University of Tennessee, Knoxville, TN 37996, USA.
Proc Natl Acad Sci U S A. 2012 Apr 17;109(16):6084-7. doi: 10.1073/pnas.1119852109. Epub 2012 Apr 2.
Thyroid hormones such as 3,3',5 triiodo-L-thyronine (T3) control numerous aspects of mammalian development and metabolism. The actions of such hormones are mediated by specific thyroid hormone receptors (TRs). TR belongs to the nuclear receptor family of modular transcription factors that binds to specific DNA-response elements within target promoters. These receptors can function as homo- or heterodimers such as TR:9-cis retinoic acid receptor (RXR). Here, we present the atomic resolution structure of the TRα•T3:RXRα•9-cis retinoic acid (9c) ligand binding domain heterodimer complex at 2.95 Å along with T3 hormone binding and dissociation and coactivator binding studies. Our data provide a structural basis for allosteric communication between T3 and 9c and negative cooperativity between their binding pockets. In this structure, both TR and RXR are in the active state conformation for optimal binding to coactivator proteins. However, the structure of TR•T3 within TR•T3:RXR•9c is in a relative state of disorder, and the observed kinetics of binding show that T3 dissociates more rapidly from TR•T3:RXR•9c than from TR•T3:RXR. Also, coactivator binding studies with a steroid receptor coactivator-1 (receptor interaction domains 1-3) fragment show lower affinities (K(a)) for TR•T3:RXR•9c than TR•T3:RXR. Our study corroborates previously reported observations from cell-based and binding studies and offers a structural mechanism for the repression of TR•T3:RXR transactivation by RXR agonists. Furthermore, the recent discoveries of multiple endogenous RXR agonists that mediate physiological tasks such as lipid biosynthesis underscore the pharmacological importance of negative cooperativity in ligand binding within TR:RXR heterodimers.
甲状腺激素，如 3,3',5-三碘-L-甲状腺素（T3），控制着哺乳动物发育和代谢的众多方面。这些激素的作用是通过特定的甲状腺激素受体（TR）介导的。TR 属于核受体家族的模块化转录因子，它与靶启动子内的特定 DNA 反应元件结合。这些受体可以作为同源或异源二聚体发挥作用，如 TR：9-顺式视黄酸受体（RXR）。在这里，我们展示了 TRα•T3：RXRα•9-顺式视黄酸（9c）配体结合域异二聚体复合物的原子分辨率结构，分辨率为 2.95Å，同时进行了 T3 激素结合和解离以及共激活剂结合研究。我们的数据为 T3 和 9c 之间的变构通讯以及它们结合口袋之间的负协同作用提供了结构基础。在这个结构中，TR 和 RXR 都处于活性构象，以最佳方式结合共激活蛋白。然而，TR•T3 内的 TR•T3：RXR•9c 的结构处于相对无序的状态，观察到的结合动力学表明，T3 从 TR•T3：RXR•9c 中的解离速度比从 TR•T3：RXR 中的解离速度更快。此外，用甾体受体共激活因子-1（受体相互作用域 1-3）片段进行的共激活因子结合研究表明，TR•T3：RXR•9c 的亲和力（K(a)）比 TR•T3：RXR 低。我们的研究证实了之前基于细胞和结合研究的观察结果，并为 RXR 激动剂对 TR•T3：RXR 转录激活的抑制提供了结构机制。此外，最近发现的多种内源性 RXR 激动剂介导脂质生物合成等生理任务，突显了配体结合中负协同作用在 TR：RXR 异二聚体中的药理学重要性。
