PHENOTYPIC MIXING BETWEEN DIFFERENT HEPADNAVIRUS NUCLEOCAPSID PROTEINS REVEALS C-PROTEIN DIMERIZATION TO BE CIS PREFERENTIAL

Hepadnaviruses encode a single core (C) protein which assembles into a nucleocapsid containing the polymerase (P) protein and pregenomic RNA during viral replication in hepatocytes. We examined the ability of h eterologous hepadnavirus C proteins to cross-oligomerize. Using a two- hybrid assay in HepG2 cells, we observed cross-oligomerization among t he core proteins from hepatitis B virus (HBV), woodchuck hepatitis vir us, and ground squirrel hepatitis virus. When expressed in Xenopus ooc ytes, in which hepadnavirus C proteins form capsids, the C polypeptide s from woodchuck hepatitis virus and ground squirrel hepatitis virus, but not duck hepatitis B virus, can efficiently coassemble with an epi tope-tagged HBV core polypeptide to form mixed capsids. However, when two different core mRNAs are coexpressed in oocytes the core monomers show a strong preference for forming homodimers rather than heterodime rs. This holds true even for coexpression of two HBV C proteins differ ing only by an epitope tag, suggesting that core monomers are not free to diffuse and associate with other monomers. Thus, mixed capsids res ult from aggregation of different species of homodimers.