Single Molecule Measurements of Interaction Free Energies Between the Proteins Within Binary and Ternary SNARE Complexes

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.

Florigen trafficking integrates photoperiod and temperature signals in Arabidopsis

The transition to flowering is the most dramatic phase change in flowering plants and is crucial for reproductive success. A complex regulatory network in plants has evolved to perceive and integrate the endogenous and environmental signals. These signals perceived, including day length and temperature, converge to regulate FLOWERING LOCUS T (FT), which encodes a mobile stimulus required for floral induction in Arabidopsis. Despite the discovery of modulation of FT messenger RNA (mRNA) expression by ambient temperature, whether the trafficking of FT protein is controlled in response to changes in growth temperature is so far unknown. Here, we show that FT transport from companion cells to sieve elements is controlled in a temperature-dependent manner. This process is mediated by multiple C2 domain and transmembrane region proteins (MCTPs) and a soluble N-ethylmaleimide-sensitive factor protein attachment protein receptor (SNARE). Our findings suggest that ambient temperatures regulate both FT mRNA expression and FT protein trafficking to prevent precocious flowering at low temperatures and ensure plant reproductive success under favorable environmental conditions.

v-SNARE function in chromaffin cells

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

Energetics, kinetics, and pathway of SNARE folding and assembly revealed by optical tweezers

Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) are universal molecular engines that drive membrane fusion. Particularly, synaptic SNAREs mediate fast calcium-triggered fusion of neurotransmitter-containing vesicles with plasma membranes for synaptic transmission, the basis of all thought and action. During membrane fusion, complementary SNAREs located on two apposed membranes (often called t- and v-SNAREs) join together to assemble into a parallel four-helix bundle, releasing the energy to overcome the energy barrier for fusion. A long-standing hypothesis suggests that SNAREs act like a zipper to draw the two membranes into proximity and thereby force them to fuse. However, a quantitative test of this SNARE zippering hypothesis was hindered by difficulties to determine the energetics and kinetics of SNARE assembly and to identify the relevant folding intermediates. Here, we first review different approaches that have been applied to study SNARE assembly and then focus on high-resolution optical tweezers. We summarize the folding energies, kinetics, and pathways of both wild-type and mutant SNARE complexes derived from this new approach. These results show that synaptic SNAREs assemble in four distinct stages with different functions: slow N-terminal domain association initiates SNARE assembly; a middle domain suspends and controls SNARE assembly; and rapid sequential zippering of the C-terminal domain and the linker domain directly drive membrane fusion. In addition, the kinetics and pathway of the stagewise assembly are shared by other SNARE complexes. These measurements prove the SNARE zippering hypothesis and suggest new mechanisms for SNARE assembly regulated by other proteins.

The Synaptic Vesicle Release Machinery

Extensive research has yielded crucial insights into the mechanism of neurotransmitter release, and working models for the functions of key proteins involved in release. The SNAREs Syntaxin-1, Synaptobrevin, and SNAP-25 play a central role in membrane fusion, forming SNARE complexes that bridge the vesicle and plasma membranes and that are disassembled by NSF-SNAPs. Exocytosis likely starts with Syntaxin-1 folded into a self-inhibited closed conformation that binds to Munc18-1. Munc13s open Syntaxin-1, orchestrating SNARE complex assembly in an NSF-SNAP-resistant manner together with Munc18-1. In the resulting primed state, with partially assembled SNARE complexes, fusion is inhibited by Synaptotagmin-1 and Complexins, which also perform active functions in release. Upon influx of Ca(2+), Synaptotagmin-1 activates fast release, likely by relieving the inhibition caused by Complexins and cooperating with the SNAREs in bringing the membranes together. Although alternative models exist and fundamental questions remain unanswered, a definitive description of the basic release mechanism may be available soon.

Different infectivity of HIV-1 strains is linked to number of envelope trimers required for entry

HIV-1 enters target cells by virtue of envelope glycoprotein trimers that are incorporated at low density in the viral membrane. How many trimers are required to interact with target cell receptors to mediate virus entry, the HIV entry stoichiometry, still awaits clarification. Here, we provide estimates of the HIV entry stoichiometry utilizing a combined approach of experimental analyses and mathematical modeling. We demonstrate that divergent HIV strains differ in their stoichiometry of entry and require between 1 to 7 trimers, with most strains depending on 2 to 3 trimers to complete infection. Envelope modifications that perturb trimer structure lead to an increase in the entry stoichiometry, as did naturally occurring antibody or entry inhibitor escape mutations. Highlighting the physiological relevance of our findings, a high entry stoichiometry correlated with low virus infectivity and slow virus entry kinetics. The entry stoichiometry therefore directly influences HIV transmission, as trimer number requirements will dictate the infectivity of virus populations and efficacy of neutralizing antibodies. Thereby our results render consideration of stoichiometric concepts relevant for developing antibody-based vaccines and therapeutics against HIV.

Our estimates of the HIV-1 entry stoichiometry, that is the number of envelope glycoprotein trimers needed to mediate fusion of viral and target cell membrane, close an important gap in our understanding of the HIV entry process. As we show, stoichiometric requirements for envelope trimers differ between HIV strains and steer virus entry efficacy and virus entry kinetics. Thus, the entry stoichiometry has important implications for HIV transmission, as demands on trimer numbers will dictate the infectivity of virus populations, target cell preferences and virus inactivation by trimer-targeting inhibitors and neutralizing antibodies. Beyond this, our data contribute to the general understanding of mechanisms and energetic requirements of protein-mediated membrane fusion, as HIV entry proved to follow similar stoichiometries as described for Influenza virus HA and SNARE protein mediated membrane fusion. In summary, our findings provide a relevant contribution towards a refined understanding of HIV-1 entry and pathogenesis with particular importance for ongoing efforts to generate neutralizing antibody based therapeutics and vaccines targeting the HIV-1 envelope trimer.

Common intermediates and kinetics, but different energetics, in the assembly of SNARE proteins

Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) are evolutionarily conserved machines that couple their folding/assembly to membrane fusion. However, it is unclear how these processes are regulated and function. To determine these mechanisms, we characterized the folding energy and kinetics of four representative SNARE complexes at a single-molecule level using high-resolution optical tweezers. We found that all SNARE complexes assemble by the same step-wise zippering mechanism: slow N-terminal domain (NTD) association, a pause in a force-dependent half-zippered intermediate, and fast C-terminal domain (CTD) zippering. The energy release from CTD zippering differs for yeast (13 k_BT) and neuronal SNARE complexes (27 k_BT), and is concentrated at the C-terminal part of CTD zippering. Thus, SNARE complexes share a conserved zippering pathway and polarized energy release to efficiently drive membrane fusion, but generate different amounts of zippering energy to regulate fusion kinetics.

DOI:http://dx.doi.org/10.7554/eLife.03348.001

Many processes in living things need molecules to be transported within, or between, cells. For example, damaged or waste molecules are transported within a cell to structures that can break the molecules down, while nerve impulses are transmitted from one neuron to the next via the release of signaling molecules.

Cells—and the compartments within cells—are surrounded by membranes that act as barriers to certain molecules. Vesicles are small, membrane-enclosed packages that are used to transport molecules between different membranes; and in order to release its cargo, a vesicle must fuse with its target membrane. To fuse like this, the forces that act to push membranes away from one another need to be overcome. Proteins called SNARES, which are embedded in both membranes, are the molecular engines that power the fusion process. Once the SNARE proteins from the vesicle and the target membrane bind, they assemble into a more compact complex that pulls the two membranes close together and allows fusion to take place.

The final shape of an assembled SNARE complex is essentially the same for all SNARE complexes; however, it is not known whether all of these complexes fold using the same method. Now Zorman et al. have used optical tweezers—an instrument that uses a highly focused laser beam to hold and manipulate microscopic objects—to observe the folding and unfolding of four different types of SNARE complex. All four SNARE complexes followed the same step-by-step process: the leading ends of the SNARE proteins slowly bound to each other; the process paused; then the rest of the proteins rapidly ‘zippered’ together.

Zorman et al. revealed that, although the steps in the processes were the same, the energy released in the last step was different when different complexes assembled. This suggests that the energy released by the ‘zippering’ of different SNARE proteins is optimized to match the required speed of different membrane fusion events. Furthermore, Zorman et al. propose that the reason why the majority of energy is released in the later stages of complex assembly is because this is when the repulsion between the two membranes is strongest.

The discoveries of Zorman et al. will now aid future efforts aimed at understanding better how the numerous other proteins that interact with SNARE proteins regulate the process of membrane fusion.

DOI:http://dx.doi.org/10.7554/eLife.03348.002

Temperature dependence of vesicular dynamics at excitatory synapses of rat hippocampus

How vesicular dynamics parameters depend on temperature and how temperature affects the parameter change during prolonged high frequency stimulation was determined by fitting a model of vesicular storage and release to the amplitudes of the excitatory post-synaptic currents (EPSC) recorded from CA1 neurons in rat hippocampal slices. The temperature ranged from low (13 °C) to higher and more physiological temperature (34 °C). Fitting the model of vesicular storage and release to the EPSC amplitudes during a single pair of brief high-low frequency stimulation trains yields the estimates of all parameters of the vesicular dynamics, and with good precision. Both fractional release and replenishment rate decrease as the temperature rises. Change of the underlying 'basic' parameters (release coupling, replenishment coupling and readily releasable pool size), which the model-fitting also yields is complex. The replenishment coupling between the readily releasable pool (RRP) and resting pool increases with temperature (which renders the replenishment rate higher), but this is more than counterbalanced by greater RRP size (which renders the replenishment rate lower). Finally, during long, high frequency patterned stimulation that leads to significant synaptic depression, the replenishment rate decreases markedly and rapidly at low temperatures (<22 °C), but at high temperatures (>28 °C) the replenishment rate rises with stimulation, making synapses better able to maintain synaptic efficacy.

Mechanical unzipping and rezipping of a single SNARE complex reveals hysteresis as a force-generating mechanism

Formation of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex provides mechanical thrust for membrane fusion, but its molecular mechanism is still unclear. Here using magnetic tweezers, we observe mechanical responses of a single neuronal SNARE complex under constant pulling force. Single SNARE complexes may be unzipped with 34 pN force. When rezipping is induced by lowering the force to 11 pN, only a partially assembled state results, with the C-terminal half of the SNARE complex remaining disassembled. Reassembly of the C-terminal half occurs only when the force is further lowered below 11 pN. Thus, mechanical hysteresis, characterized by the unzipping and rezipping cycle of a single SNARE complex, produces the partially assembled state. In this metastable state, unzipping toward the N-terminus is suppressed while zippering toward the C-terminus is initiated as a steep function of force. This ensures the directionality of SNARE-complex formation, making the SNARE complex a robust force-generating machine.

Interactions between (SNARE) proteins on vesicle and target membranes provide the force necessary to drive membrane fusion. By applying piconewton forces to single SNARE complexes, the authors identify a partially assembled intermediate state that reveals how force is generated in a consistent direction.

Single reconstituted neuronal SNARE complexes zipper in three distinct stages

Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins drive membrane fusion by assembling into a four-helix bundle in a zippering process. Here, we used optical tweezers to observe in a cell-free reconstitution experiment in real time a long-sought SNARE assembly intermediate in which only the membrane-distal amino-terminal half of the bundle is assembled. Our findings support the zippering hypothesis, but suggest that zippering proceeds through three sequential binary switches, not continuously, in the amino- and carboxyl-terminal halves of the bundle and the linker domain. The half-zippered intermediate was stabilized by externally applied force that mimicked the repulsion between apposed membranes being forced to fuse. This intermediate then rapidly and forcefully zippered, delivering free energy of 36 k(B)T (where k(B) is Boltzmann's constant and T is temperature) to mediate fusion.

Ternary SNARE complexes in parallel versus anti-parallel orientation: examination of their disassembly using single-molecule force spectroscopy

Interactions between the proteins of the ternary soluble N-ethyl maleimide-sensitive fusion protein attachment protein receptor (SNARE) complex, synaptobrevin 2 (Sb2), syntaxin 1A (Sx1A) and synaptosome-associated protein of 25 kDa (SNAP25) can be readily assessed using force spectroscopy single-molecule measurements. We studied interactions during the disassembly of the ternary SNARE complex pre-formed by binding Sb2 in parallel or anti-parallel orientations to the binary Sx1A-SNAP25B acceptor complex. We determined the spontaneous dissociation lifetimes and found that the stability of the anti-parallel ternary SNARE complex is ∼1/3 less than that of the parallel complex. While the free energies were very similar, within 0.5 k(B)T, for both orientations, the enthalpy changes (42.1 k(B)T and 39.8 k(B)T, for parallel and anti-parallel orientations, respectively) indicate that the parallel ternary complex is energetically advantageous by 2.3 k(B)T. Indeed, both ternary SNARE complex orientations were much more stable (by ∼4-13 times) and energetically favorable (by ∼9-13 k(B)T) than selected binary complexes, constituents of the ternary complex, in both orientations. We propose a model which considers the geometry for the vesicle approach to the plasma membrane with favorable energies and stability as the basis for preferential usage of the parallel ternary SNARE complex in exocytosis.

SNARE requirements en route to exocytosis: from many to few

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

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.

Munc18-1 tuning of vesicle merger and fusion pore properties

The release of hormones and neurotransmitters, mediated by regulated exocytosis, can be modified by regulation of the fusion pore. The fusion pore is considered stable and narrow initially, eventually leading to the complete merger of the vesicle and the plasma membranes. By using the high-resolution patch-clamp capacitance technique, we studied single vesicles and asked whether the Sec1/Munc18 proteins, interacting with the membrane fusion-mediating SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins, affect fusion pore properties. Munc18-1 mutants were transfected into lactotrophs to affect the interaction of Munc18-1 with syntaxin1 (Synt1) (R39C), Rab3A (E466K), and Mints (P242S). Compared with wild-type, Munc18-1 E466K increased the frequency of the fusion event. The latter two mutants increased the fusion pore dwell-time. All the mutants stabilized narrow fusion pores and increased the amplitude of fusion events, likely via preferential fusion of larger vesicles, since overexpression of Munc18-1 R39C did not affect the average size of vesicles, as determined by stimulated emission depletion (STED) microscopy. Single-molecule atomic force microscopy experiments revealed that wild-type Munc18-1, but not Munc18-1 R39C, abrogates the interaction between synaptobrevin2 (Syb2) and Synt1 binary trans-complexes. However, neither form of Munc18-1 affected the interaction of Syb2 with the preformed binary cis-Synt1A-SNAP25B complexes. This indicates that Munc18-1 performs a proofing function by inhibiting tethering of Syb2-containing vesicles solely to Synt1 at the plasmalemma and favoring vesicular tethering to the preformed binary cis-complex of Synt1A-SNAP25B. The association of Munc18-1 with the ternary SNARE complex leads to tuning of fusion pores via multiple and converging mechanisms involving Munc18-1 interactions with Synt1A, Rab3A, and Mints.

Docking and fast fusion of synaptobrevin vesicles depends on the lipid compositions of the vesicle and the acceptor SNARE complex-containing target membrane

The influence of the lipid environment on docking and fusion of synaptobrevin 2 (Syb2) vesicles with target SNARE complex membranes was examined in a planar supported membrane fusion assay with high time-resolution. Previously, we showed that approximately eight SNARE complexes are required to fuse phosphatidylcholine (PC) and cholesterol model membranes in ∼20 ms. Here we present experiments, in which phosphatidylserine (PS) and phosphatidylethanolamine (PE) were added to mixtures of PC/cholesterol in different proportions in the Syb2 vesicle membranes only or in both the supported bilayers and the Syb2 vesicles. We found that PS and PE both reduce the probability of fusion and that this reduction is fully accounted for by the lipid composition in the vesicle membrane. However, the docking efficiency increases when the PE content in the vesicle (and target membrane) is increased from 0 to 30%. The fraction of fast-activating SNARE complexes decreases with increasing PE content. As few as three SNARE complexes are sufficient to support membrane fusion when at least 5% PS and 10% PE are present in both membranes or 5% and 30% PE are present in the vesicle membrane only. Despite the smaller number of required SNAREs, the SNARE activation and fusion rates are almost as fast as previously reported in reconstituted PC/cholesterol bilayers, i.e., ~10 and ~20 ms, respectively [corrected].

SNAREs: could they be the answer to an energy landscape riddle in exocytosis?

During exocytosis, chemical transmitters stored in secretory vesicles can be released upon fusion of these intracellular organelles to the plasma membrane. In this process, SNARE proteins that form a ternary core complex play a central role. This complex could provide the means for generation/storage of energy necessary for driving the fusion of vesicular and plasma membranes. Recently, the amount of energy for (dis)assembly of the ternary complex has been measured using various experimental approaches, including atomic force microscopy, the surface force apparatus, and isothermal titration calorimetry. The obtained measurements are in good agreement with the calculated energy required for membrane fusion achieved by theoretical modeling approaches. Whether the energy expenditure to form the ternary SNARE complex can be utilized towards membrane fusion and/or docking/tethering of vesicles to the plasma membrane still remains one of the key contemporary issues in biophysics and neuroscience.

Single molecule measurements of mechanical interactions within ternary SNARE complexes and dynamics of their disassembly: SNAP25 vs. SNAP23

Regulated exocytosis is a crucial event for intercellular communication between neurons and astrocytes within the CNS. The soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor (SNARE) complex, composed of synaptobrevin 2, syntaxin and synaptosome-associated protein of 25 kDa or 23 kDa (SNAP25 or SNAP23), is essential in this process. It was reported that SNAP25 and SNAP23 have distinct roles in exocytotic release, where SNAP25, but not SNAP23, supports an exocytotic burst. It is not clear, however, whether this is due to the intrinsic properties of the ternary SNARE complex, containing either SNAP25 or SNAP23, or perhaps due to the differential association of these proteins with ancillary proteins to the complex. Here, using force spectroscopy, we show from single molecule investigations of the SNARE complex, that SNAP23A created a local interaction at the ionic layer by cuffing syntaxin 1A and synaptobrevin 2, similar to the action of SNAP25B; thus either of the ternary complexes would allow positioning of vesicles at a maximal distance of approximately 13 nm from the plasma membrane. However, the stability of the ternary SNARE complex containing SNAP23A is less than half of that for the complex containing SNAP25B. Thus, differences in the stability of the two different ternary complexes could underlie some of the SNAP25/23 differential ability to control the exocytotic burst.

Single molecule probing of SNARE proteins by atomic force microscopy

Atomic force microscopy (AFM) in force spectroscopy mode has recently emerged as a technique of choice for studying mechanical interactions between the proteins of the core soluble N-ethylmaleimide-sensitive fusion protein attachment protein attachment protein receptor (SNARE) complex. In these experiments, the rupture force, extension, spontaneous dissociation time, and interaction energy for SNARE protein-protein interactions can be obtained at the single molecule level. These measurements, which are complementary to results and conclusions drawn from other techniques, improve our understanding of the role of the SNARE complex in exocytosis.