Proposed model for cadmium induced activation of ERα.
A. Domain structure of the androgen receptor and estrogen receptor-alpha.
Estrogen receptor-alpha (ERα) and androgen receptor (AR) are divided into domains, termed A through F. The N-terminus of the receptor contains the A/B region which is a variable region that modulates transcription through a domain referred to as activation function-1 (AF-1). The DNA binding domain, region C, is a short well conserved cysteine rich region that contains two zinc fingers. In addition to DNA binding, region C plays a role in transactivation and dimerization. The C-terminus of the receptor contains regions D, E, and F. Region E is the ligand binding domain that contains the hormone inducible dimerization and activation function-2 (AF-2) domains. The ligand binding domain of ERα contains 11 alpha helices (H1, H3-H12) folded into a three layered antiparallel alpha helical sandwich surrounding the ligand binding pocket. B. Schematic model of helices H3, H4, H7 and H10-H12 of the ligand binding domain of ERα.
In the absence of estradiol (E₂), the ligand binding domain is in an inactive state. It has been proposed that, in the inactive state, the ligand binding domain of ERα is an open state with helix H10 and helix H11 in a perpendicular conformation and helix H12 positioned away from the ligand bind pocket. Upon estradiol binding, the dimerization domain of the receptor is formed by the rotation of helix H11 and the formation of a continuous bent helix with helix H10. The coactivator binding site is formed when helix H12 is repositioned under helix H4 closing the ligand binding pocket. It is proposed that, upon cadmium binding to amino acids C381, E523, H524 and D538 in the ligand binding domain, ERα undergoes similar structural changes to form the active state of the receptor.