Melen K, Kinnunen L, Julkunen I
Laboratory of Viral and Molecular Immunology, Department of Microbiology, National Public Health Institute, FIN-00300 Helsinki, Finland.
J Biol Chem. 2001 May 11;276(19):16447-55. doi: 10.1074/jbc.M008821200. Epub 2001 Jan 9.
Signal transducers and activators of transcription (STATs) are latent cytoplasmic transcription factors, which mediate interferon (IFN), interleukin, and some growth factor and peptide hormone signaling in cells. IFN stimulation results in tyrosine phosphorylation, dimerization, and nuclear import of STATs. In response to IFN-gamma stimulation, STAT1 forms homodimers, whereas IFN-alpha induction results in the formation of STAT1.STAT2 heterodimers, which assemble with p48 protein in the nucleus. Phosphorylation as such is not sufficient to target STATs into the nucleus; rather, the dimerization triggered by phosphorylation is essential. Although IFN-induced nuclear import of STATs is mediated by the importin/Ran transport system, no classic nuclear localization signal (NLS) has been found in STATs. In the three-dimensional 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 transient transfection/IFN stimulation assay that this site is essential for the nuclear import of both STAT1 and STAT2. The results suggest that two arginine/lysine-rich elements, one in each STAT monomer, are required for IFN-induced nuclear import of STAT dimers. Import-defective STAT1 and STAT2 proteins were readily phosphorylated and dimerized, but they functioned as dominant negative molecules inhibiting the nuclear import of heterologous STAT protein.
信号转导子和转录激活子（STATs）是潜在的细胞质转录因子，它们介导细胞中的干扰素（IFN）、白细胞介素以及一些生长因子和肽类激素信号传导。IFN刺激导致STATs发生酪氨酸磷酸化、二聚化并转运至细胞核。在IFN-γ刺激下，STAT1形成同源二聚体，而IFN-α诱导则导致STAT1.STAT2异源二聚体的形成，该异源二聚体在细胞核中与p48蛋白组装。磷酸化本身并不足以将STATs靶向转运至细胞核；相反，由磷酸化引发的二聚化至关重要。尽管IFN诱导的STATs核转运由输入蛋白/Ran转运系统介导，但在STATs中未发现经典的核定位信号（NLS）。在STAT1的三维结构中，我们在该分子的DNA结合结构域内观察到一个富含精氨酸/赖氨酸的结构元件。我们在STAT1和STAT2的这些元件中创建了一系列点突变，并通过瞬时转染/IFN刺激试验表明该位点对于STAT1和STAT2的核转运均至关重要。结果表明，IFN诱导的STAT二聚体核转运需要每个STAT单体中各有一个富含精氨酸/赖氨酸的元件。导入缺陷型的STAT1和STAT2蛋白很容易被磷酸化和二聚化，但它们作为显性负性分子发挥作用，抑制异源STAT蛋白的核转运。
