Inflammasomes sense intrinsic and extrinsic danger signals to trigger inflammatory responses and pyroptotic cell death. Homotypic pyrin domain (PYD) interactions of inflammasome forming Nod-like receptors with the adaptor protein ASC mediate oligomerization into helical filamentous assemblies. These supramolecular organizing centers recruit and activate caspase-1, which results in IL-1β family cytokine maturation and pyroptotic cell death. The molecular details of the critical step in signal transduction of inflammasome signaling, however, remain ill-defined. Here, we describe the cryo-EM structure of the human NLRP3 PYD filament at 3.6 Ang resolution. We identify a unique pattern of highly polar interface residues that form the homomeric interactions leading to characteristic filament ends that we designate as A- and B-end, respectively. Coupling a titration polymerization assay to cryo-EM, we demonstrate that the ASC adaptor protein elongation on NLRP3 PYD filament seeds is unidirectional, associating exclusively to the B-end of the NLRP3 filament. Notably, NLRP3 and ASC PYD filaments exhibit the same symmetry in rotation and axial rise per subunit, allowing for a continuous transition between NLRP3 as the nucleation seed and ASC as the elongator. Integrating the directionality of filament growth, we present a molecular model of the ASC speck consisting of active NLRP3-NEK7, ASC, and Caspase-1 proteins.
Inflammasomes in Liver Fibrosis
