Methods to study folding of alpha-helical membrane proteins in lipids

How alpha-helical membrane proteins fold correctly in the highly hydrophobic membrane interior is not well understood. Their folding is known to be highly influenced by the lipids within the surrounding bilayer, but the majority of folding studies have focused on detergent-solubilized protein rather than protein in a lipid environment. There are different ways to study folding in lipid bilayers, and each method has its own advantages and disadvantages. This review will discuss folding methods which can be used to study alpha-helical membrane proteins in bicelles, liposomes, nanodiscs or native membranes. These folding methods include in vitro folding methods in liposomes such as denaturant unfolding studies, and single-molecule force spectroscopy studies in bicelles, liposomes and native membranes. This review will also discuss recent advances in co-translational folding studies, which use cell-free expression with liposomes or nanodiscs or are performed in vivo with native membranes.

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.

3D printed microfluidic devices for lipid bilayer recordings

This paper verifies the single-step and monolithic fabrication of 3D structural lipid bilayer devices using stereolithography. Lipid bilayer devices are utilized to host membrane proteins in vitro for biological assays or sensing applications. There is a growing demand to fabricate functional lipid bilayer devices with a short lead-time, and the monolithic fabrication of components by 3D printing is highly anticipated. However, the prerequisites of 3D printing materials which lead to reproducible lipid bilayer formation are still unknown. Here, we examined the feasibility of membrane protein measurement using lipid bilayer devices fabricated by stereolithography. The 3D printing materials were characterized and the surface smoothness and hydrophobicity were found to be the relevant factors for successful lipid bilayer formation. The devices were comparable to the ones fabricated by conventional procedures in terms of measurement performances like the amplitude of noise and the waiting time for lipid bilayer formation. We further demonstrated the extendibility of the technology for the functionalization of devices, such as incorporating microfluidic channels for solution exchangeability and arraying multiple chambers for robust measurement.

Cell-Free Synthesis Strategies to Probe Co-translational Folding of Proteins Within Lipid Membranes

In order to comprehend the molecular basis of transmembrane protein biogenesis, methods are required that are capable of investigating the co-translational folding of these hydrophobic proteins. Equally, in artificial cell studies, controllable methods are desirable for in situ synthesis of membrane proteins that then direct reactions in the synthetic cell membrane. Here we describe a method that exploits cell-free expression systems and tunable membrane mimetics to facilitate co-translational studies. Alteration of the lipid bilayer composition improves the efficiency of the folding system. The approach also enables membrane transport proteins to be made and inserted into artificial cell platforms such as droplet interface bilayers. Importantly, this gives a new facet to the droplet networks by enabling specific transport of molecules across the synthetic bilayer against a concentration gradient. This method also includes a protocol to pause and restart translation of membrane proteins at specified positions during their co-translational folding. This stop-start strategy provides an avenue to investigate whether the proteins fold in sequence order, or if the correct fold of N-terminal regions is reliant on the synthesis of downstream residues.

Layer-by-layer assembly of multi-layered droplet interface bilayers (multi-DIBs)

Droplet interface bilayers (DIBs) have tremendous promise as platforms for fundamental biomembrane studies and in biotechnology. Being composed of a single bilayer however limits their biomimetic potential, as many cell membrane motifs are composed of multiple aligned bilayers. We describe a technology to manufacture cell-sized multi-layered DIBs (multi-DIBs) by coating giant unilamellar vesicles with a further monolayer, and allowing such structures to make contact with themselves or a monolayer coated droplet. This easily customisable strategy will pave the way for an expanded repertoire of DIB functionality, for example by facilitating the incorporation of multiple-bilayer spanning protein complexes.

Insect odorant receptor-based biosensors: Current status and prospects

Whilst the senses of vision and hearing have been successfully automated and miniaturized in portable formats (e.g. smart phone), this is yet to be achieved with the sense of smell. This is because the sensing challenge is not trivial as it involves navigating a chemosensory space comprising thousands of volatile organic compounds. Distinct aroma recognition is based on detecting unique combinations of volatile organic compounds. In natural olfactory systems this is accomplished by employing odorant receptors (ORs) with varying specificities, together with combinatorial neural coding mechanisms. Attempts to mimic the remarkable sensitivity and accuracy of natural olfactory systems has therefore been challenging. Current portable chemical sensors for odorant detection are neither sensitive nor selective, prompting research exploring artificial olfactory devices that use natural OR proteins for sensing. Much research activity to develop OR based biosensors has concentrated on mammalian ORs, however, insect ORs have not been explored as extensively. Insects possess an extraordinary sense of smell due to a repertoire of odorant receptors evolved to interpret olfactory cues vital to the insects' survival. The potential of insect ORs as sensing elements is only now being unlocked through recent research efforts to understand their structure, ligand binding mechanisms and development of odorant biosensors. Like their mammalian counterparts, there are many challenges with working with insect ORs. These include expression, purification and presentation of the insect OR in a stable display format compatible with an effective transduction methodology while maintaining OR structure and function. Despite these challenges, significant progress has been demonstrated in developing OR-based biosensors which exploit insect ORs in cells, lipid bilayers, liposomes and nanodisc formats. Ultrasensitive and highly selective detection of volatile organic compounds has been validated by coupling these insect OR display formats with transduction methodologies spanning optical (fluorescence) and electrical (field effect transistors, electrochemical impedance spectroscopy) techniques. This review summarizes the current status of insect OR based biosensors and their future outlook.

Integrating Membrane Transporter Proteins into Droplet Interface Bilayers

Droplet interface bilayers (DIBs) are an emerging tool within synthetic biology that aims to recreate biological processes in artificial cells. A critical component for the utility of these bilayers is controlled flow between compartments and, notably, uphill transport against a substrate concentration gradient. A versatile method to achieve the desired flow is to exploit the specificity of membrane proteins that regulate the movement of ions and transport of specific metabolic compounds. Methods have been in existence for some time to synthesize proteins within a droplet as well as incorporate membrane proteins into DIBS; however, there have been few reports combining synthesis and DIB incorporation for membrane transporters that demonstrate specific, uphill transport. This chapter presents two methods for the incorporation of a membrane transporter into a simple two-droplet DIB system, with the downhill and uphill transport reaction readily monitored by fluorescence microscopy.

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.

Bioluminescent detection of isothermal DNA amplification in microfluidic generated droplets and artificial cells

Microfluidic droplet generation affords precise, low volume, high throughput opportunities for molecular diagnostics. Isothermal DNA amplification with bioluminescent detection is a fast, low-cost, highly specific molecular diagnostic technique that is triggerable by temperature. Combining loop-mediated isothermal nucleic acid amplification (LAMP) and bioluminescent assay in real time (BART), with droplet microfluidics, should enable high-throughput, low copy, sequence-specific DNA detection by simple light emission. Stable, uniform LAMP–BART droplets are generated with low cost equipment. The composition and scale of these droplets are controllable and the bioluminescent output during DNA amplification can be imaged and quantified. Furthermore these droplets are readily incorporated into encapsulated droplet interface bilayers (eDIBs), or artificial cells, and the bioluminescence tracked in real time for accurate quantification off chip. Microfluidic LAMP–BART droplets with high stability and uniformity of scale coupled with high throughput and low cost generation are suited to digital DNA quantification at low template concentrations and volumes, where multiple measurement partitions are required. The triggerable reaction in the core of eDIBs can be used to study the interrelationship of the droplets with the environment and also used for more complex chemical processing via a self-contained network of droplets, paving the way for smart soft-matter diagnostics.

Preparation of cationic proteoliposomes using cell-free membrane protein synthesis: the chaperoning effect of cationic liposomes

Membrane protein reconstituted cationic liposomes are constructed using cell-free membrane protein synthesis in the presence of cationic liposomes. The chaperon effect of cationic liposomal membrane assists in folding the functional conformation of membrane protein. This preparation method enables the provision of the usage of proteoliposomes for drug delivery.

The preparation method of cationic proteoliposomes is established using a cell-free membrane protein synthesis in the presence of cationic liposomes.

Construction of a Biohybrid Odorant Sensor Using Biological Olfactory Receptors Embedded into Bilayer Lipid Membrane on a Chip

This paper describes an odorant sensor based on mosquito olfactory receptors (ORs) that is sensitive to the volatile organic compound octenol. The ORs and OR coreceptors were reconstructed in the lipid bilayer membrane in a chamber device equipped with electrodes. Using this odorant sensor, we obtained ion current changes caused by specific OR responses to octenol. We installed the odorant sensor into a mobile robot and succeeded in the demonstration of coupling octenol gas detection and robot actuation. We believe that this biohybrid odorant sensing system will be a key technology for future artificial olfaction.

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.