A Tale of an Intrinsically Disordered Tail: Learning Prion Protein Biophysics Through the Lens of Phase Separation

Cells contain membrane-enclosed organelles that compartmentalize cellular constituents and regulate biochemical processes. A growing body of exciting research now reveals that there is also an alternative mechanism of spatiotemporally-controlled intracellular compartmentalization and organization through biomolecular condensate formation via macromolecular phase separation of proteins and nucleic acids into noncanonical membraneless organelles with emergent material properties. These functional liquid-like biomolecular condensates can undergo aberrant irreversible phase transitions into gel-like or solid-like amyloid aggregates associated with a range of debilitating human diseases. Our longstanding interest in the biophysics of the prion protein (PrP) inspired us to investigate the phase behavior of the intrinsically disordered N-terminal tail of PrP that comprises an octapeptide repeat region possessing the essential sequence grammar necessary for phase separation. We showed that an intriguing disease-associated amber stop codon mutation (Y145Stop) of PrP, which yields a C-terminally truncated intrinsically disordered N-terminal fragment, spontaneously phase-separates into highly dynamic liquid droplets under physiological conditions (Agarwal et al., PNAS, 2021, 118, 45, e2100968118). Upon aging, these liquid droplets undergo a liquid-to-solid phase transition into highly ordered, beta-rich, amyloid-like aggregates that exhibit a characteristic autocatalytic self-templating behavior. The propensity for the aberrant phase transition is much lower for the full-length PrP, indicating an evolutionarily conserved protective role of the folded C-terminal domain from pathological phase transitions.