Interaction of proteins with transcriptionally active estrogen receptors

The ligand binding domain of the estrogen receptor contains a hormone-dependent transcriptional activation function. To investigate the mechanism by which it stimulates transcription, we have expressed fusion proteins containing either the wild-type or a transcriptionally defective form of this domain fused to glutathione-S-transferase and searched for proteins that specifically interact in vitro. By far-Western blotting, three proteins of 160, 140, and 80 kDa expressed in different mammalian cells (HeLa, ZR75-1, and COS-1) were shown to associate directly with the wild-type receptor in the presence of estrogen. Two additional proteins appeared to interact indirectly with the hormone binding domain since they were detected only by a pull-down assay. All of these interactions were abolished by antiestrogens, such as 4-hydroxytamoxifen, ICI 164384, or ICI 182780, which inhibit hormone-dependent transcription. Moreover, they were not observed with the transcriptionally defective form of the receptor even in the presence of estrogen. Thus, since the ability of these proteins to interact with the hormone binding domain correlates with its transcriptional activity, one or more of them may contribute to hormone-dependent transcriptional activation by the estrogen receptor.