Alpha helix content of G protein alpha subunit is decreased upon activation by receptor mimetics

Fluorescence Anisotropy Assay with Guanine Nucleotides Provides Access to Functional Analysis of Gαi1 Proteins

Gα proteins as part of heterotrimeric G proteins are molecular switches essential for G protein-coupled receptor- mediated intracellular signaling. The role of the Gα subunits has been examined for decades with various guanine nucleotides to elucidate the activation mechanism and Gα protein-dependent signal transduction. Several approaches describe fluorescent ligands mimicking the GTP function, yet lack the efficient estimation of the proteins' GTP binding activity and the fraction of active protein. Herein, we report the development of a reliable fluorescence anisotropy-based method to determine the affinity of ligands at the GTP-binding site and to quantify the fraction of active Gαi1 protein. An advanced bacterial expression protocol was applied to produce active human Gαi1 protein, whose GTP binding capability was determined with novel fluorescently labeled guanine nucleotides acting as high-affinity Gαi1 binders compared to the commonly used BODIPY FL GTPγS. This study thus contributes a new method for future investigations of the characterization of Gαi and other Gα protein subunits, exploring their corresponding signal transduction systems and potential for biomedical applications.

The Conformational Dynamics of Heterotrimeric G Proteins During GPCR-Mediated Activation

Heterotrimeric G proteins (G proteins) are essential cellular signaling proteins that mediate extracellular signals to achieve various cellular functions. G-protein-coupled receptors (GPCRs) are the major guanine nucleotide exchange factors (GEFs) that induce G proteins to release guanosine diphosphate and rapidly bind to guanosine triphosphate, resulting in G protein activation. G proteins undergo dynamic conformational changes during the activation/inactivation process, and the precise structural mechanism of GPCR-mediated G protein activation is of great interest. Over the last decade, a number of GPCR-G protein complex structures have been identified, yet an understanding of the mechanisms underlying allosteric conformational changes during receptor-mediated G protein activation and GPCR-G protein coupling selectivity is only now emerging. This review discusses recent studies on the dynamic conformational changes of G proteins and provides insight into the structural mechanism of GPCR-mediated G protein activation.

Effects of NO/cGMP inhibitors in a rat model of anaphylactoid shock

Anaphylactic shock can be defined as an acute syndrome, and it is the most severe clinical manifestation of allergic diseases. Anaphylactoid reactions are similar to anaphylactic events but differ in the pathophysiological mechanism. Nitric oxide (NO) inhibitors during anaphylaxis suggest that NO might decrease the signs and symptoms of anaphylaxis but exacerbate associated vasodilation. Therefore, blocking the effects of NO on vascular smooth muscle by inhibiting the guanylate cyclase (GC) would be a reasonable strategy. This study aimed to investigate the effects of NO/cGMP pathway inhibitors methylene blue (MB), N^ω-nitro-L-arginine methyl ester hydrochloride (L-NAME), and indigo carmine (IC) in shock induced by compound 48/80 (C48/80) in rats. The effect was assessed by invasive blood pressure measurement. Shock was initiated by C48/80 intravenous bolus injection 5 min before (prophylactic) or after (treatment) the administration of the inhibitors MB (3 mg/kg), L-NAME (1 mg/kg), and IC (3 mg/kg). Of the groups that received drugs as prophylaxis for shock, only the IC group did not present the final systolic blood pressure (SBP) better than the C48/80 group. Regarding shock treatment with the drugs tested, all groups had the final SBP similar to the C48/80group. Altogether, our results suggested that inhibition of GC and NO synthase in NO production pathway was not sufficient to revert hypotension or significantly improve survival.

Folding of Gα Subunits: Implications for Disease States

G-proteins play a central role in signal transduction by fluctuating between "on" and "off" phases that are determined by a conformational change. cAMP is a secondary messenger whose formation is inhibited or stimulated by activated G_iα1 or G_sα subunit. We used tryptophan fluorescence, UV/vis spectrophotometry, and circular dichroism to probe distinct structural features within active and inactive conformations from wild-type and tryptophan mutants of G_iα1 and G_sα. For all proteins studied, we found that the active conformations were more stable than the inactive conformations, and upon refolding from higher temperatures, activated wild-type subunits recovered significantly more native structure. We also observed that the wild-type subunits partially regained the ability to bind nucleotide. The increased compactness observed upon activation was consistent with the calculated decrease in solvent accessible surface area for wild-type G_iα1. We found that as the temperature increased, G_α subunits, which are known to be rich in α-helices, converted to proteins with increased content of β-sheets and random coil. For active conformations from wild-type and tryptophan mutants of G_iα1, melting temperatures indicated that denaturation starts around hydrophobic tryptophan microenvironments and then radiates toward tyrosine residues at the surface, followed by alteration of the secondary structure. For G_sα, however, disruption of secondary structure preceded unfolding around tyrosine residues. In the active conformations, a π-cation interaction between essential arginine and tryptophan residues, which was characterized by a fluorescence-measured red shift and modeled by molecular dynamics, was also shown to be a contributor to the stability of G_α subunits. The folding properties of G_α subunits reported here are discussed in the context of diseases associated to G-proteins.

Mitochondrial protein-derived cryptides: Are endogenous N-formylated peptides including mitocryptide-2 components of mitochondrial damage-associated molecular patterns?

Recently, much attention has been paid to "nonclassical" bioactive peptides, which are fragmented peptides simultaneously produced during maturation and degradation of various functional proteins. We identified many fragmented peptides derived from various mitochondrial proteins including mitocryptide-1 and mitocryptide-2 that efficiently activate neutrophils. These endogenous, functionally active, fragmented peptides are referred to as "cryptides." Among them, mitocryptide-2 is an N-formylated cryptide cleaved from mitochondrial cytochrome b that is encoded in mitochondrial DNA (mtDNA). It is known that 13 proteins encoded in mtDNA are translated in mitochondria as N-formylated forms, suggesting the existence of endogenous N-formylated peptides other than mitocryptide-2. Here, we investigated the effects of N-formylated peptides presumably cleaved from mtDNA-encoded proteins other than cytochrome b on the functions of neutrophilic cells to elucidate possible regulation by endogenous N-formylated cryptides. Four N-formylated cryptides derived from cytochrome c oxidase subunit I and NADH dehydrogenase subunits 4, 5, and 6 among 12 peptides from mtDNA-encoded proteins efficiently induced not only migration but also β-hexosaminidase release, which is an indicator of neutrophilic phagocytosis, in HL-60 cells differentiated into neutrophilic cells. These activities were comparable to or higher than those induced by mitocryptide-2. Although endogenous N-formylated peptides that are contained in mitochondrial damage-associated molecular patterns (DAMPs) have yet to be molecularly identified, they have been implicated in innate immunity. Thus, N-formylated cryptides including mitocryptide-2 are first-line candidates for the contents of mitochondrial DAMPs to promote innate immune responses. © 2015 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 106: 580-587, 2016.

Universal allosteric mechanism for Gα activation by GPCRs

G protein-coupled receptors (GPCRs) allosterically activate heterotrimeric G proteins and trigger GDP release. Given that there are ∼800 human GPCRs and 16 different Gα genes, this raises the question of whether a universal allosteric mechanism governs Gα activation. Here we show that different GPCRs interact with and activate Gα proteins through a highly conserved mechanism. Comparison of Gα with the small G protein Ras reveals how the evolution of short segments that undergo disorder-to-order transitions can decouple regions important for allosteric activation from receptor binding specificity. This might explain how the GPCR-Gα system diversified rapidly, while conserving the allosteric activation mechanism.

Biochemical characterization of a heterotrimeric G(i)-protein activator peptide designed from the junction between the intracellular third loop and sixth transmembrane helix in the m4 muscarinic acetylcholine receptor

Muscarinic acetylcholine receptors (mAChRs) are G-protein coupled receptors (GPCRs) that are activated by acetylcholine released from parasympathetic nerves. The mAChR family comprises 5 subtypes, m1-m5, each of which has a different coupling selectivity for heterotrimeric GTP-binding proteins (G-proteins). m4 mAChR specifically activates the Gi/o family by enhancing the guanine nucleotide exchange factor (GEF) reaction with the Gα subunit through an interaction that occurs via intracellular segments. Here, we report that the m4 mAChR mimetic peptide m4i3c(14)Gly, comprising 14 residues in the junction between the intracellular third loop (i3c) and transmembrane helix VI (TM-VI) extended with a C-terminal glycine residue, presents GEF activity toward the Gi1 α subunit (Gαi1). The m4i3c(14)Gly forms a stable complex with guanine nucleotide-free Gαi1 via three residues in the VTI(L/F) motif, which is conserved within the m2/4 mAChRs. These results suggest that this m4 mAChR mimetic peptide, which comprises the amino acid of the mAChR intracellular segments, is a useful tool for understanding the interaction between GPCRs and G-proteins.

Calnuc plays a role in dynamic distribution of Galphai but not Gbeta subunits and modulates ACTH secretion in AtT-20 neuroendocrine secretory cells

In AtT-20 cells ACTH secretion is regulated by both Ca²⁺ and G proteins. We previously demonstrated that calnuc, an EF-hand Ca²⁺ binding protein which regulates Alzheimer's β-amyloid precursor protein (APP) biogenesis, binds both Ca²⁺ as well as Gα subunits. Here we investigate calnuc's role in G protein-mediated regulation of ACTH secretion in AtT-20 neuroendocrine secretory cells stably overexpressing calnuc-GFP. Similar to endogenous calnuc, calnuc-GFP is mainly found in the Golgi, on the plasma membrane (PM), and associated with regulated secretion granules (RSG). By deconvolution immunofluorescence, calnuc-GFP partially colocalizes with Gαi1/2 and Gαi3 at the PM and on RSG. Cytosolic calnuc(ΔSS)-CFP with the signal sequence deleted also partially colocalizes with RSG and partially cosediments with Gαi1/2 in fractions enriched in RSG. Overexpression of calnuc-GFP specifically increases the distribution of Gαi1/2 on the PM whereas the distribution of Gβ subunits and synaptobrevin 2 (Vamp 2) is unchanged. Overexpression of calnuc-GFP or cytosolic calnuc(ΔSS)-CFP enhances ACTH secretion two-fold triggered by mastoparan or GTPγS but does not significantly affect glycosaminoglycan (GAG) chain secretion along the constitutive pathway or basal secretion of ACTH. Calnuc's facilitating effects on ACTH secretion are decreased after introducing anti-Gαi1/2, Gαi3, Gβ or calnuc IgG into permeabilized cells but not when Gα12 or preimmune IgG is introduced. The results suggest that calnuc binds to Gα subunits on the Golgi and on RSG and that overexpression of calnuc causes redistribution of Gαi subunits to the PM and RSG, indicating that calnuc plays a role in dynamic distribution of only Gα but not Gβ subunits. Thus calnuc may connect G protein signaling and calcium signaling during regulated secretion.

Discovery of mitocryptide-1, a neutrophil-activating cryptide from healthy porcine heart

Although neutrophils are known to migrate in response to various chemokines and complement factors, the substances involved in the early stages of their transmigration and activation have been poorly characterized to date. Here we report the discovery of a peptide isolated from healthy porcine hearts that activated neutrophils. Its primary structure is H-Leu-Ser-Phe-Leu-Ile-Pro-Ala-Gly-Trp-Val-Leu-Ser-His-Leu-Asp-His-Tyr-Lys-Arg-Ser-Ser-Ala-Ala-OH, and it was indicated to originate from mitochondrial cytochrome c oxidase subunit VIII. This peptide caused chemotaxis at concentrations lower than that inducing beta-hexosaminidase release. Such responses were observed in neutrophilic/granulocytic differentiated HL-60 cells but not in undifferentiated cells, and G(i2)-type G proteins were suggested to be involved in the peptide signaling. Moreover the peptide activated human neutrophils to induce beta-hexosaminidase secretion. A number of other amphipathic neutrophil-activating peptides presumably originating from mitochondrial proteins were also found. The present results suggest that neutrophils monitor such amphipathic peptides including the identified peptide as an initiation signal for inflammation at injury sites.

Compound 48/80 induces endothelium-dependent and histamine release-independent relaxation in rabbit aorta

Compound 48/80 (C48/80) is a synthetic condensation product of N-methyl-p-methoxyphenethylamine with formaldehyde and is an experimental drug used since the 1950s to induce anaphylactic shock through histamine release. This study was carried out to further elucidate the mechanism by which this drug induces nitric oxide (NO) release. Our specific goals were: (a) to verify if C48/80's relaxation occurs through the stimulation of histamine receptors; (b) to evaluate the endothelium-dependent relaxation induced by C48/80; (c) to identify NO as the endothelium-relaxing factor released by C48/80; (d) to identify the NO synthase (NOS) responsible for NO release; and (e) to verify if the relaxation induced by C48/80 is calcium and cyclic guanidine monophosphate (cGMP) dependent. Rabbit aorta segments, with and without endothelium, were suspended in organ chambers (25ml) filled with Krebs solution maintained at 37 degrees C, bubbled with 95% O(2)/5% CO(2) (pH 7.4). Phenylephrine was used to contract the segments. Other protocol drugs included H(1)- and H(2)-receptor antagonists, cyclooxygenase, NOS, guanylyl cyclase and phospholipase C (PLC) inhibitors. Endothelium-dependent relaxation induced by C48/80 was also studied in calcium-free Krebs solution associated with a calcium chelator. In summary, our investigation demonstrated that the C48/80 vasodilating action: (a) does not depend on H(1) and H(2) histamine receptors; (b) is NO endothelium-dependent; (c) is dependent on the endothelial constitutive NOS (NOS-3) isoform activation; (d) is cGMP-dependent; and that NOS-3 activation by C48/80: (a) is independent of PLC up to 25mug/ml and (b) is partially dependent of this lipase in higher doses.

Crystallization and preliminary X-ray crystallographic analysis of the receptor-uncoupled mutant of Galphai1

In order to understand the molecular mechanisms by which G-protein-coupled receptors (GPCRs) activate G proteins, the K349P mutant of Galpha(i1) (K349P), which is unable to couple to the muscarinic acetylcholine receptor, was prepared and its crystals were grown along with those of wild-type Galpha(i1) protein (WT). The two proteins were crystallized under almost identical conditions, thus enabling a detailed structural comparison. The crystallization conditions performed well irrespective of the identity of the bound nucleotide (GDP or GTPgammaS) and the crystals diffracted to resolutions of 2.2 A (WT.GDP), 2.8 A (WT.GTPgammaS), 2.6 A (K349P.GDP) and 3.2 A (K349P.GTPgammaS).

Molecular mechanisms for the effect of mastoparan on G proteins in tissues of vertebrates and invertebrates

The peptide toxin mastoparan increased GTP-binding activity of heterotrimeric G proteins in tissues of vertebrate and invertebrate animals, the effect of mastoparan in mussel tissues being less pronounced. The stimulatory effect of mastoparan on GTP binding was not observed after treatment of membranes with pertussis toxin that selectively modulates function of Gi proteins. Activity of mastoparan decreased in the presence of C-terminal peptide 346-355 from the Gi protein alphai2-subunit. Mastoparan dose-dependently decreased the stimulatory effect of hormones on GTP binding in tissues of rats and mussels. The influence of these hormones on the cell is realized via Gi proteins. However, mastoparan did not modulate the effect of Gs protein-activating hormones.

Mechanism of the receptor-catalyzed activation of heterotrimeric G proteins

Heptahelical receptors activate intracellular signaling pathways by catalyzing GTP for GDP exchange on the heterotrimeric G protein alpha subunit (G alpha). Despite the crucial role of this process in cell signaling, little is known about the mechanism of G protein activation. Here we explore the structural basis for receptor-mediated GDP release using electron paramagnetic resonance spectroscopy. Binding to the activated receptor (R*) causes an apparent rigid-body movement of the alpha5 helix of G alpha that would perturb GDP binding at the beta6-alpha5 loop. This movement was not observed when a flexible loop was inserted between the alpha5 helix and the R*-binding C terminus, which uncouples R* binding from nucleotide exchange, suggesting that this movement is necessary for GDP release. These data provide the first direct observation of R*-mediated conformational changes in G proteins and define the structural basis for GDP release from G alpha.

Recognition of pathogens and activation of immune responses in Drosophila and horseshoe crab innate immunity

In innate immunity, pattern recognition receptors discriminate between self- and infectious non-self-matter. Mammalian homologs of the Drosophila Toll protein, which are collectively referred to as Toll-like receptors (TLRs), recognize pathogen-associated molecular patterns (PAMPs), including lipopolysaccharides (LPS) and lipoproteins, whereas the Drosophila Toll protein does not act as a PAMP receptor, but rather binds to Spätzle, an endogenous peptide. In Drosophila, innate immune surveillance is mediated by members of the peptidoglycan recognition protein (PGRP) family, which recognize diverse bacteria-derived peptidoglycans and initiate appropriate immune reactions including the release of antimicrobial peptides and the activation of the prophenoloxidase cascade, the latter effecting localized wound healing, melanization, and microbial phagocytosis. In the horseshoe crab, LPS induces hemocyte exocytotic degranulation, resulting in the secretion of various defense molecules, such as coagulation factors, antimicrobial peptides, and lectins. Recent studies have demonstrated that the zymogen form of the serine protease factor C, a major granular component of hemocyte, also exists on the hemocyte surface and functions as a biosensor for LPS. The proteolytic activity of activated factor C initiates hemocyte exocytosis via a G protein mediated signal transduction pathway. Furthermore, it has become clear that an endogenous mechanism for the feedback amplification of the innate immune response exists and is dependent upon a granular component of the horseshoe crab hemocyte.

Structure of tightly membrane-bound mastoparan-X, a G-protein-activating peptide, determined by solid-state NMR

The structure of mastoparan-X (MP-X), a G-protein activating peptide from wasp venom, in the state tightly bound to anionic phospholipid bilayers was determined by solid-state NMR spectroscopy. Carbon-13 and nitrogen-15 NMR signals of uniformly labeled MP-X were completely assigned by multidimensional intraresidue C-C, N-CalphaCbeta, and N-Calpha-C', and interresidue Calpha-CalphaCbeta, N-CalphaCbeta, and N-C'-Calpha correlation experiments. The backbone torsion angles were predicted from the chemical shifts of 13C', 13Calpha, 13Cbeta, and 15N signals with the aid of protein NMR database programs. In addition, two 13C-13C and three 13C-15N distances between backbone nuclei were precisely measured by rotational resonance and REDOR experiments, respectively. The backbone structure of MP-X was determined from the 26 dihedral angle restraints and five distances with an average root-mean-square deviation of 0.6 A. Peptide MP-X in the bilayer-bound state formed an amphiphilic alpha-helix for residues Trp3-Leu14 and adopted an extended conformation for Asn2. This membrane-bound conformation is discussed in relation to the peptide's activities to form pores in membranes and to activate G-proteins. This study demonstrates the power of multidimensional solid-state NMR of uniformly isotope-labeled molecules and distance measurements for determining the structures of peptides bound to lipid membranes.

Gi/o proteins: Expression for direct activation enquiry

G protein-mediated pathways are fundamental mechanisms of cell signaling. In this paper, the expression and the characterization of the alphai1, alphai3, alphao1, beta1, and gamma2 subunits of the human G protein are described. This approach was developed to evaluate the G protein activation profile of new compounds. pCR-TOPO T7 vectors, engineered to contain the target sequences, were used to transform Escherichia coli competent cells. Subunits were over-expressed in a preparative scale as fusion proteins with a six-histidine tag, and subsequently purified by metal chelate chromatography. Afterward, the His-tag was removed by enterokinase digestion, and the secondary structures of the recombinant subunits were analyzed by circular dichroism. To assess the functionality of the subunits, the rate of GTP hydrolysis and GTPgammaS binding were evaluated both in the absence and in the presence of two modulators: the peptidic activator Mastoparan and the non-peptidic activator N-dodecyl-lysinamide (ML250). Tests were conducted on isolated alpha-subunit and on heterotrimeric alphabetagamma complex, alone or reconstituted in phospholipidic vesicles. Our results show that recombinant subunits are stable, properly folded and, fully active, which makes them suitable candidates for functional studies.

An antimicrobial peptide tachyplesin acts as a secondary secretagogue and amplifies lipopolysaccharide-induced hemocyte exocytosis

In the horseshoe crab, bacterial lipopolysaccharide (LPS) induces exocytosis by granular hemocytes, resulting in the secretion of various defense molecules, such as lectins and antimicrobial peptides, via a G protein-mediating signaling pathway. This response is a key component of the horseshoe crab innate immune response against infectious microorganisms. Here, we report an endogenous amplification mechanism for LPS-induced hemocytes exocytosis. The concentration of LPS required for maximal secretion decreased in proportion to the density of hemocytes, suggesting the presence of a positive feedback mechanism for secretion via a mediator secreted from hemocytes. The exocytosed fluid of hemocytes was found able to induce hemocyte exocytosis in the absence of LPS. Furthermore, tachyplesin, a major antimicrobial peptide of hemocytes, was able to trigger exocytosis in an LPS-independent manner, which was inhibited by a phospholipase C inhibitor, U-73122, and a G protein inhibitor, pertussis toxin. Surface plasmon resonance analysis showed that tachyplesin directly interacts with bovine G protein. These findings suggest that the tachyplesin-induced hemocyte exocytosis also occurs via a G protein-mediating signaling pathway. We concluded that tachyplesin functions not only as an antimicrobial substance, but also as a secondary secretagogue of LPS-induced hemocyte exocytosis, leading to the amplification of the innate immune reaction at sites of injury.

Inactivation and activation of Ras by the neurotrophin receptor p75

The neurotrophin receptor p75 induces neurotrophic and/or apoptotic signalling pathways and can also cooperate with the neurotrophic Trk receptor tyrosine kinases. Its intracellular part encloses a so-called 'death domain' with a segment similar to the wasp venom mastoparan which binds small GTPases such as Rho. To study possible interactions of p75 and Ras (and Rho) we used wild-type and mutant genes of p75 stably expressed by MDCK cells which normally have neither Trk nor p75. We found that p75 can directly bind the GTPases Ras and Rho and that the unstimulated p75 inactivates total cellular Ras through a differential influence on the dissociation of GDP and GTP from Ras and an exchange of bound Ras.GDP for free Ras.GTP. These properties of p75 could also be demonstrated in vitro and should therefore be cell type-independent. Stimulation of p75 with nerve growth factor causes Ras activation via adapter proteins known from Trk signalling and induces rapid outgrowth of cellular processes. Both inactivation and activation of Ras by p75 are controlled by the phosphorylation state of the receptor's two intracellular tyrosines. p75 also influences Rho activation and inactivation, and the combined interactions of the receptor with the two GTPases Ras and Rho can regulate neurite formation in an efficient, synergistic way.

Blockade of G protein-coupled receptors and the dopamine transporter by a transmembrane domain peptide: novel strategy for functional inhibition of membrane proteins in vivo

G protein-coupled receptors have a core consisting of seven transmembrane alpha-helices that is important in maintaining the structure of the receptor. We postulated that disruption of the transmembrane core may interfere with receptor function. In this study, the function of integral membrane proteins was disrupted in vivo using peptides mimicking their transmembrane domains. A peptide derived from transmembrane 7 of the D2 dopamine receptor injected unilaterally into caudate nucleus of rats challenged with apomorphine resulted in rotational behavior, indicating D2 receptor blockade. No rotational behavior was seen with a similar peptide based on the beta2 adrenergic receptor and the D2 transmembrane peptide did not affect the D1 dopamine receptor, indicating that the D2 receptor-derived peptide had a specific effect. The intravenous administration of a transmembrane peptide derived from the alpha1-adrenergic receptor resulted in lowered arterial blood pressure and injection of a beta1-adrenergic receptor peptide resulted in decreased heart rate. Injection of a V2 vasopressin receptor-derived transmembrane peptide resulted in increased urine output, suggesting antagonism of the effects of vasopressin. Finally, dopamine release in rat brain after cocaine administration was blocked by a transmembrane peptide based on the dopamine transporter. Circular dichroism spectroscopy of the peptides revealed alpha-helical structure similar to that of native transmembrane domains. Thus, transmembrane peptides can disrupt membrane proteins in vivo likely by competing with native transmembrane domains. The disruption of the hydrophobic core architecture of membrane proteins represents a novel mechanism of achieving functional inhibition that may be possible to exploit in developing novel therapeutics.

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.

Do mucosal mast cells contribute to the immediate asthma response?

In rat trachea, two types of mast cells have been identified, connective tissue mast cells (CTMCs) and mucosal mast cells (MMCs). We previously reported that CTMCs play an important role in tracheal contraction in vitro via 5-hydroxytryptamine (5-HT) release in a rat model. In this study, we investigated whether MMCs also play a role in tracheal contraction by employing mast cell-deficient (Ws/Ws) rats and their congenic (+/+) rats. Rats were actively sensitized with ovalbumin and challenged with it 2 weeks later. To exclude the influence of CTMCs, rats were pretreated for 7 days with compound 48/80 injected i.p. in increasing doses. Histological study confirmed that degranulation occurred in CTMCs, but MMCs still remained. Histamine levels in trachea decreased to 9.31% of control levels. Ovalbumin-specific IgE production showed a time-dependent increase in both Ws/Ws and +/+ rats after sensitization with no significantly different values between the two groups. Ovalbumin challenge caused contraction of the trachea in sensitized control (+/+) rats, but not in sensitized Ws/Ws and compound 48/80-pretreated +/+ rats. Ketanserin inhibited the contraction, but leukotriene antagonist ONO-1078 did not, indicating that the contraction was due to 5-HT, whereas leukotriene, a mediator specific derived from MMCs, has no significant effect. The results suggest that MMCs has minimal, if any, contribution to tracheal contraction and might have another function. Furthermore, Ws/Ws and the congenic rats provide a good model for studying the role of mast cells in the immunologic response in airways.

Calnuc, an EF-hand Ca(2+) binding protein, specifically interacts with the C-terminal alpha5-helix of G(alpha)i3

Calnuc (nucleobindin) was previously shown to be present both in the cytosol and in the Golgi and to be the major Golgi Ca(2+) binding protein. In this study we verified the existence of the cytosolic pool of calnuc and investigated its interaction with G(alpha)i3. Cytosolic calnuc was released by mild digitonin permeabilization. In pulse-chase experiments, the two pools of calnuc had different mobilities, suggesting different posttranslational modifications. That calnuc interacts with G(alpha)i3 in vivo was verified by the finding that G(alpha)i3 could be crosslinked intracellularly to calnuc and co-immunoprecipitated from NIH 3T3 cells stably overexpressing either activated (Q204L) or inactivated (G203A) G(alpha)i3. Binding was Ca(2+) and Mg(2+)-dependent. Calnuc and G(alpha)i3-GFP codistributed primarily in the Golgi region. By yeast two-hybrid analysis, the binding site on G(alpha)i3 for calnuc was mapped to the C-terminal region because removal of the last 12 amino acids (but not 11) abolished the interaction. Peptide competition indicated that calnuc, with its coiled-coil domain constituted by the two EF-hands, binds to G(alpha)i3's C-terminal alpha5-helix. These results demonstrate that calnuc may play an important role in G protein- and Ca(2+)-regulated signal transduction events.

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.

One-step affinity purification of the G protein betagamma subunits from bovine brain using a histidine-tagged G protein alpha subunit

An efficient one-step affinity purification of bovine brain G protein betagamma subunits (betagamma's) is described. The betagamma's, in a detergent extract of brain membranes, are first dissociated from the alpha subunits (alpha's), reassociated with decahistidine-tagged alphail produced in bacteria, and then adsorbed onto Ni2+-nitrilotriacetic acid-agarose via the histidine tag. This mild adsorption retained the high activity of the ligand alpha's, in contrast to the commonly used chemical crosslinking methods. A wash step with a buffer containing chaotropic ions (SCN-) completely removed contaminating proteins that were refractory to washes with high concentrations of detergents, after which the highly purified betagamma's were eluted with a buffer containing Al3+, Mg2+, and F- ions. The obtained betagamma's were found to be fully functional, as assessed by their ability to support pertussis toxin-catalyzed ADP-ribosylation of alphail. Since the combination of the mild adsorption via the histidine tag and the wash with chaotropic ions can be easily applied to purifying betagamma's from various animal tissues, this new chromatographic method is expected to facilitate the purification of other membrane proteins that bind to Galpha and/or Galphabetagamma.

Posttranslational modification of Galphao1 generates Galphao3, an abundant G protein in brain

Galphao, the most abundant G protein in mammalian brain, occurs at least in two subforms, i.e., Galphao1 and Galphao2, derived by alternative splicing of the mRNA. A third Galphao1-related isoform, Galphao3, has been purified, representing about 30% of total Go in brain. Initial studies revealed distinct biochemical properties of Galphao3 as compared with other Galphao isoforms. In matrix-assisted laser desorption/ionization peptide mass mapping of gel-isolated Galphao1 and Galphao3, C-terminal peptides showed a difference of +1 Da for Galphao3. Nanoelectrospray tandem mass spectrometry sequencing revealed an Asp instead of an Asn at position 346 of Galphao3. Gel electrophoretic analysis of recombinant Galphao3 showed the same mobility as native Galphao3 but distinct to Galphao1. The conversion of 346Asn-->Asp changed the signaling properties, including the velocity of the basal guanine nucleotide-exchange reaction, which points to the involvement of the C terminus in basal guanosine 5'-[gamma-thio]triphosphate binding. No cDNA coding for Galphao3 was detected, suggesting an enzymatic deamidation of Galphao1 by a yet-unidentified activity. Therefore, Galpha heterogeneity is generated not only at the DNA or RNA levels, but also at the protein level. The relative amount of Galphao1 and Galphao3 differed from cell type to cell type, indicating an additional principle of G protein regulation.

Expression, purification, and reconstitution of receptor for pituitary adenylate cyclase-activating polypeptide. large-scale purification of a functionally active G protein-coupled receptor produced in Sf9 insect cells

Human pituitary adenylate cyclase-activating polypeptide (PACAP) receptor was expressed in Sf9 insect cells and Chinese hamster ovary (CHO) cells. The recombinant receptor in Sf9 cell membranes had low affinity for 125I-PACAP27 (Kd = 155.3 pM) and was insensitive to guanosine 5'-O-3-thiotriphosphate (GTPgammaS), whereas the receptor in CHO membranes had a high affinity (Kd = 44.4 pM) and was GTPgammaS sensitive. The receptor in Sf9 membranes was converted to a high affinity state (Kd = 20-40 pM) following solubilization with digitonin. A large quantity (2 mg from 8 liters of insect cells) of the purified PACAP receptors (Bmax = 23.9 nmol/mg of protein) were obtained in a digitonin-induced high affinity state (Kd = 17.3 pM) using biotinylated ligand affinity chromatography. The apparent molecular weight of the purified receptor (Mr = 48,000) was smaller than that of the receptor from CHO cells (Mr = 58,000) due to differences in asparagine-linked sugar chains. The purified receptor reverted to a low affinity state (Kd = 182.6 pM) upon reconstitution into lipid vesicles, however, the receptor reconstituted with Gs protein had a high affinity (Kd = 40.2 pM) and was GTPgammaS sensitive. [35S]GTPgammaS binding to the reconstituted Gs protein was enhanced by PACAP27 and PACAP38 (EC50 = 42.5 and 9.4 pM, respectively) but not by antagonist PACAP(6-38), indicating that the purified receptor was functionally active.