Chromophore Structural Change during the Photocycle of a Light-Driven Cl- Pump from Mastigocladopsis repens: A Cryogenic Raman Study.

Microbial rhodopsins are the most widely distributed photoreceptors that bind a retinal Schiff base chromophore. Among them, a light-driven Cl^- pump discovered from Mastigocladopsis repens (MrHR) is distinctive in that it has the structural features of both H⁺ and Cl^- pumps. While the photocycle has been characterized by light-induced changes of the absorption spectrum, the structural changes of the retinal chromophore remain largely unknown. In this study, we examined the chromophore structural changes of MrHR by using cryogenic Raman spectroscopy. We observed five photointermediates─K, L, N1, N2, and MrHR'─that show distinct vibrational spectra, indicating atypical chromophore structures, e.g., small distortion in the K intermediate and Schiff base configurational change in the MrHR' intermediate. Based on the Raman spectra of two N intermediates (N1 and N2), we propose that N1 is the Cl^--bound state and N2 is the Cl^--unbound state, which are responsible for the Cl^- release and uptake, respectively, to achieve Cl^- pumping.