Assembly of hepadnaviral virions and subviral particles.

One of the most distinctive features of hepadnaviral infection is the production by infected cells of multiple types of virus-related particles, including large numbers of noninfections subviral particles as well as complete virions. Although this unusual property was among the first aspects of the viral life cycle to be discovered, it remains one of its least well understood aspects. Why do infected cells produce subviral particles? What factors determine whether virions or subviral particles are produced, and how are their relative levels controlled? What are the cellular pathways involved in the production of each particle type? At present our answers to all of these questions are fragmentary. However, the recent application of biochemical and genetic approaches to the study of hepadnaviral morphogenesis has reawakened interest in this process and helped to frame the overall dimensions of the problem. In the following pages I review what is known about hepadnaviral structure and assembly, with emphasis on recent mutational approaches that examine the role of individual viral gene products in the assembly process. Since the work is most advanced for the human hepatitis B virus (HBV), this review will concentrate on HBV; mention will be made of the animal hepadnaviruses only where significant differences exist or where the corresponding information for HBV is not available.