stFRET: A Worm That Changes Color When It Flexes.

stFRET is a real time optical sensor system that measures the amount of mechanical stress structural proteins undergo. Mechanical stress is central to many phenomena such as hearing, touch, and the regulation of blood pressure, however it is rarely studied at the molecular level. This was the impetus for Prof. Sachs (SUNY Buffalo) to develop a stress probe which can be inserted into structural proteins. The probe is composed of a pair of GFP-like proteins, Cerulean and Venus, linked with a stable alpha-helix. Irradiation of an isolated Cerulean molecule with 433nm light results in cyan fluorescence (475nm), however if a Venus molecule is close and in the right orientation fluorescence resonance energy transfer (FRET) occurs and the cyan fluorescence in absorbed by the Venus chromophore, which will emit yellow light (527nm). In the StFRET sensor the two fluorescent proteins are held in place by an alpha helix (molecular sprring), see below. By applying stress on the probe the distance and orientation between the Cerulean and Venus molecules is changed and the ratio of cyan to yellow emission varies thereby allowing the research to calculate the stress in the system.

 ( The height of the beta-barrel structure is 4.2 nm. The black helix is the linker, and it nominal length is 5.0 nm. Incoming arrows indicate excitation, and outgoing arrows indicate emission, with the wavelength marked next to them; the width of the arrows is proportional to the light intensity1.)
As a proof of concept Sachs and his co-workers have created a mutant C. elegans with stFRET in its muscle fibers; it changes color when it moves.
Meng, F.; Suchyna, T. M.; Sachs, F., A fluorescence energy transfer-based mechanical stress sensor for specific proteins in situ. Febs Journal 2008, 275, (12), 3072-3087.