Project Summary G protein-coupled receptors (GPCRs) are the largest family of membrane receptors in human and have emerged as the largest family of drug targets with more than 35% of all drugs on the market functioning through GPCRs. Despite their desirability, to date, many GPCR families remain undrugged, partly due to lack of mechanistic knowledge about their activation and modulation. Metabotropic glutamate receptors (mGluRs) are members of class C GPCRs and are critical modulators of glutamate signaling. Due to their widespread expression in tissue and their central role they are among the most promising drug targets for the neurological disorders such as fragile X syndrome, epilepsy, anxiety, schizophrenia as well as some cancers. Advances in protein engineering and functional, structural and computational methods in the past twenty years have provided insights into the architecture, signaling and expression patterns of mGluRs. However, a general model of how ligands change the shape of mGluRs and how this conformational change is relayed across the membrane and over 12 nm to activate specific signaling pathways is poorly understood. In this research we will develop a novel technology that will allow us to watch a single mGluR protein while functioning in physiological conditions, in real time. This will allow us to quantify the motions of different domains of the receptor that are involved in signaling. Next, we will employ this approach to study how cholesterol and synthetic modulators of mGluR signaling affect protein conformation and dynamics to affect receptor signaling. This is a multi-disciplinary proposal where state-of-the- art in vitro single-molecule FRET (smFRET) spectroscopy is complemented by live-cell imaging, protein engineering and biophysical and biochemical methods. Once accomplished, the proposed research could provide a critical step towards rational design of efficient drugs with fewer undesirable side effects. Furthermore, these studies will provide a general roadmap for quantitative high-resolution structure-function studies of mammalian membrane proteins.