Biomolecular Chemistry in Liquid Phase Separated Compartments

Abstract

Biochemical processes inside the cell take place in a complex environment that is highly crowded, heterogeneous, and replete with interfaces. The recently recognized importance of biomolecular condensates in cellular organization has added new elements of complexity to our understanding of chemistry in the cell. Many of these condensates are formed by liquid-liquid phase separation (LLPS) and behave like liquid droplets. Such droplet organelles can be reproduced and studied in vitro by using coacervates and have some remarkable features, including regulated assembly, differential partitioning of macromolecules, permeability to small molecules, and a uniquely crowded environment. Here, we review the main principles of biochemical organization in model membraneless compartments. We focus on some promising in vitro coacervate model systems that aptly mimic part of the compartmentalized cellular environment. We address the physicochemical characteristics of these liquid phase separated compartments, and their impact on biomolecular chemistry and assembly. These model systems enable a systematic investigation of the role of spatiotemporal organization of biomolecules in controlling biochemical processes in the cell, and they provide crucial insights for the development of functional artificial organelles and cells.

Keywords: artificial cells; coacervates; cytomimetic media; liquid-liquid phase separation; membraneless organelles.

Figure 1

Schematic phase diagrams for: (A) simple and symmetric complex coacervates with coexistence between a saturated dilute phase and a concentrated droplet phase. Condensation from the red cross can be induced by increasing the concentration (Δc) or lowering the temperature (ΔT), or salt concentration. Frequently, only the low-concentration branch of the binodal, or coexistence curve is shown. (B) Non-symmetric complex coacervates. Condensation can be induced by increasing the concentration of A, or reducing the concentration of B. (C) Cross-section through the two-phase region in (B) for varying mixing ratio, highlighting the re-entrant phase transition from one phase, via soluble complexes to two phases and back to one phase. (D) Control over coacervation by reversible post-translational modifications, like in Ddx4 and G3BP1. (E) RNA-dependent coacervation in FIB-1. (F) Effect of RNA on the density and viscosity of the coacervate phase of LAF-1.

Figure 2

(A) Schematic illustration of three scenarios for partitioning, depending on the relative free energy levels of the client molecule in both phases. (B) Illustration of five contributions to partitioning. (C) Possible effects of coacervate-based compartments on reaction kinetics.