Mechanism of actin-dependent activation of nucleotidyl cyclase toxins from bacterial human pathogens

Several bacterial human pathogens secrete nucleotidyl cyclase toxins, that are activated by interaction with actin of the eukaryotic host cells. However, the underlying molecular mechanism of this process which protects bacteria from self-intoxication is unclear. Here, we report structures of ExoY from Pseudomonas aeruginosa and Vibrio vulnificus in complex with their corresponding activators F-actin and profilin-G-actin. The structures reveal that in contrast to the apo state, two flexible regions become ordered and interact strongly with actin. The specific stabilization of these regions results in an allosteric stabilization of the distant nucleotide binding pocket and thereby to an activation of the enzyme. Differences in the sequence and conformation of the actin-binding regions are responsible for the selective binding to either F- or G-actin. This specificity can be biotechnologically modulated by exchanging these regions from one toxin to the other. Other bacterial nucleotidyl cyclases, such as the anthrax edema factor and CyaA from Bortedella pertussis, that bind to calmodulin undergo a similar disordered-to-ordered transition during activation, suggesting that the allosteric activation-by-stabilization mechanism of ExoY is paradigmatic for all bacterial nucleotidyl cyclase toxins.