Close is not enough: SNARE-dependent membrane fusion requires an active mechanism that transduces force to membrane anchors

Is membrane fusion an essentially passive or an active process? It could be that fusion proteins simply need to pin two bilayers together long enough, and the bilayers could do the rest spontaneously. Or, it could be that the fusion proteins play an active role after pinning two bilayers, exerting force in the bilayer in one or another way to direct the fusion process. To distinguish these alternatives, we replaced one or both of the peptidic membrane anchors of exocytic vesicle (v)- and target membrane (t)-SNAREs (soluble N-ethylmaleimide-sensitive fusion protein [NSF] attachment protein [SNAP] receptor) with covalently attached lipids. Replacing either anchor with a phospholipid prevented fusion of liposomes by the isolated SNAREs, but still allowed assembly of trans-SNARE complexes docking vesicles. This result implies an active mechanism; if fusion occurred passively, simply holding the bilayers together long enough would have been sufficient. Studies using polyisoprenoid anchors ranging from 15–55 carbons and multiple phospholipid-containing anchors reveal distinct requirements for anchors of v- and t-SNAREs to function: v-SNAREs require anchors capable of spanning both leaflets, whereas t-SNAREs do not, so long as the anchor is sufficiently hydrophobic. These data, together with previous results showing fusion is inhibited as the length of the linker connecting the helical bundle-containing rod of the SNARE complex to the anchors is increased (McNew, J.A., T. Weber, D.M. Engelman, T.H. Sollner, and J.E. Rothman, 1999. Mol. Cell. 4:415–421), suggests a model in which one activity of the SNARE complex promoting fusion is to exert force on the anchors by pulling on the linkers. This motion would lead to the simultaneous inward movement of lipids from both bilayers, and in the case of the v-SNARE, from both leaflets.     

α-SNAP Interferes with the Zippering of the SNARE Protein Membrane Fusion Machinery

Neuronal exocytosis is mediated by soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. Before fusion, SNARE proteins form complexes bridging the membrane followed by assembly toward the C-terminal membrane anchors, thus initiating membrane fusion. After fusion, the SNARE complex is disassembled by the AAA-ATPase N-ethylmaleimide-sensitive factor that requires the cofactor α-SNAP to first bind to the assembled SNARE complex. Using chromaffin granules and liposomes we now show that α-SNAP on its own interferes with the zippering of membrane-anchored SNARE complexes midway through the zippering reaction, arresting SNAREs in a partially assembled trans-complex and preventing fusion. Intriguingly, the interference does not result in an inhibitory effect on synaptic vesicles, suggesting that membrane properties also influence the final outcome of α-SNAP interference with SNARE zippering. We suggest that binding of α-SNAP to the SNARE complex affects the ability of the SNARE complex to harness energy or transmit force to the membrane.     

SNARE‐mediated membrane fusion arrests at pore expansion to regulate the volume of an organelle

Constitutive membrane fusion within eukaryotic cells is thought to be controlled at its initial steps, membrane tethering and SNARE complex assembly, and to rapidly proceed from there to full fusion. Although theory predicts that fusion pore expansion faces a major energy barrier and might hence be a rate‐limiting and regulated step, corresponding states with non‐expanding pores are difficult to assay and have remained elusive. Here, we show that vacuoles in living yeast are connected by a metastable, non‐expanding, nanoscopic fusion pore. This is their default state, from which full fusion is regulated. Molecular dynamics simulations suggest that SNAREs and the SM protein‐containing HOPS complex stabilize this pore against re‐closure. Expansion of the nanoscopic pore to full fusion can thus be triggered by osmotic pressure gradients, providing a simple mechanism to rapidly adapt organelle volume to increases in its content. Metastable, nanoscopic fusion pores are then not only a transient intermediate but can be a long‐lived, physiologically relevant and regulated state of SNARE‐dependent membrane fusion.     

Evidence that late-endosomal SNARE multimerization complex is promoted by transmembrane segments

Assembly of SNARE proteins into quaternary complexes is a critical step in membrane docking and fusion. Here, we have studied the influence of the transmembrane segments on formation of the late endosomal SNARE complex. The complex was assembled in vitro from full-length recombinant SNAREs and from mutants, where the transmembrane segments were either deleted or replaced by oligo-alanine sequences. We show that endobrevin, syntaxin 7, syntaxin 8, and vti1b readily form a complex. This complex forms a dimer as well as multimeric structures. Interestingly, the natural transmembrane segments accelerate the conversion of the quaternary complex to the dimeric form and are essential for multimerization. These in vitro results suggest that the transmembrane segments are responsible for supramolecular assembly of the endosomal SNARE complex.     

Evidence that late-endosomal SNARE multimerization complex is promoted by transmembrane segments

Assembly of SNARE proteins into quaternary complexes is a critical step in membrane docking and fusion. Here, we have studied the influence of the transmembrane segments on formation of the late endosomal SNARE complex. The complex was assembled in vitro from full-length recombinant SNAREs and from mutants, where the transmembrane segments were either deleted or replaced by oligo-alanine sequences. We show that endobrevin, syntaxin 7, syntaxin 8, and vti1b readily form a complex. This complex forms a dimer as well as multimeric structures. Interestingly, the natural transmembrane segments accelerate the conversion of the quaternary complex to the dimeric form and are essential for multimerization. These in vitro results suggest that the transmembrane segments are responsible for supramolecular assembly of the endosomal SNARE complex.     

Regulated exocytosis and SNARE function (Review)

The pairing of cognate v- and t-SNAREs between two opposing lipid bilayers drives spontaneous membrane fusion and confers specificity to intracellular membrane trafficking. These fusion events are regulated by a cascade of protein-protein interactions that locally control SNARE activity and complex assembly, determining when and where fusion occurs with high efficiency in vivo. This basic regulation occurs at all transport steps and is mediated by conserved protein families such as Rab proteins and their effectors and Sec1/unc18 proteins. Regulated exocytosis employs auxiliary components that couple the signal (which triggers exocytosis) to the fusion machinery. At the neuronal synapse, munc13 as well as munc18 control SNARE complex assembly. Synaptotagmin and complexin ensure fast synchronous calcium-evoked neurotransmitter release.     

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.     

Regulated exocytosis and SNARE function (Review)

The pairing of cognate v- and t-SNAREs between two opposing lipid bilayers drives spontaneous membrane fusion and confers specificity to intracellular membrane trafficking. These fusion events are regulated by a cascade of protein-protein interactions that locally control SNARE activity and complex assembly, determining when and where fusion occurs with high efficiency in vivo. This basic regulation occurs at all transport steps and is mediated by conserved protein families such as Rab proteins and their effectors and Sec1/unc18 proteins. Regulated exocytosis employs auxiliary components that couple the signal (which triggers exocytosis) to the fusion machinery. At the neuronal synapse, munc13 as well as munc18 control SNARE complex assembly. Synaptotagmin and complexin ensure fast synchronous calcium-evoked neurotransmitter release.     

SNARE assembly and membrane fusion, a kinetic analysis

SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) assembly may promote intracellular membrane fusion, an essential process for vesicular transport in cells. Core complex formation between vesicle-associated SNARE and target membrane SNARE perhaps drives the merging of two membranes into a single bilayer. Using spin-labeling EPR, trans-SNARE complex formation was monitored "locally" at four different core locations of recombinant yeast SNAREs, which are individually reconstituted into phospholipid vesicles. The results indicate that the time scales of core formation are virtually the same at all four locations throughout the core region, indicating the possibility of a single step core assembly, which appears to be somewhat different from what has been postulated by the "zipper" model. The EPR data were then compared with the kinetics of the lipid mixing measured with the fluorescence assay. The analysis suggests that SNARE core assembly occurs on a much faster time scale than the lipid mixing, providing a new insight into the timing of individual events in SNARE-induced membrane fusion.     

SNARE assembly and disassembly exhibit a pronounced hysteresis.

SNARE proteins are essential for intracellular membrane fusion of eukaryotes. Their assembly into stable four-helix bundles bridges membranes and may provide the energy for initiating membrane fusion. In vitro, assembly of soluble SNARE fragments is accompanied by major structural rearrangements that can be described as a folding reaction. The pathways and the thermodynamics of SNARE protein interactions, however, are not known. Here we report that assembly and dissociation of two distantly related SNARE complexes exhibit a marked hysteresis. The assembled and disassembled native states are separated by a kinetic barrier and cannot equilibrate on biologically relevant timescales. We suggest that the hysteresis is a hallmark of all SNARE complexes and that complex assembly and disassembly follow different pathways that may be independently controlled.     

Determinants of synaptobrevin regulation in membranes

Neuronal exocytosis is driven by the formation of SNARE complexes between synaptobrevin 2 on synaptic vesicles and SNAP-25/syntaxin 1 on the plasma membrane. It has remained controversial, however, whether SNAREs are constitutively active or whether they are down-regulated until fusion is triggered. We now show that synaptobrevin in proteoliposomes as well as in purified synaptic vesicles is constitutively active. Potential regulators such as calmodulin or synaptophysin do not affect SNARE activity. Substitution or deletion of residues in the linker connecting the SNARE motif and transmembrane region did not alter the kinetics of SNARE complex assembly or of SNARE-mediated fusion of liposomes. Remarkably, deletion of C-terminal residues of the SNARE motif strongly reduced fusion activity, although the overall stability of the complexes was not affected. We conclude that although complete zippering of the SNARE complex is essential for membrane fusion, the structure of the adjacent linker domain is less critical, suggesting that complete SNARE complex assembly not only connects membranes but also drives fusion.     

Is Assembly of the SNARE Complex Enough to Fuel Membrane Fusion?

The three key players in the exocytotic release of neurotransmitters from synaptic vesicles are the SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins synaptobrevin 2, syntaxin 1a, and SNAP-25. Their assembly into a tight four-helix bundle complex is thought to pull the two membranes into close proximity. It is debated, however, whether the energy generated suffices for membrane fusion. Here, we have determined the thermodynamic properties of the individual SNARE assembly steps by isothermal titration calorimetry. We found extremely large favorable enthalpy changes counterbalanced by positive entropy changes, reflecting the major conformational changes upon assembly. To circumvent the fact that ternary complex formation is essentially irreversible, we used a stabilized syntaxin-SNAP-25 heterodimer to study synaptobrevin binding. This strategy revealed that the N-terminal synaptobrevin coil binds reversibly with nanomolar affinity. This suggests that individual, membrane-bridging SNARE complexes can provide much less pulling force than previously claimed.The molecular machinery that drives the Ca²⁺-dependent release of neurotransmitters from synaptic vesicles is studied intensively. Three key players in the underlying exocytotic fusion of the vesicle with the plasma membrane are the proteins synaptobrevin 2/VAMP2 (vesicle-associated membrane protein), syntaxin 1a, and SNAP-25³ (for review, see Refs. 1-7). They belong to the so-called SNARE protein family, the members of which are involved in all vesicle fusion steps in the endocytic and secretory pathway. In general, SNARE proteins are relatively small, tail-anchored membrane proteins. Their key characteristic is the so-called SNARE motif, an extended stretch of heptad repeats that is usually connected to a single transmembrane domain by a short linker. Syntaxin and synaptobrevin each contain a single SNARE motif, whereas SNAP-25 contains two SNARE motifs connected by a palmitoylated linker region serving as a membrane anchor. The SNARE motifs of the three proteins assemble into a very tight four-helix bundle between opposing membranes; during this process the plasma membrane proteins syntaxin and SNAP-25 provide the binding site for the vesicular synaptobrevin. Formation of this complex is accompanied by extensive structural rearrangements (8-10). Based on these findings, it was put forward that the formation of the SNARE bundle provides the energy that drives membrane fusion. As the bundle is oriented in parallel, it is thought that formation of this complex starts from the membrane-proximal N termini and proceeds toward the C-terminal membrane anchors, effectively pulling the membranes together (the “zipper” model) (11). Although the zipper scenario is intuitive, it has been difficult to demonstrate directly.A decade ago it was shown that the three neuronal SNARE proteins are sufficient to fuse artificial vesicles (12). However, this reductionist approach yields rather slow fusion rates (12-14). Over the years various different end products of SNARE catalysis (complete fusion, hemifusion, and only tethered membranes) have been reported (15-19). These unsatisfactory results have fueled the debate over whether the assembly process indeed provides enough impetus to fuse bilayers. Not surprisingly, an alternative scenario has been put forward in which repulsive forces between membranes bring the SNARE assembly to a grinding halt. According to this idea, other factors like the Ca²⁺ sensor synaptotagmin or the small soluble protein complexin are needed to induce membrane merging (20-22).In simple terms, to find out whether the SNARE complex assembly is enough for membrane fusion, only the amount of energy released during complex formation and the amount of energy needed for membrane fusion need to be compared. However, the physics of membrane fusion are very complicated, and it is even more challenging to understand how proteins modulate the process. The free energy for bilayer fusion in an aqueous environment is not very high, but fusion is thought to require a large activation energy of about 40 k_BT, as two charged membranes have to be brought into close apposition. According to a theoretical model, the apposing membranes then need to be modified into a stalk-like configuration. Before fusion occurs, the process is thought to pass through a hemifusion intermediate in which only the outer monolayers are merged (for review, see Refs. 23 and 24). The role of fusion proteins is thought to lower the energy barrier for membrane fusion, but understanding how they modulate the lipid membrane and how their conformational changes are translated into a mechanical force is still in its infancy. It is not clear, for instance, whether SNARE-catalyzed fusion indeed proceeds through a stalk-like structure or just locally alters the membranes, a mechanism that might need much less activation energy.As the folding and unfolding transitions of the ternary SNARE complex exhibit a marked hysteresis (25), the question of how much energy is released during complex formation has been difficult to answer as well. To avoid the quasi-irreversibility of the process, the problem has been elegantly tackled by atomic force microscopy by two different research groups (26-28). In these experiments individual complexes affixed to solid supports were ruptured, yielding energy values of 43 and 33 k_BT. In another approach, which used a surface-force apparatus (SFA), a comparable energy of 35 k_BT has recently been determined (29). Strikingly, these values appear to correspond closely with the activation energy needed to fuse two membranes, substantiating the view that SNAREs are nano-fusion machineries. However, one should be cautious about the conclusion that these sophisticated procedures in fact yield the genuine SNARE assembly energy. For example, with the SFA approach, the number of complexes had to be deduced rather indirectly to estimate the free energy. Moreover, these approaches offered only indirect information about the reaction pathway.In this study we set out to determine the SNARE assembly energy more directly by using isothermal titration calorimetry (ITC) complemented by kinetic measurements. ITC is a powerful technique for studying the thermodynamics of macromolecular interactions by directly measuring the heat changes associated with complex formation, which at constant pressure is equal to the enthalpy change (ΔH). The titration approach also yields the stoichiometry (n), the entropy change (ΔS), and the association constant (K_A) of the reaction. We studied the consecutive reaction steps individually to gain deeper insights into the rugged energy landscape of complex formation. To study synaptobrevin binding in isolation, we used a stabilized syntaxin-SNAP-25 heterodimer, which has been shown to greatly accelerate liposome fusion rates (30). This strategy revealed that the N-terminal coil of synaptobrevin binds reversibly, making it feasible to access the free energy of SNARE assembly. Overall, our results suggest that individual SNARE complexes might provide much less pulling energy than previously claimed.     

Role of the Vam3p transmembrane segment in homodimerization and SNARE complex formation

Intracellular membrane fusion in eukaryotic cells is mediated by SNARE (soluble N-ethylmaleimide sensitive factor (NSF) attachment protein receptor) proteins and is known to involve assembly of cognate subunits to heterooligomeric complexes. For synaptic SNAREs, it has previously been shown that the transmembrane segments drive homotypic and support heterotypic interactions. Here, we demonstrate that a significant fraction of the yeast vacuolar SNARE Vam3p is a homodimer in detergent extracts of vacuolar membranes. This homodimer exists in parallel to the heterooligomeric SNARE complex. A Vam3p homodimer also formed from the isolated recombinant protein. Interestingly, homodimerization depended on the transmembrane segment. In contrast, formation of the quaternary SNARE complex from recombinant Vam3p, Nyv1p, Vti1p, and Vam7p subunits did not depend on the transmembrane segment of Vam3p nor on the transmembrane segments of its partner proteins. We conclude that Vam3p homodimerization, but not quaternary SNARE complex formation, is promoted by TMS-TMS interaction. As the transmembrane segments of Vam3p and other SNARE homologues were previously shown to be critical for membrane fusion downstream of membrane apposition, our results may shed light on the functional significance of SNARE TMS-TMS interactions.     

Kinetics of synaptotagmin responses to Ca2+ and assembly with the core SNARE complex onto membranes

The synaptic vesicle protein synaptotagmin I binds Ca2+ and is required for efficient neurotransmitter release. Here, we measure the response time of the C2 domains of synaptotagmin to determine whether synaptotagmin is fast enough to function as a Ca2+ sensor for rapid exocytosis. We report that synaptotagmin is "tuned" to sense Ca2+ concentrations that trigger neuronal exocytosis. The speed of response is unique to synaptotagmin I and readily satisfies the kinetic constraints of synaptic vesicle membrane fusion. We further demonstrate that Ca2+ triggers penetration of synaptotagmin into membranes and simultaneously drives assembly of synaptotagmin onto the base of the ternary SNARE (soluble N-ethylmaleimide-sensitive fusion protein [NSF] attachment receptor) complex, near the transmembrane anchor of syntaxin. These data support a molecular model in which synaptotagmin triggers exocytosis through its interactions with membranes and the SNARE complex.     

Coarse-Grain Simulations Reveal Movement of the Synaptobrevin C-Terminus in Response to Piconewton Forces

Fusion of neurosecretory vesicles with the plasma membrane is mediated by SNARE proteins, which transfer a force to the membranes. However, the mechanism by which this force transfer induces fusion pore formation is still unknown. The neuronal vesicular SNARE protein synaptobrevin 2 (syb2) is anchored in the vesicle membrane by a single C-terminal transmembrane (TM) helix. In coarse-grain molecular-dynamics simulations, self-assembly of the membrane occurred with the syb2 TM domain inserted, as expected from experimental data. The free-energy profile for the position of the syb2 membrane anchor in the membrane was determined using umbrella sampling. To predict the free-energy landscapes for a reaction pathway pulling syb2 toward the extravesicular side of the membrane, which is the direction of the force transfer from the SNARE complex, harmonic potentials were applied to the peptide in its unbiased position, pulling it toward new biased equilibrium positions. Application of piconewton forces to the extravesicular end of the TM helix in the simulation detached the synaptobrevin C-terminus from the vesicle's inner-leaflet lipid headgroups and pulled it deeper into the membrane. This C-terminal movement was facilitated and hindered by specific mutations in parallel with experimentally observed facilitation and inhibition of fusion. Direct application of such forces to the intravesicular end of the TM domain resulted in tilting motion of the TM domain through the membrane with an activation energy of ～70 kJ/mol. The results suggest a mechanism whereby fusion pore formation is induced by movement of the charged syb2 C-terminus within the membrane in response to pulling and tilting forces generated by C-terminal zippering of the SNARE complex.     

Synaptobrevin N-terminally bound to syntaxin–SNAP-25 defines the primed vesicle state in regulated exocytosis

Rapid neurotransmitter release depends on the ability to arrest the SNAP receptor (SNARE)–dependent exocytosis pathway at an intermediate “cocked” state, from which fusion can be triggered by Ca²⁺. It is not clear whether this state includes assembly of synaptobrevin (the vesicle membrane SNARE) to the syntaxin–SNAP-25 (target membrane SNAREs) acceptor complex or whether the reaction is arrested upstream of that step. In this study, by a combination of in vitro biophysical measurements and time-resolved exocytosis measurements in adrenal chromaffin cells, we find that mutations of the N-terminal interaction layers of the SNARE bundle inhibit assembly in vitro and vesicle priming in vivo without detectable changes in triggering speed or fusion pore properties. In contrast, mutations in the last C-terminal layer decrease triggering speed and fusion pore duration. Between the two domains, we identify a region exquisitely sensitive to mutation, possibly constituting a switch. Our data are consistent with a model in which the N terminus of the SNARE complex assembles during vesicle priming, followed by Ca²⁺-triggered C-terminal assembly and membrane fusion.     

Evidence for regulation of ER/Golgi SNARE complex formation by hsc70 chaperones

SNARE proteins control intracellular membrane fusion through formation of membrane-bridging helix bundles of amphipathic SNARE motifs. Repetitive cycles of membrane fusion likely involve repetitive folding/unfolding of the SNARE motif helical structure. Despite these conformational demands, little is known about conformational regulation of SNAREs by other proteins. Here we demonstrate that hsc70 chaperones stimulate in vitro SNARE complex formation among the ER/Golgi SNAREs syntaxin 5, membrin, rbetl and sec22b, under conditions in which assembly is normally inhibited. Thus, molecular chaperones can render the SNARE motif more competent for assembly. Partially purified hsc70 fractions from brain cytosol had higher specific activities than fully purified hsc70, suggesting the involvement of unidentified cofactors. Using chemical crosslinking of cells followed by immunoprecipitation, we found that hsc70 was associated with ER/Golgi SNAREs in vivo. Consistent with a modulatory role for hsc70 in transport, we found that excess hsc70 specifically inhibited ER-to-Golgi transport in permeabilized cells.     

The polybasic juxtamembrane region of Sso1p is required for SNARE function in vivo

Exocytosis in Saccharomyces cerevisiae requires the specific interaction between the plasma membrane t-SNARE complex (Sso1/2p;Sec9p)and a vesicular v-SNARE (Snc1/2p). While SNARE proteins drive membrane fusion, many aspects of SNARE assembly and regulation are ill defined. Plasma membrane syntaxin homologs (including Sso1p) contain a highly charged juxtamembrane region between the transmembrane helix and the "SNARE domain" or core complex domain. We examined this region in vitro and in vivo by targeted sequence modification, including insertions and replacements. These modified Sso1 proteins were expressed as the sole copy of Sso in S. cerevisiae and examined for viability. We found that mutant Sso1 proteins with insertions or duplications show limited function, whereas replacement of as few as three amino acids preceding the transmembrane domain resulted in a nonfunctional SNARE in vivo. Viability is also maintained when two proline residues are inserted in the juxtamembrane of Sso1p, suggesting that helical continuity between the transmembrane domain and the core coiled-coil domain is not absolutely required. Analysis of these mutations in vitro utilizing a reconstituted fusion assay illustrates that the mutant Sso1 proteins are only moderately impaired in fusion. These results suggest that the sequence of the juxtamembrane region of Sso1p is vital for function in vivo, independent of the ability of these proteins to direct membrane fusion.     

Synaptotagmin-1 utilizes membrane bending and SNARE binding to drive fusion pore expansion

In regulated vesicle exocytosis, SNARE protein complexes drive membrane fusion to connect the vesicle lumen with the extracellular space. The triggering of fusion pore formation by Ca²⁺ is mediated by specific isoforms of synaptotagmin (Syt), which employ both SNARE complex and membrane binding. Ca²⁺ also promotes fusion pore expansion and Syts have been implicated in this process but the mechanisms involved are unclear. We determined the role of Ca²⁺-dependent Syt-effector interactions in fusion pore expansion by expressing Syt-1 mutants selectively altered in Ca²⁺-dependent SNARE binding or in Ca²⁺-dependent membrane insertion in PC12 cells that lack vesicle Syts. The release of different-sized fluorescent peptide-EGFP vesicle cargo or the vesicle capture of different-sized external fluorescent probes was used to assess the extent of fusion pore dilation. We found that PC12 cells expressing partial loss-of-function Syt-1 mutants impaired in Ca²⁺-dependent SNARE binding exhibited reduced fusion pore opening probabilities and reduced fusion pore expansion. Cells with gain-of-function Syt-1 mutants for Ca²⁺-dependent membrane insertion exhibited normal fusion pore opening probabilities but the fusion pores dilated extensively. The results indicate that Syt-1 uses both Ca²⁺-dependent membrane insertion and SNARE binding to drive fusion pore expansion.     

Structural transitions in the synaptic SNARE complex during Ca2+-triggered exocytosis

The synaptic SNARE complex is a highly stable four-helix bundle that links the vesicle and plasma membranes and plays an essential role in the Ca(2+)-triggered release of neurotransmitters and hormones. An understanding has yet to be achieved of how this complex assembles and undergoes structural transitions during exocytosis. To investigate this question, we have mutated residues within the hydrophobic core of the SNARE complex along the entire length of all four chains and examined the consequences using amperometry to measure fusion pore opening and dilation. Mutations throughout the SNARE complex reduced two distinct rate processes before fusion pore opening to different degrees. These results suggest that two distinct, fully assembled conformations of the SNARE complex drive transitions leading to open fusion pores. In contrast, a smaller number of mutations that were scattered through the SNARE complex but were somewhat concentrated in the membrane-distal half stabilized open fusion pores. These results suggest that a structural transition within a partially disassembled complex drives the dilation of open fusion pores. The dependence of these three rate processes on position within the SNARE complex does not support vectorial SNARE complex zipping during exocytosis.