Early requirement for alpha-SNAP and NSF in the secretory cascade in chromaffin cells.

NSF and alpha-SNAP have been shown to be required for SNARE complex disassembly and exocytosis. However, the exact requirement for NSF and alpha-SNAP in vesicular traffic through the secretory pathway remains controversial. We performed a study on the kinetics of exocytosis from bovine chromaffin cells using high time resolution capacitance measurement and electrochemical amperometry, combined with flash photolysis of caged Ca2+ as a fast stimulus. alpha-SNAP, a C-terminal mutant of alpha-SNAP, and NEM were assayed for their effects on secretion kinetics. Two kinetically distinct components of catecholamine release can be observed upon fast step-like elevation of [Ca2+]i. One is the exocytotic burst, thought to represent the readily releasable pool of vesicles. Following the exocytotic burst, secretion proceeds slowly at maintained high [Ca2+]i, which may represent vesicle maturation/recruitment, i.e. some priming steps after docking. alpha-SNAP increased the amplitude of both the exocytotic burst and the slow component but did not change their kinetics, which we examined with millisecond time resolution. In addition, NEM only partially inhibited the slow component without altering the exocytotic burst, fusion kinetics and the rate of endocytosis. These results suggest a role for alpha-SNAP/NSF in priming granules for release at an early step, but not modifying the fusion of readily releasable granules.     

A novel site of action for alpha-SNAP in the SNARE conformational cycle controlling membrane fusion

Regulated exocytosis in neurons and neuroendocrine cells requires the formation of a stable soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex consisting of synaptobrevin-2/vesicle-associated membrane protein 2, synaptosome-associated protein of 25 kDa (SNAP-25), and syntaxin 1. This complex is subsequently disassembled by the concerted action of alpha-SNAP and the ATPases associated with different cellular activities-ATPase N-ethylmaleimide-sensitive factor (NSF). We report that NSF inhibition causes accumulation of alpha-SNAP in clusters on plasma membranes. Clustering is mediated by the binding of alpha-SNAP to uncomplexed syntaxin, because cleavage of syntaxin with botulinum neurotoxin C1 or competition by using antibodies against syntaxin SNARE motif abolishes clustering. Binding of alpha-SNAP potently inhibits Ca(2+)-dependent exocytosis of secretory granules and SNARE-mediated liposome fusion. Membrane clustering and inhibition of both exocytosis and liposome fusion are counteracted by NSF but not when an alpha-SNAP mutant defective in NSF activation is used. We conclude that alpha-SNAP inhibits exocytosis by binding to the syntaxin SNARE motif and in turn prevents SNARE assembly, revealing an unexpected site of action for alpha-SNAP in the SNARE cycle that drives exocytotic membrane fusion.     

Defining the SNARE complex binding surface of alpha-SNAP: implications for SNARE complex disassembly

N-Ethylmaleimide-sensitive factor (NSF) and its adaptor protein alpha-soluble NSF attachment protein (alpha-SNAP) sustain membrane trafficking by disassembling soluble NSF attachment protein receptor (SNARE) complexes that form during membrane fusion. To better understand the role of alpha-SNAP in this process, we used site-directed mutagenesis to identify residues in alpha-SNAP that interact with SNARE complexes. We find that mutations in charged residues distributed over a concave surface formed by the N-terminal nine alpha-helices of alpha-SNAP affect its ability to bind synaptic SNARE complex and promote its disassembly by NSF. Replacing basic residues on this surface with alanines reduced SNARE complex binding and disassembly, whereas replacing acidic residues with alanines enhanced alpha-SNAP efficacy in both assays. These findings show that the ability of NSF to take apart SNARE complexes depends upon electrostatic interactions between alpha-SNAP and the acidic surface of the SNARE complex and provide insight into how NSF and alpha-SNAP work together to drive disassembly.     

Assembly of SNARE core complexes prior to neurotransmitter release sets the readily releasable pool of synaptic vesicles

Precise regulation of neurotransmitter release is essential for the normal function of neural networks, but the mechanisms involved are largely unclear. Using superfused synaptosomes, we have studied the readily releasable pool of synaptic vesicles, measured as the amount of release triggered by hypertonic sucrose. We show that activation of presynaptic metabotropic glutamate receptors by dihydroxyphenylglycine and stimulation of protein kinase C by phorbol esters enhance the readily releasable pool of glutamate. Although the molecular nature of the readily releasable pool is unknown, one possibility is that during its generation, SNARE proteins form full core complexes, and that core complex formation occurs prior to neurotransmitter release. To test this possibility, we employed N-ethylmaleimide (NEM), an inhibitor of the ATPase N-ethylmaleimide-sensitive factor that dissociates core complexes, to study the relation of the readily releasable pool to core complex assembly in synaptosomes. NEM induced a dose-dependent increase in the readily releasable pool of neurotransmitters but by itself did not trigger release. Direct measurements of core complexes confirmed that NEM caused an increase in the levels of SNARE core complexes under these conditions. Our data suggest that in the readily releasable pool of synaptic vesicles, SNARE proteins are fully assembled into core complexes, and that SNARE complex assembly is a target of presynaptic regulation.     

Complexin II regulates degranulation in RBL-2H3 cells by interacting with SNARE complex containing syntaxin-3

Recent studies have revealed that SNARE proteins are involved in the exocytotic release (degranulation) in mast cells. However, the roles of SNARE regulatory proteins are poorly understood. Complexin is one such regulatory protein and it plays a crucial role in exocytotic release. In this study, we characterized the interaction between SNARE complex and complexin II in mast cells by GST pull-down assay and in vitro binding assay. We found that the SNARE complex that interacted with complexin II consisted of syntaxin-3, SNAP-23, and VAMP-2 or -8, whereas syntaxin-4 was not detected. Recombinant syntaxin-3 binds to complexin II by itself, but its affinity to complexin II was enhanced upon addition of VAMP-8 and SNAP-23. Furthermore, the region of complexin II responsible for binding to the SNARE complex, was near the central α-helix region. These results suggest that complexin II regulates degranulation by interacting with the SNARE complex containing syntaxin-3.     

SNARE proteins mediate fusion between cytosolic lipid droplets and are implicated in insulin sensitivity

The accumulation of cytosolic lipid droplets in muscle and liver cells has been linked to the development of insulin resistance and type 2 diabetes. Such droplets are formed as small structures that increase in size through fusion, a process that is dependent on intact microtubules and the motor protein dynein. Approximately 15% of all droplets are involved in fusion processes at a given time. Here, we show that lipid droplets are associated with proteins involved in fusion processes in the cell: NSF (N-ethylmaleimide-sensitive-factor), alpha-SNAP (soluble NSF attachment protein) and the SNAREs (SNAP receptors), SNAP23 (synaptosomal-associated protein of 23 kDa), syntaxin-5 and VAMP4 (vesicle-associated membrane protein 4). Knockdown of the genes for SNAP23, syntaxin-5 or VAMP4, or microinjection of a dominant-negative mutant of alpha-SNAP, decreases the rate of fusion and the size of the lipid droplets. Thus, the SNARE system seems to have an important role in lipid droplet fusion. We also show that oleic acid treatment decreases the insulin sensitivity of heart muscle cells, and this sensitivity is completely restored by transfection with SNAP23. Thus, SNAP23 might be a link between insulin sensitivity and the inflow of fatty acids to the cell.     

v-SNAREs control exocytosis of vesicles from priming to fusion

SNARE proteins (soluble NSF-attachment protein receptors) are thought to be central components of the exocytotic mechanism in neurosecretory cells, but their precise function remained unclear. Here, we show that each of the vesicle-associated SNARE proteins (v-SNARE) of a chromaffin granule, synaptobrevin II or cellubrevin, is sufficient to support Ca(2+)-dependent exocytosis and to establish a pool of primed, readily releasable vesicles. In the absence of both proteins, secretion is abolished, without affecting biogenesis or docking of granules indicating that v-SNAREs are absolutely required for granule exocytosis. We find that synaptobrevin II and cellubrevin differentially control the pool of readily releasable vesicles and show that the v-SNARE's amino terminus regulates the vesicle's primed state. We demonstrate that dynamics of fusion pore dilation are regulated by v-SNAREs, indicating their action throughout exocytosis from priming to fusion of vesicles.     

Complexin is able to bind to SNARE core complexes in different assembled states with distinct affinity

The formation of the functional SNARE complex in vivo is central to the fast neurotransmitter release at the neuronal terminal. Numerous studies revealed that this process involves progressive assembly of an α-helical bundle and is dynamically reversible. So far many proteins directly or indirectly take part in this process. Complexin, one of such factors, has revealed rapid association with the SNARE complex, however, whether or not complexin can interact with partially assembled SNARE complex is critical and yet unknown. Here, we present evidence that complexin is able to bind to various mutant versions of the SNARE complex mimicking its quaternary structure at different assembly stages. In addition, the affinity of complexin for the SNARE complex is correlated with the extent to which the SNARE complex is assembled. These results suggest that complexin is able to bind to SNARE complex before its complete formation.     

The positively charged residues in the fragment 71 - 77 of complexin is required for its binding to SNARE complex

Complexin is a cytoplasmic protein that plays an important role in the neurotransmitters release triggered by action potential. Previous studies suggested that complexin performs its functions through interaction with the SNARE complex. The crystal structure of complexin/SNARE complex revealed that complexin binds to SNARE core complex in an anti-parallel conformation with its residues 48 - 70. However, the functions of the flanking sequences are unclear. In this paper, we demonstrate that the fragment 71 - 77 of complexin is indispensable for its binding to the SNARE complex. Moreover, this interaction can be impaired by abolishing the positive charges in the fragment 71 - 77, which suggests that the positive charges in the fragment 71 - 77 are important for the interaction between complexin II and the SNARE complex.     

Synaptic vesicle mobility and presynaptic F-actin are disrupted in a N-ethylmaleimide-sensitive factor allele of Drosophila

N-ethylmaleimide sensitive factor (NSF) can dissociate the soluble NSF attachment receptor (SNARE) complex, but NSF also participates in other intracellular trafficking functions by virtue of SNARE-independent activity. Drosophila that express a neural transgene encoding a dominant-negative form of NSF2 show an 80% reduction in the size of releasable synaptic vesicle pool, but no change in the number of vesicles in nerve terminal boutons. Here we tested the hypothesis that vesicles in the NSF2 mutant terminal are less mobile. Using a combination of genetics, pharmacology, and imaging we find a substantial reduction in vesicle mobility within the nerve terminal boutons of Drosophila NSF2 mutant larvae. Subsequent analysis revealed a decrease of filamentous actin in both NSF2 dominant-negative and loss-of-function mutants. Lastly, actin-filament disrupting drugs also decrease vesicle movement. We conclude that a factor contributing to the NSF mutant phenotype is a reduction in vesicle mobility, which is associated with decreased presynaptic F-actin. Our data are consistent with a model in which actin filaments promote vesicle mobility and suggest that NSF participates in establishing or maintaining this population of actin.     

Augmentation is a potentiation of the exocytotic process

Short-term synaptic enhancement is caused by an increase in the probability with which synaptic terminals release transmitter in response to presynaptic action potentials. Since exocytosed vesicles are drawn from a readily releasable pool of packaged transmitter, enhancement must result either from an increase in the size of the pool or an elevation in the fraction of releasable vesicles that undergoes exocytosis with each action potential. We show here that at least one major component of enhancement, augmentation, is not caused by an increase in the size of the readily releasable pool but is instead associated with an increase in the efficiency with which action potentials induce the exocytosis of readily releasable vesicles.     

Imaging the assembly and disassembly kinetics of cis-SNARE complexes on native plasma membranes

Mild sonication of eukaryotic cells produces native plasma membrane sheets that retain their docked organelles, cytoskeleton structures and cytoplasmic complexes. While the delicate organization of membranous protein complexes remains undisturbed, their inner plasmalemmel leaflet can be rapidly exposed to bathing solutions, enabling specific biochemical manipulations. Here, we apply this system to track membrane-biochemistry kinetics. We monitor soluble NSF-attachment protein receptor (SNARE) complex assembly and disassembly on the plasma membrane at high time resolution. The results suggest two-phase kinetics for the assembly process and dependence of the disassembly kinetics on both N-ethyl maleimide-sensitive factor (NSF) and soluble NSF-attachment protein (alpha-SNAP) concentrations.     

Crystal structure of the amino-terminal domain of N-ethylmaleimide-sensitive fusion protein

The cytosolic ATPase N-ethylmaleimide-sensitive fusion protein (NSF) disassembles complexes of membrane-bound proteins known as SNAREs, an activity essential for vesicular trafficking. The amino-terminal domain of NSF (NSF-N) is required for the interaction of NSF with the SNARE complex through the adaptor protein alpha-SNAP. The crystal structure of NSF-N reveals two subdomains linked by a single stretch of polypeptide. A polar interface between the two subdomains indicates that they can move with respect to one another during the catalytic cycle of NSF. Structure-based sequence alignments indicate that in addition to NSF orthologues, the p97 family of ATPases contain an amino-terminal domain of similar structure.     

Dominant-negative NSF2 disrupts the structure and function of Drosophila neuromuscular synapses

N-ethylmaleimide sensitive fusion protein (NSF) is an ATPase necessary for vesicle trafficking, including exocytosis. Current models hold that NSF is required in a step that readies vesicles for fusion by disassembling postfusion SNARE protein complexes allowing them to participate in further rounds of vesicle cycling. Whereas most organisms have only one NSF isoform, Drosophila has two. dNSF1 is the predominant functional isoform in the adult nervous system. Conditional mutations in the dNSF1 gene, comatose, are paralytic and lead to disruption of synaptic transmission and the rapid accumulation of SNARE complexes in adult flies. This isoform is not required for synaptic transmission in larvae. In contrast, dNSF2 is important at earlier developmental stages, and its broad expression indicates its importance in neural and non-neural tissues alike. To study dNSF2, and to circumvent the lethality of dNSF2 null mutants, we have constructed transgenic flies carrying a dominant negative form of dNSF2. When this construct was expressed in neurons we observed suppression of synaptic transmission, activity-dependent fatigue of transmitter release, and a reduction in the number of releasable vesicles. However, we unexpectedly found that there was no accumulation of SNARE complexes accompanying these physiological phenotypes. Intriguingly, we also found that expression of mutant dNSF2 induced pronounced overgrowth of the neuromuscular junction and some misrouting of axons. These results support the idea that dNSF2 has multiple roles in cellular function and adds that not all of its functions require disassembly of the SNARE complex.     

Domains of alpha-SNAP required for the stimulation of exocytosis and for N-ethylmalemide-sensitive fusion protein (NSF) binding and activation.

The binding of alpha-SNAP to the membrane proteins syntaxin, SNAP-25, and synaptobrevin leads to the recruitment of the N-ethylmaleimide-sensitive fusion protein (NSF). ATP hydrolysis by NSF has been suggested to drive conformational changes in one or more of these membrane proteins that are essential for regulated exocytosis. Functional evidence for a role of alpha-SNAP in exocytosis in adrenal chromaffin cells comes from the ability of this protein to stimulate Ca(2+)-dependent exocytosis in digitonin-permeabilized cells. Here we examine the effect of a series of deletion mutants of alpha-SNAP on exocytosis, and on the ability of alpha-SNAP to interact with NSF, to define essential domains involved in protein-protein interactions in exocytosis. Deletion of extreme N- or C-terminal regions of alpha-SNAP produced proteins unable to bind to syntaxin or to stimulate exocytosis, suggesting that these domains participate in essential interactions. Deletion of C-terminal residues abolished the ability of alpha-SNAP to bind NSF. In contrast, deletion of up to 120 N-terminal residues did not prevent the binding of NSF to immobilized alpha-SNAP and such mutants were also able to stimulate the ATPase activity of NSF. These results suggest that the C-terminus, but not the N-terminus, of alpha-SNAP is crucial for interactions with NSF. The involvement of the C-terminus of alpha-SNAP, which contains a predicted coiled-coil domain, in the binding of both syntaxin and NSF would place the latter two proteins in proximity in a ternary complex whereupon the energy derived from ATP hydrolysis by NSF could induce a conformational change in syntaxin required for exocytosis to proceed.     

Complexin regulates the closure of the fusion pore during regulated vesicle exocytosis

Membrane fusion during exocytosis and throughout the cell is believed to involve members of the SNARE (soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptors) family of proteins. The assembly of these proteins into a four-helix bundle may be part of the driving force for bilayer fusion. Regulated exocytosis in neurons and related cell types is specialized to be fast and Ca(2+)-dependent suggesting the involvement of other regulatory proteins specific for regulated exocytosis. Among these are the complexins, two closely related proteins that bind only to the assembled SNARE complex. We have investigated the function of complexin by analysis of single vesicle release events in adrenal chromaffin cells using carbon fiber amperometry. These cells express complexin II, and overexpression of this protein modified the kinetics of vesicle release events so that their time course was shortened. This effect depended on complexin interaction with the SNARE complex as introduction of a mutation of Arg-59, a residue that interacts with synaptobrevin in the SNARE complex, abolished its effects. The data are consistent with a function for complexin in stabilizing an intermediate of the SNARE complex to allow kiss-and-run recycling of the exocytosed vesicle.     

Involvement of BNIP1 in apoptosis and endoplasmic reticulum membrane fusion

BNIP1, a member of the BH3-only protein family, was first discovered as one of the proteins that are capable of interacting with the antiapoptotic adenovirus E1B 19-kDa protein. Here we disclose a totally unexpected finding that BNIP1 is a component of the complex comprising syntaxin 18, an endoplasmic reticulum (ER)-located soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein (SNAP) receptor (SNARE). Functional analysis revealed that BNIP1 participates in the formation of the ER network structure, but not in membrane trafficking between the ER and Golgi. Notably, a highly conserved leucine residue in the BH3 domain of BNIP1 plays an important role not only in the induction of apoptosis but also in the binding of alpha-SNAP, an adaptor that serves as a link between the chaperone ATPase NSF and SNAREs. This predicts that alpha-SNAP may suppress apoptosis by competing with antiapoptotic proteins for the BH3 domain of BNIP1. Indeed, overexpression of alpha-SNAP markedly delayed staurosporine-induced apoptosis. Our results shed light on possible crosstalk between apparently independent cellular events, apoptosis and ER membrane fusion.     

Crystal structure of the vesicular transport protein Sec17: implications for SNAP function in SNARE complex disassembly.

SNAP proteins play an essential role in membrane trafficking in eukaryotic cells. They activate and recycle SNARE proteins by serving as adaptors between SNAREs and the cytosolic chaperone NSF. We have determined the crystal structure of Sec17, the yeast homolog of alpha-SNAP, to 2.9 A resolution. Sec17 is composed of an N-terminal twisted sheet of alpha-helical hairpins and a C-terminal alpha-helical bundle. The N-terminal sheet has local similarity to the tetratricopeptide repeats from protein phosphatase 5 but has a different overall twist. Sec17 also shares structural features with HEAT and clathrin heavy chain repeats. Possible models of SNAP:SNARE binding suggest that SNAPs may function as lever arms, transmitting forces generated by conformational changes in NSF/Sec18 to drive disassembly of SNARE complexes.     

Analysis of NSF mutants reveals residues involved in SNAP binding and ATPase stimulation

N-Ethylmaleimide-sensitive fusion protein (NSF) and its yeast orthologue, Sec18, are cytoplasmic AAA(+) ATPases required for most intracellular membrane fusion events. The primary function of NSF is thought to be the disassembly of cis-SNARE complexes, thus allowing trans-SNARE complex formation and subsequent membrane fusion. The importance of NSF/Sec18 in intracellular membrane traffic in vivo is highlighted by the inhibition of neurotransmission in Drosophila comatose (NSF) mutants and of constitutive secretion in yeast sec18 mutants. However, the underlying biochemical defects in these mutant proteins are largely unknown. Here, we identify the sec18-1 mutation as a G89D substitution in the N domain of Sec18p. This mutation results in an inhibition of the mutant protein's ability to bind to Sec17p (yeast alpha-SNAP). In contrast, engineering the comatose(st53)() mutation (S483L) into mammalian NSF (S491L) has no effect on alpha-SNAP binding. Instead, the stimulation of ATPase activity by alpha-SNAP required for wild-type NSF to disassemble SNARE complexes does not occur in the mutant NSF(st53) protein. This biochemical phenotype predicts a dominant negative effect, which was confirmed by engineering the st53 mutation into Sec18 (A505L), resulting in a dominant lethal phenotype in vivo. These findings suggest a biochemical basis for the block in membrane fusion observed in the mutant organisms. Furthermore, the mutants characterized here define key residues involved in two essential, but mechanistically distinct, biochemical functions of NSF: SNAP binding and SNAP-dependent ATPase stimulation.