ABSTRACT
Caspases are evolutionarily conserved cysteine proteases that are essential for regulating cell death and are involved in multiple development and disease processes, including immunity. Here, we show that the bacterial type III secretion system (T3SS) effector CopC (Chromobacterium outer protein C) from the environmental pathogen Chromobacterium violaceum attacks caspase-3/-7/-8/-9 by ADPR-deacylization to dysregulate programmed cell death, including apoptosis, necroptosis, and pyroptosis. This modification involves ADP-ribosylation- and deamination-mediated cyclization on Arg207 of caspase-3 by a mechanism that requires the eukaryote-specific protein calmodulin (CaM), leading to inhibition of caspase activity. The manipulation of cell death signaling by CopC is essential for the virulence of C. violaceum in a mouse infection model. CopC represents a family of enzymes existing in taxonomically diverse bacteria associated with a wide spectrum of eukaryotes ranging from humans to plants. The unique activity of CopC establishes a mechanism by which bacteria counteract host defenses through a previously unrecognized post-translational modification.