Minimum Membrane Bending Energies of Fusion Pores

Membranes fuse by forming highly curved intermediates, culminating in structures described as fusion pores. These hourglass-like figures that join two fusing membranes have high bending energies, which can be estimated using continuum elasticity models. Fusion pore bending energies depend strongly on shape, and the present study developed a method for determining the shape that minimizes bending energy. This was first applied to a fusion pore modeled as a single surface and then extended to a more realistic model treating a bilayer as two monolayers. For the two-monolayer model, fusion pores were found to have metastable states with energy minima at particular values of the pore diameter and bilayer separation. Fusion pore energies were relatively insensitive to membrane thickness but highly sensitive to spontaneous curvature and membrane asymmetry. With symmetrical bilayers and monolayer spontaneous curvatures of ?0.1 nm^?1 (a typical value) separated by 6 nm (closest distance determined by repulsive hydration forces), fusion pore formation required 43–65 kT. The pore radius of ~2.25 nm fell within the range estimated from conductance measurements. With bilayer separation >6 nm, fusion pore formation required less energy, suggesting that protein scaffolds can promote fusion by bending membranes toward one another. With nonzero spontaneous monolayer curvature, the shape that minimized the energy change during fusion pore formation differed from the shape that minimized its energy after it formed. Thus, a nascent fusion pore will relax spontaneously to a new shape, consistent with the experimentally observed expansion of nascent fusion pores during viral fusion.     

Exocytotic fusion pores exhibit semi-stable states

Summary Rapid-freezing/freeze-fracture electron microscopy and whole-cell capacitance techniques were used to study degranulation in peritoneal mast cells of the rat and the mutant beige mouse. These studies allowed us to create a time-resolved picture for fusion pore formation. After stimulation, a dimple in the plasma membrane formed a small contact area with the secretory granule membrane. Within this zone of apposition no ordered proteinaceous specializations were seen. Electrophysiological technique measured a small fusion pore which widened rapidly to 1 nS. Thereafter, the fusion pore remained at semi-stable conductances between 1 and 20 nS for a wide range of times, between 10 and 15,000 msec. These conductances correspond to pore diameters 25–36 nm. Ultrastructural data confirmed small pores of hourglass morphology, composed of biological membrane coplanar with both the plasma and granular membranes. Later, the fusion pore rapidly increased in conductance, consistent with the observed morphology of omega-figures. The hallmarks of channel-like behavior, instantaneous jumps in pore conductance between defined levels, and sharp peaks in histograms of conductance dwell-time, were not seen. Since the morphology of small pores shows contiguous fracture planes, the electrical data represent pores that contain lipid. These combined morphological and electrophysiological data are consistent with a lipid/protein complex mediating both the initial and later stages of membrane fusion.We would like to dedicate this paper to the memory of our friend and mentor, Alex Mauro, who emphasized to us the importance of equivalent circuits. This work was supported by National Institutes of Health grant GM-27367, and National Science Foundation grant IBN-91117509.     

Dynamics of fusion pores connecting membranes of different tensions

The energetics underlying the expansion of fusion pores connecting biological or lipid bilayer membranes is elucidated. The energetics necessary to deform membranes as the pore enlarges, in some combination with the action of the fusion proteins, must determine pore growth. The dynamics of pore growth is considered for the case of two homogeneous fusing membranes under different tensions. It is rigorously shown that pore growth can be quantitatively described by treating the pore as a quasiparticle that moves in a medium with a viscosity determined by that of the membranes. Motion is subject to tension, bending, and viscous forces. Pore dynamics and lipid flow through the pore were calculated using Lagrange's equations, with dissipation caused by intra- and intermonolayer friction. These calculations show that the energy barrier that restrains pore enlargement depends only on the sum of the tensions; a difference in tension between the fusing membranes is irrelevant. In contrast, lipid flux through the fusion pore depends on the tension difference but is independent of the sum. Thus pore growth is not affected by tension-driven lipid flux from one membrane to the other. The calculations of the present study explain how increases in tension through osmotic swelling of vesicles cause enlargement of pores between the vesicles and planar bilayer membranes. In a similar fashion, swelling of secretory granules after fusion in biological systems could promote pore enlargement during exocytosis. The calculations also show that pore expansion can be caused by pore lengthening; lengthening may be facilitated by fusion proteins.     

Membrane permeability changes at early stages of influenza hemagglutinin-mediated fusion

While biological membrane fusion is classically defined as the leak-free merger of membranes and contents, leakage is a finding in both experimental and theoretical studies. The fusion stages, if any, that allow membrane permeation are uncharted. In this study we monitored membrane ionic permeability at early stages of fusion mediated by the fusogenic protein influenza hemagglutinin (HA). HAb2 cells, expressing HA on their plasma membrane, fused with human red blood cells, cultured liver cells PLC/PRF/5, or planar phospholipid bilayer membranes. With a probability that depended upon the target membrane, an increase of the electrical conductance of the fusing membranes (leakage) by up to several nS was generally detected. This leakage was recorded at the initial stages of fusion, when fusion pores formed. This leakage usually accompanied the "flickering" stage of the early fusion pore development. As the pore widened, the leakage reduced; concomitantly, the lipid exchange between the fusing membranes accelerated. We conclude that during fusion pore formation, HA locally and temporarily increases the permeability of fusing membranes. Subsequent rearrangement in the fusion complex leads to the resealing of the leaky membranes and enlargement of the pore.     

The fusion kinetics of influenza hemagglutinin expressing cells to planar bilayer membranes is affected by HA density and host cell surface

Time-resolved admittance measurements were used to follow formation of individual fusion pores connecting influenza virus hemagglutinin (HA)- expressing cells to planar bilayer membranes. By measuring in-phase, out-of-phase, and dc components of currents, pore conductances were resolved with millisecond time resolution. Fusion pores developed in stages, from small pores flickering open and closed, to small successful pores that remained open until enlarging their lumens to sizes greater than those of viral nucleocapsids. The kinetics of fusion and the properties of fusion pores were studied as functions of density of the fusion protein HA. The consequences of treating cell surfaces with proteases that do not affect HA were also investigated. Fusion kinetics were described by waiting time distributions from triggering fusion, by lowering pH, to the moment of pore formation. The kinetics of pore formation became faster as the density of active HA was made greater or when cell surface proteins were extensively cleaved with proteases. In accord with this faster kinetics, the intervals between transient pore openings within the flickering stage were shorter for higher HA density and more extensive cell surface treatment. Whereas the kinetics of fusion depended on HA density, the lifetimes of open fusion pores were independent of HA density. However, the lifetimes of open pores were affected by the proteolytic treatment of the cells. Faster fusion kinetics correlated with shorter pore openings. We conclude that the density of fusion protein strongly affects the kinetics of fusion pore formation, but that once formed, pore evolution is not under control of fusion proteins but rather under the influence of mechanical forces, such as membrane bending and tension.     

Comparison of transient and successful fusion pores connecting influenza hemagglutinin expressing cells to planar membranes

Time-resolved admittance measurements were used to investigate the evolution of fusion pores formed between cells expressing influenza virus hemagglutinin (HA) and planar bilayer membranes. The majority of fusion pores opened in a stepwise fashion to semistable conductance levels of several nS. About 20% of the pores had measurable rise times to nS conductances; some of these opened to conductances of approximately 500 pS where they briefly lingered before opening further to semistable conductances. The fall times of closing were statistically similar to the rise times of opening. All fusion pores exhibited semistable values of conductance, varying from approximately 2-20 nS; they would then either close or fully open to conductances on the order of 1 microS. The majority of pores closed; approximately 10% fully opened. Once within the semistable stage, all fusion pores, even those that eventually closed, tended to grow. Statistically, however, before closing, transient fusion pores ceased to grow and reversed their conductance pattern: conductances decreased with a measurable time course until a final drop to closure. In contrast, pore enlargement to the fully open state tended to occur from the largest conductance values attained during a pore's semistable stage. This final enlargement was characterized by a stepwise increase in conductance. The density of HA on the cell surface did not strongly affect pore dynamics. But increased proteolytic treatment of cell surfaces did lead to faster growth within the semistable range. Transient pores and pores that fully opened had indistinguishable initial conductances and statistically identical time courses of early growth, suggesting they were the same upon formation. We suggest that transient and fully open pores evolved from common structures with stochastic factors determining their fate.     

Influenza hemagglutinin-mediated fusion pores connecting cells to planar membranes: flickering to final expansion

We have studied the fusion between voltage-clamped planar lipid bilayers and influenza virus infected MDCK cells, adhered to one side of the bilayer, using measurements of electrical admittance and fluorescence. The changes in currents in-phase and 90 degrees out-of- phase with respect to the applied sinusoidal voltage were used to monitor the addition of the cell membrane capacitance to that of the lipid bilayer through a fusion pore connecting the two membranes. When ethidium bromide was included in the solution of the cell-free side of the bilayer, increases in cell fluorescence accompanied tee admittance changes, independently confirming that these changes were due to formation of a fusion pore. Fusion required acidic pH on the cell- containing side and depended on temperature. For fusion to occur, the influenza hemagglutinin (HA) had to be cleaved into HA1 and HA2 subunits. The incorporation of gangliosides into the planar bilayers greatly augmented fusion. Fusion pores developed in four distinct stages after acidification: (a) a pre-pore, electrically quiescent stage; (b) a flickering stage, with 1-2 nS pores opening and closing repetitively; (c) an irreversibly opened stage, in which pore conductances varied between 2 and 100 nS and exhibited diverse kinetics; (d) a fully opened stage, initiated by an instantaneous, time- resolution limited, increase in conductance leveling at approximately 500 nS. The expansion of pores by stages has also been shown to occur during exocytosis in mast cells and fusion of HA-expressing cells and erythrocytes. We conclude that essential features of fusion pores are produced with proteins in just one of the two fusing membranes.     

The exocytotic fusion pore interface: a model of the site of neurotransmitter release

infrastructurel techniques have shown that an early event in the exocytotic fusion of a secretory vesicle is the formation of a narrow, water-filled pore spanning both the vesicle and plasma membranes and connecting the lumen of the secretory vesicle to the extracellular environment. Smaller precursors of the exocytotic fusion pore have been detected using electrophysio-logical techniques, which reveal a dynamic fusion pore that quickly expands to the size of the pores seen with electron microscopy. While it is clear that in the latter stages of expansion, when the size of the fusion pore is several orders of magnitude bigger than any known macromolecule, the fusion pore must be mainly made of lipids, the structure of the smaller precursors is unknown. Patch-clamp measurements of the activity of individual fusion pores in mast cells have shown that the fusion pore has some unusual and unexpected properties, namely that there is a large flux of lipid through the pore and the rate of pore closure has a discontinuous temperature dependency, suggesting a purely lipidic fusion pore. Moreover, comparisons of experimental data with theoretical fusion pores and with breakdown pores support the view that the fusion pore is initially a pore through a single bilayer, as would be expected for membrane fusion proceeding through a hemifusion mechanism. Based on these observations we present a model where the fusion pore is initially a pore through a single bilayer. Fusion pore formation is regulated by a macromolecular scaffold of proteins that is responsible for bringing the plasma membrane into a highly curved dimple very close to a tense secretory granule membrane, creating the architecture where the strongly attractive hydrophobic force causes the membranes to form a ‘hemifusion’ intermediate. Membrane fusion is completed by the formation of an aqueous pore after rupture of the shared bilayer. We also propose that the microenvironment of the interface when the pore first opens, dominated by the charged groups on the secretory vesicle matrix and phospholipids, will greatly influence the release of secretory products.     

Patch clamp studies of single intact secretory granules.

The membrane of secretory granules is involved in the molecular events that cause exocytotic fusion. Several of the proteins that have been purified from the membrane of secretory granules form ion channels when they are reconstituted in lipid bilayers and, therefore, have been thought to form part of the molecular structure of the exocytotic fusion pore. We have used the patch clamp technique to study ion conductances in single isolated secretory granules from beige mouse mast cells. We found that the membrane of the intact granule had a conductance of < 50 pS. No abrupt changes in current corresponding to the opening and closing of ion channels were observed, even under conditions where exocytotic fusion occurred. However, mechanical tension or a large voltage pulse caused the breakdown of the granule membrane resulting in the abrupt opening of a pore with an ion conductance of about 1 nS that fluctuated rapidly and could expand to an immeasurably large conductance or close completely. Surprisingly, the behavior of these pores resembled the pattern of conductance changes of exocytotic fusion pores observed in degranulating beige mast cells. This similarity supports the view that the earliest fusion pore is formed upon the breakdown of a bilayer such as that formed during hemifusion.     

Pores formed in lipid bilayer membranes by nystatin, Differences in its one-sided and two-sided action

Nystatin and amphotericin B induce a cation-selective conductance when added to one side of a lipid bilayer membrane and an anion-selective conductance when added to both sides. The concentrations of antibiotic required for the one-sided action are comparable to those employed on plasma membranes and are considerably larger than those required for the two-sided action. We propose that the two-sided effect results from the formation of aqueous pores formed by the hydrogen bonding in the middle of the bilayer of two "half pores," whereas the one-sided effect results from the half pores alone. We discuss, in terms of the flexibility of bilayer structure and its thickness, how it is possible to have conducting half pores and "complete pores" in the same membrane. The role of sterol (cholesterol and ergosterol) in pore formation is also examined.     

Persistent fusion pores but transient fusion in alveolar type II cells

The release of vesicle contents following exocytotic fusion is limited by various factors including the size of the fusion pore. Fusion pores are channel-like, narrow structures after formation and proceed through semi-stable states ('fusion pore flickering'), unless they fully expand (full fusion) or close again (transient fusion). Partial release of vesicle contents may occur during transient fusion, which was described to last between milliseconds and seconds, depending on the size of the vesicle. We studied fusion pores in a slow-secreting lung epithelial cell (type II cell) using fluorescence staining of vesicle contents (surfactant) and fluorescence recovery after photobleaching (FRAP). Surfactant is a lipidic material, which is secreted into the alveolar lumen to reduce the surface tension in the lung. We found release of surfactant to be a slow process, which can last for hours. Accordingly, fusion pores in these cells are stable structures, which appear to be a barrier for release. FRAP measurements suggest that transient fusions occasionally take place in these long-lasting fusion pores, resulting in partial release of surfactant into the extracellular space. These data suggest that postfusion mechanisms may regulate the amount of secreted surfactant.     

Hemifusion between cells expressing hemagglutinin of influenza virus and planar membranes can precede the formation of fusion pores that subsequently fully enlarge

The chronological relation between the establishment of lipid continuity and fusion pore formation has been investigated for fusion of cells expressing hemagglutinin (HA) of influenza virus to planar bilayer membranes. Self-quenching concentrations of lipid dye were placed in the planar membrane to monitor lipid mixing, and time-resolved admittance measurements were used to measure fusion pores. For rhodamine-PE, fusion pores always occurred before a detectable amount of dye moved into an HA-expressing cell. However, with DiI in the planar membrane, the relationship was reversed: the spread of dye preceded formation of small pores. In other words, by using DiI as probe, hemifusion was clearly observed to occur before pore formation. For hemifused cells, a small pore could form and subsequently fully enlarge. In contrast, for cells that express a glycosylphosphatidylinositol-anchored ectodomain of HA, hemifusion occurred, but no fully enlarged pores were observed. Therefore, the transmembrane domain of HA is required for the formation of fully enlarging pores. Thus, with the planar bilayer membranes as target, hemifusion can precede pore formation, and the occurrence of lipid dye spread does not preclude formation of pores that can enlarge fully.     

Membrane Bending Energy and Fusion Pore Kinetics in Ca-Triggered Exocytosis

A fusion pore composed of lipid is an obligatory kinetic intermediate of membrane fusion, and its formation requires energy to bend membranes into highly curved shapes. The energetics of such deformations in viral fusion is well established, but the role of membrane bending in Ca²⁺-triggered exocytosis remains largely untested. Amperometry recording showed that during exocytosis in chromaffin and PC12 cells, fusion pores formed by smaller vesicles dilated more rapidly than fusion pores formed by larger vesicles. The logarithm of 1/(fusion pore lifetime) varied linearly with vesicle curvature. The vesicle size dependence of fusion pore lifetime quantitatively accounted for the nonexponential fusion pore lifetime distribution. Experimentally manipulating vesicle size failed to alter the size dependence of fusion pore lifetime. Manipulations of membrane spontaneous curvature altered this dependence, and applying the curvature perturbants to the opposite side of the membrane reversed their effects. These effects of curvature perturbants were opposite to those seen in viral fusion. These results indicate that during Ca²⁺-triggered exocytosis membrane bending opposes fusion pore dilation rather than fusion pore formation. Ca²⁺-triggered exocytosis begins with a proteinaceous fusion pore with less stressed membrane, and becomes lipidic as it dilates, bending membrane into a highly curved shape.     

Determination of ion permeability through the channels made of porins from the outer membrane ofSalmonella typhimurium in lipid bilayer membranes

Summary The three types of porin (matrix-proteins) fromSalmonella typhimurium with molecular weights of 38,000, 39,000 and 40,000 were reconstituted with lipid bilayer membranes either as a trimer or as an oligomer (complex I). The specific conductance of the membranes increased several orders of magnitude after the addition of the porins into the aqueous phase bathing the membranes. A linear relationship between protein concentration in the aqueous phase and membrane conductance was found. In the case of lower protein concentrations (10^–12 m), the conductance increased in a stepwise fashion with a single conductance increment of 2.3 nS in 1m KCl. For a given salt the conductance increment was found to be largely independent of the particular porin (38 K, 39K or 40 K) and on the state of aggregation, although porin oligomers showed an up to 10 times smaller conductance increase in macroscopic conductance measurements. The conductance pathway has an ohmic current voltage characteristic and a poor selectivity for different alkali ions. Further information on the structure of the pores formed by the different porins fromSalmonella was obtained from the selectivity for various ions. From the permeability of the pore for large ions (Tris⁺, glucosamine⁺, Hepes^–_ a minimum pore diameter of 0.8 nm is estimated. This value is in agreement with the size of the pore as calculated from the conductance data for 1m KCl (1.4 nm for a pore length of 7.5 nm). The pore diameter may well account for the sugar permeability which has been found in reconstituted vesicles. The findings reported here are consistent with the assumption that the different porins form large aqueous channels in the lipid bilayer membranes and that the single condutance unit is a trimer. In addition, it is suggested that one trimer contains only one pore rather than a bundle of pores.     

Initial size and dynamics of viral fusion pores are a function of the fusion protein mediating membrane fusion

Background information. Protein‐mediated merger of biological membranes, membrane fusion, is an important process. To investigate the role of fusogenic proteins in the initial size and dynamics of the fusion pore (a narrow aqueous pathway, which widens to finalize membrane fusion), two different fusion proteins expressed in the same cell line were investigated: the major glycoprotein of baculovirus Autographa californica (GP64) and the HA (haemagglutinin) of influenza X31. Results. The host Sf9 cells expressing these viral proteins, irrespective of protein species, fused to human RBCs (red blood cells) upon acidification of the medium. A high‐time‐resolution electrophysiological study of fusion pore conductance revealed fundamental differences in (i) the initial pore conductance; pores created by HA were smaller than those created by GP64; (ii) the ability of pores to flicker; only HA‐mediated pores flickered; and (iii) the time required for pore formation; HA‐mediated pores took much longer to form after acidification. Conclusion. HA and GP64 have divergent electrophysiological phenotypes even when they fuse identical membranes, and fusion proteins play a crucial role in determining initial fusion pore characteristics. The structure of the initial fusion pore detected by electrical conductance measurements is sensitive to the nature of the fusion protein.     

A molecular dynamics study of the pores formed by Escherichia coli OmpF porin in a fully hydrated palmitoyloleoylphosphatidylcholine bilayer.

In this paper we study the properties of pores formed by OmpF porin from Escherichia coli, based on a molecular dynamics simulation of the OmpF trimer, 318 palmitoyl-oleoyl-phosphatidylethanolamine lipids, 27 Na+ ions, and 12,992 water molecules. After equilibration and a nanosecond production run, the OmpF trimer exhibits a C-alpha root mean square deviation from the crystal structure of 0.23 nm and a stable secondary structure. No evidence is found for large-scale motions of the L3 loop. We investigate the pore dimensions, conductance, and the properties of water inside the pore. This water forms a complicated pattern, even when averaged over 1 ns of simulation time. Around the pore constriction zone the water dipoles are highly structured in the plane of the membrane, oriented by the strong transversal electric field. In addition, there is a net orientation along the pore axis pointing from the extracellular to the intracellular side of the bilayer. The diffusion coefficients of water inside the pore are greatly reduced compared to bulk. We compare our results to results from model pores (Breed et al., 1996. Biophys. J. 70:1 643-1 661; Sansom et al. 1997. Biophys. J. 73:2404-241 5) and discuss implications for further theoretical work.     

Single-channel analysis of the conductance fluctuations induced in lipid bilayer membranes by complement proteins C5b-9

Summary Single-channel analysis of electrical fluctuations induced in planar bilayer membranes by the purified human complement proteins C5b6, C7, C8, and C9 have been analyzed. Reconstitution experiments with lipid bilayer membranes showed that the C5b-9 proteins formed pores only if all proteins were present at one side of the membrane. The complement pores had an average single-channel conductance of 3.1 nS at 0.15m KCl. The histogram of the complement pores suggested a substantial variation of the size of the single channel. The linear relationship between single-channel conductance at fixed ionic strength and the aqueous mobility of the ions in the bulk aqueous phase indicated that the ions move inside the complement pore in a manner similar to the way they move in the aqueous phase. The minimum diameter of the pores as judged from the conductance data is approximately 3 nm. The complement channels showed no apparent voltage control or regulation up to transmembrane potentials of 100 mV. At neutral pH the pore is three to four times more permeable for alkali ions than for chloride, which may be explained by the existence of fixed negatively charged groups in or near the pore. The significance of these observations to current molecular models of the membrane lesion formed by these cytolytic serum proteins is considered.     

Identification of a new pore in the mitochondrial outer membrane of a porin-deficient yeast mutant

Reconstitution experiments were performed on lipid bilayer membranes in the presence of detergent-solubilized mitochondrial outer membranes of a porin-free yeast mutant and of its parent strain. The addition of the detergent-solubilized material resulted in a strong increase in the membrane conductance which was not observed if only the detergent was added to the aqueous phase. Surprisingly, the membrane conductance induced by the detergent extracts of the mutant membrane was only a factor of 20 less than that caused by the outer membrane of the parent strain under otherwise identical conditions. Single-channel recordings of lipid bilayer membranes in the presence of mitochondrial outer membranes of the yeast mutant suggested the presence of a transient pore. The reconstituted pores had a single-channel conductance of 0.21 nS in 0.1 M KCl and the characteristics of general diffusion pores with an estimated effective diameter of 1.2 nm. The pores present in the mitochondrial outer membranes of the yeast mutant shared some similarities with the pores formed by mitochondrial and bacterial porins although their effective diameter is much smaller than those of the 'normal' mitochondrial porins which have a single-channel conductance of about 0.4 nS in 0.1 M KCl, corresponding to an effective diameter of 1.7 nm. Zero-current membrane-potential measurements suggested that the second mitochondrial porin is slightly cation-selective. Its possible role in the metabolism of mitochondria is discussed.     

An amphipathic peptide from the C-terminal region of the human immunodeficiency virus envelope glycoprotein causes pore formation in membranes.

The peptide fragment of the carboxy-terminal region of the human immunodeficiency virus (HIV) transmembrane protein (gp41) has been implicated in T-cell death. This positively charged, amphipathic helix (amino acids 828 to 848) of the envelope protein is located within virions or cytoplasm. We studied the interaction of the isolated, synthetic amphipathic helix of gp41 with planar phospholipid bilayer membranes and with Sf9 cells using voltage clamp, potentiodynamic, and single-cell recording techniques. We found that the peptide binds strongly to planar membranes, especially to the negatively charged phosphatidylserine bilayer. In the presence of micromolar concentrations of peptide sufficient to make its surface densities comparable with those of envelope glycoprotein molecules in HIV virions, an increase in bilayer conductance and a decrease in bilayer stability were observed, showing pore formation in the planar lipid bilayers. These pores were permeable to both monovalent and divalent cations, as well as to chloride. The exposure of the inner leaflet of cell membranes to even 25 nM peptide increased membrane conductance. We suggest that the carboxy-terminal fragment of the HIV type 1 envelope protein may interact with the cell membrane of infected T cells to create lipidic pores which increase membrane permeability, leading to sodium and calcium flux into cells, osmotic swelling, and T-cell necrosis or apoptosis.