Fission and Internal Fusion of Protocell with Membraneless "Organelles" Formed by Liquid-Liquid Phase Separation.

Abstract

Construction of protocells with hierarchical structures and living functions is still a great challenge. Growing evidence demonstrates that the membraneless organelles, which facilitate many essential cellular processes, are formed by RNA, protein, and other biopolymers via liquid-liquid phase separation (LLPS). The formation of the protocell in the early days of Earth could follow the same principle. In this work, we develop a novel coacervate-based protocell containing membraneless subcompartments via spontaneous liquid-liquid phase separation by simply mixing single-stranded oligonucleotides (ss-oligo), quaternized dextran (Q-dextran), and poly(l-lysine) (PLL) together. The resulting biphasic droplet, with PLL/ss-oligo phase being the internal subcompartments and Q-dextran/ss-oligo phase as the surrounding medium, is capable of sequestering and partitioning biomolecules into distinct regions. When the droplet is exposed to salt or Dextranase, the surrounding Q-dextran/ss-oligo phase will be gradually dissociated, resulting in the chaotic movement and fusion of internal subcompartments. Besides, the external electric field at a lower strength can drive the biphasic droplet to undergo a deviated circulation concomitant with the fusion of localized subcompartments, while a high-strength electric field can polarize the whole droplet, resulting in the release of daughter droplets in a controllable manner. Our study highlights that liquid-liquid phase separation of biopolymers is a powerful strategy to construct hierarchically structured protocells resembling the morphology and functions of living cells and provides a step toward a better understanding of the transition mechanism from nonliving to living matter under prebiotic conditions.