Genetic analysis of p53 nuclear importation.

A key step in activation of the p53 tumor suppressor is its transport into the nucleus; however, despite intensive study of p53, the regulation of its subcellular localization is still poorly understood. Here we examined the p53 nuclear importation using a series of mutant cell lines that were resistant to the growth inhibitory effects of temperature-sensitive murine p53 (tsp53). Examination of the p53 subcellular localization in these cell lines showed that the protein was cytoplasmic in most of them. Using a digitonin-permeabilized cell in vitro nuclear import system, we show that cytosols from these cell lines do not support nuclear translocation of a p53 nuclear localization signal (NLS)-containing substrate protein, but promote nuclear localization of a SV40TAgNLS-containing substrate. Complementation assays and use of the mutant cells themselves in the in vitro assays demonstrate that both soluble and insoluble protein components are involved in p53 nuclear import. Collectively, our results suggest that there is a p53 NLS-selective nuclear import pathway and that both soluble and insoluble proteins are involved in its function.