Targeted mutation of key residues at the start of helix 12 in the hERalpha ligand-binding domain identifies the role of hydrogen-bonding and hydrophobic interactions in the activity of the protein.

Estradiol (E(2)) and tamoxifen exert their effects through two members of the nuclear receptor superfamily, estrogen receptor (ER)-alpha and -beta. We want to identify the key interactions linking ligand-binding and activity of the ERalpha. Asp-351 and Leu-536 participate in hydrogen bond (Asp-351) and hydrophobic (Leu-536) interactions at the start of helix 12 in the ligand-binding domain (LBD) of the ERalpha. Mutations at each position alter ER activity, but we do not know which is more important. We mutated these residues in combination and individually and assessed the activity of the mutated ERs in the absence and presence of E(2) and 4-OHT on an ERE-driven and an AP-1-driven promoter, as well as their ability to interact with coregulators. On an ERE-driven promoter, the residue at position 351 determined whether E(2) stimulated or reduced the activity of the ER, as well as the level of activity in the presence of 4-OHT. Surprisingly, mutation of both residues generally did not produce cumulative deleterious effects, and they exerted counterbalancing effects on the basal activity on both promoters. Our results identify the contributions of specific interactions to the activity of the hERalpha, and support the concept that this region couples ligand-binding with ER activity.