Central domain of diphtheria toxin is the translocation (T) domain. pH induced conformational change in this domain triggers insertion into the endosomal membrane and facilitates the transfer of the catalytic domain into the cytoplasm.

1: Refined structure of monomeric diphtheria toxin at 2.3 A resolution. Bennett MJ, Eisenberg D; Protein Sci 1994;3:1464-1475. PMID:7833808

2: Crystal structure of diphtheria toxin bound to nicotinamide adenine dinucleotide. Bell CE, Eisenberg D; Biochemistry 1996;35:1137-1149. PMID:8573568