Phosphorylation of calf uterus 17 beta-estradiol receptor by endogenous Ca2+-stimulated kinase activating the hormone binding of the receptor

Direct evidence is presented that uterus 17_β-estradiol receptor is phosphorylated $\text{in}\text{,}\text{vitro}$ by an endogenous kinase. Nuclear phosphatase, cytosol Ca²⁺-stimulated kinase (the former inactivating and the latter reactivating the hormone binding of the 17_β-estradiol receptor) and receptor were purified from calf uterus. 17_β-estradiol binding was inactivated by phosphatase, then reactivated by kinase in the presence of _γ-³²P] ATP, Ca²⁺ and calmodulin, and the receptor was examined by various methods. The results of gel electrophoresis in non denaturating and denaturating conditions, and of centrifugation through sucrose gradients of receptor preincubated with monoclonal antibodies showed that the receptor is phosphorylated.