Identification of a Novel Fatty Acylated Protein That Partitions between the Plasma Membrane and Cytosol and Is Deacylated in Response to Serum and Growth Factor Stimulation

Protein depalmitoylases

Protein depalmitoylation describes the removal of thioester-linked long chain fatty acids from cysteine residues in proteins. For many S-palmitoylated proteins, this process is promoted by acyl protein thioesterase enzymes, which catalyze thioester hydrolysis to solubilize and displace substrate proteins from membranes. The closely related enzymes acyl protein thioesterase 1 (APT1; LYPLA1) and acyl protein thioesterase 2 (APT2; LYPLA2) were initially identified from biochemical assays as G protein depalmitoylases, yet later were shown to accept a number of S-palmitoylated protein and phospholipid substrates. Leveraging the development of isoform-selective APT inhibitors, several studies report distinct roles for APT enzymes in growth factor and hormonal signaling. Recent crystal structures of APT1 and APT2 reveal convergent acyl binding channels, suggesting additional factors beyond acyl chain recognition mediate substrate selection. In addition to APT enzymes, the ABHD17 family of hydrolases contributes to the depalmitoylation of Ras-family GTPases and synaptic proteins. Overall, enzymatic depalmitoylation ensures efficient membrane targeting by balancing the palmitoylation cycle, and may play additional roles in signaling, growth, and cell organization. In this review, we provide a perspective on the biochemical, structural, and cellular analysis of protein depalmitoylases, and outline opportunities for future studies of systems-wide analysis of protein depalmitoylation.

Concepts and advances in cancer therapeutic vulnerabilities in RAS membrane targeting

For decades oncogenic RAS proteins were considered undruggable due to a lack of accessible binding pockets on the protein surfaces. Seminal early research in RAS biology uncovered the basic paradigm of post-translational isoprenylation of RAS polypeptides, typically with covalent attachment of a farnesyl group, leading to isoprenyl-mediated RAS anchorage at the plasma membrane and signal initiation at those sites. However, the failure of farnesyltransferase inhibitors to translate to the clinic stymied anti-RAS therapy development. Over the past ten years, a more complete picture has emerged of RAS protein maturation, intracellular trafficking, and location, positioning and retention in subdomains at the plasma membrane, with a corresponding expansion in our understanding of how these properties of RAS contribute to signal outputs. Each of these aspects of RAS regulation presents a potential vulnerability in RAS function that may be exploited for therapeutic targeting, and inhibitors have been identified or developed that interfere with RAS for nearly all of them. This review will summarize current understanding of RAS membrane targeting with a focus on highlighting development and outcomes of inhibitors at each step.

Regulation of Ras signaling and function by plasma membrane microdomains

Together H-, N- and KRAS mutations are major contributors to ~30% of all human cancers. Thus, Ras inhibition remains an important anti-cancer strategy. The molecular mechanisms of isotypic Ras oncogenesis are still not completely understood. Monopharmacological therapeutics have not been successful in the clinic. These disappointing outcomes have led to attempts to target elements downstream of Ras, mainly targeting either the Phosphatidylinositol 3-Kinase (PI3K) or Mitogen-Activated Protein Kinase (MAPK) pathways. While several such approaches are moderately effective, recent efforts have focused on preclinical evaluation of combination therapies to improve efficacies. This review will detail current understanding of the contributions of plasma membrane microdomain targeting of Ras to mitogenic and tumorigenic signaling and tumor progression. Moreover, this review will outline novel approaches to target Ras in cancers, including targeting schemes for new drug development, as well as putative re-purposing of drugs in current use to take advantage of blunting Ras signaling by interfering with Ras plasma membrane microdomain targeting and retention.

Tetrahydrobiopterin in cardiovascular health and disease

Tetrahydrobiopterin (BH4) functions as a cofactor for several important enzyme systems, and considerable evidence implicates BH4 as a key regulator of endothelial nitric oxide synthase (eNOS) in the setting of cardiovascular health and disease. BH4 bioavailability is determined by a balance of enzymatic de novo synthesis and recycling, versus degradation in the setting of oxidative stress. Augmenting vascular BH4 levels by pharmacological supplementation has been shown in experimental studies to enhance NO bioavailability. However, it has become more apparent that the role of BH4 in other enzymatic pathways, including other NOS isoforms and the aromatic amino acid hydroxylases, may have a bearing on important aspects of vascular homeostasis, inflammation, and cardiac function. This article reviews the role of BH4 in cardiovascular development and homeostasis, as well as in pathophysiological processes such as endothelial and vascular dysfunction, atherosclerosis, inflammation, and cardiac hypertrophy. We discuss the therapeutic potential of BH4 in cardiovascular disease states and attempt to address how this modulator of intracellular NO-redox balance may ultimately provide a powerful new treatment for many cardiovascular diseases.

Analysis of protein acylation

Proteins can be acylated with a variety of fatty acids attached by different covalent bonds, influencing, among other things, their function and intracellular localization. This unit describes methods to analyze protein acylation, both levels of acylation and also the identification of the fatty acid and the type of bond present in the protein of interest. Protocols are provided for metabolic labeling of proteins with tritiated fatty acids, for exploitation of the differential sensitivity to cleavage of different types of bonds, in order to distinguish between them, and for thin-layer chromatography to separate and identify the fatty acids associated with proteins.

Metabolic labeling with fatty acids

Covalent attachment of radiolabeled fatty acids (e.g., [(3)H]myristate or palmitate) is an alternative method for labeling proteins. This unit contains methods for biosynthetic labeling with fatty acids, analysis of the fatty acid linkage with protein, analysis of total protein-bound fatty acid level in cell extracts, and analysis of the identity of the bound fatty acid.

Safety assessment of myristic acid as a food ingredient

Myristic acid is used in the food industry as a flavor ingredient. It is found widely distributed in fats throughout the plant and animal kingdom, including common human foodstuffs, such as nutmeg. Myristic acid (a 14-carbon, straight-chain saturated fatty acid) has been shown to have a low order of acute oral toxicity in rodents. It may be irritating in pure form to skin and eyes under exaggerated exposure conditions, but is not known or predicted to induce sensitization responses. Myristic acid did not induce a mutagenic response in either bacterial or mammalian systems in vitro. Relevant subchronic toxicity data are available on closely related fatty acid analogs. In particular, a NOEL of >6000mg/kg was reported for lauric acid (a 12-carbon, straight-chain saturated fatty acid) following dietary exposure to male rats for 18 weeks and a NOEL of >5000mg/kg was reported for palmitic acid (a 16-carbon, straight-chain saturated fatty acid) following dietary exposure to rats for 150 days. The data and information that are available indicate that at the current level of intake, food flavoring use of myristic acid does not pose a health risk to humans.

Lipid–protein interactions in GPCR-associated signaling

Signal transduction via G-protein-coupled receptors (GPCRs) is a fundamental pathway through which the functions of an individual cell can be integrated within the demands of a multicellular organism. Since this family of receptors first discovered, the proteins that constitute this signaling cascade and their interactions with one another have been studied intensely. In parallel, the pivotal role of lipids in the correct and efficient propagation of extracellular signals has attracted ever increasing attention. This is not surprising given that most of the signal transduction machinery is membrane-associated and therefore lipid-related. Hence, lipid-protein interactions exert a considerable influence on the activity of these proteins. This review focuses on the post-translational lipid modifications of GPCRs and G proteins (palmitoylation, myristoylation, and isoprenylation) and their significance for membrane binding, trafficking and signaling. Moreover, we address how the particular biophysical properties of different membrane structures may regulate the localization of these proteins and the potential functional consequences of this phenomenon in signal transduction. Finally, the interactions that occur between membrane lipids and GPCR effector enzymes such as PLC and PKC are also considered.

Vinblastine, a chemotherapeutic drug, inhibits palmitoylation of tubulin in human leukemic lymphocytes

BACKGROUND

We have previously shown that tubulin, the major protein of microtubules, is posttranslationally modified by palmitoylation. In addition, we demonstrated that palmitoylation of tubulin is inhibited in vitro by stoichiometric levels of the chemotherapeutic drug, vinblastine. Here, we sought to determine whether a clinically relevant dose of vinblastine inhibits palmitoylation of tubulin in vivo.

METHODS

Human CEM leukemic lymphocytes were incubated with [3H]palmitate in the presence and absence of a low, clinically relevant dose of vinblastine. [3H]palmitoylated tubulin was identified by two-dimensional PAGE and autoradiography.

RESULTS

We found, first, that tubulin was palmitoylated in CEM cells. Second, the clinically relevant dose of vinblastine inhibited palmitoylation of tubulin in vivo in CEM cells. In addition, microtubules were disassembled and cells became apoptotic.

CONCLUSION

This study identifies a previously unknown mechanism of action of vinblastine, the depalmitoylation of tubulin, and suggests that depalmitoylation of tubulin may be a target for new chemotherapeutic drugs.

Identification of Golgi-localized acyl transferases that palmitoylate and regulate endothelial nitric oxide synthase

Lipid modifications mediate the subcellular localization and biological activity of many proteins, including endothelial nitric oxide synthase (eNOS). This enzyme resides on the cytoplasmic aspect of the Golgi apparatus and in caveolae and is dually acylated by both N-myristoylation and S-palmitoylation. Palmitoylation-deficient mutants of eNOS release less nitric oxide (NO). We identify enzymes that palmitoylate eNOS in vivo. Transfection of human embryonic kidney 293 cells with the complementary DNA (cDNA) for eNOS and 23 cDNA clones encoding the Asp-His-His-Cys motif (DHHC) palmitoyl transferase family members showed that five clones (2, 3, 7, 8, and 21) enhanced incorporation of [³H]-palmitate into eNOS. Human endothelial cells express all five of these enzymes, which colocalize with eNOS in the Golgi and plasma membrane and interact with eNOS. Importantly, inhibition of DHHC-21 palmitoyl transferase, but not DHHC-3, in human endothelial cells reduces eNOS palmitoylation, eNOS targeting, and stimulated NO production. Collectively, our data describe five new Golgi-targeted DHHC enzymes in human endothelial cells and suggest a regulatory role of DHHC-21 in governing eNOS localization and function.

Multiple forms of 22kDa caveolin-1 alpha present in bovine lens cells could reflect variable palmitoylation

Two-dimensional immunoblots of immunoprecipitated caveolin-1 from cultured bovine lens epithelial cells revealed four to five-22 kDa forms of caveolin-1 alpha with isoelectric points of between pH values 5.5 and 6.6. Fibre cell membrane recovered from fresh bovine lenses displayed an even greater number of multiforms, some with isoelectric point pH values as low as about 4. Caveolin-1 can be both phosphorylated and palmitoylated. None of the caveolin-1 alpha multiforms were labelled following culture of the lens epithelial cells with 32P-orthophosphate nor were they recognized by either caveolin-specific phosphotyrosine antibody or protein anti-phosphoserine antibody and treatment of lens fibre cell membrane with phosphatase did not alter the two-dimensional profile of immunoreactive caveolins. However, short-term incubation of BLEC with 3H-palmitate labelled some of the immunoprecipitated caveolin-1 multiforms. We suggest that the observed spectrum of caveolin multiforms could reflect variable palmitoylation of its three cysteine residues and result in populations of caveolin-1 alpha molecules with separate physical and functional properties.

Role of HER receptors family in development and differentiation

Members of the epidermal growth factor receptor family of receptor tyrosine kinases play a critical role in both development and oncogenesis. The latter is suggested by the frequent overexpression of HER-2, EGFR, and HER-3 in some human carcinomas, primarily breast and squamous cancer. The biological activities of the EGFR family are exerted through various ligand-receptor and receptor-receptor interactions. One receptor that plays a central role in this signaling network is HER-2/Neu, which is considered the preferred heterodimerization partner for other members of the EGFR family. The role of these receptors and their ligands in development is discussed, with particular emphasis on their ability to mediate a variety of pathways and cellular responses, including proliferation, differentiation, and apoptosis.

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.

Analysis of protein acylation

Protein acylation is the covalent attachment of fatty acids to a protein; the most commonly added fatty acids are myristate (14:0) and palmitate (16:0). In this unit, protocols describe the use of radiolabeled fatty acids to label eukaryotic cells in vitro. The radiolabeled material produced can then be analyzed by the various methods described here: determination of the type of fatty acid linkage, checking for interconversion by determining the nature of the protein-bound label, and identification of the protein-bound fatty acid.

Intense myristoylation of a single protein in the ocular lens

A single protein of the ocular lens was intensely myristoylated following short term incubation of cultured bovine lens epithelial cells and intact rat lenses with 3H-myristic acid. It was acidic (pI <5), about 19 kDa and present exclusively in the cytosol of both cultured epithelial cells and the epithelium of the young rat lens. Fiber cell proteins were not labeled. The myristoylated protein was not seen in the epithelium of the adult rat. Essentially no protein mass was evident in the 19-20 kDa range when samples of the labeled-soluble protein were fractionated by either HPLC coupled with SDS-PAGE or 2D-electrophoresis. These findings suggest that the myristoylated-soluble protein of 19 kDa in lens (p19L) is a rapidly-turning over minor protein likely associated with lens growth. The absence of any apparent membrane association for a myristoylated protein appears unusual. The trace nature of p19L has frustrated attempts at its identification by MALDI-MS.

Formation of the 67-kDa laminin receptor by acylation of the precursor

Even though the involvement of the 67-kDa laminin receptor (67LR) in tumor invasiveness has been clearly demonstrated, its molecular structure remains an open problem, since only a full-length gene encoding a 37-kDa precursor protein (37LRP) has been isolated so far. A pool of recently obtained monoclonal antibodies directed against the recombinant 37LRP molecule was used to investigate the processing that leads to the formation of the 67-kDa molecule. In soluble extracts of A431 human carcinoma cells, these reagents recognize the precursor molecule as well as the mature 67LR and a 120-kDa molecule. The recovery of these proteins was found to be strikingly dependent upon the cell solubilization conditions: the 67LR is soluble in NP-40-lysis buffer whereas the 37LRP is NP-40-insoluble. Inhibition of 67LR formation by cerulenin indicates that acylation is involved in the processing of the receptor. It is likely a palmitoylation process, as indicated by sensitivity of NP-40-soluble extracts to hydroxylamine treatment. Immunoblotting assays performed with a polyclonal serum directed against galectin3 showed that both the 67- and the 120-kDa proteins carry galectin3 epitopes whereas the 37LRP does not. These data suggest that the 67LR is a heterodimer stabilized by strong intramolecular hydrophobic interactions, carried by fatty acids bound to the 37LRP and to a galectin3 cross-reacting molecule.

Palmitoylation of proteolipid protein from rat brain myelin using endogenously generated 18O-fatty acids

Proteolipid protein (PLP), the major protein of central nervous system myelin, contains covalently bound fatty acids, predominantly palmitic acid. This study adapts a stable isotope technique (Kuwae, T., Schmid, P. C., Johnson, S. B., and Schmid, H. O. (1990) J. Biol. Chem. 265, 5002-5007) to quantitatively determine the minimal proportion of PLP molecules which undergo palmitoylation. In these experiments, brain white matter slices from 20-day-old rats were incubated for up to 6 h in a physiological buffer containing 50% H218O. The uptake of 18O into the carbonyl groups of fatty acids derived from PLP, phospholipids, and the free fatty acid pool was measured by gas-liquid chromatography/mass spectrometry of the respective methyl esters. Palmitic acid derived from PLP acquired increasing amounts of 18O, ending with 2.9% 18O enrichment after 6 h of incubation. 18O incorporation into myelin free palmitic acid also increased over the course of the incubation (67.2% 18O enrichment). After correcting for the specific activity of the 18O-enriched free palmitic acid pool, 7.6% of the PLP molecules were found to acquire palmitic acid in 6 h. This value is not only too large to be the result of the palmitoylation of newly synthesized PLP molecules, it was also unchanged upon the inhibition of protein synthesis with cycloheximide. 18O enrichment in less actively myelinating 60-day-old rats was significantly reduced. In conclusion, our experiments suggest that a substantial proportion of PLP molecules acquire palmitic acid via an acylation/deacylation cycle and that this profile changes during development.

The effects of palmitoylation on membrane association of Semliki forest virus RNA capping enzyme

The nonstructural protein Nsp1 of Semliki Forest virus has guanine-7-methyltransferase and guanylyltransferase-like activities, required in the capping of viral mRNAs. It is palmitoylated and tightly associated with the cytoplasmic surface of the plasma membrane, endosomes, and lysosomes. To localize the acylation site(s) and the putative membrane-targeting domain, a number of deletions were made in the nsp1 gene. Most deletions resulted in the expression of nonpalmitoylated, enzymatically inactive, cytoplasmic protein. Palmitate could be released from Nsp1 with neutral hydroxylamine, indicating a thioester linkage to a cysteine residue. Therefore we mutated the conserved cysteine residues of Nsp1 to alanine. Triple mutation of Cys418, Cys419, and Cys420 resulted in nonpalmitoylated Nsp1, which was enzymatically active and still associated with membranes. However, it could be released from the membranes with 1 M NaCl, whereas 50 mM sodium carbonate (pH 12) was required to release wild type Nsp1, suggesting a conversion from an integral to a peripheral membrane protein. Indirect confocal immunofluorescence microscopy showed that the nonpalmitoylated Nsp1 colocalized with the plasma membrane marker, concanavalin A. However, it was not detected in filopodia, which were heavily stained in cells expressing wild type Nsp1. These results indicate that the acylation of Nsp1 was not needed for its targeting to the plasma membrane, but it was necessary for the migration to the filopodial extensions of the plasma membrane.

Myristoylation-dependent and electrostatic interactions exert independent effects on the membrane association of the myristoylated alanine-rich protein kinase C substrate protein in intact cells

The myristoylated alanine-rich protein kinase C substrate (MARCKS) is a widely expressed, prominent substrate for protein kinase C. MARCKS is largely associated with membranes in cells, and hydrophobic interactions involving the amino-terminal myristoyl moiety are thought to play a role in anchoring MARCKS to cellular membranes. In addition, experiments in cell-free systems have suggested that electrostatic interactions between the positively charged phosphorylation site/calmodulin binding domain (PSD) of MARCKS and negatively charged membrane lipids are also involved in this association. Although it has been inferred from phosphorylation experiments, the electrostatic nature of the interaction between the PSD and membranes has not been demonstrated directly in intact cells. We expressed human MARCKS mutated in the myristoylation site and the PSD in REF52 cells; the cells were then fractionated by ultracentrifugation. Both nonmyristoylatable MARCKS and MARCKS in which the four serines in the PSD were mutated to aspartic acids, mimicking phosphorylation, exhibited decreased membrane affinity when compared to the fully myristoylated, wild-type, tetra-Ser protein or a myristoylated, tetra-Asn mutant. A double mutant, nonmyristoylatable protein in which the four serines in the PSD were mutated to aspartic acids exhibited negligible membrane association. Similar results were obtained in 293 cells that stably expressed chicken MARCKS mutated in the same domains. The double mutant, nonmyristoylatable tetra-Asp chicken protein exhibited little membrane association as determined by both subcellular fractionation and immunoelectron microscopy. These results indicate that myristoylation and electrostatic interactions involving the PSD exert independent, essentially additive effects on the membrane association of MARCKS in intact cells.

Regulation of cellular signalling by fatty acid acylation and prenylation of signal transduction proteins

Covalent modification by fatty acylation and prenylation occurs on a wide variety of cellular signalling proteins. The enzymes that catalyze attachment of these lipophilic moieties to proteins have recently been identified and characterized. Each lipophilic group confers unique properties to the modified protein, resulting in alterations in protein/protein interactions, membrane binding and targeting, and intracellular signalling. The biochemistry and cell biology of protein myristoylation, farnesylation and geranylgeranylation is reviewed here, with emphasis on the Src family of tyrosine kinases, Ras proteins and G protein coupled signalling systems.

Acylation of pulmonary surfactant protein-C is required for its optimal surface active interactions with phospholipids

This study investigates the importance of thioester-linked acyl groups in lung surfactant protein C (SP-C) in facilitating interactions with phospholipids that yield functionally important surface active behaviors. Native SP-C, palmitoylated at cysteine residues at positions 5 and 6, was isolated from bovine lung surfactant by liquid chromatography. Deacylated SP-C (dSP-C), unchanged in composition and sequence from SP-C but having a decreased alpha-helical content in films with dipalmitoyl phosphatidylcholine (DPPC) of 52 versus 70%, was obtained by treatment with 0.1 M sodium carbonate buffer at pH 10. Surface activity was studied for SP-C and dSP-C combined with column-purified phospholipids (PPL) from calf lung surfactant or with synthetic phospholipids (DPPC or a synthetic phospholipid mixture (SPL) containing 50:35:15, DPPC:egg phosphatidylcholine:egg phosphatidylglycerol). Interfacial measurements included surface pressure time adsorption isotherms for dispersed surfactants with diffusion minimized, dynamic surface pressure area isotherms and respreading for films in the Wilhelmy balance, and overall surface tension lowering at physiologic cycling rate in oscillating bubble experiments. Dispersions of PPL:SP-C and SPL:SP-C rapidly adsorbed to high equilibrium surface pressures of 47-48 mN/m, significantly better than corresponding dispersions containing dSP-C. The adsorption of PPL:dSP-C was essentially unchanged from that of PPL alone, and the adsorption of SPL:dSP-C was improved only slightly over SPL alone. In Wilhelmy balance studies, dynamic respreading was significantly improved over phospholipids alone in films of SP-C plus PPL, SPL, or DPPC. Respreading was improved less markedly by dSP-C in corresponding films with SPL or DPPC and not at all in films with PPL. Maximum surface pressures were also higher in cycled films of SP-C versus dSP-C combined with PPL or SPL. In bubble experiments (37 degrees C, 20 cycles/min), dispersions of PPL:SP-C and SPL:SP-C reached low minimum surface tensions of <1 and 5 mN/m, respectively, whereas PPL:dSP-C and SPL:dSP-C only reached minima of approximately 20 mN/m as did PPL and SPL alone. Acylation in SP-C is crucial for its interactions with phospholipids over the full spectrum of adsorption and dynamic surface behaviors important for lung surfactant.

Association of Drosophila cysteine string proteins with membranes

Cysteine string proteins are putative synaptic vesicle proteins that lack a transmembrane domain. Our analysis shows that Drosophila cysteine string proteins are extensively modified by hydroxylamine-sensitive fatty acylation. This modification could be responsible for association of csp's with membranes. Extensive deacylation of Dcsp's by a 20 h incubation in 1 M hydroxylamine, pH 7.0, or methanolic KOH produces a protein of 6-7 kDa lower mass than untreated Dcsp's. Surprisingly, the hydroxylamine treatment does not cause release of Dcsp's from membranes. On the other hand, alkaline stripping of membranes isolated from Drosophila brain by 0.1 M sodium carbonate, pH 11.5, causes a significant release of Dcsp's from membranes into the cytosol. These results indicate that fatty acylation may not form the main anchor of Dcsp's in membranes. Taking advantage of the endocytotic block in the Drosophila mutant shibire ts1, we analyzed the acylation states of Dcsp's in two stages during synaptic vesicle recycling and found no evidence for an acylation/deacylation cycle of Dcsp's in the brain nerve terminals.

Membrane association of the myristoylated alanine-rich C kinase substrate (MARCKS) protein. Mutational analysis provides evidence for complex interactions

The myristoylated alanine-rich C kinase substrate (MARCKS) protein, a prominent cellular substrate for protein kinase C, is associated with membranes in various cell types. MARCKS is myristoylated at its amino terminus; this modification is thought to play the major role in anchoring MARCKS to cellular membranes. Recent studies have suggested that the protein's basic phosphorylation site/calmodulin binding domain may also be involved in the membrane association of MARCKS through electrostatic interactions. The present studies used mutations in the primary structure of the protein to investigate the nature of the association between MARCKS and cell membranes. In chick embryo fibroblasts, activation of protein kinase C led to a decrease in MARCKS membrane association as determined by cell fractionation techniques. Cell-free assays revealed that nonmyristoylated MARCKS exhibited almost no affinity for fibroblast membranes, despite readily demonstrable binding of the wild-type protein. Similar experiments in which the four serines in the phosphorylation site domain were mutated to aspartic acids, mimicking phosphorylation, decreased, but did not eliminate, membrane binding when compared to either the wild-type protein or a comparable tetra-asparagine mutant. Addition of calmodulin in the presence of Ca2+ also inhibited binding of the wild-type protein to membranes, presumably by neutralizing the phosphorylation site domain, or by physically interfering with its membrane association. Surprisingly, expression of a nonmyristoylatable mutant form of MARCKS in intact cells led to only a 46% decrease in its plasma membrane association, as determined by cell fractionation and immunoelectron microscopy. These results are consistent with a complex model of the interaction of MARCKS with cellular membranes, in which the myristoyl moiety, the positively charged phosphorylation site domain, and possibly other domains make independent contributions to membrane binding in intact cells.

Determination of the structural requirements for palmitoylation of p63

Palmitoylation of p63, a type II membrane protein localized in the endoplasmic reticulum, is induced in a reversible manner by the drug brefeldin A. To study the requirements for palmitoylation, mutant forms of p63 were expressed in COS cells and analyzed by metabolic labeling with [3H]palmitate, immunoprecipitation, and SDS-polyacrylamide gel electrophoresis. By investigating deletion and point mutations, Cys100 in the 106-amino acid cytoplasmic tail of p63 has been identified as the site of acylation. Site-directed mutagenesis of residues 99-105 together with cytoplasmic tail truncation mutants showed that the amino acids surrounding Cys100 are not critical for palmitoylation of this residue. Analysis of a chimeric construct between p63 and the plasma membrane protein dipeptidylpeptidase IV further revealed that p63 palmitoylation is not dependent on its transmembrane domain. In contrast, the six-amino acid distance between the end of the predicted transmembrane domain and the palmitoylation site was found to be essential for proper acylation of p63.

On the surface activity of surfactant-associated protein C (SP-C): effects of palmitoylation and pH

The effect of palmitoylation on the surface activity of bovine surfactant-associated protein C (SP-C) in lipid mixtures was investigated. Native and chemically depalmitoylated SP-C were reconstituted with dipalmitoylphosphatidylcholine/egg phosphatidylglycerol (7:3) using two different procedures, one of which included lyophilization and sonication. When tested using a pulsating bubble surfactometer, no significant changes in the surface activity of these mixtures were observed upon the hydrolysis of the palmitates. Since the purification and deacylation procedures of SP-C included the use of acid and alkali, the effect of pH was examined. The surface activity of the mixtures was found to vary with pH. At low pH values (approx. 2.5), surface tensions between 3 and 10 mN/m at minimum bubble radius were reached within 5 pulsations, while at neutral and slightly alkaline pH, surface tension reduction was much slower and near zero (< 5 mN/m) values at minimum bubble radius were not reached by the fiftieth pulsation. Protein-free lipid samples that were exposed to acid exhibited enhanced surface activity over similar non-treated samples. It is therefore concluded that low surface tension measurements recorded for acidic samples are secondary to a pH effect and do not reflect the surface activity at physiological conditions.

Palmitoylation of the glucose transporter in blood-brain barrier capillaries

Palmitoylation of GLUT1 was investigated in brain capillaries. The glucose transporter was shown to be palmitoylated using [3H]palmitate labeling and immunoprecipitation. The labeling was sensitive to methanolic KOH or hydroxylamine hydrolysis, indicating the presence of an ester or thioester bond. The released fatty acid was analyzed by reverse-phase HPLC and was identified as [3H]palmitate. Specificity of the immunoprecipitation was assessed by competitive inhibition of anti-GLUT1 binding with a synthetic C-terminal peptide against which the antibody was raised. In vivo studies were performed using capillaries isolated from control rats, streptozotocin-induced diabetic rats and diet-induced hyperglycemic rats. Glycemia was increased 2- and 5-fold in the hyperglycemic and diabetic groups, respectively. GLUT1 expression was evaluated in the three groups by Western blot analysis. A 36% decrease in GLUT1 expression was observed in the diabetic group, while there was no significant variation in GLUT1 expression in the hyperglycemic group. Palmitoylation of GLUT1 was increased in both diet-induced hyperglycemic and diabetic groups. These results suggest that palmitoylation may be involved in the regulation of glucose transport activity in hyperglycemia.

Depalmitoylation of rhodopsin with hydroxylamine

Intrinsic membrane proteins that to date have been investigated with respect to the function of palmitoylation are the beta-adrenergic receptor, rhodopsin, the alpha 2A-adrenergic receptor, and the influenza virus spike glycoprotein. As described above, the studies have led to differing conclusions with respect to the influence of palmitoylation on physiological activity. The basis of the differences remains unclear, but it may relate at least in part to the membrane environment of the protein during these studies, that is, the presence of a native membrane, the membrane composition of the expression cell line (in the case of mutant proteins), or the absence of membrane (in the case of detergent-purified proteins). For example, in the case of rhodopsin, the composition of the ROS disk membrane differs from that of the rod plasma membrane, and presumably also from the plasma membranes of cell lines in which mutant rhodopsins are expressed. Variation in membrane composition is known to have marked effects on the ability of rhodopsin to mediate the photic activation of PDE. Thus, although Karnik et al. clearly demonstrated the absence of an absolute requirement for palmitate in activating transducin, the influence of detergent on tertiary protein structure may have masked the full effect of the elimination of palmitate on the transducin-activating property of rhodopsin. Alternatively, the differing results obtained in the studies of rhodopsin could be a consequence of differences in amino acid sequences of the proteins studied. The precise functional role of the palmitate groups of rhodopsin remains an important question for further research. It was suggested by Ovchinnikov et al. that the hydrolysis of covalently bound palmitate might occur during the process of rhodopsin bleaching, but more recent data argue against this hypothesis. Experiments using synthetic peptides (representing cytoplasmic loop regions of rhodopsin) to identify the sites of interaction of R* and transducin provide support for an alternative possibility, namely, that palmitoylation and the resulting cytoplasmic loop play a role in the coupling of rhodopsin with transducin. The finding that the binding of transducin to R* occurs independently of the presence of palmitate argues against an essential requirement of palmitoylation on the binding step itself. However, available data indicate an enhancement, by depalmitoylation, of light-dependent GTPase activity in ROS preparations, although not in assays of unpalmitoylated, purified mutant rhodopsins (see above).(ABSTRACT TRUNCATED AT 400 WORDS)

Overview: protein palmitoylation in the nervous system: current views and unsolved problems

Palmitoylation refers to a dynamic post-translational modification of proteins involving the covalent attachment of long-chain fatty acids to the side chains of cysteine, threonine or serine residues. In recent years, palmitoylation has been identified as a widespread modification of both viral and cellular proteins. Because of its dynamic nature, protein palmitoylation, like phosphorylation, appears to have a crucial role in the functioning of the nervous system. Several important questions regarding the post-translational acylation of cysteine residues in proteins are briefly discussed: (a) What are the molecular mechanisms involved in dynamic acylation? (b) What are the determinants of the fatty acid specificity and the structural requirements of the acceptor proteins? (c) What are the physiological signals regulating this type of protein modification, and (d) What is the biological role(s) of this reaction with respect to the functioning of specific nervous system proteins? We also present the current experimental obstacles that have to be overcome to fully understand the biology of this dynamic modification.

Keratinocyte transglutaminase membrane anchorage: analysis of site-directed mutants

Keratinocyte transglutaminase is anchored on the cytosolic side of the plasma membrane by fatty acid thioesterification near the amino terminus, a process which is seen to occur within 30 min of synthesis. The importance of a cluster of five cysteines (residues 47, 48, 50, 51, and 53) where acylation was presumed to occur is now demonstrated by site-directed mutagenesis. Transglutaminase mutants in which the cluster is deleted or the cysteines are all converted to alanine or serine are cytosolic. Partial replacement of the cluster, leaving two contiguous cysteines, is sufficient to confer membrane anchorage, while a single cysteine is only partially effective. As demonstrated with a soluble transglutaminase mutant, membrane anchorage confers susceptibility of the amino-terminal region to phorbol ester-stimulated phosphorylation. Attachment of 105 residues from the transglutaminase amino terminus to involucrin, a highly soluble protein, results in membrane anchorage of the hybrid protein. Attachment of the cysteine cluster alone does not result in membrane attachment of involucrin, but a 32-residue segment containing this cluster is sufficient. Stable transfectants of the human transglutaminase in mouse 3T3 cells are membrane-bound, indicating the fatty acid transacylation is not keratinocyte-specific.

A reversibly palmitoylated resident protein (p63) of an ER-Golgi intermediate compartment is related to a circulatory shock resuscitation protein

The recently identified 63 kDa membrane protein, p63, is a resident protein of a membrane network interposed in between rough ER and Golgi apparatus. To characterize p63 at the molecular level a 2.91 kb cDNA encoding p63 has been isolated from a human placenta lambda gt10 cDNA library. Sequence analysis of tryptic peptides prepared from isolated p63 confirmed the identify of the cloned gene. The translated amino acid sequence consists of 601 amino acids (65.8 kDa) with a single putative membrane-spanning region and a N-terminal cytoplasmic domain of 106 amino acids. The human p63 cDNA exhibits a high level of sequence identify to the pig hepatic cDNA 3AL (accession number M27092) whose expression is enhanced after resuscitation from circulatory shock. An additional remarkable feature of p63 is that it becomes reversibly palmitoylated when intracellular protein transport is blocked by the drug brefeldin A. Overexpression of p63 in COS cells led to the development of a striking tubular membrane network in the cytoplasm. This suggests that the protein may be determinant for the structure of the p63 compartment.

Antagonism of [3H]fatty acid incorporation into vimentin by sodium pyruvate: pitfalls of protein acylation

In the course of studying possible fatty acid acylation of vimentin by cultured bovine lens epithelial cells, several potential pitfalls of protein-fatty acid acylation were recognized. Even exhaustive delipidation of vimentin with organic solvents failed to remove all noncovalently associated [3H]palmitate and [3H]myristate. Hydroxylamine treatment of vimentin, separated by sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE), failed to remove either palmitate or myristate derived radiolabel. Hydroxylamine treatment did remove palmitate label from a group of lower molecular weight proteins. The myristate radiolabel associated with vimentin recovered after SDS-PAGE and subjected to acid hydrolysis was shown due to incorporated [3H]amino acids, mainly glutamic acid, generated from the fatty acid. Adding excess sodium pyruvate to labeling media has been used by others to reduce the metabolic conversion of fatty acids to amino acids; however, no direct evidence in support of this antagonism was presented. We observed that inclusion of sodium pyruvate at between 5 and 20 mM in the labeling medium produced a dramatic decrease in incorporation of myristic acid radiolabel into vimentin. However, inclusion of even 20 mM pyruvate did not completely antagonize the metabolic conversion of fatty acid label to amino acids. Furthermore, the sodium pyruvate antagonism could be totally obscured if the exposure of X-ray film by fluorography was even slightly prolonged. The results illustrate the danger in assuming that solvent extraction totally delipidates proteins and that adding sodium pyruvate to labeling media prevents the transfer of fatty acid label to amino acids.(ABSTRACT TRUNCATED AT 250 WORDS)

Different effects of A23187 and PMA on palmitoylated platelet proteins

The effect of agonists on palmitoylated proteins was examined in platelets prelabeled with [3H]palmitic acid. Non-reduced gels revealed major labeled proteins with masses from 30-38 kDa. One of these proteins was modified by A23187, which led to a loss of radioactivity, and PMA, which altered its electrophoretic mobility. A possible link between the A23187-induced loss of label associated with the protein and the activation of calpain was suggested by the following experiments. (1) There was a good correlation between the loss of label and the proteolysis of proteins in A23187-activated platelets. (2) The permeant calpain inhibitor, E64d, blocked the loss of label as well as the proteolysis of proteins. (3) The loss of label also occurred in a Triton lysate, where calpain was known to be activated. The effect of PMA on the palmitoylated protein was observed only in prelabeled platelets. The protein kinase inhibitor, staurosporine, abolished the PMA-induced platelet aggregation as well as the mobility shift of the labeled protein.

Myristoylation of heparan sulfate proteoglycan and proteins occurs post-translationally in human colon carcinoma cells

We have recently shown that the heparan sulfate proteoglycan of human colon carcinoma cells is acylated with both myristate and palmitate, two long-chain saturated fatty acids. In this study we show that cycloheximide did not significantly inhibit the incorporation of myristic acid into either proteoglycan or total protein pool. This lack of inhibition occurred under a condition in which protein synthesis was inhibited greater than 90%. Cycloheximide, on the other hand, did not affect the incorporation of [3H]myristic acid into fatty acid nor the intracellular interconversion of myristate to palmitate. Characterization of fatty acyl moiety in the proteoglycan and protein by reverse-phase HPLC revealed that approximately 60% of the covalently bound fatty acids was myristate and the remaining 40% was palmitate. These results indicate that in human colon carcinoma cells myristoylation of heparan sulfate proteoglycan and proteins occurs post-translationally, presumably in the Golgi complex.

The fats of life: the importance and function of protein acylation

Interest in the study of the direct attachment of fatty acids to cellular proteins, termed protein acylation, has been greatly stimulated by recent experimentation that has increased our understanding of the function of the attached lipid. These developments are described, and the possibility that inhibitors of protein acylation might provide new drugs is discussed.

Fatty acylated proteins as components of intracellular signaling pathways

From the studies presented above, it is obvious that fatty acylation is a common modification among proteins involved in cellular regulatory pathways, and in certain cases mutational analyses have demonstrated the importance of covalent fatty acids in the functioning of these proteins. Indeed, certain properties provided by fatty acylation make it an attractive modification for regulatory proteins that might interact with many different substrates, particularly those found at or near the plasma membrane/cytosol interface. In the case of intracellular fatty acylated proteins, the fatty acyl moiety allows tight binding to the plasma membrane without the need for cotranslational insertion through the bilayer. For example, consider the tight, salt-resistant interaction of myristoylated SRC with the membrane, whereas its nonmyristoylated counterpart is completely soluble. Likewise for the RAS proteins, which associate weakly with the membrane in the absence of fatty acylation, while palmitoylation increases their affinity for the plasma membrane and their biological activity. Fatty acylation also permits reversible membrane association in some cases, particularly for several myristoylated proteins, thus conferring plasticity on their interactions with various signaling pathway components. Finally, although this has not been demonstrated, it is conceivable that covalent fatty acid may allow for rapid mobility of proteins within the membrane. Several questions remain to be answered concerning requirements for fatty acylation by regulatory proteins. The identity of the putative SRC "receptor" will provide important clues as to the pathways in which normal SRC functions, as well as into the process of transformation by oncogenic tyrosine kinases. The possibility that other fatty acylated proteins associate with the plasma membrane in an analogous manner also needs to be investigated. An intriguing observation that can be made from the information presented here is that at least three different families of proteins involved in growth factor signaling pathways encode both acylated and nonacylated members, suggesting that selective fatty acylation may provide a means of determining the specificity of their interactions with other regulatory molecules. Further studies of fatty acylated proteins should yield important information concerning the regulation of intracellular signaling pathways utilized during growth and differentiation.