The fusion of two membranes into one is an event shared by intracellular trafficking, fertilization, tissue formation, and viral infection (Earp et al., 2005; Jahn et al., 2003; Mohler et al., 2002). On a structural level, fusion results in the unification of the lipid and protein components of the two membranes and the intermixing of the volumes initially bound by them. During membrane fusion, the proximal leaflets of the two membranes merge first, whereas the distal membrane leaflets remain separate until the opening of a fusion pore (Chernomordik and Kozlov, 2005). This intermediate stage, called hemifusion, is a critical event shared by exocytosis, protein trafficking, and viral entry.  The topic of viral fusion peptides has been reviewed (Apellániz et al. 2014).

Three distinct classes of viral membrane fusion proteins have been identified based on structural criteria. In addition, there are at least four distinct mechanisms by which viral fusion proteins can be triggered to undergo fusion-inducing conformational changes. Viral fusion proteins also contain different types of fusion peptides and vary in their reliance on accessory proteins. These differing features combine to yield a rich diversity of fusion proteins. Yet despite this staggering diversity, all characterized viral fusion proteins convert from a fusion-competent state (dimers or trimers, depending on the class) to a membrane-embedded homotrimeric prehairpin, and then to a trimer-of-hairpins that brings the fusion peptide, attached to the target membrane, and the transmembrane domain, attached to the viral membrane, into close proximity, thereby facilitating the union of viral and target membranes. During these conformational conversions, the fusion proteins induce membranes to progress through stages of close apposition, hemifusion, and then the formation of small, and finally large, fusion pores. Clearly, highly divergent proteins have converged on the same overall strategy to mediate fusion, an essential step in the life cycle of every enveloped virus. (White et al., 2008).

White et al 2008 summarize the properties of fusion proteins from fusion proteins of different families of enveloped viruses. The native proteins are proteolytically processed to generate the pore-forming fusion proteins. The native proteins are provided below. Information on these proteins is provided in Table 1 (White et al., 2008).