Abstract
The identification of potential endogenous or synthetic ligands for orphan receptors in the steroid receptor superfamily is important both for discerning endogenous regulatory pathways and for designing receptor inhibitors. The insect nuclear receptor Ultraspiracle (USP), an ortholog of vertebrate RXR, has long been treated as an orphan receptor. We have tested here the fit of terpenoid ligands to the JH III-binding site of monomeric and homo-oligomeric USP from Drosophila melanogaster (dUSP). dUSP specifically bound juvenile hormone III (JH III), but not control farnesol or JH III acid, and also specifically changed in conformation upon binding of JH III in a fluorescence binding assay. Juvenile hormone III binding caused intramolecular changes in receptor conformation, and stabilized the receptor's dimeric/oligomeric quaternary structure. In both a radiometric competition assay and the fluorescence binding assay the synthetic JH III agonist methoprene specifically competed with JH III for binding to dUSP, the first demonstration of specific binding of a biologically active JH III analog to an insect nuclear receptor. The recombinant dUSP bound with specificity to a DR12 hormone response element in a gel shift assay. The same DR12 element conferred enhanced transcriptional responsiveness of a transfected juvenile hormone esterase core promoter to treatment of transfected cells with JH III, but not to treatment with retinoic acid or T3. The activity of JH III or JH III-like structures, but not structures without JH III biological activity, to bind specifically to dUSP and activate its conformational change, provide evidence of a terpenoid endogenous ligand for Ultraspiracle, and offer the prospect that synthetic, terpenoid structures may be discovered that can agonize or antagonize USP function in vivo.
