Simultaneous intracellular chloride and pH measurements using a GFP-based sensor

Chloride and protons perform important closely related roles in many cellular responses. Here we developed a ratiometric biosensor, ClopHensor, based on a highly chloride-sensitive Aequorea victoria GFP variant that is suited for the combined real-time optical detection of pH changes and chloride fluxes in live cells. We detected high chloride concentration in large dense-core exocytosis granules by targeting ClopHensor to these intracellular compartments.