PHOSPHORYLATION OF HUMAN PROGESTERONE-RECEPTOR BY CYCLIN-DEPENDENT KINASE-2 ON 3 SITES THAT ARE AUTHENTIC BASAL PHOSPHORYLATION SITES IN-VIVO

The human progesterone receptor (hPR) in T47D breast cancer cells is p hosphorylated on at least nine different serine residues. We have prev iously reported the identification of five sites; three are hormone in ducible (Ser(102), Ser(294) and Ser(345)), and their phosphorylation c orrelates with the timing of the change in receptor mobility on gel el ectrophoresis in response to hormone treatment. The other two sites, S e-81 and Ser(162), along with the remaining sites, are basally phospho rylated and exhibit a general increase in phosphorylation in response to hormone. With the exception of Ser(81), all of these sites are in S er-Pro motifs, suggesting that proline-directed kinases are responsibl e for their phosphorylation. We now report that cyclin A-cyclin-depend ent kinase-2 complexes phosphorylate hPR-B in vitro with a high stoich iometry on three sites that are authentic basal sites in vivo. One of these is Ser(162), which has been described previously. The other two sites are identified here as Ser(190) and Ser(400). The specificity an d stoichiometry of the in vitro phosphorylation suggest that hPR phosp horylation may be regulated in a cell cycle-dependent manner in vivo.