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Cryo-EM structure of the activated NAIP2-NLRC4 inflammasome reveals nucleated polymerization

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Science  23 Oct 2015:
Vol. 350, Issue 6259, pp. 404-409
DOI: 10.1126/science.aac5789

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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

Abstract

The NLR family apoptosis inhibitory proteins (NAIPs) bind conserved bacterial ligands, such as the bacterial rod protein PrgJ, and recruit NLR family CARD-containing protein 4 (NLRC4) as the inflammasome adapter to activate innate immunity. We found that the PrgJ-NAIP2-NLRC4 inflammasome is assembled into multisubunit disk-like structures through a unidirectional adenosine triphosphatase polymerization, primed with a single PrgJ-activated NAIP2 per disk. Cryo–electron microscopy (cryo-EM) reconstruction at subnanometer resolution revealed a ~90° hinge rotation accompanying NLRC4 activation. Unlike in the related heptameric Apaf-1 apoptosome, in which each subunit needs to be conformationally activated by its ligand before assembly, a single PrgJ-activated NAIP2 initiates NLRC4 polymerization in a domino-like reaction to promote the disk assembly. These insights reveal the mechanism of signal amplification in NAIP-NLRC4 inflammasomes.

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