Model organisms lead the way to protein palmitoyltransferases

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

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

Repositioning Lomitapide to block ZDHHC5-dependant palmitoylation on SSTR5 leads to anti-proliferation effect in preclinical pancreatic cancer models

Palmitoylation of proteins plays important roles in various physiological processes, such as cell proliferation, inflammation, cell differentiation etc. However, inhibition of protein palmitoylation has led to few new drugs to date. ZDHHC5 serves as a key enzyme to catalyze palmitoylation on SSTR5 (a proven anti-proliferation receptor in pancreatic cells). Herein, we compare single-cell transcriptome data between pancreatic cancer tissues and normal pancreas tissues and identify that ZDHHC5 is a potential target to inhibit proliferation of pancreatic cancer cells. In addition, we report the repositioning of an orphan drug (Lomitapide) as an inhibitor of ZDHHC5, and we speculate that this inhibitor may be able to block palmitylation on SSTR5. Pharmacological blockade of ZDHHC5 with Lomitapide results in attenuated cancer cell growth and proliferation which collectively contributes to antitumor responses in vitro and in vivo. This is the first study, to our knowledge, to demonstrate the utility of a pharmacological inhibitor of ZDHHC5 in pancreatic cancer, representing a new class of palmitoylation targeted therapy and laying a framework for paradigm-shifting therapies targeting cancer cell palmitoylation.

BCAT-induced autophagy regulates Aβ load through an interdependence of redox state and PKC phosphorylation-implications in Alzheimer's disease

Leucine, nutrient signal and substrate for the branched chain aminotransferase (BCAT) activates the mechanistic target of rapamycin (mTORC1) and regulates autophagic flux, mechanisms implicated in the pathogenesis of neurodegenerative conditions such as Alzheimer's disease (AD). BCAT is upregulated in AD, where a moonlighting role, imparted through its redox-active CXXC motif, has been suggested. Here we demonstrate that the redox state of BCAT signals differential phosphorylation by protein kinase C (PKC) regulating the trafficking of cellular pools of BCAT. We show inter-dependence of BCAT expression and proteins associated with the P13K/Akt/mTORC1 and autophagy signalling pathways. In response to insulin or an increase in ROS, BCATc is trafficked to the membrane and docks via palmitoylation, which is associated with BCATc-induced autophagy through PKC phosphorylation. In response to increased levels of BCATc, as observed in AD, amyloid β (Aβ) levels accumulate due to a shift in autophagic flux. This effect was diminished when incubated with leucine, indicating that dietary levels of amino acids show promise in regulating Aβ load. Together these findings show that increased BCATc expression, reported in human AD brain, will affect autophagy and Aβ load through the interdependence of its redox-regulated phosphorylation offering a novel target to address AD pathology.

Chemotherapy-induced pyroptosis is mediated by BAK/BAX-caspase-3-GSDME pathway and inhibited by 2-bromopalmitate

Many chemotherapy treatments induce apoptosis or pyroptosis through BAK/BAX-dependent mitochondrial pathway. BAK/BAX activation causes the mitochondrial outer membrane permeabilization (MOMP), which induces the activation of pro-apoptotic caspase cascade. GSDME cleavage by the pro-apoptotic caspases determines whether chemotherapy drug treatments induce apoptosis or pyroptosis, however, its regulation mechanisms are not clear. In this study, we showed that TNFα+CHX and navitoclax-induced cancer cell pyroptosis through a BAK/BAX-caspase-3-GSDME signaling pathway. GSDME knockdown inhibited the pyroptosis, suggesting the essential role of GSDME in this process. Interestingly, GSDME was found to be palmitoylated on its C-terminal (GSDME-C) during chemotherapy-induced pyroptosis, while 2-bromopalmitate (2-BP) could inhibit the GSDME-C palmitoylation and chemotherapy-induced pyroptosis. Mutation of palmitoylation sites on GSDME also diminished the pyroptosis induced by chemotherapy drugs. Moreover, 2-BP treatment increased the interaction between GSDME-C and GSDME-N, providing a potential mechanism of this function. Further studies indicated several ZDHHC proteins including ZDHHC-2,7,11,15 could interact with and palmitoylate GSDME. Our findings offered new targets to achieve the transformation between chemotherapy-induced pyroptosis and apoptosis.

Proteomic Analysis of S-Palmitoylated Proteins in Ocular Lens Reveals Palmitoylation of AQP5 and MP20

Purpose

The purpose of this study was to characterize the palmitoyl-proteome in lens fiber cells. S-palmitoylation is the most common form of protein S-acylation and the reversible nature of this modification functions as a molecular switch to regulate many biological processes. This modification could play important roles in regulating protein functions and protein–protein interactions in the lens.

Methods

The palmitoyl-proteome of bovine lens fiber cells was investigated by combining acyl-biotin exchange (ABE) chemistry and mass-spectrometry analysis. Due to the possibility of false-positive results from ABE experiment, a method was also developed for direct detection of palmitoylated peptides by mass spectrometry for validating palmitoylation of lens proteins MP20 and AQP5. Palmitoylation levels on AQP5 in different regions of the lens were quantified after iodoacetamide (IAA)-palmitate exchange.

Results

The ABE experiment identified 174 potential palmitoylated proteins. These proteins include 39 well-characterized palmitoylated proteins, 92 previously reported palmitoylated proteins in other tissues, and 43 newly identified potential palmitoylated proteins including two important transmembrane proteins in the lens, AQP5 and MP20. Further analysis by direct detection of palmitoylated peptides confirmed palmitoylation of AQP5 on C6 and palmitoylation of MP20 on C159. Palmitoylation of AQP5 was found to only occur in a narrow region of the inner lens cortex and does not occur in the lens epithelium, in the lens outer cortex, or in the lens nucleus.

Conclusions

AQP5 and MP20 are among 174 palmitoylated proteins found in bovine lens fiber cells. This modification to AQP5 and MP20 may play a role in their translocation from the cytoplasm to cell membranes during fiber cell differentiation.

Nutrient-Dependent Changes of Protein Palmitoylation: Impact on Nuclear Enzymes and Regulation of Gene Expression

Diet is the main environmental stimulus chronically impinging on the organism throughout the entire life. Nutrients impact cells via a plethora of mechanisms including the regulation of both protein post-translational modifications and gene expression. Palmitoylation is the most-studied protein lipidation, which consists of the attachment of a molecule of palmitic acid to residues of proteins. S-palmitoylation is a reversible cysteine modification finely regulated by palmitoyl-transferases and acyl-thioesterases that is involved in the regulation of protein trafficking and activity. Recently, several studies have demonstrated that diet-dependent molecules such as insulin and fatty acids may affect protein palmitoylation. Here, we examine the role of protein palmitoylation on the regulation of gene expression focusing on the impact of this modification on the activity of chromatin remodeler enzymes, transcription factors, and nuclear proteins. We also discuss how this physiological phenomenon may represent a pivotal mechanism underlying the impact of diet and nutrient-dependent signals on human diseases.

Protein Palmitoylation by ZDHHC13 Protects Skin against Microbial-Driven Dermatitis

Atopic dermatitis is a complex chronic inflammatory skin disorder that results from intimate interactions among genetic predisposition, host environment, skin barrier defects, and immunological factors. However, a clear genetic roadmap leading to atopic dermatitis remains to be fully explored. From a genome-wide mutagenesis screen, deficiency of ZDHHC13, a palmitoylacyl transferase, has previously been associated with skin and multitissue inflammatory phenotypes. Here, we report that ZDHHC13 is required for skin barrier integrity and that deficiency of ZDHHC13 renders mice susceptible to environmental bacteria, resulting in persistent skin inflammation and an atopic dermatitis-like disease. This phenotype is ameliorated in a germ-free environment and is also attenuated by antibiotic treatment, but not by deletion of the Rag1 gene, suggesting that a microbial factor triggers inflammation rather than intrinsic adaptive immunity. Furthermore, skin from ZDHHC13-deficient mice has both elevated levels of IL-33 and type 2 innate lymphoid cells, reinforcing the role of innate immunity in the development of atopic dermatitis. In summary, our study suggests that loss of ZDHHC13 in skin impairs the integrity of multiple barrier functions and leads to a dermatitis lesion in response to microbial encounters.

Palmitoyl acyltransferase DHHC21 mediates endothelial dysfunction in systemic inflammatory response syndrome

Endothelial dysfunction is a hallmark of systemic inflammatory response underlying multiple organ failure. Here we report a novel function of DHHC-containing palmitoyl acyltransferases (PATs) in mediating endothelial inflammation. Pharmacological inhibition of PATs attenuates barrier leakage and leucocyte adhesion induced by endothelial junction hyperpermeability and ICAM-1 expression during inflammation. Among 11 DHHCs detected in vascular endothelium, DHHC21 is required for barrier response. Mice with DHHC21 function deficiency (Zdhhc21dep/dep) exhibit marked resistance to injury, characterized by reduced plasma leakage, decreased leucocyte adhesion and ameliorated lung pathology, culminating in improved survival. Endothelial cells from Zdhhc21dep/dep display blunted barrier dysfunction and leucocyte adhesion, whereas leucocytes from these mice did not show altered adhesiveness. Furthermore, inflammation enhances PLCβ1 palmitoylation and signalling activity, effects significantly reduced in Zdhhc21dep/dep and rescued by DHHC21 overexpression. Likewise, overexpression of wild-type, not mutant, PLCβ1 augments barrier dysfunction. Altogether, these data suggest the involvement of DHHC21-mediated PLCβ1 palmitoylation in endothelial inflammation.

Whi2 enhances methylmercury toxicity in yeast via inhibition of Akr1 palmitoyltransferase activity

BACKGROUND

We have previously reported that Whi2 enhances the toxicity of methylmercury in yeast. In the present study we examined the proteins known to interact with Whi2 to find those that influence the toxicity of methylmercury.

METHODS

Gene disruption and site-directed mutagenesis were employed to examine the relationship of mercury toxicity and palmitoylation. Protein palmitoylation was examined using the acyl-biotinyl exchange method. Protein-protein interactions were detected by immunoprecipitation and immunoblotting.

RESULTS

We found that deletion of Akr1, a palmitoyltransferase, rendered yeast cells highly sensitive to methylmercury, and Akr1 is necessary for the methylmercury resistance of Whi2-deleted yeast. Palmitoyltransferase activity of Akr1 has an important role in the alleviation of methylmercury toxicity. Whi2 deletion or methylmercury treatment enhanced the palmitoyltransferase activity of Akr1, and methylmercury treatment reduced the binding between Akr1 and Whi2.

CONCLUSIONS

Whi2 bonds to Akr1 (a protein that is able to alleviate methylmercury toxicity) and thus inhibits Akr1's palmitoyltransferase activity, which leads to enhanced methylmercury toxicity. In contrast, methylmercury might break the bond between Whi2 and Akr1, which enhances the palmitoyltransferase activity of Akr1 to alleviate methylmercury toxicity.

GENERAL SIGNIFICANCE

This study's findings propose that the Whi2/Akr1 system can be regarded as a defense mechanism that detects methylmercury incorporation of yeast cells and alleviates its toxicity.

The Protein Acyl Transferase ZDHHC21 Modulates α1 Adrenergic Receptor Function and Regulates Hemodynamics

OBJECTIVE

Palmitoylation, the reversible addition of the lipid palmitate to a cysteine, can alter protein localization, stability, and function. The ZDHHC family of protein acyl transferases catalyzes palmitoylation of numerous proteins. The role of ZDHHC enzymes in intact tissue and in vivo is largely unknown. Herein, we characterize vascular functions in a mouse that expresses a nonfunctional ZDHHC21 (F233Δ).

APPROACH AND RESULTS

Physiological studies of isolated aortae and mesenteric arteries from F233Δ mice revealed an unexpected defect in responsiveness to phenylephrine, an α1 adrenergic receptor agonist. In vivo, F233Δ mice displayed a blunted response to infusion of phenylephrine, and they were found to have elevated catecholamine levels and elevated vascular α1 adrenergic receptor gene expression. Telemetry studies showed that the F233Δ mice were tachycardic and hypotensive at baseline, consistent with diminished vascular tone. In biochemical studies, ZDHHC21 was shown to palmitoylate the α1D adrenoceptor and to interact with it in a molecular complex, thus suggesting a possible molecular mechanism by which the receptor can be regulated by ZDHHC21.

CONCLUSIONS

Together, the data support a model in which ZDHHC21 F233Δ diminishes the function of vascular α1 adrenergic receptors, leading to reduced vascular tone, which manifests in vivo as hypotension and tachycardia. This is to our knowledge the first demonstration of a ZDHHC isoform affecting vascular function in vivo and identifies a novel molecular mode of regulation of vascular tone and blood pressure.

Palmitoyl acyltransferase Aph2 in cardiac function and the development of cardiomyopathy

Protein palmitoylation regulates many aspects of cell function and is carried out by acyl transferases that contain zf-DHHC motifs. The in vivo physiological function of protein palmitoylation is largely unknown. Here we generated mice deficient in the acyl transferase Aph2 (Ablphilin 2 or zf-DHHC16) and demonstrated an essential role for Aph2 in embryonic/postnatal survival, eye development, and heart development. Aph2(-/-) embryos and pups showed cardiomyopathy and cardiac defects including bradycardia. We identified phospholamban, a protein often associated with human cardiomyopathy, as an interacting partner and a substrate of Aph2. Aph2-mediated palmitoylation of phospholamban on cysteine 36 differentially alters its interaction with PKA and protein phosphatase 1 α, augmenting serine 16 phosphorylation, and regulates phospholamban pentamer formation. Aph2 deficiency results in phospholamban hypophosphorylation, a hyperinhibitory form. Ablation of phospholamban in Aph2(-/-) mice histologically and functionally alleviated the heart defects. These findings establish Aph2 as a critical in vivo regulator of cardiac function and reveal roles for protein palmitoylation in the development of other organs including eyes.

Overexpression of ZDHHC14 promotes migration and invasion of scirrhous type gastric cancer

Scirrhous type gastric cancer is highly aggressive and has a poorer prognosis than many other types of gastric carcinoma, due to its characteristic rapid cancer cell infiltration and proliferation, extensive stromal fibrosis, and frequent peritoneal dissemination. The aim of the present study was to identify novel prognostic markers or therapeutic targets for scirrhous type gastric cancer. We reviewed a list of genes with upregulated expression in scirrhous type gastric cancer and compared their expression with that in normal stomach from our previous Escherichia coli (E. coli) ampicillin secretion-trap (CAST) analysis. We focused on the ZDHHC14 gene, which encodes zinc finger, DHHC-type containing 14 protein. qRT-PCR analysis of ZDHHC14 in 41 gastric cancer cases revealed that compared to mRNA levels in normal non-neoplastic gastric mucosa, ZDHHC14 mRNA was overexpressed in 27% of gastric cancer tissue samples. The overexpression of ZDHHC14 was significantly associated with depth of tumor invasion, undifferentiated histology and scirrhous pattern. The invasiveness of ZDHHC14-knockdown HSC-44PE and 44As3 gastric cancer cells was decreased in comparison with that of the negative control siRNA-transfected cells, together with downregulation of MMP-17 mRNA. Integrins α5 and β1 were also downregulated in ZDHHC14-knockdown 44As3 cells. Forced expression of ZDHHC14 activated gastric cancer cell migration and invasion in vitro. These results indicate that ZDHHC14 is involved in tumor progression in patients with scirrhous type gastric cancer.

Palmitoyl acyltransferase, Zdhhc13, facilitates bone mass acquisition by regulating postnatal epiphyseal development and endochondral ossification: a mouse model

ZDHHC13 is a member of DHHC-containing palmitoyl acyltransferases (PATs) family of enzymes. It functions by post-translationally adding 16-carbon palmitate to proteins through a thioester linkage. We have previously shown that mice carrying a recessive Zdhhc13 nonsense mutation causing a Zdhcc13 deficiency develop alopecia, amyloidosis and osteoporosis. Our goal was to investigate the pathogenic mechanism of osteoporosis in the context of this mutation in mice. Body size, skeletal structure and trabecular bone were similar in Zdhhc13 WT and mutant mice at birth. Growth retardation and delayed secondary ossification center formation were first observed at day 10 and at 4 weeks of age, disorganization in growth plate structure and osteoporosis became evident in mutant mice. Serial microCT from 4-20 week-olds revealed that Zdhhc13 mutant mice had reduced bone mineral density. Through co-immunoprecipitation and acyl-biotin exchange, MT1-MMP was identified as a direct substrate of ZDHHC13. In cells, reduction of MT1-MMP palmitoylation affected its subcellular distribution and was associated with decreased VEGF and osteocalcin expression in chondrocytes and osteoblasts. In Zdhhc13 mutant mice epiphysis where MT1-MMP was under palmitoylated, VEGF in hypertrophic chondrocytes and osteocalcin at the cartilage-bone interface were reduced based on immunohistochemical analyses. Our results suggest that Zdhhc13 is a novel regulator of postnatal skeletal development and bone mass acquisition. To our knowledge, these are the first data to suggest that ZDHHC13-mediated MT1-MMP palmitoylation is a key modulator of bone homeostasis. These data may provide novel insights into the role of palmitoylation in the pathogenesis of human osteoporosis.

Protein palmitoylation in signal transduction of hematopoietic cells

The covalent attachment of palmitate to proteins can alter protein-lipid and protein-protein interactions thereby influencing protein function. Palmitoylation is a reversible post-translational modification. Thus, like protein phosphorylation, protein palmitoylation can function in activation-dependent signaling pathways. This review will provide an overview of the mechanisms and regulation of protein palmitoylation and focus on the role of palmitoylation in signal transduction pathways of lymphocytes and platelets.

Palmitoylation of influenza virus proteins

Influenza viruses contain two palmitoylated (S-acylated) proteins: the major spike protein HA (haemagglutinin) and the proton-channel M2. The present review describes the fundamental biochemistry of palmitoylation of HA: the location of palmitoylation sites and the fatty acid species bound to HA. Finally, the functional consequences of palmitoylation of HA and M2 are discussed regarding association with membrane rafts, entry of viruses into target cells by HA-mediated membrane fusion as well as the release of newly assembled virus particles from infected cells.

Global analysis of apicomplexan protein S-acyl transferases reveals an enzyme essential for invasion

The advent of techniques to study palmitoylation on a whole proteome scale has revealed that it is an important reversible modification that plays a role in regulating multiple biological processes. Palmitoylation can control the affinity of a protein for lipid membranes, which allows it to impact protein trafficking, stability, folding, signalling and interactions. The publication of the palmitome of the schizont stage of Plasmodium falciparum implicated a role for palmitoylation in host cell invasion, protein export and organelle biogenesis. However, nothing is known so far about the repertoire of protein S-acyl transferases (PATs) that catalyse this modification in Apicomplexa. We undertook a comprehensive analysis of the repertoire of Asp-His-His-Cys cysteine-rich domain (DHHC-CRD) PAT family in Toxoplasma gondii and Plasmodium berghei by assessing their localization and essentiality. Unlike functional redundancies reported in other eukaryotes, some apicomplexan-specific DHHCs are essential for parasite growth, and several are targeted to organelles unique to this phylum. Of particular interest is DHHC7, which localizes to rhoptry organelles in all parasites tested, including the major human pathogen P. falciparum. TgDHHC7 interferes with the localization of the rhoptry palmitoylated protein TgARO and affects the apical positioning of the rhoptry organelles. This PAT has a major impact on T. gondii host cell invasion, but not on the parasite's ability to egress.

Reduced expression of ZDHHC2 is associated with lymph node metastasis and poor prognosis in gastric adenocarcinoma

BACKGROUND

Zinc finger, DHHC-type containing 2 (ZDHHC2), originally named as reduced expression associated with metastasis protein (REAM), has been proposed as a putative tumor/metastasis suppressor gene and is often aberrantly decreased in human cancers. However ZDHHC2 expression pattern and its clinical significance have not yet been investigated in gastric adenocarcinoma.

METHODOLOGY/PRINCIPAL FINDINGS

Quantitative Real-Time PCR (qRT-PCR) and immunostaining were performed to detect ZDHHC2 expression in gastric adenocarcinoma, and then the correlation between ZDHHC2 expression and clinicpathologic parameters, and patient survival was analyzed. Compared to the adjacent normal tissues, ZDHHC2 expression was significantly reduced in gastric tumor tissues as shown by qRT-PCR and immunostaining. Low expression of ZDHHC2 was observed in 44.7% (211/472) of gastric adenocarcinoma patients, and was associated significantly with lymph node metastasis (p<0.001) and histological grade (p<0.001). Multivariate Cox regression analysis indicated that ZDHHC2 expression had a significant, independent predictive value for survival of gastric cancer patients (HR = 0.627, p = 0.001).

CONCLUSIONS/SIGNIFICANCE

Our data suggest that reduced ZDHHC2 expression is associated with lymph node metastasis and independently predicts an unfavorable prognosis in gastric adenocarcinoma patients.

Human DHHC proteins: a spotlight on the hidden player of palmitoylation

Palmitoylation is one of the most common posttranslational lipid modifications of proteins and we now know quite a lot about it. However, the state of knowledge about the enzymes that catalyze this process is clearly insufficient. This review is focused on 23 human DHHC genes and their products - protein palmitoyltransferases. Here we describe mainly the structure and function of these proteins, but also, to a lesser degree, what the substrates of the enzymes are and whether they are related to various diseases. The main aim of this review was to catalogue existing information concerning the human DHHC family of genes/proteins, making them and their functions easier to understand.

The 22q11.2 microdeletion: fifteen years of insights into the genetic and neural complexity of psychiatric disorders

Over the last fifteen years it has become established that 22q11.2 deletion syndrome (22q11DS) is a true genetic risk factor for schizophrenia. Carriers of deletions in chromosome 22q11.2 develop schizophrenia at rate of 25-30% and such deletions account for as many as 1-2% of cases of sporadic schizophrenia in the general population. Access to a relatively homogeneous population of individuals that suffer from schizophrenia as the result of a shared etiological factor and the potential to generate etiologically valid mouse models provides an immense opportunity to better understand the pathobiology of this disease. In this review we survey the clinical literature associated with the 22q11.2 microdeletions with a focus on neuroanatomical changes. Then, we highlight results from work modeling this structural mutation in animals. The key biological pathways disrupted by the mutation are discussed and how these changes impact the structure and function of neural circuits is described.

Subcellular Golgi localization of stathmin family proteins is promoted by a specific set of DHHC palmitoyl transferases

Palmitoylation of the neuronal microtubule regulatory stathmin phosphoproteins is crucial for their Golgi and vesicle localization and trafficking along neurites. It is promoted by a specific set of palmitoyl transferases, which suggests that palmitoylation can be viewed as a crucial regulatory process for neuronal stathmin functions.

 Protein palmitoylation is a reversible lipid modification that plays critical roles in protein sorting and targeting to specific cellular compartments. The neuronal microtubule-regulatory phosphoproteins of the stathmin family (SCG10/stathmin 2, SCLIP/stathmin 3, and RB3/stathmin 4) are peripheral proteins that fulfill specific and complementary roles in the formation and maturation of the nervous system. All neuronal stathmins are localized at the Golgi complex and at vesicles along axons and dendrites. Their membrane anchoring results from palmitoylation of two close cysteine residues present within their homologous N-terminal targeting domains. By preventing palmitoylation with 2-bromopalmitate or disrupting the integrity of the Golgi with brefeldin A, we were able to show that palmitoylation of stathmins 2 and 3 likely occurs at the Golgi and is crucial for their specific subcellular localization and trafficking. In addition, this membrane binding is promoted by a specific set of palmitoyl transferases that localize with stathmins 2 and 3 at the Golgi, directly interact with them, and enhance their membrane association. The subcellular membrane–associated microtubule-regulatory activity of stathmins might then be fine-tuned by extracellular stimuli controlling their reversible palmitoylation, which can be viewed as a crucial regulatory process for specific and local functions of stathmins in neurons.

Dynamic palmitoylation and the role of DHHC proteins in T cell activation and anergy

Although protein S-palmitoylation was first characterized >30 years ago, and is implicated in the function, trafficking, and localization of many proteins, little is known about the regulation and physiological implications of this posttranslational modification. Palmitoylation of various signaling proteins required for TCR-induced T cell activation is also necessary for their proper function. Linker for activation of T cells (LAT) is an essential scaffolding protein involved in T cell development and activation, and we found that its palmitoylation is selectively impaired in anergic T cells. The recent discovery of the DHHC family of palmitoyl acyl transferases and the establishment of sensitive and quantitative proteomics-based methods for global analysis of the palmitoyl proteome led to significant progress in studying the biology and underlying mechanisms of cellular protein palmitoylation. We are using these approaches to explore the palmitoyl proteome in T lymphocytes and, specifically, the mechanistic basis for the impaired palmitoylation of LAT in anergic T cells. This chapter reviews the history of protein palmitoylation and its role in T cell activation, the DHHC family and new methodologies for global analysis of the palmitoyl proteome, and summarizes our recent work in this area. The new methodologies will accelerate the pace of research and provide a greatly improved mechanistic and molecular understanding of the complex process of protein palmitoylation and its regulation, and the substrate specificity of the novel DHHC family. Reversible protein palmitoylation will likely prove to be an important posttranslational mechanism that regulates cellular responses, similar to protein phosphorylation and ubiquitination.

DHHC protein-dependent palmitoylation protects regulator of G-protein signaling 4 from proteasome degradation

Regulator of G-protein signaling 4 (RGS4), an intracellular modulator of G-protein coupled receptor (GPCR)-mediated signaling, is regulated by multiple processes including palmitoylation and proteasome degradation. We found that co-expression of DHHC acyltransferases (DHHC3 or DHHC7), but not their acyltransferase-inactive mutants, increased expression levels of RGS4 but not its Cys2 to Ser mutant (RGS4C2S). DHHC3 interacts with and palmitoylates RGS4 but not RGS4C2S in vivo. Palmitoylation prolongs the half-life of RGS4 by over 8-fold and palmitoylated RGS4 blocked α(1A)-adrenergic receptor-stimulated intracellular Ca(2+) mobilization. Together, our findings revealed that DHHC proteins could regulate GPCR-mediated signaling by increasing RGS4 stability.

dHIP14-dependent palmitoylation promotes secretion of the BMP antagonist Sog

Analysis of diverse signaling systems has revealed that one important level of control is regulation of membrane trafficking of ligands and receptors. The activities of some ligands are also regulated by whether they are membrane bound or secreted. In Drosophila, several morphogenetic signals that play critical roles in development have been found to be subject to such regulation. For example, activity of the Hedgehog (Hh) is regulated by Raspberry, which palmitoylates Hh. Similarly, the palmitoylases Porcupine and Raspberry increase the activities of Wingless (Wg) and the EGF-ligand Spitz (Spi), respectively. In contrast to its vertebrate homologues, which have typical N-terminal signal sequences, the precursor form of Drosophila Hh contains an internal type-II secretory signal motif. The Short Gastrulation (Sog) protein is another secreted Drosophila protein that contains a type-II signal and differs from its vertebrate ortholog Chordin which contains a standard signal peptide. In this study, we examine the regulation of Sog secretion and regulation by dHIP14, the ortholog of a mammalian palmitoylase first identified as Huntington Interacting Protein (HIP). We show that dHIP14 binds to Sog and that Sog is palmitoylated. In S2 cells, dHIP14 promotes secretion of Sog as well as stabilizing a membrane associated form of Sog. We examined the requirement for candidate cysteine residues in the N-terminal predicted cytoplasmic domain of Sog and find that Cys27, one of two adjacent cysteines (Cys27 and Cys28), is essential for the full activity of dHIP14 and its effect on Sog. Finally, we find that dHIP14 promotes the activity of Sog in vivo. These studies highlight the growing importance of lipid modification in regulating signaling at the level of ligand production and localization.

Mice with alopecia, osteoporosis, and systemic amyloidosis due to mutation in Zdhhc13, a gene coding for palmitoyl acyltransferase

Protein palmitoylation has emerged as an important mechanism for regulating protein trafficking, stability, and protein-protein interactions; however, its relevance to disease processes is not clear. Using a genome-wide, phenotype driven N-ethyl-N-nitrosourea-mediated mutagenesis screen, we identified mice with failure to thrive, shortened life span, skin and hair abnormalities including alopecia, severe osteoporosis, and systemic amyloidosis (both AA and AL amyloids depositions). Whole-genome homozygosity mapping with 295 SNP markers and fine mapping with an additional 50 SNPs localized the disease gene to chromosome 7 between 53.9 and 56.3 Mb. A nonsense mutation (c.1273A>T) was located in exon 12 of the Zdhhc13 gene (Zinc finger, DHHC domain containing 13), a gene coding for palmitoyl transferase. The mutation predicted a truncated protein (R425X), and real-time PCR showed markedly reduced Zdhhc13 mRNA. A second gene trap allele of Zdhhc13 has the same phenotypes, suggesting that this is a loss of function allele. This is the first report that palmitoyl transferase deficiency causes a severe phenotype, and it establishes a direct link between protein palmitoylation and regulation of diverse physiologic functions where its absence can result in profound disease pathology. This mouse model can be used to investigate mechanisms where improper palmitoylation leads to disease processes and to understand molecular mechanisms underlying human alopecia, osteoporosis, and amyloidosis and many other neurodegenerative diseases caused by protein misfolding and amyloidosis.

Palmitoylation of hepatitis C virus core protein is important for virion production

Hepatitis C virus core protein is the viral nucleocapsid of hepatitis C virus. Interaction of core with cellular membranes like endoplasmic reticulum (ER) and lipid droplets (LD) appears to be involved in viral assembly. However, how these interactions with different cellular membranes are regulated is not well understood. In this study, we investigated how palmitoylation, a post-translational protein modification, can modulate the targeting of core to cellular membranes. We show that core is palmitoylated at cysteine 172, which is adjacent to the transmembrane domain at the C-terminal end of core. Site-specific mutagenesis of residue Cys(172) showed that palmitoylation is not involved in the maturation process carried out by the signal peptide peptidase or in the targeting of core to LD. However, palmitoylation was shown to be important for core association with smooth ER membranes and ER closely surrounding LDs. Finally, we demonstrate that mutation of residue Cys(172) in the J6/JFH1 virus genome clearly impairs virion production.

Palmitoylation of nicotinic acetylcholine receptors

It is well established that nicotinic acetylcholine receptors (nAChRs) undergo a number of different posttranslational modifications, such as disulfide bond formation, glycosylation, and phosphorylation. Recently, our laboratory has developed more sensitive assays of protein palmitoylation that have allowed us and others to detect the palmitoylation of relatively low abundant proteins such as ligand-gated ion channels. Here, we present evidence that palmitoylation is prevalent on many subunits of different nAChR subtypes, both muscle-type nAChRs and the neuronal "alpha(4)beta(2)" and "alpha(7)" subtypes most abundant in brain. The loss of ligand binding sites that occurs when palmitoylation is blocked with the inhibitor bromopalmitate suggests that palmitoylation of alpha(4)beta(2) and alpha(7) subtypes occurs during subunit assembly and regulates the formation of ligand binding sites. However, additional experiments are needed to test whether nAChR subunit palmitoylation is involved in other aspects of nAChR trafficking or whether palmitoylation regulates nAChR function. Further investigation would be aided by identifying the sites of palmitoylation on the subunits, and here we propose a mass spectrometry strategy for identification of these sites.

Mobile DHHC palmitoylating enzyme mediates activity-sensitive synaptic targeting of PSD-95

Protein palmitoylation is the most common posttranslational lipid modification; its reversibility mediates protein shuttling between intracellular compartments. A large family of DHHC (Asp-His-His-Cys) proteins has emerged as protein palmitoyl acyltransferases (PATs). However, mechanisms that regulate these PATs in a physiological context remain unknown. In this study, we efficiently monitored the dynamic palmitate cycling on synaptic scaffold PSD-95. We found that blocking synaptic activity rapidly induces PSD-95 palmitoylation and mediates synaptic clustering of PSD-95 and associated AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid)-type glutamate receptors. A dendritically localized DHHC2 but not the Golgi-resident DHHC3 mediates this activity-sensitive palmitoylation. Upon activity blockade, DHHC2 translocates to the postsynaptic density to transduce this effect. These data demonstrate that individual DHHC members are differentially regulated and that dynamic recruitment of protein palmitoylation machinery enables compartmentalized regulation of protein trafficking in response to extracellular signals.

Analysis of DHHC acyltransferases implies overlapping substrate specificity and a two-step reaction mechanism

Asp-His-His-Cys (DHHC) cysteine-rich domain (CRD) acyltransferases are polytopic transmembrane proteins that are found along the endomembrane system of eukaryotic cells and mediate palmitoylation of peripheral and integral membrane proteins. Here, we address the in vivo substrate specificity of five of the seven DHHC acyltransferases for peripheral membrane proteins by an overexpression approach. For all analysed DHHC proteins we detect strongly overlapping substrate specificity. In addition, we now show acyltransferase activity for Pfa5. More importantly, the DHHC protein Pfa3 is able to trap several substrates at the vacuole. For Pfa3 and its substrate Vac8, we can distinguish two consecutive steps in the acylation reaction: an initial binding that occurs independently of its central cysteine in the DHHC box, but requires myristoylation of its substrate Vac8, and a DHHC-motif dependent acylation. Our data also suggest that proteins can be palmitoylated on several organelles. Thus, the intracellular distribution of DHHC proteins provides an acyltransferase network, which may promote dynamic membrane association of substrate proteins.

The specific targeting of guidance receptors within neurons: who directs the directors?

Guidance molecules present in both axonal and dendritic growth cones mediate neuronal responses to extracellular cues thereby ensuring correct neurite pathfinding and development of the nervous system. Little is known though about the mechanisms employed by neurons to deliver these receptors, specifically and efficiently, to the extending growth cone. A deeper understanding of this process is crucial if guidance receptors are to be manipulated to promote nervous system repair. Studies in other polarised cells, notably epithelial, have elucidated fundamental routes to the intracellular segregation of molecules mediated by endosomal pathways. Due to their extreme complexity and specialisation, neurons appear to have built upon these generic systems to evolve sophisticated trafficking networks. A striking feature is the axon initial segment which acts like a valve to tightly regulate the flux of molecules both entering and leaving the axon. Once in the growth cone, further controls operate to enhance the retention or rejection, as appropriate, of membrane receptors. We discuss the current state of knowledge regarding the intracellular trafficking of axon guidance receptors and how this relates to their developmental roles. We highlight the various facets still to be properly elucidated and by building on existing data regarding neuronal polarity and intracellular sorting mechanisms suggest ways to fill these gaps.

Palmitoylation-dependent neurodevelopmental deficits in a mouse model of 22q11 microdeletion

Individuals with 22q11.2 microdeletions have cognitive deficits and a high risk of developing schizophrenia. Here we provide evidence that primary hippocampal neurons from a mouse model of 22q11.2 deletion (Df(16)A(+/-) mice) have decreased density of dendritic spines and glutamatergic synapses, as well as impaired dendritic growth. These deficits were prevented by introduction of the enzymatically active ZDHHC8 palmitoyltransferase encoded by a gene in the 22q11.2 locus, and they were also observed in primary cultures from Zdhhc8-deficient mice. Many of these deficits were also present in the hippocampi of adult Df(16)A(+/-) and Zdhhc8-deficient mice. Finally, we provide evidence that PSD95 is one of the substrates of ZDHHC8. Our analysis reveals that 22q11.2 microdeletion results in deficits in neuronal development and suggests that impaired neuronal protein palmitoylation contributes to many of these deficits.

The Drosophila protein palmitoylome: characterizing palmitoyl-thioesterases and DHHC palmitoyl-transferases

Palmitoylation is the post-translational addition of a palmitate moiety to a cysteine residue through a covalent thioester bond. The addition and removal of this modification is controlled by both palmitoyl acyl-transferases and thioesterases. Using bioinformatic analysis, we identified 22 DHHC family palmitoyl acyl-transferase homologs in the Drosophila genome. We used in situ hybridization,RT-PCR, and published FlyAtlas microarray data to characterize the expression patterns of all 22 fly homologs. Our results indicate that all are expressed genes, but several, including CG1407, CG4676, CG5620, CG6017/dHIP14, CG6618, CG6627 and CG17257 appear to be enriched in neural tissues suggesting that they are important for neural function. Furthermore, we have found that several may be expressed in a sex-specific manner with adult male specific expression of CG4483 and CG17195. Using tagged versions of the DHHC genes, we demonstrate that fly DHHC proteins are primarily located in either the Golgi Apparatus or Endoplasmic Reticulum in S2 cells, except for CG1407, which was found on the plasma membrane. We also characterized the subcellular localization and expression of the three known thioesterases: Palmitoyl-protein Thioesterase 1 (Ppt1), Palmitoyl-protein Thioesterase 2 (Ppt2)and Acyl-protein Thioesterase 1 (APT1). Our results indicate that Ppt1 and Ppt2 are the major lysosomal thioesterases while APT1 is the likely cytoplasmic thioesterase. Finally, in vivo rescue experiments show that Ppt2 expression cannot rescue the neural inclusion phenotypes associated with loss of Ppt1, further supporting distinct functions and substrates for these two thioesterases. These results will serve as the basis for a more complete understanding of the protein palmitoylome's normal cellular functions in the fly and will lead to further insights into the molecular etiology of diseases associated with the mis-regulation of palmitoylation.

Hhat is a palmitoylacyltransferase with specificity for N-palmitoylation of Sonic Hedgehog

Palmitoylation of Sonic Hedgehog (Shh) is critical for effective long- and short-range signaling. Genetic screens uncovered a potential palmitoylacyltransferase (PAT) for Shh, Hhat, but the molecular mechanism of Shh palmitoylation remains unclear. Here, we have developed and exploited an in vitro Shh palmitoylation assay to purify Hhat to homogeneity. We provide direct biochemical evidence that Hhat is a PAT with specificity for attaching palmitate via amide linkage to the N-terminal cysteine of Shh. Other palmitoylated proteins (e.g. PSD95 and Wnt) are not substrates for Hhat, and Porcupine, a putative Wnt PAT, does not palmitoylate Shh. Neither autocleavage nor cholesterol modification is required for Shh palmitoylation. Both the Shh precursor and mature protein are N-palmitoylated by Hhat, and the reaction occurs during passage through the secretory pathway. This study establishes Hhat as a bona fide Shh PAT and serves as a model for understanding how secreted morphogens are modified by distinct PATs.

Analysis of S-acylation of proteins

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

An acyltransferase controls the generation of hematopoietic and endothelial lineages in zebrafish

Hematopoietic and endothelial cells develop from a common progenitor, the hemangioblast, or directly from mesodermal cells. The molecular pathway that regulates the specification of both cell lineages remains elusive. Here, we show that a lysocardiolipin acyltransferase, lycat, is critical for the establishment of both hematopoietic and endothelial lineages. We isolated lycat from the deletion interval of cloche, a zebrafish mutant that has dramatically reduced hematopoietic and endothelial cell lineages. Reduction of lycat mRNA levels in wild-type zebrafish embryos decreases both endothelial and hematopoietic lineages. Lycat mRNA rescues blood lineages in zebrafish cloche mutant embryos. E165R and G166L mutations in the highly conserved catalytic domain in lycat abolish its function in zebrafish hematopoiesis. Epistasis analysis supports that lycat acts upstream of scl and etsrp in zebrafish hemangioblast development. These data indicate that lycat is the earliest known player in the generation of both endothelial and hematopoietic lineages.

Mouse lysocardiolipin acyltransferase controls the development of hematopoietic and endothelial lineages during in vitro embryonic stem-cell differentiation

The blast colony-forming cell (BL-CFC) was identified as an equivalent to the hemangioblast during in vitro embryonic stem (ES) cell differentiation. However, the molecular mechanisms underlying the generation of the BL-CFC remain largely unknown. Here we report the isolation of mouse lysocardiolipin acyltransferase (Lycat) based on homology to zebrafish lycat, a candidate gene for the cloche locus. Mouse Lycat is expressed in hematopoietic organs and is enriched in the Lin(-)C-Kit(+)Sca-1(+) hematopoietic stem cells in bone marrow and in the Flk1(+)/hCD4(+)(Scl(+)) hemangioblast population in embryoid bodies. The forced Lycat transgene leads to increased messenger RNA expression of hematopoietic and endothelial genes as well as increased blast colonies and their progenies, endothelial and hematopoietic lineages. The Lycat small interfering RNA transgene leads to a decrease expression of hematopoietic and endothelial genes. An unbiased genomewide microarray analysis further substantiates that the forced Lycat transgene specifically up-regulates a set of genes related to hemangioblasts and hematopoietic and endothelial lineages. Therefore, mouse Lycat plays an important role in the early specification of hematopoietic and endothelial cells, probably acting at the level of the hemangioblast.

Cellular palmitoylation and trafficking of lipidated peptides

Many important signaling proteins require the posttranslational addition of fatty acid chains for their proper subcellular localization and function. One such modification is the addition of palmitoyl moieties by enzymes known as palmitoyl acyltransferases (PATs). Substrates for PATs include C-terminally farnesylated proteins, such as H- and N-Ras, as well as N-terminally myristoylated proteins, such as many Src-related tyrosine kinases. The molecular and biochemical characterization of PATs has been hindered by difficulties in developing effective methods for the analysis of PAT activity. In this study, we describe the use of cell-permeable, fluorescently labeled lipidated peptides that mimic the PAT recognition domains of farnesylated and myristoylated proteins. These PAT substrate mimetics are accumulated by SKOV3 cells in a saturable and time-dependent manner. Although both peptides are rapidly palmitoylated, the SKOV3 cells have a greater capacity to palmitoylate the myristoylated peptide than the farnesylated peptide. Confocal microscopy indicated that the palmitoylated peptides colocalized with Golgi and plasma membrane markers, whereas the corresponding nonpalmitoylatable peptides accumulated in the Golgi but did not traffic to the plasma membrane. Overall, these studies indicate that the lipidated peptides provide useful cellular probes for quantitative and compartmentalization studies of protein palmitoylation in intact cells.

Regulation of dendritogenesis via a lipid-raft-associated Ca2+/calmodulin-dependent protein kinase CLICK-III/CaMKIgamma

Ca(2+) signaling plays a central role in activity-dependent regulation of dendritic arborization, but key molecular mechanisms downstream of calcium elevation remain poorly understood. Here we show that the C-terminal region of the Ca(2+)/calmodulin-dependent protein kinase CLICK-III (CL3)/CaMKIgamma, a membrane-anchored CaMK, was uniquely modified by two sequential lipidification steps: prenylation followed by a kinase-activity-regulated palmitoylation. These modifications were essential for CL3 membrane anchoring and targeting into detergent-resistant lipid microdomains (or rafts) in the dendrites. We found that CL3 critically contributed to BDNF-stimulated dendritic growth. Raft insertion of CL3 specifically promoted dendritogenesis of cortical neurons by acting upstream of RacGEF STEF and Rac, both present in lipid rafts. Thus, CL3 may represent a key element in the Ca(2+)-dependent and lipid-raft-delineated switch that turns on extrinsic activity-regulated dendrite formation in developing cortical neurons.

Molecular cloning and characterization of human Aph2 gene, involved in AP-1 regulation by interaction with JAB1

A human Aph2 gene (hAph2) was identified and cloned from a human placenta cDNA library. Bioinformatics analysis revealed hAPH2 protein shares 96% identity with mouse APH2 and contains a zf-DHHC domain (148-210aa), which is always involved in protein-protein or protein-DNA interaction. Differential expression patterns of hAph2 mRNA were observed in normal human tissues. Yeast two-hybrid screening found another hAPH2-interacting protein JAB1. The zf-DHHC domain of hAPH2 and the C-terminal of JAB1 were confirmed to be critical for the interaction. Fused with GFP and expressed in COS-7, NIH/3T3 and SMMC-7721 cell lines, hAPH2 showed predominant distribution in the cytoplasm and co-localized with JAB1 around the nucleus. Furthermore, overexpression of hAPH2 could increase apoptosis of COS-7 cells and negatively regulate JAB1-induced activation of AP-1 in a concentration dependent manner. The expression level of c-jun was also down-regulated by overexpression of hAPH2 in COS-7 cells. These data showed some basic characterization and function of hAph2 (hAPH2), dependent or independent with JAB1.

Systematic screening for palmitoyl transferase activity of the DHHC protein family in mammalian cells

Posttranslational modifications, including phosphorylation, ubiquitination and lipid modifications, provide proteins with additional functions and regulation beyond genomic information. Palmitoylation is a reversible lipid modification with palmitic acid that plays critical roles in protein trafficking and function. However, the enzymes that mediate palmitoyl acyl transferase (PAT) have been elusive. Recent genetic analysis in yeast revealed that members of cysteine-rich DHHC domain containing proteins (DHHC proteins) mediate palmitoylation. In mammalian genomes, 23 DHHC proteins are predicted raising the possibility of a large family of PAT enzymes. Here, we describe a systematic method to examine which of the DHHC family members is responsible for palmitoylation of a substrate.

SNAREs — engines for membrane fusion

Since the discovery of SNARE proteins in the late 1980s, SNAREs have been recognized as key components of protein complexes that drive membrane fusion. Despite considerable sequence divergence among SNARE proteins, their mechanism seems to be conserved and is adaptable for fusion reactions as diverse as those involved in cell growth, membrane repair, cytokinesis and synaptic transmission. A fascinating picture of these robust nanomachines is emerging.

How does cholesterol affect the way Hedgehog works?

Members of the Hedgehog (Hh) family of proteins are conserved morphogens that spread and modulate cell fates in target tissue. Mature Hh carries two lipid adducts, a palmitoyl group at the N terminus and cholesterol at the C terminus. Recent findings have addressed how these lipid modifications affect the function and transport of Hh in Drosophila. In contrast to the palmitoyl adduct, cholesterol appears not to be essential for signalling. However, the absence of the cholesterol adduct affects the spread of Hh within tissues. As we discuss here, the exact nature of this effect is controversial.

Comparative genome-wide profiling of post-transplant lymphoproliferative disorders and diffuse large B-cell lymphomas

Post-transplant lymphoproliferative disorders (PTLD) are a major complication of solid organ transplantation, representing a cause of severe morbidity and mortality. Apart from Epstein-Barr virus infection, knowledge of the pathogenesis of monoclonal PTLD is limited. Powerful analysis techniques, such as whole genomic DNA profiling (array comparative genomic hybridisation), can improve our understanding of PTLD pathogenesis. Whole genome profiling using the Affymetrix GeneChip Human Mapping 10 k 2.0 was performed on 20 PTLD cases and 25 cases of diffuse large B-cell lymphoma (DLBCL) from immunocompetent patients as a control group. Recurrent lesions were detected among all the samples. Chromosome 18q, 7q, 3q and 12 were the most common gains in the control group. Chromosomes 5p and 11p were commonly gained in PTLD-DLBCL. The latter had frequent losses of 6q, 17p, 1p and 9p. Chromosome 12p was the most frequent target of deletions among PTLD-DLBCL cases. Loss of heterozygosity (LOH) did not always match DNA loss: chromosome 10 seemed to be targeted by uniparental disomy in PTLD. Small deletions and gains, involving both known (BCL2 and PAX5) and unknown genes (ZDHHC14), were identified. These data suggest that PTLD share, at a lower frequency, common genetic aberrations with DLBCL from immunocompetent patients. The demonstration of 9p13 amplification emphasises the importance of PAX5 in PTLD. The combination of DNA copy number and LOH assessment lead to the hypothesis that uniparental disomy may be a potential mechanism in B-cell lymphomagenesis.

Association of Arabidopsis type-II ROPs with the plasma membrane requires a conserved C-terminal sequence motif and a proximal polybasic domain

Plant ROPs (or RACs) are soluble Ras-related small GTPases that are attached to cell membranes by virtue of the post-translational lipid modifications of prenylation and S-acylation. ROPs (RACs) are subdivided into two major subgroups called type-I and type-II. Whereas type-I ROPs terminate with a conserved CaaL box and undergo prenylation, type-II ROPs undergo S-acylation on two or three C-terminal cysteines. In the present work we determined the sequence requirement for association of Arabidopsis type-II ROPs with the plasma membrane. We identified a conserved sequence motif, designated the GC-CG box, in which the modified cysteines are flanked by glycines. The GC-CG box cysteines are separated by five to six mostly non-polar residues. Deletion of this sequence or the introduction of mutations that change its nature disrupted the association of ROPs with the membrane. Mutations that changed the GC-CG box glycines to alanines also interfered with membrane association. Deletion of a polybasic domain proximal to the GC-CG box disrupted the plasma membrane association of AtROP10. A green fluorescent protein fusion protein containing the C-terminal 25 residues of AtROP10, including its polybasic domain and GC-CG box, was primarily associated with the plasma membrane but a similar fusion protein lacking the polybasic domain was exclusively localized in the soluble fraction. These data provide evidence for the minimal sequence required for plasma membrane association of type-II ROPs in Arabidopsis and other plant species.