Plasma membrane localization of G alpha z requires two signals

From fat to fire: The lipid-inflammasome connection

Inflammasomes are multiprotein complexes that play a crucial role in regulating immune responses by governing the activation of Caspase-1, the secretion of pro-inflammatory cytokines, and the induction of inflammatory cell death, pyroptosis. The inflammasomes are pivotal in effective host defense against a range of pathogens. Yet, overt activation of inflammasome signaling can be detrimental. The most well-studied NLRP3 inflammasome has the ability to detect a variety of stimuli including pathogen-associated molecular patterns, environmental irritants, and endogenous stimuli released from dying cells. Additionally, NLRP3 acts as a key sensor of cellular homeostasis and can be activated by disturbances in diverse metabolic pathways. Consequently, NLRP3 is considered a key player linking metabolic dysregulation to numerous inflammatory disorders such as gout, diabetes, and atherosclerosis. Recently, compelling studies have highlighted a connection between lipids and the regulation of NLRP3 inflammasome. Lipids are integral to cellular processes that serve not only in maintaining the structural integrity and subcellular compartmentalization, but also in contributing to physiological equilibrium. Certain lipid species are known to define NLRP3 subcellular localization, therefore directly influencing the site of inflammasome assembly and activation. For instance, phosphatidylinositol 4-phosphate plays a crucial role in NLRP3 localization to the trans Golgi network. Moreover, new evidence has demonstrated the roles of lipid biosynthesis and trafficking in activation of the NLRP3 inflammasome. This review summarizes and discusses these emerging and varied roles of lipid metabolism in inflammasome activation. A deeper understanding of lipid-inflammasome interactions may open new avenues for therapeutic interventions to prevent or treat chronic inflammatory and autoimmune conditions.

Recapitulating the potential contribution of protein S-palmitoylation in cancer

Protein S-palmitoylation is a reversible form of protein lipidation in which the formation of a thioester bond occurs between a cysteine (Cys) residue of a protein and a 16-carbon fatty acid chain. This modification is catalyzed by a family of palmitoyl acyl transferases, the DHHC enzymes, so called because of their Asp-His-His-Cys (DHHC) catalytic motif. Deregulation of DHHC enzymes has been linked to various diseases, including cancer and infections. Cancer, a major cause of global mortality, is characterized by features like uncontrolled cell growth, resistance to cell death, angiogenesis, invasion, and metastasis. Several of these processes are controlled by DHHC-mediated S-palmitoylation of oncogenes or tumor suppressors, including growth factor receptors (e.g., EGFR), kinases (e.g., AKT), and transcription factors (e.g., β-catenin). Dynamic regulation of S-palmitoylation is also governed by protein depalmitoylases. These enzymes balance the cycling of palmitoylation and regulate cellular signaling, cell growth, and its organization. Given the significance of S-palmitoylation in cancer, the DHHCs and protein depalmitoylases are promising targets for cancer therapy. Here we summarize the catalytic mechanisms of DHHC enzymes and depalmitoylases, their role in cancer progression and prevention, as well as the crosstalk of palmitoylation with other post-translational modifications. Additionally, we discuss the methods to detect S-palmitoylation, the limitations of available DHHC-targeting inhibitors, and ongoing research efforts to address these obstacles.

Fatty acid synthesis promotes inflammasome activation through NLRP3 palmitoylation

Despite its significance, the role of lipid metabolism in NLRP3 inflammasome remains elusive. Here, we reveal a critical role for fatty acid synthase (FASN) in NLRP3 inflammasome activation. We demonstrate that pharmacological or genetic depletion of FASN dampens NLRP3 activation in primary mouse and human macrophages and in mice. This disruption in NLRP3 activation is contingent upon FASN activity. Accordingly, abolishing cellular palmitoylation, a post-translational modification in which the FASN product palmitate is reversibly conjugated to cysteine residues of target proteins, blunts inflammasome signaling. Correspondingly, an acyl-biotin exchange assay corroborated NLRP3 palmitoylation. Mechanistically, Toll-like receptor (TLR) ligation introduces palmitoylation at NLRP3 Cys898, permitting NLRP3 translocation to dispersed trans-Golgi network (dTGN) vesicles, the site of inflammasome assembly, upon NLRP3 activation. Accordingly, the NLRP3 Cys898 mutant exhibits reduced palmitoylation, limited translocation to the dTGN compartment, and diminished inflammasome activation. These results underscore mechanistic insights through which lipid metabolism licenses NLRP3 inflammasome assembly and activation.

G protein-coupled receptor signaling: transducers and effectors

G protein-coupled receptors (GPCRs) are of considerable interest due to their importance in a wide range of physiological functions and in a large number of Food and Drug Administration (FDA)-approved drugs as therapeutic entities. With continued study of their function and mechanism of action, there is a greater understanding of how effector molecules interact with a receptor to initiate downstream effector signaling. This review aims to explore the signaling pathways, dynamic structures, and physiological relevance in the cardiovascular system of the three most important GPCR signaling effectors: heterotrimeric G proteins, GPCR kinases (GRKs), and β-arrestins. We will first summarize their prominent roles in GPCR pharmacology before transitioning into less well-explored areas. As new technologies are developed and applied to studying GPCR structure and their downstream effectors, there is increasing appreciation for the elegance of the regulatory mechanisms that mediate intracellular signaling and function.

Gαi protein subunit: A step toward understanding its non-canonical mechanisms

The heterotrimeric G protein family plays essential roles during a varied array of cellular events; thus, its deregulation can seriously alter signaling events and the overall state of the cell. Heterotrimeric G-proteins have three subunits (α, β, γ) and are subdivided into four families, Gαi, Gα12/13, Gαq, and Gαs. These proteins cycle between an inactive Gα-GDP state and active Gα-GTP state, triggered canonically by the G-protein coupled receptor (GPCR) and by other accessory proteins receptors independent also known as AGS (Activators of G-protein Signaling). In this review, we summarize research data specific for the Gαi family. This family has the largest number of individual members, including Gαi1, Gαi2, Gαi3, Gαo, Gαt, Gαg, and Gαz, and constitutes the majority of G proteins α subunits expressed in a tissue or cell. Gαi was initially described by its inhibitory function on adenylyl cyclase activity, decreasing cAMP levels. Interestingly, today Gi family G-protein have been reported to be importantly involved in the immune system function. Here, we discuss the impact of Gαi on non-canonical effector proteins, such as c-Src, ERK1/2, phospholipase-C (PLC), and proteins from the Rho GTPase family members, all of them essential signaling pathways regulating a wide range of physiological processes.

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.

Gγ and Gα Identity Dictate a G-Protein Heterotrimer Plasma Membrane Targeting

Heterotrimeric G-proteins along with G-protein-coupled receptors (GPCRs) regulate many biochemical functions by relaying the information from the plasma membrane to the inside of the cell. The lipid modifications of Gα and Gγ subunits, together with the charged regions on the membrane interaction surface, provide a peculiar pattern for various heterotrimeric complexes. In a previous study, we found that Gαs and Gαi3 prefer different types of membrane-anchor and subclass-specific lipid domains. In the present report, we examine the role of distinct Gγ subunits in the membrane localization and spatiotemporal dynamics of Gαs and Gαi3 heterotrimers. We characterized lateral diffusion and G-protein subunit interactions in living cells using fluorescence recovery after photobleaching (FRAP) microscopy and fluorescence resonance energy transfer (FRET) detected by fluorescence lifetime imaging microscopy (FLIM), respectively. The interaction of Gγ subunits with specific lipids was confirmed, and thus the modulation of heterotrimeric G-protein localization. However, the Gα subunit also modulates trimer localization, and so the membrane distribution of heterotrimeric G-proteins is not dependent on Gγ only.

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.

Caveolins and cavins in the trafficking, maturation, and degradation of caveolae: implications for cell physiology

Caveolins (Cavs) are ~20 kDa scaffolding proteins that assemble as oligomeric complexes in lipid raft domains to form caveolae, flask-shaped plasma membrane (PM) invaginations. Caveolae ("little caves") require lipid-lipid, protein-lipid, and protein-protein interactions that can modulate the localization, conformational stability, ligand affinity, effector specificity, and other functions of proteins that are partners of Cavs. Cavs are assembled into small oligomers in the endoplasmic reticulum (ER), transported to the Golgi for assembly with cholesterol and other oligomers, and then exported to the PM as an intact coat complex. At the PM, cavins, ~50 kDa adapter proteins, oligomerize into an outer coat complex that remodels the membrane into caveolae. The structure of caveolae protects their contents (i.e., lipids and proteins) from degradation. Cellular changes, including signal transduction effects, can destabilize caveolae and produce cavin dissociation, restructuring of Cav oligomers, ubiquitination, internalization, and degradation. In this review, we provide a perspective of the life cycle (biogenesis, degradation), composition, and physiologic roles of Cavs and caveolae and identify unanswered questions regarding the roles of Cavs and cavins in caveolae and in regulating cell physiology.¹.

G protein-membrane interactions I: Gαi1 myristoyl and palmitoyl modifications in protein-lipid interactions and its implications in membrane microdomain localization

G proteins are fundamental elements in signal transduction involved in key cell responses, and their interactions with cell membrane lipids are critical events whose nature is not fully understood. Here, we have studied how the presence of myristic and palmitic acid moieties affects the interaction of the Gαi1 protein with model and biological membranes. For this purpose, we quantified the binding of purified Gαi1 protein and Gαi1 protein acylation mutants to model membranes, with lipid compositions that resemble different membrane microdomains. We observed that myristic and palmitic acids not only act as membrane anchors but also regulate Gαi1 subunit interaction with lipids characteristics of certain membrane microdomains. Thus, when the Gαi1 subunit contains both fatty acids it prefers raft-like lamellar membranes, with a high sphingomyelin and cholesterol content and little phosphatidylserine and phosphatidylethanolamine. By contrast, the myristoylated and non-palmitoylated Gαi1 subunit prefers other types of ordered lipid microdomains with higher phosphatidylserine content. These results in part explain the mobility of Gαi1 protein upon reversible palmitoylation to meet one or another type of signaling protein partner. These results also serve as an example of how membrane lipid alterations can change membrane signaling or how membrane lipid therapy can regulate the cell's physiology.

Protein palmitoylation in signal transduction of hematopoietic cells

The covalent attachment of palmitate to proteins can alter protein-lipid and protein-protein interactions thereby influencing protein function. Palmitoylation is a reversible post-translational modification. Thus, like protein phosphorylation, protein palmitoylation can function in activation-dependent signaling pathways. This review will provide an overview of the mechanisms and regulation of protein palmitoylation and focus on the role of palmitoylation in signal transduction pathways of lymphocytes and platelets.

G protein trafficking

The classical view of heterotrimeric G protein signaling places G -proteins at the cytoplasmic surface of the cell's plasma membrane where they are activated by an appropriate G protein-coupled receptor. Once activated, the GTP-bound Gα and the free Gβγ are able to regulate plasma membrane-localized effectors, such as adenylyl cyclase, phospholipase C-β, RhoGEFs and ion channels. Hydrolysis of GTP by the Gα subunit returns the G protein to the inactive Gαβγ heterotrimer. Although all of these events in the G protein cycle can be restricted to the cytoplasmic surface of the plasma membrane, G protein localization is dynamic. Thus, it has become increasingly clear that G proteins are able to move to diverse subcellular locations where they perform non-canonical signaling functions. This chapter will highlight our current understanding of trafficking pathways that target newly synthesized G proteins to the plasma membrane, activation-induced and reversible translocation of G proteins from the plasma membrane to intracellular locations, and constitutive trafficking of G proteins.

Preferential assembly of G-αβγ complexes directed by the γ subunits

Assembly of the G-αβγ heterotrimer is required for receptor signaling. Although much has been learned about the assembly process itself, the identities of the G-αβγ combinations that actually exist in physiological setting are largely unknown. Moreover, there is uncertainty regarding whether the individual subunits associate by a random process, or combine by a regulated process to form quasi-stable G-αβγ complexes. In this chapter, we will focus on emerging genetic -evidence that supports the latter model. Specifically, we will discuss how use of gene targeted mice has revealed preferential assembly of the striatal-specific Gα(olf)β(2)γ(7) complex occurs by a sequential process that is directed by the γ(7) subunit. The existence of specific G-αβγ complexes responsible for transducing the signals from different receptors may have profound implications by providing a possible explanation for biased agonism.

Chemical and genetic probes for analysis of protein palmitoylation

Reversible protein palmitoylation is one of the most important posttranslational modifications that has been implicated in the regulation of protein signaling, trafficking, localizing and enzymatic activities in cells and tissues. In order to achieve a precise understanding of mechanisms and functions of protein palmitoylation as well as its roles in physiological processes and disease progression, it is necessary to develop techniques that can qualitatively and quantitatively monitor the dynamic protein palmitoylation in vivo and in vitro. This review will highlight recent advances in both chemical and genetic encoded probes that have been developed for accurate analysis of protein palmitoylation, including identification and quantification of acyl moieties and palmitoylated proteins, localization of amino acid residues on which acyl moieties are attached, and imaging of cellular distributions of palmitoylated proteins. The role of major techniques of fluorescence microscopy and mass spectrometry in facilitating the analysis of protein palmitoylation will also be explored.

Site-specific analysis of protein S-acylation by resin-assisted capture

Protein S-acylation is a major posttranslational modification whereby a cysteine thiol is converted to a thioester. A prototype is S-palmitoylation (fatty acylation), in which a protein undergoes acylation with a hydrophobic 16 carbon lipid chain. Although this modification is a well-recognized determinant of protein function and localization, current techniques to study cellular S-acylation are cumbersome and/or technically demanding. We recently described a simple and robust methodology to rapidly identify S-nitrosylation sites in proteins via resin-assisted capture (RAC) and provided an initial description of the applicability of the technique to S-acylated proteins (acyl-RAC). Here we expand on the acyl-RAC assay, coupled with mass spectrometry-based proteomics, to characterize both previously reported and novel sites of endogenous S-acylation. Acyl-RAC should therefore find general applicability in studies of both global and individual protein S-acylation in mammalian cells.

N-terminal polybasic motifs are required for plasma membrane localization of Galpha(s) and Galpha(q)

Heterotrimeric G proteins typically localize at the cytoplasmic face of the plasma membrane where they interact with heptahelical receptors. For G protein alpha subunits, multiple membrane targeting signals, including myristoylation, palmitoylation, and interaction with betagamma subunits, facilitate membrane localization. Here we show that an additional membrane targeting signal, an N-terminal polybasic region, plays a key role in plasma membrane localization of non-myristoylated alpha subunits. Mutations of N-terminal basic residues in alpha(s) and alpha(q) caused defects in plasma membrane localization, as assessed through immunofluorescence microscopy and biochemical fractionations. In alpha(s), mutation of four basic residues to glutamine was sufficient to cause a defect, whereas in alpha(q) a defect in membrane localization was not observed unless nine basic residues were mutated to glutamine or if three basic residues were mutated to glutamic acid. betagamma co-expression only partially rescued the membrane localization defects; thus, the polybasic region is also important in the context of the heterotrimer. Introduction of a site for myristoylation into the polybasic mutants of alpha(s) and alpha(q) recovered strong plasma membrane localization, indicating that myristoylation and polybasic motifs may have complementary roles as membrane targeting signals. Loss of plasma membrane localization coincided with defects in palmitoylation. The polybasic mutants of alpha(s) and alpha(q) were still capable of assuming activated conformations and stimulating second messenger production, as demonstrated through GST-RGS4 interaction assays, cAMP assays, and inositol phosphate assays. Electrostatic interactions with membrane lipids have been found to be important in plasma membrane targeting of many proteins, and these results provide evidence that basic residues play a role in localization of G protein alpha subunits.

Cellular palmitoylation and trafficking of lipidated peptides

Many important signaling proteins require the posttranslational addition of fatty acid chains for their proper subcellular localization and function. One such modification is the addition of palmitoyl moieties by enzymes known as palmitoyl acyltransferases (PATs). Substrates for PATs include C-terminally farnesylated proteins, such as H- and N-Ras, as well as N-terminally myristoylated proteins, such as many Src-related tyrosine kinases. The molecular and biochemical characterization of PATs has been hindered by difficulties in developing effective methods for the analysis of PAT activity. In this study, we describe the use of cell-permeable, fluorescently labeled lipidated peptides that mimic the PAT recognition domains of farnesylated and myristoylated proteins. These PAT substrate mimetics are accumulated by SKOV3 cells in a saturable and time-dependent manner. Although both peptides are rapidly palmitoylated, the SKOV3 cells have a greater capacity to palmitoylate the myristoylated peptide than the farnesylated peptide. Confocal microscopy indicated that the palmitoylated peptides colocalized with Golgi and plasma membrane markers, whereas the corresponding nonpalmitoylatable peptides accumulated in the Golgi but did not traffic to the plasma membrane. Overall, these studies indicate that the lipidated peptides provide useful cellular probes for quantitative and compartmentalization studies of protein palmitoylation in intact cells.

Assembly and trafficking of heterotrimeric G proteins

To be activated by cell surface G protein-coupled receptors, heterotrimeric G proteins must localize at the cytoplasmic surface of plasma membranes. Moreover, some G protein subunits are able to traffic reversibly from the plasma membrane to intracellular locations upon activation. This current topic will highlight new insights into how nascent G protein subunits are assembled and how they arrive at plasma membranes. In addition, recent reports have increased our knowledge of activation-induced trafficking of G proteins. Understanding G protein assembly and trafficking will lead to a greater understanding of novel ways that cells regulate G protein signaling.

Differential modulation of type 1 and type 2 cannabinoid receptors along the neuroimmune axis

Endocannabinoid-signaling chains have been implicated in a variety of pathophysiological functions, including memory, coordination, vasoregulation, reproduction, neurodegeneration, and inflammation. These activities were thought to be mediated by the activation of two G-protein-coupled receptors (GPCRs), type 1 and type 2 cannabinoid receptors (CB(1)R and CB(2)R). These two CBR subtypes share common agonists and trigger similar signaling pathways, yet they present several important differences in structure and cell distribution. In particular, recent research has shown that the CB(1)R and CB(2)R are differentially linked to lipid rafts, specialized microdomains of the plasma membrane involved in the signaling of many other GPCRs. We present an overview of the current literature on the effects that lipid raft perturbation have on CBRs activities, and provide a mechanistic model to interpret these data in terms of structural and functional aspects. These findings may also have important implications for the development of new therapeutic approaches, including lipid raft perturbing drugs, aimed to selectively modulate CB(1)R signaling in a variety of pathological conditions.

Palmitoylation of ligands, receptors, and intracellular signaling molecules

Palmitate, a 16-carbon saturated fatty acid, is attached to more than 100 proteins. Modification of proteins by palmitate has pleiotropic effects on protein function. Palmitoylation can influence membrane binding and membrane targeting of the modified proteins. In particular, many palmitoylated proteins concentrate in lipid rafts, and enrichment in rafts is required for efficient signal transduction. This Review focuses on the multiple effects of palmitoylation on the localization and function of ligands, receptors, and intracellular signaling proteins. Palmitoylation regulates the trafficking and function of transmembrane proteins such as ion channels, neurotransmitter receptors, heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors, and integrins. In addition, immune receptor signaling relies on protein palmitoylation at many levels, including palmitoylated co-receptors, Src family kinases, and adaptor or scaffolding proteins. The localization and signaling capacities of Ras and G proteins are modulated by dynamic protein palmitoylation. Cycles of palmitoylation and depalmitoylation allow H-Ras and G protein alpha subunits to reversibly bind to and signal from different intracellular cell membranes. Moreover, secreted ligands such as Hedgehog, Wingless, and Spitz use palmitoylation to regulate the extent of long- or short-range signaling. Finally, palmitoylation can alter signaling protein function by direct effects on enzymatic activity and substrate specificity. The identification of the palmitoyl acyltransferases has provided new insights into the biochemistry of this posttranslational process and permitted new substrates to be identified.

Biosynthesis and trafficking of seven transmembrane receptor signalling complexes

Recent studies have shown that 7-transmembrane receptors (7TM-Rs), their associated signalling molecules and scaffolding proteins are often constitutively associated under basal conditions. These studies highlight that receptor ontogeny and trafficking are likely to play key roles in the determination of both signalling specificity and efficacy. This review highlights information about how 7TM-Rs and their associated signalling molecules are trafficked to the cell surface as well as other intracellular destinations.

Discovery and characterization of inhibitors of human palmitoyl acyltransferases

The covalent attachment of palmitate to specific proteins by the action of palmitoyl acyltransferases (PAT) plays critical roles in the biological activities of several oncoproteins. Two PAT activities are expressed by human cells: type 1 PATs that modify the farnesyl-dependent palmitoylation motif found in H- and N-Ras, and type 2 PATs that modify the myristoyl-dependent palmitoylation motif found in the Src family of tyrosine kinases. We have previously shown that the type 1 PAT HIP14 causes cellular transformation. In the current study, we show that mRNA encoding HIP14 is up-regulated in a number of types of human tumors. To assess the potential of HIP14 and other PATs as targets for new anticancer drugs, we developed three cell-based assays suitable for high-throughput screening to identify inhibitors of these enzymes. Using these screens, five chemotypes, with activity toward either type 1 or type 2 PAT activity, were identified. The activity of the hits were confirmed using assays that quantify the in vitro inhibition of PAT activity, as well as a cell-based assay that determines the abilities of the compounds to prevent the localization of palmitoylated green fluorescent proteins to the plasma membrane. Representative compounds from each chemotype showed broad antiproliferative activity toward a panel of human tumor cell lines and inhibited the growth of tumors in vivo. Together, these data show that PATs, and HIP14 in particular, are interesting new targets for anticancer compounds, and that small molecules with such activity can be identified by high-throughput screening.

Atorvastatin desensitizes beta-adrenergic signaling in cardiac myocytes via reduced isoprenylation of G-protein gamma-subunits

Statins exert pleiotropic, cholesterol-independent effects by reducing isoprenylation of monomeric GTPases. Here we examined whether statins also reduce isoprenylation of gamma-subunits of heterotrimeric G-proteins and thereby affect beta-adrenergic signaling and regulation of force in cardiac myocytes. Neonatal rat cardiac myocytes (NRCM) were treated with atorvastatin (0.1-10 micromol/l; 12-48 h) and examined for adenylyl cyclase regulating G-protein alpha- (Galpha), beta- (Gbeta), and gamma- (Ggamma) subunits and cAMP accumulation. Engineered heart tissue (EHT) from NRCM was used to evaluate contractile consequences. In atorvastatin-treated NRCM, a second band of Ggamma3 with a lower apparent molecular weight appeared in cytosol and particulate fractions that was absent in vehicle-treated NRCM, but also seen after GGTI-298, a geranylgeranyl transferase inhibitor. In parallel, Gbeta accumulated in the cytosol and total cellular content of Galphas was reduced. In atorvastatin-treated NRCM, the cAMP-increasing effect of isoprenaline was reduced. Likewise, the positive inotropic effect of isoprenaline was desensitized and reduced after treatment with atorvastatin. The effects of atorvastatin were abolished by mevalonate and/or geranylgeranyl pyrophosphate, but not by farnesyl pyrophosphate or squalene. Taken together, the results of this study show that atorvastatin desensitizes NRCM to beta-adrenergic stimulation by a mechanism that involves reduced isoprenylation of Ggamma and subsequent reductions in the cellular content of Galphas.

Galpha subunit Gpa2 recruits kelch repeat subunits that inhibit receptor-G protein coupling during cAMP-induced dimorphic transitions in Saccharomyces cerevisiae

All eukaryotic cells sense extracellular stimuli and activate intracellular signaling cascades via G protein-coupled receptors (GPCR) and associated heterotrimeric G proteins. The Saccharomyces cerevisiae GPCR Gpr1 and associated Galpha subunit Gpa2 sense extracellular carbon sources (including glucose) to govern filamentous growth. In contrast to conventional Galpha subunits, Gpa2 forms an atypical G protein complex with the kelch repeat Gbeta mimic proteins Gpb1 and Gpb2. Gpb1/2 negatively regulate cAMP signaling by inhibiting Gpa2 and an as yet unidentified target. Here we show that Gpa2 requires lipid modifications of its N-terminus for membrane localization but association with the Gpr1 receptor or Gpb1/2 subunits is dispensable for membrane targeting. Instead, Gpa2 promotes membrane localization of its associated Gbeta mimic subunit Gpb2. We also show that the Gpa2 N-terminus binds both to Gpb2 and to the C-terminal tail of the Gpr1 receptor and that Gpb1/2 binding interferes with Gpr1 receptor coupling to Gpa2. Our studies invoke novel mechanisms involving GPCR-G protein modules that may be conserved in multicellular eukaryotes.

Cell signalling diversity of the Gqalpha family of heterotrimeric G proteins

Many receptors for neurotransmitters and hormones rely upon members of the Gqalpha family of heterotrimeric G proteins to exert their actions on target cells. Galpha subunits of the Gq class of G proteins (Gqalpha, G11alpha, G14alpha and G15/16alpha) directly link receptors to activation of PLC-beta isoforms which, in turn, stimulate inositol lipid (i.e. calcium/PKC) signalling. Although Gqalpha family members share a capacity to activate PLC-beta, they also differ markedly in their biochemical properties and tissue distribution which predicts functional diversity. Nevertheless, established models suggest that Gqalpha family members are functionally redundant and that their cellular responses are a result of PLC-beta activation and downstream calcium/PKC signalling. Growing evidence, however, indicates that Gqalpha, G11alpha, G14alpha and G15/16alpha are functionally diverse and that many of their cellular actions are independent of inositol lipid signalling. Recent findings show that Gqalpha family members differ with regard to their linked receptors and downstream binding partners. Reported binding partners distinct from PLC-beta include novel candidate effector proteins, various regulatory proteins, and a growing list of scaffolding/adaptor proteins. Downstream of these signalling proteins, Gqalpha family members exhibit unexpected differences in the signalling pathways and the gene expression profiles they regulate. Finally, genetic studies using whole animal models demonstrate the importance of certain Gqalpha family members in cardiac, lung, brain and platelet functions among other physiological processes. Taken together, these findings demonstrate that Gqalpha, G11alpha, G14alpha and G15/16alpha regulate both overlapping and distinct signalling pathways, indicating that they are more functionally diverse than previously thought.

Lipids, lipid rafts and caveolae: Their importance for GPCR signaling and their centrality to the endocannabinoid system

Scientific views of cell membrane organization are presently changing. Rather than serving only as the medium through which membrane proteins diffuse, lipid bilayers have now been shown to form compartmentalized domains with different biophysical properties (rafts/caveolae). For membrane proteins such as the G protein coupled receptors (GPCRs), a raft domain provides a platform for the assembly of signaling complexes and prevents cross-talk between pathways. Lipid composition also has a strong influence on the conformational activity of GPCRs. For certain GPCRs, such as the cannabinoid receptors, the lipid bilayer has additional significance. Endocannabinoids such as anandamide (AEA) are created in a lipid bilayer from lipid and act at the membrane embedded CB1 receptor. Endocannabinoids exiting the CB1 receptor are transported either by a carrier-mediated or a simple diffusion process to the membrane of the postsynaptic cell. Following cellular uptake, perhaps via caveolae/lipid raft-related endocytosis, AEA is rapidly metabolized by a membrane-associated enzyme, fatty acid amide hydrolase (FAAH) located in the endoplasmic reticulum. The entry point for AEA into FAAH appears to be from the lipid bilayer. This review explores the importance of lipid composition and lipid rafts to GPCR signaling and then focuses on the intimate relationship that exists between the lipid environment and the endocannabinoid system.

Palmitoylation regulates GDP/GTP exchange of G protein by affecting the GTP-binding activity of Goalpha

The effect of palmitoylation on the GTP-binding activity and conformation of Goalpha protein in hydrophobic and hydrophilic environments was studied. The binding assay was performed with an isotope labeled analog of GTP, GTP-gamma-35S, and its fluorescent analog, BODIPY FL-GTPgammaS was used to detect conformational change in the GTP-binding domain of Goalpha. Investigation of the GTP-gamma-35S binding activity of Goalpha shows that in a hydrophobic environment, mimicked by the presence of detergent, the apparent dissociation constant for palmitoylated Goalpha (K(D)=25.5x10(-9)+/-1.7x10(-9)M) increased threefold compared with that of non-palmitoylated Goalpha (K(D)=9.9x10(-9)+/-0.8x10(-9)M), while in an aqueous environment without detergent there is no significant difference between palmitoylated (K(D)=50.1 x 10(-9)+/-5.2x10(-9)M) and non-palmitoylated (K(D)=65.5x10(-9)+/-7.6x10(-9)M) Go(. This indicates that in a membrane environment palmitoylation may weaken the GTPgammaS binding ability of Go(. Fluorescent quenching studies using BODIPY FL-GTPgammaS as a probe showed that the conformation of the GTP-binding domain of Go( tends to become more compact after palmitoylation. These results imply that palmitoylation may regulate the GTP/GDP exchange of Goalpha by influencing the GTP-binding activity of Goalpha and facilitating the on-off switch function of the G protein in G protein-coupled signal transduction.

Huntingtin interacting protein 14 is an oncogenic human protein: palmitoyl acyltransferase

Protein palmitoyltransferases (PATs) represent an exciting new target for anticancer drug design due to their pivotal roles in the subcellular localization of a number of oncogenes. We show that the Huntingtin interacting protein 14 (HIP14) is a PAT with a preference for the farnesyl-dependent palmitoylation motif found in H- and N-RAS. Characterization of HIP14 in mouse cells has revealed that it has the ability to induce colony formation in cell culture, anchorage-independent growth, and tumors in mice. Activity of the enzyme and its ability to transform cells is dependent on critical residues in the active site of the enzyme.

Galphaz inhibits serum response factor-dependent transcription by inhibiting Rho signaling

Galpha12/13 or Galphaq signals induce activation of Rho GTPase, leading to serum response factor (SRF)-mediated gene transcription and actin cytoskeletal organization; however, less is known regarding how Rho pathway signals are down-regulated. Here we report that Galphaz signals inhibit serum response factor (SRF)-dependent transcription. Galphaz expression inhibits Galpha12/13-, Galphaq-, and Rho guanine nucleotide exchange factor (GEF)-induced serum response element (SRE) reporter activation in human embryonic kidney 293T and PC-12 cells. Expression of Galphaz mutants with defective fatty acylation has no inhibitory effect. Expression of Galphaz, but not Galphai, attenuates serum-induced SRE reporter activation, suggesting that Galphaz can down-regulate endogenous signals leading to SRF. Whereas Galphaz also blocks SRE reporter induction by the activated mutant RhoAL63, it does not affect Galpha12- or Rho GEF-induced RhoA activation or RhoAL63-GTP binding in vivo. Moreover, Galphaz does not inhibit SRE reporter induction by an activated form of Rho kinase. Because Galphaz inhibits RhoAL63/A188-induced reporter activation, phosphorylation of RhoA on serine 188 does not seem to be involved; furthermore, RhoA subcellular localization was not affected. Use of pharmacologic inhibitors implies that Galphaz-induced reduction of SRE reporter activation occurs via a mechanism other than adenylate cyclase modulation. These findings suggest that Galphaz signals may attenuate Rho-induced stimulation of SRF-mediated transcription.

Palmitoylation of intracellular signaling proteins: regulation and function

Protein S-palmitoylation is the thioester linkage of long-chain fatty acids to cysteine residues in proteins. Addition of palmitate to proteins facilitates their membrane interactions and trafficking, and it modulates protein-protein interactions and enzyme activity. The reversibility of palmitoylation makes it an attractive mechanism for regulating protein activity, and this feature has generated intensive investigation of this modification. The regulation of palmitoylation occurs through the actions of protein acyltransferases and protein acylthioesterases. Identification of the protein acyltransferases Erf2/Erf4 and Akr1 in yeast has provided new insight into the palmitoylation reaction. These molecules work in concert with thioesterases, such as acyl-protein thioesterase 1, to regulate the palmitoylation status of numerous signaling molecules, ultimately influencing their function. This review discusses the function and regulation of protein palmitoylation, focusing on intracellular proteins that participate in cell signaling or protein trafficking.

Desensitization of 5-HT1A receptors by 5-HT2A receptors in neuroendocrine neurons in vivo

An imbalance between serotonin-2A (5-HT2A) and 5-HT1A receptors may underlie several mood disorders. The present studies determined whether 5-HT2A receptors interact with 5-HT1A receptors in the rat hypothalamic paraventricular nucleus (PVN). The sensitivity of the hypothalamic 5-HT1A receptors was measured as oxytocin and adrenocorticotropic hormone (ACTH) responses to the 5-HT1A receptor agonist (+)-8-hydroxy-2-(di-n-propylamino) tetralin hydrobromide [(+)8-OH-DPAT] (40 microg/kg s.c.). The 5-HT(2A/2C) receptor agonist (-)DOI [(-)-1-(2,5-dimethoxy-4-iodophenyl)2-aminopropane HCl] (1 mg/kg s.c.) injected 2 h prior to (+)8-OH-DPAT significantly reduced the oxytocin and ACTH responses to (+)8-OH-DPAT, producing a heterologous desensitization of the 5-HT1A receptors. Microinjection of the 5-HT2A receptor antagonist MDL100,907 [(+)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidinemethanol; 0, 10, or 20 nmol, 15 min prior to (-)DOI] into the PVN dose-dependently prevented the desensitization of 5-HT1A receptors induced by the 5-HT2A receptor agonist (-)DOI. Double-label immunocytochemistry revealed a high degree of colocalization of 5-HT1A and 5-HT2A receptors in the oxytocin and corticotropin-releasing factor neurons of the PVN. Thus, activation of 5-HT2A receptors in the PVN may directly induce a heterologous desensitization of 5-HT1A receptors within individual neuroendocrine cells. These findings may provide insight into the long-term adaptation of 5-HT1A receptor signaling after changes in function of 5-HT2A receptors; for example, during pharmacotherapy of mood disorders.

Membrane targeting of lipid modified signal transduction proteins

Covalent attachment of lipophilic moieties to proteins influences interaction with membranes and membrane microdomains, as well as signal transduction. The most common forms of fatty acylation include modification of the N-terminal glycine of proteins by N-myristoylation and/or attachment of palmitate to internal cysteine residues. Protein prenylation involves attachment of farnesyl or geranylgeranyl moieties via thio-ether linkage to cysteine residues at or near the C-terminus. Attachment of each of these lipophilic groups is catalyzed by a distinct enzyme or set of enzymes: N-myristoyl transferase for N-myristoylation, palmitoyl acyl transferases for palmitoylation, and farnesyl or geranylgeranyl transferases for prenylation. The distinct nature of the lipid modification determines the strength of membrane interaction of the modified protein as well as the specificity of membrane targeting. Clusters of basic residues can also synergize with the lipophilic group to promote membrane binding and targeting. The final destination of the modified protein is influenced by multiple factors, including the localization of the modifying enzymes, protein/protein interactions, and the lipid composition of the acceptor membrane. In particular, much interest has been focused on the ability of fatty acylated proteins to preferentially partition into membrane rafts, subdomains of the plasma membrane that are enriched in cholesterol and glycosphingolipids. Lipid raft localization is necessary for efficient signal transduction in a wide variety of systems, including signaling by T and B cell receptors, Ras, and growth factor receptors. However, certain membrane subdomains, such as caveolae, can serve as reservoirs for inactive signaling proteins. Heterogeneity in the types of membrane subdomains, as well as in the types of lipophilic groups that are attached to proteins, provide an additional level of complexity in the regulation of signaling by membrane bound proteins.

Palmitoylation is required for membrane association of activated but not inactive invertebrate visual Gqalpha

The invertebrate visual G protein, iGqalpha plays a central role in invertebrate phototransduction by relaying signals from rhodopsin to phospholipase C leading to membrane depolarization. Previous studies have shown reversible association of iGqalpha with rhabdomeric membranes regulated by light. To address the mechanism of membrane association we cloned iGqalpha from a Loligo pealei photoreceptor cDNA library and expressed it in HEK293T cells. Mutations were introduced to eliminate putative sites for palmitoylation at cysteines in positions 3 and 4. Membrane and soluble fractions were prepared from cells where iGqalpha was either activated or maintained in the GDP-bound form, followed by identification of iGqalpha through immunoblot analysis. The wild-type iGqalpha was entirely membrane-bound and shown to be post-translationally modified by palmitoylation. The mutant iGqalpha (C3,4A) was not palmitoylated yet it was found to be membrane-associated in the inactive state, however, approximately half of the protein became soluble when activated. These results suggest that palmitoylation is not required for membrane association of iGqalpha in the inactive state but is important in maintaining the stable membrane association of activated iGqalpha-GTP. The mechanism by which iGqalpha moves away from the membrane into the cytosol in response to prolonged light-stimulation in the native squid eye appears, therefore, to involve both activation and depalmitoylation processes.

Subcellular targeting of nine calcium-dependent protein kinase isoforms from Arabidopsis

Calcium-dependent protein kinases (CDPKs) are specific to plants and some protists. Their activation by calcium makes them important switches for the transduction of intracellular calcium signals. Here, we identify the subcellular targeting potentials for nine CDPK isoforms from Arabidopsis, as determined by expression of green fluorescent protein (GFP) fusions in transgenic plants. Subcellular locations were determined by fluorescence microscopy in cells near the root tip. Isoforms AtCPK3-GFP and AtCPK4-GFP showed a nuclear and cytosolic distribution similar to that of free GFP. Membrane fractionation experiments confirmed that these isoforms were primarily soluble. A membrane association was observed for AtCPKs 1, 7, 8, 9, 16, 21, and 28, based on imaging and membrane fractionation experiments. This correlates with the presence of potential N-terminal acylation sites, consistent with acylation as an important factor in membrane association. All but one of the membrane-associated isoforms targeted exclusively to the plasma membrane. The exception was AtCPK1-GFP, which targeted to peroxisomes, as determined by covisualization with a peroxisome marker. Peroxisome targeting of AtCPK1-GFP was disrupted by a deletion of two potential N-terminal acylation sites. The observation of a peroxisome-located CDPK suggests a mechanism for calcium regulation of peroxisomal functions involved in oxidative stress and lipid metabolism.

Palmitoylation regulates regulators of G-protein signaling (RGS) 16 function. I. Mutation of amino-terminal cysteine residues on RGS16 prevents its targeting to lipid rafts and palmitoylation of an internal cysteine residue

Regulators of G-protein signaling (RGS) proteins down-regulate signaling by heterotrimeric G-proteins by accelerating GTP hydrolysis on the G alpha subunits. Palmitoylation, the reversible addition of palmitate to cysteine residues, occurs on several RGS proteins and is critical for their activity. For RGS16, mutation of Cys-2 and Cys-12 blocks its incorporation of [3H]palmitate and ability to turn-off Gi and Gq signaling and significantly inhibited its GTPase activating protein activity toward aG alpha subunit fused to the 5-hydroxytryptamine receptor 1A, but did not reduce its plasma membrane localization based on cell fractionation studies and immunoelectron microscopy. Palmitoylation can target proteins, including many signaling proteins, to membrane microdomains, called lipid rafts. A subpopulation of endogenous RGS16 in rat liver membranes and overexpressed RGS16 in COS cells, but not the nonpalmitoylated cysteine mutant of RGS16, localized to lipid rafts. However, disruption of lipid rafts by treatment with methyl-beta-cyclodextrin did not decrease the GTPase activating protein activity of RGS16. The lipid raft fractions were enriched in protein acyltransferase activity, and RGS16 incorporated [3H]palmitate into a peptide fragment containing Cys-98, a highly conserved cysteine within the RGS box. These results suggest that the amino-terminal palmitoylation of an RGS protein promotes its lipid raft targeting that allows palmitoylation of a poorly accessible cysteine residue that we show in the accompanying article (Osterhout, J. L., Waheed, A. A., Hiol, A., Ward, R. J., Davey, P. C., Nini, L., Wang, J., Milligan, G., Jones, T. L. Z., and Druey, K. M. (2003) J. Biol. Chem. 278, 19309-19316) was critical for RGS16 and RGS4 GAP activity.

Heterotrimer formation, together with isoprenylation, is required for plasma membrane targeting of Gbetagamma

Nascent beta and gamma subunits of heterotrimeric G proteins need to be targeted to the cytoplasmic face of the plasma membrane (PM) in order to transmit signals. We show that beta(1)gamma(2) is poorly targeted to the PM and predominantly localized to endoplasmic reticulum (ER) membranes when expressed in HEK293 cells, but co-expression of a G protein alpha subunit allows strong PM localization of the beta(1)gamma(2). Furthermore, C-terminal isoprenylation of the gamma subunit is necessary but not sufficient for PM localization of beta(1)gamma(2). Isoprenylation of gamma(2) and localization of beta(1)gamma(2) to the ER occurs independently of alpha expression. Efficient PM localization of beta(1)gamma(2) in the absence of co-expressed alpha is observed when a site for palmitoylation, a putative second membrane targeting signal, is introduced into gamma(2). When a mutant of alpha(s) is targeted to mitochondria, beta(1)gamma(2) follows, consistent with an important role for alpha in promoting subcellular localization of betagamma. Furthermore, we directly demonstrate the requirement for alpha by showing that disruption of heterotrimer formation by the introduction of alpha binding mutations into beta(1) impedes PM targeting of beta(1)gamma(2). The results indicate that two membrane targeting signals, lipid modification and alpha binding, make concerted contributions to PM localization of betagamma.

Partial rescue of functional interactions of a nonpalmitoylated mutant of the G-protein G alpha s by fusion to the beta-adrenergic receptor

Most heterotrimeric G-protein alpha subunits are posttranslationally modified by palmitoylation, a reversible process that is dynamically regulated. We analyzed the effects of Galpha(s) palmitoylation for its intracellular distribution and ability to couple to the beta-adrenergic receptor (betaAR) and stimulate adenylyl cyclase. Subcellular fractionation and immunofluorescence microscopy of stably transfected cyc(-) cells, which lack endogenous Galpha(s), showed that wild-type Galpha(s) was predominantly localized at the plasma membrane, but the mutant C3A-Galpha(s), which does not incorporate [(3)H]palmitate, was mostly associated with intracellular membranes. In agreement with this mislocalization, C3A-Galpha(s) showed neither isoproterenol- or GTPgammaS-stimulated adenylyl cyclase activation nor GTPgammaS-sensitive high-affinity agonist binding, all of which were present in the wild-type Galpha(s) expressing cells. Fusion of C3A-Galpha(s) with the betaAR [betaAR-(C3A)Galpha(s)] partially rescued its ability to induce high-affinity agonist binding and to stimulate adenylyl cyclase activity after isoproterenol or GTPgammaS treatment. In comparison to results with the WT-Galpha(s) and betaAR (betaAR-Galpha(s)) fusion protein, the betaAR-(C3A)Galpha(s) fusion protein was about half as efficient at coupling to the receptor and effector. Chemical depalmitoylation by hydroxylamine of membranes expressing betaAR-Galpha(s) reduced the high-affinity agonist binding and adenylyl cyclase activation to a similar degree as that observed in betaAR-(C3A)Galpha(s) expressing membranes. Altogether, these findings indicate that palmitoylation ensured proper localization of Galpha(s) and facilitated bimolecular interactions of Galpha(s) with the betaAR and adenylyl cyclase.

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.

Specificity of plasma membrane targeting by the rous sarcoma virus gag protein

Budding of C-type retroviruses begins when the viral Gag polyprotein is directed to the plasma membrane by an N-terminal membrane-binding (M) domain. While dispersed basic amino acids within the M domain are critical for stable membrane association and consequent particle assembly, additional residues or motifs may be required for specific plasma membrane targeting and binding. We have identified an assembly-defective Rous sarcoma virus (RSV) Gag mutant that retains significant membrane affinity despite having a deletion of the fourth alpha-helix of the M domain. Examination of the mutant protein's subcellular distribution revealed that it was not localized to the plasma membrane but instead was mistargeted to intracytoplasmic membranes. Specific plasma membrane targeting was restored by the addition of myristate plus a single basic residue, by multiple basic residues, or by the heterologous hydrophobic membrane-binding domain from the cellular Fyn protein. These results suggest that the fourth alpha-helix of the RSV M domain promotes specific targeting of Gag to the plasma membrane, either through a direct interaction with plasma membrane phospholipids or a membrane-associated cellular factor or by maintaining the conformation of Gag to expose specific plasma membrane targeting sequences.

Activation of Gz attenuates Rap1-mediated differentiation of PC12 cells

We previously identified a specific activation-dependent interaction between the alpha subunit of the heterotrimeric G protein, G(z), and a regulator of Rap1 signaling, Rap1GAP (Meng, J., Glick, J. L., Polakis, P., and Casey, P. J. (1999) J. Biol. Chem. 274, 36663-36669). We now demonstrate that activated forms of Galpha(z) are able to recruit Rap1GAP from a cytosolic location to the membrane. Using PC12 cells as a model for neuronal differentiation, the influence of G(z) activation on Rap1-mediated cell differentiation was examined. Introduction of constitutively-activated Galpha(z) into PC12 cells markedly attenuated the differentiation process of these cells induced by a cAMP analogue. Treatment of PC12 cells expressing wild type Galpha(z) with a specific agonist to the alpha(2A)-adrenergic receptor also attenuated cAMP-induced PC12 cell differentiation, demonstrating that receptor-mediated activation of G(z) was also effective in this regard. Furthermore, activation of G(z) decreased the ability of the cAMP analogue to trigger both Rap1 and extracellular-regulated kinase (ERK) activation. Differentiation of PC12 cells induced by nerve growth factor (NGF) is also thought to be a Rap1-mediated process, and G(z) activation was found to attenuate this process as well. Rap1 activation, ERK phosphorylation, and PC12 cell differentation induced by NGF treatment were all significantly attenuated by either transfection of constitutively activated Galpha(z) or receptor-mediated G(z) activation. Based on these findings, a model is proposed in which activation of G(z) results in recruitment of Rap1GAP to the membrane where it can effectively down-regulate Rap1 signaling. The implications of these findings in regard to a possible role for G(z) in neuronal development are discussed.

Membrane trafficking of heterotrimeric G proteins via the endoplasmic reticulum and Golgi

Membrane targeting of G-protein alphabetagamma heterotrimers was investigated in live cells by use of Galpha and Ggamma subunits tagged with spectral mutants of green fluorescent protein. Unlike Ras proteins, Gbetagamma contains a single targeting signal, the CAAX motif, which directed the dimer to the endoplasmic reticulum. Endomembrane localization of farnesylated Ggamma(1), but not geranylgeranylated Ggamma(2), required carboxyl methylation. Targeting of the heterotrimer to the plasma membrane (PM) required coexpression of all three subunits, combining the CAAX motif of Ggamma with the fatty acyl modifications of Galpha. Galpha associated with Gbetagamma on the Golgi and palmitoylation of Galpha was required for translocation of the heterotrimer to the PM. Thus, two separate signals, analogous to the dual-signal targeting mechanism of Ras proteins, cooperate to target heterotrimeric G proteins to the PM via the endomembrane.

Activation-induced subcellular redistribution of G alpha(s) is dependent upon its unique N-terminus

The heterotrimeric G protein subunit, alpha(s), can move reversibly from plasma membranes to cytoplasm in response to activation by GPCRs or activating mutations. We examined the importance of the unique N-terminus of alpha(s) in this translocation in cultured cells. alpha(s) contains a single site for palmitoylation in its N-terminus, and this was replaced by different plasma membrane targeting motifs. These N-terminal alpha(s) mutants were targeted properly to plasma membranes, capable of coupling activated GPCRs to effectors, and able to constitutively stimulate cAMP production when they also contained an activating mutation. However, when activated by a constitutively activating mutation or by agonist-activated beta-AR, these N-terminal alpha(s) mutants failed, for the most part, to undergo redistribution from plasma membranes to cytoplasm, as assayed by immunofluorescence microscopy, or from a particulate to soluble fraction, as assayed by subcellular fractionation. These results highlight the importance of the extreme N-terminus of alpha(s) and its single site of palmitoylation for facilitating activation-induced translocation and provide insight into the mechanism of this G protein trafficking event.

Dissecting receptor-G protein specificity using G alpha chimeras

In conclusion, by taking advantage of the overall sequence homology and structural similarity of G alpha subunits, functional chimeric G alpha subunits can be generated and used as tools for the identification of sequence-specific factors that mediate receptor: G protein specificity. The [35S]GTP gamma S binding assay and the affinity shift activity assay are two sensitive biochemical approaches that can be used to assess receptor: G protein coupling in vitro. These in vitro assays limit confounding influences from cellular proteins and allow for the strict control of receptor: G protein ratios.

An Arabidopsis calcium-dependent protein kinase is associated with the endoplasmic reticulum

Arabidopsis contains 34 genes that are predicted to encode calcium-dependent protein kinases (CDPKs). CDPK enzymatic activity previously has been detected in many locations in plant cells, including the cytosol, the cytoskeleton, and the membrane fraction. However, little is known about the subcellular locations of individual CDPKs or the mechanisms involved in targeting them to those locations. We investigated the subcellular location of one Arabidopsis CDPK, AtCPK2, in detail. Membrane-associated AtCPK2 did not partition with the plasma membrane in a two-phase system. Sucrose gradient fractionation of microsomes demonstrated that AtCPK2 was associated with the endoplasmic reticulum (ER). AtCPK2 does not contain transmembrane domains or known ER-targeting signals, but does have predicted amino-terminal acylation sites. AtCPK2 was myristoylated in a cell-free extract and myristoylation was prevented by converting the glycine at the proposed site of myristate attachment to alanine (G2A). In plants, the G2A mutation decreased AtCPK2 membrane association by approximately 50%. A recombinant protein, consisting of the first 10 amino acids of AtCPK2 fused to the amino-terminus of beta-glucuronidase, was also targeted to the ER, indicating that the amino terminus of AtCPK2 can specify ER localization of a soluble protein. These results indicate that AtCPK2 is localized to the ER, that myristoylation is likely to be involved in the membrane association of AtCPK2, and that the amino terminal region of AtCPK2 is sufficient for correct membrane targeting.

Potentiation of GPCR-signaling via membrane targeting of G protein alpha subunits

Different assay technologies are available that allow ligand occupancy of G protein coupled receptors to be converted into robust functional assay signals. Of particular interest are universal screening systems such that activation of any GPCR can be detected with a common assay end point. The promiscuous G protein Galpha16 and chimeric G proteins are broadly used tools for setting up almost universal assay systems. Many efforts focused on making G proteins more promiscuous, however no attempts have been made to make promiscuos G proteins more sensitive by interfering with their cellular protein distribution. As a model system, we used a promiscuous G protein alphaq subunit, that lacks the highly conserved six amino acid N-terminal extension and bears four residues of alphai sequence at its C-terminus replacing the corresponding alphaq sequence (referred to as delta6qi4). When expressed in COS7 cells, delta6qi4 undergoes palmitoylation at its N-terminus. Cell fractionation and immunoblotting analysis indicated localization in the particulate and cytosolic fraction. Interestingly, introduction of a consensus site for N-terminal myristoylation (the resulting mutant referred to as delta6qi4myr) created a protein that was dually acylated and exclusively located in the particulate fraction. As a measure of G protein activation delta6qi4 and delta6qi4myr were coexpressed (in CHO cells) with a series of different Gi/o coupled receptors and ligand induced increases in intracellular Ca2+ release were determined with the FLIPR technology (Fluorescence plate imaging reader from Molecular Devices Corp.). All of the receptors interacted more efficiently with delta6qi4myr as compared with delta6qi4. It could be shown that increased functional responses of agonist activated GPCRs are due to the higher content of delta6qi4myr in the plasma membrane. Our results indicate that manipulation of subcellular localization of G protein alpha subunits-moving them from the cytosol to the plasma membrane-potentiates signaling of agonist activated GPCRs. It is concluded that addition of myristoylation sites into otherwise exclusively palmitoylated G proteins is a new and sensitive approach and may be applicable when functional assays are expected to yield weak signals as is the case when screening extracts of tissues for biologically active GPCR ligands.

Characterization of the functional heterologous desensitization of hypothalamic 5-HT(1A) receptors after 5-HT(2A) receptor activation

Desensitization of 5-HT(1A) receptors could be involved in the long-term therapeutic effect of anxiolytic and antidepressant drugs. Pretreatment of rats with the 5-HT(2A/2C) agonist DOI induces an attenuation of hypothalamic 5-HT(1A) receptor-G(z)-protein signaling, measured as the ACTH and oxytocin responses to an injection of the 5-HT(1A) agonist 8-OH-DPAT. We characterized this functional heterologous desensitization of 5-HT(1A) receptors in rats and examined some of the mechanisms that are involved. A time course experiment revealed that DOI produces a delayed and reversible reduction of the ACTH and oxytocin responses to an 8-OH-DPAT challenge. The maximal desensitization occurred at 2 hr, and it disappeared 24 hr after DOI injection. The desensitization was dose-dependent, and it shifted the oxytocin and ACTH dose-response curves of 8-OH-DPAT to the right (increased ED(50)) with no change in their maximal responses (E(max)). The 5-HT(2A) receptor antagonist MDL 100,907 prevented the DOI-induced desensitization, indicating that 5-HT(2A) receptors mediate the effect of DOI. Analysis of the components of the 5-HT(1A) receptor-G(z)-protein signaling system showed that DOI did not alter the level of membrane-associated G(z)-proteins in the hypothalamus. Additionally, DOI did not alter the binding of [(3)H]8-OH-DPAT or the inhibition by GTPgammaS of [(3)H]8-OH-DPAT binding in the hypothalamus. In conclusion, the activation of 5-HT(2A) receptors induces a transient functional desensitization of 5-HT(1A) receptor signaling in the hypothalamus, which may occur distal to the 5-HT(1A) receptor-G(z)-protein interface.

Characterization of the functional heterologous desensitization of hypothalamic 5-HT(1A) receptors after 5-HT(2A) receptor activation

Desensitization of 5-HT(1A) receptors could be involved in the long-term therapeutic effect of anxiolytic and antidepressant drugs. Pretreatment of rats with the 5-HT(2A/2C) agonist DOI induces an attenuation of hypothalamic 5-HT(1A) receptor-G(z)-protein signaling, measured as the ACTH and oxytocin responses to an injection of the 5-HT(1A) agonist 8-OH-DPAT. We characterized this functional heterologous desensitization of 5-HT(1A) receptors in rats and examined some of the mechanisms that are involved. A time course experiment revealed that DOI produces a delayed and reversible reduction of the ACTH and oxytocin responses to an 8-OH-DPAT challenge. The maximal desensitization occurred at 2 hr, and it disappeared 24 hr after DOI injection. The desensitization was dose-dependent, and it shifted the oxytocin and ACTH dose-response curves of 8-OH-DPAT to the right (increased ED(50)) with no change in their maximal responses (E(max)). The 5-HT(2A) receptor antagonist MDL 100,907 prevented the DOI-induced desensitization, indicating that 5-HT(2A) receptors mediate the effect of DOI. Analysis of the components of the 5-HT(1A) receptor-G(z)-protein signaling system showed that DOI did not alter the level of membrane-associated G(z)-proteins in the hypothalamus. Additionally, DOI did not alter the binding of [(3)H]8-OH-DPAT or the inhibition by GTPgammaS of [(3)H]8-OH-DPAT binding in the hypothalamus. In conclusion, the activation of 5-HT(2A) receptors induces a transient functional desensitization of 5-HT(1A) receptor signaling in the hypothalamus, which may occur distal to the 5-HT(1A) receptor-G(z)-protein interface.

Mutant G protein alpha subunit activated by Gbeta gamma: a model for receptor activation?

How receptors catalyze exchange of GTP for GDP bound to the Galpha subunit of trimeric G proteins is not known. One proposal is that the receptor uses the G protein's betagamma heterodimer as a lever, tilting it to pull open the guanine nucleotide binding pocket of Galpha. To test this possibility, we designed a mutant Galpha that would bind to betagamma in the tilted conformation. To do so, we excised a helical turn (four residues) from the N-terminal region of alpha(s), the alpha subunit of G(S), the stimulatory regulator of adenylyl cyclase. In the presence, but not in the absence, of transiently expressed beta(1) and gamma(2), this mutant (alpha(s)Delta), markedly stimulated cAMP accumulation. This effect depended on the ability of the coexpressed beta protein to interact normally with the lip of the nucleotide binding pocket of alpha(s)Delta. We substituted alanine for an aspartate in beta(1) that binds to a lysine (K206) in the lip of the alpha subunit's nucleotide binding pocket. Coexpressed with alpha(s)Delta and gamma(2), this mutant, beta(1)-D228A, elevated cAMP much less than did beta(1)-wild type; it did bind to alpha(s)Delta normally, however, as indicated by its unimpaired ability to target alpha(s)Delta to the plasma membrane. We conclude that betagamma can activate alpha(s) and that this effect probably involves both a tilt of betagamma relative to alpha(s) and interaction of beta with the lip of the nucleotide binding pocket. We speculate that receptors use a similar mechanism to activate trimeric G proteins.