Fig. 5.

Representative Förster resonance energy transfer (FRET)-based biosensors with RFPs. (a) Intermolecular biosensors for protein–protein interaction. Unlike FP complementation-based biosensors, the FRET-based biosensors of protein–protein interactions are reversible. (b) Ion/small molecule biosensors. An intramolecular protein complex is formed, or a conformation changed, upon the binding of a specific ion or small molecule. (c) Protease biosensors where the two FPs are initially linked by a protease substrate sequence.