Ligand-dependent stabilization of the estrogen receptor (ER) is often postulated, with limited support from experimental data. We studied the thermal unfolding of recombinant ERα by circular dichroism (CD) spectroscopy. The TM of unfolding of ERα was 38 ± 2.4 °C, and the van't Hoff enthalpy of unfolding was 31.7 ± 3.4 kcal/mol in the absence of ligands. Addition of estradiol (E2) increased the TM to 43.6 ± 2.3 °C, while addition of E2 and an oligonucleotide harboring the estrogen response element (ERE) increased the TM to 47.9 ± 1.6 °C. Addition of the antiestrogen 4-hydroxytamoxifen (HT) alone did not increase the TM; however, a combination of HT and the ERE increased the TM to 48.9 ± 1.0 °C. The ERE alone increased the TM to 46.1 ± 0.9 °C. Addition of E2 alone had no effect on the apparent enthalpy of unfolding; however, the ERE alone increased the apparent enthalpy from 31.7 to 36.1 kcal/ mol. ERα samples containing the ERE also exhibited an increase in the negative ellipticity at 208 and 222 nm, relative to that of ligand-free ERα, suggesting a stabilization of the α-helix. CD data analysis further showed that the presence of the ERE caused a large increase in α-helical content of ERα in both the presence and absence of the ligands. This increase in α-helical content of ERα was not observed in the presence of a nonspecific oligonucleotide. These results show that the ERE can increase the thermal stability of ERα, enhance its α-helical content, and facilitate the cooperativity of the folding transition.
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