Figure 1.

Two general classes of molecular biosensors. (a) An intramolecular biosensor contains both the target and molecular recognition element, connected by a linker. The inactive target T is activated by an endogenous intracellular process to produce the active, unoccupied target T^∗; the activated state is bound by the molecular recognition element to form the complex C, generating the biosensor readout. (Illustration) Common scenario where intramolecular binding brings a Förster resonance energy transfer pair into close proximity. (b) An intermolecular biosensor, B, contains a molecular recognition element that binds to an endogenous target to form a bimolecular complex, C. (Illustration) Common scenario where complex formation results in membrane translocation of the tagged biosensor.