Reverse transcriptase- and RNA packaging signal-dependent incorporation of APOBEC3G into hepatitis B virus nucleocapsids.

APOBEC3G (A3G) is a cytidine deaminase that can inhibit a wide range of retroviruses, including the para-retrovirus hepatitis B virus (HBV). The antiviral function of A3G depends on its incorporation into assembling viral particles. However, it remains enigmatic how A3G is specifically packaged into a variety of unrelated viruses. By adopting a native agarose gel electrophoresis assay that can specifically measure the levels of A3G incorporation into HBV nucleocapsids, we found that A3G is specifically packaged into replication-competent HBV nucleocapsids in a fashion that is dependent on both the viral reverse transcriptase (RT) and viral RNA packaging signal, epsilon. In contrast, A3G is not incorporated into empty capsids formed in the absence of RT or epsilon. We demonstrated that the packaged A3G was protected from protease digestion by the nucleocapsids, thus confirming its interior localization. We also showed that A3G could bind RT specifically in an RNA-independent manner, which may be responsible for mediating the specific incorporation of A3G into replication-competent nucleocapsids. Finally, we provide evidence that the N-terminal domain of A3G is required for packaging into HBV nucleocapsids.