Heterogeneity Even at the Speed Limit of Folding: Large-scale Molecular Dynamics Study of a Fast-folding Variant of the Villin Headpiece

D. Ensign, P. M. Kasson, and V. S. Pande. Journal of Molecular Biology (2007)

SUMMARY: This paper describes the first set of results generated using the SMP clients. The main advantage of using SMP for these sorts of calculations is that the amount of computation that one client can do is several times larger than the traditional clients. This means that our simulations can get many times longer that before; in fact, this has allowed us to generate several hundred folding trajectories of the fastest-folding protein known, the HP35-NleNle variant of the villin headpiece subdomain. In this paper, because our simulation time scales compare well to the 700-nanosecond experimental folding time of this protein, AND we’ve generated enough trajectories to get good statistics, we can shed some light on the experimental results. To summarize the result, the first helix of the protein was thought to be highly structured in the unfolded state of the protein; we’ve suggested that structure in this part of the molecule is not enough to lead to fast folding, and that longer time scales than the 700-ns mark may be present in this system.

Check out the movie: it shows some simulation we did for this work, although watching one trajectory is emphatically NOT statistically significant! Some more visualizations of villin from our earlier work can be found on this page.

We have also made the raw data available to researchers on a SimTk.org page. This site includes the raw data, as well as scripts to automate the process and a VMD plugin to allow for browsing of the data. Please contact simbiosfeedback@foldingathome.org if you need help with doing this.

ABSTRACT: We have performed molecular dynamics simulations on a set of nine unfolded conformations of the fastest-folding protein yet discovered, a variant of the villin headpiece subdomain (HP-35 NleNle). The simulations were generated using a new distributed computing method, yielding hundreds of trajectories each on a time scale comparable to the experimental folding time, despite the large (10,000 atom) size of the simulation system. This strategy eliminates the need to assume a two-state kinetic model or to build a Markov state model. The relaxation to the folded state at 300 K from the unfolded configurations (generated by simulation at 373 K) was monitored by a method intended to reflect the experimental observable (quenching of tryptophan by histidine). We also monitored the relaxation to the native state by directly comparing structural snapshots with the native state. The rate of relaxation to the native state and the number of resolvable kinetic time scales both depend upon starting structure. Moreover, starting structures with folding rates most similar to experiment show some native-like structure in the N-terminal helix (helix 1) and the phenylalanine residues constituting the hydrophobic core, suggesting that these elements may exist in the experimentally relevant unfolded state. Our large-scale simulation data reveal kinetic complexity not resolved in the experimental data. Based on these findings, we propose additional experiments to further probe the kinetics of villin folding.