Movement of the retinylidene Schiff base counterion in rhodopsin by one helix turn reverses the pH dependence of the metarhodopsin I to metarhodopsin II transition

The G protein-coupled receptor rhodopsin: a historical perspective

Rhodopsin is a key light-sensitive protein expressed exclusively in rod photoreceptor cells of the retina. Failure to express this transmembrane protein causes a lack of rod outer segment formation and progressive retinal degeneration, including the loss of cone photoreceptor cells. Molecular studies of rhodopsin have paved the way to understanding a large family of cell-surface membrane proteins called G protein-coupled receptors (GPCRs). Work started on rhodopsin over 100 years ago still continues today with substantial progress made every year. These activities underscore the importance of rhodopsin as a prototypical GPCR and receptor required for visual perception-the fundamental process of translating light energy into a biochemical cascade of events culminating in vision.

Rapid release of retinal from a cone visual pigment following photoactivation

As part of the visual cycle, the retinal chromophore in both rod and cone visual pigments undergoes reversible Schiff base hydrolysis and dissociation following photobleaching. We characterized light-activated release of retinal from a short-wavelength-sensitive cone pigment (VCOP) in 0.1% dodecyl maltoside using fluorescence spectroscopy. The half-time (t(1/2)) of release of retinal from VCOP was 7.1 s, 250-fold faster than that of rhodopsin. VCOP exhibited pH-dependent release kinetics, with the t(1/2) decreasing from 23 to 4 s with the pH decreasing from 4.1 to 8, respectively. However, the Arrhenius activation energy (E(a)) for VCOP derived from kinetic measurements between 4 and 20 °C was 17.4 kcal/mol, similar to the value of 18.5 kcal/mol for rhodopsin. There was a small kinetic isotope (D(2)O) effect in VCOP, but this effect was smaller than that observed in rhodopsin. Mutation of the primary Schiff base counterion (VCOP(D108A)) produced a pigment with an unprotonated chromophore (λ(max) = 360 nm) and dramatically slowed (t(1/2) ~ 6.8 min) light-dependent retinal release. Using homology modeling, a VCOP mutant with two substitutions (S85D and D108A) was designed to move the counterion one α-helical turn into the transmembrane region from the native position. This double mutant had a UV-visible absorption spectrum consistent with a protonated Schiff base (λ(max) = 420 nm). Moreover, the VCOP(S85D/D108A) mutant had retinal release kinetics (t(1/2) = 7 s) and an E(a) (18 kcal/mol) similar to those of the native pigment exhibiting no pH dependence. By contrast, the single mutant VCOP(S85D) had an ~3-fold decreased retinal release rate compared to that of the native pigment. Photoactivated VCOP(D108A) had kinetics comparable to those of a rhodopsin counterion mutant, Rho(E113Q), both requiring hydroxylamine to fully release retinal. These results demonstrate that the primary counterion of cone visual pigments is necessary for efficient Schiff base hydrolysis. We discuss how the large differences in retinal release rates between rod and cone visual pigments arise, not from inherent differences in the rate of Schiff base hydrolysis but rather from differences in the properties of noncovalent binding of the retinal chromophore to the protein.

Mutations at position 125 in transmembrane helix III of rhodopsin affect the structure and signalling of the receptor

Mutation of L125R in trasmembrane helix III of rhodopsin, associated with the retinal degenerative disease retinitis pigmentosa, was previously shown to cause structural misfolding of the mutant protein. Also, conservative mutations at this position were found to cause partial misfolding of the mutant receptors. We report here on a series of mutations at position 125 to further investigate the role of Leu125 in the correct folding and function of rhodopsin. In particular, the effect of the size of the substituted amino-acid side chain in the functionality of the receptor, measured as the ability of the mutant rhodopsins to activate the G protein transducin, has been analysed. The following mutations have been studied: L125G, L125N, L125I, L125H, L125P, L125T, L125D, L125E, L125Y and L125W. Most of the mutant proteins, expressed in COS-1 cells, showed reduced 11-cis-retinal binding, red-shifts in the wavelength of the visible absorbance maximum, and increased reactivity towards hydroxylamine in the dark. Thermal stability in the dark was reduced, particularly for L125P, L125Y and L125W mutants. The ability of the mutant rhodopsins to activate the G protein transducin was significantly reduced in a size dependent manner, especially in the case of the bulkier L125Y and L125W substitutions, suggesting a steric effect of the substituted amino acid. On the basis of the present and previous results, Leu125 in transmembrane helix III of rhodopsin, in the vicinity of the beta-ionone ring of 11-cis-retinal, is proposed to be an important residue in maintaining the correct structure of the chromophore binding pocket. Thus, bulky substitutions at this position may affect the structure and signallling of the receptor by altering the optimal conformation of the retinal binding pocket, rather than by direct interaction with the chromophore, as seen from the recent crystallographic structure of rhodopsin.

Effect of NADPH on formation and decay of human metarhodopsin III at physiological temperatures

Difference absorption spectra were recorded during the formation and decay of metarhodopsin III after sonicated membrane suspensions of rhodopsin were bleached at 37 degrees C. The data were analyzed using SVD, spectral decomposition and global exponential fitting. By comparison of the results in the presence or absence of 70 microM NADPH and those for bovine or human rhodopsin, a single comprehensive scheme was fit to all the data, including reduction of retinal to retinol by the intrinsic retinol dehydrogenase. On the time scale studied the mechanism involves two 382 nm absorbing species and two 468 nm, absorbing species, supporting the notion that human metarhodopsin III is not a homogeneous species. The results confirm that metarhodopsin III forms and persists sufficiently long in the human retina under physiological conditions that it could undergo secondary photoisomerization.

Angiotensin II type 1 receptor-function affected by mutations in cytoplasmic loop CD

To explore peptide hormone-induced conformational changes, we attempted to engineer a metal-ion binding site between the cytoplasmic loops CD and EF in the angiotensin II type 1 (AT(1)) receptor. We constructed 12 double and six triple histidine mutant receptors, and tested the ability of each mutant and the wild-type to activate inositol phosphate (IP) production with and without ZnCl(2). Inhibition by ZnCl(2) in the double and triple His mutant receptors was not significant, but these mutations directly decreased the IP production. Systematic analysis of single His mutants demonstrated that the loop CD-mutants displayed 52-74% inhibition of IP production, whereas the loop EF-mutants did not affect IP production. These results indicate that the cytoplasmic loop CD-segment from Tyr(127) to Ile(130) is important for G(q/11) activation by the AT(1) receptor.

Combined biophysical and biochemical information confirms arrangement of transmembrane helices visible from the three-dimensional map of frog rhodopsin

The electron density projection map of frog rhodopsin at 6 A resolution had been until recently the most direct evidence for the three-dimensional structure of a transmembrane domain of any G-protein-coupled receptor. Only three out of seven transmembrane helices are clearly defined, whilst the other four are hidden in a patch of unresolved electron density. A model of the seven-helix bundle has been created by generating positions and orientations for the four unresolved helices through performing a conformational search directed by structural restraints derived from other experimental data. These four helices are significantly tilted with respect to the membrane normal, and the cytosolic end of helix C is inserted between helices D and E. These calculations produce positions and orientations for these additional helices that are consistent with the recently published low-resolution three-dimensional map, and provide a template for more detailed modelling of rhodopsin structure and function.

Role of the C9 methyl group in rhodopsin activation: characterization of mutant opsins with the artificial chromophore 11-cis-9-demethylretinal

Activation of the visual pigment rhodopsin involves both steric and electrostatic interactions between the chromophore and opsin within the retinal-binding site. Removal of the C9 methyl group of 11-cis-retinal inhibits light-dependent activation of the G protein, transducin, suggesting a direct steric contact. More recently, we have shown that steric interactions lead to receptor activation when Gly121 in the middle of transmembrane helix 3 is replaced by larger hydrophobic residues. In order to understand in more detail the role of the C9 methyl group of retinal in the structure and function of rhodopsin, we first studied the properties of recombinant 9-dm-Rho (opsin reconstituted with 11-cis-9-demethylretinal). The 9-dm-Rho pigment displayed a blue-shifted lambdamax, increased hydroxylamine reactivity, and decreased ability to activate transducin. These properties are consistent with the hypothesis that the C9 methyl group is a crucial structural anchor for the correct docking of the chromophore in its binding site. Next, we investigated the possible interaction between Gly121 of opsin and the C9 methyl group of retinal by characterizing recombinant pigments produced by combining mutant opsins (G121A, -V, -I, -L, and -W) with 11-cis-9-demethylretinal. Mutant opsins G121I, -L, and -W failed to bind the chromophore. However, the double mutant G121L/F261A bound 11-cis-9-demethylretinal to form a stable pigment with a lambdamax of 451 nm. When activity was assayed in membranes, the reduction in transducin activation by 9-dm-Rho caused by the lack of a C9 methyl group on the chromophore could be partially restored by replacing Gly121 with a bulky residue (leucine, isoleucine, or tryptophan). These results support a model of receptor activation that involves steric interaction between the C9 methyl group of the chromophore and the opsin in the vicinity of Gly121 on transmembrane helix 3.

Second-site suppressor mutations for the Arg70 substitution mutants of the Tn10-encoded metal-tetracycline/H+ antiporter of Escherichia coli

The positive charge of the Arg70 residue in the cytoplasmic loop of the Tn10-encoded metal-tetracycline/H+ antiporter (Tet(B)) of Escherichia coli is essential for the tetracycline transport function (Y. Someya and A. Yamaguchi, Biochemistry 35, 9385-9391 (1996)). In this study, we found that the R70A mutation was suppressed by the second-site mutation of Thr171 to Ser. The T171S mutation suppressed any mutations at position 70 regardless of the amino acid residue introduced. The R70A and R70C mutations were also suppressed by the T171A or T171C mutations, but not by the T171Y mutation, indicating that the decrease in the side chain volume at position 171 is essential for the suppression. Tetracycline transport activity of the R70C mutant was stimulated by Hg2+ because mercaptide formed between the SH group of Cys70 and Hg2+ worked as a functional positively-charged side chain. The activity of the R70A/R71C/T171S mutant was also stimulated by Hg2+, whereas those of the R70A/R71C, R71C, and R71C/T171S mutants were not, indicating that the T171S mutation causes the switching of the functional positive charge at position 70 to 71. Since Thr171 is in the middle of the transmembrane helix VI, the switching may be based on a remote conformational effect.

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.

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.

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

The effects of ionic strength on formation and decay of metarhodopsin II (MII), the active photointermediate of bovine rhodopsin, were studied in the native membrane environment by means of ultraviolet/ visible and Fourier-transform infrared (FTIR) spectroscopy. By increasing the concentration of KCl in the range from hypotonic to 4 M, the apparent pKa of the metarhodopsin I(MI)/MII equilibrium is shifted by approximately pH three, in favor of the MII intermediate. In addition, the apparent rate of MII formation is enhanced by an increase in ionic strength (about twofold in the presence of 2 M KCl). MIII decay is independent of the salt concentration. Attenuated-total-reflectance/FTIR data show that the high-salt conditions have no effect on the rigidity of the membrane matrix and do not induce structural changes in the intermediates themselves. Different salts were tested for their ability to shift the MI/MII equilibrium; however, no clear ion dependence was observed. We interpret these results as an indication for direct involvement of the cytosolic surface charge in the regulation of the photochemical activity of bovine rhodopsin.

Spectroscopic evidence for altered chromophore--protein interactions in low-temperature photoproducts of the visual pigment responsible for congenital night blindness

The replacement of Gly90 by Asp in human rhodopsin causes congenital night blindness. It has been suggested that the molecular origin for the trait is an altered electrostatic environment of the protonated retinal Schiff base chromophore. We have investigated the corresponding recombinant bovine rhodopsin mutant G90D, as well as the related mutants E113A and G90D/E113A, using spectroscopy at low temperature. This allows the assessment of chromophore-protein interactions under conditions where conformational changes are mainly restricted to the retinal-binding site. Each of the mutant pigments formed bathorhodopsin- and isorhodopsin-like intermediates, but the concomitant visible absorption changes reflected differences in the electrostatic environment of the protonated Schiff base in each pigment. Fourier transform infrared-difference spectroscopy revealed effects on the chromophore fingerprint and hydrogen-out-of-plane vibrational modes, which were indicative of the removal of an electrostatic perturbation near C12 of the retinal chromophore in all three mutants. A comparison of the UV-visible and infrared-difference spectra of the mutant pigments strongly suggests that Glu113 is stably protonated in G90D. The corresponding carbonyl-stretching mode is assigned to a band at 1727 cm-1. In contrast to the case in native bathorhodopsin, the all-trans-retinal chromophores in the primary photoproducts of the mutant pigments are essentially relaxed. The peptide carbonyl vibrational changes in mutants G90D and G90D/ E113A suggest that this is due to a more flexible retinal-binding site. Therefore, the steric strain exerted on the chromophore in native bathorhodopsin may be caused by electrostatic forces that specifically involve glutamate 113.

Rhodopsin activation blocked by metal-ion-binding sites linking transmembrane helices C and F

A large superfamily of receptors containing seven transmembrane (TM) helices transmits hormonal and sensory signals across the plasma membrane to heterotrimeric G proteins at the cytoplasmic face of the membrane. To investigate how G-protein-coupled receptors work at the molecular level, we have engineered metal-ion-binding sites between TM helices to restrain activation-induced conformational change in specific locations. In rhodopsin, the photoreceptor of retinal rod cells, we substituted histidine residues for natural amino acids at the cytoplasmic ends of the TM helices C and F. The resulting mutant proteins were able to activate the visual G protein transducin in the absence but not in the presence of metal ions. These results indicate that the TM helices C and F are in close proximity and suggest that movements of these helices relative to one another are required for transducin activation. Thus a change in the orientations of TM helices C and F is likely to be a key element in the mechanism for coupling binding of ligands (or isomerization of retinal) to the activation of G-protein-coupled receptors.

Specific tryptophan UV-absorbance changes are probes of the transition of rhodopsin to its active state

The difference of rhodopsin and metarhodopsin II (MII) absorption spectra exhibits a characteristic pattern in the UV wavelength range, consisting of peaks at 278, 286, 294, 302 nm. These difference bands are thought to result from the perturbation of the environments of tryptophan and/or tyrosine residues. We used site-directed mutagenesis to investigate the contribution of tryptophan absorption to these spectral features. Each of the five tryptophan residues in bovine rhodopsin was replaced by either a phenylalanine or a tyrosine. The mutant pigments (W35F, W126F, W161F, W175F, W265F/Y) were prepared and studied by UV-visible photobleaching difference spectroscopy. The difference spectra of the W35F and W175F mutants were identical to that of rhodopsin, whereas in the W161F mutant, the magnitudes of the 294- and 302-nm bands were slightly lowered. The differential absorbance at 294 nm was reduced by over 50% in the W126F and W265F/Y mutants. The difference peak at 302 nm was reduced in the W265F/Y mutants, but was almost completely absent in the W126F mutant. These data indicate that the difference bands at 294 and 302 nm originate from the perturbations of Trp126 and Trp265 environments resulting from a general conformational change concomitant with MII formation and receptor activation. Model studies on tryptophan absorption indicate that the difference peak at 294 nm is due to the differential shift of the Lb absorption of the indole side chain as a result of decreased hydrophobicity or polarizability of the Trp126 and Trp265 environments. The resolution of the 302-nm band, assigned to the differential shift of the indole La absorption, is consistent with hydrogen-bonding interactions of the indole N-H groups of Trp126 and Trp265 becoming weaker in MII. These results suggest that the photoactivation of rhodopsin involves a change in the relative disposition of transmembrane helices 3 and 6, which contain Trp126 and Trp265 respectively, within the alpha-helical bundle of the receptor.

Characterization of the mutant visual pigment responsible for congenital night blindness: a biochemical and Fourier-transform infrared spectroscopy study

A mutation in the gene for the rod photoreceptor molecule rhodopsin causes congenital night blindness. The mutation results in a replacement of Gly90 by an aspartic acid residue. Two molecular mechanisms have been proposed to explain the physiology of affected rod cells. One involves constitutive activity of the G90D mutant opsin [Rao, V. R., Cohen, G. B., & Oprian, D. D. (1994) Nature 367, 639-642]. A second involves increased photoreceptor noise caused by thermal isomerization of the G90D pigment chromophore [Sieving, P. A., Richards, J. E., Naarendorp F., Bingham, E. L., Scott, K., & Alpern, M. (1995) Proc. Natl. Acad. Sci. U.S.A. 92, 880-884]. Based on existing models of rhodopsin and in vitro biochemical studies of site-directed mutants, it appears likely that Gly90 is in the immediate proximity of the Schiff base chromophore linkage. We have studied in detail the mutant pigments G90D and G90D/E113A using biochemical and Fourier-transform infrared (FTIR) spectroscopic methods. The photoproduct of mutant pigment G90D, which absorbs maximally at 468 nm and contains a protonated Schiff base linkage, can activate transducin. However, the active photoproduct decays rapidly to opsin and free all-trans-retinal. FTIR studies of mutant G90D show that the dark state of the pigment has several structural features of metarhodopsin II, the active form of rhodopsin. These include a protonated carboxylic acid group at position Glu113 and increased hydrogen-bond strength of Asp83. Additional results, which relate to the structure of the active G90D photoproduct, are also reported. Taken together, these results may be relevant to understanding the molecular mechanism of congenital night blindness caused by the G90D mutation in human rhodopsin.

Photoactivated state of rhodopsin and how it can form

A variety of spectroscopic and biochemical studies of the photoreceptor rhodopsin have revealed conformation changes which occur upon its photoactivation. Assignment of these molecular alterations to specific regions in the receptor has been attempted by studying native opsin regenerated with synthetic retinal analogs or recombinant opsins regenerated with 11-cis retinal. We propose a model for the photoactivation mechanism which defines 'off' and 'on' states for individual molecular groups. These groups have been identified to undergo structural alterations during photoactivation. Analysis of mutant pigments in which specific groups are locked into their respective 'on' or 'off' states provides a framework to identify determinants of the active conformation as well as the minimal number of intramolecular transitions to switch to this conformation. The simple model proposed 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.

Characterization of rhodopsin mutants that bind transducin but fail to induce GTP nucleotide uptake. Classification of mutant pigments by fluorescence, nucleotide release, and flash-induced light-scattering assays

The photoreceptor rhodopsin is a seven-transmembrane helix receptor that activates the G protein transducin in response to light. Several site-directed rhodopsin mutants have been reported to be defective in transducin activation. Two of these mutants bound transducin in response to light, but failed to release the bound transducin in the presence of GTP (Franke, R. R., König, B., Sakmar, T. P., Khorana, H. G., and Hofmann, K. P. (1990) Science 250, 123-125). The present study was carried out to determine the nucleotide-binding state of transducin as it interacts with rhodopsin mutants. Five mutant bovine opsin genes were prepared by site-specific mutagenesis. Three mutant genes had deletions from one cytoplasmic loop each: AB delta 70-71; CD delta 143-150; and EF delta 237-249. Two additional loop CD mutant genes were prepared: E134R/R135E had a reversal of a conserved charge pair, and CD r140-152 had a 13-amino acid sequence replaced by a sequence derived from the amino-terminal tail. Three types of assays were carried out: 1) a fluorescence assay of photoactivated rhodopsin (R*)-dependent guanosine 5'-O-(3-thiotriphosphate) uptake by transducin, 2) an assay of R*-dependent release of labeled GDP from the alpha-subunit of transducin holoenzyme (Gt alpha).GDP, and 3) a light-scattering assay of R*.Gt complex formation and dissociation. We show that the mutant pigments, which are able to bind transducin in a light-dependent manner but lack the ability to activate transducin, most likely form R*.Gt alpha beta gamma.GDP complexes that are impaired in GDP release.

Identification of glutamic acid 113 as the Schiff base proton acceptor in the metarhodopsin II photointermediate of rhodopsin

In order to investigate the molecular mechanism of rhodopsin photoactivation, site-directed mutants of bovine rhodopsin were studied by Fourier-transform infrared (FTIR) difference spectroscopy. Rhodopsin mutants E113D and E113A were prepared in which the retinylidene Schiff base counterion, Glu113, was replaced by Asp and Ala, respectively. FTIR difference spectra were recorded and compared with spectra of recombinant native rhodopsin. Both mutant pigments formed photoproducts at 0 degrees C with vibrational absorption bands typical of the metarhodopsin II (MII) state of rhodopsin. The FTIR difference spectrum of E113D was nearly identical to that of rhodopsin. A positive band at 1712 cm-1 caused by the protonation of an internal carboxylic acid in rhodopsin was shifted slightly to 1709 cm-1 in mutant E113D. E113A was studied at acidic pH in the presence of chloride as an inorganic counterion to the protonated Schiff base. The 1712-cm-1 (1709-cm-1) band was absent in the FTIR difference spectrum of mutant E113A. Therefore, we have assigned the 1712-cm-1 absorbance band to the C = O stretching vibration of protonated Glu113 in MII of rhodopsin. These results show that the Schiff base counterion of rhodopsin, the carboxylate side chain of Glu113, becomes protonated during MII formation.