Small Molecule–Protein Hybrid for Voltage Imaging via Quenching of Bioluminescence

We report a small-molecule enzyme pair for optical voltage sensing via quenching of bioluminescence. This quenching bioluminescent voltage indicator, or Q-BOLT, pairs the dark absorbing, voltage-sensitive dipicrylamine with membrane-localized bioluminescence from the luciferase NanoLuc (NLuc). As a result, bioluminescence is quenched through resonance energy transfer (QRET) as a function of membrane potential. Fusion of HaloTag to NLuc creates a two-acceptor bioluminescence resonance energy transfer (BRET) system when a tetramethylrhodamine (TMR) HaloTag ligand is ligated to HaloTag. In this mode, Q-BOLT is capable of providing direct visualization of changes in membrane potential in live cells via three distinct readouts: change in QRET, BRET, and the ratio between bioluminescence emission and BRET. Q-BOLT can provide up to a 29% change in bioluminescence (ΔBL/BL) and >100% ΔBRET/BRET per 100 mV change in HEK 293T cells, without the need for excitation light. In cardiac monolayers derived from human-induced pluripotent stem cells (hiPSCs), Q-BOLT readily reports on membrane potential oscillations. Q-BOLT is the first example of a hybrid small molecule–protein voltage indicator that does not require excitation light and may be useful in contexts where excitation light is limiting.