The Diversity of Light-Driven Ion Pumps and their Conversion into Ion Channels

Microbial rhodopsins are integral seven-transmembrane helix proteins, which covalently bind all-trans-retinal as light-sensitive chromophore. They play an extremely important role in neuroscience as optogenetic tools for the modulation of neuronal activity. These rhodopsins are subdivided into sensory rhodopsins, ion channels and ion pumps.Light-driven ion pumps transport protons, sodium or chloride across the plasma membrane against an electrochemical gradient. We analyzed a variety of ion pumps using two-microelectrode voltage-clamp measurements (TEVC) in Xenopus leavis oocytes. We found that all pumps show different electrophysiological behaviors. Especially light-driven proton pumps can be subdivided according to their characteristics at high electrochemical load, i.e. low extracellular pH and negative voltage. Photocurrents of Bacteriorhodopsin and the rhodopsin from the eukaryotic microalga Coccomyxa subellipsoidea (CsR) are always outward directed or inactivate at high load. In contrast, the rhodopsins from Exiguobacterium sibiricum (ESR) and from the cyanobacterium Gloeobacter violaceus (GR) show inward directed photocurrents at high load. The well-established rhodopsin Arch3 from the archaeon Halorubrum sodomense shows weak inward directed photocurrents at high load.Most recently, the class of sodium pumps was discovered, but so far their underlying mechanism and ion selectivity is poorly understood. Here, we show the first more comprehensive electrophysiological characterization of the light-driven sodium-pumps in native membranes. They are characterized by an unexpected strong pH-dependency. In earlier work we have shown that proton pumps can be converted into operational proton channels via the introduction of selected mutations. Now, we transferred this approach to the sodium pumps and attained also inward directed photocurrents. This observation opens the opportunity to engineer new optogenetic tools with high ion-selectivity.