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

Area of Science:

• Biochemistry
• Molecular Biology
• Microbiology

Background:

• Bacterial pathogens utilize nucleotidyl cyclase toxins (e.g., ExoY) that are initially inactive.
• These toxins require binding to host cell actin for activation into potent enzymes.
• The precise molecular mechanism of this actin-mediated activation remains poorly understood.

Purpose of the Study:

• To elucidate the structural basis of nucleotidyl cyclase activation by actin.
• To investigate the mechanism by which bacterial toxins gain enzymatic activity upon host cell interaction.

Main Methods:

• X-ray crystallography was employed to determine the structures of ExoY from Pseudomonas aeruginosa and Vibrio vulnificus.
• Structures were solved in complex with their respective actin activators: F-actin and profilin-G-actin.

Main Results:

• The structures revealed that two previously flexible regions of ExoY become ordered upon actin binding.
• This ordering leads to allosteric stabilization of the nucleotide-binding pocket, resulting in enzyme activation.
• Sequence and conformational differences in actin-binding regions explain selective binding to F-actin versus G-actin.

Conclusions:

• The activation of ExoY by actin follows an allosteric "activation-by-stabilization" mechanism.
• This mechanism, involving a disordered-to-ordered transition, appears conserved in other nucleotidyl cyclase toxins that bind different activators like calmodulin.