Functional and structural characterization of zebrafish ASC.

The zebrafish genome encodes homologs for most of the proteins involved in inflammatory pathways; however, the molecular components and activation mechanisms of fish inflammasomes are largely unknown. ASC [apoptosis-associated speck-like protein containing a caspase-recruitment domain (CARD)] is the only adaptor involved in the formation of multiple types of inflammasomes. Here, we demonstrate that zASC is also involved in inflammasome activation in zebrafish. When overexpressed in vitro and in vivo in zebrafish, both the zASC and zASC pyrin domain (PYD) proteins form speck and filament structures. Importantly, the crystal structures of the N-terminal PYD and C-terminal CARD of zebrafish ASC were determined independently as two separate entities fused to maltose-binding protein. Structure-guided mutagenesis revealed the functional relevance of the PYD hydrophilic surface found in the crystal lattice. Finally, the fish caspase-1 homolog Caspy, but not the caspase-4/11 homolog Caspy2, interacts with zASC through homotypic PYD-PYD interactions, which differ from those in mammals. These observations establish the conserved and unique structural/functional features of the zASC-dependent inflammasome pathway. DATABASE: Structural data are available in the PDB under accession numbers 5GPP and 5GPQ.