Identification of residues critical for the extension of Munc18-1 domain 3a

BACKGROUND

Neurotransmitter release depends on the fusion of synaptic vesicles with the presynaptic membrane and is mainly mediated by SNARE complex assembly. During the transition of Munc18-1/Syntaxin-1 to the SNARE complex, the opening of the Syntaxin-1 linker region catalyzed by Munc13-1 leads to the extension of the domain 3a hinge loop, which enables domain 3a to bind SNARE motifs in Synaptobrevin-2 and Syntaxin-1 and template the SNARE complex assembly. However, the exact mechanism of domain 3a extension remains elusive.

RESULTS

Here, we characterized residues on the domain 3a hinge loop that are crucial for the extension of domain 3a by using biophysical and biochemical approaches and electrophysiological recordings. We showed that the mutation of residues T323/M324/R325 disrupted Munc13-1-mediated SNARE complex assembly and membrane fusion starting from Munc18-1/Syntaxin-1 in vitro and caused severe defects in the synaptic exocytosis of mouse cortex neurons in vivo. Moreover, the mutation had no effect on the binding of Synaptobrevin-2 to isolated Munc18-1 or the conformational change of the Syntaxin-1 linker region catalyzed by the Munc13-1 MUN domain. However, the extension of the domain 3a hinge loop in Munc18-1/Syntaxin-1 was completely disrupted by the mutation, leading to the failure of Synaptobrevin-2 binding to Munc18-1/Syntaxin-1.

CONCLUSIONS

Together with previous results, our data further support the model that the template function of Munc18-1 in SNARE complex assembly requires the extension of domain 3a, and particular residues in the domain 3a hinge loop are crucial for the autoinhibitory release of domain 3a after the MUN domain opens the Syntaxin-1 linker region.

A non-canonical target-binding site in Munc18-1 domain 3b for assembling the Mint1-Munc18-1-syntaxin-1 complex

As the prototype of Sec1/Munc18 (SM) family proteins, Munc18-1 can manipulate the distinct conformations of syntaxin-1 for controlling intracellular membrane fusion. The Munc18-1-interacting domain of Mint1 (Mint1-MID) binds to Munc18-1 together with syntaxin-1 to form a Mint1-Munc18-1-syntaxin-1 complex, but the mechanism underlying the complex assembly remains unclear. Here, we determine the structure of the Mint1-MID-Munc18-1-syntaxin-1 complex. Unexpectedly, Munc18-1 recognizes Mint1-MID and syntaxin-1 simultaneously via two opposite sites. The canonical central cavity between domains 1 and 3a of Munc18-1 embraces closed syntaxin-1, whereas the non-canonical basic pocket in domain 3b captures the acidic Mint1-MID helix. The domain 3b-mediated recognition of an acidic-helical motif is distinct from other target-recognition modes of Munc18-1. Mutations in the interface between domain 3b and Mint1-MID disrupt the assembly of the Mint1-Munc18-1-syntaxin-1 complex. This work reveals a non-canonical target-binding site in Munc18-1 domain 3b for assembling the Mint1-Munc18-1-syntaxin-1 complex.

Probing single molecule mechanical interactions of syntaxin 1A with native synaptobrevin 2 residing on a secretory vesicle

Interactive mechanical forces between pairs of individual SNARE proteins synaptobrevin 2 (Sb2) and syntaxin 1A (Sx1A) may be sufficient to mediate vesicle docking. This notion, based on force spectroscopy single molecule measurements probing recombinant Sx1A an Sb2 in silico, questioned a predominant view of docking via the ternary SNARE complex formation, which includes an assembly of the intermediate cis binary complex between Sx1A and SNAP25 on the plasma membrane to engage Sb2 on the vesicle. However, whether a trans binary Sx1A-Sb2 complex alone could mediate vesicle docking in a cellular environment remains unclear. To address this issue, we used atomic force microscopy (AFM) in the force spectroscopy mode combined with fluorescence imaging. Using AFM tips functionalized with the full Sx1A cytosolic domain, we probed native Sb2 studding the membrane of secretory vesicles docked at the plasma membrane patches, referred to as "inside-out lawns", identified based on fluorescence stains and prepared from primary culture of lactotrophs. We recorded single molecule Sx1A-Sb2 mechanical interactions and obtained measurements of force (∼183 pN) and extension (∼21.6 nm) necessary to take apart Sx1A-Sb2 binding interactions formed at tip-vesicle contact. Measured interactive force between a single pair of Sx1A-Sb2 molecules is sufficient to hold a single secretory vesicle docked at the plasma membrane within distances up to that of the measured extension. This finding further advances a notion that native vesicle docking can be mediated by a single trans binary Sx1A-Sb2 complex in the absence of SNAP25.

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.

Munc13 binds and recruits SNAP25 to chaperone SNARE complex assembly

Synaptic vesicle fusion is mediated by SNARE proteins-VAMP2 on the vesicle and Syntaxin-1/SNAP25 on the presynaptic membrane. Chaperones Munc18-1 and Munc13-1 cooperatively catalyze SNARE assembly via an intermediate 'template' complex containing Syntaxin-1 and VAMP2. How SNAP25 enters this reaction remains a mystery. Here, we report that Munc13-1 recruits SNAP25 to initiate the ternary SNARE complex assembly by direct binding, as judged by bulk FRET spectroscopy and single-molecule optical tweezer studies. Detailed structure-function analyses show that the binding is mediated by the Munc13-1 MUN domain and is specific for the SNAP25 'linker' region that connects the two SNARE motifs. Consequently, freely diffusing SNAP25 molecules on phospholipid bilayers are concentrated and bound in ~ 1 : 1 stoichiometry by the self-assembled Munc13-1 nanoclusters.

Botulinum Toxins A and E Inflict Dynamic Destabilization on t-SNARE to Impair SNARE Assembly and Membrane Fusion

Botulinum toxins (BoNTs) A and E block neurotransmitter release by specifically cleaving the C- terminal ends of SNAP-25, a plasma membrane SNARE protein. Here, we find that SNAP-25A and E, the cleavage products of BoNT A and E, respectively, terminate membrane fusion via completely different mechanisms. Combined studies of single-molecule FRET and single-vesicle fusion assays reveal that SNAP-25E is incapable of supporting SNARE pairing and thus, vesicle docking. In contrast, SNAP-25A facilitates robust SNARE pairing and vesicle docking with somewhat reduced SNARE zippering, which leads to severe impairment of fusion pore opening. The electron paramagnetic resonance results show that the discrepancy between SNAP-25A and E might stem from the extent of the dynamic destabilization of the t-SNARE core at the N-terminal half, which plays a pivotal role in nucleating SNARE complex formation. Thus, the results provide insights into the structure/dynamics-based mechanism by which BoNT A and E impair membrane fusion.

Structural characterization of the Rabphilin-3A-SNAP25 interaction

Membrane fusion is essential in a myriad of eukaryotic cell biological processes, including the synaptic transmission. Rabphilin-3A is a membrane trafficking protein involved in the calcium-dependent regulation of secretory vesicle exocytosis in neurons and neuroendocrine cells, but the underlying mechanism remains poorly understood. Here, we report the crystal structures and biochemical analyses of Rabphilin-3A C2B-SNAP25 and C2B-phosphatidylinositol 4,5-bisphosphate (PIP2) complexes, revealing how Rabphilin-3A C2 domains operate in cooperation with PIP2/Ca2+ and SNAP25 to bind the plasma membrane, adopting a conformation compatible to interact with the complete SNARE complex. Comparisons with the synaptotagmin1-SNARE show that both proteins contact the same SNAP25 surface, but Rabphilin-3A uses a unique structural element. Data obtained here suggest a model to explain the Ca2+-dependent fusion process by membrane bending with a myriad of variations depending on the properties of the C2 domain-bearing protein, shedding light to understand the fine-tuning control of the different vesicle fusion events.

Using ApoE Nanolipoprotein Particles To Analyze SNARE-Induced Fusion Pores

Here we introduce ApoE-based nanolipoprotein particle (NLP)-a soluble, discoidal bilayer mimetic of ∼23 nm in diameter, as fusion partners to study the dynamics of fusion pores induced by SNARE proteins. Using in vitro lipid mixing and content release assays, we report that NLPs reconstituted with synaptic v-SNARE VAMP2 (vNLP) fuse with liposomes containing the cognate t-SNARE (Syntaxin1/SNAP25) partner, with the resulting fusion pore opening directly to the external buffer. Efflux of encapsulated fluorescent dextrans of different sizes show that unlike the smaller nanodiscs, these larger NLPs accommodate the expansion of the fusion pore to at least ∼9 nm, and dithionite quenching of fluorescent lipid introduced in vNLP confirms that the NLP fusion pores are short-lived and eventually reseal. The NLPs also have capacity to accommodate larger number of proteins and using vNLPs with defined number of VAMP2 protein, including physiologically relevant copy numbers, we find that 3-4 copies of VAMP2 (minimum 2 per face) are required to keep a nascent fusion pore open, and the SNARE proteins act cooperatively to dilate the nascent fusion pore.

Syntaxin-3 Binds and Regulates Both R- and L-Type Calcium Channels in Insulin-Secreting INS-1 832/13 Cells

Syntaxin (Syn)-1A mediates exocytosis of predocked insulin-containing secretory granules (SGs) during first-phase glucose-stimulated insulin secretion (GSIS) in part via its interaction with plasma membrane (PM)-bound L-type voltage-gated calcium channels (Cav). In contrast, Syn-3 mediates exocytosis of newcomer SGs that accounts for second-phase GSIS. We now hypothesize that the newcomer SG Syn-3 preferentially binds and modulates R-type Cav opening, which was postulated to mediate second-phase GSIS. Indeed, glucose-stimulation of pancreatic islet β-cell line INS-1 induced a predominant increase in interaction between Syn-3 and Cavα1 pore-forming subunits of R-type Cav2.3 and to lesser extent L-type Cavs, while confirming the preferential interactions between Syn-1A with L-type (Cav1.2, Cav1.3) Cavs. Consistently, direct binding studies employing heterologous HEK cells confirmed that Syn-3 preferentially binds Cav2.3, whereas Syn-1A prefers L-type Cavs. We then used siRNA knockdown (KD) of Syn-3 in INS-1 to study the endogenous modulatory actions of Syn-3 on Cav channels. Syn-3 KD enhanced Ca2+ currents by 46% attributed mostly to R- and L-type Cavs. Interestingly, while the transmembrane domain of Syn-1A is the putative functional domain modulating Cav activity, it is the cytoplasmic domain of Syn-3 that appears to modulate Cav activity. We conclude that Syn-3 may mimic Syn-1A in the ability to bind and modulate Cavs, but preferring Cav2.3 to perhaps participate in triggering fusion of newcomer insulin SGs during second-phase GSIS.

A Coarse Grained Model for a Lipid Membrane with Physiological Composition and Leaflet Asymmetry

The resemblance of lipid membrane models to physiological membranes determines how well molecular dynamics (MD) simulations imitate the dynamic behavior of cell membranes and membrane proteins. Physiological lipid membranes are composed of multiple types of phospholipids, and the leaflet compositions are generally asymmetric. Here we describe an approach for self-assembly of a Coarse-Grained (CG) membrane model with physiological composition and leaflet asymmetry using the MARTINI force field. An initial set-up of two boxes with different types of lipids according to the leaflet asymmetry of mammalian cell membranes stacked with 0.5 nm overlap, reliably resulted in the self-assembly of bilayer membranes with leaflet asymmetry resembling that of physiological mammalian cell membranes. Self-assembly in the presence of a fragment of the plasma membrane protein syntaxin 1A led to spontaneous specific positioning of phosphatidylionositol(4,5)bisphosphate at a positively charged stretch of syntaxin consistent with experimental data. An analogous approach choosing an initial set-up with two concentric shells filled with different lipid types results in successful assembly of a spherical vesicle with asymmetric leaflet composition. Self-assembly of the vesicle in the presence of the synaptic vesicle protein synaptobrevin 2 revealed the correct position of the synaptobrevin transmembrane domain. This is the first CG MD method to form a membrane with physiological lipid composition as well as leaflet asymmetry by self-assembly and will enable unbiased studies of the incorporation and dynamics of membrane proteins in more realistic CG membrane models.

Dynamical Organization of Syntaxin-1A at the Presynaptic Active Zone

Synaptic vesicle fusion is mediated by SNARE proteins forming in between synaptic vesicle (v-SNARE) and plasma membrane (t-SNARE), one of which is Syntaxin-1A. Although exocytosis mainly occurs at active zones, Syntaxin-1A appears to cover the entire neuronal membrane. By using STED super-resolution light microscopy and image analysis of Drosophila neuro-muscular junctions, we show that Syntaxin-1A clusters are more abundant and have an increased size at active zones. A computational particle-based model of syntaxin cluster formation and dynamics is developed. The model is parametrized to reproduce Syntaxin cluster-size distributions found by STED analysis, and successfully reproduces existing FRAP results. The model shows that the neuronal membrane is adjusted in a way to strike a balance between having most syntaxins stored in large clusters, while still keeping a mobile fraction of syntaxins free or in small clusters that can efficiently search the membrane or be traded between clusters. This balance is subtle and can be shifted toward almost no clustering and almost complete clustering by modifying the syntaxin interaction energy on the order of only 1 kBT. This capability appears to be exploited at active zones. The larger active-zone syntaxin clusters are more stable and provide regions of high docking and fusion capability, whereas the smaller clusters outside may serve as flexible reserve pool or sites of spontaneous ectopic release.

Allosteric control of syntaxin 1a by Munc18-1: characterization of the open and closed conformations of syntaxin

Syntaxin 1a is a plasma membrane soluble N-ethylmaleimide-sensitive factor attachment receptor protein (SNARE) that contains an H3 domain (SNARE motif) and a regulatory Habc domain. These regions associate to produce a closed state, which is generally thought to suppress assembly of syntaxin into the SNARE complex. However, the molecular nature of the closed and open states of syntaxin is not well defined. Here, we use electron paramagnetic resonance spectroscopy to characterize conformational exchange in syntaxin. The data indicate that the H3 segment is in equilibrium between ordered and disordered states that have significant populations. In solution, the central region of the H3 segment is positioned close to the Habc domain and the configuration of syntaxin 1a is dominated by a closed state. However, an open state is enhanced in full-length membrane reconstituted syntaxin. Munc18-1 binding alters the equilibrium along H3 to favor the ordered, folded state. Munc18 also suppresses the minor open population and narrows the distance distributions between H3 and Habc. The allosteric control exhibited by Munc18 on the H3 segment and the suppression of the minor open component may both play a role in regulating membrane fusion by controlling the assembly of syntaxin into the SNARE complex.

Syntaxin1a variants lacking an N-peptide or bearing the LE mutation bind to Munc18a in a closed conformation

In neurons, soluble N-ethylmaleimide-sensitive factor attachment receptor (SNARE) proteins drive the fusion of synaptic vesicles to the plasma membrane through the formation of a four-helix SNARE complex. Members of the Sec1/Munc18 protein family regulate membrane fusion through interactions with the syntaxin family of SNARE proteins. The neuronal protein Munc18a interacts with a closed conformation of the SNARE protein syntaxin1a (Syx1a) and with an assembled SNARE complex containing Syx1a in an open conformation. The N-peptide of Syx1a (amino acids 1-24) has been implicated in the transition of Munc18a-bound Syx1a to Munc18a-bound SNARE complex, but the underlying mechanism is not understood. Here we report the X-ray crystal structures of Munc18a bound to Syx1a with and without its native N-peptide (Syx1aΔN), along with small-angle X-ray scattering (SAXS) data for Munc18a bound to Syx1a, Syx1aΔN, and Syx1a L165A/E166A (LE), a mutation thought to render Syx1a in a constitutively open conformation. We show that all three complexes adopt the same global structure, in which Munc18a binds a closed conformation of Syx1a. We also identify a possible structural connection between the Syx1a N-peptide and SNARE domain that might be important for the transition of closed-to-open Syx1a in SNARE complex assembly. Although the role of the N-peptide in Munc18a-mediated SNARE complex assembly remains unclear, our results demonstrate that the N-peptide and LE mutation have no effect on the global conformation of the Munc18a-Syx1a complex.

SNARE proteins: one to fuse and three to keep the nascent fusion pore open

Neurotransmitters are released through nascent fusion pores, which ordinarily dilate after bilayer fusion, preventing consistent biochemical studies. We used lipid bilayer nanodiscs as fusion partners; their rigid protein framework prevents dilation and reveals properties of the fusion pore induced by SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor). We found that although only one SNARE per nanodisc is required for maximum rates of bilayer fusion, efficient release of content on the physiologically relevant time scale of synaptic transmission apparently requires three or more SNARE complexes (SNAREpins) and the native transmembrane domain of vesicle-associated membrane protein 2 (VAMP2). We suggest that several SNAREpins simultaneously zippering their SNARE transmembrane helices within the freshly fused bilayers provide a radial force that prevents the nascent pore from resealing during synchronous neurotransmitter release.

Filtered FCS: species auto- and cross-correlation functions highlight binding and dynamics in biomolecules

An analysis method of lifetime, polarization and spectrally filtered fluorescence correlation spectroscopy, referred to as filtered FCS (fFCS), is introduced. It uses, but is not limited to, multiparameter fluorescence detection to differentiate between molecular species with respect to their fluorescence lifetime, polarization and spectral information. Like the recently introduced fluorescence lifetime correlation spectroscopy (FLCS) [Chem. Phys. Lett. 2002, 353, 439-445], fFCS is based on pulsed laser excitation. However, it uses the species-specific polarization and spectrally resolved fluorescence decays to generate filters. We determined the most efficient method to generate global filters taking into account the anisotropy information. Thus, fFCS is able to distinguish species, even if they have very close or the same fluorescence lifetime, given differences in other fluorescence parameters. fFCS can be applied as a tool to compute species-specific auto- (SACF) and cross- correlation (SCCF) functions from a mixture of different species for accurate and quantitative analysis of their concentration, diffusion and kinetic properties. The computed correlation curves are also free from artifacts caused by unspecific background signal. We tested this methodology by simulating the extreme case of ligand-receptor binding processes monitored only by differences in fluorescence anisotropy. Furthermore, we apply fFCS to an experimental single-molecule FRET study of an open-to-closed conformational transition of the protein Syntaxin-1. In conclusion, fFCS and the global analysis of the SACFs and SCCF is a key tool to investigate binding processes and conformational dynamics of biomolecules in a nanosecond-to-millisecond time range as well as to unravel the involved molecular states.

Single-molecule measurements of dissociation rates and energy landscapes of binary trans snare complexes in parallel versus antiparallel orientation

Interactions between synaptobrevin 2 (Sb2) and syntaxin 1A (Sx1A) can be readily isolated and studied with the use of force spectroscopy single-molecule measurements. We studied interactions between Sx1A and Sb2 in two different orientations (parallel and antiparallel) using four different terminus configurations of these proteins. Force-loading experiments indicated that protein pairs in any configuration/orientation are zippered. We measured the extension and force for disassembly of these interactions, calculated the spontaneous dissociation lifetimes, and determined their free energies, enthalpies, and entropies. Although the free energies were very similar for all four configurations (∼28 k(B)T (Eyring model) and ∼20 k(B)T (Kramers model)), the enthalpy changes of binary Sx1A-Sb2 interactions varied between 24.7 k(B)T and 33.1 k(B)T. This variation is consistent with the conformation changes that occur during disassembly of the various protein terminus configurations, as verified by alterations in the extension. The parallel interactions appear to be energetically somewhat advantageous over antiparallel configurations/orientation, especially when the N-termini of Sx1A-Sb2 are left to interact freely.

Syntaxin-1A inhibits KATP channels by interacting with specific conserved motifs within sulfonylurea receptor 2A

We previously demonstrated that syntaxin (Syn)-1A is present in the sarcolemma of rat cardiomyocytes and binds sulfonylurea receptor (SUR) 2A nucleotide binding folds (NBFs) to inhibit ATP-sensitive potassium (K(ATP)) channel. Here, we examined for the precise domains within the NBFs of SUR2A that may interact with Syn-1A. Specifically, we tested truncated NBF protein segments encompassing the conserved motifs Walker A (W(A)), signature/Linker (L), and Walker B (W(B)). In vitro binding results indicate that the domains encompassing W(A) and L of NBF-1 and all three conserved motifs of NBF-2 bound Syn-1A. Electrophysiological studies, employing inside-out patch-clamp recordings from SUR2A/Kir6.2 expressing HEK cells and mouse cardiomyocytes, show that W(B) and L of NBF-1 and all three NBF-2 truncated protein segments reduced Syn-1A inhibition of SUR2A/K(ATP) channels. Remarkably, these same NBF-1 and -2 truncated proteins could independently disrupt the intimate FRET interactions of full length SUR2A (-mCherry) and Syn-1A (-EGFP). These results taken together indicate that Syn-1A possibly maintains inhibition of cardiac ventricular K(ATP) channels by binding to large regions of NBF-1 and NBF-2 to stabilize the NBF-1-NBF-2 heterodimer formation and prevent ATP-binding and ATP hydrolysis. Since K(ATP) channels are closely coupled to metabolic states, we postulate that these very intimate Syn-1A-SUR2A interactions are critically important for myocardial protection during stress, in which profound changes in metabolic factors (pH, ATP) could modulate these Syn-1A-SUR2A interactions.

Synaptic vesicles studied by dynamic light scattering

The size polydispersity distribution of synaptic vesicles (SVs) is characterized under quasi-physiological conditions by dynamic light scattering (DLS). Highly purified fractions of SVs obtained from rat brain still contain a small amount of larger contaminant structures, which can be quantified by DLS and further reduced by asymmetric-flow field-flow (AFFF) fractionation. The intensity autocorrelation functions g (2)(τ) recorded from these samples are analyzed by a constrained regularization method as well as by an alternative direct modeling approach. The results are in quantitative agreement with the polydispersity obtained from cryogenic electron microscopy of vitrified SVs. Next, different vesicle fusion assays based on samples composed of SVs and small unilamellar proteoliposomes with the fusion proteins syntaxin 1 and SNAP-25A are characterized by DLS. The size increase of the proteoliposomes due to SNARE-dependent fusion with SVs is quantified by DLS under quasi-physiological conditions.

Inositol 1, 4, 5-trisphosphate receptor interacts with the SNARE domain of syntaxin 1B

Inositol 1, 4, 5-trisphosphate receptors (IP(3)Rs) are intracellular ligand-gated Ca(2+) channels that mediate Ca(2+) release from the endoplasmic reticulum (ER) into the cytosol and function in diverse cellular processes including fertilization, muscle contraction, apoptosis, secretion, and synaptic plasticity. The Ca(2+) release activity of IP(3)Rs is tightly regulated by many factors including IP(3)R-binding proteins. We show that IP(3)Rs interact with syntaxin 1 (Syx1), a membrane trafficking protein that regulates various plasma-membrane ion channels including N-, P/Q, and L-type voltage-gated Ca(2+) channels, voltage-gated potassium channels, and an epithelial sodium channel. We found that a SNARE-domain of Syx1B, one of the two Syx1 isoforms, directly interacted with the type1 IP(3)R (IP(3)R1) internal coupling domain, a known modulator for channel opening. These results indicate that Syx1B is an IP(3)R-interacting protein and that its interaction may play a crucial role in regulating the channel activity of IP(3)Rs in Syx1B-expressing cells.

NMR analysis of the closed conformation of syntaxin-1

The Sec1/Munc18 (SM) protein Munc18-1 and the SNAREs syntaxin-1, SNAP-25 and synaptobrevin form the core of the membrane fusion machinery that triggers neurotransmitter release. Munc18-1 binds to syntaxin-1 folded into a closed conformation and to the SNARE complex formed by the three SNAREs, which involves an open syntaxin-1 conformation. The former interaction is likely specialized for neurotransmitter release, whereas SM protein/SNARE complex interactions are likely key for all types of intracellular membrane fusion. It is currently unclear whether the closed conformation is highly or only marginally populated in isolated syntaxin-1, and whether Munc18-1 stabilizes the close conformation or helps to open it to facilitate SNARE complex formation. A detailed NMR analysis now suggests that the closed conformation is almost quantitatively populated in isolated syntaxin-1 in the absence of oligomerization, and indicates that its structure is very similar to that observed previously in the crystal structure of the Munc18-1/syntaxin-1 complex. Moreover, we demonstrate that Munc18-1 binding prevents opening of the syntaxin-1 closed conformation. These results support a model whereby the closed conformation constitutes a key intrinsic property of isolated syntaxin-1 and Munc18-1 binding stabilizes this conformation; in this model, Munc18-1 plays in addition an active role in downstream events after another factor(s) helps to open the syntaxin-1 conformation.

Depolarization-evoked secretion requires two vicinal transmembrane cysteines of syntaxin 1A

Background

The interactions of the voltage-gated Ca²⁺ channel (VGCC) with syntaxin 1A (Sx 1A), Synaptosome-associated protein of 25 kD (SNAP-25), and synaptotagmin, couple electrical excitation to evoked secretion. Two vicinal Cys residues, Cys 271 and Cys 272 in the Sx 1A transmembrane domain, are highly conserved and participate in modulating channel kinetics. Each of the Sx1A Cys mutants, differently modify the kinetics of Cav1.2, and neuronal Cav2.2 calcium channel.

Methodology/Principle Findings

We examined the effects of various Sx1A Cys mutants and the syntaxin isoforms 2, 3, and 4 each of which lack vicinal Cys residues, on evoked secretion, monitoring capacitance transients in a functional release assay. Membrane capacitance in Xenopus oocytes co-expressing Cav1.2, Sx1A, SNAP-25 and synaptotagmin, which is Bot C- and Bot A-sensitive, was elicited by a double 500 ms depolarizing pulse to 0 mV. The evoked-release was obliterated when a single Cys Sx1A mutant or either one of the Sx isoforms were substituted for Sx 1A, demonstrating the essential role of vicinal Cys residues in the depolarization mediated process. Protein expression and confocal imaging established the level of the mutated proteins in the cell and their targeting to the plasma membrane.

Conclusions/Significance

We propose a model whereby the two adjacent transmembranal Cys residues of Sx 1A, lash two calcium channels. Consistent with the necessity of a minimal fusion complex termed the excitosome, each Sx1A is in a complex with SNAP-25, Syt1, and the Ca²⁺ channel. A Hill coefficient >2 imply that at least three excitosome complexes are required for generating a secreting hetero-oligomer protein complex. This working model suggests that a fusion pore that opens during membrane depolarization could be lined by alternating transmembrane segments of Sx1A and VGCC. The functional coupling of distinct amino acids of Sx 1A with VGCC appears to be essential for depolarization-evoked secretion.

Structural analysis of SNARE motifs from sea perch, Lateolabrax japonicus by computerized approaches

Three cDNA sequences encoding four SNARE (N-ethylmaleimide-sensitive fusion protein attachment protein receptors) motifs were cloned from sea perch, and the deduced peptide sequences were analyzed for structural prediction by using 14 different web servers and softwares. The "ionic layer" structure, the three dimensional extension and conformational characters of the SNARE 7S core complex by using bioinformatics approaches were compared respectively with those from mammalian X-ray crystallographic investigations. The result suggested that the formation and stabilization of fish SNARE core complex might be driven by hydrophobic association, hydrogen bond among R group of core amino acids and electrostatic attraction at molecular level. This revealed that the SNARE proteins interaction of the fish may share the same molecular mechanism with that of mammal, indicating the universality and solidity of SNARE core complex theory. This work is also an attempt to get the protein 3D structural information which appears to be similar to that obtained through X-ray crystallography, only by using computerized approaches.

Anatomy and Dynamics of a Supramolecular Membrane Protein Cluster

Most plasmalemmal proteins organize in submicrometer-sized clusters whose architecture and dynamics are still enigmatic. With syntaxin 1 as an example, we applied a combination of far-field optical nanoscopy, biochemistry, fluorescence recovery after photobleaching (FRAP) analysis, and simulations to show that clustering can be explained by self-organization based on simple physical principles. On average, the syntaxin clusters exhibit a diameter of 50 to 60 nanometers and contain 75 densely crowded syntaxins that dynamically exchange with freely diffusing molecules. Self-association depends on weak homophilic protein-protein interactions. Simulations suggest that clustering immobilizes and conformationally constrains the molecules. Moreover, a balance between self-association and crowding-induced steric repulsions is sufficient to explain both the size and dynamics of syntaxin clusters and likely of many oligomerizing membrane proteins that form supramolecular structures.

Modulation of the Kv4.3 channel by syntaxin 1A

The SNARE protein syntaxin 1A (Syn1A) is known to inhibit delayed rectifier K(+) channels of the K(v)1 and K(v)2 families with heterogeneous effects on their gating properties. In this study, we explored whether Syn1A could directly modulate K(v)4.3, a rapidly inactivating K(v) channel with important roles in neuroendocrine cells and cardiac myocytes. Immunoprecipitation studies in HEK293 cells coexpressing Syn1A and K(v)4.3 revealed a direct interaction with increased trafficking to the plasma membrane without a change in channel synthesis. Paradoxically, Syn1A inhibited K(v)4.3 current density. In particular, Syn1A produced a left-shift in steady-state inactivation of K(v)4.3 without affecting either voltage dependence of activation or gating kinetics, a pattern distinct from other K(v) channels. Combined with our previous reports, our results further verify the notion that the mechanisms involved in Syn1A-K(v) interactions vary significantly between K(v) channels, thus providing a wide scope for Syn1A modulation of exocytosis and membrane excitability.

Role of Habc domain in membrane trafficking and targeting of syntaxin 1A

Membrane syntaxin plays essential roles in exocytosis in eukaryotic cells. The conservative H(abc) domain in plasma membrane syntaxins implies important roles for syntaxin targeting and function. Our previous study showed H(abc) domain was necessary for the trafficking and cluster distribution of syntaxin 1A on the plasma membrane. Here we identified which of the three domains (H(a), H(b) and H(c)) was essential for Stx1A trafficking and clustering. We found that, in INS-1 cells, the mutant truncated with either H(a), H(b) or H(c) domain could be sorted to the cell surface by a different mechanism compared to that of whole H(abc) truncated mutant. In contrast to wild type Stx1A, none of the mutants showed cluster distribution at the functional sites, suggesting that the physiological localization of Stx1A relies on intact H(abc) domain. Furthermore Munc18-1 is found not to be essential for Stx1A cluster distribution, despite important role in stabilizing membrane delivery of Stx1A.

Receptor-mediated regulation of tomosyn-syntaxin 1A interactions in bovine adrenal chromaffin cells

Tomosyn, a soluble R-SNARE protein identified as a binding partner of the Q-SNARE syntaxin 1A, is thought to be critical in setting the level of fusion-competent SNARE complexes for neurosecretion. To date, there has been no direct evaluation of the dynamics in which tomosyn transits through tomosyn-SNARE complexes or of the extent to which tomosyn-SNARE complexes are regulated by secretory demand. Here, we employed biochemical and optical approaches to characterize the dynamic properties of tomosyn-syntaxin 1A complexes in live adrenal chromaffin cells. We demonstrate that secretagogue stimulation results in the rapid translocation of tomosyn from the cytosol to plasma membrane regions and that this translocation is associated with an increase in the tomosyn-syntaxin 1A interaction, including increased cycling of tomosyn into tomosyn-SNARE complexes. The secretagogue-induced interaction was strongly reduced by pharmacological inhibition of the Rho-associated coiled-coil forming kinase, a result consistent with findings demonstrating secretagogue-induced activation of RhoA. Stimulation of chromaffin cells with lysophosphatidic acid, a nonsecretory stimulus that strongly activates RhoA, resulted in effects on tomosyn similar to that of application of the secretagogue. In PC-12 cells overexpressing tomosyn, secretagogue stimulation in the presence of lysophosphatidic acid resulted in reduced evoked secretory responses, an effect that was eliminated upon inhibition of Rho-associated coiled-coil forming kinase. Moreover, this effect required an intact interaction between tomosyn and syntaxin 1A. Thus, modulation of the tomosyn-syntaxin 1A interaction in response to secretagogue activation is an important mechanism allowing for dynamic regulation of the secretory response.

SNAP-23 and syntaxin-2 localize to the extracellular surface of the platelet plasma membrane

SNARE proteins direct membrane fusion events required for platelet granule secretion. These proteins are oriented in cell membranes such that most of the protein resides in a cytosolic compartment. Evaluation of SNARE protein localization in activated platelets using immunonanogold staining and electron microscopy, however, demonstrated expression of SNAP-23 and syntaxin-2 on the extracellular surface of the platelet plasma membrane. Flow cytometry of intact platelets confirmed trypsin-sensitive SNAP-23 and syntaxin-2 localization to the extracellular surface of the plasma membrane. Acyl-protein thioesterase 1 and botulinum toxin C light chain released SNAP-23 and syntaxin-2, respectively, from the surface of intact platelets. When resting platelets were incubated with both acyl-protein thioesterase 1 and botulinum toxin C light chain, a complex that included both SNAP-23 and syntaxin-2 was detected in supernatants, indicating that extracellular SNARE proteins retain their ability to bind one another. These observations represent the first description of SNARE proteins on the extracellular surface of a cell.

Selective Activation of Cognate SNAREpins by Sec1/Munc18 Proteins

Sec1/Munc18 (SM) proteins are required for every step of intracellular membrane fusion, but their molecular mechanism of action has been unclear. In this work, we demonstrate a fundamental role of the SM protein: to act as a stimulatory subunit of its cognate SNARE fusion machinery. In a reconstituted system, mammalian SNARE pairs assemble between bilayers to drive a basal fusion reaction. Munc18-1/nSec1, a synaptic SM protein required for neurotransmitter release, strongly accelerates this reaction through direct contact with both t- and v-SNAREs. Munc18-1 accelerates fusion only for the cognate SNAREs for exocytosis, therefore enhancing fusion specificity.

Syntaxin 1A promotes the endocytic sorting of EAAC1 leading to inhibition of glutamate transport

The neuronal glutamate transporter, excitatory amino-acid carrier 1 (EAAC1), plays an important role in the modulation of neurotransmission and contributes to synthesis of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) and to epileptogenesis. However, the mechanisms that regulate EAAC1 endocytic sorting and function remain largely unknown. Here, we first demonstrate that EAAC1 undergoes internalization through the clathrin-mediated pathway and further show that syntaxin 1A, a key molecule in synaptic exocytosis, potentiates EAAC1 internalization, thus leading to the functional inhibition of EAAC1. In the presence of the transmembrane domain of syntaxin 1A, its H3 coiled-coil domain of syntaxin 1A is necessary and sufficient for the inhibition of EAAC1. Furthermore, specific suppression of endogenous syntaxin 1A significantly blocked EAAC1 endocytic sorting and lysosomal degradation promoted by kainic acid, a drug for kindling the animal model of human temporal lobe epilepsy in rat, indicating a potential role of syntaxin 1A in epileptogenesis. These findings provide new evidence that syntaxin 1A serves as an intrinsic enhancer to EAAC1 endocytic sorting and further suggest that syntaxin 1A is conversant with both ;ins' and ;outs' of synaptic neurotransmission.

Distinct domain-dependent effect of syntaxin1A on amiloride-sensitive sodium channel (ENaC) currents in HT-29 colonic epithelial cells

The amiloride-sensitive epithelial sodium channel (ENaC), a plasma membrane protein mediates sodium reabsorption in epithelial tissues, including the distal nephron and colon. Syntaxin1A, a trafficking protein of the t-SNARE family has been reported to inhibit ENaC in the Xenopus oocyte expression and artificial lipid bilayer systems. The present report describes the regulation of the epithelial sodium channel by syntaxin1A in a human cell line that is physiologically relevant as it expresses both components and also responds to aldosterone stimulation. In order to evaluate the physiological significance of syntaxin1A interaction with natively expressed ENaC, we over-expressed HT-29 with syntaxin1A constructs comprising various motifs. Unexpectedly, we observed the augmentation of amiloride-sensitive currents with wild-type syntaxin1A full-length construct (1-288) in this cell line. Both γENaC and neutralizing syntaxin1A antibodies blocked native expression as amiloride-sensitive sodium currents were inhibited while munc18-1 antibody reversed this effect. The coiled-coiled domain H3 (194-266) of syntaxin1A inhibited, however the inclusion of the transmembrane domain to this motif (194-288) augmented amiloride sensitive currents. More so, data suggest that ENaC interacts with multiple syntaxin1A domains, which differentially regulate channel function. This functional modulation is the consequence of the physical enhancement of ENaC at the cell surface in cells over-expressed with syntaxin(s). Our data further suggest that syntaxin1A up-regulates ENaC function by multiple mechanisms that include PKA, PLC, PI3 and MAP Kinase (p42/44) signaling systems. We propose that syntaxin1A possesses distinct inhibitory and stimulatory domains that interact with ENaC subunits, which critically determines the overall ENaC functionality/regulation under distinct physiological conditions.

Apical targeting of syntaxin 3 is essential for epithelial cell polarity

In polarized epithelial cells, syntaxin 3 localizes to the apical plasma membrane and is involved in membrane fusion of apical trafficking pathways. We show that syntaxin 3 contains a necessary and sufficient apical targeting signal centered around a conserved FMDE motif. Mutation of any of three critical residues within this motif leads to loss of specific apical targeting. Modeling based on the known structure of syntaxin 1 revealed that these residues are exposed on the surface of a three-helix bundle. Syntaxin 3 targeting does not require binding to Munc18b. Instead, syntaxin 3 recruits Munc18b to the plasma membrane. Expression of mislocalized mutant syntaxin 3 in Madin-Darby canine kidney cells leads to basolateral mistargeting of apical membrane proteins, disturbance of tight junction formation, and loss of ability to form an organized polarized epithelium. These results indicate that SNARE proteins contribute to the overall specificity of membrane trafficking in vivo, and that the polarity of syntaxin 3 is essential for epithelial cell polarization.

Syntenin-1 is a new component of tetraspanin-enriched microdomains: mechanisms and consequences of the interaction of syntenin-1 with CD63

Tetraspanins are clustered in specific microdomains (named tetraspanin-enriched microdomains, or TERM) in the plasma membrane and regulate the functions of associated transmembrane receptors, including integrins and receptor tyrosine kinases. We have identified syntenin-1, a PDZ domain-containing protein, as a new component of TERM and show that syntenin-1 specifically interacts with the tetraspanin CD63. Detailed biochemical and heteronuclear magnetic resonance spectroscopy (NMR) studies have demonstrated that the interaction is mediated by the C-terminal cytoplasmic region of the tetraspanin and the PDZ domains of syntenin-1. Upon interaction, NMR chemical shift perturbations were predominantly localized to residues around the binding pocket of PDZ1, indicating a specific mode of recognition of the cytoplasmic tail of CD63. In addition, the C terminus of syntenin-1 has a stabilizing role in the CD63-syntenin-1 association, as deletion of the last 17 amino acids abolished the interaction. The CD63-syntenin-1 complex is abundant on the plasma membrane, and the elevated expression of the wild-type syntenin-1 slows down constitutive internalization of the tetraspanin. Furthermore, internalization of CD63 was completely blocked in cells expressing a syntenin-1 mutant lacking the first 100 amino acids. Previous results have shown that CD63 is internalized via AP-2-dependent mechanisms. Hence, our data indicate that syntenin-1 can counteract the AP-2-dependent internalization and identify this tandem PDZ protein as a new regulator of endocytosis.

SNAP-25/syntaxin 1A complex functionally modulates neurotransmitter gamma-aminobutyric acid reuptake

Neurotransmitter gamma-aminobutyric acid (GABA) release to the synaptic clefts is mediated by the formation of a soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex, which includes two target SNAREs syntaxin 1A and SNAP-25 and one vesicle SNARE VAMP-2. The target SNAREs syntaxin 1A and SNAP-25 form a heterodimer, the putative intermediate of the SNARE complex. Neurotransmitter GABA clearance from synaptic clefts is carried out by the reuptake function of its transporters to terminate the postsynaptic signaling. Syntaxin 1A directly binds to the neuronal GABA transporter GAT-1 and inhibits its reuptake function. However, whether other SNARE proteins or SNARE complex regulates GABA reuptake remains unknown. Here we demonstrate that SNAP-25 efficiently inhibits GAT-1 reuptake function in the presence of syntaxin 1A. This inhibition depends on SNAP-25/syntaxin 1A complex formation. The H3 domain of syntaxin 1A is identified as the binding sites for both SNAP-25 and GAT-1. SNAP-25 binding to syntaxin 1A greatly potentiates the physical interaction of syntaxin 1A with GAT-1 and significantly enhances the syntaxin 1A-mediated inhibition of GAT-1 reuptake function. Furthermore, nitric oxide, which promotes SNAP-25 binding to syntaxin 1A to form the SNARE complex, also potentiates the interaction of syntaxin 1A with GAT-1 and suppresses GABA reuptake by GAT-1. Thus our findings delineate a further molecular mechanism for the regulation of GABA reuptake by a target SNARE complex and suggest a direct coordination between GABA release and reuptake.

The cysteine string protein multimeric complex

Cysteine string protein (CSPalpha) is a member of the cellular folding machinery that is located on regulated secretory vesicles. We have previously shown that CSPalpha in association with Hsc70 (70kDa heat shock cognate protein) and SGT (small glutamine-rich tetratricopeptide repeat domain protein) is a guanine nucleotide exchange factor (GEF) for G(alphas). Association of this CSPalpha complex with N-type calcium channels, a channel key in coupling calcium influx with synaptic vesicle exocytosis, triggers tonic G protein inhibition of the channels. Syntaxin 1A, a plasma membrane SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) critical for neurotransmission, coimmunoprecipitates with the CSPalpha/G protein/N-type calcium channel complex, however the significance of syntaxin 1A as a component of this complex remains unknown. In this report, we establish that syntaxin 1A interacts with CSPalpha, Hsc70 as well as the synaptic protein interaction (synprint) region of N-type channels. We demonstrate that huntingtin(exon1), a putative biologically active fragment of huntingtin, displaces both syntaxin 1A and CSPalpha from N-type channels. Identification of the protein components of the CSPalpha/GEF system is essential in establishing its precise role in synaptic transmission.

Domain Requirement for the Membrane Trafficking and Targeting of Syntaxin 1A

Syntaxin plays a key role in intracellular membrane fusion in eukaryotic cells. The function of syntaxin relies on its proper trafficking to and targeting at the target membrane. The mechanisms underlying the trafficking and targeting of syntaxin to its physiological sites remain poorly understood. Here we have analyzed the trafficking of syntaxin 1A in INS-1 and CHO cells. We have identified the transmembrane domain together with several flanking positive-charged amino acids as the minimal domain required for the membrane delivery. Interestingly, we found that SNARE motif-exposed syntaxin 1A mutants were retained in endoplasmic reticulum (ER) and failed to transport to the cell surface in the absence of SNAP-25, suggesting that the exposure of the SNARE motif causes ER retention and complexation with SNAP-25 helps the ER escape. Finally, our data propose two key roles for the H(abc) domain: to protect nonspecific interaction by masking the SNARE motif and to participate in the clustering of syntaxin 1A to the fusion sites in the plasma membrane.

The role of SNARE proteins in trafficking and function of neurotransmitter transporters

The SNARE hypothesis of vesicle fusion proposes that a series of protein-protein interactions governs the delivery of vesicles to various membrane targets such as the Golgi network and the plasma membrane. Key players in this process include members of the syntaxin family of membrane proteins. The first member identified in this family, syntaxin 1A, plays an essential role in the docking and fusion of neurotransmitter-containing vesicles to the presynaptic membrane of neurons. Syntaxin 1A and other syntaxin family members have also been shown to interact with, and directly regulate, a variety of ion channels. More recently, the family of plasma membrane neurotransmitter transporters, proteins that function in part to control transmitter levels in brain, have been shown to be direct targets of syntaxin 1A regulation. This regulation involves both the trafficking of transporters as well as the control of ion and transmitter flux through transporters. In this chapter, the functional effects of syntaxin-transporter interactions are reviewed, and how such interactions may regulate neuronal signaling are considered.

Slp4-a/granuphilin-a interacts with syntaxin-2/3 in a Munc18-2-dependent manner

Slp4-a/granuphilin-a was originally described as a protein specifically associated with insulin-containing granules in pancreatic beta-cells, but it was subsequently found to be present on amylase-containing granules in parotid acinar cells. Although Slp4-a has been suggested to control insulin secretion through interaction with syntaxin-1a and/or Munc18-1, nothing is known about the binding partner(s) of Slp4-a during amylase release from parotid acinar cells, which do not endogenously express either syntaxin-1a or Munc18-1. In this study we systematically investigated the interaction between syntaxin-1-5 and Munc18-1-3 by co-immunoprecipitation assay using COS-7 cells and discovered that Slp4-a interacts with a closed conformation of syntaxin-2/3 in a Munc18-2-dependent manner, whereas Munc18-2 itself hardly interacts with Slp4-a at all. By contrast, Slp4-a was found to strongly interact with Munc18-1 regardless of the presence of syntaxin-2/3, and syntaxin-2/3 co-immunoprecipitated with Slp4-a only in the presence of Munc18-1/2. Deletion analysis showed that the syntaxin-2/3 (or Munc18-1/2)-binding site is a linker domain of Slp4-a (amino acid residues 144-354), a previously uncharacterized region located between the N-terminal Rab27A binding domain and the C2A domain. We also found that the Slp4-a.syntaxin-2 complex is actually present in rat parotid glands and that introduction of the antibody against Slp4-a linker domain into streptolysin O-permeabilized parotid acinar cells severely attenuates isoproterenol-stimulated amylase release, possibly by disrupting the interaction between Slp4-a and syntaxin-2/3 (or Munc18-2). These results suggest that Slp4-a modulates amylase release from parotid acinar cells through interaction with syntaxin-2/3 on the apical plasma membrane.