DNA allosterically modulates the steroid binding domain of the estrogen receptor

The ability of DNA to allosterically alter the conformation of the estrogen receptor's (ER) steroid binding domain was investigated. Using dissociation kinetics we observed that when DNA was bound to the DNA binding domain of the rat uterine ER the rate of estrogen dissociation from the steroid binding domain increased almost 2-fold. This change in the rate of ertrogen dissociation depended on the concentration of DNA used and correlated with the thermodynamic binding affinities (K_d) of the ER for two different DNA se7 quences. We were unable to detect a DNA-induced change in the trypsin cleavage pattern of the amino terminal end of the ER. Using a whole cell dissociation kinetics assay with MCF-7 breast cancer cells we observed a 7-fold slower rate of estrogen dissociation from the ER within the cell than from the ER in vitro. This suggests that additional factors, other than DNA binding, may modify the steroid binding domain within the cell. We conclude that DNA can allosterically modulate the structure of the steroid binding domain of the ER and we hypothesize that this conformational change may be necessary for the full transcriptional activity of the ER.