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

Abstract

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.

Copper drives prion protein phase separation and modulates aggregation

“…In addition to the PM-bound main pool, we observed a small fraction of PrP C -YFP-GPI as highly mobile, small (diameter <1μm), round structures in the cytosol, often near to the PM ( Extended Data Fig. 1b ; Supplementary Video 1 ), which is consistent with previously reported cytosolic PrP C 29 . Time lapse imaging showed coalescence of cytosolic PrP C -YFP-GPI and the exchange of PrP C -YFP-GPI signal between cytosolic structures and the PM within seconds - in both directions ( Fig.…”

“…Protein condensation is driven by weak multivalent intra- and intermolecular interactions between residue side chains or protein structural elements 29 . Clients of protein condensates, such as other proteins, nucleic acids, or ions, can modulate condensate stability by promoting or disturbing these interactions.…”

“…Interestingly, PrP C :Cu 2+ condensates formed preferentially at cell-cell junctions, where PrP:Cu 2 ’ condensation could also be involved in copper exchange between cells. PrP was reported to interact with proteins in adherens junctions and desmosomal junctions 72,73 , and we recently suggested that the multivalency of these interactions could result in LLPS at cell-cell contacts 29 .…”

Targeting Biomolecular Condensation and Protein Aggregation against Cancer

“…It seems LARKS have evolved as a structural motif because they are present in different organisms, including H. sapiens , D. melanogaster , and M. tuberculosis . Sequence evaluation of other proteins, including the prion protein (PrP), brought speculations about the participation of LARKS in the PS of the PrP, which is one of the human proteome proteins enriched in these kinked segments …”

Preface: Special issue “Brain Proteostasis in Health and Disease”

“…The following two articles relate to an important biophysical property of cells that has attracted considerable interest recently: phase separation. The review article by Mariana Amaral, Yraima Cordeiro, and colleagues provides a comprehensive biochemical landscape of phase separation of the prion protein (PrP) (do Amaral et al, 2023). PrP is a scaffold protein enriched in the brain, whose cellular form (PrPc) works as a versatile platform for plasma membrane signaling transduction.…”