Heterotrimeric G-protein α-Subunit Adopts a “Preactivated” Conformation When Associated with βγ-Subunits

Mapping the conformational landscape of the stimulatory heterotrimeric G protein

Heterotrimeric G proteins serve as membrane-associated signaling hubs, in concert with their cognate G-protein-coupled receptors. Fluorine nuclear magnetic resonance spectroscopy was employed to monitor the conformational equilibria of the human stimulatory G-protein α subunit (G_sα) alone, in the intact G_sαβ₁γ₂ heterotrimer or in complex with membrane-embedded human adenosine A_2A receptor (A_2AR). The results reveal a concerted equilibrium that is strongly affected by nucleotide and interactions with the βγ subunit, the lipid bilayer and A_2AR. The α1 helix of G_sα exhibits significant intermediate timescale dynamics. The α4β6 loop and α5 helix undergo membrane/receptor interactions and order-disorder transitions respectively, associated with G-protein activation. The αN helix adopts a key functional state that serves as an allosteric conduit between the βγ subunit and receptor, while a significant fraction of the ensemble remains tethered to the membrane and receptor upon activation.

Structural mechanism of G protein activation by G protein-coupled receptor

G protein-coupled receptors (GPCRs) are a family of membrane receptors that regulate physiology and pathology of various organs. Consequently, about 40% of drugs in the market targets GPCRs. Heterotrimeric G proteins are composed of α, β, and γ subunits, and act as the key downstream signaling molecules of GPCRs. The structural mechanism of G protein activation by GPCRs has been of a great interest, and a number of biochemical and biophysical studies have been performed since the late 80's. These studies investigated the interface between GPCR and G proteins and the structural mechanism of GPCR-induced G protein activation. Recently, arrestins are also reported to be important molecular switches in GPCR-mediated signal transduction, and the physiological output of arrestin-mediated signal transduction is different from that of G protein-mediated signal transduction. Understanding the structural mechanism of the activation of G proteins and arrestins would provide fundamental information for the downstream signaling-selective GPCR-targeting drug development. This review will discuss the structural mechanism of GPCR-induced G protein activation by comparing previous biochemical and biophysical studies.

A conserved phenylalanine as a relay between the α5 helix and the GDP binding region of heterotrimeric Gi protein α subunit

G protein activation by G protein-coupled receptors is one of the critical steps for many cellular signal transduction pathways. Previously, we and other groups reported that the α5 helix in the G protein α subunit plays a major role during this activation process. However, the precise signaling pathway between the α5 helix and the guanosine diphosphate (GDP) binding pocket remains elusive. Here, using structural, biochemical, and computational techniques, we probed different residues around the α5 helix for their role in signaling. Our data showed that perturbing the Phe-336 residue disturbs hydrophobic interactions with the β2-β3 strands and α1 helix, leading to high basal nucleotide exchange. However, mutations in β strands β5 and β6 do not perturb G protein activation. We have highlighted critical residues that leverage Phe-336 as a relay. Conformational changes are transmitted starting from Phe-336 via β2-β3/α1 to Switch I and the phosphate binding loop, decreasing the stability of the GDP binding pocket and triggering nucleotide release. When the α1 and α5 helices were cross-linked, inhibiting the receptor-mediated displacement of the C-terminal α5 helix, mutation of Phe-336 still leads to high basal exchange rates. This suggests that unlike receptor-mediated activation, helix 5 rotation and translocation are not necessary for GDP release from the α subunit. Rather, destabilization of the backdoor region of the Gα subunit is sufficient for triggering the activation process.

Interaction of the muscarinic acetylcholine receptor M₂ subtype with G protein Gα(i/o) isotypes and Gβγ subunits as studied with the maltose-binding protein-M₂-Gα(i/o) fusion proteins expressed in Escherichia coli

We expressed the fusion proteins of the muscarinic acetylcholine receptor M2 subtype (M2 receptor) with a maltose-binding protein (MBP) and various G protein α subunits (Gα(i1-i3/o)) at its N- and C-terminals, respectively (MBP-M2-Gα(i/o)), in Escherichia coli, and examined the effect of G protein βγ subunits (Gβγ) on the receptor-Gα interaction as assessed by agonist- and GDP-dependent [(35)S]GTPγS binding of the fusion proteins. We found that (i) Gβγ promoted both the agonist-dependent and -independent [(35)S]GTPγS binding with little effect on the guanine nucleotide-sensitive high-affinity agonist binding, (ii) the specific [(35)S]GTPγS binding activity was much greater for MBP-M2-Gα(oA) than for MBP-M2-Gα(i1-i3) in the absence of Gβγ, whereas Gβγ preferentially promoted the agonist-dependent decrease in the affinity for GDP of MBP-M2-Gα(i1-i3) rather than of MBP-M2-Gα(oA), and (iii) the proportion of agonist-dependent [(35)S]GTPγS binding was roughly 50% irrespective of species of Gα and the presence or absence of Gβγ. These results demonstrate that receptor-Gα fusion proteins expressed in E. coli could be useful for studies of receptor-G interaction.

A constitutively active Gα subunit provides insights into the mechanism of G protein activation

The activation of Gα subunits of heterotrimeric G proteins by G protein-coupled receptors (GPCRs) is a critical event underlying a variety of biological responses. Understanding how G proteins are activated will require structural and biochemical analyses of GPCRs complexed to their G protein partners, together with structure-function studies of Gα mutants that shed light on the different steps in the activation pathway. Previously, we reported that the substitution of a glycine for a proline at position 56 within the linker region connecting the helical and GTP-binding domains of a Gα chimera, designated αT*, yields a more readily exchangeable state for guanine nucleotides. Here we show that GDP-GTP exchange on αT*(G56P), in the presence of the light-activated GPCR, rhodopsin (R*), is less sensitive to the β1γ1 subunit complex than to wild-type αT*. We determined the X-ray crystal structure for the αT*(G56P) mutant and found that the G56P substitution leads to concerted changes that are transmitted to the conformationally sensitive switch regions, the α4-β6 loop, and the β6 strand. The α4-β6 loop has been proposed to be a GPCR contact site that signals to the TCAT motif and weakens the binding of the guanine ring of GDP, whereas the switch regions are the contact sites for the β1γ1 complex. Collectively, these biochemical and structural data lead us to suggest that αT*(G56P) may be adopting a conformation that is normally induced within Gα subunits by the combined actions of a GPCR and a Gβγ subunit complex during the G protein activation event.

One-step purification of a functional, constitutively activated form of visual arrestin

Desensitization of agonist-activated G protein-coupled receptors (GPCRs) requires phosphorylation followed by the binding of arrestin, a ~48 kDa soluble protein. While crystal structures for the inactive, 'basal' state of various arrestins are available, the conformation of 'activated' arrestin adopted upon interaction with activated GPCRs remains unknown. As a first step towards applying high-resolution structural methods to study arrestin conformation and dynamics, we have utilized the subtilisin prodomain/Profinity eXact™ fusion-tag system for the high-level bacterial expression and one-step purification of wild-type visual arrestin (arrestin 1) as well as a mutant form (R175E) of the protein that binds to non-phosphorylated, light-activated rhodopsin (Rho∗). The results show that both prodomain/Profinity eXact™ fusion-tagged wild-type and R175E arrestins can be expressed to levels approaching 2-3 mg/l in Luria-Bertani media, and that the processed, tag-free mature forms can be purified to near homogeneity using a Bio-Scale™ Mini Profinity eXact™ cartridge on the Profinia™ purification system. Functional analysis of R175E arrestin generated using this approach shows that it binds to non-phosphorylated rhodopsin in a light-dependent manner. These findings should facilitate the structure determination of this 'constitutively activated' state of arrestin 1 as well as the monitoring of conformational changes upon interaction with Rho∗.

The nucleotide exchange factor Ric-8A is a chaperone for the conformationally dynamic nucleotide-free state of Gαi1

Heterotrimeric G protein α subunits are activated upon exchange of GDP for GTP at the nucleotide binding site of Gα, catalyzed by guanine nucleotide exchange factors (GEFs). In addition to transmembrane G protein-coupled receptors (GPCRs), which act on G protein heterotrimers, members of the family cytosolic proteins typified by mammalian Ric-8A are GEFs for Gi/q/12/13-class Gα subunits. Ric-8A binds to Gα•GDP, resulting in the release of GDP. The Ric-8A complex with nucleotide-free Gαi1 is stable, but dissociates upon binding of GTP to Gαi1. To gain insight into the mechanism of Ric-8A-catalyzed GDP release from Gαi1, experiments were conducted to characterize the physical state of nucleotide-free Gαi1 (hereafter referred to as Gαi1[ ]) in solution, both as a monomeric species, and in the complex with Ric-8A. We found that Ric-8A-bound, nucleotide-free Gαi1 is more accessible to trypsinolysis than Gαi1•GDP, but less so than Gαi1[ ] alone. The TROSY-HSQC spectrum of [(15)N]Gαi1[ ] bound to Ric-8A shows considerable loss of peak intensity relative to that of [(15)N]Gαi1•GDP. Hydrogen-deuterium exchange in Gαi1[ ] bound to Ric-8A is 1.5-fold more extensive than in Gαi1•GDP. Differential scanning calorimetry shows that both Ric-8A and Gαi1•GDP undergo cooperative, irreversible unfolding transitions at 47° and 52°, respectively, while nucleotide-free Gαi1 shows a broad, weak transition near 35°. The unfolding transition for Ric-8A:Gαi1[ ] is complex, with a broad transition that peaks at 50°, suggesting that both Ric-8A and Gαi1[ ] are stabilized within the complex, relative to their respective free states. The C-terminus of Gαi1 is shown to be a critical binding element for Ric-8A, as is also the case for GPCRs, suggesting that the two types of GEF might promote nucleotide exchange by similar mechanisms, by acting as chaperones for the unstable and dynamic nucleotide-free state of Gα.

Trp fluorescence reveals an activation-dependent cation-pi interaction in the Switch II region of Galphai proteins

Crystal structures of Galpha(i) (and closely related family member Galpha(t)) reveal much of what we currently know about G protein structure, including changes which occur in Switch regions. Galpha(t) exhibits a low rate of basal (uncatalyzed) nucleotide exchange and an ordered Switch II region in the GDP-bound state, unlike Galpha(i), which exhibits higher basal exchange and a disordered Switch II region in Galpha(i)GDP structures. Using purified Galpha(i) and Galpha(t), we examined the intrinsic tryptophan fluorescence of these proteins, which reports conformational changes associated with activation and deactivation of Galpha proteins. In addition to the expected enhancement in tryptophan fluorescence intensity, activation of GalphaGDP proteins was accompanied by a modest but notable red shift in tryptophan emission maxima. We identified a cation-pi interaction between tryptophan and arginine residues in the Switch II of Galpha(i) family proteins that mediates the observed red shift in emission maxima. Furthermore, amino-terminal myristoylation of Galpha(i) resulted in a less polar environment for tryptophan residues in the GTPase domain, consistent with an interaction between the myristoylated amino terminus and the GTPase domain of Galpha proteins. These results reveal unique insights into conformational changes which occur upon activation and deactivation of G proteins in solution.

Structural evidence for a sequential release mechanism for activation of heterotrimeric G proteins

Heptahelical G-protein (heterotrimeric guanine nucleotide-binding protein)-coupled receptors couple to heterotrimeric G proteins to relay extracellular signals to intracellular signaling networks, but the molecular mechanism underlying guanosine 5'-diphosphate (GDP) release by the G protein alpha-subunit is not well understood. Amino acid substitutions in the conserved alpha5 helix of G(i), which extends from the C-terminal region to the nucleotide-binding pocket, cause dramatic increases in basal (receptor-independent) GDP release rates. For example, mutant Galpha(i1)-T329A shows an 18-fold increase in basal GDP release rate and, when expressed in culture, it causes a significant decrease in forskolin-stimulated cAMP accumulation. The crystal structure of Galpha(i1)-T329A.GDP shows substantial conformational rearrangement of the switch I region and additional striking alterations of side chains lining the catalytic pocket that disrupt the Mg(+2) coordination sphere and dislodge bound Mg(+2). We propose a "sequential release" mechanism whereby a transient conformational change in the alpha5 helix alters switch I to induce GDP release. Interestingly, this mechanistic model for heterotrimeric G protein activation is similar to that suggested for the activation of the plant small G protein Rop4 by RopGEF8.

Interactions between the amyloid precursor protein C-terminal domain and G proteins mediate calcium dysregulation and amyloid beta toxicity in Alzheimer's disease

Alzheimer's disease is characterized by neuropathological accumulations of amyloid beta(1-42) [A beta(1-42)], a cleavage product of the amyloid precursor protein (APP). Recent studies have highlighted the role of APP in A beta-mediated toxicity and have implicated the G-protein system; however, the exact mechanisms underlying this pathway are as yet undetermined. In this context, we sought to investigate the role of calcium upregulation following APP-dependent, A beta-mediated G-protein activation. Initial studies on the interaction between APP, A beta and Go proteins demonstrated that the interaction between APP, specifically its C-terminal -YENPTY- region, and Go was reduced in the presence of A beta. Cell death and calcium influx in A beta-treated cells were shown to be APP dependent and to involve G-protein activation because these effects were blocked by use of the G-protein inhibitor, pertussis toxin. Collectively, these results highlight a role for the G-protein system in APP-dependent, A beta-induced toxicity and calcium dysregulation. Analysis of the APP:Go interaction in human brain samples from Alzheimer's disease patients at different stages of the disease revealed a decrease in the interaction, correlating with disease progression. Moreover, the reduced interaction between APP and Go was shown to correlate with an increase in membrane A beta levels and G-protein activity, showing for first time that the APP:Go interaction is present in humans and is responsive to A beta load. The results presented support a role for APP in A beta-induced G-protein activation and suggest a mechanism by which basal APP binding to Go is reduced under pathological loads of A beta, liberating Go and activating the G-protein system, which may in turn result in downstream effects including calcium dysregulation. These results also suggest that specific antagonists of G-protein activity may have a therapeutic relevance in Alzheimer's disease.

Signal transducing membrane complexes of photoreceptor outer segments

Signal transduction in outer segments of vertebrate photoreceptors is mediated by a series of reactions among multiple polypeptides that form protein-protein complexes within or on the surface of the disk and plasma membranes. The individual components in the activation reactions include the photon receptor rhodopsin and the products of its absorption of light, the three subunits of the G protein, transducin, the four subunits of the cGMP phosphodiesterase, PDE6 and the four subunits of the cGMP-gated cation channel. Recovery involves membrane complexes with additional polypeptides including the Na(+)/Ca(2+), K(+) exchanger, NCKX2, rhodopsin kinases RK1 and RK7, arrestin, guanylate cyclases, guanylate cyclase activating proteins, GCAP1 and GCAP2, and the GTPase accelerating complex of RGS9-1, G(beta5L), and membrane anchor R9AP. Modes of membrane binding by these polypeptides include transmembrane helices, fatty acyl or isoprenyl modifications, polar interactions with lipid head groups, non-polar interactions of hydrophobic side chains with lipid hydrocarbon phase, and both polar and non-polar protein-protein interactions. In the course of signal transduction, complexes among these polypeptides form and dissociate, and undergo structural rearrangements that are coupled to their interactions with and catalysis of reactions by small molecules and ions, including guanine nucleotides, ATP, Ca(2+), Mg(2+), and lipids. The substantial progress that has been made in understanding the composition and function of these complexes is reviewed, along with the more preliminary state of our understanding of the structures of these complexes and the challenges and opportunities that present themselves for deepening our understanding of these complexes, and how they work together to convert a light signal into an electrical signal.

A Comprehensive Structure-Function Map of the Intracellular Surface of the Human C5a Receptor

Within any given cell many G protein-coupled receptors are expressed in the presence of multiple G proteins, yet most receptors couple to a specific subset of G proteins to elicit their programmed response. Numerous studies demonstrate that the carboxyl-terminal five amino acids of the Galpha subunits are a major determinant of specificity, however the receptor determinants of specificity are less clear. We have used a collection of 133 functional mutants of the C5a receptor obtained in a mutagenesis screen targeting the intracellular loops and the carboxyl terminus (Matsumoto, M. L., Narzinski, K., Kiser, P. D., Nikiforovich, G. V., and Baranski, T. J. (2007) J. Biol. Chem. 282, 3105-3121) to investigate how specificity is encoded. Each mutant, originally selected for its ability to signal through a nearly full-length Galpha(i) in yeast, was tested to see whether it could activate three versions of chimeric Galpha subunits consisting of Gpa1 fused to the carboxyl-terminal five amino acids of Galpha(i), Galpha(q), or Galpha(s) in yeast. Surprisingly the carboxyl-terminal tail of the C5a receptor is the most important specificity determinant in that nearly all mutants in this region showed a gain in coupling to Galpha(q) and/or Galpha(s). More than half of the receptors mutated in the second intracellular loop also demonstrated broadened G protein coupling. Given a lack of selective advantage for this broadened signaling in the initial screen, we propose a model in which the carboxyl-terminal tail acts together with the intracellular loops to generate a specificity filter for receptor-G protein interactions that functions primarily to restrict access of incorrect G proteins to the receptor.

Some mechanistic insights into GPCR activation from detergent-solubilized ternary complexes on beads

The binding of full and partial agonist ligands (L) to G protein-coupled receptors (GPCRs) initiates the formation of ternary complexes with G proteins [ligand-receptor-G protein (LRG) complexes]. Cyclic ternary complex models are required to account for the thermodynamically plausible complexes. It has recently become possible to assemble solubilized formyl peptide receptor (FPR) and beta(2)-adrenergic receptor (beta(2)AR) ternary complexes for flow cytometric bead-based assays. In these systems, soluble ternary complex formation of the receptors with G proteins allows direct quantitative measurements which can be analyzed in terms of three-dimensional concentrations (molarity). In contrast to the difficulty of analyzing comparable measurements in two-dimensional membrane systems, the output of these flow cytometric experiments can be analyzed via ternary complex simulations in which all of the parameters can be estimated. An outcome from such analysis yielded lower affinity for soluble ternary complex assembly by partial agonists compared with full agonists for the beta(2)AR. In the four-sided ternary complex model, this behavior is consistent with distinct ligand-induced conformational states for full and partial agonists. Rapid mix flow cytometry is used to analyze the subsecond dynamics of guanine nucleotide-mediated ternary complex disassembly. The modular breakup of ternary complex components is highlighted by the finding that the fastest step involves the departure of the ligand-activated GPCR from the intact G protein heterotrimer. The data also show that, under these experimental conditions, G protein subunit dissociation does not occur within the time frame relevant to signaling. The data and concepts are discussed in the context of a review of current literature on signaling mechanism based on structural and spectroscopic (FRET) studies of ternary complex components.

The receptor-bound "empty pocket" state of the heterotrimeric G-protein alpha-subunit is conformationally dynamic

Heterotrimeric G-protein activation by a G-protein-coupled receptor (GPCR) requires the propagation of structural signals from the receptor-interacting surfaces to the guanine nucleotide-binding pocket. To probe conformational changes in the G-protein alpha-subunit (G(alpha)) associated with activated GPCR (R*) interactions and guanine nucleotide exchange, high-resolution solution NMR methods are being applied in studying signaling of the G-protein, transducin, by light-activated rhodopsin. Using these methods, we recently demonstrated that an isotope-labeled G(alpha) reconstituted heterotrimer forms functional complexes under NMR experimental conditions with light-activated, detergent-solubilized rhodopsin and a soluble mimic of R*, both of which trigger guanine nucleotide exchange [Ridge, K. D., et al. (2006) J. Biol. Chem. 281, 7635-7648]. Here, it is shown that both light-activated rhodopsin and the soluble mimic of R form trapped intermediate complexes with a GDP-released "empty pocket" state of the heterotrimer in the absence of GTP (or GTPgammaS). In contrast to guanine nucleotide-bound forms of G(alpha), the NMR spectra of the GDP-released, R-bound empty pocket state of G(alpha) display severe line broadening suggestive of a dynamic intermediate state. Interestingly, the conformation of a GDP-depleted, Mg(2+)-bound state of G(alpha) generated in a manner independent of R* does not exhibit a similar degree of line broadening but rather appears structurally similar to the GDP/Mg(2+)-bound form of the protein. Taken together, these results suggest that R*-mediated changes in the receptor-interacting regions of G(alpha), and not the absence of bound guanine nucleotide, are the predominant factors which dictate G(alpha) conformation and dynamics in this R*-bound state of the heterotrimer.

NMR analysis of rhodopsin–transducin interactions

Heterotrimeric G-protein activation by an agonist-stimulated G-protein coupled receptor (R*) requires the propagation of structural signals from the receptor interacting surfaces to the guanine nucleotide-binding pocket. Employing high-resolution NMR methods, we are probing heterotrimer-associated and rhodopsin-stimulated changes in an isotope-labeled G-protein alpha-subunit (G(alpha)). A key aspect of the work involves the trapping and interrogation of discrete R*-bound conformations of G(alpha). Our results demonstrate that functionally important changes in G(alpha) structure and dynamics can be detected and characterized by NMR, enabling the generation of robust models for the global and local structural changes accompanying signal transfer from R* to the G-protein.

Structural and dynamical changes in an alpha-subunit of a heterotrimeric G protein along the activation pathway

The Galpha subunits of heterotrimeric G proteins (Galphabetagamma) mediate signal transduction via activation by receptors and subsequent interaction with downstream effectors. Crystal structures indicate that conformational changes in "switch" sequences of Galpha, controlled by the identity of the bound nucleotide (GDP and GTP), modulate binding affinities to the Gbetagamma subunits, receptor, and effector proteins. To investigate the solution structure and dynamics of Galphai1 through the G protein cycle, nitroxide side chains (R1) were introduced at sites in switch II and at a site in helix alpha4, a putative effector binding region. In the inactive Galphai1(GDP) state, the EPR spectra are compatible with conformational polymorphism in switch II. Upon complex formation with Gbetagamma, motions of R1 are highly constrained, reflecting direct contact interactions at the Galphai1-Gbeta interface; remarkably, the presence of R1 at the sites investigated does not substantially affect the binding affinity. Complex formation between the heterotrimer and activated rhodopsin leads to a dramatic change in R1 motion at residue 217 in the receptor-binding alpha2/beta4 loop and smaller allosteric changes at the Galphai1-Gbetagamma interface distant from the receptor binding surface. Upon addition of GTPgammaS, the activated Galphai1(GTP) subunit dissociates from the complex, and switch II is transformed to a unique conformation similar to that in crystal structures but with a flexible backbone. A previously unreported activation-dependent change in alpha4, distant from the interaction surface, supports a role for this helix in effector binding.

Signal Transfer from GPCRs to G Proteins

Catalysis of nucleotide exchange in heterotrimeric G proteins (Galphabetagamma) is a key step in cellular signal transduction mediated by G protein-coupled receptors. The Galpha N terminus with its helical stretch is thought to be crucial for G protein/activated receptor (R(*)) interaction. The N-terminal fatty acylation of Galpha is important for membrane targeting of G proteins. By applying biophysical techniques to the rhodopsin/transducin model system, we studied the effect of N-terminal truncations in Galpha. In Galphabetagamma, lack of the fatty acid and Galpha truncations up to 33 amino acids had little effect on R(*) binding and R(*)-catalyzed nucleotide exchange, implying that this region is not mandatory for R(*)/Galphabetagamma interaction. However, when the other hydrophobic modification of Galphabetagamma, the Ggamma C-terminal farnesyl moiety, is lacking, R(*) interaction requires the fatty acylated Galpha N terminus. This suggests that the two hydrophobic extensions can replace each other in the interaction of Galphabetagamma with R(*). We propose that in native Galphabetagamma, these two terminal regions are functionally redundant and form a microdomain that serves both to anchor the G protein to the membrane and to establish an initial docking complex with R(*). Accordingly, we find that the native fatty acylated Galpha is competent to interact with R(*) even in the absence of Gbetagamma, whereas nonacylated Galpha requires Gbetagamma for interaction. Experiments with N-terminally truncated Galpha subunits suggest that in the second step of the catalytic process, the receptor binds to the alphaN/beta1-loop region of Galpha to reduce nucleotide affinity and to make the Galpha C terminus available for subsequent interaction with R(*).

One-step purification of bacterially expressed recombinant transducin alpha-subunit and isotopically labeled PDE6 gamma-subunit for NMR analysis

Interactions between the transducin alpha-subunit (Galpha(t)) and the cGMP phosphodiesterase gamma-subunit (PDEgamma) are critical not only for turn-on but also turn-off of vertebrate visual signal transduction. Elucidation of the signaling mechanisms dominated by these interactions has been restrained by the lack of atomic structures for full-length Galpha(t)/PDEgamma complexes, in particular, the signaling-state complex represented by Galpha(t).GTPgammaS/PDEgamma. As a preliminary step in our effort for NMR structural analysis of Galpha(t)/PDEgamma interactions, we have developed efficient protocols for the large-scale production of recombinant Galpha(t) (rGalpha(t)) and homogeneous and functional isotopically labeled PDEgamma from Escherichia coli cells. One-step purification of rGalpha(t) was achieved through cobalt affinity chromatography in the presence of glycerol, which effectively removed the molecular chaperone DnaK that otherwise persistently co-purified with rGalpha(t). The purified rGalpha(t) was found to be functional in GTPgammaS/GDP exchange upon activation of rhodopsin and was used to form a signaling-state complex with labeled PDEgamma, rGalpha(t). GTPgammaS/[U-13C,15N]PDEgamma. The labeled PDEgamma sample yielded a well-resolved 1H-15N HSQC spectrum. The methods described here for large-scale production of homogeneous and functional rGalpha(t) and isotope-labeled PDEgamma should support further NMR structural analysis of the rGalpha(t)/PDEgamma complexes. In addition, our protocol for removing the co-purifying DnaK contaminant may be of general utility in purifying E. coli-expressed recombinant proteins.

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.