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[摘要]：The estrogen receptor alpha (ER-alpha) regulates expression of target genes implicated in development, metabolism, and breast cancer. Calcium-dependent regulation of ER-alpha is critical for activating gene expression and is controlled by calmodulin (CaM). Here, we present the NMR structures for the two lobes of CaM each bound to a localized region of ER-alpha (residues 287-305). A model of the complete CaM alpha ER-alpha complex was constructed by combining these two structures with additional data. The two lobes of CaM both compete for binding at the same site on ER-alpha (residues 292, 296, 299, 302, and 303), which explains why full-length CaM binds two molecules of ER-alpha in a 1:2 complex and stabilizes ER-alpha dimerization. Exposed glutamate residues in CaM (Glu(11), Glu(14), Glu(84), and Glu(87)) form salt bridges with key lysine residues in ER-alpha (Lys(299), Lys(302), and Lys(303)), which are likely to prevent ubiquitination at these sites and inhibit degradation of ER-alpha. Mutants of ER-alpha at the CaM-binding site (W292A and K299A) weaken binding to CaM, and I298E/K299D disrupts estrogen-induced transcription. CaM facilitates dimerization of ER-alpha in the absence of estrogen, and stimulation of ER-alpha by either Ca2+ and/or estrogen may serve to regulate transcription in a combinatorial fashion. |
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