Steroid binding to hepatoma tissue culture cell glucocorticoid receptors involves at least two sulfhydryl groups

The presence of a thiol in the steroid binding cavity of glucocorticoid receptors has recently been proved by our affinity labeling of Cys-656 in the steroid binding domain of rat receptors (Simons, S. S., Jr., Pumphrey, J. G., Rudikoff, S., and Eisen, H. J. (1987) J. Biol. Chem. 262, 9676-9680). Studies with the sterically small, thiol-specific reagent methyl methanethiolsulfonate (MMTS) now reveal the involvement of at least two sulfhydryl groups in steroid binding. While the dose-response curves for [3H]dexamethasone binding versus thiol reagent are normally sigmoidal, an unusual bimodal curve is obtained with MMTS in which dexamethasone binding is eliminated at low, but maintained at intermediate, MMTS concentrations. This bimodal dose-response curve demands the involvement of two (or more) thiol groups. Those receptors pretreated with intermediate concentrations of MMTS retain approximately 70% of the initial binding capacity and one-fifth the affinity for dexamethasone. Solutions of this low affinity form of receptor contain essentially no accessible -SH groups, and all of the usual covalent labeling by dexamethasone 21-mesylate of various proteins, including the receptor, is blocked. The facts, that this low affinity form of the receptor is not affected by added iodoacetamide, cannot be produced from the nonsteroid binding form of receptor simply by adding more MMTS, and displays different kinetics of formation than does the nonsteroid binding form of receptor all argue that reaction of the receptor with intermediate and low MMTS, concentrations occurs via different pathways. Nevertheless, the effects of both concentrations of MMTS on the receptor are fully reversible with added dithiothreitol. The kinetics of inhibition of [3H]dexamethasone binding at low MMTS concentrations are independent of receptor concentration, indicating an intramolecular reaction. Collectively these data suggest a model of steroid binding involving two thiols, one of which appears to be Cys-656. Low concentrations of MMTS induce the formation of an intramolecular disulfide, which prevents steroid binding, while the intermediate MMTS concentrations convert both thiols directly to mixed disulfides, and steroid binding persists. Thus, reduced thiols do not appear to be required for steroid binding if the steric bulk of the oxidized thiols is small.