MUTATIONAL ANALYSIS OF DELTA-ANTIGEN - EFFECT ON ASSEMBLY AND REPLICATION OF HEPATITIS-DELTA VIRUS

Hepatitis delta virus requires a helper function from hepatitis B viru s far packaging, release, and infection of hepatocytes. The assembly o f large delta antigen (HDAg) is mediated by copackaging with the small surface antigen of hepatitis B virus (HBsAg), and the assembly of sma ll HDAg requires interactions with large HDAg. To examine the molecula r mechanisms by which small HBsAg, large HDAg, and small HDAg interact , we have established a virion assembly system in COS7 cells by cotran sfecting plasmids encoding the smell HBsAg, the small HDAg, and large HDAg mutants. Results indicate that sequences within the C-terminal 19 -amino-acid domain flanking the Cxxx isoprenylation motif are importan t for the assembly of large NDAg. In addition, a large HDAg mutant bea ring extra sequences separating the C-terminal U-amino-acid domain fro m the common regions of the small and large HDAgs is capable, like the wild-type large HDAg, of copackaging with small HBsAg. The ability of assembly is also demonstrated for a large HDAg mutant from which nucl ear localization signals have been removed. Furthermore, a cryptic sig nal within the N-terminal 50 amino acid residues other than the putati ve N-terminal coiled-coil structure and a subdomain between amino acid residues 50 and 65 of the large HDAg are important for the assembly o f small HDAg as well as trans-dominant negative regulation of large HD Ag in hepatitis delta virus replication.