Phase separation of the mammalian prion protein: Physiological and pathological perspectives

Abnormal phase transitions have been implicated in the occurrence of proteinopathies. Disordered proteins with nucleic acidbinding ability drive the formation of reversible micron-sized condensates capable of controlling nucleic acid processing/transport. This mechanism, achieved via liquid-liquid phase separation (LLPS), underlies the formation of long-studied membraneless organelles (e.g., nucleolus) and various transient condensates formed by driver proteins. The prion protein (PrP) is not a classical nucleic acid-binding protein. However, it binds nucleic acids with high affinity, undergoes nucleocytoplasmic shuttling, contains a long intrinsically disordered region rich in glycines and evenly spaced aromatic residues, among other biochemical/biophysical properties of bona fide drivers of phase transitions. Because of this, our group and others have characterized LLPS of recombinant PrP. In vitro phase separation of PrP is modulated by nucleic acid aptamers, and depending on the aptamer conformation, the liquid droplets evolve to solid-like species. Herein, we discuss recent studies and previous evidence supporting PrP phase transitions. We focus on the central role of LLPS related to PrP physiology and pathology, with a special emphasis on the interaction of PrP with different ligands, such as proteins and nucleic acids, which can play a role in prion disease pathogenesis. Finally, we comment on therapeutic strategies directed at the non-functional phase separation that could potentially tackle prion diseases or other protein misfolding disorders.

Automated summary

Abnormal phase transitions are implicated in proteinopathies. Disordered proteins with nucleic acid-binding ability can drive the formation of reversible condensates, controlling nucleic acid processing/transport. The prion protein (PrP), although not a classical nucleic acid-binding protein, has properties similar to bona fide drivers of phase transitions. This article discusses recent studies and evidence supporting PrP phase transitions, emphasizing the central role of liquid-liquid phase separation (LLPS) in PrP physiology and pathology. The interaction of PrP with different ligands, including proteins and nucleic acids, is also explored, highlighting its potential role in prion disease pathogenesis. Therapeutic strategies targeting non-functional phase separation for the treatment of prion diseases and protein misfolding disorders are also discussed.