Signal transfer from rhodopsin to the G-protein: evidence for a two-site sequential fit mechanism

Sequence of Interactions in Receptor-G Protein Coupling

Guanine nucleotide exchange in heterotrimeric G proteins catalyzed by G protein-coupled receptors (GPCRs) is a key event in many physiological processes. The crystal structures of the GPCR rhodopsin and two G proteins as well as binding sites on both catalytically interacting proteins are known, but the temporal sequence of events leading to nucleotide exchange remains to be elucidated. We employed time-resolved near infrared light scattering to study the order in which the Galpha and Ggamma C-terminal binding sites on the holo-G protein interact with the active state of the GPCR rhodopsin (R*) in native membranes. We investigated these key binding sites within mass-tagged peptides and G proteins and found that their binding to R* is mutually exclusive. The interaction of the holo-G protein with R* requires at least one of the lipid modifications of the G protein (i.e. myristoylation of the Galpha N terminus and/or farnesylation of the Ggamma C terminus). A holo-G protein with a high affinity Galpha C terminus shows a specific change of the reaction rate in the GDP release and GTP uptake steps of catalysis. We interpret the data by a sequential fit model where (i) the initial encounter between R* and the G protein occurs with the Gbetagamma subunit, and (ii) the Galpha C-terminal tail then interacts with R* to release bound GDP, thereby decreasing the affinity of R* for the Gbetagamma subunit. The mechanism limits the time in which both C-terminal binding sites of the G protein interact simultaneously with R* to a short lived transitory state.

Rhodopsin photoproducts in 2D crystals

The published electron microscope and X-ray structures of rhodopsin have made available a detailed picture of the inactive dark state of rhodopsin. Yet, the photointermediates of rhodopsin that ultimately lead to the activated receptor species still await a similar analysis. Such an analysis first requires the generation and characterization of the photoproducts that can be obtained in crystals of rhodopsin. We therefore studied with Fourier-transform infrared (FTIR) difference spectroscopy the photoproducts in 2D crystals of bovine rhodopsin in a p22(1)2(1) crystal form. The spectra obtained by cryotrapping revealed that in this crystal form the still inactive early intermediates batho, lumi, and meta I are similar to those obtained from rhodopsin in native disk membranes, although the transition from lumi to meta I is shifted to a higher temperature. However, at room temperature, the formation of the active state, meta II, is blocked in the crystalline environment. Instead, an intermediate state is formed that bears some features of meta II but lacks the specific conformational changes required for activity. Despite being unable to activate its cognate G protein, transducin, to a significant extent, this intermediate state is capable of interacting with functional transducin-derived peptides to a limited extent. Therefore, while unable to support formation of rhodopsin's active state meta II, 2D p22(1)2(1) crystals proved to be very suitable for determining 3D structures of its still inactive precursors, batho, lumi, and meta I. In future studies, FTIR spectroscopy may serve as a sensitive assay to screen crystals grown under altered conditions for potential formation of the active state, meta II.

Truncation of the A1 adenosine receptor reveals distinct roles of the membrane-proximal carboxyl terminus in receptor folding and G protein coupling

The carboxyl terminus (C-tail) of G protein-coupled receptors is divergent in length and structure and may represent an individualized cytoplasmic domain. By progressively truncating the A1 adenosine receptor, a Gi/o-coupled receptor with short cytoplasmic stretches, we identify two inherent functions of the C-tail, namely a role in receptor export from the endoplasmic reticulum (ER) and a role in G protein coupling. Deletion of the last 22 and 26 amino acids (of 36) reduced and completely abolished surface expression of the receptor, respectively. The severely truncated receptors were retained in the ER and failed to bind ligands. If overexpressed, even a substantial portion of the full-length receptor was retained in the ER in a form that was not functional. These data indicate that folding is rate limiting in export from the ER and that the proximal segment of the carboxyl terminus provides a docking site for the machinery involved in folding and quality control. In addition, the proximal portion is also important in G protein coupling. This latter role was unmasked when the distal portion of the C-tail (the extreme 18 amino acids, including a palmitoylated cysteine) had been removed; the resulting receptor was functional and transferred the agonist-mediated signal more efficiently than the full-length receptor. Signaling was enhanced because the coupling affinity increased (by 3-fold), which translated into a higher agonist potency. Thus, the distal portion of the carboxyl terminus provides for an autoinhibitory restraint, presumably by folding back and preventing G protein access to the proximal part of the C-tail.

The C-terminal tail preceding the CAAX box of a yeast G protein gamma subunit is dispensable for receptor-mediated G protein activation in vivo

The gamma subunits of heterotrimeric G proteins are required for receptor-G protein coupling. The C-terminal domains of Ggamma subunits can contact receptors and influence the efficiency of receptor-G protein coupling in vitro. However, it is unknown whether receptor interaction with the C terminus of Ggamma is required for signaling in vivo. To address this question, we cloned Ggamma homologs with diverged C-terminal sequences from five species of budding yeast. Each Ggamma homolog functionally replaced the Ggamma subunit of the yeast Saccharomyces cerevisiae (STE18 gene product). Mutagenesis of S. cerevisiae Ste18 likewise indicated that specific C-terminal sequence motifs are not required for signaling. Strikingly, an internal in-frame deletion removing sequences preceding the C-terminal CAAX box of Ste18 did not impair signaling by either of its cognate G protein-coupled pheromone receptors. Therefore, receptor interaction with the C-terminal domain of yeast Ggamma is not required for receptor-mediated G protein activation in vivo. Because the mechanism of G protein activation by receptors is conserved from yeast to humans, mammalian receptors may not require interaction with the tail of Ggamma for G protein activation in vivo. However, receptor-Ggamma interaction may modulate the efficiency of receptor-G protein coupling or promote activation of Gbetagamma effectors that co-cluster with receptors.

Promiscuous coupling at receptor-Galpha fusion proteins. The receptor of one covalent complex interacts with the alpha-subunit of another

Fusion proteins between heptahelical receptors (GPCR) and G protein alpha-subunits show enhanced signaling efficiency in transfected cells. This is believed to be the result of molecular proximity, because the interaction between linked modules of one protein chain, if not constrained by structure, should be strongly favored compared with the same in which partners react as free species. To test this assumption we made a series of fusion proteins (type 1 and 4 opioid receptors with G(o) and beta(2) adrenergic and dopamine 1 receptors with G(sL)) and some mutated analogs carrying different tags and defective GPCR or Galpha subunits. Using cotransfection experiments with readout protocols able to distinguish activation at fused and non-fused alpha-subunits, we found that both the GPCR and the Galpha limb of one fusion protein can freely interact with non-fused proteins and the tethered partners of a neighboring fusion complex. Moreover, a bulky polyanionic inhibitor can suppress with identical potency receptor-Galpha interaction, either when occurring between latched domains of a fused system or separate elements of distinct molecules, indicating that the binding surfaces are equally accessible in both cases. These data demonstrate that there is no entropy drive from the linked condition of fusion proteins and suggest that their signaling may result from the GPCR of one complex interacting with the alpha-subunit of another. Moreover, the enhanced coupling efficiency commonly observed for fusion proteins is not due to the receptor tether, but to the transmembrane helix that anchors Galpha to the membrane.

Rhodopsin controls a conformational switch on the transducin gamma subunit

Rhodopsin, a prototypical G protein-coupled receptor, catalyzes the activation of a heterotrimeric G protein, transducin, to initiate a visual signaling cascade in photoreceptor cells. The betagamma subunit complex, especially the C-terminal domain of the transducin gamma subunit, Gtgamma(60-71)farnesyl, plays a pivotal role in allosteric regulation of nucleotide exchange on the transducin alpha subunit by light-activated rhodopsin. We report that this domain is unstructured in the presence of an inactive receptor but forms an amphipathic helix upon rhodopsin activation. A K65E/E66K charge reversal mutant of the gamma subunit has diminished interactions with the receptor and fails to adopt the helical conformation. The identification of this conformational switch provides a mechanism for active GPCR utilization of the betagamma complex in signal transfer to G proteins.

Signaling states of rhodopsin. Formation of the storage form, metarhodopsin III, from active metarhodopsin II

Vertebrate rhodopsin consists of the apoprotein opsin and the chromophore 11-cis-retinal covalently linked via a protonated Schiff base. Upon photoisomerization of the chromophore to all-trans-retinal, the retinylidene linkage hydrolyzes, and all-trans-retinal dissociates from opsin. The pigment is eventually restored by recombining with enzymatically produced 11-cis-retinal. All-trans-retinal release occurs in parallel with decay of the active form, metarhodopsin (Meta) II, in which the original Schiff base is intact but deprotonated. The intermediates formed during Meta II decay include Meta III, with the original Schiff base reprotonated, and Meta III-like pseudo-photoproducts. Using an intrinsic fluorescence assay, Fourier transform infrared spectroscopy, and UV-visible spectroscopy, we investigated Meta II decay in native rod disk membranes. Up to 40% of Meta III is formed without changes in the intrinsic Trp fluorescence and thus without all-trans-retinal release. NADPH, a cofactor for the reduction of all-trans-retinal to all-trans-retinol, does not accelerate Meta II decay nor does it change the amount of Meta III formed. However, Meta III can be photoconverted back to the Meta II signaling state. The data are described by two quasi-irreversible pathways, leading in parallel into Meta III or into release of all-trans-retinal. Therefore, Meta III could be a form of rhodopsin that is stored away, thus regulating photoreceptor regeneration.

G protein-coupled receptor rhodopsin: a prospectus

Rhodopsin is a retinal photoreceptor protein of bipartite structure consisting of the transmembrane protein opsin and a light-sensitive chromophore 11-cis-retinal, linked to opsin via a protonated Schiff base. Studies on rhodopsin have unveiled many structural and functional features that are common to a large and pharmacologically important group of proteins from the G protein-coupled receptor (GPCR) superfamily, of which rhodopsin is the best-studied member. In this work, we focus on structural features of rhodopsin as revealed by many biochemical and structural investigations. In particular, the high-resolution structure of bovine rhodopsin provides a template for understanding how GPCRs work. We describe the sensitivity and complexity of rhodopsin that lead to its important role in vision.

Structure and orientation of a G protein fragment in the receptor bound state from residual dipolar couplings

Residual dipolar couplings for a ligand that is in fast exchange between a free state and a state where it is bound to a macroscopically ordered membrane protein carry precise information on the structure and orientation of the bound ligand. The couplings originate in the bound state but can be detected on the free ligand using standard high resolution NMR. This approach is used to study an analog of the C-terminal undecapeptide of the alpha-subunit of the heterotrimeric G protein transducin when bound to photo-activated rhodopsin. Rhodopsin is the major constituent of disk-shaped membrane vesicles from rod outer segments of bovine retinas, which align spontaneously in the NMR magnet. Photo-activation of rhodopsin triggers transient binding of the peptide, resulting in measurable dipolar contributions to 1J(NH) and 1J(CH) splittings. These dipolar couplings report on the time-averaged orientation of bond vectors in the bound peptide relative to the magnetic field, i.e. relative to the membrane normal. Approximate distance restraints of the bound conformation were derived from transferred NOEs, as measured from the difference of NOESY spectra recorded prior to and after photo-activation. The N-terminal eight residues of the bound undecapeptide adopt a near-ideal alpha-helical conformation. The helix is terminated by an alpha(L) type C-cap, with Gly9 at the C' position in the center of the reverse turn. The angle between the helix axis and the membrane normal is 40 degrees (+/-4) degrees. Peptide protons that make close contact with the receptor are identified by analysis of the NOESY cross-relaxation pattern and include the hydrophobic C terminus of the peptide.

Conformational analysis of the Galpha(s) protein C-terminal region

The C-terminal domain of the heterotrimeric G protein a-subunits plays a key role in selective activation of G proteins by their cognate receptors. Several C-terminal fragments of Galpha(s) (from 11 to 21 residues) were recently synthesized. The ability of these peptides to stimulate agonist binding was found to be related to their size. Galpha(s)(380-394) is a 15-mer peptide of intermediate length among those synthesized and tested that displays a biological activity surprisingly weak compared with that of the corresponding 21-mer peptide, shown to be the most active. In the present investigation, Galpha(s)(380-394) was subjected to a conformational NMR analysis in a fluorinated isotropic environment. An NMR structure, calculated on the basis of the data derived from conventional 1D and 2D homonuclear experiments, shows that the C-terminal residues of Galpha(s)(380-394) are involved in a helical arrangement whose length is comparable to that of the most active 21 -mer peptide. A comparative structural refinement of the NMR structures of Galpha(s)(380-394) and Galpha(s)(374-394)C379A was performed using molecular dynamics calculations. The results give structural elements to interpret the role played by both the backbone conformation and the side chain arrangement in determining the activity of the G protein C-terminal fragments. The orientation of the side chains allows the peptides to assume contacts crucial for the G protein/receptor interaction. In the 15-mer peptide the lack as well as the disorder of some N-terminal residues could explain the low biological activity observed.

Rhodopsin: insights from recent structural studies

The recent report of the crystal structure of rhodopsin provides insights concerning structure-activity relationships in visual pigments and related G protein-coupled receptors (GPCRs). The seven transmembrane helices of rhodopsin are interrupted or kinked at multiple sites. An extensive network of interhelical interactions stabilizes the ground state of the receptor. The ligand-binding pocket of rhodopsin is remarkably compact, and several chromophore-protein interactions were not predicted from mutagenesis or spectroscopic studies. The helix movement model of receptor activation, which likely applies to all GPCRs of the rhodopsin family, is supported by several structural elements that suggest how light-induced conformational changes in the ligand-binding pocket are transmitted to the cytoplasmic surface. The cytoplasmic domain of the receptor includes a helical domain extending from the seventh transmembrane segment parallel to the bilayer surface. The cytoplasmic surface appears to be approximately large enough to bind to the transducin heterotrimer in a one-to-one complex. The structural basis for several unique biophysical properties of rhodopsin, including its extremely low dark noise level and high quantum efficiency, can now be addressed using a combination of structural biology and various spectroscopic methods. Future high-resolution structural studies of rhodopsin and other GPCRs will form the basis to elucidate the detailed molecular mechanism of GPCR-mediated signal transduction.

Structure of rhodopsin and the superfamily of seven-helical receptors: the same and not the same

The crystal structure of rhodopsin provides significant insights concerning structure/activity relationships in visual pigments and related G-protein-coupled receptors. The specific arrangement of seven-transmembrane helices is stabilized by a series of intermolecular interactions that appear to be conserved among Family A receptors. However, the potential for structural and functional diversity among members of the superfamily of seven-helical receptors presents a significant future challenge.

Activation and inhibition of G proteins by lipoamines

We have previously shown that alkyl-substituted amino acid derivatives directly activate G(i/o) proteins. N-Dodecyl-N(alpha),N(epsilon)-(bis-l-lysinyl)-l-lysine amide (FUB132) is a new representative of this class of compounds with increased efficacy. Here, we characterized the molecular mechanism of action of this class of compounds. FUB132 and its predecessor FUB86 were selective receptomimetics for G(i/o) because they stimulated the guanine nucleotide exchange reaction of purified G(i/o) as documented by an increased rate of GDP release, GTP gamma S binding, and GTP hydrolysis. In contrast to the receptomimetic peptide mastoparan, stimulation of G proteins by lipoamines required the presence of neither G beta gamma-dimers nor lipids. On the contrary, G beta gamma-dimers suppressed the stimulatory effect of FUB132. The stimulation of G(i/o) by lipoamines and by mastoparan was not additive. A peptide derived from the C terminus of G alpha(o3), but not a corresponding G alpha(q)-derived peptide, quenched the FUB132-induced activation of G alpha(o). In membranes prepared from human embryonic kidney 293 cells that stably expressed the G(i/o)-coupled human A(1)-adenosine receptor, lipoamines impeded high-affinity agonist binding. In contrast, antagonist binding was not affected. We conclude that alkyl-substituted amines target a site, most likely at the C terminus of G alpha(i/o)-subunits, that is also contacted by receptors. However, because G beta gamma-dimers blunt rather than enhance their efficacy, their mechanism of action differs fundamentally from that of a receptor. Thus, despite their receptomimetic effect in vitro, alkyl-substituted amines and related polyamines are poor direct G protein activators in vivo. In the presence of G beta gamma, they rather antagonize G protein-coupled receptor signaling.

[To see from light--biophysics of visual signal transduction]

To perform their functions within an organism, or to adapt to the environment as single cells, living cells react to signals detected by highly specialized receptor proteins. These include the G-protein coupled receptors (GPCRs), a receptor family, which comprises more than 1000 members, and is of outstanding significance in basic research and medical application. An archetype of a GPCR is the visual pigment rhodopsin, the photoreceptor of the retinal rod cell. Biophysical methods have largely contributed to the elucidation of rhodopsin structure and function, as well as of the corresponding signal cascade. This article discusses some of the more recent developments.

Peptide interactions with G-protein coupled receptors

Peptide recognition by G-protein coupled receptors (GPCRs) is reviewed with an emphasis on the indirect approach used to determine the receptor-bound conformation of peptide ligands. This approach was developed in response to the lack of detailed structural information available for these receptors. Recent advances in the structural determination of rhodopsin (the GPCR of the visual system) by crystallography have provided a scaffold for homology modeling of the inactive state of a wide variety of GPCRs that interact with peptide messages. Additionally, the ability to mutate GPCRs and assay compounds of similar chemical structure to test a common binding site on the receptor provides a firm experimental basis for structure-activity studies. Recognition motifs, common in other well-studied systems such as proteolytic enzymes and major histocompatibility class receptors (MHC) are reviewed briefly to provide a basis of comparison. Finally, the development of true peptidomimetics is contrasted with nonpeptide ligands, discovered through combinatorial chemistry. In many systems, the evidence suggests that the peptide ligands bind at the interface between the transmembrane segments and the extracellular loops, while nonpeptide antagonists bind within the transmembrane segments. Plausible models of GPCRs and the mechanism by which they activate G-proteins on binding peptides are beginning to emerge.

Adenosine receptors: G protein-mediated signalling and the role of accessory proteins

Ever since the discovery of the effects of adenosine in the circulation, adenosine receptors continue to represent a promising drug target. Firstly, this is due to the fact that the receptors are expressed in a large variety of cells; in particular, the actions of adenosine (or, respectively, of the antagonistic methylxanthines) in the central nervous system, in the circulation, on immune cells and on other tissues can be beneficial in certain disorders. Secondly, there exists a large number of ligands, which have been generated by introducing several modifications in the structure of the lead compounds (adenosine and methylxanthine), some of them highly specific. Four adenosine receptor subtypes have been identified by molecular cloning; they belong to the family of G protein-coupled receptors, which transfer signals by activating heterotrimeric G proteins. It has been appreciated recently that accessory proteins impinge on the receptor/G protein interaction and thus modulate the signalling reaction. These accessory components may be thought as adaptors that redirect the signalling pathway to elicit a cell-specific response. Here, we review the recent literature on adenosine receptors and place a focus on the role of accessory proteins in the organisation of adenosine receptor signalling. These components have been involved in receptor sorting, in the control of signal amplification and in the temporal regulation of receptor activity, while the existence of others is postulated on the basis of atypical cellular reactions elicited by receptor activation.

Dimerization of G-protein-coupled receptors

The evolutionary trace (ET) method, a data mining approach for determining significant levels of amino acid conservation, has been applied to over 700 aligned G-protein-coupled receptor (GPCR) sequences. The method predicted the occurrence of functionally important clusters of residues on the external faces of helices 5 and 6 for each family or subfamily of receptors; similar clusters were observed on helices 2 and 3. The probability that these clusters are not random was determined using Monte Carlo techniques. The cluster on helices 5 and 6 is consistent with both 5,6-contact and 5,6-domain swapped dimer formation; the possible equivalence of these two types of dimer is discussed because this relates to activation by homo- and heterodimers. The observation of a functionally important cluster of residues on helices 2 and 3 is novel, and some possible interpretations are given, including heterodimerization and oligomerization. The application of the evolutionary trace method to 113 aligned G-protein sequences resulted in the identification of two functional sites. One large, well-defined site is clearly identified with adenyl cyclase, beta/gamma and regulator of G-protein signaling (RGS) binding. The other G-protein functional site, which extends from the ras-like domain onto the helical domain, has the correct size and electrostatic properties for GPCR dimer binding. The implications of these results are discussed in terms of the conformational changes required in the G-protein for activation by a receptor dimer. Further, the implications of GPCR dimerization for medicinal chemistry are discussed in the context of these ET results.

Rhodopsin-transducin interface: studies with conformationally constrained peptides

To probe the interaction between transducin (G(t)) and photoactivated rhodopsin (R*), 14 analog peptides were designed and synthesized restricting the backbone of the R*-bound structure of the C-terminal 11 residues of G(t)alpha derived by transferred nuclear Overhauser effect (TrNOE) NMR. Most of the analogs were able to bind R*, supporting the TrNOE structure. Improved affinities of constrained peptides indicated that preorganization of the bound conformation is beneficial. Cys347 was found to be a recognition site; particularly, the free sulfhydryl of the side chain seems to be critical for R* binding. Leu349 was another invariable residue. Both Ile and tert-leucine (Tle) mutations for Leu349 significantly reduced the activity, indicating that the Leu side chain is in intimate contact with R*. The structure of R* was computer generated by moving helix 6 from its position in the crystal structure of ground-state rhodopsin (R) based on various experimental data. Seven feasible complexes were found when docking the TrNOE structure with R* and none with R. The analog peptides were modeled into the complexes, and their binding affinities were calculated. The predicted affinities were compared with the measured affinities to evaluate the modeled structures. Three models of the R*/G(t)alpha complex showed strong correlation to the experimental data.

Chemical modification of transducin with iodoacetic acid: transducin-alpha carboxymethylated at Cys(347) allows transducin binding to Light-activated rhodopsin but prevents its release in the presence of GTP

Modification of transducin (T) with iodoacetic acid (IAA) inhibited its light-dependent guanine nucleotide-binding activity. Approximately 1 mol of [(3)H]IAA was incorporated per mole of T. Cys(347), located on the alpha-subunit of T (T(alpha)), was identified as the major labeled residue in the [(3)H]IAA-modified holoenzyme. In contrast, Cys(135) and Cys(347) were modified with [(3)H]IAA in the isolated T(alpha). IAA-modified T was able to bind tightly to photoexcited rhodopsin (R*), but GTP did not promote the dissociation of the complex between alkylated T and R*. In addition, R* protected against the inhibition of T by IAA. A comparable inactivation of T and analogous interactions between T and R* were observed when 2-nitro 5-thiocyanobenzoic acid (NTCBA) was used as the modifying reagent (J. O. Ortiz and J. Bubis, 2001, Effects of differential sulfhydryl group-specific labeling on the rhodopsin and guanine nucleotide binding activities of transducin, Arch. Biochem. Biophys. 387, 233-242). However, while carboxymethylated T was capable of liberating GDP in the presence of R*, NTCBA-modified T was unable to release the guanine nucleotide diphosphate upon incubation with the photoactivated receptor. Thus, IAA-labeling stabilized a T:R* complex intermediate carrying the empty nucleotide pocket conformation of T. On the other hand, NTCBA-modified T seemed to be "locked" in the GDP-bound state of T, even in the presence of R*.

Rhodopsin: structural basis of molecular physiology

The crystal structure of rod cell visual pigment rhodopsin was recently solved at 2.8-A resolution. A critical evaluation of a decade of structure-function studies is now possible. It is also possible to begin to explain the structural basis for several unique physiological properties of the vertebrate visual system, including extremely low dark noise levels as well as high gain and color detection. The ligand-binding pocket of rhodopsin is remarkably compact, and several apparent chromophore-protein interactions were not predicted from extensive mutagenesis or spectroscopic studies. The transmembrane helices are interrupted or kinked at multiple sites. An extensive network of interhelical interactions stabilizes the ground state of the receptor. The helix movement model of receptor activation, which might apply to all G protein-coupled receptors (GPCRs) of the rhodopsin family, is supported by several structural elements that suggest how light-induced conformational changes in the ligand-binding pocket are transmitted to the cytoplasmic surface. The cytoplasmic domain of the receptor is remarkable for a carboxy-terminal helical domain extending from the seventh transmembrane segment parallel to the bilayer surface. Thus the cytoplasmic surface appears to be approximately the right size to bind to the transducin heterotrimer in a one-to-one complex. Future high-resolution structural studies of rhodopsin and other GPCRs will form a basis to elucidate the detailed molecular mechanism of GPCR-mediated signal transduction.

Independent and synergistic interaction of retinal G-protein subunits with bovine rhodopsin measured by surface plasmon resonance

We have used surface plasmon resonance (SPR) measurements for the kinetic analysis of G-protein-receptor interaction monitored in real time. Functionally active rhodopsin was immobilized on an SPR surface, with full retention of biochemical specific activity for catalysis of nucleotide exchange on the retinal G-protein alpha subunit, via binding to immobilized concanavalin A. The binding interactions of bovine retinal alpha(t) and beta(1)gamma(1) subunits with rhodopsin measured by SPR were profoundly synergistic. Synergistic binding of the retinal G-protein subunits to rhodopsin was not observed for guanosine 5'-[gamma-thio]triphosphate-bound Galpha(t), nor was binding observed with squid retinal Galpha(q), which is not activated by bovine rhodopsin. The binding affinity (336+/-171 nM; mean value+/-S.D.) of retinal betagamma for rhodopsin in the presence of retinal alpha subunit measured by SPR confirmed the apparent affinity of 254 nM determined previously by nucleotide exchange assays. Binding of beta(1)gamma(1), beta(1)gamma(2), and beta(1)gamma(8-olf) dimers to rhodopsin, independently of the alpha subunit, was readily observable by SPR. Further, these dimers, differing only in their gamma subunit compositions, displayed markedly distinct binding affinities and kinetics. The beta(1)gamma(2) dimer bound with a kinetically determined K(d) of 13+/-3 nM, a value nearly identical with the biochemically determined K(1/2) of 10 nM. The physiologically appropriate beta(1)gamma(1) displayed rapid association and dissociation kinetics, whereas the other beta(1)gamma dimers dissociated at a rate less than 1/100 as fast. Thus rhodopsin interaction with its native signalling partners is both rapid and transient, whereas the interaction of rhodopsin with heterologous Gbetagamma dimers is markedly prolonged. These results suggest that the duration of a G-protein-coupled receptor signalling event is an intrinsic property of the G-protein coupling partners; in particular, the betagamma dimer.

Mutant G protein alpha subunit activated by Gbeta gamma: a model for receptor activation?

How receptors catalyze exchange of GTP for GDP bound to the Galpha subunit of trimeric G proteins is not known. One proposal is that the receptor uses the G protein's betagamma heterodimer as a lever, tilting it to pull open the guanine nucleotide binding pocket of Galpha. To test this possibility, we designed a mutant Galpha that would bind to betagamma in the tilted conformation. To do so, we excised a helical turn (four residues) from the N-terminal region of alpha(s), the alpha subunit of G(S), the stimulatory regulator of adenylyl cyclase. In the presence, but not in the absence, of transiently expressed beta(1) and gamma(2), this mutant (alpha(s)Delta), markedly stimulated cAMP accumulation. This effect depended on the ability of the coexpressed beta protein to interact normally with the lip of the nucleotide binding pocket of alpha(s)Delta. We substituted alanine for an aspartate in beta(1) that binds to a lysine (K206) in the lip of the alpha subunit's nucleotide binding pocket. Coexpressed with alpha(s)Delta and gamma(2), this mutant, beta(1)-D228A, elevated cAMP much less than did beta(1)-wild type; it did bind to alpha(s)Delta normally, however, as indicated by its unimpaired ability to target alpha(s)Delta to the plasma membrane. We conclude that betagamma can activate alpha(s) and that this effect probably involves both a tilt of betagamma relative to alpha(s) and interaction of beta with the lip of the nucleotide binding pocket. We speculate that receptors use a similar mechanism to activate trimeric G proteins.

Activation of rhodopsin: new insights from structural and biochemical studies

G-protein-coupled receptors (GPCRs) are involved in a vast variety of cellular signal transduction processes from visual, taste and odor perceptions to sensing the levels of many hormones and neurotransmitters. As a result of agonist-induced conformation changes, GPCRs become activated and catalyze nucleotide exchange within the G proteins, thus detecting and amplifying the signal. GPCRs share a common heptahelical transmembrane structure as well as many conserved key residues and regions. Rhodopsins are prototypical GPCRs that detect photons in retinal photoreceptor cells and trigger a phototransduction cascade that culminates in neuronal signaling. Biophysical and biochemical studies of rhodopsin activation, and the recent crystal structure determination of bovine rhodopsin, have provided new information that enables a more complete mechanism of vertebrate rhodopsin activation to be proposed. In many aspects, rhodopsin might provide a structural and functional template for other members of the GPCR family.

Maximal rate and nucleotide dependence of rhodopsin-catalyzed transducin activation: initial rate analysis based on a double displacement mechanism

Despite the growing structural information on receptors and G proteins, the information on affinities and kinetics of protein-protein and protein-nucleotide interactions is still not complete. In this study on photoactivated rhodopsin (R*) and the rod G protein, G(t), we have used kinetic light scattering, backed by direct biochemical assays, to follow G protein activation. Our protocol includes the following: (i) to measure initial rates on the background of rapid depletion of the G(t)GDP substrate; (ii) to titrate G(t)GDP, GTP, and GDP; and (iii) to apply a double displacement reaction scheme to describe the results. All data are simultaneously fitted by one and the same set of parameters. We obtain values of K(m) = 2200 G(t)/microm(2) for G(t)GDP and K(m) = 230 microm for GTP; dissociation constants are K(d) = 530 G(t)/microm(2) for R*-G(t)GDP dissociation and K(d) = 270 microm for GDP release from R*G(t)GDP, once formed. Maximal catalytic rates per photoexcited rhodopsin are 600 G(t)/s at 22 degrees C and 1300 G(t)/s at 34 degrees C. The analysis provides a tool to allocate and quantify better the effects of chemical or mutational protein modifications to individual steps in signal transduction.

Light-induced reorganization of phospholipids in rod disc membranes

The transbilayer redistribution of spin-labeled phospholipid analogues (SL-PL) with choline, serine, and ethanolamine head groups (PC, PS, and PE, respectively) was studied on intact disc vesicles of bovine rod outer segment membranes in the dark and after illumination. Redistribution was measured by the extraction of spin-labeled lipid analogues from the outer leaflet of membrane using the bovine serum albumin back-exchange assay. In the dark, PS was distributed asymmetrically, favoring the outer leaflet, whereas PC and PE showed small if any asymmetry. Green illumination for 1 min caused lipid head group-specific reorganization of SL-PL. Extraction of SL-PS by bovine serum albumin showed a fast transient (<10 min) enhancement, which was further augmented by a peptide stabilizing the active metarhodopsin II conformation. The data suggest a direct release of 1 molecule of bound PS per rhodopsin into the outer leaflet and subsequent redistribution between the two leaflets. SL-PE and SL-PC showed more complex kinetics, in both cases consistent with a prolonged period of reduced extraction (2 phospholipids per rhodopsin in each case). The different phases of SL-PL reorganization after illumination may be related to the formation and decay of the active rhodopsin species and to their subsequent regeneration process.

Interactions of metarhodopsin II. Arrestin peptides compete with arrestin and transducin

Arrestin blocks the interaction of rhodopsin with the G protein transducin (G(t)). To characterize the sites of arrestin that interact with rhodopsin, we have utilized a spectrophotometric peptide competition assay. It is based on the stabilization of the active intermediates metarhodopsin II (MII) and phosphorylated MII by G(t) and arrestin, respectively (extra MII monitor). The protocol involves native disc membranes and three sets of peptides 10-30 amino acids in length spanning the arrestin sequence. In the absence of arrestin, not one of the peptides by itself had an effect on the amount of MII formed. However, inhibition of arrestin-dependent extra MII was found for the peptides at residues 11-30 and 51-70 (IC(50) < 100 microm) and residues 231-260 (IC(50) < 200 microm). A similar pattern of inhibition by arrestin peptides was seen when arrestin was replaced by G(t) or the farnesylated G(t)gamma C-terminal peptide. Only arrestin-(11-30) inhibited MII.G(t) less (IC(50) = 300 microm) than phosphorylated MII.arrestin. We interpreted the data by competition of the arrestin peptides for interaction sites at rhodopsin, exposed in the MII conformation and specific for both arrestin and G(t). The arrestin sites are located in both the C- and N-terminal domains of the arrestin structure.

FTIR spectroscopy of complexes formed between metarhodopsin II and C-terminal peptides from the G-protein alpha- and gamma-subunits

Metarhodopsin II (MII) provides the active conformation of rhodopsin for interaction with the G-protein, Gt. Fourier transform infrared spectra from samples prepared by centrifugation reflect the pH dependent equilibrium between MII and inactive metarhodopsin I. C-terminal synthetic peptides (Gtalpha(340-350) and Gtgamma(60-71)farnesyl) stabilize MII. We find that both peptides cause similar spectral changes not seen with control peptides (Gtalpha (K341R, L349A) and non-farnesylated Gtgamma). The spectra reflect all the protonation dependent bands normally observed when MII is formed at acidic pH. Beside the protonation dependent bands, additional features, similar with both peptides, appear in the amide I and II regions.

Diffusible ligand all-trans-retinal activates opsin via a palmitoylation-dependent mechanism

In rhodopsin's function as a photoreceptor, 11-cis-retinal is covalently bound to Lys(296) via a protonated Schiff base. 11-cis/all-trans photoisomerization and relaxation through intermediates lead to the metarhodopsin II photoproduct, which couples to transducin (G(t)). Here we have analyzed a different signaling state that arises from noncovalent binding of all-trans-retinal (atr) to the aporeceptor opsin and enhances the very low opsin activity by several orders of magnitude. Like with metarhodopsin II, coupling of G(t) to opsin-atr is sensitive to competition by synthetic peptides from the COOH termini of both G(t)alpha and G(t)gamma. However, atr does not compete with 11-cis-retinal incorporation into the Lys(296) binding site and formation of the light-sensitive pigment. Blue light illumination fails to photorevert opsin-atr to the ground state. Thus noncovalently bound atr has no access to the light-dependent binding site and reaction pathway. Moreover, in contrast to light-dependent signaling, removal of the palmitoyl anchors at Cys(322) and Cys(323) in the rhodopsin COOH terminus impairs the atr-stimulated activity. Repalmitoylation by autoacylation with palmitoyl-coenzyme A restores most of the original activity. We hypothesize that the palmitoyl moieties are part of a second binding pocket for the chromophore, mediating hydrophobic interactions that can activate a large part of the catalytic receptor/G-protein interface.

The transbilayer distribution of phospholipids in disc membranes is a dynamic equilibrium evidence for rapid flip and flop movement

We studied the transbilayer redistribution of phospholipids in bovine rod outer segment membranes on thoroughly washed, Ficoll-floated osmotically intact disc vesicles; freshly prepared membranes separated from the disc stack by osmotic shock; and intact disc stacks with a permeabilized plasma membrane (A-discs, B-discs C-discs, respectively). In all cases, spin-labelled phospholipid analogues (SL-PL) with choline, serine and ethanolamine head groups (PtdCho, PtdSer and PtdEtn, respectively) were taken up into the outer leaflet of the membranes by > 90% and within less than 30 s after SL-PL addition, as deduced from the disappearance of spin-label from the suspension medium and from the specific ESR spectrum of membrane-associated spin-label. Using BSA extraction, the amount of SL-PL in the outer leaflet of the bilayer was determined. It decreased with a mean half-time of < 5 min at 25 degrees C, indicating rapid redistribution of all spin-labelled phospholipids into the inner leaflet of the disc membranes. After 1 h, PtdCho and PtdEtn were distributed almost symmetrically, whereas PtdSer was 35 : 65% (in/out). Using subsequent incubation with BSA, the outward movement (flop) of the analogues was observed directly, demonstrating that inward and outward movements proceed in thermodynamic equilibrium. No effect of N-ethylmaleimide or ATP on the redistribution could be measured, which makes it unlikely that energy-consuming translocase or flippase processes are involved in the redistribution in the dark. We reason that the solubilization zone around the photoreceptor rhodopsin may be the locus of rapid redistribution of the highly unsaturated disc phospholipid.

Mutation of the fourth cytoplasmic loop of rhodopsin affects binding of transducin and peptides derived from the carboxyl-terminal sequences of transducin alpha and gamma subunits

The role of the putative fourth cytoplasmic loop of rhodopsin in the binding and catalytic activation of the heterotrimeric G protein, transducin (G(t)), is not well defined. We developed a novel assay to measure the ability of G(t), or G(t)-derived peptides, to inhibit the photoregeneration of rhodopsin from its active metarhodopsin II state. We show that a peptide corresponding to residues 340-350 of the alpha subunit of G(t), or a cysteinyl-thioetherfarnesyl peptide corresponding to residues 50-71 of the gamma subunit of G(t), are able to interact with metarhodopsin II and inhibit its photoconversion to rhodopsin. Alteration of the amino acid sequence of either peptide, or removal of the farnesyl group from the gamma-derived peptide, prevents inhibition. Mutation of the amino-terminal region of the fourth cytoplasmic loop of rhodopsin affects interaction with G(t) (Marin, E. P., Krishna, A. G., Zvyaga T. A., Isele, J., Siebert, F., and Sakmar, T. P. (2000) J. Biol. Chem. 275, 1930-1936). Here, we provide evidence that this segment of rhodopsin interacts with the carboxyl-terminal peptide of the alpha subunit of G(t). We propose that the amino-terminal region of the fourth cytoplasmic loop of rhodopsin is part of the binding site for the carboxyl terminus of the alpha subunit of G(t) and plays a role in the regulation of betagamma subunit binding.

A novel Ggamma isolated from Drosophila constitutes a visual G protein gamma subunit of the fly compound eye

Visual transduction in the compound eye of flies is a well established model system for the study of G protein-coupled transduction pathways. To characterize key components of the phototransduction cascade we performed substractive hybridization screening. We cloned the cDNA coding for the visual Ggamma (Ggamma(e)) subunit from Drosophila which had so far eluded identification at the molecular level. Northern blot analysis revealed the presence of a major, 1.4-kilobase(kb) Ggamma(e) transcript and two minor transcripts of 1.8 and 6 kb in size. The major 1.4-kb mRNA is expressed preferentially in the eye. The spatial expression pattern determined for Ggamma(e) as well as co-immunoprecipitation experiments demonstrated that Ggamma(e) dimerizes with Gbeta(e) to form the heterodimeric Gbetagamma subunit which functions in visual transduction in the Drosophila compound eye. Ggamma(e) shares common characteristics with the visual Ggamma subunits of human rod and cone photoreceptors although different classes of Galpha subunits are employed in vertebrate and invertebrate phototransduction. By the molecular cloning and characterization of the visual gamma subunit of Drosophila one of the few missing links in the well studied Drosophila phototransduction cascade has been characterized to complete our knowledge about the Drosophila visual transduction pathway.

Signalling states of photoactivated rhodopsin

In microseconds after photoexcitation, rhodopsin forms the Meta I intermediate from lumirhodopsin. In this conversion, contacts between retinal and the apoprotein are formed, which result in a defined arrangement of donor and acceptor groups for proton translocations. A system of protonation-dependent coupled equilibria is now adopted, comprising Meta intermediates I, II and III, and their isospectral subforms. Some Meta states were identified as signalling states, in which the receptor interacts with transducin (Gt), rhodopsin kinase (RK) and arrestin. The binding of Gt or arrestin shifts the equilibrium to Meta II, while RK does not, indicating exposure of the RK binding site(s) before Meta II is formed. On contact with the activated receptor, each signalling protein responds with a conformational change, which transforms it into a functionally active state. The bell-shaped pH/rate profiles which are seen for the activation of both the G protein and the receptor kinase, indicate the necessary protonation and deprotonation of groups with different pKa. The right wing of the profile reflects the formation of the protonated subconformation (termed MIIb) of Meta II. For the interaction with Gt, recent work suggests a 'sequential fit' mechanism, involving the recognition of the C-terminal peptide of the Gt alpha subunit and of the farnesylated C-terminus of the gamma subunit. Isolated peptides derived from these portions of the G protein mimic the left wing of the pH/rate profile. We discuss the sequential fit as a time-ordered sequence of microscopic recognition and conformational interlocking in the interaction with the G protein.

A low resolution model for the interaction of G proteins with G protein-coupled receptors

A model is presented for the interaction between G proteins and G protein-coupled receptors. The model is based on the fact that this interaction shows little specificity and thus conserved parts of the G proteins have to interact with conserved parts of the receptors. These parts are a conserved negative residue in the G protein, a fully conserved arginine in the receptor and a series of residues that are not conserved but always hydrophobic like the hydrophobic side of the C-terminal helix of the G protein and the hydrophobic side of a helix in the C-terminal domain of the receptor. Other, mainly cytosolic, factors determine the specificity and regulation of this interaction. The relation between binding and activation will be shown. A large body of experimental evidence supports this model. Despite the fact that the model does not provide atomic resolution, it can be used to explain some experimental data that would otherwise seem inexplicable, and it suggests experiments for its falsification or verification.