Lessons for the study of membrane fusion from membrane interactions in phospholipid systems.

'Fusion' in model systems usually refers to the decay of membrane configurations that are inherently unstable because of the method of preparation. Natural fusion is a controlled event during which the underlying forces and instabilities are subject to the additional effects of biochemical reactions. To understand biological fusion one must be able first to assess the interplay among these physical and chemical factors. This paper reviews traditional measurements of electrostatic double layer and electrodynamic van der Waals forces acting between bilayer membranes. It also describes the much stronger hydration forces that have now been systematically studied. An essential part of any fusion event is the ability of membrane surfaces to overcome or circumvent the hydration barrier in order to make contact. This may be accomplished through applied force, through bridging substances that displace water from the membrane surface, or through biochemical modification of surfaces. In model systems, destruction of the hydration layer can cause violent adhesion, membrane deformation, and rupture. Natural fusion proceeds by more subtle processes whereby interfacial forces are harnessed in ways not yet understood.