Self-propagating amyloid as a critical regulator for diverse cellular functions.

Amyloids are β-sheet-rich fibrillar protein aggregates characterized by structural properties of self-propagation and strong resistance to detergent and proteinase. Although a number of causative proteins for neurodegenerative disorders are known to undergo amyloid formation, recent studies have revealed that amyloids may also play beneficial roles in cells. Cellular processes that could be regulated by amyloids are diverse and include translational regulation, programmed cell death and protein storage. Yeast prions of Mod5 and Mot3, non-Mendelian extra-chromosomal factors, also show amyloid-like biophysical properties and have recently been shown to confer host cells resistant to environmental stressors. Furthermore, yeast cells actively respond to environmental stress for fitness adaptation to environmental changes by converting soluble yeast prion proteins into their amyloid forms, allowing cells to survive under stress conditions. Therefore, amyloids are not simply the terminal end-products of protein misfolding but a growing body of evidence suggests that they may possess physiological roles by using their self-propagating properties. Here, we present an overview on recent progress of the research on such functional amyloids.