CHARACTERIZATION OF THE AMINO-TERMINAL TRANSCRIPTIONAL ACTIVATION FUNCTION OF THE HUMAN ESTROGEN-RECEPTOR IN ANIMAL AND YEAST-CELLS

We have previously reported that the transcriptional activation function AF-1, located in the A/B region of the human estrogen receptor, exhibits cell type and promoter context specificity in both animal cells and yeast. To further characterize AF-1, we have constructed a number of deletion mutants spanning the A/B region in the context of either the whole human estrogen receptor or the A/B region linked to the GAL4 DNA binding do main, and tested their transcriptional activity in chicken embryo fibroblasts and in yeast cells, two cell types in which AF-1 efficiently activates transcription on its own. Additionally, we utilized HeLa cells in which AF-1 is poorly active but can synergize with the transcriptional activation function AF-2 located in the hormone binding domain. We show that in animal cells the ''independent'' activity of AF-1 is embodied in a rather hydrophobic proline-rich 99-amino acid activating domain (amino acids 51-149), whereas amino acids 51-93 and 102-149 can independently synergize with AF-2. Interestingly, in yeast, three discrete activating domains (amino acids 1-62, 80-113, and 118-149) are almost as active on their own as the whole A/B region, indicating that multiple activating domains can operate independently in yeast. Our study also demonstrates that, within the context of the whole human estrogen receptor, the same AF-1 activating domains are ''induced'' by either estradiol or 4-hydroxytamoxifen.