The assessment of local concentrations of extracellular ATP (eATP) at the site of receptor binding remains a challenge in the field of purinergic signaling. In many cases, biosensors exploiting the principle of Förster resonance energy transfer (FRET) have provided useful tools to visualize local concentrations of metabolites. A series of FRET-based biosensors based on the epsilon subunits of bacterial ATP synthases have been described for the visualisation of ATP. These sensors carry ATP-sensing units with different affinities for ATP, permitting imaging of ATP under the widely different concentration conditions found in subcellular locations such as the cytoplasm and the membrane-proximal extracellular space.
