Differential palmitoylation of the endosomal SNAREs syntaxin 7 and syntaxin 8

Attenuating ABHD17 Isoforms Augments the S-acylation and Function of NOD2 and a Subset of Crohn's Disease-associated NOD2 Variants

BACKGROUND & AIMS

NOD2 is an intracellular innate immune receptor that detects bacterial peptidoglycan fragments. Although nominally soluble, some NOD2 is associated with the plasma membrane and endosomal compartments for microbial surveillance. This membrane targeting is achieved through post-translational S-acylation of NOD2 by the protein acyltransferase ZDHHC5. Membrane attachment is necessary to initiate a signaling cascade in response to cytosolic peptidoglycan fragments. Ultimately, this signaling results in the production of antimicrobial peptides and proinflammatory cytokines. In most cases, S-acylation is a reversible post-translational modification with removal of the fatty acyl chain catalyzed by one of several acyl protein thioesterases. Deacylation of NOD2 by such an enzyme will displace it from the plasma membrane and endosomes, thus preventing signaling.

METHODS

To identify the enzymes responsible for NOD2 deacylation, we used engineered cell lines with RNA interference and small-molecule inhibitors. These approaches were combined with confocal microscopy, acyl-resin-assisted capture, immunoblotting, and cytokine multiplex assays.

RESULTS

We identified α/β-hydrolase domain-containing protein 17 isoforms (ABHD17A, ABHD17B, and ABHD17C) as the acyl protein thioesterases responsible for NOD2 deacylation. Inhibiting ABHD17 increased the plasma membrane localization of wild-type NOD2 and a subset of poorly acylated Crohn's disease-associated variants. This enhanced NOD2 activity, increasing NF-κB activation and pro-inflammatory cytokine production in epithelial cells.

CONCLUSIONS

These findings demonstrate that ABHD17 isoforms are negative regulators of NOD2. The results also suggest that targeting ABHD17 isoforms could restore functionality to specific Crohn's disease-associated NOD2 variants, offering a potential therapeutic strategy.

Rab3gap1 palmitoylation cycling modulates cardiomyocyte exocytosis and atrial natriuretic peptide release

Rab3 GTPase-activating protein 1 (Rab3gap1) hydrolyzes GTP on Rab3 to inactivate it and reinitiate the Rab3 cycle, which regulates exocytic release of neuropeptides and hormones from neuroendocrine cells and atrial natriuretic peptide (ANP) secretion by cardiomyocytes. Cysteine palmitoylation of Rab3gap1 by the Golgi-localized S-acyltransferase zDHHC9 was recently shown to hinder ANP release by impairing Rab3gap1-mediated nucleotide cycling on Rab3a. Here, we interrogate the cysteine residues of Rab3gap1 modified by palmitoylation and impacts on ANP secretion in cardiomyocytes. Although mutation of the previously identified cysteine (Cys)-678 site of Rab3gap1 alone was insufficient to elicit complete loss of Rab3gap1 palmitoylation in cardiomyocytes, combinatorial mutation of Cys-509, 510, 521, 522, and 678 (Rab3gap15CS) dramatically reduced Rab3gap1 palmitoylation. Notably, total cellular GTPase-activating protein (GAP) activity in cardiomyocytes was maintained with mutation of the Rab3gap1 palmitoylation sites as the Rab3gap15CS mutant substantially reduced steady-state Rab3a-GTP levels in cardiomyocytes similar to wild-type Rab3gap1. However, although expression of wild-type Rab3gap1 induced robust secretion of ANP and greatly enhanced phenylephrine-stimulated ANP release, the Rab3gap15CS palmitoylation-deficient mutant was incapable of promoting exocytosis and ANP release by cardiomyocytes. These data suggest Rab3gap1 cysteine palmitoylation may target Rab3gap1 to Rab3a for regulated GAP-mediated inactivation at specific intracellular membrane domains to modulate the Rab3 cycle and exocytosis. Collectively, these data support a role for Rab3gap1 palmitoylation cycling in spatiotemporal control of the Rab3 cycle to regulate exocytosis and ANP secretion by cardiomyocytes.

Attenuating ABHD17 isoforms augments the S-acylation and function of NOD2 and a subset of Crohn's disease-associated NOD2 variants

BACKGROUND AND AIMS

NOD2 is an intracellular innate immune receptor that detects bacterial peptidoglycan fragments. Although nominally soluble, some NOD2 is associated with the plasma membrane and endosomal compartments for microbial surveillance. This membrane targeting is achieved through post-translational S-acylation of NOD2 by the protein acyltransferase ZDHHC5. Membrane attachment is necessary to initiate a signaling cascade in response to cytosolic peptidoglycan fragments. Ultimately, this signaling results in the production of antimicrobial peptides and pro-inflammatory cytokines. In most cases, S-acylation is a reversible post-translational modification with removal of the fatty acyl chain catalyzed by one of several acyl protein thioesterases. Deacylation of NOD2 by such an enzyme will displace it from the plasma membrane and endosomes, thus preventing signaling.

METHODS

To identify the enzymes responsible for NOD2 deacylation, we used engineered cell lines with RNA interference and small-molecule inhibitors. These approaches were combined with confocal microscopy, acyl-resin-assisted capture, immunoblotting, and cytokine multiplex assays.

RESULTS

We identified α/β-hydrolase domain-containing protein 17 isoforms (ABHD17A, ABHD17B, and ABHD17C) as the acyl protein thioesterases responsible for NOD2 deacylation. Inhibiting ABHD17 increased the plasma membrane localization of wild-type NOD2 and a subset of poorly acylated Crohn's disease-associated variants. This enhanced NOD2 activity, increasing NF-κB activation and pro-inflammatory cytokine production in epithelial cells.

CONCLUSIONS

These findings demonstrate that ABHD17 isoforms are negative regulators of NOD2. The results also suggest that targeting ABHD17 isoforms could restore functionality to specific Crohn's disease-associated NOD2 variants, offering a potential therapeutic strategy.

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

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

Activity-dependent post-translational regulation of palmitoylating and depalmitoylating enzymes in the hippocampus

Activity-induced changes in protein palmitoylation can regulate the plasticity of synaptic connections, critically impacting learning and memory. Palmitoylation is a reversible post-translational modification regulated by both palmitoyl-acyl transferases that mediate palmitoylation and palmitoyl thioesterases that depalmitoylate proteins. However, it is not clear how fluctuations in synaptic activity can mediate the dynamic palmitoylation of neuronal proteins. Using primary hippocampal cultures, we demonstrate that synaptic activity does not impact the transcription of palmitoylating and depalmitoylating enzymes, changes in thioesterase activity, or post-translational modification of the depalmitoylating enzymes of the ABHD17 family and APT2 (also known as LYPLA2). In contrast, synaptic activity does mediate post-translational modification of the palmitoylating enzymes ZDHHC2, ZDHHC5 and ZDHHC9 (but not ZDHHC8) to influence protein-protein interactions, enzyme stability and enzyme function. Post-translational modifications of the ZDHHC enzymes were also observed in the hippocampus following fear conditioning. Taken together, our findings demonstrate that signaling events activated by synaptic activity largely impact activity of the ZDHHC family of palmitoyl-acyl transferases with less influence on the activity of palmitoyl thioesterases.

Rerouting trafficking circuits through posttranslational SNARE modifications

Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) are membrane-associated trafficking proteins that confer identity to lipid membranes and facilitate membrane fusion. These functions are achieved through the complexing of Q-SNAREs with a specific cognate target R-SNARE, leading to the fusion of their associated membranes. These SNARE complexes then dissociate so that the Q-SNAREs and R-SNAREs can repeat this cycle. Whilst the basic function of SNAREs has been long appreciated, it is becoming increasingly clear that the cell can control the localisation and function of SNARE proteins through posttranslational modifications (PTMs), such as phosphorylation and ubiquitylation. Whilst numerous proteomic methods have shown that SNARE proteins are subject to these modifications, little is known about how these modifications regulate SNARE function. However, it is clear that these PTMs provide cells with an incredible functional plasticity; SNARE PTMs enable cells to respond to an ever-changing extracellular environment through the rerouting of membrane traffic. In this Review, we summarise key findings regarding SNARE regulation by PTMs and discuss how these modifications reprogramme membrane trafficking pathways.

Examining the Underappreciated Role of S-Acylated Proteins as Critical Regulators of Phagocytosis and Phagosome Maturation in Macrophages

Phagocytosis is a receptor-mediated process used by cells to engulf a wide variety of particulates, including microorganisms and apoptotic cells. Many of the proteins involved in this highly orchestrated process are post-translationally modified with lipids as a means of regulating signal transduction, membrane remodeling, phagosome maturation and other immunomodulatory functions of phagocytes. S-acylation, generally referred to as S-palmitoylation, is the post-translational attachment of fatty acids to a cysteine residue exposed topologically to the cytosol. This modification is reversible due to the intrinsically labile thioester bond between the lipid and sulfur atom of cysteine, and thus lends itself to a variety of regulatory scenarios. Here we present an overview of a growing number of S-acylated proteins known to regulate phagocytosis and phagosome biology in macrophages.

Protein Palmitoylation in Leukocyte Signaling and Function

Palmitoylation is a post-translational modification (PTM) based on thioester-linkage between palmitic acid and the cysteine residue of a protein. This covalent attachment of palmitate is reversibly and dynamically regulated by two opposing sets of enzymes: palmitoyl acyltransferases containing a zinc finger aspartate-histidine-histidine-cysteine motif (PAT-DHHCs) and thioesterases. The reversible nature of palmitoylation enables fine-tuned regulation of protein conformation, stability, and ability to interact with other proteins. More importantly, the proper function of many surface receptors and signaling proteins requires palmitoylation-meditated partitioning into lipid rafts. A growing number of leukocyte proteins have been reported to undergo palmitoylation, including cytokine/chemokine receptors, adhesion molecules, pattern recognition receptors, scavenger receptors, T cell co-receptors, transmembrane adaptor proteins, and signaling effectors including the Src family of protein kinases. This review provides the latest findings of palmitoylated proteins in leukocytes and focuses on the functional impact of palmitoylation in leukocyte function related to adhesion, transmigration, chemotaxis, phagocytosis, pathogen recognition, signaling activation, cytotoxicity, and cytokine production.

Endosomal and Phagosomal SNAREs

The soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein family is of vital importance for organelle communication. The complexing of cognate SNARE members present in both the donor and target organellar membranes drives the membrane fusion required for intracellular transport. In the endocytic route, SNARE proteins mediate trafficking between endosomes and phagosomes with other endosomes, lysosomes, the Golgi apparatus, the plasma membrane, and the endoplasmic reticulum. The goal of this review is to provide an overview of the SNAREs involved in endosomal and phagosomal trafficking. Of the 38 SNAREs present in humans, 30 have been identified at endosomes and/or phagosomes. Many of these SNAREs are targeted by viruses and intracellular pathogens, which thereby reroute intracellular transport for gaining access to nutrients, preventing their degradation, and avoiding their detection by the immune system. A fascinating picture is emerging of a complex transport network with multiple SNAREs being involved in consecutive trafficking routes.

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.

Genome-wide differential expression of synaptic long noncoding RNAs in autism spectrum disorder

A genome-wide differential expression of long noncoding RNAs (lncRNAs) was identified in blood specimens of autism spectrum disorder (ASD). A total of 3929 lncRNAs were found to be differentially expressed in ASD peripheral leukocytes, including 2407 that were upregulated and 1522 that were downregulated. Simultaneously, 2591 messenger RNAs (mRNAs), including 1789 upregulated and 821 downregulated, were also identified in ASD leukocytes. Functional pathway analysis of these lncRNAs revealed neurological pathways of the synaptic vesicle cycling, long-term depression and long-term potentiation to be primarily involved. Thirteen synaptic lncRNAs, including nine upregulated and four downregulated, and 19 synaptic mRNAs, including 12 upregulated and seven downregulated, were identified as being differentially expressed in ASD. Our identification of differential expression of synaptic lncRNAs and mRNAs suggested that synaptic vesicle transportation and cycling are important for the delivery of synaptosomal protein(s) between presynaptic and postsynaptic membranes in ASD. Finding of 19 lncRNAs, which are the antisense, bi-directional and intergenic, of HOX genes may lead us to investigate the role of HOX genes involved in the development of ASD. Discovery of the lncRNAs of SHANK2-AS and BDNF-AS, the natural antisense of genes SHANK2 and BDNF, respectively, indicates that in addition to gene mutations, deregulation of lncRNAs on ASD-causing gene loci presents a new approach for exploring possible epigenetic mechanisms underlying ASD. Our study also opened a new avenue for exploring the use of lncRNA(s) as biomarker(s) for the early detection of ASD.

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.

The importance of lipid modified proteins in plants

Membranes have long been known to act as more than physical barriers within and between plant cells. Trafficking of membrane proteins, signalling from and across membranes, organisation of membranes and transport through membranes are all essential processes for plant cellular function. These processes rely on a myriad array of proteins regulated in a variety of manners and are frequently required to be directly associated with membranes. For integral membrane proteins, the mode of membrane association is readily apparent, but many peripherally associated membrane proteins are outwardly soluble proteins. In these cases the proteins are frequently modified by the addition of lipids allowing direct interaction with the hydrophobic core of membranes. These modifications include N-myristoylation, S-acylation (palmitoylation), prenylation and GPI anchors but until recently little was truly known about their function in plants. New data suggest that these modifications are able to act as more than just membrane anchors, and dynamic S-acylation in particular is emerging as a means of regulating protein function in a similar manner to phosphorylation. This review discusses how these modifications occur, their impact on protein function, how they are regulated, recent advances in the field and technical approaches for studying these modifications.

The Gos28 SNARE protein mediates intra-Golgi transport of rhodopsin and is required for photoreceptor survival

SNARE proteins play indispensable roles in membrane fusion events in many cellular processes, including synaptic transmission and protein trafficking. Here, we characterize the Golgi SNARE protein, Gos28, and its role in rhodopsin (Rh1) transport through Drosophila photoreceptors. Mutations in gos28 lead to defective Rh1 trafficking and retinal degeneration. We have pinpointed a role for Gos28 in the intra-Golgi transport of Rh1, downstream from α-mannosidase-II in the medial- Golgi. We have confirmed the necessity of key residues in Gos28's SNARE motif and demonstrate that its transmembrane domain is not required for vesicle fusion, consistent with Gos28 functioning as a t-SNARE for Rh1 transport. Finally, we show that human Gos28 rescues both the Rh1 trafficking defects and retinal degeneration in Drosophila gos28 mutants, demonstrating the functional conservation of these proteins. Our results identify Gos28 as an essential SNARE protein in Drosophila photoreceptors and provide mechanistic insights into the role of SNAREs in neurodegenerative disease.

Identification of Giardia lamblia DHHC proteins and the role of protein S-palmitoylation in the encystation process

Protein S-palmitoylation, a hydrophobic post-translational modification, is performed by protein acyltransferases that have a common DHHC Cys-rich domain (DHHC proteins), and provides a regulatory switch for protein membrane association. In this work, we analyzed the presence of DHHC proteins in the protozoa parasite Giardia lamblia and the function of the reversible S-palmitoylation of proteins during parasite differentiation into cyst. Two specific events were observed: encysting cells displayed a larger amount of palmitoylated proteins, and parasites treated with palmitoylation inhibitors produced a reduced number of mature cysts. With bioinformatics tools, we found nine DHHC proteins, potential protein acyltransferases, in the Giardia proteome. These proteins displayed a conserved structure when compared to different organisms and are distributed in different monophyletic clades. Although all Giardia DHHC proteins were found to be present in trophozoites and encysting cells, these proteins showed a different intracellular localization in trophozoites and seemed to be differently involved in the encystation process when they were overexpressed. dhhc transgenic parasites showed a different pattern of cyst wall protein expression and yielded different amounts of mature cysts when they were induced to encyst. Our findings disclosed some important issues regarding the role of DHHC proteins and palmitoylation during Giardia encystation.

Giardiasis is a major cause of non-viral/non-bacterial diarrheal disease worldwide and has been included within the WHO Neglected Disease Initiative since 2004. Infection begins with the ingestion of Giardia lamblia in cyst form, which, after exposure to gastric acid in the host stomach and proteases in the duodenum, gives rise to trophozoites. The inverse process is called encystation and begins when the trophozoites migrate to the lower part of the small intestine where they receive signals that trigger synthesis of the components of the cyst wall. The cyst form enables the parasite to survive in the environment, infect a new host and evade the immune response. In this work, we explored the role of protein S-palmitoylation, a unique reversible post-translational modification, during Giardia encystation, because de novo generation of endomembrane compartments, protein sorting and vesicle fusion occur in this process. Our findings may contribute to the design of therapeutic agents against this important human pathogen.

Analysis of familial hemophagocytic lymphohistiocytosis type 4 (FHL-4) mutant proteins reveals that S-acylation is required for the function of syntaxin 11 in natural killer cells

Natural killer (NK) cell secretory lysosome exocytosis and cytotoxicity are impaired in familial hemophagocytic lymphohistiocytosis type 4 (FHL-4), a disorder caused by mutations in the gene encoding the SNARE protein syntaxin 11. We show that syntaxin 11 binds to SNAP23 in NK cells and that this interaction is reduced by FHL-4 truncation and frameshift mutation proteins that delete all or part of the SNARE domain of syntaxin 11. In contrast the FHL-4 mutant proteins bound to the Sec-1/Munc18-like (SM) protein Munc18-2. We demonstrate that the C-terminal cysteine rich region of syntaxin 11, which is deleted in the FHL-4 mutants, is S-acylated. This posttranslational modification is required for the membrane association of syntaxin 11 and for its polarization to the immunological synapse in NK cells conjugated to target cells. Moreover, we show that Munc18-2 is recruited by syntaxin 11 to intracellular membranes in resting NK cells and to the immunological synapse in activated NK cells. This recruitment of Munc18-2 is abolished by deletion of the C-terminal cysteine rich region of syntaxin 11. These results suggest a pivotal role for S-acylation in the function of syntaxin 11 in NK cells.

The zinc-binding region of IL-2 inducible T cell kinase (Itk) is required for interaction with Gα13 and activation of serum response factor

Tec family kinases play critical roles in the activation of immune cells. In particular, Itk is important for the activation of T cells via the T cell Receptor (TcR), however, molecules that cooperate with Itk to activate downstream targets remain little explored. Here we show that Itk interacts with the heterotrimeric G-protein α subunit Gα13 during TcR triggering. This interaction requires membrane localization of both partners, and is partially dependent on GDP- and GTP-bound states of Gα13. Furthermore, we find that Itk interacts with Gα13 via the zinc binding regions within its Tec homology domain. The interaction between Itk and Gα13 also results in tyrosine phosphorylation of Gα13, however this is not required for the interaction. Itk enhances Gα13 mediated activation of serum response factor (SRF) transcriptional activity dependent on its ability to interact with Gα13, but its kinase activity is not required to enhance SRF activity. These data reveal a new pathway regulated by Itk in cells, and suggest cross talk between Itk and G-protein signaling downstream of the TcR.

A proteomic approach identifies many novel palmitoylated proteins in Arabidopsis

S-acylation (palmitoylation) is a poorly understood post-translational modification of proteins involving the addition of acyl lipids to cysteine residues. S-acylation promotes the association of proteins with membranes and influences protein stability, microdomain partitioning, membrane targeting and activation state. No consensus motif for S-acylation exists and it therefore requires empirical identification. Here, we describe a biotin switch isobaric tagging for relative and absolute quantification (iTRAQ)-based method to identify S-acylated proteins from Arabidopsis. We use these data to predict and confirm S-acylation of proteins not in our dataset. We identified c. 600 putative S-acylated proteins affecting diverse cellular processes. These included proteins involved in pathogen perception and response, mitogen-activated protein kinases (MAPKs), leucine-rich repeat receptor-like kinases (LRR-RLKs) and RLK superfamily members, integral membrane transporters, ATPases, soluble N-ethylmaleimide-sensitive factor-activating protein receptors (SNAREs) and heterotrimeric G-proteins. The prediction of S-acylation of related proteins was demonstrated by the identification and confirmation of S-acylation sites within the SNARE and LRR-RLK families. We showed that S-acylation of the LRR-RLK FLS2 is required for a full response to elicitation by the flagellin derived peptide flg22, but is not required for localization to the plasma membrane. Arabidopsis contains many more S-acylated proteins than previously thought. These data can be used to identify S-acylation sites in related proteins. We also demonstrated that S-acylation is required for full LRR-RLK function.

Emerging roles for protein S-palmitoylation in immunity from chemical proteomics

The activation of innate and adaptive immune signaling pathways and effector functions often occur at cellular membranes and are regulated by complex mechanisms. Here we review the growing number of proteins which are known to be regulated by S-palmitoylation in immune cells emerging from recent advances in chemical proteomics. These chemical proteomic studies have highlighted the roles of this dynamic lipid modification in regulating the specificity and strength of immune responses in different lymphocyte populations.

Tyrosine phosphorylation of a SNARE protein, syntaxin 17: implications for membrane trafficking in the early secretory pathway

The T-cell protein tyrosine phosphatase is expressed as two splice variants - TC45, a nuclear protein, and TC48, which is localized predominantly in the ER (endoplasmic reticulum). Yeast two-hybrid screening revealed direct interaction of TC48 with Syntaxin17, a SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) protein localized predominantly in the ER and to some extent in the ER-Golgi intermediate compartment. Syntaxin 17 did not interact with TC45. C-terminal 40 amino acids of TC48 were sufficient for interaction with syntaxin 17. Overexpressed syntaxin 17 was phosphorylated at tyrosine upon pervanadate treatment (a tyrosine phosphatase inhibitor/tyrosine kinase activator) of COS-1 cells. Mutational analysis identified Tyr156 in the cytoplasmic domain as the major site of phosphorylation. Endogenous syntaxin 17 was phosphorylated by pervanadate treatment in CHO and MIN6 cells but was not phosphorylated in a variety of other cell lines tested. c-Abl was identified as one of the kinases, which phosphorylates syntaxin 17 in MIN6 cells. Phosphorylation of endogenous and overexpressed syntaxin 17 was reduced in the presence of IGF receptor and EGF receptor kinase inhibitors. Serum depletion reduced pervanadate-induced phosphorylation of endogenous syntaxin 17. TC48 coexpression reduced phosphorylation of syntaxin 17 by pervanadate and purified TC48 directly dephosphorylated syntaxin 17. β-COP dispersal by overexpressed syntaxin 17 was reduced after pervanadate-induced phosphorylation. A phospho-mimicking mutant (Y156E) of syntaxin 17 showed reduced interaction with COPI vesicles. These results suggest that tyrosine phosphorylation of syntaxin 17 is likely to have a role in regulating syntaxin 17 dependent membrane trafficking in the early secretory pathway.

Specificity of transmembrane protein palmitoylation in yeast

Many proteins are modified after their synthesis, by the addition of a lipid molecule to one or more cysteine residues, through a thioester bond. This modification is called S-acylation, and more commonly palmitoylation. This reaction is carried out by a family of enzymes, called palmitoyltransferases (PATs), characterized by the presence of a conserved 50- aminoacids domain called “Asp-His-His-Cys- Cysteine Rich Domain” (DHHC-CRD). There are 7 members of this family in the yeast Saccharomyces cerevisiae, and each of these proteins is thought to be responsible for the palmitoylation of a subset of substrates. Substrate specificity of PATs, however, is not yet fully understood. Several yeast PATs seem to have overlapping specificity, and it has been proposed that the machinery responsible for palmitoylating peripheral membrane proteins in mammalian cells, lacks specificity altogether.

Here we investigate the specificity of transmembrane protein palmitoylation in S. cerevisiae, which is carried out predominantly by two PATs, Swf1 and Pfa4. We show that palmitoylation of transmembrane substrates requires dedicated PATs, since other yeast PATs are mostly unable to perform Swf1 or Pfa4 functions, even when overexpressed. Furthermore, we find that Swf1 is highly specific for its substrates, as it is unable to substitute for other PATs. To identify where Swf1 specificity lies, we carried out a bioinformatics survey to identify amino acids responsible for the determination of specificity or Specificity Determination Positions (SDPs) and showed experimentally, that mutation of the two best SDP candidates, A145 and K148, results in complete and partial loss of function, respectively. These residues are located within the conserved catalytic DHHC domain suggesting that it could also be involved in the determination of specificity. Finally, we show that modifying the position of the cysteines in Tlg1, a Swf1 substrate, results in lack of palmitoylation, as expected for a highly specific enzymatic reaction.

Gi/o signaling and the palmitoyltransferase DHHC2 regulate palmitate cycling and shuttling of RGS7 family-binding protein

R7BP (RGS7 family-binding protein) has been proposed to function in neurons as a palmitoylation-regulated protein that shuttles heterodimeric, G(i/o)α-specific GTPase-activating protein (GAP) complexes composed of Gβ5 and RGS7 (R7) isoforms between the plasma membrane and nucleus. To test this hypothesis we studied R7BP palmitoylation and localization in neuronal cells. We report that R7BP undergoes dynamic, signal-regulated palmitate turnover; the palmitoyltransferase DHHC2 mediates de novo and turnover palmitoylation of R7BP; DHHC2 silencing redistributes R7BP from the plasma membrane to the nucleus; and G(i/o) signaling inhibits R7BP depalmitoylation whereas G(i/o) inactivation induces nuclear accumulation of R7BP. In concert with previous evidence, our findings suggest that agonist-induced changes in palmitoylation state facilitate GAP action by (i) promoting Giα depalmitoylation to create optimal GAP substrates, and (ii) inhibiting R7BP depalmitoylation to stabilize membrane association of R7-Gβ5 GAP complexes. Regulated palmitate turnover may also enable R7BP-bound GAPs to shuttle between sites of low and high G(i/o) activity or the plasma membrane and nucleus, potentially providing spatio-temporal control of signaling by G(i/o)-coupled receptors.

Cytosol-dependent membrane fusion in ER, nuclear envelope and nuclear pore assembly: biological implications

Endoplasmic reticulum and nuclear envelope rearrangements after mitosis are often studied in the reconstitution system based on Xenopus egg extract. In our recent work we partially replaced the membrane vesicles in the reconstitution mix with protein-free liposomes to explore the relative contributions of cytosolic and transmembrane proteins. Here we discuss our finding that cytosolic proteins mediate fusion between membranes lacking functional transmembrane proteins and the role of membrane fusion in endoplasmic reticulum and nuclear envelope reorganization. Cytosol-dependent liposome fusion has allowed us to restore, without adding transmembrane nucleoporins, functionality of nuclear pores, their spatial distribution and chromatin decondensation in nuclei formed at insufficient amounts of membrane material and characterized by only partial decondensation of chromatin and lack of nuclear transport. Both the mechanisms and the biological implications of the discovered coupling between spatial distribution of nuclear pores, chromatin decondensation and nuclear transport are discussed.

Detergent-free isolation and characterization of cholesterol-rich membrane domains from trans-Golgi network vesicles

Cholesterol is an abundant lipid of the trans-Golgi network (TGN) and of certain endosomal membranes where cholesterol-rich microdomains are important in the organization and compartmentalization of vesicular trafficking. Here we describe the development of a rapid method to isolate a cholesterol-rich endomembrane fraction. We show that widely used subcellular fractionation techniques incompletely separate cholesterol-rich membranes, such as the TGN, from organelles, such as late endosomes and lysosomes. To address this issue, we devised a new subcellular fractionation scheme involving two rounds of velocity centrifugation, membrane sonication, and discontinuous sucrose density gradient centrifugation. This strategy resulted in the isolation of a cholesterol and GM1 glycosphingolipid-enriched membrane fraction that was completely cleared of plasma membrane, endoplasmic reticulum, and mitochondria. This buoyant fraction was enriched for the TGN and recycling endosome proteins Rab11 and syntaxin-6, and it was well resolved from cis-Golgi and early and late endosomal membranes. We demonstrate that this technique can give useful insights into the compartmentation of phosphoinositide synthesis, and it facilitates the isolation of cholesterol-rich membranes from a population of TGN-trafficking vesicles.

Exocytosis mechanisms underlying insulin release and glucose uptake: conserved roles for Munc18c and syntaxin 4

Type 2 diabetes has been coined "a two-hit disease," as it involves specific defects of glucose-stimulated insulin secretion from the pancreatic beta cells in addition to defects in peripheral tissue insulin action required for glucose uptake. Both of these processes, insulin secretion and glucose uptake, are mediated by SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) protein core complexes composed of syntaxin, SNAP-23/25, and VAMP proteins. The SNARE core complex is regulated by the Sec1/Munc18 (SM) family of proteins, which selectively bind to their cognate syntaxin isoforms with high affinity. The process of insulin secretion uses multiple Munc18-syntaxin isoform pairs, whereas insulin action in the peripheral tissues appears to use only the Munc18c-syntaxin 4 pair. Importantly, recent reports have linked obesity and Type 2 diabetes in humans with changes in protein levels and single nucleotide polymorphisms (SNPs) of Munc18 and syntaxin isoforms relevant to these exocytotic processes, although the molecular mechanisms underlying the observed phenotypes remain incomplete (5, 104, 144). Given the conservation of these proteins in two seemingly disparate processes and the need to design and implement novel and more effective clinical interventions, it will be vitally important to delineate the mechanisms governing these conserved SNARE-mediated exocytosis events. Thus, we provide here an up-to-date historical review of advancements in defining the roles and molecular mechanisms of Munc18-syntaxin complexes in the pathophysiology of Type 2 diabetes.

Palmitoylation of the synaptic vesicle fusion machinery

The fusion of synaptic vesicles with the pre-synaptic plasma membrane mediates the secretion of neurotransmitters at nerve terminals. This pathway is regulated by an array of protein-protein interactions. Of central importance are the soluble NSF (N-ethylmaleimide-sensitive factor) attachment protein receptor (SNARE) proteins syntaxin 1 and SNAP25, which are associated with the pre-synaptic plasma membrane and vesicle-associated membrane protein (VAMP2), a synaptic vesicle SNARE. Syntaxin 1, SNAP25 and VAMP2 interact to form a tight complex bridging the vesicle and plasma membranes, which has been suggested to represent the minimal membrane fusion machinery. Synaptic vesicle fusion is stimulated by a rise in intraterminal Ca2+ levels, and a major Ca2+ sensor for vesicle fusion is synaptotagmin I. Synaptotagmin is likely to couple Ca2+ entry to vesicle fusion via Ca2+-dependent and independent interactions with membrane phospholipids and the SNARE proteins. Intriguingly, syntaxin 1, SNAP25, VAMP2 and synaptotagmin I have all been reported to be modified by palmitoylation in neurons. In this review, we discuss the mechanisms and dynamics of palmitoylation of these proteins and speculate on how palmitoylation might contribute to the regulation of synaptic vesicle fusion.