Nature
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Inflammasomes are large cytoplasmic complexes that sense microbial infections/danger molecules and induce caspase-1 activation-dependent cytokine production and macrophage inflammatory death. The inflammasome assembled by the NOD-like receptor (NLR) protein NLRC4 responds to bacterial flagellin and a conserved type III secretion system (TTSS) rod component. How the NLRC4 inflammasome detects the two bacterial products and the molecular mechanism of NLRC4 inflammasome activation are not understood. ⋯ Genetic analysis of Chromobacterium violaceum infection revealed that the TTSS needle protein CprI can stimulate NLRC4 inflammasome activation in human macrophages. Similarly, CprI is specifically recognized by human NAIP, the sole NAIP family member in human. The finding that NAIP proteins are inflammasome receptors for bacterial flagellin and TTSS apparatus components further predicts that the remaining NAIP family members may recognize other unidentified microbial products to activate NLRC4 inflammasome-mediated innate immunity.
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Inflammasomes are a family of cytosolic multiprotein complexes that initiate innate immune responses to pathogenic microbes by activating the caspase 1 protease. Although genetic data support a critical role for inflammasomes in immune defence and inflammatory diseases, the molecular basis by which individual inflammasomes respond to specific stimuli remains poorly understood. The inflammasome that contains the NLRC4 (NLR family, CARD domain containing 4) protein was previously shown to be activated in response to two distinct bacterial proteins, flagellin and PrgJ, a conserved component of pathogen-associated type III secretion systems. ⋯ We dissected the biochemical mechanism underlying the requirement for NAIP proteins by use of a reconstituted NLRC4 inflammasome system. We found that NAIP proteins control ligand-dependent oligomerization of NLRC4 and that the NAIP2-NLRC4 complex physically associates with PrgJ but not flagellin, whereas NAIP5-NLRC4 associates with flagellin but not PrgJ. Our results identify NAIPs as immune sensor proteins and provide biochemical evidence for a simple receptor-ligand model for activation of the NAIP-NLRC4 inflammasomes.