Human estrogen receptor forms multiple protein-DNA complexes.

A baculovirus expression system was used to overproduce the human estrogen receptor in insect cells. The estrogen receptor made in this system is full-length, binds estrogen specifically, and is recognized by a monoclonal antibody to the human estrogen receptor. The recombinant estrogen receptor binds the estrogen response element (ERE) in both the absence and presence of estrogen if the binding is carried out in the absence of Mg2+. In the presence of Mg2+, the estrogen receptor binds the ERE in a hormone-dependent fashion. This effect is more pronounced at higher temperatures. Tamoxifen, a nonsteroidal anti-estrogen, is able to stimulate ERE binding to the same extent and under the same conditions as estradiol. Estradiol stimulates formation of an estrogen receptor-ERE complex with an increased mobility in native gels as compared with the complex formed without hormone or with tamoxifen. These results demonstrate that specific DNA binding of the estrogen receptor is not absolutely dependent on the presence of hormone and that estradiol but not tamoxifen is able to induce a change in the estrogen receptor. This differential effect of estradiol and tamoxifen may be important in understanding the role of the receptor to activate target genes differentially.