The pH-Dependent Trigger in Diphtheria Toxin T Domain Comes with a Safety Latch

Protein-side-chain protonation, coupled to conformational rearrangements, is one way of regulating physiological function caused by changes in protein environment. Specifically, protonation of histidine residues has been implicated in pH-dependent conformational switching in several systems, including the diphtheria toxin translocation (T) domain, which is responsible for the toxin's cellular entry via the endosomal pathway. Our previous studies a) identified protonation of H257 as a major component of the T domain's conformational switch and b) suggested the possibility of a neighboring H223 acting as a modulator, affecting the protonation of H257 and preventing premature conformational changes outside the endosome. To verify this "safety-latch" hypothesis, we report here the pH-dependent folding and membrane interactions of the T domain of the wild-type and that of the H223Q mutant, which lacks the latch. Thermal unfolding of the T domain, measured by circular dichroism, revealed that the reduction in the transition temperature for helical unfolding for an H223Q mutant starts at less acidic conditions (pH <7.5) relative to the wild-type protein (pH <6.5). Hydrogen-deuterium-exchange mass spectrometry demonstrates that the H223Q replacement results in a loss of stability of the amphipathic helices TH1-3 and the hydrophobic core helix TH8 at pH 6.5. That this destabilization occurs in solution correlates well with the pH-range shift for the onset of the membrane permeabilization and translocation activity of the T domain, confirming our initial hypothesis that H223 protonation guards against early refolding. Taken together, these results demonstrate that histidine protonation can fine-tune pH-dependent switching in physiologically relevant systems.