 A flying butterfly is one of the most beautiful creatures that the nature presented on the Earth. As a child we were all curious how it flied in such an elegant way. We could easily figure out how the butterfly moved its wings to fly -- flipping them in and out -- simply by watching it fly. What if the size of the butterfly is so small that we cannot see them directly with naked eyes? What if the size is that of molecules such as proteins whose dimension is smaller than the diffraction limit of visible light (~100nm)? How will we be able to track its flight and figure out the motion of its wings?
 FRET (Forster Resonance Energy Transfer) is a powerful molecular biology tool used for detecting conformational dynamics of macro-molecules such as DNA, RNA and proteins with nanometer resolution. We need two fluorescence dyes whose spectra are similar to each other. Once we label the two dyes onto each wing of the "butterfly protein", the change in color reports in real time how the butterfly changes its conformation for its flight (see Techniques).
 When the butterfly stays sitting on grass, it is likely to remain in one conformation, say, its wings closed. One steady FRET signal will be observed -- high FRET therefore red signal. When it flies, the FRET efficiency will periodically change between high and low states reflecting the way it flies -- flipping its wings in and out over and over again.
 Unfortunately, detection methods used in conventional molecular biology require to have trillions of proteins emitting their signals altogether. Therefore, it is impossible to observe each single butterfly flying and such ensemble measurements average out all the signals erasing off dynamic signature of individual proteins. That is, instead of detecting fluctuating signal of red and green, all we get is the averaged color somewhere between green and red, ie. yellow.
To deduce what is happening, scientists have used several tricks. One is to synchronize the dynamics. If we can put all the butterflies in one conformation, for exampling, making them take a nap, initially the FRET signal will be extremely high since all of them will have their wings folded. At t = 0, we will wake them up to fly and the FRET signal will drop since the FRET efficiency will be an average of folded and unfolded conformations.
 Here comes the single molecule (sm) study. The beauty of sm-study is to track and observe single butterflies simply by watching them. There is no need of synchronization. The hidden details of the dynamics are visible effortless. The heterogeneity between proteins is caught easily. See Techniques for more details.
 Update on Jul 6, 2009 |