Estrogen Receptors in the Rat Uterus. Relationship between Cytoplasmic and Nuclear Forms of the Estrogen Binding Protein

In the presence of 4 M urea, 1 M KCl, and reducing agent, both the cytoplasmic and nuclear forms of the uterine estrogen receptor sediment at 3.6 S, suggesting that both forms contain a common subunit which binds estrogens. In the presence of 0.4 M KCl, mixing experiments with cytosol preparations and nuclear extracts do not produce any change in the sedimentation of either form of the receptor. However, if mixing experiments are performed in dilute buffer solutions without added salt, the nuclear “5S” receptor sediments as an 8S species similar to that observed for the cytoplasmic form under the same conditions.