Phosphorylation of the chicken progesterone receptor

We have examined the phosphorylation of the chicken progesterone receptor in tissue slices and in vitro. The receptor is phosphorylated in tissue slices and this phosphorylation is stimulated by progesterone. As others have reported, partially purified receptor preparations contain a kinase activity which phosphorylates histones and receptor. We have shown that this activity can be separated from the receptor. The receptor is a substrate for several kinases, including the catalytic subunit of the cAMP-dependent protein kinase and PPdPK, a polypeptide-dependent protein kinase. Phosphorylation by the cAMP-dependent protein kinase results in an apparent increase in the molecular weight of the receptor when the receptor is analyzed by SDS-PAGE. These results are consistent with apparent changes in molecular weight observed for rabbit and human progesterone receptor upon treatment of tissue or cells with hormone.