How proteins produce cellular membrane curvature

The plant endoplasmic reticulum: a cell-wide web

The ER (endoplasmic reticulum) in higher plants forms a pleomorphic web of membrane tubules and small cisternae that pervade the cytoplasm, but in particular form a polygonal network at the cortex of the cell which may be anchored to the plasma membrane. The network is associated with the actin cytoskeleton and demonstrates extensive mobility, which is most likely to be dependent on myosin motors. The ER is characterized by a number of domains which may be associated with specific functions such as protein storage, or with direct interaction with other organelles such as the Golgi apparatus, peroxisomes and plastids. In the present review we discuss the nature of the network, the role of shape-forming molecules such as the recently described reticulon family of proteins and the function of some of the major domains within the ER network.

Simulations of membrane tubulation by lattices of amphiphysin N-BAR domains

Membrane compartments of manifold shapes are found in cells, often sculpted by cellular proteins. In particular, proteins of the BAR domain superfamily participate in membrane-sculpting processes in vivo and reshape also in vitro low-curvature membrane liposomes into high-curvature tubes and vesicles. Here we show by means of computer simulations totaling over 1 millisecond, how lattices involving parallel rows of amphiphysin N-BAR domains sculpt flat membranes into tubes. A highly detailed, dynamic picture of the 100-microsecond formation of membrane tubes by lattices of N-BAR domains is obtained. Lattice types inducing a wide range of membrane curvatures, with radii approximately 15-100 nm, are explored. The results suggest that multiple lattice types are viable for efficient membrane bending.

Membrane insertion of the pleckstrin homology domain variable loop 1 is critical for dynamin-catalyzed vesicle scission

The GTPase dynamin catalyzes the scission of deeply invaginated clathrin-coated pits at the plasma membrane, but the mechanisms governing dynamin-mediated membrane fission remain poorly understood. Through mutagenesis, we have altered the hydrophobic nature of the membrane-inserting variable loop 1 (VL1) of the pleckstrin homology (PH) domain of dynamin-1 and demonstrate that its stable insertion into the lipid bilayer is critical for high membrane curvature generation and subsequent membrane fission. Dynamin PH domain mutants defective in curvature generation regain function when assayed on precurved membrane templates in vitro, but they remain defective in the scission of clathrin-coated pits in vivo. These results demonstrate that, in concert with dynamin self-assembly, PH domain membrane insertion is essential for fission and vesicle release in vitro and for clathrin-mediated endocytosis in vivo.

Undulating tubular liposomes through incorporation of a synthetic skin ceramide into phospholipid bilayers

Nonspherical liposomes were prepared by doping L-alpha-phosphatidylcholine (PC) with ceramide VI (a skin lipid). Cryo-transmission electron microscopy shows the liposome shape changing from spherical to an undulating tubular morphology, when the amount of ceramide VI is increased. The formation of tubular liposomes is energetically favorable and is attributed to the association of ceramide VI with PC creating regions of lower curvature. Since ceramides are the major component of skin lipids in the stratum corneum, tubular liposomes containing ceramide may potentially serve as self-enhanced nanocarriers for transdermal delivery.

Group IV phospholipase A(2)alpha controls the formation of inter-cisternal continuities involved in intra-Golgi transport

The organization of intra-Golgi trafficking and the nature of the transport intermediates involved (e.g., vesicles, tubules, or tubular continuities) remain incompletely understood. It was recently shown that successive cisternae in the Golgi stack are interconnected by membrane tubules that form during the arrival of transport carriers from the endoplasmic reticulum. Here, we examine the mechanisms of generation and the function of these tubules. In principle, tubule formation might depend on several protein- and/or lipid-based mechanisms. Among the latter, we have studied the phospholipase A(2) (PLA(2))-mediated generation of wedge-shaped lysolipids, with the resulting local positive membrane curvature. We show that the arrival of cargo at the Golgi complex induces the recruitment of Group IVA Ca(2+)-dependent, cytosolic PLA(2) (cPLA(2)alpha) onto the Golgi complex itself, and that this cPLA(2)alpha is required for the formation of the traffic-dependent intercisternal tubules and for intra-Golgi transport. In contrast, silencing of cPLA(2)alpha has no inhibitory effects on peri-Golgi vesicles. These findings identify cPLA(2)alpha as the first component of the machinery that is responsible for the formation of intercisternal tubular continuities and support a role for these continuities in transport through the Golgi complex.

How curved membranes recruit amphipathic helices and protein anchoring motifs

Lipids and several specialized proteins are thought to be able to sense the curvature of membranes (MC). Here we used quantitative fluorescence microscopy to measure curvature-selective binding of amphipathic motifs on single liposomes 50-700 nm in diameter. Our results revealed that sensing is predominantly mediated by a higher density of binding sites on curved membranes instead of higher affinity. We proposed a model based on curvature-induced defects in lipid packing that related these findings to lipid sorting and accurately predicted the existence of a new ubiquitous class of curvature sensors: membrane-anchored proteins. The fact that unrelated structural motifs such as alpha-helices and alkyl chains sense MC led us to propose that MC sensing is a generic property of curved membranes rather than a property of the anchoring molecules. We therefore anticipate that MC will promote the redistribution of proteins that are anchored in membranes through other types of hydrophobic moieties.

Fusion peptide from influenza hemagglutinin increases membrane surface order: an electron-spin resonance study

A spin-labeling study of interactions of a fusion peptide from the hemagglutinin of the influenza virus, wt20, and a fusion-inactive mutant DeltaG1 with dimyristoylphosphatidylcholine (DMPC) and 1-palmitoyl-2-oleoyl-phosphatdylcholine bilayers was performed. We found that upon binding of wt20, the ordering of headgroups and the ordering of acyl chains near the headgroup increased significantly, in a manner consistent with a cooperative phenomenon. However, changes in the order at the end of the acyl chains were negligible. The ordering effect of wt20 on the headgroup was much stronger at pH 5 than at pH 7. No effect of DeltaG1 binding on the order of bilayers was evident. We also found that 1-palmitoyl-2-hydroxyl phosphatidylcholine, a membrane-fusion inhibitor, decreased the ordering of DMPC headgroups, whereas arachidonic acid, a membrane-fusion promoter, increased the ordering of DMPC headgroups. These results suggest that increases in headgroup ordering may be important for membrane fusion. We propose that upon binding of wt20, which is known to affect only the outer leaflet of the bilayer, this outer leaflet becomes more ordered, and thus more solid-like. Then the coupling between the hardened outer leaflet and the softer inner leaflet generates bending stresses in the bilayer, which tend to increase the negative curvature of the bilayer. We suggest that the increased ordering in the headgroup region enhances dipolar interactions and lowers electrostatic energy, which may provide an energy source for membrane fusion. Possible roles of bending stresses in promoting membrane fusion are discussed.

Connexin channels and phospholipids: association and modulation

BACKGROUND

For membrane proteins, lipids provide a structural framework and means to modulate function. Paired connexin hemichannels form the intercellular channels that compose gap junction plaques while unpaired hemichannels have regulated functions in non-junctional plasma membrane. The importance of interactions between connexin channels and phospholipids is poorly understood.

RESULTS

Endogenous phospholipids most tightly associated with purified connexin26 or connexin32 hemichannels or with junctional plaques in cell membranes, those likely to have structural and/or modulatory effects, were identified by tandem electrospray ionization-mass spectrometry using class-specific interpretative methods. Phospholipids were characterized by headgroup class, charge, glycerol-alkyl chain linkage and by acyl chain length and saturation. The results indicate that specific endogenous phospholipids are uniquely associated with either connexin26 or connexin32 channels, and some phospholipids are associated with both. Functional effects of the major phospholipid classes on connexin channel activity were assessed by molecular permeability of hemichannels reconstituted into liposomes. Changes to phospholipid composition(s) of the liposome membrane altered the activity of connexin channels in a manner reflecting changes to the surface charge/potential of the membrane and, secondarily, to cholesterol content. Together, the data show that connexin26 and connexin32 channels have a preference for tight association with unique anionic phospholipids, and that these, independent of headgroup, have a positive effect on the activity of both connexin26 and connexin32 channels. Additionally, the data suggest that the likely in vivo phospholipid modulators of connexin channel structure-function that are connexin isoform-specific are found in the cytoplasmic leaflet. A modulatory role for phospholipids that promote negative curvature is also inferred.

CONCLUSION

This study is the first to identify (endogenous) phospholipids that tightly associate with connexin channels. The finding that specific phospholipids are associated with different connexin isoforms suggests connexin-specific regulatory and/or structural interactions with lipid membranes. The results are interpreted in light of connexin channel function and cell biology, as informed by current knowledge of lipid-protein interactions and membrane biophysics. The intimate involvement of distinct phospholipids with different connexins contributes to channel structure and/or function, as well as plaque integrity, and to modulation of connexin channels by lipophilic agents.

Golgi-associated cPLA2alpha regulates endothelial cell-cell junction integrity by controlling the trafficking of transmembrane junction proteins

In endothelial cells specifically, cPLA2alpha translocates from the cytoplasm to the Golgi complex in response to cell confluence. Considering the link between confluence and cell-cell junction formation, and the emerging role of cPLA2alpha in intracellular trafficking, we tested whether Golgi-associated cPLA2alpha is involved in the trafficking of junction proteins. Here, we show that the redistribution of cPLA2alpha from the cytoplasm to the Golgi correlates with adherens junction maturation and occurs before tight junction formation. Disruption of adherens junctions using a blocking anti-VE-cadherin antibody reverses the association of cPLA2alpha with the Golgi. Silencing of cPLA2alpha and inhibition of cPLA2alpha enzymatic activity using various inhibitors result in the diminished presence of the transmembrane junction proteins VE-cadherin, occludin, and claudin-5 at cell-cell contacts, and in their accumulation at the Golgi. Altogether, our data support the idea that VE-cadherin triggers the relocation of cPLA2alpha to the Golgi and that in turn, Golgi-associated cPLA2alpha regulates the transport of transmembrane junction proteins through or from the Golgi, thereby controlling the integrity of endothelial cell-cell junctions.

Quantification of nano-scale intermembrane contact areas by using fluorescence resonance energy transfer

Nanometer-scale intermembrane contact areas (CAs) formed between single small unilamellar lipid vesicles (SUVs) and planar supported lipid bilayers are quantified by measuring fluorescence resonance energy transfer (FRET) between a homogenous layer of donor fluorophores labeling the supported bilayer and acceptor fluorophores labeling the SUVs. The smallest CAs detected in our setup between biotinylated SUVs and dense monolayers of streptavidin were approximately 20 nm in radius. Deformation of SUVs is revealed by comparing the quenching of the donors to calculations of FRET between a perfectly spherical shell and a flat surface containing complementary fluorophores. These results confirmed the theoretical prediction that the degree of deformation scales with the SUV diameter. The size of the CA can be controlled experimentally by conjugating polyethylene glycol polymers to the SUV or the surface and thereby modulating the interfacial energy of adhesion. In this manner, we could achieve secure immobilization of SUVs under conditions of minimal deformation. Finally, we demonstrate that kinetic measurements of CA, at constant adhesion, can be used to record in real-time quantitative changes in the bilayer tension of a nano-scale lipid membrane system.

Curvature-dependent lateral distribution of raft markers in the human erythrocyte membrane

The distribution of raft markers in curved membrane exvaginations and invaginations, induced in human erythrocytes by amphiphile-treatment or increased cytosolic calcium level, was studied by fluorescence microscopy. Cholera toxin subunit B and antibodies were used to detect raft components. Ganglioside GM1 was enriched in membrane exvaginations (spiculae) induced by cytosolic calcium and amphiphiles. Stomatin and the cytosolic proteins synexin and sorcin were enriched in spiculae when induced by cytosolic calcium, but not in spiculae induced by amphiphiles. No enrichment of flotillin-1 was detected in spiculae. Analyses of the relative protein content of released exovesicles were in line with the microscopic observations. In invaginations induced by amphiphiles, the enrichment of ganglioside GM1, but not of the integral membrane proteins flotillin-1 and stomatin, was observed. Based on the experimental results and theoretical considerations we suggest that membrane skeleton-detached, laterally mobile rafts may sort into curved or flat membrane regions dependent on their intrinsic molecular shape and/or direct interactions between the raft elements.

Protein-induced membrane curvature investigated through molecular dynamics flexible fitting

In the photosynthetic purple bacterium Rhodobacter (Rba.) sphaeroides, light is absorbed by membrane-bound light-harvesting (LH) proteins LH1 and LH2. LH1 directly surrounds the reaction center (RC) and, together with PufX, forms a dimeric (RC-LH1-PufX)2 protein complex. In LH2-deficient Rba. sphaeroides mutants, RC-LH1-PufX dimers aggregate into tubular vesicles with a radius of approximately 250-550 A, making RC-LH1-PufX one of the few integral membrane proteins known to actively induce membrane curvature. Recently, a three-dimensional electron microscopy density map showed that the Rba. sphaeroides RC-LH1-PufX dimer exhibits a prominent bend at its dimerizing interface. To investigate the curvature properties of this highly bent protein, we employed molecular dynamics simulations to fit an all-atom structural model of the RC-LH1-PufX dimer within the electron microscopy density map. The simulations reveal how the dimer produces a membrane with high local curvature, even though the location of PufX cannot yet be determined uniquely. The resulting membrane curvature agrees well with the size of RC-LH1-PufX tubular vesicles, and demonstrates how the local curvature properties of the RC-LH1-PufX dimer propagate to form the observed long-range organization of the Rba. sphaeroides tubular vesicles.

Calculation of free energies in fluid membranes subject to heterogeneous curvature fields

We present a computational methodology for incorporating thermal effects and calculating relative free energies for elastic fluid membranes subject to spatially dependent intrinsic curvature fields using the method of thermodynamic integration. Based on a simple model for the intrinsic curvature imposed only in a localized region of the membrane, we employ thermodynamic integration to calculate the free-energy change as a function of increasing strength of the intrinsic curvature field and a thermodynamic cycle to compute free-energy changes for different sizes of the localized region. By explicitly computing the free-energy changes and by quantifying the loss of entropy accompanied with increasing membrane deformation, we show that the membrane stiffness increases with increasing intrinsic field, thereby, renormalizing the membrane bending rigidity. The second main conclusion of this work is that the entropy of the membrane decreases with increasing size of the localized region subject to the curvature field. Our results help to quantify the free-energy change when a planar membrane deforms under the influence of curvature-inducing proteins at a finite temperature.

Physical model for the width distribution of axons

The distribution of widths of axons was recently investigated, and was found to have a distinct peak at an optimized value. The optimized axon width at the peak may arise from the conflicting demands of minimizing energy consumption and assuring signal transmission reliability. The distribution around this optimized value is found to have a distinct non-Gaussian shape, with an exponential "tail". We propose here a mechanical model whereby this distribution arises from the interplay between the elastic energy of the membrane surrounding the axon core, the osmotic pressure induced by the neurofilaments inside the axon bulk, and active processes that remodel the microtubules and neurofilaments inside the axon. The axon's radius of curvature can be determined by the cell's control of the osmotic pressure difference across the membrane, the membrane tension or by changing the composition of the different components of the membrane. We find that the osmotic pressure, determined by the neurofilaments, seems to be the dominant control parameter.

Sorting of lipids and proteins in membrane curvature gradients

The sorting of lipids and proteins in cellular trafficking pathways is a process of central importance in maintaining compartmentalization in eukaryotic cells. However, the mechanisms behind these sorting phenomena are currently far from being understood. Among several mechanistic suggestions, membrane curvature has been invoked as a means to segregate lipids and proteins in cellular sorting centers. To assess this hypothesis, we investigate the sorting of lipid analog dye trace components between highly curved tubular membranes and essentially flat membranes of giant unilamellar vesicles. Our experimental findings indicate that intracellular lipid sorting, contrary to frequent assumptions, is unlikely to occur by lipids fitting into membrane regions of appropriate curvature. This observation is explained in the framework of statistical mechanical lattice models that show that entropy, rather than curvature energy, dominates lipid distribution in the absence of strongly preferential lateral intermolecular interactions. Combined with previous findings of curvature induced phase segregation, we conclude that lipid cooperativity is required to enable efficient sorting. In contrast to lipid analog dyes, the peripheral membrane binding protein Cholera toxin subunit B is effectively curvature-sorted. The sorting of Cholera toxin subunit B is rationalized by statistical models. We discuss the implications of our findings for intracellular sorting mechanisms.

Mechanisms for the formation of membranous nanostructures in cell-to-cell communication

Cells interact by exchanging material and information. Two methods of cell-to-cell communication are by means of microvesicles and by means of nanotubes. Both microvesicles and nanotubes derive from the cell membrane and are able to transport the contents of the inner solution. In this review, we describe two physical mechanisms involved in the formation of microvesicles and nanotubes: curvature-mediated lateral redistribution of membrane components with the formation of membrane nanodomains; and plasmamediated attractive forces between membranes. These mechanisms are clinically relevant since they can be affected by drugs. In particular, the underlying mechanism of heparin's role as an anticoagulant and tumor suppressor is the suppression of microvesicluation due to plasma-mediated attractive interaction between membranes.

Drug interactions with lipid membranes

The field of drug-membrane interactions is one that spans a wide range of scientific disciplines, from synthetic chemistry, through biophysics to pharmacology. Cell membranes are complex dynamic systems whose structures can be affected by drug molecules and in turn can affect the pharmacological properties of the drugs being administered. In this tutorial review we aim to provide a guide for those new to the area of drug-membrane interactions and present an introduction to areas of this topic which need to be considered. We address the lipid composition and structure of the cell membrane and comment on the physical forces present in the membrane which may impact on drug interactions. We outline methods by which drugs may cross or bind to this membrane, including the well understood passive and active transport pathways. We present a range of techniques which may be used to study the interactions of drugs with membranes both in vitro and in vivo and discuss the advantages and disadvantages of these techniques and highlight new methods being developed to further this field.

Molecular mechanism of membrane constriction and tubulation mediated by the F-BAR protein Pacsin/Syndapin

Peripheral membrane proteins of the Bin/amphiphysin/Rvs (BAR) and Fer-CIP4 homology-BAR (F-BAR) family participate in cellular membrane trafficking and have been shown to generate membrane tubules. The degree of membrane bending appears to be encoded in the structure and immanent curvature of the particular protein domains, with BAR and F-BAR domains inducing high- and low-curvature tubules, respectively. In addition, oligomerization and the formation of ordered arrays influences tubule stabilization. Here, the F-BAR domain-containing protein Pacsin was found to possess a unique activity, creating small tubules and tubule constrictions, in addition to the wide tubules characteristic for this subfamily. Based on crystal structures of the F-BAR domain of Pacsin and mutagenesis studies, vesiculation could be linked to the presence of unique structural features distinguishing it from other F-BAR proteins. Tubulation was suppressed in the context of the full-length protein, suggesting that Pacsin is autoinhibited in solution. The regulated deformation of membranes and promotion of tubule constrictions by Pacsin suggests a more versatile function of these proteins in vesiculation and endocytosis beyond their role as scaffold proteins.

SDPR induces membrane curvature and functions in the formation of caveolae

Caveolae are plasma membrane invaginations with a characteristic flask shaped morphology. They function in diverse cellular processes, including endocytosis. The mechanism by which caveolae are generated is not fully understood, but both caveolin proteins and Polymerase I and Transcript Release Factor (PTRF, also called cavin) are important. Here we show that loss of SDPR, a caveolar protein homologous to PTRF, causes loss of caveolae. SDPR binds directly to PTRF and recruits PTRF to caveolar membranes. Over-expression of SDPR, unlike PTRF, induces deformation of caveolae and extensive tubulation of the plasma membrane. The B-subunit of shiga toxin (STB) also induces membrane tubulation, and these membrane tubes also originate from caveolae. STB co-localizes extensively with both SDPR and caveolin 1. Loss of caveolae reduces the propensity of STB to induce membrane tubulation. We conclude that SDPR is a membrane-curvature inducing component of caveolae, and that STB-induced membrane tubulation is facilitated by caveolae.

Hepatitis C virus NS4B carboxy terminal domain is a membrane binding domain

Background

Hepatitis C virus (HCV) induces membrane rearrangements during replication. All HCV proteins are associated to membranes, pointing out the importance of membranes for HCV. Non structural protein 4B (NS4B) has been reported to induce cellular membrane alterations like the membranous web. Four transmembrane segments in the middle of the protein anchor NS4B to membranes. An amphipatic helix at the amino-terminus attaches to membranes as well. The carboxy-terminal domain (CTD) of NS4B is highly conserved in Hepaciviruses, though its function remains unknown.

Results

A cytosolic localization is predicted for the NS4B-CTD. However, using membrane floatation assays and immunofluorescence, we now show targeting of the NS4B-CTD to membranes. Furthermore, a profile-profile search, with an HCV NS4B-CTD multiple sequence alignment, indicates sequence similarity to the membrane binding domain of prokaryotic D-lactate dehydrogenase (d-LDH). The crystal structure of E. coli d-LDH suggests that the region similar to NS4B-CTD is located in the membrane binding domain (MBD) of d-LDH, implying analogy in membrane association. Targeting of d-LDH to membranes occurs via electrostatic interactions of positive residues on the outside of the protein with negative head groups of lipids. To verify that anchorage of d-LDH MBD and NS4B-CTD is analogous, NS4B-CTD mutants were designed to disrupt these electrostatic interactions. Membrane association was confirmed by swopping the membrane contacting helix of d-LDH with the corresponding domain of the 4B-CTD. Furthermore, the functionality of these residues was tested in the HCV replicon system.

Conclusion

Together these data show that NS4B-CTD is associated to membranes, similar to the prokaryotic d-LDH MBD, and is important for replication.

Modeling morphological instabilities in lipid membranes with anchored amphiphilic polymers

Anchoring molecules, like amphiphilic polymers, are able to dynamically regulate membrane morphology. Such molecules insert their hydrophobic groups into the bilayer, generating a local membrane curvature. In order to minimize the elastic energy penalty, a dynamic shape instability may occur, as in the case of the curvature-driven pearling instability or the polymer-induced tubulation of lipid vesicles. We review recent works on modeling of such instabilities by means of a mesoscopic dynamic model of the phase-field kind, which take into account the bending energy of lipid bilayers.

A microfluidic platform for probing single cell plasma membranes using optically trapped Smart Droplet Microtools (SDMs)

We recently introduced a novel platform based upon optically trapped lipid coated oil droplets (Smart Droplet Microtools-SDMs) that were able to form membrane tethers upon fusion with the plasma membrane of single cells. Material transfer from the plasma membrane to the droplet via the tether was seen to occur. Here we present a customised version of the SDM approach based upon detergent coated droplets deployed within a microfluidic format. These droplets are able to differentially solubilise the plasma membrane of single cells with spatial selectivity and without forming membrane tethers. The microfluidic format facilitates separation of the target cells from the bulk SDM population and from downstream analysis modules. Material transfer from the cell to the SDM was monitored by tracking membrane localized EGFP.

Eps15 homology domain 1-associated tubules contain phosphatidylinositol-4-phosphate and phosphatidylinositol-(4,5)-bisphosphate and are required for efficient recycling

The C-terminal Eps15 homology domain (EHD) 1/receptor-mediated endocytosis-1 protein regulates recycling of proteins and lipids from the recycling compartment to the plasma membrane. Recent studies have provided insight into the mode by which EHD1-associated tubular membranes are generated and the mechanisms by which EHD1 functions. Despite these advances, the physiological function of these striking EHD1-associated tubular membranes remains unknown. Nuclear magnetic resonance spectroscopy demonstrated that the Eps15 homology (EH) domain of EHD1 binds to phosphoinositides, including phosphatidylinositol-4-phosphate. Herein, we identify phosphatidylinositol-4-phosphate as an essential component of EHD1-associated tubules in vivo. Indeed, an EHD1 EH domain mutant (K483E) that associates exclusively with punctate membranes displayed decreased binding to phosphatidylinositol-4-phosphate and other phosphoinositides. Moreover, we provide evidence that although the tubular membranes to which EHD1 associates may be stabilized and/or enhanced by EHD1 expression, these membranes are, at least in part, pre-existing structures. Finally, to underscore the function of EHD1-containing tubules in vivo, we used a small interfering RNA (siRNA)/rescue assay. On transfection, wild-type, tubule-associated, siRNA-resistant EHD1 rescued transferrin and beta1 integrin recycling defects observed in EHD1-depleted cells, whereas expression of the EHD1 K483E mutant did not. We propose that phosphatidylinositol-4-phosphate is an essential component of EHD1-associated tubules that also contain phosphatidylinositol-(4,5)-bisphosphate and that these structures are required for efficient recycling to the plasma membrane.

Phase transition induced fission in lipid vesicles

In this work we demonstrate how the first order phase transition in giant unilamellar vesicles (GUVs) can function as a trigger for membrane fission. When driven through their gel-fluid phase transition GUVs exhibit budding or pearl formation. These buds remain connected to the mother vesicle presumably by a small neck. Cooling these vesicles from the fluid phase (T>T(m)) through the phase transition into the gel state (T<T(m)), leads to complete rupture and fission of the neck, while the mother vesicle remains intact. Pearling tubes which formed upon heating break-up and decay into multiple individual vesicles which then diffuse freely. Finally we demonstrate that mimicking the intracellular bulk viscosity by increasing the bulk viscosity to 40 cP does not affect the overall fission process, but leads to a significant decrease in the size of the released vesicles.

Biophysics and function of phosphatidic acid: a molecular perspective

Phosphatidic acid is the simplest (diacyl)glycerophospholipid present in cells and is now a well established second messenger with direct biological functions. It is specifically recognized by diverse proteins and plays an important role in cellular signaling and membrane dynamics in all eukaryotes. An important determinant of the biological functions of phosphatidic acid is its anionic headgroup. In this review we will focus on the peculiar ionization properties of phosphatidic acid and their crucial role in lipid-protein interactions. We will take a molecular approach focusing entirely on the physical chemistry of the lipid and develop a model explaining the ionization properties of phosphatidic acid, termed the electrostatic-hydrogen bond switch model. Diverse examples from recent literature in support of this model will be presented and the broader implications of our findings will be discussed.

Syndapin promotes formation of a postsynaptic membrane system in Drosophila

Syndapins belong to the F-BAR domain protein family whose predicted functions in membrane tubulation remain poorly studied in vivo. At Drosophila neuromuscular junctions, syndapin is associated predominantly with a tubulolamellar postsynaptic membrane system known as the subsynaptic reticulum (SSR). We show that syndapin overexpression greatly expands this postsynaptic membrane system. Syndapin can expand the SSR in the absence of dPAK and Dlg, two known regulators of SSR development. Syndapin's N-terminal F-BAR domain, required for membrane tubulation in cultured cells, is required for SSR expansion. Consistent with a model in which syndapin acts directly on postsynaptic membrane, SSR expansion requires conserved residues essential for membrane binding in vitro. However, syndapin's Src homology (SH) 3 domain, which negatively regulates membrane tubulation in cultured cells, is required for synaptic targeting and strong SSR induction. Our observations advance knowledge of syndapin protein function by 1) demonstrating the in vivo relevance of membrane remodeling mechanisms suggested by previous in vitro and structural analyses, 2) showing that SH3 domains are necessary for membrane expansion observed in vivo, and 3) confirming that F-BAR proteins control complex membrane structures.

Lipid bilayer disruption by oligomeric alpha-synuclein depends on bilayer charge and accessibility of the hydrophobic core

Soluble oligomeric aggregates of alpha-synuclein have been implicated to play a central role in the pathogenesis of Parkinson's disease. Disruption and permeabilization of lipid bilayers by alpha-synuclein oligomers is postulated as a toxic mechanism, but the molecular details controlling the oligomer-membrane interaction are still unknown. Here we show that membrane disruption strongly depends on the accessibility of the hydrophobic membrane core and that charge interactions play an important but complex role. We systematically studied the influence of the physical membrane properties and solution conditions on lipid bilayer disruption by oligomers using a dye release assay. Varying the lipid headgroup composition revealed that membrane disruption only occurs for negatively charged bilayers. Furthermore, the electrostatic repulsion between the negatively charged alpha-synuclein and the negative surface charge of the bilayer inhibits vesicle disruption at low ionic strength. The disruption of negatively charged vesicles further depends on lipid packing parameters. Bilayer composition changes that result in an increased lipid headgroup spacing make vesicles more prone to disruption, suggesting that the accessibility of the bilayer hydrocarbon core modulates oligomer-membrane interaction. These data shed important new insights into the driving forces governing the highly debated process of oligomer-membrane interactions.

SNX9 - a prelude to vesicle release

The sorting nexin SNX9 has, in the past few years, been singled out as an important protein that participates in fundamental cellular activities. SNX9 binds strongly to dynamin and is partly responsible for the recruitment of this GTPase to sites of endocytosis. SNX9 also has a high capacity for modulation of the membrane and might therefore participate in the formation of the narrow neck of endocytic vesicles before scission occurs. Once assembled on the membrane, SNX9 stimulates the GTPase activity of dynamin to facilitate the scission reaction. It has also become clear that SNX9 has the ability to activate the actin regulator N-WASP in a membrane-dependent manner to coordinate actin polymerization with vesicle release. In this Commentary, we summarize several aspects of SNX9 structure and function in the context of membrane remodeling, discuss its interplay with various interaction partners and present a model of how SNX9 might work in endocytosis.

Spatiotemporal dynamics of the ER-derived peroxisomal endomembrane system

Recent studies have provided evidence that peroxisomes constitute a multicompartmental endomembrane system. The system begins to form with the targeting of certain peroxisomal membrane proteins to the ER and their exit from the ER via preperoxisomal carriers. These carriers undergo a multistep maturation into metabolically active peroxisomes containing the entire complement of peroxisomal membrane and matrix proteins. At each step, the import of a subset of proteins and the uptake of certain membrane lipids result in the formation of a distinct, more mature compartment of the peroxisomal endomembrane system. Individual peroxisomal compartments proliferate by undergoing one or several rounds of division. Herein, we discuss various strategies that evolutionarily diverse organisms use to coordinate compartment formation, maturation, and division in the peroxisomal endomembrane system. We also critically evaluate the molecular and cellular mechanisms governing these processes, outline the most important unanswered questions, and suggest directions for future research.

Spatial localization of PtdInsP2 in phase-separated giant unilamellar vesicles with a fluorescent PLC-delta 1 PH domain

This chapter describes a method for the preparation of giant unilamellar vesicles containing phosphatidylinositol 4,5-bisphosphate that are larger than 20 microm in size. The phospholipids composition of the vesicular membrane is such that fluid lamellar and liquid-ordered or gel phases are formed and separate within the confines of one vesicle. It outlines the preparation of a protein fluorescent label, pleckstrin homology domain from phospholipase C-delta 1, that binds specifically to phosphatidylinositol 4,5-bisphosphate. Using fluorescence microscopy, the presence and spatial position of this phosphorylated phosphatidylinositol lipid on the lipid membrane have been located with the pleckstrin homology domain. We show that phosphatidylinositol 4,5-bisphosphate and the phospholipase C-delta 1 pleckstrin homology domain are located to the fluid phase of the vesicle membrane. This approach can therefore show how membrane physical properties can affect enzyme binding to phosphatidylinositol 4,5-bisphosphate and thus further the understanding of important membrane processes such as endocytosis.

Molecular mechanisms of membrane deformation by I-BAR domain proteins

BACKGROUND

Generation of membrane curvature is critical for the formation of plasma membrane protrusions and invaginations and for shaping intracellular organelles. Among the central regulators of membrane dynamics are the BAR superfamily domains, which deform membranes into tubular structures. In contrast to the relatively well characterized BAR and F-BAR domains that promote the formation of plasma membrane invaginations, I-BAR domains induce plasma membrane protrusions through a poorly understood mechanism.

RESULTS

We show that I-BAR domains induce strong PI(4,5)P(2) clustering upon membrane binding, bend the membrane through electrostatic interactions, and remain dynamically associated with the inner leaflet of membrane tubules. Thus, I-BAR domains induce the formation of dynamic membrane protrusions to the opposite direction than do BAR and F-BAR domains. Strikingly, comparison of different I-BAR domains revealed that they deform PI(4,5)P(2)-rich membranes through distinct mechanisms. IRSp53 and IRTKS I-BARs bind membranes mainly through electrostatic interactions, whereas MIM and ABBA I-BARs additionally insert an amphipathic helix into the membrane bilayer, resulting in larger tubule diameter in vitro and more efficient filopodia formation in vivo. Furthermore, FRAP analysis revealed that whereas the mammalian I-BAR domains display dynamic association with filopodia, the C. elegans I-BAR domain forms relatively stable structures inside the plasma membrane protrusions.

CONCLUSIONS

These data define I-BAR domain as a functional member of the BAR domain superfamily and unravel the mechanisms by which I-BAR domains deform membranes to induce filopodia in cells. Furthermore, our work reveals unexpected divergence in the mechanisms by which evolutionarily distinct groups of I-BAR domains interact with PI(4,5)P(2)-rich membranes.

Lipid-engineered Escherichia coli membranes reveal critical lipid headgroup size for protein function

Escherichia coli membranes have a substantial bilayer curvature stress due to a large fraction of the nonbilayer-prone lipid phosphatidylethanolamine, and a mutant (AD93) lacking this lipid is severely crippled in several membrane-associated processes. Introduction of four lipid glycosyltransferases from Acholeplasma laidlawii and Arabidopsis thaliana, synthesizing large amounts of two nonbilayer-prone, and two bilayer-forming gluco- and galacto-lipids, (i) restored the curvature stress with the two nonbilayer lipids, and (ii) diluted the high negative lipid surface charge in all AD93 bilayers. Surprisingly, the bilayer-forming diglucosyl-diacylglycerol was almost as good in improving AD93 membrane processes as the two nonbilayer-prone glucosyl-diacylglycerol and galactosyl-diacylglycerol lipids, strongly suggesting that lipid surface charge dilution by these neutral lipids is very important for E. coli. Increased acyl chain length and unsaturation, plus cardiolipin (nonbilayer-prone) content, were probably also beneficial in the modified strains. However, despite a correct transmembrane topology for the transporter LacY in the diglucosyl-diacylglycerol clone, active transport failed in the absence of a nonbilayer-prone glycolipid. The corresponding digalactosyl-diacylglycerol bilayer lipid did not restore AD93 membrane processes, despite analogous acyl chain and cardiolipin contents. Chain ordering, probed by bis-pyrene lipids, was substantially lower in the digalactosyl-diacylglycerol strain lipids due to its extended headgroup. Hence, a low surface charge density of anionic lipids is important in E. coli membranes, but is inefficient if the headgroup of the diluting lipid is too large. This strongly indicates that a certain magnitude of the curvature stress is crucial for the bilayer in vivo.

Structure and plasticity of Endophilin and Sorting Nexin 9

Endophilin and Sorting Nexin 9 (Snx9) play key roles in endocytosis by membrane curvature sensing and remodeling via their Bin/Amphiphysin/Rvs (BAR) domains. BAR and the related F-BAR domains form dimeric, crescent-shaped units that occur N- or C-terminally to other lipid-binding, adaptor, or catalytic modules. In crystal structures, the PX-BAR unit of Snx9 (Snx9(PX-BAR)) adopts an overall compact, moderately curved conformation. SAXS-based solution studies revealed an alternative, more curved state of Snx9(PX-BAR) in which the PX domains are flexibly connected to the BAR domains, providing a model for how Snx9 exhibits lipid-dependent curvature preferences. In contrast, Endophilin appears to be rigid in solution, and the SH3 domains are located at the distal tips of a BAR domain dimer with fixed curvature. We also observed tip-to-tip interactions between the BAR domains in a trigonal crystal form of Snx9(PX-BAR) reminiscent of functionally important interactions described for F-BAR domains.

The N-terminal amphipathic alpha-helix of viperin mediates localization to the cytosolic face of the endoplasmic reticulum and inhibits protein secretion

Viperin is an evolutionarily conserved interferon-inducible protein that localizes to the endoplasmic reticulum (ER) and inhibits a number of DNA and RNA viruses. In this study, we report that viperin specifically localizes to the cytoplasmic face of the ER and that an amphipathic alpha-helix at its N terminus is necessary for the ER localization of viperin and sufficient to promote ER localization of a reporter protein, dsRed. Overexpression of intact viperin but not the amphipathic alpha-helix fused to dsRed induced crystalloid ER. Consistent with other proteins that induce crystalloid ER, viperin self-associates, and it does so independently of the amphipathic alpha-helix. Viperin expression also affected the transport of soluble but not membrane-associated proteins. Expression of intact viperin or an N-terminal alpha-helix-dsRed fusion protein significantly reduced secretion of soluble alkaline phosphatase and reduced its rate of ER-to-Golgi trafficking. Similarly, viperin expression inhibited bulk protein secretion and secretion of endogenous alpha(1)-antitrypsin and serum albumin from HepG2 cells. Converting hydrophobic residues in the N-terminal alpha-helix to acidic residues partially or completely restored normal transport of soluble alkaline phosphatase, suggesting that the extended amphipathic nature of the N-terminal alpha-helical domain is essential for inhibiting protein secretion.

Diffusion of macromolecules on lipid vesicles

Diffusion of macromolecules on a surface of lipid vesicles of different reduced volume and geometry is investigated. It is assumed that the macromolecules deform the surface of the vesicles by inducing the spontaneous curvature which is proportional to their concentration. We study how nonuniform distribution of macromolecules is reflected in the shape of the vesicles and how the shape of the vesicles influences the diffusion process.

Fusion-pore expansion during syncytium formation is restricted by an actin network

Cell-cell fusion in animal development and in pathophysiology involves expansion of nascent fusion pores formed by protein fusogens to yield an open lumen of cell-size diameter. Here we explored the enlargement of micron-scale pores in syncytium formation, which was initiated by a well-characterized fusogen baculovirus gp64. Radial expansion of a single or, more often, of multiple fusion pores proceeds without loss of membrane material in the tight contact zone. Pore growth requires cell metabolism and is accompanied by a local disassembly of the actin cortex under the pores. Effects of actin-modifying agents indicate that the actin cortex slows down pore expansion. We propose that the growth of the strongly bent fusion-pore rim is restricted by a dynamic resistance of the actin network and driven by membrane-bending proteins that are involved in the generation of highly curved intracellular membrane compartments.

Spatially selective sampling of single cells using optically trapped fusogenic emulsion droplets: a new single-cell proteomic tool

We present a platform for the spatially selective sampling of the plasma membrane of single cells. Optically trapped lipid-coated oil droplets (smart droplet microtools, SDMs), typically 0.5-5 microm in size, composed of a hexadecane hydrocarbon core and fusogenic lipid outer coating (mixture of 1,2-dioleoyl-phosphatidylethanolamine and 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine) were brought into controlled contact with target colon cancer cells leading to the formation of connecting membrane tethers. Material transfer from the cell to the SDM across the membrane tether was monitored by tracking membrane-localized enhanced green fluorescent protein.

EFC/F-BAR proteins and the N-WASP-WIP complex induce membrane curvature-dependent actin polymerization

Extended Fer-CIP4 homology (EFC)/FCH-BAR (F-BAR) domains generate and bind to tubular membrane structures of defined diameters that are involved in the formation and fission of endocytotic vesicles. Formin-binding protein 17 (FBP17) and Toca-1 contain EFC/F-BAR domains and bind to neural Wiskott-Aldrich syndrome protein (N-WASP), which links phosphatidylinositol (4,5)-bisphosphate (PIP(2)) and the Rho family GTPase Cdc42 to the Arp2/3 complex. The N-WASP-WASP-interacting protein (WIP) complex, a predominant form of N-WASP in cells, is known to be activated by Toca-1 and Cdc42. Here, we show that N-WASP-WIP complex-mediated actin polymerization is activated by phosphatidylserine-containing membranes depending on membrane curvature in the presence of Toca-1 or FBP17 and in the absence of Cdc42 and PIP(2). Cdc42 further promoted the activation of actin polymerization by N-WASP-WIP. Toca-1 or FBP17 recruited N-WASP-WIP to the membrane. Conserved acidic residues near the SH3 domain of Toca-1 and FBP17 positioned the N-WASP-WIP to be spatially close to the membrane for activation of actin polymerization. Therefore, curvature-dependent actin polymerization is stimulated by spatially appropriate interactions of EFC/F-BAR proteins and the N-WASP-WIP complex with the membrane.

Topology and membrane anchoring of the coronavirus replication complex: not all hydrophobic domains of nsp3 and nsp6 are membrane spanning

Coronaviruses express two very large replicase polyproteins, the 16 autoproteolytic cleavage products of which collectively form the membrane-anchored replication complexes. How these structures are assembled is still largely unknown, but it is likely that the membrane-spanning members of these nonstructural proteins (nsps) are responsible for the induction of the double-membrane vesicles and for anchoring the replication complexes to these membranes. For 3 of the 16 coronavirus nsps-nsp3, nsp4, and nsp6-multiple transmembrane domains are predicted. Previously we showed that, consistent with predictions, nsp4 occurs in membranes with both of its termini exposed in the cytoplasm (M. Oostra et al., J. Virol. 81:12323-12336, 2007). Strikingly, however, for both nsp3 and nsp6, predictions based on a multiple alignment of 27 coronavirus genome sequences indicate an uneven number of transmembrane domains. As a consequence, the proteinase domains present in nsp3 and nsp5 would be separated from their target sequences by the lipid bilayer. To look into this incongruity, we studied the membrane disposition of nsp3 and nsp6 of the severe acute respiratory syndrome coronavirus and murine hepatitis virus by analyzing tagged forms of the proteins expressed in cultured cells. Contrary to the predictions, in both viruses, both proteins had their amino terminus, as well as their carboxy terminus, exposed in the cytoplasm. We established that two of the three hydrophobic domains in nsp3 and six of the seven in nsp6 are membrane spanning. Subsequently, we verified that in nsp4, all four hydrophobic domains span the lipid bilayer. The occurrence of conserved non-membrane-spanning hydrophobic domains in nsp3 and nsp6 suggests an important function for these domains in coronavirus replication.

A role for phosphatidic acid in COPI vesicle fission yields insights into Golgi maintenance

Proteins essential for vesicle formation by the Coat Protein I (COPI) complex are being identified, but less is known about the role of specific lipids. Brefeldin-A ADP-ribosylated substrate (BARS) functions in the fission step of COPI vesicle formation. Here, we show that BARS induces membrane curvature in cooperation with phosphatidic acid. This finding has allowed us to further delineate COPI vesicle fission into two sub-stages: 1) an earlier stage of bud-neck constriction, in which BARS and other COPI components are required, and 2) a later stage of bud-neck scission, in which phosphatidic acid generated by phospholipase D2 (PLD2) is also required. Moreover, in contrast to the disruption of the Golgi seen on perturbing the core COPI components (such as coatomer), inhibition of PLD2 causes milder disruptions, suggesting that such COPI components have additional roles in maintaining Golgi structure other than through COPI vesicle formation.

Snake phospholipase A2 neurotoxins enter neurons, bind specifically to mitochondria, and open their transition pores

Snake presynaptic neurotoxins with phospholipase A(2) activity are potent inducers of paralysis through inhibition of the neuromuscular junction. These neurotoxins were recently shown to induce exocytosis of synaptic vesicles following the production of lysophospholipids and fatty acids and a sustained influx of Ca(2+) from the medium. Here, we show that these toxins are able to penetrate spinal cord motor neurons and cerebellar granule neurons and selectively bind to mitochondria. As a result of this interaction, mitochondria depolarize and undergo a profound shape change from elongated and spaghetti-like to round and swollen. We show that snake presynaptic phospholipase A(2) neurotoxins facilitate opening of the mitochondrial permeability transition pore, an inner membrane high-conductance channel. The relative potency of the snake neurotoxins was similar for the permeability transition pore opening and for the phospholipid hydrolysis activities, suggesting a causal relationship, which is also supported by the effect of phospholipid hydrolysis products, lysophospholipids and fatty acids, on mitochondrial pore opening. These findings contribute to define the cellular events that lead to intoxication of nerve terminals by these snake neurotoxins and suggest that mitochondrial impairment is an important determinant of their toxicity.

The Gaussian curvature elastic energy of intermediates in membrane fusion

The Gaussian curvature elastic energy contribution to the energy of membrane fusion intermediates has usually been neglected because the Gaussian curvature elastic modulus, kappa, was unknown. It is now possible to measure kappa for phospholipids that form bicontinuous inverted cubic (Q(II)) phases. Here, it is shown that one can estimate kappa for lipids that do not form Q(II) phases by studying the phase behavior of lipid mixtures. The method is used to estimate kappa for several lipid compositions in excess water. The values of kappa are used to compute the curvature elastic energies of stalks and catenoidal fusion pores according to recent models. The Gaussian curvature elastic contribution is positive and similar in magnitude to the bending energy contribution: it increases the total curvature energy of all the fusion intermediates by 100 units of k(B)T or more. It is important to note that this contribution makes the predicted intermediate energies compatible with observed lipid phase behavior in excess water. An order-of-magnitude fusion rate equation is used to estimate whether the predicted stalk energies are consistent with the observed rates of stalk-mediated processes in pure lipid systems. The current theory predicts a stalk energy that is slightly too large, by approximately 30 k(B)T, to rationalize the observed rates of stalk-mediated processes in phosphatidylethanolamine or N-monomethylated dioleoylphosphatidylethanolamine systems. Despite this discrepancy, the results show that models of fusion intermediate energy are accurate enough to make semiquantitative predictions about how proteins mediate biomembrane fusion. The same rate model shows that for proteins to drive biomembrane fusion at observed rates, they have to perform mediating functions corresponding to a reduction in the energy of a purely lipidic stalk by several tens of k(B)T. By binding particular peptide sequences to the monolayer surface, proteins could lower fusion intermediate energies by altering the elastic constants of the patches of lipid monolayer that form the stalk. Here, it is shown that if peptide binding changes kappa or some other combinations of local elastic constants by only tens of percents, the stalk energy and the energy of catenoidal fusion pores would decrease by tens of k(B)T relative to the pure lipid value. This is comparable to the required mediating effect. The curvature energies of stalks and catenoidal fusion pores have almost the same dependence on monolayer elastic constants as the curvature energies of the rhombohedral and Q(II) phases; respectively. The effects of isolated fusion-relevant peptides on the energies of these intermediates can be determined by studying the effects of the peptides on the stability of rhombohedral and Q(II) phases.

Retroviruses human immunodeficiency virus and murine leukemia virus are enriched in phosphoinositides

Retroviruses acquire a lipid envelope during budding from the membrane of their hosts. Therefore, the composition of this envelope can provide important information about the budding process and its location. Here, we present mass spectrometry analysis of the lipid content of human immunodeficiency virus type 1 (HIV-1) and murine leukemia virus (MLV). The results of this comprehensive survey found that the overall lipid content of these viruses mostly matched that of the plasma membrane, which was considerably different from the total lipid content of the cells. However, several lipids are enriched in comparison to the composition of the plasma membrane: (i) cholesterol, ceramide, and GM3; and (ii) phosphoinositides, phosphorylated derivatives of phosphatidylinositol. Interestingly, microvesicles, which are similar in size to viruses and are also released from the cell periphery, lack phosphoinositides, suggesting a different budding mechanism/location for these particles than for retroviruses. One phosphoinositide, phosphatidylinositol 4,5-bisphosphate [PI(4,5)P(2)], has been implicated in membrane binding by HIV Gag. Consistent with this observation, we found that PI(4,5)P(2) was enriched in HIV-1 and that depleting this molecule in cells reduced HIV-1 budding. Analysis of mutant virions mapped the enrichment of PI(4,5)P(2) to the matrix domain of HIV Gag. Overall, these results suggest that HIV-1 and other retroviruses bud from cholesterol-rich regions of the plasma membrane and exploit matrix/PI(4,5)P(2) interactions for particle release from cells.

Intrinsic curvature properties of photosynthetic proteins in chromatophores

In purple bacteria, photosynthesis is carried out on large indentations of the bacterial plasma membrane termed chromatophores. Acting as primitive organelles, chromatophores are densely packed with the membrane proteins necessary for photosynthesis, including light harvesting complexes LH1 and LH2, reaction center (RC), and cytochrome bc(1). The shape of chromatophores is primarily dependent on species, and is typically spherical or flat. How these shapes arise from the protein-protein and protein-membrane interactions is still unknown. Now, using molecular dynamics simulations, we have observed the dynamic curvature of membranes caused by proteins in the chromatophore. A membrane-embedded array of LH2s was found to relax to a curved state, both for LH2 from Rps. acidophila and a homology-modeled LH2 from Rb. sphaeroides. A modeled LH1-RC-PufX dimer was found to develop a bend at the dimerizing interface resulting in a curved shape as well. In contrast, the bc(1) complex, which has not been imaged yet in native chromatophores, did not induce a preferred membrane curvature in simulation. Based on these results, a model for how the different photosynthetic proteins influence chromatophore shape is presented.