Modulation of the metarhodopsin I/metarhodopsin II equilibrium of bovine rhodopsin by ionic strength--evidence for a surface-charge effect

Rhodopsins: An Excitingly Versatile Protein Species for Research, Development and Creative Engineering

The first member and eponym of the rhodopsin family was identified in the 1930s as the visual pigment of the rod photoreceptor cell in the animal retina. It was found to be a membrane protein, owing its photosensitivity to the presence of a covalently bound chromophoric group. This group, derived from vitamin A, was appropriately dubbed retinal. In the 1970s a microbial counterpart of this species was discovered in an archaeon, being a membrane protein also harbouring retinal as a chromophore, and named bacteriorhodopsin. Since their discovery a photogenic panorama unfolded, where up to date new members and subspecies with a variety of light-driven functionality have been added to this family. The animal branch, meanwhile categorized as type-2 rhodopsins, turned out to form a large subclass in the superfamily of G protein-coupled receptors and are essential to multiple elements of light-dependent animal sensory physiology. The microbial branch, the type-1 rhodopsins, largely function as light-driven ion pumps or channels, but also contain sensory-active and enzyme-sustaining subspecies. In this review we will follow the development of this exciting membrane protein panorama in a representative number of highlights and will present a prospect of their extraordinary future potential.

Membrane Curvature Revisited-the Archetype of Rhodopsin Studied by Time-Resolved Electronic Spectroscopy

G-protein-coupled receptors (GPCRs) comprise the largest and most pharmacologically targeted membrane protein family. Here, we used the visual receptor rhodopsin as an archetype for understanding membrane lipid influences on conformational changes involved in GPCR activation. Visual rhodopsin was recombined with lipids varying in their degree of acyl chain unsaturation and polar headgroup size using 1-palmitoyl-2-oleoyl-sn-glycero- and 1,2-dioleoyl-sn-glycerophospholipids with phosphocholine (PC) or phosphoethanolamine (PE) substituents. The receptor activation profile after light excitation was measured using time-resolved ultraviolet-visible spectroscopy. We discovered that more saturated POPC lipids back shifted the equilibrium to the inactive state, whereas the small-headgroup, highly unsaturated DOPE lipids favored the active state. Increasing unsaturation and decreasing headgroup size have similar effects that combine to yield control of rhodopsin activation, and necessitate factors beyond proteolipid solvation energy and bilayer surface electrostatics. Hence, we consider a balance of curvature free energy with hydrophobic matching and demonstrate how our data support a flexible surface model (FSM) for the coupling between proteins and lipids. The FSM is based on the Helfrich formulation of membrane bending energy as we previously first applied to lipid-protein interactions. Membrane elasticity and curvature strain are induced by lateral pressure imbalances between the constituent lipids and drive key physiological processes at the membrane level. Spontaneous negative monolayer curvature toward water is mediated by unsaturated, small-headgroup lipids and couples directly to GPCR activation upon light absorption by rhodopsin. For the first time to our knowledge, we demonstrate this modulation in both the equilibrium and pre-equilibrium evolving states using a time-resolved approach.

Insight into the chromophore of rhodopsin and its Meta-II photointermediate by 19F solid-state NMR and chemical shift tensor calculations

19F nuclei are useful labels in solid-state NMR studies, since their chemical shift and tensor elements are very sensitive to the electrostatic and space-filling properties of their local environment. In this study we have exploited a fluorine substituent, strategically placed at the C-12-position of 11-cis retinal, the chromophore of visual rhodopsins. This label was used to explore the local environment of the chromophore in the ground state of bovine rhodopsin and its active photo-intermediate Meta II. In addition, the chemical shift and tensor elements of the chromophore in the free state in a membrane environment and the bound state in the protein were determined. Upon binding of the chromophore into rhodopsin and Meta II, the isotropic chemical shift changes in the opposite direction by +9.7 and -8.4 ppm, respectively. An unusually large isotropic shift difference of 35.9 ppm was observed between rhodopsin and Meta II. This partly originates in the light-triggered 11-cis to all-trans isomerization of the chromophore. The other part reflects the local conformational rearrangements in the chromophore and the binding pocket. These NMR data were correlated with the available X-ray structures of rhodopsin and Meta II using bond polarization theory. For this purpose hydrogen atoms have to be inserted and hereto a family of structures were derived that best correlated with the well-established 13C chemical shifts. Based upon these structures, a 12-F derivative was obtained that best corresponded with the experimentally determined 19F chemical shifts and tensor elements. The combined data indicate strong changes in the local environment of the C-12 position and a substantially different interaction pattern with the protein in Meta II as compared to rhodopsin.

Interplay of G Protein-Coupled Receptors with the Membrane: Insights from Supra-Atomic Coarse Grain Molecular Dynamics Simulations

G protein-coupled receptors (GPCRs) are central to many fundamental cellular signaling pathways. They transduce signals from the outside to the inside of cells in physiological processes ranging from vision to immune response. It is extremely challenging to look at them individually using conventional experimental techniques. Recently, a pseudo atomistic molecular model has emerged as a valuable tool to access information on GPCRs, more specifically on their interactions with their environment in their native cell membrane and the consequences on their supramolecular organization. This approach uses the Martini coarse grain (CG) model to describe the receptors, lipids, and solvent in molecular dynamics (MD) simulations and in enough detail to allow conserving the chemical specificity of the different molecules. The elimination of unnecessary degrees of freedom has opened up large-scale simulations of the lipid-mediated supramolecular organization of GPCRs. Here, after introducing the Martini CGMD method, we review these studies carried out on various members of the GPCR family, including rhodopsin (visual receptor), opioid receptors, adrenergic receptors, adenosine receptors, dopamine receptor, and sphingosine 1-phosphate receptor. These studies have brought to light an interesting set of novel biophysical principles. The insights range from revealing localized and heterogeneous deformations of the membrane bilayer at the surface of the protein, specific interactions of lipid molecules with individual GPCRs, to the effect of the membrane matrix on global GPCR self-assembly. The review ends with an overview of the lessons learned from the use of the CGMD method, the biophysical-chemical findings on lipid-protein interplay.

A Usual G-Protein-Coupled Receptor in Unusual Membranes

G-protein-coupled receptors (GPCRs) are the largest family of membrane-bound receptors and constitute about 50% of all known drug targets. They offer great potential for membrane protein nanotechnologies. We report here a charge-interaction-directed reconstitution mechanism that induces spontaneous insertion of bovine rhodopsin, the eukaryotic GPCR, into both lipid- and polymer-based artificial membranes. We reveal a new allosteric mode of rhodopsin activation incurred by the non-biological membranes: the cationic membrane drives a transition from the inactive MI to the activated MII state in the absence of high [H(+)] or negative spontaneous curvature. We attribute this activation to the attractive charge interaction between the membrane surface and the deprotonated Glu134 residue of the rhodopsin-conserved ERY sequence motif that helps break the cytoplasmic "ionic lock". This study unveils a novel design concept of non-biological membranes to reconstitute and harness GPCR functions in synthetic systems.

Structural Insights into the Dynamic Process of β2-Adrenergic Receptor Signaling

G-protein-coupled receptors (GPCRs) transduce signals from the extracellular environment to intracellular proteins. To gain structural insight into the regulation of receptor cytoplasmic conformations by extracellular ligands during signaling, we examine the structural dynamics of the cytoplasmic domain of the β2-adrenergic receptor (β2AR) using (19)F-fluorine NMR and double electron-electron resonance spectroscopy. These studies show that unliganded and inverse-agonist-bound β2AR exists predominantly in two inactive conformations that exchange within hundreds of microseconds. Although agonists shift the equilibrium toward a conformation capable of engaging cytoplasmic G proteins, they do so incompletely, resulting in increased conformational heterogeneity and the coexistence of inactive, intermediate, and active states. Complete transition to the active conformation requires subsequent interaction with a G protein or an intracellular G protein mimetic. These studies demonstrate a loose allosteric coupling of the agonist-binding site and G-protein-coupling interface that may generally be responsible for the complex signaling behavior observed for many GPCRs.

Coarse-grained molecular dynamics provides insight into the interactions of lipids and cholesterol with rhodopsin

Protein function is a complicated interplay between structure and dynamics, which can be heavily influenced by environmental factors and conditions. This is particularly true in the case of membrane proteins, such as the visual receptor rhodopsin. It has been well documented that lipid headgroups, polyunsaturated tails, and the concentration of cholesterol in membranes all play a role in the function of rhodopsin. Recently, we used all-atom simulations to demonstrate that different lipid species have preferential interactions and possible binding sites on rhodopsin's surface, consistent with experiment. However, the limited timescales of the simulations meant that the statistical uncertainty of these results was substantial. Accordingly, we present here 32 independent 1.6 μs coarse-grained simulations exploring lipids and cholesterols surrounding rhodopsin and opsin, in lipid bilayers mimicking those found naturally. Our results agree with those found experimentally and in previous simulations, but with far better statistical certainty. The results demonstrate the value of combining all-atom and coarse-grained models with experiment to provide a well-rounded view of lipid-protein interactions.

Cyclopropyl and isopropyl derivatives of 11-cis and 9-cis retinals at C-9 and C-13: subtle steric differences with major effects on ligand efficacy in rhodopsin

Retinal is the natural ligand (chromophore) of the vertebrate rod visual pigment. It occurs in either the 11-cis (rhodopsin) or the 9-cis (isorhodopsin) configuration. In its evolution to a G protein coupled photoreceptor, rhodopsin has acquired exceptional photochemical properties. Illumination isomerizes the chromophore to the all-trans isomer, which acts as a full agonist. This process is extremely efficient, and there is abundant evidence that the C-9 and C-13 methyl groups of retinal play a pivotal role in this process. To examine the steric limits of the C-9 and C-13 methyl binding pocket of the binding site, we have prepared C-9 and C-13 cyclopropyl and isopropyl derivatives of its native ligands and of α-retinal at C-9. Most isopropyl analogues show very poor binding, except for 9-cis-13-isopropylretinal. Most cyclopropyl derivatives exhibit intermediate binding activity, except for 9-cis-13-cyclopropylretinal, which presents good binding activity. In general, the binding site shows preference for the 9-cis analogues over the 11-cis analogues. In fact, 13-isopropyl-9-cis-retinal acts as a superagonist after illumination. Another surprising finding was that 9-cyclopropylisorhodopsin is more like native rhodopsin with respect to spectral and photochemical properties, whereas 9-cyclopropylrhodopsin behaves more like native isorhodopsin in these aspects.

Methyl Substituents at the 11 or 12 Position of Retinal Profoundly and Differentially Affect Photochemistry and Signalling Activity of Rhodopsin

The C-11=C-12 double bond of the retinylidene chromophore of rhodopsin holds a central position in its light-induced photoisomerization and hence the photosensory function of this visual pigment. To probe the local environment of the HC-11=C-12H element we have prepared the 11-methyl and 12-methyl derivatives of 11-Z retinal and incorporated these into opsin to generate the rhodopsin analogs 11-methyl and 12-methyl rhodopsin. These analog pigments form with much slower kinetics and lower efficiency than the native pigment. The initial photochemistry and the signaling activity of the analog pigments were investigated by UV-vis and FTIR spectroscopy, and by a G protein activation assay. Our data indicate that the ultrafast formation of the first photointermediate is strongly perturbed by the presence of an 11-methyl substituent, but much less by a 12-methyl substituent. These results support the current concept of the mechanism of the primary photoisomerization event in rhodopsin. An important stronghold of this concept is an out-of-plane movement of the C-12H element, which is facilitated by torsion as well as extended positive charge delocalization into the C-10-C-13 segment of the chromophore. We argue that this mechanism is maintained principally with a methyl substituent at C-12. In addition, we show that both an 11-methyl and a 12-methyl substitutent perturb the photointermediate cascade and finally yield a low-activity state of the receptor. The 11-methyl pigment retains about 30% of the G protein activation rate of native rhodopsin, while the 12-methyl chromophore behaves like an inverse agonist up to at least 20 degrees C, trapping the protein in a perturbed Meta-I-like conformation. We conclude that the isomerization region of the chromophore and the spatial structure of the binding site are finely tuned, in order to achieve a high photosensory potential with an efficient pathway to a high-activity state.

Constraints of the 9-Methyl Group Binding Pocket of the Rhodopsin Chromophore Probed by 9-Halogeno Substitution

Sterical constraints of the 9-methyl-binding pocket of the rhodopsin chromophore are probed using retinal analogues carrying substituents of increasing size at the 9 position (H, F, Cl, Br, CH(3), and I). The corresponding 11-Z retinals were employed to investigate formation of photosensitive pigment, and the primary photoproduct was identified by Fourier transform infrared difference spectroscopy. In addition, any effects of cumulative strain were studied by introduction of the 9-Z configuration and/or the alpha-retinal ring structure. Our results show that the 9-F analogue still can escape from the 9-methyl-binding pocket and that its photochemistry behaves very similar to the 9-demethyl analogue. The 9-Cl and 9-Br analogues behave very similar to the native 9-methyl pigments, but the 9-I retinal does not fit very well and shows poor pigment formation. This puts an upper limit on the radial dimension of the 9-methyl pocket at 0.45-0.50 nm. Introduction of the alpha-retinal ring constraint in the 11-Z series results in cumulative strain, because the 9-I and 9-Br derivatives cannot bind to generate a photopigment. The 9-Z configuration can partially compensate for the additional alpha-retinal strain. The corresponding 9-Br analogue does form a photopigment, and the other derivatives give increased photopigment yields compared to the corresponding 11-Z derivatives. In fact, 9-Z-alpha-retinal would be an interesting candidate for retinal supplementation studies. Our data provide direct support for the concept that the 9-methyl group is an important determinant in ligand anchoring and activation of the protein and in general agree with a three-point interaction model involving the ring, 9-methyl group, and aldehyde function.

Fourier transform IR spectroscopy study for new insights into molecular properties and activation mechanisms of visual pigment rhodopsin

Fourier transform IR (FTIR) spectroscopy has been successfully applied in recent years to examine the functional and structural properties of the membrane protein rhodopsin, a prototype G protein coupled receptor. Unlike UV-visible spectroscopy, FTIR spectroscopy is structurally sensitive. It may give us both global information about the conformation of the protein and very detailed information about the retinal chromophore and all other functional groups, even when these are not directly related to the chromophore. Furthermore, it can be successfully applied to the photointermediates of rhodopsin, including the active receptor species, metarhodopsin II, and its decay products, which is not expected presently or even in the near future from crystallographic approaches. In this review we show how FTIR spectroscopy has significantly contributed to the understanding of very different aspects of rhodopsin, comprising both structural properties and the mechanisms leading to receptor activation and deactivation.

Zinc-induced decrease of the thermal stability and regeneration of rhodopsin

Zinc is present at high concentrations in the photoreceptor cells of the retina where it has been proposed to play a role in the visual phototransduction process. In order to obtain more information about this role, the study of the effect of zinc on several properties of the visual photoreceptor rhodopsin has been investigated. A specific effect of Zn(2+) on the thermal stability of rhodopsin, obtained from bovine retinas and solubilized in dodecyl maltoside detergent, in the dark is reported. The thermal stability of rhodopsin in its ground state (dark state) is clearly reduced with increasing Zn(2+) concentrations (0-50 microm Zn(2+)). The thermal bleaching process is accelerated in the presence of Zn(2+) with k rate constants, at 55 degrees C, of 0.028 +/- 0.002 min(-1) (0 microm Zn(2+)) and 0.056 +/- 0.003 min(-1) (50 microm Zn(2+)), corresponding to t(12) values of 24.4 +/- 1.6 min and 11.8 +/- 0.1 min, respectively. Thermodynamic parameters derived from Arrhenius plots show a significant E(a) increase at 50 microm Zn(2+) for the process, with deltaG++ decrease and increase in deltaH++ and deltaS++ possibly reflecting conformational rearrangements and reordering of water molecules. The stability of the metarhodopsin II intermediate is also decreased and changes in the metarhodopsin II decay pathway are also detected. The extent of rhodopsin regeneration in vitro is also reduced by zinc. These effects, specific for zinc, are also seen for rhodopsin in native disc membranes, and may be relevant to the suggested role of Zn(2+) in normal and pathological retinal function.

A green cone-like pigment in the ‘blind’ mole-rat Spalax ehrenbergi: functional expression and photochemical characterization

The degenerate subcutaneous eye of the blind mole rat belonging to the Spalax ehrenbergi superspecies has been shown to contain a long wavelength sensitive (LWS) cone pigment. Baculovirus expression of this LWS pigment and subsequent IMAC purification yields a photosensitive protein, that according to absorbance maximum (530 +/- 2 nm), kinetics of late phototransitions, and transducin activation, has all characteristics of a functional green cone pigment. The absorbance spectrum of the Spalax pigment is strongly red-shifted relative to the very homologous mouse, rabbit and rat green cone pigments (508-510 nm). Also in contrast to the rodent pigments, the Spalax pigment exhibits anion-dependent spectral properties, displaying a 12 nm blue-shift upon substitution of chloride ions by nitrate ions. Finally, the slow part of the photocascade deviates in some aspects from that of sighted mammals. The possible relevance of these findings for the evolutionary adaptation of Spalax to a subterranean ecotope is discussed.

Rhodopsin with 11-cis-locked chromophore is capable of forming an active state photoproduct

The visual pigment rhodopsin is characterized by an 11-cis retinal chromophore bound to Lys-296 via a protonated Schiff base. Following light absorption the C(11)=C(12) double bond isomerizes to trans configuration and triggers protein conformational alterations. These alterations lead to the formation of an active intermediate (Meta II), which binds and activates the visual G protein, transducin. We have examined by UV-visible and Fourier transform IR spectroscopy the photochemistry of a rhodopsin analogue with an 11-cis-locked chromophore, where cis to trans isomerization around the C(11)=C(12) double bond is prevented by a 6-member ring structure (Rh(6.10)). Despite this lock, the pigment was found capable of forming an active photoproduct with a characteristic protein conformation similar to that of native Meta II. This intermediate is further characterized by a protonated Schiff base and protonated Glu-113, as well as by its ability to bind a transducin-derived peptide previously shown to interact efficiently with native Meta II. The yield of this active photointermediate is pH-dependent and decreases with increasing pH. This study shows that with the C(11)=C(12) double bond being locked, isomerization around the C(9)=C(10) or the C(13)=C(14) double bonds may well lead to an activation of the receptor. Additionally, prolonged illumination at pH 7.5 produces a new photoproduct absorbing at 385 nm, which, however, does not exhibit the characteristic active protein conformation.

Conformation and stability of alpha-helical membrane proteins. 1. Influence of salts on conformational equilibria between active and Inactive states of rhodopsin

We studied the influence of salts on the pH-dependent conformational equilibria between the active and the inactive photoproduct states of rhodopsin, Meta II and Meta I, respectively, and between the active and inactive conformations of the apoprotein opsin. In both equilibria, the active species is favored in the presence of medium to high concentration of salt. The ion selectivity for the Meta I/Meta II equilibrium is particularly pronounced for the anions and follows the series trichloroacetate > thiocyanate > iodide > bromide > sulfate > chloride > acetate. The Hill coefficient of this salt-induced transition is close to 2.0. Both ion selectivity and Hill coefficient suggest that the transition is mainly regulated by ion binding to two specific charged binding sites in the protein with smaller contributions being due to the Hofmeister effect. We propose that these putative ion binding sites are identical to those sites that are titrated in the corresponding pH-dependent conformational transition. They presumably function as ionic locks, which keep the receptor in an inactive conformation, and which may be disrupted either by pH-dependent protonation or by salt-dependent ion binding.

Anions stabilize a metarhodopsin II-like photoproduct with a protonated Schiff base

In rhodopsin, the retinal chromophore is covalently bound to the apoprotein by a protonated Schiff base, which is stabilized by the negatively charged counterion Glu113, conferring upon it a pK(a) of presumably >16. Upon photoexcitation and conformational relaxation of the initial photoproducts, the Schiff base proton neutralizes the counterion, a step that is considered a prerequisite for formation of the active state of the receptor, metarhodopsin II (MII). We show that the pK(a) of the Schiff base drops below 2.5 in MII. In the presence of solute anions, however, it may be increased considerably, thereby leading to the formation of a MII photoproduct with a protonated Schiff base (PSB) absorbing at 480 nm. This PSB is not stabilized by Glu113, which is shown to be neutral, but by stoichiometric binding of an anion near the Schiff base. Protonation of the Schiff base in MII changes neither coupling to G protein, as assessed by binding to a transducin-derived peptide, nor the conformation of the protein, as judged by FTIR and UV spectroscopy. A PSB and an active state conformation are therefore compatible, as suggested previously by mutants of rhodopsin. The anion specificity of the stabilization of the PSB follows the series thiocyanate > iodide > nitrate > bromide > chloride > sulfate in order of increasing efficiency. This specificity correlates inversely with the strength of hydration of the respective anion species in solution and seems therefore to be determined mainly by its partitioning into the considerably less polar protein interior.

A structural role for Asp83 in the photoactivation of rhodopsin

Asp83 is a highly conserved residue in the second transmembrane domain of visual pigments and many members of other G protein-coupled receptor subfamilies. Upon illumination, the rod visual pigment rhodopsin proceeds through various intermediate states (Batho<-->BSI<-->Lumi<-->Meta I<-->Meta II). Meta II represents the active state of rhodopsin, which binds and activates the G protein transducin. Evidence has been presented that Asp83 participates in the formation of Meta II and undergoes a change in H-bonding. To investigate whether this role of Asp83 requires its proton-donating capacity and/or its H-bonding capability, we constructed the mutants D83C and D83N. Both mutants appear to effectively activate transducin, indicating that Asp83 is not essential for signal transduction. Differential effects of the mutations D83C and D83N are observed in the spectral properties and the pH sensitivity of the Meta I-->Meta II transition. In general, D83C behaves much more like wild-type than D83N. We conclude that the structural role of Asp83 also involves the acidic nature of its carboxyl group. In addition, the participation in Meta II formation of Cys83 in D83C manifests itself as a change in the vibrational properties of the sulfhydryl group, demonstrating that the -SH group can be used as a non-invasive probe for local structural changes.

Salt dependence of the formation and stability of the signaling state in G protein-coupled receptors: evidence for the involvement of the Hofmeister effect

We studied the salt dependence of both the stability and the equilibrium of the late photoproducts metarhodopsin I (MI) and II (MII) of the artificial visual pigment 9-demethyl rhodopsin (9dm-Rho). In the photoproducts of 9dm-Rho, all-trans-9-demethyl retinal acts only as a partial agonist, enabling us to study the photoproduct equilibrium of the pigment both in membranes and in detergent micelles. Chloride, bromide, and phosphate salts shift this equilibrium from the inactive MI to the active MII receptor conformation both in native membranes and even more with purified pigment in detergent micelles. In the presence of these salts, the induced MII state seems to be structurally intact, as judged by Fourier transform infrared (FTIR) and UV-vis spectroscopy. In the long term, however, we observe an increased instability of the photoproducts and a change in the decay pathways. Both MII enhancement and destabilization are particularly pronounced with the strong chaotropic salts KI and KSCN. The results fit into the framework of the Hofmeister effect and are assigned to an increased solvation of the peptide moiety of the solvent-exposed domains, their resulting partial disordering favoring MII over MI. In this picture, increased solvation also affects helix-helix interactions, thereby leading to a structural instability of the protein in the long term. The reported influences of salts on conformation and stability of this membrane protein are likely to be general and may therefore also apply to other transmembrane proteins and particularly to other G protein-coupled receptors.

A fully functional rod visual pigment in a blind mammal. A case for adaptive functional reorganization?

In the blind subterranean mole rat Spalax ehrenbergi superspecies complete ablation of the visual image-forming capability has been accompanied by an expansion of the bilateral projection from the retina to the suprachiasmatic nucleus. We have cloned the open reading frame of a visual pigment from Spalax that shows >90% homology with mammalian rod pigments. Baculovirus expression yields a membrane protein with all functional characteristics of a rod visual pigment (lambda(max) = 497 +/- 2 nm; pK(a) of meta I/meta II equilibrium = 6.5; rapid activation of transducin in the light). We not only provide evidence that this Spalax rod pigment is fully functional in vitro but also show that all requirements for a functional pigment are present in vivo. The physiological consequences of this unexpected finding are discussed. One attractive option is that during adaptation to a subterranean lifestyle, the visual system of this mammal has undergone mosaic reorganization, and the visual pigments have adapted to a function in circadian photoreception.

Rhodopsin activation affects the environment of specific neighboring phospholipids: an FTIR spectroscopic study

Rhodopsin is a member of a superfamily of G-protein-coupled receptors that transduce signals across membranes. We used Fourier-transform infrared (FTIR) difference spectroscopy to study the interaction between rhodopsin and lipid bilayer upon receptor activation. A difference band at 1744 cm(-1) (+)/1727 cm(-1) (-) was identified in the FTIR-difference spectrum of rhodopsin mutant D83N/E122Q in which spectral difference bands arising from the carbonyl stretching frequencies of protonated carboxylic acid groups were removed by mutation. As the band was abolished by detergent delipidation, we suggested that it arose from carbonyl groups of phospholipid fatty acid esters. Rhodopsin and the D83N/E122Q mutant were reconstituted into various (13)C-labeled 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine vesicles and probed. The 1744-cm(-1) (+)/1727 cm(-1) (-) band could be unequivocally assigned to a change in the lipid ester carbonyl stretch upon receptor activation, with roughly equal contribution from both lipid esters. The band intensity scaled with the amount of rhodopsin but not with the amount of lipid, excluding the possibility that it was due to the bulk lipid phase. We also excluded the possibility that the lipid band represents a change in the number of boundary lipids or a general alteration in the boundary lipid environment upon formation of metarhodopsin II. Instead, the data suggest that the lipid band represents the change of a specific lipid-receptor interaction that is coupled to protein conformational changes.

The molecular origin of the inhibition of transducin activation in rhodopsin lacking the 9-methyl group of the retinal chromophore: a UV-Vis and FTIR spectroscopic study

The formation of the active rhodopsin state metarhodopsin II (MII) is believed to be partially governed by specific steric constraints imposed onto the protein by the 9-methyl group of the retinal chromophore. We studied the properties of the synthetic pigment 9-demethyl rhodopsin (9dm-Rho), consisting of the rhodopsin apoprotein regenerated with synthetic retinal lacking the 9-methyl group, by UV-vis and Fourier transform infrared difference spectroscopy. Low activation rates of the visual G-protein transducin by the modified pigment reported in previous studies are shown to not be caused by the reduced activity of its MII state, but to be due to a dramatic equilibrium shift from MII to its immediate precursor, MI. The MII state of 9dm-Rho displays only a partial deprotonation of the retinal Schiff base, leading to the formation of two MII subspecies absorbing at 380 and 470 nm, both of which seem to be involved in transducin activation. The rate of MII formation is slowed by 2 orders of magnitude compared to rhodopsin. The dark state and the MI state of 9dm-Rho are distinctly different from their respective states in the native pigment, pointing to a more relaxed fit of the retinal chromophore in its binding pocket. The shifted equilibrium between MI and MII is therefore discussed in terms of an increased entropy of the 9dm-Rho MI state due to changed steric interactions.

Phosphorylation alters the pH-dependent active state equilibrium of rhodopsin by modulating the membrane surface potential

Phosphorylation reduces the lifetime and activity of activated G protein-coupled receptors, yet paradoxically shifts the metarhodopsin I-II (MI-MII) equilibrium (K(eq)) of light-activated rhodopsin toward MII, the conformation that activates G protein. In this report, we show that phosphorylation increases the apparent pK for MII formation in proportion to phosphorylation stoichiometry. Decreasing ionic strength enhances this effect. Gouy-Chapman theory shows that the change in pK is quantitatively explained by the membrane surface potential, which becomes more negative with increasing phosphorylation stoichiometry and decreasing ionic strength. This lowers the membrane surface pH compared to the bulk pH, increasing K(eq) and the rate of MII formation (k(1)) while decreasing the back rate constant (k(-)(1)) of the MI-MII relaxation. MII formation has been observed to depend on bulk pH with a fractional stoichiometry of 0.6-0.7 H(+)/MII. We find that the apparent fractional H(+) dependence is an artifact of altering the membrane surface charge during a titration, resulting in a fractional change in membrane surface pH compared to bulk pH. Gouy-Chapman calculations of membrane pH at various phosphorylation levels and ionic strengths suggest MII formation behavior consistent with titration of a single H(+) binding site with 1:1 stoichiometry and an intrinsic pK of 6.3 at 0.5 degrees C. We show evidence that suggests this same site has an intrinsic pK of 5.0 prior to light activation and its protonation before activation greatly enhances the rate of MII formation.

Kinetics of the light-induced proton translocation associated with the pH-dependent formation of the metarhodopsin I/II equilibrium of bovine rhodopsin

The kinetics of the formation of the metaII (MII) state of bovine rhodopsin was investigated by time-resolved electrical and absorption measurements with rod outer segment (ROS) fragments. Photoexcitation leads to proton transfer in the direction from the cytosolic to the intradiscal side of the membrane, probably from the Schiff base to the acceptor glutamate 113. Two components of comparable amplitude are required to describe the charge movement with exponential times of 1.1 (45%) and 3.0 ms (55%) (pH 7.8, 22 degreesC, 150 mM KCl). The corresponding activation energies are 86 and 123 kJ/mol, respectively (150 mM KCl). The time constants and amplitudes depend strongly on pH. Between pH 7.1 and 3.8 the kinetics becomes much faster, with the faster and slower components accelerating by factors of about 8 and 2, respectively. Complementary single-flash absorption experiments at 380 nm and 10 degreesC show that the formation of MII also occurs with two components with similar time constants and pH dependence. This suggests that both signals monitor the same molecular events. The pH dependence of the two apparent time constants and amplitudes of the optical data can be described well over the pH range 4-7.5 by two coupled equilibria between MI and two isochromic MII species MIIa and MIIb: MI MIIa(380) MIIb(380), with k0 proportional to the proton concentration. This model implies that deprotonation of the Schiff base and proton uptake are tightly coupled in ROS membranes. Models with k2 proportional to the proton concentration cannot describe the data. Photoreversal of MII by blue flashes (420 nm) leads to proton transfer in a direction opposite to that of the signal associated with MII formation. In this transition the Schiff base is reprotonated, most likely from glutamate 113. At pH 7.3, 150 mM KCl, 22 degreesC, this electrical charge reversal has an exponential time constant of about 30 ms and is about 10 times slower than the forward charge motion.

Evidence for the specific interaction of a lipid molecule with rhodopsin which is altered in the transition to the active state metarhodopsin II

Comparing the FTIR difference spectra of the rhodopsin --> metarhodopsin II transition in membranes and in dodecylmaltoside detergent, characteristic variations are observed between 1715 and 1750 cm(-1). By repeating the measurements with the rhodopsin mutant D83N/E122Q, the spectral variation between the samples in membranes versus detergent could be assigned to a difference band at 1743(+)/1724(-) cm(-1), which does not exhibit a deuteration-induced downshift. We provide evidence that this band is probably caused by the C=O stretch of only one ester group of one lipid molecule. This group interacts with the dark state of rhodopsin, whereas in metarhodopsin II, the lipid molecule behaves as if it were in the bulk lipid phase.

Tyrosine structural changes detected during the photoactivation of rhodopsin

We present the first Fourier transform infrared (FTIR) analysis of an isotope-labeled eukaryotic membrane protein. A combination of isotope labeling and FTIR difference spectroscopy was used to investigate the possible involvement of tyrosines in the photoactivation of rhodopsin (Rho). Rho --> MII difference spectra were obtained at 10 degrees C for unlabeled recombinant Rho and isotope-labeled L-[ring-2H4]Tyr-Rho expressed in Spodoptera frugiperda cells grown on a stringent culture medium containing enriched L-[ring-2H4]Tyr and isolated using a His6 tag. A comparison of these difference spectra revealed reproducible changes in bands that correspond to tyrosine and tyrosinate vibrational modes. A similar pattern of tyrosine/tyrosinate bands has also been observed in the bR --> M transition in bacteriorhodopsin, although the sign of the bands is reversed. In bacteriorhodopsin, these bands were assigned to Tyr-185, which along with Pro-186 in the F-helix, may form a hinge that facilitates alpha-helix movement.

Binding of transducin and transducin-derived peptides to rhodopsin studies by attenuated total reflection-Fourier transform infrared difference spectroscopy

Fourier transform infrared difference spectroscopy combined with the attenuated total reflection technique allows the monitoring of the association of transducin with bovine photoreceptor membranes in the dark. Illumination causes infrared absorption changes linked to formation of the light-activated rhodopsin-transducin complex. In addition to the spectral changes normally associated with meta II formation, prominent absorption increases occur at 1735 cm-1, 1640 cm-1, 1550 cm-1, and 1517 cm-1. The D2O sensitivity of the broad carbonyl stretching band around 1735 cm-1 indicates that a carboxylic acid group becomes protonated upon formation of the activated complex. Reconstitution of rhodopsin into phosphatidylcholine vesicles has little influence on the spectral properties of the rhodopsin-transducin complex, whereas pH affects the intensity of the carbonyl stretching band. AC-terminal peptide comprising amino acids 340-350 of the transducin alpha-subunit reproduces the frequencies and isotope sensitivities of several of the transducin-induced bands between 1500 and 1800 cm-1, whereas an N-terminal peptide (aa 8-23) does not. Therefore, the transducin-induced absorption changes can be ascribed mainly to an interaction between the transducin-alpha C-terminus and rhodopsin. The 1735 cm-1 vibration is also seen in the complex with C-terminal peptides devoid of free carboxylic acid groups, indicating that the corresponding carbonyl group is located on rhodopsin.

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.

Large-scale production and purification of the human green cone pigment: characterization of late photo-intermediates

We present the first characterization of the late photo-intermediates (Meta I, Meta II and Meta III) of a vertebrate cone pigment in a lipid environment. Marked differences from the same pathway in the rod pigment were observed. The histidine-tagged human green cone pigment was functionally expressed in large-scale suspension cultures in Sf9 insect cells using recombinant baculovirus. The recombinant pigment was extensively purified in a single step by immobilized metal affinity chromatography and displays the expected spectral characteristics. The purified pigment was able to activate the rod G-protein transducin at about half the rate of the rod pigment. Following reconstitution into bovine retina lipid proteoliposomes, identification and analysis of the photo-intermediates Meta I, Meta II and Meta III was accomplished. Similar to the rod pigment, our results indicate the existence of a Meta I-Meta II equilibrium, but we find no evidence for pH dependence. Replacement of native Cl- by NO3- in the anion-binding site of the cone pigment affected the spectral position of the pigment itself and of the Meta I intermediate, but not that of Meta II and Meta III. The decay rate of the 'active' intermediate Meta II did not differ for the Cl- and NO3- state. However, in qualitative agreement with results reported before for chicken cone pigments, the rate of Meta II decay was significantly higher in the human cone pigment than in the rod pigment.

Selective detergent-extraction from mixed detergent/lipid/protein micelles, using cyclodextrin inclusion compounds: a novel generic approach for the preparation of proteoliposomes

A novel generic approach is described for the selective extraction of detergents from mixed detergent/lipid/protein micelles for the preparation of proteoliposomes of defined lipid-protein ratio. The approach is based on the much higher affinity of inclusion compounds of the cyclodextrin type for detergents in comparison with bilayer-forming lipids. This approach has distinct advantages over other procedures currently in use. It produces good results with all detergents tested, independent of type and critical micelle concentration, and appears to be generally applicable. It yields nearly quantitative recovery of membrane protein in the proteoliposome fraction. Finally, no large excess of lipid is required; a molar ratio of lipid to protein of 100 to 1 already produces proteoliposomes with functional membrane protein, but higher ratios are well tolerated. The size of the vesicles thus obtained depends on the detergent used. Separation of the resulting proteoliposomes from the detergent-cyclodextrin complexes was most easily achieved by centrifugation through a discontinuous sucrose gradient. A variety of detergents was tested in this procedure on the bovine rod visual pigment rhodopsin in combination with retina lipids. In all cases good yields of proteoliposomes were obtained, which contained fully functional rhodopsin.

An additional methyl group at the 10-position of retinal dramatically slows down the kinetics of the rhodopsin photocascade

The present study focuses on ligand-protein interactions in a rhodopsin analogue generated from bovine opsin and the 10-methyl homologue of 11-cis-retinal. The analogue pigment displays a reduced alpha-band at 506 +/- 2 and a stronger beta-band at 325 nm. Remarkably, the rotational strength of these bands observed in visible circular dichroism spectra was found to be similar for both native and 10-methyl rhodopsin. The quantum yield of the analogue pigment was determined to be 0.55. All photointermediates were analyzed by Fourier transform infrared difference spectroscopy. At the batho stage, strong hydrogen-out-of-plane vibrations were observed, indicating that the 10-methyl chromophore also adopts a distorted all-trans conformation at this stage. In contrast to native rhodopsin, the batho intermediate of the 10-methyl pigment is stable up to 180 K and only slowly decays to the next intermediate between 180 and 210 K. As in native rhodopsin, the 10-methyl metarhodopsin I intermediate is generated at about 220 K, but its transition to the metarhodopsin II state is again shifted to a much higher temperature (> 293 K) than for the native pigment (> 260 K). Infrared analysis, nevertheless, shows that the conformational changes in the photointermediates of the 10-methyl pigment are basically identical with those observed in the native pigment. This is supported by a signal function assay, showing that the analogue pigment is able to activate transducin. The dual effect of the 10-methyl group on the photocascade is attributed to steric interactions which, initially, hamper the relaxation of strain in the polyene chain of the chromophore and, eventually, interfere with the conformational rearrangements of the protein moiety required to adopt the active conformation of the receptor. Our data provide direct support for the concept that the relaxation of strain in the retinal polyene chain acts as the major driving force of the photocascade dark reaction.