Microbial Rhodopsins: Receptors, Channels, and Pumps from a Single Design

Abstract

The microbial rhodopsin family is comprised of ~4800 homologous proteins containing 7 transmembrane helices forming a pocket for the chromophore retinal. Most are light‐driven ion pumps ("transport rhodopsins") and others are photosensory receptors ("sensory rhodopsins"). Since we identified the first sensory rhodopsin in 1982, homologous photosensors have been found to be widely distributed in archaea, eubacteria, and unicellular eukaryotes, and to signal with remarkably diverse mechanisms. Some of the best studied are the phototaxis receptors in haloarchaeal prokaryotes (SRI and SRII) and in eukaryotic algae (channelrhodopsins). SRI and SRII transmit signals by protein‐protein interaction to control a cytoplasmic phosphorylation cascade that modulates the prokaryotic flagellar motors. Mediating algal phototaxis, channelrhodopsins are light‐gated cation channels that depolarize the membrane to trigger Ca++‐influx into the eukaryotic flagellar axoneme. Crystallography, spectroscopy, and genetic engineering have begun to clarify how modifications of the same architecture enable the microbial rhodopsins to carry out their distinctly different molecular functions. Interconversions of their functions by mutation reveal the elegant simplicity by which evolution uses existing genes to create proteins with novel functions.