A protein-coupled fluorescent probe for organelle-specific imaging of Na⁺

Intracellular sodium concentration plays important roles in many cellular processes, but methods to analyze dynamic change of [Na⁺] in organelles of interest are still lacking. To address this problem, controlled localization of fluorescent Na⁺ probes is a key issue. In this work, we developed a protein-coupled Na⁺-sensitive fluorescent probe, HaloNa-1, which covalently labelled a HaloTag protein expressed at specific organelles in living cells. Fluorescence intensity of HaloNa-1 increased by approximately three-fold when complexed with Na⁺, and the dissociation constant was within physiological range. Notably, by conjugating a fluorescent protein to the HaloTag protein and using it as internal standard, one can reduce unwanted variation of probe fluorescence due to different cell volume, HaloTag expression level, etc. With this technique, we could observe elevation of intracellular [Na⁺] after ionophore stimulation with higher reliability than conventional sensors. We believe our probe is potentially useful to understand Na⁺ dynamics inside cells.