Fig. 3: NOD-like receptors (NLRs) and NLR-like proteins form a transkingdom gene superfamily with conserved roles in immunity and regulated cell death.

NLRs are defined by their central nucleotide-binding (NB) and oligomerization (NOD) domains. NODs generally belong to either the NB-ARC family or the NACHT family. A Founding members of the NB-ARC family include mammalian APAF-1 (Apoptotic protease activating factor 1), its homolog CED-4 (Cell death protein 4) from the roundworm Caenorhabditis elegans, and plant R (resistance) proteins. B Founding members of the NACHT family include three animal proteins – NAIP (Neuronal apoptosis inhibitory protein), CIITA (class II, major histocompatibility complex transactivator), and TEP1 (Telomerase associated protein 1) – and HET-E (heterokaryon gene), which is a protein encoded in the genome of the mold Podospora anserina. C NOD-like receptors encoded in the genome of the ascomycete Podospora anserina, a fungal model organism for the study of regulated cell death and NLR biology. The hnwd (HET-NACHT-WD40) gene family comprises five paralogs, three of which (het-r, het-d, and het-e) control heterokaryon incompatibility (HI). The family is characterized by the presence of a HET domain—likely related to the TIR (Toll/interleukin-1 receptor/resistance protein) domain—at the N-termini of its five members. The WD40 pseudo-repeats of the hnwd genes exhibit strong internal conservation and are proposed to evolve in a concerted manner. Repeats can be exchanged between alleles of the same gene or between different members of the gene family. D Interallelic genetic interactions define the het-e/het-c HI system in P. anserina. Different alleles of het-e (e.g., het-e1, het-e2, het-e3), which differ almost exclusively in the number and composition of their WD40 repeats, trigger HI in the presence of distinct alleles of the het-c gene. The het-c gene encodes a glycolipid transfer protein that is broadly conserved across fungi. Some functionally characterized het-e alleles (e.g., het-e4) appear inactive in heterokaryon incompatibility (HI) and typically carry a low number of WD40 repeats, while others (e.g., het-e2) can induce RCD in the presence of different het-c alleles. Experimental data have demonstrated that the specificity of recognition between het-e and het-c allelic variants depends on the WD40 domains of the NLRs, and the cell-death reaction relies on a functional HET/TIR domain. The het-D paralog is involved in a very similar HI system with het-c. E Cartoon representation of 45 P. anserina NOD-like receptors. Genes directly involved in heterokaryon incompatibility (HI) are highlighted in red, and those indirectly involved are shown in orange. Abbreviations: HEL (HeLo-like in fungal proteins), PAT (Patatin-like phospholipase), SBL (SesB-like lipase), GBL (Goodbye-like domain), UDP (UDP-glucose phosphorylase domain), HET (Heterokaryon determinants domain), ASM (Amyloid Signaling Motif), CARD (Caspase recruitment domain), BIR (Baculovirus IAP repeat), TR (Telomerase, Ro and Vault or TROVE domain), vWA (von Willebrand A domain), TPR (Tetratricopeptide repeat), ANK (Ankyrin repeat), and Unk (unknown).