Kurnikov IV, Kyrychenko A, Flores-Canales JC, Rodnin MV, Simakov N, Vargas-Uribe M, Posokhov YO, Kurnikova M, Ladokhin AS. pH-triggered conformational switching of the diphtheria toxin T-domain: the roles of N-terminal histidines.
J Mol Biol 2013;
425:2752-64. [PMID:
23648837 DOI:
10.1016/j.jmb.2013.04.030]
[Citation(s) in RCA: 34] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/25/2012] [Revised: 03/13/2013] [Accepted: 04/27/2013] [Indexed: 01/28/2023]
Abstract
pH-induced conformational switching is essential for functioning of diphtheria toxin, which undergoes a membrane insertion/translocation transition triggered by endosomal acidification as a key step of cellular entry. In order to establish the sequence of molecular rearrangements and side-chain protonation accompanying the formation of the membrane-competent state of the toxin's translocation (T) domain, we have developed and applied an integrated approach that combines multiple techniques of computational chemistry [e.g., long-microsecond-range, all-atom molecular dynamics (MD) simulations; continuum electrostatics calculations; and thermodynamic integration (TI)] with several experimental techniques of fluorescence spectroscopy. TI calculations indicate that protonation of H257 causes the greatest destabilization of the native structure (6.9 kcal/mol), which is consistent with our early mutagenesis results. Extensive equilibrium MD simulations with a combined length of over 8 μs demonstrate that histidine protonation, while not accompanied by the loss of structural compactness of the T-domain, nevertheless results in substantial molecular rearrangements characterized by the partial loss of secondary structure due to unfolding of helices TH1 and TH2 and the loss of close contact between the C- and N-terminal segments. The structural changes accompanying the formation of the membrane-competent state ensure an easier exposure of the internal hydrophobic hairpin formed by helices TH8 and TH9, in preparation for its subsequent transmembrane insertion.
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