Spontaneous Liquid Droplet-to-Gel Transition of Citrulline Polypeptide Complexed with Nucleic Acids.

Inside cells, proteins complex with nucleic acids to form liquid droplets resulting from liquid-liquid phase separation. The presence of mutated proteins can change the state of these liquid droplets to solids or gels, triggering neurodegenerative diseases. The mechanism of the liquid to solid or gel transition is still unclear. Solutions of poly(l-ornithine--l-citrulline) (POC) copolymers, which exhibit upper critical solution temperature-type behavior, change state upon cooling. In this study, we evaluated the effect of nucleic acids complexed with POC on phase changes. In the presence of nucleic acids, such as polyC and polyU, POC formed liquid droplets at low temperatures. The droplets dissolved at temperatures above the phase separation temperature. The phase separation temperature depended on the chemical structure of the nucleobase, implying that electrostatic and hydrogen bonding interactions between the nucleic acid and POC influenced phase separation. Furthermore, the liquid droplets spontaneously changed to gel-like precipitates due to spontaneous release of nucleic acids from the complex. The rate of the liquid droplet-to-gel transition depended on the magnitude of electrostatic and hydrogen bonding interactions between POC and nucleic acid. POC complexed with mRNA also underwent a liquid droplet-to-gel transition upon the release of mRNA. This work provides insights into the mechanism of pathogenic transitions of the cellular droplets.