Influence of obstacles on lipid lateral diffusion: computer simulation of FRAP experiments and application to proteoliposomes and biomembranes

The Utility of Fluorescence Recovery after Photobleaching (FRAP) to Study the Plasma Membrane

The plasma membrane of mammalian cells is involved in a wide variety of cellular processes, including, but not limited to, endocytosis and exocytosis, adhesion and migration, and signaling. The regulation of these processes requires the plasma membrane to be highly organized and dynamic. Much of the plasma membrane organization exists at temporal and spatial scales that cannot be directly observed with fluorescence microscopy. Therefore, approaches that report on the membrane's physical parameters must often be utilized to infer membrane organization. As discussed here, diffusion measurements are one such approach that has allowed researchers to understand the subresolution organization of the plasma membrane. Fluorescence recovery after photobleaching (or FRAP) is the most widely accessible method for measuring diffusion in a living cell and has proven to be a powerful tool in cell biology research. Here, we discuss the theoretical underpinnings that allow diffusion measurements to be used in elucidating the organization of the plasma membrane. We also discuss the basic FRAP methodology and the mathematical approaches for deriving quantitative measurements from FRAP recovery curves. FRAP is one of many methods used to measure diffusion in live cell membranes; thus, we compare FRAP with two other popular methods: fluorescence correlation microscopy and single-particle tracking. Lastly, we discuss various plasma membrane organization models developed and tested using diffusion measurements.

Bacterial protein listeriolysin O induces nonmonotonic dynamics because of lipid ejection and crowding

Membrane-bound protein complexes involving pore forming toxins (PFTs) released by virulent bacteria are known to form transmembrane pores leading to host cell lysis. Developing alternative strategies against PFT mediated bacterial virulence factors requires an understanding of the cellular membrane response. However, membrane disruption and related lipid reorganization events during attack by PFTs remain largely unexplored. We report counterintuitive and nonmonotonic variations in lipid diffusion, measured using confocal fluorescence correlation spectroscopy, due to interplay of lipid ejection and crowding by membrane-bound oligomers of a prototypical cholesterol-dependent cytolysin, listeriolysin O (LLO). The observed dynamical crossover is correlated with concentration dependent transitions of LLO oligomeric state populations from rings to arc-like pore complexes, predicted using a proposed two-state free area-based diffusion model. At low PFT concentrations, a hitherto unexplored regime of increased lipid diffusivity is attributed to lipid ejection events because of a preponderance of ring-like pore states. At higher protein concentrations in which membrane-inserted arc-like pores dominate, lipid ejection is less efficient and the ensuing crowding results in a lowering of lipid diffusion. These variations in lipid dynamics are corroborated by macroscopic rheological response measurements of PFT bound vesicles. Our study correlates PFT oligomeric state transitions, membrane remodeling, and mechanical property variations, providing unique insights into the pore forming mechanisms of cholesterol-dependent cytolysins.

Promoted Aggregation of Aβ on Lipid Bilayers in an Open Flowing System

Self-assembly of amyloid-β (Aβ) peptides in nonequilibrium, flowing conditions is associated with pathogenesis of Alzheimer's disease. We examined the role of biologically relevant, nonequilibrium, flowing conditions in the desorption, diffusion, and integration of Aβ-lipid assemblies at the membrane surface using a microchannel connected with microsyringes. A 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) bilayer was formed on a glass substrate and incubated in Aβ solution under either a quiescent condition (no flow) or flowing condition for 24 h. Although dot-like aggregates (<1 μm) comprising Aβ fibrils formed on the DMPC membrane under the quiescent condition, larger plaque-like aggregates formed under the flowing condition, suggesting that nonequilibrium continuous flow governs the cytotoxicity of Aβ species. We propose that Aβ adsorption on the membrane surface involves spontaneous desorption of Aβ-lipid to form self-assembling aggregates, with this accelerated by surface shear forces. These findings suggest that nonequilibrium, flowing conditions influence inter/intra-molecular Aβ-fibril formation to trigger formation of amyloid plaques.

Characterization of the Effect of Sphingolipid Accumulation on Membrane Compactness, Dipole Potential, and Mobility of Membrane Components

Communication between cells and their environment is carried out through the plasma membrane including the action of most pharmaceutical drugs. Although such a communication typically involves specific binding of a messenger to a membrane receptor, the biophysical state of the lipid bilayer strongly influences the outcome of this interaction. Sphingolipids constitute an important part of the lipid membrane, and their mole fraction modifies the biophysical characteristics of the membrane. Here, we describe methods that can be used for measuring how sphingolipid accumulation alters the compactness, microviscosity, and dipole potential of the lipid bilayer and the mobility of membrane components.

Assessing the extent of the structural and dynamic modulation of membrane lipids due to pore forming toxins: insights from molecular dynamics simulations

Infections caused by many virulent bacterial strains are triggered by the release of pore forming toxins (PFTs), which form oligomeric transmembrane pore complexes on the target plasma membrane. The spatial extent of the perturbation to the surrounding lipids during pore formation is relatively unexplored. Using all-atom molecular dynamics simulations, we investigate the changes in the structure and dynamics of lipids in a 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) lipid bilayer in the presence of contrasting PFTs. Cytolysin A (ClyA), an α toxin with its inserted wedge shaped bundle of inserted α helices, induces significant asymmetry across the membrane leaflets in comparison with α hemolysin (AHL), a β toxin. Despite the differences in hydrophobic mismatch and uniquely different topologies of the two oligomers, perturbations to lipid order as reflected in the tilt angle and order parameters and membrane thinning are short ranged, lying within ∼2.5 nm from the periphery of either pore complex, and commensurate with distances typically associated with van der Waals forces. In contrast, the spatial extent of perturbations to the lipid dynamics extends outward to at least 4 nm for both proteins, and the continuous survival probabilities reveal the presence of a tightly bound shell of lipids in this region. Displacement probability distributions show long tails and the distinctly non-Gaussian features reflect the induced dynamic heterogeneity. A detailed profiling of the protein-lipid contacts with tyrosine, tryptophan, lysine and arginine residues shows increased non-polar contacts in the cytoplasmic leaflet for both PFTs, with a higher number of atomic contacts in the case of AHL in the extracellular leaflet due to the mushroom-like topology of the pore complex. The short ranged nature of the perturbations observed in this simple one component membrane suggests inherent plasticity of membrane lipids enabling the recovery of the structure and membrane fluidity even in the presence of these large oligomeric transmembrane protein assemblies. This observation has implications in membrane repair processes such as budding or vesicle fusion events used to mitigate PFT virulence, where the underlying lipid dynamics and fluidity in the vicinity of the pore complex are expected to play an important role.

Numerical Simulation and FRAP Experiments Show That the Plasma Membrane Binding Protein PH-EFA6 Does Not Exhibit Anomalous Subdiffusion in Cells

The fluorescence recovery after photobleaching (FRAP) technique has been used for decades to measure movements of molecules in two-dimension (2D). Data obtained by FRAP experiments in cell plasma membranes are assumed to be described through a means of two parameters, a diffusion coefficient, D (as defined in a pure Brownian model) and a mobile fraction, M. Nevertheless, it has also been shown that recoveries can be nicely fit using anomalous subdiffusion. Fluorescence recovery after photobleaching (FRAP) at variable radii has been developed using the Brownian diffusion model to access geometrical characteristics of the surrounding landscape of the molecule. Here, we performed numerical simulations of continuous time random walk (CTRW) anomalous subdiffusion and interpreted them in the context of variable radii FRAP. These simulations were compared to experimental data obtained at variable radii on living cells using the pleckstrin homology (PH) domain of the membrane binding protein EFA6 (exchange factor for ARF6, a small G protein). This protein domain is an excellent candidate to explore the structure of the interface between cytosol and plasma membrane in cells. By direct comparison of our numerical simulations to the experiments, we show that this protein does not exhibit anomalous diffusion in baby hamster kidney (BHK) cells. The non Brownian PH-EFA6 dynamics observed here are more related to spatial heterogeneities such as cytoskeleton fence effects.

Highly hydrated deformable polyethylene glycol-tethered lipid bilayers

The realization of a solid-supported lipid bilayer acting as a workbench for the study of membrane processes is a difficult task. For robustness, the bilayer has to be tethered to the substrate. At the same time, diffusion of the lipids and plastic deformations of the membrane should not be obstructed. Furthermore, a highly hydrated surrounding is mandatory. Here, we show that grafting of a polyethylene glycol-lipid construct (PEG2000-DSPE) to a silicon oxide surface via multiple-step silane chemistry and subsequent deposition of lipids by spin-coating result in a cushioned membrane that has the desired properties. Neutron and X-ray reflectometry measurements are combined to access thickness, density, and hydration of the bilayer and the PEG cushion. We observe a spacer of 55 Å thickness between lipid bilayer and silicon-oxide surface with a rather high hydration of up to 90 ± 3% water. While 11.5 ± 3% of the lipids are grafted to the surface, as determined from the neutron data, the diffusion constant of the lipids, as probed by diffusion of 0.5% Texas Red labeled lipids, remains rather large (D = 2.1 ± 0.1 μm(2)/s), which is a reduction of only 12% compared to a supported lipid bilayer reference without immobilized lipids. Finally, AFM indentation confirms the plastic behavior of the membrane against deformation. We show that rupture of the bilayer does not occur before the deformation exceeds 40 Å. Altogether, the presented PEG-tethered lipid bilayer mimics the deformability of natural cell membranes much better than standard solid-supported lipid bilayers.

Polymerized lipid bilayers on a solid substrate: morphologies and obstruction of lateral diffusion

Substrate supported planar lipid bilayers (SPBs) are versatile models of the biological membrane in biophysical studies and biomedical applications. We previously developed a methodology for generating SPBs composed of polymeric and fluid phospholipid bilayers by using a photopolymerizable diacetylene phospholipid (DiynePC). Polymeric bilayers could be generated with micropatterns by conventional photolithography, and the degree of polymerization could be controlled by modulating UV irradiation doses. After removing nonreacted monomers, fluid lipid membranes could be integrated with polymeric bilayers. Herein, we report on a quantitative study of the morphology of polymeric bilayer domains and their obstruction toward lateral diffusion of membrane-associated molecules. Atomic force microscopy (AFM) observations revealed that polymerized DiynePC bilayers were formed as nanometer-sized domains. The ratio of polymeric and fluid bilayers could be modulated quantitatively by changing the UV irradiation dose for photopolymerization. Lateral diffusion coefficients of lipid molecules in fluid bilayers were measured by fluorescence recovery after photobleaching (FRAP) and correlated with the amount of polymeric bilayer domains on the substrate. Controlled domain structures, lipid compositions, and lateral mobility in the model membranes should allow us to fabricate model membranes that mimic complex features of biological membranes with well-defined structures and physicochemical properties.

Voltage-dependent anion channel transports calcium ions through biomimetic membranes

The mitochondrial outer membrane channel (VDAC), a central player in mitochondria and cell death, was reconstituted in polymer-supported phospholipid bilayers. Highly purified VDAC was first reconstituted in vesicles; channel properties and NADH-ferricyanide reductase activity were ascertained before deposition onto solid substrates. 1-Palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine/1,2-distearoyl-sn-glycero-3-phosphoethanolamine-poly(ethylene glycol)-N-hydroxysuccinimide mixed vesicles containing VDAC were linked onto amine-grafted surfaces (glass and gold) and disrupted to form a VDAC-containing polymer-tethered planar bilayer. Surface plasmon spectroscopy, fluorescence microscopy, and atomic force microscopy measurements ascertained the membrane thickness, fluidity, and continuity. VDAC reconstituted in bilayers efficiently transported calcium ions and was modulable by two channel blockers, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid and l-glutamate. The novel setup may allow the study of the assembly of a polyprotein complex centered on VDAC and its role in mitochondrial biology, calcium fluxes, and apoptosis.

Dynamics and diffusion in photosynthetic membranes from rhodospirillum photometricum

Photosynthetic organisms drive their metabolism by converting light energy into an electrochemical gradient with high efficiency. This conversion depends on the diffusion of quinones within the membrane. In purple photosynthetic bacteria, quinones reduced by the reaction center (RC) diffuse to the cytochrome bc(1) complex and then return once reoxidized to the RC. In Rhodospirillum photometricum the RC-containing core complexes are found in a disordered molecular environment, with fixed light-harvesting complex/core complex ratio but without a fixed architecture, whereas additional light-harvesting complexes synthesized under low-light conditions pack into large paracrystalline antenna domains. Here, we have analyzed, using time-lapse atomic force microscopy, the dynamics of the protein complexes in the different membrane domains and find that the disordered regions are dynamic whereas ordered antennae domains are static. Based on our observations we propose, and analyze using Monte Carlo simulations, a model for quinone diffusion in photosynthetic membranes. We show that the formation of large static antennae domains may represent a strategy for increasing electron transfer rates between distant complexes within the membrane and thus be important for photosynthetic efficiency.

Effects of organelle shape on fluorescence recovery after photobleaching

The determination of diffusion coefficients from fluorescence recovery data is often complicated by geometric constraints imposed by the complex shapes of intracellular compartments. To address this issue, diffusion of proteins in the lumen of the endoplasmic reticulum (ER) is studied using cell biological and computational methods. Fluorescence recovery after photobleaching (FRAP) experiments are performed in tissue culture cells expressing GFP-KDEL, a soluble, fluorescent protein, in the ER lumen. The three-dimensional (3D) shape of the ER is determined by confocal microscopy and computationally reconstructed. Within these ER geometries diffusion of solutes is simulated using the method of particle strength exchange. The simulations are compared to experimental FRAP curves of GFP-KDEL in the same ER region. Comparisons of simulations in the 3D ER shapes to simulations in open 3D space show that the constraints imposed by the spatial confinement result in two- to fourfold underestimation of the molecular diffusion constant in the ER if the geometry is not taken into account. Using the same molecular diffusion constant in different simulations, the observed speed of fluorescence recovery varies by a factor of 2.5, depending on the particular ER geometry and the location of the bleached area. Organelle shape considerably influences diffusive transport and must be taken into account when relating experimental photobleaching data to molecular diffusion coefficients. This novel methodology combines experimental FRAP curves with high accuracy computer simulations of diffusion in the same ER geometry to determine the molecular diffusion constant of the solute in the particular ER lumen.

Self-assembly of solid-supported membranes using a triggered fusion of phospholipid-enriched proteoliposomes prepared from the inner mitochondrial membrane

A general procedure for the formation ofsolid-supported artificial membranes containing transmembrane proteins is reported. The main objective was to directly use the pool of proteins of the native biomembrane (here the inner membrane from mitochondria of human carcinogenic hepatic cells) and to avoid purification steps with detergent. Proteoliposomes of phospholipid-enriched inner membranes from mitochondria were tethered and fused onto a tailored surface via a streptavidin link. The failure of some preliminary experiments on membrane formation was attributed to strong nonspecific interactions between the solid surface and the protuberant hydrophilic parts of the transmembrane complexes. The correct loading of uniform membranes was performed after optimization of a tailored surface, covered with a grafted short-chain poly(ethylene glycol), so that nonspecific interactions are reduced. Step-by-step assembly of the structure and triggered fusion of the immobilized proteoliposomes were monitored by surface plasmon resonance and fluorescence photobleaching recovery, respectively. The long-range lateral diffusion coefficient (at 22 degrees C) for a fluorescent lipid varies from 2.5 x 10(-8) cm2 s(-1) for a tethered lipid bilayer without protein to 10(-9) cm2 s(-1) for a tethered membrane containing the transmembrane proteins of the respiratory chain at a protein area fraction of about 15%. The decrease in the diffusion coefficient in the tethered membrane with increase in protein area fraction was too pronounced to be fully explained by the theoretical models of obstructed lateral diffusion. Covalent tethering links with the solid are certainly involved in the decrease of the overall lateral mobility of the components in the supported membrane at the highest protein-to-lipid ratios.

Anomalous diffusion in a gel-fluid lipid environment: a combined solid-state NMR and obstructed random-walk perspective

Lateral diffusion is an essential process for the functioning of biological membranes. Solid-state nuclear magnetic resonance (NMR) is, a priori, a well-suited technique to study lateral diffusion within a heterogeneous environment such as the cell membrane. Moreover, restriction of lateral motions by lateral heterogeneities can be used as a means to characterize their geometry. The goal of this work is to understand the advantages and limitations of solid-state NMR exchange experiments in the study of obstructed lateral diffusion in model membranes. For this purpose, simulations of lateral diffusion on a sphere with varying numbers and sizes of immobile obstacles and different percolation properties were performed. From the results of these simulations, two-dimensional 31P NMR exchange maps and time-dependent autocorrelation functions were calculated. The results indicate that the technique is highly sensitive to percolation properties, total obstacle area, and, within certain limits, obstacle size. A practical example is shown, namely the study of the well-characterized DMPC-DSPC binary mixture. The comparison of experimental and simulated results yielded obstacle sizes in the range of hundreds of nanometers, therefore bridging the gap between previously published NMR and fluorescence recovery after photobleaching results. The method could also be applied to the study of membrane protein lateral diffusion in model membranes.

A two-photon FRAP analysis of the cytoskeleton dynamics in the microvilli of intestinal cells

The molecular structure of the brush-border of enterocytes has been investigated since the 1980s, but the dynamics of this highly specialized subcellular domain have been difficult to study due to its small size. To perform a detailed analysis of the dynamics of cytoskeleton proteins in this domain, we developed two-photon fluorescence recovery after photobleaching and a theoretical framework for data analysis. With this method, fast dynamics of proteins in the microvilli of the brush border of epithelial intestinal cells can be measured on the millisecond timescale in volumes smaller than 1 microm3. Two major proteins of the cytoskeleton of the microvilli, actin and myosin 1a (Myo1a; formerly named brush border myosin I), are mobile in the brush-border of Caco-2 cells, an enterocyte-like cellular model. However, the mobility of actin is very different from that of Myo1a and they appear to be unrelated (diffusion coefficient of 15 microm2 s(-1) with a mobile fraction of 60% for actin, and 4 microm2 s(-1) with a mobile fraction of 90% for Myo1a). Furthermore, we show for the first time, in vivo, that the dynamics of Myo1a in microvilli reflect its motor activity.

Dynamic Confinement of NK2 Receptors in the Plasma Membrane

A functional fluorescent neurokinin NK2 receptor, EGFP-NK2, was previously used to follow, by fluorescence resonance energy transfer measurements in living cells, the binding of its fluorescently labeled agonist, bodipy-neurokinin A (NKA). Local agonist application suggested that the activation and desensitization of the NK2 receptors were compartmentalized at the level of the plasma membrane. In this study, fluorescence recovery after photobleaching experiments are carried out at variable observation radius (vrFRAP) to probe EGFP-NK2 receptor mobility and confinement. Experiments are carried out at 20 degrees C to maintain the number of receptors constant at the cell surface during recordings. In the absence of agonist, 35% EGFP-NK2 receptors diffuse within domains of 420 +/- 80 nm in radius with the remaining 65% of receptors able to diffuse with a long range lateral diffusion coefficient between the domains. When cells are incubated with a saturating concentration of NKA, 30% EGFP-NK2 receptors become immobilized in small domains characterized by a radius equal to 170 +/- 50 nm. Biochemical experiments show that the confinement of EGFP-NK2 receptor is not due to its association with rafts at any given time. Colocalization of the receptor with beta-arrestin and transferrin supports that the small domains, containing 30% of activated EGFP-NK2, correspond to clathrin-coated pre-pits. The similar amount of confined EGFP-NK2 receptors found before and after activation (30-35%) is discussed in term of putative transient interactions of the receptors with preexisting scaffolds of signaling molecules.

SP-B and SP-C Alter Diffusion in Bilayers of Pulmonary Surfactant

The hydrophobic proteins SP-B and SP-C promote rapid adsorption of pulmonary surfactant to an air/water interface by an unknown mechanism. We tested the hypothesis that these proteins accelerate adsorption by disrupting the structure of the lipid bilayer, either by a generalized increase in fluidity or by a focal induction of interfacial boundaries within the bilayer. We used fluorescence recovery after photobleaching to measure diffusion of nitrobenzoxadiazolyl-dimyristoyl-phosphatidylethanolamine between 11 and 54 degrees C in multilayers containing the complete set of lipids and proteins in calf lung surfactant extract (CLSE), or the complete set of neutral and phospholipids without the proteins. Above 35 degrees C, Arrhenius plots of diffusion were parallel for CLSE and neutral and phospholipids, but shifted to lower values for CLSE, suggesting that the proteins rigidify the lipid bilayer rather than producing the proposed increase in membrane fluidity. The slopes of the Arrhenius plots for CLSE were steeper below 35 degrees C, suggesting that the proteins induce phase separation at that temperature. The mobile fraction fell below 27 degrees C, consistent with a percolation threshold of coexisting gel and liquid-crystal phases. The induction of lateral phase separation in CLSE, however, does not correlate with apparent changes in adsorption kinetics at this temperature. Our results suggest that SP-B and SP-C accelerate adsorption through a mechanism other than the disruption of surfactant bilayers, possibly by stabilizing a high-energy, highly curved adsorption intermediate.

Obstructed diffusion in phase-separated supported lipid bilayers: a combined atomic force microscopy and fluorescence recovery after photobleaching approach

Proteins and other macromolecules are believed to hinder molecular lateral diffusion in cellular membranes. We have constructed a well-characterized model system to better understand how obstacles in lipid bilayers obstruct diffusion. Fluorescence recovery after photobleaching was used to measure the lateral diffusion coefficient in single supported bilayers composed of mixtures of 1,2-dilauroylphosphotidylcholine (DLPC) and 1,2-distearoylphosphotidylcholine (DSPC). Because these lipids are immiscible and phase separate at room temperature, a novel quenching technique allowed us to construct fluid DLPC bilayers containing small disk-shaped gel-phase DSPC domains that acted as obstacles to lateral diffusion. Our experimental setup enabled us to analyze the same samples with atomic force microscopy and exactly characterize the size, shape, and number of gel-phase domains before measuring the obstacle-dependent diffusion coefficient. Lateral obstructed diffusion was found to be dependent on obstacle area fraction, size, and geometry. Analysis of our results using a free area diffusion model shows the possibility of unexpected long-range ordering of fluid-phase lipids around the gel-phase obstacles. This lipid ordering has implications for lipid-mediated protein interactions in cellular membranes.

Measurement of the lateral diffusion of human MHC class I molecules on HeLa cells by fluorescence recovery after photobleaching using a phycoerythrin probe

The mobility of cell surface MHC class I molecules on HeLa cells was measured by fluorescence recovery after photobleaching (FRAP). The probe used for these studies was the phycobiliprotein R-phycoerythrin coupled to Fab fragments of a monoclonal antibody specific for human monomorphic MHC class I molecules. It was found that the recovery curves could be equally well fitted by either a random diffusion model with an immobile component or by an anomalous diffusion model. In the latter case, the anomalous diffusion exponent was consistent with that previously determined by single-particle tracking (SPT) experiments using the same probe (P. R. Smith, I. E. G. Morrison, K. M. Wilson, N. Fernandez, and R. J. Cherry. 1999. Biophys. J. 76:3331-3344). The FRAP experiments, however, yielded a considerably higher value of D(0), the diffusion coefficient for a time interval of 1 s. To determine whether the results were probe dependent, FRAP measurements were also performed with the same monoclonal antibody labeled with Oregon Green. These experiments gave similar results to those obtained with the phycoerythrin probe. FRAP experiments with the lipid probe 5-N-(octadecanoyl) aminofluoroscein (ODAF) bound to HeLa cells gave typical results for lipid diffusion. Overall, our observations and analysis are consistent with anomalous diffusion of MHC class I diffusion on HeLa cells, but quantitative differences between FRAP and SPT data remain to be explained.

Lateral distribution of cholesterol in membranes probed by means of a pyrene-labelled cholesterol: effects of acyl chain unsaturation

The lateral distribution of cholesterol in membranes in the fluid state was investigated by studying the variation of the molar absorption coefficient of pyrene-labelled cholesterol (Py-chol) vs. its concentration in vesicles made of phosphatidylcholine, with variable acyl chain unsaturations. Absorption measurements indicated non-ideal mixing of Py-chol in unsaturated lipids, a process mainly controlled by the cholesterol moiety of the probe. Similar abilities of cholesterol and Py-chol in perturbing the phase properties of pure saturated phosphatidylcholine were observed by DSC experiments. Immiscibility of sterols was corroborated by fluorescence polarization measurements, which indicated a weaker ordering effect of cholesterol in unsaturated membranes. The sizes and the quantities of sterol oligomers formed were calculated. A model for the lateral distribution of cholesterol in membranes is proposed and is applied to known cholesterol/phosphatidylcholine phase diagrams. Finally, the results are discussed with regard to recent models of biological membrane organization, (i.e. rafts).

Anomalous subdiffusion in fluorescence photobleaching recovery: a Monte Carlo study

Anomalous subdiffusion is hindered diffusion in which the mean-square displacement of a diffusing particle is proportional to some power of time less than one. Anomalous subdiffusion has been observed for a variety of lipids and proteins in the plasma membranes of a variety of cells. Fluorescence photobleaching recovery experiments with anomalous subdiffusion are simulated to see how to analyze the data. It is useful to fit the recovery curve with both the usual recovery equation and the anomalous one, and to judge the goodness of fit on log-log plots. The simulations show that the simplest approximate treatment of anomalous subdiffusion usually gives good results. Three models of anomalous subdiffusion are considered: obstruction, fractional Brownian motion, and the continuous-time random walk. The models differ significantly in their behavior at short times and in their noise level. For obstructed diffusion the approach to the percolation threshold is marked by a large increase in noise, a broadening of the distribution of diffusion coefficients and anomalous subdiffusion exponents, and the expected abrupt decrease in the mobile fraction. The extreme fluctuations in the recovery curves at and near the percolation threshold result from extreme fluctuations in the geometry of the percolation cluster.

Probing the lateral organization of membranes: fluorescence repercussions of pyrene probe distribution

Phospholipids pyrene labeled are widely used to investigate dynamics and organizations of membranes. We studied pyrene probe lateral distribution by analyzing the variations of the molar absorption coefficient (epsilon) versus probe concentrations, in small unilamellar vesicles (SUV) made of phospholipids and/or glycolipids, with pyrene labeled phosphatidylcholine (PyPC) or phosphatidylglycerol (PyPG). The results were interpreted according to an infinite associative model. They indicated that an effective self-association process corresponding to K ranging from 30 to 100 M(-1) occurred with those probes incorporated in dimannosyl diacylglycerol (DMDG). In contrast, after SUV labeling of egg yolk phosphatidylcholine (EggPC) or phosphatidylglycerol (EggPG), K values < 1 M(-1) were determined. The corresponding percentages of various stacked forms of pyrene probes were calculated. They indicated that, for a 3% PyPG labeling, the monomer represented 21% of n-mers in DMDG and 94% in EggPC. The analysis of fluorescence experiments carried out on the same samples indicated that: (i) the fluorescence process of pyrene probes was generated by the monomers: and (ii) the excimer forming resulted from a diffusional encounter between one excited and one non-excited monomer. A correction of fluorescence data allowing a more correct interpretation of fluorescence measurements was proposed.

The transmembrane protein bacterioopsin affects the polarity of the hydrophobic core of the host lipid bilayer

Influence of the transmembrane protein bacterioopsin (the retinal-free form of bacteriorhodopsin) on the polarity of egg-phosphatidylcholine bilayers was studied by means of a steady-state and time-resolved fluorescence approach exploiting the solvatochromic properties of the 2-anthroyl fluorophore. Introduced in phosphatidylcholine molecules in the form of 8-(2-anthroyl)octanoic acid, this fluorophore probed the hydrocarbon core of the lipid bilayer. As previously shown (E. Pérochon et al., Biochemistry 31 (1992) 7672-7682), water molecules were detected in this region of the terminal part of the lipid acyl chains. Their number was considerably reduced upon addition of bacterioopsin to the lipids. This was assessed by a blue shift in the fluorescence emission spectra of the probe and a marked decrease in the fractional population of fluorophores interacting with water, to the benefit of those experiencing a hydrophobic environment. In agreement with current theories, this decrease in the hydration of the bilayer may be linked to an increase in the acyl chain order and a decrease in the lateral diffusion coefficient of lipids near the protein. The data obtained at high protein concentration accounts for a protein/lipid interface which is much less hydrated than the hydrophobic core of a protein-free lipid bilayer.

Alteration of the lateral organization of the plasma membrane of Chinese hamster ovary cells by synthetic lipopeptide, Pam3Cys-Ser-Lys4

The cationic lipohexapeptide (S)-[2, 3-bis(palmitoyloxy)-(2RS)-propyl]-N-palmitoyl-(R)-Cys-(S)-Ser-(S)- Lys 4-OH, trihydrochloride (Pam3Cys-Ser-Lys4) is a synthetic analog of the triacylated N-terminal part of bacterial lipoproteins. In this study we addressed the question of whether Pam3Cys-Ser-Lys4 could modify the organization of the plasma membrane of Chinese hamster ovary cells. 1-Acyl-2-[6-(7-nitro-2-1, 3-benzoxadiazol-4-yl)amino]caproyl]-sn-glycero-3-phosphocholine (C6-NBD-PC) diffusion was followed by fluorescence recovery after photobleaching experiments carried out on the plasma membrane of Chinese hamster ovary cells. Incubation of cells in the presence of Pam3Cys-Ser-Lys4 induced an increase in the lateral diffusion coefficient and in the immobile fraction of C6-NBD-PC probes. Various control experiments have shown that the increase in the immobile fraction was not due to probe internalization induced by Pam3Cys-Ser-Lys4. Back-exchange experiments showed that a good correlation exists between the fractions of immobilized probes and nonextractable probes in the plasma membrane of Chinese hamster ovary cells. A useful way to analyze the origin of probe immobilization (micrometer-sized domains or aggregated patches of proteins) is to carry out fluorescence recovery after photobleaching experiments at variable observation radii. This type of experiment, carried out on the plasma membrane of Chinese hamster ovary cells incubated with Pam3Cys-Ser-Lys4, confirmed that the lipopeptide induced the aggregation of proteins of Chinese hamster ovary plasma membrane. Lipids which were trapped inside these aggregates were thus prevented from diffusing at long range in the plasma membrane plane and behave as an immobile fraction.

Anomalous diffusion of major histocompatibility complex class I molecules on HeLa cells determined by single particle tracking

Single-particle tracking (SPT) was used to determine the mobility characteristics of MHC (major histocompatibility complex) class I molecules at the surface of HeLa cells at 22 degrees C and on different time scales. MHC class I was labeled using the Fab fragment of a monoclonal antibody (W6/32), covalently bound to either R-phycoerythrin or fluorescent microspheres, and the particles were tracked using high-sensitivity fluorescence imaging. Analysis of the data for a fixed time interval suggests a reasonable fit to a random diffusion model. The best fit values of the diffusion coefficient D decreased markedly, however, with increasing time interval, demonstrating the existence of anomalous diffusion. Further analysis of the data shows that the diffusion is anomalous over the complete time range investigated, 4-300 s. Fitting the results obtained with the R-phycoerythrin probe to D = D0talpha-1, where Do is a constant and t is the time, gave D0 = (6.7 +/- 4.5) x 10(-11) cm2 s-1 and alpha = 0.49 +/- 0.16. Experiments with fluorescent microspheres were less reproducible and gave slower anomalous diffusion. The R-phycoerythrin probe is considered more reliable for fluorescent SPT because it is small (11 x 8 nm) and monovalent. The type of motion exhibited by the class I molecules will greatly affect their ability to migrate in the plane of the membrane. Anomalous diffusion, in particular, greatly reduces the distance a class I molecule can travel on the time scale of minutes. The present data are discussed in relation to the possible role of diffusion and clustering in T-cell activation.

Organization and dynamics of the proteolipid complexes formed by annexin V and lipids in planar supported lipid bilayers

The consequences of the binding of annexin V on its lateral mobility and that of lipids were investigated by means of experimental and simulated FRAP experiments. Experiments were carried out on planar supported bilayers (PC/PS 9:1 mol/mol mixtures) in the presence of 1 mM CaCl2 in the subphase. The probes C12-NBD-PS and fluorescein-labeled annexin V were used and the data compared with that previously obtained for C12-NBD-PC [Saurel, O., Cézanne, L., Milon, A., Tocanne, J. F., & Demange, P. (1998) Biochemistry 37, 1403-1410]. At complete coverage of the lipid bilayer by the protein (Cannexin = 80 nM), the lateral mobility of C12-NBD-PC was reduced by 40% while C12-NBD-PS and bound annexin V molecules were nearly immobilized (D < 10(-)11 cm2/s). At moderate protein concentration (20 nM < Cannexin < 80 nM), best fitting of the lipid and protein probe recoveries was achieved with one single diffusion coefficient and a mobile fraction close to 100%, indicating homogeneous lipid and protein populations. In contrast, at low protein concentration (Cannexin < 20 nM), C12-NBD-PS showed a two-component diffusion. The slow PS population at Cannexin < 20 nM and the single PS population at Cannexin > 20 nM moved at the same rate that bound annexin V (mobile fraction close to 100%), indicating strong PS/protein interactions. With the aid of computer simulations of the lateral motion of PC molecules, based on the 2-D crystalline networks formed by annexin V in contact with the lipid bilayer, these FRAP results may be accounted for by considering a rather simple model of a proteolipidic complex consisting of an extended 2-D crystalline protein network facing the lipid bilayer and stabilized by strong interactions between annexin V and PS molecules. In this model, immobilization of annexin V and PS molecules originates from their mutual interactions. The slowing down of PC molecules is due to various obstacles to their lateral diffusion which can be described as: the four PS molecules bound to the protein, the tryptophan 187 which presumably interacts with the lipids at the level of their polar headgroups and probably the three other hydrophobic amino acid residues located on the AB calcium-binding loops of the protein.

Monte Carlo analysis of obstructed diffusion in three dimensions: application to molecular diffusion in organelles

Molecular transport in the aqueous lumen of organelles involves diffusion in a confined compartment with complex geometry. Monte Carlo simulations of particle diffusion in three dimensions were carried out to evaluate the influence of organelle structure on diffusive transport and to relate experimental photobleaching data to intrinsic diffusion coefficients. Two organelle structures were modeled: a mitochondria-like long closed cylinder containing fixed luminal obstructions of variable number and size, and an endoplasmic reticulum-like network of interconnected cylinders of variable diameter and density. Trajectories were computed in each simulation for >10(5) particles, generally for >10(5) time steps. Computed time-dependent concentration profiles agreed quantitatively with analytical solutions of the diffusion equation for simple geometries. For mitochondria-like cylinders, significant slowing of diffusion required large or wide single obstacles, or multiple obstacles. In simulated spot photobleaching experiments, a approximately 25% decrease in apparent diffusive transport rate (defined by the time to 75% fluorescence recovery) was found for a single thin transverse obstacle occluding 93% of lumen area, a single 53%-occluding obstacle of width 16 lattice points (8% of cylinder length), 10 equally spaced 53% obstacles alternately occluding opposite halves of the cylinder lumen, or particle binding to walls (with mean residence time = 10 time steps). Recovery curve shape with obstacles showed long tails indicating anomalous diffusion. Simulations also demonstrated the utility of measurement of fluorescence depletion at a spot distant from the bleach zone. For a reticulum-like network, particle diffusive transport was mildly reduced from that in unobstructed three-dimensional space. In simulated photobleaching experiments, apparent diffusive transport was decreased by 39-60% in reticular structures in which 90-97% of space was occluded. These computations provide an approach to analyzing photobleaching data in terms of microscopic diffusive properties and support the paradigm that organellar barriers must be quite severe to seriously impede solute diffusion.

Molecular sorting of lipids by bacteriorhodopsin in dilauroylphosphatidylcholine/distearoylphosphatidylcholine lipid bilayers

A combined experimental and theoretical study is performed on binary dilauroylphosphatidylcholine/distearoylphosphatidylcholine (DLPC/DSPC) lipid bilayer membranes incorporating bacteriorhodopsin (BR). The system is designed to investigate the possibility that BR, via a hydrophobic matching principle related to the difference in lipid bilayer hydrophobic thickness and protein hydrophobic length, can perform molecular sorting of the lipids at the lipid-protein interface, leading to lipid specificity/selectivity that is controlled solely by physical factors. The study takes advantage of the strongly nonideal mixing behavior of the DLPC/DSPC mixture and the fact that the average lipid acyl-chain length is strongly dependent on temperature, particularly in the main phase transition region. The experiments are based on fluorescence energy transfer techniques using specifically designed lipid analogs that can probe the lipid-protein interface. The theoretical calculations exploit a microscopic molecular interaction model that embodies the hydrophobic matching as a key parameter. At low temperatures, in the gel-gel coexistence region, experimental and theoretical data consistently indicate that BR is associated with the short-chain lipid DLPC. At moderate temperatures, in the fluid-gel coexistence region, BR remains in the fluid phase, which is mainly composed of short-chain lipid DLPC, but is enriched at the interface between the fluid and gel domains. At high temperatures, in the fluid phase, BR stays in the mixed lipid phase, and the theoretical data suggest a preference of the protein for the long-chain DSPC molecules at the expense of the short-chain DLPC molecules. The combined results of the experiments and the calculations provide evidence that a molecular sorting principle is active because of hydrophobic matching and that BR exhibits physical lipid selectivity. The results are discussed in the general context of membrane organization and compartmentalization and in terms of nanometer-scale lipid-domain formation.

Influence of the intrinsic membrane protein bacteriorhodopsin on gel-phase domain topology in two-component phase-separated bilayers

We have investigated the effect of the intrinsic membrane protein bacteriorhodopsin of Halobacterium halobium on the lateral organization of the lipid phase structure in the coexistence region of an equimolar mixture of dimyristoylphos-phatidylcholine and distearoylphosphatidylcholine. The fluorescence recovery after photobleaching (FRAP) technique was used to monitor the diffusion of both a lipid analog (N-(7-nitrobenzoxa-2,3-diazol-4-yl)-dimyristoylphosphatidyle thanolamine, NBD-DMPE) and fluorescein-labeled bacteriorhodopsin (Fl-BR). In the presence of bacteriorhodopsin, the mobile fractions of the two fluorescent probes display a shift of the percolation threshold toward lower temperatures (larger gel-phase fractions), independent of the protein concentration, from 43 degrees C (without bacteriorhodopsin) to 39 degrees C and 41 degrees C for NBD-DMPE and Fl-BR, respectively. Moreover, in the presence of bacteriorhodopsin, the gel-phase domains are much less efficient in restricting the diffusion of both probes than they are in the absence of the protein in the two-phase coexistence region. Bacteriorhodopsin itself, however, obstructs diffusion of NBD-DMPE and Fl-BR to about the same extent in the fluid phase of the two-phase region as it does in the homogeneous fluid phase. These observations suggest that 1) the protein induces the formation of much larger and/or more centrosymmetrical gel-phase domains than those formed in its absence, and 2) bacteriorhodopsin partitions almost equally between the coexisting fluid and gel phases. Although the molecular mechanisms involved are not clear, this phenomenon is fully consistent with the effect of the transmembrane peptide pOmpA of Escherichia coli investigated by electron spin resonance in the same lipid system.

Phase topology and percolation in two-component lipid bilayers: a monte Carlo approach

Monte Carlo simulations of fluorescence recovery after photobleaching (FRAP) experiments on two-component lipid bilayers systems in the solid-fluid phase coexistence region were carried out to study the geometry and size of fluid domains in these bilayers. The gel phase was simulated by superposable elliptical domains, which were either of predetermined dimensions, increasing in number with increasing gel phase fraction, or of predetermined number, increasing in dimensions with increasing gel phase fraction. The simulations were done from two perspectives: 1) a time-independent analysis of fractional fluorescence recovery as a function of fractional fluid phase in the system; and 2) a time-dependent analysis of fractional fluorescence recovery as a function of time at a given fraction of fluid phase in the system. The time-dependent simulations result in recovery curves that are directly comparable to experimental FRAP curves and provide topological and geometrical models for the coexisting phases that are consistent with the experimental result.

Single-particle tracking: the distribution of diffusion coefficients

In single-particle tracking experiments, the diffusion coefficient D may be measured from the trajectory of an individual particle in the cell membrane. The statistical distribution of single-trajectory diffusion coefficients is examined by Monte Carlo calculations. The width of this distribution may be useful as a measure of the heterogeneity of the membrane and as a test of models of hindered diffusion in the membrane. For some models, the distribution of the short-range diffusion coefficient is much narrower than the observed distribution for proteins diffusing in cell membranes. To aid in the analysis of single-particle tracking measurements, the distribution of D is examined for various definitions of D and for various trajectory lengths.

Topology of gel-phase domains and lipid mixing properties in phase-separated two-component phosphatidylcholine bilayers

The influence of the lipid mixing properties on the lateral organization in a two-component, two-phase phosphatidylcholine bilayer was investigated using both an experimental (fluorescence recovery after photobleaching (FRAP)) and a simulated (Monte Carlo) approach. With the FRAP technique, we have examined binary mixtures of 1-stearoyl-2-capryl-phosphatidylcholine/1,2-distearoyl-phosphat idylcholine (C18C10PC/DSPC), and 1-stearoyl-2-capryl-phosphatidylcholine/1,2-dipalmitoyl-phospha tid ylcholine (C18C10PC/DPPC). Comparison with the 1,2-dimyristoyl-phosphatidylcholine/1,2-distearoyl-phosphatidylcholine (DMPC/DSPC) previously investigated by FRAP by Vaz and co-workers (Biophys. J., 1989, 56:869-876) shows that the gel phase domains become more effective in restricting the diffusion coefficient when the ideality of the mixture increases (i.e., in the order C18C10PC/DSPC-->C18C10PC/DPPC-->DMPC/DSPC). However, an increased lipid miscibility is accompanied by an increasing compositional dependence: the higher the proportion of the high-temperature melting component, the less efficient the gel phase is in compartmentalizing the diffusion plane, a trend that is best accounted for by a variation of the gel phase domain shape rather than size. Computer-simulated fluorescence recoveries obtained in a matrix obstructed with obstacle aggregates of various fractal dimension demonstrate that: 1) for a given obstacle size and area fraction, the relative diffusion coefficient increases linearly with the obstacle fractal dimension and 2) aggregates with a lower fractal dimension are more efficient in compartmentalizing the diffusion plane. Comparison of the simulated with the experimental mobile fractions strongly suggests that the fractal dimension of the gel phase domains increases with the proportion of high-temperature melting component in DMPC/DSPC and (slightly) in C18C10PC/DPPC.

Lateral diffusion in planar lipid bilayers: a fluorescence recovery after photobleaching investigation of its modulation by lipid composition, cholesterol, or alamethicin content and divalent cations

In spite of the fact that planar lipid bilayers are still the best-suited artificial membrane system for the study of reconstituted ion channels and receptors, data dealing with their physical characterization, especially as regards dynamics, are scanty. A combined electrical and optical chamber was designed and allowed fluorescence recovery after photobleaching recovery curves to be recorded from stable virtually solvent-free bilayers. D, the lateral diffusion coefficient of N-(7-nitrobenzoyl-2-oxa-1,3-diazol-4-yl)-1,2-dihexadecanoyl-sn- glycero-3-phosphoethanolamine, was found to be relatively insensitive to the phospholipid composition (headgroup, chain unsaturation, etc.), whereas inclusion of 33-50% cholesterol in the membrane reduced D by a factor of 2. Divalent cations significantly reduced D of negatively charged bilayers. These results compare well with data gathered on other model and natural systems. In addition, the incorporation of the voltage-dependent pore-former alamethicin did slightly reduce lipid lateral mobility. This study demonstrates the feasibility of such experiments with planar bilayers, which are amenable to physical constraints, and thus offers new opportunities for systematic studies of structure-function relationships in membrane-associating molecules.

Fluorescence-quenching study of percolation and compartmentalization in two-phase lipid bilayers

Fluorescence quenching of a lipid-labeled fluorophore by a lipid spin-labeled quencher has been studied experimentally in two-component, two-phase phosphatidylcholine bilayers to examine the effect of phase connection and disconnection on quenching. Both fluorophore and quencher prefer the fluid phase. At the percolation threshold, the point at which the fluid phase becomes subdivided into may small disconnected domains, the quenching drops abruptly. This decrease in quenching is a function of the fluid-phase fraction and is due to the heterogeneous distribution of fluorophores and quenchers over the fluid-phase domains. Computer simulations of the system were carried out with a triangular lattice divided into closed compartments of variable size and reactant occupancy. The simulations demonstrate that the degree of quenching is reduced in the disconnected systems and that the reduction is correlated with the size of the disconnected domains. The combination of experimental data with simulations leads to the conclusion that at constant temperature the size of fluid-phase domains, nfluid, in the region of the coexistence of the fluid and gel phases is proportional to the fluid fraction, Xfluid. This is in a qualitative agreement with a previous electron spin resonance study of interlipid spin-spin interactions in the same two-component, two-phase bilayer system.

7-nitrobenz-2-oxa-1,3-diazole-4-yl-labeled phospholipids in lipid membranes: differences in fluorescence behavior

Steady-state and time-resolved fluorescence properties of the 7-nitrobenz-2-oxa-1, 3-diazole-4-yl (NBD) fluorophore attached either to the sn-2 acyl chain of various phospholipids (phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and phosphatidic acid) or to the polar headgroup of phosphatidylethanolamine were studied after insertion of these NBD-labeled lipid probes into unilamellar vesicles of phosphatidylcholine, phosphatidylglycerol, phosphatidic acid, and phosphatidylserine. The fluorescence response of the NBD group was observed to strongly depend on the chemical structure and physical state of the host phospholipids and on the chemical structure of the lipid probe itself. Among the various fluorescence parameters studied, i.e., Stokes' shifts, lifetimes, and quantum yields, the quantum yields were by far the most affected by these structural and environmental factors, whereas the Stokes' shifts were practically unaffected. Thus, depending on the phospholipid probe and the host phospholipid, the fluorescence emission of the NBD group was found to vary by a factor of up to 5. Careful analysis of the data shows that for the various couples of probe and host lipid molecules studied, deexcitation of the fluorophore was dominated by nonradiative deactivation processes. This great sensitivity of the NBD group to environmental factors originates from its well-known solvatochromic properties, and comparison of these knr values with those obtained for n-propylamino-NBD in a set of organic solvents covering a large scale of polarity indicates that in phospholipids, the NBD fluorophore experiences a dielectric constant of around 27-41, corresponding to a medium of relatively high polarity. From these epsilon values and on the basis of models of the dielectric transition that characterizes any water-phospholipid interface, it can be inferred that for all of the phospholipid probes and host phospholipids tested, the NBD group is located in the region of the polar headgroups, near the phosphoglycerol moiety of the lipids.

The spatial distribution of phospholipids and glycolipids in the membrane of the bacterium Micrococcus luteus varies during the cell cycle

Recently, we have developed a photocrosslinking approach which uses anthracene as a photoactivatable group and which allows us to determine the lateral distribution of lipids in membranes quantitatively. In synchronous cultures of the gram-positive bacterium Micrococcus luteus, this approach shows that the spatial distribution of phosphatidylglycerol and dimannosyldiacylglycerol, the two major lipids in the bacterial membrane, varies greatly during the cell cycle. Minimum heterogeneity was observed during cell growth while maximum heterogeneity was detected during cell division.

Anomalous diffusion due to binding: a Monte Carlo study

In classical diffusion, the mean-square displacement increases linearly with time. But in the presence of obstacles or binding sites, anomalous diffusion may occur, in which the mean-square displacement is proportional to a nonintegral power of time for some or all times. Anomalous diffusion is discussed for various models of binding, including an obstruction/binding model in which immobile membrane proteins are represented by obstacles that bind diffusing particles in nearest-neighbor sites. The classification of binding models is considered, including the distinction between valley and mountain models and the distinction between singular and nonsingular distributions of binding energies. Anomalous diffusion is sensitive to the initial conditions of the measurement. In valley models, diffusion is anomalous if the diffusing particles start at random positions but normal if the particles start at thermal equilibrium positions. Thermal equilibration leads to normal diffusion, or to diffusion as normal as the obstacles allow.