Structure and Function of Activated Rhodopsin

Glutamic acid 181 is negatively charged in the bathorhodopsin photointermediate of visual rhodopsin

Assignment of the protonation state of the residue Glu-181 is important to our understanding of the primary event, activation processes and wavelength selection in rhodopsin. Despite extensive study, there is no general agreement on the protonation state of this residue in the literature. Electronic assignment is complicated by the location of Glu-181 near the nodal point in the electrostatic charge shift that accompanies excitation of the chromophore into the low-lying, strongly allowed ππ* state. Thus, the charge on this residue is effectively hidden from electronic spectroscopy. This situation is resolved in bathorhodopsin, because photoisomerization of the chromophore places Glu-181 well within the region of negative charge shift following excitation. We demonstrate that Glu-181 is negatively charged in bathorhodopsin on the basis of the shift in the batho absorption maxima at 10 K [λ(max) band (native) = 544 ± 2 nm, λ(max) band (E181Q) = 556 ± 3 nm] and the decrease in the λ(max) band oscillator strength (0.069 ± 0.004) of E181Q relative to that of the native protein. Because the primary event in rhodopsin does not include a proton translocation or disruption of the hydrogen-bonding network within the binding pocket, we may conclude that the Glu-181 residue in rhodopsin is also charged.

Membrane model for the G-protein-coupled receptor rhodopsin: hydrophobic interface and dynamical structure

Rhodopsin is the only member of the pharmacologically important superfamily of G-protein-coupled receptors with a known structure at atomic resolution. A molecular dynamics model of rhodopsin in a POPC phospholipid bilayer was simulated for 15 ns, revealing a conformation significantly different from the recent crystal structures. The structure of the bilayer compared with a protein-free POPC control indicated hydrophobic matching with the nonpolar interface of the receptor, in agreement with deuterium NMR experiments. A new generalized molecular surface method, based on a three-dimensional Voronoi cell construction for atoms with different radii, was developed to quantify cross-sectional area profiles for the protein, lipid acyl chains and headgroups, and water. Thus, it was possible to investigate the bilayer deformation due to curvature of the individual lipid monolayers. Moreover, the generalized molecular surface derived hydrophobic interface allowed benchmarking of the hydropathy sequence analysis, an important structural genomics tool. Five water molecules diffused into internal hydration sites during the simulation, yielding a total of 12 internal waters. The cytoplasmic loops and the C-terminal tail, containing the G-protein recognition and protein sorting sequences, exhibited a high mobility, in marked contrast to the extracellular and transmembrane domains. The proposed functional coupling of the highly conserved ERY motif to the lipid-water interface via the cytoplasmic loops provides insight into lipid effects on G-protein-coupled receptor activation in terms of a flexible surface model, involving the spontaneous monolayer curvature.

Resonance Raman analysis of the mechanism of energy storage and chromophore distortion in the primary visual photoproduct

The vibrational structure of the chromophore in the primary photoproduct of vision, bathorhodopsin, is examined to determine the cause of the anomalously decoupled and intense C(11)=C(12) hydrogen-out-of-plane (HOOP) wagging modes and their relation to energy storage in the primary photoproduct. Low-temperature (77 K) resonance Raman spectra of Glu181 and Ser186 mutants of bovine rhodopsin reveal only mild mutagenic perturbations of the photoproduct spectrum suggesting that dipolar, electrostatic, or steric interactions with these residues do not cause the HOOP mode frequencies and intensities. Density functional theory calculations are performed to investigate the effect of geometric distortion on the HOOP coupling. The decoupled HOOP modes can be simulated by imposing approximately 40 degrees twists in the same direction about the C(11)=C(12) and C(12)-C(13) bonds. Sequence comparison and examination of the binding site suggests that these distortions are caused by three constraints consisting of an electrostatic anchor between the protonated Schiff base and the Glu113 counterion, as well as steric interactions of the 9- and 13-methyl groups with surrounding residues. This distortion stores light energy that is used to drive the subsequent protein conformational changes that activate rhodopsin.

Primary events in dim light vision: a chemical and spectroscopic approach toward understanding protein/chromophore interactions in rhodopsin

The visual pigment rhodopsin (bovine) is a 40 kDa protein consisting of 348 amino acids, and is a prototypical member of the subfamily A of G protein-coupled receptors (GPCRs). This remarkably efficient light-activated protein (quantum yield = 0.67) binds the chromophore 11-cis-retinal covalently by attachment to Lys296 through a protonated Schiff base. The 11-cis geometry of the retinylidene chromophore keeps the partially active opsin protein locked in its inactive state (inverse agonist). Several retinal analogs with defined configurations and stereochemistry have been incorporated into the apoprotein to give rhodopsin analogs. These incorporation results along with the spectroscopic properties of the rhodopsin analogs clarify the mode of entry of the chromophore into the apoprotein and the biologically relevant conformation of the chromophore in the rhodopsin binding site. In addition, difference UV, CD, and photoaffinity labeling studies with a 3-diazo-4-oxo analog of 11-cis-retinal have been used to chart the movement of the retinylidene chromophore through the various intermediate stages of visual transduction.

Retinal counterion switch in the photoactivation of the G protein-coupled receptor rhodopsin

The biological function of Glu-181 in the photoactivation process of rhodopsin is explored through spectroscopic studies of site-specific mutants. Preresonance Raman vibrational spectra of the unphotolyzed E181Q mutant are nearly identical to spectra of the native pigment, supporting the view that Glu-181 is uncharged (protonated) in the dark state. The pH dependence of the absorption of the metarhodopsin I (Meta I)-like photoproduct of E181Q is investigated, revealing a dramatic shift of its Schiff base pKa compared with the native pigment. This result is most consistent with the assignment of Glu-181 as the primary counterion of the retinylidene protonated Schiff base in the Meta I state, implying that there is a counterion switch from Glu-113 in the dark state to Glu-181 in Meta I. We propose a model where the counterion switch occurs by transferring a proton from Glu-181 to Glu-113 through an H-bond network formed primarily with residues on extracellular loop II (EII). The resulting reorganization of EII is then coupled to movements of helix III through a conserved disulfide bond (Cys110-Cys187); this process may be a general element of G protein-coupled receptor activation.

FTIR spectroscopy of the M photointermediate in pharaonis rhoborhodopsin

pharaonis phoborhodopsin (ppR; also called pharaonis sensory rhodopsin II, psR-II) is a photoreceptor for negative phototaxis in Natronobacterium pharaonis. During the photocycle of ppR, the Schiff base of the retinal chromophore is deprotonated upon formation of the M intermediate (ppR(M)). The present FTIR spectroscopy of ppR(M) revealed that the Schiff base proton is transferred to Asp-75, which corresponds to Asp-85 in a light-driven proton-pump bacteriorhodopsin (BR). In addition, the C==O stretching vibrations of Asn-105 were assigned for ppR and ppR(M). The common hydrogen-bonding alterations in Asn-105 of ppR and Asp-115 of BR were found in the process from photoisomerization (K intermediate) to the primary proton transfer (M intermediate). These results implicate similar protein structural changes between ppR and BR. However, BR(M) decays to BR(N) accompanying a proton transfer from Asp-96 to the Schiff base and largely changed protein structure. In the D96N mutant protein of BR that lacks a proton donor to the Schiff base, the N-like protein structure was observed with the deprotonated Schiff base (called M(N)) at alkaline pH. In ppR, such an N-like (M(N)-like) structure was not observed at alkaline pH, suggesting that the protein structure of the M state activates its transducer protein.

Electrostatic properties of membrane lipids coupled to metarhodopsin II formation in visual transduction

Changes in lipid composition have recently been shown to exert appreciable influences on the activities of membrane-bound proteins and peptides. We tested the hypothesis that the conformational states of rhodopsin linked to visual signal transduction are related to biophysical properties of the membrane lipid bilayer. For bovine rhodopsin, the meta I-meta II conformational transition was studied in egg phosphatidylcholine (PC) recombinants versus the native rod outer segment (ROS) membranes by means of flash photolysis. Formation of metarhodopsin II was observed by the change in absorbance at 478 nm after a single actinic flash was delivered to the sample. The meta I/meta II ratio was investigated as a function of both temperature and pH. The data clearly demonstrated thermodynamic reversibility of the transition for both the egg PC recombinants and the native ROS membranes. A significant shift of the apparent pK(a) for the acid-base equilibrium to lower values was evident in the egg PC recombinant, with little meta II produced under physiological conditions. Calculations of the membrane surface pH using a Poisson-Boltzmann model suggested the free energies of the meta I and meta II states were significantly affected by electrostatic properties of the bilayer lipids. In the ROS membranes, phosphatidylserine (PS) is needed for full formation of meta II, in combination with phosphatidylethanolamine (PE) and polyunsaturated docosahexaenoic acid (DHA; 22:6omega3) chains. We propose that the PS surface potential leads to an accumulation of hydronium ions, H(3)O(+), in the electrical double layer, which drive the reaction together with the large negative spontaneous curvature (H(0)) conferred by PE plus DHA chains. The elastic stress/strain of the bilayer arises from an interplay of the approximately zero H(0) from PS and the negative H(0) due to the PE headgroups and polyunsaturated chains. The lipid influences are further explained in terms of matching of the bilayer spontaneous curvature to the curvature at the lipid/rhodopsin interface, as formulated by the Helfrich bending energy. These new findings guide current ideas as to how bilayer properties govern the conformational energetics of integral membrane proteins. Moreover, they yield knowledge of how membrane lipid-protein interactions involving acidic phospholipids such as PS and neutral polyunsaturated DHA chains are implicated in key biological functions such as vision.

Conformational energetics of rhodopsin modulated by nonlamellar-forming lipids

Rhodopsin is an important example of a G protein-coupled receptor (GPCR) in which 11-cis-retinal is the ligand and acts as an inverse agonist. Photolysis of rhodopsin leads to formation of the activated meta II state from its precursor meta I. Various mechanisms have been proposed to explain how the membrane composition affects the meta I-meta II conformational equilibrium in the visual process. For rod disk membranes and recombinant membranes containing rhodopsin, the lipid properties have been discussed in terms of elastic deformation of the bilayer. Here we have investigated the relation of nonlamellar-forming lipids, such as dioleoylphosphatidylethanolamine (DOPE), together with dioleoylphosphatidylcholine (DOPC), to the photochemistry of membrane-bound rhodopsin. We conducted flash photolysis experiments for bovine rhodopsin recombined with DOPE/DOPC mixtures (0:100 to 75:25) as a function of pH to explore the dependence of the photochemical activity on the monolayer curvature free energy of the membrane. It is well-known that DOPC forms bilayers, whereas DOPE has a propensity to adopt the nonlamellar, reverse hexagonal (H(II)) phase. In the case of neutral DOPE/DOPC recombinants, calculations of the membrane surface pH confirmed that an increase in DOPE favored the meta II state. Moreover, doubling the PE headgroup content versus the native rod membranes substituted for the polyunsaturated, docosahexaenoic acyl chains (22:6 omega 3), suggesting rhodopsin function is associated with a balance of forces within the bilayer. The data are interpreted by applying a flexible surface model, in which the meta II state is stabilized by lipids tending to form the H(II) phase, with a negative spontaneous curvature. A simple theory, based on principles of surface chemistry, for coupling the energetics of membrane proteins to material properties of the bilayer lipids is described. For rhodopsin, the free energy balance of the receptor and the lipids is altered by photoisomerization of retinal and involves curvature stress/strain of the membrane (frustration). A new biophysical principle is introduced: matching of the spontaneous curvature of the lipid bilayer to the mean curvature of the lipid/water interface adjacent to the protein, which balances the lipid/protein solvation energy. In this manner, the thermodynamic driving force for the meta I-meta II conformational change of rhodopsin is tightly controlled by mixtures of nonlamellar-forming lipids having distinctive material properties.

Bleaching kinetics of artificial visual pigments with modifications near the ring-polyene chain connection

Absorbance difference spectra were recorded at 20 degrees C from 30 ns to milliseconds after photolysis of lauryl maltoside suspensions of artificial visual pigments derived from 9-cis isomers of 5-ethylretinal, 8,16-methanoretinal (a 6-s-trans-bicyclic analogue), or 5-demethyl-8-methylretinal. In all three pigments, the earliest intermediate that was detected had the characteristics of a mixture of bathorhodopsin and a blue-shifted intermediate, BSI, which is the first decay product of bathorhodopsin in bovine rhodopsin. The first decays resolved on the nanosecond time scale were the formation of the lumirhodopsin analogues. Subsequent decays were able to be fit with a mechanistic scheme which has been shown to apply to both membrane and detergent suspensions of rhodopsin. Large increases were seen in the amount of metarhodopsin I which appeared after photolysis of 5-ethylisorhodopsin and the bicyclic isorhodopsin analogue, while 5-demethyl-8-methylisorhodopsin more closely followed native rhodopsin in decaying through meta I380, a 380 nm absorbing precursor to metarhodopsin II. In addition to forming more metarhodopsin I, the bicyclic analogue stabilized the metarhodopsin I-metarhodopsin II equilibrium similarly to what has been previously reported for 9-demethylrhodopsin in detergent, introducing the possibility that the bicyclic analogue could similarly be defective in transducin activation. These observations support the idea that long after initial photolysis, structural details of the retinylidene chromophore continue to play a decisive role in processes leading to the activated form, metarhodopsin II.

On the bioactive conformation of the rhodopsin chromophore: absolute sense of twist around the 6-s-cis bond

Incubation of opsin with synthetic 6-s-locked retinoids 2a and 2b only led to pigment formation from the alpha-locked 2a, the CD spectrum of which was similar to that of native rhodopsin (Rh). This establishes that the 6-s-bond of the chromophore in rhodopsin is cis, and that its helicity is negative. Earlier cross-linking studies showed that the 11-cis to all-trans photoisomerization occurring in the batho-Rh to lumi-Rh conversion induces a flip over of the side carrying the ring moiety. The GTP-binding assay of pigment Rh-(2a), incorporating retinal analogue 2a, has shown that its activity is 80% that of the native pigment. That is, the overall conformation around the 6-s bond is retained in the steps leading to G-protein activation.

Rhodopsin: structural basis of molecular physiology

The crystal structure of rod cell visual pigment rhodopsin was recently solved at 2.8-A resolution. A critical evaluation of a decade of structure-function studies is now possible. It is also possible to begin to explain the structural basis for several unique physiological properties of the vertebrate visual system, including extremely low dark noise levels as well as high gain and color detection. The ligand-binding pocket of rhodopsin is remarkably compact, and several apparent chromophore-protein interactions were not predicted from extensive mutagenesis or spectroscopic studies. The transmembrane helices are interrupted or kinked at multiple sites. An extensive network of interhelical interactions stabilizes the ground state of the receptor. The helix movement model of receptor activation, which might apply to all G protein-coupled receptors (GPCRs) of the rhodopsin family, is supported by several structural elements that suggest how light-induced conformational changes in the ligand-binding pocket are transmitted to the cytoplasmic surface. The cytoplasmic domain of the receptor is remarkable for a carboxy-terminal helical domain extending from the seventh transmembrane segment parallel to the bilayer surface. Thus the cytoplasmic surface appears to be approximately the right size to bind to the transducin heterotrimer in a one-to-one complex. Future high-resolution structural studies of rhodopsin and other GPCRs will form a basis to elucidate the detailed molecular mechanism of GPCR-mediated signal transduction.

Simple and efficient preparation of [10,20-13C2]- and [10-CH3,13-13C2]-10-methylretinal: introduction of substituents at the 2-position of 2,3-unsaturated nitriles

In this paper, we present the synthesis of [10,20-13C2]-10-methylretinal and [10-CH3,13-13C2]-10-methylretinal, two doubly 13C-labeled chemically modified retinals that have been recently used to study the structural and functional details behind the photocascade of bovine rhodopsin (Verdegem et al. Biochemistry 1999, 38, 11316; de Lange et al. Biochemistry 1998, 37, 1411). To obtain both doubly 13C-labeled compounds, we developed a novel synthetic method to directly and regiospecifically introduce a methyl substituent on the 2-position of 3-methyl-5-(2',6',6'-trimethyl-1'-cyclohexen-1'-yl)-2,4-pentadienenitrile. Encouraged by these results, we investigated the scope of this novel reaction by developing a general method for the introduction of a variety of substituents to the 2-position of 3-methyl-2,3-unsaturated nitriles, paving the way for simple and efficient synthesis of a wide variety of 10-, 14-, and 10,14-substituted chemically modified retinals, and other biologically important compounds.

Vertebrate photoreceptors

The basis of the duplex theory of vision is examined in view of the dazzling array of data on visual pigment sequences and the pigments they form, on the microspectrophotometry measurements of single photoreceptor cells, on the kinds of photoreceptor cascade enzymes, and on the electrophysiological properties of photoreceptors. The implications of the existence of five distinct visual pigment families are explored, especially with regard to what pigments are in what types of photoreceptors, if there are different phototransduction enzymes associated with different types of photoreceptors, and if there are electrophysiological differences between different types of cones.

Structural constraints imposed by a non-native disulfide cause reversible changes in rhodopsin photointermediate kinetics

Suspensions of bovine rhodopsin in 2% lauryl maltoside detergent were treated with Cu(phen)(3)(2+) to form a disulfide bridge between cysteines 140 and 222 which occur naturally in the bovine rhodopsin sequence. Absorption difference spectra were collected after excitation with a pulse of 477 nm light on the time scale from 1 micros to 690 ms, and the results were analyzed using global exponential fitting. Only two exponentials could be fit to data from the Cu(phen)(3)(2+)-treated rhodopsin, while three exponentials were needed to fit data either from untreated rhodopsin or from Cu(phen)(3)(2+)-oxidized rhodopsin after further dithiothreitol reduction. Dithiothreitol treatment of rhodopsin which had not been previously oxidized with Cu(phen)(3)(2+) had no effect on the observed kinetics. Since the 140-222 disulfide has previously been shown to block transducin activation, its effects on rhodopsin activation are of considerable interest. Cu(phen)(3)(2+) treatment favors formation of the meta I(380) intermediate relative to meta I(480) and slows formation of meta II from meta I(380). This suggests that the protein change involved in meta I(380) formation is similar to the structural constraint introduced by the 140-222 disulfide. These results show that formation of disulfides in rhodopsin has potential as a tool for discriminating between the three isochromic, 380 nm absorbing intermediates involved in rhodopsin activation and for gaining insight into how their structures differ.

Signalling states of photoactivated rhodopsin

In microseconds after photoexcitation, rhodopsin forms the Meta I intermediate from lumirhodopsin. In this conversion, contacts between retinal and the apoprotein are formed, which result in a defined arrangement of donor and acceptor groups for proton translocations. A system of protonation-dependent coupled equilibria is now adopted, comprising Meta intermediates I, II and III, and their isospectral subforms. Some Meta states were identified as signalling states, in which the receptor interacts with transducin (Gt), rhodopsin kinase (RK) and arrestin. The binding of Gt or arrestin shifts the equilibrium to Meta II, while RK does not, indicating exposure of the RK binding site(s) before Meta II is formed. On contact with the activated receptor, each signalling protein responds with a conformational change, which transforms it into a functionally active state. The bell-shaped pH/rate profiles which are seen for the activation of both the G protein and the receptor kinase, indicate the necessary protonation and deprotonation of groups with different pKa. The right wing of the profile reflects the formation of the protonated subconformation (termed MIIb) of Meta II. For the interaction with Gt, recent work suggests a 'sequential fit' mechanism, involving the recognition of the C-terminal peptide of the Gt alpha subunit and of the farnesylated C-terminus of the gamma subunit. Isolated peptides derived from these portions of the G protein mimic the left wing of the pH/rate profile. We discuss the sequential fit as a time-ordered sequence of microscopic recognition and conformational interlocking in the interaction with the G protein.

Plasmon resonance spectroscopy: probing molecular interactions within membranes

Surface plasmon resonance (SPR) has become a popular method for investigating biomolecular interactions. A new variant of this technique, coupled plasmon-waveguide resonance (CPWR) spectroscopy, allows the characterization of anisotropic biological membranes. Plasmon resonance can therefore be used to study the molecular events involved in a wide variety of membrane processes, including energy conversion and signal transduction.

Signal transfer from rhodopsin to the G-protein: evidence for a two-site sequential fit mechanism

Photoactivation of the retinal photoreceptor rhodopsin proceeds through a cascade of intermediates, resulting in protein-protein interactions catalyzing the activation of the G-protein transducin (Gt). Using stabilization and photoregeneration of the receptor's signaling state and Gt activation assays, we provide evidence for a two-site sequential fit mechanism of Gt activation. We show that the C-terminal peptide from the Gt gamma-subunit, Gtgamma(50-71)farnesyl, can replace the holoprotein in stabilizing rhodopsin's active intermediate metarhodopsin II (MII). However, the peptide cannot replace the Gtbeta gamma complex in direct activation assays. Competition by Gtgamma(50-71)farnesyl with Gt for the active receptor suggests a pivotal role for Gtbeta gamma in signal transfer from MII to Gt. MII stabilization and competition is also found for the C-terminal peptide from the Gt alpha-subunit, Gtalpha(340-350), but the capacity of this peptide to interfere in MII-Gt interactions is paradoxically low compared with its activity to stabilize MII. Besides this disparity, the pH profiles of competition with Gt are characteristically different for the two peptides. We propose a two-site sequential fit model for signal transfer from the activated receptor, R*, to the G-protein. In the center of the model is specific recognition of conformationally distinct sites of R* by Gtalpha(340-350) and Gtgamma(50-71)farnesyl. One matching pair of domains on the proteins would, on binding, lead to a conformational change in the G-protein and/or receptor, with subsequent binding of the second pair of domains. This process could be the structural basis for GDP release and the formation of a stable empty site complex that is ready to receive the activating cofactor, GTP.

The hydrogen-bonding network of water molecules and the peptide backbone in the region connecting Asp83, Gly120, and Glu113 in bovine rhodopsin

Difference Fourier transform infrared spectra were recorded between mutants of rhodopsin and their batho products. The pigments studied were single and combined mutants of intramembrane residues of bovine rhodopsin: Asp83, Glu113, Gly120, Gly121, and Glu122. Previous studies [Nagata, T., Terakita, A., Kandori, H., Kojima, D., Shichida, Y., and Maeda, A. (1997) Biochemistry 36, 6164-6170] showed that one of the water molecules which undergoes structural changes in this process forms hydrogen bonds with Glu113 and the Schiff base, and that another water molecule is linked to this structure through the peptide backbone. The present results show that this water molecule is located at the place that is affected by the replacements of Asp83 and Gly120 but only slightly by Gly121 and not at all by Glu122. Asp83 and Gly120 are close to each other, in view of the observations that the carboxylic C=O stretching vibration of Asp83 is affected by the G120A replacement and that each replacement affects the common peptide carbonyl groups. Our results suggest that these residues in the middle of helices B and C are linked-through a hydrogen-bonding network composed of water and the peptide backbone-with the region around Glu113.

Molecular basis of dark adaptation in rod photoreceptors

Following exposure of the eye to an intense light that 'bleaches' a significant fraction of the rhodopsin, one's visual threshold is initially greatly elevated, and takes tens of minutes to recover to normal. The elevation of visual threshold arises from events occurring within the rod photoreceptors, and the underlying molecular basis of these events and of the rod's recovery is now becoming clearer. Results obtained by exposing isolated toad rods to hydroxylamine solution indicate that, following small bleaches, the primary intermediate causing elevation of visual threshold is metarhodopsin II, in its phosphorylated and arrestin-bound form. This product activates transduction with an efficacy about 100 times greater than that of opsin.

Palmitoylation of rhodopsin with S-protein acyltransferase: enzyme catalyzed reaction versus autocatalytic acylation

Protein palmitoylation in vitro was studied using bovine rhodopsin as the substrate and a partially purified acylating enzymatic activity (PAT) from placental membranes. PAT incorporates fatty acid into rhodopsin with higher efficiency (10 times higher initial rate), as compared to autoacylation. The activity is sensitive to heat and trypsin, indicating a protein-mediated enzymatic process and requires the native conformation of rhodopsin. The presence of deacylated, free cysteine residues in dark-adapted rhodopsin increases palmitoylation via PAT. The sites for non-enzymatic and enzymatic palmitoylation could not be distinguished by peptide mapping. The reversible palmitoylation described here will provide a tool for the study of the role of palmitoylation in photoreceptor function.

Structure of the interhelical loops and carboxyl terminus of bacteriorhodopsin by X-ray diffraction using site-directed heavy-atom labeling

The positions of single amino acids in the interhelical loop regions and the C-terminal tail of bacteriorhodopsin (bR) were investigated by X-ray diffraction using site-directed heavy-atom labeling. Since wild-type bR does not contain any cysteines, appropriate cysteine mutants were produced with a unique sulfhydryl group at specific positions. These sites were then labeled with mercury using the sulfhydryl specific reagent p-chloromercuribenzoate (p-CMB). The cysteine mutants D96A/V101C, V130C, A160C, and G231C were derivatized with labeling stoichiometries of 0.93 +/- 5%, 0.85 +/- 5%, 0.79 +/- 7%, and 0.77 +/- 8%, respectively (Hg per bR). No incorporation was observed with wild-type bR under the same conditions. All mutants and heavy-atom derivatives were fully active as judged by the kinetics of the photocycle and of the proton release and uptake. Moreover, the unit cell dimensions of the two-dimensional P3 lattice were unchanged by the mutations and the derivatization. This allowed the position of the mercury atoms, projected onto the plane of the membrane, to be calculated from the intensity differences in the X-ray diffraction pattern between labeled and unlabeled samples using Fourier difference methods. The X-ray diffraction data were collected at room temperature from oriented purple membrane films at 100% relative humidity without the use of dehydrating solvents. These native conditions of temperature, humidity, and solvent are expected to preserve the structure of the surface-exposed loops. Sharp maxima corresponding to a single mercury atom were found in the difference density maps for D96A/V101C and V130C. Residues 101 and 130 are in the short loops connecting helices C/D and D/E, respectively. No localized difference density was found for A160C and G231C. Residue 160 is in the longer loop connecting helices E and F, whereas residue 231 is in the C-terminal tail. Residues 160 and 231 are apparently in a more disordered and mobile part of the structure.

Variations on a molecular switch: transport and sensory signalling by archaeal rhodopsins

The archaeal rhodopsins are a family of seven-transmembrane-helix, visual pigment-like proteins found in Halobacterium salinarum and related halophilic Archaea. Two, bacteriorhodopsin (BR) and halorhodopsin (HR), are transport rhodopsins that carry out light-driven electrogenic translocation of protons and chloride, respectively, across the cell membrane. The other two, sensory rhodopsins I and II (SRI and SRII), are phototaxis receptors that send signals to tightly bound transducer proteins that in turn control a phosphorylation cascade modulating the cell's flagellar motors. Recent progress has cast light on how nature has modified the common design of these proteins to carry out their distinctly different functions: electrogenic ion transport and non-electrogenic signal transduction. A key shared mechanism between BR and SRII appears to be an interhelical salt bridge locked conformational switch that is released by photoisomerization of retinal. In BR disruption of the lock opens a cytoplasmic half-channel that ensures uptake of the transported proton from the cytoplasmic side of the membrane at a critical time in the pumping cycle. Transducer-free SRI uses the same mechanism to carry out light-driven proton transport, but interaction with its transducer blocks the cytoplasmic half-channel thereby interrupting the transport cycle. In SRI, transducer interaction also disrupts the salt bridge in the dark, poising the receptor in an intermediate conformation able to produce opposite signals depending on the colour of the stimulus light. A model for signalling is proposed in which the salt bridge-controlled half-channel is used to modulate interaction with the Htr proteins when the receptor signalling states are formed.

Effects of pH on rhodopsin photointermediates from lumirhodopsin to metarhodopsin II

Time-resolved absorption difference spectra of membrane suspensions of bovine rhodopsin at pH 5, 6, 7, 8, 9, and 10 were collected in the time range from 1 micro s to 200 ms after laser photolysis with 7-ns pulses of 477-nm light. The data were analyzed using singular value decomposition (SVD) and global exponential fitting. At pH 7 the data agree well with previously obtained data (Thorgeirsson et al. (1993) Biochemistry 32, 13861-13872) with fits improved at all pH's by inclusion of a small component due to an absorbance change caused by rotational diffusion which is detectable even at magic angle polarization. A "square scheme" suggested to best explain the previous data, which involves two branches following decay of the lumi intermediate with pathways (1) lumi --> MI480 right harpoon over left harpoon MII and (2) lumi right harpoon over left harpoon MI380 --> MII, could be confirmed throughout the entire pH range. However, to account for the increased rate of the MII --> MI480 reaction in path 1 for rising pH values, we propose that the MII in the square scheme consists of deprotonated MII and protonated MIIH+ forms in rapid equilibrium with each other, resulting in an extended square scheme and increasing the number of 380-nm products from two to three. In addition to the kinetic processes described by the extended square scheme, above pH 8 fast ( approximately 10 micro s) and slow ( approximately 50 ms) components were found. The fast component was assigned to the decay of a blue-shifted lumi intermediate, and the slow component, resolvable only at pH 10, was assigned to formation of a 450 nm absorbing photoproduct.

A model for the recovery kinetics of rod phototransduction, based on the enzymatic deactivation of rhodopsin

We propose a model for the recovery of the retinal rod photoresponse after a short stimulus. The approach describes the enzymatic deactivation of the photoactivated receptor, rhodopsin, by simple enzyme kinetics. An important feature of this description is that the R* deactivation obeys different time laws, depending on the numbers of R* formed per disc membrane and available enzyme molecules. If the enzyme works below substrate saturation, the rate of deactivation depends linearly on the number of R*, whereas for substrate saturation a hyperbolic relation--the well-known Michaelis-Menten equation--applies. This dichotomy is used to explain experimental finding that the relation between the saturation time of the photoresponse after short illumination and the flash strength has two sharply separated branches for low and high flash intensities (up to approximately 10% bleaching). By relating both branches to properties of the enzymatic rhodopsin deactivation, the new model transcends the classical notion of a constant characteristic lifetime of activated rhodopsin. With parameters that are plausible in the light of the available data and the additional information that the deactivating enzyme, rhodopsin kinase, and the signaling G-protein, transducin, compete for the active receptor, the slopes of the saturation function are correctly reproduced.

Rhodopsin: a prototypical G protein-coupled receptor

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.

Incorporation of rhodopsin in laterally structured supported membranes: observation of transducin activation with spatially and time-resolved surface plasmon resonance

Rhodopsin-transducin coupling was used as an assay to investigate a laterally patterned membrane reconstituted with a receptor and its G protein. It served as a model system to show the feasibility to immobilize G protein-coupled receptors on solid supports and investigate receptor activation and interaction with G proteins by one-dimensional imaging surface plasmon resonance. Supported membranes were formed by the self-assembly of lipids and rhodopsin from detergent solution onto functionalized gold surfaces. They formed micrometer-sized alternating regions of pure fluid phospholipid bilayers separated by bilayers composed of an outer phospholipid leaflet on a gold-attached inner thiolipid. Rhodopsin was found to incorporate preferentially into the phospholipid bilayer regions, whereas transducin was uniformly distributed over the entire outer surface of the supported patterned membrane. The influence of rhodopsin on the dark binding of transducin to lipid membranes was described quantitatively and compared with previously published data. Coupling reactions with transducin resembled closely the native system, indicating that the native functionality of rhodopsin was preserved in the supported membranes. The spatially varying properties of the membranes resulted in a pattern of rhodopsin activity on the surface. This combination of techniques is very promising for the investigation of the lateral diffusion of transducin, can be extended to include signalling proteins downstream of the G protein, and may be applied to functional screening of other G protein-coupled receptors. In the future, it may also serve as a basis for constructing biosensors based on receptor proteins.

Ultraviolet resonance Raman examination of the light-induced protein structural changes in rhodopsin activation

Ultraviolet resonance Raman (UVRR) spectra of rhodopsin and its metarhodopsin I and metarhodopsin II photointermediates have been obtained to examine the molecular mechanism of G-protein-coupled receptor activation. Spectra were acquired using a single-pass capillary flow technique in combination with a Littrow prism UV prefilter detection system. The UVRR difference spectra between rhodopsin and mearhodopsin I exhibit small differences assignalbe to tyrosine residues and no differences due to tryptophan. The UVRR difference spectra between rhodopsin and metarhodopsin II exhibit significant differences for vibrations of both tryptophan and tyrosine residues. Most importantly, there is and intensity decrease of the totally symmetric tryptophan modes at 759, 1008, and 1545 cm-1, an intensity decrease of the tryptophan W7 band at 1357 cm-1, and a frequency shift of the tryptophan W17 ban from 885 to 892 cm-1. These difference features are assigned to one or more tryptophan residues that reside in a hydrophobic, weakly hydrogen-bonding environment in rhodopsin and that are transferred to a less hydrophobic, non-hydrogen-bonding environment during rhodopsin activation. The available evidence suggests that Trp265 makes a dominant contribution to the tryptophan features in this difference spectrum. These results are interpreted with a model for rhodopsin activation in which retinal isomerization alters the interaction of Trp265 with the ionone ring of the retinal chromophore.

GTP-binding-protein-coupled receptor kinases--two mechanistic models

Six vertebrate protein kinases (G-protein-coupled receptor kinases; GRKs) that regulate the function of G-protein-coupled receptors (GPCRs) were recently cloned; several distinct properties set them apart from conventional second-messenger regulated protein kinases. It appears that GRKs bind GPCR* through two separate sites: a high-affinity site, which involves intracellular loops of the activated receptor, and the lower-affinity site, encompassing the phosphorylation region. The high-affinity interaction may involve complementary structural elements of GRKs and GPCRs* rather than precise amino acid alignment, thus allowing broad and overlapping specificities of these kinases, in spite of differences in the sequences of GPCRs. In addition, GRK structures are modified by several posttranslational modifications, including phosphorylation, autophosphorylation, prenylation, carboxymethylation, and palmitoylation, probably affecting properties of these enzymes. While GRKs phosphorylate and inactivate receptor molecules which are engaged in G-protein activation, controversy surrounds whether GRKs might be activated and phosphorylate unstimulated GPCRs, leading to a desensitization of a larger population of the receptors. In this review, mechanistic aspects of GPCR* phosphorylation related to the distinct properties, regulation and modes of action of GRKs are described.

Chromophore structural changes in rhodopsin from nanoseconds to microseconds following pigment photolysis

Rhodopsin is a prototypical G protein-coupled receptor that is activated by photoisomerization of its 11-cis-retinal chromophore. Mutant forms of rhodopsin were prepared in which the carboxylic acid counterion was moved relative to the positively charged chromophore Schiff base. Nanosecond time-resolved laser photolysis measurements of wild-type recombinant rhodopsin and two mutant pigments then were used to determine reaction schemes and spectra of their early photolysis intermediates. These results, together with linear dichroism data, yielded detailed structural information concerning chromophore movements during the first microsecond after photolysis. These chromophore structural changes provide a basis for understanding the relative movement of rhodopsin's transmembrane helices 3 and 6 required for activation of rhodopsin. Thus, early structural changes following isomerization of retinal are linked to the activation of this G protein-coupled receptor. Such rapid structural changes lie at the heart of the pharmacologically important signal transduction mechanisms in a large variety of receptors, which use extrinsic activators, but are impossible to study in receptors using diffusible agonist ligands.

Effect of hydroxylamine on photon-like events during dark adaptation in toad rod photoreceptors

1. The suction pipette technique was used to investigate the recovery of toad rod photoreceptors following small bleaches of 0.2-3% of the rhodopsin. 2. The reduction in sensitivity and the increase in noise elicited by bleaches were measured, and from these measurements the underlying rate of occurrence of photon-like events was calculated as a function of time after the bleach. 3. Exposure to hydroxylamine solution was used to hasten the decomposition of the metarhodopsin photoproducts. The outer segment was exposed to 110 mM hydroxylamine in a low-Ca2+ Ringer solution for a period of 10-50 s beginning 10-17 min after the bleaching exposure. 4. By the time of the hydroxylamine exposure, the flash sensitivity and response kinetics had returned almost to normal, and were not significantly altered by the exposure. 5. Following hydroxylamine exposure, the rate of spontaneous photon-like events in the rods declined rapidly to near pre-bleach levels. 6. We conclude that hydroxylamine reduces the rate of occurrence of photon-like events induced by a bleach, and we postulate that this reduction results from the removal of metarhodopsin (most likely metarhodopsin II) from the outer segment. 7. Our results are consistent with a model in which photon-like events result from reversal of the reactions (phosphorylation and capping by arrestin) that lead to inactivation of the activated form of rhodopsin, Rh*.

Time-resolved spectroscopy of the early photolysis intermediates of rhodopsin Schiff base counterion mutants

Time-resolved absorption difference spectra of COS-cell expressed rhodopsin and rhodopsin mutants (E113D, E113A/A117E, and G90D), solubilized in detergent, were collected from 20 ns to 510 ms after laser photolysis with 7 ns pulses (lambda(max) = 477 nm). The data were analyzed using a global exponential fitting procedure following singular value decomposition (SVD). Over the entire time range excellent agreement was achieved between results for COS-cell and rod outer segment rhodopsin both in kinetics and in the lambda(max) values of the intermediates. The Schiff base counterion mutant E113D showed strong similarities to rhodopsin up to lumi, following the established scheme: batho <==> bsi --> lumi. Including late delay times (past 1 micros), the mutant E113D lumi decayed to metarhodopsin II (MII), showing that the detergent strongly favors MII over metarhodopsin I (MI). However, a back-reaction from MII to lumi was observed that was not seen for rhodopsin. The kinetic schemes for the mutants E113A/A117E and G90D were significantly different from that of rhodopsin. In both mutants batho decay into an equilibrium with bsi was too fast to resolve (<20 ns). The batho/bsi mixtures decayed with the following reaction scheme: batho/bsi <==> lumi <==> MI-like <==> MII-like. However, the back-reaction from MI-like to lumi was not seen in G90D. MI-like spectral intermediates absorbing around 460 nm appeared in both mutants. They have been shown to be the transducin-activating species (R*). These data, interpreted in the context of previous NMR, FTIR, and Raman data, are consistent with a picture in which the kinetics of batho decay is dependent on a protein-induced perturbation near C12-C13 of the retinal chromophore. The lambda(max) values of the bsi and lumi intermediates in the mutant pigments are interpreted in terms of movement of the Schiff base relative to its counterion.