Calmodulin Lobes Facilitate Dimerization and Activation of Estrogen Receptor-{alpha} [Protein Structure and Folding]

Estrogen receptor alpha (ER-α) is a nuclear hormone receptor that controls selected genes, thereby regulating proliferation and differentiation of target tissues, such as breast. Gene expression controlled by ER-α is modulated by Ca2+ via calmodulin (CaM). Here we present the NMR structure of Ca2+-CaM bound to two molecules of ER-α (residues 287-305). The two lobes of CaM bind to the same site on two separate ER-α molecules (residues 292, 296, 299, 302 and 303), which explains why CaM binds two molecules of ER-α in a 1:2 complex and stabilizes ER-α dimerization. Exposed glutamate residues in CaM (E11, E14, E84 and E87) form salt bridges with key lysine residues in ER-α (K299, K302 and K303), which is likely to prevent ubiquitination at these sites and inhibit degradation of ER-α. Transfection of cells with full-length CaM slightly enhanced the ability of estrogen to enhance transcriptional activation by ER-α of endogenous estrogen-responsive genes. By contrast, expression of either the N- or C-lobe of CaM (which individually bind to ER-α) abrogated estrogen-stimulated transcription of the estrogen responsive genes pS2 and progesterone receptor. These data suggest that CaM-induced dimerization of ER-α is required for estrogen stimulated transcriptional activation by the receptor. In light of the critical role of ER-α in breast carcinoma, our data suggest that small molecules that selectively disrupt the interaction of ER-α with CaM may be useful in the therapy of breast carcinoma.

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