Fusogens

What are the key players in membrane fusion? Membrane fusion is essential to life. It is required for the trafficking of material between cells and cellular compartments, for the mixing of genetic information between organisms and for the sculpting of tissues during development. Membrane fusion is also necessary for infection by enveloped viruses (Figure 1). If membranes could fuse spontaneously, chaos would rule. Trillions of intracellular vesicles and organelles would merge and cells would promiscuously fuse, eliminating all cellular compartments as well as cellularity itself. Luckily, even close and long-lived contact between biological membranes does not result in fusion. Membrane fusion is inhibited by the dense packing of proteins at the contact sites between biological membranes and by the high energy barriers associated with membrane deformation, lipid mixing and fusion-pore expansion. For fusion to become energetically favorable the membranes must overcome the repulsion forces generated by the charged hydrated phospholipids and mix with minimal exposure of their hydrophobic cores. Only proteins are sufficiently complex to coordinate, execute and control such an event, and so began the quest for the missing fusogens of intracellular membranes, cells and viruses.