Human caspase-1 autoproteolysis is required for ASC-dependent and -independent inflammasome activation
Pathogen-related signals induce a number of cytosolic pattern-recognition receptors (PRRs) to form canonical inflammasomes, which activate pro-caspase-1 and trigger pyroptotic cell death. All well-studied PRRs oligomerize with the pro-caspase-1-adapter protein ASC to generate a single large structure in the cytosol, which induces the autoproteolysis and activation of the pro-caspase-1 zymogen. However, several PRRs can also directly interact with pro-caspase-1 without ASC, forming much smaller \"ASC-independent\" inflammasomes. It is currently thought that pro-caspase-1 autoproteolysis does not occur during, and is not required for, ASC-independent inflammasome activation. Here, we show that the related human PRRs NLRP1 and CARD8 exclusively form ASC-dependent and ASC-independent inflammasomes, respectively, identifying CARD8 as the first PRR that cannot form an ASC-containing signaling platform. Despite their different structures, we discovered that both the NLRP1 and CARD8 inflammasomes require pro-caspase-1 autoproteolysis between the small and large catalytic subunits to induce pyroptosis. Thus, pro-caspase-1 self-cleavage is an obligate regulatory step in the activation of human canonical inflammasomes.