The proton sensitivity of Fluorescent Proteins: towards intracellular pH indicators

On account of the presence of an ionizable phenol group on their chromophore, most fluorescent proteins display pH-sensitive absorption and fluo- rescence emission bands. This property has great implications for most applications that involve FPs as intracellular markers of target proteins. The photophysical modulation of pH sensitivity relies upon a complex network of exchange reaction pathways involving the chromophore and funneling protons in and out the protein structure. In parallel with the fast-growing use of FPs as intracellular probes, many groups reported on the use of FP variants as genetically-encodable pH indicators of intracellular compartments that cannot be probed using conventional pH-sensitive dyes. These pH indicators shed light on many cell functions strongly coupled with local pH. Here, we shall review both the properties that make FPs so efficient as intracellular pH indicators and the probes that are more utilized by the scientific community.