Expression of hepatitis B virus polymerase in Ty1-his3AI retroelement of Saccharomyces cerevisiae

Expression and purification of a functional human hepatitis B virus polymerase

AIM: To identify a method for efficient large-scale purification of functional hepatitis B virus polymerase (HBV-Pol) without addition of cellular factors.

METHODS: Full-length HBV-Pol (843 amino acids) tagged with 5’ end Polyhistidine was expressed at a high level in an Escherichia coli (E. coli) system. Sodium dodecyl sulfate lysis buffer was utilized to dissolve insoluble HBV-Pol, and Ni-NTA resin affinity chromatography was utilized for HBV-Pol purification. Most recombinant HBV-Pol was eluted with 100 mmol/L imidazole in the presence of NP-40, a weak detergent that keeps HBV-Pol in solution. A reducing agent was utilized throughout the purification steps to keep soluble HBV-Pol from redundant disulfide bond formation.

RESULTS: The large-scale production of functional intact human HBV-Pol was achieved in an E. coli expression system. Purified HBV-Pol showed stable reverse transcriptase activity and DNA polymerase activity. The purified protein was of high purity and had stable reverse transcriptase activity.

CONCLUSION: Large-scale production of HBV-Pol in pure form should facilitate crystallization and detailed analysis of the structure and mechanism of HBV-Pol. Ability of this purification approach to obtain human HBV-Pol in an enzymatically active form should be helpful for development of drugs for treatment of chronic hepatitis B.

Efficacies of β-L-D4A against hepatitis B virus in 2.2.15 cells

AIM: To investigate the antiviral effect of beta-L-enantiomer of 2’,3’-didehydro-2’,3’-dideoxyadenosine (β-L-D4A) on 2.2.15 cells transfected with the hepatitis B virus (HBV) genome.

METHODS: Lamivudine (3TC) as a positive control. Then, HBV DNA in treated 2.2.15 cells and the Hepatitis B surface antigen (HBsAg) in the culture supernatants were detected to determine the inhibitory effect of β-L-D4A. At the same time, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) was used to detect the survival ratio of 2.2.15 cells.

RESULTS: β-L-D4A has a dose-dependent inhibitory effect on HBV DNA replication; this effect was apparent when the concentration was above 1 mol/L. When β-L-D4A was at the highest concentration, 100 mol/L, the HBsAg inhibition ratio was above 50%. The Therapeutic index (TI) of β-L-D4A was above 2.1.

CONCLUSION: β-L-D4A has a dose-dependent inhibitory effect on the replication of HBV DNA and the secretion of HBsAg at low toxicity.

Genome evolution mediated by Ty elements in Saccharomyces

How mobile genetic elements molded eukaryotic genomes is a key evolutionary question that gained wider popularity when mobile DNA sequences were shown to comprise about half of the human genome. Although Saccharomyces cerevisiae does not suffer such "genome obesity", five families of LTR-retrotransposons, Ty1, Ty2, Ty3, Ty4, and Ty5 elements, comprise about 3% of its genome. The availability of complete genome sequences from several Saccharomyces species, including members of the closely related sensu stricto group, present new opportunities for analyzing molecular mechanisms for chromosome evolution, speciation, and reproductive isolation. In this review I present key experiments from both the pre- and current genomic sequencing eras suggesting how Ty elements mediate genome evolution.

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Drug discovery and development of antiviral agents for the treatment of chronic hepatitis B virus infection

A safe and effective vaccine for hepatitis B virus (HBV) has been available for nearly twenty years and currently campaigns to provide universal vaccination in developing countries are underway. Nevertheless, chronic HBV infection remains a leading cause of chronic hepatitis worldwide and there is a strong need for safe and effective antiviral therapies. Attempts to identify and develop antiviral agents to treat chronic HBV infection remains focused on nucleoside analogs such as 3TC (lamivudine), adefovir dipivoxil, (bis-POMPMEA), and others. However, advances in our understanding of the molecular biology of HBV and the development of new assays for HBV polymerase activity, such as the reconstitution of active HBV polymerase in vitro, should facilitate large screening efforts for non-nucleoside reverse transcriptase inhibitors. Recent advances have furthered our understanding of clinical resistance to lamivudine, have provided new approaches to treatment, and have offered new perspectives on the major challenges to the identification and development of antiviral agents for chronic HBV infection. Here, in an update to our previous review article that appeared in this series [59a], we focus on recent advances that have occurred in the areas of virus structure and replication, in vitro viral polymerase assays, cell culture systems, and animal models.