Structural and functional characterization of a novel type of ligand-independent RXR-USP receptor.
Retinoid X receptor (RXR) and Ultraspiracle (USP) play a central role as ubiquitous heterodimerization partners of many nuclear receptors. While it has long been accepted that a wide range of ligands can activate vertebrate/mollusc RXRs, the existence and necessity of specific endogenous ligands activating RXR-USP in vivo is still matter of intense debate. Here we report the existence of a novel type of RXR-USP with a ligand-independent functional conformation. Our studies involved Tribolium USP (TcUSP) as representative of most arthropod RXR-USPs, with high sequence homology to vertebrate/mollusc RXRs. The crystal structure of the ligand-binding domain of TcUSP was solved in the context of the functional heterodimer with the ecdysone receptor (EcR). While EcR exhibits a canonical ligand-bound conformation, USP adopts an original apo structure. Our functional data demonstrate that TcUSP is a constitutively silent partner of EcR, and that none of the RXR ligands can bind and activate TcUSP. These findings together with a phylogenetic analysis suggest that RXR-USPs have undergone remarkable functional shifts during evolution and give insight into receptor-ligand binding evolution and dynamics.
IGBMC (Institut de Génétique et de Biologie Moléculaire et Cellulaire), UMR7104 CNRS, U596 INSERM, ULP, Département de Biologie et de Génomique Structurales, Illkirch, France.
Thomas
T
Iwema
Isabelle M L
IM
Billas
Yannick
Y
Beck
François
F
Bonneton
Hélène
H
Nierengarten
Arnaud
A
Chaumot
Geoff
G
Richards
Vincent
V
Laudet
Dino
D
Moras
D000818:Animals; D030801:Animals, Genetically Modified; D001665:Binding Sites; D004268:DNA-Binding Proteins / Q000737:chemistry* / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D019281:Dimerization; D029721:Drosophila Proteins; D004331:Drosophila melanogaster / Q000196:embryology / Q000378:metabolism; D019143:Evolution, Molecular; D017930:Genes, Reporter; D006801:Humans; D019476:Insect Proteins / Q000737:chemistry* / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D008024:Ligands; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010802:Phylogeny; D020836:Protein Structure, Quaternary; D011987:Receptors, Steroid / Q000235:genetics / Q000378:metabolism; D047490:Retinoid X Receptor alpha / Q000737:chemistry* / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D014157:Transcription Factors / Q000737:chemistry* / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D014231:Tribolium
D004268:DNA-Binding Proteins; D029721:Drosophila Proteins; D019476:Insect Proteins; D008024:Ligands; D011987:Receptors, Steroid; D047490:Retinoid X Receptor alpha; D014157:Transcription Factors; C066030:USP protein, Drosophila; C055469:ecdysone receptor