Intracellular and viral membrane fusion: a uniting mechanism

Structural and functional analyses have revealed remarkable mechanistic similarities between viral and intracellular fusion. Both fusion processes are driven by an orchestrated cascade of protein binding and folding reactions. After an initial tethering step, activation of the fusion machinery links the opposing membranes and protein folding pulls the membranes in close proximity; fusion pores form, open and dilate, and the process culminates in the complete merging of the lipid bilayers. Viral fusion is mediated by a single fusion protein, whereas the intracellular fusion machinery is split into matching halves, the v- and t-SNAREs. SNAREs, together with synaptotagmins, emerge as the key machinery for regulated exocytosis.