cDNA cloning and functional analysis of a truncated STAT5a protein from autonomously growing FDCP-1 cells.

The transcription factor STAT5 is activated by multiple hematopoietic cytokine receptors and has been implicated in the induction of cellular processes such as differentiation, proliferation and antiapoptotic activities. Here, we report cloning of the cDNA and characterization of a mutant STAT5a protein that is expressed in interleukin-3 (IL-3)-independently growing FDCP-1 cells. Analysis of the cDNA revealed a deletion of both the transactivation and the SH2 domains. Stable expression of the protein in parental IL-3-dependent cells results in elevated DNA binding activity of wild type (WT)-STAT5 in the nucleus, enhanced growth rates and a reduced susceptibility to undergo apoptosis after withdrawal of IL-3. Although the protein is not present in DNA/protein complexes in the nucleus, we observed pronounced effects on IL-3-induced signal transduction. The results suggest competition of the mutant protein with cytosolic mechanisms regulating STAT5 activity. In conclusion, the data support the hypothesis of an involvement of STAT5 in mitogenic and antiapoptotic signaling.