New Tethered Phospholipid Bilayers Integrating Functional G-Protein-Coupled Receptor Membrane Proteins

Membrane proteins exhibiting extra- and intracellular domains require an adequate near-native lipid platform for their functional reconstitution. With this aim, we developed a new technology enabling the formation of a peptide-tethered bilayer lipid membrane (pep-tBLM), a lipid bilayer grafted onto peptide spacers, by way of a metal-chelate interaction. To this end, we designed an original peptide spacer derived from the natural α-laminin thiopeptide (P19) possessing a cysteine residue in the N-terminal extremity for grafting onto gold and a C-terminal extremity modified by four histidine residues (P19-4H). In the presence of nickel, the use of this anchor allowed us to bind liposomes of variable compositions containing a 2% molar ratio of a chelating lipid, 1,2-dioleoyl-sn-glycero-3-[(N-(5-amino-1-carboxypentyl)iminodiacetic acid)succinyl] so-called DOGS-NTA, and to form the planar bilayer by triggering liposome fusion by an α-helical (AH) peptide derived from the N-terminus of the hepatitis C virus NS5A protein. The formation of pep-tBLMs was characterized by surface plasmon resonance imaging (SPRi), and their continuity, fluidity, and homogeneity were demonstrated by fluorescence recovery after photobleaching (FRAP), with a diffusion coefficient of 2.5 × 10^-7 cm²/s, and atomic force microscopy (AFM). By using variable lipid compositions including phosphatidylcholine (PC), phosphatidylserine (PS), phosphatidylethanolamine (PE), phosphatidylinositol 4,5-bisphosphate (PIP₂), sphingomyelin (SM), phosphatidic acid (PA), and cholesterol (Chol) in various ratios, we show that the membrane can be formed independently from the lipid composition. We made the most of this advantage to reincorporate a transmembrane protein in an adapted complex lipid composition to ensure its functional reinsertion. For this purpose, a cell-free expression system was used to produce proteoliposomes expressing the functional C-X-C motif chemokine receptor 4 (CXCR4), a seven-transmembrane protein belonging to the large superfamily of G-protein-coupled receptors (GPCRs). We succeeded in reinserting CXCR4 in pep-tBLMs formed on P19-4H by the fusion of tethered proteoliposomes. AFM and FRAP characterization allowed us to show that pep-tBLMs inserting CXCR4 remained fluid, homogeneous, and continuous. The value of the diffusion coefficient determined in the presence of reinserted CXCR4 was 2 × 10^-7 cm²/s. Ligand binding assays using a synthetic CXCR4 antagonist, T22 ([Tyr5,12, Lys7]-polyphemusin II), revealed that CXCR4 can be reinserted in pep-tBLMs with functional folding and orientation. This new approach represents a method of choice for investigating membrane protein reincorporation and a promising way of creating a new generation of membrane biochips adapted for screening agonists or antagonists of transmembrane proteins.

Effect of Supporting Polyelectrolyte Multilayers and Deposition Conditions on the Formation of 1-Palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine/1-Palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine Lipid Bilayers

The formation of complete supported lipid bilayers by vesicle adsorption and rupture was studied in relation to deposition conditions of vesicles and underlying cushion formed from various polyelectrolytes. Lipid vesicles were formed from zwitterionic 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and negatively charged 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) in phosphate buffer of various pH with or without NaCl addition. Polyelectrolyte multilayer films (PEM) were constructed by sequential adsorption of alternately charged polyelectrolytes from their solutions-layer-by-layer deposition (LBL). The mechanism of the formation of supported lipid bilayer on polyelectrolyte films was studied by quartz crystal microbalance with dissipation monitoring (QCM-D) and atomic force microscopy (AFM). QCM-D allowed following the adsorption kinetics while AFM measurements verified the morphology of lipid vesicles and isolated bilayer patches on the PEM cushions providing local topological images in terms of lateral organization. Additionally, polyelectrolyte cushions were characterized with ellipsometry to find thickness and swelling properties, and their roughness was determined using AFM. It has been demonstrated that the pH value and an addition of NaCl in the buffer solution as well as the type of the polyelectrolyte cushion influence the kinetics of bilayer formation and the quality of formed bilayer patches.

Probing the association of triblock copolymers with supported lipid membranes using microcantilevers

Pluronics are a class of amphiphilic triblock copolymers that are known to interact with cellular membranes in interesting ways. The solubility of these triblock copolymers in free lipid membranes can be altered with temperature, allowing the possibility of tuning their membrane insertion. However, for supported lipid membranes, the asymmetric local environment and the strong influence of the solid support can alter the solubility of these triblock copolymers in lipid membranes. Here, we probe the interactions of these copolymers with supported lipid membranes using microcantilevers and fluorescence recovery after photobleaching (FRAP) measurements. We measure the solubility and interactions of triblock copolymers (F68 and F98) in supported lipid bilayers as a function of temperature and the length of the copolymer lipophilic block. A Langmuir isotherm model and a free mean area theory are applied to describe the polymer-lipid interactions at the microcantilever surface, determine association constants, and analyze the effect of triblock copolymers on lateral lipid diffusion.

Influence of phase separating lipids on supported lipid bilayer formation at SiO2 surfaces

The importance of the lipid phase on the formation of supported lipid bilayers (SLBs) via vesicle fusion and on the resulting SLB homogeneity at SiO(2) surfaces has been studied by the quartz crystal microbalance with dissipation (QCM-D) monitoring technique. Physiologically relevant lipid compositions were chosen to correspond to different regions (l(d), l(o) and coexistence of phases) in established phase diagrams of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), N-palmitoyl-D-erythro-sphingosylphosphorylcholine (PSM) and cholesterol. For most compositions, SLBs formed through vesicle rupture in a critical-surface-coverage dependent manner. Inclusion of PSM and cholesterol into POPC vesicles significantly impaired the vesicle rupture process such that a higher critical concentration of vesicles on the surface was needed before the rupture process started. When increasing the cholesterol content the vesicles formed SLBs containing more defects in the form of intact vesicles adsorbed on the surface up to a point (l(o) phase) where vesicles did not break at all but formed supported vesicular layers. The hampering of vesicle rupture is interpreted in terms of the ability of cholesterol to accommodate vesicle deformation. Experiments using elevated temperatures to alter the lipid membrane into a more fluid phase significantly improved the quality of the SLB showing the importance of both cholesterol content and the lipid phase on SLB homogeneity.

Sensing based on assessment of non-monotonous effect determined by target analyte: case study on pore-forming compounds

A new and exciting biosensing avenue based on assessment of the non-monotonous, concentration dependent effect of pore formation is discussed. A novel kinetic model is advanced to relate surface plasmon resonance (SPR) data with actual concentrations of interacting partners. Lipid modified L1 sensor chip provide the accessible platform for SPR exploration of peptide-membrane interaction, with POPC and melittin as model systems. We show that quantitative assessment of the interaction between an antimicrobial peptide and lipid modified sensors is capable to provide both sensing avenues and detailed mechanistic insights into effects of pore-forming compounds. The proposed model combined with appropriate design of the experimental protocol adds a new depth to the classic SPR investigation of peptide-lipid interaction offering a quantitative platform for detection, improved understanding of the manifold facets of the interaction and for supporting the controlled design of novel antimicrobial compounds. This biosensing approach can be applied to an entire set of pore-forming compounds including antimicrobial peptides and exo-toxins.

Manipulation and charge determination of proteins in photopatterned solid supported bilayers

This work demonstrates the use of deep UV micropatterned chlorotrimethylsilane (TMS) monolayers to support lipid membranes on SiO(2) surfaces. After immersing such a patterned surface into a solution containing small unilamellar vesicles of egg PC, supported bilayer lipid membranes were formed on the hydrophilic, photolyzed regions and lipid monolayer over the hydrophobic, non-photolyzed regions. A barrier between the lipid monolayer and bilayer regions served to stop charged lipids migrating between the two. This allows the system to be used to separate charged lipids or proteins by electrophoresis. Either oppositely charged fluorescence labeled lipids [Texas Red DHPE (negative charge) and D291 (positive charge)] or lipids with different charge numbers [Texas Red DHPE (one negative charge) and NBD PS (two negative charges)] can be separated. We have also studied the migration of streptavidin attached to a biotinylated lipid. Negatively charged streptavidin responds to the applied electric field by moving in the direction of electroosmotic flow, i.e. towards the negative electrode. However the direction of streptavidin movement can be controlled by altering the difference in zeta potential between that of the streptavidin (zeta(1)) and the lipid membrane (zeta(2)). If zeta(1) > zeta(2), streptavidin moves to the negative electrode, while if zeta(1) < zeta(2), streptavidin moves to the positive electrode. This balance was manipulated by adding positively charged lipid DOTAP to the membrane. After measuring the average drift velocity of streptavidin as a function of DOTAP concentration, the point where zeta(1) approximately zeta(2) was found. At this point zeta(1) was calculated to be -9.8 mV which is in good agreement with the value of -13 mV from force measurements and corresponds to a charge of -2e per streptavidin, thus demonstrating the applicability of this method for determining protein charge.