Characterization of regulated nucleo-cytoplasmic localization of the STAT1 transcription factor

A large number of cytokines use the JAK/STAT pathway to transduce signals into the cell. Janus kinases (JAKs) are tyrosine kinases associated with the intracellular domains of cytokine receptors. Cytokine binding to receptors causes JAK activation which phosphorylate and activate Signal Transducers and Activators of Transcription (STATs). The STATs are intracellular signaling transcription factors able to bind DNA and activate target gene transcription. STATs serve as a direct link between activated cell surface receptor complexes and target gene expression. Latent STATs reside in the cytoplasm but rapidly accumulate in the nucleus following cytokine stimulation. Nuclear accumulation requires specific tyrosine phosphorylation by JAKs and subsequent STAT dimerization. In general, protein entrance and exit from the nucleus has been characterized as a tightly regulated process requiring soluble transport receptor recognition.;In this study, it is demonstrated that after cytokine stimulation, STAT1 accumulation in the nucleus is facilitated by the gain of a recognizable nuclear localization signal (NLS). A member of the Importinalpha family of nuclear import receptors, NPI-1, binds to STAT1 after activation and appears to facilitate nuclear import. NPI-1 can recognize tyrosine phosphorylated STAT1 and STAT1-STAT2 complexes, but not tyrosine phosphorylated STAT6, demonstrating specificity of recognition by NPI-1. STAT1 interaction with NPI-1 is competed by DNA binding in vitro. This finding supports a model of STAT1 release from NPI-1 in the nucleus to allow STAT1 dimers to bind DNA and recycling of NPI-1 back to the cytoplasm.;The presence of STATs in the nucleus is transient and within hours the STATs reappear in the cytoplasm. Results indicate that STAT1 can be tyrosine dephosphorylated in the nucleus and actively exported by the CRM1 export receptor. CRM1 recognizes a specific amino acid sequence located within the DNA binding domain of STAT1 This region shares sequence and functional properties of characterized nuclear export signals (NESs). The location of this sequence within STAT1 suggests that it is not accessible to CRM1 when STAT1 is bound to DNA. Evidence is presented to support a model in which STAT1 is tyrosine dephosphorylated in the nucleus and dissociates from DNA, allowing recognition by CRM1 and nuclear export. The regulated export of STAT1 may contribute to silencing of the signal pathway and/or to re-establishing STAT1 in the cytoplasm to monitor activity of receptor/kinase signals.