Dipeptidyl peptidase 9 sets a threshold for CARD8 inflammasome formation by sequestering its active C-terminal fragment

CARD8 detects intracellular danger signals and forms a caspase-1 activating inflammasome. Like the related inflammasome sensor NLRP1, CARD8 autoprocesses into noncovalently associated N-terminal (NT) and C-terminal (CT) fragments and binds the cellular dipeptidyl peptidases DPP8 and 9 (DPP8/9). Certain danger-associated signals, including the DPP8/9 inhibitor Val-boroPro (VbP) and HIV protease, induce proteasome-mediated NT degradation and thereby liberate the inflammasome-forming CT. Here, we report cryoelectron microscopy (cryo-EM) structures of CARD8 bound to DPP9, revealing a repressive ternary complex consisting of DPP9, full-length CARD8, and CARD8-CT. Unlike NLRP1-CT, CARD8-CT does not interact with the DPP8/9 active site and is not directly displaced by VbP. However, larger DPP8/9 active-site probes can directly weaken this complex in vitro, and VbP itself nevertheless appears to disrupt this complex, perhaps indirectly, in cells. Thus, DPP8/9 inhibitors can activate the CARD8 inflammasome by promoting CARD8 NT degradation and by weakening ternary complex stability. [Display omitted] •cryo-EM reveals a CARD8-DPP9 ternary complex that can sequester CARD8CT•Binding to DPP9 is necessary for proper regulation of the CARD8 inflammasome•DPP9 complex assembly is a checkpoint for CARD8 downstream of the proteasome•Both N-terminal degradation and disruption of the ternary complex can activate CARD8 Inflammasomes are multiprotein complexes that execute pyroptosis in response to cytosolic danger signals. DPP9 regulates the CARD8 inflammasome by unknown mechanisms. Here, Sharif et al. report cryo-EM structures of CARD8 bound to DPP9 and show that this complex inhibits CARD8 inflammasome activation downstream of the proteasome.