High-Performance Chemigenetic Potassium Ion Indicator.

Potassium ion (K) is the most abundant metal ion in cells and plays an indispensable role in practically all biological systems. Although there have been reports of both synthetic and genetically encoded fluorescent K indicators, there remains a need for an indicator that is genetically targetable, has high specificity for K versus Na, and has a high fluorescent response in the red to far-red wavelength range. Here, we introduce a series of chemigenetic K indicators, designated as the HaloKbp1 series, based on the bacterial K-binding protein (Kbp) inserted into HaloTag7 self-labeled with environmentally sensitive rhodamine derivatives. This series of high-performance indicators features high brightness in the red to far-red region, large intensiometric fluorescence changes, and a range of values. We demonstrate that they are suitable for the detection of physiologically relevant K concentration changes such as those that result from the Ca-dependent activation of the BK potassium channel.