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In 1974, Ashburner and colleagues postulated a model to explain the control of the puffing sequence on Drosophila polytene chromosomes initiated by the molting hormone 20-hydroxyecdysone. This model inspired a generation of molecular biologists to clone and characterize elements of the model, thereby providing insights into the control of gene networks by steroids, diatomic gases, and other small molecules. It led to the first cloning of the EcR subunit of the heterodimeric EcR-USP ecdysone receptor. X-ray diffraction studies of the ligand-binding domain of the receptor are elucidating the specificity of receptor-ecdysteroid interactions, the selectivity of some environmentally friendly insecticides, the evolution of the EcR-USP heterodimer, and indeed Ashburner's classical biochemical evidence for the central role of the ecdysone receptor in his model.
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Supplemental Video 1: This movie depicts schematically the structural transitions within the ligand binding domain of the Heliothis virescens ecdysone receptor EcR from the conformation binding ecdysteroid (PonA, green carbon and red oxygen atoms) to the conformation binding a dibenzoylhydrazine insecticide (BYI06830, black carbon, red oxygen, and blue nitrogen atoms), and then back again. The N and C termini of the receptor domain are denoted by blue and red spheres, respectively. Most features of secondary structure remain effectively unchanged during the transition with the following notable exceptions. The β-sheet (gold) between helices H5 and H6 is disrupted as a concomitant of rotation of two aromatic residues (phenylalanine and tyrosine) to fill the space vacated by the steroid nucleus when the ecdysteroid ligand is replaced by the dibenzoylhydrazine insecticide. Helix H2 (pink) unwinds while helices H7 and H10 (purple) move apart to accommodate the insecticide. To aid interpretation, the two ligands are depicted as transitioning from opaque (present) to fully transparent (absent). Download high-resolution version (MOV)