The Light-Activated Proton Pump Bop I of The Archaeon Haloquadratum walsbyi

We have isolated and characterized the light-driven proton pump Bop I from the ultrathin square archaeon Haloquadratum walsbyi, the most abundant component of the dense microbial community inhabiting hypersaline environments. The disruption of cells by hypo-osmotic shock yielded Bop I retinal protein highly enriched membranes, which contain one main 27 kDa protein band together with a high content of the carotenoid bacterioruberin. Light-induced pH changes were observed in suspensions of Bop I retinal protein-enriched membranes under sustained illumination. Solubilization of H. walsbyi cells with Triton X-100, followed by phenyl-Sepharose chromatography, resulted in isolation of two purified Bop I retinal protein bands; mass spectrometry analysis revealed that the Bop I was present as only protein in both the bands. The study of light/dark adaptations, M-decay kinetics, responses to titration with alkali in the dark and endogenous lipid compositions of the two Bop I retinal protein bands showed functional differences that could be attributed to different protein aggregation states. Proton-pumping activity of Bop I during the photocycle was observed in liposomes constituted of archaeal lipids. Similarities and differences of Bop I with other archaeal proton-pumping retinal proteins will be discussed. Haloquadratum walsbyi is a peculiar organism which often dominates the microbial communities of the hypersaline ecosystems, such as salt lakes and solar saltern crystallizer ponds. It is extremely thin and possesses a unique square-like shape, with sharp edges and acute straight corners. The flat cells form large sheets similar to solar panels, able to efficiently collect light as an energy source for metabolism. Like other archaeal extremely halophilic microorganisms, square cells encode light-activated retinal-proteins to survive in hypersaline environments. In this study we describe the biochemical properties and the photochemistry of the light-activated proton pump Bop I of H. walsbyi, grown in laboratory.