The length of the flexible SNAREpin juxtamembrane region is a critical determinant of SNARE-dependent fusion

Molecular mechanism underlying SNARE-mediated membrane fusion enlightened by all-atom molecular dynamics simulations

The SNAP receptor (SNARE) proteins syntaxin-1, SNAP-25, and synaptobrevin mediate neurotransmitter release by forming tight SNARE complexes that fuse synaptic vesicles with the plasma membranes in microseconds. Membrane fusion is generally explained by the action of proteins on macroscopic membrane properties such as curvature, elastic modulus, and tension, and a widespread model envisions that the SNARE motifs, juxtamembrane linkers, and C-terminal transmembrane regions of synaptobrevin and syntaxin-1 form continuous helices that act mechanically as semirigid rods, squeezing the membranes together as they assemble ("zipper") from the N to the C termini. However, the mechanism underlying fast SNARE-induced membrane fusion remains unknown. We have used all-atom molecular dynamics simulations to investigate this mechanism. Our results need to be interpreted with caution because of the limited number and length of the simulations, but they suggest a model of membrane fusion that has a natural physicochemical basis, emphasizes local molecular events over general membrane properties, and explains extensive experimental data. In this model, the central event that initiates fast (microsecond scale) membrane fusion occurs when the SNARE helices zipper into the juxtamembrane linkers which, together with the adjacent transmembrane regions, promote encounters of acyl chains from both bilayers at the polar interface. The resulting hydrophobic nucleus rapidly expands into stalk-like structures that gradually progress to form a fusion pore, aided by the SNARE transmembrane regions and without clearly discernible intermediates. The propensity of polyunsaturated lipids to participate in encounters that initiate fusion suggests that these lipids may be important for the high speed of neurotransmitter release.

The stability of the primed pool of synaptic vesicles and the clamping of spontaneous neurotransmitter release rely on the integrity of the C-terminal half of the SNARE domain of syntaxin-1A

The SNARE proteins are central in membrane fusion and, at the synapse, neurotransmitter release. However, their involvement in the dual regulation of the synchronous release while maintaining a pool of readily releasable vesicles remains unclear. Using a chimeric approach, we performed a systematic analysis of the SNARE domain of STX1A by exchanging the whole SNARE domain or its N- or C-terminus subdomains with those of STX2. We expressed these chimeric constructs in STX1-null hippocampal mouse neurons. Exchanging the C-terminal half of STX1's SNARE domain with that of STX2 resulted in a reduced RRP accompanied by an increased release rate, while inserting the C-terminal half of STX1's SNARE domain into STX2 leads to an enhanced priming and decreased release rate. Additionally, we found that the mechanisms for clamping spontaneous, but not for Ca2+-evoked release, are particularly susceptible to changes in specific residues on the outer surface of the C-terminus of the SNARE domain of STX1A. Particularly, mutations of D231 and R232 affected the fusogenicity of the vesicles. We propose that the C-terminal half of the SNARE domain of STX1A plays a crucial role in the stabilization of the RRP as well as in the clamping of spontaneous synaptic vesicle fusion through the regulation of the energetic landscape for fusion, while it also plays a covert role in the speed and efficacy of Ca2+-evoked release.

Syntaxin-1A modulates vesicle fusion in mammalian neurons via juxtamembrane domain dependent palmitoylation of its transmembrane domain

SNAREs are undoubtedly one of the core elements of synaptic transmission. Contrary to the well characterized function of their SNARE domains bringing the plasma and vesicular membranes together, the level of contribution of their juxtamembrane domain (JMD) and the transmembrane domain (TMD) to the vesicle fusion is still under debate. To elucidate this issue, we analyzed three groups of STX1A mutations in cultured mouse hippocampal neurons: (1) elongation of STX1A’s JMD by three amino acid insertions in the junction of SNARE-JMD or JMD-TMD; (2) charge reversal mutations in STX1A’s JMD; and (3) palmitoylation deficiency mutations in STX1A’s TMD. We found that both JMD elongations and charge reversal mutations have position-dependent differential effects on Ca²⁺-evoked and spontaneous neurotransmitter release. Importantly, we show that STX1A’s JMD regulates the palmitoylation of STX1A’s TMD and loss of STX1A palmitoylation either through charge reversal mutation K260E or by loss of TMD cysteines inhibits spontaneous vesicle fusion. Interestingly, the retinal ribbon specific STX3B has a glutamate in the position corresponding to the K260E mutation in STX1A and mutating it with E259K acts as a molecular on-switch. Furthermore, palmitoylation of post-synaptic STX3A can be induced by the exchange of its JMD with STX1A’s JMD together with the incorporation of two cysteines into its TMD. Forced palmitoylation of STX3A dramatically enhances spontaneous vesicle fusion suggesting that STX1A regulates spontaneous release through two distinct mechanisms: one through the C-terminal half of its SNARE domain and the other through the palmitoylation of its TMD.

Fusion with wild-type SNARE domains is controlled by juxtamembrane domains, transmembrane anchors, and Sec17

Membrane fusion requires tethers, SNAREs of R, Qa, Qb, and Qc families, and chaperones of the SM, Sec17/SNAP, and Sec18/NSF families. SNAREs have N-domains, SNARE domains that zipper into 4-helical RQaQbQc coiled coils, a short juxtamembrane (Jx) domain, and (often) a C-terminal transmembrane anchor. We reconstitute fusion with purified components from yeast vacuoles, where the HOPS protein combines tethering and SM functions. The vacuolar Rab, lipids, and R-SNARE activate HOPS to bind Q-SNAREs and catalyze trans-SNARE associations. With SNAREs initially disassembled, as they are on the organelle, we now report that R- and Qa-SNAREs require their physiological juxtamembrane (Jx) regions for fusion. Swap of the Jx domain between the R- and Qa-SNAREs blocks fusion after SNARE association in trans. This block is bypassed by either Sec17, which drives fusion without requiring complete SNARE zippering, or transmembrane-anchored Qb-SNARE in complex with Qa. The abundance of the trans-SNARE complex is not the sole fusion determinant, as it is unaltered by Sec17, Jx swap, or the Qb-transmembrane anchor. The sensitivity of fusion to Jx swap in the absence of a Qb transmembrane anchor is inherent to the SNAREs, because it remains when a synthetic tether replaces HOPS.

Structural Roles for the Juxtamembrane Linker Region and Transmembrane Region of Synaptobrevin 2 in Membrane Fusion

Formation of the trans-SNARE complex is believed to generate a force transfer to the membranes to promote membrane fusion, but the underlying mechanism remains elusive. In this study, we show that helix-breaking and/or length-increasing insertions in the juxtamembrane linker region of synaptobrevin-2 exert diverse effects on liposome fusion, in a manner dependent on the insertion position relative to the two conserved tryptophan residues (W⁸⁹/W⁹⁰). Helical extension of synaptobrevin-2 to W⁸⁹/W⁹⁰ is a prerequisite for initiating membrane merger. The transmembrane region of synaptobrevin-2 enables proper localization of W⁸⁹/W⁹⁰ at the membrane interface to gate force transfer. Besides, our data indicate that the SNARE regulatory components Munc18-1 and Munc13-1 impose liposome fusion strong demand on tight coupling between the SNARE motif and the transmembrane region of synaptobrevin-2.

Multiple factors maintain assembled trans-SNARE complexes in the presence of NSF and αSNAP

Neurotransmitter release requires formation of trans-SNARE complexes between the synaptic vesicle and plasma membranes, which likely underlies synaptic vesicle priming to a release-ready state. It is unknown whether Munc18-1, Munc13-1, complexin-1 and synaptotagmin-1 are important for priming because they mediate trans-SNARE complex assembly and/or because they prevent trans-SNARE complex disassembly by NSF-αSNAP, which can lead to de-priming. Here we show that trans-SNARE complex formation in the presence of NSF-αSNAP requires both Munc18-1 and Munc13-1, as proposed previously, and is facilitated by synaptotagmin-1. Our data also show that Munc18-1, Munc13-1, complexin-1 and likely synaptotagmin-1 contribute to maintaining assembled trans-SNARE complexes in the presence of NSF-αSNAP. We propose a model whereby Munc18-1 and Munc13-1 are critical not only for mediating vesicle priming but also for precluding de-priming by preventing trans-SNARE complex disassembly; in this model, complexin-1 also impairs de-priming, while synaptotagmin-1 may assist in priming and hinder de-priming.

SNARE-mediated membrane fusion is a two-stage process driven by entropic forces

SNARE proteins constitute the core of the exocytotic membrane fusion machinery. Fusion occurs when vesicle-associated and target membrane-associated SNAREs zipper into trans-SNARE complexes ('SNAREpins'), but the number required is controversial and the mechanism of cooperative fusion is poorly understood. We developed a highly coarse-grained molecular dynamics simulation to access the long fusion timescales, which revealed a two-stage process. First, zippering energy was dissipated and cooperative entropic forces assembled the SNAREpins into a ring; second, entropic forces expanded the ring, pressing membranes together and catalyzing fusion. We predict that any number of SNAREs fuses membranes, but fusion is faster with more SNAREs.

Membrane-Fusogen Distance Is Critical for Efficient Coiled-Coil-Peptide-Mediated Liposome Fusion

We have developed a model system for membrane fusion that utilizes lipidated derivatives of a heterodimeric coiled-coil pair dubbed E3 (EIAALEK)3 and K3 (KIAALKE)3. In this system, peptides are conjugated to a lipid anchor via a poly(ethylene glycol) (PEG) spacer, and this contribution studies the influence of the PEG spacer length, coupled with the type of lipid anchor, on liposome-liposome fusion. The effects of these modifications on peptide secondary structure, their interactions with liposomes, and their ability to mediate fusion were studied using a variety of different content mixing experiments and CD spectroscopy. Our results demonstrate the asymmetric role of the peptides in the fusion process because alterations to the PEG spacer length affect E3 and K3 differently. We conclude that negatively charged E3 acts as a "handle" for positively charged K3 and facilitates liposome docking, the first stage of the fusion process, through coiled-coil formation. The efficacy of this E3 handle is enhanced by longer spacer lengths. K3 directs the fusion process via peptide-membrane interactions, but the length of the PEG spacer plays two competing roles: a PEG4/PEG8 spacer length is optimal for membrane destabilization; however, a PEG12 spacer increases the fusion efficiency over time by improving the peptide accessibility for successive fusion events. Both the anchor type and spacer length affect the peptide structure; a cholesterol anchor appears to enhance K3-membrane interactions and thus mediates fusion more efficiently.

REEP6 mediates trafficking of a subset of Clathrin-coated vesicles and is critical for rod photoreceptor function and survival

In retinal photoreceptors, vectorial transport of cargo is critical for transduction of visual signals, and defects in intracellular trafficking can lead to photoreceptor degeneration and vision impairment. Molecular signatures associated with routing of transport vesicles in photoreceptors are poorly understood. We previously reported the identification of a novel rod photoreceptor specific isoform of Receptor Expression Enhancing Protein (REEP) 6, which belongs to a family of proteins involved in intracellular transport of receptors to the plasma membrane. Here we show that loss of REEP6 in mice (Reep6^−/−) results in progressive retinal degeneration. Rod photoreceptor dysfunction is observed in Reep6^−/− mice as early as one month of age and associated with aberrant accumulation of vacuole-like structures at the apical inner segment and reduction in selected rod phototransduction proteins. We demonstrate that REEP6 is detected in a subset of Clathrin-coated vesicles and interacts with the t-SNARE, Syntaxin3. In concordance with the rod degeneration phenotype in Reep6^−/− mice, whole exome sequencing identified homozygous REEP6-E75K mutation in two retinitis pigmentosa families of different ethnicities. Our studies suggest a critical function of REEP6 in trafficking of cargo via a subset of Clathrin-coated vesicles to selected membrane sites in retinal rod photoreceptors.

A cascade of multiple proteins and lipids catalyzes membrane fusion

Recent studies suggest revisions to the SNARE paradigm of membrane fusion. Membrane tethers and/or SNAREs recruit proteins of the Sec 1/Munc18 family to catalyze SNARE assembly into trans-complexes. SNARE-domain zippering draws the bilayers into immediate apposition and provides a platform to position fusion triggers such as Sec 17/α-SNAP and/or synaptotagmin, which insert their apolar "wedge" domains into the bilayers, initiating the lipid rearrangements of fusion.

v-SNARE function in chromaffin cells

Vesicle fusion is elementary for intracellular trafficking and release of signal molecules, thus providing the basis for diverse forms of intercellular communication like hormonal regulation or synaptic transmission. A detailed characterization of the mechanisms underlying exocytosis is key to understand how the nervous system integrates information and generates appropriate responses to stimuli. The machinery for vesicular release employs common molecular players in different model systems including neuronal and neuroendocrine cells, in particular members of the SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) protein family, Sec1/Munc18-like proteins, and other accessory factors. To achieve temporal precision and speed, excitable cells utilize specialized regulatory proteins like synaptotagmin and complexin, whose interplay putatively synchronizes vesicle fusion and enhances stimulus-secretion coupling. In this review, we aim to highlight recent progress and emerging views on the molecular mechanisms, by which constitutively forming SNAREpins are organized in functional, tightly regulated units for synchronized release. Specifically, we will focus on the role of vesicle associated membrane proteins, also referred to as vesicular SNAREs, in fusion and rapid cargo discharge. We will further discuss the functions of SNARE regulators during exocytosis and focus on chromaffin cell as a model system of choice that allows for detailed structure-function analyses and direct measurements of vesicle fusion under precise control of intracellular [Ca]i.

Suppressor Analysis of the Fusogenic Lambda Spanins

The final step of lysis in phage λ infections of Escherichia coli is mediated by the spanins Rz and Rz1. These proteins form a complex that bridges the cell envelope and that has been proposed to cause fusion of the inner and outer membranes. Accordingly, mutations that block spanin function are found within coiled-coil domains and the proline-rich region, motifs essential in other fusion systems. To gain insight into spanin function, pseudorevertant alleles that restored plaque formation for lysis-defective mutants of Rz and Rz1 were selected. Most second-site suppressors clustered within a coiled-coil domain of Rz near the outer leaflet of the cytoplasmic membrane and were not allele specific. Suppressors largely encoded polar insertions within the hydrophobic core of the coiled-coil interface. Such suppressor changes resulted in decreased proteolytic stability of the Rz double mutants in vivo Unlike the wild type, in which lysis occurs while the cells retain a rod shape, revertant alleles with second-site suppressor mutations supported lysis events that were preceded by spherical cell formation. This suggests that destabilization of the membrane-proximal coiled coil restores function for defective spanin alleles by increasing the conformational freedom of the complex at the cost of its normal, all-or-nothing functionality.IMPORTANCECaudovirales encode cell envelope-spanning proteins called spanins, which are thought to fuse the inner and outer membranes during phage lysis. Recent genetic analysis identified the functional domains of the lambda spanins, which are similar to class I viral fusion proteins. While the pre- and postfusion structures of model fusion systems have been well characterized, the intermediate structure(s) formed during the fusion reaction remains elusive. Genetic analysis would be expected to identify functional connections between intermediates. Since most membrane fusion systems are not genetically tractable, only few such investigations have been reported. Here, we report a suppressor analysis of lambda spanin function. To our knowledge this is the first suppression analysis of a class I-like complex and also the first such analysis of a prokaryote membrane fusion system.

Entropic forces drive self-organization and membrane fusion by SNARE proteins

SNARE proteins are the core of the cell's fusion machinery and mediate virtually all known intracellular membrane fusion reactions on which exocytosis and trafficking depend. Fusion is catalyzed when vesicle-associated v-SNAREs form trans-SNARE complexes ("SNAREpins") with target membrane-associated t-SNAREs, a zippering-like process releasing ∼65 kT per SNAREpin. Fusion requires several SNAREpins, but how they cooperate is unknown and reports of the number required vary widely. To capture the collective behavior on the long timescales of fusion, we developed a highly coarse-grained model that retains key biophysical SNARE properties such as the zippering energy landscape and the surface charge distribution. In simulations the ∼65-kT zippering energy was almost entirely dissipated, with fully assembled SNARE motifs but uncomplexed linker domains. The SNAREpins self-organized into a circular cluster at the fusion site, driven by entropic forces that originate in steric-electrostatic interactions among SNAREpins and membranes. Cooperative entropic forces expanded the cluster and pulled the membranes together at the center point with high force. We find that there is no critical number of SNAREs required for fusion, but instead the fusion rate increases rapidly with the number of SNAREpins due to increasing entropic forces. We hypothesize that this principle finds physiological use to boost fusion rates to meet the demanding timescales of neurotransmission, exploiting the large number of v-SNAREs available in synaptic vesicles. Once in an unfettered cluster, we estimate ≥15 SNAREpins are required for fusion within the ∼1-ms timescale of neurotransmitter release.

The Multifaceted Role of SNARE Proteins in Membrane Fusion

Membrane fusion is a key process in all living organisms that contributes to a variety of biological processes including viral infection, cell fertilization, as well as intracellular transport, and neurotransmitter release. In particular, the various membrane-enclosed compartments in eukaryotic cells need to exchange their contents and communicate across membranes. Efficient and controllable fusion of biological membranes is known to be driven by cooperative action of SNARE proteins, which constitute the central components of the eukaryotic fusion machinery responsible for fusion of synaptic vesicles with the plasma membrane. During exocytosis, vesicle-associated v-SNARE (synaptobrevin) and target cell-associated t-SNAREs (syntaxin and SNAP-25) assemble into a core trans-SNARE complex. This complex plays a versatile role at various stages of exocytosis ranging from the priming to fusion pore formation and expansion, finally resulting in the release or exchange of the vesicle content. This review summarizes current knowledge on the intricate molecular mechanisms underlying exocytosis triggered and catalyzed by SNARE proteins. Particular attention is given to the function of the peptidic SNARE membrane anchors and the role of SNARE-lipid interactions in fusion. Moreover, the regulatory mechanisms by synaptic auxiliary proteins in SNARE-driven membrane fusion are briefly outlined.

Phosphatidylserine-Dependent Catalysis of Stalk and Pore Formation by Synaptobrevin JMR-TMD Peptide

Although the importance of a SNARE complex in neurotransmitter release is widely accepted, there exist different views on how the complex promotes fusion. One hypothesis is that the SNARE complex's ability to bring membranes into contact is sufficient for fusion, another points to possible roles of juxtamembrane regions (JMRs) and transmembrane domains (TMDs) in catalyzing lipid rearrangement, and another notes the complex's presumed ability to bend membranes near the point of contact. Here, we performed experiments with highly curved vesicles brought into contact using low concentrations of polyethylene glycol (PEG) to investigate the influence of the synaptobrevin (SB) TMD with an attached JMR (SB-JMR-TMD) on the rates of stalk and pore formation during vesicle fusion. SB-JMR-TMD enhanced the rates of stalk and fusion pore (FP) formation in a sharply sigmoidal fashion. We observed an optimal influence at an average of three peptides per vesicle, but only with phosphatidylserine (PS)-containing vesicles. Approximately three SB-JMR-TMDs per vesicle optimally ordered the bilayer interior and excluded water in a similar sigmoidal fashion. The catalytic influences of hexadecane and SB-JMR-TMD on fusion kinetics showed little in common, suggesting different mechanisms. Both kinetic and membrane structure measurements support the hypotheses that SB-JMR-TMD 1) catalyzes initial intermediate formation as a result of its basic JMR disrupting ordered interbilayer water and permitting closer interbilayer approach, and 2) catalyzes pore formation by forming a membrane-spanning complex that increases curvature stress at the circumference of the hemifused diaphragm of the prepore intermediate state.

A non-zipper-like tetrameric coiled coil promotes membrane fusion

A parallel heterodimeric coiled coil can be mutated to an antiparallel tetrameric species by reversing the sequences of one of the peptides. This tetramer is capable of facilitating fast, efficient, membrane fusion of liposomes.

Lipid-anchored Synaptobrevin Provides Little or No Support for Exocytosis or Liposome Fusion

SNARE proteins catalyze many forms of biological membrane fusion, including Ca(2+)-triggered exocytosis. Although fusion mediated by SNAREs generally involves proteins anchored to each fusing membrane by a transmembrane domain (TMD), the role of TMDs remains unclear, and previous studies diverge on whether SNAREs can drive fusion without a TMD. This issue is important because it relates to the question of the structure and composition of the initial fusion pore, as well as the question of whether SNAREs mediate fusion solely by creating close proximity between two membranes versus a more active role in transmitting force to the membrane to deform and reorganize lipid bilayer structure. To test the role of membrane attachment, we generated four variants of the synaptic v-SNARE synaptobrevin-2 (syb2) anchored to the membrane by lipid instead of protein. These constructs were tested for functional efficacy in three different systems as follows: Ca(2+)-triggered dense core vesicle exocytosis, spontaneous synaptic vesicle exocytosis, and Ca(2+)-synaptotagmin-enhanced SNARE-mediated liposome fusion. Lipid-anchoring motifs harboring one or two lipid acylation sites completely failed to support fusion in any of these assays. Only the lipid-anchoring motif from cysteine string protein-α, which harbors many lipid acylation sites, provided support for fusion but at levels well below that achieved with wild type syb2. Thus, lipid-anchored syb2 provides little or no support for exocytosis, and anchoring syb2 to a membrane by a TMD greatly improves its function. The low activity seen with syb2-cysteine string protein-α may reflect a slower alternative mode of SNARE-mediated membrane fusion.

A highly tilted membrane configuration for the prefusion state of synaptobrevin

The SNARE complex plays a vital role in vesicle fusion arising during neuronal exocytosis. Key components in the regulation of SNARE complex formation, and ultimately fusion, are the transmembrane and linker regions of the vesicle-associated protein, synaptobrevin. However, the membrane-embedded structure of synaptobrevin in its prefusion state, which determines its interaction with other SNARE proteins during fusion, is largely unknown. This study reports all-atom molecular-dynamics simulations of the prefusion configuration of synaptobrevin in a lipid bilayer, aimed at characterizing the insertion depth and the orientation of the protein in the membrane, as well as the nature of the amino acids involved in determining these properties. By characterizing the structural properties of both wild-type and mutant synaptobrevin, the effects of C-terminal additions on tilt and insertion depth of membrane-embedded synaptobrevin are determined. The simulations suggest a robust, highly tilted state for membrane-embedded synaptobrevin with a helical connection between the transmembrane and linker regions, leading to an apparently new characterization of structural elements in prefusion synaptobrevin and providing a framework for interpreting past mutation experiments.

Efficient Exploration of Membrane-Associated Phenomena at Atomic Resolution

Biological membranes constitute a critical component in all living cells. In addition to providing a conducive environment to a wide range of cellular processes, including transport and signaling, mounting evidence has established active participation of specific lipids in modulating membrane protein function through various mechanisms. Understanding lipid-protein interactions underlying these mechanisms at a sufficiently high resolution has proven extremely challenging, partly due to the semi-fluid nature of the membrane. In order to address this challenge computationally, multiple methods have been developed, including an alternative membrane representation termed highly mobile membrane mimetic (HMMM) in which lateral lipid diffusion has been significantly enhanced without compromising atomic details. The model allows for efficient sampling of lipid-protein interactions at atomic resolution, thereby significantly enhancing the effectiveness of molecular dynamics simulations in capturing membrane-associated phenomena. In this review, after providing an overview of HMMM model development, we will describe briefly successful application of the model to study a variety of membrane processes, including lipid-dependent binding and insertion of peripheral proteins, the mechanism of phospholipid insertion into lipid bilayers, and characterization of optimal tilt angle of transmembrane helices. We conclude with practical recommendations for proper usage of the model in simulation studies of membrane processes.

Sec17 can trigger fusion of trans-SNARE paired membranes without Sec18

Sec17 [soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein; α-SNAP] and Sec18 (NSF) perform ATP-dependent disassembly of cis-SNARE complexes, liberating SNAREs for subsequent assembly of trans-complexes for fusion. A mutant of Sec17, with limited ability to stimulate Sec18, still strongly enhanced fusion when ample Sec18 was supplied, suggesting that Sec17 has additional functions. We used fusion reactions where the four SNAREs were initially separate, thus requiring no disassembly by Sec18. With proteoliposomes bearing asymmetrically disposed SNAREs, tethering and trans-SNARE pairing allowed slow fusion. Addition of Sec17 did not affect the levels of trans-SNARE complex but triggered sudden fusion of trans-SNARE paired proteoliposomes. Sec18 did not substitute for Sec17 in triggering fusion, but ADP- or ATPγS-bound Sec18 enhanced this Sec17 function. The extent of the Sec17 effect varied with the lipid headgroup and fatty acyl composition of the proteoliposomes. Two mutants further distinguished the two Sec17 functions: Sec17(L291A,L292A) did not stimulate Sec18 to disassemble cis-SNARE complex but triggered the fusion of trans-SNARE paired membranes. Sec17(F21S,M22S), with diminished apolar character to its hydrophobic loop, fully supported Sec18-mediated SNARE complex disassembly but had lost the capacity to stimulate the fusion of trans-SNARE paired membranes. To model the interactions of SNARE-bound Sec17 with membranes, we show that Sec17, but not Sec17(F21S,M22S), interacted synergistically with the soluble SNARE domains to enable their stable association with liposomes. We propose a model in which Sec17 binds to trans-SNARE complexes, oligomerizes, and inserts apolar loops into the apposed membranes, locally disturbing the lipid bilayer and thereby lowering the energy barrier for fusion.

Vacuolar SNARE protein transmembrane domains serve as nonspecific membrane anchors with unequal roles in lipid mixing

Membrane fusion is induced by SNARE complexes that are anchored in both fusion partners. SNAREs zipper up from the N to C terminus bringing the two membranes into close apposition. Their transmembrane domains (TMDs) might be mere anchoring devices, deforming bilayers by mechanical force. Structural studies suggested that TMDs might also perturb lipid structure by undergoing conformational transitions or by zipping up into the bilayer. Here, we tested this latter hypothesis, which predicts that the activity of SNAREs should depend on the primary sequence of their TMDs. We replaced the TMDs of all vacuolar SNAREs (Nyv1, Vam3, and Vti1) by a lipid anchor, by a TMD from a protein unrelated to the membrane fusion machinery, or by artificial leucine-valine sequences. Individual exchange of the native SNARE TMDs against an unrelated transmembrane anchor or an artificial leucine-valine sequence yielded normal fusion activities. Fusion activity was also preserved upon pairwise exchange of the TMDs against unrelated peptides, which eliminates the possibility for specific TMD-TMD interactions. Thus, a specific primary sequence or zippering beyond the SNARE domains is not a prerequisite for fusion. Lipid-anchored Vti1 was fully active, and lipid-anchored Nyv1 permitted the reaction to proceed up to hemifusion, and lipid-anchored Vam3 interfered already before hemifusion. The unequal contribution of proteinaceous TMDs on Vam3 and Nyv1 suggests that Q- and R-SNAREs might make different contributions to the hemifusion intermediate and the opening of the fusion pore. Furthermore, our data support the view that SNARE TMDs serve as nonspecific membrane anchors in vacuole fusion.

Fast, Ca2+-dependent exocytosis at nerve terminals: shortcomings of SNARE-based models

Investigations over the last two decades have made major inroads in clarifying the cellular and molecular events that underlie the fast, synchronous release of neurotransmitter at nerve endings. Thus, appreciable progress has been made in establishing the structural features and biophysical properties of the calcium (Ca2+) channels that mediate the entry into nerve endings of the Ca2+ ions that trigger neurotransmitter release. It is now clear that presynaptic Ca2+ channels are regulated at many levels and the interplay of these regulatory mechanisms is just beginning to be understood. At the same time, many lines of research have converged on the conclusion that members of the synaptotagmin family serve as the primary Ca2+ sensors for the action potential-dependent release of neurotransmitter. This identification of synaptotagmins as the proteins which bind Ca2+ and initiate the exocytotic fusion of synaptic vesicles with the plasma membrane has spurred widespread efforts to reveal molecular details of synaptotagmin's action. Currently, most models propose that synaptotagmin interfaces directly or indirectly with SNARE (soluble, N-ethylmaleimide sensitive factor attachment receptors) proteins to trigger membrane fusion. However, in spite of intensive efforts, the field has not achieved consensus on the mechanism by which synaptotagmins act. Concurrently, the precise sequence of steps underlying SNARE-dependent membrane fusion remains controversial. This review considers the pros and cons of the different models of SNARE-mediated membrane fusion and concludes by discussing a novel proposal in which synaptotagmins might directly elicit membrane fusion without the intervention of SNARE proteins in this final fusion step.

Investigation of the juxtamembrane region of neuronal-Synaptobrevin in synaptic transmission at the Drosophila neuromuscular junction

In this study, the juxtamembrane region of the Drosophila SNARE protein neuronal-Synaptobrevin (n-Syb) was tested for its role in synaptic transmission. A transgenic approach was used to express n-Syb mutant genes. The transgenes carried engineered point mutations that alter the amino acid sequence of the conserved tryptophan residues in the juxtamembrane sequence. Such transgenes were expressed in an n-syb hypomorphic background, which produces little endogenous protein. On their own, hypomorphic flies displayed severe motor inhibition, limited life span, reduced evoked junctional potentials (EJPs), decreased synchronicity in EJP time to peak, and potentiation of EJPs with 10-Hz stimulation. All of these deficits were restored to wild-type levels with the expression of wild-type transgenic n-syb, regulated by the endogenous promoter (n-syb(WT)). We created transgenic mutants with one additional tryptophan (n-syb(WW)) or one less tryptophan (n-syb(AA)) than the wild-type sequence. While n-syb(WW) resembled n-syb(WT) in all variables listed, n-syb(AA) exhibited decreased EJP amplitude, synchronicity, and quantal content. To determine whether the n-syb juxtamembrane region is important for transduction of force arising from SNARE complex assembly during membrane fusion, we introduced short 6-amino acid (n-syb(L6)) or long 24-amino acid (n-syb(L24)) flexible linkers into the n-syb transgene. We observed a reduced EJP amplitude in n-syb(L6) but not n-syb(L24), while both linker mutants showed a decreased quantal content and an effect on the readily releasable and recycling vesicle pools. In conclusion, mutation of the juxtamembrane region of n-syb deleteriously affected synaptic transmission at the Drosophila neuromuscular junction.

In vitro assay using engineered yeast vacuoles for neuronal SNARE-mediated membrane fusion

Intracellular membrane fusion requires not only SNARE proteins but also other regulatory proteins such as the Rab and Sec1/Munc18 (SM) family proteins. Although neuronal SNARE proteins alone can drive the fusion between synthetic liposomes, it remains unclear whether they are also sufficient to induce the fusion of biological membranes. Here, through the use of engineered yeast vacuoles bearing neuronal SNARE proteins, we show that neuronal SNAREs can induce membrane fusion between yeast vacuoles and that this fusion does not require the function of the Rab protein Ypt7p or the SM family protein Vps33p, both of which are essential for normal yeast vacuole fusion. Although excess vacuolar SNARE proteins were also shown to mediate Rab-bypass fusion, this fusion required homotypic fusion and vacuole protein sorting complex, which bears Vps33p and was accompanied by extensive membrane lysis. We also show that this neuronal SNARE-driven vacuole fusion can be stimulated by the neuronal SM protein Munc18 and blocked by botulinum neurotoxin serotype E, a well-known inhibitor of synaptic vesicle fusion. Taken together, our results suggest that neuronal SNARE proteins are sufficient to induce biological membrane fusion, and that this new assay can be used as a simple and complementary method for investigating synaptic vesicle fusion mechanisms.

Familial hemophagocytic lymphohistiocytosis: when rare diseases shed light on immune system functioning

The human immune system depends on the activity of cytotoxic T lymphocytes (CTL), natural killer (NK) cells, and NKT cells in order to fight off a viral infection. Understanding the molecular mechanisms during this process and the role of individual proteins was greatly improved by the study of familial hemophagocytic lymphohistiocytosis (FHL). Since 1999, genetic sequencing is the gold standard to classify patients into different subgroups of FHL. The diagnosis, once based on a clinical constellation of abnormalities, is now strongly supported by the results of a functional flow-cytometry screening, which directs the genetic study. A few additional congenital immune deficiencies can also cause a resembling or even identical clinical picture to FHL. As in many other rare human disorders, the collection and analysis of a relatively large number of cases in registries is crucial to draw a complete picture of the disease. The conduction of prospective therapeutic trials allows investigators to increase the awareness of the disease and to speed up the diagnostic process, but also provides important functional and genetic confirmations. Children with confirmed diagnosis may undergo hematopoietic stem cell transplantation, which is the only cure known to date. Moreover, detailed characterization of these rare patients helped to understand the function of individual proteins within the exocytic machinery of CTL, NK, and NKT cells. Moreover, identification of these genotypes also provides valuable information on variant phenotypes, other than FHL, associated with biallelic and monoallelic mutations in the FHL-related genes. In this review, we describe how detailed characterization of patients with genetic hemophagocytic lymphohistiocytosis has resulted in improvement in knowledge regarding contribution of individual proteins to the functional machinery of cytotoxic T- and NK-cells. The review also details how identification of these genotypes has provided valuable information on variant phenotypes.

Milk secretion: The role of SNARE proteins

During lactation, polarized mammary epithelial secretory cells (MESCs) secrete huge quantities of the nutrient molecules that make up milk, i.e. proteins, fat globules and soluble components such as lactose and minerals. Some of these nutrients are only produced by the MESCs themselves, while others are to a great extent transferred from the blood. MESCs can thus be seen as a crossroads for both the uptake and the secretion with cross-talks between intracellular compartments that enable spatial and temporal coordination of the secretion of the milk constituents. Although the physiology of lactation is well understood, the molecular mechanisms underlying the secretion of milk components remain incompletely characterized. Major milk proteins, namely caseins, are secreted by exocytosis, while the milk fat globules are released by budding, being enwrapped by the apical plasma membrane. Prolactin, which stimulates the transcription of casein genes, also induces the production of arachidonic acid, leading to accelerated casein transport and/or secretion. Because of their ability to form complexes that bridge two membranes and promote their fusion, SNARE (Soluble N-ethylmaleimide-Sensitive Factor Attachment Protein Receptor) proteins are involved in almost all intracellular trafficking steps and exocytosis. As SNAREs can bind arachidonic acid, they could be the effectors of the secretagogue effect of prolactin in MESCs. Indeed, some SNAREs have been observed between secretory vesicles and lipid droplets suggesting that these proteins could not only orchestrate the intracellular trafficking of milk components but also act as key regulators for both the coupling and coordination of milk product secretion in response to hormones.

Membranes linked by trans-SNARE complexes require lipids prone to non-bilayer structure for progression to fusion

Like other intracellular fusion events, the homotypic fusion of yeast vacuoles requires a Rab GTPase, a large Rab effector complex, SNARE proteins which can form a 4-helical bundle, and the SNARE disassembly chaperones Sec17p and Sec18p. In addition to these proteins, specific vacuole lipids are required for efficient fusion in vivo and with the purified organelle. Reconstitution of vacuole fusion with all purified components reveals that high SNARE levels can mask the requirement for a complex mixture of vacuole lipids. At lower, more physiological SNARE levels, neutral lipids with small headgroups that tend to form non-bilayer structures (phosphatidylethanolamine, diacylglycerol, and ergosterol) are essential. Membranes without these three lipids can dock and complete trans-SNARE pairing but cannot rearrange their lipids for fusion.

DOI:http://dx.doi.org/10.7554/eLife.01879.001

All cells are enclosed with a membrane that is made of phospholipid molecules, and many of the structures found inside cells—such as the vacuoles in plant and fungal cells—are also enclosed with a phospholipid membrane. To form a membrane, the phospholipid molecules—which have a phosphate head and two fatty acid tails—arrange themselves in two layers, with the fatty acid tails pointing into the membrane, and the phosphate heads pointing outwards. This structure is known as a phospholipid bilayer.

Vacuoles are filled with water that contains various proteins and molecules in solution, and adjust their volume to keep the concentrations of substances in the cell in balance. To do this, the vacuoles fuse with each other. This fusion process requires dramatic spatial rearrangements of the phospholipid molecules.

The SNARE family of proteins plays a key role in membrane fusion. As the two membranes come together, SNARE proteins located on each membrane form a complex known as a trans-SNARE complex. This docks the vacuole in place beside another vacuole while the phospholipid molecules in the two membranes rearrange. However, much less is known about the phospholipid molecules that are involved in the fusion process.

Now, Zick et al. have shown that three types of phospholipid molecules must be present for membrane fusion to be completed. These have in common that their phosphate ‘headgroups’ are small and they do not tend to form bilayers. The vacuoles can dock beside each other if these small headgroup phospholipid molecules are not present, but the bilayer lipids in the vacuole membranes cannot rearrange themselves in the absence of these particular lipids.

The importance of these nonbilayer lipid molecules had not previously been established, as the majority of experiments investigating membrane fusion used concentrations of SNARE proteins that were much higher than those found physiologically. At such high concentrations, fusion can go ahead without the nonbilayer lipid molecules being present.

DOI:http://dx.doi.org/10.7554/eLife.01879.002

Synthesis of ω-hydroxy dodecanoic acid based on an engineered CYP153A fusion construct

A bacterial P450 monooxygenase-based whole cell biocatalyst using Escherichia coli has been applied in the production of ω-hydroxy dodecanoic acid from dodecanoic acid (C12-FA) or the corresponding methyl ester. We have constructed and purified a chimeric protein where the fusion of the monooxygenase CYP153A from Marinobacter aquaeloei to the reductase domain of P450 BM3 from Bacillus megaterium ensures optimal protein expression and efficient electron coupling. The chimera was demonstrated to be functional and three times more efficient than other sets of redox components evaluated. The established fusion protein (CYP153A_{M. aq.}-CPR) was used for the hydroxylation of C12-FA in in vivo studies. These experiments yielded 1.2 g l^–1 ω-hydroxy dodecanoic from 10 g l^–1 C12-FA with high regioselectivity (> 95%) for the terminal position. As a second strategy, we utilized C12-FA methyl ester as substrate in a two-phase system (5:1 aqueous/organic phase) configuration to overcome low substrate solubility and product toxicity by continuous extraction. The biocatalytic system was further improved with the coexpression of an additional outer membrane transport system (AlkL) to increase the substrate transfer into the cell, resulting in the production of 4 g l^–1 ω-hydroxy dodecanoic acid. We further summarized the most important aspects of the whole-cell process and thereupon discuss the limits of the applied oxygenation reactions referring to hydrogen peroxide, acetate and P450 concentrations that impact the efficiency of the production host negatively.

Preparation and characterization of SNARE-containing nanodiscs and direct study of cargo release through fusion pores

This protocol describes an assay that uses suspended nanomembranes called nanodiscs to analyze fusion events. A nanodisc is a lipid bilayer wrapped by membrane scaffold proteins. Fluorescent lipids and a protein that is part of a fusion machinery, VAMP2 in the example detailed herein, are included in the nanodiscs. Upon fusion of a nanodisc with a nonfluorescent liposome containing cognate proteins (for instance, the VAMP2 cognate syntaxin1/SNAP-25 complex), the fluorescent lipids are dispersed in the liposome and the increase in fluorescence, initially quenched in the nanodisc, is monitored on a plate reader. Because the scaffold proteins restrain pore expansion, the fusion pore eventually reseals. A reducing agent, such as dithionite, which can quench the fluorescence of accessible lipids, can then be used to determine the number of fusion events. A fluorescence-based approach can also be used to monitor the release of encapsulated cargo. From data on the total cargo release and the number of the much faster lipid-mixing events, the researcher may determine the amount of cargo released per fusion event. This assay requires 3 d for preparation and 4 h for data acquisition and analysis.

Distinct Initial SNARE Configurations Underlying the Diversity of Exocytosis

The dynamics of exocytosis are diverse and have been optimized for the functions of synapses and a wide variety of cell types. For example, the kinetics of exocytosis varies by more than five orders of magnitude between ultrafast exocytosis in synaptic vesicles and slow exocytosis in large dense-core vesicles. However, in all cases, exocytosis is mediated by the same fundamental mechanism, i.e., the assembly of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. It is often assumed that vesicles need to be docked at the plasma membrane and SNARE proteins must be preassembled before exocytosis is triggered. However, this model cannot account for the dynamics of exocytosis recently reported in synapses and other cells. For example, vesicles undergo exocytosis without prestimulus docking during tonic exocytosis of synaptic vesicles in the active zone. In addition, epithelial and hematopoietic cells utilize cAMP and kinases to trigger slow exocytosis of nondocked vesicles. In this review, we summarize the manner in which the diversity of exocytosis reflects the initial configurations of SNARE assembly, including trans-SNARE, binary-SNARE, unitary-SNARE, and cis-SNARE configurations. The initial SNARE configurations depend on the particular SNARE subtype (syntaxin, SNAP25, or VAMP), priming proteins (Munc18, Munc13, CAPS, complexin, or snapin), triggering proteins (synaptotagmins, Doc2, and various protein kinases), and the submembraneous cytomatrix, and they are the key to determining the kinetics of subsequent exocytosis. These distinct initial configurations will help us clarify the common SNARE assembly processes underlying exocytosis and membrane trafficking in eukaryotic cells.

Effect of juxtamembrane tryptophans on the immersion depth of Synaptobrevin, an integral vesicle membrane protein

Proper positioning of membrane proteins in the host membrane is often critical to successful protein function. While hydrophobic considerations play a dominant role in determining the topology of a protein in the membrane, amphiphilic residues, such as tryptophan, may 'anchor' the protein near the water-membrane interface. The SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) family of membrane proteins mediates intracellular membrane fusion. Correct positioning of the SNAREs is necessary if fusion is to occur. Synaptobrevins are integral vesicle membrane proteins that are well conserved across species. Interestingly, mammalian Synaptobrevins typically contain two adjacent tryptophans near the water-membrane interface whereas the Drosophila, neuronal-Synaptobrevin (n-Syb), contains a single tryptophan in this same region. To explore the role of these tryptophan residues in membrane positioning, we prepared a peptide containing residues 75-121 of D. melanogaster n-Syb in DPC micelles, biosynthetically labeled with 4-fluorophenylalanine and 5-fluorotryptophan for the examination by ¹⁹F NMR spectroscopy. Mutations of this construct containing zero and two tryptophan residues near the water-membrane interface resulted in changes in the positioning of n-Syb in the micelle. Moreover, the addition of a second tryptophan appears to slow dynamic motions of n-Syb near the micelle-water interface. These data therefore indicate that juxtamembrane tryptophan residues are important determinants of the position of Synaptobrevin in the membrane.

Membrane fusion intermediates via directional and full assembly of the SNARE complex

Cellular membrane fusion is thought to proceed through intermediates including docking of apposed lipid bilayers, merging of proximal leaflets to form a hemifusion diaphragm, and fusion pore opening. A membrane-bridging four-helix complex of soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) mediates fusion. However, how assembly of the SNARE complex generates docking and other fusion intermediates is unknown. Using a cell-free reaction, we identified intermediates visually and then arrested the SNARE fusion machinery when fusion was about to begin. Partial and directional assembly of SNAREs tightly docked bilayers, but efficient fusion and an extended form of hemifusion required assembly beyond the core complex to the membrane-connecting linkers. We propose that straining of lipids at the edges of an extended docking zone initiates fusion.

Export requirements of pneumolysin in Streptococcus pneumoniae

Streptococcus pneumoniae is a major causative agent of otitis media, pneumonia, bacteremia, and meningitis. Pneumolysin (Ply), a member of the cholesterol-dependent cytolysins (CDCs), is produced by virtually all clinical isolates of S. pneumoniae, and ply mutant strains are severely attenuated in mouse models of colonization and infection. In contrast to all other known members of the CDC family, Ply lacks a signal peptide for export outside the cell. Instead, Ply has been hypothesized to be released upon autolysis or, alternatively, via a nonautolytic mechanism that remains undefined. We show that an exogenously added signal sequence is not sufficient for Sec-dependent Ply secretion in S. pneumoniae but is sufficient in the surrogate host Bacillus subtilis. Previously, we showed that Ply is localized primarily to the cell wall compartment in the absence of detectable cell lysis. Here we show that Ply released by autolysis cannot reassociate with intact cells, suggesting that there is a Ply export mechanism that is coupled to cell wall localization of the protein. This putative export mechanism is capable of secreting a related CDC without its signal sequence. We show that B. subtilis can export Ply, suggesting that the export pathway is conserved. Finally, through truncation and domain swapping analyses, we show that export is dependent on domain 2 of Ply.

SNARE requirements en route to exocytosis: from many to few

Although it has been known for almost two decades that the ternary complex of N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) constitutes the functional unit driving membrane fusion, our knowledge about the dynamical arrangement and organization of SNARE proteins and their complexes before and during vesicle exocytosis is still limited. Here, we review recent progress in this expanding field with emphasis on the question of fusion complex stoichiometry, i.e., how many SNARE proteins and complexes are needed for the fusion of a vesicle with the plasma membrane.

Beyond anchoring: the expanding role of the hendra virus fusion protein transmembrane domain in protein folding, stability, and function

While work with viral fusion proteins has demonstrated that the transmembrane domain (TMD) can affect protein folding, stability, and membrane fusion promotion, the mechanism(s) remains poorly understood. TMDs could play a role in fusion promotion through direct TMD-TMD interactions, and we have recently shown that isolated TMDs from three paramyxovirus fusion (F) proteins interact as trimers using sedimentation equilibrium (SE) analysis (E. C. Smith, et al., submitted for publication). Immediately N-terminal to the TMD is heptad repeat B (HRB), which plays critical roles in fusion. Interestingly, addition of HRB decreased the stability of the trimeric TMD-TMD interactions. This result, combined with previous findings that HRB forms a trimeric coiled coil in the prefusion form of the whole protein though HRB peptides fail to stably associate in isolation, suggests that the trimeric TMD-TMD interactions work in concert with elements in the F ectodomain head to stabilize a weak HRB interaction. Thus, changes in TMD-TMD interactions could be important in regulating F triggering and refolding. Alanine insertions between the TMD and HRB demonstrated that spacing between these two regions is important for protein stability while not affecting TMD-TMD interactions. Additional mutagenesis of the C-terminal end of the TMD suggests that β-branched residues within the TMD play a role in membrane fusion, potentially through modulation of TMD-TMD interactions. Our results support a model whereby the C-terminal end of the Hendra virus F TMD is an important regulator of TMD-TMD interactions and show that these interactions help hold HRB in place prior to the triggering of membrane fusion.

Expression of the Longin domain of TI-VAMP impairs lysosomal secretion and epithelial cell migration

BACKGROUND INFORMATION

TI-VAMP (tetanus neurotoxin-insensitive vesicle-associated membrane protein; also called VAMP7) belongs to the Longin subfamily of v-SNAREs (vesicular soluble N-ethylmaleimide-sensitive fusion protein-attachment protein receptors). The regulatory N-terminal extension, called the Longin domain, of TI-VAMP has been shown previously to have a dual biochemical function: it inhibits the capacity of TI-VAMP to form SNARE complexes and it binds to the delta subunit of the AP-3 (adaptor protein 3) complex in early endosomes, thereby targeting TI-VAMP to late endosomes.

RESULTS

We have generated MDCK (Madin-Darby canine kidney) cell lines expressing the Longin domain of TI-VAMP coupled to GFP (green fluorescent protein) in a doxycycline-dependent manner. As expected, AP-3delta (AP-3 delta subunit) is not properly localized in Longin-expressing cells. We have shown that the expression of the Longin domain impairs lysosomal secretion, as determined by the release of a pre-internalized fluorescent fluid-phase marker and by electron microscopy of the membrane-associated released particles. Membrane repair following mechanical wounding, a process requiring lysosomal secretion, is also impaired in cells expressing the Longin domain. Furthermore, cell migration, assessed by wound healing of MDCK monolayers, is also inhibited.

CONCLUSIONS

The results of the present study suggest that the expression of the Longin domain of TI-VAMP regulates lysosomal secretion of epithelial cells and provide molecular evidence for a role of the late endocytic system in cell migration.

A lipid-anchored SNARE supports membrane fusion

Intracellular membrane fusion requires R-SNAREs and Q-SNAREs to assemble into a four-helical parallel coiled-coil, with their hydrophobic anchors spanning the two apposed membranes. Based on the fusion properties of chemically defined SNARE- proteoliposomes, it has been proposed that the assembly of this helical bundle transduces force through the entire bilayer via the transmembrane SNARE anchor domains to drive fusion. However, an R-SNARE, Nyv1p, with a genetically engineered lipid anchor that spans half of the bilayer suffices for the fusion of isolated vacuoles, although this organelle has other R-SNAREs. To demonstrate unequivocally the fusion activity of lipid-anchored Nyv1p, we reconstituted proteoliposomes with purified lipid-anchored Nyv1p as the only protein. When these proteoliposomes were incubated with those bearing cognate Q-SNAREs, there was trans-SNARE complex assembly but, in accord with prior studies of the neuronal SNAREs, little lipid mixing. However, the addition of physiological fusion accessory proteins (HOPS, Sec17p, and Sec18p) allows lipid-anchored Nyv1p to support fusion, suggesting that trans-SNARE complex function is not limited to force transduction across the bilayers through the transmembrane domains.

GTP-dependent packing of a three-helix bundle is required for atlastin-mediated fusion

The mechanisms governing atlastin-mediated membrane fusion are unknown. Here we demonstrate that a three-helix bundle (3HB) within the middle domain is required for oligomerization. Mutation of core hydrophobic residues within these helices inactivates atlastin function by preventing membrane tethering and the subsequent fusion. GTP binding induces a conformational change that reorients the GTPase domain relative to the 3HB to permit self-association, but the ability to hydrolyze GTP is required for full fusion, indicating that nucleotide binding and hydrolysis play distinct roles. Oligomerization of atlastin stimulates its ability to hydrolyze GTP, and the energy released drives lipid bilayer merger. Mutations that prevent atlastin self-association also abolish oligomerization-dependent stimulation of GTPase activity. Furthermore, increasing the distance of atlastin complex formation from the membrane inhibits fusion, suggesting that this distance is crucial for atlastin to promote fusion.

Docking, not fusion, as the rate-limiting step in a SNARE-driven vesicle fusion assay

In vitro vesicle fusion assays that monitor lipid mixing between t-SNARE and v-SNARE vesicles in bulk solution exhibit remarkably slow fusion on the nonphysiological timescale of tens of minutes to several hours. Here, single-vesicle, fluorescence resonance energy transfer-based assays cleanly separate docking and fusion steps for individual vesicle pairs containing full-length SNAREs. Docking is extremely inefficient and is the rate-limiting step. Of importance, the docking and fusion kinetics are comparable in the two assays (one with v-SNARE vesicles tethered to a surface and the other with v-SNARE vesicles free in solution). Addition of the V(C) peptide synaptobrevin-2 (syb(57-92)) increases the docking efficiency by a factor of ∼30, but docking remains rate-limiting. In the presence of V(C) peptide, the fusion step occurs on a timescale of ∼10 s. In previous experiments involving bulk fusion assays in which the addition of synaptotagmin/Ca(2+), Munc-18, or complexin accelerated the observed lipid-mixing rate, the enhancement may have arisen from the docking step rather than the fusion step.

Fusing a lasting relationship between ER tubules

Atlastin is an integral membrane GTPase localized to the endoplasmic reticulum (ER). In vitro and in vivo analyses indicate that atlastin is a membrane fusogen capable of driving membrane fusion, suggesting a role in ER structure and maintenance. Interestingly, mutations in the human atlastin-1 gene, SPG3A, cause a form of autosomal dominant hereditary spastic paraplegia (HSP). The etiology of HSP is unclear, but two predominant forms of the disorder are caused by mutant proteins that affect ER structure, formation and maintenance in motor neurons. In this review, we describe the current knowledge about the molecular mechanism of atlastin function and its potential role in HSP. Greater understanding of the function of atlastin and associated proteins should provide important insight into normal ER biogenesis and maintenance, as well as the pathology of disease.

Endophilin functions as a membrane-bending molecule and is delivered to endocytic zones by exocytosis

Two models have been proposed for endophilin function in synaptic vesicle (SV) endocytosis. The scaffolding model proposes that endophilin's SH3 domain recruits essential endocytic proteins, whereas the membrane-bending model proposes that the BAR domain induces positively curved membranes. We show that mutations disrupting the scaffolding function do not impair endocytosis, whereas those disrupting membrane bending cause significant defects. By anchoring endophilin to the plasma membrane, we show that endophilin acts prior to scission to promote endocytosis. Despite acting at the plasma membrane, the majority of endophilin is targeted to the SV pool. Photoactivation studies suggest that the soluble pool of endophilin at synapses is provided by unbinding from the adjacent SV pool and that the unbinding rate is regulated by exocytosis. Thus, endophilin participates in an association-dissociation cycle with SVs that parallels the cycle of exo- and endocytosis. This endophilin cycle may provide a mechanism for functionally coupling endocytosis and exocytosis.

SNARE force synchronizes synaptic vesicle fusion and controls the kinetics of quantal synaptic transmission

Neuronal communication relies on rapid and discrete intercellular signaling but neither the molecular mechanisms of the exocytotic machinery that define the timing of the action potential-evoked response nor those controlling the kinetics of transmitter release from single synaptic vesicles are known. Here, we investigate how interference with the putative force transduction between the complex-forming SNARE (soluble N-ethylamide-sensitive factor attachment protein receptor) domain and the transmembrane anchor of synaptobrevin II (SybII) affects action potential-evoked currents and spontaneous, quantal transmitter release at mouse hippocampal synapses. The results indicate that SybII-generated membrane stress effectively determines the kinetics of the action potential-evoked response and show that SNARE force modulates the concentration profile of cleft glutamate by controlling the rate of transmitter release from the single synaptic vesicle. Thus, multiple SybII actions determine the exquisite temporal regulation of neuronal signaling.

Protein-driven membrane stresses in fusion and fission

Cellular membranes undergo continuous remodeling. Exocytosis and endocytosis, mitochondrial fusion and fission, entry of enveloped viruses into host cells and release of the newly assembled virions, cell-to-cell fusion and cell division, and budding and fusion of transport carriers all proceed via topologically similar, but oppositely ordered, membrane rearrangements. The biophysical similarities and differences between membrane fusion and fission become more evident if we disregard the accompanying biological processes and consider only remodeling of the lipid bilayer. The forces that determine the bilayer propensity to undergo fusion or fission come from proteins and in most cases from membrane-bound proteins. In this review, we consider the mechanistic principles underlying the fusion and fission reactions and discuss the current hypotheses on how specific proteins act in the two types of membrane remodeling.

Opposing functions of two sub-domains of the SNARE-complex in neurotransmission

The SNARE-complex consisting of synaptobrevin-2/VAMP-2, SNAP-25 and syntaxin-1 is essential for evoked neurotransmission and also involved in spontaneous release. Here, we used cultured autaptic hippocampal neurons from Snap-25 null mice rescued with mutants challenging the C-terminal, N-terminal and middle domains of the SNARE-bundle to dissect out the involvement of these domains in neurotransmission. We report that the stabilities of two different sub-domains of the SNARE-bundle have opposing functions in setting the probability for both spontaneous and evoked neurotransmission. Destabilizing the C-terminal end of the SNARE-bundle abolishes spontaneous neurotransmitter release and reduces evoked release probability, indicating that the C-terminal end promotes both modes of release. In contrast, destabilizing the middle or deleting the N-terminal end of the SNARE-bundle increases both spontaneous and evoked release probabilities. In both cases, spontaneous release was affected more than evoked neurotransmission. In addition, the N-terminal deletion delays vesicle priming after a high-frequency train. We propose that the stability of N-terminal two-thirds of the SNARE-bundle has a function for vesicle priming and limiting spontaneous release.

SNARE complex zipping as a driving force in the dilation of proteinaceous fusion pores

The assembly of SNARE proteins into a tight complex has been hypothesized to drive membrane fusion. A model of the initial fusion pore as a proteinaceous channel formed by SNARE proteins places their membrane anchors in separate membranes. This leaves the possibility of a final assembly step that brings the membrane anchors together and drives fusion pore expansion. The present study develops a model for expansion in which the final SNARE complex zipping step drives a transition from a proteinaceous fusion pore to a lipidic fusion pore. An estimate of the energy released upon merger of the helical segments of the SNARE motifs with the helical segments of the membrane anchors indicates that completing the assembly of a few SNARE complexes can overcome the elastic energy that opposes lipid bilayer deformation into a narrow fusion pore. The angle between the helical axes of the membrane anchor and SNARE motif serves as a useful reaction coordinate for this transition. Energy was calculated as a function of this angle, incorporating contributions from membrane bending, SNARE complex assembly, membrane anchor flexing and hydrophobic interactions. The rate of this transition was evaluated as a process of diffusion over the barrier imposed by these combined energies, and the rates estimated were consistent with experimental measurements.