Estrogen Receptor of Cow Uterus

II. Characterization of a Cytoplasmic Factor which Binds with Native “4S” Estrogen Receptor to Give “8S” Estrogen Receptor

Abstract

Partial purification and characterization of a cytoplasmic component of cow uterus, which specifically binds with native “4S” estrogen receptor (ER) to give “8S” ER was carried out. The component was designated as “8S” ER-forming factor [(“8S”ER)-FF]. (“8S”ER)-FF did not bind estradiol by itself. The Stokes radius of (“8S”ER)-FF was estimated to be approximately 51 A. (“8S”ER)-FF sedimented at around 6.9S in sucrose gradient centrifugation under hypotonic conditions. The molecular weight and frictional ratio of (“8S”ER)-FF were calculated to be 145, 000 and 1.47, respectively. (“8S”ER)-FF was degraded by trypsin treatment, but was not affected by DNase or RNase. The Stokes radius of “8S” ER was estimated to be approximately 68 A. The molecular weight and frictional ratio of “8S” ER were calculated to be 225, 000 and 1.69, respectively. “8S” ER was estimated to be a 1:1 complex between native “4S” ER and (“8S”ER)-FF. It was estimated that in the cow uterine cytosol, there was at least a 3-fold molar excess of (“8S”ER)-FF over the amount of “4S” ER. (“8S”ER)-FF inhibited both the Ca²⁺-dependent and -independent modifications of native “4S” ER by the cytoplasmic protease.