Ultrasensitive detection of protein translocated through toxin pores in droplet-interface bilayers

Encapsulated droplet interface bilayers as a platform for high-throughput membrane studies

Whilst it is highly desirable to produce artificial lipid bilayer arrays allowing for systematic high-content screening of membrane conditions, it remains a challenge due to the combined requirements of scaled membrane production, simple measurement access, and independent control over individual bilayer experimental conditions. Here, droplet bilayers encapsulated within a hydrogel shell are output individually into multi-well plates for simple, arrayed quantitative measurements. The afforded experimental throughput is used to conduct a 2D concentration screen characterising the synergistic pore-forming peptides Magainin2 and PGLa. Maximal enhanced activity is revealed at equimolar peptide concentrations via a membrane dye leakage assay, a finding consistent with models proposed from NMR data. The versatility of the platform is demonstrated by performing in situ electrophysiology, revealing low conductance pore activity (∼15 to 20 pA with 4.5 pA sub-states). In conclusion, this array platform addresses the aforementioned challenges and provides new and flexible opportunities for high-throughput membrane studies. Furthermore, the ability to engineer droplet networks within each construct paves the way for "lab-in-a-capsule" approaches accommodating multiple assays per construct and allowing for communicative reaction pathways.

Physicochemical characteristics of droplet interface bilayers

Droplet interface bilayer (DIB) is a lipid bilayer formed when two lipid monolayer-coated aqueous droplets are brought in contact within an oil phase. DIBs, especially post functionalization, are a facile model system to study the biophysics of the cell membrane. Continued advances in enhancing and functionalizing DIBs to be a faithful cell membrane mimetic requires a deep understanding of the physicochemical characteristics of droplet interface bilayers. In this review, we provide a comprehensive overview of the current scientific understanding of DIB characteristics starting with the key experimental frameworks for DIB generation, visualization and functionalization. Subsequently we report experimentally measured physical, electrical and transport characteristics of DIBs across physiologically relevant lipids. Advances in simulations and mathematical modelling of DIBs are also discussed, with an emphasis on revealing principles governing the key physicochemical characteristics. Finally, we conclude the review with important outstanding questions in the field.

Solution Structures of Bacillus anthracis Protective Antigen Proteins Using Small Angle Neutron Scattering and Protective Antigen 63 Ion Channel Formation Kinetics

We are studying the structures of bacterial toxins that form ion channels and enable macromolecule transport across membranes. For example, the crystal structure of the Staphylococcus aureus α-hemolysin (α-HL) channel in its functional state was confirmed using neutron reflectometry (NR) with the protein reconstituted in membranes tethered to a solid support. This method, which provides sub-nanometer structural information, could also test putative structures of the Bacillus anthracis protective antigen 63 (PA63) channel, locate where B. anthracis lethal factor and edema factor toxins (LF and EF, respectively) bind to it, and determine how certain small molecules can inhibit the interaction of LF and EF with the channel. We report here the solution structures of channel-forming PA63 and its precursor PA83 (which does not form channels) obtained with small angle neutron scattering. At near neutral pH, PA83 is a monomer and PA63 a heptamer. The latter is compared to two cryo-electron microscopy structures. We also show that although the α-HL and PA63 channels have similar structural features, unlike α-HL, PA63 channel formation in lipid bilayer membranes ceases within minutes of protein addition, which currently precludes the use of NR for elucidating the interactions between PA63, LF, EF, and potential therapeutic agents.

A skin-inspired soft material with directional mechanosensation

Lessons about artificial sensor design may be taken from evolutionarily perfected physiological systems. Mechanosensory cells in human skin are exquisitely sensitive to gentle touch and enable us to distinguish objects of different stiffnesses and textures. These cells are embedded in soft epidermal layers of gel-like consistency. Reproducing these mechanosensing capabilities in new soft materials may lead to the development of adaptive mechanosensors which will further enhance the abilities of engineered membrane-based structures with bioinspired sensing strategies. This strategy is explored here using droplet interface bilayers embedded within a thermoreversible organogel. The interface between two lipid-coated aqueous inclusions contained within a soft polymeric matrix forms a lipid bilayer resembling the lipid matrix of cell membranes. These interfaces are functionalized with bacterial mechanosensitive channels (V23T MscL) which convert membrane tension into changes in membrane conductance, mimicking mechanosensitive channel activation in mammalian mechanosensory cells. The distortion of encapsulated adhered droplets by cyclical external forces are first explored using a finite element composite model illustrating the directional propagation of mechanical disturbances imposed by a piston. The model predicts that the orientation of the droplet pair forming the membrane relative to the direction of the compression plays a role in the membrane response. The directional dependence of mechanosensitive channel activation in response to gel compression is confirmed experimentally and shows that purely compressive perturbations normal to the interface invoke different channel activities as compared to shearing displacement along a plane of the membrane. The developed system containing specially positioned pairs of droplets functionalized with bacterial mechanosensitive channels and embedded in a gel creates a skin-inspired soft material with a directional response to mechanical perturbation.

Microfluidic platform enables tailored translocation and reaction cascades in nanoliter droplet networks

In the field of bottom-up synthetic biology, lipid membranes are the scaffold to create minimal cells and mimic reactions and processes at or across the membrane. In this context, we employ here a versatile microfluidic platform that enables precise positioning of nanoliter droplets with user-specified lipid compositions and in a defined pattern. Adjacent droplets make contact and form a droplet interface bilayer to simulate cellular membranes. Translocation of molecules across membranes are tailored by the addition of alpha-hemolysin to selected droplets. Moreover, we developed a protocol to analyze the translocation of non-fluorescent molecules between droplets with mass spectrometry. Our method is capable of automated formation of one- and two-dimensional droplet networks, which we demonstrated by connecting droplets containing different compound and enzyme solutions to perform translocation experiments and a multistep enzymatic cascade reaction across the droplet network. Our platform opens doors for creating complex artificial systems for bottom-up synthetic biology.

Simon Bachler et al. present a new microfluidic platform to control the precise position and patterns of nanoliter droplets with various lipid materials. They show their platform enables monitoring of droplets and subsequent label-free mass spectrometry, which represents an important advance for the synthetic biology community.

Continuous and Rapid Solution Exchange in a Lipid Bilayer Perfusion System Based on Droplet-Interface Bilayer

Because of the high sensitivity of lipid bilayers to external pressure fluctuations, a major challenge in functional studies of biological pores or ion channels is the difficulty in exchanging solutions rapidly while maintaining the stability of the lipid bilayer in a model membrane. Here we describe a droplet-interface bilayer-based perfusion system that has been routinely used in our research and is currently the most robust and stable perfusion system that provides prompt solution exchange surrounding a lipid bilayer. In this model membrane system, solutions can be completely exchanged within 1-2 s to obtain prompt responses of a lipid bilayer or membrane pores to the membrane environments. Also, our system is stable enough to sustain continuous perfusions up to at least dozens of minutes. To demonstrate, we show that acidification-induced protein channel insertion, substrate binding to protein channels, and pH gradient-driven protein translocation of anthrax toxin can be sequentially initiated by continuous perfusions in our system. Moreover, by rapidly switching the solutions, the protein translocation based on ratchet mechanisms can be paused and reinitiated iteratively in our system. Overall, this perfusion system provides a controllable and reliable solution exchange platform for investigations of pores and translocations on lipid bilayers.

Probing membrane protein properties using droplet interface bilayers

IMPACT STATEMENT

The paper presents a comprehensive review of integral membrane protein studies utilizing droplet interface bilayers. Droplet interface bilayers are a novel method of constructing artificial lipid bilayers with enhanced stability and physicochemical complexity compared to existing methods. Their unique morphology also suggests applications in the construction of synthetic biological systems and protocells. As well as serving as a guide to in vitro membrane protein functional studies using droplet interface bilayers in the literature to date, a novel in vitro study of a flippase protein in a droplet interface bilayer is presented.

Drop-in-well chamber for droplet interface bilayer with built-in electrodes

Various methods have been developed for the formation of planar lipid bilayers, and recent techniques using water-in-oil droplets, such as droplet interface bilayer (DIB) and contact bubble bilayer (CBB) methods, allow the ready formation of bilayers with arbitrary lipid compositions. Here, we developed a simple and portable DIB system using drop-in-wells, shaping two merging wells for settling electrolyte droplets. An aliquot of the electrolyte solution (1μL) is dropped into an organic solvent, and the droplet sinks to the drop-in-well at the bottom, where two monolayer-lined droplets come in contact to form the bilayer. Pre-installed electrodes allow electrophysiological measurements. The detailed drop-in-well method is presented, and some variations of the method, such as the use of microelectrodes and a sheet with a small hole for low-noise recordings, are extended. Examples of single channel current recordings of the KcsA potassium channel are demonstrated.

Single molecule fluorescence for membrane proteins

The cell membrane is a complex milieu of lipids and proteins. In order to understand the behaviour of individual molecules is it often desirable to examine them as purified components in in vitro systems. Here, we detail the creation and use of droplet interface bilayers (DIBs) which, when coupled to TIRF microscopy, can reveal spatiotemporal and kinetic information for individual membrane proteins. A number of steps are required including modification of the protein sequence to enable the incorporation of appropriate fluorescent labels, expression and purification of the membrane protein and subsequent labelling. Following creation of DIBs, proteins are spontaneously incorporated into the membrane where they can be imaged via conventional single molecule TIRF approaches. Using this strategy, in conjunction with step-wise photobleaching, FRET and/or single particle tracking, a host of parameters can be determined such as oligomerisation state and dynamic information. We discuss advantages and limitations of this system and offer guidance for successful implementation of these approaches.

Exploring Molecular-Biomembrane Interactions with Surface Plasmon Resonance and Dual Polarization Interferometry Technology: Expanding the Spotlight onto Biomembrane Structure

The molecular analysis of biomolecular-membrane interactions is central to understanding most cellular systems but has emerged as a complex technical challenge given the complexities of membrane structure and composition across all living cells. We present a review of the application of surface plasmon resonance and dual polarization interferometry-based biosensors to the study of biomembrane-based systems using both planar mono- or bilayers or liposomes. We first describe the optical principals and instrumentation of surface plasmon resonance, including both linear and extraordinary transmission modes and dual polarization interferometry. We then describe the wide range of model membrane systems that have been developed for deposition on the chips surfaces that include planar, polymer cushioned, tethered bilayers, and liposomes. This is followed by a description of the different chemical immobilization or physisorption techniques. The application of this broad range of engineered membrane surfaces to biomolecular-membrane interactions is then overviewed and how the information obtained using these techniques enhance our molecular understanding of membrane-mediated peptide and protein function. We first discuss experiments where SPR alone has been used to characterize membrane binding and describe how these studies yielded novel insight into the molecular events associated with membrane interactions and how they provided a significant impetus to more recent studies that focus on coincident membrane structure changes during binding of peptides and proteins. We then discuss the emerging limitations of not monitoring the effects on membrane structure and how SPR data can be combined with DPI to provide significant new information on how a membrane responds to the binding of peptides and proteins.

Toward Realistic Large-Area Cell Membrane Mimics: Excluding Oil, Controlling Composition, and Including Ion Channels

Capacitance measurements provide unique insights into the thickness, compressibility, and composition of large-area membrane bilayers and are used here in addition to demonstrate the successful incorporation of model ion channels. The simultaneous ability to control the bilayer size, manipulate tension, and optically monitor and electrically stimulate freestanding membranes enables precise determination of their specific capacitance and thickness across a wide range of areas. We confirm that membranes formed by this recently developed technique have capacitive properties similar to those formed by existing protocols, including solvent-free approaches, and discuss the effect using either hexadecane or squalene as the oil solvent. The results obtained here are relevant for other methods where lipid membranes are reconstituted from a bulk oil solvent. Because biological membranes have a diverse phospholipid profile, we show that the technique can successfully reconstitute membranes with binary composition mixtures. As an outlook, we show the capability of model membrane proteins, specifically α-hemolysin and alamethicin, to be incorporated into the formed bilayers and measure ion transport.

Functional aqueous droplet networks

Droplet interface bilayers (DIBs), comprising individual lipid bilayers between pairs of aqueous droplets in an oil, are proving to be a useful tool for studying membrane proteins. Recently, attention has turned to the elaboration of networks of aqueous droplets, connected through functionalized interface bilayers, with collective properties unachievable in droplet pairs. Small 2D collections of droplets have been formed into soft biodevices, which can act as electronic components, light-sensors and batteries. A substantial breakthrough has been the development of a droplet printer, which can create patterned 3D droplet networks of hundreds to thousands of connected droplets. The 3D networks can change shape, or carry electrical signals through defined pathways, or express proteins in response to patterned illumination. We envisage using functional 3D droplet networks as autonomous synthetic tissues or coupling them with cells to repair or enhance the properties of living tissues.

An Automated Microfluidic System for the Generation of Droplet Interface Bilayer Networks

Networks of droplets, in which aqueous compartments are separated by lipid bilayers, have shown great potential as a model for biological transmembrane communication. We present a microfluidic system which allows for on-demand generation of droplets that are hydrodynamically locked in a trapping structure. As a result, the system enables the formation of a network of four droplets connected via lipid bilayers and the positions of each droplet in the network can be controlled thanks to automation of microfluidic operations. We perform electrophysiological measurements of ionic currents indicating interactions between nanopores and small molecules to prove the potential of the device in screening of the inhibitors acting on membrane proteins. We also demonstrate, for the first time, a microfluidic droplet interface bilayer (DIB) system in which the testing of inhibitors can be performed without direct contact between the tested sample and the electrodes recording picoampere currents.

Hydrodynamic trapping for rapid assembly and in situ electrical characterization of droplet interface bilayer arrays

The droplet interface bilayer (DIB) is a modular technique for assembling planar lipid membranes between water droplets in oil. The DIB method thus provides a unique capability for developing digital, droplet-based membrane platforms for rapid membrane characterization, drug screening and ion channel recordings. This paper demonstrates a new, low-volume microfluidic system that automates droplet generation, sorting, and sequential trapping in designated locations to enable the rapid assembly of arrays of DIBs. The channel layout of the device is guided by an equivalent circuit model, which predicts that a serial arrangement of hydrodynamic DIB traps enables sequential droplet placement and minimizes the hydrodynamic pressure developed across filled traps to prevent squeeze-through of trapped droplets. Furthermore, the incorporation of thin-film electrodes fabricated via evaporation metal deposition onto the glass substrate beneath the channels allows for the first time in situ, simultaneous electrical interrogation of multiple DIBs within a sealed device. Combining electrical measurements with imaging enables measurements of membrane capacitance and resistance and bilayer area, and our data show that DIBs formed in different trap locations within the device exhibit similar sizes and transport properties. Simultaneous, single channel recordings of ion channel gating in multiple membranes are obtained when alamethicin peptides are incorporated into the captured droplets, qualifying the thin-film electrodes as a means for measuring stimuli-responsive functions of membrane-bound biomolecules. This novel microfluidic-electrophysiology platform provides a reproducible, high throughput method for performing electrical measurements to study transmembrane proteins and biomembranes in low-volume, droplet-based membranes.

Millimeter-area, free standing, phospholipid bilayers

Minimal model biomembrane studies have the potential to unlock the fundamental mechanisms of cellular function that govern the processes upon which life relies. However, existing methods to fabricate free-standing model membranes currently have significant limitations. Bilayer sizes are often tens of micrometers, decoupling curvature or substrate effects, orthogonal control over tension, and solvent exchange combined with microscopy techniques is not possible, which restricts the studies that can be performed. Here, we describe a versatile platform to generate free standing, planar, phospholipid bilayers with millimeter scale areas. The technique relies on an adapted thin-film balance apparatus allowing for the dynamic control of the nucleation and growth of a planar black lipid membrane in the center of an orifice surrounded by microfluidic channels. Success is demonstrated using several different lipid types, including mixtures that show the same temperature dependent phase separation as existing protocols, moreover, membranes are highly stable. Two advantages unique to the proposed method are the dynamic control of the membrane tension and the possibility to make extremely large area membranes. We demonstrate this by showing how a block polymer, F68, used in drug delivery increases the membrane compliance. Together, the results demonstrate a new paradigm for studying the mechanics, structure, and function of model membranes.

Studies of RNA Sequence and Structure Using Nanopores

Nanopores are powerful single-molecule sensors with nanometer scale dimensions suitable for detection, quantification, and characterization of nucleic acids and proteins. Beyond sequencing applications, both biological and solid-state nanopores hold great promise as tools for studying the biophysical properties of RNA. In this review, we highlight selected landmark nanopore studies with regards to RNA sequencing, microRNA detection, RNA/ligand interactions, and RNA structural/conformational analysis.

Direct in situ measurement of specific capacitance, monolayer tension, and bilayer tension in a droplet interface bilayer

Thickness and tension are important physical parameters of model cell membranes. However, traditional methods to measure these quantities require multiple experiments using separate equipment. This work introduces a new multi-step procedure for directly accessing in situ multiple physical properties of droplet interface bilayers (DIB), including specific capacitance (related to thickness), lipid monolayer tension in the Plateau-Gibbs border, and bilayer tension. The procedure employs a combination of mechanical manipulation of bilayer area followed by electrowetting of the capacitive interface to examine the sensitivities of bilayer capacitance to area and contact angle to voltage, respectively. These data allow for determining the specific capacitance of the membrane and surface tension of the lipid monolayer, which are then used to compute bilayer thickness and tension, respectively. The use of DIBs affords accurate optical imaging of the connected droplets in addition to electrical measurements of bilayer capacitance, and it allows for reversibly varying bilayer area. After validating the accuracy of the technique with diphytanoyl phosphatidylcholine (DPhPC) DIBs in hexadecane, the method is applied herein to quantify separately the effects on membrane thickness and tension caused by varying the solvent in which the DIB is formed and introducing cholesterol into the bilayer. Because the technique relies only on capacitance measurements and optical images to determine both thickness and tension, this approach is specifically well-suited for studying the effects of peptides, biomolecules, natural and synthetic nanoparticles, and other species that accumulate within membranes without altering bilayer conductance.

Droplet interface bilayer reconstitution and activity measurement of the mechanosensitive channel of large conductance from Escherichia coli

Droplet interface bilayers (DIBs) provide an exciting new platform for the study of membrane proteins in stable bilayers of controlled composition. To date, the successful reconstitution and activity measurement of membrane proteins in DIBs has relied on the use of the synthetic lipid 1,2-diphytanoyl-sn-glycero-3-phosphocholine (DPhPC). We report the functional reconstitution of the mechanosensitive channel of large conductance (MscL) into DIBs composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), a lipid of significantly greater biological relevance than DPhPC. MscL functionality has been demonstrated using a fluorescence-based assay, showing that dye flow occurs across the DIB when MscL is gated by the cysteine reactive chemical 2-(trimethylammonium)ethyl methane thiosulfonate bromide (MTSET). MscL has already been the subject of a number of studies investigating its interaction with the membrane. We propose that this method will pave the way for future MscL studies looking in detail at the effects of controlled composition or membrane asymmetry on MscL activity using biologically relevant lipids and will also be applicable to other lipid–protein systems, paving the way for the study of membrane proteins in DIBs with biologically relevant lipids.

Nanopore sensing of botulinum toxin type B by discriminating an enzymatically cleaved Peptide from a synaptic protein synaptobrevin 2 derivative

Botulinum neurotoxins (BoNTs) are the most lethal toxin known to human. Biodefense requires early and rapid detection of BoNTs. Traditionally, BoNTs can be detected by looking for signs of botulism in mice that receive an injection of human material, serum or stool. While the living animal assay remains the most sensitive approach, it is costly, slow and associated with legal and ethical constrains. Various biochemical, optical and mechanical methods have been developed for BoNTs detection with improved speed, but with lesser sensitivity. Here, we report a novel nanopore-based BoNT type B (BoNT-B) sensor that monitors the toxin's enzymatic activity on its substrate, a recombinant synaptic protein synaptobrevin 2 derivative. By analyzing the modulation of the pore current caused by the specific BoNT-B-digested peptide as a marker, the presence of BoNT-B at a subnanomolar concentration was identified within minutes. The nanopore detector would fill the niche for a much needed rapid and highly sensitive detection of neurotoxins, and provide an excellent system to explore biophysical mechanisms for biopolymer transportation.

Heating-enabled formation of droplet interface bilayers using Escherichia coli total lipid extract

Droplet interface bilayers (DIBs) serve as a convenient platform to study interactions between synthetic lipid membranes and proteins. However, a majority of DIBs have been assembled using a single lipid type, diphytanoylphosphatidylcholine (DPhPC). The work described herein establishes a new method to assemble DIBs using total lipid extract from Escherichia coli (eTLE); it is found that incubating oil-submerged aqueous droplets containing eTLE liposomes at a temperature above the gel-fluid phase transition temperature (Tg) promotes monolayer self-assembly that does not occur below Tg. Once monolayers are properly assembled via heating, droplets can be directly connected or cooled below Tg and then connected to initiate bilayer formation. This outcome contrasts immediate droplet coalescence observed upon contact between nonheated eTLE-infused droplets. Specific capacitance measurements confirm that the interface between droplets containing eTLE lipids is a lipid bilayer with thickness of 29.6 Å at 25 °C in hexadecane. We observe that bilayers formed from eTLE or DPhPC survive cooling and heating between 25 and 50 °C and demonstrate gigaohm (GΩ) membrane resistances at all temperatures tested. Additionally, we study the insertion of alamethicin peptides into both eTLE and DPhPC membranes to understand how lipid composition, temperature, and membrane phase influence ion channel formation. Like in DPhPC bilayers, alamethicin peptides in eTLE exhibit discrete, voltage-dependent gating characterized by multiple open channel conductance levels, though at significantly lower applied voltages. Cyclic voltammetry measurements of macroscopic channel currents confirm that the voltage-dependent conductance of alamethicin channels in eTLE bilayers occurs at lower voltages than in DPhPC bilayers at equivalent peptide concentrations. This result suggests that eTLE membranes, via composition, fluidity, or the presence of subdomains, offer an environment that enhances alamethicin insertion. For both membrane compositions, increasing temperature reduces the lifetimes of single channel gating events and increases the voltage required to cause an exponential increase in channel current. However, the fact that alamethicin insertion in eTLE exhibits significantly greater sensitivity to temperature changes through its Tg suggests that membrane phase plays an important role in channel formation. These effects are much less severe in DPhPC, where heating from 25 to 50 °C does not induce a phase change. The described technique for heating-assisted monolayer formation permits the use of other high transition temperature lipids in aqueous droplets for DIB formation, thereby increasing the types of lipids that can be considered for assembling model membranes.

Anthrax toxin-induced rupture of artificial lipid bilayer membranes

We demonstrate experimentally that anthrax toxin complexes rupture artificial lipid bilayer membranes when isolated from the blood of infected animals. When the solution pH is temporally acidified to mimic that process in endosomes, recombinant anthrax toxin forms an irreversibly bound complex, which also destabilizes membranes. The results suggest an alternative mechanism for the translocation of anthrax toxin into the cytoplasm.

Programming nanopore ion flow for encoded multiplex microRNA detection

Many efforts are being made in translating the nanopore into an ultrasensitive single-molecule platform for various genetic and epigenetic detections. However, compared with current approaches including PCR, the low throughput limits the nanopore applications in biological research and clinical settings, which usually requires simultaneous detection of multiple biomarkers for accurate disease diagnostics. Herein we report a barcode probe approach for multiple nucleic acid detection in one nanopore. Instead of directly identifying different targets in a nanopore, we designed a series of barcode probes to encode different targets. When the probe is bound with the target, the barcode group polyethylene glycol attached on the probe through click chemistry can specifically modulate nanopore ion flow. The resulting signature serves as a marker for the encoded target. Therefore counting different signatures in a current recording allows simultaneous analysis of multiple targets in one nanopore. The principle of this approach was verified by using a panel of cancer-derived microRNAs as the target, a type of biomarker for cancer detection.

Dynamic morphologies of microscale droplet interface bilayers

Droplet interface bilayers (DIBs) are a powerful platform for studying the dynamics of synthetic cellular membranes; however, very little has been done to exploit the unique dynamical features of DIBs. Here, we generate microscale droplet interface bilayers (μDIBs) by bringing together femtoliter-volume water droplets in a microfluidic oil channel, and characterize morphological changes of the μDIBs as the droplets shrink due to evaporation. By varying the initial conditions of the system, we identify three distinct classes of dynamic morphology. (1) Buckling and fission: when forming μDIBs using the lipid-out method (lipids in oil phase), lipids in the shrinking monolayers continually pair together and slide into the bilayer to conserve their mass. As the bilayer continues to grow, it becomes confined, buckles, and eventually fissions one or more vesicles. (2) Uniform shrinking: when using the lipid-in method (lipids in water phase) to form μDIBs, lipids uniformly transfer from the monolayers and bilayer into vesicles contained inside the water droplets. (3) Stretching and unzipping: finally, when the droplets are pinned to the wall(s) of the microfluidic channel, the droplets become stretched during evaporation, culminating in the unzipping of the bilayer and droplet separation. These findings offer a better understanding of the dynamics of coupled lipid interfaces.

Bordetella pertussis adenylate cyclase toxin translocation across a tethered lipid bilayer

Numerous bacterial toxins can cross biological membranes to reach the cytosol of mammalian cells, where they exert their cytotoxic effects. Our model toxin, the adenylate cyclase (CyaA) from Bordetella pertussis, is able to invade eukaryotic cells by translocating its catalytic domain directly across the plasma membrane of target cells. To characterize its original translocation process, we designed an in vitro assay based on a biomimetic membrane model in which a tethered lipid bilayer (tBLM) is assembled on an amine-gold surface derivatized with calmodulin (CaM). The assembled bilayer forms a continuous and protein-impermeable boundary completely separating the underlying calmodulin (trans side) from the medium above (cis side). The binding of CyaA to the tBLM is monitored by surface plasmon resonance (SPR) spectroscopy. CyaA binding to the immobilized CaM, revealed by enzymatic activity, serves as a highly sensitive reporter of toxin translocation across the bilayer. Translocation of the CyaA catalytic domain was found to be strictly dependent on the presence of calcium and also on the application of a negative potential, as shown earlier in eukaryotic cells. Thus, CyaA is able to deliver its catalytic domain across a biological membrane without the need for any eukaryotic components besides CaM. This suggests that the calcium-dependent CyaA translocation may be driven in part by the electrical field across the membrane. This study's in vitro demonstration of toxin translocation across a tBLM provides an opportunity to explore the molecular mechanisms of protein translocation across biological membranes in precisely defined experimental conditions.

Robust reagent addition and perfusion strategies for droplet-interface bilayers

We have designed two novel devices which extend the applications for the droplet-interface bilayer (DIB) as a model membrane system. The add-chip allows successive reagent additions to one side of the lipid bilayer during an experiment while maintaining a simple setup with much lower volumes than in planar bilayer systems. The flow-chip is capable of multiple complete solution perfusions concurrently with electrophysiology measurements. Both devices preserve all of the key advantages that DIBs have relative to planar membranes, including low volume, leaflet asymmetry and the ability to separate the monolayers prior to further analysis of a droplet's contents. As a demonstration, we use these devices to monitor and quantitate molecular transport across DIBs.

Ratcheting up protein translocation with anthrax toxin

Energy-consuming nanomachines catalyze the directed movement of biopolymers in the cell. They are found both dissolved in the aqueous cytosol as well as embedded in lipid bilayers. Inquiries into the molecular mechanism of nanomachine-catalyzed biopolymer transport have revealed that these machines are equipped with molecular parts, including adjustable clamps, levers, and adaptors, which interact favorably with substrate polypeptides. Biological nanomachines that catalyze protein transport, known as translocases, often require that their substrate proteins unfold before translocation. An unstructured protein chain is likely entropically challenging to bind, push, or pull in a directional manner, especially in a way that produces an unfolding force. A number of ingenious solutions to this problem are now evident in the anthrax toxin system, a model used to study protein translocation. Here we highlight molecular ratchets and current research on anthrax toxin translocation. A picture is emerging of proton-gradient-driven anthrax toxin translocation, and its associated ratchet mechanism likely applies broadly to other systems. We suggest a cyclical thermodynamic order-to-disorder mechanism (akin to a heat-engine cycle) is central to underlying protein translocation: peptide substrates nonspecifically bind to molecular clamps, which possess adjustable affinities; polypeptide substrates compress into helical structures; these clamps undergo proton-gated switching; and the substrate subsequently expands regaining its unfolded state conformational entropy upon translocation.