Fig. 6: Necroptotic and pyroptotic pathways draw several evolutionary parallels between fungal and metazoan RCD machineries.

Fungal genomes encode a variety of putative molecular sensors controlling pore-forming proteins, such as fGSDMs. Some of these sensors belong to the NOD-like receptor (NLR) family. A subset of NLRs use signaling amyloids – some of which appear related to the metazoan necroptosis-controlling RHIM (RIP homotypic interaction motif) – to activate downstream membrane-targeting proteins. The latter are characterized by a helical bundle domain, named HeLo-like (HELL) or HeLo, evolutionary related to the four-helix bundle (4HB) of the necroptosis executioner protein MLKL (Mixed lineage kinase domain-like pseudokinase). The signaling mechanism is based on ‘amyloid templating’ or the transmission of structural information from the NLR-based signalosome to the RCD executioner protein. Certain NLR proteins carry a HeLo/HELL domain at their N-terminal end. This ‘all-in-one’ architecture is analogous to some plant NLRs, which carry a coiled-coil (CC) domain as an N-terminal effector/signaling module. Remarkably, some HELL domains are integral to proteins with kinase/pseudokinase domain, reminiscent of the original MLKL architecture. Similar diversity of molecular receptors has been uncovered in the fungal pyroptotic-like pathways. A subset of the fGSDM-controlling proteins are NLR sensors, carrying an S8 serine protease or a CHAT caspase-like domain. The protease domains can frequently be directly fused to superstructure-forming repeats (WD40, TPR or ANK) or other predicted kinase-like domains. Notably, the fungal necroptotic-like and pyroptotic-like RCD pathways are often encoded in two- or three-gene clusters, sharing thus some characteristics with prokaryotes. The number of such clusters can vary widely between species. A major open question regarding these RCD pathways in fungi is related to the nature of stimuli that activate the identified diverse putative sensors.