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cells, core mainly exists as soluble dimers, or in T = 3 or T = 4 as a template for synthesis of the (+)-strand of the
icosahedral capsids. About 95% of mature nucleocapsids DNA. This domain shares significant homology to the
[67]
isolated from Dane particles contain T = 4 capsids made RT of other retroviruses. The RT domain is the only
[12]
up of 120 core dimers, with the remaining 5% being the current anti-HBV therapeutic target, which is based on
smaller T = 3 with 90 dimers. Core is translated from the efficacy of nucleoside analogs to inhibit the human
[47]
[68]
the pgRNA and the first 149aa of core form the assembly immunodeficiency virus (HIV) RT. In fact, portions of
domain, which is sufficient for in vitro formation of capsids HBV RT can be replaced by homologous portions of HIV
that are indistinguishable from capsids isolated from RT; this can generate an active RT that can function to
Dane particles. The remaining 34-36aa makes up the produce mature HBV virions. [69]
[48]
arginine-rich C-terminal domain (CTD); phosphorylation
of various aa in the CTD regulates multiple stages of the The final domain, P, is the RNase H domain. This domain
HBV life cycle. [49-52] is responsible for degrading the pgRNA template
during synthesis of the (-)-strand of the DNA genome.
While the best-described role for core protein is Coordination of metal ion binding, which is important
assembling the nucleocapsid, the results of recent for RNase H activity, is achieved through 4 conserved
studies also suggest that the core protein does more carboxylates. Studies of the RNase H domain have also
[70]
than simply act as an inert container for the HBV genome. shown that purified recombinant RNase H domain is
In fact, core protein binds to HBV covalently closed functional in vitro and that the RNase H domain of P is
circular DNA (cccDNA), potentially to regulate spacing of important for pgRNA packaging. Further information
[71]
nucleosomes on cccDNA; cccDNA is a nuclear-localized on the HBV RT/pol, including a detailed description of the
replication intermediate of hepadnaviruses that forms RT domain active sites and binding motifs, can be found
a minichromosome (described in more detail below). [53] in the literature. [68,72]
In addition, the CTD is required for pgRNA packaging,
[54]
and core protein also plays an active role in initiating X protein
reverse transcription [55-57] and in mature nucleocapsid HBx is the only regulatory protein encoded by HBV. It is a
[58]
envelopment. The many potential roles of core protein 154aa, 17 kD protein that is encoded by the smallest HBV
in the HBV life cycle were recently reviewed, along ORF. Various studies have provided considerable evidence
with a detailed description of the mechanism of capsid that HBx plays an essential role during HBV replication.
assembly. [59] Specifically, studies have shown that HBx is bound to
cccDNA, that HBx is required for transcription from
[73]
Polymerase/reverse transcriptase cccDNA, [28,74] and that downstream HBx-mediated effects
Not long after the identification of an HBV-like virus in are required for HBV replication. Importantly, studies of
[60]
ducks, the DHBV model was used to demonstrate that other mammalian hepadnaviruses have also supported
DHBV genome replication utilizes an RNA intermediate, the role of their respective X proteins in viral replication.
implying that hepadnaviruses replicate via reverse For example, two different studies demonstrated that
[61]
transcription. While reverse transcription is a the WHV X protein is required for WHV replication in
mechanism employed by many viruses, hepadnaviruses vivo, [25,27] although another study did show a low level
approach genome replication with a number of unique of viral replication from a WHx-deficient WHV mutant
features. The 90 kD, 838aa polymerase protein of in infected woodchucks. [75] Similarly, viral replication
HBV (reverse transcriptase/RT/Pol/P) is made up of 3 was detected from tg mice expressing either wild-type
functional domains and a variable spacer region. At the or an HBx-null HBV mutant; however, when the HBx-
N-terminus is the terminal protein (TP) domain, which is null mice were crossed with HBx-tg mice, levels of HBV
important for multiple facets of the initiation of genome replication surpassed those seen in wild-type HBV-tg
[76]
replication. This region, despite its important role in mice. A similar experiment using hydrodynamic tail
P binding to the pgRNA, RNA packaging, and protein- vein injection of a plasmid encoding either the wild type
priming, [62-64] is a unique domain that is not shared by any HBV genome or an HBx-deficient mutant HBV showed a
non-hepadnavirus RTs. A variable spacer separates the significant decrease in the levels of HBV replication in
TP domain from the RT domain, and studies have shown the absence of HBx, which could be restored through
that nearly all aa within the variable spacer region can co-injection of the HBx-deficient mutant HBV and a
[26]
be mutated without altering P function. In fact, only plasmid encoding HBx. This indicates that while HBx
[65]
3 cysteine residues within the C-terminal end of the may not be absolutely required for HBV replication in
spacer region, along with a fourth in the N-terminal side these systems, it undoubtedly enhances the levels of
of the RT domain, are thought to be important for RT/pol replication. Moreover, studies of direct HBV infection
function. [66] of mice with humanized livers demonstrated that only
infection with wild type HBV, and not HBx-deficient virus,
The RT domain is responsible for genome replication by could result in HBV replication. [29,77] A similar requirement
reverse transcribing the pgRNA to form the (-)-strand of for HBx in HBV replication has been shown in human
the DNA genome and subsequent use of the (-)-strand HepG2 hepatoblastoma cells [78-82] and in primary rat
Hepatoma Research | Volume 2 | July 1, 2016 167