Green Fluorescent Protein Calcium Biosensors: Calcium Imaging with GFP Cameleons

Cytosolic and organellar free (Ca2+) concentrations ([Ca2+]) are among the most important and dynamic intracellular signals. Ca2+signals are most often measured using Ca2+sensitive fluorescent dyes (1–3), such as Fura-2 or Indo-1, or the bioluminescent protein, aequorin (4–6). Whereas synthetic fluorescent chelators are easily imaged, these are difficult to target precisely to specific subcellular locations Table 1). By contrast, aequorin is easily targeted, but requires the incorporation of a cofactor, coelenterazine. Moreover, the photon intensity from aequorin is extremely low, so that single cell imaging requires specialized photon-counting systems (4) Table 1).