The Transformed Glucocorticoid Receptor Has a Lower Steroid-Binding Affinity Than the Nontransformed Receptor

High-salt treatment of cytosolic glucocorticoid receptor (GR) preparations reduces the steroid-binding ability of the receptor and induces the conversion of the receptor from a nontransformed (non-DNA-binding) 9S form to a transformed (DNA-binding) 4S entity. Therefore, we decided to investigate the possible relationship between these two phenomena. Steroid-free GR was converted from a 9S to a 4S form by exposure to 0.4 M NaCl. The binding of [³H] triamcinolone acetonide ([³H]TA) to the 9S form was almost saturated at a concentration of 20 nM, whereas [³H]TA was hardly bound to the 4S form at this concentration. The 4S form was efficiently labeled at 200 nM. Scatchard analysis of the GR exposed to 0.4 M NaCl in the presence of 10 mM molybdate showed the presence of two types of binding sites with apparent dissociation constants of 0.52 ± 0.07 and 64.1 ± 16.2 nM, respectively. In the absence of molybdate, the ratio of the lower affinity site was increased, but the total number of binding sites was not modified. The GR with the low [³H]TA-binding affinity bound to DNA-cellulose even in its unliganded state, whereas the form with the high affinity did not. Immunoblot analysis using anti-GR monoclonal antibody revealed no difference in molecular size (M_r 94 000) between the high- and low-affinity entities. These results indicate that the transformed GR has a reduced [³H]TA-binding affinity as compared to the nontransformed GR. The steroid-binding domain (amino acids 477-777) and the DNA- and steroid-binding domains (amino acids 415-777) of the human GR were expressed in Escherichia coli as protein A fused proteins. The apparent dissociation constant of these proteins (70 nM) was similar to that of the native transformed rat GR. Taken together, these results suggest that the component(s) associating with the nontransformed GR, possibly the heat shock protein hsp 90, play(s) an important role in stabilizing the GR in a high-affinity state for steroids.