Insertion of Nanoluc into the Extracellular Loops as a Complementary Method To Establish BRET-Based Binding Assays for GPCRs

Luminescence-based techniques play an increasingly important role in all areas of biochemical research, including investigations on G protein-coupled receptors (GPCRs). One quite recent and popular addition has been made by introducing bioluminescence resonance energy transfer (BRET)-based binding assays for GPCRs, which are based on the fusion of nanoluciferase (Nluc) to the N-terminus of the receptor and the occurring energy transfer via BRET to a bound fluorescent ligand. However, being based on BRET, the technique is strongly dependent on the distance/orientation between the luciferase and the fluorescent ligand. Here we describe an alternative strategy to establish BRET-based binding assays for GPCRs, where the N-terminal fusion of Nluc did not result in functioning test systems with our fluorescent ligands (e.g., for the neuropeptide Y Y1 receptor (Y1R) and the neurotensin receptor type 1 (NTS1R)). Instead, we introduced Nluc into their second extracellular loop and we obtained binding data for the fluorescent ligands and reported standard ligands (in saturation and competition binding experiments, respectively) comparable to data from the literature. The strategy was transferred to the angiotensin II receptor type 1 (AT1R) and the M1 muscarinic acetylcholine receptor (M1R), which led to affinity estimates comparable to data from radioligand binding experiments. Additionally, an analysis of the binding kinetics of all fluorescent ligands at their respective target was performed using the newly described receptor/Nluc-constructs.