A ubiquitin-independent proteasome pathway controls the CARD8 inflammasome

Abstract

CARD8 is a pattern-recognition receptor that forms a caspase-1-activating inflammasome. CARD8 undergoes autoproteolysis, generating an N-terminal (NT) fragment with a disordered region and a ZU5 domain and a C-terminal (CT) fragment with UPA and CARD domains. DPP8 and DPP9 (DPP8/9) inhibitors, including Val-boroPro (VbP), accelerate the degradation of the NT fragment via a poorly characterized proteasome-mediated pathway, thereby releasing the inflammatory CT fragment from autoinhibition. Here, we show that the core 20S proteasome, which degrades disordered and misfolded proteins independent of ubiquitin, controls CARD8 activation. In unstressed cells, the 20S proteasome degrades just the NT disordered region, leaving behind the folded ZU5, UPA, and CARD domains to act as an inhibitor of inflammasome assembly. In VbP-stressed cells, the 20S proteasome degrades the entire NT fragment, perhaps due to ZU5 domain unfolding, freeing the CT fragment from autoinhibition. Overall, this work shows that CARD8 NTs susceptibility to 20S proteasome-mediated degradation controls inflammasome activation.