Unified Polymerization Mechanism for the Assembly of ASC-Dependent Inflammasomes

Abstract

SUMMARY Inflammasomes elicit host defense inside cells by activating caspase-1 for cytokine maturation and cell death. AIM2 and NLRP3 are representative sensor proteins in two major families of inflammasomes. The adaptor protein ASC bridges the sensor proteins and caspase-1 to form ternary inflammasome complexes, achieved through pyrindomain (PYD) interactions between sensors and ASC, and caspase activation and recruitment domain (CARD) interactions between ASC and caspase-1. We found that PYD and CARD both form filaments. Activated AIM2 and NLRP3 nucleate PYD filaments of ASC, which in turn cluster the CARD of ASC. ASC thus nucleates CARD filaments of caspase-1 leading to proximity-induced activation. Endogenous NLRP3 inflammasome is also filamentous. The cryo-EM structure of ASCPYD filament at near-atomic resolution provides a template for homo- and hetero-PYD/PYD associations, as confirmed by structure-guided mutagenesis. We propose that ASC-dependent inflammasomes in both families share a unified assembly mechanism that involves two successive steps of nucleation-induced polymerization.

The emerging role of human PYHIN proteins in innate immunity: Implications for health and disease

“…It is becoming more apparent that innate host sensors frequently utilise aggregation dependent amplification mechanisms for signal activation [43,58]. Indeed, the HCMV protein pUL83 has been shown to interact with the PYD of IFI16, blocking its oligomerisation [29], and thus antagonising a critical step required for receptor activation.…”

“…Interestingly, amino acids in the IFI16PYD which are critical for IFI16 oligomerisation are not well conserved in AIM2 [47], raising the possibility that AIM2PYD either oligomerises differently or not at all, although overexpression of the AIM2PYD does result in filament formation [29]. Since AIM2 can nucleate ASCPYD, leading to the polymerisation of ASC in the AIM2 inflammasome [43,58], it remains a possibility that AIM2PYDs which are in very close proximity to each other rather than polymerised per se may be sufficient to nucleate ASC for inflammasome formation.…”

Higher‐order oligomerization promotes localization of SPOP to liquid nuclear speckles

“…Higher‐order complexes, that is, oligomers in which the number of monomers in a complex is broadly distributed and can be large, have important functions in signal transduction and cell fate decisions (Credle et al , 2005; Korennykh et al , 2009; Yin et al , 2009; Li et al , 2012a; Banjade & Rosen, 2014; Lu et al , 2014; Xu et al , 2014). The inherent size heterogeneity of such higher‐order assemblies generates challenges for their structural and mechanistic characterization, but recent progress has provided insight into their ability to mediate signal amplification, filter noise, and regulate signaling in time and space (Li et al , 2012b; Wu, 2013).…”

“…The inherent size heterogeneity of such higher‐order assemblies generates challenges for their structural and mechanistic characterization, but recent progress has provided insight into their ability to mediate signal amplification, filter noise, and regulate signaling in time and space (Li et al , 2012b; Wu, 2013). Some of these assemblies form via nucleation‐driven polymerization and result in stable complexes, and we are starting to understand the structural basis for their proximity‐enhanced activation (Korennykh et al , 2009; Yin et al , 2009; Li et al , 2012a; Lu et al , 2014). Other assemblies depend on transient interactions and are more labile, generating oligomers with broad size distributions (Rivas et al , 2000; Errington et al , 2012; Canzio et al , 2013).…”

NLRP3, un inflammasome sous contrôle

“…Les inflammasomes canoniques, qui sont eux, associés à la caspase-1, sont assemblés par des récepteurs cytosoliques de la famille des NLR (nucleotide-binding domain leucin-rich repeat) ou ALR (Aim2-like receptors). Ces récepteurs s'oligomérisent pour former une plateforme de nucléation pour des filaments formés par la protéine adaptatrice ASC (apoptosis-associated speck-like protein containing a CARD domain, aussi appelé PYCARD, CARD5, TMS1, Q9ULZ3), dont la surface permet, à son tour, la nucléation de filaments de pro-caspase-1 [4]. Ces inflammasomes forment des specks (ou structures ponctiformes) d'une taille de l'ordre du micron, visibles en microscopie, et qui sont uniques pour une cellule.…”