Opening-up of liposomal membranes by talin

Characterization and regulation of 2D-3D convertible lipid membrane transformation

Micro-nanomaterials that can adopt different structures are powerful tools in the fields of biological and medical sciences. We previously developed a lipid membrane that can convert between 2D nanosheet and 3D vesicle forms using cationic copolymer polyallylamine-graft-polyethylene glycol and the anionic peptide E5. The properties of the membrane during conversion have been characterized only by confocal laser scan microscopy. Furthermore, due to the 2D symmetry of the lipid nanosheet, the random folding of the lipid bilayer into either the original or the reverse orientation occurs during sheet-to-vesicle conversion, compromising the structural consistency of the membrane. In this study, flow cytometry was applied to track the conversion of more than 5000 lipid membranes from 3D vesicles to 2D nanosheets and back to 3D vesicles, difficult with microscopies. The lipid nanosheets exhibited more side scattering intensity than 3D vesicles, presumably due to free fluctuation and spin of the sheets in the suspension. Furthermore, by immobilizing bovine serum albumin as one of the representative proteins on the outer leaflet of giant unilamellar vesicles at a relatively low coverage, complete restoration of lipid membranes to the original 3D orientation was obtained after sheet-to-vesicle conversion. This convertible membrane system should be applicable in a wide range of fields. Our findings also provide experimental evidence for future theoretical studies on membrane behavior.

2D-3D-Convertible, pH-Responsive Lipid Nanosheets

2D nanosheets self-assembled with amphiphilic molecules are promising tools for biomedical applications; yet, there are challenges to form and stabilize these nanosheets under complex physiological conditions. Here, the development of lipid nanosheets with high structural stability that can be reversibly converted to cell-sized vesicles by changes in pH within the physiological range robustly, are described. The system is controlled by the membrane disruptive peptide E5 and a cationic copolymer anchored on lipid membranes. It is envisioned that nanosheets formed using the dual anchoring peptide/cationic copolymer system can be employed in dynamic lipidic nanodevices, such as the vesosomes described here, drug delivery systems, and artificial cells.

The Art of Viral Membrane Fusion and Penetration

As obligate pathogens, viruses have developed diverse mechanisms to deliver their genome across host cell membranes to sites of virus replication. While enveloped viruses utilize viral fusion proteins to accomplish fusion of their envelope with the cellular membrane, non-enveloped viruses rely on machinery that causes local membrane ruptures and creates an opening through which the capsid or viral genome is released. Both membrane fusion and membrane penetration take place at the plasma membrane or in intracellular compartments, often involving the engagement of the cellular machinery and antagonism of host restriction factors. Enveloped and non-enveloped viruses have evolved intricate mechanisms to enable virus uncoating and modulation of membrane fusion in a spatiotemporally controlled manner. This chapter summarizes and discusses the current state of understanding of the mechanisms of viral membrane fusion and penetration. The focus is on the role of lipids, viral scaffold uncoating, viral membrane fusion inhibitors, and host restriction factors as physicochemical modulators. In addition, recent advances in visualizing and detecting viral membrane fusion and penetration using cryo-electron microscopy methods are presented.

Autonomous Vesicle/Sheet Transformation of Cell-Sized Lipid Bilayers by Hetero-Grafted Copolymers

Lipid bilayer transformations are involved in biological phenomena including cell division, autophagy, virus infection, and vesicle transport. Artificial materials to manipulate membrane dynamics play a vital role in cellular engineering and drug delivery technology that accesses the membranes of cells or liposomes. Transformation from 3D lipid vesicles to 2D nanosheets is thermodynamically prohibited because the apolar/polar interfaces between the hydrophobic bilayer edges and water are energetically unfavorable. We recently reported that cell-sized lipid vesicles (or giant vesicles) can be thoroughly transformed to 2D nanosheets by the addition of the amphiphilic E5 peptide and a cationic graft copolymer. Here, to understand the mechanisms underlying the lipid nanosheet formation, we systematically investigated the structural effects of the cationic copolymers on nanosheet formation. We found that lipid nanosheet formation is controlled in an all-or-nothing manner when the graft content of the copolymer is increased from 5.7 mol % to 7.7 mol %. This finding prompted us to obtain autonomous 2D/3D transformation system. A newly designed hetero-grafted cationic copolymers with thermoresponsive poly(N-isopropylacrylamide) grafts enables spontaneous 3D vesicle/2D nanosheet transformation in response to temperature. These findings would enable us to obtain smart nanointerfaces that trigger cell-sized lipid membrane dynamics in response to diverse stimuli and to create 2D-3D convertible lipid-based biomaterials.

Axisymmetric membranes with edges under external force: buckling, minimal surfaces, and tethers

We use theory and numerical computation to determine the shape of an axisymmetric fluid membrane with a resistance to bending and constant area. The membrane connects two rings in the classic geometry that produces a catenoidal shape in a soap film. In our problem, we find infinitely many branches of solutions for the shape and external force as functions of the separation of the rings, analogous to the infinite family of eigenmodes for the Euler buckling of a slender rod. Special attention is paid to the catenoid, which emerges as the shape of maximal allowable separation when the area is less than a critical area equal to the planar area enclosed by the two rings. A perturbation theory argument directly relates the tension of catenoidal membranes to the stability of catenoidal soap films in this regime. When the membrane area is larger than the critical area, we find additional cylindrical tether solutions to the shape equations at large ring separation, and that arbitrarily large ring separations are possible. These results apply for the case of vanishing Gaussian curvature modulus; when the Gaussian curvature modulus is nonzero and the area is below the critical area, the force and the membrane tension diverge as the ring separation approaches its maximum value. We also examine the stability of our shapes and analytically show that catenoidal membranes have markedly different stability properties than their soap film counterparts.

To Close or to Collapse: The Role of Charges on Membrane Stability upon Pore Formation

Resealing of membrane pores is crucial for cell survival. Membrane surface charge and medium composition are studied as defining regulators of membrane stability. Pores are generated by electric field or detergents. Giant vesicles composed of zwitterionic and negatively charged lipids mixed at varying ratios are subjected to a strong electric pulse. Interestingly, charged vesicles appear prone to catastrophic collapse transforming them into tubular structures. The spectrum of destabilization responses includes the generation of long-living submicroscopic pores and partial vesicle bursting. The origin of these phenomena is related to the membrane edge tension, which governs pore closure. This edge tension significantly decreases as a function of the fraction of charged lipids. Destabilization of charged vesicles upon pore formation is universal-it is also observed with other poration stimuli. Disruption propensity is enhanced for membranes made of lipids with higher degree of unsaturation. It can be reversed by screening membrane charge in the presence of calcium ions. The observed findings in light of theories of stability and curvature generation are interpreted and mechanisms acting in cells to prevent total membrane collapse upon poration are discussed. Enhanced membrane stability is crucial for the success of electroporation-based technologies for cancer treatment and gene transfer.

Shapes of fluid membranes with chiral edges

We carry out Monte Carlo simulations of a colloidal fluid membrane with a free edge and composed of chiral rodlike viruses. The membrane is modeled by a triangular mesh of beads connected by bonds in which the bonds and beads are free to move at each Monte Carlo step. Since the constituent viruses are experimentally observed to twist only near the membrane edge, we use an effective energy that favors a particular sign of the geodesic torsion of the edge. The effective energy also includes the membrane bending stiffness, edge bending stiffness, and edge tension. We find three classes of membrane shapes resulting from the competition of the various terms in the free energy: branched shapes, chiral disks, and vesicles. Increasing the edge bending stiffness smooths the membrane edge, leading to correlations among the membrane normals at different points along the edge. The normalized power spectrum for edge displacements shows a peak with increasing preferred geodesic torsion. We also consider membrane shapes under an external force by fixing the distance between two ends of the membrane and finding the shape for increasing values of the distance between the two ends. As the distance increases, the membrane twists into a ribbon, with the force eventually reaching a plateau.

Cationic Copolymer-Chaperoned 2D-3D Reversible Conversion of Lipid Membranes

Nanosheets have thicknesses on the order of nanometers and planar dimensions in the micrometer range. Nanomaterials that are capable of converting reversibly between 2D nanosheets and 3D structures in response to specific triggers can enable construction of nanodevices. Supra-molecular lipid nanosheets and their triggered conversions to 3D structures including vesicles and cups are reported. They are produced from lipid vesicles upon addition of amphiphilic peptides and cationic copolymers that act as peptide chaperones. By regulation of the chaperoning activity of the copolymer, 2D to 3D conversions are reversibly triggered, allowing tuning of lipid bilayer structures and functionalities.

Boundary behaviour of open vesicles in axisymmetric case

A continuous transformation from a closed vesicle to an open vesicle requires that the area of open hole enlarges from zero. Since the shape equation and boundary conditions of lipid open vesicles with free edges have been obtained, we want to know whether this process can be achieved with valid parameters. By studying the boundary conditions in the axisymmetric case, the analytic expression of the boundary edges is obtained generally. It reveals that the radius and line tension of boundary edges are confined strongly by bending moduli. In some cases, there is the minimal nonzero boundary radius and the line tension needs to surmount the maxim following the increase of boundary radius. Without the spontaneous curvature, the line tension will trend to infinite when the boundary radius shrinks to zero. The continuous opening up process requires that the spontaneous curvature is nonzero and the ratio between the bending moduli of Gauss curvature and mean curvature satisfies [Formula: see text], which is smaller than the value [Formula: see text] from experiments and simulations. This result indicates that the opening up process is discontinuous.

Hydrophobic Polyampholytes and Nonfreezing Cold Temperature Stimulate Internalization of Au Nanoparticles to Zwitterionic Liposomes

Nanomedicine relies on the effective internalization of nanoparticles combined with polymeric nanocarriers into living cells. Thus, exploration of internalization is essential for improving the efficacy of nanoparticle-based strategies in clinical practice. Here, we investigated the physicochemical internalization of gold nanoparticles (AuNPs) conjugated with hydrophobic polyampholytes into cell-sized liposomes at a low but nonfrozen temperature. The hydrophobic polyampholytes localized in the disordered phase of the membrane, and internalization of AuNPs was enhanced in the presence of hydrophobic polyampholytes together with incubation at -3 °C as compared to 25 °C. These results contribute toward a mechanistic understanding for developing a model nanomaterials-driven delivery system based on hydrophobic polyampholytes and low temperature.

Toward a unified picture of the exocytotic fusion pore

Neurotransmitter and hormone release involve calcium-triggered fusion of a cargo-loaded vesicle with the plasma membrane. The initial connection between the fusing membranes, called the fusion pore, can evolve in various ways, including rapid dilation to allow full cargo release, slow expansion, repeated opening-closing and resealing. Pore dynamics determine the kinetics of cargo release and the mode of vesicle recycling, but how these processes are controlled is poorly understood. Previous reconstitutions could not monitor single pores, limiting mechanistic insight they could provide. Recently developed nanodisc-based fusion assays allow reconstitution and monitoring of single pores with unprecedented detail and hold great promise for future discoveries. They recapitulate various aspects of exocytotic fusion pores, but comparison is difficult because different approaches suggested very different exocytotic fusion pore properties, even for the same cell type. In this Review, I discuss how most of the data can be reconciled, by recognizing how different methods probe different aspects of the same fusion process. The resulting picture is that fusion pores have broadly distributed properties arising from stochastic processes which can be modulated by physical constraints imposed by proteins, lipids and membranes.

Solubilization kinetics determines the pulsatory dynamics of lipid vesicles exposed to surfactant

We establish a biophysical model for the dynamics of lipid vesicles exposed to surfactants. The solubilization of the lipid membrane due to the insertion of surfactant molecules induces a reduction of membrane surface area at almost constant vesicle volume. This results in a rate-dependent increase of membrane tension and leads to the opening of a micron-sized pore. We show that solubilization kinetics due to surfactants can determine the regime of pore dynamics: either the pores open and reseal within a second (short-lived pore), or the pore stays open up to a few minutes (long-lived pore). First, we validate our model with previously published experimental measurements of pore dynamics. Then, we investigate how the solubilization kinetics and membrane properties affect the dynamics of the pore and construct a phase diagram for short and long-lived pores. Finally, we examine the dynamics of sequential pore openings and show that cyclic short-lived pores occur with a period inversely proportional to the solubilization rate. By deriving a theoretical expression for the cycle period, we provide an analytical tool to estimate the solubilization rate of lipid vesicles by surfactants. Our findings shed light on some fundamental biophysical mechanisms that allow simple cell-like structures to sustain their integrity against environmental stresses, and have the potential to aid the design of vesicle-based drug delivery systems. This article is part of a Special Issue entitled: Emergence of Complex Behavior in Biomembranes edited by Marjorie Longo.

Formation and size distribution of self-assembled vesicles

When detergents and phospholipid membranes are dispersed in aqueous solutions, they tend to self-assemble into vesicles of various shapes and sizes by virtue of their hydrophobic and hydrophilic segments. A clearer understanding of such vesiculation processes holds promise for better elucidation of human physiology and disease, and paves the way to improved diagnostics, drug development, and drug delivery. Here we present a detailed analysis of the energetics and thermodynamics of vesiculation by recourse to nonlinear elasticity, taking into account large deformation that may arise during the vesiculation process. The effects of membrane size, spontaneous curvature, and membrane stiffness on vesiculation and vesicle size distribution were investigated, and the critical size for vesicle formation was determined and found to compare favorably with available experimental evidence. Our analysis also showed that the critical membrane size for spontaneous vesiculation was correlated with membrane thickness, and further illustrated how the combined effects of membrane thickness and physical properties influenced the size, shape, and distribution of vesicles. These findings shed light on the formation of physiological extracellular vesicles, such as exosomes. The findings also suggest pathways for manipulating the size, shape, distribution, and physical properties of synthetic vesicles, with potential applications in vesicle physiology, the pathobiology of cancer and other diseases, diagnostics using in vivo liquid biopsy, and drug delivery methods.

Role of the Helix in Talin F3 Domain (F3 Helix) in Talin-Mediated Integrin Activation

Increases in ligand binding to cellular integrins (activation) play an important role in platelet and leukocyte function. Talin is necessary in vivo and sufficient in vitro for integrin αIIbβ3 activation. The precise mechanisms by which talin activates integrin are still being elucidated. In particular, talin undergoes conformational changes (around the F3 helix) and inserts the F3 helix into lipid bilayer; however, the connection between this lipid-inserting mechanism of talin and talin's capacity to activate integrin has never been explored before. In this work, we used rational mutagenesis, modeled cell systems, and structural modeling to study the potential role of membrane-induced talin conformational changes in talin-mediated integrin activation. Mutations of the residues critical for talin F3 helix to insert into membrane completely abolished talin-mediated integrin activation without affecting the binding of talin to integrins. Furthermore, mutations of the lipid-binding sequences in talin F3 helix significantly reduced the capacity of talin to activate integrin. Our results suggest that the F3 helix may contribute to talin-mediated integrin activation.

Free energy of the edge of an open lipid bilayer based on the interactions of its constituent molecules

Lipid-bilayers are the fundamental constituents of the walls of most living cells and lipid vesicles, giving them shape and compartment. The formation and growing of pores in a lipid bilayer have attracted considerable attention from an energetic point of view in recent years. Such pores permit targeted delivery of drugs and genes to the cell, and regulate the concentration of various molecules within the cell. The formation of such pores is caused by various reasons such as changes in cell environment, mechanical stress or thermal fluctuations. Understanding the energy and elastic behaviour of a lipid-bilayer edge is crucial for controlling the formation and growth of such pores. In the present work, the interactions in the molecular level are used to obtain the free energy of the edge of an open lipid bilayer. The resulted free-energy density includes terms associated with flexural and torsional energies of the edge, in addition to a line-tension contribution. The line tension, elastic moduli, and spontaneous normal and geodesic curvatures of the edge are obtained as functions of molecular distribution, molecular dimensions, cutoff distance, and the interaction strength. These parameters are further analyzed by implementing a soft-core interaction potential in the microphysical model. The dependence of the elastic free-energy of the edge to the size of the pore is reinvestigated through an illustrative example, and the results are found to be in agreement with the previous observations.

Investigating Hydrophilic Pores in Model Lipid Bilayers Using Molecular Simulations: Correlating Bilayer Properties with Pore-Formation Thermodynamics

Cell-penetrating and antimicrobial peptides show a remarkable ability to translocate across physiological membranes. Along with factors such as electric-potential-induced perturbations of membrane structure and surface tension effects, experiments invoke porelike membrane configurations during the solute transfer process into vesicles and cells. The initiation and formation of pores are associated with a nontrivial free-energy cost, thus necessitating a consideration of the factors associated with pore formation and the attendant free energies. Because of experimental and modeling challenges related to the long time scales of the translocation process, we use umbrella sampling molecular dynamics simulations with a lipid-density-based order parameter to investigate membrane-pore-formation free energy employing Martini coarse-grained models. We investigate structure and thermodynamic features of the pore in 18 lipids spanning a range of headgroups, charge states, acyl chain lengths, and saturation. We probe the dependence of pore-formation barriers on the area per lipid, lipid bilayer thickness, and membrane bending rigidities in three different lipid classes. The pore-formation free energy in pure bilayers and peptide translocating scenarios are significantly coupled with bilayer thickness. Thicker bilayers require more reversible work to create pores. The pore-formation free energy is higher in peptide-lipid systems than in peptide-free lipid systems due to penalties to maintain the solvation of charged hydrophilic solutes within the membrane environment.

Model systems for studying cell adhesion and biomimetic actin networks

Many cellular processes, such as migration, proliferation, wound healing and tumor progression are based on cell adhesion. Amongst different cell adhesion molecules, the integrin receptors play a very significant role. Over the past decades the function and signalling of various such integrins have been studied by incorporating the proteins into lipid membranes. These proteolipid structures lay the foundation for the development of artificial cells, which are able to adhere to substrates. To build biomimetic models for studying cell shape and spreading, actin networks can be incorporated into lipid vesicles, too. We here review the mechanisms of integrin-mediated cell adhesion and recent advances in the field of minimal cells towards synthetic adhesion. We focus on reconstituting integrins into lipid structures for mimicking cell adhesion and on the incorporation of actin networks and talin into model cells.

Recent theoretical advances in elasticity of membranes following Helfrich's spontaneous curvature model

Recent theoretical advances in elasticity of membranes following Helfrich's famous spontaneous curvature model are summarized in this review. The governing equations describing equilibrium configurations of lipid vesicles, lipid membranes with free edges, and chiral lipid membranes are presented. Several analytic solutions to these equations and their corresponding configurations are demonstrated.

The single GUV method for revealing the functions of antimicrobial, pore-forming toxin, and cell-penetrating peptides or proteins

The single GUV method provides detailed information on the elementary processes of peptide/protein-induced pore formation in lipid membranes and the entry of peptides into a GUV; specifically, the GUV method provides the rate constants of these processes.

Emergence and stability of intermediate open vesicles in disk-to-vesicle transitions

The transition between two basic structures, a disk and an enclosed vesicle, of a finite membrane is studied by examining the minimum energy path (MEP) connecting these two states. The MEP is constructed using the string method applied to continuum elastic membrane models. The results reveal that, besides the commonly observed disk and vesicle, open vesicles (bowl-shaped vesicles or vesicles with a pore) can become stable or metastable shapes. The emergence, stability, and probability distribution of these open vesicles are analyzed. It is demonstrated that open vesicles can be stabilized by higher-order elastic energies. The estimated probability distribution of the different structures is in good agreement with available experiments.

Multiple membrane interactions and versatile vesicle deformations elicited by melittin

Melittin induces various reactions in membranes and has been widely studied as a model for membrane-interacting peptide; however, the mechanism whereby melittin elicits its effects remains unclear. Here, we observed melittin-induced changes in individual giant liposomes using direct real-time imaging by dark-field optical microscopy, and the mechanisms involved were correlated with results obtained using circular dichroism, cosedimentation, fluorescence quenching of tryptophan residues, and electron microscopy. Depending on the concentration of negatively charged phospholipids in the membrane and the molecular ratio between lipid and melittin, melittin induced the “increasing membrane area”, “phased shrinkage”, or “solubilization” of liposomes. In phased shrinkage, liposomes formed small particles on their surface and rapidly decreased in size. Under conditions in which the increasing membrane area, phased shrinkage, or solubilization were mainly observed, the secondary structure of melittin was primarily estimated as an α-helix, β-like, or disordered structure, respectively. When the increasing membrane area or phased shrinkage occurred, almost all melittin was bound to the membranes and reached more hydrophobic regions of the membranes than when solubilization occurred. These results indicate that the various effects of melittin result from its ability to adopt various structures and membrane-binding states depending on the conditions.

Equilibrium shape equation and geometrically permissible condition for two-component lipid bilayer vesicles

Equilibrium shapes of vesicles composed of a mixture of partially miscible amphiphiles are investigated. To take into account the influences of the composition, a simple phenomenological coupling between the co mposition and the curvatures, including the mean curvature and the Gauss curvature of the membrane surface, is suggested. By minimizing the potential functional, the general shape equation is obtained and solved analytically for vesicles with simple shapes. Besides, the geometrical constraint equation and geometrically permissible condition for the two-component lipid vesicles are put forward. The influences of physical parameters on the geometrically permissible phase diagrams are predicted. The close relations between the predictions and existing experimental phenomena published recently are shown.

Morphological and topological transformation of membrane vesicles

Liposomes are micro-compartments made of lipid bilayer membranes withcharacteristics quite similar to those of biological membranes. To formartificial cell-like structures, we generated liposomes that containedsubunit proteins of cytoskeletons: tubulin or actin. Spherical liposomeswere transformed into bipolar or cell-like shapes by mechanical forcesgenerated by the polymerization of encapsulated subunits of microtubules.Disk- or dumbbell-shaped liposomes were developed by the polymerizationof encapsulated actin. Dynamic processes of morphological transformationsof liposomes were visualized by high intensity dark-field lightmicroscopy.Topological changes, such as fusion and division of membrane vesicles,play an essential role in cellular activities. To investigate themechanism of these processes, we visualized in real time the liposomesundergoing topological transformation. A variety of novel topologicaltransformations were found, including the opening-up of liposomes and thedirect expulsion of inner vesicles.

Kinetic process of formation and reconstruction of small unilamellar vesicles consisting of long- and short-chain lipids

Whereas the phase separation of normal phospholipids induces formation of microdomains on the surface of spherical vesicles, the separation of a long- and short-chain lipids can induce perforation of small unilamellar vesicles (SUVs) and transformation into bilayered micelles (bicelles) because the edges of the bilayers are stabilized by the short-chain lipid microdomain. In this study, the effect of the phase separation of lipids on the transformation behavior of SUVs consisting of a mixture of long- and short-chain lipids was investigated using small-angle neutron scattering. At the temperature jump from below to above the chain melting temperature of the long-chain lipid, T(c), bicelles fused together and transformed into SUVs when their size reached a critical radius. In contrast, a sequential transformation of small SUVs to small bicelles, small bicelles to large bicelles, and large bicelles to large SUVs occurred when the temperature jumped from a value far above T(c) to one slightly above T(c). To the best of my knowledge, this is the first report of such reconstruction of vesicles. By considering the bending energy of the membrane, the line tension of the rim, and the perforation due to the phase separation, the mechanism of the transformation processes was clarified.

Cell-Sized Liposomes and Droplets: Real-World Modeling of Living Cells

Recent developments in studies concerning cell-sized vesicles, such as liposomes with a lipid bilayer and water-in-oil droplets covered by a lipid monolayer, aim to realize the real-world modeling of living cells. Compartmentalization with a membrane boundary is essential for the organization of living systems. Due to the relatively large surface/volume ratio in microconfinement, the membrane interface influences phenomena related to biological functions. In this article, we mainly focus on the following subjects: (i) conformational transition of biopolymers in a confined space; (ii) molecular association on the membrane surface; and (iii) remote control of cell-sized membrane morphology.

Mechanism of structural transformations induced by antimicrobial peptides in lipid membranes

It has long been suggested that pore formation is responsible for the increase in membrane permeability by antimicrobial peptides (AMPs). To better understand the mechanism of AMP activity, the disruption of model membrane by protegrin-1 (PG-1), a cationic antimicrobial peptide, was studied using atomic force microscopy. We present here the direct visualization of the full range of structural transformations in supported lipid bilayer patches induced by PG-1 on zwitterionic 1,2-dimyristoyl-snglycero-phospho-choline (DMPC) membranes. When PG-1 is added to DMPC, the peptide first induces edge instability at low concentrations, then pore-like surface defects at intermediate concentrations, and finally wormlike structures with a specific length scale at high concentrations. The formation of these structures can be understood using a mesophase framework of a binary mixture of lipids and peptides, where PG-1 acts as a line-active agent. Atomistic molecular dynamics simulations on lipid bilayer ribbons with PG-1 molecules placed at the edge or interior positions are carried out to calculate the effect of PG-1 in reducing line tension. Further investigation of the placement of PG-1 and its association with defects in the bilayer is carried out using unbiased assembly of a PG-1 containing bilayer from a random mixture of PG-1, DMPC, and water. A generalized model of AMP induced structural transformations is also presented in this work. This article is part of a Special Issue entitled: Membrane protein structure and function.

A FERM domain in a class XIV myosin interacts with actin and tubulin and localizes to the cytoskeleton, phagosomes, and nucleus in Tetrahymena thermophila

Previous studies have shown that Myo1(myosin class XIV) localizes to the cytoskeleton and is involved in amitosis of the macronucleus and trafficking of phagosomes. Myo1 contains a FERM domain that could be a site for interaction between Myo1 and the cytoskeleton. Here, we explore the function of FERM by investigating its cytoskeleton binding partners and involvement in localization of Myo1. Alignment of Myo1 FERM with a talin actin-binding sequence, a MAP-2 tubulin-binding sequence, the radixin FERM dimerization motif, and the SV40 nuclear localization sequence (NLS) revealed putative actin- and tubulin-binding sequences, a putative FERM dimerization motif, and NLS-like sequences in both the N-terminal and C-terminal regions of Myo1 FERM. Alignment of Myo1 with an ERM C-terminal motif revealed a similar sequence in the Myo1 motor domain. GFP-FERM and two truncated FERM domains were separately expressed in Tetrahymena. GFP-FERM contained the entire Myo1 FERM. Truncated Myo1 FERM domains contained either the N-terminal or the C-terminal region of FERM and one putative sequence for actin-binding, one for tubulin-binding, a putative dimerization motif, and a NLS-like sequence. Actin antibody coprecipitated GFP-fusion polypeptides and tubulin from lysate of cells expressing GFP-fusions. Cosedimentation assays performed with either whole cell extracts or anti-actin immunoprecipitation pellets revealed that F-actin (independent of ATP) and microtubules cosedimented with GFP-fusion polypeptides. GFP-FERM localized to the cytoskeleton, phagosomes, and nucleus. Truncated GFP-FERM domains localized to phagosomes but not to the cytoskeleton or nucleus.

Membrane rupture generates single open membrane sheets during vaccinia virus assembly

The biogenesis and dynamics of cellular membranes are governed by fusion and fission processes that ensure the maintenance of closed compartments. These principles also apply to viruses during acquisition of their envelope. Based on conventional electron microscopy (EM), however, it has been proposed that poxviruses assemble from membranes made de novo with "free" ends in the cytoplasm. Here, we analyze the origin and structure of poxvirus membranes in a close-to-native state and in three dimensions by using cryopreservation and electron tomography (ET). By cryo-EM, the precursor membrane of poxviruses appears as an open membrane sheet stabilized by a protein scaffold. ET shows that this membrane is derived from pre-existing cellular membranes that rupture to generate an open compartment, rather than being made de novo. Thus, poxvirus infection represents an excellent system to study how cytoplasmic membranes can form open sheets by a process distinct from well-defined mechanisms of membrane biogenesis.

Dynamic behavior of giant liposomes at desired osmotic pressures

To apply accurate and uniform osmotic pressures to liposomes, they can be formed using the spontaneous transfer method in solutions with different osmolarities. The majority of liposomes unexpectedly opened large holes (several micrometers in diameter) in response to the osmotic pressure regardless of its strength, that is, the difference between the outside and inside solute (sucrose or KCl) concentrations. However, the lag time for any response, including the opening of a hole, after the formation of the liposome decreased with increasing osmotic pressure.

Equilibrium theory and geometrical constraint equation for two-component lipid bilayer vesicles

This paper aims at the general mathematical framework for the equilibrium theory of two-component lipid bilayer vesicles. To take into account the influences of the local compositions together with the mean curvature and Gaussian curvature of the membrane surface, a general potential functional is constructed. We introduce two kinds of virtual displacement modes: the normal one and the tangential one. By minimizing the potential functional, the equilibrium differential equations and the boundary conditions of two-component lipid vesicles are derived. Additionally, the geometrical constraint equation and geometrically permissible condition for the two-component lipid vesicles are presented. The physical, mathematical, and biological meanings of the equilibrium differential equations and the geometrical constraint equations are discussed. The influences of physical parameters on the geometrically permissible phase diagrams are predicted. Numerical results can be used to explain recent experiments.

Biochemical and structural properties of the integrin-associated cytoskeletal protein talin

Interaction of cells with the extracellular matrix is fundamental to a wide variety of biological processes, such as cell proliferation, cell migration, embryogenesis, and organization of cells into tissues, and defects in cell-matrix interactions are an important element in many diseases. Cell-matrix interactions are frequently mediated by the integrin family of cell adhesion molecules, transmembrane alphabeta-heterodimers that are typically linked to the actin cytoskeleton by one of a number of adaptor proteins including talin, alpha-actinin, filamin, tensin, integrin-linked kinase, melusin, and skelemin. The focus of this review is talin, which appears unique among these proteins in that it also induces a conformational change in integrins that is propagated across the membrane, and increases the affinity of the extracellular domain for ligand. Particular emphasis is given to recent progress on the structure of talin, its interaction with binding partners, and its mode of regulation.

Magainin 2-induced pore formation in the lipid membranes depends on its concentration in the membrane interface

Antimicrobial peptide magainin 2 forms pores in lipid membranes to induce leakage of internal contents of cells, which is a main cause of its bactericidal activity. However, the conditions and the mechanism of its pore formation remain unclear. In this report, to reveal the effect of the surface charge density of membranes on magainin 2-induced pore formation, we investigated the interaction of magainin 2 with giant unilamellar vesicles (GUVs) composed of a mixture of electrically neutral dioleoylphosphatidylcholine (DOPC) and negatively charged dioleoylphosphatidylglycerol (DOPG) in various ratios, using the single GUV method. We found that magainin 2 induced pores in the membranes of all kinds of single GUVs. For GUVs with the same charge density, the rate of the pore formation increased with magainin 2 concentration. The magainin 2 concentrations in a buffer required to induce the same rate of the pore formation greatly increased with a decrease in the surface charge density; e.g., the magainin 2 concentrations required for the pore formation in 30% DOPG/70% DOPC-GUVs were 50 times higher than those in 60% DOPG/40% DOPC-GUVs. However, after we converted the magainin 2 concentration in the buffer into that in the membrane interface, Xbmag, we found that Xbmag mainly determines the rate of the pore formation in various GUVs. These data support our model of two-state transition from the binding state to the pore state of the GUV for magainin 2-induced pore formation.

Interactions of surfactants with lipid membranes

Surfactants are surface-active, amphiphilic compounds that are water-soluble in the micro- to millimolar range, and self-assemble to form micelles or other aggregates above a critical concentration. This definition comprises synthetic detergents as well as amphiphilic peptides and lipopeptides, bile salts and many other compounds. This paper reviews the biophysics of the interactions of surfactants with membranes of insoluble, naturally occurring lipids. It discusses structural, thermodynamic and kinetic aspects of membrane-water partitioning, changes in membrane properties induced by surfactants, membrane solubilisation to micelles and other phases formed by lipid-surfactant systems. Each section defines and derives key parameters, mentions experimental methods for their measurement and compiles and discusses published data. Additionally, a brief overview is given of surfactant-like effects in biological systems, technical applications of surfactants that involve membrane interactions, and surfactant-based protocols to study biological membranes.

Modelling and simulations of multi-component lipid membranes and open membranes via diffuse interface approaches

Diffuse interface (phase field) models are developed for multi-component vesicle membranes with different lipid compositions and membranes with free boundary. These models are used to simulate the deformation of membranes under the elastic bending energy and the line tension energy with prescribed volume and surface area constraints. By comparing our numerical simulations with recent biological experiments, it is demonstrated that the diffuse interface models can effectively capture the rich phenomena associated with the multi-component vesicle transformation and thus offering great functionality in their simulation and modelling.

Single GUV method reveals interaction of tea catechin (-)-epigallocatechin gallate with lipid membranes

Tea catechins, which are flavonoids and the main components of green tea extracts, are thought to have antibacterial and antioxidant activity. Several studies indicate that lipid membranes are one of the targets of the antibacterial activity of catechins. Studies using a suspension of large unilamellar vesicles (LUVs) indicate that catechin causes gradual leakage of internal contents from LUVs. However, the detailed characteristics of the interaction of catechins with lipid membranes remain unclear. In this study, we investigated the interaction of (-)-epigallocatechin gallate (EGCg), a major catechin in tea extract, with single giant unilamellar vesicles (GUVs) of egg phosphatidylcholine (egg PC) using phase-contrast fluorescence microscopy and the single GUV method. We prepared GUVs of lipid membranes of egg PC in a physiological ion concentration ( approximately 150 mM NaCl) using the polyethylene glycol-lipid method. Low concentrations of EGCg at and above 30 muM induced rapid leakage of a fluorescent probe, calcein, from the inside of single egg PC-GUVs; after the leakage, the GUVs changed into small lumps of lipid membranes. On the other hand, phase-contrast microscopic images revealed the detailed process of the EGCg-induced burst of GUVs, the decrease in their diameter, and their transformation into small lumps. The dependence of the fraction of burst GUVs on EGCg concentration was almost the same as that of the fraction of leaked GUV. This correlation strongly indicates that the leakage of calcein from the inside to the outside of the GUV occurred as a result of the burst of the GUV. The fraction of completely leaked GUV and the fraction of the burst GUV increased with time and also increased with increasing EGCg concentration. We compared the EGCg-induced leakage from single GUVs with EGCg-induced leakage from a LUV suspension. The analysis of the EGCg-induced shape changes shows that the binding of EGCg to the external monolayer of the GUV increases its membrane area, inducing an increase in its surface pressure. Small angle x-ray scattering experiments indicate that the intermembrane distance of multilamellar vesicles of PC membrane greatly decreased at EGCg concentrations above the threshold, suggesting that neighboring membranes came in close contact with each other. On the basis of these results, we discuss the mechanism of the EGCg-induced bursting of vesicles.

Temperature-Responsive Phase Transition of Polymer Vesicles: Real-Time Morphology Observation and Molecular Mechanism

Novel thermosensitive polymer vesicles with controlled temperature-responsive phase transition at the lower critical solution temperature (LCST) varying from 8 to 81 degrees C were prepared via self-assembly of amphiphilic hyperbranched star copolymers having a hydrophobic hyperbranched poly[3-ethyl-3-(hydroxymethyl)oxetane] (HBPO) core and many hydrophilic polyethylene oxide (PEO) arms. Real-time optical microscopic observation revealed that the polymer vesicles have undergone sequential morphology changes including enrichment, aggregation, fusion, and vesicle-to-membrane transformation near the LCST. Molecular-level investigation indicates that the LCST transition results from the decreasing water solubility of the polymer vesicles with increasing temperature based on the partial dehydration of the PEO vesicle corona. On the basis of these results, a LCST transition mechanism, in view of the molecular configuration, balance of hydrophilic and hydrophobic moieties, and the vesicle morphology transformations, was proposed. As far as we know, the work presented here is the first demonstration of thermosensitive vesicles based on PEO, and the finding may be useful to design the thermosensitive core-shell structures by introducing the PEO segments.