Nutrient and heat stress induce changes to the solubility of predicted prion-like proteins in Dictyostelium discoideum

Dictyostelium discoideum has a unique proteome among sequenced organisms that encodes nearly 10,000 homopolymeric amino acid tracts longer than 10 amino acids long. These repeats are composed of every amino acid except tryptophan with asparagine and glutamine being the most prevalent. Interestingly, asparagine and glutamine-rich regions of proteins are the hallmark of prion-like domains and nearly 25% of the Dictyostelium proteome is predicted to be prion-like in nature. Here we assessed the insoluble proteome upon heat stress or during nutrient stress which induces Dictyostelium development. We found that both heat and nutrient stress induce the accumulation of predicted prion-like proteins in the insoluble fraction; however, the overall amino acid composition of insoluble proteins is different depending on the stress with a greater percentage of predicted prion-like proteins becoming insoluble upon nutrient stress. We further confirmed that endogenous polyglutamine proteins accumulate in the insoluble fraction over the developmental time course and demonstrate that exogenously expressed polyglutamine tracts form puncta and have reduced solubility in a polyglutamine length dependent manner upon nutrient stress. Finally, using a proximity labeling approach, we found that exogenously expressed polyglutamine tracts have enhanced proximity to glutamine-rich proteins. During heat and nutrient stress Dictyostelium have numerous proteins that have decreased solubility. Among these proteins, predicted prion-like proteins are enriched and the presence of a polyglutamine tract is sufficient to cause decreased solubility in a polyglutamine length dependent manner.