Control of Aggregation, Coaggregation, and Liquid Droplet of Proteins Using Small Additives

This review article presents the concepts of aggregates, coaggregates, and a liquid droplet of proteins and compares the concentrated states in the presence of small additives to control their formation and dissociation. The aggregates composed of single protein molecules result mainly from hydrophobic interactions between unfolded protein molecules. Thus, the aggregation of protein can be effectively suppressed by small additives that increase the solubility of hydrophobic solutes, typically arginine (Arg) and chaotropes. In contrast, coaggregation that is composed of two or more types of proteins results from both hydrophobic and attractive electrostatic interactions between even partially unfolded protein molecules. Accordingly, coaggregation is more controllable than simple aggregation using various types of small additives, such as ions and osmolytes, as well as Arg and chaotropes. The liquid droplets of proteins observed in living cells and a protein-polyelectrolyte complex (PPC) undergo liquid-liquid phase separation driven only by electrostatic interactions. Thus, the liquid droplets and PPCs are redissolved when the concentration of ions is increased. The properties of electrostatic or hydrophobic interactions, solid-like or liquid-like states, and apparently spherical and amorphous structures are simple but valuable criteria that can be used to control protein aggregation and condensation using small additives.