Figure 1.

Protein phase transition states and interactions with C9orf72 arginine rich dipeptide repeat proteins (DPRs) and GGGGCC (G₄C₂) repeat RNA that lead to aberrant phase separation. (A) Dispersed—soluble proteins are freely dispersed in a dilute phase. (B) Proteins with intrinsically disordered regions such as low complexity domains (LCDs) can de-mix from a dilute phase often in the presence of RNA through liquid-liquid phase separation (LLPS) to form dynamic liquid droplets in which molecules can still diffuse in and out and disperse rapidly. (C) Liquid droplets can transition into more solid-like hydrogels which consist of amyloid-like fibrils, which are less dynamic but still reversable. (D) Aberrant phase separation facilitates the formation of pathological permanent amyloid fibrils which are the constituents of aggregates found in neurodegenerative disease brain. (E) Physiological interactions that mediate LLPS include pi (Π)–Π, cation–Π, polar (dipole-dipole), charge-charge interactions in addition to hydrophobic and Π-sp2 interactions, and hydrogen bonding (not shown). (F) The arginine-rich DPRs mimic physiological interactions driving aberrant phase separation with the arginine residues acting as the cation in electrostatic interactions, both charge-charge and with aromatic side chains within LCDs (cation-Π). The G₄C₂ repeat RNA can also undergo charge-charge interactions with LCDs; further G₄C₂ G-quadruplexes may enhance RNA-RNA and RNA-LCD interactions and nucleate droplet formation by cellular RNA.