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Interaction of the Halobacterial Transducer to a Halorhodopsin Mutant Engineered so as to Bind the Transducer: Cl Circulation Within the Extracellular Channel


  • This paper is part of the Proceedings of the 12th International Conference on Retinal Proteins held at Awaji Island, Hyogo, Japan on 4–8 June 2006.

*email: (N. Kamo)


An alkali-halophilic archaeum, Natronomonas pharaonis, contains two rhodopsins that are halorhodopsin (phR), a light-driven inward Cl pump and phoborhodopsin (ppR), the receptor of negative phototaxis functioning by forming a signaling complex with a transducer, pHtrII (Sudo Y. et al., J. Mol. Biol. 357 [2006] 1274). Previously, we reported that the phR double mutant, P240T/F250YphR, can bind with pHtrII. This mutant itself can transport Cl, while the net transport was stopped upon formation of the complex. The flash-photolysis data were analyzed by a scheme in which phR→P1→P2→P3→P4phR. The P3 of the wild-type and the double mutant contained two components, X- and O-intermediates. After the complex formation, however, the P3 of the double mutant lacked the X-intermediate. These observations imply that the X-intermediate (probably the N-intermediate) is the state having Cl in the cytoplasmic binding site and that the complex undergoes an extracellular Cl circulation because of the inhibition of formation of the X-intermediate.