Membrane lipids lateral diffusion

Cell membranes are two-dimensional fluids that exhibit a wide range of dynamic behaviors. Recent technical advances have enabled unprecedented views of membrane dynamics in living cells. In this technical review, we provide a brief overview of three well-studied examples of membrane dynamics lateral diffusion of proteins and lipids in the plane of the membrane, vesicular trafficking between intracellular compartments, and exchange of proteins on and off membranes. We then discuss experimental approaches to monitor membrane protein and lipid dynamics, and we place a special emphasis on the use of genetically encoded fluorescent probes and live cell-imaging techniques. 

Diffusion is the random movement of a particle because of an exchange of thermal energy with its environment. Membrane lipids and proteins participate in highly anisotropic translational and rotational diffusion motion. Translational diffusion in the plane of the membrane is described by the mean square lateral displacement after a time At (r ) = TD At. Lipid lateral diffusion coefficients in fluid phase bilayers are typically in the range Dj 10 to 10 cm /s (3). 

G. Lindblom and G. Oraedd, Lipid Lateral Diffusion and Membrane Heterogeneity , Biochim. Biophys. Acta, Biomembr., 2009, 1788, 234. 

The interaction between bacterial lipopolysaccharides (EPS) and phospholipid cell membranes was studied by various physical methods of deep rough mutant EPS (ReEPS) of Escherichia coH incorporated in phospholipid bilayers as simple models of cell membranes. SS P-NMR spectroscopic analysis suggested that a substantial part of ReEPS is incorporated into l,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) lipid bilayers when mixed multilamellar vesicles were prepared. Furthermore the lipid lateral diffusion coefficients measurements at various molar ratios of ReEPS/egg-PC/POPG indicated that the incorporated ReEPS reduces the diffusion coefficients of the phospholipids in the membrane. EUV formed by the ReEPS from Salmonella enterica, eventually in mixture with dilauroyl phosphatidylcholine (DEPC), have been prepared and characterized by DES, SANS and EPR. PFGSE NMR measurements have shown that water permeability through the lipid bilayer is low at room temperature. However, above a transition temperature centered at 30-35 °C, the water permeability increases. ... 

Lindblom, G., Oradd, G., Rilfors, L. and Morein, S. (2002) Regulation of lipid composition in Acholeplasma laidlawii and Escherichia coli membranes NMR studies of lipid lateral diffusion at different growth temperatures. Biochemistry 41, 11512-11515. 

Incorporation of cholesterol into model membranes increase the order parameter of the hydrocarbon chains but leaves the lipid lateral diffusion almost unaffected (12). Therefore it can be concluded that the effect of cholesterol on the packing properties of the bilayers is more important than its influence on lipid bilayer dynamics. 

Moreover, properties such as lipid lateral diffusion, lipid flip/flop, membrane elasticity and surface tension can be heavily biased in MD simulations by too small boundaries. The study in ref. 68 reported a model made of over 60 different lipid types, in a stoichiometric ratio compatible to that experimentally determined using lipidomics approaches, for a total of 20 000 lipid molecules, and simulated for 40 microseconds using coarse-grained potentials. 

A continuous lipidic cubic phase is obtained by mixing a long-chain lipid such as monoolein with a small amount of water. The result is a highly viscous state where the lipids are packed in curved continuous bilayers extending in three dimensions and which are interpenetrated by communicating aqueous channels. Crystallization of incorporated proteins starts inside the lipid phase and growth is achieved by lateral diffusion of the protein molecules to the nucleation sites. This system has recently been used to obtain three-dimensional crystals 20 x 20 x 8 pm in size of the membrane protein bacteriorhodopsin, which diffracted to 2 A resolution using a microfocus beam at the European Synchrotron Radiation Facility. 

Protein lateral motion is much slower than that of lipids because proteins are larger than lipids. Also, some membrane proteins can diffuse freely through the membrane, whereas others are bound or anchored to other protein structures in the membrane. The diffusion constant for the membrane protein fibronectin is approximately 0.7 X 10 cmVsec, whereas that for rhodopsin is about 3 X 10 cmVsec. 

Two principal routes of passive diffusion are recognized transcellular (la —> lb —> lc in Fig. 2.7) and paracellular (2a > 2b > 2c). Lateral exchange of phospholipid components of the inner leaflet of the epithelial bilayer seems possible, mixing simple lipids between the apical and basolateral side. However, whether the membrane lipids in the outer leaflet can diffuse across the tight junction is a point of controversy, and there may be some evidence in favor of it (for some lipids) [63]. In this book, a third passive mechanism, based on lateral diffusion of drug molecules in the outer leaflet of the bilayer (3a > 3b > 3c), wih be hypothesized as a possible mode of transport for polar or charged amphiphilic molecules. 

To diffuse rapidly in the plane of the membrane (lateral diffusion), a molecule must simply move around in the lipid environment (including the polar head groups). It need not change how it interacts with phospholipids or with water since it is constantly exposed to pretty much the same environment. Lateral diffusion can be slowed (or prevented) by interactions between membrane proteins and the cellular cytoskeleton. This spatially restricts a plasma membrane protein to a localized environment. 

Alkyl chain heterogeneities cause cell membrane bilayers to remain in the fluid state over a broad temperature range. This permits rapid lateral diffusion of membrane lipids and proteins within the plane of the bilayer. The lateral diffusion rate for an unconstrained phospholipid in a bilayer is of the order of 1 mm2 s 1 an integral membrane protein such as rhodopsin would diffuse 40nm2 s 1. 

The prototype of a small pore-forming toxin is the S. aureus a-toxin, also called ct-hemolysin, that has been extensively investigated hy Bhakdi and coworkers. Monomers of ct-hemolysin (33 kDa) hind to the surface of erythrocytes, and after lateral diffusion within the lipid hilayer, seven monomers oligomerize to form pores in the cell membrane. The ct-hemolysin forms mushroom-shaped pores with an outer diameter of lOnm and an inner diameter of approximately 2.5 nm. Small molecules can pass through the pore and diffuse into/out of the cytosol, along with water. As a consequence of such movement, cell homeostasis is greatly disturbed and pushed into an unhealthy state. In animals, the a-hemolysin represents a major virulence factor of S. aureus which causes hemolysis as well as tissue destruction. ... 

The vast majority of biological membranes are in the liquid-crystalline phase. There are many experimental studies on model bilayer phase behavior [3]. Briefly, at low temperatures lipid bilayers form a gel phase, characterized by high order and rigidity and slow lateral diffusion. There is a main phase transition, as the temperature is increased, to the liquid-crystalline phase. The liquid-crystalline phase has more fluidity and fast lateral diffusion. 

A very brief description of biological membrane models, and model membranes, is given. Studies of lateral diffusion in model membranes (phospholipid bilayers) and biological membranes are described, emphasizing magnetic resonance methods. The relationship of the rates of lateral diffusion to lipid phase equilibria is discussed. Experiments are reported in which a membrane-dependent immunochemical reaction, complement fixation, is shown to depend on the rates of diffusion of membrane-bound molecules. It is pointed out that the lateral mobilities and distributions of membrane-bound molecules may be important for cell surface recognition. 

The rates of lateral diffusion of phospholipids in lipid bilayer membranes, and in biological membranes, were first measured using spin-labeled lipids.26 50 10 11 9 In general, these rates have been determined by incorporating spin-labeled lipids such as (V) and (VI) in phospholipid bilayers, or multilayers. The paramagnetic resonance spectra of labels such as (V), as well as the nuclear resonance spectra of other lipids in membranes containing (V), depend on the concentration c of the label in the membrane and the rate of lateral motion of the lipids. Two methods... 

As indicated in my report, we now know the rates of lateral diffusion of phospholipids in lipid bilayers in the fluid state, and in a few cases the rates of lateral diffusion of proteins in fluid lipids are also known. At the present time nothing is known about the rates of lateral diffusion of phospholipids in the crystalline, solid phases of the substances. As mentioned in my report, there are reasons to suspect that the rates of lateral diffusion of phospholipids in the solid solution crystalline phases of binary mixtures of phospholipids may be appreciable on the experimental time scale. Professor Ubbelohde may well be correct in pointing out the possibility of diffusion caused by defects. However, such defects, if present, apparently do not lead to significant loss of the membrane permeability barrier, except at domain boundaries. 

Individual lipid molecules can move laterally in the plane of the membrane by changing places with neighboring lipid molecules (Fig. 11-16c). A molecule in one mono-layer, or leaflet, of the lipid bilayer—the outer leaflet of the eiythrocyte plasma membrane, for example—can diffuse laterally so fast that it circumnavigates the erythrocyte in seconds. This rapid lateral diffusion within the plane of the bilayer tends to randomize the positions of individual molecules in a few seconds. 

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