Figure 4. Caging of head allows for extension of HR1 helix.

(A) Distribution of $Q_{\mathrm{HR1}}$ (number of postfusion-specific HR1 contacts) values when glycans are present. Distribution describes the first frame in each simulation for which the head is outside of the steric cage. In all but three simulations, nearly all HR1 contacts (>1420 of 1489) are formed upon exit of the cage. (B) Distribution when glycans are not included. When glycans are absent, it is common that HR1 is not completely formed (i.e. $Q_{\mathrm{HR1}}<$ 1350) prior to HG escape. (C) Representative snapshot of a caged structure in which HR1 is fully formed and extended toward the host. (D) Representative snapshot of a glycan-free case where the head escapes prior to fully forming HR1. As a result, HR1 can adopt bent configurations.