Reconstitution of a Kv Channel into Lipid Membranes for Structural and Functional Studies

To study the lipid-protein interaction in a reductionistic fashion, it is necessary to incorporate the membrane proteins into membranes of well-defined lipid composition. We are studying the lipid-dependent gating effects in a prototype voltage-gated potassium (Kv) channel, and have worked out detailed procedures to reconstitute the channels into different membrane systems. Our reconstitution procedures take consideration of both detergent-induced fusion of vesicles and the fusion of protein/detergent micelles with the lipid/detergent mixed micelles as well as the importance of reaching an equilibrium distribution of lipids among the protein/detergent/lipid and the detergent/lipid mixed micelles. Our data suggested that the insertion of the channels in the lipid vesicles is relatively random in orientations, and the reconstitution efficiency is so high that no detectable protein aggregates were seen in fractionation experiments. We have utilized the reconstituted channels to determine the conformational states of the channels in different lipids, record electrical activities of a small number of channels incorporated in planar lipid bilayers, screen for conformation-specific ligands from a phage-displayed peptide library, and support the growth of 2D crystals of the channels in membranes. The reconstitution procedures described here may be adapted for studying other membrane proteins in lipid bilayers, especially for the investigation of the lipid effects on the eukaryotic voltage-gated ion channels.     

The Influence of Natural Lipid Asymmetry upon the Conformation of a Membrane-inserted Protein (Perfringolysin O)

Eukaryotic membrane proteins generally reside in membrane bilayers that have lipid asymmetry. However, in vitro studies of the impact of lipids upon membrane proteins are generally carried out in model membrane vesicles that lack lipid asymmetry. Our recently developed method to prepare lipid vesicles with asymmetry similar to that in plasma membranes and with controlled amounts of cholesterol was used to investigate the influence of lipid composition and lipid asymmetry upon the conformational behavior of the pore-forming, cholesterol-dependent cytolysin perfringolysin O (PFO). PFO conformational behavior in asymmetric vesicles was found to be distinct both from that in symmetric vesicles with the same lipid composition as the asymmetric vesicles and from that in vesicles containing either only the inner leaflet lipids from the asymmetric vesicles or only the outer leaflet lipids from the asymmetric vesicles. The presence of phosphatidylcholine in the outer leaflet increased the cholesterol concentration required to induce PFO binding, whereas phosphatidylethanolamine and phosphatidylserine in the inner leaflet of asymmetric vesicles stabilized the formation of a novel deeply inserted conformation that does not form pores, even though it contains transmembrane segments. This conformation may represent an important intermediate stage in PFO pore formation. These studies show that lipid asymmetry can strongly influence the behavior of membrane-inserted proteins.     

Influence of Membrane Lipid Composition on a Transmembrane Bacterial Chemoreceptor

Most bacterial chemoreceptors are transmembrane proteins. Although less than 10% of a transmembrane chemoreceptor is embedded in lipid, separation from the natural membrane environment by detergent solubilization eliminates most receptor activities, presumably because receptor structure is perturbed. Reincorporation into a lipid bilayer can restore these activities and thus functionally native structure. However, the extent to which specific lipid features are important for effective restoration is unknown. Thus we investigated effects of membrane lipid composition on chemoreceptor Tar from Escherichia coli using Nanodiscs, small (∼10-nm) plugs of lipid bilayer rendered water-soluble by an annulus of “membrane scaffold protein.” Disc-enclosed bilayers can be made with different lipids or lipid combinations. Nanodiscs carrying an inserted receptor dimer have high protein-to-lipid ratios approximating native membranes and in this way mimic the natural chemoreceptor environment. To identify features important for functionally native receptor structure, we made Nanodiscs using natural and synthetic lipids, assaying extents and rates of adaptational modification. The proportion of functionally native Tar was highest in bilayers closest in composition to E. coli cytoplasmic membrane. Some other lipid compositions resulted in a significant proportion of functionally native receptor, but simply surrounding the chemoreceptor transmembrane segment with a lipid bilayer was not sufficient. Membranes effective in supporting functionally native Tar contained as the majority lipid phosphatidylethanolamine or a related zwitterionic lipid plus a rather specific proportion of anionic lipids, as well as unsaturated fatty acids. Thus the chemoreceptor is strongly influenced by its lipid environment and is tuned to its natural one.     

SNAREs in opposing bilayers interact in a circular array to form conducting pores

The process of fusion at the nerve terminal is mediated via a specialized set of proteins in the synaptic vesicles and the presynaptic membrane. Three soluble N-ethylmaleimide-sensitive factor (NSF)-attachment protein receptors (SNAREs) have been implicated in membrane fusion. The structure and arrangement of these SNAREs associated with lipid bilayers were examined using atomic force microscopy. A bilayer electrophysiological setup allowed for measurements of membrane conductance and capacitance. Here we demonstrate that the interaction of these proteins to form a fusion pore is dependent on the presence of t-SNAREs and v-SNARE in opposing bilayers. Addition of purified recombinant v-SNARE to a t-SNARE-reconstituted lipid membrane increased only the size of the globular t-SNARE oligomer without influencing the electrical properties of the membrane. However when t-SNARE vesicles were added to a v-SNARE membrane, SNAREs assembles in a ring pattern and a stepwise increase in capacitance, and increase in conductance were observed. Thus, t- and v-SNAREs are required to reside in opposing bilayers to allow appropriate t-/v-SNARE interactions leading to membrane fusion.     

Lipid-destabilizing properties of the hydrophobic helices H8 and H9 from colicin E1

Colicins are toxic proteins produced by Escherichia coli that must cross the membrane to exert their activity. The lipid insertion of their pf domain is linked to a conformational change which enables the penetration of a hydrophobic hairpin. They provide useful models to more generally study insertion of proteins, channel formation and protein translocation in and across membranes. In this paper, we study the lipid-destabilizing properties of helices H8 and H9 forming the hydrophobic hairpin of colicin E1. Modelling analysis suggests that those fragments behave like tilted peptides. The latter are characterized by an asymmetric distribution of their hydrophobic residues when helical. They are able to interact with a hydrophobic/hydrophilic interface (such as a lipid membrane) and to destabilize the organized system into which they insert. Fluorescence techniques using labelled liposomes clearly show that H9, and H8 to a lesser extent, destabilize lipid particles, by inducing fusion and leakage. AFM assays clearly indicate that H8 and especially H9 induce membrane fragilization. Holes in the membrane are even observed in the presence of H9. This behaviour is close to what is seen with viral fusion peptides. Those results suggest that the peptides could be involved in the toroidal pore formation of colicin E1, notably by disturbing the lipids and facilitating the insertion of the other, more hydrophilic, helices that will form the pore. Since tilted, lipid-destabilizing fragments are also common to membrane proteins and to signal sequences, we suggest that tilted peptides should have an ubiquitous role in the mechanism of insertion of proteins into membranes.     

Differential Association of Syntrophin Pairs with the Dystrophin Complex

While the specificity and timing of membrane fusion in diverse physiological reactions, including virus–cell fusion, is determined by proteins, fusion always involves the merger of membrane lipid bilayers. We have isolated a lipid-dependent stage of cell–cell fusion mediated by influenza hemagglutinin and triggered by cell exposure to mildly acidic pH. This stage preceded actual membrane merger and fusion pore formation but was subsequent to a low pH–induced change in hemagglutinin conformation that is required for fusion. A low pH conformation of hemagglutinin was required to achieve this lipid-dependent stage and also, downstream of it, to drive fusion to completion. The lower the pH of the medium applied to trigger fusion and, thus, the more hemagglutinin molecules activated, the less profound was the dependence of fusion on lipids. Membrane-incorporated lipids affected fusion in a manner that correlated with their dynamic molecular shape, a characteristic that determines a lipid monolayer's propensity to bend in different directions. The lipid sensitivity of this stage, i.e., inhibition of fusion by inverted cone–shaped lysophosphatidylcholine and promotion by cone-shaped oleic acid, was consistent with the stalk hypothesis of fusion, suggesting that fusion proteins begin membrane merger by promoting the formation of a bent, lipid-involving, stalk intermediate.     

Characterization of membrane domains by frap experiments at variable observation areas

In this paper we show that FRAP experiments at variable beam radii provide an experimental approach for investigating membrane organization and dynamics, with great potential for identifying micrometer-sized domains and determining their size and the diffusion coefficient of the lipid and protein molecules they contain. Monte Carlo simulations of FRAP experiments at variable beam radii R on models of compartmentalized membranes have allowed us to establish the relationships (i) between the mobile fraction M of a diffusing particle and the size r of the domains, and (ii) between the apparent diffusion coefficient D_app and the real diffusion coefficient D₀ of this particle inside the domains. Furthermore, in its present stage of development, this approach allows us to specify whether these domains are strictly closed or not. This approach was first validated on an experimental model of a strictly compartmentalized membrane consisting of a monolayer of apposed spherical phospholipid bilayers supported by silica beads of known radius (0.83 μm). To prevent fusion between the spherical bilayers 5 mol% of a polymer-grafted phospholipid was added to the lipids. Analysis of the M versus R data yielded a radius r of 0.92±0.09 μm for the spherical bilayers, close to that of the supporting silica beads. When applied to the experimental data available for lipids and proteins in the plasma membrane of living cells, this approach suggests the existence of domains within these membranes with a radius of about 0.4 – 0.7 μm for the lipids and 0.25 μm for the proteins. These domains are not strictly closed and they are believed to be delineated by fluctuating barriers which are more or less permeable to lipid and protein molecules. Received: 4 September 1997 / Revised version: 19 January 1998 / Accepted: 19 January 1998     

Shallow Boomerang-shaped Influenza Hemagglutinin G13A Mutant Structure Promotes Leaky Membrane Fusion

Our previous studies showed that an angled boomerang-shaped structure of the influenza hemagglutinin (HA) fusion domain is critical for virus entry into host cells by membrane fusion. Because the acute angle of ∼105° of the wild-type fusion domain promotes efficient non-leaky membrane fusion, we asked whether different angles would still support fusion and thus facilitate virus entry. Here, we show that the G13A fusion domain mutant produces a new leaky fusion phenotype. The mutant fusion domain structure was solved by NMR spectroscopy in a lipid environment at fusion pH. The mutant adopted a boomerang structure similar to that of wild type but with a shallower kink angle of ∼150°. G13A perturbed the structure of model membranes to a lesser degree than wild type but to a greater degree than non-fusogenic fusion domain mutants. The strength of G13A binding to lipid bilayers was also intermediate between that of wild type and non-fusogenic mutants. These membrane interactions provide a clear link between structure and function of influenza fusion domains: an acute angle is required to promote clean non-leaky fusion suitable for virus entry presumably by interaction of the fusion domain with the transmembrane domain deep in the lipid bilayer. A shallower angle perturbs the bilayer of the target membrane so that it becomes leaky and unable to form a clean fusion pore. Mutants with no fixed boomerang angle interacted with bilayers weakly and did not promote any fusion or membrane perturbation.     

Calcium drives fusion of SNARE-apposed bilayers

N-ethylmalemide-sensitive factor attachment protein receptor (SNARE) has been proposed to play a critical role in the membrane fusion process. The SNARE complex was suggested to be the minimal fusion machinery. However, there is mounting evidence for a major role of calcium in membrane fusion. Hence, the role of calcium in SNARE-induced membrane fusion was the focus of this study. It revealed that recombinant v-SNARE and t-SNARE, reconstituted into separate liposomes, interact to bring lipid vesicles into close proximity, enabling calcium to drive fusion of opposing bilayers. Exposure to calcium triggered vesicle fusion at both, high potency and efficacy. The half-time for calcium-induced fusion of SNARE-reconstituted vesicles was determined to be approximately 10 s, which is two orders of magnitude faster than in its absence. Calcium acts downstream of SNAREs, since the presence of SNAREs in bilayers increases the potency of calcium-induced vesicle fusion, without significantly influencing its efficacy. Hence, this study suggests that in the physiological state in cells, both SNAREs and calcium operate as the minimal fusion machinery.     

Structural basis of membrane-induced cardiotoxin A3 oligomerization

Cobra cardiotoxins (CTXs) have previously been shown to induce membrane fusion of vesicles formed by phospholipids such as cardiolipin or sphingomyelin. CTX can also form a pore in membrane bilayers containing a anionic lipid such as phosphatidylserine or phosphatidylglycerol. Herein, we show that the interaction of CTX with negatively charged lipids causes CTX dimerization, an important intermediate for the eventual oligomerization of CTX during the CTX-induced fusion and pore formation process. The structural basis of the lipid-induced oligomerization of CTX A3, a major CTX from Naja atra, is then illustrated by the crystal structure of CTX A3 in complex with SDS; SDS likely mimics anionic lipids of the membrane under micelle conditions at 1.9-A resolution. The crystal packing reveals distinct SDS-free and SDS-rich regions; in the latter two types of interconnecting CTX A3 dimers, D1 and D2, and several SDS molecules can be identified to stabilize D1 and D2 by simultaneously interacting with residues at each dimer interface. When the three CTXSDS complexes in the asymmetric unit are overlaid, the orientation of CTX A3 monomers relative to the SDS molecules in the crystal is strikingly similar to that of the toxin with respect to model membranes as determined by NMR and Fourier transform infrared methods. These results not only illustrate how lipid-induced CTX dimer formation may be transformed into oligomers either as inverted micelles of fusion intermediates or as membrane pore of anionic lipid bilayers but also underscore a potential role for SDS in x-ray diffraction study of protein-membrane interactions in the future.     

The Gbetagamma dimer drives the interaction of heterotrimeric Gi proteins with nonlamellar membrane structures

Heterotrimeric G proteins are peripheral membrane proteins that propagate signals from membrane receptors to regulatory proteins localized in distinct cellular compartments. To facilitate signal amplification, G proteins are in molar excess with respect to G protein-coupled receptors. Because G proteins are capable of translocating from membrane to cytosol, protein-lipid interactions play a crucial role in signal transduction. Here, we studied the binding of heterotrimeric G proteins (Galphabetagamma) to model membranes (liposomes) and that of the entities formed upon receptor-mediated activation (Galpha and Gbetagamma). The model membranes used were composed of defined membrane lipids capable of organizing into either lamellar or nonlamellar (hexagonal H(II)) membrane structures. We demonstrated that although heterotrimeric G(i) proteins and Gbetagamma dimers can bind to lipid bilayers of phosphatidylcholine, their binding to membranes was markedly and significantly enhanced by the presence of nonlamellar phases of phosphatidylethanolamine. Conversely, activated G protein alpha subunits showed an opposite membrane binding behavior with a marked preference for lamellar membranes. These results have important consequences in cell signaling. First, the binding characteristics of the Gbetagamma dimer account for the lipid binding behavior and the cellular localization of heterotrimeric G proteins. Second, the distinct protein-lipid interactions of heterotrimeric G proteins, Gbetagamma dimers, and Galpha subunits with membrane lipids explain, in part, their different cellular mobilizations during signaling upon receptor activation. Finally, their differential interactions with lipids suggest an active role of the membrane lipid secondary structure in the propagation of signals through G protein-coupled receptors.     

Integration and Oligomerization of Bax Protein in Lipid Bilayers Characterized by Single Molecule Fluorescence Study

Bax is a pro-apoptotic Bcl-2 family protein. The activated Bax translocates to mitochondria, where it forms pore and permeabilizes the mitochondrial outer membrane. This process requires the BH3-only activator protein (i.e. tBid) and can be inhibited by anti-apoptotic Bcl-2 family proteins such as Bcl-x_L. Here by using single molecule fluorescence techniques, we studied the integration and oligomerization of Bax in lipid bilayers. Our study revealed that Bax can bind to lipid membrane spontaneously in the absence of tBid. The Bax pore formation undergoes at least two steps: pre-pore formation and membrane insertion. The activated Bax triggered by tBid or BH3 domain peptide integrates on bilayers and tends to form tetramers, which are termed as pre-pore. Subsequent insertion of the pre-pore into membrane is highly dependent on the composition of cardiolipin in lipid bilayers. Bcl-x_L can translocate Bax from membrane to solution and inhibit the pore formation. The study of Bax integration and oligomerization at the single molecule level provides new evidences that may help elucidate the pore formation of Bax and its regulatory mechanism in apoptosis.     

How calcium may cause exocytosis in sea urchin eggs

The process of secretory granule-plasma membrane fusion can be studied in sea urchin eggs. Micromolar calcium concentrations are all that is required to bring about exocytosisin vitro. I discuss recent experiments with sea urchin eggs that concentrate on the biophysical aspects of granule-membrane fusion. The backbone of biological membranes is the lipid bilayer. Sea urchin egg membrane lipids have negatively charged head groups that give rise to an electrical potential at the bilayer-water interface. We have found that this surface potential can affect the calcium required for exocytosis. Effects on the surface potential may also explain why drugs like trifluoperazine and tetracaine inhibit exocytosis: they absorb to the bilayer and reduce the surface potential. The membrane lipids may also be crucial to the formation of the exocytotic pore through which the secretory granule contents are released. We have measured calcium-induced production of the lipid, diacylglycerol. This lipid can induce a phase transition that will promote fusion of apposed lipid bilayers. The process of exocytosis involves the secretory granule core as well as the lipids of the membrane. The osmotic properties of the granule contents lead to swelling of the granule during exocytosis. Swelling promotes the dispersal of the contents as they are extruded through the exocytotic pore. The movements of water and ions during exocytosis may also stabilize the transient fusion intermediate and consolidate the exocytotic pore as fusion occurs.     

Membrane fusion in cells: molecular machinery and mechanisms

Membrane fusion is a sine qua non process for cell physiology. It is critical for membrane biogenesis, intracellular traffic, and cell secretion. Although investigated for over a century, only in the last 15 years, the molecular machinery and mechanism of membrane fusion has been deciphered. The membrane fusion event elicits essentially three actors on stage: anionic phospholipids - phosphatidylinositols, phosphatidyl serines, specific membrane proteins, and the calcium ions, all participating in a well orchestrated symphony. Three soluble N-ethylmaleimide-sensitive factor (NSF)-attachment protein receptors (SNAREs) have been implicated in membrane fusion. Target membrane proteins, SNAP-25 and syntaxin (t- SNARE) and secretory vesicle-associated membrane protein (v-SNARE) or VAMPwere discovered in the 1990's and suggested to be the minimal fusion machinery. Subsequently, the molecular mechanism of SNARE-induced membrane fusion was discovered. It was demonstrated that when t-SNARE-associated lipid membrane is exposed to v-SNARE-associated vesicles in the presence of Ca(2+), the SNARE proteins interact in a circular array to form conducting channels, thus establishing continuity between the opposing bilayers. Further it was proved that SNAREs bring opposing bilayers close to within a distance of 2-3 Angstroms, allowing Ca(2+) to bridge them. The bridging of bilayers by Ca(2+) then leads to the expulsion of water between the bilayers at the contact site, allowing lipid mixing and membrane fusion. Calcium bridging of opposing bilayers leads to the release of water, both from the water shell of hydrated Ca(2+) ions, as well as the displacement of loosely coordinated water at the phosphate head groups in the lipid membrane. These discoveries provided for the first time, the molecular mechanism of SNARE-induced membrane fusion in cells. Some of the seminal discoveries are briefly discussed in this minireview.     

The Basic Protein of CNS Myelin: Its Structure and Ligand Binding

Consideration of the evidence presented in this review leads to the following conclusions: (a) Isolated MBP in aqueous solution has little ordered secondary or tertiary structure. (b) In this state, the protein can associate with a wide range of hydrophobic and amphiphilic compounds, these interactions involving limited sections of the protein. (c) The strength of binding to bilayers and the accompanying conformational changes in the protein are greatest for systems containing acidic lipids, presumably because of the involvement of ionic interactions. (d) When bound to bilayers of acidic lipids, MBP will have substantially more ordered secondary structure than it manifests in aqueous solution, and it is likely to be oligomeric (possibly hexameric). (e) MBP does affect the organization of lipid aggregates. It influences strongly the separation of bilayers in multilayers of purified lipids, and at present this must be viewed as its prime role within myelin. The greatest impediment to our understanding of MBP is the lack of an assayable biological activity. In contrast to the situation with enzymes, for example, we have no functional test for changes in protein structure or changes accompanying interactions with other molecules. Current evidence suggests that the protein has a structural role within myelin and that its own three-dimensional structure is strongly dependent on the molecules with which it is associated. If this picture is correct, studies of the isolated protein or of the protein in reconstituted lipid systems may yield, at best, a rough guide to the structure within its biological environment. Further clarification of the structure and function of MBP may have to await development of more powerful techniques for studying proteins bound to large molecular aggregates, such as lipid bilayers. The paucity of generally applicable methods is reflected in the fact that even low resolution structures are known for only a handful of intrinsic membrane proteins, and even more limited information exists for proteins associated with membrane surfaces. However, the increasing use of a combination of electron microscopy and diffraction on two-dimensional arrays of proteins formed on lipid bilayers (Henderson et al., 1990) offers the hope that it may not be too long before it will be possible to study at moderate resolution the three-dimensional structure of MBP bound to a lipid membrane.     

Synip Arrests Soluble N-Ethylmaleimide-sensitive Factor Attachment Protein Receptor (SNARE)-dependent Membrane Fusion as a Selective Target Membrane SNARE-binding Inhibitor

The vesicle fusion reaction in regulated exocytosis requires the concerted action of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) core fusion engine and a group of SNARE-binding regulatory factors. The regulatory mechanisms of vesicle fusion remain poorly understood in most exocytic pathways. Here, we reconstituted the SNARE-dependent vesicle fusion reaction of GLUT4 exocytosis in vitro using purified components. Using this defined fusion system, we discovered that the regulatory factor synip binds to GLUT4 exocytic SNAREs and inhibits the docking, lipid mixing, and content mixing of the fusion reaction. Synip arrests fusion by binding the target membrane SNARE (t-SNARE) complex and preventing the initiation of ternary SNARE complex assembly. Although synip also interacts with the syntaxin-4 monomer, it does not inhibit the pairing of syntaxin-4 with SNAP-23. Interestingly, synip selectively arrests the fusion reactions reconstituted with its cognate SNAREs, suggesting that the defined system recapitulates the biological functions of the vesicle fusion proteins. We further showed that the inhibitory function of synip is dominant over the stimulatory activity of Sec1/Munc18 proteins. Importantly, the inhibitory function of synip is distinct from how other fusion inhibitors arrest SNARE-dependent membrane fusion and therefore likely represents a novel regulatory mechanism of vesicle fusion.     

Individual Vesicle Fusion Events Mediated by Lipid-Anchored DNA

Membrane fusion consists of a complex rearrangement of lipids and proteins that results in the merger of two lipid bilayers. We have developed a model system that employs synthetic DNA-lipid conjugates as a surrogate for the membrane proteins involved in the biological fusion reaction. We previously showed that complementary DNA-lipids, inserted into small unilamellar vesicles, can mediate membrane fusion in bulk. Here, we use a model membrane architecture developed in our lab to directly observe single-vesicle fusion events using fluorescence microscopy. In this system, a planar tethered membrane patch serves as the target membrane for incoming vesicles. This allows us to quantify the kinetics and characteristics of individual fusion events from the perspective of the lipids or the DNA-lipids involved in the process. We find that the fusion pathways are heterogeneous, with an arrested hemi-fusion state predominating, and we quantitate the outcome and rate of fusion events to construct a mechanistic model of DNA-mediated vesicle fusion. The waiting times between docking and fusion are distributed exponentially, suggesting that fusion occurs in a single step. Our analysis indicates that when two lipid bilayers are brought into close proximity, fusion occurs spontaneously, with little or no dependence on the number of DNA hybrids formed.     

Do Lipids Show State-dependent Affinity to the Voltage-gated Potassium Channel KvAP?

As all integral membrane proteins, voltage-gated ion channels are embedded in a lipid matrix that regulates their channel behavior either by physicochemical properties or by direct binding. Because manipulation of the lipid composition in cells is difficult, we investigated the influence of different lipids on purified KvAP channels reconstituted in planar lipid bilayers of known composition. Lipids developed two distinct and independent effects on the KvAP channels; lipids interacting with the pore lowered the energy barriers for the final transitions, whereas voltage sensor-bound lipids shifted the midpoint of activation dependent on their electrostatic charge. Above all, the midpoint of activation was determined only by those lipids the channels came in contact with first after purification and can seemingly only be exchanged if the channel resides in the open state. The high affinity of the bound lipids to the binding site has implications not only on our understanding of the gating mechanism but also on the general experimental design of any lipid dependence study.     

The final conformation of the complete ectodomain of the HA2 subunit of influenza hemagglutinin can by itself drive low pH-dependent fusion

One of the best characterized fusion proteins, the influenza virus hemagglutinin (HA), mediates fusion between the viral envelope and the endosomal membrane during viral entry into the cell. In the initial conformation of HA, its fusogenic subunit, the transmembrane protein HA2, is locked in a metastable conformation by the receptor-binding HA1 subunit of HA. Acidification in the endosome triggers HA2 refolding toward the final lowest energy conformation. Is the fusion process driven by this final conformation or, as often suggested, by the energy released by protein restructuring? Here we explored structural properties as well as the fusogenic activity of the full sized trimeric HA2(1–185) (here called HA2*) that presents the final conformation of the HA2 ectodomain. We found HA2* to mediate fusion between lipid bilayers and between biological membranes in a low pH-dependent manner. Two mutations known to inhibit HA-mediated fusion strongly inhibited the fusogenic activity of HA2*. At surface densities similar to those of HA in the influenza virus particle, HA2* formed small fusion pores but did not expand them. Our results confirm that the HA1 subunit responsible for receptor binding as well as the transmembrane and cytosolic domains of HA2 is not required for fusion pore opening and substantiate the hypothesis that the final form of HA2 is more important for fusion than the conformational change that generates this form.     

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.