A DNA-Binding (R-I) and a Non-DNA-Binding (R-II) Estrogen Receptor in the Goat Uterine Nucleus: Purification and Characterization

Two forms of nuclear estrogen receptors have been isolated and purified from the goat uterus. The biochemical characteristics of the proteins imply that the receptors may be identified as the type I and type II nuclear estrogen receptors. Nevertheless, we felt a necessity to exercise caution in using this nomenclature and, therefore, decided to identify them instead as R-I and R-II, respectively. While R-I is the DNA-binding form, R-II is a non-DNA-binding protein. The two proteins are totally dissimilar in other physical characteristics like the Stokes radii (36 Å for R-I and 21 Å for R-II), sedimentation coefficients (4.8 S for R-I and 3.8 S for R-II), the Kd (1 nM for R-I and 2 nM for R-II), and the nature of the CNBr fragmentation of the proteins. The two proteins, however, cross-react with polyclonal antibodies raised against goat uterine estrogen receptor activation factor (E-RAF), a DNA-binding protein with no capacity to bind estradiol, originally discovered by T. N. R. V. Thampan and J. H. Clark (1981, Nature 290, 152-154). A major feature of the R-II isolation procedure is the chromatography of the protein on a heat shock protein 90-Sepharose column in the presence of molybdate ions and elution using a molybdate-free buffer. While estradiol-17β (E2) binding to R-II was inhibited by the presence of dithiothreitol and quercetin in the medium, E2-R-I interaction remained unaffected by these exposures.