The active metabolite of tamoxifen, 4-hydroxytamoxifen (4-OHT), is used in the laboratory for mechanistic studies of antiestrogen action. This compound binds to the estrogen receptor alpha (ER) and silences activating function 2 (AF2) in the ligand binding domain, but activating function 1 (AF1) at the other end of the ER remains constitutive and is considered to be ligand independent. Amino acid D351 in the ligand binding domain appears to be critical for interactions with the antiestrogenic side chain of antiestrogens. We have devised an assay to evaluate the biological activity of 351 mutant ERs and antiestrogens at the transforming growth factor alpha (TGFalpha) gene in situ (J. I. MacGregor Schafer et al., Cancer Res., 59: 4308-4313, 1999). The substitution of glycine for aspartate at position 351 results in the conversion of the 4-OHT:ER complex from estrogen-like to completely antiestrogenic. In cells stably expressing D351G ER, the ER retains responsiveness to estradiol (E2) and also retains antiestrogenic responsiveness to both raloxifene and ICI 182,780. The relative binding affinity of E2 for D351G ER (0.77 +/- 0.17 x 10(-9) M) is comparable with wild-type ER (0.42 +/- 0.08 x 10(-9) M). In addition, the D351G ER retains the ability to bind SRC-1 in the presence of E2, thus D351G ER AF2 activity has not been compromised. We also used a cell line stably expressing an ER with a triple mutation in helix 12 (D538A, E542A, and D545A) that ablated AF2 activity, which resulted in decreased effects of E2, suggesting that both AF1 and AF2 activity are required for maximal estrogen activity in MDA-MB-231 cells. Interestingly, the triple mutation also completely reduced the estrogen-like actions of 4-OHT. We propose that a specific mutation at amino acid 351 can allosterically silence AF1 in the 4-OHT:ER complex by either preventing the binding of coactivators or encouraging the binding of a corepressor molecule. We suggest that the 4-OHT-specific site responsible for estrogen-like actions can be referred to as AF2b. This binding site would consist of at least four carboxylic acids at amino acids 351 and 538, 542 and 545 in helix 12 to permit coactivator docking for gene activation. The AF2b site is distinct from AF2 for E2 action. Further studies will provide insight into the estrogen-like actions of tamoxifen in select tissues and breast tumors and identify a significant mechanism of drug resistance to tamoxifen.