Fluorescent-Based Strategies to Investigate G Protein-Coupled Receptors: Evolution of the Techniques to a Better Understanding

G protein-coupled receptors are key proteins in the regulation of most of the physiological responses. Their conformations are generally oscillating between inactive and active forms leading to the activation of no, a few, or many signaling pathways. Although receptors can spontaneously adopt these various conformations, their interactions with ligands, other G protein-coupled receptors, or intracellular proteins (G proteins, arrestins, etc.) can stabilize one of these conformations, leading to specific cellular responses. The identification of the partners interacting with the G protein-coupled receptors and the dynamics of these interactions is therefore crucial to fully understand receptor functioning. Although it is crucial, it remains nevertheless ambitious and difficult to achieve this goal. In the last two decades, various technical strategies have been developed to investigate molecular complexes and their dynamics. In this review, we will focus on recent technological breakthroughs in fluorescent-based techniques and their impact on the understanding of G protein-coupled receptor functioning. We will give particular attention to resonance energy transfer-based strategies, their advantages, and drawbacks and to other microscopy based techniques which are efficient to investigate stability, mobility, and dynamics of molecular complexes at the cell surface.