore, inhibition of the cyclin-dependent kinase 2 (CDK2), which is packaged into viral capsids, could block CCC DNA formation. These results prompted us to propose a model whereby rephosphorylation of HBc at both NTD and CTD by the packaged CDK2, following CTD dephosphorylation during NC maturation, facilitates uncoating and CCC DNA formation by destabilizing mature NCs.
Author summary
Hepatitis B virus (HBV) persistently infects hundreds of millions of people worldwide, causing viral hepatitis, cirrhosis and liver cancer. The basis of HBV persistence is the viral covalently closed circular (CCC) DNA, a nuclear episome, that drives all viral gene expression to sustain viral replication. CCC DNA is derived from the relaxed circular (RC) DNA, which is formed inside a proteinaceous shell, the viral capsid, but has to be released from the capsid in order to be converted to CCC DNA by host cell factors. We report here that the phosphorylation state of the capsid protein, regulated by host cell enzymes including one that is packaged inside the viral capsid, plays a critical role in regulating the release of RC DNA and thus controlling CCC DNA formation. Intense ongoing efforts are being directed at developing novel antiviral strategies to eliminate the HBV CCC DNA for curing persistence HBV infection, including those targeting the capsid protein. Our results should inspire novel approaches to targeting the HBV capsid and CCC DNA. Furthermore, uncoating is an essential step in the infection process for virtually all viruses that remains ill-understood. Thus, our results have broad implications for understanding viral infection in general.
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发表于 2020-4-3 19:42