Voltage imaging with engineered proton-pumping Rhodopsins: Insights from the proton transfer pathway

Voltage imaging using genetically encoded voltage indicators (GEVI) has taken the field of neuroscience by storm in the past decade. Its ability to create subcellular and network level readouts of electrical dynamics depends critically on the kinetics of the response to voltage of the indicator used. Engineered Microbial Rhodopsins form a GEVI sub-class known for their high voltage sensitivity and fast response kinetics. Here we review the essential aspects of mi-crobial rhodopsin photocycles that are critical to understanding the mechanisms of voltage sensitivity in these pro-teins and link them to insights from efforts to create faster, brighter and more sensitive Microbial Rhodopsin-based GEVIs.