The Role of Calcium and Osmosis in Membrane Fusion

Abstract

Although exocytosis is triggered by a rise in intracellular levels of Ca²⁺, it is not known how Ca²⁺ produces this effect. Because Ca²⁺ regulates so many cellular functions, it is difficult to establish what role any one of these plays in exocytosis. An approach for determining the mechanisms underlying fusion is to study those aspects of the process that can be attributed solely to phospholipid bilayer interactions, independent of membrane proteins and cytoplasmic factors. Thus, considerable effort has gone into studying fusion between artificial phospholipid bilayer membranes, with most of this effort directed to fusion between vesicles [8,9,11]. On the other hand, we have studied the fusion of phospholipid vesicles to planar phospholipid bilayer membranes. Here, we describe the physical principles that govern fusion in this system and discuss their possible applicability to fusion and exocytosis as they occur biologically.