LIGAND AND DNA-DEPENDENT PHOSPHORYLATION OF HUMAN PROGESTERONE-RECEPTOR INVITRO

The progesterone receptor (PR), like other members of the steroid rece ptor family, is a ligand-induced transcription factor. We have demonst rated previously that progesterone-induced binding of PR to a progeste rone response element (PRE)-linked promoter stimulates RNA synthesis f rom that promoter in a cell-free transcription extract. It has been es tablished that a hormone-mediated activation of PR beyond the removal of associated heat shock proteins is essential for efficient transacti vation of the target gene. We now report that treatment with hormone l eads rapidly to multiple phosphorylations of both the A and B forms of human PR in a HeLa nuclear extract. The putative kinase is present in the transcriptional extract but fails to phosphorylate the receptor s ignificantly in the absence of specific hormone or DNA. Efficient phos phorylation of the PR occurs only in the presence of PREs, indicating that ligand-induced binding of PR to its cognate DNA response element makes it a preferred substrate for the kinase. The kinetics of the pho sphorylation reaction overlap the kinetics of hormone-dependent RNA sy nthesis from a PRE-containing target promoter in vitro. We postulate t hat ligand and DNA-dependent phosphorylation of PR is an important fun ctional event in the process leading to receptor-mediated transactivat ion of target genes.