Concerted protonation of key histidines triggers membrane interaction of the diphtheria toxin T domain

The translocation domain (T domain) of the diphtheria toxin contributes to the transfer of the catalytic domain from the cell endosome to the cytosol, where it blocks protein synthesis. Translocation is initiated when endosome acidification induces the interaction of the T domain with the membrane of the compartment. We found that the protonation of histidine side chains triggers the conformational changes required for membrane interaction. All histidines are involved in a concerted manner, but none is indispensable. However, the preponderance of each histidine varies according to the transition observed. The pair His(223) - His(257) and His(251) are the most sensitive triggers for the formation of the molten globule state in solution, whereas His(322) - His(323) and His(251) are the most sensitive triggers for membrane binding. Interestingly, the histidines are located at key positions throughout the structure of the protein, in hinges and at the interface between each of the three layers of helices forming the domain. Their protonation induces local destabilizations, disrupting the tertiary structure and favoring membrane interaction. We propose that the selection of histidine residues as triggers of membrane interaction enables the T domain to initiate translocation at the rather mild pH found in the endosome, contributing to toxin efficacy.