Homologous and heterologous complementation of HBV and WHV capsid and polymerase functions in RNA encapsidation.

Successful encapsidation of hepadnaviral pregenomic RNA requires the orchestrated interaction of the viral capsid and polymerase proteins with each other and with the RNA packaging substrate. The early steps of this process involve binding of the polymerase to the encapsidation signal, epsilon, and are already understood in some detail. However, the underlying macromolecular interactions resulting in the subsequent encapsidation of this polymerase-epsilon complex by capsid proteins are less clearly defined. To approach this issue we have examined the ability of two different hepadnaviruses to encapsidate each other's pregenomic RNA. H. Okamoto et al. ((1990) J. Gen. Virol. 71, 959-963) have previously demonstrated that WHV polymerase could encapsidate an HBV pregenome, but the origin of the capsid proteins (i.e., HBV- or WHV-derived) required for this reaction was not clear; some evidence suggested that heterologous capsid and polymerase proteins might not be capable of interaction. To clarify this, we analyzed encapsidated RNA isolated from cytoplasmic cores produced following transient transfection of HepG2 cells with different combinations of plasmids encoding HBV or WHV core and polymerase genes. We found that (i) the essential encapsidation signal of WHV is comprised of a short region including epsilon, as in HBV; (ii) HBV and WHV polymerases are each competent to recognize both HBV and WHV packaging signals; therefore the encapsidation signals are functionally interchangeable; and (iii) HBV capsids encapsidate a WHV polymerase-epsilon complex, and vice versa, although the efficiency of heterologous packaging is slightly lower than that of homologous encapsidation. Our results underscore the close relationship of these two mammalian hepadnaviruses.