FIG. 6.

States and transitions of S-e. (A) S-e monomer. S1, blue oval; S2, yellow rectangle, drawn here with curved lines to indicate flexibility. (B) S-e trimer. A low pH triggers trimerization and rigidification. (C) Cleaved S-e trimer. The arrows indicate the trypsin cleavage site at the junction of S1 and S2. (D) Rearranged S2 trimer. Dissociation of S1 occurs spontaneously, and it can be accelerated by exposure to 1 M urea. In this diagram, we depict the clustered fusion peptides of the three subunits as an irregularly outlined region at one end of the rearranged S2 shaft and the regions between HR1 and HR2 of the subunits as three ovals at the other end.