Characterization of progesterone receptor binding to the 90- and 70-kDa heat shock proteins

In this study a model system for expression of the chicken progesterone receptor in cultured cells was developed using a quail fibroblast cell line, QT6. The chicken progesterone receptor form A expressed in QT6 cells was evaluated and determined to have a number of similarities to receptor isolated from chicken oviduct. These include hormone binding, sedimentation profile, phosphorylation pattern, heat shock protein (hsp) 70 and hsp90 associations and the ability to stimulate a reporter gene construct. Therefore, the receptor expressed in this system functioned adequately for further evaluation of the particular region (or regions) involved in hsp70 and hsp90 binding. Several receptor deletion mutants were tested for hsp70/ hsp90 binding; only the d369-659 mutant, which has the entire steroid-binding domain deleted, was unable to bind hsp90 and hsp70. Three separate regions of the steroid-binding domain were found to partially restore hsp90 and hsp70 binding to the d369-659 mutant protein. However, hsp binding was not abolished when these or other regions of the steroid binding domain were deleted individually. These findings indicate that hsp90 and hsp70 both bind to the steroid-binding domain of the receptor through interactions at multiple locations or through some structural quality that is distributed throughout this region of the protein.