AN EXTREME-XANTHORHODOPSIN FROM SALINIVIBRIO SOCOMPENSIS, A HALOPHILIC STRAIN ISOLATED FROM MODERN STROMATOLITES

GORRITI M.; BAMANN, C.; ALONSO REYES D.; WOOD, P.G.; BAMBERG, E; FARIAS M.E.; GARTNER, WOLFGANG; ALBARRACÍN V. H.

Photochemical and Photobiological Sciences

ROYAL SOC CHEMISTRY

Lugar: CAMBRIDGE; Año: 2023

1474-905X

A putative xanthorhodopsin-encoding gene, XR34, was found in the genome of themoderately halophilic gammaproteobacterium Salinivibrio socompensis S34, isolatedfrom modern stromatolites found on the shore of Laguna Socompa (3,570 m),Argentina Puna. XR-encoding genes were clustered together with genes encoding Xcarotene,retinal (vitamin-A aldehyde), and carotenoid biosynthesis enzymes while thecarotene ketolase gene critical for the salinixanthin antenna compound was absent. Toidentify its functional behaviour, we herein overexpressed and characterized thisintriguing microbial rhodopsin. Recombinant XR34 showed all the salient features ofcanonical microbial rhodopsin and covalently bound retinal as a functionalchromophore with max = 561 nm (max ca. 60´000 M-1 cm-1). Two canonicalcounterions with pK values of around 6 and 3 were identified by pH titration on therecombinant protein. With a recovery time of approximately half an hour in the dark,XR34 shows light-dark adaptation shifting the absorption maximum from 551 nm to561 nm. Laser-flash induced photochemistry at pH 9 (deprotonated primarycounterion) identified a photocycle starting with a K-like intermediate, followed by a M state(λmax ca. 400 nm, deprotonated Schiff base), and a final long wavelength absorbingN- or O-like intermediate before returning to the parental 561 nm state.Initiating the photocycle at pH 5 (protonated counterion) yields only bathochromicintermediates, due to the lacking capacity of the counterion to accept the Schiff baseproton. Illumination of the membrane-embedded protein yielded a capacitive transportcurrent. The presence of the M-intermediate under these conditions was demonstratedby a blue light-induced shunt process.