Single-molecule diffusion-based estimation of GPCR activity

G protein-coupled receptors (GPCRs) are major drug targets and have high potential for drug discovery. The development of a method for measuring the activities of GPCRs is essential for pharmacology and drug screening. However, it is difficult to measure the effects of a drug by monitoring the receptor on the cell surface, and changes in the concentrations of downstream signaling molecules, which are specific to each receptor, are used as an index of the receptor activity. Here, we show that single-molecule imaging analysis provides an alternative method for assessing GPCR activity. We monitored the dynamics of the diffusion of metabotropic glutamate receptor 3 (mGluR3), a class C GPCR, under various ligand conditions by using total internal reflection fluorescence microscopy (TIRFM). The single-molecule tracking analysis demonstrates that changes in the average diffusion coefficient of mGluR3 quantitatively reflect the ligand-dependent activity. Then, we reveal that the diffusion of receptor molecules is altered by the common physiological events associated with GPCRs, including G protein binding or accumulation in clathrin-coated pits, by inhibitor assay and dual-color single-molecule imaging analysis. We also confirm the general applicability of the method to class A and B GPCRs, demonstrating that the diffusion coefficient is a good index for estimating the activities of many GPCRs regardless of the phylogenetic groups, chemical properties of the ligands, and G protein-coupling selectivity.