bulk cell-free expression reactions with small unilamellar vesicles (suvs) for synthetic cell applications

Cell-Free Expression of Membrane Proteins in Giant Unilamellar Vesicles

Bottom-up synthetic biology has gained increasing interest over the past decade as an emerging field with numerous potential applications in bioengineering, drug delivery, and regenerative medicine1,2. The development of synthetic cells as a cornerstone of bottom-up synthetic biology, in particular, has attracted a wide range of scientific communities due to the promising applications of synthetic cells as well as their cell-like physical and biochemical properties that facilitate in vitro biophysical studies3,4,5,6. Synthetic cells are often engineered in cell-sized giant unilamellar vesicles (GUVs) in which different biological processes are recreated. Reconstitution of cell cytoskeleton7,8, light-dependent energy regeneration9, cellular communication10,11, and biosensing12 are examples of efforts made to reconstruct cell-like behaviors in synthetic cells.
While some cellular processes rely on soluble proteins, many characteristics of natural cells, such as sensing and communication, often utilize membrane proteins, including ion channels, receptors, and transporters. A major challenge in synthetic cell development is the reconstitution of membrane proteins. Although traditional methods of membrane protein reconstitution in lipid bilayers rely on detergent-mediated purification, such methods are laborious, ineffective for proteins that are toxic to the expression host, or are often not suited for membrane protein reconstitution in GUVs13.
An alternative method for protein expression is cell-free expression (CFE) systems. CFE systems have been a powerful tool in synthetic biology that allows in vitro expression of various proteins using either cell lysate or purified transcription-translation machinery14. CFE systems can also be encapsulated in GUVs, thus allowing compartmentalized protein synthesis reactions that can be programmed for various applications, such as the creation of light-harvesting synthetic cells9 or mechanosensitive biosensors15,16. Analogous to recombinant protein expression methods, membrane protein expression is challenging in CFE systems17. Aggregation, misfolding, and lack of post-translational modification in CFE systems are major bottlenecks that hinder successful membrane protein synthesis using CFE systems. The difficulty of bottom-up membrane protein reconstitution using CFE systems is due in part to the absence of a complex membrane protein biogenesis pathway that relies on signal peptides, signal recognition particles, translocons, and chaperoning molecules. However, recently, multiple studies have suggested that the presence of membranous structures such as microsomes or liposomes during translation promotes successful membrane protein expression18,19,20,21. Additionally, Eaglesfield et al. and Steink&#252;her et al. have found that the inclusion of specific hydrophobic domains known as signal peptides in the N-terminus of the membrane protein can significantly improve its expression22,23. Altogether, these studies suggest that the challenge of membrane protein reconstitution in synthetic cells can be overcome if the protein translation occurs in the presence of the GUV membrane and if proper N-terminal signal peptide is utilized.
Here, a protocol for encapsulation of the protein synthesis using recombinant elements (PURE) CFE reactions for membrane protein reconstitution in GUVs is presented. Bacterial glutamate receptor24 (GluR0) is selected as the model membrane protein, and the effect of its N-terminal signal peptide on its membrane reconstitution is studied. The effect of proteorhodopsin signal peptide, which was shown to improve membrane protein reconstitution efficiency by Eaglesfield et al.22, is investigated by constructing a mutated variant of GluR0 denoted as PRSP-GluR0 and its expression and membrane localization with wild-type GluR0 (referred to as WT-GluR0 hereafter) that harbors its native signal peptide is compared. This protocol is based on the inverted emulsion method25 with modifications that make it robust for CFE encapsulation. In the presented method, the CFE reactions are first emulsified using a lipid-in-oil solution that generates micron-sized droplets that contain the CFE system and are stabilized by the lipid monolayer. The emulsion droplets are then layered on top of an oil-water interface that is saturated with another lipid monolayer. The emulsion droplets are then forced to travel across the oil-water interface via centrifugal force. Through this process, the droplets obtain another monolayer, thus generating a bilayer lipid vesicle. The GUVs containing the CFE reaction are then incubated, during which the membrane protein is expressed and incorporated into the GUV membrane. Although this protocol is specified for cell-free expression of GluR0, it can be used for cell-free synthesis of other membrane proteins or different synthetic cell applications such as cytoskeleton reconstitution or membrane fusion studies26.

Author Spotlight: Membrane Protein Reconstitution in Synthetic Cells

Reconstitution of the Bacterial Glutamate Receptor Channel by Encapsulation of a Cell-Free Expression System

Our protocol addresses the reconstitution of a membrane protein within the lipid bilayer of a synthetic cell made using the inverted droplet emulsion method. The advantage of using the inverted droplet emulsion method is its ability to generate synthetic cells that provide a stable environment for the cell-free expression of membrane proteins. Our protocol offers a robust method for researchers in the field to synthesize membrane proteins inside synthetic cells using cell-free expression system.My lab is working on reconstituting various ion channels in synthetic cells through the reconstitution of the action potential by a well-defined set of ion channels.

Bulk Cell-Free Expression Reactions with Small Unilamellar Vesicles (SUVs) for Synthetic Cell Applications

To begin, blow a gentle stream of Argonne into a glass vial containing POPC lipids while gently rotating the vial. When the lipids have dried as a film at the bottom of the vial, loosely cap it, then transfer it to a desiccater. After one hour add 0.5 milliliters of ultrapure deionized water to dissolve the lipid film.Vortex the solution for two minutes. Next, soak two filter supports and deionized water. Place them on each of the internal membrane supports.Then place a 100 nanometer polycarbonate filter between the two internal membrane supports, held together by the extruder outer casing and retainer nut. Place the setup in the extruder stand to finish setting up the mini extrusion apparatus. Next, load the lipid water mix into a one milliliter gast tight syringe.Use swing arm clips to secure the syringe into one end of the mini extruder. Insert the second syringe into the other end of the mini extruder, making sure that it is fully depressed. Now pass the lipid water mix from the original syringe into the empty one through the apparatus.After repeating the extrusion 11 times, transfer the SUVs to a 1.5 milliliter micro centrifuge tube. To assemble the cell-free expression protocol, pipette solutions one to 3 DNA plasmid encoding for soluble SFGFP or variants of glutamate receptor SFGFP, murine RNAse, and SUV solution into a vial. Add ultrapure deionized water to bring the reaction volume to 20 microliters.Transfer the cell-free expression solution to a 96 well conical V bottom plate. Incubate the plate in a plate reader at 37 degrees Celsius for four to five hours. Set up the plate reader with a gain of 100, excitation of 485 nanometers, emission of 528 nanometers, a runtime of five hours, and an interval measurement of one read for every two minutes.Measure the GFP signal every two minutes to monitor the cell-free reaction in real time. Soluble SFGFP had the highest expression among all three proteins.

bulk-cell-free-expression-reactions-with-small-unilamellar-vesicles

Research

JoVE Journal

Bioengineering

Cell-free expression (CFE) systems are powerful tools in synthetic biology that allow biomimicry of cellular functions like biosensing and energy regeneration in synthetic cells. Reconstruction of a wide range of cellular processes, however, requires successful reconstitution of membrane proteins into the membrane of synthetic cells. While the expression of soluble proteins is usually successful in common CFE systems, the reconstitution of membrane proteins in lipid bilayers of synthetic cells has proven to be challenging. Here, a method for reconstitution of a model membrane protein, bacterial glutamate receptor (GluR0), in giant unilamellar vesicles (GUVs) as model synthetic cells based on encapsulation and incubation of the CFE reaction inside synthetic cells is demonstrated. Utilizing this platform, the effect of substituting the N-terminal signal peptide of GluR0 with proteorhodopsin signal peptide on successful cotranslational translocation of GluR0 into membranes of hybrid GUVs is demonstrated. This method provides a robust procedure that will allow cell-free reconstitution of various membrane proteins in synthetic cells.

This protocol describes the inverted emulsion method used to encapsulate a cell-free expression (CFE) system within a giant unilamellar vesicle (GUV) for the investigation of the synthesis and incorporation of a model membrane protein into the lipid bilayer.

Cell-free expression (CFE) systems are powerful tools in synthetic biology that allow biomimicry of cellular functions like biosensing and energy ...

Cell-Free Reactions in Giant Unilamellar Vesicles (GUVs) for Synthetic Cell Applications

To begin, prepare the GUV outer buffer solution in a 1.5-milliliter microcentrifuge tube. Mixed together spermidine, ATP, GTP, CTP, UTP, creatine phosphate, magnesium acetate, potassium glutamate, HEPES, potassium hydroxide, folinic acid. glucose, and amino acid stock.Add ultra purified deionized water to bring the final volume of the solution to one milliliter. Next, in a fume hood, add 17.3 microliters of POPC stock solution to a 15-milliliter glass vial. To this, pipette 1.08 microliters of PEO-b-PBD copolymer.Carefully blow a gentle stream of argon gas into the glass vial while rotating the vial to evaporate the chloroform. Then pipette 1.2 milliliters of light mineral oil into the glass vial. Vortex the lipid and oil at maximum speed for 10 to 20 seconds.Place the glass vial in an oven at around 50 degrees Celsius for 20 minutes before vortexing for an additional 10 to 20 seconds at maximum speed. Next, pipette 270 microliters of the GUV outer solution into a 1.5-milliliter microcentrifuge tube. To this, pipette 15 microliters each of five molar sodium chloride and 4.5 molar potassium chloride.Gently pipette 300 microliters of lipid and oil mixture on top of the GUV outer solution. After incubation, assemble a CFE reaction by pipetting Solutions 1 to 3, DNA plasmid encoding for soluble sfGFP or variants of glutamate receptor sfGFP, murine RNAse, and one molar sucrose into a vial. Add ultrapure deionized water to bring the reaction volume to 20 microliters.Add 600 microliters of the lipid-oil mixture to the microcentrifuge tube containing the 20-microliter reaction mixture. Pipette up and down vigorously for one minute to emulsify the reaction. Gently layer the inner emulsion on top of the oil layer in the tube.Centrifuge for 10 minutes at 2000 g at four degrees Celsius. Next, carefully pipette out the excess oil and outer solution from the microcentrifuge tube. Gently mix the GUV pellet in the remaining solution to resuspend it.Transfer the GUV solution to a clean 96-well clear flat bottom plate for incubation. Transfer the sealed plate into a plate reader to incubate at 37 degrees Celsius for five to six hours. After incubation, place the plate on an inverted confocal microscope.Focus on a region of interest containing the GUVs, then capture the images at an excitation wavelength of 488 nanometers. Save the images in the TIFF format. Open the images in an image processing software.Click on the Brightness/Contrast setting panel to open it. Then adjust the brightness and contrast to make the fluorescent proteins visible. The wild-type glutamate receptor cell-free expression reactions encapsulated in GUV demonstrated excellent expression and membrane localization.PRSP-glutamate receptor was prone to aggregation and punctate formation. The soluble sfGFP was expressed in GUVS and stayed in the GUV lumen.