Abstract
The solution properties of amino acids determine the folding, aggregation, and liquid–liquid phase separation (LLPS) behaviors of proteins. Various indices of amino acids, such as solubility, hydropathy, and conformational parameter, describe the behaviors of protein folding and solubility both in vitro and in vivo. However, understanding the propensity of LLPS and aggregation is difficult due to the multiple interactions among different amino acids. Here, the solubilities of aromatic amino acids (SAs) were investigated in solution containing 20 types of amino acids as amino acid solvents. The parameters of SAs in amino acid solvents (PSASs) were varied and dependent on the type of the solvent. Specifically, Tyr and Trp had the highest positive values while Glu and Asp had the lowest. The PSAS values represent soluble and insoluble interactions, which collectively are the driving force underlying the formation of droplets and aggregates. Interestingly, the PSAS of a soluble solvent reflected the affinity between amino acids and aromatic rings, while that of an insoluble solvent reflected the affinity between amino acids and water. These findings suggest that the PSAS can distinguish amino acids that contribute to droplet and aggregate formation, and provide a deeper understanding of LLPS and aggregation of proteins.
Highlights
Phase Separation of Proteins and Amino Acid CharacteristicsLiquid–liquid phase separation (LLPS) of proteins is a phenomenon is characterized by the formation of condensates consisting of homogeneously dispersed proteins with a composition different from that of the bulk material due to a stimulus or environmental change (Uversky, 2017; Boeynaems et al, 2018)
A soluble interaction represented by the parameters of SAs in amino acid solvents (PSASs) indicates that an amino acid is highly soluble in an amino acid solvent
There is an attractive interaction of the solute amino acid with the solvent amino acid, meaning that the water molecules are involved in the interaction
Summary
Liquid–liquid phase separation (LLPS) of proteins is a phenomenon is characterized by the formation of condensates consisting of homogeneously dispersed proteins with a composition different from that of the bulk material due to a stimulus or environmental change (Uversky, 2017; Boeynaems et al, 2018). The propensity of proteins to form droplets and aggregates, which is defined as the “propensity of phase separation (PPS)” in this paper, can be predicted to some extent by the length and types of amino acids in the IDR (Liu et al, 2019; Schuster et al, 2020). It is known that when Arg is present in a solvent, the solubility of solute molecules with aromatic rings increases because of the cation-π interaction between Arg in the solvent and the aromatic molecules of the solute (Arakawa et al, 2008; Hirano et al, 2010a,b; Ariki et al, 2011) In other words, it is the SA as a solute in a solvent containing Arg. the SA in an amino acid solvent is an index to predict the PPS because the interactions among amino acids influence solubility
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