Structures of distant diphtheria toxin homologs reveal functional determinants of an evolutionarily conserved toxin scaffold

Diphtheria toxin (DT) is the archetype for bacterial exotoxins implicated in human diseases and has played a central role in defining the field of toxinology since its discovery in 1888. Despite being one of the most extensively characterized bacterial toxins, the origins and evolutionary adaptation of DT to human hosts remain unknown. Here, we determined the first high-resolution structures of DT homologs outside of the Corynebacterium genus. DT homologs from Streptomyces albireticuli (17% identity to DT) and Seinonella peptonophila (20% identity to DT), despite showing no toxicity toward human cells, display significant structural similarities to DT sharing both the overall Y-shaped architecture of DT as well as the individual folds of each domain. Through a systematic investigation of individual domains, we show that the functional determinants of host range extend beyond an inability to bind cellular receptors; major differences in pH-induced pore-formation and cytosolic release further dictate the delivery of toxic catalytic moieties into cells, thus providing multiple mechanisms for a conserved structural fold to adapt to different hosts. Our work provides structural insights into the expanding DT family of toxins, and highlights key transitions required for host adaptation. Toxic proteins are responsible for the disease symptoms accompanying bacterial infections. Sugiman-Marangos et al. report crystal structures of two diphtheria toxin homologs, therefore providing structural insights into this family of toxins and the evolutionary adaptation of these toxins to human hosts.

Automated summary

Diphtheria toxin (DT) is a bacterial exotoxin that plays a central role in toxinology. However, the origins and evolutionary adaptation of DT to human hosts are still unknown. In this study, the researchers determined the high-resolution structures of DT homologs from Streptomyces albireticuli and Seinonella peptonophila. Despite not being toxic to human cells, these DT homologs displayed significant structural similarities to DT. The researchers found that the functional determinants of host range extended beyond cellular receptor binding, and differences in pH-induced pore formation and cytosolic release played a major role in toxin delivery into cells. This work provides insights into the expanding family of DT toxins and the mechanisms of host adaptation.