Sub-second turnover of transducin GTPase in bovine rod outer segments. A light scattering study

A fast, regenerative light scattering signal from bovine ROS, the PA-signal, reflects the light-induced, transient activation of transducin. Its rate of recovery depends on the number of photolysed rhodopsin molecules, indicating that rhodopsin deactivation and not GTPase activity is rate limiting in our in vitro system. When rhodopsin deactivation is accelerated (in the presence of NH2OH), PA-signal recovery is also accelerated. A GTPase turnover number of more than 2 s-1 (at 37 degrees C) can be derived from these experiments. This is more than one order of magnitude faster than the GTPase rates so far described in the literature and is rapid enough for a physiological shut-off mechanism. The fast GTPase is attributed to a highly intact disk stack, which never releases transducin into the free aqueous space.     

Modeling of the complex between transducin and photoactivated rhodopsin, a prototypical G-protein-coupled receptor

Obtaining a reliable 3D model for the complex formed by photoactivated rhodopsin (R*) and its G-protein, transducin (Gtalphabetagamma), would significantly benefit the entire field of structural biology of G-protein-coupled receptors (GPCRs). In this study, we have performed extensive configurational sampling for the isolated C-terminal fragment of the alpha-subunit of transducin, Gtalpha 340-350, within cavities of photoactivated rhodopsin formed by different energetically feasible conformations of the intracellular loops. Our results suggested a new 3D model of the rhodopsin-transducin complex that fully satisfied all available experimental data on site-directed mutagenesis of rhodopsin and Gtalphabetagamma as well as data from disulfide-linking experiments. Importantly, the experimental data were not used as a priori constraints in model building. We performed a thorough comparison of existing computational models of the rhodopsin-transducin complex with each other and with current experimental data. It was found that different models suggest interactions with different molecules in the rhodopsin oligomer, that providing valuable guidance in design of specific novel experimental studies of the R*-Gtalphabetagamma complex. Finally, we demonstrated that the isolated Gtalpha 340-350 fragment does not necessarily bind rhodopsin in the same binding mode as the same segment in intact Gtalpha.     

Allosteric behavior in transducin activation mediated by rhodopsin. Initial rate analysis of guanine nucleotide exchange

Photolyzed rhodopsin acts in a catalytic manner to mediate the exchange of GTP for GDP bound to transducin. We have analyzed the steady-state kinetics of this activation process in order to determine the molecular mechanism of interactions between rhodopsin, transducin, and guanine nucleotides. Initial velocities (Vo) of the exchange reaction catalyzed by rhodopsin were measured for various transducin concentrations at several fixed levels of the GTP analog, [35S]guanosine 5'-(3-O-thio)triphosphate (GTP gamma S). The initial rate data analysis rigorously demonstrates that rhodopsin mediates the activation of transducin by a double-displacement catalytic mechanism. The Michaelis-Menten curves determined as a function of [transducin] reveal remarkable allosteric behavior; analysis of this data yields a Hill coefficient of 2. Lineweaver-Burk plots of Vo-1 versus [transducin]-1 display curvilinearity indicative of positive cooperativity and a series of parallel lines are generated by plotting Vo-1 as a function of [transducin]-2. The plots of Vo-1 versus [GTP gamma S]-1 show no evidence of allosterism and are a parallel series. Furthermore, the allosteric behavior observed in the activation of transducin is also witnessed in the rhodopsin-catalyzed guanine nucleotide exchange of the G protein's purified alpha subunit in the absence of the beta X gamma subunit complex. The latter observation implies that the molecular basis for allosterism in the activation process resides in the interactions between the photoreceptor and transducin's alpha subunit.     

Effect of detergents and lipids on transducin photoactivation by rhodopsin.

Rhodopsin samples, isolated using four different extraction procedures, were used to investigate the photodependent activation of the GTPase activity of transducin. A complete inhibition of transducin light-dependent GTP hydrolytic activity was observed when rhodopsin purified in the presence of 1% digitonin, following rod outer segment (ROS) solubilization with 1% 3-[(3-cholamidopropyl) dimethylammonio]-1-propane-sulfonate (CHAPS), was used for its activation [0 pmol of inorganic phosphate (Pi) released/min/pmol of rhodopsin]. Rhodopsin, isolated in the presence of 1% digitonin following ROS solubilization with 1% digitonin, was capable of stimulating slightly transducin GTPase activity, with an initial rate of 1 pmol of GTP hydrolyzed/min/pmol of rhodopsin. However, rhodopsin purified in the presence of 0.2% n-dodecyl-beta-D-maltoside (DM), following ROS solubilization with either 1% CHAPS or 1% DM, stimulated the enzymatic activity of transducin in a light-dependent manner, with an initial rate of 5 pmol of Pi released/min/pmol of rhodopsin. Addition of 0.075% egg phosphatidylcholine (PC) to the four different solubilized rhodopsin samples significantly enhanced light-stimulated GTP hydrolysis by transducin, with initial rates increasing from 0 to 1, 1 to 2, and 5 to 30 pmol of Pi released/min/pmol of rhodopsin, respectively. Furthermore, DM-solubilized rhodopsin induced the hydrolysis of the maximum amount of GTP by transducin at 0.0075% PC, while digitonin-solubilized rhodopsin only stimulated the GTPase activity of transducin to a similar value, when the amount of the photoreceptor protein was increased 4-fold and 0.15% PC was added to the assay mixture. These results suggest that the effective photoactivation of transducin by rhodopsin requires phospholipids, which seem to be differentially eliminated with the detergent extraction procedure utilized during ROS membranes solubilization and photopigment isolation.     

Reconstitution of the vertebrate visual cascade using recombinant heterotrimeric transducin purified from Sf9 cells

For reconstitution studies with rhodopsin and cGMP phosphodiesterase (PDE), all three subunits of heterotrimeric transducin (T alpha beta gamma) were simultaneously expressed in Sf9 cells at high levels using a baculovirus expression system and purified to homogeneity. Light-activated rhodopsin catalyzed the loading of purified recombinant T alpha with GTP gamma S. In vitro reconstitution of rhodopsin, recombinant transducin, and PDE in detergent solution resulted in cGMP hydrolysis upon illumination, demonstrating that recombinant transducin was able to activate PDE. The rate of cGMP hydrolysis by PDE as a function of GTP gamma S-loaded recombinant transducin (T(*)) concentration gave a Hill coefficient of approximately 2, suggesting that the activation of PDE by T(*) was cooperatively regulated. Furthermore, the kinetic rate constants for the activation of PDE by T(*) suggested that only the complex of PDE with two T(*) molecules, PDE. T(2)(*), was significantly catalytically active under the conditions of the assay. We conclude that the model of essential coactivation best describes the activation of PDE by T(*) in a reconstituted vertebrate visual cascade using recombinant heterotrimeric transducin.     

Signaling states of rhodopsin. Retinal provides a scaffold for activating proton transfer switches

The G-protein-coupled receptor rhodopsin is activated by photoconversion of its covalently bound ligand 11-cis-retinal to the agonist all-trans-retinal. After light-induced isomerization and early photointermediates, the receptor reaches a G-protein-dependent equilibrium between active and inactive conformations distinguished by the protonation of key opsin residues. In this report, we study the role of the 9-methyl group of retinal, one of the crucial steric determinants of light activation. We find that when this group is removed, the protonation equilibrium is strongly shifted to the inactive conformation. The residually formed active species is very similar to the active form of normal rhodopsin, metarhodopsin II. It has a deprotonated Schiff base, binds to the retinal G-protein transducin, and is favored at acidic pH. Our data show that the normal proton transfer reactions are inhibited in 9-demethyl rhodopsin but are still mandatory for receptor activation. We propose that retinal and its 9-methyl group act as a scaffold for opsin to adjust key proton donor and acceptor side chains for the proton transfer reactions that stabilize the active conformation. The mechanism may also be applicable to related receptors and may thus explain the partial agonism of certain ligands.     

Effect of GDP on transducin interaction with cyclic nucleotide phosphodiesterase and rhodopsin from bovine retinal rods

In the presence of guanyl nucleotides and rhodopsin-containing retinal rod outer segment membranes, transducin stimulates the light-sensitive cyclic nucleotide phosphodiesterase 5.5-7 times. The activation constant (Ka) for GTP and Gpp(NH)p is 0.25 microM, that for GDP and GDP beta S is 14 and 110 microM, respectively. GDP purified from other nucleotide contaminations at concentrations up to 1 mM does not stimulate phosphodiesterase but binds to transducin and inhibits the Gpp(NH)p-dependent activation of phosphodiesterase. The mode of transducin interaction with bleached rhodopsin also depends on the nature of the bound guanyl nucleotide: in the presence of GDP rhodopsin-containing membranes bind 70-100% of transducin, whereas in the presence of Gpp(NH)p the membranes bind only 13% of the protein. The experimental results suggest that GDP and GTP convert transducin into two different functional states, i.e., the transducin X GTP complex binds to phosphodiesterase causing its stimulation, while the transducin X GDP complex is predominantly bound to rhodopsin.     

Functional reconstitution of the alpha 2-adrenergic receptor with guanine nucleotide regulatory proteins in phospholipid vesicles

We describe the successful reconstitution of functional interactions between an inhibitory adenylate cyclase-coupled receptor and various nucleotide-binding regulatory proteins in phospholipid vesicles. The receptor is the alpha 2-adrenergic receptor (alpha 2AR) which has been partially purified (approximately 500-5000-fold) from human platelet membranes. The nucleotide-binding regulatory proteins include purified preparations of human erythrocyte Ni and Ns, bovine retinal transducin and the recently discovered bovine brain No. Addition of the physiologic ligand, epinephrine, to vesicles containing the alpha 2AR and Ni results in stimulation of the GTPase activity in Ni. This stimulation of GTPase activity by epinephrine is prevented in the presence of the alpha-adrenergic antagonist, phentolamine, which indicates that a functional reconstitution of the alpha 2AR and Ni has been established. The maximum turnover number for the alpha 2AR-mediated epinephrine-stimulated GTPase activity in Ni is similar to the maximal turnover numbers obtained for the beta-adrenergic receptor-mediated isoproterenol-stimulated GTPase activity in Ns and the rhodopsin-mediated light-stimulated GTPase activity in transducin (0.5-1.5 mol of Pi released per min per mol of nucleotide regulatory protein). Functional similarities between the alpha 2AR and rhodopsin are observed in their interactions with the various nucleotide-binding regulatory proteins. Thus, both of these receptor proteins are capable of promoting the maximal activation of Ni and No while being much less effective in promoting the activation of Ns. However, there are differences between the alpha 2AR and rhodopsin in their interactions with transducin. Specifically, while rhodopsin will maximally activate transducin, the alpha 2AR is much less effective in promoting this activation (i.e. approximately 20% as effective as rhodopsin). Overall, these results suggest the following specificities of interaction: for rhodopsin, transducin approximately equal to Ni approximately equal to No much greater than Ns; while for alpha 2AR, Ni approximately equal to No greater than transducin greater than or equal to Ns.     

Chemical modification of bovine transducin: effect of fluorescein 5'-isothiocyanate labeling on activities of the transducin alpha subunit

Fluorescein 5'-isothiocyanate (FITC) was used to modify the lysine residues of bovine transducin (T), a GTP-binding protein involved in phototransduction of rod photoreceptor cells. The incorporation of FITC showed a stoichiometry of approximately 1 mol of FITC/mol of transducin. The labeling was specific for the T alpha subunit. There was no significant incorporation on the T beta gamma subunit. The modification had no effect on the transducin-rhodopsin interaction or on the binding of guanosine 5'-(beta, gamma-imidotriphosphate) [Gpp(NH)p] to transducin in the presence of photolyzed rhodopsin. The dissociation of the FITC-transducin-Gpp(NH)p complex from rhodopsin membrane remained unchanged. However, the intrinsic GTPase activity of T alpha and its ability to activate the cGMP phosphodiesterase were diminished by FITC modification. The rate of FITC labeling of the transducin-Gpp(NH)p complex was about 3-fold slower than that of transducin. Limited tryptic digestion and peptide mapping were used to localize the FITC labeling site. The majority of the FITC label was on the 23-kilodalton fragment, and a minor amount was on the 9-kilodalton fragment of the T alpha subunit. These results indicate that FITC labeling does not alter the activation of transducin by photolyzed rhodopsin but does affect the GTP hydrolytic activity as well as the GTP-induced conformational change of T alpha, which ultimately leads to the activation of cGMP phosphodiesterase.     

Rhodopsin/transducin interactions. I. Characterization of the binding of the transducin-beta gamma subunit complex to rhodopsin using fluorescence spectroscopy.

In this work we have used fluorescence spectroscopic approaches to examine the binding of the beta gamma T subunit complex of transducin to the photoreceptor, rhodopsin. To do this, we have covalently labeled the beta gamma T subunit complex with the environmentally sensitive fluorescent cysteine reagent 2-(4'-maleimidylanilino)naphthalene-6-sulfonic acid (MIANS). By using the MIANS moiety as a fluorescent reporter group, we were able to monitor directly the binding of the MIANS-beta gamma T complex to light-activated rhodopsin, which was reconstituted into phosphatidylcholine vesicles, through an enhancement (30-50%) in the MIANS fluorescence. Phosphatidylcholine vesicles, alone, elicited only minor changes in the MIANS-beta gamma T fluorescence (i.e. less than 10% enhancement), whereas the addition of rhodopsin in the absence of lipid vesicles and in minimal detergent fully mimicked the effects of reconstituted rhodopsin and caused a significant enhancement of the MIANS fluorescence. The interactions between the MIANS-beta gamma T complex and rhodopsin also resulted in a quenching of the rhodopsin tryptophan fluorescence (approximately 30%), which most likely reflected resonance energy transfer between the tryptophan residues and the MIANS moieties. The binding of the MIANS-beta gamma T species to the alpha T subunit was accompanied by an enhancement of the MIANS fluorescence (30-50%) and a slight blue shift of the emission maximum, as described previously (Phillips, W. J., and Cerione, R. A. (1991) J. Biol. Chem. 266, 11017-11024). However, the alpha T-induced enhancement of the MIANS-beta gamma T fluorescence was not additive with the enhancement elicited by rhodopsin. Conditions which resulted in the activation of the alpha T subunit reversed the alpha T-induced enhancement of the MIANS emission, whereas the rhodopsin-induced enhancement persisted, thereby suggesting that the rhodopsin-beta gamma T complex can remain intact throughout the G protein activation event. Studies with synthetic peptides representing different regions of the cytoplasmic domain of rhodopsin demonstrated that a portion of the putative carboxyl-terminal tail (amino acid residues 310-324) was capable of eliciting changes in the MIANS-beta gamma T fluorescence as well as inhibiting the MIANS-beta gamma T-induced quenching of the rhodopsin tryptophan fluorescence. These results suggest that this region of the rhodopsin molecule may constitute a portion of the binding domain for the beta gamma T complex.     

Reconstitution of the Vertebrate Visual Cascade Using Recombinant Heterotrimeric Transducin Purified from Sf9 Cells

For reconstitution studies with rhodopsin and cGMP phosphodiesterase (PDE), all three subunits of heterotrimeric transducin (Tαβγ) were simultaneously expressed in Sf9 cells at high levels using a baculovirus expression system and purified to homogeneity. Light-activated rhodopsin catalyzed the loading of purified recombinant Tα with GTPγS. In vitro reconstitution of rhodopsin, recombinant transducin, and PDE in detergent solution resulted in cGMP hydrolysis upon illumination, demonstrating that recombinant transducin was able to activate PDE. The rate of cGMP hydrolysis by PDE as a function of GTPγS-loaded recombinant transducin (T*) concentration gave a Hill coefficient of approximately 2, suggesting that the activation of PDE by T* was cooperatively regulated. Furthermore, the kinetic rate constants for the activation of PDE by T* suggested that only the complex of PDE with two T* molecules, PDE · T₂^*, was significantly catalytically active under the conditions of the assay. We conclude that the model of essential coactivation best describes the activation of PDE by T* in a reconstituted vertebrate visual cascade using recombinant heterotrimeric transducin.     

The dynamics of phosphodiesterase activation in rods and cones

Phototransduction starts with the activation of a rhodopsin (respectively, coneopsin) molecule, located in the outer segment of rod (respectively, cone) photoreceptors. The subsequent amplification pathway proceeds via the G-protein transducin to the activation of phosphodiesterase (PDE), a G-protein coupled effector enzyme. In this article, we study the dynamics of PDE activation by constructing a Markov model that is based on the underlying chemical reactions including multiple rhodopsin phosphorylations. We derive explicit equations for the mean and the variance of activated PDE. Our analysis reveals that a low rhodopsin lifetime variance is neither necessary nor sufficient to achieve reliable PDE activation. The numerical simulations show that during the rising phase the variability of PDE activation is much lower compared to the recovery phase, and this property depends crucially on the transducin activation rates. Furthermore, we find that the dynamics of the activation process greatly differs depending on whether rhodopsin or PDE deactivation limits the recovery of the photoresponse. Finally, our simulations for cones show that only very few PDEs are activated by an excited photopigment, which might explain why in S-cones no single photon response can be observed.     

Characterization of transducin from bovine retinal rod outer segments. Mechanism and effects of cholera toxin-catalyzed ADP-ribosylation

Transducin, a guanine nucleotide-binding protein consisting of two subunits (T alpha and T beta gamma), mediates the signal coupling between rhodopsin and a membrane-bound cyclic GMP phosphodiesterase in retinal rod outer segments. The T alpha subunit is an activator of the phosphodiesterase, and the function of the T beta gamma subunit is to physically link T alpha with photolyzed rhodopsin. In this study, the mechanism of cholera toxin-catalyzed ADP-ribosylation of T alpha has been examined in a reconstituted system consisting of purified transducin and stripped rod outer segment membranes. Limited proteolysis of the labeled T alpha with trypsin indicated that the inserted ADP-ribose is located exclusively on a single proteolytic fragment with an apparent molecular weight of 23,000. Maximal incorporation of ADP-ribose was achieved when guanosine 5'-(beta, gamma-imido)triphosphate (Gpp(NH)p) and T beta gamma were present at concentrations equal to that of T alpha and when rhodopsin was continuously irradiated with visible light in the 400-500 nm region. The stimulating effect of illumination was related to the direct interaction of the retinal chromophore with opsin. These findings strongly suggest that a transient protein complex consisting of T alpha X Gpp(NH)p, T beta gamma, and a photointermediate of rhodopsin is the required substrate for cholera toxin. Single turnover kinetic measurements demonstrated that the ADP-ribosylation of T alpha coincided with the appearance of a population of transducin molecules having a very slow rate of GTP hydrolysis. The hydrolysis rate of the bound GTP for this population was 1.1 X 10(-3)/s, which was 22-fold slower than the rate for the unmodified transducin.     

Structure and function in rhodopsin. Studies of the interaction between the rhodopsin cytoplasmic domain and transducin.

Structural requirements for the activation of transducin by rhodopsin have been studied by site-specific mutagenesis of bovine rhodopsin. A variety of single amino acid replacements and amino acid insertions and deletions of varying sizes were carried out in the two cytoplasmic loops CD (amino acids 134-151) and EF (amino acids 231-252). Except for deletion mutant delta 137-150, all the mutants bound 11-cis-retinal and displayed normal spectral characteristics. Deletion mutant delta 236-239 in loop EF caused a 50% reduction of transducin activation, whereas deletion mutant delta 244-249 and the larger deletions in loop EF abolished transducin activation. An 8-amino acid deletion in the cytoplasmic loop CD as well as a replacement of 13 amino acids with an unrelated sequence showed no transducin activation. Several single amino acid substitutions also caused significant reduction in transducin activation. The conserved charged pair Glu-134/Arg-135 in the cytoplasmic loop CD was required for transducin activation; its reversal or neutralization abolished transducin activation. Three amino acid replacements in loop EF (S240A, T243V, and K248L) resulted in significant reduction in transducin activation. We conclude that 1) both the cytoplasmic loops CD and EF are required for transducin activation, and 2) effective functional interaction between rhodopsin and transducin involves relatively large peptide sequences in the cytoplasmic loops.     

Regulation of retinal cGMP cascade by phosducin in bovine rod photoreceptor cells. Interaction of phosducin and transducin.

Photoexcitation of retinal rod photoreceptor cells involves the activation of cGMP enzyme cascade in which sequential activation of rhodopsin, transducin, and the cGMP phosphodiesterase in the rod outer segment constitutes the signal amplification mechanism. Phosducin, a 33-kDa phosphoprotein, has been shown to form a tight complex with the T beta gamma subunit of transducin. In this study, we examined the interaction of phosducin-T beta gamma and the possible regulatory role of phosducin on the cGMP cascade. Addition of phosducin to photolyzed rod outer segment (ROS) membrane reduced the GTP hydrolysis activity of transducin as well as the subsequent activation of the cGMP phosphodiesterase. Phosducin also inhibited the pertussis toxin-catalyzed ADP-ribosylation of transducin, indicating that the interaction between the T alpha and T beta gamma subunits of transducin was interrupted upon binding of phosducin. The inhibitory effects of phosducin were reversed by the addition of exogenous T beta gamma. These results suggest that phosducin is capable of regulating the amount of T beta gamma available to interact with T alpha to form the active transducin complex and thereby functions as a negative regulator of the cGMP cascade. The phosducin-induced alteration of the subunit organization of transducin was examined by chemical cross-linking method using para-phenyl dimaleimide as cross-linker. It was found that the cross-linking among T alpha and T beta gamma was blocked in the presence of phosducin. This result implies that T beta gamma may undergo a conformational change upon phosducin binding which leads to the release of T alpha. Since phosducin is a soluble protein, the interaction with transducin only occurs when transducin is dissociated from ROS disc membrane. Indeed, phosducin failed to dissociate membrane-bound transducin and did not inhibit the initial cycle of transducin activation as measured by the presteady state GTP hydrolysis. However, phosducin interacts effectively with transducin released into solution after the initial activation and blocks the re-binding of T alpha. T beta gamma to ROS membrane by forming a tight complex with T beta gamma. This interaction may play an important role in regulating the turnover of the cGMP cascade in photoreceptor cells.     

Calcium regulates the rate of rhodopsin disactivation and the primary amplification step in visual transduction

The kinetics of the light-induced activation of transducin as well as the subsequent disactivation process can be monitored by means of a specific light scattering transient PA. In this communication it is demonstrated that the rate of transducin disactivation is calcium dependent, increasing when the calcium concentration is decreased. As a consequence of the accelerated recovery in low calcium, the time to the peak of the transducin activation process is shortened and the gain of the primary amplification step, i.e. the number of transducin molecules activated per bleached rhodopsin, is reduced. Experiments using hydroxylamine as an artificial quencher of rhodopsin activity suggest that calcium acts upon rhodopsin kinase and not upon the rate of the GTPase. This would indicate that calcium may control visual adaptation not only by regulating guanine cyclase activity, but also by affecting the primary step in the transduction cascade, the rhodopsin-transducin coupling.     

Mechanism of inhibition of transducin guanosine triphosphatase activity by vanadate

The visual excitation system of the retinal rod outer segments and the hormone-sensitive adenylate cyclase complex are regulated through guanine nucleotide-binding proteins, transducin in the former and inhibitory and stimulatory regulatory components, Gi and Gs, in the latter. These proteins are functionally and structurally similar; all are heterotrimers composed of alpha, beta, and gamma subunits and exhibit guanosine triphosphatase activity stimulated by light-activated rhodopsin or the agonist-receptor complex. Adenylate cyclase can be stimulated by vanadate, which, like NaF, probably acts through Gs. Effects of vanadate on the function of a guanine nucleotide-binding protein were investigated in a reconstituted model system consisting of purified transducin subunits (T alpha, T beta gamma) and rhodopsin in phosphatidylcholine vesicles. Vanadate (decameric) inhibited [3H]GTP binding to T alpha and noncompetitively inhibited GTP hydrolysis in a concentration-dependent manner with maximal inhibition of approximately 90% at 3-5 mM. Vanadate also inhibited release of bound GDP but did not affect the rate of hydrolysis of bound GTP (single turnover rate), indicating that vanadate did not interfere with the intrinsic GTPase activity of T alpha. Binding of T alpha to rhodopsin and the ADP-ribosylation of T alpha by pertussis toxin, both of which are enhanced in the presence of T beta gamma, were inhibited by vanadate. These findings are consistent with the conclusion that vanadate can cause the dissociation of T alpha from T beta gamma, resulting in the inhibition of GDP-GTP exchange and thereby GTP hydrolysis. Adenylate cyclase activation could result from a similar effect of vanadate on Gs.     

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.     

Cyclic GMP and photoreceptor function.

A single photon can be detected by a rod photoreceptor cell. The absorption of light by rhodopsin triggers a cascade of reactions that amplifies the photon signal and results in ion channel closure with hyperpolarization of the rod photoreceptor cell. Light-induced conformational changes in rhodopsin facilitate the binding of a guanosine nucleotide-binding protein, transducin, which then undergoes a GTP-GDP exchange reaction and dissociation of the transducin complex. A subunit of transducin then activates a phosphodiesterase complex that hydrolyzes cyclic GMP. In darkness, cyclic GMP binds to cation channels of the photoreceptor plasma membrane, maintaining them in an open configuration. The light-induced reduction in cyclic GMP concentration dissociates the bound cyclic GMP, resulting in channel closure and hyperpolarization. Down-regulation of the cascade involves other proteins that block the interaction of transducin with rhodopsin and another protein that may interfere with transducin recycling. Cone photoreceptors possess a light-activated cascade that follows the rod format, but it is composed of proteins that are homologous to those of rod photoreceptors. Phototransduction in invertebrate photoreceptors uses rhodopsin to activate a cascade that uses phosphoinositides and calcium ion to regulate membrane polarization.     

Preparation of an activated rhodopsin/transducin complex using a constitutively active mutant of rhodopsin

The interaction of rhodopsin and transducin has been the focus of study for more than 30 years, but only recently have efforts to purify an activated complex in detergent solution materialized. These efforts have used native rhodopsin isolated from bovine retina and employed either sucrose density gradient centrifugation or size exclusion chromatography to purify the complex. While there is general agreement on most properties of the activated complex, subunit stoichiometry is not yet settled, with rhodopsin/transducin molar ratios of both 2/1 and 1/1 reported. In this report, we introduce methods for preparation of the complex that include use of recombinant rhodopsin, so as to take advantage of mutations that confer constitutive activity and enhanced thermal stability on the protein, and immunoaffinity chromatography for purification of the complex. We show that chromatography on ConA-Sepharose can substitute for the immunoaffinity column and that bicelles can be used instead of detergent solution. We demonstrate the following: that rhodopsin has a covalently bound all-trans-retinal chromophore and therefore corresponds to the active metarhodopin II state; that transducin has an empty nucleotide-binding pocket; that the isolated complex is active and dissociates upon addition of guanine nucleotide; and finally that the stoichiometry corresponds reproducibly to a 1/1 molar ratio of rhodopsin to transducin.