Truncated form of importin alpha identified in breast cancer cell inhibitsnuclear import of p53

Disruption of the function of tumor suppressor proteins occasionally can be dependent on their subcellular localization. In about 40% of the breast ca ncer tissues, p53 is found in the cytoplasm as opposed to the nucleus, wher e it resides in normal breast cells. This means that the regulation of subc ellular location of p53 is an important mechanism in controlling its functi on. The transport factors required for the nuclear export of p53 and the me chanisms of their nuclear export have been extensively characterized. Howev er, little is known about the mechanism of nuclear import of p53, p53 conta ins putative nuclear localization signals (NLSs) which would interact with a nuclear transport factor, importin alpha. In this report we demonstrate t hat importin a binds to NLSI in p53 and mediates the nuclear import of p53, Reverse transcriptase-polymerase chain reaction and sequencing analyses sh owed that a truncated importin alpha deleted the region encoding the putati ve NLS-binding domain of p53, suggesting that it could not bind to NLSs of p53 proteins. Binding of importin alpha to p53 was confirmed by using yeast two-hybrid assay. When expressed in CHO-K1 cells, the truncated importin a lpha predominantly localized to the cytoplasm, In truncated importin alpha expressing cells, p53 preferentially localized to cytoplasmic sites as well , A significant increase in the p21(waf1/cip1) mRNA level and induction of apoptosis were also observed in importin alpha overexpressing cells. These results strongly suggest that importin alpha functions as a component of th e NLS receptor for p53 and mediates nuclear import of p53.