A targeting signal in VAMP regulating transport to synaptic vesicles

A comprehensive review of the influence of Epigallocatechin gallate on Sjögren's syndrome associated molecular regulators of exocytosis (Review)

Sjögren's syndrome (SS) is an autoimmune disorder that affects the salivary glands, leading to reduced secretory functions and oral and ocular dryness. The salivary glands are composed of acinar cells that are responsible for the secretion and production of secretory granules, which contain salivary components, such as amylase, mucins and immunoglobulins. This secretion process involves secretory vesicle trafficking, docking, priming and membrane fusion. A failure during any of the steps in exocytosis in the salivary glands results in the altered secretion of saliva. Soluble N-ethylmaleimide-sensitive-factor attachment protein receptors, actin, tight junctions and aquaporin 5 all serve an important role in the trafficking regulation of secretory vesicles in the secretion of saliva via exocytosis. Alterations in the expression and distribution of these selected proteins leads to salivary gland dysfunction, including SS. Several studies have demonstrated that green tea polyphenols, most notably Epigallocatechin gallate (EGCG), possess both anti-inflammatory and anti-apoptotic properties in normal human cells. Molecular, cellular and animal studies have indicated that EGCG can provide protective effects against autoimmune and inflammatory reactions in salivary glands in diseases such as SS. The aim of the present article is to provide a comprehensive and up-to-date review on the possible therapeutic interactions between EGCG and the selected molecular mechanisms associated with SS.

A reference library for assigning protein subcellular localizations by image-based machine learning

Confocal micrographs of EGFP fusion proteins localized at key cell organelles in murine and human cells were acquired for use as subcellular localization landmarks. For each of the respective 789,011 and 523,319 optically validated cell images, morphology and statistical features were measured. Machine learning algorithms using these features permit automated assignment of the localization of other proteins and dyes in both cell types with very high accuracy. Automated assignment of subcellular localizations for model tail-anchored proteins with randomly mutated C-terminal targeting sequences allowed the discovery of motifs responsible for targeting to mitochondria, endoplasmic reticulum, and the late secretory pathway. Analysis of directed mutants enabled refinement of these motifs and characterization of protein distributions in within cellular subcompartments.

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.

Functional mapping of brain synapses by the enriching activity-marker SynaptoZip

Ideally, elucidating the role of specific brain circuits in animal behavior would require the ability to measure activity at all involved synapses, possibly with unrestricted field of view, thus even at those boutons deeply located into the brain. Here, we introduce and validate an efficient scheme reporting synaptic vesicle cycling in vivo. This is based on SynaptoZip, a genetically encoded molecule deploying in the vesicular lumen a bait moiety designed to capture upon exocytosis a labeled alien peptide, Synbond. The resulting signal is cumulative and stores the number of cycling events occurring at individual synapses. Since this functional signal is enduring and measurable both online and ex post, SynaptoZip provides a unique method for the analysis of the history of synaptic activity in regions several millimeters below the brain surface. We show its broad applicability by reporting stimulus-evoked and spontaneous circuit activity in wide cortical fields, in anesthetized and freely moving animals.

Visualization of synaptic activity in the living brain is challenging. This study devises a simple and efficient scheme that reports synaptic vesicle recycling in vivo using SynaptoZip, a genetically encoded sensor of past synaptic activities.

Safeguards of Neurotransmission: Endocytic Adaptors as Regulators of Synaptic Vesicle Composition and Function

Communication between neurons relies on neurotransmitters which are released from synaptic vesicles (SVs) upon Ca²⁺ stimuli. To efficiently load neurotransmitters, sense the rise in intracellular Ca²⁺ and fuse with the presynaptic membrane, SVs need to be equipped with a stringently controlled set of transmembrane proteins. In fact, changes in SV protein composition quickly compromise neurotransmission and most prominently give rise to epileptic seizures. During exocytosis SVs fully collapse into the presynaptic membrane and consequently have to be replenished to sustain neurotransmission. Therefore, surface-stranded SV proteins have to be efficiently retrieved post-fusion to be used for the generation of a new set of fully functional SVs, a process in which dedicated endocytic sorting adaptors play a crucial role. The question of how the precise reformation of SVs is achieved is intimately linked to how SV membranes are retrieved. For a long time both processes were believed to be two sides of the same coin since Clathrin-mediated endocytosis (CME), the proposed predominant SV recycling mode, will jointly retrieve SV membranes and proteins. However, with the recent proposal of Clathrin-independent SV recycling pathways SV membrane retrieval and SV reformation turn into separable events. This review highlights the progress made in unraveling the molecular mechanisms mediating the high-fidelity retrieval of SV proteins and discusses how the gathered knowledge about SV protein recycling fits in with the new notions of SV membrane endocytosis.

Indication for differential sorting of the rat v-SNARE splice isoforms VAMP-1a and -1b

Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins are essential constituents of the intracellular trafficking machinery. The variable C-terminus in the 2 rat VAMP-1 splice isoforms VAMP-1a and -1b potentially acts as a sorting signal, because similar changes at the C-terminal end of a human VAMP-1 splice isoform resulted in its sorting to mitochondria. To evaluate the differences in the subcellular localization of these two v-SNARE proteins, VAMP-1a and -1b proteins tagged with green fluorescent protein (GFP) and red fluorescent protein (RFP) were expressed in HeLa, COS-7, and MDCK cells and evaluated by conventional confocal as well as total internal reflection fluorescence microscopy. Regions consistent with the endoplasmic reticulum and Golgi apparatus demonstrated a major overlap of both signals. In the periphery, vesicular structures were observed that mainly expressed one of the 2 isoforms. Within our experimental settings, we could not observe sorting of any of the 2 isoforms to mitochondria or peroxisomes, whereas both isoforms were found expressed in a minor subset of singular vesicles, which sporadically appeared to co-localize with the exocyst marker EXOC3/Sec6. Because vesicular structures were seen that expressed only one of the two splice variants, it is possible that VAMP-1a and VAMP-1b are sorted to distinct cellular compartments that require further characterization.

CRISPR/Cas-Mediated Knockin in Human Pluripotent Stem Cells

Fluorescent reporter and epitope-tagged human pluripotent stem cells (hPSCs) greatly facilitate studies on the pluripotency and differentiation characteristics of these cells. Unfortunately traditional procedures to generate such lines are hampered by a low targeting efficiency that necessitates a lengthy process of selection followed by the removal of the selection cassette. Here we describe a procedure to generate fluorescent reporter and epitope tagged hPSCs in an efficient one-step process using the CRISPR/Cas technology. Although the method described uses our recently developed iCRISPR platform, the protocols can be adapted for general use with CRISPR/Cas or other engineered nucleases. The transfection procedures described could also be used for additional applications, such as overexpression or lineage tracing studies.

Synaptic vesicle recycling: steps and principles

Synaptic vesicle recycling is one of the best-studied cellular pathways. Many of the proteins involved are known, and their interactions are becoming increasingly clear. However, as for many other pathways, it is still difficult to understand synaptic vesicle recycling as a whole. While it is generally possible to point out how synaptic reactions take place, it is not always easy to understand what triggers or controls them. Also, it is often difficult to understand how the availability of the reaction partners is controlled: how the reaction partners manage to find each other in the right place, at the right time. I present here an overview of synaptic vesicle recycling, discussing the mechanisms that trigger different reactions, and those that ensure the availability of reaction partners. A central argument is that synaptic vesicles bind soluble cofactor proteins, with low affinity, and thus control their availability in the synapse, forming a buffer for cofactor proteins. The availability of cofactor proteins, in turn, regulates the different synaptic reactions. Similar mechanisms, in which one of the reaction partners buffers another, may apply to many other processes, from the biogenesis to the degradation of the synaptic vesicle.

A conditional transgenic reporter of presynaptic terminals reveals novel features of the mouse corticospinal tract

In many neurodegenerative disorders, including amyotrophic lateral sclerosis (ALS), synaptic alterations precede the demise of the neuronal cell, making synapses a useful vantage point from which to monitor the onset and progression of clinical signs and pathological changes. While murine models of ALS display many features in common with the clinical picture observed in patients, corticospinal tract (CST) involvement is usually less severe in mice than the picture observed in humans. In this paper we describe the characterization of a new conditional transgenic line obtained by targeted integration of a GFP-VAMP2 fusion gene into the Rosa26 locus, and devised to permit the detection of genetically defined presynaptic terminals in wild type mice and murine models of neural disorders. This reporter molecule is selectively enriched in presynaptic boutons, significantly reducing the background signal produced by fibers of passage. The specific features of this reporter line allow us to strongly support the view that murine CST terminals give rise to very few direct contacts with spinal motor neurons. Moreover, the evidence described here reveals the existence of previously uncharacterized, putative direct connections between CST presynaptic boutons and Renshaw neurons in the spinal cord. These results constitute a proof of concept for the potential application of this indicator line to morphological analyses of wild type and diseased synapses.

Use of SNAREs for the immobilization of poly-3-hydroxyalkanoate polymerase type II of Pseudomonas putida CA-3 in secretory vesicles of Saccharomyces cerevisiae ATCC 9763

Polyhydroxyalkanoate (PHA) synthase, the key enzyme in polyester biosynthesis of bacteria, has been targeted to various organelles in yeasts and plants using respective signal peptides. Here, we report that the sequences derived from SNARE domains efficiently target and integrate the PHA synthase from Pseudomonas putida CA-3 to the membrane of secretory vesicles in Saccharomyces cerevisiae. The studies with the enhanced green fluorescent protein confirm the localization of synthase enzyme in the vesicles of S. cerevisiae.

The molecular basis for the endocytosis of small R-SNAREs by the clathrin adaptor CALM

SNAREs provide a large part of the specificity and energy needed for membrane fusion and, to do so, must be localized to their correct membranes. Here, we show that the R-SNAREs VAMP8, VAMP3, and VAMP2, which cycle between the plasma membrane and endosomes, bind directly to the ubiquitously expressed, PtdIns4,5P(2)-binding, endocytic clathrin adaptor CALM/PICALM. X-ray crystallography shows that the N-terminal halves of their SNARE motifs bind the CALM(ANTH) domain as helices in a manner that mimics SNARE complex formation. Mutation of residues in the CALM:SNARE interface inhibits binding in vitro and prevents R-SNARE endocytosis in vivo. Thus, CALM:R-SNARE interactions ensure that R-SNAREs, required for the fusion of endocytic clathrin-coated vesicles with endosomes and also for subsequent postendosomal trafficking, are sorted into endocytic vesicles. CALM's role in directing the endocytosis of small R-SNAREs may provide insight into the association of CALM/PICALM mutations with growth retardation, cognitive defects, and Alzheimer's disease.

SNARE motif-mediated sorting of synaptobrevin by the endocytic adaptors clathrin assembly lymphoid myeloid leukemia (CALM) and AP180 at synapses

Neurotransmission depends on the exo-endocytosis of synaptic vesicles at active zones. Synaptobrevin 2 [also known as vesicle-associated membrane protein 2 (VAMP2)], the most abundant synaptic vesicle protein and a major soluble NSF attachment protein receptor (SNARE) component, is required for fast calcium-triggered synaptic vesicle fusion. In contrast to the extensive knowledge about the mechanism of SNARE-mediated exocytosis, little is known about the endocytic sorting of synaptobrevin 2. Here we show that synaptobrevin 2 sorting involves determinants within its SNARE motif that are recognized by the ANTH domains of the endocytic adaptors AP180 and clathrin assembly lymphoid myeloid leukemia (CALM). Depletion of CALM or AP180 causes selective surface accumulation of synaptobrevin 2 but not vGLUT1 at the neuronal surface. Endocytic sorting of synaptobrevin 2 is mediated by direct interaction of the ANTH domain of the related endocytic adaptors CALM and AP180 with the N-terminal half of the SNARE motif centered around M46, as evidenced by NMR spectroscopy analysis and site-directed mutagenesis. Our data unravel a unique mechanism of SNARE motif-dependent endocytic sorting and identify the ANTH domain proteins AP180 and CALM as cargo-specific adaptors for synaptobrevin endocytosis. Defective SNARE endocytosis may also underlie the association of CALM and AP180 with neurodevelopmental and cognitive defects or neurodegenerative disorders.

GRASP-1 regulates endocytic receptor recycling and synaptic plasticity

Remodeling of synapses is a fundamental mechanism for information storage and processing in the brain. Previous studies showed that the endosomal pathway plays a central role in synapse formation and plasticity. A popular model holds that recycling endosomes in dendrites provide the local intracellular pool of postsynaptic receptors for long-term potentiation (LTP), a widely studied cellular model for learning and memory formation. However, we are far from a complete understanding how endocytic receptor sorting and recycling is organized and coordinated in dendrites. Especially, the molecular mechanisms that couple specific endosomal trafficking routes during LTP are poorly understood. In a recent paper we discovered that the coiled-coil protein GRIP-associated protein-1 (GRASP-1) is a neuron-specific effector of the small GTPase Rab4 and key component of AMPA receptor recycling machinery in dendrites.1 GRASP-1 is essential for maintenance of spine morphology and important for LTP. GRASP-1 connects Rab4 and Rab11 recycling endosomal domains through the interaction with target (t)-SNARE syntaxin 13, which constitutes a new principle for regulating endosomal recycling. Here, we summarize our recently reported observations and further discuss their possible implications.

SNARE protein expression in synaptic terminals and astrocytes in the adult hippocampus: a comparative analysis

Several evidences suggest that astrocytes release small transmitter molecules, peptides, and protein factors via regulated exocytosis, implying that they function as specialized neurosecretory cells. However, very little is known about the molecular and functional properties of regulated secretion in astrocytes in the adult brain. Establishing these properties is central to the understanding of the communication mode(s) of these cells and their role(s) in the control of synaptic functions and of cerebral blood flow. In this study, we have set-up a high-resolution confocal microscopy approach to distinguish protein expression in astrocytic structures and neighboring synaptic terminals in adult brain tissue. This approach was applied to investigate the expression pattern of core SNARE proteins for vesicle fusion in the dentate gyrus and CA1 regions of the mouse hippocampus. Our comparative analysis shows that astrocytes abundantly express, in their cell body and main processes, all three protein partners necessary to form an operational SNARE complex but not in the same isoforms expressed in neighbouring synaptic terminals. Thus, SNAP25 and VAMP2 are absent from astrocytic processes and typically concentrated in terminals, while SNAP23 and VAMP3 have the opposite expression pattern. Syntaxin 1 is present in both synaptic terminals and astrocytes. These data support the view that astrocytes in the adult hippocampus can communicate via regulated exocytosis and also indicates that astrocytic exocytosis may differ in its properties from action potential-dependent exocytosis at neuronal synapses, as it relies on a distinctive set of SNARE proteins.

Protein trafficking in immune cells

The majority of cells of the immune system are specialized secretory cells, whose function depends on regulated exocytosis. The latter is mediated by vesicular transport involving the sorting of specialized cargo into the secretory granules (SGs), thereby generating the transport vesicles; their transport along the microtubules and eventually their signal-dependent fusion with the plasma membrane. Each of these steps is tightly controlled by mechanisms, which involve the participation of specific sorting signals on the cargo proteins and their recognition by cognate adaptor proteins, posttranslational modifications of the cargo proteins and multiple GTPases and SNARE proteins. In some of the cells (i.e. mast cells, T killer cells) an intimate connection exists between the secretory system and the endocytic one, whereby the SGs are lysosome related organelles (LROs) also referred to as secretory lysosomes. Herein, we discuss these mechanisms in health and disease states.

A coiled coil trigger site is essential for rapid binding of synaptobrevin to the SNARE acceptor complex

Exocytosis from synaptic vesicles is driven by stepwise formation of a tight alpha-helical complex between the fusing membranes. The complex is composed of the three SNAREs: synaptobrevin 2, SNAP-25, and syntaxin 1a. An important step in complex formation is fast binding of vesicular synaptobrevin to the preformed syntaxin 1.SNAP-25 dimer. Exactly how this step relates to neurotransmitter release is not well understood. Here, we combined different approaches to gain insights into this reaction. Using computational methods, we identified a stretch in synaptobrevin 2 that may function as a coiled coil "trigger site." This site is also present in many synaptobrevin homologs functioning in other trafficking steps. Point mutations in this stretch inhibited binding to the syntaxin 1.SNAP-25 dimer and slowed fusion of liposomes. Moreover, the point mutations severely inhibited secretion from chromaffin cells. Altogether, this demonstrates that the trigger site in synaptobrevin is crucial for productive SNARE zippering.

Transmembrane-domain determinants for SNARE-mediated membrane fusion

Neurosecretion involves fusion of vesicles with the plasma membrane. Such membrane fusion is mediated by the SNARE complex, which is composed of the vesicle-associated protein synaptobrevin (VAMP2), and the plasma membrane proteins syntaxin-1A and SNAP-25. Although clearly important at the point of membrane fusion, the precise structural and functional requirements for the transmembrane domains (TMDs) of SNAREs in bringing about neurosecretion remain largely unknown. Here, we used a bimolecular fluorescence complementation (BiFC) approach to study SNARE protein interactions involving TMDs in vivo. VAMP2 molecules were found to dimerise through their TMDs in intact cells. Dimerisation was abolished when replacing a glycine residue in the centre of the TMD with residues of increasing molecular volume. However, such mutations still were fully competent in bringing about membrane-fusion events, suggesting that dimerisation of the VAMP2 TMDs does not have an important functional role. By contrast, a series of deletion or insertion mutants in the C-terminal half of the TMD were largely deficient in supporting neurosecretion, whereas mutations in the N-terminal half did not display severe secretory deficits. Thus, structural length requirements, largely confined to the C-terminal half of the VAMP2 TMD, seem to be essential for SNARE-mediated membrane-fusion events in cells.

Rearrangements in the relative orientation of cytoplasmic domains induced by a membrane-anchored protein mediate modulations in Kv channel gating

Interdomain interactions between intracellular N and C termini have been described for various K(+) channels, including the voltage-gated Kv2.1, and suggested to affect channel gating. However, no channel regulatory protein directly affecting N/C interactions has been demonstrated. Most Kv2.1 channel interactions with regulatory factors occur at its C terminus. The vesicular SNARE that is also present at a high concentration in the neuronal plasma membrane, VAMP2, is the only protein documented to affect Kv2.1 gating by binding to its N terminus. As its binding target has been mapped near a site implicated in Kv2.1 N/C interactions, we hypothesized that VAMP2 binding to the N terminus requires concomitant conformational changes in the C terminus, which wraps around the N terminus from the outside, to give VAMP2 access. Here, we first determined that the Kv2.1 N terminus, although crucial, is not sufficient to convey functional interaction with VAMP2, and that, concomitant to its binding to the "docking loop" at the Kv2.1 N terminus, VAMP2 binds to the proximal part of the Kv2.1 C terminus, C1a. Next, using computational biology approaches (ab initio modeling, docking, and molecular dynamics simulations) supported by molecular biology, biochemical, electrophysiological, and fluorescence resonance energy transfer analyses, we mapped the interaction sites on both VAMP2 and Kv2.1 and found that this interaction is accompanied by rearrangements in the relative orientation of Kv2.1 cytoplasmic domains. We propose that VAMP2 modulates Kv2.1 inactivation by interfering with the interaction between the docking loop and C1a, a mechanism for gating regulation that may pertain also to other Kv channels.

Coiled-coil interactions are required for post-Golgi R-SNARE trafficking

The sorting of post-Golgi R-SNAREs (vesicle-associated membrane protein (VAMP)1, 2, 3, 4, 7 and 8) is still poorly understood. To address this, we developed a system to investigate their localization, trafficking and cell-surface levels. Here, we show that the distribution and internalization of VAMPs 3 and 8 are determined solely through a new conserved mechanism that uses coiled-coil interactions, and that VAMP4 does not require these interactions for its trafficking. We propose that VAMPs 3 and 8 are trafficked while in a complex with Q-SNAREs. We also show that the dileucine motif of VAMP4 is required for both its internalization and retrieval to the trans-Golgi network. However, when the dileucine motif is mutated, the construct can still be internalized potentially through coiled-coil interactions with Q-SNAREs.

Deletion mutants of AP-1 adaptin subunits display distinct phenotypes in fission yeast

Adaptins are subunits of the heterotetrameric (beta/mu/gamma/sigma) adaptor protein (AP) complexes that are involved in clathrin-mediated membrane trafficking. Here, we show that in Schizosaccharomyces pombe the deletion strains of each individual subunit of the AP-1 complex [Apl2 (beta), Apl4 (gamma), Apm1 (mu) and Aps1 (sigma)] caused distinct phenotypes on growth sensitivity to temperature or drugs. We also show that the Deltaapm1 and Deltaapl2 mutants displayed similar but more severe phenotypes than those of Deltaaps1 or Deltaapl4 mutants. Furthermore, the Deltaapl2Deltaaps1 and Deltaapl2Deltaapl4 double mutants displayed synthetic growth defects, whereas the Deltaaps1Deltaapl4 and Deltaapl2Deltaapm1 double mutants did not. In pull-down assay, Apm1 binds Apl2 even in the absence of Aps1 and Apl4, and Apl4 binds Aps1 even in the absence of Apm1 and Apl2. Consistently, the deletion of any subunit generally caused the disassociation of the heterotetrameric complex from endosomes, although some subunits weakly localized to endosomes. In addition, the deletion of individual subunits caused similar endosomal accumulation of v-SNARE synaptobrevin Syb1. Altogether, results suggest that the four subunits are all essential for the heterotetrameric complex formation and for the AP-1 function in exit transport from endosomes.

SLC30A3 (ZnT3) oligomerization by dityrosine bonds regulates its subcellular localization and metal transport capacity

Non-covalent and covalent homo-oligomerization of membrane proteins regulates their subcellular localization and function. Here, we described a novel oligomerization mechanism affecting solute carrier family 30 member 3/zinc transporter 3 (SLC30A3/ZnT3). Oligomerization was mediated by intermolecular covalent dityrosine bonds. Using mutagenized ZnT3 expressed in PC12 cells, we identified two critical tyrosine residues necessary for dityrosine-mediated ZnT3 oligomerization. ZnT3 carrying the Y372F mutation prevented ZnT3 oligomerization, decreased ZnT3 targeting to synaptic-like microvesicles (SLMVs), and decreased resistance to zinc toxicity. Strikingly, ZnT3 harboring the Y357F mutation behaved as a “gain-of-function” mutant as it displayed increased ZnT3 oligomerization, targeting to SLMVs, and increased resistance to zinc toxicity. Single and double tyrosine ZnT3 mutants indicate that the predominant dimeric species is formed between tyrosine 357 and 372. ZnT3 tyrosine dimerization was detected under normal conditions and it was enhanced by oxidative stress. Covalent species were also detected in other SLC30A zinc transporters localized in different subcellular compartments. These results indicate that covalent tyrosine dimerization of a SLC30A family member modulates its subcellular localization and zinc transport capacity. We propose that dityrosine-dependent membrane protein oligomerization may regulate the function of diverse membrane protein in normal and disease states.

Roles of BLOC-1 and adaptor protein-3 complexes in cargo sorting to synaptic vesicles

Neuronal lysosomes and their biogenesis mechanisms are primarily thought to clear metabolites and proteins whose abnormal accumulation leads to neurodegenerative disease pathology. However, it remains unknown whether lysosomal sorting mechanisms regulate the levels of membrane proteins within synaptic vesicles. Using high-resolution deconvolution microscopy, we identified early endosomal compartments where both selected synaptic vesicle and lysosomal membrane proteins coexist with the adaptor protein complex 3 (AP-3) in neuronal cells. From these early endosomes, both synaptic vesicle membrane proteins and characteristic AP-3 lysosomal cargoes can be similarly sorted to brain synaptic vesicles and PC12 synaptic-like microvesicles. Mouse knockouts for two Hermansky-Pudlak complexes involved in lysosomal biogenesis from early endosomes, the ubiquitous isoform of AP-3 (Ap3b1(-/-)) and muted, defective in the biogenesis of lysosome-related organelles complex 1 (BLOC-1), increased the content of characteristic synaptic vesicle proteins and known AP-3 lysosomal proteins in isolated synaptic vesicle fractions. These phenotypes contrast with those of the mouse knockout for the neuronal AP-3 isoform involved in synaptic vesicle biogenesis (Ap3b2(-/-)), in which the content of select proteins was reduced in synaptic vesicles. Our results demonstrate that lysosomal and lysosome-related organelle biogenesis mechanisms regulate steady-state synaptic vesicle protein composition from shared early endosomes.

Synaptic vesicle protein trafficking at the glutamate synapse

Expression of the integral and associated proteins of synaptic vesicles is subject to regulation over time, by region, and in response to activity. The process by which changes in protein levels and isoforms result in different properties of neurotransmitter release involves protein trafficking to the synaptic vesicle. How newly synthesized proteins are incorporated into synaptic vesicles at the presynaptic bouton is poorly understood. During synaptogenesis, synaptic vesicle proteins sort through the secretory pathway and are transported down the axon in precursor vesicles that undergo maturation to form synaptic vesicles. Changes in protein content of synaptic vesicles could involve the formation of new vesicles that either mix with the previous complement of vesicles or replace them, presumably by their degradation or inactivation. Alternatively, new proteins could individually incorporate into existing synaptic vesicles, changing their functional properties. Glutamatergic vesicles likely express many of the same integral membrane proteins and share certain common mechanisms of biogenesis, recycling, and degradation with other synaptic vesicles. However, glutamatergic vesicles are defined by their ability to package glutamate for release, a property conferred by the expression of a vesicular glutamate transporter (VGLUT). VGLUTs are subject to regional, developmental, and activity-dependent changes in expression. In addition, VGLUT isoforms differ in their trafficking, which may target them to different pathways during biogenesis or after recycling, which may in turn sort them to different vesicle pools. Emerging data indicate that differences in the association of VGLUTs and other synaptic vesicle proteins with endocytic adaptors may influence their trafficking. These observations indicate that independent regulation of synaptic vesicle protein trafficking has the potential to influence synaptic vesicle protein composition, the maintenance of synaptic vesicle pools, and the release of glutamate in response to changing physiological requirements.

Evolution of insect proteomes: insights into synapse organization and synaptic vesicle life cycle

BACKGROUND

The molecular components in synapses that are essential to the life cycle of synaptic vesicles are well characterized. Nonetheless, many aspects of synaptic processes, in particular how they relate to complex behaviour, remain elusive. The genomes of flies, mosquitoes, the honeybee and the beetle are now fully sequenced and span an evolutionary breadth of about 350 million years; this provides a unique opportunity to conduct a comparative genomics study of the synapse.

RESULTS

We compiled a list of 120 gene prototypes that comprise the core of presynaptic structures in insects. Insects lack several scaffolding proteins in the active zone, such as bassoon and piccollo, and the most abundant protein in the mammalian synaptic vesicle, namely synaptophysin. The pattern of evolution of synaptic protein complexes is analyzed. According to this analysis, the components of presynaptic complexes as well as proteins that take part in organelle biogenesis are tightly coordinated. Most synaptic proteins are involved in rich protein interaction networks. Overall, the number of interacting proteins and the degrees of sequence conservation between human and insects are closely correlated. Such a correlation holds for exocytotic but not for endocytotic proteins.

CONCLUSION

This comparative study of human with insects sheds light on the composition and assembly of protein complexes in the synapse. Specifically, the nature of the protein interaction graphs differentiate exocytotic from endocytotic proteins and suggest unique evolutionary constraints for each set. General principles in the design of proteins of the presynaptic site can be inferred from a comparative study of human and insect genomes.

Dileucine motif is sufficient for internalization and synaptic vesicle targeting of vesicular acetylcholine transporter

Efficient cholinergic transmission requires accurate targeting of vesicular acetylcholine transporter (VAChT) to synaptic vesicles (SVs). However, the signals that regulate this vesicular targeting are not well characterized. Although previous studies suggest that the C-terminus of the transporter is required for its SV targeting, it is not clear whether this region is sufficient for this process. Furthermore, a synaptic vesicle-targeting motif (SVTM) within this sequence remains to be identified. Here we use a chimeric protein, TacA, between an unrelated plasma membrane protein, Tac, and the C-terminus of VAChT to demonstrate the sufficiency of the C-terminus for targeting to synaptic vesicle-like vesicles (SVLVs) in PC12 cells. TacA shows colocalization and cosedimentation with the SV marker synaptophysin. Deletion mutation analysis of TacA demonstrates that a short, dileucine motif-containing sequence is required and sufficient to direct this targeting. Dialanine mutation analysis within this sequence suggests indistinguishable signals for both internalization and SV sorting. Using additional chimeras as controls, we confirm the specificity of this region for SVLVs targeting. Therefore, we suggest that the dileucine-containing motif is sufficient as a dual signal for both internalization and SV targeting during VAChT trafficking.

Synaptophysin I selectively specifies the exocytic pathway of synaptobrevin 2/VAMP2

Biogenesis and recycling of synaptic vesicles are accompanied by sorting processes that preserve the molecular composition of the compartments involved. In the present study, we have addressed the targeting of synaptobrevin 2/VAMP2 (vesicle-associated membrane protein 2), a critical component of the synaptic vesicle--fusion machinery, in a heterotypic context where its sorting is not confounded by the presence of other neuron-specific molecules. Ectopically expressed synaptophysin I interacts with VAMP2 and alters its default surface targeting to a prominent vesicular distribution, with no effect on the targeting of other membrane proteins. Protein-protein interaction is not sufficient for the control of VAMP2 sorting, which is mediated by the C-terminal domain of synaptophysin I. Synaptophysin I directs the sorting of VAMP2 to vesicles before surface delivery, without influencing VAMP2 endocytosis. Consistent with this, dynamin and alpha-SNAP (soluble N-ethylmaleimide-sensitive fusion protein-attachment protein) mutants which block trafficking at the plasma membrane do not abrogate the effect of synaptophysin I on VAMP2 sorting. These results indicate that the sorting determinants of synaptic vesicle proteins can operate independently of a neuronal context and implicate the association of VAMP2 with synaptophysin I in the specification of the pathway of synaptic vesicle biogenesis.

A flexible protein linker improves the function of epitope-tagged proteins in Saccharomyces cerevisiae

Epitope tagging permits the detection of proteins when protein-specific antibodies are not available. However, the epitope tag can reduce the function of the tagged protein. Here we describe a cassette that can be used to introduce an eight amino acid flexible linker between multiple Myc epitopes and the open reading frame of a given gene. We show that inserting the linker improves the in vivo ability of the telomerase subunits Est2p and Est1p to maintain telomere length. The methods used here are generally applicable to improve the function of tagged proteins in both Saccharomyces cerevisiae and Schizosaccharomyces pombe.

Protein sorting in the synaptic vesicle life cycle

At early stages of differentiation neurons already contain many of the components necessary for synaptic transmission. However, in order to establish fully functional synapses, both the pre- and postsynaptic partners must undergo a process of maturation. At the presynaptic level, synaptic vesicles (SVs) must acquire the highly specialized complement of proteins, which make them competent for efficient neurotransmitter release. Although several of these proteins have been characterized and linked to precise functions in the regulation of the SV life cycle, a systematic and unifying view of the mechanisms underlying selective protein sorting during SV biogenesis remains elusive. Since SV components do not share common sorting motifs, their targeting to SVs likely relies on a complex network of protein-protein and protein-lipid interactions, as well as on post-translational modifications. Pleiomorphic carriers containing SV proteins travel and recycle along the axon in developing neurons. Nevertheless, SV components appear to eventually undertake separate trafficking routes including recycling through the neuronal endomembrane system and the plasmalemma. Importantly, SV biogenesis does not appear to be limited to a precise stage during neuronal differentiation, but it rather continues throughout the entire neuronal lifespan and within synapses. At nerve terminals, remodeling of the SV membrane results from the use of alternative exocytotic pathways and possible passage through as yet poorly characterized vacuolar/endosomal compartments. As a result of both processes, SVs with heterogeneous molecular make-up, and hence displaying variable competence for exocytosis, may be generated and coexist within the same nerve terminal.

Distinct endocytic pathways control the rate and extent of synaptic vesicle protein recycling

Synaptic vesicles have been proposed to form through two mechanisms: one directly from the plasma membrane involving clathrin-dependent endocytosis and the adaptor protein AP2, and the other from an endosomal intermediate mediated by the adaptor AP3. However, the relative role of these two mechanisms in synaptic vesicle recycling has remained unclear. We now find that vesicular glutamate transporter VGLUT1 interacts directly with endophilin, a component of the clathrin-dependent endocytic machinery. In the absence of its interaction with endophilin, VGLUT1 recycles more slowly during prolonged, high-frequency stimulation. Inhibition of the AP3 pathway with brefeldin A rescues the rate of recycling, suggesting a competition between AP2 and -3 pathways, with endophilin recruiting VGLUT1 toward the faster AP2 pathway. After stimulation, however, inhibition of the AP3 pathway prevents the full recovery of VGLUT1 by endocytosis, implicating the AP3 pathway specifically in compensatory endocytosis.

Apical targeting of syntaxin 3 is essential for epithelial cell polarity

In polarized epithelial cells, syntaxin 3 localizes to the apical plasma membrane and is involved in membrane fusion of apical trafficking pathways. We show that syntaxin 3 contains a necessary and sufficient apical targeting signal centered around a conserved FMDE motif. Mutation of any of three critical residues within this motif leads to loss of specific apical targeting. Modeling based on the known structure of syntaxin 1 revealed that these residues are exposed on the surface of a three-helix bundle. Syntaxin 3 targeting does not require binding to Munc18b. Instead, syntaxin 3 recruits Munc18b to the plasma membrane. Expression of mislocalized mutant syntaxin 3 in Madin-Darby canine kidney cells leads to basolateral mistargeting of apical membrane proteins, disturbance of tight junction formation, and loss of ability to form an organized polarized epithelium. These results indicate that SNARE proteins contribute to the overall specificity of membrane trafficking in vivo, and that the polarity of syntaxin 3 is essential for epithelial cell polarization.

SNAREs — engines for membrane fusion

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

Factors regulating the abundance and localization of synaptobrevin in the plasma membrane

After synaptic vesicle fusion, vesicle proteins must be segregated from plasma membrane proteins and recycled to maintain a functional vesicle pool. We monitored the distribution of synaptobrevin, a vesicle protein required for exocytosis, in Caenorhabditis elegans motor neurons by using a pH-sensitive synaptobrevin GFP fusion protein, synaptopHluorin. We estimated that 30% of synaptobrevin was present in the plasma membrane. By using a panel of endocytosis and exocytosis mutants, we found that the majority of surface synaptobrevin derives from fusion of synaptic vesicles and that, in steady state, synaptobrevin equilibrates throughout the axon. The surface synaptobrevin was enriched near active zones, and its spatial extent was regulated by the clathrin adaptin AP180. These results suggest that there is a plasma membrane reservoir of synaptobrevin that is supplied by the synaptic vesicle cycle and available for retrieval throughout the axon. The size of the reservoir is set by the relative rates of exo- and endocytosis.

Structural transitions in the synaptic SNARE complex during Ca2+-triggered exocytosis

The synaptic SNARE complex is a highly stable four-helix bundle that links the vesicle and plasma membranes and plays an essential role in the Ca²⁺-triggered release of neurotransmitters and hormones. An understanding has yet to be achieved of how this complex assembles and undergoes structural transitions during exocytosis. To investigate this question, we have mutated residues within the hydrophobic core of the SNARE complex along the entire length of all four chains and examined the consequences using amperometry to measure fusion pore opening and dilation. Mutations throughout the SNARE complex reduced two distinct rate processes before fusion pore opening to different degrees. These results suggest that two distinct, fully assembled conformations of the SNARE complex drive transitions leading to open fusion pores. In contrast, a smaller number of mutations that were scattered through the SNARE complex but were somewhat concentrated in the membrane-distal half stabilized open fusion pores. These results suggest that a structural transition within a partially disassembled complex drives the dilation of open fusion pores. The dependence of these three rate processes on position within the SNARE complex does not support vectorial SNARE complex zipping during exocytosis.

A prohormone convertase cleavage site within a predicted alpha-helix mediates sorting of the neuronal and endocrine polypeptide VGF into the regulated secretory pathway

Distinct intracellular pathways are involved in regulated and constitutive protein secretion from neuronal and endocrine cells, yet the peptide signals and molecular mechanisms responsible for targeting and retention of soluble proteins in secretory granules are incompletely understood. By using confocal microscopy and subcellular fractionation, we examined trafficking of the neuronal and endocrine peptide precursor VGF that is stored in large dense core vesicles and undergoes regulated secretion. VGF cofractionated with secretory vesicle membranes but was not detected in detergent-resistant lipid rafts. Deletional analysis using epitope-tagged VGF suggested that the C-terminal 73-amino acid fragment of VGF, containing two predicted alpha-helical loops and four potential prohormone convertase (PC) cleavage sites, was necessary and sufficient with an N-terminal signal peptide-containing domain, for large dense core vesicle sorting and regulated secretion from PC12 and INS-1 cells. Further transfection analysis identified the sorting sequence as a compact C-terminal alpha-helix and embedded 564RRR566 PC cleavage site; mutation of the 564RRR566 PC site in VGF-(1-65): GFP:VGF-(545-617) blocked regulated secretion, whereas disruption of the alpha-helix had no effect. Mutation of the adjacent 567HFHH570 motif, a charged region that might enhance PC cleavage in acidic environments, also blocked regulated release. Finally, inhibition of PC cleavage in PC12 cells using the membrane-permeable synthetic peptide chloromethyl ketone (decanoyl-RVKR-CMK) blocked regulated secretion of VGF. Our studies define a critical RRR-containing C-terminal domain that targets VGF into the regulated pathway in neuronal PC12 and endocrine INS-1 cells, providing additional support for the proposed role that PCs and their cleavage sites play in regulated peptide secretion.

Subcellular localization of neural-specific NPDC-1 protein

NPDC-1 is a gene specifically expressed in neural cells when they stop to divide and begin to differentiate. Immunocytochemical study analysis of differentiated PC12 cells transfected with NPDC-tag vectors showed that NPDC-1 is transported in vesicles from the Golgi apparatus to the cell membrane and is then likely internalized into endosomes. The protein colocalized, at least partially, with synaptic vesicle proteins: synaptophysin, synaptobrevin 2, and Rab3 GEP (Rab3 GTP/GDP exchange protein). Moreover, subcellular fractionation of rat brain showed that crude synaptic membrane and crude synaptic vesicle fractions were enriched in NPDC-1. Although NPDC-1 bound Rab3 GEP in vitro, it seems unlikely to be involved in Ca2+-dependent exocytosis and, thus, in synaptic vesicle trafficking.

Vglut1 and ZnT3 co-targeting mechanisms regulate vesicular zinc stores in PC12 cells

The lumenal ionic content of an organelle is determined by its complement of channels and transporters. These proteins reach their resident organelles by adaptor-dependent mechanisms. This concept is illustrated in AP-3 deficiencies, in which synaptic vesicle zinc is depleted because the synaptic-vesicle-specific zinc transporter 3 does not reach synaptic vesicles. However, whether zinc transporter 3 is the only membrane protein defining synaptic-vesicle zinc content remains unknown. To address this question, we examined whether zinc transporter 3 and the vesicular glutamate transporter Vglut1 (a transporter that coexists with zinc transporter 3 in brain nerve terminals) were co-targeted to synaptic-like microvesicle fractions in PC12 cells. Deconvolution microscopy and subcellular fractionation demonstrated that these two transporters were present on the same vesicles in PC12 cells. Vglut1 content in synaptic-like microvesicle fractions and brain synaptic vesicles was partially sensitive to pharmacological and genetic perturbation of AP-3 function. Whole-cell flow-cytometry analysis of PC12 cell lines expressing zinc transporter 3, Vglut1 or both showed that vesicular zinc uptake was increased by Vglut1 expression. Conversely, production of zinc transporter 3 increased the vesicular uptake of glutamate in a zinc-dependent fashion. Our results suggest that the coupling of zinc transporter 3 and Vglut1 transport mechanisms regulates neurotransmitter content in secretory vesicles.

Cellugyrin induces biogenesis of synaptic-like microvesicles in PC12 cells

The four-transmembrane domain proteins synaptophysin and synaptogyrin represent the major constituents of synaptic vesicles. Our previous studies in PC12 cells demonstrated that synaptogyrin or its nonneuronal paralog cellugyrin targets efficiently to synaptic-like microvesicles (SLMVs) and dramatically increases the synaptophysin content of SLMVs (Belfort, G. M., and Kandror, K. V. (2003) J. Biol. Chem. 278, 47971-47978). Here, we explored the mechanism of these phenomena and found that ectopic expression of cellugyrin increases the number of SLMVs in PC12 cells. Mutagenesis studies revealed that cellugyrin's hydrophilic cytoplasmic domains are not involved in vesicle biogenesis, whereas small conserved hydrophobic hairpins in the first luminal loop and the carboxyl terminus of cellugyrin were found to be critical for the formation of SLMVs. In addition, the length but not the primary sequence of the second luminal loop was essential for SLMV biogenesis. We suggest that changing the length of this loop similar to disruption of the short hydrophobic hairpins alters the position of the vicinal transmembrane domains that may be crucial for protein function.

AP-3-dependent mechanisms control the targeting of a chloride channel (ClC-3) in neuronal and non-neuronal cells

Adaptor protein (AP)-2 and AP-3-dependent mechanisms control the sorting of membrane proteins into synaptic vesicles. Mouse models deficient in AP-3, mocha, develop a neurological phenotype of which the central feature is an alteration of the luminal synaptic vesicle composition. This is caused by a severe reduction of vesicular levels of the zinc transporter 3 (ZnT3). It is presently unknown whether this mocha defect is restricted to ZnT3 or encompasses other synaptic vesicle proteins capable of modifying synaptic vesicle contents, such as transporters or channels. In this study, we identified a chloride channel, ClC-3, whose level in synaptic vesicles and hippocampal mossy fiber terminals was reduced in the context of the mocha AP-3 deficiency. In PC-12 cells, ClC-3 was present in transferrin receptor-positive endosomes, where it was targeted to synaptic-like microvesicles (SLMV) by a mechanism sensitive to brefeldin A, a signature of the AP-3-dependent route of SLMV biogenesis. ClC-3 was packed in SLMV along with the AP-3-targeted synaptic vesicle protein ZnT3. Co-segregation of ClC-3 and ZnT3 to common intracellular compartments was functionally significant as revealed by increased vesicular zinc transport with increased ClC3 expression. Our work has identified a synaptic vesicle protein in which trafficking to synaptic vesicles is regulated by AP-3. In addition, our findings indicate that ClC-3 and ZnT3 reside in a common vesicle population where they functionally interact to determine vesicle luminal composition.

Reticulon 1-C/neuroendocrine-specific protein-C interacts with SNARE proteins

Reticulons are proteins of neuroendocrine cells localized primarily to the endoplasmic reticulum membrane. Despite their implication in cellular processes like apoptosis or axonal regeneration, their intracellular molecular function is still largely unknown. Here, we show that reticulon 1-C can be detected in a protein complex of 150-200 kDa, and that a number of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins, i.e. syntaxin 1, syntaxin 7, syntaxin 13 and VAMP2, can be co-immunoprecipitated with reticulon 1-C. Moreover, it localizes to a nocodazole-sensitive, but calreticulin-negative domain of the endoplasmic reticulum. Finally, overexpression in PC12 cells of a reticulon 1-C fragment which binds to SNAREs, significantly enhances human growth hormone secretion. These results suggest that reticulons are involved in vesicle trafficking events, including regulated exocytosis.

Regulated membrane trafficking of the insulin-responsive glucose transporter 4 in adipocytes

Since the discovery of insulin roughly 80 yr ago, much has been learned about how target cells receive, interpret, and respond to this peptide hormone. For example, we now know that insulin activates the tyrosine kinase activity of its cell surface receptor, thereby triggering intracellular signaling cascades that regulate many cellular processes. With respect to glucose homeostasis, these include the function of insulin to suppress hepatic glucose production and to increase glucose uptake in muscle and adipose tissues, the latter resulting from the translocation of the glucose transporter 4 (GLUT4) to the cell surface membrane. Although simple in broad outline, elucidating the molecular intricacies of these receptor-signaling pathways and membrane-trafficking processes continues to challenge the creative ingenuity of scientists, and many questions remain unresolved, or even perhaps unasked. The identification and functional characterization of specific molecules required for both insulin signaling and GLUT4 vesicle trafficking remain key issues in our pursuit of developing specific therapeutic agents to treat and/or prevent this debilitating disease process. To this end, the combined efforts of numerous research groups employing a range of experimental approaches has led to a clearer molecular picture of how insulin regulates the membrane trafficking of GLUT4.

The zinc transporter ZnT3 interacts with AP-3 and it is preferentially targeted to a distinct synaptic vesicle subpopulation

Synaptic vesicles (SV) are generated by two different mechanisms, one AP-2 dependent and one AP-3 dependent. It has been uncertain, however, whether these mechanisms generate SV that differ in molecular composition. We explored this hypothesis by analyzing the targeting of ZnT3 and synaptophysin both to PC12 synaptic-like microvesicles (SLMV) as well as SV isolated from wild-type and AP-3-deficient mocha brains. ZnT3 cytosolic tail interacted selectively with AP-3 in cell-free assays. Accordingly, pharmacological disruption of either AP-2- or AP-3-dependent SLMV biogenesis preferentially reduced synaptophysin or ZnT3 targeting, respectively; suggesting that these antigens were concentrated in different vesicles. As predicted, immuno-isolated SLMV revealed that ZnT3 and synaptophysin were enriched in different vesicle populations. Likewise, morphological and biochemical analyses in hippocampal neurons indicated that these two antigens were also present in distinct but overlapping domains. ZnT3 SV content was reduced in AP-3-deficient neurons, but synaptophysin was not altered in the AP-3 null background. Our evidence indicates that neuroendocrine cells assemble molecularly heterogeneous SV and suggests that this diversity could contribute to the functional variety of synapses.

Synaptophysin I controls the targeting of VAMP2/synaptobrevin II to synaptic vesicles

Synaptic vesicle (SV) proteins are synthesized at the level of the cell body and transported down the axon in membrane precursors of SVs. To investigate the mechanisms underlying sorting of proteins to SVs, fluorescent chimeras of vesicle-associated membrane protein (VAMP) 2, its highly homologous isoform VAMP1 and synaptotagmin I (SytI) were expressed in hippocampal neurons in culture. Interestingly, the proteins displayed a diffuse component of distribution along the axon. In addition, VAMP2 was found to travel in vesicles that constitutively fuse with the plasma membrane. Coexpression of VAMP2 with synaptophysin I (SypI), a major resident of SVs, restored the correct sorting of VAMP2 to SVs. The effect of SypI on VAMP2 sorting was dose dependent, being reversed by increasing VAMP2 expression levels, and highly specific, because the sorting of the SV proteins VAMP1 and SytI was not affected by SypI. The cytoplasmic domain of VAMP2 was found to be necessary for both the formation of VAMP2-SypI hetero-dimers and for VAMP2 sorting to SVs. These data support a role for SypI in directing the correct sorting of VAMP2 in neurons and demonstrate that a direct interaction between the two proteins is required for SypI in order to exert its effect.

Cellugyrin and synaptogyrin facilitate targeting of synaptophysin to a ubiquitous synaptic vesicle-sized compartment in PC12 cells

Cellugyrin represents a ubiquitously expressed four-transmembrane domain protein that is closely related to synaptic vesicle protein synaptogyrin and, more remotely, to synaptophysin. We report here that, in PC12 cells, cellugyrin is localized in synaptic-like microvesicles (SLMVs), along with synaptogyrin and synaptophysin. Upon overexpression of synaptophysin in PC12 cells, it is localized in rapidly sedimenting membranes and practically is not delivered to the SLMVs. On the contrary, the efficiency of the SLMV targeting of exogenously expressed cellugyrin and synaptogyrin is high. Moreover, expression of cellugyrin (or synaptogyrin) in PC12 cells dramatically and specifically increases SLMV targeting of endogenous synaptophysin. Finally, we utilized the SLMV purification scheme on a series of non-neuroendocrine cell types including the mouse fibroblast cell line 3T3-L1, the Chinese hamster ovary cell line CHO-K1, and the monkey kidney epithelial cell line COS7 and found that a cellugyrin-positive microvesicular compartment was present in all cell types tested. We suggest that synaptic vesicles have evolved from cellugyrin-positive ubiquitous microvesicles and that neuroendocrine SLMVs represent a step along that pathway of evolution.

A Complete Set of SNAREs in Yeast

Trafficking of cargo molecules through the secretory pathway relies on packaging and delivery of membrane vesicles. These vesicles, laden with cargo, carry integral membrane proteins that can determine with which target membrane the vesicle might productively fuse. The membrane fusion process is highly conserved in all eukaryotes and the central components driving membrane fusion events involved in vesicle delivery to target membranes are a set of integral membrane proteins called SNAREs. The yeast Saccharomyces cerevisiae has served as an extremely useful model for characterizing components of membrane fusion through genetics, biochemistry and bioinformatics, and it is now likely that the complete set of SNAREs is at hand. Here, we present the details from the searches for SNAREs, summarize the domain structures of the complete set, review what is known about localization of SNAREs to discrete membranes, and highlight some of the surprises that have come from the search.

Endocytosis of synaptotagmin 1 is mediated by a novel, tryptophan-containing motif

The rate at which a membrane protein is internalized from the plasma membrane can be regulated by revealing a latent internalization signal in response to an appropriate stimulus. Internalization of the synaptic vesicle membrane protein, synaptotagmin 1, is controlled by two distinct regions of its intracytoplasmic C2B domain, an internalization signal present in the 29 carboxyterminal (CT) amino acids and a separate regulatory region. We have now characterized the internalization motif by mutagenesis and found that it involves an essential tryptophan in the last beta strand of the C2B domain, a region that is distinct from the AP2-binding site previously described. Internalization through the tryptophan-based motif is sensitive to eps15 and dynamin mutants and is therefore likely to be clathrin mediated. A tryptophan-to-phenylalanine mutation had no effect on internalization of the CT domain alone, but completely inhibited endocytosis of the folded C2B domain. This result suggests that recognition of sorting motifs can be influenced by their structural context. We conclude that endocytosis of synaptotagmin 1 requires a novel type of internalization signal that is subject to regulation by the rest of the C2B domain.

The cytoplasmic domain of Vamp4 and Vamp5 is responsible for their correct subcellular targeting: the N-terminal extenSion of VAMP4 contains a dominant autonomous targeting signal for the trans-Golgi network

SNAREs represent a superfamily of proteins responsible for the last stage of docking and subsequent fusion in diverse intracellular membrane transport events. The Vamp subfamily of SNAREs contains 7 members (Vamp1, Vamp2, Vamp3/cellubrevin, Vamp4, Vamp5, Vamp7/Ti-Vamp, and Vamp8/endobrevin) that are distributed in various post-Golgi structures. Vamp4 and Vamp5 are distributed predominantly in the trans-Golgi network (TGN) and the plasma membrane, respectively. When C-terminally tagged with enhanced green fluorescent protein, the majority of Vamp4 and Vamp5 is correctly targeted to the TGN and plasma membrane, respectively. Swapping the N-terminal cytoplasmic region and the C-terminal membrane anchor domain between Vamp4 and Vamp5 demonstrates that the N-terminal cytoplasmic region of these two SNAREs contains the correct subcellular targeting information. As compared with Vamp5, Vamp4 contains an N-terminal extension of 51 residues. Appending this 51-residue N-terminal extension onto the N terminus of Vamp5 results in targeting of the chimeric protein to the TGN, suggesting that this N-terminal extension of Vamp4 contains a dominant and autonomous targeting signal for the TGN. Analysis of deletion mutants of this N-terminal region suggests that this TGN-targeting signal is encompassed within a smaller region consisting of a di-Leu motif followed by two acidic clusters. The essential role of the di-Leu motif and the second acidic cluster was then established by site-directed mutagenesis.

The SNARE motif contributes to rbet1 intracellular targeting and dynamics independently of SNARE interactions

The endoplasmic reticulum/Golgi SNARE rbet1 cycles between the endoplasmic reticulum and Golgi and is essential for cargo transport in the secretory pathway. Although the quaternary SNARE complex containing rbet1 is known to function in membrane fusion, the structural role of rbet1 is unclear. Furthermore, the structural determinants for rbet1 targeting and its cyclical itinerary have not been investigated. We utilized protein interaction assays to demonstrate that the rbet1 SNARE motif plays a structural role similar to the carboxyl-terminal helix of SNAP-25 in the synaptic SNARE complex and demonstrated the importance to SNARE complex assembly of a conserved salt bridge between rbet1 and sec22b. We also examined the potential role of the rbet1 SNARE motif and SNARE interactions in rbet1 localization and dynamics. We found that, in contrast to what has been observed for syntaxin 5, the rbet1 SNARE motif was essential for proper targeting. To test whether SNARE interactions were important for the targeting function of the SNARE motif, we used charge repulsion mutations at the conserved salt bridge position that rendered rbet1 defective for binary, ternary, and quaternary SNARE interactions. We found that heteromeric SNARE interactions are not required at any step in rbet1 targeting or dynamics. Furthermore, the heteromeric state of the SNARE motif does not influence its interaction with the COPI coat or efficient recruitment onto transport vesicles. We conclude that protein targeting is a completely independent function of the rbet1 SNARE motif, which is capable of distinct classes of protein interactions.