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Signal transductions within the cell as well as between different cells are indispensible for the existence of multi-cellular organisms as they allow a sufficient adaptation of each cell to changing environmental conditions. A variety of mechanisms that enable the cell to transmit extracellular signals into the cells are described in literature. One of these extensively investigated pathways is the JAK-STAT signal transduction. It enables the cell to detect extracellular information and to transduce them into transcriptional responses. First, receptors in the cell membrane recognize extracellular signals like interleukins or interferons. These receptors activate receptor-bound janus kinases (JAKs) in the cytoplasm by tyrosine phosphorylation. The JAKs phosphorylate proteins from the family of signal transducers and activators of transcription (STATs). The phosphorylated STAT proteins dimerize and bind to specific promoter regions in cytokine-driven target genes and modify their transcription.
In this PhD thesis, I investigated the role of conserved amino acid residues in the amino-terminal coiled-coil domain of STAT1 in interferon signaling. For this purpose several point mutations at positions 136, 137, 138, 139 and 143 as well as a double mutation at position 136/137 were introduced into a STAT1-coding plasmid by site-directed mutagenesis. The resulting mutants were expressed in HeLa cells and STAT1-negative U3A cells and characterized by means of fluorescence microscopy, western blotting, gel shift assays and transcriptional assays.
The results of these experiments showed no significant difference between the tested mutants and the wild type protein. Thus, the exchange of the above-mentioned amino acids in the amino-terminal coiled-coil domain of STAT1 does not restrict the functional integrity of STAT1 in interferon signal transduction.
These findings were also confirmed by the work of other authors demonstrating that the precise order of amino acid residues in this sequence is not of critical importance (Mertens et al., 2006). Even an extensive exchange of amino acid residues in the coiled-coil domain does not affect signal propagation by STAT1. In contrast, truncation of the sequence is leading to a persistent phosphorylation of STAT1. Hence, it can be assumed that the functional intact coiled-coil domain is required for a shift between two configurations of phosphorylated STAT1 dimer, which makes the anti-parallel dimer conformation accessible for dephosporylation. For this function, the coiled-coil domain has to be of sufficient length and flexibility. The exchange of single amino acids, however, seems to be without any effects on the amplitude and duration of interferon signals.