Structure and topology of diphtheria toxin R domain in lipid membranes.

The interaction of the receptor-binding domain (R domain) of diphtheria toxin with a pure lipid membrane has been characterized by several approaches. Using a photoactivatable lipid, the R domain has been shown to deeply insert in the lipid membrane. Three regions of the R domain (residues 380-421, 422-441, and 442 to about 483) are protected by their interaction with the membrane from externally added proteases. At least one of these regions is deeply interacting with the lipid membrane, as evidenced by the location of Cys 461 and 471 determined by fluorescence experiments. Binding of the R domain to the lipid membrane is characterized by the appearance of an alpha-helical component whose orientation is compatible with a transmembrane orientation.