Parameters affecting the fusion of unilamellar phospholipid vesicles with planar bilayer membranes.

TLDR

Multilamellar phospholipid vesicles fuse with decane‑containing planar bilayers only when an osmotic gradient exists across the membrane. The study investigates whether unilamellar vesicles can fuse with hydrocarbon‑free planar bilayers under the same osmotic conditions that enable multilamellar vesicle fusion. Unilamellar vesicles fuse with hydrocarbon‑free planar bilayers under the same osmotic gradient, requiring divalent Ca²⁺ for negatively charged lipids, while uncharged lipids fuse readily; phosphatidylethanolamine membranes fuse more efficiently than phosphatidylcholine, and the presence of hydrocarbon accelerates fusion and introduces stirring dependence.

Abstract

It was previously shown (Cohen, F. S., J. Zimmerberg, and A. Finkelstein, 1980, J. Gen. Physiol., 75:251-270) that multilamellar phospholipid vesicles can fuse with decane-containing phospholipid bilayer membranes. An essential requirement for fusion was an osmotic gradient across the planar membrane, with the vesicle-containing (cis) side hyperosmotic with respect to the opposite (trans) side. We now report that unilamellar vesicles will fuse with "hydrocarbon-free" membranes subject to these same osmotic conditions. Thus the same conditions that apply to fusion of multilamellar vesicles with planar bilayer membranes also apply to fusion of unilamellar vesicles with these membranes, and hydrocarbon is not required for the fusion process. If the vesicles and/or planar membrane contain negatively charged lipids, divalent cation (approximately 15 mM Ca++) is required in the cis compartment (in addition to the osmotic gradient across the membrane) to obtain substantial fusion rates. On the other hand, vesicles made from uncharged lipids readily fuse with planar phosphatidylethanolamine planar membranes in the near absence of divalent cation with just an osmotic gradient. Vesicles fuse much more readily with phosphatidylethanolamine-containing than with phosphatidylcholine-containing planar membranes. Although hydrocarbon (decane) is not required in the planar membrane for fusion, it does affect the rate of fusion and causes the fusion process to be dependent on stirring in the cis compartment.