Octyl glucoside extracts GTP-binding regulatory proteins from rat brain "synaptoneurosomes" as large, polydisperse structures devoid of beta gamma complexes and sensitive to disaggregation by guanine nucleotides

Direct Capture of Functional Proteins from Mammalian Plasma Membranes into Nanodiscs

Mammalian plasma membrane proteins make up the largest class of drug targets yet are difficult to study in a cell free system because of their intransigent nature. Herein, we perform direct encapsulation of plasma membrane proteins derived from mammalian cells into a functional nanodisc library. Peptide fingerprinting was used to analyze the proteome of the incorporated proteins in nanodiscs and to further demonstrate that the lipid composition of the nanodiscs directly affects the class of protein that is incorporated. Furthermore, the functionality of the incorporated membrane proteome was evaluated by measuring the activity of membrane proteins: Na(+)/K(+)-ATPase and receptor tyrosine kinases. This work is the first report of the successful establishment and characterization of a cell free functional library of mammalian membrane proteins into nanodiscs.

Correlation of transducin photoaffinity labeling with the specific formation of intermolecular disulfide linkages in its α-subunit

Transducin (T) is a heterotrimer of Tα, Tβ, and Tγ subunits. In the presence of light-activated rhodopsin, 8-azidoguanosine triphosphate (8-N3GTP) was covalently incorporated into T in a UV-light photodependent manner, with a low stoichiometry of 0.02 mol of 8-N3GTP per mol of T. Although Tα was preferentially labeled by 8-N3GTP, Tβ and Tγ were also modified. Photolabeling of T was specifically inhibited by GDP and GTP, but not by β,γ-imido-guanosine 5'-triphosphate (GMP-PNP), indicating that 8-N3GTP was modifying the GDP binding site of the holoenzyme. This was consistent with the observation that the photoaffinity probe was completely hydrolyzed to 8-N3GDP by T activated by illuminated rhodopsin. The formation of intermolecular disulfide associations in T was also determined because photolabeling of T was performed under non-reducing conditions. We established that Cys-347 of Tα was the major residue involved in the formation of disulfide-linked T oligomers. Other cysteines of Tα, such as Cys-321, also participated in the formation of disulfide bonds, revealing a complex pattern of intermolecular disulfide cross-links that led to the polymerization of T. The spontaneous generation of these cystines in Tα inhibited the light-dependent GTPase and GMP-PNP binding activities of T. A model was constructed illustrating that when two heterotrimers dimerize through the formation of disulfide bridges between the Cys-347 of their Tα subunits, the guanine ring of the 8-N3GDP bound to one T molecule might approach to the Tβγ-complex of the other heterotrimer. This model provides an explanation for the additional photolabeling of Tβ and Tγ by 8-N3GTP.

Palmitoylation participates in G protein coupled signal transduction by affecting its oligomerization

Much in vivo and in vitro evidence has shown that the alpha subunits of heterotrimeric GTP-binding proteins (G proteins) exist as oligomers in their base state and disaggregate when being activated. In this article, the influence of palmitoylation modification of Galpha(o) on its oligomerization was explored extensively. Galpha(o) protein was expressed and purified from Escherichia coli strain JM109 cotransformed with pQE60(Galpha(o)) and pBB131(N-myristoyltransferase). Non-denaturing gel electrophoresis analysis revealed that Galpha(o) existed to a small extent as monomers but mostly as oligomers including dimers, trimers, tetramers and pentamers which could disaggregate completely into monomers by GTPgammaS stimulation. Palmitoylated Galpha(o), on the other hand, only present as oligomers that were difficult to disaggregate into monomers. The effect of palmitoylation on oligomerization of Galpha(o) was further investigated by several other biochemical and biophysical methods including gel filtration chromatography, analytical ultracentrifugation and atomic force microscopy analysis. The results consistently demonstrated that palmitoylation facilitated oligomerization of the Galpha(o) protein. Autoradiography indicated that [(14)C]-palmitoylated Galpha(o) would in no case disaggregate into monomers after treatment with GTPgammaS. [(35)S]-GTPgammaS binding activity assay showed that palmitoylated Galpha(o) was saturated at only 7.8 nmol/mg compared to 21.8 nmol/mg for non-palmitoylated Galpha(o). Fluorescent quenching studies using BODIPY FL-GTPgammaS as a probe showed that the conformation of GTP-binding domain of Galpha(o) tended to become more compact after palmitoylation. These results implied that palmitoylation may regulate the GDP/GTP exchange of Galpha(o) by influencing the oligomerization state of Galpha(o) and thereby modulate the on-off switch of the G protein in G protein-coupled signal transduction.

Kinetic analysis of M2 muscarinic receptor activation of Gi in Sf9 insect cell membranes

A steady-state kinetic mechanism describing the interaction of M(2) muscarinic acetylcholine receptors and the guanine nucleotide-binding protein G(i)alpha(2)beta(1)gamma(3) are presented. Data are consistent with two parallel pathways of agonist-promoted GTPase activity arising from receptor coupled to a single or multiple guanine nucleotide-binding proteins. An aspartate 103 to asparagine receptor mutation resulted in a receptor lacking the ability to catalyze the binding of guanosine-5'-O-(3-thiotriphosphate) or guanosine triphosphate hydrolysis by the G protein. An aspartate 69 to asparagine receptor mutant was able to catalyze agonist-specific guanine nucleotide exchange and GTPase activity. A threonine 187 to alanine receptor mutation resulted in a receptor that catalyzed guanine nucleotide exchange comparable with wild-type receptors but had reduced ability to stimulate GTP hydrolysis. A tyrosine 403 to phenylalanine receptor mutation resulted in an increase in agonist-promoted GTPAse activity compared with wild type. The observation that the threonine 187 and tyrosine 403 mutants promote guanine nucleotide exchange similarly to wild type but alter GTPase activity compared with wild type suggests that the effects of the mutations arise downstream from guanine nucleotide exchange and may result from changes in receptor-G protein dissociation.

Regulation and role of adenylyl cyclase isoforms

At least nine closely related isoforms of adenylyl cyclases (ACs), the enzymes responsible for the synthesis of cyclic AMP (cAMP) from ATP, have been cloned and characterized in mammals. Depending on the properties and the relative levels of the isoforms expressed in a tissue or a cell type at a specific time, extracellular signals received through the G-protein-coupled receptors can be differentially integrated. The present review deals with various aspects of such regulations, emphasizing the role of calcium/calmodulin in activating AC1 and AC8 in the central nervous system, the potential inhibitory effect of calcium on AC5 and AC6, and the changes in the expression pattern of the isoforms during development. A particular emphasis is given to the role of cAMP during drug and ethanol dependency and to some experimental limitations (pitfalls in the interpretation of cellular transfection, scarcity of the invalidation models, existence of complex macromolecular structures, etc).

Nuclear and cytoskeletal translocation and localization of heterotrimeric G-proteins

Heterotrimeric GTP-binding proteins (G-proteins) are involved in a diverse array of signalling pathways. They are generally thought to be membrane-bound proteins, which disassociate on receptor activation and binding of GTP. A model to explain this has been proposed, which is often described as 'the G-protein cycle'. The 'G-protein cycle' is discussed in the present paper in relation to evidence that now exists regarding the non- membranous localization of G-proteins. Specifically, the experimental evidence demonstrating association of G-proteins with the cytoskeleton and the nucleus, and the mechanisms by which G-proteins translocate to these sites are reviewed. Furthermore, the possible effector pathways and the physiological function of G-proteins at these sites are discussed.

Ligand association with the rabbit kidney and brain Y1, Y2 and Y5-like neuropeptide Y (NPY) receptors shows large subtype-related differences in sensitivity to chaotropic and alkylating agents

The binding to rabbit kidney or hypothalamic particulates of the subtype-selective neuropeptide Y (NPY) receptor ligands [125I](Leu31,Pro34)hPYY (as Y1 site label at 2 nM human pancreatic polypeptide (hPP)), [125I]-hPYY(3-36) (Y2 label), and [125I]-hPP (Y5 label) displayed great differences in sensitivity to alkylators and chaotropic agents. Sensitivity to a nonionic chaotrope, urea, was much higher for the Y1 binding than for the Y5-like binding or the Y2 binding. The non-selective alkylator N-ethylmaleimide (NEM) and several alkylators selective for aminergic receptors were much more efficacious against the Y1 relative to the Y2 binding. Similar differences could be confirmed with the attachment of Y1 and Y2-selective tracers to CHO cells expressing the cloned guinea-pig Y1 or Y2 receptors. The Y5-like binding was quite insensitive to NEM, but sensitive to chloroethylclonidine (CEC) and prazobind, which were less potent at the Y1, and especially at the Y2 site. The unrestricted-access alkylator 2-aminoethyl methanethiosulfonate inhibited the binding to all subtypes, while the restricted-access agent 2-(trimethylammonium)ethylmethanethiosulfonate poorly inhibited the Y5-like binding, or the guanine nucleotide-insensitive Y2 binding. These results are compatible with an active conformation of the Y5-like site dependent on maintenance of a shared hydrophobic cavity. The Y2 sites resistant to guanosine polyphosphates and restricted-access alkylators were detected mainly in particulates slowly solubilized by cholate at 0-5 degrees C; these sites could be clustered.

Visual transduction in Drosophila

The Drosophila phototransduction cascade has emerged as an attractive paradigm for understanding the molecular mechanisms underlying visual transduction, as well as other G protein-coupled signaling cascades that are activated and terminated with great rapidity. A large collection of mutants affecting the fly visual cascade have been isolated, and the nature and function of many of the affected gene products have been identified. Virtually all of the proteins, including those that were initially classified as novel, are highly related to vertebrate homologs. Recently, it has become apparent that most of the proteins central to Drosophila phototransduction are coupled into a supramolecular signaling complex, signalplex, through association with a PDZ-containing scaffold protein. The characterization of this complex has led to a re-evaluation of the mechanisms underlying the activation and deactivation of the phototransduction cascade.

Analysis of the N-terminal binding domain of Go alpha

Signalling from membrane receptors through heterotrimeric G-proteins (G alpha and G beta gamma) to intracellular effectors is a highly regulated process. Receptor activation causes exchange of GTP for GDP on G alpha and dissociation of G alpha from G beta gamma. Both subunits remain membrane-associated and interact with a series of other molecules throughout the cycle of activation. The N-terminal binding domain of G alpha subunits interacts with the membrane by several partially defined mechanisms: the anchoring of G alpha to the more hydrophobic G beta gamma subunits, the interaction of N-terminal lipids (palmitate and/or myristate) with the membrane, and attachment of amino acid regions to the membrane {amino acids 11-14 of Go alpha (D[11-14]); Busconi, Boutin and Denker (1997) Biochem. J. 323, 239-244}. We characterized N-terminal mutants of Go alpha with known G beta gamma-binding properties for the ability to interact with phospholipid vesicles and membranes prepared from cultured cells (acceptor membranes). In vitro analysis allows membrane interactions that are important to the activated and depalmitoylated state of G alpha to be characterized. Subcellular localization was also determined in transiently transfected COS cells. All of the mutant proteins are myristoylated, and differences in myristoylation do not account for changes in membrane binding. Disrupting the N-terminal alpha-helix of Go alpha with a proline point mutation at Arg-9 (R9P) does not affect interactions with G beta gamma on sucrose-density gradients but significantly reduces acceptor membrane binding. Deletion of amino acids 6-15 (D[6-15]; reduced G beta gamma binding) or deletion of amino acids 3-21 (D[3-21]); no detectable G beta gamma binding) further reduces acceptor membrane binding. When expressed in COS cells, R9P and D[6-15] are localized in the membrane similar to wild-type Go alpha as a result of the contribution from palmitoylation. In contrast, D[3-21] is completely soluble in COS cells, and no palmitoylation is detected. The binding of Go alpha and mutants translated in vitro to liposomes indicates that Go alpha preferentially binds to neutral phospholipids (phosphatidylcholine). R9P and D[11-14] bind to phosphatidylcholine liposomes like Go alpha, but D[6-15] exhibits no detectable binding. Taken together, these studies suggest that interactions of the N-terminus of G alpha subunits with the membrane may be affected by both membrane proteins and lipids. A detailed understanding of G alpha-membrane interactions may reveal unique mechanisms for regulating signal transduction.

Is signal transduction modulated by an interaction between heterotrimeric G-proteins and tubulin?

Although it is generally accepted that tubulin plays an important role in G-protein-mediated signal transduction in a variety of systems, the mechanism of this phenomenon is not completely understood. G-protein-tubulin interaction at the cell membrane and the cytosol, and the influence of such an interaction on cellular signaling are discussed in this review article. Because the diameter of a microtubule is 25 nm and the plasma membrane is 9-11 nm thick, it is not possible for membrane-associated tubulin to assemble into a complete microtubule in the membrane environment. However, tubulin heterodimers may be able to function in the membrane environment as individual heterodimers or as polymers arranged into short protofilaments. At the cell membrane, membrane-associated tubulin may influence hormone-receptor interaction, receptor-G-protein coupling, and G-protein-effector coupling. Structural proteins, such as tubulin, can participate in cellular signaling by communicating through physical forces. By virtue of its interaction with the submembranous network of cytoskeletal proteins, tubulin, when perturbed in one locus, can transmit large changes in conformations to other points. Thus, GTP binding to membrane-associated tubulin might lead to a conformational change in either receptors or G proteins. This may, in turn, influence the binding of an agonist to its receptor. On the other hand, in the cell cytosol, subsequent to agonist-induced translocation of G-proteins from the membrane compartment to the cytosol, G-proteins may affect microtubule formation. In GH3 and AtT-20 cells (stably expressing TRH receptor), transiently transfected with Gq alpha cDNA, soluble tubulin levels decreased in Gq alpha-transfected GH3 and AtT-20 cells, by 33% and 52%, respectively. These results suggest that G-proteins may have a direct effect on the microtubule function in vivo. Because tubulin and G-protein families are ubiquitous and highly conserved, an interaction between these two protein families may occur in vivo, and this, in turn, can have an impact on signal transduction. However, the physiological significance of this interaction remains to be demonstrated.

N-terminal binding domain of Galpha subunits: involvement of amino acids 11-14 of Galphao in membrane attachment

Heterotrimeric guanine nucleotide binding proteins (G-proteins) transmit signals from membrane receptors to a variety of intracellular effectors. G-proteins reversibly associate with components of the signal transduction system, yet remain membrane attached throughout the cycle of activation. The Galpha subunits remain attached to the plasma membrane through a combination of factors that are only partially defined. We now demonstrate that amino acids within the N-terminal domain of Galpha subunits are involved in membrane binding. We used in vitro translation, a technique widely utilized to characterize functional aspects of G-proteins, and interactions with donor-acceptor membranes to demonstrate that amino acids 11-14 of Galphao contribute to membrane binding. The membrane binding of Galphao lacking amino acids 11-14 (D[11-14]) was significantly reduced at all membrane concentrations in comparison with wild-type Galphao. Several other N-terminal mutants of Galphao were characterized as controls, and these results indicate that differences in myristoylation, palmitoylation and betagamma interactions do not account for the reduced membrane binding of D[11-14]. Furthermore, when membrane attachment of Galphao and mutants was characterized in transiently transfected 35S-labelled and [3H]myristate-labelled COS cells, amino acids 11-14 contributed to membrane binding. These studies reveal that membrane binding of Galpha subunits occurs by a combination of factors that include lipids and amino acid sequences. These regions may provide novel sites for interaction with membrane components and allow additional modulation of signal transduction.

Distinct biochemical properties of the native members of the G12 G-protein subfamily. Characterization of G alpha 12 purified from rat brain

G12 and G13 are insufficiently characterized pertussis toxin-insensitive G-proteins. Here, we describe the isolation of G alpha 12 from rat brain membranes. G alpha 12 was purified to apparent homogeneity by three steps of conventional chromatography, followed by two cycles of subunit-exchange chromatography on immobilized G subunits. Purified G alpha 12 bound guanosine 5'-[gamma-thio]triphosphate slowly and substoichiometrically. For isolation of functionally active G alpha 12, it was mandatory to use sucrose monolaurate as a detergent. Comparative studies of both rat-brain-derived members of the G12 subfamily revealed differences in the affinity of G alpha 12 and G alpha 13 for G beta gamma. G alpha 12 required a higher Mg2+ concentration for AlF4- -induced dissociation from immobilized G beta gamma than did G alpha 13. In addition, the G12 subfamily members differed in their sedimentation velocities, as determined by sucrose-density-gradient centrifugation. Analysis of sedimentation coefficients revealed a higher tendency of G12 to form supramolecular structures in comparison to G13 and other G-proteins. These G13 structures were stabilized by sucrose monolaurate, which in turn may explain the necessity for this detergent for purification of functionally active G alpha 12. Despite these distinct biochemical characteristics of G12 and G13, both purified G-proteins coupled to a recombinant thromboxane A2 (TXA2) receptor reconstituted into phospholipid vesicles. These data indicate, (1) significant differences in the biochemical properties of native members of the G12 subfamily, and (2) their specific coupling to TXA2 receptors.

Modulation of the hepatic alpha 1-adrenoceptor responsiveness by colchicine: dissociation of free cytosolic Ca(2+)-dependent and independent responses

1. The cytoskeletal depolymerizing agent, colchicine, prevents the hepatic alpha 1-adrenoceptor-mediated stimulation of respiration, H+ and Ca2+ release to the effluent perfusate, intracellular alkalosis, and glycogenolysis. Unlike the other parameters, colchicine does not perturb the alpha 1-agonist-induced stimulation of gluconeogenesis or phosphorylase 'a' activation, and enhances the increase in portal pressure response. The lack of effect of colchicine on the hepatic alpha 2-adrenoceptor-mediated effects indicates that its actions are alpha 1-specific. 2. Colchicine enhances the acute alpha 1-adrenoceptor-mediated intracellular Ca2+ mobilization and prevents the activation of protein kinase C. This differential effect on the two branches of the alpha 1-adrenoceptor signalling pathway is a distinctive feature of the colchicine action. 3. The lack of effect of colchicine in altering the alpha 1-adrenoceptor ligand binding affinity suggests that it might interact with some receptor-coupled regulatory element(s). 4. The acuteness of the colchicine effect and the ability of its isomer beta-lumicolchicine to prevent all the alpha 1-adrenoceptor-mediated responses but the increase in vascular resistance, indicate that its action cannot be merely ascribed to its effects in depolymerizing tubulin. 5. Colchicine perturbs the hepatic responses to vasoactive peptides. It enhances the vasopressin-induced rise of cytosolic free Ca2+ in isolated hepatocytes and prevents the sustained decrease of Ca2+ in the effluent perfusate. It also inhibits the stimulation of glycogenolysis, without altering the stimulation of gluconeogenesis. 6. It is concluded that there are at least two major alpha 1-adrenoceptor signalling pathways. One is colchicine-sensitive, independent of variations in free cytosolic Ca2+, and protein kinase C-dependent; the other one is colchicine-insensitive, dependent on variations in free cytosolic Ca2+, and protein kinase C-independent.

Electric fields at the plasma membrane level: a neglected element in the mechanisms of cell signalling

Membrane proteins possess certain features that make them susceptible to the electric fields generated at the level of the plasma membrane. A reappraisal of cell signalling, taking into account the protein interactions with the membrane electrostatic profile, suggests that an electrical dimension is deeply involved in this fundamental aspect of cell biology. At least three types of potentials can contribute to this dimension: (1) the potential across the compact layer of water adherent to membrane surfaces; this potential is affected by classical inducers of cell differentiation, like dimethylsulfoxide and hexamethylenebisacetamide; (2) the potential across the Gouy-Chapman double layer, which accounts for the effects of extracellular cations in the modulation of differentiation; and (3) the resting potential. This last potential and its governing ion currents can be exploited in localised mechanisms of cell signalling centred on the functional association of integrin receptors with ion channels.

Tertiary and quaternary structural changes in Gi alpha 1 induced by GTP hydrolysis

Crystallographic analysis of 2.2 angstrom resolution shows that guanosine triphosphate (GTP) hydrolysis triggers conformational changes in the heterotrimeric G-protein alpha subunit, Gi alpha 1. The switch II and switch III segments become disordered, and linker II connecting the Ras and alpha helical domains moves, thus altering the structures of potential effector and beta gamma binding regions. Contacts between the alpha-helical and Ras domains are weakened, possibly facilitating the release of guanosine diphosphate (GDP). The amino and carboxyl termini, which contain receptor and beta gamma binding determinants, are disordered in the complex with GTP, but are organized into a compact microdomain on GDP hydrolysis. The amino terminus also forms extensive quaternary contacts with neighboring alpha subunits in the lattice, suggesting that multimers of alpha subunits or heterotrimers may play a role in signal transduction.

Factors determining specificity of signal transduction by G-protein-coupled receptors. Regulation of signal transfer from receptor to G-protein

Among subfamilies of G-protein-coupled receptors, agonists initiate several cell signaling events depending on the receptor subtype (R) and the type of G-protein (G) or effector molecule (E) expressed in a particular cell. Determinants of signaling specificity/efficiency may operate at the R-G interface, where events are influenced by cell architecture or accessory proteins found in the receptor's microenvironment. This issue was addressed by characterizing signal transfer from R to G following stable expression of the alpha 2A/D adrenergic receptor in two different membrane environments (NIH-3T3 fibroblasts and the pheochromocytoma cell line, PC-12). Receptor coupling to endogenous G-proteins in both cell types was eliminated by pertussis toxin pretreatment and R-G signal transfer restored by reconstitution of cell membranes with purified brain G-protein. Thus, the receptor has access to the same population of G-proteins in the two different environments. In this signal restoration assay, agonist-induced activation of G was 3-9-fold greater in PC-12 as compared with NIH-3T3 alpha 2-adrenergic receptor transfectants. The cell-specific differences in signal transfer were observed over a range of receptor densities or G-protein concentration. The augmented signal transfer in PC-12 versus NIH-3T3 transfectants occurred despite a 2-3-fold lower level of receptors existing in the R-G-coupled state (high affinity, guanyl-5'-yl imidodiphosphate-sensitive agonist binding), suggesting the existence of other membrane factors that influence the nucleotide binding behavior of G-protein in the two cell types. Detergent extraction of PC-12 but not NIH-3T3 membranes yielded a heat-sensitive, macromolecular entity that increased 35S-labeled guanosine 5'-O-(thiotriphosphate) binding to brain G-protein in a concentration-dependent manner. These data indicate that the transfer of signal from R to G is regulated by a cell type-specific, membrane-associated protein that enhances the agonist-induced activation of G.

Nobel Lecture. Signal transduction: evolution of an idea

“In general there is no set of observations conceivable which can give enough information about the past of a system to give complete information as to its future”: Norbert Wiener. “Think simplicity; then discard it”: Alfred North Whitehead

Signal transduction: evolution of an idea

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Neutrophil chemoattractant receptors and the membrane skeleton

Signal transduction via receptors for N-formylmethionyl peptide chemoattractants (FPR) on human neutrophils is a highly regulated process which involves participation of cytoskeletal-elements. Evidence exists suggesting that the cytoskeleton and/or the membrane skeleton controls the distribution of FPR in the plane of the plasma membrane, thus controlling the accessibility of FPR to different proteins in functionally distinct domains. In desensitized cells, FPR are restricted to domains which are depleted of G proteins but enriched in cytoskeletal proteins such as actin and fodrin. Thus, the G protein signal transduction partners of FPR become inaccessible to the agonist-occupied receptor, preventing cell activation. The mechanism of interaction of FPR with the membrane skeleton is poorly understood but evidence is accumulating that suggests a direct binding of FPR (and other receptors) to cytoskeletal proteins such as actin.

Cytoskeletal regulation of chemotactic receptors: molecular complexation of N-formyl peptide receptors with G proteins and actin

Signal transduction via receptors for N-formylmethionyl peptide chemoattractants (FPR) on human neutrophils is a highly regulated process. It involves direct interaction of receptors with heterotrimeric G-proteins and may be under the control of cytoskeletal elements. Evidence exists suggesting that the cytoskeleton and/or the membrane skeleton determines the distribution of FPR in the plane of the plasma membrane, thus controlling FPR accessibility to different proteins in functionally distinct membrane domains. In desensitized cells, FPR are restricted to domains which are depleted of G proteins but enriched in cytoskeletal proteins such as actin and fodrin. Thus, the G protein signal transduction partners of FPR become inaccessible to the agonist-occupied receptor, preventing cell activation. We are investigating the molecular basis for the interaction of FPR with the membrane skeleton, and our results suggest that FPR, and possibly other receptors, may directly bind to cytoskeletal proteins such as actin.

The disaggregation theory of signal transduction revisited: further evidence that G proteins are multimeric and disaggregate to monomers when activated

We have compared the sedimentation rates on sucrose gradients of the heterotrimeric GTP-binding regulatory (G) proteins Gs, G(o), Gi, and Gq extracted from rat brain synaptoneurosomes with Lubrol and digitonin. The individual alpha and beta subunits were monitored with specific antisera. In all cases, both subunits cosedimented, indicating that the subunits are likely complexed as heterotrimers. When extracted with Lubrol all of the G proteins sedimented with rates of about 4.5 S (consistent with heterotrimers) whereas digitonin extracted 60% of the G proteins with peaks at 11 S; 40% pelleted as larger structures. Digitonin-extracted Gi was cross-linked by p-phenylenedimaleimide, yielding structures too large to enter polyacrylamide gels. No cross-linking of Lubrol-extracted Gi occurred. Treatment of the membranes with guanosine 5'-[gamma-thio]triphosphate and Mg2+ yielded digitonin-extracted structures with peak sedimentation values of 8.5 S--i.e., comparable to that of purified G(o) in digitonin and considerably larger than the Lubrol-extracted 2S structures representing the separated alpha and beta gamma subunits formed by the actions of guanosine 5'-[gamma-thio]triphosphate. It is concluded that the multimeric structures of G proteins in brain membranes are at least partially preserved in digitonin and that activation of these structures in membranes yields monomers of G proteins rather than the disaggregated products (alpha and beta gamma complexes) observed in Lubrol. It is proposed that hormones and GTP affect the dynamic interplay between multimeric G proteins and receptors in a fashion analogous to the actions of ATP on the dynamic interactions between myosin and actin filaments. Signal transduction is mediated by activated monomers released from the multimers during the activation process.

G protein binding and G protein activation by nucleotide transfer involve distinct domains on tubulin: regulation of signal transduction by cytoskeletal elements

Cytoskeletal proteins have been demonstrated to interact with elements which mediate neuronal signal transduction. This laboratory has shown that tubulin forms complexes with certain G proteins and transfers GTP to them, and such nucleotide transfer correlates well with the inhibition of adenylyl cyclase. A previous study showed that tubulin-G protein complex formation occurred at regions of tubulin which were likely to be involved in binding to other tubulin dimers during the process of microtubule polymerization [Wang, N., & Rasenick, M. M. (1991) Biochemistry 30, 10957-10965]. This study represents an attempt to investigate the regions of tubulin which orchestrate the transfer of GTP from tubulin to G protein. It is demonstrated that two monoclonal antibodies, with determinants in similar regions of alpha (DM1a) or beta (DM1b) tubulin, blocked (by 70-80%) the ability of tubulin dimers (with GppNHp bound) to promote a stable inhibition of adenylyl cyclase. Under the same conditions, a polyclonal antitubulin antibody caused only a slight reduction in adenylyl cyclase inhibition. None of the antibodies altered the inhibition of adenylyl cyclase induced by GppNHp in the absence of tubulin. Under conditions where tubulin-GppNHp inhibited synaptic membrane adenylyl cyclase, tubulin dimers with the photoaffinity GTP analog azidoanilido-GTP (32P-AAGTP) bound transferred that nucleotide to G alpha i1 on the membrane. DM1a and DM1b blocked this nucleotide transfer. Similarly, tubulin-AAGTP transferred AAGTP to purified G proteins in solution and DM1a and DM1b blocked this process as well. Despite their ability to block the activation of Gi1 by tubulin, neither antibody altered the ability of tubulin to bind to native Gi alpha affixed to nitrocellulose.(ABSTRACT TRUNCATED AT 250 WORDS)

The alpha subunit of the stimulatory guanine-nucleotide-binding regulatory protein forms high-molecular-mass aggregates, concomitant with iloprost-induced desensitization of human platelet adenylyl cyclase

Prolonged treatment of human platelets with the prostacyclin analog iloprost led to desensitization of the response to various prostaglandin derivatives. However, basal adenylyl cyclase activity and stimulation by agents acting directly via Gs, the stimulatory guanine-nucleotide-binding regulatory protein of adenylyl cyclase, were likewise decreased. Reconstitution of desensitized membranes with purified Gs from turkey erythrocytes indicated no alteration in the catalyst itself. However, the function of Gs (in cholate extracts) appeared to be severely impaired when reconstituted with adenylyl cyclase catalyst. Modification of Gs was also indicated by its altered sedimentation in sucrose density gradients. From Western blots, the alpha subunit of Gs, alpha s, from control platelets sedimented as a 5.6S species, while that from desensitized cells appeared at higher S values (in a polydisperse distribution). Activation by guanosine 5'-[gamma-thio]triphosphate of Gs from control platelets shifted alpha s to 3.5-3.7S, while activation of Gs from desensitized platelets induced such shift only for a minor portion of alpha s. This small fraction alone appeared to be susceptible to ADP-ribosylation by cholera toxin/[32P]NAD. Furthermore, an antibody directed against the C-terminal hexadecapeptide of alpha s precipitated much less alpha s from cholate extracts derived from desensitized platelets. Modification of alpha s during desensitization was also suggested from cross-linking experiments using the homobifunctional agent bismaleimidohexane: alpha s from desensitized platelets formed a single product of 80 kDa, while that from untreated platelets yielded a doublet (100 kDa and 110 kDa).

Heterotrimeric G proteins in synaptoneurosome membranes are crosslinked by p-phenylenedimaleimide, yielding structures comparable in size to crosslinked tubulin and F-actin

We have treated rat brain synaptoneurosomes with the crosslinking agent N,N'-1,4-phenylenedimaleimide under conditions that cause extensive crosslinking of tubulin, F-actin, and the alpha and beta subunits of three major types of heterotrimeric GTP-binding regulatory proteins (G(o), Gs, Gi) present in brain membranes. The major crosslinked products are coeluted from Bio-Gel sizing columns as very large structures that do not penetrate stacking gels during SDS/PAGE. The alpha subunits but not the beta subunits of Gs, G(o) and Gi also yield crosslinked products of intermediate sizes. None of the products are as small as the heterotrimeric G proteins extracted from brain by cholate or Lubrol. However, the large and intermediate crosslinked structures are strikingly similar to the large, polydisperse structures of the alpha subunits of Gs, Gi, and G(o) extracted from synaptoneurosomes by the detergent octyl glucoside, which have sedimentation properties of multimeric proteins. Several ways in which multimeric forms of G proteins can explain the dynamic and pleiotropic actions of hormones and GTP on signal-transducing systems are discussed.

Dissociation of guanosine 5'-[gamma-thio]triphosphate from guanine-nucleotide-binding regulatory proteins in native cardiac membranes. Regulation by nucleotides and muscarinic acetylcholine receptors

Binding of the poorly hydrolyzable GTP analog, guanosine 5'-[gamma-thio]triphosphate (GTP[S]), to purified guanine-nucleotide-binding regulatory proteins (G proteins) has been shown to be nonreversible in the presence of millimolar concentrations of Mg2+. In porcine atrial membranes, binding of [35S]GTP[S] to G proteins was stable in the presence of 1 mM Mg2+. However, either large dilution or, even more strongly, addition of unlabelled guanine nucleotides, in the potency order, GTP[S] greater than GTP greater than or equal to guanosine 5'-[beta,gamma-imino]triphosphate greater than GDP greater than or equal to guanosine 5'-[beta-thio]diphosphate greater than GMP, markedly enhanced the observed dissociation, with 20-30% of bound [35S]GTP[S] being released by unlabelled guanine nucleotide within 20 min at 25 degrees C. Most interestingly, dissociation of [35S]GTP[S] was rapidly and markedly stimulated by agonist (carbachol) activation of cardiac muscarinic acetylcholine receptors. Carbachol-stimulated release of [35S]GTP[S] was strictly dependent on the presence of Mg2+ and an unlabelled guanine nucleotide. Although having different potency and efficiency in releasing [35S]GTP[S] from the membranes by themselves, the guanine nucleoside triphosphates and diphosphates studied, at maximally effective concentrations, promoted the carbachol-induced dissociation to the same extent, while GMP and ATP were ineffective. GTP[S]-binding-saturation experiments indicated that one agonist-activated muscarinic acetylcholine receptor can cause release of bound GTP[S] from three to four G proteins. The data presented indicate that binding of GTP[S] to G proteins in intact membranes, in contrast to purified G proteins, is reversible, and that agonist-activated receptors can even, either directly or indirectly, interact with GTP[S]-bound G proteins, resulting in release of bound guanine nucleoside triphosphate.

Amino acids 367-376 of the Gs alpha subunit induce membrane association when fused to soluble amino-terminal deleted Gi1 alpha subunit

Signal transduction GTP-binding proteins are tightly associated with plasma membrane. In the resting state, the anchorage of the alpha subunit could be indirect by means of the other beta gamma subunits or polydisperse multimers. In the activated state, although the alpha subunit is dissociated from other subunits, it is not released from the membrane and therefore is likely to contain information necessary to remain associated with the plasma membrane. Previous proteolytic experiments suggested that, in contrast to other G proteins alpha subunits, the C-terminal domain of Gs alpha (the G protein involved in adenylate cyclase stimulation) is essential for membrane association of the activated form. To better define the crucial residues involved in membrane attachment, we constructed chimeras between a soluble core and various parts of the Gs alpha C-terminal domain. We first deleted codons 2-6 of Gi1 alpha (the inhibitory G protein of the i1 subtype) to generate a soluble GTP-binding protein, delta N-Gi1 alpha. We then replaced the last 14 C-terminal codons of delta N-Gi1 alpha by different domains of the Gs alpha C terminus and looked for the membrane association of chimeric proteins after in vitro transcription, in vitro translation, and interaction with S49 cyc- membranes (obtained from a mutant cell line that does not express Gs alpha). Our results showed that addition of amino acids 367-376 of Gs alpha is sufficient to promote membrane association of the soluble N-terminal deleted Gi1 alpha.

Tubulin-G protein interactions involve microtubule polymerization domains

It has been suggested that elements of the cytoskeleton contribute to the signal transduction process and that they do so in association with one or more members of the signal-transducing G protein family. Relatively high-affinity binding between dimeric tubulin and the alpha subunits of Gs and Gi1 has also been reported. Tubulin molecules, which exist in solution as alpha beta dimers, have binding domains for microtubule-associated proteins as well as for other tubulin dimers. This study represents an attempt to ascertain whether the association between G proteins and tubulin occurs at one of these sites. Removal of the binding site for MAP2 and tau from tubulin by subtilisin proteolysis did not influence the association of tubulin with G protein, as demonstrated in overlay studies with [125I]tubulin. A functional consequence of that association, the stable inhibition of synaptic membrane adenylyl cyclase, was also unaffected by subtilisin treatment of tubulin. However, ring structures formed from subtilisin-treated tubulin were incapable of effecting such inhibition. Stable G protein-tubulin complexes were formed, and these were separated from free tubulin by Octyl-Sepharose chromatography. Using this methodology, it was demonstrated that assembled microtubules bound G protein quite weakly compared with tubulin dimers. The alpha subunit of Gi1 and, to a lesser extent, that of Go were demonstrated to inhibit microtubule polymerization. In aggregate, these data suggest that dimeric tubulin binds to the alpha subunits of G protein at the sites where it binds to other tubulin dimers during microtubule polymerization. Interaction with signal-transducing G proteins, thus, might represent a role for tubulin dimers which is independent of microtubule formation.

Colchicine inhibits acetylcholine receptor stimulation of G protein GTPase activity in rat striatum

Colchicine, which is known to influence tubulin function, was used to delineate a possible role of tubulin in signal transduction in the rat striatal membranes. Low Km GTPase activity (EC 3.6.1.-) was assayed in 10 micrograms membrane protein using [gamma-32P]GTP at 37 degrees in an ATP-regenerating buffer containing 1 microM unlabeled GTP. At 10 and 100 microM, colchicine inhibited the GTPase activity by 18% and 40%, respectively. Colchicine (100 microM) inhibited the enzymatic activity by 30-40% at all the time points the reaction was monitored. Acetylcholine (ACh) stimulated the low Km GTPase activity in a concentration-dependent manner, by up to 57%. ACh-stimulated activity was accepted as reflecting GTP hydrolysis catalyzed by receptor-coupled transducer G proteins. In the presence of 100 microM colchicine, the ability of ACh to stimulate G protein GTPase activity was inhibited. For example, at 10 microM ACh the enzyme activity was stimulated up to 52%; in the presence of 100 microM colchicine, 10 microM ACh stimulated the activity by only up to 33%. These results suggest that colchicine disrupts ACh receptor-G protein coupling as a result of its interaction with tubulin or G protein(s) or both.

Glucagon induces disaggregation of polymer-like structures of the alpha subunit of the stimulatory G protein in liver membranes

The hydrodynamic behavior of G alpha s, the alpha subunit of the stimulatory guanine nucleotide-binding regulatory protein (G protein), in octyl glucoside extracts of rat liver membranes was investigated. As was previously shown for G proteins similarly extracted from brain synaptoneurosomes, G alpha s behaved as polydisperse structures with S values higher than that of heterotrimeric G proteins. At concentrations of guanosine 5'-[gamma-thio]triphosphate (GTP[gamma S]) greater than 100 microM, incubation with membranes led to smaller structures having S values in the range of 4-5 S. Incubation of liver membranes with glucagon also caused a marked increase in structures having these S values; glucagon action required the presence of low concentrations of GTP[gamma S] (maximal, 10 microM), was rapid (within 10 sec), and was not observed with vasopressin, angiotensin II, or glucagon-(19-29). When G alpha s in its membrane-bound form was [32P]ADP-ribosylated by cholera toxin and the treated membranes were extracted with octyl glucoside, greater than 35% of the labeled G alpha s was found in material that sedimented through sucrose gradients and contained relatively low levels of immunoreactive G alpha s. Glucagon selectively converted the apparently large molecular weight structures to the 4-5 S structures in the presence of GTP[gamma S], even at 1 mM (the maximal effect of the nucleotide alone), when incubated with the toxin-treated membranes. These findings suggest that the glucagon receptor selectively interacts with polymer-like structures of G alpha s and that activation by GTP[gamma S] results in disaggregation. The role of the beta and gamma subunits of G proteins in the hormone-induced process is not clear since the polymer-like structures extracted with octyl glucoside are devoid of beta and gamma subunits.

Gap junctional permeability is affected by cell volume changes and modulates volume regulation

Isolated pancreatic acinar cell pairs became electrically uncoupled by exposure to a mild hypotonic shock. Reduction of bath osmolarity caused a delayed closure of gap junctional channels in the minute range. Dialysis of cell pairs by GTP[S] in the double whole-cell patch-clamp mode shortened the latency and shifted the hypotonically induced electrical uncoupling to lower osmolarity changes. Cellular treatment with cytochalasin B catalyzed electrical uncoupling by a hypotonic shock. In all cases, electrical uncoupling could be blocked completely by the protein kinase C (PKC) inhibitor polymyxin B. These results provide the first evidence suggesting that changes of cell volume and gap junctional permeability are correlated and that a G-protein dependent mechanism is involved. Evidence is presented that gap junctional coupling modulates volume regulation.