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 (HBcAg) were found to accumulate abundant core particles in the same way as human hepatocytes infected with hepatitis B virus (HBV) by the present 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 expression of HBcAg is highly inducible by deprivation of phosphate in the culture medium. At regular intervals after induction, cells were cryo-fixed and processed for transmission electron microscopy by ultrathin sectioning. After induction, HBcAg activity rapidly increased, becoming several hundred times higher than the initial level after 25 h. The core particles first appeared in the nucleus, then in the cytoplasm, and finally in the vacuole. Core particles passing through nuclear pores from the nucleus to the cytoplasm could be seen. Core particles were either incorporated directly in the vacuole or indirectly by first forming an autophagosome. The core particles were then released into the vacuolar sap, and were digested there. Together with the previous studies, our results suggest that, in human hepatocytes, HBcAg polypeptides are synthesized in the cytoplasm, but are assembled into core particles in the nucleus. The assembled core particles are then transported from the nucleus to the cytoplasm through nuclear pores.