Development of a Genetically Encoded Sensor for Arginine

) of 3. ArgS1 responds to Arg in both the cytoplasm and the subcellular organelles. ArgS1 monitored Arg levels in MDA-MB-231 cells, a breast cancer cell line deficient in a key enzyme for Arg synthesis (arginino-succinate synthetase1, ASS1) and amenable to Arg depletion therapy. We found that Arg levels in MDA-MB-231 cells decreased after depletion of extracellular Arg with a concomitant decline in cell viability. When ASS1 was overexpressed in the cells, Arg levels increased and cell viability was also enhanced. Thus, ArgS1 is an effective tool for real-time monitoring of Arg in human cells over a dynamic range of physiological and pathological relevance.