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 indicator, enable dual-color imaging of K and Ca dynamics in neurons and glial cells. These results demonstrate that KIRIN1 and GINKO1 are useful tools for imaging intracellular K dynamics.