Molecular Zoology Team (J.G.-M., V.L.), Institut de Génomique Fonctionnelle de Lyon, Unité Mixte de Recherche 5242, Université Lyon 1, Centre National de la Recherche Scientifique, Ecole Normale Supérieure de Lyon, 69364 Lyon Cedex 07, France; Institut National de la Santé et de la Recherche Médicale Unité 1054 (E.K.N., W.B.), Centre de Biochimie Structurale, Centre National de la Recherche Scientifique Unité Mixte de Recherche 5048, Universités Montpellier 1 and 2, 34967 Montpellier, France; CAS in Crystallography and Biophysics (E.K.N.), University of Madras, 600-005 Chennai, India; Centre of Marine and Environmental Research/Interdisciplinary Centre of Marine and Environmental Research (D.L., M.M.S., L.F.C.C.), FCUP-Department of Biology, Faculty of Sciences, University of Porto, 4050-123 Porto, Portugal; Department of Pharmaceutical Sciences (K.P., J.W.J., M.K.), School of Pharmacy, University of Maryland, Baltimore, Maryland 21201; Laboratory of Health Sciences (J.-I.N.), School of Pharmacy and Pharmaceutical Sciences, Mukogawa Women's University, Koshien, Nishinomiya, Hyogo 663-8179, Japan; Laboratory of Hygienic Chemistry and Molecular Toxicology (Y.H., T.N.), Gifu Pharmaceutical University, Gifu 501-1196, Japan; and Laboratoire de Biologie du Développement de Villefranche-sur-Mer, Unité Mixte de Recherche 7009, Sorbonne Universités, Université Pierre et Marie Curie Paris 06, Centre National de la Recherche Scientifique, Observatoire Océanologique de Villefranche-sur-Mer, 06230 Villefranche-sur-Mer, France.
Endocrinology. 2014 Nov;155(11):4275-86. doi: 10.1210/en.2014-1181. Epub 2014 Aug 13.
Nuclear receptors are transcription factors that regulate networks of target genes in response to small molecules. There is a strong bias in our knowledge of these receptors because they were mainly characterized in classical model organisms, mostly vertebrates. Therefore, the evolutionary origins of specific ligand-receptor couples still remain elusive. Here we present the identification and characterization of a retinoic acid receptor (RAR) from the mollusk Nucella lapillus (NlRAR). We show that this receptor specifically binds to DNA response elements organized in direct repeats as a heterodimer with retinoid X receptor. Surprisingly, we also find that NlRAR does not bind all-trans retinoic acid or any other retinoid we tested. Furthermore, NlRAR is unable to activate the transcription of reporter genes in response to stimulation by retinoids and to recruit coactivators in the presence of these compounds. Three-dimensional modeling of the ligand-binding domain of NlRAR reveals an overall structure that is similar to vertebrate RARs. However, in the ligand-binding pocket (LBP) of the mollusk receptor, the alteration of several residues interacting with the ligand has apparently led to an overall decrease in the strength of the interaction with the ligand. Accordingly, mutations of NlRAR at key positions within the LBP generate receptors that are responsive to retinoids. Altogether our data suggest that, in mollusks, RAR has lost its affinity for all-trans retinoic acid, highlighting the evolutionary plasticity of its LBP. When put in an evolutionary context, our results reveal new structural and functional features of nuclear receptors validated by millions of years of evolution that were impossible to reveal in model organisms.
核受体是转录因子，可调节小分子响应的靶基因网络。我们对这些受体的了解存在强烈的偏见，因为它们主要在经典模式生物中，主要是脊椎动物中进行了表征。因此，特定配体-受体对的进化起源仍然难以捉摸。在这里，我们介绍了从软体动物 Nucella lapillus（NlRAR）中鉴定和表征视黄酸受体（RAR）。我们表明，这种受体特异性地与 DNA 反应元件结合，这些元件作为与视黄酸 X 受体的异二聚体组织在直接重复中。令人惊讶的是，我们还发现 NlRAR 不与全反式视黄酸或我们测试的任何其他视黄醇结合。此外，NlRAR 无法在存在这些化合物的情况下激活报告基因的转录，也无法募集共激活因子。NlRAR 配体结合域的三维建模揭示了与脊椎动物 RAR 相似的整体结构。然而，在软体动物受体的配体结合口袋（LBP）中，与配体相互作用的几个残基的改变显然导致与配体的相互作用整体减弱。因此，LBP 内 NlRAR 的关键位置的突变产生了对类视黄醇有反应的受体。总的来说，我们的数据表明，在软体动物中，RAR 失去了对全反式视黄酸的亲和力，突出了其 LBP 的进化灵活性。从进化的角度来看，我们的结果揭示了核受体的新的结构和功能特征，这些特征经过了数百万年的进化验证，在模型生物中是不可能揭示的。
