title: Secondary structure of diphtheria toxin and its fragments interacting with acidic liposomes studied by polarized infrared spectroscopy.
abstract: We used infrared attenuated total reflection spectroscopy to study the structure of diphtheria toxin (DT) and its fragments A, B, CB1, and CB4 as a function of the pH in the absence and in the presence of phospholipid vesicles. Binding of DT to asolectin or DL-alpha-dipalmitoylphosphatidylcholine-DL-alpha-dipalmitoylphosphatidic acid liposomes at pH 7.3 results in a 10% increase in its alpha-helix content. At pH 4, in the presence of liposomes, the secondary structure of DT is characterized by the appearance of a beta-sheet structure with strengthened hydrogen bonds which did not exist before pH lowering. DT fragment B displays little conformational change upon pH lowering in the presence of liposomes. However, the alpha-helix content of CB1 increases by 10%, and polarization measurements indicate that the alpha-helices of CB1 at pH 4 are oriented parallel to the lipid acyl chains. On the other hand, the alpha-helix content of CB4 decreases dramatically while the low frequency beta-sheet content increases. Dichroism measurements demonstrate that this sheet lies close to a parallel to the bilayer surface. The fragment A of DT experiences a large conformational change upon pH lowering and binds to the liposome membrane even in the absence of DT fragment B. The conformational modification of DT fragment A is fully reversed when pH is brought back to 7.3.
authors.affiliation: Laboratoire de Chimie Physique des Macromolécules aux Interfaces, Université Libre de Bruxelles, Belgium.
authors.forename: V
authors.initials: V
authors.lastname: Cabiaux
authors.forename: R
authors.initials: R
authors.lastname: Brasseur
authors.forename: R
authors.initials: R
authors.lastname: Wattiez
authors.forename: P
authors.initials: P
authors.lastname: Falmagne
authors.forename: J M
authors.initials: JM
authors.lastname: Ruysschaert
authors.forename: E
authors.initials: E
authors.lastname: Goormaghtigh
mesh_terms: D015060:1,2-Dipalmitoylphosphatidylcholine; D000577:Amides; D004167:Diphtheria Toxin; D005583:Fourier Analysis; D006863:Hydrogen-Ion Concentration; D008081:Liposomes; D010446:Peptide Fragments; D010712:Phosphatidic Acids; D011487:Protein Conformation; D013055:Spectrophotometry, Infrared
chemical_list: D000577:Amides; D004167:Diphtheria Toxin; D008081:Liposomes; D010446:Peptide Fragments; D010712:Phosphatidic Acids; D015060:1,2-Dipalmitoylphosphatidylcholine