A Superfolder Green Fluorescent Protein-Based Biosensor Allows Monitoring of Chloride in the Endoplasmic Reticulum

[Image: see text] Though the concentration of chloride has been measured in the cytoplasm and in secretory granules of live cells, it cannot be measured within the endoplasmic reticulum (ER) due to poor fluorescence of existing biosensors. We developed a fluorescent biosensor composed of a chloride-sensitive superfolder GFP and long Stokes-shifted mKate2 for simultaneous chloride and pH measurements that retained fluorescence in the ER lumen. Using this sensor, we showed that the chloride concentration in the ER is significantly lower than that in the cytosol. This improved biosensor enables dynamic measurement of chloride in the ER and may be useful in other environments where protein folding is challenging.