Molecular Insights into the Assembly and Functional Diversifification of Typhoid Toxin

Abstract: Typhoid toxin is an A₂B₅ protein toxin and an important virulence factor for the human-adapted bacterial pathogen Salmonella enterica serovar Typhi, the causative agent of typhoid fever. Typhoid toxin contains two enzymatic subunits, PltA and CdtB, which dock onto a pentameric delivery platform composed of the protein PltB. It was recently reported that the same enzymatic subunits can assemble with a different delivery platform composed of the protein PltC, forming a distinct version of typhoid toxin. However, the differences in structure and receptor specificity between the PltC and PltB typhoid toxins remain unknown. Here, we determined atomic-level structures of the pentameric PltC subunit, the fully assembled PltC typhoid toxin, and the PltC pentamers in complex with glycan receptors. Biochemical and structural analyses indicate that PltB and PltC are unable to form heteromeric delivery complexes due to electrostatic repulsion at the subunit interface and thus form separate toxins only. We further observed that, despite low sequence similarity between PltB and PltC, they interact with PltA in a similar manner but that PltC exhibits stronger electrostatic interactions with PltA, enabling it to outcompete PltB in toxin assembly. The ligand-bound atomic structures of PltC show an additional glycan binding site not found in PltB and glycan array analysis indicates that PltB and PltC exhibit significant differences in glycan binding specificity. Collectively, this study offers atomic-level insights into how S. Typhi produces two distinct versions of typhoid toxin, thereby generating functional diversity in this key virulence factor.

Keywords: Salmonella Typhi, typhoid fever, bacterial pathogenesis, bacterial toxins, glycobiology, AB₅ toxins

Xiaoyu Liu, Zhe Chen, Xuyao Jiao, Xukai Jiang, Jicheng Qiu, Fuping You, Hongan Long, Hongzhi Cao, Casey C. Fowler, Xiang Gao* (2021) Molecular Insights into the Assembly and Functional Diversification of Typhoid Toxin. mBio 13(1): e0191621 (*, Corresponding author)