ABSTRACT
The fusion of membrane compartments within living cells is well-understood to underlie the basic mechanisms of transfer, processing, release, and reuptake of materials. The fusion processes can be modeled using various preparations of membrane bilayers composed of phospholipids incorporating various other biological components. An assay for fusion of lipid vesicles with cells was developed in which transfer of contents can be visualized. This assay is based on self-quenching properties of the water-soluble dye carboxyfluorescein and monitors transfer of contents of liposomes to cells. Moreover, fluorescence photobleaching showed that fluorescent lipid probes incorporated into lipid vesicles remained immobilized on the cell surface. Most fusion assays rely upon a change in the strength of the fluorescent signal by quenching or resonance energy transfer. Membrane-mixing assays depend upon the fluid mosaic nature of the bilayer to allow mixing of the probe molecules. However, it is well-understood that other properties of fluid mosaic bilayers can change the properties of fluorescent probes associated with them.
