A complexin/synaptotagmin 1 switch controls fast synaptic vesicle exocytosis

Solution and membrane-bound conformations of the tandem C2A and C2B domains of synaptotagmin 1: Evidence for bilayer bridging

Synaptotagmin 1 (syt1) is a synaptic vesicle membrane protein that functions as the Ca(2)(+) sensor in neuronal exocytosis. Here, site-directed spin labeling was used to generate models for the solution and membrane-bound structures of a soluble fragment of syt1 containing its two C2 domains, C2A and C2B. In solution, distance restraints between the two C2 domains of syt1 were measured using double electron-electron resonance and used in a simulated annealing routine to generate models for the structure of the tandem C2A-C2B fragment. The data indicate that the two C2 domains are flexibly linked and do not interact with each other in solution, with or without Ca(2+). However, the favored orientation is one where the Ca(2+)-binding loops are oriented in opposite directions. A similar approach was taken for membrane-associated C2A-C2B, combining both distances and bilayer depth restraints with simulated annealing. The restraints can only be satisfied if the Ca(2+) and membrane-binding surfaces of the domains are oriented in opposite directions so that C2A and C2B are docked to opposing bilayers. The result suggests that syt1 functions to bridge across the vesicle and plasma membrane surfaces in a Ca(2+)-dependent manner.

Conformation and membrane position of the region linking the two C2 domains in synaptotagmin 1 by site-directed spin labeling

Synaptotagmin 1 (syt1) is an integral membrane protein localized on the synaptic vesicle that acts as the Ca(2+) sensor for neuronal exocytosis. Synaptotagmin 1 contains two C2 domains, C2A and C2B, which bind Ca(2+) ions, membranes, and SNAREs. Here, site-directed spin labeling (SDSL) was used to determine the position and dynamics of the region that links the two C2 domains in a water soluble construct encompassing the two C2 domains (syt1C2AB). An analysis of the EPR line shapes from this region indicates that the linker is flexible and unstructured when syt1 is in solution or bound to lipid bilayers. The nanosecond dynamics of the linker does not change, in the presence or absence of Ca(2+), suggesting that there is no Ca(2+)-dependent intramolecular association between the two domains. When syt1C2AB is membrane-bound, the position of the linker relative to the membrane interface was determined by measuring parameters for the collision of the spin-labeled syt1C2AB mutants with both soluble and membrane-bound Ni(II) chelates. These data indicate that the linker does not penetrate the membrane surface but lies approximately 7-10 A from the bilayer surface. In addition, the linker remains flexible when syt1C2AB binds to the SNARE complex, indicating that direct interactions between this linker and the SNAREs do not mediate association. These data suggest that the two C2 domains of syt1 interact independently on the membrane interface, or when bound to SNAREs.

Microarray analysis of hepatic gene expression identifies new genes involved in steatotic liver

Trans-10, cis-12-conjugated linoleic acid (CLA)-enriched diets promote fatty liver in mice, while cis-9, trans-11-CLA ameliorates this effect, suggesting regulation of multiple genes. To test this hypothesis, apoE-deficient mice were fed a Western-type diet enriched with linoleic acid isomers, and their hepatic gene expression was analyzed with DNA microarrays. To provide an initial screening of candidate genes, only 12 with remarkably modified expression between both CLA isomers were considered and confirmed by quantitative RT-PCR. Additionally mRNA expression of 15 genes involved in lipid metabolism was also studied. Ten genes (Fsp27, Aqp4, Cd36, Ly6d, Scd1, Hsd3b5, Syt1, Cyp7b1, and Tff3) showed significant associations among their expressions and the degree of hepatic steatosis. Their involvement was also analyzed in other models of steatosis. In hyperhomocysteinemic mice lacking Cbs gene, only Fsp27, Cd36, Scd1, Syt1, and Hsd3b5 hepatic expressions were associated with steatosis. In apoE-deficient mice consuming olive-enriched diet displaying reduction of the fatty liver, only Fsp27 and Syt1 expressions were found associated. Using this strategy, we have shown that expression of these genes is highly associated with hepatic steatosis in a genetic disease such as Cbs deficiency and in two common situations such as Western diets containing CLA isomers or a Mediterranean-type diet. Conclusion: The results highlight new processes involved in lipid handling in liver and will help to understand the complex human pathology providing new proteins and new strategies to cope with hepatic steatosis.

Munc18-1 binding to the neuronal SNARE complex controls synaptic vesicle priming

Munc18-1 and soluble NSF attachment protein receptors (SNAREs) are critical for synaptic vesicle fusion. Munc18-1 binds to the SNARE syntaxin-1 folded into a closed conformation and to SNARE complexes containing open syntaxin-1. Understanding which steps in fusion depend on the latter interaction and whether Munc18-1 competes with other factors such as complexins for SNARE complex binding is critical to elucidate the mechanisms involved. In this study, we show that lentiviral expression of Munc18-1 rescues abrogation of release in Munc18-1 knockout mice. We describe point mutations in Munc18-1 that preserve tight binding to closed syntaxin-1 but markedly disrupt Munc18-1 binding to SNARE complexes containing open syntaxin-1. Lentiviral rescue experiments reveal that such disruption selectively impairs synaptic vesicle priming but not Ca²⁺-triggered fusion of primed vesicles. We also find that Munc18-1 and complexin-1 bind simultaneously to SNARE complexes. These results suggest that Munc18-1 binding to SNARE complexes mediates synaptic vesicle priming and that the resulting primed state involves a Munc18-1–SNARE–complexin macromolecular assembly that is poised for Ca²⁺ triggering of fusion.

Pulling force generated by interacting SNAREs facilitates membrane hemifusion

In biological systems, membrane fusion is mediated by specialized proteins. Although soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP) receptors (SNAREs) provide the minimal molecular machinery required to drive membrane fusion, the precise mechanism for SNARE-mediated fusion remains to be established. Here, we used atomic force microscope (AFM) spectroscopy to determine whether the pulling force generated by interacting SNAREs is directly coupled to membrane fusion. The mechanical strength of the SNARE binding interaction was determined by single molecule force measurements. It was revealed that the forced unbinding of the SNARE complex formed between opposing (trans) bilayers involves two activation barriers; where the steep inner barrier governs the transition from the bound to an intermediate state and the outer barrier governs the transition between the intermediate and the unbound state. Moreover, truncation of either SNAP-25 or VAMP 2 reduced the slope of the inner barrier significantly and, consequently, reduced the pulling strength of the SNARE complex; thus, suggesting that the inner barrier determines the binding strength of the SNARE complex. In parallel, AFM compression force measurements revealed that truncated SNAREs were less efficient than native SNAREs in facilitating hemifusion of the apposed bilayers. Together, these findings reveal a mechanism by which a pulling force generated by interacting trans-SNAREs reduces the slope of the hemifusion barrier and, subsequently, facilitates hemifusion and makes the membranes more prone to fusion.

Concurrent binding of complexin and synaptotagmin to liposome-embedded SNARE complexes

Synaptotagmin and complexin regulate SNARE-mediated synaptic vesicle exocytosis. It has been proposed that complexin clamps membrane fusion and that Ca²⁺-synaptotagmin displaces complexin from SNARE complexes to relieve this clamping activity. Using a reconstituted system, we demonstrate that complexin and synaptotagmin simultaneously bind to neuronal SNARE complexes and that both apo-synaptotagmin and complexin inhibit SNARE-mediated membrane fusion. Moreover, the clamping ability of apo-synaptotagmin occluded the clamping activity of complexin until the arrival of a Ca²⁺ trigger, at which point synaptotagmin accelerated fusion while high concentrations of complexin inhibited fusion. Thus, the inhibitory patterns of synaptotagmin and complexin are different, suggesting that SNAREs assemble into distinct states along the fusion pathway. These data also suggest that during synaptotagmin-regulated vesicle−vesicle fusion, complexin does not function as a fusion clamp that is relieved by Ca²⁺-synaptotagmin.

Snapin facilitates the synchronization of synaptic vesicle fusion

Synaptic vesicle (SV) fusion is a fine-tuned process requiring a concert of fusion machineries. Using cortical neurons from snapin-deficient mice, we reveal a role for Snapin in facilitating synchronous release. In addition to reduced frequency of miniature excitatory postsynaptic currents (mini-EPSCs) and smaller release-ready vesicle pool (RRP) size, snapin deficiency results in EPSCs with multiple peaks and increased rise and decay times, reflecting "desynchronized" SV fusion. These defects impair both synaptic precision and efficacy during sustained neurotransmission. Transient expression of Snapin not only rescues the slowed kinetics of EPSCs, but also further accelerates the rate found in wild-type neurons. Furthermore, expression of Snapin-C66A, a dimerization-defective mutant with impaired interactions with SNAP-25 and Synaptotagmin, reduces the RRP size but exhibits less effect on synchronized fusion. Our studies provide mechanistic insights into a dual role of Snapin in enhancing the efficacy of SV priming and in fine-tuning synchronous SV fusion.

Alternative zippering as an on-off switch for SNARE-mediated fusion

Membrane fusion between vesicles and target membranes involves the zippering of a four-helix bundle generated by constituent helices derived from target- and vesicle-soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs). In neurons, the protein complexin clamps otherwise spontaneous fusion by SNARE proteins, allowing neurotransmitters and other mediators to be secreted when and where they are needed as this clamp is released. The membrane-proximal accessory helix of complexin is necessary for clamping, but its mechanism of action is unknown. Here, we present experiments using a reconstituted fusion system that suggest a simple model in which the complexin accessory helix forms an alternative four-helix bundle with the target-SNARE near the membrane, preventing the vesicle-SNARE from completing its zippering.

Complexin controls the force transfer from SNARE complexes to membranes in fusion

Trans-SNAP receptor (SNARE, where SNAP is defined as soluble NSF attachment protein, and NSF is defined as N-ethylmaleimide-sensitive factor) complexes catalyze synaptic vesicle fusion and bind complexin, but the function of complexin binding to SNARE complexes remains unclear. Here we show that in neuronal synapses, complexin simultaneously suppressed spontaneous fusion and activated fast calcium ion-evoked fusion. The dual function of complexin required SNARE binding and also involved distinct amino-terminal sequences of complexin that localize to the point where trans-SNARE complexes insert into the fusing membranes, suggesting that complexin controls the force that trans-SNARE complexes apply onto the fusing membranes. Consistent with this hypothesis, a mutation in the membrane insertion sequence of the v-SNARE synaptobrevin/vesicle-associated membrane protein (VAMP) phenocopied the complexin loss-of-function state without impairing complexin binding to SNARE complexes. Thus, complexin probably activates and clamps the force transfer from assembled trans-SNARE complexes onto fusing membranes.

The carboxy-terminal domain of complexin I stimulates liposome fusion

Regulated exocytosis requires tight coupling of the membrane fusion machinery to a triggering signal and a fast response time. Complexins are part of this regulation and, together with synaptotagmins, control calcium-dependent exocytosis. Stimulatory and inhibitory functions have been reported for complexins. To test if complexins directly affect membrane fusion, we analyzed the 4 known mammalian complexin isoforms in a reconstituted fusion assay. In contrast to complexin III (CpxIII) and CpxIV, CpxI and CpxII stimulated soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)-pin assembly and membrane fusion. This stimulatory effect required a preincubation at low temperature and was specific for neuronal t-SNAREs. Stimulation of membrane fusion was lost when the carboxy-terminal domain of CpxI was deleted or serine 115, a putative phosphorylation site, was mutated. Transfer of the carboxy-terminal domain of CpxI to CpxIII resulted in a stimulatory CpxIII-I chimera. Thus, the carboxy-terminal domains of CpxI and CpxII promote the fusion of high-curvature liposomes.

Identification of a minimal segment of complexin II essential for preferential distribution in axons

Complexin II (CPLX2) is a soluble pre-synaptic protein believed to regulate neurotransmitter release from pre-synaptic terminals. CPLX2 is localized in pre-synaptic terminals in mature brain, but the mechanism of selective localization remains unclear. Here we identified an essential segment of CPLX2 for preferential axonal distribution. Myc-tagged CPLX2 was expressed in cultured rat hippocampal neurons and its distribution between axons and dendrites was compared by immunocytochemistry and image analysis. Fluorescence signals were detected in both axons and dendrites; however, their respective distribution varied significantly. Despite the fact that signal intensity decreased almost linearly from the base to the tip of the dendrite, a substantial level was sustained along the axon, even at a position near the tip. Image analyses using a series of mutants indicated that the deletion of 19 amino acid residues, G71-P89, within the 'central core' for binding to soluble N-ethylmaleimide sensitive factor attachment protein receptor proteins resulted in the loss of preferential axonal distribution. The enhanced green fluorescent protein derivative fused with the G71-P89 fragment exhibited a similar localization to that of wild type CPLX2, indicating that the G71-P89 region of CPLX2 is essential and sufficient for preferential axonal distribution.

Membrane fusion: grappling with SNARE and SM proteins

The two universally required components of the intracellular membrane fusion machinery, SNARE and SM (Sec1/Munc18-like) proteins, play complementary roles in fusion. Vesicular and target membrane-localized SNARE proteins zipper up into an alpha-helical bundle that pulls the two membranes tightly together to exert the force required for fusion. SM proteins, shaped like clasps, bind to trans-SNARE complexes to direct their fusogenic action. Individual fusion reactions are executed by distinct combinations of SNARE and SM proteins to ensure specificity, and are controlled by regulators that embed the SM-SNARE fusion machinery into a physiological context. This regulation is spectacularly apparent in the exquisite speed and precision of synaptic exocytosis, where synaptotagmin (the calcium-ion sensor for fusion) cooperates with complexin (the clamp activator) to control the precisely timed release of neurotransmitters that initiates synaptic transmission and underlies brain function.

Direct interaction of SNARE complex binding protein synaphin/complexin with calcium sensor synaptotagmin 1

Although the binding of synaphin (also called complexin) to the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex is critical for synaptic vesicle exocytosis, the exact role of synaphin remains unclear. Here, we show that synaphin directly binds to synaptotagmin 1, a major Ca(2+) sensor for fast neurotransmitter release, in a 1:1 stoichiometry. Mapping of the synaphin site involved in synaptotagmin 1 binding revealed that the C-terminal region is essential for the interaction between these two proteins. Binding was sensitive to ionic strength, suggesting the involvement of charged residues in the C-terminus region. Mutation of the seven consecutive glutamic acid residues (residues 108-114) at the C-terminal region of synaphin to alanines or glutamines resulted in a dramatic reduction in synaptotagmin 1 binding activity. Furthermore, a peptide from the C-terminus of synaphin (residues 91-124) blocked the binding of synaptotagmin 1 to synaphin, an effect that was abolished by mutating the consecutive glutamic acid residues to alanine. Immunoprecipitation experiments with brain membrane extracts showed the presence of a complex consisting of synaphin, synaptotagmin 1, and SNAREs. We propose that synaphin recruits synaptotagmin 1 to the SNARE-based fusion complex and synergistically functions with synaptotagmin 1 in mediating fast synaptic vesicle exocytosis.

Understanding synapses: past, present, and future

Classical physiological work by Katz, Eccles, and others revealed the central importance of synapses in brain function, and characterized the mechanisms involved in synaptic transmission. Building on this work, major advances in the past two decades have elucidated how synapses work molecularly. In the present perspective, we provide a short description of our personal view of these advances, suggest a series of important future questions about synapses, and discuss ideas about how best to achieve further progress in the field.

Arabidopsis synaptotagmin 1 is required for the maintenance of plasma membrane integrity and cell viability

Plasma membrane repair in animal cells uses synaptotagmin 7, a Ca(2+)-activated membrane fusion protein that mediates delivery of intracellular membranes to wound sites by a mechanism resembling neuronal Ca(2+)-regulated exocytosis. Here, we show that loss of function of the homologous Arabidopsis thaliana Synaptotagmin 1 protein (SYT1) reduces the viability of cells as a consequence of a decrease in the integrity of the plasma membrane. This reduced integrity is enhanced in the syt1-2 null mutant in conditions of osmotic stress likely caused by a defective plasma membrane repair. Consistent with a role in plasma membrane repair, SYT1 is ubiquitously expressed, is located at the plasma membrane, and shares all domains characteristic of animal synaptotagmins (i.e., an N terminus-transmembrane domain and a cytoplasmic region containing two C2 domains with phospholipid binding activities). Our analyses support that membrane trafficking mediated by SYT1 is important for plasma membrane integrity and plant fitness.

Complexin II plays a positive role in Ca2+-triggered exocytosis by facilitating vesicle priming

SNARE-mediated exocytosis is a multistage process central to synaptic transmission and hormone release. Complexins (CPXs) are small proteins that bind very rapidly and with a high affinity to the SNARE core complex, where they have been proposed recently to inhibit exocytosis by clamping the complex and inhibiting membrane fusion. However, several other studies also suggest that CPXs are positive regulators of neurotransmitter release. Thus, whether CPXs are positive or negative regulators of exocytosis is not known, much less the stage in the vesicle life cycle at which they function. Here, we systematically dissect the vesicle stages leading up to exocytosis using a knockout-rescue strategy in a mammalian model system. We show that adrenal chromaffin cells from CPX II knockout mice exhibit markedly diminished releasable vesicle pools (comprising the readily and slowly releasable pools), while showing no change in the kinetics of fusion pore dilation or morphological vesicle docking. Overexpression of WT CPX II-but not of SNARE-binding-deficient mutants-restores the size of the the releasable pools in knockout cells, and in WT cells it markedly enlarges them. Our results show that CPXs regulate the size of the primed vesicle pools and have a positive role in Ca(2+)-triggered exocytosis.

Direct interaction of otoferlin with syntaxin 1A, SNAP-25, and the L-type voltage-gated calcium channel Cav1.3

The molecular mechanisms underlying synaptic exocytosis in the hair cell, the auditory and vestibular receptor cell, are not well understood. Otoferlin, a C2 domain-containing Ca2+-binding protein, has been implicated as having a role in vesicular release. Mutations in the OTOF gene cause nonsyndromic deafness in humans, and OTOF knock-out mice are deaf. In the present study, we generated otoferlin fusion proteins containing two of the same amino acid substitutions detected in DFNB9 patients (P1825A in C2F and L1011P in C2D). The native otoferlin C2F domain bound syntaxin 1A and SNAP-25 in a Ca2+-dependent manner (with optimal 61 microm free Ca2+ required for binding). These interactions were greatly diminished for C2F with the P1825A mutation, possibly because of a reduction in tertiary structural change, induced by Ca2+, for the mutated C2F compared with the native C2F. The otoferlin C2D domain also bound syntaxin 1A, but with weaker affinity (Kd = 1.7 x 10(-5) m) than for the C2F interaction (Kd = 2.6 x 10(-9) m). In contrast, it was the otoferlin C2D domain that bound the Cav1.3 II-III loop, in a Ca2+-dependent manner. The L1011P mutation in C2D rendered this binding insensitive to Ca2+ and considerably diminished. Overall, we demonstrated that otoferlin interacts with two main target-SNARE proteins of the hair-cell synaptic complex, syntaxin 1A and SNAP-25, as well as the calcium channel, with the otoferlin C2F and C2D domains of central importance for binding. Because mutations in the otoferlin C2 domains that cause deafness in humans impair the ability of otoferlin to bind syntaxin, SNAP-25, and the Cav1.3 calcium channel, it is these interactions that may mediate regulation by otoferlin of hair cell synaptic exocytosis critical to inner ear hair cell function.

The Janus-faced nature of the C(2)B domain is fundamental for synaptotagmin-1 function

Synaptotagmin-1 functions as a Ca2+ sensor in neurotransmitter release and was proposed to act on both the synaptic vesicle and plasma membranes through interactions involving the Ca2+ binding top loops of its C(2) domains and the Ca2+-independent bottom face of the C(2)B domain. However, the functional importance of the C(2)B domain bottom face is unclear. We now show that mutating two conserved arginine residues at the C(2)B domain bottom face practically abolishes synchronous release in hippocampal neurons. Reconstitution experiments reveal that Ca2+-synaptotagmin-1 can dramatically stimulate the rate of SNARE-dependent lipid mixing, and that the two-arginine mutation strongly impairs this activity. These results demonstrate that synaptotagmin-1 function depends crucially on the bottom face of the C(2)B domain and strongly support the notion that synaptotagmin-1 triggers membrane fusion and neurotransmitter release by bringing the vesicle and plasma membranes together, much like the SNAREs do but in a Ca2+-dependent manner.

Membrane lysis during biological membrane fusion: collateral damage by misregulated fusion machines

In the canonical model of membrane fusion, the integrity of the fusing membranes is never compromised, preserving the identity of fusing compartments. However, recent molecular simulations provided evidence for a pathway to fusion in which holes in the membrane evolve into a fusion pore. Additionally, two biological membrane fusion models-yeast cell mating and in vitro vacuole fusion-have shown that modifying the composition or altering the relative expression levels of membrane fusion complexes can result in membrane lysis. The convergence of these findings showing membrane integrity loss during biological membrane fusion suggests new mechanistic models for membrane fusion and the role of membrane fusion complexes.

The role of the C terminus of the SNARE protein SNAP-25 in fusion pore opening and a model for fusion pore mechanics

Formation of a fusion pore between a vesicle and its target membrane is thought to involve the so-called SNARE protein complex. However, there is no mechanistic model explaining how the fusion pore is opened by conformational changes in the SNARE complex. It has been suggested that C-terminal zipping triggers fusion pore opening. A SNAP-25 mutant named SNAP-25Delta9 (lacking the last nine C-terminal residues) should lead to a less-tight C-terminal zipping. Single exocytotic events in chromaffin cells expressing this mutant were characterized by carbon fiber amperometry and cell-attached patch capacitance measurements. Cells expressing SNAP-25Delta9 displayed smaller amperometric "foot-current" currents, reduced fusion pore conductances, and lower fusion pore expansion rates. We propose that SNARE/lipid complexes form proteolipid fusion pores. Fusion pores involving the SNAP-25Delta9 mutant will be less tightly zipped and may lead to a longer fusion pore structure, consistent with the observed decrease of fusion pore conductance.

How does synaptotagmin trigger neurotransmitter release?

Neurotransmitter release at synapses involves a highly specialized form of membrane fusion that is triggered by Ca(2+) ions and is optimized for speed. These observations were established decades ago, but only recently have the molecular mechanisms that underlie this process begun to come into view. Here, we summarize findings obtained from genetically modified neurons and neuroendocrine cells, as well as from reconstituted systems, which are beginning to reveal the molecular mechanism by which Ca(2+)-acting on the synaptic vesicle (SV) protein synaptotagmin I (syt)-triggers rapid exocytosis. This work sheds light not only on presynaptic aspects of synaptic transmission, but also on the fundamental problem of membrane fusion, which has remained a puzzle that has yet to be solved in any biological system.

Complexin-I-deficient sperm are subfertile due to a defect in zona pellucida penetration

Upon adhesion to the zona pellucida, sperm undergo regulated exocytosis of the acrosome. Although it is necessary for sperm to penetrate the zona pellucida and fertilize an egg, the acrosomal membrane fusion process is poorly understood. Complexins I and II are small, cytosolic proteins that bind to a complex of proteins termed the solubleN-ethylmaleimide-sensitive factor attachment protein receptor complex to regulate synaptic vesicle exocytosis. Complexin-II-deficient mice are fertile but the fertility of sperm from complexin-I-deficient male mice is unclear because the mice have ataxia and cannot mate. Here, we show that the genes encoding complexins I and II are expressed in primary spermatocytes and spermatids. Complexin proteins were found in/near the developing acrosome in spermatids and in or around the acrosome of mature sperm. Cell fractionation demonstrated that complexins I and II were predominantly found in the cytosolic fraction. Furthermore, sperm from complexin-I-deficient mice had normal morphology, number, and only small differences in motility, as assessed by computer-assisted semen analysis. Complexin-I-deficient sperm capacitated normally and bound to the zona pellucida. But when sperm from complexin-I-deficient mice were inseminated into females, a defect in fertility was observed, in concordance with previous data showing thatin vitrofertilization rate was also reduced. If the zona pellucida was removed prior toin vitrofertilization, fertility was normal, demonstrating that zona pellucida penetration was defective, a step requiring acrosomal exocytosis. Therefore, complexin-I-deficient sperm are subfertile due to faulty zona pellucida penetration.

Membrane fusion

Subcellular compartmentalization, cell growth, hormone secretion and neurotransmission require rapid, targeted, and regulated membrane fusion. Fusion entails extensive lipid rearrangements by two apposed (docked) membrane vesicles, joining their membrane proteins and lipids and mixing their luminal contents without lysis. Fusion of membranes in the secretory pathway involves Rab GTPases; their bound ‘effector’ proteins, which mediate downstream steps; SNARE proteins, which can ‘snare’ each other, in cis (bound to one membrane) or in trans (anchored to apposed membranes); and SNARE-associated proteins (SM proteins; NSF or Sec18p; SNAP or Sec17p; and others) cooperating with specific lipids to catalyze fusion. In contrast, mitochondrial and cell-cell fusion events are regulated by and use distinct catalysts.

Synaptic vesicle fusion

The core of the neurotransmitter release machinery is formed by SNARE complexes, which bring the vesicle and plasma membranes together and are key for fusion, and by Munc18-1, which controls SNARE-complex formation and may also have a direct role in fusion. In addition, SNARE complex assembly is likely orchestrated by Munc13s and RIMs, active-zone proteins that function in vesicle priming and diverse forms of presynaptic plasticity. Synaptotagmin-1 mediates triggering of release by Ca²⁺, probably through interactions with SNAREs and both membranes, as well as through a tight interplay with complexins. Elucidation of the release mechanism will require a full understanding of the network of interactions among all these proteins and the membranes.

Distinct domains of complexins bind SNARE complexes and clamp fusion in vitro

In regulated exocytosis, the core membrane fusion machinery proteins, the SNARE proteins, are assisted by a group of regulatory factors in order to couple membrane fusion to an increase of intracellular calcium ion (Ca(2+)) concentration. Complexin-I and synaptotagmin-I have been shown to be key elements for this tightly regulated process. Many studies suggest that complexin-I can arrest the fusion reaction and that synaptotagmin-I can release the complexin-I blockage in a calcium-dependent manner. Although the actual molecular mechanism by which they exert their function is still unknown, recent in vivo experiments postulate that domains of complexin-I produce different effects on neurotransmitter release. Herein, by using an in vitro flipped SNARE cell fusion assay, we have identified and characterized the minimal functional domains of complexin-I necessary to couple calcium and synaptotagmin-I to membrane fusion. Moreover, we provide evidence that other isoforms of complexin, complexin-II, -III, and -IV, can also be functionally coupled to synaptotagmin-I and calcium. These correspond closely to results from in vivo experiments, providing further validation of the physiological relevance of the flipped SNARE system.

The endocytic recycling protein EHD2 interacts with myoferlin to regulate myoblast fusion

Skeletal muscle is a multinucleated syncytium that develops and is maintained by the fusion of myoblasts to the syncytium. Myoblast fusion involves the regulated coalescence of two apposed membranes. Myoferlin is a membrane-anchored, multiple C2 domain-containing protein that is highly expressed in fusing myoblasts and required for efficient myoblast fusion to myotubes. We found that myoferlin binds directly to the eps15 homology domain protein, EHD2. Members of the EHD family have been previously implicated in endocytosis as well as endocytic recycling, a process where membrane proteins internalized by endocytosis are returned to the plasma membrane. EHD2 binds directly to the second C2 domain of myoferlin, and EHD2 is reduced in myoferlin null myoblasts. In contrast to normal myoblasts, myoferlin null myoblasts accumulate labeled transferrin and have delayed recycling. Introduction of dominant negative EHD2 into myoblasts leads to the sequestration of myoferlin and inhibition of myoblast fusion. The interaction of myoferlin with EHD2 identifies molecular overlap between the endocytic recycling pathway and the machinery that regulates myoblast membrane fusion.

Synaptotagmin arrests the SNARE complex before triggering fast, efficient membrane fusion in response to Ca2+

Neuronal communication is mediated by Ca(2+)-triggered fusion of transmitter-filled synaptic vesicles with the presynaptic plasma membrane. Synaptotagmin I functions as a Ca(2+) sensor that regulates exocytosis, whereas soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP) receptor (SNARE) proteins in the vesicle and target membrane assemble into complexes that directly catalyze bilayer fusion. Here we report that, before the Ca(2+) trigger, synaptotagmin interacts with SNARE proteins in the target membrane to halt SNARE complex assembly at a step after donor vesicles attach, or dock, to target membranes. This results in fusion complexes that, when subsequently triggered by Ca(2+), drive rapid, highly efficient lipid mixing. Ca(2+)-independent interactions with SNAREs also predispose synaptotagmin to selectively penetrate the target membrane in response to Ca(2+); we demonstrate that Ca(2+)-synaptotagmin must insert into the target membrane to accelerate SNARE-catalyzed fusion. These findings demonstrate that Ca(2+) converts synaptotagmin from a clamp to a trigger for exocytosis.

The cytoplasmic C2A domain of synaptotagmin shows sequence specific interaction with its own mRNA

Synaptotagmin-1 (Syt1) is essential in Ca(2+)-dependent neurotransmitter release, but its expression regulation is unknown. Here we report that the cytoplasmic Syt1 fragment forms ribonucleoprotein complex by interacting with the 3' untranslated region (3(')UTR) of its own mRNA. Two protein-binding domains, GU(15) repeat and GUCAAUG, within the Syt 3'UTR and the C2 domains in Syt1, especially C2A, are essential in this ribonucleoprotein complex formation. Furthermore, in in vitro assay the translation efficiency of Syt1 mRNA was downregulated in presence of 3'UTR. These results demonstrate for the fist time that the soluble fraction of Syt1 can interact with its own mRNA in a highly sequence specific manner.

Complexin and Ca2+ stimulate SNARE-mediated membrane fusion

Ca²⁺-triggered, synchronized synaptic vesicle fusion underlies interneuronal communication. Complexin is a major binding partner of the SNARE complex, the core fusion machinery at the presynapse. The physiological data on complexin, however, have been at odds with each other, making delineation of its molecular function difficult. Here we report direct observation of two-faceted functions of complexin using the single-vesicle fluorescence fusion assay and EPR. We show that complexin I has two opposing effects on trans-SNARE assembly: inhibition of SNARE complex formation and stabilization of assembled SNARE complexes. Of note, SNARE-mediated fusion is markedly stimulated by complexin, and it is further accelerated by two orders of magnitude in response to an externally applied Ca²⁺ wave. We suggest that SNARE complexes, complexins and phospholipids collectively form a complex substrate for Ca²⁺ and Ca²⁺-sensing fusion effectors in neurotransmitter release.

Regulation of complexin 1 and complexin 2 in the developing human prefrontal cortex

Complexin 1 (CX1) and complexin 2 (CX2) are presynaptic proteins that modulate neurotransmitter release and are used as markers of inhibitory and excitatory synapses, respectively. The aim of this study was to gain insight into the development of inhibitory and excitatory synapses in human prefrontal cortex (PFC) by examining the expression of CX1 and CX2 in postmortem tissues. Relative complexin protein levels were measured by Western blotting in postmortem dorsolateral prefrontal cortex (DLPFC) of 42 subjects without neurological or psychiatric disease ranging in age from 18 gestational weeks to 25 years. Samples were batched a priori into fetal, 0-12 month, 1-5 years, 6-10 years, 11-15 years, 16-20 years, and 21-25 years age groups. CX1 and CX2 expression and CX2/CX1 demonstrated a significant effect of age group by ANOVA. Group CX1 level increased progressively across development and was lowest in the fetal group and highest in the young adult group, whereas group CX2 level increased between the fetal and the 6-10 years groups and then plateaued. Consistent with these divergent patterns, there was a significant effect of age group on CX2/CX1, which was higher in fetal and infant groups than in the young adult group. Furthermore, regression analysis demonstrated linear relationships of CX1 and CX2/CX1 with age, whereas CX2 was better described as having a curvilinear relationship with age. These data indicate that complexin expression increases during synaptic maturation in human DLPFC and that an increase in the influence of inhibitory synapses relative to that of excitatory synapses occurs during development in this cortical region.

A role for soluble N-ethylmaleimide-sensitive factor attachment protein receptor complex dimerization during neurosecretion

The interactions underlying the cooperativity of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complexes during neurotransmission are not known. Here, we provide a molecular characterization of a dimer formed between the cytoplasmic portions of neuronal SNARE complexes. Dimerization generates a two-winged structure in which the C termini of cytosolic SNARE complexes are in apposition, and it involves residues from the vesicle-associated SNARE synaptobrevin 2 that lie close to the cytosol-membrane interface within the full-length protein. Mutation of these residues reduces stability of dimers formed between SNARE complexes, without affecting the stability of each individual SNARE complex. These mutations also cause a corresponding decrease in the ability of botulinum toxin-resistant synaptobrevin 2 to rescue regulated exocytosis in toxin-treated neuroendocrine cells. Moreover, such synaptobrevin 2 mutants give rise to a dominant-negative inhibition of exocytosis. These data are consistent with an important role for SNARE complex dimers in neurosecretion.

Complexins facilitate neurotransmitter release at excitatory and inhibitory synapses in mammalian central nervous system

Complexins (Cplxs) are key regulators of synaptic exocytosis, but whether they act as facilitators or inhibitors is currently being disputed controversially. We show that genetic deletion of all Cplxs expressed in the mouse brain causes a reduction in Ca(2+)-triggered and spontaneous neurotransmitter release at both excitatory and inhibitory synapses. Our results demonstrate that at mammalian central nervous system synapses, Cplxs facilitate neurotransmitter release and do not simply act as inhibitory clamps of the synaptic vesicle fusion machinery.

Regulated airway goblet cell mucin secretion

Major advances in understanding regulated mucin secretion from airway goblet cells have been made in the past decade in the areas of pharmacology and basic cell biology. For instance, it is now appreciated that nucleotide agonists acting locally through P2Y purinoceptors on apical membranes of surface goblet cells provide the major regulatory system for mucin secretion. Similarly, Clara cells, the primary secretory cell in the mouse airways (and human small airways), are now recognized as major mucin-secreting cells. In Clara cells, the relative lack of staining for mucosubstances reflects essentially equal baseline rates of mucin synthesis and secretion, with little to no accumulation of mucin granules in storage pools. During mucous metaplasia induced under inflammatory conditions, mucin synthesis is massively upregulated in Clara cells, and stored mucin granules come to dominate the secretory cell phenotype. More importantly, we have seen a transition in the past few years from a pharmacological focus on regulated mucin secretion to a more molecular mechanistic focus that has great promise going forward. In part, these advances are occurring through the use of well-differentiated primary human bronchial epithelial cell cultures, but recent work in mouse models perhaps has had the most important impact. Emerging data from Munc13-2- and synaptotagmin 2-deficient mouse models represent the first direct, molecular-level manipulations of proteins involved in regulated secretory cell mucin secretion. These new data indicate that Munc13-2 is responsible for regulating a baseline mucin secretory pathway in the airways and is not essential for purinergic agonist-induced mucin secretion. In contrast, synaptotagmin 2, a fast Ca2+ sensor for the SNARE complex, is essential for regulated secretion. Interestingly, these early results suggest that there are two pathways for excocytic mucin release from goblet cells.

For better or for worse: complexins regulate SNARE function and vesicle fusion

In contrast to constitutive secretion, SNARE-mediated synaptic vesicle fusion is controlled by multiple regulatory proteins, which determine the Ca(2+) sensitivity of the vesicle fusion process and the speed of excitation-secretion coupling. Complexins are among the best characterized SNARE regulators known to date. They operate by binding to trimeric SNARE complexes consisting of the vesicle protein synaptobrevin and the plasma membrane proteins syntaxin and SNAP-25. The question as to whether complexins facilitate or inhibit SNARE-mediated fusion processes is currently a matter of significant controversy. This is mainly because of the fact that biochemical experiments in vitro and studies on vertebrate complexins in vivo have yielded apparently contradictory results. In this review, I provide a summary of available data on the role of complexins in SNARE-mediated vesicle fusion and attempt to define a model of complexin function that incorporates evidence for both facilitatory and inhibitory roles of complexins in SNARE-mediated fusion.

Accessory proteins stabilize the acceptor complex for synaptobrevin, the 1:1 syntaxin/SNAP-25 complex

Syntaxin/SNAP-25 interactions precede assembly of the ternary SNARE complex that is essential for neurotransmitter release. This binary complex has been difficult to characterize by bulk methods because of the prevalence of a 2:1 dead-end species. Here, using single-molecule fluorescence, we find the structure of the 1:1 syntaxin/SNAP-25 binary complex is variable, with states changing on the second timescale. One state corresponds to a parallel three-helix bundle, whereas other states show one of the SNAP-25 SNARE domains dissociated. Adding synaptobrevin suppresses the dissociated helix states. Remarkably, upon addition of complexin, Munc13, Munc18, or synaptotagmin, a similar effect is observed. Thus, the 1:1 binary complex is a dynamic acceptor for synaptobrevin binding, and accessory proteins stabilize this acceptor. In the cellular environment the binary complex is actively maintained in a configuration where it can rapidly interact with synaptobrevin, so formation is not likely a limiting step for neurotransmitter release.

Leaky synapses: regulation of spontaneous neurotransmission in central synapses

The mechanisms underlying spontaneous neurotransmitter release are not well understood. Under physiological as well as pathophysiological circumstances, spontaneous fusion events can set the concentration of ambient levels of neurotransmitter within the synaptic cleft and in the extracellular milieu. In the brain, unregulated release of excitatory neurotransmitters, exacerbated during pathological conditions such as stroke, can lead to neuronal damage and death. In addition, recent findings suggest that under physiological circumstances spontaneous release events can trigger postsynaptic signaling events independent of evoked neurotransmitter release. Therefore, elucidation of mechanisms underlying spontaneous neurotransmission may help us better understand the functional significance of this form of release and provide tools for its selective manipulation. For instance, our recent investigations indicate that the level of cholesterol in the synapse plays a critical role in limiting spontaneous synaptic vesicle fusion. Therefore, alterations in synaptic cholesterol metabolism can be a critical determinant of glutamatergic neurotransmission at rest. This article aims to provide a closer look into our current understanding of the mechanisms underlying spontaneous neurotransmission and the signaling triggered by these unitary release events.

Vesicle priming and recruitment by ubMunc13-2 are differentially regulated by calcium and calmodulin

Ca2+ regulates multiple processes in nerve terminals, including synaptic vesicle recruitment, priming, and fusion. Munc13s, the mammalian homologs of Caenorhabditis elegans Unc13, are essential vesicle-priming proteins and contain multiple regulatory domains that bind second messengers such as diacylglycerol and Ca2+/calmodulin (Ca2+/CaM). Binding of Ca2+/CaM is necessary for the regulatory effect that allows Munc13-1 and ubMunc13-2 to promote short-term synaptic plasticity. However, the relative contributions of Ca2+ and Ca2+/CaM to vesicle priming and recruitment by Munc13 are not known. Here, we investigated the effect of Ca2+/CaM binding on ubMunc13-2 activity in chromaffin cells via membrane-capacitance measurements and a detailed simulation of the exocytotic machinery. Stimulating secretion under various basal Ca2+ concentrations from cells overexpressing either ubMunc13-2 or a ubMunc13-2 mutant deficient in CaM binding enabled a distinction between the effects of Ca2+ and Ca2+/CaM. We show that vesicle priming by ubMunc13-2 is Ca2+ dependent but independent of CaM binding to ubMunc13-2. However, Ca2+/CaM binding to ubMunc13-2 specifically promotes vesicle recruitment during ongoing stimulation. Based on the experimental data and our simulation, we propose that ubMunc13-2 is activated by two Ca2+-dependent processes: a slow activation mode operating at low Ca2+ concentrations, in which ubMunc13-2 acts as a priming switch, and a fast mode at high Ca2+ concentrations, in which ubMunc13-2 is activated in a Ca2+/CaM-dependent manner and accelerates vesicle recruitment and maturation during stimulation. These different Ca2+ activation steps determine the kinetic properties of exocytosis and vesicle recruitment and can thus alter plasticity and efficacy of transmitter release.

Impaired insulin secretion and glucose intolerance in synaptotagmin-7 null mutant mice

Vertebrates express at least 15 different synaptotagmins with the same domain structure but diverse localizations and tissue distributions. Synaptotagmin-1,-2, and -9 act as calcium sensors for the fast phrase of neurotransmitter release, and synaptotagmin-12 acts as a calcium-independent modulator of release. The exact functions of the remaining 11 synaptotagmins, however, have not been established. By analogy to the role of synaptotagmin-1, -2, and -9 in neurotransmission, these other synaptotagmins may serve as Ca(2+) transducers regulating other Ca(2+)-dependent membrane processes, such as insulin secretion in pancreatic beta-cells. Of these other synaptotagmins, synaptotagmin-7 is one of the most abundant and is present in pancreatic beta-cells. To determine whether synaptotagmin-7 regulates Ca(2+)-dependent insulin secretion, we analyzed synaptotagmin-7 null mutant mice for glucose tolerance and insulin release. Here, we show that synaptotagmin-7 is required for the maintenance of systemic glucose tolerance and glucose-stimulated insulin secretion. Mutant mice have normal insulin sensitivity, insulin production, islet architecture and ultrastructural organization, and metabolic and calcium responses but exhibit impaired glucose-induced insulin secretion, indicating a calcium-sensing defect during insulin-containing secretory granule exocytosis. Taken together, our findings show that synaptotagmin-7 functions as a positive regulator of insulin secretion and may serve as a calcium sensor controlling insulin secretion in pancreatic beta cells.

Synaptotagmin-1 and -7 are functionally overlapping Ca2+ sensors for exocytosis in adrenal chromaffin cells

Synaptotagmin-1, the canonical isoform of the synaptotagmin family, is a Ca(2+) sensor for fast synchronous neurotransmitter release in forebrain neurons and chromaffin cells. Even though deletion of synaptotagmin-1 abolishes fast exocytosis in chromaffin cells, it reduces overall secretion by only 20% because of the persistence of slow exocytosis. Therefore, another Ca(2+) sensor dominates release in these cells. Synaptotagmin-7 has a higher Ca(2+) affinity and slower binding kinetics than synaptotagmin-1, matching the proposed properties for the second, slower Ca(2+) sensor. Here, we examined Ca(2+)-triggered exocytosis in chromaffin cells from KO mice lacking synaptotagmin-7, and from knockin mice containing normal levels of a mutant synaptotagmin-7 whose C(2)B domain does not bind Ca(2+). In both types of mutant chromaffin cells, Ca(2+)-triggered exocytosis was decreased dramatically. Moreover, in chromaffin cells lacking both synaptotagmin-1 and -7, only a very slow release component, accounting for approximately 30% of WT exocytosis, persisted. These data establish synaptotagmin-7 as a major Ca(2+) sensor for exocytosis in chromaffin cells, which, together with synaptotagmin-1, mediates almost all of the Ca(2+) triggering of exocytosis in these cells, a surprising result, considering the lack of a role of synaptotagmin-7 in synaptic vesicle exocytosis.

Core proteins of the secretory machinery

Members of the Rab, SM- and SNARE-protein families play key roles in all intracellular membrane trafficking steps. While SM- and SNARE-proteins become directly involved in the fusion reaction at a late stage, Rabs and their effectors mediate upstream steps such as vesicle budding, delivery, tethering, and transport. Exocytosis of synaptic vesicles and regulated secretory granules are among the best-studied fusion events and involve the Rab3 isoforms Rab3A-D, the SM protein munc18-1, and the SNAREs syntaxin 1A, SNAP-25, and synaptobrevin 2. According to the current view, syntaxin 1A and SNAP-25 at the presynaptic membrane form a complex with synaptic vesicle-associated synaptobrevin 2. As complex formation proceeds, the opposed membranes are pulled tightly together, enforcing the fusion reaction. Munc18-1 is essential for regulated exocytosis and interacts with syntaxin 1A alone or with SNARE complexes, suggesting a role for munc18-1 in controlling the SNARE-assembly reaction. Compared to other intracellular fusion steps, special adaptations evolved in the synapse to allow for the tight regulation and high membrane turnover rates required for synaptic transmission. Synaptic vesicle fusion is triggered by the intracellular second messenger calcium, with members of the synaptotagmin protein family being prime candidates for linking calcium influx to fusion in the fast phase of exocytosis. To compensate for the massive incorporation of synaptic vesicles into the plasma membrane during exocytosis, special adaptations to endocytic mechanisms have evolved at the synapse to allow for efficient vesicle recycling.

Neurotransmitter release

Neurons send out a multitude of chemical signals, called neurotransmitters, to communicate between neurons in brain, and between neurons and target cells in the periphery. The most important of these communication processes is synaptic transmission, which accounts for the ability of the brain to rapidly process information, and which is characterized by the fast and localized transfer of a signal from a presynaptic neuron to a postsynaptic cell. Other communication processes, such as the modulation of the neuronal state in entire brain regions by neuromodulators, provide an essential component of this information processing capacity. A large number of diverse neurotransmitters are used by neurons, ranging from classical fast transmitters such as glycine and glutamate over neuropeptides to lipophilic compounds and gases such as endocannabinoids and nitric oxide. Most of these transmitters are released by exocytosis, the i.e. the fusion of secretory vesicles with the plasma membrane, which exhibits distinct properties for different types of neurotransmitters. The present chapter will provide an overview of the process of neurotransmitter release and its historical context, and give a reference point for the other chapters in this book.

Tag team action at the synapse

Communication between neurons relies on chemical synapses and the release of neurotransmitters into the synaptic cleft. Neurotransmitter release is an exquisitely regulated membrane fusion event that requires the linking of an electrical nerve stimulus to Ca(2+) influx, which leads to the fusion of neurotransmitter-filled vesicles with the cell membrane. The timing of neurotransmitter release is controlled through the regulation of the soluble N-ethylmaleimide sensitive factor attachment receptor (SNARE) proteins-the core of the membrane fusion machinery. Assembly of the fusion-competent SNARE complex is regulated by several neuronal proteins, including complexin and the Ca(2+)-sensor synaptotagmin. Both complexin and synaptotagmin bind directly to SNAREs, but their mechanism of action has so far remained unclear. Recent studies revealed that synaptotagmin-Ca(2+) and complexin collaborate to regulate membrane fusion. These compelling new results provide a molecular mechanistic insight into the functions of both proteins: complexin 'clamps' the SNARE complex in a pre-fusion intermediate, which is then released by the action of Ca(2+)-bound synaptotagmin to trigger rapid fusion.

RNA editing in regulating gene expression in the brain

Adenosine to inosine RNA editing, catalyzed by Adenosine Deaminases Acting on RNA (ADARs), represents an evolutionary conserved post-transcriptional mechanism which harnesses RNA structures to produce proteins that are not literally encoded in the genome. The species-specific alteration of functionally important residues in a multitude of neuronal ion channels and pre-synaptic proteins through RNA editing has been shown to have profound importance for normal nervous system function in a wide range of invertebrate and vertebrate model organisms. ADARs have also been shown to regulate neuronal gene expression through a remarkable variety of disparate processes, including modulation of the RNAi pathway, the creation of alternative splice sites, and the abolition of stop codons. In addition, ADARs have recently been revealed to have a novel role in the primate lineage: the widespread editing of Alu elements, which comprise approximately 10% of the human genome. Thus, as well as enabling the cell-specific regulation of RNAi and selfish genetic elements, the unshackling of the proteome from the constraints of the genome through RNA editing may have been fundamental to the evolution of complex behavior.

The structure of the yeast plasma membrane SNARE complex reveals destabilizing water-filled cavities

SNARE proteins form a complex that leads to membrane fusion between vesicles, organelles, and plasma membrane in all eukaryotic cells. We report the 1.7A resolution structure of the SNARE complex that mediates exocytosis at the plasma membrane in the yeast Saccharomyces cerevisiae. Similar to its neuronal and endosomal homologues, the S. cerevisiae SNARE complex forms a parallel four-helix bundle in the center of which is an ionic layer. The S. cerevisiae SNARE complex exhibits increased helix bending near the ionic layer, contains water-filled cavities in the complex core, and exhibits reduced thermal stability relative to mammalian SNARE complexes. Mutagenesis experiments suggest that the water-filled cavities contribute to the lower stability of the S. cerevisiae complex.

Synaptotagmin C2A loop 2 mediates Ca2+-dependent SNARE interactions essential for Ca2+-triggered vesicle exocytosis

Synaptotagmins contain tandem C2 domains and function as Ca(2+) sensors for vesicle exocytosis but the mechanism for coupling Ca(2+) rises to membrane fusion remains undefined. Synaptotagmins bind SNAREs, essential components of the membrane fusion machinery, but the role of these interactions in Ca(2+)-triggered vesicle exocytosis has not been directly assessed. We identified sites on synaptotagmin-1 that mediate Ca(2+)-dependent SNAP25 binding by zero-length cross-linking. Mutation of these sites in C2A and C2B eliminated Ca(2+)-dependent synaptotagmin-1 binding to SNAREs without affecting Ca(2+)-dependent membrane binding. The mutants failed to confer Ca(2+) regulation on SNARE-dependent liposome fusion and failed to restore Ca(2+)-triggered vesicle exocytosis in synaptotagmin-deficient PC12 cells. The results provide direct evidence that Ca(2+)-dependent SNARE binding by synaptotagmin is essential for Ca(2+)-triggered vesicle exocytosis and that Ca(2+)-dependent membrane binding by itself is insufficient to trigger fusion. A structure-based model of the SNARE-binding surface of C2A provided a new view of how Ca(2+)-dependent SNARE and membrane binding occur simultaneously.

Unsolved mysteries in membrane traffic

Remarkable strides have been made over the past 20 years in elucidating the molecular basis of membrane trafficking. Indeed, a combination of biochemical and genetic approaches have determined the identity and function of many of the core constituents needed for protein secretion and endocytosis. But much remains to be learned. This review highlights underlying themes in membrane traffic to help us refocus and solve many remaining and newly emerging issues that are fundamental to mammalian cell biology and human physiology.