A novel N-terminal motif for palmitoylation of G-protein alpha subunits

Recruitment, regulation, and release: Control of signaling enzyme localization and function by reversible S-acylation

An ever-growing number of studies highlight the importance of S-acylation, a reversible protein-lipid modification, for diverse aspects of intracellular signaling. In this review, we summarize the current understanding of how S-acylation regulates perhaps the best-known class of signaling enzymes, protein kinases. We describe how S-acylation acts as a membrane targeting signal that localizes certain kinases to specific membranes, and how such membrane localization in turn facilitates the assembly of signaling hubs consisting of an S-acylated kinase's upstream activators and/or downstream targets. We further discuss recent findings that S-acylation can control additional aspects of the function of certain kinases, including their interactions and, surprisingly, their activity, and how such regulation might be exploited for potential therapeutic gain. We go on to describe the roles and regulation of de-S-acylases and how extracellular signals drive dynamic (de)S-acylation of certain kinases. We discuss how S-acylation has the potential to lead to "emergent properties" that alter the temporal profile and/or salience of intracellular signaling events. We close by giving examples of other S-acylation-dependent classes of signaling enzymes and by discussing how recent biological and technological advances should facilitate future studies into the functional roles of S-acylation-dependent signaling.

Local and substrate-specific S-palmitoylation determines subcellular localization of Gαo

Peripheral membrane proteins (PMPs) associate with cellular membranes through post-translational modifications like S-palmitoylation. The Golgi apparatus is generally viewed as the transitory station where palmitoyl acyltransferases (PATs) modify PMPs, which are then transported to their ultimate destinations such as the plasma membrane (PM). However, little substrate specificity among the many PATs has been determined. Here we describe the inherent partitioning of Gαo - α-subunit of heterotrimeric Go proteins - to PM and Golgi, independent from Golgi-to-PM transport. A minimal code within Gαo N-terminus governs its compartmentalization and re-coding produces G protein versions with shifted localization. We establish the S-palmitoylation at the outer nuclear membrane assay ("SwissKASH") to probe substrate specificity of PATs in intact cells. With this assay, we show that PATs localizing to different membrane compartments display remarkable substrate selectivity, which is the basis for PMP compartmentalization. Our findings uncover a mechanism governing protein localization and establish the basis for innovative drug discovery.

Machine Learning Methods in Prediction of Protein Palmitoylation Sites: A Brief Review

Protein palmitoylation is a fundamental and reversible post-translational lipid modification that involves a series of biological processes. Although a large number of experimental studies have explored the molecular mechanism behind the palmitoylation process, the computational methods has attracted much attention for its good performance in predicting palmitoylation sites compared with expensive and time-consuming biochemical experiments. The prediction of protein palmitoylation sites is helpful to reveal its biological mechanism. Therefore, the research on the application of machine learning methods to predict palmitoylation sites has become a hot topic in bioinformatics and promoted the development in the related fields. In this review, we briefly introduced the recent development in predicting protein palmitoylation sites by using machine learningbased methods and discussed their benefits and drawbacks. The perspective of machine learning-based methods in predicting palmitoylation sites was also provided. We hope the review could provide a guide in related fields.

Myristoylation, an Ancient Protein Modification Mirroring Eukaryogenesis and Evolution

N-myristoylation (MYR) is a crucial fatty acylation catalyzed by N-myristoyltransferases (NMTs) that is likely to have appeared over 2 billion years ago. Proteome-wide approaches have now delivered an exhaustive list of substrates undergoing MYR across approximately 2% of any proteome, with constituents, several unexpected, associated with different membrane compartments. A set of <10 proteins conserved in eukaryotes probably represents the original set of N-myristoylated targets, marking major changes occurring throughout eukaryogenesis. Recent findings have revealed unexpected mechanisms and reactivity, suggesting competition with other acylations that are likely to influence cellular homeostasis and the steady state of the modification landscape. Here, we review recent advances in NMT catalysis, substrate specificity, and MYR proteomics, and discuss concepts regarding MYR during evolution.

Targeting G protein-coupled receptors in cancer therapy

As basic research into GPCR signaling and its association with disease has come into fruition, greater clarity has emerged with regards to how these receptors may be amenable to therapeutic intervention. As a diverse group of receptor proteins, which regulate a variety of intracellular signaling pathways, research in this area has been slow to yield tangible therapeutic agents for the treatment of a number of diseases including cancer. However, recently such research has gained momentum based on a series of studies that have sought to define GPCR proteins dynamics through the elucidation of their crystal structures. In this chapter, we define the approaches that have been adopted in developing better therapeutics directed against the specific parts of the receptor proteins, such as the extracellular and the intracellular domains, including the ligands and auxiliary proteins that bind them. Finally, we also briefly outline how GPCR-derived signaling transduction pathways hold great potential as additional targets.

Post-translational Regulation of GLT-1 in Neurological Diseases and Its Potential as an Effective Therapeutic Target

Glutamate transporter-1 (GLT-1) is a Na⁺-dependent transporter that plays a key role in glutamate homeostasis by removing excess glutamate in the central nervous system (CNS). GLT-1 dysregulation occurs in various neurological diseases including Huntington's disease (HD), Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and epilepsy. Downregulation or dysfunction of GLT-1 has been a common finding across these diseases but how this occurs is still under investigation. This review aims to highlight post-translational regulation of GLT-1 which leads to its downregulation including sumoylation, palmitoylation, nitrosylation, ubiquitination, and subcellular localization. Various therapeutic interventions to restore GLT-1, their proposed mechanism of action and functional effects will be examined as potential treatments to attenuate the neurological symptoms associated with loss or downregulation of GLT-1.

GPCR Modulation in Breast Cancer

Breast cancer is the most prevalent cancer found in women living in developed countries. Endocrine therapy is the mainstay of treatment for hormone-responsive breast tumors (about 70% of all breast cancers) and implies the use of selective estrogen receptor modulators and aromatase inhibitors. In contrast, triple-negative breast cancer (TNBC), a highly heterogeneous disease that may account for up to 24% of all newly diagnosed cases, is hormone-independent and characterized by a poor prognosis. As drug resistance is common in all breast cancer subtypes despite the different treatment modalities, novel therapies targeting signaling transduction pathways involved in the processes of breast carcinogenesis, tumor promotion and metastasis have been subject to accurate consideration. G protein-coupled receptors (GPCRs) are the largest family of cell-surface receptors involved in the development and progression of many tumors including breast cancer. Here we discuss data regarding GPCR-mediated signaling, pharmacological properties and biological outputs toward breast cancer tumorigenesis and metastasis. Furthermore, we address several drugs that have shown an unexpected opportunity to interfere with GPCR-based breast tumorigenic signals.

Analysis of the brain palmitoyl-proteome using both acyl-biotin exchange and acyl-resin-assisted capture methods

Palmitoylation is a reversible post-translational protein modification in which palmitic acid is added to cysteine residues, allowing association with different cellular membranes and subdomains. Recently, techniques have been developed to identify palmitoylation on a proteome-wide scale in order to reveal the full cellular complement of palmitoylated proteins. However, in the studies reported to date, there is considerable variation between the sets of identified palmitoyl-proteins and so there remains some uncertainty over what constitutes the definitive complement of palmitoylated proteins even in well-studied tissues such as brain. To address this issue, we used both acyl-biotin exchange and acyl-resin-assisted capture approaches using rat brain as a common protein source. The palmitoyl-proteins identified from each method by mass spectrometry were then compared with each other and previously published studies. There was generally good agreement between the two methods, although many identifications were unique to one method, indicating that at least some of the variability in published palmitoyl proteomes is due to methodological differences. By combining our new data with previous publications using mammalian cells/tissues, we propose a high confidence set of bona fide palmitoylated proteins in brain and provide a resource to help researchers prioritise candidate palmitoyl-proteins for investigation.

Multifactorial Regulation of G Protein-Coupled Receptor Endocytosis

Endocytosis is a process by which cells absorb extracellular materials via the inward budding of vesicles formed from the plasma membrane. Receptor-mediated endocytosis is a highly selective process where receptors with specific binding sites for extracellular molecules internalize via vesicles. G protein-coupled receptors (GPCRs) are the largest single family of plasma-membrane receptors with more than 1000 family members. But the molecular mechanisms involved in the regulation of GPCRs are believed to be highly conserved. For example, receptor phosphorylation in collaboration with β-arrestins plays major roles in desensitization and endocytosis of most GPCRs. Nevertheless, a number of subsequent studies showed that GPCR regulation, such as that by endocytosis, occurs through various pathways with a multitude of cellular components and processes. This review focused on i) functional interactions between homologous and heterologous pathways, ii) methodologies applied for determining receptor endocytosis, iii) experimental tools to determine specific endocytic routes, iv) roles of small guanosine triphosphate-binding proteins in GPCR endocytosis, and v) role of post-translational modification of the receptors in endocytosis.

Optical manipulation of the alpha subunits of heterotrimeric G proteins using photoswitchable dimerization systems

Alpha subunits of heterotrimeric G proteins (Gα) are involved in a variety of cellular functions. Here we report an optogenetic strategy to spatially and temporally manipulate Gα in living cells. More specifically, we applied the blue light-induced dimerization system, known as the Magnet system, and an alternative red light-induced dimerization system consisting of Arabidopsis thaliana phytochrome B (PhyB) and phytochrome-interacting factor 6 (PIF6) to optically control the activation of two different classes of Gα (Gα_q and Gα_s). By utilizing this strategy, we demonstrate successful regulation of Ca²⁺ and cAMP using light in mammalian cells. The present strategy is generally applicable to different kinds of Gα and could contribute to expanding possibilities of spatiotemporal regulation of Gα in mammalian cells.

Functional compartmentalization of the plasma membrane of neurons by a unique acyl chain composition of phospholipids

In neurons, the plasma membrane is functionally separated into several distinct segments. Neurons form these domains by delivering selected components to and by confining them within each segment of the membrane. Although some mechanisms of the delivery are elucidated, that of the confinement is unclear. We show here that 1-oleoyl-2-palmitoyl-phosphatidylcholine (OPPC), a unique molecular species of phospholipids, is concentrated at the protrusion tips of several neuronal culture cells and the presynaptic area of neuronal synapses of the mouse brain. In PC12 cells, NGF-stimulated neuronal differentiation induces a phospholipase A1 activity at the protrusion tips, which co-localizes with the OPPC domain. Inhibition of the phospholipase A1 activity leads to suppression of phospholipid remodeling in the tip membrane and results in disappearance of the OPPC at the tips. In these cells, confinement of dopamine transporter and Gαo proteins to the tip was also disrupted. These findings link the lateral distribution of the molecular species of phospholipids to the formation of functional segments in the plasma membrane of neurons and to the mechanism of protein confinement at the synapse.

Structural basis of G protein-coupled receptor-Gi protein interaction: formation of the cannabinoid CB2 receptor-Gi protein complex

In this study, we applied a comprehensive G protein-coupled receptor-Gαi protein chemical cross-linking strategy to map the cannabinoid receptor subtype 2 (CB2)-Gαi interface and then used molecular dynamics simulations to explore the dynamics of complex formation. Three cross-link sites were identified using LC-MS/MS and electrospray ionization-MS/MS as follows: 1) a sulfhydryl cross-link between C3.53(134) in TMH3 and the Gαi C-terminal i-3 residue Cys-351; 2) a lysine cross-link between K6.35(245) in TMH6 and the Gαi C-terminal i-5 residue, Lys-349; and 3) a lysine cross-link between K5.64(215) in TMH5 and the Gαi α4β6 loop residue, Lys-317. To investigate the dynamics and nature of the conformational changes involved in CB2·Gi complex formation, we carried out microsecond-time scale molecular dynamics simulations of the CB2 R*·Gαi1β1γ2 complex embedded in a 1-palmitoyl-2-oleoyl-phosphatidylcholine bilayer, using cross-linking information as validation. Our results show that although molecular dynamics simulations started with the G protein orientation in the β2-AR*·Gαsβ1γ2 complex crystal structure, the Gαi1β1γ2 protein reoriented itself within 300 ns. Two major changes occurred as follows. 1) The Gαi1 α5 helix tilt changed due to the outward movement of TMH5 in CB2 R*. 2) A 25° clockwise rotation of Gαi1β1γ2 underneath CB2 R* occurred, with rotation ceasing when Pro-139 (IC-2 loop) anchors in a hydrophobic pocket on Gαi1 (Val-34, Leu-194, Phe-196, Phe-336, Thr-340, Ile-343, and Ile-344). In this complex, all three experimentally identified cross-links can occur. These findings should be relevant for other class A G protein-coupled receptors that couple to Gi proteins.

Extra-long Gαs variant XLαs protein escapes activation-induced subcellular redistribution and is able to provide sustained signaling

Murine models indicate that Gαs and its extra-long variant XLαs, both of which are derived from GNAS, markedly differ regarding their cellular actions, but these differences are unknown. Here we investigated activation-induced trafficking of Gαs and XLαs, using immunofluorescence microscopy, cell fractionation, and total internal reflection fluorescence microscopy. In transfected cells, XLαs remained localized to the plasma membrane, whereas Gαs redistributed to the cytosol after activation by GTPase-inhibiting mutations, cholera toxin treatment, or G protein-coupled receptor agonists (isoproterenol or parathyroid hormone (PTH)(1-34)). Cholera toxin treatment or agonist (isoproterenol or pituitary adenylate cyclase activating peptide-27) stimulation of PC12 cells expressing Gαs and XLαs endogenously led to an increased abundance of Gαs, but not XLαs, in the soluble fraction. Mutational analyses revealed two conserved cysteines and the highly charged domain as being critically involved in the plasma membrane anchoring of XLαs. The cAMP response induced by M-PTH(1-14), a parathyroid hormone analog, terminated quickly in HEK293 cells stably expressing the type 1 PTH/PTH-related peptide receptor, whereas the response remained maximal for at least 6 min in cells that co-expressed the PTH receptor and XLαs. Although isoproterenol-induced cAMP response was not prolonged by XLαs expression, a GTPase-deficient XLαs mutant found in certain tumors and patients with fibrous dysplasia of bone and McCune-Albright syndrome generated more basal cAMP accumulation in HEK293 cells and caused more severe impairment of osteoblastic differentiation of MC3T3-E1 cells than the cognate Gαs mutant (gsp oncogene). Thus, activated XLαs and Gαs traffic differently, and this may form the basis for the differences in their cellular actions.

The intracellular dynamic of protein palmitoylation

S-palmitoylation describes the reversible attachment of fatty acids (predominantly palmitate) onto cysteine residues via a labile thioester bond. This posttranslational modification impacts protein functionality by regulating membrane interactions, intracellular sorting, stability, and membrane micropatterning. Several recent findings have provided a tantalizing insight into the regulation and spatiotemporal dynamics of protein palmitoylation. In mammalian cells, the Golgi has emerged as a possible super-reaction center for the palmitoylation of peripheral membrane proteins, whereas palmitoylation reactions on post-Golgi compartments contribute to the regulation of specific substrates. In addition to palmitoylating and depalmitoylating enzymes, intracellular palmitoylation dynamics may also be controlled through interplay with distinct posttranslational modifications, such as phosphorylation and nitrosylation.

Greasing their way: lipid modifications determine protein association with membrane rafts

Increasing evidence suggests that biological membranes can be laterally subdivided into domains enriched in specific lipid and protein components and that these domains may be involved in the regulation of a number of vital cellular processes. An example is membrane rafts, which are lipid-mediated domains dependent on preferential association between sterols and sphingolipids and inclusive of a specific subset of membrane proteins. While the lipid and protein composition of rafts has been extensively characterized, the structural details determining protein partitioning to these domains remain unresolved. Here, we review evidence suggesting that post-translation modification by saturated lipids recruits both peripheral and transmembrane proteins to rafts, while short, unsaturated, and/or branched hydrocarbon chains prevent raft association. The most widely studied group of raft-associated proteins are glycophosphatidylinositol-anchored proteins (GPI-AP), and we review a variety of evidence supporting raft-association of these saturated lipid-anchored extracellular peripheral proteins. For transmembrane and intracellular peripheral proteins, S-acylation with saturated fatty acids mediates raft partitioning, and the dynamic nature of this modification presents an exciting possibility of enzymatically regulated raft association. The other common lipid modifications, that is, prenylation and myristoylation, are discussed in light of their likely role in targeting proteins to nonraft membrane regions. Finally, although the association between raft affinity and lipid modification is well-characterized, we discuss several open questions regarding regulation and remodeling of these post-translational modifications as well as their role in transbilayer coupling of membrane domains.

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.

Analysis of S-acylation of proteins

Palmitoylation or S-acylation is the post-translational attachment of fatty acids to cysteine residues and is common among integral and peripheral mem brane proteins. Palmitoylated proteins have been found in every eukaryotic cell type examined (yeast, insect, and vertebrate cells), as well as in viruses grown in these cells. The exact functions of protein palmitoylation are not well understood. Intrin sically hydrophilic proteins, especially signaling molecules, are anchored by long chain fatty acids to the cytoplasmic face of the plasma membrane. Palmitoylation may also promote targeting to membrane subdomains enriched in glycosphingolip ids and cholesterol or affect protein-protein interactions. This chapter describes (1) a standard protocol for metabolic labeling of palmitoylated proteins and also the procedures to prove a covalent and ester-type linkage of the fatty acids, (2) a simple method to analyze the fatty acid content of S-acylated proteins, (3) two methods to analyze dynamic palmitoylation for a given protein and (4) protocolls to study cell-free palmitoylation of proteins.

Palmitoylation participates in G protein coupled signal transduction by affecting its oligomerization

Much in vivo and in vitro evidence has shown that the alpha subunits of heterotrimeric GTP-binding proteins (G proteins) exist as oligomers in their base state and disaggregate when being activated. In this article, the influence of palmitoylation modification of Galpha(o) on its oligomerization was explored extensively. Galpha(o) protein was expressed and purified from Escherichia coli strain JM109 cotransformed with pQE60(Galpha(o)) and pBB131(N-myristoyltransferase). Non-denaturing gel electrophoresis analysis revealed that Galpha(o) existed to a small extent as monomers but mostly as oligomers including dimers, trimers, tetramers and pentamers which could disaggregate completely into monomers by GTPgammaS stimulation. Palmitoylated Galpha(o), on the other hand, only present as oligomers that were difficult to disaggregate into monomers. The effect of palmitoylation on oligomerization of Galpha(o) was further investigated by several other biochemical and biophysical methods including gel filtration chromatography, analytical ultracentrifugation and atomic force microscopy analysis. The results consistently demonstrated that palmitoylation facilitated oligomerization of the Galpha(o) protein. Autoradiography indicated that [(14)C]-palmitoylated Galpha(o) would in no case disaggregate into monomers after treatment with GTPgammaS. [(35)S]-GTPgammaS binding activity assay showed that palmitoylated Galpha(o) was saturated at only 7.8 nmol/mg compared to 21.8 nmol/mg for non-palmitoylated Galpha(o). Fluorescent quenching studies using BODIPY FL-GTPgammaS as a probe showed that the conformation of GTP-binding domain of Galpha(o) tended to become more compact after palmitoylation. These results implied that palmitoylation may regulate the GDP/GTP exchange of Galpha(o) by influencing the oligomerization state of Galpha(o) and thereby modulate the on-off switch of the G protein in G protein-coupled signal transduction.

The Importance of N-terminal Polycysteine and Polybasic Sequences for G14α and G16α Palmitoylation, Plasma Membrane Localization, and Signaling Function

Plasma membrane targeting of G protein alpha (Galpha) subunits is essential for competent receptor-to-G protein signaling. Many Galpha are tethered to the plasma membrane by covalent lipid modifications at their N terminus. Additionally, it is hypothesized that Gq family members (Gqalpha,G11alpha,G14alpha, and G16alpha) in particular utilize a polybasic sequence of amino acids in their N terminus to promote membrane attachment and protein palmitoylation. However, this hypothesis has not been tested, and nothing is known about other mechanisms that control subcellular localization and signaling properties of G14alpha and G16alpha. Here we report critical biochemical factors that mediate membrane attachment and signaling function of G14alpha and G16alpha. We find that G14alpha and G16alpha are palmitoylated at distinct polycysteine sequences in their N termini and that the polycysteine sequence along with the adjacent polybasic region are both important for G16alpha-mediated signaling at the plasma membrane. Surprisingly, the isolated N termini of G14alpha and G16alpha expressed as peptides fused to enhanced green fluorescent protein each exhibit differential requirements for palmitoylation and membrane targeting; individual cysteine residues, but not the polybasic regions, determine lipid modification and subcellular localization. However, full-length G16alpha, more so than G14alpha, displays a functional dependence on single cysteines for membrane localization and activity, and its full signaling potential depends on the integrity of the polybasic sequence. Together, these findings indicate that G14alpha and G16alpha are palmitoylated at distinct polycysteine sequences, and that the adjacent polybasic domain is not required for Galpha palmitoylation but is important for localization and functional activity of heterotrimeric G proteins.

Plant G protein heterotrimers require dual lipidation motifs of Gα and Gγ and do not dissociate upon activation

In plants one bona fide Gα subunit has been identified, as well as a single Gβ and two Gγ subunits. To study the roles of lipidation motifs in the regulation of subcellular location and heterotrimer formation in living plant cells, GFP-tagged versions of the Arabidopsis thaliana heterotrimeric G protein subunits were constructed. Mutational analysis showed that the Arabidopsis Gα subunit, GPα1, contains two lipidation motifs that were essential for plasma membrane localization. The Arabidopsis Gβ subunit, AGβ1, and the Gγ subunit, AGG1, were dependent upon each other for tethering to the plasma membrane. The second Gγ subunit, AGG2, did not require AGβ1 for localization to the plasma membrane. Like AGG1, AGG2 contains two putative lipidation motifs, both of which were necessary for membrane localization. Interaction between the subunits was studied using fluorescence resonance energy transfer (FRET) imaging by means of fluorescence lifetime imaging microscopy (FLIM). The results suggest that AGβ1 and AGG1 or AGβ1 and AGG2 can form heterodimers independent of lipidation. In addition, FLIM-FRET revealed the existence of GPα1-AGβ1-AGG1 heterotrimers at the plasma membrane. Importantly, rendering GPα1 constitutively active did not cause a FRET decrease in the heterotrimer, suggesting no dissociation upon GPα1 activation.

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.

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.

Expression pattern in the antennae of a newly isolated lepidopteran Gq protein alpha subunit cDNA

From the antennae of the moth Mamestra brassicae, we have identified a lepidopteran G protein alpha subunit belonging to the Gq family, through immunological detection in crude antennal extract and antennal primary cell cultures, followed by molecular cloning. The complete cDNA sequence (1540 bp) contains an open reading frame encoding a protein of 353 amino acids. This deduced sequence possesses all of the characteristics of the Gq family and shares a very high degree of amino-acid sequence identity with vertebrate (80% with mouse or human Gqalpha) and invertebrate subunits (varying between 60 and 87% for Gqalpha from organisms as diverse as sponge and Drosophila). The expression pattern of the Gq subunit in adult antennae was associated with the olfactory sensilla suggesting a specific role in olfaction. These data provide molecular evidence for a component of the phosphoinositide signaling pathway in moth antennae: this G protein alpha subunit may be involved in the olfaction transduction process through interaction with G-protein-coupled receptors, stimulating the phospholipase C mediated second messenger pathway.

Heterogeneous fatty acylation of Src family kinases with polyunsaturated fatty acids regulates raft localization and signal transduction

Fatty acylation of Src family kinases is essential for localization of the modified proteins to the plasma membrane and to plasma membrane rafts. It has been suggested that the presence of saturated fatty acyl chains on proteins is conducive for their insertion into liquid ordered lipid domains present in rafts. The ability of unsaturated dietary fatty acids to be attached to Src family kinases has not been investigated. Here we demonstrate that heterogeneous fatty acylation of Src family kinases occurs and that the nature of the attached fatty acid influences raft-mediated signal transduction. By using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, we show that in addition to 14:0 (myristate), 14:1 and 14:2 fatty acids can be attached to the N-terminal glycine of the Src family kinase Fyn when the growth media are supplemented with these dietary fatty acids. Moreover, we synthesized novel iodinated analogs of oleate and stearate, and we showed that heterogeneous S-acylation can occur on cysteine residues within Fyn as well as Galpha, GAP43, and Ras. Modification of Fyn with unsaturated or polyunsaturated fatty acids reduced its raft localization and resulted in decreased T cell signal transduction. These studies establish that heterogeneous fatty acylation is a widespread occurrence that serves to regulate signal transduction by membrane-bound proteins.

Cysteine residues of SNAP-25 are required for SNARE disassembly and exocytosis, but not for membrane targeting

The release of neurotransmitter at a synapse occurs via the regulated fusion of synaptic vesicles with the plasma membrane. The fusion of the two lipid bilayers is mediated by a protein complex that includes the plasma membrane target soluble N-ethylmaleimide-sensitive fusion protein (NSF) attachment protein (SNAP) receptors (t-SNAREs), syntaxin 1A and synaptosome-associated protein of 25 kDa (SNAP-25), and the vesicle SNARE (v-SNARE), vesicle-associated membrane protein (VAMP). Whereas syntaxin 1A and VAMP are tethered to the membrane by a C-terminal transmembrane domain, SNAP-25 has been suggested to be anchored to the membrane via four palmitoylated cysteine residues. We demonstrate that the cysteine residues of SNAP-25 are not required for membrane localization when syntaxin 1A is present. Analysis of the 7 S and 20 S complexes formed by mutants that lack cysteine residues demonstrates that the cysteines are required for efficient SNARE complex dissociation. Furthermore, these mutants are unable to support exocytosis, as demonstrated by a PC12 cell secretion assay. We hypothesize that syntaxin 1A serves to direct newly synthesized SNAP-25 through the Golgi transport pathway to the axons and synapses, and that palmitoylation of cysteine residues is not required for targeting, but to optimize interactions required for SNARE complex dissociation.

Membrane association and conformational change of palmitoylated G(o)alpha

Bovine brain G(o)alpha was specifically palmitoylated in vitro. The apparent dissociation constant for depalmitoylated G(o)alpha (dG(o)alpha) was 0.273 microM, while that for palmitoylated G(o)alpha (pG(o)alpha) was 5.77 nM. The dissociation rate constant (K(21)) and dissociation half-life for dG(o)alpha were 8.4x10(-4) min and 825 min respectively, while no significant dissociation of pG(o)alpha was detected. The limiting membrane insertion pressures for pG(o)alpha and dG(o)alpha were 44.4 mN/m and 41.3 mN/m respectively. These data suggested that palmitoylation facilitated the membrane association of G(o)alpha. Conformational changes of dG(o)alpha and pG(o)alpha detected by monitoring fluorescence spectra and fluorescence quenching were significantly different after they were associated with the membrane. It was suggested that conformational changes of G(o)alpha upon membrane association might be related to regulation of G(o)alpha signaling by palmitoylation.

Thioacylation is required for targeting G-protein subunit G(o1alpha) to detergent-insoluble caveolin-containing membrane domains

alpha-Subunits of heterotrimeric G(i)-like proteins (alpha(i), alpha(o) and alpha(z)) associate with the cytoplasmic leaflet of the plasma membrane by means of N-terminally linked myristic acid and palmitic acid. An additional role for palmitate has been recently suggested by the observation that fusion with the palmitoylated N-terminus of alpha(i1) relocalizes cytosolic green-fluorescent-protein reporter to low buoyancy, Triton-insoluble membrane domains (TIFF; Triton-insoluble floating fraction), enriched with caveolin-1 [Galbiati, Volonté, Meani, Milligan, Lublin, Lisanti and Parenti (1999) J. Biol. Chem 274, 5843-5850]. Here we show that, upon transient expression in transfected COS-7 cells, myristoylated and palmitoylated alpha(o) (alpha(o)wt, where wt is wild-type) is exclusively found in TIFF, from where non-palmitoylated alpha(o)wt and alpha(o)C3S (Cys(3)-->Ser) mutant are excluded. Moreover, alpha(o) fused to N-terminally truncated human vasopressin V2 receptor (V2TR-alpha(o)), lacking myristate and palmitate, still localizes at the plasma membrane by means of first transmembrane helix of V2R, but is excluded from TIFF. Likewise, alpha(o)C3S does not partition into TIFF, even when its membrane avidity is enhanced by co-expression of betagamma-subunits. Thus membrane association, in the absence of added palmitate, is not sufficient to confer partitioning of alpha(o) within TIFF, suggesting that palmitoylation is a signal for membrane compartmentalization of dually acylated alpha-subunits.

Effects of prenatal exposure to cocaine on the developing brain: anatomical, chemical, physiological and behavioral consequences

Earlier studies of human infants and studies employing animal models had indicated that prenatal exposure to cocaine produced developmental changes in the behavior of the offspring. The present paper reports on the results obtained in a rabbit model of in utero exposure to cocaine using intravenous injections (4 mg/kg, twice daily) that mimic the pharmacokinetics of crack cocaine in humans. At this dose, cocaine had no effect on the body weight gain of dams, time to delivery, litter size and body weight or other physical characteristics of the offspring. In spite of an otherwise normal appearance, cocaine-exposed neonates displayed a permanent impairment in signal transduction via the D1 dopamine receptor in caudate nucleus, frontal cortex and cingulate cortex due to an uncoupling of the receptor from its associated Gs protein. This uncoupling in the caudate nucleus was shown to have behavioral consequences in that young or adult rabbits, exposed to cocaine in utero, failed to demonstrate amphetamine-elicited motor responses normally seen after activation of D1 receptors in the caudate. The cocaine progeny also demonstrated permanent morphological abnormalities in the anterior cingulate cortex due to uncoupling of the D1 receptor and the consequent inability of dopamine to regulate neurite outgrowth during neuronal development. Consistent with the known functions of the anterior cingulate cortex, adult cocaine progeny demonstrated deficits in attentional processes. This was reflected by impairment in discrimination learning during classical conditioning that was due to an inability to ignore salient stimuli even when these were not relevant to the task. The impairment in discrimination learning also occurred in an instrumental avoidance task and could be shown to be due to an impairment of cingulothalamic learning-related neuronal coding. It was proposed that the selective loss of D1-related neurotransmission in the anterior cingulate cortex prevented an appropriate activation of GABA neurons and thus a loss of inhibitory regulation that is necessary for processes involved in associative attention. Taken together, these findings suggest that the uncoupling of the D1 receptor from its G protein may be the fundamental source of the anatomic, cognitive and motor disturbances seen in rabbits exposed to cocaine in utero. Moreover, the long-term cognitive and motor deficits observed in the rabbit model are in agreement with the recent reports indicating that persistent attentional and other behavioral deficits may be evident in cocaine-exposed children as they grow older and are challenged to master more complex cognitive tasks.

Regulation of galpha i palmitoylation by activation of the 5-hydroxytryptamine-1A receptor

Nearly all alpha subunits of heterotrimeric GTP-binding regulatory proteins (G proteins) are palmitoylated at cysteine residues near the N terminus. A regulated cycle of palmitoylation could provide a mechanism for modulating G protein signaling by affecting protein interactions and localization of the subunit. In the present studies we utilized both [(3)H]palmitate incorporation and pulse-chase techniques to address the dynamics of alpha(i) palmitoylation in Chinese hamster ovary cells. Both techniques demonstrated a dose- and time-dependent change in [(3)H]palmitate labeling of alpha(i) upon activation of stably expressed 5-hydroxytryptamine-1A receptors by the agonist (+/-)-2-dipropylamino-8-hydroxy-1,2,3, 4-tetrahydronaphthalene hydrobromide (DPAT), with an EC(50) of approximately 10 nm. For the incorporation assay, DPAT elicited an approximate doubling in labeling at the earliest time point measured. For the pulse-chase assay, DPAT promoted a significant loss of radiolabel almost equally as fast. These data demonstrate that the exchange of palmitate on alpha(i) is increased upon stimulation of 5-hydroxytryptamine-1A receptors through the combined processes of depalmitoylation and palmitoylation. These results provide the basis for extending the concept of regulated exchange of palmitate beyond G(s) and provide a framework for exploring the specific functional attributes of the palmitoylated and depalmitoylated forms of subunit.

The regulator of G protein signaling RGS4 selectively enhances alpha 2A-adreoreceptor stimulation of the GTPase activity of Go1alpha and Gi2alpha

Agonist-stimulated high affinity GTPase activity of fusion proteins between the alpha(2A)-adrenoreceptor and the alpha subunits of forms of the G proteins G(i1), G(i2), G(i3), and G(o1), modified to render them insensitive to the action of pertussis toxin, was measured following transient expression in COS-7 cells. Addition of a recombinant regulator of G protein signaling protein, RGS4, did not significantly affect basal GTPase activity nor agonist stimulation of the fusion proteins containing Galpha(i1) and Galpha(i3) but markedly enhanced agonist-stimulation of the proteins containing Galpha(i2) and Galpha(o1.) The effect of RGS4 on the alpha(2A)-adrenoreceptor-Galpha(o1) fusion protein was concentration-dependent with EC(50) of 30 +/- 3 nm and the potency of the receptor agonist UK14304 was reduced 3-fold by 100 nm RGS4. Equivalent reconstitution with Asn(88)-Ser RGS4 failed to enhance agonist function on the alpha(2A)-adrenoreceptor-Galpha(o1) or alpha(2A)-adrenoreceptor-Galpha(i2) fusion proteins. Enzyme kinetic analysis of the GTPase activity of the alpha(2A)-adrenoreceptor-Galpha(o1) and alpha(2A)-adrenoreceptor-Galpha(i2) fusion proteins demonstrated that RGS4 both substantially increased GTPase V(max) and significantly increased K(m) of the fusion proteins for GTP. The increase in K(m) for GTP was dependent upon RGS4 amount and is consistent with previously proposed mechanisms of RGS function. Agonist-stimulated GTPase turnover number in the presence of 100 nm RGS4 was substantially higher for alpha(2A)-adrenoreceptor-Galpha(o1) than for alpha(2A)-adrenoreceptor-Galpha(i2). These studies demonstrate that although RGS4 has been described as a generic stimulator of the GTPase activity of G(i)-family G proteins, selectivity of this interaction and quantitative variation in its function can be monitored in the presence of receptor activation of the G proteins.

Acylation of Galpha(13) is important for its interaction with thrombin receptor, transforming activity and actin stress fiber formation

Palmitoylation of alpha-subunits in heterotrimeric G proteins has become a research object of growing attention. Following our recent report on the acylation of the mono-palmitoylated Galpha(12) [Ponimaskin et al., FEBS Lett. 429 (1998) 370-374], we report here on the identification of three palmitoylation sites in the second member of the G(12) family, Galpha(13), and on the biological significance of fatty acids on the particular sites. Using mutants of alpha(13) in which the potentially palmitoylated cysteine residues (Cys) were replaced by serine residues, we find that Cys-14, Cys-18 and Cys-37 all serve as palmitoylation sites, and that the mutants lacking fatty acids are functionally defective. The following biological functions of Galpha(13) were found to be inhibited: coupling to the PAR1 thrombin receptor, cell transformation and actin stress fiber formation. Results from established assays for the above functions with a series of mutants, including derivatives of the constitutively active mutant Galpha(13)Q226L, revealed a graded inhibitory response on the above mentioned parameters. As a rule, it appears that palmitoylation of the N-proximal sites (e.g. Cys-14 and Cys-18) contributes more effectively to biological function than of the acylation site located more internally (Cys-37). However, the mutant with Cys-37 replaced by serine is more severely inhibited in stress fiber formation (80%) than in cell transformation (50%), pointing to the possibility of a differential involvement of the three palmitoylation sites in Galpha(13).

Galpha 13 requires palmitoylation for plasma membrane localization, Rho-dependent signaling, and promotion of p115-RhoGEF membrane binding

Most heterotrimeric G protein alpha subunits are covalently modified by palmitate attached to one or more N-terminal cysteine residues. Although a wide variety of proteins undergo palmitoylation, the role of this fatty acid modification in G protein signaling is not well understood. Thus, we examined the role of palmitoylation of alpha(13), a G protein alpha subunit that regulates many pathways involved in cell growth. Both N-terminal cysteines at positions 14 and 18 were required for palmitoylation. Mutant alpha(13), in which both cysteines were changed to serines, failed to localize to plasma membranes in transfected cells and failed to activate Rho-dependent serum response factor-mediated transcription and actin stress fiber formation. However, nonpalmitoylated, cysteine to serine mutant alpha(13) retained the ability to co-immunoprecipitate with a direct effector, p115-RhoGEF. Finally, we report the novel observation that activated alpha(13) induces a redistribution of p115-RhoGEF from the cytoplasm to plasma membranes, but non-palmitoylated mutants of alpha(13) fail to cause p115-RhoGEF translocation. These findings identify palmitoylation of alpha(13) as critical for its proper membrane localization and signaling and provide insight into the mechanism of activation of Rho-dependent signaling pathways by alpha(13).

Interaction between HIV-1 NEF and G(o) proteins in transfected COS-7 cells

Nef protein of HIV/SIV lentiviruses affects G-protein-mediated signaling, and physically associates to Lck, a myristoylated and palmitoylated Src-like tyrosine kinase. To assess whether Nef interacts with alpha-subunits of heterotrimeric G proteins (Galpha), carrying the same lipidation motif as Lck, we transiently expressed Nef and G(o)alpha (wild-type or nonpalmitoylated C3S mutant), individually or in combination, in transfected COS-7 cells. Recombinant Nef was mostly recovered in particulate fractions, and a Nef-Green Fluorescent Protein chimera was localized at the plasmalemma by in vivo fluorescence imaging. Moreover, Nef and C3S were entirely solubilized by cold Triton X-100, and excluded from low buoyant density sucrose gradient fractions, containing caveolin-1, whereas wild-type G(o)alpha was partially resistant to Triton extraction, and colocalized with caveolin-1. After coexpression, Nef recruited soluble C3S to membranes, and the two proteins were coimmunoprecipitated by G(o)alpha and Nef antisera. We conclude that Nef interacts with nonpalmitoylated G(o)alpha, presumably outside caveolin-rich microdomains of the plasma membrane.

Interaction with Gbetagamma is required for membrane targeting and palmitoylation of Galpha(s) and Galpha(q)

Peripheral membrane proteins utilize a variety of mechanisms to attach tightly, and often reversibly, to cellular membranes. The covalent lipid modifications, myristoylation and palmitoylation, are critical for plasma membrane localization of heterotrimeric G protein alpha subunits. For alpha(s) and alpha(q), two subunits that are palmitoylated but not myristoylated, we examined the importance of interacting with the G protein betagamma dimer for their proper plasma membrane localization and palmitoylation. Conserved alpha subunit N-terminal amino acids predicted to mediate binding to betagamma were mutated to create a series of betagamma binding region mutants expressed in HEK293 cells. These alpha(s) and alpha(q) mutants were found in soluble rather than particulate fractions, and they no longer localized to plasma membranes as demonstrated by immunofluorescence microscopy. The mutations also inhibited incorporation of radiolabeled palmitate into the proteins and abrogated their signaling ability. Additional alpha(q) mutants, which contain these mutations but are modified by both myristate and palmitate, retained their localization to plasma membranes and ability to undergo palmitoylation. These findings identify binding to betagamma as a critical membrane attachment signal for alpha(s) and alpha(q) and as a prerequisite for their palmitoylation, while myristoylation can restore membrane localization and palmitoylation of betagamma binding-deficient alpha(q) subunits.

Inhibition of protein palmitoylation, raft localization, and T cell signaling by 2-bromopalmitate and polyunsaturated fatty acids

The ability of the Src family kinases Fyn and Lck to participate in signaling through the T cell receptor is critically dependent on their dual fatty acylation with myristate and palmitate. Here we identify a palmitate analog, 2-bromopalmitate, that effectively blocks Fyn fatty acylation in general and palmitoylation in particular. Treatment of COS-1 cells with 2-bromopalmitate blocked myristoylation and palmitoylation of Fyn and inhibited membrane binding and localization of Fyn to detergent-resistant membranes (DRMs). In Jurkat T cells, 2-bromopalmitate blocked localization of the endogenous palmitoylated proteins Fyn, Lck, and LAT to DRMs. This resulted in impaired signaling through the T cell receptor as evidenced by reductions in tyrosine phosphorylation, calcium release, and activation of mitogen-activated protein kinase. We also examined the ability of long chain polyunsaturated fatty acids (PUFAs) to inhibit protein fatty acylation. PUFAs have been reported to inhibit T cell signaling by excluding Src family kinases from DRMs. Here we show that the PUFAs arachidonic acid and eicosapentaenoic acid inhibit Fyn palmitoylation and consequently block Fyn localization to DRMs. We propose that inhibition of protein palmitoylation represents a novel mechanism by which PUFAs exert their immunosuppressive effects.

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

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

Functional roles for fatty acylated amino-terminal domains in subcellular localization

Several membrane-associating signals, including covalently linked fatty acids, are found in various combinations at the N termini of signaling proteins. The function of these combinations was investigated by appending fatty acylated N-terminal sequences to green fluorescent protein (GFP). Myristoylated plus mono/dipalmitoylated GFP chimeras and a GFP chimera containing a myristoylated plus a polybasic domain were localized similarly to the plasma membrane and endosomal vesicles, but not to the nucleus. Myristoylated, nonpalmitoylated mutant chimeric GFPs were localized to intracellular membranes, including endosomes and the endoplasmic reticulum, and were absent from the plasma membrane, the Golgi, and the nucleus. Dually palmitoylated GFP was localized to the plasma membrane and the Golgi region, but it was not detected in endosomes. Nonacylated GFP chimeras, as well as GFP, showed cytosolic and nuclear distribution. Our results demonstrate that myristoylation is sufficient to exclude GFP from the nucleus and associate with intracellular membranes, but plasma membrane localization requires a second signal, namely palmitoylation or a polybasic domain. The similarity in localization conferred by the various myristoylated and palmitoylated/polybasic sequences suggests that biophysical properties of acylated sequences and biological membranes are key determinants in proper membrane selection. However, dual palmitoylation in the absence of myristoylation conferred significant differences in localization, suggesting that multiple palmitoylation sites and/or enzymes may exist.

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

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

Structure-function analysis of muscarinic receptors and their associated G proteins

Each member of the muscarinic receptor family (M1-M5) can interact only with a limited subset of the many structurally closely related heterotrimeric G proteins expressed within a cell. To understand how this selectivity is achieved at a molecular level, we have used the G(i/0)-coupled M2 and the Gq/11-coupled M3 muscarinic receptors as model systems. We developed a genetic strategy involving the coexpression of wild type or mutant muscarinic receptors with hybrid or mutant G protein alpha subunits to identify specific, functionally relevant receptor/G protein contact sites. This approach led to the identification of N- and C-terminal amino acids on alpha(q) and alpha(i) that are critical for maintaining proper receptor/G protein coupling. Moreover, several receptor sites were identified that are likely to be contacted by these functionally critical G alpha residues. To gain deeper insight into muscarinic receptor structure, we recently developed a cysteine disulfide cross-linking strategy, using the M3 muscarinic receptor as a model system. Among other structural modifications, this approach involves the removal of most native cysteine residues by site-directed mutagenesis, the insertion of three factor Xa cleavage sites into the third intracellular loop, and systematic 'reintroduction' of pairs of cysteine residues. Following treatment of receptor-containing membrane preparations with factor Xa and oxidizing agents, disulfide cross-linked products can be identified by immunoprecipitation and immunoblotting studies. This approach should greatly advance our knowledge of the molecular architecture of muscarinic and other G protein-coupled receptors.

The dually acylated NH2-terminal domain of gi1alpha is sufficient to target a green fluorescent protein reporter to caveolin-enriched plasma membrane domains. Palmitoylation of caveolin-1 is required for the recognition of dually acylated g-protein alpha subunits in vivo

Here we investigate the molecular mechanisms that govern the targeting of G-protein alpha subunits to the plasma membrane. For this purpose, we used Gi1alpha as a model dually acylated G-protein. We fused full-length Gi1alpha or its extreme NH2-terminal domain (residues 1-32 or 1-122) to green fluorescent protein (GFP) and analyzed the subcellular localization of these fusion proteins. We show that the first 32 amino acids of Gi1alpha are sufficient to target GFP to caveolin-enriched domains of the plasma membrane in vivo, as demonstrated by co-fractionation and co-immunoprecipitation with caveolin-1. Interestingly, when dual acylation of this 32-amino acid domain was blocked by specific point mutations (G2A or C3S), the resulting GFP fusion proteins were localized to the cytoplasm and excluded from caveolin-rich regions. The myristoylated but nonpalmitoylated (C3S) chimera only partially partitioned into caveolin-containing fractions. However, both nonacylated GFP fusions (G2A and C3S) no longer co-immunoprecipitated with caveolin-1. Taken together, these results indicate that lipid modification of the NH2-terminal of Gi1alpha is essential for targeting to its correct destination and interaction with caveolin-1. Also, a caveolin-1 mutant lacking all three palmitoylation sites (C133S, C143S, and C156S) was unable to co-immunoprecipitate these dually acylated GFP-G-protein fusions. Thus, dual acylation of the NH2-terminal domain of Gi1alpha and palmitoylation of caveolin-1 are both required to stabilize and perhaps regulate this reciprocal interaction at the plasma membrane in vivo. Our results provide the first demonstration of a functional role for caveolin-1 palmitoylation in its interaction with signaling molecules.

1,4-benzoquinone reductase from Phanerochaete chrysosporium: cDNA cloning and regulation of expression

A cDNA clone encoding a quinone reductase (QR) from the white rot basidiomycete Phanerochaete chrysosporium was isolated and sequenced. The cDNA consisted of 1,007 nucleotides and a poly(A) tail and encoded a deduced protein containing 271 amino acids. The experimentally determined eight-amino-acid N-terminal sequence of the purified QR protein from P. chrysosporium matched amino acids 72 to 79 of the predicted translation product of the cDNA. The Mr of the predicted translation product, beginning with Pro-72, was essentially identical to the experimentally determined Mr of one monomer of the QR dimer, and this finding suggested that QR is synthesized as a proenzyme. The results of in vitro transcription-translation experiments suggested that QR is synthesized as a proenzyme with a 71-amino-acid leader sequence. This leader sequence contains two potential KEX2 cleavage sites and numerous potential cleavage sites for dipeptidyl aminopeptidase. The QR activity in cultures of P. chrysosporium increased following the addition of 2-dimethoxybenzoquinone, vanillic acid, or several other aromatic compounds. An immunoblot analysis indicated that induction resulted in an increase in the amount of QR protein, and a Northern blot analysis indicated that this regulation occurs at the level of the qr mRNA.

Persistent membrane association of activated and depalmitoylated G protein alpha subunits

Heterotrimeric signal-transducing G proteins are organized at the inner surface of the plasma membrane, where they are positioned to interact with membrane-spanning receptors and appropriate effectors. G proteins are activated when they bind GTP and inactivated when they hydrolyze the nucleotide to GDP. However, the topological fate of activated G protein alpha subunits is disputed. One model declares that depalmitoylation of alpha, which accompanies activation by a receptor, promotes release of the protein into the cytoplasm. Our data suggest that activation of G protein alpha subunits causes them to concentrate in subdomains of the plasma membrane but not to be released from the membrane. Furthermore, alpha subunits remained bound to the membrane when they were activated with guanosine 5'-(3-O-thio)triphosphate and depalmitoylated with an acyl protein thioesterase. Limitation of alpha subunits to the plasma membrane obviously restricts their mobility and may contribute to the efficiency and specificity of signaling.

Immunohistochemical distribution of RGS7 protein and cellular selectivity in colocalizing with Galphaq proteins in the adult rat brain

Regulators of G protein signaling (RGS) proteins serve as potent GTPase-activating proteins for the heterotrimeric G proteins alphai/o and aq/11. This study describes the immunohistochemical distribution of RGS7 throughout the adult rat brain and its cellular colocalization with Galphaq/11, an important G protein-coupled receptor signal transducer for phospholipase Cbeta-mediated activity. In general, both RGS7 and Galphaq/11 displayed a heterogeneous and overlapping regional distribution. RGS7 immunoreactivity was observed in cortical layers I-VI, being most intense in the neuropil of layer I. In the hippocampal formation, RGS7 immunoreactivity was concentrated in the strata oriens, strata radiatum, mossy fibers, and polymorphic cells, with faint to nondetectable immunolabeling within the dentate gyrus granule cells and CA1-CA3 subfield pyramidal cells. Numerous diencephalic and brainstem nuclei also displayed dense RGS7 immunostaining. Dual immunofluorescence labeling studies with the two protein-specific antibodies indicated a cellular selectivity in the colocalization between RGS7 and Galphaq/11 within many discrete brain regions, such as the superficial cortical layer I, hilus area of the hippocampal formation, and cerebellar Golgi cells. To assess the ability of Galphaq/11-mediated signaling pathways to modulate dynamically RGS expression, primary cortical neuronal cultures were incubated with phorbol 12,13-dibutyrate, a selective protein kinase C activator. A time-dependent increase in levels of mRNA for RGS7, but not RGS4, was observed. Our results provide novel information on the region- and cell-specific pattern of distribution of RGS7 with the transmembrane signal transducer, Galphaq/11. We also describe a possible RGS7-selective neuronal feedback adaptation on Galphaq/11-mediated pathway function, which may play an important role in signaling specificity in the brain.