Cryoelectron microscopy and freeze-fracture analysis of lipid polymorphism

Frederik, PM Peter; Stuart, M.C.A.; Burger, Koert N.J.; Verkleij, A.J.

doi:10.1016/0304-3991(89)90365-3

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“…Within the context of the present study, the notion that fatty acids may promote vesicle fusion through the induction of inverted structures is of particular interest, since such structures have been suggested to play a crucial role in the Ca2+-induced fusion of CL-containing vesicles (Verkleij et al, 1979;Verkleij, 1984;De Kruijff et al, 1982;Wilschut et al, 1982Wilschut et al, , 1985Frederik et al, 1989). Clearly, the bilayer-destabilizing effect of the OA anion in the present system requires Ca2+, the OA anion as such being not destabilizing to the vesicles at all, as discussed above.…”

Section: Stimulation Of Phospholipid Vesicle Fusion By Free Fattymentioning

confidence: 68%

Calcium-induced fusion of phospholipid vesicles containing free fatty acids: modulation by transmembrane pH-gradients

Wilschut

Scholma²,

Eastman³

et al. 1992

Biochemistry

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The influence of a transmembrane pH gradient on the Ca(2+)-induced fusion of phospholipid vesicles, containing free fatty acids, has been investigated. Large unilamellar vesicles composed of an equimolar mixture of cardiolipin, dioleoylphosphatidylcholine, and cholesterol, containing 20 mol % oleic acid, were employed. Fusion was measured using a kinetic assay for lipid mixing, based on fluorescence resonance energy transfer. At pH 7.5, but not at pH 6.0, in the absence of a pH gradient, oleic acid stimulates the fusion of the vesicles by shifting the Ca2+ threshold concentration required for aggregation and fusion of the vesicles from about 13 mM to 10 mM. In the presence of a pH gradient (at an external pH of 7.5 and a vesicle interior pH of 10.5), the vesicles exhibit fusion characteristics similar to vesicles that do not contain oleic acid at all, consistent with an effective sequestration of the fatty acid to the inner monolayer of the vesicle bilayer induced by the imposed pH gradient. The kinetics of the fusion process upon simultaneous generation of the pH gradient across the vesicle bilayer and initiation of the fusion reaction show that the inward movement of oleic acid in response to the pH gradient is extremely fast, occurring well within 1 s. Conversely, dissipation of an imposed pH gradient, by addition of a proton ionophore during the course of the fusion process, results in a rapid enhancement of the rate of fusion due to reequilibration of the oleic acid between the two bilayers leaflets.

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