Reconstitution of the Bacterial Glutamate Receptor Channel by Encapsulation of a Cell-Free Expression System
Summary
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.
Abstract
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.
Introduction
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 Steinküher 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.
Protocol
Representative Results
Discussion
Virtually any cellular process that depends on the transfer of molecules or information across the cell membrane, like cell signaling or cell excitation, requires membrane proteins. Thus, the reconstitution of membrane proteins has become the main bottleneck in realizing various synthetic cell designs for different applications. Traditional detergent-mediated reconstitution of membrane proteins in biological membranes requires GUV generation methods such as gentle swelling or electroformation. Swelling approaches usually…
Divulgazioni
The authors have nothing to disclose.
Acknowledgements
APL acknowledges support from the National Science Foundation (EF1935265), the National Institutes of Health (R01-EB030031 and R21-AR080363), and the Army Research Office (80523-BB)
Materials
| 100 nm polycarbonate filter | STERLITECH | 1270193 | |
| 96 Well Clear Bottom Plate | ThermoFisher Scientific | 165305 | |
| BioTek Synergy H1M Hybrid Multi-Mode Reader | Agilent | 11-120-533 | |
| Creatine phosphate | Millipore Sigma | 10621714001 | |
| CSU-X1 Confocal Scanner Unit | Yokogawa | CSU-X1 | |
| Density gradient medium (Optiprep) | Millipore Sigma | D1556 | Optional to switch with sucrose in inner solution |
| Filter supports | Avanti | 610014 | |
| Fisherbrand microtubes (1.5 mL) | Fisher Scientific | 05-408-129 | |
| Folinic acid calcium salt hydrate | Millipore Sigma | F7878 | |
| Glucose | Millipore Sigma | 158968 | |
| HEPES | Millipore Sigma | H3375 | |
| iXon X3 camera | Andor | DU-897E-CS0 | |
| L-Glutamic acid potassium salt monohydrate | Millipore Sigma | G1501 | |
| Light mineral oil | Millipore Sigma | M5904 | |
| Magnesium acetate tetrahydrate | Millipore Sigma | M5661 | |
| Mini-extruder kit (including syringe holder and extruder stand) | Avanti | 610020 | |
| Olympus IX81 Inverted Microscope | Olympus | IX21 | |
| Olympus PlanApo N 60x Oil Microscope Objective | Olympus | 1-U2B933 | |
| PEO-b-PBD | Polymer Source | P41745-BdEO | |
| pET28b-PRSP-GluR0-sfGFP plasmid DNA | Homemade | N/A | |
| pET28b-sfGFP-sfCherry(1-10) plasmid DNA | Homemade | N/A | |
| pET28b-WT-GluR0-sfGFP plasmid DNA | Homemade | N/A | |
| POPC lipid in chloroform | Avanti | 850457C | |
| Potassium chloride | Millipore Sigma | P9541 | |
| PUREfrex 2.0 | Cosmo Bio USA | GFK-PF201 | |
| Ribonucleotide Solution Set | New England BioLabs | N0450 | |
| RNase Inhibitor, Murine | New England BioLabs | M0314S | |
| RTS Amino Acid Sampler | Biotechrabbit | BR1401801 | |
| Sodium chloride | Millipore Sigma | S9888 | |
| Spermidine | Millipore Sigma | S2626 | |
| Sucrose | Millipore Sigma | S0389 | |
| VAPRO Vapor Pressure Osmometer Model 5600 | ELITechGroup | VAPRO 5600 |
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