Neutral mutations to three acidic AF2 residues in the mouse estrogen receptor confer agonist activity to A-ring isomers of estradiol.
The ability of the estrogen receptor (ER) to function as a ligand-mediated transcription factor involves the activation function-2 (AF2) in the hormone binding domain (HBD). Although several types of ligand bind to the ER, AF2 functions selectively, as it is activated by estradiol (E2) but not by antiestrogens. The mechanism used by AF2 to interpret the chemical and structural information encoded in the bound ligand, and to transfer this information to other transcriptional regulatory proteins on the promoters of estrogen-regulated genes, is unknown at present. To address this issue, we have examined the activities of two mouse ERs with mutations in the AF2 region. One incorporated changes in three acidic residues (pJ3MOR, D542N/E546Q/D549N) on the polar face of the putative AF2 alpha-helix, whereas the other contained alterations in two hydrophobic amino acids (pJ3MOR, L543D/L544A) on the non-polar face. Transcriptional activity was measured with chloramphenicol acetyltransferase (CAT) reporter genes including a minimal (JA12) or a complex (pS2tkCAT) promoter. In transient cotransfection assays using COS-1 cells, it was found that A-ring isomers of E2, which are inactive or behave as antagonists with the wild-type ER, acted as agonists when the neutral AF2 mutant ER and the pS2tkCAT promoter were tested. On the other hand, when the mutant ER with changes in key hydrophobic residues was used with this same promoter, the estrogen antagonist, ICI 164,384, acted as an agonist. The findings in this report establish a role for acidic AF2 amino acids, and confirm the contributions made by hydrophobic residues, in the interpretation of ligand identity and in the transmission of this information to the transcriptional regulatory apparatus. Critical residues on both sides of the AF2 alpha-helix are, therefore, likely to be involved in distinguishing between ligands with either extensive or subtle alterations in structure, and in mediating this information to the regulatory proteins on estrogen-responsive genes.
Schwartz
J A
JA
Department of Biochemistry, Wayne State University, School of Medicine, Detroit, MI 48202, U.S.A.
Brooks
S C
SC
D000818:Animals; D001665:Binding Sites; D004958:Estradiol / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D009154:Mutation; D011960:Receptors, Estrogen / Q000819:agonists / Q000235:genetics* / Q000378:metabolism; D014158:Transcription, Genetic
D011960:Receptors, Estrogen; D004958:Estradiol