Membrane lipid bilayers fusion

The adliesion and fiision mechanisms between bilayers have also been studied with the SEA [M, 100]. Kuhl et al [17] found that solutions of short-chained polymers (PEG) could produce a short-range depletion attraction between lipid bilayers, which clearly depends on the polymer concentration (fignre Bl.20.1 It. This depletion attraction was found to mduce a membrane fusion widiin 10 minutes that was observed, in real-time, using PECO fringes. There has been considerable progress in the preparation of fluid membranes to mimic natural conditions in the SEA [ ], which promises even more exciting discoveries in biologically relevant areas. 

While recent attention has been largely on proteins, it should be borne in mind that membrane fusion ultimately involves the merger of phospholipid bilayers. However, little is known about the specific membrane lipid requirements. When membranes fuse, energetically unfavorable transition states are generated that may require specific lipids and lipid domains for stabilization. Although there is some evidence for a specific influence of lipids on exocytosis, it is still unclear whether specific lipid metabolites are needed or even generated at the site of membrane merger. 

Q Studies using v- and t-SNARE ptoteins reconsti-mted into separate lipid bilayer vesicles have indicated that they form SNAREpins, ie, SNARE complexes that hnk two membranes (vesicles). SNAPs and NSF are required for formation of SNAREpins, but once they have formed they can apparently lead to spontaneous fusion of membranes at physiologic temperamre, suggesting that they are the minimal machinery required for membrane fusion. 

Several glycoproteins, which are present in the lipid bilayer of the virus, are necessary for infection. One is known as GP120. It binds to the CD4 protein on the surface of the Th lymphocyte (i.e. the CD4-I- ceU). This initiates fusion with the plasma membrane of the CD4-I- cell so that the viral RNA and its proteins enter the cell (i.e. it infects the CD4-t cell). The original infection probably occurs in the peripheral circulation but the lymphocytes will be transported by the blood to the spleen, other lymph nodes and the brain, where the microglia become infected (Figure 17.46). 

Membranes of plant and animal cells are typically composed of 40-50 % lipids and 50-60% proteins. There are wide variations in the types of lipids and proteins as well as in their ratios. Arrangements of lipids and proteins in membranes are best considered in terms of the fluid-mosaic model, proposed by Singer and Nicolson % According to this model, the matrix of the membrane (a lipid bilayer composed of phospholipids and glycolipids) incorporates proteins, either on the surface or in the interior, and acts as permeability barrier (Fig. 2). Furthermore, other cellular functions such as recognition, fusion, endocytosis, intercellular interaction, transport, and osmosis are all membrane mediated processes. 

Fig. 52a-c. Scheme of the fusion process of giant liposomes and the formation of small unilamellar vesicles (SUV) at the interface, a) lipid bilayers in contact b) pores generated by electric breakdown and lipid reorientation forming SUVs c) reconstitution of lipid membranes formation of a fused giant liposome and SUVs . 

Figure 16.3 Neurotransmitter release, (a) Presynaptic nerve terminal containing vesicles and other organelles, (b) Neurotransmitter-containing vesicles are made of lipid bilayers. Associated proteins participate in the release process, (c) The vesicle associates with the presynaptic membrane via protein complexes that mediate release, (d) Release of neurotransmitter into the synapse is by protein-mediated fusion of vesicle and presynaptic membranes.

Ungermann C, Langosch D (2005) Functions of SNAREs in intracellular membrane fusion and lipid bilayer mixing. J Cell Sci 118 3819-28... 

Other biomolecules that are of interest in p.CP are lipids and lipid bilayers. Supported lipid bilayers are very fragile assemblies that are formed by lipids that are organized into two opposing leaflets on hydrophilic surfaces, such as glass or mica substrates. These structures can be also patterned on solid substrates but the p.CP technique differs slightly from the ones that were applied for proteins or DNA. First, the bilayer has to be formed on the oxidized PDMS stamp from the buffer solution by lipid vesicle fusion. Second, printing has to be carried out in water, otherwise the bilayer will lose its structure.99 This method allows efficient and reliable transfer of membrane patches to glass surfaces. 

The structure of the HA2 N-terminal fusion peptide has been probed by a combination of NMR and electron paramagnetic resonance (EPR) in detergent micelles that mimic the lipid bilayer, at both acid and neutral pHs (Han et al, 2001). At both acidic and neutral pH the structure is predominantly helical with a kink where it rises most prominently to the presumptive membrane surface. At lower pH the kink is stronger, there is additional 3io helix, and two charged residues are rotated out of the membrane plane. The stronger kink likely allows the peptide to become more deeply immersed, perhaps disrupting the membrane and facilitating fusion. 

The first steps in viral infection are binding of the virus to the cell surface, and, for viruses enclosed by a Upid membrane, the subsequent fusion of the virus and cell membranes to introduce the virus genetic material into the cell cytoplasm. Proteins embedded in the virus lipid bilayer envelope carry out cell surface receptor binding and membrane... 

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