Structural organization of the synaptic exocytosis core complex

Functional and physical coupling of voltage-sensitive calcium channels with exocytotic proteins: ramifications for the secretion mechanism

The secretion of neurotransmitters is a rapid Ca(2+)-regulated process that brings about vesicle fusion with the plasma membrane. This rapid process (< 100 microseconds) involves multiple proteins located at the plasma and vesicular membranes. Because of their homology to proteins participating in constitutive secretion and protein trafficking, they have been characterized extensively. The sequential events that lead these proteins to vesicle docking and fusion are still unclear. We will review recent studies that demonstrate the operative role played by voltage-sensitive Ca(2+) channels and discuss the relevance for the process of evoked transmitter release. The regulation of Ca(2+) influx by syntaxin, synaptosome-associated protein of 25 kDa (SNAP-25) and synaptotagmin, and the reciprocity of these proteins in controlling the kinetic properties of the channel will be discussed. Calcium channel and synaptic proteins expressed in Xenopus oocytes demonstrate a strong functional interaction, which could be pertinent to the mechanism of secretion. First, the voltage-sensitive Ca(2+) channels are negatively modulated by syntaxin: this inhibition is reversed by synaptotagmin. Second, the modulation of N-type Ca(2+) channel activation kinetics strongly suggests that the vesicle could be docked at the plasma membrane through direct interaction with synaptotagmin. Finally, these interactions provide evidence for the assembly of the voltage-sensitive Ca(2+) channel with syntaxin 1A, SNAP-25 and synaptotagmin into an excitosome complex: a putative fusion complex with a potential role in the final stages of secretion. Studies suggest that cross-talk between the synaptic proteins and the channel in a tightly organized complex may enable a rapid secretory response to an incoming signal such as membrane depolarization.

Normal and abnormal secretion by haemopoietic cells

The secretory lysosomes found in haemopoietic cells provide a very efficient mechanism for delivering the effector proteins of many immune cells in response to antigen recognition. Although secretion shows some similarities to the secretion of specialized granules in other secretory cell types, some aspects of secretory lysosome release appear to be unique to melanocytes and cells of the haemopoietic lineage. Mast cells and platelets have provided excellent models for studying secretion, but recent advances in characterizing the immunological synapse allow a very fine dissection of the secretory process in T lymphocytes. These studies show that secretory lysosomes are secreted from the centre of the talin ring at the synapse. Proper secretion requires a series of Rab and cytoskeletal elements which play critical roles in the specialized secretion of lysosomes in haemopoietic cells.

Molecular mechanisms of neurotransmitter release

The release of neurotransmitter from neurons represents one of the pivotal events in synaptic transmission. Neurotransmitters are released from synaptic vesicles in presynaptic neurons in response to neural activity, diffuse across the synaptic cleft, and bind specific receptors in order to bring about changes in postsynaptic neurons. Some of the molecular processes that govern neurotransmitter release are now becoming better understood. The steps involved can be broken down into two partially overlapping presynaptic cycles, the neurotransmitter cycle and the synaptic vesicle cycle. The neurotransmitter cycle involves transmitter biosynthesis, storage, reuptake, and degradation. The synaptic vesicle cycle involves targeting to the nerve terminal, docking, fusion, endocytosis, and recycling. Biochemical and structural studies have yielded important insight into our understanding of each of these two cycles. Further, both pharmacological and genetic interference with either of these cycles results in profound alterations in synaptic transmission and behavior, demonstrating the crucial role of neurotransmitter release.

Phosphorylation of the N-ethylmaleimide-sensitive factor is associated with depolarization-dependent neurotransmitter release from synaptosomes

Critical to SNARE protein function in neurotransmission are the accessory proteins, soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein (SNAP), and NSF, that play a role in activation of the SNAREs for membrane fusion. In this report, we demonstrate the depolarization-induced, calcium-dependent phosphorylation of NSF in rat synaptosomes. Phosphorylation of NSF is coincident with neurotransmitter release and requires an influx of external calcium. Phosphoamino acid analysis of the radiolabeled NSF indicates a role for a serine/threonine-specific kinase. Synaptosomal phosphorylation of NSF is stimulated by phorbol esters and is inhibited by staurosporine, chelerythrine, bisindolylmaleimide I, calphostin C, and Ro31-8220 but not the calmodulin kinase II inhibitor, Kn-93, suggesting a role for protein kinase C (PKC). Indeed, NSF is phosphorylated by PKC in vitro at Ser-237 of the catalytic D1 domain. Mutation of this residue to glutamic acid or to alanine eliminates in vitro phosphorylation. Molecular modeling studies suggest that Ser-237 is adjacent to an inter-subunit interface at a position where its phosphorylation could affect NSF activity. Consistently, mutation of Ser-237 to Glu, to mimic phosphorylation, results in a hexameric form of NSF that does not bind to SNAP-SNARE complexes, whereas the S237A mutant does form complex. These data suggest a negative regulatory role for PKC phosphorylation of NSF.

Sequential SNARE assembly underlies priming and triggering of exocytosis

Changes in SNARE conformations during MgATP-dependent priming of cracked PC12 cells were probed by their altered accessibility to various inhibitors. Dominant negative soluble syntaxin and, to a much lesser extent, VAMP coil domains inhibited exocytosis more efficiently after priming. Neurotoxins and an anti-SNAP25 antibody inhibited exocytosis less effectively after priming. We propose that SNAREs partially and reversibly assemble during priming, and that the syntaxin H3 domain is prevented from fully joining the complex until the arrival of the Ca2+ trigger. Furthermore, we find that mutation of hydrophobic residues of the SNAP25 C-terminal coil that contribute to SNARE core interactions affects the maximal rate of exocytosis, while mutation of charged residues on the surface of the complex affects the apparent affinity of the coil domain for the partially assembled complex.

Structural insights into the molecular mechanism of calcium-dependent vesicle-membrane fusion

The fusion of vesicles with target membranes is controlled by a complex network of protein-protein and protein-lipid interactions. Recently determined structures of the SNARE complex, synaptotagmin III, nSec1, domains of the NSF chaperone and its adaptor (SNAP), and Rab3 and some of its effectors provide the framework for developing molecular models of vesicle fusion and for designing experiments to test these models. Ultimately, knowledge of the structures of higher-order complexes and their dynamic behavior will be required to obtain a full understanding of the vesicle fusion protein machinery.

Mechanisms of synaptic vesicle exocytosis

Chemical synaptic transmission serves as the main form of cell to cell communication in the nervous system. Neurotransmitter release occurs through the process of regulated exocytosis, in which a synaptic vesicle releases its contents in response to an increase in calcium. The use of genetic, biochemical, structural, and functional studies has led to the identification of factors important in the synaptic vesicle life cycle. Here we focus on the prominent role of SNARE (soluble NSF attachment protein receptor) proteins during membrane fusion and the regulation of SNARE function by Rab3a, nSec1, and NSF. Many of the proteins important for transmitter release have homologs involved in intracellular vesicle transport, and all forms of vesicle trafficking share common basic principles. Finally, modifications to the synaptic exocytosis pathway are very likely to underlie certain forms of synaptic plasticity and therefore contribute to learning and memory.

SNARE-mediated membrane fusion

SNARE proteins have been proposed to mediate all intracellular membrane fusion events. There are over 30 SNARE family members in mammalian cells and each is found in a distinct subcellular compartment. It is likely that SNAREs encode aspects of membrane transport specificity but the mechanism by which this specificity is achieved remains controversial. Functional studies have provided exciting insights into how SNARE proteins interact with each other to generate the driving force needed to fuse lipid bilayers.

Coiled coils: a highly versatile protein folding motif

The alpha-helical coiled coil is one of the principal subunit oligomerization motifs in proteins. Its most characteristic feature is a heptad repeat pattern of primarily apolar residues that constitute the oligomer interface. Despite its simplicity, it is a highly versatile folding motif: coiled-coil-containing proteins exhibit a broad range of different functions related to the specific 'design' of their coiled-coil domains. The architecture of a particular coiled-coil domain determines its oligomerization state, rigidity and ability to function as a molecular recognition system. Much progress has been made towards understanding the factors that determine coiled-coil formation and stability. Here we discuss this highly versatile protein folding and oligomerization motif with regard to its structural architecture and how this is related to its biological functions.

N-ethylmaleimide sensitive factor (NSF) structure and function

Our understanding of the molecular mechanisms of membrane trafficking advanced at a rapid rate during the 1990s. As one of the initial protein components of the trafficking machinery to be identified, N-ethylmaleimide sensitive factor (NSF) has served as a reference point in many of these recent studies. This hexameric ATPase is essential for most of the membrane-trafficking events in a cell. Initially, due to its ATPase activity, NSF was thought to be the motor that drove membrane fusion. Subsequent studies have shown that NSF actually plays the role of a chaperone by activating SNAP receptor proteins (SNAREs) so that they can participate in membrane fusion. In this review we will examine the initial characterization of NSF, its role in membrane fusion events, and what new structural information can tell us about NSF's mechanism of action.

SNARE proteins contribute to calcium cooperativity of synaptic transmission

A hallmark of calcium-triggered synaptic transmission is the cooperative relationship between calcium and the amount of transmitter released. This relationship is thought to be important for improving the efficiency of synaptic vesicle exocytosis. Although it is generally held that cooperativity arises from the interaction of multiple calcium ions with a single calcium-sensing molecule, the precise molecular basis of this phenomenon is not known. The SNARE proteins are known to be critical for synaptic vesicle exocytosis. We therefore tested for a contribution of SNARE proteins to cooperativity by genetically reducing the levels of syntaxin IA and neuronal-synaptobrevin in Drosophila. Surprisingly, we found that reducing these SNARE proteins also reduced Ca(2+) cooperativity. Thus, SNARE proteins are important for determining the cooperative relationship between calcium and synaptic transmission.

Nadrin, a novel neuron-specific GTPase-activating protein involved in regulated exocytosis

It has been proposed that the cortical actin filament networks act as a cortical barrier that must be reorganized to enable docking and fusion of the synaptic vesicles with the plasma membranes. We identified a novel neuron-associated developmentally regulated protein, designated as Nadrin. Expression of Nadrin is restricted to neurons and correlates well with the differentiation of neurons. Nadrin has a unique structure; it contains a GTPase-activating protein (GAP) domain for Rho family GTPases, a potential coiled-coil domain, and a succession of 29 glutamines. In vitro the GAP domain activates RhoA, Rac1, and Cdc42 GTPases. Expression of Nadrin in NIH3T3 cells markedly reduced the number of the actin stress fibers and the formation of the ruffled membranes, suggesting that Nadrin regulates actin filament reorganization. In PC12 cells, Nadrin colocalized with synaptotagmin in the neurite termini and also with cortical actin filaments in the subplasmalemmal regions. Expression of Nadrin or its mutant composed of the coiled-coil and GAP domain enhanced Ca(2+)-dependent exocytosis of PC12 cells, but a mutant lacking the GAP domain inhibited exocytosis. These results suggest that Nadrin plays a role in regulating Ca(2+)-dependent exocytosis, most likely by catalyzing GTPase activity of Rho family proteins and by inducing the reorganization of the cortical actin filaments.

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.

Syntaxin 11 is an atypical SNARE abundant in the immune system

Several classes of proteins have been identified that mediate and regulate membrane dynamics throughout the eukaryotic cell. One class of membrane-trafficking proteins, referred to as soluble N-ethylmaleimide sensitive factor attachment protein receptors (SNAREs), have been implicated in mediating membrane fusion. Here we characterize syntaxin 11, an atypical syntaxin family member lacking a transmembrane domain. Syntaxin 11 was found to be enriched in tissues of the immune system including thymus, spleen and lymphnodes; however, lower levels of the protein are found in other tissues. Using immunofluorescence and electron microscopy techniques, we demonstrate that syntaxin 11 associates with intermediate compartment (IC) and post-Golgi membranes through a putative palmitoylation domain, as well as through formation of the 100-kDa complex with, as of yet, unidentified proteins. The coiled-coil forming H3 domain is required for the formation of the 100-kDa complex, and this complex can be dissociated upon addition of alphaSNAP. Thus, while the precise function of syntaxin 11 remains to be elucidated, it may be particularly important in regulating membrane dynamics of the immune system.

Differential sorting of SNAP-25a and SNAP-25b proteins in neuroblastoma cells

Exocytosis is mediated by high-affinity interactions between different SNARE proteins. The existence of several variants of each SNARE protein suggests that the specificity of fusion may be directed by unique combination of SNARE family members. We examined if two alternatively spliced variants of synaptosomal-associated protein of 25 kD, SNAP-25a and SNAP-25b, possessed distinct cellular distribution if coexpressed within the same neuroblastoma cell. Double-labelling immunofluorescence histochemistry in combination with confocal laser microscopy of individual cell clones revealed a different subcellular localisation pattern for the two SNAP-25 variants. Sucrose density gradient centrifugation of cell homogenates followed by Western blotting showed that the SNAP-25 protein variants associated with intracellular organelles of different density. Taken together, this study shows that two alternatively spliced variants of SNAP-25, differing in only nine amino acids, possess distinct properties at the level of intracellular trafficking, suggesting that the cellular localisation of SNAP-25 protein is regulated at the level of mRNA splicing.

Ordering the final events in yeast exocytosis

In yeast, assembly of exocytic soluble N-ethylmaleimide-sensitive fusion protein (NSF) attachment protein receptor (SNARE) complexes between the secretory vesicle SNARE Sncp and the plasma membrane SNAREs Ssop and Sec9p occurs at a late stage of the exocytic reaction. Mutations that block either secretory vesicle delivery or tethering prevent SNARE complex assembly and the localization of Sec1p, a SNARE complex binding protein, to sites of secretion. By contrast, wild-type levels of SNARE complexes persist in the sec1-1 mutant after a secretory block is imposed, suggesting a role for Sec1p after SNARE complex assembly. In the sec18-1 mutant, cis-SNARE complexes containing surface-accessible Sncp accumulate in the plasma membrane. Thus, one function of Sec18p is to disassemble SNARE complexes on the postfusion membrane.

Modulation of the SNARE core complex by dopamine

Communication between nerve cells in the brain occurs primarily through specialized junctions called synapses. Recently, many details of synaptic transmission have emerged. The identities of specific proteins important for synaptic vesicle release have now been established. We have investigated three synaptic proteins, VAMP (vesicle associated membrane protein; also called synaptobrevin), syntaxin, and SNAP25 (synaptosomal associated protein of 25kDa) as possible targets in the dopamine-mediated modulation of synaptic function in rat striatal slices. These three proteins form a SNARE (soluble N-ethylmalemide-sensitive factor attachment protein receptors) core complex that is known to be essential for synaptic transmission. Although it is envisioned that the SNAREs undergo dynamic and cyclic interactions to elicit synaptic vesicle release, their precise functions in neurotransmission remains unknown. We have examined SNARE complexes in intact rat striatal slices. Cellular proteins were solubilized, separated electrophoretically by SDS-PAGE, and then identified immunologically. Application of dopamine to striatal slices results in SNAREs favoring the SNARE core complex, a complex which forms spontaneously in the absence of crosslinking agents, rather than the monomer form. In addition, rapid crosslinking of dopamine-treated striatal slices demonstrates that the SNARE complex is increased 4 fold in dopamine treated striatal slices compared with control slices. Haloperidol blocked the dopamine-induced change in the core complex. These results suggest that changes in the activities of SNAREs may be involved in the underlying cellular mechanisms(s) of dopamine-regulated synaptic plasticity of the striatum.Key words: dopamine, striatium, VAMP, syntaxin, SNAP25.

Interaction of syntaxins with epithelial ion channels

Recent evidence suggests that some of the syntaxin isoforms may physically interact with and regulate the transport activity of a defined set of membrane transport proteins. This review examines recent studies of the cystic fibrosis transmembrane conductance regulator and the epithelial sodium channel which define distinct roles of syntaxin 1A and syntaxin 3 in the regulation of surface expression as well as intrinsic properties of these epithelial ion transporters.

Membrane tethering and fusion in the secretory and endocytic pathways

Studies of intracellular trafficking over the past decade or so have led to striking advances in our understanding of the molecular processes by which transport intermediates dock and fuse. SNARE proteins play a central role, assembling into complexes that bridge membranes and may catalyze membrane fusion directly. In general, different SNARE proteins operate in different intracellular trafficking pathways, so recent reports that SNARE assembly in vitro is promiscuous have come as something of a surprise. We propose a model in which proper SNARE assembly is under kinetic control, orchestrated by members of the Sec1 protein family, small GTP-binding Rab proteins, and a diverse assortment of tethering proteins.

Syntaxin 17 is abundant in steroidogenic cells and implicated in smooth endoplasmic reticulum membrane dynamics

The endoplasmic reticulum (ER) consists of subcompartments that have distinct protein constituents, morphological appearances, and functions. To understand the mechanisms that regulate the intricate and dynamic organization of the endoplasmic reticulum, it is important to identify and characterize the molecular machinery involved in the assembly and maintenance of the different subcompartments. Here we report that syntaxin 17 is abundantly expressed in steroidogenic cell types and specifically localizes to smooth membranes of the ER. By immunoprecipitation analyses, syntaxin 17 exists in complexes with a syntaxin regulatory protein, rsly1, and/or two intermediate compartment SNARE proteins, rsec22b and rbet1. Furthermore, we found that syntaxin 17 is anchored to the smooth endoplasmic reticulum through an unusual mechanism, requiring two adjacent hydrophobic domains near its carboxyl terminus. Converging lines of evidence indicate that syntaxin 17 functions in a vesicle-trafficking step to the smooth-surfaced tubular ER membranes that are abundant in steroidogenic cells.

Participation of syntaxin 1A in membrane trafficking involving neurite elongation and membrane expansion

Syntaxin 1A has been implicated to play an important role in neurotransmitter release by regulating synaptic vesicle fusion. The protein is also suggested to be required for other types of membrane fusion such as cellularization during embryonic development. In the current study, we overexpressed syntaxin 1A, SNAP-25b, and VAMP-2 in PC12 cells using recombinant adenoviruses, and determined their effects on membrane trafficking involving neurite outgrowth. It was found that overexpression of syntaxin 1A inhibited NGF-induced neurite extension, and the expressed syntaxin was localized to the plasma membrane, intracellular membranes, and the neurite tips. SNAP-25 overexpression slightly enhanced neurite elongation, whereas no significant changes in neurite growth was observed in VAMP-overexpressing cells. The effect of syntaxin 1A in general membrane trafficking was further studied by transient transfection of non-neuronal cells. Syntaxin 1A expression in HEK 239 and NIH3T3-L1 caused the cells to lose their normal morphology, leading to round and smaller cells. Deletion of the C-terminal sequence containing the H3 helix and the membrane anchoring domains of syntaxin abolished its ability to induce cell morphology changes, whereas removal of the N-terminal 1-170 amino acid sequence did not affect this activity. These findings suggest that in addition to its well documented role in synaptic vesicle fusion, syntaxin may function in other non-synaptic membrane trafficking such as neurite outgrowth and membrane expansion.

Beta-granule transport and exocytosis

Regulated beta -granule exocytosis is critical for the ability of the beta -cell to finely control body glucose homeostasis. This is now understood to be a multistage process whereby beta -granules are transported from biosynthetic/storage sites in the cell cytoplasm and targeted to specific regions of the plasma membrane. Exocytosis is achieved when these granules are triggered to fuse with the membrane by an elevated cytosolic Ca(2+). Dramatic advances have been made recently in our understanding of the protein-protein interactions and regulatory signals that govern intracellular transport and fusion. Although best understood for exocytosis from neurons and neuroendocrine cells, similar processes are thought to be conserved in the beta -cell.

The X-ray crystal structure of neuronal Sec1 from squid sheds new light on the role of this protein in exocytosis

BACKGROUND

Sec1-like molecules have been implicated in a variety of eukaryotic vesicle transport processes including neurotransmitter release by exocytosis. They regulate vesicle transport by binding to a t-SNARE from the syntaxin family. This process is thought to prevent SNARE complex formation, a protein complex required for membrane fusion. Whereas Sec1 molecules are essential for neurotransmitter release and other secretory events, their interaction with syntaxin molecules seems to represent a negative regulatory step in secretion.

RESULTS

Here we report the X-ray crystal structure of a neuronal Sec1 homologue from squid, s-Sec1, at 2.4 A resolution. Neuronal s-Sec1 is a modular protein that folds into a V-shaped three-domain assembly. Peptide and mutagenesis studies are discussed with respect to the mechanism of Sec1 regulation. Comparison of the structure of squid s-Sec1 with the previously determined structure of rat neuronal Sec1 (n-Sec1) bound to syntaxin-1a indicates conformational rearrangements in domain III induced by syntaxin binding.

CONCLUSIONS

The crystal structure of s-Sec1 provides the molecular scaffold for a number of molecular interactions that have been reported to affect Sec1 function. The structural differences observed between s-Sec1 and the structure of a rat n-Sec1-syntaxin-1a complex suggest that local conformational changes are sufficient to release syntaxin-1a from neuronal Sec1, an active process that is thought to involve additional effector molecule(s).

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.

SNARE protein trafficking in polarized MDCK cells

A key feature of polarized epithelial cells is the ability to maintain the specific biochemical composition of the apical and basolateral plasma membrane domains. This polarity is generated and maintained by the continuous sorting of apical and basolateral components in the secretory and endocytic pathways. Soluble N-ethyl maleimide-sensitive factor attachment protein receptors (SNARE) proteins of vesicle-associated membrane protein (VAMP) and syntaxin families have been suggested to play a role in the biosynthetic transport to the apical and basolateral plasma membranes of polarized cells, where they likely mediate membrane fusion. To investigate the involvement of SNARE proteins in membrane trafficking to the apical and basolateral plasma membrane in the endocytic pathway we have monitored the recycling of various VAMP and syntaxin molecules between intracellular compartments and the two plasma membrane domains in Madin-Darby canine kidney (MDCK) cells. Here we show that VAMP8/endobrevin cycles through the apical but not through the basolateral plasma membrane. Furthermore, we found that VAMP8 localizes to apical endosomal membranes in nephric tubule epithelium and in MDCK cells. This asymmetry in localization and cycling behavior suggests that VAMP8/endobrevin may play a role in apical endosomal trafficking in polarized epithelium cells.

Phosphorylated syntaxin 1 is localized to discrete domains along a subset of axons

Syntaxin 1 is a SNARE protein that plays a central role in synaptic vesicle (SV) exocytosis. We generated an antibody that specifically recognizes a casein kinase II-mediated phosphorylation on serine-14 of syntaxin 1. In this report we show that this phosphorylation occurs in vivo and is developmentally regulated in the rat brain, rising to a level of 40% of the total syntaxin in adult animals. Phosphorylated syntaxin is preferentially associated with SNAP-25 and localizes to discrete domains of the axonal plasma membrane that do not colocalize with pools of synaptic vesicles. These phosphosyntaxin domains may define fusion sites for a novel class of vesicles outside classical active zones.

A conserved membrane-spanning amino acid motif drives homomeric and supports heteromeric assembly of presynaptic SNARE proteins

Assembly of the SNARE proteins synaptobrevin/VAMP, syntaxin, and SNAP-25 to binary and ternary complexes is important for docking and/or fusion of presynaptic vesicles to the neuronal plasma membrane prior to regulated neurotransmitter release. Despite the well characterized structure of their cytoplasmic assembly domains, little is known about the role of the transmembrane segments in SNARE protein assembly and function. Here, we identified conserved amino acid motifs within the transmembrane segments that are required for homodimerization of synaptobrevin II and syntaxin 1A. Minimal motifs of 6-8 residues grafted onto an otherwise monomeric oligoalanine host sequence were sufficient for self-interaction of both transmembrane segments in detergent solution or membranes. These motifs constitute contiguous areas of interfacial residues assuming alpha-helical secondary structures. Since the motifs are conserved, they also contributed to heterodimerization of synaptobrevin II and syntaxin 1A and therefore appear to constitute interaction domains independent of the cytoplasmic coiled coil regions. Interactions between the transmembrane segments may stabilize the SNARE complex, cause its multimerization to previously observed multimeric superstructures, and/or be required for the fusogenic activity of SNARE proteins.

The secretory pathway from history to the state of the art

Maintenance of the structural and functional organization of a eucaryotic cell requires the correct targeting of proteins and lipids to their destinations. This is achieved by the delivery of newly synthesized material along the secretory pathway on one hand and by the retrieval of membranes on the other hand. Various models have been suggested over the years to explain traffic flow within the secretory pathway. The only two models that are under discussion to date are the "vesicular model" and the "cisternal maturation model". A wealth of information from various experimental approaches, strongly supports the vesicular model as the general mode of intracellular transport. Three major types of protein-coated transport vesicles are characterized in molecular detail, and have been attributed to various steps of the secretory pathway: COPII-coated vesicles allow exit from the endoplasmic reticulum (ER), COPI-coated vesicles carry proteins within the early secretory pathway, i.e. between ER and Golgi apparatus, and clathrin-coated vesicles mediate transport from the trans-Golgi network (TGN). In this review we will give an overview of the route of a protein along the secretory pathway and summarize the progress that was made within the last decades in the characterization of distinct intracellular transport steps. We will discuss the current models for the formation and fusion of vesicular carriers with a major focus on the mechanism underlying budding of a COPI-coated vesicle.

Structural insights into the molecular mechanism of Ca(2+)-dependent exocytosis

The fusion of vesicles with target membranes is controlled by a complex network of protein-protein and protein-lipid interactions. Recent structures of the SNARE complex, synaptotagmin III, nSec1, domains of NSF and its adaptor SNAP, along with Rab3 and some of its effectors, provide the framework for developing molecular models of vesicle fusion and for designing experiments to test these models. Ultimately, this knowledge of the structures of higher-order complexes and their dynamic behavior will allow us to obtain a full understanding of the vesicle fusion protein machinery.

SNAREpins are functionally resistant to disruption by NSF and alphaSNAP

SNARE (SNAP [soluble NSF (N-ethylmaleimide-sensitive fusion protein) attachment protein] receptor) proteins are required for many fusion processes, and recent studies of isolated SNARE proteins reveal that they are inherently capable of fusing lipid bilayers. Cis-SNARE complexes (formed when vesicle SNAREs [v-SNAREs] and target membrane SNAREs [t-SNAREs] combine in the same membrane) are disrupted by the action of the abundant cytoplasmic ATPase NSF, which is necessary to maintain a supply of uncombined v- and t-SNAREs for fusion in cells. Fusion is mediated by these same SNARE proteins, forming trans-SNARE complexes between membranes. This raises an important question: why doesn't NSF disrupt these SNARE complexes as well, preventing fusion from occurring at all? Here, we report several lines of evidence that demonstrate that SNAREpins (trans-SNARE complexes) are in fact functionally resistant to NSF, and they become so at the moment they form and commit to fusion. This elegant design allows fusion to proceed locally in the face of an overall environment that massively favors SNARE disruption.

SNAREs contribute to the specificity of membrane fusion

Intracellular membrane fusion is mediated by the formation of a four-helix bundle comprised of SNARE proteins. Every cell expresses a large number of SNARE proteins that are localized to particular membrane compartments, suggesting that the fidelity of vesicle trafficking might in part be determined by specific SNARE pairing. However, the promiscuity of SNARE pairing in vitro suggests that the information for membrane compartment organization is not encoded in the inherent ability of SNAREs to form complexes. Here, we show that exocytosis of norepinephrine from PC12 cells is only inhibited or rescued by specific SNAREs. The data suggest that SNARE pairing does underlie vesicle trafficking fidelity, and that specific SNARE interactions with other proteins may facilitate the correct pairing.

Nerve growth factor-induced phosphorylation of SNAP-25 in PC12 cells: a possible involvement in the regulation of SNAP-25 localization

Synaptosomal-associated protein of 25 kDa (SNAP-25), a t-SNARE protein essential for neurotransmitter release, is phosphorylated at Ser187 following activation of cellular protein kinase C by treatment with phorbol 12-myristate 13-acetate. However, it remains unclear whether neuronal activity or an endogenous ligand induces the phosphorylation of SNAP-25. Here we studied the phosphorylation of SNAP-25 in PC12 cells using a specific antibody for SNAP-25 phosphorylated at Ser187. A small fraction of SNAP-25 was phosphorylated when cells were grown in the absence of nerve growth factor (NGF). A brief treatment with NGF that was enough to activate the mitogen-activated protein kinase signal transduction pathway did not increase the phosphorylation of SNAP-25; however, phosphorylation was up-regulated after a prolonged incubation with NGF. Up-regulation was transitory, and maximum phosphorylation (a fourfold increase over basal phosphorylation) was achieved between 36 and 48 h after the addition of NGF. Immunofluorescent microscopy showed that SNAP-25 was localized primarily in the plasma membrane, although a significant population was also present in the cytoplasm. Quantitative microfluorometry revealed that prolonged treatment with NGF resulted in a preferential localization of SNAP-25 in the plasma membrane. A mutational study using a fusion protein with green fluorescent protein as a tag indicated that the point mutation of Ser187 to Ala abolished the NGF-dependent relocalization. A population of SNAP-25 in the plasma membrane was not increased by a point mutation at Ser187 to Glu; however, it was increased by prolonged treatment with NGF, indicating that the SNAP-25 phosphorylation is essential, but not sufficient, for the NGF-induced relocation to the plasma membrane. Our results suggest a close temporal relationship between the up-regulation of SNAP-25 phosphorylation and its relocation, and NGF-induced differentiation of PC12 cells.

Measurement of exocytosis by amperometry in adrenal chromaffin cells: effects of clostridial neurotoxins and activation of protein kinase C on fusion pore kinetics

We have used carbon-fibre amperometry to examine the kinetics of individual secretory granule fusion/release events in bovine adrenal chromaffin cells. Transfection with plasmids encoding the light chains of botulinum neurotoxins (BoNTs) was used to investigate the effects of cleavage of syntaxin or SNAP-25 on exocytosis. Expression of BoNT/C1 or BoNT/E inhibited the extent of exocytosis that was evoked by application of digitonin/Ca(2+) to permeabilise and stimulate single chromaffin cells. Following neurotoxin expression, the residual release events were no different from those of control cells in their magnitude and kinetics from analysis of the amperometric spikes. In contrast, activation of protein kinase C (PKC) resulted in a modification of the kinetics of single granule release events. Following phorbol ester treatment, the amperometric spikes showed a significant decrease in their total charge due to a decrease in their mean half-width with increases in the rate of the initial rise and also the fall to baseline of the spikes. These changes were prevented by pre-treatment with the PKC inhibitor bisindolylmaleimide. These results suggest that PKC regulates the rate of fusion pore expansion and also subsequent pore closure or granule retrieval. A PKC-mediated regulation of kiss-and-run fusion may, therefore, control the extent of catecholamine release from single secretory granules. The experimental approach used here may provide further information on the protein constituents and regulation of the fusion pore machinery.

Asymmetric requirements for a Rab GTPase and SNARE proteins in fusion of COPII vesicles with acceptor membranes

Soluble NSF attachment protein receptor (SNARE) proteins are essential for membrane fusion in transport between the yeast ER and Golgi compartments. Subcellular fractionation experiments demonstrate that the ER/Golgi SNAREs Bos1p, Sec22p, Bet1p, Sed5p, and the Rab protein, Ypt1p, are distributed similarly but localize primarily with Golgi membranes. All of these SNARE proteins are efficiently packaged into COPII vesicles and suggest a dynamic cycling of SNARE machinery between ER and Golgi compartments. Ypt1p is not efficiently packaged into vesicles under these conditions. To determine in which membranes protein function is required, temperature-sensitive alleles of BOS1, BET1, SED5, SLY1, and YPT1 that prevent ER/Golgi transport in vitro at restrictive temperatures were used to selectively inactivate these gene products on vesicles or on Golgi membranes. Vesicles bearing mutations in Bet1p or Bos1p inhibit fusion with wild-type acceptor membranes, but acceptor membranes containing these mutations are fully functional. In contrast, vesicles bearing mutations in Sed5p, Sly1p, or Ypt1p are functional, whereas acceptor membranes containing these mutations block fusion. Thus, this set of SNARE proteins is symmetrically distributed between vesicle and acceptor compartments, but they function asymmetrically such that Bet1p and Bos1p are required on vesicles and Sed5p activity is required on acceptor membranes. We propose the asymmetry in SNARE protein function is maintained by an asymmetric distribution and requirement for the Ypt1p GTPase in this fusion event. When a transmembrane-anchored form of Ypt1p is used to restrict this GTPase to the acceptor compartment, vesicles depleted of Ypt1p remain competent for fusion.

Exocytotic mechanism studied by truncated and zero layer mutants of the C-terminus of SNAP-25

The highly conserved SNARE proteins, SNAP-25, syntaxin and synaptobrevin, form a tight ternary complex, which is essential for exocytosis. Crystallization of this complex revealed a four-helix bundle with an unusual hydrophilic layer (zero layer) in its center. In order to evaluate the role of this layer in different kinetic components of secretion, we used the Semliki Forest virus (SFV) system to infect adrenal chromaffin cells with SNAP-25 Q174L, a point mutant in the zero layer. Using combined flash photolysis of caged calcium and membrane capacitance measurements, we investigated its effect on the exocytotic burst and sustained phase of exocytosis with high time resolution. Cells expressing SNAP-25 Q174L displayed a selective reduction in the sustained phase, while the two components of the exocytotic burst remained unaffected. Furthermore, the exocytotic response to the second flash was significantly reduced, indicating a decrease in refilling kinetics. We therefore conclude that the zero layer is critical for the formation of SNARE complexes, but that it plays no role in the dynamic equilibrium between the two exocytosis-competent vesicle pools.

The C terminus of SNAP25 is essential for Ca(2+)-dependent binding of synaptotagmin to SNARE complexes

The plasma membrane soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins syntaxin and synaptosome-associated protein of 25 kDa (SNAP25) and the vesicle SNARE protein vesicle-associated membrane protein (VAMP) are essential for a late Ca(2+)-dependent step in regulated exocytosis, but their precise roles and regulation by Ca(2+) are poorly understood. Botulinum neurotoxin (BoNT) E, a protease that cleaves SNAP25 at Arg(180)-Ile(181), completely inhibits this late step in PC12 cell membranes, whereas BoNT A, which cleaves SNAP25 at Gln(197)-Arg(198), is only partially inhibitory. The difference in toxin effectiveness was found to result from a reversal of BoNT A but not BoNT E inhibition by elevated Ca(2+) concentrations. BoNT A treatment essentially increased the Ca(2+) concentration required to activate exocytosis, which suggested a role for the C terminus of SNAP25 in the Ca(2+) regulation of exocytosis. Synaptotagmin, a proposed Ca(2+) sensor for exocytosis, was found to bind SNAP25 in a Ca(2+)-stimulated manner. Ca(2+)-dependent binding was abolished by BoNT E treatment, whereas BoNT A treatment increased the Ca(2+) concentration required for binding. The C terminus of SNAP25 was also essential for Ca(2+)-dependent synaptotagmin binding to SNAP25. syntaxin and SNAP25.syntaxin.VAMP SNARE complexes. These results clarify classical observations on the Ca(2+) reversal of BoNT A inhibition of neurosecretion, and they suggest that an essential role for the C terminus of SNAP25 in regulated exocytosis is to mediate Ca(2+)-dependent interactions between synaptotagmin and SNARE protein complexes.

Paired-pulse modulation at individual GABAergic synapses in rat hippocampus

1. Unitary inhibitory postsynaptic currents (uIPSCs) were recorded in synaptically coupled pairs of CA1 hippocampal interneurons and pyramidal neurons in rat brain slices by using dual patch-clamp techniques. Paired-pulse modulation of uIPSCs at individual GABAergic synapses was tested. 2. GABAergic synapses could be divided into two subgroups, high and low failure, depending on their failure rate. 3. The external Ca2+ levels modulate the failure rate of uIPSCs. In 0.51 mM Ca2+, low-failure pairs had a high-failure characteristic, whereas high-failure pairs had a low-failure characteristic in 8 mM Ca2+. The results suggest that uIPSC failures result from the Ca2+-dependent release mechanism rather than axon propagation failures. 4. Paired-pulse facilitation (PPF) occurred in high-failure pairs when the interspike interval was 20 ms. Paired-pulse depression (PPD) was not predominant in high-failure pairs. 5. Potency of uIPSCs, the average amplitude of non-failure events, was enhanced by PPF, suggesting that multiple synapses connect each pair. Differing numbers of activated synapses contributed to the variable amplitude of uIPSCs from a given pair. 6. PPD occurred in low-failure pairs at the tested range of interspike intervals (20-200 ms). The uIPSC2 after a large uIPSC1 was smaller than the uIPSC2 after a small uIPSC1, suggesting that PPD is use dependent and due to a decrease in the quantal content (m) after the first release. 7. In 8 mM Ca2+, PPD occurred in high-failure pairs and was larger in low-failure pairs, suggesting that the occurrence of PPF or PPD depends on the baseline release probability. 8. The GABAB receptor antagonist CGP 55845A (5 microM) decreased the baseline release probability of inhibitory synapses and attenuated PPD indirectly, rather than by blocking presynaptic GABAB autoreceptors.

Comparison of cysteine string protein (Csp) and mutant alpha-SNAP overexpression reveals a role for csp in late steps of membrane fusion in dense-core granule exocytosis in adrenal chromaffin cells

Assembly of the SNARE complex and its disassembly caused by the action of soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein (SNAP) and NSF is crucial for the maintenance of vesicular traffic, including fusion of regulated exocytotic vesicles. Various other proteins may also have important roles in the processes leading to membrane fusion via interaction with the SNARE proteins, including the secretory vesicle cysteine string protein (Csp). Here we have examined the effect of overexpression of a dominant negative alpha-SNAP mutant or Csp on exocytosis of dense-core granules in single chromaffin cells monitored using amperometry to detect released catecholamine. Exocytosis of trans-Golgi network (TGN)-derived dense-core granules was substantially inhibited by expression of alpha-SNAP(L294A). The amplitude and characteristics of the individual release events were unaffected by expression of alpha-SNAP(L294A), consistent with an essential role for alpha-SNAP in early steps of priming but not in the fusion process. In contrast, Csp overexpression, which also inhibited the extent of exocytosis, also modified the kinetics of the individual release events seen as an increase in the rise time and a broadening of the residual amperometric spikes in Csp-transfected cells. These results suggest that unlike alpha-SNAP, Csp plays a key role in the protein interactions close to the fusion process or fusion pore opening during Ca(2+)-regulated exocytosis.

SNIP, a novel SNAP-25-interacting protein implicated in regulated exocytosis

Synaptosome-associated protein of 25 kDa (SNAP-25) is a presynaptic membrane protein that has been clearly implicated in membrane fusion in both developing and mature neurons, although its mechanisms of action are unclear. We have now identified a novel SNAP-25-interacting protein named SNIP. SNIP is a hydrophilic, 145-kDa protein that comprises two predicted coiled-coil domains, two highly charged regions, and two proline-rich domains with multiple PPXY and PXXP motifs. SNIP is selectively expressed in brain where it co-distributes with SNAP-25 in most brain regions. Biochemical studies have revealed that SNIP is tightly associated with the brain cytoskeleton. Subcellular fractionation and immunofluorescence localization studies have demonstrated that SNIP co-localizes with SNAP-25 as well as the cortical actin cytoskeleton, suggesting that SNIP serves as a linker protein connecting SNAP-25 to the submembranous cytoskeleton. By using deletion analysis, we have mapped the binding domains of SNIP and SNAP-25, and we have demonstrated that the SNIP-SNAP-25 association is mediated via coiled-coil interactions. Moreover, we have shown that overexpression of SNIP or its SNAP-25-interacting domain inhibits Ca(2+)-dependent exocytosis from PC12 cells. These results indicate that SNIP is involved in regulation of neurosecretion, perhaps via its interaction with SNAP-25 and the cytoskeleton.

Secretory protein trafficking and organelle dynamics in living cells

Green fluorescent protein chimerae acting as reporters for protein localization and trafficking within the secretory membrane system of living cells have been used in a wide variety of applications, including time-lapse imaging, double-labeling, energy transfer, quantitation, and photobleaching experiments. Results from this work are clarifying the steps involved in the formation, translocation, and fusion of transport intermediates; the organization and biogenesis of organelles; and the mechanisms of protein retention, sorting, and recycling in the secretory pathway. In so doing, they are broadening our thinking about the temporal and spatial relationships among secretory organelles and the membrane trafficking pathways that operate between them.

Inhibition of SNARE complex assembly differentially affects kinetic components of exocytosis

In chromaffin cells, an increase in intracellular Ca2+ leads to an exocytotic burst followed by sustained secretion. The burst can be further resolved into two kinetically distinct components, which suggests the presence of two separate pools of vesicles. To investigate how these components relate to SNARE complex formation, we introduced an antibody that blocks SNARE assembly but not disassembly. In the presence of the antibody, the sustained component was largely blocked, the burst was slightly reduced, and one of its kinetic components was eliminated. We conclude that SNARE complexes form before Ca(2+)-triggered membrane fusion and exist in a dynamic equilibrium between a loose and a tight state, both of which support exocytosis. Interaction of the antibody with preformed SNARE complexes favors the loose state.

Dynamics of tubulovesicular recycling endosomes in hippocampal neurons

Neurons are polarized cells, the activity of which relies on the morphological and functional differences between their axonal and somatodendritic domains. One mechanism for establishing and maintaining neuronal polarity is via the selective targeting of proteins to these domains. The endocytic pathway plays a major role in the generation and maintenance of cellular polarity by selectively sorting and recycling endocytosed plasma membrane proteins. In this study we first show that endogenous syntaxin 13 localizes to tubulovesicular organelles that are present in the somatodendritic and axonal domains of neurons. These organelles contain and actively recycle transferrin receptor and are sensitive to brefeldin A, suggesting that they are analogous to the tubulovesicular recycling endosomes in non-neuronal cells. We next use a syntaxin 13-GFP fusion protein transiently expressed in hippocampal neurons, together with time-lapse microscopy, to study the dynamics of the endosomal system in neurons. The analysis revealed the presence of two distinct classes of syntaxin 13-labeled endosomes: round-oval stationary organelles and highly mobile tubulovesicular structures. The dynamic population of tubulovesicular endosomes travels in both directions along microtubules in dendrites and axons. The mobile organelles appear to fuse with and bud from the stationary endosomes, possibly as a means of delivering and picking up their cargo.

Botulinum neurotoxin A blocks synaptic vesicle exocytosis but not endocytosis at the nerve terminal

The supply of synaptic vesicles in the nerve terminal is maintained by a temporally linked balance of exo- and endocytosis. Tetanus and botulinum neurotoxins block neurotransmitter release by the enzymatic cleavage of proteins identified as critical for synaptic vesicle exocytosis. We show here that botulinum neurotoxin A is unique in that the toxin-induced block in exocytosis does not arrest vesicle membrane endocytosis. In the murine spinal cord, cell cultures exposed to botulinum neurotoxin A, neither K(+)-evoked neurotransmitter release nor synaptic currents can be detected, twice the ordinary number of synaptic vesicles are docked at the synaptic active zone, and its protein substrate is cleaved, which is similar to observations with tetanus and other botulinal neurotoxins. In marked contrast, K(+) depolarization, in the presence of Ca(2+), triggers the endocytosis of the vesicle membrane in botulinum neurotoxin A-blocked cultures as evidenced by FM1-43 staining of synaptic terminals and uptake of HRP into synaptic vesicles. These experiments are the first demonstration that botulinum neurotoxin A uncouples vesicle exo- from endocytosis, and provide evidence that Ca(2+) is required for synaptic vesicle membrane retrieval.

Differential roles of syntaxin 7 and syntaxin 8 in endosomal trafficking

To understand molecular mechanisms that regulate the intricate and dynamic organization of the endosomal compartment, it is important to establish the morphology, molecular composition, and functions of the different organelles involved in endosomal trafficking. Syntaxins and vesicle-associated membrane protein (VAMP) families, also known as soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein receptors (SNAREs), have been implicated in mediating membrane fusion and may play a role in determining the specificity of vesicular trafficking. Although several SNAREs, including VAMP3/cellubrevin, VAMP8/endobrevin, syntaxin 13, and syntaxin 7, have been localized to the endosomal membranes, their precise localization, biochemical interactions, and function remain unclear. Furthermore, little is known about SNAREs involved in lysosomal trafficking. So far, only one SNARE, VAMP7, has been localized to late endosomes (LEs), where it is proposed to mediate trafficking of epidermal growth factor receptor to LEs and lysosomes. Here we characterize the localization and function of two additional endosomal syntaxins, syntaxins 7 and 8, and propose that they mediate distinct steps of endosomal protein trafficking. Both syntaxins are found in SNARE complexes that are dissociated by alpha-soluble NSF attachment protein and NSF. Syntaxin 7 is mainly localized to vacuolar early endosomes (EEs) and may be involved in protein trafficking from the plasma membrane to the EE as well as in homotypic fusion of endocytic organelles. In contrast, syntaxin 8 is likely to function in clathrin-independent vesicular transport and membrane fusion events necessary for protein transport from EEs to LEs.

Crystal structure of the cytosolic C2A-C2B domains of synaptotagmin III. Implications for Ca(+2)-independent snare complex interaction

Synaptotagmins are synaptic vesicle-associated, phospholipid-binding proteins most commonly associated with Ca(+2)-dependent exocytotic and Ca(+2)- independent endocytotic events. Synaptotagmin III is a 63.2-kD member of the synaptotagmin homology group; one of its characteristic properties is the ability to bind divalent cations and accessory proteins promiscuously. In the cytosolic portion of this protein, a flexible seven-amino acid linker joins two homologous C2 domains. The C2A domain binds to phospholipid membranes and other accessory proteins in a divalent cation-dependent fashion. The C2B domain promotes binding to other C2B domains, as well as accessory proteins independent of divalent cations. The 3.2 A crystal structure of synaptotagmin III, residues 295-566, which includes the C2A and C2B domains, exhibits differences in the shape of the Ca(+2)-binding pocket, the electrostatic surface potential, and the stoichiometry of bound divalent cations for the two domains. These observations may explain the disparate binding properties of the two domains. The C2A and the C2B domains do not interact; synaptotagmin, therefore, covalently links two independent C2 domains, each with potentially different binding partners. A model of synaptotagmin's involvement in Ca(+2)-dependent regulation of membrane fusion through its interaction with the SNARE complex is presented.

Content mixing and membrane integrity during membrane fusion driven by pairing of isolated v-SNAREs and t-SNAREs

Membrane bilayer fusion has been shown to be mediated by v- and t-SNAREs initially present in separate populations of liposomes and to occur with high efficiency at a physiologically meaningful rate. Lipid mixing was demonstrated to involve both the inner and the outer leaflets of the membrane bilayer. Here, we use a fusion assay that relies on duplex formation of oligonucleotides introduced in separate liposome populations and report that SNARE proteins suffice to mediate complete membrane fusion accompanied by mixing of luminal content. We also find that SNARE-mediated membrane fusion does not compromise the integrity of liposomes.

Rapid and efficient fusion of phospholipid vesicles by the alpha-helical core of a SNARE complex in the absence of an N-terminal regulatory domain

A protease-resistant core domain of the neuronal SNARE complex consists of an alpha-helical bundle similar to the proposed fusogenic core of viral fusion proteins [Skehel, J. J. & Wiley, D. C. (1998) Cell 95, 871-874]. We find that the isolated core of a SNARE complex efficiently fuses artificial bilayers and does so faster than full length SNAREs. Unexpectedly, a dramatic increase in speed results from removal of the N-terminal domain of the t-SNARE syntaxin, which does not affect the rate of assembly of v-t SNARES. In the absence of this negative regulatory domain, the half-time for fusion of an entire population of lipid vesicles by isolated SNARE cores ( approximately 10 min) is compatible with the kinetics of fusion in many cell types.