Inhibition of the receptor-mediated endocytosis of diferric transferrin is associated with the covalent modification of the transferrin receptor with palmitic acid

O-GlcNAcylation regulates the stability of transferrin receptor (TFRC) to control the ferroptosis in hepatocellular carcinoma cells

Ferroptosis is an iron-dependent programmed cell death (PCD) enforced by lipid peroxidation accumulation. Transferrin receptor (TFRC), one of the signature proteins of ferroptosis, is abundantly expressed in hepatocellular carcinoma (HCC). However, post-translational modification (PTM) of TFRC and the underlying mechanisms for ferroptosis regulation remain less understood. In this study, we found that TFRC undergoes O-GlcNAcylation, influencing Erastin-induced ferroptosis sensitivity in hepatocytes. Further mechanistic studies found that Erastin can trigger de-O-GlcNAcylation of TFRC at serine 687 (Ser687), which diminishes the binding of ubiquitin E3 ligase membrane-associated RING-CH8 (MARCH8) and decreases polyubiquitination on lysine 665 (Lys665), thereby enhancing TFRC stability that favors labile iron accumulation. Therefore, our findings report O-GlcNAcylation on an important regulatory protein of ferroptosis and reveal an intriguing mechanism by which HCC ferroptosis is controlled by an iron metabolism pathway.

Regulation of cardiomyocyte intracellular trafficking and signal transduction by protein palmitoylation

Despite the well-established functions of protein palmitoylation in fundamental cellular processes, the roles of this reversible post-translational lipid modification in cardiomyocyte biology remain poorly studied. Palmitoylation is catalyzed by a family of 23 zinc finger and Asp-His-His-Cys domain-containing S-acyltransferases (zDHHC enzymes) and removed by select thioesterases of the lysophospholipase and α/β-hydroxylase domain (ABHD)-containing families of serine hydrolases. Recently, studies utilizing genetic manipulation of zDHHC enzymes in cardiomyocytes have begun to unveil essential functions for these enzymes in regulating cardiac development, homeostasis, and pathogenesis. Palmitoylation co-ordinates cardiac electrophysiology through direct modulation of ion channels and transporters to impact their trafficking or gating properties as well as indirectly through modification of regulators of channels, transporters, and calcium handling machinery. Not surprisingly, palmitoylation has roles in orchestrating the intracellular trafficking of proteins in cardiomyocytes, but also dynamically fine-tunes cardiomyocyte exocytosis and natriuretic peptide secretion. Palmitoylation has emerged as a potent regulator of intracellular signaling in cardiomyocytes, with recent studies uncovering palmitoylation-dependent regulation of small GTPases through direct modification and sarcolemmal targeting of the small GTPases themselves or by modification of regulators of the GTPase cycle. In addition to dynamic control of G protein signaling, cytosolic DNA is sensed and transduced into an inflammatory transcriptional output through palmitoylation-dependent activation of the cGAS-STING pathway, which has been targeted pharmacologically in preclinical models of heart disease. Further research is needed to fully understand the complex regulatory mechanisms governed by protein palmitoylation in cardiomyocytes and potential emerging therapeutic targets.

Competition for cysteine acylation by C16:0 and C18:0 derived lipids is a global phenomenon in the proteome

S-acylation is a reversible posttranslational protein modification consisting of attachment of a fatty acid to a cysteine via a thioester bond. Research over the last few years has shown that a variety of different fatty acids, such as palmitic acid (C16:0), stearate (C18:0), or oleate (C18:1), are used in cells to S-acylate proteins. We recently showed that GNAI proteins can be acylated on a single residue, Cys3, with either C16:0 or C18:1, and that the relative proportion of acylation with these fatty acids depends on the level of the respective fatty acid in the cell's environment. This has functional consequences for GNAI proteins, with the identity of the acylating fatty acid affecting the subcellular localization of GNAIs. Unclear is whether this competitive acylation is specific to GNAI proteins or a more general phenomenon in the proteome. We perform here a proteome screen to identify proteins acylated with different fatty acids. We identify 218 proteins acylated with C16:0 and 308 proteins acylated with C18-lipids, thereby uncovering novel targets of acylation. We find that most proteins that can be acylated by C16:0 can also be acylated with C18-fatty acids. For proteins with more than one acylation site, we find that this competitive acylation occurs on each individual cysteine residue. This raises the possibility that the function of many different proteins can be regulated by the lipid environment via differential S-acylation.

Repression of the iron exporter ferroportin may contribute to hepatocyte iron overload in individuals with type 2 diabetes

OBJECTIVE

Hyperferremia and hyperferritinemia are observed in patients and disease models of type 2 diabetes mellitus (T2DM). Likewise, patients with genetic iron overload diseases develop diabetes, suggesting a tight link between iron metabolism and diabetes. The liver controls systemic iron homeostasis and is a central organ for T2DM. Here, we investigate how the control of iron metabolism in hepatocytes is affected by T2DM.

METHODS

Perls Prussian blue staining was applied to analyze iron distribution in liver biopsies of T2DM patients. To identify molecular mechanisms underlying hepatocyte iron accumulation we established cellular models of insulin resistance by treatment with palmitate and insulin.

RESULTS

We show that a subset of T2DM patients accumulates iron in hepatocytes, a finding mirrored in a hepatocyte model of insulin resistance. Iron accumulation can be explained by the repression of the iron exporter ferroportin upon palmitate and/or insulin treatment. While during palmitate treatment the activation of the iron regulatory hormone hepcidin may contribute to reducing ferroportin protein levels in a cell-autonomous manner, insulin treatment decreases ferroportin transcription via the PI3K/AKT and Ras/Raf/MEK/ERK signaling pathways.

CONCLUSION

Repression of ferroportin at the transcriptional and post-transcriptional level may contribute to iron accumulation in hepatocytes observed in a subset of patients with T2DM.

The S-palmitoylome and DHHC-PAT interactome of Drosophila melanogaster S2R+ cells indicate a high degree of conservation to mammalian palmitoylomes

Protein S-palmitoylation, the addition of a long-chain fatty acid to target proteins, is among the most frequent reversible protein modifications in Metazoa, affecting subcellular protein localization, trafficking and protein-protein interactions. S-palmitoylated proteins are abundant in the neuronal system and are associated with neuronal diseases and cancer. Despite the importance of this post-translational modification, it has not been thoroughly studied in the model organism Drosophila melanogaster. Here we present the palmitoylome of Drosophila S2R+ cells, comprising 198 proteins, an estimated 3.5% of expressed genes in these cells. Comparison of orthologs between mammals and Drosophila suggests that S-palmitoylated proteins are more conserved between these distant phyla than non-S-palmitoylated proteins. To identify putative client proteins and interaction partners of the DHHC family of protein acyl-transferases (PATs) we established DHHC-BioID, a proximity biotinylation-based method. In S2R+ cells, ectopic expression of the DHHC-PAT dHip14-BioID in combination with Snap24 or an interaction-deficient Snap24-mutant as a negative control, resulted in biotinylation of Snap24 but not the Snap24-mutant. DHHC-BioID in S2R+ cells using 10 different DHHC-PATs as bait identified 520 putative DHHC-PAT interaction partners of which 48 were S-palmitoylated and are therefore putative DHHC-PAT client proteins. Comparison of putative client protein/DHHC-PAT combinations indicates that CG8314, CG5196, CG5880 and Patsas have a preference for transmembrane proteins, while S-palmitoylated proteins with the Hip14-interaction motif are most enriched by DHHC-BioID variants of approximated and dHip14. Finally, we show that BioID is active in larval and adult Drosophila and that dHip14-BioID rescues dHip14 mutant flies, indicating that DHHC-BioID is non-toxic. In summary we provide the first systematic analysis of a Drosophila palmitoylome. We show that DHHC-BioID is sensitive and specific enough to identify DHHC-PAT client proteins and provide DHHC-PAT assignment for ca. 25% of the S2R+ cell palmitoylome, providing a valuable resource. In addition, we establish DHHC-BioID as a useful concept for the identification of tissue-specific DHHC-PAT interactomes in Drosophila.

Drug Repositioning in Friedreich Ataxia

Friedreich ataxia is a rare neurodegenerative disorder caused by insufficient levels of the essential mitochondrial protein frataxin. It is a severely debilitating disease that significantly impacts the quality of life of affected patients and reduces their life expectancy, however, an adequate cure is not yet available for patients. Frataxin function, although not thoroughly elucidated, is associated with assembly of iron-sulfur cluster and iron metabolism, therefore insufficient frataxin levels lead to reduced activity of many mitochondrial enzymes involved in the electron transport chain, impaired mitochondrial metabolism, reduced ATP production and inefficient anti-oxidant response. As a consequence, neurons progressively die and patients progressively lose their ability to coordinate movement and perform daily activities. Therapeutic strategies aim at restoring sufficient frataxin levels or at correcting some of the downstream consequences of frataxin deficiency. However, the classical pathways of drug discovery are challenging, require a significant amount of resources and time to reach the final approval, and present a high failure rate. Drug repositioning represents a viable alternative to boost the identification of a therapy, particularly for rare diseases where resources are often limited. In this review we will describe recent efforts aimed at the identification of a therapy for Friedreich ataxia through drug repositioning, and discuss the limitation of such strategies.

Contribution of Membrane Lipids to Postsynaptic Protein Organization

The precise subsynaptic organization of proteins at the postsynaptic membrane controls synaptic transmission. In particular, postsynaptic receptor complexes are concentrated in distinct membrane nanodomains to optimize synaptic signaling. However, despite the clear functional relevance of subsynaptic receptor organization to synaptic transmission and plasticity, the mechanisms that underlie the nanoscale organization of the postsynaptic membrane remain elusive. Over the last decades, the field has predominantly focused on the role of protein-protein interactions in receptor trafficking and positioning in the synaptic membrane. In contrast, the contribution of lipids, the principal constituents of the membrane, to receptor positioning at the synapse remains poorly understood. Nevertheless, there is compelling evidence that the synaptic membrane is enriched in specific lipid species and that deregulation of lipid homeostasis in neurons severely affects synaptic functioning. In this review we focus on how lipids are organized at the synaptic membrane, with special emphasis on how current models of membrane organization could contribute to protein distribution at the synapse and synaptic transmission. Finally, we will present an outlook on how novel technical developments could be applied to study the dynamic interplay between lipids and proteins at the postsynaptic membrane.

Defective palmitoylation of transferrin receptor triggers iron overload in Friedreich ataxia fibroblasts

Friedreich ataxia (FRDA) is a frequent autosomal recessive disease caused by a GAA repeat expansion in the FXN gene encoding frataxin, a mitochondrial protein involved in iron-sulfur cluster (ISC) biogenesis. Resulting frataxin deficiency affects ISC-containing proteins and causes iron to accumulate in the brain and heart of FRDA patients. Here we report on abnormal cellular iron homeostasis in FRDA fibroblasts inducing a massive iron overload in cytosol and mitochondria. We observe membrane transferrin receptor 1 (TfR1) accumulation, increased TfR1 endocytosis, and delayed Tf recycling, ascribing this to impaired TfR1 palmitoylation. Frataxin deficiency is shown to reduce coenzyme A (CoA) availability for TfR1 palmitoylation. Finally, we demonstrate that artesunate, CoA, and dichloroacetate improve TfR1 palmitoylation and decrease iron overload, paving the road for evidence-based therapeutic strategies at the actionable level of TfR1 palmitoylation in FRDA.

A Not-So-Ancient Grease History: Click Chemistry and Protein Lipid Modifications

Protein lipid modification involves the attachment of hydrophobic groups to proteins via ester, thioester, amide, or thioether linkages. In this review, the specific click chemical reactions that have been employed to study protein lipid modification and their use for specific labeling applications are first described. This is followed by an introduction to the different types of protein lipid modifications that occur in biology. Next, the roles of click chemistry in elucidating specific biological features including the identification of lipid-modified proteins, studies of their regulation, and their role in diseases are presented. A description of the use of protein-lipid modifying enzymes for specific labeling applications including protein immobilization, fluorescent labeling, nanostructure assembly, and the construction of protein-drug conjugates is presented next. Concluding remarks and future directions are presented in the final section.

Glia maturation factor-γ regulates murine macrophage iron metabolism and M2 polarization through mitochondrial ROS

In macrophages, cellular iron metabolism status is tightly integrated with macrophage phenotype and associated with mitochondrial function. However, how molecular events regulate mitochondrial activity to integrate regulation of iron metabolism and macrophage phenotype remains unclear. Here, we explored the important role of the actin-regulatory protein glia maturation factor-γ (GMFG) in the regulation of cellular iron metabolism and macrophage phenotype. We found that GMFG was downregulated in murine macrophages by exposure to iron and hydrogen peroxide. GMFG knockdown altered the expression of iron metabolism proteins and increased iron levels in murine macrophages and concomitantly promoted their polarization toward an anti-inflammatory M2 phenotype. GMFG-knockdown macrophages exhibited moderately increased levels of mitochondrial reactive oxygen species (mtROS), which were accompanied by decreased expression of some mitochondrial respiration chain components, including the iron-sulfur cluster assembly scaffold protein ISCU as well as the antioxidant enzymes SOD1 and SOD2. Importantly, treatment of GMFG-knockdown macrophages with the antioxidant N-acetylcysteine reversed the altered expression of iron metabolism proteins and significantly inhibited the enhanced gene expression of M2 macrophage markers, suggesting that mtROS is mechanistically linked to cellular iron metabolism and macrophage phenotype. Finally, GMFG interacted with the mitochondrial membrane ATPase ATAD3A, suggesting that GMFG knockdown-induced mtROS production might be attributed to alteration of mitochondrial function in macrophages. Our findings suggest that GMFG is an important regulator in cellular iron metabolism and macrophage phenotype and could be a novel therapeutic target for modulating macrophage function in immune and metabolic disorders.

S-Palmitoylation Sorts Membrane Cargo for Anterograde Transport in the Golgi

While retrograde cargo selection in the Golgi is known to depend on specific signals, it is unknown whether anterograde cargo is sorted, and anterograde signals have not been identified. We suggest here that S-palmitoylation of anterograde cargo at the Golgi membrane interface is an anterograde signal and that it results in concentration in curved regions at the Golgi rims by simple physical chemistry. The rate of transport across the Golgi of two S-palmitoylated membrane proteins is controlled by S-palmitoylation. The bulk of S-palmitoylated proteins in the Golgi behave analogously, as revealed by click chemistry-based fluorescence and electron microscopy. These palmitoylated cargos concentrate in the most highly curved regions of the Golgi membranes, including the fenestrated perimeters of cisternae and associated vesicles. A palmitoylated transmembrane domain behaves similarly in model systems.

Treatment of Trypanosoma cruzi with 2-bromopalmitate alters morphology, endocytosis, differentiation and infectivity

Background

The palmitate analogue 2-bromopalmitate (2-BP) is a non-selective membrane tethered cysteine alkylator of many membrane-associated enzymes that in the last years emerged as a general inhibitor of protein S-palmitoylation. Palmitoylation is a post-translational protein modification that adds palmitic acid to a cysteine residue through a thioester linkage, promoting membrane localization, protein stability, regulation of enzymatic activity, and the epigenetic regulation of gene expression. Little is known on such important process in the pathogenic protozoan Trypanosoma cruzi, the etiological agent of Chagas disease.

Results

The effect of 2-BP was analyzed on different developmental forms of Trypanosoma cruzi. The IC₅₀/48 h value for culture epimastigotes was estimated as 130 μM. The IC₅₀/24 h value for metacyclic trypomastigotes was 216 nM, while for intracellular amastigotes it was 242 μM and for cell derived trypomasigotes was 262 μM (IC₅₀/24 h). Our data showed that 2-BP altered T. cruzi: 1) morphology, as assessed by bright field, scanning and transmission electron microscopy; 2) mitochondrial membrane potential, as shown by flow cytometry after incubation with rhodamine-123; 3) endocytosis, as seen after incubation with transferrin or albumin and analysis by flow cytometry/fluorescence microscopy; 4) in vitro metacyclogenesis; and 5) infectivity, as shown by host cell infection assays. On the other hand, lipid stress by incubation with palmitate did not alter epimastigote growth, metacyclic trypomastigotes viability or trypomastigote infectivity.

Conclusion

Our results indicate that 2-BP inhibits key cellular processes of T. cruzi that may be regulated by palmitoylation of vital proteins and suggest a metacyclic trypomastigote unique target dependency during the parasite development.

Electronic supplementary material

The online version of this article (10.1186/s12860-018-0170-3) contains supplementary material, which is available to authorized users.

Impaired Transferrin Receptor Palmitoylation and Recycling in Neurodegeneration with Brain Iron Accumulation

Neurodegeneration with brain iron accumulation (NBIA) is a genetically heterogeneous condition characterized by progressive dystonia with iron accumulation in the basal ganglia. How NBIA-associated mutations trigger iron overload remains poorly understood. After studying fibroblast cell lines from subjects carrying both known and unreported biallelic mutations in CRAT and REPS1, we ascribe iron overload to the abnormal recycling of transferrin receptor (TfR1) and the reduction of TfR1 palmitoylation in NBIA. Moreover, we describe palmitoylation as a hitherto unreported level of post-translational TfR1 regulation. A widely used antimalarial agent, artesunate, rescued abnormal TfR1 palmitoylation in cultured fibroblasts of NBIA subjects. These observations suggest therapeutic strategies aimed at targeting impaired TfR1 recycling and palmitoylation in NBIA.

Fatty acylation of proteins: The long and the short of it

Long, short and medium chain fatty acids are covalently attached to hundreds of proteins. Each fatty acid confers distinct biochemical properties, enabling fatty acylation to regulate intracellular trafficking, subcellular localization, protein-protein and protein-lipid interactions. Myristate and palmitate represent the most common fatty acid modifying groups. New insights into how fatty acylation reactions are catalyzed, and how fatty acylation regulates protein structure and function continue to emerge. Myristate is typically linked to an N-terminal glycine, but recent studies reveal that lysines can also be myristoylated. Enzymes that remove N-terminal myristoyl-glycine or myristate from lysines have now been identified. DHHC proteins catalyze S-palmitoylation, but the mechanisms that regulate substrate recognition by individual DHHC family members remain to be determined. New studies continue to reveal thioesterases that remove palmitate from S-acylated proteins. Another area of rapid expansion is fatty acylation of the secreted proteins hedgehog, Wnt and Ghrelin, by Hhat, Porcupine and GOAT, respectively. Understanding how these membrane bound O-acyl transferases recognize their protein and fatty acyl CoA substrates is an active area of investigation, and is punctuated by the finding that these enzymes are potential drug targets in human diseases.

Molecular characterization of HIV-1 Nef and ACOT8 interaction: insights from in silico structural predictions and in vitro functional assays

HIV-1 Nef interacts with several cellular proteins, among which the human peroxisomal thioesterase 8 (ACOT8). This interaction may be involved in the endocytosis regulation of membrane proteins and might modulate lipid composition in membrane rafts. Nef regions involved in the interaction have been experimentally characterized, whereas structural details of the ACOT8 protein are unknown. The lack of structural information hampers the comprehension of the functional consequences of the complex formation during HIV-1 infection. We modelled, through in silico predictions, the ACOT8 structure and we observed a high charge complementarity between Nef and ACOT8 surfaces, which allowed the identification of the ACOT8 putative contact points involved in the interaction. The predictions were validated by in vitro assays through the development of ACOT8 deletion mutants. Coimmunoprecipitation and immunofluorescence analyses showed that ACOT8 Arg⁴⁵-Phe⁵⁵ and Arg⁸⁶-Pro⁹³ regions are involved in Nef association. In addition, K91S mutation abrogated the interaction with Nef, indicating that Lys⁹¹ plays a key role in the interaction. Finally, when associated with ACOT8, Nef may be preserved from degradation. These findings improve the comprehension of the association between HIV-1 Nef and ACOT8, helping elucidating the biological effect of their interaction.

Hydrophobic myristic acid modified PAMAM dendrimers augment the delivery of tamoxifen to breast cancer cells

In the present study, cationic generation 5 polyamido amine (G5 PAMAM) dendrimers were hydrophobically modified by grafting the surface with lipid-like myristic acid (My) tails to augment their potential as a drug delivery vector in vitro.

SwissPalm: Protein Palmitoylation database

Protein S-palmitoylation is a reversible post-translational modification that regulates many key biological processes, although the full extent and functions of protein S-palmitoylation remain largely unexplored. Recent developments of new chemical methods have allowed the establishment of palmitoyl-proteomes of a variety of cell lines and tissues from different species.  As the amount of information generated by these high-throughput studies is increasing, the field requires centralization and comparison of this information. Here we present SwissPalm ( http://swisspalm.epfl.ch), our open, comprehensive, manually curated resource to study protein S-palmitoylation. It currently encompasses more than 5000 S-palmitoylated protein hits from seven species, and contains more than 500 specific sites of S-palmitoylation. SwissPalm also provides curated information and filters that increase the confidence in true positive hits, and integrates predictions of S-palmitoylated cysteine scores, orthologs and isoform multiple alignments. Systems analysis of the palmitoyl-proteome screens indicate that 10% or more of the human proteome is susceptible to S-palmitoylation. Moreover, ontology and pathway analyses of the human palmitoyl-proteome reveal that key biological functions involve this reversible lipid modification. Comparative analysis finally shows a strong crosstalk between S-palmitoylation and other post-translational modifications. Through the compilation of data and continuous updates, SwissPalm will provide a powerful tool to unravel the global importance of protein S-palmitoylation.

Curcumin Prevents Palmitoylation of Integrin β4 in Breast Cancer Cells

Curcumin has been shown to mitigate cancer phenotypes such as invasive migration, proliferation, and survival by disrupting numerous signaling pathways. Our previous studies showed that curcumin inhibits integrin β4 (ITG β4)-dependent migration by blocking interaction of this integrin with growth factor receptors in lipid rafts. In the current study, we investigated the possibility that curcumin inhibits ITG β4 palmitoylation, a post-translational modification required for its lipid raft localization and signaling activity. We found that the levels of ITG β4 palmitoylation correlated with the invasive potential of breast cancer cells, and that curcumin effectively reduced the levels of ITG β4 palmitoylation in invasive breast cancer cells. Through studies of ITG β4 palmitoylation kinetics, we concluded curcumin suppressed palmitoylation independent of growth factor-induced phosphorylation of key ITG β4 Ser and Tyr residues. Rather, curcumin blocked autoacylation of the palmitoyl acyltransferase DHHC3 that is responsible for ITG β4 palmitoylation. Moreover, these data reveal that curcumin is able to prevent the palmitoylation of a subset of proteins, but not indiscriminately bind to and block all cysteines from modifications. Our studies reveal a novel paradigm for curcumin to account for much of its biological activity, and specifically, how it is able to suppress the signaling function of ITG β4 in breast cancer cells.

Cowpox virus protein CPXV012 eludes CTLs by blocking ATP binding to TAP

CD8(+) CTLs detect virus-infected cells through recognition of virus-derived peptides presented at the cell surface by MHC class I molecules. The cowpox virus protein CPXV012 deprives the endoplasmic reticulum (ER) lumen of peptides for loading onto newly synthesized MHC class I molecules by inhibiting the transporter associated with Ag processing (TAP). This evasion strategy allows the virus to avoid detection by the immune system. In this article, we show that CPXV012, a 9-kDa type II transmembrane protein, prevents peptide transport by inhibiting ATP binding to TAP. We identified a segment within the ER-luminal domain of CPXV012 that imposes the block in peptide transport by TAP. Biophysical studies show that this domain has a strong affinity for phospholipids that are also abundant in the ER membrane. We discuss these findings in an evolutionary context and show that a frameshift deletion in the CPXV012 gene in an ancestral cowpox virus created the current form of CPXV012 that is capable of inhibiting TAP. In conclusion, our findings indicate that the ER-luminal domain of CPXV012 inserts into the ER membrane, where it interacts with TAP. CPXV012 presumably induces a conformational arrest that precludes ATP binding to TAP and, thus, activity of TAP, thereby preventing the presentation of viral peptides to CTLs.

Mechanistic effects of protein palmitoylation and the cellular consequences thereof

S-palmitoylation involves the attachment of a 16-carbon long fatty acid chain to the cysteine residues of proteins. The process is enzymatic and dynamic with DHHC enzymes mediating palmitoylation and acyl-protein thioesterases reverting the reaction. Proteins that undergo this modification span almost all cellular functions. While the increase in hydrophobicity generated by palmitoylation has the obvious consequence of triggering membrane association, the effects on transmembrane proteins are less intuitive and span a vast range. We review here the current knowledge on palmitoylating and depalmitoylating enzymes, the methods that allow the study of this lipid modification and which drugs can affect it, and finally we focus on four cellular processes for which recent studies reveal an involvement of palmitoylation: endocytosis, reproduction and cell growth, fat and sugar homeostasis and signal transduction at the synapse.

S-palmitoylation represents a novel mechanism regulating the mitochondrial targeting of BAX and initiation of apoptosis

The intrinsic pathway of apoptotic cell death is mainly mediated by the BCL-2-associated X (BAX) protein through permeabilization of the mitochondrial outer membrane (MOM) and the concomitant release of cytochrome c into the cytosol. In healthy, non-apoptotic cells, BAX is predominantly localized in the cytosol and exhibits a dynamic shuttle cycle between the cytosol and the mitochondria. Thus, the initial association with mitochondria represents a critical regulatory step enabling BAX to insert into MOMs, promoting the release of cytochrome c and ultimately resulting in apoptosis. However, the molecular mode of how BAX associates with MOMs and whether a cellular regulatory mechanism governs this process is poorly understood. Here we show that in both primary tissues and cultured cells, the association with MOMs and the proapoptotic action of BAX is controlled by its S-palmitoylation at Cys-126. A lack of BAX palmitoylation reduced BAX mitochondrial translocation, BAX oligomerization, caspase activity and apoptosis. Furthermore, ectopic expression of specific palmitoyl transferases in cultured healthy cells increases BAX S-palmitoylation and accelerates apoptosis, whereas malignant tumor cells show reduced BAX S-palmitoylation consistent with their reduced BAX-mediated proapoptotic activity. Our findings suggest that S-palmitoylation of BAX at Cys126 is a key regulatory process of BAX-mediated apoptosis.

The transferrin receptor-1 membrane stub undergoes intramembrane proteolysis by signal peptide peptidase-like 2b

The successive events of shedding and regulated intramembrane proteolysis are known to comprise a fundamental biological process of type I and II membrane proteins (e.g. amyloid precursor protein, Notch receptor and pro-tumor necrosis factor-α). Some of the resulting fragments were shown to be involved in important intra- and extracellular signalling events. Although shedding of the human transferrin receptor-1 (TfR1) has been known for > 30 years and soluble TfR1 is an accepted diagnostic marker, the fate of the remaining N-terminal fragment (NTF) remains unknown. In the present study, we demonstrate for the first time that TfR1-NTF is subject to regulated intramembrane proteolysis and, using MALDI-TOF-TOF-MS, we have identified the cleavage site as being located C-terminal from Gly-84. We showed that the resulting C-terminal peptide is extracellularly released after regulated intramembrane proteolysis and it was detected as a monomer with an internal disulfide bridge. We further identified signal peptide peptidase-like 2a and mainly signal peptide peptidase-like 2b as being responsible for the intramembrane proteolysis of TfR1-NTF.

Mammalian iron metabolism

ABSTRACT Iron is an essential transition metal for mammalian cellular and tissue viability. It is critical to supplying oxygen through heme, the mitochondrial respiratory chain, and enzymes such as ribonucleotide reductase. Mammalian organisms have evolved with the means of regulating the metabolism of iron, because if left unregulated, the resulting excess amounts of iron may induce chronic toxicities affecting multiple organ systems. Several homeostatic mechanisms exist to control the amount of intestinal dietary iron uptake, cellular iron uptake, distribution, and export. Within these processes, numerous molecular participants have been identified because of advancements in basic cell biology and efforts in disease-based research of iron storage abnormalities. For example, dietary iron uptake across the intestinal duodenal mucosa is mediated by an intramembrane divalent metal transporter 1 (DMT1), and cellular iron efflux involves ferroportin, the only known iron exporter. In addition to duodenal enterocytes, ferroportin is present in other cell types, and exports iron into plasma. Ferroportin was recently discovered to be regulated by the expression of the circulating hormone hepcidin, a small peptide synthesized in hepatocytes. These recent studies on the role of hepcidin in the regulation of dietary, cellular, and extracellular iron have led to a better understanding of the pathways by which iron balance in humans is influenced, especially its involvement in human genetic diseases of iron overload. Other important molecular pathways include iron binding to transferrin in the bloodstream for cellular delivery through the plasma membrane transferrin receptor (TfR1). In the cytosol, iron regulatory proteins 1 and 2 (IRP1 and IRP2) play a prominent role in sensing the presence of iron in order to posttranscriptionally regulate the expression of TfR1 and ferritin, two important participants in iron metabolism. From a toxicological standpoint, posttranscriptional regulation of these genes aids in the sequestration, control, and hence prevention of cytotoxic effects from free-floating nontransferrin-bound iron. Given the importance of dietary iron in normal physiology, its potential to induce chronic toxicity, and recent discoveries in the regulation of human iron metabolism by hepcidin, this review will address the regulatory mechanisms of normal iron metabolism in mammals with emphasis on dietary exposure. It is the goal of this review that this information may provide in a concise format our current understanding of major pathways and mechanisms involved in mammalian iron metabolism, which is a basis for control of iron toxicity. Such a discussion is intended to facilitate the identification of deficiencies so that future metabolic or toxicological studies may be appropriately focused. A better knowledge of iron metabolism from normal to pathophysiological conditions will ultimately broaden the spectrum of the usefulness of this information in biomedical and toxicological sciences for improving and protecting human health.

Palmitoylation by DHHC3 is critical for the function, expression, and stability of integrin α6β4

The laminin-binding integrin α6β4 plays key roles in both normal epithelial and endothelial cells and during tumor cell progression, metastasis, and angiogenesis. Previous cysteine mutagenesis studies have suggested that palmitoylation of α6β4 protein supports a few integrin-dependent functions and molecular associations. Here we took another approach and obtained strikingly different results. We used overexpression and RNAi knockdown in multiple cell types to identify protein acyl transferase DHHC3 as the enzyme responsible for integrin β4 and α6 palmitoylation. Ablation of DHHC3 markedly diminished integrin-dependent cellular cable formation on Matrigel, integrin signaling through Src, and β4 phosphorylation on key diagnostic amino acids (S1356 and 1424). However, unexpectedly, and in sharp contrast to prior α6β4 mutagenesis results, knockdown of DHHC3 accelerated the degradation of α6β4, likely due to an increase in endosomal exposure to cathepsin D. When proteolytic degradation was inhibited (by Pepstatin A), rescued α6β4 accumulated intracellularly, but was unable to reach the cell surface. DHHC3 ablation effects were strongly selective for α6β4. Cell-surface levels of ~10 other proteins (including α3β1) were not diminished, and the appearance of hundreds of other palmitoylated proteins was not altered. Results obtained here demonstrate a new substrate for the DHHC3 enzyme and provide novel opportunities for modulating α6β4 expression, distribution, and function.

The proprotein convertase PC7: unique zymogen activation and trafficking pathways

The zymogen activation mechanism and physiological functions of the most ancient and highly conserved basic amino acid-specific proprotein convertase 7 (PC7) are not known. Herein, we characterized the biosynthesis, subcellular localization, and trafficking of the membrane-bound full-length rat and human PC7. The prosegment of PC7 is primarily secreted alone as a non-inhibitory protein via the conventional, Golgi-dependent, secretory pathway. Mature PC7 is partially sulfated and thus reaches the cell surface via the conventional route. However, a fraction of PC7 reaches the cell surface through a brefeldin A- and COPII-independent unconventional secretory pathway. The latter trafficking may explain the rapid (<10 min) transit of a fraction of PC7 from the ER to the cell surface. Electron microscopy further confirmed the localization of PC7 to the cell surface of HEK293 cells. Within the cytosolic tail, only two cysteines (Cys(699) and Cys(704)) are palmitoylated, but this modification does not affect the choice of trafficking pathway. Swapping the transmembrane-cytosolic tail (TMCT) sequences of the convertases Furin and PC7 revealed that PC7(TMCT-Furin) is much more sulfated and hence traffics more efficiently through the conventional secretory pathway. In contrast, the Furin(TMCT-PC7) is no longer sulfated and thus reaches the cell surface by the unconventional pathway. Because trafficking of PC7(CT-Furin) and Furin(CT-PC7) resemble their wild type counterparts, we deduce that the transmembrane domain of PC7 regulates the sorting of PC7 toward the unconventional secretory pathway. In conclusion, PC7 is distinct from other proprotein convertases in its zymogen activation, subcellular localization, and trafficking.

Proteomic analysis of fatty-acylated proteins in mammalian cells with chemical reporters reveals S-acylation of histone H3 variants

Bioorthogonal chemical reporters are useful tools for visualizing and identifying post-translational modifications on proteins. Here we report the proteomic analysis of mammalian proteins targeted by a series of fatty acid chemical reporters ranging from myristic to stearic acid. The large-scale analysis of total cell lysates from fully solubilized Jurkat T cells identified known fatty-acylated proteins and many new candidates, including nuclear proteins and in particular histone H3 variants. We demonstrate that histones H3.1, H3.2, and H3.3 are modified with fatty acid chemical reporters and identify the conserved cysteine 110 as a new site of S-acylation on histone H3.2. This newly discovered modification of histone H3 could have implications for nuclear organization and chromatin regulation. The unbiased proteomic analysis of fatty-acylated proteins using chemical reporters has revealed a greater diversity of lipid-modified proteins in mammalian cells and identified a novel post-translational modification of histones.

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.

Cys palmitoylation of the beta subunit modulates gating of the epithelial sodium channel

The epithelial Na(+) channel (ENaC) is comprised of three homologous subunits (α, β, and γ) that have a similar topology with two transmembrane domains, a large extracellular region, and cytoplasmic N and C termini. Although ENaC activity is regulated by a number of factors, palmitoylation of its cytoplasmic Cys residues has not been previously described. Fatty acid-exchange chemistry was used to determine whether channel subunits were Cys-palmitoylated. We observed that only the β and γ subunits were modified by Cys palmitoylation. Analyses of ENaCs with mutant β subunits revealed that Cys-43 and Cys-557 were palmitoylated. Xenopus oocytes expressing ENaC with a β C43A,C557A mutant had significantly reduced amiloride-sensitive whole cell currents, enhanced Na(+) self-inhibition, and reduced single channel P(o) when compared with wild-type ENaC, while membrane trafficking and levels of surface expression were unchanged. Computer modeling of cytoplasmic domains indicated that β Cys-43 is in proximity to the first transmembrane α helix, whereas β Cys-557 is within an amphipathic α-helix contiguous with the second transmembrane domain. We propose that β subunit palmitoylation modulates channel gating by facilitating interactions between cytoplasmic domains and the plasma membrane.

Receptor-mediated therapeutic transport across the blood–brain barrier

The blood–brain barrier (BBB) hinders drug delivery to the brain parenchyma. The ultimate goal of brain drug targeting technology is to deliver therapeutics across the BBB with a diverse collection of molecular transport systems. Receptor-mediated transcytosis (RMT) is one such class of transport system. Insulin and transferrin, as well as other endogenous peptides, employ the vesicular trafficking machinery of the endothelium to transport substances between the blood and the brain. In addition to vector development, strategies for coupling drugs to the vector that give high-efficiency coupling are the other important element for RMT. After the BBB-targeting vector–therapeutic conjugates have crossed the BBB, there may still be a need to target them to a specific population of cells in the brain. This review will focus on two major aspects of RMT brain drug delivery: new advances of existing RMT systems and development of new BBB transport vectors and specific RMT targets.

Palmitoylation of interferon-alpha (IFN-alpha) receptor subunit IFNAR1 is required for the activation of Stat1 and Stat2 by IFN-alpha

Type I interferons (IFNs) bind IFNAR receptors and activate Jak kinases and Stat transcription factors to stimulate the transcription of genes downstream from IFN-stimulated response elements. In this study, we analyze the role of protein palmitoylation, a reversible post-translational lipid modification, in the functional properties of IFNAR. We report that pharmacological inhibition of protein palmitoylation results in severe defects of IFN receptor endocytosis and signaling. We generated mutants of the IFNAR1 subunit of the type I IFN receptor, in which each or both of the two cysteines present in the cytoplasmic domain are replaced by alanines. We show that cysteine 463 of IFNAR1, the more proximal of the two cytoplasmic cysteines, is palmitoylated. A thorough microscopic and biochemical analysis of the palmitoylation-deficient IFNAR1 mutant revealed that IFNAR1 palmitoylation is not required for receptor endocytosis, intracellular distribution, or stability at the cell surface. However, the lack of IFNAR1 palmitoylation affects selectively the activation of Stat2, which results in a lack of efficient Stat1 activation and nuclear translocation and IFN-alpha-activated gene transcription. Thus, receptor palmitoylation is a previously undescribed mechanism of regulating signaling activity by type I IFNs in the Jak/Stat pathway.

Palmitoylation of membrane proteins (Review)

S-Palmitoylation is a reversible post-translational modification that results in the addition of a C16-carbon saturated fatty acyl chain to cytoplasmic cysteine residues. This modification is mediated by Palmitoyl-acyl Transferases that are starting to be investigated, and reversed by Protein Palmitoyl Thioesterases, which remain enigmatic. Palmitoylation of cytoplasmic proteins has been well described to regulate the interaction of these soluble proteins with specific membranes or membrane domains. Less is known about the consequences of palmitoylation in transmembrane proteins not only due to the dual difficulty of following a lipid modification and dealing with membrane proteins, but also due to the complexity of the palmitoylation-induced behavior. Moreover, possibly because the available data set is limited, the change in behavior induced by palmitoylation of a transmembrane protein is currently not predictable. We here review the various consequences reported for the palmitoylation of membrane proteins, which include improper folding in the endoplasmic reticulum, retention in the Golgi, inability to assemble into protein platforms, altered signaling capacity, premature endocytosis and missorting in the endocytic pathway. We then discuss the possible underlying mechanisms, in particular the ability of palmitoylation to control the conformation of transmembrane segments, to modify the affinity of a membrane protein for specific membrane domains and to control protein-protein interactions.

Palmitoylation of the P2X7 receptor, an ATP-gated channel, controls its expression and association with lipid rafts

The P2X7 receptor (P2X7R) is an ATP-gated cationic channel expressed by hematopoietic, epithelial, and neuronal cells. Prolonged ATP exposure leads to the formation of a nonselective pore, which can result in cell death. We show that P2X7R is associated with detergent-resistant membranes (DRMs) in both transfected human embryonic kidney (HEK) cells and primary macrophages independently from ATP binding. The DRM association requires the posttranslational modification of P2X7R by palmitic acid. Treatment of cells with the palmitic acid analog 2-bromopalmitate as well as mutations of cysteine to alanine residues abolished P2X7R palmitoylation. Substitution of the 17 intracellular cysteines of P2X7R revealed that 4 regions of the carboxyl terminus domain are involved in palmitoylation. Palmitoylation-defective P2X7R mutants showed a dramatic decrease in cell surface expression because of their retention in the endoplasmic reticulum and proteolytic degradation. Taken together, our data demonstrate that P2X7R palmitoylation plays a critical role in its association with the lipid microdomains of the plasma membrane and in the regulation of its half-life.

Slow endocytosis of the LDL receptor-related protein 1B: implications for a novel cytoplasmic tail conformation

The LDL receptor-related protein 1B (LRP1B) is a putative tumor suppressor homologous to LRP1. Both LRP1 and LRP1B contain cytoplasmic tails with several potential endocytosis motifs. Although the positions of these endocytic motifs are similar in both receptors, LRP1B is internalized at a 15-fold slower rate than LRP1. To determine whether the slow endocytosis of LRP1B is due to the utilization of an endocytosis motif other than the YATL motif used by LRP1, we tested minireceptors with mutations in each of the five potential motifs in the LRP1B tail. Only mutation of both NPXY motifs together abolished LRP1B endocytosis, suggesting that LRP1B can use either of these motifs for internalization. LRP1B contains a unique insertion of 33 amino acids not present in LRP1 that could lead to altered recognition of trafficking motifs. Surprisingly, deletion of this insertion had no effect on the endocytosis rate of LRP1B. However, replacing either half of the LRP1B tail with the corresponding LRP1 sequence markedly accelerated LRP1B endocytosis. From these data, we propose that both halves of the LRP1B cytoplasmic tail contribute to a unique global conformation, which results in less efficient recognition by endocytic adaptors and a slow endocytosis rate.

TNF trafficking to human mast cell granules: mature chain-dependent endocytosis

Mast cells play a crucial role at the early stages of immune response against bacteria and parasites where their functionality is based on their capability of releasing highly bioactive compounds, among them TNF. Mast cells are considered the only cells storing preformed TNF, which allows for the immediate release of this cytokine upon contact with pathogens. We approached the question of mechanisms and amino acid motifs directing newly synthesized TNF for storage in cytoplasmic granules by analyzing the trafficking of a series of TNF-enhanced GFP fusion proteins in human mast cell lines HMC-1 and LAD2. Protein covering the full TNF sequence was successfully sorted into secretory granules in a process involving transient exposure on the outer membrane and re-endocytosis. In human cells, contrary to results previously obtained in a rodent model, TNF seems not to be glycosylated and, thus, trafficking is carbohydrate independent. In an effort to localize the amino acid motif responsible for granule targeting, we constructed additional fusion proteins and analyzed their trafficking, concluding that granule-targeting sequences are localized in the mature chain of TNF and that the cytoplasmic tail is expendable for endocytotic sorting of this cytokine, thus excluding direct interactions with intracellular adaptor proteins.

Palmitoylation: policing protein stability and traffic

Palmitate modifies both peripheral and integral membrane proteins and its addition can be permanent or transient, which makes it unique among the lipid modifications of proteins. The presence of palmitate on a protein affects how the protein interacts with lipids and proteins in a membrane compartment, and the reversibility of palmitoylation allows different modes of trafficking between membrane compartments. Here, we review recent studies that have provided insights into the mechanisms that mediate the functional consequences of this versatile modification.

Molecular Evolution of the Transferrin Receptor/Glutamate Carboxypeptidase II Family

The transferrin receptor family is represented by at least seven different homologous proteins in primates. Transferrin receptor (TfR1) is a type II membrane glycoprotein that, as a cell surface homodimer, binds iron-loaded transferrin as part of the process of iron transfer and uptake. Other family members include transferrin receptor 2 (TfR2), glutamate carboxypeptidase II (GCP2 or PSMA), N-acetylated alpha-linked acidic dipeptidase-like protein (NLDL), N-acetylated alpha-linked acidic dipeptidase 2 (NAALAD2), and prostate-specific membrane antigen-like protein (PMSAL/GCPIII). We compared 86 different sequences from 24 different species, from mammals to fungi. Through this comparison, we have identified several highly conserved residues specific to each family not previously associated with clinical mutations. The evolutionary history of the TfR/GCP2 family shows repeated episodes of duplications consistent with recent theories that nondispensable, slowly evolving genes are more likely to form multiple gene families.

Palmitoyl protein thioesterase 1 (PPT1) deficiency causes endocytic defects connected to abnormal saposin processing

Infantile neuronal ceroid lipofuscinosis (INCL) is a severe neurodegenerative disorder of the childhood caused by mutations in the gene encoding palmitoyl protein thioesterase 1 (PPT1). PPT1 localizes to late endosomes/lysosomes of non-neuronal cells and in neurons also to presynaptic areas. PPT1-deficiency causes massive death of cortical neurons and most tissues show an accumulation of saposins A and D. We have here studied endocytic pathways, saposin localization and processing in PPT1-deficient fibroblasts to elucidate the cellular defects resulting in accumulation of specific saposins. We show that PPT1-deficiency causes a defect in fluid-phase and receptor-mediated endocytosis, whereas marker uptake and recycling endocytosis remain intact. Furthermore, we show that saposins A and D are more abundant and relocalized in PPT-deficient fibroblasts and mouse primary neurons. Metabolic labeling and immunoprecipitation analyses revealed hypersecretion and abnormal processing of prosaposin, implying that the accumulation of saposins may result from endocytic defects. We show for the first time a connection between saposin storage and a defect in the endocytic pathway of INCL cells. These data provide new insights into the metabolism of PPT1-deficient cells and offer a basis for further studies on cellular processes causing neuronal death in INCL and other neurodegenerative diseases.

Identification and characterization of p63 (CKAP4/ERGIC-63/CLIMP-63), a surfactant protein A binding protein, on type II pneumocytes

Surfactant protein A (SP-A) binds to alveolar type II cells through a specific high-affinity cell membrane receptor, although the molecular nature of this receptor is unclear. In the present study, we have identified and characterized an SP-A cell surface binding protein by utilizing two chemical cross-linkers: profound sulfo-SBED protein-protein interaction reagent and dithiobis(succinimidylpropionate) (DSP). Sulfo-SBED-biotinylated SP-A was cross-linked to the plasma membranes isolated from rat type II cells, and the biotin label was transferred from SP-A to its receptor by reduction. The biotinylated SP-A-binding protein was identified on blots by using streptavidin-labeled horseradish peroxidase. By using DSP, we cross-linked SP-A to intact mouse type II cells and immunoprecipitated the SP-A-receptor complex using anti-SP-A antibody. Both of the cross-linking approaches showed a major band of 63 kDa under reduced conditions that was identified as the rat homolog of the human type II transmembrane protein p63 (CKAP4/ERGIC-63/CLIMP-63) by matrix-assisted laser desorption ionization and nanoelectrospray tandem mass spectrometry of tryptic fragments. Thereafter, we confirmed the presence of p63 protein in the cross-linked SP-A-receptor complex by immunoprobing with p63 antibody. Coimmunoprecipitation experiments and functional assays confirmed specific interaction between SP-A and p63. Antibody to p63 could block SP-A-mediated inhibition of ATP-stimulated phospholipid secretion. Both intracellular and membrane localized pools of p63 were detected on type II cells by immunofluorescence and immunobloting. p63 colocalized with SP-A in early endosomes. Thus p63 closely interacts with SP-A and may play a role in the trafficking or the biological function of the surfactant protein.

Recycling of MUC1 is dependent on its palmitoylation

MUC1 is a mucin-like transmembrane protein expressed on the apical surface of epithelia, where it protects the cell surface. The cytoplasmic domain has numerous sites for phosphorylation and docking of proteins involved in signal transduction. In a previous study, we showed that the cytoplasmic YXXphi motif Y20HPM and the tyrosine-phosphorylated Y60TNP motif are required for MUC1 clathrin-mediated endocytosis through binding AP-2 and Grb2, respectively (Kinlough, C. L., Poland, P. A., Bruns, J. B., Harkleroad, K. L., and Hughey, R. P. (2004) J. Biol. Chem. 279, 53071-53077). Palmitoylation of transmembrane proteins can affect their membrane trafficking, and the MUC1 sequence CQC3RRK at the boundary of the transmembrane and cytoplasmic domains mimics reported site(s) of S-palmitoylation. [3H]Palmitate labeling of Chinese hamster ovary cells expressing MUC1 with mutations in CQC3RRK revealed that MUC1 is dually palmitoylated at the CQC motif independent of RRK. Lack of palmitoylation did not affect the cold detergent solubility profile of a chimera (Tac ectodomain and MUC1 transmembrane and cytoplasmic domains), the rate of chimera delivery to the cell surface, or its half-life. Calculation of rate constants for membrane trafficking of wild-type and mutant Tac-MUC1 indicated that the lack of palmitoylation blocked recycling, but not endocytosis, and caused the chimera to accumulate in a EGFP-Rab11-positive endosomal compartment. Mutations CQC/AQA and Y20N inhibited Tac-MUC1 co-immunoprecipitation with AP-1, although mutant Y20N had reduced rates of both endocytosis and recycling, but a normal subcellular distribution. The double mutant chimera AQA+Y20N had reduced endocytosis and recycling rates and accumulated in EGFP-Rab11-positive endosomes, indicating that palmitoylation is the dominant feature modulating MUC1 recycling from endosomes back to the plasma membrane.

Receptor palmitoylation and ubiquitination regulate anthrax toxin endocytosis

The anthrax toxin is composed of three independent polypeptide chains. Successful intoxication only occurs when heptamerization of the receptor-binding polypeptide, the protective antigen (PA), allows binding of the two enzymatic subunits before endocytosis. We show that this tailored behavior is caused by two counteracting posttranslational modifications in the cytoplasmic tail of PA receptors. The receptor is palmitoylated, and this unexpectedly prevents its association with lipid rafts and, thus, its premature ubiquitination. This second modification, which is mediated by the E3 ubiquitin ligase Cbl, only occurs in rafts and is required for rapid endocytosis of the receptor. As a consequence, cells expressing palmitoylation-defective mutant receptors are less sensitive to anthrax toxin because of a lower number of surface receptors as well as premature internalization of PA without a requirement for heptamerization.

Swf1-dependent palmitoylation of the SNARE Tlg1 prevents its ubiquitination and degradation

Protein palmitoylation is a post-translational modification that affects a great number of proteins. In most cases, the enzymes responsible for this modification have not been identified. Some proteins use palmitoylation to attach themselves to membranes; however, palmitoylation also occurs in transmembrane proteins, and the function of this palmitoylation is less clear. Here we identify Swf1, a member of the DHHC-CDR family of palmitoyltransferases, as the protein responsible for modifying the yeast SNAREs Snc1, Syn8 and Tlg1, at cysteine residues close to the cytoplasmic end of their single transmembrane domains (TMDs). In an swf1Delta mutant, Tlg1 is mis-sorted to the vacuole. This occurs because unpalmitoylated Tlg1 is recognised by the ubiquitin ligase Tul1, resulting in its targeting to the multivesicular body pathway. Our results suggest that one role of palmitoylation is to protect TMDs from the cellular quality control machinery, and that Swf1 may be the enzyme responsible for most, if not all, TMD-associated palmitoylation in yeast.

Internalization and trafficking of the human and rat growth hormone-releasing hormone receptor

Internalization and intracellular trafficking of the growth hormone-releasing hormone receptor (GHRH-R) were studied in rat anterior pituitary and human (h) and rat (r) GHRH-R-transfected BHK cells, with the GHRH agonist, [N(alpha)-5-carboxyfluoresceinyl-D-Ala(2), Ala(8), Ala(15), Lys(22)]hGHRH(1-29)NH(2) (Fluo-GHRH). Time- and temperature-dependent internalization of stimulated GHRH-R was blocked by phenyl arsine oxide (PAO) in both cell types. In anterior pituitary and rGHRH-R-transfected BHK cells, only filipin III and cerulenin blocked receptor-mediated internalization of Fluo-GHRH while in hGHRH-R-transfected BHK cells, only hyperosmolar sucrose inhibited this process. These results suggest that hGHRH-R internalization is clathrin-dependent, while fatty acid acylation of rGHRH-R appears to be a prerequisite to caveolin-dependent internalization. Experiments in anterior pituitary using Bodipy-FL-C(5) ganglioside GM1, a specific marker of lipid rafts such as caveolae, confirmed this latter pathway. Co-localization of Fluo-GHRH with LysoTracker indicated that Fluo-GHRH was directed to acidic organelles in both cell types. Finally, studies using cycloheximide and monensin showed that upon stimulation with GHRH, an optimal concentration of functional GHRH-R was maintained at the plasma membrane due to de novo synthesis and recycling in pituitary cells and to de novo synthesis solely in hGHRH-R-transfected BHK cells. This first study on the dynamics of the GHRH/GHRH-R complexes using fluorescence imaging in a native environment compared to cell system models, revealed that both receptor primary structure and concentration at the plasma membrane play important roles in internalization and trafficking of specific G-protein-coupled receptors (GPCR).

Transferrin receptor 1

With the discovery that transferrin serves as the iron source for hemoglobin-synthesizing immature red blood cells came the demonstration that a cell surface receptor, now known as transferrin receptor 1, is required for iron delivery from transferrin to cells. (A recently described second transferrin receptor, with as yet poorly understood function, will not be discussed in this brief review.) In succeeding years transferrin receptor 1 was established as a gatekeeper for regulating iron uptake by most cells, and the transferrin-to-cell endocytic pathway characterized in detail. HFE, the protein incriminated in the pathogenesis of hereditary hemochromatosis, a disorder of progressive and toxic iron overload, competes with transferrin for binding to receptor, thereby impeding the uptake of iron from transferrin. Mutation of HFE destroys this competition, thus facilitating access of transferrin and its iron to cells. Availability of the crystal structure of transferrin receptor 1, along with those of transferrin and HFE, opened research on molecular mapping of the transferrin-HFE- transferrin receptor interfaces by correlated synchrotron-generated hydroxyl radical footprinting and cryo-electron microscopy. The emerging challenge is to relate structure to the functional effects of receptor binding on the iron-binding and iron-releasing properties of transferrin within the iron-dependent cell.

The effects of HIV-1 Nef on CD4 surface expression and viral infectivity in lymphoid cells are independent of rafts

The HIV-1 Nef protein is a critical virulence factor that exerts multiple effects during viral replication. Nef modulates surface expression of various cellular proteins including CD4 and MHC-I, enhances viral infectivity, and affects signal transduction pathways. Nef has been shown to partially associate with rafts, where it can prime T cells for activation. The contribution of rafts during Nef-induced CD4 down-regulation and enhancement of viral replication remains poorly understood. We show here that Nef does not modify the palmitoylation state of CD4 or its partition within rafts. Moreover, CD4 mutants lacking palmitoylation or unable to associate with rafts are efficiently down-regulated by Nef. In HIV-infected cells, viral assembly and budding occurs from rafts, and Nef has been suggested to increase this process. However, using T cells acutely infected with wild-type or nef-deleted HIV, we did not observe any impact of Nef on raft segregation of viral structural proteins. We have also designed a palmitoylated mutant of Nef (NefG3C), which significantly accumulates in rafts. Interestingly, the efficiency of NefG3C to down-regulate CD4 and MHC-I, and to promote viral replication was not increased when compared with the wild-type protein. Altogether, these results strongly suggest that rafts are not a key element involved in the effects of Nef on trafficking of cellular proteins and on viral replication.

Palmitoylation of the V2 vasopressin receptor carboxyl tail enhances beta-arrestin recruitment leading to efficient receptor endocytosis and ERK1/2 activation

A large number of G protein-coupled receptors are palmitoylated on cysteine residues located in their carboxyl tail, but the general role of this post-translational modification remains poorly understood. Here we show that preventing palmitoylation of the V2 vasopressin receptor, by site-directed mutagenesis of cysteines 341 and 342, significantly delayed and decreased both agonist-promoted receptor endocytosis and mitogen-activated protein kinase activation. Pharmacological blockade of receptor endocytosis is without effect on the vasopressin-stimulated mitogen-activated protein kinase activity, excluding the possibility that the reduced kinase activation mediated by the palmitoylation-less mutant could result from altered receptor endocytosis. In contrast, two dominant negative mutants of beta-arrestin which inhibit receptor endocytosis also attenuated vasopressin-stimulated mitogen-activated protein kinase activity, suggesting that the scaffolding protein, beta-arrestin, represents the common link among receptor palmitoylation, endocytosis, and kinase activation. Coimmunoprecipitation and bioluminescence resonance energy transfer experiments confirmed that inhibiting receptor palmitoylation considerably reduced the vasopressin-stimulated recruitment of beta-arrestin to the receptor. Interestingly, the changes in beta-arrestin recruitment kinetics were similar to those observed for vasopressin-stimulated receptor endocytosis and mitogen-activated protein kinase activation. Taken together the results indicate that palmitoylation enhances the recruitment of beta-arrestin to the activated V2 vasopressin receptor thus facilitating processes requiring the scaffolding action of beta-arrestin.

The position of cysteine relative to the transmembrane domain is critical for palmitoylation of H1, the major subunit of the human asialoglycoprotein receptor

The mammalian hepatic asialoglycoprotein receptor (ASGP-R) is an endocytic recycling receptor that mediates the internalization of desialylated glycoproteins and their delivery to lysosomes where they are degraded. The human ASGP-R is a hetero-oligomeric complex composed of two subunits designated H1 and H2. Both subunits are palmitoylated at the cytoplasmic Cys residues near their transmembrane domains (TMD). The cytoplasmic Cys(36) in H1 is located at a position that is five amino acids from the transmembrane junction. Because the sequences of subunits in all mammalian ASGP-R species are highly conserved especially at the region near the palmitoylated Cys, we sought to identify a recognition signal for the palmitoylation of H1. Various types of H1 mutants were created by site-directed or deletion mutagenesis including alteration of the amino acids surrounding Cys(36), replacing portions of the TMD with that of a different protein and partial deletion of the cytoplasmic domain as well as transposing the palmitoylated Cys to positions further away from the TMD. Mutant H1 cDNAs were transiently expressed in COS-7 cells, and the H1 proteins were analyzed after metabolic labeling with [(3)H]palmitate. The results indicate that neither the native amino acid sequence surrounding Cys(36) nor the majority of the cytoplasmic domain sequence is critical for palmitoylation. Palmitoylation was also not dependent on the native TMD of H1. In contrast, the attachment of palmitate was abolished if the Cys residue was transposed to a position that was 30 amino acids away from the transmembrane border. We conclude that the spacing of a Cys residue relative to the TMD in the primary protein sequence of H1 is the major determinant for successful palmitoylation.