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 K_d values. We demonstrate that they are suitable for the detection of physiologically relevant K⁺ concentration changes such as those that result from the Ca²⁺-dependent activation of the BK potassium channel.