Formation of versus Recruitment to RNA-Rich Condensates: Controlling Effects Exerted by Peptide Side Chain Identity

Liquid-liquid phase separation (LLPS), the spontaneous formation of contiguous liquid phases with distinct compositions, has been long known in chemical systems and more recently recognized as a ubiquitous feature of cell biology. We describe a system involving biologically relevant components, synthetic peptides, and total yeast RNA, that has enabled us to explore factors that underlie phase separation. Coulombic complementarity between a cationic peptide and anionic RNA is necessary but not sufficient for formation of a condensed phase in our system. In addition to a net positive charge, the peptide must present the proper type of cationic moiety. Guanidinium groups, as found in the Arg side chain, support phase separation, but ammonium groups, as found in the Lys side chain, or dimethylguanidinium groups, as found in post-translationally modified Arg side chains, do not support phase separation in our system. However, the cationic groups that do not support phase separation via interaction with RNA can nevertheless enable recruitment to a condensed phase, which reveals that the network of forces governing condensed phase formation can differ from the network of forces governing recruitment to such a phase. We introduce a new method for measuring the concentrations of components in condensed phases based on fluorine-containing additives and F-19 NMR.

Automated summary

Liquid-liquid phase separation is a phenomenon where distinct liquid phases with different compositions spontaneously form, which has been known in chemical systems for a long time and recently recognized in cell biology as well. By using synthetic peptides and total yeast RNA, researchers have investigated the factors underlying phase separation and discovered the importance of Coulombic complementarity and the specific type of cationic moiety in the peptides.