Endoplasmic reticulum localization of DHHC palmitoyltransferases mediated by lysine-based sorting signals

Palmitoylation of synaptic proteins: roles in functional regulation and pathogenesis of neurodegenerative diseases

Palmitoylation is a type of lipid modification that plays an important role in various aspects of neuronal function. Over the past few decades, several studies have shown that the palmitoylation of synaptic proteins is involved in neurotransmission and synaptic functions. Palmitoyl acyltransferases (PATs), which belong to the DHHC family, are major players in the regulation of palmitoylation. Dysregulated palmitoylation of synaptic proteins and mutated/dysregulated DHHC proteins are associated with several neurodegenerative diseases, such as Alzheimer's disease (AD), Huntington's disease (HD), and Parkinson's disease (PD). In this review, we summarize the recent discoveries on the subcellular distribution of DHHC proteins and analyze their expression patterns in different brain cells. In particular, this review discusses how palmitoylation of synaptic proteins regulates synaptic vesicle exocytotic fusion and the localization, clustering, and transport of several postsynaptic receptors, as well as the role of palmitoylation of other proteins in regulating synaptic proteins. Additionally, some of the specific known associations of these factors with neurodegenerative disorders are explored, with a few suggestions for the development of therapeutic strategies. Finally, this review provides possible directions for future research to reveal detailed and specific mechanisms underlying the roles of synaptic protein palmitoylation.

Inhibition of fatty acid uptake by TGR5 prevents diabetic cardiomyopathy

Diabetic cardiomyopathy is characterized by myocardial lipid accumulation and cardiac dysfunction. Bile acid metabolism is known to play a crucial role in cardiovascular and metabolic diseases. Takeda G-protein-coupled receptor 5 (TGR5), a major bile acid receptor, has been implicated in metabolic regulation and myocardial protection. However, the precise involvement of the bile acid-TGR5 pathway in maintaining cardiometabolic homeostasis remains unclear. Here we show decreased plasma bile acid levels in both male and female participants with diabetic myocardial injury. Additionally, we observe increased myocardial lipid accumulation and cardiac dysfunction in cardiomyocyte-specific TGR5-deleted mice (both male and female) subjected to a high-fat diet and streptozotocin treatment or bred on the diabetic db/db genetic background. Further investigation reveals that TGR5 deletion enhances cardiac fatty acid uptake, resulting in lipid accumulation. Mechanistically, TGR5 deletion promotes localization of CD36 on the plasma membrane through the upregulation of CD36 palmitoylation mediated by the palmitoyl acyltransferase DHHC4. Our findings indicate that the TGR5-DHHC4 pathway regulates cardiac fatty acid uptake, which highlights the therapeutic potential of targeting TGR5 in the management of diabetic cardiomyopathy.

Mechanisms and functions of protein S-acylation

Over the past two decades, protein S-acylation (often referred to as S-palmitoylation) has emerged as an important regulator of vital signalling pathways. S-Acylation is a reversible post-translational modification that involves the attachment of a fatty acid to a protein. Maintenance of the equilibrium between protein S-acylation and deacylation has demonstrated profound effects on various cellular processes, including innate immunity, inflammation, glucose metabolism and fat metabolism, as well as on brain and heart function. This Review provides an overview of current understanding of S-acylation and deacylation enzymes, their spatiotemporal regulation by sophisticated multilayered mechanisms, and their influence on protein function, cellular processes and physiological pathways. Furthermore, we examine how disruptions in protein S-acylation are associated with a broad spectrum of diseases from cancer to autoinflammatory disorders and neurological conditions.

Protein lipidation in health and disease: molecular basis, physiological function and pathological implication

Posttranslational modifications increase the complexity and functional diversity of proteins in response to complex external stimuli and internal changes. Among these, protein lipidations which refer to lipid attachment to proteins are prominent, which primarily encompassing five types including S-palmitoylation, N-myristoylation, S-prenylation, glycosylphosphatidylinositol (GPI) anchor and cholesterylation. Lipid attachment to proteins plays an essential role in the regulation of protein trafficking, localisation, stability, conformation, interactions and signal transduction by enhancing hydrophobicity. Accumulating evidence from genetic, structural, and biomedical studies has consistently shown that protein lipidation is pivotal in the regulation of broad physiological functions and is inextricably linked to a variety of diseases. Decades of dedicated research have driven the development of a wide range of drugs targeting protein lipidation, and several agents have been developed and tested in preclinical and clinical studies, some of which, such as asciminib and lonafarnib are FDA-approved for therapeutic use, indicating that targeting protein lipidations represents a promising therapeutic strategy. Here, we comprehensively review the known regulatory enzymes and catalytic mechanisms of various protein lipidation types, outline the impact of protein lipidations on physiology and disease, and highlight potential therapeutic targets and clinical research progress, aiming to provide a comprehensive reference for future protein lipidation research.

S-acylation: an orchestrator of the life cycle and function of membrane proteins

S-acylation is a covalent post-translational modification of proteins with fatty acids, achieved by enzymatic attachment via a labile thioester bond. This modification allows for dynamic control of protein properties and functions in association with cell membranes. This lipid modification regulates a substantial portion of the human proteome and plays an increasingly recognized role throughout the lifespan of affected proteins. Recent technical advancements have propelled the S-acylation field into a 'molecular era', unveiling new insights into its mechanistic intricacies and far-reaching implications. With a striking increase in the number of studies on this modification, new concepts are indeed emerging on the roles of S-acylation in specific cell biology processes and features. After a brief overview of the enzymes involved in S-acylation, this viewpoint focuses on the importance of S-acylation in the homeostasis, function, and coordination of integral membrane proteins. In particular, we put forward the hypotheses that S-acylation is a gatekeeper of membrane protein folding and turnover and a regulator of the formation and dynamics of membrane contact sites.

CPT1A induction following epigenetic perturbation promotes MAVS palmitoylation and activation to potentiate antitumor immunity

Targeting epigenetic regulators to potentiate anti-PD-1 immunotherapy converges on the activation of type I interferon (IFN-I) response, mimicking cellular response to viral infection, but how its strength and duration are regulated to impact combination therapy efficacy remains largely unknown. Here, we show that mitochondrial CPT1A downregulation following viral infection restrains, while its induction by epigenetic perturbations sustains, a double-stranded RNA-activated IFN-I response. Mechanistically, CPT1A recruits the endoplasmic reticulum-localized ZDHHC4 to catalyze MAVS Cys79-palmitoylation, which promotes MAVS stabilization and activation by inhibiting K48- but facilitating K63-linked ubiquitination. Further elevation of CPT1A incrementally increases MAVS palmitoylation and amplifies the IFN-I response, which enhances control of viral infection and epigenetic perturbation-induced antitumor immunity. Moreover, CPT1A chemical inducers augment the therapeutic effect of combined epigenetic treatment with PD-1 blockade in refractory tumors. Our study identifies CPT1A as a stabilizer of MAVS activation, and its link to epigenetic perturbation can be exploited for cancer immunotherapy.

Palmitoylation of the hemagglutinin of influenza B virus by ER-localized DHHC enzymes 1, 2, 4, and 6 is required for efficient virus replication

IMPORTANCE

Influenza viruses are a public health concern since they cause seasonal outbreaks and occasionally pandemics. Our study investigates the importance of a protein modification called "palmitoylation" in the replication of influenza B virus. Palmitoylation involves attaching fatty acids to the viral protein hemagglutinin and has previously been studied for influenza A virus. We found that this modification is important for the influenza B virus to replicate, as mutating the sites where palmitate is attached prevented the virus from generating viable particles. Our experiments also showed that this modification occurs in the endoplasmic reticulum. We identified the specific enzymes responsible for this modification, which are different from those involved in palmitoylation of HA of influenza A virus. Overall, our research illuminates the similarities and differences in fatty acid attachment to HA of influenza A and B viruses and identifies the responsible enzymes, which might be promising targets for anti-viral therapy.

Diverse Roles of Protein Palmitoylation in Cancer Progression, Immunity, Stemness, and Beyond

Protein S-palmitoylation, a type of post-translational modification, refers to the reversible process of attachment of a fatty acyl chain-a 16-carbon palmitate acid-to the specific cysteine residues on target proteins. By adding the lipid chain to proteins, it increases the hydrophobicity of proteins and modulates protein stability, interaction with effector proteins, subcellular localization, and membrane trafficking. Palmitoylation is catalyzed by a group of zinc finger DHHC-containing proteins (ZDHHCs), whereas depalmitoylation is catalyzed by a family of acyl-protein thioesterases. Increasing numbers of oncoproteins and tumor suppressors have been identified to be palmitoylated, and palmitoylation is essential for their functions. Understanding how palmitoylation influences the function of individual proteins, the physiological roles of palmitoylation, and how dysregulated palmitoylation leads to pathological consequences are important drivers of current research in this research field. Further, due to the critical roles in modifying functions of oncoproteins and tumor suppressors, targeting palmitoylation has been used as a candidate therapeutic strategy for cancer treatment. Here, based on recent literatures, we discuss the progress of investigating roles of palmitoylation in regulating cancer progression, immune responses against cancer, and cancer stem cell properties.

Palmitoylation in apoptosis

Palmitoylation, a critical lipid modification of proteins, is involved in various physiological processes such as altering protein localization, transport, and stability, which perform essential roles in protein function. Palmitoyltransferases are specific enzymes involved in the palmitoylation modification of substrates. S-palmitoylation, as the only reversible palmitoylation modification, is able to be deacylated by deacyltransferases. As an important mode of programmed cell death, apoptosis functions in the maintenance of organismal homeostasis as well as being associated with inflammatory and immune diseases. Recently, studies have found that palmitoylation and apoptosis have been demonstrated to be related in many human diseases. In this review, we will focus on the role of palmitoylation modifications in apoptosis.

Astrocyte Elevated Gene-1 Cys75 S-Palmitoylation by ZDHHC6 Regulates Its Biological Activity

Nonalcoholic fatty liver disease is a major risk factor for hepatocellular carcinoma (HCC). Astrocyte elevated gene-1/Metadherin (AEG-1/MTDH) augments lipid accumulation (steatosis), inflammation, and tumorigenesis, thereby promoting the whole spectrum of this disease process. Targeting AEG-1 is a potential interventional strategy for nonalcoholic steatohepatitis (NASH) and HCC. Thus, proper understanding of the regulation of this molecule is essential. We found that AEG-1 is palmitoylated at residue cysteine 75 (Cys75). Mutation of Cys75 to serine (Ser) completely abolished AEG-1 palmitoylation. We identified ZDHHC6 as a palmitoyltransferase catalyzing the process in HEK293T cells. To obtain insight into how palmitoylation regulates AEG-1 function, we generated knock-in mice by CRISPR/Cas9 in which Cys75 of AEG-1 was mutated to Ser (AEG-1-C75S). No developmental or anatomical abnormality was observed between AEG-1-wild type (AEG-1-WT) and AEG-1-C75S littermates. However, global gene expression analysis by RNA-sequencing unraveled that signaling pathways and upstream regulators, which contribute to cell proliferation, motility, inflammation, angiogenesis, and lipid accumulation, were activated in AEG-1-C75S hepatocytes compared to AEG-1-WT. These findings suggest that AEG-1-C75S functions as dominant positive and that palmitoylation restricts oncogenic and NASH-promoting functions of AEG-1. We thus identify a previously unknown regulatory mechanism of AEG-1, which might help design new therapeutic strategies for NASH and HCC.

Regulation of T cell function by protein S-acylation

S-acylation, the reversible lipidation of free cysteine residues with long-chain fatty acids, is a highly dynamic post-translational protein modification that has recently emerged as an important regulator of the T cell function. The reversible nature of S-acylation sets this modification apart from other forms of protein lipidation and allows it to play a unique role in intracellular signal transduction. In recent years, a significant number of T cell proteins, including receptors, enzymes, ion channels, and adaptor proteins, were identified as S-acylated. It has been shown that S-acylation critically contributes to their function by regulating protein localization, stability and protein-protein interactions. Furthermore, it has been demonstrated that zDHHC protein acyltransferases, the family of enzymes mediating this modification, also play a prominent role in T cell activation and differentiation. In this review, we aim to highlight the diversity of proteins undergoing S-acylation in T cells, elucidate the mechanisms by which reversible lipidation can impact protein function, and introduce protein acyltransferases as a novel class of regulatory T cell proteins.

Structural Exploration on Palmitoyltransferase DHHC3 from Homo sapiens

DHHC3 belongs to a family of DHHC palmitoyltransferase, which catalyzes the S-palmitoylation of target proteins by attaching a fatty acyl group to a cysteine. Recently, DHHC3 has been demonstrated to be a promising antitumor target in cancer therapeutics. However, the detailed structure and catalysis mechanism of DHHC3 remain elusive, considering its sequence diversity from the DHHC homologues with known crystal structures. Here, we described the expression and purification of human DHHC3 (hDHHC3) and truncated hDHHC3 with the flexible N-terminal domain (NTD) removed. Purified hDHHC3 proteins were used under various conditions for protein crystallization. LAMTOR1, one of the interacting proteins of hDHHC3 to facilitate the crystallization, was further identified by mass spectrometry and co-immunoprecipitation assay. The structural exploration using cryogenic electronic microscopy (cryo-EM) on the inactive hDHHS3 mutant showed a typical sideview of membrane proteins. These results provide a preliminary guidance for the structural determination of DHHC3.

Redirecting Chemotherapeutics to the Endoplasmic Reticulum Increases Tumor Immunogenicity and Potentiates Anti-PD-L1 Therapy

The endoplasmic reticulum (ER) in cancer cells has been considered as a pharmacological target. Still, the effects of a ER-targeted system remain less investigated, due to the fact that most chemo-drugs take actions in the nucleus. Here, it is demonstrated that ER-targeted delivery of doxorubicin (DOX), a typically nucleus-tropic-and-acting agent, attenuates its original effect on cytotoxicity while generating new functions favorable for immune activation. First, a library of DOX derivatives with variable ER-targeting abilities is synthesized. The results reveal that higher ER-targeting efficiency correlates with greater ER stress. As compared with naïve drug, ER-targeted DOX considerably alters the mode of action from nuclear DNA damage-associated cytotoxicity to ER stress-mediated calreticulin exposure. Consequently, ER-targeted DOX decreases cytotoxicity but increases the capability to induce immunogenic cell death (ICD). Therefore, a platform combining naïve and ER-targeted DOX is constructed for in vivo application. Conventional polymer-DOX conjugate inhibits tumor growth by exerting a direct killing effect, and ER-targeted polymer-DOX conjugate suppresses residual tumors by eliciting ICD-associated immunity, together resulting in considerable tumor regression. In addition, simultaneous inhibition of adaptive PD-L1 enrichment (due to negative-feedback to ICD induction) further leads to greater therapeutic outcome. Collectively, ER-targeted therapy can enhance anticancer efficacy by promoting ICD-associated immunotherapy, and potentiating chemotherapy and checkpoint blockade therapy.

Golgi Apparatus Regulates Plasma Membrane Composition and Function

Golgi apparatus is the central component of the mammalian secretory pathway and it regulates the biosynthesis of the plasma membrane through three distinct but interacting processes: (a) processing of protein and lipid cargoes; (b) creation of a sharp transition in membrane lipid composition by non-vesicular transport of lipids; and (c) vesicular sorting of proteins and lipids at the trans-Golgi network to target them to appropriate compartments. We discuss the molecules involved in these processes and their importance in physiology and development. We also discuss how mutations in these molecules affect plasma membrane composition and signaling leading to genetic diseases and cancer.

Depalmitoylation rewires FLT3-ITD signaling and exacerbates leukemia progression

Internal tandem duplication within FLT3 (FLT3-ITD) is one of the most frequent mutations in acute myeloid leukemia (AML) and correlates with a poor prognosis. Whereas the FLT3 receptor tyrosine kinase is activated at the plasma membrane to transduce PI3K/AKT and RAS/MAPK signaling, FLT3-ITD resides in the endoplasmic reticulum and triggers constitutive STAT5 phosphorylation. Mechanisms underlying this aberrant FLT3-ITD subcellular localization or its impact on leukemogenesis remain poorly established. In this study, we discovered that FLT3-ITD is S-palmitoylated by the palmitoyl acyltransferase ZDHHC6. Disruption of palmitoylation redirected FLT3-ITD to the plasma membrane and rewired its downstream signaling by activating AKT and extracellular signal-regulated kinase pathways in addition to STAT5. Consequently, abrogation of palmitoylation increased FLT3-ITD-mediated progression of leukemia in xenotransplant-recipient mouse models. We further demonstrate that FLT3 proteins were palmitoylated in primary human AML cells. ZDHHC6-mediated palmitoylation restrained FLT3-ITD surface expression, signaling, and colonogenic growth of primary FLT3-ITD+ AML. More important, pharmacological inhibition of FLT3-ITD depalmitoylation synergized with the US Food and Drug Administration-approved FLT3 kinase inhibitor gilteritinib in abrogating the growth of primary FLT3-ITD+ AML cells. These findings provide novel insights into lipid-dependent compartmentalization of FLT3-ITD signaling in AML and suggest targeting depalmitoylation as a new therapeutic strategy to treat FLT3-ITD+ leukemias.

ZDHHC11 Positively Regulates NF-κB Activation by Enhancing TRAF6 Oligomerization

Tumor necrosis factor receptor-associated factor 6 (TRAF6) is a RING domain ubiquitin ligase that plays an important role in nuclear factor-κB (NF-κB) signaling by regulating activation of the TAK1 and IKK complexes. However, the molecular mechanisms that regulate TRAF6 E3 activity remain unclear. Here, we found that ZDHHC11, a member of the DHHC palmitoyl transferase family, functions as a positive modulator in NF-κB signaling. ZDHHC11 overexpression activated NF-κB, whereas ZDHHC11 deficiency impaired NF-κB activity stimulated by IL-1β, LPS, and DNA virus infection. Furthermore, Zdhhc11 knockout mice had a lower level of serum IL6 upon treatment with LPS and D-galactosamine or HSV-1 infection than control mice. Mechanistically, ZDHHC11 interacted with TRAF6 and then enhanced TRAF6 oligomerization, which increased E3 activity of TRAF6 for synthesis of K63-linked ubiquitination chains. Collectively, our study indicates that ZDHHC11 positively regulates NF-κB signaling by promoting TRAF6 oligomerization and ligase activity, subsequently activating TAK1 and IKK complexes.

ZDHHC19 Is Dispensable for Spermatogenesis, but Is Essential for Sperm Functions in Mice

Spermatogenesis is a complicated process involving mitotically proliferating spermatogonial cells, meiotically dividing spermatocytes, and spermatid going through maturation into spermatozoa. The post-translational modifications of proteins play important roles in this biological process. S-palmitoylation is one type of protein modifications catalyzed by zinc finger Asp-His-His-Cys (ZDHHC)-family palmitoyl S-acyltransferases. There are 23 mammalian ZDHHCs that have been identified in mouse. Among them, Zdhhc19 is highly expressed in adult testis. However, the in vivo function of Zdhhc19 in mouse spermatogenesis and fertility remains unknown. In this study, we knocked out the Zdhhc19 gene by generating a 2609 bp deletion from exon 3 to exon 6 in mice. No differences were found in testis morphology and testis/body weight ratios upon Zdhhc19 deletion. Spermatogenesis was not disrupted in Zdhhc19 knockout mice, in which properly developed TRA98+ germ cells, SYCP3+ spermatocytes, and TNP1+ spermatids/spermatozoa were detected in seminiferous tubules. Nevertheless, Zdhhc19 knockout mice were male infertile. Zdhhc19 deficient spermatozoa exhibited multiple defects including abnormal morphology of sperm tails and heads, decreased motility, and disturbed acrosome reaction. All of these led to the inability of Zdhhc19 mutant sperm to fertilize oocytes in IVF assays. Taken together, our results support the fact that Zdhhc19 is a testis enriched gene dispensable for spermatogenesis, but is essential for sperm functions in mice.

Post-Translational Modification and Subcellular Compartmentalization: Emerging Concepts on the Regulation and Physiopathological Relevance of RhoGTPases

Cells and tissues are continuously exposed to both chemical and physical stimuli and dynamically adapt and respond to this variety of external cues to ensure cellular homeostasis, regulated development and tissue-specific differentiation. Alterations of these pathways promote disease progression-a prominent example being cancer. Rho GTPases are key regulators of the remodeling of cytoskeleton and cell membranes and their coordination and integration with different biological processes, including cell polarization and motility, as well as other signaling networks such as growth signaling and proliferation. Apart from the control of GTP-GDP cycling, Rho GTPase activity is spatially and temporally regulated by post-translation modifications (PTMs) and their assembly onto specific protein complexes, which determine their controlled activity at distinct cellular compartments. Although Rho GTPases were traditionally conceived as targeted from the cytosol to the plasma membrane to exert their activity, recent research demonstrates that active pools of different Rho GTPases also localize to endomembranes and the nucleus. In this review, we discuss how PTM-driven modulation of Rho GTPases provides a versatile mechanism for their compartmentalization and functional regulation. Understanding how the subcellular sorting of active small GTPase pools occurs and what its functional significance is could reveal novel therapeutic opportunities.

Regulation of Dynamic Protein S-Acylation

Protein S-acylation is the reversible addition of fatty acids to the cysteine residues of target proteins. It regulates multiple aspects of protein function, including the localization to membranes, intracellular trafficking, protein interactions, protein stability, and protein conformation. This process is regulated by palmitoyl acyltransferases that have the conserved amino acid sequence DHHC at their active site. Although they have conserved catalytic cores, DHHC enzymes vary in their protein substrate selection, lipid substrate preference, and regulatory mechanisms. Alterations in DHHC enzyme function are associated with many human diseases, including cancers and neurological conditions. The removal of fatty acids from acylated cysteine residues is catalyzed by acyl protein thioesterases. Notably, S-acylation is now known to be a highly dynamic process, and plays crucial roles in signaling transduction in various cell types. In this review, we will explore the recent findings on protein S-acylation, the enzymatic regulation of this process, and discuss examples of dynamic S-acylation.

Palmitoylation of the envelope membrane proteins GP5 and M of porcine reproductive and respiratory syndrome virus is essential for virus growth

Porcine reproductive and respiratory syndrome virus (PRRSV), an enveloped positive-strand RNA virus in the Arteiviridae family, is a major pathogen affecting pigs worldwide. The membrane (glyco)proteins GP5 and M form a disulfide-linked dimer, which is a major component of virions. GP5/M are required for virus budding, which occurs at membranes of the exocytic pathway. Both GP5 and M feature a short ectodomain, three transmembrane regions, and a long cytoplasmic tail, which contains three and two conserved cysteines, respectively, in close proximity to the transmembrane span. We report here that GP5 and M of PRRSV-1 and -2 strains are palmitoylated at the cysteines, regardless of whether the proteins are expressed individually or in PRRSV-infected cells. To completely prevent S-acylation, all cysteines in GP5 and M have to be exchanged. If individual cysteines in GP5 or M were substituted, palmitoylation was reduced, and some cysteines proved more important for efficient palmitoylation than others. Neither infectious virus nor genome-containing particles could be rescued if all three cysteines present in GP5 or both present in M were replaced in a PRRSV-2 strain, indicating that acylation is essential for virus growth. Viruses lacking one or two acylation sites in M or GP5 could be rescued but grew to significantly lower titers. GP5 and M lacking acylation sites form dimers and GP5 acquires Endo-H resistant carbohydrates in the Golgi apparatus suggesting that trafficking of the membrane proteins to budding sites is not disturbed. Likewise, GP5 lacking two acylation sites is efficiently incorporated into virus particles and these viruses exhibit no reduction in cell entry. We speculate that multiple fatty acids attached to GP5 and M in the endoplasmic reticulum are required for clustering of GP5/M dimers at Golgi membranes and constitute an essential prerequisite for virus assembly.

Porcine reproductive and respiratory syndrome virus (PRRSV), an arterivirus in the order Nidovirales, is an important pathogen for pigs. Despite its importance in veterinary medicine, basic structural and functional features of its membrane proteins have not been elucidated. Here, we provide evidence for palmitoylation of the PRRSV major membrane proteins GP5 and M at a cluster of membrane-near cysteines. Fatty acid attachment is required for virus growth, since removal of all acylation sites from either M or GP5 prevents recue of infectious particles. Furthermore, viruses lacking individual acylation sites in M and GP5 grow to significantly lower titers in cell culture. The specific infectivity and cell entry of viruses lacking two acylation sites in Gp5 is, however, not reduced. Likewise, these viruses revealed no effect on dimerization of GP5 with M, its transport to budding sites, and incorporation into virus particles. Since cells transfected with a cDNA expressing non-acylated GP5, or non-acylated M release no virus-like particles into the supernatant we propose that the fatty acids are required for the budding process. They might trigger assembly of GP5/M dimers to form a coat inside the lipid bilayer that induces membrane curvature.

Substrate recruitment by zDHHC protein acyltransferases

Protein palmitoylation is the post-translational attachment of fatty acids, most commonly palmitate (C16 : 0), onto a cysteine residue of a protein. This reaction is catalysed by a family of integral membrane proteins, the zDHHC protein acyltransferases (PATs), so-called due to the presence of an invariant Asp-His-His-Cys (DHHC) cysteine-rich domain harbouring the catalytic centre of the enzyme. Conserved throughout eukaryotes, the zDHHC PATs are encoded by multigene families and mediate palmitoylation of thousands of protein substrates. In humans, a number of zDHHC proteins are associated with human diseases, including intellectual disability, Huntington's disease, schizophrenia and cancer. Key to understanding the physiological and pathophysiological importance of individual zDHHC proteins is the identification of their protein substrates. Here, we will describe the approaches and challenges in assigning substrates for individual zDHHCs, highlighting key mechanisms that underlie substrate recruitment.

Insights into the molecular basis of the palmitoylation and depalmitoylation of NCX1

Catalyzed by zDHHC-PAT enzymes and reversed by thioesterases, protein palmitoylation is the only post-translational modification recognized to regulate the sodium/calcium exchanger NCX1. NCX1 palmitoylation occurs at a single site at position 739 in its large regulatory intracellular loop. An amphipathic ɑ-helix between residues 740-756 is a critical for NCX1 palmitoylation. Given the rich background of the structural elements involving in NCX1 palmitoylation, the molecular basis of NCX1 palmitoylation is still relatively poorly understood. Here we found that (1) the identity of palmitoylation machinery of NCX1 controls its spatial organization within the cell, (2) the NCX1 amphipathic ɑ-helix directly interacts with zDHHC-PATs, (3) NCX1 is still palmitoylated when it is arrested in either Golgi or ER, indicating that NCX1 is a substrate for multiple zDHHC-PATs, (4) the thioesterase APT1 but not APT2 as a part of NCX1-depalmitoylation machinery governs subcellular organization of NCX1, (5) APT1 catalyzes NCX1 depalmitoylation in the Golgi but not in the ER. We also report that NCX2 and NCX3 are dually palmitoylated, with important implications for substrate recognition and enzyme catalysis by zDHHC-PATs. Our results could support new molecular or pharmacological strategies targeting the NCX1 palmitoylation and depalmitoylation machinery.

Roles of palmitoylation in structural long-term synaptic plasticity

Long-term potentiation (LTP) and long-term depression (LTD) are important cellular mechanisms underlying learning and memory processes. N-Methyl-d-aspartate receptor (NMDAR)-dependent LTP and LTD play especially crucial roles in these functions, and their expression depends on changes in the number and single channel conductance of the major ionotropic glutamate receptor α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) located on the postsynaptic membrane. Structural changes in dendritic spines comprise the morphological platform and support for molecular changes in the execution of synaptic plasticity and memory storage. At the molecular level, spine morphology is directly determined by actin cytoskeleton organization within the spine and indirectly stabilized and consolidated by scaffold proteins at the spine head. Palmitoylation, as a uniquely reversible lipid modification with the ability to regulate protein membrane localization and trafficking, plays significant roles in the structural and functional regulation of LTP and LTD. Altered structural plasticity of dendritic spines is also considered a hallmark of neurodevelopmental disorders, while genetic evidence strongly links abnormal brain function to impaired palmitoylation. Numerous studies have indicated that palmitoylation contributes to morphological spine modifications. In this review, we have gathered data showing that the regulatory proteins that modulate the actin network and scaffold proteins related to AMPAR-mediated neurotransmission also undergo palmitoylation and play roles in modifying spine architecture during structural plasticity.

The palmitoyl acyltransferases ZDHHC5 and ZDHHC8 are uniquely present in DRG axons and control retrograde signaling via the Gp130/JAK/STAT3 pathway

Palmitoylation, the modification of proteins with the lipid palmitate, is a key regulator of protein targeting and trafficking. However, knowledge of the roles of specific palmitoyl acyltransferases (PATs), which catalyze palmitoylation, is incomplete. For example, little is known about which PATs are present in neuronal axons, although long-distance trafficking of palmitoyl-proteins is important for axon integrity and for axon-to-soma retrograde signaling, a process critical for axon development and for responses to injury. Identifying axonally targeted PATs might thus provide insights into multiple aspects of axonal biology. We therefore comprehensively determined the subcellular distribution of mammalian PATs in dorsal root ganglion (DRG) neurons and, strikingly, found that only two PATs, ZDHHC5 and ZDHHC8, were enriched in DRG axons. Signals via the Gp130/JAK/STAT3 and DLK/JNK pathways are important for axonal injury responses, and we found that ZDHHC5 and ZDHHC8 were required for Gp130/JAK/STAT3, but not DLK/JNK, axon-to-soma signaling. ZDHHC5 and ZDHHC8 robustly palmitoylated Gp130 in cotransfected nonneuronal cells, supporting the possibility that Gp130 is a direct ZDHHC5/8 substrate. In DRG neurons, Zdhhc5/8 shRNA knockdown reduced Gp130 palmitoylation and even more markedly reduced Gp130 surface expression, potentially explaining the importance of these PATs for Gp130-dependent signaling. Together, these findings provide new insights into the subcellular distribution and roles of specific PATs and reveal a novel mechanism by which palmitoylation controls axonal retrograde signaling.

Regulation of T cell receptor signaling by protein acyltransferase DHHC21

S-acylation reversible-post-translational lipidation of cysteine residues-is emerging as an important regulatory mechanism in T cell signaling. Dynamic S-acylation is critical for protein recruitment into the T cell receptor complex and initiation of the subsequent signaling cascade. However, the enzymatic control of protein S-acylation in T cells remains poorly understood. Here, we report a previously uncharacterized role of DHHC21, a member of the mammalian family of DHHC protein acyltransferases, in regulation of the T cell receptor pathway. We found that loss of DHHC21 prevented S-acylation of key T cell signaling proteins, resulting in disruption of the early signaling events and suppressed expression of T cell activation markers. Furthermore, downregulation of DHHC21 prevented activation and differentiation of naïve T cells into effector subtypes. Together, our study provides the first direct evidence that DHHC protein acyltransferases can play an essential role in regulation of T cell-mediated immunity.

Spatial organization of palmitoyl acyl transferases governs substrate localization and function

Protein palmitoylation is a critical posttranslational modification that regulates protein trafficking, localization, stability, sorting and function. In mammals, addition of this lipid modification onto proteins is mediated by a family of 23 palmitoyl acyl transferases (PATs). PATs often palmitoylate substrates in a promiscuous manner, precluding our understanding of how these enzymes achieve specificity for their substrates. Despite generous efforts to identify consensus motifs defining PAT-substrate specificity, it remains to be determined whether additional factors beyond interaction motifs, such as local palmitoylation, participate in PAT-substrate selection. In this review, we emphasize the role of local palmitoylation, in which substrates are palmitoylated and trapped in the same subcellular compartments as their PATs, as a mechanism of enzyme-substrate specificity. We focus here on non-Golgi-localized PATs, as physical proximity to their substrates enables them to engage in local palmitoylation, compared to Golgi PATs, which often direct trafficking of their substrates elsewhere. PAT subcellular localization may be an under-recognized, yet important determinant of PAT-substrate specificity that may work in conjunction or completely independently of interaction motifs. We also discuss some current hypotheses about protein motifs that contribute to localization of non-Golgi-localized PATs, important for the downstream targeting of their substrates.

S-Palmitoylation of the sodium channel Nav1.6 regulates its activity and neuronal excitability

S-Palmitoylation is a reversible post-translational lipid modification that dynamically regulates protein functions. Voltage-gated sodium channels are subjected to S-palmitoylation and exhibit altered functions in different S-palmitoylation states. Our aim was to investigate whether and how S-palmitoylation regulates Nav1.6 channel function and to identify S-palmitoylation sites that can potentially be pharmacologically targeted. Acyl-biotin exchange assay showed that Nav1.6 is modified by S-palmitoylation in the mouse brain and in a Nav1.6 stable HEK 293 cell line. Using whole-cell voltage clamp, we discovered that enhancing S-palmitoylation with palmitic acid increases Nav1.6 current, whereas blocking S-palmitoylation with 2-bromopalmitate reduces Nav1.6 current and shifts the steady-state inactivation in the hyperpolarizing direction. Three S-palmitoylation sites (Cys¹¹⁶⁹, Cys¹¹⁷⁰, and Cys¹⁹⁷⁸) were identified. These sites differentially modulate distinct Nav1.6 properties. Interestingly, Cys¹⁹⁷⁸ is exclusive to Nav1.6 among all Nav isoforms and is evolutionally conserved in Nav1.6 among most species. Cys¹⁹⁷⁸ S-palmitoylation regulates current amplitude uniquely in Nav1.6. Furthermore, we showed that eliminating S-palmitoylation at specific sites alters Nav1.6-mediated excitability in dorsal root ganglion neurons. Therefore, our study reveals S-palmitoylation as a potential isoform-specific mechanism to modulate Nav activity and neuronal excitability in physiological and diseased conditions.

PtdIns4P-mediated electrostatic forces influence S-acylation of peripheral proteins at the Golgi complex

Protein S-acylation is a reversible post-translational modification involving the addition of fatty acids to cysteines and is catalyzed by transmembrane protein acyltransferases (PATs) mainly expressed at the Golgi complex. In case of soluble proteins, S-acylation confers stable membrane attachment. Myristoylation or farnesylation of many soluble proteins constitutes the initial transient membrane adsorption step prior to S-acylation. However, some S-acylated soluble proteins, such as the neuronal growth-associated protein Growth-associated protein-43 (GAP-43), lack the hydrophobic modifications required for this initial membrane interaction. The signals for GAP-43 S-acylation are confined to the first 13 amino acids, including the S-acylatable cysteines 3 and 4 embedded in a hydrophobic region, followed by a cluster of basic amino acids. We found that mutation of critical basic amino acids drastically reduced membrane interaction and hence S-acylation of GAP-43. Interestingly, acute depletion of phosphatidylinositol 4-phosphate (PtdIns4P) at the Golgi complex reduced GAP-43 membrane binding, highlighting a new, pivotal role for this anionic lipid and supporting the idea that basic amino acid residues are involved in the electrostatic interactions between GAP-43 and membranes of the Golgi complex where they are S-acylated.

Toward the identification of ZDHHC enzymes required for palmitoylation of viral protein as potential drug targets

Introduction: S-acylation is the attachment of fatty acids not only to cysteines of cellular, but also of viral proteins. The modification is often crucial for the protein´s function and hence for virus replication. Transfer of fatty acids is mediated by one or several of the 23 members of the ZDHHC family of proteins. Since their genes are linked to various human diseases, they represent drug targets.Areas covered: The authors explore whether targeting acylation of viral proteins might be a strategy to combat viral diseases. Many human pathogens contain S-acylated proteins; the ZDHHCs involved in their acylation are currently identified. Based on the 3D structure of two ZDHHCs, the regulation and the biochemistry of the palmitolyation reaction and the lipid and protein substrate specificities are discussed. The authors then speculate how ZDHHCs might recognize S-acylated membrane proteins of Influenza virus.Expert opinion: Although many viral diseases can now be treated, the available drugs bind to viral proteins that rapidly mutate and become resistant. To develop inhibitors for the genetically more stable cellular ZDHHCs, their binding sites for viral substrates need to be identified. If only a few cellular proteins are recognized by the same binding site, development of specific inhibitors may have therapeutic potential.

Computational Nanoscopy of Tight Junctions at the Blood-Brain Barrier Interface

The selectivity of the blood-brain barrier (BBB) is primarily maintained by tight junctions (TJs), which act as gatekeepers of the paracellular space by blocking blood-borne toxins, drugs, and pathogens from entering the brain. The BBB presents a significant challenge in designing neurotherapeutics, so a comprehensive understanding of the TJ architecture can aid in the design of novel therapeutics. Unraveling the intricacies of TJs with conventional experimental techniques alone is challenging, but recently developed computational tools can provide a valuable molecular-level understanding of TJ architecture. We employed the computational methods toolkit to investigate claudin-5, a highly expressed TJ protein at the BBB interface. Our approach started with the prediction of claudin-5 structure, evaluation of stable dimer conformations and nanoscale assemblies, followed by the impact of lipid environments, and posttranslational modifications on these claudin-5 assemblies. These led to the study of TJ pores and barriers and finally understanding of ion and small molecule transport through the TJs. Some of these in silico, molecular-level findings, will need to be corroborated by future experiments. The resulting understanding can be advantageous towards the eventual goal of drug delivery across the BBB. This review provides key insights gleaned from a series of state-of-the-art nanoscale simulations (or computational nanoscopy studies) performed on the TJ architecture.

S-acylated Golga7b stabilises DHHC5 at the plasma membrane to regulate cell adhesion

S-acylation (palmitoylation) is the only fully reversible lipid modification of proteins; however, little is known about how protein S-acyltransferases (PATs) that mediate it are regulated. DHHC5 is a PAT that is mainly localised at the plasma membrane with roles in synaptic plasticity, massive endocytosis and cancer cell growth/invasion. Here, we demonstrate that DHHC5 binds to and palmitoylates a novel accessory protein Golga7b. Palmitoylation of Golga7b prevents clathrin-mediated endocytosis of DHHC5 and stabilises it at the plasma membrane. Proteomic analysis of the composition of DHHC5/Golga7b-associated protein complexes reveals a striking enrichment in adhesion proteins, particularly components of desmosomes. We show that desmoglein-2 and plakophilin-3 are substrates of DHHC5 and that DHHC5 and Golga7b are required for localisation of desmoglein-2 to the plasma membrane and for desmosomal patterning. Loss of DHHC5/Golga7b causes functional impairments in cell adhesion, suggesting these proteins have a wider role in cell adhesion beyond desmosome assembly. This work uncovers a novel mechanism of DHHC5 regulation by Golga7b and demonstrates a role for the DHHC5/Golga7b complex in the regulation of cell adhesion.

Toll-like receptor mediated inflammation requires FASN-dependent MYD88 palmitoylation

Toll-like receptor (TLR)/myeloid differentiation primary response protein (MYD88) signaling aggravates sepsis by impairing neutrophil migration to infection sites. However, the role of intracellular fatty acids in TLR/MYD88 signaling is unclear. Here, inhibition of fatty acid synthase by C75 improved neutrophil chemotaxis and increased the survival of mice with sepsis in cecal ligation puncture and lipopolysaccharide-induced septic shock models. C75 specifically blocked TLR/MYD88 signaling in neutrophils. Treatment with GSK2194069 that targets a different domain of fatty acid synthase, did not block TLR signaling or MYD88 palmitoylation. De novo fatty acid synthesis and CD36-mediated exogenous fatty acid incorporation contributed to MYD88 palmitoylation. The binding of IRAK4 to the MYD88 intermediate domain and downstream signal activation required MYD88 palmitoylation at cysteine 113. MYD88 was palmitoylated by ZDHHC6, and ZDHHC6 knockdown decreased MYD88 palmitoylation and TLR/MYD88 activation upon lipopolysaccharide stimulus. Thus, intracellular saturated fatty acid-dependent palmitoylation of MYD88 by ZDHHC6 is a therapeutic target of sepsis.

Functional interactions between herpes simplex virus pUL51, pUL7 and gE reveal cell-specific mechanisms for epithelial cell-to-cell spread

Herpes simplex virus spread between epithelial cells is mediated by virus tegument and envelope protein complexes including gE/gI and pUL51/pUL7. pUL51 interacts with both pUL7 and gE/gI in infected cells. We show that amino acids 30-90 of pUL51 mediate interaction with pUL7. We also show that deletion of amino acids 167-244 of pUL51, or ablation of pUL7 expression both result in failure of gE to concentrate at junctional surfaces of Vero cells. We also tested the hypothesis that gE and pUL51 function on the same pathway for cell-to-cell spread by analyzing the phenotype of a double gE/UL51 mutant. In HaCaT cells, pUL51 and gE function on the same spread pathway, whereas in Vero cells they function on different pathways. Deletion of the gE gene strongly enhanced virus release to the medium in Vero cells, suggesting that the gE-dependent spread pathway may compete with virion release to the medium.

Protein palmitoylation and cancer

Protein S-palmitoylation is a reversible post-translational modification that alters the localization, stability, and function of hundreds of proteins in the cell. S-palmitoylation is essential for the function of both oncogenes (e.g., NRAS and EGFR) and tumor suppressors (e.g., SCRIB, melanocortin 1 receptor). In mammalian cells, the thioesterification of palmitate to internal cysteine residues is catalyzed by 23 Asp-His-His-Cys (DHHC)-family palmitoyl S-acyltransferases while the removal of palmitate is catalyzed by serine hydrolases, including acyl-protein thioesterases (APTs). These enzymes modulate the function of important oncogenes and tumor suppressors and often display altered expression patterns in cancer. Targeting S-palmitoylation or the enzymes responsible for palmitoylation dynamics may therefore represent a candidate therapeutic strategy for certain cancers.

Protein Lipidation: Occurrence, Mechanisms, Biological Functions, and Enabling Technologies

Protein lipidation, including cysteine prenylation, N-terminal glycine myristoylation, cysteine palmitoylation, and serine and lysine fatty acylation, occurs in many proteins in eukaryotic cells and regulates numerous biological pathways, such as membrane trafficking, protein secretion, signal transduction, and apoptosis. We provide a comprehensive review of protein lipidation, including descriptions of proteins known to be modified and the functions of the modifications, the enzymes that control them, and the tools and technologies developed to study them. We also highlight key questions about protein lipidation that remain to be answered, the challenges associated with answering such questions, and possible solutions to overcome these challenges.

Identification and dynamics of the human ZDHHC16-ZDHHC6 palmitoylation cascade

S-Palmitoylation is the only reversible post-translational lipid modification. Knowledge about the DHHC palmitoyltransferase family is still limited. Here we show that human ZDHHC6, which modifies key proteins of the endoplasmic reticulum, is controlled by an upstream palmitoyltransferase, ZDHHC16, revealing the first palmitoylation cascade. The combination of site specific mutagenesis of the three ZDHHC6 palmitoylation sites, experimental determination of kinetic parameters and data-driven mathematical modelling allowed us to obtain detailed information on the eight differentially palmitoylated ZDHHC6 species. We found that species rapidly interconvert through the action of ZDHHC16 and the Acyl Protein Thioesterase APT2, that each species varies in terms of turnover rate and activity, altogether allowing the cell to robustly tune its ZDHHC6 activity.

Substrate selectivity in the zDHHC family of S-acyltransferases

S-acylation is a reversible lipid modification occurring on cysteine residues mediated by a family of membrane-bound 'zDHHC' enzymes. S-acylation predominantly results in anchoring of soluble proteins to membrane compartments or in the trafficking of membrane proteins to different compartments. Recent work has shown that although S-acylation of some proteins may involve very weak interactions with zDHHC enzymes, a pool of zDHHC enzymes exhibit strong and specific interactions with substrates, thereby recruiting them for S-acylation. For example, the ankyrin-repeat domains of zDHHC17 and zDHHC13 interact specifically with unstructured consensus sequences present in some proteins, thus contributing to substrate specificity of these enzymes. In addition to this new information on zDHHC enzyme protein substrate specificity, recent work has also identified marked differences in selectivity of zDHHC enzymes for acyl-CoA substrates and has started to unravel the underlying molecular basis for this lipid selectivity. This review will focus on the protein and acyl-CoA selectivity of zDHHC enzymes.

Ganglioside glycosyltransferases are S-acylated at conserved cysteine residues involved in homodimerisation

Ganglioside glycosyltransferases (GGTs) are type II membrane proteins bearing a short N-terminal cytoplasmic tail, a transmembrane domain (TMD), and a lumenal catalytic domain. The expression and activity of these enzymes largely determine the quality of the glycolipids that decorate mammalian cell membranes. Many glycosyltransferases (GTs) are themselves glycosylated, and this is important for their proper localisation, but few if any other post-translational modifications of these proteins have been reported. Here, we show that the GGTs, ST3Gal-V, ST8Sia-I, and β4GalNAcT-I are S-acylated at conserved cysteine residues located close to the cytoplasmic border of their TMDs. ST3Gal-II, a GT that sialylates glycolipids and glycoproteins, is also S-acylated at a conserved cysteine located in the N-terminal cytoplasmic tail. Many other GTs also possess cysteine residues in their cytoplasmic regions, suggesting that this modification occurs also on these GTs. S-acylation, commonly known as palmitoylation, is catalysed by a family of palmitoyltransferases (PATs) that are mostly localised at the Golgi complex but also at the endoplasmic reticulum (ER) and the plasma membrane. Using GT ER retention mutants, we found that S-acylation of β4GalNAcT-I and ST3Gal-II takes place at different compartments, suggesting that these enzymes are not substrates of the same PAT. Finally, we found that cysteines that are the target of S-acylation on β4GalNAcT-I and ST3Gal-II are involved in the formation of homodimers through disulphide bonds. We observed an increase in ST3Gal-II dimers in the presence of the PAT inhibitor 2-bromopalmitate, suggesting that GT homodimerisation may be regulating S-acylation

Protein palmitoylation: Palmitoyltransferases and their specificity

A plethora of novel information has emerged over the past decade regarding protein lipidation. The reversible attachment of palmitic acid to cysteine residues, termed S-palmitoylation, has focused a special attention. This is mainly due to the unique role of this modification in the regulation of protein trafficking and function. A large family of protein acyltransferases (PATs) containing a conserved aspartate-histidine-histidine-cysteine motif use ping-pong kinetic mechanism to catalyze S-palmitoylation of a substrate protein. Here, we discuss the topology of PAT proteins and their cellular localization. We will also give an overview of the mechanism of protein palmitoylation and how it is regulated. New information concerning the recent discovery of depalmitoylating enzymes belonging to the family of α/β-hydrolase domain-containing protein 17 (ABHD17A) is included. Considering the recent advances that have occurred in understanding the mechanisms underlying the interplay between palmitoylation and depalmitoylation, it is clear that we are beginning to understand the fundamental nature of how cellular signal-transduction mediates membrane-level organization in health and disease. Impact statement Protein palmitoylation is one of most important reversible post-translational modifications of protein function in cell-signaling systems. This review gathers the latest information on the molecular mechanism of protein palmitoyl transferase action. It also discusses the issue of substrate specificity of palmitoyl transferases. Another important question is the role of depalmitoylation enzymes. This review should help to formulate questions concerning the regulation of activity of particular PATs as well as of depalmitoylating enzymes (APT).

Molecular basis of fatty acid selectivity in the zDHHC family of S-acyltransferases revealed by click chemistry

S-acylation is a major posttranslational modification, catalyzed by the zinc finger DHHC domain containing (zDHHC) enzyme family. S-acylated proteins can be modified by different fatty acids; however, very little is known about how zDHHC enzymes contribute to acyl chain heterogeneity. Here, we used fatty acid-azide/alkyne labeling of mammalian cells, showing their transformation into acyl-CoAs and subsequent click chemistry-based detection, to demonstrate that zDHHC enzymes have marked differences in their fatty acid selectivity. This difference in selectivity was apparent even for highly related enzymes, such as zDHHC3 and zDHHC7, which displayed a marked difference in their ability to use C18:0 acyl-CoA as a substrate. Furthermore, we identified isoleucine-182 in transmembrane domain 3 of zDHHC3 as a key determinant in limiting the use of longer chain acyl-CoAs by this enzyme. This study uncovered differences in the fatty acid selectivity profiles of cellular zDHHC enzymes and mapped molecular determinants governing this selectivity.

Specific Palmitoyltransferases Associate with and Activate the Epithelial Sodium Channel

The epithelial sodium channel (ENaC) has an important role in regulating extracellular fluid volume and blood pressure, as well as airway surface liquid volume and mucociliary clearance. ENaC is a trimer of three homologous subunits (α, β, and γ). We previously reported that cytoplasmic residues on the β (βCys-43 and βCys-557) and γ (γCys-33 and γCys-41) subunits are palmitoylated. Mutation of Cys that blocked ENaC palmitoylation also reduced channel open probability. Furthermore, γ subunit palmitoylation had a dominant role over β subunit palmitoylation in regulating ENaC. To determine which palmitoyltransferases (termed DHHCs) regulate the channel, mouse ENaCs were co-expressed in Xenopus oocytes with each of the 23 mouse DHHCs. ENaC activity was significantly increased by DHHCs 1, 2, 3, 7, and 14. ENaC activation by DHHCs was lost when γ subunit palmitoylation sites were mutated, whereas DHHCs 1, 2, and 14 still activated ENaC lacking β subunit palmitoylation sites. β subunit palmitoylation was increased by ENaC co-expression with DHHC 7. Both wild type ENaC and channels lacking β and γ palmitoylation sites co-immunoprecipitated with the five activating DHHCs, suggesting that ENaC forms a complex with multiple DHHCs. RT-PCR revealed that transcripts for the five activating DHHCs were present in cultured mCCD_cl1 cells, and DHHC 3 was expressed in aquaporin 2-positive principal cells of mouse aldosterone-sensitive distal nephron where ENaC is localized. Treatment of polarized mCCD_cl1 cells with a general inhibitor of palmitoylation reduced ENaC-mediated Na⁺ currents within minutes. Our results indicate that specific DHHCs have a role in regulating ENaC.

Progress toward Understanding Protein S-acylation: Prospective in Plants

S-acylation, also known as S-palmitoylation or palmitoylation, is a reversible post-translational lipid modification in which long chain fatty acid, usually the 16-carbon palmitate, covalently attaches to a cysteine residue(s) throughout the protein via a thioester bond. It is involved in an array of important biological processes during growth and development, reproduction and stress responses in plant. S-acylation is a ubiquitous mechanism in eukaryotes catalyzed by a family of enzymes called Protein S-Acyl Transferases (PATs). Since the discovery of the first PAT in yeast in 2002 research in S-acylation has accelerated in the mammalian system and followed by in plant. However, it is still a difficult field to study due to the large number of PATs and even larger number of putative S-acylated substrate proteins they modify in each genome. This is coupled with drawbacks in the techniques used to study S-acylation, leading to the slower progress in this field compared to protein phosphorylation, for example. In this review we will summarize the discoveries made so far based on knowledge learnt from the characterization of protein S-acyltransferases and the S-acylated proteins, the interaction mechanisms between PAT and its specific substrate protein(s) in yeast and mammals. Research in protein S-acylation and PATs in plants will also be covered although this area is currently less well studied in yeast and mammalian systems.

The zDHHC family of S-acyltransferases

The discovery of the zDHHC family of S-acyltransferase enzymes has been one of the major breakthroughs in the S-acylation field. Now, more than a decade since their discovery, major questions centre on profiling the substrates of individual zDHHC enzymes (there are 24 ZDHHC genes and several hundred S-acylated proteins), defining the mechanisms of enzyme-substrate specificity and unravelling the importance of this enzyme family for cellular physiology and pathology.

Local Palmitoylation Cycles and Specialized Membrane Domain Organization

Palmitoylation is an evolutionally conserved lipid modification of proteins. Dynamic and reversible palmitoylation controls a wide range of molecular and cellular properties of proteins including the protein trafficking, protein function, protein stability, and specialized membrane domain organization. However, technical difficulties in (1) detection of palmitoylated substrate proteins and (2) purification and enzymology of palmitoylating enzymes have prevented the progress in palmitoylation research, compared with that in phosphorylation research. The recent development of proteomic and chemical biology techniques has unexpectedly expanded the known complement of palmitoylated proteins in various species and tissues/cells, and revealed the unique occurrence of palmitoylated proteins in membrane-bound organelles and specific membrane compartments. Furthermore, identification and characterization of DHHC (Asp-His-His-Cys) palmitoylating enzyme-substrate pairs have contributed to elucidating the regulatory mechanisms and pathophysiological significance of protein palmitoylation. Here, we review the recent progress in protein palmitoylation at the molecular, cellular, and in vivo level and discuss how locally regulated palmitoylation machinery works for dynamic nanoscale organization of membrane domains.

Selenoprotein K and protein palmitoylation

SIGNIFICANCE

Selenoprotein K (SelK) is an endoplasmic reticulum (ER) membrane protein, and its expression is sensitive to dietary selenium levels. A recently described role for SelK as a cofactor in catalyzing protein palmitoylation reactions provides an important link between low dietary selenium intake and suboptimal cellular functions that depend on this selenoprotein for palmitoylation.

RECENT ADVANCES

A recent breakthrough provided insight into the contribution of SelK to calcium (Ca(2+)) flux in immune cells. In particular, SelK is required for palmitoylation of the Ca(2+) channel protein, inositol-1,4,5-triphosphate receptor (IP3R) in the ER membrane. Without this post-translational modification, expression and function of the IP3R is impaired. SelK is required for palmitoylation of another transmembrane protein, CD36, and very likely other proteins. SelK serves as a cofactor during protein palmitoylation by binding to the protein acyltransferase, DHHC6, thereby facilitating addition of the palmitate via a thioester bond to the sulfhydryl group of cysteine residues of target proteins.

CRITICAL ISSUES

The association of DHHC6 and SelK is clearly important for immune cell functions and possibly other cell types. The step in the DHHC6 catalyzed S-acylation reaction on which SelK acts remains unclear and possible mechanisms of how the kinetics of the reaction are impacted by SelK binding to DHHC6 are presented here.

FUTURE DIRECTIONS

Uncovering the specific role of SelK in promoting DHHC6 catalyzed protein palmitoylation may open a new line of inquiry into other selenoproteins playing similar roles as cofactors for different enzymatic processes.