Fig. 2.

Comparison of folding energies and kinetics of different gp41 complexes. (A) Force-extension curves (FECs) of gp41-ll, gp41-sl, and gp41-ll-N554D. FECs were obtained by pulling (black) and then relaxing (gray) single gp41 complexes and fit by the worm-like chain model (red) in the continuous regions, revealing different gp41 folding states (insets). Data associated with all FECs shown in this work were mean-filtered using a 10-ms time window. A close-up view of the transition region is shown as an inset. (B) Extension-time trajectories of gp41-ll under three different constant mean forces revealing the reversible transition between the 6HB and the 5HB states. The idealized state transitions based on HMM are shown in red. (C) Force-dependent unfolding probabilities (Upper) and transition rates (Lower) for gp41-ll (black), gp41-sl (purple), and gp41-ll-N554D (red). Solid and hollow symbols indicate unfolding and folding rates, respectively. The measurements (symbols) were fit with a theoretical model (lines, see Materials and Methods). (D) Extension-time trajectories of gp41-sl under three constant forces. (E) Extension-time trajectory of gp41-ll at pH 6.0. (F) Loop size-dependent equilibrium lifetimes and the extrapolation to the wild-type loop size of the gp41 hairpin (red dot). The traces in B, D, and E shared the same scale bar.