luciferase, which oxidises luciferin (coelenterazin) by simultaneously emitting light at
lexc= 480 nm. This light directly excites green fluorescent protein (GFP), which, in
turn, emits fluorescence atlem= 509 nm.
Fluorescence labelling of proteins by other proteins presents a useful approach to
study various processesin vivo. Labelling can be done at the genetic level by generating
fusion proteins. Monitoring of protein expression by GFP is an established technique
and further development of ‘living colours’ will lead to promising new tools.
While nucleic acids have been the main players in the genomic era, the postgenomic/
proteomic era focusses on gene products, the proteins. New proteins are being dis-
covered and characterised, others are already used within biotechnological processes. In
particular for classification and evaluation of enzymes and receptors, reaction systems
can be designed such that the reaction of interest is detectable quantitatively using FRET
donor and acceptor pairs.
For instance, detection methods for protease activity can be developed based on BRET
applications. A protease substrate is fused to a GFP variant on the N-terminal side and
dsRED on the C-terminal side. The latter protein is a red fluorescing FRET acceptor
and the GFP variant acts as a FRET donor. Once the substrate is cleaved by a protease,
the FRET effect is abolished. This is used to directly monitor protease activity. With a
combination of FRET analysis and two-photon excitation spectroscopy it is also possible
to carry out a kinetic analysis.
A similar idea is used to label human insulin receptor (see Section 17.4.4) in order
to quantitatively assess its activity. Insulin receptor is a glycoprotein with twoaand
twobsubunits, which are linked by dithioether bridges. The binding of insulin
induces a conformational change and causes a close spatial arrangement of bothb
subunits. This, in turn, activates tyrosine kinase activity of the receptor.
In pathological conditions such as diabetes, the tyrosine kinase activity is different
than in healthy conditions. Evidently, it is of great interest to find compounds that
stimulate the same activity as insulin. By fusing thebsubunit of human insulin receptor
toRenilla reniformisluciferase and yellow fluorescent protein (YFP) a FRET donor–
acceptor pair is obtained, which reports theligand-induced conformational change and
precedes the signal transduction step. This reporter system is able to detect the effects of
insulin and insulin-mimicking ligands in order to assess dose-dependent behaviour.Fluorescence recovery after photo bleaching (FRAP)
If a fluorophore is exposed to high intensity radiation it may be irreversibly damaged and
lose its ability to emit fluorescence. Intentional bleaching of a fraction of fluorescently
labelled molecules in a membrane can be used to monitor the motion of labeled molecules
in certain (two-dimensional) compartments. Moreover, the time-dependent monitoring
allows determination of the diffusion coefficient. A well-established application is the
usage of phospholipids labelled with NBD(e.g. NBD-phosphatidylethanolamine, Fig.
12.13b) which are incorporated into a biological or artificial membrane. The specimen
is subjected to a pulse of high-intensity light (photo bleaching), which causes a sharp drop
of fluorescence in the observation area (Fig. 12.13). Re-emergence of fluorescence emis-
sion in this area is monitored as unbleached molecules diffused into the observation area.
From the time-dependent increase of fluorescence emission, the rate of diffusion of the503 12.3 Fluorescence spectroscopy