Glucocorticoid receptor of X. laevis: possible effect of phosphorylation on hormone binding

The cytoplasmic glucocorticoid receptor of X. laevis liver has a high affinity for [3H]dexamethasone (Kd, 0.3 x 10(-8) M), and its binding specificity for a variety of steroids is similar to that found for mammalian glucocorticoid receptors. The ability of this receptor to bind [3H]-dexamethasone is stable at 0 degrees C but is rapidly lost at 10 and 20 degrees C. Alkaline phosphatase increases, whereas molybdate and tungstate decrease, the rate at which the binding activity is lost. These results are consistent with the loss of binding activity being due to dephosphorylation of the receptor. Binding of [3H]dexamethasone to the receptor does not alter the rate at which the binding activity is lost but does increase the stabilizing effect of molybdate. 100 mM molybdate lowers the apparent affinity of the receptor for [3H]dexamethasone, suggesting that molybdate can interact with the X. laevis glucocorticoid receptor. Addition of UTP, but not ATP, GTP or CTP, reactivates the receptor-binding activity, which indicates that the receptor may be phosphorylated by a UTP-dependent protein kinase.