Structural Response of the Estrogen Receptor Ligand Binding Domain to Selective Ligand Binding by Spin Label Distance Measurements

Abstract

Estrogen Receptor‐α (ER‐α) is a nuclear hormone receptor that has become an important therapeutic target for tissue‐specific inhibitors. It has been suggested that the dynamics and conformations of the helix 12 (H12) region of the ERα ligand binding domain (LBD) are affected by ligand and coregulator binding. In agonist form, H12 is stabilized by folding over the binding pocket, forming a surface where coregulators bind. Antagonists disrupt this folding, inhibiting the coregulator binding that is necessary for ER‐α to carry out its downstream effects. Selective estrogen receptor modulators (SERMs) show tissue‐selective anti‐estrogenic activity. It has been suggested that SERMs partially disrupt coactivator binding by displacing H12. To investigate this possibility, H12 was spin‐labeled at residue 543 and at residue 530 in the H12 hinge. Changes in structure induced by binding a series of agonist to antagonist ligands and coregulator peptide SRC1 were observed by measuring interspin distances using double electron‐electron resonance (DEER). DEER showed that ligand binding induces dynamic changes that directly correlate with the ligand's biological activity. These results demonstrate the importance of characterizing receptor‐ligand dynamics that are unavailable from standard structural methods.