Phenotypic mixing between different hepadnavirus nucleocapsid proteins reveals C protein dimerization to be cis preferential

Deep mutational scanning of hepatitis B virus reveals a mechanism for cis-preferential reverse transcription

Hepatitis B virus (HBV) is a small double-stranded DNA virus that chronically infects 296 million people. Over half of its compact genome encodes proteins in two overlapping reading frames, and during evolution, multiple selective pressures can act on shared nucleotides. This study combines an RNA-based HBV cell culture system with deep mutational scanning (DMS) to uncouple cis- and trans-acting sequence requirements in the HBV genome. The results support a leaky ribosome scanning model for polymerase translation, provide a fitness map of the HBV polymerase at single-nucleotide resolution, and identify conserved prolines adjacent to the HBV polymerase termination codon that stall ribosomes. Further experiments indicated that stalled ribosomes tether the nascent polymerase to its template RNA, ensuring cis-preferential RNA packaging and reverse transcription of the HBV genome.

The hepatitis B virus polymerase

Hepatitis B virus (HBV) is a hepatotropic, partially double-stranded DNA virus that replicates by reverse transcription and is a major cause of chronic liver disease and hepatocellular carcinoma. Reverse transcription is catalyzed by the four-domain multifunctional HBV polymerase (P) protein that has protein-priming, RNA- and DNA-dependent DNA synthesis (i.e., reverse transcriptase), and ribonuclease H activities. P also likely promotes the three strand transfers that occur during reverse transcription, and it may participate in immune evasion by HBV. Reverse transcription is primed by a tyrosine residue in the amino-terminal domain of P, and P remains covalently attached to the product DNA throughout reverse transcription. The reverse transcriptase activity of P is the target for the nucleos(t)ide analog drugs that dominate HBV treatment, and P is the target of ongoing efforts to develop new drugs against both the reverse transcriptase and ribonuclease H activities. Despite the unusual reverse transcription pathway catalyzed by P and the importance of P to HBV therapy, understanding the enzymology and structure of HBV P severely lags that of the retroviral reverse transcriptases due to substantial technical challenges to studying the enzyme. Obtaining a better understanding of P will broaden our appreciation of the diversity among reverse transcribing elements in nature, and will help improve treatment for people chronically infected with HBV.

The hepatitis B virus ribonuclease H is sensitive to inhibitors of the human immunodeficiency virus ribonuclease H and integrase enzymes

Nucleos(t)ide analog therapy blocks DNA synthesis by the hepatitis B virus (HBV) reverse transcriptase and can control the infection, but treatment is life-long and has high costs and unpredictable long-term side effects. The profound suppression of HBV by the nucleos(t)ide analogs and their ability to cure some patients indicates that they can push HBV to the brink of extinction. Consequently, more patients could be cured by suppressing HBV replication further using a new drug in combination with the nucleos(t)ide analogs. The HBV ribonuclease H (RNAseH) is a logical drug target because it is the second of only two viral enzymes that are essential for viral replication, but it has not been exploited, primarily because it is very difficult to produce active enzyme. To address this difficulty, we expressed HBV genotype D and H RNAseHs in E. coli and enriched the enzymes by nickel-affinity chromatography. HBV RNAseH activity in the enriched lysates was characterized in preparation for drug screening. Twenty-one candidate HBV RNAseH inhibitors were identified using chemical structure-activity analyses based on inhibitors of the HIV RNAseH and integrase. Twelve anti-RNAseH and anti-integrase compounds inhibited the HBV RNAseH at 10 µM, the best compounds had low micromolar IC₅₀ values against the RNAseH, and one compound inhibited HBV replication in tissue culture at 10 µM. Recombinant HBV genotype D RNAseH was more sensitive to inhibition than genotype H. This study demonstrates that recombinant HBV RNAseH suitable for low-throughput antiviral drug screening has been produced. The high percentage of compounds developed against the HIV RNAseH and integrase that were active against the HBV RNAseH indicates that the extensive drug design efforts against these HIV enzymes can guide anti-HBV RNAseH drug discovery. Finally, differential inhibition of HBV genotype D and H RNAseHs indicates that viral genetic variability will be a factor during drug development.

Current therapy for HBV blocks DNA synthesis by the viral reverse transcriptase and can control the infection indefinitely, but treatment rarely cures patients. More patients could be cured by suppressing HBV replication further using a new drug in combination with the existing ones. The HBV RNAseH is a logical drug target because it is the second of only two viral enzymes that are essential for viral replication, but it has not been exploited, primarily because it is very difficult to produce active enzyme. We expressed active recombinant HBV RNAseHs and demonstrated that it was suitable for antiviral drug screening. Twenty-one candidate HBV RNAseH inhibitors were identified based on antagonists of the HIV RNAseH and integrase enzymes. Twelve of these compounds inhibited the HBV RNAseH in enzymatic assays, and one inhibited HBV replication in cell-based assays. The high percentage of compounds developed against the HIV RNAseH and integrase that were also active against the HBV RNAseH indicates that the extensive drug design efforts against these HIV enzymes can be used to guide anti-HBV RNAseH drug discovery.

Molecular biology of the hepatitis B virus for clinicians

Hepatitis B virus (HBV) infection is one of the major global health problems, especially in economically under-developed or developing countries. HBV infection can lead to a number of clinical outcomes including chronic infection, cirrhosis and liver cancer. It ranks among the top 10 causes of death, being responsible for around 1 million deaths every year. Despite the availability of a highly efficient vaccine and potent antiviral agents, HBV infection still remains a significant clinical problem, particularly in those high endemicity areas where vaccination of large populations has not been possible due to economic reasons. Although HBV is among the smallest viruses in terms of virion and genome size, it has numerous unique features that make it completely distinct from other DNA viruses. It has a partially double stranded DNA with highly complex genome organization, life cycle and natural history. Remarkably distinct from other DNA viruses, it uses an RNA intermediate called pregenomic RNA (pgRNA) and reverse transcriptase for its genome replication. Genome replication is accomplished by a complex mechanism of primer shifting facilitated by direct repeat sequences encoded in the genome. Further, the genome has evolved in such a manner that every single nucleotide of the genome is used for either coding viral proteins or used as regulatory regions or both. Moreover, it utilizes internal in-frame translation initiation codons, as well as different reading frames from the same RNA to generate different proteins with diverse functions. HBV also shows considerable genetic variability which has been related with clinical outcomes, replication potential, therapeutic response etc. This review aims at reviewing fundamental events of the viral life cycle including viral replication, transcription and translation, from the molecular standpoint, as well as, highlights the clinical relevance of genetic variability of HBV.

Human papillomavirus type 18 chimeras containing the L2/L1 capsid genes from evolutionarily diverse papillomavirus types generate infectious virus

Papillomaviruses (PVs) comprise a large family of viruses infecting nearly all vertebrate species, with more than 100 human PVs identified. Our previous studies showed that a mutant chimera HPV18/16 genome, consisting of the upper regulatory region and early ORFs of HPV18 and the late ORFs of HPV16, was capable of producing infectious virus in organotypic raft cultures. We were interested in determining whether the ability of this chimeric genome to produce infectious virus was the result of HPV18 and HPV16 being similarly oncogenic, anogenital types and whether more disparate PV types could also interact functionally. To test this we created a series of HPV18 chimeric genomes where the ORFs for the HPV18 capsid genes were replaced with the capsid genes of HPV45, HPV39, HPV33, HPV31, HPV11, HPV6b, HPV1a, CRPV, and BPV1. All chimeras were able to produce infectious chimeric viral particles, although with lower infectivity than wild-type HPV18. Steps in the viral life cycle and characteristics of the viral particles were examined to identify potential causes for the decrease in infectivity.

Assembly and export determine the intracellular distribution of hepatitis B virus core protein subunits

Little is known about the parameters and factors that determine the intracellular distribution of the hepatitis B virus core protein (HBc). In order to study HBc in living cells, HBc was tagged with green fluorescent protein (GFP). Being assembly-incompetent, the GFP-fusion protein was distributed equally throughout the cell. Mutational inactivation of known serine-phosphorylation sites within the C-terminal region led to predominantly intranuclear localization. Phosphorylation of these targets, presumably by an SR domain protein kinase, resulted in a predominantly cytoplasmic localization, which suggests active cytoplasmic export or retention. The phosphoserine itself, and not its negative charge, appears essential for the underlying mechanism. In addition, the arginine-rich, protamine-like domain surrounding these phosphorylation sites does not function as the dominant nuclear-localization signal, as had been assumed previously, because neither deleting nor altering these sequences led to a change in intracellular HBc subunit distribution. Restoring the capability of the fusion protein to form capsids by co-assembly with assembly-competent, sterically uncompromised HBc subunits provided a second assay that gave insight into the effects resulting from capsid formation. Assembly was found to be the dominant factor in the cytoplasmic retention of the GFP-HBc fusion protein. Furthermore, the stability of these empty capsids was influenced by the cell-cycle inhibitor nocodazole. Thus, the intracellular distribution of HBc is dominated by cytoplasmic assembly, which is supported by the active nuclear export of HBc subunits, and modulated during the cell cycle by the instability of capsids.

A novel method for characterizing the multi-functional C-terminal domain of the Hepadnavirus core protein

The Hepadnavirus core protein is a viral structural protein with an N-terminal self-assembling domain and a C-terminal protamine-like arginine-rich domain (ARD). The ARD contains four clusters of arginine residues involved in RNA binding, genome DNA synthesis, and nuclear localization. Characterization of the multi-functions of ARD has been impeded due to the insoluble nature of the core protein expressed in vitro. A GST (glutathione-S-transferase) and ARD fusion protein, GST-ARD, was expressed and purified in this study. Gel mobility shift assays using purified GST-ARD fusion proteins demonstrated that, similar to protamine, the ARD domain of the core protein bound to oligonucleotides without sequence preference. In vitro affinity chromatography binding assays showed further that the ARD bound to tested random plasmid DNA in a sequence-independent manner. The GST-ARD fusion protein-based approach can be employed further to study other biochemical properties of the core protein.

Hepatitis B virus morphogenesis

The hepatitis B virus (HBV) particle consists of an envelope containing three related surface proteins and probably lipid and an icosahedral nucleocapsid of approximately 30 nm diameter enclosing the viral DNA genome and DNA polymerase. The capsid is formed in the cytosol of the infected cell during packaging of an RNA pregenome replication complex by multiple copies of a 21-kDa C protein. The capsid gains the ability to bud during synthesis of the viral DNA genome by reverse transcription of the pregenome in the lumen of the particle. The three envelope proteins S, M, and L shape a complex transmembrane fold at the endoplasmic reticulum, and form disulfide-linked homo- and heterodimers. The transmembrane topology of a fraction of the large envelope protein L changes post-translationally, therefore, the N terminal domain of L (preS) finally appears on both sides of the membrane. During budding at an intracellular membrane, a short linear domain in the cytosolic preS region interacts with binding sites on the capsid surface. The virions are subsequently secreted into the blood. In addition, the surface proteins can bud in the absence of capsids and form subviral lipoprotein particles of 20 nm diameter which are also secreted.

Clathrin-mediated endocytosis and lysosomal cleavage of hepatitis B virus capsid-like core particles

The hepatitis B virus (HBV) core particle serves as a protective capsid shell for the viral genome and is highly immunogenic. Recombinant capsid-like core particles are used as effective carriers of foreign T and B cell epitopes and as delivery vehicles for oligonucleotides. The core monomer contains an arginine-rich C terminus that directs core particle attachment to cells via membrane heparan sulfate proteoglycans. Here we investigated the mechanism of recombinant core particle uptake and its intracellular fate following heparan sulfate binding. We found that the core particles are internalized in an energy-dependent manner. Core particle uptake is inhibited by chlorpromazine and by cytosol acidification known to block clathrin-mediated endocytosis but not by nystatin, which blocks lipid raft endocytosis. Particle uptake is abolished by expression of dominant negative forms of eps15 and Rab5, adaptors involved in clathrin-mediated endocytosis and early endosome transport, respectively. Endocytosed particles are transported to lysosomes where the core monomer is endoproteolytically cleaved into its distinct domains. Using protease inhibitors, cathepsin B was identified as the enzyme responsible for core monomer cleavage. Finally we found that monomer cleavage promotes particle dissociation within cells. Together, our results show that HBV capsid-like core particles are internalized through clathrin-mediated endocytosis, leading to lysosomal cleavage of the core monomer and particle dissociation.

Hepatitis B virus DNA replication is coordinated by core protein serine phosphorylation and HBx expression

The hepatitis B virus (HBV) core protein forms the capsid of viral particles and is essential for viral genome DNA replication and maturation. The C terminus of core protein contains three serines at positions 155, 162, and 170, phosphorylation of which is important for viral DNA replication. We demonstrate that the phosphorylation of these serines is stimulated by the viral HBx protein, a regulatory protein that activates signal transduction pathways and viral replication. HBx is therefore shown to stimulate HBV replication by increasing core serine phosphorylation. Mutational, biochemical, and mixing studies of C-terminal core serine mutants demonstrate that multiple serine phosphorylations occur on the same core protein. Mutation of individual core protein serines is shown to inhibit HBV replication at distinct stages corresponding to encapsidation of viral pregenomic RNA, reverse transcription, and restriction to synthesis of specific DNA replicative intermediates. We therefore demonstrate that a primary target of HBV replication that is regulated by HBx protein corresponds to increased phosphorylation of the viral core protein. We also demonstrate that core phosphorylation mediated by HBx promotes sequential progression of viral replication through the assembly of capsids primed for different stages of DNA synthesis.

Analysis of self-association of West Nile virus capsid protein and the crucial role played by Trp 69 in homodimerization

The understanding of capsid (C) protein interactions with itself would provide important data on how the core is organized in flaviviruses during assembly. In this study, West Nile (WN) virus C protein was shown to form homodimers using yeast two-hybrid analysis in conjunction with mammalian two-hybrid and in vivo co-immunoprecipitation assays. To delineate the region on the C protein which mediates C-C dimerization, truncation studies were carried out. The results obtained clearly showed that the internal hydrophobic segment flanked by helix I and helix III of WN virus C protein is essential for the self-association of C protein. The crucial role played by Trp 69 in stabilizing the self-association of C protein was also demonstrated by mutating Trp to Gly/Arg/Phe. Substitution of the Trp residue with Gly/Arg abolished the dimerization, whereas substitution with Phe decreased the self-association significantly. The results of this study pinpoint a critical residue in the C protein that potentially plays a role in stabilizing the homotypic interaction.

Antivirals interacting with hepatitis B virus core protein and core mutations may misdirect capsid assembly in a similar fashion

Recently, heteroarylpyrimidines (HAP) have been identified as potent inhibitors of capsid maturation. Here we discuss the HAP mode of action comparing the aggregation phenotype of wild-type and mutant core proteins with the respective phenotype imposed by HAP or other agents interacting with core protein. Pertinent tests include core fusion protein-mediated transactivation in a two-hybrid system and capsid formation. The finding that transactivation appeared to be unaffected by HAP, or by mutations preventing assembly, is surprising and raises the question for the structure of the interacting hybrid core proteins: Are they monomers, dimers or even oligomers? A direct activity of core fusion monomers is not excluded but considered to be highly unlikely due to rapid homodimerisation. A role of core fusion dimers in transactivation would indicate distinct interactions with a differential sensitivity to HAP. Regarding significance of data gained in two-hybrid systems, caution is necessary, since the site of transactivation is the nucleus, whereas the real site of the core protein interactions during replication is the cytoplasm. Apparently, HAP leave the monomer-monomer interface of HBV core protein unaffected but prevent capsid maturation by interacting with a region known to be crucial for dimer multimerisation and formation of stable capsids. It is suggested to use antivirals as tools for the elucidation of early steps in genome replication and capsid assembly. A frame for this could be the hypothesis that the virus uses soluble core protein, namely intracellular maturation intermediates of HbeAg for a core targeted self-restriction of replication.

Mosaic particles formed by wild-type hepatitis B virus core protein and its deletion variants consist of both homo- and heterodimers

Co-expression in Escherichia coli of wild-type (wt) hepatitis B virus core protein (HBc) and its naturally occurring variants with deletions at amino acid positions 77-93 or 86-93 leads to formation of mosaic particles, which consist of three dimer subunit compositions. These compositions are wt/variant HBc heterodimers and two types of homodimers, formed by wt HBc or the variant HBc themselves. Mosaic particles were found also when both HBc deletion variants 77-93 and 86-93 were co-expressed in E. coli. These findings are discussed in terms of their significance for hepatitis B virus pathogenesis and prospective use of mosaic particles in vaccine development.

Conversion of poorly immunogenic malaria repeat sequences into a highly immunogenic vaccine candidate

The recent success of a Plasmodium falciparum malaria vaccine consisting of circumsporozoite protein (CSP) T and B cell epitopes has rekindled interest in the development of a pre-erythrocytic vaccine. In order to optimize immunogenicity, well-characterized CSP-specific neutralizing B cell epitopes and a universal T cell epitope were combined with an efficient and flexible particulate carrier platform, the hepatitis B core antigen (HBcAg), to produce a novel pre-erythrocytic vaccine candidate. The vaccine candidate, V12.PF3.1, is a potent immunogen in mice eliciting unprecedented levels (greater than 10(6) titers) of sporozoite-binding antibodies after only two doses. The anti-sporozoite antibodies are long lasting, represent all IgG isotypes, and antibody production is not genetically restricted. CSP-specific CD4+ T cells are also primed by V12.PF3.1 immunization in a majority of murine strains. Furthermore, the hybrid HBcAg-CS particles can be produced inexpensively in bacterial expression systems. These and other characteristics suggest that V12.PF3.1 represents an efficient and economical P. falciparum vaccine candidate for use separately or in combination with other formulations.

Interaction of wild-type and naturally occurring deleted variants of hepatitis B virus core polypeptides leads to formation of mosaic particles

The simultaneous presence of hepatitis B virus (HBV) genomes carrying wild-type (wt) and in-frame deleted variants of the HBV core gene has been identified as a typical feature of HBV-infected renal transplant patients with severe liver disease. To investigate possible interactions of wt and deleted core polypeptides a two-vector Escherichia coli expression system ensuring their concomitant synthesis has been developed. Co-expression of wt and a mutant core lacking 17 amino acid residues (77-93) within the immunodominant region led to the formation of mosaic particles, whereas the mutant alone was incapable of self-assembly.

Hepatitis B virus biology

Hepadnaviruses (hepatitis B viruses) cause transient and chronic infections of the liver. Transient infections run a course of several months, and chronic infections are often lifelong. Chronic infections can lead to liver failure with cirrhosis and hepatocellular carcinoma. The replication strategy of these viruses has been described in great detail, but virus-host interactions leading to acute and chronic disease are still poorly understood. Studies on how the virus evades the immune response to cause prolonged transient infections with high-titer viremia and lifelong infections with an ongoing inflammation of the liver are still at an early stage, and the role of the virus in liver cancer is still elusive. The state of knowledge in this very active field is therefore reviewed with an emphasis on past accomplishments as well as goals for the future.

Hepatitis B virus maturation is affected by the incorporation of core proteins having a C-terminal substitution of arginine or lysine stretches

Assembly of replication-competent hepadnavirus nucleocapsids requires interaction of core protein, polymerase and encapsidation signal (epsilon) with viral pregenomic RNA. The N-terminal portion (aa 1-149) of the core protein is able to self-assemble into nucleocapsids, whereas the C-terminal portion (aa 150-183) is known to interact with pregenomic RNA. In this study, two hepatitis B virus (HBV) core mutants (C144Arg and C144Lys) in which the C-terminal SPRRR (Ser-Pro-Arg-Arg-Arg) motif was replaced by a stretch of arginine or lysine residues were generated to test their role in pregenome encapsidation and virus maturation. Mutant or wild-type core-expression plasmids were co-transfected with a core-negative plasmid into human hepatoma HuH-7 cells to compare trans-complementation efficiency for virus replication. Both low- and high-density capsids were present in -the cytoplasm and culture medium of HuH-7 cells in all transfections. Nucleocapsids formed by C144Arg and C144Lys, however, lost the endogenous polymerase activity to repair HBV DNA. Furthermore, in co-transfection of pHBVC144Arg or pHBVC144Lys with a plasmid which produces replication-competent nucleocapsids, the HBV DNA repairing signal was reduced 40- to 80-fold. This is probably due to formation of mosaic particles of wild-type and mutant cores. Results indicated that the SPRRR motif at the core protein C terminus is important for HBV DNA replication and maturation. Additionally, triple-plasmid transfection experiments showed that nucleocapsids containing various amounts of C144Arg and wild-type core proteins exhibited a bias in selecting a shorter pregenome for encapsidation and DNA replication. It is therefore suggested that unknown factors are also involved in HBV pregenome packaging.

Hepatitis B core particles as a universal display model: a structure-function basis for development

Because it exhibits a remarkable capability to accept mutational intervention and undergo correct folding and self-assembly in all viable prokaryotic and eukaryotic expression systems, hepatitis B core (HBc) protein has been favored over other proposed particulate carriers. Structurally, the unusual alpha-helical organization of HBc dimeric units allows introduction of foreign peptide sequences into several areas of HBc shells, including their most protruding spikes. Progress toward full resolution of the spatial structure as well as accumulation of chimeric HBc-based structures has brought closer the knowledge-based design of future vaccines, gene therapy tools and other artificial particulate objects.

Mapping of homologous interaction sites in the hepatitis B virus core protein

Hepatitis B virus consists of an outer envelope and an inner capsid, or core, that wraps around the small genome plus the viral replication enzyme. The icosahedrally symmetric nucleocapsid is assembled from multiple dimeric subunits of a single 183-residue capsid protein, which must therefore contain interfaces for monomer dimerization and for dimer multimerization. The atomic structure of the protein is not known, but electron microscopy-based image reconstructions suggested a hammerhead shape for the dimer and, very recently, led to a tentative model for the main chain trace. Here we used a combination of interaction screening techniques and functional analyses of core protein variants to define, at the primary sequence level, the regions that mediate capsid assembly. Both the two-hybrid system and the pepscan technique identified a strongly interacting region I between amino acids (aa) 78 and 117 that probably forms part of the dimer interface. Surprisingly, mutations in this region, in the context of a C-terminally truncated but assembly-competent core protein variant, had no detectable effect on assembly. By contrast, mutations in a second region, bordered by aa 113 and 143, markedly influenced capsid stability, strongly suggesting that this region II is the main contributor to dimer multimerization. Based on the electron microscopic data, it must therefore be located at the basal tips of the dimer, experimentally supporting the proposed main chain trace.

A hydrophobic heptad repeat of the core protein of woodchuck hepatitis virus is required for capsid assembly

The capsid particle of hepadnaviruses is assembled from its dimer precursors. However, the mechanism of the protein-protein interaction is still poorly understood. A small region in the capsid protein of woodchuck hepatitis virus (WHV) contains four hydrophobic residues, including leucine 101, leucine 108, valine 115, and phenylalanine 122, that are conserved and spaced every seventh residue in the primary sequence to form a hydrophobic heptad repeat (hhr). A hydrophobic force often plays an important role in the interaction of proteins. Therefore, to investigate the role of this region in capsid assembly, we individually changed the codons specifying these four hydrophobic amino acids to codons specifying alanine or proline. In addition, we examined the in vivo infectivity of a WHV genome bearing a naturally occurring single amino acid change (histidine 104-->proline) in the hhr region. The phenotype of each altered genome was determined in both eukaryotic and prokaryotic systems by a capsid protein assay and electron microscopic examination. We show that replacement of any one of the four hydrophobic residues with alanine did not prevent capsid assembly. However, assembled capsid particles were not detected if combinations of any two of the four residues were substituted with alanines or if the spacing of these four hydrophobic residues was changed. An individual introduction of a proline (which dramatically changes the secondary structure of proteins) into different positions of this small region also abolished capsid assembly in vitro or viral replication in vivo. These results suggested that the hhr region of the core protein of WHV was critical for capsid assembly.

Evolutionary conservation in the hepatitis B virus core structure: comparison of human and duck cores

BACKGROUND

Hepatitis B virus is a major human pathogen which has been extensively studied, yet its structure is unknown. Cryo-electron microscopy of the viral cores expressed in Escherichia coli or isolated from infected liver provides a means for determining the structure of the hepatitis B nucleocapsid.

RESULTS

Using cryo-electron microscopy and three-dimensional image reconstruction, we have determined the structures of duck and human hepatitis B virus cores and find that they have similar dimer-clustered T = 3 and T = 4 icosahedral organizations. The duck virus core protein sequence differs from the human in both length and amino acid content; however, the only significant structural differences observed are the lobes of density on the lateral edges of the projecting (distal) domain of the core protein dimer. The different cores contain varying amounts of nucleic acid, but exhibit similar contacts between the core protein and the nucleic acid. Immunoelectron microscopy of intact cores has localized two epitopes on the core surface corresponding to residues 76-84 and 129-132.

CONCLUSIONS

The bacterial expression system faithfully reproduces the native hepatitis B virus core structure even in the absence of the complete viral genome. This confirms that proper assembly of the core is independent of genome packaging. Difference imaging and antibody binding map three sequence positions in the structure: the C terminus and the regions near amino acids 80 and 130. Finally, we suggest that the genome-core interactions and the base (proximal) domain of the core dimer are evolutionarily conserved whereas the projecting domain, which interacts with the envelope proteins, is more variable.

Identification of two separable modules in the duck hepatitis B virus core protein

Hepadnavirus replication requires the concerted action of the polymerase and core proteins to ensure packaging of the RNA pregenome and DNA maturation. The arginine-rich C terminus of the core protein plays an essential role in both of these steps while being dispensable for nucleocapsid formation. In an attempt to identify other functional domains of the core protein, we performed a series of trans-complementation experiments analyzing the ability of duck and human hepatitis B virus (DHBV and HBV) core protein subunits to support the replication of a core-defective DHBV genome. Plasmids expressing the N-terminal amino acids 1 to 67 or the remaining C-terminal portion, amino acids 67 to 262, of the DHBV core protein were cotransfected into LMH cells along with a replication-deficient construct coding for the DHBV pregenome and polymerase. Neither the N nor the C terminus alone yielded replication-competent core particles. However, cotransfection of plasmids that separately expressed both regions restored a normal replication pattern. Furthermore, the DHBV C terminus but not the N terminus could be replaced by the corresponding domain of the HBV core protein in this assay. Finally, coexpression of the complete HBV core protein and the N terminus from DHBV resulted in DHBV replication, while the HBV core protein alone was not functional. Taken together, these findings suggest a modular organization of the DHBV core protein in which the C terminus is functionally conserved among different hepadnaviruses.

Phenotypic mixing of rodent but not avian hepadnavirus surface proteins into human hepatitis B virus particles

The virus family Hepadnaviridae comprises two genera: orthohepadnaviruses isolated from humans (hepatitis B virus [HBV]) and rodents (e.g., woodchuck hepatitis virus [WHV]) and avihepadnaviruses isolated from birds (e.g., duck hepatitis B virus [DHBV]). They carry in their envelopes two (DHBV) or three (HBV and WHV) coterminal proteins referred to as small (S), middle (M), or large (L) surface protein. These proteins are also secreted from infected cells as subviral particles consisting of surface protein and lipid (e.g., 20-nm hepatitis B surface antigen for HBV). To investigate the assembly of these proteins, we asked whether surface proteins from different hepadnaviruses are able to mix phenotypically with each other. By coexpression and coimmunoprecipitation with species-specific antibodies, we could show the formation of mixed subviral particles and disulfide-linked heterodimers between the WHV S and HBV M proteins whereas the DHBV and HBV surface proteins did not coassemble. Complementation of HBV genomes defective in expressing the S or L protein and therefore incompetent to form virions was possible with the closely related WHV S protein or a WHV pre-S-HBV S chimera, respectively, but not with the less related DHBV S or L protein or with a DHBV L-HBV S chimera. The results suggest that the assembly of HBV subviral particles and virion envelopes requires relatively precise molecular interactions of their surface proteins, which are not conserved between the two hepadnavirus genera. This contrasts with the ability of, e.g., rhabdoviruses or retroviruses, to incorporate envelope proteins even from unrelated viruses.