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Stimulation of Hepatitis B Virus Genome Replication by HBx is Linked to Both Nuclear and Cytoplasmic HBx Expression

Yonsei University

¹ E-mail: wsryu{at}yonsei.ac.kr

HBx, a small regulatory protein of hepatitis B virus, plays an important role in stimulating viral genome replication. HBx was shown to be associated with diverse subcellular locations, such as the nucleus, cytoplasm, and mitochondria. Some studies have linked the stimulation of genome replication by HBx to its cytoplasmic function, while other reports have attributed this function to its nuclear component. To clarify this discrepancy, we measured viral genome replication by complementing a HBx-null replicon in two different ways: (i) co-transfecting an increasing amount of HBx expression plasmid, and (ii) co-transfecting retargeted variants of HBx that are confined to either the nucleus or cytoplasm due to the either NLS (nuclear localization signal) or NES (nuclear export signal) tags, respectively. Intriguingly, immunostaining analysis indicated that the subcellular localization of HBx primarily is influenced by its abundance, where HBx is confined to the nucleus at low levels but usually is detected in the cytoplasm at high levels. Importantly, HBx, whether retargeted either by the NLS- or NES-tag, stimulates viral genome replication to the level comparable to that of the wild-type. Further, similar to the wild-type, the stimulation of viral genome replication by the retargeted HBx occurred at the transcription level. Thus, we concluded that the stimulation of viral genome replication by HBx is linked to both nuclear and cytoplasmic HBx, although the underlying mechanism of stimulation most likely differs.

Received 22 December 2008; accepted 5 January 2009.

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