Resonance Raman Determination of Chromophore Structures of Heliorhodopsin Photointermediates.

Light is utilized as energy or information by rhodopsins (membrane proteins that contain a retinal chromophore). Heliorhodopsins (HeRs) are a new class of rhodopsins with low sequence identity (<15%) to microbial and animal rhodopsins. Their physiological roles remain unknown, although the involvement of a long-lived intermediate in the photocycle suggests a light-sensor function. Characterization of the molecular structures of the intermediates is essential to an understanding of the roles and mechanisms of HeRs. We determined the chromophore structures of the intermediates in HeR 48C12 by time-resolved resonance Raman spectroscopy and observed that the hydrogen bond of the protonated Schiff base strengthened prior to deprotonation. The chromophore is photoisomerized from the all-trans to the 13-cis form and is reisomerized in the transition from the O intermediate to the unphotolyzed state. Our results demonstrate that the chromophore structure evolves similarly to microbial rhodopsins, despite the dissimilarity in amino acid residues surrounding the chromophore.