12Stat1 is one of the seven members of a family of STATs – latent cytoplasmic proteins activated by various stimuli (cytokines and growth factors) and involved in the regulation of cell growth and differentiation, immune response and homeostasis.13 Stimulation with IFN-γ results in the activation of Janus kinases (Jak) 1 and 2. Activated Jaks phosphorylate tyrosine residues on the IFN-γ receptor, which serve as STAT1 docking sites. Following phosphorylation of tyrosine 701 (Y701) Ceritinib mw STAT1 monomers homodimerize, translocate to the nucleus and activate the transcription of target genes14–16 through binding to γ-activated sequence elements (GAS).17 The promoters of IFN-γ-activated

genes usually contain GAS.13 Two putative GAS sequences have been identified in the GILT promoter at 130 and 510 bp upstream of exon 1 of the GILT gene. There are two naturally occurring forms of STAT1: STAT1α and the alternatively spliced isoform STAT1β. STAT1β lacks the 38 amino acid residues in the C-terminal transcriptional activation domain that can bind the histone acetyltransferases p300/CBP.18,19 STAT1 is primarily activated through phosphorylation at tyrosine 701.20 A secondary,

independent, phosphorylation event occurs at serine 727, which is needed for maximal transcriptional activity.21 In addition to its role in regulating the expression of target genes upon stimulation with IFN, STAT1 has also been shown to play a role in the constitutive expression of certain genes: see more low Molecular mass Polypeptide 2 (LMP2),22,23 caspases24 and major histocompatibility complex (MHC) class I.25 In this study, we investigated whether STAT1 interacts with the GILT

promoter in the absence of IFN-γ. Our data suggest that the presence of Stat1 in a mouse fibroblast cell line correlates with decreased activity of the GILT promoter and decreased constitutive expression of GILT protein. The DNA affinity precipitation assay (DAPA) showed that STAT1 binds with high specificity to putative GAS motifs in the GILT promoter in the absence of IFN-γ stimulation. We also showed that STAT1 residues Y701 and S727 are not required for constitutive STAT1 CYTH4 binding to the GILT promoter. Therefore, phosphorylation of Y701, thought to be necessary for STAT1 homodimerization, is not required for constitutive binding of STAT1 to the GILT promoter. The absence of C-terminal amino acids from the alternatively spliced form of STAT1β does not prevent the binding of STAT1 to the GILT promoter. The remaining N-terminal portion of STAT1 seems to be crucial for binding of STAT1 to the GILT promoter, independently of IFN-γ stimulation. Our experiments indicate that STAT1 residues 426/427 are required for constitutive interaction of STAT1 with the GILT promoter.