Figure 1.

Designs for voltage-sensor domain-based voltage indicators: (a) Upper panel, schematic of FRET-based voltage-sensitive probes of the VSFP2 family. The voltage-sensor domain, consisting of four segments (S1–S4) crossing the plasma membrane (PM), is fused to a pair of fluorescent proteins (FP, D FRET donor; FP, A FRET acceptor). A change in membrane potential induces a rearrangement of the two fluorescent proteins that is optically reported as a change in the ratio between donor and acceptor fluorescence. Lower panels, example recording from cultured hippocampal cells showing spontaneous action potential firing. The three sweeps of optical recordings shown in black, red, and blue color correspond to the superimposed microelectrode recording traces of same color. (b) Single fluorescent protein probes of the VSFP3 family. (c) VSFPs incorporating a circularly permuted fluorescent protein. (d) FRET-based voltage sensitive probe of the VSFP-Butterfly family, where the voltage-sensor domain is sandwiched between two fluorescent proteins.