N-terminal degradation activates the Nlrp1b inflammasome

Intracellular pathogens and danger signals trigger the formation of inflammasomes, which activate inflammatory caspases and induce pyroptotic cell death. The anthrax lethal factor metalloprotease and small molecule DPP8/9 inhibitors both activate the Nlrp1b inflammasome, but the molecular mechanism of Nlrp1b activation is not known. Here, we used genome-wide CRISPR/Cas9 knockout screens to identify genes required for Nlrp1b-mediated pyroptosis, and discovered that lethal factor induces cell death via the N-end rule proteasomal degradation pathway. Lethal factor directly cleaves Nlrp1b, which induces the N-end rule-mediated degradation of the Nlrp1b N-terminus and thereby frees the Nlrp1b C-terminus to activate caspase-1. DPP8/9 inhibitors also induce proteasomal degradation of the Nlrp1b N-terminus, but, in contrast, not through the N-end rule pathway. Overall, these data reveal that N-terminal degradation is the common mechanism for activation of this innate immune sensor protein.\n\nOne Sentence SummaryProteasome-mediated degradation of the Nlrp1b N-terminus releases the Nlrp1b C-terminus to activate caspase-1 and induce pyroptotic cell death.