PACKAGING OF HEPATITIS-DELTA VIRUS-RNA VIA THE RNA-BINDING DOMAIN OF HEPATITIS-DELTA ANTIGENS - DIFFERENT ROLES FOR THE SMALL AND LARGE DELTA-ANTIGENS

Hepatitis delta virus (HDV) is composed of four specific components. T he first component is envelope protein which contains hepatitis B surf ace antigens. The second and third components are nucleocapsid protein s, referred to as small and targe hepatitis delta antigens (HDAgs). Th e final component is a single-stranded circular RNA molecule known as the viral genome. In order to study the mechanism of HDV RNA packaging , a four-plasmid cotransfection system in which each viral component w as provided by a separate plasmid was employed. Virus-like particles r eleased from Huh-7 cells receiving such a cotransfection were found to contain HDV RNA along with three proteins. Therefore, the four-plasmi d cotransfection system could lead to successful HDV RNA packaging in vitro. The system was then used to show that the large HDAg alone was able to achieve a low level of HDV RNA packaging. Analysis of a variet y of large HDAg mutants revealed that the RNA-binding domain was essen tial for viral RNA packaging. By increasing the incorporation of small HDAg into virus-like particles, we found a three- to fourfold enhance ment of HDV RNA packaging. This effect was probably through a direct b inding of HDV RNA, independent from that of large HDAg, with the small HDAg. The subsequent RNA-protein complex was packaged into particles. The results provided insight into the roles and functional domains of small and large HDAgs in HDV RNA packaging.