Surfactant bilayers maintain transmembrane protein activity

Kinetic study of membrane protein interactions: from three to two dimensions

Molecular interactions are contingent upon the system's dimensionality. Notably, comprehending the impact of dimensionality on protein-protein interactions holds paramount importance in foreseeing protein behaviour across diverse scenarios, encompassing both solution and membrane environments. Here, we unravel interactions among membrane proteins across various dimensionalities by quantifying their binding rates through fluorescence recovery experiments. Our findings are presented through the examination of two protein systems: streptavidin-biotin and a protein complex constituting a bacterial efflux pump. We present here an original approach for gauging a two-dimensional binding constant between membrane proteins embedded in two opposite membranes. The quotient of protein binding rates in solution and on the membrane represents a metric denoting the exploration distance of the interacting sites-a novel interpretation.

Simulating Bilayers of Nonionic Surfactants with the GROMOS-Compatible 2016H66 Force Field

Polyoxyethylene glycol alkyl ether amphiphiles (C_iE_j) are important nonionic surfactants, often used for biophysical and membrane protein studies. In this work, we extensively test the GROMOS-compatible 2016H66 force field in molecular dynamics simulations involving the lamellar phase of a series of C_iE_j surfactants, namely C₁₂E₂, C₁₂E₃, C₁₂E₄, C₁₂E₅, and C₁₄E₄. The simulations reproduce qualitatively well the monitored structural properties and their experimental trends along the surfactant series, although some discrepancies remain, in particular in terms of the area per surfactant, the equilibrium phase of C₁₂E₅, and the order parameters of C₁₂E₃, C₁₂E₄, and C₁₂E₅. The polar head of the C_iE_j surfactants is highly hydrated, almost like a single polyethyleneoxide (PEO) molecule at full hydration, resulting in very compact conformations. Within the bilayer, all C_iE_j surfactants flip-flop spontaneously within tens of nanoseconds. Water-permeation is facilitated, and the bending rigidity is 4 to 5 times lower than that of typical phospholipid bilayers. In line with another recent theoretical study, the simulations show that the lamellar phase of C_iE_j contains large hydrophilic pores. These pores should be abundant in order to reproduce the comparatively low NMR order parameters. We show that their contour length is directly correlated to the order parameters, and we estimate that they should occupy approximately 7-10% of the total membrane area. Due to their highly dynamic nature (rapid flip-flops, high water permeability, observed pore formation), C_iE_j surfactant bilayers are found to represent surprisingly challenging systems in terms of modeling. Given this difficulty, the results presented here show that the 2016H66 parameters, optimized independently considering pure-liquid as well as polar and nonpolar solvation properties of small organic molecules, represent a good starting point for simulating these systems.

Characterization of a Biomimetic Mesophase Composed of Nonionic Surfactants and an Aqueous Solvent

We have investigated the physical and biomimetic properties of a sponge (L₃) phase composed of pentaethylene glycol monododecyl ether (C₁₂E₅), a nonionic surfactant, an aqueous solvent, and a cosurfactant. The following cosurfactants, commonly used for solubilizing membrane proteins, were incorporated: n-octyl-β-d-glucopyranoside (β-OG), n-dodecyl-β-d-maltopyranoside (DDM), 4-cyclohexyl-1-butyl-β-d-maltoside (CYMAL-4), and 5-cyclohexyl-1-pentyl-β-d-maltoside (CYMAL-5). Partial phase diagrams of these systems were created. The L₃ phase was characterized using crossed polarizers, diffusion of a fluorescent probe by fluorescence recovery after pattern photobleaching (FRAPP), and freeze fracture electron microscopy (FFEM). By varying the hydration of the phase, we were able to tune the distance between adjacent bilayers. The characteristic distance (d_b) of the phase was obtained from small angle scattering (SAXS/SANS) as well as from FFEM, which yielded complementary d_b values. These d_b values were neither affected by the nature of the cosurfactant nor by the addition of membrane proteins. These findings illustrate that a biomimetic surfactant sponge phase can be created in the presence of several common membrane protein-solubilizing detergents, thus making it a versatile medium for membrane protein studies.

ATPase Activity Measurements by an Enzyme-Coupled Spectrophotometric Assay

Enzymatic coupled assays are usually based on the spectrophotometric registration of changes in NADH/NAD(+) or NADPH/NADP(+) absorption at 340 nm accompanying the oxidation/reduction of reactants that by dehydrogenases and other helper enzymes are linked to the activity of the enzymatic reaction under study. The present NADH-ATP-coupled assay for ATPase activity is a seemingly somewhat complicated procedure, but in practice adaptation to performance is easily acquired. It is a more safe and elegant method than colorimetric methods, but not suitable for handling large number of samples, and also presupposes that the activity of the helper enzymes is not severely affected by the chemical environment of the sample in which it is tested.

FRAP to Characterize Molecular Diffusion and Interaction in Various Membrane Environments

Fluorescence recovery after photobleaching (FRAP) is a standard method used to study the dynamics of lipids and proteins in artificial and cellular membrane systems. The advent of confocal microscopy two decades ago has made quantitative FRAP easily available to most laboratories. Usually, a single bleaching pattern/area is used and the corresponding recovery time is assumed to directly provide a diffusion coefficient, although this is only true in the case of unrestricted Brownian motion. Here, we propose some general guidelines to perform FRAP experiments under a confocal microscope with different bleaching patterns and area, allowing the experimentalist to establish whether the molecules undergo Brownian motion (free diffusion) or whether they have restricted or directed movements. Using in silico simulations of FRAP measurements, we further indicate the data acquisition criteria that have to be verified in order to obtain accurate values for the diffusion coefficient and to be able to distinguish between different diffusive species. Using this approach, we compare the behavior of lipids in three different membrane platforms (supported lipid bilayers, giant liposomes and sponge phases), and we demonstrate that FRAP measurements are consistent with results obtained using other techniques such as Fluorescence Correlation Spectroscopy (FCS) or Single Particle Tracking (SPT). Finally, we apply this method to show that the presence of the synaptic protein Munc18-1 inhibits the interaction between the synaptic vesicle SNARE protein, VAMP2, and its partner from the plasma membrane, Syn1A.

Stability of C(12)E(j) Bilayers Probed with Adhesive Droplets

The stability of model surfactant bilayers from the poly(ethylene glycol) mono-n-dodecyl ether (C12Ej) family was probed. The surfactant bilayers were formed by the adhesion of emulsion droplets. We generated C12Ej bilayers by forming water-in-oil (w/o) emulsions with saline water droplets, covered by the surfactant, in a silicone and octane oil mixture. Using microfluidics, we studied the stability of those bilayers. C12E1 allowed only short-lived bilayers whereas C12E2 bilayers were stable over a wide range of oil mixtures. At high C12E2 concentration, a two-phase region was displayed in the phase diagram: bilayers formed by the adhesion of two water droplets and Janus-like particles consisting of adhering aqueous and amphiphilic droplets. C12E8 and C12E25 did not mediate bilayer formation and caused phase inversion leading to o/w emulsion. With intermediate C12E4 and C12E5 surfactants, both w/o and o/w emulsions were unstable. We provided the titration of the C12E2 bilayer with C12E4 and C12E5 to study and predict their stability behavior.