[show abstract][hide abstract] ABSTRACT: The mutation of a single amino acid in the ligand binding domain of the human androgen receptor (AR) can induce functional abnormalities; for example, in androgen binding or interactions with coregulators. We report here on the structure/function analysis of the ARE709K substitution that is associated with partial androgen insensitivity syndrome. We introduced several mutations at position 709 and tested the consequences of these changes on AR structure and activity in the presence of androgen and antiandrogens. Our results demonstrate that a strong interaction between helix H12 and residue 709 in H3 is required to obtain a fully functional AR. We show that glutamic acid 709 can be replaced by a bulky tyrosine residue without significant effect on the activation by agonists. In contrast, smaller or linear residues that are unable to maintain a tight interaction with H12 induce a substantial loss of androgen-induced AR activity. We also show that the agonist activity of partial antiandrogens is dependent on the side-chain residue at position 709. Strikingly, the ARE709Y substitution causes the conversion of cyproterone acetate into a pure antiandrogen and bicalutamide into a partial agonist. Together, our structural and functional data reveal the key role of glutamic acid 709 in androgenic and antiandrogenic activities.