Conserved alpha-helical segments on yeast homologs of the synaptobrevin/VAMP family of v-SNAREs mediate exocytic function

SNARE Protein Snc1 Is Essential for Vesicle Trafficking, Membrane Fusion and Protein Secretion in Fungi

Fungi are an important group of microorganisms that play crucial roles in a variety of ecological and biotechnological processes. Fungi depend on intracellular protein trafficking, which involves moving proteins from their site of synthesis to the final destination within or outside the cell. The soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNARE) proteins are vital components of vesicle trafficking and membrane fusion, ultimately leading to the release of cargos to the target destination. The v-SNARE (vesicle-associated SNARE) Snc1 is responsible for anterograde and retrograde vesicle trafficking between the plasma membrane (PM) and Golgi. It allows for the fusion of exocytic vesicles to the PM and the subsequent recycling of Golgi-localized proteins back to the Golgi via three distinct and parallel recycling pathways. This recycling process requires several components, including a phospholipid flippase (Drs2-Cdc50), an F-box protein (Rcy1), a sorting nexin (Snx4-Atg20), a retromer submit, and the COPI coat complex. Snc1 interacts with exocytic SNAREs (Sso1/2, Sec9) and the exocytic complex to complete the process of exocytosis. It also interacts with endocytic SNAREs (Tlg1 and Tlg2) during endocytic trafficking. Snc1 has been extensively investigated in fungi and has been found to play crucial roles in various aspects of intracellular protein trafficking. When Snc1 is overexpressed alone or in combination with some key secretory components, it results in enhanced protein production. This article will cover the role of Snc1 in the anterograde and retrograde trafficking of fungi and its interactions with other proteins for efficient cellular transportation.

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.

Components of the SNARE-containing regulon are co-regulated in root cells undergoing defense

The term regulon has been coined in the genetic model plant Arabidopsis thaliana, denoting a structural and physiological defense apparatus defined genetically through the identification of the penetration (pen) mutants. The regulon is composed partially by the soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor (SNARE) syntaxin PEN1. PEN1 has homology to a Saccharomyces cerevisae gene that regulates a Secretion (Sec) protein, Suppressor of Sec 1 (Sso1p). The regulon is also composed of the β-glucosidase (PEN2) and an ATP binding cassette (ABC) transporter (PEN3). While important in inhibiting pathogen infection, limited observations have been made regarding the transcriptional regulation of regulon genes until now. Experiments made using the model agricultural Glycine max (soybean) have identified co-regulated gene expression of regulon components. The results explain the observation of hundreds of genes expressed specifically in the root cells undergoing the natural process of defense. Data regarding additional G. max genes functioning within the context of the regulon are presented here, including Sec 14, Sec 4 and Sec 23. Other examined G. max homologs of membrane fusion genes include an endosomal bromo domain-containing protein1 (Bro1), syntaxin6 (SYP6), SYP131, SYP71, SYP8, Bet1, coatomer epsilon (ϵ-COP), a coatomer zeta (ζ-COP) paralog and an ER to Golgi component (ERGIC) protein. Furthermore, the effectiveness of biochemical pathways that would function within the context of the regulon ave been examined, including xyloglucan xylosyltransferase (XXT), reticuline oxidase (RO) and galactinol synthase (GS). The experiments have unveiled the importance of the regulon during defense in the root and show how the deposition of callose relates to the process.

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 tip growth apparatus of Aspergillus nidulans

Hyphal tip growth in fungi is important because of the economic and medical importance of fungi, and because it may be a useful model for polarized growth in other organisms. We have investigated the central questions of the roles of cytoskeletal elements and of the precise sites of exocytosis and endocytosis at the growing hyphal tip by using the model fungus Aspergillus nidulans. Time-lapse imaging of fluorescent fusion proteins reveals a remarkably dynamic, but highly structured, tip growth apparatus. Live imaging of SYNA, a synaptobrevin homologue, and SECC, an exocyst component, reveals that vesicles accumulate in the Spitzenkörper (apical body) and fuse with the plasma membrane at the extreme apex of the hypha. SYNA is recycled from the plasma membrane by endocytosis at a collar of endocytic patches, 1-2 mum behind the apex of the hypha, that moves forward as the tip grows. Exocytosis and endocytosis are thus spatially coupled. Inhibitor studies, in combination with observations of fluorescent fusion proteins, reveal that actin functions in exocytosis and endocytosis at the tip and in holding the tip growth apparatus together. Microtubules are important for delivering vesicles to the tip area and for holding the tip growth apparatus in position.

Sphingolipids and membrane biology as determined from genetic models

The importance of sphingolipids in membrane biology was appreciated early in the twentieth century when several human inborn errors of metabolism were linked to defects in sphingolipid degradation. The past two decades have seen an explosion of information linking sphingolipids with cellular processes. Studies have unraveled mechanistic details of the sphingolipid metabolic pathways, and these findings are being exploited in the development of novel therapies, some now in clinical trials. Pioneering work in yeast has laid the foundation for identifying genes encoding the enzymes of the pathways. The advent of the era of genomics and bioinformatics has led to the identification of homologous genes in other species and the subsequent creation of animal knock-out lines for these genes. Discoveries from these efforts have re-kindled interest in the role of sphingolipids in membrane biology. This review highlights some of the recent advances in understanding sphingolipids' roles in membrane biology as determined from genetic models.

Spatially Segregated SNARE Protein Interactions in Living Fungal Cells

The machinery for trafficking proteins through the secretory pathway is well conserved in eukaryotes, from fungi to mammals. We describe the isolation of the snc1, sso1, and sso2 genes encoding exocytic SNARE proteins from the filamentous fungus Trichoderma reesei. The localization and interactions of the T. reesei SNARE proteins were studied with advanced fluorescence imaging methods. The SSOI and SNCI proteins co-localized in sterol-independent clusters on the plasma membrane in subapical but not apical hyphal regions. The vesicle SNARE SNCI also localized to the apical vesicle cluster within the Spitzenkörper of the growing hyphal tips. Using fluorescence lifetime imaging microscopy and Foerster resonance energy transfer analysis, we quantified the interactions between these proteins with high spatial resolution in living cells. Our data showed that the site of ternary SNARE complex formation between SNCI and SSOI or SSOII, respectively, is spatially segregated. SNARE complex formation could be detected between SNCI and SSOI in subapical hyphal compartments along the plasma membrane, but surprisingly, not in growing hyphal tips, previously thought to be the main site of exocytosis. In contrast, SNCI.SSOII complexes were found exclusively in growing apical hyphal compartments. These findings demonstrate spatially distinct sites of plasma membrane SNARE complex formation in fungi and the existence of multiple exocytic SNAREs, which are functionally and spatially segregated. This is the first demonstration of spatially regulated SNARE interactions within the same membrane.

A yeast assay probes the interaction between botulinum neurotoxin serotype B and its SNARE substrate

The seven functionally distinct serotypes (A-G) of botulinum neurotoxin (BoNT) are dichains consisting of light chain (LC) with zinc-dependent endoprotease activity connected by one disulfide bond to heavy chain with neuronal-cell translocation and receptor-binding domains. LC-mediated proteolysis of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins and consequent inhibition of synaptic vesicle fusion to the presynaptic membrane of human motor neurons are responsible for flaccid paralysis associated with botulism. LC endoproteolysis is complex, requiring highly extended SNARE sequences at the surface of intracellular membranes and prompting our development of a genetically amenable assay to monitor the interaction between BoNT/LC and its SNARE substrate. Using BoNT serotype B as a model, the assay employs a chimeric SNARE protein where a portion of neuronal synaptobrevin (Sb) is fused to Snc2p, a Sb ortholog required for protein secretion from yeast cells. Regulated expression of serotype B-LC in yeast leads to cleavage of the chimera and a conditional growth defect. To assess utility of this assay for monitoring SNARE protein cleavage, we growth-selected chimeric SNARE mutations that inhibited proteolysis. When these mutations were introduced into Sb and examined for cleavage, substrate residues located near and distal to the cleavage site were important, including residues positioned near the Sb transmembrane domain, an unexplored aspect of BoNT cell intoxication. Additional mutations were positioned in a nine-residue SNARE motif, supporting a previously assigned role for this motif in LC recognition and providing proof of principle for the application of yeast-based technology to study intracellular BoNT/LC endoproteases.

Detection of protein–protein interactions by a combination of a novel cytoplasmic membrane targeting system of recombinant proteins and fluorescence resonance energy transfer

A novel protein molecular targeting system was created using a cytoplasmic face of a plasma membrane-targeting system in Saccharomyces cerevisiae. The technique involves a molecular display for the creation of a novel reaction site and interaction sites in the field of biotechnology. In a model system, a fluorescent protein was targeted as a reporter to the cytoplasmic face of the plasma membrane. The C-terminal transmembrane domain (CTM) of Ras2p and Snc2p was examined as the portions with anchoring ability to the cytoplasmic face of the plasma membrane. We found that the CTM of Snc2p targeted the enhanced cyan fluorescent protein (ECFP)-protein A fusion protein on the cytoplasmic face of the plasma membrane more strongly than that of Ras2p. To develop it for use as a detection system for protein-protein interactions, the Fc fragment of IgG (Fc) was genetically fused with the enhanced yellow fluorescent protein (EYFP) and expressed in the cytoplasm of the ECFP-protein A-anchored cell. A microscopic analysis showed that fluorescence resonance energy transfer (FRET) between ECFP-protein A and EYFP-Fc occurred, and the change in fluorescence was observed on the cytoplasmic face of the plasma membrane. The detection of protein-protein interactions at the cytoplasmic face of a plasma membrane using FRET combined with a cytoplasmic face-targeting system for proteins provides a novel method for examining the molecular interactions of cytoplasmic proteins, in addition to conventional methods, such as the two-hybrid method in the nuclei.

The Gcs1 Arf-GAP mediates Snc1,2 v-SNARE retrieval to the Golgi in yeast

Gcs1 is an Arf GTPase-activating protein (Arf-GAP) that mediates Golgi-ER and post-Golgi vesicle transport in yeast. Here we show that the Snc1,2 v-SNAREs, which mediate endocytosis and exocytosis, interact physically and genetically with Gcs1. Moreover, Gcs1 and the Snc v-SNAREs colocalize to subcellular structures that correspond to the trans-Golgi and endosomal compartments. Studies performed in vitro demonstrate that the Snc-Gcs1 interaction results in the efficient binding of recombinant Arf1Delta17N-Q71L to the v-SNARE and the recruitment of purified coatomer. In contrast, the presence of Snc had no effect on Gcs1 Arf-GAP activity in vitro, suggesting that v-SNARE binding does not attenuate Arf1 function. Disruption of both the SNC and GCS1 genes results in synthetic lethality, whereas overexpression of either SNC gene inhibits the growth of a distinct subset of COPI mutants. We show that GFP-Snc1 recycling to the trans-Golgi is impaired in gcs1Delta cells and these COPI mutants. Together, these results suggest that Gcs1 facilitates the incorporation of the Snc v-SNAREs into COPI recycling vesicles and subsequent endosome-Golgi sorting in yeast.

Cdc50p, a protein required for polarized growth, associates with the Drs2p P-type ATPase implicated in phospholipid translocation in Saccharomyces cerevisiae

Cdc50p, a transmembrane protein localized to the late endosome, is required for polarized cell growth in yeast. Genetic studies suggest that CDC50 performs a function similar to DRS2, which encodes a P-type ATPase of the aminophospholipid translocase (APT) subfamily. At low temperatures, drs2Delta mutant cells exhibited depolarization of cortical actin patches and mislocalization of polarity regulators, such as Bni1p and Gic1p, in a manner similar to the cdc50Delta mutant. Both Cdc50p and Drs2p were localized to the trans-Golgi network and late endosome. Cdc50p was coimmunoprecipitated with Drs2p from membrane protein extracts. In cdc50Delta mutant cells, Drs2p resided on the endoplasmic reticulum (ER), whereas Cdc50p was found on the ER membrane in drs2Delta cells, suggesting that the association on the ER membrane is required for transport of the Cdc50p-Drs2p complex to the trans-Golgi network. Lem3/Ros3p, a homolog of Cdc50p, was coimmunoprecipitated with another APT, Dnf1p; Lem3p was required for exit of Dnf1p out of the ER. Both Cdc50p-Drs2p and Lem3p-Dnf1p were confined to the plasma membrane upon blockade of endocytosis, suggesting that these proteins cycle between the exocytic and endocytic pathways, likely performing redundant functions. Thus, phospholipid asymmetry plays an important role in the establishment of cell polarity; the Cdc50p/Lem3p family likely constitute potential subunits specific to unique P-type ATPases of the APT subfamily.

Identification and characterization of Snapin as a ubiquitously expressed SNARE-binding protein that interacts with SNAP23 in non-neuronal cells

Members of the SNARE (soluble N -ethylmaleimide-sensitive fusion protein attachment protein receptor) superfamily [syntaxins, VAMPs (vesicle-associated membrane proteins) and SNAP25 (synaptosome-associated protein-25)-related proteins] are required for intracellular membrane-fusion events in eukaryotes. In neurons, assembly of SNARE core complexes comprising the presynaptic membrane-associated SNAREs syntaxin 1 and SNAP25, and the vesicle-associated SNARE VAMP2, is necessary for synaptic vesicle exocytosis. Several accessory factors have been described that associate with the synaptic SNAREs and modulate core complex assembly or mediate Ca2+ regulation. One such factor, Snapin, has been reported to be a brain-specific protein that interacts with SNAP25, and regulates association of the putative Ca2+-sensor synaptotagmin with the synaptic SNARE complex [Ilardi, Mochida and Sheng (1999) Nat. Neurosci. 2, 119-124]. Here we demonstrate that Snapin is expressed ubiquitously in neuronal and non-neuronal cells. Furthermore, using protein-protein-interaction assays we show that Snapin interacts with SNAP23, the widely expressed homologue of SNAP25, and that the predicted C-terminal helical domain of Snapin contains the SNAP23-binding site. Subcellular localization experiments revealed that Snapin is a soluble protein that exists in both cytosolic and peripheral membrane-bound pools in adipocytes. Moreover, association of Snapin with the plasma membrane was detected in cells overexpressing a Snapin-green fluorescent protein fusion protein. Finally, we show that Snapin is able to form a ternary complex with SNAP23 and syntaxin 4, suggesting that it is a component of non-neuronal SNARE complexes. An important implication of our results is that Snapin is likely to perform a general role in SNARE-mediated vesicle fusion events in non-neuronal cells in addition to its participation in Ca2+-regulated neurosecretion.

Characterization of temperature-sensitive mutations in the yeast syntaxin 1 homologues Sso1p and Sso2p, and evidence of a distinct function for Sso1p in sporulation

The duplicated genes SSO1 and SSO2 encode yeast homologues of syntaxin 1 and perform an essential function during fusion of secretory vesicles at the plasma membrane. We have used in vitro mutagenesis to obtain a temperature-sensitive SSO2 allele, sso2-1, in which a conserved arginine has been changed to a lysine. A yeast strain that lacks SSO1 and carries the sso2-1 allele ceases growth and accumulates secretory vesicles at the restrictive temperature. Interestingly, the strain also has a pronounced phenotype at the permissive temperature, causing a defect in bud neck closure that prevents separation of mother and daughter cells. The same mutation was introduced into SSO1, producing the sso1-1 allele, which also has a temperature-sensitive phenotype, although less pronounced than sso2-1. A screen for high copy number suppressors of sso2-1 yielded three genes that are involved in the terminal step of secretion: SNC1, SNC2 and SEC9. The sso1-1 mutation interacts synthetically with a disruption of the MSO1 gene, which encodes a Sec1p interacting protein. Interestingly, we further found that both MSO1 and SSO1, but not SSO2, are required for sporulation. This difference is not due to differential expression, since SSO2 expressed from the SSO1 promoter failed to restore sporulation. We conclude that a functional difference exists between the Sso1 and Sso2 proteins, with the former being specifically required during sporulation.

A t-SNARE of the endocytic pathway must be activated for fusion

The t-SNARE in a late Golgi compartment (Tlg2p) syntaxin is required for endocytosis and localization of cycling proteins to the late Golgi compartment in yeast. We show here that Tlg2p assembles with two light chains, Tlg1p and Vti1p, to form a functional t-SNARE that mediates fusion, specifically with the v-SNAREs Snc1p and Snc2p. In vitro, this t-SNARE is inert, locked in a nonfunctional state, unless it is activated for fusion. Activation can be mediated by a peptide derived from the v-SNARE, which likely bypasses additional regulatory proteins in the cell. Locking t-SNAREs creates the potential for spatial and temporal regulation of fusion by signaling processes that unleash their fusion capacity.

Vesicle-associated membrane protein of Arabidopsis suppresses Bax-induced apoptosis in yeast downstream of oxidative burst

Programmed cell death (PCD) in many systems is controlled by relative amounts of the apoptosis-regulating proteins Bax and Bcl-2 through homo- or heterodimerization. Here we show that Bax-induced PCD of yeast was suppressed by transformation with a vesicle-associated membrane protein from Arabidopsis (AtVAMP), which was isolated by screening a cDNA expression library against sugar-induced cell death in yeast. AtVAMP expression blocked Bax-induced PCD downstream of oxidative burst. AtVAMP also prevented H(2)O(2)-induced apoptosis in yeast and in Arabidopsis cells. Reduced oxidation of lipids and plasma membrane proteins was detected in the AtVAMP-transformed yeast, suggesting improved membrane repair. Inhibition of intracellular vesicle trafficking by brefeldin A induced apoptosis from a sublethal concentration of H(2)O(2). No protection occurred by overexpression of the yeast homolog SCN2. However, efficient suppression of yeast PCD occurred by expression of a chimeric gene, composed of the conserved domains from yeast, fused to the variable N-terminal domain from Arabidopsis, resulting in exchange of the proline-rich N-terminal domain of SCN2 with a proline-poor Arabidopsis sequence. Our results suggest that intracellular vesicle traffic can regulate execution of apoptosis by affecting the rate of membrane recycling and that the proline-rich N-terminal domain of VAMP inhibited this process.

Longins: a new evolutionary conserved VAMP family sharing a novel SNARE domain

This article describes the discovery of a novel SNARE domain that might be involved in the regulation of membrane fusion. This domain is shared by a novel family of VAMPs called long VAMPs or longins. Members of this family are more conserved among eukaryotes than are classical VAMPs, possibly because of their underlying basic SNARE function.

t-SNARE dephosphorylation promotes SNARE assembly and exocytosis in yeast

The role of protein phosphorylation in secretion is not well understood. Here we show that yeast lacking the Snc1,2 v-SNAREs, or bearing a temperature-sensitive mutation in the Sso2 t-SNARE, are rescued at restrictive conditions by the addition of ceramide precursors and analogs to the growth medium. Rescue results from dephosphorylation of the Sso t-SNAREs by a ceramide-activated type 2A protein phosphatase (Sit4) involved in cell cycle control. Sso t-SNARE dephosphorylation correlated with its assembly into complexes with the Sec9 t-SNARE, both in vitro and in vivo, and with an increase in protein trafficking and secretion in cells. SNARE complexes isolated under these conditions contained only Sso and Sec9, suggesting that a t-t-SNARE fusion complex is sufficient to confer exocytosis. Mutation of a single PKA site (Ser79 to Ala79) in Sso1 resulted in a decrease in phosphorylation and was sufficient to confer growth to snc cells at restrictive conditions. Thus, modulation of t-SNARE phosphorylation regulates SNARE complex assembly and membrane fusion in vivo.

Exocytosis requires asymmetry in the central layer of the SNARE complex

Assembly of SNAREs (soluble N:-ethylmaleimide- sensitive factor attachment protein receptors) mediates membrane fusions in all eukaryotic cells. The synaptic SNARE complex is represented by a twisted bundle of four alpha-helices. Leucine zipper-like layers extend through the length of the complex except for an asymmetric and ionic middle layer formed by three glutamines (Q) and one arginine (R). We have examined the functional consequences of Q-R exchanges in the conserved middle layer using the exocytotic SNAREs of yeast as a model. Exchanging Q for R in Sso2p drastically reduces cell growth and protein secretion. When a 3Q/1R ratio is restored by a mirror R-->Q substitution in the R-SNARE Snc2p, wild-type functionality is observed. Secretion is near normal when all four helices contain Q, but defects become apparent when additional mutations are present in other layers. Using molecular dynamics free energy perturbation simulations, these findings are rationalized in structural and energetic terms. We conclude that the asymmetric arrangement of the polar amino acids in the central layer is essential for normal function of SNAREs in membrane fusion.

Yeast exocytic v-SNAREs confer endocytosis

In yeast, homologues of the synaptobrevin/VAMP family of v-SNAREs (Snc1 and Snc2) confer the docking and fusion of secretory vesicles at the cell surface. As no v-SNARE has been shown to confer endocytosis, we examined whether yeast lacking the SNC genes, or possessing a temperature-sensitive allele of SNC1 (SNC1(ala43)), are deficient in the endocytic uptake of components from the cell surface. We found that both SNC and temperature-shifted SNC1(ala43) yeast are deficient in their ability to deliver the soluble dye FM4-64 to the vacuole. Under conditions in which vesicles accumulate, FM4-64 stained primarily the cytoplasm as well as fragmented vacuoles. In addition, alpha-factor-stimulated endocytosis of the alpha-factor receptor, Ste2, was fully blocked, as evidenced using a Ste2-green fluorescent protein fusion protein as well as metabolic labeling studies. This suggests a direct role for Snc v-SNAREs in the retrieval of membrane proteins from the cell surface. Moreover, this idea is supported by genetic and physical data that demonstrate functional interactions with t-SNAREs that confer endosomal transport (e.g., Tlg1,2). Notably, Snc1(ala43) was found to be nonfunctional in cells lacking Tlg1 or Tlg2. Thus, we propose that synaptobrevin/VAMP family members are engaged in anterograde and retrograde protein sorting steps between the Golgi and the plasma membrane.

Compartmental specificity of cellular membrane fusion encoded in SNARE proteins

Membrane-enveloped vesicles travel among the compartments of the cytoplasm of eukaryotic cells, delivering their specific cargo to programmed locations by membrane fusion. The pairing of vesicle v-SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) with target membrane t-SNAREs has a central role in intracellular membrane fusion. We have tested all of the potential v-SNAREs encoded in the yeast genome for their capacity to trigger fusion by partnering with t-SNAREs that mark the Golgi, the vacuole and the plasma membrane. Here we find that, to a marked degree, the pattern of membrane flow in the cell is encoded and recapitulated by its isolated SNARE proteins, as predicted by the SNARE hypothesis.

Membrane fusion and exocytosis

Membrane fusion involves the merger of two phospholipid bilayers in an aqueous environment. In artificial lipid bilayers, fusion proceeds by means of defined transition states, including hourglass-shaped intermediates in which the proximal leaflets of the fusing membranes are merged whereas the distal leaflets are separate (fusion stalk), followed by the reversible opening of small aqueous fusion pores. Fusion of biological membranes requires the action of specific fusion proteins. Best understood are the viral fusion proteins that are responsible for merging the viral with the host cell membrane during infection. These proteins undergo spontaneous and dramatic conformational changes upon activation. In the case of the paradigmatic fusion proteins of the influenza virus and of the human immunodeficiency virus, an amphiphilic fusion peptide is inserted into the target membrane. The protein then reorients itself, thus forcing the fusing membranes together and inducing lipid mixing. Fusion of intracellular membranes in eukaryotic cells involves several protein families including SNAREs, Rab proteins, and Sec1/Munc-18 related proteins (SM-proteins). SNAREs form a novel superfamily of small and mostly membrane-anchored proteins that share a common motif of about 60 amino acids (SNARE motif). SNAREs reversibly assemble into tightly packed helical bundles, the core complexes. Assembly is thought to pull the fusing membranes closely together, thus inducing fusion. SM-proteins comprise a family of soluble proteins that bind to certain types of SNAREs and prevent the formation of core complexes. Rab proteins are GTPases that undergo highly regulated GTP-GDP cycles. In their GTP form, they interact with specific proteins, the effector proteins. Recent evidence suggests that Rab proteins function in the initial membrane contact connecting the fusing membranes but are not involved in the fusion reaction itself.

Structures of yeast vesicle trafficking proteins

In protein transport between organelles, interactions of v- and t-SNARE proteins are required for fusion of protein-containing vesicles with appropriate target compartments. Mammalian SNARE proteins have been observed to interact with NSF and SNAP, and yeast SNAREs with yeast homologues of NSF and SNAP proteins. This observation led to the hypothesis that, despite low sequence homology, SNARE proteins are structurally similar among eukaryotes. SNARE proteins can be classified into two groups depending on whether they interact with SNARE binding partners via conserved glutamine (Q-SNAREs) or arginine (R-SNAREs). Much of the published structural data available is for SNAREs involved in exocytosis (either in yeast or synaptic vesicles). This paper describes circular dichroism, Fourier transform infrared spectroscopy, and dynamic light scattering data for a set of yeast v- and t-SNARE proteins, Vti1p and Pep12p, that are Q-SNAREs involved in intracellular trafficking. Our results suggest that the secondary structure of Vti1p is highly alpha-helical and that Vti1p forms multimers under a variety of solution conditions. In these respects, Vti1p appears to be distinct from R-SNARE proteins characterized previously. The alpha-helicity of Vti1p is similar to that of Q-SNARE proteins characterized previously. Pep12p, a Q-SNARE, is highly alpha-helical. It is distinct from other Q-SNAREs in that it forms dimers under many of the solution conditions tested in our experiments. The results presented in this paper are among the first to suggest heterogeneity in the functioning of SNARE complexes.

Yeast VSM1 encodes a v-SNARE binding protein that may act as a negative regulator of constitutive exocytosis

We have screened for proteins that interact with v-SNAREs of the late secretory pathway in the yeast Saccharomyces cerevisiae. A novel protein, designated Vsm1, binds tightly to the Snc2 v-SNARE in the two-hybrid system and can be coimmunoprecipitated with Snc1 or Snc2 from solubilized yeast cell extracts. Disruption of the VSM1 gene results in an increase of proteins secreted into the medium but does not affect the processing or secretion of invertase. In contrast, VSM1 overexpression in cells which bear a temperature-sensitive mutation in the Sec9 t-SNARE (sec9-4 cells) results in the accumulation of non-invertase-containing low-density secretory vesicles, inhibits cell growth and the secretion of proteins into the medium, and blocks rescue of the temperature-sensitive phenotype by SNC1 overexpression. Yet, VSM1 overexpression does not affect yeast bearing a sec9-7 allele which, in contrast to sec9-4, encodes a t-SNARE protein capable of forming a stable SNARE complex in vitro at restrictive temperatures. On the basis of these results, we propose that Vsm1 is a novel v-SNARE-interacting protein that appears to act as negative regulator of constitutive exocytosis. Moreover, this regulation appears specific to one of two parallel exocytic paths which are operant in yeast cells.

Assembly of a ternary complex by the predicted minimal coiled-coil-forming domains of syntaxin, SNAP-25, and synaptobrevin. A circular dichroism study

The assembly of target (t-SNARE) and vesicle-associated SNAP receptor (v-SNARE) proteins is a critical step for the docking of synaptic vesicles to the plasma membrane. Syntaxin-1A, SNAP-25, and synaptobrevin-2 (also known as vesicle-associated membrane protein, or VAMP-2) bind to each other with high affinity, and their binding regions are predicted to form a trimeric coiled-coil. Here, we have designed three peptides, which correspond to sequences located in the syntaxin-1A H3 domain, the C-terminal domain of SNAP-25, and a conserved central domain of synaptobrevin-2, that exhibit a high propensity to form a minimal trimeric coiled-coil. The peptides were synthesized by solid phase methods, and their interactions were studied by CD spectroscopy. In aqueous solution, the peptides were unstructured and showed no interactions with each other. In contrast, upon the addition of moderate amounts of trifluoroethanol (30%), the peptides adopted an alpha-helical structure and displayed both homomeric and heteromeric interactions. The interactions observed in ternary mixtures induce a stabilization of peptide structure that is greater than that predicted from individual binary interactions, suggesting the formation of a higher order structure compatible with the assembly of a trimeric coiled-coil.

Involvement of long chain fatty acid elongation in the trafficking of secretory vesicles in yeast

Members of the synaptobrevin/VAMP family of v-SNAREs are thought to be essential for vesicle docking and exocytosis in both lower and higher eukaryotes. Here, we describe yeast mutants that appear to bypass the known v-SNARE requirement in secretion. Recessive mutations in either VBM1 or VBM2, which encode related ER-localized membrane proteins, allow yeast to grow normally and secrete in the absence of Snc v-SNAREs. These mutants show selective alterations in protein transport, resulting in the differential trafficking and secretion of certain protein cargo. Yet, processing of the vacuolar marker, carboxypeptidase Y, and the secreted protein, invertase, appear normal in these mutants indicating that general protein trafficking early in the pathway is unaffected. Interestingly, VBM1 and VBM2 are allelic to ELO3 and ELO2, two genes that have been shown recently to mediate the elongation of very long chain fatty acids and subsequent ceramide and inositol sphingolipid synthesis. Thus, the v-SNARE requirement in constitutive exocytosis is abrogated by mutations in early components of the secretory pathway that act at the level of lipid synthesis to affect the ability of secretory vesicles to sort and deliver protein cargo.

Genetic and morphological analyses reveal a critical interaction between the C-termini of two SNARE proteins and a parallel four helical arrangement for the exocytic SNARE complex

In a screen for suppressors of a temperature-sensitive mutation in the yeast SNAP-25 homolog, Sec9, we have identified a gain-of-function mutation in the yeast synaptobrevin homolog, Snc2. The genetic properties of this suppression point to a specific interaction between the C-termini of Sec9 and Snc2 within the SNARE complex. Biochemical analysis of interactions between the wild-type and mutant proteins confirms this prediction, demonstrating specific effects of these mutations on interactions between the SNAREs. The location of the mutations suggests that the C-terminal H2 helical domain of Sec9 is likely to be aligned in parallel with Snc2 in the SNARE complex. To test this prediction, we examined the structure of the yeast exocytic SNARE complex by deep-etch electron microscopy. Like the neuronal SNARE complex, it is a rod approximately 14 nm long. Using epitope tags, antibodies and maltose-binding protein markers, we find that the helical domains of Sso, Snc and both halves of Sec9 are all aligned in parallel within the SNARE complex, suggesting that the yeast exocytic SNARE complex consists of a parallel four helix bundle. Finally, we find a similar arrangement for SNAP-25 in the neuronal SNARE complex. This provides strong evidence that the exocytic SNARE complex is a highly conserved structure composed of four parallel helical domains whose C-termini must converge in order to bring about membrane fusion.

Structural organization of the synaptic exocytosis core complex

Syntaxin, vesicle-associated membrane protein (VAMP), and synaptosome-associated protein of 25 kDa (SNAP-25) form a ternary "core complex" central to the process of synaptic vesicle docking and fusion. Several lines of evidence support the hypothesis that the proteins assemble in a coiled-coil structure, but the alignment of alpha helices in this coil and the overall conformation of the coil are unknown. We employ the technique of fluorescence resonance energy transfer (FRET) to investigate the alignment between syntaxin and VAMP. With the acceptor probe coupled to the amino-terminal end of the VAMP coiled-coil domain, the donor probe fluorescence is quenched to a greater extent when it is on the amino-terminal end of the syntaxin H3 domain than when it is on the carboxy-terminal end. The data indicate that syntaxin and VAMP bind primarily in a parallel arrangement and suggest a coiled-coil structure that is bent rather than fully extended. We propose a model in which binding of SNAP receptor (SNARE) protein coiled-coil domains helps drive vesicle fusion.