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Abstract
Although G-protein–coupled receptors (GPCRs) control vast physiological pathways, their activation remains chemically and physically enigmatic. Our osmotic stress studies of the visual receptor rhodopsin have redefined the standard model of GPCR signaling by revealing the essential role of bulk water. We show results consistent with a large number of water molecules flooding the rhodopsin interior during activation to stabilize the effector binding conformation. These results suggest a model of GPCR activation in which the receptor becomes solvent-swollen upon formation of the active state. We thus demonstrate the mechanism whereby water acts as a powerful allosteric modulator of a pharmacologically important membrane protein family. The Rhodopsin family of G-protein–coupled receptors (GPCRs) comprises the targets of nearly a third of all pharmaceuticals. Despite structural water present in GPCR X-ray structures, the physiological relevance of these solvent molecules to rhodopsin signaling remains unknown. Here, we show experimental results consistent with the idea that rhodopsin activation in lipid membranes is coupled to bulk water movements into the protein. To quantify hydration changes, we measured reversible shifting of the metarhodopsin equilibrium due to osmotic stress using an extensive series of polyethylene glycol (PEG) osmolytes. We discovered clear evidence that light activation entails a large influx of bulk water (∼80–100 molecules) into the protein, giving insight into GPCR activation mechanisms. Various size polymer osmolytes directly control rhodopsin activation, in which large solutes are excluded from rhodopsin and dehydrate the protein, favoring the inactive state. In contrast, small osmolytes initially forward shift the activation equilibrium until a quantifiable saturation point is reached, similar to gain-of-function protein mutations. For the limit of increasing osmolyte size, a universal response of rhodopsin to osmotic stress is observed, suggesting it adopts a dynamic, hydrated sponge-like state upon photoactivation. Our results demand a rethinking of the role of water dynamics in modulating various intermediates in the GPCR energy landscape. We propose that besides bound water, an influx of bulk water plays a necessary role in establishing the active GPCR conformation that mediates signaling.
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2
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Guérin DMA, Digilio A, Branda MM. Dimeric Rhodopsin R135L Mutant-Transducin-like Complex Sheds Light on Retinitis Pigmentosa Misfunctions. J Phys Chem B 2021; 125:12958-12971. [PMID: 34793169 DOI: 10.1021/acs.jpcb.1c06348] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Rhodopsin (RHO) is a light-sensitive pigment in the retina and the main prototypical protein of the G-protein-coupled receptor (GCPR) family. After receiving a light stimulus, RHO and its cofactor retinylidene undergo a series of structural changes that initiate an intricate transduction mechanism. Along with RHO, other partner proteins play key roles in the signaling pathway. These include transducin, a GTPase, kinases that phosphorylate RHO, and arrestin (Arr), which ultimately stops the signaling process and promotes RHO regeneration. A large number of RHO genetic mutations may lead to very severe retinal dysfunction and eventually to impaired dark adaptation disease called autosomal dominant retinitis pigmentosa (adRP). In this study, we used molecular dynamics (MD) simulations to evaluate the different behaviors of the dimeric form of wild-type RHO (WT dRHO) and its mutant at position 135 of arginine to leucine (dR135L), both in the free (noncomplexed) and in complex with the transducin-like protein (Gtl). Gtl is a heterotrimeric model composed of a mixture of human and bovine G proteins. Our calculations allow us to explain how the mutation causes structural changes in the RHO dimer and how this can affect the signal that transducin generates when it is bound to RHO. Moreover, the structural modifications induced by the R135L mutation can also account for other misfunctions observed in the up- and downstream signaling pathways. The mechanism of these dysfunctions, together with the transducin activity reduction, provides structure-based explanations of the impairment of some key processes that lead to adRP.
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Affiliation(s)
- Diego M A Guérin
- Department of Biochemistry and Molecular Biology, University of the Basque Country (EHU) and Instituto Biofisika (CSIC, UPV/EHU), Barrio Sarriena S/N, 48940 Leioa, Vizcaya, Spain
| | - Ayelen Digilio
- Department of Physics, National University of San Luis (UNSL), Av. Ejército de los Andes 950, 5700 San Luis, Argentina
| | - María Marta Branda
- Institute of Applied Physics (CONICET-UNSL), Av. Ejercito de los Andes 950, 5700 San Luis, Argentina
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3
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Abstract
Infrared difference spectroscopy probes vibrational changes of proteins upon their perturbation. Compared with other spectroscopic methods, it stands out by its sensitivity to the protonation state, H-bonding, and the conformation of different groups in proteins, including the peptide backbone, amino acid side chains, internal water molecules, or cofactors. In particular, the detection of protonation and H-bonding changes in a time-resolved manner, not easily obtained by other techniques, is one of the most successful applications of IR difference spectroscopy. The present review deals with the use of perturbations designed to specifically change the protein between two (or more) functionally relevant states, a strategy often referred to as reaction-induced IR difference spectroscopy. In the first half of this contribution, I review the technique of reaction-induced IR difference spectroscopy of proteins, with special emphasis given to the preparation of suitable samples and their characterization, strategies for the perturbation of proteins, and methodologies for time-resolved measurements (from nanoseconds to minutes). The second half of this contribution focuses on the spectral interpretation. It starts by reviewing how changes in H-bonding, medium polarity, and vibrational coupling affect vibrational frequencies, intensities, and bandwidths. It is followed by band assignments, a crucial aspect mostly performed with the help of isotopic labeling and site-directed mutagenesis, and complemented by integration and interpretation of the results in the context of the studied protein, an aspect increasingly supported by spectral calculations. Selected examples from the literature, predominately but not exclusively from retinal proteins, are used to illustrate the topics covered in this review.
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4
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Dušeková E, Garajová K, Yavaşer R, Varhač R, Sedlák E. Hofmeister effect on catalytic properties of chymotrypsin is substrate-dependent. Biophys Chem 2018; 243:8-16. [DOI: 10.1016/j.bpc.2018.10.002] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/16/2018] [Revised: 09/30/2018] [Accepted: 10/09/2018] [Indexed: 12/26/2022]
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5
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Qi B, Guo X, Gao Y, Li D, Luo J, Li H, Eghtesadi SA, He C, Duan C, Liu T. Strong Co-Ion Effect via Cation−π Interaction on the Self-Assembly of Metal–Organic Cationic Macrocycles. J Am Chem Soc 2017; 139:12020-12026. [DOI: 10.1021/jacs.7b06564] [Citation(s) in RCA: 27] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Affiliation(s)
- Bo Qi
- Department
of Polymer Science, The University of Akron, Akron, Ohio 44325-3909, United States
- State
Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, P. R. China
| | - Xiangyang Guo
- State
Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, P. R. China
| | - Yunyi Gao
- Department
of Polymer Science, The University of Akron, Akron, Ohio 44325-3909, United States
| | - Dong Li
- Department
of Polymer Science, The University of Akron, Akron, Ohio 44325-3909, United States
| | - Jiancheng Luo
- Department
of Polymer Science, The University of Akron, Akron, Ohio 44325-3909, United States
| | - Hui Li
- Department
of Polymer Science, The University of Akron, Akron, Ohio 44325-3909, United States
| | - Seyed Ali Eghtesadi
- Department
of Polymer Science, The University of Akron, Akron, Ohio 44325-3909, United States
| | - Cheng He
- State
Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, P. R. China
| | - Chunying Duan
- State
Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, P. R. China
| | - Tianbo Liu
- Department
of Polymer Science, The University of Akron, Akron, Ohio 44325-3909, United States
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6
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Kirchberg K, Kim TY, Haase S, Alexiev U. Functional interaction structures of the photochromic retinal protein rhodopsin. Photochem Photobiol Sci 2010; 9:226-33. [PMID: 20126799 DOI: 10.1039/b9pp00134d] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
We studied functional interaction structures of the vertebrate membrane photoreceptor rhodopsin containing retinal as a chromophore. Using time-resolved fluorescence depolarization we analyzed real-time dynamics and conformational changes of the cytoplasmic helix 8 (H8) preceding the long C-terminal tail of rhodopsin. H8 runs parallel to the membrane surface and extends from transmembrane helix 7 whose highly conserved NPxxY(x)F motif connects that region of rhodopsin with the retinal binding pocket. Our measurements indicate that photo-induced retinal isomerization from 11-cis to all-trans provokes conformational changes of H8, including slower motion and reduced flexibility, that are specific for the active metarhodopsin-II photo-intermediate. These conformational changes are absent in the retinal-devoid state opsin and in the phosphorylated metarhodopsin-II state upon receptor deactivation. Furthermore we show that membrane rim effects can influence interfacial reactions at the cytoplasmic rhodopsin surface such as proton transfer reactions between surface and aqueous bulk phase or binding of the signaling protein transducin visualized with single-molecule widefield microscopy. These findings are important for an understanding of the effects of membrane structure on the photo-transduction mechanism.
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Affiliation(s)
- Kristina Kirchberg
- Freie Universität Berlin, Inst. für. Experimentalphysik, Arnimallee 14, D-14195 Berlin, Germany
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7
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Li H, Xu YY, Weng YX. Infrared Absorption Intensity Analysis as a New Tool for Investigation of Salt Effect on Proteins. CHINESE J CHEM PHYS 2009. [DOI: 10.1088/1674-0068/22/06/556-562] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
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8
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Varhač R, Tomášková N, Fabián M, Sedlák E. Kinetics of cyanide binding as a probe of local stability/flexibility of cytochrome c. Biophys Chem 2009; 144:21-6. [DOI: 10.1016/j.bpc.2009.06.001] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/09/2009] [Revised: 05/30/2009] [Accepted: 06/01/2009] [Indexed: 10/20/2022]
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9
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Effect of Hofmeister ions on protein thermal stability: Roles of ion hydration and peptide groups? Arch Biochem Biophys 2008; 479:69-73. [DOI: 10.1016/j.abb.2008.08.013] [Citation(s) in RCA: 84] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/23/2008] [Revised: 08/17/2008] [Accepted: 08/20/2008] [Indexed: 11/22/2022]
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10
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Nucci NV, Vanderkooi JM. Effects of Salts of the Hofmeister Series on the Hydrogen Bond Network of Water. J Mol Liq 2008; 143:160-170. [PMID: 19847287 PMCID: PMC2748947 DOI: 10.1016/j.molliq.2008.07.010] [Citation(s) in RCA: 77] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
Abstract
The effect of salts on water behavior has been a topic of interest for many years; however, some recent reports have suggested that ions do not influence the hydrogen bonding behavior of water. Using an effective two-state hydrogen bonding model to interpret the temperature excursion infrared response of the O-H stretch of aqueous salt solutions, we show a strong correlation between salt effects on water hydrogen bonding and the Hofmeister order. These data clearly show that salts do have a measurable impact on the equilibrium hydrogen bonding behavior of water and support models which explain Hofmeister effects on the basis of solute charge density.
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Affiliation(s)
- Nathaniel V. Nucci
- Department of Biochemistry and Biophysics, University of Pennsylvania. 422 Curie Boulevard, Philadelphia, PA 19104, 1*(215)898-8783, fax: 1*(215)573-2085,
| | - Jane M. Vanderkooi
- Department of Biochemistry and Biophysics, University of Pennsylvania. 422 Curie Boulevard, Philadelphia, PA 19104, 1*(215)898-8783, fax: 1*(215)573-2085,
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11
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Alexandrov AI, Mileni M, Chien EY, Hanson MA, Stevens RC. Microscale Fluorescent Thermal Stability Assay for Membrane Proteins. Structure 2008; 16:351-9. [DOI: 10.1016/j.str.2008.02.004] [Citation(s) in RCA: 345] [Impact Index Per Article: 21.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/04/2007] [Revised: 02/06/2008] [Accepted: 02/06/2008] [Indexed: 11/25/2022]
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12
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Tóth K, Sedlák E, Sprinzl M, Zoldák G. Flexibility and enzyme activity of NADH oxidase from Thermus thermophilus in the presence of monovalent cations of Hofmeister series. BIOCHIMICA ET BIOPHYSICA ACTA-PROTEINS AND PROTEOMICS 2008; 1784:789-95. [PMID: 18339331 DOI: 10.1016/j.bbapap.2008.01.022] [Citation(s) in RCA: 39] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/06/2007] [Revised: 01/26/2008] [Accepted: 01/29/2008] [Indexed: 11/25/2022]
Abstract
Recently, we have shown that anions of Hofmeister series affect the enzyme activity through modulation of flexibility of its active site. The enzyme activity vs. anion position in Hofmeister series showed an unusual bell-shaped dependence. In the present work, six monovalent cations (Na(+), Gdm(+), NH(4)(+), Li(+), K(+) and Cs(+)) of Hofmeister series with chloride as a counterion have been studied in relation to activity and stability of flavoprotein NADH oxidase from Thermus thermophilus (NOX). With the exception of strongly chaotropic guanidinium cation, cations are significantly less effective in promoting the Hofmeister effect than anions mainly due to repulsive interactions of positive charges around the active site. Thermal denaturations of NOX reveal unfavorable electrostatic interaction at the protein surface that may be shielded to different extent by salts. Michaelis-Menten constants for NADH, accessibility of the active site as reflected by Stern-Volmer constants and activity of NOX at high cation concentrations (1-2 M) show bell-shaped dependences on cation position in Hofmeister series. Our analysis indicates that in the presence of kosmotropic cations the enzyme is more stable and possibly more rigid than in the presence of chaotropic cations. Molecular dynamic (MD) simulations of NOX showed that active site switches between open and closed conformations [J. Hritz, G. Zoldak, E. Sedlak, Cofactor assisted gating mechanism in the active site of NADH oxidase from Thermus thermophilus, Proteins 64 (2006) 465-476]. Enzyme activity, as well as substrate binding, can be regulated by the salt mediated perturbation of the balance between open and closed forms. We propose that compensating effect of accessibility and flexibility of the enzyme active site leads to bell-shaped dependence of the investigated parameters.
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Affiliation(s)
- Kamil Tóth
- Department of Biochemistry, Faculty of Sciences P. J. Safárik University, Kosice, Slovakia
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13
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Hoersch D, Otto H, Joshi CP, Borucki B, Cusanovich MA, Heyn MP. Role of a conserved salt bridge between the PAS core and the N-terminal domain in the activation of the photoreceptor photoactive yellow protein. Biophys J 2007; 93:1687-99. [PMID: 17496031 PMCID: PMC1948051 DOI: 10.1529/biophysj.107.106633] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The effect of ionic strength on the conformational equilibrium between the I(2) intermediate and the signaling state I(2)' of the photoreceptor PYP and on the rate of recovery to the dark state were investigated by time-resolved absorption and fluorescence spectroscopy. With increasing salt concentration up to approximately 600 mM, the recovery rate k(3) decreases and the I(2)/I(2)' equilibrium (K) shifts in the direction of I(2)'. At higher ionic strength both effects reverse. Experiments with mono-(KCl, NaBr) and divalent (MgCl(2), MgSO(4)) salts show that the low salt effect depends on the ionic strength and not on the cation or anion species. These observations can be described over the entire ionic strength range by considering the activity coefficients of an interdomain salt bridge. At low ionic strength the activity coefficient decreases due to counterion screening whereas at high ionic strength binding of water by the salt leads to an increase in the activity coefficient. From the initial slopes of the plots of log k(3) and log K versus the square root of the ionic strength, the product of the charges of the interacting groups was found to be -1.3 +/- 0.2, suggesting a monovalent ion pair. The conserved salt bridge K110/E12 connecting the beta-sheet of the PAS core and the N-terminal domain is a prime candidate for this ion pair. To test this hypothesis, the mutants K110A and E12A were prepared. In K110A the salt dependence of the I(2)/I(2)' equilibrium was eliminated and of the recovery rate was greatly reduced below approximately 600 mM. Moreover, at low salt the recovery rate was six times slower than in wild-type. In E12A significant salt dependence remained, which is attributed to the formation of a novel salt bridge between K110 and E9. At high salt reversal occurs in both mutants suggesting that salting out stabilizes the more compact I(2) structure. However, chaotropic anions like SCN shift the I(2)/I(2)' equilibrium toward the partially unfolded I(2)' form. The salt linkage K110/E12 stabilizes the photoreceptor in the inactive state in the dark and is broken in the light-induced formation of the signaling state, allowing the N-terminal domain to detach from the beta-scaffold PAS core.
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Affiliation(s)
- Daniel Hoersch
- Biophysics Group, Department of Physics, Freie Universität Berlin, Berlin, Germany
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14
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Borucki B, Joshi CP, Otto H, Cusanovich MA, Heyn MP. The transient accumulation of the signaling state of photoactive yellow protein is controlled by the external pH. Biophys J 2006; 91:2991-3001. [PMID: 16829563 PMCID: PMC1578485 DOI: 10.1529/biophysj.106.086645] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The signaling state of the photoreceptor photoactive yellow protein is the long-lived intermediate I(2)'. The pH dependence of the equilibrium between the transient photocycle intermediates I(2) and I(2)' was investigated. The formation of I(2)' from I(2) is accompanied by a major conformational change. The kinetics and intermediates of the photocycle and of the photoreversal were measured by transient absorption spectroscopy from pH 4.6 to 8.4. Singular value decomposition (SVD) analysis of the data at pH 7 showed the presence of three spectrally distinguishable species: I(1), I(2), and I(2)'. Their spectra were determined using the extrapolated difference method. I(2) and I(2)' have electronic absorption spectra, with maxima at 370 +/- 5 and 350 +/- 5 nm, respectively. Formation of the signaling state is thus associated with a change in the environment of the protonated chromophore. The time courses of the I(1), I(2), and I(2)' intermediates were determined from the wavelength-dependent transient absorbance changes at each pH, assuming that their spectra are pH-independent. After the formation of I(2)' ( approximately 2 ms), these three intermediates are in equilibrium and decay together to the initial dark state. The equilibrium between I(2) and I(2)' is pH dependent with a pK(a) of 6.4 and with I(2)' the main species above this pK(a). Measurements of the pH dependence of the photoreversal kinetics with a second flash of 355 nm at a delay of 20 ms confirm this pK(a) value. I(2) and I(2)' are photoreversed with reversal times of approximately 55 micros and several hundred microseconds, respectively. The corresponding signal amplitudes are pH dependent with a pK(a) of approximately 6.1. Photoreversal from I(2)' dominates above the pK(a). The transient accumulation of I(2)', the active state of photoactive yellow protein, is thus controlled by the proton concentration. The rate constant k(3) for the recovery to the initial dark state also has a pK(a) of approximately 6.3. This equality of the equilibrium and kinetic pK(a) values is not accidental and suggests that k(3) is proportional to [I(2)'].
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Affiliation(s)
- Berthold Borucki
- Biophysics Group, Department of Physics, Freie Universität Berlin, 14195 Berlin, Germany
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15
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Janz JM, Farrens DL. Role of the Retinal Hydrogen Bond Network in Rhodopsin Schiff Base Stability and Hydrolysis. J Biol Chem 2004; 279:55886-94. [PMID: 15475355 DOI: 10.1074/jbc.m408766200] [Citation(s) in RCA: 57] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Little is known about the molecular mechanism of Schiff base hydrolysis in rhodopsin. We report here our investigation into this process focusing on the role of amino acids involved in a hydrogen bond network around the retinal Schiff base. We find conservative mutations in this network (T94I, E113Q, S186A, E181Q, Y192F, and Y268F) increase the activation energy (E(a)) and abolish the concave Arrhenius plot normally seen for Schiff base hydrolysis in dark state rhodopsin. Interestingly, two mutants (T94I and E113Q) show dramatically faster rates of Schiff base hydrolysis in dark state rhodopsin, yet slower hydrolysis rates in the active MII form. We find deuterium affects the hydrolysis process in wild-type rhodopsin, exhibiting a specific isotope effect of approximately 2.5, and proton inventory studies indicate that multiple proton transfer events occur during the process of Schiff base hydrolysis for both dark state and MII forms. Taken together, our study demonstrates the importance of the retinal hydrogen bond network both in maintaining Schiff base integrity in dark state rhodopsin, as well as in catalyzing the hydrolysis and release of retinal from the MII form. Finally, we note that the dramatic alteration of Schiff base stability caused by mutation T94I may play a causative role in congenital night blindness as has been suggested by the Oprian and Garriga laboratories.
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Affiliation(s)
- Jay M Janz
- Department of Biochemistry and Molecular Biology, Oregon Health and Science University, 3181 SW Sam Jackson Park Road, Portland, OR 97239, USA
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16
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Influence of salts on rhodopsin photoproduct equilibria and protein stability. Curr Opin Colloid Interface Sci 2004. [DOI: 10.1016/j.cocis.2004.05.018] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
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17
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Boström M, Williams DRM, Ninham BW. Specific ion effects: Role of salt and buffer in protonation of cytochrome c. THE EUROPEAN PHYSICAL JOURNAL. E, SOFT MATTER 2004; 13:239-245. [PMID: 15103518 DOI: 10.1140/epje/i2003-10072-0] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/24/2023]
Abstract
Changes in background salt and buffer are known to influence the properties of proteins. The reasons have remained obscure. The challenge posed by many such problems is this. Can physical chemistry contribute any predictive quantitative insights to what is in effect the simplest macromolecular solution behavior? Or must all remain specific? Our thesis is that it can. For definiteness we consider here as an illustrative example: surface pH and protonation equilibria of cytochrome c. We demonstrate an important role for ionic dispersion forces, missing from previous theoretical treatments. Unlike charge interactions these are different for each ionic species, and act between a protein and both salt and buffer ions. The charge of proteins depends not only on pH, ionic charge, and salt concentration. Taking ionic dispersion forces into account goes some way towards explaining the dependence on ionic species. We demonstrate why the addition of buffer can have profound effects, including reversal of the salt dependence of the protein charge.
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Affiliation(s)
- M Boström
- Department of Physics and Measurement Technology, Linköping University, Sweden.
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18
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Nair VD, Sealfon SC. Agonist-specific transactivation of phosphoinositide 3-kinase signaling pathway mediated by the dopamine D2 receptor. J Biol Chem 2003; 278:47053-61. [PMID: 12970364 DOI: 10.1074/jbc.m303364200] [Citation(s) in RCA: 82] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Bromocriptine, acting through the dopamine D2 receptor, provides robust protection against apoptosis induced by oxidative stress in PC12-D2R and immortalized nigral dopamine cells. We now report the characterization of the D2 receptor signaling pathways mediating the cytoprotection. Bromocriptine caused protein kinase B (Akt) activation in PC12-D2R cells and the inhibition of either phosphoinositide (PI) 3-kinase, epidermal growth factor receptor (EGFR), or c-Src eliminated the Akt activation and the cytoprotective effects of bromocriptine against oxidative stress. Co-immunoprecipitation studies showed that the D2 receptor forms a complex with the EGFR and c-Src that was augmented by bromocriptine, suggesting a cross-talk between these proteins in mediating the activation of Akt. EGFR repression by inhibitor or by RNA interference eliminated the activation of Akt by bromocriptine. D2 receptor stimulation by bromocriptine induced c-Src tyrosine 418 phosphorylation and EGFR phosphorylation specifically at tyrosine 845, a known substrate of Src kinase. Furthermore, Src tyrosine kinase inhibitor or dominant negative Src interfered with Akt translocation and phosphorylation. Thus, the predominant signaling cascade mediating cytoprotection by the D2 receptor involves c-Src/EGFR transactivation by D2 receptor, activating PI 3-kinase and Akt. We also found that the agonist pramipexole failed to stimulate activation of Akt in PC12-D2R cells, providing an explanation for our previous observations that, despite efficiently activating G-protein signaling, this agonist had little cytoprotective activity in this experimental system. These results support the hypothesis that specific dopamine agonists stabilize distinct conformations of the D2 receptor that differ in their coupling to G-proteins and to a cytoprotective c-Src/EGFR-mediated PI-3 kinase/Akt pathway.
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Affiliation(s)
- Venugopalan D Nair
- Department of Neurology, Mount Sinai School of Medicine, One Gustave L. Levy Place, New York, NY 10029, USA
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19
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Vogel R, Siebert F. Fourier transform IR spectroscopy study for new insights into molecular properties and activation mechanisms of visual pigment rhodopsin. Biopolymers 2003; 72:133-48. [PMID: 12722110 DOI: 10.1002/bip.10407] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
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.
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Affiliation(s)
- Reiner Vogel
- Biophysics Group, Institut für Molekulare Medizin und Zellforschung, Albert-Ludwigs-Universität Freiburg, Hermann-Herder-Strasse 9, D-79104 Freiburg, Germany
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Janz JM, Fay JF, Farrens DL. Stability of dark state rhodopsin is mediated by a conserved ion pair in intradiscal loop E-2. J Biol Chem 2003; 278:16982-91. [PMID: 12547830 DOI: 10.1074/jbc.m210567200] [Citation(s) in RCA: 68] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The rhodopsin crystal structure reveals that intradiscal loop E-2 covers the 11-cis-retinal, creating a "retinal plug." Recently, we noticed the ends of loop E-2 are linked by an ion pair between residues Arg-177 and Asp-190, near the highly conserved disulfide bond. This ion pair appears biologically significant; it is conserved in almost all vertebrate opsins and may occur in other G-protein-coupled receptors. We report here that the Arg-177/Asp-190 ion pair is critical for the folding and stability of dark state rhodopsin. We find ion pair mutants that regenerate with retinal are functionally and spectrally wild-type-like yet thermally unstable in their dark state because of rapid hydrolysis of the retinal Schiff base linkage. Surprisingly, Arrhenius analysis indicates that the activation energies for the hydrolysis process are similar between the ion pair mutants and wild-type rhodopsin. Furthermore, the ion pair mutants do not show increased reactivity toward hydroxylamine, suggesting that their instability is not caused by an increased exposure to bulk solvent. Our results indicate that the loop E-2 ion pair is important for rhodopsin stability and thus suggest that retinitis pigmentosa observed in patients with Asp-190 mutations may in part be the result of thermally unstable rhodopsin proteins.
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Affiliation(s)
- Jay M Janz
- Department of Biochemistry and Molecular Biology, Oregon Health and Science University, Portland, Oregon 97201, USA
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Alexiev U, Rimke I, Pöhlmann T. Elucidation of the nature of the conformational changes of the EF-interhelical loop in bacteriorhodopsin and of the helix VIII on the cytoplasmic surface of bovine rhodopsin: a time-resolved fluorescence depolarization study. J Mol Biol 2003; 328:705-19. [PMID: 12706727 DOI: 10.1016/s0022-2836(03)00326-7] [Citation(s) in RCA: 78] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
The conformation of the AB-loop and EF-loop of bacteriorhodopsin and of the fourth cytoplasmic loop (helix VIII) of bovine rhodopsin were assessed by a combination of time-resolved fluorescence depolarization and site-directed fluorescence labeling. The fluorescence anisotropy decays were measured employing a tunable Ti:sapphire laser/microchannel plate based single-photon counting apparatus with picosecond time resolution. This method allows measurement of the diffusional dynamics of the loops directly on a nanosecond time-scale. We implemented the method to study model peptides and two-helix systems representing sequences of bacteriorhodopsin. Thus, we systematically analyzed the anisotropic behavior of four different fluorescent dyes covalently bound to a single cysteine residue on the protein surface and assigned the anisotropy decay components to the modes of motion of the protein and its segments. We have identified two mechanisms of loop conformational changes in the functionally intact proteins bacteriorhodopsin and bovine rhodopsin. First, we found a surface potential-dependent transition between two conformational states of the EF-loop of bacteriorhodopsin, detected with the fluorescent dye bound to position 160. A transition between the two conformational states at 150mM KCl and 20 degrees C requires a surface potential change that corresponds to Deltasigma approximately -1.0e(-)/bacteriorhodopsin molecule. We suggest, that the surface potential-based switch of the EF-loop is the missing link between the movement of helix F and the transient surface potential change detected during the photocycle of bacteriorhodopsin. Second, in the visual pigment rhodopsin, with the fluorescent dye bound to position 316, a particularly striking pH-dependent conformational change of the fourth loop on the cytoplasmic surface was analyzed. The loop mobility increased from pH 5 to 8. The midpoint of this transition is at pH 6.2 and correlates with the midpoint of the pH-dependent equilibrium between the active metarhodopsin II and the inactive metarhodopsin I state.
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Affiliation(s)
- U Alexiev
- Department of Physics, Freie Universität Berlin, Arnimallee 14, D-14195 Berlin, Germany.
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Fan G, Siebert F, Sheves M, Vogel R. Rhodopsin with 11-cis-locked chromophore is capable of forming an active state photoproduct. J Biol Chem 2002; 277:40229-34. [PMID: 12177057 DOI: 10.1074/jbc.m205033200] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
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.
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Affiliation(s)
- Guibao Fan
- Department of Organic Chemistry, Weizmann Institute of Science, Rehovot 76100, Israel
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Prioleau C, Visiers I, Ebersole BJ, Weinstein H, Sealfon SC. Conserved helix 7 tyrosine acts as a multistate conformational switch in the 5HT2C receptor. Identification of a novel "locked-on" phenotype and double revertant mutations. J Biol Chem 2002; 277:36577-84. [PMID: 12145300 DOI: 10.1074/jbc.m206223200] [Citation(s) in RCA: 101] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Studies in many rhodopsin-like G-protein-coupled receptors are providing a general scheme of the structural processes underlying receptor activation. Microdomains in several receptors have been identified that appear to function as activation switches. However, evidence is emerging that these receptor proteins exist in multiple conformational states. To study the molecular control of this switching process, we investigated the function of a microdomain involving the conserved helix 7 tyrosine in the serotonin 5HT2C receptor. This tyrosine of the NPXXY motif was substituted for all naturally occurring amino acids. Three distinct constitutively active receptor phenotypes were found: moderate, high, and "locked-on" constitutive activity. In contrast to the activity of the other receptor mutants, the high basal signaling of the locked-on Y7.53N mutant was neither increased by agonists nor decreased by inverse agonists. The Y7.53F mutant was uncoupled. Computational modeling based on the rhodopsin crystal structure suggested that Y7.53 interacts with the conserved aromatic ring at position 7.60 in the recently identified helix 8 domain. This provided a basis for seeking revertant mutations to correct the defective function of the Y7.53F receptor. When the Y7.53F receptor was mutated at position 7.60, the wild-type phenotype was restored. These results suggest that Y7.53 and Y7.60 contribute to a common functional microdomain connecting helices 7 and 8 that influences the switching of the 5HT2C receptor among multiple active and inactive conformations.
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Affiliation(s)
- Cassandra Prioleau
- Department of Neurology, Mount Sinai School of Medicine, New York, New York 10029, USA
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