Research Article: Calpain-Mediated Processing of Adenylate Cyclase Toxin Generates a Cytosolic Soluble Catalytically Active N-Terminal Domain

Date Published: June 26, 2013

Publisher: Public Library of Science

Author(s): Kepa B. Uribe, Aitor Etxebarria, César Martín, Helena Ostolaza, Mikael Skurnik.


Bordetella pertussis, the whooping cough pathogen, secretes several virulence factors among which adenylate cyclase toxin (ACT) is essential for establishment of the disease in the respiratory tract. ACT weakens host defenses by suppressing important bactericidal activities of the phagocytic cells. Up to now, it was believed that cell intoxication by ACT was a consequence of the accumulation of abnormally high levels of cAMP, generated exclusively beneath the host plasma membrane by the toxin N-terminal catalytic adenylate cyclase (AC) domain, upon its direct translocation across the lipid bilayer. Here we show that host calpain, a calcium-dependent Cys-protease, is activated into the phagocytes by a toxin-triggered calcium rise, resulting in the proteolytic cleavage of the toxin N-terminal domain that releases a catalytically active “soluble AC”. The calpain-mediated ACT processing allows trafficking of the “soluble AC” domain into subcellular organella. At least two strategic advantages arise from this singular toxin cleavage, enhancing the specificity of action, and simultaneously preventing an indiscriminate activation of cAMP effectors throughout the cell. The present study provides novel insights into the toxin mechanism of action, as the calpain-mediated toxin processing would confer ACT the capacity for a space- and time-coordinated production of different cAMP “pools”, which would play different roles in the cell pathophysiology.

Partial Text

Bordetella pertussis, the bacterium causative of whooping cough, is one of the important pathogens among infants and toddlers because despite the high vaccine coverage throughout the world, the bacterium is still circulating and killing newborns in many countries [1], [2]. The adenylate cyclase toxin (ACT) is essential in the pathogenesis of B. pertussis[3]–[6].

We report here on two novel and relevant findings: first we demonstrate that after translocation ACT is cleaved off inside the phagocytes at its N-terminal domain by calcium-activated cellular calpain, and second, that the released enzymatically active “soluble” AC domain migrates intracellularly to colocalize to the nucleus and mitochondria. The proteolytic processing of a bacterial toxin by host calpains represents a unique mechanism that confers to ACT the possibility to generate the important second messenger cAMP either underneath the plasma membrane or in various subcellular organelles.