Estrogen receptor of cow uterus. I. Characterization of native and proteolyzed "4S" estrogen receptors.

Abstract

The estrogen receptor (ER) was separated from the cytoplasmic components of the cow uterus through successive gel filtrations of the cytosol in the presence of antipain, a protease inhibitor. The partially purified ER (designated as native "4S" ER) sedimented at 4.5S under both hypotonic (low salt) and hypertonic (0.4 M KCl) conditions. The molecular weight of native "4S" ER was estimated to be approximately 82,000 with a Stokes radius of 44 A. If purification was performed in the absence of antipain, native "4S" ER was very labile to a protease present in the cytoplasm. The modified ER (designated as modified "4S" ER) sedimented at 4.5S like native "4S" ER, but had a smaller stokes radius (35 A) and a smaller molecular weight (65,000). In the presence of 0.4 M NaSCN, native "4S" ER remained intact, but modified "4S" ER dissociated and gave a fragment which retained the estradiol binding site. This fragment sedimented at 2.9S and had a Stokes radius of approximately 24 A (molecular weight, approximately 29,000). In the presence of 0.4 M NaSCN, native "4S" ER remained intact, but modified "4S" ER dissociated and gave a fragment which retained the estradiol binding site. This fragment sedimented at 2.9S and had a Stokes radius of approximately 24 A (molecular weight, approximately 29,000). In the presence of Ca2+ ions and in the absence of antipain, the native ER was modified by the cytoplasmic protease to give ERs sedimenting at 3.8-4.5S. The presence of a cytoplasmic component which specifically binds with native "4S" ER to give "8S" ER under hypotonic conditions was indicated. Modified "4S" ER was unable to bind with this component.