BioPHYSICAL chemistry

located at very similar positions, the loops connecting the
helices differ. The domains in the extramembraneous
regions are thought to shift during the photocycle in
a manner that is not fully established, to bind proteins
such as tranducin during the signaltransduction process
(Figure 17.3).
Members of the rhodopsin family are found in all three
domains of life, the Archaea, Eubacteria, and Eukaryota.
The strong structural homology between rhodopsin and
bacteriorhodopsin is consistent with the concept that
all opsins have similar folds with seven transmembrane
helices surrounding a retinal molecule (Figure 17.12).
Despite the structural homology, the different opsins have
significantly different functions involving light-driven ion
transport or photosensing signaling. Rhodopsin serves as
a G-coupled protein whereas bacteriorhodopsin serves
as a proton pump. Halorhodopsin is found in halophilic
archaea and serves primarily as a chloride pump, although
it can also transport other ions such as bromide. Sensory
rhodopsins (SRI and SRII) are found in the membrane
with other proteins, such as kinases, which serve to trans-
mit the light-induced structural changes into cellular
signals.
The overall structure of sensory rhodopsin is very
similar to that of bacteriorhodopsin, with the presence
of the transmembrane helices. The loops connecting
the helices are more extensive and serve as part of the
binding domain for theG-proteins. Sensory rhodopsin
contains an 11-cis-retinal chromatophore that isomerizes
to an all-trans-configuration upon light absorption. The
largest difference compared to bacteriorhodopsin and
rhodopsin is the presence of additional subunits, the
bound transducer proteins. In response tolight-induced
structural changes of the helices surrounding the retinal,
which have not yet been delineated, these transducer
proteins initialize a phosphorylation cascade that regulates the flagellar
motors. Although the structures of the transducer proteins have not yet
been determined, structures of soluble domains suggest that the proteins
are remarkably long, with a total length of about 400 Å and the bulk of
the protein on the cytoplasmic side of the membrane, formed by long
helices that are responsible for the signaling activity.
Halorhodopsin transports chloride ions rather protons but still possesses
many of the same structural features as bacteriorhodopsin. Both have seven
transmembrane helices surrounding the retinal, although halorhodopsin
is a trimer in the membrane rather than a monomer. Halorhodopsin has

CHAPTER 17 SIGNAL TRANSDUCTION 385

1

2

3

4

5

H

H

Asp-96

Asp-85

Arg-82

Glu-204 Glu-194

Retinal

Figure 17.11Proton transfer can

be considered to be driven by a

light-activated switch that activates a

series of steps (1–5). 1, the sequential

deprotonation of the Schiff base and

protonation of Asp-85; 2, proton

release; 3, reprotonation of the Schiff

base and deprotonation of Asp-96;

4, reprotonation of Asp-96;

5, deprotonation of Asp-85 and

reprotonation of the release site.

Modified from Luecke et al. (1999).