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INSERM U 547, Institut Pasteur, 1 rue du Professeur Calmette, 59019-Lille, France
¹ CNRS UMR 49, École Normale Supérieure de Lyon, 46 allée d’Italie, 69364-Lyon, France
² INSERM U 545 Institut Pasteur, 1 rue du Professeur Calmette, 59019-Lille, France
³ Laboratoire d’Écologie Microbienne de la Rhizosphère (LEMiR), Département d’Écophysiologie Végétale et de Microbiologie (DEVM), CEA Cadarache, 13108 Saint Paul Lez Durance, France

(Requests for offprints should be addressed to R J Pierce; Email: Raymond.Pierce{at}pasteur-lille.fr)

(D Bouton is now at Department for Biosciences at Novum, Karolinska Institute, 141 57 Huddinge, Sweden)
(R L de Mendonça is now at Université Libre de Bruxelles, Service de Microbiologie, Hôpital Erasme, Route du Lennik, 1070-Bruxelles, Belgium)
(C Noël is now at School of Biology, Institute for Research on Environment and Sustainability, Devonshire Building, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, UK)
(M Robinson-Rechavi is now at Joint Center for Structural Genomics, UCSD and Burnham Institute, 10901 North Torrey Pines Road, La Jolla, CA 92037, USA)

Retinoid X receptors (RXR) are members of the nuclear receptor superfamily of ligand-activated transcription factors that have been characterized in a wide variety of metazoan phyla. They act as heterodimer partners of other nuclear receptors, and in vertebrates also activate transcription as homodimers in the presence of a ligand, 9-cis retinoic acid. In order to test the hypothesis that retinoic acid signaling pathways involving RXRs are present in the Lophotrochozoa, we have sought to isolate conserved members of this family from the platyhelminth parasite Schistosoma mansoni and its intermediate host, the mollusk Biomphalaria glabrata. Here we report that an RXR ortholog from B. glabrata (BgRXR) is better conserved, compared with mouse RXR, both in the DNA-binding domain (89% identity) and in the ligand-binding domain (LBD) (81% identity), than are arthropod homologs. In EMSA, BgRXR binds to the direct repeat response element DR1 as a homodimer or as a heterodimer with mammalian RAR, LXR, FXR or PPAR. When transfected alone into mammalian cell lines, BgRXR transactivated transcription of a reporter gene from the Apo-A1 promoter in the presence of 9-cis retinoic acid or DHA. Constructs with the Gal4 DNA binding domain fused to the hinge and LBDs of BgRXR were used to show that ligand-dependent activation of transcription by BgRXR required its intact AF-2 activation domain, and that the LBD can form homodimers. Finally, the binding of 9-cis retinoic acid preferentially protected the LBD of BgRXR from degradation by trypsin in a proteolysis protection assay. Our results show that BgRXR binds and is activated by retinoids and suggest that retinoid signaling pathways are conserved in the Lophotrochozoa. The nucleotide sequence reported in this paper has been submitted to the GenBank/EBI Data Bank with accession no. AY048663.

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