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Time-Resolved Detection of Sensory Rhodopsin II-Transducer Interaction

Keiichi Inoue ^*, Jun Sasaki , Masayo Morisaki , Fumio Tokunaga  and Masahide Terazima ^*

^* Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto, 606-8502, Japan; Center for Membrane Biology, Department of Biochemistry and Molecular Biology, University of Texas Health Science Center, Houston, Texas 77030-1503 USA; and Department of Earth and Space Science, Graduate School of Science, Osaka University, Osaka, 560-0055 Japan

Correspondence: Address reprint requests to Masahide Terazima, Tel.: 81-75-753-4026; Fax: 81-75-753-4000; E-mail: mterazima{at}kuchem.kyoto-u.ac.jp.

The dynamics of protein conformational change of Natronobacterium pharaonis sensory rhodopsin II (NpSRII) and of NpSRII fused to cognate transducer (NpHtrII) truncated at 159 amino acid sequence from the N-terminus (NpSRII-NpHtrII) are investigated in solution phase at room temperature by the laser flash photolysis and the transient grating methods in real time. The diffusion coefficients of both species indicate that the NpSRII-NpHtrII exists in the dimeric form in 0.6% dodecyl-ß-maltopyranoside (DM) solution. Rate constants of the reaction processes in the photocycles determined by the transient absorption and grating methods agree quite well. Significant differences were found in the volume change and the molecular energy between NpSRII and NpSRII-NpHtrII samples. The enthalpy of the second intermediate (L) of NpSRII-NpHtrII is more stabilized compared with that of NpSRII. This stabilization indicates the influence of the transducer to the NpSRII structure in the early intermediate species by the complex formation. Relatively large molecular volume expansion and contraction were observed in the last two steps for NpSRII. Additional volume expansion and contraction were induced by the presence of NpHtrII. This volume change, which should reflect the conformational change induced by the transducer protein, suggested that this is the signal transduction process of the NpSRII-NpHtrII.

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