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Open AccessArticle

Role of Acidic Residues in Helices TH8–TH9 in Membrane Interactions of the Diphtheria Toxin T Domain

Department of Biochemistry and Molecular Biology, University of Kansas Medical Center, Kansas City, KS 66160, USA
Department of Microbiology and Immunobiology, Harvard Medical School, Boston, MA 02115, USA
Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213, USA
Author to whom correspondence should be addressed.
These authors contributed equally and are listed alphabetically.
Academic Editor: Holger Barth
Toxins 2015, 7(4), 1303-1323;
Received: 5 February 2015 / Revised: 6 April 2015 / Accepted: 7 April 2015 / Published: 14 April 2015
(This article belongs to the Section Bacterial Toxins)
The pH-triggered membrane insertion of the diphtheria toxin translocation domain (T domain) results in transferring the catalytic domain into the cytosol, which is relevant to potential biomedical applications as a cargo-delivery system. Protonation of residues is suggested to play a key role in the process, and residues E349, D352 and E362 are of particular interest because of their location within the membrane insertion unit TH8–TH9. We have used various spectroscopic, computational and functional assays to characterize the properties of the T domain carrying the double mutation E349Q/D352N or the single mutation E362Q. Vesicle leakage measurements indicate that both mutants interact with the membrane under less acidic conditions than the wild-type. Thermal unfolding and fluorescence measurements, complemented with molecular dynamics simulations, suggest that the mutant E362Q is more susceptible to acid destabilization because of disruption of native intramolecular contacts. Fluorescence experiments show that removal of the charge in E362Q, and not in E349Q/D352N, is important for insertion of TH8–TH9. Both mutants adopt a final functional state upon further acidification. We conclude that these acidic residues are involved in the pH-dependent action of the T domain, and their replacements can be used for fine tuning the pH range of membrane interactions. View Full-Text
Keywords: pH-trigger; conformational switching; membrane insertion pH-trigger; conformational switching; membrane insertion
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Ghatak, C.; Rodnin, M.V.; Vargas-Uribe, M.; McCluskey, A.J.; Flores-Canales, J.C.; Kurnikova, M.; Ladokhin, A.S. Role of Acidic Residues in Helices TH8–TH9 in Membrane Interactions of the Diphtheria Toxin T Domain. Toxins 2015, 7, 1303-1323.

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