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

Glucagon Receptor Antagonism Ameliorates Progression of Heart Failure

Mice were treated with a fully human monoclonal glucagon receptor antagonistic antibody REMD2.59 following myocardial infarction or pressure overload. REMD2.59 treatment blunted cardiac hypertrophy and fibrotic remodeling, and attenuated contractile dysfunction at 4 weeks after myocardial infarction. In addition, REMD2.59 treatment at the onset of pressure overload significantly suppressed cardiac hypertrophy and chamber dilation with marked preservation of cardiac systolic and diastolic function. Initiation of REMD2.59 treatment 2 weeks after pressure overload significantly blunted the progression of cardiac pathology. These results provide the first in vivo proof-of-concept evidence that glucagon receptor antagonism is a potentially efficacious therapy to ameliorate both onset and progression of heart failure.

Ligand-Induced Coupling between Oligomers of the M2 Receptor and the Gi1 Protein in Live Cells

Uncertainty over the mechanism of signaling via G protein-coupled receptors (GPCRs) relates in part to questions regarding their supramolecular structure. GPCRs and heterotrimeric G proteins are known to couple as monomers under various conditions. Many GPCRs form oligomers under many of the same conditions, however, and the biological role of those complexes is unclear. We have used dual-color fluorescence correlation spectroscopy to identify oligomers of the M₂ muscarinic receptor and of G_i1 in purified preparations and live Chinese hamster ovary cells. Measurements on differently tagged receptors (i.e., eGFP-M₂ and mCherry-M₂) and G proteins (i.e., eGFP-Gα_i1β₁γ₂ and mCherry-Gα_i1β₁γ₂) detected significant cross-correlations between the two fluorophores in each case, both in detergent micelles and in live cells, indicating that both the receptor and G_i1 can exist as homo-oligomers. Oligomerization of differently tagged G_i1 decreased upon the activation of co-expressed wild-type M₂ receptor by an agonist. Measurements on a tagged M₂ receptor (M₂-mCherry) and eGFP-Gα_i1β₁γ₂ co-expressed in live cells detected cross-correlations only in the presence of an agonist, which therefore promoted coupling of the receptor and the G protein. The effect of the agonist was retained when a fluorophore-tagged receptor lacking the orthosteric site (i.e., M₂(D103A)-mCherry) was co-expressed with the wild-type receptor and eGFP-Gα_i1β₁γ₂, indicating that the ligand acted via an oligomeric receptor. Our results point to a model in which an agonist promotes transient coupling of otherwise independent oligomers of the M₂ receptor on the one hand and of G_i1 on the other and that an activated complex leads to a reduction in the oligomeric size of the G protein. They suggest that GPCR-mediated signaling proceeds, at least in part, via oligomers.

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 modification of Galpha(o) depresses its susceptibility to GAP-43 activation

Interaction between GAP-43 (growth associated protein-43) and Galpha(o) (alpha subunit of Go protein) influences the signal transduction pathways leading to differentiation of neural cells. GAP-43 is known to increase guanine nucleotide exchange by Galpha(o), which is a major component of neuronal growth cone membranes. However, it is not clear whether GAP-43 stimulation is related to the Galpha(o) palmitoylation or the conversion of Galpha(o) from oligmers to monomers, which was shown to be a necessary regulatory factor in GDP/GTP exchange of Galpha(o). Here we expressed and purified GAP-43, GST-GAP-43 and Galpha(o) proteins, detected their stimulatory effect on [(35)S]-GTPgammaS binding of Galpha(o). It was found that the EC(50) of both GAP-43 and GST-GAP-43 activation were tenfold lower in case of depalmitoylated Galpha(o) than palmitoylated Galpha(o). Non-denaturing gel electrophoresis and p-PDM cross-linking analysis revealed that addition of GST-GAP-43 induced disassociation of depalmitoylated Galpha(o) from oligomers to monomers, but did not influence the oligomeric state of palmitoylated Galpha(o), which suggests that palmitoylation is a key regulatory factor in GAP-43 stimulation on Galpha(o). These results indicated the interaction of GAP-43 and Galpha(o) could accelerate conversion of depalmitoylated Galpha(o) but not palmitoylated Galpha(o) from oligomers to monomers, so as to increase the GTPgammaS binding activity of Galpha(o). Results here provide new evidence about how signaling protein palmitoylation is involved in the G-protein-coupled signal transduction cascade, and give a useful clue on the participation of GAP-43 in G-protein cycle by its preferential activation of depalmitoylated Galpha(o).

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.

Interaction of glyceraldehyde-3-phosphate dehydrogenase in the light-induced rod alpha-transducin translocation

The light-dependent subcellular translocation of rod alpha-transducin (GNAT-1, or rod Talpha) has been well documented. In dark-adapted animals, rod Talpha (rTalpha) is predominantly located in the rod outer segment (ROS) and translocates into the rod inner segment (RIS) upon exposure to the light. Neither the molecular participants nor the mechanism(s) involved in this protein trafficking are known. We hypothesized that other proteins must interact with rTalpha to affect the translocations. Using the MBP-rTalpha fusion pulldown assay, the yeast two-hybrid assay and the co-immunoprecipitation assay, we identified glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and rTalpha as interacting proteins. Immunoprecipitation also showed beta-actin associates with rTalpha in the dark but not in the light. To further investigate the involvement of GAPDH in light-induced rod Talpha translocation, GAPDH mRNA was knocked down in vivo by transient expression of siRNAs in rat photoreceptor cells. Under completely dark- and light-adapted conditions, the translocation of rTalpha was not significantly different within the 'GAPDH knock-down photoreceptor cells' compared to the non-transfected control cells. However, under partial dark-adaptation, rTalpha translocated more slowly in the 'GAPDH knock-down cells' supporting the conclusion that GAPDH is involved in rTalpha translocation from the RIS to the ROS during dark adaptation.

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.

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.

Rate zonal sedimentation of proteins in one hour or less

Rate zonal sedimentation gives information about the shape and size of proteins, and is useful for investigating protein-protein interactions. However, rate zonal sedimentation experiments typically last approximately 1 day. In contrast, this report describes a rate zonal sedimentation method requiring 1 h or less. This was accomplished by centrifuging small density gradients (200 microliters) prepared with sucrose or OptiPrep in a fixed-angle rotor at high relative centrifugal force. By using small gradient volumes, the sample dilution that occurs with larger gradients and with many chromatographic techniques was also avoided. For a variety of proteins, plots of S20,w versus distance sedimented during centrifugation in a TLA 120.2 rotor were linear. As a practical application, sedimentation of the heterotrimeric stimulatory G protein and its dissociated alpha-subunit were determined. The results were similar to those obtained with 17- to 22-h centrifugations in an SW 50.1 rotor and agreed with previously published values. Long periods of centrifugation might preclude the study of some unstable proteins or the investigation of protein-protein interactions whose affinities are to low to survive the lengthy centrifugations required to carry out traditional rate zonal sedimentation experiments. A rate zonal sedimentation technique that rivals many chromatographic methods in celerity will help to circumvent these problems.

G protein mechanisms: insights from structural analysis

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

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.

The photoactivatable NAD+ analogue [32P]2-azido-NAD+ defines intra- and inter-molecular interactions of the C-terminal domain of the G-protein G alpha t

Recently, we reported the synthesis and use of [32P]2-azido-NAD+ as a probe to study the structural organization of G-proteins. Pertussis toxin was used to 'tether' [32P]2-azido-ADP-ribose of [32P]2-azido-NAD+ to Cys347 of the alpha subunit of the G-protein Gt. Light activation of the azide moiety covalently cross-linked the domain containing Cys347 at the C-terminus of alpha t with neighbouring intra- and inter-molecular domains of holo-transducin. The radiolabel from [32P]2-azido-ADP-ribose was then transferred to the 'acceptor' domain by cleaving the thioglycosidic bond between Cys347 and [32P]2-azido-ADP- ribose with mercuric acetate. ADP-ribosylation followed by photocross-linking of holo-transducin indicated intramolecular interactions of the C-terminal domain with other alpha t domains and intermolecular interactions with holotransducin alpha and gamma subunits. The radiolabelled peptides, which were radiolabelled because of the transfer of the photoactive moiety, were identified by utilizing 2-(2'-nitrophenylsulphenyl)-3-methyl-3'- bromoindolenine ('BNPS-skatole') and CNBr. The results indicate that the C-terminus of alpha t interacts with both N-terminal and C-terminal domains within the alpha t molecular. Mapping the interacting sites between cross-linked alpha dimers and alpha trimers indicates that the C-terminal domain of alpha t is involved in the formation of alpha t homopolymers in solution. In addition, our studies place the beta gamma subunit in close proximity to Cys347 of alpha t, as indicated by the transfer of [32P]2-azido-ADP-ribose from Cys347 to the gamma subunit, which was further localized to the C-terminal half of gamma t. The studies presented here identify the C-terminal intra- and inter-molecular interactions of the alpha subunit of holo-transducin.

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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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.

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.