Polypeptide components of two 8 S forms of chicken oviduct progesterone receptor

Two 8 S forms of progesterone receptor from the chicken oviduct were purified to near homogeneity and analyzed for peptide composition by gel electrophoresis. Form I contains two major peptides with molecular weights of 90,000 and 75,000. Form II also contains two major peptides with molecular weights of 90,000 and 110,000. In glycerol gradients containing molybdate, the 90,000, 110,000, and 75,000 molecular weight peptides co-sediment with the [3H]progesterone peak at 8 S. In high salt gradients lacking molybdate, the [3H]progesterone peak co-sediments with the 110,000 and 75,000 molecular weight peptides at 4 S, while the 90,000 molecular weight peptide sediments at 6-7 S. On photoactivation, the synthetic progestin R5020 binds covalently to the 110,000 and 75,000 molecular weight peptides. Thus, both 8 S forms of progesterone receptor contain 90,000 molecular weight peptides which do not bind progesterone, and each 8 S form contains a separate form of progesterone-binding peptide. When receptor is purified from oviduct minces incubated with [32P]orthophosphate, autoradiography indicates the presence of 32P in the 90,000 and 110,000 molecular weight peptides and in a peptide which appears to be slightly larger than 75,000 and may be a more highly phosphorylated fraction of the 75,000 molecular weight peptide. Thus, three separate peptides have been identified as components of the 8 S form of progesterone receptor, and all three appear to exist as phosphoproteins.