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We designed a fluorescence resonance energy transfer (FRET)-based approach to study the ligand binding constants of the adenosine A2A receptor (A2AR). Our assay is based in the interaction of a fluorescent A2AR agonist ligand (MRS5424) with an A2AR tagged with the cyan fluorescent protein (CFP) at the N-terminus (i.e. A2ARCFP) and expressed in living cells. Thus, upon fast superfusion of the A2ARCFP expressing cells with MRS5424, the ligand-receptor interaction is determined by single-cell FRET in a real-time mode. Accordingly, our approach allowed immediate ‘real-time’ readout of the ligand-receptor interaction, thus allowing kinetic binding experiments, a feature impossible to achieve using conventional radioisotope-labelled ligands. In addition, since our assay permitted the visual confirmation of receptor localization it also allowed localized saturation binding experiments.