Direct association and nuclear import of the hepatitis B virus X protein with the NF-kappa B inhibitor I kappa B alpha

The X protein of hepatitis B virus (HBV) is a transcriptional activator whi ch is required for infection and may play an important role in HBV-associat ed hepatocarcinogenesis. It has been suggested that X acts as a nuclear coa ctivator or stimulates several signal transduction pathways by acting in th e cytoplasm. One of these pathways leads to the nuclear translocation bf NF -kappa B. A recent report indicates that X activates NF-kappa B by acting o n two cytoplasmic inhibitors of this family of transcription factors: I kap pa B alpha and the precursor/inhibitor p105. We demonstrate here that X dir ectly interacts with I kappa B alpha, which is able to transport it to the nucleus by a piggyback mechanism. This transport requires a region of I kap pa B alpha (the second ankyrin repeat) which has been demonstrated to be in volved in its nuclear import following NF-kappa B activation. Using deletio n mutants, we showed that amino acids 249 to 253 of I kappa B alpha (locate d in the C-terminal part of the sixth ankyrin repeat) play a critical role in the interaction with X. This small region overlaps one of the domains of I kappa B alpha mediating the interaction with the p50 and p65 subunits of NF-kappa B and is also close to the nuclear export sequence of I kappa B a lpha, therefore providing a potential explanation for the nuclear accumulat ion of I kappa B alpha with X. This association can also be observed upon t he induction of endogenous I kappa B alpha by tumor necrosis factor alpha ( TNF-alpha) treatment of Chang cells expressing X. In accordance with this o bservation, band shift analysis indicates that X induces a sustained NF-kap pa B activation following TNF-alpha treatment, probably by preventing the r eassociation of newly synthesized nuclear I kappa B alpha With DNA-bound NF -kappa B complexes.