Constitutive and IFN-gamma-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-gamma (IFN-gamma)-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-gamma 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-alpha-mediated induction of apoptosis proceed unaltered. Thus, tyrosine-phosphorylated and unphosphorylated STAT1 molecules shuttle via independent pathways to distinct sets of target genes.
Meyer
Thomas
T
Nachwuchsgruppe Zelluläre Signalverarbeitung, Forschungsinstitut für Molekulare Pharmakologie and Institut für Kristallographie, Freie Universität Berlin, Robert-Rössle-Strasse 10, D-13125 Berlin, Germany.
Begitt
Andreas
A
Lödige
Inga
I
van Rossum
Marleen
M
Vinkemeier
Uwe
U
D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites / Q000235:genetics; D002467:Cell Nucleus / Q000378:metabolism*; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D006367:HeLa Cells; D006801:Humans; D007371:Interferon-gamma / Q000494:pharmacology; D051379:Mice; D008969:Molecular Sequence Data; D010766:Phosphorylation; D021381:Protein Transport / Q000187:drug effects / Q000502:physiology*; D050794:STAT1 Transcription Factor; D016415:Sequence Alignment; D015398:Signal Transduction / Q000502:physiology; D015534:Trans-Activators / Q000235:genetics / Q000378:metabolism*
D004268:DNA-Binding Proteins; D050794:STAT1 Transcription Factor; C494079:STAT1 protein, human; C494080:Stat1 protein, mouse; D015534:Trans-Activators; D007371:Interferon-gamma