Intracellular pH levels can now be measured with a protein that glows red.
The red fluorescent protein has been dubbed pHRed by its designers, Gary Yellen and his colleagues at Harvard Medical School in Boston, Massachusetts. Light-detecting molecules of pHRed are preferentially excited by light at a wavelength of either 440 nanometres in basic conditions or 585 nanometres in acidic conditions, and the excitation ratio of the two gives a read-out of pH.
pHRed's fluorescence lifetime — the time between excitation and emission — also responds to pH. The authors used this feature in two-photon microscopy to track second-by-second pH changes in living cells.
A red pH sensor leaves the often-used green fluorescence wavelength available for simultaneous study of other cellular properties, such as energy metabolism, during multicolour imaging.
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A microscopist's litmus test. Nature 474, 423 (2011). https://doi.org/10.1038/474423b
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DOI: https://doi.org/10.1038/474423b