ABSTRACT
Membrane fusion occurs commonly in nature, although we do not yet understand the molecular mechanism. Even at the ultrastructural level, the morphological sequence of fusion between membranes is understood only in general terms. It is believed that adjacent membranes come in close contact before fusion occurs. The chronicle description and timed measurements of changes of membrane morphology during fusion are possible only recently, using synchronized fusion triggering and rapid cryofixation techniques. The close contact between fusion partners usually precedes fusion. The adjacent membranes are held together by electrostatic or hydration forces, sometimes via molecular bridges. The actual fusion event starts when the membrane integrity of both partners is locally and momentarily disrupted. The membrane planes of the fusion partners are adjoined momentarily through point or line defects in the bilayers. Membrane fusion may be triggered using natural or artificial means. Examples of natural triggers include secretion initiators and electric excitation for synaptic vesicle release.
