Plasticity in PYD assembly revealed by cryo-EM structure of the PYD filament of AIM2

Absent in melanoma 2 (AIM2) is an essential cytosolic double-stranded DNA receptor that assembles with the adaptor, apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC), and caspase-1 to form the AIM2 inflammasome, which leads to proteolytic maturation of cytokines and pyroptotic cell death. AIM2 contains an N-terminal Pyrin domain (PYD) that interacts with ASC through PYD/PYD interactions and nucleates ASC(PYD) filament formation. To elucidate the molecular basis of AIM2-induced ASC(PYD) polymerization, we generated AIM2(PYD) filaments fused to green fluorescent protein (GFP) and determined its cryo-electron microscopic (cryo-EM) structure. The map showed distinct definition of helices, allowing fitting of the crystal structure. Surprisingly, the GFP-AIM2(PYD) filament is a 1-start helix with helical parameters distinct from those of the 3-start ASC(PYD) filament. However, despite the apparent symmetry difference, helical net and detailed interface analyses reveal minimal changes in subunit packing. GFP-AIM2(PYD) nucleated ASC(PYD) filament formation in comparable efficiency as untagged AIM2(PYD), suggesting assembly plasticity in both AIM2(PYD) and ASC(PYD). The DNA-binding domain of AIM2 is able to form AIM2/DNA filaments, within which the AIM2(PYD) is brought into proximity to template ASC(PYD) filament assembly. Because ASC is able to interact with many PYD-containing receptors for the formation of inflammasomes, the observed structural plasticity may be critically important for this versatility in the PYD/PYD interactions.