Fig. 1.

BRET is dependent on the distance between the donor luciferase and the acceptor fluorophore. Addition of the cell permeant Renilla luciferase substrate coelenterazine (ctz) results in oxidation of the substrate to coelenteramide, which produces blue light at 482 nm. When protein-protein interactions between Protein X and Protein Y bring the donor luciferase (RLuc) and acceptor fluorophore (YFP) in close proximity (<10 nm), the energy from the donor can be transferred to the acceptor and light is produced at 527 nm. When the BRET tags are not in close enough proximity, light is only emitted at 482 nm