Kinetics of complexin binding to the SNARE complex: correcting single molecule FRET measurements for hidden events

Complexins: Ubiquitously Expressed Presynaptic Regulators of SNARE-Mediated Synaptic Vesicle Fusion

Neurotransmitter release is a spatially and temporally tightly regulated process, which requires assembly and disassembly of SNARE complexes to enable the exocytosis of transmitter-loaded synaptic vesicles (SVs) at presynaptic active zones (AZs). While the requirement for the core SNARE machinery is shared by most membrane fusion processes, SNARE-mediated fusion at AZs is uniquely regulated to allow very rapid Ca2+-triggered SV exocytosis following action potential (AP) arrival. To enable a sub-millisecond time course of AP-triggered SV fusion, synapse-specific accessory SNARE-binding proteins are required in addition to the core fusion machinery. Among the known SNARE regulators specific for Ca2+-triggered SV fusion are complexins, which are almost ubiquitously expressed in neurons. This chapter summarizes the structural features of complexins, models for their molecular interactions with SNAREs, and their roles in SV fusion.

Stabilization of the SNARE Core by Complexin-1 Facilitates Fusion Pore Expansion

In the neuron, neurotransmitter release is an essential function that must be both consistent and tightly regulated. The continuity of neurotransmitter release is dependent in large part on vesicle recycling. However, the protein factors that dictate the vesicle recycling pathway are elusive. Here, we use a single vesicle-to-supported bilayer fusion assay to investigate complexin-1 (cpx1)’s influence on SNARE-dependent fusion pore expansion. With total internal reflection (TIR) microscopy using a 10 kDa polymer fluorescence probe, we are able to detect the presence of large fusion pores. With cpx1, however, we observe a significant increase of the probability of the formation of large fusion pores. The domain deletion analysis reveals that the SNARE-binding core domain of cpx1 is mainly responsible for its ability to promote the fusion pore expansion. In addition, the results show that cpx1 helps the pore to expand larger, which results in faster release of the polymer probe. Thus, the results demonstrate a reciprocal relationship between event duration and the size of the fusion pore. Based on the data, a hypothetical mechanistic model can be deduced. In this mechanistic model, the cpx1 binding stabilizes the four-helix bundle structure of the SNARE core throughout the fusion pore expansion, whereby the highly curved bilayer within the fusion pore is stabilized by the SNARE pins.

Cell fusion in cancer hallmarks: Current research status and future indications

Cell fusion is involved in several physiological processes, such as reproduction, development and immunity. Although cell fusion in tumors was reported 130 years ago, it has recently attracted great interest, with recent progress in tumorigenesis research. However, the role of cell fusion in tumor progression remains unclear. The pattern of cell fusion and its role under physiological conditions are the basis for our understanding of the pathological role of cell fusion. However, the role of cell fusion in tumors and its functions are complicated. Cell fusion can directly increase tumor heterogeneity by forming polyploids or aneuploidies. Several studies have reported that cell fusion is associated with tumorigenesis, metastasis, recurrence, drug resistance and the formation of cancer stem cells. Given the diverse roles cell fusion plays in different tumor phenotypes, methods based on targeted cell fusion have been designed to treat tumors. Research on cell fusion in tumors may provide novel ideas for further treatment.

Super-Resolution Imaging of Competitive Unlabeled DNA Hybridization Reveals the Influence of Fluorescent Labels on Duplex Formation and Dissociation Kinetics

Single-molecule fluorescence imaging is a powerful method to measure reversible reaction kinetics, allowing one to monitor the bound state of individual probe molecules with fluorescently labeled targets. In the case of DNA hybridization, previous studies have shown that the presence of a fluorescent label on a target strand can exhibit significant influence on the stability of a DNA duplex that is formed. In this work, we have developed a super-resolution imaging method to measure the hybridization kinetics of unlabeled target DNA that compete with a fluorescently labeled tracer DNA strand to hybridize with an unlabeled probe DNA immobilized at a surface. The hybridization of an unlabeled DNA target cannot be detected directly, but its presence blocks the immobilized probe DNA, influencing the measured time intervals between labeled DNA hybridization events. We derive a model for competitive hybridization kinetics to extract the association and dissociation rate constants of the unlabeled species from the distribution of time intervals between hybridization events of the labeled tracer DNA at individual localized DNA probe sites. We use this methodology to determine the hybridization kinetics of a model 11-mer unlabeled target DNA strand and then determine how five different fluorescent labels attached to the same target DNA strand impact the hybridization kinetics. Compared to the unlabeled target, these labels can slow the association and dissociation rates by as much as a factor of 5. The super-resolution time-interval methodology provides a unique approach to determining fundamental (label-free) rates of DNA hybridization, revealing the significant influence of fluorescent labels on these kinetics. This measurement concept can be extended to studies of other reversible reaction systems, where kinetics of unlabeled species can be determined from their influence on the reaction of a labeled species with localized probe molecules on a surface.

Photobleaching statistics in single-molecule on-/off-time distributions

The on- and and off-time distributions from fluorescence single-molecule experiments are widely used to extract kinetics parameters with the goal to provide a quantitative description for the molecule's behavior on the ensemble level. Such experiments are inevitably influenced by photobleaching, where the fluorescent probe transitions to a nonemissive state. Yet, it appears that few reports went beyond acknowledging this unavoidable complication; in fact, it has so far been ignored when evaluating off-time distributions. Here, we present a theoretical framework that allows the derivation of analytical equations in which photobleaching kinetics are rigorously incorporated. Unexpectedly, our results indicate that the off-time distribution should be nonexponential even when all the rate processes are single exponential. With the analytical theory understood and demonstrated as easy to implement, such ubiquitous photochemical processes can now be readily included in routine experimental analyses.

Biochemical studies of membrane fusion at the single-particle level

To study membrane fusion mediated by synaptic proteins, proteoliposomes have been widely used for in vitro ensemble measurements with limited insights into the fusion mechanism. Single-particle techniques have proven to be powerful in overcoming the limitations of traditional ensemble methods. Here, we summarize current single-particle methods in biophysical and biochemical studies of fusion mediated by soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) and other synaptic proteins, together with their advantages and limitations.

Focused clamping of a single neuronal SNARE complex by complexin under high mechanical tension

Neuronal soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) catalyze synaptic vesicle fusion with presynaptic membranes through the formation of SNARE complexes. Complexin (Cpx) is the only presynaptic protein that tightly binds to SNAREs and regulates membrane fusion, but how it modulates the energy landscape of SNARE complex assembly, especially under mechanical tension on the complex, remains unclear. Here, using magnetic tweezers, we report how Cpx interacts with single SNARE complexes. The effects of Cpx manifest only under high mechanical tensions above 13 pN. Cpx stabilizes the central four-helix bundle of SNARE motifs and, at the same time, prevents the complete zippering of SNAREs by inhibiting linker-domain assembly. These results suggest that Cpx generates a focused clamp for the neuronal SNARE complex in a linker-open conformation. Our results provide a hint as to how Cpx cooperates with neuronal SNAREs to prime synaptic vesicles in preparation for synchronous neurotransmitter release.

The SNARE complex enables the fusion of synaptic vesicles with presynaptic membrane via a zippering process that is modulated by the protein complexin, though the precise mechanism remains unclear. Here, the authors used magnetic tweezers to show how complexin prepares a SNARE complex for fusion under mechanical tension.

NSF-mediated disassembly of on- and off-pathway SNARE complexes and inhibition by complexin

SNARE complex disassembly by the ATPase NSF is essential for neurotransmitter release and other membrane trafficking processes. We developed a single-molecule FRET assay to monitor repeated rounds of NSF-mediated disassembly and reassembly of individual SNARE complexes. For ternary neuronal SNARE complexes, disassembly proceeds in a single step within 100 msec. We observed short- (<0.32 s) and long-lived (≥0.32 s) disassembled states. The long-lived states represent fully disassembled SNARE complex, while the short-lived states correspond to failed disassembly or immediate reassembly. Either high ionic strength or decreased αSNAP concentration reduces the disassembly rate while increasing the frequency of short-lived states. NSF is also capable of disassembling anti-parallel ternary SNARE complexes, implicating it in quality control. Finally, complexin-1 competes with αSNAP binding to the SNARE complex; addition of complexin-1 has an effect similar to that of decreasing the αSNAP concentration, possibly differentially regulating cis and trans SNARE complexes disassembly.

The AFF-1 exoplasmic fusogen is required for endocytic scission and seamless tube elongation

Many membranes must merge during cellular trafficking, but fusion and fission events initiating at exoplasmic (non-cytosolic) membrane surfaces are not well understood. Here we show that the C. elegans cell-cell fusogen anchor-cell fusion failure 1 (AFF-1) is required for membrane trafficking events during development of a seamless unicellular tube. EGF-Ras-ERK signaling upregulates AFF-1 expression in the excretory duct tube to promote tube auto-fusion and subsequent lumen elongation. AFF-1 is required for scission of basal endocytic compartments and for apically directed exocytosis to extend the apical membrane. Lumen elongation also requires the transcytosis factor Rab11, but occurs independently of dynamin and clathrin. These results support a transcytosis model of seamless tube lumen growth and show that cell-cell fusogens also can play roles in intracellular membrane trafficking events.

Combining gold nanoparticle antennas with single-molecule fluorescence resonance energy transfer (smFRET) to study DNA hairpin dynamics

The association of a plasmonic nano-antenna with single-molecule FRET technique presents new prospects to investigate the dynamics of biological molecules. However, the presence of a plasmonic nano-antenna significantly modifies the FRET rate and efficiency; this makes its applicability to the prevalent single-molecule FRET experiments unclear. Herein, using gold nanoparticle antennas of different sizes and DNA hairpins labelled with FRET pairs (Cy3 and Cy5) as the model system, we performed experiments to study the folding dynamics of single DNA hairpins at various salt concentrations. Our results indicate that gold nanoparticle antennas can enhance single-molecule fluorescence of Cy3 and Cy5 up to 3-5 folds, substantially reduce the FRET efficiency, and alter the obtained FRET efficiency histograms. However, the folding dynamics of DNA hairpins remains unaffected, and the correct kinetic and dynamic information can still be extracted from the seriously modified FRET efficiencies. Therefore, our experiments demonstrate the feasibility and compatibility for applying plasmonic nano-antennas to the mostly used single-molecule FRET assays, which provide a broad range of possibilities for the future applications of these nano-antennas in studying various essential biological processes.

The hallmarks of cell-cell fusion

Cell-cell fusion is essential for fertilization and organ development. Dedicated proteins known as fusogens are responsible for mediating membrane fusion. However, until recently, these proteins either remained unidentified or were poorly understood at the mechanistic level. Here, we review how fusogens surmount multiple energy barriers to mediate cell-cell fusion. We describe how early preparatory steps bring membranes to a distance of ∼10 nm, while fusogens act in the final approach between membranes. The mechanical force exerted by cell fusogens and the accompanying lipidic rearrangements constitute the hallmarks of cell-cell fusion. Finally, we discuss the relationship between viral and eukaryotic fusogens, highlight a classification scheme regrouping a superfamily of fusogens called Fusexins, and propose new questions and avenues of enquiry.

Complexin induces a conformational change at the membrane-proximal C-terminal end of the SNARE complex

Complexin regulates spontaneous and activates Ca²⁺-triggered neurotransmitter release, yet the molecular mechanisms are still unclear. Here we performed single molecule fluorescence resonance energy transfer experiments and uncovered two conformations of complexin-1 bound to the ternary SNARE complex. In the cis conformation, complexin-1 induces a conformational change at the membrane-proximal C-terminal end of the ternary SNARE complex that specifically depends on the N-terminal, accessory, and central domains of complexin-1. The complexin-1 induced conformation of the ternary SNARE complex may be related to a conformation that is juxtaposing the synaptic vesicle and plasma membranes. In the trans conformation, complexin-1 can simultaneously interact with a ternary SNARE complex via the central domain and a binary SNARE complex consisting of syntaxin-1A and SNAP-25A via the accessory domain. The cis conformation may be involved in activation of synchronous neurotransmitter release, whereas both conformations may be involved in regulating spontaneous release.

DOI:http://dx.doi.org/10.7554/eLife.16886.001

Nerve cells communicate via electrical signals that travel at high speeds. However, these signals cannot pass across the gaps – called synapses – that separate one nerve cell from the next. Instead, signals pass between nerve cells via molecules called neurotransmitters that are released from the membrane of the first cell and recognized by receptors in the membrane of the next. Prior to being released, neurotransmitters are packaged inside bubble-like structures called vesicles. The synaptic vesicles must fuse with the cell membrane in order to release their contents into the synaptic cleft. Proteins called SNAREs work together with other proteins to allow this membrane fusion to occur rapidly after the electrical signal arrives.

Complexin is a synaptic protein that binds tightly to a complex of SNARE proteins to regulate membrane fusion. This protein activates the quick release of neurotransmitters, which is triggered by an increase in calcium ions as the electrical signal reachess the synapse. Complexin also regulates a different type of neurotransmitter release, which is known as “spontaneous release”. The complexin protein is made up of different regions, each of which is required for one or more of the protein’s activities. However, it is not clear how these regions, or domains, interact with SNAREs and other proteins to enable complexin to perform these roles.

Choi et al. have now investigated whether the different activities of mammalian complexin are related to the structure that it adopts when it interacts with the SNARE complex. Complexes of SNARE proteins were assembled with one of the SNARE proteins tethered to a surface for imaging. Next, a light-based imaging technique called single molecule Förster resonance energy transfer (or FRET) was used to monitor how complexin interacts with the SNARE complex. This technique allows individual proteins that have been labeled with fluorescent markers to be followed under a microscope and can show how they interact in real-time.

Using this approach, Choi et al. showed that complexin could adopt two different shapes or conformations when it binds to the SNARE complex. In one, complexin interacted closely with the SNARE complex so that it made part of the complex change shape. In the other, complexin was able to bridge two SNARE complexes. Complexin can therefore interact with SNARE complexes in different ways by using different regions of the protein.

These findings provide insight into how complexin may regulate membrane fusion via the SNARE complex. In the future, single molecule FRET could be used to study other proteins found at synapses and understand the other steps that regulate the release of neurotransmitters.

DOI:http://dx.doi.org/10.7554/eLife.16886.002

Complexins: small but capable

Despite intensive research, it is still unclear how an immediate and profound acceleration of exocytosis is triggered by appropriate Ca(2+)-stimuli in presynaptic terminals. This is due to the fact that the molecular mechanisms of "docking" and "priming" reactions, which set up secretory vesicles to fuse at millisecond time scale, are extremely hard to study. Yet, driven by a fruitful combination of in vitro and in vivo analyses, our mechanistic understanding of Ca(2+)-triggered vesicle fusion has certainly advanced in the past few years. In this review, we aim to highlight recent progress and emerging views on the molecular mechanisms, by which constitutively forming SNAREpins are organized in functional, tightly regulated units for synchronized release. In particular, we will focus on the role of the small regulatory factor complexin whose function in Ca(2+)-dependent exocytosis has been controversially discussed for more than a decade. Special emphasis will also be laid on the functional relationship of complexin and synaptotagmin, as both proteins possibly act as allies and/or antagonists to govern SNARE-mediated exocytosis.

Differential interaction of tomosyn with syntaxin and SNAP25 depends on domains in the WD40 β-propeller core and determines its inhibitory activity

Neuronal exocytosis depends on efficient formation of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complexes and is regulated by tomosyn, a SNARE-binding protein. To gain new information about tomosyn's activity, we characterized its mobility and organization on the plasma membrane (PM) in relation to other SNARE proteins and inhibition of exocytosis. By using direct stochastic optical reconstruction microscopy (dSTORM), we found tomosyn to be organized in small clusters adjacent to syntaxin clusters. In addition, we show that tomosyn is present in both syntaxin-tomosyn complexes and syntaxin-SNAP25-tomosyn complexes. Tomosyn mutants that lack residues 537-578 or 897-917 from its β-propeller core diffused faster on the PM and exhibited reduced binding to SNAP25, suggesting that these mutants shift the equilibrium between tomosyn-syntaxin-SNAP25 complexes on the PM to tomosyn-syntaxin complexes. As these deletion mutants impose less inhibition on exocytosis, we suggest that tomosyn inhibition is mediated via tomosyn-syntaxin-SNAP25 complexes and not tomosyn-syntaxin complexes. These findings characterize, for the first time, tomosyn's dynamics at the PM and its relation to its inhibition of exocytosis.

Dynamics of MutS-mismatched DNA complexes are predictive of their repair phenotypes

MutS recognizes base–base mismatches and base insertions/deletions (IDLs) in newly replicated DNA. Specific interactions between MutS and these errors trigger a cascade of protein–protein interactions that ultimately lead to their repair. The inability to explain why different DNA errors are repaired with widely varying efficiencies in vivo remains an outstanding example of our limited knowledge of this process. Here, we present single-molecule Förster resonance energy transfer measurements of the DNA bending dynamics induced by Thermus aquaticus MutS and the E41A mutant of MutS, which is known to have error specific deficiencies in signaling repair. We compared three DNA mismatches/IDLs (T-bulge, GT, and CC) with repair efficiencies ranging from high to low. We identify three dominant DNA bending states [slightly bent/unbent (U), intermediately bent (I), and significantly bent (B)] and find that the kinetics of interconverting among states varies widely for different complexes. The increased stability of MutS–mismatch/IDL complexes is associated with stabilization of U and lowering of the B to U transition barrier. Destabilization of U is always accompanied by a destabilization of B, supporting the suggestion that B is a “required” precursor to U. Comparison of MutS and MutS-E41A dynamics on GT and the T-bulge suggests that hydrogen bonding to MutS facilitates the changes in base–base hydrogen bonding that are required to achieve the U state, which has been implicated in repair signaling. Taken together with repair propensities, our data suggest that the bending kinetics of MutS–mismatched DNA complexes may control the entry into functional pathways for downstream signaling of repair.

Cancer/testis antigen PAGE4, a regulator of c-Jun transactivation, is phosphorylated by homeodomain-interacting protein kinase 1, a component of the stress-response pathway

Prostate-associated gene 4 (PAGE4) is a cancer/testis antigen that is typically restricted to the testicular germ cells but is aberrantly expressed in cancer. Furthermore, PAGE4 is developmentally regulated with dynamic expression patterns in the developing prostate and is also a stress-response protein that is upregulated in response to cellular stress. PAGE4 interacts with c-Jun, which is activated by the stress-response kinase JNK1, and plays an important role in the development and pathology of the prostate gland. Here, we have identified homeodomain-interacting protein kinase 1 (HIPK1), also a component of the stress-response pathway, as a kinase that phosphorylates PAGE4 at T51. We show that phosphorylation of PAGE4 is critical for its transcriptional activity since mutating this T residue abolishes its ability to potentiate c-Jun transactivation. In vitro single molecule FRET indicates phosphorylation results in compaction of (still) intrinsically disordered PAGE4. Interestingly, however, while our previous observations indicated that the wild-type nonphosphorylated PAGE4 protein interacted with c-Jun [RajagopalanK. et al. (2014) Biochim, Biophys. Acta1842, 154−16324263171], here we show that phosphorylation of PAGE4 weakens its interaction with c-Jun in vitro. These data suggest that phosphorylation induces conformational changes in natively disordered PAGE4 resulting in its decreased affinity for c-Jun to promote interaction of c-Jun with another, unidentified, partner. Alternatively, phosphorylated PAGE4 may induce transcription of a novel partner, which then potentiates c-Jun transactivation. Regardless, the present results clearly implicate PAGE4 as a component of the stress-response pathway and uncover a novel link between components of this pathway and prostatic development and disease.

Small-molecule photostabilizing agents are modifiers of lipid bilayer properties

Small-molecule photostabilizing or protective agents (PAs) provide essential support for the stability demands on fluorescent dyes in single-molecule spectroscopy and fluorescence microscopy. These agents are employed also in studies of cell membranes and model systems mimicking lipid bilayer environments, but there is little information about their possible effects on membrane structure and physical properties. Given the impact of amphipathic small molecules on bilayer properties such as elasticity and intrinsic curvature, we investigated the effects of six commonly used PAs--cyclooctatetraene (COT), para-nitrobenzyl alcohol (NBA), Trolox (TX), 1,4-diazabicyclo[2.2.2]octane (DABCO), para-nitrobenzoic acid (pNBA), and n-propyl gallate (nPG)--on bilayer properties using a gramicidin A (gA)-based fluorescence quench assay to probe for PA-induced changes in the gramicidin monomer↔dimer equilibrium. The experiments were done using fluorophore-loaded large unilamellar vesicles that had been doped with gA, and changes in the gA monomer↔dimer equilibrium were assayed using a gA channel-permeable fluorescence quencher (Tl⁺). Changes in bilayer properties caused by, e.g., PA adsorption at the bilayer/solution interface that alter the equilibrium constant for gA channel formation, and thus the number of conducting gA channels in the large unilamellar vesicle membrane, will be detectable as changes in the rate of Tl⁺ influx-the fluorescence quench rate. Over the experimentally relevant millimolar concentration range, TX, NBA, and pNBA, caused comparable increases in gA channel activity. COT, also in the millimolar range, caused a slight decrease in gA channel activity. nPG increased channel activity at submillimolar concentrations. DABCO did not alter gA activity. Five of the six tested PAs thus alter lipid bilayer properties at experimentally relevant concentrations, which becomes important for the design and analysis of fluorescence studies in cells and model membrane systems. We therefore tested combinations of COT, NBA, and TX; the combinations altered the fluorescence quench rate less than would be predicted assuming their effects on bilayer properties were additive. The combination of equimolar concentrations of COT and NBA caused minimal changes in the fluorescence quench rate.

The Stress-response protein prostate-associated gene 4, interacts with c-Jun and potentiates its transactivation

The Cancer/Testis Antigen (CTA), Prostate-associated Gene 4 (PAGE4), is a stress-response protein that is upregulated in prostate cancer (PCa) especially in precursor lesions that result from inflammatory stress. In cells under stress, translocation of PAGE4 to mitochondria increases while production of reactive oxygen species decreases. Furthermore, PAGE4 is also upregulated in human fetal prostate, underscoring its potential role in development. However, the proteins that interact with PAGE4 and the mechanisms underlying its pleiotropic functions in prostatic development and disease remain unknown. Here, we identified c-Jun as a PAGE4 interacting partner. We show that both PAGE4 and c-Jun are overexpressed in the human fetal prostate; and in cell-based assays, PAGE4 robustly potentiates c-Jun transactivation. Single-molecule Förster resonance energy transfer experiments indicate that upon binding to c-Jun, PAGE4 undergoes conformational changes. However, no interaction is observed in presence of BSA or unilamellar vesicles containing the mitochondrial inner membrane diphosphatidylglycerol lipid marker cardiolipin. Together, our data indicate that PAGE4 specifically interacts with c-Jun and that, conformational dynamics may account for its observed pleiotropic functions. To our knowledge, this is the first report demonstrating crosstalk between a CTA and a proto-oncogene. Disrupting PAGE4/c-Jun interactions using small molecules may represent a novel therapeutic strategy for PCa.

Stabilization of spontaneous neurotransmitter release at ribbon synapses by ribbon-specific subtypes of complexin

Ribbon synapses of tonically releasing sensory neurons must provide a large pool of releasable vesicles for sustained release, while minimizing spontaneous release in the absence of stimulation. Complexins are presynaptic proteins that may accomplish this dual task at conventional synapses by interacting with the molecular machinery of synaptic vesicle fusion at the active zone to retard spontaneous vesicle exocytosis yet facilitate release evoked by depolarization. However, ribbon synapses of photoreceptor cells and bipolar neurons in the retina express distinct complexin subtypes, perhaps reflecting the special requirements of these synapses for tonic release. To investigate the role of ribbon-specific complexins in transmitter release, we combined presynaptic voltage clamp, fluorescence imaging, electron microscopy, and behavioral assays of photoreceptive function in zebrafish. Acute interference with complexin function using a peptide derived from the SNARE-binding domain increased spontaneous synaptic vesicle fusion at ribbon synapses of retinal bipolar neurons without affecting release triggered by depolarization. Knockdown of complexin by injection of an antisense morpholino into zebrafish embryos prevented photoreceptor-driven migration of pigment in skin melanophores and caused the pigment distribution to remain in the dark-adapted state even when embryos were exposed to light. This suggests that loss of complexin function elevated spontaneous release in illuminated photoreceptors sufficiently to mimic the higher release rate normally associated with darkness, thus interfering with visual signaling. We conclude that visual system-specific complexins are required for proper illumination-dependent modulation of the rate of neurotransmitter release at visual system ribbon synapses.

Molecular mechanisms of COMPLEXIN fusion clamp function in synaptic exocytosis revealed in a new Drosophila mutant

The COMPLEXIN (CPX) proteins play a critical role in synaptic vesicle fusion and neurotransmitter release. Previous studies demonstrated that CPX functions in both activation of evoked neurotransmitter release and inhibition/clamping of spontaneous synaptic vesicle fusion. Here we report a new cpx mutant in Drosophila melanogaster, cpx(1257), revealing spatially defined and separable pools of CPX which make distinct contributions to the activation and clamping functions. In cpx(1257), lack of only the last C-terminal amino acid of CPX is predicted to disrupt prenylation and membrane targeting of CPX. Immunocytochemical analysis established localization of wild-type CPX to active zone (AZ) regions containing neurotransmitter release sites as well as broader presynaptic membrane compartments including synaptic vesicles. Parallel biochemical studies confirmed CPX membrane association and demonstrated robust binding interactions of CPX with all three SNAREs. This is in contrast to the cpx(1257) mutant, in which AZ localization of CPX persists but general membrane localization and, surprisingly, the bulk of CPX-SNARE protein interactions are abolished. Furthermore, electrophysiological analysis of neuromuscular synapses revealed interesting differences between cpx(1257) and a cpx null mutant. The cpx null exhibited a marked decrease in the EPSC amplitude, slowed EPSC rise and decay times and an increased mEPSC frequency with respect to wild-type. In contrast, cpx(1257) exhibited a wild-type EPSC with an increased mEPSC frequency and thus a selective failure to clamp spontaneous release. These results indicate that spatially distinct and separable interactions of CPX with presynaptic membranes and SNARE proteins mediate separable activation and clamping functions of CPX in neurotransmitter release.

Large conformational changes in MutS during DNA scanning, mismatch recognition and repair signalling

MutS protein recognizes mispaired bases in DNA and targets them for mismatch repair. Little is known about the transient conformations of MutS as it signals initiation of repair. We have used single-molecule fluorescence resonance energy transfer (FRET) measurements to report the conformational dynamics of MutS during this process. We find that the DNA-binding domains of MutS dynamically interconvert among multiple conformations when the protein is free and while it scans homoduplex DNA. Mismatch recognition restricts MutS conformation to a single state. Steady-state measurements in the presence of nucleotides suggest that both ATP and ADP must be bound to MutS during its conversion to a sliding clamp form that signals repair. The transition from mismatch recognition to the sliding clamp occurs via two sequential conformational changes. These intermediate conformations of the MutS:DNA complex persist for seconds, providing ample opportunity for interaction with downstream proteins required for repair.

Single-molecule FRET study of SNARE-mediated membrane fusion

Membrane fusion is one of the most important cellular processes by which two initially distinct lipid bilayers merge their hydrophobic cores, resulting in one interconnected structure. Proteins, called SNARE (soluble N-ethylmaleimide-sensitive factor-attachment protein receptor), play a central role in the fusion process that is also regulated by several accessory proteins. In order to study the SNARE-mediated membrane fusion, the in vitro protein reconstitution assay involving ensemble FRET (fluorescence resonance energy transfer) has been used over a decade. In this mini-review, we describe several single-molecule-based FRET approaches that have been applied to this field to overcome the shortage of the bulk assay in terms of protein and fusion dynamics.

Four-color single-molecule fluorescence with noncovalent dye labeling to monitor dynamic multimolecular complexes

To enable studies of conformational changes within multimolecular complexes, we present a simultaneous, four-color single molecule fluorescence methodology implemented with total internal reflection illumination and camera-based, wide-field detection. We further demonstrate labeling histidine-tagged proteins noncovalently with Tris-nitrilotriacetic acid (Tris-NTA)-conjugated dyes to achieve single molecule detection. We combine these methods to colocalize the mismatch repair protein MutSα on DNA while monitoring MutSα-induced DNA bending using Förster resonance energy transfer (FRET) and to monitor assembly of membrane-tethered SNARE protein complexes.

Binding of the complexin N terminus to the SNARE complex potentiates synaptic-vesicle fusogenicity

Complexins facilitate and inhibit neurotransmitter release through distinct domains, and their function was proposed to be coupled to the Ca(2+) sensor synaptotagmin-1 (Syt1). However, the mechanisms underlying complexin function remain unclear. We now uncover an interaction between the complexin N terminus and the SNARE complex C terminus, and we show that disrupting this interaction abolishes the facilitatory function of complexins in mouse neurons. Analyses of hypertonically induced exocytosis show that complexins enhance synaptic-vesicle fusogenicity. Genetic experiments crossing complexin- and Syt1-null mice indicate a functional interaction between these proteins but also show that complexins can promote Ca(2+)-triggered release in the absence of Syt1. We propose that the complexin N terminus stabilizes the SNARE complex C terminus and/or helps release the inhibitory function of complexins, thereby activating the fusion machinery in a manner that may cooperate with Syt1 but does not require Syt1.

Real-time visualization of complexin during single exocytic events

Understanding the fundamental role of soluble NSF attachment protein receptor (SNARE) complexes in membrane fusion requires knowledge of the spatiotemporal dynamics of their assembly. We visualized complexin (cplx), a cytosolic protein that binds assembled SNARE complexes, during single exocytic events in live cells. We found that cplx appeared briefly during full fusion. However, a truncated version of cplx containing only the SNARE-complex binding region persisted at fusion sites for seconds and caused fusion to be transient. Resealing pores with the mutant cplx only partially released transmitter and lipid probes, indicating that the pores are narrow and not purely lipidic in structure. Depletion of cplx similarly caused secretory cargo to be retained. These data suggest that cplx is recruited at a late step in exocytosis and modulates fusion pores composed of SNARE complexes.

Single-molecule FRET-derived model of the synaptotagmin 1-SNARE fusion complex

Synchronous neurotransmission is triggered when Ca(2+) binds to synaptotagmin 1 (Syt1), a synaptic-vesicle protein that interacts with SNAREs and membranes. We used single-molecule fluorescence resonance energy transfer (FRET) between synaptotagmin's two C2 domains to determine that their conformation consists of multiple states with occasional transitions, consistent with domains in random relative motion. SNARE binding results in narrower intrasynaptotagmin FRET distributions and less frequent transitions between states. We obtained an experimentally determined model of the elusive Syt1-SNARE complex using a multibody docking approach with 34 FRET-derived distances as restraints. The Ca(2+)-binding loops point away from the SNARE complex, so they may interact with the same membrane. The loop arrangement is similar to that of the crystal structure of SNARE-induced Ca(2+)-bound Syt3, suggesting a common mechanism by which the interaction between synaptotagmins and SNAREs aids in Ca(2+)-triggered fusion.

Detecting the conformation of individual proteins in live cells

We combined single molecule fluorescence resonance energy transfer (smFRET) with single particle tracking in live cells to detect the in vivo conformation of individual proteins. We site-specifically labeled recombinant SNARE proteins with a FRET donor and acceptor before microinjecting them into cultured cells. We observed that individual proteins rapidly incorporated into folded complexes at the cell membrane, demonstrating the potential of this method to reveal dynamic interactions within cells.

Vesicle-associated membrane protein (VAMP) cleavage by a new metalloprotease from the Brazilian scorpion Tityus serrulatus

We present evidence that venom from the Brazilian scorpion Tityus serrulatus and a purified fraction selectively cleave essential SNARE proteins within exocrine pancreatic tissue. Western blotting for vesicle-associated membrane protein type v-SNARE proteins (or synaptobrevins) reveals characteristic alterations to venom-treated excised pancreatic lobules in vitro. Immunocytochemistry by electron microscopy confirms both the SNARE identity as VAMP2 and the proteolysis of VAMP2 as a marked decrease in secondary antibody-conjugated colloidal gold particles that are predominantly associated with mature zymogen granules. Studies with recombinant SNARE proteins were used to determine the specific cleavage site in VAMP2 and the susceptibility of VAMP8 (endobrevin). The VAMP2 cleavage site is between the transmembrane anchor and the SNARE motif that assembles into the ternary SNARE complex. Inclusion of divalent chelating agents (EDTA) with fraction nu, an otherwise active purified component from venom, eliminates SNARE proteolysis, suggesting the active protein is a metalloprotease. The unique cleavages of VAMP2 and VAMP8 may be linked to pancreatitis that develops following scorpion envenomation as both of these v-SNARE proteins are associated with zymogen granule membranes in pancreatic acinar cells. We have isolated antarease, a metalloprotease from fraction nu that cleaves VAMP2, and report its amino acid sequence.

Tilting the balance between facilitatory and inhibitory functions of mammalian and Drosophila Complexins orchestrates synaptic vesicle exocytosis

SNARE-mediated synaptic exocytosis is orchestrated by facilitatory and inhibitory mechanisms. Genetic ablations of Complexins, a family of SNARE-complex-binding proteins, in mice and Drosophila cause apparently opposite effects on neurotransmitter release, leading to contradictory hypotheses of Complexin function. Reconstitution experiments with different fusion assays and Complexins also yield conflicting results. We therefore performed cross-species rescue experiments to compare the functions of murine and Drosophila Complexins in both mouse and fly synapses. We found that murine and Drosophila Complexins employ conserved mechanisms to regulate exocytosis despite their strikingly different overall effects on neurotransmitter release. Both Complexins contain distinct domains that facilitate or inhibit synaptic vesicle fusion, and the strength of each facilitatory or inhibitory function differs significantly between murine and Drosophila Complexins. Our results show that a relative shift in the balance of facilitatory and inhibitory functions results in differential regulation of neurotransmitter release by murine and Drosophila Complexins in vivo, reconciling previous incompatible findings.

Single-molecule studies of the neuronal SNARE fusion machinery

SNAREs are essential components of the machinery for Ca(2+)-triggered fusion of synaptic vesicles with the plasma membrane, resulting in neurotransmitter release into the synaptic cleft. Although much is known about their biophysical and structural properties and their interactions with accessory proteins such as the Ca(2+) sensor synaptotagmin, their precise role in membrane fusion remains an enigma. Ensemble studies of liposomes with reconstituted SNAREs have demonstrated that SNAREs and accessory proteins can trigger lipid mixing/fusion, but the inability to study individual fusion events has precluded molecular insights into the fusion process. Thus, this field is ripe for studies with single-molecule methodology. In this review, we discuss applications of single-molecule approaches to observe reconstituted SNAREs, their complexes, associated proteins, and their effect on biological membranes. Some of the findings are provocative, such as the possibility of parallel and antiparallel SNARE complexes or of vesicle docking with only syntaxin and synaptobrevin, but have been confirmed by other experiments.

Single-molecule fluorescence studies of a PH domain: new insights into the membrane docking reaction

Proteins containing membrane targeting domains play essential roles in many cellular signaling pathways. However, important features of the membrane-bound state are invisible to bulk methods, thereby hindering mechanistic analysis of membrane targeting reactions. Here we use total internal reflection fluorescence microscopy (TIRFM), combined with single particle tracking, to probe the membrane docking mechanism of a representative pleckstrin homology (PH) domain isolated from the general receptor for phosphoinositides, isoform 1 (GRP1). The findings show three previously undescribed features of GRP1 PH domain docking to membranes containing its rare target lipid, phosphatidylinositol (3,4,5)-trisphosphate [PI(3,4,5)P(3)]. First, analysis of surface diffusion kinetics on supported lipid bilayers shows that in the absence of other anionic lipids, the PI(3,4,5)P(3)-bound protein exhibits the same diffusion constant as a single lipid molecule. Second, the binding of the anionic lipid phosphatidylserine to a previously unidentified secondary binding site slows both diffusion and dissociation kinetics. Third, TIRFM enables direct observation of rare events in which dissociation from the membrane surface is followed by transient diffusion through solution and rapid rebinding to a nearby, membrane-associated target lipid. Overall, this study shows that in vitro single-molecule TIRFM provides a new window into the molecular mechanisms of membrane docking reactions.

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

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

Molecular form follows function: (un)snaring the SNAREs

Exocytotic release of transmitters is mediated by the ternary SNARE complex. The form of this complex is consistent with its function in the positioning of vesicles to the plasma membrane and their fusion to it. Recent advances in single-molecule techniques, however, bring an additional layer of complexity to this process, implicating that there might be various modes of operation. For example, the binary syntaxin-synaptobrevin 2 complex, in addition to the ternary complex containing SNAP25, might enable vesicular docking. Single-molecule techniques allow direct measurements of the distance/extension, rupture force, spontaneous dissociation times and interaction energy for SNARE protein-protein interactions. These measurements are complementary to results and conclusions drawn from other techniques. Consequently, single-molecule techniques promise tremendous opportunities for in vitro investigations of SNARE proteins to improve our understanding of their role in exocytosis.

The F-techniques: advances in receptor protein studies

Recent developments in advanced microscopy techniques, the so-called F-techniques, including Förster resonance energy transfer, fluorescence correlation spectroscopy and fluorescence lifetime imaging, have led to a wide range of novel applications in biology. The F-techniques provide quantitative information on biomolecules and their interactions and give high spatial and temporal resolution. In particular, their application to receptor protein studies has led to new insights into receptor localization, oligomerization, activation and function in vivo. This review focuses on the application of the F-techniques to the study of receptor molecules and mechanisms in the last three years and provides information on new modalities that will further improve their applicability and widen the range of biological questions that can be addressed.

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

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

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

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

Tag team action at the synapse

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