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 photointermediatesK,
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.