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

Structural aspects of heterotrimeric G-protein signaling

Recently, structures of heterotrimeric G-protein subunits have been determined in isolation, in conjunction with each other, and in complex with their regulators. Along with biochemical information, these structures suggest how G-protein subunits are oriented relative to the membrane surface and relative to seven-transmembrane helix receptors. They also suggest mechanisms for receptor-catalyzed nucleotide exchange.

Molecular discrimination between Campylobacter jejuni, Campylobacter coli, Campylobacter lari and Campylobacter upsaliensis by polymerase chain reaction based on a novel putative GTPase gene

Polymerase chain reaction (PCR) mediated DNA fingerprinting has resulted in the identification of a novel Campylobacter jejuni gene, encoding a GTPase protein. The gene, consisting of 383 amino acids contained semi-conserved GTP-binding sites (designated G-1 to G-4), that are characteristic for members of the GTPase protein superfamily. Remarkably, this gene from C. Jejuni appears to encode a member of a novel family of GTP-binding proteins, containing two separate putative GTP-binding domains, each comprising a series of semi-conserved GTP-binding motifs. Spacing between these motifs is highly conserved. Based on this novel gene, a general PCR strategy for the identification of C. jejuni, C. coli, C. lari and C. upsaliensis was developed. PCR primers were deduced from GTP-binding motifs G-1 and G-3 of the first GTP-binding domain. These GTP-binding sites flank a variable region of precisely 117 bp in the four Campylobacter spp. that allowed the development of species-specific probes. This PCR-hybridization assay offers a novel tool for rapid molecular detection and specific identification of the thermophilic Campylobacter spp.

Conditional activation defect of a human Gsalpha mutant

Hormonal signals activate trimeric G proteins by promoting exchange of GTP for GDP bound to the G protein's alpha subunit (Galpha). Here we describe a point mutation that impairs this activation mechanism in the alpha subunit of Gs, producing an inherited disorder of hormone responsiveness. Biochemical analysis reveals an activation defect that is paradoxically intensified by hormonal and other stimuli. By substituting histidine for a conserved arginine residue, the mutation removes an internal salt bridge (to a conserved glutamate) that normally acts as an intramolecular hasp to maintain tight binding of the gamma-phosphate of GTP. In its basal, unperturbed state, the mutant alphas binds guanosine 5'-[gamma-thio]triphosphate (GTP[gammaS]), a GTP analog, slightly less tightly than does normal alphas, but (in the GTP[gammaS]-bound form) can stimulate adenylyl cyclase. The activation defect becomes prominent only under conditions that destabilize binding of guanine nucleotide (receptor stimulation) or impair the ability of alphas to bind the gamma-phosphate of GTP (cholera toxin, AlF4- ion). Although GDP release is usually the rate-limiting step in nucleotide exchange, the biochemical phenotype of this mutant alphas indicates that efficient G protein activation by receptors and other stimuli depends on the ability of Galpha to clasp tightly the GTP molecule that enters the binding site.

Biochemical characteristics of a rice (Oryza sativa L., IR36) G-protein alpha-subunit expressed in Escherichia coli

A cDNA encoding the alpha-subunit of the heterotrimeric G-protein in rice (RGA1) was overexpressed in Escherichia coli and then isolated by Ni2+-nitrilotriacetic acid affinity chromatography. The molecular mass of RGA1 bearing a His tag was approx. 49 kDa. Immunoblot analysis using anti-RGA1 revealed that the RGA1 protein is most abundant in seedling leaves and least abundant in mature roots. It exists at particularly high levels in the immature embryo after pellicle extrusion. In addition, the RGA1 antiserum exhibited a difference in binding affinity for Galpha proteins from monocots (maize and rice) and dicots (Arabidopsis, pea, soya bean and tomato); whereas it cross-reacted with Galpha proteins of monocots, it did not with those of dicot plants. When bound to guanosine 5'-(gamma-thio)triphosphate (GTP[S]), the RGA1 protein was partially protected from tryptic proteolysis. In the presence of GTP[S], trypsin cleaved the RGA1 protein into four fragments 24, 14, 11 and 5 kDa in size. When RGA1 was bound to GDP, only the 5 kDa polypeptide was seen on SDS/PAGE after trypsin digestion. Photoaffinity labelling with [alpha-32P]GTP and a GTP[S]-binding assay revealed that RGA1 incorporated 32P and showed specific binding to a guanine nucleotide. Guanidine binding of RGA1 was affected by the concentration of MgCl2 (maximum at 2 mM). The rate of guanine nucleotide binding of RGA1 (kon,GTP[S]=0.0141+/-0.0014 min-1) and, at steady state, the kcat value for GTP hydrolysis (0.0075+/-0.0001 min-1) were very low even at 2 mM MgCl2. The binding affinity for the nucleotides examined was in the order GTP-S- >/= GTP > GDP > CTP > ATP >/= dTTP.

Structure of RGS4 bound to AlF4--activated G(i alpha1): stabilization of the transition state for GTP hydrolysis

RGS proteins are GTPase activators for heterotrimeric G proteins. We report here the 2.8 A resolution crystal structure of the RGS protein RGS4 complexed with G(i alpha1)-Mg2+-GDP-AlF4 . Only the core domain of RGS4 is visible in the crystal. The core domain binds to the three switch regions of G(i alpha1), but does not contribute catalytic residues that directly interact with either GDP or AlF4-. Therefore, RGS4 appears to catalyze rapid hydrolysis of GTP primarily by stabilizing the switch regions of G(i alpha1), although the conserved Asn-128 from RGS4 could also play a catalytic role by interacting with the hydrolytic water molecule or the side chain of Gln-204. The binding site for RGS4 on G(i alpha1) is also consistent with the activity of RGS proteins as antagonists of G(alpha) effectors.

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.

A new family of G-protein regulators - the RGS proteins

Genetic experiments have recently been used to identify a family of 'regulator of G-protein signaling' (RGS) proteins, which downregulate signaling by heterotrimeric G proteins. The first biochemical studies of RGS proteins have shown that they accelerate the GTPase activities of G-protein alpha subunits, thus driving G proteins into their inactive GDP-bound forms. The physiological significance of the large number of different RGS proteins remains to be explored.

G protein beta gamma subunits

Guanine nucleotide binding (G) proteins relay extracellular signals encoded in light, small molecules, peptides, and proteins to activate or inhibit intracellular enzymes and ion channels. The larger G proteins, made up of G alpha beta gamma heterotrimers, dissociate into G alpha and G beta gamma subunits that separately activate intracellular effector molecules. Only recently has the G beta gamma subunit been recognized as a signal transduction molecule in its own right; G beta gamma is now known to directly regulate as many different protein targets as the G alpha subunit. Recent X-ray crystallography of G alpha, G beta gamma, and G alpha beta gamma subunits will guide the investigation of structure-function relationships.

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+.

Functional and structural complexity of signal transduction via G-protein-coupled receptors

A prerequisite for the maintenance of homeostasis in a living organism is fine-tuned communication between different cells. The majority of extracellular signaling molecules, such as hormones and neurotransmitters, interact with a three-protein transmembrane signaling system consisting of a receptor, a G protein, and an effector. These single components interact sequentially and reversibly. Considering that hundreds of G-protein-coupled receptors interact with a limited repertoire of G proteins, the question of coupling specificity is worth considering. G-protein-mediated signal transduction is a complex signaling network with diverging and converging transduction steps at each coupling interface. The recent realization that classical signaling pathways are intimately intertwined with growth-factor-signaling cascades adds another level of complexity. Elaborate studies have significantly enhanced our knowledge of the functional anatomy of G-protein-coupled receptors, and the concept has emerged that receptor function can be modulated with high specificity by coexpressed receptor fragments. These results may have significant clinical impact in the future.

Structure of the GDP-Pi complex of Gly203-->Ala gialpha1: a mimic of the ternary product complex of galpha-catalyzed GTP hydrolysis

BACKGROUND

G proteins play a vital role in transmembrane signalling events. In their inactive form G proteins exist as heterotrimers consisting of an alpha subunit, complexed with GDP and a dimer of betagamma subunits. Upon stimulation by receptors, G protein alpha subunits exchange GDP for GTP and dissociate from betagamma . Thus activated, alphasubunits stimulate or inhibit downstream effectors. The duration of the activated state corresponds to the single turnover rate of GTP hydrolysis, which is typically in the range of seconds. In Gialpha1, the Gly203-->Ala mutation reduces the affinity of the substrate for Mg2+, inhibits a key conformational step that occurs upon GTP binding and consequently inhibits the release of betagamma subunits from the GTP complex. The structure of the Gly203-->Ala mutant of Gialpha1 (G203AGialpha1) bound to the slowly hydrolyzing analog of GTP (GTPgammaS) has been determined in order to elucidate the structural changes that take place during hydrolysis.

RESULTS

We have determined the three dimensional structure of a Gly203-->Ala mutant of Gialpha1 at 2.6 A resolution. Although crystals were grown in the presence of GTPgammaS and Mg2+, the catalytic site contains a molecule of GDP and a phosphate ion, but no Mg2+. The phosphate ion is bound to a site near that occupied by the gamma-phosphate of GTPgammaS in the activated wild-type alpha subunit. A region of the protein, termed the Switch II helix, twists and bends to adopt a conformation that is radically different from that observed in other Gialpha1 subunit complexes.

CONCLUSIONS

Under the conditions of crystallization, the Gly203-->Ala mutation appears to stabilize a conformation that may be similar, although perhaps not identical, to the transient ternary product complex of Gialpha1-catalyzed GTP hydrolysis. The rearrangement of the Switch II helix avoids a potential steric conflict caused by the mutation. However, it appears that dissociation of the gamma-phosphate from the pentacoordinate intermediate also requires a conformational change in Switch II. Thus, a conformational rearrangement of the Switch II helix may be required in Galpha-catalyzed GTP hydrolysis.

Localization of the effector-specifying regions of Gi2alpha and Gqalpha

Heterotrimeric G proteins transmit hormonal and sensory signals received by cell surface receptors to effector proteins that regulate cellular processes. Members of the highly conserved family of alpha subunits specifically modulate the activities of a diverse array of effector proteins. To investigate the determinants of alpha subunit-effector specificity, we localized the effector-specifying regions of alphai2, which inhibits adenylyl cyclase, and alphaq, which stimulates phosphoinositide phospholipase C using chimeric alpha subunits. The chimeras were generated using an in vivo recombination method in Escherichia coli. The effector-specifying regions of both alphai2 and alphaq were localized within the GTPase domain. An alphaq/alphai2/alphaq chimera containing only 78 alphai2 residues within the GTPase domain robustly inhibited adenylyl cyclase. This alphai2 segment includes regions corresponding to two of the three regions of alphas that activate adenylyl cyclase, but does not include any of the alpha subunit regions that switch conformation upon binding GTP. Replacement of the alphaq residues that comprise the helical domain with the homologous alphai2 residues resulted in a chimeric alpha subunit that activated phospholipase C. Combined with previous studies of the effector-specifying residues of alphas and alphat, our results suggest that the effector specificity of alpha subunits is generally determined by the GTPase and not the helical domain.

Molecular switch of F0F1-ATP synthase, G-protein, and other ATP-driven enzymes

Exchange-out of amide tritium from labeled gamma-subunit of alpha 3 beta 3 gamma complex of F0F1-ATP synthase was not accelerated by ATP, suggesting that hemagglutinin-type transition of coiled-coil structure did not occur in gamma-subunit. Local topology of nucleotide binding site and "switch II" region of G-protein alpha resemble those of F1-beta subunit and other proteins which catalyze ATP-triggered reactions. Probably, binding of nucleotide to F0F1-ATP synthase induces conformational change of the switch II-like region with transforming beta subunit structure from "open" to "close" for and this transformation results in loss of hydrogen bonds with gamma subunit, thus enabling the gamma subunit to move.

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.

Pseudohypoparathyroidism, a novel mutation in the betagamma-contact region of Gsalpha impairs receptor stimulation

Pseudohypoparathyroidism, type Ia (PHP-Ia), is a dominantly inherited endocrine disorder characterized by resistance to hormones that act by stimulating adenylyl cyclase. It is caused by inheritance of an autosomal mutation that inactivates the alpha subunit (alphas) of Gs, the stimulatory regulator of adenylyl cyclase. In three members of a family, the PHP-Ia phenotype is associated with a mutation (R231H) that substitutes histidine for an arginine at position 231 in alphas. We assessed signaling function of alphas-WT versus alphas-R231H transiently transfected in HEK293 cells. Hormone receptor-dependent stimulation of cAMP accumulation in cells expressing alphas-R231H is reduced by approximately 75% in comparison to cAMP accumulation in cells expressing alphas-WT. A second mutation, alphas-R201C, inhibits the GTPase turnoff reaction of alphas, thus producing receptor-independent stimulation of cAMP accumulation. The double mutant, alphas-R231H/R201C, stimulates cAMP accumulation almost as well (approximately 80%) as does alphas-R201C itself, indicating that the R231H mutation selectively impairs receptor-dependent signaling. In three-dimensional structures of G protein heterotrimers, Arg-231 is located in a region, switch 2, that is thought to interact with the betagamma subunit rather than with the hormone receptor. Thus, the R231H phenotype suggests that switch 2 (perhaps in concert with betagamma) mediates G protein activation by receptors at a site distant from the receptor-G protein contact surface.

A peptide derived from a beta2-adrenergic receptor transmembrane domain inhibits both receptor dimerization and activation

One of the assumptions of the mobile receptor hypothesis as it relates to G protein-coupled receptors is that the stoichiometry of receptor, G protein, and effector is 1:1:1 (Bourne, H. R., Sanders, D. A., and McCormick, F.(1990) Nature 348, 125-132). Many studies on the cooperativity of agonist binding are incompatible with this notion and have suggested that both G proteins and their associated receptors can be oligomeric. However, a clear physical demonstration that G protein-coupled receptors can indeed interact as dimers and that such interactions may have functional consequences was lacking. Here, using differential epitope tagging we demonstrate that beta2-adrenergic receptors do form SDS-resistant homodimers and that transmembrane domain VI of the receptor may represent part of an interface for receptor dimerization. The functional importance of dimerization is supported by the observation that a peptide derived from this domain that inhibits dimerization also inhibits beta-adrenergic agonist-promoted stimulation of adenylyl cyclase activity. Moreover, agonist stimulation was found to stabilize the dimeric state of the receptor, while inverse agonists favored the monomeric species, which suggests that interconversion between monomeric and dimeric forms may be important for biological activity.

Protein kinase C phosphorylates G12 alpha and inhibits its interaction with G beta gamma

Of nine G protein alpha subunits examined, only alpha 12 and alpha z served as substrates for phosphorylation by various isoforms of protein kinase C in vitro. A close homolog of alpha 12, alpha 12 was not phosphorylated. Exposure of NIH 3T3 cells that stably express alpha 12 to phorbol 12-myristate 13-acetate also resulted in phosphorylation of the protein. Phosphorylation in vitro occurred near the amino terminus (probably Ser38), and approximately 1 mol of phosphate was incorporated per mol of alpha 12. Although G protein heterotrimers containing either alpha 12 or a z were poor substrates for phosphorylation, the isolated alpha subunits were phosphorylated equally well in their GDP- or GTP gamma S-bound forms. The guanine nucleotide binding properties of purified alpha 12 and alpha z were unaltered by phosphorylation, as was the capacity of alpha z to inhibit type V adenylyl cyclase. However, phosphorylation of either protein greatly reduced its affinity for G protein beta gamma subunits, consistent with the newly determined crystal structure of a G protein heterotrimer. We suggest that protein kinase C regulates alpha 12- and alpha z-mediated signaling pathways by preventing their association with beta gamma.

Kinesin and myosin: molecular motors with similar engines

Structure determination of the catalytic domains of two members of the kinesin superfamily reveals that this class of molecular motor exhibits the same architecture as myosin and suggests that these microtubule- and actin-based motors arose from a common ancestor.

Deciphering the alphabet of G proteins: the structure of the alpha, beta, gamma heterotrimer

The recent independent structure elucidations of two heterotrimeric G proteins represent a milestone in our understanding of the regulation of this important class of signal switch molecules. The results show how the introduction of GTP into the heterotrimer produces two signalling molecules: the G alpha-GTP and G beta, gamma subunits.

Active site comparisons highlight structural similarities between myosin and other P-loop proteins

The phosphate binding loop (P-loop) is a common feature of a large number of enzymes that bind nucleotide whose consensus sequence is often used as a fingerprint for identifying new members of this group. We review here the binding sites of nine purine nucleotide binding proteins, with a focus on their relationship to the active site of myosin. This demonstrates that there is considerable conversation in the distribution and nature of the ligands that coordinate the triphosphate moiety. This comparison further suggests that at least myosin and the G-proteins utilize a similar mechanism for nucleotide hydrolysis.

Heterotrimeric G proteins

Over the past year, the thrust of work in the field of heterotrimeric G proteins has been primarily in the following areas: first, resolution of their three-dimensional structures by X-ray crystallography; second, elucidation of the effect of lipid modifications on the Galpha and Ggamma subunits; third, understanding the role of the Gbetagamma dimer in stimulation of a variety of effectors following receptor activation; and fourth, identification of the points of contact among the Galpha, Gbeta, and Ggamma subunits, and between these subunits and their cognate receptor or effector molecules.

The 2.0 A crystal structure of a heterotrimeric G protein

The structure of a heterotrimeric G protein reveals the mechanism of the nucleotide-dependent engagement of the alpha and beta gamma subunits that regulates their interaction with receptor and effector molecules. The interaction involves two distinct interfaces and dramatically alters the conformation of the alpha but not of the beta gamma subunits. The location of the known sites for post-translational modification and receptor coupling suggest a plausible orientation with respect to the membrane surface and an activated heptahelical receptor.

Functional importance of the amino terminus of Gq alpha

Gq alpha is palmitoylated at residues Cys9 and Cys10. Removal of palmitate from purified Gq alpha with palmitoylthioesterase in vitro failed to alter interactions of Gq alpha with phospholipase C-beta 1, the G protein beta gamma subunit complex, or m1 muscarinic cholinergic receptors. Mutants C9A, C10A, C9A/C10A, C9S/C10S, and truncated Gq alpha (removal of residues 1-6) were synthesized in Sf9 cells and purified. Loss of both Cys residues or truncation prevented palmitoylation of Gq alpha. However, truncated Gq alpha and the single Cys mutants activated phospholipase C-beta 1 normally, while the double Cys mutants were poor activators. Loss of both Cys residues impaired but did not abolish interaction of Gq alpha with m1 receptors. These Cys residues are thus important regardless of their state of palmitoylation. When expressed in HEK-293 or Sf9 cells, all of the proteins studied associated entirely or predominantly with membranes, although a minor fraction of nonpalmitoylated Gq alpha proteins accumulated in the cytosol of HEK-293 cells. When subjected to TX-114 phase partitioning, a significant fraction of all of the proteins, including those with no palmitate, was found in the detergent-rich phase. Removal of residues 1-34 of Gq alpha caused a loss of surface hydrophobicity as evidenced by complete partitioning into the aqueous phase. The Cys residues at the amino terminus of Gq alpha are thus important for its interactions with effector and receptor, and the amino terminus conveys a hydrophobic character to the protein distinct from that contributed by palmitate.

The structure of the G protein heterotrimer Gi alpha 1 beta 1 gamma 2

The crystallographic structure of the G protein heterotrimer Gi alpha 1(GDP)beta 1 gamma 2 (at 2.3 A) reveals two nonoverlapping regions of contact between alpha and beta, an extended interface between beta and nearly all of gamma, and limited interaction of alpha with gamma. The major alpha/beta interface covers switch II of alpha, and GTP-induced rearrangement of switch II causes subunit dissociation during signaling. Alterations in GDP binding in the heterotrimer (compared with alpha-GDP) explain stabilization of the inactive conformation of alpha by beta gamma. Repeated WD motifs in beta form a circularized sevenfold beta propeller. The conserved cores of these motifs are a scaffold for display of their more variable linkers on the exterior face of each propeller blade.