Cryo-EM structure of the NLRC4^CARD filament provides insights into how symmetric and asymmetric supramolecular structures drive inflammasome assembly

Inflammasomes are supramolecular signaling platforms integral to innate immune defense against invading pathogens. The NOD-like receptor (NLR) family apoptosis inhibitory protein (NAIP)䡠NLR family caspase-recruiting domain (CARD) domain-containing 4 (NLRC4) inflammasome recognizes intracellular bacteria and induces the polymerization of the caspase-1 protease, which in turn executes maturation of interleukin-1 (IL-1) and pyroptosis. Several high-resolution structures of the fully assembled NAIP䡠NLRC4 complex are available, but these structures do not resolve the architecture of the CARD filament in atomic detail. Here, we present the cryo-EM structure of the filament assembled by the CARD of human NLRC4 (NLRC4^CARD) at 3.4 Å resolution. The structure revealed that the helical architecture of the NLRC4^CARD filament is essentially identical to that of the downstream filament assembled by the CARD of caspase-1 (casp1^CARD), but deviates from the split washer-like assembly of the NAIP䡠NLRC4 oligomer. Our results suggest that architectural complementarity is a major driver for the recognition between upstream and downstream CARD assemblies in inflammasomes. Furthermore, a Monte Carlo simulation of the NLRC4^CARD filament assembly rationalized why an (un)decameric NLRC4 oligomer is optimal for assembling the helical base of the NLRC4^CARD filament. Together, our results explain how symmetric and asymmetric supramolecular assemblies enable high-fidelity signaling in inflammasomes.