Differential Requirements of Hsp90 and DNA for the Formation of Estrogen Receptor Homodimers and Heterodimers

The two estrogen receptor (ER) subforms, ER alpha and ER beta, are capable of forming DNA-binding homodimers and heterodimers. Although binding to DNA is thought to stabilize ER dimers, how ER alpha/alpha, ER beta/beta, and ER alpha/beta dimerization is regulated by DNA and the chaperone protein Hsp90 is poorly understood. Using our highly optimized bioluminescence resonance energy transfer assays in conjunction with assays for transcriptional activation of ERs, we determined that DNA binding appears to play a minor role in the stabilization of ER dimers, especially in the case of ER beta/beta homodimers. These findings suggest that ER dimers form before they associate with chromatin and that DNA binding plays a minor role in stabilizing ER dimers. Additionally, although Hsp90 is essential for the proper dimerization of ER beta/beta and ER alpha/beta, it is not required for the proper dimerization of ER alpha/alpha. Despite this, Hsp90 is critical for the estrogen-dependent transcriptional activity of the ER beta/beta homodimer. Thus, Hsp90 is implicated as an important regulator of distinct aspects of ER alpha and ER beta action.