Investigations addressing the cellular localization and interaction partners of the serine727-phosphorylated form of the transcription factor STAT1

The transcription factor STAT1 plays an essential role in the cellular response to interferons, cytokines that are indispensable for activation of the innate immunity. Consequently, STAT1 deficient mice are extremely susceptible to infections. Two posttranslational modifications of STAT1, the phosphorylation of the residues tyrosine 701 and serine 727, have been shown to be required for proper antiproliferative and antiviral effects of interferons. The residue serine 727 is also needed for those functions of STAT1 that are independent of interferons. For example, the programmed cell death induced by the tumor necrosis factor alpha (TNFalpha) was demonstrated to be influenced by the residue serine 727 but not on the residue tyrosine 701. While it is known that the phosphorylation of the tyrosine 701 leads to dimerization, translocation to the nucleus and DNA binding of STAT1, the molecular mechanisms of function of the serine 727 phosphorylation are less well understood. One important step towards understanding the nature of processes depending on the serine 727 of STAT1, is the analysis of the regulation of the serine 727 phosphorylation. In recent years, considerable knowledge has been accumulated on stimuli leading to serine 727 phosphorylation, though the interferon induced serine kinase and its subcellular localization remain unknown. The restriction of phosphorylation events to specific cellular compartments is important for the maintenance of specificity of the signal transduction pathways. In case of STAT1 it seems likely that such an compartmentalization of the interferon induced serine 727 phosphorylation occurs, and will be analyzed in one part of this project proposal. The subject of the other part of this proposal is the characterization of proteins that interact with STAT1 in a serine 727-dependent manner. Such proteins are expected to increase or decrease the transcriptional activity of STAT1, depending on the promoter context. They are also instrumental in helping to understand the novel interferon- independent mode of STAT1 activity, that affects the apoptotic behavior of cells in response to TNFalpha.