Membrane Fusion: The Influenza Paradigm

Membrane fusion reactions are central to the lives of all eukaryotic cells. In addition, the fusion events that join two or more previously separate cells, such as sperm-egg fusion and myoblast fusion, provide key developmental switches. Topologically, fusion events can be considered in two classes. Ectoplasmic fusion events, alternatively referred to as periplasmic fusion events, are those in which the bilayer leaflets that first contact one another face the extracellular space. Endoplasmic fusion events, alternatively referred to as cytoplasmic fusion events, are those in which the bilayer leaflets that first contact one another face the cytoplasm. Hence, virus-cell and cell-cell fusion events are ectoplasmic events, whereas fusion events that deliver material along both the endocytotic and secretory pathways are endoplasmic (White 1992; Rothman and Warren 1994).