Novel reagents for labeling proteins in live and fixed cells

FAP reporters consist of a polypeptide unit (the FAP) and an organic small molecule unit (the fluorogen). Neither the FAP nor the fluorogen is fluorescent by itself, but together they form a complex that exhibits strong fluorescence due to conformational constraint of the fluorogen when bound by the FAP. Because the same FAP can be used with a variety of fluorogens of different spectral properties, the system allows one to do very informative experiments (e.g., pulse-chase experiments or differential labeling of the same protein in different cellular compartments) that are not possible using standard fluorescent proteins or chemical fluorophores. In this issue of B&B, Gallo, Vasilev, and Jarvik introduce new FAP-based immunodetection reagents that dramatically expand the range of the FAP-fluorogen system, making it possible to FAP tag almost any cellular protein to which a standard antibody is available.