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  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→P 4→phR. 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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry