To determine whether accessory proteins mediate the ligand- and DNA sequence-dependent specificity of estrogen receptor (ER) interaction with DNA, the binding of partly purified vs highly purified bovine ER to various estrogen response elements (EREs) was measured in the presence of different ER ligands. Partly purified estradiol-liganded ER (E2-ER) binds cooperatively to stereoaligned tandem EREs flanked by naturally occurring AT-rich sequences, with a stoichiometry of one E2-ER dimer per ERE. In contrast, highly purified E2-ER binds with a 10-fold lower affinity and non-cooperatively to EREs flanked by the AT-rich region. Moreover, the binding stoichiometry of highly purified E2-ER was 0.5 E2-ER dimer, or one monomer per ERE, independent of the ERE flanking sequence. Interestingly, the binding of ER liganded with the antiestrogen 4-hydroxytamoxifen (4-OHT-ER) was non-cooperative with an apparent stoichiometry of 0.5 4-OHT-ER dimer per ERE, regardless of ER purity or ERE flanking sequence. We recently showed that when 4-OHT-ER binds DNA, one molecule of 4-OHT dissociates from the dimeric 4-OHT-ER-ERE complex, accounting for the reduced apparent binding stoichiometry. In contrast, ER covalently bound by tamoxifen aziridine (TAz) gave an ERE binding stoichiometry of one TAz-ER dimer per ERE, and TAz-ER binds cooperatively to multiple AT-rich EREs, regardless of the purity of the receptor. We have obtained evidence that purification of ER removes an accessory protein(s) that interacts with ER in a sequence- and/or DNA conformational-dependent manner, resulting in stabilization of E2, but not 4-OHT, in the ligand binding domain when the receptor binds to DNA. We postulate that retention of ligand by ER maintains the receptor in a conformation necessary to achieve high-affinity, cooperative ERE binding.
J.H.A. and C.M.K. should be considered equal first authors on this work. Current address: Dept. Biochemistry, University of Louisville School of Medicine, Louisville, KY 40292, U.S.A.
