Signal transduction via membrane protein complex : photo-signal transduction of Halobacterium salinarum

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Many biological functions are performed by membrane protein complexes, one example of which is the photo-reception/taxis of halobacterium. In this study, we investigated the photo-reception via the signaling complex of pharaonis phoborhodopsin (ppR, or pharaonis sensory rhodopsin) and its transducer (pHtrII) in Natronomonas pharaonis. Results obtained are :1.ppR is a retinal membrane protein. Recently many papers report the existence of retinal proteins in many micro-organisms, and these retinal proteins are called microbial rhodopsin. Arginine residues near the retinal Schiff base are conserved among these rhodopsins. The arginine residue in ppR was mutated and its role was investigated.2.Illumination to ppR induced conformational changes of the F-helix and hydrogen bonding networks.3.The conformational changes were also viewed by a solid-state NMR.4.Illumination elicited a proton release/uptake of ppR, which was monitored by a transparent SnO_2 electrode.5.Interaction between a cytoplasmic part of pHtrII and a loop of ppR as well as interaction within the membrane were shown.6.The intra-membranous three hydrogen bonds were verified using various mutants.7.On illumination, ppR alone can transport protons from inside to outside, and its pumping activity loses when it combined with pHtrll. Halorhodopsin (hR) is inwardly light-driven Cl^- pump, and when hR was engineered so as to be able to bind with pHtrII, the complex lost the pumping activity while the mutant alone retained the activity.8.The thermodynamic parameters of Cl- binding of hR were determined by isothermal titration calorimetry.