Issue 5, 2018

Spectroscopic characteristics of Rubricoccus marinus xenorhodopsin (RmXeR) and a putative model for its inward H+ transport mechanism

Abstract

A new group of microbial rhodopsins named xenorhodopsins (XeR), which are closely related to the cyanobacterial Anabaena sensory rhodopsin, show a light-driven “inward” proton transport activity, as reported for one representative of this group from Parvularcula oceani (PoXeR). In this study, we functionally and spectroscopically characterized a new member of the XeR clade from a marine bacterium Rubricoccus marinus SG-29T (RmXeR). Escherichia coli cells expressing recombinant RmXeR showed a light-induced alkalization of the cell suspension, which was strongly impaired by a protonophore, suggesting that RmXeR is a light-driven “inward” proton pump as is PoXeR. The spectroscopic properties of purified RmXeR were investigated and compared with those of PoXeR and a light-driven “outward” proton pump, bacteriorhodopsin (BR) from the archaeon Halobacterium salinarum. Action spectroscopy revealed that RmXeR with all-trans retinal is responsible for the light-driven inward proton transport activity, but not with 13-cis retinal. From pH titration experiments and mutational analysis, we estimated the pKa values for the protonated Schiff base of the retinal chromophore and its counterion as 11.1 ± 0.07 and 2.1 ± 0.07, respectively. Of note, the direction of both the retinal composition change upon light–dark adaptation and the acid-induced spectral shift was opposite that of BR, which is presumably related to the opposite directions of ion transport (from outside to inside for RmXeR and from inside to outside for BR). Flash photolysis experiments revealed the appearances of three intermediates (L, M and O) during the photocycle. The proton uptake and release were coincident with the formation and decay of the M intermediate, respectively. Together with associated findings from other microbial rhodopsins, we propose a putative model for the inward proton transport mechanism of RmXeR.

Graphical abstract: Spectroscopic characteristics of Rubricoccus marinus xenorhodopsin (RmXeR) and a putative model for its inward H+ transport mechanism

Supplementary files

Article information

Article type
Paper
Submitted
02 ذو القعدة 1438
Accepted
19 محرم 1439
First published
19 محرم 1439

Phys. Chem. Chem. Phys., 2018,20, 3172-3183

Spectroscopic characteristics of Rubricoccus marinus xenorhodopsin (RmXeR) and a putative model for its inward H+ transport mechanism

S. Inoue, S. Yoshizawa, Y. Nakajima, K. Kojima, T. Tsukamoto, T. Kikukawa and Y. Sudo, Phys. Chem. Chem. Phys., 2018, 20, 3172 DOI: 10.1039/C7CP05033J

To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page.

If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given.

If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page.

Read more about how to correctly acknowledge RSC content.

Social activity

Spotlight

Advertisements