Itinerary profiling to analyze a large number of protein-folding trajectories

Understanding how proteins fold through a vast number of unfolded states is a major subject in the study of protein folding. Herein, we present itinerary profiling as a simple method to analyze molecular dynamics trajectories, and apply this method to Trp-cage. In itinerary profiling, structural clusters included in a trajectory are represented by a bit sequence, and a number of trajectories, as well as the structural clusters, can be compared and classified. As a consequence, the structural clusters that characterize the foldability of trajectories were able to be identified. The connections between the clusters were then illustrated as a network and the structural features of the clusters were examined. We found that in the true folding funnel, Trp-cage formed a left-handed main-chain topology and the Trp6 side-chain was located at the front of the main-chain ring, even in the initial unfolded states. In contrast, in the false folding funnel of the pseudo-native states, in which the Trp6 side-chain is upside down in the protein core, Trp-cage had a right-handed main-chain topology and the Trp side-chain was at the back. The initial topological partition, as determined by the main-chain handedness and the location of the Trp residue, predetermines Trp-cage foldability and the destination of the trajectory to the native state or the pseudo-native states.