Functional Prions in Mammalian Innate Immune Signaling
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Pathogens and cellular danger signals activate mammalian cytosolic sensors such as RIG-I and NLRP3 which signal through respective adaptor proteins MAVS and ASC to produce robust innate immune and inflammatory responses. MAVS and ASC harbor N-terminal CARD and PYRIN domains, respectively, essential for their signaling ability. Using the Sup35 based yeast prion assay, we show that CARD and PYRIN function as bona fide prions in yeast when fused to Sup35C. In response to respective upstream sensors RIG-I and NLRP3, both CARD and PYRIN form self-perpetuating, SDS-resistant polymers that are inherited cytoplasmically through multiple cell divisions. Similar to other cases of prion switch, CARD exhibits nucleation- and polymerization-dependent prion conversion in yeast. Likewise, a yeast prion domain (NM) can functionally replace CARD and PYRIN in mammalian innate immune and inflammasome signaling. Mutations in MAVS and ASC that disrupt their prion activities in yeast also abrogate their ability to signal in mammalian cells. Furthermore, fibers of recombinant PYRIN can convert ASC into functional polymers capable of activating caspase-1. Remarkably, homologous domains from a conserved NOD- like receptor (NWD2) and classic prion (HET-s) in fungi can functionally reconstitute signaling of NLRP3 and ASC PYRINs in mammalian cells. These results indicate that prion- like polymerization is a conserved signal transduction mechanism in innate immunity and inflammation.