Identification of a hormone-dependent phosphorylation site adjacent to the DNA-binding domain of the chicken progesterone receptor

We have previously identified three phosphorylation sites in chicken progesterone receptor (PR) isolated from the cytosol of chicken oviducts. Several lines of evidence suggested that there might be additional phosphorylations that occur only in the nuclear compartment. We have reexamined the phosphorylation of the PR using whole cell extracts and have identified a second hormone-dependent phosphorylation site (Ser367) which is located on the amino-terminal side of the DNA-binding domain. This site also contains a Ser-Pro consensus sequence, as do the three previously reported sites. There are four Ser-Pro sequences in chicken PR, and all four are phosphorylated. Two of the sites showed substantial phosphorylation in the absence of hormone, with some enhancement in response to hormone, whereas two of the sites showed very little phosphorylation in the absence of hormone. This suggests either that despite the Ser-Pro consensus sequences, the sites are phosphorylated by different enzymes, or that binding hormone changes the conformation of the protein sufficiently to expose the two hormone-dependent phosphorylation sites.