Estrogen-binding Protein of the Rat Uterus

When 3H-estradiol is administered to rats or incubated with rat uteri, the bulk of the labeled estrogen is found in the nuclear fraction. In previous work, nuclei prepared in media containing low salt (0.1 m or less KCl) were then extracted with a high salt medium (0.4 m KCl) to obtain an estrogen-protein complex sedimenting at approximately 5 S. We have now found that homogenizing the uteri directly in the high salt media results in the extraction of an estrogen-protein complex that sediments at ∼6 S. The cytoplasmic uterine binding protein, when treated with high salt media, is principally 4 S but contains approximately 20% 6 S. The 6 S, 5 S, and 4 S entities all have approximately the same affinity for estrogen (Kd, 2 to 4 x 10-10). These and other experiments suggest that, although the relationship between the 8 S, 6 S, 5 S, and 4 S forms of estrogen-binding protein is not clear, they all contain a common estrogen binding site and likely represent different states of aggregation of one or more types of protein subunits.