Distribution, lateral mobility and function of membrane proteins incorporated into giant unilamellar vesicles

Dynamic microcompartmentalization of giant unilamellar vesicles by sol-gel transition and temperature induced shrinking/swelling of poly(N-isopropyl acrylamide)

Giant unilamellar vesicles (GUVs) were microinjected with aqueous solutions of poly(-isopropyl acrylamide) (PNIPAAm). Temperature-dependent sol-gel phase transitions of the solutions, followed by shrinking and swelling of the resulting hydrogel, were studied in the presence of a variety of co-solutes within the GUV. Reversible formation of a dense, spherical hydrogel structure (compartment) was observed in all cases with defined shrinking/swelling behaviour at temperatures above the lower critical solution temperatures (LCSTs). Nanotube-mediated merging of two vesicles with thus formed compartments resulted in a single GUV with two internalized hydrogel structures. As an application example, we demonstrate how fluorescent nanoparticles can be immobilized in such gel structures.

Protein mobility and diffusive barriers inEscherichia coli: consequences of osmotic stress

The effect of osmotic stress on the intracellular diffusion of proteins in Escherichia coli was studied, using a pulsed version of fluorescence recovery after photo-bleaching, pulsed-FRAP. This method employs sequences of laser pulses which only partly bleach the fluorophores in a cell. Because the cell size and geometry are taken into account, pulsed-FRAP enables to measure diffusion in very small cells of different shapes. We found that upon an osmotic upshock from 0.15 to 0.6 Osm, imposed by NaCl or sorbitol, the apparent intracellular diffusion (D) of mobile green fluorescent protein (GFP) decreased from 3.2 to 0.4 microm(2) s(-1), whereas the membrane permeable glycerol had no effect. Exposing E. coli cells to higher osmolalities (> 0.6 Osm) led to compartmentalization of the GFP into discrete pools, from where the GFP could not escape. Although free diffusion through the cell was hindered, the mobility of GFP in these pools was still relatively high (D approximately 0.4 microm(2) s(-1)). The presence of osmoprotectants restored the effect of osmotic stress on the protein mobility and apparent compartmentalization. Also, lowering the osmolality from 0.6 Osm back to 0.15 Osm restored the mobility of GFP. The implications of these findings in terms of heterogeneities and diffusive barriers inside the cell are discussed.

On the decrease in lateral mobility of phospholipids by sugars

Upon cold and drought stress, sucrose and trehalose protect membrane structures from fusion and leakage. Similarly, these sugars protect membrane proteins from inactivation during dehydration. We studied the interactions between sugars and phospholipid membranes in giant unilamellar vesicles with the fluorescent lipid analog 3,3'-dioctadecyloxacarbocyanine perchlorate incorporated. Using fluorescence correlation spectroscopy, it was found that sucrose decreased the lateral mobility of phospholipids in the fully rehydrated, liquid crystalline membrane more than other sugars did, including trehalose. To describe the nature of the difference in the interaction of phospholipids with sucrose and trehalose, atomistic molecular dynamics studies were performed. Simulations up to 100 ns showed that sucrose interacted with more phospholipid headgroups simultaneously than trehalose, resulting in a larger decrease of the lateral mobility. Using coarse-grained molecular dynamics, we show that this increase in interactions can lead to a relatively large decrease in lateral phospholipid mobility.

Dual-color fluorescence-burst analysis to probe protein efflux through the mechanosensitive channel MscL

The mechanosensitive channel protein of large conductance, MscL, from Escherichia coli has been implicated in protein efflux, but the passage of proteins through the channel has never been demonstrated. We used dual-color fluorescence-burst analysis to evaluate the efflux of fluorescent labeled compounds through MscL. The method correlates the fluctuations in intensity of fluorescent labeled membranes and encapsulated (macro)molecules (labeled with second fluorophore) for each liposome diffusing through the observation volume. The analysis provides quantitative information on the concentration of macromolecules inside the liposomes and the fraction of functional channel proteins. For MscL, reconstituted in large unilamellar vesicles, we show that insulin, bovine pancreas trypsin inhibitor, and other compounds smaller than 6.5 kDa can pass through MscL, whereas larger macromolecules cannot.

Direct reconstitution of plasma membrane lipids and proteins in nanotube-vesicle networks

We demonstrate here that nanotube-vesicle networks can be constructed directly from plasma membranes of cultured cells. We used a combination of dithiothreitol (DTT) and formaldehyde to produce micron-sized plasma membrane vesicles that were subsequently shaped into networks using micromanipulation methods previously used on purely synthetic systems. Only a single cell is required to derive material sufficient to build a small network. This protocol covers the advantages of reconstitution in vesicles, such as full control over the solution environment, while keeping the proteins in their original surroundings with the proper orientation. Furthermore, control of membrane protein and lipid content in the networks is achievable by employing different cell types, for example, by overexpression of a desired protein or the use of specialized cell-types as sources for rare proteins and lipids. In general, the method provides simple accessibility for functional studies of plasma membrane constituents. Specifically, it provides a direct means to functionalize nanotube-vesicle networks with desired proteins and lipids for studies of transport activity both across membranes (protein-mediated) and across nanotubes (diffusion), and substrate conversion down to the single-molecule limit. Nanotube-vesicle networks can adopt different geometries and topologies and undergo shape changes at will, providing a flexible system for changing the physical and chemical environment around, for example, a membrane protein. Furthermore, the method offers unique possibilities for extracting membrane and protein material for nanotechnological sensor and analytical devices based on lipid membrane networks.

Lipid diffusion in giant unilamellar vesicles is more than 2 times faster than in supported phospholipid bilayers under identical conditions

The lateral diffusion coefficients of a BODIPY tail-labeled lipid in two model systems, namely, free-standing giant unilamellar vesicles (GUVs) and supported phospholipid bilayers (SPBs), were determined by fluorescence correlation spectroscopy (FCS) using the Z-scan approach. For the first time, the performed measurements on 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) bilayers maintain exactly the same experimental conditions for both systems, which allows for a quantitative comparison of lipid diffusion in these two commonly used model membranes. The results obtained revealed that the lipid mobility in free-standing bilayers (D=7.8+/-0.8 microm2 s-1) is significantly higher than in the bilayer created on the solid support (mica) (D=3.1+/-0.3 microm2 s-1).

Lateral mobility of proteins in liquid membranes revisited

The biological function of transmembrane proteins is closely related to their insertion, which has most often been studied through their lateral mobility. For >30 years, it has been thought that hardly any information on the size of the diffusing object can be extracted from such experiments. Indeed, the hydrodynamic model developed by Saffman and Delbrück predicts a weak, logarithmic dependence of the diffusion coefficient D with the radius R of the protein. Despite widespread use, its validity has never been thoroughly investigated. To check this model, we measured the diffusion coefficients of various peptides and transmembrane proteins, incorporated into giant unilamellar vesicles of 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (SOPC) or in model bilayers of tunable thickness. We show in this work that, for several integral proteins spanning a large range of sizes, the diffusion coefficient is strongly linked to the protein dimensions. A heuristic model results in a Stokes-like expression for D, (D proportional, variant 1/R), which fits literature data as well as ours. Diffusion measurement is then a fast and fruitful method; it allows determining the oligomerization degree of proteins or studying lipid-protein and protein-protein interactions within bilayers.

Seeing spots: Complex phase behavior in simple membranes

Liquid domains in model lipid bilayers are frequently studied as models of raft domains in cell plasma membranes. Micron-scale liquid domains are easily produced in vesicles composed of ternary mixtures of a high melting temperature lipid, a low melting temperature lipid, and cholesterol. Here, we describe the rich phase behavior observed in binary and ternary systems. We then discuss experimental challenges inherent in mapping phase diagrams of even simple lipid systems. For example, miscibility behavior varies with lipid type, lipid ratio, lipid oxidation, and level of impurity. Liquid domains are often circular, but can become noncircular when membranes are near critical points. Finally, we reflect on applications of phase diagrams in model systems to rafts in cell membranes.

Functional analysis of detergent-solubilized and membrane-reconstituted ATP-binding cassette transporters

ATP-binding cassette (ABC) transporters are vital to any living system and are involved in the translocation of a wide variety of substances, from ions and nutrients to high molecular weight proteins. This chapter describes methods used to purify and membrane reconstitute ABC transporters in a fully functional state. The procedures are largely based on our experience with substrate-binding protein-dependent ABC uptake systems from bacteria, but the approaches should be applicable to multisubunit membrane complexes in general. Also, we present simple methods, based on substrate binding or translocation, to follow the activity of the protein complexes in detergent-solubilized and/or membrane-reconstituted state(s).