Phosphorylation of chicken oviduct progesterone receptor by cAMP-dependent protein kinase

Phosphorylation of immunopurified chicken oviduct progesterone receptor (PR) was studied in intact cells and under cell-free conditions. Cytosol PR was isolated by incubation with anti-PR monoclonal antibody αPR22 adsorbed to protein A-Sepharose and suspended in a reaction mixture containing 10 mm Mg²⁺, 0.1 mm [γ-³²P]ATP, and the catalytic subunit of cAMP-dependent protein kinase (cAMP-PK) from bovine heart. All three major proteins of avian PR (PR-A, 79 kDa; PR-B, 110 kDa; 90 kDa) incorporated ³²P-radioactivity on serine residues. The phosphorylation reaction was inhibited by synthetic inhibitors of protein kinases, H-8 and 20-residue peptide IP₂₀. A 40 °C preexposure of PR oligomer increased phosphorylation of the 90-kDa protein, known to be a heat-shock protein (hsp-90). The extent of the phosphorylation reaction was temperature-dependent as the ³²P-incorporation into PR-A and PR-B increased gradually, showing a maximum at 37 °C. Multiple phosphopeptides (4-7) were resolved by two-dimensional electrophoresis chromatography following cleavage of ³²P-labeled peptides with trypsin. Both A and B forms of receptor showed similar phosphorylation patterns with B receptor digestion exhibiting two to three additional peptides. Under physiological conditions, preincubation of oviduct mince with forskolin, a regulator of intracellular cAMP levels, caused a greater extent of phosphorylation of PR-A and PR-B proteins. The results of this study demonstrate that chicken oviduct PR is an excellent substrate for the action of cAMP-PK in vitro and that this enzyme may be a physiological regulator of progesterone action in the oviduct.