FUSION OF UNILAMELLAR VESICLES TO PLANAR LIPID BILAYERS: A MODEL FOR EXOCYTOSIS

Abstract

Exocytosis is a ubiquitous process in eukaryotic cells. Despite this, little is known about the mechanism by which this process takes place. A model system, the fusion of phospholipid vesicles with planar bilayer membranes, has been developed to study this process. Fusion was assayed by monitoring the incorporation of a vesicular membrane marker, an ion channel (porin), into the planar membrane. The overall fusion process can be experimentally separated into two steps. In the first step, the vesicle attaches to the planar membrane and forms a stable pre-fusion state. The influence of lipid composition and divalent cations on the formation of this state are examined. In the second step osmotically driven water flow into the vesicles causes vesicular swelling. If the swelling is sufficient it destabilizes the vesicle membrane in the region of contact resulting in rupture of the membrane and subsequent fusion with the planar membrane. Osmotic water flow is thus the driving force for fusion. Water flow into the vesicles can be induced either directly by classical colloid osmotic processes or indirectly by a transplanar membrane osmotic gradient, with vesicle-containing side hyperosmotic. The effects of osmoticant permeability and the magnitude of the osmotic gradient on fusion are studied. Finally the implications of these results for biological fusion are discussed.