High-Performance Chemigenetic\nPotassium Ion Indicator

Potassium ion (K<sup>+</sup>) is the most abundant metal\nion in\ncells and plays an indispensable role in practically all biological\nsystems. Although there have been reports of both synthetic and genetically\nencoded fluorescent K<sup>+</sup> indicators, there remains a need\nfor an indicator that is genetically targetable, has high specificity\nfor K<sup>+</sup> versus Na<sup>+</sup>, and has a high fluorescent\nresponse in the red to far-red wavelength range. Here, we introduce\na series of chemigenetic K<sup>+</sup> indicators, designated as the\nHaloKbp1 series, based on the bacterial K<sup>+</sup>-binding protein\n(Kbp) inserted into HaloTag7 self-labeled with environmentally sensitive\nrhodamine derivatives. This series of high-performance indicators\nfeatures high brightness in the red to far-red region, large intensiometric\nfluorescence changes, and a range of <i>K</i><sub>d</sub> values. We demonstrate that they are suitable for the detection\nof physiologically relevant K<sup>+</sup> concentration changes such\nas those that result from the Ca<sup>2+</sup>-dependent activation\nof the BK potassium channel.