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

Abstract

In the fungus Podospora anserina, the [Het-s] prion induces programmed cell death by activating the HET-S pore-forming protein. The HET-s β-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 β-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 β-solenoid fold. Here, we analyze the ability of NWD2 to induce formation of the β-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 β-solenoid fold abolish templating activity. In vitro, the N-terminal motif assembles into infectious prion amyloids that display a structure resembling the β-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 β-solenoid fold in fungal NLR function further highlights the general importance of amyloid and prion-like signaling in immunity-related cell fate pathways.

On the evolutionary trajectories of signal-transducing amyloids in fungi and beyond

“…The R0 motif is essential for signal transducing by NWD2. 10 In the current model, when the NOD-like receptor is activated by the detection of a specific molecular signal, it oligomerizes bringing together the R0 motifs, which cooperatively adopt the b-solenoid fold. The molecular complex of activated NWD2 molecules then templates the unstructured PFD of HET-S to induce cell death.…”

“…15 The PFD of HET-S can be templated to adopt the b-solenoid fold either by the [Het-s] prion -a reaction occurring during the non-self recognition process -or by a NOD-like receptor (NLR) termed NWD2. 10,18 NLRs are intracellular molecular receptors involved in innate immunity in metazoans and are proposed to carry the same function in fungi. 19 NWD2 is encoded by the gene adjacent to het-S in the genome of P. anserina and this genomic clustering is conserved in a large number of ascomycetes.…”

“…In this context, recent reports expose a new functional role for amyloids as signal-transducing domains controlling programmed cell death. 10,11 Signal-transducing amyloids can be defined as motifs or domains adopting an amyloid fold exclusively in response to precise endogenous or exogenous signals and functioning as structural template or scaffold to bind and/or activate downstream proteins. Despite the uncertain phylogenetic relations between the amyloids involved in signal transducing, these domains represent the first described class of functional amyloids to be conserved in different kingdoms of life.…”

As a toxin dies a prion comes to life: A tentative natural history of the [Het-s] prion

“…19 This activity depends on the N-terminal HET-s-like motif and point mutations predicted to affect the b-solenoid fold affect prion inducing activity of NWD2. The NWD2 HET-s-like motif alone is sufficient to induce [Het-s] formation and is able to convert HET-S to the pore-forming state.…”

“…In all hets strains, the nwd2 gene is in an inactivated pseudo-gene form. 19 One possible explanation is that in the absence of a functional HET-S effector, NWD2 becomes useless. Effectively from this point on, duality emerged.…”