Residues in the ligand binding domain that confer progestin or glucocorticoid specificity and modulate the receptor transactivation capacity

To localize regions conferring ligand binding specificity of the human glucocorticoid (hGR) and progesterone (hPR) receptors, we constructed chimeras comprising the DNA-binding domain of the yeast transcription factor GAL4, linked to the ligand binding domain of hGR or hPR. Replacement of a sequence of hGR encompassing helices H6 and H7 with the homologous sequence from hPR creates a chimeric protein GP3, which binds the progestin RU 27987 with high affinity, and results in a concomitant loss of glucocorticoid binding [dexamethasone (DEX), RU 43044]. Moreover, GP3 is not able to mediate RU 27987-induced transactivation. A detailed mutational analysis of this sequence and the study of the recently solved hPR crystal structure revealed five residues that confer progestin responsiveness to GR or modulate ligand binding and transcriptional activation. Notably, the simultaneous presence of residues Ser637 and Phe639 on GP3, lining the ligand binding pocket, is specifically involved in RU 27987 binding. The absence of residues Asp641, Gln642, and Leu647 on GP3 is accountable for the lack of glucocorticoids binding on GP3. Unlike residue 642, residues 641 and 647 are not in direct contact with the ligand and most likely act through steric-mediated interactions. The presence of Gln642 and Leu647 are determinant for transcriptional activation in response to DEX and RU 27987, respectively. DEX-dependent transactivation is further enhanced by the presence of Leu647.