Structure and function of proteins in G-protein-coupled signal transfer

Assays for activation of opsin by all-trans-retinal

The data collected with the techniques discussed in this chapter suggest significant differences between the active conformation(s) of the opsin/atr complex, which are reversibly formed in the dark, and the active conformation (R*) of the meta-II photoproduct. First, there is good evidence for noncovalent opsin/atr complexes with considerable activity (although covalent binding of atr is found in mutant opsins. Even more intriguing, all-trans-retinal in an amount that saturates the activity of the opsin/atr complex toward Gt does not measurably inhibit the access of 11-cis-retinal to the light-sensitive binding site during regeneration (Fig. 2C). On the other hand, forced protonation at or near Glu-134 appears to be an integral mechanism for both the meta-II and the opsin-like activities (Fig. 4). Thus, it is not inconceivable that these two activities of the receptor arise from two fundamentally different conformations, one meta-II-like and one opsin-like. They would be similar with respect to the Gt (or RK) protein-protein interaction but different in their mode of retinal-protein interaction.

Unfolding pathways of individual bacteriorhodopsins

Atomic force microscopy and single-molecule force spectroscopy were combined to image and manipulate purple membrane patches from Halobacterium salinarum. Individual bacteriorhodopsin molecules were first localized and then extracted from the membrane; the remaining vacancies were imaged again. Anchoring forces between 100 and 200 piconewtons for the different helices were found. Upon extraction, the helices were found to unfold. The force spectra revealed the individuality of the unfolding pathways. Helices G and F as well as helices E and D always unfolded pairwise, whereas helices B and C occasionally unfolded one after the other. Experiments with cleaved loops revealed the origin of the individuality: stabilization of helix B by neighboring helices.

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.

The role of nitric oxide synthase/nitric oxide in vascular smooth muscle control

Vascular compliance is dependent on endogenous and exogenous sources of nitric oxide (NO). In a discussion of therapeutics and NO derived via nitric oxide synthase (NOS) enzymes, it is necessary to examine the pathways of each drug to provide the clinical perfusionist with a greater understanding of the role of NOS/NO in vascular function. Endothelial-derived NO is a contributor in the vasoregulation of vascular smooth muscle. Therapeutics seek to mimic the vasodilatory effects of the endogenous NO. The therapeutics included in this review are nitroglycerin, nitroprusside, amyl nitrite, and inhalation of NO. L-Arginine supplementation provides additional substrate for the endogenous pathway that can augment NO production. NO is a small bioactive molecule involved in various biochemical pathways. Dysregulation of NO production can impair normal physiologic control of vascular compliance. Therefore, the purpose of this review is to provide the perfusionist with an understanding of the biochemical and pharmacological aspects of NOS/NO associated with vascular function.

The amino terminus of the fourth cytoplasmic loop of rhodopsin modulates rhodopsin-transducin interaction

Rhodopsin is a seven-transmembrane helix receptor that binds and catalytically activates the heterotrimeric G protein transducin (G(t)). This interaction involves the cytoplasmic surface of rhodopsin, which comprises four putative loops and the carboxyl-terminal tail. The fourth loop connects the carboxyl end of transmembrane helix 7 with Cys(322) and Cys(323), which are both modified by membrane-inserted palmitoyl groups. Published data on the roles of the fourth loop in the binding and activation of G(t) are contradictory. Here, we attempt to reconcile these conflicts and define a role for the fourth loop in rhodopsin-G(t) interactions. Fluorescence experiments demonstrated that a synthetic peptide corresponding to the fourth loop of rhodopsin inhibited the activation of G(t) by rhodopsin and interacted directly with the alpha subunit of G(t). A series of rhodopsin mutants was prepared in which portions of the fourth loop were replaced with analogous sequences from the beta(2)-adrenergic receptor or the m1 muscarinic receptor. Chimeric receptors in which residues 310-312 were replaced could not efficiently activate G(t). The defect in G(t) interaction in the fourth loop mutants was not affected by preventing palmitoylation of Cys(322) and Cys(323). We suggest that the amino terminus of the fourth loop interacts directly with G(t), particularly with Galpha(t), and with other regions of the intracellular surface of rhodopsin to support G(t) binding.

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.

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.

Activation of the rod G-protein Gt by the thrombin receptor (PAR1) expressed in Sf9 cells

Functional coupling of the human thrombin receptor PAR1 (protease-activated receptor 1) with the retinal rod G-protein transducin (Gt, a member of the Gi family) was studied in a reconstituted system of membranes from Sf9 cells expressing the thrombin receptor and purified Gt from bovine rod outer segments. TRAP6-agonist-activated PAR1 interacts productively with the distant G-protein. Agonist-dependent Gt activation was measured using a real-time fluorimetric GTP[S]-binding assay and membranes from Sf9 cells. To characterize nucleotide-exchange catalysis by PAR1, we analyzed dependence on nucleotides, temperature and pH. Activation was inhibited by low GDP concentrations (IC50 = 5.2 +/- 1.5 microM at 5 microM GTP[S]), indicating that receptor-Gt coupling, followed by instantaneous GDP release, is rate limiting under the conditions (25 degrees C). Arrhenius plots of the temperature dependence reflect an apparent Ea of 60 +/- 3.5 kJ.mol-1. Evaluation of the pH/rate profiles of Gt activation indicates that the activating conformation of the receptor is determined by protonation of a titratable group with an apparent pKa of 6.4. This supports the idea that the active state of agonist-bound PAR1 depends on forced protonation, indicating possible analogies to the scheme established for rhodopsin.

Kinetics versus equilibrium: the importance of GTP in GPCR activation

Agonist-bound G-protein-coupled receptors (GPCRs) facilitate GDP-GTP exchange on their cognate G proteins. The binding properties of GPCRs are adequately described by the ternary complex model. However, in this article a more realistic (steady-state) model, which is necessary to describe the catalytic effect of agonist-bound receptors on G-protein activation, will be discussed. This model predicts that agonist potency and efficacy might vary from tissue to tissue, depending on the G-protein concentration and can be extended to explain why an agonist's ability to increase the receptor's affinity for empty G proteins (in the absence of GTP) is related to the agonist's efficacy.

Micropatterned immobilization of a G protein-coupled receptor and direct detection of G protein activation

G protein-coupled receptors (GPCRs) constitute an abundant family of membrane receptors of high pharmacological interest. Cell-based assays are the predominant means of assessing GPCR activation, but are limited by their inherent complexity. Functional molecular assays that directly and specifically report G protein activation by receptors could offer substantial advantages. We present an approach to immobilize receptors stably and with defined orientation to substrates. By surface plasmon resonance (SPR), we were able to follow ligand binding, G protein activation, and receptor deactivation of a representative GPCR, bovine rhodopsin. Microcontact printing was used to produce micrometer-sized patterns with high contrast in receptor activity. These patterns can be used for local referencing to enhance the sensitivity of chip-based assays. The immobilized receptor was stable both for hours and during several activation cycles. A ligand dose-response curve with the photoactivatable agonist 11-cis-retinal showed a half-maximal signal at 120 nM. Our findings may be useful to develop novel assay formats for GPCRs based on receptor immobilization to solid supports, particularly to sensor surfaces.

Enhanced expression, native purification, and characterization of CCR5, a principal HIV-1 coreceptor

Seven-transmembrane segment, G protein-coupled receptors (GPCRs) play important roles in many biological processes in which pharmaceutical intervention may be useful. High level expression and native purification of GPCRs are important steps in the biochemical and structural characterization of these molecules. Here, we describe enhanced mammalian cell expression and purification of a codon-optimized variant of the chemokine receptor CCR5, a GPCR that plays a central role in the entry of the human immunodeficiency virus-1 (HIV-1) into immune cells. CCR5 could be solubilized in its native state as determined by its ability to be precipitated by 2D7, a conformation-dependent anti-CCR5 antibody, and by the HIV-1 gp120 envelope glycoprotein. The 2D7 antibody recognized immature and mature forms of CCR5 equally, whereas gp120 preferentially recognized the mature form, a result that underscores a role for posttranslational modification of CCR5 in its HIV-1 coreceptor function. The methods described herein contribute to the analysis of CCR5 and are likely to be applicable to many other GPCRs.

cGMP binding to noncatalytic sites on mammalian rod photoreceptor phosphodiesterase is regulated by binding of its gamma and delta subunits

The binding of cGMP to the noncatalytic sites on two isoforms of the phosphodiesterase (PDE) from mammalian rod outer segments has been characterized to evaluate their role in regulating PDE during phototransduction. Nonactivated, membrane-associated PDE (PDE-M, alpha beta gamma2) has one exchangeable site for cGMP binding; endogenous cGMP remains nonexchangeable at the second site. Non-activated, soluble PDE (PDE-S, alpha beta gamma2 delta) can release and bind cGMP at both noncatalytic sites; the delta subunit is likely responsible for this difference in cGMP exchange rates. Removal of the delta and/or gamma subunits yields a catalytic alphabeta dimer with identical catalytic and binding properties for both PDE-M and PDE-S as follows: high affinity cGMP binding is abolished at one site (KD >1 microM); cGMP binding affinity at the second site (KD approximately 60 nM) is reduced 3-4-fold compared with the nonactivated enzyme; the kinetics of cGMP exchange to activated PDE-M and PDE-S are accelerated to similar extents. The properties of nonactivated PDE can be restored upon addition of gamma subunit. Occupancy of the noncatalytic sites by cGMP may modulate the interaction of the gamma subunit with the alphabeta dimer and thereby regulate cytoplasmic cGMP concentration and the lifetime of activated PDE during visual transduction in photoreceptor cells.

Molecular determinants of the reversible membrane anchorage of the G-protein transducin

Transducin is a heterotrimer formed by a fatty acylated alpha-subunit and a farnesylated betagamma-subunit. The role of these two covalent modifications and of adjacent hydrophobic and charged amino acid residues in reversible anchoring at disk model membranes is investigated at different pH values, salt concentrations, and lipid packing densities using the monolayer expansion technique and CD spectroscopy. The heterotrimer only binds if the acetylated alpha-subunit is transformed into its surface-active form by divalent cations. In the presence of salts the alpha(GDP)-subunit, the betagamma-complex, and the heterotrimer bind to POPC monolayers at 30 mN/m, estimated to mimic the lateral packing density of disk membranes, with apparent binding constants of Kapp = (1.1 +/- 0.3) x 10(6) M-1 (reflecting the penetration of the fatty acyl chain together with approximately three adjacent hydrophobic amino acid residues), Kapp = (3.5 +/- 0.5) x 10(6) M-1 (reflecting the penetration of the farnesyl chain), and Kapp = (1.6 +/- 0.3) x 10(6) M-1 (reflecting a major contribution of the alpha(GDP)-subunit with only a minor contribution from the betagamma-complex). The apparent binding constant of the alpha(GTP)-subunit is distinctly smaller than that of the alpha(GDP)-subunit. Binding to negatively charged POPC/POPG (75/25 mole/mole) monolayers is reinforced by 2-3 cationic residues for the betagamma-complex. The alpha-subunit shows no electrostatic attraction and the heterotrimer shows even a slight electrostatic repulsion which becomes the dominating force in the absence of salts.

Structural basis of G protein-coupled receptor function

The vast majority of extracellular signaling molecules, like hormones and neurotransmitters, interact with a class of membranous receptors characterized by a uniform molecular architecture of seven transmembrane alpha-helices linked by extra- and intracelluar peptide loops. In a reversible manner, binding of diverse agonists to heptahelical receptors leads to activation of a limited repertoire of heterotrimeric guanine nucleotide-binding proteins (G proteins) forwarding the signal to intracellular effectors such as enzymes and ion channels. Proper functioning of a G protein-coupled receptor is based on a complex interplay of structural determinants which are ultimately responsible for receptor folding, trafficking and transmembrane signaling. Applying novel biochemical and molecular biological methods interesting insights into receptor structure/function relationships became available. These studies have a significant impact on our understanding of the molecular basis of human diseases and may eventually lead to novel therapeutic strategies.

An arginine residue is essential for stretching and binding of the substrate on UDP-glucose-4-epimerase from Escherichia coli. Use of a stacked and quenched uridine nucleotide fluorophore as probe

In the previous paper we demonstrated that uridine-5'-beta-1-(5-sulfonic acid) naphthylamidate (UDPAmNS) is a stacked and quenched fluorophore that shows severalfold enhancement of fluorescence in a stretched conformation. UDPAmNS was found to be a powerful competitive inhibitor (Ki = 0.2 mM) for UDP-glucose-4-epimerase from Escherichia coli. This active site-directed fluorophore assumed a stretched conformation on the enzyme surface, as was evidenced by full enhancement of fluorescence in saturating enzyme concentration. Complete displacement of the fluorophore by UDP suggested it to bind to the substrate binding site of the active site. Analysis of inactivation kinetics in presence of alpha,beta-diones such as phenylglyoxal, cyclohaxanedione, and 2,3-butadione suggested involvement of the essential arginine residue in the overall catalytic process. From spectral analysis, loss of activity could also be directly correlated with modification of only one arginine residue. Protection experiments with UDP showed the arginine residue to be located in the uridyl phosphate binding subsite. Unlike the native enzyme, the modified enzyme failed to show any enhancement of fluorescence with UDPAmNS clearly demonstrating the role of the essential arginine residue in stretching and binding of the substrate. The potential usefulness of such stacked and quenched nucleotide fluorophores has been discussed.

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

Photoactivation of the retinal photoreceptor rhodopsin proceeds through a cascade of intermediates, resulting in protein-protein interactions catalyzing the activation of the G-protein transducin (Gt). Using stabilization and photoregeneration of the receptor's signaling state and Gt activation assays, we provide evidence for a two-site sequential fit mechanism of Gt activation. We show that the C-terminal peptide from the Gt gamma-subunit, Gtgamma(50-71)farnesyl, can replace the holoprotein in stabilizing rhodopsin's active intermediate metarhodopsin II (MII). However, the peptide cannot replace the Gtbeta gamma complex in direct activation assays. Competition by Gtgamma(50-71)farnesyl with Gt for the active receptor suggests a pivotal role for Gtbeta gamma in signal transfer from MII to Gt. MII stabilization and competition is also found for the C-terminal peptide from the Gt alpha-subunit, Gtalpha(340-350), but the capacity of this peptide to interfere in MII-Gt interactions is paradoxically low compared with its activity to stabilize MII. Besides this disparity, the pH profiles of competition with Gt are characteristically different for the two peptides. We propose a two-site sequential fit model for signal transfer from the activated receptor, R*, to the G-protein. In the center of the model is specific recognition of conformationally distinct sites of R* by Gtalpha(340-350) and Gtgamma(50-71)farnesyl. One matching pair of domains on the proteins would, on binding, lead to a conformational change in the G-protein and/or receptor, with subsequent binding of the second pair of domains. This process could be the structural basis for GDP release and the formation of a stable empty site complex that is ready to receive the activating cofactor, GTP.

The specificity of interaction of archaeal transducers with their cognate sensory rhodopsins is determined by their transmembrane helices

Chimeras of the Halobacterium salinarum transducers HtrI and HtrII were constructed to study the structural determinants for their specific interaction with the phototaxis receptors sensory rhodopsins I and II (SRI and SRII), respectively. Interaction of receptors and transducers was assessed by two criteria: phototaxis responses by the cells and transducer-modulation of receptor photochemical reaction kinetics in membranes. Coexpression of HtrI with SRII or HtrII with SRI did not result in interaction by either criterion. Each receptor was coexpressed with chimeric transducers in which various domains of the two transducers were interchanged. The results show that the presence of the two transmembrane helices of HtrI in a chimera is necessary and sufficient for functional transducer complexation with SRI, i.e., for wild-type SRI photoreactions and attractant and 2-photon repellent phototaxis responses. Additionally, a previously demonstrated chaperone-like facilitation of SRI folding or stability by HtrI was shown to depend only on the two transmembrane helices of HtrI in chimeric transducers. Similarly, the two transmembrane helices of HtrII specify interaction with the repellent receptor SRII according to motility analysis and laser-flash spectroscopy. The results support a model in which the membrane domains of the receptor/transducer complexes, consisting of the seven helices of the receptor interacting with the four-helix bundle of the transducer dimer, produce SRI- and SRII-specific signals to the flagellar motor by means of interchangeable cytoplasmic domains.

Palmitoylation of rhodopsin with S-protein acyltransferase: enzyme catalyzed reaction versus autocatalytic acylation

Protein palmitoylation in vitro was studied using bovine rhodopsin as the substrate and a partially purified acylating enzymatic activity (PAT) from placental membranes. PAT incorporates fatty acid into rhodopsin with higher efficiency (10 times higher initial rate), as compared to autoacylation. The activity is sensitive to heat and trypsin, indicating a protein-mediated enzymatic process and requires the native conformation of rhodopsin. The presence of deacylated, free cysteine residues in dark-adapted rhodopsin increases palmitoylation via PAT. The sites for non-enzymatic and enzymatic palmitoylation could not be distinguished by peptide mapping. The reversible palmitoylation described here will provide a tool for the study of the role of palmitoylation in photoreceptor function.

Binding of transducin and transducin-derived peptides to rhodopsin studies by attenuated total reflection-Fourier transform infrared difference spectroscopy

Fourier transform infrared difference spectroscopy combined with the attenuated total reflection technique allows the monitoring of the association of transducin with bovine photoreceptor membranes in the dark. Illumination causes infrared absorption changes linked to formation of the light-activated rhodopsin-transducin complex. In addition to the spectral changes normally associated with meta II formation, prominent absorption increases occur at 1735 cm-1, 1640 cm-1, 1550 cm-1, and 1517 cm-1. The D2O sensitivity of the broad carbonyl stretching band around 1735 cm-1 indicates that a carboxylic acid group becomes protonated upon formation of the activated complex. Reconstitution of rhodopsin into phosphatidylcholine vesicles has little influence on the spectral properties of the rhodopsin-transducin complex, whereas pH affects the intensity of the carbonyl stretching band. AC-terminal peptide comprising amino acids 340-350 of the transducin alpha-subunit reproduces the frequencies and isotope sensitivities of several of the transducin-induced bands between 1500 and 1800 cm-1, whereas an N-terminal peptide (aa 8-23) does not. Therefore, the transducin-induced absorption changes can be ascribed mainly to an interaction between the transducin-alpha C-terminus and rhodopsin. The 1735 cm-1 vibration is also seen in the complex with C-terminal peptides devoid of free carboxylic acid groups, indicating that the corresponding carbonyl group is located on rhodopsin.

Expression of G-protein subtypes in cultured cerebral endothelial cells

This paper describes Western-blotting evidence for the presence of various guanine nucleotide binding proteins, G-proteins in cultured rat cerebral endothelial cells (CECs) and two immortalized cerebral endothelial cell lines, RBE4 and GP8. By using specific antibodies raised against known sequences of appropriate G-protein types that were previously characterized, we demonstrated the presence of Gsalpha, Gi2alpha, Gi3alpha, Gq/11alpha, Goalpha and Gbeta in cell lysates of primary cultures of CECs, and plasma membranes of RBE4 and GP8 cells. The appearance of Goalpha proteins in CECs might be of special importance, since they were not detected in peripheral endothelial cells in previous studies. Isoproterenol and bradykinin displayed significant, dose-dependent stimulation of [35S]GTPgammaS binding above basal values. This assay, reflecting the GDP-GTP exchange reaction on Galpha-subunits by receptor agonists, suggested that there were functional, G-protein coupled beta-adrenergic and bradykinin receptors in these systems. No significant stimulation of [35S]GTP7gammaS binding was noted with serotonin under our experimental conditions. Since stimulation of [35S]GTPgammaS binding by isoproterenol and bradykinin was additive, it was concluded that different Galpha proteins were activated by these two ligands. In analogy to other systems, activation of Gs is most likely by isoproterenol, while Gi and/or Gq/11 proteins might be activated by bradykinin receptors. The possible significance of the receptors and G-proteins detected is being discussed in the functioning of cerebral endothelium, and thus the blood-brain barrier.

The G-protein betagamma complex

The vast majority of signalling pathways in mammalian cells are mediated by heterotrimeric (alpha betagamma) G proteins. Reviewed here is regulation of signal transduction by the betagamma complex at different protein interfaces: subunit-subunit, receptor-G protein and G protein-effector. The role of diverse beta and gamma subunit types in achieving specificity in signalling and potentially unidentified functions for these subunits also are discussed.

TSH Receptor Mutations and Thyroid Disease

Mutations of the thyrotropin receptor (TSHr) can be loss of function or gain of function. Loss-of-function mutations can affect a variety of loci in the TSHr gene. Their most common manifestation is resistance to TSH; they may also be the cause of a subset of cases of congenital hypothyroidism. Gain-of-function mutations are of greater theoretical interest. Somatic mutations constitutively activating the TSHr are the major cause of benign toxic thyroid adenomas, and of some cases of multinodular goiters. They underlie hereditary toxic thyroid hyperplasia, and have been found in cases of sporadic congenital non-autoimmune hyperthyroidism. A role for TSHr polymorphisms in Graves' disease has not been documented.

Light-activated rhodopsin induces structural binding motif in G protein alpha subunit

A large superfamily of transmembrane receptors control cellular responses to diverse extracellular signals by catalyzing activation of specific types of heterotrimeric GTP-binding proteins. How these receptors recognize and promote nucleotide exchange on G protein alpha subunits to initiate signal amplification is unknown. The three-dimensional structure of the transducin (Gt) alpha subunit C-terminal undecapeptide Gtalpha(340-350) IKENLKDCGLF was determined by transferred nuclear Overhauser effect spectroscopy while it was bound to photoexcited rhodopsin. Light activation of rhodopsin causes a dramatic shift from a disordered conformation of Gtalpha(340-350) to a binding motif with a helical turn followed by an open reverse turn centered at Gly-348, a helix-terminating C capping motif of an alphaL type. Docking of the NMR structure to the GDP-bound x-ray structure of Gt reveals that photoexcited rhodopsin promotes the formation of a continuous helix over residues 325-346 terminated by the C-terminal helical cap with a unique cluster of crucial hydrophobic side chains. A molecular mechanism by which activated receptors can control G proteins through reversible conformational changes at the receptor-G protein interface is demonstrated.

Structural features of the noncatalytic cGMP binding sites of frog photoreceptor phosphodiesterase using cGMP analogs

The cGMP-specific phosphodiesterase (PDE) of retinal photoreceptors is a central regulatory enzyme in the visual transduction pathway of vertebrate vision. Although the mechanism of activation of PDE by transducin is well understood, the role of the noncatalytic cGMP binding sites located on the catalytic subunits of PDE remains obscure. We report here for the first time the molecular basis of the noncovalent interactions between cGMP and the high affinity, noncatalytic cGMP binding sites of frog photoreceptor PDE. None of the tested cGMP analogs were able to bind with greater affinity than cGMP itself, and the noncatalytic sites were unable to bind cAMP. The major determinant for discrimination of cGMP over cAMP is in the N-1/C-6 region of the purine ring of cGMP where hydrogen bonding probably stabilizes the selective binding of cGMP. Substitutions at the C-2 position demonstrate that this region of the molecule plays a secondary but significant role in stabilizing cGMP binding to PDE through hydrogen bond interactions. The unaltered hydrogen at the C-8 position is also important for high affinity binding. A significant interaction between the binding pocket and the ribose ring of cGMP occurs at the 2'-hydroxyl position. Steric constraints were greatest in the C-8 and possibly the C-6/N-1 regions, whereas the C-2/N-3 and C-2' regions tolerated bulky substituents better. Several lines of evidence indicate that the noncatalytic site binds cGMP in the anti-conformation. The numerous noncovalent interactions between cGMP and the noncatalytic binding pocket of the photoreceptor PDE described in this study account for both the high affinity for cGMP and the high level of discrimination of cGMP from other cyclic nucleotides at the noncatalytic site.

Modulation of GTPase activity of G proteins by fluid shear stress and phospholipid composition

Mechanical forces arising from strain, pressure, and fluid shear stress are sensed by cells through an unidentified mechanoreceptor(s) coupled to intracellular signaling pathways. In vascular endothelial cells, fluid shear stress is transduced via pathway(s) involving heterotrimeric guanine nucleotide-binding proteins (G proteins) by molecular mechanisms that are unknown. In the present study, we investigated the activation of purified G proteins reconstituted into phospholipid vesicles. Vesicles containing G proteins were loaded with [gamma-32P]GTP and subjected to physiological levels of fluid shear stress in a cone-and-plate viscometer. Steady-state GTP hydrolysis was measured as an index of G protein function. Shear stress (0-30 dynes/cm2) activated G proteins in dose-dependent manner (0.48-4.6 pmol/min per microg of protein). Liposomes containing lysophosphatidylcholine (30 mol %) or treated with benzyl alcohol (40 mM), conditions that increase bilayer fluidity, exhibited 3- to 5-fold enhancement of basal GTPase activity. Conversely, incorporation of cholesterol (24 mol %) into liposomes reduced the activation of G proteins by shear. These results demonstrate the ability of the phospholipid bilayer to mediate the shear stress-induced activation of membrane-bound G proteins in the absence of protein receptors and that bilayer physical properties modulate this response.

A model for the recovery kinetics of rod phototransduction, based on the enzymatic deactivation of rhodopsin

We propose a model for the recovery of the retinal rod photoresponse after a short stimulus. The approach describes the enzymatic deactivation of the photoactivated receptor, rhodopsin, by simple enzyme kinetics. An important feature of this description is that the R* deactivation obeys different time laws, depending on the numbers of R* formed per disc membrane and available enzyme molecules. If the enzyme works below substrate saturation, the rate of deactivation depends linearly on the number of R*, whereas for substrate saturation a hyperbolic relation--the well-known Michaelis-Menten equation--applies. This dichotomy is used to explain experimental finding that the relation between the saturation time of the photoresponse after short illumination and the flash strength has two sharply separated branches for low and high flash intensities (up to approximately 10% bleaching). By relating both branches to properties of the enzymatic rhodopsin deactivation, the new model transcends the classical notion of a constant characteristic lifetime of activated rhodopsin. With parameters that are plausible in the light of the available data and the additional information that the deactivating enzyme, rhodopsin kinase, and the signaling G-protein, transducin, compete for the active receptor, the slopes of the saturation function are correctly reproduced.

Photoreceptor phosphodiesterase: interaction of inhibitory gamma subunit and cyclic GMP with specific binding sites on catalytic subunits

The photoreceptor phosphodiesterase (PDE6) is the central effector enzyme in the phototransduction cascade of photoreceptor cells. It is the only known PDE isoform the activity of which is regulated by interaction with a heterotrimeric G protein. The rod PDE6 holoenzyme is a tetrameric protein consisting of two large catalytic alpha and beta subunits and two small gamma subunits, which serve as potent inhibitors of PDE6. In dark-adapted photoreceptors, the gamma subunits maintain PDE6 activity at a low level. When exposed to light the visual pigment rhodopsin activates the retinal G protein, transducin, leading to release of the inhibitory action of the gamma subunits. In addition to the active sites where cGMP is hydrolyzed, the alpha and beta catalytic subunits have high-affinity, noncatalytic cGMP binding sites. These noncatalytic sites do not directly regulate cGMP catalysis at the active site, but rather can modulate the affinity with which the gamma subunits bind to the catalytic subunits. This article describes a number of experimental approaches that have recently been developed for studying the interactions between catalytic and inhibitory subunits of PDE6, as well as the dynamics of cGMP binding to and dissociation from the PDE6 noncatalytic sites.

Flow-mediated regulation of G-protein expression in cocultured vascular smooth muscle and endothelial cells

G-proteins have been implicated in the transduction of a number of flow-induced responses. We determined whether flow can modulate vascular endothelial or smooth muscle cell G-protein signaling. By use of a perfused transcapillary coculture system that permits the chronic exposure of cultured endothelial cells (ECs) and smooth muscle cells (SMCs) to physiological shear stresses, cocultures were exposed to stepwise increases in flow up to (1) 2 mL/min (low flow: 0.5 dyne/cm2), or (2) 44 mL/min (high flow: 15 dyne/cm2) and maintained for 72 hours before SMCs and ECs were harvested separately. Using Western blot analysis, EC Gi alpha3 expression was significantly increased (41+/-2.9%) by high-flow conditions compared with low-flow. The changes in G-protein expression were associated with a significant increase in endothelial nitric oxide synthase (eNOS) activity, elevated prostacyclin levels in the perfusing media, increased pertussis toxin-catalyzed ADP ribosylation of Gi alpha substrates, and enhanced agonist-stimulated GTPase activity in cocultured ECs. In contrast, high flow induced a significant decrease in Gi alpha1-2 expression (57 5%) in SMCs cocultured with ECs, an effect that was endothelium dependent, inhibited by indomethacin, and correlated with a decrease in pertussis toxin-catalyzed ADP ribosylation of Gi alpha substrates, reduced agonist-stimulated GTPase activity, and enhanced basal and G-protein-stimulated adenylyl cyclase activity. These data demonstrate that flow mediates selective changes in EC and SMC G-protein expression concomitant with changes in G-protein functionality and cellular signaling capacity. Moreover, flow-induced changes in SMC G-protein signaling capacity are endothelium dependent and require a cyclooxygenase product. G-protein modulation may thus represent an important mechanism whereby hemodynamic forces regulate vessel wall function.

Structure and function in rhodopsin: rhodopsin mutants with a neutral amino acid at E134 have a partially activated conformation in the dark state

The Glu-134-Arg-135 residues in rhodopsin, located near the cytoplasmic end of the C helix, are involved in G protein binding, or activation, or both. Furthermore, the charge-neutralizing mutation Glu-134 to Gln-134 produces hyperactivity in the activated state and produces constitutive activity in opsin. The Glu/Asp-Arg charge pair is highly conserved in equivalent positions in other G protein-coupled receptors. To investigate the structural consequences of charge-neutralizing mutations at Glu-134 and Arg-135 in rhodopsin, single spin-labeled side chains were introduced at sites in the cytoplasmic domains of helices C (140), E (227), F (250), or G (316) to serve as "molecular sensors" of the local helix bundle conformation. In each of the spin-labeled rhodopsins, a Gln substitution was introduced at either Glu-134 or Arg-135, and the electron paramagnetic resonance spectrum of the spin label was used to monitor the structural response of the helix bundle. The results indicate that a Gln substitution at Glu-134 induces a photoactivated conformation around helices C and G even in the dark state, an observation of potential relevance to the hyperactivity and constitutive activity of the mutant. In contrast, little change is induced in helix F, which has been shown to undergo a dominant motion upon photoactivation. This result implies that the multiple helix motions accompanying photoactivation are not strongly coupled and can be induced to take place independently. Gln substitution at Arg-135 produces only minor structural changes in the dark- or light-activated conformation, suggesting that this residue is not a determinant of structure in the regions investigated, although it may be functionally important.

G protein mechanisms: insights from structural analysis

This review is concerned with the structures and mechanisms of a superfamily of regulatory GTP hydrolases (G proteins). G proteins include Ras and its close homologs, translation elongation factors, and heterotrimeric G proteins. These proteins share a common structural core, exemplified by that of p21ras (Ras), and significant sequence identity, suggesting a common evolutionary origin. Three-dimensional structures of members of the G protein superfamily are considered in light of other biochemical findings about the function of these proteins. Relationships among G protein structures are discussed, and factors contributing to their low intrinsic rate of GTP hydrolysis are considered. Comparison of GTP- and GDP-bound conformations of G proteins reveals how specific contacts between the gamma-phosphate of GTP and the switch II region stabilize potential effector-binding sites and how GTP hydrolysis results in collapse (or reordering) of these surfaces. A GTPase-activating protein probably binds to and stabilizes the conformation of its cognate G protein that recognizes the transition state for hydrolysis, and may insert a catalytic residue into the G protein active site. Inhibitors of nucleotide release, such as the beta gamma subunit of a heterotrimeric G protein, bind selectively to and stabilize the GDP-bound state. Release factors, such as the translation elongation factor, Ts, also recognize the switch regions and destabilize the Mg(2+)-binding site, thereby promoting GDP release. G protein-coupled receptors are expected to operate by a somewhat different mechanism, given that the GDP-bound form of many G protein alpha subunits does not contain bound Mg2+.