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Interaction between Estrogen Receptor and Subcellular Structures of Target Cells: Nuclear Localization of Unoccupied Receptor and Its Modification Induced by Estradiol a

Experimental conditions affecting the partitioning of the estrogen receptor were studied. Homogenization of rat uteri at 25 degrees C resulted in a particulate partitioning of the estrogen receptor. The use of frozen tissue (-70 degrees C) or pre-exposure of the tissue to 0 degrees C prior to 25 degrees C homogenization, homogenization at 0 degrees C and tissue dilution all induced soluble partitioning of the receptor. The estrogen receptor found in the particulate fraction was mostly associated with the nuclei, even in the absence of hormone. The interaction between estradiol and the estrogen receptor induced modification in the receptor's charge and size that promoted its cold-insensitive association with the nuclei of target cells. These modifications were studied in a cell-free in vitro system and were reversibly blocked by molybdate. Similar changes occurred in vivo when estradiol interacted with the receptor in the nuclei of target cells.