Constitutive and IFN-γ-induced nuclear import of STAT1 proceed through independent pathways

STAT1 functions as both a constitutive transcriptional regulator and, in response to cytokine stimulation of cells, as an inducible tyrosine-phosphorylated transcription factor. Here, we identify and characterize a non-transferable nuclear targeting sequence in the STAT1 DNA-binding domain. This conserved signal is critical for the interferon-γ (IFN-γ)-induced nuclear import of phosphorylated STAT1 dimers and requires adjacent positively charged and hydrophobic residues for functioning. Additionally, the constitutive nucleocytoplasmic shuttling of STAT1 in the absence of IFN-γ stimulation is revealed. Nuclear import and export of unphosphorylated STAT1 are demonstrated to be sensitive towards wheat germ agglutinin and to occur independently of the import receptor p97. Loss-of-function mutations of the dimer-specific import signal block nuclear entry of tyrosine-phosphorylated STAT1, which in turn also prevents induction of cytokine-inducible target genes. Nevertheless, nuclear import of unphosphorylated STAT1 continues and the STAT1-dependent constitutive expression of caspases and the tumor necrosis factor-α-mediated induction of apoptosis proceed unaltered. Thus, tyrosine-phosphorylated and unphosphorylated STAT1 molecules shuttle via independent pathways to distinct sets of target genes.