Fusion, of membranes

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. 

Fusion events may be homotypic or heterotypic. Homotypic fusion indicates fusion between membranes that originate from the same compartment (i.e. fusion of ER-derived vesicles to form tubular vesicular clusters, see below). Heterotypic membrane fusion indicates fusion of membranes originating from different compartments (i.e. synaptic vesicles and the plasma membrane). Triggered fusion like neurotransmitter release is typically heterotypic. 

Partial lipid extraction is considered as biologically important in several processes involving membranes, for example the fusion of membranes or enzymatic reactions with participation of phospholipids. To simulate this process, non-equilibrium or steered MD simulations can be used. This type of simulation was employed for the first time by Grubmuller et al. [96] to study the rupture of the binding in the strepta-vidin-biotin complex. Inspired by this pioneering work, similar studies have been undertaken to characterize the extraction of a phospholipid from the membrane. 

The fusion of membranes at several stages in the vesicle-mediated transport of materials between Golgi compartments requires a specific protein known as the N-ethylmaleimide-sensitive fusion protein... 

Fusion of membranes is a very important biological event which, in its detailed mechanisms, is still poorly understood Also here, time-resolved X-ray diffraction offers attractive possibilities. This shall be illustrated by the first study that has recently been undertaken in this direction... 

The whole virus or its genetic material alone (DNA or RNA) enters the cell s cytoplasm (penetration and uncoating). A virus may have different penetration mechanisms in the host cell. For enveloped virus, fusion of membrane sometimes occurs. Most viruses are introduced into the cell by a kind of phagocytosis named viropexis. Virus particles are transported along the network of cytoplasmic microtubules to a specific cell site where subsequent replication takes place. Uncoating results in the liberation of viral nucleic acids into the cell which makes them sensitive to nucleases. 

The interaction of vesicles with molecules in the surrounding solution is a particularly fascinating topic. Small and large molecules can bind to the bilayer membrane and mediate the interactions between vesicles. In this respect, synthetic vesicles are versatile model systems for the protein- and carbohydrate-mediated recognition, adhesion, and fusion of membranes that occur during endocytosis, viral infection, cell adhesion, and the growth of tissue from individual cells. 

Glaser, P.E., Gross, R.W. (1995) Rapid plasmenylethanolamine-selective fusion of membrane bilayers catalyzed by an isoform of glyceraldehyde-3-phosphate dehydrogenase discrimination between glycolytic and fusogenic roles of individual isoforms. Biochemistry 34, 12193-12203. 

Examination of the developing capillaiy system in high Pb-exposed animals revealed aberrations in the developing capillaiy buds, which were either dilated or lacked apical processes. Electron microscopic examination has revealed capillaiy damage, with vessel walls containing numerous varicosities, the fusion of membranes periodically interrupted, hypertrophied capillaries and endothelial cells, and a few necrotic endothelieil cefis. Lead appears to be concentrated, at least initially, in bredn capillaries in animals exposed to Pb developmentaUy and is localized in the cytoplasm of the capillaiy endothelium cells eifter an intraperitoneal (ip) injection of ra oactive Pb. ... 

One recent paper has referred to the effect of surfactants on immune responses in mice [163]. Both stimulation and inhibition of the immune responses was revealed depending on the substance studied and the duration of the exposure. Triton X-100 causes stimulation of immunological activity. According to Ahkong et al. [88] surfactant adjuvants may facilitate the secretion of mediators by affecting fusion of membranes, e.g. of lymphocytes and macrophages. But no clear indication of mechanism of action was adduced. 

The same type of narrow resonance was observed during fusion of membrane vesicles (Verkleij et ai, 1979, 1980). This was interpreted in terms of a structure allowing rapid isotropic averaging (Cullis et ai, 1978). A variety of possibilities can lead to such averaging—small vesicles, micelles, inverted micelles, even cubic or rhombic phases. In some cases the narrow resonances were correlated with the appearance of lipidic particles in freeze-fracture electron micrographs (Verkleij et ai, 1979 de Kniijff et ai, 1979) and interpreted as intramembrane inverted micelles. 

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