Identification of charged residues in an N-terminal portion of the hormone-binding domain of the human estrogen receptor important in transcriptional activity of the receptor.

We have shown that charged amino acids near C530 of the human estrogen receptor (ER) are involved in receptor discrimination between estrogen and antiestrogen. We now examine the role of charged residues, adjacent to the three other cysteines (381, 417, and 447) in the hormone-binding domain of the human ER, in the hormone-binding, DNA-binding, and transcription activation abilities of the receptor. Mutation of the one charged amino acid nearest to C381 gave a mutant receptor (E380Q) requiring two to three times less estradiol (E2) than wild type (WT) ER to achieve maximal activity and having activity in the absence of added estrogen that was 6-fold higher than that of WT receptor. The enhanced ability of this mutant to bind to estrogen response element DNA in the absence and presence of estrogen may, at least in part, explain its elevated, seemingly constitutive trans-activation activity and its increased sensitivity to estrogen. While more sensitive to E2, this E380Q mutant was less sensitive than WT ER to antiestrogen for suppression of transcriptional activity. Mutation of all three charged residues nearest to C381 (the triple mutant D374N, E380Q, and E385Q) resulted in a greatly reduced potency of the receptor in trans-activation with no change in estrogen-binding affinity. When K449 (near C447), highly conserved among steroid receptors, was mutated to Q, 400-fold more E2 was required for maximal reporter gene trans-activation due to an unstable, temperature-sensitive hormone-receptor complex. In contrast, the mutant K416Q (near C417) was unaltered in E2-binding or receptor transcriptional activity. These studies reveal a region in the N-terminal portion of the hormone-binding domain (ca. amino acids 374-385) where alterations in charged residues result in either increases or decreases in receptor transcriptional activity with no change in receptor affinity for hormone. Our findings suggest that this region may be important in DNA binding and protein-protein interactions that modulate transcriptional activity of the ER. In addition, the region near C447, which is well conserved among steroid receptors, appears to be important in maintaining the receptor in a conformation that is stable at physiological (37 C) temperatures. To our knowledge, this is the first report of an ER (E380Q) with a sensitivity to E2 for trans-activation greater than that of WT receptor and having high trans-activation activity in the absence of added hormone.