FIGURE 13-15 Schematic model of estrogen receptor action. Although the ER is predominantly found in the cell nucleus, it constantly shuttles between the nucleus and cytoplasmic compartments. After the ER binds to the ligand, it then dissociates from the inactive hsp 90-ER complex and forms a homodimer, (ER)2. The liganded ER homodimer then seeks out estrogen-specific hormone response elements, SRE, and forms a DNA-receptor complex that, in some fashion, stabilizes the formation of a transcription complex (TC). The TC comprises RNA polymerase II (pol II), transcription factors IIA, IIB, IID, and HE/ F, and a TATA box, all of which are located on the promoter of the gene that will be transcribed. The A/B, C, and E shown on the receptor (top portion of figure) refer to the domains of the ER (see Figures 1-26 and 13-14). [For a detailed discussion, see Tsai, M.-J., and O'Malley, B. W. (1994). Molecular mechanisms of actions of steroid/thyroid receptor superfamily members. Ann. Rev. Biochem. 63,451-486.]

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