Estrogen Receptors in the Rat Uterus. Multiple Forms Produced by Concentration-Dependent Aggregation

The sedimentation coefficient of the estradiol receptor proteins from rat uterine cytosol is highly variable and concentration dependent. In dilute buffer solutions without added salt the value of the sedimentation coefficient varies from 7 to 9 S, in 0.15 M KCl solutions the values vary from 3.8 to 7 S, and in 0.4 M KCl solutions the values vary from 3.8 to 5.2 S. The observed concentration dependence seems to result from a slow aggregation of estradiol binding proteins following tissue homogenization. The aggregation does not appear to be self-association of estrogen binding proteins, but seems rather to represent an interaction of estrogen binding units with other proteins present in uterine cytosol. Protein present in the soluble fraction of nontarget tissues appears to interact with uterine estradiol receptors to a more limited extent. The time-dependent aggregation can be minimized by working with dilute solutions. This suggests that in vivo the uterine estradiol receptor may exist as a 3.8-4.8S species, rather than 8 S as previously believed.