Structural Disorder in Action in a Bacterial Toxin: Secretion, Folding and Host Cell Hijacking

Abstract

The adenylate cyclase toxin (CyaA, 1706 residues) is an RTX protein that plays an essential role in the early stages of respiratory tract colonization by Bordetella pertussis, the causative agent of whooping cough. Its cell intoxication process, however, is still poorly understood. After its secretion through a dedicated type 1 secretion system, CyaA intoxicates human cells via a unique mechanism of translocation of its catalytic domain (AC) directly across the plasma membrane of target cells. Once in the cytosol, AC interacts with calmodulin (CaM) and produces supraphysiological levels of cAMP, leading to cell death. Our results, based on SAXS, HDX-MS and SR-CD data, illustrate how the structural flexibility of this bacterial toxin contributes to various biological processes, including its secretion, its folding, its translocation into target cells, and its activation by CaM. All of these steps involve disorder-to-order structural transitions that are finely tuned to the specific environmental conditions that the toxin successively experiences along its journey from the host bacterium to the eukaryotic target cell. Finally, CyaA is known for its propensity to aggregate into multimeric forms in the absence of a chaotropic agent in vitro. We have recently shown that calcium and molecular confinement are required for CyaA folding into a monomeric and functional species. This opens up new avenues for both basic science, as well as biotechnological applications of recombinant CyaA as an antigen delivery vehicle, and as a potential protective antigen in the next-generation of pertussis vaccines.