(a) Optical step responses of cultured HEK293T cells transfected with MacQ, and cultured neurons transfected with MacQ-mOrange2 and MacQ-mCitrine (440 Hz frame rate). The MacQ VSD increased its fluorescence intensity with increased voltage depolarization (left). The donor fluorescent proteins of the FRET-opsin sensors exhibited decreases in fluorescence intensity with increased depolarization (middle, right). We held neurons at −70 mV at the start of each trace and stepped to command voltages ranging from −140 mV to +100 mV.
(b) Steady state fluorescence responses of MacQ-mOrange2 and MacQ-mCitrine as a function of neuronal membrane voltage.
Illumination at the specimen plane was 1400 mW mm−2 (λ = 633 nm) for the MacQ studies, and 15 mW mm−2 (λ = 530 nm and 505 nm, for MacQ-mOrange and MacQ-mCitrine, respectively) for the FRET-opsin sensors. Error bars are s.e.m.