pH-insensitive glucose indicators.

There is an urgent need for developing a biosensor that can real-time and noninvasively determine glucose concentration within living cells. In our previous study, we have engineered a glucose indicator protein (GIP) that can provide continuous glucose monitoring through a conformation change-induced Förster resonance-energy transfer measurement. Because of the pH-sensitivity of the fluorescent proteins used in the GIP construction, the GIP made from these fluorescent proteins is less tolerant to a pH change, especially to the acidic environment. It has been well documented that intracellular pH does not always remain the same, and it fluctuates in metabolism and other cellular activities and also differs between cellular compartments. To address these issues, we developed a GIP that can tolerate to pH change. This GIP was constructed by flanking a glucose binding protein with a cyan fluorescent protein and a pH-insensitive yellow fluorescent protein. Our experimental results indicated that the new GIP is more tolerant to pH change. The glucose response of this new GIP kept almost unchanged from pH 7.3 to 5.3, suggesting its capability of tolerating to acidic environment. This capability is desirable for intracellular glucose measurement.