Fourier transform infrared studies of active-site-methylated rhodopsin. Implications for chromophore-protein interaction, transducin activation, and the reaction pathway

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.

Key issues in the photochemistry and signalling-state formation of photosensor proteins

Four families of photosensors (i.e., rhodopsins, phytochromes, xanthopsins and cryptochromes) exist, which vary widely in the degree to which we understand the molecular basis of their activity. Some of their members are ideal model systems for studying the structure-function relation of proteins, and the role of dynamics therein. The photochemistry of photosensor activation is based upon the cis <--> trans isomerization of the chromophore. This configurational transition leads to the formation of a signalling state of sufficient stability to communicate the presence of photons to a downstream signal-transduction partner. In the xanthopsins it has been demonstrated that the exact nature of this signalling state is strongly dependent on the mesoscopic context of the sensor protein. The cryptochromes appear to challenge the photoisomerization rule.

Vibrational spectrum of the lumi intermediate in the room temperature rhodopsin photo-reaction

The vibrational spectrum (650-1750 cm(-1)) of the lumi-rhodopsin (lumi) intermediate formed in the microsecond time regime of the room-temperature rhodopsin (RhRT) photoreaction is measured for the first time using picosecond time-resolved coherent anti-Stokes Raman spectroscopy (PTR/CARS). The vibrational spectrum of lumi is recorded 2.5 micros after the 3-ps, 500-nm excitation of RhRT. Complementary to Fourier transform infrared spectra recorded at Rh sample temperatures low enough to freeze lumi, these PTR/CARS results provide the first detailed view of the vibrational degrees of freedom of room-temperature lumi (lumiRT) through the identification of 21 bands. The exceptionally low intensity (compared to those observed in bathoRT) of the hydrogen out-of-plane (HOOP) bands, the moderate intensity and absolute positions of C-C stretching bands, and the presence of high-intensity C==C stretching bands suggest that lumiRT contains an almost planar (nontwisting), all-trans retinal geometry. Independently, the 944-cm(-1) position of the most intense HOOP band implies that a resonance coupling exists between the out-of-plane retinal vibrations and at least one group among the amino acids comprising the retinal binding pocket. The formation of lumiRT, monitored via PTR/CARS spectra recorded on the nanosecond time scale, can be associated with the decay of the blue-shifted intermediate (BSI(RT)) formed in equilibrium with the bathoRT intermediate. PTR/CARS spectra measured at a 210-ns delay contain distinct vibrational features attributable to BSI(RT), which suggest that the all-trans retinal in both BSI(RT) and lumiRT is strongly coupled to part of the retinal binding pocket. With regard to the energy storage/transduction mechanism in RhRT, these results support the hypothesis that during the formation of lumiRT, the majority of the photon energy absorbed by RhRT transfers to the apoprotein opsin.

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.

Mechanisms of opsin activation

Rhodopsin is constrained in an inactive conformation by interactions with 11-cis-retinal including formation of a protonated Schiff base with Lys296. Upon photoisomerization, major structural rearrangements that involve protonation of the active site Glu113 and cytoplasmic acidic residues, including Glu134, lead to the formation of the active form of the receptor, metarhodopsin II b, which decays to opsin. However, an activated receptor may be generated without illumination by addition of all-trans-retinal or its analogues to opsin, as measured in this study by the increased phosphorylation of opsin by rhodopsin kinase. The potency of stimulation depended on the chemical and isomeric nature of the analogues and the length of the polyene chain with all-trans-C17 aldehyde and all-trans-retinal being the most active and trans-C12 aldehyde being the least active. Certain cis-isomers, 11-cis-13-demethyl-retinal and 9-cis-C17 aldehyde, were also active. Most of the retinal analogues tested did not regenerate a spectrally identifiable pigment, and many were incapable of Schiff base formation (ketone, stable oximes, and Schiff base-derivatives of retinal). Thus, receptor activation resulted from formation of non-covalent complexes with opsin. pH titrations suggested that an equilibrium exists between partially active (protonated) and inactive (deprotonated) forms of opsin. These findings are consistent with a model in which protonation of one or more cytoplasmic carboxyl groups of opsin is essential for activity. Upon addition of retinoids, the partially active conformation of opsin is converted to a more active intermediate similar to metarhodopsin II b. The model provides an understanding of the structural requirements for opsin activation and an interpretation of the observed activities of natural and experimental opsin mutants.

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.

Changes in structure of the chromophore in the photochemical process of bovine rhodopsin as revealed by FTIR spectroscopy for hydrogen out-of-plane vibrations

The hydrogen out-of-plane bending (HOOP) vibrations were studied in the difference Fourier transform infrared spectra of lumirhodopsin and metarhodopsin I by use of a series of specifically deuterated retinal derivatives of bovine rod outer segments. The 947 cm-1 band of lumirhodopsin and the 950 cm-1 band of metarhodopsin I were assigned to the mode composed of both 11-HOOP and 12-HOOP vibrations. This result suggests that the perturbation near C12-H of the retinal in the earlier intermediate, bathorhodopsin (Palings, van den Berg, Lugtenburg and Mathies, Biochemistry, 28 (1989) 1498-1507), is extinguished in lumirhodopsin and metarhodopsin I. Unphotolyzed rhodopsin and metarhodopsin I exhibited the 14-HOOP bands in the 12-D derivatives at 901 and 886 cm-1, respectively. Lumirhodopsin, however, did not show the 14-HOOP in the 12-D derivatives. The result suggests a change in geometrical alignment of the C14-H bond in lumirhodopsin with respect to the N-H bond of the Schiff base.

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.

Opsins with mutations at the site of chromophore attachment constitutively activate transducin but are not phosphorylated by rhodopsin kinase

More than 70 mutations in the gene encoding the visual pigment rhodopsin have been identified in patients with autosomal dominant retinitis pigmentosa. Most of these mutations are thought to interfere with proper folding of the membrane protein. However, families with a severe phenotype of retinitis pigmentosa have been identified and shown to carry a mutation at the site of chromophore attachment, Lys-296. This mutation disrupts the inactive conformation of opsin and results in a constitutively active protein that can activate the rod-specific GTP-binding protein, transducin, in the absence of light and in the absence of the chromophore 11-cis-retinal. It has been suggested that this mutant opsin molecule may cause rod degeneration by depletion of the components used to inactivate rhodopsin, such as rhodopsin kinase. In this work we test this idea by determining whether two constitutively active opsin mutants are phosphorylated by rhodopsin kinase. We found that opsin mutants where Lys-296 is replaced either by Glu (K296E) or by Gly (K296G) are not substrates of rhodopsin kinase in the absence of chromophore. However, when K296G is regenerated with a Schiff base complex of 11-cis-retinal and n-propylamine and exposed to illumination, phosphorylation of opsin occurs. These experiments suggest that in the rod photoreceptors of patients with retinitis pigmentosa carrying a mutation at Lys-296, there is persistent activation of the GTP-binding protein-mediated cascade. This may result in a situation that mimics long-term exposure to continuous illumination and results in the degeneration of photoreceptors.

Protonation states of membrane-embedded carboxylic acid groups in rhodopsin and metarhodopsin II: a Fourier-transform infrared spectroscopy study of site-directed mutants

A method was developed to measure Fourier-transform infrared (FTIR) difference spectra of detergent-solubilized rhodopsin expressed in COS cells. Experiments were performed on native bovine rhodopsin, rhodopsin expressed in COS cells, and three expressed rhodopsin mutants with amino acid replacements of membrane-embedded carboxylic acid groups: Asp-83-->Asn (D83N), Glu-122-->Gln (E122Q), and the double mutant D83N/E122Q. Each of the mutant opsins bound 11-cis-retinal to yield a visible light-absorbing pigment. Upon illumination, each of the mutant pigments formed a metarhodopsin II-like species with maximal absorption at 380 nm that was able to activate guanine nucleotide exchange by transducin. Rhodopsin versus metarhodopsin II-like photoproduct FTIR-difference spectra were recorded for each sample. The COS-cell rhodopsin and mutant difference spectra showed close correspondence to that of rhodopsin from disc membranes. Difference bands (rhodopsin/metarhodopsin II) at 1767/1750 cm-1 and at 1734/1745 cm-1 were absent from the spectra of mutants D83N and E122Q, respectively. Both bands were absent from the spectrum of the double mutant D83N/E122Q. These results show that Asp-83 and Glu-122 are protonated both in rhodopsin and in metarhodopsin II, in agreement with the isotope effects observed in spectra measured in 2H2O. A photoproduct band at 1712 cm-1 was not affected by either single or double replacements at positions 83 and 122. We deduce that the 1712 cm-1 band arises from the protonation of Glu-113 in metarhodopsin II.

Interaction of rhodopsin with the G-protein, transducin

Rhodopsin, upon activation by light, transduces the photon signal by activation of the G-protein, transducin. The well-studied rhodopsin/transducin system serves as a model for the understanding of signal transduction by the large class of G-protein-coupled receptors. The interactive form of rhodopsin, R*, is conformationally similar or identical to rhodopsin's photolysis intermediate Metarhodopsin II (MII). Formation of MII requires deprotonation of rhodopsin's protonated Schiff base which appears to facilitate some opening of the rhodopsin structure. This allows a change in conformation at rhodopsin's cytoplasmic surface that provides binding sites for transducin. Rhodopsin's 2nd, 3rd and putative 4th cytoplasmic loops bind transducin at sites including transducin's 5 kDa carboxyl-terminal region. Site-specific mutagenesis of rhodopsin is being used to distinguish sites on rhodopsin's surface that are important in binding transducin from those that function in activating transducin. These observations are consistent with and extend studies on the action of other G-protein-coupled receptors and their interactions with their respective G proteins.