Engineering orthogonal ligand-receptor pairs from "near drugs"

Cell-permeable small molecules are powerful tools for unraveling complex ce llular pathways. We demonstrate that nuclear hormone receptors can be engin eered through mutagenesis to create orthogonal ligand-receptor pairs to con trol transcription. Mutated residues in the retinoid X receptor (RXR) were chosen from structural analysis of RXR and the retinoic acid receptor (RAR) ligand binding domains. The potential ligands screened for activation of v ariant receptors are "near drugs"-compounds synthesized during structure-ac tivity studies that are structurally similar to an approved drug yet inacti ve on the wild-type receptor. One variant, Q275C;I310M;F3131, is poorly act ivated by ligands for the wild-type receptor but is activated by a "near dr ug", fulfilling the criteria of an orthogonal ligand-receptor pair. These e xperiments demonstrate that nuclear hormone receptors are well suited to su pply orthogonal ligand-receptor pairs for experimental biology, biotechnolo gy, and gene therapy. Our findings also demonstrate the general principle t hat inactive compounds synthesized during drug discovery can be combined wi th mutant proteins to rapidly create new tools for controlling cellular pro cesses.