Truncated Form of Importin α Identified in Breast Cancer Cell Inhibits Nuclear Import of p53

Abstract Disruption of the function of tumor suppressor proteins occasionally can be dependent on their subcellular localization. In about 40% of the breast cancer tissues, p53 is found in the cytoplasm as opposed to the nucleus, where it resides in normal breast cells. This means that the regulation of subcellular location of p53 is an important mechanism in controlling its function. The transport factors required for the nuclear export of p53 and the mechanisms of their nuclear export have been extensively characterized. However, little is known about the mechanism of nuclear import of p53. p53 contains putative nuclear localization signals (NLSs) which would interact with a nuclear transport factor, importin α. In this report we demonstrate that importin α binds to NLSI in p53 and mediates the nuclear import of p53. Reverse transcriptase-polymerase chain reaction and sequencing analyses showed that a truncated importin α deleted the region encoding the putative NLS-binding domain of p53, suggesting that it could not bind to NLSs of p53 proteins. Binding of importin α to p53 was confirmed by using yeast two-hybrid assay. When expressed in CHO-K1 cells, the truncated importin α predominantly localized to the cytoplasm. In truncated importin α expressing cells, p53 preferentially localized to cytoplasmic sites as well. A significant increase in the p21waf1/cip1 mRNA level and induction of apoptosis were also observed in importin α overexpressing cells. These results strongly suggest that importin α functions as a component of the NLS receptor for p53 and mediates nuclear import of p53.