Trends in Biochemical Sciences
ReviewActivation of rhodopsin: new insights from structural and biochemical studies
Section snippets
G-protein-coupled receptors
Signal detection and transmission across biological membranes is initiated by the interaction of a chemical or physical stimulus with a specific membrane receptor which, in turn, becomes activated and initiates a chain of intracellular reactions that result in modulation of target protein activity. GPCRs are a superfamily of such membrane proteins that transmit a signal by coupling to heterotrimeric guanine nucleotide-binding proteins (known as large G proteins), which consist of three subunits
Topology of rhodopsin
Vertebrate rhodopsin is located in the membranes of discs – flat vesicles that fill the outer segment of rod cells. The extracellular (intradiscal) and intracellular regions of rhodopsin each consist of three interhelical loops (given the prefix E or I, for extracellular and intracellular, respectively) and a terminal (COOH or NH2) tail region. A fourth cytoplasmic loop (H-VIII) is formed by anchoring the C-terminal tail to the membrane via two Cys residues (Fig. 1), which carry palmitates in
Rhodopsin structure and retinal diseases
Visual pigments contain the 11-cis-retinal chromophore, bound via a protonated Schiff base linkage to a Lys side chain in the middle of H-VII. Because the retinal protonated Schiff base model compounds absorb ∼440 nm light, additional interactions between the chromophore and amino acids of the protein moiety of visual pigments cause the desirable shift of the optimal absorption (opsin shift), for instance ∼500 nm for rhodopsin and ∼400–600 nm for color pigments. Several studies on visual
Ground state
The activity of ligand-free opsin is equal to 10−6 of the activity of the all-trans-retinal-bound active metarhodopsin II state 17. However, the 11-cis-retinal-bound rhodopsin ground state exhibits an even lower level of activity against Gt, which shows that the 11-cis-retinal acts as an inverse agonist and imposes further structural constraints. The interactions can now be seen in the ground-state structure and explain why the binding of the chromophore is concomitant with a decrease in
Photochemistry and early intermediates
Absorption of a photon provides rhodopsin with the energy to form the active state. As shown in Fig. 5, three phases of the activation process can be distinguished: (1) light induced cis–trans isomerization of the retinal; (2) thermal relaxation of the retinal–protein complex; and (3) the late equilibria that are affected by the interaction of rhodopsin with the G protein.
Two-thirds of the energy of 57 kcal mol−1 (238 kJ mol−1) taken up by light absorption are stored in the photoisomerized
The meta states
The intermediate MII is the signaling state capable of interacting with the G protein 6. Formation of MII from its predecessor, metarhodopsin I (MI), accompanies a large shift in the absorption maximum (first ‘bleached’ product of rhodopsin 6), the breakage of the stabilizing salt bridge between the negatively charged side chain of Glu113 and the protonated Schiff base between Lys296 and retinal 31, 32, and the motion of transmembrane helices 33, 34. All of these events occur within the
Formation of the signaling state
The positive enthalpy (ΔH, Fig. 5a) of MII formation indicates that molecular interactions built up in MI are lost upon transition to MII. To drive the conversion, the entropy, and thus the overall disorder in the protein, must increase. This observation would be consistent with the idea that formation of the active state is merely a release of constraints in the helix bundle, thus exposing cytoplasmic binding sites. Experimentally, it was indeed found that the insertion of successively
Rhodopsin as a model for other GPCRs
In the rhodopsin family of GPCRs, biogenic amine ligands are analogs of the retinal structure, with cationic ammonium groups at one end and a ring-like structure at the other 49. The inverse agonist 11-cis-retinal is covalently linked to the protein via a flexible Lys side chain. The ground state structure of rhodopsin provides clues as to how this ‘safety cord’ ensures a permanent occupancy of the ligand-binding site and stabilizing interactions, including a salt bridge-like stable apposition
Concluding remarks
Photoreceptors are designed for the extreme conditions of very low and high activities in dark and light, respectively. The strong constraints seen in the ground-state structure 4 and the early energy-rich intermediates, which can only be reached by uptake of photonic energy, reveal the necessary barriers against spontaneous activation. However, many of the structural details are similar to those in other GPCRs, and the late meta intermediates show close analogies to the low- and high-affinity
Acknowledgements
We wish to thank Thaddeus Dryja for the list of retinitis pigmentosa mutations, HartmutLücke, TorstenSchöneberg and John Spudich for discussions. Supported by grants from the Deutsche Forschungsgemeinschaft (SFB 449), the Fonds der Chemischen Industrie, National Eye Institute (EY09339), Research to Prevent Blindness, Inc. (RPB) to the Dept of Ophthalmology at the University of Washington, Foundation Fighting Blindness, Inc., the Ruth and Milton Steinbach Fund, and the E.K. Bishop Foundation.
References (54)
An α-carbon template for the transmembrane helices in the rhodopsin family of G-protein-coupled receptors
J. Mol. Biol.
(1997)Structural basis of G-protein-coupled receptor function
Mol. Cell. Endocrinol.
(1999)Late photoproducts andsignaling states of bovine rhodopsin
Folding and assembly in rhodopsin. Effect of mutations in the sixth transmembrane helix on the conformation of the third cytoplasmic loop
J. Biol. Chem.
(1999)Mutation of the fourth cytoplasmic loop of rhodopsin affects binding of transducin and peptides derived from the carboxyl-terminal sequences of transducin α and γ subunits
J. Biol. Chem.
(2000)- et al.
Transient dichroism in photoreceptor membranes indicates that stable oligomers of rhodopsin do not form during excitation
Biophys. J.
(1985) How color visual pigments are tuned
Trends Biochem. Sci.
(1999)A comparison of the efficiency of G-protein activation by ligand-free and light-activated forms of rhodopsin
Biophys. J.
(1997)Constitutively active mutants of rhodopsin
Neuron
(1992)- et al.
The functional topography of transmembrane domain 3 of the M1 muscarinic acetylcholine receptor, revealed by scanning mutagenesis
J. Biol. Chem.
(1999)
The dynamic aspects of proton transfer processes
Biochim. Biophys. Acta
A conserved carboxylic acid group mediates light-dependent proton uptake and signaling by rhodopsin
J. Biol. Chem.
Protein–protein interaction converts a proton pump into a sensory receptor
Cell
Time-resolved detection of transient movement of helix F in spin-labelled pharaonis sensory rhodopsin II
J. Mol. Biol.
Structure and mechanism of vertebrate visual pigments
Functionally discrete mimics of light-activated rhodopsin identified through expression of soluble cytoplasmic domains
J. Biol. Chem.
Signaling states of rhodopsin: retinal provides a scaffold for activating proton transfer switches
J. Biol. Chem.
FTIR spectroscopy of complexes formed between metarhodopsin IIand C-terminal peptides from the G-protein α-and γ-subunits
FEBS Lett.
Raster3D: photorealistic molecular graphics
Methods Enzymol.
Projection structure of rhodopsin
Nature
Arrangement of rhodopsin transmembrane α-helices
Nature
Crystal structure of rhodopsin: a G protein-coupled receptor
Science
Control of rhodopsin multiple phosphorylation
Biochemistry
Regulation of sorting and post-Golgi trafficking of rhodopsin by its C-terminal sequence QVS(A)PA
Proc. Natl. Acad. Sci. U. S. A.
AT1-receptor heterodimers show enhanced G-protein activation and altered receptor sequestration
Nature
Signal transfer from rhodopsin to the G-protein: evidence for a two-site sequential fit mechanism
Proc. Natl. Acad. Sci. U. S. A.
Identification of the Cl(−)-binding site in the human red and green color vision pigments
Biochemistry
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