The Unusual Binding Properties of the Third Distinct Teleost Estrogen Receptor Subtype ERβa Are Accompanied by Highly Conserved Amino Acid Changes in the Ligand Binding Domain

Three forms of estrogen receptor: ERα, ERβ (ERβb), and a second ERβ, ERβa (formerly ERγ) are present in teleost fish. All ERβas share amino acid changes in the ligand binding domain that may influence ligand specificity and receptor function. We compared binding specificities of the three ERs of the teleost fish, Atlantic croaker Micropogonias undulatus. Bacterially expressed Atlantic croaker (ac) ERα, -βb, and -βa fusion proteins showed specific, high affinity binding to 17β-[3H]estradiol, with Kd values of 0.61 ± 0.013, 0.40 ± 0.006, and 0.38 ± 0.059 nm, respectively. Rank orders of binding were: diethylstilbestrol ≫ ICI182780 > 4-hydroxytamoxifen > ICI164384 > estradiol ≥ zearalenone > moxestrol > tamoxifen > estrone ≥ 17α-estradiol > estriol > 2-hydroxyestrone = genistein ≫ RU486 for acERα; ICI182780 > diethylstilbestrol > 4-hydroxytamoxifen > estradiol > ICI164384 > genistein > moxestrol > tamoxifen > zearalenone = estrone > estriol = 17α-estradiol > 2-hydroxyestrone ≫ RU486 for acERβb; and estradiol...