The avian progesterone receptor: isolation and characterization of phosphorylated forms

Methods have been developed for isolation of the avian progesterone receptor in the nontransformed, molybdate-stabilized state. The final step in this procedure. DEAE-Sephadex chromatography, resolves the receptor into two forms, components I and II. Analysis of these components by polyacrylamide gel electrophoresis under denaturing conditions shows that both contain a peptide with Mr = 90,000. These peptides contain phosphorylated serine residues, as shown by [32P]orthophosphate incorporation studies. When the cytosol receptor is treated with alkaline phosphatase, its steroid binding capacity is abolished. These studies show that the nontransformed progesterone receptor is a phosphoprotein and indicate that receptor phosphorylation may be important to the maintenance of steroid binding capacity.