Glucocorticoids, a class of steroid hormones, associate specifically with intracellular receptors, facilitating a conformational change that converts the receptor in vitro to a DNA-binding protein1 and in vivo to a nuclear species2 that activates a class of transcriptional enhancers termed glucocorticoid response elements3,4 (GREs). The DNA sequences recognized specifically by the hormone-receptor complex correspond directly to those required for GRE enhancement4,5. The structural transition that accompanies steroid binding, 'receptor transformation', has been monitored by changes in receptor chromatographic properties6, accessibility to monoclonal antibodies7, association with other receptor subunits or with heterologous proteins8,9, add aqueous two-phase partition coefficient10,11. However, the significance of the structural change for the biological activity of the receptor is not understood. We have used cloned rat glucocorticoid-receptor coding sequences12 to produce and characterize a novel class of receptor mutants that elicit GRE enhancer function in transfected cells even in the absence of hormone. The constitutive activity of those receptor derivatives, together with mapping studies that distinguish between the DNA- and hormone-binding domains of the receptor, imply that the conformational change corresponding to receptor transformation may simply unmask pre-existing functional domains for DNA binding, enhancer activation, or both.
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