Molecular Mechanism Behind the Fast Folding/Unfolding Transitions of Villin Headpiece Subdomain: Hierarchy and Heterogeneity

Proteins involve motions over a wide range of spatial and temporal scales. While the large conformational changes, such as folding and functioning, are slow and appear to occur in a highly cooperative manner, how the hierarchical dynamics over different time scales play a role during these slow transitions has been of great interest over the decades. Here we study the folding mechanism of the villin headpiece subdomain (HP35) to understand the molecular mechanism behind this prototypical fast-folding protein. The ∼400 μs molecular dynamics (MD) trajectories obtained by Piana et al. [Piana, S.; Lindorff-Larsen, K.; Shaw, D. E. Proc. Natl. Acad. Sci. U.S.A. 2012, 109, 17845] are analyzed in detail. By extracting the slowest mode from the trajectories, which is responsible for the folding/unfolding transitions, and by analyzing the transition events along this mode, we find that the transitions occur in a heterogeneous manner. Detailed analysis of the individual transition events shows that the folding/unfolding transitions occur via two qualitatively different pathways, i.e., the unfolding triggered from the C-terminal (α₃ helix) and from the N-terminal (α₁-α₂ loop). Non-native contacts are also found to contribute in slowing down the transitions. The folding of HP35 thus proceeds in a segmental manner rather than cooperatively at the submicrosecond time scale. The Lys→Nle mutation is found to speed up the transitions by rigidifying the α₃ helix, i.e., suppressing one transition pathway. The analysis of the microsecond dynamics in the single-molecule Förster resonance energy transfer efficiency trajectories, which are calculated from the MD data, reveals that the folding/unfolding transitions in the NleNle mutant can be fitted with a two-state model, whereas those in WT appear to be more complex and involves multiple time scales. This is due to the coupling between the folding/unfolding transitions and conformational transitions within the unfolded and intermediate states. The present study demonstrates that a protein as small as HP35 already involves heterogeneous characters during folding/unfolding transitions when the hierarchical dynamics at the molecular level is considered, thus heterogeneity can be a general characteristic in protein folding.