Acute stress and multicellular development alter the solubility of the <i>Dictyostelium</i> Sup35 ortholog ERF3

Among sequenced organisms, the genome of <i>Dictyostelium discoideum</i> is unique in that it encodes for a massive amount of repeat-rich sequences in the coding region of genes. This results in the <i>Dictyostelium</i> proteome encoding for thousands of repeat-rich proteins, with nearly 24% of the <i>Dictyostelium</i> proteome encoding Q/N-rich regions that are predicted to be prion like in nature. To begin investigating the role of prion-like proteins in <i>Dictyostelium</i>, we decided to investigate ERF3, the <i>Dictyostelium</i> ortholog of the well-characterized yeast prion protein Sup35. ERF3 lacks the Q/N-rich region required for prion formation in yeast, raising the question of whether this protein aggregates and has prion-like properties in <i>Dictyostelium</i>. Here, we found that ERF3 formed aggregates in response to acute cellular stress. However, unlike <i>bona fide</i> prions, we were unable to detect transmission of aggregates to progeny. We further found that aggregation of this protein is driven by the ordered C-terminal domain independently of the disordered N-terminal domain. Finally, we also observed aggregation of ERF3 under conditions that induce multicellular development, suggesting that this phenomenon may play a role in <i>Dictyostelium</i> development. Together, these findings suggest a role for regulated protein aggregation in <i>Dictyostelium</i> cells under stress and during development.IMPORTANCEPrion-like proteins have both beneficial and deleterious effects on cellular health, and many organisms have evolved distinct mechanisms to regulate the behaviors of these proteins. The social amoeba <i>Dictyostelium discoideum</i> contains the highest proportion of proteins predicted to be prion like and has mechanisms to suppress their aggregation. However, the potential roles and regulation of these proteins remain largely unknown. Here, we demonstrate that aggregation of the <i>Dictyostelium</i> translation termination factor ERF3 is induced by both acute cellular stress and by multicellular development. These findings imply that protein aggregation may have a regulated and functional role in the <i>Dictyostelium</i> stress response and during multicellular development.