Transport and Sensory Rhodopsins in Microorganisms

Authored by: Yuki Sudo

CRC Handbook of Organic Photochemistry and Photobiology

Print publication date:  March  2012
Online publication date:  March  2012

Print ISBN: 9781439899335
eBook ISBN: 9781466561250
Adobe ISBN:

10.1201/b12252-50

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Abstract

Rhodopsin molecules are photochemically reactive membrane-embedded proteins, with seven transmembrane α-helices which bind the chromophore retinal (vitamin A aldehyde). 1,2 Rhodopsins are classified into two groups, microbial (type 1) and mammalian (type 2). 3 Type 2 rhodopsins, such as visual pigments, are G-protein-coupled receptors (GPCRs) that are widespread in vertebrates and invertebrates. 2,4,5 This chapter reviews type 1 rhodopsins functioning as light-driven ion transporters or photosensory receptors in microorganisms. They are widespread in the microbial world in prokaryotes (bacteria and archaea) and in eukaryotes (fungi and algae). 3,6,7 A striking characteristic of these photoactive proteins is their wide range of seemingly dissimilar functions. Some are light-driven transporters, such as the proton pump bacteriorhodopsin (BR) and the chloride pump halorhodopsin (HR) (Figure 49.1). 3,8 Others are light sensors, such as the phototaxis receptors sensory rhodopsins I and II (SRI and SRII). 9 SRI and SRII relay signals by protein–protein interactions to integral membrane transducer proteins HtrI and HtrII, respectively, and SRI–HtrI and SRII–HtrII complexes control the flagellar motor rotation through kinases (Figure 49.1). The microbial rhodopsins have become a focus of interest, in part because of their importance to the general understanding of ion flux across membranes, and communication between integral membrane proteins, about which little is known.

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