Domain Mobility in the ORF2p Complex Revealed by Molecular Dynamics Simulations and Big Data Analysis.

ORF2p (open reading frame 2 protein) is a multifunctional multidomain enzyme that demonstrates both reverse transcriptase and endonuclease activities and is associated with the pathophysiology of cancer. The 3D structure of the entire seven-domain ORF2p complex was revealed with the recent achievements in structural studies. The different arrangements of the CTD (carboxy-terminal domain) and tower domains were identified as the "closed-ring" and "open-ring" conformations, which differed by the hairpin position of the tower domain, but the structural diversity of these complexes has the potential to be more extensive. To study this, we performed sub-microsecond all-atom molecular dynamics simulations of the entire ORF2p complex with different starting configurations. The obtained molecular dynamic trajectories frames were assigned to several clusters following the dimension reduction to three principal components of the 1275 distances feature matrix. Five and six clusters were obtained for the "open" and "closed" ring models, respectively. While the fingers-palm-thumb core retains its rigid configuration during the MD (molecular dynamics) simulations, all other domains display the complicated dynamic behavior not observed in the experimental structures. The EN (endonuclease) and CTD domains display significant translations and rotations while their internal structures stay rigid. The CTD domain can either form strong contacts with the tower or be far apart from it for both formal "open" and "closed" ring states because the tower hairpin position is not the only determining factor of the protein complex configuration. While only the "thumb up" conformation is observed in all the trajectories, the active site can be obstructed by the movement of the CTD domain. Thus, molecular modeling and machine learning techniques provide valuable insights into the dynamical behavior of the ORF2p complex, which is hard to uncover with experimental methods, given the complexity and size of the object.