Direct evidence of in vitro phosphorylation-dephosphorylation of the estradiol-17 beta receptor. Role of Ca2+-calmodulin in the activation of hormone binding sites.

The calf uterus estradiol-17 beta receptor is a phosphoprotein and its hormone binding activity is regulated by two endogenous enzymes both of which have been purified and characterized in detail: a nuclear phosphatase that inactivates the hormone binding sites of the receptor, and a cytosol kinase that activates these sites. Here we report that the kinase is stimulated by Ca2+ and calmodulin. Direct evidence is presented that the receptor is phosphorylated by the kinase and dephosphorylated by the phosphatase.