Classification of proteins inducing liquid–liquid phase separation: sequential, structural and functional characterization

Liquid–liquid phase separation (LLPS) within the cell can form biological condensates, which are increasingly recognized to play important roles in various biological processes. Most proteins involved in LLPS are known to be intrinsically disordered proteins containing intrinsically disordered regions (IDRs) with low complexity regions (LCRs). The proteins driving LLPS were selected from databases of LLPS-related proteins and then classified into three classes according to the components in the condensates. Through in silico analyses, we found that proteins in the homo class, those that induce LLPS without partner molecules, have different IDRs and LCRs compared with the reference proteome. In contrast, proteins in the other classes, those that induce LLPS with partner proteins (the hetero class) or nucleic acids (the mixed class), did not show a clear difference to the reference proteome in IDRs and LCRs. The hetero-class proteins contained structural domains to serve protein–protein interactions, and the mixed-class ones had the structural domains associated with nucleic acids. These results suggest that IDRs in the homo-class proteins have unique IDRs, which provide multivalent interaction sites required for LLPS, whereas the hetero- and mixed-class proteins can induce LLPS through the combination of the interaction among LCRs, structural domains and nucleic acids.