In regulating gene expression in mammalian tissues, steroid hormones bind to cytoplasmic receptors and the resulting complex associates with the nucleus. In an attempt to study this nuclear association under more precisely controlled conditions, we examined the binding of receptor-[(3)H]steroid complexes to isolated nuclei. Two systems were used: the glucocorticoid-responsive hepatoma cell and the estrogen-sensitive immature rat uterus. Cell-free nuclear binding resembles that observed in the intact cell; it requires the receptor-steroid complex in the appropriate form (“active” complex); it is of high affinity; and it appears to involve about 4000 nuclear acceptor sites per haploid genome. Furthermore, whether binding occurs in intact cells or in cell-free extracts, complexes dissociate from nuclei at similar rates, and the ability of NaCl to extract either type of nuclear-bound complex and the sedimentation velocities of the extracted complexes are identical. Despite these similarities, acceptor sites detected in isolated nuclei differ from those of intact cells, since in neither hepatoma cells nor in the uterus do complexes bound to the nuclei of intact cells prevent further cell-free binding. We conclude that receptor-steroid complexes bind to acceptor sites in isolated nuclei that are chemically similar to, but topographically different from, those of the intact cell
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