Localization and activity of the SNARE Ykt6 determined by its regulatory domain and palmitoylation

Lipid Regulated Intramolecular Conformational Dynamics of SNARE-Protein Ykt6

Cellular informational and metabolic processes are propagated with specific membrane fusions governed by soluble N-ethylmaleimide sensitive factor attachment protein receptors (SNARE). SNARE protein Ykt6 is highly expressed in brain neurons and plays a critical role in the membrane-trafficking process. Studies suggested that Ykt6 undergoes a conformational change at the interface between its longin domain and the SNARE core. In this work, we study the conformational state distributions and dynamics of rat Ykt6 by means of single-molecule Förster Resonance Energy Transfer (smFRET) and Fluorescence Cross-Correlation Spectroscopy (FCCS). We observed that intramolecular conformational dynamics between longin domain and SNARE core occurred at the timescale ~200 μs. Furthermore, this dynamics can be regulated and even eliminated by the presence of lipid dodecylphoshpocholine (DPC). Our molecular dynamic (MD) simulations have shown that, the SNARE core exhibits a flexible structure while the longin domain retains relatively stable in apo state. Combining single molecule experiments and theoretical MD simulations, we are the first to provide a quantitative dynamics of Ykt6 and explain the functional conformational change from a qualitative point of view.

TANGO1 recruits ERGIC membranes to the endoplasmic reticulum for procollagen export

Previously we showed that membrane fusion is required for TANGO1-dependent export of procollagen VII from the endoplasmic reticulum (ER) (Nogueira, et al., 2014). Along with the t-SNARE Syntaxin 18, we now reveal the complete complement of SNAREs required in this process, t-SNAREs BNIP1 and USE1, and v-SNARE YKT6. TANGO1 recruits YKT6-containing ER Golgi Intermediate Compartment (ERGIC) membranes to procollagen VII-enriched patches on the ER. Moreover residues 1214-1396, that include the first coiled coil of TANGO1, specifically recruit ERGIC membranes even when targeted to mitochondria. TANGO1 is thus pivotal in concentrating procollagen VII in the lumen and recruiting ERGIC membranes on the cytoplasmic surface of the ER. Our data reveal that growth of a mega transport carrier for collagen export from the ER is not by acquisition of a larger patch of ER membrane, but instead by addition of ERGIC membranes to procollagen-enriched domains of the ER by a TANGO1-mediated process.

DOI:http://dx.doi.org/10.7554/eLife.10982.001

Structure and function of longin SNAREs

Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins constitute the core membrane fusion machinery of intracellular transport and intercellular communication. A little more than ten years ago, it was proposed that the long N-terminal domain of a subset of SNAREs, henceforth called the longin domain, could be a crucial regulator with multiple functions in membrane trafficking. Structural, biochemical and cell biology studies have now produced a large set of data that support this hypothesis and indicate a role for the longin domain in regulating the sorting and activity of SNAREs. Here, we review the first decade of structure-function data on the three prototypical longin SNAREs: Ykt6, VAMP7 and Sec22b. We will, in particular, highlight the conserved molecular mechanisms that allow longin domains to fold back onto the fusion-inducing SNARE coiled-coil domain, thereby inhibiting membrane fusion, and describe the interactions of longin SNAREs with proteins that regulate their intracellular sorting. This dual function of the longin domain in regulating both the membrane localization and membrane fusion activity of SNAREs points to its role as a key regulatory module of intracellular trafficking.

TLR signals induce phagosomal MHC-I delivery from the endosomal recycling compartment to allow cross-presentation

Adaptation of the endoplasmic reticulum (ER) pathway for MHC class I (MHC-I) presentation in dendritic cells enables cross-presentation of peptides derived from phagocytosed microbes, infected cells, or tumor cells to CD8 T cells. How these peptides intersect with MHC-I molecules remains poorly understood. Here, we show that MHC-I selectively accumulate within phagosomes carrying microbial components, which engage Toll-like receptor (TLR) signaling. Although cross-presentation requires Sec22b-mediated phagosomal recruitment of the peptide loading complex from the ER-Golgi intermediate compartment (ERGIC), this step is independent of TLR signaling and does not deliver MHC-I. Instead, MHC-I are recruited from an endosomal recycling compartment (ERC), which is marked by Rab11a, VAMP3/cellubrevin, and VAMP8/endobrevin and holds large reserves of MHC-I. While Rab11a activity stocks ERC stores with MHC-I, MyD88-dependent TLR signals drive IκB-kinase (IKK)2-mediated phosphorylation of phagosome-associated SNAP23. Phospho-SNAP23 stabilizes SNARE complexes orchestrating ERC-phagosome fusion, enrichment of phagosomes with ERC-derived MHC-I, and subsequent cross-presentation during infection.

Soluble NSF attachment protein receptor molecular mimicry by a Legionella pneumophila Dot/Icm effector

Upon infection, Legionella pneumophila uses the Dot/Icm type IV secretion system to translocate effector proteins from the Legionella-containing vacuole (LCV) into the host cell cytoplasm. The effectors target a wide array of host cellular processes that aid LCV biogenesis, including the manipulation of membrane trafficking. In this study, we used a hidden Markov model screen to identify two novel, non-eukaryotic soluble NSF attachment protein receptor (SNARE) homologs: the bacterial Legionella SNARE effector A (LseA) and viral SNARE homolog A proteins. We characterized LseA as a Dot/Icm effector of L. pneumophila, which has close homology to the Qc-SNARE subfamily. The lseA gene was present in multiple sequenced L. pneumophila strains including Corby and was well distributed among L. pneumophila clinical and environmental isolates. Employing a variety of biochemical, cell biological and microbiological techniques, we found that farnesylated LseA localized to membranes associated with the Golgi complex in mammalian cells and LseA interacted with a subset of Qa-, Qb- and R-SNAREs in host cells. Our results suggested that LseA acts as a SNARE protein and has the potential to regulate or mediate membrane fusion events in Golgi-associated pathways.

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.

Lipid regulated conformational dynamics of the longin SNARE protein Ykt6 revealed by molecular dynamics simulations

The conformation and subcellular localization of R-SNARE protein Ykt6 are regulated by the lipidation state of its C-terminal CCAIM motif. Biochemical and crystallography studies showed that lipid molecules binding at a hydrophobic pocket at the interface between the longin domain and the SNARE core can lock Ykt6 at a closed conformation and mimic the farnesylated state of Ykt6. In this study, we performed in silico farnesylation of Ykt6 and explored the conformational dynamics of Ykt6 using conventional and steered MD simulations. We found that the farnesylated Ykt6 model structure is stable during the 2 μs simulation and the farnesyl group adopts conformations similar to those of the DPC molecule bound to Ykt6. Both DPC binding and farnesylation were found to reduce the conformational flexibility of Ykt6 and hinder the dissociation of SNARE core from the longin domain. The dissociation of the αF-αG segment is the rate-limiting step during the putative closed-to-open conformational transition of Ykt6, and the key residues involved in this process are consistent with the experimental mutagenesis study.

A comparative analysis of trypanosomatid SNARE proteins

The Kinetoplastida are flagellated protozoa evolutionary distant and divergent from yeast and humans. Kinetoplastida include trypanosomatids, and a number of important pathogens. Trypanosoma brucei, Trypanosoma cruzi and Leishmania spp. inflict significant morbidity and mortality on humans and livestock as the etiological agents of human African trypanosomiasis, Chagas' disease and leishmaniasis respectively. For all of these organisms, intracellular trafficking is vital for maintenance of the host–pathogen interface, modulation/evasion of host immune system responses and nutrient uptake. Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) are critical components of the intracellular trafficking machinery in eukaryotes, mediating membrane fusion and contributing to organelle specificity. We asked how the SNARE complement evolved across the trypanosomatids. An in silico search of the predicted proteomes of T. b. brucei and T. cruzi was used to identify candidate SNARE sequences. Phylogenetic analysis, including comparisons with yeast and human SNAREs, allowed assignment of trypanosomatid SNAREs to the Q or R subclass, as well as identification of several SNAREs orthologous with those of opisthokonts. Only limited variation in number and identity of SNAREs was found, with Leishmania major having 27 and T. brucei 26, suggesting a stable SNARE complement post-speciation. Expression analysis of T. brucei SNAREs revealed significant differential expression between mammalian and insect infective forms, especially within R and Qb-SNARE subclasses, suggesting possible roles in adaptation to different environments. For trypanosome SNAREs with clear orthologs in opisthokonts, the subcellular localization of TbVAMP7C is endosomal while both TbSyn5 and TbSyn16B are at the Golgi complex, which suggests conservation of localization and possibly also function. Despite highly distinct life styles, the complement of trypanosomatid SNAREs is quite stable between the three pathogenic lineages, suggesting establishment in the last common ancestor of trypanosomes and Leishmania. Developmental changes to SNARE mRNA levels between blood steam and procyclic life stages suggest that trypanosomes modulate SNARE functions via expression. Finally, the locations of some conserved SNAREs have been retained across the eukaryotic lineage.

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SNARE proteins are essential components of intracellular transport.

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These proteins exhibit considerable conservation across pathogenic trypanosomes.

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Some trypanosome SNARE families are expanded or lost.

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Developmental changes in trypanosome SNARE expression are apparent.

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Orthologous SNAREs demonstrate conserved locations and hence function.

Functional redundancy between trans-Golgi network SNARE family members in Arabidopsis thaliana

Background

Vesicle fusion is an essential process for maintaining the structure and function of the endomembrane system. Fusion is mediated by t-SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) fusion proteins on the target membrane and v-SNAREs on the vesicle membrane; v-and t-SNAREs interact with each other, driving vesicle fusion with the target membrane. The Arabidopsis thaliana trans-Golgi network resident SNAREs SYP41 and VTI12, along with YKT61/62, have been shown to function in vesicle fusion in vitro, consistent with immunoprecipitation results showing their interaction in Arabidopsis cell extracts. Conflicting published results have indicated that SYP4 family members are either functionally redundant or have distinct and essential functions; the reason for this discrepancy is unclear.

Results

Here we used a proteoliposome fusion assay to demonstrate that SYP42 and SYP43 can substitute for SYP41 in driving lipid mixing, providing support for functional overlap between family members. Previous reports have also suggested that VTI11 and VTI12 SNAREs show partial overlap in function, despite having mostly distinct localizations and binding partners. We show that VTI11 can substitute for VTI12 in in vitro lipid mixing reactions, providing molecular support for the genetic evidence for partial functional redundancy in vivo.

Conclusions

Our data provide biochemical evidence for functional overlap in membrane fusion between members of the SYP4 or VTI1 SNARE groups, supporting previous genetic data suggesting redundancy.

Selective control of SNARE recycling by Golgi retention

Two distinct sets of soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNARE) catalyze membrane fusion in the cis-Golgi and trans-Golgi. The mechanism that controls Golgi localization of SNAREs remains largely unknown. Here we tested three potential mechanisms, including vesicle recycling between the Golgi and the endoplasmic reticulum, partitioning in Golgi lipid microdomains, and selective intra-Golgi retention. Recycling rates showed a linear relationship with intra-Golgi mobility of SNAREs. The cis-Golgi SNAREs had higher mobility than intra-Golgi SNAREs, whereas vesicle SNAREs had higher mobility than target membrane SNAREs. The differences in SNARE mobility were not due to preferential partitioning into detergent-resistant membrane microdomains. We propose that intra-Golgi retention precludes entropy-driven redistribution of SNAREs to the endoplasmic reticulum and endocytic compartments.

Specificity of botulinum protease for human VAMP family proteins

The botulinum neurotoxin light chain (BoNT-LC) is a zinc-dependent metalloprotease that cleaves neuronal SNARE proteins such as SNAP-25, VAMP2, and Syntaxin1. This cleavage interferes with the neurotransmitter release of peripheral neurons and results in flaccid paralysis. SNAP, VAMP, and Syntaxin are representative of large families of proteins that mediate most membrane fusion reactions, as well as both neuronal and non-neuronal exocytotic events in eukaryotic cells. Neuron-specific SNARE proteins, which are target substrates of BoNT, have been well studied; however, it is unclear whether other SNARE proteins are also proteolyzed by BoNT. Herein, we define the substrate specificity of BoNT-LC/B, /D, and /F towards recombinant human VAMP family proteins. We demonstrate that LC/B, /D, and /F are able to cleave VAMP1, 2, and 3, but no other VAMP family proteins. Kinetic analysis revealed that all LC have higher affinity and catalytic activity for the non-neuronal SNARE isoform VAMP3 than for the neuronal VAMP1 and 2 isoforms. LC/D in particular exhibited extremely low catalytic activity towards VAMP1 relative to other interactions, which we determined through point mutation analysis to be a result of the Ile present at residue 48 of VAMP1. We also identified the VAMP3 cleavage sites to be at the Gln 59-Phe 60 (LC/B), Lys 42-Leu 43 (LC/D), and Gln 41-Lys 42 (LC/F) peptide bonds, which correspond to those of VAMP1 or 2. Understanding the substrate specificity and kinetic characteristics of BoNT towards human SNARE proteins may aid in the development of novel therapeutic uses for BoNT.

Organization of SNAREs within the Golgi stack

Antero- and retrograde cargo transport through the Golgi requires a series of membrane fusion events. Fusion occurs at the cis- and trans-side and along the rims of the Golgi stack. Four functional SNARE complexes have been identified mediating lipid bilayer merger in the Golgi. Their function is tightly controlled by a series of reactions involving vesicle tethering and SM proteins. This network of protein interactions spatially and temporally determines the specificity of transport vesicle targeting and fusion within the Golgi.

SNARE proteins are required for macroautophagy

Macroautophagy mediates the degradation of long-lived proteins and organelles via the de novo formation of double-membrane autophagosomes that sequester cytoplasm and deliver it to the vacuole/lysosome; however, relatively little is known about autophagosome biogenesis. Atg8, a phosphatidylethanolamine-conjugated protein, was previously proposed to function in autophagosome membrane expansion, based on the observation that it mediates liposome tethering and hemifusion in vitro. We show here that with physiological concentrations of phosphatidylethanolamine, Atg8 does not act as a fusogen. Rather, we provide evidence for the involvement of exocytic Q/t-SNAREs in autophagosome formation, acting in the recruitment of key autophagy components to the site of autophagosome formation, and in regulating the organization of Atg9 into tubulovesicular clusters. Additionally, we found that the endosomal Q/t-SNARE Tlg2 and the R/v-SNAREs Sec22 and Ykt6 interact with Sso1-Sec9, and are required for normal Atg9 transport. Thus, multiple SNARE-mediated fusion events are likely to be involved in autophagosome biogenesis.

Mechanisms of protein retention in the Golgi

The protein composition of the Golgi is intimately linked to its structure and function. As the Golgi serves as the major protein-sorting hub for the secretory pathway, it faces the unique challenge of maintaining its protein composition in the face of constant influx and efflux of transient cargo proteins. Much of our understanding of how proteins are retained in the Golgi has come from studies on glycosylation enzymes, largely because of the compartment-specific distributions these proteins display. From these and other studies of Golgi membrane proteins, we now understand that a variety of retention mechanisms are employed, the majority of which involve the dynamic process of iterative rounds of retrograde and anterograde transport. Such mechanisms rely on protein conformation and amino acid-based sorting signals as well as on properties of transmembrane domains and their relationship with the unique lipid composition of the Golgi.

Genome-wide identification of Phytophthora sojae SNARE genes and functional characterization of the conserved SNARE PsYKT6

Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) are central components of the machinery mediating membrane fusion and key factors for vesicular trafficking in all eukaryotic cells. Taking advantage of the available whole genome sequence of the oomycete plant pathogen Phytophthora sojae, 35 genes encoding putative SNARE proteins were identified in the genome of this organism. PsYKT6, one of the most conserved SNARE proteins, was functionally characterized by homology-dependent gene silencing. The phenotype analysis showed that PsYKT6 is important for proper asexual development, sexual reproduction, and pathogenesis on host soybean cultivars.

Combination of NMR spectroscopy and X-ray crystallography offers unique advantages for elucidation of the structural basis of protein complex assembly

NMR spectroscopy and X-ray crystallography are two premium methods for determining the atomic structures of macro-biomolecular complexes. Each method has unique strengths and weaknesses. While the two techniques are highly complementary, they have generally been used separately to address the structure and functions of biomolecular complexes. In this review, we emphasize that the combination of NMR spectroscopy and X-ray crystallography offers unique power for elucidating the structures of complicated protein assemblies. We demonstrate, using several recent examples from our own laboratory, that the exquisite sensitivity of NMR spectroscopy in detecting the conformational properties of individual atoms in proteins and their complexes, without any prior knowledge of conformation, is highly valuable for obtaining the high quality crystals necessary for structure determination by X-ray crystallography. Thus NMR spectroscopy, in addition to answering many unique structural biology questions that can be addressed specifically by that technique, can be exceedingly powerful in modern structural biology when combined with other techniques including X-ray crystallography and cryo-electron microscopy.

Proteomic analysis of male 4C germ cell proteins involved in mouse meiosis

Male meiosis is a specialized type of cell division that gives rise to sperm. Errors in this process can result in the generation of aneuploid gametes, which are associated with birth defects and infertility in humans. Until now, there has been a lack of a large-scale identification of proteins involved in male meiosis in mammals. In this study, we report the high-confidence identification of 3625 proteins in mouse male germ cells with 4C DNA content undergoing meiosis I. Of these, 397 were found to be testis specific. Bioinformatics analysis of the proteome led to the identification of 28 proteins known to be essential for male meiosis in mice. We also found 172 proteins that had yeast orthologs known to be essential for meiosis. Chromosome distribution analysis of the proteome showed underrepresentation of the identified proteins on the X chromosome, which may be due to meiotic sex chromosome inactivation. Characterization of the proteome of 4C germ cells from mouse testis provides an inventory of proteins, which is useful for understanding meiosis and the mechanisms of male infertility.

Alpha-synuclein delays endoplasmic reticulum (ER)-to-Golgi transport in mammalian cells by antagonizing ER/Golgi SNAREs

This work demonstrates that α-synuclein inhibits the biosynthetic secretory pathway of mammalian cells potently and directly under nontoxic conditions and in the absence of insoluble α-synuclein aggregates. A potential mechanism involving α-synuclein binding to ER/Golgi SNAREs and inhibiting fusogenic SNARE complex assembly is elucidated.

Toxicity of human α-synuclein when expressed in simple organisms can be suppressed by overexpression of endoplasmic reticulum (ER)-to-Golgi transport machinery, suggesting that inhibition of constitutive secretion represents a fundamental cause of the toxicity. Whether similar inhibition in mammals represents a cause of familial Parkinson's disease has not been established. We tested elements of this hypothesis by expressing human α-synuclein in mammalian kidney and neuroendocrine cells and assessing ER-to-Golgi transport. Overexpression of wild type or the familial disease-associated A53T mutant α-synuclein delayed transport by up to 50%; however, A53T inhibited more potently. The secretory delay occurred at low expression levels and was not accompanied by insoluble α-synuclein aggregates or mistargeting of transport machinery, suggesting a direct action of soluble α-synuclein on trafficking proteins. Co-overexpression of ER/Golgi arginine soluble N-ethylmaleimide-sensitive factor attachment protein receptors (R-SNAREs) specifically rescued transport, indicating that α-synuclein antagonizes SNARE function. Ykt6 reversed α-synuclein inhibition much more effectively than sec22b, suggesting a possible neuroprotective role for the enigmatic high expression of ykt6 in neurons. In in vitro reconstitutions, purified α-synuclein A53T protein specifically inhibited COPII vesicle docking and fusion at a pre-Golgi step. Finally, soluble α-synuclein A53T directly bound ER/Golgi SNAREs and inhibited SNARE complex assembly, providing a potential mechanism for toxic effects in the early secretory pathway.

Analysis of protein S-acylation by gas chromatography-coupled mass spectrometry using purified proteins

S-acylation, also known as palmitoylation, involves the attachment of acyl fatty acids to thiol groups of cysteine residues through a reversible thioester bond. Owing to its reversibility, S-acylation is important in regulation of diverse signaling cascades, including Ras-associated cancers in mammals, stress response and metabolic regulation. Here we describe a simple protocol for analysis of protein S-acylation using gas chromatography-coupled mass spectrometry. Analysis can be carried out with as little as 1 microg of purified protein and allows chemical identification and, potentially, quantification of the acyl moieties. The method is based on cleavage of the fatty acids from proteins by hydrogenation with platinum (IV) oxide. This causes an acid transesterification of the acyl groups, adding an ethyl group to the carboxyl head of the fatty acid. The addition of the ethyl group reduces the polarity of the fatty acids, allowing their efficient separation by gas chromatography.

Lipid-Induced conformational switch controls fusion activity of longin domain SNARE Ykt6

While most SNAREs are permanently anchored to membranes by their transmembrane domains, the dually lipidated SNARE Ykt6 is found both on intracellular membranes and in the cytosol. The cytosolic Ykt6 is inactive due to the autoinhibition of the SNARE core by its longin domain, although the molecular basis of this inhibition is unknown. Here, we demonstrate that unlipidated Ykt6 adopts multiple conformations, with a small population in the closed state. The structure of Ykt6 in complex with a fatty acid suggests that, upon farnesylation, the Ykt6 SNARE core forms four alpha helices that wrap around the longin domain, forming a dominantly closed conformation. The fatty acid, buried in a hydrophobic groove formed between the longin domain and its SNARE core, is essential for maintaining the autoinhibited conformation of Ykt6. Our study reveals that the posttranslationally attached farnesyl group can actively regulate Ykt6 fusion activity in addition to its anticipated membrane-anchoring role.

Transmembrane protein-free membranes fuse into xenopus nuclear envelope and promote assembly of functional pores

Post-mitotic reassembly of nuclear envelope (NE) and the endoplasmic reticulum (ER) has been reconstituted in a cell-free system based on interphase Xenopus egg extract. To evaluate the relative contributions of cytosolic and transmembrane proteins in NE and ER assembly, we replaced a part of native membrane vesicles with ones either functionally impaired by trypsin or N-ethylmaleimide treatments or with protein-free liposomes. Although neither impaired membrane vesicles nor liposomes formed ER and nuclear membrane, they both supported assembly reactions by fusing with native membrane vesicles. At membrane concentrations insufficient to generate full-sized functional nuclei, addition of liposomes and their fusion with membrane vesicles resulted in an extensive expansion of NE, further chromatin decondensation, restoration of the functionality, and spatial distribution of the nuclear pore complexes (NPCs), and, absent newly delivered transmembrane proteins, an increase in NPC numbers. This rescue of the nuclear assembly by liposomes was inhibited by wheat germ agglutinin and thus required active nuclear transport, similarly to the assembly of full-sized functional NE with membrane vesicles. Mechanism of fusion between liposomes and between liposomes and membrane vesicles was investigated using lipid mixing assay. This fusion required interphase cytosol and, like fusion between native membrane vesicles, was inhibited by guanosine 5'-3-O-(thio)triphosphate, soluble N-ethylmaleimide-sensitive factor attachment protein, and N-ethylmaleimide. Our findings suggest that interphase cytosol contains proteins that mediate the fusion stage of ER and NE reassembly, emphasize an unexpected tolerance of nucleus assembly to changes in concentrations of transmembrane proteins, and reveal the existence of a feedback mechanism that couples NE expansion with NPC assembly.

Multiple roles of the vesicular-SNARE TI-VAMP in post-Golgi and endosomal trafficking

SNARE (Soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins are the core machinery of membrane fusion. Vesicular SNAREs (v-SNAREs) interact with their target SNAREs (t-SNAREs) to form SNARE complexes which mediate membrane fusion. Here we review the basic properties and functions of the v-SNARE TI-VAMP/VAMP7 (Tetanus neurotoxin insensitive-vesicle associated membrane protein). TI-VAMP interacts with its t-SNARE partners, particularly plasmalemmal syntaxins, to mediate membrane fusion and with several regulatory proteins especially via its amino-terminal regulatory Longin domain. Partners include AP-3, Hrb/(Human immunodeficiency virus Rev binding) protein, and Varp (Vps9 domain and ankyrin repeats containing protein) and regulate TI-VAMP's function and targeting. TI-VAMP is involved both in secretory and endocytic pathways which mediate neurite outgrowth and synaptic transmission, plasma membrane remodeling and lysosomal secretion.

The Longin Domain Regulates the Steady-State Dynamics of Sec22 in Plasmodium falciparum

The specificity of vesicle-mediated transport is largely regulated by the membrane-specific distribution of SNARE (soluble N -ethylmaleimide-sensitive factor attachment protein receptor) proteins. However, the signals and machineries involved in SNARE protein targeting to the respective intracellular locations are not fully understood. We have identified a Sec22 ortholog in Plasmodium falciparum (PfSec22) that contains an atypical insertion of the Plasmodium export element within the N-terminal longin domain. This Sec22 protein partially associates with membrane structures in the parasitized erythrocytes when expressed under the control of the endogenous promoter element. Our studies indicate that the atypical longin domain contains signals that are required for both endoplasmic reticulum (ER)/Golgi apparatus recycling of PfSec22 and partial export beyond the ER/Golgi apparatus interface. ER exit of PfSec22 is regulated by motifs within the α3 segment of the longin domain, whereas the recycling and export signals require residues within the N-terminal hydrophobic segment. Our data suggest that the longin domain of PfSec22 exhibits major differences from the yeast and mammalian orthologs, perhaps indicative of a novel mechanism for Sec22 trafficking in malaria parasites.

Sec22b is a negative regulator of phagocytosis in macrophages

The endoplasmic reticulum (ER) is proposed to be a membrane donor for phagosome formation. In support of this, we have previously shown that the expression level of syntaxin 18, an ER-localized SNARE protein, correlates with phagocytosis activity. To obtain further insights into the involvement of the ER in phagocytosis we focused on Sec22b, another ER-localized SNARE protein that is also found on phagosomal membranes. In marked contrast to the effects of syntaxin 18, we report here that phagocytosis was nearly abolished in J774 macrophages stably expressing mVenus-tagged Sec22b, without affecting the cell surface expression of the Fc receptor or other membrane proteins related to phagocytosis. Conversely, the capacity of the parental J774 cells for phagocytosis was increased when endogenous Sec22b expression was suppressed. Domain analyses of Sec22b revealed that the R-SNARE motif, a selective domain for forming a SNARE complex with syntaxin18 and/or D12, was responsible for the inhibition of phagocytosis. These results strongly support the ER-mediated phagocytosis model and indicate that Sec22b is a negative regulator of phagocytosis in macrophages, most likely by regulating the level of free syntaxin 18 and/or D12 at the site of phagocytosis.

SNAREs in neurons – beyond synaptic vesicle exocytosis (Review)

The paradigm for soluble N-ethylmaleimide sensitive factor attachment protein receptor (SNARE) function in mammalian cells has been built on advancements in our understanding of structural and biochemical aspects of synaptic vesicle exocytosis, involving specifically synaptobrevin, syntaxin 1 and SNAP25. Interestingly, a good number of SNAREs which are not directly involved in neurotransmitter exocytosis, are either brain-enriched or have distinct neuron-specific functions. Syntaxins 12/13 regulates glutamate receptor recycling via its interaction with neuron-enriched endosomal protein of 21 kDa (NEEP21). TI-VAMP/VAMP7 is essential for neuronal morphogenesis and mediates the vesicular transport processes underlying neurite outgrowth. Ykt6 is highly enriched in the cerebral cortex and hippocampus and is targeted to a novel compartment in neurons. Syntaxin 16 has a moderate expression level in many tissues, but is rather enriched in the brain. Here, we review and discuss the neuron-specific physiology and possible pathology of these and other (such as SNAP-29 and Vti1a-beta) members of the SNARE family.

Blocking the secretion of saliva by silencing the HlYkt6 gene in the tick Haemaphysalis longicornis

Soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptors (SNAREs) have been identified as the key components of the protein complexes that facilitate vesicle traffic, of which Ykt6 (from Saccharomyces cerevisiae, v-SNARE) is proved to be a multifunctional protein in the membrane fusion. In the present study, a tick homologue of Ykt6 (HlYkt6, predicted 22.6 kDa), was isolated from the ixodid tick Haemaphysalis longicornis. RT-PCR and Western blot analysis indicated that the gene and the encoded protein were expressed ubiquitously in different tissues of the partially fed adult tick. Silencing of the HlYkt6 gene resulted in a significant decrease of the engorged body weight (82.9 +/- 26.8 mg vs. 232.17 +/- 59.1 mg in the PBS-injected control group and 178.7 +/- 57.0 mg in the GFP dsRNA-injected control group) and high mortality of replete ticks (100% in tested group vs. 4.8% in the PBS and 20.4% in GFP dsRNA-injected control groups). Disruption of HlYkt6 mRNA led to the suppression of saliva secretion, and a lower anticoagulant activity of the released liquid from the glands (APTT time: 25.25 +/- 1.50 s) than that of the control groups (39.25 +/- 0.50 s in the PBS-treated group and 40.0 +/- 1.41 s in the GFP dsRNA-treated group). These results suggest the vital role of the HlYkt6 protein in the exocytosis of saliva proteins, the feeding and survival of ticks.

Differential palmitoylation of the endosomal SNAREs syntaxin 7 and syntaxin 8

Palmitoylation is a posttranslational modification that regulates protein trafficking and stability. In this study we investigated whether the endosomal soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNARE) proteins syntaxin 7 and syntaxin 8 are modified with palmitate. Using metabolic labeling and site-directed mutagenesis, we show that human syntaxins 7 and 8 are modified with palmitate through a thioester linkage. Palmitoylation is dependent upon cysteine 239 of human syntaxin 7 and cysteine 214 of syntaxin 8, residues that are located on the cytoplasmic face of the transmembrane domain (TMD). Palmitoylation of syntaxin 8 is minimally affected by the Golgi-disturbing agent brefeldin A (BFA), whereas BFA dramatically inhibits palmitoylation of syntaxin7. The differential effect of BFA suggests that palmitoylation of syntaxins 7 and 8 occurs in distinct subcellular compartments. Palmitoylation does not affect the rate of protein turnover of syntaxins 7 and 8 nor does it influence the steady-state localization of syntaxin 8 in late endosomes. Syntaxin 7 actively cycles between endosomes and the plasma membrane. Palmitoylation-defective syntaxin 7 is selectively retained on the plasma membrane, suggesting that palmitoylation is important for intercompartmental transport of syntaxin 7.

Depalmitoylation of Ykt6 prevents its entry into the multivesicular body pathway

The dually lipidated SNARE Ykt6 is found on intracellular membranes and in the cytosol. In this study, we show that Ykt6 localizes to the Golgi as well as endosomal and vacuolar membranes in vivo. The ability of Ykt6 to cycle between the cytosol and the membranes depends on the intramolecular interaction of the N-terminal longin and C-terminal SNARE domains and not on either domain alone. A mutant deficient in this interaction accumulates on membranes and--in contrast to the wild-type protein--does not get released from vacuoles. Our data also indicate that Ykt6 is a substrate of the DHHC (Asp-His-His-Cys) acyltransferase network. Overexpression of the vacuolar acyltransferase Pfa3 drives the F42S mutant not only to the vacuole but also into the vacuolar lumen. Thus, depalmitoylation and release of Ykt6 are needed for its recycling and to circumvent its entry into the endosomal multivesicular body pathway.

Farnesylation of the SNARE protein Ykt6 increases its stability and helical folding

The evolutionarily conserved soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins are involved in the fusion of vesicles with their target membranes. While most SNAREs are permanently anchored to membranes by their transmembrane domains, the vesicle-associated SNARE Ykt6 has been found both in soluble and in membrane-bound pools. The R-SNARE Ykt6 is thought to mediate interactions between various Q-SNAREs by a reversible membrane-targeting cycle. Membrane attachment of Ykt6 is achieved by its C-terminal prenylation and palmitoylation motif succeeding the SNARE motif. In this study, we have analyzed full-length farnesylated Ykt6 from yeast and humans by biochemical and structural means. In vitro farnesylation of the C-terminal CAAX box of recombinant full-length Ykt6 resulted in stabilization of the native protein and a more compactly folded structure, as shown by size exclusion chromatography and limited proteolysis. Circular dichroism spectroscopy indicated a specific increase in the helical content of the farnesylated Ykt6 compared to the nonlipidated form or the single-longin domain, which correlated with a marked increase in stability as observed by heat denaturation experiments. Although highly soluble, farnesylated Ykt6 is capable of lipid membrane binding independent of the membrane charge, as shown by surface plasmon resonance. The crystal structure of the N-terminal longin domain of yeast Ykt6 (1-140) was determined at 2.5 A resolution. As similarly found in a previous NMR structure, the Ykt6 longin domain contains a hydrophobic patch at its surface that may accommodate the lipid moiety. In the crystal structure, this hydrophobic surface is buried in a crystallographic homomeric dimer interface. Together, these observations support a previously suggested closed conformation of cytosolic Ykt6, where the C-terminal farnesyl moiety folds onto a hydrophobic groove in the N-terminal longin domain.

Systematic analysis of SNARE localization in the filamentous fungus Aspergillus oryzae

In spite of their great importance for both applied and basic biology, studies on vesicular trafficking in filamentous fungi have been so far very limited. Here, we identified 21 genes, which might be a total set, encoding putative SNARE proteins that are key factors for vesicular trafficking, taking advantage of available whole genome sequence in the filamentous fungus Aspergillus oryzae. The subsequent systematic analysis to determine the localization of putative SNAREs using EGFP-fused chimeras revealed that most putative SNAREs show similar subcellular distribution to their counterparts in the budding yeast. However, there existed some characteristic features of SNAREs in A. oryzae, such as SNARE localization at/near the septum and the presence of apparently non-redundant plasma membrane Qa-SNAREs. Overall, this analysis allowed us to provide an overview of vesicular trafficking and organelle distribution in A. oryzae.

The SNARE protein family of Leishmania major

Background

Leishmania major is a protozoan parasite with a highly polarised cell shape that depends upon endocytosis and exocytosis from a single area of the plasma membrane, the flagellar pocket. SNAREs (soluble N-ethylmaleimide-sensitive factor adaptor proteins receptors) are key components of the intracellular vesicle-mediated transports that take place in all eukaryotic cells. They are membrane-bound proteins that facilitate the docking and fusion of vesicles with organelles. The recent availability of the genome sequence of L. major has allowed us to assess the complement of SNAREs in the parasite and to investigate their location in comparison with metazoans.

Results

Bioinformatic searches of the L. major genome revealed a total of 27 SNARE domain-containing proteins that could be classified in structural groups by phylogenetic analysis. 25 of these possessed the expected features of functional SNAREs, whereas the other two could represent kinetoplastid-specific proteins that might act as regulators of the SNARE complexes. Other differences of Leishmania SNAREs were the absence of double SNARE domain-containing and of the brevin classes of these proteins. Members of the Qa group of Leishmania SNAREs showed differential expressions profiles in the two main parasite forms whereas their GFP-tagging and in vivo expression revealed localisations in the Golgi, late endosome/lysosome and near the flagellar pocket.

Conclusion

The early-branching eukaryote L. major apparently possess a SNARE repertoire that equals in number the one of metazoans such as Drosophila, showing that the machinery for vesicle fusion is well conserved throughout the eukaryotes. However, the analysis revealed the absence of certain types of SNAREs found in metazoans and yeast, while suggesting the presence of original SNAREs as well as others with unusual localisation. This study also presented the intracellular localisation of the L. major SNAREs from the Qa group and reveals that these proteins could be useful as organelle markers in this parasitic protozoon.

Unique self-palmitoylation activity of the transport protein particle component Bet3: a mechanism required for protein stability

Bet3 is a component of the transport protein particle complex involved in vesicular trafficking to and through the Golgi complex. X-ray structural analysis of human and mouse Bet3 revealed a hydrophobic tunnel inside the protein, which is occupied by a fatty acid linked to cysteine-68. We show here that Bet3 has strong self-palmitoylating activity. Incubation of purified Bet3 with [3H]palmitoyl-CoA (Pal-CoA) leads to a rapid and stoichiometric attachment of fatty acids to cysteine-68. Bet3 has an intrinsic affinity for Pal-CoA, and the palmitoylation reaction occurs at physiological pH values and Pal-CoA concentrations. Moreover, Bet3 is also efficiently palmitoylated at cysteine-68 inside vertebrate cells. Palmitoylation can occur late after Bet3 synthesis, but once the fatty acids are bound they are not removed, not even by disassembly of the Golgi complex. Narrowing the hydrophobic tunnel by exchange of alanine-82 with bulkier amino acids inhibits palmitoylation, both in vitro and inside cells, indicating that the fatty acid must insert into the tunnel for stable attachment. Finally, we show that palmitoylation of Bet3 plays a structural role. CD spectroscopy reveals that chemically deacylated Bet3 has a reduced melting temperature. As a consequence of its structural defect nonacylated Bet3 does not bind to TPC6, a further subunit of the transport protein particle complex, and is degraded inside cells.

Global analysis of protein palmitoylation in yeast

Protein palmitoylation is a reversible lipid modification that regulates membrane tethering for key proteins in cell signaling, cancer, neuronal transmission, and membrane trafficking. Palmitoylation has proven to be a difficult study: Specifying consensuses for predicting palmitoylation remain unavailable, and first-example palmitoylation enzymes--i.e., protein acyltransferases (PATs)--were identified only recently. Here, we use a new proteomic methodology that purifies and identifies palmitoylated proteins to characterize the palmitoyl proteome of the yeast Saccharomyces cerevisiae. Thirty-five new palmitoyl proteins are identified, including many SNARE proteins and amino acid permeases as well as many other participants in cellular signaling and membrane trafficking. Analysis of mutant yeast strains defective for members of the DHHC protein family, a putative PAT family, allows a matching of substrate palmitoyl proteins to modifying PATs and reveals the DHHC family to be a family of diverse PAT specificities responsible for most of the palmitoylation within the cell.

Identification of the yeast R-SNARE Nyv1p as a novel longin domain-containing protein

Using nuclear magnetic resonance spectroscopy, we establish that the N-terminal domain of the yeast vacuolar R-SNARE Nyv1p adopts a longin-like fold similar to those of Sec22b and Ykt6p. Nyv1p is sorted to the limiting membrane of the vacuole via the adaptor protein (AP)3 adaptin pathway, and we show that its longin domain is sufficient to direct transport to this location. In contrast, we found that the longin domains of Sec22p and Ykt6p were not sufficient to direct their localization. A YXX phi-like adaptin-dependent sorting signal (Y31GTI34) unique to the longin domain of Nyv1p mediates interactions with the AP3 complex in vivo and in vitro. We show that amino acid substitutions to Y31GTI34 (Y31Q;I34Q) resulted in mislocalization of Nyv1p as well as reduced binding of the mutant protein to the AP3 complex. Although the sorting of Nyv1p to the limiting membrane of the vacuole is dependent upon the Y31GTI34 motif, and Y31 in particular, our findings with structure-based amino acid substitutions in the mu chain (Apm3p) of yeast AP3 suggest a mechanistically distinct role for this subunit in the recognition of YXX phi-like sorting signals.

A Multigene Family Encoding R-SNAREs in the Ciliate Paramecium tetraurelia

SNARE proteins (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) mediate membrane interactions and are conventionally divided into Q-SNAREs and R-SNAREs according to the possession of a glutamine or arginine residue at the core of their SNARE domain. Here, we describe a set of R-SNAREs from the ciliate Paramecium tetraurelia consisting of seven families encoded by 12 genes that are expressed simultaneously. The complexity of the endomembrane system in Paramecium can explain this high number of genes. All P. tetraurelia synaptobrevins (PtSybs) possess a SNARE domain and show homology to the Longin family of R-SNAREs such as Ykt6, Sec22 and tetanus toxin-insensitive VAMP (TI-VAMP). We localized four exemplary PtSyb subfamilies with GFP constructs and antibodies on the light and electron microscopic level. PtSyb1-1, PtSyb1-2 and PtSyb3-1 were found in the endoplasmic reticulum, whereas PtSyb2 is localized exclusively in the contractile vacuole complex. PtSyb6 was found cytosolic but also resides in regularly arranged structures at the cell cortex (parasomal sacs), the cytoproct and oral apparatus, probably representing endocytotic compartments. With gene silencing, we showed that the R-SNARE of the contractile vacuole complex, PtSyb2, functions to maintain structural integrity as well as functionality of the osmoregulatory system but also affects cell division.

A live cell, image-based approach to understanding the enzymology and pharmacology of 2-bromopalmitate and palmitoylation

The addition of a lipid moiety to a protein increases its hydrophobicity and subsequently its attraction to lipophilic environments like membranes. Indeed most lipid-modified proteins are localized to membranes where they associate with multiprotein signaling complexes. Acylation and prenylation are the two common categories of lipidation. The enzymology and pharmacology of prenylation are well understood but relatively very little is known about palmitoylation, the most common form of acylation. One distinguishing characteristic of palmitoylation is that it is a dynamic modification. To understand more about how palmitoylation is regulated, we fused palmitoylation substrates to fluorescent proteins and reported their subcellular distribution and trafficking. We used automated high-throughput fluorescence microscopy and a specialized computer algorithm to image and measure the fraction of palmitoylation reporter on the plasma membrane versus the cytoplasm. Using this system we determined the residence half-life of palmitate on the dipalmitoyl substrate peptide from GAP43 as well as the EC(50) for 2-bromopalmitate, a common inhibitor of palmitoylation.

Functions of SNAREs in intracellular membrane fusion and lipid bilayer mixing

Intracellular membrane fusion occurs with exquisite coordination and specificity. Each fusion event requires three basic components: Rab-GTPases organize the fusion site; SNARE proteins act during fusion; and N-ethylmaleimide-sensitive factor (NSF) plus its cofactor alpha-SNAP are required for recycling or activation of the fusion machinery. Whereas Rab-GTPases seem to mediate the initial membrane contact, SNAREs appear to lie at the center of the fusion process. It is known that formation of complexes between SNAREs from apposed membranes is a prerequisite for lipid bilayer mixing; however, the biophysics and many details of SNARE function are still vague. Nevertheless, recent observations are shedding light on the role of SNAREs in membrane fusion. Structural studies are revealing the mechanisms by which SNARES form complexes and interact with other proteins. Furthermore, it is now apparent that the SNARE transmembrane segment not only anchors the protein but engages in SNARE-SNARE interactions and plays an active role in fusion. Recent work indicates that the fusion process itself may comprise two stages and proceed via a hemifusion intermediate.

Modulation of neuronal protein trafficking and function by palmitoylation

Modification of proteins with the lipid palmitate regulates targeting to specific vesicular compartments and synaptic membranes. Mounting evidence indicates that this lipid modification modulates diverse aspects of neuronal development and synaptic transmission. In particular, palmitoylation regulates the function of proteins that control neuronal differentiation, axonal pathfinding and filopodia formation. In addition, trafficking of numerous proteins associated with synaptic vesicle release machinery requires protein palmitoylation. Remarkably, reversible palmitoylation of specific scaffolding proteins and signaling molecules dynamically regulates ion channel clustering and synaptic strength. The recent discovery of enzymes that palmitoylate specific subsets of synaptic proteins suggests that this process is tightly controlled in neurons.

The SNARE Ykt6 is released from yeast vacuoles during an early stage of fusion

The farnesylated SNARE (N-ethylmaleimide-sensitive factor attachment protein receptor) Ykt6 mediates protein palmitoylation at the yeast vacuole by means of its amino-terminal longin domain. Ykt6 is localized equally to membranes and the cytosol, although it is unclear how this distribution is mediated. We now show that Ykt6 is released efficiently from vacuoles during an early stage of yeast vacuole fusion. This release is dependent on the disassembly of vacuolar SNAREs (priming). In recent literature, it had been demonstrated for mammalian Ykt6 that the membrane-bound form is both palmitoylated and farnesylated at its carboxy-terminal CAAX box, whereas soluble Ykt6 is only farnesylated. In agreement with this, we find that yeast Ykt6 becomes palmitoylated in vitro at its C-terminal CAAX motif. Mutagenesis of the potential palmitoylation site in yeast Ykt6 prevents stable membrane association and is lethal. On the basis of these and other findings, we speculate that Ykt6 is released from membranes by depalmitoylation. Such a mechanism could enable recycling of this lipid-anchored SNARE from the vacuole independent of retrograde transport.

The Golgi apparatus: defining the identity of Golgi membranes

The Golgi apparatus is a stack of compartments that serves as a central junction for membrane traffic, with carriers moving through the stack as well as arriving from, and departing toward, many other destinations in the cell. This requires that the different compartments in the Golgi recruit from the cytosol a distinct set of proteins to mediate accurate membrane traffic. This recruitment appears to reflect recognition of small GTPases of the Rab and Arf family, or of lipid species such as PtdIns(4)P and diacylglycerol, which provide a unique "identity" for each compartment. Recent work is starting to reveal the mechanisms by which these labile landmarks are generated in a spatially restricted manner on specific parts of the Golgi.

Intra-Golgi transport: A way to a new paradigm?

The morpho-functional principles of intra-Golgi transport are, surprisingly, still not clear, which is in marked contrast to our advanced knowledge of the underlying molecular machineries. Recently, the conceptual and technological hindrances that had delayed progress in this area have been disappearing, and a cluster of powerful morphological techniques has been revealing new glimpses of the organization of traffic in intact cells. Here, we discuss the new concepts around the present models of intra-Golgi transport.

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

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

Palmitoylation regulates plasma membrane-nuclear shuttling of R7BP, a novel membrane anchor for the RGS7 family

The RGS7 (R7) family of RGS proteins bound to the divergent Gβ subunit Gβ5 is a crucial regulator of G protein–coupled receptor (GPCR) signaling in the visual and nervous systems. Here, we identify R7BP, a novel neuronally expressed protein that binds R7–Gβ5 complexes and shuttles them between the plasma membrane and nucleus. Regional expression of R7BP, Gβ5, and R7 isoforms in brain is highly coincident. R7BP is palmitoylated near its COOH terminus, which targets the protein to the plasma membrane. Depalmitoylation of R7BP translocates R7BP–R7–Gβ5 complexes from the plasma membrane to the nucleus. Compared with nonpalmitoylated R7BP, palmitoylated R7BP greatly augments the ability of RGS7 to attenuate GPCR-mediated G protein–regulated inward rectifying potassium channel activation. Thus, by controlling plasma membrane nuclear–shuttling of R7BP–R7–Gβ5 complexes, reversible palmitoylation of R7BP provides a novel mechanism that regulates GPCR signaling and potentially transduces signals directly from the plasma membrane to the nucleus.

The human SNARE protein Ykt6 mediates its own palmitoylation at C-terminal cysteine residues

The yeast SNARE (soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor) protein Ykt6 was shown to mediate palmitoylation of the fusion factor Vac8 in a reaction essential for the fusion of vacuoles. Here I present evidence that hYkt6 (human Ykt6) has self-palmitoylating activity. Incubation of recombinant hYkt6 with [3H]Pal-CoA ([3H]palmitoyl-CoA) leads to covalent attachment of palmitate to C-terminal cysteine residues. The N-terminal domain of human Ykt6 contains a Pal-CoA binding site and is required for the reaction.

The longin domain regulates subcellular targeting of VAMP7 in Arabidopsis thaliana

SNAREs (soluble N-ethyl-maleimide sensitive factor attachment protein receptors) which locate on the specific organelle membrane assure the correct vesicular transport by mediating specific membrane fusions. SNAREs are referred to as R- or Q-SNAREs on the basis of the amino acid sequence similarities and specific conserved residues. All of the Arabidopsis R-SNAREs have a N-terminal domain, called the longin domain (LD). In this study, we investigated the vacuolar targeting mechanism of Arabidopsis R-SNAREs. The vacuolar localized AtVAMP711 was used as the mother protein of GFP-tagged chimeric proteins joined to several domains such as the LD, the SNARE motif (SNM) and the transmembrane domain (TMD) of other organelle-localized R-SNAREs. The results showed that, whereas the TMD is not relevant for the vacuolar targeting, a complete LD is essential for the vacuolar and subcellular targeting.