DHHC17 palmitoylates ClipR-59 and modulates ClipR-59 association with the plasma membrane

The ZDHHC13/ZDHHC17 subfamily: From biological functions to therapeutic targets of diseases

The ZDHHC13/ZDHHC17 subfamily belongs to the zinc finger DHHC-domain containing (ZDHHC) family, including ZDHHC13 and ZDHHC17. Recent studies have shown that the ZDHHC13/ZDHHC17 subfamily is involved in various pathological and physiological processes, including S-palmitoylation, Mg2+ transport, and CALCOCO1-mediated Golgiphagy. Moreover, the ZDHHC13/ZDHHC17 subfamily plays a crucial role in the occurrence and development of many diseases, including Huntington disease (HD), osteoporosis, atopic dermatitis, diabetes, and cancer. In the present review, we describe the distribution, structure, and post-translational modifications (PTMs) of the ZDHHC13/ZDHHC17 subfamily. Moreover, we effectively summarize the biological functions and associated diseases of this subfamily. Given the pleiotropy of the ZDHHC13/ZDHHC17 subfamily, it is imperative to conduct further research on its members to comprehend the pertinent pathophysiological mechanisms and to devise tactics for managing and controlling various diseases.

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.

ZDHHC17 participates in the pathogenesis of polycystic ovary syndrome by affecting androgen conversion to estrogen in granulosa cells

Polycystic ovary syndrome (PCOS) is a prevalent endocrine disorder affecting women of reproductive age and is a significant cause of female subfertility. Our previous research demonstrated that the abnormal palmitoylation of heat shock protein-90α (HSP90α) plays a role in the development of PCOS. However, the palmitoyl acyltransferases in HSP90α palmitoylation remain poorly understood. Herein, we identified ZDHHC17 as a major palmitoyl acyltransferase for HSP90α palmitoylation in granulosa cells. ZDHHC17 protein expression was diminished under excess androgen conditions in vitro and in vivo. Consistently, ovarian ZDHHC17 expression was found to be attenuated in patients with PCOS. ZDHHC17 depletion decreased HSP90α palmitoylation levels and hampered the conversion of androgen to estrogen via CYP19A1. Furthermore, ZDHHC17-mediated regulation of CYP19A1 expression was dependent on HSP90α palmitoylation. Our findings reveal that the regulatory role of HSP90α palmitoylation by ZDHHC17 is critical in PCOS pathophysiology and provide insights into the role of ZDHHC17 in reproductive endocrinology.

Control of mitochondria-associated endoplasmic reticulum membranes by protein S-palmitoylation: Novel therapeutic targets for neurodegenerative diseases

Mitochondria-associated endoplasmic reticulum membranes (MAMs) are dynamic coupling structures between mitochondria and the endoplasmic reticulum (ER). As a new subcellular structure, MAMs combine the two critical organelle functions. Mitochondria and the ER could regulate each other via MAMs. MAMs are involved in calcium (Ca²⁺) homeostasis, autophagy, ER stress, lipid metabolism, etc. Researchers have found that MAMs are closely related to metabolic syndrome and neurodegenerative diseases (NDs). The formation of MAMs and their functions depend on specific proteins. Numerous protein enrichments, such as the IP3R-Grp75-VDAC complex, constitute MAMs. The changes in these proteins govern the interaction between mitochondria and the ER; they also affect the biological functions of MAMs. S-palmitoylation is a reversible protein post-translational modification (PTM) that mainly occurs on protein cysteine residues. More and more studies have shown that the S-palmitoylation of proteins is closely related to their membrane localization. Here, we first briefly describe the composition and function of MAMs, reviewing the component and biological roles of MAMs mediated by S-palmitoylation, elaborating on S-palmitoylated proteins in Ca²⁺ flux, lipid rafts, and so on. We try to provide new insight into the molecular basis of MAMs-related diseases, mainly NDs. Finally, we propose potential drug compounds targeting S-palmitoylation.

DHHC17 Is a New Regulator of AMPK Signaling

The association of AMP-activated protein kinase (AMPK) with membranes plays a critical role in the regulation of AMPK activation and function. Protein lipid modification, including palmitoylation, myristoylation, and farnesyation, constitutes a crucial mechanism in the regulation of protein dynamic interactions with membranes. Among the three subunits of the AMPK heterotrimeric complex, the structural subunit AMPKβ is myristoylated and the catalytic subunit AMPKα is palmitoylated. Here, we report the characterization of AMPKα palmitoylation. We found that AMKPα was palmitoylated at Cys209 and Cys543, and this was required for AMPK activation and subcellular membrane compartmentalization. To understand the regulation of AMPKα palmitoylation, we have identified DHHC17 as a candidate palmitoyltransferase for AMPKα and found that DHHC17, by palmitoylating AMPKα, modulated AMPK membrane association and activation in response to energy stress. To determine the role of DHHC17 in cell function, we generated DHHC17 liver-specific knockout mice and found that inactivation of DHHC17 in the mouse liver impaired AMPK activation and hepatic autophagy and caused a type 2 diabetes-like syndrome. Overall, our studies demonstrate that AMPKα palmitoylation plays a critical role in AMPK activation and that DHHC17, through its modulation of AMPK signaling, constitutes a new regulator of hepatic metabolism.

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.

Regulatory effects of protein S-acylation on insulin secretion and insulin action

Post-translational modifications (PTMs) such as phosphorylation and ubiquitination are well-studied events with a recognized importance in all aspects of cellular function. By contrast, protein S-acylation, although a widespread PTM with important functions in most physiological systems, has received far less attention. Perturbations in S-acylation are linked to various disorders, including intellectual disability, cancer and diabetes, suggesting that this less-studied modification is likely to be of considerable biological importance. As an exemplar, in this review, we focus on the newly emerging links between S-acylation and the hormone insulin. Specifically, we examine how S-acylation regulates key components of the insulin secretion and insulin response pathways. The proteins discussed highlight the diverse array of proteins that are modified by S-acylation, including channels, transporters, receptors and trafficking proteins and also illustrate the diverse effects that S-acylation has on these proteins, from membrane binding and micro-localization to regulation of protein sorting and protein interactions.

Putative Role of Protein Palmitoylation in Cardiac Lipid-Induced Insulin Resistance

In the heart, inhibition of the insulin cascade following lipid overload is strongly associated with contractile dysfunction. The translocation of fatty acid transporter CD36 (SR-B2) from intracellular stores to the cell surface is a hallmark event in the lipid-overloaded heart, feeding forward to intracellular lipid accumulation. Yet, the molecular mechanisms by which intracellularly arrived lipids induce insulin resistance is ill-understood. Bioactive lipid metabolites (diacyl-glycerols, ceramides) are contributing factors but fail to correlate with the degree of cardiac insulin resistance in diabetic humans. This leaves room for other lipid-induced mechanisms involved in lipid-induced insulin resistance, including protein palmitoylation. Protein palmitoylation encompasses the reversible covalent attachment of palmitate moieties to cysteine residues and is governed by protein acyl-transferases and thioesterases. The function of palmitoylation is to provide proteins with proper spatiotemporal localization, thereby securing the correct unwinding of signaling pathways. In this review, we provide examples of palmitoylations of individual signaling proteins to discuss the emerging role of protein palmitoylation as a modulator of the insulin signaling cascade. Second, we speculate how protein hyper-palmitoylations (including that of CD36), as they occur during lipid oversupply, may lead to insulin resistance. Finally, we conclude that the protein palmitoylation machinery may offer novel targets to fight lipid-induced cardiomyopathy.

Regulatory effects of post-translational modifications on zDHHC S-acyltransferases

The human zDHHC S-acyltransferase family comprises 23 enzymes that mediate the S-acylation of a multitude of cellular proteins, including channels, receptors, transporters, signaling molecules, scaffolds, and chaperones. This reversible post-transitional modification (PTM) involves the attachment of a fatty acyl chain, usually derived from palmitoyl-CoA, to specific cysteine residues on target proteins, which affects their stability, localization, and function. These outcomes are essential to control many processes, including synaptic transmission and plasticity, cell growth and differentiation, and infectivity of viruses and other pathogens. Given the physiological importance of S-acylation, it is unsurprising that perturbations in this process, including mutations in ZDHHC genes, have been linked to different neurological pathologies and cancers, and there is growing interest in zDHHC enzymes as novel drug targets. Although zDHHC enzymes control a diverse array of cellular processes and are associated with major disorders, our understanding of these enzymes is surprisingly incomplete, particularly with regard to the regulatory mechanisms controlling these enzymes. However, there is growing evidence highlighting the role of different PTMs in this process. In this review, we discuss how PTMs, including phosphorylation, S-acylation, and ubiquitination, affect the stability, localization, and function of zDHHC enzymes and speculate on possible effects of PTMs that have emerged from larger screening studies. Developing a better understanding of the regulatory effects of PTMs on zDHHC enzymes will provide new insight into the intracellular dynamics of S-acylation and may also highlight novel approaches to modulate S-acylation for clinical gain.

Acylation - A New Means to Control Traffic Through the Golgi

The Golgi is well known to act as center for modification and sorting of proteins for secretion and delivery to other organelles. A key sorting step occurs at the trans-Golgi network and is mediated by protein adapters. However, recent data indicate that sorting also occurs much earlier, at the cis-Golgi, and uses lipid acylation as a novel means to regulate anterograde flux. Here, we examine an emerging role of S-palmitoylation/acylation as a mechanism to regulate anterograde routing. We discuss the critical Golgi-localized DHHC S-palmitoyltransferase enzymes that orchestrate this lipid modification, as well as their diverse protein clients (e.g., MAP6, SNAP25, CSP, LAT, β-adrenergic receptors, GABA receptors, and GLUT4 glucose transporters). Critically, for integral membrane proteins, S-acylation can act as new a “self-sorting” signal to concentrate these cargoes in rims of Golgi cisternae, and to promote their rapid traffic through the Golgi or, potentially, to bypass the Golgi. We discuss this mechanism and examine its potential relevance to human physiology and disease, including diabetes and neurodegenerative diseases.

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.

DNA-Methylation and Body Composition in Preschool Children: Epigenome-Wide-Analysis in the European Childhood Obesity Project (CHOP)-Study

Adiposity and obesity result from the interaction of genetic variation and environmental factors from very early in life, possibly mediated by epigenetic processes. Few Epigenome-Wide-Association-Studies have identified DNA-methylation (DNAm) signatures associated with BMI and body composition in children. Body composition by Bio-Impedance-Analysis and genome-wide DNAm in whole blood were assessed in 374 pre-school children from four European countries. Associations were tested by linear regression adjusted for sex, age, centre, education, 6 WBC-proportions according to Houseman and 30 principal components derived from control probes. Specific DNAm variants were identified to be associated with BMI (212), fat-mass (230), fat-free-mass (120), fat-mass-index (24) and fat-free-mass-index (15). Probes in genes SNED1(IRE-BP1), KLHL6, WDR51A(POC1A), CYTH4-ELFN2, CFLAR, PRDM14, SOS1, ZNF643(ZFP69B), ST6GAL1, C3orf70, CILP2, MLLT4 and ncRNA LOC101929268 remained significantly associated after Bonferroni-correction of P-values. We provide novel evidence linking DNAm with (i) altered lipid and glucose metabolism, (ii) diabetes and (iii) body size and composition in children. Both common and specific epigenetic signatures among measures were also revealed. The causal direction with phenotypic measures and stability of DNAm variants throughout the life course remains unclear and longitudinal analysis in other populations is required. These findings give support for potential epigenetic programming of body composition and obesity.

Peptide array-based screening reveals a large number of proteins interacting with the ankyrin-repeat domain of the zDHHC17 S-acyltransferase

zDHHC S-acyltransferases are enzymes catalyzing protein S-acylation, a common post-translational modification on proteins frequently affecting their membrane targeting and trafficking. The ankyrin repeat (AR) domain of zDHHC17 (HIP14) and zDHHC13 (HIP14L) S-acyltransferases, which is involved in both substrate recruitment and S-acylation-independent functions, was recently shown to bind at least six proteins, by specific recognition of a consensus sequence in them. To further refine the rules governing binding to the AR of zDHHC17, we employed peptide arrays based on zDHHC AR-binding motif (zDABM) sequences of synaptosomal-associated protein 25 (SNAP25) and cysteine string protein α (CSPα). Quantitative comparisons of the binding preferences of 400 peptides allowed us to construct a position-specific scoring matrix (PSSM) for zDHHC17 AR binding, with which we predicted and subsequently validated many putative zDHHC17 interactors. We identified 95 human zDABM sequences with unexpected versatility in amino acid usage; these sequences were distributed among 90 proteins, of which 62 have not been previously implicated in zDHHC17/13 binding. These zDABM-containing proteins included all family members of the SNAP25, sprouty, cornifelin, ankyrin, and SLAIN-motif containing families; seven endogenous Gag polyproteins sharing the same binding sequence; and several proteins involved in cytoskeletal organization, cell communication, and regulation of signaling. A dozen of the zDABM-containing proteins had more than one zDABM sequence, whereas isoform-specific binding to the AR of zDHHC17 was identified for the Ena/VASP-like protein. The large number of zDABM sequences within the human proteome suggests that zDHHC17 may be an interaction hub regulating many cellular processes.

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).

Protein palmitoylation in the development and plasticity of neuronal connections

Protein palmitoylation, or the reversible addition of the fatty acid, palmitate, onto substrate proteins, can impact the structure and stability of proteins as well as regulate protein-protein interactions and the trafficking and localization of proteins to cell membranes. This posttranslational modification is mediated by palmitoyl-acyltransferases, consisting of a family of 23 zDHHC proteins in mammals. This review focuses on the subcellular distribution of zDHHC proteins within the neuron and the regulation of zDHHC trafficking and function by synaptic activity. We review recent studies identifying actin binding proteins, cell adhesion molecules and synaptic scaffolding proteins as targets of palmitoylation, and examine the implications of activity-mediated palmitoylation in the establishment and plasticity of neuronal connections.

DHHC7 Palmitoylates Glucose Transporter 4 (Glut4) and Regulates Glut4 Membrane Translocation

Insulin-dependent translocation of glucose transporter 4 (Glut4) to the plasma membrane plays a key role in the dynamic regulation of glucose homeostasis. We recently showed that this process is critically dependent on palmitoylation of Glut4 at Cys-223. To gain further insights into the regulation of Glut4 palmitoylation, we set out to identify the palmitoyl acyltransferase (PAT) involved. Here we report that among 23 mammalian DHHC proteins, DHHC7 is the major Glut4 PAT, based on evidence that ectopic expression of DHHC7 increased Glut4 palmitoylation, whereas DHHC7 knockdown in 3T3-L1 adipocytes and DHHC7 KO in adipose tissue and muscle decreased Glut4 palmitoylation. Moreover, inactivation of DHHC7 suppressed insulin-dependent Glut4 membrane translocation in both 3T3-L1 adipocytes and primary adipocytes. Finally, DHHC7 KO mice developed hyperglycemia and glucose intolerance, thereby confirming that DHHC7 represents the principal PAT for Glut4 and that this mechanism is essential for insulin-regulated glucose homeostasis.

ClipR-59 plays a critical role in the regulation of body glucose homeostasis

By regulating Akt membrane compartmentalization, ClipR-59 modulates adipocyte glucose transport. To elucidate the role of ClipR-59 in the regulation of whole body glucose homeostasis, we have generated adipose tissue specific transgenic mice and examined how forcing expression of ClipR-59 in adipose tissue affects body glucose homeostasis. We found that ClipR-59 adipose transgenic mice showed lower blood glucose level with increased glucose tolerance and enhanced insulin sensitivity. Moreover, ClipR-59 adipose transgenic mice were lean with reduced fat mass and against diet induced obesity. Finally, we examined the potential impact of ClipR-59 on adipose endocrine function and found that ClipR-59 expression enhanced adiponectin secretion in both 3T3-L1 adipocytes and adipose tissue, accompanied with increased circulating adiponectin and enhanced AMPKα phosphorylation at Thr172 in adipose tissue and skeletal muscle. Overall, these studies demonstrate that ClipR-59 is likely an important regulator of body glucose homeostasis and adipocyte function.

Identification of a Novel Sequence Motif Recognized by the Ankyrin Repeat Domain of zDHHC17/13 S-Acyltransferases

S-Acylation is a major post-translational modification affecting several cellular processes. It is particularly important for neuronal functions. This modification is catalyzed by a family of transmembrane S-acyltransferases that contain a conserved zinc finger DHHC (zDHHC) domain. Typically, eukaryote genomes encode for 7-24 distinct zDHHC enzymes, with two members also harboring an ankyrin repeat (AR) domain at their cytosolic N termini. The AR domain of zDHHC enzymes is predicted to engage in numerous interactions and facilitates both substrate recruitment and S-acylation-independent functions; however, the sequence/structural features recognized by this module remain unknown. The two mammalian AR-containing S-acyltransferases are the Golgi-localized zDHHC17 and zDHHC13, also known as Huntingtin-interacting proteins 14 and 14-like, respectively; they are highly expressed in brain, and their loss in mice leads to neuropathological deficits that are reminiscent of Huntington's disease. Here, we report that zDHHC17 and zDHHC13 recognize, via their AR domain, evolutionary conserved and closely related sequences of a [VIAP][VIT]XXQP consensus in SNAP25, SNAP23, cysteine string protein, Huntingtin, cytoplasmic linker protein 3, and microtubule-associated protein 6. This novel AR-binding sequence motif is found in regions predicted to be unstructured and is present in a number of zDHHC17 substrates and zDHHC17/13-interacting S-acylated proteins. This is the first study to identify a motif recognized by AR-containing zDHHCs.

Curation of the Mammalian Palmitoylome Indicates a Pivotal Role for Palmitoylation in Diseases and Disorders of the Nervous System and Cancers

Palmitoylation involves the reversible posttranslational addition of palmitate to cysteines and promotes membrane binding and subcellular localization. Recent advancements in the detection and identification of palmitoylated proteins have led to multiple palmitoylation proteomics studies but these datasets are contained within large supplemental tables, making downstream analysis and data mining time-consuming and difficult. Consequently, we curated the data from 15 palmitoylation proteomics studies into one compendium containing 1,838 genes encoding palmitoylated proteins; representing approximately 10% of the genome. Enrichment analysis revealed highly significant enrichments for Gene Ontology biological processes, pathway maps, and process networks related to the nervous system. Strikingly, 41% of synaptic genes encode a palmitoylated protein in the compendium. The top disease associations included cancers and diseases and disorders of the nervous system, with Schizophrenia, HD, and pancreatic ductal carcinoma among the top five, suggesting that aberrant palmitoylation may play a pivotal role in the balance of cell death and survival. This compendium provides a much-needed resource for cell biologists and the palmitoylation field, providing new perspectives for cancer and neurodegeneration.

Protein localization is essential for mediating protein function within the cellular context. Mislocalization of proteins can offset cellular balance, influencing whether a cell lives or dies. Many proteins are directed to cellular membranes through the addition of fats, or lipidation. In particular, palmitoylation involves the reversible addition of the fatty acid palmitate to cysteines. Its reversibility makes it a unique form of lipidation allowing its dynamic regulation. Recent advancements in fast, sensitive, non-radioactive methods to detect palmitoylation have led to an explosion in the identification of palmitoylated proteins through proteomics studies. However, the data is hidden in large supplemental tables in various formats. Thus, we curated a list of palmitoylated proteins revealing that approximately 10 percent of the human genome encodes for a proteoform that is palmitoylated. Computational analysis confirmed that palmitoylation is involved in protein localization and indicated a new role in metabolism. Importantly, we found that palmitoylation was enriched at neuronal synapses and in disorders of the nervous system, including Schizophrenia and Huntington disease. Interestingly, palmitoylation was equally enriched in cancers. Consequently, we suggest that palmitoylation plays a critical role in cell fate and our compendium provides a plethora of targets for neurodegeneration and cancer.

The canonical DHHC motif is not absolutely required for the activity of the yeast S-acyltransferases Swf1 and Pfa4

Protein S-acyltransferases, also known as palmitoyltransferases (PATs), are characterized by the presence of a 50-amino acid domain called the DHHC domain. Within this domain, these four amino acids constitute a highly conserved motif. It has been proposed that the palmitoylation reaction occurs through a palmitoyl-PAT covalent intermediate that involves the conserved cysteine in the DHHC motif. Mutation of this cysteine results in lack of function for several PATs, and DHHA or DHHS mutants are used regularly as catalytically inactive controls. In a genetic screen to isolate loss-of-function mutations in the yeast PAT Swf1, we isolated an allele encoding a Swf1 DHHR mutant. Overexpression of this mutant is able to partially complement a swf1Δ strain and to acylate the Swf1 substrates Tlg1, Syn8, and Snc1. Overexpression of the palmitoyltransferase Pfa4 DHHA or DHHR mutants also results in palmitoylation of its substrate Chs3. We also investigated the role of the first histidine of the DHHC motif. A Swf1 DQHC mutant is also partially active but a DQHR is not. Finally, we show that Swf1 substrates are differentially modified by both DHHR and DQHC Swf1 mutants. We propose that, in the absence of the canonical mechanism, alternative suboptimal mechanisms take place that are more dependent on the reactivity of the acceptor protein. These results also imply that caution must be exercised when proposing non-canonical roles for PATs on the basis of considering DHHC mutants as catalytically inactive and, more generally, contribute to an understanding of the mechanism of protein palmitoylation.

The physiology of protein S-acylation

Protein S-acylation, the only fully reversible posttranslational lipid modification of proteins, is emerging as a ubiquitous mechanism to control the properties and function of a diverse array of proteins and consequently physiological processes. S-acylation results from the enzymatic addition of long-chain lipids, most typically palmitate, onto intracellular cysteine residues of soluble and transmembrane proteins via a labile thioester linkage. Addition of lipid results in increases in protein hydrophobicity that can impact on protein structure, assembly, maturation, trafficking, and function. The recent explosion in global S-acylation (palmitoyl) proteomic profiling as a result of improved biochemical tools to assay S-acylation, in conjunction with the recent identification of enzymes that control protein S-acylation and de-acylation, has opened a new vista into the physiological function of S-acylation. This review introduces key features of S-acylation and tools to interrogate this process, and highlights the eclectic array of proteins regulated including membrane receptors, ion channels and transporters, enzymes and kinases, signaling adapters and chaperones, cell adhesion, and structural proteins. We highlight recent findings correlating disruption of S-acylation to pathophysiology and disease and discuss some of the major challenges and opportunities in this rapidly expanding field.

Glut4 palmitoylation at Cys223 plays a critical role in Glut4 membrane trafficking

Recently, we identified Glut4 as a palmitoylated protein in adipocytes. To understand the role of Glut4 palmitoylation in Glut4 membrane trafficking, a process that is essential for maintenance of whole body glucose homeostasis, we have characterized Glut4 palmitoylation. We found that Glut4 is palmitoylated at Cys223 and Glut4 palmitoylation at Cys223 is essential for insulin dependent Glut4 membrane translocation as substitution of Cys223 with a serine residue in Glut4 (C223S Glut4) diminished Glut4 responsiveness to insulin in membrane translocation in both adipocytes and CHO-IR cells. We have examined C223S Glut4 subcellular localization and observed that it was absence from tubular-vesicle structure, where insulin responsive Glut4 vesicles were presented. Together, our studies uncover a novel mechanism under which Glut4 palmitoylation regulates Glut4 sorting to insulin responsive vesicles, thereby insulin-dependent Glut4 membrane translocation.

ClipR-59 interacts with Elmo2 and modulates myoblast fusion

Recent studies using ClipR-59 knock-out mice implicated this protein in the regulation of muscle function. In this report, we have examined the role of ClipR-59 in muscle differentiation and found that ClipR-59 knockdown in C2C12 cells suppressed myoblast fusion. To elucidate the molecular mechanism whereby ClipR-59 regulates myoblast fusion, we carried out a yeast two-hybrid screen using ClipR-59 as the bait and identified Elmo2, a member of the Engulfment and cell motility protein family, as a novel ClipR-59-associated protein. We showed that the interaction between ClipR-59 and Elmo2 was mediated by the atypical PH domain of Elmo2 and the Glu-Pro-rich domain of ClipR-59 and regulated by Rho-GTPase. We have examined the impact of ClipR-59 on Elmo2 downstream signaling and found that interaction of ClipR-59 with Elmo2 enhanced Rac1 activation. Collectively, our studies demonstrate that formation of an Elmo2·ClipR-59 complex plays an important role in myoblast fusion.

The Golgi S-acylation machinery comprises zDHHC enzymes with major differences in substrate affinity and S-acylation activity

S-acylation, the attachment of fatty acids onto cysteine residues, regulates protein trafficking and function and is mediated by a family of zDHHC enzymes. The S-acylation of peripheral membrane proteins has been proposed to occur at the Golgi, catalyzed by an S-acylation machinery that displays little substrate specificity. To advance understanding of how S-acylation of peripheral membrane proteins is handled by Golgi zDHHC enzymes, we investigated interactions between a subset of four Golgi zDHHC enzymes and two S-acylated proteins-synaptosomal-associated protein 25 (SNAP25) and cysteine-string protein (CSP). Our results uncover major differences in substrate recognition and S-acylation by these zDHHC enzymes. The ankyrin-repeat domains of zDHHC17 and zDHHC13 mediated strong and selective interactions with SNAP25/CSP, whereas binding of zDHHC3 and zDHHC7 to these proteins was barely detectable. Despite this, zDHHC3/zDHHC7 could S-acylate SNAP25/CSP more efficiently than zDHHC17, whereas zDHHC13 lacked S-acylation activity toward these proteins. Overall the results of this study support a model in which dynamic intracellular localization of peripheral membrane proteins is achieved by highly selective recruitment by a subset of zDHHC enzymes at the Golgi, combined with highly efficient S-acylation by other Golgi zDHHC enzymes.

The palmitoyl acyltransferase HIP14 shares a high proportion of interactors with huntingtin: implications for a role in the pathogenesis of Huntington's disease

HIP14 is the most highly conserved of 23 human palmitoyl acyltransferases (PATs) that catalyze the post-translational addition of palmitate to proteins, including huntingtin (HTT). HIP14 is dysfunctional in the presence of mutant HTT (mHTT), the causative gene for Huntington disease (HD), and we hypothesize that reduced palmitoylation of HTT and other HIP14 substrates contributes to the pathogenesis of the disease. Here we describe the yeast two-hybrid (Y2H) interactors of HIP14 in the first comprehensive study of interactors of a mammalian PAT. Unexpectedly, we discovered a highly significant overlap between HIP14 interactors and 370 published interactors of HTT, 4-fold greater than for control proteins (P = 8 × 10(-5)). Nearly half of the 36 shared interactors are already implicated in HD, supporting a direct link between HIP14 and the disease. The HIP14 Y2H interaction set is significantly enriched for palmitoylated proteins that are candidate substrates. We confirmed that three of them, GPM6A, and the Sprouty domain-containing proteins SPRED1 and SPRED3, are indeed palmitoylated by HIP14; the first enzyme known to palmitoylate these proteins. These novel substrates functions might be affected by reduced palmitoylation in HD. We also show that the vesicular cargo adapter optineurin, an established HTT-binding protein, co-immunoprecipitates with HIP14 but is not palmitoylated. mHTT leads to mislocalization of optineurin and aberrant cargo trafficking. Therefore, it is possible that optineurin regulates trafficking of HIP14 to its substrates. Taken together, our data raise the possibility that defective palmitoylation by HIP14 might be an important mechanism that contributes to the pathogenesis of HD.

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.