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

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 132, the homolog of Asp 96 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 85 and Asp 96 (Asp 121 and Glu 132). We assigned a pair of FTIR bands (positive at 1749 cm −1 and negative at 1734 cm −1) to the protonation and deprotonation, respectively, of these carboxylic acids.