Lipid mixing and content release in single-vesicle, SNARE-driven fusion assay with 1-5 ms resolution

Docking, not fusion, as the rate-limiting step in a SNARE-driven vesicle fusion assay

In vitro vesicle fusion assays that monitor lipid mixing between t-SNARE and v-SNARE vesicles in bulk solution exhibit remarkably slow fusion on the nonphysiological timescale of tens of minutes to several hours. Here, single-vesicle, fluorescence resonance energy transfer-based assays cleanly separate docking and fusion steps for individual vesicle pairs containing full-length SNAREs. Docking is extremely inefficient and is the rate-limiting step. Of importance, the docking and fusion kinetics are comparable in the two assays (one with v-SNARE vesicles tethered to a surface and the other with v-SNARE vesicles free in solution). Addition of the V(C) peptide synaptobrevin-2 (syb(57-92)) increases the docking efficiency by a factor of ∼30, but docking remains rate-limiting. In the presence of V(C) peptide, the fusion step occurs on a timescale of ∼10 s. In previous experiments involving bulk fusion assays in which the addition of synaptotagmin/Ca(2+), Munc-18, or complexin accelerated the observed lipid-mixing rate, the enhancement may have arisen from the docking step rather than the fusion step.

In vitro system capable of differentiating fast Ca2+-triggered content mixing from lipid exchange for mechanistic studies of neurotransmitter release

Understanding the molecular principles of synaptic vesicle fusion is a long-sought goal. It requires the development of a synthetic system that allows manipulations and observations not possible in vivo. Here, we report an in vitro system with reconstituted synaptic proteins that meets the long-sought goal to produce fast content release in the millisecond time regime upon Ca(2+) triggering. Our system simultaneously monitors both content and lipid exchange, and it starts from stable interacting pairs of donor and acceptor vesicles, mimicking the readily releasable pool of synaptic vesicles prior to an action potential. It differentiates between single-vesicle interaction, hemifusion, and complete fusion, the latter mimicking quantized neurotransmitter release upon exocytosis of synaptic vesicles. Prior to Ca(2+) injection, the system is in a state in which spontaneous fusion events between donor and acceptor vesicles are rare. Upon Ca(2+) injection, a rapid burst of complete fusion events emerges, followed by a biphasic decay. The present study focuses on neuronal SNAREs, the Ca(2+) sensor synaptotagmin 1, and the modulator complexin. However, other synaptic proteins could be added and their function examined. Ca(2+) triggering is cooperative, requiring the presence of synaptotagmin, whereas SNAREs alone do not produce a fast fusion burst. Manipulations of the system mimic effects observed in vivo. These results also show that neuronal SNAREs alone do not efficiently produce complete fusion, that the combination of SNAREs with synaptotagmin lowers the activation barriers to full fusion, and that complexin enhances this kinetic control.

Single vesicle assaying of SNARE-synaptotagmin-driven fusion reveals fast and slow modes of both docking and fusion and intrasample heterogeneity

Lipid mixing between vesicles functionalized with SNAREs and the cytosolic C2AB domain of synaptotagmin-1 recapitulates the basic Ca(2+) dependence of neuronal exocytosis. However, in the conventional ensemble lipid mixing assays it is not possible to discriminate whether Ca(2+) accelerates the docking or the fusion of vesicles. Here we report a fluorescence microscopy-based assay to monitor SNARE-mediated docking and fusion of individual vesicle pairs. In situ measurement of the concentration of diffusing particles allowed us to quantify docking rates by a maximum-likelihood approach. This analysis showed that C2AB and Ca(2+) accelerate vesicle-vesicle docking with more than two orders of magnitude. Comparison of the measured docking rates with ensemble lipid mixing kinetics, however, suggests that in most cases bilayer fusion remains the rate-limiting step. Our single vesicle results show that only ∼60% of the vesicles dock and only ∼6% of docked vesicles fuse. Lipid mixing on single vesicles was fast (t(mix) < 1 s) while an ensemble assay revealed two slow mixing processes with t(mix) ∼ 1 min and t(mix) ∼ 20 min. The presence of several distinct docking and fusion pathways cannot be rationalized at this stage but may be related to intrasample heterogeneities, presumably in the form of lipid and/or protein composition.

Membrane fusion: five lipids, four SNAREs, three chaperones, two nucleotides, and a Rab, all dancing in a ring on yeast vacuoles

Although fusion mechanisms are highly conserved in evolution and among organelles of the exocytic and endocytic pathways, yeast vacuole homotypic fusion offers unique technical advantages: excellent genetics, clear organelle cytology, in vitro colorimetric fusion assays, and reconstitution of fusion from all-pure components, including a Rab GTPase, HOPS (homotypic fusion and vacuole protein sorting complex), four SNAREs [soluble N-ethylmaleimide-sensitive factor (NSF) attachment receptors] that snare (bind) each other, SNARE-complex disassembly chaperones, and vacuolar lipids. Vacuole fusion studies offer paradigms of the interdependence of lipids and fusion proteins to assemble a fusion microdomain, distinct lipid functions, SNARE complex proofreading through the synergy between HOPS and the SNARE disassembly chaperones, and the role of each fusion protein in promoting radical bilayer restructuring for fusion without lysis.

Model lipid bilayer with facile diffusion of lipids and integral membrane proteins

A model membrane system is formed by the rupture of giant unilamellar vesicles (GUVs) onto a passivating layer comprising a PEG polymer cushion anchored in a lipid bilayer supported on glass. The novel use of pH-dependent electrostatic interactions between NeutrAvidin in the passivating layer and anionic lipids in the GUV drives vesicle rupture. The resulting "GUV pancakes" are single, planar lipid bilayer patches whose diameters vary from approximately 20 to 50 microm. The pancakes have several potential advantages for the in vitro study of protein-lipid interactions and integral membrane protein function. All components are commercially available. The pancakes resist nonspecific binding of vesicles containing protein. Both lipids and integral membrane proteins exhibit good lateral mobility in the GUV pancakes, as evidenced by single-particle tracking (SPT) of the DiD double-tailed fluorescent probe and of the integral membrane protein syntaxin-1A, labeled with AlexaFluor 633 (AF633-Syx). At least 80% of both probes exhibit free, homogeneous diffusion with a diffusion coefficient of approximately 5.5 microm(2) s(-1), which is more than 10 times faster than diffusion in a GUV pancake supported on bare glass. Atomic force microscopy (AFM) suggests that the polymer cushion has a height of approximately 4 nm. The mobility of a large fraction of the AF633-Syx probe suggests that even integral membrane proteins with large domains on both sides of the lipid bilayer should exhibit free diffusion within a GUV pancake.

Negative potentials across biological membranes promote fusion by class II and class III viral proteins

Fusion of virions pseudotyped with a class II, SFV E1 or VEEV E, or a class III protein, VSV G is promoted by negative potentials and hindered by positive potentials across the target cell. Hemifusion is independent of polarity. Reversion of hemifused membranes into two distinct ones is responsible for voltage-dependence and inhibition of fusion.

Voltage was investigated as a factor in the fusion of virions. Virions, pseudotyped with a class II, SFV E1 or VEEV E, or a class III protein, VSV G, were prepared with GFP within the core and a fluorescent lipid. This allowed both hemifusion and fusion to be monitored. Voltage clamping the target cell showed that fusion is promoted by a negative potential and hindered by a positive potential. Hemifusion occurred independent of polarity. Lipid dye movement, in the absence of content mixing, ceased before complete transfer for positive potentials, indicating that reversion of hemifused membranes into two distinct membranes is responsible for voltage dependence and inhibition of fusion. Content mixing quickly followed lipid dye transfer for a negative potential, providing a direct demonstration that hemifusion induced by class II and class III viral proteins is a functional intermediate of fusion. In the hemifused state, virions that fused exhibited slower lipid transfer than did nonfusing virions. All viruses with class II or III fusion proteins may utilize voltage to achieve infection.

Lipid membrane adhesion and fusion driven by designed, minimally multivalent hydrogen-bonding lipids

Cyanuric acid (CA) and melamine (M) functionalized lipids can form membranes that exhibit robust hydrogen-bond driven surface recognition in water, facilitated by multivalent surface clustering of recognition groups and variable hydration at the lipid-water interface. Here we describe a minimal lipid recognition cluster: three CA or M recognition groups are forced into proximity by covalent attachment to a single lipid headgroup. This trivalent lipid system guides recognition at the lipid-water interface using cyanurate-melamine hydrogen bonding when incorporated at 0.1-5 mol percent in fluid phospholipid membranes, inducing both vesicle-vesicle binding and membrane fusion. Fusion was accelerated when the antimicrobial peptide magainin was used to anchor trivalent recognition, or when added exogenously to a preassembled lipid vesicle complex, underscoring the importance of coupling recognition with membrane disruption in membrane fusion. Membrane apposition and fusion were studied in vesicle suspensions using light scattering, FRET assays for lipid mixing, surface plasmon resonance, and cryo-electron microscopy. Recognition was found to be highly spatially selective as judged by vesicular adhesion to surface patterned supported lipid bilayers (SLBs). Fusion to SLBs was also readily observed by fluorescence microscopy. Together, these studies indicate effective and functional recognition of trivalent phospholipids, despite low mole percentage concentration, solvent competition for hydrogen bond donor/acceptor sites, and simplicity of structure. This novel designed molecular recognition motif may be useful for directing aqueous-phase assembly and biomolecular interactions.

Phosphoinositides and SNARE chaperones synergistically assemble and remodel SNARE complexes for membrane fusion

Yeast vacuole fusion requires 4 SNAREs, 2 SNARE chaperone systems (Sec17p/Sec18p/ATP and the HOPS complex), and 2 phosphoinositides, phosphatidylinositol 3-phosphate [PI(3)P] and phosphatidylinositol 4,5-bisphosphate [PI(4,5)P(2)]. By reconstituting proteoliposomal fusion with purified components, we now show that phosphoinositides have 4 distinct roles: PI(3)P is recognized by the PX domain of the SNARE Vam7p; PI(3)P enhances the capacity of membrane-bound SNAREs to drive fusion in the absence of SNARE chaperones; either PI(3)P or PI(4,5)P(2) can activate SNARE chaperones for the recruitment of Vam7p into fusion-competent SNARE complexes; and either PI(3)P or PI(4,5)P(2) strikingly promotes synergistic SNARE complex remodeling by Sec17p/Sec18p/ATP and HOPS. This ternary synergy of phosphoinositides and 2 SNARE chaperone systems is required for rapid fusion.