Lipid modifications of G protein subunits

Binding of G alpha(o) N terminus is responsible for the voltage-resistant inhibition of alpha(1A) (P/Q-type, Ca(v)2.1) Ca(2+) channels

G-protein-mediated inhibition of presynaptic voltage-dependent Ca(2+) channels is comprised of voltage-dependent and -resistant components. The former is caused by a direct interaction of Ca(2+) channel alpha(1) subunits with G beta gamma, whereas the latter has not been characterized well. Here, we show that the N terminus of G alpha(o) is critical for the interaction with the C terminus of the alpha(1A) channel subunit, and that the binding induces the voltage-resistant inhibition. An alpha(1A) C-terminal peptide, an antiserum raised against G alpha(o) N terminus, and a G alpha(o) N-terminal peptide all attenuated the voltage-resistant inhibition of alpha(1A) currents. Furthermore, the N terminus of G alpha(o) bound to the C terminus of alpha(1A) in vitro, which was prevented either by the alpha(1A) channel C-terminal or G alpha(o) N-terminal peptide. Although the C-terminal domain of the alpha(1B) channel showed similar ability in the binding with G alpha(o) N terminus, the above mentioned treatments were ineffective in the alpha(1B) channel current. These findings demonstrate that the voltage-resistant inhibition of the P/Q-type, alpha(1A) channel is caused by the interaction between the C-terminal domain of Ca(2+) channel alpha(1A) subunit and the N-terminal region of G alpha(o).

Goalpha regulates volatile anesthetic action in Caenorhabditis elegans

To identify genes controlling volatile anesthetic (VA) action, we have screened through existing Caenorhabditis elegans mutants and found that strains with a reduction in Go signaling are VA resistant. Loss-of-function mutants of the gene goa-1, which codes for the alpha-subunit of Go, have EC(50)s for the VA isoflurane of 1.7- to 2.4-fold that of wild type. Strains overexpressing egl-10, which codes for an RGS protein negatively regulating goa-1, are also isoflurane resistant. However, sensitivity to halothane, a structurally distinct VA, is differentially affected by Go pathway mutants. The RGS overexpressing strains, a goa-1 missense mutant found to carry a novel mutation near the GTP-binding domain, and eat-16(rf) mutants, which suppress goa-1(gf) mutations, are all halothane resistant; goa-1(null) mutants have wild-type sensitivities. Double mutant strains carrying mutations in both goa-1 and unc-64, which codes for a neuronal syntaxin previously found to regulate VA sensitivity, show that the syntaxin mutant phenotypes depend in part on goa-1 expression. Pharmacological assays using the cholinesterase inhibitor aldicarb suggest that VAs and GOA-1 similarly downregulate cholinergic neurotransmitter release in C. elegans. Thus, the mechanism of action of VAs in C. elegans is regulated by Goalpha, and presynaptic Goalpha-effectors are candidate VA molecular targets.

Inhibition of GDP/GTP exchange on G alpha subunits by proteins containing G-protein regulatory motifs

A novel Galpha binding consensus sequence, termed G-protein regulatory (GPR) or GoLoco motif, has been identified in a growing number of proteins, which are thought to modulate G-protein signaling. Alternative roles of GPR proteins as nucleotide exchange factors or as GDP dissociation inhibitors for Galpha have been proposed. We investigated the modulation of the GDP/GTP exchange of Gialpha(1), Goalpha, and Gsalpha by three proteins containing GPR motifs (GPR proteins), LGN-585-642, Pcp2, and RapIGAPII-23-131, to elucidate the mechanisms of GPR protein function. The GPR proteins displayed similar patterns of interaction with Gialpha(1) with the following order of affinities: Gialpha(1)GDP >> Gialpha(1)GDPAlF(4)(-) > or = Gialpha(1)GTPgammaS. No detectable binding of the GPR proteins to Gsalpha was observed. LGN-585-642, Pcp2, and RapIGAPII-23-131 inhibited the rates of spontaneous GTPgammaS binding and blocked GDP release from Gialpha(1) and Goalpha. The inhibitory effects of the GPR proteins on Gialpha(1) were significantly more potent, indicating that Gi might be a preferred target for these modulators. Our results suggest that GPR proteins are potent GDP dissociation inhibitors for Gialpha-like Galpha subunits in vitro, and in this capacity they may inhibit GPCR/Gi protein signaling in vivo.

Regulation of G proteins by covalent modification

Heterotrimeric G protein alpha,beta, and gamma subunits are subject to several kinds of co- and post-translational covalent modifications. Among those relevant to G protein-coupled receptor signaling in normal cell function are lipid modifications and phosphorylation. N-myristoylation is a co-translational modification occurring for members of the G(i) family of Galpha subunits, while palmitoylation is a post-translational modification that occurs for these and most other Galpha subunits. One or both modifications are required for plasma membrane targeting and contribute to regulating strength of interaction with the Gbetagamma heterodimer, effectors, and regulators of G protein signaling (RGS proteins). Galpha subunits, including those with transforming activity, are often inactive when unable to be modified with lipids. The reversible nature of palmitoylation is intriguing in this regard, as it lends itself to a regulation integrated with the activation state of the G protein. Several Galpha subunits are substrates for phosphorylation by protein kinase C and at least one is a substrate for phosphorylation by the p21-activated protein kinase. Phosphorylation in both instances inhibits the interactions of these subunits with the Gbetagamma heterodimer and RGS proteins. Several Galpha subunits are also substrates for tyrosine phosphorylation. A Ggamma subunit is phosphorylated by protein kinase C, with the consequence that it interacts more tightly with a Galpha subunit but less well with an effector.

Reconstitution of the vertebrate visual cascade using recombinant heterotrimeric transducin purified from Sf9 cells

For reconstitution studies with rhodopsin and cGMP phosphodiesterase (PDE), all three subunits of heterotrimeric transducin (T alpha beta gamma) were simultaneously expressed in Sf9 cells at high levels using a baculovirus expression system and purified to homogeneity. Light-activated rhodopsin catalyzed the loading of purified recombinant T alpha with GTP gamma S. In vitro reconstitution of rhodopsin, recombinant transducin, and PDE in detergent solution resulted in cGMP hydrolysis upon illumination, demonstrating that recombinant transducin was able to activate PDE. The rate of cGMP hydrolysis by PDE as a function of GTP gamma S-loaded recombinant transducin (T(*)) concentration gave a Hill coefficient of approximately 2, suggesting that the activation of PDE by T(*) was cooperatively regulated. Furthermore, the kinetic rate constants for the activation of PDE by T(*) suggested that only the complex of PDE with two T(*) molecules, PDE. T(2)(*), was significantly catalytically active under the conditions of the assay. We conclude that the model of essential coactivation best describes the activation of PDE by T(*) in a reconstituted vertebrate visual cascade using recombinant heterotrimeric transducin.

Src tyrosine kinase is a novel direct effector of G proteins

Heterotrimeric G proteins transduce signals from cell surface receptors to modulate the activity of cellular effectors. Src, the product of the first characterized proto-oncogene and the first identified protein tyrosine kinase, plays a critical role in the signal transduction of G protein-coupled receptors. However, the mechanism of biochemical regulation of Src by G proteins is not known. Here we demonstrate that Galphas and Galphai, but neither Galphaq, Galpha12 nor Gbetay, directly stimulate the kinase activity of downregulated c-Src. Galphas and Galphai similarly modulate Hck, another member of Src-family tyrosine kinases. Galphas and Galphai bind to the catalytic domain and change the conformation of Src, leading to increased accessibility of the active site to substrates. These data demonstrate that the Src family tyrosine kinases are direct effectors of G proteins.

Adenylyl cyclase regulates signal onset via the inhibitory GTP-binding protein, Gi

Adenylyl cyclase, the enzyme that converts ATP to cAMP, is regulated by its stimulatory and inhibitory GTP-binding proteins, G(s) and G(i), respectively. Recently, we demonstrated that besides catalyzing the synthesis of cAMP, type V adenylyl cyclase (ACV) can act as a GTPase-activating protein for Galpha(s) and also enhance the ability of activated receptors to stimulate GTP-GDP exchange on heterotrimeric G(s) (Scholich, K., Mullenix, J. B., Wittpoth, C., Poppleton, H. M., Pierre, S. C., Lindorfer, M. A., Garrison, J. C., and Patel, T. B. (1999) Science 283, 1328-1331). This latter action of ACV would facilitate the rapid onset of signaling via G(s). Because the C1 region of ACV interacts with the inhibitory GTP-binding protein Galpha(i), we investigated whether the receptor-mediated activation of heterotrimeric G(i) was also regulated by ACV and its subdomains. Our data show that ACV and its C1 domain increased the ability of a muscarinic receptor mimetic peptide (MIII-4) to enhance activation of heterotrimeric G(i) such that the amount of peptide required to stimulate G(i) in steady-state GTPase activity assays was 3-4 orders of magnitude less than without the C1 domain. Additionally, the MIII-4-mediated binding of guanosine 5'-(gamma-thio)triphosphate (GTPgammaS) to G(i) was also markedly increased in the presence of ACV or its C1 domain. In contrast, the C2 domain of ACV was not able to alter either the GTPase activity or the GTPgammaS binding to G(i) in the presence of MIII-4. Furthermore, in adenylyl cyclase assays employing S49 cyc(-) cell membranes, the C1 (but not the C2) domain of ACV enhanced the ability of peptide MIII-4 as well as endogenous somatostatin receptors to activate endogenous G(i) and to inhibit adenylyl cyclase activity. These data demonstrate that adenylyl cyclase and its C1 domain facilitate receptor-mediated activation of G(i).

Slow modal gating of single G protein-activated K+ channels expressed in Xenopus oocytes

The slow kinetics of G protein-activated K+ (GIRK) channels expressed in Xenopus oocytes were studied in single-channel, inside-out membrane patches. Channels formed by GIRK1 plus GIRK4 subunits, which are known to form the cardiac acetylcholine (ACh)-activated GIRK channel (KACh), were activated by a near-saturating dose of G protein betagamma subunits (Gbetagamma; 20 nM). The kinetic parameters of the expressed GIRK1/4 channels were similar to those of cardiac KACh. GIRK1/4 channels differed significantly from channels formed by GIRK1 with the endogenous oocyte subunit GIRK5 (GIRK1/5) in some of their kinetic parameters and in a 3-fold lower open probability, Po. The unexpectedly low Po (0.025) of GIRK1/4 was due to the presence of closures of hundreds of milliseconds; the channel spent approximately 90 % of the time in the long closed states. GIRK1/4 channels displayed a clear modal behaviour: on a time scale of tens of seconds, the Gbetagamma-activated channels cycled between a low-Po mode (Po of about 0.0034) and a bursting mode characterized by an approximately 30-fold higher Po and a different set of kinetic constants (and, therefore, a different set of channel conformations). The available evidence indicates that the slow modal transitions are not driven by binding and unbinding of Gbetagamma. The GTPgammaS-activated Galphai1 subunit, previously shown to inhibit GIRK channels, substantially increased the time spent in closed states and apparently shifted the channel to a mode similar, but not identical, to the low-Po mode. This is the first demonstration of slow modal transitions in GIRK channels. The detailed description of the slow gating kinetics of GIRK1/4 may help in future analysis of mechanisms of GIRK gating.

Molecular cloning, genomic organization, and biochemical characterization of myristoyl-CoA:protein N-myristoyltransferase from Arabidopsis thaliana

Myristoyl-CoA:protein N-myristoyltransferase (NMT, EC 2.3.1.97) catalyzes the co-translational addition of myristic acid to the amino-terminal glycine residue of a number of important proteins of diverse functions. We have isolated a full-length Arabidopsis thaliana cDNA encoding NMT (AtNMT1), the first described from a higher plant. This AtNMT1 cDNA clone has an open reading frame of 434 amino acids and a predicted molecular mass of 48,706 Da. The primary structure is 50% identical to the mammalian NMTs. Analyses of Southern blots, genomic clones, and database sequences suggested that the A. thaliana genome contains two copies of NMT gene, which are present on different chromosomes and have distinct genomic organizations. The recombinant AtNMT1 expressed in Escherichia coli exhibited a high catalytic efficiency for the peptides derived from putative plant myristoylated proteins AtCDPK6 and Fen kinase. The AtNMT was similar to the mammalian NMTs with respect to a relative specificity for myristoyl CoA among the acyl CoA donors and also inhibition by the bovine brain NMT inhibitor NIP(71). The AtNMT1 expression profile indicated ubiquity in roots, stem, leaves, flowers, and siliques (approximately 1.7 kb transcript and approximately 50 kDa immunoreactive polypeptide) but a greater level in the younger tissue, which are developmentally very active. NMT activity was also evident in all these tissues. Subcellular distribution studies indicated that, in leaf extracts, approximately 60% of AtNMT activity was associated with the ribosomal fractions, whereas approximately 30% of the activity was observed in the cytosolic fractions. The NMT is biologically important to plants, as noted from the stunted development when the AtNMT1 was down-regulated in transgenic Arabidopsis under the control of an enhanced CaMV 35S promoter. The results presented in this study provide the first direct molecular evidence for plant protein N-myristoylation and a mechanistic basis for understanding the role of this protein modification in plants.

Heterotrimeric G proteins in heart disease

Guanine nucleotide binding proteins (G proteins) are largely grouped into three classes: heterotrimeric G proteins, ras-like or small molecular weight GTP binding proteins, and others like Gh. In the heart G proteins transduce signals from a variety of membrane receptors to generate diverse effects on contractility, heart rate, and myocyte growth. This central position of G proteins forming a switchboard between extracellular signals and intracellular effectors makes them candidates possibly involved in the pathogenesis of cardiac hypertrophy, heart failure, and arrhythmia. This review focuses primarily on discoveries of heterotrimeric G protein alterations in heart diseases that help us to understand the pathogenesis and pathophysiology. We also discuss the underlying molecular mechanisms of heterotrimeric G protein signalling.Key words: G proteins, signal transduction, adrenergic system, heart failure, hypertrophy.

N-Myristoylation and betagamma play roles beyond anchorage in the palmitoylation of the G protein alpha(o) subunit

Many of the alpha subunits of heterotrimeric GTP-binding regulatory proteins (G proteins) are palmitoylated, a modification proposed to play a key role in the stable anchorage of the subunits to the plasma membrane. Palmitoylation of alpha subunits from the G(i) family is preceded by N-myristoylation, which alone or together with betagamma probably supports a reversible interaction of the alpha subunit with membrane as a prerequisite to the eventual incorporation of palmitate. Previous studies have not addressed, however, the question of whether membrane association alone, carried out through N-myristoylation, interaction with betagamma, or other events, is sufficient for palmitoylation. We report here for alpha(o) that it is not. We found that N-myristoylation is required for palmitoylation at least in part because it supports events subsequent to membrane attachment. Mutants of alpha(o) designed to target the subunit to membrane without an N-myristoyl group are unable to be palmitoylated as evaluated by incorporation of [(3)H]palmitate. Mutants of alpha(o) unable to interact normally with betagamma yet still attach to membrane demonstrate that betagamma, in contrast, is not required for palmitoylation. betagamma becomes necessary, however, when the N-myristoyl group is absent. Our results suggest that N-myristoylation and betagamma, while almost certainly relevant to the reversible interaction of alpha(o) with membrane, also play at least partly overlapping, post-anchorage roles in palmitoylation.

Functional interaction between Galpha(z) and Rap1GAP suggests a novel form of cellular cross-talk

G(z) is a member of the G(i) family of trimeric G proteins whose primary role in cell physiology is still unknown. In an ongoing effort to elucidate the cellular functions of G(z), the yeast two-hybrid system was employed to identify proteins that specifically interact with a mutationally activated form of Galpha(z). One of the molecules uncovered in this screen was Rap1GAP, a previously identified protein that specifically stimulates GTP hydrolytic activity of the monomeric G protein Rap1 and thus is believed to function as a down-regulator of Rap1 signaling. Like G(z), the precise role of Rap1 in cell physiology is poorly understood. Biochemical analysis using purified recombinant proteins revealed that the physical interaction between Galpha(z) and Rap1GAP blocks the ability of RGSs (regulators of G protein signaling) to stimulate GTP hydrolysis of the alpha subunit, and also attenuates the ability of activated Galpha(z) to inhibit adenylyl cyclase. Structure-function analyses indicate that the first 74 amino-terminal residues of Rap1GAP, a region distinct from the catalytic core domain responsible for the GAP activity toward Rap1, is required for this interaction. Co-precipitation assays revealed that Galpha(z), Rap1GAP, and Rap1 can form a stable complex. These data suggest that Rap1GAP acts as a signal integrator to somehow coordinate and/or integrate G(z) signaling and Rap1 signaling in cells.

Regions on adenylyl cyclase that are necessary for inhibition of activity by beta gamma and G(ialpha) subunits of heterotrimeric G proteins

The two large cytoplasmic domains (C1 and C2) of adenylyl cyclases (AC), when expressed separately and mixed together, reconstitute enzyme activity that can be regulated by various modulators. Therefore, we have used the C1 or its C1a subdomain and C2 regions from type I AC (ACI) and type V AC (ACV) to identify the region on ACI that interacts with beta gamma subunits of heterotrimeric G proteins. In addition, we also used a chimeric C1 domain (VC1aIC1b) in which the C1a region was derived from ACV and the C1b region was from ACI. By mixing the C1 or C1a or VC1aIC1b domains with C2 regions of ACI or ACV, we have shown that the C1a region (amino acids 236-471) of ACI is sufficient to observe beta gamma-mediated inhibition of enzyme activity, which is stimulated by either constitutively active G(salpha) (G(salpha)*) or Ca(2+)/calmodulin (CaM). Although the C1b region and C2 domain of ACI were by themselves not sufficient for inhibition of activity by beta gamma subunits, the presence of both of these regions formed another beta gamma interaction site that was sufficient to observe G(salpha)*- or Ca(2+)/CaM-stimulated activity. Inhibition of AC activity attributable to interaction of beta gamma subunits at either of the two sites was blocked by a peptide (QEHA) that has previously been shown to inhibit the effects of beta gamma on various effectors. Moreover, the C1 region of ACI was sufficient to observe G(ialpha1)-elicited inhibition of Ca(2+)/CaM-stimulated activity. Although the C1a region of ACV was sufficient for inhibition of activity by G(ialpha1), the presence of C1b region from either ACI or ACV increased sensitivity to inhibition by the inhibitory G protein. Thus, the inhibitory influences of G(ialpha1) are mediated on the C1 regions of both ACI and ACV. The effects of beta gamma on ACI can be mediated by interactions with the C1a region and a beta gamma interacting site formed by the C1b and C2 domains of this enzyme.

Structural and functional differences of two forms of GTP-binding protein, Gq, in the cephalopod retina

The major GTP-binding protein (G-protein) in the rhabdomeric photoreceptor membranes of the squid (Watasenia scintillans) has been identified as a Gq-class G-protein. Anti-Gq alpha antibodies recognized a protein not only in the photoreceptor membranes but also in soluble fractions of the retina. The 42 kD protein in the soluble fractions (soluble Gq alpha) had the same molecular mass and the same reactivities to anti-Gq antibodies as those of membrane-bound Gq alpha. The G beta subunit was scarcely detected in the soluble fractions, being found mostly in the membrane fraction, indicating soluble Gq alpha exists in monomeric form. Soluble Gq alpha had no effect on the GTPase activity of the photoreceptor membranes, suggesting that it does not interact with photoactivated rhodopsin or G beta gamma. Soluble Gq alpha would be an inactive form of Gq alpha. In the retina of Octopus fangsiao, soluble Gq alpha was scarcely detected after dark adaptation, but increased during subsequent light exposure and decreased on returning to dark adaptation. These results with Octopus suggest that functional membrane-bound Gq alpha is converted to soluble Gq alpha on exposure to light. Transformation of membrane-bound Gq alpha into the soluble form by hydroxylamine suggests that the difference between membrane-bound and soluble Gq alpha is associated with the attachment of fatty acid(s).

Molecular determinants of the reversible membrane anchorage of the G-protein transducin

Transducin is a heterotrimer formed by a fatty acylated alpha-subunit and a farnesylated betagamma-subunit. The role of these two covalent modifications and of adjacent hydrophobic and charged amino acid residues in reversible anchoring at disk model membranes is investigated at different pH values, salt concentrations, and lipid packing densities using the monolayer expansion technique and CD spectroscopy. The heterotrimer only binds if the acetylated alpha-subunit is transformed into its surface-active form by divalent cations. In the presence of salts the alpha(GDP)-subunit, the betagamma-complex, and the heterotrimer bind to POPC monolayers at 30 mN/m, estimated to mimic the lateral packing density of disk membranes, with apparent binding constants of Kapp = (1.1 +/- 0.3) x 10(6) M-1 (reflecting the penetration of the fatty acyl chain together with approximately three adjacent hydrophobic amino acid residues), Kapp = (3.5 +/- 0.5) x 10(6) M-1 (reflecting the penetration of the farnesyl chain), and Kapp = (1.6 +/- 0.3) x 10(6) M-1 (reflecting a major contribution of the alpha(GDP)-subunit with only a minor contribution from the betagamma-complex). The apparent binding constant of the alpha(GTP)-subunit is distinctly smaller than that of the alpha(GDP)-subunit. Binding to negatively charged POPC/POPG (75/25 mole/mole) monolayers is reinforced by 2-3 cationic residues for the betagamma-complex. The alpha-subunit shows no electrostatic attraction and the heterotrimer shows even a slight electrostatic repulsion which becomes the dominating force in the absence of salts.

Ribozyme approach identifies a functional association between the G protein beta1gamma7 subunits in the beta-adrenergic receptor signaling pathway

The complex role that the heterotrimeric G proteins play in signaling pathways has become increasingly apparent with the cloning of countless numbers of receptors, G proteins, and effectors. However, in most cases, the specific combinations of alpha and betagamma subunits comprising the G proteins that participate in the most common signaling pathways, such as beta-adrenergic regulation of adenylyl cyclase activity, are not known. The extent of this problem is evident in the fact that the identities of the betagamma subunits that combine with the alpha subunit of Gs are only now being elucidated almost 20 years after its initial purification. In a previous study, we described the first use of a ribozyme strategy to suppress specifically the expression of the gamma7 subunit of the G proteins, thereby identifying a specific role of this protein in coupling the beta-adrenergic receptor to stimulation of adenylyl cyclase activity in HEK 293 cells. In the present study, we explored the potential utility of a ribozyme approach directed against the gamma7 subunit to identify functional associations with a particular beta and alphas subunit of the G protein in this signaling pathway. Accordingly, HEK 293 cells were transfected with a ribozyme directed against the gamma7 subunit, and the effects of this manipulation on levels of the beta and alphas subunits were determined by immunoblot analysis. Among the five beta alphas subunits detected in these cells, only the beta1 subunit was coordinately reduced following treatment with the ribozyme directed against the gamma7 subunit, thereby demonstrating a functional association between the beta1 and gamma7 subunits. The mechanism for coordinate suppression of the beta1 subunit was due to a striking change in the half-life of the beta1 monomer versus the beta1 heterodimer complexed with the gamma7 subunit. Neither the 52- nor 45-kDa subunits were suppressed following treatment with the ribozyme directed against the gamma7 subunit, thereby providing insights into the assembly of the Gs heterotrimer. Taken together, these data show the utility of a ribozyme approach to identify the role of not only the gamma subunits but also the beta subunits of the G proteins in signaling pathways.

G protein alpha subunits activate tubulin GTPase and modulate microtubule polymerization dynamics

G proteins serve many functions involving the transfer of signals from cell surface receptors to intracellular effector molecules. Considerable evidence suggests that there is an interaction between G proteins and the cytoskeleton. In this report, G protein alpha subunits Gi1alpha, Gsalpha, and Goalpha are shown to activate the GTPase activity of tubulin, inhibit microtubule assembly, and accelerate microtubule dynamics. Gialpha inhibited polymerization of tubulin-GTP into microtubules by 80-90% in the absence of exogenous GTP. Addition of exogenous GTP, but not guanylylimidodiphosphate, which is resistant to hydrolysis, overcame the inhibition. Analysis of the dynamics of individual microtubules by video microscopy demonstrated that Gi1alpha increases the catastrophe frequency, the frequency of transition from growth to shortening. Thus, Galpha may play a role in modulating microtubule dynamic instability, providing a mechanism for the modification of the cytoskeleton by extracellular signals.

One-step affinity purification of the G protein betagamma subunits from bovine brain using a histidine-tagged G protein alpha subunit

An efficient one-step affinity purification of bovine brain G protein betagamma subunits (betagamma's) is described. The betagamma's, in a detergent extract of brain membranes, are first dissociated from the alpha subunits (alpha's), reassociated with decahistidine-tagged alphail produced in bacteria, and then adsorbed onto Ni2+-nitrilotriacetic acid-agarose via the histidine tag. This mild adsorption retained the high activity of the ligand alpha's, in contrast to the commonly used chemical crosslinking methods. A wash step with a buffer containing chaotropic ions (SCN-) completely removed contaminating proteins that were refractory to washes with high concentrations of detergents, after which the highly purified betagamma's were eluted with a buffer containing Al3+, Mg2+, and F- ions. The obtained betagamma's were found to be fully functional, as assessed by their ability to support pertussis toxin-catalyzed ADP-ribosylation of alphail. Since the combination of the mild adsorption via the histidine tag and the wash with chaotropic ions can be easily applied to purifying betagamma's from various animal tissues, this new chromatographic method is expected to facilitate the purification of other membrane proteins that bind to Galpha and/or Galphabetagamma.

Molecular characterisation of plant cDNAs BnMAP4Kalpha1 and BnMAP4Kalpha2 belonging to the GCK/SPS1 subfamily of MAP kinase kinase kinase kinase

Several yeast and mammal MAP kinase modules require, upstream of their MAP kinase kinase kinase (MAP3K), a MAP3K kinase (MAP4K). An Arabidopsis thaliana EST clone, sharing identity to MAP4Ks from yeast and mammals, has been used to isolate cDNA clones from a Brassica napus microspore-derived embryo cDNA library. The BnMAP4Kalpha1 and BnMAP4K-alpha2 clones encode putative proteins possessing the 12 subdomains of the serine/threonine protein kinase catalytic domain. A detailed analysis showed that they belong to the GCK/SPS1 subfamily of MAP4K proteins which possess an amino terminal catalytic domain and a long carboxy terminal tail. A Southern blot analysis suggested that the two proteins are encoded by a small multigene family. Expression studies revealed the presence of BnMAP4Kalpha1 and -alpha2 transcripts in all the tissues examined; however, they are most abundant in roots, siliques and flower buds. The expression of BnMAP4Kalpha1 and -alpha2 at the three main developmental stages of microspore-derived embryos (i.e., globular/heart, torpedo and cotyledonary) was confirmed by northern blot and RT-PCR analysis. An expression analysis of the above genes using synchronised Arabidopsis thaliana cell suspensions showed that the homologues genes are cell cycle regulated.

Signalling functions of protein palmitoylation

Covalent lipid modifications anchor numerous signalling proteins to the cytoplasmic face of the plasma membrane. These modifications mediate protein-membrane and protein-protein interactions and are often essential for function. Protein palmitoylation, due to its reversible nature, may be particularly important for modulating protein function during cycles of activation and deactivation. Despite intense investigation, the exact functions of protein palmitoylation are not well understood. However, it is clear that palmitoylation can affect a protein's affinity for membranes, subcellular localization, and interactions with other proteins. In this review, recent advances in understanding the functions and mechanisms of protein palmitoylation are discussed, with particular emphasis on how this lipid affects the biochemistry and cell biology of signalling proteins.

Mutagenesis of putative acylation sites alters function, localization, and accumulation of a Gi alpha subunit of the chestnut blight fungus Cryphonectria parasitica

Targeted disruption of cpg-1, a gene encoding the G protein Gi alpha subunit, CPG-1, in the chestnut blight fungus, Cryphonectria parasitica, results in reduced mycelial growth, reduced orange pigmentation, loss of virulence, loss of asexual sporulation, and female infertility. We report the development of a complementation system for cpg-1 null mutants and its use to evaluate the in vivo consequences of mutating conserved putative CPG-1 myristoylation (G2) and palmitoylation (C3) sites. Independent mutations of the two putative acylation sites differentially altered complex fungal biological processes, including virulence, and modified CPG-1 membrane association. Results of combined Northern (RNA) and Western (immunoblot) analysis also indicated a role for lipid modification in post-transcriptional regulation of CPG-1 accumulation.

Identification of a Gialpha binding site on type V adenylyl cyclase

The stimulatory G protein alpha subunit Gsalpha binds within a cleft in adenylyl cyclase formed by the alpha1-alpha2 and alpha3-beta4 loops of the C2 domain. The pseudosymmetry of the C1 and C2 domains of adenylyl cyclase suggests that the homologous inhibitory alpha subunit Gialpha could bind to the analogous cleft within C1. We demonstrate that myristoylated guanosine 5'-3-O-(thio)triphosphate-Gialpha1 forms a stable complex with the C1 (but not the C2) domain of type V adenylyl cyclase. Mutagenesis of the membrane-bound enzyme identified residues whose alteration either increased or substantially decreased the IC50 for inhibition by Gialpha1. These mutations suggest binding of Gialpha within the cleft formed by the alpha2 and alpha3 helices of C1, analogous to the Gsalpha binding site in C2. Adenylyl cyclase activity reconstituted by mixture of the C1 and C2 domains of type V adenylyl cyclase was also inhibited by Gialpha. The C1b domain of the type V enzyme contributed to affinity for Gialpha, but the source of C2 had little effect. Mutations in this soluble system faithfully reflected the phenotypes observed with the membrane-bound enzyme. The pseudosymmetrical structure of adenylyl cyclase permits bidirectional regulation of activity by homologous G protein alpha subunits.

RGSZ1, a Gz-selective regulator of G protein signaling whose action is sensitive to the phosphorylation state of Gzalpha

Regulators of G protein signaling (RGS) are a family of proteins that attenuate the activity of the trimeric G proteins. RGS proteins act as GTPase-activating proteins (GAPs) for the alpha subunits of several trimeric G proteins, much like the GAPs that regulate the activity of monomeric G proteins such as Ras. RGS proteins have been cloned from many eukaryotes, and those whose biochemical activity has been characterized regulate the members of the Gi family of G proteins; some forms can also act on Gq proteins. In an ongoing effort to elucidate the role of Gzalpha in cell signaling, the yeast two-hybrid system was employed to identify proteins that could interact with a mutationally activated form of Gzalpha. A novel RGS, termed RGSZ1, was identified that is most closely related to two existing RGS proteins termed RetRGS1 and GAIP. Northern blot analysis revealed that expression of RGSZ1 was limited to brain, and expression was particularly high in the caudate nucleus. Biochemical characterization of recombinant RGSZ1 protein revealed that RGSZ1 was indeed a GAP and, most significantly, showed a marked preference for Gzalpha over other members of the Gialpha family. Phosphorylation of Gzalpha by protein kinase C, an event known to occur in cells and that was previously shown to influence alpha-betagamma interactions of Gz, rendered the G protein much less susceptible to RGSZ1 action.

Structural basis of activity and subunit recognition in G protein heterotrimers

BACKGROUND

Inactive heterotrimeric G proteins are composed of a GDP-bound alpha subunit (Galpha) and a stable heterodimer of Gbeta and Ggamma subunits. Upon stimulation by a receptor, Galpha subunits exchange GDP for GTP and dissociate from Gbetagamma, both Galpha and Gbetagamma then interact with downstream effectors. Isoforms of Galpha, Gbeta and Ggamma potentially give rise to many heterotrimeric combinations, limited in part by amino acid sequence differences that lead to selective interactions. The mechanism by which GTP promotes Gbetagamma dissociation is incompletely understood. The Gly203-->Ala mutant of Gialpha1 binds and hydrolyzes GTP normally but does not dissociate from Gbetagamma, demonstrating that GTP binding and activation can be uncoupled. Structural data are therefore important for understanding activation and subunit recognition in G protein heterotrimers.

RESULTS

The structures of the native (Gialpha1beta1gamma2) heterotrimer and that formed with Gly203-->AlaGialpha1 have been determined to resolutions of 2.3 A and 2.4 A, respectively, and reveal previously unobserved segments at the Ggamma2 C terminus. The Gly203-->Ala mutation alters the conformation of the N terminus of the switch II region (Val201-Ala203), but not the global structure of the heterotrimer. The N termini of Gbeta and Ggamma form a rigid coiled coil that packs at varying angles against the beta propeller of Gbeta. Conformational differences in the CD loop of beta blade 2 of Gbeta mediate isoform-specific contacts with Galpha.

CONCLUSIONS

The Gly203-->Ala mutation in Gialpha1 blocks the conformational changes in switch II that are required to release Gbetagamma upon binding GTP. The interface between the ras-like domain of Galpha and the beta propeller of Gbeta appears to be conserved in all G protein heterotrimers. Sequence variation at the Gbeta-Galpha interface between the N-terminal helix of Galpha and the CD loop of beta blade 2 of Gbeta1 (residues 127-135) could mediate isoform-specific contacts. The specificity of Gbeta and Ggamma interactions is largely determined by sequence variation in the contact region between helix 2 of Ggamma and the surface of Gbeta.

Acylation of Escherichia coli hemolysin: a unique protein lipidation mechanism underlying toxin function

The pore-forming hemolysin (HlyA) of Escherichia coli represents a unique class of bacterial toxins that require a posttranslational modification for activity. The inactive protoxin pro-HlyA is activated intracellularly by amide linkage of fatty acids to two internal lysine residues 126 amino acids apart, directed by the cosynthesized HlyC protein with acyl carrier protein as the fatty acid donor. This action distinguishes HlyC from all bacterial acyltransferases such as the lipid A, lux-specific, and nodulation acyltransferases, and from eukaryotic transferases such as N-myristoyl transferases, prenyltransferases, and thioester palmitoyltransferases. Most lipids directly attached to proteins may be classed as N-terminal amide-linked and internal ester-linked acyl groups and C-terminal ether-linked isoprenoid groups. The acylation of HlyA and related toxins does not equate to these but does appear related to a small number of eukaryotic proteins that include inflammatory cytokines and mitogenic and cholinergic receptors. While the location and structure of lipid moieties on proteins vary, there are common effects on membrane affinity and/or protein-protein interactions. Despite being acylated at two residues, HlyA does not possess a "double-anchor" motif and does not have an electrostatic switch, although its dependence on calcium binding for activity suggests that the calcium-myristoyl switch may have relevance. The acyl chains on HlyA may provide anchorage points onto the surface of the host cell lipid bilayer. These could then enhance protein-protein interactions either between HlyA and components of a host signal transduction pathway to influence cytokine production or between HlyA monomers to bring about oligomerization during pore formation.

Sequestration of the G protein beta gamma subunit complex inhibits receptor-mediated endocytosis

Cell surface receptors that mediate endocytosis cluster into clathrin-coated pits, which pinch off to form vesicles that transport the receptors and their ligands. This multi-step process requires the coordinated action of many factors, including GTP-hydrolyzing proteins such as dynamin and regulators of actin cytoskeleton assembly. We note herein that sequestration of heterotrimeric G protein beta gamma subunits in intact cells strongly inhibits clathrin-coated pit-mediated endocytosis and causes rearrangement of the actin cytoskeleton. Our results suggest that cells contain a pool of free beta gamma and that it functions constitutively to permit endocytosis.

Determinants of gi1alpha and beta gamma binding. Measuring high affinity interactions in a lipid environment using flow cytometry

G protein heterocomplex undergoes dissociation and association during its functional cycle. Quantitative measurements of alpha and betagamma subunit binding have been difficult due to a very high affinity. We used fluorescence flow cytometry to quantitate binding of fluorescein-labeled Gi1alpha (F-alpha) to picomolar concentrations of biotinylated G beta gamma. Association in Lubrol solution was rapid (kon = 0.7 x 10(6) M-1 s-1), and equilibrium binding revealed a Kd of 2.9 +/- 0.8 nM. The binding showed a complex dependence on magnesium concentration, but activation of F-alpha with either GDP/aluminum fluoride or guanosine 5'-O-(3-thiotriphosphate) completely prevented formation of the heterocomplex (Kd > 100 nM). The binding was also influenced by the detergent or lipid environment. Unlabeled betagamma reconstituted in biotinylated phospholipid vesicles (pure phosphatidylcholine or mixed brain lipids) bound F-alpha approximately 2-3-fold less tightly (Kd = 6-9 nM) than in Lubrol. In contrast, beta gamma in ionic detergents such as cholate and 3-[(cholamidopropyl)diethylammonio]-1-propanesulfonate exhibited substantially lower affinities for F-alpha. Dissociation of F-alpha from beta gamma reconstituted in lipid vesicles was observed upon addition of aluminum fluoride or excess unlabeled alpha subunit, indicating that myristoylated alpha subunit has only a weak interaction with lipids without the beta gamma subunit. The kinetics of aluminum fluoride-stimulated dissociation were slower than those of the alpha subunit conformational change detected by intrinsic fluorescence. These results quantitatively demonstrate G protein subunit dissociation upon activation and provide a simple but powerful new approach for studying high affinity protein/protein interactions in solution or in a lipid environment.

Galpha12 requires acylation for its transforming activity

The alpha subunit of the heterotrimeric G protein G12, harboring a mutation in the GTP binding domain (Q229L), behaves as a potent oncogene in NIH 3T3 cells. This alpha subunit, like most other G protein alpha subunits, undergoes palmitoylation, the reversible posttranslational addition of palmitate to cysteine residues. We investigated the role of palmitoylation of alpha12 in membrane localization and transformation efficiency and whether another lipid modification, myristoylation, could substitute for palmitoylation. NIH 3T3 cells were stably transfected with plasmids that expressed the wild-type alpha12, the constitutively active Q229L (QL) mutant, and mutants in which C11 was changed to S (C11S) and S2 and R6 were changed to G and S, respectively (S2G). Incorporation of [3H]palmitate was found in the endogenous and expressed alpha12 but not in the C11S mutants. Incorporation of [3H]myristate was found only in the S2G mutants. The wild type, QL mutant, and all the acylation mutants were found in the particulate fraction. Cells expressing the nonpalmitoylated C11S,QL mutant did not undergo transformation. The S2G mutation in the nonpalmitoylated C11S,QL mutant restored the transformation efficiency to a greater level than that of the palmitoylated QL mutant as measured by foci formation, growth in soft agar, and growth rate. Palmitoylation was critical for the transformation efficiency of alpha12 but not specifically required because myristoylation could substitute for these functions.

Mechanism of RGS4, a GTPase-activating protein for G protein alpha subunits

GTP hydrolysis by guanine nucleotide-binding proteins, an essential step in many biological processes, is stimulated by GTPase-activating proteins (GAPs). The mechanisms whereby GAPs stimulate GTP hydrolysis are unknown. We have used mutational, biochemical, and structural data to investigate how RGS4, a GAP for heterotrimeric G protein alpha subunits, stimulates GTP hydrolysis. Many of the residues of RGS4 that interact with Gi alpha 1 are important for GAP activity. Furthermore, optimal GAP activity appears to require the additive effects of interactions along the RGS4-G alpha interface. GAP-defective RGS4 mutants invariably were defective in binding G alpha subunits in their transition state; furthermore, the apparent strengths of GAP and binding defects were correlated. Thus, none of these residues of RGS4, including asparagine 128, the only residue positioned at the active site of Gi alpha 1, is required exclusively for catalyzing GTP hydrolysis. These results and structural data (Tesmer, J. G. G., Berman, D. M., Gilman, A. G., and Sprang, S. R. (1997) Cell 89, 251-261) indicate that RGS4 stimulates GTP hydrolysis primarily by stabilizing the transition state conformation of the switch regions of the G protein, favoring the transition state of the reactants. Therefore, although monomeric and heterotrimeric G proteins are related, their GAPs have evolved distinct mechanisms of action.

Met-Gly-Cys motif from G-protein alpha subunit cannot direct palmitoylation when fused to heterologous protein

To determine whether the N-terminal Met-Gly-Cys motif from G-protein alpha subunit can direct palmitoylation of protein, we have generated heterologous fusion proteins containing the first 10 amino acids of Gi1 alpha and Gs alpha using tumor necrosis factor as a model protein and determined their ability to incorporate palmitate using in vitro and in vivo expression systems. DNA sequences coding for the N-terminal 10 amino acids of Gi1 alpha and Gs alpha were fused to the 5'-end of the cDNA coding for the mature domain of tumor necrosis factor (TNF) to give Gi1 alpha-TNF and Gs alpha-TNF cDNA. In vitro translation of the mRNA coding for the Gi1 alpha-TNF cDNA gave rise to an N-myristoylated fusion TNF with a molecular mass of 18 kDa as determined by the incorporation of [3H]myristic acid and by immunoprecipitation with anti-TNF antibody. In contrast, no incorporation of fatty acid was detected for Gs alpha-TNF. Baculovirus expression of the Gi1 alpha-TNF cDNA in Sf-9 cells gave rise to an N-myristoylated but not palmitoylated fusion TNF. This myristoylation was inhibited by replacement of Gly-2 with Ala but not Cys-3 with Ala, indicating the acylation reaction is entirely dependent on the N-myristoylation signal (Met-Gly-X-X-X-Ser) and Cys-3 is not involved. As is the case with in vitro translation, no incorporation of fatty acid was detected for Gs alpha-TNF. These results indicated that unlike the myristoylation signal Met-Gly-X-X-X-Ser/Thr/Cys, the Met-Gly-Cys motif found in G-protein alpha subunits itself is not sufficient to direct palmitoylation even if Gly-2 is myristoylated after removal of initiating Met. Thus, another structural determinant is implicated in this modification.

Plasma membrane localization of G alpha z requires two signals

Three covalent attachments anchor heterotrimeric G proteins to cellular membranes: the alpha subunits are myristoylated and/or palmitoylated, whereas the gamma chain is prenylated. Despite the essential role of these modifications in membrane attachment, it is not clear how they cooperate to specify G protein localization at the plasma membrane, where the G protein relays signals from cell surface receptors to intracellular effector molecules. To explore this question, we studied the effects of mutations that prevent myristoylation and/or palmitoylation of an epitope-labeled alpha subunit, alpha z. Wild-type alpha z (alpha z-WT) localizes specifically at the plasma membrane. A mutant that incorporates only myristate is mistargeted to intracellular membranes, in addition to the plasma membrane, but transduces hormonal signals as well as does alpha z-WT. Removal of the myristoylation site produced a mutant alpha z that is located in the cytosol, is not efficiently palmitoylated, and does not relay the hormonal signal. Coexpression of beta gamma with this myristoylation defective mutant transfers it to the plasma membrane, promotes its palmitoylation, and enables it to transmit hormonal signals. Pulse-chase experiments show that the palmitate attached to this myristoylation-defective mutant turns over much more rapidly than does palmitate on alpha z-WT, and that the rate of turnover is further accelerated by receptor activation. In contrast, receptor activation does not increase the slow rate of palmitate turnover on alpha z-WT. Together these results suggest that myristate and beta gamma promote stable association with membranes not only by providing hydrophobicity, but also by stabilizing attachment of palmitate. Moreover, palmitoylation confers on alpha z specific localization at the plasma membrane.

Inhibition of brain Gz GAP and other RGS proteins by palmitoylation of G protein alpha subunits

Palmitoylation of the alpha subunit of the guanine nucleotide-binding protein Gz inhibited by more than 90 percent its response to the guanosine triphosphatase (GTPase)-accelerating activity of Gz GAP, a Gz-selective member of the regulators of G-protein signaling (RGS) protein family of GTPase-activating proteins (GAPs). Palmitoylation both decreased the affinity of Gz GAP for the GTP-bound form of Galphaz by at least 90 percent and decreased the maximum rate of GTP hydrolysis. Inhibition was reversed by removal of the palmitoyl group by dithiothreitol. Palmitoylation of Galphaz also inhibited its response to the GAP activity of Galpha-interacting protein (GAIP), another RGS protein, and palmitoylation of Galphai1 inhibited its response to RGS4. The extent of inhibition of Gz GAP, GAIP, RGS4, and RGS10 correlated roughly with their intrinsic GAP activities for the Galpha target used in the assay. Reversible palmitoylation is thus a major determinant of Gz deactivation after its stimulation by receptors, and may be a general mechanism for prolonging or potentiating G-protein signaling.

A cysteine-3 to serine mutation of the G-protein Gi1 alpha abrogates functional activation by the alpha 2A-adrenoceptor but not interactions with the beta gamma complex

Pertussis toxin-resistant (C351G) and also palmitoylation-negative (C3S/C351G), myristoylation-negative (G2A/C351G) and combined acylation-negative (G2A/C3S/C351G) forms of the G-protein Gi1 alpha were expressed in COS-7 cells along with the porcine alpha 2A-adrenoceptor. G2A/C3S/C351G Gi1 alpha and G2A/C351G Gi1 alpha were largely cytosolic and failed to interact with the agonist-occupied alpha 2A-adrenoceptor in membrane preparations. In contrast, C351G Gi1 alpha was almost entirely particulate and the alpha 2-adrenoceptor agonist UK14304 caused a marked stimulation of its GTPase activity and binding of [35S]GTP gamma S which was not prevented by pertussis toxin treatment of the cells. C3S/C351G Gi1 alpha was present in both the particulate and cytosolic fractions but the GTPase activity of the membrane bound fraction was only slightly activated by the alpha 2A-adrenoceptor. Coexpression of C3S/C351G Gi1 alpha and the alpha 2A-adrenoceptor along with beta 1 and gamma 2 subunits increased the P2 membrane complement of the alpha subunit and increased substantially the ratio of membrane bound to cytosolic protein. However, this also failed to allow marked stimulation of high-affinity GTPase activity by the alpha 2A-adrenoceptor despite the increased proportion of G-protein in the P2 membrane fraction. Despite the low fractional activation of C3S/C351G Gi1 alpha by the alpha 2A-adrenoceptor compared to C351G Gi1 alpha, the palmitoylation-resistant G-protein caused a marked reduction in pertussis toxin-resistant, agonist (UK14304)-mediated stimulation of adenylyl cyclase activity. UK14304 caused the same degree of effect on adenylyl cyclase activity in pertussis toxin-treated cells following transfection of the same amounts of C351G Gi1 alpha and C3S/C351G Gi1 alpha, as both appear to act to sequester beta gamma subunits. By contrast, neither G2A/C351G Gi1 alpha nor G2A/C3S/C351G Gi1 alpha resulted in effective regulation of adenylyl cyclase activity.

Role of subunit diversity in signaling by heterotrimeric G proteins

The heterotrimeric G proteins are extensively involved in the regulation of cells by extracellular signals. The receptors that control them are often the targets of drugs. There are many isoforms of each of the three subunits that make up these proteins. Thus far, genes for at least sixteen alpha subunits, five beta subunits, and eleven gamma subunits have been identified. In addition, some of these proteins have splice variants or are differentially modified. Based upon what is already known, there are well over a thousand possible G protein heterotrimer combinations. The role of subunit diversity in heterotrimer formation and its effect on signaling by G proteins are still not well understood. However, many current lines of research are leading toward an understanding of these roles. The functional significance of subunit heterogeneity is related to the mechanisms used by G proteins to transmit and integrate the many signals coming into cells through this system. Described here are the basic mechanisms by which G proteins integrate cellular responses, the possible role of subunit heterogeneity in these mechanisms, and the evidence for and against their physiological significance. Recent studies suggest the likely possibility that subunit heterogeneity plays an important role in signaling by G proteins. This role has the potential to extend substantially the flexibility of G proteins in mediating cellular responses to extracellular signals. However, the details of this are yet to be worked out, and they are the subject of many different avenues of research.

Fatty acid acylation of platelet proteins

A variety of fatty acids can become covalently attached to platelet proteins by thioester linkage. These fatty acids include palmitate, myristate, stearate, arachidonate, and eicosapentaenoate. More than 20 platelet proteins can be acylated by fatty acids. Several of the acylated platelet proteins have been identified, including glycoprotein Ib beta, glycoprotein IX, P-selectin, G-protein alpha subunits, and CD9. This report reviews the fatty acid acylation of platelet proteins.

Gsalpha contains an unidentified covalent modification that increases its affinity for adenylyl cyclase

Many G protein alpha subunits are dually acylated with myristate and palmitate or are palmitoylated on more than one cysteine residue near their N termini. The Galpha protein that activates adenylyl cyclase, alphas, is not myristoylated but can be reversibly palmitoylated. It appears that alphas contains another, as-yet-unidentified covalent modification that decreases its apparent dissociation constant for adenylyl cyclase from 50 nM to <0. 5 nM. This modification is at or near the N terminus of the protein and is hydrophobic. Palmitoylation of native alphas does not account for its high affinity for adenylyl cyclase.

The stoichiometry of G alpha(s) palmitoylation in its basal and activated states

Palmitoylation is the dynamic modification of proteins by the addition of palmitate to cysteine residues. The alpha subunits of heterotrimeric G proteins undergo palmitoylation on their amino terminus, and activation of alpha(s) accelerates its palmitate turnover. In previous studies, palmitoylation was assessed by incorporation or turnover of [3H]palmitate. These studies did not determine the fraction of alpha(s) that is palmitoylated because the specific activity of [3H]palmitoyl-CoA within cells is indeterminate. We developed an HPLC method to determine the fraction of alpha(s) that was palmitoylated in the basal and activated states. COS and S49 cells were radiolabeled with [35S]methionine, and alpha(s) was immunoprecipitated from the particulate fraction. The immunoprecipitated proteins were separated by reverse phase HPLC into two peaks that were determined to contain the modified and unmodified forms of alpha(s). Approximately 77% of the endogenous alpha(s) in COS cells and 70% in S49 lymphoma cells were palmitoylated. The fraction of alpha(s) that was modified did not change after treatment with isoproterenol, a beta-adrenergic receptor agonist that causes turnover of palmitate on alpha(s). These results suggest that receptor activation of alpha(s) caused a rapid turnover of palmitate to maintain most of alpha(s) in its palmitoylated form.

Comparison of mRNA expression of two regulators of G-protein signaling, RGS1/BL34/1R20 and RGS2/G0S8, in cultured human blood mononuclear cells

RGS1 and RGS2 are members of a new class of regulators of G-protein signaling identified by their selective mRNA expression either in phorbol ester (TPA)-stimulated human B lymphocytes (RGS1/1R20/BL34) or in blood mononuclear cells treated with the T-cell lectin concanavalin A (ConA) and cycloheximide (RGS2/G0S8). The RGS1 gene shows low basal mRNA expression in freshly purified blood mononuclear cells, which increases upon incubation for a day. In contrast, RGS2 initially shows high basal levels of mRNA expression, which subsequently decrease. Expression of both genes increases in response to ConA, with RGS2 mRNA levels increasing briskly to a maximum between 0.5 and 1 hr and decreasing to baseline by 6 hr, whereas the RGS1 mRNA increase is delayed reaching a maximum between 1 and 2 hr. RGS1 mRNA levels increase much more in response to a protein kinase C activator (TPA), than to a calcium ionophore (ionomycin), whereas the opposite is true for RGS2. We suggest that ConA elevates RGS2 on the basis of its ability to increase intracellular calcium, and that RGS2 may be involved in the regulation of intracellular calcium. The distinction between RGS1 and RGS2 is further emphasized by studies indicating that recombinant RGS2 does not bind in vitro to two members of the G(i) subfamily of G-protein alpha-subunits for which recombinant RGS1 has high affinity.

Assembly and intracellular targeting of the betagamma subunits of heterotrimeric G proteins

The assembly in living cells of heterotrimeric guanine nucleotide binding proteins from their constituent alpha, beta, and gamma subunits is a complex process, compounded by the multiplicity of the genes that encode them, and the diversity of receptors and effectors with which they interact. Monoclonal anti-beta antibodies (ARC5 and ARC9), raised against immunoaffinity purified beta gamma complexes, recognize beta subunits when not associated with gamma and can thus be used to monitor assembly of beta gamma complexes. Complex formation starts immediately after synthesis and is complete within 30 min. Assembly occurs predominantly in the cytosol, and association of beta gamma complexes with the plasma membrane fraction starts between 15-30 min of chase. Three pools of beta subunits can be distinguished based on their association with gamma subunits, their localization, and their detergent solubility. Association of beta and alpha subunits with detergent-insoluble domains occurs within 1 min of chase, and increases to reach a plateau of near complete detergent resistance within 30 min of chase. Brefeldin A treatment does not interfere with delivery of beta gamma subunits to detergent-insoluble domains, suggesting that assembly of G protein subunits with their receptors occurs distally from the BFA-imposed block of intracellular membrane trafficking and may occur directly at the plasma membrane.

A GTPase-activating protein for the G protein Galphaz. Identification, purification, and mechanism of action

A GTPase-activating protein (GAP) specific for Galphaz was identified in brain, spleen, retina, platelet, C6 glioma cells, and several other tissues and cells. Gz GAP from bovine brain is a membrane protein that is refractory to solubilization with most detergents but was solubilized with warm Triton X-100 and purified up to 50,000-fold. Activity is associated with at least two separate proteins of Mr approximately 22,000 and 28,000, both of which have similar specific activities. In an assay that measures the rate of hydrolysis of GTP pre-bound to detergent-soluble Galphaz, the GAP accelerates hydrolysis over 200-fold, from 0.014 to 3 min -1 at 15 degrees C, or to >/=20 min-1 at 30 degrees C. It does not alter rates of nucleotide association or dissociation. When co-reconstituted into phospholipid vesicles with trimeric Gz and m2 muscarinic receptor, Gz GAP accelerates agonist-stimulated steady-state GTP hydrolysis as predicted by its effect on the hydrolytic reaction. In the single turnover assay, the Km of the GAP for Galphaz-GTP is 2 nM. Its activity is inhibited by Galphaz-guanosine 5'-O-thiotriphosphate (Galphaz-GTPgammaS) or by Galphaz-GDP/AlF4 with Ki approximately 1.5 nM for both species; Galphaz-GDP does not inhibit. G protein betagamma subunits inhibit Gz GAP activity, apparently by forming a GTP-Galphazbetagamma complex that is a poor GAP substrate. Gz GAP displays little GAP activity toward Galphai1 or Galphao, but its activity with Galphaz is competitively inhibited by both Galphai1 and Galphao at nanomolar concentrations when they are bound to GTPgammaS but not to GDP. Neither phospholipase C-beta1 (a Gq GAP) nor several adenylyl cyclase isoforms display Gz GAP activity.

Does subunit dissociation necessarily accompany the activation of all heterotrimeric G proteins?

Heterotrimeric (alpha beta gamma) G proteins mediate a variety of signal transduction events in virtually every cell of every eukaryotic organism. The predominant hypothesis is that dissociation of the alpha-subunit from the G beta gamma-subunit complex necessarily accompanies the activation of these proteins, and that the alpha-subunit is primarily responsible for regulating the response of effector molecules. However, there is increasing evidence that both the alpha-subunit and the beta gamma-subunit complex function in regulating effector activity. Furthermore, data for some G proteins suggest that they function as activated heterotrimers rather than as dissociated subunits.

RGS4 and GAIP are GTPase-activating proteins for Gq alpha and block activation of phospholipase C beta by gamma-thio-GTP-Gq alpha

RGS proteins constitute a newly appreciated and large group of negative regulators of G protein signaling. Four members of the RGS family act as GTPase-activating proteins (GAPs) with apparent specificity for members of the Gi alpha subfamily of G protein subunits. We demonstrate here that two RGS proteins, RGS4 and GAIP, also act as GAPs for Gq alpha, the G alpha protein responsible for activation of phospholipase C beta. Furthermore, these RGS proteins block activation of phospholipase C beta by guanosine 5'-(3-O-thio) triphosphate-Gq alpha. GAP activity does not explain this effect, which apparently results from occlusion of the binding site on G alpha for effector. Inhibitory effects of RGS proteins on G protein-mediated signaling pathways can be demonstrated by simple mixture of RGS4 or GAIP with plasma membranes.

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+.

Myristoylation

N-myristoylation is an acylation process absolutely specific to the N-terminal amino acid glycine in proteins. This maturation process concerns about a hundred proteins in lower and higher eukaryotes involved in oncogenesis, in secondary cellular signalling, in infectivity of retroviruses and, marginally, of other virus types. Thy cytosolic enzyme responsible for this activity, N-myristoyltransferase (NMT), studied since 1987, has been purified from different sources. However, the studies of the specificities of the various NMTs have not progressed in detail except for those relating to the yeast cytosolic enzyme. Still to be explained are differences in species specificity and between various putative isoenzymes, also whether the data obtained from the yeast enzyme can be transposed to other NMTs. The present review discusses data on the various addressing processes subsequent to myristoylation, a patchwork of pathways that suggests myristoylation is only the first step of the mechanisms by which a protein associates with the membrane. Concerning the enzyme itself, there are evidences that NMT is also present in the endoplasmic reticulum and that its substrate specificity is different from that of the cytosolic enzyme(s). These differences have major implications for their differential inhibition and for their respective roles in several pathologies. For instance, the NMTs from mammalians are clearly different from those found in several microorganisms, which raises the question whether the NMT may be a new targets for fungicides. Finally, since myristoylation has a central role in virus maturation and oncogenesis, specific NMT inhibitors might lead to potent antivirus and anticancer agents.

Inhibition of an inward rectifier potassium channel (Kir2.3) by G-protein betagamma subunits

The molecular basis of G-protein inhibition of inward rectifier K+ currents was examined by co-expression of G-proteins and cloned Kir2 channel subunits in Xenopus oocytes. Channels encoded by Kir2.3 (HRK1/HIR/BIRK2/BIR11) were completely suppressed by co-expression with G-protein betagamma subunits, whereas channels encoded by Kir2. 1 (IRK1), which shares 60% amino acid identity with Kir2.3, were unaffected. Co-expression of Galphai1 and Galphaq subunits also partially suppressed Kir2.3 currents, but Galphat, Galphas, and a constitutively active mutant of Galphail (Q204L) were ineffective. Gbetagamma and Kir2.3 subunits were co-immunoprecipitated using an anti-Kir2.3 antibody. Direct binding of G-protein betagamma subunits to fusion proteins containing Kir2.3 N terminus, but not to fusion proteins containing Kir2.1 N terminus, was also demonstrated. The results are consistent with suppression of Kir2.3 currents resulting from a direct protein-protein interaction between the channel and G-protein betagamma subunits. When Kir2.1 and Kir2.3 subunits were coexpressed, the G-protein inhibitory phenotype of Kir2.3 was dominant, suggesting that co-expression of Kir2.3 with other Kir subunits might give rise to novel G-protein-inhibitable inward rectifier currents.