NMR backbone and methyl resonance assignments of an inhibitory G-alpha subunit in complex with GDP

G-proteins are essential switch points at the cell membrane that control downstream signaling by their ability to adopt an inactive, GDP-bound or an active, GTP-bound state. Among other exchange factors, G-protein coupled receptors (GPCRs) induce exchange of GDP to GTP and thus promote the active state of the G-protein. The nucleotide-binding α subunit of the G-protein undergoes major conformational changes upon nucleotide binding. Thus, an NMR analysis of the two distinct nucleotide-bound states is essential for a more detailed understanding of associated structural changes. Here, we provide an NMR backbone as well as methyl group resonance assignment of an inhibitory G-alpha subunit subtype 1 (Gα_i,1) in the GDP-bound form and show that, in contrast to the GTP-bound form, large parts of the protein are mobile, presumably caused by a loose arrangement of the two subdomains in Gα that tightly interact with each other only in the GTP-bound state. As the GDP-bound form represents the GPCR-binding-competent state, the presented NMR data will be essential for further studies on G-protein-GPCR interactions and dynamics in solution for receptor systems that couple to G-proteins containing an inhibitory Gα,1 subunit.

Direct regulation of p190RhoGEF by activated Rho and Rac GTPases

Rho family GTPases regulate a wide range of cellular processes. This includes cellular dynamics where three subfamilies, Rho, Rac, and Cdc42, are known to regulate cell shape and migration though coordinate action. Activation of Rho proteins largely depends on Rho Guanine nucleotide Exchange Factors (RhoGEFs) through a catalytic Dbl homology (DH) domain linked to a pleckstrin homology (PH) domain that subserves various functions. The PH domains from Lbc RhoGEFs, which specifically activate RhoA, have been shown to bind to activated RhoA. Here, p190RhoGEF is shown to also bind Rac1·GTP. Crystal structures reveal that activated Rac1 and RhoA use their effector-binding surfaces to associate with the same hydrophobic surface on the PH domain. Both activated RhoA and Rac1 can stimulate exchange of nucleotide on RhoA by localization of p190RhoGEF to its substrate, RhoA·GDP, in vitro. The binding of activated RhoA provides a mechanism for positive feedback regulation as previously proposed for the family of Lbc RhoGEFs. In contrast, the novel interaction between activated Rac1 and p190RhoGEF reveals a potential mechanism for cross-talk regulation where Rac can directly effect stimulation of RhoA. The greater capacity of Rac1 to stimulate p190RhoGEF among the Lbc RhoGEFs suggests functional specialization.

Mouse Neuroblastoma CB1 Cannabinoid Receptor-Stimulated [35S]GTPɣS Binding: Total and Antibody-Targeted Gα Protein-Specific Scintillation Proximity Assays

G protein-coupled receptors (GPCRs) are important regulators of cellular signaling functions and therefore are a major target for drug discovery. The CB1 cannabinoid receptor is among the most highly expressed GPCRs in neurons, where it regulates many differentiated neuronal functions. One model system for studying the biochemistry of neuronal responses is the use of neuroblastoma cells originating from the C1300 tumor in the A/J mouse, including cloned cell lines NS20, N2A, N18TG2, N4TG1, and N1E-115, and various immortalized hybrids of neurons with N18TG2 cells. GPCR signal transduction is mediated through interaction with multiple types and subtypes of G proteins that transduce the receptor stimulus to effectors. The [35S]GTPɣS assay provides a valuable pharmacological method to evaluate efficacy and potency in the first step in GPCR signaling. Here, we present detailed protocols for the [35S]GTPɣS-binding assay to measure the total G protein binding and the antibody-targeted scintillation proximity assay to measure specific Gα proteins in neuroblastoma cell membrane preparations. This chapter presents step-by-step methods from cell culture, membrane preparation, assay procedures, and data analysis.

Protocols and Good Operating Practices in the Study of Cannabinoid Receptors

With the approach of the 30th year since the pioneering discovery of a cannabinoid receptor in rat brain (Devane et al., 1988), the field of cannabinoid pharmacology and physiology has impacted human physiology at multiple levels. The development of highly specific and potent orthosteric ligands, as well as the blossoming field of allosteric ligand development, has placed the endocannabinoid system in the forefront as a modulator of a multitude of physiologic processes. Reproducibility among laboratories is especially important due to the development of novel tools to investigate the role(s) of the endocannabinoid system in human physiology, and to clarify the roles for medicinal marijuana. Any definitive role in normal, or diseased states, must be satisfied through the demonstration of a specific receptor-mediated event. This chapter provides working protocols for the study of cannabinoid receptor-ligand binding, as well as immediate and downstream G protein-dependent signaling assays to assess receptor function.

Evaluating the Activity of Smoothened Toward G Proteins Using [³⁵S]Guanosine 5'-(3-O-thio)triphosphate ([³⁵S]GTPγS)

The utilization of heterotrimeric G protein, and in particular those of the Gi, family, by Hedgehogs through Smoothened has become increasingly clear. We describe here a method for evaluating the activity of Smoothened toward G proteins in membranes derived from human embryonic kidney-293 (HEK293) cells. The assay relies on receptor-promoted exchange of GDP for [(35)S]GTPγS on the Gα subunit. The assay is best suited for analysis of the constitutive activity of Smoothened, inverse agonism superimposed on this activity, and neutral antagonism superimposed on inverse agonism. The assay would also be suitable for several other applications requiring a proximal, quantifiable readout of Smoothened activity.

Binding Selectivity of Abaloparatide for PTH-Type-1-Receptor Conformations and Effects on Downstream Signaling

The PTH receptor type 1 (PTHR1) mediates the actions of two endogenous polypeptide ligands, PTH and PTHrP, and thereby plays key roles in bone biology. Based on its capacity to stimulate bone formation, the peptide fragment PTH (1-34) is currently in use as therapy for osteoporosis. Abaloparatide (ABL) is a novel synthetic analog of human PTHrP (1-34) that holds promise as a new osteoporosis therapy, as studies in animals suggest that it can stimulate bone formation with less of the accompanying bone resorption and hypercalcemic effects that can occur with PTH (1-34). Recent studies in vitro suggest that certain PTH or PTHrP ligand analogs can distinguish between two high-affinity PTHR1 conformations, R(0) and RG, and that efficient binding to R(0) results in prolonged signaling responses in cells and prolonged calcemic responses in animals, whereas selective binding to RG results in more transient responses. As intermittent PTH ligand action is known to favor the bone-formation response, whereas continuous ligand action favors the net bone-resorption/calcemic response, we hypothesized that ABL binds more selectively to the RG vs the R(0) PTHR1 conformation than does PTH (1-34), and thus induces more transient signaling responses in cells. We show that ABL indeed binds with greater selectivity to the RG conformation than does PTH (1-34), and as a result of this RG bias, ABL mediates more transient cAMP responses in PTHR1-expressing cells. The findings provide a plausible mechanism (ie, transient signaling via RG-selective binding) that can help account for the favorable anabolic effects that ABL has on bone.

Backbone resonance assignments for G protein α(i3) subunit in the GDP-bound state

Guanine-nucleotide binding proteins (G proteins) serve as molecular switches in signaling pathways, by coupling the activation of G protein-coupled receptors (GPCRs) at the cell surface to intracellular responses. In the resting state, G protein forms a heterotrimer, consisting of the G protein α subunit with GDP (Gα·GDP) and the G protein βγ subunit (Gβγ). Ligand binding to GPCRs promotes the GDP-GTP exchange on Gα, leading to the dissociation of the GTP-bound form of Gα (Gα·GTP) and Gβγ. Then, Gα·GTP and Gβγ bind to their downstream effector enzymes or ion channels and regulate their activities, leading to a variety of cellular responses. Finally, Gα hydrolyzes the bound GTP to GDP and returns to the resting state by re-associating with Gβγ. The G proteins are classified with four major families based on the amino acid sequences of Gα: i/o, s, q/11, and 12/13. Here, we established the backbone resonance assignments of human Gαi3, a member of the i/o family with a molecular weight of 41 K, in complex with GDP. The chemical shifts were compared with those of Gα(i3) in complex with a GTP-analogue, GTPγS, which we recently reported, indicating that the residues with significant chemical shift differences are mostly consistent with the regions with the structural differences between the GDP- and GTPγS-bound states, as indicated in the crystal structures. The assignments of Gα(i3)·GDP would be useful for the analyses of the dynamics of Gα(i3) and its interactions with various target molecules.

Solubilization and reconstitution of the mu-opioid receptor expressed in human neuronal SH-SY5Y and CHO cells

The zwitterionic detergent CHAPS was used to solubilize the human mu-opioid receptor (hMOR) from SH-SY5Y neuroblastoma cells and recombinant hMOR-CHO (CHO-T7-hMOR) and hMOR-SH-SY5Y (SH-SY5Y-T7-hMOR) cell membranes. Agonist stimulation and G-protein activation by the mu-selective opioid agonist DAMGO ([D-Ala2, N-MePhe4, Gly-ol]-enkephalin) were recovered after removing of CHAPS after polyethylene glycol (PEG) precipitation. Binding assays show that hMOR solubilized and reconstituted this way was functional and able to interact with both agonist peptides and with G-protein. The effective solubilization and reconstitution of hMOR from mammalian cells, without truncation and extensive modification, represent an essential step toward the purification of a receptor bearing important post-translational modifications.

Seeded microtubule growth for cryoelectron microscopy of end-binding proteins

End-binding proteins (EBs) have the ability to autonomously track the ends of growing microtubules, where they recruit several proteins that control various aspects of microtubule cytoskeleton organization and function. The structural nature of the binding site recognized by EBs at growing microtubule ends has been a subject of debate. Recently, a fluorescence microscopy assay used for the study of dynamic end tracking in vitro was adapted for cryoelectron microscopy (cryo-EM). In combination with single-particle reconstruction methods, this modified assay was used to produce the first subnanometer-resolution model of how the microtubule-binding domain of EBs binds to microtubules grown in the presence of GTPγS. A GTPγS microtubule can be considered a static mimic of the transiently existing binding region of EBs at a microtubule end growing in the presence of GTP. Here we describe in detail the procedure used to generate these samples. It relies on the polymerization of microtubules from preformed stabilized and quantum dot-labeled microtubule seeds. This allows the cryo-EM analysis of proteins bound to paclitaxel-free microtubules. It provides freedom for using different GTP analogues during microtubule elongation independent of their nucleation properties. This assay could also be useful for the cryo-EM analysis of other microtubule-associated proteins.

[Dmt1]N/OFQ(1-13)-NH2: a potent nociceptin/orphanin FQ and opioid receptor universal agonist

BACKGROUND AND PURPOSE

Intrathecally (i.t.) administered nociceptin/orphanin FQ (N/OFQ) evokes antinociceptive effects in rodents. Recent studies in monkeys demonstrated that i.t. co-application of N/OFQ and morphine elicits synergistic antinociceptive actions suggesting mixed N/OFQ peptide (NOP) and μ opioid receptor agonists as innovative spinal analgesics. Thus, novel N/OFQ related peptides were synthesized in order to identify and pharmacologically characterize a mixed NOP/ μ opioid receptor agonist.

EXPERIMENTAL APPROACH

The following in vitro assays were used: calcium mobilization in cells expressing the human NOP or classical opioid receptors and chimeric G proteins, receptor and [(35)S]-GTPγS binding, [(35)S]-GTPγS binding in rat spinal cord membranes, guinea pig ileum bioassay. In vivo experiments were performed in monkeys using the tail withdrawal assay.

KEY RESULTS

From calcium mobilization studies [Dmt(1)]N/OFQ(1-13)-NH(2) was selected as the most potent and least selective compound. The mixed NOP/opioid full agonist activity and high affinity of [Dmt(1)]N/OFQ(1-13)-NH(2) was confirmed at human recombinant receptors in receptor binding, calcium mobilization and/or [(35)S]-GTPγS binding studies, at rat spinal cord receptors in [(35)S]-GTPγS binding experiments, and at guinea pig receptors inhibiting neurogenic contractions in the ileum. In vivo in the tail withdrawal assay in monkeys i.t. [Dmt(1) ]N/OFQ(1-13)-NH(2) was able to elicit robust and long-lasting antinociceptive effects.

CONCLUSIONS AND IMPLICATIONS

Collectively, these results demonstrate that [Dmt(1)]N/OFQ(1-13)-NH(2) behaves as NOP/opioid receptor universal agonist and substantiate the suggestion that such mixed ligands are worthy of development as innovative spinal analgesics.

Structural flexibility of the G alpha s alpha-helical domain in the beta2-adrenoceptor Gs complex

The active-state complex between an agonist-bound receptor and a guanine nucleotide-free G protein represents the fundamental signaling assembly for the majority of hormone and neurotransmitter signaling. We applied single-particle electron microscopy (EM) analysis to examine the architecture of agonist-occupied β(2)-adrenoceptor (β(2)AR) in complex with the heterotrimeric G protein Gs (Gαsβγ). EM 2D averages and 3D reconstructions of the detergent-solubilized complex reveal an overall architecture that is in very good agreement with the crystal structure of the active-state ternary complex. Strikingly however, the α-helical domain of Gαs appears highly flexible in the absence of nucleotide. In contrast, the presence of the pyrophosphate mimic foscarnet (phosphonoformate), and also the presence of GDP, favor the stabilization of the α-helical domain on the Ras-like domain of Gαs. Molecular modeling of the α-helical domain in the 3D EM maps suggests that in its stabilized form it assumes a conformation reminiscent to the one observed in the crystal structure of Gαs-GTPγS. These data argue that the α-helical domain undergoes a nucleotide-dependent transition from a flexible to a conformationally stabilized state.

Progesterone signals through membrane progesterone receptors (mPRs) in MDA-MB-468 and mPR-transfected MDA-MB-231 breast cancer cells which lack full-length and N-terminally truncated isoforms of the nuclear progesterone receptor

The functional characteristics of membrane progesterone receptors (mPRs) have been investigated using recombinant mPR proteins over-expressed in MDA-MB-231 breast cancer cells. Although these cells do not express the full-length progesterone receptor (PR), it is not known whether they express N-terminally truncated PR isoforms which could possibly account for some progesterone receptor functions attributed to mPRs. In the present study, the presence of N-terminally truncated PR isoforms was investigated in untransfected and mPR-transfected MDA-MB-231 cells, and in MDA-MB-468 breast cancer cells. PCR products were detected in PR-positive T47D Yb breast cancer cells using two sets of C-terminus PR primers, but not in untransfected and mPR-transfected MDA-MB-231 cells, nor in MDA-MB-468 cells. Western blot analysis using a C-terminal PR antibody, 2C11F1, showed the same distribution pattern for PR in these cell lines. Another C-terminal PR antibody, C-19, detected immunoreactive bands in all the cell lines, but also recognized α-actinin, indicating that the antibody is not specific for PR. High affinity progesterone receptor binding was identified on plasma membranes of MDA-MB-468 cells which was significantly decreased after treatment with siRNAs for mPRα and mPRβ. Plasma membranes of MDA-MB-468 cells showed very low binding affinity for the PR agonist, R5020, ≤1% that of progesterone, which is characteristic of mPRs. Progesterone treatment caused G protein activation and decreased production of cAMP in MDA-MB-468 cells, which is also characteristic of mPRs. The results indicate that the progestin receptor functions in these cell lines are mediated through mPRs and do not involve any N-terminally truncated PR isoforms.

Comparative biochemical and pharmacological characterization of a novel, NOP receptor selective hexapeptide, Ac-RYYRIR-ol

Nociceptin/orphanin FQ (N/OFQ) is an endogenous neuropeptide, which is widely distributed in central and peripheral nervous system. Some N/OFQ sequence unrelated hexapeptides can effectively bind to the N/OFQ peptide (NOP) receptor and they were used as template for structure-activity studies that lead to discovery of the new NOP selective ligands. In the present study, the pharmacological profile of the novel hexapeptide Ac-RYYRIR-ol was investigated using various in vitro assays including receptor binding and G-protein activation in rat brain membranes, mouse and rat vas deferens, guinea pig ileum, mouse colon and Ca(2+) mobilization assay in chinese hamster ovary (CHO) cells co-expressing the human recombinant NOP receptor and the C-terminally modified Galpha(qi5) protein. In rat brain membranes Ac-RYYRIR-ol displaced both [(3)H]nociceptin/OFQ and [(3)H]Ac-RYYRIK-ol with high affinity (pK(i) 9.35 and 8.81, respectively) and stimulated [(35)S]GTPgammaS binding showing however lower maximal effects than N/OFQ (alpha=0.28). The stimulatory effect of Ac-RYYRIR-ol was antagonized by the selective NOP receptor antagonist UFP-101. In the electrically stimulated mouse vas deferens Ac-RYYRIR-ol displayed negligible agonist activity while antagonizing in a competitive manner (pA(2) 7.99) the inhibitory effects of N/OFQ. Similar results were obtained in the rat vas deferens. In the mouse colon Ac-RYYRIR-ol produced concentration dependent contractile effects with similar potency and maximal effects as N/OFQ. Finally, in the Ca(2+) mobilization assay performed with CHO-hNOP-Galpha(qi5) cells Ac-RYYRIR-ol displayed lower potency and maximal effects (alpha=0.87) compared with N/OFQ. In conclusion, the novel NOP receptor selective hexapeptide Ac-RYYRIR-ol has been shown to have fine selectivity, high potency, furthermore agonist and antagonist effects toward the NOP receptors were measured in various assays; this is likely due to its partial agonist pharmacological activity.

Measurement of one and two bond N-C couplings in large proteins by TROSY-based J-modulation experiments

Residual dipolar couplings (RDCs) between NC' and NC(alpha) atoms in polypeptide backbones of proteins contain information on the orientation of bond vectors that is complementary to that contained in NH RDCs. The (1)J(NC)(alpha) and (2)J(NC)(alpha) scalar couplings between these atoms also display a Karplus relation with the backbone torsion angles and report on secondary structure. However, these N-C couplings tend to be small and they are frequently unresolvable in frequency domain spectra having the broad lines characteristic of large proteins. Here a TROSY-based J-modulated approach for the measurement of small (15)N-(13)C couplings in large proteins is described. The cross-correlation interference effects inherent in TROSY methods improve resolution and signal to noise ratios for large proteins, and the use of J-modulation to encode couplings eliminates the need to remove frequency distortions from overlapping peaks during data analysis. The utility of the method is demonstrated by measurement of (1)J(NC'), (1)J(NC)(alpha) , and (2)J(NC)(alpha) scalar couplings and (1)D(NC') and D(NC)(alpha) residual dipolar couplings for the myristoylated yeast ARF1.GTPgammas protein bound to small lipid bicelles, a system with an effective molecule weight of approximately 70kDa.

Mapping allosteric connections from the receptor to the nucleotide-binding pocket of heterotrimeric G proteins

Heterotrimeric G proteins function as molecular relays that mediate signal transduction from heptahelical receptors in the cell membrane to intracellular effector proteins. Crystallographic studies have demonstrated that guanine nucleotide exchange on the Galpha subunit causes specific conformational changes in three key "switch" regions of the protein, which regulate binding to Gbetagamma subunits, receptors, and effector proteins. In the present study, nitroxide side chains were introduced at sites within the switch I region of Galphai to explore the structure and dynamics of this region throughout the G protein cycle. EPR spectra obtained for each of the Galpha(GDP), Galpha(GDP)betagamma heterotrimer and Galpha(GTPgammaS) conformations are consistent with the local environment observed in the corresponding crystal structures. Binding of the heterotrimer to activated rhodopsin to form the nucleotide-free (empty) complex, for which there is no crystal structure, causes prominent changes relative to the heterotrimer in the structure of switch I and contiguous sequences. The data identify a putative pathway of allosteric changes triggered by receptor binding and, together with previously published data, suggest elements of a mechanism for receptor-catalyzed nucleotide exchange.

Synthesis and biological activity of new melatonin dimeric derivatives

A new series of melatonin (MLT) dimers were obtained by linking together two melatonin units with a linear alkyl chain through the MLT acetamido group or through a C-2 carboxyalkyl function. The binding properties of these ligands were evaluated in in vivo experiments on cloned human MT(1) and MT(2) receptors expressed in NIH3T3 rat fibroblast cells. The class of 2-carboxyalkyl dimers was the most interesting one with compounds having good MT(1)/MT(2) nanomolar affinity. The data obtained suggest that the spacer length is crucial for optimal interaction at both receptor subtypes as well as to determine functional activity of the resulting dimers.

Structural basis for the interaction of CCR5 with a small molecule, functionally selective CCR5 agonist

The chemokine receptor CCR5 is an attractive target for HIV-1 drug development, as individuals whose cells lack surface CCR5 expression are highly resistant to HIV-1 infection. CCR5 ligands, such as CCL5/RANTES, effectively inhibit HIV-1 infection by competing for binding opportunities to the CCR5 and inducing its internalization. However, the inherent proinflammatory activity of the chemotactic response of CCR5 ligands has limited their clinical use. In this study, we found that a novel small molecule, functionally selective CCR5 agonist, 2,2-dichloro-1-(triphenylphosphonio)vinyl formamide perchlorate (YM-370749), down-modulates CCR5 from the cell surface without inducing a chemotactic response and inhibits HIV-1 replication. In molecular docking studies of YM-370749 and a three-dimensional model of CCR5 based on the rhodopsin crystal structure as well as binding and functional studies using various CCR5 mutants, the amino acid residues necessary for interaction with YM-370749 were marked. These results provide a structural basis for understanding the activation mechanism of CCR5 and for designing functionally selective agonists as a novel class of anti-HIV-1 agents.

NMR structure of an intracellular loop peptide derived from prostaglandin EP3alpha receptor

We found that a peptide (EP3a: TIKALVSRCRAKAAV) corresponding to the N-terminal site of the intracellular third loop of human prostaglandin EP3alpha receptor could activate G protein alpha-subunit directly. The activity was almost same as Mastoparan-X, a G protein activating peptide from wasp venom. The three-dimensional molecular structure of the peptide in SDS-d(25) micelles was determined by 2D (1)H NMR spectroscopy. The structure of EP3a consists of a positive charge cluster on the C-terminal helical site. The cluster was also found in several corresponding receptor peptides. Therefore, the positive charge cluster on the helical structure might play a crucial role in activation of G protein.

[(35)S]GTPgammaS binding stimulated by endomorphin-2 and morphiceptin analogs

The ability of several mu-selective opioid peptides to activate G-proteins was measured in rat thalamus membrane preparations. The mu-selective ligands used in this study were three structurally related peptides, endomorphin-1, endomorphin-2 and morphiceptin, and their analogs modified in position 3 or 4 by introducing 3-(1-naphthyl)-d-alanine (d-1-Nal) or 3-(2-naphthyl)-d-alanine (d-2-Nal). The results obtained for these peptides in [(35)S]GTPgammaS binding assay were compared with those obtained for a standard mu-opioid agonist DAMGO. [d-1-Nal(3)]Morphiceptin was more potent in G-protein activation (EC(50) value of 82.5+/-4.5 nM) than DAMGO (EC(50)=105+/-9 nM). [d-2-Nal(3)]Morphiceptin, as well as endomorphin-2 analogs substituted in position 4 by either d-1-Nal or d-2-Nal failed to stimulate [(35)S]GTPgammaS binding and were shown to be potent antagonists against DAMGO. It seems that the topographical location of the aromatic ring of position 3 and 4 amino acid residues can result in a completely different mode of action, producing either agonists or antagonists.

Synthesis, cannabinoid receptor activity, and enzymatic stability of reversed amide derivatives of arachidonoyl ethanolamide

Retroanandamide (2f) and its 10 analogues (1a-e, 2a-e) were synthesized and evaluated for the cannabinoid receptor activation by a [35S]GTPgammaS binding assay using rat cerebellar membranes, and Chinese hamster ovary cell membranes expressing human CB2 receptors. The primary goal of the study was to develop cannabinoid receptor agonists having improved enzymatic stability compared to endogenous N-arachidonoyl ethanolamide (AEA). Furthermore, by reversing the amide bond of AEA, the formation of arachidonic acid would be prevented. Finally, an effect of the carbonyl carbon position on the cannabinoid receptor activity was explored by synthesizing retroanandamide analogues having different chain lengths (1a-e, C19; 2a-f, C20). All the synthesized compounds, except 2c, behaved as partial agonists for the both cannabinoid receptors. In rat brain homogenate, the reversed amides possessed significantly higher stability against FAAH induced degradation than AEA. Therefore, the reversed amide analogues of AEA may serve as enzymatically stable structural basis for the drug design based on the endogenous cannabinoids.

Conformational changes associated with receptor-stimulated guanine nucleotide exchange in a heterotrimeric G-protein alpha-subunit: NMR analysis of GTPgammaS-bound states

Solution NMR studies of a (15)N-labeled G-protein alpha-subunit (G(alpha)) chimera ((15)N-ChiT)-reconstituted heterotrimer have shown previously that G-protein betagamma-subunit (G(betagamma)) association induces a "pre-activated" conformation that likely facilitates interaction with the agonist-activated form of a G-protein-coupled receptor (R*) and guanine nucleotide exchange (Abdulaev, N. G., Ngo, T., Zhang, C., Dinh, A., Brabazon, D. M., Ridge, K. D., and Marino, J. P. (2005) J. Biol. Chem. 280, 38071-38080). Here we demonstrated that the (15)N-ChiT-reconstituted heterotrimer can form functional complexes under NMR experimental conditions with light-activated, detergent-solubilized rhodopsin (R*), as well as a soluble mimic of R*. NMR methods were used to track R*-triggered guanine nucleotide exchange and release of guanosine 5'-O-3-thiotriphosphate (GTPgammaS)/Mg(2+)-bound ChiT. A heteronuclear single quantum correlation (HSQC) spectrum of R*-generated GTPgammaS/Mg(2+)-bound ChiT revealed (1)HN, (15)N chemical shift changes relative to GDP/Mg(2+)-bound ChiT that were similar, but not identical, to those observed for the GDP.AlF(4)(-)/Mg(2+)-bound state. Line widths observed for R*-generated GTPgammaS/Mg(2+)-bound (15)N-ChiT, however, indicated that it is more conformationally dynamic relative to the GDP/Mg(2+)- and GDP.AlF(4)(-)/Mg(2+)-bound states. The increased dynamics appeared to be correlated with G(betagamma) and R* interactions because they are not observed for GTPgammaS/Mg(2+)-bound ChiT generated independently of R*. In contrast to R*, a soluble mimic that does not catalytically interact with G-protein (Abdulaev, N. G., Ngo, T., Chen, R., Lu, Z., and Ridge, K. D. (2000) J. Biol. Chem. 275, 39354-39363) is found to form a stable complex with the GTPgammaS/Mg(2+)-exchanged heterotrimer. The HSQC spectrum of (15)N-ChiT in this complex displays a unique chemical shift pattern that nonetheless shares similarities with the heterotrimer and GTPgammaS/Mg(2+)-bound ChiT. Overall, these results demonstrated that R*-induced changes in G(alpha) can be followed by NMR and that guanine nucleotide exchange can be uncoupled from heterotrimer dissociation.

The structures of exocyst subunit Exo70p and the Exo84p C-terminal domains reveal a common motif

The exocyst is a large complex that is required for tethering vesicles at the final stages of the exocytic pathway in all eukaryotes. Here we present the structures of the Exo70p subunit of this complex and of the C-terminal domains of Exo84p, at 2.0-A and 2.85-A resolution, respectively. Exo70p forms a 160-A-long rod with a novel fold composed of contiguous alpha-helical bundles. The Exo84p C terminus also forms a long rod (80 A), which unexpectedly has the same fold as the Exo70p N terminus. Our structural results and our experimental observations concerning the interaction between Exo70p and other exocyst subunits or Rho3p GTPase are consistent with an architecture wherein exocyst subunits are composed of mostly helical modules strung together into long rods.

Interaction of calmodulin with the serotonin 5-hydroxytryptamine2A receptor. A putative regulator of G protein coupling and receptor phosphorylation by protein kinase C

The 5-hydroxytryptamine2A (5-HT2A) receptor is a G(q/11)-coupled serotonin receptor that activates phospholipase C and increases diacylglycerol formation. In this report, we demonstrated that calmodulin (CaM) co-immunoprecipitates with the 5-HT2A receptor in NIH-3T3 fibroblasts in an agonist-dependent manner and that the receptor contains two putative CaM binding regions. The putative CaM binding regions of the 5-HT2A receptor are localized to the second intracellular loop and carboxyl terminus. In an in vitro binding assay peptides encompassing the putative second intracellular loop (i2) and carboxyl-terminal (ct) CaM binding regions bound CaM in a Ca2+-dependent manner. The i2 peptide bound with apparent higher affinity and shifted the mobility of CaM in a nondenaturing gel shift assay. Fluorescence emission spectral analyses of dansyl-CaM showed apparent K(D) values of 65 +/- 30 nM for the i2 peptide and 168 +/- 38 nM for the ct peptide. The ct CaM-binding domain overlaps with a putative protein kinase C (PKC) site, which was readily phosphorylated by PKC in vitro. CaM binding and phosphorylation of the ct peptide were found to be antagonistic, suggesting a putative role for CaM in the regulation of 5-HT2A receptor phosphorylation and desensitization. Finally, we showed that CaM decreases 5-HT2A receptor-mediated [35S]GTPgammaS binding to NIH-3T3 cell membranes, supporting a possible role for CaM in regulating receptor-G protein coupling. These data indicate that the serotonin 5-HT2A receptor contains two high affinity CaM-binding domains that may play important roles in signaling and function.

Identification and characterisation of a novel splice variant of the human CB1 receptor

Cannabinoid ligands are implicated in many physiological processes and to date two receptors have been identified. However, a growing body of evidence exists that suggests the presence of additional receptors. Whilst cloning the previously described hCB1a, we have identified a novel variant that we call hCB1b. Characterising these two splice variants demonstrates that they have a unique pharmacological profile and that their RNA's are expressed at low levels in a variety of tissues.

Use of a native affinity ligand for the detection of G proteins by capillary isoelectric focusing with laser-induced fluorescence detection

Affinity probe capillary isoelectric focusing (CIEF) with laser-induced fluorescence was explored for detection of Ras-like G proteins. In the assay, a fluorescent BODIPY FL GTP analogue (BGTPgammaS) and G protein were incubated resulting in formation of BGTPgammaS-G protein complex. Excess BGTPgammaS was separated from BGTPgammaS-G protein complex by CIEF using a 3-10 pH gradient and detected in whole-column imaging mode. In other cases, a single point detector was used to detect zones during the focusing step of CIEF using a 2.5-5 pH gradient. In this case, analyte peaks passed the detector in approximately 5 min at an electric field of 350 V/cm. Detection during focusing allowed for more reproducible assays at shorter times but with a sacrifice in sensitivity compared to detection during mobilization. Resolution was adequate to separate BGTPgammaS-Ras and BGTPgammaS-Rab3A complexes. Formation of specific complexes was confirmed by adding GTPgammaS to samples containing BGTPgammaS-G protein. GTPgammaS competed with BGTPgammaS for G protein binding sites resulting in decreased BGTPgammaS-G protein peak heights. The concentrating effect of CIEF enabled detection limits of 30 pM.

Phosphatidylethanolamine enhances rhodopsin photoactivation and transducin binding in a solid supported lipid bilayer as determined using plasmon-waveguide resonance spectroscopy

Flash photolysis studies have shown that the membrane lipid environment strongly influences the ability of rhodopsin to form the key metarhodopsin II intermediate. Here we have used plasmon-waveguide resonance (PWR) spectroscopy, an optical method sensitive to both mass and conformation, to probe the effects of lipid composition on conformational changes of rhodopsin induced by light and due to binding and activation of transducin (G(t)). Octylglucoside-solubilized rhodopsin was incorporated by detergent dilution into solid-supported bilayers composed either of egg phosphatidylcholine or various mixtures of a nonlamellar-forming lipid (dioleoylphosphatidylethanolamine; DOPE) together with a lamellar-forming lipid (dioleoylphosphatidylcholine; DOPC). Light-induced proteolipid conformational changes as a function of pH correlated well with previous flash photolysis studies, indicating that the PWR spectral shifts monitored metarhodopsin II formation. The magnitude of these effects, and hence the extent of the conformational transition, was found to be proportional to the DOPE content. Our data are consistent with previous suggestions that lipids having a negative spontaneous curvature favor elongation of rhodopsin during the activation process. In addition, measurements of the G(t)/rhodopsin interaction in a DOPC/DOPE (25:75) bilayer at pH 5 demonstrated that light activation increased the affinity for G(t) from 64 nM to 0.7 nM, whereas G(t) affinity for dark-adapted rhodopsin was unchanged. By contrast, in DOPC bilayers the affinity of G(t) for light-activated rhodopsin was only 18 nM at pH 5. Moreover exchange of GDP for GTP gamma S was also monitored by PWR spectroscopy. Only the light-activated receptor was able to induce this exchange which was unaffected by DOPE incorporation. These findings demonstrate that nonbilayer-forming lipids can alter functionally linked conformational changes of G-protein-coupled receptors in membranes, as well as their interactions with downstream effector proteins.

Development of displacement binding and GTPgammaS scintillation proximity assays for the identification of antagonists of the micro-opioid receptor

This article describes the development of micro-opioid receptor (MOR) binding and GTPgammaS functional SPAs as improved screening tools for the identification of MOR antagonists. Opioid receptors are members of the seven-transmembrane G protein-coupled receptor (GPCR) family and are involved in the control of various aspects of human physiology, including pain, stress, reward, addiction, respiration, gastric motility, and pituitary hormone secretion. Activation of the MOR initiates intracellular signaling pathways leading to a reduction in intracellular cyclic AMP levels, inhibition of calcium channels, and activation of potassium channels resulting in a reduction of the excitability of neurons. Characterization of opioid receptor ligand binding has traditionally been accomplished through the use of low throughput filtration-based binding assays, whereas functional activity has been based upon cyclic AMP measurements or filtration-based GTPgammaS functional assays. This report describes the development of a MOR displacement binding SPA using the radiolabeled antagonist [(3)H]diprenorphine ((3)H-DPN). The assay was optimized using statistical experimental design and demonstrates the stability and robustness necessary for HTS. The assay was biased toward the identification of MOR antagonists through the addition of Na(+). Our assay conditions also minimized the phenomenon of ligand depletion, a problem commonly observed in low-volume assays using high receptor-expressing cell lines. The optimized procedure revealed (3)H-DPN affinity constants at the MOR that were consistent with results obtained using filtration methods (K(D) (SPA) = 1.89 +/- 0.24 nM, K(D) (filtration) = 1.88 +/- 0.35 nM). The binding SPA identified known opioid receptor modulators contained within the Library of Pharmacological Active Compounds (LOPAC) cassette, and the GTPgammaS scintillation proximity assay (SPA) was used to confirm the functional activity of the LOPAC antagonists acting at the MOR. Conversion of the ligand binding and GTPgammaS functional assays to a homogeneous SPA generated a simple assay with dramatically increased throughput. Data from the development and implementation of the displacement binding and GTPgammaS functional SPAs are presented.

Crystal structure of transglutaminase 3 in complex with GMP: structural basis for nucleotide specificity

Epidermal-type Transglutaminase 3 (TGase 3) is a Ca(2+)-dependent enzyme involved in the cross-linking of structural proteins required in the assembly of the cell envelope. We have recently shown that calcium-activated TGase 3, like TGase 2, can bind, hydrolyze, and is inhibited by GTP despite lacking structural homology with other GTP-binding proteins. Here we report the crystal structure determined at 2.0 A resolution of TGase 3 in complex with GMP to elucidate the structural features required for nucleotide recognition. Binding affinities for various nucleotides were found by fluorescence displacement to be as follows: guanosine 5'-3-O-(thio)triphosphate (GTPgammaS) (0.4 microm), GTP (0.6 microm), GDP (1.0 microm), GMP (0.4 microm), and ATP (28.0 microm). Furthermore, we found that GMP binds as a reversible, noncompetitive inhibitor of TGase 3 transamidation activity, similar to GTPgammaS and GDP. A genetic algorithm similarity program (GASP) approach (virtual ligand screening) identified three compounds from the Lead Quest trade mark data base (Tripos Inc.) based on superimposition of GTPgammaS, GDP, and GMP guanine nucleotides from our crystal structures to generate the minimum align flexible fragment. These three were nucleotide analogs without a phosphate group containing the minimal binding motif for TGase 3 that includes a nucleoside recognition groove. Binding affinities were measured as follows: TP349915 (K(d) = 4.1 microm), TP395289 (K(d) = 38.5 microm), TP394305 (K(d) = 1.0 mm). Remarkably, these compounds do not inhibit but instead activate TGase 3 transamidation by about 10-fold. These results suggest that the nucleotide binding pocket in TGase 3 may be exploited to either enhance or inhibit the enzymatic activity as required for different therapeutic approaches.

Structural basis for the coordinated regulation of transglutaminase 3 by guanine nucleotides and calcium/magnesium

Transglutaminase 3 (TGase 3) is a member of a family of Ca2+-dependent enzymes that catalyze covalent cross-linking reactions between proteins or peptides. TGase 3 isoform is widely expressed and is important for effective epithelial barrier formation in the assembly of the cell envelope. Among the nine TGase enzyme isoforms known in the human genome, only TGase 2 is known to bind and hydrolyze GTP to GDP; binding GTP inhibits its transamidation activity but allows it to function in signal transduction. Here we present biochemical and crystallographic evidence for the direct binding of GTP/GDP to the active TGase 3 enzyme, and we show that the TGase 3 enzyme undergoes a GTPase cycle. The crystal structures of active TGase 3 with guanosine 5'-O-(thiotriphosphate) (GTPgammaS) and GDP were determined to 2.1 and 1.9 A resolution, respectively. These studies reveal for the first time the reciprocal actions of Ca2+ and GTP with respect to TGase 3 activity. GTPgammaS binding is coordinated with the replacement of a bound Ca2+ with Mg2+ and conformational rearrangements that together close a central channel to the active site. Hydrolysis of GTP to GDP results in two stable conformations, resembling both the GTP state and the non-nucleotide bound state, the latter of which allows substrate access to the active site.

A putative consensus sequence for the nucleotide-binding site of annexin A6

Reaction-induced infrared difference spectroscopy (RIDS) has been used to investigate the nature of interactions of human annexin A6 (ANXA6) with nucleotides. RIDS results for ANXA6, obtained after the photorelease of GTP-gamma-S, ATP, or P(i) from the respective caged compounds, were identical, suggesting that the interactions between the nucleotide and ANXA6 were dominated by the phosphate groups. Phosphate-induced structural changes in ANXA6 were small and affected only seven or eight amino acid residues. The GTP fluorescent analogue, 2'(3')-O-(2,4,6-trinitrophenyl)guanosine 5'-triphosphate (TNP-GTP), quenched tryptophan fluorescence of ANXA6 when bound to the protein. A binding stoichiometry of 1 mol of nucleotide/mol ANXA6 was established with a K(D) value of 2.8 microM for TNP-GTP. The bands observed on RIDS of ANXA6 halves (e.g., N-terminal half, ANXA6a, and C-terminal half, ANXA6b) were similar to those of the whole molecule. However, their amplitudes were smaller by a factor of 2 compared to those of whole ANXA6. TNP-GTP bound to both fragments of ANXA6 with a stoichiometry of 0.5 mol/mol. However, the binding affinities of ANXA6a and ANXA6b differed from that of ANXA6. Simulated molecular modeling revealed a nucleotide-binding site which was distributed in two distinct domains. Residues K296, Y297, K598, and K644 of ANXA6 were less than 3 A from the bound phosphate groups of either GTP or ATP. The presence of two identical sequences in ANXA6 with the F-X-X-K-Y-D/E-K-S-L motif, located in the middle of ANXA6, at residues 293-301 (within ANXA6a) and at 641-649 (within ANXA6b), suggested that the F-X-X-K-Y-D/E-K-S-L motif was the putative sequence in ANXA6 for nucleotide binding.

Serine 232 of the alpha(2A)-adrenergic receptor is a protein kinase C-sensitive effector coupling switch

alpha(2)-adrenergic receptors (alpha(2)AR) couple to multiple effectors including adenylyl cyclase and phospholipase C. We hypothesized that signaling selectivity to these effectors is dynamically directed by kinase-sensitive domains within the third intracellular loop of the receptor. Substitution of Ala for Ser232, which is in the N-terminal region of this loop in the alpha(2A)AR, resulted in a receptor that was markedly uncoupled ( approximately 82% impairment) from stimulation of inositol phosphate accumulation while the capacity to inhibit adenylyl cyclase remained relatively intact. In S232A alpha(2A)AR transfected cell membranes, agonist-promoted [(35)S]GTPgammaS binding was reduced by approximately 50%. Coexpression of modified G proteins rendered insensitive to pertussis toxin revealed that the S232A receptor was uncoupled from both G(i) and G(o). S232 is a potential PKC phosphorylation site, and whole cell phosphorylation studies showed that the mutant had depressed phosphorylation compared to wild type (1.3- vs 2.1-fold/basal). Consistent with S232 directing coupling to phospholipase C, PMA exposure resulted in approximately 67% desensitization of agonist-promoted inositol phosphate accumulation without significantly affecting inhibition of adenylyl cyclase. The dominant effect of mutation or phosphorylation at this site on inositol phosphate as compared to cAMP signaling was found to most likely be due to the low efficiency of signal transduction via phospholipase C vs adenylyl cyclase. Taken together, these results indicate that S232 acts as a selective, PKC-sensitive, modulator of effector coupling of the alpha(2A)AR to inositol phosphate stimulation. This represents one mechanism by which cells route stimuli directed to multifunctional receptors to selected effectors so as to attain finely targeted signaling.

Different domains in the third intracellular loop of the GLP-1 receptor are responsible for Galpha(s) and Galpha(i)/Galpha(o) activation

It has previously been shown that the GLP-1 receptor is primarily coupled to the adenylate cyclase pathway via activation of Galpha(s) proteins. Recent studies have shown that the third intracellular loop of the receptor is important in the stimulation of cAMP production. We have studied the effect of three synthetic peptide sequences derived from the third intracellular loop of the GLP-1 receptor on signal transduction in Rin m5F cell membranes. The whole third intracellular loop strongly stimulates both pertussis toxin and cholera toxin-sensitive G proteins, while the N-terminal half exclusively stimulates cholera toxin-sensitive G proteins and the C-terminal half only stimulates pertussis toxin-sensitive G-proteins as demonstrated by measurements of GTPase activity. These data confirm that the principal stimulatory G-protein interaction site resides in the third intracellular loop, but also suggest that the GLP-1 receptor is not only coupled to the Galpha(s) but also to the Galpha(i)/Galpha(o) type of G proteins and that distinct domains within the third intracellular loop are responsible for the activation of the different G-protein subfamilies.

[Identification and isolation of GTP-binding regulator protein from plasma membranes of oocytes from the starfish Asterias amurensis]

A method for isolating a GTP-binding regulatory protein from starfish oocytes is described. The protein consists of three subunits with molecular weights of 40, 37, and about 8 kDa. It is shown that the 40-kDa subunit has a high GTPase activity and is susceptible to ADP-ribosylation by pertussis toxin. The latter property of this subunit proved to decrease upon its incubation with nonhydrolyzable GTP analogues. These data provide evidence that the plasma membrane of starfish oocytes contains a 40-kDa GTP-binding protein with properties characteristic of the alpha subunit of the inhibitory Gi protein. The role of this protein in the transmembrane signal transmission from the 1-methyladenine receptor to intracellular effectors is discussed.

Crystal structure and functional analysis of Ras binding to its effector phosphoinositide 3-kinase gamma

Ras activation of phosphoinositide 3-kinase (PI3K) is important for survival of transformed cells. We find that PI3Kgamma is strongly and directly activated by H-Ras G12V in vivo or by GTPgammaS-loaded H-Ras in vitro. We have determined a crystal structure of a PI3Kgamma/Ras.GMPPNP complex. A critical loop in the Ras binding domain positions Ras so that it uses its switch I and switch II regions to bind PI3Kgamma. Mutagenesis shows that interactions with both regions are essential for binding PI3Kgamma. Ras also forms a direct contact with the PI3Kgamma catalytic domain. These unique Ras/PI3Kgamma interactions are likely to be shared by PI3Kalpha. The complex with Ras shows a change in the PI3K conformation that may represent an allosteric component of Ras activation.

Potential role for protein kinases in regulation of bidirectional endoplasmic reticulum-to-Golgi transport revealed by protein kinase inhibitor H89

Recent evidence suggests a regulatory connection between cell volume, endoplasmic reticulum (ER) export, and stimulated Golgi-to-ER transport. To investigate the potential role of protein kinases we tested a panel of protein kinase inhibitors for their effect on these steps. One inhibitor, H89, an isoquinolinesulfonamide that is commonly used as a selective protein kinase A inhibitor, blocked both ER export and hypo-osmotic-, brefeldin A-, or nocodazole-induced Golgi-to-ER transport. In contrast, H89 did not block the constitutive ER Golgi-intermediate compartment (ERGIC)-to-ER and Golgi-to-ER traffic that underlies redistribution of ERGIC and Golgi proteins into the ER after ER export arrest. Surprisingly, other protein kinase A inhibitors, KT5720 and H8, as well as a set of protein kinase C inhibitors, had no effect on these transport processes. To test whether H89 might act at the level of either the coatomer protein (COP)I or the COPII coat protein complex we examined the localization of betaCOP and Sec13 in H89-treated cells. H89 treatment led to a rapid loss of Sec13-labeled ER export sites but betaCOP localization to the Golgi was unaffected. To further investigate the effect of H89 on COPII we developed a COPII recruitment assay with permeabilized cells and found that H89 potently inhibited binding of exogenous Sec13 to ER export sites. This block occurred in the presence of guanosine-5'-O-(3-thio)triphosphate, suggesting that Sec13 recruitment is inhibited at a step independent of the activation of the GTPase Sar1. These results identify a requirement for an H89-sensitive factor(s), potentially a novel protein kinase, in recruitment of COPII to ER export sites, as well as in stimulated but not constitutive Golgi-to-ER transport.

Volatile anesthetics modulate the binding of guanine nucleotides to the alpha subunits of heterotrimeric GTP binding proteins

The effects of volatile anesthetics on guanine nucleotide binding to the purified alpha subunits of heterotrimeric GTP binding (G) proteins were studied. At sub-anesthetic doses, halothane, isoflurane, enflurane and sevoflurane inhibit exchange of GTPgammaS for GDP bound to Galpha subunits and markedly enhance the dissociation of GTPgammaS, but fail to suppress GDPbetaS release. Nucleotide exchange from non-myristoylated Galpha(i1) is similarly inhibited in the absence of any membrane lipid or detergent. The degrees of inhibition of GDP/GTPgammaS exchange and enhancement of GTPgammaS dissociation are in the same order: alpha(i2)alpha(i1)alpha(i3)alpha(s). By contrast, Galpha(o), which is closely related to Galpha(i), is completely insensitive to anesthetics. We conclude that volatile agents, at clinically relevant doses, have a direct effect on the conformation and stability of the GTP/Mg(2+) bound state of some, but not all Galpha subunits. By destabilizing this state, volatile agents may uncouple metabotropic and other heptahelical receptors from pathways modulating neuronal excitation.

Biochemical and crystallographic characterization of a Rho effector domain of the protein serine/threonine kinase N in a complex with RhoA

The effector domain of human protein serine/threonine kinase N (PKN), an effector protein for the small GTP-binding protein Rho, was expressed and purified for protein characterization and crystallization in a complex form with human RhoA. In solution, RhoA binds to the PKN effector domain with 1:2 stoichiometry in a GTP-dependent manner. The obtained complex crystals diffract to 2.2 A resolution.

Structure of Gialpha1.GppNHp, autoinhibition in a galpha protein-substrate complex

The structure of the G protein Gialpha1 complexed with the nonhydrolyzable GTP analog guanosine-5'-(betagamma-imino)triphosphate (GppNHp) has been determined at a resolution of 1.5 A. In the active site of Gialpha1. GppNHp, a water molecule is hydrogen bonded to the side chain of Glu43 and to an oxygen atom of the gamma-phosphate group. The side chain of the essential catalytic residue Gln204 assumes a conformation which is distinctly different from that observed in complexes with either guanosine 5'-O-3-thiotriphosphate or the transition state analog GDP.AlF4-. Hydrogen bonding and steric interactions position Gln204 such that it interacts with a presumptive nucleophilic water molecule, but cannot interact with the pentacoordinate transition state. Gln204 must be released from this auto-inhibited state to participate in catalysis. RGS proteins may accelerate the rate of GTP hydrolysis by G protein alpha subunits, in part, by inserting an amino acid side chain into the site occupied by Gln204, thereby destabilizing the auto-inhibited state of Galpha.

Determination of the affinities between heterotrimeric G protein subunits and their phospholipase C-beta effectors

Phosphatidylinositide-specific phospholipase C-betas play a key role in Ca2+ signaling and are specifically activated by the alphaq family of heterotrimeric G proteins and as well as betagamma subunits. We have determined the affinity between Gbetagamma subunits and GTPgammaS and GDP-liganded Galphaq subunits on membrane surfaces, and their respective affinities to PLC-beta1, -beta2 and -beta3 effectors by fluorescence spectroscopy. We find that activation of Galphaq by GTPgammaS decreases its affinity for Gbetagamma subunits at least 36-fold compared to the GDP-liganded form, but increases its affinity for PLC-betas at least 40-200-fold depending on the PLC-beta isoform. The affinity of Galphaq(GTPgammaS) is similar for PLC-beta1 and -beta3 and 10-fold stronger for PLC-beta2, which corresponds to the reported relationship between the concentration of Galphaq(GTPgammaS) and PLC-beta activation on lipid bilayers. We find that a large portion of the PLC-beta-Galphaq association energy lies within the 400 residue C-terminal region of PLC-beta1 since truncating this region reduces its Galphaq affinity. In contrast, the isolated N-terminal region does not interact with Galphaq. Gbetagamma subunits interact with all three PLC-beta isotypes, but only showed strong binding to PLC-beta2, and activation of the three PLC-betas by Gbetagamma subunits parallels this behavior. We also tested the possibility that both Galphaq and Gbetagamma can simultaneously bind PLC-beta2. Our data argue against simultaneous binding and show that Galphaq and Gbetagamma independently regulate this effector.

Entamoeba histolytica: signaling through G proteins

The intracellular signaling pathways of Entamoeba histolytica are largely unknown. Although the expression of guanine nucleotide binding proteins (G proteins) is expected from functional studies, their biochemical characterization remains elusive in this protozoan. Using a combination of biochemical and immunological studies, we provide strong evidence for the presence of a Gs protein in amoeba. Our results strengthen our understanding of the signal transduction mechanisms in E. histolytica as potential sites of a new therapeutic strategy.

Identifying RNA minor groove tertiary contacts by nucleotide analogue interference mapping with N2-methylguanosine

Nucleotide analogue interference mapping (NAIM) is a general biochemical method that rapidly identifies the chemical groups important for RNA function. In principle, NAIM can be extended to any nucleotide that can be incorporated into an in vitro transcript by an RNA polymerase. Here we report the synthesis of 5'-O-(1-thio)-N2-methylguanosine triphosphate (m2GalphaS) and its incorporation into two reverse splicing forms of the Tetrahymena group I intron using a mutant form of T7 RNA polymerase. This analogue replaces one proton of the N2 exocyclic amine with a methyl group, but is as stable as guanosine (G) for secondary structure formation. We have identified three sites of m2GalphaS interference within the Tetrahymena intron: G22, G212, and G303. All three of these guanosine residues are known to utilize their exocyclic amino groups to participate in tertiary hydrogen bonds within the ribozyme structure. Unlike the interference pattern with the phosphorothioate of inosine (IalphaS, an analogue that deletes the N2 amine of G), m2GalphaS substitution did not cause interference at positions attributable to secondary structural stability effects. Given that the RNA minor groove is likely to be widely used for helix packing, m2GalphaS provides an especially valuable reagent to identify RNA minor groove tertiary contacts in less well-characterized RNAs.

Regulation of phospholipase C-delta by GTP-binding proteins-rhoA as an inhibitory modulator

The regulation of Phospholipase C (PLC)delta activity remains obscure. These studies show that PLCdelta1 activity is significantly enhanced by both guanosine thiotriphosphate (GTPgammaS) and Clostridium botulinum exoenzyme C3 (C3) but not by aluminium fluoride. C3 ADP ribosylated a 21-kDa protein in the PLCdelta1 preparation and Western blotting identified rhoA in these samples. RhoA acts as an inhibitory modulator of PLCdelta activity.

Differential neurotoxicity of etorphine-like opiates: lack of correlation with their ability to activate opiate receptors

The present study was undertaken to compare the neurotoxic effects of three etorphine-like opiates (etorphine, dihydroetorphine, and another derivative of oripavine) and heroin with their ability to activate opiate receptors in human neuroblastoma cell line SK-N-SH as well as in two other neuronal cell lines. Neurotoxicity was measured by using [3H]-thymidine incorporation analysis, cell viability measurement and Cytosensor microphysiometry. It was found that, in spite of the very similar molecular structures of these opiates, they displayed significant differences in cytotoxicity, with etorphine and another derivative of oripavine possessing high potency but dihydroetorphine and heroin little effect. However, neurotoxic potency of the opiates was not directly correlated to their ability to activate opioid receptors, as determined by [35S]-guanylyl-5'-O-(gamma-tho)-triphosphate binding assay. These findings provide clear evidence of differential neurotoxicity of etorphine-like opiates, and suggest that the neurotoxicity is not closely related to the molecular configuration required as opioid receptor agonist but is probably associated with the presence of a double bond in the structure.

Identification of a new intermediate state that binds but not activates transducin in the bleaching process of bovine rhodopsin

Using time-resolved low-temperature spectroscopy, we have examined whether or not bovine rhodopsin has a unique transducin-binding state, meta Ib, previously detected from chicken rhodopsin. Unlike chicken meta Ib, bovine meta Ib was detected only by detailed kinetics analysis of the bleaching process, but it was stabilized by transducin and visualized in the observed spectral changes. From the effect of GTPgammaS, it was revealed that meta Ib induced no GDP-GTP exchange reaction in transducin. Thus meta Ib is a common intermediate of vertebrate rhodopsin and transducin is activated in two steps by meta Ib and meta II.

Detection and quantitation of heterotrimeric G proteins by fluorescence resonance energy transfer

N-Methyl-3'-O-anthranoyl (mant) guanine nucleotide analogs are useful environmentally sensitive fluorescent probes for detection of heterotrimeric guanine nucleotide binding proteins. The mant derivative of GTP gamma S, mGTP gamma S, is synthesized and purified by modification of a method initially described by Hiratsuka (1983, Biochim. Biophys. Acta 742, 496-508). The binding affinity of mGTP gamma S for G proteins Gi and G(o) is comparable to that of GTP gamma S. The rate of binding is determined by the dissociation rate of the endogenously bound GDP. The large fluorescence increase observed upon mGTP gamma S binding to G protein is due, in part, to resonance energy transfer from tryptophans in the G protein to the mant guanine nucleotide. The magnitude of the fluorescence increase is dependent upon the concentration of G protein. Therefore, mGTP gamma S binding can be used to quantitate and locate G proteins during the protein purification process. This method is rapid compared to the [35S]GTP gamma S binding assay in that (i) the bound ligand does not need to be separated from the free ligand thus avoiding vacuum filtration and (ii) the time required to measure fluorescence in each sample is less than that required for scintillation counting. In addition, the use of radioactivity can be avoided. Thus, the mGTP gamma S binding assay for the detection of Gi and G(o) represents a rapid, reliable alternative to assays based on radiolabeled GTP gamma S binding or ADP-ribosylation with pertussis toxin.

Small GTP-binding proteins in the brain-corpus cardiacum-corpus allatum complex of the silkworm, Bombyx mori: involvement in the secretion of prothoracicotropic hormone

At least three GTP-binding proteins (G-proteins), 28, 25, and 21 kDa, were found in the brain-corpus cardiacum-corpus allatum complex (BR-CC-CA) of the silkworm, Bombyx mori. They bound to GTP and GDP specifically among nucleotides tested, indicating that these proteins are small G-proteins. The 25 kDa G-protein showed a cross-reactivity to anti-rab3A antibody, while it did not cross-react with anti-rhoA, rab3B, and anti-ras antibodies. On the other hand, the 28 and 21 kDa G-proteins showed no cross-reactivity to any of those antibodies tested. Immunoblot analysis using the anti-rab3A antibody demonstrated that the 25 kDa G-protein was detected preferentially in the BR-CC-CA, and to some extent in the suboesophageal ganglion, but not in the salivary gland, fat body, prothoracic gland, and oesophagus. These results suggested that the 25 kDa G-protein was a member of the rab family of G-proteins. Furthermore, 1 mM GTP gamma S capable of activating G-proteins induced BR-CC-CA to release PTTH under the conditions that stimulation of the PTTH release with hetero-trimeric G-protein was suppressed. These results indicated that the small G-proteins may possibly contribute to PTTH release in Bombyx mori.

Crystal structure of the catalytic domains of adenylyl cyclase in a complex with Gsalpha.GTPgammaS

The crystal structure of a soluble, catalytically active form of adenylyl cyclase in a complex with its stimulatory heterotrimeric G protein alpha subunit (Gsalpha) and forskolin was determined to a resolution of 2.3 angstroms. When P-site inhibitors were soaked into native crystals of the complex, the active site of adenylyl cyclase was located and structural elements important for substrate recognition and catalysis were identified. On the basis of these and other structures, a molecular mechanism is proposed for the activation of adenylyl cyclase by Gsalpha.

Crystal structure of the adenylyl cyclase activator Gsalpha

The crystal structure of Gsalpha, the heterotrimeric G protein alpha subunit that stimulates adenylyl cyclase, was determined at 2.5 A in a complex with guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS). Gsalpha is the prototypic member of a family of GTP-binding proteins that regulate the activities of effectors in a hormone-dependent manner. Comparison of the structure of Gsalpha.GTPgammaS with that of Gialpha.GTPgammaS suggests that their effector specificity is primarily dictated by the shape of the binding surface formed by the switch II helix and the alpha3-beta5 loop, despite the high sequence homology of these elements. In contrast, sequence divergence explains the inability of regulators of G protein signaling to stimulate the GTPase activity of Gsalpha. The betagamma binding surface of Gsalpha is largely conserved in sequence and structure to that of Gialpha, whereas differences in the surface formed by the carboxyl-terminal helix and the alpha4-beta6 loop may mediate receptor specificity.

Regional polysterism in the GTP-bound form of the human c-Ha-Ras protein

The backbone 1H, 13C, and 15N resonances of the c-Ha-Ras protein [a truncated version consisting of residues 1-171, Ras(1-171)] bound with GMPPNP (a slowly hydrolyzable analogue of GTP) were assigned and compared with those of the GDP-bound Ras(1-171). The backbone amide resonances of amino acid residues 10-13, 21, 31-39, 57-64, and 71 of Ras(1-171).GMPPNP, but not those of Ras(1-171).GDP, were extremely broadened, whereas other residues of Ras(1-171).GMPPNP exhibited amide resonances nearly as sharp as those of Ras(1-171). GDP. The residues exhibiting the extreme broadening, except for residues 21 and 71, are localized in three functional loop regions [loops L1, L2 (switch I), and L4 (switch II)], which are involved in hydrolysis of GTP and interactions with other proteins. From the temperature and magnetic field strength dependencies of the backbone amide resonance intensities, the extreme broadening was ascribed to the exchange at an intermediate rate on the NMR time scale. It was shown that the Ras(1-171) protein bound with GTP or GTPgammaS (another slowly hydrolyzable analogue of GTP) exhibits the same type of broadening. Therefore, it is a characteristic feature of the GTP-bound form of Ras that the L1, L2, and L4 loop regions, but not other regions, are in a rather slow interconversion between two or more stable conformers. This phenomenon, termed a "regional polysterism", of these loop regions may be related with their multifunctionality: the GTP-dependent interactions with several downstream target groups such as the Raf and RalGDS families and also with the GTPase activating protein (GAP) family. In fact, the binding of Ras(1-171).GMPPNP with the Ras-binding domain (residues 51-131) of c-Raf-1 was shown to eliminate the regional polysterism nearly completely. It was indicated, therefore, that each target/regulator selects its appropriate conformer among those presented by the "polysteric" binding interface of Ras. As the downstream target groups exhibit no apparent sequence homology to each other, it is possible that one target group prefers a conformer different from that preferred by another group. The involvement of loop L1 in the regional polysterism might suggest that the negative regulators, GAPs, bind to the polysteric binding interface (loops L2 and L4) of Ras and cooperatively select a conformer suitable for transition of the GTPase catalytic center, involving loops L1 and L4, into the highly active state.