ABSTRACT
Highly purified progesterone receptor has been shown to be a phosphoprotein containing covalently labeled phosphoserine. Uterine estradiol receptor is reported to be phosphorylated on tyrosine residues in vitro by a purified calmodulin-stimulated nuclear kinase. The mechanisms of phosphorylation/dephosphorylation of steroid receptors have been linked to both its ligand binding activity and receptor activation. The two less well-studied steroid receptors in the context of receptor phosphorylation, the androgen and estrogen receptors, have also been shown to be phosphoproteins. It is increasingly clear that sex-steroid hormone receptors function as regulatory proteins within responsive cells such as those of the breast and uterus. Uterine estradiol receptor is reported to be phosphorylated on tyrosine residues in vitro by a purified calmodulin-stimulated nuclear kinase. The protein kinase (PK) activity exhibited by this preparation was highly dependent on adenosine triphosphate (ATP), since guanosine triphosphate (GTP) did not serve as a phosphate donor.
