Background Hepatitis B Virus (HBV) infection is a worldwide biomedical problem and an improved understanding of the assembly and structure of the virus may help develop new antiviral therapies. The HBV core gene codes for precore protein (pre-C) which is either partially processed to form a secreted non-particulate protein called e-antigen (HBeAg) or fully processed to produce core antigen (HBcAg). HBcAg is a 183-residue protein that encapsidate around a RNA-reverse transcriptase complex (HBV polymerase). HBcAg has been expressed in E.coli were it assembles in the bacterial cytoplasm into icosahedral capsids, which contain bound host nucleic acid. Deletion of the polybasic C-terminal 34 residues also produces assembly competent protein. The C-terminal truncated protein (Cpe: residues 1-149) does not contain nucleic acid, can be highly purified and is more suitable for structural studies than the full-length protein. Native HBeAg is also C-terminally truncated at position 149 and in addition contains a 10 residue N-terminal extension derived from partial processing of pre-C. Although the function of HBeAg is unclear it is an important serological marker. Results To localize regions of functional and structural importance, we have introduced by site-directed mutagenesis reactive cysteine residues at various sites in the protein that can be specially labeled with electron dense reagents. Other successful approaches have included the use of mutants with inserted or deleted amino acids (7-10 residues) and the labeling with antibodies. The structural determinations have been made by high-resolution electron microscopy and image analysis (A. Steven). Based on structural information, mutants have been constructed which allow the production of unassembled capsid subunits. These protein constructs have been shown to be ideally suited for structural determination by high-resolution NMR. Other work includes: study of the HBV capsid assembly, results suggests that interfering with assembly could be an important target for antiviral therapeutics; expression of the full-length HBV polymerase has been achieved and conditions for production of protein suitable for structural studies are being sort. Significance and future direction: More detailed structural information on the structure of the HBV capsid subunit has been achieved and work in progress will help in understanding its assembly into capsids and the concomitant conformation changes that occur. Other targets of HBV for structural determination include the HBV polymerase. This protein, and functional regions thereof, has been expressed in E.coli and structure determinations will be performed. Summary The Hepatitis B Virus (HBV) is the major worldwide cause of cancer. Although a vaccine is available, chronic HBV is often acquired in childhood. The HBV nucleocapsid plays an important structural role and metabolic role in the life cycle of the virus. An understanding of the molecular structure of the HBV nucleocapsid would allow targeted drug discovery with the aim of preventing the assembly and formation of the virus.
