title: The structure of the ultraspiracle ligand-binding domain reveals a nuclear receptor locked in an inactive conformation.
abstract: Ultraspiracle (USP) is the invertebrate homologue of the mammalian retinoid X receptor (RXR). RXR plays a uniquely important role in differentiation, development, and homeostasis through its ability to serve as a heterodimeric partner to many other nuclear receptors. RXR is able to influence the activity of its partner receptors through the action of the ligand 9-cis retinoic acid. In contrast to RXR, USP has no known high-affinity ligand and is thought to be a silent component in the heterodimeric complex with partner receptors such as the ecdysone receptor. Here we report the 2.4-A crystal structure of the USP ligand-binding domain. The structure shows that a conserved sequence motif found in dipteran and lepidopteran USPs, but not in mammalian RXRs, serves to lock USP in an inactive conformation. It also shows that USP has a large hydrophobic cavity, implying that there is almost certainly a natural ligand for USP. This cavity is larger than that seen previously for most other nuclear receptors. Intriguingly, this cavity has partial occupancy by a bound lipid, which is likely to resemble the natural ligand for USP.
authors.affiliation: Medical Research Council, Laboratory of Molecular Biology, Cambridge CB2 2QH, United Kingdom.
authors.forename: G M
authors.initials: GM
authors.lastname: Clayton
authors.forename: S Y
authors.initials: SY
authors.lastname: Peak-Chew
authors.forename: R M
authors.initials: RM
authors.lastname: Evans
authors.forename: J W
authors.initials: JW
authors.lastname: Schwabe
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D018360:Crystallography, X-Ray; D004268:DNA-Binding Proteins / Q000737:chemistry* / Q000378:metabolism; D029721:Drosophila Proteins; D004331:Drosophila melanogaster; D006801:Humans; D008024:Ligands; D050356:Lipid Metabolism; D051379:Mice; D008958:Models, Molecular; D008969:Molecular Sequence Data; D017433:Protein Structure, Secondary; D011987:Receptors, Steroid / Q000737:chemistry* / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D014157:Transcription Factors / Q000737:chemistry* / Q000378:metabolism
chemical_list: D004268:DNA-Binding Proteins; D029721:Drosophila Proteins; D008024:Ligands; D011987:Receptors, Steroid; D014157:Transcription Factors; C066030:USP protein, Drosophila