DYNAMICS OF HEPATITIS-B VIRUS CORE ANTIGEN IN A TRANSFORMED YEAST-CELL - ANALYSIS WITH AN INDUCIBLE SYSTEM

Transformed yeast cells expressing hepatitis B virus core antigen (HBc Ag) were found to accumulate abundant core particles in the same way a s human hepatocytes infected with hepatitis B virus (HBV) by the prese nt authors. We, therefore, offer a good model system for studying the dynamics of assembly of HBcAg into core particles. To investigate this problem, we have developed a transformed yeast cell in which expressi on of HBcAg is highly inducible by deprivation of phosphate in the cul ture medium, At regular intervals after induction, cells were cryo-fix ed and processed for transmission electron microscopy by ultrathin sec tioning. After induction, HBcAg activity rapidly increased, becoming s everal hundred times higher than the initial level after 25 h. The cor e particles first appeared in the nucleus, then in the cytoplasm, and finally in the vacuole. Core particles passing through nuclear pores f rom the nucleus to the cytoplasm could be seen. Core particles were ei ther incorporated directly in the vacuole or indirectly by first formi ng an autophagosome. The core particles were then released into the va cuolar sap, and were digested there. Together with the previous studie s, our results suggest that, in human hepatocytes, HBcAg polypeptides are synthesized in the cytoplasm, but are assembled into core particle s in the nucleus. The assembled core particles are then transported fr om the nucleus to the cytoplasm through nuclear pores.