LIGAND-DEPENDENT, TRANSCRIPTIONALLY PRODUCTIVE ASSOCIATION OF THE AMINO-TERMINAL AND CARBOXYL-TERMINAL REGIONS OF A STEROID-HORMONE NUCLEARRECEPTOR

The estrogen receptor (ER), a 66-kDa protein that mediates the actions of estrogens in estrogen-responsive tissues, is a member of a large s uperfamily of nuclear hormone receptors that function as ligand-activa ted transcription factors, ER shares a conserved structural and functi onal organization with other members of this superfamily, including tw o transcriptional activation functions (AFs), one located in its amino -terminal region (AF-1) and the second located in its carboxyl-termina l, ligand-binding region (AF-2), In most promoter contexts, synergism between AF-1 and AF-2 is required for full ER activity, In these studi es, we demonstrate a functional interaction of the two AF-containing r egions of ER, when expressed as separate polypeptides in mammalian cel ls, in response to 17 beta-estradiol (E(2)) and antiestrogen binding, The interaction was transcriptionally productive only in response to E (2), and was eliminated by point or deletion mutations that destroy AF -1 or AF-2 activity or E(2) binding, Our results suggest a definitive mechanistic role for E(2) in the activity of ER-namely, to alter recep tor conformation to promote an association of the amino- and carboxyl- terminal regions, leading to transcriptional synergism between AF-1 an d AF-2. The productive reassembly of two portions of ER expressed in c ells as separate polypeptides demonstrates the evolutionarily conserve d modular structural and functional organization of the nuclear hormon e receptors, The ligand-dependent interaction of the two AF-containing regions of ER allows for the assembly of a complete activation functi on from two distinct regions within the same protein, providing a mech anism for hormonally regulated transcription.