Genetically encoded fluorescent indicators for imaging intracellular potassium ion concentration

Potassium ion (K(+)) homeostasis and dynamics play critical roles in biological activities. Here we describe three genetically encoded K(+) indicators. KIRIN1 (potassium (K) ion ratiometric indicator) and KIRIN1-GR are Förster resonance energy transfer (FRET)-based indicators with a bacterial K(+) binding protein (Kbp) inserting between the fluorescent protein FRET pairs mCerulean3/cp173Venus and Clover/mRuby2, respectively. GINKO1 (green indicator of K(+) for optical imaging) is a single fluorescent protein-based K(+) indicator constructed by insertion of Kbp into enhanced green fluorescent protein (EGFP). These indicators are suitable for detecting K(+) at physiologically relevant concentrations in vitro and in cells. KIRIN1 enabled imaging of cytosolic K(+) depletion in live cells and K(+) efflux and reuptake in cultured neurons. GINKO1, in conjunction with red fluorescent Ca(2+) indicator, enable dual-color imaging of K(+) and Ca(2+) dynamics in neurons and glial cells. These results demonstrate that KIRIN1 and GINKO1 are useful tools for imaging intracellular K(+) dynamics.