Phosphorylation of Gz in human platelets. Selectivity and site of modification

Every Detail Matters. That Is, How the Interaction between Gα Proteins and Membrane Affects Their Function

In highly organized multicellular organisms such as humans, the functions of an individual cell are dependent on signal transduction through G protein-coupled receptors (GPCRs) and subsequently heterotrimeric G proteins. As most of the elements belonging to the signal transduction system are bound to lipid membranes, researchers are showing increasing interest in studying the accompanying protein-lipid interactions, which have been demonstrated to not only provide the environment but also regulate proper and efficient signal transduction. The mode of interaction between the cell membrane and G proteins is well known. Despite this, the recognition mechanisms at the molecular level and how the individual G protein-membrane attachment signals are interrelated in the process of the complex control of membrane targeting of G proteins remain unelucidated. This review focuses on the mechanisms by which mammalian Gα subunits of G proteins interact with lipids and the factors responsible for the specificity of membrane association. We summarize recent data on how these signaling proteins are precisely targeted to a specific site in the membrane region by introducing well-defined modifications as well as through the presence of polybasic regions within these proteins and interactions with other components of the heterocomplex.

Gα12/13 signaling in metabolic diseases

As the key governors of diverse physiological processes, G protein-coupled receptors (GPCRs) have drawn attention as primary targets for several diseases, including diabetes and cardiovascular disease. Heterotrimeric G proteins converge signals from ~800 members of the GPCR family. Among the members of the G protein α family, the Gα₁₂ family members comprising Gα₁₂ and Gα₁₃ have been referred to as gep oncogenes. Gα_12/13 levels are altered in metabolic organs, including the liver and muscles, in metabolic diseases. The roles of Gα_12/13 in metabolic diseases have been investigated. In this review, we highlight findings demonstrating Gα_12/13 amplifying or dampening regulators of phenotype changes. We discuss the molecular basis of G protein biology in the context of posttranslational modifications to heterotrimeric G proteins and the cell signaling axis. We also highlight findings providing insights into the organ-specific, metabolic and pathological roles of G proteins in changes associated with specific cells, energy homeostasis, glucose metabolism, liver fibrosis and the immune and cardiovascular systems. This review summarizes the currently available knowledge on the importance of Gα_12/13 in the physiology and pathogenesis of metabolic diseases, which is presented according to the basic understanding of their metabolic actions and underlying cellular and molecular bases.

Understanding the activities of two members of a vital category of proteins called G proteins, which initiate metabolic changes when signaling molecules bind to cells, could lead to new therapies for many diseases. Researchers in South Korea and Japan, led by Sang Geon Kim at Seoul National University, review the significance of the Gα12 and Gα13 proteins in diseases characterised by significant changes in metabolism, including liver conditions and disorders of the cardiovascular and immune systems. Specific roles for the proteins have been identified by a variety of methods, including studying the effect of disabling the genes that code for them in mice. Recent insights suggest that drugs interfering with the activity of these Gα proteins might help treat many conditions in which the molecular signalling networks involving the proteins are disrupted.

A Differential Hypofunctionality of Gαi Proteins Occurs in Adolescent Idiopathic Scoliosis and Correlates with the Risk of Disease Progression

Adolescent idiopathic scoliosis is the most prevalent spine deformity and the molecular mechanisms underlying its pathophysiology remain poorly understood. We have previously found a differential impairment of melatonin receptor signaling in AIS osteoblasts allowing the classification of patients into three biological endophenotypes or functional groups (FG1, FG2 and FG3). Here, we provide evidence that the defect characterizing each endophenotype lies at the level of Gαi proteins leading to a systemic and generalized differential impairment of Gi-coupled receptor signaling. The three Gαi isoforms exhibited a selective serine phosphorylation patterns for each AIS endophenotype resulting in a differential reduction in Gαi protein activity as determined by cellular dielectric spectroscopy and small interfering RNA methods. We found that one endophenotype (FG2) with phosphorylated Gαi₁ and Gαi₂ was consistently associated with a significantly high risk of spinal deformity progression when compared to the other two endophenotypes (FG1 and FG3). We further demonstrated that each endophenotype is conserved among affected family members. This study expands our understanding of the mechanism underlying the Gi-coupled receptor signaling dysfunction occurring in AIS and provides the first evidence for its hereditary nature. Collectively, our findings offers a new perspective on Gαi hypofunctionality in a human disease by revealing specific serine phosphorylation signatures of Gαi isoforms that may facilitate the identification of AIS patients at risk of spinal deformity progression.

G protein subunit phosphorylation as a regulatory mechanism in heterotrimeric G protein signaling in mammals, yeast, and plants

Heterotrimeric G proteins composed of Gα, Gβ, and Gγ subunits are vital eukaryotic signaling elements that convey information from ligand-regulated G protein-coupled receptors (GPCRs) to cellular effectors. Heterotrimeric G protein-based signaling pathways are fundamental to human health [Biochimica et Biophysica Acta (2007) 1768, 994-1005] and are the target of >30% of pharmaceuticals in clinical use [Biotechnology Advances (2013) 31, 1676-1694; Nature Reviews Drug Discovery (2017) 16, 829-842]. This review focuses on phosphorylation of G protein subunits as a regulatory mechanism in mammals, budding yeast, and plants. This is a re-emerging field, as evidence for phosphoregulation of mammalian G protein subunits from biochemical studies in the early 1990s can now be complemented with contemporary phosphoproteomics and genetic approaches applied to a diversity of model systems. In addition, new evidence implicates a family of plant kinases, the receptor-like kinases, which are monophyletic with the interleukin-1 receptor-associated kinase/Pelle kinases of metazoans, as possible GPCRs that signal via subunit phosphorylation. We describe early and modern observations on G protein subunit phosphorylation and its functional consequences in these three classes of organisms, and suggest future research directions.

Regulators of G protein signaling in neuropsychiatric disorders

Regulators of G protein signaling (RGS) comprise a diverse group of about 40 proteins which determine signaling amplitude and duration via modulation of receptor/G protein or receptor/effector coupling. Several members of the RGS family are expressed in the brain, where they have precise roles in regulation of important physiological processes. The unique functions of each RGS can be attributed to its structure, distinct pattern of expression, and regulation, and its preferential interactions with receptors, Galpha subunits and other signaling proteins. Evidence suggests dysfunction of RGS proteins is related to several neuropathological conditions. Moreover, clinical and preclinical work reveals that the efficacy and/or side effects of treatments are highly influenced by RGS activity. This article summarizes findings on RGS proteins in vulnerability to several neuropsychiatric disorders, the mechanism via which RGS proteins control neuronal responses and their potential use as drug targets.

Molecular determinants of melatonin signaling dysfunction in adolescent idiopathic scoliosis

Presently, the genetic cause of adolescent idiopathic scoliosis (AIS), the most common form of scoliosis, remains unclear. Among many hypotheses, the neuroendocrine hypothesis involving a melatonin deficiency as the source for AIS generated the greatest interest and controversy since no decrease in circulating melatonin level has been observed in a majority of studies. Previously, we have reconciled the role of melatonin in AIS by demonstrating a melatonin signaling dysfunction occurring in osteoblasts derived from AIS patients, which contrasted with similar cells isolated from healthy subjects. We found that this difference is caused in AIS cells by increased phosphorylation of serine residues affecting the activity of G inhibitory proteins normally associated with melatonin cell surface receptors. Here we propose a preliminary molecular classification of patients with AIS based on the cellular response to the melatonin (cAMP) and distinct protein-protein interactions. These interactions include those between protein kinase C delta (PKCdelta) and MT2 melatonin receptors or PKCdelta and the receptor for activated protein C kinase 1. This finding could help in future molecular classification of patients with AIS.

RGSZ1 interacts with protein kinase C interacting protein PKCI-1 and modulates mu opioid receptor signaling

Protein kinase C interacting protein (PKCI-1) was identified among the potential interactors from a yeast two hybrid screen of human brain library using N terminal of RGSZ1 as a bait. The cysteine string region, unique to the RZ subfamily, contributes to the observed interaction because PKCI-1 interacted with N-terminus of RGS17 and GAIP, but not with that of RGS2 or RGS7 where cysteine string motif is absent. The interaction between RGSZ1 and PKCI-1 was confirmed by coimmunoprecipitation and immunofluorescence. PKCI-1 and RGSZ1 could be detected by coimmunoprecipitation using 14-3-3 antibody in cells transfected with PKCI-1 or RGSZ1 respectively, but when transfected with PKCI-1 and RGSZ1 together, only RGSZ1 could be detected. Phosphorylation of Galphaz by protein kinase C (PKC) reduces the ability of the RGS to effectively function as GTPase accelerating protein for Galphaz, and interferes with ability of Galphaz to interact with betagamma complex. We investigated the roles of 14-3-3 and PKCI-1 in phosphorylation of Galphaz. Phosphorylation of Galphaz by PKC was inhibited by 14-3-3 and the presence of PKCI-1 did not provide any further inhibition. PKCI-1 interacts with mu opioid receptor and suppresses receptor desensitization and PKC related mu opioid receptor phosphorylation [W. Guang, H. Wang, T. Su, I.B. Weinstein, J.B. Wang, Mol. Pharmacol. 66 (2004) 1285.]. Previous studies have also shown that mu opioid receptor co-precipitates with RGSZ1 and influence mu receptor signaling by acting as effector antagonists [J. Garzon, M. Rodriguez-Munoz, P. Sanchez-Blazquez, Neuropharmacology 48 (2005) 853., J. Garzon, M. Rodriguez-Munoz, A. Lopez-Fando, P. Sanchez-Blazquez Neuropsychopharmacology 30 (2005) 1632.]. Inhibition of cAMP by mu opioid receptor was significantly reduced by RGSZ1 and this effect was enhanced in combination with PKCI-1. Our studies thus provide a link between the previous observations mentioned above and indicate that the major function of PKCI-1 is to modulate mu opioid receptor signaling pathway along with RGSZ1, rather than directly mediating the Galphaz RGSZ1 interaction.

The RGSZ2 protein exists in a complex with mu-opioid receptors and regulates the desensitizing capacity of Gz proteins

The regulator of G-protein signaling RGS17(Z2) is a member of the RGS-Rz subfamily of GTPase-activating proteins (GAP) that efficiently deactivate GalphazGTP subunits. We have found that in the central nervous system (CNS), the levels of RGSZ2 mRNA and protein are elevated in the hypothalamus, midbrain, and pons-medulla, and that RGSZ2 is glycosylated in synaptosomal membranes isolated from CNS tissue. In analyzing the function of RGSZ2 in the CNS, we found that when the expression of RGSZ2 was impaired, the antinociceptive response to morphine and [D-Ala2, N-MePhe4, Gly-ol5]-enkephalin (DAMGO) augmented. This potentiation involved mu-opioid receptors and increased tolerance to further doses of these agonists administered 24 h later. High doses of morphine promoted agonist desensitization even within the analgesia time-course, a phenomenon that appears to be related to the great capacity of morphine to activate Gz proteins. In contrast, the knockdown of RGSZ2 proteins did not affect the activity of delta receptor agonists, [D-Pen2,5]-enkephalin (DPDPE), and [D-Ala2] deltorphin II. In membranes from periaqueductal gray matter (PAG), both RGSZ2 and the related RGS20(Z1) co-precipitated with mu-opioid receptors. While a morphine challenge reduced the association of Gi/o/z with mu receptors, it increased their association with the RGSZ2 and RGSZ1 proteins. However, only Galphaz subunits co-precipitated with RGSZ2. Doses of morphine that produced acute tolerance maintained the association of Galpha subunits with RGSZ proteins even after the analgesic effects had ceased. These results indicate that both RGSZ1 and RGSZ2 proteins influence mu receptor signaling by sequestering Galpha subunits, therefore behaving as effector antagonists.

Regulation of hematopoietic-specific G-protein Galpha15 and Galpha16 by protein kinase C

Heterotrimeric G proteins mediate cell growth and differentiation by coupling cell surface receptors to intracellular effector enzymes. The G-protein alpha subunit, Galpha(16), and its murine homologue Galpha(15), are expressed specifically in hematopoietic cells and their expression is highly regulated during differentiation of normal and leukemic cells. In this study, we examined the phosphorylation of Galpha(15)/Galpha(16) and its role in receptor and effector coupling. We observed a PMA-stimulated intact cell phosphorylation of Galpha(15) in COS7 cells transfected with Galpha(15) and protein kinase Calpha (PKCalpha), and phosphorylation of endogenous Galpha(16) in HL60 cells. We also showed that peptides derived from the two G-proteins were phosphorylated in vitro using purified brain PKC. Furthermore, we identified the putative phosphorylation site and showed that mutation or deletion of this PKC phosphorylation site inhibited phospholipase C (PLC) activation. The behavior of double mutants with the constitutively active G-protein mutation (QL-mutant) and mutation in the putative phosphorylation site suggests that the phosphorylation site of Galpha(15/16) is essential for receptor-coupled activation of PLC, but not for direct interaction of the G-protein with PLC-beta.

Characterization and channel coupling of the P2Y(12) nucleotide receptor of brain capillary endothelial cells

Rat brain capillary endothelial (B10) cells express an unidentified nucleotide receptor linked to adenylyl cyclase inhibition. We show that this receptor in B10 cells is identical in sequence to the P2Y(12) ADP receptor ("P2Y(T)") of platelets. When expressed heterologously, 2-methylthio-ADP (2-MeSADP; EC(50), 2 nm), ADP, and adenosine 5'-O-(2-thio)diphosphate were agonists of cAMP decrease, and 2-propylthio-D-beta,gamma-difluoromethylene-ATP was a competitive antagonist (K(B), 28 nm), as in platelets. However, 2-methylthio-ATP (2-MeSATP) (EC(50), 0.4 nm), ATP (1.9 microm), and 2-chloro-ATP (190 nm), antagonists in the platelet, were also agonists. 2-MeSADP activated (EC(50), 0.1 nm) GIRK1/GIRK2 inward rectifier K(+) channels when co-expressed with P2Y(12) receptors in sympathetic neurons. Surprisingly, P2Y(1) receptors expressed likewise gave that response; however, a full inactivation followed, absent with P2Y(12) receptors. A new P2Y(12)-mediated transduction was found, the closing of native N-type Ca(2+) channels; again both 2-MeSATP and 2-MeSADP are agonists (EC(50), 0.04 and 0.1 nm, respectively). That action, like their cAMP response, was pertussis toxin-sensitive. The Ca(2+) channel inhibition and K(+) channel activation are mediated by beta gamma subunit release from a heterotrimeric G-protein. G alpha subunit types in B10 cells were also identified. The presence in the brain capillary endothelial cell of the P2Y(12) receptor is a significant extension of its functional range.

G(z alpha) deficient mice: enzyme levels in the autonomic nervous system, neuronal survival and effect of genetic background

Our laboratory has generated a genetically mutant mouse in which the alpha subunit of the heterotrimeric GTP binding protein, G(z) has been made dysfunctional by homologous recombination to determine its in vivo function. These animals show a characteristic failure to thrive phenotype. G(z alpha) is expressed in a variety of nervous system tissues as well as in the adrenal medulla. We therefore examined the autonomic nervous system of the G(z alpha) deficient mouse by measuring the activity of tyrosine hydroxylase and choline acetyltransferase in the superior cervical ganglia, submaxillary gland and the adrenal medulla. Preliminary results using animals of mixed BALB/c and C57BL/6 strains gave inconsistent results. Further experiments demonstrated differences in the activity of tyrosine hydroxylase and choline acetyltransferase between BALB/c and C57BL/6 mouse strains. The analysis of the pure strains showed a reduction in the size and enzyme levels of the adrenal gland and submaxillary glands of the G(z alpha) deficient mouse suggesting a role for adrenal insufficiency and/or nutritional disorders for the failure to thrive phenotype. The survival of sympathetic and sensory neurons was also examined in the G(z alpha) deficient mouse and in the presence of pertussis toxin, sympathetic but not sensory neuronal survival in G(z alpha) deficient mice was significantly attenuated. This suggests that in vivo other pertussis toxin sensitive G proteins may be recruited to compensate for the loss of G(z alpha).

G(z) signaling: emerging divergence from G(i) signaling

A large variety of neurotransmitters, hormones, and chemokines regulate cellular functions via cell surface receptors that are coupled to guanine nucleotide-binding regulatory proteins (G proteins) belonging to the G(i) subfamily. All members of the G(i) subfamily, with the sole exception of G(z), are substrates for the pertussis toxin ADP-ribosyl transferase. G(z) also exhibits unique biochemical and regulatory properties. Initial portrayals of the cellular functions of G(z) bear high resemblance to those of other G(i) proteins both in terms of the receptors and effectors linked to G(z). However, recent discoveries have begun to insinuate a distinct role for G(z) in cellular communication. Functional interactions of the alpha subunit of G(z) (Galpha(z)) with the NKR-P1 receptor, Galpha(z)-specific regulator of G protein signaling, p21-activated kinase, G protein-regulated inducers of neurite outgrowth, and the Eya2 transcription cofactor have been demonstrated. These findings provide possible links for G(z) to participate in cellular development, survival, proliferation, differentiation and even apoptosis. In this review, we have drawn a sketch of a signaling network with G(z) as the centerpiece. The emerging picture is one that distinguishes G(z) from other members of the G(i) subfamily.

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.

Heterologous desensitization of response mediated by selective PKC-dependent phosphorylation of G(i-1) and G(i-2)

This study examined the ability of protein kinase C (PKC) to induce heterologous desensitization by targeting specific G proteins and limiting their ability to transduce signals in smooth muscle. Activation of PKC by pretreatment of intestinal smooth muscle cells with phorbol 12-myristate 13-acetate, cholecystokinin octapeptide, or the phosphatase 1 and phosphatase 2A inhibitor, calyculin A, selectively phosphorylated Galpha(i-1) and Galpha(i-2), but not Galpha(i-3) or Galpha(o), and blocked inhibition of adenylyl cyclase mediated by somatostatin receptors coupled to G(i-1) and opioid receptors coupled to G(i-2), but not by muscarinic M(2) and adenosine A(1) receptors coupled to G(i-3). Phosphorylation of Galpha(i-1) and Galpha(i-2) and blockade of cyclase inhibition were reversed by calphostin C and bisindolylmaleimide, and additively by selective inhibitors of PKCalpha and PKCepsilon. Blockade of inhibition was prevented by downregulation of PKC. Phosphorylation of Galpha-subunits by PKC also affected responses mediated by betagamma-subunits. Pretreatment of muscle cells with cANP-(4-23), a selective agonist of the natriuretic peptide clearance receptor, NPR-C, which activates phospholipase C (PLC)-beta3 via the betagamma-subunits of G(i-1) and G(i-2), inhibited the PLC-beta response to somatostatin and [D-Pen(2,5)]enkephalin. The inhibition was partly reversed by calphostin C. Short-term activation of PKC had no effect on receptor binding or effector enzyme (adenylyl cyclase or PLC-beta) activity. We conclude that selective phosphorylation of Galpha(i-1) and Galpha(i-2) by PKC partly accounts for heterologous desensitization of responses mediated by the alpha- and betagamma-subunits of both G proteins. The desensitization reflects a decrease in reassociation and thus availability of heterotrimeric G proteins.

Loss of signaling through the G protein, Gz, results in abnormal platelet activation and altered responses to psychoactive drugs

Heterotrimeric G proteins mediate the earliest step in cell responses to external events by linking cell surface receptors to intracellular signaling pathways. G(z) is a member of the G(i) family of G proteins that is prominently expressed in platelets and brain. Here, we show that deletion of the alpha subunit of G(z) in mice: (i) impairs platelet aggregation by preventing the inhibition of cAMP formation normally seen at physiologic concentrations of epinephrine, and (ii) causes the mice to be more resistant to fatal thromboembolism. Loss of G(zalpha) also results in greatly exaggerated responses to cocaine, reduces the analgesic effects of morphine, and abolishes the effects of widely used antidepressant drugs that act as catecholamine reuptake inhibitors. These changes occur despite the presence of other G(ialpha) family members in the same cells and are not accompanied by detectable compensatory changes in the level of expression of other G protein subunits. Therefore, these results provide insights into receptor selectivity among G proteins and a model for understanding platelet function and the effects of psychoactive drugs.

Hypertolerance to morphine in G(z alpha)-deficient mice

Our laboratory has generated a mouse deficient in the alpha (alpha) subunit of the G protein, G(z), (G(z alpha)) gene and we have examined the involvement of G(z alpha) in spinal and supraspinal analgesia and tolerance mechanisms. Spinal analgesia was tested by the response times to heat or cold tail flick times in a water bath at 50 degrees C or -5 degrees C and supraspinal analgesia was tested by the times for paw licking and jumping from a plate at 52 degrees C or 0.5 degrees C. Tolerance to morphine was induced in wild type and G(z alpha)-deficient mice over a 5 day period and the behavioral tests were performed daily. The tail flick reaction times to both hot and cold stimuli did not differ between the wild type and G(z alpha)-deficient mice. Analysis of the reaction times from the hot and cold plate tests showed the G(z alpha)-deficient mice developed tolerance to morphine to a greater degree and at a faster rate than wild type mice. Opioid binding assays were performed on synaptic membranes prepared from naive and morphine tolerant wild type and G(z alpha)-deficient brains. No changes in the affinity of morphine for its receptor or in the density of mu and delta opioid receptors were found between the two groups of mice in the naive or morphine tolerant state. This indicates that the absence of G(z alpha) does not affect opioid receptor affinity or receptor up or down regulation. Our results suggest that the presence of G(z alpha) delays the development of morphine tolerance and represents a possible therapeutic target for improving the clinical use of morphine.

Evidence for transduction of mu but not kappa opioid modulation of extracellular signal-regulated kinase activity by G(z) and G(12) proteins

Chronic treatment with micro or kappa opioid agonists (>/=2 h) inhibits EGF-induced ERK activation in opioid receptor overexpressing COS-7 cells. Although acute mu and kappa opioids activate ERK via a pertussis toxin-sensitive G protein, pertussis toxin insensitivity of the chronic mu (but not kappa) action was observed. Here, we tested several pertussis toxin-insensitive G proteins as candidates to transduce acute and/or chronic opioid modulation of ERK. Overexpressed Galpha(z) (but not Galpha(12)) transduced acute mu (but not kappa) ERK activation in pertussis toxin-treated COS-7 cells. Chronic mu (but not kappa) inhibited EGF stimulation of ERK in pertussis toxin-treated cells overexpressing Galpha(z) or Galpha(12). Transfection of Galpha(13) or Galpha(q) blocked inhibition under the same conditions. Overexpressed interfering and non-interfering Galpha(z) mutants differentially affected mu inhibition of ERK consistent with G(z) transduction. In this and prior studies, Galpha(z) and Galpha(12) immunoreactivity were detected in untransfected COS-7 cells, suggesting that these G proteins may be endogenous mediators of chronic mu inhibitory actions on ERK.

The role of heterotrimeric G proteins in platelet activation

Activation of platelets plays a central role in hemostasis as well as in various thromboembolic diseases like myocardial infarction or stroke. Most platelet activating stimuli function through receptors which couple to heterotrimeric G proteins of the Gi, Gq and G12 families. Recent studies have elucidated the roles of individual G proteins in the regulation of platelet functions like shape change, aggregation and granule secretion. The signaling pathways mediated by heterotrimeric G proteins operate synergistically to induce a full activation of platelets. This review summarizes recent progress in the understanding of upstream regulation of platelet activation through G protein-coupled receptors.

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.

Reciprocal signaling between heterotrimeric G proteins and the p21-stimulated protein kinase

p21-activated protein kinase (PAK)-1 phosphorylated Galpha(z), a member of the Galpha(i) family that is found in the brain, platelets, and adrenal medulla. Phosphorylation approached 1 mol of phosphate/mol of Galpha(z) in vitro. In transfected cells, Galpha(z) was phosphorylated both by wild-type PAK1 when stimulated by the GTP-binding protein Rac1 and by constitutively active PAK1 mutants. In vitro, phosphorylation occurred only at Ser(16), one of two Ser residues that are the major substrate sites for protein kinase C (PKC). PAK1 did not phosphorylate other Galpha subunits (i1, i2, i3, o, s, or q). PAK1-phosphorylated Galpha(z) was resistant both to RGSZ1, a G(z)-selective GTPase-activating protein (GAP), and to RGS4, a relatively nonselective GAP for the G(i) and G(q) families of G proteins. Phosphorylation of Ser(27) by PKC did not alter sensitivity to either GAP. The previously described inhibition of G(z) GAPs by PKC is therefore mediated by phosphorylation of Ser(16). Phosphorylation of either Ser(16) by PAK1 or Ser(27) by PKC decreased the affinity of Galpha(z) for Gbetagamma; phosphorylation of both residues by PKC caused no further effect. PAK1 thus regulates Galpha(z) function by attenuating the inhibitory effects of both GAPs and Gbetagamma. In this context, the kinase activity of PAK1 toward several protein substrates was directly inhibited by Gbetagamma, suggesting that PAK1 acts as a Gbetagamma-regulated effector protein. This inhibition of mammalian PAK1 by Gbetagamma contrasts with the stimulation of the PAK homolog Ste20p in Saccharomyces cerevisiae by the Gbetagamma homolog Ste4p/Ste18p.

Physiological regulation of G protein-linked signaling

Heterotrimeric G proteins in vertebrates constitute a family molecular switches that transduce the activation of a populous group of cell-surface receptors to a group of diverse effector units. The receptors include the photopigments such as rhodopsin and prominent families such as the adrenergic, muscarinic acetylcholine, and chemokine receptors involved in regulating a broad spectrum of responses in humans. Signals from receptors are sensed by heterotrimeric G proteins and transduced to effectors such as adenylyl cyclases, phospholipases, and various ion channels. Physiological regulation of G protein-linked receptors allows for integration of signals that directly or indirectly effect the signaling from receptor-->G protein-->effector(s). Steroid hormones can regulate signaling via transcriptional control of the activities of the genes encoding members of G protein-linked pathways. Posttranscriptional mechanisms are under physiological control, altering the stability of preexisting mRNA and affording an additional level for regulation. Protein phosphorylation, protein prenylation, and proteolysis constitute major posttranslational mechanisms employed in the physiological regulation of G protein-linked signaling. Drawing upon mechanisms at all three levels, physiological regulation permits integration of demands placed on G protein-linked signaling.

Cyclic AMP-dependent phosphorylation of thromboxane A(2) receptor-associated Galpha(13)

Although it is well established that cAMP inhibits platelet activation induced by all agonists, the thromboxane A(2) signal transduction pathway was found to be particularly sensitive to such inhibition. Therefore, we examined whether cAMP-dependent kinase mediates phosphorylation of the thromboxane A(2) receptor-G-protein complex. It was found that cAMP induces protein kinase A-dependent [gamma-(32)P]ATP labeling of solubilized membrane proteins in the region of Galpha subunits, i.e. 38-45 kDa. Moreover, ligand affinity chromatography purification of thromboxane A(2) receptor-G-protein complexes from these membranes revealed that 38-45-kDa phosphoproteins co-purify with thromboxane A(2) receptors. Immunoprecipitation of the affinity column eluate with a Galpha(13) antibody demonstrated that 8-Br-cAMP increased phosphorylation of thromboxane A(2) receptor-associated Galpha(13) by 87 +/- 27%. In separate experiments, immunopurification of Galpha(13) on microtiter wells coated with a different Galpha(13) antibody revealed that 8-Br-cAMP increased Galpha(13) phosphorylation by 53 +/- 19%. Finally, treatment of (32)P-labeled whole platelets with prostacyclin resulted in a 90 +/- 14% increase in phosphorylated Galpha(13) that was abolished by pretreatment with the adenylate cyclase inhibitor MDL-12. These results provide the first evidence that protein kinase A mediates phosphorylation of Galpha(13) both in vitro and in vivo and provides a basis for the preferential inhibition of thromboxane A(2)-mediated signaling in platelets by cAMP.

A Critical Role for N-ethylmaleimide–Sensitive Fusion Protein (NSF) in Platelet Granule Secretion

The molecular mechanisms that regulate membrane targeting/fusion during platelet granule secretion are not yet understood.N-ethylmaleimide-sensitive fusion protein (NSF), soluble NSF attachment proteins (SNAPs), and SNAREs (SNAP receptors) are elements of a conserved molecular machinery for membrane targeting/fusion that have been detected in platelets. We examined whether NSF, an ATPase that has been shown to play a critical role in membrane targeting/fusion in many cell types, is necessary for platelet granule secretion. Peptides that mimic NSF sequence motifs inhibited both -granule and dense-granule secretion in permeabilized human platelets. This inhibitory effect was sequence-specific, because neither proteinase K-digested peptides nor peptides containing similar amino acids in a scrambled sequence inhibited platelet secretion. The peptides that inhibited platelet granule secretion also inhibited the human recombinant -SNAP–stimulated ATPase activity of recombinant NSF. It was also found that anti-NSF antibodies, which inhibited recombinant -SNAP–stimulated ATPase activity of NSF, inhibited platelet granule secretion in permeabilized cells. The inhibition by anti-NSF antibodies was abolished by the addition of recombinant NSF. These data provide the first functional evidence that NSF plays an important role in platelet granule secretion.

A Critical Role for N-ethylmaleimide–Sensitive Fusion Protein (NSF) in Platelet Granule Secretion

The molecular mechanisms that regulate membrane targeting/fusion during platelet granule secretion are not yet understood.N-ethylmaleimide-sensitive fusion protein (NSF), soluble NSF attachment proteins (SNAPs), and SNAREs (SNAP receptors) are elements of a conserved molecular machinery for membrane targeting/fusion that have been detected in platelets. We examined whether NSF, an ATPase that has been shown to play a critical role in membrane targeting/fusion in many cell types, is necessary for platelet granule secretion. Peptides that mimic NSF sequence motifs inhibited both -granule and dense-granule secretion in permeabilized human platelets. This inhibitory effect was sequence-specific, because neither proteinase K-digested peptides nor peptides containing similar amino acids in a scrambled sequence inhibited platelet secretion. The peptides that inhibited platelet granule secretion also inhibited the human recombinant -SNAP–stimulated ATPase activity of recombinant NSF. It was also found that anti-NSF antibodies, which inhibited recombinant -SNAP–stimulated ATPase activity of NSF, inhibited platelet granule secretion in permeabilized cells. The inhibition by anti-NSF antibodies was abolished by the addition of recombinant NSF. These data provide the first functional evidence that NSF plays an important role in platelet granule secretion.

Protein kinase C-promoted inhibition of Galpha(11)-stimulated phospholipase C-beta activity

The effects of protein kinase C (PKC) activation on inositol lipid signaling were examined. Using the turkey erythrocyte model of receptor-regulated phosphoinositide hydrolysis, we developed a membrane reconstitution assay to study directly the effects of activation of PKC on the activities of Galpha(11), independent of potential effects on the receptor or on PLC-beta. Membranes isolated from erythrocytes pretreated with 4beta-phorbol-12beta-myristate-13alpha-acetate (PMA) exhibited a decreased capacity for Galpha(11)-mediated activation of purified, reconstituted PLC-beta1. This inhibitory effect was dependent on both the time and concentration of PMA incubation and occurred as a decrease in the efficacy of GTPgammaS for activation of PLC-beta1, both in the presence and absence of agonist; no change in the apparent affinity for the guanine nucleotide occurred. Similar inhibitory effects were observed after treatment with the PKC activator phorbol-12,13-dibutyrate but not after treatment with an inactive phorbol ester. The inhibitory effects of PMA were prevented by coaddition of the PKC inhibitor bisindolylmaleimide. Although the effects of PKC could be localized to the membrane, no phosphorylation of Galpha(11) occurred either in vitro in the presence of purified PKC or in intact erythrocytes after PMA treatment. These results support the hypothesis that a signaling protein other than Galpha(11) is the target for PKC and that PKC-promoted phosphorylation of this protein results in a phosphorylation-dependent suppression of Galpha(11)-mediated PLC-beta1 activation.

An EGF receptor-mediated signal attenuates the inhibitory effect of LPA on an adenylate cyclase activity

A tyrosine kinase receptor-mediated and a heterotrimeric G protein-coupled receptor-mediated signals have been shown to evoke distinct intracellular signaling events. There has been increasing evidence that cross-talk exists between a tyrosine kinase receptor-mediated and a heterotrimeric G protein-coupled receptor-mediated signal transduction pathways. In the present study, we have studied effects of EGF receptor activation on activities of inhibitory G protein (Gi). We show that the amounts of Gi/Go ADP-ribosylated by islet-activating protein (IAP) increased by 30-40% in the membranes of Rat 1 fibroblast cells pretreated with EGF compared with those without pretreatment. When an effect of lysophosphatidic acid (LPA) stimulation on an adenylate cyclase activity was examined, LPA partly attenuated forskolin-stimulated adenylate cyclase activity via Gi because IAP pretreatment blocked the inhibitory effect of LPA. Pretreatment with EGF reduced the ability of LPA to inhibit the forskolin-stimulated adenylate cyclase activity, while the pretreatment did not have any effects on the forskolin-stimulated activity. Thus, the EGF receptor-mediated signal appears to cause the impairment of Gi function in Rat 1 fibroblast cells.

G protein alpha subunit G alpha z couples neurotransmitter receptors to ion channels in sympathetic neurons

The functional roles subserved by G(alpha)z, a G protein alpha subunit found predominantly in neuronal tissues, have remained largely undefined. Here, we report that G(alpha)z coupled neurotransmitter receptors to N-type Ca2+ channels when transiently overexpressed in rat sympathetic neurons. The G(alpha)z-mediated inhibition was voltage dependent and PTX insensitive. Recovery from G(alpha)z-mediated inhibition was extremely slow but accelerated by coexpression with RGS proteins. G(alpha)z selectively interacted with a subset of receptors that ordinarily couple to N-type Ca2+ channels via PTX-sensitive Go/i proteins. In addition, G(alpha)z rescued the activation of heterologously expressed GIRK channels in PTX-treated neurons. These results suggest that G(alpha)z is capable of coupling receptors to ion channels and might underlie PTX-insensitive ion channel modulation observed in neurons under physiological and pathological conditions.

RGSZ1, a Gz-selective RGS protein in brain. Structure, membrane association, regulation by Galphaz phosphorylation, and relationship to a Gz gtpase-activating protein subfamily

We cloned the cDNA for human RGSZ1, the major Gz-selective GTPase-activating protein (GAP) in brain (Wang, J., Tu, Y., Woodson, J., Song, X., and Ross, E. M. (1997) J. Biol. Chem. 272, 5732-5740) and a member of the RGS family of G protein GAPs. Its sequence is 83% identical to RET-RGS1 (except its N-terminal extension) and 56% identical to GAIP. Purified, recombinant RGSZ1, RET-RGS1, and GAIP each accelerated the hydrolysis of Galphaz-GTP over 400-fold with Km values of approximately 2 nM. RGSZ1 was 100-fold selective for Galphaz over Galphai, unusually specific among RGS proteins. Other enzymological properties of RGSZ1, brain Gz GAP, and RET-RGS1 were identical; GAIP differed only in Mg2+ dependence and in its slightly lower selectivity for Galphaz. RGSZ1, RET-RGS1, and GAIP thus define a subfamily of Gz GAPs within the RGS proteins. RGSZ1 has no obvious membrane-spanning region but is tightly membrane-bound in brain. Its regulatory activity in membranes depends on stable bilayer association. When co-reconstituted into phospholipid vesicles with Gz and m2 muscarinic receptors, RGSZ1 increased agonist-stimulated GTPase >15-fold with EC50 <12 nM, but RGSZ1 added to the vesicle suspension was <0.1% as active. RGSZ1, RET-RGS1, and GAIP share a cysteine string sequence, perhaps targeting them to secretory vesicles and allowing them to participate in the proposed control of secretion by Gz. Phosphorylation of Galphaz by protein kinase C inhibited the GAP activity of RGSZ1 and other RGS proteins, providing a mechanism for potentiation of Gz signaling by protein kinase C.

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.

Developmental expression of messenger RNA levels of the alpha subunit of the GTP-binding protein, Gz, in the mouse nervous system

There has been recent evidence that Gz may play a role in the transmission of the neurotrophic signal from nerve terminals to the cell bodies [Johanson, S.O., Crouch, M.F., Hendry, I.A., Signal transduction from membrane to nucleus: the special case for neurons, Neurochem. Res. 21 (1996) 779-785]. We examined the developmental expression of the alpha subunit of Gz (Gzalpha) in the peripheral and central nervous systems of the mouse. Our laboratory has developed a quantitative reverse transcription-polymerase chain reaction (RT-PCR) for Gzalpha which makes use of a fragment of the PCR product shortened by 107 base pairs creating a standard which mimics the original RNA. Serial dilutions of the mouse RNA with a constant concentration of mimic RNA were made and the point where equal amounts of product are formed allows accurate measurement of Gzalpha mRNA in the tissue. We have demonstrated that in the developing mouse superior cervical ganglion (SCG), dorsal root ganglion (DRG) and trigeminal ganglion the expression of Gzalpha mRNA is highest perinatally. From 3 weeks of age, in all tissues with the exception of the SCG, Gzalpha mRNA levels fall to lower levels in the adult animal. The developmental pattern of expression of Gzalpha in both the cerebellum and the brain differs from the peripheral nervous system. In the cerebellum, Gzalpha mRNA expression is highest around birth and in the brain it is highest around third postnatal week and then the levels decline as adulthood is approached. These results suggest that the highest level of Gzalpha mRNA is expressed at the time when target tissue innervation is occurring. This further strengthens the hypothesis that Gzalpha is important in the transfer of information from target tissues to the innervating nerve cells.

Agonist-independent activation of Gz by the 5-hydroxytryptamine1A receptor co-expressed in Spodoptera frugiperda cells. Distinguishing inverse agonists from neutral antagonists

The human 5-hydroxytryptamine1A receptor, when expressed in Spodoptera frugiperda (Sf9) cells, facilitates the binding of [35S]GTPgammaS to a co-expressed GTP-binding regulatory protein, Gz, consistent with constitutive activity. The antagonists 4-(2'-methoxyphenyl)-1-[2'(n-2"-pyridinyl)-p-iodobenzamido]ethyl-p ipe razine (p-MPPI) and the related fluorobenzamido analogue p-MPPF had little (p-MPPI) or no (p-MPPF) effect on this activity. In contrast, a third antagonist, the neuroleptic spiperone, produced an almost complete suppression. Thus, using G protein activation as an index of receptor activity, p-MPPF was classified as a neutral antagonist, p-MPPI as a partial inverse agonist, and spiperone as essentially a full inverse agonist. As predicted, spiperone displayed properties consistent with a special form of noncompetitive antagonism when used to displace the agonist [125I]R-(+)-trans-8-hydroxy-2-[N-n-propyl-N-(3'-iodo-2'-propenyl)amin o]tetralin. Our data profile Sf9 cells as a unique vehicle for the characterization of inverse agonists, as these cells support a systematic pairing of mammalian receptors and G proteins, quantitative assays of G protein activation, and unambiguously labeled populations of coupled and uncoupled receptors.

Signal transduction through trimeric G proteins in megakaryoblastic cell lines

The biogenesis of trimeric G proteins was investigated by measurement of the expression of alpha-subunits in the megakaryoblastic cell lines MEG-01, DAMI, and CHRF-288-11, representing stages of increasing maturation, and compared with platelets. Megakaryoblasts and platelets contained approximately equal amounts of Gi alpha-1/2, Gi alpha-3, Gq alpha, and G12 alpha protein. Maturation was accompanied by (1) downregulation of mRNA for Gs alpha and disappearance of iloprost-induced Ca2+ mobilization, (2) upregulation of the long form of Gs alpha protein (Gs alpha-L) and an increase in iloprost-induced cAMP formation, and (3) upregulation of G16 alpha mRNA and G16 alpha protein and appearance of thromboxane A2-induced signaling (Ca2+ mobilization and stimulation of prostaglandin I2-induced cAMP formation). Gz alpha protein was absent in the megakaryoblasts despite weak expression of Gz alpha mRNA in DAMI and relatively high levels of Gz alpha mRNA and Gz alpha protein in platelets. These findings reveal major changes in G protein-mediated signal transduction during megakaryocytopoiesis and indicate that G16 alpha couples the thromboxane receptor to phospholipase C beta.

Reconstitution of receptors and GTP-binding regulatory proteins (G proteins) in Sf9 cells. A direct evaluation of selectivity in receptor.G protein coupling

The selectivity in coupling of various receptors to GTP-binding regulatory proteins (G proteins) was examined directly by a novel assay entailing the use of proteins overexpressed in Spodoptera frugiperda (Sf9) cells. Activation of G proteins was monitored in membranes prepared from Sf9 cells co-expressing selected pairs of receptors and G proteins (i.e. alpha, beta1, and gamma2 subunits). Membranes were incubated with [35S]guanosine 5'-(3-O-thio)triphosphate (GTPgammaS) +/- an agonist, and the amount of radiolabel bound to the alpha subunit was quantitated following immunoprecipitation. When expressed without receptor (but with beta1gamma2), the G protein subunits alphaz, alpha12, and alpha13 did not bind appreciable levels of [35S]GTPgammaS, consistent with a minimal level of GDP/[35S]GTPgammaS exchange. In contrast, the subunits alphas and alphaq bound measurable levels of the nucleotide. Co-expression of the 5-hydroxytryptamine1A (5-HT1A) receptor promoted binding of [35S]GTPgammaS to alphaz but not to alpha12, alpha13, or alphas. Binding to alphaz was enhanced by inclusion of serotonin in the assay. Agonist activation of both thrombin and neurokinin-1 receptors promoted a modest increase in [35S]GTPgammaS binding to alphaz and more robust increases in binding to alphaq, alpha12, and alpha13. Binding of [35S]GTPgammaS to alphas was strongly enhanced only by the activated beta1-adrenergic receptor. Our data identify interactions of receptors and G proteins directly, without resort to measurements of effector activity, confirm the coupling of the 5-HT1A receptor to Gz and extend the list of receptors that interact with this unique G protein to the receptors for thrombin and substance P, imply constitutive activity for the 5-HT1A receptor, and demonstrate for the first time that the cloned receptors for thrombin and substance P activate G12 and G13.

Galpha12 and galpha13 are phosphorylated during platelet activation

The ubiquitously expressed G-proteins G12 and G13 whose function is currently not clear have been shown to be activated in platelet membranes through receptors that stimulate platelet aggregation. We used intact human platelets to determine whether alpha subunits of both G-proteins can be phosphorylated under physiological conditions. Activation of human platelets by thrombin and the thromboxane A2 receptor agonist U46619 lead to phosphorylation of Galpha12 and Galpha13. Phosphorylation occurred rapidly after addition of thrombin and was not mediated by glycoprotein IIb/IIIa (integrin alphaIIbbeta3) activation. Phosphorylation of Galpha12 and Galpha13 could be mimicked by phorbol 12-myristate 13-acetate, and thrombin-induced phosphorylation was inhibited by the protein kinase C inhibitor calphostin C indicating an involvement of protein kinase C in Galpha12/13 phosphorylation induced by thrombin in human platelets. The phosphorylation of both G protein alpha subunits was reconstituted in COS-7 cells cotransfected with Galpha12 or Galpha13 and different protein kinase C isoforms. Among the protein knase C isoforms tested, protein kinase C beta, delta, and epsilon were most effective in promoting phosphorylation of Galpha12 and Galpha13 in a phorbol 12-myristate 13-acetate-dependent manner. These data demonstrate that Galpha12 and Galpha13 are phosphorylated under in vivo conditions and that this phosphorylation involves protein kinase C.

Activation of Gsalpha by the epidermal growth factor receptor involves phosphorylation

Previous studies from our laboratory have shown that epidermal growth factor (EGF) stimulates cAMP accumulation in the heart via a process involving Gsalpha and the EGF receptor (EGFR) protein tyrosine kinase activity (Nair, B. G., Parikh, B., Milligan, G., and Patel, T. B. (1990) J. Biol. Chem. 265, 21317-21322; Nair, B. G., and Patel, T. B. (1993) Biochem. Pharmacol. 46, 1239-1245). Therefore, studies were performed to investigate the hypothesis that the EGFR protein tyrosine kinase phosphorylates Gsalpha and activates this protein. Employing purified EGFR and Gsalpha, we have demonstrated that the EGFR kinase phosphorylates Gsalpha in a time-dependent manner with a stoichiometry of 2 mol of phosphate incorporated/mol of Gsalpha. As determined by phosphoamino acid analysis, the phosphorylation of Gsalpha by the EGFR kinase was exclusively on tyrosine residues. Interestingly, GDP and guanosine 5'-3-O-(thio)triphosphate (GTPgammaS) inhibited the phosphorylation of Gsalpha without altering EGFR autophosphorylation. However, G protein betagamma subunits protected against GDP- and GTPgammaS-mediated inhibition of phosphorylation of Gsalpha. In functional studies, phospho-Gsalpha demonstrated a greater GTPase activity and also a greater capacity to bind GTPgammaS as compared to the nonphosphorylated Gsalpha. Moreover, the phospho-Gsalpha augmented adenylyl cyclase activity in S49 cyc- cell membranes to a greater extent than its nonphosphorylated counterpart. Therefore, we conclude that phosphorylation of Gsalpha on tyrosine residues by the EGFR kinase activates this G protein and increases its ability to stimulate adenylyl cyclase.

Phosphoinositide 3-kinase gamma and p85/phosphoinositide 3-kinase in platelets. Relative activation by thrombin receptor or beta-phorbol myristate acetate and roles in promoting the ligand-binding function of alphaIIbbeta3 integrin

Platelets exposed to thrombin or thrombin receptor agonist peptide (SFLLRN) activate phospholipase C and protein kinase C (PKC), and accumulate 3-phosphorylated phosphoinositides (3-PPI) as a function of the activation and relocalization of two cytoskeletally-associated phosphoinositide 3-kinases (PI 3-K): p85/PI 3-K and PI 3-Kgamma. We now report that exposure of platelets to PKC-activating beta-phorbol myristate acetate (betaPMA) does not stimulate PI 3-Kgamma, but rather stimulates p85/PI 3-K, which associates with the cytoskeleton. Wortmannin is an inhibitor of both PI 3-Ks, known to act with more potency on p85/PI 3-K. betaPMA-stimulated 3-PPI accumulation is more sensitive to wortmannin (IC50 = 1.3 nM) than is SFLLRN- or thrombin-stimulated 3-PPI accumulation (IC50 = 10 nM). The activity of p85/PI 3-K in immunoprecipitates or in cytoskeletal fractions is inhibited more potently by exposure of platelets to wortmannin than is the activity of PI 3-Kgamma. betaPMA or SFLLRN promotes the conversion of platelet integrin alphaIIb/beta3 into a fibrinogen-binding form required for platelet aggregation. Activation of alphaIIb/beta3 in response to betaPMA or SFLLRN is inhibited by wortmannin with an IC50 of 1 nM in each case. Wortmannin inhibits neither activation of alphaIIb/beta3 by ligand-induced binding site antibody (anti-LIBS6 Fab) nor anti-LIBS6 Fab-induced platelet aggregation in the presence of fibrinogen, indicating that this type of "outside-in" signaling by alphaIIb/beta3 is largely PI 3-K-independent. We conclude that p85/PI 3-K, in preference to PI 3-Kgamma, contributes to activation of alphaIIb/beta3 when the thrombin receptor or PKC is stimulated.

Arachidonate and related unsaturated fatty acids selectively inactivate the guanine nucleotide-binding regulatory protein, Gz

Gz is a member of the family of trimeric guanine nucleotide-binding regulatory proteins (G proteins), which plays a crucial role in signaling across cell membranes. The expression of Gz is predominately confined to neuronal cells and platelets, suggesting an involvement in a neuroendocrine process. Although the signaling pathway in which Gz participates is not yet known, it has been linked to inhibition of adenylyl cyclase. We have found that arachidonate and related unsaturated fatty acids suppress guanine nucleotide binding to the alpha subunit of Gz. This inhibition of nucleotide binding by cis-unsaturated fatty acids is specific for Gz alpha; other G protein alpha subunits are relatively insensitive to these lipids. The IC50 for inhibition by the lipids closely corresponds to their critical micellar concentrations, suggesting that the interaction of the lipid micelle with Gzalpha is the primary event leading to inhibition. The presence of the acidic group of the fatty acid is critical for inhibition, as no effect is observed with the corresponding fatty alcohol. While arachidonic acid produces near-complete inhibition of both GDP and guanosine 5-(3-O-thio)triphosphate binding by Gzalpha, release of GDP from the protein was unaffected. Furthermore, the rate of inactivation of Gzalpha by arachidonate is essentially identical to the rate of GDP release from the protein, indicating that GDP release is required for inactivation. These observations indicate that the mechanism of inactivation of Gzalpha by unsaturated fatty acids is through an interaction of an acidic lipid micelle with the nucleotide-free form of the protein. Although the physiologic significance of this finding is unclear, similar effects of unsaturated fatty acids on other proteins involved in cell signaling indicate potential roles for these lipids in signal modulation. Additionally, the ability of arachidonate to inactivate this adenylyl cyclase-inhibitory G protein provides a molecular mechanism for previous findings that treatment of platelets with arachidonate results in elevated cAMP levels.

Phosphorylation of Gz alpha by protein kinase C blocks interaction with the beta gamma complex

Gz alpha is a G protein alpha subunit with biochemical properties that distinguish it from other members of the G protein alpha subunit family. One such property is its ability to be stoichiometrically phosphorylated by protein kinase C (PKC), both in vitro and in intact cells. The site of this phosphorylation has been mapped to a region near the N terminus of Gz alpha, but no functional significance of the modification has been established. To investigate this question, we have developed a baculovirus/Sf9 cell expression system to produce Gz alpha. The protein purified from Sf9 cells is functional as assessed by its ability both to bind guanine nucleotide in a Mg(2+)-sensitive fashion and to serve as a substrate for phosphorylation by PKC. Furthermore, addition of the G protein beta gamma complex purified from bovine brain inhibits phosphorylation of Gz alpha in a dose-dependent manner. Conversely, phosphorylation of Gz alpha inhibits its ability to interact with beta gamma subunits. These results establish a functional consequence for PKC-catalyzed phosphorylation of Gz alpha and suggest a mechanism for regulation of signaling through Gz by preventing reassociation of its subunits.

Cyclic GMP-dependent protein kinase blocks pertussis toxin-sensitive hormone receptor signaling pathways in Chinese hamster ovary cells

cGMP-dependent protein kinase (cGMP kinase) has been implicated in the regulation of the cytosolic calcium level ([Ca2+]i). In Chinese hamster ovary (CHO) cells stably transfected with the cGMP kinase I alpha (CHO-cGK cells), cGMP kinase suppressed the thrombin-induced increase in inositol 1,4,5-trisphosphate and [Ca2+]i (Ruth, P., Wang, G.-X., Boekhoff, I., May, B., Pfeifer, A., Penner, R., Korth, M., Breer, H., and Hofmann, F. (1993) Proc. Natl. Acad. Sci. U. S. A. 90, 2623-2627). Cholecystokinin activated intracellular calcium release via a pertussis toxin (PTX)-insensitive pathway in CHO-cGK cells. cGMP kinase did not attenuate the CCK-stimulated [Ca2+]i. In contrast, cGMP kinase suppressed calcium influx stimulated by insulin-like growth factors 1 and 2 (IGF-1 and IGF-2) via PTX-sensitive pathways. The effects of PTX and cGMP kinase on [Ca2+]i were not additive. 8-Bromo-cGMP had no effect on [Ca2+]i stimulated by IGF-1 or IGF-2 in wild type CHO cells. These results suggested that cGMP kinase inhibited the different signaling pathways by the phosphorylation of a PTX-sensitive G protein. cGMP kinase phosphorylated the alpha subunits of Gi1, Gi2, and Gi3 in vitro. Phosphorylation stoichiometry was 0.4 mol of phosphate/mol of G alpha i1 after reconstitution of heterotrimeric Gi1 in phospholipid vesicles. The alpha subunit of Gi was also phosphorylated in vivo. These results show that cGMP kinase blocks transduction of distinct hormone pathways that signal via PTX-sensitive Gi proteins.

Receptors and G proteins as primary components of transmembrane signal transduction. Part 2. G proteins: structure and function

Seven-transmembrane receptors signal through nucleotide-binding proteins (G proteins) into the cell. G proteins are membrane-associated proteins composed of three subunits termed alpha, beta and gamma, of which the G alpha subunit classifies the heterotrimer. So far, 23 different mammalian G alpha subunits are known, which are grouped in four subfamilies (Gs, Gi, Gq, G12) on the basis of their amino acid similarity. They carry an endogenous GTPase activity allowing reversible functional coupling between ligand-bound receptors and effectors such as enzymes and ion channels. In addition, five G beta and seven G gamma subunits have been identified which form tightly associated beta gamma heterodimers. Upon activation by a ligand-bound receptor the G protein dissociates into G alpha and G beta gamma, which both transmit signal by interacting with effectors. On the G protein level, specificity and selectivity of the incoming signal is accomplished by G protein trimers composed of distinct subunits. On the other hand, many receptors have been shown to activate different G proteins, thereby regulating diverse signal transduction pathways.

Chronic morphine induces tolerance and desensitization of mu-opioid receptor but not down-regulation in rabbit

Tolerance to chronic morphine treatment was studied in adult rabbits and modifications in the number and the state of coupling of the mu-opioid receptors were investigated in the cerebellum. Tolerance was induced by the subcutaneous injection of progressively increasing doses of morphine (5-100 mg/kg/injection) over 6 days and its occurrence was controlled by a nociceptive test: electrical stimulation of the dental pulp. At the end of the treatment, the rabbits were tolerant to the analgesic effects of morphine and the tolerance phenomenon correlated well with a significant decrease in the adenylate cyclase inhibition (approximately 60%). The functional uncoupling between the enzyme and the mu-opioid receptor was accompanied neither by a decrease in the number of high affinity receptors measured by equilibrium binding techniques (Kd = 0.19 +/- 0.03 in control vs. 0.11 +/- 0.04 nM in tolerant animals; Bmax = 322 +/- 62 vs. 362 +/- 58 fmol/mg of protein), nor by a modification of the physical coupling between the receptor and its G-protein. It can be concluded that desensitization, under our experimental conditions, can be clearly distinguished from down-regulation.