Arginine/lysine-rich structural element is involved in interferon-induced nuclear import of STATs

Signal transducers and activators of transcription (STATs) are latent cytop lasmic transcription factors, which mediate interferon (IFN), interleukin, and some growth factor and peptide hormone signaling in cells, IFN stimulat ion results in tyrosine phosphorylation, dimerization, and nuclear import o f STATs. In response to IFN-gamma stimulation, STAT1 forms homodimers, wher eas IFN-alpha induction results in the formation of STAT1.STAT2 heterodimer s, which assemble with p48 protein in the nucleus. Phosphorylation as such is not sufficient to target STATs into the nucleus; rather, the dimerizatio n triggered by phosphorylation is essential. Although IFN-induced nuclear i mport of STATs is mediated by the importin/Ran transport system, no classic nuclear localization signal (NLS) has been found in STATs. In the three-di mensional structure of STAT1, we observed a structural arginine/lysine-rich element within the DNA-binding domain of the molecule. We created a series of point mutations in these elements of STAT1 and STAT2 and showed by tran sient transfection/IFN stimulation assay that this site is essential for th e nuclear import of both STAT1 and STAT2. The results suggest that two argi nine/lysine-rich elements, one in each STAT monomer, are required for IFN-i nduced nuclear import of STAT dimers, import-defective STAT1 and STAT2 prot eins were readily phosphorylated and dimerized, but they functioned as domi nant negative molecules inhibiting the nuclear import of heterologous STAT protein.