PHOSPHORYLATION OF SER(530) FACILITATES HORMONE-DEPENDENT TRANSCRIPTIONAL ACTIVATION OF THE CHICKEN PROGESTERONE-RECEPTOR

Phosphorylation regulates the activities of many proteins, including transcription factors. However, the evidence for the significance of phosphorylation in steroid hormone receptor action is mainly indirect. In this study, one of the hormone-induced phosphorylation sites of chicken progesterone receptor, Ser(530), was mutated to alanine, a nonphosphorylatable amino acid, and the transcriptional activity of the mutant receptor was compared with that of wild type in a transient cotransfection assay. The results showed that this mutation resulted in reduced transcriptional activity of chicken progesterone receptor at low hormone concentrations but did not affect the maximal activity of the receptor at saturating levels of hormone, suggesting that the phosphorylation at Ser(530) influences the response of the receptor to its ligand. The decreased sensitivity of the mutant receptor is not due to a decrease in hormone-binding affinity, leading to our hypothesis that Ser(530) phosphorylation stabilizes the receptor in its active state, perhaps by preventing its reassociation with heat shock proteins or by maintaining a conformation suitable for interaction with other transcription factors.