Li, Y. & Tsien, R.W. Nat. Neurosci. 15, 1047–1053 (2012).

Our understanding of the complex physiology of neurons can be greatly aided by the use of genetically encoded, fluorescent reporters. The turnover of vesicles that occurs in neuronal synapses, for example, signals the transmission of electrochemical signals between neurons. This process can be monitored using a pH-sensitive variant of GFP called pHluorin. But because many other activity-based reporters are also based on GFP, they cannot easily be used together. Li and Tsien engineered a pH-sensitive red fluorescent protein, dubbed pHTomato, which can be used in combination with other probes based on GFP. They used it to make a reporter of activity-dependent vesicle membrane fusions. By coupling pHTomato and GFP-based probes, they demonstrate the possibility of all-optical neurophysiology studies.