The hepatitis B virus X protein plays an important role in the regulation of viral genome expression and has also been implicated in the development of liver cancer associated with chronic viral infection. Several effects have been attributed to X but their biological relevance remains elusive. One of the confusing issues has been so far the uncertaintys concerning its cellular location. To gain insight into the mechanism(s) how X exerts its effects, we have analysed its subcellular distribution and its dependency on the cell cycle. We used two complementary approaches namely, immunolocalization using a cell line stably expressing X, and characterization of the dynamics of X location in living cells by means of the reporter gene GFP. Our data clearly define the cytosol as the prime location of X, irrespectively of the cell cycle and show in addition the close attachment of a fraction of X to the nuclear membrane. However, X does not associate with any cytoplasmic vesicles and organelles so far tested. In contrast, our study provides strong evidence for the codistribution of X with the cytosolic fraction of proteasomes. In pulse-chase experiments, X decayed with a half-life of less than 30 min and proteasome-inhibitors did not modify its turnover, suggesting that X colocalization with the proteasome does not simply point to its degradation pathway. The proteolytic processing of the p105 precursor of the p50 subunit of the NF-κB transcription factor, which has been shown to be proteasome-dependent, is markedly slow down in the presence of X. These findings suggest that X modulates the processing rate of p105 by acting presumably at the level of the proteasome. Thus, targeting of proteasomes by X might be one of the pathways employed by this viral protein to subvert cellular functions.
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