HEPATITIS-DELTA ANTIGEN MEDIATES THE NUCLEAR IMPORT OF HEPATITIS-DELTA VIRUS-RNA

Hepatitis delta virus (HDV) RNA replicates in the nuclei of virus-infe cted cells. The mechanism of nuclear import of HDV RNA is so far unkno wn. Using a fluorescein-labeled HDV RNA introduced into partially perm eabilized HeLa cells, we found that HDV RNA accumulated only in the cy toplasm, However, in the presence of hepatitis delta antigen (HDAg), w hich is the only protein encoded by HDV RNA, the HDV RNA was transloca ted into the nucleus, suggesting that nuclear import of HDV RNA is med iated by HDAg. Deletion of the nuclear localization signal (NLS) or RN A-binding motifs of HDAg resulted in the failure of nuclear import of HDV RNA, indicating that both the NLS and an RNA-binding motif of HDAg are required for the RNA-transporting activity of HDAg. Surprisingly, any one of the three previously identified RNA-binding motifs was suf ficient to confer the RNA-transporting activity. We have further shown that HDAg, via its NLS, interacts with karyopherin alpha 2 in vitro, suggesting that nuclear import of the HDAg-HDV RNA complex is mediated by the karyopherin alpha 2 beta heterodimer. The nuclear import of HD V RNA may be the first biological function of HDAg in the HDV life cyc le.