Ion-transporting mechanism in microbial rhodopsins: Mini-review relating to the session 5 at the 19th International Conference on Retinal Proteins
Abstract
Microbial rhodopsin is basically composed of seven transmembrane helices and binds an all-trans retinal as the chromophore through a protonated Schiff base linkage with a specific lysin residue on the seventh helix [1,2].Upon specific wavelength of light activation, isomerization of the retinal chromophore from all-trans to 13-cis configuration initiates the cyclic photoreaction (Figure 1a).Initially, four kinds of microbial rhodopsins were found from an extremely halophilic archaeon, Halobacterium salinarum, and extensive studies were conducted; they are bacteriorhodpsin (BR), halorhodopsin (HR), sensory rhodopsin I (SRI), and sensory rhodopsin II (SRII).BR functions as an outward proton pump [3], while HR functions as inward chloride ion pump.They both are regarded as ion transporting rhodopsins.On the other hand, SRI and SRII each functions as light sensors regulating the positive and negative phototactic behavior of a bacterial cell, respectively.Since 2000, the metagenomic analysis opened new worlds of microbial rhodopsins in the ocean, fresh water, soils, etc. Nowadays, various kinds of functions, including but not limited to light-driven ion pump, light sensor, light-gated channel, light-regulated enzyme, and more are identified in new microbial rhodopsins (Figure 1b) [2,4].In this brief review, ion-transporting rhodopsins are main focus.An outward proton-pumping rhodopsin generates proton gradient and forms electrochemical potential or proton mobile force (PMF)…
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