Chaperone-mediated heterotypic phase separation regulates liquid-to-solid phase transitions into amyloid fibrils

Abstract

Biomolecular condensates formed via the phase separation of proteins and nucleic acids are thought to regulate a myriad of cellular processes with exquisite spatiotemporal precision. However, such highly dynamic, viscoelastic, mesoscopic, intracellular membraneless bodies can undergo aberrant liquid-to-solid transitions into a range of amyloid-like species. The formation of such pathological assemblies necessitates their clearance by the cellular protein quality control machinery comprising molecular chaperones. Nonetheless, the mechanism underlying the chaperone-mediated regulation of protein homeostasis within biomolecular condensates remains elusive. Here, we present a unique case demonstrating that a heat shock protein 40 (Hsp40), Ydj1, promotes the heterotypic phase separation of intrinsically disordered tau via intermolecular electrostatic and hydrophobic interactions. Through a diverse array of tools involving high-resolution fluorescence imaging, single-droplet steady-state and picosecond time-resolved fluorescence anisotropy, and single-molecule FRET (Forster resonance energy transfer), we elucidate the diverse structural conformations of tau present within phase-separated heterotypic condensates that are otherwise predisposed to aggregation. Our vibrational Raman spectroscopy and electron microscopy data show that the presence of Ydj1 in tau-Ydj1 condensates abolishes the formation of amyloid fibrils, unlike tau-only droplets. By sequentially deleting segments, we identify amyloidogenic hexapeptide motifs located in the hydrophobic microtubule-binding region of tau that foster contacts with the peptide-binding regions of Ydj1, promoting the formation of tau-Ydj1 binary condensates. Additionally, we show that the underlying network of interactions governing these condensates can be further tuned by RNA. Our results underscore an intriguing interplay of molecular drivers that govern chaperone-associated phase separation, with broader implications for the chaperoning of a wide range of intrinsically disordered proteins involved in physiology and disease.