Activation of the dioxin and glucocorticoid receptors to a DNA binding state under cell-free conditions.

The activation in vitro of dioxin and glucocorticoid receptors from a non-DNA binding to a DNA binding state was characterized. Ligand-free dioxin and glucocorticoid receptors were partially co-purified from rat liver cytosol, and both receptors sedimented at 9 S following labeling with the respective ligand. The 9 S forms of the dioxin and glucocorticoid receptors have previously been shown to represent heteromeric complexes containing the Mr approximately equal to 90,000 heat shock protein. The 9 S ligand-free or ligand-bound glucocorticoid receptor was converted to the monomeric 4-5 S form upon exposure to 0.4 M NaCl even in the presence of the stabilizing agent molybdate. Under identical conditions, the 9 S ligand-free and ligand-bound dioxin receptor forms remained essentially intact. However, in the absence of molybdate, the dioxin receptor could be converted to a 4-5 S form upon exposure to high concentrations of salt. These results indicate that the glucocorticoid receptor readily dissociates from the 9 S to the 4-5 S form even in the absence of hormone, whereas both the ligand-free and ligand-occupied 9 S dioxin receptor forms represent more stable species. Gel mobility shift experiments revealed that the 4-5 S glucocorticoid receptor interacted with a glucocorticoid response element both in the absence and presence of ligand. On the other hand, occupation of the dioxin receptor by ligand greatly enhanced the ability of the receptor to be activated to a form that binds to its target enhancer element. Once dissociated, the monomeric form of the dioxin receptor was also able to interact with its DNA target sequences even in the absence of ligand. Thus, ligand binding efficiently facilitates subunit dissociation of the dioxin receptor but is not a prerequisite for DNA binding per se. Given the apparent stability of its non-DNA binding 9 S form, the dioxin receptor system might be a useful model for the investigation of the mechanism of activation of soluble receptor proteins.