Location of the Second Steroid-Binding Site on the Glucocorticoid Receptor*

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Abstract
The ability of nonradioactive progesterone to accelerate the dissociation of tritiated dexamethasone from the agonist-binding site of the glucocorticoid receptor was used as a probe for the existence of a second antagonist steroid site. The goal was to evaluate if this second site was on the glucocorticoid receptor and, if so, on what region of that protein. The magnitude of acceleration of dissociation was assessed during purification of the receptor as well as with the untransformed (multimeric), transformed (monomeric), and mero-receptor. In each preparation progesterone caused a statistically significant increase in the rate of agonist dissociation. The documentation of acceleration of dissociation during purification suggests that progesterone is interacting with a site on the glucocorticoid receptor and not with another protein. Evidence for the second binding site was found using each form of the receptor, untransformed, transformed, and mero-receptor, suggesting that the second binding site resides on the steroid-binding domain of the receptor which is on the carboxy-terminus, topographically close to the agonist-binding site.