FIG. 7.

Model for the role of free thiols in virion-associated F protein. (A) F protein in prefusion conformation. HN protein binds to the receptor and facilitates interaction between F protein and target cell isomerases. This binding leads to cleavage of disulfide bond in F protein (B) in prefusion or open stalk conformation, which has been shown to be present at lower temperatures. (C) F protein in prehairpin conformation, which has been shown to require 37°C. (D) F protein in postfusion conformation. HR1 and HR2 mutants are proposed to be defective in attaining form C and are arrested in form B, an open stalk conformation.