5Institut de Biologie Structurale, Centre National de la Recherche Scientifique, 38044 Grenoble, France
6Institut de Biologie Structurale, Direction des Sciences du Vivant, Commissariat à l'Énergie Atomique, 38044 Grenoble, France
7Laboratory for Advanced Studies of Membrane Proteins, Moscow Institute of Physics and Technology, 141700 Dolgoprudniy, Moscow Region, Russia
8Institute of Complex Systems (ICS), ICS-6: Structural Biochemistry, Research Centre Jülich, 52425 Jülich, Germany
* To whom correspondence should be addressed.
Received January 12, 2015; Revision received March 10, 2015
This review covers the properties of a retinal protein (ESR) from the psychrotrophic bacterium Exiguobacterium sibiricum that functions as a light-driven proton pump. The presence of a lysine residue at the position corresponding to intramolecular proton donor for the Schiff base represents a unique structural feature of ESR. We have shown that Lys96 successfully facilitates delivery of protons from the cytoplasmic surface to the Schiff base, thus acting as a proton donor in ESR. Since proton uptake during the photocycle precedes Schiff base reprotonation, we conclude that this residue is initially in the uncharged state and acquires a proton for a short time after Schiff base deprotonation and M intermediate formation. Involvement of Lys as a proton donor distinguishes ESR from the related retinal proteins – bacteriorhodopsin (BR), proteorhodopsin (PR), and xanthorhodopsin (XR), in which the donor function is performed by residues with a carboxyl side chain. Like other eubacterial proton pumps (PR and XR), ESR contains a histidine residue interacting with the proton acceptor Asp85. In contrast to PR, this interaction leads to shift of the acceptor’s pKa to more acidic pH, thus providing its ability to function over a wide pH range. The presence of a strong H-bond between Asp85 and His57, the structure of the proton-conducting pathways from cytoplasmic surface to the Schiff base and to extracellular surface, and other properties of ESR were demonstrated by solving its three-dimensional structure, which revealed several differences from known structures of BR and XR. The structure of ESR, its photocycle, and proton transfer reactions are discussed in comparison with homologous retinal proteins.
KEY WORDS: retinal protein, proteorhodopsin, Exiguobacterium sibiricum, Schiff base, proton acceptor, proton donor, photocycle