Pathogens and cause pyroptosis and danger signals activate the formation of inflammasomes, which trigger caspases. Small-molecule DPP8/9 and the anthrax lethal factor metalloprotease inhibitors equally trigger the NLRP1B inflammasome, but the mechanism of activation that is NLRP1B is unknown. We used knockout displays to identify genes necessary for NLRP1B-mediated pyroptosis. We discovered that factor induces cell death via the N-end rule proteasomal degradation. NLRP1B is directly cleft by lethal factor, causing the N-end rule degradation of the NLRP1B N terminus and arming the NLRP1B C terminus to activate caspase-1. Thus, N-terminal degradation is the frequent activation mechanism of this innate sensor.