POSTTRANSCRIPTIONAL REGULATION OF HEPATITIS-B VIRUS-REPLICATION BY THE PRECORE PROTEIN

Hepadnaviruses encode two core-related open reading frames. One direct s the synthesis of the p21 core protein, which subsequently becomes a structural component of the viral nucleocapsid. The other produces a p 25 precore protein that is targeted by a signal peptide to a cell secr etory pathway where N-terminal processing will create a p22 species. T his molecule will be further modified at the C-terminal region to gene rate p17, and the truncated protein is secreted from the cell as hepat itis B e antigen (HBeAg), The function of the precore gene in the biol ogy of hepadnaviruses is unknown. We found that ablation of the precor e gene resulted in the generation of a hepatitis B virus (HBV) species ,vith a high-replication-level phenotype. More important, expression i n trans of physiologic levels of p25 restored viral replication to wil d-type levels, Moreover, transient or stable overexpression of the pre core gene resulted in striking inhibition of HBV replication, The mole cular species responsible for this viral inhibitory effect was identif ied as the p22 nonsecreted HBeAg precursor protein. By sucrose gradien t sedimentation analysis, we determined that expression of p22 leads t o the formation of nucleocapsids similar to those made with wild-type p21 core protein, Immunoprecipitation experiments revealed that the p2 1 and p22 physically interact and form hybrid nucleocapsid structures devoid of pregenomic viral RNA. These experiments suggest that express ion of the precore gene may be important in the regulation of HBV repl ication and describe a possible molecular mechanism(s) for this effect .