Structural analyses of reaction states of ion-pumping rhodopsins

2019 Fiscal Year Final Research Report

• Project/Area Number: 16K07267
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Multi-year Fund
• Section: 一般
• Research Field: Structural biochemistry
• Research Institution: Nagoya University
• Principal Investigator: Kouyama Tsutomu 名古屋大学, 理学研究科, 名誉教授 (30170210)
• Co-Investigator(Kenkyū-buntansha): 井原 邦夫 名古屋大学, 遺伝子実験施設, 准教授 (90223297)
• Project Period (FY): 2016-04-01 – 2020-03-31
• Keywords: イオン能動輸送 / 光駆動塩素イオンポンプ / 光駆動プロトンポンプ / レチナールタンパク質 / レチナール異性体 / X線結晶構造解析 / ハロロドプシン

Outline of Final Research Achievements

The anion pumping cycle of halorhodopsin is initiated when the all-trans/15-anti isomer of retinal is photo-isomerized into the 13-cis/15-anti configuration. A previous study suggested that a reaction state with 13-cis/15-syn retinal occurred during the anion release process, i.e., after the N state with the 13-cis/15-anti retinal and before the O state with all-trans/15-anti retinal. In this study, we found that the 13-cis isomer (HR'), in which the absorption spectrum was blue-shifted by 8 nm as compared with that of the trans isomer (HR), accumulated significantly when pHR-rich claret membranes in 4 M NaBr was illuminated with continuous light. It was suggested that the branching of the trans photocycle into the 13-cis isomer (HR’) occurs during the decay of an O-like state (O’) with 13-cis/15-syn retinal. At a high bromide ion concentration, the anion pumping cycle is described by the scheme: HR -(hν)-> K -> L1 -> N -> N’ -> O’ -> HR’-> HR.

Free Research Field

生物物理

Academic Significance and Societal Importance of the Research Achievements

ハロロドプシンの陰イオン輸送サイクル中に3段階のレチナール異性化反応（13-trans/15-anti―→13-cis/15-anti―→13-cis/15-syn―→13-trans/15-anti）が起こることを明らかにした。タンパク質部分も反応サイクルの間に3種類の構造変化を起こすことが示されており、これらの結果を基に、レチナールの異性化→タンパク質の構造変化→イオン移動→レチナールの異性化、…、という具合に反応が進行することで、イオンの一方向の輸送が保証される、という作業仮説を提唱した。これは、ロドプシン群タンパク質の光反応機構に関する従来の基本的な考えを大きく変えることになる。