The juxtamembrane linker of full-length synaptotagmin 1 controls oligomerization and calcium-dependent membrane binding

Synaptotagmin 1 (Syt1) is the calcium sensor for synchronous neurotransmitter release. The two C2 domains of Syt1, which may mediate fusion by bridging the vesicle and plasma membranes, are connected to the vesicle membrane by a 60-residue linker. Here, we use site-directed spin labeling and a novel total internal reflection fluorescence vesicle binding assay to characterize the juxtamembrane linker and to test the ability of reconstituted full-length Syt1 to interact with opposing membrane surfaces. EPR spectroscopy demonstrates that the majority of the linker interacts with the membrane interface, thereby limiting the extension of the C2A and C2B domains into the cytoplasm. Pulse dipolar EPR spectroscopy provides evidence that purified full-length Syt1 is oligomerized in the membrane, and mutagenesis indicates that a glycine zipper/GXXXG motif within the linker helps mediate oligomerization. The total internal reflection fluorescence-based vesicle binding assay demonstrates that full-length Syt1 that is reconstituted into supported lipid bilayers will capture vesicles containing negatively charged lipid in a Ca(2+)-dependent manner. Moreover, the rate of vesicle capture increases with Syt1 density, and mutations in the GXXXG motif that inhibit oligomerization of Syt1 reduce the rate of vesicle capture. This work demonstrates that modifications within the 60-residue linker modulate both the oligomerization of Syt1 and its ability to interact with opposing bilayers. In addition to controlling its activity, the oligomerization of Syt1 may play a role in organizing proteins within the active zone of membrane fusion.

Regulation of Rac1 translocation and activation by membrane domains and their boundaries

The activation of Rac1 and related Rho GTPases involves dissociation from Rho GDP-dissociation inhibitor proteins and translocation to membranes, where they bind effectors. Previous studies have suggested that the binding of Rac1 to membranes requires, and colocalizes with, cholesterol-rich liquid-ordered (lo) membrane domains (lipid rafts). Here, we have developed a fluorescence resonance energy transfer (FRET) assay that robustly detects Rac1 membrane targeting in living cells. Surprisingly, FRET with acceptor constructs that were targeted to either raft or non-raft areas indicated that Rac1 was present in both regions. Functional studies showed that Rac1 localization to non-raft regions decreased GTP loading as a result of inactivation by GTPase-activating proteins. In vitro, Rac1 translocation to supported lipid bilayers also required lo domains, yet Rac1 was concentrated in the liquid-disordered (ld) phase. Single-molecule analysis demonstrated that translocation occurred preferentially at lo-ld boundaries. These results, therefore, suggest that Rac1 translocates to the membrane at domain boundaries, then diffuses into raft and non-raft domains, which controls interactions. These findings resolve discrepancies in our understanding of Rac biology and identify novel mechanisms by which lipid rafts modulate Rho GTPase signaling.

Rapid fusion of synaptic vesicles with reconstituted target SNARE membranes

Neurotransmitter release at neuronal synapses occurs on a timescale of 1 ms or less. Reconstitution of vesicle fusion from purified synaptic proteins and lipids has played a major role in elucidating the synaptic exocytotic fusion machinery with ever increasing detail. However, one limitation of most reconstitution approaches has been the relatively slow rate of fusion that can be produced in these systems. In a related study, a notable exception is an approach measuring fusion of single reconstituted vesicles bearing the vesicle fusion protein synaptobrevin with supported planar membranes harboring the presynaptic plasma membrane proteins syntaxin and SNAP-25. Fusion times of ∼20 ms were achieved in this system. Despite this advance, an important question with reconstituted systems is how well they mimic physiological systems they are supposed to reproduce. In this work, we demonstrate that purified synaptic vesicles from rat brain fuse with acceptor-SNARE containing planar bilayers equally fast as equivalent reconstituted vesicles and that their fusion efficiency is increased by divalent cations. Calcium boosts fusion through a combined general electrostatic and synaptotagmin-specific mechanism.

Partitioning of synaptotagmin I C2 domains between liquid-ordered and liquid-disordered inner leaflet lipid phases

Synaptotagmin I is the calcium sensor in synchronous neurotransmitter release caused by fusion of synaptic vesicles with the presynaptic membrane in neurons. Synaptotagmin I interacts with acidic phospholipids, but also with soluble N-ethylmaleimide-sensitive factor attachment receptors (SNAREs), at various stages in presynaptic membrane fusion. Because SNAREs can be organized into small cholesterol-dependent clusters in membranes, it is important to determine whether the C2 domains of synaptotagmin target membrane domains with different cholesterol contents. To address this question, we used a previously developed asymmetric two-phase lipid bilayer system to investigate the membrane binding and lipid phase targeting of soluble C2A and C2AB domains of synaptotagmin. We found that both domains target more disordered cholesterol-poor domains better than highly ordered cholesterol-rich domains. The selectivity is greatest (∼3-fold) for C2A binding to disordered domains that are formed in the presence of 5 mol % PIP(2) and 15 mol % PS. It is smallest (∼1.4-fold) for C2AB binding to disordered domains that are formed in the presence of 40 mol % PS. In the course of these experiments, we also found that C2A domains in the presence of Ca(2+) and C2AB domains in the absence of Ca(2+) are quite reliable reporters of the acidic lipid distribution between ordered and disordered lipid phases. Accordingly, PS prefers the liquid-disordered phase over the liquid-ordered phase by ∼2-fold, but PIP(2) has an up to 3-fold preference for the liquid-disordered phase.

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].

Morphological changes induced by the action of antimicrobial peptides on supported lipid bilayers

We utilized epifluorescence microscopy to investigate the morphological changes in labeled lipid bilayers supported on quartz surfaces (SLBs) induced by the interaction of cationic antimicrobial peptides with the lipid membranes. The SLBs were prepared from 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) and mixtures thereof as well as from Escherichia coli lipid extract. We succeeded in the preparation of POPG and POPG-rich SLBs without the necessity to use fusogenic agents such as calcium by using the Langmuir-Blodgett/Langmuir-Schaefer transfer method. The adsorption of the peptides to the SLBs was initially driven by electrostatic interactions with the PG headgroups and led to the formation of lipid protrusions bulging out from the lipid layer facing the bulk, originating particularly from domain boundaries and membrane defects. The shape, size, and frequency of the lipid protrusions are mainly controlled by the peptide macroscopic properties and the membrane composition. A restructuring of the lipid protrusions into other structures can also occur over time.

Single SNARE-mediated vesicle fusion observed in vitro by polarized TIRFM

Single-vesicle fusion assays in vitro are useful tools for examining mechanisms of membrane fusion at the molecular level mediated by soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs). This approach allows the experimentalist to define the lipid and protein composition of the two fusing membranes and perform experiments under highly controlled conditions. In previous experiments, in which we reconstituted a SNARE acceptor complex into supported membranes and observed the docking and fusion of fluorescently labeled synaptobrevin proteoliposomes by total internal reflection fluorescence microscopy with millisecond time resolution, we were able to determine the optimal number of SNARE complexes needed for fast fusion. Here, we utilize this assay in combination with polarized total internal reflection fluorescence microscopy to investigate topology changes that vesicles undergo after the onset of fusion. The theory that describes the fluorescence intensity during the transformation of a single vesicle from a spherical particle to a flat membrane patch is developed and confirmed by experiments with three different fluorescent probes. Our results show that on average, the fusing vesicles flatten and merge into the planar membrane within 8 ms after fusion starts.

Single vesicle millisecond fusion kinetics reveals number of SNARE complexes optimal for fast SNARE-mediated membrane fusion

SNAREs mediate membrane fusion in intracellular vesicle traffic and neuronal exocytosis. Reconstitution of membrane fusion in vitro proved that SNAREs constitute the minimal fusion machinery. However, the slow fusion rates observed in these systems are incompatible with those required in neurotransmission. Here we present a single vesicle fusion assay that records individual SNARE-mediated fusion events with millisecond time resolution. Docking and fusion of reconstituted synaptobrevin vesicles to target SNARE complex-containing planar membranes are distinguished by total internal reflection fluorescence microscopy as separate events. Docking and fusion are SNAP-25-dependent, require no Ca(2+), and are efficient at room temperature. Analysis of the stochastic data with sequential and parallel multi-particle activation models reveals six to nine fast-activating steps. Of all the tested models, the kinetic model consisting of eight parallel reaction rates statistically fits the data best. This might be interpreted by fusion sites consisting of eight SNARE complexes that each activate in a single rate-limiting step in 8 ms.

Transbilayer effects of raft-like lipid domains in asymmetric planar bilayers measured by single molecule tracking

Cell membranes have complex lipid compositions, including an asymmetric distribution of phospholipids between the opposing leaflets of the bilayer. Although it has been demonstrated that the lipid composition of the outer leaflet of the plasma membrane is sufficient for the formation of raft-like liquid-ordered (l(o)) phase domains, the influence that such domains may have on the lipids and proteins of the inner leaflet remains unknown. We used tethered polymer supports and a combined Langmuir-Blodgett/vesicle fusion (LB/VF) technique to build asymmetric planar bilayers that mimic plasma membrane asymmetry in many ways. We show that directly supported LB monolayers containing cholesterol-rich l(o) phases are inherently unstable when exposed to water or vesicle suspensions. However, tethering the LB monolayer to the solid support with the lipid-anchored polymer 1,2-dimyristoyl phophatidylethanolamine-N-[poly(ethylene glycol)-triethoxysilane] significantly improves stability and allows for the formation of complex planar-supported bilayers that retain >90% asymmetry for 1-2 h. We developed a single molecule tracking (SPT) system for the study of lipid diffusion in asymmetric bilayers with coexisting liquid phases. SPT allowed us to study in detail the diffusion of individual lipids inside, outside, or directly opposed to l(o) phase domains. We show here that l(o) phase domains in one monolayer of an asymmetric bilayer do not induce the formation of domains in the opposite leaflet when this leaflet is composed of palmitoyl-oleoyl phosphatidylcholine and cholesterol but do induce domains when this leaflet is composed of porcine brain phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and cholesterol. The diffusion of lipids is similar in l(o) and liquid-disordered phase domains and is not affected by transbilayer coupling, indicating that lateral and transverse lipid interactions that give rise to the domain structure are weak in the biological lipid mixtures that were employed in this work.

Measuring lipid asymmetry in planar supported bilayers by fluorescence interference contrast microscopy

There is substantial scientific and practical interest in engineering supported lipid bilayers with asymmetric lipid distributions as models for biological cell membranes. In principle, it should be possible to make asymmetric supported lipid bilayers by either the Langmuir-Blodgett/Schafer (LB/LS) or Langmuir-Blodgett/vesicle fusion (LB/VF) techniques (Kalb et al. Biochim. Biophys. Acta 1992, 1103, 307-316). However, the retention of asymmetry in biologically relevant lipid bilayers has never been experimentally examined in any of these systems. In the present work, we developed a technique that is based on fluorescence interference contrast (FLIC) microscopy to measure lipid asymmetry in supported bilayers. We compared the final degree of lipid asymmetry in LB/LS and LB/VF bilayers with and without cholesterol in liquid-ordered (l(o)) and liquid-disordered (l(d)) phases. Of five different fluorescent lipid probes that were examined, 1,2-dipalmitoyl-phosphatidylethanolamine-N-[lissamine rhodamine B] was the best for studying supported bilayers of complex composition and phase by FLIC microscopy. An asymmetrically labeled bilayer made by the LB/LS method was found to be at best 70-80% asymmetric once completed. In LB/LS bilayers of either l(o) or l(d) phase, cholesterol increased the degree of lipid mixing between the opposing monolayers. The use of a tethered polymer support for the initial monolayer did not improve lipid asymmetry in the resulting bilayer. However, asymmetric LB/VF bilayers retained nearly 100% asymmetric label, with or without the use of a tethered polymer support. Finally, lipid mixing across the center of LB/LS bilayers was found to have drastic effects on the appearance of l(d)-l(o) phase coexistence as shown by epifluorescence microscopy.

Potassium channel gating in adhesion: from an oocyte-silicon to a neuron-astrocyte adhesion contact

In a neuron-astrocyte adhesion contact the ionic current due to the opening of voltage-dependent potassium channels has to flow along a narrow intercellular cleft, generating there an extracellular voltage. This voltage might be large enough to affect significantly the dependence of channel gating from the intracellular voltage. In order to test this hypothesis, we considered a Xenopus oocyte expressing voltage-dependent potassium channels adhering to a layer of silicon oxide as a simplified model of cell-cell adhesion; here the cell membrane and silicon oxide are separated by a narrow cleft and form a junction of circular shape. We measured directly the extracellular voltage along the diameter of the cleft and investigated its effect on channel gating using a linear array of field effect transistors integrated in the silicon substrate. On this experimental basis we demonstrated that the voltage dependence of potassium channels is strongly affected by adhesion, as can be predicted using a model of a two-dimensional cable and electrodiffusion theory. Computations based on the model showed that along a neuron-astrocyte adhesion contact the opening of voltage-dependent Kv2.1 potassium channels is significantly reduced with respect to identical channels facing an open extracellular space.

Membrane fusion: a structural perspective on the interplay of lipids and proteins

The fusion of biological membranes is governed by the carefully orchestrated interplay of membrane proteins and lipids. Recently determined structures of fusion proteins, individual domains of fusion proteins and their complexes with regulatory proteins and membrane lipids have yielded much suggestive insight into how viral and intracellular membrane fusion might proceed. These structures may be combined with new knowledge on the fusion of pure lipid bilayer membranes in an attempt to begin to piece together the complex puzzle of how biological membrane fusion machines operate on membranes.

Measuring distances in supported bilayers by fluorescence interference-contrast microscopy: polymer supports and SNARE proteins

Fluorescence interference-contrast (FLIC) microscopy is a powerful new technique to measure vertical distances from reflective surfaces. A pattern of varying intensity is created by constructive and destructive interference of the incoming and reflected light at the surface of an oxidized silicon chip. Different levels of this pattern are probed by manufacturing silicon chips with terraces of oxide layers of different heights. Fluorescence collected from membranes that are deposited on these terraces is then used to measure the distance of the fluorescent probes from the silicon oxide surface. Here, we applied the method to measure the distance between supported lipid bilayers and the surface of oxidized silicon chips. For plain fluid phosphatidylcholine bilayers, this distance was 1.7 +/- 1.0 nm. The cleft distance was increased to 3.9 +/- 0.9 nm in bilayers that were supported on a 3400-Da polyethylene glycol cushion. This distance is close to the Flory distance (4.8 nm) that would be expected for a grafted random coil of this polymer. In a second application, the distance of a membrane-bound protein from the membrane surface was measured. The integral membrane protein syntaxin1A/SNAP25 (t-SNARE) was reconstituted into tethered polymer-supported bilayers. A soluble form of the green fluorescent protein/vesicle-associated membrane protein (GFP-VAMP) was bound to the reconstituted t-SNAREs. The distance of the GFP from the membrane surface was 16.5 +/- 2.8 nm, indicating an upright orientation of the rod-shaped t-SNARE/v-SNARE complex from the membrane surface.