Novel functions of nuclear transport factors : stress-response mechanism of cell nucleus

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Various cellular stresses induce the nuclear accumulation of importinα. However, it is unknown whether accumulated importinαperforms roles in the nucleus. In this study, we found that nuclear-retained importinα2binds with DNase I-sensitive nuclear component(s) and exhibits selective up-regulation of mRNA encoding Serine/ threonine kinase35(STK35). Chromatin immunoprecipitation and promoter analysis demonstrated that importinα2binds to the promoter region of STK35and accelerates its transcription. Furthermore, constitutive overexpression of STK35proteins enhances caspase-independent cell death under oxidative stress conditions. These results reveal that nuclear-localized importinα2influences gene expression and contributes directly to cell fate outcomes including non-apoptotic cell death. This is the first evidence showing that importinαhas novel functions other than nuclear protein import in the nucleus.