Engineering Protein Molecular Switches To Regulate Gene Expression with Small Molecules
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Small molecule dependent molecular switches that control gene expression are important tool in understanding biological cellular processes and for regulating gene therapy. Nuclear receptors are ligand activated transcription factors that have been engineered to selectively respond to synthetic ligands and used as regulators of gene expression. In this work the retinoid X receptor (RXR), has been used to develop an inducible molecular switch with a near drug like compound LG335. Three RXR variants (Q275C; I310M; F313I), (I268A; I310A; F313A; L436F), (I268V; A272V; I310M; F313S; L436M) were created via site-directed mutagenesis and a structure based approach, such that they preferentially bind to the synthetic ligand LG335 and not its natural ligand, 9-cis retinoic acid. These variants show reverse ligand specificity as designed and have an EC50 for LG335 of 80 nM, 30 nM, 180 nM, respectively. The ligand binding domains of the RXR variants were fused to a yeast transcription factor Gal4 DNA binding domain. This modified chimeric fusion protein showed reverse response element specificity as designed and recognized the Gal4 response element instead of the RXR response element. The modified RXR protein did not heterodimerize with wild type RXR or with other nuclear receptor such as retinoic acid receptor. These RXR-based molecular switches were tested in retroviral vectors using firefly luciferase and green fluorescence protein and they maintain their inducible behavior with LG335. These experiments demonstrate the orthogonality of RXR variants and their possible use in regulating gene therapy.