Rhodopsin
Rhodopsin and phototransduction.
Pepe IM
Institute of Biophysics, Faculty of Medicine, University of Genoa, Italy. pepe@ibf.unige.it
Recent studies on rhodopsin structure and function are reviewed and the properties of vertebrate as well as invertebrate rhodopsin described. Open issues such as the 'red shift' of the absorbance spectra are emphasized in the light of the present model of the retinal-binding pocket. The processes that restore the rhodopsin content in photoreceptors are also presented with a comparison between vertebrate and invertebrate visual systems. The central role of rhodopsin in the phototransduction cascade becomes evident by examining the main reports on light-activated conformational changes of rhodopsin and its interaction with transducin. Shut-off mechanisms are considered by reporting the studies on the sites of rhodopsin phosphorylation and arrestin binding. Furthermore, recent findings on the energetics of phototransduction point out that the ATP needed for photoreception in vertebrates is synthesized in the outer segments where phototransduction events take place.
How vertebrate and invertebrate visual pigments differ in their mechanism of photoactivation.
Nakagawa M, Iwasa I, Kikkawa S, Tsuda M, and Ebrey TG
Structure of rhodopsin.
Schertler GF
MRC Laboratory of Molecular Biology, Cambridge, UK. gfx@mrc-lmb.cam.ac.uk
Two-dimensional crystals of rhodopsin were studied to determine the arrangement of the transmembrane alpha helices. A combination of electron cryo-microscopy, image processing and electron crystallography was used to extract amplitudes and phases from images, and a three-dimensional map to a resolution of 7.5 A was calculated. Density peaks for all seven transmembrane helices were observed and the helix axes for all seven helices could be estimated. Near the intracellular side, which interacts with the G protein transducin, we observed three layers of helices arranged differently from bacteriorhodopsin. The arrangement opens up towards the extracellular side forming a cavity that serves as the binding pocket for the retinal. This cavity is closed towards the intracellular side by the long and highly tilted helix 3, and must be closed towards the extracellular side by the loop linking helices 4 and 5 that is linked by a disulphide bridge to the extracellular end of helix 3.
Structure of rhodopsin.
Schertler GF
MRC Laboratory of Molecular Biology, Cambridge, UK. gfx@mrc-lmb.cam.ac.uk
Two-dimensional crystals of rhodopsin were studied to determine the arrangement of the transmembrane alpha helices. A combination of electron cryo-microscopy, image processing and electron crystallography was used to extract amplitudes and phases from images, and a three-dimensional map to a resolution of 7.5 A was calculated. Density peaks for all seven transmembrane helices were observed and the helix axes for all seven helices could be estimated. Near the intracellular side, which interacts with the G protein transducin, we observed three layers of helices arranged differently from bacteriorhodopsin. The arrangement opens up towards the extracellular side forming a cavity that serves as the binding pocket for the retinal. This cavity is closed towards the intracellular side by the long and highly tilted helix 3, and must be closed towards the extracellular side by the loop linking helices 4 and 5 that is linked by a disulphide bridge to the extracellular end of helix 3.
Rhodopsin phosphorylation and its role in photoreceptor function.
Hurley JB, Spencer M, Niemi GA
Department of Biochemistry, University of Washington, Seattle 98195, USA. jbhhh@u.washington.edu
Light-stimulated phosphorylation of rhodopsin was first described 25 years ago. This paper reviews the progress that has been made towards (i) understanding the nature of the enzymes that phosphorylate and dephosphorylate rhodopsin (ii) identifying the sites of phosphorylation on rhodopsin and (iii) understanding the physiological importance of rhodopsin phosphorylation. Many important questions related to rhodopsin phosphorylation remain unanswered and new strategies and methods are needed to address issues such as the roles of Ca2+ and recoverin. We present one such method that uses mass spectrometry to quantitate rhodopsin phosphorylation in intact mouse retinas.
Rhodopsin: a prototypical G protein-coupled receptor.
Sakmar TP
Howard Hughes Medical Institute, Laboratory of Molecular Biology and Biochemistry, Rockefeller University, New York, New York 10021, USA.
A variety of spectroscopic and biochemical studies of recombinant site-directed mutants of rhodopsin and related visual pigments have been reported over the past 9 years. These studies have elucidated key structural elements common to visual pigments. In addition, systematic analysis of the chromophore-binding pocket in rhodopsin and cone pigments has led to an improved understanding of the mechanism of the opsin shift, and of particular molecular determinants underlying color vision in humans. Identification of the conformational changes that occur on rhodopsin photoactivation has been of particular recent concern. Assignments of light-dependent molecular alterations to specific regions of the chromophore have also been attempted by studying native opsins regenerated with synthetic retinal analogs. Site-directed mutagenesis of rhodopsin has also provided useful information about the retinal-binding pocket and the molecular mechanism of rhodopsin photoactivation. Individual molecular groups have been identified to undergo structural alterations or environmental changes during photoactivation. Analysis of particular mutant pigments in which specific groups are locked into their respective "off" or "on" states has provided a framework to identify determinants of the active conformation, as well as the minimal number of intramolecular transitions required to switch between inactive and active conformations. A simple model for the active state of rhodopsin can be compared to structural models of its ground state to localize chromophore-protein interactions that may be important in the photoactivation mechanism. This review focuses on the recent functional characterization of site-directed mutants of bovine rhodopsin and some cone pigments. In addition, an attempt is made to reconcile previous key findings and existing structural models with information gained from the analysis of site-directed mutant pigments.
Physiological activity of retinoids in natural and artificial visual pigments.
Corson DW, Crouch RK
Department of Pathology, Medical University of South Carolina, Charleston 29425, USA. CORSONDW@MUSC.EDU
Invertebrate visual pigments.
Gartner W, Towner P
Max-Planck-Institut fur Strahlenchemie, Mulheim an der Ruhr, Germany.
Modulation of rhodopsin function by properties of the membrane bilayer.
Brown MF
Department of Chemistry, University of Arizona, Tucson 85721.
A prevalent model for the function of rhodopsin centers on the metarhodopsin I (MI) to metarhodopsin II (MII) conformational transition as the triggering event for the visual process. Flash photolysis techniques enable one to determine the [MII]/[MI] ratio for rhodopsin in various recombinant membranes, and thus investigate the roles of the phospholipid head groups and the lipid acyl chains systematically. The results obtained to date clearly show that the pK for the acid-base MI-MII equilibrium of rhodopsin is modulated by the lipid environment. In bilayers of phosphatidylcholines the MI-MII equilibrium is shifted to the left; whereas in the native rod outer segment membranes it is shifted to the right, i.e., at neutral pH near physiological temperature. The lipid mixtures sufficient to yield full photochemical function of rhodopsin include a native-like head group composition, viz, comprising phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylserine (PS), in combination with polyunsaturated docosahexaenoic acid (DHA; 22:6 omega 3) chains. Yet such a native-like lipid mixture is not necessary for the MI-MII conformational transition of rhodopsin; one can substitute other lipid compositions having similar properties. The MI-MII transition is favored by relatively small head groups which produce a condensed bilayer surface, viz, a comparatively small interfacial area as in the case of PE, together with bulky acyl chains such as DHA which prefer a relatively large cross sectional area. The resulting force imbalance across the layer gives rise to a curvature elastic stress of the lipid/water interface, such that the lipid mixtures yielding native-like behavior form reverse hexagonal (HII) phases at slightly higher temperatures. A relatively unstable membrane is needed: lipids tending to form the lamellar phase do not support full native-like photochemical function of rhodopsin. Thus chemically specific properties of the various lipids are not required, but rather average or material properties of the entire assembly, which may involve the curvature free energy of the membrane-lipid water interface. These findings reveal that the membrane lipid bilayer has a direct influence on the energetics of the conformational states of rhodopsin in visual excitation.
Interrelations of bioenergetic and sensory functions of the retinal proteins.
Skulachev VP
Department of Bioenergetics, A. N. Belozersky Institute of Physico-Chemical Biology, Moscow State University, Russia.
Rhodopsins are intrinsic membrane retinal-containing proteins composed of 7 hydrophobic alpha-helical transmembrane columns and hydrophilic sequences of various length connecting the helices and localized at N- and C-ends of the polypeptide. The chromophore (retinal) forms a Schiff base with a lysine residue in the middle part of the last alpha-helix. Absorption of a photon results in isomerization of retinal which gives rise to a conformational change in the protein moiety. Rhodopsins can be involved in two entirely different types of activities, i.e. ion pumping and photosensing. Recent observations concerning the pumping and sensory mechanisms allowed both these events to be explained in terms of one and the same unitary concept, which postulates the formation of a hydrophilic cleft in the hydrophobic part of the protein molecule as a crucial step in energy conservation and photosensing.
Rhodopsin: structure, function, and genetics.
Nathans J
Howard Hughes Medical Institute, Department of Molecular Biology and Genetics, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205.
Nature of the primary photochemical events in rhodopsin and bacteriorhodopsin.
Birge RR
Department of Chemistry, Syracuse University, NY 13244.
Eye 1998;12 ( Pt 3b):521-5
Control of rhodopsin activity in vision.
Baylor DA, Burns ME
Department of Neurobiology, Stanford University School of Medicine, CA 94305, USA.
Although rhodopsin's role in activating the phototransduction cascade is well known, the processes that deactivate rhodopsin, and thus the rest of the cascade, are less well understood. At least three proteins appear to play a role: rhodopsin kinase, arrestin and recoverin. Here we review recent physiological studies of the molecular mechanisms of rhodopsin deactivation. The approach was to monitor the light responses of individual mouse rods in which rhodopsin was altered or arrestin was deleted by transgenic techniques. Removal of rhodopsin's carboxy-terminal residues which contain phosphorylation sites implicated in deactivation, prolonged the flash response 20-fold and caused it to become highly variable. In rods that did not express arrestin the flash response recovered partially, but final recovery was slowed over 100-fold. These results are consistent with the notion that phosphorylation initiates rhodopsin deactivation and that arrestin binding completes the process. The stationary night blindness of Oguchi disease, associated with null mutations in the genes for arrestin or rhodopsin kinase, presumably results from impaired rhodopsin deactivation, like that revealed by the experiments on transgenic animals.