BRET is based on the fact that the energy derived from a luciferase reaction can be used to excite a fluorescent protein if the latter is in close proximity to the luciferase enzyme.

Especially in the field of G-protein coupled receptor research, BRET technology offers the opportunity to establish a homogeneous, universal and functional assay, taking advantage of the fact that ß-arrestin (which is naturally playing a role in the desensitation of the receptors) binds to the intracellular part of the activated receptor.
G-protein coupled receptors, also referred to as 7-transmembrane (7TM) receptors, comprise the largest and most diverse superfamily of proteins known. To a minor part only the ligands are known.

There are several advantages of BRET over other methods.
It is a non-radioactive and homogeneous technology. The ratiometric signal minimises interferences from assay conditions. There is no auto-fluorescence coming from compounds or cell and buffer components as no light source is required.

To get some idea of how important GPCRs are in drug discovery:

• Currently ~ 30 % of drugs are targeted against GPCRs
• Only 5 % of the known receptors are targeted with drugs
• To only 20 % of the rest the corresponding ligands are known

And – the Mithras LB 940 and TriStar LB 941 are currently considered the only instruments meeting the needs for sensitivity and consistency which offers injection possibilities.

Over the last years different BRET methods have been developed. All of them have their limitations and benefits.
Various donor and acceptor pairs and their corresponding wavelengths can be found in the table below:

Method	Acceptor	Substrate	Acceptor Emission [nm]	Donor	Donor Emission [nm]
BRET 1	RLuc	Coelenterazine	480	eYFP	530
BRET 2	RLuc	Deep Blue C™	395	GFP2	510
eBRET 2	RLuc8	Deep Blue C™	395	GFP2	510
BRET 3	Firefly	Luciferin	565	DsRed	583
QD-BRET	RLuc/RLuc8	Coelenterazine	480	Qdot	605

The original BRET method using Coelenterazine as substrate is nowadays called BRET 1. It is characterized by strong signals and long life-time.

BRET 2 in comparison has better separated donor and acceptor emission peaks. This makes BRET 2 a better choice for screening assays where high signal to noise ratios are required. A clear limitation of BRET 2 is the low light emission and the short life-time.

Enhanced BRET 2 (eBRET) – leads to approximately 5-fold better signal as in the original BRET 2 version. eBRET uses a new Renilla luciferase mutant, Rluc8.

The Firefly luciferase in BRET 3 shows lower cellular autofluorescence at the emission wavelength (565 nm) but disadavantages are weak signals and overlap between donor and acceptor emission peaks.

A brand new BRET version is the Quantum Dot-BRET (QD-BRET). The emission peaks are clearly separated which makes QD-BRET ideal for screening applications. Disadvantages are the large size of the QD molecules (1.5 - 6nm) and the fact that genetical coding of QD-proteins is not possible. QD proteins cannot be expressed in living cells but must be added.

Literature: Bacart et al.(2008): The BRET technology and its application to screening assays, Biotechnol. J. 2008, 3, 311–324