J. Biol. Chem., Vol. 263, Issue 30, 15369-15377, 10, 1988

Conformation and model membrane interactions of diphtheria toxin fragment A

JM Zhao and E London
Department of Biochemistry, State University of New York, Stony Brook 11794-5215.

Low pH is believed to play a critical role in the penetration of membranes by diphtheria toxin in vivo. In this report, the pH dependence of the conformation of fragment A of diphtheria toxin has been studied using fluorescence techniques. As pH is decreased, fragment A in solution undergoes a reversible conformational change beginning below pH 5. The conformational change occurs rapidly upon exposure to low pH. It involves both an increase in the exposure of tryptophanyl residues to solution and a switch from a hydrophilic state to a hydrophobic state as judged by fragment A binding to micelles of a mild detergent (Brij 96). At low pH fragment A also rapidly and tightly binds to and penetrates model membranes. Binding is reversed when pH is neutralized. The transition pH, the apparent midpoint of the change between the hydrophilic state and the membrane-penetrating hydrophobic state, occurs at about pH 3.5 in the presence of Brij 96 micelles, pH 4 in the presence of small unilamellar vesicles (SUV) composed of zwitterionic phosphatidylcholine, and pH 5 in the presence of SUV composed of 25 mol % anionic phosphatidylglycerol and 75% phosphatidylcholine. The effects of high temperature provide an important clue as to the nature of the changes at low pH. At neutral pH and high temperature, i.e. in the thermally denatured state, a conformational change similar to that observed at low pH occurs, although fragment A does not become hydrophobic. In addition, the effects of low pH and high temperature on the stability of the native state are cumulative. This indicates that the changes in fragment A both at high temperature and at low pH involve denaturation, although there appears to be only partial unfolding under these conditions. Based on the results of this study, the role of fragment A in diphtheria toxin membrane penetration and translocation is evaluated.
CiteULike    Complore    Connotea    Del.icio.us    Digg    Reddit    Technorati    What's this?

This article has been cited by other articles:

				Z. Wu, K. S. Jakes, B. S. Samelson-Jones, B. Lai, G. Zhao, E. London, and A. Finkelstein Protein Translocation by Bacterial Toxin Channels: A Comparison of Diphtheria Toxin and Colicin Ia Biophys. J., November 1, 2006; 91(9): 3249 - 3256. [Abstract] [Full Text] [PDF]

				P. R. D'Silva and A. K. Lala Organization of Diphtheria Toxin in Membranes. A HYDROPHOBIC PHOTOLABELING STUDY J. Biol. Chem., April 14, 2000; 275(16): 11771 - 11777. [Abstract] [Full Text] [PDF]

				J. Ren, K. Kachel, H. Kim, S. E. Malenbaum, R. J. Collier, and E. London Interaction of Diphtheria Toxin T Domain with Molten Globule-Like Proteins and Its Implications for Translocation Science, May 7, 1999; 284(5416): 955 - 957. [Abstract] [Full Text]

				D. Tortorella, D. Sesardic, C. S. Dawes, and E. London Immunochemical Analysis of the Structure of Diphtheria Toxin Shows All Three Domains Undergo Structural Changes at Low pH J. Biol. Chem., November 17, 1995; 270(46): 27439 - 27445. [Abstract] [Full Text] [PDF]

				D. Tortorella, D. Sesardic, C. S. Dawes, and E. London Immunochemical Analysis Shows All Three Domains of Diphtheria Toxin Penetrate across Model Membranes J. Biol. Chem., November 17, 1995; 270(46): 27446 - 27452. [Abstract] [Full Text] [PDF]