Microsecond Molecular Dynamics Simulations of Diphtheria Toxin Translocation T-Domain pH-Dependent Unfolding in Solution

Abstract

Diphtheria toxin is a multi-domain protein that invades cells by using their own endocytosis mechanism. In endocytosis, an endosome, a lipid bilayer vesicle, is formed to encapsulate an extracellular molecule. Subsequent acidification of endosome internal solution induces conformational rearrangements and membrane insertion of such encapsulated diphtheria toxin translocation domain (T-domain). In solution at neutral pH, a stand-alone T-domain adopts an all alpha-helical globular structure; however, atomistic details of the pH-dependent conformational changes of the protein are not completely understood. We model structural rearrangements in T-domain in 18 {micro}s long molecular dynamics (MD) simulations of neutral and low pH T-domain models in explicit solvent. At low pH, six histidine residues of the protein were protonated. Two independent MD trajectories resulted in partial protein unfolding at low pH, in which similar regions of the protein conformational subspace were explored. Notably, a pH induced unfolding transition was initiated by partial unfolding of helix TH4 followed by unfolding of helix TH1. Helix TH2 repeatedly unfolds in the low pH T-domain model, which is consequently predicted to be disordered by a consensus of disorder prediction algorithms. Protonation of histidines disrupted a hydrophobic core containing a putative transmembrane helix TH8, which is encircled by hydrophobic surfaces of helices TH3, TH5 and TH9. Afterwards, the low pH T-domain model was reorganized into an ensemble of partially unfolded structures with increased solvent exposure of hydrophobic and charged sites. Thus, MD simulations suggest the destabilizing role of protonation of histidines, in the neutral pH conformation in solution, which may facilitate the initial stages of T-domain membrane binding. The simulation at neutral pH samples conformations in the vicinity of the native structure of the protein. However, significant fluctuations of the protein, including unfolding and refolding of -helices were observed at these simulation time-scales.