Abstract

Microbial rhodopsin proteins are found in aquatic organisms throughout the world and in all kingdoms of life. In the wild, these proteins transduce sunlight into changes in membrane potential, which their host organisms use for phototaxis and solar energy capture. We engineered several microbial rhodopsins to run backward: to transduce changes in membrane potential into an optical signal. Upon expression in E. coli, we observed a dramatic blinking behavior, reminiscent of action potentials in neurons. These electrical spikes were correlated with efflux of cytoplasmic contaminants. We also expressed voltage indicators in rat neurons and human induced pluripotent stem cell (iPS) cardiomyocytes. In both cell types we recorded electrical dynamics with sub-millisecond temporal resolution and sub-micron spatial resolution. These experiments provide the first movies of electrical waves propagating within a single cell.