The Cytoplasmic Estradiol Receptors of Bovine Uterus: THEIR OCCURRENCE, INTERCONVERSION, AND BINDING PROPERTIES

Abstract

Abstract The interconversions and properties of uterine cytoplasmic receptor proteins have been studied. In general the properties analyzed are those pertaining to the binding of the cytosol receptors to membranes or nucleohistone. Specific cytoplasmic estradiol receptors are found in both the endometrial and myometrial layers of both mature and immature heifers. The level of receptors is highest in immature tissues. The 4 S and 9 S receptors have similar binding properties and probably represent different levels of molecular complexity of the same receptor. The 4 S and 5 S receptors have very different binding properties in that the 4 S estradiol-receptor binds membranes in a temperature dependent process requiring the presence of divalent cations, whereas the 5 S receptor binds nucleohistone in a very rapid reaction even at 4°, in the absence of divalent cations. A small amount of 4 S receptor also binds to nucleohistone but only at elevated temperatures (25°) and in the absence of divalent cations. A small measure of specificity for target organ material is seen in the binding of 4 S estradiol-receptor to membranes. No specificity for target tissue was noted for 5 S estradiol-receptor binding to nuclear material. The 4 S form of the estradiol-receptor complex is converted into the 5 S estradiol-receptor in a reaction which does not require the presence of chromatin, but is dependent upon an elevated temperature and the presence of estradiol. Immature uterine tissue does not contain the 5 S receptor if assayed directly, but if the tissue is preincubated with estradiol, the formation of the 5 S receptor can be demonstrated, arguing that the 4 S-5 S conversion occurs within the cell in a reaction requiring the presence of estradiol. Since after incubation the 5 S estradiol-receptor is present in the cytoplasm in vivo and can bind nuclear material instantaneously at 4°, we have raised the possibility that the 5 S estradiol-receptor found associated with nuclear material could have been translocated and bound during the homogenization of the tissue.