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

Analysis of Protein Cysteine Acylation Using a Modified Suspension Trap (Acyl-Trap)

Proteins undergo reversible S-acylation via a thioester linkage in vivo. S-palmitoylation, modification by C16:0 fatty acid, is a common S-acylation that mediates critical protein-membrane and protein-protein interactions. The most widely used S-acylation assays, including acyl-biotin exchange and acyl resin-assisted capture, utilize blocking of free Cys thiols, hydroxylamine-dependent cleavage of the thioester and subsequent labeling of nascent thiol. These assays generally require >500 μg of protein input material per sample and numerous reagent removal and washing steps, making them laborious and ill-suited for high throughput and low input applications. To overcome these limitations, we devised "Acyl-Trap", a suspension trap-based assay that utilizes a thiol-reactive quartz to enable buffer exchange and hydroxylamine-mediated S-acyl enrichment. We show that the method is compatible with protein-level detection of S-acylated proteins (e.g., H-Ras) as well as S-acyl site identification and quantification using "on trap" isobaric labeling and LC-MS/MS from as little as 20 μg of protein input. In mouse brain, Acyl-Trap identified 279 reported sites of S-acylation and 1298 previously unreported putative sites. Also described are conditions for long-term hydroxylamine storage, which streamline the assay. More generally, Acyl-Trap serves as a proof-of-concept for PTM-tailored suspension traps suitable for both traditional protein detection and chemoproteomic workflows.

Analysis of Protein Cysteine Acylation Using a Modified Suspension Trap (Acyl-Trap)

Proteins undergo reversible S-acylation via a thioester linkage in vivo. S-palmitoylation, modification by C16:0 fatty acid, is a common S-acylation that mediates critical protein-membrane and protein-protein interactions. The most widely used S-acylation assays, including acyl-biotin exchange and acyl resin-assisted capture, utilize blocking of free Cys thiols, hydroxylamine-dependent cleavage of the thioester and subsequent labeling of nascent thiol. These assays generally require >500 micrograms of protein input material per sample and numerous reagent removal and washing steps, making them laborious and ill-suited for high throughput and low input applications. To overcome these limitations, we devised "Acyl-Trap", a suspension trap-based assay that utilizes a thiol-reactive quartz to enable buffer exchange and hydroxylamine-mediated S-acyl enrichment. We show that the method is compatible with protein-level detection of S-acylated proteins (e.g. H-Ras) as well as S-acyl site identification and quantification using "on trap" isobaric labeling and LC-MS/MS from as little as 20 micrograms of protein input. In mouse brain, Acyl-Trap identified 279 reported sites of S-acylation and 1298 previously unreported putative sites. Also described are conditions for long-term hydroxylamine storage, which streamlines the assay. More generally, Acyl-Trap serves as a proof-of-concept for PTM-tailored suspension traps suitable for both traditional protein detection and chemoproteomic workflows.

Effects of Post-translational Modifications on Membrane Localization and Signaling of Prostanoid GPCR-G Protein Complexes and the Role of Hypoxia

G protein-coupled receptors (GPCRs) play a pivotal role in the adaptive responses to cellular stresses such as hypoxia. In addition to influencing cellular gene expression profiles, hypoxic microenvironments can perturb membrane protein localization, altering GPCR effector scaffolding and altering downstream signaling. Studies using proteomics approaches have revealed significant regulation of GPCR and G proteins by their state of post-translational modification. The aim of this review is to examine the effects of post-translational modifications on membrane localization and signaling of GPCR-G protein complexes, with an emphasis on vascular prostanoid receptors, and to highlight what is known about the effect of cellular hypoxia on these mechanisms. Understanding post-translational modifications of protein targets will help to define GPCR targets in treatment of disease, and to inform research into mechanisms of hypoxic cellular responses.

Cellular N-myristoyltransferases play a crucial picornavirus genus-specific role in viral assembly, virion maturation, and infectivity

In nearly all picornaviruses the precursor of the smallest capsid protein VP4 undergoes co-translational N-terminal myristoylation by host cell N-myristoyltransferases (NMTs). Curtailing this modification by mutation of the myristoylation signal in poliovirus has been shown to result in severe assembly defects and very little, if any, progeny virus production. Avoiding possible pleiotropic effects of such mutations, we here used pharmacological abrogation of myristoylation with the NMT inhibitor DDD85646, a pyrazole sulfonamide originally developed against trypanosomal NMT. Infection of HeLa cells with coxsackievirus B3 in the presence of this drug decreased VP0 acylation at least 100-fold, resulting in a defect both early and late in virus morphogenesis, which diminishes the yield of viral progeny by about 90%. Virus particles still produced consisted mainly of provirions containing RNA and uncleaved VP0 and, to a substantially lesser extent, of mature virions with cleaved VP0. This indicates an important role of myristoylation in the viral maturation cleavage. By electron microscopy, these RNA-filled particles were indistinguishable from virus produced under control conditions. Nevertheless, their specific infectivity decreased by about five hundred fold. Since host cell-attachment was not markedly impaired, their defect must lie in the inability to transfer their genomic RNA into the cytosol, likely at the level of endosomal pore formation. Strikingly, neither parechoviruses nor kobuviruses are affected by DDD85646, which appears to correlate with their native capsid containing only unprocessed VP0. Individual knockout of the genes encoding the two human NMT isozymes in haploid HAP1 cells further demonstrated the pivotal role for HsNMT1, with little contribution by HsNMT2, in the virus replication cycle. Our results also indicate that inhibition of NMT can possibly be exploited for controlling the infection by a wide spectrum of picornaviruses.

Picornaviruses are important human and animal pathogens. Protective vaccines are only available against very few representatives. Furthermore, antiviral drugs have not made it to the market because of serious side effects and viral mutational escape. We here show that pharmacological inhibition of cellular myristoyltransferases severely decreased myristoylation of enteroviral structural proteins as exemplified by coxsackievirus B3, a prominent pathogen causing virus-induced acute and chronic heart disease. The drug DDD85646 substantially diminished virus yield and almost abolished the infectivity of the residual progeny virus. It is highly effective against several other picornaviruses, except those two included in our study that naturally do not process VP0. Our work provides new insight into the role of myristoylation in the life cycle of picornaviruses and identifies the responsible cellular enzyme as a promising candidate for antiviral therapy.

Enhanced calcium cycling and contractile function in transgenic hearts expressing constitutively active G alpha o* protein

In contrast to the other heterotrimeric GTP-binding proteins (G proteins) Gs and Gi, the functional role of G o is still poorly defined. To investigate the role of G alpha o in the heart, we generated transgenic mice with cardiac-specific expression of a constitutively active form of G alpha o1* (G alpha o*), the predominant G alpha o isoform in the heart. G alpha o expression was increased 3- to 15-fold in mice from 5 independent lines, all of which had a normal life span and no gross cardiac morphological abnormalities. We demonstrate enhanced contractile function in G alpha o* transgenic mice in vivo, along with increased L-type Ca2+ channel current density, calcium transients, and cell shortening in ventricular G alpha o*-expressing myocytes compared with wild-type controls. These changes were evident at baseline and maintained after isoproterenol stimulation. Expression levels of all major Ca2+ handling proteins were largely unchanged, except for a modest reduction in Na+/Ca2+ exchanger in transgenic ventricles. In contrast, phosphorylation of the ryanodine receptor and phospholamban at known PKA sites was increased 1.6- and 1.9-fold, respectively, in G alpha o* ventricles. Density and affinity of beta-adrenoceptors, cAMP levels, and PKA activity were comparable in G alpha o* and wild-type myocytes, but protein phosphatase 1 activity was reduced upon G alpha o* expression, particularly in the vicinity of the ryanodine receptor. We conclude that G alpha o* exerts a positive effect on Ca2+ cycling and contractile function. Alterations in protein phosphatase 1 activity rather than PKA-mediated phosphorylation might be involved in hyperphosphorylation of key Ca2+ handling proteins in hearts with constitutive G alpha o activation.

Live cell analysis of G protein beta5 complex formation, function, and targeting

The G protein beta(5) subunit differs from other beta subunits in having divergent sequence and subcellular localization patterns. Although beta(5)gamma(2) modulates effectors, beta(5) associates with R7 family regulators of G protein signaling (RGS) proteins when purified from tissues. To investigate beta(5) complex formation in vivo, we used multicolor bimolecular fluorescence complementation in human embryonic kidney 293 cells to compare the abilities of 7 gamma subunits and RGS7 to compete for interaction with beta(5). Among the gamma subunits, beta(5) interacted preferentially with gamma(2), followed by gamma(7), and efficacy of phospholipase C-beta2 activation correlated with amount of beta(5)gamma complex formation. beta(5) also slightly preferred gamma(2) over RGS7. In the presence of coexpressed R7 family binding protein (R7BP), beta(5) interacted similarly with gamma(2) and RGS7. Moreover, gamma(2) interacted preferentially with beta(1) rather than beta(5). These results suggest that multiple coexpressed proteins influence beta(5) complex formation. Fluorescent beta(5)gamma(2) labeled discrete intracellular structures including the endoplasmic reticulum and Golgi apparatus, whereas beta(5)RGS7 stained the cytoplasm diffusely. Coexpression of alpha(o) targeted both beta(5) complexes to the plasma membrane, and alpha(q) also targeted beta(5)gamma(2) to the plasma membrane. The constitutively activated alpha(o) mutant, alpha(o)R179C, produced greater targeting of beta(5)RGS7 and less of beta(5)gamma(2) than did alpha(o). These results suggest that alpha(o) may cycle between interactions with beta(5)gamma(2) or other betagamma complexes when inactive, and beta(5)RGS7 when active. Moreover, the ability of beta(5)gamma(2) to be targeted to the plasma membrane by alpha subunits suggests that functional beta(5)gamma(2) complexes can form in intact cells and mediate signaling by G protein-coupled receptors.

Shuttling of G protein subunits between the plasma membrane and intracellular membranes

Heterotrimeric G proteins (alphabetagamma) mediate the majority of signaling pathways in mammalian cells. It is long held that G protein function is localized to the plasma membrane. Here we examined the spatiotemporal dynamics of G protein localization using fluorescence recovery after photobleaching, fluorescence loss in photobleaching, and a photoswitchable fluorescent protein, Dronpa. Unexpectedly, G protein subunits shuttle rapidly (t1/2 < 1 min) between the plasma membrane and intracellular membranes. We show that consistent with such shuttling, G proteins constitutively reside in endomembranes. Furthermore, we show that shuttling is inhibited by 2-bromopalmitate. Thus, contrary to present thought, G proteins do not reside permanently on the plasma membrane but are constantly testing the cytoplasmic surfaces of the plasma membrane and endomembranes to maintain G protein pools in intracellular membranes to establish direct communication between receptors and endomembranes.

Subunit dissociation and diffusion determine the subcellular localization of rod and cone transducins

Activation of rod photoreceptors by light induces a massive redistribution of the heterotrimeric G-protein transducin. In darkness, transducin is sequestered within the membrane-enriched outer segments of the rod cell. In light, it disperses throughout the entire neuron. We show here that redistribution of rod transducin by light requires activation, but it does not require ATP. This observation rules out participation of molecular motors in the redistribution process. In contrast to the light-stimulated redistribution of rod transducin in rods, cone transducin in cones does not redistribute during activation. Remarkably, when cone transducin is expressed in rods, it does undergo light-stimulated redistribution. We show here that the difference in subcellular localization of activated rod and cone G-proteins correlates with their affinity for membranes. Activated rod transducin releases from membranes, whereas activated cone transducin remains bound to membranes. A synthetic peptide that dissociates G-protein complexes independently of activation facilitates dispersion of both rod and cone transducins within the cells. This peptide also facilitates detachment of both G-proteins from the membranes. Together, these results show that it is the dissociation state of transducin that determines its localization in photoreceptors. When rod transducin is stimulated, its subunits dissociate, leave outer segment membranes, and equilibrate throughout the cell. Cone transducin subunits do not dissociate during activation and remain sequestered within the outer segment. These findings indicate that the subunits of some heterotrimeric G-proteins remain associated during activation in their native environments.

Bring your own G protein

G protein-coupled receptor (GPCR)-Galpha fusion proteins were first characterized more than 10 years ago as a strategy for studying receptor-G protein signaling. A large number of studies have used this approach to characterize receptor coupling to members of the Gs, Gi, and Gq families of Galpha subunits, but this strategy has not been widely used to study Galpha12 and Galpha13. As described in the article by Zhang et al. in this issue of Molecular Pharmacology (p. 1433) characterization of the signaling properties of thromboxane A2 receptor (TPalpha) -Galpha12 and -Galpha13 fusion constructs demonstrates the applicability of this strategy to members of this unique family of Galpha subunits, and how this strategy can be used to resolve otherwise difficult problems of receptor pharmacology associated with these proteins. The general strategy of making receptor-Galpha fusion constructs has wide applicability to a number of research problems, but there are perhaps also "hidden messages" in how different receptor-Galpha subunit fusion pairs behave.

Loss of association between activated Galpha q and Gbetagamma disrupts receptor-dependent and receptor-independent signaling

The G protein subunit, betagamma, plays an important role in targeting alpha subunits to the plasma membrane and is essential for binding and activation of the heterotrimer by heptahelical receptors. Mutation of residues in the N-terminal alpha-helix of alpha s and alpha q that contact betagamma in the crystal structure of alpha i reduces binding between alpha and betagamma, inhibits plasma membrane targeting and palmitoylation of the alpha subunit, and results in G proteins that fail to couple receptor activation to stimulation of effector. Overexpression of betagamma can recover this loss of signaling through Gs but not Gq. In fact, a single mutation (I25A) in alpha q can block alpha q-mediated generation of inositol phosphates. Function is not recovered by betagamma overexpression nor myristoylation directed plasma membrane localization. Introduction of a Q209L activating mutation with I25A results in a constitutively active alpha q as expected, but surprisingly a R183C activating mutation does not result in constitutive activity when present with I25A. Examination of binding between alpha and betagamma via a pull down assay shows that the N-terminal betagamma-binding mutations inhibit alpha-betagamma binding significantly more than the R183C or Q209L activating mutations do. Moreover, introduction of the I25A mutation into alpha q RC disrupts co-immunoprecipitation with PLCbeta1. Taken together, results presented here suggest that alpha-betagamma binding is necessary at a point downstream from receptor activation of the heterotrimeric G protein for signal transduction by alpha q.

Palmitoylation and Plasma Membrane Targeting of RGS7 Are Promoted by αo

Regulator of G protein signaling (RGS) proteins modulate G protein signaling by acting as GTPase-activating proteins for G protein alpha-subunits. RGS7 belongs to a subfamily of RGS proteins that exist as dimers with the G protein beta(5)-subunit. In this report, we addressed the mechanisms of plasma membrane localization of beta(5)RGS7. When expressed in human embryonic kidney 293 cells, beta(5)RGS7 was found to be cytoplasmic and soluble. Expression of alpha(o) promoted a strong redistribution of beta(5)RGS7 to the plasma membrane. Expression of alpha(q), however, failed to affect the subcellular localization of beta(5)RGS7. The constitutively active mutant alpha(o)R179C, like wild-type alpha(o), strongly recruited beta(5)RGS7 to plasma membranes; however, inactive alpha(o)G204A, RGS-insensitive alpha(o)G184S, and lipidation-deficient alpha(o)G2A were all defective in the ability to promote plasma membrane localization of beta(5)RGS7. In addition, palmitoylation of RGS7 was demonstrated, and palmitoylation required expression of alpha(o) or alpha(o)R179C. To examine potential palmitoylation sites of RGS7, several cysteines were substituted with serines. beta(5)RGS7C133S failed to localize to plasma membranes when coexpressed with alpha(o), suggesting cysteine 133 of RGS7 as a putative palmitoylation site. Finally, deletion of amino acids 76 to 128 of RGS7, which includes part of the disheveled, EGL-10, pleckstrin (DEP) domain, prevented alpha(o)-mediated plasma membrane recruitment of beta(5)RGS7. These findings are the first to demonstrate Galpha-regulated plasma membrane localization and palmitoylation of beta(5)RGS7 and suggest that membrane targeting of beta(5)RGS7 is a complex process requiring at least RGS domain-mediated interaction with alpha(o) and RGS7 palmitoylation.

Mutation of cysteine 214 in Gi1 alpha subunit abolishes its endogenous GTPase activity

The functional consequences of the mutation of a conserved Cys-214 in Galpha(i1) have been investigated. We reported herein that substitutions of Cys-214 of Galpha(i1) to either alanine or tryptophan abolished the intrinsic GTPase activity. Free phosphate release from [32P]GTP-bound Galpha(i1) C214A or [32P]GTP-bound Galpha(i1) C214W was at least 30-fold lower than that of the wild-type Galpha(i1) in single-turnover GTPase assays. Consistently, tryptic proteolysis of C214A and C214W proteins showed that they were partially protected by GTP, further confirming that the GTPase activity in both mutant proteins was impaired. Expression of C214A or C214W mutants in Chinese hamster ovary K1 cells caused significant inhibition of forskolin-stimulated adenylate cyclase activity. However, the mutations did not significantly affect the GTP[S] (guanosine 5'-[gamma-[35S]thio]triphosphate)-binding activity. Both C214A and C214W mutants serve as good substrates for pertussis toxin-catalysed ADP ribosylation, indicating that they interact well with betagamma subunits. Moreover, RGS4 protein, a GTPase-activating protein for Galpha(i1), cannot interact with Cys-214 mutants even in the presence of AlF4-, which induces the transition state of Galpha. In summary, our findings suggest that C214A or C214W are GTPase-deficient mutants and can functionally serve as constitutively active forms of Galpha(i1) in cells.

Unexpected protein families including cell defense components feature in the N-myristoylome of a higher eukaryote

N-Myristoylation is an irreversible modification that affects the membrane binding properties of crucial cytoplasmic proteins from signal transduction cascades. We characterized the two putative N-myristoyltransferases of Arabidopsis thaliana as a means of investigating the entire N-myristoylation proteome (N-myristoylome) in a higher eukaryote. AtNMT1 compensated for the nmt1 defect in yeast, whereas AtNMT2 and chimeras of the two genes did not. Only AtNMT1 modified known N-myristoylated proteins in vitro. AtNMT1 is therefore responsible for the A. thaliana N-myristoylome, whereas AtNMT2 does not seem to have usual myristoylation activity. We began with the whole set of N-myristoylated G proteins in the A. thaliana proteome. We then used a reiterative approach, based on the in vitro N-myristoylation of more than 60 different polypeptides, to determine the substrate specificity of AtNMT1. We found that the positive charge on residue 7 of the substrate was particularly important in substrate recognition. The A. thaliana N-myristoylome consists of 437 proteins, accounting for 1.7% of the complete proteome. We demonstrated the N-myristoylation of several unexpected protein families, including innate immunity proteins, thioredoxins, components of the protein degradation pathway, transcription factors, and a crucial regulatory enzyme of glycolysis. The role of N-myristoylation is discussed in each case; in particular, this process may underlie the "guard" hypothesis of innate immunity.

Promiscuous coupling at receptor-Galpha fusion proteins. The receptor of one covalent complex interacts with the alpha-subunit of another

Fusion proteins between heptahelical receptors (GPCR) and G protein alpha-subunits show enhanced signaling efficiency in transfected cells. This is believed to be the result of molecular proximity, because the interaction between linked modules of one protein chain, if not constrained by structure, should be strongly favored compared with the same in which partners react as free species. To test this assumption we made a series of fusion proteins (type 1 and 4 opioid receptors with G(o) and beta(2) adrenergic and dopamine 1 receptors with G(sL)) and some mutated analogs carrying different tags and defective GPCR or Galpha subunits. Using cotransfection experiments with readout protocols able to distinguish activation at fused and non-fused alpha-subunits, we found that both the GPCR and the Galpha limb of one fusion protein can freely interact with non-fused proteins and the tethered partners of a neighboring fusion complex. Moreover, a bulky polyanionic inhibitor can suppress with identical potency receptor-Galpha interaction, either when occurring between latched domains of a fused system or separate elements of distinct molecules, indicating that the binding surfaces are equally accessible in both cases. These data demonstrate that there is no entropy drive from the linked condition of fusion proteins and suggest that their signaling may result from the GPCR of one complex interacting with the alpha-subunit of another. Moreover, the enhanced coupling efficiency commonly observed for fusion proteins is not due to the receptor tether, but to the transmembrane helix that anchors Galpha to the membrane.

Role of a conserved amino-terminal sequence in the ecotropic MLV receptor mCAT1

The amino-terminus of mCAT1 and homologous proteins is predicted to form a positively charged, amphipathic alpha helix on the cytoplasmic side of the plasma membrane. Peptides with similar sequence motifs often provide membrane anchors, protein-protein interaction domains, or intracellular transport-targeting signals. Deleting most of the cytoplasmic N-terminal sequence of mCAT1 led to reduced expression on the cell surface and accumulation in the endoplasmic reticulum but did not abrogate receptor function. Surprisingly, when the N-terminal 36 or 18 amino acids of mCAT1 were fused to green fluorescent protein (gfp), gfp accumulated almost exclusively in mitochondria. Mitochondrial targeting depended on arginines at positions 15 and 16 and was inhibitable by downstream transmembrane sequences. Although the full-length mCAT1 was not detected in mitochondria, the mitochondrial-targeting property of the N-terminal sequence fused to gfp is conserved in orthologous and paralogous proteins that diverged approximately 80 million years ago, suggesting a conserved biological function. We propose that the conserved N-terminal motif of CAT proteins provides a regulatable signal for transport to, or retention in, different cell membrane compartments.

Galpha(s) is palmitoylated at the N-terminal glycine

Covalent lipid attachments are essential co- and post-translational modifications for signalling proteins. Galpha(s), the alpha-subunit of the heterotrimeric G protein that activates adenylyl cyclase, is known to be palmitoylated at the third N-terminal amino acid, a cysteine. Palmitoylation is involved in anchoring Galpha(s) to the membrane by increasing its intrinsic hydrophobicity. We identified by mass spectrometry a second, functionally even more important, covalent modification. It consists of another palmitoyl residue attached to the preceding glycine (Gly(2)). Palmitoylation at this position has profound consequences for levels of signal transduction. It sensitizes the cell up to 200-fold for adenylyl cyclase-stimulating agents. The inhibitory inputs mediated by Galpha(i) are downregulated to <10%. Thereby, Gly(2)-palmitoylation of Galpha(s) relieves cellular stimulation at the level of adenylyl cyclase whereas it renders the inhibitory modulation via Galpha(i) more difficult.

Role of palmitoylation/depalmitoylation reactions in G-protein-coupled receptor function

G-protein-coupled receptors (GPCRs) constitute one of the largest protein families in the human genome. They are subject to numerous post-translational modifications, including palmitoylation. This review highlights the dynamic nature of palmitoylation and its role in GPCR expression and function. The palmitoylation of other proteins involved in GPCR signaling, such as G-proteins, regulators of G-protein signaling, and G-protein-coupled receptor kinases, is also discussed.

CB(1) cannabinoid receptor-G protein association: a possible mechanism for differential signaling

Effects of cannabinoid compounds on neurons are predominantly mediated by the CB(1) cannabinoid receptor. Onset of signaling cascades in response to cannabimimetic drugs is triggered by the interaction of the cannabinoid receptor with G(i/o) proteins. Much work has been done to delineate the cannabinoid agonist-induced downstream signaling events; however, it remains to define the molecular basis of cannabinoid receptor-G protein interactions that stimulate these signaling pathways. In this review, we discuss several signal transduction pathways, focusing on studies that demonstrate the efficacy of CB(1) receptor agonists through G protein mediated pathways.

Characterization of subcellular localization and stability of a splice variant of G alpha i2

BACKGROUND

Alternative mRNA splicing of alpha(i2), a heterotrimeric G protein alpha subunit, has been shown to produce an additional protein, termed salpha(i2). In the salpha(i2) splice variant, 35 novel amino acids replace the normal C-terminal 24 amino acids of alpha(i2). Whereas alpha(i2) is found predominantly at cellular plasma membranes, salpha(i2) has been localized to intracellular Golgi membranes, and the unique 35 amino acids of salpha(i2) have been suggested to constitute a specific targeting signal.

RESULTS

This paper proposes and examines an alternative hypothesis: disruption of the normal C-terminus of alpha(i2) produces an unstable protein that fails to localize to plasma membranes. salpha(i2) is poorly expressed upon transfection of cultured cells; however, radiolabeling indicated that alpha(i2) and salpha(i2) undergo myristoylation, a co-translational modification, equally well suggesting that protein stability rather than translation is affected. Indeed, pulse-chase analysis indicates that salpha(i2) is more rapidly degraded compared to alpha(i2). Co-expression of betagamma rescues PM localization and increases expression of salpha(i2). In addition, alpha(i2)A327S, a mutant previously shown to be unstable and defective in guanine-nucleotide binding, and alpha(i2)(1-331), in which the C-terminal 24 amino acids of alpha(i2) are deleted, show a similar pattern of subcellular localization as salpha(i2) (i.e., intracellular membranes rather than plasma membranes). Finally, salpha(i2) displays a propensity to localize to potential aggresome-like structures.

CONCLUSIONS

Thus, instead of the novel C-terminus of salpha(i2) functioning as a specific Golgi targeting signal, the results presented here indicate that the disruption of the normal C-terminus of alpha(i2) causes mislocalization and rapid degradation of salpha(i2).

Characterization of a G protein coupling "YL" motif of the human VPAC1 receptor, equivalent to the first two amino acids in the "DRY" motif of the rhodopsin family

The conserved residues Y239 and L240 of human VPAC1 receptor are predicted to be at the same location as the asparagine and arginine in the "DRY" motif in the Rhodopsin family of G protein-coupled receptors. By comparing vasoactive intestinal peptide (VIP) binding with or without the presence of GTP-gamma-S, it was found that the deltadelta G(o) for the endogenous G-protein coupling was 1.5 kJ/mol, 0.95 kJ/mol, and 3.4 kJ/mol for theY239A, L240A, and wild-type receptor, respectively. VIP-induced cAMP production in whole cells support the results of the binding studies, as Y239A had a moderate and L240A a pronounced impaired ability to produce cAMP. The mutants had a minor influence on the intrinsic "low affinity to high affinity equilibrium," suggesting that the dominating effect of these mutants is a perturbation of the G protein-binding site. Thus, the highly diverged chemical properties of the hydrophobic "YL" motif and charged "DR(Y)" motif could be a crucial difference between the Secretin Receptor Family and the Rhodopsin Family with respect to receptor activation and G-protein coupling.

Gbeta gamma isoforms selectively rescue plasma membrane localization and palmitoylation of mutant Galphas and Galphaq

Mutation of Galpha(q) or Galpha(s) N-terminal contact sites for Gbetagamma resulted in alpha subunits that failed to localize at the plasma membrane or undergo palmitoylation when expressed in HEK293 cells. We now show that overexpression of specific betagamma subunits can recover plasma membrane localization and palmitoylation of the betagamma-binding-deficient mutants of alpha(s) or alpha(q). Thus, the betagamma-binding-defective alpha is completely dependent on co-expression of exogenous betagamma for proper membrane localization. In this report, we examined the ability of beta(1-5) in combination with gamma(2) or gamma(3) to promote proper localization and palmitoylation of mutant alpha(s) or alpha(q). Immunofluorescence localization, cellular fractionation, and palmitate labeling revealed distinct subtype-specific differences in betagamma interactions with alpha subunits. These studies demonstrate that 1) alpha and betagamma reciprocally promote the plasma membrane targeting of the other subunit; 2) beta(5), when co-expressed with gamma(2) or gamma(3), fails to localize to the plasma membrane or promote plasma membrane localization of mutant alpha(s) or alpha(q); 3) beta(3) is deficient in promoting plasma membrane localization of mutant alpha(s) and alpha(q), whereas beta(4) is deficient in promoting plasma membrane localization of mutant alpha(q); 4) both palmitoylation and interactions with betagamma are required for plasma membrane localization of alpha.

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.

Myristoylation alters retinoic acid-induced down-regulation of MARCKS in immortalized hippocampal cells

The myristoylated alanine-rich C kinase substrate (MARCKS) is a prominent PKC-substrate in the brain, which has been implicated in brain development, cytoskeletal remodeling, calcium/calmodulin signaling, and neuroplasticity. The sequence of the Macs gene codes for a protein that has three highly conserved domains including a 5' myristoylation region and a 25-amino-acid phosphorylation site domain (PSD), which are involved in anchoring MARCKS to the cellular membrane. In this study, we examined the role of the myristoylation signal in the regulation of MARCKS in transfected rat hippocampal cells (H19-7) following retinoic acid (RA) treatment. A mutant MARCKS lacking the myristoylation signal was engineered by substitution of alanine for glycine at position 2 of the Macs gene and was found to be exclusively expressed in the cytosol fraction of transfected cells. Exposure of the wild-type MARCKS-transfected cells to RA resulted in an apparent shift of MARCKS from the membrane to the cytosol, while the total protein of wild-type MARCKS was not significantly changed. In contrast, RA-exposed cells transfected with the mutant MARCKS revealed a dramatic reduction of expression of MARCKS protein in both cytosol and total protein fractions. These data suggest that the absence of the myristoyl moiety may not only alter the anchoring of the protein to the membrane but also play a novel role in modulating cellular levels of MARCKS protein in response to RA.

S-Nitrosocysteine increases palmitate turnover on Ha-Ras in NIH 3T3 cells

Ha-Ras is modified by isoprenoid on Cys(186) and by reversibly attached palmitates at Cys(181) and Cys(184). Ha-Ras loses 90% of its transforming activity if Cys(181) and Cys(184) are changed to serines, implying that palmitates make important contributions to oncogenicity. However, study of dynamic acylation is hampered by an absence of methods for acutely manipulating Ha-Ras palmitoylation in living cells. S-nitrosocysteine (SNC) and, to a more modest extent, S-nitrosoglutathione were found to rapidly increase [(3)H]palmitate incorporation into cellular or oncogenic Ha-Ras in NIH 3T3 cells. In contrast, SNC decreased [(3)H]palmitate labeling of the transferrin receptor and caveolin. SNC accelerated loss of [(3)H]palmitate from Ha-Ras, implying that SNC stimulated deacylation and permitted subsequent reacylation of Ha-Ras. SNC also decreased Ha-Ras GTP binding and inhibited phosphorylation of the kinases ERK1 and ERK2 in NIH 3T3 cells. Thus, SNC altered two important properties of Ha-Ras activation state and lipidation. These results identify SNC as a new tool for manipulating palmitate turnover on Ha-Ras and for studying requirements of repalmitoylation and the relationship between palmitate cycling, membrane localization, and signaling by Ha-Ras.