The Photocycle and Proton Translocation Pathway in a Cyanobacterial Ion-Pumping Rhodopsin

다국어 초록 (Multilingual Abstract)

The genome of thylakoidless cyanobacterium Gloeobacter violaceus encodes a fast-cycling rhodopsin capable of light-driven proton transport. We characterize the dark state, the photocycle, and the proton translocation pathway of GR spectroscopically. The dark state of GR contains predominantly all-trans-retinal and, similar to proteorhodopsin, does not show the light/dark adaptation. We found an unusually strong coupling between the conformation of the retinal and the site of Glu<SUP>132</SUP>, the homolog of Asp<SUP>96</SUP> of BR. Although the photocycle of GR is similar to that of proteorhodopsin in general, it differs in accumulating two intermediates typical for BR, the L-like and the N-like states. The latter state has a deprotonated cytoplasmic proton donor and is spectrally distinct from the strongly red-shifted N intermediate known for proteorhodopsin. The proton uptake precedes the release and occurs during the transition to the O intermediate. The proton translocation pathway of GR is similar to those of other proton-pumping rhodopsins, involving homologs of BR Schiff base proton acceptor and donor Asp<SUP>85</SUP> and Asp<SUP>96</SUP> (Asp<SUP>121</SUP> and Glu<SUP>132</SUP>). We assigned a pair of FTIR bands (positive at 1749 cm<SUP>-1</SUP> and negative at 1734 cm<SUP>-1</SUP>) to the protonation and deprotonation, respectively, of these carboxylic acids.