Inflammasomes take the wheel: Cells require microbial ligand binding to sense pathogens (see the Perspective by Liu and Xiao). Binding to the family of NOD-like receptors triggers the assembly of large protein signaling complexes called inflammasomes, leading infected cells to die and produce inflammatory mediators. Hu et al. and Zhang et al. use cryo–electron microscopy to uncover the structural and biochemical basis of two such receptors: NAIP2, which directly binds microbial ligands, and NLRC4, a protein functioning directly downstream. A self-propagating activation mechanism of downstream inflammasome signaling starts with one molecule of NAIP4 directly binding its microbial ligand. NAIP4 then catalyzes the activation of 10 to 12 NLRC4 molecules to form a wheel-like structure. Science , this issue p. 399 , 404 ; see also p. 376

The CARD-only protein INCA inhibits inflammasome assembly by capping caspase-1 CARD oligomers and preventing their further polymerization. Inflammasomes are cytosolic caspase-1-activation complexes that sense intrinsic and extrinsic danger signals, and trigger inflammatory responses and pyroptotic cell death. Homotypic interactions among Pyrin domains and caspase recruitment domains (CARDs) in inflammasome-complex components mediate oligomerization into filamentous assemblies. Several cytosolic proteins consisting of only interaction domains exert inhibitory effects on inflammasome assembly. In this study, we determined the structure of the human caspase-1 CARD domain (caspase-1CARD) filament by cryo-electron microscopy and investigated the biophysical properties of two caspase-1-like CARD-only proteins: human inhibitor of CARD (INCA or CARD17) and ICEBERG (CARD18). Our results reveal that INCA caps caspase-1 filaments, thereby exerting potent inhibition with low-nanomolar Ki on caspase-1CARD polymerization in vitro and inflammasome activation in cells. Whereas caspase-1CARD uses six complementary surfaces of three types for filament assembly, INCA is defective in two of the six interfaces and thus terminates the caspase-1 filament.

Significance: The proteasome holoenzyme is an ATP-dependent protease in eukaryotes that degrades ubiquitylated substrates. It is involved in numerous important biological processes, such as cell division, differentiation, innate immunity, adaptive immunity, regulation of gene expression, and response to proteotoxic stress. Using cryoelectron microscopy, we have examined multiple conformational states of the human proteasome at medium to high resolution. Our results reveal that the substrate-conducting channel in the core particle is transiently opened and accompanied by dynamic changes in structure of the particle. These observations provide new insights into how the proteasome recognizes ubiquitylated substrates and translocates them through a channel and gate to degradation sites in the core particle.