Conformation of the Diphtheria Toxin T Domain in Membranes: A Site-Directed Spin-Labeling Study of the TH8 Helix and TL5 Loop

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Abstract
The isolated T domain of diphtheria toxin was mutated by cysteine-scanning mutagenesis at 28 consecutive sites (residues 328−355) that comprise the TH8 helix and the TL5 interhelical loop in the native toxin. After derivatizing the mutant proteins with a sulfhydryl-selective nitroxide reagent, we examined the mobility of each nitroxide and its accessibility to polar and nonpolar paramagnetic reagents, before and after insertion into phospholipid bilayers. The data obtained with the proteins in solution at pH 8 are generally consistent with predictions from the crystal structure of the toxin. Upon membrane binding at pH 4.6, a major structural reorganization of the domain was seen, which dramatically reduced the accessibility of most residues in this region to the polar reagent nickel(II)−ethylenediaminediacetate complex (NiEDDA). Many of these residues also showed reduced accessibility to the nonpolar reagent O2. Periodic accessibility of the nitroxide side chains along the sequence to these reagents shows that TH8 remains largely helical in the membrane-bound state, with one surface associated with protein and the other facing the hydrophobic interior of the bilayer. In addition, the TL5 loop also appears to become α-helical in the membrane, with one surface in contact with protein and the other in contact with the bilayer interior. These findings provide a structural framework for understanding how the T domain forms a transmembrane channel and mediates translocation of diphtheria toxin's enzymic moiety across a membrane.