Conformational and electrostatic properties of unoccupied and liganded estrogen receptors determined by aqueous two-phase partitioning.

The technique of aqueous two-phase partitioning (ATPP) has been used to characterize conformational and electrostatic properties of unoccupied and liganded rat uterine estrogen receptors. The adaptation of the hydroxylapatite receptor assay with ATPP systems has permitted estrogen receptor (ER) partition coefficients to be accurately determined, even when the partitioning process results in significant loss of ER binding capacity. The pH and salt dependences of estrogen receptor partition coefficients indicate that the theory governing partitioning behavior can be accurately applied to partitioning data obtained with crude cytosols. This technique has revealed a ligand-induced change in the properties of the unoccupied receptor that precedes the process of heat-induced transformation in vitro. The difference in partitioning behavior between unoccupied and nontransformed estrogen receptor is observed in all combinations of buffers and salts tested and is of equal magnitude as the difference between partition coefficients of nontransformed and transformed ER. The partition coefficients of both unoccupied and nontransformed ER are constant over the ER concentration range in which binding cooperativity has been previously demonstrated. The combined effects of salt and pH on ER partition coefficients indicate a pI of approximately 5.5 for both unoccupied and nontransformed estrogen receptors. However, the partition coefficients at the pI differ. It is concluded that estradiol binding to its unoccupied receptor results in a change in surface properties of the ER monomer that is independent of receptor transformation and makes the receptor less hydrophobic.