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.
Photoreceptor proteins play a critical
role in light utilization
for energy conversion and environmental sensing. Rhodopsin is a prototypical
photoreceptor protein containing a retinal group that functions as
a light-receptive site. It is essential to characterize the structure
of the retinal chromophore because the chromophore structure, along
with retinal–protein interactions, regulates which wavelengths
of light are absorbed. Resonance Raman spectroscopy is a powerful
tool to characterize chromophore structures in proteins. The resonance
Raman spectra of heliorhodopsins, a recently discovered rhodopsin
family, were previously reported to exhibit two intense ethylenic
CC stretching bands never observed for type-1 rhodopsins.
Here, we show that the double-band feature in the ethylenic CC
stretching modes is not due to structural inhomogeneity but rather
to the retinal polyene chain’s linear structure. It contrasts
with bent all-trans chromophore in type-1 rhodopsins.
The linear structure of the chromophore results from weak atomic contacts
between the 13-methyl group and a nearby Trp side chain, which can
slow thermal reisomerization in the photocycle. It is possible that
the deceleration of reisomerization increases the lifetime of the
signaling intermediate for photosensory function.
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