Nonequilibrium fluctuations of lipid membranes by the rotating motor protein F1F0-ATP synthase

Modeling membranes in situ

Molecular dynamics simulations of cellular membranes have come a long way-from simple model lipid bilayers to multicomponent systems capturing the crowded and complex nature of real cell membranes. In this opinionated minireview, we discuss the current challenge to simulate the dynamics of membranes in their native environment, in situ, with the prospect of reaching the level of whole cells and cell organelles using an integrative modeling framework.

Kinetically Controlled and Nonequilibrium Assembly of Block Copolymers in Solution

Covalent polymers are versatile macromolecules that have found widespread use in society. Contemporary methods of polymerization have made it possible to construct sequence polymers, including block copolymers, with high precision. Such copolymers assemble in solution when the blocks have differing solubilities. This produces nano- and microparticles of various shapes and sizes. While it is straightforward to draw an analogy between such amphiphilic block copolymers and phospholipids, these two classes of molecules show quite different assembly characteristics. In particular, block copolymers often assemble under kinetic control, thus producing nonequilibrium structures. This leads to a rich variety of behaviors being observed in block copolymer assembly, such as pathway dependence (e.g., thermal history), nonergodicity and responsiveness. The dynamics of polymer assemblies can be readily controlled using changes in environmental conditions and/or integrating functional groups situated on polymers with external chemical reactions. This perspective highlights that kinetic control is both pervasive and a useful attribute in the mechanics of block copolymer assembly. Recent examples are highlighted in order to show that toggling between static and dynamic behavior can be used to generate, manipulate and dismantle nonequilibrium states. New methods to control the kinetics of block copolymer assembly will provide endless unanticipated applications in materials science, biomimicry and medicine.

Red blood cell flickering activity locally controlled by holographic optical tweezers

Red blood cells possess a singular mechanobiology, enabling efficient navigation through capillaries smaller than their own size. Their plasma membrane exhibits non-equilibrium shape fluctuation, often reported as enhanced flickering activity. Such active membrane motion is propelled by motor proteins that mediate interactions between the spectrin skeleton and the lipid bilayer. However, modulating the flickering in living red blood cells without permanently altering their mechanical properties represents a significant challenge. In this study, we developed holographic optical tweezers to generate a force field distributed along the equatorial membrane contour of individual red blood cells. In free-standing red blood cells, we observed heterogeneous flickering activity, attributed to localized membrane kickers. By employing holographic optical forces, these active kickers can be selectively halted under minimal invasion. Our findings shed light on the dynamics of membrane flickering and established a manipulation tool that could open new avenues for investigating mechanotransduction processes in living cells.

Electron videography of a lipid-protein tango

Biological phenomena, from enzymatic catalysis to synaptic transmission, originate in the structural transformations of biomolecules and biomolecular assemblies in liquid water. However, directly imaging these nanoscopic dynamics without probes or labels has been a fundamental methodological challenge. Here, we developed an approach for "electron videography"-combining liquid phase electron microscopy with molecular modeling-with which we filmed the nanoscale structural fluctuations of individual, suspended, and unlabeled membrane protein nanodiscs in liquid. Systematic comparisons with biochemical data and simulation indicate the graphene encapsulation involved can afford sufficiently reduced effects of the illuminating electron beam for these observations to yield quantitative fingerprints of nanoscale lipid-protein interactions. Our results suggest that lipid-protein interactions delineate dynamically modified membrane domains across unexpectedly long ranges. Moreover, they contribute to the molecular mechanics of the nanodisc as a whole in a manner specific to the protein within. Overall, this work illustrates an experimental approach to film, quantify, and understand biomolecular dynamics at the nanometer scale.

Entropy-driven nonequilibrium phonon-stimulated electron-phonon coupling in tin dioxide nanorods

Nonequilibrium (NEQ) phonon fluctuation in a nanosystem has been studied through the statistical assessment of the entropy-production and -consumption events in ultrasmall tin dioxide (SnO_{2}) nanorods. Size- and shape-dependent alteration in free energy leading to modulation of the probability distribution function of the phonon dynamics has been observed from the x-ray diffraction and Raman scattering characterizations. The Gallavotti-Cohen nonequilibrium fluctuation theorem has been utilized to qualitatively describe the aforementioned behaviors under the influence of a global flux. The observation of entropy consumption and thermodynamically favorable entropy-production events indicates the presence of NEQ fluctuations in the phonon modes. The effective energy scale of fluctuation in driven phonon modes, dissipating energy faster than relaxation time, is quantified on the order of nanojoules. From optical absorption and photoluminescence studies, the observation of the electron-phonon coupled state confirms the interaction of the NEQ phonons with electrons. The strength of the coupling has been estimated from the temperature-independent Barry center shift and found to be enhanced to 5.35. Valence band x-ray photoelectron spectroscopy and Fourier transformed infrared spectroscopy analyses reconcile NEQ phonon mediated alteration of the valence band density of states, activation of silent phonon modes, and superior excitonic transitions, suitable for the new generation of ultrafast quantum device applications.

Synthetic Cells Revisited: Artificial Cells Construction Using Polymeric Building Blocks

The exponential growth of research on artificial cells and organelles underscores their potential as tools to advance the understanding of fundamental biological processes. The bottom-up construction from a variety of building blocks at the micro- and nanoscale, in combination with biomolecules is key to developing artificial cells. In this review, artificial cells are focused upon based on compartments where polymers are the main constituent of the assembly. Polymers are of particular interest due to their incredible chemical variety and the advantage of tuning the properties and functionality of their assemblies. First, the architectures of micro- and nanoscale polymer assemblies are introduced and then their usage as building blocks is elaborated upon. Different membrane-bound and membrane-less compartments and supramolecular structures and how they combine into advanced synthetic cells are presented. Then, the functional aspects are explored, addressing how artificial organelles in giant compartments mimic cellular processes. Finally, how artificial cells communicate with their surrounding and each other such as to adapt to an ever-changing environment and achieve collective behavior as a steppingstone toward artificial tissues, is taken a look at. Engineering artificial cells with highly controllable and programmable features open new avenues for the development of sophisticated multifunctional systems.

Experimental determination and mathematical modeling of standard shapes of forming autophagosomes

The formation of autophagosomes involves dynamic morphological changes of a phagophore from a flat membrane cisterna into a cup-shaped intermediate and a spherical autophagosome. However, the physical mechanism behind these morphological changes remains elusive. Here, we determine the average shapes of phagophores by statistically investigating three-dimensional electron micrographs of more than 100 phagophores. The results show that the cup-shaped structures adopt a characteristic morphology; they are longitudinally elongated, and the rim is catenoidal with an outwardly recurved shape. To understand these characteristic shapes, we establish a theoretical model of the shape of entire phagophores. The model quantitatively reproduces the average morphology and reveals that the characteristic shape of phagophores is primarily determined by the relative size of the open rim to the total surface area. These results suggest that the seemingly complex morphological changes during autophagosome formation follow a stable path determined by elastic bending energy minimization.

Structure and Elasticity of Mitochondrial Membranes: A Molecular Dynamics Simulation Study

Mitochondria are known as the powerhouse of the cell because they produce energy in the form of adenosine triphosphate. They also have other crucial functions such as regulating apoptosis, calcium homeostasis, and reactive oxygen species production. To perform these diverse functions, mitochondria adopt specific structures and frequently undergo dynamic shape changes, indicating that their mechanical properties play an essential role in their functions. To gain a detailed understanding at the molecular level of the structure and mechanical properties of mitochondria, we carry out atomistic molecular dynamics simulations for three inner mitochondrial membranes and three outer mitochondrial membrane models. These models take into account variations in cardiolipin and cholesterol concentrations as well as the symmetry/asymmetry between the two leaflets. Our simulations allow us to calculate various structural quantities and the bending, twisting, and tilting elastic moduli of the membrane models. Our results indicate that the structures of the inner and outer mitochondrial membranes are quite similar and do not depend much on the variation in lipid compositions. However, the bending modulus of the membranes increases with increasing concentrations of cardiolipin or cholesterol but decreases with a membrane asymmetry. Notably, we found that the dipole potential of the membrane increases with an increasing cardiolipin concentration. Finally, possible roles of cardiolipin in regulating ion and proton currents and maintaining the cristate are discussed in some details.

Light-Driven Membrane Assembly, Shape-Shifting, and Tissue Formation in Chemically Responsive Synthetic Cells

Living systems create remarkable complexity from a limited repertoire of biological building blocks by controlling assembly dynamics at the molecular, cellular, and multicellular level. An open question is whether simplified synthetic cells can gain similar complex functionality by being driven away from equilibrium. Here, we describe a dynamic synthetic cell system assembled using artificial lipids that are responsive to both light and chemical stimuli. Irradiation of disordered aggregates of lipids leads to the spontaneous emergence of giant cell-like vesicles, which revert to aggregates when illumination is turned off. Under irradiation, the synthetic cell membranes can interact with chemical building blocks, remodeling their composition and forming new structures that prevent the membranes from undergoing retrograde aggregation processes. The remodeled light-responsive synthetic cells reversibly alter their shape under irradiation, transitioning from spheres to rodlike shapes, mimicking energy-dependent functions normally restricted to living materials. In the presence of noncovalently interacting multivalent polymers, light-driven shape changes can be used to trigger vesicle cross-linking, leading to the formation of functional synthetic tissues. By controlling light and chemical inputs, the stepwise, one-pot transformation of lipid aggregates to multivesicular synthetic tissues is feasible. Our results suggest a rationale for why even early protocells may have required and evolved simple mechanisms to harness environmental energy sources to coordinate hierarchical assembly processes.

Rotation of the c-Ring Promotes the Curvature Sorting of Monomeric ATP Synthases

ATP synthases are proteins that catalyse the formation of ATP through the rotatory movement of their membrane-spanning subunit. In mitochondria, ATP synthases are found to arrange as dimers at the high-curved edges of cristae. Here, a direct link is explored between the rotatory movement of ATP synthases and their preference for curved membranes. An active curvature sorting of ATP synthases in lipid nanotubes pulled from giant vesicles is found. Coarse-grained simulations confirm the curvature-seeking behaviour of rotating ATP synthases, promoting reversible and frequent protein-protein contacts. The formation of transient protein dimers relies on the membrane-mediated attractive interaction of the order of 1.5 kB T produced by a hydrophobic mismatch upon protein rotation. Transient dimers are sustained by a conic-like arrangement characterized by a wedge angle of θ ≈ 50°, producing a dynamic coupling between protein shape and membrane curvature. The results suggest a new role of the rotational movement of ATP synthases for their dynamic self-assembly in biological membranes.

Activity modulation of the Escherichia coli F1FO ATP synthase by a designed antimicrobial peptide via cardiolipin sequestering

Most antimicrobial peptides (AMPs) exert their microbicidal activity through membrane permeabilization. The designed AMP EcDBS1R4 has a cryptic mechanism of action involving the membrane hyperpolarization of Escherichia coli, suggesting that EcDBS1R4 may hinder processes involved in membrane potential dissipation. We show that EcDBS1R4 can sequester cardiolipin, a phospholipid that interacts with several respiratory complexes of E. coli. Among these, F₁F_O ATP synthase uses membrane potential to fuel ATP synthesis. We found that EcDBS1R4 can modulate the activity of ATP synthase upon partition to membranes containing cardiolipin. Molecular dynamics simulations suggest that EcDBS1R4 alters the membrane environment of the transmembrane F_O motor, impairing cardiolipin interactions with the cytoplasmic face of the peripheral stalk that binds the catalytic F₁ domain to the F_O domain. The proposed mechanism of action, targeting membrane protein function through lipid reorganization may open new venues of research on the mode of action and design of other AMPs.

Function Investigations and Applications of Membrane Proteins on Artificial Lipid Membranes

Membrane proteins play an important role in key cellular functions, such as signal transduction, apoptosis, and metabolism. Therefore, structural and functional studies of these proteins are essential in fields such as fundamental biology, medical science, pharmacology, biotechnology, and bioengineering. However, observing the precise elemental reactions and structures of membrane proteins is difficult, despite their functioning through interactions with various biomolecules in living cells. To investigate these properties, methodologies have been developed to study the functions of membrane proteins that have been purified from biological cells. In this paper, we introduce various methods for creating liposomes or lipid vesicles, from conventional to recent approaches, as well as techniques for reconstituting membrane proteins into artificial membranes. We also cover the different types of artificial membranes that can be used to observe the functions of reconstituted membrane proteins, including their structure, number of transmembrane domains, and functional type. Finally, we discuss the reconstitution of membrane proteins using a cell-free synthesis system and the reconstitution and function of multiple membrane proteins.

Capsule self-oscillating gels showing cell-like nonthermal membrane/shape fluctuations

A primary interest in cell membrane and shape fluctuations is establishing experimental models reflecting only nonthermal active contributions. Here we report a millimeter-scaled capsule self-oscillating gel model mirroring the active contribution effect on cell fluctuations. In the capsule self-oscillating gels, the propagating chemical signals during a Belousov-Zhabotinsky (BZ) reaction induce simultaneous local deformations in the various regions, showing cell-like shape fluctuations. The capsule self-oscillating gels do not fluctuate without the BZ reaction, implying that only the active chemical parameter induces the gel fluctuations. The period and amplitude depend on the gel layer thickness and the concentration of the chemical substrate for the BZ reaction. Our results allow for a solid experimental platform showing actively driven cell-like fluctuations, which can potentially contribute to investigating the active parameter effect on cell fluctuations.

C. Pereira I, López-Montero I, Vélez M, Pita M, De Lacey AL. Electro-enzymatic ATP regeneration coupled to biocatalytic phosphorylation reactions

Adenosine-5-triphosphate (ATP) is the main energy vector in biological systems, thus its regeneration is an important issue for the application of many enzymes of interest in biocatalysis and synthetic biology. We have developed an electroenzymatic ATP regeneration system consisting in a gold electrode modified with a floating phospholipid bilayer that allows coupling the catalytic activity of two membrane-bound enzymes: NiFeSe hydrogenase from Desulfovibrio vulgaris and F₁F_o-ATP synthase from Escherichia coli. Thus, H₂ is used as a fuel for producing ATP. This electro-enzymatic assembly is studied as ATP regeneration system of phosphorylation reactions catalysed by kinases, such as hexokinase and NAD⁺-kinase for respectively producing glucose-6-phosphate and NADP⁺.

Mitochondrial membrane models built from native lipid extracts: Interfacial and transport properties

The mitochondrion is an essential organelle enclosed by two membranes whose functionalities depend on their very specific protein and lipid compositions. Proteins from the outer mitochondrial membrane (OMM) are specialized in mitochondrial dynamics and mitophagy, whereas proteins of the inner mitochondrial membrane (IMM) have dedicated functions in cellular respiration and apoptosis. As for lipids, the OMM is enriched in glycerophosphatidyl choline but cardiolipin is exclusively found within the IMM. Though the lipid topology and distribution of the OMM and IMM are known since more than four decades, little is known about the interfacial and dynamic properties of the IMM and OMM lipid extracts. Here we build monolayers, supported bilayers and giant unilamellar vesicles (GUVs) of native OMM and IMM lipids extracts from porcine heart. Additionally, we perform a comparative analysis on the interfacial, phase immiscibility and mechanical properties of both types of extract. Our results show that IMM lipids form more expanded and softer membranes than OMM lipids, allowing a better understanding of the physicochemical and biophysical properties of mitochondrial membranes.

Experimental platform for the functional investigation of membrane proteins in giant unilamellar vesicles

Giant unilamellar vesicles (GUVs) are micrometer-sized model membrane systems that can be viewed directly under the microscope. They serve as scaffolds for the bottom-up creation of synthetic cells, targeted drug delivery and have been widely used to study membrane related phenomena in vitro. GUVs are also of interest for the functional investigation of membrane proteins that carry out many key cellular functions. A major hurdle to a wider application of GUVs in this field is the diversity of existing protocols that are optimized for individual proteins. Here, we compare PVA assisted and electroformation techniques for GUV formation under physiologically relevant conditions, and analyze the effect of immobilization on vesicle structure and membrane tightness towards small substrates and protons. There, differences in terms of yield, size, and leakage of GUVs produced by PVA assisted swelling and electroformation were found, dependent on salt and buffer composition. Using fusion of oppositely charged membranes to reconstitute a model membrane protein, we find that empty vesicles and proteoliposomes show similar fusion behavior, which allows for a rapid estimation of protein incorporation using fluorescent lipids.

Molecular Shape Solution for Mesoscopic Remodeling of Cellular Membranes

Cellular membranes self-assemble from and interact with various molecular species. Each molecule locally shapes the lipid bilayer, the soft elastic core of cellular membranes. The dynamic architecture of intracellular membrane systems is based on elastic transformations and lateral redistribution of these elementary shapes, driven by chemical and curvature stress gradients. The minimization of the total elastic stress by such redistribution composes the most basic, primordial mechanism of membrane curvature-composition coupling (CCC). Although CCC is generally considered in the context of dynamic compositional heterogeneity of cellular membrane systems, in this article we discuss a broader involvement of CCC in controlling membrane deformations. We focus specifically on the mesoscale membrane transformations in open, reservoir-governed systems, such as membrane budding, tubulation, and the emergence of highly curved sites of membrane fusion and fission. We reveal that the reshuffling of molecular shapes constitutes an independent deformation mode with complex rheological properties.This mode controls effective elasticity of local deformations as well as stationary elastic stress, thus emerging as a major regulator of intracellular membrane remodeling.

Membrane rigidity regulates E. coli proliferation rates

Combining single cell experiments, population dynamics and theoretical methods of membrane mechanics, we put forward that the rate of cell proliferation in E. coli colonies can be regulated by modifiers of the mechanical properties of the bacterial membrane. Bacterial proliferation was modelled as mediated by cell division through a membrane constriction divisome based on FtsZ, a mechanically competent protein at elastic interaction against membrane rigidity. Using membrane fluctuation spectroscopy in the single cells, we revealed either membrane stiffening when considering hydrophobic long chain fatty substances, or membrane softening if short-chained hydrophilic molecules are used. Membrane stiffeners caused hindered growth under normal division in the microbial cultures, as expected for membrane rigidification. Membrane softeners, however, altered regular cell division causing persistent microbes that abnormally grow as long filamentous cells proliferating apparently faster. We invoke the concept of effective growth rate under the assumption of a heterogeneous population structure composed by distinguishable individuals with different FtsZ-content leading the possible forms of cell proliferation, from regular division in two normal daughters to continuous growing filamentation and budding. The results settle altogether into a master plot that captures a universal scaling between membrane rigidity and the divisional instability mediated by FtsZ at the onset of membrane constriction.

How rotating ATP synthases can modulate membrane structure

F1Fo-ATP synthase (ATP synthase) is a central membrane protein that synthetizes most of the ATP in the cell through a rotational movement driven by a proton gradient across the hosting membrane. In mitochondria, ATP synthases can form dimers through specific interactions between some subunits of the protein. The dimeric form of ATP synthase provides the protein with a spontaneous curvature that sustain their arrangement at the rim of the high-curvature edges of mitochondrial membrane (cristae). Also, a direct interaction with cardiolipin, a lipid present in the inner mitochondrial membrane, induces the dimerization of ATP synthase molecules along cristae. The deletion of those biochemical interactions abolishes the protein dimerization producing an altered mitochondrial function and morphology. Mechanically, membrane bending is one of the key deformation modes by which mitochondrial membranes can be shaped. In particular, bending rigidity and spontaneous curvature are important physical factors for membrane remodelling. Here, we discuss a complementary mechanism whereby the rotatory movement of the ATP synthase might modify the mechanical properties of lipid bilayers and contribute to the formation and regulation of the membrane invaginations.

En route to dynamic life processes by SNARE-mediated fusion of polymer and hybrid membranes

A variety of artificial cells springs from the functionalization of liposomes with proteins. However, these models suffer from low durability without repair and replenishment mechanisms, which can be partly addressed by replacing the lipids with polymers. Yet natural membranes are also dynamically remodeled in multiple cellular processes. Here, we show that synthetic amphiphile membranes also undergo fusion, mediated by the protein machinery for synaptic secretion. We integrated fusogenic SNAREs in polymer and hybrid vesicles and observed efficient membrane and content mixing. We determined bending rigidity and pore edge tension as key parameters for fusion and described its plausible progression through cryo-EM snapshots. These findings demonstrate that dynamic membrane phenomena can be reconstituted in synthetic materials, thereby providing new tools for the assembly of synthetic protocells.

Mitochondrial Cristae Architecture and Functions: Lessons from Minimal Model Systems

Mitochondria are known as the powerhouse of eukaryotic cells. Energy production occurs in specific dynamic membrane invaginations in the inner mitochondrial membrane called cristae. Although the integrity of these structures is recognized as a key point for proper mitochondrial function, less is known about the mechanisms at the origin of their plasticity and organization, and how they can influence mitochondria function. Here, we review the studies which question the role of lipid membrane composition based mainly on minimal model systems.

Undulation of a moving fluid membrane pushed by filament growth

Biomembranes experience out-of-equilibrium conditions in living cells. Their undulation spectra are different from those in thermal equilibrium. Here, we report on the undulation of a fluid membrane pushed by the stepwise growth of filaments as in the leading edge of migrating cells, using three-dimensional Monte Carlo simulations. The undulations are largely modified from equilibrium behavior. When the tension is constrained, the low-wave-number modes are suppressed or enhanced at small or large growth step sizes, respectively, for high membrane surface tensions. In contrast, they are always suppressed for the tensionless membrane, wherein the wave-number range of the suppression depends on the step size. When the membrane area is constrained, in addition to these features, a specific mode is excited for zero and low surface tensions. The reduction of the undulation first induces membrane buckling at the lowest wave-number, and subsequently, other modes are excited, leading to a steady state.

Fluctuation spectroscopy of giant unilamellar vesicles using confocal and phase contrast microscopy

A widely used method to measure the bending rigidity of bilayer membranes is fluctuation spectroscopy, which analyses the thermally-driven membrane undulations of giant unilamellar vesicles recorded with either phase-contrast or confocal microscopy. Here, we analyze the fluctuations of the same vesicle using both techniques and obtain consistent values for the bending modulus. We discuss the factors that may lead to discrepancies.

Constructing artificial respiratory chain in polymer compartments: Insights into the interplay between bo3 oxidase and the membrane

Cytochrome bo3 ubiquinol oxidase is a transmembrane protein, which oxidizes ubiquinone and reduces oxygen, while pumping protons. Apart from its combination with F1Fo-ATPase to assemble a minimal ATP regeneration module, the utility of the proton pump can be extended to other applications in the context of synthetic cells such as transport, signaling, and control of enzymatic reactions. In parallel, polymers have been speculated to be phospholipid mimics with respect to their ability to self-assemble in compartments with increased stability. However, their usability as interfaces for complex membrane proteins has remained questionable. In the present work, we optimized a fusion/electroformation approach to reconstitute bo3 oxidase in giant unilamellar vesicles made of PDMS-g-PEO and/or phosphatidylcholine (PC). This enabled optical access, while microfluidic trapping allowed for online analysis of individual vesicles. The tight polymer membranes and the inward oriented enzyme caused 1 pH unit difference in 30 min, with an initial rate of 0.35 pH·min-1 To understand the interplay in these composite systems, we studied the relevant mechanical and rheological membrane properties. Remarkably, the proton permeability of polymer/lipid hybrids decreased after protein insertion, while the latter also led to a 20% increase of the polymer diffusion coefficient in polymersomes. In addition, PDMS-g-PEO increased the activity lifetime and the resistance to free radicals. These advantageous properties may open diverse applications, ranging from cell-free biotechnology to biomedicine. Furthermore, the presented study serves as a comprehensive road map for studying the interactions between membrane proteins and synthetic membranes, which will be fundamental for the successful engineering of such hybrid systems.

Development of Artificial Cell Models Using Microfluidic Technology and Synthetic Biology

Giant lipid vesicles or liposomes are primarily composed of phospholipids and form a lipid bilayer structurally similar to that of the cell membrane. These vesicles, like living cells, are 5-100 μm in diameter and can be easily observed using an optical microscope. As their biophysical and biochemical properties are similar to those of the cell membrane, they serve as model cell membranes for the investigation of the biophysical or biochemical properties of the lipid bilayer, as well as its dynamics and structure. Investigation of membrane protein functions and enzyme reactions has revealed the presence of soluble or membrane proteins integrated in the giant lipid vesicles. Recent developments in microfluidic technologies and synthetic biology have enabled the development of well-defined artificial cell models with complex reactions based on the giant lipid vesicles. In this review, using microfluidics, the formations of giant lipid vesicles with asymmetric lipid membranes or complex structures have been described. Subsequently, the roles of these biomaterials in the creation of artificial cell models including nanopores, ion channels, and other membrane and soluble proteins have been discussed. Finally, the complex biological functions of giant lipid vesicles reconstituted with various types of biomolecules has been communicated. These complex artificial cell models contribute to the production of minimal cells or protocells for generating valuable or rare biomolecules and communicating between living cells and artificial cell models.

Potentiometric detection of ATP based on the transmembrane proton gradient generated by ATPase reconstituted on a gold electrode

Adenosine triphosphate (ATP) is a key molecule as energy vector for living organisms, therefore its detection reveals the presence of microbial colonies. Environments where the existence of microbial pathogens suppose a health hazard can benefit from real time monitoring of such molecule. We report a potentiometric biosensor based on ATP-synthase from Escherichia coli reconstituted in a floating phospholipid bilayer over gold electrodes modified with a 4-aminothiophenol self-assembled monolayer. The use of a pH-dependent redox probe on the electrode surface allows a simple, specific and reliable on site determination of ATP concentration from 1 μM to 1 mM. The broad range ATP biosensor can offer an alternative way of measuring in a few minutes the presence of microbial contamination.

Rotating Membrane Inclusions Crystallize Through Hydrodynamic and Steric Interactions

We show that rotating membrane inclusions can crystallize due to combined hydrodynamic and steric interactions. Alone, steric repulsion of unconfined particles, even with thermal fluctuations, does not lead to crystallization, nor do rotational hydrodynamic interactions which allow only a marginally stable lattice. Hydrodynamic interactions enable particles to explore states inaccessible to a nonrotational system, yet, unlike Brownian motion, Hamiltonian conservation confines the ensemble which, when combined with steric interactions, anneals into a stable crystal state.

Membrane curvature induces cardiolipin sorting

Cardiolipin is a cone-shaped lipid predominantly localized in curved membrane sites of bacteria and in the mitochondrial cristae. This specific localization has been argued to be geometry-driven, since the CL's conical shape relaxes curvature frustration. Although previous evidence suggests a coupling between CL concentration and membrane shape in vivo, no precise experimental data are available for curvature-based CL sorting in vitro. Here, we test this hypothesis in experiments that isolate the effects of membrane curvature in lipid-bilayer nanotubes. CL sorting is observed with increasing tube curvature, reaching a maximum at optimal CL concentrations, a fact compatible with self-associative clustering. Observations are compatible with a model of membrane elasticity including van der Waals entropy, from which a negative intrinsic curvature of -1.1 nm-1 is predicted for CL. The results contribute to understanding the physicochemical interplay between membrane curvature and composition, providing key insights into mitochondrial and bacterial membrane organization and dynamics.

Supramolecular zippers elicit interbilayer adhesion of membranes producing cell death

Background

The fluorescent dye 10-N-nonyl acridine orange (NAO) is widely used as a mitochondrial marker. NAO was reported to have cytotoxic effects in cultured eukaryotic cells when incubated at high concentrations. Although the biochemical response of NAO-induced toxicity has been well identified, the underlying molecular mechanism has not yet been explored in detail.

Methods

We use optical techniques, including fluorescence confocal microscopy and lifetime imaging microscopy (FLIM) both in model membranes built up as giant unilamellar vesicles (GUVs) and cultured cells. These experiments are complemented with computational studies to unravel the molecular mechanism that makes NAO cytotoxic.

Results

We have obtained direct evidence that NAO promotes strong membrane adhesion of negatively charged vesicles. The attractive forces are derived from van der Waals interactions between anti-parallel H-dimers of NAO molecules from opposing bilayers. Semi-empirical calculations have confirmed the supramolecular scenario by which anti-parallel NAO molecules form a zipper of bonds at the contact region. The membrane remodeling effect of NAO, as well as the formation of H-dimers, was also confirmed in cultured fibroblasts, as shown by the ultrastructure alteration of the mitochondrial cristae.

Conclusions

We conclude that membrane adhesion induced by NAO stacking accounts for the supramolecular basis of its cytotoxicity.

General significance

Mitochondria are a potential target for cancer and gene therapies. The alteration of the mitochondrial structure by membrane remodeling agents able to form supramolecular assemblies via adhesion properties could be envisaged as a new therapeutic strategy.

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NAO promotes interbilayer adhesion of negatively charged lipid vesicles.

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Membrane adhesion derives from the self-assembly of NAO into antiparallel H-dimers.

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The adhesion strength promoted by antiparallel H-aggregates is 10⁻⁶ J/m².

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The formation of NAO H-aggregates produces cell death in fibroblasts.

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The molecular mechanism of NAO cytotoxicity relies on the adhesion ability of H-dimers.

Mastering Complexity: Towards Bottom-up Construction of Multifunctional Eukaryotic Synthetic Cells

With the ultimate aim to construct a living cell, bottom-up synthetic biology strives to reconstitute cellular phenomena in vitro - disentangled from the complex environment of a cell. Recent work towards this ambitious goal has provided new insights into the mechanisms governing life. With the fast-growing library of functional modules for synthetic cells, their classification and integration become increasingly important. We discuss strategies to reverse-engineer and recombine functional parts for synthetic eukaryotes, mimicking the characteristics of nature's own prototype. Particularly, we focus on large outer compartments, complex endomembrane systems with organelles, and versatile cytoskeletons as hallmarks of eukaryotic life. Moreover, we identify microfluidics and DNA nanotechnology as two technologies that can integrate these functional modules into sophisticated multifunctional synthetic cells.