Internally ratiometric fluorescent sensors for evaluation of intracellular GTP levels and distribution

GTP is a major regulator of multiple cellular processes, but tools for quantitative evaluation of GTP levels in live cells have not been available. Here we report characterization of genetically encoded GTP sensors, constructed by inserting cpYFP into a region of the bacterial FeoB G-protein that undergoes a GTP-driven conformational change. GTP binding to these sensors results in a ratiometric change in their fluorescence, thereby providing an internally normalized response to changes in GTP levels while minimally perturbing those levels. Mutations introduced into FeoB to alter its affinity for GTP allowed generation of sensors with a wide dynamic range. Critically, in mammalian cells the sensors show consistent changes in fluorescence intensity ratios upon depletion or restoration of GTP pools. These sensors are suitable for detecting spatio-temporal changes in GTP levels in living cells, and for the development of high throughput screenings of molecules modulating intracellular GTP levels.