For centuries, the threat of prion disease has plagued populations – whether it be in the form of scrapie ravaging through the sheep populations of Spain in the eighteenth century, fatal familial insomnia afflicting families in Italy, or an outbreak of Creutzfeldt-Jakob disease in the UK triggered by the consumption of contaminated beef. The ability of prions to spread with such pathogenic intent whilst remaining incurable fuels the fire of public concern - yet delving beyond that classical view of prions has led to an enlightened appreciation of the role of such proteins throughout the biota of life.

Prion-like proteins subvert the dogma of the one-to-one structure-function relationship traditionally held onto when considering the physiology of proteins. Instead, these proteins display structural polymorphism, capable of adopting different conformations under different conditions; this expands the capabilities of protein function and represents a mechanism for epigenetic coding, with protein conformation possessing the ability convey and propagate specific states for biological advantage without the need for DNA modification. The properties observed in many of these proteins, including those of multivalency and low complexity, predispose them towards phase separation, whereby localised concentration of biomolecules compartmentalise cellular activity. This ability confers upon an organism the ability to respond to stimuli in dynamic and transient fashions without needing to use cumbersome membrane-bound organelles.

Using the Prion-Like Amino Acid Composition program, we have identified the stress-responsive protein ABU-13 as a potential prion in the nematode species, Caenorhabditis elegans. Phenotypic analysis of ABU-13 knockout animals has identified a non-redundant role for this protein in ER stress and innate immune responses – in keeping with previous evidence that this family of proteins is involved with a non-canonical unfolded protein response pathway of the endoplasmic reticulum. These knockout animals do not display defects in the activation of the stress-response gene hsp-4 upon tunicamycin treatment, suggesting that these effects occur independently of the canonical UPR pathway.

In vivo and in vitro characterisation of ABU-13 has demonstrated a propensity for coalescence, reminiscent of a phase-separated organelle. This is further supported by the ability of this protein to bind RNA – a common feature of prion-like proteins involved with such transitions. Co-immunoprecipitation of ABU-13 confirmed an enrichment of ER and RNA-related protein interactions, supporting a role for this protein in ER stress responses. In addition to this, a number of glycosylation related terms were identified, pointing towards a role for ABU-13 in the folding quality control of such proteins.

These punctated structures do, however, appear less mobile than traditional liquid-like condensates, as demonstrated by a slow FRAP recovery, indicative of a more hydrogel-like structure. To further investigate this, we extracted hydrogel-forming proteins from N2 animals using a biotinylated isoxazole precipitation, identifying a number of novel proteins that may be involved with in the formation of such solid-like structures. ABU-13, however, was not amongst this dataset, suggesting that if it were indeed involved in a physiological hydrogel, this transition is driven by another protein component.

Overall, we propose that ABU-13 represents a phase-separating prion-like protein capable of modulating ER stress and immune responses, potentially via the regulation of RNA processing.