DEER Analysis of GPCR Conformational Heterogeneity

G protein-coupled receptors (GPCRs) represent a large class of transmembrane helical proteins which are involved in numerous physiological signaling pathways and therefore represent crucial pharmacological targets. GPCR function and the action of therapeutic molecules are defined by only a few parameters, including receptor basal activity, ligand affinity, intrinsic efficacy and signal bias. These parameters are encoded in characteristic receptor conformations existing in equilibrium and their populations, which are thus of paramount interest for the understanding of receptor (mal-)functions and rational design of improved therapeutics. To this end, the combination of site-directed spin labeling and EPR spectroscopy, in particular double electron-electron resonance (DEER), is exceedingly valuable as it has access to sub-Angstrom spatial resolution and provides a detailed picture of the number and populations of conformations in equilibrium. This review gives an overview of existing DEER studies on GPCRs with a focus on the delineation of structure/function frameworks, highlighting recent developments in data analysis and visualization. We introduce conformational efficacy as a parameter to describe ligand-specific shifts in the conformational equilibrium, taking into account the loose coupling between receptor segments observed for different GPCRs using DEER.

Automated summary

GPCRs are crucial pharmacological targets involved in various physiological signaling pathways, with their function and the action of therapeutic molecules defined by parameters such as receptor basal activity, ligand affinity, intrinsic efficacy, and signal bias. The combination of site-directed spin labeling and EPR spectroscopy, particularly DEER, provides detailed insight into the populations of receptor conformations in equilibrium, offering valuable information for understanding receptor functions and designing improved therapeutics. Recent studies using DEER have focused on delineating structure/function frameworks and introducing conformational efficacy as a parameter to describe ligand-specific shifts in the conformational equilibrium.