FIGURE 2.

General mechanism of fusion process employed by Class I fusion proteins. An environmental trigger such as an acidic pH or the binding to a coreceptor (represented figuratively by the scissors) induces a conformational change that exposes the fusion peptide (A). The fusion peptide then inserts into the host cell membrane causing the fusion proteins to fold back on themselves, inducing the bending of apposed membranes (B). The folding creates a contact between the membranes, leading first to hemifusion (C). Finally, the refolding leads to the formation of the fusion pore (D) and subsequent mixing of contents.