Résumé
Mutation of isoleucine 747 to threonine in the C-terminal part of the ligand-binding domain (LBD) of the human glucocorticoid receptor (GR) alters the ligand specificity for transactivation. Natural glucocorticoids such as cortisol or corticosterone were completely inactive with the mutant 1747T, whereas synthetic steroids like dexamethasone efficiently stimulated GR 1747T-mediated transactivation. However, the corresponding ligand dose-response curve for dexamethasone-induced transactivation was shifted to higher concentrations when compared with that obtained with the wild type GR. Neither this shift nor the inability of cortisol to activate the 1747T mutant was due to an altered in vitro ligand-binding affinity. In the canonical three-dimensional structure of nuclear receptor LBDs, isoleucine 747 is in the direct vicinity of residues that contribute to the ligand-binding pocket. Moreover, it is located in the C-terminal LBD region, which harbors the conserved core of the activation function AF-2 and undergoes a ligand-induce transconformation, required to generate the surface interacting with putative transcriptional intermediary factors/coactivators of AF-2. The phenotype of 1747T mutant is discussed in view of the possible consequences of the mutation on the various events which, according to the model, lead to a transcriptionally competent AF-2.