Creation of "super" glucocorticoid receptors by point mutations in the steroid binding domain

Almost all modifications of the steroid binding domain of glucocorticoid receptors are known to cause a reduction or loss of steroid binding activity. Nonetheless, we now report that mutations of cysteine 656 of the rat receptor, which was previously suspected to be a crucial amino acid for the binding process, have produced "super" receptors. These receptors displayed an increased affinity for glucocorticoid steroids and a decreased relative affinity for cross-reacting steroids such as progesterone and aldosterone. The increased in vitro affinity of the super receptors was maintained in a whole cell bioassay. These results indicate that additional modifications of the glucocorticoid receptor, and probably the other steroid receptors, may further increase the binding affinity and/or specificity.