Single molecule observation of liposome-bilayer fusion thermally induced by soluble N-ethyl maleimide sensitive-factor attachment protein receptors (SNAREs)

Abstract

A single molecule fluorescence assay is presented for studying the mechanism of soluble N-ethyl maleimide sensitive-factor attachment protein receptors (SNAREs)-mediated liposome fusion to supported lipid bilayers. The three neuronal SNAREs syntaxin-1A, synaptobrevin-II (VAMP), and SNAP-25A were expressed separately, and various dye-labeled combinations of the SNAREs were tested for their ability to dock liposomes and induce fusion. Syntaxin and synaptobrevin in opposing membranes were both necessary and sufficient to dock liposomes to supported bilayers and to induce thermally activated fusion. As little as one SNARE interaction was sufficient for liposome docking. Fusion of docked liposomes with the supported bilayer was monitored by the dequenching of soluble fluorophores entrapped within the liposomes. Fusion was stimulated by illumination with laser light, and the fusion probability was enhanced by raising the ambient temperature from 22 to 37 degrees C, suggesting a thermally activated process. Surprisingly, SNAP-25 had little effect on docking efficiency or the probability of thermally induced fusion. Interprotein fluorescence resonance energy transfer experiments suggest the presence of other conformational states of the syntaxin*synaptobrevin interaction in addition to those observed in the crystal structure of the SNARE complex. Furthermore, although SNARE complexes involved in liposome docking preferentially assemble into a parallel configuration, both parallel and antiparallel configurations were observed.

FIGURE 1

SNARE-dependent docking of liposomes to supported bilayers. (A) Number of docked liposomes as a function of protein concentration in the supported bilayer. Supported bilayers were prepared from egg PC with unlabeled syntaxin at the concentration indicated below the graph and exposed to 100 nM SNAP-25 for 40 min. Liposomes reconstituted by dialysis with 10–30 Cy3 labeled Ser²⁸Cys synaptobrevin molecules were then introduced above the bilayers for 40 min at a concentration of 0.3 mg/ml (lipid) (corresponding to 10 nM liposome concentration) and rinsed away. Solid circles report the average of the number of docked liposomes per 4050 μm² field of view sampled at many locations on the bilayer (error bars are the standard deviation of the average). Open circles are the docking results for identical experiments using protein-free supported bilayers. The inset table shows docking results for supported bilayers containing syntaxin, synaptobrevin (100 molecules/μm²), or no protein and with/without SNAP-25 pretreatment of 250 nM for 1 h. For the inset, synaptobrevin liposomes incubation was 0.3 mg/ml for 1 h for the syntaxin and synaptobrevin experiment and 0.15 mg/ml for 100 min for the protein-free control. Membrane protein concentrations are derived from initial lipid/protein ratios during reconstitution. The emission intensity of synaptobrevin liposomes docked to syntaxin-containing bilayers was similar to that measured for spatially resolved liposomes adsorbed to a quartz surface at sufficient dilution such that bilayers do not form (Johnson et al., 2002). Thus, the emission intensity for a single docked liposome could be determined. The experiment was conducted at 22°C. (B) Real-time washing of liposomes reconstituted with Cy3 labeled synaptobrevin (Ser²⁸Cys) docked to syntaxin•SNAP-25 egg PC bilayers. The deposited bilayer contains 200 unlabeled syntaxin/μm² in Egg PC incubated with 60 nM SNAP-25 for 20 min before docking. Incubation with liposomes was as in A. Panels are the raw output from the CCD camera showing emission between 550 and 650 nm (Cy3) for a 45 μm × 45 μm area of the deposited bilayer. The labels along the bottom axis indicate the time, relative to the initial onset of flow in the movie, for the particular movie frame. The first frame shows 10-nM liposomes in solution before rinsing has commenced. By the second frame, an automated buffer exchanger begins to rinse the bilayer with liposome free buffer (flow is from the upper left to the lower right). The last frame shows the view field after washing has concluded. Note the intensity of these docked liposomes is greater than a single Cy3 dye and some photobleaching has occurred by the end of the movie (see supplementary movie S1). C is the same as B, but for a protein-free egg PC bilayer (see supplementary movie S2).

FIGURE 2

The Botulinum serotype B light chain can digest excess synaptobrevin from liposomes but does not undock them. (A) Conditions are similar as in Fig. 1 A, but at a concentration of 270 syntaxin per μm², and exposure of the syntaxin•egg PC bilayers with Cy5 labeled Ser²⁸Cys synaptobrevin liposomes for 15 min, followed by rinsing with buffer containing 250 nM SNAP-25 for 2 h. The sample was then rinsed with SNAP-25 free buffer and illuminated with blue and red light to observe fluorescence from the content and protein dyes. Shown in A is the intensity distribution (immediately after SNAP-25 is rinsed away) of the synaptobrevin Cy5 dye for the locations with docked liposomes normalized by the independently measured intensity of single Cy5 dyes in this apparatus. The average number of docked liposomes per field of view (4050 μm² in the microscope) was 74 ± 18 as measured by content fluorescence as sampled at many locations on the bilayer. (B) BoNT/B protease was then flowed into the chamber at 1 mg/ml in 10 mM BisTris pH 6.8, 100 mM NaCl, and 1 mM DTT for 1 h and rinsed away. The resulting distribution of the intensity of synaptobrevin Cy5 dyes for locations with docked liposomes is shown. After BoNT/B treatment, the density of docked liposomes changed very little (74 ± 18 per field of view before BoNT/B treatment and 64 ± 8 per field of view after BoNT/B treatment as measured from content spots) but the amount of synaptobrevin present at each docked liposome was significantly reduced. All data were acquired at 22°C and corrected for the 53% labeling efficiency of synaptobrevin.

FIGURE 3

Examples of undocking and bursting events. Supported bilayers of egg PC were prepared with 100 molecules/μm² syntaxin (A and B) or 180 molecules/μm² (C) and exposed to 250 nM SNAP-25 for 1 h (A and B) or no SNAP-25 (C). Liposomes containing 200 mM calcein were reconstituted with 10–30 Cy5-labeled Ser₂₈Cys synaptobrevin molecules. They were then introduced above the bilayers for 80 min (A and B) or 15 min (C) at a concentration of 0.15 mg/ml (lipid) (corresponding to a liposome concentration of 5 nM) and rinsed away. Calcein at 200 mM is highly self-quenched and illumination of docked liposomes loaded with calcein by 488-nm light (½ mW/0.02 mm²) lead to a number of different behaviors. A shows a stable, docked liposome, B shows an undocking liposome, and C shows a docked liposome that bursts above the bilayer. All data were acquired at 22°C.

FIGURE 4

Simultaneous recording of fluorescence emission from liposome content and labeled protein during liposome undocking and bursting events. Supported bilayers of 10% brain PS/90% egg PC were prepared with syntaxin at 100 molecules/μm² without SNAP-25. Liposomes containing 200 mM calcein were reconstituted with 10–30 Cy5 labeled Ser²⁸Cys synaptobrevin molecules and then introduced above the bilayers at a concentration of 0.3 mg/ml (lipid) (corresponding to a liposome concentration of 10 nM). After 80 min, the bilayers were well rinsed. The data were acquired at 22°C. The bilayer was illuminated with ½ mW 488-nm light and 5 mW 635-nm light. The green trace is the emission from the calcein dye. The red trace is the emission from the Cy5 synaptobrevin dye. The intensity level for a single Cy5 dye is estimated to be 100–150 (see B, inset). The Cy5 dye was photobleaching rapidly because no oxygen scavenger enzymes were used. (A) An undocking event where the content and synaptobrevin simultaneously vanish. (B) A bursting event where the content signal vanishes but the synaptobrevin molecules remain in the same location. The inset in B is a detail of the final Cy5 photobleaching in the full trace presumably corresponding to a single labeled synaptobrevin molecule.

FIGURE 5

Simultaneous recording of fluorescence emission from liposome content and labeled protein during liposome fusion events. Supported bilayers of 10% brain PS in 90% egg PC were prepared with 200 syntaxin/μm² and exposed to 250 nM SNAP-25 for 1 h. Liposomes containing 200 mM calcein were reconstituted with 10–30 Cy5 labeled Ser²⁸Cys synaptobrevin molecules, introduced above the bilayers for 1 h at a concentration of 0.3 mg/ml (lipid), and then were rinsed away. (A) The images represent a single 11 μm × 11 μm patch of membrane with docked liposomes observed in two different spectral ranges: emission between 515–580 nm (lower row) from calcein in liposome content, and emission at wavelengths >650 nm (upper row) from Cy5 on synaptobrevin. The times above the frames indicate relative times of extraction from raw data movie (see supplementary movie S4). (B) Time trace for content and protein emission extracted from the same fusion event. Calcein emission shows a rapid increase due to dequenching consistent with liposome fusion, whereas the Cy5 emission shows an exponential decay due to photobleaching. The data were acquired at 22°C.

FIGURE 6

The fusion probability of docked liposomes is independent of their size or synaptobrevin (SB) concentration. Liposomes containing 200 mM calcein were reconstituted with 10–30 Cy5-labeled Ser²⁸Cys synaptobrevin, docked to supported bilayers as described in Fig. 8, and observed with ½ mW 488-nm light and 5 mW 635-nm light simultaneously for ∼2 min (data at 23°C). The liposomes were divided into two populations: those that fuse and those that do not fuse during the observation period. The value of the initial content intensity and the initial synaptobrevin-Cy5 intensity were measured and plotted in A. The subpopulation of liposomes with a fusion event is shown as large red circles, and that with no fusion event is represented by small blue dots. In B and C the magnitude of the jump in intensity during a fusion event (content intensity postjump minus content intensity prejump) for the subpopulation with fusion was extracted and is plotted against the initial content intensity and the initial synaptobrevin-Cy5 intensity.

FIGURE 7

Added Ca²⁺ has no effect on the distribution of fusion events over time. Histogram shows the time of the onset of a fusion signal relative to the start of illumination and data recording. Supported bilayers consisting of 10% brain PS in 90% egg PC were prepared with 100 molecules/μm² syntaxin and exposed to 250 nM SNAP-25 for 1 h. Liposomes containing 200 mM calcein and reconstituted with 10–30 Cy5-labeled Ser²⁸Cys synaptobrevin molecules were then introduced above the bilayers at 22°C for 40 min at a lipid concentration of 0.3 mg/ml (corresponding to a liposome density of 10 nM) and rinsed away. The temperature of the bilayer was then increased to 37°C. Fluorescence emission from the calcein content dye was recorded during excitation with 488-nm laser light. The “skyline” histogram shows fusion events recorded in the absence of Ca²⁺. In three replicate experiments, a total of 75 movies were recorded at different locations of the deposited bilayers before introduction of calcium-containing buffer. A total of 64 fusion events were observed in a population of 640 docked liposomes. The shaded histogram shows fusion events in the presence of 2 mM Ca²⁺. For these movies, after 1 s an automated buffer exchange apparatus began to pump TBS buffer augmented with 2 mM calcium chloride over the membrane (pump active for duration of the green bar). The experimental chamber held ∼25 μl of fluid and the pump maintained flow at ∼25 μl/s for the subsequent 4 s to insure complete buffer exchange. Three replicate experiments yielded three movies because each bilayer could only be flushed with Ca²⁺ once. A total of eight fusion events was observed in a population of 57 docked liposomes. The distributions are similar to the Ca²⁺-free population. No fusion events were observed within 5 s of the onset of Ca²⁺ addition, and fusion persisted throughout the observation period.

FIGURE 8

Probe laser and ambient temperature stimulate fusion with no requirement for SNAP-25. Supported bilayers of 10% brain PS, 90% egg PC were prepared with syntaxin at 100 molecules/μm² and exposed to 250 nM SNAP-25 for 1 h where indicated. Liposomes containing 200 mM calcein were reconstituted with 10–30 Cy5-labeled Ser²⁸Cys synaptobrevin molecules and then introduced above the bilayers at a lipid concentration of 0.3 mg/ml (corresponding to liposome concentration of 10 nM). After 80 min, the bilayers were well rinsed. The bilayers were maintained at room temperature throughout the docking process. Separate fields on the same bilayer were viewed with ½ mW 488-nm light for 2 min. (A) Time distribution, relative to onset of illumination, for fusion events (see Fig. 5 B) occurring at 23°C with a syntaxin bilayer lacking SNAP-25. (B) Time distribution, relative to onset of illumination, for fusion events occurring at 23°C with a syntaxin bilayer preincubated with SNAP-25. (C) Time distribution, relative to onset of illumination, for fusion events at 37°C with a syntaxin bilayer lacking SNAP-25. (D) Time distribution, relative to onset of illumination, for fusion events at 37°C with a syntaxin bilayer preincubated with SNAP-25. The temperature of the slides used for A and B was increased to 37°C and the observations were repeated on unobserved areas of the bilayer to yield histograms C and D. The total fraction of docked liposomes observed to fuse was 15% for the experiment without SNAP-25 at 23°C, and 5% for all other cases.

FIGURE 9

Single molecule FRET observation of the SNARE complex configuration for a single docked liposome. Cy3-labeled Ser¹⁹³Cys syntaxin and SNAP-25 were associated with the deposited bilayer. Cy5-labeled Ser²⁸Cys synaptobrevin was reconstituted in the liposomes. Supported bilayers of egg PC were prepared with labeled syntaxin at low enough surface density that individual molecules could be optically separated (typically 100 per field of view or 4020 μm²). Liposomes loaded with 50 mM calcein and reconstituted with 10–30 labeled synaptobrevin molecules were then introduced above the bilayer for 2 h at a concentration of 1 mg/ml (lipid) and rinsed away. SNAP-25 was added at 100 nM for 2 h, and then rinsed away. BoNT/B protease was added at 1 mg/ml for 80 min at room temperature in 10 mM BisTris buffer (pH6.8), 100 mM NaCl, and 1 mM DTT followed by rinsing into TBS oxygen-scavenger buffer. The bar at the top of the graph indicates the laser illumination sequence. For the first 1.1 s, ½ mW 488-nm light was used, which excited the content dye but not the Cy3 or Cy5 dyes. Between 1.l s and 15.1 s, 8 mW of 532-nm light is used. 532-nm light excites the Cy3 dye directly, the Cy5 dye very little, and calcein to about the same intensity as with the ½ mW of 488-nm light. After 15.1 s, 635-nm light is used to directly excite the Cy5 dye. In this graph, the green trace shows emission in the short wavelength detection path (calcein and Cy3 dye emission are passed) and the red trace shows emission in the long wavelength (emission of the Cy5 dye only; see Materials and Methods). The experiment was carried out at 22°C. Note that the individual liposome is docked by one labeled SNARE complex.