Invagination of Giant Unilamellar Vesicles upon Membrane Mixing with Native Vesicles.

We demonstrate rapid membrane mixing between GUVs of pure lipid compositions and membrane vesicles (MVs) isolated from the plasma membrane of Vero cells, resulting in the transfer of native lipids and proteins to the GUVs. The steps involved in the membrane mixing are docking followed by membrane fusion. We show that positively charged lipids of the GUVs are essential for the docking, and the native membrane components of MVs drive the fusion. The interleaflet and lateral asymmetry and a change in the membrane tension upon the membrane mixing trigger membrane invagination. We detected outward and inward invagination sites at the rim of the GUVs within 10-40 min of the membrane mixing. The extent of the invaginations depends on the cholesterol and sphingomyelin (SM) contents in the GUVs. Cholesterol content above a critical concentration disfavors membrane invaginations, and the SM lipid is an essential molecular factor for membrane invagination.