Signal Transduction by a Fungal NOD-Like Receptor Based on Propagation of a Prion Amyloid Fold

In the fungus Podospora anserina, the [Het-s] prion induces programmed cell death by activating the HET-S pore-forming protein. The HET-s beta-solenoid prion fold serves as a template for converting the HET-S prion-forming domain into the same fold. This conversion, in turn, activates the HET-S pore-forming domain. The gene immediately adjacent to het-S encodes NWD2, a Nod-like receptor (NLR) with an N-terminal motif similar to the elementary repeat unit of the beta-solenoid fold. NLRs are immune receptors controlling cell death and host defense processes in animals, plants and fungi. We have proposed that, analogously to [Het-s], NWD2 can activate the HET-S pore-forming protein by converting its prion-forming region into the beta-solenoid fold. Here, we analyze the ability of NWD2 to induce formation of the beta-solenoid prion fold. We show that artificial NWD2 variants induce formation of the [Het-s] prion, specifically in presence of their cognate ligands. The N-terminal motif is responsible for this prion induction, and mutations predicted to affect the beta-solenoid fold abolish templating activity. In vitro, the N-terminal motif assembles into infectious prion amyloids that display a structure resembling the beta-solenoid fold. In vivo, the assembled form of the NWD2 N-terminal region activates the HET-S pore-forming protein. This study documenting the role of the beta-solenoid fold in fungal NLR function further highlights the general importance of amyloid and prion-like signaling in immunity-related cell fate pathways.