Identification of shallow and deep membrane-penetrating forms of diphtheria toxin T domain that are regulated by protein concentration and bilayer width

The α-helix-rich, hydrophobic transmembrane (T) domain of diphtheria toxin is believed to play a central role in membrane insertion by the toxin and in the translocation of its catalytic domain across membranes. In this report, T domain structure was studied using site-directed single-Cys mutants. The residues chosen, 322 (near the amino-terminal end of helix TH8), 333 (within helix TH8), and 356 (within helix TH9) were substituted with Cys and labeled with the fluorescent probe bimane. (Residues 333 and 356 should be located within the bilayer in the transmembrane state, and residue 322 should not penetrate the bilayer.) After insertion of T domain into model membrane vesicles, the location of bimane label relative to the lipid bilayer was characterized by its fluorescence emission and by its quenching with nitroxide-labeled phospholipids. It was found that when the T domain is added to dioleoylphosphatidylcholine-containing vesicles, all three residues reside close to the outer surface. However, at high T domain concentration or in thinner dimyristoleoylphosphatidylcholine-containing vesicles, a large fraction of residues 333 and 356 penetrate deeply into the membrane. In contrast, residue 322 remains exposed to aqueous solution under these conditions. These conclusions were confirmed by a novel antibody binding method. Antibodies that quench the fluorescence of 4,4-difluoro-5,7-dimethyl- 4-bora-3a,4a-diaza-3-indacene (BODIPY) groups were used to evaluate the exposure of BODIPY-labeled 322, 333, and 356. Maximum exposure of residues 333 and 356 to externally added antibody was only observed under conditions in which bimane fluorescence showed that these residues do not penetrate the bilayer. In contrast, residue 322 remained exposed under all conditions. We propose that the deeply penetrating T domain conformation represents a transmembrane or near-transmembrane state the regulation of the transmembrane/nontransmembrane equilibrium should be a key to understanding diphtheria toxin membrane insertion and translocation. Our results suggest that toxin-toxin interactions may play an important role in regulating this behavior.