Mechanisms of unphosphorylated STAT3 transcription factor binding to DNA

Olga A. Timofeeva, Sergey Chasovskikh, Irina Lonskaya, Nadya I. Tarasova, Lyuba Khavrutskii, Sergey G. Tarasov, Xueping Zhang, Valeriy R. Korostyshevskiy, Amrita Cheema, Lihua Zhang, Sivanesan Dakshanamurthy, Milton L. Brown and Anatoly Dritschilo

Journal of Biological Chemistry
2012 vol: 287 pp: 14192-14200doi: 10.1074/jbc.M111.323899

Abstract

Phosphorylation of signal transducer and activator of transcription 3 (STAT3) on a single tyrosine residue in response to growth factors, cytokines, interferons, and oncogenes activates its dimerization, translocation to the nucleus, binding to the interferon γ (gamma)-activated sequence (GAS) DNA-binding site and activation of transcription of target genes. STAT3 is constitutively phosphorylated in various cancers and drives gene expression from GAS-containing promoters to promote tumorigenesis. Recently, roles for unphosphorylated STAT3 (U-STAT3) have been described in response to cytokine stimulation, in cancers, and in maintenance of heterochromatin stability. However, the mechanisms underlying U-STAT3 binding to DNA has not been fully investigated. Here, we explore STAT3-DNA interactions by atomic force microscopy (AFM) imaging. We observed that U-STAT3 molecules bind to the GAS DNA-binding site as dimers and monomers. In addition, we observed that U-STAT3 binds to AT-rich DNA sequence sites and recognizes specific DNA structures, such as 4-way junctions and DNA nodes, within negatively supercoiled plasmid DNA. These structures are important for chromatin organization and our data suggest a role for U-STAT3 as a chromatin/genome organizer. Unexpectedly, we found that a C-terminal truncated 67.5-kDa STAT3 isoform recognizes single-stranded spacers within cruciform structures that also have a role in chromatin organization and gene expression. This isoform appears to be abundant in the nuclei of cancer cells and, therefore, may have a role in regulation of gene expression. Taken together, our data highlight novel mechanisms by which U-STAT3 binds to DNA and supports U-STAT3 function as a transcriptional activator and a chromatin/genomic organizer.

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Topics: Atomic force microscopy, DNA structure, STAT transcription factor, STAT3 transcription factors, DNA binding, Monolith– MicroScale Thermophoresis, MST, Proteins, Publications