Abstract
The ligand-free estrogen receptor (ER), like other steroid receptors, interacts with the 90-kDa heat shock protein hsp90 in vitro. Analysis of the effect of potential ER-hsp90 interactions in vivo on receptor function is complicated by the fact that hsp90 binds to ER domains required for hormone binding and stable DNA binding. ER chimeras were therefore created by replacing the ER DNA binding domain with that of GAL4. In addition, the N-terminal AF-1 domain of the ER was replaced with the strong constitutive activation domain of VP16 to create VP16-GAL-ERs. These chimeras bind DNA in a ligand-independent manner, but, importantly, are ligand-dependent transactivators, unlike VP16-GAL, which displays strong constitutive activity under the same conditions. Hormone induces transactivation by VP16-GAL-ERs to levels similar to the constitutive activity of VP16-GAL. Glycerol gradient and coimmunoprecipitation experiments showed that, unlike the wild-type ER, VP16-GAL-ER chimeras do not interact with hsp90. Deletion analyses indicate that a region of the ER, primarily between amino acids 370 and 470, is responsible for repressed transcription. Our results suggest that interaction with hsp90 is not necessary for controlling hormone-dependent transcription by the ER and provide evidence for repressor factors that interact with the N-terminal portion of the receptor's ligand binding domain in the absence of hormone.
Highlights
The ligand-free estrogen receptor (ER), like other steroid receptors, interacts with the 90-kDa heat shock protein hsp90 in vitro
We have created ER chimeras that are functional in vivo, and that do not interact with hsp90 in vitro, by replacing the ER DNA binding domain with that of the yeast transactivator GAL4
Any hormone-free GAL-ER chimeras, which do not interact with hsp90, would be expected to be nuclear and possibly DNA-bound
Summary
The ligand-free estrogen receptor (ER), like other steroid receptors, interacts with the 90-kDa heat shock protein hsp in vitro. Our results suggest that interactionwithhsp90isnotnecessaryforcontrollinghormonedependent transcription by the ER and provide evidence for repressor factors that interact with the N-terminal portion of the receptor’s ligand binding domain in the absence of hormone. Gene transfer experiments have shown that the receptor can be nuclear in the absence of hormone, and can bind DNA, providing evidence for the presence of ligand-free ER homodimers [13, 14]. We have created ER chimeras that are functional in vivo, and that do not interact with hsp in vitro, by replacing the ER DNA binding domain with that of the yeast transactivator GAL4. Our results suggest that interaction with hsp is not necessary for controlling hormone-dependent transcription by the ER and, provide evidence for transcriptional repressors that interact with the ligand binding domain of the receptor in the absence of hormone
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