Munc18/Syntaxin interaction kinetics control secretory vesicle dynamics

Serine-129 phosphorylation of α-synuclein is an activity-dependent trigger for physiologic protein-protein interactions and synaptic function

Phosphorylation of α-synuclein at the serine-129 site (α-syn Ser129P) is an established pathologic hallmark of synucleinopathies and a therapeutic target. In physiologic states, only a fraction of α-syn is phosphorylated at this site, and most studies have focused on the pathologic roles of this post-translational modification. We found that unlike wild-type (WT) α-syn, which is widely expressed throughout the brain, the overall pattern of α-syn Ser129P is restricted, suggesting intrinsic regulation. Surprisingly, preventing Ser129P blocked activity-dependent synaptic attenuation by α-syn-thought to reflect its normal function. Exploring mechanisms, we found that neuronal activity augments Ser129P, which is a trigger for protein-protein interactions that are necessary for mediating α-syn function at the synapse. AlphaFold2-driven modeling and membrane-binding simulations suggest a scenario where Ser129P induces conformational changes that facilitate interactions with binding partners. Our experiments offer a new conceptual platform for investigating the role of Ser129 in synucleinopathies, with implications for drug development.

Regulation of Syntaxin3B-Mediated Membrane Fusion by T14, Munc18, and Complexin

Retinal neurons that form ribbon-style synapses operate over a wide dynamic range, continuously relaying visual information to their downstream targets. The remarkable signaling abilities of these neurons are supported by specialized presynaptic machinery, one component of which is syntaxin3B. Syntaxin3B is an essential t-SNARE protein of photoreceptors and bipolar cells that is required for neurotransmitter release. It has a light-regulated phosphorylation site in its N-terminal domain at T14 that has been proposed to modulate membrane fusion. However, a direct test of the latter has been lacking. Using a well-controlled in vitro fusion assay, we found that a phosphomimetic T14 syntaxin3B mutation leads to a small but significant enhancement of SNARE-mediated membrane fusion following the formation of the t-SNARE complex. While the addition of Munc18a had only a minimal effect on membrane fusion mediated by SNARE complexes containing wild-type syntaxin3B, a more significant enhancement was observed in the presence of Munc18a when the SNARE complexes contained a syntaxin3B T14 phosphomimetic mutant. Finally, we showed that the retinal-specific complexins (Cpx III and Cpx IV) inhibited membrane fusion mediated by syntaxin3B-containing SNARE complexes in a dose-dependent manner. Collectively, our results establish that membrane fusion mediated by syntaxin3B-containing SNARE complexes is regulated by the T14 residue of syntaxin3B, Munc18a, and Cpxs III and IV.

Super-resolved trajectory-derived nanoclustering analysis using spatiotemporal indexing

Single-molecule localization microscopy techniques are emerging as vital tools to unravel the nanoscale world of living cells by understanding the spatiotemporal organization of protein clusters at the nanometer scale. Current analyses define spatial nanoclusters based on detections but neglect important temporal information such as cluster lifetime and recurrence in "hotspots" on the plasma membrane. Spatial indexing is widely used in video games to detect interactions between moving geometric objects. Here, we use the R-tree spatial indexing algorithm to determine the overlap of the bounding boxes of individual molecular trajectories to establish membership in nanoclusters. Extending the spatial indexing into the time dimension allows the resolution of spatial nanoclusters into multiple spatiotemporal clusters. Using spatiotemporal indexing, we found that syntaxin1a and Munc18-1 molecules transiently cluster in hotspots, offering insights into the dynamics of neuroexocytosis. Nanoscale spatiotemporal indexing clustering (NASTIC) has been implemented as a free and open-source Python graphic user interface.

Clinical variants in Caenorhabditis elegans expressing human STXBP1 reveal a novel class of pathogenic variants and classify variants of uncertain significance

Purpose

Modeling disease variants in animals is useful for drug discovery, understanding disease pathology, and classifying variants of uncertain significance (VUS) as pathogenic or benign.

Methods

Using Clustered Regularly Interspaced Short Palindromic Repeats, we performed a Whole-gene Humanized Animal Model procedure to replace the coding sequence of the animal model's unc-18 ortholog with the coding sequence for the human STXBP1 gene. Next, we used Clustered Regularly Interspaced Short Palindromic Repeats to introduce precise point variants in the Whole-gene Humanized Animal Model-humanized STXBP1 locus from 3 clinical categories (benign, pathogenic, and VUS). Twenty-six phenotypic features extracted from video recordings were used to train machine learning classifiers on 25 pathogenic and 32 benign variants.

Results

Using multiple models, we were able to obtain a diagnostic sensitivity near 0.9. Twenty-three VUS were also interrogated and 8 of 23 (34.8%) were observed to be functionally abnormal. Interestingly, unsupervised clustering identified 2 distinct subsets of known pathogenic variants with distinct phenotypic features; both p.Tyr75Cys and p.Arg406Cys cluster away from other variants and show an increase in swim speed compared with hSTXBP1 worms. This leads to the hypothesis that the mechanism of disease for these 2 variants may differ from most STXBP1-mutated patients and may account for some of the clinical heterogeneity observed in the patient population.

Conclusion

We have demonstrated that automated analysis of a small animal system is an effective, scalable, and fast way to understand functional consequences of variants in STXBP1 and identify variant-specific intensities of aberrant activity suggesting a genotype-to-phenotype correlation is likely to occur in human clinical variations of STXBP1.

Functional regulation of syntaxin-1: An underlying mechanism mediating exocytosis in neuroendocrine cells

The fusion of the secretory vesicle with the plasma membrane requires the assembly of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein complexes formed by synaptobrevin, syntaxin-1, and SNAP-25. Within the pathway leading to exocytosis, the transitions between the "open" and "closed" conformations of syntaxin-1 function as a switch for the fusion of vesicles with the plasma membranes; rapid assembly and disassembly of syntaxin-1 clusters on the plasma membrane provide docking and fusion sites for secretory vesicles in neuroendocrine cells; and the fully zippered trans-SNARE complex, which requires the orderly, rapid and accurate binding of syntaxin-1 to other SNARE proteins, play key roles in triggering fusion. All of these reactions that affect exocytosis under physiological conditions are tightly regulated by multiple factors. Here, we review the current evidence for the involvement of syntaxin-1 in the mechanism of neuroendocrine cell exocytosis, discuss the roles of multiple factors such as proteins, lipids, protein kinases, drugs, and toxins in SNARE complex-mediated membrane fusion, and present an overview of syntaxin-1 mutation-associated diseases with a view to developing novel mechanistic therapeutic targets for the treatment of neuroendocrine disorders.

Vesicle trafficking and vesicle fusion: mechanisms, biological functions, and their implications for potential disease therapy

Intracellular vesicle trafficking is the fundamental process to maintain the homeostasis of membrane-enclosed organelles in eukaryotic cells. These organelles transport cargo from the donor membrane to the target membrane through the cargo containing vesicles. Vesicle trafficking pathway includes vesicle formation from the donor membrane, vesicle transport, and vesicle fusion with the target membrane. Coat protein mediated vesicle formation is a delicate membrane budding process for cargo molecules selection and package into vesicle carriers. Vesicle transport is a dynamic and specific process for the cargo containing vesicles translocation from the donor membrane to the target membrane. This process requires a group of conserved proteins such as Rab GTPases, motor adaptors, and motor proteins to ensure vesicle transport along cytoskeletal track. Soluble N-ethyl-maleimide-sensitive factor (NSF) attachment protein receptors (SNARE)-mediated vesicle fusion is the final process for vesicle unloading the cargo molecules at the target membrane. To ensure vesicle fusion occurring at a defined position and time pattern in eukaryotic cell, multiple fusogenic proteins, such as synaptotagmin (Syt), complexin (Cpx), Munc13, Munc18 and other tethering factors, cooperate together to precisely regulate the process of vesicle fusion. Dysfunctions of the fusogenic proteins in SNARE-mediated vesicle fusion are closely related to many diseases. Recent studies have suggested that stimulated membrane fusion can be manipulated pharmacologically via disruption the interface between the SNARE complex and Ca²⁺ sensor protein. Here, we summarize recent insights into the molecular mechanisms of vesicle trafficking, and implications for the development of new therapeutics based on the manipulation of vesicle fusion.

Conformational change of Syntaxin-3b in regulating SNARE complex assembly in the ribbon synapses

Neurotransmitter release of synaptic vesicles relies on the assembly of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex, consisting of syntaxin and SNAP-25 on the plasma membrane and synaptobrevin on the synaptic vesicle. The formation of the SNARE complex progressively zippers towards the membranes, which drives membrane fusion between the plasma membrane and the synaptic vesicle. However, the underlying molecular mechanism of SNARE complex regulation is unclear. In this study, we investigated the syntaxin-3b isoform found in the retinal ribbon synapses using single-molecule fluorescence resonance energy transfer (smFRET) to monitor the conformational changes of syntaxin-3b that modulate the SNARE complex formation. We found that syntaxin-3b is predominantly in a self-inhibiting closed conformation, inefficiently forming the ternary SNARE complex. Conversely, a phosphomimetic mutation (T14E) at the N-terminal region of syntaxin-3b promoted the open conformation, similar to the constitutively open form of syntaxin LE mutant. When syntaxin-3b is bound to Munc18-1, SNARE complex formation is almost completely blocked. Surprisingly, the T14E mutation of syntaxin-3b partially abolishes Munc18-1 regulation, acting as a conformational switch to trigger SNARE complex assembly. Thus, we suggest a model where the conformational change of syntaxin-3b induced by phosphorylation initiates the release of neurotransmitters in the ribbon synapses.

CK2 Phosphorylation Is Required for Regulation of Syntaxin 1A Activity in Ca2+-Triggered Release in Neuroendocrine Cells

The polybasic juxtamembrane region (5RK) of the plasma membrane neuronal SNARE, syntaxin1A (Syx), was previously shown by us to act as a fusion clamp in PC12 cells, as charge neutralization of 5RK promotes spontaneous and inhibits Ca²⁺-triggered release. Using a Syx-based FRET probe (CSYS), we demonstrated that 5RK is required for a depolarization-induced Ca⁺²-dependent opening (close-to-open transition; CDO) of Syx, which involves the vesicular SNARE synaptobrevin2 and occurs concomitantly with Ca²⁺-triggered release. Here, we investigated the mechanism underlying the CDO requirement for 5RK and identified phosphorylation of Syx at Ser-14 (S14) by casein kinase 2 (CK2) as a crucial molecular determinant. Thus, following biochemical verification that both endogenous Syx and CSYS are constitutively S14 phosphorylated in PC12 cells, dynamic FRET analysis of phospho-null and phospho-mimetic mutants of CSYS and the use of a CK2 inhibitor revealed that the S14 phosphorylation confers the CDO requirement for 5RK. In accord, amperometric analysis of catecholamine release revealed that the phospho-null mutant does not support Ca²⁺-triggered release. These results identify a functionally important CK2 phosphorylation of Syx that is required for the 5RK-regulation of CDO and for concomitant Ca²⁺-triggered release. Further, also spontaneous release, conferred by charge neutralization of 5RK, was abolished in the phospho-null mutant.

Reexamination of N-terminal domains of syntaxin-1 in vesicle fusion from central murine synapses

Syntaxin-1 (STX1) and Munc18-1 are two requisite components of synaptic vesicular release machinery, so much so synaptic transmission cannot proceed in their absence. They form a tight complex through two major binding modes: through STX1’s N-peptide and through STX1’s closed conformation driven by its H_abc- domain. However, physiological roles of these two reportedly different binding modes in synapses are still controversial. Here we characterized the roles of STX1’s N-peptide, H_abc-domain, and open conformation with and without N-peptide deletion using our STX1-null mouse model system and exogenous reintroduction of STX1A mutants. We show, on the contrary to the general view, that the H_abc-domain is absolutely required and N-peptide is dispensable for synaptic transmission. However, STX1A’s N-peptide plays a regulatory role, particularly in the Ca²⁺-sensitivity and the short-term plasticity of vesicular release, whereas STX1’s open conformation governs the vesicle fusogenicity. Strikingly, we also show neurotransmitter release still proceeds when the two interaction modes between STX1A and Munc18-1 are presumably intervened, necessitating a refinement of the conceptualization of STX1A–Munc18-1 interaction.

Protein Phosphorylation in Depolarized Synaptosomes: Dissecting Primary Effects of Calcium from Synaptic Vesicle Cycling

Synaptic transmission is mediated by the regulated exocytosis of synaptic vesicles. When the presynaptic membrane is depolarized by an incoming action potential, voltage-gated calcium channels open, resulting in the influx of calcium ions that triggers the fusion of synaptic vesicles (SVs) with the plasma membrane. SVs are recycled by endocytosis. Phosphorylation of synaptic proteins plays a major role in these processes, and several studies have shown that the synaptic phosphoproteome changes rapidly in response to depolarization. However, it is unclear which of these changes are directly linked to SV cycling and which might regulate other presynaptic functions that are also controlled by calcium-dependent kinases and phosphatases. To address this question, we analyzed changes in the phosphoproteome using rat synaptosomes in which exocytosis was blocked with botulinum neurotoxins (BoNTs) while depolarization-induced calcium influx remained unchanged. BoNT-treatment significantly alters the response of the synaptic phoshoproteome to depolarization and results in reduced phosphorylation levels when compared with stimulation of synaptosomes by depolarization with KCl alone. We dissect the primary Ca2+-dependent phosphorylation from SV-cycling-dependent phosphorylation and confirm an effect of such SV-cycling-dependent phosphorylation events on syntaxin-1a-T21/T23, synaptobrevin-S75, and cannabinoid receptor-1-S314/T322 on exo- and endocytosis in cultured hippocampal neurons.

Phosphorylation of the Retinal Ribbon Synapse Specific t-SNARE Protein Syntaxin3B Is Regulated by Light via a Ca2 +-Dependent Pathway

Neurotransmitter release at retinal ribbon-style synapses utilizes a specialized t-SNARE protein called syntaxin3B (STX3B). In contrast to other syntaxins, STX3 proteins can be phosphorylated in vitro at T14 by Ca^2+/calmodulin-dependent protein kinase II (CaMKII). This modification has the potential to modulate SNARE complex formation required for neurotransmitter release in an activity-dependent manner. To determine the extent to which T14 phosphorylation occurs in vivo in the mammalian retina and characterize the pathway responsible for the in vivo phosphorylation of T14, we utilized quantitative immunofluorescence to measure the levels of STX3 and STX3 phosphorylated at T14 (pSTX3) in the synaptic terminals of mouse retinal photoreceptors and rod bipolar cells (RBCs). Results demonstrate that STX3B phosphorylation at T14 is light-regulated and dependent upon the elevation of intraterminal Ca²⁺. In rod photoreceptor terminals, the ratio of pSTX3 to STX3 was significantly higher in dark-adapted mice, when rods are active, than in light-exposed mice. By contrast, in RBC terminals, the ratio of pSTX3 to STX3 was higher in light-exposed mice, when these terminals are active, than in dark-adapted mice. These results were recapitulated in the isolated eyecup preparation, but only when Ca²⁺ was included in the external medium. In the absence of external Ca²⁺, pSTX3 levels remained low regardless of light/dark exposure. Using the isolated RBC preparation, we next showed that elevation of intraterminal Ca²⁺ alone was sufficient to increase STX3 phosphorylation at T14. Furthermore, both the non-specific kinase inhibitor staurosporine and the selective CaMKII inhibitor AIP inhibited the Ca²⁺-dependent increase in the pSTX3/STX3 ratio in isolated RBC terminals, while in parallel experiments, AIP suppressed RBC depolarization-evoked exocytosis, measured using membrane capacitance measurements. Our data support a novel, illumination-regulated modulation of retinal ribbon-style synapse function in which activity-dependent Ca²⁺ entry drives the phosphorylation of STX3B at T14 by CaMKII, which in turn, modulates the ability to form SNARE complexes required for exocytosis.

Phosphorylation of Syntaxin-1a by casein kinase 2α regulates pre-synaptic vesicle exocytosis from the reserve pool

The t-soluble NSF-attachment protein receptor protein Syntaxin-1a (Stx-1a) is abundantly expressed at pre-synaptic terminals where it plays a critical role in the exocytosis of neurotransmitter-containing synaptic vesicles. Stx-1a is phosphorylated by Casein kinase 2α (CK2α) at Ser14, which has been proposed to regulate the interaction of Stx-1a and Munc-18 to control of synaptic vesicle priming. However, the role of CK2α in synaptic vesicle dynamics remains unclear. Here, we show that CK2α over-expression reduces evoked synaptic vesicle release. Furthermore, shRNA-mediated knockdown of CK2α in primary hippocampal neurons strongly enhanced vesicle exocytosis from the reserve pool, with no effect on the readily releasable pool of primed vesicles. In neurons in which endogenous Stx-1a was knocked down and replaced with a CK2α phosphorylation-deficient mutant, Stx-1a(D17A), vesicle exocytosis was also increased. These results reveal a previously unsuspected role of CK2α phosphorylation in specifically regulating the reserve synaptic vesicle pool, without changing the kinetics of release from the readily releasable pool.

Imaging the rapid yet transient accumulation of regulatory lipids, lipid kinases, and protein kinases during membrane fusion, at sites of exocytosis of MMP-9 in MCF-7 cells

Background

The regulation of exocytosis is physiologically vital in cells and requires a variety of distinct proteins and lipids that facilitate efficient, fast, and timely release of secretory vesicle cargo. Growing evidence suggests that regulatory lipids act as important lipid signals and regulate various biological processes including exocytosis. Though functional roles of many of these regulatory lipids has been linked to exocytosis, the dynamic behavior of these lipids during membrane fusion at sites of exocytosis in cell culture remains unknown.

Methods

Total internal reflection fluorescence microscopy (TIRF) was used to observe the spatial organization and temporal dynamics (i.e. spatial positioning and timing patterns) of several lipids, and accessory proteins, like lipid kinases and protein kinases, in the form of protein kinase C (PRKC) associated with sites of exocytosis of matrix metalloproteinase-9 (MMP-9) in living MCF-7 cancer cells.

Results

Following stimulation with phorbol myristate acetate (PMA) to promote exocytosis, a transient accumulation of several distinct regulatory lipids, lipid kinases, and protein kinases at exocytic sites was observed. This transient accumulation centered at the time of membrane fusion is followed by a rapid diffusion away from the fusion sites. Additionally, the synthesis of these regulatory lipids, degradation of these lipids, and the downstream effectors activated by these lipids, are also achieved by the recruitment and accumulation of key enzymes at exocytic sites (during the moment of cargo release). This includes key enzymes like lipid kinases, protein kinases, and phospholipases that facilitate membrane fusion and exocytosis of MMP-9.

Conclusions

This work suggests that these regulatory lipids and associated effector proteins are locally synthesized and/or recruited to sites of exocytosis, during membrane fusion and cargo release. More importantly, their enrichment at fusion sites serves as an important spatial and temporal organizing “element” defining individual exocytic sites.

Exogenous carbon monoxide suppresses LPS-Induced platelet SNAREs complex assembly and α-granule exocytosis via integrin αIIbβ3-Mediated PKCθ/Munc18a pathway

Activation of coagulation occurs in sepsis and contributes to the development of thrombosis. Platelet α-granule exocytosis plays an important role in septic coagulation abnormalities. The present study aimed to investigate the effects and the underlying mechanisms of exogenous carbon monoxide, carbon monoxide-releasing molecules II (CORM-2)-liberated CO, on suppressing platelet α-granule exocytosis in sepsis. It was shown that CORM-2 weakened α-granule membrane fusion with platelet plasma membrane and attenuated α-granule contents exocytosis in LPS-Induced platelet. Further studies revealed that CORM-2 suppressed the expression of integrin α_IIbβ₃ in platelets stimulated by LPS. This was accompanied by a decrease in production and phosphorylation of PKCθ and Munc18a, SNARE complex assembly and subsequently platelet α-granule exocytosis. Taken together, we suggested that the potential mechanism of suppressive effect of CORM-2 on LPS-induced platelet SNAREs complex assembly and α-Granule Exocytosis might involve integrin α_IIbβ₃-mediated PKCθ/Munc18a pathway activation.

The selective disruption of presynaptic JNK2/STX1a interaction reduces NMDA receptor-dependent glutamate release

The neuronal loss caused by excessive glutamate release, or ‘excitotoxicity’, leads to several pathological conditions, including cerebral ischemia, epilepsy, and neurodegenerative diseases. Over-stimulation of presynaptic N-methyl-D-aspartate (NMDA) receptors is known to trigger and support glutamate spillover, while postsynaptic NMDA receptors are responsible for the subsequent apoptotic cascade. Almost all molecules developed so far are unable to selectively block presynaptic or postsynaptic NMDA receptors, therefore a deeper knowledge about intracellular NMDA pathways is required to design more specific inhibitors. Our previous work showed that presynaptic c-Jun N-terminal kinase 2 (JNK2) specifically regulates NMDA-evoked glutamate release and here we demonstrate that an interaction between Syntaxin-1a and JNK2 is fundamental to this mechanism. Based on this evidence, a new cell permeable peptide (CPP), “JGRi1”, has been developed to disrupt the JNK2/STX1a interaction to indirectly, but specifically, inhibit presynaptic NMDA receptor signaling. JGRi1 reduces the NMDA-evoked release of glutamate both in in-vitro and ex-vivo experiments while also being able to widely diffuse throughout brain tissue via intraperitoneal administration. In conclusion, the JNK2/STX1 interaction is involved in presynaptic NMDA-evoked glutamate release and the novel CPP, JGRi1, acts as a pharmacological tool that promotes neuroprotection.

A VPS33A-binding motif on syntaxin 17 controls autophagy completion in mammalian cells

Autophagy is an intracellular degradation pathway that transports cytoplasmic material to the lysosome for hydrolysis. It is completed by SNARE-mediated fusion of the autophagosome and endolysosome membranes. This process must be carefully regulated to maintain the organization of the membrane system and prevent mistargeted degradation. As yet, models of autophagosomal fusion have not been verified within a cellular context because of difficulties with assessing protein interactions in situ Here, we used high-resolution fluorescence lifetime imaging (FLIM)-FRET of HeLa cells to identify protein interactions within the spatiotemporal framework of the cell. We show that autophagosomal syntaxin 17 (Stx17) heterotrimerizes with synaptosome-associated protein 29 (SNAP29) and vesicle-associated membrane protein 7 (VAMP7) in situ, highlighting a functional role for VAMP7 in autophagosome clearance that has previously been sidelined in favor of a role for VAMP8. Additionally, we identified multimodal regulation of SNARE assembly by the Sec1/Munc18 (SM) protein VPS33A, mirroring other syntaxin-SM interactions and therefore suggesting a unified model of SM regulation. Contrary to current theoretical models, we found that the Stx17 N-peptide appears to interact in a positionally conserved, but mechanistically divergent manner with VPS33A, providing a late "go, no-go" step for autophagic fusion via a phosphoserine master-switch. Our findings suggest that Stx17 fusion competency is regulated by a phosphosite in its N-peptide, representing a previously unknown regulatory step in mammalian autophagy.

Endosomal and Phagosomal SNAREs

The soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein family is of vital importance for organelle communication. The complexing of cognate SNARE members present in both the donor and target organellar membranes drives the membrane fusion required for intracellular transport. In the endocytic route, SNARE proteins mediate trafficking between endosomes and phagosomes with other endosomes, lysosomes, the Golgi apparatus, the plasma membrane, and the endoplasmic reticulum. The goal of this review is to provide an overview of the SNAREs involved in endosomal and phagosomal trafficking. Of the 38 SNAREs present in humans, 30 have been identified at endosomes and/or phagosomes. Many of these SNAREs are targeted by viruses and intracellular pathogens, which thereby reroute intracellular transport for gaining access to nutrients, preventing their degradation, and avoiding their detection by the immune system. A fascinating picture is emerging of a complex transport network with multiple SNAREs being involved in consecutive trafficking routes.

Diverse exocytic pathways for mast cell mediators

Mast cells play pivotal roles in innate and adaptive immunities but are also culprits in allergy, autoimmunity, and cardiovascular diseases. Mast cells respond to environmental changes by initiating regulated exocytosis/secretion of various biologically active compounds called mediators (e.g. proteases, amines, and cytokines). Many of these mediators are stored in granules/lysosomes and rely on intricate degranulation processes for release. Mast cell stabilizers (e.g. sodium cromoglicate), which prevent such degranulation processes, have therefore been clinically employed to treat asthma and allergic rhinitis. However, it has become increasingly clear that different mast cell diseases often involve multiple mediators that rely on overlapping but distinct mechanisms for release. This review illustrates existing evidence that highlights the diverse exocytic pathways in mast cells. We also discuss strategies to delineate these pathways so as to identify unique molecular components which could serve as new drug targets for more effective and specific treatments against mast cell-related diseases.

Application of High-Throughput Assays to Examine Phospho-Modulation of the Late Steps of Regulated Exocytosis

Abstract: Regulated exocytosis enables a range of physiological functions including neurotransmission, and the late steps (i.e., docking, priming and Ca²⁺-triggered membrane fusion) are modulated by a highly conserved set of proteins and lipids. Many of the molecular components and biochemical interactions required have been identified; the precise mechanistic steps they modulate and the biochemical interactions that need to occur across steps are still the subject of intense investigation. Particularly, although the involvement of phosphorylation in modulating exocytosis has been intensively investigated over the past three decades, it is unclear which phosphorylation events are a conserved part of the fundamental fusion mechanism and/or serve as part of the physiological fusion machine (e.g., to modulate Ca²⁺ sensitivity). Here, the homotypic fusion of cortical vesicles was monitored by utilizing new high-throughput, cost-effective assays to assess the influence of 17 small molecule phospho-modulators on docking/priming, Ca²⁺ sensitivity and membrane fusion. Specific phosphatases and casein kinase 2 are implicated in modulating the Ca²⁺ sensitivity of fusion, whereas sphingosine kinase is implicated in modulating the ability of vesicles to fuse. These results indicate the presence of multiple kinases and phosphatases on the vesicles and critical phosphorylation sites on vesicle membrane proteins and lipids that directly influence late steps of regulated exocytosis.

Analysis of protein phosphorylation in nerve terminal reveals extensive changes in active zone proteins upon exocytosis

Neurotransmitter release is mediated by the fast, calcium-triggered fusion of synaptic vesicles with the presynaptic plasma membrane, followed by endocytosis and recycling of the membrane of synaptic vesicles. While many of the proteins governing these processes are known, their regulation is only beginning to be understood. Here we have applied quantitative phosphoproteomics to identify changes in phosphorylation status of presynaptic proteins in resting and stimulated nerve terminals isolated from the brains of Wistar rats. Using rigorous quantification, we identified 252 phosphosites that are either up- or downregulated upon triggering calcium-dependent exocytosis. Particularly pronounced were regulated changes of phosphosites within protein constituents of the presynaptic active zone, including bassoon, piccolo, and RIM1. Additionally, we have mapped kinases and phosphatases that are activated upon stimulation. Overall, our study provides a snapshot of phosphorylation changes associated with presynaptic activity and provides a foundation for further functional analysis of key phosphosites involved in presynaptic plasticity.

DOI:http://dx.doi.org/10.7554/eLife.14530.001

The human nervous system contains more than a hundred billion neurons that are connected with each other via junctions called synapses. When an electrical impulse travelling along a neuron arrives at a synapse, it triggers bubble-like packages called synaptic vesicles within the neuron to merge with the neuron’s surface membrane. The contents of these vesicles – chemical messengers called neurotransmitters – are then released into the synapse and carry the signal to the next neuron.

Complex molecular machines made from many different proteins control the release of neurotransmitters. Quite a few of these proteins are regulated by the addition of phosphate groups at specific sites. However, not all of the proteins involved in the release of neurotransmitters have been studied in detail and it is largely unclear how most of them are regulated.

Now, Kohansal-Nodehi et al. have used techniques involving mass spectrometry to find out which proteins have phosphate groups added or removed in neurons that are releasing neurotransmitters. The experiments used pinched-off synapses isolated from rat brains. These structures, referred to as “synaptosomes”, lend themselves to this kind of study because they can be induced to continuously release neurotransmitters for several minutes. Kohansal-Nodehi et al. identified over 250 specific sites on proteins in the synaptosomes where phosphate groups are attached, including many on the key proteins known to operate in neurotransmitter release. Moreover, some proteins were modified at multiple sites, especially the proteins that form a scaffold to capture synaptic vesicles close to the membrane and prepare them for release. The data also revealed important clues about the enzymes that either attach or remove the phosphate groups.

Together, these findings provide new insights into the regulatory networks that control many proteins at the same time. The next challenge is to sort out which of these modifications change the interactions between the proteins that control neurotransmitter release, and to understand how these changes influence the trafficking of synaptic vesicles.

DOI:http://dx.doi.org/10.7554/eLife.14530.002

Phosphorylation of syntaxin-3 at Thr 14 negatively regulates exocytosis in RBL-2H3 mast cells

Recent studies have revealed that soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins interact with each other, forming a SNARE complex that induces exocytosis in mast cells. Previously, we reported that syntaxin-3, a SNARE protein, regulates mast cell exocytosis and is constantly phosphorylated. In this study, we tried to identify the amino acid residue that is phosphorylated in mast cells, and to elucidate the regulatory mechanism of exocytosis by phosphorylation in syntaxin-3. We found that Thr 14 of syntaxin-3 was a phosphorylation site in mast cells. In addition, the overexpression of a constitutively dephosphorylated syntaxin-3 (T14A) mutant enhanced mast cell exocytosis. We also showed that the phosphomimetic mutation of syntaxin-3 at Thr 14 (T14E) induced structural changes in syntaxin-3, and this mutation inhibited binding of syntaxin-3 to Munc18-2. These results suggest that phosphorylated syntaxin-3 at Thr 14 negatively regulates mast cell exocytosis by impairing the interaction between syntaxin-3 and Munc18-2.

Membrane Trafficking in Neuronal Development: Ins and Outs of Neural Connectivity

During development, neurons progress through rapid yet stereotypical shape changes to achieve proper neuronal connectivity. This morphological progression requires carefully orchestrated plasma membrane expansion, insertion of membrane components including receptors for extracellular cues into the plasma membrane and removal and trafficking of membrane materials and proteins to specific locations. This review outlines the cellular machinery of membrane trafficking that play an integral role in neuronal cell shape change and function from initial neurite formation to pathway navigation and synaptogenesis.

A molecular toggle after exocytosis sequesters the presynaptic syntaxin1a molecules involved in prior vesicle fusion

Neuronal synapses are among the most scrutinized of cellular systems, serving as a model for all membrane trafficking studies. Despite this, synaptic biology has proven difficult to interrogate directly in situ due to the small size and dynamic nature of central synapses and the molecules within them. Here we determine the spatial and temporal interaction status of presynaptic proteins, imaging large cohorts of single molecules inside active synapses. Measuring rapid interaction dynamics during synaptic depolarization identified the small number of syntaxin1a and munc18-1 protein molecules required to support synaptic vesicle exocytosis. After vesicle fusion and subsequent SNARE complex disassembly, a prompt switch in syntaxin1a and munc18-1-binding mode, regulated by charge alteration on the syntaxin1a N-terminal, sequesters monomeric syntaxin1a from other disassembled fusion complex components, preventing ectopic SNARE complex formation, readying the synapse for subsequent rounds of neurotransmission.

An extended helical conformation in domain 3a of Munc18-1 provides a template for SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complex assembly

Munc18-1, a SEC1/Munc18 protein and key regulatory protein in synaptic transmission, can either promote or inhibit SNARE complex assembly. Although the binary inhibitory interaction between Munc18-1 and closed syntaxin 1 is well described, the mechanism of how Munc18-1 stimulates membrane fusion remains elusive. Using a reconstituted assay that resolves vesicle docking, priming, clamping, and fusion during synaptic exocytosis, we show that helix 12 in domain 3a of Munc18-1 stimulates SNAREpin assembly and membrane fusion. A single point mutation (L348R) within helix 12 selectively abolishes VAMP2 binding and the stimulatory function of Munc18-1 in membrane fusion. In contrast, targeting a natural switch site (P335A) at the start of helix 12, which can result in an extended α-helical conformation, further accelerates lipid-mixing. Together with structural modeling, the data suggest that helix 12 provides a folding template for VAMP2, accelerating SNAREpin assembly and membrane fusion. Analogous SEC1/Munc18-SNARE interactions at other transport steps may provide a general mechanism to drive lipid bilayer merger. At the neuronal synapse, Munc18-1 may convert docked synaptic vesicles into a readily releasable pool.

Secrets of platelet exocytosis - what do we really know about platelet secretion mechanisms?

Upon activation by extracellular matrix components or soluble agonists, platelets release in excess of 300 active molecules from intracellular granules. Those factors can both activate further platelets and mediate a range of responses in other cells. The complex microenvironment of a growing thrombus, as well as platelets' roles in both physiological and pathological processes, require platelet secretion to be highly spatially and temporally regulated to ensure appropriate responses to a range of stimuli. However, how this regulation is achieved remains incompletely understood. In this review we outline the importance of regulated secretion in thrombosis as well as in 'novel' scenarios beyond haemostasis and give a detailed summary of what is known about the molecular mechanisms of platelet exocytosis. We also discuss a number of theories of how different cargoes could be released in a tightly orchestrated manner, allowing complex interactions between platelets and their environment.

Imaging large cohorts of single ion channels and their activity

As calcium is the most important signaling molecule in neurons and secretory cells, amongst many other cell types, it follows that an understanding of calcium channels and their regulation of exocytosis is of vital importance. Calcium imaging using calcium dyes such as Fluo3, or FRET-based dyes that have been used widely has provided invaluable information, which combined with modeling has estimated the subtypes of channels responsible for triggering the exocytotic machinery as well as inferences about the relative distances away from vesicle fusion sites these molecules adopt. Importantly, new super-resolution microscopy techniques, combined with novel Ca(2+) indicators and imaginative imaging approaches can now define directly the nano-scale locations of very large cohorts of single channel molecules in relation to single vesicles. With combinations of these techniques the activity of individual channels can be visualized and quantified using novel Ca(2+) indicators. Fluorescently labeled specific channel toxins can also be used to localize endogenous assembled channel tetramers. Fluorescence lifetime imaging microscopy and other single-photon-resolution spectroscopic approaches offer the possibility to quantify protein-protein interactions between populations of channels and the SNARE protein machinery for the first time. Together with simultaneous electrophysiology, this battery of quantitative imaging techniques has the potential to provide unprecedented detail describing the locations, dynamic behaviors, interactions, and conductance activities of many thousands of channel molecules and vesicles in living cells.

Cyclin-dependent kinase-5 and p35/p25 activators in schizophrenia and major depression prefrontal cortex: basal contents and effects of psychotropic medications

Cyclin-dependent kinase-5 (CDK5) and p35/p25 activators, interacting with the exocytotic machinery (e.g. munc18-1 and syntaxin-1A), play critical roles in neurosecretion. The basal status of CDK5/p35/p25 and the effect of psychotropic drugs (detected in blood/urine samples) were investigated in post-mortem prefrontal cortex (PFC)/Brodmann's area 9 of schizophrenia (SZ) and major depression (MD) subjects. In SZ (all subjects, n = 24), CDK5 and p35, but not p25, were reduced (-28 to -58%) compared to controls. In SZ antipsychotic-free (n = 12), activator p35 was decreased (-52%). In SZ antipsychotic-treated (n = 12), marked reductions of CDK5 (-47%), p35 (-76%) and p25 (-36%) were quantified. In MD (n = 13), including antidepressant-free/treated subgroups, CDK5, p35 and p25 were unaltered. In SZ (n = 24), CDK5, p35 or p25 correlated with munc18-1a, but not with syntaxin-1A. The results demonstrate reduced p35 basal content and down-regulation of CDK5/p35/p25 by antipsychotics in SZ. The suggested CDK5/munc18-1a functional interaction may lead to dysregulated neurosecretion in SZ PFC.

Increased activity of the vesicular soluble N-ethylmaleimide-sensitive factor attachment protein receptor TI-VAMP/VAMP7 by tyrosine phosphorylation in the Longin domain

Vesicular (v)- and target (t)-SNAREs play essential roles in intracellular membrane fusion through the formation of cytoplasmic α-helical bundles. Several v-SNAREs have a Longin N-terminal extension that, by promoting a closed conformation, plays an autoinhibitory function and decreases SNARE complex formation and membrane fusion efficiency. The molecular mechanism leading to Longin v-SNARE activation is largely unknown. Here we find that exocytosis mediated by the Longin v-SNARE TI-VAMP/VAMP7 is activated by tonic treatment with insulin and insulin-like growth factor-1 but not by depolarization and intracellular calcium rise. In search of a potential downstream mechanism, we found that TI-VAMP is phosphorylated in vitro by c-Src kinase on tyrosine 45 of the Longin domain. Accordingly, a mutation of tyrosine 45 into glutamate, but not phenylalanine, activates both t-SNARE binding and exocytosis. Activation of TI-VAMP-mediated exocytosis thus relies on tyrosine phosphorylation.

A feasible add-on upgrade on a commercial two-photon FLIM microscope for optimal FLIM-FRET imaging of CFP-YFP pairs

Fluorescence lifetime imaging microscopy (FLIM) based on time-correlated single photon counting (TCSPC) is a widely used method for fluorescence resonance energy transfer (FRET). Here we report a feasible add-on approach to upgrade a commercial two-photon FLIM microscope into a single-photon FLIM microscope which provides optimal FLIM-FRET imaging of FRET pairs consisting of cyan fluorescent proteins (CFPs) as the donor and yellow fluorescent proteins (YFPs) as the acceptor. The capability of the upgraded system is evaluated and discussed, and the imaging performance of the system is demonstrated using FLIM-FRET experiments with a representative CFP-YFP FRET pair (mCerulean-mCitrine).

Munc18-1 protein molecules move between membrane molecular depots distinct from vesicle docking sites

Four evolutionarily conserved proteins are required for mammalian regulated exocytosis: three SNARE proteins, syntaxin, SNAP-25, and synaptobrevin, and the SM protein, Munc18-1. Here, using single-molecule imaging, we measured the spatial distribution of large cohorts of single Munc18-1 molecules correlated with the positions of single secretory vesicles in a functionally rescued Munc18-1-null cellular model. Munc18-1 molecules were nonrandomly distributed across the plasma membrane in a manner not directed by mode of interaction with syntaxin1, with a small mean number of molecules observed to reside under membrane resident vesicles. Surprisingly, we found that the majority of vesicles in fully secretion-competent cells had no Munc18-1 associated within distances relevant to plasma membrane-vesicle SNARE interactions. Live cell imaging of Munc18-1 molecule dynamics revealed that the density of Munc18-1 molecules at the plasma membrane anticorrelated with molecular speed, with single Munc18-1 molecules displaying directed motion between membrane hotspots enriched in syntaxin1a. Our findings demonstrate that Munc18-1 molecules move between membrane depots distinct from vesicle morphological docking sites.

Secretory vesicles are preferentially targeted to areas of low molecular SNARE density

Intercellular communication is commonly mediated by the regulated fusion, or exocytosis, of vesicles with the cell surface. SNARE (soluble N-ethymaleimide sensitive factor attachment protein receptor) proteins are the catalytic core of the secretory machinery, driving vesicle and plasma membrane merger. Plasma membrane SNAREs (tSNAREs) are proposed to reside in dense clusters containing many molecules, thus providing a concentrated reservoir to promote membrane fusion. However, biophysical experiments suggest that a small number of SNAREs are sufficient to drive a single fusion event. Here we show, using molecular imaging, that the majority of tSNARE molecules are spatially separated from secretory vesicles. Furthermore, the motilities of the individual tSNAREs are constrained in membrane micro-domains, maintaining a non-random molecular distribution and limiting the maximum number of molecules encountered by secretory vesicles. Together our results provide a new model for the molecular mechanism of regulated exocytosis and demonstrate the exquisite organization of the plasma membrane at the level of individual molecular machines.

Phosphatidylinositol 4,5-biphosphate (PIP(2)) lipids regulate the phosphorylation of syntaxin N-terminus by modulating both its position and local structure

Syntaxin (STX) is a N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein that binds to the plasma membrane and regulates ion channels and neurotransmitter transporters. Experiments have established the involvement of the N-terminal segment of STX in direct protein-protein interactions and have suggested a critical role for the phosphorylation of serine 14 (S14) by casein kinase-2 (CK2). Because the organization of STX in the plasma membrane was shown to be regulated by phosphatidylinositol 4,5-biphosphate (PIP(2)) lipids, we investigated the mechanistic involvement of PIP(2) lipids in modulating both the membrane interaction and the phosphorylation of STX, using a computational strategy that integrates mesoscale continuum modeling of protein-membrane interactions, with all-atom molecular dynamics (MD) simulations. Iterative applications of this protocol produced quantitative evaluations of lipid-type demixing due to the protein and identified conformational differences between STX immersed in PIP(2)-containing and PIP(2)-depleted membranes. Specific sites in STX were identified to be important for the electrostatic interactions with the PIP(2) lipids attracted to the protein, and the segregation of PIP(2) lipids near the protein is shown to have a dramatic effect on the positioning of the STX N-terminal segment with respect to the membrane/water interface. This PIP(2)-dependent repositioning is shown to modulate the extent of exposure of S14 to large reagents representing the CK2 enzyme and hence the propensity for phosphorylation. The prediction of STX sites involved in such PIP(2)-dependent regulation of STX phosphorylation at S14 offers experimentally testable probes of the mechanisms and models presented in this study, through structural modifications that can modulate the effects.

Regulation of munc18-1 and syntaxin-1A interactive partners in schizophrenia prefrontal cortex: down-regulation of munc18-1a isoform and 75 kDa SNARE complex after antipsychotic treatment

Munc18-1 and syntaxin-1 are crucial interacting molecules for synaptic membrane fusion and neurotransmitter release. Contrasting abnormalities of several proteins of the exocytotic machinery, including the formation of SNARE (synaptobrevin, SNAP-25 and syntaxin-1) complexes, have been reported in schizophrenia. This study quantified in the dorsolateral prefrontal cortex (PFC, Brodmann area 9) the immunocontent of munc18-1a/b isoforms, syntaxin-1A, other presynaptic proteins (synaptotagmin, synaptophysin), and SNARE complexes, as well as the effects of psychoactive drug exposure, in schizophrenia (SZ, n=24), non-schizophrenia suicide (SD, n=13) and major depression (MD, n=15) subjects compared to matched controls (n=39). SZ was associated with normal expression of munc18-1a/b and increased syntaxin-1A (+44%). The presence of antipsychotic drugs reduced the basal content of munc18-1a isoform (-23%) and synaptobrevin (-32%), and modestly reduced that of up-regulated syntaxin-1A (-16%). Munc18-1a and syntaxin-1A protein expression correlated positively in controls but showed a markedly opposite pattern in SZ, regardless of antipsychotic treatment. Thus, the ratio of syntaxin-1A to munc18-1a showed a net increase in SZ (+53/114%). The SNARE complex (75 kDa) was found unaltered in antipsychotic-free and reduced (-28%) in antipsychotic-treated SZ subjects. None of these abnormalities were observed in SD and MD subjects, unexposed or exposed to psychoactive drugs. The results reveal some exocytotic dysfunctions in SZ that are probably related to an imbalance of the interaction between munc18-1a and SNARE (mainly syntaxin-1A) complex. Moreover, antipsychotic drug treatment is associated with lower content of key proteins of the exocytotic machinery, which could result in a destabilization/impairment of neurosecretion.

Resolving the function of distinct Munc18-1/SNARE protein interaction modes in a reconstituted membrane fusion assay

Sec1p/Munc18 proteins and SNAP receptors (SNAREs) are key components of the intracellular membrane fusion machinery. Compartment-specific v-SNAREs on a transport vesicle pair with their cognate t-SNAREs on the target membrane and drive lipid bilayer fusion. In a reconstituted assay that dissects the sequential assembly of t-SNARE (syntaxin 1·SNAP-25) and v-/t-SNARE (VAMP2·syntaxin 1·SNAP-25) complexes, and finally measures lipid bilayer merger, we resolved the inhibitory and stimulatory functions of the Sec1p/Munc18 protein Munc18-1 at the molecular level. Inhibition of membrane fusion by Munc18-1 requires a closed conformation of syntaxin 1. Remarkably, the concurrent preincubation of Munc18-1-inhibited syntaxin 1 liposomes with both VAMP2 liposomes and SNAP-25 at low temperature releases the inhibition and effectively stimulates membrane fusion. VAMP8 liposomes can neither release the inhibition nor exert the stimulatory effect, demonstrating the need for a specific Munc18-1/VAMP2 interaction. In addition, Munc18-1 binds to the N-terminal peptide of syntaxin 1, which is obligatory for a robust stimulation of membrane fusion. In contrast, this interaction is neither required for the inhibitory function of Munc18-1 nor for the release of this block. These results indicate that Munc18-1 and the neuronal SNAREs already have the inherent capability to function as a basic stage-specific off/on switch to control membrane fusion.

Non fitting based FRET-FLIM analysis approaches applied to quantify protein-protein interactions in live cells

New imaging methodologies in quantitative fluorescence microscopy and nanoscopy have been developed in the last few years and are beginning to be extensively applied to biological problems, such as the localization and quantification of protein interactions. Fluorescence resonance energy transfer (FRET) detected by fluorescence lifetime imaging microscopy (FLIM) is currently employed not only in biophysics or chemistry but also in bio-medicine, thanks to new advancements in technology and also new developments in data treatment. FRET-FLIM can be a very useful tool to ascertain protein interactions occurring in single living cells. In this review, we stress the importance of increasing the acquisition speed when working in vivo employing Time-Domain FLIM. The development of the new mathematical-based non-fitting methods allows the determining of the fraction of interacting donor without the requirement of high count statistics, and thus allows the performing of high speed acquisitions in FRET-FLIM to still be quantitative.

Possible roles for Munc18-1 domain 3a and Syntaxin1 N-peptide and C-terminal anchor in SNARE complex formation

Munc18-1 and Syntaxin1 are essential proteins for SNARE-mediated neurotransmission. Munc18-1 participates in synaptic vesicle fusion via dual roles: as a docking/chaperone protein by binding closed Syntaxin1, and as a fusion protein that binds SNARE complexes in a Syntaxin1 N-peptide dependent manner. The two roles are associated with a closed-open Syntaxin1 conformational transition. Here, we show that Syntaxin N-peptide binding to Munc18-1 is not highly selective, suggesting that other parts of the SNARE complex are involved in binding to Munc18-1. We also find that Syntaxin1, with an N peptide and a physically anchored C terminus, binds to Munc18-1 and that this complex can participate in SNARE complex formation. We report a Munc18-1-N-peptide crystal structure that, together with other data, reveals how Munc18-1 might transit from a conformation that binds closed Syntaxin1 to one that may be compatible with binding open Syntaxin1 and SNARE complexes. Our results suggest the possibility that structural transitions occur in both Munc18-1 and Syntaxin1 during their binary interaction. We hypothesize that Munc18-1 domain 3a undergoes a conformational change that may allow coiled-coil interactions with SNARE complexes.

Protein kinase CK2 associates to lipid rafts and its pharmacological inhibition enhances neurotransmitter release

In the present work we report the presence of protein kinase CK2 in lipid raft preparations from rat brain synaptosomes, obtained after detergent extraction and subsequent isolation of detergent-resistant membranes using sucrose gradient ultracentrifugation. Moreover, the phosphorylation of syntaxin-1 at Ser14, a specific CK2 target, has been detected in lipid rafts, as assessed by a phospho-specific antibody. Treatment with DMAT, a specific CK2 inhibitor, results in a decrease of syntaxin-1 Ser14 phosphorylation in lipid rafts, while the glutamate release from synaptosomes is enhanced. In conclusion, CK2 might control neurotransmitter release by acting on SNARE proteins attached to cholesterol-enriched microdomains.

Munc18-1 as a key regulator of neurosecretion

Munc18-1 plays essential roles in neurosecretion by interacting with syntaxin-1 and controlling the formation of the soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNARE) complex. At least three important functions of Munc18-1 have been proposed: (i) molecular chaperone of syntaxin-1 for appropriate localization and expression of syntaxin-1, (ii) priming/stimulation of the SNARE-mediated membrane fusion, and (iii) docking of large dense-core vesicles to the plasma membrane. Similarly, at least two different binding modes have been proposed for the interaction between Munc18-1 and syntaxin-1: (i) binary binding to a 'closed' conformation of syntaxin-1, and (ii) binding to the N-terminal peptide of syntaxin-1, which is thought to enable an interaction with the quaternary SNARE complex and/or further stabilize the binary interaction between Munc18-1 and closed syntaxin-1. Recent structural analyses have identified critical Munc18-1 residues implicated in these different modes of binding. These have recently been tested functionally in rescue experiments using Munc18-1 null neurons, chromaffin cells and Munc18-1/-2 knockdown PC12 cells, allowing remarkable progress to be made in the structural/functional understanding of Munc18-1. In this review, we summarize these recent advances and attempt to propose an updated model of the pleiotropic functions of Munc18-1 in neuroexocytosis.

Vesicle fusion probability is determined by the specific interactions of munc18

Mammalian-regulated secretion is absolutely dependent on four evolutionarily conserved proteins: three SNARE proteins and munc18. Dissecting the functional outcomes of the spatially organized protein interactions between these factors has been difficult because of the close interrelationship between different binding modes. Here, we investigated the spatial distribution of single munc18 molecules at the plasma membrane of cells and the underlying interactions between syntaxin and munc18. Disruption of munc18 binding to the N-terminal peptide motif of syntaxin did not alter munc18 localization on the plasma membrane but had a pronounced influence on the behavior of secretory vesicles and their likelihood to undergo fusion. We therefore conclude that interaction with the syntaxin N-peptide can confer differential release probabilities to secretory vesicles and may contribute to the delineation of secretory vesicle pools.

Membrane protein clusters at nanoscale resolution: more than pretty pictures

Fluorescence microscopy is powerful for analyzing the composition and dynamics of cellular elements, but studying precise molecule patterns is precluded due to diffraction limited resolution. This barrier has been lifted now through several superresolution microscopy techniques. They revealed that proteins assemble in defined groups (clusters). A new challenge thus appears for the biologist: to find out whether clusters are molecular machines, stabilizers of defined protein conformations, or simply protein reservoirs.

SNARE bundle and syntaxin N-peptide constitute a minimal complement for Munc18-1 activation of membrane fusion

Whittling away SNARE complex components reveals essential domains for Munc18-1–mediated membrane fusion.

Sec1/Munc18 (SM) proteins activate intracellular membrane fusion through binding to cognate SNAP receptor (SNARE) complexes. The synaptic target membrane SNARE syntaxin 1 contains a highly conserved H_abc domain, which connects an N-peptide motif to the SNARE core domain and is thought to participate in the binding of Munc18-1 (the neuronal SM protein) to the SNARE complex. Unexpectedly, we found that mutation or complete removal of the H_abc domain had no effect on Munc18-1 stimulation of fusion. The central cavity region of Munc18-1 is required to stimulate fusion but not through its binding to the syntaxin H_abc domain. SNAP-25, another synaptic SNARE subunit, contains a flexible linker and exhibits an atypical conjoined Q_bc configuration. We found that neither the linker nor the Q_bc configuration is necessary for Munc18-1 promotion of fusion. As a result, Munc18-1 activates a SNARE complex with the typical configuration, in which each of the SNARE core domains is individually rooted in the membrane bilayer. Thus, the SNARE four-helix bundle and syntaxin N-peptide constitute a minimal complement for Munc18-1 activation of fusion.